Liquid purification or separation – Filter – Material
Reexamination Certificate
2001-02-09
2003-08-05
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500390, C210S500270, C210S640000, C096S013000, C095S045000, C095S054000
Reexamination Certificate
active
06602415
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to polymeric membranes. Specifically, rigid polymeric membranes that go through a selectivity maximum as a function of copolymer composition and/or operating conditions, such as elevated temperature and/or feed pressure are described.
II. Brief Description of the Prior Art
The separation of one or more gases from a multicomponent mixture of gases is necessary in a large number of industries. Such separations currently are undertaken commercially by processes such as cryogenics, pressure swing adsorption, and membrane separations. In certain types of gas separations, membrane separations have been found to be economically more viable than other processes.
In a pressure-driven gas membrane separation process, one side of the gas separation membrane is contacted with a multicomponent gas mixture. Certain of the gases of the mixture permeate through the membrane faster than the other gases. Gas separation membranes thereby allow some gases to permeate through them while serving as a relative barrier to other gases. The relative gas permeation rate through the membrane is a property of the membrane material composition and its morphology.
It has been suggested in the prior art that the intrinsic permeability of a polymer membrane is a function of both gas diffusion through the membrane, controlled in part by the packing and molecular free volume of the material, and gas solubility within the material. Selectivity may be determined by the ratio of the permeabilities of two gases being separated by a material.
Transport of gases in polymers and molecular sieve materials occurs via a well known sorption-diffusion mechanism. The permeability coefficient (P
A
) of a particular gas is the flux (N
A
) normalized to the pressure difference across the membrane (&Dgr;p
A
), and the membrane thickness (l).
P
A
=
N
A
l
Δ
p
A
(
1
)
The permeability coefficient of a particular penetrant gas is also equal to the product of the diffusion coefficient (D
A
) and the solubility coefficient (S
A
).
P
A
=D
A
S
A
(2)
The permselectivity (&agr;
A/B
) of a membrane material (also ideal selectivity) is the ratio of the permeability coefficients of a penetrant pair for the case where the downstream pressure is negligible relative to the upstream feed pressure. Substituting equation (2), the ideal permselectivity is also a product of the diffusivity selectivity and solubility selectivity of the particular gas pair.
α
A
/
B
=
P
A
P
B
=
D
A
D
B
·
S
A
S
B
(
3
)
The variation of gas permeability with pressure in glassy polymers is often represented by the dual mode model. Petropulos (1970); Vieth, et al. (1976); Koros, et al. (1977). The model accounts for the differences in gas transport properties in an idealized Henry's law and Langmuir domains of a glassy polymer,
P
=
k
D
D
D
+
C
H
′
D
H
b
1
+
bp
(
4
)
where k
D
is the Henry's law constant, C′
H
is the Langmuir capacity constant, p is pressure, and b is the Langmuir affinity constant. This model can be further extended to mixed gas permeability:
P
A
=
k
DA
D
DA
+
C
HA
′
b
A
D
HA
1
+
b
A
p
A
+
b
B
p
B
(
5
)
where p
A
and p
B
are the partial pressures of gasses A and B respectively. This model is valid for a binary gas mixture of components A and B, and it only accounts for competitive sorption.
The temperature dependence of permeability for a given set of feed partial pressures is typically represented by an Arrhenius relationship:
P
=
P
o
exp
[
-
E
p
RT
]
(
6
)
where P
o
is a pre-exponential factor, E
p
is the apparent activation energy for permeation, T is the temperature of permeation in Kelvin, and R is the universal gas constant. The permeability can further be broken up into temperature dependent diffusion and sorption coefficients from equation (2). The temperature dependence of the penetrant diffusion coefficient can also be represented by an Arrhenius relationship:
D
=
D
o
exp
[
-
E
d
RT
]
(
7
)
Again D
o
is a pre-exponential factor, and E
d
is the activation energy for diffusion. The activation energy for diffusion represents the energy required for a penetrant to diffuse or “jump” from one equilibrium site within the matrix to another equilibrium site. The activation energy is related to the size of the penetrant, the rigidity of the polymer chain, as well as polymeric chain packing. The temperature dependence of sorption in polymers may be described using a thermodynamic van't Hoff expression:
S
=
S
o
exp
[
-
H
s
RT
]
(
8
)
where S
o
is a pre-exponential factor, and H
s
is the apparent heat of sorption as it combines the temperature dependence of sorption in both the Henry's law and Langmuir regions.
From transition state theory the pre-exponential for diffusion can be represented by
D
o
=
e
λ
2
kT
h
exp
[
S
d
R
]
(
9
)
Here, S
d
is the activation entropy, &lgr; is the diffusive jump length, k is Boltzmann's constant, and h is Planck's constant. Substituting (9) into (3) (neglecting small differences in the jump length of similarly sized penetrants) results in the diffusive selectivity as the product of energetic and entropic terms:
D
A
D
B
=
exp
[
-
Δ
E
d
,
A
,
B
RT
]
exp
[
Δ
S
d
,
A
,
B
R
]
(
10
)
The diffusivity selectivity is determined by the ability of the polymer to discriminate between the penetrants on the basis of their sizes and shapes, and is governed primarily by intrasegmental motions and intersegmental packing. The diffusive selectivity will be based on both the difference in activation energy for both penetrants, &Dgr;E
d
, as well as the difference in activation entropy for both penetrants, &Dgr;S
d
.
Significant increases in diffusivity and diffusivity selectivity can be obtained in conventional polymers by simultaneously inhibiting intrasegmental motions and intersegmental chain packing. These results can be summarized as two principles for tailoring membrane materials:
1. Structural moieties which inhibit chain packing while simultaneously inhibiting torsional motion about flexible linkages on the polymer backbone tend to increase permeability while maintaining permselectivity;
2. Structural moieties which decrease the concentration of mobile linkages in the polymer backbone and do not significantly change intersegmental packing tend to increase permselectivity without decreasing permeability significantly.
The ratio of specific free volume to polymer specific volume, the fractional free volume, is representative of the degree of openness of the matrix. This index takes into account the filling of space by bulky side groups, but is not experimentally determined. The specific free volume is typically estimated by a group contribution method such as that of Bondi (1968) or Van Krevelen et al. (1976). The polymer specific volume is determined by dividing the molecular weight of the repeat unit by the bulk polymer density. The fractional free volume gives a measure of the degree of openness of the polymeric matrix. A relatively high fractional free volume is indicative of an open matrix, while a relatively low fractional free volume indicates a closed matrix. Materials with larger free fractional volumes are expected to have greater diffusivities (and sorption coefficients) and thus greater permeabilities than materials with smaller fractional free volumes.
Much of the work in the field has been directed to developing membranes that optimize the separation factor and total flux of a given system. It is disclosed in U.S. Pat. No. 4,717,394 to Hayes that aromatic polyimides containing the residue of alkylated aromatic diamines are useful in separating a variety of gases. Moreover, it has been reported in the literature that other polyimides, polycarbonates, polyurethanes, polysulfones and polyphenyleneoxides are useful for like purposes. U.S. P
Burns Ryan L.
Koros William J.
Board of Regents The University of Texas
Fortuna Ana
Meyertons Eric B.
Meyertons Hood Kivlin Kowert & Goetzel P.C.
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