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

critical temperatures, but they are also above their critical pressures. There-

fore, air will probably deviate from ideal-gas behavior, and thus it should be

treated as a real-gas mixture.

The energy balance for this steady-flow system can be expressed on a unit

mole basis as

where the enthalpy change for either component can be determined from the

generalized enthalpy departure chart (Fig. A–29) and Eq. 12–58:

The first two terms on the right-hand side of this equation represent the

ideal-gas enthalpy change of the component. The terms in parentheses repre-

sent the deviation from the ideal-gas behavior, and their evaluation requires a

knowledge of reduced pressure PRand reduced temperature TR, which are

calculated at the mixture temperature Tmand mixture pressure Pm.

(a) If the N 2 and O 2 mixture is assumed to behave as an ideal gas, the

enthalpy of the mixture will depend on temperature only, and the enthalpy

values at the initial and the final temperatures can be determined from the

ideal-gas tables of N 2 and O 2 (Tables A–18 and A–19):

(b) Kay’s rule is based on treating a gas mixture as a pseudopure substance

whose critical temperature and pressure are

and

Then,

TR,2

Tm,2

Tcr,m

160 K

132.2 K

1.21

PR

Pm

Pcr,m

10 MPa

3.74 MPa

2.67

TR,1

Tm,1

Tcr,m

220 K

132.2 K

1.66

 1 0.79 21 3.39 MPa 2  1 0.21 21 5.08 MPa 2 
3.74 MPa

P¿cr,m 
ayi Pcr,i
yN 2 Pcr,N 2 yO 2 Pcr,O 2

 1 0.79 21 126.2 K 2  1 0.21 21 154.8 K 2 
132.2 K

T¿cr,m
ayiTcr,i
yN 2 Tcr,N 2 yO 2 Tcr,O 2

1744 kJ/kmol

 1 0.79 216391  46482 kJ>kmol 1 0.21 216404  46572 kJ>kmol

qout
yN 21 h 1 h 22 N 2 yO 21 h 1 h 22 O 2

h2,ideal,O 2 
4657 kJ>kmol

T 2 
160 KSh2,ideal,N 2 
4648 kJ>kmol

h1,ideal,O 2 
6404 kJ>kmol

T 1 
220 KSh1,ideal,N 2 
6391 kJ>kmol

h 1 h 2 
h1,idealh2,idealRuTcr 1 Zh 1 Zh 22

qout
h 1 h 2 
yN 21 h 1 h 22 N 2 yO 21 h 1 h 22 O 2

eineout
¢esystem
 0 S ein
eout S h 1 
h 2 qout

Chapter 13 | 695

f Zh 1 ,m
1.0

f Zh 2 ,m
2.6

→

0

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