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Biotechnology is now accepted as an attractive means of improving the efficiency of many industrial processes, and resolving serious environmental problems. One of the reasons for this is the extraordinary metabolic capability that exists within the bacterial world. Microbial enzymes are capable of biotransforming a wide range of compounds, and the worldwide increase in attention being paid to this concept can be attributed to several factors, including the presence of a wide variety of catabolic enzymes and the ability of many microbial enzymes to transform a broad range of unnatural compounds (xenobiotics) as well as natural compounds. Biotransformation processes have several advantages compared with chemical processes, including: (i) microbial enzyme reactions are often more selective; (ii) biotransformation processes are often more energy-efficient; (iii) microbial enzymes are active under mild conditions; and (iv) microbial enzymes are environment-friendly biocatalysts. Although many biotransformation processes have been described, only a few of these have been used as part of an industrial process. Many opportunities remain in this area.
Sulfur level in oil fractions and legislative regulations
The sulfur content of crude oil can vary from 0.03 to 7.89 % (w/w) (Kilbane & Le Borgne, 2004). The API gravity of oil is decreasing and sulfur content is increasing (Swaty, 2005), resulting in an increase in the sulfur concentrations in finished petroleum products. Sulfur is preferentially associated with the higher molecular mass components of crude oils. When crude oil is refined the sulfur concentrates into the high molecular mass fractions.
All crude oils are composed primarily of hydrocarbons of paraffinic, naphthenic and aromatic classes with a very broad range of molecular masses. Refining is the physical, thermal and chemical separation of crude oil into its major distillation fractions, which are then further processed through a series of separation and conversion steps into finished petroleum products. The primary products of the industry fall into three major categories: fuels (such as gasoline, diesel oil, jet fuel and kerosene), finished non-fuel products (such as solvents, lubricating oils, greases, petroleum wax, asphalt and coke), and chemical industry feed stocks (such as naphtha, ethane, propane, butane, benzene, toluene and xylene) (EPA, 1995).
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Conventional HDS is a high-pressure (150–200 psig) and high-temperature (200–450 °C) catalytic process that converts organic sulfur to hydrogen sulfide gas by reacting crude oil fractions with hydrogen in the presence of an efficient inorganic catalyst. The reactivity of organosulfur compounds varies widely depending on their structure and the local sulfur atom environment. The conditions depend upon the level of desulfurization required (Gupta et al., 2005).
Biocatalytic desulfurization (BDS)
BDS is often considered as a potential alternative to the conventional deep HDS processes used in refineries. In this process, bacteria remove organosulfur from petroleum fractions without degrading the carbon skeleton of the organosulfur compounds. During a BDS process, alkylated dibenzothiophenes (Cx-DBTs) are converted to non-sulfur compounds, for example 2-hydroxybiphenyl (2-HBP), and sulfate. BDS offers mild processing conditions and reduces the need for hydrogen. Both these features would lead to high energy savings in the refinery. Further, significant reductions in greenhouse gas emissions have also been predicted if BDS is used (Linguist & Pacheco, 1999).
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BDS as a complementary technology
As already mentioned, HDS is not equally effective in desulfurizing all classes of sulfur compounds present in fossil fuels. The BDS process, on the other hand, is effective regardless of the position of alkyl substitutions (Pacheco, 1999). However, the HDS process conditions are sufficient not only to desulfurize sensitive (labile) organosulfur compounds, but also to (i) remove nitrogen and metals from organic compounds, (ii) induce saturation of at least some carbon–carbon double bonds, (iii) remove substances having an unpleasant smell or colour, (iv) clarify the product by drying it, and (v) improve the cracking characteristics of the material (McFarland, 1999; Monticello, 1996; Swaty, 2005).