Patent Publication Number: US-8536368-B2

Title: Processes for the production of acrylic acids and acrylates from a trioxane feed

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 13/251,623, which was filed on Oct. 3, 2011. The entirety of this application is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the production of acrylic acid via the aldol condensation reaction of an alkanoic acid and an alkylenating agent. More specifically, the present invention relates to the use of a specific alkylenating agent. 
     BACKGROUND OF THE INVENTION 
     α,β-unsaturated acids, particularly acrylic acid and methacrylic acid, and the ester derivatives thereof are useful organic compounds in the chemical industry. These acids and esters are known to readily polymerize or co-polymerize to form homopolymers or copolymers. Often the polymerized acids are useful in applications such as superabsorbents, dispersants, flocculants, and thickeners. The polymerized ester derivatives are used in coatings (including latex paints), textiles, adhesives, plastics, fibers, and synthetic resins. 
     Because acrylic acid and its esters have long been valued commercially, many methods of production have been developed. One exemplary acrylic acid ester production process utilizes: (1) the reaction of acetylene with water and carbon monoxide; and/or (2) the reaction of an alcohol and carbon monoxide, in the presence of an acid, e.g., hydrochloric acid, and nickel tetracarbonyl, to yield a crude product comprising the acrylate ester as well as hydrogen and nickel chloride. Another conventional process involves the reaction of ketene (often obtained by the pyrolysis of acetone or acetic acid) with formaldehyde, which yields a crude product comprising acrylic acid and either water (when acetic acid is used as a pyrolysis reactant) or methane (when acetone is used as a pyrolysis reactant). These processes have become obsolete for economic, environmental, or other reasons. 
     More recent acrylic acid production processes have relied on the gas phase oxidation of propylene, via acrolein, to form acrylic acid. The reaction can be carried out in single- or two-step processes but the latter is favored because of higher yields. The oxidation of propylene produces acrolein, acrylic acid, acetaldehyde and carbon oxides. Acrylic acid from the primary oxidation can be recovered while the acrolein is fed to a second step to yield the crude acrylic acid product, which comprises acrylic acid, water, small amounts of acetic acid, as well as impurities such as furfural, acrolein, and propionic acid. Purification of the crude product may be carried out by azeotropic distillation. Although this process may show some improvement over earlier processes, this process suffers from production and/or separation inefficiencies. In addition, this oxidation reaction is highly exothermic and, as such, creates an explosion risk. As a result, more expensive reactor design and metallurgy are required. Also, the cost of propylene is often prohibitive. 
     The aldol condensation reaction of formaldehyde and acetic acid and/or carboxylic acid esters has been disclosed in literature. This reaction forms acrylic acid and is often conducted over a catalyst. For example, condensation catalysts consisting of mixed oxides of vanadium and phosphorus were investigated and described in M. Ai,  J. Catal.,  107, 201 (1987); M. Ai,  J. Catal.,  124, 293 (1990); M. Ai,  Appl. Catal.,  36, 221 (1988); and M. Ai,  Shokubai,  29, 522 (1987). 
     US Patent Publication No 2012/0071688 discloses a process for preparing acrylic acid from methanol and acetic acid in which the methanol is partially oxidized to formaldehyde in a heterogeneously catalyzed gas phase reaction. The product gas mixture thus obtained and an acetic acid source are used to obtain a reaction gas input mixture that comprises acetic acid and formaldehyde. The acetic acid is used in excess over the formaldehyde. The formaldehyde present in reaction gas input mixture is aldol-condensed with the acetic acid via heterogeneous catalysis to form acrylic acid. Unconverted acetic acid still present alongside the acrylic acid in the product gas mixture is removed therefrom and is recycled to the reaction gas input mixture. The acetic acid conversions in the disclosed references, however, may leave room for improvement. 
     Although the aldol condensation reaction is disclosed, there has been little if any disclosure relating to: 1) the effects that the use of alternative alkylenating agents may have on overall reaction/separation system efficiency; or 2) separation schemes that may be employed to effectively provide purified acrylic acid from the aldol condensation crude product. 
     Thus, the need exists for a process for producing purified acrylate product, e.g., acrylic acid, which provides improvements in reaction and/or separation efficiencies. 
     The references mentioned above are hereby incorporated by reference. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention is described in detail below with reference to the appended drawings, wherein like numerals designate similar parts. 
         FIG. 1  is a schematic of an acrylic acid reaction/separation system in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic of an acrylic acid reaction/separation system in accordance with an embodiment of the present invention. 
         FIG. 3  is a schematic of an acrylic acid reaction/separation system in accordance with an embodiment of the present invention. 
         FIG. 4  is a graph showing conversions achieved using the process of the present invention. 
         FIG. 5  is a graph showing yields achieved using the process of the present invention. 
         FIG. 6  is a graph showing selectivities achieved using the process of the present invention. 
         FIG. 7  is a graph showing space time yields achieved using the process of the present invention. 
     
    
    
