SU1225485A3 - Method of producing urea - Google Patents

Abstract

Urea is produced form a gaseous mixture of H2, N2 and CO2, e.g. that obtained by the steam reforming of hydrocarbons. Carbon dioxide is absorbed from the mixture by the action, firstly, of an aqueous ammonia solution (e.g. above 70% by weight) and, secondly, of an ammonia-containing solution of ammonium carbonate, a solution of ammonium carbamate thus being obtained, together with a gaseous stream of nitrogen and hydrogen. The carbamate solution is passed to a urea reactor to obtain a urea solution containing unconverted carbamate and excess ammonia. The carbamate is decomposed and the evolved ammonia is recycled to the urea reactor along with carbon dioxide. The urea solution, now containing a reduced amount of carbamate (e.g. 50% of its original content), is passed to an adiabatic stripper in which the stripping gas used comprises hydrogen and nitrogen obtained from the CO2-absorption stage. Decomposition products together with water are removed from the adiabatic stripper, and are condensed in the presence of ammoniacal ammonium carbonate and the condensate so obtained is passed to the CO2-absorption step. <IMAGE>

Description

The invention relates to an improvement in the method of producing urea.

The purpose of the invention is to simplify the process.

The proposed method comprises the following stages:

a) the direction of the gas stream obtained in a reforming unit with steam or in an incomplete oxidation unit for liquid or gaseous hydrocarbons, which replenishes the newly introduced gas stream for ammonia synthesis and

mainly from Hj, Nj and CO, into the CO absorption installation, which uses concentrated: aqueous ammonia solution, the absorption installation includes two successive absorption steps: in the first, the absorbing solution used is a concentrated aqueous ammonia solution (with ammonia concentration more than 70 wt.%, preferably with a concentration of 80 wt.%), in a second - aqueous solution of ammonium carbonate saturated with ammonia, obtained in the decomposition stage under reduced pressure, or, in the absence of the latter, in this ne decomposition of the urea solution in vacuum;

b) removal from the CO absorption section. gas flow consisting mainly of

, |, (with possible traces of NHj and COj), along with a liquid stream, consisting mainly of an aqueous solution of carbs of ammonium carbonate;

c) feeding an aqueous solution of ammonium carbamate to the urea synthesis reactor, in which ammonium carbamate is partially converted to urea; ,

d) withdrawing from the reactor the synthesis of urea of an aqueous solution of urea containing not subjected to chemical conversion of ammonium and ammonium ammonium carbamine in an amount exceeding biometric and possible removal from the upper part of the reactor of a gas stream consisting mainly of inert gases with small quantities NH

And CO

a

e) supplying an aqueous solution of urea from the above step (d) to a decomposition unit in which: ammonium arbammate decomposes into ammonia and carbon dioxide, the latter being removed from this decomposition unit along with water (which evaporates), being sent as parto

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25485

phase in urine synthesis reactor;

f) withdrawing from the plant a decomposition of an aqueous solution of urea containing approximately 50% of a carbamicate, originally from a solution of urea leaving the synthesis reactor, and feeding this aqueous solution of urea to an distillation (or steam) column operating in an adiabatic a mode in which a gas stream from said step (c) is used as a stripping agent;

g) removing from the bottom of the adiabatic regime an urea solution that does not contain ammonium carbamate,

and removing the stripping agent (t + Hj) from the upper part of this stripping column along with the decomposed products;

nor ammonium carbamate N H ,, - i-CO, j

and

together with evaporated water;

