Thermodynamics

Chapter 15 | 779(a) the heat transfer per kmol of CH 4 , (b) the entropy generation, and (c) thereversible work and exergy destruction. Assume that T 0 298 K and theproducts leave the combustion chamber at 1 atm pressure.Solution This is the same combustion process we discussed in Example15–10, except that the combustion products are brought to the state of thesurroundings by transferring heat from them. Thus the combustion equationremains the same:At 25°C, part of the water will condense. The amount of water vapor thatremains in the products is determined from (see Example 15–3)andCH 4 1g2 3 1O 2 3.76N 2 2 S CO 2 2H 2 O O 2 11.28N 2N vN gasP vP totalTherefore, 1.57 kmol of the H 2 O formed is in the liquid form, which isremoved at 25°C and 1 atm. When one is evaluating the partial pressures ofthe components in the product gases, the only water molecules that need tobe considered are those that are in the vapor phase. As before, all thegaseous reactants and products are treated as ideal gases.(a) Heat transfer during this steady-flow combustion process is determinedfrom the steady-flow energy balance E out E in on the combustion chamber,since all the reactants and products are at the standard reference of 25°Cand the enthalpy of ideal gases depends on temperature only. Solving forQ out and substituting the h – f ° values, we have 871,400 kJ/kmol CH 43.1698 kPa101.325 kPa 0.03128N v a P vP total b N gas 10.031282 113.28 N v 2 S N v 0.43 kmolQ out a N p h ° f,p a N r h ° f,rQ out 11 kmol CH 4 2174,850 kJ>kmol CH 4 2 11 kmol CO 2 21393,520 kJ>kmol CO 2 2 30.43 kmol H 2 O 1g243241,820 kJ>kmol H 2 O 1g24 31.57 kmol H 2 O 1243285.830 kJ>kmol H 2 O 124(b) The entropy of the reactants was evaluated in Example 15–10 and wasdetermined to be S react 3023.69 kJ/kmol · K CH 4 . By following a similarapproach, the entropy of the products is determined to beN i y i s – ° i (T, 1 atm) R u ln y i P m N i s – iH 2 O() 1.57 1.0000 69.92 — 109.77H 2 O 0.43 0.0314 188.83 28.77 93.57CO 2 1 0.0729 213.80 21.77 235.57O 2 1 0.0729 205.04 21.77 226.81N 2 11.28 0.8228 191.61 1.62 2179.63S prod 2845.35