Biodesulfurization

Biodesulfurization is a process that removes sulfur from fossil fuels using a series of enzyme-catalyzed reactions.Biodesulfurization is another alternative processing method that has some similarity to the above-mentioned oxidative desulfurization, in that both methods oxidize sulfur atoms in the sulfur-containing compounds. Certain microbial biocatalysts have been identified that can biotransform sulfur compounds found in fuels, including ones that selectively remove sulfur from dibenzothiophene-type heterocyclic compounds. Biocatalytic sulfur removal from fuels may have applicability for producing low sulfur gasoline and diesel fuels. 

A biodesulfurization process has been reported by Energy Biosystems It involves the removal of sulfur-containing hydrocarbon compounds from distillate (diesel) fuel or naphtha (gasoline) streams using bacteria. The distillate stream is first mixed with an aqueous media containing the bacteria. 

Piddington et al. studied the sequence and molecular characterization of a DNA region encoding the DBT desulfurization operon of Rhodococcus sp. strain IGTS8. DBT can be desulfiirized to 2-hydroxybiphenyl (2-HBP) by 

Lee et al. studied microbial desulfurization of DBTs bearing alkyl substitutions adjacent to the sulfur atom, such as 4,6-diethyldibenzothiophene (4,6-DEDBT), which are referred to as sterically hindered with regard to access to the sulfur moiety. By using enrichment cultures with 4,6-DEDBT as the sole sulfur source, bacterial isolates which selectively remove sulfur from sterically hindered DBTs were obtained. The isolates were tentatively identified as Artiirobacter species, 1,6-DEDBT sulfone was shown to be an 

As sirmmarized by McFarland et al. , microbial sulfur-specific transformations have been identified that selectively desulfurize organic sulfur compoimds in fossil fuels. Recent discoveries related to biodesulfurization mechanisms may lead to commercial applications of biodesulfurization through engineering recombinant strains for overexpression of biodesulfurization genes, removal of end product repression, 

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Macpherson T, CW Greer, E Zhou, AM Jones, G Wisse, PCK Lau, B Sankey, MJ Grossman, J Hawari (1998) Application of SPME/GC-MS to characterize metabolitres in the biodesulfurization of organosulur model compounds in bitumen. Environ Sci Technol 32 421 26. 

The application of biocatalytic technologies in the refining industry will be possible only if it can improve product yields and produce cleaner fuels economically. The hurdle to commercialization of the biodesulfurization process is still the activity of the biocatalyst. The reasons for this will be evident from the discussion in Chapter 3. 

Biodesulfurization (BDS) is the excision (liberation or removal) of sulfur from organosul-fur compounds, including sulfur-bearing heterocycles, as a result of the selective cleavage of carbon-sulfur bonds in those compounds by the action of a biocatalyst. Biocatalysts capable of selective sulfur removal, without significant conversion of other components in the fuel are desirable. BDS can either be an oxidative or a reductive process, resulting in conversion of sulfur to sulfate in an oxidative process and conversion to hydrogen sulfide in a reductive process. However, the reductive processes have been rare and mostly remained elusive to development due to lack of reproducibility of the results. Moderate reaction conditions are employed, in both processes, such as ambient temperature (about 30°C) and pressure. 

Table 6. Comparison of properties of enzymes involved in biodesulfurization...

The Keasling group [175] demonstrated that the flavin reductase from Vibrio harveyi, when expressed in E. coli along with the dsz cassette, enhanced biodesulfurization rate by about six-fold. However, it should be noted that the overexpression of the flavin oxidoreductase results only in increase in DBT removal rate, but not HBP production rate. This is expected since the flavin reductase is only required for the first two steps of desulfurization. Another interesting result from this study was the reduced rate of HBP production when the flavin reductase was co-expressed. This was probably due... 

Table 8. Comparison of flavin reductase from various organisms capable of supporting biodesulfurization...

The specificity of DszC enzyme is more clear from studies on diesel biodesulfurization [70,75,86], These studies show a very broad specificity for the DszC. These results are discussed in detail in the diesel biodesulfurization Section 2.3.6. 

Apart from biocatalyst activity, several other parameters are important in development of a biodesulfurization process. These parameters include oil/water ratio, composition of aqueous phase used for biocatalyst suspension during desulfurization, biocatalyst loading, oil/water separation following completion of desulfurization, potential for biocatalyst recycle, recycle of aqueous phase to reduce fresh water usage and wastewater minimization, as well as secondary oil separation and purification operations. 

The process steps have not received as much attention, except the third step. As of now, a commercial process has not been developed although one pilot scale study has been conducted. This section will describe the research efforts conducted to date and report on the process options considered for biodesulfurization. 

