Guangren Yu

A review of extractive desulfurization of fuel oils using ionic liquids

Hydrodesulfurization (HDS), a widely employed method in industries for the desulfurization of fuel oils, such as gasoline and diesel fuel is faced with the challenge of producing lower-sulfur or sulfur-free fuel oils, which are required by more and more countries. However, HDS is not very effective for the removal of thiophenic sulfur compounds due to sterically-hindered adsorption on the catalyst surface, unless operated under harsh conditions, such as high temperature, high pressure, and requirement of a noble catalyst and hydrogen. Extractive desulfurization (EDS) of fuel oils using ionic liquids (ILs) has been intensively studied in recent decades and has a good future as an alternative or complementary method to HDS. In this review, we reviewed the research results of EDS using ILs and provided comprehensive discussions on diverse factors, which influence desulfurization, such as the IL species, IL–oil mass ratio, initial sulfur content, temperature, time, mutual solubility, multiple extractions and regeneration. Potential problems or shortcomings were also stated. Some other desulfurization methods currently under study, such as extraction, oxidation, adsorption and biodesulfurization were also briefly outlined. It can be inferred that ILs remain a class of ideal solvents to realize clean fuel oil in the near future because of their desirable physiochemical properties, which are lacking in molecular organic solvents, while there are possible challenges, such as relatively high viscosity and low efficiency.

Extractive desulfurization of fuel oils with low-viscosity dicyanamide-based ionic liquids

Four low-viscosity ionic liquids (ILs) based on the dicyanamide anion ([N(CN)2]−), i.e., 1-butyl-3-methylimdazolium ([BMI][N(CN)2]), 1-ethyl-3-methylimdazolium ([EMI][N(CN)2]), ethylated tetrahydrothiophenium ([S2][N(CN)2]) and ethyldimethylsulfonium ([EtMe2S][N(CN)2]), have been investigated to determine their extraction capability for thiophene (TS) and dibenzothiophene (DBT) from model fuel oils. Aromatic imidazolium is more efficient than cyclic thiophenium and tetrahedral trialkylsulfonium; specifically, the S-extraction ability follows the order [BMI][N(CN)2] > [EMI][N(CN)2] > [S2][N(CN)2] > [EtMe2S][N(CN)2], with DBT being more efficiently extracted than TS. The S-extraction of [BMI][N(CN)2] has been investigated as a representative with respect to the influence of extraction temperature, IL:oil mass ratio, initial S-content, multiple extractions and reusability, along with its mutual solubility in oil. The percentage of S-removal from gasoline and diesel fuel were 48.5 and 68.7%, respectively, in a single extraction at 25 °C, 1:1(w/w) IL:oil, 5 min; the S-content in gasoline decreased from 599 ppm to 4 ppm after 5 extraction cycles and in diesel fuel decreased from 606 ppm to an undetectable value after 4 cycles. The mutual solubility is not pronounced and the extraction efficiency is not conspicuously changed after 6 regeneration cycles. It is worth noting that a short extraction time of < 5 min is observed for all the ILs at room temperature, which is understood by their low viscosities and effective mass transfer. This work may offer a new option for the deep desulfurization of fuel oils.

Extractive Desulfurization of Fuel Oils with Thiazolium-Based Ionic Liquids

A new class of green solvents, known as ionic liquids (ILs), has recently been the subject of intensive research on the extractive desulfurization of fuel oils because of the limitation of the traditional hydrodesulfurization method in catalytically removing thiophenic sulfur compounds. In this work, four thiazolium-based ILs, that is, 3-butyl-4-methylthiazolium dicyanamide ([BMTH][DCA]), 3-butyl-4-methylthiazolium thiocyanate ([BMTH][SCN]), 3-butyl-4-methylthiazolium hexafluorophosphate ([BMTH][PF6]), and 3-butyl-4-methylthiazolium tetrafluoroborate ([BMTH][BF4]), are synthesized. The extractive capability of these ILs in removing thiophene (TS) and dibenzothiophene (DBT) from model fuel oils is investigated. [BMTH][DCA] and [BMTH][SCN] present better extractive desulfurization capability than [BMTH][BF4] and [BMTH][PF6], which may be ascribed to the additional π−π interaction between –C≡N (in [BMTH][DCA] and [BMTH][SCN]) and thiophenic ring (in TS and DBT); DBT in diesel fuel is more efficiently extracted than TS in gasoline. [BMTH][DCA] offers the best desulfurization results, where 64% and 45% sulfur removal are obtained for DBT and TS, respectively, at IL:oil mass ratio of 1:1, 25°C, 20 min. [BMTH][DCA] is thus selected to systematically investigate the effects of temperature, IL:oil mass ratio, initial sulfur content, multiple-extraction, and IL regeneration on desulfurization. The mutual solubility of [BMTH][DCA] with fuel oil is also determined. It is observed that the desulfurization capability is not too sensitive to temperature and initial sulfur content, which is desired in industrial application; the sulfur contents in gasoline and diesel fuel are reduced from 558 ppm to 20 ppm (after 5 cycles) and from 547 ppm to 8 ppm (after 4 cycles), respectively. This work may show a new option for deep desulfurization of fuel oils.

