WO1983000861A1

WO1983000861A1 - Process for the preparation of urea

Info

Publication number: WO1983000861A1
Application number: PCT/EP1982/000187
Authority: WO
Inventors: Van Kunstmestfabrieken B.V. Unie
Original assignee: Van Nassau, Petrus, Johannes, Marie; Biermans, Andreas, Johannes; Jonckers, Kees; De Cooker, Mario, Gustaaf, Roger, Tilly
Priority date: 1981-09-01
Filing date: 1982-08-31
Publication date: 1983-03-17
Also published as: GB2107704A; KR840001132A; JPS58501374A; DD204250A5; IT1149070B; PL238093A1; ES515378A0; NO831532L; BR8207840A; NL8104039A; EP0086804A1; OA07201A; ZA825789B; ES8306104A1; IT8249046A0; GR76909B

Abstract

A process for the preparation of urea from ammonia and carbon dioxide at an elevated temperature and pressure having a reaction zone and a stripping zone. In the reaction zone, carbon dioxide and a portion of the ammonia are converted to ammonium carbamate, and a portion of the ammonium carbamate is converted to urea, the combined conversions resulting in a net formation of heat. In the stripping zone, a urea product stream containing unconverted ammonium carbamate is heated by heat exchange with the reaction zone to decompose a portion of ammonium carbamate. The reaction zone is maintained at a pressure of between about 125 and 250 bar, and the stripping zone is maintained at a pressure lower than the pressure in the reaction zone.

Description

PROCESS FOR THE PREPARATION OF UREA

This invention relates to a process for preparing urea from ammonia and carbon dioxide at elevated temperatures and pressures whereby the net heat produced by the urea synthesis reaction can be more efficiently and effectively utilized.

One process for the preparation of urea which has found wide use in practical applications is described in European Chemical News urea Supplement of January 17, 1969, at pages 17-20. In the process there disclosed, the urea synthesis solution is formed in a reaction zone maintained at a high pressure and temperature, and is thereafter subjected to a stripping treatment at the synthesis pressure by heating this solution and contacting it countercurrently with a carbon dioxide stripping gas so as to decompose a major portion of the ammonium carbamate contained therein. The gas mixture thus formed, containing ammonia and carbon dioxide together with the stripping gas and a small quantity of water vapor, is removed .from the remaining urea product stream and introduced into a condensation zone wherein it is condensed to form an aqueous ammonium carbamate solution. This aqueous carbamate solution, as well as the remaining non-condensed gas mixture, is recycled to the reaction zone for conversion to urea. The condensation of this gas mixture returned to the reaction zone provides the heat required for the conversion of ammonium carbamate into urea, and'no heat need be supplied to the reaction zone from tne outside. The heat required for the stripping treatment is provided by the condensation of high- pressure steam on the outside of tubes in a vertical heat exchanger in which the stripping takes place. According to this publication, approximately 1,000 kg of steam at a pressure of about 25 bar is required per ton of urea. In practice, the consumption of high pressure steam (25 bar) has been reduced to about 850 kg per ton of urea, and the heat used in the stripping treatment can be partially recovered by condensation of the resulting gas mixture. This condensation, however, takes place at a relatively low temperature level with the result that steam of only 3-5 bar is produced, for which there is relatively little use in either this process or outside the process. For this reason, and particularly in view of continually increasing energy prices, it is highly desirable to reduce the consumption of high-pressure steam as much as possible and, depending on local conditions and needs, it may be desirable as well to avoid or greatly reduce the production of surplus low- pressure steam.

