JPS59122452A - Urea synthesis tube - Google Patents

Abstract

PURPOSE:To decrease the height of a urea synthesis plant and mitigate the problems accompanied to a huge plant, and to produce urea in an efficiency comparable to or higher than that of the vertical synthesis tube, by using a synthesis tube placed horizontally or nearly horizontally and having the inner space divided into the sections connected with each other in series. CONSTITUTION:The synthesis tube 101 is divided into plural sections by partition walls, and furnished with a means for releasing and dispersing the raw material gas and a means for introducing the raw material liquid at the bottom of the tube. A carbamate solution is introduced to the bottom of the horizontally placed synthesis tube 101 from the carbamate condenser 103 through the line 3, and the uncondensed gas discharged from the condenser 103 is supplied to each section through the line 3. The gas is absorbed in the solution in the course of flowing upward concurrently with the solution, and the liquid containing the synthesized product obtained by the reaction is flowed over the partition walls to the next section. The unabsorbed gas is transferred through the connecting hole bored above the liquid level, discharged from the end section, and introduced to the scrubber 104 through the line 7. The synthesis liquid passed through all sections is sent to the stripper 102 through the line 5, and the urea solution is obtained from the line 11.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a urea synthesis tube, and more particularly to a synthesis tube installed horizontally or substantially horizontally. Industrial synthesis of urea using ammonia and carbon dioxide as raw materials is widely practiced. The urea synthesis tubes used for this are all vertical, and basically both raw materials are fed from below, and the reaction is carried out in parallel flow of both raw materials. A product stream containing reactants, inert gas, water, etc. is removed. Urea is extracted as an aqueous solution from the product stream and becomes the final product through processes such as concentration.

Although some of the remaining components in the product stream are discharged as necessary, they are usually recovered by an appropriate method, circulated to the synthesis tube, and sent to the lower part of the synthesis tube. At the same time, ammonia and Carbon dioxide, etc. are supplied to the lower part of the synthesis tube from outside the system.

The conditions for the synthesis reaction are approximately a temperature of 160 to 210°C and a pressure of 10°C.
0-300kO/cwrG, NH3/CO2 molar ratio 2.
5-6.0, residence time for one pass in synthesis tube 15-70
It takes about a minute.

God's methods are being put into practice to recover and circulate reusable components in product streams economically and effectively with as little energy loss as possible, and one of the most representative is the stripping method. That is, the product stream is generally stripped with raw carbon dioxide gas or ammonia gas in a stripper under the same high pressure as in the urea synthesis reaction, after or with appropriate decomposition as necessary, and the useful material entrained in this stripping gas is stripped. Typically, the components are at least partially absorbed in an aqueous solution mainly containing ammonium carbamate in a carbamate condenser, and then the remaining unabsorbed gas is sent to the lower part of the synthesis tube as a circulating fluid. Ru. The effluent from the lower part of the stripper undergoes appropriate separation and purification steps to become a urea melt. The useful components recovered from this separated and purified culm are generally absorbed into water or a so-called urea solution, which is further used as an absorption liquid for the carbamate condenser.

Today, it is typical to absorb the useful gases obtained, inter alia, by stripping with carbon dioxide gas, in carbamate condensers. In such a method,
It is convenient to arrange the carbamate condenser, synthetic tube, and stripper in this order from top to bottom, since it is possible to assemble a system using gravity without using any special power.

However, as mentioned above, these systems are 100-300/c
Since it has to withstand the high pressure of dG, if the production volume is large, it must be heavy and huge, causing various physical, economic, and personnel problems in construction, maintenance, and inspection. Ta. To give a specific example, when starting synthesis, it is necessary to reach the top of the system for inspection in the event of an abnormality, water injection in an emergency, periodic repairs, etc. Structures such as scaffolding, stairs, and handrails are required, and such work requires work at heights, which is inefficient and dangerous. For example, in a Urea Nissan 20001-N plant, the above three systems are approximately 70 m in height, including the necessary scrubber, as will be explained later with drawings.
It reaches the level of Most of this, 40m, comes from synthetic pipes. In the synthesis tube, it is necessary for the raw material flow in which gas and liquid coexist to flow in parallel, so it is essential that the synthesis tube be of an upright vertical type.

Especially in the stripping method, which is often carried out at lower pressures, there are concerns such as upward blow-through of the raw material I gas component and back mixing of the liquid in the synthesis tube from the top to the bottom in an upright synthesis tube, and these should be prevented. This had the disadvantage of requiring careful consideration.

