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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to new composite membranes, processes for their
preparation and their use for removing benzenes optionally substituted by
lower alkyl radicals, hydroxyl, chlorine or bromine from their mixtures
with aliphatic and/or cycloaliphatic hydrocarbons, alcohols, ethers,
ketones and/or carboxylic acid esters or from effluent.
Membranes can be used for removal of substance mixtures by permeation. A
procedure can be followed here in which, for example, a substance mixture
in the liquid phase (feed solution) is brought to one side of the membrane
and one substance therefrom, a certain group of substances therefrom or a
mixture enriched in the one substance or in he certain group of substances
is removed, also in the liquid form, on the one side of the membrane
(permeation in the narrower sense). The substance which has passed through
the membrane and has been collected again on the other side or the
substance mixture described is called the permeate. However, it is also
possible to follow the procedure in which, for example, the feed is
brought to the one side of the membrane in liquid or gaseous form,
preferably in liquid form, and the permeate is removed in the form of a
vapour on the other side and is then condensed (pervaporation).
Such permeation processes are useful additions to other processes of
substance removal, such as distillation or absorption. Permeation,
specifically pervaporation, can be of useful service in particular in the
removal of substance mixtures which boil as azeotropes.
2. Description of the State of the Art
There have previously been many attempts to adapt membranes of various
polymer materials to individual specific purposes. It is thus known from
U.S. Pat. No. 2,953,520 to enrich benzene in the permeate and in this way
substantially to separate it off from an azeotropic benzene/methanol
mixture with the aid of a non-porous plastic membrane of polyethylene. It
is furthermore known from U.S. Pat. No. 3,776,970 to separate the two
aromatic compounds styrene and ethylbenzene with the aid of a membrane of
certain polyurethane elastomers such that styrene is enriched in the
permeate. It is furthermore known from German Patent Specification No.
2,627,629 to remove benzene and alkylbenzenes from aliphatic hydrocarbons,
cycloaliphatic hydrocarbons, alcohols, ethers and carboxylic acid esters
with the aid of polyurethane membranes.
SUMMARY OF THE INVENTION
It has now been found, surprisingly, that the removal of benzenes
optionally substituted by lower alkyl radicals, hydroxyl, chlorine or
bromine from their mixtures with aliphatic and/or cycloaliphatic
hydrocarbons, alcohols, ethers, ketones and/or carboxylic acid esters or
from effluent can be substantially improved using the composite membrane
described below in comparison with the polyurethane membranes described in
German Patent Specification No. 2,627,629, these improved removal effects
becoming particularly clear in the field of mixtures of low aromatic
content.
The invention thus relates to composite membranes consisting of
(i) a macroporous filler-containing membrane of at least two incompatible
polymers and
(ii) a pore-free polyurethane (PU) membrane applied to (i).
DETAILED DESCRIPTION OF THE INVENTION
The macroporous membrane according to (i) consists of at least two polymers
which are incompatible in solution, that is to say, lead to phase
separation in a common solution. Further details on incompatible polymer
systems which demix are to be found in the monograph by Paul J. Flory,
Principles of Polymer Chemistry, Ithaca, N.Y., (1953). By dispersing
insoluble fillers into this unstable mixture, this mixture is converted
into a stable homogeneous dispersion. This dispersion is then applied to a
substrate as a casting solution. The macroporous filler-containing
membrane according to (i) is produced from this casting solution by
precipitation coagulation, which is also called phase inversion. This
technology of phase inversion is known, for example from H. Strathmann,
Trennungen von molekularen Mischungen mit Hilfe synthetischer Membranen
(Separations of Molecular Mixtures with the aid of synthetic membranes),
Steinkopf-Verlag, Darmstadt (1979) and D. R. Lloyds, Materials Science of
Synthetic Membranes, ACS Symp. Ser. No. 269, Washington D.C. (1985).
These publications also describe the typical membrane structures obtained
during precipitation coagulation. These are always asymmetric membrane
structures with a denser polymer skin on the membrane surface and higher
porosities inside the membrane. The pore structure can be finger-like or
foamlike, depending on the recipe of the casting solution. By forming the
denser polymer skin on the membrane surface, the pore diameters of the
conventional membranes are limited and as a rule
do not exceed values of about 8-10 .mu.m.
