JPH08301834A - Synthesis of urea - Google Patents

Abstract

PURPOSE: To prevent leakage of a mixed gas from a shaft seal apparatus by reducing installing space of a compressor and power. CONSTITUTION: Ammonia and unreacted ammonium carbamate contained in urea synthesizing liquid from an urea synthesizing area are separated as ammonia, carbon dioxide and a mixed gas by carbon dioxide stripping and the pressure of the mixed gas is increased and the mixed gas is introduced into a condensation area in a mixed gas compressing part 4 of a centrifugal compressor constituting of carbon dioxide compressing part 5 and a mixed gas compressing part 4, providing the former impeller and latter impeller on a common shaft and having a structure blowing out the mixed gas to shaft seal apparatus and the produced condensate or gas-liquid mixture is circulated into the urea synthesizing area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved method for synthesizing cyclic urea from ammonia and carbon dioxide, and more particularly to synthesizing urea in a gaseous unreacted ammonia and carbon dioxide separated from a urea synthesis solution through a condensation step. It relates to a urea synthesis method including an improved method for recycling to the process.

[0002]

Urea synthesis from ammonia and carbon dioxide is a pressure suitable for urea synthesis, for example, a gauge pressure of 140-
250 kg / cm ² , and temperature, for example 170-21
Carried out in excess ammonia at 0 ° C. The urea synthesis solution from such a urea synthesis step contains, in addition to urea and water, excessively supplied ammonia and unreacted ammonium carbamate. Therefore, how these unreacted substances are recovered and reused for urea synthesis. The superiority or inferiority of the method to be used determines the superiority or inferiority of the whole urea synthesis method. The method currently widely used as a method for recovering unreacted substances is to remove unreacted substances by stripping the urea synthesis liquid from the urea synthesis step with carbon dioxide at a pressure almost equal to or slightly lower than the urea synthesis pressure. Separated as a mixed gas of ammonia, carbon dioxide and water, this mixed gas is mixed with water,
A method is widely used in which an aqueous solution of ammonia or an aqueous solution of ammonium carbamate is brought into contact with the solution to condense it, and the resulting condensate or gas-liquid mixture is recycled to the urea synthesis step.

This method will be described in detail with reference to FIG. Liquid ammonia that has been boosted to the urea synthesis pressure from line 109 and a condensate or gas-liquid mixture consisting of ammonia, carbon dioxide and water produced in condenser 103 from line 114 are introduced into urea synthesis tube 101, Subject to urea synthesis under conditions.

Urea from the urea synthesis tube via line 110,
A urea synthesis solution containing water, ammonia and ammonium carbamate is taken out, introduced at the top of the stripper 102, and flows down while being heated, while line 11 is being supplied.
1 and is contacted with carbon dioxide which is compressed by the compressor 105 and introduced from the bottom of the stripper 102, and most of ammonia and ammonium carbamate are separated from the urea synthesis solution as a mixed gas of ammonia, carbon dioxide and water. To be done. Aqueous urea solution containing urea, water and residual ammonia and ammonium carbamate is withdrawn from the bottom of the stripper, depressurized via a pressure reducing valve 106 and sent to the process step at low pressure, where residual ammonia and ammonium carbamate is removed from the aqueous urea solution with ammonia, carbon dioxide and It is separated as a mixed gas consisting of water, absorbed in an absorption medium such as water, diluted ammonia water, or an aqueous urea solution and recovered as a low-pressure absorbent, while urea is obtained from the aqueous urea solution by a known method.

The mixed gas taken out from the top of the stripper 102 is passed through the line 112 to the carbamate condenser 1
Introduced in 03. On the other hand, the inert gas contained in the liquid ammonia from the line 109 and the carbon dioxide from the line 111 is separated from the urea synthesis liquid at the top of the urea synthesis tube 101, and passes through the line 115 to the absorption tower 107.
Is contacted with the low-pressure absorption liquid introduced through the line 116 at a pressure substantially equal to the urea synthesis pressure, and ammonia and carbon dioxide accompanying the inert gas are absorbed to obtain a high-pressure absorption liquid. This high-pressure absorbent is on line 113
It is introduced into the top of the carbamate condenser 103 via the and under cooling (it is usually cooled by steam generation).
In contact with the mixed gas from line 112, at least a portion of the mixed gas is condensed and absorbed to produce a condensed liquid or a gas-liquid mixing part of ammonia, carbon dioxide and water. The inert gas from which ammonia and carbon dioxide have been separated is decompressed by the decompression valve 108 and discharged.

