Axial Distribution of Oxygen Concentration in Different Airlift Bioreactor Scales: Mathematical Modeling and Simulation

Abstract

Steady and unsteady state oxygen concentration distributions in the liquid and gas phases along the axial direction of different airlift bioreactor scales have been simulated for various gas flow rates and oxygen consumption rates by applying the axial dispersion model to the riser and the downcomer, and a complete mixing model for the top (separator) and the bottom sections of the bioreactor. The results show that the dissolved oxygen concentration is very low at the lower part of the downcomer when the rate of oxygen consumption by microorganisms is very high. Furthermore, the shorter (small) bioreactor shows relatively more uniform axial dissolved oxygen concentrations than the longer (large) bioreactor, due to the effect of the hydrostatic pressure along the bioreactor. One of the most important geometric factors for mass transfer is the reactor height, which dominates the mean pressure and thus influences the saturation concentration and mass transfer driving force. The presented model can be applied for modeling and scale-up of practical airlift bioreactors.

Modeling of Mass Transfer in an Internal Loop Airlift Reactor

“…Similar to other studies 5, 19, the following assumptions were made to derive the mathematical mass transfer model: (1) there are no radial gradients of velocity and density in liquid and gas phases; (2) ideal behaviors of the gas phase; (3) gas holdup and mass transfer coefficient in the reactor are constant; (4) axial dispersion coefficients of liquid and gas phases in riser and downcomer are constant, respectively; (5) isothermal conditions in the reactor. Note that the third and fourth assumptions may be invalid for industrial‐scale reactors.…”

“…Airlift reactors are generally divided into two categories: the external‐loop airlift reactor (ELALR) composed of two conduits connected at the top and the bottom, and the internal‐loop airlift reactor (ILALR) constructed with two concentric cylinders 4. A typical airlift reactor involves four distinct sections: riser, downcomer, base (bottom section), and gas separator (top section) 1, 5. The liquid circulation in the reactor is driven by the gas holdup difference between the riser and the downcomer 2.…”

“…The former consists of a set of partial differential equations with boundary conditions, while the latter is composed of a set of first‐order differential equations 18. In some studies 5, 7, 18–20, the ADM or the TSM is also combined with the continuously stirred‐tank reactor (CSTR) model to describe mass transfer processes in different sections of airlift reactors. Because of the extreme complexity of flow patterns in the reactor, simplification has to be made to establish these mass transfer models, such as the assumption of plug flow and perfect mixing 13, 14.…”

Bioreactors: Airlift Reactors

“…While in a short equipment the oxygen concentration may change very little, and many times can be considered as constant, this is not so in taller columns, especially at low J G , when the gas stream depletion is greater. Distributions of gas composition have been modeled and presented and can be of use to the designer (247)(248)(249).…”