Role of liquid bridge forces in cohesive fluidization
The roles of liquid bridging and particle surface washing with a surfactant have been demonstrated using Geldart D particles undergoing artificially induced cohesive gas-solid fluidization with thin-oil films. It has been shown that a number of factors, including the amount of liquid present and the liquid-solid contact angle, influence the interparticle bonding. The fluidization behaviour, including minimum fluidization velocity, has been found to depend on liquid bonding, which is a function of both the liquid and particle surface properties. Treatment of particles with a surfactant raises the liquid-solid contact angle, thereby decreasing the capillary forces. The results of this work demonstrate that this effect may be more significant than viscosity at conditions between minimum bubbling and minimum fluidization. This confirms that static bridgeforces, dominated by capillary pressures and surface tension, control fluidization under low velocity conditions. Surface tension forces also play a significant role during bubbling and slugging behaviour, but at these higher gas velocities, binder viscosity increases in importance. The disadvantage of gas by-passing in fluidized beds has been shown to be a phenomenon which is subject to manipulation. It has been demonstrated that controlled, and minimal liquid addition to a gas-solid fluidized bed reduces the bubble size and average bed voidage with little apparent agglomeration. These results confirm that more investigation is required in the design of fluidized beds where intimate gas-solid contacting is required.