## 4 Electrical Tomography Applications
### Applications of ET in the Pulp Suspensions Domain  
As already mentioned, due to low cost and ease of transport, and despite their low resolution in comparison with other tomographic techniques, ET has been used in pulp suspensions analysis. In the next paragraphs, a review of some of applications found in the literature is carried out. [PERSON] et al. [123] defined a gas mixing index derived from the standard deviation of cross-sectional local gas holdup obtained from electrical resistance tomography and used it to investigate the dispersion of gas into pulp-suspension horizontal flow downstream of 90\({}^{\circ}\) tees as affected by the fibre mass concentrations in the range 0-3.0% (w/w), superficial liquid/pulp velocities (0.5-5.0 m/s) and superficial gas velocities (0.11-0.44 m/s). The mixing in the turbulent flow of dilute pulp suspensions was similar to that for water but differed significantly at higher suspension concentrations. Mixing worsened with increasing fibre mass concentration for the bubble flow regime, which was likely due to a dense plug of fibres in the core of the pipe causing bubbles to congregate near the wall. When buoyancy was significant, gas uniformity improved with increasing pulp concentration, since robust fibre networks caused liquid/pulp slug slugs to flow at the top of the pipe, whereas stratified flow was approached at lower concentrations. Mixing was less dependent on superficial liquid/pulp velocity at higher pulp concentrations, due to there being less variation in flow regimes. Using the same modified mixing index and electrical resistance tomography, [PERSON] et al. [54] pursued their research and presented results about the quality of liquid mixing into the mainstream for an in-line mechanical mixer. The experiments were conducted in a pilot-scale flow loop facility, consisting of two 4000 L tanks, a 40 HP centrifugal pump and a 40 m length of 76.2 mm PVC pipe; the acrylic test section, 1.88 m in length, included eight ERT sensor planes spaced at 250 mm intervals. The first sensor plane was located 67 mm upstream of the side-stream injection, and the impeller was 104 mm downstream of the injection point. The mechanical mixer system consisted of a 0.5 HP motor, a 1.41 N-m rotating torque sensor with an integral optical encoder, a 12.7-mm-diameter stainless steel shaft, and a 63.5-mm-wide octagonal impeller with a 38.1-mm-diameter hole in the middle to prevent excessive force on the impeller when it was perpendicular to the flow. In the experiments were used: aqueous pulp suspensions with concentrations in the range 0-3.0% (w/w); jet velocities ranging from 3.8 to 12.6 m/s; main-stream velocities in between 0.5-3.0 m/s; rotational speeds of 0 to 800 rpm. Some examples of images obtained in this study are presented in Figure 3. It was observed for all evaluated pulp concentrations that the mixing quality diminished when the jet was far from the wall of the pipe. For higher impeller speeds, the effect of the water flow on the residence time of the mixing was significant. When using the impeller, the quality of the mixed pulp suspensions improved noticeably; it was found that the quality was analogous to the situation in which water flow approaches a turbulent regime, but with a much smaller main-stream velocity needed for the mixing when compared to the case where only the tee mixer is used. For higher pulp concentrations, however, the supplied energy to the process, even when the highest main-stream velocity and impeller speed were used, was not enough to equal the turbulence level attained in water. Improvements on the mixing quality were observed to occur in the high-shear zone around the impeller when the impeller speed increased; however, it was simultaneously observed that turbulence decayed rapidly downstream due also to reflocculation. ERT was used by [PERSON] et al. [124] to evaluate the performance of a pilot-scale medium-consistency mixer: Additionally, they compared the obtained results with those from temperature measurements.