Homogeneous polymer casting solutions are used as the starting substances
in the production of precipitation coagulation membranes of the
conventional type, since otherwise unstable membranes are obtained. For
this reason, typical membrane casting solutions are formed from a polymer
and a solvent or solvent mixture (for example polyamide in
dimethylacetamide or cellulose acetate in acetone/formamide).
There have already been attempts to produce membranes having increased
permeabilities by specific recipes of the polymer casting solutions.
Membranes are described in Chem. Pro. Res. Dev. 22 (1983), 320-326 or in
DE-OS (German Published Specification) No. 3,149,976 which have been
produced using polymer casting solutions containing water-soluble
polymers, such as polyvinylpyrrolidone, which are dissolved out during the
coagulation in water and in this way lead to enlarged pores. Membranes of
polymer mixtures have also been described. However, the recipes of the
corresponding casting solutions are built up in such a way that
homogeneous polymer solutions are obtained on the basis of the solubility
parameters. For example, EP No. 66,408 describes membranes of a mixture of
cellulose acetate and polymethyl methacrylate which have increased
permeabilities in comparison with the conventional membranes of one
polymer. However, polymer combinations with similar solubility parameters
and certain very narrow mixing ratios are depended upon here.
It has now been found, surprisingly, that macroporous membranes of polymers
which are incompatible and immiscible per se and which can be processed in
any desired mixing ratio to give homogeneous casting solutions if certain
insoluble fillers are dispersed in them display the abovementioned better
removal effects in association with pore-free polyurethane (PU) membranes
applied to them.
For example, if a 20% strength by weight solution of polyurethane in
dimethylformamide (PU/DMF solution) and a 20% strength by weight solution
of polyacrylonitrile in dimethylformamide (PAN/DMF solution) are mixed,
while stirring, phase separation occurs after the mixture has stood for a
short while. Such mixtures are unstable and are unsuitable as casting
solutions for production of membranes. In contrast, if the same
polymer/DMF solutions are combined with simultaneous or subsequent
dispersing in of fillers, for example talc, homogeneous stable casting
solutions which are suitable for membrane production by the precipitation
coagulation method are obtained.
In comparison with the known membranes, the membranes produced from such
casting solutions have significantly larger pores on the surface and a
very much higher overall porosity.
As electron microscopy photographs of the cross-section of these polymer
membranes shows, these are structures with a felt-like build-up, whereas
the asymmetric structure build-up of the denser polymer skin on the
membrane surface is almost completely suppressed. Average pore diameters
of up to 30 .mu.m can to be detected on the membrane surface of a membrane
of the above recipe.
The polymer casting solutions required for production of such membrane
matrices must fulfil the following conditions:
(a) The solutions of the individual polymer components should not be
miscible with one another. With miscible systems, analogously to
conventional casting solutions, membrane structures of fine porosity and
pronounced asymmetric structure are obtained.
(b) The solvents of the individual polymer components must be miscible with
one another.
(c) To convert the immiscible polymer components into homogeneous casting
solutions, suitable insoluble fillers, for example inorganic fillers, must
be dispersed in them.
The nature of the filler can in some cases be important for the stability
and homogeneity of the casting solution. Whereas, for example, casting
solutions of PU/PAN mixtures containing titanium dioxide (TiO.sub.2
RKB2.RTM., Bayer AG) or barium sulphate (Blanc Fixe Mikron.RTM.,
Sachtleben) having specific surface areas of about 3 m.sup.2 /g (particle
size about 0.5-1.0 .mu.m) are less favourable in respect of stability and
homogeneity, solutions of the same polymer mixture containing talc (Talc
AT 1, Norwegian Talc) show a good homogeneity and dispersion stability.
Similarly good results could also be obtained with very fine-grained
fillers of high specific surface area, for example with the titanium
dioxide Degussa P25 (about 40 m.sup.2 /g) or the silicon dioxide Aerosil
200.RTM. Degussa (200 m.sup.2 /g). Mixtures of talc with barium sulphate
or talc with TiO.sub.2 RKB2.RTM. or titanium dioxide P25.RTM., Degussa,
with barium sulphate lead to suitable casting solutions. It was also
possible to prepare suitable casting solutions by dispersing in
microcrystalline cellulose (for example Arbocel B E 600/30.RTM., J.
Rettenmaier & Soohne). Other suitable fillers are CaCO.sub.3, MgCO.sub.3,
ZnO and iron oxides.
The function and action of the filler is conversion of the unstable
inhomogeneous polymer solution into stable and homogeneous casting
solutions; the mechanism of this "solubilization" is unknown.
