Phase equilibrium solvent selection

No other solvent extraction process other than the CO2 technology allows such a strong influence on loading, phase equilibrium, and selectivity. Unfortunately, the solubility of extracted substances in CO2 is relatively low, compared with the usual solvents which give absolute miscibility with the extracted valuable materials in most cases. The determination of solubility and solvent ratios is therefore important for the economy of the process. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

The non-random two-liquid segment activity coefficient model is a recent development of Chen and Song at Aspen Technology, Inc., [1], It is derived from the polymer NRTL model of Chen [26], which in turn is developed from the original NRTL model of Renon and Prausznitz [27]. The NRTL-SAC model is proposed in support of pharmaceutical and fine chemicals process and product design, for the qualitative tasks of solvent selection and the first approximation of phase equilibrium behavior in vapour liquid and liquid systems, where dissolved or solid phase pharmaceutical solutes are present. The application of NRTL-SAC is demonstrated here with a case study on the active pharmaceutical intermediate Cimetidine, and the design of a suitable crystallization process. 

Mixture property Define the model to be used for liquid activity coefficient calculation, specify the binary mixture (composition, temperature, pressure), select the solute to be extracted, the type of phase equilibrium calculation (VLE or LLE) and finally, specify desired solvent performance related properties (solvent power, selectivity, etc.)... 

Hence, a very important factor of preferential dianion formation is the decrease in electrostatic repulsion between anion-radicals. By changing the ion-pair stability, particularly, by solvent selection, one can manage the equilibrium of liquid-phase electron-transfer reactions. 

Physical Equilibria and Solvent Selection. In nrder lor two separale liquid phases to exist in equilibrium, there must be a considerable degree of thermodynamically nonideal behavior. If the Gibbs free energy. G. nf a mixture of two solutions exceeds the energies of the initial solutions, mixing does not occur and the system remains in iwo phases. For the binary system containing only components A and B. the condition for the formation of two phases is... 

The partition coefficient (k/sw) is the ratio of concentrations of a specific compound in an immiscible solvent and water at equilibrium. Solvents with high partition coefficients can sequester the target compound, thus limiting its availability for the enzyme action in the aqueous phase [129], The partition coefficient for anthracene was determined in 15 solvents from different nature mineral oils, vegetable oils, alcohols, hydrocarbons, and others. The values of log K w obtained ranged from 3.7 (silicone oil) to 5.2 (undecanone). Among the solvents evaluated, two were selected for a further study the solvent with the lowest partition coefficient (silicone oil, log Ksw 3.7) and a solvent with an intermediate value (dodecane, log Ksw 4.5) [53],... 

At a selected alloy temperature the vapor pressure of cadmium is determined as a function of alloy composition the cerium solvent has a negligible vapor pressure. The alloy, located in one leg of a sealed inverted U-tube, is subjected to various specific pressures of cadmium from a supply of pure cadmium at selected temperatures in the second leg of the tube. The U-tube is freely suspended at its midpoint and connected to a balance, so that the transfer of cadmium from one leg of the tube to the other can be measured. This gives information as to the change in alloy composition and phase equilibrium. 

Solvent Loading. The solvent circulation rate is a function of the reflux ratio in the primary tower and the liquid-phase concentration of the solvent. For a given solvent selectivity, as the solvent concentration rises, the propane-propylene relative volatility increases and hence the required reflux rate falls. The increased relative volatility results in a decreased number of equilibrium stages required for the desired separation. Figure 4 shows the effect of solvent concentration on the number... 

For the right choice of the selective solvent and for the development and design of separation processes, a reliable knowledge of the phase equilibrium behavior (extractive distillation vapor-liquid equilibrium (VLE) extraction liquid-liquid equilibrium (LLE), absorption gas-liquid equilibrium (GLE)) is required. This information is available from phase equilibrium thermodynamics. 

Solvent selection for the micronization of drugs is crucial because the molecules may be multifunctional and polar with a tendency toward hydrogen bonding, resulting in specific solute-solvent interactions (61). However, it is generally believed that the solute-antisolvent interaction is negligible compared with the solvent-antisolvent interaction for the S-F-V equilibrium in antisolvent crystallization systems. Thus the solvent-C02 interaction is solely responsible for the reduction of solvent power of the mixed solvent. Accordingly, the PMVF of solvent in a binary (solvent-antisolvent) mixture depicts the solute mole fraction in the ternary (solute-solvent-antisolvent) liquid phase. 

A simple experimanc in which a proposed solvent is mixed with the feed mixture can be used to guide solvent selection evan if the equilibrium phases are not analyzed to determine concentrations. If lahotmory facilities are available, batch shakeout rests are often the mosi attractive way lo screen solvents and quickly identity a fow suitable candidates for further study. 

The two basic parameters thel must be estimated for design work are the solute distribalion coefficient and the solvent selectivity. At equilibrium, the activity of end) component in lhaB phase is equal to its activity in the C phase that is. 

It should be noted that the partition ratio at equlibrium predicts the optimum desorption effiency attainable, and other experiments may be necessary to detect nonequlibrium situations. Desorption efficiency should not be taken as the recovery since other factors may have a significant effect. After a solvent/sorbent system is selected and tested using the phase equilibrium technique, direct injections of the test compound are made into collection tubes with and without air being pulled through. If the desorption efficiencies as determined by direct injection are considerably lower than phase equilibrium values, interaction or reaction on the sorbent surface is indicated. If the total recovery from the simulated air collection is lower than the direct injection efficiency (even through no breakthrough has occurred) hydrolysis, oxidation, or another reaction may indicated. 

When we apply thermodynamics to industrial and research problems, we should draw fundamental ideas from Parts 1 and 11, devise an appropriate solution strategy, as in Chapter 10, and combine those with a computational technique, as in Chapter 11. Such a procedme provides values for measurables that can be used to interpret novel phenomena, to design new processes, and to improve existing processes. The procedure is illustrated in this chapter for several well-developed situations. They include conventional phase-equilibrium calculations for vapor-liquid, liquid-liquid, and solid-solid equilibria ( 12.1) solubility calculations for gases in liquids, solids in liquids, and solutes in near-critical solvents ( 12.2) independent variables in steady-flow processes ( 12.3) heat effects for flash separators, absorbers, and chemical rectors ( 12.4) and effects of changes of state on selected properties ( 12.5). 

The problem of solvent selection is relatively complex and a thorough treatment requires considerable information. In addition to basic liquid-liquid equilibrium data, knowledge of the phase densities, viscosities, and the liquid-liquid interfacial tension is also important. Moreover, the economics of IXE systems are often dominated by the solvent regeneration costs. If, for example, solvent regeneration b to be accomplished by extractive or azeotropic distillation, then vapor-liquM equilibrinm data for the ternary system must also be available. Insofar as the most interesting LLE systems ate often ffiose whidi are least ideal, the generation of a physical property data base to complete cost analysis is usually a sigruficant problem. 

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