Page 64 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

The inlet gas to the compressor is precooled by the acid gas removed.
Cryogenic removal is most often used when the content of carbon dioxide is
high, typically around 50%.

Membrane based removal is based on certain materials that allow the acid
gases, but not the hydrocarbons, to diffuse through the membrane. This
procedure can be performed alone or in combination with absorption liquid.

Sulfur Unit. The H2S-rich stripped gas stream is then fed to a Claus
process - a multistage process with two main sections: A thermal section
fires H 2S with air or oxygen to produce SO 2 and elemental sulfur which is
released when cooled. A catalytic section allows more H 2S to react with SO 2
with alumina or titanium dioxide (TiO 2) to produce water and elemental sulfur
(the Claus reaction: 2H 2S + SO 2 → 3S + 2H 2O). The Claus process can
recover 95-97% of the sulfur in the feed gases.

A Tail Gas Treatment unit serves to reduce the sulfur content to below 250
ppm, corresponding to a total sulfur recovery of 99.9%. More complex
solutions can reduce total sulfur down to 10 ppm. Some important processes
include SCOT (Shell Claus Offgas Treatment) which removes SO 2 by
combustion with hydrogen over catalysts to produce H 2S and water. H 2S is
recycled to the Claus unit. Other solutions are the Beavon Sulfur Removal
(BSR) process based on amine solvent and catalysts.

5.1.2 Dehydration
Dehydration is either glycol-based scrubbers as described in chapter 4.3.2 or
based on Pressure Swing Adsorption (PSA). Newer processes also use
membranes.

5.1.3 Mercury removal

Mercury removal is generally based on molecular sieves. A molecular sieve
is a substance containing a material with tiny pores to achieve a large
surface area, such as activated carbon. The surface of the material allows
certain molecules to bind by surface tension. The molecules can later be
extracted and the sieve material regenerated by heating, pressure and/or
purging with a carrier gas.

A molecular sieve is commonly cyclic with one active unit and one (or more)
units in regeneration.

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