The pore size is controlled via the choice of polymers and the particular
quantities. The fillers have only a minor influence, if any, on the pore
size. The particle diameters of the fillers are of a much smaller order of
size (.ltoreq.5 .mu.m) than the pore diameters of the polymer membrane
(.ltoreq.30 .mu.m). The process of precipitation coagulation in
combination with the type of casting solutions described here is
responsible for the pore formation of the membranes according to the
invention. The range of the average pore size of the macroporous membranes
according to the invention is 10 to 30 .mu.m, preferably 15 to 25 .mu.m.
Such an average pore size does not exclude the occurrence of pores in a
range below (for example from 1 .mu.m) and in a range above (for example
up to 50 .mu.m).
The following polymer classes, for example, can be used to produce the
macroporous filler-containing membrane according to (i): cellulose esters,
polyvinyl esters, polyurethanes, polyacrylic derivatives and acrylic
copolymers, polycarbonates and their copolymers, polysulphones,
polyamides, polyimides, polyhydantoins, polystyrene and styrene
copolymers, poly(para-dimethyl-phenylene oxide), polyvinylidene fluoride,
polyacrylonitrile and ethylene/vinyl acetate copolymers containing at
least 50% by weight of vinyl acetate.
Preferably, two or three incompatible polymers from the class of
polyurethanes, polyacrylonitrile, polyvinyl acetate, polystyrene,
polysulphone, polyvinylidene fluoride, polyamide, polyhydantoin and
ethylene/vinyl acetate copolymers containing at least 50% by weight of
vinyl acetate are employed. Examples of binary incompatible polymer
systems are:
cellulose esters/polyvinyl esters (such as the cellulose acetate Cellidor
CP.RTM./the polyvinyl acetate Mowili.RTM.)
polyurethane/polyacrylic derivatives (such as Desmoderm KBH.RTM./the
polyacrylonitrile Dralon T.RTM. or Desmoderm KBH.RTM./amino-modified
Dralon or Desmoderm KBH.RTM./anionically modified Dralon U.RTM., that is
to say provided with sulphate groups)
polycarbonate copolymers/polyurethane (such as polyether
polycarbonate/Desmoderm KBH.RTM.)
polyvinyl derivatives/polysulphones (such as polyvinylidene fluoride/the
polysulphone Udel P 1700.RTM.)
polyamides or polyimides/polystyrene or styrene copolymers
poly(para-dimethyl-phenylene oxide)/polyvinylidene fluoride and
polyhydantoin/polystyrene.
Other two-component combinations which may be mentioned are: Dralon
U.RTM./Mowilith.RTM. and Cellidor CP.RTM./Dralon U.RTM.; examples of
ternary polymer mixtures are Cellidor CP.RTM./Dralon U.RTM./polystyrene,
Mowilith R.RTM./Desmoderm KBH.RTM./polyvinyl chloride and Desmoderm
KBH.RTM./Mowilith R.RTM./Dralon T.RTM., it also being possible for Dralon
T.RTM. to be replaced by Dralon A.RTM..
Preferred binary and ternary polymer systems are: Desmoderm KBH.RTM./Dralon
T.RTM., Desmoderm KBH.RTM./Dralon A.RTM., Desmoderm
KBH.RTM./Mowilith/Dralon T.RTM., it also being possible for Dralon T.RTM.
to be replaced by Dralon A.RTM. or Dralon U.RTM..
The chemical structures of the polymers preferably employed are described
in the appendix to the embodiment examples.
The ratio of the amounts of the polymers, which is required for the pore
diameters, in the particular combinations can be determined by appropriate
experiments.
If the polymers, of which there are at least two, are mixed in
approximately the same amounts, as a rule higher values for the average
pore sizes are obtained; if the amounts differ relatively widely, lower
values are obtained. The polymer casting solution should contain at least
10% by weight of one polymer based on the total amount of all the
polymers.
Dimethylformamide (DMF) is a particularly suitable solvent for the
preparation of casting solutions of the preferred polymer combinations.
Other suitable solvents are, depending on the polymers used:
N-methylpyrrolidone (NMP), dimethyl sulphoxide (DMSO), dimethylacetamide,
dioxolane, dioxane, acetone, methyl ethyl ketone or Cellosolve.RTM..
The amount of solvent is chosen such that a viscosity of the casting
solution which reaches the range from 500 to 25,000 mPas is achieved. As a
rule, this corresponds to a polymer content of 10 to 40% by weight in the
overall casting solution.
