JPH10120643A - Improved urea synthsis - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce urea by a method that has high adaptability to variations of operating condition and reduces energy consumption. SOLUTION: This production of urea, which consists of a reaction heat recovery process that formats ammonium carbamate from ammonia and carbon dioxide, first and second urea synthesizing processes which differ in reaction conditions, a stripping process, and a purification process for obtaining urea from the urea solution delivered from the stripping process, comprises subjecting the ammonium carbamate after heat recovery to urea synthesis in the first urea synthesizing process, treating in the stripping process the synthesized urea solution coming from the first synthetic process together with the urea solution coming from the second urea synthesizing process mentioned below to separate the urea solution and a gaseous mixture of ammonia and carbon dioxide, sending the separated gaseous mixture through a condensation process to the second urea synthesizing process, and leading the second synthesized urea solution into the stripping process where the second urea solution is treated together with the first synthesize urea solution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an improvement in a two-stage urea synthesis method. In particular, it relates to an improved urea synthesis method suitable for increasing the production capacity of an existing urea synthesis plant.

[0002]

BACKGROUND OF THE INVENTION The present applicant has previously disclosed a urea synthesis method suitable for enhancing the production of a new or especially an existing urea synthesis plant for the purpose of reducing energy consumption as disclosed in Japanese Patent Application Laid-Open No. Hei 8-198838. In the proposed. This proposal,
In the method of synthesizing urea from excess ammonia and carbon dioxide, (1) a step of removing and recovering the heat of reaction between ammonia and carbon dioxide, and (2) a reaction product of removing and recovering the heat of reaction of ammonia and carbon dioxide. Urea synthesis step of treating a product, ammonium carbamate, with urea synthesis pressure and temperature to obtain a first urea synthesis solution comprising urea, excess ammonia, unreacted ammonium carbamate and water, (3) first urea The synthesis solution is decompressed, and the ammonium carbamate in the urea synthesis solution is thermally decomposed into gaseous ammonia and carbon dioxide, and by blowing in carbon dioxide, these gases are mixed with excess ammonia as a mixed gas of ammonia, carbon dioxide, etc. A stripping step of separating from a urea solution containing residual unreacted ammonium carbamate, ( A) a second urea synthesis solution comprising urea, excess ammonia, unreacted ammonium carbamate and water by treating the mixed gas separated in the stripping step and the ammonium carbamate aqueous solution from the recovery step described below at a urea synthesis pressure and temperature; (5) combining the urea solution from the stripping step with the second urea synthesis solution to remove unreacted ammonium carbamate and excess ammonia from the urea solution to ammonia and carbon dioxide And (6) the urea liquid from the recovery step is further processed by removing the intermediate pressure mixed gas into an absorbing medium and recovering it as an aqueous solution of ammonium carbamate. And a purification step for obtaining

[0003]

However, in the urea synthesis method described above, the mixture of the urea solution from the stripping step and the second urea synthesis solution introduced into the recovery step has an upstream side composition. It was difficult to return to the original composition when adversely affected by fluctuations in the operating conditions.

[0004] The pressure and temperature of the second urea synthesis step are lower than those of the first urea synthesis step, and H /
Since C is high, the urea synthesis rate is low, and the residual ammonia concentration is high. Since such a urea synthesis solution is directly sent to the downstream recovery step, the load of the recovery step is likely to increase, and this may cause a problem particularly in the case of increasing the capacity of an existing plant.

Accordingly, an object of the present invention is to solve these problems and to provide an improved method for synthesizing urea, which has high adaptability to driving fluctuations and consumes little energy.

[0006]

SUMMARY OF THE INVENTION The improved urea synthesis method of the present invention comprises mixing carbon dioxide and a stoichiometric excess of ammonia in a mixing step at urea synthesis pressure to form ammonium carbamate. The heat of reaction generated at this time is removed, and the ammonium carbamate is then subjected to urea synthesis in a first urea synthesis step to produce a first urea synthesis solution comprising urea, excess ammonia, unreacted ammonium carbamate and water. Let
Treating the first urea synthesis solution in a stripping step to separate the urea solution from a mixed gas of ammonia, carbon dioxide and water, and further treating the urea solution to obtain urea;
On the other hand, the mixed gas is condensed by contacting it with an absorbing medium under cooling in a condensation step, and the obtained condensate is subjected to urea synthesis in a second urea synthesis step to carry out urea, excess ammonia, unreacted ammonium carbamate. And a second urea synthesis solution comprising water and water, and treating the second urea synthesis solution to obtain urea. In the urea synthesis method, the pressure in the stripping step is set higher than the pressure in the first urea synthesis step. And the pressure of the second urea synthesis step is set to a pressure substantially equal to the pressure of the stripping step, and the second urea synthesis liquid is introduced into the stripping step to remove the first urea. Treating the excess ammonia and unreacted ammonium carbamate contained in the first and second urea synthesis solutions with the synthesis solution as a mixed gas of ammonia, carbon dioxide and water Separated from the gas mixture is characterized in that introduced into the condensation step.

