KR790001193B1 - Process for separating and recovering unreacted materials in urea synthesis - Google Patents

Abstract

CO2 and NH3 were reacted under urea synthesis temp. and pressure to obtain effluent contg. urea, an excess of NH3, ammonium carbamate, and water, subjecting effluent to two decomposition stages a high pressure(above 10kg/cm2) and a low pressure stage (below 5kg/cm2). Separating from effluent to gaseous mixts- -NH3, CO2, water, and contacting each mixt with absorbent, and recycling the obtained absorbate to the urea synthesis zone.

Description

Separation and Recovery Method of Unreacted Material in Urea Synthesis

1 is a system diagram showing one embodiment of the method of the present invention,

2 is a system diagram showing an example of one embodiment of the present invention method in the case of using stripping with carbon dioxide in combination.

The present invention relates to a method for recovering unreacted materials when synthesizing urea with carbon dioxide and ammonia. In detail, the present invention provides a method for recovering unreacted materials from urea synthesis liquid. The present invention relates to a method for improving the method of reducing the moisture accompanying ammonia and carbon dioxide.

The ammonia and carbon dioxide are reacted at high temperature and high pressure, the obtained urea synthesis liquid is depressurized sequentially, and stripping, stripping or distillation are carried out in a plurality of stages. The mixture was separated as a mixed gas, and the unreacted substance flowing out in the low pressure distillation was absorbed into the absorbent liquid to obtain an absorbent liquid. The absorbent liquid was then sequentially boosted and used as an absorbent liquid of the mixed gas separated from the separation zone of the higher pressure step. The solution circulating urea synthesis method which absorbs the mixed gas separated in each stage and circulates the absorption liquid which finally came out of the absorption zone of the highest pressure stage to the urea synthesis zone is well known.

The urea synthesis reaction, in which ammonia and carbon dioxide react to form urea and water, is a parallel reaction. The higher the water content in the urea synthesis system, the lower the rate of urea synthesis. It is necessary to make the moisture in the water as small as possible.

To this end, it is preferable to absorb the small amount of absorbent liquid as high as possible in each absorption stage and to reduce the amount of water evaporated according to the mixed gas of the unreacted substance separated in each separation stage, and to prevent the absorption liquid from diluting. .

The highest amount of water contained in the mixed gas of the unreacted material separated in the low pressure separation zone operated at a pressure of 0-5 kg / cm 2 (gauge) among the water in the unreacted material separated in the separation zone of each pressure stage, It becomes a problem.

In particular, in order to completely separate the unreacted material in the low pressure stage, when carbon dioxide is introduced from the bottom of the separation zone in the low pressure stage to form the unreacted material, only water that evaporates accompanying the carbon dioxide is separated. Since the amount of water in this mixed gas increases, a means for preventing this is necessary.

In order to solve these problems, Patent Publication No. 49-19,259 (USP 3,725,210) (Taiwan Patent No. 6,909) discloses the temperature at the top of the rectifying region at the low pressure stage at a pressure of 0-5 kg / cm 2 (gauge). Is proposed at 60-120 ° C. and a method of heating the bottom temperature to 100-140 ° C., but the present invention provides a method for improving the same.

By holding the government temperature of this rectification zone at 60-120 degreeC, it is possible to lower the vapor pressure of water and to reduce the moisture content of a mixed gas.

In order to lower the temperature of the government, in the conventional method, the mixed gas from the government of the rectification station is led to the reflux condenser, cooled to 50-100 ° C. to condense the water vapor to reflux to this rectification station. The amount of heat lost is a loss and increases the amount of heat required to heat this separation zone by that amount.

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

Another object of the present invention is to provide a method of increasing the urea synthesis rate by reducing the moisture in the unreacted mixed gas separated from the urea synthesis liquid.

Still another object of the present invention is to provide an effect of effectively using the calories of the urea synthesis liquid and reducing the calories required for the separation of unreacted materials.