In addition to the fillers already mentioned above, there may also be
mentioned zeolites and bentonites, and furthermore mixtures of TiO.sub.2
with BaSO.sub.3 or talc with BaSO.sub.4, and furthermore mixtures of
TiO.sub.2 of large and small specific surface area, such as TiO.sub.2
RKB2.RTM. Bayer/TiO.sub.2 P 25.RTM. Degussa. Preferred fillers are: talc,
microcrystalline cellulose, zeolites, bentonites, BaSO.sub.4, TiO.sub.2
and SiO.sub.2.
The overall process for the preparation of content (i) in the composite
membranes according to the invention can be described with the aid of a
preferred example as follows: The DMF polymer solutions, in each case
about 20% strength by weight, of Desmoderm KBH.RTM., Mowilith.RTM. and
Dralon T.RTM. were mixed with the aid of a high-speed stirrer (dissolver)
to give a homogeneous polymer casting solution, talc being dispersed in.
After degassing in vacuo, this casting solution was applied in a layer
thickness of, for example, 150 .mu.m with the aid of a doctor blade to a
carrier substrate and was dipped in the coagulation bath, for example pure
water. After a residence time of about 2 minutes, the microporous
filler-containing membrane formed in this way was removed from the
coagulation bath and dried with warm air.
Surfactants, for example dioctyl sodium sulphosuccinate or
dodecylbenzenesulphonates, can also be used to prepare the casting
solution. Water-soluble polymers, such as cellulose ethers, polyethylene
glycols, polyvinyl alcohol or polyvinylpyrrolidone can also be a
constituent of the polymer casting solution. Other possible additives are
so-called coagulation auxiliaries, such as, for example, cationic
polyurethane dispersions (such as Desmoderm Koagulant KPK.RTM.).
The carrier substrates used for application of the casting solution can be
one which merely serves for the production of the macroporous
filler-containing membrane according to (i) and is therefore peeled off
again after the coagulation operation on (i). For this purpose, the
carrier substrate must be smooth and is, for example, glass, a
polyethylene terephthalate film or a siliconized carrier material.
However, if the composite membrane according to the invention of (i) and
(ii) is to be provided with a support material for improving the
mechanical stability, materials which are permeable to liquid, such as
woven polymer fabric or polymer non-wovens, to which the macroporous
filler-containing membrane (i) shows good adhesion are used as the carrier
substrate. The co-use of such a support material (woven fabric or
non-woven) is preferred for the composite membranes according to the
invention.
It is furthermore known, for increasing the surface area of membranes, also
to use these in the form of tubes, hoses or hollow fibres, as well as in
the form of films, production of which has just been described. These
tubes, hoses or hollow fibres can be arranged and used in special
separation units, which are called modules, in order to achieve maximum
membrane surface areas with the minimum possible apparatus volumes. Such
tubes, hoses or hollow fibres can be produced, for example, by forcing the
filler-containing and in this way stabilized casting solution described
above through the outer annular gap of a concentric two-component die,
whilst a coagulating agent, such as water, is forced through the central
die opening and the casting solution which issues moreover enters a
coagulation bath, such as water; coagulation is in this way performed from
the inside and from the outside.
After coagulation and drying, a pore-free polyurethane (PU) membrane is
applied to the macroporous filler-containing membrane (i) by the casting
technique.
The thickness of this pore-free PU membrane is 0.5-500 .mu.m, preferably
5-50 .mu.m.
Polyurethanes for this pore-free PU membrane (ii) and their preparation are
known. Polyurethanes are in general prepared by reaction of higher
molecular weight di- or polyhydroxy compounds and aliphatic, araliphatic
or aromatic di- or polyisocyanates and if appropriate so-called
chain-lengthening agents.
Examples which may be mentioned of starting materials containing OH end
groups are: polyesters of carbonic acid and aliphatic dicarboxylic acids
having 2-10 C atoms, preferably of adipic and sebacic acid, with aliphatic
dialcohols having 2-10 C atoms, preferably those having 2 to 6 C atoms, it
also being possible for the dialcohols to be used as a mixture in order to
lower the melting points of the polyesters; polyesters of low molecular
weight aliphatic lactones and .omega.-hydroxycarboxylic acids, preferably
of caprolactone or .omega.-hydroxycapric acid, the carboxyl groups of
which have been reacted with diols; and furthermore polyalkylkene
etherdiols, specifically polytetramethylene etherdiols, polytrimethylene
etherdiols, polypropylene glycol or corresponding copolyethers.
