Integrated process for the production of formaldehyde-stabilised urea

Abstract

A process for producing formaldehyde-stabilised urea is described comprising generating a synthesis gas, subjecting the synthesis gas to water-gas shift to form a shifted gas; recovering carbon dioxide from the shifted gas; synthesising methanol from the carbon dioxide-depleted synthesis gas; subjecting recovered methanol to oxidation; subjecting the methanol synthesis off-gas to methanation; synthesising ammonia from the ammonia synthesis gas and recovering the ammonia; reacting ammonia and recovered carbon dioxide stream to form a urea stream; and stabilising the urea by mixing the urea stream and a stabiliser prepared using the recovered formaldehyde, wherein a portion of the synthesis gas by-passes either the one or more of the water-gas shift reactors, carbon dioxide removal unit, or water-gas shift reactors and the carbon dioxide removal unit used in the process.

Claims

1. A process for producing formaldehyde-stabilised urea comprising the steps of: (A) generating a synthesis gas comprising hydrogen, nitrogen, carbon monoxide, carbon dioxide and steam in a synthesis gas generation unit; (B) subjecting the synthesis gas to one or more stages of water-gas shift in one or more water-gas shift reactors to form a shifted gas; (C) recovering carbon dioxide from the shifted gas in a carbon dioxide removal unit to form a carbon dioxide-depleted synthesis gas; (D) passing the carbon dioxide-depleted synthesis gas through a methanol synthesis unit to synthesize a crude methanol and recovering the crude methanol and a methanol synthesis off-gas stream comprising nitrogen, hydrogen and residual carbon monoxide; (E) oxidizing at least a portion of the recovered crude methanol with air in a formaldehyde production unit to produce an oxidised gas containing formaldehyde and a formaldehyde vent gas, the crude methanol having been adjusted to have a water content in a range of from 5 wt % to 20 wt %; (F) methanating the methanol synthesis off-gas in a methanation reactor containing a methanation catalyst to form an ammonia synthesis gas; (G) synthesising ammonia from the ammonia synthesis gas in an ammonia production unit and recovering the ammonia; (H) reacting a portion of the ammonia and at least a portion of the recovered carbon dioxide in a urea production unit to form a urea stream; and (I) stabilising the urea by mixing the urea stream and a stabiliser prepared using formaldehyde recovered from the formaldehyde production unit, wherein: (i) a portion of the synthesis gas generated by the synthesis gas generation unit by-passes (a) one or more water-gas shift reactors; or (b) one or more water-gas shift reactors and the carbon dioxide removal unit; or (ii) a portion of the shifted gas by-passes the carbon dioxide removal unit; and wherein the crude methanol recovered from step (D) is fed to oxidizing step (E).

2. The process according to claim 1 wherein the synthesis gas is generated by steam reforming a hydrocarbon or gasifying a carbonaceous feedstock.

3. The process according to claim 1, wherein the synthesis gas is generated from a hydrocarbon by adiabatic pre-reforming, primary reforming in a fired or gas-heated steam reformer and secondary or autothermal reforming with air or oxygen-enriched air, or a combination thereof.

4. The process according to claim 3, wherein a portion of the hydrocarbon by-passes the primary fired or gas-heated steam reformer.

5. The process according to claim 1, further comprising compressing and dividing a source of air into a first portion that is provided to the formaldehyde production unit of step (E) for oxidizing methanol and a second portion that is further compressed and provided to the synthesis gas generation unit of step (A).

6. The process according to claim 1, wherein the one or more stages of water-gas shift of step (B) comprise one or more stages of high temperature shift, low temperature shift, medium temperature shift, isothermal shift, or sour shift.

7. The process according to claim 1, wherein the one or more stages of water-gas shift in step (B) consists of a high temperature shift reactor and a low temperature shift reactor and the portion of the synthesis gas by-passes either the high temperature shift reactor, or both the high temperature shift reactor and the low temperature shift reactor, or a portion of a high-temperature shifted gas by-passes the low temperature shift reactor.

8. The process according to claim 1, wherein carbon dioxide is removed in step (C) using absorption or adsorption.

9. The process according to claim 1, wherein the methanol synthesis of step (D) is performed on a once-through basis, or on a recycle basis wherein unreacted gases, after methanol condensate removal, are returned to the methanol synthesis unit.

10. The process according to claim 1, wherein the methanol synthesis unit of step (D) is operated in a single stage at an inlet temperature in the range of from 200-320 C.

11. The process according to claim 1, wherein the formaldehyde production unit of step (E) comprises an oxidation reactor containing a bed of oxidation catalyst.

