Phase equilibrium measurements and thermodynamic modeling of {CO₂ + diethyl succinate + cosolvent} systems

Abstract

This work reports experimental phase equilibrium data for the binary systems {CO₂(1) + diethyl succinate(2)} and {CO₂(1) + ethyl acetate(2)}, and for the ternary systems {CO₂(1) + diethyl succinate(2) + ethanol(3)} and {CO₂(1) + diethyl succinate(2) + ethyl acetate(3)}. The experiments for the binary system {CO₂(1) + diethyl succinate(2)} were carried out at temperatures ranging from 308 K to 358 K, and temperatures ranging from 303 K to 343 K for {CO₂(1) + ethyl acetate(2)} as well as for all ternary systems. Only vapor-liquid equilibria were observed for all systems. The experimental measurements were carried out in a high-pressure variable-volume view cell containing a movable piston, which allows pressure control inside the cell. Binary and ternary systems were modeled with the Peng-Robinson cubic equation of state with the van der Waals quadratic mixing rule (vdW2). The presence of a cosolvent (ethanol and ethyl acetate) decreased the saturation pressures of either bubble or dew points compared to the binary system CO₂ + diethyl succinate, increasing the solubility of diethyl succinate in CO₂.

Introduction

Diethyl succinate (DES) is a diester that occurs naturally in plants. It can be used as a flavoring agent, in fragrances, synthesis of polyesters, plasticizers, food additives, chemical intermediates [[1], [2], [3], [4]], and as a solvent to recover carboxylic acids [5]. A potential method of recovery of DES from plants is via extraction with supercritical carbon dioxide (scCO₂). On the other hand, esters such as DES can be synthesized from the esterification of carboxylic acids either catalyzed by enzymes or chemical catalysts in scCO₂ medium [[6], [7], [8], [9]]. Another very promising process involving is the CO₂ capture with DES as solvent [10]. In all these processes mentioned, a cosolvent use can be an interesting technical strategy in order to enhance the DES solubility in CO₂ at high pressure conditions or to promote a synergic effect of mixed solvents. However, phase equilibrium data are imperative for the design of a larger scale process. Phase equilibrium data at high pressures for supercritical fluid systems are scarce and as supercritical fluid applications are increasing, the collection of such data has becoming more important. Supercritical carbon dioxide in particular has been widely studied as a promising alternative solvent for chemical reaction and separation processes because it is nontoxic, inexpensive, easily recycled and has low critical temperature and pressure (T_c = 304.2 K, p_c = 7.38 MPa, respectively) [11]. Favorable large-scale application solvents should ideally be highly selective, non-viscous, chemically stable and non-corrosive. CO₂ fulfills these requirements. Moreover, as opposed to organic chemical solvents such as methanol, propylene carbonate, and polyethylene glycol dimethyl ether (Selexol) the recovery of which in the desorption stage can be problematic and/or energy intensive, CO₂ can be easily recovered by a change in temperature or pressure in the desorption process [10]. Thus, it forms an effective and low-cost physical absorbent, suitable for large-scale industrial applications.

Literature has reported that diethyl succinate (DES) is considered an environmentally adequate solvent because of its low volatility and nearly zero solvent loss, as stated by Li et al. [10]. Feng et al. [3] presented vapor-liquid equilibrium (VLE) data from 308.15 to 328.15 K and pressures up to 13 MPa for the system CO₂+DES using a semi-flow type apparatus. Li et al. [10] used a gas-phase recirculation method to determine the VLE data from 288.15 to 318.15 K and pressures up to 2.99 MPa. Gui et al. [12] used the constant-volume method to perform VLE experiments at different temperatures in the range of 285.19–313.26 K and up to 6 MPa. However, a high discrepancy between the VLE data of CO₂ + DES presented by those authors can be noted; the most likely reason for such differences in the phase equilibrium measured is because they used a technique with phase sampling. To the best of our knowledge, VLE data of CO₂ + DES have not been reported in the literature so far using a synthetic variable-volume view cell (without sampling). Furthermore, no experimental information has been reported on ternary systems involving CO₂ + DES + cosolvents.

In this context, the main goal of this work is to report experimental phase equilibrium data for the binary systems {CO₂(1)+DES(2)} and {CO₂(1)+ ethyl acetate(2)} and for the ternary systems {CO₂(1) + DES(2) + ethanol(3)} and {CO₂(1) + DES(2) + ethyl acetate(3)}. The experimental results for the systems investigated were modeled using the Peng-Robinson equation of state (PR-EoS) with the conventional quadratic van der Waals mixing rule (vdW2).