Control of an Evaporator. Consider the evaporator of Fig. 13-3.4. The level in the evaporator is…

Control of an Evaporator. Consider the evaporator of Fig. 13-3.4. The level in the evaporator is controlled very tightly by manipulating a valve on the feed line, because this is the largest of the inlet and outlet streams (LC). It is desired to control the feed flow, wF (FC) and the product composition, x, (AC), by manipulating the signals to the valves on the product, mP, and steam, ms, streams. The evaporator economy is approximately constant at E lb of vapor per lb of steam, and the inlet composition is x,, weight % solute. (a) Show, from steady-state balances on the evaporator, that the relationships between the variables are

(b) Derive the general formulas for the steady-state open-loop gains and for the relative gains. Is the interaction positive or negative? (c) Develop a general pairing strategy that will minimize the effect of interaction for any evaporator depending on the design values of the process variables

 

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