GB2073234A - Purification of Gas for the Production of Methanol by Synthesis - Google Patents

Abstract

A crude gas mainly consisting of hydrogen and carbon monoxide derived from the gasification of coal or crude oil fractions is cooled under 0 DEG C, methanol is injected to remove water and other condensable components, further cooled and scrubbed by m-Xylene at a pressure between 25 ata and 100 ata at a temperature between {5 DEG C and {35 DEG C so that all sulphur compounds are removed. A minor part of the sulphur-free gas is treated in a CO- conversion plant to produce H2 and CO2, and then scrubbed with methanol to reduce the CO2 content to ca. 2%. The resulting converted and methanol-scrubbed gas is then mixed with the main gas stream, to form the synthesis gas having a ratio H2:CO=2:1 and a CO2 content of about 5%. <IMAGE>

Description

SPECIFICATION Purification of Gas for the Production of Methanol by Synthesis It is known to purify a crude gas obtained by a suitable coal gasification process for instance the Koppers-Totzek process or by a process involving gasification of crude oil for instance the Shell gasification process or the Texaco gasification process, by a physically operated low temperature gas scrubbing process followed by a monoxide conversion (shift conversion) for instance as disclosed in Patent Specification No. 1,164,407. In the known process, the crude gas is purified by a single scrubbing agent, i.e. methanol, to remove its sulphur compounds H2S and COS and to remove CO2.It is an object of the present invention to provide a process for the purification of gas for the synthesis of methanol by which a substantial saving in energy expenditure as compared with the known process can be obtained, and in respect of which the plant installation cost required is reduced as compared with the known process.

According to the invention there is provided a process for the purification of a crude gas mainly consisting of hydrogen and carbon monoxide to be used for the production of methanol, the crude gas deriving from the gasification of coal or crude oil fractions, in which the crude gas obtained is cooled under OOC, and wherein, by the injection of methanol, water and other condensable components in the crude gas are removed and wherein the gas, after further cooling, is scrubbed by m-Xylene in a scrubber under a pressure between 25 ata and 100 ata at a temperature between -50C and -350C so that in the scrubber all sulphur compounds are removed from the gas whereafter a minor part of the sulphur-free gas is led off as a minor gas stream, the remainder proceeding as a main gas stream, the sulphur-free gas in the minor gas stream being treated in a CO-conversion plant, in which the carbon monoxide and water are converted to H2 and CO2, the sulphur-free converted gas from the COconversion plant being then treated in a small plant in which it is scrubbed with methanol to reduce the CO2 content in the gas to ca. 2%, the resulting converted and methanol-scrubbed gas being then mixed with the main gas stream, which does not flow through the CO-conversion plant, to form the synthesis gas having the ratio H2:CO=2:1 and a CO2 content of about 5% according to the requirements of the methanol synthesis process.

Thus, by contrast with the known process, the process of the present invention uses not one single scrubbing agent to remove the sulphur compounds and the CO2, which also has to be removed, but two scrubbing agents, one which is selective for sulphur compounds and one which is more favourable for the removal of CO2.

The use of the selective scrubbing agent m-Xylene makes sure that substantially all the H2S and COS will be removed from the gas so treated. There is a small quantity of CO2 which is removed simultaneously.

When the crude gas is treated with methanol a rather large part of the CO2 content in the crude gas will be removed. If m-Xylene is used in place of methanol, only a smaller portion of the CO2 content is removed. The scrubbing agent, methanol in the prior art process, m-Xylene in the process of the invention, has to be regenerated. When the methanol is regenerated in a steam heated column a gas is obtained in which the concentration of H2S is not sufficient for the operation of a Claus kiln. If m Xylene is used for scrubbing out the sulphur compounds the H2S concentration in the gas driven off in the regenerator is sufficiently concentrated to operate the Claus kiln favourably and economically.

