Patent Publication Number: US-2013253224-A1

Title: Process for Producing Acrylic Acids and Acrylates

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the production of acrylics, including acrylic acid and/or acrylates. More specifically, the present invention relates to the separation of acrylic acid from formaldehyde formed via the condensation of acetic acid and formaldehyde. 
     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 super absorbents, 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). The acetic acid conversions in these reactions, however, may leave room for improvement. Although this reaction is disclosed, there has been little if any disclosure relating to separation schemes that may be employed to effectively provide purified acrylic acid from the aldol condensation crude product. 
     Thus, the need exists for processes for producing purified acrylic acid and, in particular, for separation schemes to effectively purify unique aldol condensation crude products to form the purified acrylic acid. 
     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 process flowsheet showing an acrylic acid reaction/separation system in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of an alkylenating split of an acrylic acid reaction/separation system in accordance with one embodiment of the present invention. 
         FIG. 3  is a schematic diagram of an acrylic acid reaction/separation system in accordance with one embodiment of the present invention. 
         FIG. 4  is a graph showing the effect of dilution in a liquid-liquid extraction column for the purification of acrylic acid. 
     
    
    
     SUMMARY OF THE INVENTION 
     In one embodiment, the present invention is to a process for producing an acrylate product. The process comprises the steps of providing a crude product stream comprising acrylics, alkylenating agent and water and diluting the crude product stream with at least one diluents to form a diluted crude product stream. The process further comprises the step of contacting the diluted crude product stream with at least one extraction agent to form an extract stream comprising acrylate and extraction agent, and a raffinate stream comprising alkylening agent and at least one diluent. 
     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 an alkylenating agent, e.g., 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 separation schemes that may be employed to effectively purify the unique crude product that is produced. Other conventional reactions, e.g., propylene oxidation or ketene/formaldehyde, do not yield crude products that comprise higher amounts of formaldehyde. The primary reactions and the side reactions in propylene oxidation do not create formaldehyde. In the reaction of ketene and formaldehyde, a two-step reaction is employed and the formaldehyde is confined to the first stage. Also, the ketene is highly reactive and converts substantially all of the reactant formaldehyde. As a result of these features, very little, if any, formaldehyde remains in the crude product exiting the reaction zone. Because no formaldehyde is present in crude products formed by these conventional reactions, the separation schemes associated therewith have not addressed the problems and unpredictability that accompany crude products that have higher formaldehyde content. 
     In one embodiment, the present invention is 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 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 products does not require the presence of acrylic acid, methacrylic acid, and the salts and esters thereof. 
     The inventive process, in one embodiment, includes the step of providing a crude product stream comprising the acrylic acid and/or other acrylate products. The crude product stream of the present invention, unlike most conventional acrylic acid-containing crude products, further comprises a significant portion of at least one alkylenating agent. Preferably, the at least one alkylenating agent is formaldehyde. For example, the crude product stream 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 stream 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 stream may comprise less than 50 wt. % alkylenating agent(s), e.g., less than 45 wt. %, less than 25 wt. %, or less than 10 wt. %. 
     In one embodiment, the crude product stream of the present invention further comprises water. For example, the crude product stream may comprise less than 60 wt. % water, e.g., less than 50 wt. %, less than 40 wt. %, or less than 30 wt. %. In terms of ranges, the crude product stream may comprise from 1 wt. % to 60 wt. % water, e.g., from 5 wt. % to 50 wt. %, from 10 wt. % to 40 wt. %, or from 15 wt. % to 40 wt. %. In terms of upper limits, the crude product stream may comprise at least 1 wt. % water, e.g., at least 5 wt. %, at least 10 wt. %, or at least 15 wt. %. 
     In one embodiment, the crude product stream of the present invention comprises very little, if any, of the impurities found in most conventional acrylic acid crude product streams. For example, the crude product stream 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 stream of the present invention comprises very little, if any, furfural and/or acrolein. In one embodiment, the crude product stream comprises substantially no furfural and/or acrolein, e.g., no furfural and/or acrolein. In one embodiment, the crude product stream 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 crude product stream comprises 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 stream 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. Exemplary compositional data for the crude product stream are shown in Table 1. Components other than those listed in Table 1 may also be present in the crude product stream. 
     
       
         
           
               
             
               
                 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 60 
                     5 to 50 
                     10 to 40 
                     15 to 40 
               
               
                 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 
               
               
                   
               
            
           
         
       
     
