Effect of static bed height

Figure II. Effect of static bed height on vohimetric physical moss transfer coefficient.

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Again, as in the case of jet attrition, attention must be paid in the experimental determination of Ra bub to the isolation of the attrition that is due to bubbles. There are basically two ways to do this. The one is to use a porous plate distributor in order to avoid any grid jets. The other is the procedure suggested by Ghadiri et al. (1992a) which is depicted in Fig. 7 the measurement of the production rate of fines at different values of the static bed height permits to eliminate the grid jet effects. 

Experiments were performed at room temperature to study the effect of liquid and gas velocities, particle diameter and static bed height on the mass transfer coefficient and interfacial area. Gas and liquid flow rates were varied from 1.03x10 to 1.82x10 m /s and 3.3x10 to 25x10 m /s, respectively. 

The effect of particle diameter, superficial liquid velocity, superficial gas velocity and static bed height on the rate of mass transfer across the gas-liquid interface in a three-phase fluidized bed were investigated. 

The tube exchanges heat with its surrounding particles and fluid. The local HTC has a distribution closely related to these observed flow patterns. The distribution and magnitude of HTC are two factors commonly used to describe the heat transfer in such a system (Botterill et al., 1984 Schmidt and Renz, 2005 Wong and Seville, 2006). The effects of the gas velocity and the tube position are examined, showing consistent results with those reported in the Hterature (BotteriU et al., 1984 Kim et al., 2003 Fig. 23). The local HTC is high at sides of the tube aroimd 90° and 270°, while it is low at the upstream and downstream of the tube aroimd 0° and 180°. With the increase of gas velocity, the local HTC increases first and then decreases (Fig. 23A). The local HTC is also affected by tube positions and increases with the increase of tube level within the bed static height as shown in Fig. 23B. 

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