Ideal Flow Reactors - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 222

f Ae þ

17 ln 21 17f Ae

0:9 þ

17 ln 21 0 : 9 17

19
17f Ae

4 19

0:9 17

4

S R = A ¼ ¼ ¼ 0:73661

21 21

The product stream concentration can be computed as.

C Re ¼ S R = A n 1R

n 1 A ðC A 0 C A e Þ¼6:6295 mol

= L

C Qe ¼ð1 S R = A Þ n 2Q

n 2 A ðC A 0 C Ae Þ¼2:3705 mol

= L

As a comparison, here is how the three types of reactor arrangement will do to the final
product mixture for the particular problem:

PFR S R/A ¼ 0.19066, C Re ¼ 1.716 mol/L

CSTR S R/A ¼ 0.33333, C Re ¼ 3 mol/L

PFR with optimum feed S R/A ¼ 0.73661, C Re ¼ 6.6295 mol/L

The reactor feed strategy greatly affects the product mixture. PFR with optimum (distrib-
uted) feed yields significantly more desired product.

5.7.2. Reactive Distillation
Separation of one or more products from the reaction stream via a different phase is an
effective way of increasing the process efficiency. Figure 5.17 shows a sketch of a conceptual
reactive distillation tower, which looks identical to a tray tower when idealized.
Figure 5.17 a shows the overall flow schematic with the liquid reaction mixture being fed
from the tower top. The liquid is flowing down the column with a number of stopping stages
where catalysts present. While each particular case could have slightly different arrange-
ment, this conceptual column is general enough to describe most of the systems.
Performance analysis of the reactive distillation tower can be done in a manner similar to
what we have learned so far for reactor analysis. This is made clear by examining an indi-
vidual stage i as isolated in Fig. 5.17 b and c. This illustration shows that each stage may be
treated as a CSTR that is coupled with mass transfer to remove one or more volatile compo-
nents. Therefore, this idealization of the reactive distillation tower renders it to a CSTR train.
Mole balance of A in the liquid (or reactive) stream in the i -th stage yields:

Q L i1 C A i1 Q L i C A i þ r A i V i J A i V i a i ¼ 0 (5.81)

where a i is the specific liquid e vapor contacting area (area divided by total liquid volume) at
stage i , Q L i 1 is the volumetric flow rate of liquid stream flowing into the i -th stage, Q L i is the
volumetric flow rate of liquid stream flowing out of the i -th stage, C A i is the concentration of
A in liquid phase at stage i , r A i is the rate of formation of A in stage i , V i is the total volume of
liquid in stage i , and J A i is the mass transfer flux of A out of liquid phase in stage i . Mole
balance of A in the vapor stream leads to

F Viþ1 y A iþ1 F Vi y A i þ J A i V i a i ¼ 0 (5.82)