Ammonia-urea integrated process and plant

Abstract

A process for the production of ammonia and urea in an ammonia-urea integrated plant comprising an ammonia section and a tied-in urea section, wherein a hydrocarbon is reformed to produce ammonia make-up synthesis gas; said make-up gas is purified by shift conversion and removal of carbon dioxide; carbon dioxide is removed from the make-up gas by a first and a second CO2 removal sections; the first section removes CO2 by absorption with a suitable medium, and the second section removes CO2 by washing with a carbamate solution taken from the urea section; the make-up gas is reacted to produce ammonia; the CO2 removed from the make-up gas and at least part of the ammonia are used to produce urea.

Claims

1. A process for the production of ammonia and urea in an ammonia-urea integrated plant comprising: reforming a hydrocarbon source to obtain a make-up gas containing hydrogen, CO.sub.2, and nitrogen, wherein said make-up gas, after purification, is converted into ammonia in an ammonia synthesis section, at least part of the synthesized ammonia provides an ammonia feed of a urea synthesis process, said urea synthesis process also receiving a carbon dioxide feed, the urea synthesis process comprising the reaction of ammonia and carbon dioxide in a urea synthesis section to form a urea aqueous solution, and subsequent treatment of said solution in a urea recovery section that produces a carbamate solution, wherein the purification of the make-up gas comprises a first step of CO.sub.2 removal in a first CO.sub.2 removal unit and a second step of CO.sub.2 removal in a second CO.sub.2 removal unit, said first and second CO.sub.2 removal units not being part of said urea synthesis section, one of said first and second CO.sub.2 removal steps comprises washing the CO.sub.2-containing make-up gas with the carbamate solution taken from said urea recovery section, said carbon dioxide feed of the urea synthesis process comprises at least part of the carbon dioxide separated from said make-up gas in the CO.sub.2 removal steps, and wherein said CO.sub.2 removal steps are carried out in series.

2. The process according to claim 1, further comprising introducing liquid or gaseous ammonia into the carbamate washing step.

3. The process according to claim 1, wherein the other of said CO.sub.2 removal steps comprises absorption of CO.sub.2 into an absorbing medium.

4. The process according to claim 1, wherein a portion of the CO.sub.2-containing make-up gas admitted to said CO.sub.2 removal steps bypasses the first CO.sub.2 removal step of the series, and is sent directly to the subsequent CO.sub.2 removal step.

5. The process according to claim 1, wherein the carbamate washing step is the first of the series.

6. The process according to claim 1, comprising a step of compression of the CO.sub.2-containing make-up gas prior to said CO.sub.2 removal steps, said make-up gas being compressed to an intermediate pressure lower than the ammonia synthesis pressure, wherein said CO.sub.2 removal steps are carried out under said intermediate pressure.

7. The process according to claim 1, further comprising introducing an aqueous solution of ammonia into the carbamate washing step.

8. The process according to claim 7, wherein said aqueous solution of ammonia is obtained from treatment of a purge gas drawn from the ammonia synthesis section of the integrated ammonia-urea plant.

9. The process according to claim 1, wherein the conversion of the purified make-up gas into ammonia is carried out in at least a first synthesis and a second synthesis, wherein said first synthesis produces a first amount of ammonia and a first stream of unreacted make-up gas, and said unreacted make-up gas is converted in said second synthesis obtaining a second amount of ammonia and a second stream of unreacted make-up gas, and at least a portion of said second stream of unreacted make-up gas is recycled to said first synthesis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scheme of an embodiment of the invention.

(2) FIG. 2 is a scheme of a second embodiment of the invention.

(3) FIG. 3 is a scheme of a third embodiment of the invention.

(4) FIG. 4 is a scheme of an integrated ammonia-urea plant according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) Referring to FIG. 1, ammonia make-up gas MUG e.g. from a shift converter of a reforming section (not shown) is directed to a CO2 removal section comprising CO2 removal stages CDR1 and CDR2. Said stages are arranged in series, so that the effluent of the stage CDR1 feeds the subsequent stage CDR2.

(6) The make-up gas leaving the second stage CDR2 is further purified by methanation MET and feeds ammonia synthesis AS.

(7) The ammonia A feeds directly a urea section US together with carbon dioxide CO2 removed from the syngas in stage CDR2. A part of the ammonia produced in the ammonia section (stream A1) is fed to the CO2 removal section CDR1. The urea synthesis US produces urea U.

(8) The first stage CDR1 operates by washing the syngas with a carbamate solution CS taken from a urea recovery section within the urea synthesis US. For example the solution CS is obtained after decomposition of an aqueous solution of urea produced in a urea reactor or urea synthesis loop. The carbamate solution CS is added with ammonia A1 in the section CDR1 in order to increase the CO2 absorption capability.

(9) The carbamate solution CS, plus the carbon dioxide removed from the syngas, are withdrawn from the stage CDR1 (i.e. after washing the syngas) and are sent again to the urea section US. Preferably all the carbon dioxide removed from the syngas in said stage CDR1 is recycled to the urea section.

(10) The second stage CDR2 operates for example by absorption of carbon dioxide in a suitable medium which is then conveniently regenerated to desorb the gaseous carbon dioxide.

(11) FIG. 2 shows an embodiment similar to FIG. 1, including a bypass line BP to allow some of the make-up gas MUG to bypass the first stage CDR1 and be admitted directly to the second stage CDR2.