Aromatic diisocyanates, such as toluylene diisocyanate and m-xylylene
diisocyanate, araliphatic diisocyanates, such as diphenylmethane
4,4'-diisocyanate, or aliphatic and cycloaliphatic diisocyanates, such as
hexamethylene diisocyanate and dicyclohexylmethane 4,4'-diisocyanate, as
well as isophorone diisocyanate, are used as the diisocyanates.
If appropriate, these starting materials can also be reacted with
dialcohols which are additionally employed, to give so-called prepolymers,
and these can then be polymerized again with further di- or polyhydroxy
compounds and di- or polyisocyanates and if appropriate further
chain-lengthening agents. In addition to the two-dimensionally crosslinked
polyurethanes obtainable by using diols and diisocyanates,
three-dimensionally crosslinked polyurethanes can also be obtained if
trihydroxy compounds and/or polyols and/or tris- and/or polyisocyanates
are simultaneously used as starting materials in the polymerization.
Three-dimensional crosslinking can also be achieved, however, if
two-dimensionally crosslinked polyurethanes which still contain free
hydroxyl and/or isocyanate groups are subsequently further reacted with
trifunctional alcohols and/or isocyanates. Such three-dimensionally
crosslinked polyurethanes can likewise be obtained by subsequent reaction
of two-dimensionally crosslinked polyurethanes containing free isocyanate
end groups with small amounts of polymers having end groups containing
reactive hydrogen atoms, such as formaldehyde resins or melamine resins.
Film-forming elastic polyurethanes are preferably used for the pore-free
PU membranes (ii), these being prepared as so-called "one-component PU"
with a characteristic number(equivalent)
##EQU1##
of about 1.0, for example in the range from 0.95 to 1.1. Butane-1,4-diol
adipic acid polyester, hexamethylene 1,6-glycol adipic acid polyester and
hexane-1,6-diol poly-carbonate, in particular, are employed here as diols.
Preferred diisocyanates are isophorone diisocyanate,
4,4'-diisocyanato-diphenylmethane and toluylene diisocyanate. Ethylene
glycol, butane-1,4-diol, ethanolamine and diamino-dicyclohexyl-methane are
preferably used as chain-lengthening agents.
This group also includes polyurethanes which are prepared from a prepolymer
having free hydroxyl groups, a diol and a diisocyanate with a
characteristic number
##EQU2##
Another preferred group of such film-forming polyurethanes are so-called
"two-component PUs" of one of the abovementioned polyurethanes, which have
been crosslinked by subsequent further polymerization with a polyol, such
as trimethylolpropane, and if appropriate a chain-lengthener, such as
butylene 1,3-glycol, and a diisocyanate. This group of "two-component PUs"
also includes those polyurethanes which have subsequently been further
crosslinked with formaldehyde resins or melamine resins.
Other polyurethanes can of course also be used for the production of the
pore-free PU membranes (ii) such as are used in the composite membranes
according to the invention; only those polyurethanes which dissolve in the
aromatic and aliphatic or cycloaliphatic hydrocarbons to be separated are
unsuitable.
In addition to the abovementioned casting technique for application of the
pore-free PU membrane (ii) onto the microporous filler-containing membrane
(i), application by extrusion, calendering or the injection moulding
technique is in principle also conceivable. However, application by the
casting technique is preferred.
Within the casting technique, a possible embodiment is to add acrylates to
the PU casting solution. These added acrylates enable the pore-free PU
membrane (ii) to crosslink within the composite membranes according to the
invention by UV irradiation or Y radiation or electron beams and in this
way to be stabilized mechanically.
Possible acrylates are acrylic acid esters and/or methacrylic acid esters
of diols having 4-12 C atoms or of tri- or tetraalcohols, in particular
butane-1,4-diol acrylate, butanediol bis-methacrylate, and in particular
trimethylolpropane trisacrylate, trimethylolpropane trimethacrylate,
pentaerythritol tetraacrylate or pentaerythritol tetramethacrylate, or
urethane acrylates (for example reaction products of trimethylolpropane,
isophorone diisocyanate and hydroxyethyl acrylate). Their amount is 4-24%
by weight, based on the total amount of polyurethane and acrylates. A
crosslinkable acrylate/polyurethane blend is thus obtained for (ii).