     The unique crude product stream of the present invention may be separated in a separation zone to form a purified product, e.g., a purified acrylic acid product. In one embodiment, the inventive process comprises the step of separating at least a portion of the crude product stream to form an alkylenating agent stream and an intermediate product stream. This separating step may be referred to as an “alkylenating agent split.” 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 wt. % 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 product 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 product stream of the present invention. 
     The present invention, surprisingly and unexpectedly, achieves effective separation of alkylenating agent(s) from the inventive crude product stream to yield a purified product comprising acrylate products and very low amounts of other impurities. 
     It has now been discovered that the separation of the crude product stream, e.g., the alkylenating agent split, may be, at least in part, enhanced by diluting the crude product stream prior to directing the crude product stream to the separation zone. Accordingly, the inventive process, in one embodiment, comprises the step of diluting the crude product stream with at least one diluent. The inventors have found that the dilution of the crude acrylic product stream, surprisingly and unexpectedly, reduces the amount of alkylenating agent and acetic acid extracted into the organic extract stream. As such, the process of separating acetic acid and acrylic acid from the organic extract stream is greatly improved. 
     In an embodiment, the crude acrylic product stream may be diluted by one or more diluents. Exemplary diluents may be water, acetic acid, methanol, ethanol, acetone, or a combination thereof. The crude product stream may be diluted by at least one diluents in an amount greater than 50%, e.g., greater than 80%, greater than 100%, greater than 150%, or greater than 200%. 
     The process may further comprise the step of contacting the diluted crude product stream with at least one extraction agent to form an extract stream and a raffinate stream. The extract stream comprises acrylate products and the raffinate stream comprises alkylenating agent and at least a portion of the diluent. Both the extract stream and the raffinate stream may comprise a portion of the extraction agent and the extraction agent may be recovered and reused in the liquid-liquid extraction unit. The extract stream may be treated to remove residual extraction agent therefrom, thereby yielding an intermediate product stream. The raffinate stream may be treated to remove residual extraction agent therefrom thus yielding an alkylenating agent stream. 
     In some embodiments, when the crude product stream is diluted as discussed above, the extract stream comprises less than 10 wt. % alkylenating agent, e.g., less than 5 wt. %, less than 2 wt. %, or less than 1 wt. %. In terms of ranges, the extract stream comprises from 1 ppm to 10 wt. % alkylenating agent, e.g., from 0.05 wt. % to 5 wt. %, from 0.05 wt. % to 2 wt. %, or from 0.05 wt. % to 1 wt. %. In one embodiment, the extract stream is substantially free of the alkylenating agent, i.e., less than 7 wt. %, less than 4 wt. %, or less than 1 wt. %. In one embodiment, the amount of acetic acid extracted from the crude product stream to the extract stream reduces from over 90% to less than 80%, to less than 70%, to less than 60%. In one embodiment, the extract stream comprises less than 7 wt. % water, i.e., less than 4 wt. %, less than 1 wt. %, or less than 0.1 wt. %. In terms of ranges, the extract stream comprises from 0.1 wt. % to 10 wt. % water, e.g., from 0.1 wt. % to 7 wt. % or from 1 wt. % to 4 wt. %. In one embodiment, the extract stream comprises at least 1 wt. % acrylate products, e.g., at least 5 wt. %, or at least 10 wt. %. In terms of ranges, the extract stream comprises from 1 wt. % to 25 wt. % acrylate products, e.g., 1 wt. % to 15 wt. % or from 1 wt. % to 10 wt. %. In one embodiment, the extract stream comprises less than 25 wt % acetic acid, e.g., less than 15 wt. %, or less than 10 wt. %. In terms of ranges, the extract stream comprises from 1 wt. % to 25 wt. % acetic acid, e.g., from 1 wt. % to 15 wt. %, or from 10 wt. % to 15 wt. %. In one embodiment, the extract stream comprises at least 20 wt. % acrylate products and acetic acid combined, e.g., at least 30 wt. %, or at least 50 wt. %. 
     In some embodiments, the alkylenating split is performed such that a lower amount of acetic acid is present in the resulting extract stream. Preferably, the extract stream comprises little or no acetic acid. As an example, the extract stream, in some embodiments, comprises less than 90% of the acetic acid, e.g., less than 70%, less than 50%, less than 30%, or less than 15%. Surprisingly and unexpectedly, the present invention provides for the lower amounts of acetic acid in the extract stream, which, beneficially reduces or eliminates the need for further treatment of the extract stream to remove acetic acid. In some embodiments, the extract 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 via extraction, e.g., via contact with an extraction agent. In some embodiments, other separation methods, may be employed in combination with the extraction. For example, using precipitation methods, e.g., crystallization, and/or azeotropic distillation may also be employed with the extraction. Of course, other suitable separation methods may be employed in combination with the extraction. 
     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 acrylate 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 acrylate product stream, in one embodiment, comprises little if any alkylenating agent. For example, the intermediate acrylate product stream may comprise less than 10 wt. % alkylenating agent, e.g., less than 8 wt. % alkylenating agent, less than 6 wt. %, or less than 4 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 some cases, depending on the weight ratio of extraction agent(s) to the crude product stream, the intermediate acrylate product stream may comprise a large amount of extraction agent(s). As a result, the intermediate acrylate product stream may comprise lower amounts of alkylenating agent. For example, in one embodiment, the intermediate acrylate product stream comprises less than 5 wt. % alkylenating agent, e.g., less than 1 wt. % or less than 0.1 wt. %. 
     In one embodiment, the crude 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 one embodiment, the crude product 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 product 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=2N HAcA /[N HOAc +N HCHO +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, formaldehyde, and water. 
     Production of Acrylate Products 
     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, for example formaldehyde, under conditions effective to form the crude acrylate product stream. 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 formaldehyde, which is conducted over a catalyst comprising vanadium and titanium. In one embodiment, the crude product stream is the product of a reaction in wherein methanol with acetic acid are combined to generate formaldehyde in situ. The aldol condensation then follows. In one embodiment, a methanol-formaldehyde solution is 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. 6,232,352, 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. 
     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 syngas. The syngas 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 syngas, 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 an aldehyde or precursor to an aldehyde 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 preferred embodiments, the alkylenating agent comprises 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, an alcohol may serve as a source of the alkylenating agent. For example, the alcohol may be reacted in situ to form the alkylenating agent, e.g., the aldehyde. 
     The alkylenating agent, e.g., formaldehyde, may be derived from any suitable source. Exemplary sources may include, for example, aqueous formaldehyde solutions, anhydrous formaldehyde derived from a formaldehyde drying procedure, trioxane, diether of methylene glycol, and paraformaldehyde. In a preferred embodiment, the formaldehyde is produced via a methanol oxidation process, which reacts methanol and oxygen to yield the formaldehyde. 
     In other embodiments, the alkylenating agent is a compound that is a source of formaldehyde. Where forms of formaldehyde that are not as freely or weakly complexed are used, the formaldehyde will 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 350° C. or over an acid catalyst at over 100° C. to form the formaldehyde. 
     In one embodiment, the alkylenating agent corresponds to Formula I. 
     
