JPH0141622B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an improved process for producing urea from ammonia and carbon dioxide.

Various methods are known for producing urea from ammonia and urea. One method is to introduce the urea synthesis liquid containing unconverted ammonium carbamate (hereinafter referred to as "unconverted material") discharged from the urea synthesis tube into a stripper at substantially the same pressure as the urea synthesis pressure. At least a portion of the newly supplied carbon dioxide is used to strip unconverted substances under heating, and the unconverted substances separated as gaseous ammonia and carbon dioxide in the stripper are led to a condenser for heat recovery. A method for producing urea is known, which consists of condensing and circulating the resulting condensate through a urea synthesis tube (see Japanese Patent Publication No. 19964/1983). In this method, the pressure in the condenser is slightly lower than the pressure in the urea synthesis tube due to the pressure drop, so in order to circulate condensate from the condenser to the urea synthesis tube, the condenser must be installed higher than the stripper. , or it is necessary to use a booster.

The method of installing the condenser higher than the stripper and using gravity to feed the condensate into the urea synthesis tube does not require power, but in the case of large equipment, a strong mount is required to install the heavy condenser at a high position. A foundation for this is required, which increases equipment costs.

In addition, one method using a booster is to pressurize the newly supplied liquid ammonia to a pressure higher than that of the urea synthesis tube, supply this to the ejector as a driving fluid, and use the suction action of the ejector to remove the condensate from the condenser. It is known that the pressure of urea is increased and circulated to the urea synthesis tube (Special Publication No. 1973-
(See No. 42085). However, in this method, it is necessary to increase the pressure of liquid ammonia, which is the driving fluid, to a pressure higher than 60 kg/cm ² than when directly feeding the synthesis tube, and for this purpose, extra energy is required from outside the urea synthesis system. must be supplied.

The present invention aims to provide a method for producing urea in which unconverted products can be recycled to urea synthesis without using energy from outside the urea synthesis system.

The method for producing urea according to this invention involves reacting ammonia and carbon dioxide at a urea synthesis temperature and pressure, heating the resulting urea synthesis liquid at a pressure substantially equal to the urea synthesis pressure, and then heating the resulting urea synthesis liquid at a pressure substantially equal to the urea synthesis pressure. decomposing at least a portion of the converted product into gaseous ammonia and carbon dioxide; and separating the gaseous ammonia and carbon dioxide as a gaseous mixture from the aqueous urea solution containing residual unconverted product; and converting the separated gaseous mixture into the urea. condensing at a pressure substantially equal to the synthesis pressure and recycling the resulting condensate to the urea synthesis, while the aqueous urea solution is depressurized and heated to decompose substantially all of the residual unconverted material; When the aqueous urea solution thus obtained, which is substantially free of unconverted products, is subjected to a finishing treatment to obtain urea, the gaseous mixture is supplied to the eductor as a driving fluid without pressurization. , supplying a medium capable of absorbing the gaseous mixture, which has approximately the same pressure as the urea synthesis pressure and a lower temperature than the gaseous mixture, as a suction fluid to the ejector, and bringing the gaseous mixture into contact with the medium; The method is characterized in that the condensate thus obtained is pressurized by the absorption pressurization effect of the ejector and recycled to the urea synthesis.

The method of this invention maintains a circulating flow by creating locally a low temperature region and a high temperature region in a quasi-cyclic system under approximately the same pressure, and generating work in the low temperature region by the above-mentioned easy dissolution phenomenon. This is the way to do it. In other words, the circulation flow is maintained not by power but by heat exchange.

Embodiments of the invention will be described below with reference to FIGS. 1 and 2. The embodiment shown in FIG. 1 shows an example in which the condenser is provided separately from the urea synthesis tube;
The figure shows an example in which the condenser is integrated into the lower part of the urea synthesis tube. 1 and 2, the urea synthesis tube 1 is supplied with liquid ammonia from line 9, carbon dioxide from line 10, and the condensate described below from condensate 3 (via line 20), preferably under pressure. The reaction is carried out at 140-250 Kg/cm ² G and at a temperature of 160-210°C. The total NH ₃ /total CO ₂ molar ratio supplied to the urea synthesis tube 1 is 2~
5, particularly preferably 3 to 4.5.

Urea synthesis liquid (urea, water,
The unconverted product and excess ammonia (if used) are introduced via line 11 into a high-pressure decomposer 2 maintained at a pressure substantially equal to the urea synthesis pressure, and are passed through line 12 in a heating tube 4. The high-pressure steam introduced and discharged from line 13 is heated, preferably to 170-220°C, to decompose at least a portion of the unconverted material and, together with excess ammonia, form a gaseous mixture of ammonia and carbon dioxide (hereinafter simply "gaseous"). The mixture is separated from the aqueous urea solution containing the remainder of the unconverted product. Note that it is also possible to accelerate the decomposition of unconverted products by blowing a portion of the make-up carbon dioxide into the bottom of the high-pressure decomposer through line 21 (if excess ammonia is used, the excess ammonia is mainly driven out and the unconverted decomposition of converted products remains relatively small).

