WO1998017635A1

WO1998017635A1 - Process for the urea synthesis from ammonia and carbon dioxide

Info

Publication number: WO1998017635A1
Application number: PCT/IT1997/000252
Authority: WO
Inventors: Vincenzo Lagana'
Original assignee: Eurotecnica Contractors And Engineers S.P.A.
Priority date: 1996-10-17
Filing date: 1997-10-16
Publication date: 1998-04-30
Also published as: AU4638197A; IT1290423B1; ITMI962152A1

Abstract

Process for the manufacture of urea from carbon dioxide and ammonia with high conversion yields including a urea synthesis section and an ammonia stripping section of carbamate obtained as by-product, characterized in that the urea synthesis zone is operated at a pressure higher than the one existing in the stripping section.

Description

Description "PROCESS FOR THE UREA SYNTHESIS FROM AMMONIA AND CARBON DIOXIDE" Technical field The present invention refers to an improved process for the urea manufacture from ammonia and carbon dioxide. More particularly the present invention is directed to a urea manufacturing process wherein carbamate, resulting as a by-product of the urea synthesis, is decomposed by stripping with ammonia to recover urea as well as to recycle the stripping section outcoming gases to the synthesis zone. Background art

It is well known that the urea synthesis processes nowadays in operation comprise a carbamate decomposition section located downstream of the synthesis zone in order to obtain, respectively, a urea solution to be possibly sent to further purification steps and a mixture of stripping gases consisting of ammonia, carbon dioxide and water to be recycled to the synthesis zone. In the above processes carbamate decomposition is obtained by reacting (stripping) carbamate with one of the reactants, (ammonia or carbon dioxide), or, in accordance to a more recent technology by consecutive carbamate strippings with both reactants. It has been suggested to carry out synthesis reaction and carbamate stripping essentially at same pressure to avoid pumping gas mixture coming out of the stripping section and recondensed in form of carbamate to the urea synthesis zone.

It has been thought indeed that by operating under isobaric conditions a double benefit is obtained: an energy saving together with avoiding the use of pumps to recycle a highly aggressive liquid such as carbamate. On the other hand earring out the reaction in isobaric conditions as in the stripping section has the drawback that to maintain the same synthesis pressure the ammonia - carbon dioxide ratio cannot exceed certain values corresponding to a low synthesis yield, owing to the fact that, as it is well known to those skilled in the art, the higher is such ratio the higher is the reaction yield. Disclosure of invention

It has been now found, and this is an object of the present invention, that it is possible to manufacture urea from ammonia and carbon dioxide with high yields and consequently with an energy saving, by a process in which the urea synthesis zone is operated at a pressure higher than the one existing in the stripping section. More particularly in the process according to the present invention the urea synthesis reaction is carried out with an ammonia to urea molar reaction in the range of 4 to 5, under a pressure in the range of 180 to 250.10² KPa and at a temperature of from 170 to 250°C, the stripping section operating with ammonia at a pressure comprised between 1 10 and 160.10² KPa. In the process according to the present invention the conversion to urea is higher than 70%, while in the conventional isobaric processes the conversion to urea does not reach 65% Such higher conversion yields obtained by the process according to the present invention, and the associated reduction of carbamate amount to be decomposed and recycled, largely counterbalance, as it will be seen hereinafter, any drawback connected with the necessary carbamate solution pumping from the stripping and condensation section to urea synthesis reactor. Moreover it has to be taken into account that the presently available centrifugal pumps used for carbamate recycling are quite reliable and do not exhibit any unreliability inconvenient as it was experienced in the past with reciprocating pumps.

High conversion obtained by the process according to the present invention allows to reduce steam consumption for the carbamate decomposition if compared with isobaric process steam consumption. Moreover by carrying out the ammonia stripping step at considerably lower pressure than the synthesis, and even lower than the one conventionally employed in isobaric stripping processes, carbamate decomposition is made easier leaving

SUBSTITUTE SHEET RUL carbamate solutions with a carbamate concentration of few percents by reducing therefore the downstream stripping step treatment complications. The process according to the present invention may be schematized as follows. Urea synthesis is carried out, as aforesaid, under a pressure in the range of from 180.10² KPa to 250.10² KPa, while the ammonia stripping is performed at a pressure of from 110.10² to l60.10² KPa.

Vapors coming out of the stripping section are mixed with carbamate liquid solution resulting from the carbamate decomposition section located downstream of the synthesis zone. Said carbamate decomposition is well known in the art and generally operates at pressure lower than or equal to 22.10² KPa.

