Supercritical fluid separations phase equilibria

In this paper, we describe the apparatus we use to make phase equilibrium measurements on mixtures of conqponents with greatly differing volatilities, putting particular emphasis on recent inqprove-ments over the previous version (6-7). We also describe quantitative measurements of the solubility of methyl oleate in supercritical fluids which can provide a basis for choosing a solvent to separate fatty acids in edible oils. In the following paper (JB.) we explore the utility of cubic equations of state to describe the results of supercritical fluid - liquid phase equilibrium measurements. Some additional experimental results on the mutual solubility of methyl linoleate and carbon dioxide are presented there also. 

The basic for developing a high pressure liquid extraction unit is the phase equilibrium for the (at least) ternary system, made up of compound A and compound B, which have to be separated by the supercritical fluid C. Changing pressure and temperature influences on one hand the area of the two phase region, where extraction takes place, and on the other hand the connodes, representing the equilibrium between extract and raffinate phase. 

We refer to Fig. 6.7-1. Reaching once equilibrium between the supercritical fluid SCF1 and the feed in the extractor El is enough for separation. By changing pressure and temperature the produced extract EX1 and raffinate R1 concentrations can be varied following the ternary phase equilibrium. The supercritical solvent-to-feed flow rate ratio affects the amounts of products obtained from a given feed. The apparatus required to apply this method are a normal stirred reactor, where contact of the two phases takes place, followed by a separator eliminating the extract from the extraction gas, which is recycled back to the extractor. 

For any pure chemical species, there exists a critical temperature (Tc) and pressure (Pc) immediately below which an equilibrium exists between the liquid and vapor phases (1). Above these critical points a two-phase system coalesces into a single phase referred to as a supercritical fluid. Supercritical fluids have received a great deal of attention in a number of important scientific fields. Interest is primarily a result of the ease with which the chemical potential of a supercritical fluid can be varied simply by adjustment of the system pressure. That is, one can cover an enormous range of, for example, diffusivities, viscosities, and dielectric constants while maintaining simultaneously the inherent chemical structure of the solvent (1-6). As a consequence of their unique solvating character, supercritical fluids have been used extensively for extractions, chromatographic separations, chemical reaction processes, and enhanced oil recovery (2-6). 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

A model based on a modified mixing rule for the Peng-Robinson equation of state was able to reproduce quantitatively all features of the observed phase equilibrium behavior, with model parameters determined from binary data only. The use of such models may substantially facilitate the task of process design and optimization for separations that utilize supercritical fluids. 

We now want to consider the extent to which a solid is soluble in a liquid, a gas. or a supercritical fluid. (This last case is of interest for supercritical extraction, a new separation method.) To analyze these phenomena we again start with the equality of the species fugacities in each phase. However, since the fluid (either liquid, gas, or supercritical fluid) is not present in the solid, two simplifications arise. First, the equilibrium criterion applies only to the solid solute, which we denote by the subscript 1 and second, the solid phase fugacity of the solute is that of the pure solid. Thus we have the single equilibrium relation... 

Table I. lists a few possible advantages for conducting chemical synthesis in a supercritical environment. For example, supercritical fluids might provide a means to manipulate reaction environments by altering density and temperature to influence both reaction rate and selectivity. Futhermore, in a homogeneous supercritical phase, one can, in principle, eliminate interfacial transport limitations. Effectively, temperature and pressure are used to alter density in a way to influence both solvation dynamics and equilibrium solubility. With supercritical fluid solvents, there is the possibility of integrating both reaction and separation processes, which could lead to economic advantages over conventional synthetic processes carried out in liquid solvents.

In recent years, supercritical technology, especially supercritical carbon dioxide (scCCb), has been widely applied in the processing of polymer nanocomposites. A supercritical fluid is defined as "any substance, the temperature and pressure of which are higher than their critical values, and which has a density close to, or higher than, its critical density" (Darr Poliakoff, 1999). Fig. 3 shows a schematic representation of the density and organization of molecules of a pure fluid in solid state, gas state, liquid state and the supercritical domain. No phase separation occurs for any substance at pressures or temperatures above its critical values. In other words, the critical point represents the highest temperature and pressure at which gas and liquid can coexist in equilibrium. 

In separation equilibrium involving a supercritical fluid (SCF), the two systems commonly used are solid-SCF and liquid-SCF. A solute or solutes are distributed between the two immiscible phases. A supercritical fluid is normally... 

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