KR790001171B1 - Method of recovering unreacted materials and heat in urea synthesis - Google Patents

Abstract

Ammonia and carbon dioxide were reacted under urea synthesis conditions to obtain effluent contg. urea, unreacted ammonium carbamate, an excess of ammonia and water. Effluent pressure was reduced in two stages, and decomposed to gaseous mixts. ammonia, carbon dioxide. Contacting each gaseous mixt. with an absorbent under decomposition pressure, recycling the obtained absorbate to the urea synthesis zone, Ammonia was separated by stripping with water vapor.

Description

Recovery method of unreacted substance and heat in urea synthesis

The figure shows a flow sheet showing one sphere of the method of the present invention.

The present invention relates to a method for recovering unreacted ammonia and carbon dioxide contained in urea synthesis liquid in synthesizing urea from carbon dioxide and ammonia, and more specifically, the present invention relates to heat recovery steam in urea synthesis. The present invention relates to a method for efficiently recovering ammonia from condensed water vapor that has evaporated upon concentration of an aqueous urea solution and advantageously performing heat recovery.

In the urea synthesis by the so-called solution complete circulation method, urea is synthesized by reacting ammonia and carbon dioxide under high temperature and high pressure, and the urea synthesis liquid containing unreacted ammonia and carbon dioxide is sequentially depressurized to obtain a plurality of steps. By stripping or distillation, the unreacted product is a mixed gas of ammonia carbon dioxide and water vapor, separated from this urea synthesis liquid, and at about the same pressure as the pressure in each step, the mixed gas is lowered at a lower pressure stage. The absorbent liquid to be introduced is absorbed, and the absorbent liquid is used as the absorbent liquid in a higher pressure step.

In this way, the mixed gas separated in each step is absorbed sequentially, and the absorbing liquid which exited the absorption zone of the highest pressure step is boosted by urea synthesis pressure, and circulated through the urea synthesis zone.

In the separation zone of each pressure step, in order to decompose unreacted ammonium carbamate and separate it as ammonia and carbon dioxide, the urea synthesis liquid is indirectly heated by steam or the like, but the amount of heat required for heating It is a big problem to occupy most of the calories required for synthesis, and to reduce any reasonably.

The decomposition reaction of unreacted ammonium carbamate in the urea synthesis liquid is a large endothermic reaction, and when the mixed gas containing ammonia and carbon dioxide separated by decomposing the ammonium carbamate is absorbed into the absorbent liquid, the heat of absorption is largely reversed. Emits.

In order to recover this heat of absorption, it is advantageous to perform absorption under the high temperature and high pressure possible in each absorption stage.

In the case of recovering the heat of absorption in the form of steam, in order to effectively use the steam, heat recovery is preferably performed as steam of high quality and high pressure.

For this reason, conventionally, the heat of absorption of the high-pressure decomposition step is introduced into the urea synthesis region as the sensible heat of the recovered ammonia carbamate solution which is circulated to the urea synthesis region without heat recovery from the absorption stage. As shown in Korean Patent Application Publication No. 49-102620, Korean Patent Application No. 74-1167, and Taiwan Patent Application No. 6310196, the heat generated in the high pressure absorption zone was boosted to the urea synthesis pressure by a heat exchanger installed in the high pressure absorption zone. 5 kg / cm 2 of excess heat is transferred to the urea synthesis temperature by transferring to a high pressure liquid ammonia, introducing it into the urea synthesis zone, and installing a heat exchanger in a urea synthesis zone or a mixer installed at its front end. Efforts have been made to recover as high-quality steam of (gauge) or higher pressure.

These methods are advantageous for obtaining steam at high temperature and high pressure, but since the heat exchange is performed under high temperature and high pressure in the urea synthesis zone and under intense corrosive environment, the heat exchanger requires high pressure resistance, heat resistance, and corrosion resistance, and thus the construction cost is very high. It becomes many and becomes disadvantageous in this point.

On the other hand, with the increase in the size of the urea synthesis apparatus, there is no big problem in the past.

The small amount of ammonia discharged along with the condensate of the vapor evaporated in the urea condensation process was greatly closed.

