IE48099B1 - Process for 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.

Description

This invention relates to a process for producing urea.

A number of processes for producing urea are known. One of these processes is the process described in British Patent Specification No. 1,560,174, which process comprises the following stages:5 (i) feeding to a urea-synthesis reactor a stream of anhydrous ammonia and/or of aqueous ammonia solution and a stream of an ammonium carbamate solution, to produce an aqueous solution of urea contaminated with ammonium carbamate; (ii) subjecting to decomposition in a decomposer the ammonium carbamate contained in the urea solution produced in the urea synthesis reactor, to produce ammonia and carbon dioxide; (iii) recycling in the gaseous state to the urea synthesis reactor the ammonia and carbon dioxide produced in stage (ii); (iv) feeding the aqueous solution of urea emerging from the decomposer, which - 2 48099 solution still contains some of the ammonium carbamate which was contained in the urea solution emerging from the urea synthesis reactor, to an adiabatic stripping column in which the stripping stream is a gaseous stream comprising C02, N2 and H2, to strip off the majority of the ammonia; (v) feeding the stripping stream and the stripped gases emerging from the stripping column to a C02-absorption column in which as a liquid absorbent there is used an aqueous solution of ammonium carbonate which is rich in ammonia, to produce the stream of ammonium carbamate solution which is fed in stage (i) to the urea synthesis reactor; and (vi) feeding the non-absorbed gaseous stream which is rich in ammonia, emerging from the C02-absorption column, to a scrubbing column, where that stream is contacted with an aqueous solution of ammonium carbonate, to produce (a) the aqueous solution of ammonium carbonate which is rich in ammonia and is used as said liquid absorbent and (b) an ammonia synthesis gas comprising nitrogen and hydrogen.

A disadvantage of the above process is that the content of C02 of the urea solution obtained from the bottom of the adiabatic column is rather high (e.g. 10% to 15% by weight) and that consequently the urea solution must be subjected to low pressure decomposition. The operation of the adiabatic stripping column according to the above process necessitates however the use of C02 as a stripping agent, since the C02 is used, by partial reaction thereof with the ammonia contained in the urea solution, to supply at least partially the heat required for decomposition of the ammonium carbamate contained in the urea solution. Thus, in view of the aforementioned high C02 content, the decomposer in which the urea solution is subjected to low pressure decomposition must be rather large.

It has been surprisingly found in accordance with the invention that it is - 3 48099 possible to dispense with C02 as the stripping agent, and consequently to obtain an end product substantially deprived of C02 without dispensing with the use of adiabatic stripping. This achievement is possible by selecting appropriate ratios of H20 to C0a and of NH3 to C02 on the urea synthesis reactor.

According to the present invention, there is provided a process for producing urea, comprising the following stages: (a) passing a gaseous stream comprising H2, N2 and C02 to a C02-absorption system in which C02 is absorbed by an aqueous solution of ammonia; (b) discharging from the C02-absorption system a gaseous stream comprising H2 and N? and a liquid stream comprising an aqueous solution of ammonium carbamate; (c) feeding with the aqueous solution of ammonium carbamate a urea-synthesis reactor wherein the carbamate is partially converted into urea; (d) discharging from the urea-synthesis reactor an aqueous solution of urea containing unconverted ammonium carbamate and the excess of ammonia relative to the stoichiometric amount; (e) feeding the aqueous solution of urea which contains the unconverted carbamate to a decomposer and thermally decomposing therein a portion of the carbamate to form (i) a solution of urea containing carbamate and (ii) decomposition products, the latter being recycled in the vapour phase to the urea synthesis reactor; (f) decomposing the carbamate contained in the solution of urea formed in the decomposer used in stage (e) in an adiabatic stripper in which the gaseous stream formed in stage (b) is used as the stripping agent, a urea solution substantially devoid of carbamate being obtained at the stripper bottom, and decomposition products, stripping agent and evaporated water being obtained at the stripper head; - 4 48099 (g) condensing the decomposition products and the evaporated water in a condenser in the presence of an ammonia-containing solution of ammonium carbonate and discharg’ig an inert gas comprising H2 and N2 from the condenser head; (h) passing the condensate produced in stage (g), namely an aqueous ammoniacontaining solution of ammonium carbonate, to the C02-absorption system; and (i) discharging the solution of urea from the bottom of the adiabatic stripper.

