JPS6110546A - Production of urea - Google Patents

Abstract

PURPOSE:The two-stage stripping decomposition of a urea synthesis mixture is effected by keeping the pressure in the first stage at almost the same level as in the urea synthesis and lowering it in the second stage to a substantial extent and heating the second stage with the heat of condensation of the gas separated in the first stage whereby the consumption of high-pressure steam is reduced. CONSTITUTION:The urea synthesis mixture from the urea synthesizer 101 is fed to the top of the first stripping docomposer 102 and subjected to stripping decomposition by introducing NH3 a part of which is gasified by heating with the heater 105, into the lower part and effecting indirect heating of the decomposer 102 with high-pressure steam 10. The separated gas is introduced into the second stripping decomposer 103 on the high-pressure side (line 9), while the urea solution is a little lowered in its pressure, introduced into the second decomposer 103 on the lower-pressure side (line 8). Further, the gas separated in the first stage is made to condense by the absorption medium which is sent with pressure from the carbamate condenser 104 to the second decomposer 103 on the high- pressure side and the condensation heat released is fed to the lower-pressure side to effect stripping decomposition with the urea solution introduced from the upper part and CO2 introduced to the lower part.

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention This invention relates to improvements in the process of urea synthesis and separation and recovery of unreacted substances. That is, it relates to a process for producing urea using a stripping cracking process with low thermal energy requirements.

[Prior Art] A stripping process is known as a method for producing urea that requires a small amount of thermal energy. In this process, unreacted ammonium carbamate and excess ammonia (hereinafter referred to as unreacted substances) in the urea synthesis solution are removed under a pressure equal to the urea synthesis pressure using carbon dioxide or ammonia to be supplied as a raw material, using a heating medium. (Usually, high-pressure steam of Q/CdQ or higher is used.) While heating, stripping is performed to decompose and separate. The separated ammonia and carbon dioxide, and the mixed gas consisting of carbon dioxide or ammonia used to decompose and separate unreacted substances, are condensed under almost the same pressure, and the heat of condensation generated at this time is transferred to the urea liquid. used for thermal decomposition of carbon dioxide, or recovered as low-pressure steam.

In this way, a considerable portion of unreacted substances is recovered under a pressure approximately equal to the synthesis pressure, and the heat of condensation can be effectively extracted from the separated mixed gas, making this method an excellent method with low energy requirements. It is something that

The gas used for stripping is carbon dioxide or ammonia. Usually one of these gases (often carbon dioxide) is used.

However, methods are also known in which these two gases are used simultaneously. In this case, the stripping and decomposition process is divided into two parts, in which stripping is carried out using ammonia in the first stage and using carbon dioxide in the second stage.

The cracked gas flowing out from the first stage is recycled as is to the urea synthesis step, and the separated gas from the second stage is condensed in the carbamate condensation step, as in the case of conventional carbon dioxide stripping, and then processed into urea. It is recycled to the synthesis process.

By stripping in two stages in this way, the steam consumption rate can be considerably improved, but since heating is performed using high-pressure steam in all stripping decomposers, the amount of high-pressure steam required still remains. Quite a lot.

The methods we provide herein significantly reduce this steam requirement, particularly high pressure steam requirements.

[Disclosure of the Invention] Our method consists of subjecting the urea synthesis liquid, which flows out from the urea synthesis process, to stripping decomposition in two stages (first with ammonia and then with carbon dioxide), and the first stripping and decomposition step is essentially a synthesis process. pressure, and the next cracking step involves stripping at a pressure well below this. The decomposition heating in the first stage is performed by high pressure steam, while the decomposition heating in the second stage is performed by the heat of condensation of the separated gas from the first stage. In other words, the separated gas flowing out from the first stage decomposition process is
It is introduced into the high pressure side of the second stage stripping decomposer and condensed at approximately synthetic pressure by the absorption medium pumped from the carbamate condenser. The heat of condensation generated at this time covers the amount of heat required for the second stage stripping.

If the two-stage decomposition process is carried out at the same pressure as in the usual method,
Although there cannot be a temperature difference between the condensing temperature and the stripping temperature, by performing the second stage at a considerably lower pressure, a sufficient temperature difference can be obtained for heat exchange.

The separated gas leaving the second stage stripping decomposer is condensed by an absorption medium in a carbamate condenser. The absorption medium obtained here is introduced into the high pressure side of the second stage, where it comes into contact with the separated gas from the first stage and condenses. However, the pressure on the high pressure side of this second stage is higher than that of the carbamate condenser, so a pump is used to pressurize it.

