PROCESS FOR THE PRODUCTION OF FORMALDEYDE-STABILIZED UREA

Abstract

A process for the production of formaldehyde-stabilised urea is described comprising the steps of: (a) generating a synthesis gas; (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) cooling the shifted gas to below the dew point and recovering condensate to form a dried shifted gas; (d) recovering carbon dioxide from the dried shifted gas in a carbon dioxide removal unit to form a carbon dioxide-depleted synthesis gas; (e) synthesising methanol from the carbon dioxide-depleted synthesis gas in a methanol synthesis unit and recovering the methanol and a methanol synthesis off-gas; (f) subjecting at least a portion of the recovered methanol to oxidation with air to form formaldehyde in a stabiliser production unit; (g) subjecting the methanol synthesis off-gas to methanation in a methanation reactor containing a methanation catalyst to form an ammonia synthesis gas; (h) synthesising ammonia from the ammonia synthesis gas in an ammonia production unit and recovering the ammonia; (i) 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 (j) stabilising the urea by mixing the urea stream and a stabiliser prepared using the formaldehyde produced in the stabiliser production unit, wherein the carbon dioxide removal unit operates by means of absorption using a liquid absorbent and comprises an absorbent regeneration unit, wherein the process includes recovering a carbon dioxide-containing gas stream from the absorbent regeneration unit, compressing at least a portion of the recovered carbon dioxide-containing gas stream to form a compressed carbon dioxide-containing gas stream and passing the compressed carbon dioxide-containing gas stream to the methanol synthesis unit.

Claims

1. A process for the production of 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) cooling the shifted gas to below the dew point and recovering condensate to form a dried shifted gas; (d) recovering carbon dioxide from the dried shifted gas in a carbon dioxide removal unit to form a recovered carbon dioxide stream and carbon dioxide-depleted synthesis gas; (e) 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; (f) subjecting at least a portion of the recovered methanol to oxidation with air to form formaldehyde in a stabiliser production unit; (g) subjecting the methanol synthesis off-gas to methanation in a methanation reactor containing a methanation catalyst to form an ammonia synthesis gas; (h) synthesising ammonia from the ammonia synthesis gas in an ammonia production unit and recovering the ammonia; (i) reacting a portion of the ammonia and a portion of the recovered carbon dioxide stream in a urea production unit to form a urea stream; and (j) stabilising the urea by mixing the urea stream and a stabiliser prepared using the formaldehyde produced in the stabiliser production unit, wherein the carbon dioxide removal unit operates by means of absorption using a liquid absorbent and comprises an absorbent regeneration unit, wherein the process includes recovering a carbon dioxide-containing gas stream from the absorbent regeneration unit, compressing a portion of the recovered carbon dioxide-containing gas stream to form a compressed carbon dioxide-containing gas stream and passing the compressed carbon dioxide-containing gas stream to the methanol synthesis unit.

2. The process of claim 1, wherein the synthesis gas generation stage comprises steam reforming of a hydrocarbon or the gasification of a carbonaceous feedstock.

3. The process of claim 1, wherein the synthesis gas generation stage comprises primary reforming in a fired or gas-heated steam reformer and secondary reforming in a secondary reformer with air or oxygen-enriched air.

4. The process of claim 1, wherein the one or more stages of water-gas shift comprise one or more stages of high temperature shift, low temperature shift, medium temperature shift, isothermal shift and sour shift.

5. The process of claim 1, wherein carbon dioxide removal unit comprises an absorption unit comprising one or more absorption vessels to which the dried shifted gas and an absorbent liquid are fed, and an absorbent regeneration unit comprising one or more absorbent regeneration vessels in which a carbon dioxide-laden absorbent liquid is regenerated by heating and/or reducing the pressure, to produce the carbon dioxide-containing gas stream.

6. The process of claim 1, wherein the carbon dioxide-containing gas stream comprises a vent gas stream containing carbon dioxide, hydrogen and other absorbed gases, an essentially pure carbon dioxide stream, or a mixture of these.

7. The process of claim 6, wherein the vent gas stream comprises 10-45 mol % hydrogen.

8. The process of claim 6, wherein the essentially pure carbon dioxide stream comprises <10 mole % hydrogen.

9. The process of claim 6, wherein the amount of vent gas passed to the methanol synthesis unit is in the range of from 0.1-2.5% by volume of the dried shifted gas fed to the carbon dioxide removal unit.

10. The process of claim 6, wherein the amount of pure carbon dioxide passed to the methanol synthesis unit is in the range of from 0.1% to 10% by volume of the total pure carbon dioxide separated in the carbon dioxide removal unit.

