Annular reactors are typically translucent large diameter containers filled with algae suspended in a liquid medium, in which gases are bubbled from the bottom of the container. Since no precisely defined flow lines are reproducibly formed, it can be difficult to control the mixing properties of the system which can lead to low mass transfer coefficients, poor photomodulation, and low productivity. Moreover, to work with sufficient volume, the large diameter leads to a considerable dark fraction in the middle of the cylinder. This part does not contribute to productivity or may even have detrimental effects on growth.
The tubular PBR is one of the most suitable types for outdoor mass cultures. Most of them are usually constructed with either glass or plastic tubes. They consist of straight, coiled or looped transparent tubing arranged in various ways for maximizing sunlight capture. Properly designed tubular PBRs completely isolate the culture from potentially contaminating external environments, allowing extended duration algal cultures. Mixing of the cultures in tubular PBRs is usually done either with air-pump or airlift systems. This design is very suitable for outdoor mass cultures of algae since they have a large illumination surface area. On the other hand, one of the major limitations of tubular PBRs is their poor mass transfer. It should be noted that mass transfer (oxygen build-up) becomes a problem when tubular PBRs are scaled up. For instance, some studies have shown that very high dissolved oxygen levels are easily reached in tubular PBRs.
Comparing the three ways of engineering closed PBRs, the tubular ones facilitate better control of many culture environment parameters such as the carbon dioxide supply, the water supply, the optimal temperature, the efficient exposure to sunlight, the culture density, the pH level, the gas supply rate, or the mixing regime.