STUDIES ON SOLVENT EXTRACTIVE DESULFURIZATION OF GAS OIL

Abstract

Gas oil is extensively used in the transportation vehicles such as cars, buses, trucks, locomotives, marines, etc.; and static equipment such as generators, farm pumping set, engines, etc. Diesel share among transportation fuels and its demand has grown significantly over the last few years and this trend is expected to continue for the coming years due to inherent benefits, such as low market price and higher calorific value and higher engine thermal efficiency of diesel in comparison to gasoline. However, its combustion contributes significant harmful emission of nitrogen and sulfur oxides, unburnt hydrocarbons and particulate matter to the environment. This leads to serious environmental and health concerns such as smog, global warming and water pollution, acid rain, cancer, neurotoxicity, etc. [Srivastava, 2012]. Quantities of these emissions increase with an increase in concentration of sulfur, nitrogen and aromatic compounds in gas oil. Therefore, environmental regulations have been implemented across the globe to limit the sulfur and aromatic content of gas oil for improving the air quality. Thus, deep removal of sulfur and poly aromatics from gas oil is the need of the hour to minimize the emissions of oxides of sulfur, poly aromatic hydrocarbons (PAH), particulate fines, etc.; to minimize corrosion and wear of engine systems; and to improve the performance of emission control technologies. Currently, refining industry is facing a serious challenge of meeting the increasing demand of gas oil with required stringent specifications. This challenge shall become more serious in future due to necessity of processing the sour and heavy crude. Hydro-treatment is the well established process for removal for sulfur and poly aromatics from gas oil in the refineries. It is known that benzothiophene (BT), dibenzothiophene (DBT) and their alkylated derivates are refractory sulfur compounds which remain in hydrotreated gas oil as well. The condensed polyaromatics in gas oil act as inhibitors during hydrotreatment of refractive sulfur compounds because of strong competition among aromatic and sulfur compounds for adsorption on catalyst active sites. Nitrogen compounds such as carbazole and acridine (dibenzo[b,e]pyridine) retard the performance of hydrotreatment even at low concentrations . Deep removal of these impurities using hydrotreatment requires very severe operating conditions of temperature and pressure, expensive catalysts, revamp of existing hydrogen plant for capacity enhancement or installation of new H2 generation plant so as to meet the significantly increased H2 consumption and H2S-free H2 recirculation. These activities require significant increase in operating cost and huge capital investment requirement to revamp the existing facilities [Srivastava, 2012]. Therefore, in this competitive world, when refinery margin for processing a barrel of crude is decreasing, refineries are looking for optimizing the conversion and investment cost for meeting the new stringent ultra low sulfur specification in gas oil. Abstract iii Various researchers are working in the area of development of nonhydrodesulfurization methods such as oxidative desulfurizataion (ODS) [Arellano et al., 2014; Yu et al., 2013; Maity et al., 2012; Bhasarkar et al., 2013], selective adsorptive desulfurization (ADS) [Nair and Tatarchuk, 2011; Shalaby et al., 2009], biodesulfurization (BDS) [Agarwal and Sharma, 2010; Mukhopadhyaya et al., 2007], and solvent extraction desulfurization (SEDS) [Rodríguez-Cabo et al., 2012] that have the potential to be used as either stand alone or as complementary method with hydrodesulfurization for cost effective production of ultra clean gas oil. Desulfurization process can be made more economical by using combination of processes. SEDS process can remove the refractive sulfur compounds, polyaromatics and nitrogen compounds to a very large extent [Gaile et al., 2010] and can be used prior to conventional hydrotreatment process for attaining the goal of deep desulfurization at lower capital and operating costs than required in standalone hydrotreatment method. Although, solvent extraction is widely adapted process in refining industries for production/removal of aromatics, however, its application for gas oil desulfurization is in research and development stage. Some computational studies for solvent screening using the capacity, selectivity and performance index (capacity × selectivity) at infinite dilution as performance indicators for removal of model sulfur, aromatic and nitrogen compounds have been reported [Anantharaj and Banerjee, 2011a,b]. In these studies, target compounds are thiophenes, BT and DBT sulfur compounds and the solvent is ionic