The invention relates to a particular use of a tubular membrane, to
a method for manufacturing a particular membrane, and to a separation process
wherein such membrane is used. More in particular, the invention is in the field
of selective membranes and their use in the separation of components from a liquid
mixture on the basis of pervaporation and pressure traction.
In the case of pervaporation, a liquid mixture to be separated is
in contact with a separating top layer of a membrane. On the other side of the
membrane, a reduced pressure is created. Because of this applied difference in
pressure and the difference in thermodynamic activity related thereto, permeation
of one or more components from the liquid mixture takes place. On the permeate
side, these components are recondensed through cooling and/or increased pressure.
The effectiveness of pervaporation can be increased by applying liquid phases
of higher temperatures and/or higher pressures on the feed side, and/or by applying
lower pressures or by cooling deeper on the permeate side.
Pressure traction differs from pervaporation in that for pressure
traction no mechanical pressure difference is applied over the separation membrane,
but a difference in concentration or activity is created by flowing past a gas,
such as nitrogen, or a liquid, such as glycerol, on the permeate side.
Pervaporation and pressure traction are techniques that are used
for removing substances from liquid mixtures. These techniques are in particular
suitable for removing impurities or relatively small amounts of a specific component
or specific components from a liquid mixture and for separating azeotropic mixtures.
These known separation techniques are typically carried out utilizing a membrane
on the basis of (organic) polymers. Such processes are described in, inter alia,
Mulder, 'Basic principles of membrane technology', Kluwer Dordrecht (1991), and
the ScD dissertation of H. Nijhuis, titled "Removal of trace organics from water
by pervaporation', University of Twente (1991).
Unlike distillation methods, pervaporation and pressure traction
can be used for the direct separation of the most volatile or the least volatile
component from liquid mixtures. This can be set through a suitable choice of the
In the field to which the present invention relates, it has thus
far been assumed that these separation techniques are based on differences in the
membrane permeability and the vapor pressure of the components to be separated,
see for instance Boddeker and Bengtson, 'Pervaporation membrane separation processes',
Elsevier Amsterdam (1989), in particular Chapter 12. The membrane permeability
is a function of the solubility of the permeant and the diffusion coefficient
in the membrane polymer under non-isotropic swelling conditions. In fact, the membrane
permeability depends on the interactions of the components of a liquid mixture
to be separated with the membrane material.
This general assumption is based on the fact that in the known pervaporation/pressure
traction membrane - and also in gas separation membranes, for that matter - a rising
temperature involves an increase of the flux through the membrane, but a decrease
of the selectivity.
From a scientific point of view, this points to the importance of
the swelling of the membrane.
Membranes on the basis of - usually organic - polymers have a number
of drawbacks. For instance, such membranes exhibit a great variation in performance
when they are alternately used for different mixtures of liquids. Moreover, the
stability of organic polymer-based membranes decreases substantially over time.
In fact, this effect strongly depends on the solvent that is contacted.with the
Apart from the problems in chemical stability, the known polymeric
membranes are also sensitive to effects of temperature and pressure, while, further,
concentration differences of the components to be separated in the liquid mixture
flowing past can have a great influence on the separation performance.
In the prior art literature it has been suggested that membranes
on the basis of zeolites can be used for liquid separations on the basis of pervaporation
and pressure traction. However, the manufacture of zeolite membranes is highly
complicated, inter alia because of the requirement that the membrane surface between
the molecular sieve crystals must be completely liquid-tight and the molecular
sieve crystals must often be oriented in a particular direction, depending on
the pore structure.
EP-A-0 320 033 relates to a composite ceramic micropermeable membrane
suitable for separation and filtration, and particularly for micro-, ultra- or
hyperfiltration. Said membrane comprises a support and a microporous layer, which
microporous layer is applied to said support from a suspension of inorganic material.
Before applying said suspension the affinity between support and suspension is
altered by altering the hydrophilicity and particularly by making the support more
hydrophobic. In a preferred embodiment this is achieved by treating the support
with a silane composition.
In EP-A-0 471 910 a ceramic filter is described which is made from
an aggregate comprising coarse alumina particles and a sintering aid of mixed
alumina and zirconium particles.
