Using Electrical Resistance Tomography Images to Analyze the Batch and Continuous-Flow Mixing of Pseudoplastic Fluids Possessing Yield Stress in the Agitated Tanks
Non-ideal flows, such as dead zones and channeling, are commonly occurred in the mixing of nonNewtonian fluids possessing yield stress. It is a challenging task to visualize such phenomena occurring inside the mixing vessels due to the opacity of the fluids. Numerous conventional techniques have been used to study the mixing operations in both batch and continuous modes; however, these techniques have their own limitations as they are not suitable for opaque fluids or they might interfere with the local flow pattern. Therefore, the core objective of this study was to employ electrical resistance tomography (ERT), which is an efficient and nonintrusive flow visualization technique for both opaque and transparent fluids, to study the batch and continuousflow mixing of non-Newtonian fluids possessing yield stress in a stirred vessel. In this study the ERT technique was employed to: (i) measure the mixing time in the batch mode, (ii) measure the cavern size in the batch mode, (iii) analyze the cavern deformation in the continuous mode, and (iv) visualize the flow pattern generated by the impeller inside the stirred tank in the continuous mode. The 2D and 3D tomography images were used to identify the existence of the non-ideal flows encountered inside the mixing vessel. The ERT data were also used to validate the CFD model. Applying the outcomes of this research can enhance the design criteria of the mixing of nonNewtonian fluids in both batch and continuous modes.
Tomography images to analyze the deformation of the cavern in the continuous‐flow mixing of non‐Newtonian fluids
Tomography, an efficient nonintrusive technique, was employed to visualize the flow in continuous-flow mixing and to measure the cavern volume (Vc) in batch mixing. This study has demonstrated an efficient method for flow visualization in the continuous-flow mixing of opaque fluids using two-dimensional (2-D) and 3-D tomograms. The main objective of this study was to explore the effects of four inlet-outlet configurations, fluid rheology (0.5–1.5% xanthan gum concentration), high-velocity jet (0.317–1.660 m s−1), and feed flow rate (5.3 × 10−5−2.36 × 10−4 m3 s−1) on the deformation of the cavern. Dynamic tests were also performed to estimate the fully mixed volume (Vfully mixed) for the RT, A310, and 3AM impellers in a continuous-flow mixing system, and it was found that Vfully mixed was greater than Vc. Incorporating the findings of this study into the design criteria will minimize the extent of nonideal flows in the continuous-flow mixing of complex fluids and eventually improve the quality of end-products. © 2013 American Institute of Chemical Engineers AIChE J, 60: 315–331, 2014
Effects of Equipment Dimensions on Liquid Mixing Time of Bubble Column with Draft Tube
The liquid mixing time of a bubble column with a draft tube is experimentally studied. The diameter and height of the column, the diameter and length of the draft tube, and the mode of sparging gas were changed and the liquid mixing time was measured.The lengths of bottom and top clearances have significant effects on the liquid mixing time. The liquid mixing time increases with increasing column aspect ratio of height to diameter. The liquid mixing time of the bubble column with a draft tube is longer than that of a standard bubble column when the column aspect ratio is relatively high. The liquid mixing time has a minimum when the diameter ratio of draft tube to column is 0.5 to 0.6. These effects of equipment dimensions are expressed by an equation. In the experiment using different solutions, the liquid mixing time becomes shorter with increasing liquid surface tension, or with increasing liquid viscosity.