The possibility of producing certain value-added compounds such as surfactants, which can be derived from intermediates produced in petroleum biodesulfurization processes, has been evaluated. HPBS is a molecule with amphiphilic characteristics desirable for surfactant applications [243], Several oxidation reactions, from the 4S pathway are considered before reaching the final product. The compounds of the invention include acyloxybiphenylsulfinates, acyloxybiphenylsulfonates, alkyl sulfinatobiphenyl ethers, and alkyl sulfonatobiphenyl ethers. The invention also provides methods of producing these compounds. 

An important parameter that has to be considered during desulfurization as well as for subsequent biocatalyst separation and recycle is the impact of the oil phase on the biocatalyst activity and half-life. Additionally, the effect of the biocatalyst on forma-tion/breakage of the oil-water emulsions is also important. The latter will be discussed in Section 2.3.3. It becomes important for lower boiling feedstocks such as gasoline, which offers the most toxic solvent environment for the biodesulfurization catalyst. The effect of solvents on biocatalysts has been investigated in very few reports. A study by the Monot group reported effect of two solvents on several Rhodococcus strains [254], The strains contacted with the solvents and their desulfurization activity, growth, and... 

The effect of oil/water ratio has been studied extensively for various catalysts. Patel et al. [258] reported effect of oil/water ratio on rate of desulfurization by IGTS8. They used freeze-dried cells reconstituted with water to do the studies. They found that a minimum of 1.25 mL of water per gram of freeze-dried cells is necessary to enable biodesulfurization. At a W/O ratio of 1 9, about 82% of the maximum desulfurization activity was achieved. The rate of desulfurization was reported to be similar between the W/O ratio of 1 1 and 4 1, but decreased upon increasing oil content further. The effect of the ratio was also studied by Shan et al. using diesel oil as the oil phase and P. delafieldii R-8 as the biocatalyst [259], The water content was varied to obtain a W/O ratio between 0 1 and 20 1, using a fixed amount of biocatalyst and oil. The authors found that the desulfurization rate increased up to a W/O ratio of 2 1, after which it remained constant. 

The effect of the cell density was studied in biodesulfurization of diesel oil by P. delafieldii R-8 [259], An optimum was reported to exist for this biocatalyst as well. Above 25g/L cell density, the specific desulfurization rate decreased. In this case a statistical analysis was not performed to identify the point of mass transfer limitation. 

The high sulfur-containing feedstock and the biocatalyst, usually suspended in the aqueous phase have to be contacted with each other in a bioreactor. A homogeneous, continuous phase would be preferred, which would imply formation of an emulsion, preferably a microemulsion. Several bioreactor designs have been suggested for biodesulfurization of petroleum feedstocks including impeller-mixed systems [65,202], electro-spray bioreactor [220,261,262], and draft tube air-lift bioreactor [263],... 

Design and development of an electro spray bioreactor was reported and its suitability for biodesulfurization applications was demonstrated [261], Mixing oil with water in large bioreactors requires significant energy input for classical impeller-based reactors. The electro spray bioreactor (ESB) was developed from an emulsion phase contactor... 

The draft-tube airlift bioreactor was studied using water-in-kerosene microemulsions [263], The effect of draft tube area vs. the top-section area on various parameters was studied. The effect of gas flow rates on recirculation and gas carry over due to incomplete gas disengagement were studied [264], Additionally, the effect of riser to downcomer volume was also studied. The effect of W/O ratio and viscosity was tested on gas hold-up and mass transfer coefficient [265], One limitation of these studies was the use of plain water as the aqueous phase in the cold model. The absence of biocatalyst or any fermentation broth from the experiments makes these results of little value. The effect of the parameters studied will greatly depend on the change in viscosity, hold-up, phase distribution caused due to the presence of biocatalyst, such as IGTS8, due to production of biosurfactants, etc., by the biocatalyst. Thus, further work including biocatalyst is necessary to truly assess the utility of the draft-tube airlift bioreactor for biodesulfurization. 

In a biodesulfurization process, there are actually three phases. For a liquid mixture containing the three phases - liquid fossil fuel, water, and the biocatalyst, more than one filter would be required. One filter will preferentially collect either the liquid fossil fuel or aqueous phase as the filtrate. The retentate will then flow to the second filter, which will collect the component not removed before. The remaining retentate, containing the biocatalyst, can then, preferably, be recycled. The process can be used to resolve an emulsion or microemulsion of the liquid fossil fuel and aqueous phase resulting from a... 

A bench-scale settler was used in an experimental study to assess continuous biodesulfurization operation to separate oil and water + biocatalyst [260], The design was based on a viton tubing settler surface placed at an angle to allow separation of the multi phase mixture. The device was reported to operate with over 95% efficiency for first 24 hours after which the performance reduced drastically. 

Most of the biodesulfurization microorganisms studied are best suitable for the diesel fraction of the crude oil, since this contains significant levels of DBT and its derivatives, which are the substrates of the Rhodococcus IGTS8 and related biocatalysts. A summary of biocatalysts tested is given in Table 13. 

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