STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
GAS PERMEATION TROUGH POLYAMIDES AND POLYIMIDES
POLYMERS MEMBRANES
T.D. Silipas1, V. Tosa1, Dana Garganciuc2, Gh. Batrinescu2, Gabriela Roman2,
B. Albu2
1
National Institute for R&D of Isotopic and Molecular
Technologies, P.O. Box 700, Cluj-Napoca, Romania
2
'CCMMM, CP 15-43, 206 Splaiul Independentei,
Bucarest, Romania
ABSTRACT. We measured the permeation rate and the separation factor
for different gases. After finding the permeation rates, a corelation
between the obtained data and the type of polymer and method used for
membrane preparation, has been performed. The results have been
compared to an asymmetric ACA membrane produced by SEPAREX.
Membrane based gas separation processes [1], over the last three decades,
have proved their potential as better alternatives to traditional separation processes.
The conventional processes, for example absorption, cryogenic distillation, and
pressure swing adsorption (PSA) are energy intensive, as well as responsible for
some environmental pollution. An illustrative cost comparison of an ethyl cellulose
membrane system with a standard PSA approach for 35% oxygen enrichment of air
shows a reduction of 47% in capital costs, and 38% in operating costs for the
membrane based process. The membrane based separation processes are not only
cost effective and environmentally friendly, but also, with many novel polymeric
materials available, offer much more versatility and simplicity in customized
system designs.
Gas transport properties of polymer membranes [2] are not only important
to industrial production of high purity gases, but also plays a role in the application
of membranes as barrier materials for food packaging and beverage industry. It is
therefore important to know the permeation rates of atmospheric gases through
these membranes.
The polyamides and polyimides membranes were prepared at CCMMM
Bucharest by using various conditions, e.g. with or without support, having a
symmetric or asymmetric structure. The aim was to explore and find the best
conditions for the gas separation process. To characterize the polymer membranes
to permeation we used the manometric method as specified in ASTM D-1434/82.
We used an experimental set-up designed and built [3] in the NIRDIMT Cluj. In
principle we measured the increase of pressure in a given volume, as a result of gas
permeation through the membrane.
In order to have an efficient separation process, when compared to other
separation methods,
the membrane must fulfill simultaneously the two
T.D. SILIPAS
performance criteria, namely high flux and high selectivity for the gas in question.
Once these criteria are fulfilled the membrane must have good chemical stability
with respect to the gases involved and good mechanical properties in order to be
proof against as high as possible pressure gradients between the two sides of the
membrane. The requirement to have the above conditions fulfilled simultaneously
restrict in a considerable manner the area of the available methods to prepare the
membrane.
The measurement of the membrane permeability for various pure gases
means a characterization of its performance in terms of flux and selectivity. This
measurement is in fact a quality test, which is done immediately after membrane
preparation. That is why it is recommended to test the membrane for at least one
"fast" gas, such as H2 or He, and a "slow" gas, N2 usually. If the permeation is big
for He or H2, typical values being in the range 5*10-5 cm3/cm2.sec.cmHg, and the
selectivity for H2-N2 is also big, say above 25, then the membrane can be accepted
as good, and can be considered as a starting point for the scaling-up in a separation
module.
The membrane we prepared are in the class of heterocyclic polymers, the
choice of this class confers solubility in organic solvent, that is, a higher and easier
processing. We used two different kinds of thermostable heterocyclic polymers:
florurated polyamides and polyimides. We obtained membranes with a good
mechanical resistance, uniform in appearance. We could divide the prepared
membranes in 6 categories: asymmetric membranes with and without support, with
adjuvant, obtained by reticulation, composites and obtained using controlled
evaporation (symmetric membranes).
Table 1Permeation flux Q at 50oC through
The SEPAREX asymmetric membrane (ACA)
He
CO2
O2
N2
At.
mass
(uam)
4.06
44.0
32.0
28.0
GAS
Boiling
temp.
