GB2196965A - Process for the separation of unconverted raw materials in urea synthesis - Google Patents
Process for the separation of unconverted raw materials in urea synthesis Download PDFInfo
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- GB2196965A GB2196965A GB08725321A GB8725321A GB2196965A GB 2196965 A GB2196965 A GB 2196965A GB 08725321 A GB08725321 A GB 08725321A GB 8725321 A GB8725321 A GB 8725321A GB 2196965 A GB2196965 A GB 2196965A
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- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C273/00—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups
- C07C273/02—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds
- C07C273/04—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds from carbon dioxide and ammonia
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Abstract
In a process for the synthesis of urea the improvement is described in which the decomposition and separation of unconverted raw materials which are conventionally performed at a pressure equal to the pressure of the synthesis reaction by stripping and formation of ammonium carbamate, are partially replaced by decomposition and separation thereof and absorption thereof under a medium pressure which is lower than the pressure of synthesis reaction, the unconverted materials being separated as a first gaseous mixture under the synthesis pressure, a second gaseous mixture under a medium pressure which is lower than the pressure of synthesis reaction and a third gaseous mixture under a third pressure which is lower than the said medium pressure, and the separated unconverted materials recycled by absorption of the second gaseous mixture in a solution of the third gaseous mixture at the medium pressure and absorption in the resulting solutions at the synthesis pressure of a portion of the first gaseous mixture, the resulting aqueous solution and unabsorbed first gaseous mixture being recycled to the synthesis section, and in which the heat generated in the said absorption is utilized in the overall process to reduce the amount of high pressure steam required in the overall process and reduce the amount of low pressure steam generated, the ratio of total ammonia to total carbon dioxide in the synthesis section being maintained within the range of from 2.8 to 5.0.
Description
SPECIFICATION
Process for the separation of unconverted raw materials
The present invention relates to an improvement in a process for the synthesis of urea.
Particularly, it relates to an improvement in the thermal efficiency of the main steps of the urea process.
More particularly, it relates to an overall improvement in the pressurized system for the synthesis of urea involving CO2 stripping wherein part of the make-up carbon dioxide is used to separate the materials unconverted to urea, which is characterized by the effective recovery and utilization of the heat generated by the re-condensation of the off gas discharged from a separator for unconverted materials.
In the main stages of the urea process in voiving conventional CO2 stripping, raw materials which have not been converted into urea are decomposed into a gaseous mixture by both heating with high-pressure steam of at least about 10 atm and stripping with make-up gaseous carbon dioxide raw material under a pressure equal to the synthesis pressure which is the maximum pressure in the process.The gaseous mixture is stripped from the liquid phase and condensed under a pressure substantially equal to the synthesis pressure to form ammonium carbamate, while the heat generated by the formation is generally recovered as a low-pressure steam of at most several atm and utilized in the low-pressure stages of urea synthesis, for example, decomposition of unconverted materials under low pressure or concentration of the aqueous solution of urea remaining after the separation of unconverted materials.
Although the recovered low-pressure steam is utilized in some stages of the urea process, excessive low-pressure steam still remains and is at present used in suitable fields outside the urea process, for example, general heating.
It has generally been thought that the thermal efficiency of a urea process is high, even though a large amount of high-pressure steam is used, because the amount of the low-pressure steam recovered by the formation of ammonium carbamate is large and the value obtained by offsetting the amount of the highpressure steam used by that of the excessive recovered low-pressure steam is small, thus making the total amount of steam used in the urea process appear to be small.
For example, it is disclosed in "Kagaku Binran, Applied Section", the third edition, p. 20 that the urea process which involves the recycling of a solution but does not involve recovery of low-pressure steam requires 0.9 to 1.2 ton of high-pressure steam per ton of urea product, while the process involving conventional CO2 stripping requires 1.0 to 1.1 ton of high-pressure steam.
In the latter process, the amount of the excessive low-pressure steam discharged from the system is 0.25 ton, and this amount has generally been subtracted from the total amount of high pressure steam required to give an apparent figure for overall steam requirement of 0.75 to 0.85 ton.
The cost of high-pressure steam is of course higher than that of low-pressure steam.
Furthermore, it is difficult in practice to find suitable uses for the excess low-pressure steam produced so that it can be effectively utilized.
