Using tomography to visualize the continuous-flow mixing of biopolymer solutions inside a stirred tank reactor

• Continuous-flow mixing of the biopolymer solutions was studied using 2D and 3D tomograms.
• Effects of six various operating conditions and design parameters were investigated.
• An efficient flow visualization method for the opaque fluids was demonstrated.
• Non-ideal flows such as channeling and dead zones inside the reactor were identified.
• The rheology of the biopolymer solution had a significant effect on the mixing quality.

The vast majority of non-Newtonian fluids are naturally opaque; therefore, visualizing the flow field of such fluids inside a reactor is a challenging task. To identify non-ideal flows, such as dead volumes and channeling, in a continuous-flow mixing system, the flow field inside a stirred tank reactor was visualized using electrical resistance tomography (ERT), an efficient non-intrusive measurement technique. 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 (5.3 × 10−5–2.36 × 10−4 m3 s−1), impeller type (the Rushton turbine and Maxblend impellers), and impeller speed (54–250 rpm) on the flow patterns generated in the continuous-flow mixing of the xanthan gum solution, which is a pseudoplastic fluid exhibiting yield stress. Using 2D and 3D tomography images, this article effectively presents a competent method to visualize the flow of opaque fluids in laminar and transitional regions inside a reactor. In this study, the existence of non-ideal flows in a stirred tank reactor for opaque fluids was identified using ERT. The quantitative results showed that the tracer distribution (or mixing quality) inside stirred vessel was enhanced by decreasing the fluid yield stress, increasing the impeller speed, increasing the jet velocity, and using the close clearance impeller. The results also showed that the location of the inlet and outlet streams has a significant effect on the mixing quality. To improve the design of the continuous-flow mixing of non-Newtonian fluids, the findings of this study can be integrated into the design criteria to achieve optimal mixing results.