Abstract:
An apparatus and process are described for recovery of a carboxylic acid by hydrolysis of an ester in a mixture comprising the ester, an alcohol and water. The apparatus comprises a catalytic distillation column containing an acidic catalyst and a distillation column. Simultaneously and interdependently the alcohol is catalytically dehydrated to the corresponding ether and water, and said water reacts with the ester to generate a carboxylic acid rich stream from the catalytic distillation column. The acid is recovered by distillation in the distillation column. The process requires no added water. A second embodiment of the apparatus and process has means to co-feed one or both of added methanol and/or water with the feed to maintain substantially optimum operation independent of feed composition.

Description:
FIELD 
       [0001]    The present invention relates to recovery of carboxylic acids by hydrolysis of esters, and in particular to recovery of acetic acid by hydrolysis of methyl acetate containing mixtures from polyvinyl alcohol production processes. 
       BACKGROUND 
       [0002]    Several industrial processes produce streams containing one or more esters of carboxylic acids, RO.CO.R′, where R and R′ are selected from among hydrocarbyl radicals or substituted hydrocarbyl radicals. Examples of such industrial processes include those for production of vinyl alcohol polymers or terephthalic acid. Carboxylic acid esters can be hydrolyzed to generate the corresponding acid and alcohol, as shown in Equation 1. Equation 1 is an equilibrium reaction and requires an excess of water to drive the reaction well to the right hand side. 
         [0000]      RO.CO.R′+H 2 O         ROH+HO.CO.R′  [1] 
         [0003]    Processes for production of polyvinyl alcohol (PVA) and its derivatives are described by Marten in “Vinyl Alcohol Polymers” in Kirk-Othmer Encyclopedia of Technology, John Wiley &amp; Sons, Inc. A variety of vinyl ester monomers can be polymerized to form a polymer, of which polyvinyl acetate (PVAc) is preferred. PVAc then is further reacted to manufacture PVA. Commonly, PVAc is reacted with methanol (MeOH) to form PVA and methyl acetate (MeOAc). The components of the polymerization reaction mixture are continuously separated. Unreacted monomer can be stripped from the reactor using, for example, methanol vapor. The overhead fraction from the stripper comprises a mixture of vinyl ester monomer and at least one solvent such as methanol. The vinyl ester monomer is then extracted for recycle to the polymerization reactor. In the production of polyvinyl alcohol (PVA) MeOAc is produced as a by-product at a ratio of 1.68 tons of MeOAc per ton of PVA. 
         [0004]    One outlet stream typically comprises a mixture including MeOAc, MeOH and a small amount of water. The weight ratio of these components varies over a range of relative concentrations, among which a typical composition is approximately 75% MeOAc, 23% MeOH and 2% water. Among these components MeOH and water have relatively low value when compared to the values of MeOAc and acetic acid (HOAc). 
         [0005]    The MeOH and MeOAc can be distilled off and, at the same time, water can be added in order to obtain an aqueous PVA solution. However, there are disadvantages to this approach. The resulting PVA suspension is fine, difficult to filter, and so the process is uneconomical. Further, this approach requires time-consuming, energy intensive and hence expensive distillation of large amounts of solvents requiring a plurality of distillation columns. Several approaches have been undertaken to improve the chemical efficiency and economics of processes for production of PVA. In particular, efforts have been directed to recycle of solvents and processing of the outlet streams to recover valuable by-products. 
         [0006]    Kowaka et al. in U.S. Pat. No. 6,743,859 issued in 2004 describe a method for production of high-strength PVA with a high degree of saponification. The apparatus for the process of &#39;859 includes an outlet line for recovery of MeOH and MeOAc identified in  FIGS. 1 through 3  by the reference numeral  7 , however no details are presented for the process for the separation and recovery of those components. 
         [0007]    Bauer et al. in U.S. Pat. No. 6,576,720 issued in 2003 describe an alternative approach in which a liquid phase comprising MeOH, MeOAc and HOAc is recycled for use in further polyvinyl ester transesterification. The process of &#39;720 can be used for other alcohols and esters. The mixture of alcohol and corresponding ester is recycled for use as the polymerization reaction medium. Make-up comprising one or both of MeOH and HOAc is provided to maintain the composition of the recycle mixture. The PVA is produced and isolated using saponification with KOH and then neutralization, preferably with a strong acid such as HCl. Thus water, less than 1% by weight in the initial reaction mixture, is produced by both the reaction of MeOH and HOAc and the neutralization process. 
         [0008]    MeOAc may be sold or further hydrolysed to recover HOAc. 
         [0009]    Kim et al. in U.S. Pat. No. 5,770,770 issued in 1998 describe a reactive distillation process for the well known equilibrium reaction 2 for the recovery of MeOH and HOAc from catalytic hydrolysis of MeOAc. Reaction 2 is a specific example of the type of reaction shown in Equation 1. 
         [0000]      MeOAc+H 2 O         MeOH+HOAc   [2] 