It may be mentioned that the gas leaving the regenerator, if methanol alone is used, could also be made usable in a Claus kiln but it would require a series of pieces of costly equipment. In the process of the invention, part of the gas which has undergone the removal of its sulphur compounds treated in a CO-shift conversion plant in order to make it possible to produce a synthesis gas in which the ratio CO:H2=1 :2 can be obtained. The larger part of the gas stream is not treated in the shift conversion plant and mixed, downstream of the CO2 removal plant, with the converted CO2- free gas. (The converted gas contains about 34% CO2 by volume which in a scrubbing plant operated at a low temperature on methanol will be removed).

In the process disclosed in the previously mentioned Patent Specification No. 1,164,407 the scrubbing agent is first used for the removal of CO2 and part of the scrubbing agent is then used to remove sulphur compounds.

In the same Patent Specification, besides methanol, the compounds Xylene and Toluene are mentioned as suitable scrubbing agents but in all cases the same scrubbing agent is used for the removal of CO2 and sulphur compounds.

The present invention makes use of the fact that for the removal of sulphur compounds m-Xylene is most selective and guarantees an optimum of sulphur removal. On the other hand methanol is more suitable for the removal of CO2 than m-Xylene and can be regarded as an optimum scrubbing agent for CO2 in physical scrubbing processes.

The following comparison will show this in figures: 100 000 Nm3/h crude gas deriving from an oil gasification underwent a physical wash at 60 at. abs. and at a scrubbing temperature of 200 C, under which conditions H2S and COS were removed down to 1 ppm in order not to damage the catalyst of the shift conversion plant.

From the converted gas, ca. 51000 Nm3/h, the CO2 was removed under 56,5 at. abs. and at a scrubbing temperature of 00C down to 2% by volume.

The crude gas had the following analysis: CO2 6.85% CO 46.01 % H2 45.29 % H2S 0.69 % % ali per volume COS 0.03 % CH4 0.2% N2 0.38 % Ar 0.55% 100.00% Below the figures for methanol are listed in the left column, those for m-Xylene in the right column: Methanol Solubility for H2S 50 Solubility for COS 33 Solubility for CO2 11,4 CO2-partial pressure 0.0685.6=4.11 ata Selectivity 33::11.4=2.9 Calculated quantity of scrubbing agent, (methanol), to remove COS 1.2 . 100 000 =60 m /h 60.33 Methanol Gases dissolved in the 60 m3/h scrubbing agent Nm /h Vol% CO2 4.11 . 11.4 . 60 = 2810 69 CO 27.6 . 0.21 . 60 = 350 8.6 H2 27.2 . 0.11 . 60 = 180 4.4 H2S complete = 690 17.0 COS complete = 30 0.72 CH4 0.12 . 0.58 . 60 = 4.2 0.1 N2 0.23 . 0.16 . 60 = 2.2 0.04 Ar 0.33.0.3 .60 = 6.0 0.14 4072.4 100.00 The gas leaving the methanol regeneration column has only 17% H2S and is not suitable for a Claus kiln to produce sulphur.

Pressure 60 at. abs.

Scrubbing temperature -20 C m-Xylene Solubility for H2S 48 Solubility for COS 55 Solubility for CO2 4,9 CO2-partial pressure 0.0685.60=4.11 ata.

Selectivity 33:11.4=9.8 m-Xylene is more than three times more selective for the sulphur compounds than methanol.

Calculated quantity of scrubbing agent (m-Xylene), to remove H2S 1.33 . 100 000 =45 m /h 60.48 m-Xylene Gases dissolved in the 46 m3/h scrubbing agent Nm /h Vol% 4.11 . 4.9 . 46 = 925 42.4 27.6 . 0.36 . 46 = 458 21.4 27.2 . 0.02 . 46 = 25 1.17 complete = 690 32.3 complete = 30 1.4 0.12 . 0.98 . 46 = 5.4 0.25 0.23 . 0.14 . 46 = 1.5 0.07 0.330.3 . 46 = 4.5 0.21 2139.4 100.00 The gas leaving the m-Xylene regeneration column has 32.3% H2S and is without further treatment suitable for the Claus kiln for the production of sulphur.

