Holdup, liquid

Liquid holdup in a bioreactor can be measured by recording the residence time distribution (RTD) of a tracer that is injected into the bioreactor (Boyer et al., 2002). By following the tracer, the liquid mixing can also be characterized. The most common RTD method involves injecting a small amount of salt tracer and then measuring 

The volumetric tray liquid holdup can be considered consisting of two main parts the liquid on the active tray area and the liquid in the downcomer. These holdups can be estimated from the liquid heights, hi and h, and the corresponding areas  

An absorber column is to be designed for lowering the concentration of acetone 

The absorber will be a trayed column. using sieve trays with the following  

Froth density in the downcomer Fraction of flood velocity 

Since the vapor rate is highest at the top, the column will be sized based on the top tray conditions. The liquid and vapor on that tray will be represented by the absorbent and the overhead vapor. 

For most random packings, the pressure drop suffered by the gas is influenced by the gas and liquid flow rates. At a fixed gas velocity, the gas-pressure drop 

The specific liquid holdup (i.e., volume of liquid holdup/volume of packed bed) in the preloading region has been found from extensive experiments by Billet and Schultes (1995) for a wide variety of random and structured packings and for a number of gas-liquid systems to depend on packing characteristics, and the viscosity, density, and superficial velocity of the liquid according to the dimensionless expression 

Values of a and Ch are characteristic of the particular type and size of packing, as listed together with packing void fraction, e, and other packing constants in Table 4.1. Because the specific liquid holdup in the preloading region is constant, equation (4-3) does not involve gas-phase properties or gas velocity. 

At low liquid velocities, liquid holdup can be so small that the packing is no longer completely wetted. When this happens, packing mass-transfer efficiency decreases dramatically, particularly for aqueous systems of high surface tension. To ensure complete wetting of packing, proven liquid distributors and redistributors should be used and superficial liquid velocities should exceed the following values (Seader and Henley, 1998)  

Example 4.2 Specific Liquid Holdup and Void Fraction in Second- and Third-Generation Random Packings 

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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)]. 

Although a number of studies were made and approximate methods developed for predicting the effect of liquid holdup in the period of the 1950s and 1960s, as summarized in the 6th edition of Peny .s Chemical Engineers Handbook, the complexity of the effect of liqmd holdup is such that it is now best to use computer-based batch-distillation algorithms to determine the effect of holdup on a case-bycase basis. 

For preliminary studies of batch rectification of multicomponent mixtures, shortcut methods that assume constant molal overflow and negligible vapor and liquid holdup are useful. The method of Diwekar and Madhaven [Ind. Eng. Chem. Res., 30, 713 (1991)] can be used for constant reflux or constant overhead rate. The method of Sundaram and Evans [Ind. Eng. Chem. Res., 32, 511 (1993)] applies only to the case of constant remix, but is easy to apply. Both methods employ the Fenske-Uuderwood-GiUilaud (FUG) shortcut procedure at successive time steps. Thus, batch rectification is treated as a sequence of continuous, steady-state rectifications. 

Derivatives or rates of change of tray and condenser-reflux drum hquid holdup with respecl to time are sufficiently small compared with total flow rates that these derivatives can be approximated by incremental changes over the previous time step. Derivatives of liquid enthalpy with respect to time eveiywhere can oe approximated in the same way. The derivative of the liquid holdup in the reboiler can likewise be approximated in the same way except when reflux ratios are low. 

At the bottom of the column, a liquid sump of constant and perfectly mixed molar liquid holdup Mg is provided. A portion of the hq-uid flowing from this sump passes to a thermosiphon reboiler, with the... 

The liquid holdup on each of the Nt eqmlibrium trays is assumed to be perfectly mixed but will vary as liquid rates leaving the trays vary. Vapor holdup is assumed to be negligible everywhere. Tray molar vapor rates V vary with time but at any instant in time are eveiy-where equal. 

Open-loop behavior of multicomponent distillation may be studied by solving modifications of the multicomponent equations of Distefano [Am. Inst. Chem. Eng. J., 14, 190 (1968)] as presented in the subsection Batch Distillation. One frequent modification is to include an equation, such as the Francis weir formula, to relate liquid holdup on a tray to liquid flow rate leaving the tray. Applications to azeotropic-distillation towers are particularly interesting because, as discussed by and ihustrated in the Following example from Prokopalds and Seider... 

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

More often than not the rate at which residual absorbed gas can be driven from the liqmd in a stripping tower is limited by the rate of a chemical reaction, in which case the liquid-phase residence time (and hence, the tower liquid holdup) becomes the most important design factor. Thus, many stripper-regenerators are designed on the basis of liquid holdup rather than on the basis of mass transfer rate. 

We note that when the second term in the denominator of Eq. (14-64) is small, the liquid holdup in the tower can have a significant influence upon the rate of absorption if an extremely slow chemical reaction is involved. 

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. 

Region III, P < 0.02. Reaction is slow and occurs in the bulk hquid. Interfacial area and liquid holdup should be high, especially the latter. Bubble columns will be suitable. 

Three criteria for scale-up are that the laboratory and industrial units have the same mass-transfer coefficients /cg and E/cl and the same ratio of the specific interfacial surface and liquid holdup Tables 23-9 and 23-10 give order-of-magnitude values of some parameters that may be expected in common types of liquid/gas contactors. 

TABLE 23-9 Mass-Transfer Coefficients/ Interfacial Areas and Liquid Holdup in Gas/Liquid Reactions... 

Liquid holdup is made up of a dynamic fraction, 0.03 to 0.25, and a stagnant fraction, 0.01 to 0.05. The high end of the stagnant fraction includes the hquid that partially fills the pores of the catalyst. The effective gas/liquid interface is 20 to 50 percent of the geometric surface of the particles, but it can approach 100 percent at high hquid loads with a consequent increase of reaction rate as the amount of wetted surface changes. 

Liquid Holdup The major factor influencing this property is the liquia flow rate, but the shape, size, and wetting characteristics of the particles and the gas rate and the initial distribution of liquid also enter in. One of the simpler correlations is that of Midoux et al. (J. 

Assume an accumulator having a liquid holdup of 2,000gal. Assume that the liquid leaving as reflux plus distillate is 40GPM. Further assume that the accumulator liquid starts at 2.0 ol % of a key component and that the test conditions produce 0.0 vol. % of the key component. How long will it take for the concentration to reach... 

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. 

Fractionators and Other Towers - An equivalent "tower dumped" level is calculated by adding the liquid holdup on the trays to the liquid at normal tower bottom (high liquid level). The surface that is wetted by this equivalent level and which is within 7.5 m of grade is used. 

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. 

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