Packings liquid holdup

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 ... 

FIG. 14-59 Typical vendor data for liquid holdup of a structured packing, Gempak 2A. [Cou7tesy Glitsch, Inc., Dallas, Texas.]... 

FIG. 14-60 Comp arison of measured and calculated values of liquid holdup for Gempak 2A structured packing, air-water system. [Rocha et al., Ind. Eng. Chem., 32, 641 (1.9.93).] Reproduced with permission. Copyright 199.3 American Chemical Society. 

Pressure drop through gauze and sheet metal structured packings [115] applies for the region below the loading point and cannot predict the flood point because liquid holdup vs. gas velocity is not included. The latest version of the equation is in Reference 108 ... 

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... 

The central difficulty in applying Equations (11.42) and (11.43) is the usual one of estimating parameters. Order-of-magnitude values for the liquid holdup and kiA are given for packed beds in Table 11.3. Empirical correlations are unusually difficult for trickle beds. Vaporization of the liquid phase is common. From a formal viewpoint, this effect can be accounted for through the mass transfer term in Equation (11.42) and (11.43). In practice, results are specific to a particular chemical system and operating mode. Most models are proprietary. 

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). 

Note that the parameter VL/VR in a slurry reactor is the fractional liquid holdup. In the packed bubble bed reactor and the trickle-bed reactor, under complete recycling of the liquid stream, VL/VR is the ratio of total volume of the liquid that is processed (recycled) to the volume of reactor, and is always greater than 1. By recycling, it is possible to process a larger volume of liquid than the reactor volume by having a surge tank in the recycle line. 

Inglezakis, V.J., Zorpas, A.A., Grigoropoulou, H.P., Arapoglou, D., Liquid holdup measurements using tracing techniques in zeolite packed beds, 5th International Scientific and Technical Conference Water Supply and Water Quality, Poznan, Poland, 430-440 (2002)... 

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. 

If the liquid holdup is too low, fractionation efficiency will be bad. We say that the height equivalent to a theoretical plate (HETP) will be high. If the liquid holdup is too high, fractionation efficiency will also be poor. We again say that the HETP will be high. This idea is expressed in Fig. 7.6. When the holdup rises above the point that corresponds to the minimum HETP, we can say that the packing is beginning to flood. The minimum HETP point on Fig. 7.6 corresponds to the point of incipient flood, discussed in Chap. 1. 

For structured-type packing, a liquid holdup of 4 to 5 percent corresponds to this optimum packing fractionation efficiency. For 1-in Raschig rings, this optimum holdup would be roughly 10 to 12 percent. 

The amount of liquid holdup in the packing is of interest when the liquid is unstable or when a desirable reaction is to be carried out in the vessel. A correlation for Raschig rings, Berl saddles, and intalox saddles is due to Leva (Tower Packings and Packed Tower Design, U.S. Stoneware Co., Akron, OH, (1953) ... 

Stichlmair and Fair (Distillation Principles and Practice, Wiley -VCH, New York, 1998) show that liquid holdup is primarily a function of the liquid Froude number, specific surface area of the packing, and physical properties. They recommend the correction by Engel, Stichlmair, and Geipel [Ind. Eng. Chem. Symp. Ser. 142, 939 (1997)]. 

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