Packed columns liquid holdup

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... 

High recovery of a volatile component by a batch operation is required. Liquid holdup is much lower in a packed column. 

Liquid Holdup Three modes of liquid holdup in packed columns are recognized ... 

Piret et al. measured liquid holdup in a column of 2J-ft diameter and 6-ft packed height, packed with graded round gravel of lj-in. size, the total voidage of the bed being 38.8%. The fluid media, air and water, were in countercurrent flow. The liquid holdup was found to increase markedly with liquid flow rate, but was independent of gas flow rate below the loading point. Above the loading point, an increase of liquid hold-up with gas flow rate was observed. 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

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... 

Fig. 5.2.4 Plot of the liquid holdup as a function of increasing liquid velocity. An increase in the liquid holdup is observed with increasing liquid velocity. The gas superficial velocity is constant at 66 mm s-1. Data are shown for a bed of 5-mm diameter glass spheres packed within a column of inner diameter 40 mm [40]. Reproduced from Ref. [40], with kind permission from Elsevier, Copyright Liquid velocity (mm s-1) (2001).

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

The concept of column void volume (Vg) is important for several reasons. Void volume is the volume of the empty column minus the volume occupied by the solid packing materials. It is the liquid holdup volume of the column that each analyte must elute from. Note that the void volume is equal to the void time multiplied by the flow rate (T). 

The older tall oil distillation columns used bubble cap trays. In new columns, structured packing is preferred. Because of the low pressure drop of structured packing, steam injection is no longer necessary. The low liquid holdup of this packing minimizes the reactions of the fatty and resin acids. A specific distillation sequence for vacuum columns using structured packing of Sulzer has been described (26). Depitching is carried out at a vacuum of... 

To increase product recovery in batch distillations, as a result to the lower liquid holdup in a packed column. 

FIG. 14-73 Liquid holdup, air-water data by Billet ( Packed Column Design and Analysis, Ruhr University, Bochum, Germany), preloading regime. (From Kister H. Z., Distillation Design, copyright by McGraw-Hill reprinted with permission.)... 

The measured cross-sectionally averaged liquid holdup (i.e. measured liquid saturation times the bed voidage) are consistent with previously reported results [4], For a similar packing structure, Illiuta and Larachi [4] found liquid holdups, as calculated by their mechanistic model and compared with the reported experimental results, slightly lower than the values obtained in this study. However, there are some differences between the two studies this work was performed in a smaller diameter column ( 30 cm versus... 

I. Iliuta, F. Larachi, Mechanistic model for structured-packing containing columns Irrigated pressure drop, liquid holdup and packing fractional wetted area, Ind. Eng. Chem. Res. 40 (2000) 5140-5146. 

D. Toye, P. Marchot, M. Crine, M. Pelsser, G. L Homme, Local measurements of void fraction and liquid holdup in packed columns using X-ray computed tomography, Chem. Eng. Process. 37 (6) (1998) 511-520. 

F. Yin, A. Afacan, K. Nandakumar, K.T. Chuang, Liquid holdup distribution in packed columns gamma ray tomography and CFD simulation, Chem. Eng. Process. 41 (5) (2002) 473-483. 

The liquid holdup of the packing section decreases, which leads to a lower conversion of the kinetically controlled reactions of C02 and a reduction in the C02 absorption rate. As a consequence, the solvent mole fractions of HC()3 and carbamate decreases whereas the relative fraction of HS increases. The selectivity of the absorption process toward the H2S and HCN reduction is enhanced by minimizing the liquid holdup of the column. At the same time, a larger interfacial area improves the performance of the plant. Therefore, modem industrial sour gas scrubbers should be equipped with structured packings. 

It is shown, that the performance of a pulsing packed column can be split up into its two component parts, the pulses and the zones in between pulses. The pulses can be described as parts of the bed already in the dispersed bubble flow regime the zones-in between the pulses as parts of the bed still in the gas-continuous regime. The pulse frequency is linearly dependent upon the real liquid velocity. The properties of the pulse, like holdup, velocity and height are quite independent upon all the parameters except gas flow rate. 

Several final points remain to be considered, Fir, t, all of the discussions above ignored the effect of liquid holdup in the column and condenser, but because the combined volumes of these two are quite small as compared to the volumes in the still and receiver (particularly because so many batch stills include packed sections rather than trays), this assumption does not seem unwarranted, particularly for binary mixtures. 

Batch distillation. Because of the smaller liquid holdup of packed columns, a higher percentage of the liquid can be recovered as top product. 

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... 

Low weight and low liquid holdup. The total weight of the column and the resultant foundation load is low if the weight of the packing and the liquid holdup in the tower are low. The amount of liquid holdup, however, must be sufficiently great to retain an effective driving force for mass transfer. 

The effect of liquid flow rate on (AP/AZ)LG is taken into consideration by its effect on the liquid holdup. The form of Eq. (6-19) is taken as the same as the Ergun. equations25-60 for flow through packed beds using an effective voidage available fox gas flow defined as (1 — c — fi0L — k), where c is. the vo.lume fraction occupied by solid. /i0L is the liquid holdup per unit volume of column, and k is the effective deadspace volume per unit volume of column (taken here to be 0). 

Two forms of Eq. (6-21) have been proposed. For low values of the Reynolds number of liquid flow, liquid holdup in a packed column can be predicted by consideration of laminar liquid flow down inclined surfaces against a pressure gradient In this viscous flow regime,... 

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... 

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