Consider a neutralization system shown in Fig. E22.7, where a certain amount of acid feed is added to a tank, chemically treated, and then sent to the next tank. pH analyzer (AT) indicates when the neutralization is complete. Level switches LS1, LS2, and LS3 are activated when the level in the tank is at or above a given level. The neutralization process proceeds with the following steps:
(a) Initially, all the valves are closed, the mixer is off, and the neutralization tank is empty.
(b) When the start button (not shown) is pressed, V1 opens and LS 2 is activated. These actions fill the tank with the solution to be neutralized.
(c) When the solution level rises above LS2, start mixer M. When the level drops below LS1, stop the mixer.
(d) Whenever the pH of the solution is too low, open V 2 to add neutralizer.
(e) If the tank becomes full before the acid feed is neutralized, indicated by the activation of LS3, close V2 to stop the inflow of neutralizer. Next, open V4 to reduce the liquid level to the point indicated by LS2; this solution will be reprocessed later. Then close V4 and proceed with step (d) again.
(f) When the pH of the solution is correct, close V 2 and open V 3 to drain the tank. When the tank is empty, indicated by the deactivation of LS1, close V3 and proceed with step (a). Draw information flow and ladder logic diagrams and a sequential function chart.