       
         
         
             
             
         
       
     
     In this formula, 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. 
     The term “formalin,” refers to a mixture of formaldehyde, methanol, and water. In one embodiment, formalin comprises from 25 wt. % to 65 wt. % formaldehyde; from 0.01 wt. % to 25 wt. % methanol; and from 25 wt. % to 70 wt. % water. 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 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 products, 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 acrylates product per liter catalyst per hour, e.g., at least 40 grams of acrylates per liter catalyst per hour or at least 100 grams of acrylates per liter catalyst per hour, is preferred. In terms of ranges, the productivity preferably is from 20 to 500 grams of acrylates per liter catalyst per hour, e.g., from 20 to 200 grams of acrylates per liter catalyst per hour or from 40 to 140 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. 
     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 29%, e.g., less than 25% or less than 15%. More preferably, these undesirable products are not detectable. 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. 
     The alkanoic acid or ester thereof and alkylenating agent may be fed independently or after prior mixing to a reactor containing the catalyst. The reactor may be any suitable reactor or combination of reactors. Preferably, the reactor comprises a fixed bed reactor or a series of fixed bed reactors. In one embodiment, the reactor is a packed bed reactor or a series of packed bed reactors. In one embodiment, the reactor is a fixed bed reactor. Of course, other reactors such as a continuous stirred tank reactor or a fluidized bed reactor may be employed. 
     In some embodiments, the alkanoic acid, e.g., acetic acid, and the alkylenating agent, e.g., formaldehyde, are fed to the 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 300° C., or at least 350° 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 350° 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. 
     Water may be present in the reactor in amounts up to 60 wt. %, by weight of the reaction mixture, e.g., up to 50 wt. % or up to 40 wt. %. Water, however, is preferably reduced due to its negative effect on process rates and separation costs. 
     In one embodiment, an inert or reactive gas is supplied to the reactant stream. Examples of inert gases include, but are not limited to, nitrogen, helium, argon, and methane. Examples of reactive gases or vapors include, but are not limited to, oxygen, carbon oxides, sulfur oxides, and alkyl halides. When reactive gases such as oxygen 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. 
     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 to the reactor after sufficient separation from the desired product. 
     When the desired product is an unsaturated ester made by reacting an ester of an alkanoic acid ester with formaldehyde, the alcohol corresponding to the ester may also be fed to the reactor either with or separately to 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 has relatively high molar ratios of oxide additive to titanium. 
     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 of Acrylic Acid and Formaldehyde 
     As discussed above, the crude product stream is separated to yield an intermediate acrylate product stream.  FIG. 1  is a flow diagram depicting the formation of the crude product stream and the separation thereof to obtain an intermediate acrylate product stream. 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 feed, e.g., formaldehyde feed  110 , and vaporizer  112 . 
     Acetic acid and formaldehyde 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 first 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 product stream, which is withdrawn, preferably continuously, from reactor  106  via line  116 . Although  FIG. 1  shows the crude product stream being withdrawn from the bottom of reactor  106 , the crude product stream may be withdrawn from any portion of reactor  106 . Exemplary composition ranges for the crude product stream 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/recycle streams. Such guard beds may be employed in the vapor or liquid 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 product stream in line  116  is fed to alkylenating agent split unit  104 . In an embodiment, the crude product stream in line  116  may be cooled using condensers prior to feeding to alkylenating agent split unit  104 . Alkylenating agent split unit  104  may comprise one or more separation units, e.g., two or more or three or more. In one example, separation zone contains multiple columns, as shown in  FIG. 2 . Alkylenating agent split unit  104  separates the crude product stream into at least one intermediate acrylate product stream, which exits via line  118  and at least one alkylenating agent stream, which exits via line  120 . 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 
                 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 to 1   
               
               
                   
               
            
           
         
       
     
