Bed efficiencies

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

To speed up a separation process one must accelerate the migration rate of the effluent band. The migration rate of a band depends on two factors One factor is the linear flow rate, which is directly proportional to the migration rate of the band. The other factor is the distribution ratio, the ratio between the equilibrium concentration of an ion in the resin and aqueous phases, which is inversely proportional to the migration rate of the band. In a CIEC process, the linear flow rate cannot be increased because it may increase the magnitude of the HETP, and thus lower bed efficiency. ... 

The conversion efficiency of the 100 B/D plant was better than the 4 B/D unit, which in turn was better than the bench-scale reactor. Such improvements in fluid-bed efficiency as its size increased were also observed for the MTG process. 

We begin in Chapter 2 with a discussion of the chromatographic process, developing the separate concepts of (1) equilibrium distribution of sample between adsorbent and solvent (or gas) and (2) bed efficiency or theoretical plate number. These two factors are then related in a general way to the problem of separation, and the various techniques of adsorption chromatography are introduced in terms of the different separation problems they are intended to solve. Chapter 3 provides a general discussion of adsorption, emphasizing those fundamental concepts which will be necessary in the discussions of later chapters. The effect of sample size on separation is treated in Chapter 4, particularly the factors which affect isotherm linearity. Chapter 5 provides a complete treatment of bed efficiency in liquid-solid systems. The distribution of sample... 

Fig. 5-1. Maximum bed efficiency as a function of column pressure, adsorbent particle size, and separation time (column length unlimited). Assumes separation conditions corresponding to Eq. (5-8)- 5-8b) and /C, = 2V jW. Reprinted from Analytical Chemistry, 39, 705 (1967), copyright 1967, by the American Chemical Society. Reprinted by permission of the copyright owner.

The theory of bed efficiency in thin-layer chromatography (TLC) can be developed from the same basic premises which apply to separations in columns. The dependence of bed efficiency on separation conditions might therefore be expected to be quite similar for separations on columns and "plates. Actually there are some important differences. In the following discussion we shall follow the treatment of Stewart (25,26). 

Limited experimental data [see discussion of Ref. (22)] suggest that the A term of Eq. (5-15) for TLC separation is commonly much larger than are the second two terms (2 and 3), so that /t to a first approximation. This simplifies any discussion of the dependence of bed efficiency in TLC on separation conditions. The value of A in TLC separations appears to be given by the same relationship as for column elution separations Eq. (5-8). Thus TLC adsorbents normally fall in the particle size range, for which A (and hence H ) is predicted equal to... 

Bases, on different adsorbents, 285 Basic eluent anomaly, 223-224 Basicity vs. absorptivity, 305 Bed development, 10-16, 24-26 Bed efficiency, see also Theoretical plate number, 19... 

---