Equilibrium solvent absorption

A resolution of nanograms could be realized by Mikkilineni et al. Measurements were also made as a function of time to obtain diffusion coefficients. Comparison with gravimetric sorption measurements demonstrated the accuracy of the experiment. Ref presents some details about the electronic circuit, the mounting arrangements for the quartz crystals and the sorption cell. Because very thin films are applied, equilibrium solvent absorption also can be obtained at polymer mass fractions approaching 1 (i.e., for small sol-... 

In all cases, the selective solvents (entrainers) have the task of altering the partition coefficients in a way that high separation factors and selectivities for the different phase equilibria (extractive distillation vapor-liquid equilibrium (VLE), extraction liquid-liquid equilibrium (LLE), absorption gas-liquid equilibrium (GLE)) are achieved, resulting in a separation of compounds. The required partition coefficients, separation factors and selectivities can be calculated with the help of thermodynamic models (g -models, equations of state). 

Cotnplax Donor Solvent Absorption range (nm) Extinction ooafficiant E X 10 l.mola xri7 Equilibrium constant (molas.1) KxlO Enthalpy of formation (J/mole) AH Xir ... 

For infinite dilution operation the carrier gas flows directly to the column which is inserted into a thermostated oil bath (to get a more precise temperature control than in a conventional GLC oven). The output of the column is measured with a flame ionization detector or alternately with a thermal conductivity detector. Helium is used today as carrier gas (nitrogen in earlier work). From the difference between the retention time of the injected solvent sample and the retention time of a non-interacting gas (marker gas), the thermodynamic equilibrium behavior can be obtained (equations see below). Most experiments were made up to now with packed columns, but capillary columns were used, too. The experimental conditions must be chosen so that real thermodynamic data can be obtained, i.e., equilibrium bulk absorption conditions. Errors caused by unsuitable gas flow rates, unsuitable polymer loading percentages on the solid support material and support surface effects as well as any interactions between the injected sample and the solid support in packed columns, unsuitable sample size of the injected probes, carrier gas effects, and imprecise knowledge of the real amount of polymer in the column, can be sources of problems, whether data are nominally measured under real thermodynamic equilibrium conditions or not, and have to be eliminated. The sizeable pressure drop through the column must be measured and accounted for. 

Since an increase in the temperature from i,a to i.co occurs, this causes a decrease in the solvent absorption capacity of the soluted substance according to Henry s law. The absorption efficiency reduces due to an unfavorable equilibrium position at the higher temperature, (see Chapter 1.4.3.3). Consequently, isothermic absorption is aimed for in practice. The heat flux to be removed under isothermal absorption at (t / a = di u) follows from Eq. (3-7). 

The addition of a source of molybdate anions to the hybrid precursor solution gave rise to the formation of a denser siloxane network and to a substantial increase in Tg of the organic phase. This also resulted in a large increase in the rubberyplateau modulus and in a substantial reduction in both the rate of solvent uptake and the equilibrium amount of solvent absorption. These effects are illustrated in Figures 24.8 and 24.9. 

Figure 14.12 Effective diffusion coefficient of Irganox 1076 as a function of maximum solvent absorption. Error bars are the SD for the determination of solvent absorption obtained for at least five measurements at equilibrium. Reproduced with permission from Helmroth and co-workers. Food Additives and Contaminants, 2002, 19, 2, 176 [38], 2002, Taylor Francis)...

It should be noted that the highest possible absorption rates will occur under conditions in which the hquid-phase resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. Such situations would exist, for instance, for NH3 absorption into an acid solution, for SO9 absorption into an alkali solution, for vaporization of water into air, and for H9S absorption from a dilute-gas stream into a strong alkali solution, provided there is a large excess of reagent in solution to consume all the dissolved gas. This is known as the gas-phase mass-transfer limited condition, wrien both the hquid-phase resistance and the back pressure of the gas equal zero. Even when the reaction is sufficiently reversible to allow a small back pres-... 

For the special case of absorption from lean gases with relatively large amounts of solvent, the equilibrium hues are defined for each component by the relation... 

With a reactive solvent, the mass-transfer coefficient may be enhanced by a factor E so that, for instance. Kg is replaced by EKg. Like specific rates of ordinary chemical reactions, such enhancements must be found experimentally. There are no generalized correlations. Some calculations have been made for idealized situations, such as complete reaction in the liquid film. Tables 23-6 and 23-7 show a few spot data. On that basis, a tower for absorption of SO9 with NaOH is smaller than that with pure water by a factor of roughly 0.317/7.0 = 0.045. Table 23-8 lists the main factors that are needed for mathematical representation of KgO in a typical case of the absorption of CO9 by aqueous mouethauolamiue. Figure 23-27 shows some of the complex behaviors of equilibria and mass-transfer coefficients for the absorption of CO9 in solutions of potassium carbonate. Other than Henry s law, p = HC, which holds for some fairly dilute solutions, there is no general form of equilibrium relation. A typically complex equation is that for CO9 in contact with sodium carbonate solutions (Harte, Baker, and Purcell, Ind. Eng. Chem., 25, 528 [1933]), which is... 

For an absorption system in which the solute is highly soluble in the solvent, such that the equilibrium pressure of the solute over the absorbing liquid is very low, we may write ... 

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