Back-mixing Studies in the Presence of an Unstable Density Gradient in a Reciprocating Plate Extraction Column
The efficient design of extraction columns calIs for the accounting of flow non idealities, collectively termed axial mixing, in the phases involved. Axial mixing occurs mainly through back-mixing caused by circulation, wake transport with dispersed drops and induced turbulence due to mechanical agitation. Non uniform velocity profiles and forward mixing due to drops of different sizes are the other factors. A new factor identified to significantly enhance back-mixing, is the increase in the density of the continuous phase with column height termed as the unstable density gradient. In this thesis, attention was focussed on this factor using a 5 cm reciprocating plate extraction column (RPC).
A new approach based on the Kohnogoroff isotropic turbulence theory was developed to model the continuous phase back-mixing under two phase flow conditions. The contribution due to mechanical agitation, dispersed phase flow and the unstable density gradient were accounted in the model in terms of their respective energy dissipation rates and mixing lengths. The parameters were estimated by fitting the model to the experimental data obtained through the steady state tracer injection technique under non mass transfer conditions. The results showed that the unstable density gradient played an important role in enhancing the back-mixing even though its energy dissipation rate was very small relative to the other two contributing factors. This was due to the large mixing length associated with the unstable density gradient effect. At high mechanical agitations, the contributions of the dispersed phase and the density gradient declined.
A preliminary study was also performed to investigate the unstable density gradient effect under mass transfer condition. Water was used as the solvent to extract i-propanol from lsopar M drops in the extractor. As increasing amount of alcohol was extracted, the density of water decreased leading to an unstable density gradient. To ensure the reliability of the estimation of the back-mixing coefficient (Ec), a non transferring tracer dye was also injected steadily into the continuous phase. Ec values were determined simultaneously from the concentration profiles of the tracer and the solute, thereby providing two independent estimates for each experiment. The results were comparable under most operating conditions thereby validating the tracer technique. The dispersed phase hold-up, drop size and the mass transfer coefficient were also estimated and compared critically with the models available in literature.
The hydrodynamics in the column were significantly altered by the interfacial effects associated with the transfer of solute from the dispersed phase to the continuous phase. In particular coalescence was promoted leading to large drops. Under these conditions the Kolmogoroff model could not isolate the influence of the density gradient from other contributing factors. However the experimental results clearly showed an enhancement in the back-mixing relative to the non mass transfer case.
Based on these observations it is concluded that care must be taken to avoid the unstable density gradient created either inadvertently during tracer measurements involving ionic compounds or during mass transfer.