Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
1995-06-07
2003-11-25
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S576200, C423S576800, C423S245100, C095S143000
Reexamination Certificate
active
06652826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a cyclic process for direct conversion of low concentrations of hydrogen sulfide (H
2
S) in gas mixtures to elemental sulfur by catalytic oxidation over an activated carbon catalyst, sorption of the sulfur product by the said carbon and subsequent desorption and recovery of the sulfur during regeneration of the catalyst.
2. Prior Art
Current practice in the sour natural gas processing industry is to remove the acid components (H
2
S and CO
2
) from the natural gas by a sweetening process. The removed hydrogen sulfide, if present in small quantities, is incinerated to sulfur dioxide (SO
2
) and vented to the atmosphere provided that the amount of released SO
2
is acceptable according to regulations with respect to the environment. If the quantity of H
2
S removed by a sweetening process is sufficiently large, it is generally fed to a Claus plant and recovered as elemental sulfur.
Recent more stringent regulations in certain jurisdictions concerning the release of sulfur as SO
2
to the atmosphere have made the sour gas processing industries aware that they will be required to reduce sulfur-containing emissions to the environment substantially. With the increasing demand for elemental sulfur and the need to meet the existing environmental regulations, considerable attention has been given to the development of inexpensive and effective methods for the recovery of elemental sulfur from natural gases containing H
2
S.
It has been known for some time that hydrogen sulfide in natural gas or other gases can be oxidized in the presence of various catalysts to sulfur dioxide or sulfur. Examples of some of these processes may be found in the patent literature.
In Canadian Patent 1,172,428 by R. F. Jagodzinski and R. K. Kerr issued on Aug. 14, 1984, a process is disclosed whereby hydrogen sulfide in sour gas is reacted with oxygen at pressures greater than 5 atmospheres over an activated alumina or a vanadium pentoxide catalyst. The catalyst is continuously soaked and submerged in liquid sulfur in a reactor at temperatures between 250 and 550° C. Elemental sulfur is produced along with a substantial fraction of SO
2
. The unreacted H
2
S from this first reactor is then reacted with the produced SO
2
to produce elemental sulfur and water in a second reactor.
In Canadian Patent 1,063,321 by W. H. Powlesland and J. W. Smith issued on Oct. 2, 1979, a process is disclosed whereby H
2
S from a “fouled gas” is removed by passing the gas through hydrated hematite (Fe
2
O
3
) pellets in a chamber, thereby producing water and forming elemental sulfur which coats the pellets. Pellets are continuously withdrawn from the bottom of the chamber to a tumbler where continuous tumbling of the pellets abrades the elemental sulfur from their surface. The sulfur is recovered and the abraded pellets are then continuously returned to the top of the chamber. The process is complex and the composition of the product stream is not given. It is stated that during low temperature regeneration of the product Fe
2
S
3
by oxidation, the possibility of SO
2
production is high because of the high temperature rise in the following reaction (1) which in turn can initiate reaction (2)
Fe
2
S
3
+1.5O
2
=Fe
2
O
3
+3S+144 kcal. (1)
Fe
2
S
3
+4.5O
2
=Fe
2
O
3
+3SO
2
+347 kcal. (2)
It is advised in the patent that reaction (2) be avoided if possible because of SO
2
production.
In Canadian Patent 722,113 issued on Nov. 23, 1965, E. E. Baker and W. A. Duncan describe a process in which hydrogen sulfide in natural gas is oxidized in a bed of molecular sieve (crystalline zeolite) pellets having an apparent pore size of at least 4.6 Angstrom units (AU) at temperatures below 150° F. and at a “first higher pressures” (100-1000 psig) thereby adsorbing the hydrogen sulfide. A hydrogen sulfide-depleted natural gas stream is discharged from the first bed. The pressure in the first bed is then reduced to a “second lower pressure” (50 psig) at which hydrogen sulfide and other gases are desorbed. The released gases are then adsorbed in a second molecular sieve bed of crystalline zeolite again having an apparent pore size of at least 4.6 A.U. at temperatures in the range of 350°-750° F. The sorption is conducted in the presence of free oxygen so as to produce and recover elemental sulfur. This is a two-step process. There is no mention of SO
2
production.
In Canadian Patent 1,117,276 by K. D. Henning et al. issued on Feb. 2, 2002, a process is disclosed for elimination of sulfur compounds, in particular hydrogen sulfide, from gases containing the same, by reaction with oxygen and/or SO
2
in the presence of activated carbon at elevated temperatures to produce elemental sulfur. The process is performed at temperatures between 120 and 240° C. and at pressures ranging from 1 to 50 bars and with O
2
/H
2
S molar ratios from 1.53 to 2.2 (i.e. 3.06 to 4.4 times the stoichiometric ratio). A two-step process is necessary if the H
2
S content in the feed gas exceeds 1318 ppm (i.e. 2 g H
2
S per m
3
of feed gas). The regeneration of the carbon in the first adsorber is less frequent than that in the second adsorber because of its autoregeneration. When regeneration is required, it is carried out with a hot inert gas. The preferred particle size of the activated carbon is 3 to 6 mm. In the first adsorber the activated carbon has a medium pore radius between 7-12 A.U. while in the second adsorber it is 5-8 A.U. There is no mention of the effect of pressure on H
2
S conversion and SO
2
production.
The disadvantages of the above-mentioned processes are that they are complicated and that some of them produce substantial amounts of SO
2
. None, except the last one, uses activated carbon as a catalyst during the catalytic oxidation of H
2
S. Although Patent 1,117,276 describes a process which is similar to the process being disclosed herein, it fails to recognize the positive effect of elevated pressure operation to achieve (a) high hydrogen sulfide conversions to elemental sulfur and (b) reduced SO
2
production. In addition, it has been found that by use of lower O
2
/H
2
S ratios than the above patent describes, lower SO
2
production can be achieved at high H
2
S conversion levels.
The above mentioned patents fail to recognize the deleterious effects of having traces of heavy hydrocarbons in the feed gas. Unless these components are removed by means of cryogenic equipment or a guard bed, the overall life of the catalyst will be reduced, the time between regenerations of the catalyst will be shortened and the quality of the product sulfur will deteriorate.
Canadian Patent 1,117,276 fails to recognize the value of operation at pressures beyond the range specified (1 to 50 bars), in terms of being able to use lower O
2
/H
2
S molar ratios which favor lower SO
2
production, in terms of more effective utilization of the activated carbon catalyst between regenerations and in terms of reduced energy requirements in processing the gas ready for delivery to a pipeline for sale at pressures of, for example, 65 to 70 bars. Furthermore, the use of higher O
2
/H
2
S ratios than are required leaves more unconverted oxygen and the associated nitrogen in the gas causing dilution and a lower calorific value of the product.
SUMMARY OF THE INVENTION
The objectives of the present invention are to provide a simple and efficient process to oxidize H
2
S in gas mixtures catalytically in the presence of air and an activated carbon catalyst so as to produce substantially pure elemental sulfur and, simultaneously, to reduce the production of SO
2
to acceptable levels so that the product gas can be fed directly into pipelines ready for use by consumers in the case of natural gas or it can be burned, flared or otherwise vented to the atmosphere. According to this invention:
Sour natural gas or other gas mixtures containing low concentrations (preferably in the range from 4 ppm to 5 mol %) of H
2
S is mixed with air/oxygen at (O
2
Chowdhury Aminul Islam
Ghosh Tushar Kanti
Tollefson Eric Lars
Holt William H.
Langel Wayne A.
Vanoy Timothy C.
Xergy Processing Inc.
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