On the other hand, if the synthesis tube is simply provided horizontally or substantially horizontally, the height can be reduced, but the parallel flow of gas and liquid becomes poor, leading to a decrease in the synthesis rate.

Using ammonia and carbon dioxide as starting materials, urea is synthesized by flowing liquid and gaseous raw materials in the same direction in a urea synthesis tube at high temperatures and high pressures suitable for urea synthesis. The present invention was obtained as a result of research to allow the urea synthesis reaction to proceed as well or better than conventional methods even when the tube is horizontal or substantially horizontal.

That is, it is installed horizontally or approximately horizontally, and its interior space consists of a plurality of sections connected in series, with at least a part of the raw material flow being supplied to the section at the inlet end, and at least part of the raw material flow in the remaining part being supplied to the section at the inlet end. A urea synthesis tube is provided which is fed into section 4 of the section, the feed stream being directed entirely to an outlet end, and a product stream comprising urea exiting from an outlet end opposite the inlet end.

Each section of the synthesis tube of the present invention usually has a raw material gas release and dispersion mechanism and a raw material liquid introduction mechanism at its bottom, and the raw material gas and liquid rise within the section in parallel flow while being brought into sufficient contact with each other. , the liquid overflows and flows down from the upper end of the weir provided in the section at the height of the upper liquid level, and the lower end of the partition wall between the section and the adjacent section on the outlet end side or the It flows into the adjacent section from an inlet provided nearby. Generally, it is preferable that the weir and the partition wall be provided perpendicularly or substantially perpendicularly to the axis of the synthetic tube in accordance with the inclination of the axis of the synthetic tube, but the invention is not necessarily limited thereto. Gas discharged from the bottom of the section onto the upper liquid level may be vented from the top of each section, but usually a device to prevent liquid or droplets from getting mixed in is installed at a portion of each bulkhead higher than the liquid level as necessary. The liquid level in the adjacent section closer to the outlet end flows into the space through the communication hole with the
Everything collects in the exit end section and exits from there. The circulating solution, which also often contains components derived from by-product carbamates, is introduced from the introduction mechanism at the bottom of the inlet end section, while the synthesis solution is introduced from the open end section, generally at the end of the synthesis tube. The overflow flows between the weir or overflow wall provided near the wall or side wall and these walls, and is then taken out of the synthesis tube.

According to the present invention, even in a plant with a production capacity of 2,000 tons, the height of the plant derived from the urea synthesis pipe can be reduced from 40 m to only about 7 m, and even including the scrubber, carbamate condenser, stripper, etc., it is approximately 34 m. Reduce by half. Therefore, the various problems mentioned above are significantly alleviated. Moreover, since the synthesis tube is divided into a plurality of sections, there is no need to worry about upward blow-through of the previously mentioned raw material gas components or back mixing toward the liquid inlet in the synthesis tube.

In the present invention, the number of sections in the synthesis tube is not particularly limited, but is generally from several to about 20, and the volumes of liquid in the individual sections are usually equal to each other, but may be different if necessary. Circulating fluid is introduced from the inlet end section;
Carbon dioxide gas or a gas mixture containing further ammonia, water, and liquid ammonia are generally introduced in at least one of a plurality of sections, including the inlet end, in appropriate portions as necessary, and each raw material or other feed is Gas is introduced from the bottom of each section and absorption and/or reaction takes place in parallel flow upwards within the section; movement to the next section is brought about by overflow of the liquid downwards; The unabsorbed inert gas is the main stream, but it moves through the communication hole in the part above the liquid level of the partition wall between each section, and finally the synthetic liquid and unabsorbed gas are released from the outlet end section. , generally separated into liquid and gas and discharged. Each raw material is introduced into at least one section at an appropriate position where the reaction etc. can preferably take place, and in an amount so that the reaction etc. can take place preferably depending on the section position. In the case of liquid ammonia, in particular,
It is preferable to carry out the introduction in a later section when this reacts sufficiently and 7- is absorbed so that almost no gasification occurs, or when it starts to contribute to improving the synthesis rate. The gas is dispersed from the bottom of each section into cells by a suitable dispersion device such as a sparger, and then emptied into the liquid flowing upward, and virtually all of the gas is absorbed by the time it reaches the liquid level above the section. Most of the gas is absorbed by the liquid. It is also preferable to install two or three perforated plates or the like in each section as necessary to promote contact between gas and liquid, and this is particularly effective when the operating pressure is low compared to the synthesis temperature.

