Separation Process Principles- 2n - Seader & Henley - Solutions Manual

F 100 kmolh d b 53 62 115 kmolh therefore it

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Unformatted text preview: . Find: Most probable cause for abnormal operation. Recommended further tests. If 90 mol% methanol distillate could be obtained by increasing the reflux ratio for a constant vapor rate. Analysis: First determine whether the normal operation can be verified by the McCabe-Thiele method. With L/D = R = 1, from Eq. (7-7), the slope of the rectifying operating line = L/V = R/(1 + R) = 1/2 = 0.5. Also, xF = 0.5, xD = 0.90, and xB = 0.05. What is not known is the phase condition of the feed. If a saturated liquid feed is assumed, giving a vertical q-line as shown in the plot below, stepping stages up from the bottom, with the feed stage to plate 3 from the bottom, less than 2 theoretical plates are needed in the rectifying section, while 4 are present. The construction is shown on the next page. Therefore, it appears that the feed is not a saturated liquid, but is partially vaporized. Exercise 7.19 (continued) Analysis: Normal Operation (continued) By trial and error, using q-lines of various slopes, the following McCabe-Thiele diagram is consistent with the given data. It shows a q-line with a slope of -0.34. slope = q /(q − 1) Therefore, q = slope/(slope-1)=-0.34/(-0.34-1.0)=0.25 From Eq. (7-19), molar fraction vaporized = 1 − q = 1 − 0.25 = 0.75 Exercise 7.19 (continued) Analysis: Normal operation (continued) The material balance for the normal operation is as follows, using the overall balances, F = 100 = D + B and 0.5F = 0.5(100) = 50 = xDD + xBB = 0.90D + 0.05B. Stream Feed Bottoms Distillate Reflux kmol/h 100 47.06 52.94 52.94 mol% methanol 50 5 90 90 Exercise 7.19 (continued) Analysis: Abnormal operation For the abnormal operation, first check the overall total material balance using the given data. F = 100 kmol/h. D + B = 53 + 62 = 115 kmol/h. Therefore, it appears that we have 115 - 100 = 15 kmol/h more flow out of the distillation system. Now check the methanol overall material balance using the given data. Methanol flow rate in = 0.51(100) = 51 kmol/h. Methanol flow rate out = 0.80(53) + 0.12(62) = 49.84 kmol/h. Therefore, the methanol balance is close, with only about a 2% discrepancy. Now check the water overall material balance using the given data. Water flow in = 0.49(100) = 49 kmol/h. Water flow out = 0.20(53) + 0.88(62) = 65.16 kmol/h. Therefore, we have 65.16 - 49 = 16.16 kmol/h more water out than in. This is a significant discrepancy. It appears certain that water is leaking into the distillation system. Two possibilities are: (1) leakage of condenser cooling water into the condensate, or (2) leakage of reboiler steam into the boilup vapor. A reboiler steam leak may not be serious because the steam might not get to the top of the column to dilute the methanol product. A condenser cooling water leak could be very serious because part of it would end up in the distillate, thereby diluting the methanol product. Because of the impure methanol distillate for the abnormal operation, it appears that a condenser cooling water leak is the fault. Check thi...
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