That is, in the conventional finishing process, a small amount of ammonia and carbon dioxide remaining in the urea aqueous solution cannot be recovered economically and disposed of together with the drainage.

In the large-scale urea apparatus, this discarded ammonia and carbon dioxide are insignificantly lost, and in addition to the extremely low concentration of ammonia contained in the drainage, in the large-scale apparatus, the total discharge amount is large. Due to the rich nutrition of the ocean, which is the cause of environmental pollution, it was desirable to develop an improved ammonia recovery method that can prevent the loss of valuable ammonia and solve the pollution problem at the same time.

A method for separating and recovering ammonia and carbon dioxide, which is used for regeneration of water containing ammonia and carbon dioxide, wherein the aqueous solution is subjected to rectification or by stripping with water vapor, thereby ammonia, carbon dioxide and water vapor. It was proposed to separate a mixed gas consisting of the two, and to absorb and recover the mixed gas together with the discharge gas from the low pressure decomposition of the unreacted ammonium carbamate.

However, in this conventional method, the amount of heat of steam used for heating or stripping at the time of rectifying becomes lost when cooling and absorbing this mixed gas, and it cannot be effectively used, and also the heat of condensation of water vapor is removed. In order to do that, you need extra coolant.

Moreover, since the amount of water in the mixed gas is large, the absorbent liquid obtained by the absorption of the mixed gas is excessively diluted and introduced into the urea synthesis system, resulting in a disadvantage in that the urea synthesis rate is lowered. In Application 75-2429, a method is proposed in which a thin aqueous solution containing ammonia and carbon dioxide is added to rectification, introduced into a decomposition zone, and ammonia and carbon dioxide are recovered, and the amount of heat required for rectification is recovered. The present invention seeks to provide a method for carrying out these unreacted substances and heat recovery more advantageously.

On the other hand, conventionally, the heating of the low-pressure decomposition zone of the unreacted ammonium carbamate in urea synthesis has been performed by heating by indirect heating by steam using a heater installed in this decomposition zone, but indirect heat exchange In the case of heating by steam, steam at a pressure of 5 kg / cm 2 (gauge) or more is required, and low pressure steam cannot be used.

Patent Publication No. 45-15015 discloses a method of directly blowing, thermally decomposing and stripping an unreacted ammonium carbamate in urea synthesis at a low pressure decomposition zone, but when concentrating the urea solution as in the prior art. However, if the dilute aqueous solution containing ammonia and carbon dioxide, which is discharged during the concentration of urea aqueous solution, is discarded as it is, unreacted ammonium carbamate which has not been separated and recovered in this low pressure decomposition zone is lost. Therefore, if it is going to separate almost all of ammonia and carbon dioxide by steam stripping in the low pressure decomposition zone, the urea synthesis liquid containing no ammonia and carbon dioxide comes into contact with high temperature water vapor near the bottom of the decomposition tower. Since hydrolysis of urea takes place in this part, this method suggests that low pressure steam can be used. Despite the gateum and, in fact, it not practiced.

An object of the present invention is to provide a method for efficiently and economically recovering unreacted ammonia and carbon dioxide discharged from a urea concentration process.

It is another object of the present invention to provide a method for removing ammonia in wastewater discharged from a urea concentration process and solving environmental pollution problems.

Still another object of the present invention is to provide a method for efficiently recovering heat required for separating and recovering unreacted material from urea synthesis liquid.

It is still another object of the present invention to provide a heat recovery method using an inexpensive device having a low construction cost in urea synthesis.

In order to achieve the above object, the present inventors conducted a detailed study and found a method of recovering ammonia in wastewater by steam stripping and recovering calories at the same time from the following facts.

When no high pressure steam is required depending on the use of the heat recovery steam, for example, a process of recovering lean ammonia water generated in the concentration process of urea solution by steam stripping or low pressure decomposition of unreacted ammonium carbamate in urea synthesis solution In the case of use in a step or the like, by heat recovery of the heat of absorption of unreacted ammonia and carbon dioxide in each absorption step, it is advantageous to provide a steam having a thermal power suitable for the purpose.