Desulphurised Gas: Nm /h vol% CO2 6850-2810 = 4040 4.4 CO 46010- 350 = 45660 47.55 H2 45290- 180= 45110 47.1 CH4 200- 4= 196 0.2 N2 380- 2= 378 0.39 Ar 550- 6= 544 0.56 95928 100.00 Converted Gas: CO2 33.2 Vol % CO 3.0 Vol % H2 63.0 Vol % Inerts 0.8 Vol % Methanol 35928 Nm3/h desulphurised gas enter into the shift conversion. After shift conversion the quantity of gas is 1.433.35928=51500 Nm3/h 60 000 Nm3/h are led around the shift conversion stage and are fed into the gas stream leaving the CO2 scrubbing plant, the last-noted gas stream comprising 34 333 Nm3/h. The volume of synthesis gas is therefore 94 333 Nm3/h.

Desulphurised Gas: Nm3/h Vol% 6850-925= 5925 6.06 46010-458= 45552 46.5 45290-25= 45265 46.3 200-5 = 195 0.2 380-2 = 378 0.39 550-5 = 545 0.55 97860 100.00 Converted Gas; 34.0 Vol % 3.0 Vol % 62.2 Vol % 0.8Vol% m-Xylene 35360 Nm3/h desulphurised gas enter into the shift conversion. The quantity of the converted gas is 1.422 35360=50250 Nm3/h 62 500 Nm3/h are led around the shift conversion stage in this case and are fed into the gas stream leaving the CO2 scrubbing plant, the last-noted gas stream being 33 467 Nm3/h. The volume of synthesis gas obtained in this gas is 95 967 Nm3/h.

Comparison for CO2-scrubbing Pressure 56.5 at. abs.

Temperature --1 OOC.

CO2 partial pressure 0.332 56.5=18.8 at a.

Solubility for CO2...16 Lowest temperature by depressurizing cooling taken to be -650C.

Quantity of CO2to be removed ca. 16500 Nm3/h Quantity of scrubbing agent: 100 m3/h Pure gas: 34333 Nm3/h=100% Synthesis gas: 94333 Nm3/h=1 00% CO2 partial pressure 0.34 56.5=1 9.2 at a.

Solubility for CO2...7.35 Lowest temperature by depressurizing cooling taken to be -41 OC.

Quantity of CO2 to be removed ca. 16500 Nm3/h Quantity of scrubbing agent: 250 m3/h Pure gas: 33467 Nm3/h=100% Synthesis gas: 95967 Nm3/h-1 00% This comparison shows that the crude gas should be scrubbed by m-Xylene before the shift conversion because this scrubbing agent is considerably lower in quantity than methanol. The application of m-Xylene means a remarkable saving in installation cost and power consumption.

Furthermore the sulphur compounds recovered from the m-Xylene are of such a high concentration that without any further equipment sulphur can be produced at a high efficiency in a Claus kiln.

On the other hand the comparison indicates that for scrubbing CO2 out of the converted gas a low temperature gas scrubbing plant operated on methanol is superior to the application of m-Xylene for this special purpose because the quantity of scrubbing agent using methanol is considerably less than in the case of the use of m-Xylene.

A process embodying the invention is described below with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing a gas purification plant for performing a process embodying the invention, and Figure 2 is a flow diagram illustrating the two scrubbing procedures respectively before and after the shift conversion stage.

Referring to the drawings, the crude gas, saturated by water vapour, is fed at 60 ata. through pipe 1 into heat exchangers 2, 4 and 6 into a scrubbing tower 7. In the heat exchanger 2 the water vapour is condensed at +5 C and, through the condensation pot 3, separated from the crude gas-according to the example 100 000 Nm3/h. In order to avoid the formation of ice a small quantity of methanol is injected into the gas stream. The mixture of water and methanol is separated by the condensation pot 5.