Further, GB-A-2,277,885 suggests to make membranes at the inside of
a tube on the basis of silicon nitride materials mixed with oxides. Said method
of making is however not enablingly described. Silicon nitride materials could
- at the moment of the filing of said GB-A - only be made on a practical scale
by gas phase or gas plasma processes. These techniques result in coarse particles
and cannot provide a layer of uniform thickness on the inside of a tube.
In accordance with the invention, it has now been found that when
a relatively dense ceramic oxide layer is provided on the inner wall of a tubular
porous support, a membrane structure is formed which can perfectly be used for
pervaporation and pressure traction processes.
Hence, the invention relates in a first aspect to the use of a tubular
membrane comprising a tubular porous support and a separating top layer from a
ceramic oxide having an average pore diameter ranging between 0.1 and 10 nm on
the inside of the support for separating liquid mixtures on the basis of pervaporation
or pressure traction.
In a second aspect, the invention relates to a method for manufacturing
a tubular pervaporation or pressure traction membrane having an average pore diameter
ranging between 0.1 and 10 nm, comprising flowing a sol which comprises chain-shaped
silicon compounds through a porous tubular support which support has pores of a
diameter ranging between 1 nm and 10000 nm wherein, in the direction of the non-separating
side, pore structures of increasingly large diameter are present under such conditions
that from the sol, a layer of gel is formed on the inside of the support followed
by calcining and sintering at a temperature ranging between 300 and 500°C for 1-10
In yet another respect the invention relates to a method for separating
a liquid mixture through pervaporation or pressure traction utilising a membrane
manufactured according to the method of the invention.
Membranes of the type porous support/separating top layer are already
known for the use in gas separations. In this connection, reference is made to
the article by De Lange et al. titled 'Formation and characterization of supported
microporous ceramic membranes prepared by sol-gel modification techniques', in
J. Membrane Sci. 99 (1995), 57-75. However, as indicated hereinabove, it
is highly surprising that such membranes can also be used when they are in contact
with liquid mixtures, because it was assumed that pervaporation and pressure traction
membranes were only active if they exhibited interaction with the liquid mixture.
However, ceramic separation layers are relatively inert; they do not swell and
contain no selective channel structures like zeolites.
Because, surprisingly, an active interaction between separation layer
and liquid mixture does not seem to be necessary in the use according to the invention,
no conditioning time or acclimatizing step is necessary either, in contrast with
the known (polymeric) pervaporation or pressure traction membranes. This advantage
enables the use of the membranes according to the invention in processes wherein,
in the mixtures to be separated, substantial variations occur in, for instance,
chemical composition, polarity, pressure and/or temperature, or wherein highly
different mixtures are used or processed, as in a so-called 'multipurpose plant'.
The use is possible in batchwise and continuous processes.
Other important advantages of the membranes according to the invention
over the known pervaporation and pressure traction membranes are that they are
chemically resistant and relatively little sensitive to high temperatures up to
300°C and even higher.
Moreover, the ceramic membranes according to the invention have the
advantage that an increase of the temperature and/or pressure involves an increase
of the flux through the membranes, while the selectivity stays substantially the
Depending on the material and the structure of the separating top
layer, the membranes according to the invention can be employed for different uses.
For instance, the membranes according to the invention provide the possibility
of selectively sequestering a component from a reaction mixture continuously and
at relatively low operating costs, so that the equilibrium for a particular reaction
shifts and processes take place more specifically and/or rapidly and higher conversions
are eventually realized. In addition, the membrane can be used for purifying a
particular liquid or for selectively recovering a valuable component. In particular,
the membrane according to the invention is highly suitable for separating azeotropic
The known membranes on the basis of a porous support and a ceramic
separation layer that are used for gas separation are flat membranes. In accordance
with the invention, tubular membranes are presently manufactured, which tubular
membranes are provided, on the inside thereof, with a separating layer. This embodiment
renders it simple to construct modular systems wherein the membrane surface is
easy to scale up. In this manner, the modular size can readily be adapted to the
desired processing capacity in an industrial process. Moreover, such a system
gives hydrodynamic advantages. The mass transfer improves substantially, compared
with flat structures. This means, inter alia, that when high-viscous liquids should
be treated, it is still possible to work at a low water concentration. In tubular
or pipe-shaped channels, a slighter film thickness can easily be created at a relatively
slight energy input, so that the thickness of the boundary layer between membrane
wall and liquids is reduced. Thus, the resistance to transportation from the mixture
to be treated to the membrane decreases.
