JPS5822147B2 - Urea synthesis method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved method of urea synthesis,
More specifically, the present invention relates to an improved method of urea synthesis including a new method of supplying raw materials to a urea synthesis tube.

As is well known, the overall reaction for synthesizing urea is represented by the reaction formula: 2NH3+CO2-NH2C0NH2+H20 (1).

This reaction depends on the reaction temperature and the NH3/
There is an equilibrium defined by C02 and the H20/C02 molar ratio, and given enough reaction time the reaction will reach equilibrium.

Urea synthesis tubes used industrially are not only required to withstand high temperatures and pressures, but also require the use of expensive corrosion-resistant materials to withstand the severe corrosion of ammonium carbamate, so the capacity is as small as possible. As a result, it cannot be said that sufficient reaction time is provided for reaction (1) to reach equilibrium.

In consideration of this, various efforts have been made to increase the equilibrium attainment rate (degree of reaching equilibrium) by keeping the capacity of the urea synthesis tube at 5.

One example of these devices is to use a baffle plate that partitions the inside of a vertical cylindrical urea reactor into several stages, as shown in Japanese Patent Publication No. 45-38813.

However, as mentioned above, the method of using this baffle plate requires the use of expensive materials because it is installed inside the urea synthesis pipe, which is in a high temperature, high pressure, and severely corrosive environment. , leaving a considerable negative impact on the corrosion resistance of the materials used for drilling, cutting, etc.

In addition, these baffle plates require 5 to 20 pieces to be effective, which not only increases the cost, but also causes disadvantages in design and manufacturing to support the weight.
This causes indirect disadvantages such as installation and removal during equipment maintenance, and maintenance of the baffle plate itself.

The present invention aims to provide a urea synthesis method that can increase the rate of reaching equilibrium without increasing the capacity of the urea synthesis tube.

The present invention aims to provide a urea synthesis method in which starting materials supplied in gaseous and liquid form to a urea synthesis tube are uniformly mixed.

The urea synthesis method of the present invention involves passing carbon dioxide, ammonia and recovered unreacted carbon dioxide and ammonia in gaseous and liquid form through a gas-liquid mixing device at a liquid/gas weight ratio of at least 2.5, with the gas being passed through a gas-liquid mixing device with fine bubbles. It is characterized in that it is dispersed in a liquid to form a mixture, and this mixture is reacted in a urea synthesis zone at a urea synthesis temperature and pressure.

In this way, by dispersing the gas supplied to the urea synthesis zone in the form of fine bubbles in the liquid supplied to the urea synthesis zone, it is possible to improve the equilibrium attainment rate using urea synthesis tubes of the same capacity.

In the present invention, the term "gas" refers to carbon dioxide at 30-250°C at the urea synthesis pressure in a normal solution circulation process, but also includes, for example, unreacted ammonium carbamate and excess ammonia during urea synthesis. It also includes a mixed gas containing carbon dioxide and ammonia obtained by separating urea from the urea synthesis liquid.

In addition, "liquid" refers to the temperature at 30-250°C under the urea synthesis pressure.
Dioxide obtained by separating unreacted ammonium and excess ammonia during urea synthesis from the urea synthesis solution, and condensing or absorbing at least a portion of the resulting mixed gas containing carbon dioxide and ammonia into an absorption medium. A general term for solutions containing carbon and ammonia.

For example, in the case of carbon dioxide and ammonia, the "gas" and "liquid" mentioned above also include substances that exceed the critical state, so they cannot be accurately called gases or liquids, but they are
For convenience, we will define it like this.

In the present invention, various gas-liquid mixing devices having a structure capable of dispersing gas in the liquid as fine bubbles can be used.

However, it is preferable to use a pipe-type non-driven mixer because it is small and can disperse a large amount of gas in the liquid, and because it is small it requires less expensive corrosion-resistant material.

Such a mixer is a device for sufficiently mixing two or more fluids, for example, by filling a hollow tube with a large number of elements that divide the fluid flow path into a plurality of parts.
No. 4-8290, consisting of a hollow cylindrical tube and a number of bent sheet-like elements extending longitudinally in series within the tube, each element extending over the length of the tube. extending to the tube wall and dividing the reading tube into two separate channels, the elements being arranged alternately and in point contact with each other and curved to change the direction of fluid across the tube. The edges of each element are placed at an angle to the contacting edges of adjacent elements, and the total cross-sectional area of the two channels is substantially constant over the length of the reading tube. Chemical equipment J1973
The honeycomb structures described in the October issue of 2007, pages 99 to 105 are stacked in two layers, with the corners of the upper and lower honeycomb chambers located in the center of the stacked chambers, A honeycomb mixer or the like is preferably used in which the hexagonal shapes are arranged in the lateral (side) direction with respect to the fluid flow direction, and the hexagonal sides of the upper and lower honeycomb structures are alternated on a line.

