Urea production plant

Abstract

Disclosed is a plant for the production of urea. The plant comprises conventional sections for synthesis and recovery, for evaporation and condensation, for urea finishing, and for dust scrubbing. According to the invention, an additional evaporation and condensation loop is introduced from and to the dust scrubbing section. This loop results in a more favorable energy consumption of the plant.

Claims

1. A process for the preparation of urea comprising: (a) one or more synthesis and recovery steps wherein ammonia and carbon dioxide are reacted to form urea and wherein an aqueous solution comprising urea is formed; (b) an evaporation step wherein water is evaporated from the aqueous solution formed in (a) so as to result in a first concentrated urea-comprising liquid and an first aqueous vapor phase; (c) subjecting said first aqueous vapor phase of (b) to a first condensation step to give an aqueous solution; (d) subjecting the first concentrated urea-comprising liquid of (a) to a finishing treatment resulting in solid urea, wherein heat is removed by means of a cooling gas; (e) subjecting the cooling gas of (d) to dust scrubbing, wherein urea is recovered in an aqueous stream; (f) subjecting said aqueous stream of (e) to evaporation so as to result in a second concentrated urea-comprising liquid and a second vapor phase, wherein the evaporation is conducted in an evaporation step separate from the evaporation step (b), and wherein the second concentrated urea-comprising liquid is recirculated and subjected to the finishing step (d), and (g) subjecting the vapor phase originating from the separate evaporation step (f) to a second condensation step separate from the first condensation step (c), and (h) using the condensate from said second condensation step (g) in the dust scrubbing step (e).

2. The process of claim 1 wherein the condensate from the second condensation step (g) is subjected to hydrolysis and steam stripping.

3. The process of claim 1 wherein the dust scrubbing of (e) employs circulating urea solution as washing agent and the condensate from the second condensation step (g) is subjected to hydrolysis and steam stripping.

4. The process of claim 3 wherein the aqueous stream of (e) contains 10-60% by weight of urea.

5. The process of claim 4 wherein the condensate from the second condensation step (g) is subjected to hydrolysis and steam stripping.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of a conventional urea production plant.

(2) FIG. 2 is a schematic representation of a urea production plant according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) In a general sense, the invention is based on the judicious insight to include an additional evaporation loop in the process. The additional evaporation loop, as defined, surprisingly reduces the energy consumption downstream of the condensation section (E). Typically, downstream of the condensation section (E) one will find a process condensate treatment section (F).

(4) The additional evaporation loop is provided for by adding a second evaporation section (G) and a second condensation section (H) to the plant. Particularly, the liquids obtained in the second condensation section are used in the dust scrubbing section (D).

(5) Where, in this description, it is spoken of “fluid communication”, this refers to any connection between a first part or section of a plant and a second part or section of a plant via which fluids, notably liquids, can flow from the first part of the plant to the second part of the plant. Such fluid communication is typically provided by piping systems, hoses, or other devices well-known to the skilled person for the transportation of fluids.

(6) Where in this description it is spoken of “gas flow lines” this refers to any connection between a first part or section of a plant and a second part or section of a plant via which gas or vapours, notably aqueous vapours, can flow from the first part of the plant to the second part of the plant. Such gas flow lines typically comprise piping systems, or other devices well-known to the skilled person for the transportation of gases, if needed under above or below (vacuum) atmospheric pressures.

(7) The invention is applicable both to the construction of new urea plants (“grass root” plants) as well as in revamping existing urea plants.

(8) In case of a newly constructed urea plant, the required investment for the process condensate treatment section (F) is considerably reduced by the measures of the invention, viz. to build-in an additional evaporation section (G) and an additional evaporation section (H) in such a way as to create an evaporation and condensation loop from and to the dust scrubbing section.

(9) The second evaporation section (G) is fed with the urea solution (9) purged from the dust scrubbing section (D). This second evaporation section, just as the main evaporation section B, may comprise one or more evaporators. Therein the urea solution (9) is concentrated, generally under vacuum, to a concentration that allows feeding the concentrated solution (16) as a co-feed to the finishing section C. The off-gas or off-gasses (17) from the second evaporation section are condensed in a second condensation section (H). Like the first condensation section (E), this second condensation section (H), usually will comprise one or more shell-and-tube heat exchangers, wherein the cooling is provided by means of a cooling liquid, most commonly water, with steam driven ejectors to maintain the desired vacuum. The steam required for these vacuum ejectors is indicated as stream (S3) in FIG. 2. However, also any other known technology to condense the off-gasses (17) may be applied here. The resulting condensate (18) is substantially free of ammonia, such that this water flow is an excellent source of water to be used as water feed to the scrubbing unit (D). In the event that the amount of water (18) should be insufficient to meet the required amount of water in this dust scrubber, than additional water (10) can be added. This additional water (10) may originate from the process condensate treatment section (F), however also any other aqueous stream can be used as long as it does not contain any environmental unfriendly volatile components.

