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.

The development of a temperature gradient across the membranes enhances the flux of dual membrane by a factor of 25 with only a 3-5 °C temperature difference (Cath et al. 2005). However, this method cannot be applied to the microalgae broth culture

because the increased temperature would kill the microalgae. Operation at high or low flow rate also does not cause emulsion compared to conventional devices, which are subject to flooding at high flow rate and unloading at low flow rate, whereas the overall mass transfer is controlled by the liquid film resistance even at elevated pres­sures (Dindore et al. 2004). The flow in the liquid phase can be controlled at a steady condition, thus preventing the mass transfer reduction. The performance of the membrane contactor still can be improved when the concentration of the absor­bent and the liquid-gas pressure difference are designed properly, as shown in Yan et al. (2007). Increasing the number of capillaries and, at the same time, decreasing the gas flow rate enhanced the CO2 removal efficiency (Bottino et al. 2008). Treatment of the membrane, such as drawing in the axial direction and soaking in concentrated acid, improved the CO2 recovery rate from the absorbent (Takahashi et al. 2011).

The modification of membrane surface was also capable of enhancing the absorption flux. Bakeri et al. (2012) showed that the modification of the poly — etherimide hollow-fibre membrane showed 72% higher absorption flux than commercial polypropylene membrane contactors. The removal and mass trans­fer of CO2 from flue gas also can be increased by letting the flue gas flow inside the hollow-fibre membrane rather than inside the lumen membrane (Yanchao et al. 2012).

Membrane photobioreactor is a term that is used to describe a membrane contactor that is integrated with a photobioreactor to enhance CO2 mitigation by microalgae. The membrane module can be externally or internally integrated with the culture vessel of microalgae. The membrane photobioreactor can be considered as a new development in the research field, since it was first mentioned in 1998 by Ferreira et al. (1998). The latest study of the membrane photobioreactor was in 2008 by Fan et al. (2008). Between these years, only a very few research efforts are described in the literature, showing that there is still much work to be conducted. Photobioreactors are seen as one of the most reliable ways to reduce the emissions of CO2 to the atmosphere, and this is accomplished by using microalgae to sequester CO2 and transform it into biomass for use as biofuel feedstock.

Gas-liquid mass transfer is an important feature of photobioreactors, and the biggest challenge is to design for high biomass productivity. Poor mass transfer increases the risk of CO2 stripping in the photobioreactor, and this inhibits the growth of microalgae (Hoekema et al. 2002). This is because the higher the photo­synthesis activity, the higher the DO. The DO is significant enough to inhibit the growth of microalgae. However, this cycle can be predicted by using a mathematical equation (Hai et al. 2000). The correct light regime also is important to allow high photosynthetic rates, and this can be estimated by using modelling equations (Zijffers et al. 2008). The most favourable photosynthetic rate is eight quanta for every one molecule of O2 (Brindley et al. 2011). Another alternative that can be used to maintain the uptake light is the lumostatic operation of a bubble-column photo­bioreactor (Choi et al. 2003). Photobioreactors also have high-efficiency utilization of CO2 (Chiu et al. 2008), whereas up to 82.3% CO2 removal efficiency can be achieved in an airlift bioreactor. Airlift bioreactors also are suitable for batch, con­tinuous, and semi-continuous culturing of microalgae (Kaewpintong et al. 2007).

The basic feature of a membrane-integrated photobioreactor for CO2 utilization by microalgae culture consists of CO2 supply set, a membrane set, and photobioreactor system. The CO2 supply set consists of an air compressor, gas mixer, gas filter (optional), and a CO2 tank. The membrane set consists of a peristaltic pump and one or more mem­brane contactor modules. The photobioreactor system consists of an illumination source and a closed culture vessel (bioreactor) to culture microalgae. This system can be arranged as shown in Fig. 14.4. The CO2 cycle also is shown in Fig. 14.4. The mem­brane module can be rearranged inside the culture vessel (Fan et al. 2007), and the harvesting system can be integrated with the membrane photobioreactor system.

The most important part of a membrane photobioreactor set up is the connection of the membrane device to the photobioreactor, because it affects the CO2 supply and microalgae productivity. The CO2 gas can be saturated using a water column (Ferreira et al. 1998) or a gas mixer (Cheng et al. 2006; Fan et al. 2007). Each open­ing between the CO2 tube and the membrane fittings of the system must be sealed with rubber caps and tape to avoid gas releases to the surroundings and gas exchanges between the system and the surrounding air. The CO2 value can be measured directly by a CO2 meter or integrated with a chromatography system for direct measurement of DO and CO2 (Cheng et al. 2006; Fan et al. 2007). The indirect method is mea­sured by stoichiometric equation of pH value and NaOH solution (Carvalho and Malcata 2001; Fan et al. 2008).

