Chemistry Reference
In-Depth Information
Fig. 8 Model of the improved
CPF column.
SL
sol,
GL
gel
SL
m = 1
condenser
m = 2
reflux
of phase”
m = m
FD
FD
m = m
max
-1
m = m
max
GL
EA
translucent, or opaque [
89
]. The CPF allows fractionation according to the chemical
composition and according to the molecular weight and, hence, the tailoring of the
copolymer for a certain application purpose.
R
¨
tzsch et al. [
48
,
49
] proposed a theoretical treatment of the CPF similar to that
of the BW fractionation. The CPF column is divided into a number of stages
m
(Fig.
8
). The EA enters the column at stage
m
¼
m
Max
and leaves it as sol at stage
m
¼
1. The stationary state is calculated by repeating calculation of stages
m
¼
1,
...
,
m
Max
. At start (
i
¼
0), the column is filled with EA. For the first set
of equilibria (
i
¼
1), a certain amount of feed (FD) is added at stage
m
¼
1, and the
related phase II is transferred downwards to the next stage
m
2, filled with EA.
When phase II has left the column at
m
Max
as the first, nonstationary gel phase, all
phases I are shifted by one stage upwards, and stage
m
Max
is filled with pure EA
again. The calculation is repeated for
i
¼
¼
2 and
m
¼
1,
,
m
Max
etc. The stationary
...
state is reached when the results for
i
and
i
1 no longer change systematically.
Using this simulation procedure, some improvements could be suggested [
48
].
The pulsating sieve-bottom column was replaced by a non-pulsating column filled
with glass beads. In this manner, the number of theoretical plates could be raised
considerably. A further improvement of the fractionation efficiency results from the
reflux of part of the polymer contained in the sol phase. In practice, this situation
was realized by putting a condenser on the top of the column and introducing the
feed somewhere near of the upper third of the column (Fig.
8
). These suggestions
were verified experimentally using the system dichloromethane/diethylene glycol/
bisphenol-A polycarbonate [
48
]. Except for the lowest molecular weight fraction,
one obtains nonuniformities on the order of 0.1.