         [0010]    It has long been recognized that this reaction could be used to recover HOAc from MeOAc from a PVA manufacturing process as described, by example, by Adelman et al. in U.S. Pat. No. 4,352,940 issued in 1982. It also was recognized in &#39;940 that it was necessary to minimize the amount of water used in the process to reduce the costs of recovery and re-use of the products from the reaction in the PVA manufacture process. However, when a minimum amount of water is used, the equilibrium reaction 2 lies to the left hand side. Reaction 2 can be driven to the right hand side only by continuous removal of at least one of the products. 
         [0011]    In an alternative approach for treatment of the PVA manufacture outlet stream, MeOAc can be separated from the mixture using extractive distillation. One example of this method is described by Xiao et al. in Chemical Engineering Science, volume 56, pages 6553-6562 (2001). In the first column, water is added to the liquid stream from the PVA plant. The volatiles from the first column are then hydrolyzed in a fixed bed reactor containing a bed of an acidic catalyst which catalyzes the hydrolysis of MeOAc to MeOH and HOAc. The effluent stream from the fixed bed reactor is distilled in a second distillation column to provide a volatiles stream and a bottoms stream. The volatiles from the second column are recycled for mixing with further MeOAc feed to the first extractive distillation column. The bottoms from the second column are separated into a water rich stream and a HOAc rich stream by distillation in a third column. The bottoms from the first column are separated by distillation in a fourth column into a water rich stream and a MeOH rich stream. Thus the overall process for recovery of HOAc requires four distillation columns and a fixed bed reactor. Further, to drive well to the right hand side the well known catalytic MeOAc hydrolysis equilibrium reaction shown as Equation 2, it is necessary to use a large amount of water. Thus the process is energy intensive as that water must be volatolized in both the second and third columns. 
         [0012]    Each of the above processes requires use of a plurality of columns and reactors to react, separate and recover the components of the stream from the PVA manufacturing reactor. Consequently, capital and operating costs are high. Further, when water is added, either as reagent or for extractive distillation, that water must also be separated, which is a costly and time consuming feature. 
         [0013]    Hoyme et al. in U.S. Pat. No. 6,518,465 issued in 2003 describe another concept based process, derived from simulations using the commercial available program Aspen Plus, in which the stream containing MeOAc from PVA manufacture is reacted in a reactive distillation column to produce DME and HOAc. Water was added to hydrolyze MeOAc and thereby generate HOAc which is recovered. The molar ratio of water in the process stream is between 0.05% and 20%, and preferably is between 0.3% and 3%. In this process it is recognized that methanol also may react to generate dimethyl ether (DME) and water in the also well known acid catalyzed equilibrium reaction shown in Equation 3. The process of Hoyme et al. in &#39;465 is basically hydrolysis of MeOAc to HOAc by addition of water. 
         [0000]      2 MeOH         Me 2 O+H 2 O   [3] 
         [0014]    What is needed is an apparatus and process for separation and recovery of HOAc produced by hydrolysis of MeOAc in a stream from a reactor in which PVA is produced. 
         [0015]    The present invention is a process for recovery of HOAc by catalytic distillation of streams comprising MeOAc, MeOH and lesser amounts of water wherein the apparatus comprises only two columns: the catalytic distillation column and a distillation column for HOAc recovery, the process requiring no added water. Reactive distillation is used for conversion of a mixed MeOAc-MeOH feed to HOAc and DME, through a two-step consecutive reaction process. The first reaction involves the equilibrium dehydration of MeOH to form DME and water. The second reaction employs this water to react with MeOAc in the hydrolysis reaction that produces MeOH and HOAc. The MeOH produced during hydrolysis of MeOAc is then able to participate further in MeOH dehydration to produce more water. In effect, the two reactions are “self-feeding” and can be visually represented as follows: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0016]    The benefit of this reactive distillation process is that no additional water is added as feed and the process continues as long as there is water and/or MeOH in the column to react. Once the MeOAc-MeOH azeotrope has been broken, any further separation of MeOAc and HOAc becomes a matter of simple distillation. 
       SUMMARY 
       [0017]    The present invention relates to a process for recovery of carboxylic acids from mixtures containing a corresponding ester and an alcohol through hydrolysis in a catalytic distillation column containing an acidic catalyst. The alcohol is dehydrated to produce the corresponding ether and water, and this water is used to hydrolyze the ester to the corresponding alcohol and acid. A bottoms stream containing the acid exits the catalytic distillation column and is sent to a distillation column from which the substantially pure acid is recovered as a bottom stream. There is no requirement for water to be added to the feed for the process. An alternative embodiment of the apparatus includes means for addition of supplemental amounts of methanol and/or water to maintain optimum operation of the process independent of the composition of the ester containing mixture from which acetic acid is to be recovered. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0018]    For a more complete understanding of the present invention and for further objects and advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings. 