Again, the fact that the volume of the steam generated in the system is large means that the volume of the substance to be treated in the process is also large, thus requiring large-capacity units to be used in each of the stages.
It is thus evident that the urea process involving conventional CO2 stripping is not economical and it would be highly desirable that the amount of the expensive high-pressure steam required should be reduced, even if the amount of the excess low-pressure steam recovered is reduced.
The main factor, by which the amount of the high-pressure steam required is determined, is the conversion of ammonium carbamate into urea. The higher is the conversion, the smaller is the amount of the highpressure steam required for the decomposition and separation of unconverted raw materials, as far as the ratio of ammonia to carbon dioxide in a reactor for urea synthesis is specified.
However, the practical upper limit of the conversion is 70 to 75%, so that if the amount of the high-pressure steam required is to be reduced it must be done by means other than increasing the ratio of ammonia to carbon dioxide.
In the synthesis of urea wherein the molar ratio of the total ammonia to the total carbon dioxide present in the synthesis section is not more than about 3.5, the stripping of unconverted materials can be carried out with a high efficiency because of low synthesis pressure, so the content of ammonia and carbon dioxide in the urea synthesis solution remaining after the stripping is low.
When the molar ratio of the total ammonia to the total carbon dioxide present in the synthesis section is as close as possible to 2.8, the required synthesis pressure is correspondingly low and therefore, the stripping is more effective. In these circumstances however, the temperature at which the gaseous mixture generated by the stripping is condensed to form ammonium carbamate is too low to attain efficient recovery of heat.
As described above, in the urea process involving conventional CO2 stripping there is no necessity for liquefaction and recovery of excess ammonia and little or no necessity or apparent incentive for the employment of a stage for separating unconverted raw materials wherein the pressure of the urea synthesis solution is reduced to a medium value. The process is evaluated economically good, because the amount of low-pressure steam recovered is large, even though the amount of the highpressure steam required is large. That the amount of the high-pressure steam required can be reduced by employing a stage under a medium pressure to separate unconverted raw materials and applying the recovered heat to this step has never been contemplated.
In the synthesis of urea wherein the molar ratio of the total ammonia to the total carbon dioxide present in the synthesis section is about 3.5 or above, the higher the molar ratio, the higher becomes the necessary synthesis pressure and thus the higher the temperature needed at which the gaseous mixture formed by the stripping is condensed to form ammonium carbamate, which enables the recovery of heat to be attained at a high level.
Under these conditions however, the stripping becomes difficult.
The present invention has for its object the reduction of the amount of high-pressure steam required and improvement in the recovery of heat with respect to the main stages of the urea process including sections of synthesis, decomposition and separation of unconverted materials and formation of ammonium carbamate.
The present invention is characterized by employing a decomposition and separation, and an absorption stage of unconverted materials, which are carried out under a medium pressure, between a stripping stage under a pressure equal to synthesis pressure and a decomposition and separation stage under low pressure, transferring the heat generated by the formation of ammonium carbamate under a pressure equal to the synthesis pressure to the decomposition and separation stage of a medium pressure without the use of a heating medium such as steam, but directly through heat transfer tube walls and utilizing the thustransferred heat in the decomposition and separation under a medium pressure.Thus, according to the present invention, the optimum conditions for a series of stages carried out under a pressure not lower than the medium pressure, which significantly affect the performance of the overall process of the synthesis of urea, are also specified.
The optimum conditions have been achieved based on various data and experiences obtained by design of many urea plants and their practical operations thereof.
In the method of the present invention, the reaction of ammonia with carbon dioxide is carried out in a synthesis section in which a high-pressure reactor has a temperature and a pressure suitable for the synthesis of urea to obtain a urea synthesis solution.
This solution comprising ammonium carbamate, ammonia, water and urea is discharged from the synthesis section. From this solution is obtained a first gaseous mixture called "off gas" by stripping with part of the make-up carbon dioxide whilst heating under a pressure substantially equal to the pressure of urea synthesis and separating the first gaseous mixture from the urea synthesis solution.
The pressure of the urea synthesis solution remaining after the separation of the off gas is reduced to a medium value of from 12 to 24 kg/cm2G. Furthermore, part of the residual unconverted materials contained in the urea synthesis solution results in a gaseous mixture and is separated from the synthesis solution as a second gaseous mixture at a medium pressure.
