Page 227 - Modelling in Transport Phenomena A Conceptual Approach

7.5. CONSERVATION OF ENERGY 207

psys = 7 [ 130.3 - (12)(5)] 1'4 = 2.95 bar
130.3
Comment: Note that Eq. (11) can be rearranged in the form
Ts ys (%$'
-- -
TO
The use of the ideal gas law to express the number of moles gives

7-1 7-1
To
which is a well-known equation for a closed system undergoing a wersible adiabatic
(or, isentropic) process. Therefore, the gas remaining in the tank undergoes a
reversible adiabatic expansion throughout the process.

b) System: Contents of the tank
Assumption

1. Properties of the tank contents are uniform, i.e., Hout = Hsys.
Analysis
Equation (7.3-2) becomes

dnsys
-noUt = - -12=- dnsys
dt dt
Integration of Eq. (1 7) yields

nsys = no - 12t (18)
where no is the number of moles of air initially present in the tank, i.e.,

no=-- POV - (7)(0.5) = 130.3mol
RT, (8.314 x 10-5)(50 + 273)

In this case the process is isothermal and, a a result, the pressure of the system
can be directly calculated from the ideal gas law, i.e.,

The use of Eq. (18) in Eq. (19) results an