In order to achieve the above object, the present inventors conducted a detailed study and found a method of reducing the moisture in the unreacted mixed gas separated in the low pressure step while effectively using the calories retained by the urea synthesis liquid. Carbon oxide and ammonia are reacted at urea synthesis pressure and temperature to produce urea synthesis liquid containing urea excess ammonia unreacted ammonium carbamate and water, and the urea synthesis liquid is subjected to at least 10 kg / cm 2 (gauge) Or an ammonium carbamate decomposition process of at least two stages of two or more stages of high pressure stage and a low pressure stage of 5 kg / cm 2 (gauge) or less, wherein ammonia, carbon dioxide and water vapor are separated in the separation zone of each pressure stage. The mixed gas is separated, the mixed gas is circulated to the urea synthesis zone, and at least one part of the separation zone of this low pressure stage is rectified. In some cases, urea synthesis from the separation zone of the high pressure stage in the solution circulation synthesis method in which the urea synthesis liquid exiting the rectification zone is introduced into a further construction zone, contacted with carbon dioxide, and stripping unreacted materials. The liquid was indirectly heat exchanged with the synthesis liquid of the urea at the bottom of the rectifying zone in the low pressure stage, cooled to 100-170 ° C., and then decompressed to the pressure of this low pressure stage to lead to the government of this rectifying zone. The bottom of this rectifying zone is heated by exchange, and the temperature of this rectifying station is held at 60-120 占 폚 and the temperature of the bottom at 100-140 占 폚.

When pressure of 10-170kg / cm2 (gauge) and 140-200 ° C of urea synthesis liquid is depressurized and flashed to 0-5kg / cm2 (gauge) at high pressure, the temperature is 60- When it lowers to 120 degreeC and introduces this into the government of this low pressure rectifier, it is possible to keep the temperature of the government of this low pressure rectifier low, without losing heat, and to reduce the water in the separated unreacted substance and mixed gas.

At this time, the excess heat amount of the high temperature urea synthesis liquid obtained from the high pressure separation step can be effectively used to heat the bottom of the low pressure rectification zone by indirect heat exchange, and the amount of steam required to heat the bottom of the low pressure rectification zone. Can reduce the amount.

The reason is that the amount of water vapor in the mixed gas separated in the low pressure separation stage decreases as compared with the case where the urea synthesis liquid in the high pressure separation zone is directly pushed in the low pressure separation stage, so that the latent heat of evaporation required for evaporation of water is reduced, This is because the amount of steam required to heat the bottom of the low-pressure rectifier is reduced.

In order to complete the separation of unreacted ammonia and carbon dioxide from the urea synthesis liquid in the low pressure stage, the low pressure stage is divided into a rectification zone and a construction station, and the urea synthesis liquid from the rectification zone It is preferable to introduce the carbon dioxide from the bottom of the construction zone, to construct residual unreacted material, and to introduce the mixed gas to the bottom of the rectification zone.

In this case, since the absolute amount of water in the mixed gas obtained only at the portion corresponding to the carbon dioxide used for construction increases in the rectification region, it is necessary to lower the government temperature of the rectification region as much as possible to reduce this increase. In the present invention, since the stationary temperature of the rectifying zone is maintained in the range of 60-120 ° C. depending on the distillation pressure, even if the unreacted ammonia and carbon dioxide are formed by carbon dioxide, the increase in moisture in the mixed gas obtained is low. It is possible to suppress.

Therefore, despite the increase in the amount of mixed gas, the absolute amount of evaporated water due to the mixed gas does not increase very much.

In the present invention, when unreacted ammonia and carbon dioxide are formed in the urea synthesis liquid provided with the construction station under the separation zone in the low pressure stage, carbon dioxide is most efficient as the gas introduced into the bottom of the construction station. The amount used is minimal, so that the amount of mixed gas for building carbon dioxide and unreacted materials in the low pressure separation zone is slightly increased, and the absolute amount of water vapor accompanying is slightly increased.

