Fluidization behavior in bubbling fluidized beds has been widely studied. Solids mixing and bubble characteristics studies have shown a fluidized bed to be well mixed with bubble voidage fraction profiles that are more or less symmetrical about the central axis of the column. Recent studies have shown, however, that deep fluidized beds of Group A particles can have a severe bypassing of gas. Knowlton (2001) presented videos of fluidization tests conducted with 4 and 11% fines (<44 micron) FCC catalyst particles in a transparent 0.3-m-diameter unit. The 4% fines bed was observed to have a significant gas bypassing in a form of a high velocity air jet that preferentially flowed up near the column wall while the rest of the bed stayed defluidized or poorly fluidized. The air jet moved around the wall but it occasionally anchored or stayed longer at the cyclone dipleg return location. When the fines content was increased to 11% the bed fluidized uniformly with no gas bypassing.

Wells (2001) using FCC catalyst particles in a semi-circular Plexiglas unit, for bed heights of 2.4 to 4.9 m, observed that a “snake” of streaming gas formed a short distance above the grid and passed through the bed, bypassing the mostly stagnant bed. Later studies (Karri et al., 2004, Issangya et al., 2006, Cocco et al., 2006 and Issangya et al., 2008) identified various parameters that can influence gas bypassing. These included fines content, bed height, superficial gas velocity, baffles, system pressure and imposed solids flux. Issangya et al. (2007) suggested that the standard deviation of differential pressure fluctuations measured across short intervals around the column can diagnose gas bypassing and can also give the approximate location(s) of where gas bypassing was occurring. If gas bypassing occurs in industrial beds, it can result in poor gas/solids contacting and poorly fluidized entrances to standpipes or discharge regions of cyclone diplegs. Gas bypassing appears to be a result of gas compression by the pressure head generated in the deep beds that is significant enough to cause defluidization of solids, similar to what occurs in standpipes. Being unable to rise as well distributed bubbles through the nearly defluidized solids mass the air then flows up in a stream (or streams) of bubbles via any lesser resistive path. The objective of this study was to determine if the emulsion phase density is affected by the added pressure head in deep beds.