Ammonia and carbon dioxide in the drainage of the urea solution concentration can be almost completely separated by storytelling by low pressure steam, in which case it is advantageous that no heater is required.

The heating of the low pressure decomposition zone of the ammonium carbamate in the urea synthesis liquid is advantageous to use low pressure steam rather than indirect heating by heat exchange, and to use a low pressure steam, and without the need to install a heater. If stripping is stopped at the stage where small amounts of ammonia remain without completely separating the ammonia, hydrolysis of urea can be ignored.

The present inventors have summed up these facts and invented the method of recovering the unreacted substance in the urea synthesis method advantageously and performing heat recovery efficiently by the sophisticated steam constitution method of the high pressure system and the low pressure system in the urea synthesis method.

That is, according to the method of the present invention, ammonia and carbon dioxide are reacted under urea synthesis conditions to produce a urea synthesis liquid containing urea, unreacted ammonia carbamate, excess ammonia, and water. Sequentially depressurize, decompose the unreacted ammonium carbamate, separate the mixed gas of ammonia and carbon dioxide in each decomposition step, and mix the gas separated in each decomposition step with substantially equal isostatic pressure The sorbent solution was sequentially absorbed into the absorbent medium, and the absorbent liquid containing the obtained ammonia carbamate was circulated in the urea synthesis zone, while the urea aqueous solution containing the smaller amount of the ammonium carbamate obtained in the final decomposition step was concentrated to obtain a concentrated aqueous solution. Condensed water containing a small amount of ammonia obtained by condensing the vaporized water vapor at In the urea synthesis method for separating and recovering ammonia in the condensed water, the decomposition gas at the highest pressure stage, if necessary, is introduced into the urea synthesis zone, The mixed gas is absorbed into the absorbent liquid obtained by absorbing the absorbent medium, and the absorbed heat released at this time is recovered as steam having a pressure of 2-4 kg / cm 2 by indirect heat exchange, and the steam is ammonia in a small amount of ammonia and dioxidized. Substantially all of the carbon is built up and decomposed, and the resulting ammonia and carbon dioxide-containing water vapor is introduced into the rectifying zone of the final decomposition step to directly heat the urea synthesis liquid, Ammonia and carbon dioxide contained in this steam are recovered together with the mixed gas. It is an unreacted substance and a heat recovery method in urea synthesis characterized by combining an unrecovered recovery method and a heat recovery method.

In the present invention, decomposition of the unreacted ammonium carbamate contained in the urea synthesis liquid in the urea synthesis pipe is at least one stage of high pressure decomposition at a pressure equal to 15 kg / cm 2 (gauge) -urea synthesis pressure. Step and a low pressure decomposition step of 1-4 kg / cm 2 (gauge).

High-pressure decomposition of unreacted ammonium carbamate may be stripping by carbon dioxide or ammonia gas at a pressure substantially the same as the urea synthesis pressure, or by high-pressure rectification at 15-25 kg / cm 2 (gauge). The decomposing step of unreacted ammonium carbamate by decompression only to 40-100 kg / cm 2 (gauge) may be followed by a medium pressure rectification step of 15-25 kg / cm 2 (gauge).

In the present invention, the ammonia stripper is preferably organized at about the same pressure as the low-pressure decomposition zone, and is operated at 2-4 kg / cm 2 (gauge), and the steam blown into the ammonia stripper has a pressure almost equal to the operating pressure of the ammonia stripper. It is advantageous to use steam.

Therefore, low pressure steam of 2-4 kg / cm <2> (gauge) is sufficient as the steam blown into an ammonia stripper, and it is possible to effectively use the heat recovery steam in a high pressure absorption zone. The amount of this steam is blown into a low pressure distillation column via an ammonia stripper, and an amount sufficient to heat the low pressure distillation column is used, and 0.05-0.15 kg of steam is used per kg of urea production.

In the ammonia stripper substantially all of the ammonia and carbon dioxide in the waste water are separated.

Some of the blown steam is consumed for heating the ammonia stripper, condenses and is discharged with drainage from the ammonia stripper, and most of the steam is mixed gas with separated ammonia and carbon dioxide as it is in the distillation column of the low pressure decomposition stage. Blown into the heating source.

