Adsorption separation factor

For the PC model, adsorption can be seen as purely partitioning behavior. This model ascribes a thermodynamic partitioning coefficient ( adsorption separation factor ) to each solute to describe adsorption quantitatively. No explicit thermodynamic contribution of the surface site to the adsorption process nor competitive adsorption effects are considered. 

Equation (405) [Eq. (1) in Refs 39 and 41] defines the adsorption separation factor... 

If Eqs (407) and (408) are added and rearranged, the resulting equation gives the adsorption separation factor, 2> for solute partitioning, which can be seen to be the quotient of the two individual relative adsorbabilities ... 

The adsorption separation factor, 2> be seen as a true constant (at constant temperature) since it equates to standard state chemical potentials (and temperature). 

Equation (409) is in a form which clearly defines the adsorption separation factor. However, Eq. (409) is not a very convenient adsorption equation since it contains four unknown variables Xl> i>X2 62 ( 1 2 unknown constant). Elimination of two unknowns is possible using solution and surface adsorbed mass balance equations. Two mass balance equations over both species, 1 and 2, in the solution and surface adsorbed phases can be written, respectively, as... 

In contrast to trace impurity removal, the use of adsorption for bulk separation in the liquid phase on a commercial scale is a relatively recent development. The first commercial operation occurred in 1964 with the advent of the UOP Molex process for recovery of high purity / -paraffins (6—8). Since that time, bulk adsorptive separation of liquids has been used to solve a broad range of problems, including individual isomer separations and class separations. The commercial availability of synthetic molecular sieves and ion-exchange resins and the development of novel process concepts have been the two significant factors in the success of these processes. This article is devoted mainly to the theory and operation of these Hquid-phase bulk adsorptive separation processes. 

The separating power of a chromatographic process arises from the development of many theoretical plates to achieve adsorption equiUbrium within a column of moderate length. Even though the separation factor between two components may be small, any desired resolution may be achieved with sufficient theoretical plates. 

Separation Factor By analogy with the mass-action case and appropriate for both adsorption and ion exchange, a separation factor / can be defined based on dimensionless system variables [Eq. (16-10)] by... 

Commonly used forms of this rate equation are given in Table 16-12. For adsorption bed calculations with constant separation factor systems, somewhat improved predictions are obtained using correction factors f, and fp defined in Table 16-12 is the partition ratio... 

FIG. 16-14 Constant separation factor batch adsorption curves for external mass-transfer control with an infinite fluid volume and n j = 0. 

The treatment here is restricted to the Langmuir or constant separation factor isotherm, single-component adsorption, dilute systems, isothermal behavior, and mass-transfer resistances acting alone. References to extensions are given below. Different isotherms have been considered, and the theory is well understood for general isotherms. 

When the adsorption equihbrium is nonlinear, skewed peaks are obtained, even when N is large. For a constant separation-factor isotherm with R < 1 (favorable), the leading edge of the chromatographic peak is steeper than the trailing edge. Wmen R > 1 (unfavorable), the opposite is true. 

Additional adsorption sites are provided on open metal sites, when available. [Cu3(BTC)2] is performant in the selective adsorption and separation of olefinic compounds. The highly relevant separations of propene from propane and of isobutene from isobutane have been accomplished with separation factors of 2.0 and 2.1, respectively [101, 102]. [Cu3(BTC)2] also selectively takes up pentene isomers from aliphatic solvent in liquid phase, and even discriminates between a series of cis- and trans-olefin isomer mixtures with varying chain length, always preferring a double bond in cis-position. This behavior is ascribed to tt -complexation with the open Cu sites [100]. 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

Membranes offer a format for interaction of an analyte with a stationary phase alternative to the familiar column. For certain kinds of separations, particularly preparative separations involving strong adsorption, the membrane format is extremely useful. A 5 x 4 mm hollow-fiber membrane layered with the protein bovine serum albumin was used for the chiral separation of the amino acid tryptophan, with a separation factor of up to 6.6.62 Diethey-laminoethyl-derivatized membrane disks were used for high-speed ion exchange separations of oligonucleotides.63 Sulfonated membranes were used for peptide separations, and reversed-phase separations of peptides, steroids, and aromatic hydrocarbons were accomplished on C18-derivatized membranes. 

