Bubble formation dynamics and transport phenomena in high pressure bubble columns and slurry bubble columns.

摘要

Bubble column and slurry bubble column reactors are widely used in industry, particularly in chemical and petrochemical industries. Many industrial processes of considerable commercial interest are conducted under high pressures, such as methanol synthesis, resid hydrotreating, Fischer-Tropsch synthesis and benzene hydrogenation. Fully understanding pressure effects on flow characteristics in such processes is crucial to operation, optimization and design of industrial high-pressure reactors. In this study, bubble formation dynamics and key transport properties are investigated under a wide range of industrial-relevant conditions.; The behavior of bubble formation from a single orifice in non-aqueous liquids and liquid-solid suspensions under various gas injection conditions is studied experimentally at high pressures. A mechanistic model is developed to account for the initial bubble size in liquid-solid suspensions at high pressures. The model takes into consideration various forces acting on a bubble, including the impact forces due to solid particles such as suspension inertial force and particle-bubble collision force, as well as buoyancy, gas momentum, drag, surface tension, and Basset forces. Numerical techniques based on computational fluid dynamics (CFD) with discrete phase simulation method are also employed to simulate bubble formation dynamics at high pressures.; Key transport properties including heat transfer and liquid-phase mixing are investigated at high pressures. The variation in heat transfer coefficient with pressure is attributed to the counteracting effects of increased liquid viscosity, decreased bubble size, and increased gas holdup or frequency of bubble passage over the heating surface as the pressure increases. A consecutive film and surface renewal model is used to analyze the heat transfer results. The axial dispersion coefficients of the liquid phase in bubble columns are measured by the thermal dispersion technique. The axial dispersion coefficient increases with an increase in gas velocity, and decreases with increasing pressure. Effects of gas velocity and pressure on liquid mixing can be explained based on the combined mechanism of global liquid internal circulation and local turbulent fluctuations.