     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 stream are shown in Table 3. Components other than those listed in Table 3 may also be present in the purified alkylate product stream. Examples include methanol, methyl acetate, methyl acrylate, dimethyl ketone, carbon dioxide, carbon monoxide, oxygen, nitrogen, and acetone. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 ALKYLENATING 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 
                 0.001 to 5    
                 0.001 to 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 mentioned above, the crude product stream of the present invention comprises little, if any, furfural and/or acrolein. As such the derivative stream(s) of the crude product streams 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. 
       FIG. 2  shows an overview of a portion of a reaction/separation scheme in accordance with the present invention. Acrylate product system  200  comprises reaction zone  202  and alkylenating agent split unit  204 , which performs the alkylenating split of the separation process. Reaction zone  202  comprises reactor  206 , alkanoic acid feed, e.g., acetic acid feed,  208 , alkylenating agent feed, e.g., formaldehyde 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 . 
     Reaction zone  202  yields a crude product stream, which exits reaction zone  202  via line  216  and is directed to alkylenating split unit  204 . The components of the crude product stream are discussed above. In addition, the separation of the crude product stream further comprises acetic acid split unit and drying unit. Alkylenating split unit  204  may also comprise an optional 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 or downstream of the alkylenating agent split unit. Depending on the configuration, the light ends removal unit removes light ends from the crude product 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. 
     As shown in  FIG. 2 , alkylenating agent split unit  204  comprises extraction column  218 , and extraction agent recovery columns  220  and  222 . Alkylenating agent split unit  204  receives crude acrylic product stream in line  216  and separates same into at least one stream comprising alkylenating agent, and at least one stream comprising acrylate products. In accordance with an embodiment of the invention, the crude acrylic product is fed to liquid-liquid extraction column  218 . Extraction column  218  utilizes at least one extractive agent to effectively extract acrylic acid to the extract stream  226  and form a raffinate (i.e., aqueous) stream  224  comprising water, formaldehyde, some acetic acid and a small amount of the extractive agent(s). 
     It has now been discovered that the separation of acrylic acid and unreacted formaldehyde may be enhanced by employing one or more liquid-liquid extraction columns using one or more extraction agents. In a preferred embodiment, the crude acrylic product stream may be diluted with at least one diluent by at least 50%, e.g., at least 80%, at least 100%, at least 150%, or at least 200%. 
     As shown in  FIG. 2 , crude product stream  216  is fed to liquid-liquid extraction column  218  where the crude product stream is contacted with one or more extraction agents, e.g., organic solvents, which are fed via line  228 . Liquid-liquid extraction column  218  extracts the acrylate products, e.g., acrylic acid, from crude product stream  216  into extract stream  226 . The extract stream further comprises organic solvent. Acrylate products may be separated from extract stream  226  and collected as intermediate acrylate product stream in line  230 . Organic solvent may be separated and recycled to liquid-liquid extraction unit  218  via line  232 . 
     Raffinate stream  224  comprises water, alkylenating agent, acetic acid and organic solvent and exits liquid-liquid extraction unit  218 . A portion of raffinate stream  224  may be further treated and/or recycled. For example, the acetic acid in the extract stream and the raffinate stream may be separated then recycled and/or used in this or other processes. Similarly, organic solvent in the raffinate stream may be recovered and recycled to liquid-liquid extraction unit  218 . 
     In one embodiment, liquid-liquid extraction column  218  may be any conventional liquid-liquid extraction device, for example, a static mixer, a stirred vessel, a mixer/settler, a rotary-disc extractor, an extractor with centrifugation, a column with perforated plates or packing, agitated columns, pulsed columns, disc and donut style columns or other liquid-liquid extraction devices. In one embodiment, liquid-liquid extraction column  218  may be a tray column having from 5 to 70 trays, e.g., from 15 to 50 trays, or from 20 to 45 trays. 
     In one embodiment, extraction column  218  may operate counter-currently, meaning that the extraction agent and the crude acrylic product stream flow in opposite directions of one another. In another embodiment, extraction column  218  may operate co-currently, meaning that the extraction agent and the crude acrylic product stream flow in the same direction. 
     In one embodiment, the extraction may be carried out is a continuous manner. In another embodiment, the extraction may be carried out in a batch-wise manner. 
     In an embodiment, the extraction agent is introduced to liquid-liquid extraction column  218  via line  228 , preferably in the bottom part of the column, e.g., bottom half or bottom third. The extractive agent may comprise of one or more suitable organic solvents, including diisobutyl ketone (DIBK), cyclohexane, toluene, isopropyl acetate, o-xylene, p-xylene, m-xylene, butyl acetate, butanol, methyl acetate, methyl acrylate, diphenyl ether, ethyl acrylate, methyl acrylate, methyl acetate, butyl acetate, isopropyl acetate, ethyl propionate, hexane, benzene, diisopropyl ether, n,n-dimethyl aniline, dibutyl ether, tetralin, butyl acrylate, 2-ethylhexyl alcohol, isophorone, ditolyl ether, dimethyl phthalate, 3,3 trimethyl-cyclohexanone, biphenyl, o-dichlorobenzene, toluene, and a mixture thereof. 
     In an embodiment, the suitable extraction agent is less volatile than acrylic acid. The inventors discovered that by using extraction agent that is less volatile than acrylic acid, the energy cost is reduced because the extraction agent is not boiled overhead in a distillation column with the acrylic acid. In addition, the potential for acrylic acid to undergo polymerization is reduced. In some embodiments, the temperature at which the extraction may be carried out depends upon the extraction agent being used and the components in the crude acrylic product stream. In an embodiment, the extraction agent extracts acrylic acid from the crude acrylic product stream into the organic extract stream. In another embodiment, the organic extract stream is substantially free of water and formaldehyde. In an embodiment, the extraction is carried out at a temperature such that the organic extract stream is substantially free of water and formaldehyde. 
     In one embodiment, the extraction may be carried out at a pressure of from about 80 kPa to about 130 kPa, e.g., from about 90 kPa to about 115 kPa or from about 100 kPa to about 105 kPa. In terms of lower limits, the extraction may be carried out at a pressure greater than 80 kPa, e.g., greater than 90 kPa or greater than 100 kPa. In terms of upper limits, the extraction may be carried out at a pressure less than 130 kPa, e.g., less than 115 kPa or less than 105 kPa. 
     In one embodiment, the crude acrylic product feed may be of higher density than the extraction agent mixture. In such embodiments, the extraction agent mixture may be fed to a point in the liquid-liquid extraction column below the feed point of the crude acrylic product feed. In another embodiment, the crude acrylic product feed may be of lower density than the extraction agent. In such embodiments, the extraction agent may be fed at a point in the extraction column above the crude acrylic product feed. 
     In an embodiment, crude acrylic product feed  216  may be diluted with one or more diluents, which are fed via diluent feed line  238 . Surprisingly and unexpectedly, the inventors have discovered that the dilution of the crude acrylic product feed reduces the amount of formaldehyde and acetic acid extracted into extract stream  226 . In one embodiment, as shown in  FIG. 2 , crude acrylic product feed  216  may be diluted using one or more diluents prior to the extraction step. In another embodiment, the diluent(s) may be added to extraction column  218  as a separate feed (not shown). In one embodiment, the diluent feed and the crude product feed stream may be directed to the extraction column  218  together or separately. In one embodiment, the diluent is fed at a point above the crude acrylic product feed. In another embodiment, the diluents is fed at a point below the crude acrylic product feed. In one embodiment, the diluent may be fed at a point above the extraction agent feed. In one embodiment, the diluents may be fed at a point below the extraction agent feed. The diluents may vary widely. Exemplary diluents may be water, acetic acid, methanol, ethanol, acetone, or a combination thereof. In an embodiment, the diluents may be derivative or recycled streams from this purification process. The use of such diluents beneficially eliminates the need for an outside diluent feed. In another embodiment, the diluents may be derivative or recycled streams from other purification processes, i.e., an outside source. 
     In an embodiment, the crude acrylic product feed is diluted by at least 50%, e.g., at least 80%, at least 100%, at least 150%, or at least 200%. In other words, the diluent is added to the crude acrylic acid product in an amount that is at least 50% of the initial crude acrylic acid product, e.g., at least 80%, at least 100%, at least 150%, or at least 200%. In terms of weight ratio, the weight ratio of crude acrylic product feed to diluents ranges from 1:0 to 1:10, e.g., from 1:0 to 1:2. In a preferred embodiment, the weight ratio of crude acrylic product feed to diluents is about 1:0.25 or about 1:0.5. 
     Exemplary compositional ranges for the extract stream  226  and raffinate stream  224  of the extraction column  218  are shown in Table 4. Components other than those listed in Table 4 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 EXTRACTION COLUMN (218) 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
               
