GB1597131A

GB1597131A - Treatment of gases

Info

Publication number: GB1597131A
Application number: GB439077A
Authority: GB
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1978-02-03
Filing date: 1978-02-03
Publication date: 1981-09-03

Description

(54) IMPROVEMENTS IN OR RELATING TO TREATMENT OF GASES (71) We, BRITISH GAS CORPORATION, of 59 Bryanston Street, London W1A 2AZ, a British Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to the purification of gases, more particularly to the purification of gases by the removal of sulphur compounds therefrom.

It is well known that sulphur compounds may be removed from gases by first converting said compounds to hydrogen sulphide and absorbing the hydrogen sulphide either into liquid or solid catch systems. One method whereby hydrogen sulphide may be produced from organic sulphur compounds is by reaction with hydrogen over molybdenum based catalysts such as nickel-molybdenum (Nimox) or cobalt molybdenum (Comox). The hydrogen sulphide produced by this catalytic hydrogenation of organic sulphur compounds is conventionally absorbed by catch vessels containing zinc oxide. The hydrodesulphurisation processes have conventionally been employed for the purification of feedstocks to protect sulphur-sensitive catalysts.

More recent developments in the art of manufacturing methane-containing gases, and particularly gases which are suitable as substitutes for natural gas, have indicated that coke oven gas is a useful feedstock material.

Coke oven gas contains hydrogen sulphide, organic sulphur compounds, hydrogen cyanide, ethylene and oxygen, in addition to the principal constituents H2, CO, CO2 CH4 higher saturated hydrocarbons, and N2. Before coke oven gas can be reformed over nickel catalysts at high temperatures (700"C and above) to produce hydrogen, the H2S and organic sulphur must be removed. Before coke oven gas can be methanated over nickel catalysts at low temperatures (250 to 500"C) to produce substitute natural gas, H2S, organic sulphur, unsaturated hydrocarbons and 2 must be removed.

One process route for removing catalyst poisons comprises catalytically hydrogenating the sulphur compounds, as described above, to hydrogen sulphide and thereafter removing the hydrogen sulphide by physical absorption into a liquid absorbent, for example by the Rectisol Process, or by chemical absorption, for example by the 'Stretford Process' as described in our UK Patent No. 948270.

However, in the hydrogenation stage, reactions other than that represented by the following equation: (1) Organic Sulphur + H2 < H2S + Saturated Hydrocarbon Compounds also occur. The following reactions have been shown to occur: (2) H2 + 1/2 02 < H2O (3) C2H4 + H2 < C2H6 All three reactions are exothermic. but the heat released in 1 is generally insignificant compared with the heat released by reactions 2 and 3.

The presence of oxygen is a nuisance and its concentration in the coke oven gas varies, resulting in a wide variation of Nimox outlet temperature. If the 2 concentration is high enough, unacceptably high catalyst temperatures result.

The normal working temperature of a nimox type catalyst is in the range 300 to 500"C, depending on feedstock, preferably in the range 350 to 4500C. If coke oven gas is introduced at a temperature in this range, the exothermic reactions 2 and 3 cause very high outlet temperatures.

Reducing the inlet temperature to between 100 and 1500C to accommodate these exothermic reactions means that only part of the catalyst is at a suitable temperature for organic sulphur conversion, and therefore more catalyst needs to be used. However, at 100 to 1500C, the exothermic reaction zone moves quickly through the bed, leaving the whole bed at its inlet temperature, which is too low for effective conversion of organic sulphur. An acceptable rate of reaction zone movement was obtained at higher inlet temperatures.

It has been observed that - at a preheat temperature of 150"C, increasing the oxygen concentration in the coke oven gas results in a much sharper, shorter reaction zone, with an acceptable rate of movement, and allows a larger proportion of the catalyst to be at a temperature suitable for organic sulphur conversion.

Oxygen, which has hitherto been a nuisance, is now used to advantage since the full operating potential of the catalyst can be realised by a balance of inlet temperature with oxygen concentration.

If the coke oven gas contains sufficient H2S and organic sulphur to keep the nimox sulphided, methanation reactions do not occur.

The Stretford Process is inhibited by the presence of HCN in the gas being processed.

However, it has been observed that the hydrogen cyanide is removed during the hydrogenation stage viz: (4) 2HCN + 3H2 < 2CH4 + N2 (5) 4HCN + 502 > 2H2O + 4CO2 + 2N2 and that reaction 4 will be 99 to 99.5% complete and reaction 5 will be essentially 100% complete at normal nimox operating temperatures.

The present invention accordingly provides a process for the purification of gases containing hydrocarbons and organic sulphur compounds, which process comprises subjecting said gas, in the presence of an oxygen-containing gas, to a hydrogenation reaction by contacting said gas with a bed of a hydrogenation catalyst having a high selectivity for hydrogenating organic sulphur compounds, wherein sufficient oxygen is present such that the temperature at the outlet of said catalyst bed is maintained by the reaction of from 300 to 5000C, preferably from 350 to 450"C, and removing hydrogen sulphide from the gas thus produced.

The gas may be a coke oven gas which, further, may or may not contain oxygen.

Temperature control may be achieved by adjusting the oxygen concentration of the gas and, additionally, by adjusting the inlet temperature of the catalyst bed.

Preferably the inlet temperature is maintained at about 1500C and the outlet temperature adjusted to from 300 to 5000C by the supply of oxygen to the gases to be reacted.

Any oxygen containing gas may be employed for the process of the invention. Air is a suitable and cheap source of oxygen for the process of the invention.

The process of the invention may be effected at any convenient pressure although working above 100 psi is preferred.

