Hydrodynamic behavior of an airlift reactor with net draft tube with different configurations: Numerical evaluation using CFD technique--颗粒学报PARTICUOLOGY

Salehpour, R., Jalilnejad, E., Nalband, M., & Ghasemzadeh, K. (2020). Hydrodynamic behavior of an airlift reactor with net draft tube with different configurations: Numerical evaluation using CFD technique. Particuology, 51, 91-108. https://doi.org/10.1016/j.partic.2019.09.005

Hydrodynamic behavior of an airlift reactor with net draft tube with different configurations: Numerical evaluation using CFD technique

Reza Salehpour, Elham Jalilnejad ^*, Mehran Nalband, Kamran Ghasemzadeh

Faculty of Chemical Engineering, Urmia University of Technology, Urmia, Iran

10.1016/j.partic.2019.09.005

Volume 51, August 2020, Pages 91-108

Received 26 May 2019, Revised 22 September 2019, Accepted 23 September 2019, Available online 4 December 2019, Version of Record 11 April 2020.

E-mail: e.jalilnejad@uut.ac.ir

Highlights

• Two-fluid CFD model is used for simulation of airlift reactor with net draft tube.

• The CFD model properly accounts for geometric effects.

• Gas holdup, liquid velocity and mass transfer coefficient are evaluated in simulations.

• CFD approach leads to more accurate and reasonable results than empirical modelling.

• Complex dynamic behavior of ALR-NDTs is evaluated numerically.

Abstract

In terms of gas holdup, liquid velocity, and volumetric mass transfer coefficient for oxygen (K_La), the hydrodynamic behavior of four configurations of an airlift reactor (ALR) with a net draft tube (NDT) of different net mesh sizes (ALR-NDT-3, 6, 12, and ALR) have been numerically simulated for a range of inlet air flow rates. The effect of various levels of ratio of height (H) to inner tube diameter (D) of the net draft tube (H/D: 9.3, 10.7, 17.5, and 20) and ratio of inner cross-sectional area of the riser (A_r) to the inner cross sectional area of the downcomer (A_d) (A_d/A_r: 1.3 and 7) for different air flow rates is also evaluated for each reactor configuration operating with an air–water system. The two-fluid formulation coupled with the k–ε turbulence model is used for computational fluid dynamics (CFD) analysis of flow with Eulerian descriptions for the gas and liquid phases. Interactions between air bubbles and liquid are taken into account using momentum exchange and drag coefficient based on two different correlations. Trends in the predicted dynamical behavior are similar to those found experimentally. A good agreement was achieved suggesting that geometric effects are properly accounted for by the CFD model. After a comparison with experimental data, numerical simulations show significant enhanced gas holdup, liquid velocity, and K_La for the ALR-NDTs compared with the conventional ALR. Higher gas holdup values are achieved for ALR-NDT-3 than that for the other ALRs because it acts like a bubble column reactor as the holes present in the NDT are large. Maximum liquid velocities are seen in ALR-NDT-12, which operates like a conventional ALR. Moreover, the interaction between the NDT and upward gas flow leads to cross flow through the net, small bubbles, and high interfacial area as well as good mass transfer. This was significant in ALR-NDT-6 with maximum K_La value of 0.031 s⁻¹. The applied methodology provides an insightful understanding of the complex dynamic behavior of ALR-NDTs and may be helpful in optimizing the design and scale-up of reactors.

Graphical abstract

Keywords

Airlift reactor; Net draft tube; Hydrodynamic behavior; Computational fluid dynamics (CFD); Gas holdup; Mass transfer coefficient

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