Green And Efficient Entrainers For Distillation Of Acetic Acid From Aqueous Solution By Ionic Liquids

Acetic acid is an important bulk chemical raw material and solvent in pharmaceutical and petroleum industry, the annual demand of acetic acid is about 11 million of tons in the world. A large amount of acetic acid and water mixtures are produced in these processes, such as the production of chemical fiber material pure terephthalic acid (PTA), in which acetic acid is used as solvent and detergent. Because the oxidation reaction could produce water in the production of PTA, every year millions of tons of acetic acid need to be concentrated and purified from aqueous solutions, and return to the oxidation reactor to reuse. On the other hand, acetic acid may be a kind of common by-product in biomass utilization. For example, the global production of furfural can produce at least 5 millions of tons waste water containing low concentration of acetic acid every year. As acetic acid can cause a significant chemical oxygen demand (COD) to water discharge and bring substantial public health problems. It is demanded to remove, recover, concentrate, and purify acetic acid from aqueous solutions, which has a great significance for environmental protection and resource utilization.At present, the methods of separation of acetic acid form aqueous solution include distillation, biological degradation, adsorption, pervaporation, extraction, esterification, neutralization, and so on, in which distillation is more mature and large production capacity for industrial production. Because of the low relative volatility and close boiling points between water and acetic acid, the energy consumption and the production cost of the ordinary distillation are very high. Extractive distillation has been proved to be an efficient method for this separation, and several solvents, such as dimethyl sulfoxide (DMSO), N, N- dimethylformamide (DMF), sulfolane, adiponitrile, N-methyl pyrrolidone, and N-methyl acetamide, have been used as entrainers for the separation. But the use of a great deal of toxic or hazardous solvents may cause secondary pollutions, and developing methods in an environment-friendly way attracts much attention.Ionic liquids (ILs) are environment-friendly solvents and promising alternative of the traditional organic solvents. It can be used for a variety of chemical processes, such as separation, reaction, catalysis and extaction. Arlt and co-workers were the first group to suggest ILs as selective compounds for the separation of azeotropic mixtures, and found that it could break a variety of azeotropic systems. The effect of various ILs was reported by other researchers. However, most of these studies concentrated on the alcohol azeotropic systems containing ILs. The system of acetic acid+water with close boiling point lacked corresponding research work.Thus, the entrainer performances of forty-two kinds of hydrophilic ILs for the separation were investigated in this work. The effects of ILs anion, cation core and cation alkyl side chain length on the separation were discussed. For the entrainer performance of acetic acid from aqueous solution, ILs anion had a crucial impact on the separation, imidazolium cation core was better than piperidinium, pyrrolidinium, quaternary ammonium and pyridinium cation cores, and the growth of ILs cation alkyl side chain length could slightly increase the relative volatility. Hydrophilic ILs with imidazolium cation and alkylphosphate anion could effectively increase the relative volatility of the system which showed a remarkable entrainer performance.Isobaric vapor-liquid equilibrium (VLE) of the systems water+acetic acid containing ILs 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM][DEP]) or 1-butyl-3-methylimidazolium dibutylphosphate ([BMIM][DBP]) were determined at 101.32 kPa by using an improved Rose-Williams equilibrium still. After the addition of [BMIM][DBP], the boiling point difference of the water+acetic acid system increased from 17.88 K (0 wt% IL) to 41.13 K (about 57 wt% IL), and the relative volatility (x1’=0.9) increased from 1.60 (0 wt%IL) to 9.81 (about 57 wt% IL). The recyclability and reusability [BMIM][DBP] was also tested.Based on the chemical theory and Hayden-O’Connell (HOC) method, the parameters of NRTL model for the binary systems were obtained, and the predicted values of the ternary systems of water+acetic acid+[EMIM][DEP] or [BMIM][DBP] fitted the experimental VLE data well.At last, intermolecular interactions mechanism between ILs and acetic acid aqueous solutions was inferred. Structure of minimum energy found for [BMIM][DBP] was performed with the semi empirical optimization program PM3 of Chem3D Ultra8.0. The proton donating ability of acetic acid molecule is stronger than water molecule, so the interaction between acetic acid and [BMIM][DBP] is much higher than that of water and [BMIM][DBP], and hydrogen-bonding interaction (P11=O12…H-O) in hydrogen atom of acetic acid and oxygen atom of [BMIM][DBP] anion tends to form. As a result, it can make the relative volatility of water and acetic acid increase greatly. When adjusting [BMIM][DBP] recovery temperature and pressure, the hydrogen bond can relieve, and the original molecule restored. The interaction between acetic acid and [BMIM][DBP] can be verified through transform infrared spectroscopy, which could provide useful information for identifying the presence of certain functional groups or chemical bonds, and new carboxylic-salt function group do not formed for the mixture of acetic acid and [BMIM][DBP].This method is green for the separation of acetic acid, and also promised for the separation of other close boiling points systems.