Abstract:
A methanol/MMA azeotrope is broken or avoided by a method comprising the steps of (1) raising the pressure within a first vessel, e.g., a distillation column, that contains a methanol/MMA azeotrope, (2) collecting the azeotrope as a liquid, and then in a second, separate vessel, e.g., another distillation column, (3) raising the pressure sufficiently to allow for the breaking of or avoidance of the azeotrope and the recovery of the methanol.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to breaking a methanol/methyl methacrylate (MMA) azeotrope using pressure swing distillation. In one aspect the invention relates to the recovery and recycle of the methanol used in the manufacture of MMA. 
       BACKGROUND OF THE INVENTION 
       [0002]    Methanol is used in the manufacture of methyl methacrylate (MMA). Methanol and MMA form an azeotrope or a “near azeotrope”, otherwise known as a “tangent pinch”. This means that there is no separation possible between the methanol and MMA without a means of breaking the azeotrope. 
         [0003]    One method of breaking a methanol/MMA azeotrope is by changing the pressure in that part of the MMA manufacturing process containing the azeotrope. However, raising the pressure only results in shifting the vapor concentration slightly above the liquid concentration. Without further modifications to this arrangement, the energy penalty for this operation is prohibitive. 
         [0004]    U.S. Pat. No. 4,937,302 teaches a method for the separation of technical methanol-MMA mixtures by polymerization of the MMA. The polymerization is suitably carried out as a copolymerization, at least with long-chain aliphatic C 8  to C 20 -alkyl esters of methacrylic acid as comonomers, and as a solution polymerization, and the methanol is recovered by distillation. 
         [0005]    German patent publication DE-OS No. 32 11 901 describes a method for the separation of methanol from aqueous mixtures of MMA and methanol, such as are formed in the esterification of methacrylic acid with methanol, in which are added to the mixture azeotrope-formers which, in the presence of MMA and water, form with methanol azeotropes which have a boiling point at least 0.2 Centigrade degrees below the boiling point of the azeotrope of methanol and MMA. 
         [0006]    JP 03819419 B2 describes a methanol recovery column where the methanol and methacrolein are separated from MMA in a distillation column with no other separating agents added. The overhead composition is limited by the azeotropic composition (11 wt % of MMA in methanol). While the azeotropic composition can be approached by using a large number of trays and/or a high reflux ratio, the MMA composition in the overheads cannot be less than the azeotropic composition. This is undesirable as the MMA is the desired product, and sending it hack to the reactor requires larger equipment and, more importantly, provides the opportunity for the valuable product to react further to by-products, thereby lowering the MMA yield. 
         [0007]    U.S. Pat. No. 4,518,462 describes the removal of methanol from MMA using a C 6 -C 7  saturated hydrocarbon, e.g. hexane, cyclohexane, heptane, methyl cyclopentane or dimethylpentane, as an entrainer. No water is added to the overheads decanter, so the phases split into hydrocarbon-rich and methanol-rich layers. One of the drawbacks of this approach is the limited ability to dry the recycle stream. In addition, in order to reduce the MMA to low levels in the recycle stream, a large amount of entrainer is required, resulting in high energy usage and a large and expensive distillation column. 
         [0008]    U.S. Pat. No. 5,028,735, U.S. Pat. No. 5,435,892, and JP 02582127 B2 describe a similar entrainer process where either sufficient water is in the feed or water is added to the overhead decanter to form an organic and aqueous layer. In this case, essentially all of the hydrocarbon entrainer resides in the organic layer. The aqueous layer can be sent to a drying column to remove water from the recycle stream; however, large amounts of hexane are still required to minimize MMA in the recycle stream. For example, U.S. Pat. No. 5,028,735 describes an entrainer process using hexane as the entrainer with hexane usage of at least 17-fold the water content of the feed and 3-fold the methanol in the feed. 
         [0009]    U.S. Pat. No. 6,680,405, uses methacrolein as an entrainer. While the azeotrope composition was broken, it resulted in only a minor improvement, namely 7.4% MMA in the recycle stream. 
       SUMMARY OF THE INVENTION 
       [0010]    In one embodiment of this invention, a methanol/MMA azeotrope is broken or avoided by a method comprising the steps of (1) raising the pressure within a first vessel, e.g., a distillation column, that contains a methanol/MMA azeotrope, (2) collecting the azeotrope as a liquid, and then in a second, separate vessel, e.g., another distillation column, (3) raising the pressure sufficiently to allow for the recovery of the methanol. In one embodiment of the invention, the process is conducted without the use of an azeotropic agent. In one embodiment of the invention, the process is conducted with the use of an azeotropic agent. 
         [0011]    In one embodiment of the invention, the lower pressure vessel, i.e., the first vessel, is equipped with a reboiler, and the higher pressure vessel, i.e., the second vessel, acts as a heat pump for the reboiler of the lower pressure vessel thus reducing the energy consumption required to operate the lower pressure vessel. 
         [0012]    In one embodiment the invention is a process for breaking or minimizing a methanol/methyl methacrylate (MMA) azeotrope, the process comprising the steps of:
       (A) Feeding a liquid stream comprising methanol and MMA to a first distillation column operated at a first pressure and equipped with a reboiler;   (B) Separating the liquid stream within the first distillation column into a first distillation column overheads stream comprising a methanol/MMA azeotrope and a first distillation column bottoms stream;   (C) Transferring the first distillation column overheads stream to a second distillation column operated at a second pressure, the operating pressure of the second distillation column greater than the operating pressure of the first distillation column;   (D) Separating the first distillation column overheads stream within the second distillation column into a second distillation column overheads stream comprising an amount of methanol/MMA azeotrope that is less than the amount of methanol/MMA azeotrope in the first column overheads stream, and a second distillation column bottoms stream; and   (E) Recovering at least a part of the second distillation column overheads stream.       
 
