Page 287 - Modeling of Chemical Kinetics and Reactor Design

Industrial and Laboratory Reactors 257

Table 4-6
Design guidelines for reactors
1. Single irreversible reactions (not autocatalytic)

(A) Isothermal—always use a plug flow reactor
(B) Adiabatic
1. Plug flow if the reaction rate monotonically decreases with conversion
2. CFSTR operating at the maximum reaction rate followed by a plug flow
section
2. Single reversible reactions—adiabatic
(A) Maximum temperature—adiabatic
(B) A series of adiabatic beds with a decreasing temperature profile if exothermic
3. Parallel reactions-composition effects
(A) For A → R (desired) and A → S (waste), where the ratio of the reaction
k k C )
a
rates is r R r =( 1 2 a − 2
1
S
A
1. If a > a , keep C high
1
A
2
a. Use batch or plug flow
b. High pressure, eliminate inerts
c. Avoid recycle of products
d. Can use a small reactor
2. If a < a , keep C low
A
1
2
a. Use a CFSTR with a high conversion
b. Large recycle of products
c. Low pressure, add inerts
(B) For A + B → R (desired) and A + B → S (waste), where the ratio of the
k k C )
a
b
1
1
rates is r R r =( 1 2 a − 2 C b − 2
B
S
A
1. If a > a and b > b , both C and C high
1
2
A
2
B
1
2. If a < a and b > b , then C low, C high
2
B
1
A
1
2
3. If a > a and b < b , then C high, C low
2
1
1
A
B
2
4. If a < a and b < b , both C and C low
B
2
2
1
1
A
5. See Figure 4-26 for various reactor configuration
4. Consecutive reactions-composition effects.
(A) A → R (desired) and R → S (waste)—minimize the mixing of streams with
different compositions.
5. Parallel reactions-temperature effects r /r = (k /k )f(C , C )
R S 1 2 A B
(A) If E > E , use a high temperature
2
1
(B) If E < E , use an increasing temperature profile
1
2
6. Consecutive reactions-temperature effects A → R  → S

k
k 1
2
(A) If E > E , use a decreasing temperature profile—not very sensitive
2
1
(B) If E < E , use a low temperature
1
2
Source: J. M. Douglas, Conceptual Design of Chemical Processes, McGraw-Hill, Inc., 1988.