(K)
4.25
194.6
90.2
77.0
Q
Mol.
diameter
(Å)
2.58
3.99
3.43
3.68
(cm3/cm2*sec.cmHg)
10-5
19.2
6.9
1.4
0.29
To measure the flux of gas through the membrane we used the manometric
method, as described in ASTM D-1434/82, under the regime of stationary
permeation, operating on a home made experimental set-up. We used four gases
He, N2, O2 of purity 99,5 %, and CO2 of purity better than 99%. The area of the
membrane varied between 2.02 and 2.8 cm2, and the working pressure on one side
GAS PERMEATION TROUGH POLYAMIDES AND POLYIMIDES POLYMERS MEMBRANES
of the membrane was, depending on the type of membrane, in the range from 120
Torr to atmospheric pressure.
We considered useful to compare our membrane to a commercial one,
already used for gas separations. In this sense we also measured an asymmetric
membrane prepared from cellulose acetate, produced by SEPAREX (Table 1).
We analyzed and measured a number of over 20 membranes, the results
being synthesized as follows:
For polyamides membranes, depending on membrane type, the permeation
flux is two orders of magnitude higher for asymmetric membranes than for the
symmetric ones. Even for the asymmetric membranes the separation factor is
slightly diminished, these are superior to symmetric membranes (see Fig. 1)
6
He
CO2
O2
N2
14
13
12
11
10
9
2
7
0
5
M28 simetrica
gaze pure
-6
-6
2
8
1
He
CO2
O2
N2
15
3
9
Q*10 -6[cm 3/cm 2 sec cmHg]
10
3
Q*10 [cm /cm sec cmHg]
11
Q*10 [cm /cm sec cmHg]
He
CO2
O2
N2
12
4
8
7
6
5
3
4
2
3
2
1
1
0
M8 asymmetric
gases
Figure 1 Permeation flux Q for a
symmetric and an asymmetric
0
M20 with adjuvants
M8 whithout adjuvant
gases
Figure 2 Permeation flux Q for a
membrane with and without adjuvants
membrane
The presence of adjuvants in the membrane structure is beneficial for the
polyamide membrane, the values for the permeation flux and separation factor are
enhanced (see Fig. 2).
For polyimide membrane the values for the permeation flux are in the
range from 4,56 to 7,23 *10-6 cm3/cm2.s.cmHg, and the selectivities in the range
from 54,33 to 73,55. These values indicate that the permeation is the result of a
solving-diffusion process, not a transport process through a porous membrane. This
is a first positive result and it demonstrates that the preparation procedure was
correct. The differences between the above values could be attributed to different
preparation process. The influence in the case of using a support is not significant,
the values obtained for the flux and separation factor being in the range of
experimental errors. We illustrate in Fig. 3 and Fig. 4 a comparison between the
T.D. SILIPAS
60
60
3
40
-6
30
20
50
Factor de separare
He
CO2
O2
N2
50
2
Q*10 [cm /cm sec cmHg]
70
40
He/N2
O2/N2
30
20
10
10
0
0
M2 polyimide
M2 poliimida
M8 polyamide
M8 poliamida
gases
gases
Figure 3. Permeation flux Q through Figure 4. Separation factors through
polyamide and polyimide membranes
polyamide and polyimide membranes
polyamide and polyimide membranes in terms of flux and separation factor
respectively. The polyimide membaranes, as compared to the polyamide
membranes have permeation flux 5 times higher and a separation factor almost 7
times higher (see membrane 4, for example), for a mixture He-H2, which indicate
clearly their superiority.
References
[1] S. Ale x a nd er St er n , Journal of Membrane Science, 94(1994),1-65.
[2] E. Sad a, H. K u maz a wa, P . X u, S. T . W an g , J. Polymer Sciences:part B:
Polymers Physics, 28, (1990), 113-125
[3] A. B ar b u, T . D. Si lip as, I . B ra t u , Journal of Molecular Structure, 410-411,
(1997), 233-236.