The reference 6 indicates an ammonia evaporator in which the crude gas is cooled down to ca.

--200C. The last traces of the methanol-water mixture are separated out by the condensation pot 9.

In scrubbing tower 7 substantially all sulphur compounds are removed from the crude gas by ca.

46 m3/h m-Xylene. The heat of absorption which evolves is taken out by the ammonia evaporator 8.

The desulphurised gas, ca. 97860 Nm3/h flows through pipe 10 through the heat exchangers 4 and 2.

The pipe 10 branches into pipe 11 and 12. The smaller volume of gas, ca. 36860 Nm3/h is fed through pipe 11 into the shift conversion plant 13. The greater volume, ca. 61 000 Nm3/h flows through pipe 1 2 and is reunited with the smaller amount of gas leaving the CO2-scrubbing plant in pipe 36. The mixed synthesis gas flows through pipe 37 into the methanol synthesis installation. The mixture is performed behind the CO2-removal plant so that only the small amount of converted gas need be subjected to CO2-removal.

The m-Xylene leaving the scrubbing tower 7 is depressurized in vessel 15 to ca. 45 ata. The gas liberated contains, besides CO2, smaller volumes of CO and H2 and is therefore fed into the recompressor 27 which pumps it back into the stream of crude gas. Through pipe 1 6 m-Xylene flows through the heat exchanger 1 7 into the regeneration column 1 8. The regeneration of the m-Xylene occurs at ca. 1.5 ata by the application of condensed low pressure steam indirectly by heater 19.

The off-take of column 1 8 includes a certain proportion of methanol because of the methanol injection in the heat exchanger 4 from where small amounts of methanol are carried over into the scrubbing tower 7 by the crude gas. The gases liberated in the head of column 18 having small amounts of scrubbing agent vapours which are liquified in the condensor 22 and are collected in vessel 23. In the ammonia evaporator 25 the scrubbing agent vapours are separated from the H2S gas which flows through pipe 26 to the Claus kiln, ca. 2140 Nm3/h, H2Sand COS content ca.33.7 The pump 24 feeds the reflux to the head of column 18. The m-Xylene leaving the sump of column 1 8 in a regenerated state is pumped through pump 20 into the heat exchanger 1 7 and the ammonia evaporator 21 onto the scrubbing tower 7.

The converted gas, ca. 52500 Nm3/h which leaves the shift conversion plant 13-saturated with water-flows through pipe 14, the heat exchangers 28, 30 and 32 into the scrubbing tower 34. In the heat exchanger 28 the gas is cooled down to +50C where the biggest part of the water vapour condenses. The water is discharged by condensation pot 29. The remaining water vapour will be removed in the heat exchanger 30 in which a constant methanol injection takes place. The discharge occurs through the condensation pot 31. The last traces of the water-methanol mixture are discharged from the ammonia evaporator 32 through the condensation pot 33. The gas which has been cooled down to ca. --100C in the ammonia evaporator 32 flows into the scrubbing tower 34.In this scrubbing tower the methanol as a scrubbing agent takes out the CO2 at ca. 56 ata. down to 2% by volume, i.e.

ca. 100 m3/h which heats itself up by the absorption heat in the scrubbing tower 34 from ca. -650C to -1 00C. The heat gained in the circulation of the scrubbing agent is removed by a recirculation in the lower part of the scrubbing tower 34, cooling being effected by an ammonia water system deriving from the absorption refrigeration machine, ammonia evaporator 35; the machine is designed to serve both scrubbers, i.e. before and after the shift conversion. The heater of the refrigeration machine is heated by crude gas, saturated with water vapour, in an indirect heat exchange apparatus.

