Galvanic Applied Sciences 943-TGX, Operation Manual

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Revision 4 12 August 02, 2019
Section 1 Model 943-TGX Tail Gas Analyzer
Spectrophotometer Overview
1.1 Overview
The use of a Tail Gas Analyzer to assist in the control of combustion air to acid gas ratios in
Claus Sulfur Recovery Plants is a standard procedure in industry. The plant tail gas is
analyzed using ultraviolet spectroscopy and an output signal that corresponds to the air
requirement is determined. This observed signal is proportional to the percentage change
required in the combustion air to provide stoichiometric concentrations of the principal
reactants; H
2
S and SO
2
. When the process is optimized and the correct stoichiometric
concentrations of H
2
S and SO
2
are achieved, the feedback signal (normally referred to as Air
Demand) is zero (which means that no change is required).
The simplified Air Demand equation is:
Air Demand = F × ([H2S] - Rop [SO2])
where:
F = plant specific gain factor
[H2S ] = concentration of H
2
S
[SO2] = concentration of SO
2
Rop = operating ratio (typically 2)
When the stoichiometric concentrations of H
2
S and SO
2
([H
2
S] / [SO
2
]) are equal to 2 , the
Air Demand output becomes zero regardless of the plant specific gain factor (F) when the
operating ratio (R
op
) is two (2). When the plant specific gain factor (F) is established for a
particular process, the units of Air Demand become ‘percent change required in process air’.
( i.e., a computed Air Demand of +1.5% means that the process is 1.5% excess in air.) To
achieve optimum performance of the Claus Sulfur Recovery Plant, the Air Demand should be
kept near zero.
The sample gas obtained from the sulfur plant waste or tail gas stream may also contain
other sulfur species such as COS, CS
2
, and S
vapour
. These species, if present in significant
concentrations, must be analyzed for and a correction be made to avoid interference with the
H
2
S and SO
2
analysis.