(12) FIG. 3 shows an embodiment wherein the CO2 removal stages CDR1 and CDR2 are in parallel. Accordingly, a part of the make-up gas MUG goes to the first stage CDR1 and a remaining part goes to the second stage CDR2.

(13) FIG. 4 illustrates a parallel embodiment of the invention with a greater detail.

(14) Referring to FIG. 4, an integrated ammonia-urea plant 1 comprises an ammonia section 2 and a urea section 3. The ammonia section 2 includes a reforming front-end 4 and a synthesis loop 5.

(15) The reforming front-end 4 includes: primary reformer 7, secondary reformer 8, shift converter 9, low-pressure (LP) syngas compression stage 10, carbon dioxide removal section 11, methanator 12 and high-pressure (HP) syngas compression stage 6. The carbon dioxide removal section 11 includes stages 11a and 11b in parallel.

(16) Natural gas 20 and steam 21 catalytically react in the primary reformer 7 to provide a partially reformed gas 22. Said partially reformed gas 22 further reacts in the secondary reformer 8 fired by an oxidant (e.g. air) 23. The fully reformed gas 24 leaving the secondary reformer 8 is treated in the shift converter 9 where CO is converted to CO2.

(17) The shifted gas 25 is compressed in the LP compression stage 10. The compresses gas 26 delivered by said compression stage 10 is split into a first portions 27 and a second portion 28 which are treated respectively in the CO2 removal stages 11a and 11b.

(18) In the first CO2 removal stage 11a, carbon dioxide is absorbed in a solution of a suitable absorbent and then stripped therefrom to provide a first CO2-depleted make-up gas 29 and a CO2 stream 30. Here the term “CO2 stream” denotes a gas stream composed predominantly of CO2.

(19) In the second CO2 removal stage 11b (carbamate washing stage), the make-up gas 28 is contacted with a carbamate solution 61 taken from the tied-in urea section 3. A passivation agent (e.g. an oxygen carrier such as an oxygen-containing gas or hydrogen peroxide solution) may be added for corrosion protection and prevention.

(20) FIG. 4 illustrates a preferred embodiment wherein the carbamate solution 61 is mixed with an aqueous ammonia solution 53 collected from a purge recovery section 17 of the ammonia synthesis loop 5. Accordingly, both streams 61 and 53 provide a washing medium to remove carbon dioxide from the syngas.

(21) Additionally, an ammonia stream 62 is fed to the section 11b. This ammonia stream 62 allows to increase the absorption capability of the carbamate 61 and to deliver a CO2 depleted gas stream 31 containing only a negligible amount of unrecovered carbon dioxide.

(22) The CO2 removal stage 11b produces a second CO2-depleted gas stream 31 and discharges a carbamate solution 32 which is sent back to the urea section 3 as further explained below. Preferably all the CO2 removed from the make-up gas 28 in the stage 11b is contained in the stream 32.

(23) The second CO2-depleted gas stream 31 is cooled and sent to a washing column 33, wherein it is washed with water in order to remove traces of ammonia, thus providing a washed gas stream 34 which is joined with the above mentioned first gas portion 29 coming from the stage 11a.

(24) The so obtained syngas 35 (now comprising the gas effluent from both stages 11a and 11b) is further treated in a methanator 12 for conversion of residual amounts of CO into methane.

(25) The purified gas 36 effluent from said methanator 12 is sent to the HP compression section 6 to reach the ammonia synthesis pressure, e.g. 150 bar.

(26) The carbamate solution 32 from stage 11b is sent to the urea section 3 together with ammonia 37 from bottom of said column 33 via a mixer 38. The resulting mixed flow 39 is sent to the urea section 3, preferably to the synthesis section. The solution 32 is advantageously cooled to a temperature above the crystallization temperature of the carbamate.

(27) The synthesis loop 5 essentially comprises: a main reactor 13, a second reactor 14, a main loop HP separator 15, a second loop separator 16 and a purge recovery unit 17.

(28) The compressed syngas 40 delivered by the HP compression stage 6 is joined with a stream of unreacted gas 50 and fed to the main reactor 13 where it partially reacts to give ammonia. The product gas 41 is cooled by a gas cooler 42 and passed through the main loop separator 15 which separates liquid ammonia 43 from unreacted gas 44.

(29) Said unreacted gas 44 is fed to the second ammonia reactor 14 for further conversion. The resulting product gas 45 is cooled in a second gas cooler 46 and sent to the loop separator 16 which separates liquid ammonia 47 from unreacted synthesis gas 48.

(30) The liquid streams 43 and 47 form the ammonia output. At least part of this ammonia output feeds the urea section 3 via line 49.

(31) The unreacted gas 48 is split into portions 50 and 51. The first portion of unreacted gas 50 is recycled back to the main reactor 13 with the delivery stream 40 of the compressor 6, as above mentioned. The second portion of unreacted gas 51 is sent to the purge recovery unit 17, in particular for the recovery of hydrogen.

(32) Said recovery unit 17 produces a stream 52 containing recovered hydrogen, which is sent to the suction side of compressor 6 together with the make-up gas 36, and the aqueous ammonia solution 53.

(33) The urea section 3 receives ammonia from line 49 and carbon dioxide from lines 30 and 39, to produce urea 60. A part of the ammonia for the urea synthesis is contained in the stream 39 and it is supplied to the urea section 3 as stream 62 via the section 11b.