CFD simulation and experimental measurement of gas holdup and liquid interstitial velocity in internal loop airlift reactor M. ˇ Simc ˇı ´k a,n , A. Mota b , M.C. Ruzicka a , A. Vicente b , J. Teixeira b a Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Rozvojova 135, 16502 Prague 6, Czech Republic b Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal article info Article history: Received 30 September 2010 Received in revised form 28 January 2011 Accepted 31 January 2011 Available online 5 February 2011 Keywords: Airlift CFD Hydrodynamics Multiphase flow Multiphase reactors Simulation abstract This paper documents experiments and CFD simulations of the hydrodynamics of our two-phase (water, air) laboratory internal loop airlift reactor (40 l). The experiments and simulations were aimed at obtaining global flow characteristics (gas holdup and liquid interstitial velocity in the riser and in the downcomer) in our particular airlift configurations. The experiments and simulations were done for three different riser tubes with variable length and diameter. Gas (air) superficial velocities in riser were in range from 1 to 7.5 cm/s. Up to three circulation regimes were experimentally observed (no bubbles in downcomer, bubbles in downcomer but not circulating, and finally the circulating regime). The primary goal was to test our CFD simulation setup using only standard closures for interphase forces and turbulence, and assuming constant bubble size is able to capture global characteristics of the flow for our experimental airlift configurations for the three circulation regimes, and if the simulation setup could be later used for obtaining the global characteristic for modified geometries of our original airlift design or for different fluids. The CFD simulations were done in commercial code Fluent 6.3 using algebraic slip mixture multiphase model. The secondary goal was to test the sensitivity of the simulation results to different closures for the drag coefficient and the resulting bubble slip velocity and also for the turbulence. In addition to the simulations done in Fluent, simulation results using different code (CFX 12.1) and different model (full Euler–Euler) are also presented in this paper. The experimental measurements of liquid interstitial velocity in the riser and in the downcomer were done by evaluating the response to the injection of a sulphuric acid solution measured with pH probes. The gas holdup in the riser and downcomer was measured with the U-tube manometer. The results showed that the simulation setup works quite well when there are no bubbles present in the downcomer, and that the sensitivity to the drag closure is rather low in this case. The agreement was getting worse with the increase of gas holdup in the downcomer. The use of different multiphase model in the different code (CFX) gave almost the same results as the Fluent simulations. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Airlift reactors are pneumatically agitated vessels, and are one among different types of multiphase reactors. They possess good mixing, mass and heat transfer characteristics and they are used in a wide range of industrial applications such as waste water treatment, chemical (e.g. hydrogenations and oxidations) and biochemical processes, and others. The other advantages are simplicity of construction, absence of moving parts, and low power consumption. Their other advantageous features in case of biochemical processes are ease of long term sterile operation, and a hydrodynamic environment suitable for fragile biocatalysts, which are susceptible to physical damage by fluid turbulence or mechanical agitation (Chisti, 1998). The airlift reactor consists of two interconnected main parts, the riser and the downcomer. Gas is injected into the riser and the resulting difference between average densities in the riser and in the downcomer provides a driving force for liquid circulation. Also solid particles can be present (catalyst, biomass, etc.). There are two main groups of airlift reactors namely, the internal loop airlift reactor and the external loop airlift reactor. The internal loop airlift reactor is a bubble column divided into the two parts by a draft tube inserted into the column. The external loop airlift consists of two separate columns connected with pipes. Other important part, which may or may not be present, is the gas separator. Its purpose is to prevent bubbles from being entrained into the downcomer, which would decrease the driving force for liquid circulation. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.01.059 n Corresponding author. E-mail address: simcik@icpf.cas.cz (M. ˇ Simc ˇı ´k). Chemical Engineering Science 66 (2011) 3268–3279