Scientific Journal of Frontier Chemical Development March 2014, Volume 4, Issue 1, PP.1-5
Progress of SO2 Absorption by Immobilized Ionic Liquids Jinping Tian1 , 2, #, Haidong Guo 2, Zhiqiang Wang 2, Bingzheng Li 2, Yongkang Lv 1 1. College of Chemistry & Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, China. 2. School of Environment & Safety, Taiyuan University of Science and Technology, Taiyuan, 030024, China. #Email: jpttyust@126.com
Abstract The traditional flue gas purification technology has many drawbacks, then a new and efficient method of treating SO2 in flue gas is focused on protecting current environment. Immobilized ionic liquids has its unique advantages in gas absorption. This paper discussed the hazard of SO2 in the flue gas, current technologies pertaining to SO2 treatment, and analyzed the advantages and disadvantages of these techniques. It is obtained that immobilized ionic liquid can be applied in the treatment of SO2 in flue gas. Finally, the paper prospected SO2 recovery prospects by immobilized ionic liquids and future research directions. Keywords: Immobilized Ionic Liquid; SO2; Absorption
1 INTRODUCTION Sulphur dioxide (SO2) is one of the major air pollutants. High concentration of SO2 emitted from power factories would seriously pollute the atmosphere, resulting in haze, a large area of acid rain, corrosion construction, and endangers against humanity and the health of animal and plant. Therefore, it is the most important work for environmental researchers to effectively reduce emission amount of SO2. At present, flue-gas desulphurization (FGD) is the main technical means to control SO2 in the world, national research and development and use of flue gas desulphurization technologies have more than 200 types. Among these, about 20 types are used in industries [1]. In China, there are FGD technology [2], wet limestone-gypsum FGD technology, Spray-drying desulfurization technology, Seawater flue-gas desulphurization, Wet FGD purification technology integration, etc. Wet limestone-gypsum FGD technology and spray-drying desulfurization technology have many drawbacks. For example, machinery materials corrosion, scaling and equipment failures and other problems. It will also create the large energy consumption, waste water treatment needs, higher investment/running costs. Seawater flue-gas desulphurization technology is limited by the geographical disadvantage, so it is only suitable for coastal power plants, and on the sulfur content of fuels expectations [1]. In addition to the above-mentioned shortcomings, in the traditional desulfurization technology, there are still difficulties such as byproduct treatment, complex process, wastewater, secondary pollution and many other issues [3]. Futhermore, now it is also for the most widely used wet FGD technology how to reasonablely dispose of waste after the SO2 absorption, how to defog and other issues.
2 IONIC LIQUIDS AND IMMOBILIZED IONIC LIQUIDS 2.1 Ionic Liquids (ILs) ILs are organic liquids, which are composed entirely of ions at room temperature and the ambient, so ILs have been called room temperature ionic liquids or molten salt, also non-aqueous liquid, and the liquid organic salts, etc. Due to their low vapor pressure, high thermal stability, broad fluid process, high ionic conductivity and designing, etc., ILs have been widely used in organic synthesis, extraction and separation, electrochemical reactions [4-6]. -1www.sjfcd.org
In last ten years, ILs emerged as green materials. ILs can dissolve or absorb a variety of gases including SO2, which is a new method for gas purification (eg desulfurization), and also expand the ILs applications. Compared to conventional molecular solvents [7-13], ILs have special advantages in desulfurization process. In recent years, for absorption and separation of SO2 of ionic liquid, there are guanidine salts, alcohol amines, imidazoles, pyridines, quaternary ammonium salts, etc [14-15]. Ideal ILs for SO2 absorption should have small viscosity, high fluidity, and small gas mass transfer resistance. However, the viscosity of the ionic liquid synthesized are usually larger, and even some ILs appears to be solid. This high viscosity will make the gas-liquid contact surface relatively small, decease the diffusion rate of gas molecules into a viscous medium, etc. [16], which subsequently affects the behavior of the absorption of SO2. Therefore, ionic liquid immobilization method can be taken into account because it can effectively increase the gas-liquid contact area, reduce the viscosity effects of ionic liquid on the SO2 absorption process.
2.2 Immobilization of Ionic Liquids Immobilization of Ionic Liquids is that ionic liquid is immobilized on a solid support to give immobilized ionic liquids (IILs) or solid material having a relatively larger surface area of the ionic liquid. According to various theories of the interaction between the ionic liquid and solid sipport materials, IILs Immobilization can be divided into two types. One is chemical bond-type immobilized ionic liquids (between the ionic liquid and a solid support material by covalent bonding) and the other is physical adsorption between the ionic liquid and a solid carrier material by non-chemically bond. The latter is the easiest immobilization way. Supports are impregnated soaked by ILs, and then remove the excess ILs, which made SILs. SILs have high selectivity and good reusability. ILs is immobilized on supports, which can overcome some shortcomings, e.g. Ils’ high comsumption and low utilization rate, and difficulties in separating ILs from the reactants or products etc. Solid supports materials including silica gel, activated carbon, zeolite, etc. can be used for the Ils [17].
