Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
2001-04-05
2003-02-11
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S220000, C423S228000, C423S237000, C423S242100, C423S573100, C423S576200, C423S576800
Reexamination Certificate
active
06517801
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of and apparatus for treating a feed gas stream containing hydrogen sulphide.
So-called acid gas streams containing hydrogen sulphide and carbon dioxide are formed as waste streams in, for example, oil and gas refineries. It is necessary to treat an acid gas stream so as to remove substantially all its content of hydrogen sulphide before it is discharged to the atmosphere. This removal of hydrogen sulphide is conventionally performed by the Claus process, in which part of the hydrogen sulphide content is burned in a furnace to form sulphur dioxide and water vapour; some of the resultant sulphur dioxide reacts in the furnace with residual hydrogen sulphide to form a gas mixture containing hydrogen sulphide, sulphur dioxide, carbon dioxide, water vapour and sulphur vapour, and also typically including hydrogen, carbon monoxide, carbon oxysulphide (carbonyl sulphide) and carbon disulphide. The sulphur vapour is extracted from the gas mixture by condensation, and the resulting gas mixture substantially free of sulphur vapour is subjected to a plurality of catalytic stages of further reaction between sulphur dioxide and hydrogen sulphide so as to form further sulphur vapour. The further sulphur vapour is extracted from the gas mixture downstream of each stage of catalytic reaction. A tail gas containing typically from 2 to 6 percent of the original sulphur content of the acid gas is thereby formed. The tail gas is sent for further treatment to remove substantially all the remaining sulphur compounds.
Increasingly, the environmental standards that the above-described treatment is required to meet are becoming more stringent. In some countries it is required that no more than three out of every thousand sulphur atoms that enter the process are discharged to the atmosphere. In other words, the percentage conversion of hydrogen sulphide to sulphur needs to be at least 99.7% to meet this standard. As a result, Claus plants have tended to become more complex with a greater number of catalytic stages of reaction between hydrogen sulphide and sulphur dioxide and a more elaborate tail gas clean up unit. This elaboration has added to the cost of the plant.
EP-A-0 565 316 relates to a process which makes possible a reduction in both the number and the size of the catalytic stages of reaction between hydrogen sulphide and sulphur dioxide, and in some examples can eliminate these stages altogether. In the process according to EP-A-0 565 316 some or all of the gas mixture leaving the sulphur condenser associated with the furnace is sent to a catalytic hydrogenator in which sulphur dioxide is reduced to hydrogen sulphide. Water vapour is then removed by condensation from the gas mixture exiting the catalytic hydrogenator. The resulting gas mixture depleted of water vapour is typically divided into two streams. One stream is recycled to the furnace. The other stream may be fed to a train of further but smaller, stages of reaction between hydrogen sulphide and sulphur dioxide, or in a few examples may be sent directly to an incinerator. In the examples in which the purge gas is sent directly to an incinerator, both the feed and the recycled gas streams are preheated to a temperature in the range of 100° C. to 300° C. It is stated that the recycle to feed ratio tends rapidly to increase with increasing percentage conversions above 98%, thereby increasing the need for preheating of the recycle and adding a requirement for increasing the size of the furnace. Accordingly such examples are not suitable for meeting a demand for 99.7% conversion of hydrogen sulphide to sulphur irrespective of the composition of the feed gas stream. It is also disclosed that hydrogen sulphide may be separated from the purge stream by washing it with a suitable amine.
U.S. Pat. No. 4,919,912 also discloses processes for recovering sulphur from a hydrogen sulphide containing gas stream in which an amine separation step is used. In the process according to U.S. Pat. No. 4,919,912 a sulphur dioxide bearing feed stream is reacted in a catalytic Claus reactor with a stream containing hydrogen sulphide and, after extraction of resulting sulphur vapour, residual sulphur dioxide is hydrogenated back to hydrogen sulphide. Hydrogen sulphide is then separated by absorption in an aqueous solution of a suitable amine and recycled to the catalytic Claus reactor as the hydrogen sulphide containing gas stream. In one class of processes according to U.S. Pat. No. 4,919,912 the sulphur dioxide bearing feed stream to the catalytic reactor is formed by combustion of a hydrogen sulphide stream. Some of the so-formed sulphur dioxide reacts with residual hydrogen sulphide to form some sulphur vapour. Removal of the resulting sulphur vapour is effected by precursory condensation upstream of the catalytic Claus reactor. This class is therefore in essence a conventional Claus process with only one catalytic stage of reaction between hydrogen sulphide and sulphur dioxide and with a tail gas clean up unit that recycles hydrogen sulphide to the catalytic Claus reactor.
The additional catalytic stage of reaction between hydrogen sulphide and sulphur dioxide in this process has the effect of reducing the mole fraction of hydrogen sulphide in the feed to the absorption unit and of thereby decreasing the amount of hydrogen sulphide which is recycled, thus making it far less advantageous in comparison with the process according to EP-A-0 565 316.
EP-A-0 798 032 relates to a modification of the process according to EP-A-0 565 316 in which catalytic stages of reaction between hydrogen sulphide and sulphur dioxide are avoided and in which downstream of the water removal stage the catalytically hydrogenated gas is compressed and subjected to fractional distillation to form a first fraction rich in hydrogen sulphide and a second fraction depleted of hydrogen sulphide. A stream of the second fraction is purged from the process, typically being sent to an incinerator, and a stream of the first fraction is returned to the furnace. This process has, however, a number of disadvantages which affect its suitability for commercial use. Firstly, because the separation by fractional distillation of hydrogen sulphide from carbon dioxide needs to be performed at below 0° C., it is necessary in the water removal stage to have complete extraction of water vapour from the catalytically hydrogenated gas. This adds considerably to the complexity of the water removal stage, an adsorption unit being required. Further, the requirement for compression of the water-depleted hydrogenated gas stream adds considerably to the operating and capital costs of the process. It is also necessary to provide a suitable refrigeration system to provide condensation at the top of the distillation column. Finally, above about a mole fraction of 0.8 for carbon dioxide, the separation becomes difficult due to the formation of a mixture displaying azeotropic characteristics, and an unreasonably large number of theoretical separation stages would be needed if it were desired to extract carbon dioxide in a high state of purity, ie with a low hydrogen sulphide content.
None of the above described processes provides a simple method of achieving particularly high effective percentage conversions to sulphur of the hydrogen sulphide content of the feed gas stream. It is an aim of the invention to provide a method and apparatus which is suitable for this purpose; and which is capable of obtaining a conversion of 99.7% or higher when the hydrogen sulphide content of the feed gas stream is at least 70% by volume.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of treating a feed gas stream containing hydrogen sulphide and carbon dioxide, comprising the steps of:
(i) burning in a furnace part of the hydrogen sulphide content of the feed gas stream so as to form sulphur dioxide and water vapour, supplying oxygen gas to the furnace to support combustion of the said part of the feed gas stream, and reacting in the
Balse Vijay Ramanand
Graville Stephen Rhys
Watson Richard William
Neida Philip H. Von
Pace Salvatore P.
Silverman Stanley S.
The BOC Group plc
Vanoy Timothy C
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