Various proposals have already been made to use the heat released in the formation of carbamate from ammonia and carbon dioxide to provide at least a part of the heat requirements in the stripping treatment. For various reasons, however, these proposals have not found practical application. For example, in British patent specification 1,147,734, a process is disclosed wherein the ammonium carbamate and urea synthesis is effected in two successive reaction zones. Fresh ammonia and at least a portion of the fresh carbon dioxide are fed into the first reaction zone, and the heat released by the formation of ammonium carbamate in this first reaction zone is transferred, via built-in heat exchange elements, to the urea synthesis solution formed in the second reaction zone at a pressure equal to or lower than that in the first reaction zone, resulting in the decomposition of unreaσted ammonium carbamate. The decomposition products thus formed are stripped from the remaining urea solution by means of an inert gas. in carrying out this known process, the fresh ammonia and carbon dioxide must be pre-heated to a high temperature, for instance to the synthesis temperature maintained in the first reaction zone, in order to have sufficient heat to effect the decomposition of ammonium carbamate in the stripping zone. This pre-heating of the reactants consumes a significant quantity of high-pressure steam. Of the surplus heat formed in the urea synthesis, however, only a small portion can be recovered as steam at a pressure of about 5 bar via a cooling system for the second reaction zone. A larger portion of this surplus heat is recovered at a relatively low temperature level in the absorption column, operating at the pressure of the second reaction zone, in which column the ammonia and carbon dioxide stripped from the urea product stream are separated from the inert stripping gas. The steam so produced has a relatively low pressure of only about 2 bar, for which there is little use within the process. In another process as disclosed in U. S.

Patent 3,957,868, the urea synthesis reaction is carried out in the shell side of a vertical tubular heat exchanger wherein the tcps of the tubes open into the shell-side space of the heat exchanger. The urea synthesis solution formed in the shell-side reaction zone flows into and down the inside of the tubes wherein ammonium carbamate is decomposed by means of the heat of reaction transferred through the tube walls, and the ammonia and carbon dioxide thus released are stripped from the solution.

In order to maximize the decomposition of ammonium carbamate, it is necessary to have the heat of reaction available at as high a temperature level as possible, and to insure optimum heat transfer to the solution to be stripped. The measures proposed in this reference to accomplish this include carrying out the urea synthesis and the stripping at a temperature of 210 to 245^οC and a pressure of 250 to 600 atmospheres, maintaining a relatively high NH₃/CO₂ molar ratio in the liquid phase in the reaction zone, and recirculating a part of the gas mixture removed from the stripping zone into the bottom of the reaction zone together with an extra quantity of gas mixture which is kept available for recirculation by condensing only a portion of the gaseous ammonia and carbon dioxide supplied to the reaction zone ammonium carbamate.

Thus, the weight of urea produced will be low relative to the quantities of ammonia and carbon dioxide supplied to the reaction zone.

Because of the high temperatures employed in this process, the materials of construction to beused must satisfy high standards of corrosion resistance. Moreover, a relatively high capital investment is required for the equipment because of the higher pressure used, and the special design of the combined reactor- stripper. When all of these factors are taken into account, this known process, in practical application, has little or no economic advantage over the process described in the European Chemical News urea Supplement which has found general application.

It is the object of the present invention to provide a process for the production of urea wherein the quantity of high-pressure steam required for the decomposition of non-converted ammonium carbamate is substantially reduced while avoiding the disadvantages of the above-mentioned known processes.

These objectives are accomplished in accordance with the present invention wherein, in a reaction zone, a solution containing ammonium carbamate and non-converted ammonium is formed at a high temperature and pressure from carbon dioxide and excess ammonia and the ammonium carbamate is partly converted into urea, and at least a portion of the carbamate-containing urea synthesis solution so obtained is subjected to a stripping treatment in a stripping zone, wherein the heat required in the stripping zone is obtained by heat exchange with the reaction zone in which the carbamate is formed, and the stripped urea synthesis solution is processed to form product urea as either a solution or solid. In accordance with the invention, the stripping zone is maintained at a pressure lower than the pressure in the reaction zone exchanging heat with the stripping zone, which reaction zone is maintained at a pressure of 125-250 bar.

The condensation of ammonia and carbon dioxide in the reaction zone is preferably effected in the presence of relatively large quanities of urea and water, which causes the average temperature in the reaction zone to rise considerably. The heat of reaction from the reaction zone is exchanged with a stripping zone wherein it is utilized to decompose ammonium carbamate in the urea product stream at a pressure less than the pressure prevailing in the reaction zone. When carried out in this manner, the heat of reaction from the reaction zone is available at a high temperature level, and can be more effectively and efficiently utilized than in the prior art in the decomposition and stripping of ammonium carbamate from the urea product stream.

