Browsing by Author "Soni, Minal."

Now showing 1 - 2 of 2

The effect of solid micro particles on mass transfer in agitated dispersions.
(2008) Soni, Minal.
The industrial application of gas-liquid contactors has made effective design and optimisation of these processes a very important topic. In order to sustain a competitive advantage, rate limiting steps must be clearly understood. Hydrodynamics, heat transfer and mass transfer are complicated features of gas-liquid contactors and require a fundamental understanding. The mechanism of mass transfer in the presence of a small concentration of solid micro particles has been the subject of debate. The adsorption of gas by solid particles ("shuttle mechanism") is the traditional explanation. Recent experimental evidence suggests that the introduction of micro particles removes trace surface active impurities from the system and allows the true mass transfer coefficient to be measured. The objective of this study was to confirm the surfactant removal theory. Mass transfer is a field characterised by imprecise empirical relationships and difficult to obtain experimental parameters. This puts into context the significant challenge posed in preparing the careful set of measurements and analyses presented in this study to lend support to the surfactant removal mechanism. The study began with a review of mass transfer models. These models are based on concepts such as surface renewal and idealised turbulence. It is, however, difficult to choose between the models as they predict similar values despite being based on different mechanisms. The overall mass transfer coefficient is composed of the gas-phase coefficient (kGa) and liquid-phase coefficient (kLa). As the values of the coefficients are comparable and the solubility of oxygen or hydrogen is very Iow, the overall mass transfer coefficient is approximately equal to the liquid side coefficient. The relationship of kL with the diffusion coefficient (D) is one of the limited ways of choosing between the models. Mass transfer models predict k j • u:. D" . n is predicted to be % for a rigid surface (contaminated interface region) and Y2 for a mobile surface (clean interface region). If the surfactant removal mechanism applies, the introduction of solid particles will be accompanied by a reduction of n from % to 1/2. The effect of particles on n can be calculated from precise measurement of kL of gases with significantly different diffusion coefficients. A review of experimental methods was made to find precise methods to characterise mass transfer in the presence of solid micro particles. The chemical sulphite, gas-interchange and pressure step methods were identified as appropriate methods. These were implemented in a stirred cell (0.5 !) and an agitated tank (6 I). The chemical sulphite measurements were used to confirm that the enhancement of kLa is due to an enhancement of kL and not the specific interfacial area (a). Flat surface experiments were made using water and 0.8 M sodium sulphate batches. The reduction of n from % to Y2 was confirmed in both apparatuses after the addition of solid particles. The data were very well correlated and the dependence of kr on the energy dissipation rate per unit volume (e) is similar to the theoretically predicted value of 114 for the exponent. Observation of the reduction of n from % to Y2 was extended to agitated dispersions. The stirred cell kLa data were measured by the gas interchange method and are of excellent quality. The agitated tank results were measured by pressure step methods. The pressure dependence of the polarographic probes affected the precision of the results and the effect was within the experimental uncertainty. The effect of particles on n could not, therefore, be conclusively confirmed in the agitated tank. By relating precisely measured mass transfer coefficients to the diffusion coefficients; the surfactant removal theory is confirmed. The result is valid for a flat mass transfer area as well as for agitated dispersion where the nature of the interface region changes with time due to the accumulation of surfactants on an initially clean interface.
Vapour-liquid equilibria and infinite dilution activity coefficient measurements of systems involving diketones.
(2003) Soni, Minal.; Ramjugernath, Deresh.; Raal, Johan David.
Acetylpropionyl (2,3-pentanedione) and diacetyl (2,3-butanedione) are by-products of sugar manufacture. Both diketones have many uses, mainly food related. Vapour-liquid equilibrium data and infinite dilution activity coefficients are required to design purification processes for these chemicals. A review of available experimental methods revealed that the vapour and liquid recirculating still is most appropriate when both isobaric and isothermal VLE are required. The low-pressure dynamic still of Raal and Muhlbauer (1998) used in this study incorporates many features to ensure that measurements are of excellent quality (as demonstrated by Joseph et al., 2001). VLE measurements were made for the following systems: • Acetone with diacetyl at 30 C, 40 C, 50 C and 40 kPa • Methanol with diacetyl at 40 C, 50 C, 60 C and 40 kPa • Diacetyl with 2,3-pentanedione at 60 C, 70 C, 80 C and 40 kPa • Acetone with 2,3-pentanedione at 50 C, 30 kPa and 40 kPa. All the systems, except for methanol with diacetyl, displayed close to ideal behaviour. This was expected as they are mixtures of ketones. Solution thermodynamics allows one to perform data reduction of the measured VLE data to ensure accurate extrapolation and interpolation of the measurements. Furthermore, the quality of the data can be judged using thermodynamic consistency tests. The data were represented by the Gamma-Phi approach to VLE (the preferred method for low-pressure VLE computations). The two-term virial equation of state was used to account for vapour phase non-ideality. Second virial coefficients were calculated by the method of Hayden and 0'Connell (1975). The liquid phase non-ideality was accounted for by the Wilson, NRTL or UNIQUAC models. The best fit models are proposed for each system, as are parameters as functions of temperature for the isobaric data. The data were judged to be of high thermodynamic consistency by the stringent point test (Van Ness and Abbott, 1982) and the direct test (Van Ness, 1995) for thermodynamic consistency. The data sets were rated, at worst, "3" on the consistency index proposed by Van Ness (1995). A rating of "I" is given for a perfectly consistent data set and "10" for an unacceptable data set. For the system acetone with 2,3-pentanedione, isobars at 30 kPa and 40 kPa were measured. The results from the reduction of the 30 kPa set were used to accurately predict the 40 kPa data set. Infinite dilution activity coefficients were measured by the inert gas stripping method (based on the principle of exponential dilution). In order to specify the appropriate dilutor flask height (to ensure equilibrium is achieved), mass transfer considerations were made. These computations ensured that the gas phase was in equilibrium with the liquid phase at the gas exit point. The following infinite dilution activity coefficients were measured: • Acetone in diacetyl at 30 C • Methanol in diacetyl at 40 C • Diacetyl in 2,3-pentanedione at 60°C • Acetone in 2,3-pentanedione at 50 C. The ketone mixtures, once again, displayed close to ideal behaviour.