The urea synthesis solution remaining after the separation of the second gaseous mixture is again subjected to a pressure reduction and heating to separate part of the residual unconverted raw materials as a third gaseous mixture. The urea synthesis solution remaining after the separation of the third gaseous mixture is subjected to a final pressure reduction to separate the residual unconverted raw materials almost completely. The thus obtained aqueous urea solution is transferred to a purification and concentration stage.
In order to recycle the unconverted materials to the synthesis section, the second gaseous mixture is brought into contact with a pressurized solution containing the third gaseous mixture absorbed therein under a pressure substantially equal to the pressure of the separation of the second gaseous mixture, thus being absorbed in the solution.
The second solution formed by the contact with the second gaseous mixture is pressurized to at least a pressure substantially equal to the pressure of urea synthesis and brought into contact with a gaseous mixture discharged from the synthesis section and then with the above off gas, i.e., the first gaseous mixture. Thus, an adequate amount of the first gaseous mixture is absorbed in the second solution.
The third solution formed by the absorption of an adequate amount of the first gaseous mixture into the second solution and the unabsorbed part of the first gaseous mixture are recycled to the synthesis section.
The heat generated, when the second gaseous mixture is brought into contact with the pressurized solution containing the third gaseous mixture absorbed therein under a pressure substantially equal to the pressure of the generation of the second gaseous mixture to make the mixture absorbed in the solution, is recovered and utilized.
When the first gaseous mixture is brought into contact with the second solution under a pressure substantially equal to the pressure of urea synthesis, ammonium carbamate is formed in the solution to generate heat, part of this heat is directly transferred through heat transfer walls to the urea synthesis solution at a medium pressure to be used for the separation of the secondary gaseous mixture, while the residual part thereof is used for the generation of steam.
When the synthesis of urea is carried out at a molar ratio of the total ammonia to the total carbon dioxide in the synthesis section of from 2.8 to 3.4, the pressure of the synthesis section is controlled in the range of from 140 to 170 kg/cm2G and the temperature and pressure, with which the decomposition and separation of unconverted raw materials are carried out to form the first gaseous mixture, are so controlled as to keep the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the first gaseous mixture in the range of from 13 to 24% by weight.Further, the separation of the second gaseous mixture is carried out under a pressure of from 12 to 18 kg/cm2G and the temperature at which the decomposition and separation of unconverted materials are carried out to form the second gaseous mixture is so controlled as to keep the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the second gaseous mixture in the range of from 5 to 11% by weight.
When the synthesis of urea is carried out at a molar ratio of the total ammonia to the total carbon dioxide in the synthesis section of from more than 3.4 to 4.2, the pressure of the synthesis section is controlled in the range of from 160 to 190 kg/cm2G and the temperature and pressure, with which the decomposition and separation of unconverted raw materials are carried out to form the first gaseous mixture, are so controlled as to keep the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the first gaseous mixture in the range of from 20 to 30% by weight.Further, the separation of the second gaseous mixture is carried out under a pressure of from 14 to 20 kg/cm2G and the temperature, at which the decomposition and separation of unconverted materials are carried out to form the second gaseous mixture, is so controlled as to keep the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the second gaseous mixture in the range of from 6 to 12% by weight.
When the synthesis of urea is carried out at a molar ratio of the total ammonia to the total carbon dioxide in the synthesis section of from more than 4.2 to 5. 0, the pressure of the synthesis section is controlled in the range of from 180 to 210 kg/cm2G and the temperature and pressure, at which the decomposition and separation of unconverted raw materials are carried out to form the first gaseous mixture, are so controlled as to keep the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the first gaseous mixture in the range of from 25 to 35% by weight.Further, the separation of the second gaseous mixture is carried out under a pressure of from 16 to 24 kg/cm2G and the temperature, at which the decomposition and separation of unconverted raw materials are carried out to form the second gaseous mixture, is so controlled as to keep the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the second gaseous mixture in the range of from 7 to 13% by weight.
The use of an ordinary automatic control system of feedback type is suitable to control the synthesis system to each condition as those described above, though the use of an automatic control system of direct digital control type is also possible.