           [0019]      FIG. 1  is a schematic diagram of the apparatus comprising a catalytic distillation column and a distillation column. 
           [0020]      FIG. 2  shows the effect of changing pressure on the temperature profile in the catalytic distillation column. 
           [0021]      FIG. 3  shows the effect of excess water on product distribution in the CD column. 
           [0022]      FIG. 4  shows the effect on product distribution in the CD column when there is no water content in the feed. 
           [0023]      FIG. 5  shows the effect on the composition profile when the MeOAc-MeOH feed is in molar ratio 0.55; RR=1.8, D/F=0.475, T=120° C. and P=150 psi. 
           [0024]      FIG. 6  shows the apparatus for experiments to determine the effect of mixture composition on production of DME. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The following description comprises data obtained through laboratory experiments and simulations using ASPEN PLUS. However, in order to perform the simulations and thereby correlate experimental data with theory, it was necessary to obtain data on the physical and chemical equilibria occurring in the respective reaction mixtures. The data for individual reactions 2 and 3 are available in the open literature. However, no physical and chemical kinetic data were available for mixtures comprising all five components involved. To obtain accurate simulation results, experiments were performed to obtain kinetic data for reactions among combinations of all five components. The following brief descriptions provide background data available and the measures taken to determine the equilibria parameters. 
         [0026]    As a consequence of having experimental data on physical and chemical equilibria that were hitherto unavailable to those skilled in the art, the construction, experimental demonstration and modeling of the process comprising the invention described herein have higher reliability than those described elsewhere. For the first time, we show accurately a process comprising the two concurrent continuous reversible reactions occurring in a single reactor, one generating water and DME from MeOH and another consuming said water by reaction with MeOAc to generate both MeOH and HOAc. Water generated in one reaction is consumed in the other, and MeOH generated in the latter reaction is consumed in the former. 
         [0027]    We are for the first time able to compute fully the roles and impacts of each of the components in the reaction mixture, experimentally verify the computed data, and so establish operating parameters so that: 
         [0028]    there is essentially no water generated in the overall process, and substantially all water is consumed when there is excess MeOAc to undergo hydrolysis; 
         [0029]    DME is recovered as a volatile in the overhead; 
         [0030]    a stream that is richer in HOAc is generated for transfer to a distillation column for recovery of essentially anhydrous HOAc, separately from recovery of DME; and 
         [0031]    the process has high overall energy efficiency, as there is no requirement for consumption of energy to remove added high concentrations of water. 
         [0032]    The process of the present invention therefore offers advantages over those described in the prior art. 
         [0033]    We will show that components of the bottoms from the single reactor in which the two equilibrium reactions are occurring include a much higher concentration of HOAc, along with MeOAc and smaller amounts of additional components. For the overall process to operate with high energy efficiency and generate substantially pure HOAc, it is necessary that the bottoms from the reactor are transferred to a separate distillation column. In a modified embodiment of the process, all methyl species are converted when DME is completely removed by catalytic distillation with the consequence that the bottoms stream is substantially pure HOAc. 
         [0034]    In a further modification, using either a batch reactor or a continuous reaction system, if the feed composition is adjusted through addition of stoichiometric amounts of methanol and/or water, substantially pure HOAc can be produced in a single column because DME is very volatile and can be completely removed from the reaction mixture. 
         [0035]    Physical Equilibria 
         [0036]    An important consideration in any reactive distillation simulation is the choice of physical equilibrium model and the ability to reliably predict multicomponent vapour-liquid equilibria (VLE) and liquid-liquid equilibria (LLE). Reliable VLE and LLE are needed to establish distillation boundaries and to determine if and where azeotropes and phase separations occur. There exists an abundance of patchwork/piecemeal phase equilibrium data in the open literature on the multicomponent mixture of MeOAc-MeOH-water-HOAc-DME and the respective subsystems. The various equations used to model these systems take into account the non-ideality of the vapor phase (due to dimerization of HOAc). 
         [0037]    Chemical Equilibria: 
         [0038]    The two concurrent reversible reactions considered in the process are the hydrolysis of MeOAc and the dehydration of MeOH. The hydrolysis of MeOAc with water over an acid catalyst produces HOAc and MeOH (Equation 2). Reaction 2 is equilibrium limited with a reported equilibrium constant of 0.13 at 25° C. (Ge et al., “Kinetics of Heterogeneous Hydrolysis of Methyl Acetate”, Chemical Reaction and Engineering and Technology (Chinese Journal), Vol. 14, No. 2, 1998, pp 138-144). The dehydration of methanol, consumes two moles of methanol to produce one mole of DME and one mole of water (Equation 3). 
