Dynamic model for vacuum freeze-drying of waterlogged archaeological wooden artefacts

Abstract

Abstract A mathematical model for vacuum freeze-drying of waterlogged and PEG impregnated wooden objects has been developed. The model is based on basic physical laws for heat and mass transfer in vacuum and porous objects. The heat transfer to the object is considered to be radiation and the drying to take place as an ice front retreating parallel to the surface of the object. The model was used to determine the heat and water vapour transfer coefficient for waterlogged oak, birch and balsa wood, and for frozen aqueous solutions of PEG 4000. Based on the aforementioned data, theoretical transfer coefficients for PEG treated wood were derived. A numerical model based on linear drying in finite time intervals was developed. The model, which has as its output the position of the ice front, surface and ice temperatures, was found to predict vacuum freeze-drying processes in waterlogged wood and PEG with excellent accuracy. The model can be used as a tool for development of freeze-drying equipment especially designed for waterlogged archaeological wooden objects and for new and less resource demanding freeze-drying procedures.