Computers and Chemical Engineering 28 (2004) 2765–2777 Modeling and scale up of airlift bioreactor H. Znad a,∗ , V. B´ aleˇ s a , Y. Kawase b a Department of Chemical and Biochemical Engineering, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinsk´ eho 9, 81237 Bratislava, Slovakia b Department of Applied Chemistry, Toyo University, Japan Received 10 March 2004; received in revised form 20 August 2004; accepted 20 August 2004 Available online 6 October 2004 Abstract A previously presented mathematical model based on a tanks-in-series model with back flow for an airlift bioreactor is extended by considering the variations of the oxygen in the gas phase and the hydrostatic pressure along the bioreactor. The kinetic model used considers the effect of two substrates (glucose and dissolved oxygen) on the growth rate. A set of first order differential equations for the material balances of the micro-organism, glucose, product, dissolved oxygen, and oxygen in the gas phase around the hypothetical well mixed stages in the riser and the downcomer were solved simultaneously using Athena software package. The model has been validated with experimental data of gluconic acid fermentation in two different scales of internal loop airlift bioreactor, 10.5 dm 3 and 35 dm 3 . The scale up effects on the performance, kinetic parameters and model predictions of gluconic acid fermentation in airlift bioreactors were studied. The model is simple enough to be used in design and scale up studies and it can be adapted to other airlift system configurations and fermentation systems other than gluconic acid fermentation. © 2004 Elsevier Ltd. All rights reserved. Keywords: Airlift bioreactor; Gluconic acid; Dissolved oxygen; Modeling; Bioreactors 1. Introduction Many fermentor designs have been proposed for increas- ing the oxygen transfer rate and for minimizing the power consumption, one of the most promising was found to be the airlift bioreactor which was first patented by (Lefrancois, Mariller, & Mejane, 1955). Airlift bioreactors are comprised of four distinct zones, each with its own distinct flow pat- tern. The first zone, in which the gas is sparged, is denoted the riser, as the gas–liquid dispersion travels upward in co- current. This section has the higher fractional gas hold up and where most of the gas–liquid mass transfer takes place. The liquid leaving the top of the riser enters a gas disengagement zone, the gas–liquid separator, where, depending on its spe- cific design, some or most of the dispersed gas is removed. The gas free liquid (or a dispersion of lesser gas hold up) then ∗ Corresponding author. Tel.: +421 2 59325267; fax: +421 2 52496743. E-mail address: znad71@yahoo.com (H. Znad). flows into the downcomer and travels to the base of the de- vice, through the bottom, where it re-enters the riser. Thus, the liquid phase circulate continuously around the loop. Airlift bioreactor commonly used in aerobic fermentation processes because of their simple structure, low shear stress, and easy of maintenance in comparison to the conventional stirred tank bioreactor. An accurate description of the performance of airlift biore- actors is still difficult (Merchuk, 1993; Onken & Weiland, 1983). Mixing in airlift bioreactors is usually imperfect and mathematical models for airlift bioreactors cannot be de- scribed by neither the perfect mixing (continuous stirred tank reactors: CSTR) nor the plug flow (plug flow reactors: PFR) (Luttman, Thoma, Buchholz, & Schugerl, 1983a,b). Little work has been done on the mathematical modeling of real fermentation systems involving imperfect mixing and their optimal design in airlift bioreactors. The mixing mod- els used in most of the previous investigations dealing with airlift bioreactors are an axial dispersion model (ADM) and 0098-1354/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compchemeng.2004.08.024