Note: Descriptions are shown in the official language in which they were submitted.
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Water Treatment Process
The present invention relates to a method for the preparation of purified
water; more
particularly, it relates to a method for the preparation of purified water by
means of a
membrane.
Water purification, e.g. preparation of potable water or chemically pure water
from
water containing dissolved salts or other chemicals, is used in areas where
pure water
is not available.
Two widely used techniques are distillation and reverse osmosis. 'These
methods are
used for desalinisation of water and for purification of brackish water and
are widely
applied. However, these methods are relatively expensive in use as
distillation, even
using flash distillation and vacuum distillation techniques require large
amounts of
energy and large scale plant. Reverse osmosis requires the use of high
pressures and
membranes capable of withstanding these pressures.
Other polymeric membranes have been suggested but these have suffered defects
in
practice due to their lack of robustness, a tendency to foul and difficulty to
produce in
2 0 large sizes.
Zeolite membranes are known to be able to remove water from organic fluids and
several processes and applications have been disclosed. However, in these
previously
disclosed applications, relatively small amounts of water are removed from the
organic liquid, which is required to be dehydrated. We have found that with
previously disclosed zeolite membranes, contact with a liquid mixture which is
predominantly water in order to separate water from other compounds is not
practicable. This is thought to be due to the defects in the membrane,
allowing
contaminants to be present in the water and, whereas this is not significant
when a
dehydrated organic liquid is required, it is not acceptable when pure water is
required.
Other defects arise from the nature of the zeolite membrane; these include
cracking
and ion-exchange. When water containing salts come into contact with the
zeolite
membrane, the membrane will 'crack' and defects appear which affect the
utility of
the membrane. With salts and other ionic compounds present in the water, ion-
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exchange can take place with the zeolite membrane and the membrane loses its
effectiveness.
Hitherto with known zeolite membranes it has not proved possible to remove
contaminants from water when the contaminants are only present in small
amounts as
the volume of water which has to pass through the water makes it impractical
for use.
However we have found surprisingly that with our membranes it has proved
practical
to remove contaminants which are present in a small amount and we have devised
a
method of purifying water using a treated membrane which reduces these
problems.
According to the invention there is provided a method for obtaining purified
water
from water containing contaminants which method comprises passing the
contaminated water through a crystalline zeo-type material to separate the
contaminants from the water, characterised by the zeolite membrane having been
treated by contact with a with a silicic acid and/or polysilicic acid or a
mixture of
silicic and/or poysilicic acids or with an organic silicate.
Zeo-type materials are also known as molecular sieves which are widely known
and
2 0 used. They comprise an extended network of channels formed from
silicon/oxygen
tetrahedrons joined through the oxygen atoms. Zeolites and alumino-silicates
are the
most commonly known form of zeo-type materials and the present invention is
applicable to any membrane formed from zeo-type materials and particularly
applicable to zeolites and alumino-silicates. In the "Atlas of Zeolite
Structure Types",
Meier and Ofsen, 1987, Polycrystal Book Service, Pittsburg USA, various types
of
structure are described and, for example, those described as having LTA, MEL,
MFI
or TON structure can be used.
In "New Developments in Zeolite Science and Technology Proceedings of the 7th
International Conference, Tokyo, 1986, page 103, another class of zeo-type
materials
are disclosed as crystalline aluminophosphate, silicoalumina phosphates and
other
metallo-alumino phosphates.
Typical zeolites which can be used in the present invention are Zeolites
include but
are not limited to, 3A, 4A, SA, 13X, X, Y, ZSMS, MAPOs, SAPOs, Silicalite,
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(3, 9,etc.
The porous supports on which zeo-type membranes are formed and which can be
used in the present invention include those formed of metals, ceramics, glass,
mineral, carbon or polymer fibres or cellulosic or organic or inorganic
polymers.
