Physical aspects of liquid-impelled loop reactors

The liquid-impelled loop reactor (LLR) is a reactor that consists of two parts : the main tube and the circulation tube. Both parts are in open connection at the bottom and at the top. The reactor is filled with a liquid phase: the continuous phase. Another liquid phase is injected in the main tube by means of pumping. This liquid phase is immiscible with the continuous phase and its density is significantly different. If the density is higher than the density of the continuous phase, injection takes places at the top of the main tube. For a lower density injection takes place at the bottom. Due to the density difference the dispersed-phase droplets that are formed will fall or rise, respectively, and coalesce at the other end of the tube. The coalesced liquid is discharged from the reactor. Due to the presence of dispersed phase in the main tube a pressure difference exists which causes circulation of the continuous phase in the reactor. This results in good mixing without the use of an impeller. For biotechnological purposes it is most likely that the continuous phase is an aqueous phase that contains the biocatalyst, possibly immobilized. The work on the liquid-impelled loop reactor originates from two previous research studies. First, the physical characterisation and modelling of air-lift loop reactors for application in cultivating shear-sensitive biocatalysts. Second, the research focused on application of organic solvents in biological processes, which is a promising area for many years already.

In a review the current state of the art is given with respect to biocatalysis in media consisting of two liquid phases that are not miscible. This research area has shown much progress in recent years, however, industrial applications seem still not very numerous. To carry out two-liquid-phase experiments on a scale bigger than shake flasks, in most cases existing bioreactors are used. Adjustments are made to the reactor and the processing to make them suitable for use with two liquid phases. The liquid-impelled loop reactor can be seen as a special case, where an air-lift loop reactor is adjusted for use with two liquid phases instead of a liquid and a gas phase.

The research was started with characterisation of important physical aspects such as drop size, dispersed-phase concentration (holdup) and continuous-phase velocity as function of the dispersed-phase flow rate. Description of drop sizes that are formed in the liquid-impelled loop reactor at the liquid sparger, show good agreement with theoretical predictions. The hydrodynamic model that was used in the air-lift loop study is applied. It shows to be a good method to describe holdup and liquid velocity. The best results with this model are obtained when it is assumed that the continuous phase flows fastest in the centre of the tube and that the dispersed phase concentrates in the centre of the tube.

On the topic of hydrodynamic models for air-lift loop reactors many articles are published. Because this still continues, this literature is analysed and the basic principles of the several models are described and compared. It appears that in all models the relative velocity between dispersed-phase bubbles and continuous phase plays an essential role. This quantity is difficult to determine and shows a wide spread due to the distribution in bubble size. Furthermore, velocity profiles or turbulence can have much influence but are not taken into account in the described models. Comparison of the models by means of using literature data did not yield a clear preference for one of the models nor for a particular basic principle.

To describe mixing in the continuous phase, the one-dimensional axial dispersion model is used, which is in general suitable for flow in tubular devices. The mixing parameter is determined per reactor section. For the main tube a correlation between mixing parameter and energy dissipation is given. The mixing parameter can be used to describe the flow of the continuous phase as a plug flow with axial disturbances. Furthermore, dimensionless mixing times can be estimated. The dimensionless mixing time is the number of circulations that is necessary to achieve complete mixing of the continuous phase, where the criterium must be defined by the user.

Mass transfer is investigated in an FC40 water system. For this purpose a new method is developed based on the principle of a steady-state measurement, in stead of the most widely used dynamic measurement. Compared to a gas/liquid system at equal dispersed-phase flow rates, the mass-transfer rate in the liquid/liquid system is favorable. This is due to the larger exchange area, because the drops are smaller than the bubbles and the drop velocity relative to the continuous phase is lower than the relative velocity of the bubbles. The mass-transfer coefficient for the liquid/liquid system, derived from experimental results, however, is lower than literature values for gas/liquid systems. This is probably caused by the lower diffusion coefficient of oxygen in liquid than of oxygen in gas. The transfer capacity can often be the highest for gas/liquid systems because the maximum dispersed-phase flow rate in liquid/liquid systems is limited with respect to drop formation and coalescence. Further physical reseach must be focussed on this limitation.