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention relates to a process for producing an acrylate product. The process comprises the step of reacting a reaction mixture comprising an alkanoic acid and an alkylenating agent composition under conditions effective to form a crude acrylate product. The alkylenating agent composition comprises at least 1 wt % of a ring-structured compound comprising at least two oxygen atoms and at least one carbon atom, e.g., trioxane and/or tetraoxane. The process further comprises the step of separating at least a portion of the crude acrylate product to form at least one alkylenating agent stream and at least one purified acrylate product stream comprising acrylate product. 
     In one embodiment, the invention relates to a process for producing an acrylate product. The process comprises the step of reacting a reaction mixture comprising an alkanoic acid, an alkylenating agent, less than 45 wt % water, and less than 5 wt % methanol under conditions effective to form a crude acrylate product comprising acrylate product and alkylenating agent. The reaction mixture may further comprise trioxane. 
     In one embodiment, the invention relates to a process for producing an acrylate product. The process comprises the step of reacting trioxane under conditions effective to form a finished alkylenating agent mixture. The process further comprises the step of reacting a reaction mixture comprising an alkanoic acid and at least a portion of the finished alkylenating agent mixture under conditions effective to form a crude acrylate product comprising acrylate product. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Introduction 
     Production of unsaturated carboxylic acids such as acrylic acid and methacrylic acid and the ester derivatives thereof via most conventional processes have been limited by economic and environmental constraints. In the interest of finding a new reaction path, the aldol condensation reaction of acetic acid and formaldehyde has been investigated. This reaction may yield a unique crude product that comprises, inter alia, a higher amount of (residual) formaldehyde, which is generally known to add unpredictability and problems to separation schemes. Although the aldol condensation reaction of acetic acid and formaldehyde is known, there has been little if any disclosure relating to the use of alternative alkylenating agents, e.g., trioxane, and how these alternative alkylenating agents may affect overall reaction zone and/or separation zone efficiency. 
     Trioxane and tetraoxane are known compounds. It has now been discovered that the use of ring-structured compounds such as trioxane and/or tetraoxane as alkylenating agent precursors and/or as a components of an alkylenating agent composition provides unexpected benefits (as compared conventional alkylenating agents, e.g., formaldehyde) in both the reaction zone and in the separation zone. For example, it has now been found that a ring-structured compound feed comprises significantly less water than a conventional formaldehyde feed, e.g., a formaldehyde feed stream from a methanol oxidation process. In one embodiment, the ring-structured compound, e.g., trioxane, is in the solid phase. Without being bound by theory, it is believed that water may inhibit the aldol condensation reaction. As a result of using alkylenating agent compositions that comprise lower amounts of water, this reaction inhibition may be reduced and overall reaction efficiencies may be improved. In addition, the use of a less hydrous alkylenating agent composition provides for a crude acrylate product that comprises less water. Because the crude acrylate product comprises lower amounts of water, the burden on the separation zone to yield a low-water finished product is reduced. Also, low-water ring-structured compounds, e.g., anhydrous ring-structured compounds, can be combined with other high-water alkylenating agents to form an alkylenating agent composition. The addition of trioxane has the benefits of: 1) decreasing the overall water content; and 2) increasing the concentration of alkylenating agent, thus upgrading the quality of the alkylenating agent composition (as compared to a methanol oxidation-formed alkylenating agent stream). By upgrading the alkylenating agent stream in this manner, the need for pre-reactor evaporators may be reduced or eliminated. 
     The present invention relates to a process for producing acrylic acid, methacrylic acid, and/or the salts and esters thereof. As used herein, acrylic acid, methacrylic acid, and/or the salts and esters thereof, collectively or individually, may be referred to as “acrylate product” or “acrylate products.” The use of the terms acrylic acid, methacrylic acid, or the salts and esters thereof, individually, does not exclude the other acrylate products, and the use of the term acrylate product does not require the presence of acrylic acid, methacrylic acid, and the salts and esters thereof. 
     In one embodiment, the process comprises the step of reacting a reaction mixture comprising an alkanoic acid and an alkylenating agent composition under conditions effective to form a crude acrylate product. The alkylenating agent composition comprises at least 1 wt % of a ring-structured compound comprising at least two oxygen atoms and at least one carbon atom, e.g., at least 5 wt %, at least 10 wt %, at least 25 wt %, at least 45 wt %, at least 46 wt %, at least 50 wt %, at least 75 wt %. In terms of ranges, the alkylenating agent composition may comprise from 1 wt % to 100 wt % ring-structured compound, e.g., from 5 wt % to 99 wt %, from 10 wt % to 90 wt %, or from 25 wt % to 75 wt %. The alkylenating agent composition may comprise pure or substantially pure ring-structured compound. In terms of upper limits, the alkylenating agent composition may comprise less than 99 wt % ring-structured compound, e.g., less than 90 wt % or less than 75 wt %. In one embodiment, the ring-structured compound may be selected from the group consisting of trioxane, tetraoxane, and mixtures thereof. In one embodiment, the ring-structured compound comprises trioxane, preferably 1,3,5-trioxane. In one embodiment, the ring-structured compound comprises tetraoxane. Preferably, the alkylenating agent composition is not formed via methanol oxidation. Without being bound by theory, it is believed that the ring-structured compound may undergo a decomposition reaction to form alkylenating agent(s), e.g., formaldehyde, formaldehyde polymers, and/or formaldehyde derivatives. These compounds may be formed in situ. The ring-structured compound may serve as an alkylenating agent precursor. As a result, a finished alkylenating agent stream comprising alkylenating agent(s) may be formed, which stream may then be reacted with alkanoic acid to form the crude acrylate product. 
     Because the (initial) alkylenating agent composition comprises the ring-structured compounds and alkylenating agent composition is a component of the reaction mixture, the (initial) reaction mixture may comprise the ring-structured compounds as well. In one embodiment, the reaction mixture comprises less than 55 wt % ring-structured compounds, e.g., less than 35 wt % or less than 25 wt %. In terms of ranges, the reaction mixture may comprise from 1 wt % to 55 wt % ring-structured compounds, e.g., from 5 wt % to 35 wt % or from 5 wt % to 25 wt %. In terms of lower limits, the reaction mixture may comprise at least 1 wt % ring-structured compounds, e.g., at least 3 wt % or at least 5 wt %. Weight percentages may be based on the total weight of the reaction mixture. 
     The reaction mixture may also comprise alkylenating agent, e.g., formaldehyde. The alkylenating agent may result from the decomposition of the ring-structured compounds and/or from another source of alkylenating agent, e.g., a formaldehyde recycle stream. In one embodiment, the reaction mixture may comprise less than 55 wt % alkylenating agent, e.g., less than 35 wt % or less than 25 wt %. In terms of ranges, the reaction mixture may comprise from 0.5 wt % to 55 wt % alkylenating agent, e.g., from 1 wt % to 35 wt % or from 5 wt % to 15 wt %. In terms of lower limits, the reaction mixture may comprise at least 0.5 wt % alkylenating agent, e.g., at least 1 wt % or at least 5 wt %. 
     In one embodiment, the process comprises the step of reacting, e.g., decomposing, a ring-structured compound, e.g., trioxane, under conditions effective to form a finished alkylenating agent mixture. The resultant finished alkylenating mixture may comprise alkylenating agent, e.g., formaldehyde. In one embodiment, ring-structured compound conversion is high, e.g., at least 40%, at least 50%, or at least 60%. The finished alkylenating mixture may comprise at least 35 wt % alkylenating agent, e.g., at least 55 wt % or at least 75 wt %. In terms of ranges, the finished alkylenating agent mixture may comprise from 35 wt % to 100 wt % alkylenating agent, e.g., from 55 wt % to 90 wt % or from 70 wt % to 80 wt %. In terms of upper limits, the finished alkylenating mixture may comprise less than 100 wt % alkylenating agent, e.g., less than 90 wt % or less than 80 wt %. In some embodiments, the alkylenating agent mixture comprises all alkylenating agent. The finished alkylenating agent mixture will have little, if any water. For example the crude alkylenating agent mixture may comprise less than 30 wt % water, e.g., less than 10 wt % or less than 1 wt %. 
     The (initial) alkylenating agent composition may comprise smaller amounts of formaldehyde, as compared to a conventional methanol oxidation formaldehyde feed stream. In one embodiment, the alkylenating agent composition comprises less than 55 wt % formaldehyde and/or methanol oxidation formaldehyde precursors, e.g., less than 50 wt % or less than 45 wt %. In one embodiment, methanol oxidation formaldehyde precursors are those compounds used to form formaldehyde via methanol oxidation. Exemplary methanol oxidation formaldehyde precursors include, but are not limited to, methanol and oxygen. Methanol oxidation formaldehyde precursors do not include the ring-structured components discussed herein. In one embodiment, the alkylenating agent composition comprises less than 30 wt % methanol oxidation by-products, e.g., less than 15 wt % or less than 1 wt %. Exemplary methanol oxidation by-products include carbon monoxide and carbon dioxide. 
     The alkylenating agent composition, in some cases, may comprise water. These amounts of water will be less than the amounts of water in conventional methanol oxidation product streams. In one embodiment, the alkylenating agent composition comprises less than 50 wt % water, e.g., less than 30 wt % or less than 1 wt %. In terms of ranges, the alkylenating agent composition may comprise from 0.1 wt % to 50 wt % water, e.g., from 1 wt % to 30 wt % or from 5 wt % to 30 wt %. In some embodiments, the alkylenating agent may comprise no water. In terms of lower limits, the alkylenating agent composition may comprise at least 0.1 wt % water, e.g., at least 1 wt % or at least 5 wt %. In one embodiment, the alkylenating agent composition is substantially anhydrous. 
     Because the alkylenating agent composition comprises these low amounts of water, the reaction mixture will comprise low amounts of water as well. In one embodiment, the reaction mixture comprises less than 45 wt % water, e.g., less than 20 wt % water, less than 5 wt % or less than 1 wt %. In terms of ranges, the reaction mixture may comprise from 0.1 wt % to 45 wt % water, e.g., from 0.5 wt % to 20 wt % or from 1 wt % to 5 wt %. In some embodiments, the reaction mixture may comprise no water. In terms of lower limits, the reaction mixture may comprise at least 0.1 wt % water, e.g., at least 0.5 wt % or at least 1 wt %. Weight percentages may be based on the total weight of the reaction mixture. 
     In one embodiment, the reaction mixture will comprise methanol, e.g., low amounts of methanol. In one embodiment, the reaction mixture comprises less than 5 wt % methanol, e.g., less than 4 wt %, less than 3 wt %, less than 1 wt % or less than 0.5 wt %. In terms of ranges, the reaction mixture may comprise from 0.05 wt % to 5 wt % methanol, e.g., from 0.1 wt % to 4 wt % or from 1 wt % to 3 wt %. In some embodiment, the reaction mixture may comprise no methanol. In terms of lower limits, the reaction mixture may comprise at least 0.05 wt % methanol, e.g., at least 0.1 wt % or at least 1 wt %. The amounts of water and/or methanol in the reaction mixture, in some embodiments, will be less than the amounts of water and/or methanol in conventional in conventional methanol oxidation product streams. When conventional methanol oxidation product streams are employed as alkylenating agent feed streams, at least a portion of the water contained therein may be removed prior to the condensation reaction, e.g., via evaporation. By employing the ring-structured compound, e.g., the trioxane in accordance, with the present invention, the need for pre-reactor evaporators may be reduced or eliminated. 
     The reaction mixture, in some embodiments, may further comprise oxygen. In one embodiment, the reaction mixture comprises from 0.5 wt % to 10 wt % oxygen, e.g., from 0.5 wt % to 6 wt %, from 0.5 wt % to 5 wt %, from 0.5 wt % to 4 wt %, from 1 wt % to 3 wt %, or from 1 wt % to 2 wt %. In terms of lower limits, the reaction mixture may comprise at least 0.5 wt % oxygen, e.g., at least 1 wt % or at least 3 wt %. In terms of upper limits, the reaction mixture may comprise less than 10 wt % oxygen, e.g., less than 6 wt % or less than 5 wt %. 
     As noted above, the reaction mixture further comprises alkanoic acid, e.g., acetic acid. In one embodiment, the reaction mixture comprises from 1 wt % to 40% alkanoic acid, e.g., from 1 wt % to 30 wt % or from 5 wt % to 20 wt %; and from 0.5 wt % to 35 wt % alkylenating agent, e.g., from 0.5 wt % to 25 wt % or from 1 wt % to 15 wt %. 
     In one embodiment, a non-reactive gas e.g., an inert gas, is supplied to the reactor. Examples of non-reactive gases include, but are not limited to, nitrogen, helium, argon, and methane. For example, the reaction mixture may comprise from 25 wt % to 97.5 wt % inert gas, e.g., from 33 wt % to 90 wt %, from 40 wt % to 90 wt % or from 65 wt % to 85 wt %. Preferably, the inert gas is nitrogen. In a preferred embodiment, higher amounts of nitrogen are employed to dilute the reaction mixture, e.g., nitrogen dilution conditions. For example, the reaction mixture may comprise at least 74 wt % nitrogen, e.g., at least 80 wt %, at least 85 wt %, or at least 90 wt %. 
     In one embodiment, a reactive gas (other than the reactant gases) is supplied to the reactant stream. Examples of reactive gases or vapors include, but are not limited to, carbon oxides, sulfur oxides, and alkyl halides. When reactive gases are added to the reactor, these gases, in some embodiments, may be added in stages throughout the catalyst bed at desired levels as well as feeding with the other feed components at the beginning of the reactors. The addition of these additional components may improve reaction efficiencies. 
     Preferred embodiments of the inventive process demonstrate a low selectivity to undesirable products, such as carbon monoxide and carbon dioxide. The selectivity to these undesirable products preferably is less than 30%, e.g., less than 20%, less than 15%, less than 10%, or less than 5%. 
     As a result of the use of the ring-structured compounds of the present invention, alkanoic acid conversion is greater than 30%, e.g., greater than 40% or greater than 50%. These conversions are at least as high as conversions achieved using methanol oxidation product streams as alkylenating agent feeds. With the inventive alkylenating agent compositions, however, separations are greatly improved due to the lower amounts of water to be removed. In most cases, the conversions of the present invention are higher than conversions achieved using methanol oxidation product streams as alkylenating agent feeds. 
     In one embodiment, the process further comprises the step of separating at least a portion of the crude acrylate product. The separating may form at least one derivative stream. In one embodiment, the crude acrylate product comprises formaldehyde. The formaldehyde may be formed from the decomposition of the ring-structured compound. The formaldehyde may be formaldehyde that did not react with alkanoic acid and, thus, was carried through into the crude acrylate product. In a preferred embodiment, the crude acrylate product is separated to form a recycle stream comprising formaldehyde. At least a portion of the recycle stream may be recycled to the reactor. In a preferred embodiment, at least a portion of the recycle stream is recycled and combined with the alkylenating agent composition to form a combined alkylenating agent composition comprising ring-structured compound and formaldehyde, e.g., formaldehyde from the recycle stream. The combined alkylenating agent composition may be fed to the reactor to form acrylate product. In one embodiment the formaldehyde is fed to the reactor without being combined with the ring-structured compound. 
     The ring-structured compound may be reacted, e.g., reacted in situ, to form alkylenating agent. In one embodiment, the process comprises the step of reacting the ring-structured compound, e.g., trioxane, under conditions effective to form a finished alkylenating agent mixture. In one embodiment, the reaction is a decomposition reaction. In one embodiment, the reaction is not conducted over a catalyst. In one embodiment, the reaction may be conducted over a catalyst. The catalyst may vary widely. In a preferred embodiment, the catalyst is an acidic catalyst. In one embodiment, the reaction is conducted without employing a methanol oxidation catalyst. In one embodiment, the ring-structured compound is reacted with acetic acid to form the finished alkylenating agent mixture comprising formaldehyde resulting from the decomposition of the ring-structured compound. In one embodiment, the ring-structured compound is reacted with sulfuric acid to form the finished alkylenating agent mixture. 
     In cases where the crude acrylate product comprises residual acid, the process may further comprise the step of separating the crude acrylate product to form an acid recycle stream comprising acid. The process may further comprise the step of recycling at least a portion of the acid recycle stream to the reactor in which the decomposition reaction is conducted. 
     Crude Acrylate Product 
     The aldol condensation reaction of the present invention, unlike most conventional acrylic acid-containing crude products, yields a crude acrylate product stream comprising acrylate product, a significant portion of at least one alkylenating agent, and low amounts of water and/or methanol. Preferably, the at least one alkylenating agent is the ring-structured compound discussed herein. For example, the crude product streams may comprise at least 0.5 wt % alkylenating agent(s), e.g., at least 1 wt %, at least 5 wt %, at least 7 wt %, at least 10 wt %, or at least 25 wt %. In terms of ranges, the crude product streams may comprise from 0.5 wt % to 50 wt % alkylenating agent(s), e.g., from 1 wt % to 45 wt %, from 1 wt % to 25 wt %, from 1 wt % to 10 wt %, or from 5 wt % to 10 wt %. In terms of upper limits, the crude product streams may comprise at most 50 wt % alkylenating agent(s), e.g., at most 45 wt %, at most 25 wt %, or at most 10 wt %. 
     The crude acrylate product streams comprise at least 1 wt % acrylate product, e.g., at least 5 wt % or at least 10 wt %. In terms of ranges, the crude product streams may comprise from 1 wt % to 75 wt % acrylate product, e.g., from 1 wt % to 50 wt %, from 5 wt % to 50 wt %, or from 10 wt % to 40 wt %. In terms of upper limits, the crude product streams may comprise at most 75 wt % alkylenating agent(s), e.g., at most 50 wt %, or at most 40 wt %. Preferably the acrylate product is acrylic acid. 
     In one embodiment, the crude product streams of the present invention further comprise water. For example, the crude product streams may comprise less than 50 wt % water, e.g., less than 40 wt %, less than 30 wt %, less than 25 wt %, less than 20 wt %, or less than 10 wt %. In terms of ranges, the crude product streams may comprise from 1 wt % to 50 wt % water, e.g., from 5 wt % to 40 wt %, from 10 wt % to 30 wt %, from 5 wt % to 20 wt %, or from 15 wt % to 25 wt %. In terms of lower limits, the crude product streams may comprise at least 1 wt % water, e.g., at least 5 wt %, at least 10 wt %, or at least 15 wt %. Because of the use of ring-structured compounds in the alkylenating agent feed, the water content in the crude product stream, beneficially, will be reduced, as compared to processes in which a conventional formaldehyde feed is employed. 
     In one embodiment, the crude product streams of the present invention comprise very little, if any, of the impurities found in most conventional acrylic acid crude product streams. For example, the crude product streams of the present invention may comprise less than 1000 wppm of such impurities (either as individual components or collectively), e.g., less than 500 wppm, less than 100 wppm, less than 50 wppm, or less than 10 wppm. Exemplary impurities include acetylene, ketene, beta-propiolactone, higher alcohols, e.g., C 2+ , C 3+ , or C 4+ , and combinations thereof. Importantly, the crude product streams of the present invention comprise very little, if any, furfural and/or acrolein. In one embodiment, the crude product streams comprise substantially no furfural and/or acrolein, e.g., no furfural and/or acrolein. In one embodiment, the crude product streams comprise less than less than 500 wppm acrolein, e.g., less than 100 wppm, less than 50 wppm, or less than 10 wppm. In one embodiment, the crude product streams comprise less than less than 500 wppm furfural, e.g., less than 100 wppm, less than 50 wppm, or less than 10 wppm. Furfural and acrolein are known to act as detrimental chain terminators in acrylic acid polymerization reactions. Also, furfural and/or acrolein are known to have adverse effects on the color of purified product and/or to subsequent polymerized products. 
     In addition to the acrylic acid and the alkylenating agent, the crude product streams may further comprise acetic acid, water, propionic acid, and light ends such as oxygen, nitrogen, carbon monoxide, carbon dioxide, methanol, methyl acetate, methyl acrylate, acetaldehyde, hydrogen, and acetone. In one embodiment, because the exemplary oxygen amounts are employed, carbon monoxide and/or carbon dioxide production in inhibited. For example, the crude acrylate product may comprise less than 20 wt % carbon monoxide and/or carbon dioxide, e.g., less than 15 wt %, less than 10 wt % or less than 5 wt %. In one embodiment, a weight ratio of carbon monoxide and carbon dioxide, combined, to acrylate product is less than 0.50, e.g., less than 0.46, less than 0.4, or less than 0.25. 
     Exemplary compositional data for the crude product streams are shown in Table 1. The compositional data in Table 1 reflects the composition of the crude product stream that is fed to the separation zone if 1) nitrogen dilution conditions are not employed; or 2) nitrogen dilution conditions are employed and at least a major portion, preferably substantially all, of the nitrogen used in the nitrogen dilution is removed from the crude product stream before being fed to the separation zone. Components other than those listed in Table 1 may also be present in the crude product streams. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 CRUDE ACRYLATE PRODUCT STREAM COMPOSITIONS 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Conc. 
                 Conc. 
                 Conc. 
                 Conc. 
               
