PROCESS AND APPARATUS FOR THE REMOVAL OF HYDROGEN SULPHIDE FROM A GAS MIXTURE

Abstract: Process for the removal of hydrogen sulphide from a gas mixture comprising passing the gas mixture through a reactor (2) filled with catalyst for the conversion of hydrogen sulphide to carbonyl sulphide, and separating the gas mixture leaving reactor (2) in a separator (7) at a temperature below the boiling point of carbonyl sulphide in a lighter fraction and a heavier fraction in which the carbonyl sulphide is absorbed.

Description: The invention relates to a process for removing hydrogen sulphide from a gas mixture comprising hydrocarbons, hydrogen sulphide and carbon dioxide. Examples of such a gas mixture are natural gas, or a hydrocarbon-containing gas mixture produced during the production of petroleum. In a known process for the removal of hydrogen sulphide from a gas mixture, the gas mixture is contacted with a suitable liquid absorbent that absorbs almost all the hydrogen sulphide, yielding a purified gas mixture containing almost no hydrogen sulphide and a hydrogen sulphide-loaded absorbent, which absorbent is subsequently regenerated, yielding a regeneration off-gas with a high hydrogen sulphide content. It is also known to adsorb hydrogen sulphide on molecular sieves. The adsorbed hydrogen sulphide is then desorbed by means of regeneration gas, giving a regeneration off-gas with a high hydro­ gen sulphide content. In both cases, the regeneration off-gas is fed to a hydrogen sulphide processing installation, such as a Claus unit. The known processes are, however, too expensive for the removal of hydrogen sulphide from a gas mixture with a very low hydrogen sulphide content. The object of the invention is to provide a cheap process for the removal of hydrogen sulphide from a gas mixture. The process for the removal of hydrogen sulphide from a gas mixture containing hydrocarbons, hydrogen sulphide and carbon dioxide thereto comprises (a) passing the gas mixture through a reactor which is filled at least partly with a catalyst suitable for the conversion of hydrogen sulphide to carbonyl sulphide; (b) passing the gas mixture leaving the reactor to a separator where, at a temperature below the boiling point of carbonyl sulphide at the pressure prevailing in the separator, a lighter hydrocarbon fraction is separated from a heavier hydrocarbon fraction in which the carbonyl sulphide is absorbed; and (c) discharging from the separator separately from each other the lighter hydrocarbon fraction and the heavier hydrocarbon fraction containing the carbonyl sulphide. The invention further relates to an apparatus for the removal of hydrogen sulphide from a gas mixture comprising hydrocarbons, hydrogen sulphide and carbon dioxide, which apparatus comprises a reactor which is filled at least partly with a catalyst suitable for the conversion of hydrogen sulphide to carbonyl sulphide and provided with a supply line and a discharge line, and a separator connected to the discharge line and provided with an upper discharge line and a lower discharge line. The invention will now be discussed in greater detail with reference to the drawings, in which: - Figure 1 shows schematically a simple arrangement for the removal of hydrogen sulphide from a gas mixture; and - Figure 2 shows schematically an additional separator which can be connected to the arrangement shown in Figure 1. The arrangement according to Figure 1 comprises a supply line 1 connected to reactor 2 which is filled at least partly with a catalyst suitable for the conversion of hydrogen sulphide to carbonyl sulphide, a discharge line 4 connected to the reactor 2, a heat exchanger 5 arranged in the discharge line 4, and a separator 7 into which the discharge line 4 discharges, which separator 7 is provided with an upper discharge line 10 and a lower discharge line 11. During operation of the apparatus, a gas mixture comprising hydrocarbons and hydrogen sulphide is fed to the reactor 2 via the supply line 1. The composition of the gas will generally be 50-95% vol. methane, 2-20% vol. ethane, 0.3-20% vol. propane, 0.4-15% vol. nitrogen, 0.5-3% vol. carbon dioxide, 1-100 ppm (volume parts per million volume parts) hydrogen sulphide, and the rest comprising heavier hydrocarbon fractions, such as butane, pentane, hexane, etc. In the reactor 