CASE 6. AMMONIUM NITRATE PRILLING PLANT CONTROL SYSTEM Ammonium nitrate is a major fertilizer. Figure B-16 shows the process for its man ufacture (Reference 3). A weak solution of ammonium nitrate (NH4NO3) is pumped from a feed tank to an evaporator. At the top of the evaporator there is a steam ejector vacuum system. The air fed to the system controls the vacuum drawn. The concen trated solution is pumped to a surge tank and then fed into the top of a prilling tower. In this tower the concentrated solution of NH NO, is dropped from the top against a strong updraft of air. The air is supplied by a blower at the bottom of the tower The air chills the droplets in spherical form and removes part of the moisture, leaving damp pellets or prills. The pellets are then conveyed to a rotary drier where they are dried. They are then cooled. conveyed to a mixer for the addition of an antisticking agent (clay or diatomaceous earth), and bagged for shipping. A. How would you control the production rate of this unit? B. Design the system to implement the following: 1. Control the level in the evaporator. 2. Control the pressure in the evaporator. This can be accomplished by manip- ulating the flow of air to the exit pipe of the evaporator. Air Steam Weak NH.NO feed tank Steam ejector Surge tank Evaporator Prilling tower Steam Air To cooler and bagging Air Drier Air -W Air heat Figure B-16 Ammonium nitrate process, 3. Control the level in the surge tank. 4. Control the temperature of the dried pellets leaving the dryer. 5. Control the density of the strong solution leaving the evaporator. Be sure to specify the action of valves and controllers. C. If the flow to the prilling tower varies often, it may also be desired to vary the air flow through the tower. How would you implement this