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10.1 Rev: 12-6-03 According to Guideline 6, the manipulated variable should have a large effect on the controlled variable. Clearly, it is easier to control a liquid level by manipulating a large exit stream, rather than a small stream. Because R/D>1, the reflux flow rate R is the preferred manipulated variable. 10.2 Exit flow rate w4 has no effect on x3 or x4 because it does not change the relative amounts of materials that are blended. The bypass fraction f has a dynamic effect on x4 but no steady-state effect because it also does not change the relative amounts of materials that are blended. Thus, w2 is the best choice. 10.3 Both the steady-state and dynamic behavior needs to be considered. From a steady-state perspective, the reflux stream temperature TR would be a poor choice because it is insensitive to changes in xD, due to the small nominal value of 5 ppm. For example, even a 100% change from 5 to 10 ppm would result in a negligible change in TR. Similarly, the temperature of the top tray would be a poor choice. An intermediate tray temperature would be more sensitive to changes in the tray composition but may not be representative of xD. Ideally, the tray location should be selected to be the highest tray in the column that still has the desired degree of sensitivity to composition changes. The choice of an intermediate tray temperature offers the advantage of early detection of feed disturbances and disturbances that originate in the stripping (bottom) section of the column. However, it would be slow to respond to disturbances originating in the condenser or in the reflux drum. But on balance, an intermediate tray temperature is the best choice. 10.6 Variables : q1, q2,.... q6, h1, h2 Equations : 3 flow-head relations: NV = 8 q3 = Cv1 h1 10-2 q5 = Cv 2 h2 q 4 = K (h1 - h2 )
2 mass balances:
1A1 dh1 = 12 q1 + q6 - q3 - q4 3 dt dh 1A2 2 = 12 q2 + q4 - q5 3 dt Thus NE = 5 Degrees of freedom: NF = NV NE = 8 - 5 = 3 Disturbance variable : q6 NF = NFC + ND NFC = 3 - 1 = 2 ND = 1 ...
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- Spring '09