System and method for synthesis of dialkyl carbonates using carbon dioxide reaction with methanol and ammonia

Abstract

A method and system for membrane-assisted production of high purity concentrated dimethyl carbonate by the reaction of carbon dioxide and methanol is provided. Carbon dioxide is recovered from flue gas or other dilute streams from industrial processes by a membrane and subsequent conversion takes place to an intermediate methyl carbamate by reacting of carbon dioxide with ammonia and methanol. For high-purity carbon dioxide obtained by one of the carbon capture technologies or by a process (such as, for example, ethanol fermentation process) the membrane reactor is replaced with a catalytic reactor for direct conversion of carbon dioxide to methyl carbamate by reacting with ammonia and methanol. The methyl carbamate is further reacted with methanol for conversion to dimethyl carbonate. An integrated reactive distillation process using side reactors is used for facilitating the catalytic reaction in the subject method for producing high purity dimethyl carbonate.

Claims

1. A method for synthesis of dimethyl carbonate, comprising: (a) establishing a system including a reactor sub-system comprising a membrane reactor and a catalytic reactor operatively coupled to an output of said membrane reactor, and a distillation sub-system comprising a reaction distillation column, a product distillation column in a thermal coupling with said reaction distillation column, and a plurality of side reactors operatively coupled to said reaction distillation column of said distillation sub-system, said distillation sub-system being operatively coupled to said reactor sub-system, (b) capturing and supplying carbon dioxide (CO.sub.2) into said membrane reactor of said reactor sub-system, (c) feeding methanol and ammonia into said membrane reactor of said reactor sub-system, and (d) reacting said carbon dioxide (CO.sub.2) with said methanol and ammonia in said membrane reactor of said reactor sub-system, thus forming a membrane reactor product comprising dimethyl carbonate, unreacted methanol, unreacted ammonia, and methyl carbamate, (e) feeding a membrane reactor product stream of said membrane reactor product formed in said membrane reactor of said reactor sub-system into said reaction distillation column of said distillation sub-system, (f) distilling said membrane reactor product in said reaction distillation column of said distillation sub-system to separate the unreacted ammonia from said membrane reactor product, recovering the unreacted ammonia from said reaction distillation column of said distillation sub-system, and recycling said unreacted ammonia to said membrane reactor of said reactor sub-system, (g) recovering said dimethyl carbonate, methyl carbamate, and unreacted methanol from said reaction distillation column of said distillation sub-system, feeding said recovered dimethyl carbonate, methyl carbamate, and unreacted methanol to a plurality of side reactors to form a concentrated dimethyl carbonate at an output of said reaction distillation column of said distillation sub-system, and recycling the concentrated dimethyl carbonate to an input of said reaction distillation column of said distillation sub-system, (h) separating unreacted methanol by distillation within said reaction distillation column of said distillation sub-system, recovering the separated unreacted methanol from said reaction distillation column of said distillation sub-system, and recycling the recovered methanol to said membrane reactor of said reactor sub-system, and (i) supplying a concentrated dimethyl carbonate stream of said concentrated dimethyl carbonate from the output of said reaction distillation column of said distillation sub-system to an input of said product distillation column of said distillation sub-system, distilling said concentrated dimethyl carbonate in said product distillation column of said distillation sub-system and recycling said distilled concentrated dimethyl carbonate stream via said product distillation column of said distillation system, thus obtaining a substantially pure dimethyl carbonate product.

2. The method of claim 1, further comprising: in said step (b), capturing said carbon dioxide from combustion flue gas or a dilute industrial process stream for said reaction in said membrane reactor, and delivering high-purity carbon dioxide captured from the flue gas or the dilute industrial stream in said catalytic reactor, in said step (c), feeding a recycled ammonia and methanol into said catalytic reactor of said reactor sub-system, following said step (d), feeding a vapor phase of said membrane reactor product from said membrane reactor into said catalytic reactor of said reactor sub-system to further react an unreacted carbon dioxide remaining in said membrane reactor product with the recycled ammonia and methanol, thus producing a catalytic reactor product.

3. The method of claim 2, further comprising: in said step (a), coupling a first permeation-vaporization (PerVap) membrane to an output of said membrane reactor and to an output of said catalytic reactor of said reactor sub-system, and following said step (d), directing said membrane reactor product stream from said output of said membrane reactor and from said output of said catalytic reactor product, respectively, to said first PerVap membrane to separate byproduct water therefrom and to form a PerVap membrane product stream containing methyl carbamate and ammonia.

4. The method of claim 3, further comprising: in said step (a), operatively coupling said reaction distillation column to an output of said first PerVap membrane, and in said step (e), feeding said reaction distillation column of said distillation sub-system with said first PerVap membrane product stream from the output of said first PerVap membrane.

