Residence time distribution functions applied to skim milk concentration in falling film evaporators

Falling film evaporators (FFE) are widely used in the chemical and food industries. However, the control and design of this operation is still based on empirical knowledge. The residence time distribution (RTD) of the product provides valuable information to characterize the mixing phenomena and flow profile in FFE. The aim of this study was first to characterize RTD functions of a single stage FFE pilot, experimentally, and then to model these RTD functions and identify the flow regime in the process. Experimental runs were carried out with skim milk at 100 g·kg-1 of total inlet solids. Flow was characterized using experimental film Reynolds numbers (Ref). The RTD functions were modelled according to a combination of two reactor sets in parallel (A and B). Determination of the Ref proved to be a powerful tool for the characterization of the flow profile in FFE and provided information about the mean residence time. The velocity of wavy-laminar flow, Ref > 25, was higher than that of laminar flow. Some RTD curves exhibited shoulders at the ends, where 0.9 % to 6 % of skim milk particles passed by reactor set B. This route presented a zone with high retention of molecules, higher mean residence time and lower dispersion. Reactor set A described the path of between 94.0 % and 99.9 % of the product particles in the FFE. The model with two reactor sets in parallel was effective to model the RTD curves, in view of the very low standard deviations obtained. The future prospects are at the level of using the RTD approach with different products and evaporation temperatures in the evaporator device. These parameters can be accessed by modelling or measurement, and will make it possible to describe the time/temperature history of a product.