The “propane” tanks that are used for heating and cooking actually contain a mixture of propane and butane

The “propane” tanks that are used for heating and cooking actually contain a mixture of propane and butane. Some suppliers will tailor the composition of the fuel based on the local weather conditions. In cold weather, a higher percentage of propane is used in order to achieve a higher delivery pressure. During the summer, the propane percentage is reduced. Assume that liquid and vapor phases of propane and n-butane form ideal solutions due to their chemical similarity. Property data for the pure fluids are available in EES using the substances‘Propane’ and ‘n-Butane’, respectively. The lower heating values of propane and n-butane are 46,327 kJ/kg and 45,348 kJ/kg, respectively.

a) Suppose that a V = 0.42 m3 tank is charged on a hot summer day with a saturated mixture of propane and n-butane to 900 kPa at 40◦ C. The volume fraction of the liquid is 95% after charging. What are the total mass of fuel in the tank and the overall mole fraction of propane?

b) What is the pressure in the tank if its contents are later cooled to −30◦ C without any of the fuel having been used?

c) What will be the mass of fuel and the overall mole fraction of propane if the tank is charged with a mixture of propane and n-butane at −30◦ C, 115 kPa so that 80% of the volume is liquid?

d) If the tank contents determined in part (c) are later heated to 40◦ C without any of the fuel having been used, what will the pressure be?

e) Estimate the percent difference in energy content for a fully charged tank for cases (a) and (c)


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