5. The method of claim 4, further comprising: in said step (a), operatively coupling an ammonia rectification column to an upper output of said reaction distillation column, and in said step (f), recycling said unrecovered ammonia from the upper output of said reaction distillation column to said ammonia rectification column, thus producing rectified ammonia.

6. The method of claim 5, further comprising: recycling said rectified ammonia from said ammonia rectification column into said membrane reactor, and reacting said rectified ammonia from said ammonia rectification column with said carbon dioxide captured in said step (b) and said methanol fed in said step (c) in said membrane reactor, thus producing methyl carbamate.

7. The method of claim 5, further comprising: in said step (a), operatively coupling said plurality of side catalytic reactors to a bottom output of said reaction distillation column, and in said step (g), recycling said methyl carbonate, methyl carbamate, and unreacted methanol from said bottom output of said reaction distillation column of said distillation sub-system through said plurality of side catalytic reactors, thus producing said concentrated dimethyl carbonate at outputs of said plurality of side catalytic reactors, and circulating said concentrated dimethyl carbonate from said outputs of the plurality of side catalytic reactors to said reaction distillation column at least twice, thus recovering said concentrated dimethyl carbonate.

8. The method of claim 7, further comprising: in said step (a), configuring said reaction distillation column with a plurality of distillation stages, in said step (g), drawing a mixture of dimethyl carbonate, methyl carbamate and unreacted methanol from at least a bottom distillation stage of said plurality of distillation stages of said reaction distillation column; passing said drawn mixture of dimethyl carbonate, methyl carbamate and unreacted methanol through said plurality of side catalytic reactors for producing said concentrated dimethyl carbonate; returning a mixture of dimethyl carbonate, methyl carbamate and unreacted methanol containing said concentrated dimethyl carbonate from said bottom output of said reaction distillation column to said reaction distillation column, and producing a vapor phase of said concentrated dimethyl carbonate in said reaction distillation column; withdrawing said mixture containing said concentrated dimethyl carbonate in the vapor phase from a middle distillation stage of said plurality of distillation phases of said reaction distillation column; and recycling a bottom product from said bottom output of said reaction distillation column to said plurality of side catalytic reactors, wherein said bottom product includes unreacted methyl carbamate.

9. The method of claim 8, further comprising: in said step (a), operatively coupling at least one second PerVap membrane to said reaction distillation column, and in said step (h), condensing said mixture containing said concentrated dimethyl carbonate in the vapor phase, subsequently feeding said condensed mixture containing said concentrated dimethyl carbonate into said at least one second PerVap membrane for selective separation of said unreacted methanol from said condensed mixture containing concentrated dimethyl carbonate, recovering said separated unreacted methanol from said reaction distillation column, and recycling the recovered unreacted methanol to said membrane reactor of said reactor sub-system; feeding a second PerVap product stream containing dimethyl carbonate from said at least one second PerVap membrane into said product distillation column to produce said substantially pure dimethyl carbonate product; coupling at least one third PerVap membrane to an upper portion of said product distillation column; and condensing said second PerVap product stream and feeding said condensed second PerVap product stream from said product distillation column into said at least one third PerVap membrane for selective separation of methanol from said condensed second PerVap product stream.

10. The method of claim 9, further comprising: operating said product distillation column at a high pressure for separation of methanol and dimethyl carbonate from an azeotropic mixture thereof; and recovering said substantially pure dimethyl carbonate product as a bottom product of said product distillation column.

11. The method of claim 1, where said reaction distillation column is equipped with thermally active trays disposed at selected locations of said reaction distillation column.

12. The method of claim 1, where said membrane reactor is configured to recover and concentrate carbon dioxide from a dilute carbon dioxide stream and includes at least one membrane module selected from a group consisting of: a membrane module having a plurality of membranes defining flow passages therebetween and catalysts packed in said flow passages, and a membrane module with catalysts embedded on a membrane surface for conversion of carbon dioxide to methyl carbamate by reacting with ammonia and methanol, and wherein said catalytic reactor is selected from a group consisting of: a trickle-bed reactor, a packed-bed up-flow reactor, and a fluidized-bed reactor, said catalytic reactor being configured for conversion of captured high-concentration carbon dioxide to methyl carbamate by reacting with ammonia and methanol.

13. The method of claim 9, further comprising: in said step (a), operatively coupling a condenser unit to said reaction distillation column, in said step (g), drawing a product mixture of ammonia, unreacted methanol and dimethyl carbonate from an upper stage of said plurality of distillation stages of said reaction distillation column; condensing said ammonia and unreacted methanol in said condenser unit; in said step (f), charging said product mixture to said ammonia rectification column, and charging said rectified ammonia from said ammonia rectification unit to said membrane reactor; and in said step (h), recycling said bottom product from said reaction distillation column to at least one of said plurality of side reactors or into said product distillation column of said distillation sub-system.

