IE46700B1 - 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.

Description

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 dissociation 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 arid ammonia. As stated above, the second stream is fed to the condenser. - 2 <3 6 7 Ο Ο I 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 NH^ to CO., 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 CO,,, 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,i the molar ratio of amnonia to carbon dioxide is in the range of from 2.5:1 to 7:1.

Assuming a given total pressure and a given H^O to COg ratio, and setting a ratio of NH3 to C02 (which ratio must be comparatively high in the - 3 >i ΰ ϊ 9 0 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 installation for carrying out the invention.

Carbon dioxide 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 C02 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 dioxide reacts adibatically 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 MHj .to C02 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 T/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 C02 ratio such that the evolution of heat takes place at a high temperature, it is possible to produce, from the wafer 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 C02 ratio and the reactor temperature are adjusted by feeding a small portion of liquid ammonia into the - 4 4 ο 7 0 0 bottom of the reactor 2 via line 9 and a predominant portion of gaseous ammonia from stripper 3 via line 10.

The ΝΗ^ to COg ratio in reactor 2 is kept higher than that in absorber 1, whereby the conversion of C02 to urea is increased.

The NH3 to C02 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 decreasesas the NHg to C02 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 heat-exchanger 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. - 5 EXAMPLE To produce 72,550 kg/hour of urea, the absorber 1 was fed with' 53,167 kg/hour of C02 at a temperature of 150°C and 32,853 kg/hour of NHg at a temperature of 50°C, 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) C02 .... 10,127 kg/hour (17.64% by weight) H20 .... 19,926 kg/hour (34.73% by weight).

The absorber operates under the pressure of the entire urea? synthesis loop which has 180 kg/cm (allowing for pressure drops). The C02 and NHg 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 180°C and having the following composition:- NHg .... 50,794 kg/hour (42.23% by weight) .... 50,794 kg/hour (42,23% by weight) .... 18,699 kg/hour (15.54% by weight) This solution was passed to reactor 2.

Uncondensed vapours left the top of the absorber 1 at a temperature of 180°C. They had the following composition:NH3 .... 9,382 kg/hour (40.60% by weight) C02 .... 12,500 kg/hour (54.09% by weight) H20 .... 1,227 kg/tjour ( 5.31% by weight).

These vapours 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 100°C and with the vapours leaving the stripper 3, which have a temperature of 190°C and the following composition:- 50,218 kg/hour (59.10% by weight) 26,259 kg/hour (30.90% by weight) 8,497 kg/hour (10.00% by weight) - 6 4 6 7 U Ο The solution leaving the reactor was sent to stripper 3.. The solution has a temperature of 185°C, and the following composition:- Urea ... . 72,500 kg/hour (28.64% by weight) nh3 ... .107,768 kg/hour (42.58% by weight) 5 C02 ... . 23,886 kg/hour ( 9.44% by weight) H20 ... . 48,946 kg/hour (19.34% by weight). Through the stripper bottom, there exited a stream at 21O°C which was sent to a conventional concentration section. This stream has following composition: 10 Urea ... . 72,500 kg/hour (37.91% by weight) nh3 ... . 66,932 kg/hour (35.00% by weight) { C02 ... . 10,127 kg/hour ( 5.30% by weight) H?0 ... . 41,6/6 kg/hour (21.79% by weight). CLAIMS 15 1. A method for the production of urea from carbon dioxide and ammonia, comprising reacting the two reactants in a first reaction zone

Claims (6)

1. The solution has a temperature of 185°C, and the following composition:- Urea ... . 72,500 kg/hour (28.64% by weight) nh 3 ... .107,768 kg/hour (42.58% by weight) 5 C0 2 ... . 23,886 kg/hour ( 9.44% by weight) H 2 0 ... . 48,946 kg/hour (19.34% by weight). Through the stripper bottom, there exited a stream at 21O°C which was sent to a conventional concentration section. This stream has following composition: 10 Urea ... . 72,500 kg/hour (37.91% by weight) nh 3 ... . 66,932 kg/hour (35.00% by weight) { C0 2 ... . 10,127 kg/hour ( 5.30% by weight) H ? 0 ... . 41,6/6 kg/hour (21.79% by weight). CLAIMS 15 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 20 of urea is completed; discharging a urea solution containing unreacted y 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 25 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. - 7 2. A method according to Claim 1, wherein the aqQeous 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. 10

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

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