Adsorption mixtures

Separation of components with a low concentration. Distillation is not well suited to the separation of products which form a low concentration in the feed mixture. Adsorption and absorption are both effective alternative means. 

The role of the metal in the NO reduction by CO is clearly shown by CO, NO and CO-NO (1 1 mixture) adsorptions at room temperature and at reaction temperatures... 

The separation of micropore and mesopores plus the outer surface area was done by the combination of elution and flash thermodesorption [9]. With the latter it is possible to determine the contribution of the micropores to sorption separately as micropore desorption requires a higher activation energy because of the above mentioned effects. After injection of an organic vapour or a selection of a particular concentration of the gas mixture, adsorption took place on the sample in the column. The following carrier gas eluted the adsorbate and the peak was recorded. 

Figure 15.12. Multicomponent mixtures, adsorption, and desorption, (a) Concentrations of the components of a ternary mixture in continuous adsorption, as in a moving bed unit (Kovach, 1979). (b) Composition of a desorbed stream consisting of several components as a function of time.

Adsorption at the surface is assumed to be rapid compared to the diffusion rates. The following relationship was developed to describe the mixture adsorption equilibria (3) ... 

Theoretical predictions of ideal mixture adsorption can be made on an a priori basis if the pure component adsorption isotherms are known. 

Scamehorn et. al. expanded a single component adsorption equation ( ) to describe the adsorption of binary mixtures of anionic surfactants of a homologous series (1 1). Ideal solution theory was found to describe the system fairly well. The mixed adsorption equations worked very well in predicting the mixture adsorption, but the equations were complex and would be difficult to extend beyond a binary system. 

The method of predicting the mixture adsorption isotherms is to first select the feed mole fractions of interest and to pick an adsorption level within Region II. The pure component standard states are determined from the total equilibrium concentration that occurs at that set level of adsorption for the pure surfactant component adsorption isotherms. The total equilibrium mixture concentration corresponding to the selected adsorption level is then calculated from Equation 8. This procedure is repeated at different levels of adsorption until enough points are collected to completely descibe the mixture adsorption isotherm curve. 

The agreement between the mixture adsorption data and ideal solution theory is excellent. It is important to remember that while looking at various constant leyels of adsorption in Region II, we are looking at the CAC of the mixed admicelle that has just formed on a particular patch. By looking at different adsorption levels, we are looking at how the two surfactants interact on different energy level patches on the surface. 

Equation 8 was also used to predict the mixture adsorption isotherms in Region II of and on... 

J. U. Keller, F. Dreisbach, H. Rave, R. Staudt, M. Tomalla Measurement of gas mixture adsorption equilibria of natural gas compounds on microporous sorbents. Adsorption 5 (1999) 199-214. 

Ideal adsorbed solution theory (lAST) [4] is a widely used engineering thermodynamic method, the equivalent to the Raoult s law for adsorption, which uses the pure-gas isotherms as inputs to predict the mixture adsorption equilibrium at the temperature of interest. Before the lAST calculation, the pure gas isotherm must be fitted with a suitable isotherm equation to an acceptable degree of accuracy we used a piece-wise fit [3]. 

For the ethane/carbon dioxide mixture in MCM-41, lAST gives good predictions for the mixture adsorption at low and moderate pressures, but exhibits some deviations at high pressures. This non-ideal behaviour might be due to the chemical dissimilarity of the adsorptive species, which will be taken into account in subsequent GCMC simulations. Nevertheless, lAST gives very accurate selectivity results for this mixture over the whole pressure range. 

Simulation data on mixture adsorption can be used to screen zeolites as adsorbents, but experimental data are necessary to validate the simulations and to accurately design the separation process. The first step of the process design is to obtain such data. However, the experimental assessment of multi-component adsorption equilibria and kinetics is not straightforward and is highly time-consuming. As a result, some theories have been developed that predict adsorption behaviour for a mixture based on the pure component equilibria [1,3]. The isotherm data have to be correlated before their use in a design model... 

APPLICATION OF CHARACTERIZATION PROCEDURE FOR COMPLEX MIXTURE ADSORPTION IN WATER AND WASTEWATER TREATMENT... 

