# The Significance of the Sorption Isotherm on the Simulated Performance of Grain Driers
## 1 Introduction  
Stored grains and desiccants may be dried by ventilating them with air that has a low relative humidity. The performance of driers can be estimated accurately only if the equilibrium moisture content \(W\) of the solid phase is reliably known as a function of the temperature \(T\) and humidity \(w\) of its surrounding atmosphere. In other words, it is essential to have accurate sorption isotherms. This enables the moisture content of the solid phase in the vicinity of the air inlet to the drier to be estimated. This may be the target moisture content, or it may be a constraint imposed on the moisture content. For example, some grains, such as rice and corn, must not be over-dried because it results in their cracking during processing. If desirable properties such as germination and dough quality are to be preserved, temperature constraints must be imposed on the air used to dry grains. The overall rate of drying of hygroscopic materials, such as food grains, is a function of the differences in the humidities of the air entering and leaving the bed. To calculate thehumidity of air leaving a drier, it is important to have isotherms that are accurate when the solid material has a high moisture content. Weak physico-chemical bonds form when moisture is adsorbed by hygroscopic materials. The formation of the bonds is accompanied by the liberation of energy, which is termed the isothermal differential heat of wetting, \(h_{w}\), of the solid phase [1]. Wetting of the solid reduces the surface energy of the pores that permeate grains and desiccants; hence, the enthalpy of the solid decreases when its moisture content increases. As a result, \(h_{w}\) is a negative quantity. Isotherms provide relationships not only between \(W\), \(T\), and \(w\), but they also subsume information on the differential heat of wetting \(h_{w}\). Hence, it is important that sorption isotherms be not only accurate but expressions for \(h_{w}\) must be consistent with the isotherms. The energy required to remove a given quantity of adsorbed water is a function of the integral heat of wetting, which is the integration of \(h_{w}\) between the initial and final moisture contents. As evidenced by the work of [PERSON] et al. [2], the value of \(h_{w}\) generally increases rapidly as the moisture content of the solids tends to zero; hence, it is important that psychrometric data are gathered in the region of low relative humidity, i.e., \(0\leq r\leq 0.1\). The commercial importance of canola has resulted in its psychrometric properties receiving considerable attention in the scientific literature. For example, [PERSON] and [PERSON] [3] identify twelve publications that present experimental data on the sorption properties of canola that includes 668 experimental data points. They comprise 21 subsets that include adsorption and desorption. [PERSON] and [PERSON] [3] fitted their data to the modified [PERSON] [4], the modified Halsey reported by [PERSON] et al. [5], the modified Henderson used by [PERSON] et al. [6], and the modified Oswin [7] equations. They concluded that the modified Halsey equation provides the most accurate estimates of the relative humidities and moisture contents in the adsorption, desorption, and combined cases. [PERSON] and [PERSON] [3] report that in the case of the [PERSON] model, the mean relative deviation \(W_{MRD}\) between the modelled \(W_{model}\) and experimental \(W_{exp}\) moisture contents defined by  
\[W_{MRD}=\frac{1}{N}\sum_{i=1}^{N}\frac{\left|W_{exp,i}-W_{model,i}\right|}{W_ {exp,i}} \tag{1}\]  
is 0.084 and 0.17 for the adsorption and desorption isotherms, respectively. They recognise that this is a significant error, and they caution against fitting isotherms to only one set of experimental data. This need for caution is evident when examining the experimental results obtained by [PERSON] [8], who found that [PERSON]'s equation is clearly the least accurate, which is diametrically contrary to the finding of [PERSON] and [PERSON] [3]. [PERSON] et al.