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

1 n d system 40 21 19 the given specifications are

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Unformatted text preview: From Eq. (5-67), ND = NV - NE = 14 - 4 = 10. A typical specification for a single evaporator effect might be F, wF, TF, PF, wL, P, TS, PS, TC , and PC . For the total condenser, the variables are: (Into) V, T, P; (out of) V, PT , TT ; Q Therefore, NV = 6. The relationships among the variables are: Energy balance: Q = V(HV in - HV out) Exercise 5.35 Analysis: (continued) (continued) Therefore, NE =1 and ND = 6 - 1 = 5. Typical specs might be V, T, P, PT , TT For the pump, the variables are L, wL, TL, PL, PP with no equations and 5 degrees of freedom. Now analyze the system. From Eq. (5-70), ND system = ND all units u − interconnecting redundant variables ND all units u (1) = 3(10) + 5 + 5 = 40 The number of interconnecting redundant variables are 4 (L, wL, T, P) for each of 3 liquid streams and 3 (V, T, P) for each of 3 vapor streams, or 4(3) + 3(3) = 21 Therefore, from Eq. (1), ( N D )system = 40 - 21 = 19 The given specifications are as follows, where effects are numbered 1, 2, 3 from left to right in Fig. 5.23: wF1 , w L3 , PS1 , TS1 . Therefore, additional specifications needed = 19 - 4 = 15, which might be: Feed rate to Effect 1 = F1 Temperature and pressure of feed to Effect 1 = TF1 , PF1 Pressure in each effect = P1, P2, P3 Temperature and pressure leaving condenser = TT, PT Pressure leaving the pump = PP Pressure of condensed steam leaving each effect = pressure of steam entering the effect Temperature of condensed steam leaving each effect = saturation for the exit pressure Exercise 5.36 Subject: Degrees of freedom and additional specifications for a reboiled stripper. Given: Reboiled stripper with specifications in Fig. 5.28. Find: (a) (b) (c) (d) Number of variables Number of relationships. Number of degrees of freedom. Additional specifications, if any. Analysis: (a) and (b) For an N -1 cascade and a partial reboiler from Table 5.3 NV NE Element N -1 cascade 7(N-1)+2C(N-1)+2C+7 5(N-1)+2C(N-1)+2 Partial reboiler 3C+10 2C+6 Total 7N+2NC+3C+10 5N+2NC+3 From Eq. (5-68), From Eq. (5-69), NV NE unit = unit = NV all elements NE all elements e − NR C + 3 + N A e − NR (1) (2) Since NR = 2 and NA = 0, Eqs. (1) and (2) become, ( NV )unit = (7N +2NC +3C +10) − 2 ( C + 3) = 7 N + 2 NC + C + 4 ( N E )unit = 5N +2NC +3 − 2 = 5N +2NC + 1 (c) From Eq. (5-67), ND = NV - NE = (7N+2NC+C+4) - (5N+2NC+1) = 2N + C + 3 (d) Specifications are: All feed conditions Number of stages Total C+2 1 C+3 Therefore, can make 2N additional specifications, which might be: All stage and reboiler pressures N Adiabatic stages N -1 Have one specification left. Make it the bottoms rate, distillate rate, or boilup rate or ratio. Exercise 5.37 Subject: Degrees of freedom and specifications for thermally coupled distillation. Given: Distillation system in Fig. 5.29. Find: (a) (b) (c) (d) Number of variables Number of relationships. Number of degrees of freedom. Reasonable set of specifications. Analysis: Treat the system as one of two units with NR = 4 interconnecting...
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