Determination of various sulfur compounds in indigenous crude oil and the development of biochemical desulphurization techniques

Abstract

Sulfur in crude oil and in the products derived from it, is perhaps the most talked about subject in public because it has a pivotal bearing on the environment on one side and nuisance to the refiner on the other side. During combustion, sulfur and its analogues are oxidized to SOx which are thrown into the environment with several implications. The public awareness in term of pollutants has grown up tremendously, hence the refiners have to resort to desulfurize even residues before they are put to use. The present study was focused on to desulfurize the distillate fractions chemically as well as biologically to make such products sweet for environmentally friendly utilization and to help the refiners to remove even the least traces of sulfur and to enable them to adhere strictly with environmental codes and regulations in vogue. Crude oil sample was collected from Attock oil refinery (Pakistan) and was fractionated into three distillate fractions i.e. light fraction; b.pt 30-1000C, middle fraction; b.pt 101-2000C and heavy fraction; b.pt 201-3000C. In the first set of experiments, each of the fractions was scrubbed with four different chemical reagents e.g. HCl, NaOH, Na2HAsO4 and NH4VO3, separately. The various sulphur moieties like elemental sulfur, total sulfides, aliphatic sulfides, mercaptan sulfur, hydrogen sulfides, dibenzothiophene and benzothiophene were determined in original oil fractions and then in residual oil fractions from each chemical scrubbing. The percent reduction in sulphur anologues was also calculated to decide about the suitability of each scrubbing agent understudy for removal of a particular sulphur moiety. In the second set of experiments, oil fractions understudy were cultivated with two bacterial strains cultured in coal mine water collected fro Much coal, mine, Pakistan. The bacterial strains identified were pseudomonas putida and thiobacillus thioxidans. The inoculations were performed for time duration of 12 hr, 24 hr, 48 hr, one week, and one month. Sulfur and its analogues in the original and residual samples from chemical and biological treatments were investigated by using various analytical protocols. Elemental sulfur, total sulfides, aliphatic sulfides, mercaptan sulfur, & hydrogen sulfides were determined by gravimetric, potentiometric, titrimetric, and UV spectrophotometric methods. GC Mass specrtrophotometry was used for identification of dibenzothiophene and benzothiophene. From the results obtained with chemical desulphurization, it is inferred that ammonium vendate (NH4VO3) is proposed to be the best desulphurization reagent among the four reagents studied which caused significant sulfur depletion. Among the bacterial strains, thiobacillus thioxidans was found to be the best than pseudomonas putida in term of sulfur removal capability. T. thioxidans caused 72% sulfur depletion compared to Ps. putida which reduced nearly 63 % sulphur in one month inoculation time. The percent reduction data obtained for all inoculations performed reveals that desulphurization was enroute when the time was extended beyond 12 hr even up to one month .However, form the reduction data calculated between 00 hr-12 hr, 12 hr-24 hr, 24 hr-48 hr, 48 hr-one week, and one week- one month inoculations, it is suggested that 12 hr is the optimum time for significant removal of sulphur moieties. From the comparative study of both chemical and biochemical desulphurization attempted, it is note worthy that biodesulfurization is more easy, economical, less tedious, more environmentally friendly method than chemical desulphurization.