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8.2.4 Product Composition Performance

The results presented up to this point have only looked at flows, levels, pressures, and tray temperatures. The effects of control structure and steady-state distillate flowrate have been examined for disturbances in feed flowrate. We have not looked at feed composition disturbances.

The job of the column is to achieve the desired product purities in the face of disturbances. Recall that the design specifications are 1 mol% iC4 in the distillate and 0.5 mol% C3 in the bottoms. We are using single-end control with a temperature on a tray in the column controlled by manipulating reboiler heat input. The other degree of freedom is held constant and depends on the control structure. In CS1 it is the reflux to feed ratio, and in CS3 it is the reflux ratio. In control structure CS2, the other degree of freedom is a fixed condenser heat removal.

Feed Flowrate Changes Both CS1 and CS3 structures should handle feed rate changes, at least from a steady-state standpoint; that is, product compositions should return to their design values for feed rate changes. This is expected because we are

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