In this way, the heat source of the second stage stripping decomposer is provided by the heat of condensation of the gas separated in the first stage, and the high pressure steam is significantly reduced.

Eleventh, a flow sheet of this invention is shown.

Ammonia 1, carbon dioxide 5, and an absorption medium 6 are supplied to the urea synthesis tube 101, and after the synthesis reaction is completed, they flow out from 7 and are supplied to the upper part of the first stripping decomposer 102. On the other hand, a part of the ammonia is supplied to the ammonia heater 10.
5, it becomes gaseous and flows from the bottom of 102. The first stripping decomposer is indirectly heated by a heating medium 10 (usually high pressure steam). The gas separated here is introduced into the high pressure section of the second stripping decomposer (line 9), and the urea liquid is introduced into the low pressure side of this decomposer (line 8) after being somewhat reduced in pressure.

The high pressure side of the second stripping decomposer contains the separated gas from the first decomposer and the absorption medium 2 from the carbamate condenser.
4 is inflowed. The urea solution flowing out from the first decomposer flows into the low pressure side from above, and the carbon dioxide 4 flows from below.

On the high pressure side, the separation gas and the absorption medium come into contact and condense while causing a carbamate production reaction, generating a large amount of heat.
. This heat is given to the low pressure side and becomes the heat source for stripping decomposition.

In the second stripping decomposer, reactions on both sides proceed smoothly only when a sufficient amount of heat flows from the high pressure side to the low pressure side. That is, it is necessary that there is a sufficient temperature difference between the reaction temperatures on both sides.

As for the temperature on both sides, the higher the pressure, the higher the temperature, and the lower the pressure, the lower the temperature. In other words, the temperature difference between the two sides is
This is ensured by creating a pressure difference on both sides.

Exiting the second stripping decomposer, the separated gas 13 enters the carbamate condenser 104.

Meanwhile, the absorption medium from the recovery process enters at 22.

The two then come into contact and condense. The heat of condensation generated at this time is indirectly given to the urea solution in the decomposition process, a heat medium such as boiler water, or both.

If there are two or more heat carriers, separate condensers may be used depending on the design. In this case, we put them together 1
A set means one condenser.

All of the heat generated in this condenser is effectively used for other purposes. The pressure of this condensation step depends on the condensation temperature. If the temperature is high, the pressure must also be high. In our case, a pressure difference between the first and second stripping steps is required, and if the second pressure increases, the first pressure, and therefore the urea synthesis pressure, must also increase.

In order to lower this condensation temperature (and therefore pressure), it is effective to lower the temperature of the side to which heat is applied (the heat medium). A unique way to do this.

We developed a method that uses urea solution from the decomposition process.

This is part of our invention and will be discussed later.

The absorption medium condensed in the drain carbamate condenser is sent to the high pressure side of the second stripping decomposer as the absorption medium. However, since the pressure is high, a pump is used to pump it.

Naturally, power is required, but we have developed a method that eliminates the need for external power. This also forms part of our invention, and will be discussed later.

The above is an overview of our invention, which reduces the amount of high-pressure steam required for stripping by half compared to conventional processes, making it extremely energy-saving.

This urea production method is characterized by performing stripping in two stages and creating a considerable pressure difference between the two stages.

It is desirable to keep the pressure as low as possible in the second-stage stripping process, but there is a limit, so the pressure for vigorous urea synthesis must be quite high.Therefore, this method is rather unsuitable. It is suitable for production methods with a high ammonia excess rate and high synthesis pressure.In other words, the molar ratio of total ammonia/total carbon dioxide supplied to the synthesis process is 3.
5 to 5.0 is suitable. In this case, the synthesis pressure (therefore, the pressure of the first stripping decomposition step and the high pressure side absorption step of the second stripping) is preferably 180 to 250 kg 10 I1 g. If it is less than 180 kg/cwfo, the synthesis reaction will not be carried out sufficiently, and if it is more than 250 ka/CIIQ, it is not necessary. Particularly suitable pressures for this process are 190-23
0ka/cWlcI.

Further, the pressure in the second stripping step and the carbamate condensation step is preferably 130 to 200 kg/C1/g. If it is less than 130 to 9/cnfg, the condensation temperature is too low and the heat of condensation cannot be used effectively, and 200kQ/c-
This is because if the temperature is higher than ++f0, the temperature is too high and there is no need for it. A particularly desirable pressure is 140-180kGl/
CIIIg.

