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Poster De Conférence Année : 2015

Flow regime assessment in falling film evaporators using residence time distribution functions

Résumé

Falling film evaporators (FFE) are widely used in the chemical and food industries for their ability to process heat-sensitive products. However, the control and design of this operation is still based on empirical knowledge. As boiling and concentration take place inside the FFE, study of the mechanisms is complex, because both occur simultaneously. Rates of enzymatic and chemical changes thus, depend on solute concentration, temperature and time. 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 falling film evaporator 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 and three different feed mass flow rates. Flow was characterized using experimental film Reynolds numbers (Ref). RTD functions were based on measurement of conductivity of a tracer (NaCl at 10 g·kg-1) in the feed product. The RTD of the product particles was measured at four different sections in the vacuum evaporator equipment. These RTD functions were modelled according to a combination of two reactor sets in parallel (A and B), each being constituted of a series of perfectly mixed reactors tanks. 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, that correspond to the time when 50% of the material entering at t = 0 had passed through the equipment. When Ref < 25, the flow behavior changed from wavy-laminar to laminar flow and the mean residence time of the product increased. The velocity of wavy-laminar flow, Ref > 25, was lower than that of laminar flow. The concentration of skim milk did not affect the time that the first particles took to exit the evaporator. Increase in the concentration of skim milk induced an increase in the dispersion of its particles, that was also affected by the mass flow rate and sections (distance covered by the product). 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, probably due to the high retention of molecules, caused by product recirculation. 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. This model allowed the calculation of the amount of product that followed different paths in the FFE. These results should contribute significantly to increasing the efficiency of cleaning processes in the food and chemical industries. Appropriate instrumentation, choice and sites of the sensors in the vacuum evaporation equipment, combined with understanding of process engineering, resulted in good characterization of this process. 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, and to establish further correlations with changes in constituents and operational parameters.
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Dates et versions

hal-01173944 , version 1 (07-07-2015)

Identifiants

  • HAL Id : hal-01173944 , version 1
  • PRODINRA : 307518

Citer

Arlan Caldas Pereira Silveira, Gaëlle Tanguy-Sai, Fabrice Ducept, I.T. Perrone, Romain Jeantet, et al.. Flow regime assessment in falling film evaporators using residence time distribution functions. 7. European Meeting on Chemical Industry and Environment, Jun 2015, Tarragone, Spain. , 2015. ⟨hal-01173944⟩
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