JPH08301835A - Urea synthesis method - Google Patents

Abstract

(57) [Summary] [Structure] Ammonia and unreacted ammonium carbamate contained in the urea synthesis liquid from the urea synthesis zone are separated as ammonia, carbon dioxide and water as a mixed gas by carbon dioxide stripping. A centrifugal compressor having a dry gas seal or an oil film type shaft seal as a shaft sealing device pressurizes, condenses and circulates in the urea synthesis region. A part of carbon dioxide supplied to the stripper can be introduced as a buffer gas into the labyrinth seal portion between the shaft sealing device or the casing and the rotor. [Effect] The mixed gas can be condensed and easily circulated in the urea synthesis region, and the centrifugal compressor having the above structure can be stably operated for a long period of time without leakage of the mixed gas from the bearing.

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 widely used at present as a method for recovering unreacted substances is that the urea synthesis solution from the urea synthesis step is not subjected to stripping with carbon dioxide at a pressure almost equal to or slightly lower than the urea synthesis pressure. Separating the reactants as a mixed gas of ammonia, carbon dioxide and water,
A method is widely used in which this mixed gas is brought into contact with water, an aqueous solution of ammonia, an aqueous solution of ammonium carbamate or the like to be condensed, and the resulting condensed liquid 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.

From the urea synthesis tube via line 110 urea,
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 and the carbamate condenser are installed several tens of meters higher than the stripper placed on the ground, and the gravity of the urea synthesis tube causes the urea synthesis solution to flow down to the stripper having a higher pressure than the synthesis tube.

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.

[0010]

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

According to the above 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 pedestal. Since heavy equipment is installed, the construction becomes large and the burden of maintenance work is heavy after construction.

Further, in the above-mentioned method 2, in order to drive the ejector, the liquid ammonia has to be boosted to a pressure considerably higher than the urea synthesis pressure by a low efficiency pump, 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 solve the above-mentioned problems, and to improve the condensate or gas-liquid mixture from a carbamate condenser to easily and efficiently transfer it to a urea synthesis tube having a pressure higher than that of a stripper. To provide a urea synthesis method including the above-mentioned method.

[0014]

The urea synthesis method according to the present invention comprises urea, water, ammonia and unreacted ammonium carbamate obtained by reacting ammonia and carbon dioxide under urea synthesis conditions in a urea synthesis zone. In the separation zone, the urea synthesis liquid 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 condensed with carbamate. Contacting the absorption medium with cooling in the zone to condense and absorb at least a portion of the mixed gas to produce a condensed liquid or a gas-liquid mixture of ammonia, carbon dioxide and water,
When the condensate or gas-liquid mixture is circulated to the urea synthesis zone, while urea is obtained from the urea aqueous solution obtained in the separation zone, the mixed gas from the separation zone is used as a shaft seal device for a dry gas seal or oil. It is characterized in that a centrifugal compressor equipped with a film type shaft seal is used to pressurize and introduce the gas into the carbamate condenser.

The urea synthesis method of the present invention also includes the case where the centrifugal compressor has a structure in which a part of carbon dioxide introduced into the separation zone is introduced as a buffer gas into a dry gas seal shaft sealing device. .

The present invention also includes the case where a part of carbon dioxide introduced into the separator is introduced into the labyrinth seal portion between the impeller and the oil film seal device.

Further, in this case, the case of heating and introducing carbon dioxide is also included.

In the present invention, the mixed gas to be pressurized by the centrifugal compressor is a gas of high temperature and high pressure, which is highly corrosive and highly toxic. Therefore, the centrifugal compressor having the above-mentioned shaft seal portion is preferably used. Introduces part of the carbon dioxide that should be introduced into the separation area into the shaft seal part and the labyrinth seal part to prevent the mixed gas from leaking from the bearings, enabling safe and easy boosting of the mixed gas. Becomes