When urea synthesis liquid containing unreacted ammonium carbamate is stripped by steam, carbon dioxide is relatively easily separated from ammonia and carbon dioxide produced by decomposition of ammonia carbamate, but the ammonia is completely separated. In order to do this, a high temperature is required, and hydrolysis by water vapor of urea becomes remarkable. If steam stripping is stopped with a small amount of ammonia remaining, hydrolysis of urea is negligible.

Therefore, in the present invention, steam is introduced into the stop of the distillation column in the low pressure decomposition step, steam stripping is performed in the upper half of the distillation column, and the urea synthesis liquid is heated to 125-145 ° C., and a small amount of ammonia remains. It is preferable to strip most of the residual ammonia by stripping the urea synthesis liquid to be stripped with carbon dioxide blown from the bottom of the distillation column.

In this case, the amount of carbon dioxide blown into the bottom of the low pressure distillation column tower is 0.01-0.2 kg mole per 1 kg mole of urea production, and this carbon dioxide is discharged as a mixed gas of ammonia, carbon dioxide, and water vapor in the column of the low pressure column. Since the absorbed liquid is absorbed into the absorbent liquid in the low pressure absorption zone, the ammonia / 2 carbon oxide ratio in the absorbed zone decreases, so that the water circulated in the urea synthesis zone can be reduced by reducing the absorbed liquid.

Instead of introducing carbon dioxide into the low pressure decomposition zone as described above, the urea synthesis liquid is taken out from a low pressure distillation column into a sieve containing a small amount of unreacted ammonium carbamate, and the synthesis liquid is flushed to almost atmospheric pressure. It is also possible to remove most of this unreacted ammonium carbamate.

According to the method of the present invention, most of the steam used for stripping the lean ammonia water is directly blown directly into the distillation column of the low-pressure decomposition stage together with the separated ammonia and carbon dioxide, and can be advantageously used as the heat source.

Since most of the steam blown into the low pressure distillation column is condensed in the distillation column, discharged as urea solution from the bottom of the column, and evaporated in a subsequent urea solution concentration step, there is no fear of increasing the amount of water circulating in the entire urea synthesis system.

As a result of heat recovery of low pressure steam blown into the ammonia stripper in the high pressure absorption zone, as compared with the method described in the above-mentioned patent publication (S) 49-102620, since the high pressure liquid ammonia does not pass through the heat exchanger of the high pressure absorber, the internal pressure is at least It is easy to manufacture, and construction cost is also reduced.

In addition, as described above, the absorbent liquid containing ammonium carbamate circulating the heat of absorption in the high pressure absorption zone into the urea synthesis zone or the high temperature and high pressure of the urea synthesis zone by introducing into the urea synthesis zone as the sensible heat of liquid ammonia. According to the method of the present invention, the amount of steam recovered in the urea synthesis zone is reduced according to the method of the present invention, so that the required heat transfer area of the heat exchanger is reduced, and even a small heat exchanger is satisfied. The construction cost of an expensive heat exchanger used under high pressure is reduced.

When low pressure steam of 2-4 kg / cm 2 (gauge) can be effectively used as in the present invention, high-pressure steam of 5 kg / cm 2 (gauge) or more is generated by an expensive heat exchanger, and a part of the pressure is reduced. Using it becomes uneconomical.

When generating steam according to the required pressure as in the present invention, the construction cost is lowered and economical.

Furthermore, in the present invention, a heat exchanger for heating the low pressure distillation column becomes unnecessary, and the hanging facilities are reduced by that amount.

Next, in order to deepen the understanding of the present invention, an example of the method of the present invention will be described with reference to the drawings.

In the urea synthesis pipe (1), a urea, ammonia, carbon dioxide, water, and a burette obtained by synthesizing urea at a pressure of 200 to 260 kg / cm 2 (gauge) and a temperature of 180 to 210 ° C is lined up. Via (2), the pressure was reduced to 30-120 kg / cm 2 (gauge) through the pressure reducing valve 3 and introduced into the high pressure separator 5 via the line 4.

In the high pressure separator 5, unreacted ammonia and carbon dioxide are separated by a push, but unreacted materials may be separated by heat distillation.

