Mass transfer random packings

Boston inside-out method. 172-177, 198 Bravo, et al. rfructured packing pressure drop, 447, 499, 500 Bravo Fair et al. mass transfer random packing, 528-530 structured packings, 474, 529, 531, 532 Brown-Martin method, 109 Bryoden method, 161, 162, 175, 176. 179... 

FIGURE 5.16-1 Packing parameters for estimating vapor-phsee mass transfer, random packings (Ref, 7). 

FIGURE 5.18-2 Packing parameters for estimating liquid-phase mass transfer, random packings (Ref 7). (Note The value of 0 from the chart is in feet. (lfc> /h fl2) x 0.00136 = kg/s-m2.)... 

Mass transfer in packed columns is a continuous, differential, process, so the transfer unit method should be used to determine the column height, as used in absorption see Section 11.14.2. However, it often convenient to treat them as staged processes and use the HETS for the packing employed. For random packings the HETS will, typically, range from 0.5 to 1.5 m, depending on the type and size of packing used. 

The macroporous structure of monoliths allows the overcoming of some of the disadvantages of conventional affinity chromatographyt . Monoliths have lower mass transfer resistance and pressure drop than conventional random packed beds, and mass transfer within monolith channel rates can be substantially larger than mass transfer in packed beds used in conventional chromatography. 

The situation is very much poorer for stmctured rather than random packings, in that hardly any data on Hq and have been pubHshed. Based on a mechanistic model for mass transfer, a way to estimate HETP values for stmctured packings in distillation columns has been proposed (91), yet there is a clear need for more experimental data in this area. 

The dispersion of a solute band in a packed column was originally treated comprehensively by Van Deemter et al. [4] who postulated that there were four first-order effect, spreading processes that were responsible for peak dispersion. These the authors designated as multi-path dispersion, longitudinal diffusion, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase. Van Deemter derived an expression for the variance contribution of each dispersion process to the overall variance per unit length of the column. Consequently, as the individual dispersion processes can be assumed to be random and non-interacting, the total variance per unit length of the column was obtained from a sum of the individual variance contributions. 

There are four basic dispersion processes that can occur in a packed column that will account for the final band variance. They are namely, The Multipath Effect, Longitudinal Diffusion, the Resistance to Mass Transfer in the Mobile Phase and the Resistance to Mass Transfer in the Stationary Phase. All these processes are random and essentially noninteracting and, therefore, provide individual contributions of variance that can be summed to produce the final band variance. Each process will now be discussed individually. 

Use of HTU and K a Data In estimating the size of a commercial gas absorber or liquid stripper it is desirable to have data on the overall mass-transfer coefficients (or heights of transfer units) for the system of interest, and at the desired conditions of temperature, pressure, solute concentration, and fluid velocities. Such data should best be obtained in an apparatus of pilot-plant or semiworks size to avoid the abnormalities of scale-up. Within the packing category, there are both random and ordered (structured) packing elements. Physical characteristics of these devices will be described later. 

The correlations of Billet (66) and Onda et al. (187) are valid for various mixtures and packings and cover both absorption and distillation processes. The correlation of Kolev (133) is obtained for RA and certain random packings. In general, the mass transfer coefficient correlations need to be compared to one another and validated using experimental data. This shows, in particular, the way the mass transfer correlations influence the concentration prohles of the components and other relevant process characteristics. 

In the absence of experimental data, mass transfer coefficients (and hence heights of transfer units) can be estimated by generalized models. A popular and easy to use correlation for random packings is that of Bolles and Fair (1982). The earlier correlations of Onda et al. (1968) and Bolles and Fair are also useful for random packings. 

The plug-flow model indicates that the fluid velocity profile is plug shaped, that is, is uniform at all radial positions, fact which normally involves turbulent flow conditions, such that the fluid constituents are well-mixed [99], Additionally, it is considered that the fixed-bed adsorption reactor is packed randomly with adsorbent particles that are fresh or have just been regenerated [103], Moreover, in this adsorption separation process, a rate process and a thermodynamic equilibrium take place, where individual parts of the system react so fast that for practical purposes local equilibrium can be assumed [99], Clearly, the adsorption process is supposed to be very fast relative to the convection and diffusion effects consequently, local equilibrium will exist close to the adsorbent beads [2,103], Further assumptions are that no chemical reactions takes place in the column and that only mass transfer by convection is important. 

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