Date Published: July 28, 2017
Publisher: Springer Berlin Heidelberg
Author(s): Amir Hussain, Martin Kangwa, Marcelo Fernandez-Lahore.
The successful industrial production of ethanol and fine chemicals requires the development of new biocatalytic reactors and support materials to achieve economically viable processes. In this work, a Stirred-Catalytic-Basket-BioReactor using various immobilizing foams as support material and compared to free cells were used, focusing mainly on; (i) effect of mass-transfer on cells physiology and (ii) ethanol productivity. The performance of the reactor was further evaluated by ethanol volumetric productivity, yield and time for process completion and it was found that the variation of ethanol production and diffusion of the substrate in fermentation process are co-related with the stirrer speed and initial glucose concentration. It was also observed that the time difference for glucose consumption between free and immobilized cells (alginate and sponges) tends to increase by increasing the glucose concentration in the medium. We found that at higher stirrer speed (500 rpm) when using higher glucose concentration (200 g/l), ethanol volumetric productivity increased significantly in the sponge (85 g/l) as compared to alginate beads (79 g/l) and free cells (60 g/l). From the data obtained, it can be concluded that sponges are the best support material for attaining higher ethanol productivity. A stirred catalytic basket bioreactor with yeast cells immobilized in polyethylene sponge gives higher ethanol production at a higher glucose consumption rate, and this productivity is due to higher mixing efficiency and reduced external as well as internal mass transfer limitations. The potentials of the reactor rank it as a remarkable ethanol/fine-chemical production approach that needs further investigations.
Air-pollution currently witnessed globally, caused mainly by the extensive usage of fossil fuel has brought about devastating effects both environmentally and health, thereby encouraging extensive scientific research in finding alternative and cheaper bio-fuel like ethanol via microbial fermentation in the bioreactor. The traditional setups used in the ethanol production like membrane bioreactor, airlift bioreactor, fixed bed bioreactors and stirred tank reactors have some drawbacks of less product yield due to low mass and heat transfer, inefficient conversion of substrate, uneven mixing and shear stress on biocatalysts (Hussain et al. 2015a). To overcome these problems and to improve the efficiency of a bioreactor, four most important factors need to be put in consideration i.e. choice of the fermentation process, biocatalyst, support for immobilization and bioreactor design.
In biocatalysis, free cells reactions known as a homogeneous catalytic reaction when reactant and catalysts are in the same phase in which mass transfer effect is considered as negligible. When cells are immobilized in different matrices like alginate beads and chemically grafted sponges, the reaction is known as heterogeneous, and reactants and catalysts are in different phases. The catalyst is normally in the solid phase, and reactants are in liquid phase, and the reaction is dependent on the mass transfer (Klaewkla et al. 2011; Hussain et al. 2015b). In this instance, the reaction only takes place when reactants are transferred to the catalytic reaction site by diffusing across external fluid layer around the catalyst (external mass transfer) into pores within the catalyst (Internal mass transfer).