2 at least a part of the hydrogen sulphide is converted into carbonyl sulphide according to the reaction H₂S + CO₂ →COS + H₂O. This reaction is catalyzed by the catalyst present in the reactor 2. The gas mixture leaving the reactor 2 then passes through the discharge line 4 to the heat exchanger 5 where the gas mixture is brought to a temperature above the boiling point of ethane at the pressure prevailing in the separator 7 and below the boiling point of carbonyl sulphide at that pressure. At this temperature the mixture is passed to the separator, from which the gaseous lighter hydrocarbon fraction, methane and ethane, is discharged via the upper discharge line 10, and from which the liquid heavier hydro­ carbon fraction, comprising propane, butane, pentane, hexane, etc. together with the carbonyl sulphide, is discharged via the lower discharge line 11. The liquid is led through the lower discharge line 11 to a unit for the removal of carbonyl sulphide. As such an apparatus is known per se, it will not be shown and the process for the removal of carbonyl sulphide will not be described. In general, the gas fed to the reactor 2 will be at a temperature of between -5°C and +20°C and a pressure of between 5 MPa and 15 MPa. The quantity of gas will be between 1.5×10⁶ Nm³/day and 30×10⁶ Nm³/day (1 Nm³ gas corresponds to 1 m³ gas at a temperature of 0°C and a pressure of 0.1 MPa). The temperature of the gas mixture in the separator will be between -80°C and +20°C. The gas speed in the reactor 2 will not exceed 0.3 Nm³/s per m² surface area of the reactor 2 in an unfilled state. The catalyst in the reactor 2 should be suitable to catalyze the conversion of hydrogen sulphide to carbonyl sulphide. Suitable catalysts for this purpose are, for example, molecular sieves such as zeolites and alkali or alkaline earth aluminium silicates with pore diameters of between 3 × 10⁻¹⁰ m and 10 × 10⁻¹⁰ m. Molecular sieves are, moreover, able to adsorb water, so that the equilibrium in the above-mentioned reaction is shifted towards the carbonyl sulphide side. If the lighter hydrocarbon fraction which leaves the separator 7 via the upper discharge line 10 is only to contain methane, the gas mixture must be cooled in the heat exchanger 5 to a temperature above the boiling point of methane at the pressure prevailing in the separator and lower than the boiling point of ethane at that pressure. The mixture discharged via the lower discharge line 11 will in that case additionally contain ethane. In order to separate ethane from this mixture, the mixture is passed through the bottom discharge line 11 to an additional separator 12 (see Figure 2) via a heat exchanger 14, where it is brought to a temperature above the boiling point of ethane at the pressure prevailing in the additional separator 12 and below the boiling point of carbonyl sulphide at that pressure, which temperature is higher than that in the separator 7. This temperature will in general lie between 40°C and 80°C. It is also possible to lower the pressure of the gas before it is fed into the additional separator 12. Ethane is removed from the additional separator 12 via an upper discharge line 15, and the heavier hydrocarbon fraction with carbonyl sulphide is discharged via a lower discharge line 16. As has been discussed for Figure 1, the carbonyl sulphide can be removed from the heavier hydrocarbon fraction by a known method. Instead of directly separating carbonyl sulphide from the mixture of heavier hydrocarbon fraction and carbonyl sulphide, the carbonyl sulphide can first be separated from the mixture together with propane. To this end, the mixture is led to a second additional separator (not shown) via a heat exchanger in which the mixture is brought to a temperature below the boiling point of butane at the pressure prevailing in the second additional separator and above the boiling point of carbonyl sulphide at that pressure. The gas mixture leaving the top of the second additional separator will then contain propane and carbonyl sulphide, and the mixture leaving the bottom will contain the heavier hydrocarbon fraction comprising butane, pentane, hexane, etc. In this case as well, the pressure of the gas mixture can, if desired, be lowered before it is fed into the second additional separator. If the temperature of the gas mixture led