# Mass Transfer

Mass transfer is driven by a vapour pressure differential between the air and desiccant solution. Equation 3.8 is used to determine the mass transferred in kg s^{-1 }(Welty et al. 2008).

where *p**_{v}* and p

_{so}i are the inlet equivalent vapour pressure of the air and liquid desiccant solution respectively in Pa. kM and are the respective convective mass transfer coefficients of the air and desiccant solution in kg m

^{-2}s.

*R*

*is the molar gas constant for water (461.5 J kg*

_{w}^{-1}K).

*H*is the Henry’s Law constant—a measure of the solubility of a gas in a liquid (wt%), and is dependent on the solute, the solvent and the temperature, under atmospheric pressure (101,325 Pa). The

*H*constant for the CHKO

_{2}solution (Я

_{снк02}has been gained from experimental data presented in the literature (Stephen and Stephen 1963), the relationship to solution temperature is shown in Eq. 3.9.

The convective mass transfer coefficient of the airstream in kg m ^{2} s can be determined from Eq. 3.10 (Cengel and Ghajar 2011).

*kH* is the convective heat transfer coefficient of the airstream in W m^{-2} K, determined using Eq. 3.3. Le is the Lewis number, and is defined in Eq. 3.11 as:

The Sc is the Schmidt number and is defined in Eq. 3.12 as:

The Prandtl number is previously defined in Eq. 3.5. *D _{AB}* is the mass diffusion coefficient, and is defined in Eq. 3.15.

The convective mass transfer coefficient in kg m^{-2} s of the desiccant solution is determined using Eq. 3.13 (Liu 2008).

&H_{o1} is the heat transfer coefficient of the airstream, determined using Eq. 3.3. The thermal diffusivity of the desiccant solution in m^{2} s^{-1} is calculated using Eq. 3.14:

kS_{ol} is the thermal conductivity of the desiccant solution in W m^{-1} K. D_{AB} is the mass diffusion coefficient of water vapour into the desiccant solution and is a function or air temperature and pressure. The mass diffusion coefficient is expressed in Eq. 3.15 (Welty et al. 2008).

where *p* is equal to one under atmospheric pressure
conditions, D_{0} is the binary diffusion coefficient of vapour in air = 2.46 x 10^{-5} m^{2} s^{-1} and *T _{0} =* 273.15 K.

The overall convective mass transfer coefficient in kg m^{-2} s of the investigated heat and mass exchanger is calculated using Eq. 3.16 (Ge et al. 2014).

**Fig. 3.8 ****The heat and mass transfer process in a single air/solution channel**

Figure 3.8 provides a diagram conceptualising the flow of heat (dq) and mass *(dm) *in a single air and solution channel during the dehumidification and regeneration processes.