Simultaneous heat, moisture, and salt transfer in porous building materials considering osmosis flow: Part 2: Numerical simulation for poultice desalination using osmosis

Salt weathering of outdoor cultural properties is a significant conservation issue caused by external climatic fluctuations and groundwater uptake, and this study focuses on poultice desalination to suppress salt weathering. In a previous study, simultaneous heat, moisture, and salt transfer equations in porous materials considering osmosis were derived. Based on this theory, this study aims to evaluate the applicability and limitations of poultice desalination utilizing osmotic flow, as well as to characterize the calculation methods and challenges involved in simulating desalination processes. To achieve these objectives, the experimental results of poultice desalination reported by other researchers were reproduced by simulation. In the simulations, kaolin clay applied between the substrate and poultice was assumed to be the source of osmosis, and three calculation approaches were compared: (a) without considering osmosis, (b) as a filtration membrane that physically blocks the solute, and (c) considering the osmotic flow caused by the surface charges on kaolin. A good agreement with the experimental results was obtained in the third scenario. The key factors of this simulation are as follows: (i) anisotropy of the reflection coefficient, (ii) the calculation methodology of osmotic pressure, and (iii) introduction of a distinct equilibrium moisture content for kaolin clay. In addition, to reproduce a prolonged period of osmosis, this study demonstrates the importance of considering bidirectional solution flow and treating solute advection as a dispersion phenomenon under conditions in which osmosis and osmotic pressure are balanced. Furthermore, for the practical application of poultice desalination using osmotic flow, the interface material must be adequately supported to prevent its collapse under the applied osmotic pressure, thereby ensuring the desired effect. This research presents a simulation method capable of reproducing poultice desalination while incorporating osmotic flow, highlighting both the potential and current limitations of this approach.