For safety purposes, highly volatile or poisonous powders or materials in solution or suspension with volatile solvents require to be dried in an Inert Circuit Spray Dryer atmosphere.
Although either Carbon dioxide or Nitrogen would be suitable substitutes for the drying air, Nitrogen is the usual chosen gas.
Because of the exorbitant cost of the gas, a single Nitrogen charge would be re-circulated and used constantly between CIP cleaning shut-downs of the plant.
The ducting would be designed to continuously re-circulate the nitrogen through the circuit, condensing and re-heating and the plant would either operate on a 24 hour basis or the nitrogen remained hot during shut down otherwise Nitrogen would be wasted when the circuit cools.
Certain essential design aspects would apply, for instance there must be no powder allowed to pass into either the tubular condenser or the heater. Collection would need to be by bag filter, fitted with a secondary filter to avoid powder leaking into the gas circuit, and the back-purge gas being taken from the circuit, compressed and pulsed back through the bags to clean the powder from the bag surfaces.
The evaporated moisture vapour must be removed from the Nitrogen in order to maintain constant conditions. This is done by passing the exhaust gas through a finned tube heat exchanger, or condenser, with chilled water passing through the tubes and the exhaust Nitrogen gas over the finned outside tube surface.
The condensate removed is either passed to drain or recovered for re-use and the gas temperature is raised above the dew point by passing the gas through a fan immediately after the condenser.
Two circulating fans are normally used, each fan would need to have a sealing gland with Nitrogen connection where the shaft passes through the casing, as would any rotary valves, screw conveyors or any moving equipment
The entire circuit would operate at slightly positive pressure, as measured at the fan suction, and maintained at the same pressure by automatically controlled Nitrogen charging valves.
Essential instrumentation, in addition to the usual operating instruments would include circuit pressure Oxygen and Nitrogen meters with shut down facilities.