Ayappa K. G., Simulation of binary mixture adsorption in carbon nanotubes Transitions in adsorbed fluid composition. Langmuir 14 (1998) pp. 880-890. 

The binary mixture adsorption is simulated by assuming an ideal AB-gas at fixed T, Pa and Pb- In equilibrium there are two ways to perform a change of the system state adsorbing (desorbing) one molecule onto (from) the surfece. 

In Figures 2 and 3 the experimental excess isotherms are shown for four studied adsorption systems in the coordinates n /n" = x" - x Benzene is preferentially adsorbed as well from n-heptane as from 2-propanol. For both mixtures adsorption is higher for the carbon AC-2. 

The first examples of the P-K reaction required high temperatures and pressures to succeed. Several improvements in yield and reaction rate have resulted from continued research efforts over the past 30 years. These include the addition of silica gel200 to the reaction mixture (adsorption of the alkyne-Co complex onto silica may restrict molecular motion, allowing the ene-yne system to interact more readily also, lack of solvent would allow bimolecular reactions to proceed faster) the use of tertiary amine A-oxidcs 201-202 the application of photochemical conditions to ease departure of CO in the rate-determining step and the inclusion of various Lewis bases, which help stabilize intermediate Co complexes.203... 

With gas mixtures, enhancement of the separation factor can be obtained by preferential sorption of mobile species of one of the components of the gas mixture. Adsorption does not always lead to enhanced separation. In a mixture of light non-adsorbing molecules and heavy molecules, the heavy molecules move slower than the lighter ones but in many cases are preferentially adsorbed. Consequently the flux of the heavier molecules is better enhanced by surface diffusion and the separation factor decreases. This occurs, e.g., in CH4/CO2... 

Cerofolini and Rudzihski [43] have reviewed the theoretical principles of single gas and mixture adsorption on heterogeneous surfaces. Their review is chronologically arranged from the earliest to the latest approaches. In the same book, Tovbin [44] reported the application of lattice-gas models to explain mixed-gas adsorption equilibria on heterogeneous surfaces he also discussed [45] the kinetic aspects of adsorption-desorption on flat heterogeneous surfaces. The book [46] also contains other papers on different aspects of adsorption for the reader interested in surface diffusion processes. 

Gas mixture adsorption is a field that is still waiting for a better theory to explain the experimental data. The Ideal Adsorbed Solution Theory cannot explain aU the facts and needs to be replaced by a new model that includes nonideal effects, and adsorbent surface heterogeneity in particular. This field is acquiring increasing relevance because of its technological impHcations. 

Jiang, J. and Sandler, S.l. (2003). Monte Carlo simulation ofOj and N2 mixture adsorption in nanoporous carbon (Cj jj schwarzite). Langmuir, 19, 5936-41. 

Special examples of mixture adsorption are competitive adsorption of the different forms of the same substance, such as pH-dependent ionic and undissociated molecular forms, monomers, and associates of the same substance, as well as potential-dependent adsorption of the same compound in two different orientations in the adsorbed layer. Different orientations on the electrode surface—for example, flat and vertical—are characterized with different adsorption constants, lateral interactions, and surface concentrations at saturation. If there are strong attractive interactions between the adsorbed molecules, associates and micellar forms can be formed in the adsorbed layer even when bulk concentrations are below the critical micellar concentration (CMC). These phenomena were observed also at mineral oxide surfaces for isomerically pure anionic surfactants and their mixtures and for mixtures of nonionic and anionic surfactants (Scamehorn et al., 1982a-c). 

The present paper deals with the threedimensional diagram for description of the equilibrium adsorption of O2/N2 mixtures on clinoptilolite. This diagram shows the dependence of the amount adsorbed on the total pressure, as well as the equilibrium concentration of the gas mixtures. Adsorption from binary mixtures involves theoretical and experimental problems. Ruthven s and Loughlin s works (7,8) provide much detail and they succeed in describing fully enough the complicated systems, but all at the expense of considerable characteristic constants used and elaborate computation. The experimental... 

Separation of a wide range of drugs of abuse present in mixtures Adsorption Partisil fines Methanol/2N ammonia/IN ammonium nitrate (27 2 1)... 

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