![]() ![]() Thermal Sulphate Reduction chemical is the direct reduction of sulphate by hydrocarbons in order to produce hydrogen sulphide. This reaction was confirmed by polarisation of a platinum electrode to 0.70 V in a solution of NaCl/Na2SO3 followed by positive identification of dithionite by spectroscopic analysis. The reaction has been identified as the formation of dithioniteĮ o = 0.416 0.1182 pH + 0.0295 log (SO 3 2-) 2 / (S 2O 4 2-) ![]() The curves show a reduction reaction occurring at a potential of about 0.6 V (SCE) which is concentration dependent (in respect of reaction rate and the equilibrium potential) and substantially under diffusion control. Scan rates of 1 mv/sec and rotation rates of 10 Hz are used. The electrochemical nature of the reaction has been further investigated by potentiodynamic polarisation scans using a gold rotating disc electrode (RDE). Bisulphite concentrations from 0.3 g/l to 12.5 g/l have been used, and the presence of sulphide is the confirmed. Several qualitative tests have been performed, using mild steel coupons in deaerated 3.5% NaCl solutions, with pH adjusted to 5 or 6 with NaOH/HCl at ambient temperature. ![]() The nature of the reduction reaction has been investigated, on both NH 4HSO 3 and Na2SO3. Then a direct reaction of bisulphite/sulphite with the steel process equipment and tubing can also occur. Such method can cause sulphite scavengers to react with the residual chlorine used as the primary biocide, thereby reducing protection against SRB's. Some operations, NH 4HSO 3 is often only used during maintenance shut downs of the mechanical units. It is usual to add, continuously, small amounts of ammonium bisulphite or sodium sulphite to chemically scavenge further oxygen. These achieve reductions to around 100 50 parts per billion(ppb). Mechanical means, gas counter current stripping or vacuum deaerators, are used. To protect process and well completion of equipment against excessive corrosion, oxygen levels in sea water need to be reduced prior to injection. Reduction of SRB Protection by Bisulphite Chemicals This consumption of hydrogen is one way in which SRB are implicated in corrosion events in the oil industry. In this case, the requirement for carbon is satisfied by organic compounds or from the fixation of carbon dioxide. Some SRB’s are able to use hydrogen (via hydrogenese enzymes) rather than organic compounds as electron donors. Sulphate acts as the electron acceptor, being reduced to sulphide. It donates an electron, along an electron transport chain. SRB's utilise suitable carbon and energy sources in oxidising an organic substrate. Since SRB grow in the absence of oxygen, the oxidation of organics, such as acetic acid, is linked to the reduction of sulphate: In general, SRB, obtain energy for growth and reproduction from the oxidation of a range of organic materials which also serve as sources of carbon. That under favourable conditions SRB's will convert SO4- sources to H 2S. Under optimal conditions, it has been estimated that a sphere of porous rock approximately 7 metres in radius could support an SRB population capable of producing the 400 kg of H 2S per day observed for badly soured wells. It has a wide range of metabolic mechanisms which allows sulphate reduction to proceed under many different environmental conditions at the expense of a range of electron donors and carbon sources. SRBs are widely distributed in oil production facilities and in seawater, which makes their introduction into water-flooded reservoirs. These organisms "breathe" sulfate rather than oxygen, in a form of anaerobic respiration. Sulfate reducing bacteria (SRB) can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microorganisms, having contributed to the sulfur cycle soon after life emerged on Earth. Mechanisms of souring Sulphate Reducing Bacteria
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