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Estimation of holdup

Distillation columns have a large inventory in the reboiler, and typically, an inventory several times greater in the column itself. Column holdup may be reduced by using low holdup internals. Conventional trays and packings differ by a factor of about 10 in inventory per theoretical plate (Kletz, 1985a). An estimate of holdup per theoretical plate is shown (Lees, 1980). [Pg.93]

Other procedures based on approaches similar to those mentioned have been used for estimation of holdup time values when they are required for calculating retention indices LRI in Equation (4) (see Section 3.1.2). In [13], Hm was obtained by extrapolation from the experimental retention of an n-alkanes mixture at two temperatures, which was fitted to the number of carbon atoms using its linear relationship with log k. A linear relationship with temperature, T, was assumed in order to estimateat other temperatures. Arey et al. [34] found that ethane and propane eluted in the column with the same retention as bleed from column, using the last as a marker through the GCxGC elution process for calculation. [Pg.63]

The advantage of this concept is that it shows the strong effect to be expected of the gas snperficial velocity on mass transfer. This is certainly found experimentally. Assnming a typical rise velocity of gas bubbles of 0.3 m/s, it gives a crude estimate of holdup. As gas volume fraction increases, hindered rising and bubble swarms break down this simple relation. This relation also shows that a decrease in bubble size which leads to more surface area, and slower rise velocity results in more holdup. [Pg.792]

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. [Pg.88]

Static holdup depends upon the balance between surface-tension forces tending to hold hquiciin the bed and gravity or other forces that tend to displace the liquid out of the bed. Estimates of static holdup (for gravity drainage) may be made from the following relationship of Shulman et al. [Am. Jn.st. Chem. Eng. J., 1, 259 (1955)] ... [Pg.1393]

The power for agitation of two-phase mixtures in vessels such as these is given by the cuiwes in Fig. 15-23. At low levels of power input, the dispersed phase holdup in the vessel ((j)/ ) can be less than the value in the feed (( )df) it will approach the value in the feed as the agitation is increased. Treybal Mass Transfer Operations, 3d ed., McGraw-HiU, New York, 1980) gives the following correlations for estimation of the dispersed phase holdup based on power and physical properties for disc flat-blade turbines ... [Pg.1468]

At present, Eq. (68) only provides a simple estimate of the mass and energy transfer processes in forced-flow nucleation. The methods for evaluating the parameters must be improved by further detailed research on forced-flow nucleation. In particular, the calculation of the rate of nucleation n and of the bubble departure frequency / are the weakest points in the analysis of this heat-transfer region. Obviously, accurate prediction of the pressure drop and holdups are also needed. [Pg.42]

This correlation gives, for perfectly wettable solids, fairly good estimates of the static holdup for different particle-geometries and sizes. Saez and Carbonnel [26] used the hydraulic diameter, instead of the nominal particle diameter, as the characteristic length in the Eotvos number, to include the influence of the particle geometry on the static hold-up. However, no improvement could be obtained in correlating the data with this new representation. [Pg.283]

More recent literature regarding generalized correlational efforts for gas holdup is adequately reviewed by Tsuchiya and Nakanishi [Chem. Eng Sci., 47(13/14), 3347 (1992)] and Sotelo etal. [Inf. Chem. Eng., 34(1), 82-90 (1994)]. Sotelo et al. (op. cit.) have developed a dimensionless correlation for gas holdup that includes the effect of gas and liquid viscosity and density, interfacial tension, and diffuser pore diameter. For systems that deviate significantly from the waterlike liquids for which Fig. 14-104 is applicable, their correlation (the fourth numbered equation in the paper) should be used to obtain a more accurate estimate of gas holdup. Mersmann (op. cit.) and Deckwer et al. (op. cit.) should also be consulted. [Pg.110]

For the second case, a hollow ball was dipped inside a jar filled with water and scanned. The diameters of the ball and jar were 6.98 and 18.95 cm, respectively, as shown in Fig. 5a. A scan area of 27 cm in diameter was reproduced using the reconstruction algorithm. The dimensions of the objects as reproduced by the scan were 6.97 and 19.20 cm, respectively, as shown in Fig. 5b, which give a maximum spatial error of about 2.5 mm. This is good enough to resolve relatively small maldistribution, if it exists, inside the 30.48-cm-diameter column used in this study. The figure shows that the overall error in the estimated total holdup is within 12.8%. [Pg.63]

The volume diameter of a particle may be useful in applications where equivalent volume is of primary interest, such as in the estimation of solids holdup in a fluidized bed or in the calculation of buoyancy forces of the particles. The volume of a particle can be determined by using the weighing method. Sauter s diameter is widely used in the field of reacting gas-solid flows such as in studies of pulverized coal combustion, where the specific surface area is of most interest. [Pg.6]

X-—The fraction of (inter-particle) void space pixels containing some liquid provides an upper estimate of liquid saturation, from which values of liquid holdup are obtained. By extrapolation of the data to zero liquid superficial... [Pg.120]

Example 5 Determination of holdup time in downcomer. A valve-tray tower with 24-in. plate spacing and liquid crossflow contains straight segmental downcomers. The overflow weir at the downcomer entrance is formed by an extension of the downcomer plate. The height of this weir is 3 in. The inside diameter D of the tower is 5 ft, and the weir length is 0. 60. If liquid with a density of 55 lb/ft3 flows across die plate at a rate of 30,000 lb/h, estimate the residence or holdup time in the downcomer from this plate. [Pg.685]

Various methods for estimating KLs are described by Satterfield.150 The most conservative estimate of KLS is obtained as KI S = D/<5,, where D is the molecular diffusivity of the reactant in the liquid phase and <5L the average thickness of liquid film surrounding the particles. This estimation assumes no turbulence in the liquid film. The average thickness of the liquid film can be obtained from a knowledge of the dynamic liquid holdup and the outside area of catalyst particles per unit volume of the reactor, os. For example, if the dynamic liquid holdup is 50 percent of the void volume e, then <5L = e/2as. Various methods for estimating fcL and Ks under trickle-flow conditions are described in Chap. 6. [Pg.48]

Recommendations The gas holdup in the bubble-flow regime can be estimated using either Cq. (7-13) or F.q. (7-14). For the estimation of liquid holdup in the bubble-flow regime, use of Eq. (7-9) is recommended. In the pulsed-flow regime, the data of PERC and Eq. (7-15) would be useful. More experimental work with the hydrocarbon systems is needed. [Pg.247]

A rough estimate of the decanter volume required can be made by taking a holdup time of 5 to 10 minutes, which is usually sufficient where emulsions are not likely to form. Methods for the design of decanters are given by Hooper (1997) and Signales (1975). The general approach taken is outlined here and illustrated by Example 10.3. [Pg.585]

In bubble columns and gas-liquid stirred reactors, the estimation of parameters is more difficult than in gas-solid or liquid-solid fluidized beds. Solid particles are rigid, and hence the fluid-solid interface is nonde-formable, whereas the gas-liquid interface is deformable. In addition, the effect of surface-active agents is much more pronounced in the case of gas-liquid interfaces. This leads to uncertainties in the prediction of all major parameters, such as the terminal bubble rise velocity, the bubble diameter, the gas holdup, and the relation between the bubble diameter and the terminal bubble raise velocity. [Pg.1172]

Prakash and Bendale (10) have undertaken an extensive literature search on gas holdup and dispersion correlations in slurry bubble column reactors. They tested various correlations for the estimation of gas holdup against experimental data and found that the average absolute relative error varied from 12 to 165%. These large errors are primarily due to the fact that literature correlations for gas holdup are based on constant gas velocity along the column height. In reality. [Pg.128]


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