The microalgal reactors described above differ in features such as surface-to-volume ratio, freedom to adjust orientation and inclination, efficiency of mixing and gas supply (related to hydrodynamics and mass transfer), ease of maintenance, tempera­ture regulation, and construction materials. Table 5.4 presents a comparison of these design features in six major types of reactor. No reactor design is able to effectively control all these parameters simultaneously; therefore, any choice will be a compro­mise between the advantages and disadvantages of each system (Table 5.5).

5.4.1 The Open versus Closed System Debate

The relative merits of closed and open systems have been extensively debated in the microalgal literature (Pulz, 2001; Carvalho et al., 2006; Grobbelaar, 2009; Mata et al., 2010). There is no doubt that open ponds are the primary systems used in large-scale, outdoor microalgal cultures, but their commercial use has been limited to species that can be maintained using an extreme cultivation environment

TABLE 5.4 Comparison of Main Design Features of Various Reactor Types Reactor Type Raceway Vertical Column or Airlift Horizontal Tubular Helical Tubular Flat Plate Stirred Tank Mixing Fair Good Uniform Uniform Largely uniform Largely uniform Gas transfer Poor Good Low to high along length of tube Low to high along length of tube Good Low to high Hydrodynamic stress Low Low Low to high depending on pumping system Low to high depending on pumping system Low High Light harvesting Fair, depending Good at low volume; poor Excellent at narrow Excellent at narrow Excellent at Poor efficiency (area:volume ratio) on depth at high volume diameter diameter narrow diameter Temperature control None Good Good Good Good Good Species control Difficult Easy Easy Easy Easy Easy Sterility None Easily achievable Potentially achievable Potentially achievable Potentially achievable Easily achievable Land area required Large Small Large Medium Small Small Source: From Borowitzka (1999); Carvalho et al. (2006).

TABLE 5.5 Advantages and Disadvantages of Different Reactor Types
Reactor Type Raceway	Advantages Relatively economical Low energy input Easy to clean and maintain No O2 build-up Large production capacity	Disadvantages Little control of culture conditions Poor mixing Light limitation Readily contaminated Limited growing period Poor productivity Large land area required Limited CO2 mass transfer with large CO2 losses to atmosphere if pond sparged Temperature determined by climate
Vertical column PBR Excellent mixing High mass transfer rates Low shear stress Easy to clean and sterilize Reduced photo-inhibition Low cost Low land area requirement	Low angle to incident sunlight Size limited by area:volume ratio Large mixing energy due to gas compression
Gradients of pH, O2, and CO2 along tubes Wall growth: fouling in tubes difficult to clean Large land area required Significant water losses if evaporative cooling is used Possible hydrodynamic stress
Large illuminated surface area Good biomass productivity Relatively cheap construction materials
Tubular PBR

Large illuminated surface area Short light path Good biomass productivity Easier to clean Lower O2 build-up

Scale-up requires many modules — material intensive Temperature control critical in thin reactors

Source: From Borowitzka (1999); Pulz (2001); Chisti (2007); Ugwu et al. (2008); Brennan and Owende (2010); Mata et al. (2010).

(Lee, 2001). To expand the product range, there is significant interest in the design of closed reactors, particularly in the production of high-value, low-volume prod­ucts requiring a high degree of sterility. The essence of the debate is presented in Table 5.6 through a comparison of the key parameters of open and closed reactors.

Despite their higher cost and technical complexity, closed systems promise great improvements in enhancing control over process parameters. The challenge appears to

TABLE 5.6

Comparison of Key Design Features and Process Parameters of Open and Closed Systems

Open Systems Closed Systems

Source: From Pulz (2001); Carvalho et al. (2006); Grobbelaar (2009); Mata et al. (2010).

lie in enhancing productivity sufficiently that it outweighs the additional cost of closed reactors. Another alternative is to attempt to design PBRs that are cheap to build in terms of construction materials, as well as efficient in terms of light distribution, mixing, gas sparging, etc, which makes them cheap to operate by lowering energy requirements.

A major but rarely recognized concern, particularly for energy products such as biofuels, is the energy balance of the production system. For a process to be econom­ically viable and sustainable, the energy generated when the product is used must be greater than that involved in its manufacture. The energy inputs in microalgal reactors are particularly focused on the mixing and gas pressurization, as well as the embodied energy in reactor materials; therefore, open systems have a more favorable energy balance than closed systems (Richardson, 2011).

5.2 CONCLUSION

In the production of algal energy products, the aim is the biological conversion of sunlight to a more convenient, portable, storable, and accessible form of fuel. In the case of biodiesel production, this entails the production of algal lipids. Lipid productivity is dependent on both biomass productivity and lipid content (Griffiths and Harrison, 2009), which is determined by both the species used and the culture conditions provided by the reactor.

Most large-scale commercial algal production systems to date have been for food, feed, neutraceutical, or fine chemical production. As biofuel is a bulk commodity prod­uct, production must be on a grand scale, and costs must be extremely low. Sterility, particularly microbial contamination, is perhaps less of a concern for energy produc­tion than it would be for a product such as a neutraceutical or fine chemical for human consumption. A particular consideration with an energy product is that the energy balance must be positive; that is, the energy recovered from the product must exceed the energy input required for production. LCA (life cycle assessment) studies to date suggest that biofuel production in closed reactors is unable to achieve a net energy ratio (energy out/energy into process) of above one (Lardon et al., 2009; Richardson, 2011).

It is generally considered that closed PBRs alone will be incapable of cost-effectively producing microalgal biomass on the scale required for biofuels (Greenwell et al., 2010). While productivities will inevitably be lower in open raceways, it is envisaged that open systems, due to their lower cost, simplicity of operation, and ability to scale to large volumes, will form the basis of microalgal production for biofuels (Sheehan et al., 1998). The lipids necessary for biodiesel production are often produced under nutrient stress conditions. Therefore, it is likely that a two-phase system using closed reactors to generate contamination-free inoculum with a high biomass concentration for a second product-generating stage in open systems could be advantageous.

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