Liquid holdup pressure

TTie technological scheme of the experimental installation for measuring liquid holdup, pressure drop, loading and flooding points, effective... 

Pressure drop due to hydrostatic head can be calculated from hquid holdup B.]. For nonfoaming dilute aqueous solutions, R] can be estimated from f i = 1/[1 + 2.5(V/E)(pi/pJ ]. Liquid holdup, which represents the ratio of liqmd-only velocity to actual hquid velocity, also appears to be the principal determinant of the convective coefficient in the boiling zone (Dengler, Sc.D. thesis, MIT, 1952). In other words, the convective coefficient is that calciilated from Eq. (5-50) by using the liquid-only velocity divided by in the Reynolds number. Nucleate boiling augments conveclive heat transfer, primarily when AT s are high and the convective coefficient is low [Chen, Ind Eng. Chem. Process Des. Dev., 5, 322 (1966)]. 

Except for special sihiations, pressure relief devices are not provided for fire exposure of heat exchangers, air fins, or piping, nor are the exposed surfaces of such items included for calculating the fire exposure heat input. Special situations may be congestion and substandard spacing, or unusually large equipment with normal liquid holdup over about 4 m and/or which represents over 15% of the total wetted surface of the system to which it is directly connected for pressure relief. 

Eaton, Ben A., el al., The Prediction of Flow Patterns, Liquid Holdup and Pressure Losses Occurring During Continuous Tw o-Phase Flow in Horizontal Pipelines. /. Petrol. TechnoL, June 1967, pp. 315-328. 

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 ... 

Larkins et al. (L2) visually observed flow patterns and measured pressure drop and liquid holdup for cocurrent downflow of gas and liquid through beds of spheres, cylinders, and Raschig rings of diameters from 3 mm to f in. in experimental columns of 2- and 4-in. diameter, as well as in a commercial unit several feet in diameter. The fluid media were air, carbon dioxide, or natural gas and water, water containing methylcellulose, water containing soap, ethylene glycol, kerosene, lubricating oil, or hexane. 

The distribution of air and liquid in the pipe, and the proportion of the cross-section occupied by liquid (the holdup of the liquid) have an important beating on the flow of the two fluids, firstly because the hydrostatic pressure is affected by the liquid holdup, tind secondly because the nature of the flow affects the frictional pressure drop. Furthermore, the velocity of the air relative to the liquid is also dependent upon its pattern of distribution. 

A separated flow model for stratified flow was presented by Taitel and Dukler (1976a). They indicated analytically that the liquid holdup, R, and the dimensionless pressure drop, 4>G, can be calculated as unique f unctions of the Lockhart-Martinelli parameter, X (Lockhart and Martinelli, 1949). Considering equilibrium stratified flow (Fig. 3.37), the momentum balance equations for each phase are... 

From this discussion of parameter evaluation, it can be seen that more research must be done on the prediction of the flow patterns in liquid-liquid systems and on the development of methods for calculating the resulting holdups, pressure drop, interfacial area, and drop size. Future heat-transfer studies must be based on an understanding of the fluid mechanics so that more accurate correlations can be formulated for evaluating the interfacial and wall heat-transfer coefficients and the Peclet numbers. Equations (30) should provide a basis for analyzing the heat-transfer processes in Regime IV. 

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 column contains a total of N-p theoretical trays. The liquid holdup on each tray including the downcomer is M . The liquid on each tray is assumed to be perfectly mixed with composition x,. The holdup of the vapor is assumed to be negligible throughout the system. Although the vapor volume is large, the number of moles is usually small because the vapor density is so much smaller than the liquid density. This assumption breaks down, of course, in high-pressure columns. 

From the above equation, the variation of equilibrium disjoining pressure and the radius of curvature of plateau border with position for a concentrated emulsion can be obtained. If the polarizabilities of the oil, water and the adsorbed protein layer (the effective Hamaker constants), the net charge of protein molecule, ionic strength, protein-solvent interaction and the thickness of the adsorbed protein layer are known, the disjoining pressure II(x/7) can be related to the film thickness using equations 9 -20. The variation of equilitnium film thickness with position in the emulsion can then be calculated. From the knowledge of r and Xp, the variation of cross sectional area of plateau border Qp and the continuous phase liquid holdup e with position can then be calculated using equations 7 and 21 respectively. The results of such calculations for different parameters are presented in the next session. 

Alternatively, experimental measurement of the variation of equilibrium continuous liquid holdup with position for a concentrated oil-in-water emulsion can be employed to infer the variation of disjoining pressure with film thickness. Since the continuous phase liquid holdup e is known as a function of position, xp, Op and r can be calculated using equations 7,21 and 24. Equation 24 will then yield the disjoining pressure II at the film thickness xp. ... 

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... 

In the same way, Larachi et al. [48] evaluated with an important trickle-flow-regime database (4,000 experiments) different phenomenological models for liquid holdup and two-phase pressure drop in TBR. Table 5.2-5 summarizes the respective scatters (mean relative error and deviation) between the experimental values of pressure drop, AP/Z, and liquid holdup, fit, and their predictions by the different models. 

Statistical evaluation of models for two-phase pressure drop and liquid holdup in trickle flow... 

Figure 5.2-25. Influence of the total reactor pressure and of the liquid flow-rate on the dynamic liquid holdup for the single water operation (after Wammes et al. [34]).

Figure 6.9. Pressure drop gradient and liquid holdup in liquid-gas concurrent flow in granular beds. [Sato, Hirose, Takahashi, and Toda, J. Chem. Eng. Jpn. 6, 147-152 (1973)]. (a) Correlation of the two phase pressure drop gradient A P/L, = 1.30 + 1.85Y-0 85. (b) Correlation of frictional holdup hL of liquid in the bed a, is the specific surface, 1/mm, d is particle diameter, and D is tube diameter, h, - 0.4a]/3Y 22.

Hewitt el al. (H10), 1963 Numerous data on pressure drops and liquid holdup in vertical upward cocurrent flow of air and water films, and comparisons with published theories. 

As the liquid holdup increases, the effective orifice diameter may become so small that the liquid surface becomes continuous across the cross section of the column. Column instability occurs concomitantly with a rising continuous-phase liquid body in the column. Pressure drop shoots up with only a slight change in gas rate (condition C or C ). The phenomenon is called flooding and is analogous to entrainment flooding in a tray column. 

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