Some Effects of Particle Wettability in Agitated Solid‐Gas‐Liquid Systems: Gas‐Liquid Mass Transfer and the Dispersion of Floating Solids

Abstract

R ecent studies here shown that the addition of solid particles can have a profound effect on the characteristics of liquid/liquid dispersions (Nienow et.al., 2000; Nienow et.al., 2003). A particularly important parameter is the wettability of these particles which has been shown to strongly affect the rate of drop coalescence. On the other hand, though for gas-liquid systems the impact of the addition of particulate solids has been quite extensively studied (Nienow and Bujalski, 2002), there has been little work done related to solids wettability. This paper addresses this question for the first time in relation to stirred vessels. The new issues particularly addressed are rates of gas-liquid mass transfer in the presence of solids of different wettability and the impact of wettability on the ability to ingest or draw-down floating solids. Literature Mass TransferMany studies have been carried out on gas-liquid mass transfer, especially of oxygen to or from water and have recently been well summarised elsewhere (Nienow, 2003). Van't Riet ( 1979) was the first to show that k L a data could be related to mean specific energy dissipation rate, (e T ) g (W/kg) and superficial gas velocity, v s (m/s), by an empirical equation of the form:where a and b are about 0.5. He also concluded that, within ±25%, the literature data showed that k L a was independent of the type of impeller used. In general, moving from water to solutions of electrolytes or alcohols significantly enhances k L a whilst the addition of antifoam greatly reduces it (Martin et.al., 1994). In other words, A in Equation 1 is very sensitive to the chemical composition of the solution. Such changes in composition also affect a and b but to a much lesser extent but A, a and b are still independent of impeller type (see for example, Two-and three-phase systems have been studied using particles that are water-wetted and denser than water and with similar sized, small floating particles, both wetted and nonwetted. The parameters investigated have been the agitator speed required to disperse the different types of particles under sparged and non-sparged conditions and rates of gas-liquid mass transfer. With the wetted, dense particles, only at concentrations of > ~20% w/w did the k L a fall significantly (by ~20 to 30%) whilst solid suspension followed the trends of earlier work. With the less dense particles, even with < ~1% w/w that were not wetted, it was very difficult to disperse them throughout the liquid phase, though up-pumping impellers were more effective than radial. In addition, bubble size significantly increased and k L a fell by a factor of 2 to 3. With the wetted, less dense ones up to 2% w/w, uniform dispersion was achieved even at much lower speeds and k L a did not change compared to the case without solids.On a étudié des systèmes bi et triphasiques à l'aide de particules mouillées par l'eau et plus denses que l'eau et de petites particules flottantes de calibre semblable, mouillées et non mouillées. Les paramètres étudiés sont la vitesse ...

Volumetric Gas−Liquid Mass Transfer Coefficient in Aerated Stirred Tank Reactors with Dense Floating Solid Particles

“…The experimental results in Figure indicate that at low impeller speeds and gas flow rates gas hold-ups were almost constant and with increasing impeller speed gas hold-up significantly increased. The increase in the gas hold-up with increasing N and U g is presumably due to the enhancement of the overall liquid turbulence causing the bubble break-up . The enhancement of bubble break-up might result in the increase in the number of small bubbles and then the increase in the gas hold-up.…”

“…On the contrary, solid particles may also break up large bubbles formed by bubble coalescence. The bubble break-up causes an increase in a and hence k L a . The mechanisms for effects of the presence of floating on k L a coefficients still remain uncertain.…”

“…The increase in the gas hold-up with increasing N and U g is presumably due to the enhancement of the overall liquid turbulence causing the bubble break-up. 13 The enhancement of bubble break-up might result in the increase in the number of small bubbles and then the increase in the gas hold-up. When the impeller speed was less than the minimum impeller speed for off-surface floating-particle suspension, N jsg , the gas hold-ups were almost constant.…”

“…Despite many publications on the gasÀliquid mass transfer in stirred tank reactors with heavy solids, there is little published work on that with floating solid particles. Gentile et al 13 measured the gasÀliquid mass transfer rate for the threephase system with nonwettable polypropylene particles (d p = 425 μm, F s = 902 kg m À3 ) and wettable Fillite particles (d p = 130 μm, F s = 706 kg m À3 ). While the volumetric mass transfer coefficient with 0.1% w/w nonwetted solids was found to fall by a factor of 2 to 3 as compared with the case without solids, that with wetted solids at concentrations up to 2% w/w did not change as compared with the data for water.…”

Experimental Studies of Hydrodynamics and Regime Transition in Bubble Columns

“…There are many influencing factors that affect the flow phenomena such as column dimensions, pressure effect, particle concentrations, liquid phase properties, orifice plate configurations, as reported in several studies on homogeneous to heterogeneous regime transition (Ruzicka et al, 2001a;Vial et al, 2001a). Firstly, the effect of solid loading in the bubble column on gas hold-up has been studied both experimentally and by CFD simulation (Jamialahmadi and Müller-Steinhagen, 1993 and ref: there in;Saxena and Chen, 1994;Garcia-Ochoa et al, 1997;Gentile et al, 2003). Increasing solid concentration, particle size and solid-liquid density difference results in a decrease in gas hold-up.…”