Page 164 - Materials Chemistry, Second Edition

Mass-Balance Concept and Reactor Design 147

Discussion:
1. The total volume of the reactor(s) for each of the four configura-
tions is 4 m .
3
2. The effluent concentrations from all the configurations exceed
the cleanup level. The configurations (a), (b), and (d) have the
same effluent concentrations because the residence times of all
reactors are identical. The effluent concentration from configura-
tion (c) is the worst among the four.
3. To split the flow into reactors in parallel with the same residence
time does not have any impact on the final effluent concentra-
tion, as shown in the cases (a), (b), and (d).

Example 4.23: PFRs in Parallel
Subsurface soil at a site is impacted by diesel fuel at a concentration of 1,800
mg/kg. Aboveground remediation, using slurry bioreactors, is proposed.
The treatment system is required to handle a slurry flow rate of 0.04 m /min.
3
The required final diesel concentration in the soil is 100 mg/kg. The reaction
is first-order with a rate constant 0.1/min, as determined from a bench-scale
study.
Four different configurations of slurry bioreactors in the PFR mode are
considered. Determine the final effluent concentration from each of these
arrangements and whether it meets the cleanup requirement:

(a) One 4-m reactor
3
(b) Two 2-m reactors in parallel (each receives 0.02 m /min flow)
3
3
(c) One 1-m reactor and one 3-m reactor in parallel (each receives 0.02
3
3
m /min flow)
3
(d) One 1-m reactor and one 3-m reactor in parallel (the smaller reac-
3
3
tor receives 0.01 m /min flow while the larger one receives 0.03 m /
3
3
min flow)
Solution:
(a) For the 4-m reactor,
3
the residence time = V/Q = 4 m /(0.04 m /min) = 100 min
3
3
Q, C in Q, C out
4 m 3