Page 68 - Synthetic Fuels Handbook

56 CHAPTER TWO

The chemistry of the Claus process involves partial oxidation of hydrogen sulfide to sul-
fur dioxide and the catalytically promoted reaction of hydrogen sulfide and sulfur dioxide
to produce elemental sulfur. The reactions are staged and are as follows:
Thermal stage:
H S + 3/2 O → SO + H O
2
2
2
2
Thermal and catalytic stage:
SO + 2H S → 3S + 2H O
2
2
2
The efficiency of sulfur recovery depends upon such things as feed composition, age
of the catalyst, and number of reactor stages. Typical sulfur recovery efficiencies for Claus
plants are 90 to 96 percent for a two-stage plant and 95 to 98 percent for a three-stage
plant. Because of equilibrium limitations and other sulfur losses, overall sulfur recovery
efficiency in a Claus unit usually does not exceed 98 percent.
The off-gas leaving a Claus plant is referred to as tail gas, and, in the past was burned
to convert the unreacted hydrogen sulfide to sulfur dioxide, before discharge to the atmo-
sphere, which has a much higher toxic limit. However, the increasing standards of effi-
ciency required by the pressure from environmental protection has led to the development
of a large number of Claus tail gas clean-up units, based on different concepts, in order to
remove the last remaining sulfur species (Gall and Gadelle, 2003).
The oxygen-blown Claus process was originally developed to increase capacity at exist-
ing conventional Claus plants and to increase flame temperatures of gases having low
hydrogen sulfide content. The process has also been used to provide the capacity and oper-
ating flexibility for sulfur plants where the feed gas is variable in flow and composition
such as often found in refineries.
Liquid redox sulfur recovery processes are liquid-phase oxidation processes which use
a dilute aqueous solution of iron or vanadium to remove hydrogen sulfide selectively by
chemical absorption from sour gas streams. These processes can be used on relatively small
or dilute hydrogen sulfide stream to recover sulfur from the acid gas stream or, in some
cases, they can be used in place of an acid gas removal process. The mildly alkaline lean
liquid scrubs the hydrogen sulfide from the inlet gas stream, and the catalyst oxidizes the
hydrogen sulfide to elemental sulfur. The reduced catalyst is regenerated by contact with
air in the oxidizer(s). Sulfur is removed from the solution by flotation or settling, depend-
ing on the process.
The wet oxidation processes are based on reduction-oxidation (redox) chemistry to
oxidize the hydrogen sulfide to elemental sulfur in an alkaline solution containing an oxygen
carrier. Vanadium and iron are the two oxygen carriers that are used. The best example of a
process using the vanadium carrier is the Stretford process. The most prominent examples
of the processes using iron as a carrier are the LO-CAT process and the SulFerox process.
Both processes are capable of up to 99 percent or more sulfur recovery. However, using
the processes for Claus tail gas treating requires hydrolysis of all the sulfur dioxide in the
tail gas to hydrogen sulfide because the sulfur dioxide will react with the buffering base
potassium hydroxide (KOH) and form potassium sulfate (K SO ) which will consume
4
2
the buffering solution and quickly saturate it.
Tail gas-treating process involves the removal of the remaining sulfur compounds from
gases remaining after sulfur recovery. Tail gas from a typical Claus process, whether a
conventional Claus or one of the extended versions of the process, usually contains small
but varying quantities of carbonyl sulfide, carbon disulfide, hydrogen sulfide, and sulfur
dioxide as well as sulfur vapor. In addition, there may be hydrogen, carbon monoxide, and
carbon dioxide in the tail gas.