(413b) Bubble Sizes in Agitated Solvent/Reactant Mixtures Used in Heterogeneous Catalytic Hydrogenation

Nienow, A., Birmingham University
Stitt, H., Johnson Matthey
Hu, B., Birmingham University
Pacek, A. W., Birmingham University

Catalytic hydrogenations reactions are frequently conducted in ?dead-end' multiphase stirred reactors with the reactant dissolved in either an alcohol, water or a mixture of the two. In such systems, the rate of gas?liquid mass transfer which depends on bubble size may well be the overall rate-limiting step. However, a study of bubble sizes across the whole range of solvent compositions from entirely water to entirely organic has not been reported. Here, for the first time, a systematic investigation has been made in a 3L, ?dead-end? vessel containing 1% by volume of air dispersed by a Rushton turbine in water, isopropanol (IPA) and mixtures of the two, with and without 2-butyne-1, 4-diol simulating a reactant. In the single component solvents (water s = ~ 72 mNm-1, isopropanol s = ~ 21 mNm-1) though the interfacial tensions are very different, irregular, relatively large bubbles of similar sizes were observed (d32 ~ 300 mm in IPA, and ~ 400 mm in water,) with a wide size distribution. In the mixed aqueous/organic solvents, and especially at the lower concentrations of IPA (1%, 5%, 10%), the bubbles were spherical, much smaller (d32 from 50 to 70 mm) with a narrow size distribution. The addition of the reactant to the mixed solvents had little effect on the mean size, shape or distribution. However, addition to water (thus producing a mixed liquid phase) led to small (50 mm diameter) spherical bubbles of narrow size distribution. Neither Weber number nor surface tension were suitable for correlating bubble sizes since s decreases steadily from pure water to IPA whilst bubble size passes through a minimum at around 5% IPA. For any particular fluid composition, the functionality between d32 and is similar, i.e. . The above observations are explained in terms of the polarisation of bubble surfaces in miscible mixed aqueous/organic liquids caused by preferential directional adsorption at low concentrations of the organic component with it's OH groups at the gas/liquid interface. As a result, coalescence is heavily suppressed in the low concentration miscible alcohol ( or diol)/aqueous systems whilst strong coalescence dominates bubble sizes in water and the alcohol and at high concentrations of the latter.