As the pressure in the reaction zone increases, the condensation of carbon dioxide and ammonia to ammonium carbamate takes place at higher temperatures and, consequently, more heat will be available for the decomposition of carbamate in the stripping zone. Preferably, a pressure differential between the heat-exchanging reaction zone and the stripping zones should be at least about 20 bar, resulting in a sufficiently high average temperature difference and thus driving force between the two zones for effective heat exchange. A pressure difference of more than 120 bar is theoretically possible, but this complicates the design of the reactor-stripper apparatus. Host preferably the reaction zone should be maintained at a pressure of 40 to 60 bar higher than in the stripping zone. This provides a suitable driving force for the heat exchange without the need for equipment of a more complex design, and moreover has the advantage over the use of higher pressure differentials that the gas mixture released in the lower pressure stripping zone need not be compressed to so high a pressure to be recycled to the reaction zone. Where the pressure differential is in the range of about 40 to

60 bar, the gas mixture can be compressed without the need for intermediate cooling, while in the reaction zone the heat will yet be available at a sufficiently high temperature level.

The amount of heat transferred from the reaction zone to the stripping zone can be increased by converting, in the reaction zone, an amount of carbamate to urea and water. Due to the presence of this urea and water, in which the ammonia and carbon dioxide are absorbed and the ammonium carba made formed dissolves, the temperature in the reaction zone rises with an additional amount. Depending on the pressure maintained in the reaction zone, this temperature increase may be from about 10 to 20 °C.

In order to further increase and make maximum use of the heat released in the reaction zone, it is desirable to insure intensive mixing of the contents of the reaction zone, which has the effect of raising the temperature level and promotes an increased transfer of heat to the stripping zone. To this end, the stripping can be carried out in a known manner within the tubes of a vertical heat exchanger while the shell side of the exchanger serves as the reaction zone, and the contents of the reaction zone are sufficiently mixed so that the difference in temperature between the top and bottom of the reaction zone is limited to 5°C at most. Preferably, to maximize the heat transfer, this temperature difference should be maintained at most at about 2°C. The invention will be illustrated by means of the drawings of Figures 1-4.

Figure 1 illustrates an embodiment of the invention wherein the urea is prepared in two steps, each having the same pressure, and wherein a portion of the gas mixture obtained in the stripping zone is supplied to the reaction zone exchanging heat with the stripping zone.

Figure 2 illustrates a similar embodiment for the preparation of urea in two steps wherein the second reaction step is carried out at a lower pressure than the first.

Figure 3 illustrates an embodiment of the invention wherein the stripping is effected in two successive steps.

Figure 4 illustrates an embodiment of the invention wherein the stripping is effected in two stripping zones arranged in parallel.

In each of the figures, the apparatus identified by letter A is the reactor-stripper here illustrated as a vertical shell and tube heatexchanger wherein the reaction zone is contained in the shell side, and the stripping zone is within the tubes. In each figure, the letter B denotes a carbamate condenser, C and after-reactor, D a heater-decomposer, E a scrubber, and the equipment designated by the letters F, G, H, and K are, respectively, a carbon dioxide compressor, an ammonia heater, an ejector, and an ammonia pump. L is a carbamate pump, M is a gas compressor, and N is a further ejector.

In the embodiment of the invention illustrated at Figure 1, a urea synthesis solution containing urea, water, non-converted carbamate, and free ammonia, is introduced via line 31 into the stripping zone of reactor-stripper A wherein it is heated and passed countercurrently against a gaseous stream of carbon dioxide. The carbon dioxide is supplied through line 1, compressed to a pressure of, for instance, 140 bar by compressor F, and introduced into the stripping zone via line 2. Air or some other oxygen containing inert gas mixture is added to the carbon dioxide in order to keep the materials which come into contact with the ammonium carbamate-containing solutions at high temperatures in a passive state. The heat required for the decomposition of ammonium carbamate in the stripping zone is supplied by heat exchange from the reaction zone, and the gas mixture of ammonia and carbon dioxide thus formed, together with the fresh carbon dioxide introduced, is expelled from the urea synthesis solution and discharged from the stripping zone of reactor-stripper A through line 4. The stripped urea product stream is removed from the stripping zone through line 5 and subsequently treated in a known manner to remove the small quantity of ammonium carbamate still remaining, and is further processed to form a concentrated urea solution or solid urea. The gas mixture discharged from the stripping zone through line 4 is brought up to a higher pressure of, for instance, 190 bar by means of compressor M, and is subsequently divided into two portions. One portion is passed via line 10 into the bottom of the shell side reaction zone of reactor-stripper A, and another portion is supplied through line 12 to carbamate condenser B, which is maintained at the same pressure as the reaction zone. Ammonia required for the synthesis reaction is primarily supplied directly to the reaction zone through line 3, pump K, ammonia heater G, and line 11. However, a portion of the required ammonia is also supplied through line 39 to carbamate condenser B. In carbamate condenser B, the gas mixture supplied from the stripping zone through line 12 is partly condensed in the presence of an ammonium carbamate solution supplied through line 16 from scrubber E and fresh ammonia supplied through line 39, and the heat of condensation is removed and utilized to produce steam of, for instance, 3-6 bar. If hydrostatic pressure is insufficient to transport the carbamate solution from scrubber E to carbamate condenser B due to too small a difference in level between the two pieces of apparatus, then ejector N, driven with the ammonia supplied through line 39, can be utilized to assist this transport.

