Liquid holdup in packed columns

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

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. 

Engel V, Stichmair J, Geipel W (1997) A new model to predict liquid holdup in packed columns-using data based on crqracitance measurement techniques. In Institution of chemical engineers symposium series, pp 939-947... 

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. 

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. 

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... 

Distillation control schemes may be analyzed either on a steady-stale (sensitivity analysis) or on a dynamic basis. The latter requires a dynamic model that takes into account the dynamic response of the column and the control loope. An example of a dynamic model is described by McCune and Gailier,6 but il should be apparent from the material presented enrlier that the holdup characteristics of distillation columa devices can vary widely, and snch variation should be accommodated by the model. The development of the naw high-efficiency packings has caused a new look at the system dynamics when the liquid holdup in the column is quite low, and thus the existing models for trays may not be adjustable to application to packings. The use of a tray-type dynamic model is described in the article by Gailier and McCune 7 so work to date has been reported for packed column dynamic models. 

Packed columns are not likely to be affected by inverse response, but may in principle experience a related problem. In the loading regime, a rise in vapor flow increases liquid holdup in the column. With scheme 16.4e, raising boilup will therefore induce a drop in bottom level, which in turn will reduce boilup. If sufficiently vigorous, this phenomenon can unsettle column boilup. The expectation of this behavior is based on first principles only the author is not aware of any troublesome reports of this behavior. 

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

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. 

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. 

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,... 

Very recently, an interesting study of the liquid holdup in a packed column with cocurrent gas-liquid upflow was carried out by the Pittsburgh Energy Research Center.22 In this study, a series of radial conductivity measurements was performed on air-water flow through a 10.2-cm-i.d. clear acrylic column, with various packings. Three packing sizes (1.9-cm x 1.9-cm, 0.635-cm x 0.635-cm, and 0.32-cm x 0.32-cm cylinders) were examined. The conductivity detection system comprised two solid stainless-steel rods fitted with Teflon jackets, leaving only the... 

The experimental data were largely obtained in bubble-flow and pulsed-flow regimes. A typical radial variation in the liquid holdup obtained under pulsed-flow regime is shown in Fig. 7-11. Runs nos. 1 and 2 in this figure are duplicate runs. Although the manner in which the column was packed may have had some effect on the holdup profile, it is clear from this figure that the liquid holdup profile was relatively flat in the center of the tube and was very sharp near the wall. It should be noted that the liquid holdup in this study was defined in terms of fraction of open reactor volume occupied by the liquid. 

Holdup of liquid can be quite low in packed columns, an advantage when the liquid is thermally sensitive. 

The liquid holdup in the packed column is considerably less than in the plate column. 

Depending on the tray geometry (diameter, weir height, downcomer design, etc.) in trayed columns or on the type of packing in packed columns, a certain amount of liquid is retained within the column as liquid holdup. An additional amount of liquid holdup is associated with the condenser assembly. The remaining space within the distillation system contains the vapor holdup. In continuous columns, holdup is usually not a factor since it does not affect the separation as long as the column operates at steady state. 

For distillation columns Add sufficient trays to account for disturbances and anticipated expansion of production. The expected turndown ratios affect the choice of internals in a distillation column with a large turndown ratio, suggesting the use of bubble caps a low turndown ratio may point to structured packings. At the same time, safety and hazard analysis indicates that we want a minimum of liquid holdup in the system. 

Absorption and stripping are usually conducted in packed columns or in trayed towers. Packed columns are preferred when (1) the required column diameter is less than 60 cm (2) the pressure drop must be low, as for a vacuum service (3) corrosion considerations favor the use of ceramic or polymeric materials and/or (4) low liquid holdup is desirable. Trayed towers are preferred when (1) the liquid/gas ratio is very low, and (2) frequent cleaning is required. If there is no overriding consideration, cost is the major factor to be taken into account when choosing between packed columns and trayed towers for absorption or stripping. 

Gas-liquid reactions are sometimes carried out in packed columns. Although the reaction takes place in the liquid phase, the holdup of liquid is not measured, and the reaction rate is given per unit volume of the packed column. The rate is then a function of packing characteristics, liquid rate, and physical properties that affect the holdup as well as kinetic factors. 

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