         [0018]    In one embodiment the methanol and MMA are within a product stream of an MMA manufacturing process in which methacrylic acid and methanol are reacted. In one embodiment the process comprises the further step of recycling at least a part of the second distillation column overheads stream to the reboiler of the first distillation column. In one embodiment the process employs the use of an azeotropic agent. In one embodiment the process does not employ the use of an azeotropic agent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is schematic of one embodiment of the process of this invention. 
           [0020]      FIG. 2  is a graph reporting the concentration of methanol in the vapor and liquid phases at an elevated pressure and temperature of a binary mixture of methanol and MMA, demonstrating that a separation is achievable at an elevated pressure and temperature. 
           [0021]      FIG. 3  is a graph reporting the concentration of methanol in the vapor and liquid phases at a lower pressure and temperature of a binary mixture of methanol and MMA, demonstrating that a separation is not achievable at a lower pressure and temperature since the vapor and liquid compositions are essentially identical. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Definitions 
       [0022]    Unless stated to the contrary, or implicit from the context, all parts and percentages are based on weight and all test methods are current as of the filing date of this application. For purposes of United States patent practice, the contents of any referenced patent, patent application or publication are incorporated by reference in their entirety (or its equivalent US version is so incorporated by reference) especially with respect to the disclosure of definitions (to the extent not inconsistent with any definitions specifically provided in this disclosure) and general knowledge in the art. 
         [0023]    “A”, “an”, “the”, “at least one”, and “one or more” are used interchangeably. The terms “comprises,” “includes,” and variations thereof do not have a limiting meaning where these terms appear in the description and claims. Thus, for example, an process stream that includes “an” azeotropic agent can be interpreted to mean that the process stream includes “one or more” azeotropic agents. 
         [0024]    “Comprising,” “including,” “having,” and their derivatives, are not intended to exclude the presence of any additional component, step or procedure, whether or not the same is specifically disclosed. In order to avoid any doubt, all compositions claimed through use of the term “comprising” may include any additional additive, adjuvant, or compound, whether polymeric or otherwise, unless stated to the contrary. In contrast, the term, “consisting essentially of” excludes from the scope of any succeeding recitation any other component, step, or procedure, excepting those that are not essential to operability. The term “consisting of” excludes any component, step, or procedure not specifically delineated or listed. 
         [0025]    The recitations of numerical ranges by endpoints include all numbers subsumed in that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). For the purposes of the invention, and consistent, with what one of ordinary skill in the art would understand, a numerical range is intended to include and support all possible subranges that are included in that range. For example, the range from 1 to 100 is intended to convey from 1.01 to 100, from 1 to 99.99, from 1.01 to 99.99, from 40 to 60, from 1 to 55, etc. The recitations of numerical ranges and/or numerical values, including such recitations in the claims, can also be read to include the term “about”. In such instances the term “about” refers to numerical ranges and/or numerical values that are substantially the same as those recited. 
         [0026]    “Azeotrope”, “azeotrope mixture” and like terms mean a liquid mixture of two or more substances which behaves like a single substance in that the vapor produced by partial evaporation of the liquid mixture has the same composition as the liquid mixture, and the liquid mixture does not change in composition as it evaporates. Smith and Van Ness,  Introduction to Chemical Engineering Thermodynamics,  3 rd  Ed., p. 312, McGraw-Hill Book Co. As used in the context of this disclosure, the term “azeotrope” includes “near azeotrope” as defined below. 
