Applications of the Air Lift Fermenter

Since the first development of the airlift fermenter by Le Francois in 1955 (1), the applications have increased to cover most areas of fermentation. This configuration has likewise been developed into many different forms, including: (a) Concentric tube airlift (2); (b) external loop airlift (3); (c) jet loop reactor (4); and (d) propeller loop reactor (4). The subject for discussion in this paper will be the concentric tube airlift, as shown in Fig. (1). This consists of a vertical cylindrical vessel incorporating a concentric draught tube into which is injected a flow of air or other gas mixture suitable for oxygenation. The resulting reduction in bulk density in the central riser causes the contents to move upward thus displacing the contents of the surrounding downcomer, which moves downward, thus inducing circulation of the total contents. This has the advantage of generating liquid mixing and gas transfer without the use of mechanical agitators. The number of possible applications for the airlift fermenter has increased with the advent of genetic engineering techniques that result in new strains of unicellular organisms capable of producing many novel products. An almost universal requirement is the maintenance of absolute sterility, and the absence of agitation with associated shaft seals may be of benefit here. A typical property of some novel organisms is the sensitivity to shear stress of either the product (often a large, easily denatured protein) or the cells themselves, in which there may be an absence of a thick protective cell wall. The airlift fermenter should therefore be of use in these applications.