Extractive denitrogenation of fuel oils with dicyanamide-based ionic liquids

The removal of nitrogen-compounds (N-compounds), e.g. basic and neutral species, from fuel oils is necessary because of their inhibiting effect on the hydrodesulfurization process. In this work, the extractive denitrogenation performance of four dicyanamide-based ionic liquids (ILs) with different cationic characteristics, i.e., aromatic 1-butyl-3-methylimdazolium dicyanamide ([BMI][N(CN)2]) and 1-ethyl-3-methylimdazolium dicyanamide ([EMI][N(CN)2]), cyclic ethylated tetrahydrothiophenium dicyanamide ([S2][N(CN)2]), and tetrahedral ethyldimethylsulfonium dicyanamide ([EtMe2S][N(CN)2]), is investigated using basic pyridine and neutral carbazole as representative N-compounds. These ILs are capable of effectively extracting the N-compounds from the fuel oils with carbazole being more efficiently extracted than pyridine; also, aromatic imidazolium ILs exhibit better performance than cyclic thiophenium and tetrahedral trialkylsulfonium ILs in the order [BMI][N(CN)2] > [EMI][N(CN)2] > [S2][N(CN)2] > [EtMe2S][N(CN)2]. Under ambient conditions, 1 : 1 (w/w) IL : oil, the N-content in the raffinate phase of the carbazole-containing fuel oil is undetected after <5 min of contact with [BMI][N(CN)2] and [EMI][N(CN)2], while 96.8% and 84.3% N-extraction efficiency is obtained after contact with [S2][N(CN)2] and EtMe2S][N(CN)2] respectively; for pyridine-containing fuel oil, the N-extraction efficiency in the aforementioned ILs is 72.7%, 69.1%, 63.5% and 59.8%, respectively. Compared with other ILs reported, the extractive performance of these ILs is competitive. [BMI][N(CN)2] is selected as a representative IL to undergo a series of parallel experiments to determine the influence of IL : oil mass ratio, temperature, initial N-content, and multiple extractions; a recyclability test is also performed. This work may present a new approach to fuel denitrogenation.

Molecular simulations of phosphonium-based ionic liquid

Compared with imidazolium-based ionic liquids (ILs), phosphonium-based ILs have been proven to be more stable in thermodynamics and less expensive to manufacture. In this work, a kind of phosphonium-based IL, [PC6C6C6C14][Tf2N], was studied under several conditions using molecular dynamics simulations based on both the all-atom force field (AAFF) and the united-atom force field. Liquid density was calculated to validate the force field. Compared with experimental data, good agreement was obtained for the simulated density based on the AAFF. Heat capacities at constant pressure were calculated at several temperatures, and good linear relationships were observed. Self-diffusion coefficients, viscosities and conductivities were also calculated to study the dynamics properties of this IL. The viscosity of this IL at 293 K was also compared with experimental data, and the error was in a reasonable range. In order to depict the microstructures of the IL, centre-of-mass and site-to-site radial distribution functions were employed. In addition, spatial distribution functions were investigated to present the more intuitive features.

Computational fluid dynamics study on mixing mode and power consumption in anaerobic mono- and co-digestion

Computational fluid dynamics (CFD) was applied to investigate mixing mode and power consumption in anaerobic mono- and co-digestion. Cattle manure (CM) and corn stover (CS) were used as feedstock and stirred tank reactor (STR) was used as digester. Power numbers obtained by the CFD simulation were compared with those from the experimental correlation. Results showed that the standard k-ε model was more appropriate than other turbulence models. A new index, net power production instead of gas production, was proposed to optimize feedstock ratio for anaerobic co-digestion. Results showed that flow field and power consumption were significantly changed in co-digestion of CM and CS compared with those in mono-digestion of either CM or CS. For different mixing modes, the optimum feedstock ratio for co-digestion changed with net power production. The best option of CM/CS ratio for continuous mixing, intermittent mixing I, and intermittent mixing II were 1:1, 1:1 and 1:3, respectively.

Desulfurization of Real Fuel Oils by Extraction with Ionic Liquids

Aromatic 1-butyl-3-methylimidazolium dicyanamide ([C4mim][N(CN)2]) and 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][N(CN)2]) ionic liquids are tested for their performance in the extractive desulfurization of real FCC gasoline and diesel fuel. [C4mim][N(CN)2] has proven to be more effective than [C2mim][N(CN)2] in removing sulfur from fuels and was thus selected to undergo a series of further tests. A competitive desulfurization efficiency of nearly 40% and 30% was realized with [C4mim][N(CN)2] for diesel fuel and gasoline, respectively, in a single extraction at <1 h, 25°C, and 1:1(w/w)IL:fuel. The influence of IL:fuel mass ratio, temperature, and multiple extractions on S-extraction efficiency is investigated, and the result tends to favor large-scale industrial application. This high efficiency obtained at low temperature, together with the insensitivity of the Nernst partition coefficient on desulfurization efficiency, is industrially favorable because not much energy and cost are required. The influence of mass ratios is not obvious, but to some degree, the Nernst partition coefficients depend on the mass ratio, suggesting that this extraction is not a completely physically-determined extraction. This work offers a significant contribution to the production of clean oils by extraction with ionic liquids.

