Exergy Analysis of Reacting and Psychrometric SystemsPropane gas (C3H8) at 25°C, 1 atm and...

Exergy Analysis of Reacting and Psychrometric Systems

Propane gas (C3H8) at 25°C, 1 atm and a volumetric flow rate of 0.03 m3/min enters a furnace operating at steady state and burns completely with 200% of theoretical air entering at 25°C, 1 atm. The furnace provides energy by heat transfer at 227°C for an industrial process and combustion products at 277°C, 1 atm for cogeneration of hot water. For the furnace, perform a full exergy accounting, in kJ/min, of the exergy supplied by the fuel. Use standard chemical exergies from Table A-26 (Model II), as required, and ignore the effects of motion and gravity.

Table A-26

Standard Molar Chemical Exergy, e−ch(kJ/kmol), of Selected Substances at 298 K and p0

aJ. Ahrendts, “Die Exergie Chemisch Reaktionsfahiger Systeme,” VDI-Forschungsheft, VDI-Verlag, Dusseldorf, 579, 1977. Also see “Reference States,” Energy—The International journal, 5: 667-677, 1980. In Model I, p0= 1.019 atm. This model attempts to impose a criterion that the reference environment be in equilibrium. The reference substances are determined assuming restricted chemical equilibrium for nitric acid and nitrates and unrestricted thermodynamic equilibrium for all other chemical components of the atmosphere, the oceans, and a portion of the Earth's crust. The chemical composition of the gas phase of this model approximates the composition of the natural atmosphere. 

bJ. Szargut, D. R. Morris, and F. R. Steward, Exergy Analysis of Thermal, Chemical, and Metallurgical Processes, Hemisphere, New York, 1988. In Model II, p0 = 1.0 atm. In developing this model a reference substance is selected for each chemical element from among substances that contain the element being considered and that are abundantly present in the natural environment, even though the substances are not in completely mutual stable equilibrium. An underlying rationale for this approach is that substances found abundantly in nature have little economic value. On an overall basis, the chemical composition of the exergy reference environment of Model II is closer than Model I to the composition of the natural environment, but the equilibrium criterion is not always satisfied.