The aqueous urea solution containing residual unconverted substances separated from the gaseous mixture in the high pressure separator 2 is transferred to the pressure reducing valve 14.
The urea product is then reduced to a pressure of preferably 2 to 30 Kg/cm ² G and sent via line 15 to one or more low pressure cracking and finishing steps well known to those skilled in the art to produce the product urea.

The gaseous mixture decomposed in the high-pressure decomposer 2 is supplied via a line 16 to a drive fluid chamber 5 in the ejector section of the condenser 3, which is maintained at substantially the same pressure as the high-pressure decomposer 2. On the other hand, a medium (hereinafter simply referred to as "medium") capable of absorbing the gaseous mixture and having a lower temperature than the gaseous mixture is supplied from the line 17 to the small chamber 6 for the fluid to be sucked in the ejector section.

The condenser 3 consists of a cooling section and an ejector section. The cooling section has at least one cooling pipe 8,
This cooling pipe is supplied from line 18 and line 19
It is cooled by the cooling medium discharged from the This cooling section is well known to those skilled in the art and typically recovers the heat of condensation of the gaseous mixture (such as the heat of formation of ammonium carbamate) by generating steam. The ejector part includes a small chamber 5 for driving fluid, a small chamber 6 for sucked fluid, and a jet nozzle 7.
Regarding the relative relationship between the shape and dimensions of the cooling pipe 8 and the inlet portion of the cooling pipe 8, known ejector design methods can be applied. The cooling pipe 8 may be a straight pipe, or the inlet portion may be modified to have a shape suitable for exerting the ejector function. The cooling pipe 8 preferably has an inner diameter of 15 to 30 mm. Normally, a plurality of cooling pipes 8, especially a large number of cooling pipes 8, are used, and the jet nozzle 7
They are provided in correspondence with one another.

The medium supplied from line 17 is water, aqueous ammonia, aqueous urea solution, or a solution obtained by absorbing ammonia and carbon dioxide obtained by decomposing residual unconverted substances in the low-pressure decomposition process. . These media and line 16
_NH3 in the condensate obtained by condensing and absorbing the gaseous mixture by contacting with the gaseous mixture from:
It is preferable to adjust the amount and composition of the medium so that the molar ratio of CO ₂ :H ₂ O is in the range of 4.0 to 4.5:1.0 to 1.5:1. According to the inventors' study, in the condensate
When the NH ₃ /CO ₂ molar ratio is 3.0 to 3.5, the equilibrium pressure of the condensate becomes minimum, and the gaseous mixture can be condensed and absorbed even if the condensation temperature is 165 to 170°C. However, even if this NH ₃ /CO ₂ molar ratio is outside the above range, condensation and absorption is possible if the condensation temperature is 150 to 160°C within the range of 2.4 to 4.5. inside the urea synthesis tube
The molar ratio of _NH3 / _CO2 is particularly preferably controlled between 3 and 4.5, so that the molar ratio of _NH3 / _CO2 in the condensate is within the range of 2.5 to 4.5. Therefore, it is possible to easily condense and absorb the gaseous mixture supplied from the line 16, maintain the heat balance of the urea synthesis tube by keeping the condensation temperature as high as possible, and reduce the pressure of the steam recovered in the condenser 3. In order _{to increase}
Preferably, the amount of medium from line 17 is controlled such that the molar ratio of H ₂ O/CO ₂ is between 0.7 and 1.2. The condensation temperature of the gaseous mixture from line 16 (the temperature of the resulting condensate) is preferably between 140 and 180C.

Additionally, the composition of the condensate may be adjusted as described above to provide make-up ammonia and/or
or some of the carbon dioxide to lines 22 and 23
It is also possible to supply the fluid to be aspirated into the small chamber 6 from there. In this case, depending on the selection of the amounts of ammonia and/or carbon dioxide and the selection of the condensation temperature, it may become impossible to maintain the urea synthesis temperature. In this case, the urea synthesis temperature can be maintained by preheating the ammonia and/or carbon dioxide supplied to the urea synthesis tube 1.

The gaseous mixture supplied to the drive fluid chamber 5 and passed through the jet nozzle 7 is supplied to the suctioned fluid chamber 6 and comes into contact with a medium at a lower temperature than the gaseous mixture, which is entering the cooling pipe 8. do. Due to this contact and being cooled by the cooling medium from line 18, the gaseous mixture is condensed and absorbed into this medium while generating heat, so that the gaseous mixture has a narrow opening area. The ejector is ejected from the tip of the ejection nozzle 7 into the medium in the cooling pipe 8 at high speed in a predetermined direction, and as a result, the ejector's suction pressurization action occurs, and the gaseous mixture is condensed and absorbed into the medium, resulting in the generated condensate. The liquid is pressurized. This condensate exits the cooling pipe 8 and enters the urea synthesis tube 1 via line 20 or directly, and is subjected to urea synthesis together with make-up ammonia and carbon dioxide as described above.

