Sulphur Recovery from Oil

First, all sulphides are converted to hydrogen sulphide (H2S) by reacting with hydrogen (hydrogenation). The process also converts some unsaturated alkenes into more saturated products, lowering of the octane rating of the fuel.

formula: removing sulphur from an alkyl sulphide

This hydrogen sulphide is then separated from the other gases by dissolving in organic bases like monoethanolamine (MEA), diethanolamine (DEA) or methyldiethanolamine (MDEA).

formula: three organic bases


This example shows the reaction of hydrogen sulphide with DEA.

formula: reaction of hydrogen sulphide with DEA

The hydrogen sulphide can then be released by heating the amine to about 115ºC. Carbon dioxide also dissolves in the amine and can be removed, but high CO2 content oil or gas may have to have its sulphur recovered using another technique.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Alternative catalysts

Some plants are using titanium oxide catalysts in place of alumina. The advantages of using these catalysts are:

  • They are better able to resist sulphation and so have an extended life.
  • They are more active and achieve higher conversion rates of 90% as opposed to the 60% of alumina
  • They also catalyse the breakdown of by-products like carbon disulphide (CS2) and carbonyl sulphide (COS)


Although they are 4-5 times more expensive at present, existing users claim that there are overall benefits.

 

 

Processing hydrogen sulphide

The hydrogen sulphide is then passed through one or two stages:

Step 1: Partial oxidation to sulphur dioxide, so that it enters the next step in the desired molar ratio of 2:1 with sulphur dioxide. Some sulphur is also produced at this stage and removed.

formula:

In the KVT - Sulfox process, the sulphur dioxide produced at this stage is used to manufacture sulphuric acid, missing out the second stage.

Step 2: Sulphur dioxide reacts with the remaining hydrogen sulphide, forming sulphur and steam. This stage is run at much lower temperatures and at low pressure to maximise sulphur production.

formula:

The second, catalytic step does not proceed to completion but requires several passes over the catalyst bed to result in 94 - 97% conversion to sulphur. The catalyst needs regular replacing as it becomes clogged with sulphur ("sulphated") and carbon.

Both equilibria are highly exothermic and the heat generated at each stage is used within the plant.
photo: sulphur recovery unit at a Texas plant

Final Stage

The final stage involves the cleaning of the remaining gas, known as the "tail gas". Cobalt- molybdenum catalysts convert any remaining sulphur dioxide, and by-products like carbon disulphide (CS2) and carbonyl sulphide (COS), to hydrogen sulphide. This is then removed by amine extraction and returned to the oxidation stage.

This final step ensures an impressive 99.8% removal of sulphur from the oil and gas deposits.

 

 


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