state and the chemical energy term appears explicitly. When expressed prop-

erly, the enthalpy term should reduce to the enthalpy of formation h

• °fat the
  standard reference state. With this in mind, we express the enthalpy of a
  component on a unit mole basis as (Fig. 15–21)

where the term in the parentheses represents the sensible enthalpy relative to

the standard reference state, which is the difference between h

• the sensible
  enthalpy at the specified state) and h


• 
  

  ° (the sensible enthalpy at the standard
  reference state of 25°C and 1 atm). This definition enables us to use enthalpy
  values from tables regardless of the reference state used in their construction.
  When the changes in kinetic and potential energies are negligible, the
  steady-flow energy balance relation E

  
  

.

inE

.

outcan be expressed for a chem-

ically reacting steady-flow systemmore explicitly as

(15–8)

Rate of net energy transfer in Rate of net energy transfer out

by heat, work, and mass by heat, work, and mass

where n

.

pand n

.

r represent the molal flow rates of the product pand the reac-

tant r, respectively.

In combustion analysis, it is more convenient to work with quantities

expressed per mole of fuel. Such a relation is obtained by dividing each

term of the equation above by the molal flow rate of the fuel, yielding

(15–9)

Energy transfer in per mole of fuel Energy transfer out per mole of fuel

by heat, work, and mass by heat, work, and mass

where Nrand Nprepresent the number of moles of the reactant rand the

product p, respectively, per mole of fuel. Note that Nr1 for the fuel, and

the other Nrand Npvalues can be picked directly from the balanced

combustion equation. Taking heat transfer tothe system and work done by

the system to be positivequantities, the energy balance relation just dis-

cussed can be expressed more compactly as

(15–10)

or as

(15–11)

where

If the enthalpy of combustion h

• °Cfor a particular reaction is available, the
  steady-flow energy equation per mole of fuel can be expressed as

(15–12)

The energy balance relations above are sometimes written without the work

term since most steady-flow combustion processes do not involve any work

interactions.

Q
WhC°aNp 1 h
h° (^2) p
aNr 1 h
h° (^2) r¬¬ 1 kJ>kmol 2
HreactaNr 1 h°fh
h° (^2) r¬¬ 1 kJ>kmol fuel 2
HprodaNp 1 h°fh
h° (^2) p¬¬ 1 kJ>kmol fuel 2
Q
WHprod
Hreact¬¬ 1 kJ>kmol fuel 2
Q
WaNp^1 h°fh
h°^2 paNr^1 h°fh
h°^2 r
QinWinaNr 1 h°fh
h° (^2) rQoutWoutaNp 1 h°fh
h° (^2) p
Q

inW

inan

r^1 h°fh
h°^2 rQ

outW

outan

p^1 h°fh
h°^2 p
Enthalpyh°f 1 h
h° 2 ¬¬ 1 kJ>kmol 2
766 | Thermodynamics
Enthalpy atEnthalpy at
2525 °C, 1 atmC, 1 atm
SensibleSensible
enthalpy relative enthalpy relative
to 25to 25 °C, 1 atmC, 1 atm
H H = = N(hf° ++ h h – h h °)
FIGURE 15–21
The enthalpy of a chemical component
at a specified state is the sum of the
enthalpy of the component at 25C, 1
atm (hf°), and the sensible enthalpy of
the component relative to 25C, 1 atm.
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