Biomedical Engineering Reference
In-Depth Information
One example of interest is pulping or conversion of plant biomass into fibers via chemical
processes. There are at least three groups of substances in the woody biomass: hemicellu-
loses, lignin, and cellulose. All these substances react with sodium hydroxide at high temper-
atures, with different rates. The concentration of alkali is the key factor in cellulose/
hemicellulose degradation, which occurs while lignin is reacting with caustic to degrade
into small molecules and become soluble in aqueous solution. Modern kraft pulping
digesters are built with distributed white liquor feed as well as black liquor removal capabil-
ities. The aim is to maintain constant alkali concentration (a key reactant for pulping) and
degraded product concentration.
To illustrate how a distributed feed can affect the outcome of the reaction, we use a parallel
reaction network as example:
A
þ
B
/
C
þ
D
r
1
¼ k
1
C
A
C
B
(5.78)
r
2
¼ k
2
C
A
C
B
A
þ
B
/
E
þ
F
(5.79)
If product E is the desired product, we can write the differential selectivity of E as
k
2
C
A
C
B
k
1
C
A
C
B
k
2
C
A
C
B
d
F
A
j
due to the formation of E
d
F
A
j
total
r
2
A
r
1
A
þ r
2
A
¼
s
E
=
A
¼
¼
(5.80)
k
2
C
m1
B
k
1
þ k
2
C
m1
¼
B
Therefore, the level of concentration of B in the reactor affects the selectivity to the forma-
tion of product E. Either high or low concentration of B will change the product mix if
m
is
not exactly unity.
Figure 5.16
shows three possible selected reactor feeding schemes.
Figure 5.16
a will give the highest possible concentrations of both A and B in the reactor.
Figure 5.16
b is a case where the concentration of B is maximum in the reactor, while the
concentration A is maintained at minimum (the same as when the reaction is complete,
i.e. the outlet).
Figure 5.16
c is a case where the concentration of A is maximum in the
reactor, while the concentration B is maintained at minimum (the same as when the reac-
tion is complete, i.e. the outlet). If a CSTR was chosen, we would have the lowest concen-
trations of both A and B in the reactor, which would be equivalent to those in the reactor
outlet.
Example 5-7. Product distribution for parallel reactions.
We wish to produce R in the following reaction:
k
1
C
A
C
1=2
B
1 þ K
A
C
A
þ K
B
C
1=2
A
þ
B
/
R
;
r
1
¼
(E5-7.1)
B
Unfortunately a side reaction occurs, where product P is formed
k
2
C
B
1 þ K
A
C
A
þ K
B
C
1=2
A
þ
B
/
P
;
r
2
¼
(E5-7.2)
B
Search WWH ::
Custom Search