The carbon dioxide introduced to the construction part is absorbed and recovered by the absorption catalyst together with the unreacted ammonia and carbon dioxide separated in the rectification zone and the construction area in the low pressure absorption process.

At this time, the introduced carbon dioxide acts to reduce the equilibrium pressure in the low pressure absorption process, so that the amount of the absorption medium used in the low pressure absorption process can be reduced.

Carbon dioxide introduced into the construction zone is used one part of the raw material, the amount is preferably 0.01-0.2 mol, especially 0.02-0.1 mol per mol of urea introduced into the construction zone.

If the amount of carbon dioxide is more than 0.2 mole, the increase of the outflow is remarkable, and if it is less than 0.01 mole, the unreacted substance cannot be substantially completely constructed.

In the separation zone of the low pressure stage used in the present invention, it is composed of a rectifying zone, a heat exchange zone and a heating zone, and in some cases, also a construction zone, but these may be integrated or may be used individually. Even when the construction station is provided, it is preferable to use the rectifying station and the lower half as the construction station. The structure of the rectifying zone of the present invention is preferably a perforated plate or a seedbed shelf or a chopped end of a structure having a function corresponding thereto, but may be a filling part having the same function. .

As the construction zone, a charging section is used, and as a heating zone, a multi-tube heat exchanger, i.e., a single pass and reboiler or a thin film flow type heater, is used. Heated by the medium.

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

First, in FIG. 1, in the high pressure separation step, most of the unreacted ammonia and carbon dioxide are separated, and the urea synthesis liquid having a pressure of 10-170kg / cm2 (gauge) from the bottom of the high-pressure distillation tower and a temperature of 140-200 ° C is It is introduced into the heat exchanger 2 via the line 1. Heat exchanged with urea synthesis liquid from the bottom of the column in a heat exchanger and cooled to 100-170 ° C., preferably 120-150 ° C., 0-5 kg / to the decompression valve (4) via line (3). The pressure is reduced to 2 cm 2 (gauge), preferably 1.5-3.0 kg / cm 2 (gauge), and is pushed through the line 5 to the top of the low pressure distillation column 6. By this adiabatic expansion, the temperature of the urea synthesis region introduced into the government of the rectifying region 7 is 60-120 ° C.

In the rectification station, the urea synthesis liquid separating gas consisting of unreacted ammonia, carbon dioxide and water vapor flows down the rectification station (7) and reaches the bottom of the rectification station.

From this bottom of the rectifying zone, one part of the urea synthesis liquid is withdrawn via line (8), led to a heat exchanger (2) and heated to 100-140 ° C, preferably 115-135 ° C, and the line (9) It is circulated to the bottom of the rectification station.

Furthermore, one part of the urea synthesis liquid taken out through the line 9 is led to the reboiler 11 via the line 10 and is equally 100-140 ° C. by steam introduced from the line 15. It is heated to 115-135 ° C. and circulated through the line 12, 9 to the bottom of the rectifying station.

The urea synthesis liquid circulating from the bottom of the tower to the heat exchanger and reboiler may be passed in parallel to the heat exchanger and reboiler as shown in FIG. 1 or may be passed in series.

The urea synthesis liquid from the bottom of the rectifying zone is heated by the heat exchanger (2) and the reboiler (11), and most of the unreacted ammonia and carbon dioxide remaining in the urea synthesis liquid are separated as mixed gas, and urea synthesis is performed. The liquid enters the bottom of the rectification zone with the liquid, where it is separated from the urea synthesis liquid and rises in the rectification zone, where a part of the water vapor contained in the mixed gas condenses, subtracts moisture, and in the rectification station government, It is discharged via line 13 together with the mixed gas separated by flashing and led to a low pressure absorption zone. In the case of stripping the urea synthesis liquid at the bottom of the rectifying zone with carbon dioxide, as shown in FIG. 2, the urea synthesis liquid is guided to the building force 16, which is a packed column installed at the bottom of the distillation column 6. While maintaining this, the carbon dioxide introduced from the carbon dioxide introduction pipe 17 and the oil contact with the fragrance oil at the lower part of the construction area, substantially remaining completely separated from the remaining unreacted ammonia and carbon dioxide, Is sent to the subsequent urea solution concentration process discharged via line (14).

