|dc.description.abstract||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.||en_US