[0007]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, N, C and H
Means the number of moles of ammonia, the number of moles of carbon dioxide and the number of moles of water, respectively, and N / C and H / C indicate the molar ratio of ammonia and water to carbon dioxide, respectively, and are dimensionless numerical values. The urea synthesis rate based on carbon dioxide is
Of the total amount of carbon dioxide, the ratio of the molar amount of carbon dioxide converted to urea carbon dioxide to the total molar amount of carbon dioxide is multiplied by 100, and the unit is%. Inert gas is a general term for air for preventing corrosion of a synthesis tube, stripper, covermet condenser, and the like, and impurities originally contained in raw carbon dioxide.

The present invention will be described with reference to FIG. 1, which is a flow sheet showing one embodiment of the urea synthesis method of the present invention.
In the process shown in FIG.
Pressurize the urea synthesis solution and supply it to the stripper.
The condenser and the second urea synthesis tube can be placed on the ground.

The raw liquid ammonia is supplied to the mixer 4 via a line 21 and the raw carbon dioxide is supplied to a mixer 4 via lines 22 and 23. Here, ammonia and carbon dioxide react to form ammonium carbamate. The reaction heat generated at this time is, for example, the condensed water of steam
And heat is recovered by generating low-pressure steam in the line 51.

Unreacted ammonia and ammonium carbamate are supplied from the mixer 4 via a line 24 to a first urea synthesis tube 1 and subjected to urea synthesis. The synthesis conditions of the first urea synthesis tube were as follows: pressure 170-260 bar, temperature 1
80-210 ° C, N / C 3.0-4.5, H / C
Is 0 (no water present). The urea synthesis rate at this time is 60 to 80% (based on carbon dioxide). The first
The urea synthesis tube 1 may be a new one, or may be one used in an existing process. For example, a case where a retrofitting is performed to increase the output of an existing plant and an existing synthetic pipe is used.

From the outlet of the first urea synthesizing tube 1, a first urea synthesizing solution containing excess ammonia, unreacted ammonium carbamate, and urea and water as a reaction product passes through a line 25 and is appropriately compressed by a pressure reducing valve. , And supplied to the ejector 6 via the line 26. The suction side of the ejector is supplied via a line 35 with a second urea synthesis liquid that has flowed down by gravity through a down pipe 10 of a second urea synthesis pipe 2 described later. Above pressure reducing valve 7
That the pressure in the line 26 is
, So that the pressure in line 26 is sufficient to drive the ejector 6 so that the second urea synthesis solution flows from the line 35 into the ejector 6 as is usual. It is necessary to adjust the pressure. The first urea synthesis solution and the second urea synthesis solution that have been combined by the ejector 6 are supplied to the stripper 3 via the line 27.

Usually, a stripper 3 provided with a vertical heating tube is heated by steam entering from a line 40 and exiting from a line 41. At the top of stripper 3,
The above-mentioned first and second urea synthesis liquids are supplied, and a part of the raw carbon dioxide is supplied to the bottom via a line 28,
While ascending in the stripper, it comes into contact with the flowing urea synthesis solution to separate excess ammonia and unreacted ammonium carbamate in the urea synthesis solution as a mixed gas of ammonia, carbon dioxide and the like. The amount of this raw carbon dioxide is 0.4 tons or less per ton of urea. This raw material carbon dioxide may be heated only without being supplied in some cases.

[0013] The normal operating conditions of the stripper are 140-
180 bar, 170-220 ° C.

The composition of the urea solution discharged from the bottom of the stripper via the line 30, the pressure reducing valve 8 and the line 31 varies depending on the amount of carbon dioxide supplied to the bottom of the stripper. , Carbon dioxide 5
15%, urea 40-60%, the balance being water.

