Multi-layer membrane and the use thereof for the separation of liquid mixtures according to the pervaporation process

A multi-layer membrane having a porous backing layer of polyacrylonitrile, polysulfone or the like, and an active separating layer of polyvinyl alcohol or cellulose acetate. The membrane is particularly suitable for the separation of water-alcohol mixtures according to the pervaporation process.

BACKGROUND OF THE INVENTION 
It is known in the art that liquid mixtures can be separated in that the 
liquid mixture is placed into contact with one side of a suitable 
polymeric membrane while a vacuum is applied to the other side of the 
polymeric membrane or an inert gas stream is guided past the same. Some of 
the components of the liquid mixture that permeate more rapidly through 
the polymeric membrane are continuously removed in vaporized condition 
from the side of the polymeric membrane in communication with the gaseous 
phase either by evacuation or by the inert gas stream. Non-permeating 
components are retained in the liquid phase accumulating therein until the 
desired degree of separation of components of a rapid and of a poor 
permeation and the desired degree of purity of the retained components of 
an inferior permeation, respectively, has been attained. 
What is particularly noteworthy of this process, called liquid permeation 
or pervaporation and used for the separation of gas mixtures by means of 
membranes, is the fact that it also permits decomposition of such liquid 
mixtures into their components that cannot be separated by simple 
distillation because they either form azeotropes or the boiling points of 
the components are so close as to prevent an effective and economical 
separation. For, in pervaporation, it is no longer the partial vapor 
pressure of the components above the liquid that determines the 
composition of the mixture in the gaseous phase, but rather the different 
permeability of the membrane and, hence, the selectivity thereof for the 
various components in the liquid mixture. On laboratory scale, for 
example, mixtures of benzene/cyclohexane and isopropanol/water could be 
separated beyond the respective azeotropic mixtures by means of 
pervaporation. Similarly, it was possible to separate the xylene isomers 
o-xylene (boiling point 144.4.degree. C.), m-xylene (boiling point 
139.1.degree. C.) and p-xylene (boiling point 138.3.degree. C.) by 
pervaporation in laboratory. 
A special position is occupied by the mixtures of the simple 
oxygen-containing organic compounds, e.g. of the simple alcohols, ketones, 
ethers, aldehydes and acids, with water. On the one hand, these compounds, 
frequently, are technically important substances required in large scale 
for the most various applications in dry and anhydrous condition; 
conversely, these compounds, in general, completely or largely are 
miscible with water forming azeotropic mixtures with water so that the 
separation and recovery of the anhydrous organic substances envolve 
substantial expenditure. Many attempts have therefore been made to use 
pervaporation processes for the separation of such mixtures; however, the 
efforts so far taken have never exceeded the stage of laboratory tests. 
Admittedly, the prior art membranes of cellulose diacetate and triacetate 
that are also employed otherwise, e.g., in reversing osmosis, for some 
purposes have an adequate mechanical stability and a satisfactory flow of 
permeate; However, the selectivity thereof does not yet permit a 
large-scale industrial use for pervaporation. The present invention is now 
concerned with a multi-layer membrane having a non-porous separating layer 
from a first polymer and a backing layer from a second polymer, which is 
characterized in that the separating layer is comprised of polyvinyl 
alcohol or cellulose acetate. 
SUMMARY OF THE INVENTION 
The membrane of the invention is suitable for the separation of liquid 
mixtures by means of liquid permeation or pervaporation, especially for 
the separation of water from its mixtures with oxygen-containing, organic 
liquids, such as simple alcohols, ethers, ketones, aldehydes or acids. 
Moreover, the membrane of the invention is also suitable for the 
separation of gas mixtures. 
The membranes of the invention, owing to their multi-layer structures, on 
the one hand, have an excellent mechanical stability and, on the other 
hand, the separating layer can be applied to the mechanically stable 
backing layer with a thickness which is sufficiently thin to provide 
permeate flow and selectivity, but also to permit industrial use of such 
membranes. In technical usage, the multi-layer membranes are also 
designated as compound or composite membranes. 
In accordance with the present invention, the separating layer of the 
membrane is comprised of polyvinyl alcohol or cellulose acetate. Polyvinyl 
alcohol is usefully obtained by saponification of polyvinyl acetate. 
Preferably, a polyvinyl alcohol is used that has a high saponification 
degree, e.g., a saponification degree in excess of 98 or 99 percent. The 
molecular weights are uncritical, if only film formation and membrane 
formation, respectively, are safeguarded. Usual molecular weights are 
within the range of between 15.000 and 200.000, e.g. between 70.000 and 
120.000 (Daltons). Suitable products are commercially available. The 
cellulose acetates, in the first place, are cellulose diacetate and 
triacetate having the characteristics normally used in the production of 
membranes. Separating layers from polyvinyl alcohol are the preferred ones 
as they yield substantially improved results both as to selectivity and 
permeate flow. 
