Tubular systems are widely used as close systems in commercial production. Usually tubu­lar photobioreactors are made of transparent polypropylene acrylic or polyvinylchloride

pipes with small internal diameters to increase the penetration of light. Mixing and agitation of the culture are maintained by an air pump to provide circulation (see Figure 2.4).

The most significant characteristic of this tubular system that is different from the vertical column bioreactor is the improvement of air-residence time inside the tubular bioreactor, which can provide more dissolved CO2. These systems could use artificial light, but they are also designed based on natural light (sunlight) provided from outside of the tube. The hydrodynamic stress on the algae may vary, depending on the flow characteristics of each system (e. g., turbulent flow, pump type). Likewise, the gas transfer to the culture may vary from low to high, depending on the flow characteristics and the air-supply technique adopted. The operational difficulties are similar to other systems, including growth of microalgae on the wall of the tubes, thus blocking the light penetration; high oxygen concen­tration that can inhibit photosynthesis; and limits on the length of the tube in a single run (Briassoulis et al., 2010).

Coil-type systems are often adopted to enhance the efficiency of space utilization com­pared to the other categories. Among the most important advantages of the system and com­mon to most coil-type systems are the larger ratio of surface area to culture volume to receive illumination effectively, as well as the easy control of temperature and contaminants (Briassoulis et al., 2010). The cleaning problems of tubular systems are not easy to overcome due to the small internal tube size, which has no ready mechanical way to conduct the inside cleaning for a long tube. The scale-up of these systems is relatively easy compared with other photobioreactor designs. The increase of tubular photobioreactor working volume can easily be achieved by simply extending the tube length to the designed volume if the air pump can affordably provide enough power to pump in air bubbles.

Light

source

Microalgae tank

і

Gas mixer

CO2 tank

FIGURE 2.4 Horizontal tubular photobioreactors for microalgae cultivation.