The ammonium carbamate solution formed incarbamate condenser B is passed via line 17 into the bottom of the reaction zone of reactor-stripper A. In this reaction zone, a large portion of the ammonia and carbon dioxide supplied through lines 10 and 11 is converted into ammonium carbamate, and this ammonium carbamate, together with the ammonium carbamate contained in the carbamate solution supplied through line 17, is partially converted into urea and water. The conversion of the ammonia and carbon dioxide into ammonium carbamate is an exothermic reaction releasing heat, whereas the conversion of carbamate into urea and water is endothermic consuming heat. The net reaction, however, results in a surplus of heat which is transferred by heat exchange from the reaction zone to the stripping zone of reactor-stripper A. Inasmuch as the formation of ammonium carbamate and urea in accordance with the invention is carried out in the reaction zone at a pressure higher than that maintained in the stripping zone, the surplus heat will be available at a relatively high temperature level. Accordingly, a significant portion of the ammonium carbamate present iri the urea product solution supplied to the stripping zone through line 31 can be decomposed without additional heat having to be supplied from an external source. Moreover, by continuing the conversion of ammonium carbamate into urea and water in the reaction zone until more than half of the equilibrium quantity obtainable under the reaction conditions applied has been formed, a further temperature increase in the reaction zone is achieved, and the amount of surplus heat transferred to the stripping zone increases. This effect is increased as the degree of reaction in the synthesis zone moves further toward the equilibrium condition. For instance, if in a typical embodiment of the process according to Figure 1 at a pressure of about 240 bar, a molar ratio of ammonia to carbon dioxide of 3,40:1 and a molar ratio of water to carbon dioxide of 0,46:1 about 50 % of the equilibrium amount of urea is formed the average temperature in the reaction zone will be about 195 °C. With 60, 70, 80 and 90 % of the equilibrium amount of urea formed the temperature will be about 196, 199, 201 and 203 °C, respectively. The temperature increases resulting from higher conversions of ammonium carbamate to urea and water are substantial with respect to the temperature differential across the heat exchange surfaces in the reactor-stripper. Generally, conditions will be selected such that the amount of urea formed is greater than about 70 percent of that which would be formed at equilibrium. However, because of the strongly diminishing speed of reaction as the urea formation moves toward equilibrium, the formation of urea

