Liquid flow with a free surface inside a rotating horizontal cylinder is classified as rimming flow and the flow is generated due to external forces associated with inertia, viscosity, gravity and surface tension. This paper will discuss the hydrodynamics of thin film flow inside a rotating horizontal cylinder. Process Intensification and Clean Technology (PICT) group has been developing thin film reactors technology based on the rotating tube concept. The main aim of this study is to develop fluid dynamic understanding that will help predict liquid film thickness, residence time and heat and mass transfer intensities. Liquid film thickness is measured using an interferometric technique. There is extensive experimental as well as theoretical data available on rimming flow inside a partially-filled rotating horizontal cylinder (confined geometry) and there is no published data on rimming flow studies with a continuous flow system.