Abstract

The major technological challenges faced by modern chemical industries are non-ideal flows such as dead zones and channeling encountered in the mixing of fluids with complex rheology. These cause sub-optimal mixing and lead to low quality products and high costs of raw materials. Therefore, the core objectives of this study were to develop methodology and tools to design an efficient continuous-flow mixing system for the fluids with complex rheology using electrical resistance tomography (ERT), computational fluid dynamics (CFD), and dynamic tests. The xanthan gum solution, which is a pesudoplastic fluid with yield stress, was used to study the dynamic behavior of the continuous-flow mixing process. The power consumption, cavern size, mixing time, and the extents of channelling and the fraction of fully mixed volume were successfully determined using dynamic tests, ERT tests, and CFD simulations and used as mixing quality criteria. A novel and efficient method was developed for flow visualization in the continuous-flow mixing of opaque fluids using 2D and 3D tomograms. A unique study on identifying the sources of flow non-ideality in non-Newtonian fluids with yield stress was done by visualizing the flow pattern inside the continuous-flow mixing vessel using 2D and 3D tomograms. The deformation of the cavern was analyzed and quantified in the continuous-flow mixing system for yield-pseudoplastic fluids using ERT. Moreover, the cavern volume was compared with the fully mixed volume and it was found that the latter was higher due to the extra momentum induced by the inlet-outlet flow. A novel study on exploring the effect of the rheological parameters of the pseudoplastic fluids with yield stress on the non ideal flows in a continuous-flow mixing system was performed using CFD. The CFD results revealed that the mixing quality was improved when the degree of the shear thinning was increased. The ratio of the residence time to the batch mixing time was evaluated to achieve ideal mixing for the continuous-flow mixing of yield-pseudoplastic fluids using dynamic tests and ERT. It was found that the ratio of residence time to the batch mixing time should be at least 8.2 or higher to achieve ideal mixing.

Highlights

  • The key objective of any mixing process is to maximize the degree of homogeneity of a property such as concentration, viscosity, color, and temperature

  • The results showed that for the same operating conditions, as the jet velocity (Vj) was increased from 0.317 to 1.66 m s-1, the mixing quality was improved for both configurations

  • Further increase in the jet velocity from 1.66 to 3.24 m s-1 had an adverse effect on the mixing quality for the top inlet and bottom outlet (TI–bottom outlet (BO)) configuration, this potential problem was overcome by relocating the outlet using top inlet (TI)–top outlet (TO) configuration

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Summary

Introduction

The key objective of any mixing process is to maximize the degree of homogeneity of a property such as concentration, viscosity, color, and temperature. The key objective of this study was to employ the ERT technique in order to explore the effects of the inlet and outlet locations (four configurations: top inlet-bottom outlet, bottom inlet-top outlet, bottom inlet-bottom outlet, and top inlet-top outlet), fluid rheology (0.5–1.5% xanthan gum concentration), jet velocity (0.317–1.660 m s-1), feed flow rate (3.20–14.17 L min-1), impeller type (the Rushton turbine and Maxblend impellers), and impeller speed (54–250 rpm) on the flow patterns generated in the continuousflow mixing of the xanthan gum solution, which is a pseudoplastic fluid exhibiting yield stress. The core objective of this research was to investigate the effects of K (3–33 Pa sn), n (0.11–0.99), τy (1.7–20.6 Pa), xanthan gum concentration (0.5–1.5%), and feed flow rate (Q) (03.20–14.17 L min-1) on the extent of channeling (f) and fully mixed volume (Vfully mixed) in the continuous stirred-tank reactor using computational fluid dynamics (CFD).

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