3 replies to this topic

[Pronab]

If excess air is used in the Reaction Furnace of a Claus sulfur recovery unit (a ratio of H2S:SO2 > 2:1) is it H2S in liquid sulfur will come down? What is the optimum temperature for liquid sulfur degassing system? Currently we are maintaining 143 Deg. C and all other operating parameters are normal however H2S in liquid sulfur is off-spec (21ppm VS. 10 ppm).

Thanks in advance.
Pronab.

[sgkim]

Hi, Pronab

143 ^oC would be an optimum temperature. The higher the temperature the higher residual H2S content. At the higher temperaures, H2S content tends to increase due to the polysulfides, H2Sx, which are generated by the reaction of H2S with elemental sulfur and are not easily decomposable. Lots of degassing processes SNEA, Shell, Exxon, Texagulf empoly stripping of H2S by using stripping(sweeping) gases (like air, tail gas, nitrogen, SO2, etc) exhausted by steam ejectors.

Along with sweeping, sometimes catalysts such as NH3, urea, ammonium salts, amines, are injected to decompose polysulfides if those are not objectionable. Spraying of sulfur liquid, gas bubbling in the tank, or stripping column is used. 15 ppm and higher concentration is to be avoided due to the accumulation of H2S gas in the gas phase during transportation. Maximum 10 ppm seems to be safe cencentration determined based on experimental results.

Details can be referred to a conferencial material captioned as "Sulfur Degasification" by J.A. Lagas, Comprimo. The author recommends that air is the best sweep gas. But remember H2S concentration in the exhaust gas shall be maintained much below the explosion limit 3.5 v%.

Stefano

[Pronab]

Stefano
Thank you for your attention.
In our two similar Sulfur recovery plants (Single stage Claus process followed by a back end CBA process) one of which degassing system (using SNEA process) facing problem (please see the table). For this specific problem, I want to know whether we can use excess air in reaction furnace to bring down H2S in liquid sulfur. In other way what are the precautionary measure that we should take to control H2S in liquid sulfur. We found all the operating parameters in both the Unit are quite normal, except sweep air flow in A is less than B. We can expect higher H2S in A than B, surprising LAB result shown the reverse for several times.



Unit UNIT - A UNIT-B
Plant load % 90 90
Sweep air flow Nm3/hr. 780 1070
Residence time Day 1 day 1 day
Sulfur pit temperature Deg. C 143 143
H2S in Liquid Sulfur ppm 10.4 21.2

[sgkim]

pronab:

Anomalic results might be stem from the difference of the residual H2S content in the sulfur. If the H2S contents are the same for the two streams, check the vapor space volumes and the pressures in both tanks.

The excess air would be within +/- 1% of theoretical amount at best to avoid from deactivation of the reactor by residual oxygen. Instead, the major concern would be residual COS and CS2 concentrations at the furnace outlets of which concentration is known to be greatly dependent on the furnace temperture.

Check the analytical data of feed gas to the reactors if any special reason for the difference of H2S in sulfur is not observed.

Stefano