A pressure difference is required between the first and second stripping decomposition pressures, and the pressure difference is 25 to 75 kg/cm.
is desirable. If it is less than 25 ka/cm, the temperature difference between the high pressure side and the low pressure side of the second stripping decomposer cannot be maintained sufficiently, and the
ka/cm is too large. The most desirable pressure difference is 40
~60 to 9/cm.

ammonia in the two stripping decomposition steps;
As for the amount of carbon dioxide fed, in the first case, ammonia is 0 to 5% of the raw material ammonia in this urea synthesis.
0%, 60-100% of raw carbon dioxide in the second
is appropriate.

In the first stripping and decomposition step, there may be cases where no stripping gas is fed, and in this case, the gas is separated only by heating.

The ammonia fed in the first stripping cracking step can be replaced with gas purged from the urea synthesizer. This purge gas is mostly ammonia with low amounts of carbon dioxide and water. This gas is used as a stripping gas.

Since the amount of gas to be purged is not so large and this purge gas is used to purge inert gas, it is impossible to use the entire amount, so there is a limit to the amount of gas that can be fed. However, the amount of new ammonia sent will decrease accordingly.

Next, a specific method for lowering the pressure in the carbamate condenser mentioned above will be described.

As a way to reduce the condensation pressure of the carbamate condenser, we use the urea liquid in the decomposition process as a heating medium.

The decomposition process consists of medium-pressure and low-pressure carbamate decomposition processes, and we use the urea liquid in both processes as the heating medium.

As mentioned above, by using the urea liquid from the medium and low pressure cracking process as the heating medium in the carbamate condenser, the condenser pressure can be greatly reduced.

This condenser is divided into upper and lower parts, with high-pressure absorption medium and stripping cracked gas flowing from the upper part, and the lower pressure side containing medium-pressure cracked urea liquid in the upper part and low-pressure cracked urea liquid in the lower part. It is desirable to let it flow. Since the low pressure decomposed urea liquid has a sufficiently low temperature, it is suitable to flow this liquid to the lower part to complete the condensation in this condenser.

In our process, about half of the amount of gas separated in stripping is condensed on the high pressure side of the second stripping decomposer, so the amount condensed in this carbamate condenser is equal to the remaining half and the amount in the feed. carbon dioxide. Therefore, the amount is about 2/3 compared to the case of normal one-stage stripping.

Therefore, the amount of heat generated is approximately proportional to it. This amount of heat seems to be slightly larger than the amount of heat required for gas decomposition separation at medium and low pressures, but since there is a sufficient temperature difference and sufficient heat transfer takes place, if the amount of heat is too large, the concentration will be limited. As you move forward, you will balance out and it will not be a problem.

Now, our process incorporates things that increase power consumption, such as arm-pressure pumps of the absorption media.

In contrast, we have succeeded in realizing this process without increasing external power by effectively utilizing the energy of the urea solution that is being depressurized.

i.e., using a water turbine for power recovery, high-pressure urea liquid (in our case, flowing out of the second stripping decomposer,
The energy generated when the pressure of the urea solution (urea solution) is reduced to the next decomposition step (medium pressure decomposer) is used to drive the pressure boost pump 111 of the absorption medium.

The output of this power recovery turbine is sufficient to drive the latter. An example of these combinations is shown in FIG.

An electric motor is sandwiched in between, with a turbine connected to one end and a pump connected to the other end. The electric motor is used for starting, but it usually rotates without load.

When there is insufficient power due to fluctuations in operating conditions, power is automatically supplied from the electric motor. In other words, it is most suitable as a governor.

Suitable corrosion-resistant materials for this turbine include high-grade stainless steel, titanium, zirconium, etc., which are used in the high-pressure portion of urea.

[Example] Synthesis process pressure 210kQ/Cnfg Temperature 190°C Total ammonia/total carbon dioxide molar ratio 4.0 First stripping decomposition process pressure 210+! /CTl1g Temperature: 200°C Heating steam pressure: 25kg/cm.

2nd stripping decomposition process High pressure side Pressure 210kG/c++N] Temperature 185℃ Low pressure side Pressure 160kGl/cd! J temperature 170℃ Carbamate condensation process high pressure side pressure 160kQ/Cnf (1 temperature 150℃ low pressure side upper pressure 15KO/allfg temperature 14
0℃ Lower pressure 1kg/cdg Temperature 120℃ Medium pressure decomposition process Pressure 15ka/cmQ Temperature 140℃ Low pressure decomposition process Pressure 1 to 9/cy + fg temperature 120
°C Steam consumption Pressure (25ko/cnfg) Invention method
Conventional method 500 900kCI/ton urea

[Brief explanation of the drawing]