In order to introduce this condensate or gas-liquid mixture into the urea synthesis tube, the pressure of this condensate or gas-liquid mixture must be equal to or higher than the pressure of the urea synthesis tube, so Therefore, the stripper 102 pressure must be higher than the carbamate condenser 103. Then, the pressure of the urea synthesis tube is 10
The pressure becomes lower than 2 and the urea synthesis liquid cannot be sent from the urea synthesis tube to the stripper. In order to solve this problem, the following method has been conventionally adopted.

1. The urea synthesis tube, the carbamate condenser, and the stripper placed on the ground are installed several tens of meters above the stripper, and the urea synthesis solution flows down from the urea synthesis tube to the stripper having a higher pressure than the synthesis tube by gravity.

2. The urea synthesis solution is moved from the urea synthesis tube to the stripper with a pressure difference, and the mixed gas from the stripper is transferred to the carbamate condenser with a pressure difference. The gas-liquid mixture from the carbamate condenser whose pressure becomes lower than the urea synthesis pressure is sucked by the ejector and moves to the urea synthesis tube whose pressure is higher than that of the condenser. This ejector is driven by the raw material liquid ammonia whose pressure is raised to a pressure considerably higher than the urea synthesis pressure by the high-pressure pump.

[0009]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
There are the following problems to be solved.

According to the above-mentioned method 1, it is necessary to install a heavy urea synthesis tube and a carbamate condenser at a high position of 20 to 40 meters above the ground on a large earthquake-resistant mount, but at a high place. Since heavy equipment is installed, the construction becomes large and the burden of maintenance work is heavy after construction.

In the above method 2, the liquid ammonia has to be boosted to a pressure considerably higher than the urea synthesis pressure by a low-efficiency pump in order to drive the ejector, but most of the internal energy generated by the boosting is ejector. Lost as a pressure loss.

Therefore, an object of the present invention is to improve the above problems and to transfer the condensate or gas-liquid mixture from the carbamate condenser easily and efficiently to the urea synthesis tube at a higher pressure than the stripper. To provide a urea synthesis method including the above-mentioned method.

[0013]

The method for synthesizing urea according to the present invention comprises reacting ammonia and carbon dioxide under urea synthesis conditions in the urea synthesis zone to obtain urea, water, ammonia and unreacted ammonium carbamate. In the separation zone, the urea synthesis liquid containing is brought into contact with carbon dioxide under heating to separate the ammonia and the unreacted ammonium carbamate as a mixed gas of ammonia, carbon dioxide and water to obtain an aqueous urea solution, and the mixed gas is subjected to carbamate. At least a part of the mixed gas is condensed and absorbed by contacting with an absorption medium under cooling in a condensation zone to generate a condensed liquid or a gas-liquid mixture of ammonia, carbon dioxide and water, and the condensed liquid or the gas-liquid mixture is When the urea is circulated to the urea synthesis zone, while urea is obtained from the urea aqueous solution obtained in the separation zone, The carbon dioxide introduced into the zone is compressed using a centrifugal compressor having a shaft sealing device, and an additional impeller is provided on the same common shaft adjacent to the final stage impeller of the centrifugal compressor, and the separation is performed. The mixed gas from the zone is pressurized by the additional impeller and introduced into the condensation zone.

The centrifugal compressor used in the present invention is
In the seal part between the final impeller for carbon dioxide compression and the impeller for mixed gas compression, a structure with an inlet for introducing a part of carbon dioxide discharged from this centrifugal compressor as a buffer gas is provided. You may have.

