Abstract:
An integrated process and system for synthesis of organic-acid esters is provided. The method of synthesizing combines reaction and distillation where an organic acid and alcohol composition are passed through a distillation chamber having a plurality of zones. Side reactors are used for drawing off portions of the composition and then recycling them to the distillation column for further purification. Water is removed from a pre-reactor prior to insertion into the distillation column. An integrated heat integration system is contained within the distillation column for further purification and optimizing efficiency in the obtaining of the final product.

Description:
RELATED PATENT APPLICATIONS 
     This Application claims priority to U.S. Provisional Patent Application Ser. No. 61/961,526, filed on 17 Oct. 2013, and is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under DE-SC0008290 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This method and system relates to an integrated distillation process with side reactors and water separation units for manufacture of industrial chemicals. In particular, this system and method relates to an integrated distillation process which provides for a reactive distillation column having a multiplicity of zones through which a pre-reacted alcohol and organic acid composition is passed. 
     Still further, this invention relates to a system and method for synthesizing organic-acid esters where the distillation column includes a plurality of zones where a portion of the composition is removed in a side-draw for passage through side reactors. 
     Still further, this invention pertains to a system and method for synthesizing organic acid esters where the drawn portion of the composition reacted in side reactors is heated for recycling of a vapor stream into an overhead vapor stream of the distillation column and simultaneously returning a liquid stream to a zone of the distillation chamber below the zone from which the side draw was transported to the side reactors. Further, this invention relates to a system and method for synthesizing organic-acid esters where particular zones within the distillation column include heat integration systems where heated liquid from heaters are returned to a zone for passage through heat exchange tubes for controlling the temperature parameters of the distillation system. 
     Further, the subject system and method for synthesizing organic-acid esters includes reboilers which are contained within the distillation column in predetermined zones for further adjusting temperature parameters between zones of the distillation column. 
     RELATED PRIOR ART 
     Esters are synthesized by reacting organic acids with alcohols. Prior art esterification processes having separate chemical reaction and separation processes are generally energy and capital intensive. An integrated process of reactive distillation would combine chemical reaction and separation into a single process unit which would significantly reduce energy consumption and capital costs and is a long needed solution to the problem of synthesizing organic-acid esters. 
     Eastman Chemical has provided a commercial methyl acetate process with a single step esterification reaction using reactive distillation to perform five functions in one column which has resulted in a 60% reduction of capital expenditure and energy consumption relative to the conventional unit operation design. Application of reactive distillation to esterification of carboxylic acids with multiple acid groups such as citric acid to form tri-ethyl citrate has not been practiced commercially with success. The Michigan State University has developed conventional reactive distillation processes, however, there are challenges of slow reaction kinetics and thermodynamic limitations rendering a traditional unit operation based process with significant disadvantages, and conventional reactive distillation requires extremely large columns and significant energy usage to produce the esters. Thus, advances in reactive-distillation processing are needed to meet the low capital and energy cost requirements necessary for commercial competitiveness. 
     At the present time, most esters of organic acids are produced batch-wise from multi-step reversible reactions between multi-functional acids and one alcohol, such as triethyl citrate from ethanol or a multi-functional alcohol and one acid, such as isosorbide di-esters. The reversible reactions require an excess of lighter reactant in order to strip off water by-product and drive the reaction to completion. Such batch processes are energy inefficient and require large amounts of the excess reactant and result in a batch-to-batch produce quality variation. Thus, there is the need for a continuous process system and method which provides a uniform product quality with less raw material waste and improved thermal management. 
     It is not believed that there are any heat integrated distillation side reactor based processes for manufacturing of multifunctional esters such as triethyl citrate. Reactive distillation based multifunctional ester processes have been shown in prior systems published by Michigan State University. The subject method and system of heat integrated distillation side reactor processing with a water separation unit such as a PerVap membrane is in direct opposition with the conventional process of separate reaction and multiple separation and purification steps where the subject system and method includes the recycling of unreacted intermediate products. 
