Research article Modeling and CFD-PBE simulation of an airlift bioreactor for PHB production Poorya Mavaddat, 1 Seyyed Mohammad Mousavi, 1 * Ershad Amini, 2 Hamidreza Azargoshasb 1 and Seyed Abbas Shojaosadati 1 1 Biotechnology Group, Chemical Engineering Department, Tarbiat Modares University, PO box 14115-143, Tehran, Iran 2 School of Chemical Engineering, College of Engineering, University of Tehran, PO Box 11155-4563, Tehran, Iran Received 26 August 2013; Revised 26 November 2013; Accepted 17 December 2013 ABSTRACT: In the present attempt, a 3D simulation using a commercially available computational fluid dynamics package (FLUENT 6.3.26), and a metabolic model were used to investigate hydrodynamics and production of polyhydroxybutyrate (PHB) in an airlift bioreactor, performed by Tavares et al. (L. Z. Tavares, E. S. da Silva, J. G. C. Pradella. Biochem. Eng. J., 2004; 18, 21–31). An Eulerian approach was applied to model the gas–liquid interactions. In order to account for the combined effect of bubble breakup and coalescence in the bioreactor, a population balance model implemented in the software was used. Biosynthesis of PHB in the bioreactor was examined and maximum forward reaction rate values of thiolase, reductase, and synthase steps were determined as V 1 (thiolase) = 1.8 mM/min, V 1 (reductase) = 400 mM/min, and V 1 (synthase) = 380 mM/min to predict the experimental PHB production rate by Tavares et al. suitably. A simplified reaction was considered for PHB production. Moreover, gas holdup, liquid velocity vectors, shear stress, and volumetric oxygen transfer coefficient were investigated. Also, molar concentration profiles of PHB and glucose within the bioreactor were obtained. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd. KEYWORDS: modeling; computational fluid dynamics (CFD); polyhydroxybutyrate (PHB); airlift bioreactor INTRODUCTION Too long durability of plastics has caused several environmental problems. Using biodegradable polymers is a suitable solution for these problems. Polyhydro- xybutyrate (PHB) is a biodegradable biopolymer, with similar properties to thermoplastics. Microorganisms can accumulate PHB in response to insufficient supply of nutrients, such as nitrogen and mineral salts. PHB is accumulated as a storage material which provides a reserve of carbon and energy. [1] However, widespread use of PHB and its copolymers is limited mainly because of its relatively high cost and therefore, several studies have tackled the problem using different approaches. [2] There have been various studies using mathematical models, neural and metabolic networks to describe and improve the PHB production process. [2–12] But the topic can be approached on another way. Today computational fluid dynamics (CFD) has emerged as a powerful tool for simulation of flow behavior [13] and local hydrodynamics in bubble col- umns, membrane bioreactors, and airlift and stirred tank reactors. [14–18] This is an approach based on first principles in which the governing equations of conti- nuity, momentum, and energy for each phase are solved. An important advantage of the CFD approach is that column geometry and scale effects are automatically accounted. High cost of experiments, inaccessibility to all locations in the system and turbulent multiphase flow lead to using CFD simulation, which is a cost-effective tool. Therefore, CFD can shed light on investigation and prediction of various characteristics. [15] However, success of the CFD simulation strategy is crucially dependent on the proper modeling of the momentum exchange, or drag coefficient between gas and liquid phases. [19] There has been a remarkable prog- ress in CFD modeling of dispersed gas–liquid two-phase flows over the last decade. The Euler–Euler (E–E) model and Euler–Lagrange (E–L) model are widely used to investigate hydrodynamics of bubble columns and airlift reactors. [20] An airlift reactor is a bubble column with a draft tube to circulate the fluid in the reactor. Airlift reactors are widely used in chemical, petrochemical, and biochemical industries due to their beneficial advantageous such as mechanical simplicity, good mixing, low shear rate, high capacity handling, and low operation cost, [21] and they seem as a suitable choice for mass production of PHB. *Correspondence to: Seyyed Mohammad Mousavi, Tarbiat Modares University, PO box 14115-143, Tehran, Iran. E-mail: mousavi_m@modares.ac.ir © 2014 Curtin University of Technology and John Wiley & Sons, Ltd. Curtin University is a trademark of Curtin University of Technology ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING Asia-Pac. J. Chem. Eng. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/apj.1785