12. The process according to claim 1, wherein the formaldehyde production unit of step (E) generates a formaldehyde vent gas which is directly recycled or recycled after one or more stages of vent gas treatment in a vent-gas treatment unit.

13. The process according to claim 12, wherein the vent gas treatment unit comprises a gas-liquid separator that separates a nitrogen-rich off-gas from liquid methanol.

14. The process according to claim 12, wherein the formaldehyde vent gas is recycled to the methanol synthesis step (D) either directly without treatment or indirectly after first passing to an emission control unit comprising a catalytic combustor to convert the vent stream into carbon dioxide, nitrogen and steam.

15. The process according to claim 12, wherein the formaldehyde vent gas is recycled to the carbon dioxide removal step (C) either directly without treatment or indirectly after first passing to an emission control unit comprising a catalytic combustor to convert the vent stream into carbon dioxide, nitrogen and steam.

16. The process according to claim 12, wherein the formaldehyde vent gas is recycled to the urea synthesis stage after it has first passed to an emission control unit comprising a catalytic combustor to convert the vent stream into carbon dioxide, nitrogen and steam.

17. The process according to claim 12, wherein the formaldehyde vent gas is recycled directly to a synthesis gas generation unit comprising the synthesis gas.

18. The process according to claim 1, wherein the one or more stages of water-gas shift of step (B) comprise a single stage of high temperature shift, a combination of high temperature shift and low temperature shift, a single stage of medium temperature shift, or a combination of medium temperature shift and low temperature shift.

19. The process according to claim 1, wherein (i) a portion of the synthesis gas generated by the synthesis gas generation unit by-passes (a) one or more water-gas shift reactors.

20. The process according to claim 1, wherein (i) a portion of the synthesis gas generated by the synthesis gas generation unit by-passes (b) one or more water-gas shift reactors and the carbon dioxide removal unit.

21. The process according to claim 1, wherein (ii) a portion of the shifted gas by-passes the carbon dioxide removal unit.

Description

(1) The present invention will now be described by way of example with reference to the accompanying drawings in which;

(2) FIG. 1 is a schematic representation of a process according to a first aspect of the present invention including by-pass of the one or more water-gas shift reactors, and

(3) FIG. 2 is a schematic representation of a process according to a second aspect of the present invention including by-pass of the carbon dioxide removal unit.

(4) It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

(5) In FIG. 1, a natural gas stream 10, steam 12 and an air stream 14 are fed to a synthesis gas generation unit 18 comprising a primary reformer and secondary reformer. The natural gas is primary reformed with steam in externally-heated catalyst filled tubes and the primary reformed gas subjected to secondary reforming in the secondary reformer with the air to generate a raw synthesis gas comprising nitrogen, hydrogen, carbon dioxide, carbon monoxide and steam. A portion of the natural gas may by-pass the primary reformer and be fed along with the primary reformed gas to the secondary reformer. A flue gas 16 is discharged from the primary reformer. The steam to carbon monoxide ratio of the raw synthesis gas may be adjusted by steam addition if necessary and the gas subjected to water-gas shift in a high temperature shift reactor 20 containing a high temperature shift catalyst and then a low temperature shift reactor 22 containing a low temperature shift catalyst. The water-gas shift reaction increases the hydrogen and carbon dioxide contents and the steam and carbon monoxide contents are decreased. The shifted synthesis gas is fed to a carbon dioxide removal unit 24 operating by means of reactive absorption. A carbon dioxide and water stream is recovered from the separation unit 24 by line 26 for further use. A carbon dioxide-depleted synthesis gas 28 comprising hydrogen, carbon monoxide and nitrogen is passed from the carbon dioxide removal unit 24 to a methanol synthesis unit 30 comprising a methanol converter containing a bed of methanol synthesis catalyst. Upstream of the methanol synthesis unit 30, water in the shifted gas and the carbon dioxide-depleted synthesis gas is removed by cooling and separation of the condensate. The dried carbon dioxide-depleted synthesis gas is then heated and fed to the methanol synthesis unit. Methanol is synthesised in the converter on a once-through basis, separated from the product gas mixture and recovered from the methanol synthesis unit 30. The methanol is passed via line 32 to a formaldehyde production unit 34 comprising an oxidation reactor containing an oxidation catalyst. An air source 36 is fed to the oxidation reactor where it is reacted with the methanol to produce formaldehyde. The oxidation reactor is operated in a loop with a portion of the reacted gas fed to the inlet of the reactor. The formaldehyde production unit is fed with cooling water 38 and generates a steam stream 40 and a formaldehyde vent gas 42. The formaldehyde is recovered in an absorption tower which may be fed with water or urea via line 67 such that either aqueous formaldehyde or a urea-formaldehyde concentrate (UFC) product stream 44 may be recovered from the formaldehyde production unit 34 for further use. A portion of the formaldehyde product stream 45 can be taken for use in, for example, a separate urea-stabilisation plant or for sale, if the flow of formaldehyde produced is in excess of that required for the associated urea plant. A methanol synthesis off-gas stream 46 comprising hydrogen, nitrogen and unreacted carbon monoxide recovered from the methanol synthesis unit 30 is passed to a methanation unit 48 comprising a methanation reactor containing a bed of methanation catalyst. Carbon oxides remaining in the off-gas 46 are converted to methane and water in the methanation reactor. Water is recovered from the methanation unit 48 by line 50. The methanated off-gas is an ammonia synthesis gas comprising nitrogen, hydrogen and methane. The ammonia synthesis gas is passed from the methanation unit 48 by line 52 to an ammonia synthesis unit 54 comprising an ammonia converter containing one or more beds of ammonia synthesis catalyst. The ammonia converter is operated in a loop with a portion of the reacted gas fed to the inlet of the converter. Ammonia is produced in the converter and recovered from the ammonia synthesis unit 54 by line 56. A purge gas stream 60 comprising methane and unreacted hydrogen and nitrogen is recovered from the ammonia synthesis unit 54 and provided to the synthesis gas generation unit 18 as fuel and/or feed to the primary and/or secondary reformers. A vent gas stream 62 is also recovered from the ammonia synthesis unit 54. A portion 58 of the ammonia is separated from the product stream 56. The remaining ammonia is passed to a urea synthesis unit 64 where it is reacted with a purified carbon dioxide stream provided by stream 26 to produce a urea stream and water. Water is recovered from the urea synthesis unit 64 by line 66. The urea stream is passed by line 68 to a stabilisation unit 70 comprising a stabilisation vessel where it is treated with aqueous formaldehyde or a urea formaldehyde concentrate provided by line 44 to form a formaldehyde-stabilised urea product. The formaldehyde-stabilised urea product is recovered from the stabilisation unit 70 by line 72.