The urea solution is fed to a medium-pressure cracking column under operating conditions of usually 15 to 20 bar and 130 to 160 ° C. to separate ammonia and carbon dioxide as a mixed gas, which is further processed in a purification step. Urea is obtained as a product.

The mixed gas such as ammonia and carbon dioxide separated in the stripper is supplied to the condenser 5 through a line 29, and is brought into contact with an aqueous solution of ammonium carbamate introduced through a line 36 from the scrubber 9 under cooling, and condensed. Absorbed. The condenser 5 is 140-18
It is operated at 0 bar and 150-190 ° C. Usually, in the condenser, steam condensed water is supplied to the shell side for cooling, and the heat of condensation is recovered by the generation of steam.

In the scrubber 9, an inert gas is supplied to the bottom from the top of the second urea synthesis tube via a line 34, and ammonia and carbon dioxide separated in the medium pressure decomposition and purification step are converted into an absorption medium such as water. It is washed with an ammonium carbamate aqueous solution (recovery solution) obtained by absorption, and ammonia and carbon dioxide in the inert gas are absorbed. An aqueous solution of ammonium carbamate (recovery liquid) is passed through line 36 as described above to the condenser 5.
Supplied to The scrubber 9 is operated at a temperature substantially equal to 150-180 ° C. at a pressure substantially equal to the pressure of the second urea synthesis tube. The inert gas discharged from the top via line 37 is sent to a downstream decomposition and recovery step together with residual ammonia and carbon dioxide. In addition, the scrubber 9
It does not need to be installed. In this case, the inert gas from the top of the second urea synthesis tube is sent to the decomposition and recovery step as it is.

Condensate and uncondensed gas are supplied from the condenser 5 to the bottom of the second urea synthesis tube 2 via a line 33. Urea is synthesized in the second urea synthesis tube 2,
The second urea synthesis liquid flows down the down pipe 10 having an opening at the top by gravity, is supplied to the suction side of the ejector 6 via the line 35, and is supplied to the top of the stripper 3 as described above.

The operating conditions of the second urea synthesis tube are 140 to
180 bar, 180-200 ° C, N / C 2.5-4.
5, H / C is 0.5 to 2.0, and the urea synthesis rate is 45 to
65%. Since the second urea synthesis tube can be placed on the ground, the existing urea synthesis tube can be used as the second urea synthesis tube when increasing the production capacity of the existing plant.

Next, another embodiment of the present invention will be described with reference to the process of FIG. 2 is basically the same as the process in FIG. 1 except that the ejector 6 in FIG. 1 is not used and the installation positions of the respective devices are different.

[0021]

The present invention will be described in more detail with reference to examples. It goes without saying that the invention is not limited only to these examples. Example 1 The urea synthesis process of FIG. 1 was performed under the operating conditions of the first urea synthesis tube, the ejector, the stripper, the condenser, and the second urea synthesis tube shown in Table 1. Example 2 Under the same operating conditions as in Example 1, the urea synthesis process of FIG. 2 was performed. By placing the second urea synthesis tube, condenser and scrubber higher than the stripper,
Circulation of liquid and gas between each device was realized without using an ejector. Example 3 The urea synthesis process of FIG. 1 was performed under the operating conditions shown in Table 1 while changing the blowing ratio of the raw carbon dioxide into the mixer and the stripper.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

According to the urea synthesis method of the present invention, the following effects can be achieved.

(1) Even if the N / C of the first urea synthesis tube is increased, excess ammonia is converted to urea in the second urea synthesis tube together with the carbon dioxide and the recovery liquid used for stripping. Therefore, the urea synthesis rate can be increased as a whole.

(2) By treating the second urea synthetic solution together with the first urea synthetic solution in a stripper,
Compared with the conventional method of JP-A-8-198838, when the residual ammonia at the outlet of the stripper is the same (28%), a higher urea synthesis rate was obtained. 85% in Example 1).

(3) The urea synthesis rate (74
%, A lower residual ammonia concentration was obtained (in contrast to 28% in the conventional method, 21% in Example 3 of the present invention).
%).

(4) Since the amount of residual ammonia and carbon dioxide in the urea solution from the stripper is reduced, the heat for decomposing these at a medium pressure can be reduced by about 30% as compared with the conventional method. As a result, the required steam amount in this process is also 30
% Savings.

