Behaviour of Bubble Columns with Two and Three Phases

Abstract

Bubble columns are multi-phase contactors widely used in the process industries. For proper design, effective operation and analysis of such columns, knowledge of the fluid-dynamics, mixing and transport parameters is essential. Hitherto, most studies have been on simple two-phase systems in relatively small diameter columns at low phase through puts. A study of the fluid dynamics and mixing characteristics of bubble columns was thus undertaken, following a comprehensive literature review that confirmed the need for further research. A two-dimensional column 15.3cm x 1.5cm in section and 143cm high, and a column 15.2cm in diameter were used to study the effects of operating conditions and phase properties on the gas hold-up, flow regimes, bubble dynamics and mixing in two- and three-phase systems. Complex two- and three-phase systems were also tested. Air, water and a variety of solid particles were used as the gas, liquid and solid phases respectively. More complex systems involved the use of the following additives: alcohols, ketones, esters and electrolytes. Three methods were employed in measuring gas hold-up while flow regimes and bubble dynamics were characterised by visual observations, photographic techniques and qualitative analysis: The "stimulus-response" technique was used for mixing studies. In all cases, high flow rates corresponding to regimes of industrial importance were investigated. Experimental results were qualitatively and quantitatively analysed. Column behaviour varied with the operating conditions, nature of the phases and column design. The gas hold-up increased with gas flow rate for all systems studied and was also sensitive to the properties of the phases. Flow regimes, bubble dynamics and dispersion coefficients all depended on the gas velocity and nature of the phases. Small bubbles and thus high interfacial areas were produced by the presence of organic solutes and electrolytes. These additives affect the coalescence behaviour of aqueous solutions drastically changing from quick coalescence as in water to coa-lescence restraining. However, the effect of additives was insignificant on the three-phase systems. The effect of solids was linked to the surface property of wettability. This led to a two-way classification of behaviour: while the wettable solids tended to suppress bubble coalescence, non-wettable solids enhanced coalescence. Consequently, the former resulted in high values of gas hold-up while the latter reduced gas hold-up and dispersion when compared to systems without solids. A new concept aimed at a clearer understanding of column behaviour is presented. This is based on the analysis of molecular behaviour in bulk phases and at interfaces between phases. Using this approach, it should eventually be possible to predict the effect of various factors on the design and operation of bubble columns.