The mixed gas containing ammonia, carbon dioxide and water vapor flowing out of the high pressure separator 5 is introduced into the high pressure absorber 57 held at approximately the same pressure via the line 58.

The urea synthesis liquid at the bottom of the high pressure separator (5) is introduced into the medium pressure distillation column (9) after being decompressed to 10-20 kg / cm 2 (gauge) via the line 6, the pressure reducing valve 7, and the line 8. .

A heater 10 is provided at the bottom of the central distillation column 9 and heated by high pressure steam of 10 kg / cm 2 (gauge), and most of the unreacted ammonium carbamate in the urea synthesis liquid is ammonia and carbon dioxide. And as an exhaust gas of water vapor. The discharge gas from the medium pressure distillation column 9 is introduced into the medium pressure absorption tower 46 via the line 48.

The urea synthesis liquid from the bottom of the middle distillation column also contains up to 10% of unreacted ammonium carbamate, but is decompressed to 1.5-3.5 kg / cm 2 via line 11, pressure reducing valve 12, and line 13 Is introduced into the column top of the low pressure distillation column (14).

In the middle of the low pressure distillation column 14, steam of 2-4 kg / cm 2 (gauge) containing ammonia and carbon dioxide in an ammonia stripper described later is blown from the line 37 and heated directly. In the bottom of this distillation column, a part of carbon dioxide of the urea synthesis raw material is blown in the ratio of 0.01-0.2 kgmol / kgmol urea production amount via the line 71.

Unreacted ammonium carbamate is decomposed by stripping with carbon dioxide in the lower half of the low pressure distillation column 14 and stripping with a mixed gas of steam and carbon dioxide in the upper half, and most of the ammonia and carbon dioxide are at the top of the column. Is separated from. The low pressure distillation column 14 is held at 120-145 ° C in the 100-120 ° C steam blowing stage in the top.

The urea synthesis liquid from the bottom of the low pressure column also contains a very small amount of ammonia and carbon dioxide. This urea synthesis liquid is introduced into the vacuum concentrator 18 and concentrated through the line 15, the reduced pressure side 16, and the line 17. At this time, the vaporized water vapor is introduced into the surface condenser 23 via the line 22 and condensed entirely. First of all, the introduction of the water vapor surface capacitor 23 in the line 22 may be carried out via a suitable separator such as a cyclone separator equipped with a cooling jacket, whereby the droplet of the urea solution may be separated.

The condensed water in the surface capacitor 23 contains 2 wt% or less of ammonia and 1 wt% or less of carbon dioxide. This condensate is boosted to a pressure of 1.5-3.5 kg / cm &lt; 2 &gt; (gauge) in the pump 30 via the line 29, and is passed through the line 31, the heat exchanger 32, and the line 33 to form an ammonia stripper ( 34) is introduced.

The top bottom of the ammonia stripper 34 has a ratio of 0.05-0.15 kg per kg of urea production through a line 60 and a low pressure steam of 2-4 kg / cm 2 (gauge) obtained by heat recovery in the high pressure absorber 57. It blows in, and substantially all of ammonia and carbon dioxide in this condensate are separated, and it is recovered as a mixed gas of steam from the top of the ammonia stripper 34.

The mixed bath is blown into the stop of the low pressure distillation column via the line 37, and the carbon dioxide is recovered and used as a heat source of the low pressure distillation column.

At the bottom of the ammonia stripper, the concentrations of ammonia and carbon dioxide remaining in the drain discharged through the line 35, the heat exchanger 32, and the line 36 are 10-20 ppm and 20-50 ppm, respectively. Without fear, it is possible to discard it as it is.

The urea aqueous solution concentrated in the electroconcentrator 18 is sent to the crystal bath 19 to precipitate urea crystals and become urea slurry. The urea crystal in this slurry is separated from the mother liquor in the centrifuge 20 and sent to the subsequent finishing process via the line 21.