The process of the present invention enables a high thermal efficiency to be attained by setting the molar ratio of the total ammonia to the total carbon dioxide in the synthesis section within the range from 2.8 to 5.0. Further, the process of the present invention gives an improvement in the recovery and utilization of the heat generated when the off gas formed by stripping under a pressure substantially equal to the pressure of the synthesis section is condensed to ammonium carbamate and when the off gas formed from the urea synthesis solution of a medium pressure of 12 to 20 kg/cm2G is condensed.
According to the process of the present invention, the temperautre and pressure of each stage are controlled to thereby reduce the amount of the high-pressure steam required.
Furthermore, low-pressure steam, the possible uses for which are limited, is generated in a reduced amount.
In the process of the present invention, the decomposition and separation of unconverted raw materials by stripping and the formation of ammonium carbamate, both of which, in prior art methods, have been entirely carried out under a pressure equal to the pressure of the synthesis section, are partially replaced by the decomposition and separation thereof and the absorption thereof under a medium pressure. Thus, a heat exchanger is disposed within the condenser within which ammonium carbamate is formed and the heat exchanger can be utilized as a heater wherein the decomposition and separation of unconverted raw materials can be carried out under a medium pressure, thereby enabling the volume of the separate medium pressure heating device to be reduced.Furthermore, owing to the increase in the difference in temperature between the two fluids exchanging heat in the heat exchanger the required heat transfer area of the heat exchanger is reduced, thus bringing about a reduction in the cost of equipment.
The invention will be further illustrated by, though not limited to, the following Examples, and by reference to the accompanying drawing which is a flow sheet of a process for the synthesis of urea according to the present invention, wherein a solid line shows a pipe connecting various units in which a liquid fluid flows, while a two-dot-dash line shows a pipe connecting various units in which a gaseous fluid flows. The pipe 15 is one which feeds steam for heating the separator 16.
Example 1
Hereinafter, all amounts by kg are those per hour, while amounts by kg with respect to the ammonia and carbon dioxide contained in a gaseous mixture recovered and recycled from the top of a reactor 3 via a pipe 24 are omitted as well as in Example 2 and 3.
Referring to the drawing, 23,611 kg of liquid ammonia was fed to a reactor 3 for urea synthesis via a pipe 1, while an aqueous solution of unconverted raw materials having a temperature of 178"C and a gaseous mixture of unconverted materials having a temperature of 178"C were fed to the reactor 3 via pipes 12 and 13, respectively.
The amount of the unconverted raw materials fed to the reactor 3 via the pipes 12 and 13 was 90,266 kg, while that of water fed was 10,833 kg. The molar ratio of the total ammonia to the total carbon dioxide in the reactor 3 was 3.2.
When the amount of the ammonium carbamate formed in the reactor 3 is reduced by increasing the amount of the aqueous solution of ammonium carbamate to be fed via the pipe 12 and reducing the amount of the gaseous mixture to be fed via the pipe 13, the amount of the heat generated in the reactor 3 is reduced to iower the temperature in the reactor 3. On the other hand, when the amount of the ammonium carbamate formed in the reactor 3 is increased by reducing the amount of the aqueous solution of ammonium carbamate to be fed via the pipe 12 and increasing the amount of the gaseous mixture to be fed via the pipe 13, the amount of the heat generated in the reactor 3 is increased to enhance the temperature in the reactor 3. By these means, the temperature in the reactor 3 was kept at a specified value.
The reactor 3 for urea synthesis was kept in the state of 145 kg/cm2G and 185"C to form urea to a conversion of 60%.
A urea synthesis solution comprising 41,667 kg of urea, 59,708 kg of unconverted materials and 23,333 kg of water was taken out of the reactor 3 and fed to the top of the stripper 5 having a pressure equal to the inner pressure of the reactor 3 via a pipe 4.
Inert gases gathering in the upper part of the reactor 3, which comprised oxygen, which had been fed to the reactor 3 for the purpose of preventing the corrosion of the equipment, and other components, which had entered into the reactor 3 as impurities contained in the raw materials, were discharged together with accompanied ammonia, carbon dioxide and water, as a geseous mixture from the top of the reactor 3 via a pipe 24.
The urea synthesis solution fed to the top of the stripper 5 flowed down along the inner surface of many vertical heating tubes set in the stripper 5 as a thin film.