     Esterification of organic acids with alcohols produce water, which induces reverse reactions and thereby limits conversion to chemical equilibrium. In order to minimize such reverse actions, excess reactant with higher volatility (either alcohol or organic acid) is used to carry water to the overhead of the distillation column which requires vaporization in the column and subsequent condensation in the overhead condenser resulting in a high energy consumption and larger distillation column. The subject system is based on the application of a water separation unit such as a PerVap or molecular sieve bed or another standard type aqueous/organic phase separator to separate product water from the effluent streams of a pre-reactor as well as side reactors in an integrated process of permeation and evaporation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram of the system and method for synthesizing organic-acid esters; 
         FIG. 2  is a schematic drawing of a heat integration system contained within a distillation column of the subject invention; 
         FIG. 3A  is a schematic drawing of a heat integration system mounted on a distillation tray within a distillation column where catalyst containers are mounted between heat transfer coiled tubes; 
         FIG. 3B  is directed to an embodiment of the heat integration system showing heat transfer helically wound coiled tubes embedded within a catalyst container or structure packing which are mounted on distillation trays within the distillation column; 
         FIG. 4  is a schematic drawing showing brazed fins for holding catalyst bags or structure packing where the heat transfer fluid is passed through the catalyst containers; and, 
         FIG. 5  is a schematic flow diagram showing the system and method for a commercial plant in the synthesis of organic-acid esters. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , there is shown an integrated process for manufacturing of esters of organic acids through the reaction of organic acid esterification with alcohol. An alcohol and organic composition is contained in tank  10 . The alcohol and organic acid composition is drawn through tank line  12  by pump  14  and possibly preheated in preheater  16  prior to insertion into pre-reactor  18  on line  20 . 
     In general, the subject process and system is adaptable to a wide range of alcohols and is particularly directed for use with ethanol, butanol, hexanol, 2-ethyl hexanol, 3,5,5-trimethylhexanol, nonyl alcohol, isonyl alcohol, tridecanol, methanol, benzyl alcohol and isosorbide. 
     Similarly, the organic acid may be chosen from organic acids such as citric acid, adipic acid, succinic acid, 1,4-cyclohexane dicarboxylic acid, 1,2-cyclohexane dicarboxylic acid and a mixture of fatty acids derived from vegetable oils. 
     Reaction of the alcohol and organic acid composition is provided in pre-reactor  18  which is maintained at a predetermined temperature and pressure, dependent upon the parameters and composition of the alcohol and organic acid to provide a reaction product mixture of alcohol and organic acid. Such pre-reactors  18  are well-known in the art and provide for reaction of the inserted composition. The reaction product mixture is then passed on pre-reactor line  22  to first water separation unit  24 . 
     First water separation unit  24  may provide for a PerVap membrane, a molecular sieve bed, or another standard type aqueous/organic phase separator. First water separation unit  24  effectively separates water from the pre-reactor  18  effluent or reaction product mixture which permits a sufficient quantity of excess, unreacted reactant (alcohol) which leaves water separation unit  24  to enter the distillation column  26 , as will be described in following paragraphs. 
     First water separation unit  24  serves to separate water from pre-reactor  18  effluent and a vapor and liquid stream of water is passed on first water separation unit outflow line  28  to first condenser  30  for removal of a water-rich stream on condenser effluent line  32 . Unreacted alcohol which has not been reacted in pre-reactor  18  in combination with the organic acid is inserted into distillation column  26  on water separation unit line  34 . Distillation column  26  consists of a plurality of zones which include upper distillation zone  36  and bottom or lower distillation column zone  38  with intermediate distillation zones represented in  FIG. 1  by elements  40 ,  40 ′,  42  and  42 ′. Only two intermediate zones will be discussed in general for clarity purposes. However, it is to be clearly understood that distillation column  26  may include a plurality of first and second representative distillation column zones  40  and  42 . 
     Upon entry of unreacted alcohol and organic acid into upper distillation column zone  36  on water separation unit line  34 , there is a mixture of liquid and vapor. A column vapor stream is removed on overhead vapor line  44  and inserted into second condenser  46  which condenses the column vapor stream in overhead vapor line  44  into a liquid stream on second condenser output line  48  for reinsertion into upper distillation column zone  36  for passage through the representative first and second distillation column zones  40  and  42 . The vapor residual is passed on vapor effluent condenser line  50  for insert into second water separation unit  52  which as was the case for first water separation unit  24  may be a PerVap membrane, a molecular sieve bed, or other type of standard aqueous/organic phase separator. Water is drawn from second water separation unit  52  on output line  56  to third condenser  54  for condensing the vapor into a water rich stream on third condenser line  58 . Water separated overhead vapor line  60  passes from second water separation unit  52  to fourth condenser  62  with excess reactant alcohol being passed on fourth condenser line  64  to be passed to representative side reactors  74 ,  76 ,  100  to be further discussed in following paragraphs. 