In the separating layers it is necessary (as it is with the porous backing 
layers) that the polymers used in the present invention not be removed or 
attacked either by water or by the solvents to be separated. 
The use of polyvinyl alcohol as a separating layer exhibits quite a number 
of characteristics. Polyvinyl alcohol is easily soluble in water for which 
reason it can in simple manner be applied from an aqueous solution, 
whereas polyvinyl alcohol is insoluble in all simple organic solvents. 
Polyvinyl alcohol, chemically and thermally, is stable; polyvinyl alcohol 
layers can be after-treated by cross-linkage to the extent that, in the 
long run, they are also insoluble in hot water showing only a swelling 
with water that can be adjusted by the type of the cross-linking reaction. 
Very thin, firmly adhering separating layers of polyvinyl alcohol that 
have an adequately high permeate flow can be applied to suitable backing 
layers. Owing to the conditions of manufacture, the properties of the 
polyvinyl alcohol separating layers are widely variable so that the 
pervaporation membranes produced thereby, with respect to selectivity and 
permeate flow can be optimally adapted to the respective problem of 
separation. 
Insolubility of the polyvinyl alcohol in water is caused by cross-linkage. 
Preferably, cross-linking is performed by etherification, esterification 
or acetalization, or by a combined use of the said processes. Examples are 
esterification with dicarboxylic acids, preferably those that, in 
addition, contain hydroxyl- and/or keto-groups; etherification under 
catalytic exposure to acids, or by means of dihalogen compounds, such as 
1,3-dichloroacetone or 1,3-dichloroisopropanol, or acetalization by means 
of aldehydes or dialdehydes. Based on ethanol/water mixtures, 
esterification, in general, results in an increased selectivity and 
acetalization leads to an increase in the rate of permeate flow while the 
influence of etherification on selectivity and permeate flow rate is less 
pronounced. The afore-going, in analogy, applies to other hydrous 
mixtures. The effects of a number of different cross-links are revealed by 
the examples. 
The separating layer of polyvinyl alcohol or cellulose acetate is required 
to be non-porous and free of defects (free of holes). The thickness of the 
separation layer, in general, amounts to between 0.05 and 10 .mu.m, 
preferably to between 0.1 and 5 .mu.m, with layer strengths of about 1 to 
2 .mu.m, in practice, having proved to be particularly suitable. 
The non-porous separating layer, dissolved in a suitable solvent, is 
directly applied to the porous backing layer. The concentration in this 
respect is non-critical, however, hardly will it be possible to attain 
sufficiently thin layers by excessively viscous solutions. The preferred 
solvent for polyvinyl alcohol is water, for cellulose triacetate it is 
chloroform or trichloroethylene. 
Generally, all substances suitable for use as ultrafiltration membranes can 
be considered as porous backing layers for the multi-layer membranes of 
the invention. Owing to the desired thermal stability and the 
unsensitivity to the solvent mixtures to be separated, porous backing 
layers of polyacrylonitrile (PAN), polysulfone (PS) and hydrolyzed or 
saponified cellulose acetates will be preferred. Preferably, the porous 
backing layer has a very close distribution of pore radii with an average 
pore radius such that the macromolecules of the polymer used for the 
separating layer of the membrane, preferably polyvinyl alcohol, cannot 
penetrate into the pores of the backing layer but are rather retained on 
the surface. In this manner, very uniform, thin and efficient separating 
layers can be applied to the porous backing layer. The adjustment of the 
pore radii and of the distribution of the pore radii, on the one hand, can 
be effected by corresponding conditions in the preparation of the porous 
backing layer as such, or it can be effected by applying an intermediate 
layer to a less suitable porous backing layer, in which case, the 
separating layer is applied to the intermediate layer (see e.g. examples 5 
and 6 ). When using cellulose acetate for the separating layer, pore 
distribution and average pore radius of the backing layer are less 
critical, as the molecular weight of cellulose acetate is substantially 
higher than that of polyvinyl alcohol and, beyond that, also a different 
molecular configuration prevails. Finally, solutions from cellulose 
acetate in organic solvents are of a higher viscosity than corresponding 
aqueous polyvinyl alcohol solutions. For all these reasons, the use of 
cellulose acetate as a separating layer is less sensitive regarding the 
pore distribution and the pore radius of the porous backing layer. 
The thickness of the porous backing layer is not critical, if only an 
adequate mechanical strength of the multi-layer membrane is safeguarded. 