liquid. Removal of sulfur, aromatics and nitrogen compounds from liquid fuels using single-stage batch and multistage (set of separating funnels) continuous counter-current solvent extraction have also been reported [Gaile et al., 2010]. These studies are based on either straight run gas oil (SRGO) or hydrotreated gas oil or vacuum gas oil feed stocks. Further, either degree of sulfur removal (Dsr) or sulfur removal capacity and yield of gas oil have been used as solvent performance indicators. An exhaustive literature review reveals that there are number of aspects of extractive desulfurization of gas which need to be studied for paving the path of its successful commercialization of SEDS in the refinery. Considering various possibilities, work on various aspects was carried out in the present study to bridge some research gaps and provide the insight of extractive desulfurization capabilities for desulfurization of gas oil streams. In the first part of this work, a new strategy for a realistic and practical screening of solvents for removal of highly refractory sulfur and nitrogen compounds from gas oil has been evolved and presented. Two major class of solvents vis a vis conventional organic solvents and ILs are used for aromatic, sulfur and nitrogen compounds removal from Abstract iv hydrocarbon streams. Six most widely used industrially proven conventional organic solvents and twenty two imidazolium based IL solvents were selected for removal of BT, DBT and their alkylated derivatives, and nitrogen compounds from gas oil. The solubility parameters, molar volume, van der Waals volume of sulfur, aromatics and nitrogen compounds which can represent the gas oil were estimated using ab initio molecular dynamics method. These parameters were used for estimating the standard heat of vaporization and activity coefficients at infinite dilution of model gas oil compounds in solvents using available correlations. The capacity, selectivity and performance index (PI) of solvents were estimated for selected sulfur and nitrogen compounds. To understand the effect of complexity of solvent recovery section on their industrial utilization, two type of solvent recovery sections were conceptualized for recovering of solvents: one having boiling point lower than gas oil, and the other, for solvents having boiling point in the range gas oil distillation. Based on complexity of recovery section, a new industrial usability index (SIUI) of solvent was defined and used for their rating for sulfur and nitrogen compounds from gas oil. The solvents were ranked for removal of BT, DBT and their alkylated derivatives sulfur compounds, quinoline, indole and carbazole nitrogen compounds from gas oil. It was observed that selectivity and capacity values of solvents for nitrogen compounds are higher than most of the sulfur compounds. Rankings based on selectivity and capacity correlated well with the solubility parameter. It was also observed that ranking of solvents strongly depends on the parameter/index selected for the ranking. PI which combines the effect of both capacity and selectivity seems to be better index than individual capacity and selectivity indexes to rank the solvents. Industrial usability index (SIUI) of solvents which includes PI and process complexity factor of solvent recovery seems more practical and realistic criteria to be used for solvents assessment for a given separation. 4,6 dimethyl DBT and quinoline WEre most refractory sulfur and nitrogen compounds to be removed among selected compounds in the study. Overall, organic solvents were found to be better solvents for desulfurization and denitrogenation of gas oil as compared to ionic liquid solvents. There was no single solvent which ranked 1st for all sulfur and nitrogen compounds removal from gas oil. Therefore, the detailed sulfur component analysis of gas oil is plays an important role in selection of solvent. Moreover, it seems that the best solvent should have moderate capacity, selectivity and lower boiling point than gas oil. Further, experimental evaluation of industrially proven and viable organic solvents was carried out for extraction of sulfur and polyaromatics impurities from actual SRGO containing 1.3 wt% sulfur. Effect of extraction temperature, solvent to feed ratio, anti-solvent concentration and number of stages (during batch operation) on the Dsr and aromatics Abstract v removal and yield were evaluated in batch and continuous counter current extraction system. Performance of a solvent extraction process which is governed by Dsr and yield of extracted SRGO (ESRGO) was evaluated in terms of a performance factor (Pf,α) which had been defined in terms of weight factor (0 < α < 1) as: Pf,α=α Dsr+(1-α) yield. DMF solvent was found to be better solvent in terms of Pf,α