Determination and modeling of the isotherms of adsorption/desorption and thermodynamic properties of obeche and lotofa using nelson's sorption model

Equilibrium moisture content (EMC), net isosteric heat of sorption (NIHS), net entropy of sorption (NES) and standard free energy of Gibbs (SFEG) of Triplochiton scleroxylon (obeche) and Sterculia rhinopetala (lotofa) are modeled using Nelson's sorption isotherm model. Experimental data are determined using a dynamic vapor sorption apparatus. Air relative humidity is ranged from 0 to 90% and two air temperatures (20 °C and 40 °C) are used. Experimental data of EMCs obtained from adsorption and desorption for the mentioned woods were compared with the predicted results using Nelson's sorption model by the application of the same operating conditions (i.e. temperatures and relative humidity conditions). The results showed that the predicted ones were in good agreement with the experimental data, when the relative humidity varied from 1.2 to 90%. Both adsorption and desorption phase presented a mean relative errors value inferior to 6.3%. The parameters that define the sorption isotherm varied with the wood type and the sorption mode. Thus, Nelson's sorption isotherm model is a useful tool that can be used for prediction of the moisture change in wood under different environmental conditions and for predicting indirectly the thermodynamic parameters such as NIHS, NES and SFEG. In adsorption and desorption mode, the NIHS are estimated using Clausius-Clapeyron's equation. The desorption values were higher than the adsorption ones. NIHS versus EMC varied exponentially and the parameters of the function were presented variable with the sorption mode and the wood type. The total energy that we needed to extract and evaporate all bounded water is 12,398.6 J/mol for obeche and 11,627.7J/mol for lotofa. The total energy that we needed to condense and fix all bounded water taken from the state vapour is 11,829.34 J/mol for obeche and 11,379.39 J/mol for lotofa. Water molecules are more mobile during desorption than adsorption in the case of obeche. In the case of lotofa, these molecules are more mobile during adsorption. SFEG is higher in desorption than adsorption and it is influenced by species and less by the temperature. When the temperature increased, the SFEG increased as well. At low values of EMC, NIHS and SFEG are higher and decreased with the increase of EMC until reaching the fibre saturation point.