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

FIG. 1 is a flow sheet showing the method of the present invention.
Referring to, the liquid ammonia whose pressure has been increased to the urea synthesis pressure from the line 109, the ammonia from the line 114,
A condensate or a gas-liquid mixture of carbon dioxide and water is introduced into the urea synthesis tube 101, preferably 170 ° to 210 ° C.
A temperature of from 180 ° to 200 ° C. and an ammonia / carbon dioxide molar ratio of 3.
0-5.0, particularly preferably 3.0-4.0 and water /
The urea synthesis is carried out at a molar ratio of carbon dioxide 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. When the water / carbon dioxide molar ratio is less than 0.2, it becomes difficult to circulate the recovered unreacted ammonium carbamate into the urea synthesis tube, while when it exceeds 1.5, the conversion rate of carbon dioxide to urea is significantly reduced. To do. The urea synthesis pressure depends on the temperature, the ammonia / carbon dioxide molar ratio, and other conditions, but in principle, it is the same as or slightly higher than the pressure exhibited by the urea synthesis equilibrium composition that has reached equilibrium. In the invention, the gauge pressure is preferably 130-
250 kg / cm ² , particularly preferably 140 to 200 k
g / cm ² .

The urea synthesis solution is introduced from the urea synthesis tube 101 through the line 110 to the top of the stripper 102.
This transfer is performed by the pressure difference between the urea synthesis tube 101 and the stripper 102. 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 object of the present invention can be achieved by setting the pressure of the stripper to a pressure lower than that of the urea synthesis tube by 2-3 kg / cm ^2, but in order to further accelerate the decomposition of unreacted substances in the stripper, 10-20 kg / It is preferred that the pressure is cm ² lower.

The urea synthesis solution introduced into the stripper 102 is introduced from a line 111 while flowing down under heating, and is compressed by the compressor 105 to a pressure equal to or slightly higher than the pressure of the stripper 102 and then stripped. Contact with carbon dioxide blown from the bottom of the
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. 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 in the stripper 102 is withdrawn from the top, is introduced into the centrifugal compressor 4 via line 112, preferably 3~20kg / cm ^2, particularly 10-20 kg / cm ² boosting, line 117 And is introduced into the carbamate condenser 103. A line 1 is supplied with a condensate or a gas-liquid mixture of ammonia, carbon dioxide and water produced in the carbamate condenser 103 by this pressurization.
It can be transferred to the urea synthesis tube 101 via 14.
The structure of the centrifugal compressor 4 used in the present invention will be described in detail later.

At the top of the carvermate condenser 103,
In the same manner as in the conventional method described with reference to FIG. 4, the inert gas separated at the top of the urea synthesis tube is brought into contact with the low pressure absorbing liquid from the line 116 in the absorption tower 107, thereby being accompanied by the inert gas. A high-pressure absorption liquid obtained by absorbing ammonia and carbon dioxide is introduced, and the mixed gas from line 117 is condensed and absorbed under cooling (cooling is usually performed by steam generation) to condense ammonia, carbon dioxide and water. Alternatively, a gas-liquid mixture is produced. The temperature of the carbamate condenser 103 is 170-
190 ° C. is preferred. The condensate or gas-liquid mixture is introduced into the urea synthesis tube 101 via line 114.

Next, the centrifugal compressor used in the present invention will be described. The centrifugal compressor used in the present invention is
Since it handles a mixed gas of high temperature and high pressure, which is highly corrosive and highly toxic, it is required not only to have corrosion resistance to this mixed gas, but also to prevent leakage of the mixed gas from the shaft-sealed portion.

In the present invention, the shaft sealing device is composed of a rotating ring provided on the rotor and stationary rings provided on the casing side on both sides of the rotating ring in the minute gap, and is rotated by the rotation of the rotating ring. A pressure is generated in the gap between the ring and the stationary ring, and an oil film is formed between the dry gas seal or the rotor that always maintains a minute gap above a certain level, or the rotor attached to the casing and the stationary ring having the minute gap. Adopting an oil film type shaft seal, preferably by introducing a part of carbon dioxide to be introduced into the stripper as a buffer gas in the labyrinth seal portion between the casing and the shaft in the shaft sealing device, the above-mentioned requirements It corresponds to. FIG. 2 (in the case of a dry gas seal) showing a part of a partial cross section along the rotor and the flow of the buffer gas in the centrifugal compressor used in the present invention.
And a part of the cross section of the centrifugal compressor and the flows of the seal oil and the buffer gas are specifically described with reference to FIG. 3 (in the case of an oil film type shaft seal).

