There are sometimes procedures for drying a used drying agent. Usually that's reserved for an expensive synthetic one made of polymer beads or something like that. If the solid was just used in a drying tube, the recipe may call for gentle or moderate heating, either with or without forced air or vacuum. Sorry I can't be more specific, but it depends on the solid, and I don't know all the recipes, you will have to Google each. However, if the solid was used to dry an organic liquid, you may be called on to recrystallize first to remove impurities -- not really worthwhile for something as common as sodium sulfate.