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Investigation and optimization of intensified separation processes: treatment of aqueous organic mixtures using reactive extraction and emulsion liquid membrane techniques.

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The application of intensified processes of reactive extraction and emulsion liquid membrane technique for the separation of various low molecular weight carboxylic acids (propionic acid, malic acid and butyric acid) from very dilute aqueous solution was undertaken. The aqueous phase feed concentration ranges of the different carboxylic acids for reactive extraction [propionic, butyric (0.4-1 kmol/m3), malic (0.1-1kmol/m3) acids] and emulsion liquid membrane (propionic and malic acid (0.05 – 0.1 k mol/m3) were chosen to simulate the actual aqueous waste streams and fermentation conditions encountered in industry. Trioctylamine extractant in 1-decanol as active diluent was used as the extractant (organic) phase to perform the experiment. The effect of different process variables on the extraction efficiency expressed in terms of distribution coefficient (KD) and degree of extraction (%E) was systematically determined. Three independent process variables were chosen, including temperature (298.15-313.15 K for propionic acid and malic acid and 298.15-318.15 K for butyric acid), initial organic acid concentration in the aqueous phase and trioctylamine composition (10-30 %) in the organic phase for the reactive extraction technique. The interactive effects and optimum values of these process design variables were determined using response surface methodology (RSM) for the reactive extraction process. The statistical design analysis demonstrated that the acid concentration and trioctylamine composition had significant effect while temperature had an insignificant effect on the response value as well as interactive and quadratic effect on the response. The optimum solution led to an experimentally determined extraction efficiency of 89.79% for propionic acid, 93.25% for malic acid and 96.45% for butyric acid. The extraction efficiency in the emulsion liquid membrane process is dependent on the studied parameters such as initial acid concentration, sodium carbonate concentration, trioctylamine concentration, treat ratio and extraction time. The formulation of the liquid membrane consists of trioctylamine as carrier, 1-decanol as modifier, sorbitan monooleate (Span 80) as surfactants in heptane and sodium carbonate (Na2CO3) as a stripping agent. Response surface methodology (RSM) and artificial neural network (ANN) was employed for experimental design, optimization, construction and interpretation of response/output surface plots so as to show the effect of input variables on extraction efficiency in addition to the combined effects between variables. The optimum solution achieved by RSM led to an experimentally determined extraction efficiency of 92.28% and 85.91% in the propionic and malic acid extraction respectively by (emulsion liquid membrane) ELM process. The intrinsic kinetic studies of reactive extraction were determined for propionic and malic acid extraction using dilute solutions of the acids with concentration range of 0.2 to 0.6 kmol/m3 and trioctylamine (10%v/v) in 1 decanol as extractant at 303.15 K. The kinetic process parameters such as reaction order, mass transfer coefficient and rate constant were evaluated using the experimental data. From the results obtained, the reaction was found to be an instantaneous second-order chemical reaction occurring in the organic diffusion film. The values of the rate constants were found to be 0.430 m3/mol s and 0.332 m3/mol s respectively for propionic acid and malic acid while the mass transfer coefficient, km was also obtained for propionic acid (9 x 10-6 m/s) and malic acid (3x10-6 m/s). From the results obtained, these intensified technique represents an effective method for the recovery of low concentrations of carboxylic acids from aqueous waste streams and fermentation broths, with emulsion liquid membrane offering significantproperties/characteristics like small quantity of organic phase and extractant, very fast extraction time, increased solute transfer rate and selectivity through the membrane, high selectivity and applicability in specie removal from very low to high concentrations and governed by a non-equilibrium mass transfer. It is therefore worth investing in this process or alternatively a hybrid of both reactive extraction and emulsion liquid membrane processes.


Doctoral Degree. University of KwaZulu-Natal, Durban.