Suitable metals include titanium, chromium and alloys such as those sold under
the
Trade Marks "Fecralloy" and "Hastalloy" and stainless steels. The porous
supports
may be formed of a mesh or from sintered metal particles or a mixture of both.
These
are commonly sold in the form of filters.
Porous ceramics, glass mineral or carbon materials can be used including
porous
silicon and other carbides, clays and other silicates and porous silica. If
desired, the
support can be a zeolite formed by compression or using a binder. The shape of
the
support is not critical, for example, flat sheet, tubular, wound spiral, etc.
can be used.
If polymeric materials are used, these can optionally be film coated with
metal or
metal oxide or a silicic acid as herein defined.
The porous support can be also be a granular solid e.g. formed of particles of
a
closely packed material such as a pellitised catalyst.
The present invention can be used with porous supports of any suitable size
although,
for large flux rates through a membrane, large pore sizes are preferred.
Preferably
pore sizes of 0.01 to 2,000 microns, more preferably of 0.1 to 200 and ideally
of 1 to
20 microns are used. Pore sizes up to 300 microns can be determined by bubble
point
pressure as specified in ISO 4003. Larger pore sizes can be measured by
microscopic
methods.
The larger the relative amount of the surface which is composed of voids in
general
the more suitable the porous support.
The membranes which can be treated to be used in the method of the present
invention can be formed by any method, for example by crystallisation from a
gel or
solution, by plasma deposition or by any other method such as electro
deposition of
crystals on conducting substrates e.g. as described in DE 4109037.
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When the membrane comprising a film of zeo-type material is prepared by
crystallisation from a synthesis gel, any of the methods described in the
prior art can
be used.
The synthesis gel used in the process can be any gel which is capable of
producing
the desired crystalline zeo-type material. Gels for the synthesis of zeo-type
materials
are well known and are described in the prior art given above or, for example,
in EP-
A-57049, EP-A- 104800, EP-A-2899 and EP-A-2900. Standard text books by D W
Breck ("Zeolites Molecular Sieves, Structure Chemistry and Use") published by
John
Wiley (1974) and P.A Jacobs and J.A Martens (Studies in Surface Science and
Catalysis No. 33, Synthesis of High Silica Alumino silicate Zeolites"
published by
Elsevier (1987), describe many such synthesis gels. The process which can be
used
includes conventional syntheses of zeo-type materials, except that the
synthesis is
carried out in the presence of the porous support. Most commonly, gels are
crystallised by the application of heat.
The treated membrane which is used in the process of the invention can be
prepared
by a process which comprises deposition or crystallisation from a growth
medium, In
one embodiment of the invention the growth medium can be used in two different
2 0 methods.
In the gel method (method 1 ) for forming the membrane the gel used to form
the
membrane preferably has a molar composition in the range of
(1.5 - 3.0)NazO : (1)AlzO; : (2.0)SiO~ : (50-200)Hz0
and the method used can be used in any of the methods disclosed in the
references
listed above.
In the liquid solution method (method 2) the liquid solution used to form the
membrane preferably has a molar composition in the range of :-
(6 - 10.0)Na~O : (0.2)A120, : (1.0)SiOz : (150-250)H20
The liquid solution preferably contains a maximum amount of the compound
capable
of crystallising to form a zeo-type material whilst still remaining a liquid
solution. By
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maximum amount is meant the maximum amount which can be maintained in
solution so that no precipitation occurs before zeolite formation.
Methods ( 1 ) and (2) can be used under the conditions listed below and method
( 1 )
and method (2) can be used either on their own or with method {1) followed by
method {2) or vice versa.
The conditions which can be used for forming the membrane are with a
temperature
of the growth solution preferably in the range of 50 to 100°C and the
pH can be
adjusted e.g. to pH of 12.5 to 14 by addition of sodium hydroxide or ammonia.