to the second additional separator is below the boiling point of pentane and above the boiling point of propane, the gas mixture leaving the top of the second additional separator will contain propane, butane and carbonyl sulphide, while the gas mixture leaving the bottom of this separator will contain the heavier hydrocarbon fraction. The second additional separator can either be connected to the lower discharge line 11 (see Figure 1) or to the lower discharge line 16 (see Figure 2). If the quantity of the heavier hydrocarbon fraction in the gas mixture that is fed through the line 1 (see Figure 1) to the reactor 2 is too small to separate all the carbonyl sulphide formed in the reactor 2 from the lighter hydrocarbon fraction, or if the gas mixture only contains hydrocarbons in the form of methane and ethane, then propane can be added to the gas mixture before it is cooled in the heat exchanger 5. This propane can be added to the gas mixture before it is fed into the reactor 2 or after it leaves the reactor 2 and before it is cooled. The gas mixture is cooled to such a temperature that the gaseous lighter hydrocarbon fractions can be removed from the separator 7 via the upper discharge line 10 and the propane with the carbonyl sulphide in liquid form via the lower discharge line 11. The carbonyl sulphide can be removed from propane in a known manner, after which the propane can again be added to the gas mixture as described above. Instead of propane, any other material can be used with a higher boiling point than ethane, for example butane or pentane. Since the heavier hydrocarbon fraction has a volume that is less than about 20% of the total volume of the gas supplied per unit time, the carbonyl sulphide formed from the hydrogen sulphide will, in the process according to the invention, be absorbed in this smaller volume. This means that it is sufficient to use an apparatus according to the invention that is smaller than an apparatus for the removal of hydrogen sulphide from the total volume of gas. The invention is therefore attractive because treating of relatively small amounts of gaseous hydrocarbons contaminated with hydrogen sulphide now becomes economically feasible.

Claims: 1. A process for the removal of hydrogen sulphide from a gas mixture containing hydrocarbons, hydrogen sulphide and carbon dioxide, which process comprises (a) passing the gas mixture through a reactor which is filled at least partly with a catalyst suitable for the conversion of hydrogen sulphide to carbonyl sulphide; (b) passing the gas mixture leaving the reactor to a separator where, at a temperature below the boiling point of carbonyl sulphide at the pressure prevailing in the separator, a lighter hydrocarbon fraction is separated from a heavier hydrocarbon fraction in which the carbonyl sulphide is absorbed; and (c) discharging from the separator separately from each other the lighter hydrocarbon fraction and the heavier hydrocarbon fraction the carbonyl sulphide. 2. Process according to claim 1, wherein either before or after step (a) a material with a higher boiling point than that of ethane at the pressure prevailing in the separator is added to the gas mixture. 3. Process according to claim 1 or 2, wherein after step (c) the heavier hydrocarbon fraction with the carbonyl sulphide is passed through an additional separator where, at a temperature below the boiling point of carbonyl sulphide at the pressure in the additional separator and higher than the temperature in step (b), the fraction is separated into a lighter hydrocarbon fraction and a heavier hydrocarbon fraction with the carbonyl sulphide, which two fractions are separately removed from the additional separator. 4. Apparatus for the removal of hydrogen sulphide from a gas mixture comprising hydrocarbons, hydrogen sulphide and carbon dioxide, which apparatus comprises a reactor which is filled at least partly with a catalyst suitable for the conversion of hydrogen sulphide to carbonyl sulphide and provided with a supply line and a discharge line, and a separator connected to the discharge line and provided with an upper discharge line and a lower discharge line.

Inventors: DE KRAA, JOHANNES ANDREAS, ZOMERMAN, JANNES JAN
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1986
Main classification: C07C7
IPC codes: C07C7, C10K1