Applications of foams and foaming are found in many industries like the flotation of minerals, enhanced oil recovery, drilling in oil reservoirs, insulation, construction and refining processes such as Vacuum distillation and Delay-Coker reactors. However, foaming and defoaming are not yet understood. Foams trap gas and are not wanted in many applications. Guitian and Joseph (1997) proposed fundamental studies of their observations on foam suppression experiments they carried out in a cold slit bubble reactor. They found that foaming may be strongly suppressed by fluidizing hydrophilic particles in the bubbly mixture below the foam. They suggest that the suppression is achieved by increasing the wetted area of solids surface (walls and particles), by bed expansion and by decreasing the gas hold-up by increasing the effective density of the liquid solid mixture.
Frye and Berg (1989) studied the antifoam action of hydrophobic particles using two different tests (particle-induced film rupture and foam shake test), but they did not use a fluidized bed. Armstrong et at. (1976) observed adhesion of air bubbles to Teflon-coated glass beads fluidized in water. Tsutsumi, Dastidar and Fan (1991) studied the characteristics of water-air-solid fluidization with non-wettable (hydrophobic) particles and classified the flow pattern according to the motion of the particle-bubble aggregates. In this work, we fluidized hydrophobic and hydrophilic versions of two different sands in the same slit bubble reactor Guitian and Joseph (1997) used. We found that the hydrophobic sands suppress the foam substantially better than their hydrophilic counterparts. We also observed that, when foam is not present in the reactor (i.e. at high liquid velocities), the gas hold-up in the bubbly mixture was higher for the hydrophobic version of one sand. This result may be explained in terms of attachment of the particles onto the air bubbles, which increases the residence time of the gas phase, as suggested by Tsutsumi et al. (1991). In the other hand, the gas hold-up in the bubbly mixture for the hydrophobic version of the other sand was smaller. A possible explanation is supported by Armstrong et al. (1976) findings. They suggested that the phenomenon of bubble adhesion to the non-wettable particle leads to a decrease in the apparent density of the particle, which in turn is responsible for a larger bed expansion and smaller gas holdup compared with wettable particle systems. These results suggest that the degree of hydrophobicity matters.
Hydrophobic particles appear to break, and not only to suppress, foam; and they may have a greater application.