L) introducing a gas mixture removed from the top of the stripping column, operating in the adiabatic mode, into a condenser in which ammonia and carbon dioxide are condensed by cooling as a result of indirect heat exchange with the refrigerant in the presence of an ammonia-saturated ammonium carbonate solution from the decomposition stage under reduced pressure, while the stream containing H and N is removed from the upper part and, after being converted to methane, is fed to the ammonia synthesis stage BMeq / re with inert gases M and Hj coming out of installations bsorbtsii SB. ;

i) directing the condensate from step (h) p installation of absorption f) feeding the urea solution from step () to the evaporation stage B vacuum (directly at this stage

either pre-pass through the decomposition stage under reduced pressure (4-5 atm)), while both the decomposition unit under reduced pressure (from the upper part of the installation) and the evaporation unit in vacuum leave a gas mixture consisting of ammonia, carbon dioxide and water, which when condensed forms a solution saturated with ammonia

ammonium carbonate used in steps (a) and (h), and molten urea is formed at the bottom of the evaporation unit in vacuum.

3

The drawing shows the installation for the implementation of the proposed method.

An example: The original (newly introduced into the system) gas, consisting mainly of CO, NJ and H, after its compression is directed through pipe 1 to the CO 2 absorber, in which the absorbing solution used consists mainly of an aqueous solution of an ammonia vial, which is supplied through pipe 3 from the ammonia synthesis reactor, ammonium carbamate, leaving the absorber 2, is directed through pipe 4 to the urea synthesis reactor 5.

The gas with CO is removed from it is removed from the absorber 2, and through pipe 6 is sent to condenser 7 with desorption of NH, in which j is absorbed by an aqueous solution of carbonic ammonia supplied through pipe 8 from the low pressure solution regeneration section using a pump 9.

The ammonia-rich solution formed in condenser 7 is directed through pipe 10 to absorber 2.

The purified gas is fed through pipe 11 to an adiabatic stripping column 12, into which pipe 13 is injected with a urea solution, which is sent from the ammonium carbamate decomposition unit 14. This gas stream is withdrawn from the stripping column 12 and sent via pipe 15 to an ammonium carbamate condenser 16, in which it is mixed with an aqueous solution 17 of ammonium carbonate supplied via pump 9 through pipe 18 from the low pressure solution regeneration section. The ammonium carbamate formed in the condenser 16 is directed through pipe 19 and using pump 20 to the CO absorber 2. Two of the most valuable gases coming from the condenser 16 are sent through pipe 21 to the methane synthesis unit and which get saturated with ammonia solution fed to the absorber 2 through pipe 3, as described above,

Carbamate solution. The pipe 4 goes to the urea synthesis reactor 5, in which the carbamate is dehydrated to form urea. The urea solution thus formed goes through the pipe 22 to the carbamate decomposition unit 14, in which the carbamate is decomposed into CO and NHj. These

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254.85

decomposition products are again directed through pipe 23 to reactor 5,

A solution of urea with carbamate, not decomposed 1 to 5, is fed through pipe 13 into an adiabatic stripping column 12,

The urea solution leaving the bottom of the column 12 is fed to

) 0 section evaporated. in a vacuum.

In carrying out the method, at the outlet of the stripping column operating in the adiabatic mode, a urea solution is obtained having a concentration of urea 5 which is immediately sent to the final stage of treatment in vacuum.

The proposed method eliminates the costly decomposition operation of the ammonia carbamate, which was not decomposed, at an average (18 atm) and increased (4.5 atm) pressures, and thus the vaporized re-condensation occurs.

According to a known method, a solution directed to an evaporation step in a vacuum is obtained with a considerable expenditure of energy.

For the implementation of all stages, it is necessary that The ratio to COj in the urea synthesis reactor ranged from 0.9 to 1.3 (preferably 1.1), and the ratio of NH to C0. ranging from 4.5 to 6.5 (preferably 3.5).

35 In this system (not shown), instead of a pump 20 for recycling carbonate to the absorber 2, an ejector in which the working fluid is a nascent ammonia4Q solution with an ammonia concentration of 80% fed from the synthesis reactor is used. ammonia through the pipe 3.

