Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
2001-12-18
2004-10-05
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S576800
Reexamination Certificate
active
06800269
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention generally relates to sulfur recovery processes and apparatus for removing hydrogen sulfide from waste gas. More particularly, the invention relates to such processes that avoid thermally combusting H
2
S and to apparatus that does not include a conventional Claus thermal reactor.
2. Description of the Related Art
In many industrial situations today it is desirable to prepare elemental sulfur from H
2
S or gaseous mixtures containing moderate to high concentrations of H
2
S. Often this is done in conjunction with cleaning up gaseous petroleum feed streams that contain H
2
S, since sulfur is generally considered undesirable in most petroleum refining products and the quality of the various petroleum fractions may be upgraded by removing the sulfur content. For example, a natural gas stream containing H
2
S is treated to remove the H
2
S, and the H
2
S rich gas fed to a modified Claus sulfur recovery unit which converts the H
2
S to elemental sulfur. In the modified Claus process, hydrogen sulfide is partially combusted with air in a reaction furnace to form sulfur dioxide. The combustion gases are cooled in a waste heat boiler in which a portion of the uncombusted hydrogen sulfide reacts with sulfur dioxide to form elemental sulfur and water vapor. The partially converted mixture then flows to a condenser where the elemental sulfur is removed in molten form. The remaining gases are then heated and passed over a catalytic converter bed for further conversion to elemental sulfur and then again cooled to condense incremental sulfur. From one to four stages of reheat, conversion and condensing are typically used.
FIG. 1
is a flow diagram of a typical prior art Claus plant. A coalescer is sometimes provided to remove entrained liquids (sulfur) from the final condenser tail gas. In many cases, a “tail gas” cleanup unit such as the well-known SCOT unit is utilized to clean up the tail gas from the modified Claus process. Tail gas treatment units process the unreacted H
2
S, SO
2
, various compounds such as COS and CS
2
, and elemental sulfur vapor into H
2
S which is then recycled back to the thermal stage of the Claus process. Alternatively, the remaining sulfur containing compounds are converted to SO
2
which is absorbed in aqueous solutions to form bisulfite salts. Still other types of tail gas treatments which have been well described in the literature involve operating Claus catalyst beds at temperatures below the dew point of sulfur, or promote the direct oxidation of the remaining H
2
S to sulfur over a bed of solid catalyst or in a liquid contacting device. The waste gas emerging from the tail gas unit is typically vented into the atmosphere after incineration of the residual sulfur containing compounds to SO
2
. The thermal stage of the Claus process is a burner in a refractory lined chamber. H
2
S along with other compounds such as CO
2
, methane and light hydrocarbon gases, nitrogen, ammonia, and hydrogen, is fed to the burner. Air, pure oxygen, or a mixture of both is also fed to the burner. A flame is used to ignite the mixture of gases. In the flame, ⅓ of the H
2
S is oxidized by the reaction:
H
2
S+{fraction (3/2)}O
2
→SO
2
+H
2
O (1)
The remaining ⅔ of the H
2
S then reacts with the SO
2
generated in the flame to form elemental sulfur and water:
2H
2
S+SO
2
⇄3
/x
S
x
+2H
2
O (2)
wherein x=2, 6, or 8. Together, reaction stages (1) and (2) are referred to as the “Claus reaction.” The maximum efficiency for conversion to sulfur is given by equilibrium computations best described by Gamson and Elkins (
Chem. Eng. Prog.
(1953) 4 9:203-215) to be 70 to 75%, depending on the flame temperature. The efficiency decreases with decreasing residence time in the reactor. The sulfur formed by the thermal stage is recovered as a liquid by first cooling the hot reaction gases (typically about 1,800-2,700° F.) in a fire tube boiler, followed by condensation in the tubes of a low pressure steam generator. Removing the liquid sulfur allows the equilibrium Claus reaction in reaction (2), above, to shift to the right to form more sulfur. At low temperatures (i.e., below 500° F.) sulfur formation from the Claus reaction is about 90 to 98% efficient, but requires a catalyst to make the reaction go at an acceptable rate. The gas containing the unreacted H
2
S and SO
2
, in the 2:1 ratio required for the Claus reaction, are heated to a temperature which prevents liquid sulfur from condensing in the catalyst bed by varying means. The gas passes over the catalyst and the Claus reaction proceeds until equilibrium is reached. Reactor effluent is cooled and sulfur is again condensed out of the gas stream. The reheat of the gases, catalytic reaction, and sulfur condensation is repeated. Usually, 2 to 3 catalytic stages are employed. Any remaining H
2
S, SO
2
, sulfur, or other sulfur compounds in the Claus plant effluent are either incinerated to SO
2
and discharged to the atmosphere, or incinerated to SO
2
and absorbed by chemical reaction, or converted by hydrogen to H
2
S and recycled or absorbed by an alkanolamine solution. This is accomplished by various Claus “tail gas” treatment units, which improve the efficiency of sulfur removal from the gas discharged to the atmosphere.
Over several decades, there have been modifications of the Claus process, mainly involving improvement of the burner design, more active and durable catalysts, different types of reheaters, and in some cases replacing air with oxygen as the oxidizer. Some of the more recent improvements have been directed toward significantly increasing the processing capability of the process. (Watson, et. al., “The Successful Use of Oxygen in Claus Plants,”
HTI Quarterly: Winter
1995/1996 pp. 95-101) The basic H
2
S conversion process remains essentially the same, however, since its inception in 1883.
The greatest problem with the Claus process is the inherent equilibrium constraint of the Claus reaction caused by the necessity of generating the SO
2
intermediate. Others have addressed this problem by attempting to directly oxidize H
2
S to sulfur using alumina based catalysts and low temperature operating conditions. Typically, these processes are catalytic oxidations operating at temperatures below about 454° C., so that the reaction can be contained in ordinary carbon steel vessels. Usually these catalytic oxidation processes are limited to Claus tail gas operations or sulfur recovery from streams that have very low H
2
S content (i.e., about 1-3%). One reason for this limited use is that the heat evolved from the oxidation of a concentrated stream of H
2
S would drive the reaction temperatures well above 454° C. requiring refractory lined vessels such as the conventional Claus thermal reactor. Low concentration H
2
S streams will not produce enough energy release from oxidation to sustain a flame as in a thermal reactor stage. These existing catalytic oxidation technologies are therefore limited to low concentration streams using non-refractory lined vessels. These processes are also limited in the amount of sulfur that can be handled because the heat transfer equipment needed to remove the heat of reaction becomes extremely large due to the low temperature differential between the process and the coolant streams.
Other techniques for improving efficiency of sulfur removal that have been described in the literature include: 1) adsorbing sulfur cooled below the freezing point on a solid material followed by releasing the trapped sulfur as a liquid by heating the solid adsorbent; 2) selectively oxidizing the remaining H
2
S to sulfur using air; and 3) selectively oxidizing the H
2
S to sulfur employing aqueous redox chemistry utilizing chelated iron salts or nitrite salts in an attempt to purifying hydrogen sulfide contaminated hydrogen or gaseous light hydrocarbon resources. According to these methods, the H
2
S-contaminated hydrogen or hydrocarbon stream is contact
Allison Joe D.
Keller Alfred E.
Pruitt Terry D.
Ramani Sriram
Conley Rose PC
Conocophillips Company
Langel Wayne A.
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