Vent dryers are sometimes used to prevent the ingress of moisture into a tank storing concentrated sulphuric acid. The hygroscopic nature of concentrated sulphuric acid means that any moisture entering the tank will be quickly absorbed into the acid. Over time the top layer of acid will become more dilute and more corrosive. In tanks where the contents are not turned over frequently, the top layer of weak acid will actually corrode a ring entirely around the inside of the tank.
Desiccant dryers are the most common way of preventing the ingress of moisture into a tank. The size of the dryer required to protect a storage tank is determined by the amount of air and the rate at which the air enters the storage tank. Generally, the rate at which the air enters the tank is the pump-out rate; however, there are exceptions. A large tank that is nominally empty can have air entering the tank due to temperature and pressure changes faster than the pump-out rate. The desiccant in the dryer should be sized to last from one to six months per charge of desiccant. With shorter service cycles, the labour cost becomes prohibitive and, with longer cycles, the capital costs become higher. When the dryer desiccant capacity has reached its limit, the dryer is taken off-line and the desiccant regenerated.