Transcript lec7_printx
Lecture 7
Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place.
2
Today’s lecture
Block 1: Mole Balances Block 2: Rate Laws Block 3: Stoichiometry Block 4: Combine California Professional Engineers Exam
Exam is not curved, 75% or better to pass Problem 4-12
3
4
General Guidelines for California Problems
Some hints:
1. Group unknown parameters/values on the same side of the equation example: [unknowns] = [knowns] 2. Look for a Case 1 and a Case 2 (usually two data points) to make intermediate calculations 3. Take ratios of Case 1 and Case 2 to cancel as many unknowns as possible 4. Carry all symbols to the end of the manipulation before evaluating, UNLESS THEY ARE ZERO
5
Gas Phase PFR
The irreversible elementary reaction takes place in the gas phase in an isothermal tubular (plug-flow) reactor. Reactant A and a diluent C are fed in equimolar ratio, and conversion of A is 80%. If the molar feed rate of A is cut in half, what is the conversion of A assuming that the feed rate of C is left unchanged? Assume ideal behavior and that the reactor temperature remains unchanged.
[From California Professional Engineering/Engineers Exam.]
F A 01 ; F I 0 X 0 .
8 2
A
B
F A 02 0 .
5 F A 01 ; F I 0 X ?
Unknown: V , k , T , P , C A 0 , F A 0 , F I 0 , V 0
6
Gas Phase PFR Will the conversion increase or decrease?
INCREASE: Slower Volumetric Rate (Reactants spend more time in the reactor DECREASE: Concentration of Reactant Diluted
7
Gas Phase PFR
Assumptions: T T 0 , P P 0 , V 1 V 2 , k 1 k 2 , P 1 P 2 , C T 0 C T 02 1) Mole Balance: dX dV r A F A 0 2) Rate Law: r A kC 2 A 3) Stoichiometry: (gas phase) 0 1 X ; T T 0 ; P P 0 A ½ B
8
Gas Phase PFR
1 2 1 1 2 C A C A 1 0 1 X X 4) Combine: r A kC 2 A 0 1 1 X X 2 dX dV V kC 2 A 0 F A 0 1 1 X X 2 F A 0 kC 2 A 0 0 X 1 1 X X 2 dX
9
Gas Phase PFR
kC 2 A 0 V F A 0 2 1 1 X 2 X 1 2 X 1 X
Case 1:
kC 2 A 01 V 2 .
9 F A 01
Case 2:
kC 2 A 02 V RHS F A 02 Take ratio of Case 2 to Case 1
10
Gas Phase PFR
Most make this assumption to keep pressure the same, C T02 =C T01
kC
2
A
02
V F A
02
kC
2
A
01
V F A
01
RHS
2.9
C A
02
C A
01 2
F A
01
F A
02
y A
02
y A
01
C T
02
C T
01 2 1
F A
01
F A
02
y A
01 1 2
y A
02 1 3
F A
02
F A
01 1 2
11
Gas Phase PFR
1 3 1 2 2 1 1 2
RHS
2.9
8 9
RHS
2
y A
02 1 6 8 9 2 1 6 1 1 6
X
2 1 6 2
X
2 1 1 2 6 1
X
2
X
2 Solve with computer, X=0.758
12 Heat Effects Isothermal Design Stoichiometry Rate Laws Mole Balance
13