Consequently, the invention also relates to a tubular membrane comprising
a tubular structure from a porous material, which structure comprises, on the inside
thereof, a ceramic separation layer.
As stated, the membrane used according to the invention for pervaporation
and/or pressure traction purposes comprises a porous support. In fact, this support
can consist of any material that is inert to the liquids to be separated. Very
suitable are supports from a metal oxide, such as aluminum oxide, zirconium oxide
and silicon oxide, from metal, from carbon, and from combinations of these materials.
It is essential that the porosity of the support be greater than that of the separating
layer. Suitable results are obtained with porous materials having pores of a diameter
ranging between 1 nm and 10000 nm. In order not to render the pressure drop over
the complete membrane too great, the porous support material has, in the direction
of the non-separating side, pore structures of increasingly large diameter. Such
supports are already known and commercially available.
On the porous support, a microporous separating layer from a ceramic
material is present. This layer has a thickness preferably ranging between 0.1
nm and 100 µm, more preferably between 1 nm and 5 µm, and is substantially responsible
for the separating action of the membrane. The microporous layer has pores of
an average diameter ranging between 0.1 and 10 nm, and preferably between 0.1
and 2 nm.
The separating layer can consist of different materials of a ceramic
nature, but is preferably at least partly formed from a ceramic oxide. Suitable
materials are silicon oxide, titanium oxide, zirconium oxide and aluminum oxide,
and combinations of these oxides. In the preferred embodiment, the separating
layer is formed from silicon oxide.
The membranes according to the invention can be manufactured in manners
that are known for the manufacture of the inorganic membranes that can be used
in the separation of gases. Such methods are mentioned in, inter alia, the above-mentioned
article by De Lange et al.; in the article by Saracco et al. in J. Membrane Sci.
95 (1994) 105-123; in the article by Brinker et al. in J. Membrane Sci.
77 (1993) 165-179; and in the contribution by Keizer et al. in 'Ceramics
Today - Tomorrow's Ceramics', P. Vincenzini (ed.), Elsevier Science Publishers
B.V. (1991), 2511-2524.
In particular, the membranes used according to the invention for
pervaporation and pressure traction are manufactured by providing a layer from
a gel or sol of a suitable starting material, for instance a gel or sol on the
basis of silicon, aluminum, zirconium and/or titanium compounds, on a porous support,
and then calcining and sintering them to obtain a ceramic material. If necessary,
this treatment can be repeated some times until the ceramic layer has acquired
the desired thickness and/or a desired degree of porosity.
Although the ceramic separation layers are generally stable at high
temperatures up to about 1000°C, the calcining and sintering steps must not be
carried out at such a high temperature, because in that case, too coarse pores
are formed. The same problem presents itself when the sintering time is too long.
For instance, silicon oxide is stable up to 900°C. However, when
a silicon-containing gel is sintered at temperatures above 500°C and/or the sintering
treatment is continued longer than 10 hours, pores are obtained that can be considerably
larger than 5 nm. Separation layers having pores of such large dimensions are not,
or at least less, suitable for pervaporation and pressure traction purposes. Titanium
oxide is much less thermally stable than silicon oxide. It is stable only up to
about 350°C. Therefore, a layer from this compound is manufactured by carrying
out a sintering step at 300°C at a maximum.
Preferably, a sol of a silicon, aluminum, zirconium and/or titanium
oxide is prepared by hydrolyzing a suitable organometallic compound. Next, from
this sol, solvent is extracted through the porous support, as a result of which
a gel layer is formed on the support.
In a particular embodiment, the invention relates to a method for
manufacturing a tubular pervaporation or pressure traction membrane, comprising
flowing a sol which comprises chain-shaped silicon compounds through a porous tubular
support under such conditions that on the inside of the support, a gel layer is
formed from the sol, followed by calcining and sintering at a temperature ranging
between 300 and 500°C for 1-10 hours.
