Exploration of membrane-based dehumidification system to improve the energy efficiency of kiln drying processes: Part 1 – Factors that affect moisture removal efficiency

Abstract

Green wood drying through a steam kiln-drying technology is an energy-demanding process. This process consumes a large amount of energy to evaporate water from wood and discharge it to the atmosphere through venting. The thermal energy loss from the venting of dry kilns takes up to 20% of total energy consumed by the whole wood-drying operation because a considerably large amount of thermal energy is stored in the exhaust air. Harvesting and reusing such waster thermal energy wood improve the energy efficiency of kiln-drying process. Advanced moisture-selective membranes have been used to dehydrate humid air or gas steam because of the advantages of low energy requirements, simplicity of operation, and high specificity. However, the application of the membrane in wood-drying processes has not been addressed. Therefore, this study aimed to investigate the feasibility of using a moisture-selective membrane system to dehydrate the warm moist exhaust air to achieve an energy-saving purpose. The membrane material was polydimethylsiloxane (PDMS) with high water vapor permeability. A small membrane-based dehumidification system was constructed to evaluate the effects of four factors (temperature, airflow rate, initial RH, and vacuum pressure) on the efficiency of moisture vapor removal. Statistical analysis in terms of response surface methodology was carried out. The major findings include 1) an increase in the temperature and vacuum pressure caused a significant increase in the efficiency of moisture vapor removal, 2) the initial RH had little influence on the efficiency of moisture vapor removal, 3) increasing the airflow rate had a negative impact on the efficiency of moisture vapor removal, and 4) the regression model can be used to predict the efficiency of moisture vapor removal. This PDMS membrane would be a possible solution for a pre-drying process at relatively low operation temperatures (<45 o C), that is dehumidification process.