JPS5812270B2 - Urea synthesis method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved process for cyclic urea synthesis from carbon dioxide and a stoichiometric excess of ammonia.

Recently, in the urea synthesis process, the separation of unreacted carbon dioxide and ammonia from the urea synthesis liquid has been carried out at increasingly higher pressures, and a notable example is the separation of unreacted carbon dioxide and ammonia from the urea synthesis liquid under almost synthesis pressure with carbon dioxide gas or ammonia gas. This is the stripping process.

In such a process, inert gases such as nitrogen, hydrogen, and methane are contained in the raw material carbon dioxide and ammonia, and a small amount of air (hereinafter referred to as this air) is added to prevent corrosion of the urea synthesis tube. (referred to as inert gases) accumulate in high-pressure systems such as urea synthesis and stripping stages.

Therefore, it is necessary to extract these inert gases from the system.

There are two ways to remove inert gas: one is to remove it in a urea synthesis tube, and the other is to remove it when stripping unreacted carbon dioxide and ammonia are condensed, but in the latter case, the amount of gas accompanying the inert gas is smaller than in the former. The disadvantage is that the amount of carbon dioxide and ammonia produced is large.

Carbon dioxide and ammonia accompanying the separated inert gas must be recovered.

As one method, Sunamprogetsutei's urea synthesis process separates inert gas when condensing unreacted carbon dioxide and ammonia, which have been driven out of the urea synthesis solution by ammonia gas. It is introduced into a medium-pressure absorption tower together with unreacted carbon dioxide and ammonia separated by pressure distillation, and the carbon dioxide and ammonia accompanying the inert gas are recovered.

Alternatively, Stamicarbon's process flushes the inert gas at the top of the urea synthesis tube with ammonium carbamate solution from the carbon dioxide and ammonia recovery stage at low pressure and entrains it under urea synthesis pressure. It absorbs carbon dioxide and ammonia and releases only inert gas under reduced pressure.

In this process, the inert gas after removing carbon dioxide and ammonia has the risk of explosion, so explosion-proof measures must be taken, and at the urea synthesis pressure, and
Careful operation is required because it is operated in a device directly connected to the urea synthesis tube.

The present invention seeks to provide a new and improved method for cyclic urea synthesis.

The present invention also seeks to provide an improved cyclic urea synthesis method that includes a novel method for treating inert gas separated from the urea synthesis solution.

The urea synthesis method of the present invention is as follows.

Carbon dioxide and stoichiometrically excess ammonia are reacted at urea synthesis pressure and urea synthesis temperature to produce a urea synthesis liquid containing unreacted carbon dioxide and ammonia, and the urea synthesis liquid is The inert gas contained therein is separated from the inert gas along with the accompanying unreacted carbon dioxide and ammonia at a pressure approximately equal to the urea synthesis pressure, and then the urea is separated at a pressure approximately equal to the urea synthesis pressure. The synthesis solution is subjected to stripping with carbon dioxide or ammonia gas to separate at least a portion of the unreacted carbon dioxide and ammonia, and the urea synthesis solution from the stripping is reduced to 10 -2
The urea synthesis liquid from the high-pressure distillation is subjected to high-pressure distillation at 5 kg/cm2 (gauge) to separate most of the carbon dioxide and ammonia contained therein, and the urea synthesis liquid from the high-pressure distillation is subjected to low-pressure distillation to remove the carbon dioxide and ammonia contained therein. All carbon dioxide and ammonia are separated to obtain a urea aqueous solution, and urea is obtained from the urea aqueous solution thus obtained, while the inert gas and the high-pressure distillation are separated together with the accompanying unreacted carbon dioxide and ammonia. of carbon dioxide and ammonia are contacted with an absorption medium to absorb the carbon dioxide and ammonia to produce a high-pressure absorption liquid, and the unabsorbed ammonia gas and the inert gas are cooled to convert the liquid ammonia into the inert gas. urea, characterized in that the high-pressure absorption is brought into contact with the unreacted carbon dioxide and ammonia from the stripping and condensed, and the ammonium carbamate liquid thus obtained and the liquid ammonia are recycled to urea synthesis. Synthesis method.

The process of the invention can also be a complete cycle urea synthesis process.