14. The method of claim 7, further comprising: in said step (a), operatively coupling at least one side catalytic reactor of said plurality of side catalytic reactors to the bottom output of said reaction distillation column, and converting methyl carbamate drawn from said reaction distillation column to the concentrated dimethyl carbonate in said at least one side catalytic reactor.

15. The method of claim 14, further comprising: feeding said concentrated dimethyl carbonate composition from said at least one side catalytic reactor into the product distillation column at a location in said product distillation column below a location where the product mixture is drawn to said at least one side catalytic reactor.

16. The method of claim 7, further comprising: passing said concentrated dimethyl carbonate through a plurality of distillation stages in said product distillation column in a direction from a top distillation stage towards a lower distillation stage of said product distillation column.

17. The method of claim 14, further comprising: circulating the concentrated dimethyl carbonate composition in the vapor phase from the output of said product distillation column to an input of said product distillation column for producing a highly-concentrated substantially pure dimethyl carbonate product.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic flow diagram of the subject membrane-assisted alkyl-carbonate process for dilute sources of carbon dioxide;

(2) FIG. 2 is a schematic flow diagram of the catalytic direct conversion process for high-purity captured carbon dioxide;

(3) FIG. 3 is a schematic flow diagram of the catalytic direct conversion process for the divided-wall column;

(4) FIG. 4. is a schematic representation of the catalytic reactor with down-flow of reactants integrated with permeation-vaporization (PerVap) membrane;

(5) FIG. 5. is a schematic representation of the catalytic reactor with up-flow of reactants integrated with PerVap membrane;

(6) FIG. 6. is a membrane element with catalyst packed in the reactant flow channel of the membrane reactor;

(7) FIG. 7. is a membrane element with catalyst embedded on the membrane surface on the side of the reactant flow channel in the membrane reactor;

(8) FIG. 8. is a schematic illustration of the membrane reactor with shell-and-tube module with cross-flow configuration;

(9) FIG. 9. is a schematic representation of the membrane reactor with shell-and-tube module with parallel-flow configuration;

(10) FIG. 10. depicts the membrane reactor with plate-and-frame module with cross-flow configuration;

(11) FIG. 11. is a schematic diagram of plate heat exchanger based on which plate-and-frame membrane reactor modules can be assembled;

(12) FIG. 12. is a figure depicting kinetic test data for conversion of urea to methyl carbamate;

(13) FIG. 13. is a schematic flow diagram of the prototype test unit for measuring performance parameters of side reactor; and

(14) FIG. 14. is a schematic flow diagram of the test unit for measuring performance parameters of the PerVap membrane for selective separation of methanol from azeotrope of methanol and dimethyl carbonate.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

(15) The subject process for producing purified and concentrated dimethyl carbonate, as illustrated in FIGS. 1-3, uses carbon dioxide (CO.sub.2) as feed stock. In the present process, carbon dioxide (CO.sub.2) is either captured from a flue gas using a membrane reactor, or, alternatively, concentrated carbon dioxide is used which is captured by one of the commercial processes, such as, for example, the Amine absorption process.

(16) As shown in FIGS. 1-3, a reactive distillation column is equipped in the subject process with side reactors, and the permeation-vaporization (PerVap) membrane is integrated with either a membrane reactor or a catalytic reactor for direct conversion of carbon dioxide by reacting with ammonia and methanol.

(17) The subject system 10, as shown in FIG. 1, is designed for synthesis of alkyl carbonates using carbon dioxide recovered from flue stream 12 gases using a membrane reactor system 16. The CO.sub.2 rich flue gas 12 enters the membrane reactor 16. Subsequently, CO.sub.2 is recovered from the flue gas 12, and reacts with methanol and ammonia to form methyl carbamate, which is an intermediate for synthesis of dimethyl carbonate. The carbon dioxide lean stream 14 exits from the membrane reactor 16 after CO.sub.2 is recovered from the flue gas 12 in the membrane reactor 16.

(18) As shown in FIG. 1, the recycled ammonia (NH.sub.3) is fed to the membrane reactor 16 via the recycled ammonia feed line 20. Recycled methanol (CH.sub.3OH) is fed from lines 168 and 128 along with fresh methanol via the recycled methanol line 18 and is mixed with the recycled ammonia line 20.

(19) The mixture of streams 18 and 20 can be in the liquid or the vapor phase before entering into the membrane reactor 16. Carbon dioxide permeating though the membrane in the membrane reactor 16 reacts with ammonia and methanol entered via the streamlines 18 and 20.