In an embodiment, the process of the invention comprises the following stages: (a) sending a gaseous stream obtained by steam-reforming or partial oxidation of a liquid or gaseous hydrocarbon, which stream acts as a raw gas stream for the synthesis of ammonia and which essentially comprises H2, N2 and C02, to a C02-absorption system wherein a concentrated aqueous solution of ammonia is used and which preferably comprises two serially arranged discrete absorption stages, in the first of which the absorbing liquor is a concentrated aqueous solution of ammonia (concentration above 708 by weight ammonia, 808 being preferred), whereas in the second stage the absorption liquor is an aqueous ammonia-containing solution of ammonium carbonate obtained from a lowpressure decomposition stage, or, in the case in which the latter is not used, from a stage of decomposition under vacuum of the urea solution; (b) discharging from the C02-absorption section a gas stream which essentially comprises N2 and H2 (with possible traces of NH3 and C02) together with a liquid stream which essentially comprises an aqueous solution of ammonium carbamate; - 5 (c) feeding the aqueous solution of ammonium carbamate to a urea-synthesis reactor wherein the ammonium carbamate is partially converted into urea; (d) discharging from the urea-synthesis reactor an aqueous solution of urea which contains the un-converted ammonium carbamate and the excess of ammonia over the stoichiometric amount and possibly a gaseous stream from the reactor top, the stream essentially comprising inert material with a certain content of NH3 and C02; (e) feeding the aqueous solution of urea from stage (d) to a decomposer in which the ammonium carbamate is decomposed into ammonia and carbon dioxide, the latter being drawn from the decomposer together with the water which has evaporated off and being recycled in the vapour phase to the urea-synthesis reactor; (f) discharging from the decomposer an aqueous solution of urea which contains about 50% of the carbamate originally contained in the urea solution leaving the synthesis reactor and feeding the aqueous solution to an adiabatic stripper wherein there is used as a stripping agent the gaseous stream of stage (b); (g) discharging from the bottom of the adiabatic stripper a solution of urea substantially devoid of ammonium carbamate, and discharging from the top of the stripper the stripping agent (N2 + Hz), the products of decomposition of the carbamate (NH3 + C02) and the evaporated water; (h) introducing the gaseous mixture drawn from the top of the adiabatic stripper into a condenser wherein the ammonia and the carbon dioxide are condensed by cooling as a result of indirect heat exchange with a cold fluid in the presence of an ammonia-containing solution of ammonium carbonate and discharging a stream of H2 and N2 from the top and after methanization, feeding the product to an ammonia synthesis reactor together with the inert gases N2 and H2 leaving the C02-absorption system; (i) sending the condensate from stage (h) to the C02-absorber; and - 6 48099 (j) subjecting the urea solution of stage (g) to concentration under vacuum, either directly or after a low-pressure decomposition stage (e.g. at 4 to 5 atmospheres), there being obtained from the head of the low-pressure decomposition stage and from the concentration stage a gas mixture comprising ammonia, C02 and water, which, when condensed, is the ammonia-containing solution of ammonium carbonate used for stages (a) and (h), melted urea being obtained in the stage of concentration under vacuum.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing which shows a plant suitable for carrying out a process according to the invention.

A raw gas which consists of C02, N2 and H2 is compressed and passed along pipeline 1 to a C02-absorber 17, wherein the absorbing liquid is mainly an aqueous solution of ammonia which is supplied along a pipeline 6 from an ammonia synthesis reactor. The ammonium carbamate formed in the absorber 17 is passed along pipeline 7 to a urea-synthesis reactor 18.

The decarbonated gas formed in the absorber 17 is passed along pipeline 2 to an NH3-stripping condenser 19 in which NH3 is absorbed by an aqueous solution of ammonium carbonate supplied along a pipeline 15, by means of a pump 20, from a low-pressure liquor-recovering section.

The ammonia-containing solution formed in stripping condenser 19 is passed along pipeline 13 to the absorber 17.

The purified gas from the stripping condenser 19 is passed along pipeline 3 to an adiabatic stripping column 21 which is supplied, via the pipeline 10, with a urea solution formed in a carbamate decomposer 22. The gas stream leaving the column 21 is passed along pipeline 4 to a carbamate condenser 23 where it is combined with an aqueous solution of ammonium carbonate supplied, via pipeline 14, pump 20 and pipeline 15, from the low-pressure liquor-recovery - 7 48099 section. The carbamate produced in condenser 23 is passed along pipeline 12, by means of a pump 24, to the C02-absorber 17. The purified gases leaving the condenser 23 are passed along pipeline 5 and subjected to methanization and, subsequently, to an ammonia synthesis process in which an ammonia-containing solution is produced and passed to the absorber 17 along pipeline 6.