Although FIGS. 1 and 2 show examples in which the condenser 3 is placed horizontally and vertically with upward flow, the present invention can also be applied to cases where the condenser 3 is placed vertically with downward flow. However, in the case of a vertical downward flow, some inert gas (derived from the raw carbon dioxide) may be present in the gaseous mixture from line 16. Even when the gas is removed, a small amount of inert gas (accompanied by the gaseous mixture) is still included, and this may accumulate and make it difficult for the ejector to exert its suction and pressure boosting effect.

According to this invention, the gaseous mixture obtained by high-pressure decomposition of unconverted substances can be recycled to urea synthesis without using energy from outside the urea synthesis system. That is, it is not necessary to pressurize make-up liquid ammonia to a pressure higher than the urea synthesis pressure by 60 kg/cm ² or more, which is the case when using an ejector according to the conventional method. This corresponds to a power saving of approximately 4KWH per ton of product urea.
Furthermore, since there is no need to install the condenser and urea synthesis tube at a high position, there is no need to make their frames, foundations, etc. particularly strong, and an increase in equipment costs can be suppressed.

EXAMPLES The present invention will be explained in more detail with reference to Examples below.

EXAMPLE An experiment was carried out according to the process shown in FIG. 1 using a test device capable of producing 60 kg of urea/hr. Liquid ammonia pressurized to 180Kg/cm ² G and preheated to 150℃ 34.4
Kg/hr, carbon dioxide pressurized to 180Kg/cm ² G 22.0
Kg/hr and the condensate described later with an inner diameter of 0.35 m and a height of
The mixture was supplied to a 4.8 m urea synthesis tube and reacted at 190°C.

The urea synthesis liquid from the urea synthesis tube is supplied to the high-pressure decomposer, and at the same time, 22.0Kg/hr of carbon dioxide is supplied.
decompose the unconverted product at 195°C by contacting with
A gaseous mixture consisting of 42.6Kg/hr of ammonia, 24.3Kg/hr of carbon dioxide and 5.0Kg/hr of water was mixed with 60Kg/hr of urea.
Kg/hr, ammonia 28.8Kg/hr, carbon dioxide 23.4
Kg/hr and water was separated from an aqueous urea solution consisting of 30.1 Kg/hr. This aqueous urea solution was sequentially depressurized to 18 Kg/cm ² G and 2 Kg/cm ² G to separate residual unconverted substances and subjected to finishing treatment to obtain 60 Kg/hr of urea. The separated unconverted product was absorbed into water by a conventional method to obtain an aqueous ammonium carbamate solution containing 28.4 Kg/hr of ammonia, 23.4 Kg/hr of carbon dioxide, and 12.1 Kg/hr of water at 18 Kg/cm ² G and at 95°C. .

Using the above gaseous mixture as a driving fluid, the ejection nozzle of the ejector part of the condenser (the inner diameter of the ejection port is 2
mm, ferrite-austenite two-phase stainless steel) and cooled the cooling pipe (ferrite-austenite two-phase stainless steel with an inner diameter of 15 mm) with boiling water of 2 kg/cm ² G to 150 The ammonium carbamate aqueous solution kept at 180 Kg/cm ² G and pressurized was inhaled and pressurized. The generated condensate was recirculated smoothly to the urea synthesis tube.

[Brief explanation of drawings]

FIGS. 1 and 2 are flow sheets for explaining embodiments of the present invention. 1...Urea synthesis tube, 2...High pressure decomposer, 3...
Condenser, 4...Heating tube, 7...Blowout nozzle, 8...
...Cooling pipe.

Claims (1)

[Claims] 1. Ammonia and carbon dioxide are reacted at a urea synthesis temperature and pressure, and the resulting urea synthesis liquid is heated at a pressure substantially equal to the urea synthesis pressure to convert unconverted ammonium carbamate into urea contained therein. decompose at least a portion of the gaseous ammonia and carbon dioxide into gaseous ammonia and carbon dioxide, and separate the gaseous ammonia and carbon dioxide as a gaseous mixture from the aqueous urea solution containing residual unconverted ammonium carbamate; Condensing at a pressure substantially equal to the synthesis pressure and circulating the resulting condensate to the urea synthesis, while the aqueous urea solution is depressurized and heated to decompose substantially all of the residual unconverted ammonium carbamate. , and the resulting aqueous urea solution containing substantially unconverted ammonium carbamate is subjected to a finishing treatment to obtain urea, and the gaseous mixture is supplied as a driving fluid to an ejector without pressurization. On the other hand, a medium capable of absorbing the gaseous mixture at approximately the same pressure as the urea synthesis pressure and having a temperature lower than that of the gaseous mixture is supplied to the ejector as a suction fluid, and the gaseous mixture is mixed with the medium. A method for producing urea, which comprises: condensing the liquid by contacting the urea, and circulating the thus obtained condensate pressurized by the suction pressurization action of the ejector to the urea synthesis.