Three-phase (gas-liquid-solid) reactors are used extensively in chemical, petrochemical, refining, pharmaceutical, biotechnology, food and environmental industries. Among various types of gas-liquid-solid reactors, the co-current gas-liquid-solid fluidized bed reactor using coarser solid particles is the vital and most widely used three-phase contactor finds application in numerous industrial applications. The successful design and operation of such a system depends on the prediction of the fundamental characteristics. The gas holdup is one of the most important characteristics for analyzing the performance of a three phase fluidized bed. For chemical processes where gas-liquid mass transfer is the rate limiting step, it is important to be able to estimate the gas holdup as this relates directly to the mass transfer. The complex hydrodynamics of these reactors are not well understood due to complicated phenomena such as particle–particle, liquid–particle and particle–bubble interactions. For this reason, computational fluid dynamics (CFD) has been promoted as a useful tool for understanding multiphase reactors for precise design and scale up.
In the present investigation an attempt has been made to study the gas holdup characteristics of a three phase fluidized bed in a broader range of operation. Experiments have been conducted to examine the gas holdup of a co-current gas-liquid-solid three-phase fluidized bed with a modified air sparger using liquid as the continuous phase and gas as the discontinuous phase. Spherical glass beads have been used as the solid phase. An antenna type air sparger has been used for the generation of fine bubbles. By the use of such air sparger the pressure drop in the distributor section can be avoided. A correlation for overall gas holdup has been developed from dimensional analysis. A two dimensional transient model has been developed to simulate the phase holdup behaviour of the mentioned gas–liquid–solid three-phase fluidized bed using the computational fluid dynamics (CFD) method. An Eulerian Granular Multiphase model has been used in the present study and simulations are carried out using the commercial CFD package Fluent 6.2.16. Comparisons have been made between the overall gas holdup values calculated from the developed and the existing correlations and the experimental ones and with those obtained from CFD simulation.
Keywords: three-phase fluidization, cfd, phase holdup, multiphase flow.
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