化学反应工程Chapter2

1 dNi moles i formed ri S dt ( surface) (time)
''
(3)
化学反应工程 Based on unit volume of solid in gas-solid systems,
m oles i form ed 1 dNi ri VS dt (volum eof solid ) (tim e)
C6 H12 O6 6O2 6 CO2 6H 2 O, - H r 2816 kJ
O2 come from air and CO2 come out from the process of human breathing. Find man’s metabolic rate (the rate of living, loving, and laughing) in terms of moles of oxygen used per m3 of person per second.
''''
(5)
化学反应工程
In homogeneous systems the volume of fluid in the reactor is often identical to the volume of reactor. V VR Eqs.1 and 5 are used interchangeably.
化学反应工程
Chapter 2 Kinetics of Homogeneous Reactions
化学反应工程
2.1 Definition of Reaction Rate
How to define the rate of a reaction?
-Select one reaction component i
Therefore, So from Eq. (i)
H2 used = 6 kmol/s; O2 used = 3 kmol/s and the rate of reaction is
rH 2 rO 2 6 km ol m oleused 1 4 2 . 829 10 s 0.2121 m3 (m 3 of rocket) s
side by side
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Multiple reactions
A more complicated example is
ABR R BS
Here, reaction proceeds in parallel with respect to B, but in series with respect to A, R, and S.
In heterogeneous systems all the above definitions of reaction rate are encountered, the definition used in any particular situation often being a matter of convenience.
化学反应工程
化学反应工程 2.2 CONCENTRATION-DEPENDENT TERM OF A RATE EQUATION Experience shows that the rate of reaction is influenced by the composition and temperature, thus we can write
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SOLUTION
We want to evaluate
rH 2 1 dN H 2 V dt and rO 2 1 dNO 2 V dt
Let us evaluate terms. The reactor volume and the volume in which reaction takes place are identical. Thus,
V

4
(0.6) 2 (0.75) 0.2121m 3
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Next, let us look at the reaction occurring,
H2 1 O2 H 2O 2
molar stoichiometry molecular weight:
1 0.5 1 2gm 16gm 18gm
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Single and Multiple Reactions
When a single stoichiometric equation and single rate equation are chosen to represent the progress of the reaction, we have a single reaction.
or
V ri W ri' S ri'' VS ri''' Vr ri''''
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EXAMPLE 2.1 THE ROCKET ENGINE
A rocket engine, Fig. E1.1, burns a stoichiometric mixture of fuel (liquid hydrogen) in oxidant (liquid oxygen). The combustion chamber is cylindrical, 75 cm long and 60 cm in diameter, and the combustion process produces 108 kg/s of exhaust gases (H2O). If combustion is complete, find the rate of reaction of hydrogen and of oxygen.
化学反应工程
Next, noting that each mole of glucose consumed uses 6 moles of oxygen and releases 2816 kJ of energy. We see that we need
dNO 2 6 moleO 2 moleO 2 6000kJ / day 12.8 dt 2816kJ / mole glu cose 1 mole glu cose day
m oles i form ed 1 dNi ri W dt (m ass of solid )(tim e)
'
(2)
Based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gassolid systems,
化学反应工程
Elementary and Nonelementary Reactions
Consider a single reaction with stoichiometric equation
ABR
If we postulate(假定) that the rate-controlling mechanism involves the collision or interaction of a single molecule of A with a single molecule of B, then at a given temperature the number of collisions is proportional to the concentration of reactants in the mixture; hence, the rate of disappearance of A is given by
rA kCA CB
化学反应工程 Such reactions in which the rate equation corresponds to a stoichiometric equation are called elementary reactions. When there is no direct correspondence between stoichiometry and rate, then we have nonelementary reactions.
e concent rat ion t emperat ur rA f dependent , dependent t erm s t erms
Hold temperature constant to investigate concentrationdependent term Different types of reactions: Single reaction vs. multiple reactions.
-Based on unit volume of reacting fluid,
m oles i form ed 1 dNi ri V dt (volum eof fluid)(tim e)
(1)
化学反应工程 Based on unit mass of solid in fluid-solid systems,
'''
(4)
Based on unit volume of reactor, if different from the rate based on unit volume of fluid,
moles i formed 1 dNi ri Vr dt (volum eof reactor) (tim e)
When more than one stoichiometric equation is chosen to represent the observed changes, then more than one kinetic expression is needed to follow the changing composition of all the reaction components, and we have multiple reactions.
3 km ol m oleused 1 4 1.4150 10 3 s 0.2121 m (m 3 of rocket) s
化学反应工程
EXAMPLE 2.2 THE LIVING PERSON
A human being (75kg) consumes about 6000 kJ of food per day. Assume that the food is all glucose and that the overall reaction is
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Multiple reactions may be classified as:
series reactions,
A R S
parallel reactions, which are of two types R A S competitive and
Biblioteka Baidu
A R BS
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Relations between different rate definitions:
volume mass ' surface '' volume ''' volume '''' of fluid ri of solid ri of solid ri of solid ri of reactor ri
Inserting into Eg. (i)
12.8 moleO 2 used moleO 2 used 1 day 1 rO2 0.002 3 day 24 3600 s 0.075 m m3 s
'''
Note: compare this value with those listed in Figure 1.3.