(6) In FIG. 1, in order to adjust the carbon oxides content of the gas stream 28 fed to the methanol synthesis unit 30, one or more by-pass streams are provided around the water-gas shift reactors. Thus in one aspect, a by-pass stream 74 conveys a portion of the raw synthesis gas produced by the synthesis gas generation unit 18 around the high temperature shift reactor 20 to the feed to the low temperature shift reactor 22. In an alternative aspect, a by-pass stream 76 conveys a portion of the high temperature shifted synthesis gas around the low temperature shift reactor 22 and to the feed stream to the carbon dioxide removal unit 24. In a preferred aspect, a by-pass stream 78 conveys a portion of the raw synthesis gas produced by the synthesis gas generation unit 18 around the high temperature shift reactor 20 and the low temperature shift reactor 22 directly to the feed stream to the carbon dioxide removal unit 24.

(7) In FIG. 2, the synthesis gas generation, water-gas shift, carbon dioxide removal, methanol synthesis, methanation, formaldehyde synthesis, ammonia synthesis, urea synthesis and stabilisation step are the same as depicted in FIG. 1.

(8) However, in FIG. 2, in order to adjust the carbon oxides content of the gas stream 28 fed to the methanol synthesis unit 30, one or more by-pass streams are provided around the carbon dioxide removal unit 24. Thus in one aspect, a by-pass stream 80 conveys a portion of the shifted synthesis gas produced by the low temperature shift reactor 22 around the carbon dioxide removal unit 24 to the feed stream 28 to the methanol synthesis unit 30. In an alternative aspect, a by-pass stream 82 conveys a portion of the high temperature shifted synthesis gas around the low temperature shift reactor 22 and the carbon dioxide removal unit to the feed stream 28 to the methanol synthesis unit 30. In an alternative aspect, a by-pass stream 84 conveys a portion of the raw synthesis gas produced by the synthesis gas generation unit 18 around the high temperature shift reactor 20, the low temperature shift reactor 22 and the carbon dioxide removal unit 24 directly to the feed stream 28 to the methanol synthesis unit 30.

(9) The present invention will now be described with reference to the following example.

(10) A process according to FIG. 1 was modelled to determine the effect of by-passing a portion of the process gas 76 around the low temperature shift reactor 22. The synthesis gas generation was by primary and secondary steam reforming with air of natural gas with, both high temperature and low temperature water-gas shift. There was no hydrocarbon by-pass of the primary reformer to the secondary reformer. The formaldehyde product was produced using the air oxidation of the methanol over a particulate iron/molybdenum catalyst disposed in cooled tubes, with recycle of a portion of the unreacted gas to control the temperature within the oxidation reactor. The methanol synthesis was performed on a once though basis and the ammonia synthesis was performed with recycle of a portion of the product gas to maximise ammonia production. The compositions, pressures and temperatures for the various streams are given below.