While the urea synthesis solution flowed down as described above, the unconverted materials contained therein were decomposed to a gaseous mixture by heating of the outer surface of the heating tubes with high-pressure steam of 17 kg/cm2G fed via a pipe 6 and stripping with 30,558 kg of carbon dioxide having a temperature of 140"C and fed via a pipe 2. The gaseous mixture thus-formed was separated from the liquid phase and rose in the vertical heating tubes.
A urea synthesis solution comprising 41,667 kg of urea, 16,917 kg of unconverted raw materials comprising ammonia, carbon dioxide and the like and 20,042 kg of water and having a temperature of 170"C was discharged from the bottom of the stripper 5 and passed through a pipe 8 and subjected to pressure reduction to 15.5 kg/cm2G at a pressure reducing valve 9. Then, the resulting urea synthesis solution was heated by passing through a heat exchanger set in a condenser 10 to decompose part of unconverted materials contained in the urea synthesis solution, thus a gas-liquid mixture being formed. This mixture was fed to a separator 16 and heated with steam fed via a pipe 15 to discharge a crude aqueous solution of urea comprising 41, 667 kg of urea, 6, 000 kg (in total) of ammonia and carbon dioxide and 18,625 kg of water and having a temperature of 152"C from the bottom via a pipe 17.
The crude aqueous solution of urea was passed through the pipe 17, subjected to pressure reduction at a pressure reducing valve 18 and transferred to the successive purification and concentration stages which are not shown in the drawing.
A gaseous mixture comprising 10,917 kg (in total) of ammonia and carbon dioxide and 1,417 kg of water was discharged from the top of the separator 16 and fed to an absorber 20 kept under a medium pressure equal to that of the separator 16 via a pipe 19.
12,125 kg of an aqueous solution of unconverted raw materials of 50"C was recovered in the downstream following the pressure reducing valve 18 and fed to the absorber 20 via a pipe 21 to absorb the gaseous mixture fed via the pipe 19 at 113"C.
Most of the heat generated by this adsorp tion was utilized for the concentration of an aqueous solution of urea.
An aqueous solution comprising 16,917 kg of unconverted raw materials and 7,542 kg of water which had been formed by the above absorption was pressurized with a pump 22 to at least a pressure equal to the synthesis pressure and transferred to a scrubber 25 via a pipe 23 to absorb the ammonia and carbon dioxide contained in the inert gas fed via the pipe 24. The resulting aqueous solution was transferred to the condenser 10 via a pipe 11.
Part of the gaseous mixture which had been separated in the stripper 5 and had been transferred to the condenser 10 via a pipe 7 was condensed in the above aqueous solution of unconverted materials depending upon the temperature of the reactor 3 to form ammonium carbamate.
Part of the heat generated by this regeneration, i.e., an amount corresponding to 9,988 kg of steam was used to heat depressurized urea synthesis solution from the stripper 5, while the residual part thereof was used for the generation of steam to give 32,410 kg of low-pressure steam of 4 kg/cm2G, which was recovered via a pipe 14.
An aqueous solution of unconverted materials formed in the condenser 10 was recycled to the reactor 3 via the pipe 12, while an uncondensed part of the gaseous mixture fed to the condenser 10 was recycled to the reactor 3 via the pipe 13.
In the conventional CO2 stripping process, neither the condenser 10 for the direct heat recovery nor the separator 16 is provided.
Accordingly, high-pressure steam is consumed in the stripper 5 in an amount corresponding to the amount of heat required in the separator 16 and the amount of less valuable lowpressure steam generated is also increased by the increased amount of the high-pressure steam consumed in the stripper 5.
In selecting the structure of the condenser 10 consideration should be given to inter alia the ratio of the amount of heat required in the separator 16 to that recovered via the pipe 14, the difference in temperature between the condenser 10 and the liquid reaction mixture after passing through the pressure reducing valve 9. For example, after passing through the valve 9, the urea synthesis solution may be passed either through the inside of heat transfer tubes as shown in the drawing or through the outside of the tubes.
Example 2
23,611 kg of liquid ammonia was fed to a reactor 3 via a pipe 1, while an aqueous solution of unconverted materials at 1800C and a gaseous mixture of unconverted materials at 180"C were fed to the reactor 3 via pipes 12 and 13, respectively. The total amount of the ammonia and carbon dioxide fed via the pipes
12 and 13 was 90,766 kg, while the amount of the water fed was 11,042 kg. The reactor 3 was kept in the state of 175 kg/cm2G and 1900C to form urea in a conversion of 68% therein. A urea synthesis solution comprising 41,667 kg of urea, 60,208 kg of unconverted materials and 23,542 kg of water was discharged via a pipe 4. The molar ratio of the total ammonia to the total carbon dioxide in the reactor 3 was 4.0.