On the other hand, the mother liquor consisting of the urea aqueous solution separated by the centrifuge 20 is boosted by the pump 41 via the line 40, introduced into the low pressure absorber 39 via the line 42, and the low pressure distillation column 14. ) Absorbs substantially all of the mixed gas consisting of ammonia, carbon dioxide and water vapor introduced via line 38 at the top of the column, and then is boosted to pump 44 via line 43 and line 45. Via, it is sent to the middle pressure absorption tower tower top as the absorption liquid of the medium pressure absorption tower 46.

Most of the exhaust gas containing ammonia and carbon dioxide in the middle distillation column 9 is absorbed by the medium pressure absorption tower 46, and the unabsorbed ammonia from the top of the medium pressure absorption tower tower passes through the line 49 to form an ammonia capacitor. Cooled to 50, boosted to pump 52 via line 51, and circulated to heat recovery 64 via line 53. The absorbent liquid from the bottom of the central absorption tower is pumped via line 54, pump 55, line 56, introduced into high pressure absorber 57, ammonia from the high pressure separator 5, and deoxidation. Absorbs exhaust gases consisting of carbon and water vapor.

The heat of absorption discharged | emitted at this time is collect | recovered as steam of the pressure of 2-4 kg / cm <2> (gauge) by the heat exchanger installed in this high pressure absorber, this absorption zone is cooled, and it absorbs substantially all of this discharge gas.

The temperature of the high pressure absorber is held at 120-170 ° C., preferably 130-160 ° C.

The steam recovered by the high pressure absorber 57 is blown into the ammonia stripper 34 via the line 60.

The absorbent liquid containing urea, ammonia and carbon dioxide from the high pressure absorber 57 is boosted to the pump 64 via the line 61 and introduced into the heat recovery 64 via the line 63. Fresh carbon dioxide introduced from line 67 is compressed in a carbon dioxide gas compressor 68, fed to heat recovery 64 via line 69, mixed with fresh and recovered ammonia from line 53 and Is fed to the urea composite pipe 1 via the line 65. The heat recovery unit 64 is operated at about the same pressure as the urea synthesis pipe, and is heated at the same time as the heat generated during the production of ammonium carbamate. Furthermore, the heat generated by the excess steam is about 5 kg / cm 2 (gauge) or more. As absorbed in line 66.

In this embodiment, if there is a use of 2-4 kg / cm 2 (gauge) of steam in addition to the ammonia stripper, there is a portion of the carbon dioxide that is fed to the heat recovery 64 via the line 70 It is possible to introduce into the high pressure absorber 57, to increase the amount of heat generated in the high pressure absorber, and to increase the number of low pressure recovery teams.

In this case, it is possible to reduce the heat exchanger of the heat recovery device 64, and furthermore, to eliminate the heat recovery device in some cases, and the construction cost decreases.

Furthermore, the carbon dioxide compression power can be reduced.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The urea synthesis solution consisting of urea 1154, ammonia 1166, carbon dioxide 328, water 528, and burette 3.5 kg / hr, was carried out at 195 ° C. and operated at 230 kg / cm 2 (gauge). ) Was pushed through a high pressure separator 5 operated at a pressure of 95 kg / cm 2 (gauge) via a pressure reducing valve 3 and a line 4.

Here, the discharge gas which consists of ammonia 263, carbon dioxide 42, and 13 kg / hr of water was isolate | separated.

From the bottom of the high pressure separator 5, a urea synthesis solution composed of urea 1148, ammonia 906, carbon dioxide 290, water 513, and burette 3.7 kg / hr was taken out, and the line 6, the pressure reducing valve 7, and Via line (8), it is sent to a medium pressure distillation column (9) operated at a pressure of 17 kg / cm &lt; 2 &gt; (gauge), and most of ammonia and carbon dioxide are evaporated and separated from the urea synthesis liquid, and then the urea synthesis liquid is 11), the pressure reducing valve 12 and the line 13 were fed from the tower top into a low pressure distillation column 14 operated at a pressure of 2.3 kg / cm 2 (gauge), and the residual ammonia and carbon dioxide remaining in the urea synthesis liquid. Most were distilled off.