     Thermal heat pump  66  is used for recovery of latent heat of condensation from vapor stream on line  60  for use in intermediate heat exchangers to be discussed in following paragraphs. A commercially available refrigerant pair such as ammonia/water mixture or other organic pairs may be used in thermal heat pump  66 . Refrigerant  68  recovers the latent heat of condensation and transfers upgraded heat at higher temperatures as represented by upgraded heat transfer lines  70  to the intermediate heat exchangers. Refrigerant  68  recovers the heat and transfers the upgraded heat at higher temperature to the overall process by using the upgraded heat within the intermediate heat exchangers. Thermal heat pump  66  may be operated by waste heat transferred to it on line  72  which is generally available in a commercial plant. Thermal heat pump  66  significantly improves the overall efficiency by recovering and utilizing latent heat from the vapor stream which otherwise would be rejected to cooling water or the external environment. 
     Further referring to  FIG. 1 , there is provided top side reactor  74  and intermediate side reactor  76  which is representative of one or more side reactors in the distillation column  26 . Only two side reactors  74  and  76  are shown for clarity purposes. The reaction mixture being inserted into distillation column  26  on water separation unit line  34  passes into first representative upper distillation column zone  40  a portion of which is passed on zone output line  78  to second pump  80  for insertion into top side reactor  74 . A further reaction is provided in top side reactor  74  with the reacted composition being inserted into first heat exchanger  82  by passage on first heat exchanger input line  84 . As is seen, thermal heat pump  66  provides additional heat to first heat exchanger  82  for increasing the overall efficiency of the process. Once the effluent from top side reactor  74  is heated, heat exchanger  82  passes the vapor stream on heat exchanger output line  86  to overhead vapor line  44  for insertion into second condenser  46  in a recycling process. 
     Heat exchanger  82  heats the liquid to both a vapor and a liquid portion where the liquid portion is inserted on line  88  to a next sequentially lower zone  42  for further reaction within distillation column  26 . 
     Intermediate zones  40 ′ and  42 ′ operate in substantially the same process mode as that provided for first representative upper distillation column zone or top zone  40  with the exception that intermediate side reactors represented by intermediate side reactor  76  provides for fresh and recycled or recovered excess reactant (alcohol) being inserted on recycle line  90  for passage through a fifth condenser  92  prior to insertion into intermediate side reactor  76 . The reacted composition inside reactor  76  will then pass through line  84 ′ to heat exchanger  82 ′ and then pass the vapor on heat exchanger output line  86  back to overhead vapor line  44 . The liquid portion of the heat exchanger effluent is then passed on heat exchanger liquid line  88 ′ to second representative intermediate zone  42 ′. 
     The distilled reactant is passed to bottom or lower distillation column zone  38  subsequent to passage through distillation column  26 . Distillation reactant line  94  draws off the reacted and distilled composition through distilled reactant line  94  to be pumped by third pump  96  into line  108  where it is combined with recycle line  98  which carries fresh and recycled or recovered excess reactant (alcohol) for insertion into product side reactor  100  for passage to product heat exchanger  102  on line  108 . Vapor stream from heat exchanger  102  is passed on vapor product line  104  back to distillation column  26  into bottom or lower distillation column zone  38 . Product line  106  then provides for the product ester of high purity on line  106 . 
     Of importance is the use of first water separation unit  24  which effectively separates the water from the pre-reactor  18  effluent or reaction product mixture which allows a sufficient quantity of excess, unreacted reactant (alcohol) leaving the pre-reactor  18  to enter distillation column  26  and subsequently and initially being fed to first side reactor  74  which reduces or eliminates the requirement of additional excess reactant (alcohol) to be fed to the side reactor  74 . By effectively separating product water on line  32  subsequent to pre-reaction in pre-reactor  18 , the process energy consumption is reduced by more than 40%. Further reduction may be possible by implementing an additional water separation unit  24  in the excess reactant (alcohol) recovery process. 
     Table I represents the improvements when a water separation unit  24  (in the form of a PerVap membrane) is inserted between the pre-reactor  18  and distillation column  26 . The Table is depicted for reaction of citric acid esterification with ethanol. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                 With Separation 
                 With Water 
                   