The recovered two-phase mixture is made to condense in a process section referred to as carbamate condensation section by operating essentially at the same pressure as the stripping section; resulting carbamate is recycled by means of a pump to the synthesis zone. Carbamate condensation heat is partially employed to provide residual carbamate, still present in the urea solution coming out of the stripping section, with the necessary heat of decomposition, while the condensation heat of the carbamate decomposed in the latter step can be used for steam production.

The urea manufacturing process according to the present invention is specifically suitable for its integration with a melamine synthesis process starting from urea. As a matter of fact the melamine manufacturing process starting from urea yields, as by-products, ammonia, water and carbon dioxide which are condensed as a carbamate solution that has to be recycled back to urea synthesis zone. According to the process of the present invention it is possible to recycle said carbamate solution without introducing into the urea synthesis reactor the water present in said solution. In fact, as it will appear from the detailed description provided hereinafter, aqueous carbamate solution is added to the product sent to the stripping section and then vapors are sent after condensation to the synthesis section, while water remains into the urea solution.

Detailed description of the invention and drawing

In order to better demonstrate the invention an example is given hereinafter of an embodiment of the urea synthesis process. The example has not to be interpreted as limitative of the scope of the invention. For clarification sake the description of the process is made with reference to the annexed drawing wherein a block diagram of a plant suitable to operate the process according to the invention is reported.

CO₂ coming from the plant battery limits is compressed at a pressure of 230.10² KPa by means of compressor 1 and is sent to the bottom of the urea synthesis reactor 2. Both recycled carbamate through pump 3, and ammonia coming from pump 4 and preheated to a temperature of 90°C in preheated 5 enter the bottom of the reactor. Reactor operates at a tot pressure of 226.10² KPa and a 195°C top temperature with a CO₂ conversion yield of

73%. On top of reactor 2 a liquid phase is obtained which contains urea, residual carbamate, excess ammonia and synthesis water produced in the carbamate dehydration to urea, and a gas phase comprising inert gases which are contained in CO₂ reactant, i.e. ammonia saturated passivating air, CO₂ and water.

Liquid solution is discharged from reactor 2 trough valve 6 whereby its pressure is lowered to 130.10² KPa before entering stripper 7 operating at a pressure of 130.10² KPa.

Gases accumulated on top of reactor are discharged through valve 8 and enter the bottom part of stripper 7 so that passivating air, contained in said gases, may carry out its anticorrosion action also within stripper 7.

Solution entering the top of the falling film type stripper, is heated up to a temperature of 207°C by means of steam fed into the heating mantle; The steam provides the necessary heat to decompose most of carbamate coming out of reactor 2. The incompletely purified urea solution is expanded through valve 9 down to a pressure of 18.10² KPa to continue its purification, while effluent gases from the top section of stripper 7 enter the top section of carbamate condenser 10A through ejector 12 wherein said gases are thoroughly mixed with carbamate solution fed by pump 14 after having been preheated up to a temperature of 100°C by preheated 13.

Mixed phase coming out of ejector 12 enters the pipes of condenser 10A where the vapor phase condensation starts and carbamate formation heat is provided to the stripper effluent solution to further decompose the residual carbamate by preheating said mantle flowing solution up a temperature of 165°C and producing a large amount of gas phase products that are removed from liquid phase in separator 15.

The solution accumulated in the separator 15 after having been expanded through valve 17 down to a pressure of 4.5.10² KPa enter the mantle of carbamate condenser 10B to yield a solution at a temperature of 138°C containing about 70% urea and a very small amount of residual carbamate. Said solution is collected in separator 16 wherefrom both vapor phase and liquid phase are removed to continue the decomposition process.

Heat for this decomposition step is provided by carbamate condensation taking place in the pipes of carbamate condenser 10B.

The solution and the vapor exit through the bottom of condenser 10B to reach their complete condensation in the carbamate condenser 11 then, finally, enter separator 18 wherefrom carbamate, by means of the pump 3, is recycled to reactor 2. Ammonia, CO₂ and water saturated inert gases are injected, through valve 19 into the urea solution leaving valve 9 to employ air contained therein as a passivating agent for the mantle of carbamate condenser 10 A. Carbamate final condensation heat is used to produce 4.5.10² KPa pressure steam suitable for use in the urea solution vacuum concentration section. The urea solution accumulated in separator 16 after expansion through valve 20 down to a pressure of 0.3.10² KPa enters the bottom of evaporator 21 A to produce a 85% concentrated solution, the heat necessary to reach said concentration is provided by the condensation of vapors coming out from separator 15 which are mixed with the solution fed by pump 26.

Steam heated urea solution enter evaporator 21B at a pressure of 4.5.10² KPa to reach a concentration of 95% and a temperature of 130°C.