The mixed gas of carbon dioxide introduced into the construction station (16) and the ammonia and carbon dioxide separated from the construction station are led from the construction station to the bottom of the rectification station and separated from the reboiler (11) and the heat exchanger (2). It mixes with the mixing gas to raise the rectifying station and is discharged from the government via the line 13.

According to the present invention, before depressurizing to the low pressure of the urea synthesis liquid from the high pressure stage separation zone, if it is cooled by exchanging heat with the urea synthesis liquid at the bottom of the rectifying zone bottom of the low pressure stage separation zone, the pressure is reduced. Since the temperature lowers further, and it is possible to hold the temperature of the government of the rectifying zone at a desirable temperature, the amount of heat removed by the heat exchanger becomes a part of the heat source for heating the bottom of the rectifying region. The expansion of the urea synthesis liquid is used to transfer heat from the cold rectification zone to the hotter stationary bottom, and the amount of heat retained by the urea synthesis liquid is transferred from the rectification to the hotter stationary bottom. It is possible to effectively use the amount of heat held by the urea synthesis liquid as a heat source for rectification.

As a result, it is possible to reduce the amount of steam necessary for rectifying the urea synthesis liquid at low pressure.

In addition, a condenser for condensing and refluxing the mixed gas from the rectifier tower is unnecessary, and cooling water required for this is also unnecessary.

Furthermore, by keeping the temperature of the rectifier tower low, the amount of water vapor in the mixed gas of the unreacted material separated from the rectification zone is reduced, the amount of water circulated to the urea synthesis zone is reduced, and the urea synthesis rate in the urea synthesis zone is reduced. It is possible to raise.

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

Example 1

First, by the method of the present invention, urea 1,130, ammonia 107, dihydrocarbon 28 and water 447 kg /, taken out from a high pressure distillation column tower operated at a pressure of 19 kg / cm 2 (gauge) and a column bottom temperature of 180 ° C. Insulating and pure at the top of the low pressure distillation column operating the urea synthesis liquid consisting of hr at 2.7 kg / cm 2 (gauge) as it is, ammonia 73.8 carbon dioxide 25.4 and water 53.7 kg / hr are separated as a mixed gas. The temperature dropped to 120 ° C.

As a result, the liquid after the flush was composed of urea 1130, ammonia 33.2, carbon dioxide 2.6 and water 393.3 kg / hr. This liquid was subsequently heated to 130 ° C. with a reboiler.

The residual ammonia 2.0 and carbon dioxide in the urea synthesis liquid taken out from the bottom of the distillation column became 1.5 kg / hr, and the mixed gas obtained by combining the push gas and the gas separated from the distillation column and separated in the column top was ammonia. The composition was 105.0, carbon dioxide 26.5 and water 76.4 kg / hr.

At this time, the reboiler consumed 88 kg / hr of steam at 5 kg / cm 2 (gauge).

Next, according to the method of the present invention, the high pressure distillation column column bottom liquid is not directly decompressed, but is directly passed through observation of the multi-tubular heat exchanger, while the coaxial of the heat exchanger is subjected to heat exchange through the column bottom liquid of the low pressure column. Done.

As a result, the temperature of the high pressure urea synthesis liquid which improved the heat exchanger fell to 150 degreeC.

The urea synthesis liquid was flushed to the top of a low-pressure distillation column of 2.7 kg / cm 2 (gauge). Ammonia 56.3, 16.5 hydrocarbons, and 28.3 kg / hr of water were separated as a mixed gas, and the temperature of the liquid was lowered to 110 ° C. did. As a result, the liquid after the push was composed of urea 1130, ammonia 50.7, carbon dioxide 11.5 and 418.7.