beyond 90 percent of the equilibrium quantity requires substantially longer residence times, and hence an unattractively large volume in the reaction zone. At a given pressure in the reaction zone, amount of excess ammonia, molar ratio of H₂O/CO₂ and residence time, the temperature that can be achieved in the reaction zone is fixed. The quantity of gas mixture that is supplied through line 10 is then fixed as well inasmuch as the amount of this gas mixture effects the residence time, and thus the amount of heat transferred can no longer be increased by increasing the quantity of gas mixture introduced. The heat not utilized is recovered from the process by condensing part of the gas mixture discharged from the stripping zone of reactor- stripper A in carbamate condenser B and scrubber E and forming steam or heating process streams by the heat thereby released. It is very desirable to maintain intensive mixing in a reaction zone in order to obtain as even a temperature distribution as possible and to maximize heat transfer to the stripping zone. Some mixing is achieved by causing the gas mixture and ammonium carbamate solution to flow from the bottom to the top through the reaction zone. This mixing may be enhanced, however, by the provision of baffles, guides, or similar elements in the reaction zone resulting in a further improvement in heat transfer. The solution of urea, water, ammonia, and non-converted carbamate discharged from the reaction zone of reactor-stripper A, together with a quantity of a gas mixture consisting of ammonia, carbon dioxide, water, and inert gas, is passed through line 19 and introduced into after-reactor C, which is maintained at the same pressure as the reaction zone of reactor-stripper A. The conversion of ammonium carbamate into urea is continued in after- reactor C until at least 90 percent of the equilibrium quantity of urea obtainable under the reaction conditions prevailing in the after-reactor has been formed. The heat required for this conversion is obtained by supplying, through line 40, a non-condensed portion of the gas mixture supplied to carbamate condenser B through line 12, to the after-reactor wherein it is condensed, releasing both the heat of condensation and heat produced by the formation of ammonium carbamate. Alternatively, this non-condensed gas mixture from carbamate condenser B can be supplied to the reaction zone of reactor-stripper A together with the carbamate solution formed in carbamate condenser B. In the latter case, however, the heat required in after-reactor C must be obtained in a different manner, for instance, by supplying a larger quantity of gas mixture from the reaction zone of reactor- stripper A to the after-reactor, and condensing it therein.

The urea synthesis solution formed in after-reactor C is expanded in expansion valve 41 to the pressure of the stripping zone, for instance from 190 to 140 bar, and the gas-liquid mixture thus formed is passed to gas-liquid separator S via line 20, wherein the gas mixture consisting of ammonia, carbon dioxide, and water vapor is separated from the remaining urea product stream. This gas mixture is added, via line 42, to the gas mixture discharged from the stripping zone of reactor-stripper A through line 4. The remaining urea product stream from gas-liquid separator S is introduced through line 31 into the stripping zone of reactor-stripper A.

A gas mixture consisting of inert gases , ammonia, carbon dioxide, and water vapor is discharged from after-reactor C , and is fed through line 21 to scrubber E which is maintained at the same pressure as the reaction zone of reactor- stripper A, carbamate condenser B, and af ter-reactor C. In scrubber E, anπonia and carbon dioxide are recovered by scrubbing with water or a dilute carbamate solution supplied through line 23, while the heat of absorptio n is removed. The amnonium carbamate solution thus obtained is applied to carbama te condenser B through line 16 and if necessary, ejector N, while the remaining off-gas mixture is discharged from scrubber E through line 24. In this embodiment of the invention no high pressure steam is used for the decomposition of non-converted ammomium carbamate .

In the embodiment of the process as illustrated in Figure 2, only the reaction zone of reactor-stripper A is maintained at a high pressure of , for instance, 190 bar, whereas the stripping zone , carbamate condenser B , after-reactor C, and scrubber E are all maintained at the same lower pressure of, for instance , 140 bar . It is then necessary to compress to the higher reaction-zone pressure only that portion of the gas mixture discharged from the stripping zone that is fed to the reaction zone through line 10. Accordingly, a smaller compressor can be used as compared to the process according to Figure 1. the pressure of the urea product solution formed in the reaction zone .is lowered to the pressure prevailing in the lew pressure portion of the process comprised of the stripper, carbamate condenser after-reactor, and scrubber, and pump L is used to bring the carbaiiate solution formed in carboartate condenser B up to the pressure of the reaction zone for introduction therein. Also in this embodiment of the process according to the invention the non-converted ammonium carbamate is decomposed withouth the use of high pressure steam. Figure 3 illustrates an embodiment of the process according to. the invention in which the ammonium carbamate-containing urea synthesis solution formed in after-reactor C is passed through line 43 to heater-decomposer D wherein a portion of ammonium carbamate is decomposed and removed from the remaining urea product solution. The pressure in heater-decomposer D is preferably maintained at about the same pressure as the reaction zone and after-reactor C, but this pressure may also be lower than in the reaction zone. This heater-decomposer may be in the form of either a concurrent or a countercurrent heater, that is, the directions of flow of the liquid stream and liberated gas stream may be either cocurrent or countercurrent with respect to one another. A small quantity of air or other oxygen-containing gas mixture is supplied to heater-decomposer D via line 44 to passivate and reduce the corrosion of apparatus contacted with the high temperature ammonium carbamate solution.