The medium pressure distillation column is indirectly heated by a high pressure steam having a gauge pressure of 10 kg / cm 2 with the heater 10 at the bottom of the column, and the low pressure distillation column does not use a heater and steam containing ammonia and carbon dioxide recovered from the ammonia stripper described below. Was heated by blowing directly into the low pressure distillation column.

In the lower part of the steam inlet part of the low pressure distillation column 14, the charging part was installed in the lower part, and carbon dioxide was blown in in the tower bottom. The government of the low pressure distillation tower was 110 ° C and 120 ° C. Steam distillation was carried out in the upper half of the low pressure distillation column, stripped with dihydrocarbon at the top half, and a small amount of ammonia and dicyclic carbon remained in the urea synthesis liquid taken out from the bottom.

This synthetic liquid is introduced into the vacuum concentrator 18 via the line 15, the pressure reducing valve 16, and the line 17, and evaporates ammonia and hydrocarbon dioxide slightly dissolved in water at a pressure of 80 mmHg and a temperature of 60 ° C. The concentrate was sent to a crystallizer 19, and urea was crystallized and separated from the mother liquor by a centrifuge 20. Urea crystal was put into the assembly process (not shown) via the line 21.

Water vapor containing a small amount of ammonia and carbon dioxide evaporated in a vacuum condenser is introduced into the surface capacitor 23 via the line 22, supplied through the line 24, and discharged through the line 26. It cooled by indirect heat exchange with and condensed substantially all. The vacuum concentrator 18 and the surface capacitor 23 were connected to the steam ejector 27 driven by the high pressure steam supplied from the line 28 via the line 26, and were held under reduced pressure.

The condensate containing 0.57 wt% of ammonia, 0.2 wt% of carbon dioxide, and 0.09 wt% of urea obtained from the surface capacitor 23 is boosted to the pump 30 via the line 29, and furthermore, in the heat exchanger 32. The ammonia stripper 34 operated by heat exchange with the wastewater from the column bottom of the ammonia stripper 34 and heated up at a pressure of 2.5 kg / cm 2 (gauge) was fed. The ammonia stripper was also blown directly by heating the heat recovery steam mentioned later, without providing a heater.

Substantially all of the ammonia and carbon dioxide recovered from the surface condenser 23 together with the water tank of 53 kg / hr at the top of the ammonia stripper 34 is discharged as a discharge point, and the mixed gas is discharged. Through (37), the low pressure distillation column was introduced as a heating source.

Only 20 ppm of ammonia, 12 ppm of carbon dioxide, and 20 ppm of urea remained in the wastewater discharged through the line 35 from the bottom of the ammonia stripper 34.

On the other hand, the mother liquor separated by the centrifugal separator 2 is introduced into the low pressure absorber 39 operated at a pressure of 2 kg / cm 2 through the line 40, the pump 41, and the line 42, and the mother liquor allows the At the top of the low pressure distillation column 14, a mixed gas of ammonia, carbon dioxide and water vapor introduced through the line 38 was absorbed.

The absorbent liquid thus produced was then pumped up to a medium pressure absorption tower 46 operated at a pressure of 16.5 kg / cm 2 (gauge), and the mixed discharge gas of ammonia, carbon dioxide, and water vapor in the previous tower of the medium pressure distillation tower was lined. Into the bottom of the medium pressure absorption tower from (48), it was made to make a countercurrent contact with the absorbing liquid, and the carbon dioxide, all of water vapor, and a part of ammonia were absorbed.

In the vacuum concentrator 18, a part of the urea is circulated to the heat exchanger 47 installed at the bottom of the medium pressure absorption tower 46 (not shown), and the heat of absorption in the medium pressure absorption tower is concentrated for concentrating the urea solution. Heat recovery was performed as a circle.

In the medium pressure absorption tower, unabsorbed ammonia was introduced into the ammonia capacitor 50 through the line 49 at the top of the medium pressure absorption tower, and condensed into liquid by water cooling.

