METHOD FOR REMOVING HYDROGEN SULFIDE FROM AN ACID GAS

Abstract

A method for removing hydrogen sulfide (H.sub.2S) from an acid gas comprises feeding the gas to a membrane separation unit, collecting the product gas from the membrane unit, heating the permeate stream to the necessary inlet temperature for catalytic oxidation of H.sub.2S and feeding the heated permeate stream to a catalytic oxidation unit, where H.sub.2S is oxidized to SO.sub.2. The heating of the permeate stream is accomplished by using a fraction of the feed gas to heat the permeate stream in a separate heater or by using a steam-fired heater. The method is especially suited for use on an off-shore facility.

Claims

1. A method for removing hydrogen sulfide (H.sub.2S) from an acid gas, said method comprising the steps of: feeding the gas to a membrane separation unit, optionally after membrane pre-treatment, drying and/or extraction of higher hydrocarbons, collecting the product gas from the membrane unit, heating the permeate stream to the necessary inlet temperature for catalytic oxidation of H.sub.2S to SO.sub.2 according to the equation
H.sub.2S+1.5 O.sub.2->SO.sub.2+H.sub.2O  and feeding the heated permeate stream to a catalytic oxidation unit, where the H.sub.2S is oxidized.

2. The method according to claim 1, wherein the heating of the permeate stream is accomplished by using a fraction of the feed gas to heat the permeate stream in a separate heater.

3. The method according to claim 1, wherein pre-heating of the permeate stream is accomplished by using a steam-fired heater.

4. The method according to any of the claims 1-3, wherein the acid gas is natural gas.

5. The method according to any of the claims 1-3, wherein the acid gas is biogas.

6. The method according to any of the claims 1-3, wherein the acid gas is associated gas.

7. The method according to claim 1, wherein the membrane separation unit and the catalytic oxidation unit are placed on an off-shore facility.

8. The method according to claim 1, wherein the permeate pre-heating temperature is within the temperature range 180-400° C.

9. The method according to claim 8, wherein the permeate pre-heating temperature is within the temperature range 220-350° C.

10. The method according to claim 1, wherein the catalyst to be used in the catalytic oxidation unit is a monolithic metal oxide catalyst consisting of one or more metal oxides.

11. The method according to claim 10, wherein the metal of the metal oxide in the catalyst is selected from the group comprising V, Cr, W, Ce, Mo, Nb, Fe, Si, Ti, Al, Ca and Mg, and one or more supports taken from the group comprising Al.sub.2O.sub.3, SiO.sub.2, SiC and TiO.sub.2.

Description

EXAMPLE

[0028] A nominal gas feed stream is supplied in an amount of 400,000 Nm.sup.3/h at a pressure of 25 bar gauge and a temperature of 30° C. This gas feed stream has the following composition: 39% CO.sub.2, 1.5% N.sub.2, 44.6% methane, 6.8% ethane, 4.9% propane, 1% butane, 0.3% pentane and 1.9% higher hydrocarbons (C.sub.5+).

[0029] The gas feed stream is passed through a membrane separation unit, resulting in a product gas stream which may contain 22% CO.sub.2 and 50 ppm H.sub.2S. A normal membrane pre-treatment, drying, extra compression and extraction of natural gas liquids may have been applied.

[0030] The permeate stream is in an amount of around 70,000 Nm.sup.3/h CO.sub.2 with around 28 ppm H.sub.2S. It exits the membrane separation unit at a pressure close to ambient pressure.

[0031] In order to thermally oxidize the H.sub.2S, a temperature of 700-900° C. is needed. The exact lower temperature limit for the membrane operation is set by the hydrate formation temperature, but it is often in the range of 10-20° C. In the case of using a thermal oxidation process, an equivalent of 30-35 MW will be needed for this CO.sub.2 dominated stream to reach a temperature of 800° C. This corresponds to approximately 100 MM BTU/h or—with a gas price of 10 US$ per MM BTU—to approximately 3.5 MM US$ per year or around 3000 Nm.sup.3/h natural gas for flaring the permeate stream.

[0032] There are two ways of providing the fuel needed for the flare: [0033] to operate the membrane at a higher temperature and let fuel slip to the permeate side through the membrane, or [0034] to route dedicated fuel to the flare.

[0035] Both methods are employed in practice.

[0036] In contrast, a catalytic oxidation unit of the type used in the method of the invention will require an inlet temperature of 250° C. and 9 MW or 870 Nm.sup.3/h of fuel. This will represent a fuel saving of 2.5 MM US$ per year. In this case, the membrane should operate as cold as possible without any risk of hydrate formation, and the necessary fuel is to be routed directly to a pre-heat burner to heat the inlet of the catalytic oxidation unit. Other heat sources, such as steam, may also be used to pre-heat the feed gas stream to the catalytic oxidation unit.