Phase equilibrium purification

Phase-solubility analysis17 (sometimes referred to as phase equilibrium purification) is the quantitative determination of the purity of a substance through the application of precise solubility measurements. At a given temperature, a definite amount of a pure substance is soluble in a definite quantity of solvent. The resulting solution is saturated with respect to the particular substance, but the solution remains unsaturated with respect to other substances even though such substances may be closely related in chemical structure and physical properties to the particular substance being tested. There are examples of the use of this technique in HPLC methods development18 and in the characterization of reference standards,19 but the... 

Impurity enhancement techniques such as fraction collection and phase equilibrium purification can be used to provide enriched samples for use in the method development process.23 When using the fraction collection approach, one or more cuts (fractions) of the chromatographic separation of a bulk lot or mother liquor are isolated. The excess solvent in these fractions is then evaporated to achieve the desired concentration enhancement. These fractions typically contain extraneous peaks because of the presence of salts in the mobile phase or sample degradation during the concentration step. The salts can be removed by extraction and/or a LC cleanup step. To insure that these extraneous peaks/artifacts are not identified as key peaks for separation, the original bulk lot or mother liquor should be included in the method development sample set. The same holds true for phase-equilibrium-purification supernatants. 

The field of membrane separations is radically different from processes based on vapor-liquid or fluid-solid operations. This separation process is based on differences in mass transfer and permeation rates, rather than phase equilibrium conditions. Nevertheless, membrane separations share the same goal as the more traditional separation processes the separation and purification of products. The principles of multi-component membrane separation are discussed for membrane modules in various flow patterns. Several applications are considered, including purification, dialysis, and reverse osmosis. 

Our interest in phase equilibria is twofold to make predictions about the equilibrium state for the types of phase equilibria listed above using activity coefficient models and/or equations of state, and to use experimental phase equilibrium data to obtain activity coefficient and other partial molar property information. Also, there are brief introductions to how such information is used in the design of several different types of purification processes, including distillation (this chapter) and liquid-liquid extraction (Chapter 11). 

Henry Belin du Pont Chair and director of the Center for Molecular and Engineering Thermodynamics at the University of Delaware. His extensive research interests include applied thermodynamics and phase equilibrium, environmental engineering, and separations and purification. Dr. Sandler is a recipient of the Warren K. Lewis Award from the American Institute of Chemical Engineers and the Inaugural E.A. Mason Memorial Lecturer Award from Brown University. He is a member of the National Academy of Engineering and has published more than 250 technical articles in recognized journals and conference proceedings. 

Solubility calculations are merely phase-equilibrium calculations applied to supercritical gases in liquids, solids in liquids, and solutes in near-critical fluids. The last application has drawn substantial attention, for near-critical extraction processes are being applied, not only in the chemical and energy industries, but also in food processing, purification of biological products, and clean-up of hazardous wastes. 

A column comprises individual separation stages in which the purification of the product is carried out by means of the effect that vapor and liquid have different compositions at equilibrium. Accordingly, the column design calls for knowledge of the phase equilibria of the systems [5]. Normally, phase equilibrium calculations are based on binary parameters describing the interactions of two different molecules. If multicomponent mixtures are considered, some of these interactions might be unknown. To obtain better simulation results, they should at least be estimated. This was the main reason for the development of the UNIFAC group contribution method twenty years ago. 

The design of purification processes involving the removal of impurities by condensation from pressurized gas streams requires phase equilibrium data which are often not available. The objective of this paper is to examine several methods of predicting the composition of a gas phase in equilibrium with an essentially pure condensed phase when only the properties of the pure components are known and to compare these predictions with the limited experimental data for the system hydrogen-methane at low temperatures. 

The novel approach finally taken was to conduct the reaction and purification steps in a reactor-distillation column in which methyl acetate could be made with no additional purification steps and with no unconverted reactant streams. Since the reaction is reversible and equilibrium-limited, high conversion of one reactant can be achieved only with a large excess of the other. However, if the reacting mixture is allowed to flash, the conversion is increased by removal of the methyl acetate from the liquid phase. With the reactants flowing countercurrently in a sequence of... 