The above centrifugal compressor has a shaft sealing device,
It may have a structure provided with an inlet for introducing a part of carbon dioxide discharged from the centrifugal compressor as a buffer gas.

Further, the above buffer gas may be heated and introduced.

Since the mixed gas to be pressurized in the present invention is a gas having high temperature and high pressure and being highly corrosive and toxic, the pressure of the raw carbon dioxide and the mixed gas is increased as described above. It is carried out by one centrifugal compressor in which each of the compression parts is combined, and preferably, a part of the discharge gas from the carbon dioxide compression part is provided between the carbon dioxide compression part and the mixed gas compression part and / or in the shaft sealing device. By introducing the above into the above, it is possible to easily increase the pressure of the mixed gas by preventing the mixed gas from leaking from the shaft sealing portion, and further, it is possible to save the installation location of the centrifugal compressor and the power. Become.

The present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a flow sheet showing the method of the present invention.
1, the liquid ammonia whose pressure is increased to the urea synthesis pressure from the line 9 and the condensate or gas-liquid mixture of ammonia, carbon dioxide and water from the line 14 are the urea synthesis tube 1.
Preferably at a temperature of 170 ° to 210 ° C., particularly preferably at a temperature of 180 ° to 200 ° C. and an ammonia / carbon dioxide molar ratio of 3.0 to 5.
The urea synthesis is carried out at 0, particularly preferably 3.0 to 4.0 and a water / carbon dioxide molar ratio of preferably 0.2 to 1.5, particularly preferably 0.3 to 1.0. When the urea synthesis temperature is 170 ° C. or lower, the reaction rate decreases, so a large-capacity reactor is required, and when it is 210 ° C. or higher, the equilibrium pressure exhibited by the urea synthesis liquid becomes large, and the pressure resistance of the reactor needs to be increased. . When the ammonia / carbon dioxide molar ratio is in the above range, the amount of the urea synthesis liquid per unit of produced urea is minimized. On the other hand, if the molar ratio is less than 3.0, the conversion rate of carbon dioxide to urea decreases, and the amount of steam for stripping unreacted ammonium carbamate increases. If the molar ratio exceeds 5.0, the ammonia content in the urea synthesis solution becomes too large, and the consumption of steam for stripping this ammonia increases. water/
If the molar ratio of carbon dioxide is less than 0.2, it becomes difficult to circulate the recovered unreacted ammonium carbamate into the urea synthesis tube, while if it exceeds 1.5, the conversion rate of carbon dioxide to urea is significantly reduced. Urea synthesis pressure is temperature,
Although it depends on the ammonia / carbon dioxide molar ratio and other conditions, it is basically the same as or slightly higher than the pressure exhibited by the equilibrium synthesis equilibrium composition that has reached equilibrium, and in the present invention, it is preferably a gauge. Pressure 130-250k
g / cm ² , particularly preferably 140 to 200 kg / cm
²

The urea synthesis solution is introduced from the urea synthesis tube 1 through the line 10 to the top of the stripper 2. This transfer is performed by the pressure difference between the urea synthesis tube 1 and the stripper 2. This pressure difference may be a pressure difference sufficient for the urea synthesis liquid to flow from the urea synthesis tube to the stripper. The stripper pressure is, for example, 2-3 kg / cm higher than that of the urea synthesis tube.
⁽²⁾ If the pressure is low, the object of the present invention can be achieved.
The pressure is preferably 10 to ²⁰ kg / cm ² lower.

The urea synthesis solution introduced into the stripper 2 is introduced from the line 11 while flowing down under heating,
The carbon dioxide compression section 5 of the centrifugal compressor causes the stripper 2
Is contacted with carbon dioxide blown from the bottom of the stripper 2 after being compressed to a pressure equal to or slightly higher than the pressure of, and the contained ammonia and carbon dioxide are separated as a mixed gas of ammonia, carbon dioxide and water.
The temperature of the stripper is preferably 170 to 200 ° C.
It is said. The urea aqueous solution from the bottom of the stripper is decompressed through the pressure reducing valve 6 and treated in the same manner as in the conventional method described with reference to FIG. 4 to obtain urea and a low-pressure absorbing liquid.