               
                 Component 
                 (wt %) 
                 (wt %) 
                 (wt %) 
                 (wt %) 
               
               
                   
               
               
                 Acrylic Acid 
                 1 to 75 
                 1 to 50 
                 5 to 50 
                 10 to 40 
               
               
                 Alkylenating Agent(s) 
                 0.5 to 50 
                 1 to 45 
                 1 to 25 
                 1 to 10 
               
               
                 Acetic Acid 
                 1 to 90 
                 1 to 70 
                 5 to 50 
                 10 to 50 
               
               
                 Water 
                 1 to 50 
                 5 to 30 
                 5 to 20 
                 5 to 15 
               
               
                 Propionic Acid 
                 0.01 to 10 
                 0.1 to 10 
                 0.1 to 5 
                 0.1 to 1 
               
               
                 Oxygen 
                 0.01 to 10 
                 0.1 to 10 
                 0.1 to 5 
                 0.1 to 1 
               
               
                 Nitrogen 
                 0.1 to 20 
                 0.1 to 10 
                 0.5 to 5 
                 0.5 to 4 
               
               
                 Carbon Monoxide 
                 0.01 to 10 
                 0.1 to 10 
                 0.1 to 5 
                 0.5 to 3 
               
               
                 Carbon Dioxide 
                 0.01 to 10 
                 0.1 to 10 
                 0.1 to 5 
                 0.5 to 3 
               
               
                 Other Light Ends 
                 0.01 to 10 
                 0.1 to 10 
                 0.1 to 5 
                 0.5 to 3 
               
               
                   
               
            
           
         
       
     
     Any suitable reaction and/or separation scheme may be employed to form the crude product stream as long as the reaction provides the crude product stream components that are discussed above. For example, in some embodiments, the acrylate product stream is formed by contacting an alkanoic acid, e.g., acetic acid, or an ester thereof with an alkylenating agent, e.g., a methylenating agent, under conditions effective to form the crude acrylate product stream. The alkylenating agent may be formed, e.g., in situ, from the ring-structured compound. Preferably, the contacting is performed over a suitable catalyst. The crude product stream may be the reaction product of the alkanoic acid-alkylenating agent reaction. In a preferred embodiment, the crude product stream is the reaction product of the aldol condensation reaction of acetic acid and trioxane, which is conducted over a catalyst comprising vanadium and titanium. In one embodiment, the crude product stream is the product of a reaction where trioxane (and optionally acetic acid) generate formaldehyde in situ. The aldol condensation then follows. In one embodiment, a methanol-formaldehyde solution and the ring-structured compounds are reacted with acetic acid to form the crude product stream. 
     The alkanoic acid, or an ester of the alkanoic acid, may be of the formula R′—CH 2 —COOR, where R and R′ are each, independently, hydrogen or a saturated or unsaturated alkyl or aryl group. As an example, R and R′ may be a lower alkyl group containing for example 1-4 carbon atoms. In one embodiment, an alkanoic acid anhydride may be used as the source of the alkanoic acid. In one embodiment, the reaction is conducted in the presence of an alcohol, preferably the alcohol that corresponds to the desired ester, e.g., methanol. In addition to reactions used in the production of acrylic acid, the inventive catalyst, in other embodiments, may be employed to catalyze other reactions. 
     The alkanoic acid, e.g., acetic acid, may be derived from any suitable source including natural gas, petroleum, coal, biomass, and so forth. As examples, acetic acid may be produced via methanol carbonylation, acetaldehyde oxidation, ethylene oxidation, oxidative fermentation, and anaerobic fermentation. 
     As petroleum and natural gas prices fluctuate, becoming either more or less expensive, methods for producing acetic acid and intermediates such as methanol and carbon monoxide from alternate carbon sources have drawn increasing interest. In particular, when petroleum is relatively expensive compared to natural gas, it may become advantageous to produce acetic acid from synthesis gas (“syngas”) that is derived from any available carbon source. U.S. Pat. No. 6,632,752, which is hereby incorporated by reference, for example, teaches a method of retrofitting a methanol plant for the manufacture of acetic acid. By retrofitting a methanol plant, the large capital costs associated with carbon monoxide generation for a new acetic acid plant are significantly reduced or largely eliminated. All or part of the syngas is diverted from the methanol synthesis loop and supplied to a separator unit to recover carbon monoxide and hydrogen, which are then used to produce acetic acid. 
     In some embodiments, at least some of the raw materials for the above-described aldol condensation process may be derived partially or entirely from syngas. For example, the acetic acid may be formed from methanol and carbon monoxide, both of which may be derived from syngas. For example, the methanol may be formed by steam reforming syngas, and the carbon monoxide may be separated from syngas. In other embodiments, the methanol may be formed in a carbon monoxide unit, e.g., as described in EP2076480; EP1927380; EP2072490; EP1914219; EP1904426; EP2072487; E02072492; EP2072486; EP2060553; EP1741692; EP1907744; EP2060555; EP2186787; EP2072488; and U.S. Pat. No. 7,842,844, which are hereby incorporated by reference. Of course, this listing of methanol sources is merely exemplary and is not meant to be limiting. In addition, the above-identified methanol sources, inter alia, may be used to form the formaldehyde, e.g., in situ, which, in turn may be reacted with the acetic acid to form the acrylic acid. The syngas, in turn, may be derived from variety of carbon sources. The carbon source, for example, may be selected from the group consisting of natural gas, oil, petroleum, coal, biomass, and combinations thereof. Syngas or hydrogen may also be obtained from bio-derived methane gas, such as bio-derived methane gas produced by landfills or agricultural waste. 
     In another embodiment, in addition to the acetic acid formed via methanol carbonylation, some additional acetic acid may be formed from the fermentation of biomass and may be used in the hydrogenation step. The fermentation process preferably utilizes an acetogenic process or a homoacetogenic microorganism to ferment sugars to acetic acid producing little, if any, carbon dioxide as a by-product. The carbon efficiency for the fermentation process preferably is greater than 70%, greater than 80% or greater than 90% as compared to conventional yeast processing, which typically has a carbon efficiency of about 67%. Optionally, the microorganism employed in the fermentation process is of a genus selected from the group consisting of  Clostridium, Lactobacillus, Moorella, Thermoanaerobacter, Propionibacterium, Propionispera, Anaerobiospirillum , and  Bacteriodes , and in particular, species selected from the group consisting of  Clostridium formicoaceticum, Clostridium butyricum, Moorella thermoacetica, Thermoanaerobacter kivui, Lactobacillus delbrukii, Propionibacterium acidipropionici, Propionispera arboris, Anaerobiospirillum succinicproducens, Bacteriodes amylophilus  and  Bacteriodes ruminicola . Optionally in this process, all or a portion of the unfermented residue from the biomass, e.g., lignans, may be gasified to form hydrogen that may be used in the hydrogenation step of the present invention. Exemplary fermentation processes for forming acetic acid are disclosed in U.S. Pat. Nos. 6,509,180; 6,927,048; 7,074,603; 7,507,562; 7,351,559; 7,601,865; 7,682,812; and 7,888,082, the entireties of which are incorporated herein by reference. See also U.S. Pub. Nos. 2008/0193989 and 2009/0281754, the entireties of which are incorporated herein by reference. 
     Examples of biomass include, but are not limited to, agricultural wastes, forest products, grasses, and other cellulosic material, timber harvesting residues, softwood chips, hardwood chips, tree branches, tree stumps, leaves, bark, sawdust, off-spec paper pulp, corn, corn stover, wheat straw, rice straw, sugarcane bagasse, switchgrass, miscanthus, animal manure, municipal garbage, municipal sewage, commercial waste, grape pumice, almond shells, pecan shells, coconut shells, coffee grounds, grass pellets, hay pellets, wood pellets, cardboard, paper, plastic, and cloth. See, e.g., U.S. Pat. No. 7,884,253, the entirety of which is incorporated herein by reference. Another biomass source is black liquor, a thick, dark liquid that is a byproduct of the Kraft process for transforming wood into pulp, which is then dried to make paper. Black liquor is an aqueous solution of lignin residues, hemicellulose, and inorganic chemicals. 
     Methanol carbonylation processes suitable for production of acetic acid are described in U.S. Pat. Nos. 7,208,624, 7,115,772, 7,005,541, 6,657,078, 6,627,770, 6,143,930, 5,599,976, 5,144,068, 5,026,908, 5,001,259, and 4,994,608, all of which are hereby incorporated by reference. 
     U.S. Pat. No. RE 35,377, which is hereby incorporated by reference, provides a method for the production of methanol by conversion of carbonaceous materials such as oil, coal, natural gas and biomass materials. The process includes hydrogasification of solid and/or liquid carbonaceous materials to obtain a process gas which is steam pyrolized with additional natural gas to form syn gas. The syn gas is converted to methanol which may be carbonylated to acetic acid. U.S. Pat. No. 5,821,111, which discloses a process for converting waste biomass through gasification into syn gas, as well as U.S. Pat. No. 6,685,754 are hereby incorporated by reference. 
     In one optional embodiment, the acetic acid that is utilized in the condensation reaction comprises acetic acid and may also comprise other carboxylic acids, e.g., propionic acid, esters, and anhydrides, as well as acetaldehyde and acetone. In one embodiment, the acetic acid fed to the condensation reaction comprises propionic acid. For example, the acetic acid fed to the reaction may comprise from 0.001 wt % to 15 wt % propionic acid, e.g., from 0.001 wt % to 0.11 wt %, from 0.125 wt % to 12.5 wt %, from 1.25 wt % to 11.25 wt %, or from 3.75 wt % to 8.75 wt %. Thus, the acetic acid feed stream may be a cruder acetic acid feed stream, e.g., a less-refined acetic acid feed stream. 
     As used herein, “alkylenating agent” means a compound suitable for reacting with the alkanoic acid, e.g., acetic acid, to form an unsaturated acid, e.g., acrylic acid, or an alkyl acrylate. In one embodiment, the alkylenating agent may comprise a methylenating agent such as formaldehyde, which preferably is capable of adding a methylene group (═CH 2 ) to the organic acid. Other alkylenating agents may include, for example, acetaldehyde, propanal, butanal, aryl aldehydes, benzyl aldehydes, alcohols, and combinations thereof. This listing is not exclusive and is not meant to limit the scope of the invention. In one embodiment, the ring-structured compound may serve as a source of the alkylenating agent. For example, the trioxane may be reacted in situ to form the alkylenating agent, e.g., the aldehyde. 
     In some embodiments, alkylenating agent may be present in the alkylenating agent composition along with the ring-structured compound. The alkylenating agent may be derived from any suitable source. When formaldehyde is utilized along with the ring-structured compounds, exemplary sources may include, for example, aqueous formaldehyde solutions, anhydrous formaldehyde derived from a formaldehyde drying procedure, diether of methylene glycol, and paraformaldehyde. 
     In some embodiments, as discussed herein, the alkylenating agent composition may comprise a compound that is a source of formaldehyde. Where forms of formaldehyde that are not as freely or weakly complexed are used, the formaldehyde may form in situ in the condensation reactor or in a separate reactor prior to the condensation reactor. Thus for example, trioxane may be decomposed over an inert material or in an empty tube at temperatures over 750° C. or over an acid catalyst at over 100° C. to form the formaldehyde. 
     In one embodiment, an alkylenating agent corresponding to Formula I may be employed in conjunction with the ring-structured compounds. 
     
       
         
         
             
             
         
       
     