(10) It has now been found that the aqueous stream (18) obtained in this way, does not contain ammonia at all, or at any rate in a very low concentration that does not limit the use of the aqueous stream as a source of water for the dust scrubbing section, nor any other environmental unfriendly volatile components. As a result, this stream can be directed directly to the dust scrubbing section, in this way by-passing the process condensate treatment section (F). Some entrained urea may be present in the aqueous stream (18), however this does not harm the dust scrubbing process (D), nor does this urea (present in the liquid form) result in any contamination of the off-gas (8). As a result, the flow (12) feeding the process condensate treatment section, is reduced. As a result, the amount of steam required in this process condensate treatment section reduces as well. For a ‘grass-root’ urea plant, also the dimensions of the equipment required in the process condensate treatment section can be substantially reduced.

(11) The invention is also well applicable in a ‘revamp’ or ‘debottlenecking’ scheme of an existing plant. Processes to revamp or debottleneck urea plants are common in the art. The main objective of such a ‘revamp’ or ‘debottlenecking’ of an existing plant usually is the increase of the production capacity of such an existing plant. In those cases where the process condensate treatment section of the existing plant is the limiting factor in obtaining a maximized production, then application of the invention clearly will result in more room available in this process condensate treatment section, thus allowing for an increase of the urea production capacity without applying costly modifications to the equipment in the process condensate treatment section. In those plants where the maximum attainable production capacity is limited by other constraints, then the application of the invention can still be used to further increase the production capacity of the plant, in addition to any other measures to increase the plant capacity by removing or reducing said constraints.

(12) The invention thus also provides a method of modifying an existing urea plant, said plant comprising a synthesis and recovery section (A) which is in fluid communication with an evaporation section (B), said evaporation section being in fluid communication with a finishing section (C) and having a gas flow line to a condensation section (E); said finishing section (C) having a gas flow line to a dust scrubbing section (D); the process comprising the step of adding to the plant an additional evaporation section (G) placed downstream of the dust scrubbing section (D), said additional evaporation section (G) being placed such as to be in fluid communication with the finishing section (C); wherein the additional evaporation section (G) has a gas flow line to an additional condensation section (H), and wherein the additional condensation section (H) is in fluid communication with the dust scrubbing section (D).

(13) In another interesting embodiment, the invention can be used in order to increase the capacity of an existing urea plant. This use comprises the introduction, as described above, of an additional evaporation and condensation loop from and to the dust scrubbing section (D).

(14) The plants as devised or modified according to the invention serve to produce urea. Without necessarily altering the general urea synthesis process, the invention also pertains to a new process for the production of urea, wherein the energy benefits are enjoyed that are associated with using the plant of the invention.

(15) Thus, the invention also pertains to a process for the preparation of urea comprising the steps of (a) one or more synthesis and recovery steps wherein ammonia and carbon dioxide are reacted to form urea and wherein an aqueous solution comprising urea is formed; (b) an evaporation step wherein water is evaporated from the aqueous solution formed in (a) so as to result in a concentrated urea-comprising liquid and an aqueous vapour phase; (c) subjecting the concentrated urea-comprising liquid to a finishing treatment resulting in solid urea, wherein heat is removed by means of a cooling gas such as air; (d) subjecting the cooling gas to dust scrubbing, wherein urea is recovered in an aqueous stream; (e) subjecting said aqueous stream to evaporation so as to result in an additional concentrated urea-comprising liquid and a vapour stream, wherein the evaporation is conducted in an evaporation step separate from the evaporation step (b), wherein the additional concentrated urea-comprising liquid is further subjected to the finishing step (c), and wherein vapours originating from the separate evaporation step are subjected to a separate condensation step (f), and wherein the condensate from said separate condensations step (f) is used in the dust scrubbing step (d).