Any gas-liquid mixing device based on the same principle as thermal theory is also preferably used.

Preferably, these mixers are made of corrosion-resistant materials such as zircon or titanium.

Alternatively, it is possible to use a centrifugal pump, which has rotary blades driven by a motor or the like inside the casing, but this requires a device to seal the urea synthesis pressure and is also somewhat complicated as a device. You cannot avoid the disadvantage of becoming.

Furthermore, it is possible to use devices such as orifices that create turbulent flow to improve the mixing of gas and liquid.
This mixer is also inferior to the above-mentioned mixer in terms of mixing effect and pressure loss.

It is desirable to provide such a gas-liquid mixing device outside the urea synthesis tube as close as possible to the inlet.

This is because once the gas is dispersed into fine bubbles, while it is being transported through a long pipe, collisions between the bubbles occur and the diameter of the bubbles gradually increases, reducing the effectiveness.

Alternatively, it is also possible to install this bubble mixing device inside the urea synthesis tube.

In this case, the gas-liquid mixing device is not required to withstand pressure, so it is advantageous in terms of cost, but it is inferior to installing it outside in terms of dead space created inside the urea synthesis tube and installation and maintenance.

The installation position is preferably near the inlet of the synthetic pipe.

In order to disperse gas as fine bubbles in a liquid in a gas-liquid mixing device, especially when using a pipe-type non-driven mixer, the gas can be dispersed as fine bubbles no matter what ratio the gas and liquid are mixed. There is no one-size-fits-all ratio, and there is an appropriate range for this ratio.

This range varies depending on various physical properties of the gas and liquid, and is extremely difficult to predict in systems that involve high temperatures and pressures such as the urea synthesis conditions, and reactions such as the dissolution of the gas into the liquid and the urea synthesis reaction. It's tough.

The present inventors considered that the range in which the equilibrium attainment rate of reaction formula (1) becomes significantly high is the range in which the gas has achieved a state in which the gas is dispersed as fine particles in the liquid, and found this range.
It has been found that the effects of the present invention can be observed when the gas ratio is about 2.5 or more by weight, that is, when the liquid is in excess.

Such a liquid/gas ratio is suitable for urea synthesis in the form of a solution by a normal solution circulation process, i.e., by condensing or absorbing the entire amount of gas containing carbon dioxide and ammonia separated in the unreacted product separation zone into an absorption medium. For example, in a process in which urea is synthesized in a region where the molar ratio of NH3/C02 is 40, the molar ratio of H20/C02 is 1.0, the temperature is 190°C, and the pressure is 200 atm, the value is about 5. The gas can be sufficiently dispersed in the liquid in the form of fine bubbles using a liquid mixing device.

Even if the synthesis conditions change slightly, this ratio will only change slightly, and will not change if it is less than 2.5.

On the other hand, in a gas circulation process, that is, a process in which at least a part of the mixed gas of unreacted carbon dioxide and ammonia separated from the urea synthesis liquid is recycled to the urea reaction zone in a gaseous state, a part of the mixed gas is Unless it is circulated through the urea synthesis tube in a condensed state or in the form of a solution absorbed in an absorption medium, the liquid/gas ratio cannot be brought to the side of liquid excess of 2.5 or more.

The proportion of the gas mixture that must be in the form of a solution (
The condensation rate) is at least about 50%.

Of course, increasing the condensation rate will increase the rate of equilibrium attainment, but it is meaningless to increase the condensation rate based on this fact alone, and this should be determined after looking at the entire process.

As mentioned above, the present invention can be adopted in the urea synthesis stage of various urea production processes, that is, the solution circulation process and the gas circulation process. is stripped with raw material carbon dioxide or ammonia at substantially urea synthesis pressure,
It also includes those in which a part of the generated mixed gas of carbon dioxide and ammonia is directly converted into S without being condensed, and the remaining part is condensed and circulated to the urea synthesis tube.

The above-mentioned liquid/gas ratio is preferably in the range of 2.6-14.0.

Even if the liquid/gas weight ratio is 14.0 or more, it is sufficiently practical in terms of gas dispersion, but such an excess of liquid is unfavorable in terms of the heat balance of the urea synthesis tube and is not very practical. Not the point.

The advantages of the invention are as follows.

(1) Under the same urea synthesis conditions, the rate of equilibrium attainment can be increased without increasing the capacity of the urea synthesis tube, and therefore the conversion rate to urea can be increased.

Therefore, the energy required to separate and circulate unreacted carbon dioxide and ammonia can be reduced.