High-pressure steam of 19 kg/cm2G was fed to a stripper 5 via a pipe 6, while 30,558 kg (equal to the amount used in Example 1) of pressurized carbon dioxide of 140"C was fed thereto via a pipe 2.
A urea synthesis solution comprising 41,667 kg of urea, 21,458 kg of unconverted material and 20,667 kg of water and having a temperature of 175"C was discharged from the bottom of the stripper 5, passed through a pipe 8 and subjected to pressure reduction to 17 kg/cm2G at a pressure reducing valve 9. The resulting urea synthesis solution was fed to a heat exchanger in a condenser 10.
A urea synthesis solution comprising 41,667 kg of urea, 6,000 kg of unconverted materials and 18,625 kg of water and having a temperature of 1600C was discharged from the bottom of a separator 16 via a pipe 17 and subjected to pressure reduction at a pressure reducing valve 18.
A gaseous mixture comprising 15,458 kg of unconverted materials and 2,042 kg of water was discharged from the top of the separator 16 and fed to an absorber 20 via a pipe 19.
12, 125 kg of an aqueous solution of unconverted materials having a temperature of 55"C was recovered in a downstream following the pressure reducing valve 18 which is not shown in the drawing. This aqueous solution of unconverted materials was fed to the absorber 20 via a pipe 21 to absorb the gaseous mixture fed via the pipe 19 at 110 C.
An aqueous solution comprising 21,458 kg of unconverted materials and 8,167 kg of water, which had been formed by this absorption, was pressurized to a pressure equal to the synthesis pressure to absorb useful components contained in the gaseous mixture fed to a scrubber 25 via a pipe 24. The resulting aqueous solution was fed to the condenser 10 via a pipe 11.
A gaseous mixture transferred from the stripper 5 via a pipe 7 was condensed in the condenser 10 depending upon the temperature of the reactor 3 to generate heat. Part of the heat, i. e., an amount corresponding to
13,340 kg of steam was used for the separator 16, while the residual part thereof was recovered as 20,603 kg of low-pressure steam of 5 kg/cm2G via a pipe 14.
Example 3
23,611 kg of liquid ammonia was fed to a reactor 3 via a pipe 1, while a gaseous mixture of unconverted materials and an aqueous solution of unconverted materials, both of which have a temperature of 181"C, i.e., 101,885 kg (in total) of ammonia and carbon dioxide and 13,125 kg of water were fed to the reactor 3 via pipes 12 and 13. Urea was formed in the reactor 3 under a pressure of 200 kg/cm2G at a temperature of 190"C to a conversion of 71% to discharge a urea synthesis solution comprising 41,667 kg of urea, 71,297 kg of unconverted materials and 25,625 kg of water from the reactor 3 via a pipe 4. The molar ratio of the total ammonia to the total carbon dioxide in the reactor 3 was 5.0.
The urea synthesis solution was fed to the top of a stripper 5 having the same pressure as that of the reactor 3 via the pipe 4, heated with high-pressure steam of 21 kg/cm2G fed via a pipe 6 and stripped with 30,558 kg of pressurized carbon dioxide of 140 C fed via a pipe 2.
A urea synthesis solution comprising 41,667 kg of urea, 30,334 kg of unconverted materials and 22,667 kg of water and having a temperature of 187"C was discharged from the bottom of the stripper 5, passed through a pipe 8 and subjected to pressure reduction to 18.5 kg/cm2G at a pressure reducing valve 9. The resulting urea synthesis solution was fed to a heat exchanger in a condenser 10.
A urea synthesis solution comprising 41,667 kg of urea, 6, 000 kg of unconverted materials and 19, 458 kg of water and having a temperature of 164"C was discharged from the bottom of a separator 16 via a pipe 17.
A gaseous mixture comprising 24,334 kg of unconverted materials and 3,209 kg of water was discharged from the top of the separator 16 and fed to an absorber 20 having the same pressure as that of the separator 16 via a pipe 19.
