Non dispersive solvent extraction for removal of dissolved Organic from aqueous medium using hollow fiber membrane contactor

摘要

PUREX process is based on the principle of mass transfer by liquid liquid dispersion. Tri-n-butyl phosphate (TBP) is universal extractant for PUREX process which is employed for reprocessing the irradiated nuclear fuels for separation and recovery of fissile and fertile materials.[1] The multi cycle solvent extraction processes encompass continuous extraction and stripping operations that are invariably carried out in pulsed columns. In solvent extraction, immiscible liquid-liquid dispersion is produced. The production of a dispersion is complex in nature due to the occurrence and interaction of several phenomena, namely drop break-up and coalescence, mean flow pattern and turbulence, drop suspension, interfacial area and drop size distribution, possible phase inversion, the influence of system composition as well as of small amounts of impurities, etc. Depending on the operating conditions the aqueous phase emanating out of these columns will have significant amount of entrained organic and dissolved TBP content [2]. As the aqueous streams need to be evaporated for safe storage/disposal, the organic content associated with the aqueous phase needs to be minimized to avoid red oil formation during evaporation. Removal of such dissolved TBP is of direct interest in reprocessing processes as this would enable manifold evaporation of aqueous stream without any operational difficulties and make it suitable for safe disposal[2,3]. The non-dispersive solvent extraction is a configuration of the conventional solvent-extraction process where a microporous membrane separates both the immiscible phases, one of which impregnates the membrane, thus bringing the liquid-liquid interface to one side of the membrane. The membrane-contactors modules were assembled in the laboratory using Accurel PP50/280 hollow fibers (MEMBRANA society) in glass module and epoxy resin were used for sealing the ends. The module contains 50 polypropylene hollow fibers of an average length of 20 cm. The used polypropylene hollow fibers have internal diameter and thickness were 280 μm and 50 μm respectively. The average pore size and porosity are 0.05 μm and the 50-55% respectively. Experiments were conducted in closed system for the removal of dissolved organic from aqueous feed using n-Dodecane in organic phase. Flow rate of 5 mL /minute and O/A phase ratio of 1:1 was maintained. A simple model describing the behavior of a hollow fiber membrane contactor working as a closed system is used in order to determine an overall mass transfer coefficient that lumps together the resistances in both sides of the membrane, i.e. the resistance of the membrane itself and the reaction kinetics at the interface. Separate test run were carried out to investigate the extraction behavior of dissolved organic from aqueous feed by n-Dodecane in organic phase.Fig.1 represent the profile of decrease in dissolved organic concentration at different acidity with respect to time. The extraction of dissolves organic was studied with a function of feed acidity and the result reveals that extraction decreases with increase in feed acidity[Fig.2].This may be probably due to co extraction of acid in organic phase. Bajoria et al. has obtained the same results and mentioned that transfer of nitric acid into the organic phase increases with the concentration of the nitric acid in the aqueous phase [4]. The controlling mechanism for the transport of dissolved organic not in the aqueous phase but it is in the organic diffusivity through in the microporous membrane considering the resistance-in-series classic model. The observed extraction kinetics was fast enough for quantitative separation of dissolved organic in limited period of time. The association of the efficient membrane based non-dispersive solvent extraction technique using hollow fiber contactor looks promising for removal of dissolved organic from aqueous streams of PUREX process.