(20) The resulting product methyl carbamate (produced in the membrane reactor 16), as well as dimethyl carbonate, and the unreacted ammonia and methanol exit the membrane reactor 16 via the streamline 22. If the stream exiting via the line 22 is in the vapor phase, it is condensed by a condenser 24.

(21) The vapor-liquid phases generated in the condenser 24 are separated in the flash tank 26. The liquid phase 28 exiting the flash tank 26 is pumped by a pump 32 via a streamline 34 towards the PerVap membrane 50 to selectively separate the byproduct water via the streamline 52.

(22) The vapor phase streamline 30 from the flash tank 26 enters into a finishing catalytic reactor 36 for further reacting unreacted carbon dioxide with recycled ammonia and methanol entering via the stream 18′.

(23) The product stream 38 from the catalytic reactor 36 is fed into the flash tank 40 which is cooled by a cooler 40′ entering into tank 40 in order to maximize recovery of products in the liquid phase.

(24) A residual unreacted carbon dioxide and an inert gas, such as nitrogen, is purged via the streamline 44. The product liquid stream 42 is pumped by the pump 46 via the streamline 48 to the PerVap membrane 50 along with stream 34 (liquid phase) to selectively separate the byproduct water which is condensed and recover via the streamline 52.

(25) The product stream 54 from the PerVap membrane 50 consist of methyl carbamate and unreacted methanol and ammonia. The product stream 54 is fed to the distillation column (or the reaction column) 100. The reaction distillation system 100 includes a plurality of recycling components supporting the reactions which result in a purified and concentrated dimethyl carbonate exiting from a product distillation column 102 via the dimethyl carbonate product line 148.

(26) The methyl carbamate (which is the product of the conversion of CO.sub.2 in the membrane reactor 16) is converted to dimethyl carbonate by way of the multiple, for example, two side reactors 94 and 96. More or less than two side reactors may be used in the present system 10, including the one connected to the bottom of the distillation column (reaction distillation column) 100. As an example, only one flow process for one of the side reactors will be further described for the sake of brevity of description.

(27) With respect to the process associated with the side reactor 94, a product stream is drawn from one of the stages of the distillation column 100 which flows through the product streamline 84 to the pump 86 which enters the product stream into the heat exchanger 92. The heat exchanger 92 recovers heat from product streams for pre-heating the feed for improved energy efficiency of the overall process.

(28) Subsequent to the passage of the product stream through the heat exchanger 92, the products stream enters into the side reactor 94 along with the recycle methanol stream 98 and the recycle stream 118 from the bottom of the distillation column 100.

(29) The produced methyl carbamate is subsequently converted to dimethyl carbonate in the side reactor 94 and exits therefrom via the product line 102 for passage through the heat exchanger 92, and re-enters into the distillation column 100 on the line 104.

(30) It is to be understood that multiple side reactors may be used in the subject system for achieving a desired conversion of methyl carbamate to dimethyl carbonate. The conversion to the final product may be by the use of reactive distillation stages 170 of the distillation column 100.

(31) Although only one reactive distillation stage 170 is shown, it is to be understood that a number of stages 170 may be used. Catalysts on the reactive distillation stages 170 may either be incorporated on distillation trays or packed columns.

(32) As is seen in FIG. 1, the product streams returning via the re-entry product lines 104 are inserted into the distillation column (reaction distillation column) 100 one stage lower than the withdrawal stage represented by the product line 84. Distillation stages where the product streams are introduced into the reaction distillation column 100 are equipped with thermal devices 58. The thermal devices 58 may be incorporated on distillation trays or within packed columns. Thermal devices 58 are thermally coupled with the thermal devices 134 of the product distillation column 102 for recovering heat energy from the distillation column (or product column) 102 operating at a higher temperature than the temperature of the reaction distillation column 100. A well-known heat transfer fluid system, or a heat pipe, may be used to transfer the heat energy from the distillation column 102 to the distillation column 100.

(33) A product mixture consisting of unreacted methyl carbamate and dimethyl carbonate accumulates in the bottom portion 106 of the distillation column 100 and is fed to the side reactor via the streamline 108 for further conversion of residual methyl carbamate. The product stream is returned to the heat exchanger (reboiler) 110. Dimethyl carbonate along with unreacted methanol is vaporized in through the reboiler 110. Vapor phase dimethyl carbonate along with methanol in the vapor phase re-introduced into the distillation column 100 via the streamline 112.

(34) The liquid product stream 114 containing unreacted methyl carbamate from the reboiler 110 is fed to the pump 116 for recycling to side reactors via the streams 118 for further conversion to dimethyl carbonate.

