EFFECT OF MICROBIAL ADSORPTION AT THE GAS/LIQUID INTERFACE ON OXYGEN ENHANCEMENT FACTORS

The adsorption of micro-organisms at the gas/liquid interface causes both an increase in the oxygen respiration rate and a decrease in the oxygen diffusion coefficient near the interface. An increase in the oxygen consumption rate increases the oxygen mass transfer rate into the bacterial broth but a decreased oxygen diffusion coefficient reduces the mass transfer rate. These two factors must be taken into account when enhancement factors are calculated.     

MASS TRANSFER COEFFICIENTS FOR REACTIVE FLUIDS FLOWING IN CONDUITS WITH SEMI-PERMEABLE WALLS

The advancing front theory is an approximate solution for mass transfer into a reactive fluid when the reaction can be assumed to be very fast. The theory has had considerable use in predicting mass transfer characteristics for reactive fluids flowing in conduits. In this paper, the mass transfer coefficient, in the form of the local, fluid-side Sherwood number, is derived for reactive flow in conduits with semi-permeable walls. The local, fluid-side Sherwood number is given as a function of the Graetz number, the wall Sherwood number, and a dimensionless reaction strength parameter. The limiting cases of both the constant wall concentration boundary condition (Sh_w?∞) and the constant wall flux boundary condition (Sh_w?∞)are investigated. Comparisons of the results with the classical Graetz and Leveque theories give conclusions about the accuracy of the advancing front theory for the worst possible case.     

The Photofading of Mixtures of Azo Dyes and Mixtures of an Azo Dye and Oxygen in Aquous Solutions Containing d,l-Mandelic Acid

The reactions of the reducing species formed on photo-excitation of d, l-mandelic acid in aqueous solutions with azo dyes, oxygen, mixtures of azo dyes, and mixtures of azo dyes and oxygen have been investigated. The results indicate that all reactions can be described as reductions of the oxidising agents (azo dyes and/or oxygen) by the photochemically formed reducing species. The azo dyes used can be arranged in a sequence according to their electron affinities. The higher the electron affinity the more susceptible a dye is to reduction in d, l-mandelic acid on ultraviolet irradiation. The inhibition of the photoreduction of azo dyes in solution by oxygen is mainly caused by a competition of the azo dye present and oxygen for the reducing species. However, the rates of fading of the dyes in aerobic solutions are only partly determined by the electron affinities of dye and oxygen. Other factors, such as the relative rates of intermediate reactions and the occurrence of as yet unidentified side-reactions, determine to an important extent the sequence of stability of the azo dyes during irradiation in aerobic solutions.