3 PROGRESS IN IONIC LIQUIDS IMMOBILIZED FOR DESULFURIZATION From the relevant literature [18-24] in recent years, we have known that immobilized ionic liquids not only can be applied in oil desulfurization, but also can absorb SO2. Wang et al. [18] made ImCl-silica gel (SG-ImCl) that was prepared by directly immobilized [1-methylimidazole] [Cl] (ImCl) onto silica gel (SG) using co-condensation of ionic liquid modified organosilane and tetraethoxysilane (TEOS) as silica source in strong acid condition.SG-ImCl at SiO2/ILs ratio of 9.5:0.5 and 9:1 can be well remove sulfur in simulating gasoline that is composed of the thiophene, 2-methyl-thiophene, benzothiophene and dibenzothiophene. But when SiO2/ILs at SiO2/ILs ratio of 8:2, which showed poor removal activity, the sulfur adsorption capacity of SG-ImCl is less than that the silica (SG) that was unsupported Ionic Liquid. Esther Kuhlmann et al. [19] studied a series of imidazole phosphate ionic liquid supported on silica for desulfurization of fuel oil model system. Fuel oil model system consisted of the dibenzothiophene/dodecyl and butyl mercaptan/ecane mixture. The results showed that: (1) Due to its large surface area, a single stage extraction process reduce the sulfur content to less than 100 ppm from the initial 500 ppm. However, the only pure ionic liquids reduce to 200 ppm. (2) Multi-stage extraction process, the SILs in the second phase reduce the sulfur content to 50 ppm. While, the pure ionic liquids in the seventh stage only reduce the sulfur content in the range of 10 ppm.(3) it is very active/effective for ionic liquids used by Supported Ionic Liquid technology because of its small film thickness, large specific surface area, and can be applied to the packed bed column extraction technologies. Florian Kohler et al. [20] used equimolar amounts of imidazolium chloride and zinc (II) chloride or tin (II) chloride mixture 24 hours to synthesize dichloromethane ionic liquids at 70℃, and then the Ils immobilized on alumina supports for the removal of sulfur in simulating gasoline (containing 500ppm n-butyl mercaptan). The results showed that: during absorbing at 90 ℃ and atmospheric pressure, the immobilized ionic liquids can have the breakthrough time 830 minutes, and 500 ppm n-butyl mercaptan can be reduced to 5 ppm. Wang et al. [21] synthesized three kinds of chloromethylated polystyrene-supported metal halide/imidazole ionic liquids (M/CMPS-ImCl, M=CuCl, ZnCl2, FeCl3) for the removal of thiophene and its homologues from analog oil. -2www.sjfcd.org
The results showed that: These three SILs are very effective in removing sulfur compounds at room temperature, wherein CuCl/CMPS-ImCl fordesulfurization is best, because of π-complexation intensity between Cu+ and thiophene was stronger than those of Fe3+ and Zn2+. It is noted that, the SILs desulfurization activity after 6 cycles was not significantly reduced. It is common for SILs to desulfurize from oil, and capture of SO2 in flue gas by SILs is now reported in the literatures [23-24]. At different temperatures, various ILs were supported on materials having a high specific surface area. Then this SILs absorb the component (CO2, NO, NO2 and SO2) in flue gas. SILs significantly improved ILs adsorption capacity, and it can absorb low-concentration pollutants such as SO2 in the flue gas, so it is promising for ILs in gas purification techniques [22]. Zhang et al. [23] reported that the ionic liquid 1, 1, 3, 3-tetramethylguanidinium lactate (TMGL) was supported onto porous silica particles to adsorb SO2. TMGL-SiO2 capacity reached 0.6 g SO2/gTMGL in 15-30 min with pure SO2 gas and 0.15 g SO2/g TMGL in 17h with a N2/SO2 mixture gas that contained 2160 ppm SO2. The SO2 concentration was reduced to 12.6 ppm after sorption. Li [24] reported that the ionic liquid 1-pentyl-3-methylimidazolium Bromide ([C5MIM] Br) was supported onto silica particles to adsorb SO2.The results showed that: the absorbption capacity of silica gel for SO2 improved after being loaded by [C5MIM] Br. The internal pores of silica gel changed and the specific surface areas reduced.ILs, meanwhile, increased the SO2-absorbing capacity. [C5MIM] Br/SiO2 has the largest eqilibrium absorption amounts (0.21gSO2/g [C5MIM] Br/SiO2) when mass ratio of ILs and SiO2 is 0.5:1.Via analysis of the infrared spectrum, it was found that both ILs and silica gel interacted with SO2.
4 IMMOBILIZED IONIC LIQUIDS SO2 ADSORPTION PROSPECTS Utilization of SO2 in flue gas as a resource can not only reduce even eliminate environmental pollution resulted from SO2 released from factories into atmosphere, but also provides the raw materials for the production of sulfuric acid and other chemical products. Generally lower concentrations of SO2 in the flue gas, cannot be directly used as raw materials due to production high cost.SO2 that was captured by conventional methods cannot be resource utilization. The study shows that SILs as a novel functional material has a clear advantage in absorption of SO2: it can absorb low concentration of SO2 in the flue gas; and SO2 absorption is reversible, i.e. It can be reused; Kinetic parameters of adsorption/desorption can be designed by changing the temperature, pressure and gas concentration. Compared with the pure ionic liquid, its viscosity significantly decreases, and thus reduces the absorption resistance. Therefore, these SILs can provide with a novel technique that can achieve utilization of SO2. Currently, the researches on SO2 adsorption of SILs are still in the laboratory stage, the effect of other components of its actual flue gas on adsorption behavior is not clear, the mechanism of adsorption of SO2 should be also studied, as well as whether it will also be investigated that whether Ils affect the organisms or not. Therefore, life evaluation (toxicological evaluation) of SILs and absorption mechanism of SILs for SO2 should be taken into account in future studies. With further study of the immobilized ionic liquid, SILs will play an important role in the flue gas purification.
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AUTHOR Jinping Tian (1963- ), Senior Engineer, graduated from Taiyuan Institute of Technology and major in Polymer Chemicals (1983.07), member of ChemicalIndustry and Engineering Society of China, expert of Department of Environmental Protection of Shanxi Province. Major fields of study: Environment-friendly materials, Environmental Chemical Technology. Email: jpttyust@126.com
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