In heater-decomposer D the urea synthesis solution is heated to a temperature of, for instance, 180 to 220^οC by means of steam, decomposing a portion of the ammonium carbamate present, and a gas mixture consisting of ammonia, carbon dioxide, water vapor, and inert gas is evolved. This gas mixture is discharged through line 10 and introduced, with the assistance of ejector H, into the reaction zone of reactor- stripper A. This ejector is driven by fresh ammonia supplied by pump K through heater G and line 11. The urea product stream leaving heater- decomposer D, still containing a quantity of ammonium carbamate, is expanded in expansion valve 41, and the gas-liquid mixture thus formed is introduced into separator S via line 20 wherein the gas mixture evolved by the expansion is separated frora the urea product stream, and sent to carbamate condenser B via lines 42 and 12. The urea product stream from separator S is then fed into the stripping zone of reactor-stripper A for decomposition of a further quantity of ammonium carbamate. Because a portion of the ammonium carbamate has already been decomposed in heater-decomposer D, this decomposition in the stripping zone can now be achieved with a smaller heat transfer area, and a smaller quantity of carbon dioxide stripping gas may suffice. Thus, a part of the total quantity of carbon dioxide required for the process may optionally be supplied through line 45 directly into the reaction zone rather than all carbon dioxide being introduced into the process through the stripping zone via line 2.

The gas mixture leaving after-reactor C is passed, via expansion valve 22, to scrubber E for recovery of ammonia and carbon dioxide. In this embodiment, scrubber E is operated at the same pressure maintained in the stripping zone of reactor-stripper A and in carbamate condenser B.

It is also possible, in accordance with the embodiment of the invention shown in Figure 4, to treat only a portion of the urea synthesis solution formed in after-reactor C in the stripping zone of reactor-stripper A, and to treat the remaining portion in heater-decomposer D. Preferably, the pressure maintained in heater decomposer D will be the same as in the stripping zone. However, it is possible to use a lower pressure in heater- decomposer D in which case the gas mixture released can be supplied to carbamate condenser B by means of an ejector which may be driven by, for instance, a portion of the fresh ammonia feed. In this embodiment, as in the embodiment of Figure 3, only a portion of the ammonium carbamate leaving after- reactor C need by decomposed and removed in the stripping zone of reactor-stripper A, and thus a smaller heat transfer area and a lesser quantity of carbon dioxide stripping gas in the stripping zone may suffice.

The embodiments of the invention illustrated in Figures 1 and 2 will be described in further detail by means of the following examples.

Example I

Urea is produced in accordance with the process configuration illustrated in Figure 1. For each tonne of urea produced, 400 kg of ammonia is supplied through ammonia pump K, heated to a temperature of 110°C in heater G and introduced into the reaction zone of reactor-stripper A via line 11, and 733 kg of carbon dioxide and 29 kg of inert gases (namely air) are introduced into the stripping zone of reactor-stripper A through compressor F and line 2. The pressures maintained in the reaction zone and stripping zone of reactor-stripper A are 186 bar and 137 bar, respectively.

The reaction zone of reactor-stripper A also is fed through line 10 with a gas mixture having a temperature of 227^οC and consisting of 513 kg of ammonia, 721 kg of carbon dioxide, 43 kg water , and 21 kg inert gas , and through line 17 with a carbamate solution having a temperature of 175 ^οC, and consisting of 624 kg of ammonia, 540 kg of carbon dioxide, and 173 kg of water.

The volume of the reaction zone of reactor- stripper A, and consequently the residence time therein, is such that at the prevailing pressure and related temperature of about 193 °C, a gas-liquid mixture is formed containing 1084 kg of ammonia, 674 kg of carbon dioxide; 800 kg of urea 456 kg of water, and 21 kg of inart gas. This gas-liquid mixture discharged from the synthesis zone via lire 19, together with a gas mixture discharged from carbamate condenser B through line 40, is reacted in after-reactor C , maintained at the same pressure as the reaction zone (186 bar), to form a urea synthesis solution, having a temperature of 196 ^οC and consisting of 842 kg of ammonia, 436 kg of carbon dioxide , 1000 kg of urea, and 510 kg of water. The quantity of urea formed in the reaction zone is about 76 % of the quantity that would be obtained if the conversion of amm onium carbamate to urea would be all owed to proceed to equilibrium.