         [0039]    The equilibrium constant has been reported to range from 68.4 to 52.3 in the temperature range from 85° C. to 115° C. (Nisoli et al., “Attainable Regions for Reaction with Separation” in AIChE Journal, Vol. 43, No. 3 (2), 1997, pp 374-387). The overall reaction is given by: 
         [0000]      MeOAc+MeOH         DME+HOAc   (4) 
         [0040]    Because water is both consumed and produced stoichiometrically, it drops out of the overall reaction equation. 
         [0041]    The Invention: 
         [0042]    An exemplary embodiment of the invention, which is non-limiting, will now be described with reference to  FIG. 1  through  FIG. 5 . By way of example, the apparatus and process parameters described for the embodiment will be presented with reference to recovery of HOAc by hydrolysis of MeOAc in a mixture fed to the apparatus from a PVA manufacturing facility. It will be recognized that the method and the principles of operation of the apparatus will apply to recovery of other carboxylic acids from other mixtures, when the operating parameters are amended according to the properties of those carboxylic acids and the esters from which they are recovered. 
         [0043]    Referring to  FIG. 1 , an apparatus  10  has a catalytic distillation column  12  and a distillation column  14 . The term “catalytic distillation” as used herein refers to simultaneous reaction of the components within a mixture and separation of those components by distillation, at least one reaction occurring in the column being catalyzed by a catalyst. 
         [0044]    Catalytic distillation column  12  has a top portion (“rectifying zone”)  18 , a middle portion (“reactive zone”)  20  and a bottom portion (“stripping zone”)  22 . A fixed bed  24  containing an acidic catalyst  26  is situated within middle portion  16  of catalytic distillation column  12 . When, for example, a feed  30  is an HOAc rich stream from a chemicals manufacturing facility (not illustrated) such as a PVA manufacturing facility, a feed line  28  carries feed  30  to a location  32  part way up fixed bed  24 . A first condenser  34  is used to condense a portion of vapors in top portion  18 , from which a fraction is recycled to distillation column  12  and another fraction is recovered as first volatiles product  44  via a first volatiles outlet line  36 . A first reboiler  38  is used to reboil a portion of first liquid bottoms  40  exiting bottom portion  22  for recycle to catalytic distillation column  12 , and another portion of first liquid bottoms  40  is removed via a first liquid bottoms outlet line  42 . 
         [0045]    Acidic catalyst  26  can be one or more of several different acidic catalysts. It has been found through experiments that any one of several acidic resin catalysts is suitable for use in the catalytic hydration of MeOAc according to Equation 2, including but not limited to use of the acidic forms of commercially available resins, Amberlyst 15, 35 or 37. 
         [0046]    Distillation column  14  has a top portion  50 , a middle portion  52 , a bottom portion  54 , a second condenser  56  and a second reboiler  58 . The another portion of first liquid bottoms  40  from catalytic distillation column  12  is fed via first liquid bottoms outlet line  42  to a position  60  approximately midway up middle portion  52  of distillation column  14 . The optimum position  60  for connection of first liquid bottoms outlet line  42  and distillation column  14  has been determined experimentally using modeling and predicted operating performance data have been confirmed experimentally, as described in the EXAMPLES below. 
         [0047]    Second condenser  56  condenses a portion of vapors in top portion  50  for recycle to distillation column  14 , and another portion is recovered as a second volatiles product  61  via a second volatiles outlet line  62 . Second reboiler  58  reboils a portion of a second liquid bottoms  64  in bottom portion  54  of distillation column  14 , and another portion of second liquid bottoms  64  is recovered via a second liquid bottoms outlet line  66 . 
         [0048]    When the chemicals processing facility manufactures one or more of PVA and derivatives of PVA feed  40  is a stream from the manufacturing facility typically comprising MeOAc, MeOH and water. It is desirable to recover acetic acid, which may be recycled for use in manufacture of vinyl acetate which in turn can be polymerized for manufacture of PVA. The ether formed from dehydration of MeOH is DME and it is recovered as first volatiles product  44 . DME can be recovered for sale or for other use. 
         [0049]    The second volatiles product  61  from distillation column  14  is much richer in MeOAc than was feed  40 , and this stream can be recovered for sale or recycled to distillation column  12 . 
         [0050]    The method of use of apparatus  10  will now be illustrated with reference to  FIG. 3 through 5 , using as the example reaction hydrolysis of MeOAc for recovery of HOAc. 
         [0051]    The composition of feed  30  from a PVA manufacturing facility is shown in Table 4. Also in Table 4 are the compositions, for one exemplary set of operating parameters, of that portion of first volatile products  44  recovered via first volatiles outlet line  36  and that portion of first liquid bottoms  40  removed via first liquid bottoms outlet line  42 . 