This recovered liquid ammonia is mixed with fresh ammonia in line 72 via line 51, boosted to pump urea synthesis by pump 52, and transferred to urea synthesis pipe via line 53 (64). ) At a rate of 113 kg / hr. The absorbing liquid produced in the bottom of the medium pressure absorption tower was urea 114, ammonia 380, carbon dioxide 342, water 203, and burette 6 kg / hr, and reached the temperature of 103 degreeC. This absorbing liquid is fed to a high pressure absorber 57 operated at a temperature of 150 ° C. and a pressure of 94.5 kg / cm 2 (gauge) via a line 54, a pump 55, and a line 56, and at the same time, the high pressure separator 5 described above. A mixed gas of ammonia 263, carbon dioxide 42, and water vapor 13 kg / hr separated into a high pressure absorber 57 was introduced through a line 58 to be absorbed almost completely by an absorbent app.

At a small column bottom installed in the absorber, 1 kg / hr of ammonia was discharged together with the inert gas, which was reduced in pressure and introduced into the medium pressure absorption tower 46 through the line 59.

A saturated steam having a gauge pressure of 2.5 kg / cm 2 was generated by a high pressure absorber, the heat of absorption was removed, and recovered.

The steam recovery was 73 kg / hr.

This low pressure steam was sent to the ammonia stripper 34 described above through the line 60, of which 20 kg / hr was consumed in the ammonia stripper and condensed.

The remaining 53 kg / hr of steam was introduced into the low pressure distillation column 14 described above via the line 37.

The absorbent liquid produced by the high pressure absorber 57 is composed of urea 114, ammonia 642, carbon dioxide 384, water 216, and burette 6 kg / hr. The pressure was increased to the pressure and introduced into the heat recovery device 64 operated at a pressure of 230 kg / cm 2 (gauge) and a temperature of 180 ° C. via the line 63.

At the same time, the raw material carbon dioxide was boosted by the carbonic acid gas compressor 68 and fed to the heat recovery unit 64 at a rate of 705 kg / hr through the line 69.

In the heat recovery device 64, the absorbent liquid was heated up by the exotherm during the production of ammonium carbamate, and the resulting high temperature ammonium carbamate solution was introduced into the urea synthesis pipe via the line 65. Since the outlet temperature of the urea synthesis pipe was kept at 195 ° C., the excess calorific value of the heat recoverer 64 was recovered from the line 66 as a saturation steam at a pressure of 5 kg / cm 2 (gauge). The amount of steam generated reached 67 kg / hr.

This steam can be used as a heat source for melters for urea requiring 5 kg / cm 2 (gauge) steam or for other uses.

That is, it was possible to recover 140 kg / hr of steam together with the high pressure absorber and the heat recoverer.

Claims (1)

Ammonia carbon dioxide was reacted under urea synthesis to produce urea unreacted ammonium carbamate, excess ammonia, and water, followed by urea synthesis solution. Carbamate is decomposed, and the mixed gas of ammonia and carbon dioxide in each decomposition step is separated, and the mixed gas separated in each decomposition step is sequentially absorbed by the absorbent medium under this decomposition pressure and substantially the same pressure. The obtained absorbent liquid containing ammonium carbamate was circulated to the urea synthesis zone, while the concentrated urea aqueous solution containing a small amount of ammonium carbamate obtained in the final decomposition step was concentrated to obtain a rich aqueous solution. The condensate containing a small amount of ammonia obtained by condensation is stripped with water vapor, and the hairs in the condensate In the urea synthesis method for separating and recovering ah, a cracked gas at the highest pressure stage is optionally mixed with a portion of the raw material carbon dioxide to be introduced into the urea synthesis zone, and the mixed gas from the next decomposition stage is absorbed into the absorbent medium. Absorbed in the absorbent liquid obtained by absorption, the heat of absorption discharged at this time is recovered as steam of 2-4 kg / cm <2> by indirect heat exchange, and this steam is made to contact this condensate containing a small amount of ammonia and carbon dioxide, In addition, substantially all of the ammonia and carbon dioxide in the condensate are constructed and separated, and the resulting ammonia and carbon dioxide containing water vapor is introduced into the rectifying zone of the final decomposition step, and the urea synthesis liquid is directly heated. Recovery of the unreacted material which recovers ammonia and carbon dioxide contained in this steam together with the mixed gas generated in this decomposition step. And the unreacted and heat recovery method in a urea synthesis, characterized in that a combination of heat recovery method.

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