INTEGRATED PROCESS FOR THE PRODUCTION OF FORMALDEHYDE-STABILIZED UREA

Abstract

A process for the production of formaldehyde-stabilised urea is described 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) recovering carbon dioxide from the synthesis gas to form a carbon dioxide-depleted synthesis gas; (c) synthesising methanol from the carbon dioxide-depleted synthesis gas in a methanol synthesis unit and recovering the methanol and a methanol synthesis off-gas comprising nitrogen, hydrogen and residual carbon monoxide; (d) subjecting at least a portion of the recovered methanol to oxidation with air in a formaldehyde production unit; (e) subjecting the methanol synthesis off-gas to methanation in a methanation reactor containing a methanation catalyst to form an ammonia synthesis gas; (f) synthesising ammonia from the ammonia synthesis gas in an ammonia production unit and recovering the ammonia; (g) reacting a portion of the ammonia and at least a portion of the recovered carbon dioxide stream in a urea production unit to form a urea stream; and (h) stabilising the urea by mixing the urea stream and a stabiliser prepared using formaldehyde recovered from the formaldehyde production unit, wherein a source of air is compressed and divided into first and second portions, the first portion is provided to the formaldehyde production unit for the oxidation of methanol and the second portion is further compressed and provided to the synthesis gas generation unit.

Claims

1. A process for producing formaldehyde-stabilized 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) recovering carbon dioxide from the synthesis gas to form a carbon dioxide-depleted synthesis gas; (c) synthesizing methanol from the carbon dioxide-depleted synthesis gas in a methanol synthesis unit and recovering the methanol and a methanol synthesis off-gas comprising nitrogen, hydrogen and residual carbon monoxide; (d) subjecting at least a portion of the recovered methanol to oxidation with air in a formaldehyde production unit; (e) subjecting the methanol synthesis off-gas to methanation in a methanation reactor containing a methanation catalyst to form an ammonia synthesis gas; (f) synthesizing ammonia from the ammonia synthesis gas in an ammonia production unit and recovering the ammonia; (g) reacting a portion of the ammonia and at least a portion of the recovered carbon dioxide stream in a urea production unit to form a urea stream; and (h) stabilizing the urea by mixing the urea stream and a stabilizer prepared using formaldehyde recovered from the formaldehyde production unit, wherein a source of air is compressed and divided into first and second portions, the first portion is provided to the formaldehyde production unit for the oxidation of methanol and the second portion is further compressed and provided to the synthesis gas generation unit.

2. A process according to claim 1 wherein the synthesis gas is provided in a synthesis gas production unit comprising a synthesis gas generation stage and a water-gas shift stage.

3. A process according to claim 2 wherein the synthesis gas generation stage is based on steam reforming of a hydrocarbon; or by the gasification of a carbonaceous feedstock.

4. A process according to claim 2 wherein the synthesis gas generation stage is provided by one or more of adiabatic pre-reforming and/or primary reforming in a fired or gas-heated steam reformer or secondary or autothermal reforming with air or oxygen-enriched air.

5. A process according to claim 2 wherein the water gas shift stage comprises one or more stage of high temperature shift, low temperature shift, medium temperature shift, isothermal shift or sour shift.

6. A process according to claim 1 wherein carbon dioxide removal is effected using absorption or adsorption.

7. A process according to claim 1 wherein the methanol synthesis is operated on a once-through, or a recycle basis in which unreacted gases, after condensate removal, are returned to the methanol converter in a loop.

8. A process according to claim 1 wherein the methanol synthesis is operated in a single stage at an inlet temperature in the range 200-320 C.

9. A process according to claim 1 wherein crude methanol recovered from the methanol synthesis stage is fed without purification to an oxidation reactor within the formaldehyde production unit.

10. A process according to claim 1 wherein the formaldehyde production unit comprises an oxidation reactor containing a bed of oxidation catalyst and is operated with or without recycle of unreacted gases to the inlet of the oxidation reactor.

11. A process according to claim 1 wherein the formaldehyde production unit generates a formaldehyde vent gas which is recycled to the process, either directly or after one or more stages of vent gas treatment in a vent-gas treatment unit.

12. A process according to claim 11 wherein the vent gas treatment unit comprises a gas-liquid separator that separates the nitrogen-rich off-gas from liquid methanol.

13. A process according to claim 11 wherein the formaldehyde vent gas is recycled directly without treatment to the methanol synthesis stage or indirectly after it has first passed to an emission control system comprising a catalytic combustor to convert the vent stream into carbon dioxide, nitrogen and steam.

