Total liquid holdup

Hoogendoorn and Lips (H10) carried out residence-time distribution experiments for countercurrent trickle flow in a column of 1.33-ft diameter and 5- and 10-ft height packed with -in. porcelain Raschig rings. The fluid media were air and water, and ammonium chloride was used as tracer. The total liquid holdup was calculated from the mean residence time as found... 

Under well-mixed flow conditions, it is reasonable to assume that the mixer holdup volumes, VLmn ind Vomn will vary in direct proportion to the appropriate phase flow rate, and that the total liquid holdup in the mixer will vary as a function of the total flow rate to the mixer. 

Calculate new total liquid holdups from the sum of the component holdups. Then calculate the new liquid mole fractions from the component holdups and the total holdups. 

Liquid holdup is critical in the downflow operation of fixed beds, in contrast to the upflow operation where the liquid occupies practically the whole external free void volume of the bed. Total liquid holdup ht consists of two parts static h, and dynamic holdup liA. Static holdup is related to the volume of liquid that is adherent to the particles surface, whereas dynamic holdup is related to the flowing pari of the liquid. 

In a reactor completely filled with liquid, the wetting efficiency is 100% or, in other words, the external wetting of the catalyst is complete (Burghardt et al., 1995). While it is true that when a fixed bed is completely filled with liquid wetting is complete (wetting efficiency is unity), the opposite is not true in a trickle bed, a portion of the bed voids will be always occupied by the gas phase. Thus, while in a well-operated trickle bed the wetting efficiency could be unity, its total liquid holdup based on the void volume is always lower than the bed voidage, i.e. the bed is never completely filled with liquid. 

Residence time can be as short as 10 sec. One 750 gpm unit is said to have a total liquid holdup of 200 gal. From 3-10 stages per unit have been reported, although Table 14.6 shows a range of 1.8-7.7. A 65 in. dia casing can accommodate throughputs up to 25,000 gal/hr. An economic comparison of a Podbielniak with other... 

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 ... 

A variety of correlations have been developed for the total and the dynamic liquid holdup. For instance, the total liquid holdup has been correlated with the Lockhardt-Martinelli parameter X for spherical and cylindrical particles [Midou, Favier, and Charpentier, J. Chem. Eng. Japan, 9 350 (1976)]... 

High efficiency packing, such as oriented wire packing (Koch/Sulzer, Flexipack, Goodlow), has a thin film of liquid on the wires so that the total liquid holdup is relatively low. Other suggestions from Lees (1980) include using tall thin columns such as those used for heat sensitive materials, and... 

The average total liquid holdup in a trickle bed reactor decreases with increasing bed depth in a low pressure laboratory or pilot scale column. However, for commercial use where there is a moderate to high pressure input, the holdup is essentially constant (Figure 5). 

G(/.) system response to an impulse /ii. total liquid holdup per unit void volume... 

The liquid holdup is largely measured by a tracer technique. In this technique the total liquid holdup (dynamic -4- static) is obtained by multiplying the liquid flow rate by the mean residence time. There are a large number of holdup correlations reported in the literature. Since some correlations are for the total liquid holdup and some for the dynamic holdup, proper precautions should be taken in using these correlations. The liquid holdup has been defined in terms of either void volume or the total volume of the column. 

Basically, two types of correlation for the dynamic or total liquid holdup are reported in the literature. Some investigators have correlated the liquid holdup directly to the liquid velocity nd fluid properties by either dimensional or dimensionless relations. In more recent investigations, the liquid holdup is correlated to the Lockhart-Martinelli parameter APl/APg (or an equivalent of it. as discussed in the earlier section). 

Turpin and Huntington100 obtained their data with tabular alumina partiejes of 0.76 and 0.823 cm diameter packed in 5.1-, 10.2-, and 15.3-cm-diameter columns. The air and water flow rate ranges were 8 through 2,400 g cm-2 h-1 and 2,400 through 20,000 gcm"2h , respectively. The data for the total liquid holdup were correlated using a somewhat different type of relation ... 

Unlike Larkins et al.4 and Abbott et al.,1 Sato et al.7S assumed that the total liquid holdup depends on the specific surface area of the bed as well as on an energy parameter. They correlated their data for the air-water system obtained in 65.8- and 122-mm-i.d. columns packed with six different sized packings between 2.59 and 24.3 mm diameter by a relation... 

Figure 6-6 Comparison of total liquid holdup with predictive correlations Cl = 5 kg m-2 s 1 [from data by Charpentier and Favier14). (o) Comparison with Eq. (6-6), (h) comparison with Eq. (6-37), (e) comparison with Eq. (6-40).

This correlation was in reasonable agreement with the low gas flow (Gg < 0.01 g cm-2 s1) data of Sato et al.74 for a benzoic acid-water system with 5.5- and 12.2-mmparticle diameter. Hirose et al.38 made extensive measurements in trickle-flow, pulsed-flowrand bubble-flow regimes and correlated the enhancement factor in Ks due to parallel gas flow with the liquid velocity. They found that this enhancement factor (ratio of Ks in the presence of gas flow to Ks in the absence of gas flow) was inversely proportional to the total liquid holdup and to a first approximation has the value 2. Their data for Ks as a function of liquid velocity... 

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... 

Experimental values of the total liquid holdup were obtained by simultaneous closure of the inlet and exit solenoid valves, followed by drainage over a 15-min period. In some experiments, these values were checked by actual weighing of the section of the column in between the solenoid valves. The liquid holdup data were obtained by correlating the conductivity to the liquid/gas ratio. The details of the liquid holdup measurement technique are given in the Pittsburgh Energy Research Center s quarterly reports.22... 

Figure 7-12 Dynamic and total liquid holdup as a function of average gas interparticle pore velocity in a 10.2-cm-i.d. column packed with 1.9-cm x 1.9-cm ceramic cylinders.22...

Stiegel and Shah34 reported the liquid holdup characteristics of a packed (with 0.318-cm polyethylene or 0.44-cm equivalent diameter packing) rectangular (16.8 cm x 2.06 cm) column. The column was approximately 122 cm high. The measurements were carried out with an air (in the flow range of 0 through 0.203 kg s 1 m 2) water system. The total liquid holdup was correlated by the relation... 

The liquid holdup characteristics of a countercurrent, three-phase spouted-bed column are studied by Vukovic et al.141 They found that the total liquid holdup increased with the gas flow rate. Both the total and dynamic liquid holdups were comparable to those obtained in cocurrent-upflow three-phase fluidized-bed columns operating with similar gas and liquid flow conditions. 

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