Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
2001-10-30
2003-11-11
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S220000, C423S222000, C423S224000, C423S242100, C423S242200, C423S575000, C423S542000
Reexamination Certificate
active
06645459
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
Not Applicable
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK
Not Applicable
FIELD OF THE INVENTION
The present invention relates to a process of removing hydrogen sulfide from natural gas or other industrial gas, in an integrated system where sulfur is produced.
BACKGROUND OF THE INVENTION
One of the most common systems for processing gases containing hydrogen sulfide and producing sulfur involves the use of well-known absorber-stripper steps to separate H
2
S and the well-known Claus process to produce sulfur. In such system, in simplified form, the basic steps are usually:
(a) H
2
S removal from sour gas, using an H
2
S absorbent, to obtain sweetened product gas.
(b) Stripping H
2
S out of the H
2
S-rich absorbent to obtain H
2
S.
(c) H
2
S combustion to obtain SO
2
and H
2
S.
(d) Solid-catalyzed H
2
S reaction with SO
2
at high temperature to form and recover S and to make an off-gas containing reduced amounts of H
2
S and SO
2
.
(e) Treating the off-gas from step (d) to recover as S a major fraction of the remaining amounts of H
2
S and SO
2
and to form a stack gas that may be released to the atmosphere.
Steps (c) and (d) in combination are often regarded as the Claus process.
A system that is directed to treating sour gas but does not include reaction of H
2
S to form sulfur is shown in FIGS. 14-25 of Kohl and Riesenfeld, Gulf Publishing Co., 1985 “Gas Purification”, 3rd Edition. FIGS. 14-25 in the Kohl et al. reference shows the basic steps of (a) H
2
S removal from sour gas using an absorbent to take out the H
2
S, so as to obtain treated (sweetened gas) of reduced H
2
S content out the top of the absorber or “contactor” and H
2
S-rich absorbent out of the bottom of the absorber; and (b) stripping H
2
S out of the H
2
S-rich absorbent, by a flash regeneration technique and a heated regeneration technique to strip H
2
S from the absorbent and obtain H
2
S and regenerated (lean) absorbent for reuse in step (a).
The system illustrated in the Kohl et al. reference uses a physical absorbent, such as propylene carbonate.
A chemical solvent could be used in that basic-type system, possibly without the flash regeneration part of step (b). Examples of known chemical-type absorbents include amines, such as monoethanolamine (“MEA”).
Just as Kohl et al. reference at FIGS. 14-25 is directed to H
2
S absorption/stripping steps, also FIG. 5 from a paper by Lynn et al., “The University of California Berkeley's Sulfur Recovery Process: Claus Revisited”, 1999 Sulfur Recovery Conference, Austin, Tex., Oct. 24-27, 1999, shows the resultant H
2
S from absorption/stripping can be routed to a reactor. The reactors illustrated in the October 1999 paper are used in combination with a Claus plant (see, for example, the furnace illustrated in FIG. 4). The SO
2
-rich gas from the furnace is routed to the bottom of an SO
2
absorber column. The SO
2
is cooled in the bottom of the SO
2
absorber using a cooled organic solvent (SO
2
absorbent) that is recirculated, through a solvent quench heat exchanger, in a loop at the bottom of the SO
2
absorber.
The October 1999 paper also shows in FIG. 4 a process flow diagram for a typical Shell Claus Off-Gas Treatment (SCOT) unit. It is well known in the sulfur recovery industry that a SCOT unit may be used downstream of a Claus plant as a tail-gas clean-up unit (TGCU) to increase the recovery of sulfur from what otherwise would be achieved by only using a conventional Claus plant.