     In Formula I, R 5  and R 6  may be independently selected from C 1 -C 12  hydrocarbons, preferably, C 1 -C 12  alkyl, alkenyl or aryl, or hydrogen. Preferably, R 5  and R 6  are independently C 1 -C 6  alkyl or hydrogen, with methyl and/or hydrogen being most preferred. X may be either oxygen or sulfur, preferably oxygen; and n is an integer from 1 to 10, preferably 1 to 3. In some embodiments, m is 1 or 2, preferably 1. 
     In one embodiment, the compound of formula I may be the product of an equilibrium reaction between formaldehyde and methanol in the presence of water. In such a case, the compound of formula I may be a suitable formaldehyde source. In one embodiment, the formaldehyde source includes any equilibrium composition. Examples of formaldehyde sources include but are not restricted to methylal (1,1 dimethoxymethane); polyoxymethylenes —(CH 2 —O) i — wherein i is from 1 to 100; formalin; and other equilibrium compositions such as a mixture of formaldehyde, methanol, and methyl propionate. In one embodiment, the source of formaldehyde is selected from the group consisting of 1,1 dimethoxymethane; higher formals of formaldehyde and methanol; and CH 3 —O—(CH 2 —O) i —CH 3  where i is 2. 
     The alkylenating agent may be used with or without an organic or inorganic solvent. 
     As discussed above, in some embodiments, the alkylenating agent that is reacted with the alkanoic acid may be provided to the process in the form of a finished alkylenating agent stream, e.g., the alkylenating agent may be formed from the ring-structured compound and then condensed. In cases where alkylenating agent is fed in conjunction with the ring-structured compound, the alkylenating agent may be an impure alkylenating agent stream that comprises alkylenating agent, e.g., formaldehyde, and at least one other impurity, e.g., water and/or methanol. In one embodiment where alkylenating agent is used in the alkylenating agent composition, the alkylenating agent stream comprises formalin. The term “formalin,” refers to a mixture of formaldehyde, methanol, and water. In one embodiment, formalin comprises from 37 wt % to 55 wt % formaldehyde, from 44 wt % to 60 wt % water, and from 0.01 wt % to 25 wt % methanol. In cases where a mixture of formaldehyde, methanol, and methyl propionate is used, the mixture comprises less than 10 wt % water, e.g., less than 5 wt % or less than 1 wt %. In accordance with the present invention, the impure alkylenating agent may be dehydrated to reduce impurity content therein, e.g., to remove water from the impure alkylenating agent stream. However, as noted above, the use of the ring-structured compounds in the alkylenating agent feed composition, in many cases, will reduce or eliminate the need for pre-reactor dehydration. 
     In some embodiments, the condensation reaction may achieve favorable conversion of acetic acid and favorable selectivity and productivity to acrylates. For purposes of the present invention, the term “conversion” refers to the amount of acetic acid in the feed that is converted to a compound other than acetic acid. Conversion is expressed as a percentage based on acetic acid in the feed. The conversion of acetic acid may be at least 10%, e.g., at least 20%, at least 40%, or at least 50%. 
     Selectivity, as it refers to the formation of acrylate product, is expressed as the ratio of the amount of carbon in the desired product(s) and the amount of carbon in the total products. This ratio may be multiplied by 100 to arrive at the selectivity. Preferably, the catalyst selectivity to acrylate products, e.g., acrylic acid and methyl acrylate, is at least 40 mol %, e.g., at least 50 mol %, at least 60 mol %, or at least 70 mol %. In some embodiments, the selectivity to acrylic acid is at least 30 mol %, e.g., at least 40 mol %, or at least 50 mol %; and/or the selectivity to methyl acrylate is at least 10 mol %, e.g., at least 15 mol %, or at least 20 mol %. 
     The terms “productivity” or “space time yield” as used herein, refers to the grams of a specified product, e.g., acrylate products, formed per hour during the condensation based on the liters of catalyst used. A productivity of at least 20 grams of acrylate product per liter catalyst per hour, e.g., at least 100 grams of acrylates per liters catalyst per hour or at least 200 grams of acrylates per liter catalyst per hour, is preferred. In terms of ranges, the productivity preferably is from 20 to 1000 grams of acrylates per liter catalyst per hour, e.g., from 20 to 700 grams of acrylates per liter catalyst per hour or from 100 to 600 grams of acrylates per liter catalyst per hour or from 200 to 600 grams of acrylates per liter catalyst per hour or from 70 to 700 grams of acrylates per liter catalyst per hour. 
     In one embodiment, the inventive process yields at least 1,800 kg/hr of finished acrylic acid, e.g., at least 3,500 kg/hr, at least 18,000 kg/hr, or at least 37,000 kg/hr. 
     Formation of alkanes, e.g., ethane, may be low, and ideally less than 2%, less than 1%, or less than 0.5% of the acetic acid passed over the catalyst is converted to alkanes, which have little value other than as fuel. 
     In some embodiments, the alkanoic acid, e.g., acetic acid, and the alkylenating agent, e.g., trioxane, are fed to the respective reactor at a molar ratio of at least 0.10:1, e.g., at least 0.75:1 or at least 1:1. In terms of ranges the molar ratio of alkanoic acid to alkylenating agent may range from 0.10:1 to 10:1 or from 0.75:1 to 5:1. In some embodiments, the reaction of the alkanoic acid and the alkylenating agent is conducted with a stoichiometric excess of alkanoic acid. In these instances, acrylate selectivity may be improved. As an example, the acrylate selectivity may be at least 10% higher than a selectivity achieved when the reaction is conducted with an excess of alkylenating agent, e.g., at least 20% higher or at least 30% higher. In other embodiments, the reaction of the alkanoic acid and the alkylenating agent is conducted with a stoichiometric excess of alkylenating agent. 
     The condensation reaction may be conducted at a temperature of at least 250° C., e.g., at least 70° C., or at least 750° C. In terms of ranges, the reaction temperature may range from 200° C. to 500° C., e.g., from 250° C. to 400° C., or from 250° C. to 750° C. Residence time in the reactor may range from 1 second to 200 seconds, e.g., from 1 second to 100 seconds. Reaction pressure is not particularly limited, and the reaction is typically performed near atmospheric pressure. In one embodiment, the reaction may be conducted at a pressure ranging from 0 kPa to 4100 kPa, e.g., from 3 kPa to 345 kPa, or from 6 kPa to 103 kPa. The acetic acid conversion, in some embodiments, may vary depending upon the reaction temperature. 
     In one embodiment, the reaction is conducted at a gas hourly space velocity (“GHSV”) greater than 600 hr −1 , e.g., greater than 1000 hr −1  or greater than 2000 hr −1 . In one embodiment, the GHSV ranges from 600 hr 1  to 10000 hr −1 , e.g., from 1000 hr −1  to 8000 hr −1  or from 1500 hr −1  to 7500 hr −1 . As one particular example, when GHSV is at least 2000 hr −1 , the acrylate product STY may be at least 150 g/hr/liter. 
     In one embodiment, the unreacted components such as the alkanoic acid and formaldehyde as well as the inert or reactive gases that remain are recycled after sufficient separation from the desired product. 
     When the desired product is an unsaturated ester made by reacting an ester of an alkanoic acid with trioxane and/or formaldehyde, the alcohol corresponding to the ester may also be fed to the respective reaction zone either with or separately from the other components. For example, when methyl acrylate is desired, methanol may be fed to the reactor. The alcohol, amongst other effects, reduces the quantity of acids leaving the reactor. It is not necessary that the alcohol is added at the beginning of the reactor and it may for instance be added in the middle or near the back, in order to effect the conversion of acids such as propionic acid, methacrylic acid to their respective esters without depressing catalyst activity. In one embodiment, the alcohol may be added downstream of the reactor. 
     Catalyst Composition 
     The catalyst may be any suitable catalyst composition. As one example, condensation catalyst consisting of mixed oxides of vanadium and phosphorus have been investigated and described in M. Ai,  J. Catal.,  107, 201 (1987); M. Ai,  J. Catal.,  124, 293 (1990); M. Ai,  Appl. Catal.,  36, 221 (1988); and M. Ai,  Shokubai,  29, 522 (1987). Other examples include binary vanadium-titanium phosphates, vanadium-silica-phosphates, and alkali metal-promoted silicas, e.g., cesium- or potassium-promoted silicas. 
     In a preferred embodiment, the inventive process employs a catalyst composition comprising vanadium, titanium, and optionally at least one oxide additive. The oxide additive(s), if present, are preferably present in the active phase of the catalyst. In one embodiment, the oxide additive(s) are selected from the group consisting of silica, alumina, zirconia, and mixtures thereof or any other metal oxide other than metal oxides of titanium or vanadium. Preferably, the molar ratio of oxide additive to titanium in the active phase of the catalyst composition is greater than 0.05:1, e.g., greater than 0.1:1, greater than 0.5:1, or greater than 1:1. In terms of ranges, the molar ratio of oxide additive to titanium in the inventive catalyst may range from 0.05:1 to 20:1, e.g., from 0.1:1 to 10:1, or from 1:1 to 10:1. In these embodiments, the catalyst comprises titanium, vanadium, and one or more oxide additives and have relatively high molar ratios of oxide additive to titanium. 
     In another embodiment, the inventive process employs a catalyst comprising vanadium, titanium, bismuth, tungsten, or mixtures thereof. In some embodiments, the catalyst comprises bismuth. In other embodiments, the catalyst comprises tungsten. Exemplary catalyst compositions include vanadium/titanium/bismuth, vanadium/titanium/tungsten, bismuth/tungsten, and vanadium/bismuth/tungsten. 
     In other embodiments, the catalyst may further comprise other compounds or elements (metals and/or non-metals). For example, the catalyst may further comprise phosphorus and/or oxygen. In these cases, the catalyst may comprise from 15 wt. % to 45 wt. % phosphorus, e.g., from 20 wt. % to 35 wt. % or from 23 wt. % to 27 wt. %; and/or from 30 wt. % to 75 wt. % oxygen, e.g., from 35 wt. % to 65 wt. % or from 48 wt. % to 51 wt. %. 
     In some embodiments, the catalyst further comprises additional metals and/or oxide additives. These additional metals and/or oxide additives may function as promoters. If present, the additional metals and/or oxide additives may be selected from the group consisting of copper, molybdenum, tungsten, nickel, niobium, and combinations thereof. Other exemplary promoters that may be included in the catalyst of the invention include lithium, sodium, magnesium, aluminum, chromium, manganese, iron, cobalt, calcium, yttrium, ruthenium, silver, tin, barium, lanthanum, the rare earth metals, hafnium, tantalum, rhenium, thorium, bismuth, antimony, germanium, zirconium, uranium, cesium, zinc, and silicon and mixtures thereof. Other modifiers include boron, gallium, arsenic, sulfur, halides, Lewis acids such as BF 3 , ZnBr 2 , and SnCl 4 . Exemplary processes for incorporating promoters into catalyst are described in U.S. Pat. No. 5,364,824, the entirety of which is incorporated herein by reference. 
     If the catalyst comprises additional metal(s) and/or metal oxides(s), the catalyst optionally may comprise additional metals and/or metal oxides in an amount from 0.001 wt. % to 30 wt. %, e.g., from 0.01 wt. % to 5 wt. % or from 0.1 wt. % to 5 wt. %. If present, the promoters may enable the catalyst to have a weight/weight space time yield of at least 25 grams of acrylic acid/gram catalyst-h, e.g., least 50 grams of acrylic acid/gram catalyst-h, or at least 100 grams of acrylic acid/gram catalyst-h. 
     In some embodiments, the catalyst is unsupported. In these cases, the catalyst may comprise a homogeneous mixture or a heterogeneous mixture as described above. In one embodiment, the homogeneous mixture is the product of an intimate mixture of vanadium and titanium oxides, hydroxides, and phosphates resulting from preparative methods such as controlled hydrolysis of metal alkoxides or metal complexes. In other embodiments, the heterogeneous mixture is the product of a physical mixture of the vanadium and titanium phosphates. These mixtures may include formulations prepared from phosphorylating a physical mixture of preformed hydrous metal oxides. In other cases, the mixture(s) may include a mixture of preformed vanadium pyrophosphate and titanium pyrophosphate powders. 
     In another embodiment, the catalyst is a supported catalyst comprising a catalyst support in addition to the vanadium, titanium, oxide additive, and optionally phosphorous and oxygen, in the amounts indicated above (wherein the molar ranges indicated are without regard to the moles of catalyst support, including any vanadium, titanium, oxide additive, phosphorous or oxygen contained in the catalyst support). The total weight of the support (or modified support), based on the total weight of the catalyst, preferably is from 75 wt. % to 99.9 wt. %, e.g., from 78 wt. % to 97 wt. % or from 80 wt. % to 95 wt. %. The support may vary widely. In one embodiment, the support material is selected from the group consisting of silica, alumina, zirconia, titania, aluminosilicates, zeolitic materials, mixed metal oxides (including but not limited to binary oxides such as SiO 2 —Al 2 O 3 , SiO 2 —TiO 2 , SiO 2 —ZnO, SiO 2 —MgO, SiO 2 —ZrO 2 , Al 2 O 3 —MgO, Al 2 O 3 —TiO 2 , Al 2 O 3 —ZnO, TiO 2 —MgO, TiO 2 —ZrO 2 , TiO 2 —ZnO, TiO 2 —SnO 2 ) and mixtures thereof, with silica being one preferred support. In embodiments where the catalyst comprises a titania support, the titania support may comprise a major or minor amount of rutile and/or anatase titanium dioxide. Other suitable support materials may include, for example, stable metal oxide-based supports or ceramic-based supports. Preferred supports include silicaceous supports, such as silica, silica/alumina, a Group IIA silicate such as calcium metasilicate, pyrogenic silica, high purity silica, silicon carbide, sheet silicates or clay minerals such as montmorillonite, beidellite, saponite, pillared clays, other microporous and mesoporous materials, and mixtures thereof. Other supports may include, but are not limited to, iron oxide, magnesia, steatite, magnesium oxide, carbon, graphite, high surface area graphitized carbon, activated carbons, and mixtures thereof. These listings of supports are merely exemplary and are not meant to limit the scope of the present invention. 
     In some embodiments, a zeolitic support is employed. For example, the zeolitic support may be selected from the group consisting of montmorillonite, NH 4  ferrierite, H-mordenite-PVOx, vermiculite-1, H-ZSM5, NaY, H-SDUSY, Y zeolite with high SAR, activated bentonite, H-USY, MONT-2, HY, mordenite SAR 20, SAPO-34, Aluminosilicate (X), VUSY, Aluminosilicate (CaX), Re-Y, and mixtures thereof. H-SDUSY, VUSY, and H-USY are modified Y zeolites belonging to the faujasite family. In one embodiment, the support is a zeolite that does not contain any metal oxide modifier(s). In some embodiments, the catalyst composition comprises a zeolitic support and the active phase comprises a metal selected from the group consisting of vanadium, aluminum, nickel, molybdenum, cobalt, iron, tungsten, zinc, copper, titanium cesium bismuth, sodium, calcium, chromium, cadmium, zirconium, and mixtures thereof. In some of these embodiments, the active phase may also comprise hydrogen, oxygen, and/or phosphorus. 
     In other embodiments, in addition to the active phase and a support, the inventive catalyst may further comprise a support modifier. A modified support, in one embodiment, relates to a support that includes a support material and a support modifier, which, for example, may adjust the chemical or physical properties of the support material such as the acidity or basicity of the support material. In embodiments that use a modified support, the support modifier is present in an amount from 0.1 wt. % to 50 wt. %, e.g., from 0.2 wt. % to 25 wt. %, from 0.5 wt. % to 15 wt. %, or from 1 wt. % to 8 wt. %, based on the total weight of the catalyst composition. 
     In one embodiment, the support modifier is an acidic support modifier. In some embodiments, the catalyst support is modified with an acidic support modifier. The support modifier similarly may be an acidic modifier that has a low volatility or little volatility. The acidic modifiers may be selected from the group consisting of oxides of Group IVB metals, oxides of Group VB metals, oxides of Group VIB metals, iron oxides, aluminum oxides, and mixtures thereof. In one embodiment, the acidic modifier may be selected from the group consisting of WO 3 , MoO 3 , Fe 2 O 3 , Cr 2 O 3 , V 2 O 5 , MnO 2 , CuO, Co 2 O 3 , Bi 2 O 3 , TiO 2 , ZrO 2 , Nb 2 O 5 , Ta 2 O 5 , Al 2 O 3 , B 2 O 3 , P 2 O 5 , and Sb 2 O 3 . 
     In another embodiment, the support modifier is a basic support modifier. The presence of chemical species such as alkali and alkaline earth metals, are normally considered basic and may conventionally be considered detrimental to catalyst performance. The presence of these species, however, surprisingly and unexpectedly, may be beneficial to the catalyst performance. In some embodiments, these species may act as catalyst promoters or a necessary part of the acidic catalyst structure such in layered or sheet silicates such as montmorillonite. Without being bound by theory, it is postulated that these cations create a strong dipole with species that create acidity. 
     Additional modifiers that may be included in the catalyst include, for example, boron, aluminum, magnesium, zirconium, and hafnium. 
     As will be appreciated by those of ordinary skill in the art, the support materials, if included in the catalyst of the present invention, preferably are selected such that the catalyst system is suitably active, selective and robust under the process conditions employed for the formation of the desired product, e.g., acrylic acid or alkyl acrylate. Also, the active metals and/or pyrophosphates that are included in the catalyst of the invention may be dispersed throughout the support, coated on the outer surface of the support (egg shell) or decorated on the surface of the support. In some embodiments, in the case of macro- and meso-porous materials, the active sites may be anchored or applied to the surfaces of the pores that are distributed throughout the particle and hence are surface sites available to the reactants but are distributed throughout the support particle. 
     The inventive catalyst may further comprise other additives, examples of which may include: molding assistants for enhancing moldability; reinforcements for enhancing the strength of the catalyst; pore-forming or pore modification agents for formation of appropriate pores in the catalyst, and binders. Examples of these other additives include stearic acid, graphite, starch, cellulose, silica, alumina, glass fibers, silicon carbide, and silicon nitride. Preferably, these additives do not have detrimental effects on the catalytic performances, e.g., conversion and/or activity. These various additives may be added in such an amount that the physical strength of the catalyst does not readily deteriorate to such an extent that it becomes impossible to use the catalyst practically as an industrial catalyst. 
     Separation 
     The unique crude acrylate stream of the present invention may be separated in a separation zone to form a final product, e.g., a final acrylic acid product.  FIG. 1  is a flow diagram depicting the formation of the crude acrylate stream and the separation thereof to obtain acrylate product  118 . Acrylate product system  100  comprises reaction zone  102  and separation zone  104 . Reaction zone  102  comprises reactor  106 , alkanoic acid feed, e.g., acetic acid feed,  108 , alkylenating agent composition feed, e.g., trioxane feed  110 , and vaporizer  112 . 
     Acetic acid and trioxane are fed to vaporizer  112  via lines  108  and  110 , respectively, to create a vapor feed stream, which exits vaporizer  112  via line  114  and is directed to reactor  106 . In one embodiment, lines  108  and  110  may be combined and jointly fed to the vaporizer  112 . The temperature of the vapor feed stream in line  114  is preferably from 200° C. to 600° C., e.g., from 250° C. to 500° C. or from 340° C. to 425° C. Alternatively, a vaporizer may not be employed and the reactants may be fed directly to reactor  106 . 
     Any feed that is not vaporized may be removed from vaporizer  112  and may be recycled or discarded. In addition, although line  114  is shown as being directed to the upper half of reactor  106 , line  114  may be directed to the middle or bottom of reactor  106 . Further modifications and additional components to reaction zone  102  and separation zone  104  are described below. 
     Reactor  106  contains the catalyst that is used in the reaction to form crude acrylate product, which is withdrawn, preferably continuously, from reactor  106  via line  116 . Although  FIG. 1  shows the crude acrylate product being withdrawn from the bottom of reactor  106 , the crude acrylate product may be withdrawn from any portion of reactor  106 . Exemplary composition ranges for the crude acrylate product are shown in Table 1 above. 
     In one embodiment, one or more guard beds (not shown) may be used upstream of the reactor to protect the catalyst from poisons or undesirable impurities contained in the feed or return/purge streams. Such guard beds may be employed in the vapor or liquid acrylate streams. Suitable guard bed materials may include, for example, carbon, silica, alumina, ceramic, or resins. In one aspect, the guard bed media is functionalized, e.g., silver functionalized, to trap particular species such as sulfur or halogens. 
     The crude acrylate stream in line  116  is fed to separation zone  104 . Separation zone  104  may comprise one or more separation units, e.g., two or more or three or more. Separation zone  104  separates the liquid acrylate stream to yield a finished acrylate product, which exits via line  118 . 
       FIG. 2  shows an overview of an alternative reaction/separation scheme in accordance with the present invention. Acrylate product system  200  comprises reaction zone  202  and separation zone  204 . Reaction zone  202  comprises reactor  206 , alkanoic acid feed, e.g., acetic acid feed,  208 , alkylenating agent composition feed, e.g., trioxane feed,  210 , vaporizer  212 , and line  214 . Reaction zone  202  and the components thereof function in a manner similar to reaction zone  102  of  FIG. 1 . Reactor  206  contains the catalyst that is used in the reaction to form crude acrylate product, which is withdrawn, preferably continuously, from reactor  206  via line  216 . 
     Reaction zone  202  yields a crude acrylate product, which exits reaction zone  202  via line  216  and is directed to separation zone  204 . The components of the crude acrylate product are discussed above. 
     As shown in  FIG. 2 , separation zone  204  contains multiple columns. Separation zone  204  comprises alkylenating agent split unit  232 , acrylate product split unit  234 , drying unit  236 , and methanol removal unit  238 . In one embodiment, the inventive process comprises the step of separating at least a portion of the crude acrylate stream to form an alkylenating agent stream and an intermediate product stream. This separating step may be referred to as the “akylenating agent split.” 
     Exemplary compositional ranges for the intermediate acrylate product stream are shown in Table 2. Components other than those listed in Table 2 may also be present in the intermediate acrylate product stream. Examples include methanol, methyl acetate, methyl acrylate, dimethyl ketone, carbon dioxide, carbon monoxide, oxygen, nitrogen, and acetone. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 INTERMEDIATE ACRYLATE PRODUCT 
               
