High-Pressure Phase Equilibria of Ionic Liquids and Compressed Gases for Applications in Reactions and Absorption Refrigeration

Abstract

Environmental concerns using volatile organic compounds have attracted intensive research of replacing them with more sustainable ("greener") solvents. Ionic liquids have been promising alternatives due to their unique physical and chemical properties, especially their lack of volatility. However, using ionic liquids over common organic solvents has several challenges, i.e., higher viscosity (lower diffusivity) than common organic solvents; lower solubility of reaction gases and large number of high-melting solids not liquids at processing conditions. Coupling ionic liquids with compressed gases systems may overcome most of these difficulties for their applications in separations, reactions, materials processing and engineering applications. To further develop these processes, phase behavior and phase equilibrium knowledge are of essential significance. The main objective of this research is to investigate high pressure global phase behavior and measure phase equilibrium of ionic liquids and compressed gases for their applications as reaction media for hydrogenation and hydroformylation reaction and as working fluids for absorption air conditioning systems. This research investigates imidazolium ionic liquids with various alkyl groups and anions and two compressed gases: carbon dioxide (CO2) and 1,1,1,2-Tetrafluoroethane (R-134a). The global phase behavior and phase equilibria are measured in the temperature range from approximately 0C to 105C and pressure up to 330 bar. Binary systems of R-134a with ionic liquids of [EMIm][Tf2N], [HMIm][Tf2N], [HMIm][PF6], [HMIm][BF4], and [BMIm][PF6] indicate a Type V system according to the classification of Scott and van Konynenburg. Regions of multiphase equilibria exist, viz. vapor liquid equilibrium, vapor liquid liquid equilibrium, liquid liquid equilibrium; while Type III phase behavior for [HMIm][Br] and R-134a is observed, which provides a novel purification method to separate [HMIm][Tf2N] from [HMIm][Br] after synthesis. The phase behavior of CO2 and all ionic liquids indicate Type III systems. The effects of ionic liquid structures on the solubility, lower critical end-point (LCEP) and mixture critical points are also investigated. Complete phase behavior study with the reactant, product, CO2 and ionic liquids for hydrogenation and hydroformylation reactions are accomplished, and volume expansion and molar volume data of liquid phase with CO2 pressure are measured simultaneously. Two regions of reaction rate with different primary phenomena can be explained by phase behavior knowledge: dilution effect and reactant partitioning. The unique phase properties of IL/CO2 suggest an ease product purification process and make it a favorable biphasic solvent system. Detailed phase behavior and equilibrium help understand fully the kinetics results, determine the operating conditions, choose appropriate separation process and properly design an optimal reaction system. The understanding and quantitative modeling of the high-pressure phase behavior and equilibria data are essential for process design and simulation. The Peng-Robinson equation of state model with van der Waals 2-parameter mixing rule is chosen to correlate the experimental data and predict phase equilibrium. Thermodynamic modeling of an absorption air conditioning system using ILs and compressed gases as working fluids is developed. This work reveals that a biphasic reaction system with IL/CO2 for homogeneously catalyzed reactions provides a highly tunable, flexible and economic platform for reactions and separations. However, without understanding the phase equilibrium, the kinetics results cannot be properly interpreted. This work also demonstrates that the absorption refrigeration system using ionic liquids and compressed gases in vehicles is feasible using just the waste heat from the engine and can provide comparable cooling capacity as vapor compression system. The common vapor compression system diverts work from the engine to power the air conditioning system and this adds to fuel consumption and pollution.