Mathematical Model of Fixed-Bed Drying and Strategies for Crumb Rubber Producing STR20

Abstract

The objectives of this research were to investigate empirical and diffusion models for thin-layer crumb rubber drying for producing STR20 rubber using hot air temperatures of 110–130°C and to study the effect of drying parameters such as inlet drying temperature, volumetric flow rate, and initial moisture content on the quality of dried rubber. Finally, a mathematical drying model for predicting the drying kinetics of crumb rubber was developed using inlet air flow rates of 300–600 m³/min-m³ of crumb rubber (equivalent to 1.8–5.0 m/s) with the crumb rubber thickness fixed at 0.25 m. The average initial moisture content of samples was in the ranges of 40 and 50% dry basis while the desired final moisture content was below 5% dry basis. The results showed that the drying equation of crumb rubber was highly related to the inlet air temperature, while the drying constant value was not proportional to the initial moisture content. Consequently, the experimental data were formulated using nine empirical models and the analytical solution of moisture ratio equation was developed by Fick's law of diffusion. The result showed that the simulated data best fitted the logarithmic model and was in reasonable agreement to the experimental data. The effective diffusion coefficient of crumb rubber was in the range of 1.0 × 10⁻⁹ to 2.15 × 10⁻⁵ m²/s corresponding to drying temperatures between 40 and 150°C, respectively. The effects of air recirculation, inlet drying temperature, initial moisture contents, air flow rate, and drying strategies on specific energy consumption and quality of samples were reported. The experiments were conducted using two different drying strategies as follows: one-stage and two-stage drying conditions. The results showed that initial moisture content and air flow rates significantly affected the specific energy consumption and quality of rubber, while the volumetric air flow rate acted as dominant effect to the specific energy consumption. The simulated results concluded that the percentage of recycled air between 90 and 95% provided the lowest specific energy consumption as compared to the others.

Keywords:

• Crumb rubber
• Energy consumption
• High-temperature drying
• Hot air drying
• Mathematical model

ACKNOWLEDGMENTS

The authors express their sincere thanks to the Southland Rubber Resource factory and the AlliedTech Engineering Co. Ltd. for their preparing raw material, in-kind sponsor, and chemical testing; the Thailand Research Fund (TRF); Graduate school scholarship; Energy Policy and Planning Office (EPPO); and the Department of Physics Faculty of Science, Department of Chemical Engineering, and Department of Mechanical Engineering Faculty of Engineering, Prince of Songkla University, Thailand, for their financial support and for their research grant.

Notes

Note: SEC_exp is specific energy consumption of experimental results, MJ/kg of water evaporated; SECpredict is specific energy consumption of simulated results, MJ/kg of water evaporated; 1D means one-direction air flow drying; 2D means two-direction air flow drying.

Note: Standard STR20 block rubber must be limited as follows:

% Dirt ≤ 0.16%; ASH ≤ 0.80%; VM ≤ 0.80%; N2 ≤ 0.60%; PO > 30%; PRI > 40%.

— means no test applicable.

Dirt = dirt material or impurity inside block rubber; VM = volatile matter; PO = initial plasticity; PRI = plasticity ratio index.