(35) A product mixture consisting primarily of methanol and ammonia with some fraction of dimethyl carbonate in the top portion 56 of the distillation column 100 and fed to the heat exchanger 62 (overhead partial condenser) via the streamline 60. Volatile ammonia and methanol, in the vapor phase, are subsequently fed to the heat exchanger 68 for condensing fully before entrance into the ammonia rectification column 76 aided by the pump 72 via the streamline 74.

(36) The liquid product from the heat exchanger 62 with recovered dimethyl carbonate is returned via the line 64 to the first stage of the distillation column 100.

(37) Ammonia recovered from the ammonia rectification column 76 is recycled into the membrane reactor either in the liquid or the vapor phase via the streamline 20.

(38) The bottom product of the ammonia rectification column 76 is pumped by the pump 80 to a reservoir tank for methanol recycle or to the product distillation column 102 via the streamline 82 for recovery of dimethyl carbonate carried over by methanol.

(39) As seen in FIG. 1, a vapor-phase product stream with a high concentration of dimethyl carbonate is withdrawn from one of the intermediate stages 136 (in the reaction column 100) and is fed into the heat exchanger 122 via the streamline 120. The product stream 120 is fully condensed and is fed into the PerVap membrane 126 via the streamline 124 for a selective separation of high-concentration methanol that is recycled to the side reactors 94, 96 and the membrane reactor 16 via the streamline 128.

(40) The concentrated dimethyl carbonate stream is fed into the distillation column (product column) 102 via stream 130 on one of the intermediate stages 138. The distillation column 102 (product column) operates at higher pressure to effectively separate methanol from dimethyl carbonate (from the azeotrope of methanol and dimethyl carbonate).

(41) Product stream 142 with a high-concentration of dimethyl carbonate is withdrawn from the bottom portion 140 of the distillation column 102 and is fed into the heat exchanger (reboiler) 144 for vaporizing a small fraction of methanol that may have been carried down the distillation column 102 and fed back into the distillation column 102. The purified high-concentration dimethyl carbonate is withdrawn via the line 148 of the product column 102 as a final product.

(42) A methanol-rich product stream 150 is withdrawn from the top portion 132 of the distillation column 102 and is fully condensed by the heat exchanger (overhead condenser) 152. The condensed product stream 154 is fed into the PerVap membrane 156 for selective separation of methanol for recycling to side reactors 94 and 96 and the membrane reactor 16 via streamline 168. The stream 158 is retuned into the first stage of the distillation column 102 as a reflux.

(43) FIG. 2 is representative of another embodiment of the subject system 10′ using the catalytic reactor process for direct conversion of high-purity CO.sub.2 to DMC. The system shown in FIG. 2 thus does not use the membrane reactor 16 and associated components presented in FIG. 1. The high-purity captured carbon dioxide is captured by one of the commercial or emerging carbon capture technologies. High-purity captured carbon dioxide is fed to the top of the catalytic reactor 172, which is a downflow catalytic reactor, referred to herein as a trickle-bed reactor. The combined stream of recycled and fresh methanol stream 18 and recycle ammonia stream 20 are also fed to the top. The combined feed stream of methanol 18 and ammonia 20 can be in liquid or vapor phase. The product stream 38 containing methyl carbamate along with the unreacted ammonia and methanol are fed to the flash tank 40. The subsequent process following the catalytic reactor 172 is identical to that shown in FIG. 1.

(44) Referring to FIG. 3, the urea process used in the system presented in U.S. Pat. No. 9,796,656 has been replaced by the catalytic reactor 172 for direct conversion of the captured high-purity carbon dioxide by reacting with methanol and ammonia. The catalytic reactor 172 is identical to the one presented in FIG. 2. The remaining part of the process is identical to that shown in U.S. Pat. No. 9,796,656 with stream and component numbers having an added pre-number 3. For example, for the column 36 number 336 is used in FIG. 3 similar numbering has been used to other elements as well.

(45) Referring now to FIGS. 4 and 5, a two flow configuration of the catalytic reactor containing packed-bed catalyst are presented. FIG. 4 is representative of the catalytic reactor 400 with high-purity carbon dioxide entering via the stream 401. The reactant (methanol) on the stream 402 and ammonia is fed on the stream 403 into the catalytic reactor 400 from the top 12 in a trickle-bed reactor configuration. Reactants streams 402 and 403 can be in the liquid or the vapor phase.

(46) Product methyl carbamate along with unreacted ammonia, methanol and carbon dioxide flows downward and exit from the bottom of the catalytic reactor 400 via the stream 404, and is fed therefrom into the flash tank 405.

(47) Heat exchanger 405′ is mounted inside the flash tank 405 to condense and cool the product for recovery of methyl carbamate while purging unreacted carbon dioxide and the inert gas (such as nitrogen).