In other words, by lowering the feed liquid temperature by 30 ° C by heat exchange, the evaporation of the water in the push was actually reduced to near 1/2. The liquid after the flush was distilled in a distillation column, and the temperature of the column bottom liquid was held at 130 ° C. as before by heating by the reheater and the newly added heat exchanger.

As a result, the residual ammonia in the column bottom liquid was 2.3 and carbon dioxide was 1.7 kg / hr, and the separation gas in the column top was 104.7, carbon dioxide 26.3 and water 52.8 kg / hr. Therefore, although the ammonia and carbon dioxide remaining in the column bottom liquid were approximately the same as foreplay, the total amount of water evaporated decreased to 70%.

At this time, the reboiler consumes 64 kg / hr of steam of 5 kg / cm 2 (gauge), which saves 24 kg / hr of steam compared to the above case which does not comply with the method of the present invention.

That is, the amount of heat consumed in the reboiler is usually reduced by the difference in the amount of water evaporated. Furthermore, as a ripple effect, the water in the absorbent liquid absorbing the separated mixed gas and circulating in the urea synthesis tube was reduced, resulting in an improvement in the rate of urea synthesis in the urea synthesis tube.

Example 2

In Example 1, moreover, a construction station consisting of a packed bed having a height of 5 m connected directly to the bottom of the distillation column was installed, the urea synthesis liquid was dropped, and 33 kg / hr of carbon dioxide was blown from the bottom into the gas phase. Stripping was done.

The stripped mixed gas was raised in the packed bed, moreover, the rectified area was raised and discharged together with the push gas and the distillation gas from the top of the distillation column.

First, without using the heat exchanger of this invention in Example 1, and using the stripping method mentioned above in the conventional method, the gas composition in a tower top is ammonia 106.0, carbon dioxide 60.0 water 86.4 kg / hr, The ammonia and carbon dioxide in the urea synthesis liquid discharged from the packed packing part were 1.0 kg / hr, respectively. On the other hand, when the above-mentioned stripping method is used in combination with the method of the present invention in Example 1, the gas composition in the column top of the distillation column is ammonia 105.8, carbon dioxide 59.8, water 59.3 kg / hr, and the bottom temperature during charging is 125 ° C. The ammonia and carbon dioxide in the urea solution coming out from this bottom were 1.2 kg / hr, respectively.

That is, as in the case of Example 1, according to the method of the present invention, the amount of evaporation of water was reduced by about 70% compared with the case of the conventional method.

Claims (1)

Carbon dioxide and ammonia are reacted at urea synthesis pressure and temperature to produce urea synthesis solution containing urea, excess ammonia, unreacted ammonium carbamate and water, and the urea synthesis solution is subjected to a pressure of 10 kg / cm 2 ( Gauges) at least two stages of the Ammonium carbamate dispensing process of one or two or more stages of high pressure stages and low pressure stages of pressure less than 5 kg / cm 2 (gauge), and ammonia, carbon dioxide in the separation zone of each pressure stage. And separating the mixed gas consisting of steam, and sequentially absorbing the mixed gas into the absorbent liquid in the absorption zone under the same pressure as the separation zone and circulating the absorbent liquid in the urea synthesis zone, and separating the low pressure stage. At least a part of the station is regarded as a rectifying station, and in some cases, the urea synthesis liquid exiting the rectifying station is introduced into the constructing station and brought into contact with carbon dioxide to react with the unreacted material. In the solution circulating urea synthesis method in which the stripping is performed, the urea synthesis liquid exiting the separation zone of the high pressure stage is indirectly heat-exchanged with the urea synthesis liquid at the bottom of the rectification zone of the separation zone of the low pressure step, and then cooled to 100-170 ° C. The pressure was reduced to the pressure of the stage and introduced into the stationary part of the rectifier. At the same time, the bottom of the stationary station was heated by the heat exchange, and the temperature of the stationary part was 60-120 ° C and the temperature of the bottom was 100-140 ° C. Separation and recovery method of the unreacted substance in the urea synthesis zone characterized by holding.

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