               
                 STREAM COMPOSITION 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Acrylic Acid 
                 at least 5 
                 5 to 99 
                 35 to 65 
               
               
                 Acetic Acid 
                 less than 95 
                 5 to 90 
                 20 to 60 
               
               
                 Water 
                 less than 25 
                 &lt;20 
                 &lt;15 
               
               
                 Alkylenating Agent 
                  &lt;1 
                  &lt;0.5 
                  &lt;0.1 
               
               
                 Propionic Acid 
                 &lt;10 
                 0.01 to 5 
                 0.01 o 1 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the alkylenating agent stream comprises significant amounts of alkylenating agent(s). For example, the alkylenating agent stream may comprise at least 1 wt. % alkylenating agent(s), e.g., at least 5 wt. %, at least 10 wt. %, at least 15 wt. %, or at least 25 wt. %. In terms of ranges, the alkylenating stream may comprise from 1 wt. % to 75 wt. % alkylenating agent(s), e.g., from 3 to 50 wt. %, from 3 wt. % to 25 wt. %, or from 10 wt. % to 20 wt. %. In terms of upper limits, the alkylenating stream may comprise less than 75 wt. % alkylenating agent(s), e.g. less than 50 wt. % or less than 40 wt. %. In preferred embodiments, the alkylenating agent is formaldehyde. 
     As noted above, the presence of alkylenating agent in the crude acrylate stream adds unpredictability and problems to separation schemes. Without being bound by theory, it is believed that formaldehyde reacts in many side reactions with water to form by-products. The following side reactions are exemplary.
 
CH 2 O+H 2 O→HOCH 2 OH
 
HO(CH 2 O i-1 H+HOCH 2 OH→HO(CH 2 O) i H+H 2 O for  i&gt; 1
 
     Without being bound by theory, it is believed that, in some embodiments, as a result of these reactions, the alkylenating agent, e.g., formaldehyde, acts as a “light” component at higher temperatures and as a “heavy” component at lower temperatures. The reaction(s) are exothermic. Accordingly, the equilibrium constant increases as temperature decreases and decreases as temperature increases. At lower temperatures, the larger equilibrium constant favors methylene glycol and oligomer production and formaldehyde becomes limited, and, as such, behaves as a heavy component. At higher temperatures, the smaller equilibrium constant favors formaldehyde production and methylene glycol becomes limited. As such, formaldehyde behaves as a light component. In view of these difficulties, as well as others, the separation of streams that comprise water and formaldehyde cannot be expected to behave as a typical two-component system. These features contribute to the unpredictability and difficulty of the separation of the unique crude acrylate stream of the present invention. 
     The present invention, surprisingly and unexpectedly, achieves effective separation of alkylenating agent(s) from the inventive crude acrylate stream to yield a purified product comprising acrylate product and very low amounts of other impurities. 
     In one embodiment, the alkylenating split is performed such that a lower amount of acetic acid is present in the resulting alkylenating stream. Preferably, the alkylenating agent stream comprises little or no acetic acid. As an example, the alkylenating agent stream, in some embodiments, comprises less than 50 wt. % acetic acid, e.g., less than 45 wt. %, less than 25 wt. %, less than 10 wt. %, less than 5 wt. %, less than 3 wt. %, or less than 1 wt. %. Surprisingly and unexpectedly, the present invention provides for the lower amounts of acetic acid in the alkylenating agent stream, which, beneficially reduces or eliminates the need for further treatment of the alkylenating agent stream to remove acetic acid. In some embodiments, the alkylenating agent stream may be treated to remove water therefrom, e.g., to purge water. 
     In some embodiments, the alkylenating agent split is performed in at least one column, e.g., at least two columns or at least three columns. Preferably, the alkylenating agent is performed in a two column system. In other embodiments, the alkylenating agent split is performed via contact with an extraction agent. In other embodiments, the alkylenating agent split is performed via precipitation methods, e.g., crystallization, and/or azeotropic distillation. Of course, other suitable separation methods may be employed either alone or in combination with the methods mentioned herein. 
     The intermediate product stream comprises acrylate products. In one embodiment, the intermediate product stream comprises a significant portion of acrylate products, e.g., acrylic acid. For example, the intermediate product stream may comprise at least 5 wt. % acrylate products, e.g., at least 25 wt. %, at least 40 wt. %, at least 50 wt. %, or at least 60 wt. %. In terms of ranges, the intermediate product stream may comprise from 5 wt. % to 99 wt. % acrylate products, e.g. from 10 wt. % to 90 wt. %, from 25 wt. % to 75 wt. %, or from 35 wt. % to 65 wt. %. The intermediate product stream, in one embodiment, comprises little if any alkylenating agent. For example, the intermediate product stream may comprise less than 1 wt. % alkylenating agent, e.g., less than 0.1 wt. % alkylenating agent, less than 0.05 wt. %, or less than 0.01 wt. %. In addition to the acrylate products, the intermediate product stream optionally comprises acetic acid, water, propionic acid and other components. 
     In some cases, the intermediate acrylate product stream comprises higher amounts of alkylenating agent. For example, in one embodiment, the intermediate acrylate product stream comprises from 1 wt. % to 50 wt. % alkylenating agent, e.g., from 1 wt. % to 10 wt. % or from 5 wt. % to 50 wt. %. In terms of limits, the intermediate acrylate product stream may comprise at least 1 wt. % alkylenating agent, e.g., at least 5 wt. % or at least 10 wt. %. 
     In one embodiment, the crude acrylate stream is optionally treated, e.g. separated, prior to the separation of alkylenating agent therefrom. In such cases, the treatment(s) occur before the alkylenating agent split is performed. In other embodiments, at least a portion of the intermediate acrylate product stream may be further treated after the alkylenating agent split. As one example, the crude acrylate stream may be treated to remove light ends therefrom. This treatment may occur either before or after the alkylenating agent split, preferably before the alkylenating agent split. In some of these cases, the further treatment of the intermediate acrylate product stream may result in derivative streams that may be considered to be additional purified acrylate product streams. In other embodiments, the further treatment of the intermediate acrylate product stream results in at least one finished acrylate product stream. 
     In one embodiment, the inventive process operates at a high process efficiency. For example, the process efficiency may be at least 10%, e.g., at least 20% or at least 35%. In one embodiment, the process efficiency is calculated based on the flows of reactants into the reaction zone. The process efficiency may be calculated by the following formula.
 
Process Efficiency=2 N   HAcA   /[N   HOAc   +N   Trioxane   +N   H2O ]
 
     where: 
     N HAcA  is the molar production rate of acrylate products; and 
     N HOAC , N HCHO , and N H2O  are the molar feed rates of acetic acid, trioxane, and water, if any. 
     In other embodiments, the intermediate acrylate product stream comprises higher amounts of alkylenating agent. For example, the intermediate acrylate product stream may comprise from 1 wt. % to 10 wt. % alkylenating agent, e.g., from 1 wt. % to 8 wt. % or from 2 wt. % to 5 wt. %. In one embodiment, the intermediate acrylate product stream comprises greater than 1 wt. % alkylenating agent, e.g., greater than 5 wt. % or greater than 10 wt. %. 
     Exemplary compositional ranges for the alkylenating agent stream are shown in Table 3. Components other than those listed in Table 3 may also be present in the alkylenating agent stream. Examples include methanol, methyl acetate, methyl acrylate, dimethyl ketone, carbon dioxide, carbon monoxide, oxygen, nitrogen, and acetone. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 ALKYLENATING AGENT STREAM COMPOSITION 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Acrylic Acid 
                 less than 15 
                 0.01 to 10 
                 0.1 to 5 
               
               
                 Acetic Acid 
                 10 to 65 
                 20 to 65 
                 25 to 55 
               
               
                 Water 
                 15 to 75 
                 25 to 65 
                 30 to 60 
               
               
                 Alkylenating Agent 
                 at least 1 
                 1 to 75 
                 10 to 20 
               
               
                 Propionic Acid 
                 &lt;10 
                 0.01 5 
                 0.02 1 
               
               
                   
               
            
           
         
       
     
     In other embodiments, the alkylenating stream comprises lower amounts of acetic acid. For example, the alkylenating agent stream may comprise less than 10 wt. % acetic acid, e.g., less than 5 wt. % or less than 1 wt. %. 
     As shown in Table 3, the alkylenating agent stream may comprise significant amounts of alkylenating agent. In some embodiments, the alkylenating agent stream may be recycled to the reactor. In one embodiment, the alkylenating agent stream is recycled and combined with the alkylenating agent composition that is fed to the reactor. 
     As mentioned above, the crude acrylate product of the present invention comprises little, if any, furfural and/or acrolein. As such the derivative stream(s) of the crude acrylate products will comprise little, if any, furfural and/or acrolein. In one embodiment, the derivative stream(s), e.g., the streams of the separation zone, comprises less than less than 500 wppm acrolein, e.g., less than 100 wppm, less than 50 wppm, or less than 10 wppm. In one embodiment, the derivative stream(s) comprises less than less than 500 wppm furfural, e.g., less than 100 wppm, less than 50 wppm, or less than 10 wppm. 
     Separation zone  204  may also comprise a light ends removal unit (not shown). For example, the light ends removal unit may comprise a condenser and/or a flasher. The light ends removal unit may be configured either upstream of the alkylenating agent split unit. Depending on the configuration, the light ends removal unit removes light ends from the crude acrylate stream, the alkylenating stream, and/or the intermediate acrylate product stream. In one embodiment, when the light ends are removed, the remaining liquid phase comprises the acrylic acid, acetic acid, alkylenating agent, and/or water. 
     Alkylenating agent split unit  232  may comprise any suitable separation device or combination of separation devices. For example, alkylenating agent split unit  232  may comprise a column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In other embodiments, alkylenating agent split unit  232  comprises a precipitation unit, e.g., a crystallizer and/or a chiller. Preferably, alkylenating agent split unit  232  comprises a single distillation column. 
     In another embodiment, the alkylenating agent split is performed by contacting the crude acrylate product with a solvent that is immiscible with water. For example, alkylenating agent split unit  232  may comprise at least one liquid-liquid extraction column. In another embodiment, the alkylenating agent split is performed via azeotropic distillation, which employs an azeotropic agent. In these cases, the azeotropic agent may be selected from the group consisting of methyl isobutylketene, o-xylene, toluene, benzene, n-hexane, cyclohexane, p-xylene, and mixtures thereof. This listing is not exclusive and is not meant to limit the scope of the invention. In another embodiment, the alkylenating agent split is performed via a combination of distillations, e.g., standard distillation, and crystallization. Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 2 , alkylenating agent split unit  232  comprises first column  244 . The liquid acrylate stream in line  216 ′ is directed to first column  244 . First column  244  separates the liquid acrylate stream to form a distillate in line  240  and a residue in line  242 . The distillate may be refluxed and the residue may be boiled up as shown. Stream  240  comprises at least 1 wt % alkylenating agent. As such, stream  240  may be considered an alkylenating agent stream. The first column residue exits first column  244  in line  242  and comprises a significant portion of acrylate product. As such, stream  242  is an intermediate product stream. In one embodiment, at least a portion of stream  240  is directed to drying unit  236 . 
     Exemplary compositional ranges for the distillate and residue of first column  244  are shown in Table 4. Components other than those listed in Table 4 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 FIRST COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 less than 5 
                 less than 3 
                 0.05 to 1 
               
