Packings holdup

Keywords Hydrodynamics Structured packing Holdup Computed tomography... 

Mass median size particle in the )J,m ft flLo Packing holdup in preloading -/- -/-... 

In the case of a packed column, the terms on the right-hand side should each be divided by the voidage, ie, the volume fraction not occupied by the soHd packing (71). In unpacked columns at low values of the sHp velocity approximates the terminal velocity of an isolated drop, but the sHp velocity decreases with holdup and may also be affected by column internals such as agitators, baffle plates, etc. The sHp velocity can generally be represented by (73) ... 

Temperature-sensitive mixtures are to be separated. To avoid decomposition and/or polymerization, vacuum operation may then be necessary. The smaller liqmd holdup and pressure drop theoretical stage of a packed column may be particularly desirable. 

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

Holdup of liepikl can be epiite lox idn packed columns, an advantage x ldjen tbe licpiid is tbeiniaU) sensitive. 

Sepn. Purif., 3, 19 (1989)] takes holdup into account and applies to random as well as structured packings. It is somewhat cumbersome to use and requires three constants for each packing type and size. Such constants have been evaluated, however, For a number of commonly used packings. A more recent pressure drop and holdup model, suitable for extension to the flood point, has been pubhshed by Rocha et al. [Jnd. Eng. Chem. Research, 35, 1660 (1996)]. This model takes into account variations in surface texturing of the different brands of packing. 

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

Static holdup is the amount of liquid remaining on packing that has been fully wetted and then drained. Total holdup is the amount of liquid on the packing under dynamic conditions. Operating holdup is the amount of liquid attributed to operation and is measured experimentally as the difference between total and static holdup. Thus,... 

For other packings and for the case in which static holdup is changed by gas flowing through the bed, the method of Dombrowsld and Brownell [Jnd. Eng. Chem., 46, 1207 (1954)], which correlates static holdup with a dimensionless capillary number, should be used. 

Typical total holdup data for packings are shown in Figs. 14-58 and 14-59. It should be noted that over much of the preloading range gas rate has little effecl on holdup. 

FIG. 14-58 Typical holdup data for random packings and the air-water system. The raschig rings are of ceramic material. To convert pounds per hour per fr to Idlograms per second per m , multiply hy 0.001.356 to convert inches to millimeters, miinltiplyhy 25.4. [Shulman etal., AIChE J. i, 247 (I.9.5.5).]... 

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. 

Correlations for axial dispersion in beds packed with very small particles (<50 Im) that take into account the holdup of liquid in a bed are discussed by Horvath and Lin [J. Chromatogr, 126, 401 (1976)]. 

Region I, P > 2. Reaction is fast and occurs mainly in the liquid film so C L 0- The rate of reaction = kj aEC i will be large when a is large, but hquid holdup is not important. Packed towers or stirred tanks will be suitable. 

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 ... 

Lower holdup of liquid Less than other packings, may require special attention to operationtil controls. 

Otake, T. and K. Okada, Liquid Hold-up in Packed Towers, Operating and Holdup Without Gas Flow, Soc. Chem. Engrs. Qapan) 17, No. 7, 176 (1953). 

Shulman, H. L., C. F. Ullrich and N. Wells, Performance of Packed Columns, Total, Static and Operating Holdups, Amer. Inst. Chem. Engr. four. 1, No. 2, 247 (1955). 

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... 

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