## 4 Electrical Tomography Applications
### Applications of ET in the Pulp Suspensions Domain
[126] investigated digesters (digesters for chemical pulping are pressurised reactors that create a flow of process liquors relative to wood chips to delignify the wood and produce kraft  
Figure 3: Softwood pulp suspension electrical tomography for Up = 0.5 m/s (main stream velocity), Cm = 2.0% and Dr = 0.05 (the injection tube length-to-diameter ratio), using sequential sensing planes (the location of the planes P1–P8 is from left to right in the flow direction, with P1 located before the injection point and the impeller), with (**a**) R = 12.2 (velocity ratio between main stream velocity and jet velocity) and N = 403 rpm (rotation speed); (**b**) R = 16.5 and N = 410 rpm. In all the images the injected tracer is represented by the high-conductivity region in red and yellow, whereas the mainstream is represented by the low conductivity blue region. (Reproduced with permission from [PERSON] et al. [54]). -pulp) and observed that as their size increased to accommodate higher production rates, operating problems related to liquor flow increased (as defined by the authors, liquor flow is the relative flow of liquor through the chip mass). Among the different problems, they found decreased counter current washing efficiency, accelerated corrosion, increased pulp non-uniformity and problems with chip movement. In a column arranged to operate as the wash section of a continuous digester, they used ERT to obtain images of the flow properties. For the tests carried, the packed bed was modelled using HDPE pellets of high-density polyethylene, having diameters of around 4.44 mm and thickness of around 1.91 mm. Regarding the liquid phase, a split between liquor flows was made to obtain ratios of axial (up flow in all cases), \(Q_{A}\), to radial flow, \(Q_{R}\), of 0.5, 1.0 and 2.0. The tests with wood chips showed that liquid flow adjacent to the vessel wall was significantly greater than at the centre of the vessel. This difference was not as significant for the HDPE disks for which the wall-affected zone is much smaller and below the spatial resolution of the ERT system. When a tracer was isokinetically added, ERT reconstructed images below the down corner showed that the entire trace leaved the down corner when \(Q_{R}\) was twice the value of \(Q_{A}\); nevertheless, for a 1:1 flow ratio, some tracer was detected above the screen level. Figure 4 presents some images of the measurements conducted. [PERSON] et al. [56] used Electrical resistance tomography (ERT) to evaluate the uniformity of liquor flow through a porous media in a laboratory model digester under scaled industrial conditions: a 1:15 geometrically scaled vessel, a vessel to particle diameter ratio of 93:1 to minimise wall effects, and close approximation of liquor superficial velocity and pore Reynolds number (pore Reynolds number is based on the capillary model of flow through packed beds and accounts for the porosity, \(\varepsilon\), tortuosity, \(\tau\), and specific surface area presented to the flow). Their motivation departed from the literature, where the extent of delignification in batch digesters varied as a function of chip location in the vessel. This non-uniformity may be exacerbated by several factors but is commonly attributed to poor and/or non-uniform liquor flow through the reactor. Local interstitial flow velocities were measured using pixel-pixel cross correlation techniques. Tests were conducted to cover the range of typical liquor splits, \(Q_{T}\)/\(Q_{B}\) (the liquor volumetric flow ratio between the top and bottom of the digester) found in industrial units: 0.5, 1.0, and 2.0. The results showed that creation of uniform zones was readily achieved, although a certain flow rate was needed in the upper section to establish uniformity due to asymmetric liquor addition to the vessel. A mixing region was observed at the screen elevation in the outflow zone from the digester. The location of this interaction zone moved, depending on the relative liquor flux to the top and bottom of the vessel.