Numerical investigation of constant pressure filtration
thesisposted on 20.06.2014, 11:58 authored by Ronald M. Wairagu
The objective of this research was to perform studies on cake filtration through a modelling approach. Cake filtration is an important process in solid liquid separation. A computer code was written with the aid of Runge Kutta numerical scheme to be able to analyse existing filtration data. The first instance of the study was to compare model predictions of liquid pressure profiles to experimentally measured liquid pressure profiles and this were found to be in agreement. Internal cake properties, average porosity, permeability and specific resistance, were obtained from the model and the average values compared to those obtained from the experiment. It was found that using the Happel cell model led to the over prediction of permeability and under prediction of specific cake resistance by an order of magnitude of 2. This was then corrected by a shape factor being used to account for the different shape of talc particle as the Happel cell model assumes all particles are spheres whereas in reality very few particulate systems are spherical in nature. The effects of time and pressure on local cake properties were also investigated. It was seen that the porosity at the medium of a talc cake decreases rapidly as filtration pressure increases which was due to the compressible nature of talc suspensions. The porosity at the medium also changed very rapidly in the initial periods of filtration further highlighting the compressible nature of talc suspensions. Filter cake height is an important parameter in filter design. However it was found to be arbitrary in nature. Four different approaches of obtaining a filter cake height namely mass balance equation, modern filtration theory, experimentally measured transient liquid pressure profiles and the model in the current work were used in order to obtain and compare filter cake heights. The model over predicted filter cake height as compared to the other methods however this was deduced to be a different physical interpretation of a filter cake height and was investigated. It was found that the model over predicted cake height. The distribution of filtration pressure within the filter cake was also studied. The over prediction of filter cake height using the model was shown to be as a consequence of buffer layer which was the region between the model prediction and mass balance equation prediction. It was shown that constant pressure filtration referred to the total pressure loss across the slurry, filter cake and filter medium as opposed to it being just the forming pressure.
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- Chemical Engineering