               
                 Raffinate 
                   
                   
                   
               
               
                 Stream 
                   
                   
                   
               
               
                 Acrylic Acid 
                 100 ppm to 10 
                 1 to 5  
                 0.1 to 1   
               
               
                 Acetic Acid 
                 1 to 30 
                 5 to 30 
                  5 to 25 
               
               
                 Water 
                 50 to 90  
                 50 to 70  
                 40 to 50 
               
               
                 Alkylenating 
                 1 to 30 
                 1 to 20 
                 10 to 20 
               
               
                 Agent 
                   
                   
                   
               
               
                 Extraction 
                 1 ppm to 
                 1 ppm to 
                 1 ppm to 
               
               
                 Agent 
                 2000 ppm 
                 2000 ppm 
                 2000 ppm 
               
               
                 Extract Stream 
                   
                   
                   
               
               
                 Acrylic Acid 
                 1 to 25 
                 1 to 15 
                  1 to 10 
               
               
                 Acetic Acid 
                 1 to 25 
                 1 to 15 
                 10 to 15 
               
               
                 Water 
                 0.1 to 5   
                 1 to 5  
                 3 to 5 
               
               
                 Alkylenating 
                 0.1 to 2   
                 0.5 to 2   
                 1 to 2 
               
               
                 Agent 
                   
                   
                   
               
               
                 Extraction 
                 50 to 90  
                 50 to 70  
                 50 to 60 
               
               
                 Agent 
               
               
                   
               
            
           
         
       
     
