Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 15 | 779

(a) the heat transfer per kmol of CH 4 , (b) the entropy generation, and (c) the
reversible work and exergy destruction. Assume that T 0 298 K and the
products leave the combustion chamber at 1 atm pressure.

Solution This is the same combustion process we discussed in Example
15–10, except that the combustion products are brought to the state of the
surroundings by transferring heat from them. Thus the combustion equation
remains the same:

At 25°C, part of the water will condense. The amount of water vapor that
remains in the products is determined from (see Example 15–3)

and

Therefore, 1.57 kmol of the H 2 O formed is in the liquid form, which is
removed at 25°C and 1 atm. When one is evaluating the partial pressures of
the components in the product gases, the only water molecules that need to
be considered are those that are in the vapor phase. As before, all the
gaseous reactants and products are treated as ideal gases.

(a) Heat transfer during this steady-flow combustion process is determined
from the steady-flow energy balance EoutEinon the combustion chamber,

since all the reactants and products are at the standard reference of 25°C
and the enthalpy of ideal gases depends on temperature only. Solving for
Qoutand substituting the h

• f° values, we have

(b) The entropy of the reactants was evaluated in Example 15–10 and was
determined to be Sreact3023.69 kJ/kmol · K CH 4. By following a similar
approach, the entropy of the products is determined to be

Ni yi s–°i(T, 1 atm) 
Ruln yiPm Nis–i

H 2 O() 1.57 1.0000 69.92 — 109.77

H 2 O 0.43 0.0314 188.83 28.77 93.57

CO 2 1 0.0729 213.80 21.77 235.57

O 2 1 0.0729 205.04 21.77 226.81

N 2 11.28 0.8228 191.61 1.62 2179.63

Sprod2845.35

871,400 kJ/kmol CH 4

¬
3 1.57 kmol H 2 O 1
243
285.830 kJ>kmol H 2 O 1
24

¬
3 0.43 kmol H 2 O 1 g 243
241,820 kJ>kmol H 2 O 1 g 24

¬
1 1 kmol CO 221
393,520 kJ>kmol CO 22

Qout 1 1 kmol CH 421 
74,850 kJ>kmol CH 42

QoutaNph

°f,paNrh

°f,r

Nva

Pv

Ptotal

b Ngas 1 0.03128 21 13.28Nv 2 SNv0.43 kmol

Nv

Ngas



Pv

Ptotal



3.1698 kPa

101.325 kPa

0.03128

CH 41 g 2  31 O 2 3.76N 22 SCO 2 2H 2 OO 2 11.28N 2