The FIG. 4 illustration of the SCOT unit shows steps including (a) combining a reducing gas with the Claus tail-gas, (b) reducing (hydrogenating) the tail gas containing SO
2
, S, COS, and CS
2
in the SCOT reactor to obtain an H
2
S-rich stream, (c) quenching the H
2
S-rich stream by direct contact with water in a quench tower, (d) H
2
S absorption/stripping steps to produce an H
2
S stream, and (e) recycle of the H
2
S stream to the Claus plant. Thus, FIG. 4 of the October 1999 paper is an example of the use of a direct contact aqueous quench in a sulfur recovery process, though to cool an H
2
S-rich gas, not an SO
2
-rich gas, as in the present invention.
Another reference which illustrates a process similar to that shown in FIG. 4 from the October 1999 paper, is Naber et al. “New Shell Process Treats Claus Off-Gas”,
Chemical Engineering Progress,
Vol. 69, No. 12, page 29, December 1973.
The Claus process itself, which consists of a series of reactors in which SO
2
and H
2
S react to form water and sulfur vapor. The reaction is equilibrium-limited at temperatures above the dew point of sulfur vapor. The gas stream leaving each reactor is near chemical equilibrium. In normal operation, most of the sulfur is condensed between reactors to allow further reaction in the next stage. However, in the Claus process, the condensers operate above the dew point of water to avoid forming Wackenroder's liquid (a dilute aqueous mixture of colloidal sulfur and a solution of sulfoxy acids; see Hackh's Chemical Dictionary, Fourth Edition, 1969) and the problems that their formation would present. This is done even though the presence of water vapor in the gas stream limits the extent of reaction that can be achieved and thus necessitates the installation of a tail-gas treatment process.
My prior International patent application WO 99/12849, which is hereby incoporated herein by reference, describes a process in which gaseous hydrogen sulfide (H
2
S) reacts with gaseous sulfur dioxide (SO
2
) in the presence of an organic liquid or solvent wherein the following reaction occurs:
2H
2
S(g)+SO
2
(g)→3S(l)+2H
2
O(g) (1)
In the reactor, it is desired to operate above the melting point of sulfur. The reacting gases may flow co-currently or counter-currently to a stream of the organic liquid. A preferred example of such a reactor is a tray-type column in which the reacting gases flow counter-currently to a stream of the organic liquid. The sulfur produced by Reaction (1) in either type of reactor forms a separate liquid phase that flows co-currently with the organic liquid.
The gaseous sulfur dioxide is produced by combustion of hydrogen sulfide. Preferably this combustion is conducted fuel-rich to avoid the risk of forming SO
3
and NO
x
, both of which are undesirable. However, if the combustion is fuel-rich, then elemental sulfur forms in addition to SO
2
and will be condensed and partially dissolved in the solvent used in the SO
2
absorber, which is undesirable. On the other hand, if the combustion is carried out fuel-lean, the free oxygen that accompanies fuel-lean combustion can cause degradation of the solvent in the SO
2
absorber. Furthermore, water vapor is formed by the combustion of H
2
S and any hydrocarbons that are present in the acid gas fed to the furnace. Most of the water vapor will also condense in the solvent in the SO
2
absorber. The presence of water vapor together with the SO
2
requires additional cooling, or a higher solvent flow, in the absorber. In addition, that water must be boiled out of the solvent in the SO
2
stripper, thereby increasing the energy required in operating the stripper. Furthermore, most of this added water vapor must be condensed from the SO
2
leaving the stripper before the latter enters the reactor column to avoid an excessive vapor flow within the reactor column.
SUMMARY OF THE INVENTION
According to the present invention, a process is provided for removing H
2
S from an H
2
S-rich gas and producing sulfur, which comprises:
(a) reacting H
2
S in the H
2
S-rich gas with SO
2
to produce sulfur and a reactor off-gas containing H
2
S and H
2
O;
(b) combusting the reactor off-gas to produce a combustion gas containing SO
2
, water vapor;
(c) cooling the combustion gas from step (b) to condense water vapor and sulfur and produce an aqueous stream comprising primarily water; and
(d) introducing t
Silverman Stanley S.
The Regents of the University of California
Townsend and Townsend / and Crew LLP
Vanoy Timothy C.
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