     In another embodiment, extract stream  226  is substantially free of water or formaldehyde. Preferably, extract stream  226  comprises less than 7 wt. % water, e.g., less than 4 wt. % water or less than 1 wt. % water and the extract stream  226  comprises less than 7 wt. % formaldehyde, e.g., less than 4 wt. % formaldehyde, or less than 1 wt. % formaldehyde. 
     In another embodiment, the extract stream  226  comprises at least 50% of the acrylate product in the crude acrylic product stream, e.g., at least 65%, at least 80%, or at least 95%. In terms of selectivity, the extractive agent has a greater selectivity to acrylic acid than acetic acid. The extractive agent also has a greater selectivity to acrylic acid than formaldehyde. In an embodiment, the extraction selectivity to acrylic acid is at least 40%, e.g., at least 1%, or at least 1.1%. The extraction selectivity to the combination of acrylic acid and acetic acid is at least 1.15%, e.g., at least 1.1%, or at least 1.05%. In another embodiment, the extraction selective to formaldehyde is at most 0.5%, e.g., at most 0.2%, or at most 0.06%. The extraction selectivity to acetic acid is at most 0.6%, e.g., at most 0.5%, or at most 0.4%. 
     In another embodiment, raffinate stream  224  comprises the majority of the diluents, i.e., water. Preferably, raffinate stream  224  comprises greater than 40 wt. % water, e.g., greater than 50 wt. %, or greater than 70 wt. %. In another embodiment, the raffinate stream comprises at least 1 wt. % alkylenating agent, e.g., at least 5 wt. % or at least 10 wt. %. In another embodiment, the raffinate stream  224  comprises less than 10 wt. % actylate product, e.g., less than 5 wt. %, less than 1 wt. %. In another embodiment, the raffinate stream  224  comprises less than 50 wt. % acetic acid, e.g., less than 30 wt. % or less than 25 wt. %. In another embodiment, raffinate stream  224  comprises less than 60 wt. % acrylate products and acetic acid combined, e.g., less than 50 wt. %, or less than 40 wt. %. In an embodiment, the acrylate products in the raffinate stream is less than 50% of the acrylate products in the crude product stream, e.g., less than 40 wt. %, or less than 30%. In an embodiment, the alkylenating agent in the raffinate stream is at least 50% of the alkylenating agent in the crude product stream, e.g., at least 60% or at least 70%. 
     It is noted that  FIG. 2  is an exemplary embodiment of the liquid-liquid extractive distillation separation process. Although the organic extract stream  226  is shown as a distillate and the raffinate stream  224  is shown as a residue stream, it is noted that, depending on the extractive agent used, the organic extract stream  226  may be the residue stream and the aqueous stream  226  may be the distillate stream. 
     Continuing with  FIG. 2 , at least a portion of extract stream  226  may be fed to extractive agent recovery column  220 . Extractive agent recovery column  220  separates the at least a portion of extract stream  226  into an intermediate acrylate product stream in line  230  and a first solvent stream in line  232 . The intermediate acrylate product stream  230  may be refluxed as shown and the first solvent stream  232  may be boiled up as shown. The intermediate product stream comprises at least 1 wt. % acrylic acid. Stream  230 , like stream  226 , may be considered an acrylate product stream. In one embodiment, at least a portion of the contents of line  232  is returned, either directly or indirectly, to extraction column  218 . 
     In an embodiment, intermediate acrylate product stream  230  comprises acrylic acid and acetic acid. In an embodiment, the intermediate acrylate product stream  230  is substantially free of extractive agent, e.g., comprises of less than 5 wt. % of the extractive agent, less than 1 wt. % or less than 0.1 wt. %. 
     Exemplary compositional ranges for the acrylate product stream  230  and the first solvent stream  232  of the first solvent recovery column  220  are shown in Table 5. Components other than those listed in Table 5 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 EXTRACTIVE AGENT RECOVER COLUMN (220) 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 20 to 80 
                 30 to 70 
                 40 to 60 
               
               
                 Acetic Acid 
                 15 to 60 
                 25 to 50 
                 30 to 40 
               
               
                 Water 
                  1 to 35 
                  5 to 25 
                 10 to 15 
               
               
                 Alkylenating Agent 
                 0.01 to 20   
                 0.1 to 10  
                 &lt;5 
               
               
                 Extractive Agent 
                 &lt;10 
                 &lt;1.0 
                 &lt;0.1 
               
               
                 Residue 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.001 to 15   
                 0.01 to 10  
                 &lt;1.0 
               
               
                 Acetic Acid 
                 &lt;2.0 
                 &lt;1.0 
                 &lt;0.1 
               
               
                 Water 
                 &lt;2.0 
                 &lt;1.0 
                 &lt;0.1 
               
               
                 Alkylenating Agent 
                 &lt;2.0 
                 &lt;1.0 
                 &lt;0.1 
               
               
                 Extractive Agent 
                 &gt;99.9 
                 &gt;95 
                 &gt;90 
               
               
                   
               
            
           
         
       
     
     In an embodiment, raffinate  224  may contain some extractive agent. Similarly, the extractive agent in raffinate  224  may be separate and recycled to extraction column  218 . As shown in  FIG. 2 , at least a portion of raffinate stream  224  may be fed to a second extractive agent recovery column  222 . Second extractive agent recovery column  222  separates the at least a portion of raffinate stream  224  into an alkylenating agent stream in line  234  and a second solvent stream in line  236 . The alkylenating agent stream  234  and the second solvent stream  236  may be refluxed as shown. The alkylenating agent stream, as discussed above, comprises at least 1 wt. % alkylenating agent. In one embodiment, at least a portion of line  234  is returned, either directly or indirectly, to extraction column  218 . In one embodiment, extractive agent in line  234  may be combined with extractive agent in line  232  prior to returning to extractive column  218 . 
     Exemplary compositional ranges for the distillate  234  and residue  226  of the second solvent recovery column  222  are shown in Table 6. Components other than those listed in Table 6 may also be present in the residue and distillate. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 SOLVENT RECOVER COLUMN (222) 
               
