Photobioreactor design: Mixing, carbon utilization, and oxygen accumulation

Abstract

Photobioreactor design and operation are discussed in terms of mixing, carbon utilization, and the accumulation of photosynthetically produced oxygen. The open raceway pond is the primary type of reactor considered; however small diameter (1-5 cm) horizontal glass tubular reactors are compared to ponds in several respects. These are representative of the diversity in photobioreactor design: low capital cost, open systems and high capital cost, closed systems. Two 100-m(2) raceways were operated to provide input data and to validate analytical results. With a planktonic Chlorella sp., no significant difference in productivity was noted between one pond mixed at 30 cm/s and another mixed from 1 to 30 cm/s. Thus, power consumption or CO(2) outgassing limits maximal mixing velocities. Mixing power inputs measured in 100-m(2) ponds agreed fairly well with those calculated by the use of Manning's equation. A typically configured tubular reactor flowing full (1 cm diameter, 30 cm/s) consumes 10 times as much energy as a typical pond (20 cm deep flowing at 20 cm/s). Tubular reactors that flow only partially full would be limited by large hydraulic head losses to very short sections (as little as 2 m length at 30 cm/s flow) or very low flow velocities. Open ponds have greater CO(2) storage capacity than tubular reactors because of their greater culture volume per square meter (100-300 L/m(2) vs. 8-40 L/m(2) for 1-5-cm tubes). However, after recarbonation, open ponds tend to desorb CO(2) to the atmosphere. Thus ponds must be operated at higher pH and lower alkalinity than would be possible with tubular reactors if cost of carbon is a constraint. The mass transfer coefficient, K(L), for CO(2) release through the surface of a 100-m(2) pond was determined to be 0.10 m/h. Oxygen buildup would be a serious problem with any enclosed reactor, especially small-diameter tubes. At maximal rates of photosynthesis, a 1-cm tubular reactor would accumulate 8-10 mg O(2)/L/min. This may result in concentrations of oxygen reaching 100 mg/L, even with very frequent gas exchange. In an open pond, dissolved oxygen rises much more slowly as a consequence of the much greater volume per unit surface area and the outgassing of oxygen to the atmosphere. The maximum concentration of dissolved oxygen is typically 25-40 mg/L. The major advantage of enclosed reactors lies in the potential for aseptic operation, a product value which justifies the expense. For most products of algal mass cultivation, open ponds are the only feasible photobioreactor design capable of meeting the economic and operating requirements of such systems, provided desirable species can be maintained.

Current Status and Outlook in the Application of Microalgae in Biodiesel Production and Environmental Protection

“…Openculture systems normally cannot use supplied CO 2 effectively, which easily escaped from the culture media due to the shallow depths and poor CO 2 mass transfer efficiency, and CO 2 fixation efficiency ranged from 10 to 30% (Weissman et al, 1988). Carbon fixation would be increased only when the CO 2 residence time was increased in bioreactors (Cheng et al, 2006).…”

Biotechnology of algal biomass production: a review of systems for outdoor mass culture

“…Torzillo et al, 1986;Richmond, 1987Richmond, , 1990Richmond etal., 1993;Benemann, 1989;Benemann et al, 1987;Weissman et al, 1988;Kyle, 1989). For identical biomass productivities, the higher cell densities obtained with biophotoreactors result in reduced harvesting volumes and thus in reduced processing (partition solid/liquid) costs.…”

“…Chaumont et al (1991a) have patented a system using recycling balls for continuous cleaning of the inner surface of their tubular photobioreactor. The main problems encountered in the operation of photobioreactors are the control of oxygen concentration (Weissman et al, 1988) and overheating in summer which may require special and costly precautions (Torzillo et al, 1986). In order to combine the advantages of both systems, Richmond (1987) proposed a culture concept made of a tubular reactor, where the algae culture would heat quickly at the beginning and the end of the light phase, connected to an open-channel race-way where overheating would be controlled by circulating the culture.…”

Evaluation of Growth Yield of Spirulina maxima in Photobioreactors

“…Many advanced and complicated solutions for the cultivation of microalgae have been proposed [12][13][14][15] . If cheaper construction of a reactor for the cultivation of photoautotrophic organisms with a better utilization of nutrients could be used instead of technically advanced solutions, greater economic sustainability could be achieved [16][17][18][19] . Two reactors that differ in their construction, ratio of illuminated surface (A) to volume (V), and operating costs were used.…”