Sulphur Recovery in Natural Gas System-II |ChemFam #23|

Greetings to everyone! In my previous post, we were discussing about the recovery processes of sulphur in natural gas system. We have already studied Claus, straight-through Claus and split-Claus process. Today, we shall be learning some more processes which are used for the recovery of sulphur. This is in continuation with the previous lecture on the sulphur recovery processes.

WHAT WE SHALL LEARN?

In this particular post, we shall be learning about few other processes for the recovery of sulphur which are worth mentioning and explaining. This includes: tail gas treatment, redox process, SCOT process and use of sulphur scavengers.

Tail Gas Treatment

First let us see the tail gas treatment method. This method is used to remove the last remaining sulphur containing species to meet the environmental regulation after the sulphur recovery. These sulphur containing species are -

Carbonyl sulphide (COS)
Carbon disulphide (CS₂)
Methyl mercaptan (CH₃SH)
CO, H₂S, SO₂, H₂, CO₂ and other sulphur vapor compounds.

This process uses an amine system for the recovery of sulphur.

Process Description

The process basically comprises of 3 main steps. First step involves the generation of reducing gas to obtain H₂ and CO and tail gas preheat. Here, we heat the tail gas to 550-650°F by a reducing gas generator (RGG). So, the natural gas as well as the combustion air is inputted into the RGG.

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After this we carry out the hydrogenation and hydrolysis of the sulphur dioxide and other sulphur species to obtain H₂S. Now, let’s have a brief idea about these two terms -

Hydrogenation: It is the conversion of SO₂ and elemental sulphur (S_x) to H₂S using hydrogen.

SO₂ + 3H₂ → H₂S + 4H₂O + ΔH

S_x +x H₂ → x H₂S + ΔH

Hydrolysis: It is the conversion of COS and CS₂ to H₂S using water.

COS + H₂O → H₂S + CO₂ + ΔH

CS₂ + 2H₂O → 2H₂S + CO₂ + ΔH

The third step involves cooling the gas and recovery of sulphur by amine. The cooling of the reduced tail gas is done to about 90-100°F and then the water vapor condenses out and it gets removed. The removal of H₂S and some CO₂ from cooled gas is carried out by absorbing with NaOH and amine. Then recycling of H₂S and CO₂ to sulphur recovery unit (SRU) is done for further processing of sulphur. This is the overall description of the tail-gas treatment.

Redox Process

Let us now come to the Redox process. The name itself indicates here we are doing some oxidation and reduction reactions. It is a liquid phase process. First we remove the H₂S by scrubbing with some mild alkaline solution. Some Venturi sprays are used for scrubbing. Then we selectively remove the H₂S by chemical absorption using a dilute aqueous solution of iron or vanadium. The catalyst oxidizes the H₂S to elemental sulphur and the reduced catalyst is regenerated by contacting with air in the oxidizer(s). The removal of sulphur from the solution is done by floatation or settling for disposal or further purification.

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This redox process can be used on relatively small or dilute H₂S stream to recover sulphur from the acid gas stream or in place of an acid gas removal process. The most common examples of the processes using iron as carrier includes The LO-CAT process and SulFerox process. It is possible to recover 99% of the sulphur from the tail gas by this process.

SCOT Process

Next we come to the SCOT process and it is used to reduce SO₂ in the Claus plant back to the H₂. This process consists of two main steps: Catalytic hydrogenation/hydrolysis and amine scrubbing. Here, the uses of amine treating is done to remove the H₂S which is then recycled back to the Claus plant for conversion to elemental sulphur. This can produce an exit gas that contains very low amount of sulphur in the range of 10 to 400 ppmv and it increases the sulphur recovery upto 99.7% or higher. That is why SCOT process is very effective one for recovery of sulphur.

Process Description

The process involves various steps. First step involves heating of the feed gas to around 575°F in an inline burner along with a reducing gas such as H₂ or a CO and H₂ mixture. Then it gets catalytically reduced to get H₂S and water from SO₂, CS₂ and COS in a SCOT catalytic reactor. In the next step, cooling of the gas is done in a waste-heat exchanger to about 320°F and it produces low pressure steam. This low pressure steam can be used for the regeneration purpose and the gas is further cooled to about 100°F in a quench tower. Cooling is done as absorption is favoured at low temperature and this cooling is achieved by recycling with water in the quench tower.

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After that simultaneous absorption of H₂S and stripping of CO₂ occurs from the quenched gas using an aqueous amine solution in the absorption column. The exit gas from the top of the absorber is sent to an incinerator. The regeneration of the rich amine is carried out by steam stripping in regeneration column and recycling of recovered sulphur is sent to the Claus unit. This is the overall description of the SCOT process.

Sulphur Scavengers

The sulphur scavengers are similar to the ones which we have learnt in the acid gas removal. It is suitable for batch removal of small quantity of sulphur usually 80 kg/day or less. These sulphur scavengers are non regenerable because they react chemically with the H₂S. That is why they cannot be recovered and are confined whenever we have small amount of H₂S.

B I B L I O G R A P H Y

Sulphur Recovery and Tail Gas Treating

Upstream LNG Technology

Read my previous Blogs:

Sulphur Recovery in Natural Gas System-I |ChemFam #22|

Nitrogen Removal in Natural Gas System-II |ChemFam #21|

Nitrogen Removal in Natural Gas System-I |ChemFam #20|

Acid Gas Removal in Natural Gas System-II |ChemFam #19|

Acid Gas Removal in Natural Gas System-I |ChemFam #18|

Estimation of Water Content in Natural Gas |ChemFam #17|

Membrane Separation in Natural Gas System |ChemFam #16|

Design of distillation column |ChemFam #15|