In addition, in the exposed end section or the section near it, the synthesis reaction can be allowed to proceed without actually being supplied with gas.

Since the liquid is transferred from one section to the other section by overflow, the position of the upper liquid level within each section is lower as the section is farther from the end (closer to the outlet end). In pipes, the depth of liquid in each section is generally shallower toward the outlet end if the pipe is installed horizontally, which is the normal case when the bottom height of each section is at the same 10-degree position.

The synthetic tube can be tilted slightly to achieve the same depth, but this generally reduces the uniformity of liquid depth within the section.
In order to ensure good reaction, consideration should be given to the shape of the bottom of each section, as well as the method and equipment for dispersion and mixing of gas and liquid.

The present invention will be specifically explained in the following example, but this example is only a typical representative example of the present invention, and the present invention is not limited thereto.

Production 12 due to the highly effective stripping process
The operating and equipment design values are shown for a large-scale plant of 1,000 tons/day.

In FIG. 1, a stripper 102, a synthesis pipe 101 (drawn larger than other equipment for ease of understanding the internal structure), a carbamate condenser 103, and a scrubber 10.
4 are assembled into one frame in this order from bottom to top. A synthetic tube with an inner diameter of 3 m and a length of 38 m is installed horizontally, and its interior is divided into eight sections of equal liquid volume, from the first section at the left end, or inlet end, to the eighth section at the right end, or outlet end of the synthetic pipe 101 in the figure. It is divided into eight sections. The total height from the stripper to the scrubber including piping will be approximately 30 IIl. The depth of the pedestal is approximately 7m.

The system is operated at a pressure of 1 to 0 kfl/C11f.

Carbamate solution (NH373.95, G O273,
98, and 1''2023.65 t/kr each) enter the bottom of the synthesis tube 101 (left end in the figure) from line 3.

On the other hand, the uncondensed mixed gas in the Bamate condenser 103 (
N H327,06, C0224,05 and Hz 0
2.32 each t/#r) is divided into eight parts through line 4 and introduced into synthesis tube 101.

That is, 12.1% of that is in the first section, 13.4% in the second section, 58.9% in the third section, and 58.9% in the fourth section.
2.4% is introduced into the section, and sections 5 to 8 are each introduced in amounts ranging from 2.4 to 0.7%. The third section also contains a new liquid ammonia 41.5t/I? corresponding to the urea output from this system. r is introduced. Therefore, a large proportion of gas is introduced into this section. In this example, liquid ammonia is introduced only into the third section, but of course it may be introduced into a plurality of appropriate sections as necessary. The mixed gas introduced into each section becomes fine bubbles through a sparger installed at the bottom of each section and rises in the liquid phase, during which time substantially most of the gas is absorbed into the liquid phase. Ru. The circulating carbamate solution introduced from the bottom of the first section flows upwards and at the top it overflows the weir and flows into the bottom of the second section. A similar flow is repeated for the second to eighth sections in order. In the fifth to eighth sections, the small amount of gas introduced here results in only absorption of CO2 into the liquid phase;
Conversely, evaporation of NH3 and Hz0 occurs from the liquid phase depending on its composition. Between the first to eighth sections, the liquid temperature gradually rises due to gas absorption and urea is produced, and in the final eighth section, urea is synthesized to a value close to equilibrium. Table 1 shows the urea synthesis rate in each section in this example.

11- (Table 1) Section Temperature (℃) Synthesis rate (%) 1 1
75 33 2 184 52 3 187 60 4 188 62 5 189 63 6 189 64 7 190 65 8 190 Result of 6. From the upper part of the 8th section, a synthetic solution with an NH3/GO2 molar ratio of 0.65 and an equilibrium achievement rate of 95% is obtained. From the gas phase in this section, N accompanies the inert gas.
H34,57, CO20,32 and H200,36 each t//? r is extracted and guided through line 1 to scrubber 104. In the scrubber 104, a carbamate solution (NH32
2,44,CO229,03 Oyobi 142018.70
NH3 in the inert gas,
CO2 and 1 (20) are absorbed and removed, and the inert gas after this treatment is depressurized by a valve 502 and then treated in a medium pressure or low pressure system in order to remove the NH3 and CO2 that it still contains, although it is a very small amount.

