first law analysis for reacting system consider a constant pressure process in which n f moles of...
Post on 29-Dec-2015
216 Views
Preview:
TRANSCRIPT
First Law Analysis for Reacting System
Consider a constant pressure process in which nf moles of fuel react withna moles of air to produce np moles of product:
PnAnFn paf
Applying First Law with state 1 being the reactants at P1, T1 and state 2being products at P2, T2:
Reactants Products
ReactantsProducts
Reaction
State 1 State 2
Q
W
)()( 121221 VVPUUQ
WUQ
RRii
PpiiRP ThnThnHH
HH
VPUVPU
)()(
)()(
12
111222
HP < HR Q < 0 exothermic reaction
HP > HR Q > 0 endothermic reaction
)()( 1212 VVPUUQ
First Law Analysis for Reacting System
Enthalpy of Reaction
Consider the case where the final temperature of the products is the same as the initial temperature of the reactants (e.g., calorimeter is used to measure Q).
ReactionQ
W
P1=P2=Po
T1=T2=To
The heat released under this situation is referred to as the enthalpy of reaction, HR ,
fuel of kmolor kgper kJ :units )()(
)()(
Roii
Poii
RRii
PpiiR
ThnThn
ThnThnH
To
The maximum amount of energy is released from a fuel when reacted with astoichiometric amount of air and all the hydrogen and carbon contained in thefuel is converted to CO2 and H2O
This maximum energy is referred to as the heat of combustion or the heating value and it is typically given per mass of fuel
Heat of Combustion
22222 476.3
2)76.3(
4NOHCONOHC
HR(298K)
Heat of Combustion
There are two possible values for the heat of combustion that can be calculated depending on whether the water in the products is taken to be in a liquid or vapour state.
Tp
T
S
hghf
From steam tables: hfg = hg – hf > 0
HR = HP – HR < 0 (exothermic)
The term higher heat of combustion is used when the water in the products is taken to be in the liquid state
The term lower heat of combustion is used when the water in the productsis taken to be in the vapour state
Heat of FormationConsider the following reactions taking place at atmospheric pressure andwith TP = TR = 298K
222
2222
/ 000,394 )()()(
/ 000,286 )()()(2/1
COkmolkJQgCOgOsC
OHkmolkJQlOHgHgO
In these reactions H2O and CO2 are formed from their elements in theirnatural state at standard temperature and pressure (STP) 1 atm and 298K.
Reactions of this type are called formation reactions and the corresponding measured heat release Q is referred to as the standard heat of formationand takes the symbol so:
ofh
kmolkJh
kmolkJh
C
O
/ 000,394
/ 000,286
oO f,
oH f,
2
2
Values for standard heat of formation for different species are tabulated
Enthalpy Scale for a Reacting System
By international convention, the enthalpy of every element in its natural state(e.g., O2(g), N2(g), H2(g), C(s)) at STP has been set to zero
0)298,1( ohKatmh
We need to take into account that for a reacting system the working fluid changes molecularly from reactants to products while undergoing a process.
Consider the following identity:
)]298,1(),([)298,1(),( KatmhTPhKatmhTPh
(note the notation convention)
at STP
The enthalpy of all other substances at STP is simply the heat of formation of the substance, since it is formed from its elements, for example:
)]298,1(),([ ),( , KatmhTPhhTPh iio
ifi
sensible enthalpychemical enthalpy TK ip dTc298 ,
Therefore, the enthalpy of the i’th component in a mixture is:
Enthalpy Scale for a Reacting System
)(,)(
)(,)()()(
222
22
2222
2/1 @
)()()(2/1
lOHflOH
lOHfgHgOlOH
hh
hhhhQSTPrecall
lOHgHgO
The sensible enthalpy for different species is tabulated as a function of temperature (JANNAF Tables).
top related