RU2396253C2 - Method of obtaining urea and installation for its realisation - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to obtaining urea from ammonia and carbon dioxide. As a result of ammonia and carbon dioxide interaction at high pressure in reactor obtained is water solution, which contains urea, ammonium carbamate and ammonia. From obtained water solution carbamate and ammonia are separated by decomposing carbamate and thermal evaporation of ammonia and carbon dioxide in stripping apparatus, obtaining ammonia and carbon dioxide, which after that are again condensed in condenser obtaining carbamate, which is returned to reactor. All stages together with reaction of synthesis are carried out in fact at one and the same pressure. In addition, into reactor additionally supplied is passivating oxygen, obtained as a result of interaction waste gases, which contain carbon dioxide, ammonia and passivating oxygen which did not take part in the reaction, are removed from reactor and are supplied into bottom part of stripping apparatus for passivation of at least part of its internal surfaces. Installation for obtaining urea contains communicating with each other and forming closed high pressure contour reactor, stripping apparatus, condenser, section of final urea purification and pipelines for supply into reactor of carbon dioxide and ammonia. It is equipped with pipeline for supply into reactor of passivating oxygen and pipeline for connecting upper part of reactor with bottom part of stripping apparatus.

EFFECT: ensuring even and efficient passivation of all apparatuses of high pressure contour.

13 cl, 2 dwg

Description

FIELD OF THE INVENTION

The present invention relates to a method for producing urea from ammonia and carbon dioxide as a result of their interaction at a certain high pressure in an appropriate urea synthesis reactor. The invention relates in particular to a method of the type indicated above, in which the reaction product of ammonia and carbon dioxide, which is essentially an aqueous solution containing urea, ammonium carbamate and ammonia, is separated into ammonium carbamate and ammonia, which are returned to the reactor, and to an aqueous solution of urea, from which, upon subsequent purification, urea is obtained with a minimum amount of ammonia and carbon dioxide remaining in it. More specifically, the present invention relates to a method of the type indicated above, in which ammonia and carbon dioxide are obtained by thermal evaporation by decomposition of the carbamate in a suitable stripping apparatus, which are then condensed again in the corresponding condenser to produce carbamate, which is returned to the reactor, and all these operations together with the urea synthesis reaction are carried out at essentially the same pressure in a closed loop, known as a high pressure loop (HPC).

The present invention also relates to a plant for producing urea according to the invention.

State of the art

It is known that urea is currently produced by the method of the type indicated above, in which carbamate and ammonia are separated from an aqueous solution in the so-called HPC, which includes a urea synthesis reactor, a stripper, and a condenser, which is usually made in the form of a tubular heat exchanger. It is also known that the high corrosivity of the liquids and gases involved in the urea production process and their chemical aggressiveness reduce the service life of the corresponding HPC apparatus.

For this reason, and also taking into account the difficult operating conditions in the HPC (pressure from 135 to 175 bar, temperature in the reactor from 180 to 200 ° C, temperature in the stripper from 170 to 210 ° C), further improvement of the technology for producing urea substantially depends on the possibility of using steel of special grades, metals such as titanium and zirconium, as well as other materials with high corrosion resistance.

To protect apparatuses made from such materials from corrosion, their outer and / or inner walls, which are in the HPC in contact with aggressive liquids and / or gases, are usually passivated.

It is known, in particular, the proposal to use oxygen in the synthesis of urea and to supply air to the HPC in a certain amount (possibly enriched with oxygen), which passivates the metal walls of the apparatus.

For this, in particular, the oxygen necessary to passivate the walls of the reactor is usually added in a certain amount of air to the carbon dioxide supplied to the reactor through its lower part, and the oxygen rising in the reactor passivates all metal surfaces with which it somehow enters in contact.

The predominant part of the oxygen supplied from below to the reactor, then, together with the urea solution, is supplied to the upper part (head) of the stripping apparatus located above the tube bundle. Oxygen separated from the urea solution, together with the vapors formed in the stripping apparatus, immediately leaves the upper part of the stripping apparatus and cannot ensure proper passivation of its internal surfaces. Therefore, in order to reliably passivate the rest of the HPC, a certain regulated amount of air must be supplied to the lower part of the stripper, the oxygen contained in it rises up in the stripper, passes through the tube bundle and passivates the internal surfaces of the stripper accordingly.

With all its specific advantages, this method of producing urea also has certain disadvantages associated with the supply of additional air to the lower part of the stripping apparatus.

