Operating holdup

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,... 

Operating holdup may be estimated by the dimensionless equation of Buchanan [Jnd. Eng. Chem. Eundam., 6,400 (1967)] ... 

Operating holdup contributes effectively to mass-transfer rate, since it provides residence time for phase contact and surface regeneration via agglomeration and dispersion. Static holdup is hmited in its contribution to mass-transfer rates, as indicated by Thoenes and Kramers [Chem. Eng. ScL, 8, 271 (1958)]. In laminar regions holdup in general has a negative effecl on the efficiency of separation. 

The volume of droplets within the contactor at any time is referred to as the operational holdup of the dispersed phase, generally expressed as a fraction of the contactor volume. 

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). 

FIG. 14-74 Effect of liquid and gas rates on the operating holdup of modem random packings (25-mm NorPac ). [From R. Billet and M. Schultes, IChemE Symp. Ser. 104, p. A159, 1987. Reprinted courtesy of the Institution of Chemical Engineers (UK). ]... 

The effect of liquid and vapor rates on the operating holdup is shown in Fig. 8.21. In the preloading regime holdup is essentially independent of vapor flow (100,101), but is a strong function of liquid flow rate and packing size. Smaller-size packings and high liquid rates tend to have more holdup. 

When the reaction occurs only in the liquid phase, only dynamic or operating holdup is important for kinetic data evaluation. However, when the reaction occurs both in the liquid and gas phases, both static and dynamic liquid holdups affect the reaction rates. 

Liquid holdup is a function of liquid flow rate and column pressure drop. Two types of holdup have been defined. Static holdup is the volume of liquid per volume of packing that remains after gas and liquid flows are stopped and bed has drained. Static holdup depends on packing surface characteristics. The second type is operating holdup that is the volume of liquid per volume of packing that drains out of the bed after gas and liquid flows have been stopped. The gas flow rate has little effect on holdup below loading. 

The holdup shown in Fig. 5.8-9 Is given as a fraction of the total bed volume, It is a dyuamic, or total holdups and is given the notatiou H,. If the vapor and liquid flows were to be slopped, and the bed then allowed to drain, a certain amount or liquid would be retained in the interstices of the bed this is die static holdup, H,. The diflereace between these holdup values is the operating holdup, Ha. 

Operating holdup may be estimated from the dimensionless equation of Buchanan 1 ... 

The hist group ou the right is a film number and die second is the Froude number. Mote will be said about these dimensionless parameters when structured packings are discussed. Representative operating holdup data for a structured packing material are given in Fig. 5.8-10. 

For the flowing solids phase, a complication in interpretation of the residence time distribution is caused by the nature of the solids holdup. The dynamic holdup is usually assumed to be the operating holdup, and static holdup is often treated as a dead part of flowing solids. However, if there is an exchange between static and dynamic holdup, this affects the residence time distribution of flowing solids, and consequently, the contactor performance. 

When the packing is irrigated, the influence of the presence of liquid may be related to the operating holdup, using a form alternate to that given eariier as Fig. 5.8-6. Boner and Kallis concluded that the following simple form is adequate ... 

The operational holdup can be calculated from the dimensionless correlation of Buchanan [1967] ... 

Liquid surface tension has practically no effect on operating holdup for high surface tension liquids, such as water. For ordinary organic liquids (a about 27 dyne/cm) at low liquid rates, the operating holdup will be about 12% lower than for water. Holdup will be reduced up to 20%, for low surface tension systems (a about 13 dyne/cm) at low liquid rates. At liquid rates above 7 gpm/ft this effect of surface tension on liquid holdup diminishes. These measurements were determined at atmospheric pressure and should not be extrapolated to high-pressure distillations. 

A packed bed in concurrent flow has no conventional flooding limitation because liquid holdup tends to decrease with an increasing gas rate, as shown in Figure 1-17 [21]. The curve at zero gas flow shows the usual effect of liquid flow rate on operating holdup. As gas flows downward through the bed it accelerates the liquid velocity, thus reducing the volumetric holdup. 

---