A unified approach to the scale-up of gas - solid fluidized bed and gas - liquid bubble column reactors

Abstract

The aim of this work is to develop a unified approach to the scale-up of gas - solid (G - S) fluid bed and gas - liquid (G - L) bubble column reactors. The unified approach relies on analogies in the hydrodynamic behavior of G - S and G - L systems in both the homogeneous and heterogeneous flow regimes. The homogeneous G - S fluidization regime is to be identified with homogeneous G - L bubbly flow while the heterogeneous G - S fluidization regime is to be identified with the churnturbulent regime for G - L bubble columns. In the heterogeneous flow regime of operation, the classic two-phase theory developed for G - S fluidized beds can be applied with profit to describe also the hydrodynamics of G - L bubble columns provided the "dilute" phase is identified with the fast-rising large bubbles, and the "dense" phase is identified with the liquid phase containing entrained "small" bubbles. Quantitative analogies in the hydrodynamic behavior of G - S and G - L systems are demonstrated by the use of extensive experimental data obtained in five columns of three different diameters (2 × 0.1 m, 1× 0.19 m and 2 × 0.38 m). The total expanded bed height in the experiments varied in the range 0.5-3.5 m. About 4,000 dynamic gas disengagement experiments were carried out with various systems to determine the total gas hold-up and the hold-ups of the "dilute" and "dense" phases. The gas phases used in the experiments were air, helium, argon and sulfur hexafluoride. Fluidized cracking catalyst (FCC) was used as the solid phase in fluid bed experiments. In bubble column operations the liquid phase used was water, paraffin oil or tetradecane. Sintered plates were used for gas distribution in all the columns. The gas hold-up in the "dense" phase, ε_df was found to be practically independent of the scale of operation. The hold-up of the fast-rising "dilute" phase, ε_b, on the other hand was found to be a significant function of the column diameter,DT, and of the total dispersion height,H. The "dilute" phase gas hold-up can be modeled for both G-S and G-L systems using a theory allowing for bubble growth in the region above distributor. The bubbles are assumed to grow in diameter up to a distanceh, at which the bubbles reach their equilibrium size. The equilibration heighth was found to increase with the superficial gas velocity through the dilute phase, U - U_df). For air - FCC, the value ofh varies in the range 0.4-1.2 m. For bubble columns the values ofh are significantly smaller and lie in the range 0-0.5 m. Increasing gas density increases the total gas voidage in both G - S and G - L systems but has no significant effect on the hold-up of the dilute phase. In G - L bubble columns, the liquid properties affect the total gas hold-up but have only a minor influence on the dilute phase hold-up. The unified model to describe the bubble hydrodynamics in G - S fluid beds and G - L bubble columns is a useful tool in scaling-up these two reactor types, because of the possibilities of cross-fertilization of design data.