Gas and liquid holdups

Earlier studies on the measurements of the gas holdup in a packed bubble-column were performed by Weber,39 Stemberding,33 and Voyer and Miller.38 Weber found that the gas holdup was unaffected by the liquid flow rate, a result similar to the one observed in an unpacked bubble-column. A decrease in the surface tension of the liquid was also found to increase the gas holdup. The correlations presented by him are summarized in Table 7-1. 

Voyer and Miller38 measured the liquid holdup in a column with screen packings. The liquid holdup decreased with an increase in gas velocity however, no correlations were presented. Turpin and Huntington37 measured the liquid holdup in an air-water system and 5.1-, 10.2-, and 15.3-cm-diameter columns packed with tabular alumina particles of 0.76 and 0.82 cm in diameter. The total liquid holdup was correlated to the ratio of liquid-to-gas mass fluxes by an empirical relation 

tis is the external surface area per unit volume of column and o is the standard deviation in each variable. ReL and ReG are the liquid and gas Reynolds numbers based on the particle diameter, /,. It should be noted that, unlike the report of Weber,39 the above relation indicates a mild dependence of liquid holdup on the liquid velocity, a result similar to the one reported independently by Ohshima et al.zl Heilman s9 correlation, however, indicates that the gas holdup (or liquid holdup) is independent of the liquid flow rate as long as the liquid flow rate is low and the gas holdup remains below 0.54. 

The dimensionless correlations between the liquid holdup and the gas and liquid Reynolds numbers are also given by Ford6 and Saada.26 Ford6 suggested that for h, 0.43, a relation 

Achwal and Stepanek1 recently measured the holdup profile in a packed bubble-column by a method based on measuring thermal conductivity. They found that the gas holdup in a packed bed increased with height and related this increase to the change in pressure. Two separate correlations for the average gas holdup were derived. One, based on the homogeneous flow model, was expressed as 

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Fig. 5. Sum of gas and liquid holdup in gas-liquid fluidized bed. Experimental data of Turner (T4) and theoretical curves of 0stergaard (03).

The bed void volume available for flow and for gas and liquid holdup is determined by the particle size distribution and shape, the particle porosity, and the packing effectiveness. The total voidage and the total liquid holdup can be divided into external and internal terms corresponding to interparticle (bed) and intraparticle (porosity) voidage. The external liquid holdup is further subdivided into static holdup eLs (holdup remaining after bed draining due to surface tension forces) and dynamic holdup eLrf. Additional expressions for the liquid holdup are the pore fillup Ft and the liquid saturation SL ... 

The holdup of a phase is usually defined as the volume of the phase per unit reactor volume. However, for a fixed-bed reactor, the gas and liquid holdups are often defined on the basis of void volume of the reactor. In a fixed-bed reactor, the liquid and sometimes gas holdups are divided into two parts dynamic holdup, which depends largely on the gas and liquid flow rates and the properties of the fluids and the packing material, and static holdup, which depends to a major extent on the nature of the packing (e.g., porosity of the packing) and the fluids properties. The relationships between the holdups of various phases and the system variables for a variety of three-phase reactors are discussed in Chaps. 6 through 9. 

Recommendations All existing literature data on gas and liquid holdups for countercurrent flow systems are for packings normally used in absorption towers or gas -liquid reactors. The validity of these data for small packings that would normally be used in gas-liquid-solid catalytic reactions needs to be checked. 

Kim et al.56 studied the effects of air and water velocity and particle size ranging from 2.6 through 6.0 mm on the gas and liquid holdups in a two-dimensional column. Based on their data, they reported the following dimensionless correlations for the liquid and gas holdups ... 

Equations (9-28) and (9-29) allow the evaluation of either he or hL if the other is known. In order to predict both ha and /iL independently, another relation is needed. Darton and Harrison21 derived this relation based on the drift flux approach. The gas drift flux vCD is correlated to the gas and liquid holdups as... 

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