In Chapter 18 overall composition dynamics, based on the Rippin and Lamb model, were represented graphically in Figure 18.1. If we assume that boilup, bottom-product flow, reflux, and top-product flow are the manipulated flows, then for the system of Figure 20.1 we may prepare the closed-loop signal flow diagram of Figure 20.2 for:
1. Condensate receiver level control cascaded to distillate flow control
2. Overhead composition control cascaded to reflux flow control
3. Base level control cascaded to bottom-product flow control
4. Base composition control cascaded to steam flow control
For all four loops we assume that the secondary flow controls are fast compared with the primary level and composition loops. This permits us to ignore flow control loop dynamics. Then to go from the primary controller output to flow we need only multiply by 1/K^ where K^ is the flow-meter static gain (linear flow meters assumed).
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