11. The process of claim 1, wherein the carbon dioxide-containing gas stream further comprises steam and before compression, the carbon dioxide-containing gas stream is cooled to below the dew point to condense the steam as water which is recovered using a separator to produce a dry carbon dioxide-containing gas stream.

12. The process of claim 1, wherein the methanol synthesis unit is operated on a once-through basis, or on a recycle basis in which unreacted gases, after methanol removal, are returned to the methanol synthesis reactor in a loop.

13. The of claim 1, wherein the methanol synthesis is operated in a single stage at an inlet temperature to the catalyst in the range of from 200-320 C.

14. The process of claim 1, wherein a crude methanol product recovered from the methanol synthesis stage is fed to the oxidation reactor.

15. The process of claim 1, wherein the formaldehyde stabiliser production unit generates a stabiliser unit 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.

16. The process of claim 4, wherein the one or more stages of water-gas shift 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.

17. The process of claim 7, wherein the vent gas stream comprises 25-40 mole % hydrogen.

18. The process of claim 8, wherein the essentially pure carbon dioxide stream comprises 5 mole % hydrogen.

19. The process of claim 10, wherein the amount of pure carbon dioxide passed to the methanol synthesis unit is in the range 0.1% to 5% by volume of the total pure carbon dioxide separated in the carbon dioxide removal unit.

20. The process of claim 13, wherein the methanol synthesis is operated in a single stage at an inlet temperature to the catalyst in the range of from 200-270 C.

Description

[0064] The present invention will now be described by way of example with reference to the accompanying drawings in which;

[0065] FIG. 1 is a schematic representation of a process according to a first aspect of the present invention.

[0066] 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.

[0067] 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 in the primary reformer 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 comprising an absorption unit 24 and a regeneration unit 26. The absorption unit 24 comprises a single absorption vessel in which the shifted synthesis gas is contacted with a liquid absorbent. The liquid absorbent absorbs carbon dioxide to produce a carbon dioxide-depleted synthesis gas 32 comprising hydrogen, carbon monoxide and nitrogen. A carbon dioxide-laden absorbent liquid is fed from the absorption unit 24 to a regeneration unit 26 comprising an absorbent regeneration vessel operating in two stages, with the first stage operating at a higher pressure than the second stage. A vent gas 28 containing carbon dioxide and hydrogen is recovered from the regeneration unit 26 between the first stage and the second stage. A carbon dioxide stream 30 is recovered from the second stage of the regeneration unit 26 for further use. If desired, the vent gas 28 may be cooled to below the dew point in a heat exchanger and the condensate separated in a separator to provide a dry vent gas. The vent gas 28 (or dry vent gas) is compressed by a compressor (not shown) to form a compressed vent gas. The compressed vent gas 28 and the carbon dioxide-depleted synthesis gas 32 recovered from the absorption unit 24 are passed to a methanol synthesis unit 34 comprising a methanol converter containing a bed of methanol synthesis catalyst. Alternatively, or in addition, a portion of the recovered carbon dioxide stream 30 may be dried if necessary to remove water, compressed and fed to the methanol synthesis unit, via line 35. Although separate streams are depicted, it may be convenient to combine these streams upstream of the methanol synthesis unit. If desired, upstream of the methanol synthesis unit 34, water in the carbon dioxide-depleted synthesis gas 32 may also be removed by cooling and separation of the condensate. The vent gas 28, and/or carbon dioxide stream 35 and carbon dioxide-depleted synthesis gas 32 may be heated if necessary before feeding them to the methanol synthesis unit 34. Methanol is synthesised in a single converter on a once-through basis and separated from the product gas mixture. A crude methanol product is recovered from the methanol synthesis unit 34 by line 36 and passed to a stabiliser production unit 38 comprising an oxidation reactor containing an oxidation catalyst. An air source 40 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 stabiliser production unit is fed with cooling water 42 and generates a steam stream 44 and a stabiliser unit vent gas 46. The formaldehyde produced in the oxidation reactor is recovered in an absorption tower which may be fed with water and optionally urea via line 48 such that either aqueous formaldehyde or a urea-formaldehyde concentrate (UFC) product stream 50 may be recovered from the stabiliser production unit 38 for further use. A portion 52 of the stabiliser product stream 50 can be taken for use in, for example, a separate urea-stabilisation plant or for sale, if the flow of stabiliser produced is in excess of that required for the associated urea plant.