            
           
           
               
               
               
               
            
               
                   
                 Conc. (wt. %) 
                 Conc. (wt. %) 
                 Conc. (wt. %) 
               
               
                   
               
               
                 Distillate 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.001 to 10 
                 0.01 to 1   
                 &lt;0.1 
               
               
                 Acetic Acid 
                 0.001 to 30 
                 0.01 to 20     
                 0.1 to 10  
               
               
                 Water 
                   45 to 85 
                 55 to 85 
                 65 to 75 
               
               
                 Alkylenating Agent 
                    5 to 50 
                  5 to 40 
                 15 to 30 
               
               
                 Extractive Agent 
                  0.01 to 25 
                 0.1 to 20  
                  1 to 10 
               
               
                 Residue 
                   
                   
                   
               
               
                 Acrylic Acid 
                 0.01 to 5 
                 0.01 to 1   
                 &lt;0.1 
               
               
                 Acetic Acid 
                    5 to 40 
                 15 to 35 
                 20 to 30 
               
               
                 Water 
                   40 to 90 
                 50 to 80 
                 60 to 70 
               
               
                 Alkylenating Agent 
                  0.1 to 25 
                  1 to 20 
                  5 to 15 
               
               
                 Extractive Agent 
                 0.01 to 5 
                 0.01 to 1   
                 &lt;0.1 
               
               
                   
               
            
           
         
       
     
     In cases where 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 by maintaining a low pressure in the columns of alkylenating agent split unit  204  may inhibit and/or eliminate polymerization of the acrylate products, e.g., acrylic acid, which may contribute to fouling of the column(s). 
     The inventive process further comprises the step of separating the intermediate acrylate product stream  230  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.” 
     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.” 
     As shown in  FIG. 3 , intermediate product stream  230  exits alkylenating agent split unit  204  and is directed to acrylate product split unit  240  for further separation, e.g., to further separate the acrylate products therefrom. Acrylate product split unit  240  may comprise any suitable separation device or combination of separation devices. For example, acrylate product split unit  240  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  240  comprises a precipitation unit, e.g., a crystallizer and/or a chiller. Preferably, acrylate product split unit  240  comprises two standard distillation columns as shown in  FIG. 3 . In another embodiment, acrylate product split unit  240  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  240  comprises fourth column  246  and fifth column  248 . Acrylate product split unit  240  receives at least a portion of purified acrylic product stream in line  230  and separates same into finished acrylate product stream  256  and at least one acetic acid-containing stream. As such, acrylate product split unit  240  may yield the finished acrylate product. 
     As shown in  FIG. 3 , at least a portion of purified acrylic product stream in line  230  is directed to fourth column  246 . Fourth column  246  separates the purified acrylic product stream to form fourth distillate, e.g., line  254 , and fourth 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  254  comprises acetic acid and some acrylic acid. The fourth residue exits fourth column  246  in line  256  and comprises a significant portion of acrylate products. As such, stream  256  is a finished product stream. Exemplary compositional ranges for the distillate and residue of fourth column  246  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 (246) 
               
            
           
           
               
               
               
               
            
               
                   
                 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 
               
               
                 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 
               
               
                   
               
            
           
         
       
     
     Returning to  FIG. 3 , at least a portion of stream  254  is directed to fifth column  248 . Fifth column  248  separates the at least a portion of stream  254  into a distillate in line  258  and a residue in line  260 . The distillate may be refluxed and the residue may be boiled up as shown. The distillate comprises a major portion of acetic acid. The fifth column residue exits fifth column  248  in line  260  and comprises acetic acid and some acrylic acid. At least a portion of line  260  may be returned to fourth column  246  for further separation. In one embodiment, at least a portion of line  260  is returned, either directly or indirectly, to reactor  206 . In one embodiment, at least a portion of line  258  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  258  and  260  may be directed to an ethanol production system that utilizes the hydrogenation of acetic acid. Exemplary compositional ranges for the distillate and residue of fifth column  248  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 (248) 
               
            
           
           
               
               
               
               
            
               
                   
                 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   
               
               
                 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     
               
               
                   
               
            
           
         
       
     
     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 by 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  240  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  234  exits alkylenating agent split unit  204  and is directed to acetic acid split unit  242  for further separation, e.g., to further separate the alkylenating agent and the acetic acid therefrom. Acetic acid split unit  242  may comprise any suitable separation device or combination of separation devices. For example, acetic acid split unit  242  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  242  comprises a precipitation unit, e.g., a crystallizer and/or a chiller. Preferably, acetic acid split unit  242  comprises a standard distillation column as shown in  FIG. 3 . In another embodiment, acetic acid split unit  242  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  242  comprises sixth column  250 . Acetic acid split unit  242  receives at least a portion of alkylenating agent stream in line  234  and separates same into a sixth distillate comprising alkylenating agent in line  262 , e.g., a purified alkylenating stream, and a sixth residue comprising acetic acid in line  264 , 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  262  and/or line  264  are returned, either directly or indirectly, to reactor  206 . At least a portion of stream in line  264  may be further separated. In another embodiment, at least a portion of the acetic acid-containing stream in line  264  may be directed to an ethanol production system that utilizes the hydrogenation of acetic acid form the ethanol. 
     The stream in line  262  comprises alkylenating agent and water. The stream in line  264  comprises acetic acid and water. Exemplary compositional ranges for the distillate and residue of sixth column  250  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 (250) 
               