         [0052]    Laboratory experiments, described in EXAMPLE 1 below, showed that the rate of dehydration of MeOH to form DME (Equation 3) is affected by the concentrations of both water and MeOH in a reaction mixture initially comprising MeOH-water-HOAc-MeOAc. Thus, to maintain efficient operation, it is desirable that the mixture fed to catalytic distillation column  12  has an optimum composition profile. 
         [0053]    An alternative embodiment of the present invention enables more efficient operation of the process independent of variations that may occur from time to time in the composition of feed  30  before it is fed via feed line  28  into catalytic distillation column  12 , illustrated in  FIG. 1 . At least one additional feed line (not illustrated) is provided that independently can feed additional amounts of one or both of methanol and water to catalytic distillation column  12 . Depending on the composition of feed  30 , additional methanol and/or water may be added into feed line  28  before feed  30  or at appropriate locations on catalytic distillation column  12 . We found that the consequence of ensuring a substantially optimum concentration profile of both methanol and water along the catalytic distillation column  12  is that the operation of the process occurring therein is production of substantially pure DME and HOAc so the operating cost for the process is minimized. 
         [0054]    A benefit from use of the second embodiment of the present invention is that the apparatus is more versatile. The capability to provide amounts of one or both of methanol and water in addition to feed  30  can be used for recovery of HOAc (or other carboxylic acids) from a variety of different industrial streams containing hydrolysable esters. 
         [0055]    A benefit arising from use of either apparatus  10  or the second embodiment of the present invention is that the process can be operated so as to produce a stream of high purity DME and HOAc as products. In this modification of either embodiment, DME is continuously removed from apparatus  10  so that substantially all methyl species are converted to DME and so are removed from the system, with the consequence that first liquid bottoms  40  comprises substantially pure HOAc. 
         [0056]    The following EXAMPLES will illustrate application of apparatus and method for recovery of HOAc. Each set of conditions has been modeled using ASPEN and results from modeling have been confirmed through experiment. 
       EXAMPLES 
     Example 1 
     Batch Distillation 
       [0057]    Laboratory experiments were conducted to confirm literature data and to obtain additional insights into the two catalytic reactions for the formation of DME and HOAc, using an apparatus  100  illustrated in  FIG. 6 . Apparatus  100  for conducting batch non-equilibrium experimental reactions comprised a reaction vessel  102 , a condenser  104  and a line  106  for recovery and analysis of a DME containing stream  108 . The feed  110  comprised several different mixtures of MeOH, water and MeOAc in different ratios. Each reaction mixture  110  and catalyst  112  were mixed in reaction vessel  102  and refluxed. The amount of the stream exiting via line  106  was determined gravimetrically and its composition was determined using gas chromatography. Using apparatus  100  it was found that the rate of DME and HOAc formation depended upon the composition of reaction mixture  110  as well as the concentration of catalyst  112  therein. In particular, it was found that the formation of DME depended on the amounts of water and MeOH present in reaction mixture  110 . The data so obtained were then used to design and simulation of the process. 
         [0058]    Using the system demonstrated in these laboratory experiments and illustrated in  FIG. 6 , it has been determined that all methyl groups from MeOAc can be removed as DME, thus producing a liquid product rich in HOAc. In this further embodiment of the process, sufficient MeOH and/or water are added to reaction mixture  110  so as to hydrolytically cleave all methyl groups from MeOAc. The methyl groups are removed as DME by volatilization. The products formed are a liquid product rich in HOAc and a volatile product rich in DME. 
       Example 2 
     Model Batch Catalytic Distillation 
       [0059]    To examine the reliability and effectiveness of the physical and reaction equilibrium models, a batch catalytic distillation simulation was performed and compared to experimental data collected in the lab. 
         [0060]    For the experimental test, a batch Parr reactor was loaded with a 150 g mixture of MeOAc/MeOH feed in a 75:25 weight ratio, along with 30 g of previously dried acid catalyst (Amberlyst  35 ). The top of the batch reactor was modified with a stainless steel condenser, which allowed the most volatile component (i.e., DME) to escape. The condenser was cooled with cold water; GC analysis of the off-gas from the condenser confirmed that &gt;99% DME was leaving the reactor. The reactor was operated at 120° C. and 150 psig over a period of 13 hours until equilibrium had been reached. It was determined that equilibrium was established based on vapor and liquid samples from the reactor which showed constant concentration over several samples. Vapor and liquid samples were measured on an HP 6890 GC equipped with capillary column and TCD. A two-stage equilibrium reactor with distillation column was sufficient to model the batch distillation process in Aspen Plus. Table 1 shows the liquid composition for the batch catalytic distillation simulation and compares it to experimental batch catalytic distillation results. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Liquid product composition for batch catalytic distillation 
               
