Packings dynamic holdup

The bed void volume available for flow and for gas and liquid holdup is determined by the particle size distribution and shape, the particle porosity, and the packing effectiveness. The total voidage and the total liquid holdup can be divided into external and internal terms corresponding to interparticle (bed) and intraparticle (porosity) voidage. The external liquid holdup is further subdivided into static holdup eLs (holdup remaining after bed draining due to surface tension forces) and dynamic holdup eLrf. Additional expressions for the liquid holdup are the pore fillup Ft and the liquid saturation SL ... 

The holdup of a phase is usually defined as the volume of the phase per unit reactor volume. However, for a fixed-bed reactor, the gas and liquid holdups are often defined on the basis of void volume of the reactor. In a fixed-bed reactor, the liquid and sometimes gas holdups are divided into two parts dynamic holdup, which depends largely on the gas and liquid flow rates and the properties of the fluids and the packing material, and static holdup, which depends to a major extent on the nature of the packing (e.g., porosity of the packing) and the fluids properties. The relationships between the holdups of various phases and the system variables for a variety of three-phase reactors are discussed in Chaps. 6 through 9. 

The dynamic holdup depends mainly on the particle size and the flow rate and physical properties of the liquid. For laminar flow, the average film thickness is predicted to vary with, as in flow down a wetted-wall column or an inclined plane. In experiments with water in a string-of-spheres column, where the entire surface was wetted, the holdup did agree with theory [28]. For randomly packed beds, the dynamic holdup usually varies with a fractional power of the flow rate, but the reported exponents range from 0.3 to 0.8, and occasionally agreement with the 1/3 power predicted by theory may be fortuitous. 

According to Otake and Okada [26] for nonporous spherical packing c = 1.295, a = 0.676, p = —0.44, and y = 1.0, whereas Satterfield et al. [27] arrived at c = 1.0, a = 0.333, p — —0.33, and y = 0. Note that the dynamic holdup is independent of the gas flow rate, but varies with the liquid flow rate. Goto and Smith [28] observed agreement between their experimental data and Otake and Okada s correlation for large-particle diameters only and between their data and Satterfield et al. s correlation for small-particle diameters only. The following external or total holdup equation is proposed by Midoux [71] ... 

Stanimirovic, O.P., Prediction of Pressure Drop and Dynamic Holdup of Solid Phase in a Gas-Solid-Packed Bed Reactor, B.S. thesis, Faculty of Technology, Novi Sad, Yugoslavia, 1998. 

Liquid was delivered into the packed bed with a needle. The gas feed line was connected to the feed section with a T-junction therefore, the gas flowed through the annular area between the liquid feed line and the outer pipe. When the gas and liquid flows were stopped, the liquid entirely remained in the micro-structured bed. Thus, the bed has zero dynamic holdup and only static holdup. The static holdup (gj.) is expressed as the fraction of the space between the particles that is, on average, filled with liquid ... 

Figure 8.19 Dynamic holdup of different packings in high-pressure columns [36].

The dynamic holdup Ha is toe difference between total and static holdup of toe packing. It is toe liquid in toe packing held by toe resistance forces. In case of large industrial packings, and they are more important, toe value of toe static holdup is to be n ected. 

The dynamic holdup represents fee liquid flowing in fee packing in the form of films, drops and jets. 

To describe the dynamic holdup over the loading point using e3q>erimental data for Ihe same packings, the following equation is proposed [164] ... 

A comparison between the dynamic holdup over the loading point for the packings presented in Table 26 and the values calculated by equation (186) is presented in Fig. 57. 

The investigations of the dynamic holdup are carried out [154] for all packings presented in Table 31. Some experimental data are represented in Figs. 72 and 73. 

The value of the dynamic holdup of the packing is presented [154] as a sum of the holdup under the loading point and an addition holdup due to the forces between the gas and the liquid phase, For calculating each of them, the following equations are proposed. 

Some experimental data for foe dynamic holdup [189] of different Holpack packings for a system air-water under foe loading point are presented in Fig, 98. 

For calculating the dynamic holdup of these packings (under foe loading point), foe following equation is proposed [189] ... 

The results for the dynamic holdup of the crossed strips packing versus the gas velocity at different liquid superficial velocities are presented in Fig. 147. 

Seff- so called effective gravity, calculated by iuation (22S) packing height, m dynamic holdup of the liquid phase ... 

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,... 

In concurrent downward-flow trickle beds of 1 meter in height and with diameters of respectively 5, 10 and 20 cm, filled with different types of packing material, gas-continuous as well as pulsing flow was realized. Residence time distribution measurements gave information about the liquid holdup, its two composing parts the dynamic and stagnant holdup and the mass transfer rate between the two. 

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