Drying of Banana Pseudo-stem Fibers: Evaluation of Kinetic Models, Effective Diffusivity, Thermodynamic Properties, and Structural Characterization

ABSTRACT

Banana pseudo-stem fiber drying was studied in a vertical fixed-bed convective dryer (60, 75, and 90°C). Nine mathematical models were used to analyze the drying behavior and the effective moisture diffusivity, activation energy, and thermodynamic properties were calculated. The dry fibers were evaluated by thermogravimetric, spectroscopic, and morphological analyses. High drying initial rates (25–30%) were observed indicating rapid evaporation of the free moisture present in the fibers. At the end of the process the moisture content decreased to 2.82, 0.14, and 0.16% (dry basis, db). The diffusion approximation model best fitted the experimental data and the effective diffusion coefficient increased with increasing temperature, reaching the order 10⁻⁷ m² s⁻¹. The activation energy required to initiate moisture removal from the fibers equaled 47.61 kJ mol⁻¹, and contrary to the entropy and Gibbs free energy, the enthalpy decreased with increasing temperature, indicating that drying is an endergonic non-spontaneous process. Lignocellulosic absorption bands were identified and material degradation occurred at temperatures >190°C, according to thermogravimetric analysis. Morphological changes in the dry fibers mainly occurred at 90°C and led to structural damage. These changes were attributed to the tensile strength generated from the temperature and moisture gradients produced during drying.

摘要

研究了香蕉假茎纤维在立式固定床对流干燥机 (60°C, 75°C, 90°C) 中的干燥. 采用九种数学模型分析了干燥过程, 计算了有效水分扩散系数, 活化能和热力学性质. 干纤维通过热重分析, 光谱分析和形态分析进行评估. 观察到较高的干燥初始速率 (25–30%), 表明纤维中的游离水分迅速蒸发. 在处理结束时, 含水量降低到2.82, 0.14和0.16% (干基, db). 扩散近似模型与实验数据吻合较好, 有效扩散系数随温度升高而增大, 达到10-7m2s-1级. 纤维吸湿所需的活化能为47.61kj-mol-1, 与熵和吉布斯自由能相反, 随着温度的升高, 热焓降低, 表明干燥是一个非自发过程. 根据热重分析, 确定了木质纤维的吸收带, 并且材料在温度>190°C时发生降解. 干纤维的形态变化主要发生在90°C时, 并导致结构损伤. 这些变化归因于在干燥过程中产生的温度和湿度梯度产生的拉伸强度.

KEYWORDS:

• Drying kinetics
• banana pseudostem fibers
• effective diffusivity
• mathematical modeling
• activation energy
• characterization of natural fibers

关键词:

• 干燥动力学
• 香蕉假茎纤维
• 有效扩散系数
• 数学建模
• 活化能
• 天然纤维的特性

Acknowledgments

The authors would like to thank Professor Nielson Fernando da Paixão Ribeiro (In Memoriam) for all the teachings and guidance in the preparation of this work. Also to the National Council for Scientific and Technological Development (CNPq), Coordination for the Improvement of Higher Education Personnel (CAPES), and Amazon Foundation for Studies and Research (FAPESPA) for their financial support to this research, and PROPESP and FADESP/UFPA for providing language editing services. There are no conflicts of interest to declare.

Additional information

Funding

This work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico [No number]; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior [No number]; Fundação Amazônia Paraense de Amparo à Pesquisa [No number].