The pure gas, ca. 34600 Nm3/h flows through pipe 36, the heat exchangers 30 and 28. It is united with the desulphurised gas, ca. 61 000 Nm3/h which comes from pipe 12 and flows through pipe 37, ca. 95 600 Nm3/h synthesis gas, to the methanol synthesis installation.

The methanol, having dissolved the gases, is discharged from scrubbing tower 34 and fed into the lower part of the expansion tower 38 and expands there to ca. 45 ata. The expansion gas contains, besides mainly CO2, also some percentage of CO and H7. It is fed through pipe 39 to the recompressor 27. The methanol flows through pipe 40 onto the upper part of tower 38. Here occurs the expansion of the methanol down to ca. 1.5 ata. The expansion gas, mainly CO2, is discharged through pipe 41. The methanol flows through pipe 42 in a section of tower 38 which is operated at ca. 0.6 ata. The expansion gas is sucked off by the vacuum blower 43. The methanol flows through pipe 45 into a section of tower 38 which is operated under a pressure of ca. 0.3 ata.By the different expansion stages and the degasifying of methanol, a temperature of ca. -650C is reached. The expansion gas is sucked off by vacuum blower 44 and fed into blower 43. The whole of the gases from pipes 41 and 46 is discharged through pipe 47, this being ca. 1 7900 Nm3/h.

The so regenerated methanol after passing through pipe 48 is pumped by pump 49 onto the scrubbing tower 34. Through pipe 50 a small amount of methanol is branched off to be used in the heat exchangers 4 and 30 for injection.

The mixture of methanol and water discharged from the condensation pots 5, 9, 31 and 33 runs through pipe 51 to pump 52 through a steam heated preheater 53 and is fed onto the distillation column 54, where the separation of methanol and water occurs under a pressure of ca. 2 ata. In the preheater 55 the heat is applied by condensing low pressure steam. The water, after the separation mentioned, runs through pipe 56 whereas the methanol vapours leave the column 54 at the top. They are condensed in condenser 57 and collected in vessel 58. The recycling pump 59 pumps the methanol to column 54, a part of it through pipe 60 into the ammonia evaporator 61 finally into the section of the expansion tower 38, section operated under 1.5 ata.

Claims (5)

Claims

1. A process for the purification of a crude gas mainly consisting of hydrogen and carbon monoxide to be used for the production of methanol, the crude gas deriving from the gasification of coal or crude oil fractions, in which the crude gas obtained is cooled under OOC, and wherein, by the injection of methanol, water and other condensable components in the crude gas are removed and wherein the gas, after further cooling, is scrubbed by m-Xylene in a scrubber under a pressure between 25 ata and 100 ata at a temperature between -50C and -350C so that in the scrubber all sulphur compounds are removed from the gas whereafter a minor part of the sulphur-free gas is led off as a minor gas stream, the remainder proceeding as a main gas stream, the sulphur-free gas in the minor gas stream being treated in a CO-conversion plant, in which the carbon monoxide and water are converted to H2 and CO2, the sulphur-free converted gas from the CO-conversion plant being then treated in a small plant in which it is scrubbed with methanol to reduce the CO2 content in the gas to ca. 2%, the resulting converted and methanol-scrubbed gas being then mixed with the main gas stream, which does not flow through the CO-conversion plant, to form the synthesis gas having a ratio H2:CO=2:1 and a CO2 content of about 5% according to the requirements of the methanol synthesis process.

2. A process for the purification of a crude gas according to claim 1 wherein the low temperature optimum scrubbing method using m-Xylene for the desulphurisation of the crude gas is combined with the low temperature optimum scrubbing method using methanol for the CO2-scrubbing from the converted gas.

3. A process for the purification of crude gas according to claim 1 or claim 2 wherein the CO2removal from the sulphur-free converted gas is carried out in a small gas stream only, being less than half of the volume of a gas which would result from a mixture of the two gas streams mentioned previously immediately following the CO-conversion plant.

4. A purification method for crude gas for the production of methanol substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

5. Any novel feature or combination of features described herein.