Photobioreactor design: Mixing, carbon utilization, and oxygen accumulation

Area of Science:

• Biotechnology and Bioengineering
• Algal Cultivation Systems
• Photobioreactor Design

Background:

• Photobioreactor design and operation are critical for efficient algal cultivation.
• Key operational parameters include mixing, carbon utilization, and oxygen accumulation.
• Open raceway ponds and closed tubular reactors represent diverse design approaches.

Purpose of the Study:

• To compare the operational characteristics of open raceway ponds and tubular photobioreactors.
• To evaluate factors such as mixing efficiency, energy consumption, carbon dioxide utilization, and oxygen buildup.
• To determine the economic and operational feasibility of different photobioreactor designs for algal mass cultivation.

Main Methods:

• Operated two 100-m(2) open raceway ponds with a planktonic Chlorella sp. to collect data.
• Compared productivity and mixing parameters between ponds with different mixing velocities.
• Analyzed energy consumption using Manning's equation and compared it with tubular reactor data.
• Measured carbon dioxide mass transfer coefficient (K(L)) in open ponds.
• Modeled oxygen accumulation rates in both reactor types under maximal photosynthesis.

Main Results:

• No significant productivity difference was observed between ponds with mixing velocities of 30 cm/s and 1-30 cm/s, indicating power consumption or CO(2) outgassing limits maximal mixing.
• A 1 cm diameter tubular reactor at 30 cm/s consumes 10 times more energy than a typical open pond (20 cm deep at 20 cm/s).
• Open ponds offer greater CO(2) storage capacity but tend to desorb CO(2) and accumulate less dissolved oxygen (25-40 mg/L) compared to tubular reactors (potentially 100 mg/L).
• The CO(2) mass transfer coefficient (K(L)) for a 100-m(2) pond surface was determined to be 0.10 m/h.

Conclusions:

• Open raceway ponds are more energy-efficient and cost-effective for large-scale algal mass cultivation compared to tubular photobioreactors.
• While enclosed reactors offer aseptic operation, their higher capital and operational costs limit their feasibility for bulk algal production.
• Open ponds are the most practical photobioreactor design for meeting economic and operational requirements in algal mass cultivation, provided species stability can be maintained.