Trimethylolpropane trisacrylate is particularly preferably employed.
If aqueous PU dispersions (Angew. Makromolek. Chemie 98 (1981) 133-165) are
used for the production of the pore-free PU membrane (ii), these can be
crosslinked with carbodiimides, if appropriate, in order to improve the
mechanical strength.
Plasticizers, such as nonylphenol, or fillers, such as finely divided
SiO.sub.2 (for example silica gel or Aerosil grades from Degussa) and
zeolites, can furthermore also be used for production of the PU membrane
(ii).
The invention furthermore relates to production of composite membranes of
the abovementioned type, which is characterized in that
(a) an insoluble filler is dispersed in a solution containing at least two
incompatible polymers in amounts which lead to phase separation in the
solution, a homogeneous casting solution being formed,
(b) this solution is processed to membranes in the form of films, tubes,
hoses or hollow fibres and precipitation coagulation is carried out and
(c) a pore-free PU membrane is applied to the macroporous filler-containing
membrane obtained in this way.
In the production of the membranes in step (b) in the form of films, the
solution is applied to a carrier substrate and, after the precipitation
coagulation in the manner described above before step (c) is carried out,
the coagulate is detached from the carrier substrate.
Preferably, however, this process is modified so that the carrier substrate
is a support material of the type mentioned, which remains on the
composite membrane. The pore-free PU membrane (ii) is then applied in the
casting process in the manner described above.
In the case where the composite membranes according to the invention are
produced in the form of tubes, hoses or hollow fibres, after production of
the macroporous filler-containing membrane (i), for example by extrusion
and coagulation in the manner described above, a PU casting solution is
applied to the inside of such tubes, hoses or hollow fibres by casting in
order to produce the pore-free PU membrane (ii), the system being
subsequently flushed with an inert gas, if appropriate, for example in
order to avoid sticking of the inside in the case of hollow fibres. This
inert gas can at the same time be prewarmed in order to effect evaporation
of the solvent from the casting solution. Such a method of application of
(ii) is suitable for bringing the mixture to be separated, of benzenes
optionally substituted by lower alkyl radicals, hydroxyl, chlorine or
bromine and aliphatic and/or cycloaliphatic hydrocarbons, alcohols,
ethers, ketones and/or carboxylic acid esters, or the effluent containing
such benzenes, inside these tubes, hoses or hollow fibres and for removing
the permeate enriched in optionally substituted benzene from the outer
surface of the tubes, hoses or hollow fibres. This type of build-up of the
composite membranes according to the invention is particularly favourable
if a pressure gradient from a higher to a lower pressure is to be applied
from the mixture side to the permeate side.
In addition, the reverse use is in principle also possible, that is to say
bringing of the starting mixture onto the outer surface of the tubes,
hoses or hollow fibres and removal of the permeate from the inside
surface. For this embodiment, the PU casting solution for the production
of (ii) must be brought onto the outer surface of tubes, hoses or hollow
fibres of the macroporous filler-containing membrane (i).
The invention furthermore relates to the use of the composite membranes
described above for removing benzene, which can be mono-, di- or
trisubstituted by chlorine, bromine, C.sub.1 -C.sub.4 -alkyl or hydroxyl
from aliphatic and/or cycloaliphatic hydrocarbons, alcohols, ethers,
ketones and/or carboxylic acid esters or from effluent.
Optionally substituted benzenes are: benzene, toluene, xylene,
ethylbenzene, propylbenzene, chlorobenzene, dichlorobenzene, bromobenzene,
phenol or cresol.
Examples of aliphatic or cycloaliphatic hydrocarbons from which the
optionally substituted benzene is to be removed are, for example,
straight-chain or branched hydrocarbons having 5-14 C atoms, such as
pentane, hexane, heptane, 2-methyl- and 5-methylhexane,
2,2-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane,
straight-chain or branched tetradecane, i-octane or cycloaliphatic
hydrocarbons, in particular having 5 and 6 ring C atoms, which can also be
substituted by C.sub.1 -C.sub.8 -alkyl, preferably C.sub.1 -C.sub.4 -alkyl
and particularly preferably by methyl and ethyl. These aliphatic or
cycloaliphatic hydrocarbons can be present individually or as a mixture;
mixtures of petrochemical origin, for example for fuels, are preferably
suitable. Preferred cycloaliphatic hydrocarbons in these are
methylcyclopentane, cyclohexane and methylcyclohexane. It is also possible
for more than one optionally substituted benzene for removal to be present
in the mixture.