Obviously, at high pressure in the stripper reaching 140 bar, only compressed air can be supplied into it using sophisticated equipment, in particular, an appropriate compressor, which consumes a lot of energy and requires additional maintenance costs.

Another disadvantage of this method of producing urea is associated with the need to use a sufficiently reliable device for precisely controlling the amount of air supplied to the reactor and the stripper, the excess of which can lead to unpleasant consequences, in particular, the formation of an explosive mixture.

Another disadvantage of this method of producing urea is that together with the air that is supplied to the stripper in addition to the air supplied to the reactor, a significant amount of inert gas enters the stripper, which must be separated from the reaction products using separate and quite complex separators. So, in particular, when air is supplied to the reactor and to the stripping apparatus together with the reagents (ammonia and carbon dioxide), which are the starting materials for urea production, in order to avoid losses and to reduce the overall yield of HPC, as well as to solve environmental problems associated with their possible ingress into the surrounding atmosphere, at existing plants for the production of urea, it is necessary to use special, rather sophisticated equipment designed to isolate and trap ammonia and carbon dioxide from those leaving the steam gas apparatus.

Summary of the invention

The basis of the present invention was the task of developing a new method for producing urea of the type indicated at the beginning of the description, which would provide the possibility of effective uniform and cheap passivation of HPC devices and at the same time the possibility of effective and economical elimination of the limitations and / or disadvantages of known methods.

This problem is solved using the proposed invention in the method of producing urea from ammonia and carbon dioxide, which consists in the fact that as a result of the interaction of ammonia and carbon dioxide at a certain high pressure in an appropriate reactor, an aqueous solution containing urea, ammonium carbamate and ammonia is obtained from carbamate and ammonia are isolated in an aqueous solution, and ammonia and dioxide are obtained by decomposing the carbamate and thermally evaporating ammonia and carbon dioxide in an appropriate stripping apparatus gleroda which again is condensed in a suitable condenser to afford the carbamate that is recycled to the reactor, and all these steps together with the synthesis reaction is carried out substantially at the same pressure and which is characterized in that the rector is fed to the synthesis passivation oxygen; flue gases containing unreacted carbon dioxide and ammonia and passivating oxygen are removed from the reactor, flue gases taken from the reactor are fed to the lower part of the stripping apparatus and used to passivate at least part of its internal surfaces.

The urea preparation method according to the invention makes it possible to substantially reduce the amount of air supplied to the HPC, and accordingly to significantly increase the overall HPC output and to provide effective passivation of the internal walls of the stripping apparatus exposed to aggressive (corrosive) liquids and gases.

Other distinctive features and advantages of the urea preparation process according to the invention are described in more detail below by way of example, not limiting, but only illustrating the invention of a preferred embodiment thereof with reference to the accompanying drawings.

Brief Description of the Drawings

The accompanying description of the drawings shows:

figure 1 - diagram of the installation for producing urea proposed in the invention method and

figure 2 is a schematic view on an enlarged scale of one of the apparatus of the installation, a diagram of which is shown in figure 1.

Preferred Embodiment

In the drawings attached to the description, the urea plant according to the invention according to the invention is indicated by the general position 10.

When producing urea by the method of the type indicated at the beginning of the description, urea is obtained from ammonia and carbon dioxide, which react at a certain high pressure in the corresponding reactor 12.

In the reactor 12, an aqueous solution containing urea, ammonium carbamate and ammonia is obtained by reacting ammonia and carbon dioxide.

Carbamate and ammonia are isolated from the resulting aqueous solution, and by decomposing the carbamate and thermally evaporating ammonia and carbon dioxide in an appropriate stripping apparatus 14, ammonia and carbon dioxide are obtained, which are then condensed again in the corresponding condenser 16 to give carbamate, which is returned to reactor 12. All of these the steps, including the urea synthesis reaction, are carried out essentially at the same high pressure in the high pressure circuit (HPC).

In a preferred embodiment, an aqueous solution containing urea, ammonium carbamate and ammonia, taken from the reactor 12, is fed into the upper part 14a of the stripping apparatus 14.

When urea is produced by the method of the invention, passivating oxygen is also supplied to the reactor 12, exhaust gases containing unreacted carbon dioxide and ammonia and passivating oxygen are removed from the reactor 12, and the exhaust gases discharged from the reactor 12 are supplied to the lower part 14b of the stripping apparatus 14 for passivation of its inner metal walls or for passivation of at least part of its inner surface.

