RESEARCH ARTICLE CFD simulation of the hydrodynamics in an internal air-lift reactor with two different configurations Mona EBRAHIMIFAKHAR 1 , Elmira MOHSENZADEH 1 , Sadegh MORADI (✉) 1 , Mostafa MORAVEJI 1 , Mahmoud SALIMI 2 1 Department of Chemical Engineering, Faculty of Engineering, Arak University, Arak 38156-8-8849, Iran 2 Department of Chemical Engineering, Islamic Azad University, Arak Branch, Arak, Iran © Higher Education Press and Springer-Verlag Berlin Heidelberg 2011 Abstract Computational fluid dynamics (CFD) was used to investigate the hydrodynamic parameters of two internal airlift bioreactors with different configurations. Both had a riser diameter of 0.1 m. The model was used to predict the effect of the reactor geometry on the reactor hydrody- namics. Water was utilized as the continuous phase and air in the form of bubbles was applied as the dispersed phase. A two-phase flow model provided by the bubbly flow application mode was employed in this project. In the liquid phase, the turbulence can be described using the k-ε model. Simulated gas holdup and liquid circulation velocity results were compared with experimental data. The predictions of the simulation are in good agreement with the experimental data. Keywords airlift reactor, gas holdup, liquid circulation velocity, bubbly flow, computational fluid dynamics (CFD) 1 Introduction Airlift reactors (ALRs) are widely used in various industrial applications such as chemical and petrochemical industries, biochemical fermentation and biological wastewater treat- ment processes [1–3]. These reactors are one of the most important types of modified bubble columns (BCs) [4] and are divided into two parts by an internal baffle or a draft tube. A gas is injected into one of these parts and the gas– liquid dispersion moves upwards. This part is called the riser, and the other part is called the downcomer. Due to buoyancy, the bubbles rise, inducing a circulating motion in the liquid. Furthermore, as the bubbles rise through the water, the gas dissolves in the liquid. Airlift reactors are known to be efficient contacting devices for processes involving gases, liquids and solids and their advantages can be summarized as follows: a relatively simple mechanical structure without internal or moving parts, low shear stress, excellent heat and mass- transfer capacity and good mixing characteristics with low energy consumption [5]. Although ALRs are thought to be an improvement over BCs, they still have limitations. First, the ALR, like the BC, is not suitable for highly viscous fluids. Second, all ALRs have a minimum volume that must be maintained to ensure consistent fluid circulation within the reactor. Third, ALR design is usually reactor and application specific, limiting individual ALR usefulness to processes with minimal changes in the operating parameters because after the initial geometric parameters are set at design time, the gas velocity is the only remaining adjustable parameter [6]. Basically, there are two types of ALRs: internal loop reactors and external loop reactors. The internal loop airlift reactor (IL-ALR) is a type of airlift bioreactor that consists of concentric cylinders or split vessels [7]. External loop airlift reactors (EL-ALR) include a riser and a downcomer column which are connected by horizontal connections at the top and bottom of the reactor. The static pressure difference or the net density difference of the fluid between the riser and the downcomer is the driving force that circulates the liquid [8]. The two key hydrodynamic parameters of airlift reactors are the gas holdup and liquid circulation velocity. These factors are affected by: the geometry of the reactor, the riser to downcomer cross-sectional area ratio, the liquid height and the operational conditions which include superficial gas velocity, liquid velocity, gas bubble size and the surface tension of the water. In the design of airlift reactors, the geometry of the system plays an important role in its efficiency for mixing and mass transfer [9].The effects of geometric parameters have been considered by many researchers. Koide et al. [10] analyzed the effects of the ratio of the internal Received April 26, 2011; accepted August 18, 2011 E-mail: s-morady@araku.ac.ir Front. Chem. Sci. Eng. 2011, 5(4): 455–462 DOI 10.1007/s11705-011-1116-x