The mixed gas separated by the stripper 2 is taken out from the top and introduced into the mixed gas compression section 4 of the centrifugal compressor through the line 12, preferably 3 to 20 kg / c.
m ^2, particularly 10-20 kg / cm ² boosting, line 1
It is introduced into the carbamate condenser 3 via 3. The condensed liquid or gas-liquid mixture of ammonia, carbon dioxide and water produced in the carbamate condenser 3 by this pressurization is supplied to the line 1.
It can be transferred to the urea synthesis tube 1 via 4. In addition,
The structure of the mixed gas compression unit 4 of the centrifugal compressor used in the present invention will be described in detail later.

At the top of the carvermate condenser 3 is shown in FIG.
The inert gas separated at the top of the urea synthesis tube is absorbed in the absorption tower 7 in the same manner as in the conventional method described with reference to FIG.
A high pressure absorbing liquid obtained by absorbing ammonia and carbon dioxide accompanying an inert gas by being brought into contact with the low pressure absorbing liquid from the line 16 is introduced, and under cooling (cooling is usually performed by steam generation) line Thirteen
The mixed gas from is condensed and absorbed to produce a condensate or a gas-liquid mixture of ammonia, carbon dioxide and water. The temperature of the carbamate condenser 3 is preferably 170 to 190 ° C.
The condensate or gas-liquid mixture is introduced into the urea synthesis tube 1 via line 14.

Next, the centrifugal compressor used in the present invention will be described. The centrifugal compressor used in the present invention is
Since it handles high-temperature and high-pressure mixed gas that is highly corrosive and highly toxic, it not only has corrosion resistance to this mixed gas, but also blocks the carbon dioxide compression section and the mixed gas compression section. Moreover, it is required that the mixed gas does not leak from the shaft-sealed portion. In the present invention, the pressure of each of carbon dioxide and the mixed gas is increased by a single centrifugal compressor in which the carbon dioxide compression section and the mixed gas compression section are combined, and preferably in the shaft sealing device and mixed with the carbon dioxide compression section. The above requirements are satisfied by introducing a part of the gas discharged from the carbon dioxide compression section into the seal section between the gas compression section and the seal.

The centrifugal compressor used in the present invention will be specifically described with reference to FIG. 2 which is a partial sectional view showing a part of the cross section and the flow of gas. Adjacent to the final impeller 23 of the carbon dioxide compression section in the casing 21, an additional impeller 22 for mixed gas compression is provided on the rotor (common shaft) 24 so that the directions of suction and discharge are the same. . The dry gas seal as the shaft sealing device of the rotor 24 includes a dry gas seal rotating ring 25, a dry gas seal stationary ring 26 and a spring 26A,
Due to the rotary motion of the dry gas seal rotary ring 25, a pressure is generated in the gap between the rotary ring and the stationary ring, and a microscopic gap of a certain level or more is always maintained by itself.

A buffer gas inlet 27 is provided in the labyrinth seal section between the carbon dioxide compression section and the mixed gas compression section. The gas discharged from the carbon dioxide compression section is introduced into the bottom of the stripper 2 via a line 32, a part of the gas is diverted via a line 33, and a part of the gas is steam-heated in a heat exchanger 30 and a line 34. Then, the gas is introduced into the buffer gas inlet 27. The introduced buffer gas is blown out to the labyrinth seal between the final stage impeller of the carbon dioxide compression unit and the impeller of the mixed gas compression unit. The blowing of heated carbon dioxide creates a seal between the two compression parts, although not completely. Since the pressure of the introduced buffer gas is higher than the pressure of the mixed gas sucked in through the line 35, the buffer gas mixes with the mixed gas in the mixed gas compression section, but since carbon dioxide is a component of the mixed gas, it does not affect the process. It does not hurt at all. Since the buffer gas is preferably heated to a temperature of 155 to 180 ° C., even if it is mixed with the mixed gas, the temperature of the mixed gas is not lowered to be condensed. The mixed gas pressurized in the mixed gas compression section 4 is sent to the carbamate condenser 3 through the line 36.