               
                 Acetic Acid 
                 less than 10 
                 less than 5 
                 0.5 to 3 
               
               
                 Water 
                 40 to 90 
                 45 to 85 
                 50 to 80 
               
               
                 Alkylenating Agent 
                 at least 1 
                 1 to 75 
                 10 to 40 
               
               
                 Propionic Acid 
                 less than 10 
                 less than 5 
                 less than 1 
               
               
                 Methanol 
                 less than 5 
                 less than 1 
                 less than 0.5 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 10 to 80 
                 15 to 65 
                 20 to 50 
               
               
                 Acetic Acid 
                 40 to 80 
                 45 to 70 
                 50 to 65 
               
               
                 Water 
                 1 to 40 
                 1 to 20 
                 1 to 10 
               
               
                 Alkylenating Agent 
                 at least 1 
                 1 to 50 
                 1 to 10 
               
               
                 Propionic Acid 
                 less than 10 
                 less than 5 
                 less than 1 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the first distillate comprises smaller amounts of acetic acid, e.g., less than 25 wt %, less than 10 wt %, e.g., less than 5 wt % or less than 1 wt %. In one embodiment, the first residue comprises larger amounts of alkylenating agent. In some embodiments, the streams that contain a significant amount of alkylenating agent may be recycled to the reactor. In one embodiment, the first distillate is recycled and combined with the alkylenating agent composition that is fed to the reactor. 
     In some embodiments, the intermediate acrylate product stream comprises higher amounts of alkylenating agent, e.g., greater than 1 wt % greater than 5 wt % or greater than 10 wt %. 
     For convenience, the distillate and residue of the first column may also be referred to as the “first distillate” or “first residue.” The distillates or residues of the other columns may also be referred to with similar numeric modifiers (second, third, etc.) in order to distinguish them from one another, but such modifiers should not be construed as requiring any particular separation order. 
     In one embodiment, polymerization inhibitors and/or anti-foam agents may be employed in the separation zone, e.g., in the units of the separation zone. The inhibitors may be used to reduce the potential for fouling caused by polymerization of acrylates. The anti-foam agents may be used to reduce potential for foaming in the various streams of the separation zone. The polymerization inhibitors and/or the anti-foam agents may be used at one or more locations in the separation zone. 
     In cases where any of alkylenating agent split unit  232  comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 300 kPa, e.g., from 10 kPa to 100 kPa or from 40 kPa to 80 kPa. In preferred embodiments, the pressure at which the column(s) are operated is kept at a low level e.g., less than 100 kPa, less than 80 kPa, or less than 60 kPa. In terms of lower limits, the column(s) may be operated at a pressures of at least 1 kPa, e.g., at least 20 kPa or at least 40 kPa. Without being bound by theory, it is believed that alkylenating agents, e.g., formaldehyde, may not be sufficiently volatile at lower pressures. Thus, maintenance of the column pressures at these levels surprisingly and unexpectedly provides for efficient separation operations. In addition, it has surprisingly and unexpectedly been found that by maintaining a low pressure in the columns of alkylenating agent split unit  232  may inhibit and/or eliminate polymerization of the acrylate products, e.g., acrylic acid, which may contribute to fouling of the column(s). 
     In one embodiment, the alkylenating agent split is achieved via one or more liquid-liquid extraction units. Preferably, the one or more liquid-liquid extraction units employ one or more extraction agents. Multiple liquid-liquid extraction units may be employed to achieve the alkylenating agent split. Any suitable liquid-liquid extraction devices used for multiple equilibrium stage separations may be used. Also, other separation devices, e.g., traditional columns, may be employed in conjunction with the liquid-liquid extraction unit(s). 
     In one embodiment (not shown), the crude acrylate product is fed to a liquid-liquid extraction column where the crude acrylate product is contacted with an extraction agent, e.g., an organic solvent. The liquid-liquid extraction column extracts the acids, e.g., acrylic acid and acetic acid, from the crude acrylate product. An aqueous phase comprising water, alkylenating agent, and some acetic acid exits the liquid-liquid extraction unit. Small amounts of acrylic acid may also be present in the aqueous stream. The aqueous phase may be further treated and/or recycled. An organic phase comprising acrylic acid, acetic acid, and the extraction agent also exits the liquid-liquid extraction unit. The organic phase may also comprise water and formaldehyde. The acrylic acid may be separated from the organic phase and collected as product. The acetic acid may be separated then recycled and/or used elsewhere. The solvent may be recovered and recycled to the liquid-liquid extraction unit. 
     The inventive process further comprises the step of separating the intermediate acrylate product stream to form a finished acrylate product stream and a first finished acetic acid stream. The finished acrylate product stream comprises acrylate product(s) and the first finished acetic acid stream comprises acetic acid. The separation of the acrylate products from the intermediate product stream to form the finished acrylate product may be referred to as the “acrylate product split.” 
     Returning to  FIG. 2 , intermediate product stream  242  exits alkylenating agent split unit  232  and is directed to acrylate product split unit  234  for further separation, e.g., to further separate the acrylate products therefrom. Acrylate product split unit  234  may comprise any suitable separation device or combination of separation devices. For example, acrylate product split unit  234  may comprise at least one column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In other embodiments, acrylate product split unit  234  comprises a precipitation unit, e.g., a crystallizer and/or a chiller. Preferably, acrylate product split unit  234  comprises two standard distillation columns as shown in  FIG. 2 . In another embodiment, acrylate product split unit  234  comprises a liquid-liquid extraction unit. Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 2 , acrylate product split unit  234  comprises second column  252  and third column  254 . Acrylate product split unit  234  receives at least a portion of purified acrylic product stream in line  242  and separates same into finished acrylate product stream  256  and at least one acetic acid-containing stream. As such, acrylate product split unit  234  may yield the finished acrylate product. 
     As shown in  FIG. 2 , at least a portion of purified acrylic product stream in line  242  is directed to second column  252 . Second column  252  separates the purified acrylic product stream to form second distillate, e.g., line  258 , and second residue, which is the finished acrylate product stream, e.g., line  256 . The distillate may be refluxed and the residue may be boiled up as shown. 
     Stream  258  comprises acetic acid and some acrylic acid. The second column residue exits second column  252  in line  256  and comprises a significant portion of acrylate product. As such, stream  256  is a finished product stream. Exemplary compositional ranges for the distillate and residue of second column  252  are shown in Table 5. Components other than those listed in Table 5 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 SECOND COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.1 to 40 
                 1 to 30 
                 5 to 30 
               
               
                 Acetic Acid 
                 60 to 99 
                 70 to 90 
                 75 to 85 
               
               
                 Water 
                 0.1 to 25 
                 0.1 to 10 
                 1 to 5 
               
               
                 Alkylenating Agent 
                 0.1 to 10 
                 0.5 to 15 
                 1 to 5 
               
               
                 Propionic Acid 
                 less than 10 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 at least 85 
                 85 to 99.9 
                 95 to 99.5 
               
               
                 Acetic Acid 
                 less than 15 
                 0.1 to 10 
                 0.1 to 5 
               
               
                 Water 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Alkylenating Agent 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Propionic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                   
               
            
           
         
       
     
     Returning to  FIG. 2 , at least a portion of stream  258  is directed to third column  254 . Third column  254  separates the at least a portion of stream  258  into a distillate in line  260  and a residue in line  262 . The distillate may be refluxed and the residue may be boiled up as shown. The distillate comprises a major portion of acetic acid. In one embodiment, at least a portion of line  260  is returned, either directly or indirectly, to reactor  206 . The third column residue exits third column  254  in line  262  and comprises acetic acid and some acrylic acid. At least a portion of line  262  may be returned to second column  252  for further separation. In one embodiment, at least a portion of line  262  is returned, either directly or indirectly, to reactor  206 . In another embodiment, at least a portion of the acetic acid-containing stream in either or both of lines  260  and  262  may be directed to an ethanol production system that utilizes the hydrogenation of acetic acid to form the ethanol. In another embodiment, at least a portion of the acetic acid-containing stream in either or both of lines  260  and  262  may be directed to a vinyl acetate system that utilizes the reaction of ethylene, acetic acid, and oxygen form the vinyl acetate. Exemplary compositional ranges for the distillate and residue of third column  254  are shown in Table 6. Components other than those listed in Table 6 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 THIRD COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.01 to 10 
                 0.05 to 5 
                 0.1 to 1 
               
               
                 Acetic Acid 
                 50 to 99.9 
                 70 to 99.5 
                 80 to 99 
               
               
                 Water 
                 0.1 to 25 
                 0.1 to 15 
                 1 to 10 
               
               
                 Alkylenating Agent 
                 0.1 to 25 
                 0.1 to 15 
                 1 to 10 
               
               
                 Propionic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 5 to 50 
                 15 to 40 
                 20 to 35 
               
               
                 Acetic Acid 
                 50 to 95 
                 60 to 80 
                 65 to 75 
               
               
                 Water 
                 0.01 to 10 
                 0.01 to 5 
                 0.1 to 1 
               
               
                 Alkylenating Agent 
                 less than 1 
                 0.001 to 1 
                 0.01 to 1 
               
               
                 Propionic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the third distillate is recycled and/or combined with the alkylenating agent composition that is fed to the reactor. 
     In cases where the acrylate product split unit comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 300 kPa, e.g., from 10 kPa to 100 kPa or from 40 kPa to 80 kPa. In preferred embodiments, the pressure at which the column(s) are operated is kept at a low level e.g., less than 50 kPa, less than 27 kPa, or less than 20 kPa. In terms of lower limits, the column(s) may be operated at a pressures of at least 1 kPa, e.g., at least 3 kPa or at least 5 kPa. Without being bound by theory, it has surprisingly and unexpectedly been found that be maintaining a low pressure in the columns of acrylate product split unit  234  may inhibit and/or eliminate polymerization of the acrylate products, e.g., acrylic acid, which may contribute to fouling of the column(s). 
     It has also been found that, surprisingly and unexpectedly, maintaining the temperature of acrylic acid-containing streams fed to acrylate product split unit  234  at temperatures below 140° C., e.g., below 130° C. or below 115° C., may inhibit and/or eliminate polymerization of acrylate products. In one embodiment, to maintain the liquid temperature at these temperatures, the pressure of the column(s) is maintained at or below the pressures mentioned above. In these cases, due to the lower pressures, the number of theoretical column trays is kept at a low level, e.g., less than 10, less than 8, less than 7, or less than 5. As such, it has surprisingly and unexpectedly been found that multiple columns having fewer trays inhibit and/or eliminate acrylate product polymerization. In contrast, a column having a higher amount of trays, e.g., more than 10 trays or more than 15 trays, would suffer from fouling due to the polymerization of the acrylate products. Thus, in a preferred embodiment, the acrylic acid split is performed in at least two, e.g., at least three, columns, each of which have less than 10 trays, e.g. less than 7 trays. These columns each may operate at the lower pressures discussed above. 
     Returning to  FIG. 2 , alkylenating agent stream  240  exits alkylenating agent split unit  232  and is directed to drying unit  236  for further separation, e.g., to further separate the water therefrom. The separation of the formaldehyde from the water may be referred to as dehydration. Drying unit  236  may comprise any suitable separation device or combination of separation devices. For example, drying unit  236  may comprise at least one column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In other embodiments, drying unit  236  comprises a dryer and/or a molecular sieve unit. In a preferred embodiment, drying unit  236  comprises a liquid-liquid extraction unit. In one embodiment, drying unit  236  comprises a standard distillation column as shown in  FIG. 2 . Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 2 , drying unit  236  comprises fourth column  270 . Drying unit  236  receives at least a portion of alkylenating agent stream in line  240  and separates same into a fourth distillate comprising water, formaldehyde, and methanol in line  272  and a fourth residue comprising mostly water in line  274 . The distillate may be refluxed and the residue may be boiled up as shown. In one embodiment, at least a portion of line  272  is returned, either directly or indirectly, to reactor  206 . 
     Exemplary compositional ranges for the distillate and residue of fourth column  270  are shown in Table 7. Components other than those listed in Table 7 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 FOURTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Acetic Acid 
                 less than 2 
                 0.01 to 1 
                 0.01 to 1 
               
               
                 Water 
                 20 to 90 
                 30 to 80 
                 40 to 70 
               
               
                 Alkylenating Agent 
                 10 to 70 
                 20 to 60 
                 30 to 50 
               
               
                 Methanol 
                 0.01 to 15 
                 0.1 to 10 
                 1 to 5 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 less than 1 
                 0.001 to 1 
                 0.01 to 1 
               
               
                 Acetic Acid 
                 less than 15 
                 0.1 to 10 
                 0.1 to 5 
               
               
                 Water 
                 at least 85 
                 85 to 99.9 
                 95 to 99.5 
               
               
                 Alkylenating Agent 
                 less than 1 
                 0.001 to 1 
                 0.1 to 1 
               