The proposed method is carried out at a pressure in the range from 100 to 300 kg / cm, actually equal to the pressure at which the ammonia synthesis reactor is operated (in this case, a pump may be necessary to supply a solution saturated with ammonia from

50 working on the described cycle) or at a pressure ranging from 10 to

. 400 kg / cm, which is lower than the pressure at which the ammonia synthesis reactor is operating, in this case the pump may be excluded).

In tab. 1 and 2 are working

process conditions, concentrations of products used and rates of 30

45

currents (the numbers in the name A-raf co-columns of the distillation of the middle and lower correspond to the notation in the drawing), who is increasing and reducing the social structure. Thus, simplifying the processing of CO, in the reaction mass, the yield is reached 3 evaporation, with 3.26

decomposition of ammonium carbamate to 2.65%.

Table 1

T iWJepaTjrpu, With Pressure j SITM

H coj

fJH

,about

BUT

Urea

Lsego

140

g 00

18:;

Process conditions using products

13535

200198.5194 // 194.2

17 73 {20) 17173 (22) 17173 (12.5) 17173 (23) 53237 (62) 53237 (68) 53237 (38,, 7) 53237 (73)

$ 5458 (18) 32A7 (2.4) -37909 (30.9) 14051 (8p81)

7323 (10) 63882 (46.4) 2998 (4) 25926 (80) 68954 (56.3) 7П 7Й (Л, 63)

6AVZ (20) 15687 (12., 8) 32573 (20.43)

„..„ „„ AKe67 (2b, 13)

85368 (tOO) 73233 (100) 137539 (100) 73408 (100) 32409 (100) 122550 (100) i59469 (tCO)

T a b l to 1d a 2

...; -,

one

10 1 t I 12 Type of flow, kg / h (wt.%)

Solution MO 1 Urea solution

Chevina

TeLMnepaTypa, With Pressure ,, atm CO, NH,

but

Urea Ik: its

195 210

196.5

6698 (18.1), 7353 (6) 25835 (70) (37) 4386 (11, 9) 28187 (23) h 1667 (34)

110

60 195

933 (1.4) 7080 (11.1) 660 (18)

2177 (3.3) 48422 (75.6) 1540 (42)

.21119 (32.1) 8535 (13.3) 1467 (40) 41667 (63.2)

36919 (100) 122550 (100) 55896 (100) 64037 (100) 3667 (100)

vniigm

Order 1971/62

Productive. -Hp - TH6: j g, Uzhgorod, ul, Design, 4

140

g 00

18:;

Urea solution

Carbonate solution

110

60 195

933 (1.4) 7080 (11.1) 660 (18)

2177 (3.3) 48422 (75.6) 1540 (42)

.21119 (32.1) 8535 (13.3) 1467 (40) 41667 (63.2)

Circulation 3/9

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Claims (1)

METHOD FOR PRODUCING UREA from ammonia and carbon dioxide in a molar ratio of 5.4-5.6: 1 in the presence of water with a feed of the resulting aqueous urea solution containing 50% unconverted carbamate into the distillation column operating in adiabatic mode in the presence of a gas-dispersing agent - gas a stream obtained by reforming hydrocarbons with water vapor or their incomplete oxidation and containing CO _g , N ^, N _g at a molar ratio of Li _g and H _g 1: 3, the gas stream leaving this stage is condensed in the presence of saturated amm a solution of ammonium carbamate by indirect heat exchange with the removal of inert gases, and the urea solution leaving the distillation column is fed to the evaporation stage and the urea melt is removed, characterized in that, in order to simplify the process, the synthesis of urea from carbon dioxide and ammonia is carried out in molar the ratio of water and carbon dioxide equal to 1.08-1.12: 1, and before the supply of the distillation agent into the distillation column, carbon dioxide is preliminarily absorbed from it and an aqueous solution of ammonium carbamate is obtained, which is returned into the synthesis zone, and inert. The gases obtained during the condensation stage are discharged to the methane synthesis unit, and then to the ammonia synthesis unit. -SLLouJ 225485