GB1586629A - Production of urea - Google Patents

Abstract

In a process for producing urea with ammonia and pure carbon dioxide as starting materials, the improvement comprises feeding liquid ammonia in part to the absorber (1) with all the carbon dioxide and in part to the synthesis reactor (2). The NH3/CO2 molar ratios are 2 to 4 for the condenser and 2.5 to 7 for the reactor. Film-type absorbers are preferred. <IMAGE>

Description

(54) PRODUCTION OF UREA (71) We, SNAMPROGETTI S.p.A., an Italian company, of Corso Venezia, 16, Milan, Italy, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method for the production of urea from pure carbon dioxide and ammonia.

In a known method for the production of urea, carbon dioxide is reacted with gaseous ammonia produced by decomposition of ammonium carbamate, the gaseous ammonia having previously been used for stripping the urea solution. The gaseous mixture thus obtained, composed of the dissociation gases, carbon dioxide and ammonia, is cooled by the use of water as a coolant to form ammonium carbamate which is fed to a reaction wherein it is dehydrated to form urea. From the reactor, the mixture used for the synthesis of the urea is fed to a stripper for stripping the carbon dioxide and ammonia formed by thermal decomposition of the carbamate, such stripping being carried into effect in a dissocation apparatus through the bottom of which gaseous ammonia is introduced and through the top of which the mixture is introduced. Two streams emerge from the dissociation apparatus, the first being a solution of urea still containing a certain quantity of carbamate, and the second being dissociated gases and ammonia. As stated above, the second stream is fed to the condenser.

The most serious drawback of the above method is the high concentration of ammonium carbamate in the urea solution produced and the consequent reduced urea yield. This drawback arises from the need to use constant NH3 to CO2 ratios, both in the condenser and in the reactor.

It has surprisingly been found, according to the present invention, that urea can be produced from ammonia and carbon dioxide with improved yields by using, instead of the condensation coil, a film absorber (the operation of which will be described hereinafter) and by feeding, in addition to gaseous ammonia to the bottom stripper, liquid ammonia partly to the absorber and partly to the synthesis reactor so as to obtain different molar ratios of NH3 to CO2, namely a ratio of from 2.5:1 to 7:1 in the reactor and of from 2:1 to 4:1 in the absorber.

According to the present invention, there is provided a method for the production of urea from carbon dioxide and ammonia, comprising reacting the two reactants in a first reaction zone wherein the molar ratio of ammonia to carbon dioxide is in the range of from 2:1 to 4:1; discharging the resulting reaction mixture from the first reaction zone wherein the formation of urea is completed; discharging a urea solution containing unreacted ammonium carbamate from the second reaction zone and feeding it to a stripper wherein the unreacted carbamate is decomposed and the decomposition products are stripped with gaseous ammonia; and condensing the decomposition products and the gaseous ammonia used for stripping, in the first reaction zone, thus forming ammonium carbamate; wherein the first reaction zone is a film absorber to the top of which is fed an aqueous ammoniacal solution of ammonium carbonate and in which the ammonia and carbon dioxide react adiabatically, and whereIn, in the second reaction zone, the molar ratio of ammonia to carbon dioxide is in the range of from 2.5:1 to 7:1.

Assuming a given total pressure and a given H2O to CO2 ratio, and setting a ratio of NH3 to CO2 (which ratio must be comparatively high in the reactor and the stripper and comparatively low in the absorber), the invention will now be described by way of example with reference to the single figure of the accompanying drawing, which shows an instal lation for carrying out the invention.

Carbon diodixe is fed to the bottom of a film absorber 1, together with a portion of liquid ammonia, via line 4. To the top of the absorber, via line 5, recycled absorbing solution, namely an aqueous ammoniacal solution of ammonium carbonate, is fed. The overall NH3 to CO2 ratio is regulated by governing the rate of flow of liquid ammonia into the bottom of the absorber.

In the lower section of the absorber 1, the carbon diodixe reacts adiabatically with the ammonia to produce ammonium carbamate.

The heat evolved increases the temperature of the mixture to a value which depends upon the total pressure, the water content and the NH3 to CO2 ratio.