The carbamate solution produced in absorber 17 is passed along pipeline 7 to the urea-synthesis reactor 18 in which dehydration of the carbamate to form urea takes place. The solution of urea thus obtained is passed along pipeline 8 to the carbamate decomposer 22 wherein the carbamate is decomposed to form C02 and NH3 which are recycled, via pipeline 9, to the reactor 18.

The urea solution, along with carbamate which has not decomposed, is passed along pipeline 10 to the adiabatic column 21 (which has been described above).

The solution of urea leaving the bottom of the column 21 is passed along pipelinel1 and is subjected to evaporation under vacuum in a conventional manner.

It is quite surprising that, when operating the above apparatus, it is possible to obtain, at the exit of the adiabatic stripping column 21, a solution of urea which is so highly concentrated as to be able to be used directly in the final treatment of evaporation under vacuum. By so doing, the considerable advantage is achieved in that the expensive operations of decomposition of the undecomposed carbamate under medium pressure (e.g. 18 atmospheres) and low pressure (e.g. 4.5 atmospheres), and recondensation of the vapours produced are not required. This is the contrary of the teachings of the prior art, according to which the solution subjected to evaporation under vacuum is obtained by the use of considerable amounts of power.

In the urea synthesis reactor 18 described above, the ratio of H20 to C02 should be from 0.9:1 to 1.3:1, preferably 1.1:1, the ratio of NH3 to C02 should be from - 8 48099 4.5:1 to 6.5:1, preferably 5.5:1.

In a modification of the above apparatus, instead of the pump 24 for recycling the carbonate to the absorber 17, there is used an ejector the working fluid of which is the 80% ammonia-containing solution from the ammonia synthesis reactor from the pipeline 6.

As regards the pressures, the apparatus described above is operated under a pressure of from 100 kg/cm2, a pressure which is virtually equal to that at which the ammonia synthesis reactor operates. In this case, a pump may be required for sending the ammonia-containing solution from the ammonia synthesis reactor to the apparatus. Alternatively, there is used a pressure which is from 10 kg/cm2 to 100 kg/cm2 below the pressure at which the ammonia synthesis reactor is operated, in which case the pump is not required.

In a practical embodiment, the plant described above with reference to the drawing is operated under the conditions, concentrations and flow rates given in the following Table. In the Table, the term Syngas means Synthesis Gas. - 9 4 8 0 9 9 υβ co r«51· CO Ld <— _! Φ CQ Q. «X ·· h- OΟ CO -Ρ Ρ <— co co co co m -to- ·»— θ IO <3* CM CM fO α) $c o GO <υ o GO tn oo o o <ύ· o <— CM «3 c o ε i i ΙΛ CO OT C >» GO CM -σ ro φ ΙΛ ro OT c >» GO T0 D i— Li_ CM <=1· OT ο ίΣ o r— U_ ο in co in cn co <«in σ co o CM o ro Q_ ΙΛ Ε Φ Φ Sh- CU Cn -p SZ OT -P OT ε sz S·— 00 CQ in Γ-*. r·*. o ·«— in *$· w- CM r— r— co cn co oo o co CM co tn ot sr ro m co OT OT CO (--. oo in CO CO Xo CO co CM CO OT «51· in co CM γιο co ’«S' cn co •St cn LD o in in I CM CM OT o =3CM CO co co i «=ί· i i CM (—. co r— Γ— r CM co m OT OT CM in Γ-. «-r «5f cm oo cm co τ- co «st (Ο Ν r-. CM r— co «5jco Γ— OT CO in rco o > o tZ S- --. O co c e cfQ E -C i_ Ο o c ε CM CO I CD I CM CO Γ— Γηco CM co in co CM I C— CD CM 00 1 I CM CO <— co r(-. co <«— CM r—. co *— m co tn «4* in ΛΙ CU O 7Z. ZC O co co CM 00 r—. co CO - 10 TABLE (cont.) Pipeline in OJ o Ch Carbonate Soln t I Flow Wt. kg/h wt. % 660 18 CJ M· o in 1467 40 3667 100 c to CO o P *” in co o 1/Ϊ 2 1 I *— Γ*· 1 o ω p <0 p c Q 3 © CJ tn t-x Q 2 sz co CJ co CO i_ o m O o Kt- in O f0 to σι P“ σι r-x CO 00 «3* o U. jz i ι *3· 1 to yi «d· co CJ P co CJ co o 2 1 1 co to © r- c o P co 3 CO bx σι r*x to rt3 g5 5 XX co r-x <— to Ch © o \ Ch *— *- to co cn CJ «“ m ©1 EZ jZ 1 1 CJ ct* to P to Cx co o 2 1 1 CO CJ co o c Γ— o p to in 3 co in Cx. r-x o <0 2 XX m CO IO in © © o co co *— to in i. τ— Ch cn Cx tn co «» CM 23 cm*· Lu JZ 1 1 cf CJ •=fr CJ Ch © P 00 © «— © 2 1 1 «Γ- hx *” 1 CJ CJ P 3 ΟΟ in to Ch O 2 _c Ch co co T— f0 in o to CO co Ch V CD CD m in to 3= Lu Οέ t 1 CJ 1 co ε —xp O to 0 'χ-' Ό • w r— Ο. vi <ϋ E 0> AJ o 0) P Φ S- AJ AJ O X CM L· o lZ l— o- Z X o z X X 1—