For a proper operation of the separating layer, it is desired that
a silicon oxide layer be obtained having fewest possible pores greater than about
2 nm. Such a layer can be obtained by flowing the sol through the tubular porous
support one or more times for 1-40 seconds, followed by calcining and sintering
at 300-500°C, preferably at a temperature of 370-425°C, for 1-10 hours.
In a preferred embodiment of the method, as sol, a condensate of
an acid-hydrolized organometallic compound is used, for instance and preferably
tetraethyl orthosilicate in ethanol.
In more detail, in the last preferred embodiment, a separating layer
from silicon oxide is provided on the inside of a tubular porous support, starting
from a mixture of ethanol and tetraethyl orthosilicate. This mixture is hydrolized
with nitric acid and water. Next, the reaction mixture is refluxed for about 3
hours, so that a condensate is formed. This condensate is dispersed into a sol
in ethanol. When this sol is flown through the porous tubular support, ethanol
is extracted, as a result of which the sol concentration increases and a gel is
formed. Because of the use of the condensed, chain-shaped silicon structures, a
thin top layer having small pores can be obtained. It is further noted that the
use of other acid or base-hydrolized organometallic compounds proceeds in an analogous
In comparison with the known membranes from organic polymers, the
membranes according to the invention are chemically and thermally stable. Depending
on the ceramic material and the desired pore structure, temperatures above 300°C
can be used in the pervaporation or pressure traction processes.
In fact, the membrane according to the invention may and can be contacted
with all liquid mixtures, and in particular with all conventional solvents, without
the membrane being affected thereby. Possibly, only the sealing between the membrane
and the membrane module may present problems. A sealing material that can suitably
be used for most solvents at temperatures below 150°C is Viton®; at temperatures
up to 300°C, Calrez® is a suitable sealing material. At working temperatures
above 300°C, special compounds on the basis of ceramics, cermets or other metal-ceramic
combinations or metal should be used.
More in particular, the membranes according to the invention are
used for separating components from liquid mixtures. Particularly, all types of
mixtures of liquids or organic solvents can be separated. Polar as well as nonpolar
mixtures, and mixtures of polar and nonpolar liquids can be separated.
In a preferred embodiment, mixtures of organic solvents with water
are separated from each other.
In particular, it has proved possible to dehydrate lower alcohols,
such as methanol, ethanol and 2-propanol, utilizing the membranes according to
the invention, while selectivities of 400, 200 and 600 respectively were found
at water contents of 2 wt.% in the alcohol fraction. In this connection, the selectivity
(α) is defined as:
α = (mwater/malc)permeate/ (mwater/malc)feed,
mi:mass percent component i.
The fluxes through the membrane were 50, 150 and 160 g/m2/h
respectively. At higher water contents in the feed, the selectivities decrease
and the fluxes increase.
Accordingly, the overall performance of the membranes according to
the invention is comparable with the commercially applied organic-polymer membranes.
However, unlike the known membranes, the performance of the ceramic membranes remained
substantially the same over a period of 3 months. Moreover, the performance was
not appreciably affected by contacting the membrane according to the invention
with different alcohol mixtures and mixtures having different water contents.
Methanol could be separated from methyl-tert.-butylether (MTBE) with
a selectivity of about 19 and a flux of 41 g/m2/h, 9 wt.% methanol in
MTBE being present in the feed flow.
Presently, the invention will be specified with reference to the
examples given below.
A tubular asymmetrical Al2O3-membrane (T1-70),
produced by the firm US-Filter, was provided with an SiO2-top layer
on the inside of the tube. For that purpose, a mixture of tetraethyl orthosilicate
(TEOS) and ethanol was hydrolized with nitric acid (1M) and water (molar ratio
TEOS/ethanol/nitric acid/water = 1/3.8/0.085/6.4). 0.1 mole TEOS (20.8 g) was
started from. The mixture was refluxed at 80°C for 3 hours and then dispersed in
1 liter ethanol. Thus, a sol was obtained that was used for applying the top layer.
The tubular support was twice coated by flowing through with the sol for 4 seconds.
