Heat-transfer characteristics in slurry bubble columns at elevated pressures and temperatures

The heat-transfer behavior between an immersed heating surface and the surrounding gas-liquid-solid medium in a slurry bubble column is studied experimentally and analytically. The operating pressures and temperatures vary up to 4.2 MPa and 81 degrees C, respectively. Nitrogen, Paratherm NF heat-transfer fluid, and 53-mu m glass beads are used as the gas, liquid, and solid phases, respectively. The solids concentrations are varied up to 35 vol %, while the superficial gas velocities are varied up to 20 cm/s. The effects of gas velocity, solids concentration, pressure, and temperature on the heat-transfer coefficient are examined. It is found that pressure has a significant effect on heat-transfer characteristics. The heat-transfer coefficient in a slurry bubble column decreases appreciably with an increase in pressure. It is noted that the variation in the heat-transfer coefficient with pressure is attributed to the counteracting effects of the increased liquid viscosity, decreased bubble size, and increased gas holdup or frequency of bubble passage over the heating surface as the pressure increases. The addition of particles to the liquid phase enhances heat transfer substantially. The effect of temperature on the heat-transfer behavior is mainly determined by the change in liquid viscosity. An empirical correlation is proposed to predict the heat-transfer coefficient in a slurry bubble column under high-pressure conditions. A consecutive film and surface renewal model is used to analyze the heat-transfer results. On the basis of the model analysis, it is found that the major heat-transfer resistance in high-pressure slurry bubble columns is within a fluid film surrounding the heating surface.