               
                 Propionic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the fourth distillate is recycled and/or combined with the alkylenating agent composition that is fed to the reactor. 
     In cases where the drying unit comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 500 kPa, e.g., from 25 kPa to 400 kPa or from 100 kPa to 300 kPa. 
     Returning to  FIG. 2 , alkylenating agent stream  272  exits drying unit  236  and is directed to methanol removal unit  238  for further separation, e.g., to further separate the methanol therefrom. Methanol removal unit  238  may comprise any suitable separation device or combination of separation devices. For example, methanol removal unit  238  may comprise at least one column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In one embodiment, methanol removal unit  238  comprises a liquid-liquid extraction unit. In a preferred embodiment, methanol removal unit  238  comprises a standard distillation column as shown in  FIG. 2 . Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 2 , methanol removal unit  238  comprises fifth column  280 . Methanol removal unit  238  receives at least a portion of line  272  and separates same into a fifth distillate comprising methanol and water in line  282  and a fifth residue comprising water and formaldehyde in line  284 . The distillate may be refluxed and the residue may be boiled up (not shown). In one embodiment, at least a portion of line  284  is returned, either directly or indirectly, to reactor  206 . Fifth distillate  382  may be used to form additional formaldehyde. 
     Exemplary compositional ranges for the distillate and residue of fifth column  280  are shown in Table 8. Components other than those listed in Table 8 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 FIFTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Acetic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Water 
                 20 to 60 
                 30 to 50 
                 35 to 45 
               
               
                 Alkylenating Agent 
                 0.1 to 25 
                 0.5 to 20 
                 1 to 15 
               
               
                 Methanol 
                 20 to 70 
                 30 to 60 
                 40 to 50 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Acetic Acid 
                 less than 15 
                 0.1 to 10 
                 0.1 to 5 
               
               
                 Water 
                 40 to 80 
                 50 to 70 
                 55 to 65 
               
               
                 Alkylenating Agent 
                 20 to 60 
                 30 to 50 
                 35 to 45 
               
               
                 Methanol 
                 less than 15 
                 0.1 to 10 
                 0.1 to 5 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the firth distillate and/of the fifth residue are recycled and/or combined with the alkylenating agent composition that is fed to the reactor. 
     In cases where the methanol removal unit comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 500 kPa, e.g., from 25 kPa to 400 kPa or from 100 kPa to 300 kPa. 
       FIG. 3  shows an overview of a reaction/separation scheme in accordance with the present invention. Acrylate product system  300  comprises reaction zone  302  and separation zone  304 . Reaction zone  302  comprises reactor  306 , alkanoic acid feed, e.g., acetic acid feed,  308 , alkylenating agent composition feed, e.g., trioxane feed,  310 , vaporizer  312 , and line  314 . Reaction zone  302  and the components thereof function in a manner similar to reaction zone  102  of  FIG. 1 . 
     Reaction zone  302  yields a crude acrylate product, which exits reaction zone  302  via line  316  and is directed to separation zone  304 . The components of the crude acrylate product are discussed above. Separation zone  304  comprises alkylenating agent split unit  332 , acrylate product split unit  334 , acetic acid split unit  336 , and drying unit  338 . Separation zone  304  may also comprise a light ends removal unit (not shown). For example, the light ends removal unit may comprise a condenser and/or a flasher. The light ends removal unit may be configured either upstream of the alkylenating agent split unit. Depending on the configuration, the light ends removal unit removes light ends from the crude acrylate stream, the alkylenating stream, and/or the intermediate acrylate product stream. In one embodiment, when the light ends are removed, the remaining liquid phase comprises the acrylic acid, acetic acid, alkylenating agent, and/or water. 
     Alkylenating agent split unit  332  may comprise any suitable separation device or combination of separation devices. For example, alkylenating agent split unit  332  may comprise a column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In other embodiments, alkylenating agent split unit  332  comprises a precipitation unit, e.g., a crystallizer and/or a chiller. Preferably, alkylenating agent split unit  332  comprises two standard distillation columns. In another embodiment, the alkylenating agent split is performed by contacting the crude acrylate product with a solvent that is immiscible with water. For example alkylenating agent split unit  332  may comprise at least one liquid-liquid extraction columns. In another embodiment, the alkylenating agent split is performed via azeotropic distillation, which employs an azeotropic agent. In these cases, the azeotropic agent may be selected from the group consisting of methyl isobutylketene, o-xylene, toluene, benzene, n-hexane, cyclohexane, p-xylene, and mixtures thereof. This listing is not exclusive and is not meant to limit the scope of the invention. In another embodiment, the alkylenating agent split is performed via a combination of distillation, e.g., standard distillation, and crystallization. Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 3 , alkylenating agent split unit  332  comprises sixth column  344  and seventh column  346 . Alkylenating agent split unit  332  receives liquid acrylate stream in line  316  and separates same into at least one alkylenating agent stream, e.g., stream  348 , and at least one intermediate product stream, e.g., stream  342 . Alkylenating agent split unit  332  performs an alkylenating agent split, as discussed above. 
     In operation, as shown in  FIG. 3 , the liquid acrylate stream in line  316  is directed to sixth column  344 . Sixth column  344  separates the crude acrylate product a distillate in line  340  and a residue in line  342 . The distillate may be refluxed and the residue may be boiled up as shown. Stream  340  comprises at least 1 wt. % alkylenating agent. As such, stream  340  may be considered an alkylenating agent stream. The sixth column residue exits sixth column  344  in line  342  and comprises a significant portion of acrylate product. As such, stream  342  is an intermediate product stream. Exemplary compositional ranges for the distillate and residue of sixth column  344  are shown in Table 9. Components other than those listed in Table 9 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 SIXTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.1 to 20 
                 1 to 10 
                 1 to 5 
               
               
                 Acetic Acid 
                 25 to 65 
                 35 to 55 
                 40 to 50 
               
               
                 Water 
                 15 to 55 
                 25 to 45 
                 30 to 40 
               
               
                 Alkylenating Agent 
                 at least 1 
                 1 to 75 
                 10 to 20 
               
               
                 Propionic Acid 
                 &lt;10 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 at least 5 
                 5 to 99 
                 35 to 65 
               
               
                 Acetic Acid 
                 less than 95 
                 5 to 90 
                 20 to 60 
               
               
                 Water 
                 less than 25 
                 0.1 to 10 
                 0.5 to 7 
               
               
                 Alkylenating Agent 
                  &lt;1 
                 &lt;0.5 
                 &lt;0.1 
               
               
                 Propionic Acid 
                 &lt;10 
                 0.01 to 5 
                 0.01 o 1 
               
               
                   
               
            
           
         
       
     
     In one embodiments, the first distillate comprises smaller amounts of acetic acid, e.g., less than 25 wt. %, less than 10 wt. %, e.g., less than 5 wt. % or less than 1 wt. %. In one embodiment, the first residue comprises larger amounts of alkylenating agent, e.g., 
     In other embodiments, the intermediate acrylate product stream comprises higher amounts of alkylenating agent, e.g., greater than 1 wt. % greater than 5 wt. % or greater than 10 wt. %. 
     In one embodiment, the sixth distillate is recycled and/or combined with the alkylenating agent composition that is fed to the reactor. 
     In one embodiment, polymerization inhibitors and/or anti-foam agents may be employed in the separation zone, e.g., in the units of the separation zone. The inhibitors may be used to reduce the potential for fouling caused by polymerization of acrylates. The anti-foam agents may be used to reduce potential for foaming in the various streams of the separation zone. The polymerization inhibitors and/or the anti-foam agents may be used at one or more locations in the separation zone. 
     Returning to  FIG. 3 , at least a portion of stream  340  is directed to seventh column  346 . Seventh column  346  separates the at least a portion of stream  340  into a distillate in line  348  and a residue in line  350 . The distillate may be refluxed and the residue may be boiled up as shown. The distillate comprises at least 1 wt. % alkylenating agent. Stream  348 , like stream  340 , may be considered an alkylenating agent stream. The seventh column residue exits seventh column  346  in line  350  and comprises a significant portion of acetic acid. At least a portion of line  350  may be returned to sixth column  344  for further separation. In one embodiment, at least a portion of line  350  is returned, either directly or indirectly, to reactor  306 . Exemplary compositional ranges for the distillate and residue of seventh column  346  are shown in Table 10. Components other than those listed in Table 10 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 SEVENTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.01 to 10 
                 0.05 to 5 
                 0.1 to 0.5 
               
               
                 Acetic Acid 
                 10 to 50 
                 20 to 40 
                 25 to 35 
               
               
                 Water 
                 35 to 75 
                 45 to 65 
                 50 to 60 
               
               
                 Alkylenating Agent 
                 at least 1 
                 1 to 75 
                 10 to 20 
               
               
                 Propionic Acid 
                 0.01 to 10 
                 0.01 to 5 
                 0.01 to 0.05 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 0.1 to 25 
                 0.05 to 15 
                 1 to 10 
               
               
                 Acetic Acid 
                 40 to 80 
                 50 to 70 
                 55 to 65 
               
               
                 Water 
                 1 to 40 
                 5 to 35 
                 10 to 30 
               
               
                 Alkylenating Agent 
                 at least 1 
                 1 to 75 
                 10 to 20 
               
               
                 Propionic Acid 
                 &lt;10 
                 0.001 to 5 
                 0.001 to 1 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the seventh distillate and/or the seventh residue are recycled and/or combined with the alkylenating agent composition that is fed to the reactor. 
     In cases where any of the alkylenating agent split unit comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 300 kPa, e.g., from 10 kPa to 100 kPa or from 40 kPa to 80 kPa. In preferred embodiments, the pressure at which the column(s) are operated is kept at a low level e.g., less than 100 kPa, less than 80 kPa, or less than 60 kPa. In terms of lower limits, the column(s) may be operated at a pressures of at least 1 kPa, e.g., at least 20 kPa or at least 40 kPa. Without being bound by theory, it is believed that alkylenating agents, e.g., formaldehyde, may not be sufficiently volatile at lower pressures. Thus, maintenance of the column pressures at these levels surprisingly and unexpectedly provides for efficient separation operations. In addition, it has surprisingly and unexpectedly been found that be maintaining a low pressure in the columns of alkylenating agent split unit  332  may inhibit and/or eliminate polymerization of the acrylate products, e.g., acrylic acid, which may contribute to fouling of the column(s). 
     In one embodiment, the alkylenating agent split is achieved via one or more liquid-liquid extraction units. Preferably, the one or more liquid-liquid extraction units employ one or more extraction agents. Multiple liquid-liquid extraction units may be employed to achieve the alkylenating agent split. Any suitable liquid-liquid extraction devices used for multiple equilibrium stage separations may be used. Also, other separation devices, e.g., traditional columns, may be employed in conjunction with the liquid-liquid extraction unit(s). 
     In one embodiment (not shown), the crude acrylate product is fed to a liquid-liquid extraction column where the crude acrylate product is contacted with an extraction agent, e.g., an organic solvent. The liquid-liquid extraction column extracts the acids, e.g., acrylic acid and acetic acid, from the crude acrylate product. An aqueous stage comprising water, alkylenating agent, and some acetic acid exits the liquid-liquid extraction unit. Small amounts of acrylic acid may also be present in the aqueous stream. The aqueous phase may be further treated and/or recycled. An organic phase comprising acrylic acid, acetic acid, and the extraction agent also exits the liquid-liquid extraction unit. The organic phase may also comprise water and formaldehyde. The acrylic acid may be separated from the organic phase and collected as product. The acetic acid may be separated then recycled and/or used elsewhere. The solvent may be recovered and recycled to the liquid-liquid extraction unit. 
     The inventive process further comprises the step of separating the intermediate acrylate product stream to form a finished acrylate product stream and a first finished acetic acid stream. The finished acrylate product stream comprises acrylate product(s) and the first finished acetic acid stream comprises acetic acid. The separation of the acrylate products from the intermediate product stream to form the finished acrylate product may be referred to as the “acrylate product split.” 
     Returning to  FIG. 3 , intermediate product stream  342  exits alkylenating agent split unit  332  and is directed to acrylate product split unit  334  for further separation, e.g., to further separate the acrylate products therefrom. Acrylate product split unit  334  may comprise any suitable separation device or combination of separation devices. For example, acrylate product split unit  334  may comprise at least one column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In other embodiments, acrylate product split unit  334  comprises a precipitation unit, e.g., a crystallizer and/or a chiller. Preferably, acrylate product split unit  334  comprises two standard distillation columns as shown in  FIG. 3 . In another embodiment, acrylate product split unit  334  comprises a liquid-liquid extraction unit. Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 3 , acrylate product split unit  334  comprises eighth column  352  and ninth column  354 . Acrylate product split unit  334  receives at least a portion of purified acrylic product stream in line  342  and separates same into finished acrylate product stream  356  and at least one acetic acid-containing stream. As such, acrylate product split unit  334  may yield the finished acrylate product. 
     As shown in  FIG. 3 , at least a portion of purified acrylic product stream in line  342  is directed to eighth column  352 . Eighth column  352  separates the purified acrylic product stream to form eighth distillate, e.g., line  358 , and eighth residue, which is the finished acrylate product stream, e.g., line  356 . The distillate may be refluxed and the residue may be boiled up as shown. 
     Stream  358  comprises acetic acid and some acrylic acid. The eighth column residue exits eighth column  352  in line  356  and comprises a significant portion of acrylate product. As such, stream  356  is a finished product stream. Exemplary compositional ranges for the distillate and residue of eighth column  352  are shown in Table 11. Components other than those listed in Table 11 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 EIGHTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.1 to 40 
                 1 to 30 
                 5 to 30 
               
               
                 Acetic Acid 
                 60 to 99 
                 70 to 90 
                 75 to 85 
               
               
                 Water 
                 0.1 to 25 
                 0.1 to 10 
                 1 to 5 
               
               
                 Alkylenating Agent 
                 less than 1 
                 0.001 to 1 
                 0.1 to 1 
               