Finally urea solution enter evaporator 2 IB where vapor phase is sent to a first vacuum system 28, while urea solution is sent to final concentrator 23 operating at a pressure of 0.03.10² KPa and a temperature of 140^CC to obtain essentially pure urea at a concentration of 99.1%. The heat necessary to reach said concentration is provided by 4.5.10² KPa steam condensing on the mantle side.

Urea coming out of concentrator 23 enter the separator 24 wherefrom the vapor phase is sent to second vacuum system 29, while molten urea is fed by means of the pump 30 to the top of the prilling tower 31 wherein it is sprayed in order to obtain the final product in form of prills.

By falling down along the prilling tower, molten urea solidifies by cooling in contact with rising air.

Molten urea contains a small amount of free ammonia that when mixed with cooling air would result in an environmental pollution when discharged into the atmosphere.

In order to reduce the free ammonia content, CO₂ is injected in the delivery side of molten urea pump 30. CO₂ will react with ammonia to yield carbamate and therefore will lower the ammonia emission level of the top of tower 31.

The two-phase liquid coming out of the mantle of evaporator 21 is sent to ammonia preheated 5 to condense further and then enters the bottom section of column 32 wherein a

EET R 6 carbamate liquid phase is formed in the bottom while pure gaseous ammonia exits from the upper part of the column.

Carbamate formation heat is removed by means of a pure ammonia reflux entering on top of column 32. By means of pump 14 the bottom carbamate enters the preheat er 13 wherein the preheating heat is provided by steam coming out of the top of separator 16; the mixed phase is sent to final condenser 25 together with carbamate solution 39 coming from a process condensate treatment section that has not been illustrated in the drawing.

Carbamate coming out of condenser 25 is collected in a reservoir 27 wherefrom carbamate is recycled by means of the pump 26 as illustrated above.

Gas phase ammonia coming out of the top of column 32 enters condenser 33 wherein ammonia is condensed and collected in a reservoir 34 wherefrom it is drawn by pump 35 which delivers ammonia both on top of column 31 as a reflux and to pump 4 that will send ammonia to reactor 2 after its preheating in heat exchanger 5. Inert gases accumulated in reservoir 34 are saturated of ammonia at a temperature of 43°C and a pressure of 18.10² KPa: therefore before discharging to the atmosphere they are washed in column 36 with fresh ammonia coming from battery limits, typically at a temperature of -34°C to lower as much as possible their ammonia content.

After having described and illustrated the overall process it is important to describe the case in which a melamine manufacturing plant is operated downstream the urea plant.

It is well known that melamine plant produces certain amounts of NH₃, CO₂, H₂O gas which are condensed in the melamine plant itself, liquid carbamate obtained in such a way has to be recycled to urea plant in order to transform it into urea.

In order to avoid feeding water associated with said recycled carbamate to urea reactor, it is advisable that carbamate recycle through line 37 is mixed with urea solution coming out of the reactor 2 after expansion valve 6, the combined stream being fed to stripper 7. In such a way only about 10% of water entering with said recycle comes out with gases from the top of the stripper 7 and goes back to the reactor, while the remaining 90% is discharged from the bottom of the stripper 7 and is removed together with the urea solution passing through subsequent concentration phases as abovesaid. A quantitative example is given hereinafter. EXAMPLE

In the example reference is made to a lOOOt/d nominal plant capacity. The reactor works at the following operating conditions: Molar ratio: NH₃/CO₂ = 4.1; H₂O/CO₂ = 0.5 Temperature: 199°C; pressure 198.10² KPa

In these conditions reactor yield is 71.5%, the outgoing solution having the following composition: NH₃ 44125 kg/h 37.00% by weight

CO₂ 12204 kg/h 10.23% by weight

H₂O 21246 kg/h 17.82% by weight

UREA 41667 kε/h 34.95% by weight

Total 119242 kg/h

Carbamate solution content is 21634 kg/h.

An isobaric stripping process reactor operates in the following conditions: Molar ratio: NH₃/CO₂ = 3.5; H₂O/CO₂ = 0.5

Temperature: 189°C; pressure 145.10² KPa

In these conditions reactor yield is 64.3%, the outgoing solution having the following composition:

NH₃ 40653 kg/h 33.46% by weight CO₂ 16967 kg/h 13.96% by weight

H₂O 22221 kg/h 18.29% by weight UREA 41667 kg/h 34.29% by weight

Total 121507 kg/h

Carbamate solution content is 30078 kg/h.

It appears that in case of an isobaric stripping process, carbamate to be decomposed is 8444 kg/h more than the inventive process, resulting in a 6249000 kcal h higher energy consumption. If saturated steam is used at a pressure of 23.10² KPa (abs) the increase of steam consumption will be 14013 kg/h corresponding to 336 kg for each tons of manufactured urea.

