Must a desiccant be made anhydrous before it can be used for drying? I'm writing about the treatment of methane gas, and in one part of the process, the methane gas is dried in a tank called a scrubber. Here, a desiccant called triethylene glycol is used.
Thinking about another desiccant, magnesium sulfate heptahydrate (Epsom salt), the water must be taken away through evaporation before the salt can be used for drying. The chemical must be anhydrous, so that it'll steal away all the water from the object in need of drying.
However, is this the case with triethylene glycol? Must it be made anhydrous before it can be used for drying, or is its attraction to water molecules great enough to begin with?
In the oil and gas industry, glycols are used in a scrubber to remove water from natural gas [methane] for a number or reasons: water is very soluble in glycols, water is extracted into “dry” glycols due to equilibrium forces, and there is efficient contact between the gases and the liquid.  Another huge part of the stribber is the regeneration portion of the system, the stripper – getting to the point of your question.  Heat is applied to the water/glycol mixture after having been in contact with the “wet” methane, driving off the water and regenerating the sorbent, the glycol.  The glycol can then be recycled.  Although I have described this as a batch process, the actual process is continuous.
In an industrial setting, “throwing something away” is used as a last resort.
Below is a schematic of a glycol dehydrator and regeneration unit.

https://www.sciencedirect.com/science/article/pii/S2352484719301039#fig1