Liquid Hold-Up

Pipelines are cleaned and inspected using pigs . Pigs usually have a steel body fitted with rubber cups and brushes or scrapers to remove wax and rust deposits on the pipe wall, as the pig is pumped along the pipe. Sometimes spherical pigs are used for product separation or controlling liquid hold up. In field lines handling untreated crude may have to be insulated to prevent wax formation. 

Liquid hold up in a tower represents the liquid held in the void spaces of the packing during operating conditions. At flooding, essentially all of the voids are filled with liquid. 

Figure 9-44. Gas-liquid hold-up data for ceramic rings and saddles. Used by permission of Leva, M. Tbwer Packings and Packed Tower Design, 2nd ed., U.S. Stoneware Co. (now, Norton Chemical Process Equipment Corp.) (1953).

Total liquid hold-up in packed bed, h[ = static hold-up, hg, plus operating hold-up, ho [64, 66]. 

Figure 9-46. Physical property corrections for liquid hold-up for ceramic packing and carbon packing (as noted). Reproduced by permission of A/.C/r.E. Jour., Shulman, H. L., Wells, N., Ullrich, C. F., and Proulx, A. Z., V. 1, No. 2 (1955) p. 259 all rights reserved.

Determine (1) the tower diameter (2) pressure drop (3) liquid hold-up... 

Liquid hold-up, ft /ft packed tower volume Operating hold-up for any liquid, ft /ff packing volume... 

Jesser, B. W. and J. C. Elgin, Studies of Liquid Hold-up in Packed Towers, Trans. Amer. Inst. Chem. Engr., 39, No. 3 277 (1943). 

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). 

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. 

Equation 5.2 is found to hold well for non-Newtonian shear-thinning suspensions as well, provided that the liquid flow is turbulent. However, for laminar flow of the liquid, equation 5.2 considerably overpredicts the liquid hold-up e/,. The extent of overprediction increases as the degree of shear-thinning increases and as the liquid Reynolds number becomes progressively less. A modified parameter X has therefore been defined 16 171 for a power-law fluid (Chapter 3) in such a way that it reduces to X both at the superficial velocity uL equal to the transitional velocity (m )f from streamline to turbulent flow and when the liquid exhibits Newtonian properties. The parameter X is defined by the relation... 

Thus, in summary, liquid hold-up can be calculated using equation 5.2 for ... 

A knowledge of hold-up is particularly important for vertical flow since the hydrostatic pressure gradient, which is frequently the major component of the total pressure gradient, is directly proportional to liquid hold-up. However, in slug flow, the situation is complicated by the fact that any liquid which is in the form of an annular film surrounding the gas slug does not contribute to the hydrostatic pressure 14. ... 

The pressure drop due to acceleration is important in two-phase flow because the gas is normally flowing much faster than the liquid, and therefore as it expands the liquid phase will accelerate with consequent transfer of energy. For flow in a vertical direction, an additional term — AZ y must be added to the right hand side of equation 5.5 to account for the hydrostatic pressure attributable to the liquid in the pipe, and this may be calculated approximately provided that the liquid hold-up is known. 

A well-substantiated correlation for air-water systems taken from the trickle bed literature (Morsi and Charpentier, 1981) was used for the volumetric mass transfer coefficients in the / , and (Rewap)i terms in the model. The hi term was taken from a correlation of Kirillov et al. (1983), while the liquid hold-up term a, in Eqs. (70), (71), (74), (77), and (79) were estimated from a hold-up model of Specchia and Baldi (1977). All of these correlations require the pressure drop per unit bed length. The correlation of Rao and Drinkenburg (1985) was employed for this purpose. Liquid static hold-up was assumed invariate and a literature value was used. Gas hold-up was obtained by difference using the bed porosity. 

Specchia, V., and Baldi, G., Pressure drop and liquid hold-up for two-phase concurrent flow in packed beds. Chem. Eng. Sci. 32, 515-523 (1977). 

A horizontal separator would be selected when a long liquid hold-up time is required. 

In the design of a horizontal separator the vessel diameter cannot be determined independently of its length, unlike for a vertical separator. The diameter and length, and the liquid level, must be chosen to give sufficient vapour residence time for the liquid droplets to settle out, and for the required liquid hold-up time to be met. 

Size the vessel to restrict the carryover of liquid droplets. The liquid hold-up time need not be considered, as the liquid level will be a function of the thermal design. 

The basic theory of batch distillation is given in Volume 2, Chapter 11 and in several other texts Hart (1997), Perry et al. (1997) and Walas (1990). In the simple theoretical analysis of batch distillation columns the liquid hold-up in the column is usually ignored. This hold-up can have a significant effect on the separating efficiency and should be taken into account when designing batch distillation columns. The practical design of batch distillation columns is covered by Hengstebeck (1976), Ellerbe (1997) and Hart (1997). 

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