               
               
                   
                   
                 of Water from 
                 Separation 
                   
               
               
                   
                   
                 Pre-reactor 
                 of Pre- 
                 Without 
               
               
                   
                   
                 Product Stream 
                 reactor 
                 Water 
               
               
                   
                 Process 
                 and Overhead 
                 Product  
                 Separation 
               
               
                 Item 
                 Parameter 
                 Product 
                 Stream 
                 Units 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Production 
                 40.0 KTA 
                 40.0 KTA 
                 40.0 KTA 
               
               
                   
                 Capacity 
                   
                   
                   
               
               
                 2 
                 Feed Rate - 25% 
                 10,616 kg/hr 
                 10,460 kg/hr 
                 10,460 kg/hr 
               
               
                   
                 citric acid in 
                   
                   
                   
               
               
                   
                 ethanol 
                   
                   
                   
               
               
                 3 
                 Recycled ethanol 
                 3,063 kg/hr 
                 3,500 kg/hr 
                 6,230 kg/hr 
               
               
                   
                 feed to side 
                   
                   
                   
               
               
                   
                 reactors 
                   
                   
                   
               
               
                 4 
                 Product Stream 
                   
                   
                   
               
               
                 5 
                 Flow rate 
                 5,485 kg/hr 
                 5,495 kg/hr 
                 5,460 kg/hr 
               
               
                 6 
                 Tri-ethyl 
                 90% by wt 
                 90% by wt 
                 90% by wt. 
               
               
                   
                 concentration 
                   
                   
                   
               
               
                 7 
                 Product yield based 
                 96% by mol. 
                 96% by mol. 
                 96% by mol. 
               
               
                   
                 on citric acid 
                   
                   
                   
               
               
                 8 
                 Side Reactors 
                   
                   
                   
               
               
                 9 
                 Temperature 
                 107° C. 
                 105° C. 
                 105° C. 
               
               
                 10 
                 Pressure 
                 2.5 atm 
                 2.5 atm 
                 2.5 atm 
               
               
                 11 
                 Distillation columnn 
                   
                   
                   
               
               
                 12 
                 Reflux temperature 
                 107° C. 
                 103° C. 
                 105° C. 
               
               
                 13 
                 Bottom 
                 150° C. 
                 148° C. 
                 105° C. 
               
               
                   
                 temperature 
                   
                   
                   
               
               
                 14 
                 Energy 
                 3,080 kW 
                 5,985 kW 
                 10,255 kW 
               
               
                   
                 Consumption 
                   
                   
                   
               
               
                 15 
                 Synthesis process 
                 2,520 kW 
                 2,520 kW 
                 4,160 kW 
               
               
                 16 
                 Ethanol recovery 
                   
                 3,465 kW 
                 6,095 kW 
               
               
                   
                 unit - distillation 
               
               
                   
               
             
          
         
       
     
