The interfacial characteristics of falling film reactors
Falling film reactors are a very effective geometry in which to carry out fast exothermic gas-liquid reactions, because they allow easy removal of the heat produced. To optimise and improve falling film reactors, soundly based physical models are required. Existing models assume a flat liquid film combined with empirically-determined mass transfer coefficients, but there is growing evidence that this is unsatisfactory, since the interface is dominated by ripples and larger disturbance waves which have a significant effect on the mass and heat transfer rates through and into the film. The main objective of this study was the better understanding through experimental study of the interfacial characteristics of liquid falling films in the presence of a co-current down flow of air, with emphasis on the physical and hydrodynamic conditions encountered in current falling film reactor technology, so that the information obtained can be implemented in the enhancement of falling film reactor modelling. The liquids chosen were water, for validation of the measurement techniques through comparison with the literature, and two common falling film sulphonation reactor feedstocks; dodecylbenzene and ethoxylated alcohol. The experimental study consisted of film thickness and disturbance wave celerity measurements in a purpose built flow facility with a flat plate flow surface. A novel spatial film thickness measurement technique, the Light Absorption Imaging Technique (LAIT), was developed to give unparalleled spatial information on the interfacial characteristics of liquid films. A computer algorithm-based technique has been developed for categorisation of the spatial film data obtained by LAIT into substrate and disturbance wave regions, from which globally averaged characteristics of the interfacial structure have been obtained. Disturbance wave celerity measurements were obtained utilising the cross-correlated output signal from two light sources and a manual time-of-flight measurement technique using the images from a high-speed video camera. The study has provided a comprehensive set of data on the hydrodynamic structure of the interface, with and without the influence of a co-current down flow of air, for both water and actual sulphonation feed stocks as the liquid phase. The dramatic influence of both flow conditions and physical properties on interfacial characteristics has been demonstrated In particular, a transition has been observed of the disturbance wave structures in falling films due to the influence of a co- current airflow. Differences in the disturbance wave structure have been identified between the organic and water films after this transition.