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Two phase flow and mass transfer on perforated distillation plates

Ho, G.E.ORCID: 0000-0001-9190-8812 (1970) Two phase flow and mass transfer on perforated distillation plates. PhD thesis, The University of Queensland .

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Different two phase flow patterns occur in plate distillation columns, which result in different mass transfer characteristics. Well established two phase flow theories are used to identify and characterize these patterns.

To express the degree of contact between the vapour and liquid on a plate the Hausen plate efficiency definition is studied, and shown to possess advantages over the Murphree definitions. For the case of straight equilibrium and operating lines analytical expressions have been derived for binary systems for the relation between the Hausen point efficiency and the overall number of transfer units, and for the relation between the Hausen plate and point efficiencies.

By examining the relation between the gas superficial velocity in the column and the gas hold up fraction of the two phase mixture four flow patterns on perforated plates are distinguished:

1. Bubble flow - The gas hold up fraction increases rapidly with gas velocity from nearly zero to about O.U. The dispersion consists of swarms of bubbles of narrow size range.

2. Cellular foam - The gas hold up fraction is high (above 0.8) and decreases slightly with increasing gas velocity. The bubbles are deformed into polyhedra.

3. Froth - The gas hold up fraction increases with gas velocity. The bubbles are of widely distributed sizes and the dispersion is severely agitated with liquid circulation patterns occurring on the plate.

4. Spray - The gas hold up fraction increases gradually with gas velocity. The dispersion consists of various size droplets fluidised by the gas.

For the bubble flow regime a spherical cell model is used to derive the relation between gas superficial velocity and gas hold up fraction. This agrees well with the empirical correlation of Wallis. The mass transfer coefficients derived from the model also agree well with the correlations of Calderbank. The effects of bubble size distribution are discussed.

The cellular foam is shown to have similar structure and properties as stable foams. An analysis is described which is based on equating the flow of liquid carried up by the gas and that flowing down through the Plateau borders. This gives the relationship between gas velocity, gas hold up fraction, and bubble size. Results obtained using a 4in. diameter column and three plates (l/l6, 1/8, 3/l6in. diameter holes, approximately 9% free area) with the air-water system are satisfactorily correlated using the relationship. Additional data obtained with ethanol solutions (h, 1 and 2 mole %) also support the analysis. The cellular foam model is used to explain the transition to froth, and to derive relationships for the liquid phase mass transfer.

Finally, approximate and limiting relations are derived for the relations between gas velocity and gas hold up fraction for the froth region. The patterns and rate of liquid circulation are shown to be important in determining the gas velocity - gas hold up relation, and these have still to be quantitatively described before an accurate relationship can be derived.

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