A hydrophobic membrane contactor is preferable for CO2 dispersion in a photobio­reactor to prevent the microalgae cells clogging the pores. The removal process of dissolved gases from the liquid phase is illustrated in Fig. 14.2 . This illustration represents the basic features of the membrane module.

The literature shows that there are a few commercial types of membrane contac­tors that can be used for CO.-O2 exchange. The membrane type is microporous polypropylene hollow-fibre product of AKZO/ENKA, which was tested and found suitable for the CO2-O2 exchange process, as shown in Fig. 14.3a. This type of membrane resulted in ten times more CO2 mass transfer compared to a bubble col­umn. Both membrane contactors in Fig. 14.3a, b are vertical, microporous, hollow — fibre contactors that were made of 10 and 445 fibres with interfacial contact areas between the gases and fluid estimated to be 23.8 cm2 and 75 cm2, respectively. The membrane in Fig. 14.3b resulted in a CO2 fixation rate that was 3.25 times higher than that of a regular bubble column, and it was tested by Cheng et al. (2006) and found to be a suitable device for carbonation and deoxygenation processes in the microalgae culture broth.

Figure 14.3a, b shows vertical tubular hydrophobic membrane contactors. The DO also decreased by a factor of 30 compared to a regular photobioreactor without a membrane contactor. However, the microalgae productivity only showed slight increases. Theoretically, the higher the mass transfer of CO2 in the photobioreactor, the higher the interfacial contact area between the CO2 and microalgae cells; thus, a higher growth rate of microalgae should have occurred. It can be concluded that some factor, other than interfacial contact area, must be a limiting factor for micro­algae productivity.

Ho

Fig. 14.2 Illustration of the use of a membrane contactor for gas removal between two phases

Another membrane module that was tested and that can be used for CO2-O2 exchange in the microalgae culture broth is shown in Fig. 14.3c. Figure 14.3c shows a horizontal, tubular membrane contactor composed of 7,400 hollow fibres (Carvalho and Malcata 2001). This type of membrane also resulted in slight increases of biomass productivity, indicating that some other factor in the mem­brane photobioreactor has an equivalent effect on the CO2 fixation rate and biomass productivity.