[0068] A methanol synthesis off-gas stream 54 comprising hydrogen, nitrogen and unreacted carbon monoxide recovered from the methanol synthesis unit 34 is passed to a methanation unit 56 comprising a methanation reactor containing a bed of methanation catalyst. Carbon oxides remaining in the off-gas 54 are converted to methane and water in the methanation reactor. Water is recovered from the methanation unit 56 by line 58. The methanated off-gas is an ammonia synthesis gas comprising nitrogen and hydrogen and a small amount of methane. The ammonia synthesis gas is passed from the methanation unit 56 by line 60 to an ammonia synthesis unit 62 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 62 by line 64. A purge gas stream 66 comprising methane and unreacted hydrogen and nitrogen is recovered from the ammonia synthesis unit 62 and provided to the synthesis gas generation unit 18 as fuel. A portion 68 of the ammonia is separated from the product stream 64. The remaining ammonia is passed to a urea synthesis unit 70 where it is reacted with a purified carbon dioxide stream provided by stream 30 to produce a urea stream and water. Water is recovered from the urea synthesis unit 70 by line 72. The urea stream is passed by line 74 to a stabilisation unit 76 comprising a stabilisation vessel where it is treated with aqueous formaldehyde or a urea formaldehyde concentrate provided by line 50 to form a stabilised urea product. The stabilised urea product is recovered from the stabilisation unit 76 by line 78.

[0069] The invention will now be described with reference to the following examples in accordance with the process of FIG. 1.

EXAMPLE 1

[0070] A formaldehyde-stabilised urea process was modelled based on a shifted synthesis gas having a composition as follows;

TABLE-US-00001 CO.sub.2 13.0 mole % CO 0.1 H.sub.2 44.4 N.sub.2 14.9 Ar 0.2 CH.sub.4 0.2 H.sub.2O 27.2

[0071] The shifted synthesis gas (162479 kg/hr) was cooled, condensate removed and fed to a CO.sub.2 removal unit comprising an absorption vessel fed with MDEA and a regeneration vessel in which the CO.sub.2-laden absorbent was regenerated. The absorption vessel produced 54568 kg/h of a CO.sub.2-depleted synthesis gas for methanol synthesis having the following composition;

TABLE-US-00002 CO.sub.2 0.1 mole % CO 0.2 H.sub.2 73.6 N.sub.2 24.6 Ar 0.3 CH.sub.4 0.3 H.sub.2O 0.9

[0072] The regeneration vessel produced 1797 hg/hr of a vent gas having the following composition;

TABLE-US-00003 CO.sub.2 55.0 mole % CO 0.2 H.sub.2 33.0 N.sub.2 6.0 Ar 0.0 CH.sub.4 0.4 H.sub.2O 5.4

[0073] The vent gas was compressed from its pressure of 2.5 bar abs to 29.2 bar abs.

[0074] The CO.sub.2-depleted synthesis gas and the compressed vent gas were provided to a methanol synthesis reactor operating at 200 C. The additional methanol production by including the vent gas was calculated to be 11.6 tonnes/day. The additional methanol is able to provide additional formaldehyde stabiliser.

EXAMPLE 2

[0075] A urea plant (taking ammonia from more than one facility) was modelled based on the shifted synthesis gas for one of the ammonia facilities, having a composition as follows;

TABLE-US-00004 CO.sub.2 12.7 mole % CO 0.2 H.sub.2 42.9 N.sub.2 15.1 Ar 0.2 CH.sub.4 0.6 H.sub.2O 28.3

[0076] The shifted synthesis gas (282074 kg/hr) was cooled, condensate removed and fed to a CO.sub.2 removal unit comprising an absorption vessel and a regeneration vessel in which the CO.sub.2-laden absorbent was regenerated. The absorption vessel produced 96214 kg/hr of a CO.sub.2-depleted synthesis gas having the following composition;

TABLE-US-00005 CO.sub.2 0.2 mole % CO 0.3 H.sub.2 71.9 N.sub.2 25.3 Ar 0.3 CH.sub.4 1.0 H.sub.2O 1.0

[0077] The CO.sub.2-depleted synthesis gas was found to contain insufficient carbon oxides to produce enough methanol, and so UFC-85 stabiliser, to meet the demands of the urea plant. According to the present invention, 1460 kg/hr purified carbon dioxide (about 1.5% by volume of the overall carbon dioxide recovered) was combined with the carbon dioxide-depleted synthesis gas, resulting in a synthesis gas fed to the methanol synthesis unit having the following composition;

TABLE-US-00006 CO.sub.2 0.5 mole % CO 0.3 H.sub.2 71.7 N.sub.2 25.2 Ar 0.3 CH.sub.4 1.0 H.sub.2O 1.0

[0078] The synthesis gas was provided to a methanol synthesis reactor at 215 C. The additional methanol production by including the vent gas was calculated to be 9 tonnes/daysufficient to make enough UFC-85 stabiliser to feed the urea plant. There is a slight but acceptable reduction in the ammonia production (1.3% on a mass basis).