On the other hand, the remaining portion of carbon dioxide from the line 33 passes through the pressure control valve 29 so that the pressure is 0 to 100 kg.
/ Cm ² and then introduced into the filter 31 through the line 37, the mist and dust contained therein are removed and introduced through the buffer gas inlet 28, and the dry gas seal rotating ring 25 and the dry gas seal stationary ring 26 are finely divided. It is possible to prevent the leaked gas from leaking from the gap and the compressed mixed gas from leaking from the bearing portion (not shown) through the shaft sealing device.

The amount of carbon dioxide blown into the labyrinth seal section and the shaft sealing device between the final stage impeller of the carbon dioxide compression section and the impeller of the mixed gas compression section is 0. About 5 to 1.5% is preferable.

Corrosion-resistant materials such as austenite stainless steel, austenite-ferrite dual phase alloy, and titanium are used as the material of the casing and the impeller of the mixed compression section.

[0030]

EXAMPLES The present invention will be described in more detail below by showing an example in which the present invention is applied to a urea production apparatus having a urea production amount of 1,000 t / day.

Example Referring to FIG. 1, a gauge pressure of 180 kg / c from line 9
Liquid ammonia pressurized to m ² , 23,500 kg / h
And ammonia 5 obtained in the cover mate condenser 3
5,000 kg / h, carbon dioxide 49,500 kg / h
A condensate solution containing 13500 kg / h of water and water was introduced into the urea synthesis tube 1 through the line 14 and reacted. The reaction conditions for the urea synthesis tube are 175 kg / cm gauge pressure.
² , at a temperature of 190 ° C., the ammonia / carbon dioxide molar ratio and the water / carbon dioxide molar ratio were 4.1 and 0.64, respectively.

The urea synthesis liquid sent from the urea synthesis tube 1 to the stripper 2 through the line 10 is 44,700 kg of urea.
/ H, ammonia 49,300 kg / h, carbon dioxide 1
It contained 5,400 kg / h and 26,000 kg / h of water. The urea synthetic solution has a gauge valve of 155 kg due to the pressure reducing valve.
The pressure was reduced to / cm ² and then introduced at the top of the stripper. In the stripper, while the urea synthesis solution was flowing down, 28,800 kg of carbon dioxide introduced from the bottom through the line 11, the carbon dioxide compression section 5 of the centrifugal compressor, and a temperature of 19
Contacted at 0 ° C., ammonia and ammonium carbamate were withdrawn from the top as a mixed gas of ammonia, carbon dioxide and water. The urea aqueous solution taken out from the bottom of the stripper 2 via the pressure reducing valve 6 is sent to the low pressure treatment step, and the residual ammonia and ammonium carbamate are completely separated from the urea aqueous solution as a mixed gas of ammonia, carbon dioxide and water, and the urea 44 ，
A urea aqueous solution containing 700 kg / h was obtained. The separated mixed gas was absorbed by diluted ammonia water and recovered as a low-pressure absorption liquid.

The mixed gas from the stripper 2 is supplied to the line 1
2 into the mixed gas compression section 4 of the centrifugal compressor shown in FIG. 2, the pressure was increased to a gauge pressure of 175 kg / cm ^2, and it was introduced into the carbamate condenser 3 via line 13. The amount of carbon dioxide introduced as the buffer gas was 100 kg / hr from the buffer gas inlet 28 and 400 kg / hr from the buffer gas inlet 27. On the other hand, the absorption liquid, which has been increased in gauge pressure to 175 kg / cm ² and has been introduced into the absorption tower 7 through the line 16 and has come into contact with the inert gas from the line 15 and has absorbed the ammonia and carbon dioxide contained therein, flows through the line 17. The mixed gas introduced into the carbamate condenser 3 through the line 13
Absorbed at 5 ℃, ammonia 55,000kg / h,
Carbon dioxide 49,500 kh / h and water 13,500
A condensate consisting of kg was obtained. This condensate was circulated to the urea synthesis tube 1 via the line 14 as described above.