         [0027]    “Near azeotrope”, “tangent pinch” and like terms mean a liquid mixture of two or more substances in which the relative volatility of the components is so close as to make distillation impractical. This is generally considered to occur when the relative volatility between the components to be separated is below 1.10. 
         [0028]    “Azeotrope agent” and like terms mean a substance that when added to an azeotrope mixture comprising first and second components will form a new azeotrope mixture with one of the first and second components. The new azeotrope mixture will have a boiling point different from the original azeotrope mixture such that the first and second components of the original azeotrope mixture can be separated by distillation, i.e., one of the first and second components will remain with the new azeotrope (either as a distillation overhead or bottom) while the other will separate from the original azeotrope as a distillation overhead or bottom (whatever is the opposite of the new azeotrope). 
         [0029]    “Heat pump” and similar terms mean a device that provides heat energy from a source of heat or “heat sink” to a destination. Heat pumps are designed to move thermal energy opposite to the direction of spontaneous heat flow by absorbing heat from a cold space and releasing it to a warmer one. A heat pump uses some amount of external power to accomplish the work of transferring energy from the heat source to the heat sink. 
       MMA Process 
       [0030]    The process for producing methyl methacrylate by an esterification reaction between methacrolein and methanol is not particularly limited, and may comprise any of a suitable gas phase or liquid phase or slurry phase reaction. How to carry out the reaction is also not particularly limited, and the reaction may be carried out in any of a continuous or batch manner. For example, there can be given a process comprising carrying out the reaction using a palladium-based catalyst in a liquid phase in a continuous manner. The oxidative esterification process is well known. See, e.g., U.S. Pat. Nos. 5,969,178; 6,107,515; 6,040,472; 5,892,102; 4,249,019 and 4,518,796. 
       Methanol/MMA Azeotrope 
       [0031]    In one embodiment, the process for manufacturing MMA produces a MMA/methanol azeotrope that has a composition of 0.92 mole fraction of methanol in the vapor and liquid phases and boils at a temperature of 64.5° C. and a pressure of 1,013 millibar (101.35 kiloPascal). 
       Pressure Swing Distillation Process 
       [0032]      FIG. 1  describes one embodiment of this invention. Equipment representations are made with Aspen software. Feed stream  10  comprising the effluent from a reactor in which oxygen, methacrolein and methanol were reacted to make methyl methacrylate, contains nominally 0.13 mole fraction water, 0.75 mole fraction methanol, 0.038 mole fraction methacrolein, and 0.085 mole fraction MMA, at a temperature of approximately 80° C. (but ranging from 30° C. to 100° C.) enters first (low pressure) distillation column  11  at or near its vertical midpoint. The column is operated at atmospheric pressure, at which the azeotrope between methanol and MMA will occur at the conditions described above, i.e., 0.92 mole fraction of methanol and 0.08 mole fraction of MMA. The overhead stream from the column  11  is taken out of the top of the column in a concentration far enough removed from the azeotrope such that separation is still practical. In the bottom of tower  11 , essentially all of the methanol is removed (99.95% by weight in the example) from the incoming feed material. Column  11  distillate bottoms, or simply “bottoms”, are removed through line  12 , passed through reboiler (i.e., a heat exchanger)  13  in which the temperature of the bottoms is reduced to the condensing temperature at atmospheric pressure, or approximately 60° C. to 70° C., in the Example, and recovered through line  14 . In one embodiment a side stream of the bottoms from column  11  is recycled from reboiler  13  through line  15  to the bottom of column  11  to assist in maintaining the desired operating temperature of column  11 . 
