Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
2000-04-07
2001-09-11
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S574100, C423S576800, C423SDIG005
Reexamination Certificate
active
06287535
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for treating combustible gas streams containing hydrogen sulphide.
Hydrogen sulphide containing gas streams (sometimes referred to as “acid gas streams”) are typically formed in oil refineries and natural gas processing units. Such streams should not be vented directly to the atmosphere because hydrogen sulphide is poisonous. A conventional method of treating a hydrogen sulphide-containing gas stream (which, if desired, has been pre-concentrated) is by the Claus process. In this process a part of the hydrogen sulphide content of the gas stream is subjected to combustion in a thermal stage taking the form of a furnace so as to form sulphur dioxide. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide so as to form sulphur vapour. The reaction between hydrogen sulphide and sulphur dioxide does not go to completion. The effluent gas stream from the furnace is cooled and sulphur is extracted, typically by condensation, from the cooled effluent gas stream. The resulting gas stream, still containing residual hydrogen sulphide and sulphur dioxide, passes through a train of stages in which catalysed reaction between the residual hydrogen sulphide and the residual sulphur dioxide takes place. Resulting sulphur vapour is extracted downstream of each stage. The effluent gas from the most downstream of the sulphur extractions may be incinerated or subjected to further treatment, eg by the SCOT or Beavon process, in order to form a gas stream which can be vented safely to the atmosphere.
Air may be used to support the combustion of hydrogen sulphide in the initial part of the process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which is, of course, present in the air that supports the combustion) flow through the process and therefore place a ceiling on the rate at which the gas stream containing hydrogen sulphide can be treated in a furnace of a given size. This ceiling can be raised by using commercially produced oxygen or oxygen-enriched air to support the combustion of the hydrogen sulphide.
Generally, depending on the concentration of the hydrogen sulphide containing gas stream, supply of commercially pure oxygen instead of air will result in the creation of excessive temperatures in the furnace which are liable to cause damage, particularly to the refractory lining of the furnace. Various methods are known for increasing the degree of enrichment of the air in oxygen without creating excessive temperatures. For example, United Kingdom Patent Application 2 173 780 A discloses moderating the temperature by introducing liquid water into the flame zone of the furnace. U.S. Pat. No. 5,352,433 discloses a particularly advantageous process in which the capacity or throughput of a Claus process is increased by conducting the combustion of the hydrogen sulphide in two separate furnaces. Accordingly, the overall amount of heat generated by the combustion is allocated between the two furnaces without the need to employ an external or recycled temperature moderator. Thus, a higher degree of uprating can be achieved than in other methods.
Typically, when combustion of the hydrogen sulphide takes place in two separate furnaces, it is possible to operate a process by retro-fitting an additional furnace and appropriate heat exchange equipment to an existing plant.
It is an aim of the present invention to provide a method and apparatus for recovering sulphur from hydrogen sulphide containing gas streams which is flexible to operate, which can be effectively controlled, and which can still offer at least some of the advantages of operation with commercially pure oxygen or oxygen-enriched air.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of treating a plurality of combustible gas streams containing hydrogen sulphide, comprising the steps of:
(a) operating to recover sulphur from a first combustible gas stream containing hydrogen sulphide, a first Claus plant having a train of stages including, in sequence, a first thermal Claus stage, a first sulphur condenser, and at least one sub-train of stages including a catalytic Claus stage and a second sulphur condenser downstream thereof;
(b) burning in at least one further thermal Claus stage part of the hydrogen sulphide content of a second combustible gas stream;
(c) supplying to the further thermal Claus stage a combustion-supporting gas having an oxygen mole fraction of at least about 0.25 so as to support combustion of hydrogen sulphide therein, the combustion-supporting gas being formed of a stream of pure or impure oxygen separated from air or a mixture of a stream of air with the said stream of pure or impure oxygen;
(d) withdrawing an effluent gas stream containing hydrogen sulphide, sulphur dioxide, water vapour and sulphur vapour from the further thermal Claus stage and removing sulphur vapour from the effluent gas stream in a further sulphur condenser so as to form a sulphur-depleted effluent gas stream;
(e) mixing the sulphur-depleted effluent gas stream with first combustible gas undergoing treatment in the first Claus plant at a region thereof downstream of the first thermal Claus stage and upstream of the start of the catalytic Claus reaction in the said sub-train;
(f) generating a first control signal which is a function of the flow rate into the further thermal Claus stage of one of the second combustible gas and at least one combustible component thereof into the said further thermal Claus stage;
(g) generating a second control signal which is a function of the hydrogen sulphide/sulphur dioxide mole ratio in the said sulphur depleted effluent gas stream; and
(h) employing the control signals in setting the rate at which the combustion supporting gas is supplied to the further thermal Claus stage.
The invention also provides plant for treating a plurality of combustible gas streams containing hydrogen sulphide, comprising:
a) a first Claus plant for the recovery of sulphur from a first combustible gas stream containing hydrogen sulphide having a train of stages including, in sequence, a first thermal Claus stage, a first sulphur condenser, and at least one sub-train of stages including a catalytic Claus reaction stage and a second sulphur condenser;
b) at least one further thermal Claus stage for the combustion of part of the hydrogen sulphide content of a second combustible gas stream containing hydrogen sulphide;
c) at least one inlet to the further thermal Claus stage for a combustion-supporting gas having an oxygen mole fraction of at least about 0.25, the combustion-supporting gas being formed of a stream of pure or impure oxygen separated from air or of a mixture of a stream of air with the said stream of pure or impure oxygen;
d) an outlet from the further thermal Claus stage for an effluent gas stream containing hydrogen sulphide, sulphur dioxide, water vapour and sulphur vapour;
e) a further condenser for extracting sulphur vapour from the effluent gas stream, so as to form a sulphur-depleted effluent gas stream having an inlet communicating with the outlet from the further thermal Claus stage;
f) an outlet for the sulphur-depleted effluent gas stream communicating with a region of the first Claus plant downstream of the first thermal Claus stage and upstream of where therein the catalytic Claus reactions starts:
g) means for generating a first control signal which is a function of the flow rate into the said further Claus stage of one of the second combustible gas stream and at least one of the combustible components thereof.
h) means for generating a second control signal which is a function of the mole ratio of hydrogen sulphide to sulphur dioxide in the said sulphur-depleted effluent gas; and
i) means responsive to the control signals for setting the rate at which the combustion-supporting gas is supplied to the further thermal Claus reaction stage.
The method and plant according to the invention offer a number of advantages. Fir
Cheung Wan Yee
Griffin Steven P.
Pace Salvatore P.
The BOC Group plc
Vanoy Timothy C
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