EFFECT OF INTERNALS ON FLOW PATTERN IN BUBBLE COLUMN

Abstract

Bubble columns are an important class of contacting devices in chemical industry. Their simple construction makes them ideal reactors for two and three- phase operations such as heterogeneous catalysis. Several flow processes influence overall dynamics of gas — liquid flow and hence mixing and transport processes in bubble columns. Gas-liquid flow in two-dimensional (2D) rectangular columns has gained significant importance in the nuclear and car industries where they have been used mostly for cooling purposes. They are also an integral part of the design of compact heat exchangers in boiling and condensation in spacecraft and aircraft applications. 2D reactors could be forcibly used in the chemical industry, since they allow better control on local hydrodynamics, and associated heat and mass transfer, and experimental data is easier to obtain than in three-dimensional (3D) columns. In spite of the wide variety of contact scheme induced in bubble column equipment by means of introduction of internals as baffles, geometric irregularities or heat exchangers, most of the work in this field have been carried out in equipment lacking these internals. In the present work, we have computationally studied the effect of internals on dynamics of gas — liquid flow in a rectangular bubble column, run under bubble flow and churn turbulent conditions. The numerical simulation was done using the Computational Fluid Dynamics (CFD) based FLUENT software ( Fluent Inc., USA). The Silicon Graphics 320 visual workstation computer with Windows NT workstation 4.0 (Microsoft Inc., USA) was used for this study. The flow in the bubble column (both with and without internals) reactor was modeled using the Algebraic Slip Mixture Model (ASMM) incorporated in the FLUENT software. The simulations were performed for a 19 cm i.d. air-water bubble column. Gas superficial velocities measured were 2 cm sl and 12 cm characteristics of bubble and churn turbulent flow regime, respectively. The column contained a batch liquid with unexpanded liquid height of 95 cm. In the present simulations distributor effects have been ignored by considering it a perforated plate, which had been modeled as uniform source of the gas phase. The internals used are: i. Two plates with 11 holes, each at heights of 0.285 m and 0.570 m, with hole diameter of 0.0038 m, center-to-center distance of holes is 0.01267 m. ii. Rectangular rods, five in number with length 0.85 m and width 0.0127 m and having a center to center distance of 0.03 m. Distance from top and bottom of the rectangular domain is 0.05 m each. It is concluded that with internals flow pattern can be very well approximated by axial dispersion model with transportation lag. The introduction of internals brings in some regularity in the flow, results in greater mixing and higher gas hold up values. The presence of internals physically reduces the radial turbulent length scale since the internals restrict the flow in the radial direction. Reasonably, good agreement was obtained between the experimental data available in literature and simulations, and the flow pattern profiles obtained for various column configuration are appears to be sensible and logical. The favorable results suggest that the simple two —dimensional simulation can be used for reasonable engineering calculations of the overall flow pattern and gas holdup distributions.