The thickness of the porous backing layer amounts to e.g. 20, 50 or 100 
.mu.m, or more. 
In a preferred embodiment, the porous backing layer in the inventive 
membrane is applied to a fleece or to a fabric serving as a carrying 
layer. As are the other layers, the carrier layer, preferably, is 
resistant to temperature and chemicals. The carrier layer, on the layer 
side, preferably, is smooth to avoid a damage to the porous backing layer. 
Polyesters are the preferred ones; celulose layers, in general, are not 
sufficiently smooth. The polyamides actually suitable, in general are less 
preferred owing to the thermal resistance thereof lower than that of 
polyesters and owing to their lower solvent strength. The thickness of the 
carrier layer is not critical either; in practice, thicknesses of between 
50 and 150 .mu.m, e.g. approximately 100 .mu.m, have proved to be 
particularly suitable. 
Application and distribution of the polymeric solutions forming the porous 
backing layer and the non-porous separating layer, generally, are 
performed in a manner that the polymeric solutions, by way of a knife 
blade or glass rod, are distributed over and swept clear of the 
corresponding layer. For this, the layer thicknesses are adjusted in 
accordance with the desired layer thickness. In addition to that process, 
in practice, especially for less viscous polymeric solutions, also a 
process has proved to be suitable that in the American-language literature 
has been described as the "meniscus coating" or "dip coating". The carrier 
material to be laminated, with the side to be coated is drawn downwardly 
over a roll just touching the surface of the polymeric solution to be 
applied. A meniscus is then formed between the liquid surface and the 
carrier material, with the surface of the carrier material being wetted by 
the polymeric solution. In some cases, it is preferred to improve the 
wettability of the carrier material by the addition of wetting agents. By 
varying the viscosity of the polymeric solution, the drawing speed, the 
carrier material and of the dripping time of the polymeric solution, the 
thickness of the so applied layer of the polymeric solution can be widely 
varied and reproducibly adjusted. 
The permeate flow in kg/h.m.sup.2 under the test conditions of the 
temperature, the composition of the mixture to be separated and of the 
pressure on the permeate side, and the selectivity B of the membrane under 
the said conditions serve for characterizing of pervaporation membranes. B 
is a non-dimensional figure representing the concentration ratio of the 
binary mixture in the permeate divided by the concentration ratio in the 
feed. 
##EQU1## 
The volume of the permeate flow shows heavy dependence on the temperature. 
While in all examples of embodiment the feed of the liquid mixture was 
performed under atmospheric pressure, the pressures prevailing on the 
permeate side amounted to between 10 and 50 mbar. In that range the level 
of the permeate-sided pressure was without any noteable influence on the 
permeate flow and the selectivity of the membranes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to FIG. 1, a multi-layer membrane 1 is comprised of a polyester 
fleece carrier layer 2 having a thickness of 120 .mu.m. Provided thereon 
is a porous backing layer 3 of polyacrylonitrile having a thickness of 50 
.mu.m. The separating layer 4 is comprised of polyvinyl alcohol 
cross-linked with maleic acid and has a thickness of 1.2 .mu.m. The 
manufacture of that multi-layer membrane will be described in example 1. 
According to FIG. 2, a multi-layer membrane 21 is comprised of a polyester 
fleece carrier layer 22 having a thickness of 120 .mu.m. Provided thereon 
is a porous backing layer 23 of polyacrylonitrile having a layer thickness 
of 50 .mu.m. Provided thereon is a porous intermediate layer (another 
backing layer) 24 of saponified cellulose triacetate having a thickness of 
50 .mu.m. A non-porous separating layer 25 of polyvinyl alcohol 
cross-linked with maleic acid has a layer thickness of under 1 .mu.m. The 
production of that multi-layer membrane will be described in examples 5 
and 6. 
The examples will explain the invention. 
The carrier layer as used in the examples is a polyester fleece having a 
layer thickness of about 120 .mu.m. 
The polyvinyl alcohol (PVA) as used is a commercially available product 
having a degree of saponification of at least 99 percent and an average 
molecular weight of 115.000 (Daltons). 
EXAMPLE 1 
A 15% solution of polyacrylonitrile (PAN) in dimethyl formamide (DMF) by 
means of a knife blade is applied as a layer of a 50 .mu.m thickness to a 
polyester fleece forming a carrier layer, and is precipitated according to 
the phase inversion process at a temperature of 8.degree. C. The resulting 
porous membrane, at a pressure differences of 4 bar, shows a clear water 
discharge rate of 150 l/h.m.sup.2 and a retention capacity of more than 
99.5 percent for a 1% solution PVA in water. 
