Elementary Principles 283 - 8.12 a dC i n= p H O(l 2 = 75.4...

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8- 4 8.12 a. C p di HO ( l ) o 2 kJ / (kmol C) =⋅ 754 . =75.4 kJ/(kmol .o C) V = 1230 L , n V M == = ρ 1230 1 1 683 L kg 1 L kmol 18 kg kmol . ± .. ( ) . ` Q Q t nC d T t p T T = = z ( l ) o o 2 kmol kJ kmol C C 8 h h 3600 s kW 2 68 3 40 29 1 1967 b. ±± ± QQ Q total to the surroundings to water =+ , ± . Q to the surroundings kW = ± . . . () Q Q t d T t PHO to water to water o o kmol 3 h kJ / (kmol C) 3600 s/ h C kW = = z 2 29 40 68 3 11 5245 ± QE total total kW kW 3 h = 21.64 kW h =⇒ = × 7212 c. Cost heating up from 29 C to 40 C oo 21.64 kW h $0 / (kW h) = $2.16 = × .10 keeping temperature constant for 13 h total 1.967 kW 13 h $0.10/(kW h)=$2.56 $2.16 $2.56 $4.72 Cost Cost × =+= d. If the lid is removed, more heat will be transferred into the surroundings and lost, resulting in higher cost. 8.13 a. Δ ²² ² HH H N (25 C) N (700 C) N (700 C) N (25 C) 2 o 2 o 2 o 2 o kJ mol =− = = 2059 0 2059 bg b. Δ ² H H (800 F) H (77 F) H (77 F) H (800 F) 2 o 2 o 2 o 2 o Btu / lb - mol = = 0 5021 5021 c. Δ ² . H CO (300 C) CO (1250 C) CO (1250 C) CO (300 C) 2 o 2 o 2 o 2 o kJ mol = = 6306 1158 5148 d. Δ ² H O (970 F) O (0 F) O (0 F) O (970 F) 2 o 2 o 2 o 2 o Btu / lb - mol = = 539 6774 7313 8.14 a. ± m = 300 kg / min ± . n 300 1 1000 1 1785
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