Af ter expansion of this urea synthesis solution to 137 bar in expansion vaivg 41, the gas mixture thereby evolved, consisting of 44 kg of ammonia, 21 kg of carbon dioxide, and 9 kg of water vapor , is separated in gas-liquid separator S leaving a urea product solution having a temperature of 188 ^οC, and consisting of 798 kg of ammoni a, 415 kg of carbon dioxide , 1000 kg of urea, and 501 kg of water. This urea product stream is then introduced into the stripping zone of reactor-stripper A wherein it is heated and stripped countercurrently with f resh car bon dioxide resulting in a residual urea product containing 1000 kg of urea, 450 kg of water , 124 kg of ammonia, and 161 kg of carbon dioxide. The gases released in the stripping treatment, together with the carbon dioxide stripping gas used, form a gas mixture containing 674 kg ammonia, 987 kg of carbon dioxide, 51 kg of water vapor, and 29 kg of inert gas, which gas mixture is compressed in compressor M to a pressure of 186 bar, together with the gas mixture separated in gas-liquid separator S. A portion of this combined compressed gas mixture is supplied to the reaction zone of reactor-stripper A, and the remaining portion, consisting of 205 kg of ammonia, 287 kg of carbon dioxide, 17 kg of water vapor, and 8 kg of inert gas, is partly condensed in carbamate condenser B together with 186 kg of fresh ammonia and the ammonium carbamate solution formed in scrubber E which consists of 227 kg of ammonia, 271 of carbon dioxide, and 157 kg of water. The heat released in this condensation is recovered and used for the production of 355 kg of 3.5 bar saturated steam. In this embodiment of the invention, the consumption of high-pressure steam (20-30 bar) for the decomposition of non-converted carbamate is nil.

Example II urea is prepared in accordance with the process configuration illustrated in Figure 2. For each tonne of urea produced, the reaction zone of reactor-stripper A is fed with 400 kg of ammonia at 110^οC via the pump K, ammonia heater G, and line 11, with a gas mixture at 221°C consisting of 569 kg of ammonia, 781 kg of carbon dioxide, 45 kg of water, and 22 kg of inert gas via line 10, and with a carbamate solution at 165°C composed of 611 kg of ammonia, 516 of carbon dioxide, and 172 kg of water via line 17. In the reaction zone, a gas-liquid mixture is formed at a temperature of about 193 °C consisting of 1127 kg of ammonia , 710 kg of carbon dioxide, 800 kg of urea, 457 kg of water, and 22 kg of inert gas. The quantity of urea foπred in the reaction zone is about 67 % of the quantity of urea that would be obtained if the conversion of amronium carbamate to urea would be allowed to proceed to equilibrium. This gas-liquid mixture is expanded to a pressure of about 137 bar and, together with thejgas mixture supplied through line 40 containing 18 kg of ammonia , 15 kg of carbon dioxide, 1kg of water, and 7 kg of inert gas, is reacted in after-reactor

C to form another 200 kg of urea, resulting in 2862 kg of urea synthesis solution oohtaining 1000 kg of urea. This urea synthesis solution, having a temperature of 182 °C and a pressure of 137 baio and which, in additioh to 1000 kg of urea, contains 883 kg of ammo nia, 469 kg of carbon diokide , and 510 kg of water, is supplied, directly to the stripping zcne of reactor-stripper A wherein it is stripped at a pressure of 137 bar with 733 kg of gaseous car bon dioxide, resulting in a residual urea product stream containing 1000 kg of urea, 450 kg of water, 124 kg of ammonia, and 161 kg of carbon dioxide.

Of the gas mixture discharged from the stripping zone of reactor-stripper A, 1417 kg is compressed to a pressure of 186 bar in compressor M and subsequently fed into the reaction zone , while the remaining portion, consisting of 190 kg of ammonia, 260 kg of carbon dioxide , 15 kg of water , and 7 kg of inert gas, is partly condensed in carbamate condenser B with the aid of 167 kg of ammonia and the carbamate solution from scrubber E consisting of 272 kg of ammonia, 271 kg of carbon dioxide, and 157 kg of water. The res ul ting ammonium carbamate solution, as noted above , is fed into the reaction zone of reactor-stripper A via pump L and line 17, and the remaining non-condensed gas mixture is fed to the after-reactor, as noted above, via line 40.