         [0033]    Reboiler  13  and lines  12 ,  14  and  15  are graphical representations of a conventional reboiler circuit. In practice, in a thermosyphon reboiler, liquid leaves the bottom of tower  11  in line  12  and enters reboiler  13 . Part of the liquid is vaporized providing the heat to the tower required for the separation, and a part of the liquid is passed forward as product, i.e., bottoms, through line  14 . 
         [0034]    Distillate overheads, or simply “overheads”, are removed from near the top of column  11  via line  16  and split into two streams by any conventional means, e.g., a forked or Y-pipe. The first stream of overheads from column  11  passes through line  16 A to pump  18  and then through line  19  to second (high pressure) distillation column  17 . The second stream of overheads from column  11  passes from line  16  through line  16 B to condenser  28  from where it is recycled back to the top of column  11  through line  29 . 
         [0035]    In one embodiment distillation column  11  operates at a ratio of pressure to distillation column  17  of at least 1 to 5. In one embodiment column  11  operates at a pressure of 101.325 kilopascals of absolute pressure, where the azeotrope is present, column  17  operates at a pressure of 5 times that, or 506.625 kilopascals, where the azeotrope has been shifted because of the pressure difference. 
         [0036]    The bottoms from column  17  are removed through line  20 , passed through reboiler (i.e., a heat exchanger)  21 , and recycled to the upper half of column  11  via line  22 . In one embodiment a side stream of bottoms is recycled from reboiler  21  through line  23  to the bottom of column  17  to assist in maintaining the desired operating temperature of column  17 . 
         [0037]    In one embodiment the overheads from column  17  pass through line  24  and are split by any conventional means, e.g., a forked or Y-pipe, into lines  24 A and  24 B. The overheads in line  24 A are collected as distillate product, and the overheads in line  24 B are passed to condenser  25 . In one embodiment the overheads from column  17  are recycled to the top of column  17  by line  26 . The recycled overheads from column  17  assist in maintaining the desired operating temperatures of column  17 . 
       Reboiler 
       [0038]    In one embodiment the process of this invention transfers energy in the form of heat from high pressure column  17  to low pressure column  11 . This transfer occurs by removing methanol from condenser  25  to reboiler  13  of low pressure column  11 . This heat transfer reduces the duty required to operate the combined tower operation by 49% of the energy required without the combined heat integration. In the following example, 1.545e+08 watts are transferred from condenser  25  at the top of high pressure tower  17  to the low pressure column  11 . Thus, the external energy input required to operate the low pressure column is reduced by this same amount. This, in essence, is a 50% reduction in the energy input requirements to the separation. 
         [0039]    Although the invention has been described primarily in the context of a process for the manufacture of MMA, the invention has applicability in any circumstance in which an azeotrope of methanol and MMA is to be broken. While the practice of the invention does not require the use of an azeotropic agent, it allows for the use of such an agent if desired. 
         [0040]    The invention is further described, but not limited, by the following numerical simulation example. 
       Example 
       [0041]    The following is a numerical simulation (Aspen Version 8.0) demonstrating the removal of the MMA component from the overhead product of column  17  in  FIG. 1  with only parts per million of methanol remaining in the bottom product of column  17 . The heat can be transferred directly from the vapor leaving the top of column  17  and condensing on the shell or tube side of reboiler  13 . This is the most thermodynamically efficient manner of transferring the heat since there is no entropy loss. For the sake of convenience, the vapor from the top of column  17  may be transferred and condensed to a working fluid, such as water or another convenient heat transfer fluid, and then transferred to reboiler  13 . 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Numerical Simulation of Breaking a Methanol/MMA Azeotrope 
               
