Concentration, Fouling or Deposit Characterization

To date and according to the limits of our knowledge, there exists only one study making use of tomography for the study of fouling [7]. Other authors have used the absorption of X-rays to follow the formation of deposits 8]. We will summarily describe them by showing the limits of tomography with respect to the study of deposits and their formation. 

Frank et al., in 2000, published a paper dealing with the visualization of concentration field in hemodialyzers [7]. Using a medical X-ray scanner, they observe a hemodialysis module (1.4 m ) containing about 10000 fibers (outside diameter 255 pm). To enhance the contrast, they add some sodium iodide (Nal, 0.1 M) to the water. The voxel size is approximately 5 mm in thickness and possesses a cross-section of 0.1875 x 0.1875 mm (note that they do not use the SRpCT but a commercial scanner). That means they cannot directly observe the particle, the fiber or the fouling, all of which are smaller in size. 

The example of Frank et al. [7] is interesting because it shows some of the limits of tomography  

To circumvent these difficulties and to obtain shorter acquisition times, it is possible to take only one simple radiograph. Yeo et al. [8] applied this principle in 2006 by using a technique called X-ray microimaging (XMI). They observed the deposition of particles of iron hydroxide (sizes from 0.1 pm to 10 pm) during a dead-end filtration into the lumen of a PAN fiber (nominal size pore 0.5 pm, outer diameter 0.8 mm). The images they obtained, using a pixel size equal to 1pm, were the projections of the fiber structure (i.e. membrane plus pores filled with water) and the deposition of iron hydroxide. The acquisition time was short (1-10 s) and images were recorded every 3 min. 

Finally, they concluded that the technique is able to observe the deposition of particles as a cake inside the lumen of a hollow fiber membrane and also to detect deposition and fouling within the membrane structure and they suggested that it can also be applied to the evaluation of critical flux in cross-flow operation by observing the flux at which deposition begins. 

---