In FIG. 2, a rotor 22 is provided inside a casing 21. Rotor 22 is casing 2
1, two bearings 26 are supported and two dry gas seal rotary rings 24 are provided near the impellers with respect to the bearings 26. Corresponding to the dry gas seal rotary ring 24, two dry gas seal stationary rings 25 are provided on the casing side so as to have a minute gap with the rotary rings on both sides of the dry gas seal rotary ring 24 to provide a dry gas seal. Configure. Reference numeral 23 denotes an impeller attached to the rotor, and 28 denotes a flow of intake gas.
9 shows the flow of discharge gas.

The carbon dioxide used as the urea synthesis raw material is compressed by the compressor 105.
The pressure is increased by means of and is introduced into the stripper, and a part of the pressure is divided through a line 31 to remove mist, dust and the like through a filter 30, and then the lines 32 and 33.
Then, the gas is introduced from the buffer gas inlet 27 into the dry seal device. This carbon dioxide leaks little by little through the gap between the dry seal stationary ring and the dry seal rotating ring, and prevents the mixed gas from leaking from the bearing 26 to the outside through the dry seal device.

Oil film type shaft seal Referring to FIG.
A rotor 22 having a bearing 3 is supported by a bearing 26 and is arranged through the casing 21. A floating seal ring (gas side) 40 and a floating seal ring (atmosphere side) 41 are attached to the casing with a minute gap therebetween, and a seal oil supply port 42 is provided, which constitutes an oil film type shaft seal. The seal oil that forms an oil film between the rotor and the floating seal ring is supplied from the seal oil head tank to the lines 47 and 49 and the seal oil introduction hole 42.
Is supplied to the shaft seal device, leaks to the gas side and the atmosphere side through the seal portion, and is discharged through lines 52 and 53. The seal oil is supplied from the line 46 to the head tank 44 through the level control valve. The head tank 44 is pressure-equalized with the mixed gas by the sealing chamber pressure guiding lines 43 and 50.

A buffer gas inlet 27 is provided in the labyrinth seal portion between the impeller and the oil film seal device. The discharge gas of the carbon dioxide compressor 105 is introduced into the bottom of the stripper 102, but a part of the discharge gas is diverted via the line 31, steam-heated in the heat exchanger 35, and centrifugally compressed via the lines 48 and 51. It is introduced into the buffer gas inlet 27 of the machine 4. The introduced buffer gas is blown out to the labyrinth seal between the impeller and the oil film device. By blowing the heated carbon dioxide, the seal oil is prevented from being deteriorated by the mixed gas in the line 52, and the parts such as the seal ring 40 cooled by the seal oil come into contact with the mixed gas to cool it. Prevents carbamate from precipitating. Since the pressure of the introduced buffer gas is higher than the pressure of the mixed gas sucked from the line 28, the buffer gas is mixed into the mixed gas, but since carbon dioxide is a component of the mixed gas, it does not affect the process at all. It does not hinder. 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.

[0031]

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 from line 109
23,500 kg of liquid ammonia boosted to 1 / cm ²
/ H is 55,000 kg / h of ammonia obtained in the covermate condenser 103, carbon dioxide 49,
A condensate composed of 500 kg / h and 13,500 kg / h of water was introduced into the urea synthesis tube 101 through the line 114 and reacted. The reaction conditions for the urea synthesis tube are: 175 kg / cm ² gauge pressure, 190 ° C. temperature, ammonia /
The carbon dioxide molar ratio and the water / carbon dioxide molar ratio were 4.1 and 0.64, respectively.