Purified coke oven gas, so produced, may be used for the hydrogasification of additional hydrocarbon feedstock, e.g. naphtha or kerosine, which, itself, will have to be purified from sulphur compounds. Some or all of the coke oven gas may be used as the hydrogenating gas for hydrodesulphurisation of the hydrocarbon feedstock, which could be effected in a separate section from that in which the coke oven gas is purified.

Alternatively, the same Nimox-zinc oxide system may be used, with the coke oven gas and additional hydrocarbon feedstock being purified together. In this case, a preheat temperature within the range 200 - 300"C would be required to maintain adequate desulphurisation temperatures.

WHAT WE CLAIM IS: 1. A process for the purification of gases containing hydrocarbons and organic sulphur compounds, which process comprises subjecting said gas, in the presence of an oxygen-containing gas, to a hydrogenation reaction by contacting said gas with a bed of a hydrogenation catalyst having a high selectivity for hydrogenating organic sulphur compounds, wherein sufficient oxygen is present such that the temperature at the outlet of said catalyst bed is maintained by the reaction from 300 to 500"C, and removing hydrogen sulphide from the gas thus produced.

2. A process as claimed in Claim 1, wherein the gas to be purified is coke oven gas.

3. A process as claimed in Claim 1 or Claim 2, wherein the outlet temperature is maintained from 350 to 450"C.

4. A process as claimed in any one of Claims 1 to 3, wherein temperature control of the outlet of the catalyst bed is achieved by adjusting the oxygen concentration in the gas to be purified.

5. A process as claimed in Claim 4, wherein additional temperature control is achieved by adjusting the inlet temperature of the catalyst bed.

6. A process as claimed in Claim 5, wherein the inlet temperature is adjusted to about

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Reducing the inlet temperature to between 100 and 1500C to accommodate these exothermic reactions means that only part of the catalyst is at a suitable temperature for organic sulphur conversion, and therefore more catalyst needs to be used. However, at 100 to 1500C, the exothermic reaction zone moves quickly through the bed, leaving the whole bed at its inlet temperature, which is too low for effective conversion of organic sulphur. An acceptable rate of reaction zone movement was obtained at higher inlet temperatures. It has been observed that - at a preheat temperature of 150"C, increasing the oxygen concentration in the coke oven gas results in a much sharper, shorter reaction zone, with an acceptable rate of movement, and allows a larger proportion of the catalyst to be at a temperature suitable for organic sulphur conversion. Oxygen, which has hitherto been a nuisance, is now used to advantage since the full operating potential of the catalyst can be realised by a balance of inlet temperature with oxygen concentration. If the coke oven gas contains sufficient H2S and organic sulphur to keep the nimox sulphided, methanation reactions do not occur. The Stretford Process is inhibited by the presence of HCN in the gas being processed. However, it has been observed that the hydrogen cyanide is removed during the hydrogenation stage viz: (4) 2HCN + 3H2 < 2CH4 + N2 (5) 4HCN + 502 > 2H2O + 4CO2 + 2N2 and that reaction 4 will be 99 to 99.5% complete and reaction 5 will be essentially 100% complete at normal nimox operating temperatures. The present invention accordingly provides a process for the purification of gases containing hydrocarbons and organic sulphur compounds, which process comprises subjecting said gas, in the presence of an oxygen-containing gas, to a hydrogenation reaction by contacting said gas with a bed of a hydrogenation catalyst having a high selectivity for hydrogenating organic sulphur compounds, wherein sufficient oxygen is present such that the temperature at the outlet of said catalyst bed is maintained by the reaction of from 300 to 5000C, preferably from 350 to 450"C, and removing hydrogen sulphide from the gas thus produced. The gas may be a coke oven gas which, further, may or may not contain oxygen. Temperature control may be achieved by adjusting the oxygen concentration of the gas and, additionally, by adjusting the inlet temperature of the catalyst bed. Preferably the inlet temperature is maintained at about 1500C and the outlet temperature adjusted to from 300 to 5000C by the supply of oxygen to the gases to be reacted. Any oxygen containing gas may be employed for the process of the invention. Air is a suitable and cheap source of oxygen for the process of the invention. The process of the invention may be effected at any convenient pressure although working above 100 psi is preferred. Purified coke oven gas, so produced, may be used for the hydrogasification of additional hydrocarbon feedstock, e.g. naphtha or kerosine, which, itself, will have to be purified from sulphur compounds. Some or all of the coke oven gas may be used as the hydrogenating gas for hydrodesulphurisation of the hydrocarbon feedstock, which could be effected in a separate section from that in which the coke oven gas is purified. Alternatively, the same Nimox-zinc oxide system may be used, with the coke oven gas and additional hydrocarbon feedstock being purified together. In this case, a preheat temperature within the range 200 - 300"C would be required to maintain adequate desulphurisation temperatures. WHAT WE CLAIM IS:

1. A process for the purification of gases containing hydrocarbons and organic sulphur compounds, which process comprises subjecting said gas, in the presence of an oxygen-containing gas, to a hydrogenation reaction by contacting said gas with a bed of a hydrogenation catalyst having a high selectivity for hydrogenating organic sulphur compounds, wherein sufficient oxygen is present such that the temperature at the outlet of said catalyst bed is maintained by the reaction from 300 to 500"C, and removing hydrogen sulphide from the gas thus produced.

1500C.

7. A process as claimed in any one of the preceding Claims, wherein the oxygen containing gas is air.

8. A Process as claimed in any one of the preceding Claims, wherein the catalysts is a sulphide nickel-molybdenum or cobalt-molybdenum based catalyst.

9. A process for the purification of gases according to Claim 1 and substantially as hereinbefore described.

10. Gaese whenever purifed by the process claimed in any one of the preceding Claims.