In this case, the absorption medium is previously made to absorb carbon dioxide and ammonia from the low-pressure distillation.

In the present invention, the urea synthesis step uses a stoichiometric excess of ammonia to carbon dioxide, preferably 17
Temperature of 0-220℃, and 1o O-3 00kg
/cm2 (gauge) pressure.

The preferred NH3/CO2 molar ratio is 2.1-5, particularly preferred is 3-4.

It is preferable that the urea synthesis pressure be as high as possible than the equilibrium pressure corresponding to the urea synthesis temperature, but it is uneconomical to increase the urea synthesis pressure just to reduce the amount of carbon dioxide and ammonia accompanying the inert gas. It is preferable to employ a pressure in the above range from .

In the present invention, inert gas is extracted from the urea synthesis liquid at a pressure approximately equal to the urea synthesis pressure.

It is desirable that the amount of carbon dioxide and ammonia accompanying the inert gas be as small as possible, but the partial pressure of carbon dioxide and ammonia on the urea synthesis liquid in contact with the inert gas is approximately the same pressure as the urea synthesis pressure. It is most advantageous to extract the inert gas from the urea synthesis liquid at approximately the same pressure as the urea synthesis pressure, since this is the lowest in each of the steps being operated.

Most preferably, the inert gas is withdrawn from the top of the urea synthesis tube, but a gas-liquid separation is provided between the urea synthesis stage and the urea synthesis liquid stripping stage to separate the inert gas. You can also do it.

Here, "pressure approximately equal to the urea synthesis pressure" means "approximately equal to the urea synthesis pressure, or at most 10 k
g/cm2 high pressure.

The urea synthesis liquid separated from the inert gas is subjected to stripping with carbon dioxide or ammonia at a pressure approximately equal to the urea synthesis pressure to separate at least a portion of the unreacted carbon dioxide and ammonia contained therein.

The temperature and pressure in this case are preferably 15
0-230℃, 90-310kg/cm2 gauge)
It is.

As the stripping gas, it is preferable to use raw carbon dioxide because it has a high separation rate of unreacted carbon dioxide and ammonia and has low solubility in the urea synthesis solution.

The urea synthesis solution from the stripping stage is 10-2 5
The pressure is reduced to kg/cm2 (gauge) and introduced into a high pressure distillation stage to separate most of the unreacted carbon dioxide and ammonia contained therein.

The temperature of the high pressure distillation stage is preferably 110-170°C.

The high-pressure distillation stage may be a rectification column (eg, a multi-plate column) or a device consisting of a heater and a gas-liquid separator.

Furthermore, various other types of devices are possible.

The aqueous urea solution from the high-pressure distillation stage, which still contains small amounts of unreacted carbon dioxide and ammonia, weighs 1-5 kg/cm2.
(gauge) to remove all unreacted carbon dioxide and ammonia by conventional methods, and in a finishing step convert to crystalline urea or urea melt to form granular urea.

The carbon dioxide and ammonia separated from the aqueous urea solution at low pressure are absorbed into a suitable absorption medium such as water, aqueous ammonia, or aqueous urea solution.

The obtained absorption liquid is pressurized to the same pressure as the high-pressure distillation, and brought into contact with the carbon dioxide and ammonia separated in the high-pressure distillation, as well as the inert gas separated from the urea synthesis liquid, which has been reduced to the same pressure as the high-pressure distillation. All of the carbon dioxide and some of the ammonia are absorbed.

The temperature at this time is preferably 100-130°C.

If the absorption pressure is higher than 25 kg/cm2 (gauge), carbon dioxide and ammonia will be absorbed, resulting in concentration of inert gas and increased risk of explosion.

Since the pressure of high-pressure distillation and the absorption pressure of carbon dioxide and ammonia separated in high-pressure distillation are the same, the upper limit of the high-pressure distillation pressure is limited in terms of explosion of inert gas.

On the other hand, the lower limit of the absorption pressure is limited from the point that the amount of absorption medium must not become too large, and is 10 kg/cm2.
(gauge).

Excess ammonia not absorbed in this absorption stage is sent together with the inert gas to an ammonia condensation stage where it is cooled and liquid ammonia is separated from the inert gas.