In one study, various distinct types of polar modifiers to n-hexane were tested for 3-chloro-l-phenylpropanol (3CPP) and 1-phenylpropanol (IPP) enantiomer separation [53]. Thereby, alcohol modifiers turned out to be more effective displacers of the solutes from the adsorption places on the sorbent surface, yet aprotic polar modifiers provided higher separation factors (with ethyl acetate in n-hexane affording the best separations for these chiral alcohols). It is evident, though, that the optimal choice of polar modifier is strongly solute dependent and can therefore not be generalized. 

Surface area is by no means the only physical property which determines the extent of adsorption and catalytic reaction. Equally important is the catalyst pore structure which, although contributing to the total surface area, is more conveniently regarded as a separate factor. This is because the distribution of pore sizes in a given catalyst preparation may be such that some of the internal surface area is completely inaccessible to large reactant molecules and may also restrict the rate of conversion to products by impeding the diffusion of both reactants and products throughout the porous medium. 

When the sample introduced into the column is composed of a number of components, the different components of the sample compete for adsorption on the surface of the stationary phase. That competition will affect the individual band profiles and the resulting band shape will depend on several factors, such as the isotherm, the separation factor, the loading factor, the relative concentration of the components, elution order, etc. [1,11],... 

The parameter La is also called the separation factor and provides a quantitative description of the equilibrium regions La = 0 for irreversible, La< 1 for favorable, La = 1 for lineal-, and La > 1 for unfavorable adsorption. The same holds for Fr in Freundlich s isotherm. 

Permeability of a membrane is determined partly by gas diffusivity, but adsorption phenomena can exist at higher pressures. Separation factors of two substances are approximately in the ratios of their permeabilities, = PoA.fPoB- Some data of permeabilities and separation factors are in Table 19.7, together with a list of membranes that have been used commercially for particular separations. Similar but not entirely consistent data are tabulated in the Chemical Engineers Handbook (McGraw-Hill, New York. 1984, pp. 17.16,17.18). 

Very often the liquids to be processed may be contaminated with substances detrimental to some types of zeolites consequently a complete knowledge of the process stream composition and physical properties must be available before preliminary sieve selection can be made. In the absence of prior knowledge of separation factors, competitive co-adsorption, environmental stability, regeneration techniques, or irreversible zeolite contamination, zeoli te contamination, zeolite specification must be proceded by time-con-... 

Figures 9 and 10 illustrate changes in two dependent variables dynamic N2 adsorption capacities and CH4/N2 separation factors. Independent variables are column temperature, operating pressure, and time allowed for vacuum regeneration. This experimental series used a constant feed rate of 6.0 1/min over a time of 1.00 min into a 1" dia. x 24" long adsorber filled with 180g of zeolite. Column depressurization took place for 1.00 min. and this was followed by a variable length vacuum regeneration.

Recent developments demonstrate possibilities for inorganic C02 selective membranes. Microporous membranes with strong C02 adsorption show C02 selectivity if other gas species are hindered in accessing the pores. For instance, at intermediate temperatures, limited C02 selectivity to N2 (to about 400 °C) and H2 (to about 200 °C) is reported for MFI zeolite membranes [96]. Also, at high pressure (10-15 bars) C02 selectivity has been demonstrated in MFI membranes (C02/N2 separation factor ... 

Langmuir isotherm or model Simple mathematical representation of a favorable (type I) isotherm defined by Eq. (2) for a single component and Eq. (4) for a binary mixture. The separation factor for a Langmuir system is independent of concentration. This makes the expression particularly useful for modeling adsorption column dynamics in multicomponent systems. 

Although the multicomponent Langmuir equations account qualitatively for competitive adsorption of the mixture components, few real systems conform quantitatively to this simple model. For example, in real systems the separation factor is generally concentration dependent, and azeotrope formation (a = 1.0) and selectivity reversal (a varying from less than 1.0 to more than 1.0 over the composition range) are relatively common. Such behavior may limit the product purity attainable in a particular adsorption separation. It is sometimes possible to avoid such problems by introducing an additional component into the system which will modify the equilibrium behavior and eliminate the selectivity reversal. 

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