             
          
           
               
                   
                 10 
                 24A 
                 14 
               
               
                   
                 FEED 
                 DISTILLATE 
                 BOTTOMS 
               
               
                   
               
             
          
           
               
                 Temperature ° C. 
                 80 
                 110.5 
                 77.9 
               
               
                 Pressure bar 
                 4.9 
                 5 
                 1 
               
               
                 Mass Frac 
                   
                   
                   
               
               
                 H2O 
                 0.06 
                 293 PPM 
                 0.248 
               
               
                 MEOH 
                 0.623 
                 0.822 
                 0.001 
               
               
                 MAL 
                 0.069 
                 0.091 
                 105 PPB 
               
               
                 MMA 
                 0.22 
                 0.083 
                 0.647 
               
               
                 Mole Frac 
                   
                   
                   
               
               
                 H2O 
                 0.128 
                 585 PPM 
                 0.649 
               
               
                 MEOH 
                 0.741 
                 0.921 
                 0.002 
               
               
                 MAL 
                 0.038 
                 0.047 
                  71 PPB 
               
               
                 MMA 
                 0.084 
                 0.03 
                 0.305 
               
               
                 Mole Flow kmo1/h 
                   
                   
                   
               
               
                 H2O 
                 265.645 
                 0.977 
                 264.637 
               
               
                 MEOH 
                 1540.043 
                 1539.324 
                 0.85 
               
               
                 MAL 
                 78.226 
                 78.224 
                 &lt;0.001 
               
               
                 MMA 
                 174.108 
                 49.804 
                 124.28 
               
               
                   
               
               
                 H 2 O-Water 
               
               
                 MeOH-Methanol 
               
               
                 MAL-Methacrolein 
               
               
                 MMA-Methyl Methacrylate 
               
               
                 PPM-Parts per million 
               
               
                 Frac-Fraction 
               
               
                 Kmol/h-kilomoles per hour 
               
             
          
         
       
     
         [0042]    In  FIG. 1  and the numerical information of Table 1, the main entrance and exit streams to separation columns  11  and  17  are shown. Feed stream  10  enters column  11  from the upstream reactor. It contains unreacted methanol, water produced in the stoichiometry, and the MMA produced in the reaction, along with some unreacted MAL. This feed is introduced to low pressure column  11  where below the feed point, stripping of the light keys and anything lighter than the light key takes place. Essentially all of the methanol is removed from the feed, and the bottoms stream exits low pressure column  11  through line  14  free of methanol. The upper section of the low pressure column, the rectification section, enriches the material in methanol past the point that the azeotrope that exists at lower pressure would allow. The material is condensed in condenser  28 , and part is returned as reflux to column  11  through line  16 . The other part is pumped to high pressure tower  17 . 
         [0043]    As seen in  FIG. 2  and  FIG. 3 , the azeotrope between methanol and MMA is shifted by the increase in pressure. Thus, the enrichment of methanol can proceed by rectification at the higher pressure. 
         [0044]    While the azeotrope can be shifted by higher pressure, the relative volatility between methanol and MMA is still between 1.2 and 1.4 at a pressure of 500 kilopascals, requiring that a relatively high reflux ratio be used to rectify the remainder of the methanol. This results in a high energy consumption without the addition of the integration to utilize the heat from the high pressure column to the low pressure column.