A cultivation technique is important because it affects the CO2 utilization and bio­mass productivity of the microalgae. Thus, the choice of a batch, continuous, or semi-continuous mode of culture has a significant effect on the microalgae photosyn­thesis process. A batch culture of microalgae is defined as a culture period, whereas the cultivated microalgae cell is harvested at once. The batch culture requires a sim­pler design compared to a continuous culture. The batch operation usually remains the same despite increased dilution, and, once harvested, the entire culture is replaced with fresh microalgae cells and media. The continuous culture is conducted based on dilution rate. When the dilution reaches a certain point, half of the media are har­vested and replaced with new media to maintain the desired dilution, usually up to 0.67 per day. The dilution rate also affects microalgae productivity and its biochemi­cal contents (Moreno et al. 1998; Sanchez et al. 2008; Cuaresma et al. 2011).

The high number of measurements of microalgae analysis is one of the advan­tages of the batch culture (Pors et al. 2010). The batch culture also is capable of producing more biomass and biochemical compounds compared to a continuous culture (Yongmanltchal and Ward 1992; Gonzalez and Bashan 2000). However, in terms of productivity, a continuous culture has a greater yield of biomass (Carvalho and Malcata 2001).

Effective design on continuous culture system is capable to lower the production cost, about 40% than the traditional batch culture (Bentley et al. 2008; Sananurak et al. 2009). Most microalgae species that were cultivated in batch or continuous cultures do not have much effect on productivity compared to the nutrient feed con­centration, such as the nitrogen and carbon supplies. The growth rates of batch and continuous cultures are about the same (Pruvost et al. 2009). For a longer cultivation period, such as 4 months, increased production of microalgae can be achieved in continuous culture mode compare to batch culture mode (Rodriguez et al. 2010). The biomass productivity of microalgae cultivated in the continuous mode is com­parable to that in the batch mode (Ethier et al. 2011).

14.3 Conclusion

The membrane-integrated photobioreactor serves two major roles in biofuel pro­duction. The first role is to increase the mass transfer of exchange CO2-O2 gases in the photobioreactor, and the other is to enhance the photosynthetic rate of microal­gae, thereby increasing microalgae productivity. Although it has been proven that a membrane contactor can increase the mass transfer rate in the gas exchange process in a photobioreactor, the issue involving pressure drop due to fouling of the pores of the membrane has become a major challenge to the use of the membrane photobio­reactor. This problem is associated with the design parameters of the system and its operating conditions, such as operating pressure and the circulation process of the media culture. We are confident that this can be solved by conducting a numerical study through modelling and experimental work. The membrane photobioreactor system could be an extremely important device for both removing CO2 from the atmosphere and producing biomass by microalgae.

Acknowledgments This work was financially supported by the Research Grant LRGS/TD/2011/ UMP/PG/04 from Ministry of Higher Education of Malaysia. This work was also supported by the Borneo Marine Research Institute, Universiti Malaysia Sabah, Malaysia.

[1] OPBC is created under the National Biomass Strategy to accelerate development and commercialization of technologies for conversion of lignocellulosic biomass feedstock into higher — value-added uses such as biofuels and bio-based chemicals and the related technical, logistics, and social aspects.

N. Desmira (H) • K. Kuniyuki

EcoTopia Science Institute, Nagoya University, Furocho, Nagoya 464-8603, Japan e-mail: nelfad@gmail. com

[3] Morita

Department of Engineering Science, Osaka Electro-Communication University, 18-8 Hatsucho, Neyagawa 572-8530, Japan

R. Pogaku and R. Hj. Sarbatly (eds.), Advances in Biofuels,

DOI 10.1007/978-1-4614-6249-1_6, © Springer Science+Business Media New York 2013

A. H. Kamaruddin (H)

Chemical Engineering Department, Universiti Sains Malaysia, Penang, Malaysia e-mail: chazlina@eng. usm. my

N. A. Serri • S. R.A. Rahaman

Universiti Sains Malaysia, Penang, Malaysia

J. H. Sim

Universiti Tun Abdul Razak, Kuala Lumpur, Malaysia

[5] F. A. Halim

Chemical Engineering Department, Universiti Teknologi Mara, Penang, Malaysia

R. Pogaku and R. Hj. Sarbatly (eds.), Advances in Biofuels,

DOI 10.1007/978-1-4614-6249-1_8, © Springer Science+Business Media New York 2013