(48) The liquid stream 406 is pumped by the pump 408 though the PerVap membrane 410 for selective separation of byproduct water on the streamline 412. The product stream containing carbamate and unreacted methanol is fed to the distillation column for conversion to dimethyl carbonate.

(49) FIG. 5 is shown with the same catalytic reactor 400 with carbon dioxide and reactants methanol and ammonia fed from the bottom of the catalytic reactor 400. This catalytic reactor configuration is called flow-reactor, which can be packed-bed or fluidized-bed reactor.

(50) Referring FIGS. 6 and 7, there are two design concepts of the membrane reactor 16 which is shown in FIG. 1. FIG. 6 details an element section 500 of the membrane reactor 16. The membrane 501 consists of a membrane support and a carbon dioxide transport membrane film. Catalysts 502 are packed on the other side of the membrane through which reactants (ammonia and methanol) flow via the streamline 505. Flue gas containing carbon dioxide stream 503 flows through one side of the membrane. As carbon dioxide diffuses through the membrane 501, it reacts with methanol and ammonia in presence of a catalyst in the bulk flow region as depicted by the reaction equation 507. The resulting product stream exits via the streamline 506.

(51) FIG. 7 represents an element section of the membrane reactor 16 (of FIG. 1) and differs from the element shown in FIG. 6 by inclusion of the embedded catalyst layer 502′ instead of the packed-bed catalyst 502 of FIG. 6 on the membrane surface 501. As the carbon dioxide diffuses through the membrane 501, it reacts with methanol and ammonia at the membrane surface where catalysts 502′ are embedded on the surface. The product methyl carbamate is then carried away by flowing methanol and exit via the streamline 506.

(52) Referring to FIGS. 8, 9 and 10, there are three different configurations of the membrane modules. FIG. 8 represents a shell-and-tube module 600 equipped with tubular membranes 601 with cross-flow of carbon dioxide flow stream 603. The tubular membrane 601 may have transport membrane film either inside or outside of the tube. Membrane tubes 601 are either packed with catalyst, as shown by FIG. 6, or embedded on the membrane surface, as shown in FIG. 7. Methanol and ammonia are fed in the module 600 via the line 602.

(53) Some fraction of carbon dioxide is converted to products and the flow stream 604 exits as carbon dioxide lean flue gas.

(54) The product stream consisting of methyl carbamate, some fraction of dimethyl carbonate, and unreacted ammonia, methanol and carbon dioxide, exits via flow stream 605 for further conversion.

(55) FIG. 9 represents a shell-and-tube module 611 with tubular membranes 610, similar to that in FIG. 8, with parallel counter-flow carbon dioxide and reactants ammonia and methanol. Methanol and ammonia, either in the liquid or the vapor phase, as flow stream 612, are introduced from the top of the membrane module 611 and are distributed uniformly by a distributor 616 among all membrane tubes. Carbon dioxide is introduced via the flow streamline 613 on the shell side and it flows upward parallel to the tubular membrane tubes 610. Catalysts 617 are either packed inside tubes 610 or embedded on the membrane surface as shown in FIGS. 6 and 7. The carbon dioxide lean stream exits via the flow streamline 614, while methyl carbamate and unreacted methanol, ammonia and CO.sub.2 exit via the line 615.

(56) FIG. 10 represents an innovative concept of a plate-and-frame membrane module 620 used in the subject system. In the module 620, parallel plates 621 are assembled with alternate plate flow channels 622 which are packed with catalysts 623. Alternatively, catalysts may be embedded on the surface of the plates 621, as shown in FIGS. 6 and 7.

(57) The carbon dioxide stream 624 enters from the side of the plate-and-frame membrane module 620 as shown by FIG. 10 and exists from the opposite side as flow stream 626. The flow stream 628 of ammonia is introduced from the top of the module 620 and flows down through the channels 622 holding catalysts 623.

(58) Carbon dioxide diffusing through the membrane reacts with ammonia and methanol to produce methyl carbamate. The products stream 630 is withdrawn from the bottom of the membrane module 620.

(59) The elemental section of plate-and-frame membrane module 620 can be assembled in a commercial-scale unit based on the conventional technology suitable for plate heat exchangers as shown, as an example, in FIG. 11. It is to be understood that commercial membrane modules including spiral-wound membrane modules or hollow-fiber membrane modules can also be used in the subject system. However, loading these types of commercial membranes with catalysts is difficult and such membrane modules cannot be built on a large scale required for converting flue gas carbon dioxide to alkyl carbonates.