and regeneration point of views. Comparative analysis of degree of sulfur, di-aromatics and poly-aromatics removal during batch and continuous extraction using N-N-dimethyl formamide (DMF) as solvent at various experimental conditions reveal that water concentration in solvent changes the value of ESRGO yield and impurities removal significantly. Extraction temperature (TE) and water content in solvent gives the flexibility to adjust the yield and degree of removal of impurities to maximize the benefit in a given situation. Continuous counter current extraction is much more effective than the single stage extraction. Selection of weight factor for sulfur removal and yield affects the performance factor of extraction process and need utmost care in its value selection. To optimize the operating variables for continuous SEDA, a full factorial central composite design (CCD) method was used to design the experiments for extractive desulfurization of SRGO in packed bed extractor using DMF as solvent. The operational parameters namely anti-solvent water concentration (Wc) in main solvent, solvent to feed ratio (S/F), and extraction temperature (TE) which affect the sulfur removal and yield were used as input variables in design of experiments. Considering the trade off phenomenon between sulfur removal and yield, multi-response optimization with desirability function approach was used to estimate the optimized value of these operating parameters so as to maximize sulfur removal and yield of ESRGO. Optimum values of selected variables were: water content in solvent=2.91 vol.%, solvent to feed ratio=1.70 and extraction temperature=46.4oC. At the maximum desirability value, ESRGO yield and percent sulfur removal were 81.7% and 60.5%, respectively. Since, importance of sulfur removal and yield would depend on the secondary process to be selected for reducing the sulfur to 50-10 ppm, an analysis of goal importance effect on optimized value of operational parameters for maximum desirability was also studied. Solvent extraction works on the principle of relative solubility of feed compounds in solvent. There are various gas oil streams which have significant different composition of paraffines, naphthenes, aromatics and sulfur compounds refinery. Therefore, to illustrate the effect of sulfur compounds molecular structure on its extractability and to understand the effect of carrier phase composition on Dsr, SEDS of various synthetic sulfur compounds from synthetic carrier phases of different composition was carried out using DMF as solvent. It Abstract vi was found that extractability of sulfur compounds strongly depends on the its nature and carrier phase composition. Moreover, DMF solvent extraction of SRGO, LCO, CGO and MGO samples (having significantly different sulfur compounds and composition) further verified that performance parameters such as yield of raffinate, Dsr, distribution coefficient, extraction factor and Pf,α depend on gas oil sample composition. To understand the effect of extraction operating conditions and importance of use of water in solvent for enhancing the extractive desulfurization performance, solvent extraction SRGO, LCO, CGO and MGO was studied using industrial N-methyl-2-pyrrolidone (NMP) solvent in single stage batch extractor and continuous counter current packed bed extraction column. Effect of TE, S/F, Wc on Dsr, yield of extracted gas oil (Y%) and Pf,α was studied in single stage batch extractor for SRGO, LCO and CGO. After optimizing the operating conditions for SRGO, LCO, and CGO in single stage batch extractor, studies on mixed gas oil (MGO) were carried out in single stage batch extractor and in continuous counter current packed bed at estimated optimized values of TE, S/F and Wc. The major issues associated with solvent extraction for gas oil desulfurization are to minimize the loss of valuable hydrocarbon with extract and value addition to extract hydrocarbon. Both these issues have been addressed in the present study by generating pseudo-raffinate from the extract phase using water as antisolvent so as to minimize the loss of valuable hydrocarbon with extract and improving the quality (BMCI) of extract hydrocarbon for their utilization as CBFS in black carbon generation process. It may be mentioned that the BMCI values and sulfur content of extract hydrocarbon was found in the range for CBFS which are being marketed by refineries in India. Quantitative evaluation of distillate products from processing of the pseudo raffinate (generated from the extract phase using antisolvent) in hydrocracker and fluid catalytic cracking (FCC) processes was also carried out. The benefits and befitting of disposal of extract hydrocarbon in delayed coker as blending stream with vacuum residue (VR) was explored.