Sorbent selection parameter

A simple method for determining sorbent selectivity was proposed by Knaebel (1995), which involved taking the ratio of Henry law constants. This rough estimate for selectivity can serve as a cmde sorbent selection parameter. A more precise estimate can be obtained by comparing the isothermal binary working... 

Having defined the two contributing factors to the parameter, the PSA sorbent selection parameter (S) can be written as follows ... 

Pressure Ratio Ph/Pl Purge-to-Feed Ratio P/F PcD (atm) O2 Product Recovery (%) Sorbent Selection Parameter, S... 

Although the two sorbents have a subtle difference in performance, the better one involving the LiAgX sorbent is reflected accurately by the parameter. In both cases S was higher for LiAgX compared with LiLSX under the said operating pressures. Thus the sorbent selection parameter is shown to be sensitive to even slight differences in the adsorption isotherms. 

Figure 3.3. O2 product recovery (%) vs. sorbent selection parameter S for the comparison of LiX (LiLSX) and NaX adsorbents for air separation at different pressure ratios. Refer to Table 3.3 for additional details (from Rege and Yang, 2001).

Sorbent Selection Parameters for Other Processes. The above analysis pertains to bnlk separation. For pnrification processes, that is, PSA, and TSA, as well as liqnid phase apphcations, no stndy has been reported for establishing a sorbent selection parameter. In this case, obviously a 2 for the adsorption step... 

Table 3.4. PSA simulation operating conditions, performance results, and sorbent selection parameter S for the comparison of LiAgX (1 Ag -ion and 95 + per unit cell) vs. LiX (Si/AI = 1,100% Li -exchange) adsorbents for air separation (Ph = 1.0 atm)...

Here S is the sorbent selection parameter, component 1 is the fast diffusing component, t is the step time for adsorption, and P is the partial pressure in the feed mixture. For a given feed mixture and step time t, the parameter may be simplified as follows when comparing different sorbents ... 

In kinetic separation, both equilibrium amount and diffusion rates are important. A sorbent selection parameter (Sk) has been given in Chapter 3. It involves both diffusivity and equilibrium constant. The equilibrium isotherms and diffusion time constants for a number of important sorbates on CMS s are given below. 

As shown in Section 3.3 in Chapter 3, the PSAA/SA separation results may be correlated with the simple sorbent selection parameter S. ... 

Table 10.1. Sorbent selection parameter (S a Aq /Aqo,) for VSA/PSA performance for air separation, comparing LiX and CaX (Si/AI = 1.25 for both)...

For the process of pressure-swing adsorption of a nontrace binary gas mixture, Rege and Yang (2001) have defined a dimensionless sorbent selection parameter S based on the Langmuir isotherm parameters ( 7Aff)ma>i At,(< B,.)max Bt for species A and B, respectively (see equation (3.3.112 b)) ... 

Assume linear isotherm behavior corresponding to the lower-pressure region of the Langmuir isotherm. Show that this sorbent selection parameter S, based on the product of the selectivity ratio (( Air)max At / (<7B.j)max Bt) and the ratio of the working capacity of the two species A and B (working capacity is the difference between the adsorbed amounts at adsorption (high) pressure Ph and at desorption (low) pressure Pi, preferably with the gas mbcture) (maybe described as follows ... 

The preceding approach, in principle, also enables the effect of the valency and the activity coefficients of different displacer salts to be considered in the evaluation of the Zc or the corresponding z values for any HP-IEX system. For example, if the retention results for a series of polypeptides and/or proteins chromatographed with the same HP-IEX sorbent, column, flow rate, elution conditions, buffer composition, pH, and at the same temperature, but with NaCl or CaCl2 as the displacing salt are compared, then the relative differences between these two salt systems can be evaluated in terms of a free energy selectivity parameter r such that... 

Fig. 3-7 Graphic determination of the number of theoretical plates, N, required for the separation R = 1.0) of a pair of compounds, depending on the sorbent selectivity a and parameters, the vaiues being controtied by the capacity factor k [79].

Another important parameter for column selection is the proper choice of sorbent porosity. The pore size of the sorbent determines the fractionation range of the column. The best way of doing this is by looking at the calibration curves of the columns, which are normally documented by the column vendor (cf. Fig. 9.3 for PSS SDV column calibration curves and PSS SDV fractionation ranges) (7). 

The popularity of this extraction method ebbs and flows as the years go by. SFE is typically used to extract nonpolar to moderately polar analytes from solid samples, especially in the environmental, food safety, and polymer sciences. The sample is placed in a special vessel and a supercritical gas such as CO2 is passed through the sample. The extracted analyte is then collected in solvent or on a sorbent. The advantages of this technique include better diffusivity and low viscosity of supercritical fluids, which allow more selective extractions. One recent application of SFE is the extraction of pesticide residues from honey [27]. In this research, liquid-liquid extraction with hexane/acetone was termed the conventional method. Honey was lyophilized and then mixed with acetone and acetonitrile in the SFE cell. Parameters such as temperature, pressure, and extraction time were optimized. The researchers found that SFE resulted in better precision (less than 6% RSD), less solvent consumption, less sample handling, and a faster extraction than the liquid-liquid method [27]. 

Sander, L.C. and Wise, S.A., Influence of substrate parameters on column selectivity with alkyl bonded phase sorbents, J. Chwmatogr, 316, 163, 1984. 

Such approaches underpin the current popularity of RP-HPLC procedures for the purification of synthetic or recombinant polypeptides at the production scale, or analogous approaches employed in the HP-IEX of commercially valuable proteins. However, in some cases when linear scale-up methods are applied to higher molecular weight polypeptides or proteins, their biological activity may be lost due to unfavorable column residency effects and sorbent surface area dependencies. It is thus mandatory that the design and selection of preparative separation system specifically address the issues of recovery of bioactivity. Often some key parameters can be easily controlled, i.e., by operating the preparative separation at lower temperatures such a 4°C, or by minimizing column residency times. 

As noted, and as detailed in Table 2, a large variety of stationary-phase and mobile-phase factors influence the selectivity, recovery, and stability of proteins and other biomacromolecules in the adsorptive modes of HPLC. Batch adsorption pilot experiments provide an expedient approach to ascertain the effect of many parameters, such as the pH, nature, and concentration of organic solvent or ionic additives in the mobile phase, the temperature- or the static-binding capacity with a defined sorbent. Similarly, the influence of... 

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