Chemical and Biomolecular Engineering - Theses

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    Influence of plate wettability on the performance of a pulsed disc and doughnut column
    Gräbin, Teobaldo ( 2015)
    The pulsed disc and doughnut column (PDDC) can be seen as a variation of the pulsed perforated plate column (PPPC). It has gained attention due to its simple design, compact nature and lack of internal moving parts leading to safety and economic benefits. The PPPC hydrodynamics and mass transfer performance has been widely studied and reported and due to its similarity, applied to PDDC in a few studies. A few investigators have reported the effect of plate wettability on the performance of PPPC with contradictory results. In this study a tri-n-octylamine – sulphuric acid – water system is used in the study of the hydrodynamics and mass transfer performance of a pilot plant PDDC with mass transfer from dispersed phase to continuous phase and with both aqueous and organic dispersions at low, intermediate and high agitation intensities. To simulate different plate wettability three materials with distinct wetting properties has been selected, namely: Teflon (hydrophobic), Nylon (intermediate) and Stainless Steel (hydrophilic). It was found that Teflon behaves as a super-hydrophobic material in the liquid-liquid system used and may cause phase inversion for organic dispersion with insufficient pulsation. It was found that the magnitude of the effect of agitation intensity on holdup is influenced by the plate wettability. Holdup, measured in terms of characteristic velocity, was lower for Teflon and higher for Stainless Steel for aqueous dispersion. For organic dispersion Nylon showed the lowest characteristic velocity and Stainless Steel the highest. The effect of agitation intensity on the characteristic velocity from the low to intermediate agitation intensity was similar for intermediate and non-wetting plates in both organic and aqueous dispersions, but has a little effect on the wetting plate, i.e. Teflon with dispersed organic and Stainless Steel with dispersed aqueous. Despite the changes in the holdup, the Sauter mean diameter was independent of the plate wettability for dispersed aqueous. However, the distribution of droplet size changed with the operating parameters and three distribution functions were fitted to experimental data, namely: Lognormal, Gamma and Weibull (also known as Rosin-Rammer). The later fitted the raw and grouped data the best. For dispersed organic, the drop size in Nylon plates was significantly smaller than that of Teflon plates. The measured holdup and droplet size was compared with existing models and their accuracy assessed. Two holdup models were modified to incorporate the wetting characteristic of the plate materials. The holdup and droplet size models, suitable for this system, were used to calculate the mass transfer coefficient based on the axial dispersion model which was also compared with existing models and a new correlation developed.