(5) The amount of recovered liquid obtained by absorbing the ammonia and carbon dioxide separated in the above-mentioned medium pressure decomposition step into water, aqueous ammonia and a dilute ammonium carbamate aqueous solution is about 30 times smaller than that of the conventional method. %, Which also saved the power of the high pressure recovery liquid pump.

(6) Since there is a high adaptability to the operation fluctuation, even if there is a composition fluctuation of the mixture of the first and second urea synthesis liquids due to the fluctuation of the operation conditions on the upstream side, the fluctuated composition can be easily obtained. It can be returned to the original.

(7) The excess ammonia in the first urea synthesis solution is stripped off only by heating or by a combination of heating and blowing of carbon dioxide, so that the ammonia is effectively separated.

[Brief description of the drawings]

FIG. 1 is a flow sheet showing one embodiment of the present invention.

FIG. 2 is a flow sheet showing another embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 First urea synthesis tube 2 Second urea synthesis tube 3 Stripper 4 Mixer 5 Condenser 6 Ejector 7, 8 Pressure reducing valve 9 Scrubber 10 Down pipe

Claims (10)

[Claims] 1. At a urea synthesis pressure, carbon dioxide and a stoichiometric excess of ammonia are mixed in a mixing step to form ammonium carbamate, the heat of reaction generated at this time is removed, and the ammonium carbamate is then removed. In a first urea synthesis step, urea synthesis is performed to generate a first urea synthesis liquid composed of urea, excess ammonia, unreacted ammonium carbamate and water, and the first urea synthesis liquid is treated in a stripping step. The urea solution is separated from the mixed gas of ammonia, carbon dioxide and water by further treatment of the urea solution to obtain urea, while the mixed gas is condensed by contact with the absorbing medium under cooling in the condensation step The resulting condensate is subjected to urea synthesis in a second urea synthesis step to produce urea, excess ammonia, and unreacted ammonium. In a urea synthesis method for producing a second urea synthesis solution consisting of mucarbamate and water and treating the second urea synthesis solution to obtain urea, the pressure of the stripping step is increased by the pressure of the first urea synthesis step. Pressure, the pressure of the second urea synthesis step is made substantially equal to the pressure of the stripping step, the second urea synthesis liquid is introduced into the stripping step, and And the excess ammonia and unreacted ammonium carbamate contained in the first and second urea synthesis solutions are separated from the urea solution as a mixed gas of ammonia, carbon dioxide and water. An improved method for synthesizing urea, which is introduced into the condensation step. 2. The method according to claim 1, wherein a part of the raw material carbon dioxide is introduced into the stripping step, and the excess ammonia and unreacted ammonium carbamate are stripped under heating. 3. The method according to claim 2, wherein the amount of said raw carbon dioxide is not more than 0.4 ton / ton of product urea. 4. The first urea synthesis solution is expanded through an ejector, whereby the second urea synthesis solution is sucked and the first and second urea synthesis solutions are introduced into a stripping step. The method according to any one of claims 1 to 3. 5. The pressure in the mixing step is from 170 to 26.
The method according to claim 1, wherein the temperature is 0 bar and the temperature is 160-200 ° C. 6. The pressure of the first urea synthesis step is from 170 to 170.
260 bar, temperature 180-210 ° C., N / C (molar ratio of ammonia to carbon dioxide) 3.0-4.5, H /
The method according to claim 1, wherein C (molar ratio of water to carbon dioxide) is substantially 0 and the residence time is 10 to 50 minutes. 7. The pressure in the stripping step is from 140 to
The process according to claim 1, wherein the temperature is 180 bar, the top temperature is 180-200 ° C, and the bottom temperature is 170-215 ° C. 8. The process according to claim 1, wherein the pressure in the condensation step is 140-180 bar and the temperature is 150-190 ° C. 9. The pressure of the second synthesis step is 140 to 18
0 bar, temperature 180-200 ° C, N / C 2.5-
4.5, H / C 0.5-2.0, residence time 10-6
2. The method of claim 1, wherein the time is 0 minutes. 10. The absorption medium introduced into the condensation step,
Residual ammonia and carbon dioxide in the urea solution are decomposed and separated, and the inert gas discharged from the second synthesis step with a recovered solution containing ammonium carbamate obtained by absorption with water is discharged at 140 to 180 bar and 150 to 180 ° C.
The method according to claim 1, which is obtained by washing in step (1) and absorbing ammonia and carbon dioxide contained therein.