The heat released in carbamate condenser B is recovered and used for the production of 327 kg of 3.5 bar saturated steam. In this embodiment of the invention as well, no high-pressure steam (20-30 bar) is required for the decomposition of non- converted ammonium carbamate.

Claims

1. Process for the preparation of urea from ammonia and carbon dioxide at an elevated temperature and pressure having a reaction zone and a stripping zone wherein in said reaction zone, carbon dioxide and a portion of said ammonia are converted to ammonium carbamate, and a portion of said ammonium carbamate is converted to urea to form a reaction zone effluent containing product urea, unconverted ammonium carbamate and excess ammonia, said conversions resulting in a net formation of heat, and in said stripping zone, a urea product stream containing unconverted ammonium carbamate is heated to decompose at least a portion of said ammonium carbamate by heat exchange with said reaction zone, and stripped to remove gaseous ammonia and carbon dioxide thus formed from said urea product stream, characterized in that said reaction zone exchanging heat with said stripping zone is maintained at a pressure in the range of between about 125 and 250 bar, and said stripping zone is maintained at a pressure lower than the pressure in said reaction zone.

2. Process according to claim 1, characterized in that said stripping zone is maintained at a pressure of between about 20 and 120 bar lower than the pressure maintained in said reaction zone.

3. Process according to claim 2, characterized in that said stripping zone is maintained at a pressure of between about 40 and 60 bar lower than the pressure maintained in said reaction zone.

4. Process according to any one of claims 1-3, characterized in that in said reaction zone, the conversion of ammonium carbamate into urea is continued until the quantity of urea formed is at least 50 percent of that quantity of urea that would be obtained at equilibrium under the reaction conditions present in said reaction zone.

5. Process according to claim 1, characterized in that in said reaction zone, the conversion of ammonium carbamate into urea is continued until the quantity of urea formed is at least 70 percent of that quantity of urea that would be obtained at equilibrium under the reaction conditions present in said reaction zone.

6. Process according to any one of claims 1-5, characterized in that the contents of said reaction zone are intensively mixed.

7. Process according to claim 6, characterized in that said reaction and stripping zones are within a vertical tube and shell heat exchanger, said stripping zone being within the tubes of said heat exchanger and said reaction zone being within the shell of said heat exchanger, and wherein the temperature differential between the top and bottom of said reaction zone is limited to at most 5 °C.

8. Process according to claim 7, characterized in that the temperature differential between the top and bottom of said reaction zone is limited to at most 2 °C.

9. Process according to any one of claims 1-8, characterized in that the reaction zone effluent is introduced into an after-reaction zone wherein an additional portion of ammonium carbamate is converted to urea to form a urea product stream containing urea, in a quantity of at least 90 percent of the quantity of urea that would be formed at equilibrium under the conditions prevailing in said after-reaction zone, and unconverted ammonium carbamate, and wherein said urea product steam is thereafter introduced into said stripping zone.

10. Process according to claim 9, characterized in that the urea product stream from said after-reaction zone, prior to being introduced into said stripping zone, is treated in a heater-decomposer wherein at least a portion of the ammonium carbamate is decomposed by heating to form a gas mixture containing ammonia and carbon dioxide and a residual urea product stream of reduced carbamate content, and wherein said gas mixture is introduced into said reaction zone and said residual urea product stream is introduced intosaid stripping zone.

11. Process according to any one of claims 1-9, characterized in that the urea product stream from said stripping zone is introduced into a second stripping zone wherein additional ammonium carbamate is decomposed and removed from the urea product stream.

12. Process according to claim 9, characterized in that only a portion of the urea product stream from said after-reaction zone is introduced into said stripping zone, and a remaining portion of the urea product stream from said after-reaction zone is introduced into a second stripping zone wherein ammonium carbamate is decomposed to form a gas mixture containing ammonia and carbon dioxide, which gas mixture is separated from the residual urea product stream.

13. Process according to claim 11 or 12, characterized in that said second stripping zone is maintained at the same pressure as said stripping zone.

14. Process for the preparation of urea, substantially as described and illustrated in the drawing.

15. Process for the preparation of urea, substantially as described and explained with reference to Example I.

16. Process for the preparation of urea, substantially as described and explained with reference to Example II.

17. Urea and urea solutions obtained by the process according to any one of the preceding claims.