Distillation columns interfacial area

From the analysis given already of the diffusional nature of absorption, one of the outstanding requirements is to provide as large an interfacial area of contact as possible between the phases. For this purpose, columns similar to those used for distillation are suitable. However, whereas distillation columns are usually tall and thin absorption columns are more likely to be short and fat. In addition, equipment may be used in which gas is passed into a liquid which is agitated by a stirrer. A few special forms of units have also been used, although it is the packed column which is most frequently used for gas absorption applications. 

The mechanism of transfer of solute from one phase to the second is one of molecular and eddy diffusion and the concepts of phase equilibrium, interfacial area, and surface renewal are all similar in principle to those met in distillation and absorption, even though, in liquid-liquid extraction, dispersion is effected by mechanical means including pumping and agitation, except in standard packed columns. 

The total mass transferred is the product of the average flux and the total interfacial area (ahfA j). These expressions for the mass transfer rates in distillation are useful in the prediction of the performance of distillation columns (Chapter 14). 

Bravo and Fair s Correlation for Randomly Packed Columns Bravo and Fair (1982) developed a method of estimating the mass transfer characteristics for distillation in randomly packed columns. Their method is based on Onda s Eqs. 12.3.26 and 12.3.29 for the vapor-and liquid-phase mass transfer coefficients, but uses an alternative correlation for the interfacial area density... 

Arwickar (1981) reported some results for distillation under total reflux conditions of the system acetone-methyl acetate-methanol. The experiments were carried out in a laboratory scale column of 7.62 cm diameter packed with 0.635 cm Raschig rings. The simulation of total reflux operations using the nonequilibrium model is discussed by Krishnamurthy and Taylor (1985a). In simulations of Arwickar s experiments Taylor et al. used the correlations of Onda et al. (1968) to estimate the mass transfer coefficients in each phase and the effective interfacial area. The average absolute discrepancy between predicted and measured mole fractions was less than 2 mol% for acetone and methyl acetate and less than 4 mol% for methanol. 

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

Fractionation takes place by contacting an upward flow of vapour with a downward flow of liquid over as large and turbulent vapour/Hquid interfacial area as possible. The surface area is created either by bubbling the vapour through the liquid on distillation trays or by spreading the Hquid very thinly over column packing in the vapour stream. 

In the presentation the nonequilibrium model for the calculation of the three-phase distillation in a packed column is presented. The model considers the mass transfer between all three existing phases. A remarkable point is that by using normal two-phase methods mass transfer coefficients and interfacial areas can be calculate in a first step. The study is strongly influenced by the interaction between the development of the model and the experiments. The validation was carried out in a packed column of the institute on experiments at total and finite reflux. Two different heterogeneous azeotropic mixtures have been separated by distillation. The distillation run at finite reflux showed an unusual behaviour, which is based in the heterogeneity of the liquid phase. In this case a nonequilibrium model is needed. A comparison of the simulation results and the experimental specifications shows the performance of the developed... 

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