This paper focuses on the modeling of flow and mixing in a bubble column reactor operated at high gas velocities (up to 0.40 m/s). A dual-tip conductivity probe was used to measure local void properties such as local time-averaged gas holdup, chord length distribution, bubble velocity distribution, and interfacial area. Chord length distribution was converted to bubble size distribution, using the backward transformation method. Liquid-phase mixing time measurements were conducted using a conductivity probe. A computational fluid dynamics (CFD) model was developed to simulate the unsteady gas−liquid flow in a bubble column using commercial code FLUENT 6.2. The time-averaged flow properties predicted by CFD simulations were compared with the experimental data. The role of unsteady flow structures in mixing was studied. The “multiple snapshots” approach was used to simulate the mixing time using CFD. The mixing times that were predicted for all superficial gas velocities compared favorably to the measured values. This study of the hydrodynamic behavior of a bubble column at higher gas velocity provides a basis for understanding and simulating solid suspension (or solid mixing) in slurry bubble column reactors.

Gas-liquid/gas-liquid-solid systems are widely used in the chemical process industry for a variety of applications. Recently, gas to liquid (GTL) technologies employing gas-liquid-solid systems, are receiving a wide attention. Gas-liquid flows in bubble columns are intrinsically unsteady and are composed of several flow processes occurring at different time and space scales. The presence of solids further complicates the unsteady fluid dynamics. The unsteady fluid dynamics often govern the mixing and transport processes and is inter-related in a complex way with the design and operating parameters like sparger and reactor configuration, gas flow rate and solid loading. Therefore, it is essential to develop (and validate) computational models to simulate such complex relationships for guiding the reactor design. Most of the earlier work on modelling of the gas-liquid flows was focused on predicting timeaveraged flow properties (Ranade, 1997), where the unsteady information is lost. Recently, Buwa and Ranade (2003a) have studied the role of unsteady flow structures in the liquid phase mixing in bubble columns and have shown that three-dimensional unsteady simulations are necessary for accurate prediction of mixing time. In this work, we have studied dynamics of gas-liquid and gas-liquid-solid flows in cylindrical bubble columns.In past, most of the work done on characterization of unsteady gas-liquid flows was focused on small rectangular bubble columns (Becker et al., 1994;Pfleger et al., 1999;Pan et al. 2000;Buwa and Ranade, 2002, 2003a) because of the simple geometry (lower computational demands) and well established plume oscillations. In order to transform the developed understanding and models from rectangular bubble columns to industrial bubble columns, it is essential to study the dynamics of gas-liquid flows in cylindrical bubble columns experimentally as well as computationally. Most of the earlier work on cylindrical bubble columns was carried out with an objective of measuring and predicting the time-averaged velocity and gas hold-up profiles (experimental: Yao et al., 1991;Kumar et al., 1997;Chen et al., 2001; computational: Ranade, 1997;Sanyal et al., 1999). Even though a few attempts have been made to characterize the dynamics of gas-liquid flow in cylindrical bubbles columns experimentally (Chen et al., 1994;Becker et al., 1999) and computationally (Pfleger and Becker, 2001), the agreement between measured and predicted dynamic characteristics is still qualitative and the simulations are rather restricted to lower superficial gas velocities (< 0.02 m/s). Moreover, the effect of solids on dynamic characteristics is not clearly understood and addressed before. It is therefore essential to extend present understanding and models to predict dynamic characteristics Gas-liquid/gas-liquid-solid systems are widely used in the chemical process industry for a variety of applications. In the present work, we have characterized the dynamics of gas-liquid/gas-liquid-solid flows in cylindrical bubble columns u...

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Most laboratory bubble columns are equipped with sieve plate spargers. The sieve plate spargers are known to lead to nonuniform gas distribution. It is important to account for such nonuniform gas distribution at the sparger in the computational model before experimental data collected from such columns are used to fit the model parameters. In this article, such an attempt is made. A detailed, 3D CFD model was developed to simulate unsteady gas−liquid flows in bubble columns with sieve plate spargers. The sensitivity of the nonuniformity of gas distribution at the sparger with sparger resistance was examined. The model predictions were compared with the experimental data. The developed model and presented results will be useful for simulating industrial bubble columns.