On the fouling of falling film evaporators due to film break-up

摘要

When concentrating milk-derived products in falling film evaporators, fouling is recognized to be very detrimental to production. Fouling is likely to occur if the falling film breaks up. Thus a key design and operational criterion is that the tubes are completely wetted and film break-up is avoided. This requires an adequate peripheral flowrate (mass flowrate per unit of total tube perimeter). The minimum peripheral flowrate is product and concentration dependent and not well known. The force balance of Hartley and Murgatroyd gives a criteria for the existence of stable dry patches in a film and thus the likelihood of fouling. The minimum peripheral flowrate prediction depends upon liquid density, viscosity and surface tension and, furthermore, liquid/tube advancing contact angle. The first three properties have been well studied for most milk-derived products but there is little data on advancing contact angles in the open literature. Furthermore, the minimum peripheral flowrate has a strong dependence upon contact angle. A simple, robust experimental technique is presented to measure advancing contact angles. This enables the prediction of minimum peripheral flows. These predictions are experimentally verified by observations of the stability of dry patches created in falling films. Finally, overall heat transfer coefficient data from an industrial evaporator is used to infer the flowrate at which film break-up commences. This further validates the accuracy of the predictions.