               
                 Propionic Acid 
                 &lt;10 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 at least 85 
                 85 to 99.9 
                 95 to 99.5 
               
               
                 Acetic Acid 
                 less than 15 
                 0.1 to 10 
                 0.1 to 5 
               
               
                 Water 
                 less than 1 
                 less than 0.1 
                 less than 0.01 
               
               
                 Alkylenating Agent 
                 less than 1 
                 0.001 to 1 
                 0.1 to 1 
               
               
                 Propionic Acid 
                 0.1 to 10 
                 0.1 to 5 
                 0.5 to 3 
               
               
                   
               
            
           
         
       
     
     Returning to  FIG. 3 , at least a portion of stream  358  is directed to ninth column  354 . Ninth column  354  separates the at least a portion of stream  358  into a distillate in line  360  and a residue in line  362 . The distillate may be refluxed and the residue may be boiled up as shown. The distillate comprises a major portion of acetic acid. In one embodiment, at least a portion of line  360  is returned, either directly or indirectly, to reactor  306 . The ninth column residue exits ninth column  354  in line  362  and comprises acetic acid and some acrylic acid. At least a portion of line  362  may be returned to eighth column  352  for further separation. In one embodiment, at least a portion of line  362  is returned, either directly or indirectly, to reactor  306 . In another embodiment, at least a portion of the acetic acid-containing stream in either or both of lines  360  and  362  may be directed to an ethanol production system that utilizes the hydrogenation of acetic acid form the ethanol. In another embodiment, at least a portion of the acetic acid-containing stream in either or both of lines  360  and  362  may be directed to a vinyl acetate system that utilizes the reaction of ethylene, acetic acid, and oxygen form the vinyl acetate. Exemplary compositional ranges for the distillate and residue of ninth column  354  are shown in Table 12. Components other than those listed in Table 12 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 NINTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.01 to 10 
                 0.05 to 5 
                 0.1 to 1 
               
               
                 Acetic Acid 
                 50 to 99.9 
                 70 to 99.5 
                 80 to 99 
               
               
                 Water 
                 0.1 to 25 
                 0.1 to 15 
                 1 to 10 
               
               
                 Alkylenating Agent 
                 less than 10 
                 0.001 to 5 
                 0.01 to 5 
               
               
                 Propionic Acid 
                 0.0001 to 10 
                 0.001 to 5 
                 0.001 to 0.05 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 5 to 50 
                 15 to 40 
                 20 to 35 
               
               
                 Acetic Acid 
                 50 to 95 
                 60 to 80 
                 65 to 75 
               
               
                 Water 
                 0.01 to 10 
                 0.01 to 5 
                 0.1 to 1 
               
               
                 Alkylenating Agent 
                 less than 1 
                 0.001 to 1 
                 0.1 to 1 
               
               
                 Propionic Acid 
                 &lt;10 
                 0.001 to 5 
                 0.001 to 1 
               
               
                   
               
            
           
         
       
     
     In cases where the acrylate product split unit comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 300 kPa, e.g., from 10 kPa to 100 kPa or from 40 kPa to 80 kPa. In preferred embodiments, the pressure at which the column(s) are operated is kept at a low level e.g., less than 50 kPa, less than 27 kPa, or less than 20 kPa. In terms of lower limits, the column(s) may be operated at a pressures of at least 1 kPa, e.g., at least 3 kPa or at least 5 kPa. Without being bound by theory, it has surprisingly and unexpectedly been found that be maintaining a low pressure in the columns of acrylate product split unit  334  may inhibit and/or eliminate polymerization of the acrylate products, e.g., acrylic acid, which may contribute to fouling of the column(s). 
     It has also been found that, surprisingly and unexpectedly, maintaining the temperature of acrylic acid-containing streams fed to acrylate product split unit  334  at temperatures below 140° C., e.g., below 130° C. or below 115° C., may inhibit and/or eliminate polymerization of acrylate products. In one embodiment, to maintain the liquid temperature at these temperatures, the pressure of the column(s) is maintained at or below the pressures mentioned above. In these cases, due to the lower pressures, the number of theoretical column trays is kept at a low level, e.g., less than 10, less than 8, less than 7, or less than 5. As such, it has surprisingly and unexpectedly been found that multiple columns having fewer trays inhibit and/or eliminate acrylate product polymerization. In contrast, a column having a higher amount of trays, e.g., more than 10 trays or more than 15 trays, would suffer from fouling due to the polymerization of the acrylate products. Thus, in a preferred embodiment, the acrylic acid split is performed in at least two, e.g., at least three, columns, each of which have less than 10 trays, e.g. less than 7 trays. These columns each may operate at the lower pressures discussed above. 
     The inventive process further comprises the step of separating an alkylenating agent stream to form a purified alkylenating stream and a purified acetic acid stream. The purified alkylenating agent stream comprises a significant portion of alkylenating agent, and the purified acetic acid stream comprises acetic acid and water. The separation of the alkylenating agent from the acetic acid may be referred to as the “acetic acid split.” 
     Returning to  FIG. 3 , alkylenating agent stream  348  exits alkylenating agent split unit  332  and is directed to acetic acid split unit  336  for further separation, e.g., to further separate the alkylenating agent and the acetic acid therefrom. Acetic acid split unit  336  may comprise any suitable separation device or combination of separation devices. For example, acetic acid split unit  336  may comprise at least one column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In other embodiments, acetic acid split unit  336  comprises a precipitation unit, e.g., a crystallizer and/or a chiller. Preferably, acetic acid split unit  336  comprises a standard distillation column as shown in  FIG. 3 . In another embodiment, acetic acid split unit  336  comprises a liquid-liquid extraction unit. Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 3 , acetic acid split unit  336  comprises tenth column  364 . Acetic acid split unit  336  receives at least a portion of alkylenating agent stream in line  348  and separates same into a tenth distillate comprising alkylenating agent in line  366 , e.g., a purified alkylenating stream, and a tenth residue comprising acetic acid in line  368 , e.g., a purified acetic acid stream. The distillate may be refluxed and the residue may be boiled up as shown. In one embodiment, at least a portion of line  366  and/or line  368  are returned, either directly or indirectly, to reactor  306 . At least a portion of stream in line  368  may be further separated. In another embodiment, at least a portion of the acetic acid-containing stream in line  368  may be directed to an ethanol production system that utilizes the hydrogenation of acetic acid form the ethanol. In another embodiment, at least a portion of the acetic acid-containing stream in line  368  may be directed to a vinyl acetate system that utilizes the reaction of ethylene, acetic acid, and oxygen form the vinyl acetate. 
     The stream in line  366  comprises alkylenating agent and water. The stream in line  368  comprises acetic acid and water. Exemplary compositional ranges for the distillate and residue of tenth column  364  are shown in Table 13. Components other than those listed in Table 13 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 TENTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 less than 1 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Acetic Acid 
                 less than 1 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Water 
                 40 to 80 
                 50 to 70 
                 55 to 65 
               
               
                 Alkylenating Agent 
                 20 to 60 
                 30 to 50 
                 35 to 45 
               
               
                 Propionic Acid 
                 less than 1 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 less than 1 
                 0.01 to 5 
                 0.1 to 1 
               
               
                 Acetic Acid 
                 25 to 65 
                 35 to 55 
                 40 to 50 
               
               
                 Water 
                 35 to 75 
                 45 to 65 
                 50 to 60 
               
               
                 Alkylenating Agent 
                 less than 1 
                 0.01 to 5 
                 0.1 to 1 
               
               
                 Propionic Acid 
                 less than 1 
                 0.01 5 
                 0.02 1 
               
               
                   
               
            
           
         
       
     
     In one embodiment, the tenth distillate is recycled and/or combined with the alkylenating agent composition that is fed to the reactor. 
     In cases where the acetic acid split unit comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 500 kPa, e.g., from 25 kPa to 400 kPa or from 100 kPa to 300 kPa. 
     The inventive process further comprises the step of separating the purified acetic acid stream to form a second finished acetic acid stream and a water stream. The second finished acetic acid stream comprises a major portion of acetic acid, and the water stream comprises mostly water. The separation of the acetic from the water may be referred to as dehydration. 
     Returning to  FIG. 3 , tenth residue  368  exits acetic acid split unit  336  and is directed to drying unit  338  for further separation, e.g., to remove water from the acetic acid. Drying unit  338  may comprise any suitable separation device or combination of separation devices. For example, drying unit  338  may comprise at least one column, e.g., a standard distillation column, an extractive distillation column and/or an azeotropic distillation column. In other embodiments, drying unit  338  comprises a dryer and/or a molecular sieve unit. In a preferred embodiment, drying unit  338  comprises a liquid-liquid extraction unit. In one embodiment, drying unit  338  comprises a standard distillation column as shown in  FIG. 3 . Of course, other suitable separation devices may be employed either alone or in combination with the devices mentioned herein. 
     In  FIG. 3 , drying unit  338  comprises eleventh column  370 . Drying unit  338  receives at least a portion of finished acetic acid stream in line  368  and separates same into an eleventh distillate comprising a major portion of water in line  372  and an eleventh residue comprising acetic acid and small amounts of water in line  374 . The distillate may be refluxed and the residue may be boiled up as shown. In one embodiment, at least a portion of line  374  is returned, either directly or indirectly, to reactor  306 . In another embodiment, at least a portion of the acetic acid-containing stream in line  374  may be directed to an ethanol production system that utilizes the hydrogenation of acetic acid form the ethanol. In another embodiment, at least a portion of the acetic acid-containing stream in line  374  may be directed to a vinyl acetate system that utilizes the reaction of ethylene, acetic acid, and oxygen form the vinyl acetate. 
     Exemplary compositional ranges for the distillate and residue of eleventh column  370  are shown in Table 14. Components other than those listed in Table 14 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 14 
               
             
            
               
                   
               
               
                 ELEVENTH COLUMN 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 less than 1 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Acetic Acid 
                 less than 1 
                 0.01 to 5 
                 0.01 to 1 
               
               
                 Water 
                 90 to 99.9 
                 95 to 99.9 
                 95 to 99.5 
               
               
                 Alkylenating Agent 
                 less than 1 
                 0.01 to 5 
                 0.01 to 1 
               
               
                 Propionic Acid 
                 less than 1 
                 0.001 to 5 
                 0.001 to 1 
               
               
                 Residue 
               
               
                 Acrylic Acid 
                 less than 1 
                 0.01 to 5 
                 0.01 to 1 
               
               
                 Acetic Acid 
                 75 to 99.9 
                 85 to 99.5 
                 90 to 99.5 
               
               
                 Water 
                 25 to 65 
                 35 to 55 
                 40 to 50 
               
               
                 Alkylenating Agent 
                 less than 1 
                 less than 0.001 
                 less than 0.0001 
               
               
                 Propionic Acid 
                 less than 1 
                 0.001 to 5 
                 0.01 1 
               
               
                   
               
            
           
         
       
     
     In cases where the drying unit comprises at least one column, the column(s) may be operated at suitable temperatures and pressures. In one embodiment, the temperature of the residue exiting the column(s) ranges from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting the column(s) preferably ranges from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. The pressure at which the column(s) are operated may range from 1 kPa to 500 kPa, e.g., from 25 kPa to 400 kPa or from 100 kPa to 300 kPa.  FIG. 3  also shows tank  376 , which, collects at least one of the process streams prior to recycling same to reactor  306 . Tank  376  is an optional feature. The various purge streams that may, alternatively, be recycled directly to reactor  306  without being collected in tank  376 . 
     EXAMPLES 
     Example 1 
     Reaction mixtures comprising trioxane, oxygen, and acetic acid were reacted over a catalyst in accordance with the present invention. The molar ratio of acetic acid to trioxane was approximately 1.5:1. The gas hourly space velocity was 600 hr −1 . The reaction temperature was 370° C. The conversions, selectivities, yields were measured at various time periods. 
     Comparative Example A 
     Reaction mixtures comprising formaldehyde, oxygen, and acetic acid were reacted over a catalyst in accordance with the present invention. The molar ratio of acetic acid to formaldehyde was approximately 1.5:1. The gas hourly space velocity was 600 hr −1 . The reaction temperature was 370° C. The conversions, selectivities, yields were measured at various time periods. 
     The results of the testing are shown in  FIGS. 4-7 . As shown in  FIG. 4 , the acetic acid conversions obtained by using trioxane as an alkylenating agent are quite similar to those obtained using formaldehyde, especially as the time on stream exceeds 1.5 hours. In some cases, the conversions obtained using trioxane actually show improvements over the comparative examples. 
     As shown in  FIG. 5 , the yields obtained by using trioxane as an alkylenating agent are quite similar to those obtained using formaldehyde, especially as the time on stream exceeds 1.5 hours. In some cases, the yields obtained using trioxane actually show improvements over the comparative examples. 
     As shown in  FIG. 6 , the selectivities obtained by using trioxane as an alkylenating agent are quite similar to those obtained using formaldehyde, especially as the time on stream exceeds 1.5 hours. In some cases, the selectivities obtained using trioxane actually show improvements over the comparative examples. 
     As shown in  FIG. 7 , the space time yields obtained by using trioxane as an alkylenating agent are quite similar to those obtained using formaldehyde, especially as the time on stream exceeds 1.5 hours. In some cases, the space time yields obtained using trioxane actually show improvements over the comparative examples. 
     Beneficially, the process of the present invention maintains conversion, selectivity, and yield (as compared to conventional processes that utilize formaldehyde as an alkylenating agent), while reducing the amount of water in the resultant crude acrylate product stream. This reduction in water advantageously improves separation efficiencies. 
     While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those of skill in the art. In view of the foregoing discussion, relevant knowledge in the art and references discussed above in connection with the Background and Detailed Description, the disclosures of which are all incorporated herein by reference. In addition, it should be understood that aspects of the invention and portions of various embodiments and various features recited below and/or in the appended claims may be combined or interchanged either in whole or in part. In the foregoing descriptions of the various embodiments, those embodiments which refer to another embodiment may be appropriately combined with other embodiments as will be appreciated by one of skill in the art. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.