CFD modeling of a bubble column reactor carrying out a consecutive A _→ B → C reaction

Abstract

In this paper, we develop a CFD model for describing a bubble column reactor for carrying out a consecutive first-order reaction sequence A → B → C. Three reactor configurations, all operating in the homogeneous bubbly regime, were investigated: (I) column diameter D_T = 0.1 m, column height H_T = 1.1 m, (II) D_T = 0.1 m, H_T = 2 m, and (III) D_T = 1 m, H_T = 5 m. Eulerian simulations were carried out for superficial gas velocities U_G in the range of 0.005-0.04 m/s, assuming cylindrical axisymmetry. Additionally, for configurations I and III fully three-dimensional transient simulations were carried out for checking the assumption of cylindrical axisymmetry. For the 0.1 m diameter column (configuration I), 2-D axisymmetric and 3-D transient simulations yield nearly the same results for gas holdup ε _G, centerline liquid velocity V_L(0), conversion of A, χ _A, and selectivity to B, S_B. In sharp contrast, for the 1 m diameter column (configuration III), there are significant differences in the CFD predictions of ε_G, V_L(0), χ_A, and S_B using 2-D and 3-D simulations; the 2-D strategies tend to exaggerate V_L(0), and underpredict ε _G, χ _A, and S_B. The transient 3-D simulation results appear to be more realistic. The CFD simulation results for χ _A and S_B are also compared with a simple analytic model, often employed in practice, in which the gas phase is assumed to be in plug flow and the liquid phase is well mixed. For the smaller diameter columns (configurations I and II) the CFD simulation results for χ_A are in excellent agreement with the analytic model, but for the larger diameter column the analytic model is somewhat optimistic. There are two reasons for this deviation. Firstly, the gas phase is not in perfect plug flow and secondly, the liquid phase is not perfectly mixed. The computational results obtained in this paper demonstrate the power of CFD for predicting the performance of bubble column reactors. Of particular use is the ability of CFD to describe scale effects.