               
                 experiment and simulation at T = 120° C. and 
               
               
                 P = 150 psi for MeOAc/MeOH 
               
               
                 mass feed ratio of 75:25. 
               
             
          
           
               
                   
                   
                 Simulation 
                 Experimental 
               
               
                   
                   
                 “Product” 
                 Product 
               
               
                   
                   
                 stream 
                 stream 
               
               
                   
                 Component 
                 Weight % 
                 Weight % 
               
               
                   
                   
               
             
          
           
               
                   
                 MeOAc 
                 58.5 
                 69.25 
               
               
                   
                 MeOH 
                 2.1 
                 2.87 
               
               
                   
                 DME 
                 9.8 
                 3.5 
               
               
                   
                 Water 
                 4.0 
                 3.31 
               
               
                   
                 HOAc 
                 25.7 
                 21.06 
               
               
                   
                   
               
             
          
         
       
     
         [0061]    Comparison of data from the simulation and experimental results confirms that the predictions from our model are accurate in terms of reaction rate and equilibrium constants. 
       Example 3 
     Simulation of Continuous Catalytic Distillation Column 
       [0062]    The continuous catalytic distillation process consists of a 20 stage CD column with total condenser and partial reboiler. The parameters for the base case simulation are given in Table 2. All feed streams enter at 25° C. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Parameter values for the base case used in Aspen Plus 
               
               
                 simulations 
               
             
          
           
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
             
          
           
               
                   
                 Column pressure (atm) 
                 5 
               
               
                   
                 Distillate to feed ratio (D/F) 
                 0.45 
               
               
                   
                 Reflux ratio (RR) 
                 2.1 
               
               
                   
                 Total number of stages 
                 20 
               
               
                   
                 Reaction stage location 
                 3 to 16 
               
               
                   
                   
                 (inclusive) 
               
               
                   
                 Feed stage location (above stage) 
                 12 
               
               
                   
                 Feed composition (mass basis): 
               
               
                   
                 MeOAc 
                 0.75 
               
               
                   
                 MeOH 
                 0.23 
               
               
                   
                 H 2 O 
                 0.02 
               
               
                   
                   
               
             
          
         
       