14. A process according to claim 11 wherein the formaldehyde vent gas is recycled directly without treatment to the carbon dioxide removal stage or indirectly after it has first passed to an emission control system comprising a catalytic combustor to convert the vent stream into carbon dioxide, nitrogen and steam.

15. A process according to claim 11 wherein the formaldehyde vent gas is recycled after it has first passed to an emission control system comprising a catalytic combustor to convert the vent stream into carbon dioxide, nitrogen and steam to the urea synthesis stage.

16. A process according to claim 11 wherein the formaldehyde vent gas is recycled directly to the synthesis gas generation unit as a component of a fuel gas.

17. A process according to claim 8 wherein the methanol synthesis is operated in a single stage at an inlet temperature in the range 200-270 C.

18. A process according to claim 3 wherein the hydrocarbon is natural gas, naphtha, or a refinery off-gas.

19. A process according to claim 3 wherein the carbonaceous feedstock is coal or biomass.

Description

[0054] The present invention will now be described by way of example with reference to the accompanying drawings in which;

[0055] FIG. 1 is a schematic representation of a process according to a first aspect of the present invention;

[0056] FIG. 2 is a schematic representation of a process according to a second aspect of the present invention;

[0057] FIG. 3 is a schematic representation of a process according to a third aspect of the present invention; and

[0058] FIG. 4 is a schematic representation of a process according to a fourth aspect of the present invention.

[0059] 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.

[0060] In FIG. 1, a natural gas stream 10, steam 16 and a first portion 14 of an air stream 12 are fed to a synthesis gas generation unit 18 comprising a primary reformer, a secondary reformer and a water-gas shift unit comprising high- and low-temperature shift converters. 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 air to generate a raw synthesis gas comprising nitrogen, hydrogen, carbon dioxide, carbon monoxide and steam. The steam to carbon monoxide ratio of the raw synthesis gas may be adjusted by steam addition if necessary and the gas subjected to high temperature shift and low temperature shift in shift converters containing high and low temperature shift catalysts to generate a shifted synthesis gas mixture 22 in which the hydrogen and carbon dioxide contents are increased and the steam and carbon monoxide contents decreased. Steam 20, generated by cooling the secondary and shifted gas streams, may be exported from the synthesis gas generation unit 18. The shifted synthesis gas 22 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. If desired, upstream of the methanol synthesis unit 30, steam in the shifted gas may be removed by cooling and separation of condensate. Methanol is synthesised in the converter on a once-through basis and separated from the product gas mixture and recovered from the methanol synthesis unit 30 by line 32 and passed to a formaldehyde production unit 34 comprising an oxidation reactor containing an oxidation catalyst. A second portion 36 of the air source 12 is fed with the methanol to the oxidation reactor where they are reacted 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. Other feed streams to the formaldehyde production unit may include boiler feed water, process water and caustic (not shown). The formaldehyde is recovered in an absorption tower which may be fed with urea via line 67 such that either an aqueous formaldehyde or a urea-formaldehyde concentrate (UFC) product stream 44 may be recovered from the formaldehyde production unit 34 for further use. 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 essentially nitrogen and 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 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.

[0061] In FIG. 2, the same synthesis gas generation, carbon dioxide removal, methanol synthesis, methanation, ammonia synthesis, urea synthesis and stabilisation units 18, 24, 30, 48, 54, 64 & 70 as set out in FIG. 1 are provided. In this embodiment, the methanol stream 32 is passed to a methanol purification unit 80 where it is subjected to a distillation step heated by steam stream 40 recovered from the formaldehyde production unit 34. Purified methanol is fed from the purification unit 80 by line 82 to the formaldehyde production unit 34 where it is oxidised to produce formaldehyde. A by-product stream is recovered from the methanol purification unit 80 by line 88. In addition, the vent gas stream 42 from the formaldehyde production unit 34 is passed to a gas-liquid separation unit 84 where unreacted methanol is recovered from a nitrogen-rich gas stream. Unreacted methanol is fed from the separation unit 84 to the methanol purification unit 80 by line 86. If desired, the nitrogen-rich gas stream may be compressed and passed to the ammonia synthesis unit 54 (as shown by a dotted line 90).

[0062] In FIG. 3, the same synthesis gas generation, carbon dioxide removal, methanol synthesis, methanation, ammonia synthesis, urea synthesis and stabilisation units 18, 24, 30, 48, 54, 64 & 70 as set out in FIG. 1 are provided. In this embodiment, the vent gas stream 42 from the formaldehyde production unit 34 is passed to an emission control system (ECS) 100 comprising a catalytic combustor in which the organic vent gas components are converted to carbon dioxide and steam. The combusted gas mixture, (i.e. ECS effluent) which comprises nitrogen, carbon dioxide and steam may be suitably compressed and recycled from the emission control system 100 to the process. In one embodiment, the combusted gas mixture from the ECS unit 100 is passed by line 102 to the methanol synthesis unit 30 where the carbon dioxide may be reacted with hydrogen in the synthesis gas to generate additional methanol. Alternatively or additionally, the combusted gas mixture may be provided by line 106 to the carbon dioxide removal unit 24 where the steam and carbon dioxide are removed to provide additional nitrogen in the synthesis gas. Alternatively or additionally, the combusted gas mixture may be provided via line 104 to the urea production unit 64 where the carbon dioxide is reacted to produce additional urea.