A portion of the obtained liquid ammonia is sent to the absorption stage for cooling;
The remainder is recycled to the urea synthesis stage.

The inert gas is accompanied by some ammonia, which is further introduced into a washing tower where it is washed with water or the like to remove ammonia.

The upper limit of the pressure of the inert gas introduced into the washing tower is 25 kg.
/cm3 (gauge), there is no risk of explosion, but
This danger may exist in the very short line from the washing tower to the depressurization.

However, this line has a relatively low pressure and is connected to the process main line via a washing tower, so even if an explosion occurs, it will not cause any major damage to the process main line.

The high-pressure absorption liquid obtained by absorbing carbon dioxide and ammonia separated by high-pressure distillation is pressurized to the pressure of the stripping stage and brought into contact with the carbon dioxide and ammonia separated in the stripping stage, and the urea synthesis temperature is maintained at a predetermined value. At least a portion of the carbon dioxide and ammonia are condensed and liquefied while removing the heat of reaction so that the reaction heat becomes liquefied.

This condensation is carried out at a temperature of 140-200 DEG C. with heat recovery by steam generation of 3-10 kg/cm@2 (gauge).

The condensate thus obtained as well as the end-condensate carbon dioxide and ammonia (if present) are recycled to the urea synthesis stage.

Next, embodiments of the present invention will be described below with reference to the accompanying drawings.

Make-up ammonia from line 2 into urea synthesis tube 1 is pressurized together with circulating ammonia from line 25 by a boost pump (not shown), and then passes through line 2A.
In addition, the make-up carbon dioxide from line 3 is pressurized by a compressor (not shown), and in some cases, a portion of it passes through line 3A and is further added to the recovered ammonium carbamate liquid and uncondensed carbon dioxide and ammonia (if any). ) is introduced via line 4, with a pressure of 1 0 0 - 3 0 0 kg/cJ (gauge)
, and 170-220°C.

The molar ratio of total ammonia to total carbon dioxide at the inlet of the urea synthesis tube 10 is preferably 2.1-5, particularly preferably 3-4.

A discharge pipe 5 is provided in the urea synthesis tube so that a κ inert gas separation space 18 is formed in the upper part of the urea synthesis tube.

The urea synthesis liquid is removed from the discharge tube 5 and introduced into the top of the stripper 6 via line 5A.

The stripper 6 is heated to 150 DEG -230 DEG C. by high pressure steam introduced through line 8 and discharged through line 9.

The pressure in the stripper is usually 90-310k9/Cd (gauge), which is approximately equal to the pressure in the urea synthesis tube.

At least a portion of the make-up carbon dioxide from line 7 is introduced into the bottom of the stripper 6 and contacts the urea synthesis solution countercurrently to drive out a portion of unreacted carbon dioxide and ammonia in the urea synthesis solution. .

The urea synthesis liquid from stripper 6 passes through line 10,
The pressure is reduced to 10-25 kg/cm2 (gauge) by the pressure reducing valve 11 and introduced into the top of the high pressure distillation column 12 via the line 10A.

A heating pipe 13 is provided at the bottom of the high-pressure distillation column, which heats the urea synthesis liquid flowing down the column.

Most of the unreacted carbon dioxide and ammonia remaining in the urea synthesis liquid are decomposed in the high-pressure distillation column.

The urea aqueous solution still containing a small amount of unreacted carbon dioxide and ammonia flows from the bottom of the high-pressure distillation column 12 to the line 14.
, the pressure is reduced to 1-5 Kg/cm2 (gauge), and it is sent to a low-pressure unreacted carbon dioxide and ammonia separation step (not shown) by a conventional method to be processed to obtain an aqueous urea solution. .

This aqueous urea solution is then sent to conventional finishing steps to obtain the product urea.

The absorption liquid obtained by absorbing the carbon dioxide and ammonia separated in the low-pressure unreacted carbon dioxide and ammonia separation process into an absorption medium is pressurized to 10-25 kg/cm2 (gauge), and passes through line 15 to the high-pressure absorption tower. 16 is introduced at the top.

Unreacted carbon dioxide and ammonia separated in the high-pressure distillation column 12 are introduced into the top of the high-pressure absorption column 16 via a line 17.

