Modeling the Drying of Grass Seeds (Brachiaria brizantha) in Fluidized Beds

Abstract

A modified three-phase model is developed to simulate the drying of Brachiaria brizantha in fluidized beds. In this new model, the constitutive equation of drying kinetics is formulated including both the constant rate and the falling rate mechanisms; the seed shrinkage is taken into account during all drying operation and the transition between bubbling to slugging regime is delineated for estimating the bubble velocity and size. Such modifications improve the mathematical model to better simulate the drying of coarse particles in fluidized beds. The best estimation of the five adjustable model parameters, which are required to define heat and mass transfer mechanisms between interstitial gas and seed particles and to specify the heat loss from dryer walls to ambient air, is attained by incorporating an optimization routine into the computer model program. Having been specially designed to supply data for this model, experiments are performed in a bath laboratory-scale fluidized bed. Additional data are generated to validate the model and program routines. Results show a good agreement between simulated and experimental data, validating the approach used to describe drying kinetics and particle shrinkage.

Modeling and simulating the drying of grass seeds (Brachiaria brizantha) in fluidized beds: evaluation of heat transfer coefficients

“…Three copper-constantan thermocouples are installed inside the column to measure: the inlet air temperature, T g0 , (thermocouple inserted just before the distributor plate); the solid-gas emulsion temperature, T bed , or the solid temperature, T s , (thermocouple placed 0.10 m above the air distributor) and the outlet air temperature, T gL , (thermocouple placed at z = 0.38 m from the base of the column). The solid temperature, T s , is measured during a short time interval in which the bed is collapsed; details of this method are discussed by Rizzi Jr. et al (2005). A thermocouple adhered to the outer column wall (at z =0.38 m) records the column wall temperature, T w .…”

Modeling, simulation, and optimization of a microalgae biomass drying process

“…Based on the measured data, it was observed that the mass and heat transfer correlations of Equations , , and were not adequate to predict the required time for biomass drying. Therefore, a new correlation for the mass transfer coefficient ( k m ) that considers the solid humidity ratio and preserves the dependence on the solid‐phase temperature was derived, which is a reasonable proposition according to Rizzi . The resulting final expression is as follows: where x 1 , x 2 , and x 3 are adjustment constants and T s is given in °C and ω s in kg kg −1 .…”

“…Physically, Equation is not adequate for wet microalgae biomass because it was derived for grass seeds, which present a much lower water content than the wet microalgae biomass. Therefore, the adjusted values for x 1 and x 2 based on wet microalgae biomass experiments are quite different from the ones obtained in Equation for grass seeds, and additionally, it was necessary to include a dependency on biomass humidity ratio, as shown in Equation .…”

“…Physically, Equation is not adequate for wet microalgae biomass because it was derived for grass seeds, which present a much lower water content than the wet microalgae biomass. Therefore, the adjusted values for x 1 and x 2 based on wet microalgae biomass experiments are quite different from the ones obtained in Equation for grass seeds, and additionally, it was necessary to include a dependency on biomass humidity ratio, as shown in Equation . Analogously, the heat transfer coefficient correlation proposed by Dittus and Boelter was originally developed for internal pipe flow without the occurrence of simultaneous mass transfer, which physically explains the x 4 adjustment constant value much greater than the original (0.023) proposed in Equation .…”

“…The work of Rizzi correlated the global mass transfer coefficient ( k m ) with the solid temperature during drying, because of the strong dependence of temperature on the drying rate: where T s is given in °C.…”