[show abstract][hide abstract] ABSTRACT: A theoretical analysis using the surface-renewal and film-penetration models,
which includes gas-phase resistance to mass transfer, is presented for the rate of
absorption of a gas and its transfer to the bulk liquid in the case where the solute gas
undergoes a first-order chemical reaction in the liquid phase. It reveals that:
(a) The fraction of absorbed gas transported to the bulk liquid depends upon the Hatta
number Ha in case of the surface-renewal model and on Ha as well as a dimensionless
hydrodynamic parameter in case of the film-penetration model.
(b) The widely assumed law of addition of resistances is valid for the surface-renewal
and film-penetration models.
(c) The reaction influences both the overall mass-transfer coefficient and the nature of
the driving force, i.e. the increased rate of absorption due to the reaction is not solely
due to the enhancement factor multiplying the liquid-phase mass-transfer coefficient
for physical absorption as has been conventionally assumed in the literature.
It is also shown that in a gas-liquid reactor the film and surface-renewal models give close
predictions for both the rate of absorption and concentration of dissolved gas in the liquid
leaving the reactor. For values of Ha ≥ 0.5, the bulk-liquid concentration of dissolved gas
predicted by both models is negligible compared to its interfacial concentration.
[show abstract][hide abstract] ABSTRACT: In aerobic bioprocesses, oxygen is a key substrate; due to its low solubility in broths (aqueous solutions), a continuous supply is needed. The oxygen transfer rate (OTR) must be known, and if possible predicted to achieve an optimum design operation and scale-up of bioreactors. Many studies have been conducted to enhance the efficiency of oxygen transfer. The dissolved oxygen concentration in a suspension of aerobic microorganisms depends on the rate of oxygen transfer from the gas phase to the liquid, on the rate at which oxygen is transported into the cells (where it is consumed), and on the oxygen uptake rate (OUR) by the microorganism for growth, maintenance and production. The gas-liquid mass transfer in a bioprocess is strongly influenced by the hydrodynamic conditions in the bioreactors. These conditions are known to be a function of energy dissipation that depends on the operational conditions, the physicochemical properties of the culture, the geometrical parameters of the bioreactor and also on the presence of oxygen consuming cells. Stirred tank and bubble column (of various types) bioreactors are widely used in a large variety of bioprocesses (such as aerobic fermentation and biological wastewater treatments, among others). Stirred tanks bioreactors provide high values of mass and heat transfer rates and excellent mixing. In these systems, a high number of variables affect the mass transfer and mixing, but the most important among them are stirrer speed, type and number of stirrers and gas flow rate used. In bubble columns and airlifts, the low-shear environment compared to the stirred tanks has enabled successful cultivation of shear sensitive and filamentous cells. Oxygen transfer is often the rate-limiting step in the aerobic bioprocess due to the low solubility of oxygen in the medium. The correct measurement and/or prediction of the volumetric mass transfer coefficient, (k(L)a), is a crucial step in the design, operation and scale-up of bioreactors. The present work is aimed at the reviewing of the oxygen transfer rate (OTR) in bioprocesses to provide a better knowledge about the selection, design, scale-up and development of bioreactors. First, the most used measuring methods are revised; then the main empirical equations, including those using dimensionless numbers, are considered. The possible increasing on OTR due to the oxygen consumption by the cells is taken into account through the use of the biological enhancement factor. Theoretical predictions of both the volumetric mass transfer coefficient and the enhancement factor that have been recently proposed are described; finally, different criteria for bioreactor scale-up are considered in the light of the influence of OTR and OUR affecting the dissolved oxygen concentration in real bioprocess.
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