     
       Example 4 
     Effect of Pressure 
       [0063]    Operating pressure is one of the key elements in the design of a CD column. The choice of operating pressure for a catalytic distillation column depends on many considerations such as the overhead temperature, bottom temperature, reaction temperature and relative volatilities of the components in the system. The column pressure sets the lower and upper bounds of the temperature within the column. For the present process the operating pressure is chosen within a range such that water can be used as a coolant for the overhead condenser and steam can be used as a heating medium for the reboiler. Within this range, the reaction temperature mainly determines the operating pressure. Because the catalytic reactions take place within the liquid phase, the reaction temperature is close to the boiling point of liquid flowing around the catalyst. As a result the reaction temperature increases with column pressure.  FIG. 2  shows the effect of column pressure on the temperature profile in the column. In these simulations the parameters are those as in the base case, except for the reflux ratio. As column pressure was increased, the reflux ratio was also increased concurrently in order to achieve mathematical convergence in the simulation. The reflux ratio was increased by the minimum amount that would satisfy convergence of the CD column, with all other parameters being held constant. 
       Example 5 
     Effect of Water 
       [0064]    The amount of water in the feed is an important variable in the CD process because water is a necessary reactant in the hydrolysis reaction. However, water is also produced via MeOH dehydration. Therefore, there should be an optimum amount of water that will allow both reactions to proceed readily without accumulating a large amount of water in the reaction mixture. The optimum process achieves maximum conversion of reactants while at the same time completely removing water from any of the product streams.  FIG. 3  shows the effect of excess water content of the feed stream (relative ratio of MeOAc:MeOH in the feed remains constant). When excess water enters as a component of the feed the hydrolysis reaction is limited by the concentration of MeOAc. MeOAc is completely consumed and excess water exits with HOAc in the bottoms product. 
         [0065]    Conversely, when no water enters in the feed, only the water that is produced through MeOH dehydration is available for the hydrolysis reaction. In this case DME is again the distillate product, and a mixture of HOAc and MeOAc is now the bottoms product. All of the water and MeOH is reacted away. The resulting profile of concentrations of all CD column reaction mixture components is shown in  FIG. 4 . 
       Example 6 
     Effect of MeOAc/MeOH Ratio 
       [0066]    Based on the overall reaction equation given by equation (3), one might predict that the there should be very little influence on the process arising from changes in the ratio of MeOAc to MeOH. Increasing either MeOH or MeOAc in the feed should favour DME and HOAc product formation. However, based on the stoichiometry of the individual reactions, this is not necessarily true. The effect of increasing the MeOAc/MeOH feed ratio results in complete conversion of water and methanol in the CD column with a profile similar to that shown in  FIG. 4 . When considering only the hydrolysis reaction, one would expect that by increasing the amount of MeOAc in the feed, water could be more easily reacted to completion. When water is consumed and MeOH is produced, the equilibrium for the dehydration consequently shifts to favor the production of DME. When the MeOAc/MeOH ratio decreases below a specific stoichiometric amount, the MeOAc reactant becomes the limiting reactant. 
         [0067]    Simulation results show that column parameters can be varied to completely remove both MeOAc and MeOH from the column mixture, and a mixture of water and HOAc remains as the bottoms product.  FIG. 5  illustrates this effect when using a MeOAc/MeOH molar feed ratio of 0.55. 
       Example 7 
     Simulation of the Method of the Process 
       [0068]    Catalytic distillation column  14  has 20 stages. Feed  30  comprises a mixture containing about 75% MeOAc, 23% MeOH and 2% water, and is fed at 100 kg.h −1  into stage 12 into catalytic distillation column  12  at 100° C. The pressure under which feed  30  is supplied is 10 atm. The column operates at 5 atm. and so feed  30  is supplied via a back-pressure regulator (not illustrated). The reflux ratio is 2 and the distillate-to-feed ratio is 0.45. At steady state, the temperature at stage 3, at the top of the reaction zone is 31° C. and at the bottom of the reaction zone, stage 16, is 127° C. 
         [0069]    First volatile products  44  exiting top portion  18  of catalytic distillation column  12  comprises almost entirely DME (over 99%; Table 3). First liquid bottoms  40  exiting bottom portion  22  of catalytic distillation column  12  comprises over 92% HOAc and the balance is less than 8% MeOAc, with only trace amounts of MeOH, water and DME (Table 4). 
         [0070]    First volatiles product  44  comprises close to 100% DME. 
         [0071]    First liquid bottoms  40  comprise a mixture of about 91% HOAc, 8.9% MeOAc, 0.04% water, and traces of MeOH and DME. 
         [0072]    Distillation column  14  has 10 stages and has a reflux ratio of 7 and a distillate-to-feed ratio of 0.07. First liquid bottoms  40  from catalytic distillation column  12  is fed at stage 5 into distillation column  14  at a temperature of 170.5° C. and a pressure of 5 atm. 
         [0073]    Second liquid bottoms  64  from distillation column  14  is substantially pure HOAc (see Table 4). Second volatiles product  60  is rich in MeOAc and is recyclable. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Composition of feed and product streams for catalytic 
               
               
                 distillation column 12 in Example 7. 
               
