ubble column reactors are finding increasing use in industrial practice; this reactor technology figures prominently in processes for converting natural gas to liquid fuels and light olefins using Fischer Tropsch synthesis (Krishna et al., 1996; Maretto and Krishna, 1999; Sie and Krishna, 1999). There are considerable reactor design and scale-up problems associated with the Fischer Tropsch bubble column slurry reactor. Firstly, large gas throughputs are involved, necessitating the use of large diameter reactors, typically 5 to 8 m, often in parallel.
Secondly, the process operates under high pressure conditions, typically 4 MPa. Thirdly, in order to obtain high conversion levels, large reactor heights, typically 30 to 40 rn tall, are required along with the use of highly concentrated slurries, approaching 40 ~01%. Finally, the process is exothermic in nature, requiring heat removal by means of cooling tubes inserted in the reactor. Successful commercialisation of this technology is crucially dependent on the proper understanding of the scaling up principles of bubble columns for the above mentioned conditions fall outside the purview of most published theories and correlations (Deckwer, 1992; Fan, 1989).
The objective of the present communication is to demonstrate that the hydrodynamics of bubble columns with concentrated slurries can be mimicked using a highly viscous liquid. The advantage of this approach is that experimentation with a viscous liquid is much simpler than with concentrated slurries, and parameters such as liquid velocity profiles and liquid phase backmixing can be determined more easily with liquids rather than with slurries. A further objective of this work is to present a more fundamentally based model for the estimation of the gas hold-up in slurry bubble columns than presented in our earlier work (Krishna et al., 1997).
Experimental
Experiments were performed in polyacrylate columns with inner diameters of 0.1, 0.19 and 0.38 m. The gas distributors used in the three columns were all made of sintered bronze plate (with a mean pore size of 50 pm). All columns were equipped with quick closing valves in the gas inlet pipe in order to perform dynamic gas disengagement, or bed collapse experiments. Pressure taps were installed along the height of the columns. Validyne DP15 pressure sensors, connected to the pressure taps and coupled to a PC, allowed the transient pressure signals to be recorded during dynamic gas disengagement experiments. The gas flow rates entering the column were measured with the use of a set of rotameters, placed in parallel, as shown in Figure 1 for the 0.38 m column. This setup was typical. Air was used as the gas phase in all experiments. Firstly,