Cocurrent packed columns, mass-transfer

Cocurrent packed columns - Trickle-bed reactgr. Cocurrent gas-liquid flow in packed beds, packing being either catalytic or inert, is advantageously employed in the petroleum and chemical industries. Successful modeling of mass transfer in packed-bed reactors requires careful study of the three-pliase hydrodynamics - fluid flow patterns, pressure drops, and liquid holdup. 

In contrast to continuous packed bed columns, each stage, whether cocurrent or countercurrent, can be considered to be at equilibrium for many multi-phase mass-transfer processes such as distillation, absorption, extraction etc. Such stages are usually called ideal stages . 

The reported study on gas-liquid interphase mass transfer for upward cocurrent gas-liquid flow is fairly extensive. Mashelkar and Sharma19 examined the gas-liquid mass-transfer coefficient (both gas side and liquid side) and effective interfacial area for cocurrent upflow through 6.6-, 10-, and 20-cm columns packed with a variety of packings. The absorption of carbon dioxide in a variety of electrolytic and ronelectrolytic solutions was measured. The results showed that the introduction of gas at high nozzle velocities (>20,000 cm s ) resulted in a substantial increase in the overall mass-transfer coefficient. Packed bubble-columns gave some improvement in the mass-transfer characteristics over those in an unpacked bubble-column, particularly at lower superficial gas velocities. The value of the effective interfacial area decreased very significantly when there was a substantial decrease in the superficial gas velocity as the gas traversed the column. The volumetric gas-liquid mass-transfer coefficient increased with the superficial gas velocity. 

Most recently. Kirillov and Nasamanyan15 carried out a very interesting unsteady-state analysis of liquid-solid mass transfer for cocurrent upflow in a fixed-bed reactor. The analysis was compared and verified by the steady-state measurements of liquid-solid mass-transfer coefficients in a 10-cm x 10-cm square column with a height of 50 cm. Three types of packings, 30-mm and 8-mm... 

A set of ternary mass transfer experiments was carried out by Toor and Sebulsky (1961b) and Modine (1963) in a wetted-wall column and also in a packed column. These authors measured the simultaneous rates of transfer between a vapor-gas mixture containing acetone, benzene, and nitrogen or helium, and a binary liquid mixture of acetone and benzene. Vapor and liquid streams were in cocurrent flow in the wetted-wall column and in countercurrent flow in the packed column. Their experimental results show that diffusional interaction effects were significant in the vapor phase, especially for the experiments with helium in the wetted wall column. 

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

In packed columns, it is conceptually incorrect to use the staged model even though it works if the correct height equivalent to a theoretical plate (HETP) is used. In this chapter we will develop a physically more realistic model for packed columns that is based on mass transfer between the phases. After developing the model for distillation, we will discuss mass transfer correlations that allow us to predict the required coefficients for common packings. Next, we will repeat the analysis for both dilute and concentrated absorbers and strippers and analyze cocurrent absorbers. A simple model for mass transfer on a stage will be developed for distillation, and the estimation of stage efficiency will be considered. After a mass transfer analysis of mixer-setder extractors. Section 16.8 and the appendix to Chapter 16 will develop the rate model for distillation. 

Bubble reactors do not contain any packing and are fed by cocurrent or countercurrent gas and liquid streams, hi general the chemical process requires a catalyst and this is fed continuously. Industrial bubble reactors are of the column type and generally contain an internal heat exchanger for controlling the temperature of the reaction mixture. The complex flow pattern of both phases achieves a reasonable mass transfer rate. 

Oshima, S, T, Takemasu, Y, Kuriki, K, Shimada, M, Suzuki, and J, Kato, "Liquid Phase Mass Transfer Coefficient and Gas Holdup in a Packed Bed Cocurrent Upflow Column", Kagaku Kogaku 3, (1977) 400-404,... 

It shoiild be pointed out that in most liquefaction processes, the flow of the reactants is cocurrent i q)ward. In cocurrent operation, there is no flooding limit, and greater throu put is attained compared with co mterc irrent column of similar size. Moreover, for the same values of gas and liquid flow rates, interfacial area, liqiaid mass transfer coefficient and pressure drop values in a cocurrent upward packed column are always higher than those obtained in downflow towers (l 3). Upflow operations also give a better performance due to larger liqioid holdup and better liq iid distribution throu out the catalyst bed jlkk). 

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