12,958 kg of an aqueous solution of unconverted raw materials of 58"C was recovered in a downstream following the pressure reducing valve 18, which is not shown in the drawing, and fed to the absorber 20 via a pipe 21 to absorb the gaseous mixture fed via the pipe 19 at 107"C.
A highly concentrated aqueous solution comprising 30,334 kg of unconverted materials and 10,167 kg of water which had been formed in the absorber 20 was pressurized to at least a pressure equal to the synthesis pressure and transferred to a scrubber 25 to absorb the unconverted materials contained in a gaseous mixture fed via a pipe 24 substantially completely. The resulting aqueous solution was fed to the condenser 10. A gaseous mixture fed to the condenser 10 via a pipe 7 was condensed therein depending upon the temperature of the reactor 3 to generate heat. Part of the generated heat, i.e., an amount corresponding to 19,755 kg of steam was used in the separator 16, while the residual part thereof was recovered as 5,044 kg of low-pressure steam of'5 kg/cm2G via a pipe 14.
Claims (2)
1. A process for the separation of uncon verted materials which comprises feeding ammonia and carbon dioxide to a synthesis section having a temperature and a pressure suitable for urea synthesis to form a urea synthesis solution comprising urea, ammonium carbamate, ammonia, carbon dioxide and water, separating part of materials unconverted into urea from this synthesis solution as a first gaseous mixture by stripping using part of the make up carbon dioxide raw material and heating under a pressure substantially equal to the pressure of urea synthesis, reducing the pressure of the synthesis solution remaining after the separation of the first gaseous mixture, separating part of the unconverted raw materials as a second gaseous mixture from the resulting urea synthesis solution by heating, bringing the second gaseous mixture into contact with a first solution returning the residual unconverted materials to be recycled to the synthesis section under a pressure substantially equal to the pressure for the separation of the second gaseous mixture to form a second solution, recovering and utilizing the heat generated by the formation of the second solution within the process, bringing the first gaseous mixture into contact with the second solution under a pressure substantially equal to the pressure of the urea synthesis to form ammonium carbamate, transferring part of the heat generated by the formation of ammonium carbamate directly through heat transfer tube walls to the urea synthesis solution to separate the second gaseous mixture and utilizing the residual part thereof for the generation of steam and transferring the mixture obtained by the contact of the first gaseous mixture with the second solution to the synthesis section, characterized in that::
when the molar ratio of the total ammonia to the total carbon dioxide to be fed to the synthesis section is controlled in the range of from 2.8 to 3.4, the pressure of the synthesis section is controlled in the range of from 140 to 170 kg/cm2G, the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the first gaseous mixture is controlled in the range of from 13 to 24% by weight and the separation of the second gaseous mixture is carried out under a pressure of from 12 to 18 kg/cm2G, while the total content of ammonia and csrbon dioxide in the urea synthesis solution remaining after the separation of the second gaseous mixture is controlled in the range of from 5 to 11% by weight,
when the molar ratio of the total ammonia to the total carbon dioxide to be fed to the synthesis section is controlled in the range of from more than 3.4 to 4.2, the pressure of the synthesis section is controlled in the range of from 160 to 190 kg/cm2G, the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the first gaseous mixture is controlled in the range of from 20 to 30% by weight and the separation of the second gaseous mixture is carried out under a pressure of from 14 to 20 kg/cm2G, while the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the second gaseous mixture is controlled in the range of from 6 to 12% by weight, and
when the molar ratio of the total ammonia to the total carbon dioxide to be fed to the synthesis section is controlled in the range of from more than 4.2 to 5.0, the pressure of the synthesis section is controlled in the range of from 180 to 210 kg/cm2G, the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the first gaseous mixture is controlled in the range of from 25 to 35% by weight and the separation of the second gaseous mixture is carried out under a pressure of from 16 to 24 kg/cm2G, while the total content of ammonia and carbon dioxide in the urea synthesis solution remaining after the separation of the second gaseous mixture is controlled in the range of from 7 to 13% by weight.
