Numerical investigation of gas-driven flow in 2-D bubble columns

Homogeneous and re-circulatory gas–liquid flow in a bubble column: A numerical investigation using OpenFOAM

This work presents the influence of the sparger opening area, gas velocity, and bubble size on hydrodynamics and transition of the flow regime from uniform to re-circulatory in a rectangular bubble column using OpenFOAM. In the course of development of the model, the effect of several drag closures and lift on the predictability of the CFD model was studied by comparing the predictions with published experimental results. Reynolds number-based drag closure was found to be suitable for uniform sparger whereas Tsuchiya drag (Tsuchiya et al. in Chem Eng Sci 52:3053–3066, 1997. https://doi.org/10.1016/S0009-2509(97)00127-9) was used to simulate gas–liquid flow for other spargers. Simulations were performed for seven different spargers with opening area 18–100% (superficial gas velocity of 2.9–5.8 cm/s) and bubble size of 2–8 mm. The smaller opening area and higher gas velocity promote the re-circulatory flow in the bubble column. Change in bubble size affects the hydrodynamics due to change in lift and drag forces.

CFD Simulation of flow pattern in a bubble column reactor for forming aerobic granules and its development

The flow pattern is considered to play an important role in the formation of aerobic granular sludge in a bubble column reactor; therefore, it is necessary to understand the behavior of the flow in the reactor. A three-dimensional computational fluid dynamics (CFD) simulation for bubble column reactor was established to visualize the flow patterns of two-phase air-liquid flow and three-phase air-liquid-sludge flow under different ratios of height to diameter (H/D ratio) and superficial gas upflow velocities (SGVs). Moreover, a simulation of the three-phase flow pattern at the same SGV and different characteristics of the sludge was performed in this study. The results show that not only SGV but also properties of sludge involve the transformation of flow behaviors and relative velocity between liquid and sludge. For the original activated sludge floc to cultivate aerobic granules, the flow pattern has nothing to do with sludge, but is influenced by SGV, and the vortices is occurred and the relative velocity is increased with an increase in SGV; the two-phase flow can simplify the three-phase flow that predicts the flow pattern development in bubble column reactor (BCR) for aerobic granulation. For the aerobic granules, the liquid flow behavior developed from the symmetrical circular flow to numbers and small-size vortices with an increase in the sludge diameter, the relative velocity is amount up to u_r = 5.0, it is 29.4 times of original floc sludge.

A PBM-CFD Model with Optimized PBM-Customized Drag Equations for Chemisorption of CO2 in a Bubble Column

In this work, a computational fluid dynamics (CFD) approach based on two-fluid model (TFM) is introduced to describe the reversible two-step reactions found in the chemisorption process of ${\text{C}}{{\text{O}}_2}$ by an aqueous ${\text{NaOH}}$ solution in a lab-scale bubble column reactor. The population balance model (PBM) is applied to track the bubble size distribution with considering the coalescence and breakage terms, which then leads to a CFD-PBM model for describing the chemisorption of CO2 in an aqueous ${\text{NaOH}}$ solution. Drag force is considered for the interfacial momentum transfer and a modified PBM-customized drag model with the correction factor is subsequently adopted, in which the contribution of different bubble size groups in each computational cell is computed. The tested boundary conditions include superficial gas velocities, gas-inlet sparger and the reactor dimension. Detailed and comprehensive investigations are done in the evolution of gas holdup, pH value, concentration distribution and bubble diameter distribution which are essential in optimizing the reactor performance in terms of yield and selectivity. Importantly, the current CFD-PBM model is able to predict the entire reaction process.

Effect of Scale on Hydrodynamics of Internal Gas-Lift Loop Reactor-Type Anaerobic Digester Using CFD

This work evaluates the ability of computational fluid dynamics (CFD) to simulate the flow and predict the hydrodynamics of internal gas-lift loop reactor (IGLR)-type anaerobic digester. In addition, it also analyzes if CFD can account for the effects of operating conditions, geometry as well as scale of the reactor. For this purpose, three-dimensional two-phase CFD simulations were performed using CFX for laboratory-scale and pilot-scale IGLR. The CFD predictions were evaluated against experimental data obtained from computer automated radioactive particle tracking (CARPT). The CFD predictions provided good qualitative but only reasonable quantitative comparison. After validation of CFD model, effect of gas flow rate, draft tube diameter, sparger geometry and reactor scale on flow pattern, liquid velocity and dead volume was investigated. Higher gas flow rates did not offer any significant advantage in increasing liquid circulation in the downcomer or decreasing the dead volume. Configuration with draft tube diameter half of tank diameter, equipped with cross sparger showed comparatively better liquid circulation than other configurations. For same superficial gas velocity, increasing the scale increases the magnitude of liquid velocity but fails to match the mixing intensity observed in laboratory scale. Different interphase forces, turbulence models and closures are also evaluated to improve the predictability of CFD models.