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

# 5 69 ne system ne all units u nr 2 therefore

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Unformatted text preview: streams and N A = 0. (a) and (b) The feed unit is like an absorber with an additional intermediate feed. Therefore, from Table 5.3 for an N-stage cascade, NV = (7 N + 2 NC + 2C + 7) + (C + 3) for extra feed +1 for extra feed location = 7 N + 2 NC + 3C + 11 N E = (5 N + 2 NC + 2) + 1 for extra feed = 5 N + 2 NC + 3 The second unit is a distillation column like that in Example 5.5, except with an additional feed and two more sidestreams that are located at the two feed stages. Therefore, from Example 5.5, using M for the number of stages, including the reboiler, NV = (7 M + 2 MC + 5C + 20) + 3(C + 3) for extra feed and 2 extra sidestreams +1 for extra feed location = 7 M + 2 MC + 8C + 30 N E = (5 M + 2 MC + 4C + 9) + 2(C + 2) for 2 extra sidestreams + 1 for extra feed = 5 M + 2 MC + 6C + 14 From Eq. (5-68), NV system = NV u − N R C + 3 + N A (1) all units From Eq. (5-69), NE system = NE all units u − NR (2) Therefore, since NR = 4 and NA = 0, Eqs. (1) and (2) become, ( NV )system = (7 N + 2 NC + 3C + 11)+ ( 7 M + 2 MC + 8C + 30 ) − 4 ( C + 3) ( N E )system = 7( N + M ) + 2C ( N + M ) + 7C + 29 = (5N +2NC +3)+(5M +2MC +6C +14) − 4 = 5(N + M )+2C ( N + M ) + 6C + 13 Analysis: (d) Exercise 5.37 (continued) (c) From Eq. (5-71), ( N D )system = ( NV )system − ( N E )system = [ 7( N + M ) + 2C ( N + M ) + 7C + 29] − [5( N + M ) + 2C ( N + M ) + 6C + 13] = 2( N + M ) + C + 16 A reasonable set of specifications is as follows: Pressure at each stage, reboiler, condenser, reflux divider Heat transfer at each stage and, divider Complete feed specification Number of stages, N Number of stages, M Feed stage location Locations of 3 sidestreams Flow rates of 3 sidestreams Reflux ratio Distillate rate Saturated liquid reflux or reflux temperature if subcooled Total N+M+2 N+M C+2 1 1 1 3 3 1 1 1 2(N + M) + C + 16 Exercise 5.38 Subject: Degrees of freedom and specs for a column with a liquid sidestream. Given: Column in Fig. 5.30. Find: (a) Number of degrees of freedom. (b) Reasonable set of design variables. Analysis: (a) The column in Fig. 5.30 has a partial condenser. A similar column with total condenser is analyzed in Example 5.5, giving ND = 2N + C + 11. In Table 5.2, in operations (b) and (c), it is seen that a column with a partial condenser has 3 less degrees of freedom than a column with a total condenser. Therefore, here, ND = (2N + C + 11) - 3 = 2N + C + 8. (b) A reasonable set of specifications is: Pressures of stages, reboiler, condenser N+1 Heat transfer for stages N -1 Feed completely specified C+2 Feed stage and sidestream stage locations 2 Distillate (volatile impurities) rate 1 Reflux ratio 1 Number of stages 1 Sidestream rate 1 Total 2N + C + 8 Exercise 5.39 Subject: Degrees of freedom analysis for distillation with a sidecut reboiled stripper. Given: Distillation system in Fig. 5.31. Find: (a) (b) (c) (d) Number of variables. Number of equations. Number of degrees of freedom. Reasonable set of design variables. Analysis: The system consists of (1) distillation column with 2 feeds and a sidestream, (2) reboiled stripper, (3) cooler, and...
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