Abstract
Kiln drying of rubberwood lumbers is a complex transport phenomenon for realistic modeling and simulation. To decouple this complexity, researchers usually divide their research into two parts. The first one is single-lumber drying kinetics to describe how wood lumber responds to its surface conditions. Then they combine this drying kinetics with a lumped transport model or dispersion model or computational fluid dynamics. The mathematical models are then solved numerically to predict the industrial kiln drying behaviors. This work focuses on the drying kinetics of stacked rubberwood lumbers using hot air at different air velocity (0.5, 1.5, 2.5, 3.5, 4.0 m/s), relative humidity (6–67% relative humidity (RH)) and temperature (60–100 °C). The drying kinetics followed the conventional drying theory. However, the two drying periods, namely constant and falling rate (CRP and FRP), were not distinct. As the air velocity increased, the transition from CRP to FRP is faster. The middle of the transition period (at critical moisture content, CMC) moves closer to the fiber saturation point (FSP). The overall mass transfer coefficients in the falling rate period for stacked rubberwood drying were lower than those predicted by the Ananias correlation. Hence, a modified formula was proposed, representing the overall moisture transfer coefficients as a function of air velocity, temperature, relative humidity, and lumbers thickness for the range of variables under investigation satisfactorily. In general, the drying rate and the overall moisture transfer coefficient increased with increasing air velocity, drying temperature, and decreasing RH. Relative humidity directly affects the driving force of moisture transfer rate because higher RH is associated with higher equilibrium moisture content. A lumped parameter model for kiln drying was also developed. After being integrated with the estimated mass transfer coefficient, the model can predict the moisture profiles in lab-scale kiln drying satisfactory, although the model needs more validation data. These kinetic parameters and correlation for stacked rubberwood drying can be used in more complex models and process optimization in future research.
Highlights
Thailand is the largest natural rubber producer, followed by Malaysia and Indonesia in rubber plantation and production [1,2]
The sorption isotherm has been established for wood lumber and was verified for rubberwood lumbers in which, percentage of equilibrium moisture content (EMC) expressed by Eq (5) [10]
Falling rate period (FRP) Following the Ananias's approach/a simplified diffusion theory, we propose that a constant mass transfer coefficient is valid to describe the drying rate for the whole falling rate period, as shown in Eq (7)
Summary
Thailand is the largest natural rubber producer, followed by Malaysia and Indonesia in rubber plantation and production [1,2]. Ratanawilai et al [22] attempted to reduce an industrial kiln's drying time for drying rubberwood lumber (1, 1.5, and 2.0-inch thickness) by manipulating the air recycle ratio and achieved 23, 21.8, and 15.6 % time reduction respectively These works did not use the kinetic modeling approach in their studies. It is desirable to find a balanced approach with a moderate mathematical requirement and does not need to provide an air velocity profile within the drying chamber It can still describe and sufficiently predict rubberwood kiln driers' performance for different lumber properties and dimensions, drying temperature, air properties, velocity, and recycle ratio. We discuss the drying kinetics and drying mechanisms extensively to justify the applicability of the unified (covering a full range of drying curves) moisture transfer coefficient based on Ananias's approach
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