Passivating oxygen is fed to the HPC, preferably by adding air to the carbon dioxide fed to reactor 12, i.e. by supplying air and the oxygen contained therein directly to the reactor 12. The amount of air added to the carbon dioxide is adjusted appropriately to avoid supplying too much oxygen to the reactor, which can lead to undesirable consequences, in particular the formation of an explosive mixture in the reactor.

When urea is obtained by the method proposed in the preferred embodiment of the invention, exhaust gases containing carbon dioxide, ammonia and passivating oxygen are removed from the upper part 14a of the stripping apparatus 14, and these gases are sent to the condenser 16 to obtain liquid carbamate and passivation of the inner metal walls the capacitor 16 or at least part of the inner surface of the capacitor 16.

Below, with reference to the drawings attached to the description, the main structural features of the installation 10, intended for the synthesis of urea from ammonia and carbon dioxide, are considered.

The installation 10 consists of communicating with each other and forming a closed high-pressure circuit (HPC) of the urea synthesis reactor 12, the stripper 14 and the condenser 16. In this installation, in particular, ammonia is fed into the reactor 12 through pipeline 2, and carbon dioxide through pipeline 3.

Passivating oxygen supplied to the reactor 12 through line 5 is withdrawn from the reactor via line 6, which connects the upper part 12a of the reactor 12 to the lower part 14b of the stripper 14. The passivating oxygen from line 5 is preferably supplied to line 3, through which carbon dioxide is fed into the reactor. It is also preferable to supply passivating oxygen to the pipe 5 as part of ordinary air.

An aqueous solution withdrawn from the reactor containing urea, ammonium carbamate and ammonia is supplied via line 4 to the upper part 14a of the stripper 14.

The stripping apparatus 14 is essentially a heat exchanger 24 consisting of a bundle of vertical pipes, into which, through the pipe 14 located above it in the upper part 14a of the stripping apparatus 14, a pipe is taken out of the synthesis reactor 12 containing urea, ammonium carbamate and ammonia, which is from top to bottom passes through the tubes of the heat exchanger and is collected in the lower part of the stripper apparatus body. The exhaust gases removed from the upper part 12a of the reactor 12, containing unreacted carbon dioxide and ammonia and passivating oxygen, are supplied through the pipe 34 to the lower part 14b of the stripping apparatus 14 located under the tubular heat exchanger 24, from which they enter the lower ends of the heat exchanger tubes 24.

Figure 2 shows one of the tubes of the heat exchanger 24, indicated by 24a, and the tubes indicated by 35 and 36, respectively, designed to exhaust exhaust gases containing carbon dioxide, ammonia and passivating oxygen from the stripper, and to select a urea solution with ammonia residues and carbon dioxide.

In a preferred embodiment, the upper part 14a of the stripper 14 is connected by a conduit 7 to a capacitor 16.

Upon receipt of urea proposed in the invention method, the installation 10 operates as follows.

The aqueous solution supplied from the reactor 12 to the stripping apparatus, containing urea, ammonium carbamate and ammonia, flows in the form of a film down the tubes 24a of the heat exchanger 24; gases containing carbon dioxide, ammonia and passivating oxygen removed from the upper part of the reactor 12a and supplied to the stripping apparatus through the pipe 6, rise through the tubes 24a of the heat exchanger 24 and uniformly affect all the internal parts of the stripping apparatus 14 subject to corrosion (inner walls of the bottom housing parts 14b, inner surfaces of tubes 24a and inner walls of upper housing part 14a). It must be emphasized that since the working pressure in the reactor 12 is slightly higher than the pressure in the stripping apparatus 14 (or rather, slightly higher than the pressure in the stripping apparatus 14, which is part of the HPC, in which all the apparatuses operate at essentially the same pressure), the reactor exhaust gases enter the stripper 14 essentially by gravity without the use of any separate compressor.

The heat necessary for evaporation is obtained by condensing water vapor, for example, at a pressure of 25 bar, which passes inside the stripper 14 through the annular space of the heat exchanger 24 and heats its tubes 24a from the outside. Pipelines for supplying fresh steam to the stripping apparatus and for extracting spent steam from it are indicated in the drawings at 36 and 37, and the corresponding nozzles at 38 and 39.