(60) For process streams illustrated in FIGS. 1, 2 and 3, the methanol/dimethyl carbonate azeotrope is shown to be broken with PerVap membrane unit on the distillate stage and between the two distillation columns, and the recovered methanol is recycled and fed to either singular or multiple side reactors. PerVap membrane units, as illustrated in FIGS. 1, 2 and 3 are commercially available and PerVap membranes, such as, for example, zeolite, cross-linked chitosan and highly fluorinated polymer membranes, may be used. The PerVap membrane units shown are for illustration purposes only, and other separation technologies for separating and recycle of the excess reactant methanol from the product stream may be used. Such separation methods may include molecular-sieve separation, pressure-swing adsorption (PSA), temperature-swing adsorption (TSA), liquid-liquid separation of immiscible liquid mixtures, liquid entrainment and heat integrated distillation.

(61) The side reactors, main catalytic reactor and membrane reactors illustrated in FIGS. 1, 2 and 3 may be packed with commercial heterogeneous catalysts for each embodiment of the subject process.

(62) Alternatively, homogeneous catalyst may be used which is dissolved in methanol. Such catalysts may be provided in the form of zinc oxide, zinc acetate dihydrate, zinc carbonate, zinc hydroxide, zinc nitrate hexahydrate, zinc chloride, lead nitrate, lead oxide, dialkyl tin oxide, dialkyl tin methoxide, or zinc oxide/urea organometallic complex. Alkyl may be any saturated carbon chain having less than 10 carbons. Different catalysts may be used on the individual membrane reactor, as well as the primary catalytic reactor, for direct conversion, and the individual side reactors.

(63) The Table 1 below represents process parameters of a typical commercial plant cited in FIG. 1 with production capacity of 50,000 metric tons per year with product purity of 92 wt %.

(64) TABLE-US-00001 TABLE 1 Process Parameter Value Units Dimethyl Carbonate (DMC) 6,414 kg/hr Production Capacity 50,000 metric tons/year Pure DMC 6,408 kg/hr DMC Concentration 99% wt % Product yield based on CO.sub.2 98% Feedstock CO.sub.2 feed stream 43,667 kg/hr CO.sub.2 concentration 12% Fresh Methanol Flow Rate 4,715 kg/hr Side Reactors Temperature 170 ° C. Pressure 27 bar First Distillation column Reflux temperature 81 ° C. Bottom temperature 220 ° C. Pressure 2.0 to 4.0 bar Second Distillation column Reflux temperature 136 ° C. Bottom temperature 250 ° C. Pressure 6.0 to 10.0 bar CO.sub.2 Merit Value CO.sub.2 Consumed 0.49 kg CO.sub.2/kg DMC CO.sub.2 Generated by the process 0.15 kg CO.sub.2/kg DMC CO.sub.2 Emissions of Methanol 0.39 kg CO.sub.2/kg DMC Net CO2 emission 0.05 kg CO.sub.2/kg DMC

(65) The process consumes 0.49 kg of carbon dioxide per kg of dimethyl carbonate with net emissions of 0.05 kg carbon dioxide, as shown in Table 1. If the feed stock methanol is produced by renewable hydrogen and carbon dioxide, then there would be significant net permanent sequestration of carbon dioxide in the form of consumer product of alkyl carbonates.

(66) This is compared to emissions of 1.76 kg carbon dioxide per kg of dimethyl carbonate produced by syngas-based commercial process. Table 2 represents the estimated global demands of dimethyl carbonate and corresponding potential abatement of carbon dioxide emissions. With full implementation of this invention process by 2050, there would be significant global abatement of carbon dioxide.

(67) TABLE-US-00002 TABLE 2 DMC Market CO.sub.2 Abatement potentials, kTA* Potentials, kTA* Applications 2018 2030 2018 2030 Polycarbonate production 2,440 4,910 3,831 7,708 Lithium-ion batteries 45 350 71 550 Solvent (replacing ketones) 1,430 1,430 2,245 2,857 Polyurethane production 11,350 11,350 17,820 28,998 Diesel-engine additive** 1,580,000 2,480,000 *Thousand metric tons per year **Based on government approval for pollution control
Validation of Side Reactors

(68) The concept of side reactors has been experimentally validated in an integrated reaction column test unit. A flow redirecting device is installed in a packed column for directing liquid flowing down the packed column to the side reactor. The vapor rising from the bottom part of the column is directed to bypass of the side draw line of liquid. The product stream from the side reactor is returned to the next stage of the packing below the point of side draw. An integrated pump and surge tank system is used for controlling the liquid flow to the side reactor. The test data validated the performance of side reactor for the chemical system of conversion of CO.sub.2 to dialkyl carbonates. ASPEN Plus® process analysis is validated with the experimental test data obtained with this integrated test unit where three side reactors are connected to the reaction column.