             
          
           
               
                   
                 Flow rates (kg · h −1 ) 
               
             
          
           
               
                   
                   
                   
                   
                 First liquid 
               
               
                   
                   
                   
                 First volatiles 
                 bottoms product 
               
               
                   
                 Component 
                 Feed 30 
                 product 44 
                 40 
               
               
                   
                   
               
             
          
           
               
                   
                 MeOAc 
                 75 
                 0.093 
                 5.50 
               
               
                   
                 MeOH 
                 23 
                 &lt;0.001 
                 0.0025 
               
               
                   
                 HOAc 
                   
                 &lt;0.001 
                 56.26 
               
               
                   
                 Water 
                 2 
                 &lt;0.001 
                 0.026 
               
               
                   
                 DME 
                   
                 3.81 
                 &lt;0.001 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Composition of feed and product streams for distillation 
               
               
                 column 14 in Example 7. 
               
             
          
           
               
                   
                 Flow rates (kg · h −1 ) 
               
             
          
           
               
                   
                   
                 Feed (first 
                   
                   
               
               
                   
                   
                 liquid 
                 Second 
                 First liquid 
               
               
                   
                   
                 bottoms 
                 volatiles 
                 bottoms product 
               
               
                   
                 Component 
                 product 40) 
                 product 60 
                 64 
               
               
                   
                   
               
             
          
           
               
                   
                 MeOAc 
                 5.50 
                 5.20 
                 0.31 
               
               
                   
                 MeOH 
                 0.0025 
                 &lt;0.002 
                 &lt;0.001 
               
               
                   
                 HOAc 
                 56.26 
                 0.025 
                 56.24 
               
               
                   
                 Water 
                 0.026 
                 0.005 
                 0.021 
               
               
                   
                 DME 
                 &lt;0.001 
                 &lt;0.001 
                 &lt;0.001 
               
               
                   
                   
               
             
          
         
       
     
       REFERENCES CITED 
       [0074]      
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 U.S. Patent Documents 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 4,352,940 
                 October, 1982 
                 Adelman et al. 
                 562/607 
               
               
                 5,770,770 
                 June, 1998 
                 Kim et al. 
                 562/608 
               
               
                 6,518,465 
                 February, 2003 
                 Hoyme et al. 
                 568/698 
               
               
                 6,576,720 
                 June, 2003 
                 Bauer et al. 
                 526/70 
               
               
                 6,743,859 
                 June, 2004 
                 Kowaka et al. 
                 525/62 
               
               
                   
               
             
          
         
       
     
       OTHER REFERENCES 
       [0075]    Ge, X., Wang, Z, Yong, J., Yang D., “Kinetics of Heterogeneous Hydrolysis of Methyl Acetate” Chemical Reaction and Engineering and Technology (Chinese Journal), Vol. 14, No. 2, 1998, pp 138-144. 
         [0076]    Gmehling, J., Bölts, R., “Azeotropic Data for Binary and Ternary Systems at Moderate Pressures” Journal of Chemical Engineering Data, Vol. 41, No. 2, 1996, pp 202-209. 
         [0077]    Marten, F. L., “Vinyl Alcohol Polymers” in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley &amp; Sons, Inc. 
         [0078]    Nisoli, A., Malone, M. F., Doherty, M. F., “Attainable Regions for Reaction with Separation” American Institute for Chemical Engineering Journal, Vol. 43, No. 3 (2), 1997, pp 374-387. 
         [0079]    Teshima, T., Hiyoshi, S., Matsuda, H., Monma, S., and Iwaba, S., “Extractive distillation. II. Vapor-liquid equilibrium relation of methyl acetate-methanol-water system” Vol. 55, 1952, pp 801-803). 
         [0080]    Xiao, J., Liu, J., Juntai, L., Jiang X., and Zhang, Z., “Increase in MeOAc conversion in PVA production by replacing the fixed bed reactor with a catalytic distillation column” Chemical Engineering Science, 56, 6553-6562 (2001).