Fig. 1 Flow sheet of main parts of the present invention Fig. 2 Explanation diagram of power recovery combination Fig. 3 Flow sheet of embodiments of the present invention 101 Urea synthesis tube 102 First stripping decomposer 103 Second stripping decomposer 104 Carbamate condenser 104U Upper part 104L Lower part 105 Ammonia heater 106, 107 Gas-liquid separator 108 Recovery tower 111 Absorption medium boost pump 112 Power recovery turbine 1 Ammonia synthesis pipe inlet 2 Stripping ammonia inlet 3 Purge gas 4 Stripping carbon dioxide inlet 5 Carbon dioxide synthesis pipe inlet 6 Absorption Media synthesis pipe inlet A Urea synthesis liquid outlet 8 First decomposed urea liquid outlet 9 First
Decomposed separation gas outlet 10 High pressure steam 11 Second decomposed urea liquid outlet 12 Urea gas-liquid separator inlet 13 Second decomposer separated gas outlet 14.15.16.17.18 Urea liquid 19, medium pressure cracked gas

Claims (6)

[Claims] (1) Ammonia and carbon dioxide are reacted under the pressure and temperature of urea synthesis in the urea synthesis step, and most of the unreacted ammonium carbamate and excess ammonia are removed from the resulting urea synthesis liquid by mixing ammonia and carbon dioxide. The separated mixed gas is absorbed into an absorption medium and recycled to the urea synthesis process, while the urea liquid obtained by separation is further separated into ammonia, carbon dioxide and water in the subsequent decomposition process. In the urea production method, these separated gases are condensed and recovered in a recovery step and circulated to the mixed gas absorption step as an absorption medium, in which the flow following the urea synthesis step includes two stripping decomposers and one Two carbamate condensers are installed (first step)
In the first stripping decomposer, on one side (high pressure side), the urea synthesis liquid is fed from the top and ammonia gas is fed from the bottom at substantially the same pressure as the synthesis process, and the other A heating medium is passed through the side (low pressure side), and the separated gas obtained on the high pressure side is transferred to the high pressure side of the next second stripping decomposer at substantially the same pressure. The pressure is reduced to some extent and it flows into the low pressure side of the second decomposer. (Second step) In the second stripping decomposer,
On one side (low pressure side), the urea solution from the first step is fed from the top, carbon dioxide is fed from the bottom, and the other side (low pressure side)
On the high pressure side), the separated gas obtained in the first step,
The absorption medium sent from the third step described below is sent,
The two are brought into contact and the low-pressure side is heated by the condensation heat generated thereby, the resulting absorption medium is circulated to the urea synthesis process, and the separated gas obtained on the low-pressure side is used for the next third step. The urea liquid is depressurized and flows into the subsequent medium-pressure decomposition process. (Third step) In the carbamate condenser, the urea liquid for the subsequent decomposition process and the third heat medium are on one side (low pressure side). Or both are distributed respectively and the other side (high pressure side)
The separated gas that flowed from the second step and the absorption medium obtained in the intermediate pressure recovery step are fed into the system, the two are brought into contact, and the low pressure side is heated by the heat of condensation generated thereby. A method for producing urea, characterized in that the absorption medium is pressurized and sent to the high pressure side of the second step. (2) Total ammonia/total carbon dioxide molar ratio in the urea synthesis step 3.5 to 5.0, pressure 180 in the synthesis step, first step stripping decomposition (high pressure side) and second step stripping decomposition (high pressure side) ~250kg/cm^2g,
2nd step stripping decomposition (low pressure side) and 3rd step carbamate condensation (high pressure side) pressure 130-200kg
/cm^2g, and pressure difference between the two: 25-75kg/c
The method for producing urea according to claim 1, wherein the operating condition is m^2. (3) The amount of ammonia fed in the first step stripping cracking is 0 to 50% of the raw material ammonia, and the amount of carbon dioxide fed in the second step stripping cracking is 60 to 100% of the raw material carbon dioxide. The method for producing urea according to claim 1 or 2, wherein: (4) The method according to any one of claims 1 to 3, wherein a part of the purge gas discharged from the urea synthesis step is introduced instead of the ammonia fed to the first step stripping and decomposition. Urea production method. (5) The carbamate condenser for the third step consists of two parts, the upper and lower parts, and the fluid on the high pressure side is connected to the urea liquid from the medium pressure cracking process in the upper part, and the urea liquid from the low pressure cracking process in the lower part. Claims 1 to 4 that contact
The method for producing urea according to any of paragraphs. (6) A pump that boosts the pressure of the absorption medium obtained in the third step carbamate condensation is driven by a power recovery turbine powered by the urea liquid flowing out from the second step stripping and decomposition, as claimed in claim 1. 6. The method for producing urea according to any one of items 5 to 5.