## 4 Electrical Tomography Applications
### Pulp Suspensions Characterisation Using ET: The Authors' Approach
#### 4.3.2 Overview of the Pulp Suspension Flow Imaging  
For the tests depicted in this section, industrial pulp suspensions of _Pine_ and _Eucalyptus_ fibres, both collected in a local paper mill, were firstly squeezed and washed in water, and the fibres were then resuspended in tap water doped with NaCl with adjusted electrical conductivity. The behaviours of _Eucalyptus_ pulp suspensions (average Fibre length of 0.706 mm) and of _Pine_ pulp suspensions (average Fibre length of 2.18 mm) were compared, as in Figure 8, under the conditions listed in Table 1.  
Before analysing the obtained suspensions flow images, we must reinforce that the tomography's reflect the voltage differences measured. For fibre concentrations of around 2.35%, the fibre distributions in the cross-pipe section are diverse for the two types of fibres. Concerning the _Pine_ suspension, a plug type flow behaviour is already visible at the lowest velocity, with an increase in the plug (size and \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Pulp** & **Concentration** & **Flow Velocities (m/s)** & **Suspension Conductivity (mS-cm\({}^{-1}\))** \\ \hline _Eucalypt_ & 2.35 & 0.5, 1, 1.5, 2, 2.5, 3 & 1.460 \\  & 1.5 & 0.5, 1, 1.5, 2, 2.5, 3 & 1.150 \\  & 1 & 0.5, 1, 1.5, 2, 2.5, 3 & 1.128 \\ _Pine_ & 2.35 & 0.5, 1, 1.5, 2, 2.5, 3 & 1.471 \\  & 1.5 & 0.5, 1, 1.5, 2, 2.5, 3 & 1.461 \\  & 1 & 0.5, 1, 1.5, 2, 2.5, 3 & 1.435 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Table of concentrations and flow velocities tested. Figure 7: Image of one of the 32 Titanium electrodes test rings. intensity of the blue colour) when the velocity increases to 2 m/s (see Figure 8a,c,e). However, a further increase in the velocity up to 3 m/s leads to breakage of the plug, as in Figure 8g), as was expected for this type of long and stiff fibre. When analysing the _Eucalyptus_ fibre suspension behaviour, at the lowest velocity the fibres remain more or less dispersed in the cross-pipe section (see Figure 8b), and the plug only starts to be visible at the velocity of 1 m/s, becoming denser at 2 m/s (Figure 8d,f). Similarly to the _Pine_ fibres, a further increase in the _Eucalyptus_ suspension velocity up to 3 m/s leads to plug breakage (Figure 8h). These differences in the reconstructed EIT images for this concentration are related to the physical properties of the fibres, in particular the fibre length and stiffness of the two pulps that originate different interactions between fibres when the suspensions flow in the tube. For a fibre concentration of around 1% (EIT images depicted in Figure 9), it was not possible to collect consistent data for the _Eucalyptus_ fibres, and so Figure 9 depicts EIT images only for _Pine_ fibre suspensions. For these suspensions, and by comparison with Figure 8, it is evident that at lower concentrations and lower velocities (in this case 0.5 m/s), _Pine_ fibres migrate and become agglomerated in a central plug, while for a more a more concentrated suspension, the fibres (Figure 8) occupy a larger region of the pipe cross-section. However, at more diluted suspensions, the strength of  
Figure 8: EIT images obtained with a concentration of \(\cong\)2.35% (w/w) of _Pine_ (left column: (**a**,**c**,**e**,**g**)) and _Eucalyptus_ (right column: (**b**,**d**,**f**,**h**)) fibres with increasing flow velocity (0.5 m/s, 1m/s, 2m/s and 3m/s, top to bottom). the interconnection of the fibres is lower and the plug breakage occurs at lower velocities as the velocity increases. It must be emphasised at this point that environmental factors that were not easily kept under control may affect the colour mapping of the reconstructed images. In particular, the temperature changes significantly during continuous testing due to the energy input from the pump. Moreover, since cooling of the system was performed by passing tap water through a tube immersed in the suspension tank, the temperature of this cooling water changes significantly with respect to the seasons, which also affects the temperature in the rig.