            
           
           
               
               
               
               
            
               
                   
                 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 
               
               
                 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     
               
               
                   
               
            
           
         
       
     
     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 , sixth residue  264  exits acetic acid split unit  242  and is directed to drying unit  272  for further separation, e.g., to remove water from the acetic acid. Drying unit  272  may comprise any suitable separation device or combination of separation devices. For example, drying unit  272  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  272  comprises a dryer and/or a molecular sieve unit. In a preferred embodiment, drying unit  272  comprises a liquid-liquid extraction unit. In one embodiment, drying unit  272  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  272  comprises seventh column  252 . Drying unit  272  receives at least a portion of second finished acetic acid stream in line  264  and separates same into a seventh distillate comprising a major portion of water in line  266  and a sixth residue comprising acetic acid and small amounts of water in line  268 . The distillate may be refluxed and the residue may be boiled up as shown. In one embodiment, at least a portion of line  268  is returned, either directly or indirectly, to reactor  206 . In another embodiment, at least a portion of the acetic acid-containing stream in line  268  may be directed to an ethanol production system that utilizes the hydrogenation of acetic acid form the ethanol. 
     Exemplary compositional ranges for the distillate and residue of seventh column  252  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 (252) 
               
            
           
           
               
               
               
               
            
               
                   
                 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 
               
               
                 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 
               
               
                   
               
            
           
         
       
     
     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. 2  also shows tank  276 , which, collects at least one of the process streams prior to recycling same to reactor  206 . Tank  276  is an optional feature. The various recycle streams that may, alternatively, be recycled directly to reactor  206  without being collected in tank  276 . 
     EXAMPLES 
     Example 1 
       FIG. 4  illustrates the effect the diluents have on the liquid-liquid extraction process. In  FIG. 4 , the percentages of acrylic acid, acetic acid and formaldehyde that are extracted into the extract stream. The percentages are plotted as a function of dilution percentage of the acrylic product feed. A 90/10 diisobutyl ketone/cyclohexane was used as the extraction agent mixture and the solvent to feed ratio was 2:1. As shown in  FIG. 4 , at 0% dilution of the crude acrylic product feed, 100% of the acrylic acid, over 95% of the acetic acid, and over 27% of the formaldehyde is extracted from the crude acrylic product feed and into the extract stream. As dilution increases, the amount of acetic acid and formaldehyde extracted into the extract stream decreases. For example, at a 2:1 (i.e., 50%) dilution of crude acrylic product feed to water, the amount of acetic acid extracted decreases from about 95% to about 72%. The amount of acetic acid extracted further decreases to less than 60% when the dilution of the crude acrylic produce feed to water is at 1:1 (i.e., 100% dilution). Similarly, the amount of formaldehyde extracted decreases from about 27% to less than 5% when the crude acrylic product feed is diluted by 100% of water. Surprisingly and unexpectedly, the amount of acrylic acid extracted remains at 100% regardless of the dilution of the crude acrylic product feed. In comparison, the amount of acetic acid and formaldehyde extracted decreases as the crude acrylic product feed is diluted. Therefore, the dilution of the crude acrylic feed with a diluent beneficially favors the separation of acrylic acid from formaldehyde and acetic acid, without compromising the amount of acrylic acid extracted. 
     Example 2 
     A simulation of a process in accordance with  FIG. 2  was conducted using ASPEN™ software. The compositions of the various process streams are shown in Table 11. 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 SIMULATED COMPOSITIONAL 
               
               
                 DATA FOR PROCESS STREAMS 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Extraction 
                 Extraction 
               
               
                   
                 Extraction 
                 Agent Recovery 
                 Agent Recovery 
               
               
                   
                 Column 218 
                 Column 220 
                 Column 222 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Distil- 
                   
                 Distil- 
                   
                 Distil- 
                   
               
               
                 Comp. 
                 late 
                 Residue 
                 late 
                 Residue 
                 late 
                 Residue 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Acrylic Acid 
                 15.7 
                 0.0 
                 48.5 
                 0.5 
                 0.0 
                 0.0 
               
               
                 Acetic Acid 
                 11.0 
                 22.3 
                 34.9 
                 0.0 
                 3.3 
                 25.1 
               
               
                 Water 
                 4.5 
                 66.4 
                 14.3 
                 0.0 
                 69.3 
                 65.9 
               
               
                 Formaldehyde 
                 0.7 
                 10.7 
                 2.3 
                 0.0 
                 22.5 
                 9.0 
               
               
                 DIBK 
                 68.1 
                 0.6 
                 0.0 
                 99.5 
                 5.0 
                 0.0 
               
               
                   
               
            
           
         
       
     
     As shown in  FIG. 2 , a unique crude product stream may be formed via the aldol condensation of acetic acid and formaldehyde. This formaldehyde-containing product stream can be effectively separated in accordance with the present invention to achieve an extract stream that comprise of less than 7 wt. % water, and less than 10 wt. % formaldehyde. 
     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.