(16) The invention is not limited to any particular urea production process.

(17) A frequently used process for the preparation of urea according to a stripping process is the carbon dioxide stripping process as for example described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In this process, the synthesis section followed by one or more recovery sections. The synthesis section comprises a reactor, a stripper, a condenser and a scrubber in which the operating pressure is in between 12 and 18 MPa and preferably in between 13 and 16 MPa. In the synthesis section the urea solution leaving the urea reactor is fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution. Such a stripper can be a shell and tube heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed to the synthesis is added to the bottom part of the stripper. At the shell side, steam is added to heat the solution. The urea solution leaves the heat exchanger at the bottom part, while the vapour phase leaves the stripper at the top part. The vapour leaving said stripper contains ammonia, carbon dioxide and a small amount of water. Said vapour is condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The heat released by the exothermic carbamate condensation reaction in said condenser is usually used to produce steam that is used in a downstream urea processing section for heating and concentrating the urea solution. Since a certain liquid residence time is created in a submerged type condenser, a part of the urea reaction takes already place in said condenser. The formed solution, containing condensed ammonia, carbon dioxide, water and urea together with the non-condensed ammonia, carbon dioxide and inert vapour is sent to the reactor. In the reactor the above mentioned reaction from carbamate to urea approaches the equilibrium. The ammonia to carbon dioxide molar ratio in the urea solution leaving the reactor is generally in between 2.5 and 4 mol/mol. It is also possible that the condenser and the reactor are combined in one piece of equipment. An example of this piece of equipment as described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The formed urea solution leaving the urea reactor is supplied to the stripper and the inert vapour containing non-condensed ammonia and carbon dioxide is sent to a scrubbing section operating at a similar pressure as the reactor. In that scrubbing section the ammonia and carbon dioxide is scrubbed from the inert vapour. The formed carbamate solution from the downstream recovery system is used as absorbent in that scrubbing section. The urea solution leaving the stripper in this synthesis section requires a urea concentration of at least 45% by weight and preferably at least 50% by weight to be treated in one single recovery system downstream the stripper. The recovery section comprises a heater, a liquid/gas separator and a condenser. The pressure in this recovery section is between 200 to 600 kPa. In the heater of the recovery section the bulk of ammonia and carbon dioxide is separated from the urea and water phase by heating the urea solution. Usually steam is used as heating agent. The urea and water phase, contains a small amount of dissolved ammonia and carbon dioxide that leaves the recovery section and is sent to a downstream urea processing section where the urea solution is concentrated by evaporating the water from said solution.

(18) Other processes and plants include those that are based on technology such as the HEC process developed by Urea Casale, the ACES process developed by Toyo Engineering Corporation and the process developed by Snamprogetti. All of these processes, and others, may be used preceding the urea finishing method of the invention.

(19) Urea finishing techniques, such as prilling and granulation, are known to the skilled person. Reference is made to, e.g., Ullmann's Encyclopedia of Industrial Chemistry, 2010, chapter 4.5. on urea.

(20) The invention will hereinafter be further illustrated with reference to the following, non-limiting examples and the figures. The figures are schematic drawings indicating process sections (letters A-G) and streams (numbered). The composition of the streams is clarified in the tables below. The legend for the process sections is as follows: A. Urea synthesis and recirculation section(s); B. Main evaporation section; C. Finishing section; D. Dust scrubbing; E. condensation section; F. Process condensate treatment section; G. Second evaporation section; H. second condensation section.

EXAMPLE 1

(21) A grass root urea plant for the production of 100 ton/h of solid urea is constructed according to the scheme in FIG. 1. The flows in this plant are as indicated in table 1. The feed (12) to the process condensate treatment section is 43130 kg/h. In order to treat this process condensate in the process condensate treatment section F a total of 11708 kg/h (S2) of steam is required.