This involved the tubular support being clamped vertically. Next, the membranes
were sintered at 400°C for 3 hours. The filters used had a pore diameter of 4 nm
(40 Å) on the inside of the tube (lumen side). The pore diameter of the silica
layer provided was between 0.1 and 0.5 nm (1 Å and 5 Å). The pore diameter was
determined according to the modified Horváth-Kawatzoe model for nitrogen adsorption
in cylindrical pores, as described in chapter IV of the ScD dissertation of R.S.A.
de Lange (1993), University of Twente. The thickness of this layer varied between
50 nm and 100 nm.
To the selective silica side of this membrane, mixtures of water
with different alcohols were presented: water/methanol, water/ethanol and water/isopropanol.
At a feed composition of 2 mass% water and 98 mass% alcohol, high selectivities
were found of 400, 200 and 600 respectively. Here, the selectivity (α) is
α = (mwater/malc)permeate/(mwater/malc)feed,
mi:mass percent component i.
The permeate fluxes measured were 50, 150 and 160 g/m2 membrane/hour.
The addition of carboxylic acids such as oleic acid and erucic acid
to the above mixtures resulted in higher permeate fluxes and selectivities. This
increase proved to correspond to the composition of the mixture. At an equimolar
alcohol/acid ratio, the final fluxes were 50-100% higher than those for alcohol-water
mixtures. In the case of methanol, ethanol and isopropanol, these fluxes were 77,
155 and 260 g/m2
membrane/hour, the associated selectivities being 490,
240 and 750.
An increase of the amount of water in the feed resulted in an increase
of the permeate flux and a decrease in selectivity. For 5% water, a selectivity
of 60, 100 and 500 was achieved in the case of methanol, ethanol and isopropanol
respectively, the fluxes being 100, 200 and 210 g/m2
concentrations of water in the feed resulted in still lower selectivities and higher
The membranes were used for 13 weeks for separating alcohol/water
mixtures with different alcohols and different percentages of water (1-20 mass%).
During these three months, no significant change was found in the membrane selectivity
or size of the permeate flux.
Example 2. Dehydration of alcohols at high temperatures.
The membranes were included in a stainless-steel housing as described
in De Lange et al., 'Preparation and characterization of microporous sol-gel derived
membranes for gas separation applications' in 'Better ceramics through chemistry
V', Hampden Smith M.J.; Klemperer, W.G.; Brinker, C.J., Materials Res. Soc. Pittsburg,
U.S.A., Mat. Symp. Proc. 271 (1992), 505-510, after which a mixture of alcohol
and water was presented. The temperatures and pressures used were chosen to be
above ambient (at normal pressure (100 kPa), the boiling points of methanol, ethanol
and isopropanol are 64, 72 and 78°C respectively). At an amount of water of 9 wt.%
in methanol and at 81°C and 115 kPa excess pressure, a permeate flux of 660 g/m2
hour was measured, while the selectivity was hardly affected (α-20). At 100°C
and 250 kPa excess pressure, the flux was 1050 g/m2 hour, with the selectivity
remaining the same. For water in ethanol (2 mass% water) at 100% and 180 kPa,
a tripling in flux was measured, while here, too, the selectivity was hardly affected
(α-155). For water in isopropanol mixtures at 100°C with an increasing water
content of from 2.8 to 7.7 mass%, the flux increased from 1252 g/m2
hour to 2613 g/m2 hour.
The silica membranes described are used for separating methanol from
a methanol/methyl tertiary butylether (MTBE). MTBE is used as lead substitute in
petrol and MTBE is one of the fastest growing bulk chemicals of the last decade.
MTBE is produced via a reaction of tertiary butylether with an excess measure
of methanol. The mixture eventually produced is characterized by an azeotropic
evaporation curve, which complicates the reprocessing with distillation. The removal
of methanol is possible with a ceramic silica membrane. For this purpose, different
mixtures of MTBE and methanol were presented as feed. At a mass% of between 6 and
17 methanol in MTBE, the flux of the membranes was between 10-50 g/m2
The membrane exhibits selectivity for methanol, the selectivity was 2-19, with
a maximum 19 at 9 mass% methanol.