Abstract

This doctoral thesis addresses the mixing of highly viscous Newtonian fluids (corn syrup solutions) in a novel aerated reactor equipped with a central impeller (a pitched blade turbine in upward or downward pumping mode) and a wall scraping anchor. The non-intrusive electrical resistance tomography (ERT), dynamic gas disengagement method (DGD), design of experiments (DOE), computational fluid dynamics (CFD), and population balance model (PBM) were employed to characterize the performance of this novel aerated system. The performance criteria to be examined were mixing time, power uptake, gas holdup, and bubble size distribution. In this study, novel correlations were developed to estimate the gassed power drawn by the coaxial mixer, mixing time, and gas holdup. In addition, to obtain a master power curve, two new dimensionless correlations were proposed for the generalized power number and gas flow number by incorporating the equivalent rotational speed for the coaxial mixer, speed ratio (central impeller speed/anchor speed), and the central impeller power fraction into these two correlations. The experimental data demonstrated that gas flow affected the aerated anchor power consumption and central impeller power consumption in different manners. It was also found that at the higher fluid viscosity and beyond the critical speed ratio of 10, the anchor power consumption was increased by increasing the speed ratio (i.e. decreasing the anchor speed). It was shown that in the presence of gas, the anchor impeller in combination with the upward pumping pitched blade turbine in the co-rotating mode exhibited shorter mixing times and lower power consumption than the anchor-downward pumping pitched blade coaxial mixer. To enhance the efficiency of the aerated mixer, it is critical to investigate the influence of the gas-liquid flow within the vessel on the bubble size distribution (BSD) and the local and global gas holdup. To achieve this goal, the effects of the bubble breakup and coalescence on the BSD within the vessel were incorporated into the CFD model through the CFD-PBM coupling. The experimental and simulation results showed that beyond the critical speed ratio of 10, the volume fractions of the large bubbles decreased while the volume fractions of the small bubbles increased.

Highlights

  • Stirred vessels are indispensable parts of variety of process industries such as chemical and biochemical processes, pharmaceutical, food industries, waste water treatment, and so on for single phase or multiphase flow agitation (Labik et al, 2014; Abdullah et al, 2011; Khopkar and Tanguy, 2008; Khopkar et al, 2007; Murthy et al 2007)

  • Analysis of mixing in an aerated reactor equipped with the coaxial mixer through electrical resistance tomography and response surface method Mixing characteristics of an aerated coaxial mixer composed of an anchor and a central impeller was investigated using the non-invasive flow visualization technique called electrical resistance tomography (ERT)

  • The aerated coaxial mixing vessel consisted of two independent impellers, which can turn at different speeds

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Summary

Introduction

Stirred vessels are indispensable parts of variety of process industries such as chemical and biochemical processes, pharmaceutical, food industries, waste water treatment, and so on for single phase or multiphase flow agitation (Labik et al, 2014; Abdullah et al, 2011; Khopkar and Tanguy, 2008; Khopkar et al, 2007; Murthy et al 2007). All the gas-liquid flow conditions such as flooding, loading, and complete dispersion should be considered to study the impact of gassing on the mixing time and power consumption of the aerated coaxial mixers. The main challenge in the aerobic fermentation technologies is supplying sufficient oxygen (air) into the fluid which is essential for the growth of the microbial cells This difficulty is raised in high cell density bioreactors, at which mass transfer rate between the gas and liquid phases is a limiting step affecting cell growth and productivity. In a number of published works, a constant bubble size was assumed in modelling of aerated mixers due to the simplicity, lack of experimental data, and lesser computational time To tackle this problem a population balance model coupled with CFD is mainly utilized

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