(2) In order to obtain the same urea synthesis rate under the same urea synthesis conditions, a smaller capacity urea synthesis tube is sufficient.

Therefore, equipment costs can be reduced.

(3) Although it is a secondary effect, the air or oxygen introduced to prevent corrosion of the urea synthesis tube is also dispersed in the liquid as fine bubbles, so the air or oxygen necessary to obtain the same corrosion protection effect is also dispersed in the liquid. The amount of oxygen can be reduced.

The effects of the present invention will be further explained by showing Examples and Comparative Examples below.

Example 1 In FIG. 1, a urea synthesis tube 1 has an inner diameter of 1,200 m, an effective length of 12,500 mm, and the inside is lined with titanium.

From lines 2, 3, and 4, liquid ammonia, carbon dioxide, and recovered liquid containing unreacted carbon dioxide and ammonia are supplied, respectively, pressurized to the urea synthesis pressure.
After being collected in a hollow mixing chamber 5, the gas is introduced into a pipe-type non-driven mixer 7 through a line 6, where it is dispersed in the liquid as fine bubbles.

The gas-liquid mixture leaving the mixer 7 is led to the urea synthesis tube 1 through a very short line 8 to synthesize urea, and the urea synthesis liquid is discharged through a line 9.

The mixer 7 is filled with 20 stages of elements that divide the flow into two and change the direction by 90°C, as described in Japanese Patent Publication No. 44-8290.

This element is made of titanium.

The amounts of liquid ammonia, carbon dioxide, and recovered liquid are 12 tons/hour, 7.8 tons/hour, and 23.5 tons/hour, respectively, the urea synthesis temperature is 190°C, and the pressure is 160k.
y/i (gauge).

The composition of the recovered liquid is NH337% (weight) CO231
% (wt), and H2O32% (wt).

From the above values, the NH of all raw materials to be supplied to the urea synthesis tube
3/CO (molar ratio) is 3.55 and H20/C02 (molar ratio) is 1.22.

On the other hand, when N H3/CO2 (molar ratio) is 3.55, H2
When a liquid having a composition of 0/C02 (molar ratio) of 1.22 was reacted at 190° C. for 2 hours using an 11-volume autoclave, the conversion rate of C02 to urea was 62.5%.

This conversion rate can be considered to be approximately an equilibrium conversion rate.

The conversion rate of this example and the equilibrium attainment rate with respect to the above-mentioned equilibrium conversion rate are shown in Table 1 below.

Example 2 *The apparatus shown in FIG. 2 was operated in the same manner as in Example 1.

The apparatus shown in FIG. 2 differs from the apparatus shown in FIG. 1 in that a mixer 7 is installed inside the urea synthesis tube 1 and directly connected to the inlet of the synthesis tube.

The raw material mixture mixed in the mixer γ is directly discharged from the outlet of the mixer 7 into the urea synthesis tube.

The results are shown in Table 1.

Comparative Example 1 The apparatus shown in FIG. 3 (exactly the same as the apparatus of Example 1 except that it does not include the mixing chamber 5 and the mixer 7) was operated in the same manner as in Example 1.

The results are shown in Table 1.

Comparative Example 2 The same operation as in Example 1 was carried out using the apparatus shown in FIG.

This device does not include the mixing chamber 5 and the mixer 7, but instead consists of 8 titanium disks with a diameter of 1,200 mL and a thickness of 10 mm, each with a reinforced frame, and holes with a diameter of 20 mrn drilled all over the entire surface at a pitch of 30 mm. were installed horizontally at equal intervals as shown in the figure.

The results are shown in Table 1.

[Brief explanation of drawings]

1st. Second. FIG. 3 and FIG. 4 show Example 1, respectively.
Example 2. 1 is a diagram schematically showing a urea synthesis apparatus used in Comparative Example 1 and Comparative Example 2. 1...Urea synthesis tube, 5...Mixing chamber, 7
・・・・・・Piped type non-drive mixer.

Claims (1)

[Claims] 1. Carbon dioxide, ammonia, and recovered unreacted carbon dioxide and ammonia in a liquid/gas weight ratio of at least 2.
．． In step 5, the gaseous and liquid forms are passed through a gas-liquid mixing device;
A urea synthesis method characterized in that the gas is dispersed in a liquid as fine bubbles to form a mixture, and the mixture is reacted at a urea synthesis temperature and pressure in a urea synthesis zone. 2. The method according to claim 1, characterized in that the liquid/gas weight ratio is 2.6-14.0. 3. The method according to claim 1, wherein the gas-liquid mixing zone comprises a pipe-type non-driven mixer. 4. The method according to claim 1, wherein the gas-liquid mixing zone is provided inside or outside the urea synthesis zone, close to the inlet thereof.