Why is it necessary to filter of drying agents before distillation the substance back?

Question

example of drying agents: anhydrous magnesium sulfate and anhydrous calcium chloride

Answer 1

There are at least a couple of reasons.  First, when you distill, if you leave the drying agent in the material you are distilling, a lot of the material will remain adhered to the drying agent which has a very large surface area.  This will reduce your yield.  Second, the bonding of water to the drying agent is reversible, and, at high temperature, the water can be released again, so will end up co-distilling with your product.