On the other hand, the inert gas separated in the inert gas separation space 18 of the urea synthesis tube 1 is extracted from the line 19 together with carbon dioxide and ammonia, and the pressure reducing valve 20
The pressure is reduced to 0-25kg/cm2 (gauge), and line 1
It is introduced into the middle or bottom of the high pressure absorption tower 16 via 9A.

A cooling pipe 21 is provided at the bottom of the high-pressure absorption tower 16, thereby maintaining the bottom temperature of the high-pressure absorption tower at ioo-130°C.

In the cooling pipe 21, it is of course possible to recover the absorbed heat by steam generation, but it is most preferable to introduce urea slurry from a urea aqueous solution concentrating crystallizer into the cooling pipe 21 as a cooling medium for cooling. be.

In this way, the heat generated by recovering the carbon dioxide and ammonia introduced into the high-pressure absorption tower along with the inert gas can be easily and effectively recovered without adding any special equipment.

The mixed gas introduced from line 17 and line 19A comes into contact with the absorption liquid from line 15 and aqueous ammonia, which will be described later, from line 31, and all of the carbon dioxide and part of the ammonia in the mixed gas are absorbed.

Unabsorbed ammonia is introduced from the top of the column together with an inert gas into an ammonia condenser 23 through a line 22, cooled with water, and condensed.

A portion of the obtained liquid ammonia is circulated via line 24 to the top of the high-pressure absorption tower for cooling, and the remainder is circulated via line 25 to the urea synthesis tube 1.

The inert gas separated from the liquid ammonia in the ammonia condenser 123 is introduced into the ammonia washing tower 27 through the line 26 along with the ammonia gas, and is washed with water introduced from the line 28 to remove ammonia. 29, the pressure is reduced to normal pressure by the pressure reducing valve 30, and the liquid is taken out through the line 29A and used as appropriate.

Note that a cooler (not shown) is provided at the bottom of the ammonia washing tower 27.

The ammonia water taken out from the bottom of the ammonia washing tower 27 is supplied to the top of the high pressure absorption tower 16 via a line 31.

In addition to hydrogen and methane, the inert gas contains oxygen added to prevent corrosion of the equipment, so after the ammonia is removed, the inert gas in line 29 is at risk of explosion. be done.

However, this pressure is 10-25#/CrtiC gauge)
Therefore, there is almost no danger of explosion.

Moreover, since it is isolated from the main line of the process by the ammonia washing tower 27, there is no risk of damaging the main line even in the unlikely event of an explosion.

The high-pressure absorption liquid discharged from the bottom of the high-pressure absorption tower 16 passes through a line 32, is pressurized by a pump 33 to the same pressure as the stripper pressure, and is introduced into the top of a carbamate condenser 34.

Meanwhile, a mixed gas of unreacted carbon dioxide and ammonia from the stripper 6 is also fed via line 35 to the top of the power-bamate condenser 34, where it contacts the high-pressure absorption liquid and is at least partially condensed into ammonium carbamate. Become.

The heat generated at this time is recovered and removed by water introduced from the line 36.

From line 37, 3-
Steam of 10 k9/cm2 (gauge) is taken out.

This steam generation causes the carbamate condenser 3 to
The temperature in step 4 is maintained at 140-200°C.

The produced ammonium carbamate liquid as well as uncondensed carbon dioxide and ammonia (if present) are introduced into the urea synthesis tube 1 via line 4.

The urea synthesis method of the present invention provides various effects described below.

First, stripping of carbon dioxide or ammonia separates and recovers a significant portion of unreacted carbon dioxide and ammonia at approximately the same pressure as the urea synthesis pressure, reducing the amount of absorption media used for recovery and reducing the amount of unreacted carbon dioxide and ammonia. The amount of absorption medium entering the synthesis tube is reduced, and it is possible to obtain a correspondingly higher urea synthesis rate.

Second, the inert gas that accumulates in the synthesis system due to the inert gas in the raw material carbon dioxide and ammonia (oxygen is also mixed in to prevent equipment corrosion) is discharged during the urea synthesis stage or between it and the stripping stage. Because the process is carried out between 200 and 3000, the amount of carbon dioxide and ammonia accompanying the discharged inert gas is small.

