An original image-processing technique for obtaining the mixing time:: The box-counting with erosions method

This study shows an original method to follow the blending of two powders and of these two powders and one viscous liquid in a classical reactor, under different agitation conditions. This technique takes a series of images and analyses them. It is a non-intrusive method which avoids sampling and therefore process interruption and sample preparation necessary otherwise. However, the image analysis is not evident and needs several steps. The simple application of the box-counting method on the synthetic on real images did not lead to good results because of the lack of variation of the fractal dimension between different images with a variable homogeneity. The box-counting method was then associated with an erosion treatment, leading to a useful parameter to characterise the mixture homogeneity. This parameter is the variation of the fractal dimension of the image, from the initial crude image to the final completely eroded one. The first results showed that the mixing is completely homogeneous after 100 s of agitation. In fact, a pseudo-homogeneity is achieved much more rapidly (t < 40 s) but subsequent images show that aggregates still remain in the reactor. The method is then applied to the mixing of the two powders under different operating conditions and also to the blend of the two powders and one viscous liquid to simulate the chemical reaction evolution. The results obtained are satisfactory. They allow determination of the mixing time and quantification of the degree of homogeneity of the blend. For the solid/liquid mixture, the poor affinities between oil and solid have modified the characteristics of the mixture and made it difficult to produce. The final aim is to link these results on the mixing homogeneity and the solid/solid contact times with the reaction kinetics. (c) 2005 Elsevier B.V. All rights reserved.