State-of-the-Art Redox Chemistry Technology
The basic chemistry is identical for all three LO-CAT® system
configurations. The LO-CAT® process
absorbs H2S, ionizes H2S,
oxidizes H2S to S, reduces iron, absorbs
oxygen, then oxidizes iron. Hydrogen sulfide is thus converted to
elemental sulfur
using an environmentally safe chelated iron catalyst. The iron catalyst
is held in solution by organic chelating agents that wrap around
the iron ion in a claw-like fashion, preventing precipitation of
either iron sulfide (FeS) or iron hydroxide [Fe(OH3)].
The LO-CAT process is based on reduction-oxidation (Redox) chemistry.
Two different redox reactions take place -- one in the absorber section,
which converts H2S to elemental sulfur, and one in
the oxidizer section, which regenerates the catalyst.
Absorber reactions
In the absorber, H2S is absorbed into the slightly alkaline,
aqueous LO-CAT solution. The H2S ionizes to bisulfide,
which is oxidized to sulfur by reducing the iron ion from the ferric
to the ferrous state. The reduced iron ions are then transferred from
the absorber to the oxidizer.
The absorber reactions are as follows:
Oxidizer reactions
In the oxidizer, atmospheric oxygen is absorbed into the LO-CAT solution.
The ferrous iron is reoxidized to ferric iron, regenerating the catalyst.
The regenerated catalyst is ready for use in the absorber section.
The oxidizer reactions are as follows:
Overall reactions
The overall reaction is an isothermal, low operating cost method of
carrying out a modified Claus reaction. The chemical additions required
to maintain the above reactions are caustic for maintaining the pH,
replacement of chelated iron lost in the sulfur removal process,
and replacement of degraded chelating agents.
The overall reaction is as follows:
Side Reactions
As with any chemical process, side reactions can occur during the LO-CAT
process. For example, thiosulfate formation increases when oxygen
is present in the sour gas. This occurs when the sour gas being treated
is an air stream or when the sour gas has been contaminated with
air. Thiosulfate does have some benefits in the process, since it
stabilizes the chelating agents, reducing degradation and thereby
reducing chemical costs. On the other hand, too much thiosulfate
requires the addition of caustic to maintain pH, and blowdown may
be required to avoid salt buildup in the system.
Biocarbonate formation depends on the amount of carbon dioxide absorbed
from the sour gas, which depends on the CO2 partial
pressure and the pH of the solution. There are no benefits to biocarbonate
formation. Caustic must be added to maintain pH, and some of the
CO2 is lost.
These side reactions as shown below:
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