(22) TABLE-US-00001 TABLE 1 UREA NH.sub.3 CO.sub.2 H.sub.2O N.sub.2 O.sub.2 TOTAL TEMP Stream: phase kg/h kg/h kg/h kg/h kg/h kg/h kg/h C.  1 gas 73134 73134 40  2 liquid 56603 56603 25  3 liquid 100211 2893 1377 33262 137743 80  4 liquid 104800 4367 109167 140  5 solid 99800 200 100000 50  6 gas 651950 198050 850000 30  7 gas 5000 4167 651950 198050 859167 95  8 gas 37265 651950 198050 887265 45  9 liquid 5000 7500 12500 45 10 liquid 40598 40598 40 11 gas 411 2893 1377 36396 41077 135 12 liquid 411 2893 1377 38450 43130 40 13 liquid 3126 1678 3203 8007 60 14 liquid 46832 46832 40 15 liquid 6233 6233 40 S1 gas 2054 2054 150 (steam) S2 gas 11708 11708 150 (steam)

(23) A grass root urea plant for the same production of 100 ton/h is constructed, using the scheme according to the invention (FIG. 2). The flows in this process are as indicated in table 2. As can be seen from this table, now only 9153 kg/hr (S2) of steam is required in the process condensate treatment section (F).

(24) TABLE-US-00002 TABLE 2 Stream: UREA NH.sub.3 CO.sub.2 H.sub.2O N.sub.2 O.sub.2 TOTAL TEMP kg/h phase kg/h kg/h kg/h kg/h kg/h kg/h kg/h C.  1 gas 73134 73134 40  2 liquid 56603 56603 25  3 liquid 100136 2889 1376 33172 137573 80  4 liquid 99800 4158 103958 140  5 solid 99800 200 100000 50  6 gas 651950 198050 850000 30  7 gas 5000 4167 651950 198050 859167 95  8 gas 37265 651950 198050 887265 45  9 liquid 5075 7612 12687 45 10 liquid 32933 32933 40 11 gas 336 2889 1376 29013 33614 135 12 liquid 336 2889 1376 30694 35295 40 13 liquid 3080 1622 3134 7836 60 14 liquid 36612 36612 40 15 liquid 3679 3679 40 16 liquid 5000 208 5208 140 17 gas 75 7404 7479 135 18 liquid 75 7778 7853 40 S1 gas 1681 1681 150 (steam) S2 gas 9153 9153 150 (steam) S3 gas 374 374 150 (steam)

EXAMPLE 2

(25) An existing urea plant, constructed according to the scheme of FIG. 1, is producing 100 ton/h of solid urea. The flows in this plant are as indicated in table 1. In this plant the process condensate treatment section F is the limiting factor in achieving the maximum possible production capacity: in case the production capacity exceeds 100 ton/h, then flow 12 exceeds 43130 kg/hr, which in this plant results in flooding of one of the columns in the process condensate treatment section.

(26) In this plant a (small) second evaporation section (G) and a (small) condensation section (F) are added according to the scheme of the invention (FIG. 2). The flows in the process after this revamp are as indicated in table 3. As can be seen from the table, the plant capacity (5) can be increased to 122 ton/hr, with the same amount of process condensate flow (12) as before (43130 kg/h), such that no flooding of the column in the process condensate treatment occurred at the increased production capacity of the plant. The plant thus was debottlenecked to a production capacity equal to 122% of its original maximum capacity.

(27) TABLE-US-00003 TABLE 3 UREA NH.sub.3 CO.sub.2 H.sub.2O N.sub.2 O.sub.2 TOTAL TEMP Stream: phase kg/h kg/h kg/h kg/h kg/h kg/h kg/h C.  1 gas 89368 89368 40  2 liquid 69167 69167 25  3 liquid 122365 3530 1681 40535 168111 80  4 liquid 121954 5081 127035 140  5 solid 121954 244 122198 50  6 gas 796671 242014 1038685 30  7 gas 6110 5092 796671 242014 1049886 95  8 gas 45537 796671 242014 1084222 45  9 liquid 6201 9302 15503 45 10 liquid 40243 40243 40 11 gas 411 3530 1681 35454 41076 135 12 liquid 411 3530 1681 37508 43130 40 13 liquid 3763 1982 3830 9576 60 14 liquid 44739 44739 40 15 liquid 4496 4496 40 16 liquid 6110 255 6364 140 17 gas 91 9047 9139 135 18 liquid 91 9504 9596 40 S1 gas 2054 2054 150 (steam) S2 gas 11185 11185 150 (steam) S3 gas 457 457 150 (steam)

(28) Note to table 1, 2 and 3: As usual in urea technology, ammonium carbamate in these tables is listed in the form of its constituents (NH.sub.3 and CO.sub.2). It should be noted that in liquid streams most of the CO.sub.2 as listed in the tables actually is present as ammonium carbamate.