The development of mixture sorption kinetics becomes increasingly Important since a number of purification and separation processes involves sorption at the condition of thermodynamic non-equilibrium. For their optimization, the kinetics of multicomponent sorption are to be modelled and the rate parameters have to be identified. Especially, in microporous sorbents, due to the high density of the sorption phase and, therefore, the mutual Influences of sorbing species, a knowledge of the matrix of diffusion coefficients is needed [6]. The complexity of the phenomena demands combined experimental and theoretical research. Actual directions of the development in this field are as follows ... 

All Bi(OAlk)3 derivatives of n- and z -alcohols are volatile, which permits their purification by distillation, but the yields of the sublimates are low as the processes are accompanied by partial decomposition. A much higher thermal stability is demonstrated by Bi(OBu )3. According to the mass-spectrometry data the first members of the homologous series are dimeric in the gas phase, while Bi(OR% and Bi(0C2H40Me)3 are monomeric [967], However the solid state structure of the latter contains the polymer chains and its solutions in benzene — the dimeric molecules. The NMR spectra indicate the equilibrium in solution of aggregates with different number of chelated ligands [1069]. This obstacle hinders the crystallization of 2-methoxyethoxide from solutions (Table 12.15). 

In ion exchange, the aqueous phase ions are replaced with H and OH ions. If the aqueous phase ions are in equilibrium with the adsorbed ions, their removal from the aqueous phase causes desorption of the adsorbed ions to maintain the equilibrium until all of the adsorbed ions have been removed. In practice, this removal is quantitative (2-5). Ion exchange is rapid and easily carried out however, commercial ion exchange resins contain leachable polyelectrolytes which adsorb on latex particle surfaces these polyelectrolytes can be removed only by an arduous purification process (2-5). 

For the equilibrium measurements the dyestuffs were precipitated on the surface of selected kieselguhrs as stationary phases and filled into a chromatographic column. Then temperature and pressure were adjusted. Before each measurement series a reference spectrum was recorded. Firstly, purification from better soluble impurities of the dyestuffs was performed in a continuous flow. In the following, the dependence of the saturation concentration on the flow-rate and the time which is necessary for equilibration was determined. Within the experimental accuracy no dependence on these parameters was found. Nevertheless, for equilibration the substance was treated more than 10 min under isothermal-isobaric conditions. 

Concomitant crystallization is by no means limited to crystallization from solution, nor to preservation of constant molecular conformation. As noted in Section 2.2.5 the classic pressure vs temperature phase diagram for two solid phases (Fig. 2.6) of one material exhibits two lines corresponding to the solid/vapour equilibrium for each of two polymorphs. At any one temperature one would expect the two polymorphs to have different vapour pressures. This, in fact, is the basis for purification of solids by sublimation. Nevertheless there are examples where the two have nearly equal vapour pressures at a particular temperature and thus cosublime. This could be near the transition temperature or simply because the two curves are similar over a large range of temperatures or in close proximity at the temperature at which the sublimation is carried out. For instance, the compounds 3-VI and 3-Vn both yield two phases upon... 

Figure 1. Vacuum line and transfer system for the determination of single stage fractionation factors and composition of the phases (1) storage hulb for purified NO (2) manometer (3) silica gel tube for further purification of NO (4) gas burette containing mercury covered with a layer of a-bromonaphthalene (5) manometer (6) equilibrium vessel (7) connection for mechanical pump (8) connection for sample tube (9) stainless steel Helicoid gage (10) connection to mercury diffusion pump...

At the surface of any liquid there is a continuing process of evaporation and recondensation. At any temperature, the equilibrium concentration of the substance in the vapor phase determines the vapor pressure. When the temperature is raised sufficiently for the vapor pressure of the substance to equal the atmospheric pressure, the substance is said to boil, and that temperature is called the boili7ig pomt. Since this phenomenon is of great importance in laboratory work for the purification and transfer of liquids and for maintaining reaction mixtures at relatively constant temperatures, it is desirable to examine it fairly carefully. 

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