Solution coming out of the reactor at a pressure of 199.10² KPa is expanded down to a pressure of 13510² KPa, then is sent to stripper operating at a temperature of 205°C, and the purified solution has the following composition:

NH₃ 19151 kg/h 22.62% by weight

CO₂ 3779 kg/h 4.46% by weight

H₂O 20089 kg/h 23.72% by weight U URREEAA 4 411666677 kkgg//hh 49.20% by weight

Total 84686 kg/h

The 23.10² KPa saturated steam consumption is 11288 kg/h corresponding to 271 kg/t of urea.

The stripper out coming solution is expanded to a pressure of 18.10² KPa and is sent to the mantle of the carbamate condenser 10A wherein is heated up to a temperature of 160°C to decompose residual carbamate obtaining the following solution: NH₃ 1249 kg/h 2.16% by weight

CO₂ 309 kg/h 0.54% by weight

H₂O 14536 kg/h 25.17% by weight UREA 41667 kg/h 72.13% by weight

Total 57761 kg/h The heat necessary to carbamate decomposition is 2967000 kcal/h and is provided by stripper effluent vapor condensation.

Solution coming out of the decomposition step under a pressure of 18.10² KPa is expanded down to a pressure of 4.5.10² KPa and is sent to carbamate condenser mautle 10B wherein the solution is heated up to a temperature of 138°C to decompose residual carbamate obtaining the following composition.

The heat necessary for carbamate decomposition is 1934000 kcal h and is provided by the condensation of the stripping outcoming vapors.

Stripping effluent gases are combined with carbamate solution coming out from the 18.10² KPa section and are condensed under a pressure of 134.10² KPa and a temperature of

156°C with a heat production of 7255000 kcal/h.

Since heat requirement for both decomposers is 4901000 kcal h, the residual heat is used in the final carbamate condenser 11 to produce 4.5.10² KPa saturated steam in an amount of

4624 kg/h corresponding to 111 kg/t of urea that may be used in the urea vacuum concentration section.

Vapor coming out of the 18.10²KPa carbamate decomposition section and separator 15 are condensed in the mantle of evaporator 21 A wherein the urea solution is concentrated up to

85% by weight.

Heat provided by vapor condensation is approximately 4250000 kcal/h while the remain heat necessary to concentrate urea solution up to 95% and corresponding to 2709000 kcal/h is provided by heated steam.

In conclusion the process according to this invention whose stripping section operates at lower pressure than the synthesis zone, exhibits a total steam consumption of 470 kg/t of manufactured urea, compared with a consumption of conventional stripping processes of approximately 700kg/t of manufactured urea.

Claims

1. Process for the manufacture urea from carbon dioxide and ammonia with high conversion yields including a urea synthesis section and an ammonia stripping section of carbamate obtained as by-product, characterized in that the urea synthesis zone is operated at a pressure higher than the one existing in the stripping section.

2. Process for the manufacture urea according to claim 1, characterized in that the urea synthesis reaction is carried out with an ammonia to urea molar reaction in the range of 4 to 5, under a pressure ia the range of 180 to 250 10² KPa and at a temperature of from 170 to 250°C, the stripping section operating with ammonia at a pressure comprised between 110 and 160.10² KPa.

3. Process for the manufacture urea according to claim 2, characterized in that vapors coming out of the stripping section are mixed with carbamate liquid solution resulting from the carbamate decomposition section located downstream of the synthesis zone, said carbamate decomposition being of the type well known in the art and generally operating at pressure lower than or equal to 22.10² KPa and that the recovered two-phase mixture is condensed in a process section referred to as carbamate condensation section by operating essentially at the same pressure as the stripping section, the resulting carbamate being recycled by means of a pump to the synthesis zone.

4. Process for the manufacture urea according to claim 3, characterized in that the carbamate condensation heat is partially employed to provide residual carbamate, still present in the urea solution coming out of the stripping section, with the necessary heat of decomposition, while the condensation heat of the carbamate decomposed in the latter step can is for steam production.

5. Process for the manufacture urea according to anyone of preceding claims, characterized in that part of ammonia and carbon dioxide used as reactants derive as byproducts from a

ET RULE 26 melamine manufacturing plant integrated with the urea process and are fed in form of carbamate aqueous solution to carbamate stripping section.

6. Process according to anyone of preceding claims, characterized in that manufactured urea, sent in molten phase to a prilling tower to form prills in contact with air, is previously additioned with a certain amount of carbon dioxide to reduce the free ammonia content and the associated environmental pollution.

B TIT