Chapter 6 Self Test
Practice 1. What Type of Reaction is Taking Place?
Three species were found in a CSTR. The following concentration
data were obtained as a function of temperature. The initial
concentration of the single reactant, A, was the same at all
temperatures. Both B and C are products.
CA0 = 2moles/dm3 Run T (oC) CA (mole/dm3) CB (mole/dm3) CC
(mole/dm3)
1 30 1.7 0.01 0.29
2 50 1.4 0.03 0.57
3 70 1.0 0.1 0.90
4 100 0.5 1.25 1.25
5 120 0.1 1.80 0.1
6 130 0.01 1.98 0.01
Explain the above data, what type of reaction is taking place,
independent, complex, series or parallel?
Solution
Observations At low temperatures 1) Little conversion of A 2)
Little B formed 3) Mostly C formed (but not too much because of the
low conversion 15 to 30% - of A) At high temperatures 1) Virtually
complete conversion of A 2) Mostly B formed Data suggest 2
reactions
Reaction (1) is dominant at high temperatures
with
Reaction (2) is dominant at low temperatures
Practice 2 Maximizing the Selectivity - Parallel Reactions
Determine the instantaneous selectivity, SD/U, for the liquid
phase reactions:
Sketch the selectivity as a function of the concentration of A.
Is there an optimum and if so what is it?
Solution
Finding the optimum concentration:
Use CSTR with exit concentration C*A
Practice 3 Finding the SelectivityFor the elementary
reactions:
with k1=.1 s-1 and k2=.2 s-1 with CAO= 2 mol/dm3. Plot the
concentration of B and selectivity of B to C as a function of space
time in a CSTR.
Solution
CSTR A. Balance on Species A
B. Balance on Species B
C. Balance on Species C
The space time,, is the minimum value of V (i.e.) at which there
is an acceptable molar flow rate of the desired product B from the
CSTR.
Practice 4 Concentration-Time Trajectories1). The following
concentration-time trajectories were observed in a batch
reactor
Which of the following reaction pathways best describes the
data:
Answer
Choice B is the answer. Choices A and C are incorrect because
they show species B eventually consumed, which is clearly not the
case.2). Sketch the concentration-time trajectory for the
reaction:
Answer
(1) B virtually consumed so no more D can be produced in
reaction 2. (2) Rates of Consumption of A and Ba are virtually the
same. (3) Rate of consumption of B greater than that of A owing to
Reaction 2. Below is the polymath code typed in to produce the
graph above.
Practice 5 Writing Net Rates of FormationThe reactions:
are elementary. Write the net rates of formation for A, B, C and
D.
SolutionA.
B.
C.
D.
These net rates of reaction are now coupled with the appropriate
mole balance of A, B, C, and D and solved using a numerical
software package.For example for a PFR:
Problem 6 Write F~v relationships
Before carrying out any calculations, make a sketch as to what
you expect the molar flow rate profiles to look like
Solution
A is consumed in reactions 1 and 2, so FA will steadily
decrease
B is consumed in reaction 1, and once A is consumed, it will no
longer be able to react and the flow rate will remain constant
C is produced in reaction 1, consumed in reaction 2, and
therefore the molar flow rate of C will go through a maximum will
also no longer be consumed once A is used up
D will be produced once C is formed and will continue to
increase until A is consumed
You may want to solve for these profiles quantitatively by using
Polymath.
Problem 7 Correct the wrong solutions for a given
problemProblemConsider the elementary gas phase reactions
Set up the equations to calculate the concentration of each
species as a function of time in a constant volume batch reactor.
The reaction is carried out isothermally.
Solution A combined mole balance(1) and rate laws(2)
(3)Stoichiometry
at t = 0 then NA = NA0 and These equations will be typed into
Polymath. What five things are wrong with each of the first three
steps of the multiple reaction algorithm?
Answer(1)Mole Balance: V missing on right hand side of each of
the combined mole balance and rate law equations (1) through (4).
(2)Stoiciometry: The gas reaction occurs in a constant volume rigid
container, V = V0. Therefore the equations [5) and (6)] for the
concentration of the reacting species are not correct
(3)Rate laws, relative rates, and net rates: The net rate of
reaction is not correct for B,C, and D, i.e., equations (2), (3),
and (4).
Correct Solutions Since V = V0
(1) A 2B (2) 2B A (3) 2B 3C (4) 2B D Mole Balance (1) (2) (3)
(4) Net Rates and Rate Laws Net Rates Species A
Species B
Species C
Species D
Summary
StoichiometryThe pressure inside the vessel is 1