Possible further organic solvents from which optionally substituted
benzenes can be removed with the aid of the membrane according to the
invention are alcohols, such as ethanol; ethers, such as dioxane; ketones,
such as cyclohexanone, and carboxylic acid esters, such as ethyl acetate.
The removal is by liquid/liquid permeation, gaseous/gaseous permeation or
liquid/gaseous pervaporation, preferably by liquid/gaseous pervaporation.
The techniques needed for this are known to the expert. Preferably, a
pressure gradient in the direction of the permeate is used, for which a
reduced pressure (for example 1-500 mbar) is applied to the permeate side.
It is surprising that the composite membranes according to the invention
have a significantly improved separation factor for optionally substituted
benzenes.
The separation factor .alpha., which represents a measure of the selective
permeability of the membrane, is generally stated as a measure of the
removal effect; it is defined by the following equation:
##EQU3##
in which C.sub.Ap and C.sub.Bp denote the concentrations of substances A
and B in the permeate (p) and
C.sub.Ag and C.sub.Bg denote the corresponding concentrations in the
mixture (g) to be separated,
and wherein
A in each case denotes the component to be removed, in the present case the
optionally substituted benzene (or several benzenes) and
B denotes the other or remaining components of the mixture.
A very surprising effect of the composite membranes according to the
invention is their successful use for removal of optionally substituted
benzene from effluent.
BRIEF DESCRIPTION OF THE FIGURES
FIGS. 1 and 2 are graphs depicting the separation effect and flow
characteristics of the membranes according to Examples 1 to 3 (membrane
No. 1: pore-free (PU) membrane; membrane No. 2: composite membrane
according to the invention; membrane No. 3: pore-free PU membrane combined
with a polyamide membrane).
EXAMPLE 1
(a) Production of the macroporous filler-containing polymer blend membrane
21.6 g of a 17% strength Dralon T.RTM./DMF solution, 65.2 g of a 20%
strength KBH.RTM. polyurethane/DMF solution, 86.6 g of a 25% strength
Mowilith 50.RTM./solution, 22.5 g of sodium dioctyl sulphosuccinate, 14.8
g of talc AT 1, 59.4 g of barium sulphate (Blanc Fixe Mikron), 17.3 g of
KPK.RTM. (Bayer AG, cationic polyurethane dispersion) and 140.0 g of DMF
were processed to a homogeneous dispersion with the aid of a high-speed
stirrer (dissolver). After degassing in vacuo, this casting solution was
coated in a layer thickness of 150 .mu.m with the aid of a doctor blade
onto a polypropylene non-woven 200 .mu.m thick (type FO 2430 from
Freudenberg) and coagulated in water at 45.degree. for 3 minutes. The
polymer matrix formed in this way and resting on the carrier film was
dried by means of warm air.
(b) Application of the pore-free PU membrane: (production of the composite
membrane according to the invention)
The porous membrane matrix obtained according to (a) was coated with the
following polyurethane: 100.0 g of poly-hexanediol adipate (average
molecular weight about 850), 57.5 g of isophorone diisocyanate and 23.7 g
of isophoronediamine were reacted with one another in a known manner. A
30% strength solution (weight/volume) of this polyurethane in a mixture of
toluene and isopropanol (1:1) was filtered through a pressure filter and
the filtrate was left to stand until it was free from bubbles. This
polyurethane casting solution was applied with a wet application of 100
.mu.m onto the macroporous carrier membrane described in (a). The solvent
was removed with the aid of warm air; the composite membrane No. 2
characterized in FIGS. 1 and 2 was in this way obtained.
The membrane characterized in FIGS. 1 and 2 (for comparison) was obtained
by coating a polyamide microfiltration (MF) membrane (Pall, 0.2 .mu.m)
with the same polymer casting solution according to (b) under the same
production parameters.
EXAMPLE 2
(for comparison)
Production of the carrier-tree polyurethane pervaporation membrane
The polymer solution described in Example 1(b) was coated in a layer
thickness of 100 .mu.m onto a transparent polyethylene terephthalate film
(PET film). The solvent was removed by evaporation with warm air; the
membrane film adhering to the PET film was in this way obtained. Membrane
No. 1 characterized in FIGS. 1 and 2 was obtained by careful peeling off
from the PET film.
