Bioreactors

How do tubular reactors work?

In a transparent tank, or algal bag system for growing algae the organisms pick up light whilst in the few centimetres near the tank wall or surface where they react with carbon dioxide and nutrients and photosynthesis occurs. Once they move away from that surface then light cannot penetrate and photosynthesis ceases. However, this dark area allows more complex protein building to occur within the algae. By the very nature of a tank the ratio of light to dark area is small and so algal growth is limited.

Key benefits of a tubular reactor

	It offers maximum efficiency in using light and therefore greatly improves productivity. Typically the culture density of algae produced is 10 to 20 times greater than bag culture - and can be even greater.
	Space saving. Can be mounted vertically, horizontally or at an angle, indoors or outdoors.
	Dramatically reduces labour requirements and eliminates handling problems.
	Systems can be operated for long periods without culture crashes occurring.
	Patented tube self cleaning mechanisms can dramatically reduce fouling.
	They are closed, controlled, usually automated systems and therefore cultures may be more easily kept hygienically. Environmental parameters are simply controlled.

Tubular reactors get round this problem by using narrow diameter tubes that allow the light to penetrate to the centre of the tube. This maximises the surface area available for photosynthesis. Because there is still a requirement for algae to spend time away from the light the tubing is connected to a tank and the algae is constantly recirculated from the tank round the tube and back to the tank.

There are a number of different implementations of tubular reactors but they basically all work in this way. The tube can be a flexible transparent tube which is either laid out in a serpentine manner or coiled, or rigid, in which case tubes are either joined at the end by U joints or by manifolds

There is a substantial body of literature to show that they are an extremely efficient way of growing large quantities of many algae. An increasing number of hatcheries and algal production facilities are using tubular reactors for the production benefits that they bring in terms of yield, control of environment and labour savings.

Limitations
Pumps
In order to reduce the problems of algae fouling and to avoid frequent tube cleaning, the flow rate through the tubing needs to be sufficient to induce turbulent flow (some systems avoid the need for this by having pig based cleaners which are used relatively frequently but this means stopping the system for cleaning).

The simplest and cheapest means of achieving this is with a centrifugal pump. This is very effective with tough cell walled algae such as nannochloropsis or chlorella and these are probably the most successful species to grow in tubular reactors where they grow to extremely good densities. A centrifugal pump will however act like a liquidizer on many algal species and will destroy them completely.

Diaphragm pumps and very low shear pumps such as Discflo are good for pumping flagellate species such as tetraselmis and isochrysis. Diaphragm pumps are cheap but often require a high pressure compressed air supply which is often not available. Low shear pumps, by their nature, require larger sizes to produce the same flow as their centrifugal equivalents which means that they can be expensive. Some people have achieved good results with flexible impeller pumps.

System size
As the length of tube gets longer so the friction of the liquid in the tubing gets greater and the head pressure required to pump it gets greater. Therefore bigger and more expensive pumps are required.

In addition, as the amount of time the algae spends in the light increases so the available carbon dioxide is consumed and oxygen poisoning combined with bleaching from the excess light can occur.

Our solution
We have carried out research into tubular reactors for a number of years and developed the BioFence tubular reactor as an easy to assemble modular system.

Why the BioFence?
Expandability
If algae passes along a length of tube eventually the water becomes supersaturated with oxygen which inhibits growth. The BioFence has the tubes arranged on manifolds which dramatically increase the path length that can be taken by the algae so very large systems can be built.

Click picture to enlarge

The tubes can be joined together and the manifolds stacked so the system is highly expandable.

Click picture to enlarge

In addition, the manifold system reduces the size of pump required to recirculate the algae thus saving both cost and algal damage.

Self cleaning highly transparent tubes
The BioFence uses very tough, very clear food grade plastic tubes.
They are self cleaning thanks to patented special cleaning beads that continuously circulate with the algae.
This enables the system to be run for extended periods without culture crashes or cleaning.

Space saving
The tubes can be mounted vertically in a fence like arrangement or horizontally on the ground or at an angle on a roof. They take up a lot less room than other growing methods and of course, they are growing far more concentrated algae so even less space is required.
An array of tubes 10 m x 1.8 m produces the equivalent of 2000 litres of bag strength algae per day

Labour Saving
Because the BioFence is automatically controlled and delivers algae either continuously or at set intervals the equipment requires little attention. A cell count once a day should suffice. In addition, the algae can be delivered either directly to where it will be used or into easily handled containers.

To work out your system requirements for the amount of algae you want to grow click here