Non-invasive monitoring of segregated phases in a biogas plant: An ultrasonic approach

A non-invasive ultrasonic technique (UT) is developed to determine gas-liquid-solid composition inside digester of a biogas plant. The technique involves sweeping a contact type UT probe along the wall of pipes to determine the relative fraction of gas, liquid and solid phases under segregated conditions. The technique is validated at laboratory scale for two-phase (air-water) and three-phase (air-water-solid) systems. The average absolute error in prediction of segregated interfaces is ∼2.5%. Field trials are done on horizontal pipe sections (5 coils) of digester in the plant. More than 50% of the top most pipe run is occupied by gas. For second/third pipe runs solid fraction is around 20–25% while gas fraction is 10–15%. However, for the 4th and 5th pipe rows, the solid fraction is around 40–55%. A gradual increase of gas hold up towards the top regions and larger accumulation of solids at the bottom is expected because of waste feeding at the bottom. The described UT based system represents a novel way forward in use of ultrasound technology to detect segregated multiphase phases in digesters of biogas plants.