Performance of packed columns: VI. Mass transfer rates for dehumidification at high solute concentrations

Abstract

Air-carbon tetrachloride gas streams were dehumidified with cold liquid carbon tetrachloride in a short 4.0-in. diameter column packed with 0.5-in. Raxhig rings in u n attempt to explain reported unsuccessful attempts to correlate dehumidification data obtained a t high solute concentration.The data were in satisfactory agreement with a previously presented correlation if the Schmidt number was evaluated a t average film conditions and interfacial conditions were employed with rigorous calculation methods to evaluate the mass transfer coefficients.The previous paper (10) of this series presented a correlation for mass transfer in packed columns at low and high solute concentrations which was based on adiabatic vaporization data.Cairns and Roper (1) obtained a similar correlation for adiabatic vaporization data employing a wetted-wall column:In a later attempt to test the usefulness of Equation ( 2 ) Cairns and Roper(2) obtained mass transfer data at high humidities for dehumidification runs in a wetted-wall column. These data however could not be correlated by Equation ( 2 ) or any other satisfactory method. Yoshida and Tanaka (11) on the other hand, working with columns packed with Raschig rings, were able to obtain satisfactory correlations of both vaporization and dehumidification data obtained at low humidities. There are several possible explanations for the different conclusions reached by Cairns and Roper (2) and Yoshida and Tanaka (11 ) . The first is that correlations obtained for low solute concentrations do not apply at high concentrations for dehumidification. The second is that packed-column data lead to correlations which are not similar in form to those obtained for wetted-wall columns. The third, and most likely explanation, is that Cairns and Roper (2) used an oversimplified method of calculation to evaluate their data. They assumed the bulk liquid temperature was equal to the interfacial temperature and employed a logarithmic mean driving force to calculate the mass transfer coefficients. These simplifications are suitable for adiabatic vaporization work when the liquid temperature is held constant at the adiabatic saturation temperature but are not justified for dehumidification work where heat transfer takes place in the liquid phase and the interfacial temperature may be substantially different from the bulk liquid temperature. Yoshida and Tanaka (11) working at low humidities were able to estimate interfacial temperatures and employ a rigorous trial-and-error, graphical method of calculation to evaluate the mass transfer coefficients.In order to test the explanations offered above and the extension of the use of Equation (1) to dehumidification work an experimental investigation was undertaken to obtain suitable dehumidification data. A short 0.5-in. Raschig ring packed column was employed to dehumidify air-carbon tetrachloride gas streams at low and high solute concentrations with cold liquid carbon tetrachloride. This system has a number of advantages over a wettedwall column employing ...

Absorption methods for the determination of mass transfer parameters of packing internals: A literature review

“…In addition, using CCl 4 instead of water makes it possible to obtain more generally valid correlations, as the physical properties of the gas phase change to a greater extent with concentration. 95 For example, Sc G shows a fivefold variation with CCl 4 mole fraction ranging from 0 to 0.7 in air.…”

“…The reverse process, dehumidification, or condensation, has also been used as a method for determining k G a. Yoshida and Tanaka 92 and Shulman and Robinson 95 performed such tests, where the solute content of air was reduced by getting it into contact with a cold liquid stream, thus partially condensing the vapor. The correlations established for vaporization can only be applied here with a precise measurement or a rigorous estimation 95 of interfacial temperature. With dehumidification, the temperature of the liquid surface differs from that of the bulk liquid, as heat transfer takes place in the liquid, unlike the case of constant liquid temperature vaporization, where the constant liquid temperature means that vaporization heat is provided entirely by the gas phase.…”

Review of Mass-Transfer Correlations for Packed Columns

“…Another more exhaustive investigative work about the performance of packed column was done by Shulman and co-workers, resulting in a series of papers. − Their experimental investigations on the performance of random packed columns have been on aqueous and nonaqueous systems. They determined the gas-phase mass-transfer coefficient by vaporization processes, and then the correlation of k G was used to determine the wetted areas of packings when irrigated with water and to calculate the effective interfacial areas from data available.…”

Effect of Concentration on Gas-Liquid Mass Transfer Coefficient

“…The over-all material balances were calculated from the predicted ratios of Cg°/Cgca or Cw°/Cw'x' Since for absorption (employing Equation 12) (>+;£) 1 Vg Cg_ a. Vw cy (21) and for desorption (employing Equation 13) with a = 0.775, VJVm = 1.180, and B = 2.093, Cg°/Cgŝ hould be 1.915 and GyVCy," should be 2.093, for absorption and desorption, respectively. Actual experimental values are shown in Table II…”