EXAMPLE 3
Production of a composite membrane with a pore-free acrylate/polyurethane
blend separating layer
3.75 g of trimethylolpropane triacrylate (commercial product from Rohm) and
0.18 g of 1-hydroxycyclohexylphenyl ketone (Irgacure 184.RTM., commercial
product from Ciba-Geigy), as a photoinitiator, were added to a
polyurethane casting solution of 25.0 g of polyurethane (chemical
structure as in Example 1(b), 37.5 g of toluene and 37.5 g of isopropanol.
The mixture was homogenized by stirring and left to stand for degassing.
This casting solution was then applied in a layer thickness of 150 .mu.m
to the polymer blend membrane described in Example 1(a) and the solvent
was subsequently evaporated off. The pore-free acrylate/polyurethane blend
layer formed in this way was crosslinked with the aid of UV light.
______________________________________
Exposure conditions
Exposure apparatus Hanovia
Radiation source medium-pressure
mercury vapour lamp
Lamp output 80 W/cm
Distance between sample and lamp
11 cm
Belt speed 10 m/minute
______________________________________
The separation effect and flow characteristics of this membrane during
toluene/cyclohexane separation corresponded to those of the membrane
described in Example 1 (FIG. 1). However, improved membrane stabilities
could be observed at high temperatures.
EXAMPLE 4
Toluene/cyclohexane separation
The membranes described in Examples 1 and 2 were tested with the aid of a
pervaporator module, such as is described, for example, in DE-OS (German
Published Specification) No. 3,441,190, under the same conditions by
allowing feed solutions of various compositions to flow over. The
experimental conditions and the experimental results are shown in FIGS. 1
and 2.
The increase in selectivity when the macroporous polymer blend membrane is
used according to the invention as a composite component is striking.
Whereas the composite membrane according to the invention remained fully
functional for several days at 50.degree. C., polyurethane membrane No. 1
dissolved after a few hours under these conditions.
Explanatory note on FIGS. 1 and 2:
The composition of the substance mixture to be separated (feed) as a
function increasing toluene content is in each case shown on the abscissa.
The permeate concentration with increasing toluene content is shown on the
ordinate in FIG. 1 and the corresponding permeate flow is shown on the
ordinate in FIG. 2. Composite membrane No. 2 according to the invention
shows an unexpected increase in selectivity (increase in the separation
factor .alpha.), especially in the regin of low toluene concentrations.
The macroporous filler-containing membrane (i) of at least two
incompatible polymers thus contributes towards the selecting effect,
although it places no resistance against the feed because of the
macroporous structure and thus displays no corresponding separation action
in accordance with the concept of the solubility/diffusion model. The
composite membrane according to the invention is additionally overall more
mechanically and chemically stable, even at higher temperatures.
EXAMPLE 5
Removal of chlorobenzene from an effluent
The feed solution to be purified was an effluent which contained 10% of
ethanol and 150 ppm of chlorobenzene. Composite membrane No. 2 from
Example 1 was used. The feed solution was kept static (without flowing
over) on the membrane (temperature=30.degree. C.; permeate pressure p=11
mbar).
After 4 hours of testing, the content of chlorobenzene in the feed solution
had been reduced to 0.02 ppm.
EXAMPLE 6
Separation of benzene/cyclohexane
Composite membrane No. 2 from Example 1 was used. Composition of the feed
solution: 55% of benzene, 45% of cyclohexane.
The experiment was carried out as in Example 3. A flow of 0.6 L/m.sup.2
.times.hour was determined. Only traces (<0.5% of cyclohexane) could be
found in the permeate.
APPENDIX
Chemical structures of the polymers preferably used
Polyurethane (KBH.RTM., Bayer AG)
Thermoplastic polyadduct which was obtained by reaction of 75 parts of a
polyester of adipic acid, ethylene glycol and 1,4-butanediol (molecular
weight=2,000), 25 parts of a polyester of adipic acid and 1,4-butanediol
(molecular weight=2,250), 25 parts of 1,4-butanediol and 85 parts of
diphenylmethane 4,4'-diisocyanate.
##STR1##
Cationic polyurethane dispersion (KBK.RTM., Bayer AG)
The polyurethane dispersion serves as a coagulation auxiliary and is a
cationic emulsifier-free dispersion of a reaction product of 200 parts of
a polyester of adipic acid, phthalic acid and ethylene glycol (molecular
weight=1,700), 50 parts of toluylene diisocyanate, 20 parts of
N-methyldiethanolamine and 6 parts of p-xylylene dichloride.
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