Gases containing carbon dioxide, ammonia and passivating oxygen, taken from the upper part 14a of the stripping apparatus 14, are fed through a pipe 7 to the condenser 16 to obtain liquid carbamate and passivate the inner walls of the condenser 16.

In the condenser 16 (which essentially consists of a tube bundle of heat exchanger), in which condensation (essentially complete in the embodiment shown in FIG. 1) takes place of the gases removed from the upper part 14a of the stripping apparatus 14 and a certain amount of heat is generated, steam is generated low pressure (for example, about 3-5 bar), which is used to clean the urea in the cleaning section located behind the HPC.

The level of the urea solution in the upper part 12a of the reactor 12 is constantly regulated using a conventional device 4a, which is therefore not requiring a special description, which allows the aqueous solution containing urea, ammonium carbamate and ammonia to be separated and fed from the reactor to the upper part 14a of the stripping apparatus 14, from the off-gases containing unreacted carbon dioxide and ammonia and passivating oxygen, which are supplied from the reactor to the lower part 14b of the stripping apparatus 14.

Gases taken from the reactor also contain inert gases, in particular hydrogen (which enters the reactor 12 together with carbon dioxide from the ammonia plant located before the urea plant 10 and liquid ammonia) and nitrogen, which is contained in the air added to the carbon dioxide fed to the reactor 12.

The carbamate solution obtained in the condenser 16 is fed into a separator 28 through a pipeline 8. In the separator 28, a predominant part of inert gases and some oxygen are extracted from the carbamate solution, which are fed through a pipe 9a to the urea final treatment section 32 (the so-called urea regeneration or recovery section) .

The carbamate solution exiting the separator 28, which is practically free of inert substances, is fed through a pipe 9 to the ejector 30. From the ejector 30, through which liquid ammonia is supplied to the reactor, the carbamate solution together with ammonia enters the reactor 12.

The urea solution, which contains the residues of ammonia and carbon dioxide, is fed from the lower part 14b of the stripping apparatus 14 via a pipe 13 to the urea final cleaning section 32. The oxygen contained in the inert gases is supplied via line 9a to section 32 and is used as necessary to passivate it.

The molten urea obtained in section 32 is sent through line 1 for further processing, for example, to a granulation plant, and the inert gas stream, along with the residues of ammonia and carbon dioxide, is sent through line 9b for treatment to appropriate treatment devices not shown in the drawings. In these devices, all residues of ammonia and carbon dioxide are removed from inert gases before being discharged into the atmosphere.

In section 32, in the process of urea purification, a so-called weak carbamate solution or an aqueous carbamate solution is formed, which is returned via pipeline 11 and pipeline 7 to condenser 16 and used to absorb ammonia and carbon dioxide vapors coming into the condenser from the stripping apparatus 14.

In high-capacity urea plants, the condenser 16 included in the HPC is preferably made in the form of an immersion condenser, known as “Full Condenser” ™, with a heat exchanger consisting of a bundle of vertical tubes of a certain size to obtain the optimal residence time for the urea to form a solution carbamate. Such a capacitor 16 works as a preliminary reactor and can significantly reduce the volume of the main reactor 12.

The present invention also provides a method for passivation of the HPC of a urea plant 10, which includes urea synthesis reactor 12 connected to each other, a stripper 14 and a condenser 16. When passivating the HPC of a urea plant, the method of the invention provides passivating oxygen to the reactor 12, exhaust gases are discharged from the reactor 12, which contain unreacted carbon dioxide and ammonia and passivating oxygen, and the gases discharged from the reactor are fed to the bottom 14b of the stripping apparatus 14 for passivating at least a portion of the internal surfaces of the stripping apparatus 14.

In a preferred embodiment of the passivation method of the invention, furthermore, exhaust gases containing carbon dioxide, ammonia and passivating oxygen are removed from the upper part 14a of the stripping apparatus 14, and the gases discharged from the stripping apparatus are supplied to a condenser 16 to produce liquid carbamate and passivation of at least part of the inner walls of the capacitor 16.

The present invention also relates to the use of exhaust gases from the reactor 12 of an apparatus 10 for producing urea containing unreacted carbon dioxide and ammonia and passivating oxygen for passivating at least the inner surface of the bottom of the stripper 14 with passivating oxygen, which is supplied into the reactor 12.

From the above description it clearly follows that the urea production method proposed in the invention solves the problem posed by the invention and has a number of advantages, the main of which is the uniform passivation of all HPC devices.

In addition, the proposed invention is characterized by the possibility of simple and reliable implementation.

