Stirred Tank Reactor Bio-catalytic Degradation Of Sulfur Heterocyclic Compounds

Emission of sulfur-oxides to the atmosphere through combustion of fossil fuel is a main cause of serious environmental problem such as acid rain. Countries all over the word have respond to this problem by enacting more and more stringent regulations, demanding refiners to produce petroleum products such as diesel fuel and gasoline with lower sulfur levels. Hydrodesulfurization (HDS) is a traditional technology used to remove sulfur from petroleum, but this process is reaching the limits of its cost and technical effectiveness. Biocatalytic desulfurization (BDS) may offer an attractive alternative to conventional thermochemical treatment due to the mild operating conditions and greater reaction specificity afforded by the nature of biocatalysis.A strain, isolated from refinery sludge and identified as Corynebacterium sp. ZD-1, can metabolize dibenzothiophene (DBT) to 2-hydroxybiphenyl (2-HBP) through a sulfur-specific pathway. The ability of strain ZD-1 to metabolize DBT in the presence of glycerol was investigated. The experimental results showed that the strain has its maximal desulfurization activity in the late exponential growth phase; and that the specific production rate of 2-HBP is about 0.14 (mmol/kg dry cell/min). The strain reached its maximum growth (1.80 g dry cell/1) in shaking flask culture after 45.5 h. Some effect factors including initial pH, temperature, carbon source, sulfur source, and nitrogen source on the growth of Corynebacterium sp. ZD-1 were also studied. The optimum conditions for the growth of ZD-1 were obtained at initial pH=6.0⁷.5, 10.0g/l of glycerol, 5.0 g/l of NH₄C1, 0.20mmol/l of DBT and 30℃. Because of the poor solubility of DBT in water, the growth kinetics of ZD-1 was studied with Tween-80 existence. The growth kinetics of ZD-1 met the Monod-equation, meanwhile, μ_max= 0.134 hr^<sub>-1 and k_m =0.016 mmol/l.The behavior of DBT degradation by ZD-1 in aqueous phase was investigated. 2-HBP inhibited the growth of strain ZD-1, the production of DBT degradation enzymes, and the activity of enzymes. Sulfate inhibited the production of dibenzothiophene (DBT) degradation enzymes but had no effect on the enzymes' activity. The production rates of 2-HBP at lower cell densities were higher and the maximum amount conversion of DBT to 2-HBP (0.067mmol/l) after 8 h was gained at 9.2 (g dry cell/1) rather higher cell density.The effects of oil/water volume ratio, composition of aqueous phase, gas flow rate, and stirrer speed on the DBT degradation were studied. It was found that the optimum oil-water ratio and rotation rate were 1: 2. Growing cells could degrade DBT for 48 h, which was much longer than resting cells in potassium phosphate buffer (pH 7.0) or physiological saline (both only 7h). Meanwhile, Growing cells had higher desufurization activity than the others. As the culture media were refreshed regularly, ZD-1 could keep high desulfurization activity up to 200 h. The optimum conditions in stirred reactor were high gas flow rate and 1300 r/min.Microbacterium sp. ZD-M2, a strain isolated lately, had higher desulfurization activity thanZD-1 weather in aqueous phase or in oil-water system. Furthermore, ZD-M2 could degrade 4,6-dimethyl-DBT, thiophene, benzothiophene and 70% diphenylsulfid. Thus, more research work was done for ZD-M2.The optimum conditions for ZD-M2 were 1:1 oil/water ratio, 1140r/min stirrer speed. The cells almost had same desulfurization activity in culture media, potassium phosphate buffer (pH 7.0) or physiological saline. When the cell concentration of ZD-M2 was from 5.6 g dry cell/1 to 14.0 g dry cell/1, the specific production rate of 2-HBP were almost same, 0.200 mmol (DBT) /kg dry cell/min, or 2.21mg (DBT)/ g/ h. It was found that high concentration of DBT inhibited biodesulfurization of ZD-M2. Biochemical reaction kinetics of desulfurization by ZD-M2 met substrate inhibition model, r_max =0.806 mmol/kg dry cell/min, K_m =4.395 mmol/1, K_s=1.984 mmol/l.The effects of surfactants enhanced on the DBT degradation were researched. Among Brij, Tween-80, Triton-100X...