Modeling of the limiting step of water sorption by composite sorbents of the “calcium chloride in porous matrix” type

A nonstationary model is proposed to describe the water sorption from air by composite sorbents of the “salt in a porous matrix” type via the interaction of water with the salt to form a salt solution in pores. It is shown that the dynamics of the moisture content at the adsorber outlet cannot be described under the assumption of constancy of the effective sorption rate constant. The limiting step of water sorption is analyzed, and it is supposed that the limiting step is the water diffusion through the solution layer forming near the external surface of a granule. An expression for the sorption rate constant as a function of the moisture content of the sorbent is proposed that takes into account the monotonic decrease in this constant with an increase in the amount of water sorbed. The effective diffusion coefficient at the limiting step of sorption is determined, which turns out to be close to the effective diffusion coefficient in an aqueous calcium chloride solution.