2. A process as claimed in claim 1, substantially as described in any one of the
Examples and with reference to the accompanying drawing.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP25940486A JPS63112552A (en) | 1986-10-30 | 1986-10-30 | Separation method for unconverted substance |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8725321D0 GB8725321D0 (en) | 1987-12-02 |
| GB2196965A true GB2196965A (en) | 1988-05-11 |
| GB2196965B GB2196965B (en) | 1990-07-11 |
Family
ID=17333651
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8725321A Expired - Fee Related GB2196965B (en) | 1986-10-30 | 1987-10-29 | Urea synthesis. |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS63112552A (en) |
| CN (1) | CN1020898C (en) |
| BR (1) | BR8705723A (en) |
| GB (1) | GB2196965B (en) |
| IN (1) | IN171250B (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1203765A2 (en) * | 2000-11-01 | 2002-05-08 | Toyo Engineering Corporation | Process for the synthesis of urea |
| WO2014122894A1 (en) * | 2013-02-08 | 2014-08-14 | Toyo Engineering Corporation | Process for recovering carbon dioxide from combustion exhaust gas |
| JP2018076379A (en) * | 2012-05-03 | 2018-05-17 | スタミカーボン・ベー・フェー | Method and device for manufacturing urea from ammonia and carbon dioxide |
| US20180258033A1 (en) * | 2015-09-08 | 2018-09-13 | Toyo Engineering Corporation | Urea Manufacturing Method And Urea Manufacturing Apparatus |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1039413C (en) * | 1993-09-16 | 1998-08-05 | 中国化学工程总公司 | Stripping recirculation technology & apparatus for urea production |
| CN1041199C (en) * | 1994-06-18 | 1998-12-16 | 中国五环化学工程公司 | Energy saving device for producing urea by full-circulating process |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| IT1068268B (en) * | 1976-09-09 | 1985-03-21 | Snam Progetti | PROCEDURE FOR THE PRODUCTION OF UREA AND PURIFICATION OF WATERS |
| JPS5573651A (en) * | 1978-11-30 | 1980-06-03 | Toyo Eng Corp | Preparation of urea |
| JPS5746954A (en) * | 1980-09-05 | 1982-03-17 | Mitsui Toatsu Chem Inc | Synthesis of urea |
| JPS5750954A (en) * | 1980-09-12 | 1982-03-25 | Mitsui Toatsu Chem Inc | Synthesis of urea |
| JPS5867661A (en) * | 1981-10-16 | 1983-04-22 | Mitsui Toatsu Chem Inc | Synthesizing method of urea |
| JPS61109760A (en) * | 1984-11-02 | 1986-05-28 | Toyo Eng Corp | Production of urea |
-
1986
- 1986-10-30 JP JP25940486A patent/JPS63112552A/en active Pending
-
1987
- 1987-10-16 IN IN907/DEL/87A patent/IN171250B/en unknown
- 1987-10-27 BR BR8705723A patent/BR8705723A/en not_active IP Right Cessation
- 1987-10-29 CN CN 87107799 patent/CN1020898C/en not_active Expired - Lifetime
- 1987-10-29 GB GB8725321A patent/GB2196965B/en not_active Expired - Fee Related
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1203765A2 (en) * | 2000-11-01 | 2002-05-08 | Toyo Engineering Corporation | Process for the synthesis of urea |
| EP1203765A3 (en) * | 2000-11-01 | 2003-10-08 | Toyo Engineering Corporation | Process for the synthesis of urea |
| JP2018076379A (en) * | 2012-05-03 | 2018-05-17 | スタミカーボン・ベー・フェー | Method and device for manufacturing urea from ammonia and carbon dioxide |
| WO2014122894A1 (en) * | 2013-02-08 | 2014-08-14 | Toyo Engineering Corporation | Process for recovering carbon dioxide from combustion exhaust gas |
| US20180258033A1 (en) * | 2015-09-08 | 2018-09-13 | Toyo Engineering Corporation | Urea Manufacturing Method And Urea Manufacturing Apparatus |
| US10501407B2 (en) * | 2015-09-08 | 2019-12-10 | Toyo Engineering Corporation | Urea manufacturing method and urea manufacturing apparatus |
Also Published As
| Publication number | Publication date |
|---|---|
| IN171250B (en) | 1992-08-22 |
| BR8705723A (en) | 1988-05-31 |
| GB8725321D0 (en) | 1987-12-02 |
| JPS63112552A (en) | 1988-05-17 |
| CN1020898C (en) | 1993-05-26 |
| CN87107799A (en) | 1988-07-06 |
| GB2196965B (en) | 1990-07-11 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19981029 |