The urea synthesizer was operated continuously for two months, but no leakage was found from the bearing of the centrifugal compressor.

[0035]

According to the present invention, the condensate or gas-liquid mixture produced in the carbamate condenser can be easily circulated through the urea synthesis tube by pressurizing the mixed gas from the stripper using a centrifugal compressor. You can The centrifugal compressor used in the present invention has a structure in which an additional impeller is provided on the common shaft adjacent to the final-stage impeller of the carbon dioxide compression section, and the pressure of the mixed gas is increased by the additional impeller. It has a shaft seal device in the compression part, preferably from the mixed gas compression part through the shaft seal part by blowing a carbon dioxide compression part into the labyrinth seal between the last stage impeller and the additional impeller and the shaft seal device. The leakage of the mixed gas is prevented, and the installation place of the compressor and the power consumption can be saved.

Further, unlike the conventional method, since the equipment can be installed on the ground, there is no problem with the earthquake resistance as in the conventional method, the construction cost can be reduced, and the operation and maintenance are easy. The profits are also great.

[Brief description of drawings]

FIG. 1 is a flow sheet showing a method for synthesizing urea according to the present invention.

FIG. 2 is a partial cross-sectional view showing a part of a cross section along a rotation axis of a centrifugal compressor used in the present invention and a gas flow.

FIG. 3 is a flow sheet showing a conventional urea synthesis method involving stripping of unreacted ammonium carbamate with carbon dioxide.

[Explanation of symbols]

 1,101 Urea synthesis tube 2,102 Stripper 3,103 Carbamate condenser 4 Centrifugal compressor Mixed gas compression section 5 Centrifugal compressor Carbon dioxide compression section 105 Compressor 21 Casing 22 Impeller for mixed gas compression 23 Carbon dioxide compression section Final Impeller 24 Rotor (common shaft) 25 Dry gas seal rotating ring 26 Dry gas seal stationary ring 27 Buffer gas inlet (labyrinth seal) 28 Buffer gas inlet (dry gas seal) 29 Pressure control valve 30 Heat exchanger 31 Filter

[Procedure amendment]

[Submission date] October 12, 1995

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

FIG.

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 3

[Correction method] Change

[Correction content]

[Figure 3]

Claims (4)

[Claims] 1. A urea obtained by reacting ammonia and carbon dioxide under urea synthesis conditions in a urea synthesis zone,
A urea synthesis solution containing water, ammonia and unreacted ammonium carbamate is brought into contact with carbon dioxide under heating in the separation zone to separate the ammonia and the unreacted ammonium carbamate as a mixed gas of ammonia, carbon dioxide and water to give urea. An aqueous solution is obtained, and the mixed gas is brought into contact with the absorption medium under cooling in the carbamate condensation zone to condense and absorb at least a part of the mixed gas to produce a condensed liquid or a gas-liquid mixture of ammonia, carbon dioxide and water, The condensate or gas-liquid mixture is circulated to the urea synthesis zone, while the urea is obtained from the urea aqueous solution obtained in the separation zone, the carbon dioxide introduced into the separation zone is compressed by centrifugal compression having a shaft sealing device. Machine with an additional impeller on the same common axis adjacent to the last stage impeller of the centrifugal compressor. Synthesis of urea, characterized in that the mixed gas from the separation zone is boosted by the addition of the impeller is introduced into the condensation zone. 2. The centrifugal compressor has a seal portion between a final impeller for carbon dioxide compression and an impeller for mixed gas compression,
The method according to claim 1, having a structure provided with an inlet for introducing a part of carbon dioxide discharged from the centrifugal compressor as a buffer gas. 3. The centrifugal compressor in its shaft seal device,
The method according to claim 1 or 2, which has a structure provided with an inlet for introducing a part of carbon dioxide discharged from the centrifugal compressor as a buffer gas. 4. The method according to claim 2, wherein the buffer gas is heated and introduced.