Temperature, equilibrium-selective adsorption

When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

Keywords. Na-X type zeolite, CO selectivity, low temperature, equilibrium adsorbent, pressure-swing adsorption... 

Counterdiffusion of cumene and 1-MN occurred readily in type Y zeolite, as shown by several studies. The 1-MN is selectively adsorbed relative to cumene thus, when the zeolite was initially saturated with cumene and placed in 1-MN, essentially 100% of the cumene diffused out but when the zeolite was saturated with 1-MN and placed in cumene, only about 74% of the 1-MN diffused out. The same end point was also reached when SK-500 saturated with cumene was placed in a mixture of 1-MN and cumene in the proper ratio. This selective adsorption equilibrium value was essentially independent of temperature and, except for the cerium form of type Y, was independent of the nature of the cation... 

The concept that the removal of an undesired reaction product by selective adsorption from the reaction zone of an equilibrium-controlled reaction increases the conversion and the rate of formation of the desired component (based on Le Chatelier s principle) was used to develop a novel PSA process concept called SERP for direct production of fuel cell-grade hydrogen by steam reforming of methane (CH4 + 2H20 44 C02 + 4H2).57 61 The concept uses a physical admixture of a reforming (noble metal on alumina) catalyst and a chemisorbent (K2C03 promoted hydrotalcite), which selectively and reversibly chemisorbs C02 from a gas at a temperature of -450 °C in the presence of steam. The cyclic SERP steps consisted of the following ... 

When a drying temperature is selected, the relative humidity should not be too low so as to initiate calcination, or too high so as to promote surface adsorption and capillary condensation. In addition the drying conditions of temperature and relative humidity must not affect the chemical equilibrium. However, since each calcium sulfate compound has its own stability region in the phase diagram, the drying conditions must be in a region where all the phases present in the sample remain stable. 

A threedimensional diagram, expressing the amount adsorbed as a function of sorbate equilibrium concentration and total pressure, is drawn using both direct experimental data and computed data for C /N- mixtures on K-clinoptilolite at room temperature. The real adsorption isotherms follow the Freundlich law for Nj and Henry s law for C>2 Freundlich and henry constants are computed and correlated to sorbate equilibrium concentration. Selectivity coefficients,, equilibrium constants, K, and the changes in the standard Gibbs free energy, aG°, are determined. Selectivity towards N. is satisfactory and K-clinoptilolite could be used for industrial air enrichment. 

Since adsorption equilibrium is the most fundamental property, a number of studies have been conducted to determine I) the amount of species adsorbed under a given set of conditions (concentration and temperature) or 2) how selective adsorption takes place when two or more adsorbable components coexist. There are many empirical and theoretical approaches. Only several simple relations, however, can be applied in later treatments on kinetic description of adsorption. These relations are sometimes insufficient for predicting adsorption isotherms under a new set of operating conditions. Thus more sophisticated trials on sound thermodynamics or on substantial models have been proposed by many authors. 

The stoppers for vials contain a certain amount of water, which depends on the composition of the stoppers. De Grazio and Flynn [1.86] showed, that the selection of the polymer, the additives for the vulcanization, and the filler influence the adsorption and desorption of water. However even the best possible mixture increases the RM in 215 mg sucrose from 1.95 % to 2.65 % during 3 months storage time at room temperature. Other stopper mixtures show an increase up to 1.7 %. Pikal and Shah [1.87] demonstrated, that the desorption of water from the stopper and the absorption of water by the product depends, in the equilibrium state, on the mass and water content of the stopper and the water content and sorption behavior of the dry product. 

On the contrary, a more advanced methodology makes use of nonlinear chromatography experiments If the adsorption isotherms are measured under variable temperatures, the corresponding thermodynamic parameters for each site can be obtained in view of the van t Hoff dependency (site-selective thermodynamics measurements) [51,54]. Thus, the adsorption equilibrium constants of the distinct sites bi a = ns, s) are related to the enthalpy (A// ) and entropy (A5j) according to the following equation [54] ... 

However, if the valences of the exchanging cations are equal, the selectivity coefficient or pseudo-equilibrium constant is not affected by concentration. As already mentioned, one isotherm corresponds to a specific temperature in the case of adsorption or ion exchange of equal valence ions, whereas additionally, the same normality is required for the existence of only one isotherm in the case of ion exchange of different valence ions, due to the concentration-valence effect (Helfferich, 1962). The determination of the true equilibrium constant should be based on the thermodynamic activities (activity coefficients) of the species rather than concentrations. It is clear that the difficulties in the determination of activity coefficients also complicate the determination of the true equilibrium constant (Culfaz and Yagiz, 2004). 

Reversibility. Apparent irreversibility phenomena of ion exchange in NaX were studied with zinc and cobalt ions using a temperature-variation method described in the experimental section. In view of the high selectivity of NaX for bivalent cations at low zeolite loading, the concentration of bivalent ions in the equilibrium solution is quite sensitive to small changes in the surface composition. In fact, the adsorption removal of bivalent cations at low loading, below 0.2, is quantitative or nearly so (99.5% or better). Consequently the value of the equilibrium concentration is an ideal criterion for assessing either reversibility or equilibrium conditions. 

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