The urea synthesis solution sent from the urea synthesis tube 101 to the stripper 102 via the line 110 is urea 44,
700 kg / h, ammonia 49,300 kg / h, carbon dioxide 15,400 kg / h and water 26,000 k
It contained g / h. The urea synthesis solution was depressurized by a pressure reducing valve to a gauge pressure of 155 kg / cm ² , and then introduced into the top of the stripper. In the stripper, while the urea synthesis solution is flowing down, it comes into contact with 28,800 kg of carbon dioxide introduced from the bottom through the line 111 and the compressor 105 at a temperature of 190 ° C., and ammonia and ammonium carbamate are mixed with ammonia, carbon dioxide and water. It was taken off as gas from the top. The urea aqueous solution taken out from the bottom of the stripper 102 via the pressure reducing valve 106 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 4
An aqueous urea solution containing 4,700 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 102 enters the centrifugal compressor 4 shown in FIG. 2 through a line 112, the pressure is increased to a gauge pressure of 175 kg / cm ^2, and is introduced into a carbamate condenser 103 through a line 117. As a buffer gas, carbon dioxide, 100 kg / h, was introduced into the dry gas seal part. The amount of carbon dioxide introduced into the labyrinth was 300 kg / h. On the other hand, gauge pressure 1
The absorption liquid, which has been pressurized to 75 kg / cm ² and has been introduced into the absorption tower 107 from the line 116 and has come into contact with the inert gas from the line 115 and has absorbed the ammonia and carbon dioxide contained therein, passes through the line 117 to the carbamate condenser. The mixed gas introduced into 103 and introduced through line 113 is absorbed at 180 ° C.
A condensate consisting of 0 kg / h, carbon dioxide 49,500 kh / h and water 13,500 kg was obtained. This condensate is passed through the line 114 as described above, and the urea synthesis tube 10 is
Cycled to 1.

The urea synthesizer was continuously operated for two months, but no gas leaked from the shaft seal portion and no abnormality was found in the centrifugal compressor.

[0036]

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 uses a dry gas seal or an oil film type shaft seal in order to prevent the mixed gas in the casing from leaking to the atmosphere, and preferably uses a buffer gas between the shaft sealing portion or the casing and the rotor. Since it is introduced into the labyrinth seal part, it is possible to continuously and reliably compress the high temperature and high pressure corrosive and toxic gas mixture without leakage from the bearing for a long period of time.

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 the maintenance are easy. The profits are also great.

[Brief description of drawings]

FIG. 1 is a flow sheet showing a urea synthesis method 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 flow of a buffer gas.

FIG. 3 is a partial cross-sectional view showing another part of a centrifugal compressor used in the present invention, showing a part of a cross section taken along a rotation axis and flows of seal oil and buffer gas.

FIG. 4 is a flow sheet showing a conventional urea synthesis method including stripping of unreacted ammonium carbamate with carbon dioxide.

[Explanation of symbols]

 101 urea synthesis tube 102 stripper 103 carbamate condenser 4 centrifugal compressor (for mixed gas) 105 compressor (for carbon dioxide) 21 casing 22 rotor 23 impeller 24 dry gas seal rotating ring 25 dry gas seal stationary ring 26 bearing 27 buffer Gas inlet port 28 Intake gas 29 Discharge gas 30 Filter 35 Heat exchanger 40 Floating seal ring (gas side) 41 Floating seal ring (atmosphere side) 42 Seal oil supply port 43, 50 Seal chamber pressure line 44 Seal oil head tank 45 Level control valve

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] October 12, 1995

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Fig. 4

[Correction method] Change

[Correction content]

[Figure 4]

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 produce 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, When the condensate or gas-liquid mixture is circulated to the urea synthesis zone, while urea is obtained from the urea aqueous solution obtained in the separation zone, the mixed gas from the separation zone is used as a shaft seal device for a dry gas seal or oil. Characterized by increasing the pressure by a centrifugal compressor equipped with a film type shaft seal and introducing it into the condensation zone Urea synthesis method of. 2. The method according to claim 1, wherein the centrifugal compressor has a structure in which a part of carbon dioxide introduced into the separation zone is introduced as a buffer gas into a dry gas seal shaft sealing device. 3. When the centrifugal compressor is a centrifugal compressor equipped with an oil film type shaft seal device, a part of carbon dioxide introduced into the separation zone is further added to the casing of the centrifugal compressor. The method according to claim 1, which has a structure introduced into a labyrinth seal portion between the shaft and the shaft. 4. The method according to claim 3, wherein the carbon dioxide is heated and introduced.