Distributed moisture content in a continuous fluidized bed dryer
Systematic methods and tools for managing the complexity
Process Simulation & Optimization - I (T4-9a)
Keywords: population balances, granular material, moisture measurement, fluidized bed, drying
In chemical industry the fluidized bed drying of solid material is very common. A uniform moisture content of the product to be dried is highly desired. In continuous operation mode the particles may have a non uniform moisture content distribution due a different residence time in the apparatus. One the one hand a short residence time might cause a very high moisture content on the other hand to long exposition time might damage the product. Latter may harm the quality of material especially in such cases where the solids are highly thermo sensitive.
To gain a better understanding of the process the moisture distributions occurring in fluidized bed drying need to be investigated by means of population balances. In a first step experiments were carried out with a lab scale dryer. For different process conditions the moisture distributions were determined by single particle measurements. These measurements were conducted using the nuclear-magnetic-resonance (NMR) technique. By calibration of the NMR signal with a coloumetric moisture measurement the absolute mass of water within a single particle was determined. This approach allows to measure the moisture distribution very fast and precise. It is well known that the drying rates depend strongly on different process conditions such as gas temperature, particle flow rate and bed mass. In the framework of this study only the influence of moisture load and therefore residence time distribution has been investigated. An increased particle flow rate reduces the residence time. Consequently the moisture distribution in the product will change significantly.
Applying the concept of population balances, which takes the temporal change of number density distribution in dependence on internal coordinates the residence time , the moisture and the enthalpy of the solids, the measured moisture distributions can also be described theoretically. The three dimensional population balance equation describe only the behavior of the solid phase. To complete the model one needs to combine the population balances for the solid phase with balances for the gas phase. Therefore the mass and enthalpy balances and appropriate kinetics of heat and mass transfer respectively have to be defined. In our studies the fluidized bed model suggested by Burgschweiger and Tsotsas [1] was applied, which assumes that the gas passes the solid phase in plug flow while the solid phase is perfectly mixed.
Within this study first comparisons between experiments and simulations will be presented. By comparison of experimental mean particle moisture content with simulation results it has been impressively proven that the performance of a dryer can be described with higher accuracy using the population balance concept.
[1] Burgschweiger, J., Tsotsas, E. (2002). Experimental investigation and modelling of continuous fluidized bed drying under steady-state and dynamic conditions. Chem. Eng. Sci. Vol 57, pp. 5021-5038.
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Presented Monday 17, 11:52 to 12:11, in session Process Simulation & Optimization - I (T4-9a).