[0063] In FIG. 4, the same synthesis gas generation, carbon dioxide removal, methanol synthesis, methanation, ammonia synthesis, urea synthesis and stabilisation units 18, 24, 30, 48, 54, 64 & 70 as set out in FIG. 1 are provided. In this embodiment, the vent gas stream 42 from the formaldehyde production unit 34 is recycled directly, without treatment in an ECS or other vent gas treatment units, to the process. In one embodiment, vent gas stream is passed by line 108 to the methanol synthesis unit 30 where the carbon dioxide is reacted with hydrogen to generate methanol. Alternatively or in addition, the vent gas stream may be passed by line 110 to the carbon dioxide removal unit 24 where the steam and carbon dioxide are removed. Alternatively or in addition, the vent gas stream may be passed by line 112 to the synthesis gas generation unit 18 as a fuel.

[0064] The present invention will now be described with reference to the following example in accordance with the flow sheet depicted in FIG. 1.

[0065] A process according to FIG. 1 was modelled to determine the effects of using the compressed air feed for both the synthesis gas generation and formaldehyde production in a 3000 mtpd urea plant. The synthesis gas generation was by conventional primary and secondary steam reforming with air of natural gas with, both high-temperature and low-temperature water gas shift. The formaldehyde production was performed using air oxidation of methanol over a particulate iron/molybdenum catalyst disposed in cooled tubes, with recycle of a portion of the product gas to control the temperature of 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 maximise the conversion to produce ammonia. The compositions, pressures and temperatures for the various streams are given below.

TABLE-US-00002 Stream mole % dry 10 16 14 36 22 26 28 32 N.sub.2 1.32 78.08 78.08 20.62 0.14 25.16 O.sub.2 20.96 20.96 H.sub.2 2.81 60.45 0.70 73.71 NH.sub.3 CH.sub.4 91.81 0.30 <0.01 0.37 Ar 0.01 0.93 0.93 0.24 0.29 CO.sub.2 2.56 0.03 0.03 18.08 99.15 0.10 CO 0.30 0.01 0.37 C.sub.2H.sub.6 1.23 C.sub.3H.sub.8 0.02 C.sub.4+ 0.24 CH.sub.3OH 100.00 CH.sub.2O CO(NH.sub.2).sub.2 Dry Flow kmol/hr 2434.8 3314.4 78.6 12747.3 2314.3 10433.0 25.0 H.sub.2O kmol/hr 7216.2 4078.6 4030.5 48.1 51.4 Total flow kmol/hr 2434.8 7216.2 3314.4 78.6 16825.9 6344.8 10481.1 76.4 Temperature C. 28 328 172 25 210 70 6 Pressure bar abs 42.0 125.0 37.5 1.5 31.6 1.3 30.0 90.0 Stream mole % dry 46 52 56 26 68 67 44 42 72 N.sub.2 25.38 25.55 <0.01 0.02 91.35 O.sub.2 5.48 H.sub.2 73.72 73.54 <0.01 0.15 NH.sub.3 99.99 0.18 0.18 0.01 CH.sub.4 0.37 0.61 0.01 Ar 0.29 0.30 1.10 CO.sub.2 0.06 99.83 CO 0.14 1.51 C.sub.2H.sub.6 C.sub.3H.sub.8 C.sub.4+ CH.sub.3OH 0.04 0.24 0.51 <0.01 CH.sub.2O 0.04 0.04 82.56 0.02 1.10 CO(NH.sub.2).sub.2 99.78 99.78 17.20 98.89 Dry Flow kmol/hr 10345.0 10274.6 4921.9 2003.0 2086.1 4.9 28.1 66.4 2104.6 H.sub.2O kmol/hr 1.5 31.5 209.9 293.2 15.9 9.5 1.9 14.0 Total flow kmol/hr 10346.5 10306.1 4921.9 2212.9 2379.3 20.8 37.6 68.3 2118.7 Temperature C. 6 330 22 133 45 30 30 95 Pressure bar abs 90.0 182.0 17.3 1.3 0.9 5.0 4.0 2.5 1.0