V

—w = 5%

,1,1

,r,i

,1,1,

lAvx h"1)

lAvx h"1)

Fig. 14. Effect of liquid flowrate L on tray efficiency EmL (V= 440.59 m ' h1; weir height hw ~ 50

(2) When raising the solid concentration, tray efficiency first increases and then decreases over a wide range of liquid flowrate. The reason may be that at low concentration the added solid particles promote the disturbance of interface between gas-liquid phases, and thus dwindle the liquid film thickness. A thinner liquid film thickness means a higher liquid-side mass transfer coefficient. Since the systems discussed in this work are just governed by liquid film, the resistance to mass transfer is mainly concentrated in the liquid film. So in terms of two-film theory tray efficiency will increase as the solid concentration rises. On the other hand, the presence of particles close to the interface may hold back the mass transfer of liquid phase and reduce the contact area between gas and liquid phases. At higher solid concentrations, this effect will be dominant. Consequently, the overall result is that tray efficiency no longer increases and even seems go down. This trend is consistent with the conclusion about the effect of solid particles on mass transfer coefficients put forwarded by Alper and Beenackers [78, 79].

It is known that the factors influencing tray efficiency are very complicated and involve physical properties of the systems, tray configuration, hydrodynamics etc. An institution of U.S.A., AIChE, has ever organized many researchers to study tray efficiency. Ultimately, a set of method recommended for estimation of tray efficiency of gas-liquid two phases flow, which is now called AIChE method [80], is put forward. Herein, this method is tested with the data obtained in this study for air-water system and then modified to predict tray efficiency of air-water-solid particles system.

Firstly, this method is used for predicting tray efficiency for the air-water system. It is found that the average relative deviation (ARD) of tray efficiency is 15.7% between this investigation and the values estimated by AIChE method for 21 points, and can satisfy the general engineering requirement. At the same time, it indicates that the obtained experimental data are reliable. The ARD is defined as where n is the number of data points.

Then, this method is extended to estimate the tray efficiency of the air-water-solid particles system by means of replacing the liquid physical properties of liquid with slurry density pm and viscosity ¡im, which can be calculated from the following equations [81]:

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