     In the overall process, heat integration is an important part of the concept for increasing energy efficiency and maintaining optimum conditions while minimizing adverse impacts of process condition perturbations, such as changes in reflux ratio or condenser temperature to maximize conversion and separation. As has previously been discussed in  FIG. 1 , latent heat liberated in condensing the overhead vapor passing from upper distillation column zone  36  may be recycled into intermediate reboilers or heat exchangers  82 ,  82 ′, using the thermal heat pump  66 . 
     It is to be noted that the latent heat of condensing the overhead vapor may also be recycled using a thermal or mechanical vapor recompression heat pump. In a particular instance, the use of a thermal heat pump such as  66 , has been shown to reduce energy consumption in hydrogen peroxide distillation by up to 56%. As is always the case, the relative economics between a mechanical vapor recompression heat pump and a thermal heat pump would depend on comparative costs of electricity, steam, or recovery of available waste heat. 
     Internal heat integration units  314  focus on process controls by maintaining parameters within an optimum operating temperature envelope. Internal heat integration may be provided by providing cooling as singular or multiple heat integration units or multiple internal refluxes  314 . Internal reboilers  316  may be incorporated as shown in  FIG. 1  in one or more distillation zones of distillation column  26 . Such internal reboilers  316  provide heat as necessary for the obtaining of optimum temperature profiles of the distilling composition. 
     Prior art distillation processes are generally controlled by monitoring temperatures and pressures as well as minor fluctuation in the process parameters by compensation through use of linear programming methods. With the use of side reactors such as  74  and  76 , the interactive effects of reaction and separation require a new generation of process controls and thermal management within distillation column  26 . The subject process is unlike the governing of temperature profiles by boiling points of top and bottom products. This concept process is governed by the product streams from side reactors such as  74  and  76 . In order to maintain predetermined temperature profiles for particular compositions, internal heat integration is required with external feedback controls. 
     Referring now to  FIG. 2 , there is shown a schematic view of a representative heat integration system  300  mounted internal to distillation column  26  in a predetermined zone or zones. Heat integration system  300  may include sieve or valve distillation tray  302  with liquid flow passing in liquid flow direction  310 . Catalyst containers  308  are mounted or sandwiched between heat transfer coiled tubes  306  as provided in  FIG. 2 . Heat transfer tube headers  320  are provided with the weirs  304  established on opposing ends of the rows of heat transfer coil tubes  306  and the sandwiched catalyst containers  308 . 
       FIGS. 3A and 3B  schematically show two forms of heat integration system  300 .  FIG. 3A  provides for heat transfer coil tubes  306  for transporting heat from tubes  306  to catalyst container  308  with a liquid flow direction  310  where catalyst containers  308  are sandwiched between sequentially mounted tubes  306 . Vapor flow passes in direction  318  with liquid flow in direction  310  for passage over heat transfer coil tubes  306 . 
     In another type of heat integration system  300 ′, coil tubes  306  as shown in  FIG. 3B  are provided as being embedded within catalyst containers  308  mounted on sieve or valve distillation trays  302 . Alternatively, catalyst container  308  may be mounted internal the helically wound heat transfer tubes  306 . 
     Referring to  FIG. 4 , there is shown a further embodiment of heat integration system  300 ″ which provides for a system whereby catalyst container bags  308  are mounted on sieve distillation tray  302  and include a plurality of brazed fins  324  to hold the catalyst containers  308  while providing for an extended heat transfer area. Input flow header  320  provides for the entry of the heat transfer fluid and output flow header  322  returns the heat transfer fluid to the process system. 
     Process design of a commercial distillation plant is shown in  FIG. 5  with design parameters for a 40,000 tons per annum commercial plant as shown in Table I. Feed enters through stream line  201  and is reacted in pre-reactor  202  to produce a reaction product mixture. Reactor effluent  210  is fed to PerVap membrane  207  where water produced in the reaction in the pre-reactor  202  is removed and condensed and exits as a condensed water product on line  208 . In this example, the alcohol and organic acid composition is ethanol and citric acid. The remaining excess alcohol and citrate esters are also exiting the PerVap unit  207  and enter the distillation column in line  209 . As was presented in the process  FIG. 1 , the heavier esters travel down the column and are removed in side streams and fed to side reactors  200 ,  203 ,  204 , and  205 . In all but the top-most side-stream/side-reactor configuration  200 , fresh and/or recycled ethanol  206  is brought back on lines  206   a ,  206   b , and  206   c  which are combined with the liquid side-draw  210   a ,  210   b , and  210   c  prior to entering the reactors  203 ,  204 , and  205 . 
     The fresh and/or recycled ethanol stream may be preheated to maintain the reactor feed at desired temperatures. The effluent stream  211   a ,  211   b , and  211   c  from each of the respective side reactors  203 ,  204 , and  205  are then fed to a reboiler  212   a ,  212   b ,  212   c  where the byproduct water and unreacted ethanol are vaporized prior to the remaining liquid being returned to the column stage or zone below the corresponding side-draw. 
     For all but the final reboiler  212   c , the vapor streams  213 ,  213   a ,  213   b  are combined with the column overhead stream  215 . Any triethyl citrate which was partially vaporized from the vapor streams is condensed and returned to the distillation column on line  217  in order to optimize product recovery. The vapor stream  218  is then passed through a molecular sieve for separation of water  220  from the ethanol-rich stream  221 . Ethanol may be further concentrated by distillation for separation of by-product diethyl ether  222 . Ethanol with a purity of &gt;99 wt %  206  is then recycled into the side reactors. The reboiler  212   c  from the final side reactor  205  is employed as the column&#39;s reboiler and the vapor is returned to the bottom zone of the column through line  223 . It has been found that the product stream has a 90 wt % triethyl citrate  224  being withdrawn as a liquid stream from reboiler  212   c.    
     Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention as defined in the appended claims. For example, functionally equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular locations of elements, steps, or processes may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.