The gases which are evolved from the adiabatic bath and which are in chemical equilibrium therewith at that temperature, are partly absorbed by the recycled ammoniacal ammonium carbonate solution which flows in the form of a film from the top of the absorber.

That part of the gas which has not been absorbed leaves the absorber 1 via line 6 and passes into line 7 which carries gaseous ammonia to stripper 3.

The heat evolved by the absorption is taken up by water flowing through a jacket around the absorber. Since the gas and the absorbing liquor in equilibrium throughout the entire absorber have an NH3 to CO2 ratio such that the evolution of heat takes place at a high temperature, it is possible to produce, from the water coolant, steam having a thermal level which is high enough for it to be used subsequently in the installation.

The carbamate is passed along line 8 to the reactor 2, in which dehydration to form urea takes place. The NH3 to CO2 ratio and the reactor temperature are adjusted by feeding a small portion of liquid ammonia into the bottom of the reactor 2 via line 9 and a predominant portion of gaseous ammonia from stripper 3 via line 10.

The NH3 to CO2 ratio in reactor 2 is kept higher than that in absorber 1 ,whereby the conversion of CO2 to urea is increased.

The NH3 to CO2 ratios, the temperature and the yields are such that the absorption condensation heat of the gas from the stripper 3 enables the thermal balance of the reactor 2 at the preselected total pressure to be kept constant. In this way, it is possible to increase the yield in the reactor while reducing the consumption of steam by the stripper and increasing the thermal level of the steam produced by the absorber.

The pressure in the reactor-stripper system is virtually the same, and it is regulated by discharging from the top of the reactor the inert gases coming from the bottom of the stripper and from the absorber for carbon dioxide. The pressure can be either more or less than the pressure in the absorber, depending upon whether the transfer of the carbamate takes place by barometric pressure or by means of pumps.

Since the yield in the reactor is high and in view of the physical and chemical properties of the solution which leaves the reactor through line 11, the amounts of carbon dioxide and water separated in the stripper are small as compared to the amount of ammonia. In addition, the heat required by the system is not great, since the solution of urea is close to its critical condition, whereby ammonia can easily be distilled.

The critical temperature decreases as the NH3 to CO2 ratio increases and as the water content decreases. Therefore, the exchange surfaces required in the stripper are less than the surfaces required for conventional strippers and, in addition, lower heatexchanger temperatures are necessary, whereby steam at a lower thermal level may be used, the running costs being thereby reduced.

The invention will now be illustrated by the following Example in which reference is made to the accompanying drawing.

EXAMPLE To produce 72,550 kg/hour of urea, the absorber 1 was fed with 53,167 kg/hour of CO2 at a temperature of 1500C and 32,853 kg/hour of NH3 at a temperature of 500C, together with recycle carbonate which has a temperature of 76"C and which has the following composition: NH3 ....27,323 kg/hour (47.62% by weight) CO2 .... 10,127kg/hour (17.65% by weight) ....... 19,926 kg/hour (34.73% by weight) The absorber operates under the pressure of the entire urea-synthesis loop which was 180 kg/cm2 (allowing for pressure drops). The CO2 and NH3 react in the absorber and the reaction heat was used to produce steam.

From the bottom of the absorber, there emerged a carbamate solution having a temperature of 1800C and having the following composition: - NH3 .... 50,794 kg/hour (42.23% by weight) CO2 .... 50,794 kg/hour (42.23% by weight) H2 .... 18,699 kg/hour (15.54% by weight) This solution was passed to reactor 2.

Uncondensed vapors left the top of the absorber 1 at a temperature of 1800C. They had the following composition: NH3 .... 9,382 kglhour (40.60% by weight) CO2 .... 12,500 kg/hour (54.09% by weight) H2O.... 1,227 kg/hour ( 5.31% by weight) These vapors were fed to the bottom stripper 3.