Extractive Desulfurization of Fuel Oils with Dicyano(nitroso)methanide-based Ionic Liquids

The inability of traditional hydrodesulfurization (HDS) to effectively remove aromatic sulfur compounds such as thiophene (TS) and dibenzothiophene (DBT) has called for alternative methods to be studied, among which extractive desulfurization using ionic liquids (ILs) has attracted increasing interest. In this work, we prepared a new IL, 1-butyl-3-methylimidazolium dicyano(nitroso)methanide ([C4mim][dcnm]), and investigated its extractive desulfurization for both model oils and real FCC gasoline, where model diesel fuel was composed of n-hexane and droplets of DBT and model gasoline was composed of n-hexane, toluene and droplets of TS. Other three [dcnm]-based ILs, 1-ethyl-3-methylimidazolium dicyano(nitroso)methanide ([C2min][dcnm]), N-ethyl-N-methylpyrrolidinium dicyano(nitroso)methanide ([C2mpyr][dcnm]), and N-butyl-N-methylpyrrolidinium dicyano(nitroso)methanide ([C4mpyr][dcnm]), were also comparatively investigated. These [dcnm]-based ILs have low viscosity which favors the mass transfer and reduces the extractive equilibrium time, also are fluorine-free which avoids the corrosion by hydrogen fluoride from anion decomposition that occurs generally in fluorine-containing ILs. The desulfurization ability follows the order [C4min][dcnm] > [C4mpyr][dcnm] > [C2min][dcnm] > [C2mpyr][dcnm]. Typically, [C4min][dcnm] is capable of removing 66% DBT and 53% TS from their respective model oils after one cycle (initial 500 ppm S, 25°C, 15 min, mass ratio of IL:oil 1:1), and < 10 ppm S-content can be obtained after 4 cycles. It was observed interestingly that the S-content in real FCC gasoline can be reduced from initial 250 ppm to < 30 ppm after 6 cycles using [C4min][dcnm] as extractive reagent, which is better than some previous results for real feedstocks. Mutual solubility, extractive temperature, IL:oil mass ratio, multiple extraction, initial S-content, and regeneration were also studied. These dcnm-based ILs are competitive extractive reagents compared with some other ILs to remove those aromatic S-compounds from fuel oils.

Cu(I)-Based Ionic Liquids as Potential Absorbents to Separate Propylene and Propane

The absorption of propylene and propane in Cu(I)-based ionic liquids, i.e., 1-butyl-3-methylimidazolium chloride/CuCl ([Bmim][Cl]/CuCl), N-Methyl pyrrolidone chloride/CuCl ([HNMP][Cl]/CuCl), and tricaprylmethylammonium thiocyanate/CuSCN ([A336][SCN]/CuSCN), are investigated in this work. It is observed that such Cu(I)-based ionic liquids, especially Bmim-based ionic liquids, present good absorption capability for propylene and good selectivity over propane, e.g., 1.0 kilogram [Bmim][Cl]/CuCl is able to absorb 0.08 mol propylene while only 0.006 mol propane at 25°C and 1.3 bar with the selectivity of 13. The effects of pressure, Cu+ concentration, and temperature on the absorption are investigated; in addition, the absorption kinetics of propylene by [Bmim][Cl]/CuCl is obtained. The much higher absorption capability for propylene than propane is ascribed to the π-complexation between propylene and Cu+. This work shows that the absorption by Cu(I)-based ionic liquids is an potential alternative method for traditional cryogenic distillation with high energy cost to separate propylene and propane.

Oxidative Desulfurization of Gasoline by Ionic Liquids Coupled with Extraction by Organic Solvents

In this work, desulfurization of real fluidized catalytic cracking (FCC) gasoline was investigated in dual steps; first in oxidative desulfurization (ODS) using imidazolium and pyrrolidonium based Bronsted acidic ionic liquids (ILs) as solvent and catalyst and hydrogen peroxide as oxidant. In second step, extractive desulfurization took place using organic solvents of furfural, furfural alcohol and ethylene glycol. Variety of factors such as temperature, time, mass ratio of oil/ILs and regeneration and recycling of ILs, multiple-step desulfurization of ILs and organic solvents and solvent/oil ratio were also investigated. The S-content was significantly decreased to ca. 18 ppm from initial S-content of 260 ppm with a total S-removal of ca. 95% in one-step ODS using pyrrolidonium based ILs coupled with five-step extraction desulfurization (EDS) using furfural alcohol as extractant. This work shows that oxidative desulfurization using ionic liquids coupled with extractive desulfurization using organic solvents is a potential method to produce clean gasoline.