Another advantage of the method proposed in the invention is associated with the non-obvious at first glance the possibility of using the predominant part of the oxygen supplied to the reactor for efficient passivation of the stripping apparatus (and also the apparatus located behind it) and, as a result, with a significant compared with known methods of obtaining urea of this type by reducing oxygen consumption.

In addition, in contrast to the known methods for the production of urea and passivation of the HPC apparatus according to the invention, the method does not require the use of sophisticated equipment for the separation of inert gases and the regeneration or isolation of the starting reagents from the gases emitted into the atmosphere.

Obviously, specialists in this field can, in accordance with specific requirements, make various changes and improvements to the urea plant described above that do not go beyond the scope of the invention defined by its formula.

Claims (13)

1. A method of producing urea from ammonia and carbon dioxide, comprising feeding ammonia and carbon dioxide to the reactor and ensuring their interaction at high pressure to obtain an aqueous solution containing urea, ammonium carbamate and ammonia, feeding the resulting aqueous solution to and from this aqueous solution, the allocation of carbamate and ammonia by ensuring the decomposition of carbamate and thermally evaporated ammonia and carbon dioxide and their subsequent re-condensation in the condenser to obtain carbamate, to which is returned to the reactor, and a solution containing urea, ammonia residues and carbon dioxide is obtained from the stripper, which is sent to the urea final purification section, all stages of the process being carried out at essentially the same pressure, characterized in that the reactor is additionally passivating oxygen is fed, exhaust gases obtained during the reaction, containing unreacted carbon dioxide, ammonia and passivating oxygen, are removed from the reactor and fed to the lower part of the stripping apparatus and for passivating at least a portion of its interior surfaces. 2. The method according to claim 1, characterized in that the air used as passivating oxygen is added to the carbon dioxide supplied to the reactor. 3. The method according to claim 1, characterized in that the aqueous solution containing urea, ammonium carbamate and ammonia is fed to the top of the stripper. 4. The method according to claim 1, characterized in that the exhaust gases containing carbon dioxide, ammonia and passivating oxygen are removed from the upper part of the stripping apparatus and fed to a condenser to obtain liquid carbamate and passivate at least part of its internal surfaces . 5. Installation for the production of urea from ammonia and carbon dioxide, containing communicating with each other and forming a closed loop high pressure reactor, stripper, condenser, section for the final purification of urea and pipelines for feeding carbon dioxide and ammonia into the reactor, characterized in that it equipped with a pipe for supplying passivating oxygen to the reactor and a pipe for connecting the upper part of the reactor with the lower part of the stripping apparatus. 6. Installation according to claim 5, characterized in that the pipeline for supplying passivating oxygen to the reactor is connected to the pipeline for supplying carbon dioxide to the reactor. 7. Installation according to claim 6, characterized in that the pipeline for supplying passivating oxygen to the reactor is configured to supply air. 8. Installation according to claim 5, characterized in that it is equipped with a pipeline for supplying to the upper part of the stripping apparatus an aqueous solution taken from a reactor containing urea, ammonium carbamate and ammonia. 9. The installation according to claim 5, characterized in that it is provided with a pipeline connecting the upper part of the stripping apparatus with a condenser. 10. Installation according to claim 5, characterized in that the stripping apparatus is equipped with a heat exchanger consisting of a bundle of vertical tubes for supplying to their upper ends an aqueous solution containing urea, ammonium carbamate and ammonia taken from the reactor, while the lower ends of the tubes are made with the possibility of communication with the pipeline for connecting the upper part of the reactor with the lower part of the stripping apparatus. 11. The method of passivation of the high pressure circuit of the installation for producing urea, consisting of a reactor connected to each other, a stripper, a condenser and a final urea purification section, comprising supplying exhaust gases containing unreacted carbon dioxide, ammonia and passivating to the lower part of the stripper oxygen obtained in the method according to claim 1 for passivation of at least part of its inner surfaces. 12. The method according to claim 11, characterized in that the exhaust gases containing carbon dioxide, ammonia and passivating oxygen are removed from the upper part of the stripping apparatus and fed to a condenser to obtain liquid carbamate and passivate at least part of its internal surfaces . 13. The use of exhaust gases containing unreacted carbon dioxide, ammonia and passivating oxygen obtained in the method according to claim 1 for passivation of at least part of the internal surfaces of the stripping apparatus of the urea plant of claim 5.

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