Tubular reactors are characterized by very high area-to-volume ratios (dependent on tube diameter) but poor mass transfer, leading to O2 build-up and CO2 depletion (resulting in photorespiration, oxidative damage, and pH gradients) over the length of closed tubing. Tubes are generally manufactured from polyethylene or glass. The most important criteria for construction material are transparency, to allow good light penetration, and low cost (Ugwu et al., 2008). Additional challenges include photo-inhibition, temperature control, and fouling due to cells adhering to the inside of tube walls, leading to decreased light penetration (Ugwu et al., 2008). Narrow — diameter tubes can present a challenge to clean.

Most tubular reactors can be categorized into one of three types:

1. Vertical airlifts or bubble columns consisting of a clear vertical tube mixed by gas sparging from the bottom

2. Horizontal tubular systems with clear, thin-diameter tubing lying or stacked horizontally, usually connected to a gas transfer system

3. Helical tubular reactors consisting of thin, flexible tubing coiled around a circular framework

Airlift and bubble column reactors (Figures 5.2a and b) are examples of vertical tubular reactors. Air, or air enriched with CO2, is bubbled into the bottom, providing efficient mixing and gas transfer throughout the reactor. The simplest form of bubble column reactor is a hanging polyethylene bag, and these have frequently been used as a low-cost option. Plastic bags have a high transparency, good sterility at start-up (due to the high temperatures used in plastic extrusion), and are readily replace­able. Concentrations three times that of open ponds were obtained by culturing Porphyridium in 25 L hanging bags (Cohen and Arad, 1989). Other researchers have also found that 40 to 50 L bags are practical (Trotta, 1981; Martinez-Jeronimo and Espinosa-Chavez, 1994).

Although cultivation in plastic bags is simple, cheap, and widely employed, par­ticularly in the production of microalgae as feed for aquaculture hatcheries, scale-up is limited by the fragility of cheap plastic and light penetration, as increases in bag volume lead to decreased productivity due to mutual shading (Martinez-Jeronimo and Espinosa-Chavez, 1994). Rigid vertical tubes have also been frequently used (Carvalho et al., 2006). In an airlift reactor, an inner tube called the riser directs air bubbles up the center of the reactor and then down the outer region, called the down­comer (Figure 5.2b). This provides effective, gentle mixing and produces regular light-dark cycles.

Vertical reactors are compact, low cost, and easy to clean and keep sterile (Ugwu et al., 2008); however, their size is limited by the surface-to-volume ratio. The scale — up of any tank, container, or hanging bag becomes limited by light penetration at a

Schematic diagram of (a) vertical bubble column and (b) airlift reactor.

volume of between 50 and 100 L. Underwater lighting has been considered, either in the form of submersible lamps, or optical fibers redirecting sunlight, but these add to the cost and energy footprint of the system (Pulz, 2001). In addition, the vertical angle of reactors is not ideal for capturing incident sunlight (Carvalho et al., 2006).

Horizontal tubular reactors are designed to optimize light capture by increasing the angle to sunlight (Figure 5.3). Internal tube diameters range between 1.2 and 13 cm (Lee, 2001). The thinner the tubing used in the solar collector, the more efficient the light capture but the lower the culture volume per length of tubing. Thin tubing is also particularly susceptible to overheating, and temperature regulation mechanisms must be installed—for example, evaporative water cooling (often requiring large volumes of water), immersing the tubes in water, or shading them by covering or overlap­ping the tubes. The length of the closed tubing is constrained by O2 build-up; there­fore, reactors are usually modular, with parallel sets of shorter tubes interconnected, rather than a single long tube. Gas transfer takes place either at tube connections or in a dedicated gas exchange unit, where aeration and mixing are provided either by pump or airlift (Ugwu et al., 2008). There have been successful runs of horizontal tubular reactors with volumes of up to 8,000 to 10,000 L (Torzillo et al., 1986). One of the major disadvantages of horizontal reactors is the large land area required for horizontal tubing. The increased productivity with respect to open ponds may not be cost effective if a greater land area is necessary (Carvalho et al., 2006).

Helical tubular reactors (Figure 5.4) are a promising alternative to horizontal tubular reactors as they reduce the land area required. By expanding vertically,

the areal footprint of the reactor is smaller, but the angle to sunlight is also reduced. Placing a light in the center of the coil can improve light penetration. A conical, instead of helical, framework has also been suggested, as it improves the spatial distribution of tubes for sunlight capture (Morita et al., 2000). However, scale-up is then limited as the angle and height of the coil are defined. One of the most effective designs is the Biocoil developed by Robinson (1987). It consists of a set of polyethyl­ene plastic tubes (2.4 to 5 cm in diameter) wound helically around an open circular framework. Parallel bands of tubes connect to a gas exchange tower. A centrifugal pump is used to move culture to the top of the coil, which may not be suitable for all species due to shear stress in sensitive cells and pump fouling in filamentous species. A heat exchanger or evaporative cooling provides temperature control. The system provides good mixing; minimal cell adhesion to the inside of tubes and scale-up is easy, involving the addition of more parallel coils. Several marine species and Spirulina have been successfully cultivated for more than 4 months in a 700 L Biocoil (Borowitzka, 1999; Carvalho et al., 2006).

An а-shaped reactor, presenting an interesting alternative tube layout, was designed by Lee et al. (1995) (Figure 5.5). The symmetrical design uses airlift pumps to aerate and mix the culture in vertical tubes at either end. This then flows down tubes at a 45° angle, thus maximizing the angle of tubes to sunlight while saving space. As flow through the entire system is in the same direction, with two CO2 injection points, relatively high liquid flow and mass transfer rates can be main­tained with relatively low air supply rates (Lee et al., 1995).