Processes Based On Quinones

 

These have become the leading processes in removing low level hydrogen sulphides and is a common addition to Claus and similar plants as a final polish of the effluent.

The most commonly used is the Stretford Process. This is a direct development of the anthroquinone system, which used the redox cycle of this compound in the oxidation of the H2S.

Anthroquinone was partially sulphonated to improve water solubility.

The ADA (anthroquinone disulphonic acid) process was capable of reducing H2S to < 1ppm but it had a number of disadvantages.

  • Only 50% of the sulphide was converted to sulphur

  • 30% to thiosulphate

  • 20% to sulphate.

These side reactions were a clear problem since there was no obvious market for the products. Note that markets for bi-products can often be dominant in technology adoption decisions.

Thus by the end of the 1950s those techniques based on the use of metal oxide suspensions and on the use of a quinone had been found to be unsatisfactory.

In 1960 Dr T Nicklin of the NW Gas Board realised the possibility of using a water soluble polyvalent metal used to oxidise the sulphide to free sulphur and then using the quinone as an oxygen carrier in the re oxidation of the metal

 

THE MECHANISM -For those who would like it.

Hydrogen sulphide is absorbed in sodium carbonate solution forming bisulphide ions. The bisulphide ions are oxidised by metavanadate, which is reduced from the pentavalent to the Quadravalent State.

The ADA re-oxidises the reduced vanadium back to the metavanadate and is reduced to a phenolic form in the process.
Finally aeration of the solution oxidises the phenolic form of the ADA back to the quinone form.

INITIAL PROBLEMS:

Depositions of vanadium salts- overcome by using a sequestering agent such as citrate.

When vanadium is used the generation of sulphate and thiosulphate is almost entirely prevented.

It is not economic to totally prevent side product formation and these compounds would eventually build up to unacceptable levels in the liquid unless a continuous bleed/replacement system is used. This produces a liquid effluent, which must then be disposed of.