Third, since the present invention has a high-pressure distillation and absorption stage after the stripping stage, recovery of carbon dioxide and ammonia entrained in the inert gas can be carried out in this high-pressure absorption stage with heat recovery.

Moreover, the pressure at this stage is 10-25 kg/cm
2 (gauge), there is almost no danger of explosion of the inert gas from which carbon dioxide and ammonia have been removed.

A feature of the present invention is that the inert gas containing ammonia and carbon dioxide is treated in a medium pressure absorption region of 15 to 25 kg/cm2 (gauge). The primary treatment may be carried out in a vacuum chamber, and then the pressure may be reduced to 15 to 25 kg/cm2 (gauge).

For example, if the urea synthesis pressure is low and the temperature is high, the amount of ammonia and carbon dioxide gas entrained will also increase.

In such cases, the amount of heat generated during medium pressure absorption may be greater than the amount that can be recovered and used.

Therefore, if, for example, 20 to 60% of the entrained ammonia and carbon dioxide are absorbed under the same urea synthesis pressure prior to depressurization, the heat generated at this time can be absorbed more effectively than at intermediate pressure. In addition to being able to be used for various purposes, the above-mentioned drawbacks can also be eliminated.

As a medium for this absorption, a solution containing the carbamate conduit can be used.

1 An example according to the process of the attached drawings is shown below.

The examples show flow quantities and compositions in each line and operating conditions in each device.

Example

[Brief explanation of the drawing]

The drawings are flow sheets for explaining specific implementations of the present invention. 1: Urea synthesis tube, 6: Stripper, 12: High pressure distillation column, 16: High pressure absorption column, 23: Ammonia condenser,
27: Ammonia washing tower, 34: Carhamet condensate? 1,2A: ammonia, 3A and T: carbon dioxide,
4: Ammonium carbamate liquid and uncondensed carbon dioxide and ammonia, 5A: Urea synthesis liquid, 19A: Inert gas (including carbon dioxide and ammonia).

Claims (1)

[Claims] 1. Carbon dioxide and a stoichiometric excess of ammonia relative to carbon dioxide are reacted at a urea synthesis pressure and a urea synthesis temperature to produce a urea synthesis liquid containing unreacted carbon dioxide and ammonia. , after separating the inert gas contained therein from the urea synthesis liquid at a pressure approximately equal to the urea synthesis pressure, together with the accompanying unreacted carbon dioxide and ammonia; Below, the urea synthesis solution is subjected to stripping with carbon dioxide or ammonia gas to separate at least a portion of the unreacted carbon dioxide and ammonia, and the urea synthesis solution from the stripping is subjected to stripping with carbon dioxide or ammonia gas.
The urea synthesis liquid from the high-pressure distillation is subjected to high-pressure distillation at 5 kg/cm2 (gauge) to separate most of the carbon dioxide and ammonia contained therein, and the urea synthesis liquid from the high-pressure distillation is subjected to low-pressure distillation to remove the carbon dioxide and ammonia contained therein. All carbon dioxide and ammonia are separated to obtain a urea aqueous solution, and urea is obtained from the urea aqueous solution thus obtained, while the inert gas and the high-pressure distillation are separated together with the accompanying unreacted carbon dioxide and ammonia. of carbon dioxide and ammonia are contacted with an absorption medium to absorb the carbon dioxide and ammonia to produce a high-pressure absorption liquid, and the unabsorbed ammonia gas and the inert gas are cooled to convert the liquid ammonia into the inert gas. and contacting the high-pressure absorption liquid with the unreacted carbon dioxide and ammonia from the stripping to condense it, and recycling the thus obtained ammonium carbamate liquid and the liquid ammonia to urea synthesis. Urea synthesis method. 2 The molar ratio of the raw material ammonia and the raw material carbon dioxide is 2.
．． 1-5, and the urea synthesis pressure is 100-300 kg
/cm2 (gauge), the urea synthesis temperature is 170-220
The method according to claim 1, wherein the temperature is .degree. 3. The method of claim 1, wherein said stripping is carried out with raw carbon dioxide gas under a pressure of 90-310 kg/cm@2 (gauge) and at a temperature of 150-230 DEG C. 4. The method according to claim 1, wherein the absorption medium is previously made to absorb carbon dioxide and ammonia from the low-pressure distillation.