Mass transfer in packed columns

The mass transfer equations discussed above are now combined with a material balance on the transferred component to calculate the column or packing height required for a given separation. The column cross-sectional area A is assumed known at this point although in a complete column design A must be determined based on pressure drop considerations. The column, which is in countercurrent flow with only liquid feed and vapor product at the top, and vapor feed and liquid product at the bottom (absorber, stripper, column section), is deflned as follows  

The negative sign indicates that the component is transferred from the vapor to the liquid. Substituting the value of Na from Equations 15.17 and 15.18 into 15.19 results in the following  

Equivalent results may be obtained in terms of concentrations and partial pressures, using Equations 15.17a and 15.18a  

In equimolar counterdiffusion or unimolar dilute diffusion, V and L are assumed constant, allowing Equations 15.22 and 15.23 to be written as 

The applicability of these equations could be expanded by keeping more terms inside the integral and carrying out the integration numerically. Another technique for the unimolar diffusion case is to express liquid and vapor flow rates on a solute-free basis, so they could be assumed constant over broader operating conditions. Compositions in this case would be expressed as mole ratios instead of mole fractions, and the phase equilibrium data would be converted accordingly. 

For packed oolumns, which are dassilied as counteiflow cobama (most trays are crossflow devices), mass transfer is related to transfer rates in couiMctflow vapor-Uqmd contacting. The most-used criterion of efficiency is the AeigAr equhxUent to a AeoretiadpUae (UETP) that was first introduced by Peters  

In using this expression to determine the required height of packing, one would calculate the number of ideal stages (Section 5.3) and obtain a value of HETP (likely from a packing vendor) the height of packing would then follow directly. 

Alternately, the height of packing may be obtained by a more fundamental approach, the use of transfer units  

As is the case for a tray column, the efficiency depends on the system properties, the flow conditions, and the geometty of the device. Because of the many possible variations in the geometry of the packed bed, the last-named parameter assumes great importance in the estimation of packed column efficiency. 

Methods for predicting efficiency also parallel those for tray columns comparison against a similar installation, use of empirical methods, direct scaleup from laboratory or pilot plant, and use of theoretically derived models. Approaches by vendors of packing usually center on comparisons with similar installations (the so-called vendor experience ) and empirical approximations. Direct scaleup from small column studies is difficult with packed columns because of the unknown efiects of geometrical factors and the variations of liquid distribution that are required for practical reasons. Theoretical or semitheoretical models are difficult to validate because of the flow effects on interfacial area. It may be concluded that there is no very good way to predict packed column efficiency, at least for the random type packings. 

The approach used in the present woric is that of semitheoretical models for mass transfer prediction. The reader should remember, however, that the packing vendors should be consulted as eariy as possible. These people have a great deal of experience with their own packings and, in addition, have done a great deal of their own test work. The mt els can then be used in connection with the vendor information to arrive at a final design. 

---

Other correlations based partially on theoretical considerations but made to fit existing data also exist (71—75). A number of researchers have also attempted to separate from a by measuring the latter, sometimes in terms of the wetted area (76—78). Finally, a number of correlations for the mass transfer coefficient itself exist. These ate based on a mote fundamental theory of mass transfer in packed columns (79—82). Although certain predictions were verified by experimental evidence, these models often cannot serve as design basis because the equations contain the interfacial area as an independent variable. 

The nomographs can be used to make a quick, rough, estimate of the column height, but are an oversimplification, as they do not take into account all the physical properties and other factors that affect mass transfer in packed columns. 

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. 

Fernandez, M. A., Laughinghouse, W. S., and Carta, G. J. Chromatogr. A, 746 (1996) 184-198. Characterisation of protein adsorption by composite silicapolyacrylamide gel anion exchanges II mass transfer in packed columns and predictability of breakthrough behaviour. 

Figure 9.1 Mass transfer in packed columns, (a) Differential section (6) driving force diagram.

Arwickar, K. J., Mass Transfer in Packed Column Absorption and Multicomponent Distillation, Ph.D. Thesis in Chemical Engineering, University of California, Santa Barbara, CA, 1981. 

Steam stripping is often performed in plate columns since, in the past, few studies of mass transfer in packed columns have been made. A detailed study of stripping toluene from water in a... 

Stichlmair, J., and A. Stemmer, "Influence of Maldistribution on Mass Transfer in Packed Columns, I. Chem. E. Symp. Ser. 104, 1987, p. B213. 

By convention, mass transfer in packed columns is described by the model of transfer units since a countercurrent flow of gas and liquid prevails in packed beds. According to this model the required height H of the packing within the column is... 

At the present state of the art all models for the prediction of mass transfer in packed columns are insufficient to some degree. One problem is a poor knowledge of the interfacial area effective for mass transfer. Pubhshed studies differ significantly and their results are often contradieting. Frrrthermore, the existing models for predicting mass transfer coefficients are not suffieiently reliable. 

Shende,B.W. and M.M.Sharma. "Mass transfer in packed columns Cocurrent operation". Chem.Engng.Sci. 29 (1974) 1763. 

Charpentier.J.C. "Recent progress in two-phase gas-liquid mass transfer in packed columns". Chem.Eng.J. II (1976) 161. 

Mobile Phase Mass Transfer in Packed Columns... 

Charpentier, J,C., "Recent Progress in Two Phase Gas-Liquid Mass Transfer in Packed Columns", Chem. Eng. J. 11, (1976) 161-181,... 

N. Kolev, Kr. Semkov, R. Darakchiev, Mathematical modelling of heat and mass transfer in packed columns. New development in separation processes associated with process industries, biotechnolo and rarvironmentrJ protection, edited 1 RJohoiacld, K.W. Szewezyk, European federation of chemical engeneering Workshop party on distilation, absorption and extracktion, Warsaw. May 29-31,1996. 

---