The results of Doctor N.A. Voinov and his colleagues’ research aimed at developing gas–liquid film bioreactors are systematized. Fluid dynamics and heat and mass transfer in a liquid film flowing by gravity down a surface with artificial large-scale roughness have been investigated. Relationships based on the plug flow model are suggested for calculating mass transfer in the working zones of the bioreactor. Ways of raising the productivity of the apparatus and reducing the cost of culturing Candida scottii yeast are considered. Closed-loop gas circuit schemes are suggested for the film bioreactor.
film bioreactor, yeast biomass, heat transfer, mass transfer, artificial roughness
INTRODUCTION
Gas–liquid bioreactors are widely used in the food and related industries, including the production of enzyme preparations, baker’s yeast, biopolymers, and other microbiological synthesis products. One of the serious drawbacks of the existing industrial bioreactors is their low biomass output capacity, high stirring and concentrating costs, and large mounts of microbiological synthesis products discharged into the environment because of the low intensity of heat transfer. This imposes limits on the composition of te gaseous substrate, generates temperature distribution nonuniformities in the culture liquid, and does not ensure proper purification of the exit gas from metabolism products.
Of the wide variety of the existing fermenters, the most widely employed ones are bubblers with mechanical stirring and liquid circulation in the working space of the apparatus. However, the energy consumption per unit weight of the resulting biomass in stirred bioreactors is higher than in the other types of apparatuses [1] and is 3–4 (kW h)/kg at a comparatively low biomass concentration (Table 1). Although the stirring of the liquid increases the interfacial area owing to the breakup of gas bubbles, the liquid flow remains mainly laminar and the energy consumed does not afford an adequate increase in the rate of oxygen transport in the liquid phase. In addition, heat removal from the reaction zone of the apparatuses is slow and, as a consequence, the culturing process often takes place at a non-optimal temperature.
One way of increasing the output capacity of the gas–liquid bioreactor is by saturating the culture liquid with the gas in a turbulent liquid film. [2–4] flowing down the surface of contact devices. This technique has found application in microorganism culturing methods developed by the authors [5–10].
The introduction of a falling-film section for gas absorption in the culture liquid into the stirred apparatus makes it possible to significantly intensify heat and mass transfer and to reduce specific expenses.
Film bioreactors are next-generation apparatuses, and their introduction into industry is impeded by the poor understanding of the heat transfer processes occurring there. The oxygen transfer rate in the falling liquid film can reach 10 kg/(m3 h) or over [11], and the surface mass transfer coefficient can be up to (2–5) * 10–2 m/s, one order of magnitude larger than in the other types of fermenters. Heat transfer in a turbulent film [12] is also more intensive than in bubblers and gas-lift apparatuses. Furthermore, biomass growth in film bioreactors can be carried out without employing mechanical devices for transporting the components of the gaseous substrate, since the gas is uninvolved in the generation of the phase contact surface and in liquid turbulization. Owing to the high rates of oxygen supply and metabolite removal, film bioreactors are capable of processing concentrated nutrient media, ensure a high product output rate and fine purification of the spent gas from metabolites and substrate drops, maintain a high degree of sterility in the process, and make it possible to organize a closed-loop gas circuit and gas cleaning in the apparatus.
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