On the other hand, the above synthetic solution obtained from section 8 (urea 87
．． 70. NH394,24, CO233,39 and H
2O51,92 each j/I? r composition) is introduced into the top of the stripper 102 through line 5, and fresh CO2 gas (56, 67
t/l? r, at a temperature of 130°C), and by heating with high pressure steam from line 23, a portion of unreacted NH3 and CO2 in this synthetic liquid solution is stripped, and at the same time, a part of unreacted NH3 and CO2 is removed from the bottom of the column at a temperature of 180°C. Urea solution (Fi
j2 elementary 85.04. NH32L75, CO223,
33, and H2O44,20 each j/#r composition) is (
Line 11) is obtained.

After the pressure is reduced by the pressure reducing valve 501, the urea solution has a pressure of 18 ka (not shown).
/ at G medium pressure carbamate decomposer,
Thereafter, it is further subjected to conventional treatments such as low-pressure decomposition and purification. Unreacted NH3, which was separated after medium pressure decomposition, is added to the dilute aqueous ammonium carbonate solution obtained from the purification process.

The CO2 is absorbed to obtain the above-mentioned medium pressure absorption solution, which after backpressure to the synthetic pressure is introduced into the scrubber 104 via line 8.

Also, the mixed gas (N
H374,00, CO268,67 and l-1206
. Condenses or is absorbed while descending through the vessel. The absorbed heat generated at this time is 4.5k due to line 21.
It is recovered as low pressure steam of g/enrQ.

This heat recovery is performed so that the liquid temperature at the final stage of the synthesis tube 101, that is, the eighth section at the right end in the figure, is maintained at 190°C.

In the above example of the present invention, the heights of the individual parts of the system, including incidental parts such as piping, are approximately 7 m from the top for the scrubber, 10 m for the carbamate condenser, 9 synthetic pipes, 7
m, the stripper will be 10Ill, and the total length will be 30m.

On the other hand, in the case of a conventional vertical synthetic pipe, a system with similar capacity has a configuration as shown in Figure 2 (however, the scrubber 10
4 is drawn at a lower position than the actual position, which is higher than the carbamate condenser 103, for convenience of drawing. ), the heights of the individual parts of the system, including incidental parts such as piping, are approximately 1047 m from the top, 10 m for the scrubber, 10 m for the carbamate condenser, and 10 m for the vertical synthesis pipe.
1a 40m, stripper 102 10m, reaching a total of 7oI11.

Therefore, in addition to the various problems mentioned above, the huge weight of equipment, including synthetic pipes and other equipment, which is heavier due to its height, is concentrated in a smaller area. It is also more difficult to secure the required amount than in the case of the present invention, and there are also other problems such as construction period and cost. This conventional problem has been solved by the present invention.

It has conventionally been believed that if the urea synthesis tube is installed horizontally, the synthesis rate will drop, but by adopting the internal structure described above, a sufficiently high value can be obtained. On the other hand, even if there is a large amount of inert gas and a large amount of NH3 and CO2 accompanying it, the high pressure scrubber recovers heat and absorbs these one by one, so there is no loss.

[Brief explanation of the drawing]

FIG. 1 is a conceptual flow diagram of a system using the synthetic tube of the present invention, and FIG. 2 is a conceptual flow diagram of a system using a conventional vertical synthetic tube. 1 101.101a New NH 3 person Lorain 2
New CO2 population line 3103 to 102 Carbamate solution line 4103 to 101 Uncondensed mixed gas line 5101 to 101
Synthetic liquid line 6102 to 102 Absorption liquid inlet line 9104 to mixed gas line 8104 1101 to 104 Inert purge line 10 to 502
Absorption liquid line 11 from 104 to 103
102 to 501 core urea liquid outlet line 21 Recovery low pressure steam line 22 Coolant line 16-23 High pressure steam line 101 Synthesis tube 101a Vertical synthesis tube 102 Stripper 103 Carbamate condenser 104 Scrubber 501 Pressure reducing valve 502 Inert barge pressure reducing valve Applicant Toyo Engineering Co., Ltd. JP-A-59-1
22452 (6)

Claims (1)

[Claims] Using ammonia and carbon dioxide as starting materials, liquid and gaseous raw Fl streams are flowed in the same direction in a urea synthesis tube under high temperature and pressure suitable for urea synthesis. When the synthesis tube is synthesized, the synthesis tube is installed horizontally or substantially horizontally, and the synthesis tube interior space consists of sections connected in series, and at least a part of the raw material flow is supplied to the section at the inlet end of the synthesis tube, and the remaining part is original! a urea synthesis tube, wherein the stream is fed to at least some sections of the tube, the feed stream is all passed to the outlet end, and the compound stream containing the compound is discharged from the outlet end section of the synthesis tube.