Liquid purification or separation – Processes – Treatment by living organism
Patent
1999-02-10
2000-04-04
Wyse, Thomas G.
Liquid purification or separation
Processes
Treatment by living organism
210614, 210620, 210916, C02F 334, C02F 158
Patent
active
060456957
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to the biological treatment of spent caustics resulting from the removal of sulphides from hydrocarbon sources.
BACKGROUND
Sodium hydroxide solutions are used in petroleum refining and chemical industries to remove hydrogen sulphide from various hydrocarbon streams. When the sulphide has reacted with the sodium hydroxide, the resulting solution is usually referred to as spent sulphidic caustic. Depending on the hydrocarbon source, spent caustics may also contain phenols, mercaptans, amines and other organic compounds that are soluble or emulsified in the caustic. Spent caustics typically have a pH greater than 12 and sulphide concentrations exceeding 2 wt. % (.apprxeq.a more than 0.6 mol/l).
At the moment, spent caustics are usually treated by the "wet air oxidation", wherein sulphides and mercaptans are oxidised chemically at high pressures and temperatures. This process is expensive because of the required chemicals, and leads to residual waste in the form of gaseous sulphur dioxide and liquid sulphuric acid and sulphate. Another known method of disposal of spent caustic is deep well injection, which is also expensive.
A biological process for the treatment of spent caustics was described by Rajganesh, Sublette, Camp and Richardson, Biotechnol. Prog. 1995 (11), 228-230. In this process, sulphides are completely oxidised to sulphate by Thiobacillus denitrificans. However, the production of only sulphate is often not desirable because the pH may become too low. This known process also requires nitrate, which has to be added to the spent caustics, leading to additional costs for chemical requirements.
SUMMARY OF THE INVENTION
It has been found now that the biological treatment of spent caustics and similar waste streams can be improved by controlling the redox potential of the biological treatment medium so as to partly or predominantly produce elemental sulphur in addition to sulphate.
It has furthermore been found that the treatment of spent caustics also containing mercaptans can be accomplished by using bacteria of the novel strain Methylophaga sulfidovorans.
DETAILED DESCRIPTION OF THE INVENTION
In a first aspect, the invention pertains to a process for the biological treatment of a spent caustic solution containing sulphides and possibly other sulphur compounds, wherein the sulphides are partly converted to elemental sulphur and partly to sulphate.
The biological oxidation of sulphide by aerobic Thiobacilli can be represented by the following equations, especially (1) and (2): reaction (2) sulphate is formed with a drop in pH. In order to cause the oxidation to proceed partly or pre-dominantly through reaction (1), one could in principle use a low oxygen concentration, since the higher the oxygen concentration is, the more sulphate is formed. However, at moderate sulphide loading rates (up to about 250 mg/l.h), sulphate production is already complete at oxygen levels as low as 0.1 mg/l, which is about the detection limit. One could also use a high sulphide load, but this leads to increased mercaptan loads which are difficult to biodegrade.
It was surprisingly found, that the sulphide oxidising reaction can be controlled towards partial or predominant sulphur formation by adjusting the redox potential of the medium of the oxidation at a value below -300 mV (against an Ag/AgCl reference electrode). The following redox ranges were found: at a redox potential below -360 mV (against an Ag/AgCl reference electrode), hydrogen sulphide is completely converted to elemental sulphur; at a redox potential between -360 and -300 mV (against an Ag/AgCl reference electrode), hydrogen sulphide is partially converted to elemental sulphur and partially to sulphate; at a redox potential above -300 mV (against an Ag/AgCl reference electrode), hydrogen sulphide is completely converted to sulphate. The range of -300/-360 mV against Ag/AgCl corresponds to a range of -97/-157 mV against a H.sub.2 reference electrode at 30.degree. C. This allows an optimum sulphide removal
REFERENCES:
patent: 5366633 (1994-11-01), Buisman
patent: 5474682 (1995-12-01), Buisman
patent: 5480550 (1996-01-01), Sublette
patent: 5518619 (1996-05-01), Buisman
patent: 5637220 (1997-06-01), Buisman
Bontsema Jan
Buisman Cees Jan Nico
De Zwart Jolyn Martha Maria
Janssen Albert Joseph
Kuenen Johannes Gijsbrecht
Paques Bio Systems B.V.
Wyse Thomas G.
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