(69) Kinetic Test Data

(70) Kinetic tests were performed with zinc oxide and zinc oxide-urea complex, which is found to be sparingly soluble in methanol. FIG. 12 shows conversion of urea to methyl carbamate at three reaction temperatures. The kinetic parameters are derived from such laboratory tests are validated with prototype test unit. The validated kinetic model for flow reactor is then incorporated into the process design using a commercial design software ASPEN Plus®.

(71) Prototype Test Results

(72) FIG. 13 presents a flow diagram of the prototype test unit of a side reactor. This prototype test unit was used for conversion of methyl carbamate (MC) and urea to DMC. The kinetic parameters obtained with batch tests described above were incorporated into the ASPEN Plus® simulation model to predict the performance of side reactors. Table 3 shows the comparison of ASPEN Plus® predictions and test results of product composition. The quality and quantity of test parameter measurements were deemed adequate for validating the ASPEN Plus® model for simulation of the side reactor.

(73) TABLE-US-00003 TABLE 3 1.1.1.1 1.1.1.2 Test Run 1 Test Run 2 Test Run 3 Test Model Test Model Test Model Parameters Data Results Data Results Data Results Feed 37.6 36.8 45.3 Flowrate (g/min) Feed Composition (wt %) Methanol 76.4 74.0 69.3 Methyl 21.4 21.2 23.3 Carbamate (MC) DMC 0.37 0.51 0.36 Urea 1.83 4.31 7.09 Product Composition (wt %) Methanol 78.2 75.4 74.6 71.8 70.8 66.1 Methyl 20.6 22.9 22.8 25.4 24.8 29.9 Carbamate (MC) DMC 0.66 0.82 0.83 1.00 0.55 0.80 Urea 0.53 0.26 1.80 0.58 3.90 1.53
Performance of PerVap Membrane

(74) FIG. 14 depicts schematic flow diagram of the test unit for measuring performance parameters of PerVap membrane for selective separation of methanol from azeotrope of methanol and dimethyl carbonate. Table 4 represents a summary of the performance parameters. Two series of tests were performed with liquid phase and vapor phase feed as shown in Table 4. In general high-purity methanol was separated as permeate with high-degree of selectivity. The PerVap membrane performance parameters were incorporated into the ASPEN Plus® process model.

(75) TABLE-US-00004 TABLE 4 Liquid Feed Composition, wt % Permeation Feed Rate Temp Perm. Flux Feed Retentate Permeate Flux MeOH/DMC ID mL/min C. g/min MeOH DMC MeOH DMC MeOH DMC kg/m.sup.2/hr Selectivity Comments Liquid Feed TEST 1 4.0  95 0.51 — — 67.3% 32.7% 95.9% 4.1% 6.1 11.4 TEST 2 4.0  95 1.04 65.0% 35.0% 64.9% 35.1% 95.5% 4.5% 12.5  11.5 Broken O-ring TEST 3 4.0 105 0.68 67.8% 32.2% 66.3% 33.7% 97.9% 2.1% 8.2 23.7 TEST 4 4.0 105 0.59 65.3% 34.7% 63.4% 36.6% 97.3% 2.7% 7.1 20.8 TEST 5 4.0 105 0.65 61.1% 38.9% 57.4% 42.6% 98.0% 2.0% 7.8 36.4 Vapor Feed TEST 6 4.0 105 0.34 26.5% 73.5% 24.7% 75.3% 93.6% 6.4% 4.1 44.6 TEST 7 4.0 109 0.36 19.7% 80.3% 23.1% 76.9% 61.4% 8.6% 4.3 35.4 TEST 8 4.0 139 0.31 67.7% 32.3% 68.0% 32.0% 96.2% 3.8% 3.7 11.9 TEST 9 4.0 133 0.27 68.6% 31.4% 68.6% 31.4% 97.3% 2.7% 3.2 16.5 Membrane area 0.005 m.sup.2 Selectivity, MeOH/DMC ${\alpha }_{\mathrm{MeOH}/\mathrm{DMC}}=\frac{Y_{\mathrm{MeOH}}/Y_{\mathrm{DMC}}}{X_{\mathrm{MeOH}}/X_{\mathrm{DMC}}}$
Interfacing of Side Reactors with Distillation Column

(76) Interfacing the side reactors with the distillation column without adverse impacts on the column performance requires careful design. This invention focuses on the following key criteria in design interface: 1) vapor flow should not be disturbed; 2) total or partial liquid flow to the side reactor using flow control valves should be employed; 3) liquid is returned to the next stage to a tray or packed column; 4) heat is recovered using a feed/effluent heat exchanger for the side reactor and the column may operate at different temperatures and pressures; and 5) interfacing design is based on commercially available hardware devices for minimizing operational risks.

(77) 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.