Claims (9)

1. A process for producing urea, comprising the following stages: (a) passing a gaseous stream comprising H 2 , N 2 and C0 2 to a C0 2 -absorption system in which C0 2 is absorbed by an aqueous solution of ammonia; 5 (b) discharging from the C0 2 -absorption system a gaseous stream comprising H 2 and N 2 and a liquid stream comprising an aqueous solution of ammonium carbamate; (c) feeding with the aqueous solution of ammonium carbamate a urea-synthesis reactor wherein the carbamate is partially converted into urea; (d) discharging from the urea-synthesis reactor an aqueous solution of urea 10 containing unconverted ammonium carbamate and the excess of ammonia relative to the stoichiometric amount; (e) feeding the aqueous solution of urea which contains the unconverted carbamate to a decomposer and thermally decomposing therein a portion of the carbamate to form (i) a solution of urea containing carbamate and (ii) de15 composition products, the latter being recycled in the vapour phase to the urea synthesis reactor; (f) decomposing the carbamate contained in the solution of urea formed in the decomposer used in stage (e) in an adiabatic stripper in which the gaseous stream formed in stage (b) is used as the stripping agent, a urea solution 20 substantially devoid of carbamate being obtained at the stripper bottom, and decomposition products, stripping agent and evaporated water being obtained at the stripper head; (g) condensing the decomposition products and the evaporated water in a condenser in the presence of an ammonia-containing solution of ammonium 25 carbonate and discharging an inert gas comprising H 2 and N 2 from the condenser head; - 12 48099 (h) passing the condensate produced in stage (g) namely an aqueous ammoniacontaining solution of ammonium carbonate to the C0 2 -absorption system; and (i) discharging the solution of urea from the bottom of the adiabatic stripper.

2. A process according to Claim 1, wherein the C0 2 -absorption system comprises two serially arranged absorption stages, in the first of which the absorption liquor is said aqueous solution of ammonia used in stage (a), which solution contains more than 70% by weight of ammonia, and in the second of which the absorption liquor is said aqueous ammonia-containing solution of ammonium carbonate produced in stage (g).

3. A process according to Claim 1 or 2, wherein, in the urea-synthesis reactor, the ratio H 2 0 to C0 2 is from 0.9:1 to 1.3:1 and the ratio of NH 3 to C0 a is from 4.5:1 to 6.5:1.

4. A process according to Claim 3, wherein the ratio of H 2 0 to C0 2 is 1.1:1 and the ratio of NH 3 to C0 2 is 5.5:1.

5. A process according to any of Claims 1 to 4 wherein, in stage (e), about 50% of the carbamate is thermally decomposed in the decomposer.

6. A process according to any of Claims 1 to 5, wherein the gaseous stream used in stage (a) is one obtained by steam reforming or partially oxidising a liquid or gaseous hydrocarbon.

7. A process according to any of Claims 1 to 6, wherein the inert gas obtained from the condenser head in stage (g) is subjected to methanization, wherein an ammonia solution is produced from the product thus obtained and wherein the ammonia solution is recycled to the C0 2 -absorption system.

8. A process according to Claim 1, substantially as hereinbefore described with reference to the accompanying drawing. - 13

9. Urea when produced by a process according to any of Claims 1 to 8.