(11) TABLE-US-00002 Stream mole % dry 10 12 14 36 26 28 32 46 52 N.sub.2 1.32 78.08 78.08 0.16 23.61 0.04 24.52 24.91 O.sub.2 20.96 20.96 H.sub.2 2.81 0.65 73.69 0.01 73.90 73.48 NH.sub.3 CH.sub.4 91.81 0.01 0.62 0.01 0.64 1.22 Ar 0.01 0.93 0.93 0.36 0.38 CO.sub.2 2.56 0.03 0.03 99.15 0.10 0.01 0.03 CO 0.03 1.62 0.45 C.sub.2H.sub.6 1.23 C.sub.3H.sub.8 0.02 C.sub.4+ 0.24 CH.sub.3OH 99.92 0.08 CH.sub.2O CO(NH.sub.2).sub.2 Dry Flow 1525.0 1988.2 240.2 1359.7 6658.1 78.2 6408.8 6308.7 kmol/hr H.sub.2O 4369.4 151.1 18.3 2559.8 2.5 6.7 0.2 1.1 kmol/hr Total flow 1525.0 4369.4 2139.3 258.5 3919.5 6660.6 84.9 6409.0 6309.8 kmol/hr Temperature 365 390 25 225 10 350 103 C. Pressure 44 45 1 1 92 3 90 182 bar abs Stream mole % dry 56 68 67 44 42 72 76 N.sub.2 <0.01 92.46 20.52 O.sub.2 5.49 H.sub.2 <0.01 59.71 NH.sub.3 99.99 0.18 0.18 CH.sub.4 0.01 0.43 Ar 0.24 CO.sub.2 15.25 CO 3.85 C.sub.2H.sub.6 C.sub.3H.sub.8 C.sub.4+ CH.sub.3OH 0.24 0.18 <0.01 CH.sub.2O 0.04 0.04 82.50 0.02 1.10 CO(NH.sub.2).sub.2 99.78 99.78 17.19 98.89 Dry Flow 3067.3 1225.6 14.8 84.8 200.6 1236.5 2299.2 kmol/hr H.sub.2O 172.3 48.1 28.5 5.8 8.2 840.2 kmol/hr Total flow 3067.3 1397.8 62.9 113.3 206.4 1244.7 3139.5 kmol/hr Temperature 22 133 45 30 30 95 205 C. Pressure 17 1 4 3 1 1 32 bar abs

(12) This example relates to a 1,200 mtpd ammonia plant that makes 86 mtpd of UFC-85, for which about 60 mtpd of methanol is required. In this case (Case 1), the process characteristics are as follows;

(13) TABLE-US-00003 Units Case 1 By-pass flow rate as % of the reformed synthesis mol % 30 gas flow rate Carbon monoxide in by-pass mol % (dry) 3.85 Carbon dioxide in by-pass mo l% (dry) 15.25 Carbon monoxide inlet methanol synthesis mol % (dry) 1.62 Carbon dioxide inlet methanol synthesis mol % (dry) 0.01 Water content of crude methanol % wt 4.6 Relative size of methanol synthesis catalyst bed % 100

(14) The proportion of the gas flow that is diverted to the by-pass stream in Case 1 is relatively high as a large portion of the carbon oxides are removed in the carbon dioxide removal unit. In comparison, Case 2, modelled according to FIG. 2, refers to the situation where there is a by-pass stream 82 around the low temperature shift reactor 22 and the carbon dioxide removal unit 24. In comparison, Case 3, again modelled according to FIG. 2, refers to the situation where there is a by-pass stream 84 around the high temperature shift reactor 20, the low temperature shift reactor 22 and the carbon dioxide removal unit 24. That is, a single by-pass stream around all three units, transferring a portion of the synthesis gas to the carbon dioxide-depleted gas stream. The model results are shown below.

(15) TABLE-US-00004 Units Case 2 Case 3 By-pass flow rate as % of the reformed Mole % 7 6 synthesis gas flow rate Carbon monoxide in by-pass mol % (dry) 3.85 13.81 Carbon dioxide in by-pass mol % (dry) 15.25 7.12 Carbon monoxide inlet methanol mol % (dry) 0.70 1.27 synthesis Carbon dioxide inlet methanol synthesis mol % (dry) 1.29 0.53 Water content of crude methanol % wt 32.6 17.1 Relative size of methanol synthesis % 63 79 catalyst bed

(16) By comparison to Case 1 above, the proportion of the process gas stream being by-passed around the units is significantly smaller.