In addition to the solution of carbamate, reactor 2 was fed with 47,840 kg/hour of ammonia at 1000C and with the vapors leaving the stripper 3, which have a temperature of 1900C and the following composition: NH3 .... 50,218 kg/hour (59.10% by weight) CO2 .... 26,259 kg/hour (3090% by weight) ....... 8,497 kg/hour (10.00% by weight) The solution leaving the reactor was sent to stripper 3. The solution has a temperature of 185 C, and the following composition: Urea.... 72,500kg/hour (28.64% by weight) NH3 .... 107,768 kg/hour (42.58% by weight) CO2 .... 23,886kg/hour( 9A4%byweight) H,O .... 48,946 kg/hour (19.34% buy weight) Through the stripper bottom, there exited a stream at 2100C which was sent to a conventional concentration section. This stream has the following composition: Urea .... 72,500 kg/hour (37.91% by weight) NH3 ....66,932 kg/hour (35.00% by weight) CO2 .... 10,127kg/hour( 5.30% by weight) H2O ....41,676 kg/hour (21.79% by weight) WHAT WE CLAIM IS: 1. A method for the production of urea from carbon dioxide and ammonia, comprising reacting the two reactants in a first reaction zone wherein the molar ratio of ammonia to carbon dioxide is in the range of from 2:1 to 4:1; discharging the resulting reaction mixture from the first reaction zone and sending it to a second reaction zone wherein the formation of urea is completed; discharging a urea solution containing unreacted ammonium carbamate from the second reaction zone and feeding it to a stripper wherein the unreacted carbamate is decomposed and the decomposition products are stripped with gaseous ammonia; and condensing the decomposition products and the gaseous ammonia used for stripping, in the first reaction zone, thus forming ammonium carbamate; wherein the first reaction zone is a film absorber to the top of which is fed an aqueous ammoniacal solution of ammonium carbonate and in which the ammonia and carbon dioxide react adiabatically, and wherein, in the second reaction zone, the molar ratio of ammonia to carbon dioxide is in the range of from 2.5:1 to 7:1.

2. A method according to Claim 1, wherein the aqueous ammoniacal ammonium carbonate solution fed to the top of the first reaction zone is a recycled solution, and wherein the carbon dioxide and ammonia are fed to the bottom of the first reaction zone.

3. A method according to Claim 1 or 2, wherein part of the gaseous ammonia used to strip the decomposition products is gaseous ammonia obtained as a head product from the first reaction zone.

4. A method for the production of urea, substantially as hereinbefore described with reference to the accompanying drawing.

5. A method for the production ofurea, substantially as described in the foregoing Example.

6. Urea whenever produced by a method according to any of Claims 1 to 5.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. CO2 .... 10,127kg/hour( 5.30% by weight) H2O ....41,676 kg/hour (21.79% by weight) WHAT WE CLAIM IS:

1. A method for the production of urea from carbon dioxide and ammonia, comprising reacting the two reactants in a first reaction zone wherein the molar ratio of ammonia to carbon dioxide is in the range of from 2:1 to 4:1; discharging the resulting reaction mixture from the first reaction zone and sending it to a second reaction zone wherein the formation of urea is completed; discharging a urea solution containing unreacted ammonium carbamate from the second reaction zone and feeding it to a stripper wherein the unreacted carbamate is decomposed and the decomposition products are stripped with gaseous ammonia; and condensing the decomposition products and the gaseous ammonia used for stripping, in the first reaction zone, thus forming ammonium carbamate; wherein the first reaction zone is a film absorber to the top of which is fed an aqueous ammoniacal solution of ammonium carbonate and in which the ammonia and carbon dioxide react adiabatically, and wherein, in the second reaction zone, the molar ratio of ammonia to carbon dioxide is in the range of from 2.5:1 to 7:1.

2. A method according to Claim 1, wherein the aqueous ammoniacal ammonium carbonate solution fed to the top of the first reaction zone is a recycled solution, and wherein the carbon dioxide and ammonia are fed to the bottom of the first reaction zone.

3. A method according to Claim 1 or 2, wherein part of the gaseous ammonia used to strip the decomposition products is gaseous ammonia obtained as a head product from the first reaction zone.

4. A method for the production of urea, substantially as hereinbefore described with reference to the accompanying drawing.

5. A method for the production ofurea, substantially as described in the foregoing Example.

6. Urea whenever produced by a method according to any of Claims 1 to 5.