Subsequently applied to that PAN membrane is a solution of 7 percent by 
weight PVA in water added to which are 0.05 mole maleic acid per mole 
monomeric unit PVA. After drying, hardening and cross-linking of the PVA 
separating layer at 150.degree. C., the PVA separating layer will no 
longer be soluble either in boiling water. Tests with water/alcohol 
mixtures, at a feed temperature of 80.degree. C. and a water/alcohol ratio 
in the feed of 1/4 resulted in a selectivity of B=1400 and a rate of 
permeate flow of 0.04 kg/h.m.sup.2. 
Under otherwise identical conditions except for a water/alcohol ratio in 
the feed of 5/95, the selectivity was still 9500 at a rate of permeate 
flow of 0.01 kg/h.m.sup.2. 
EXAMPLE 2 
Example 1 is repeated, except for that a reduced PVA concentration of 5% is 
used in the preparation of the PVA separating layer. Under the conditions 
as specified, the following values are measured on the final membrane: 
Feed 12 percent by weight water, 88 percent by weight ethanol, feed 
temperature 80.degree. C., selectivity 250, rate of permeate flow 0.05 
kg/h.m.sup.2. 
Feed 20 percent by weight water, 80 percent by weight isopropanol, feed 
temperature 45.degree. C., selectivity 250, rate of permeate flow 0.3 
kg/h.m.sup.2. 
Feed 20 percent by weight water, 80 percent by weight acetone, feed 
temperature 60.degree. C., selectivity 100, rate of permeate flow 0.25 
kg/h.m.sup.2. 
EXAMPLE 3 
A PAN membrane produced in accordance with example 1, is coated with an 
aqueous solution of the following composition: PVA 5 percent by weight; 
formaldehyde 1 mole per mole PVA monomeric unit; hydrochloric acid 1 mole 
per mole PVA monomeric unit. 
After hardening at 155.degree. C., the following separating performances 
are determined for ethanol/water mixtures at a temperature of 70.degree. 
C.: 
Feed 80 percent by weight ethanol, selectivity 30, rate of permeate flow 
1.5 kg/h.m.sup.2. 
Feed 90 percent by weight ethanol, selectivity 50, rate of permeate flow 
1.0 kg/h.m.sup.2. 
Feed 99 percent by weight ethanol, selectivity 30, rate of permeate flow 
0.25 kg/h.m.sup.2. 
EXAMPLE 4 
An 18% solution of polysulfone (see Condensed Chemical Dictionnary, 8th 
edition, 1971, p. 712) in DMF, in accordance with the description in 
example 1, is applied to polyester fleece as a carrier layer and is 
precipitated in water at a temperature of 8.degree. C. by phase inversion. 
Applied to the so formed porous backing layer is an aqueous solution of 6 
percent by weight PVA containing 0.05 mole fumaric acid per mole monomeric 
unit PVA and is hardened at 150.degree. C. At a temperature of 80.degree. 
C. and at a feed concentration of 80 percent by weight ethanol in water, a 
selectivity of 350 and a permeate flow of 0.2 kg/h.m.sup.2 were measured. 
EXAMPLE 5 
A 15% solution of PAN in DMF, according to example 1 is applied to a 
polyester fleece forming the carrier layer and is precipitated in water at 
a temperature of 15.degree. C. The so obtained porous membrane shows a 
clear water discharge of more than 150 l/h.m.sup.2, a retention capacity 
for a 1 percent PVA solution of 90 percent at an average molecular weight 
of the PVA of 11,500, and of 50 percent at an average molecular weight of 
the PVA of 72,000. After drying, a 1% solution of cellulose triacetate in 
anhydrous chloroform is applied to the said membrane by "dip-coating", and 
the solvent is evaporated under the exclusion of moisture. At a 
temperature of 80.degree. C., the said membrane, at a feed concentration 
of 80 percent ethanol in water, showed a selectivity of 10 at a permeate 
rate of flow of 2 kg/h.m.sup.2. 
EXAMPLE 6 
The membrane provided with a separating layer of cellulose triacetate 
obtained according to example 5 was exposed to an aqueous ammonia solution 
of a pH-value of 10.5 until complete saponification of the cellulose 
triacetate. Now the said porous membrane, for PVA of a molecular weight of 
115,000, showed a retention capacity of in excess of 99.5 percent and, for 
PVA of a molecular weight of 72,000, showed a retention capacity of 98 
percent. Coated with a 3 percent PVA solution containing maleic acid as 
the cross-linking agent, at a feed temperature of 78.degree. C. and at an 
ethanol concentration of 80 percent, a selectivity of 250 was obtained at 
a permeate flow rate of 0.5 kg/h.m.sup.2.