If
desired the sodium ion concentration can be increased without increasing the
pH by
the addition of a sodium salt such as sodium chloride. The growth solution can
be
seeded with zeolite crystals of the desired zeolite to be synthesised. The
membrane
can be washed to pH neutral after membrane formation prior to any post-
treatment.
The porous support can be contacted with the growth medium by immersion or by
pouring the growth medium over the support with the support held substantially
horizontal, either face up at the bottom of a container, or face down at the
surface of
the growth medium, or it can be passed over one or both sides of the support,
with the
2 0 support held substantially horizontal, or it can be passed over one or
both sides of the
support with the support held substantially vertical or the support can be in
any
intermediate position.
The growth medium can be kept static, stirred, tumbled or passed over or
around the
support, alternatively the growth medium can be passed over both sides of the
support
with the support held substantially horizontal or at any intermediate
position:
Pressure may also be applied but it is usually convenient to conduct the
crystallisation under autogenous pressure. Preferably the porous support is
completely
immersed in the growth medium; alternatively, if desired, only one surface of
the
support may be in contact with the growth medium. This may be useful, for
example, if it is desired to produce a membrane in the form of a tube, where
only the
inside or outside of the tube need be in contact with the growth medium.
It may be useful if it is. desired to produce a membrane containing two
different
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zeolites, one on each side of the support. Use of such a bi-functional
membrane
would be equivalent to using two separate membranes, each carrying a different
zeolite.
If desired, the treatment with the gel or liquid solution can be repeated one
or more
times to obtain thicker membrane coatings.
Preferably the porous support is pre-treated with a zeolite initiating agent.
The zeolite
initiating agent is preferably a cobalt, molybdenum or nickel oxide or it can
be
particles of a zeolite, e.g. the zeolite which it is intended to deposit on
the porous
support, or any combination of these. Another example of an initiating agent
is a
compound which can deposit a zeo-type pre-cursor material e.g. a silicic acid
or
polysilicic acid.
The zeolite initiation agent can be contacted with the porous support by a wet
or dry
process. If a dry process is used, the particles of the zeolite initiation
agent can be
rubbed into the surface of the porous material, or the porous material surface
can be
rubbed in the particles.
Alternatively the particles of the zeolite initiation agent can be caused to
flow over
and/or through the porous support, or pulled into the support by means of a
vacuum.
If a wet process is used, a liquid suspension of powder of the zeolite
initiation agent
is formed and the liquid suspension contacted with the porous support to
deposit the
zeolite initiation agent on the support.
Before contacting the surface of the porous support with the zeolite
initiation agent
the surface is preferably wetted with wetting agent such as an alcohol, water
or a
mixture of these.
When a silicic acid is used as an initiating agent it can be a silicic acid as
herein
defined.
In the present specification by silicic acid is meant monosilicic, low, medium
and
high molecular weight polysilicic acids and mixtures thereof.
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As well as treatment with silicic acids the zeo-type materials can be treated
with
alkylorthosilicates such as tetra ethyl ortho silicate (TEOS) and tetra
isopropyl ortho
silicate (TIPOT) akoxyorthosilicates such as tetramethoxyortho silicate which
form a
polysilicic acid when applied to the zeo-type material. These
alkylorthosilicates and
alkoxyorthosilicates form mesoporous silica compounds which consist
essentially of
a series of polysilicic acid units linked together, each unit comprising a
polysilicic
acid molecule as described in GB Patent Application 9316350.9 and comprising a
plurality of three dimensional species linked together with each species
either having
silicon atom bridges with an oxygen atom between each silicon atom or hydroxyl
groups on the silicon atoms.
The treatment of zeo-type materials by silicic acids is described in WO
96/09110.
Methods of making silicic acids are described in GB Patent Application 2269377
and
a preferred method is by acidification of a sodium silicate solution followed
by
separation of the silicic acid by phase separation using an organic solvent
such as
tetrahydrofuran. The organic phase can then be dried and anhydrous silicic
acid
separated e.g. by addition of n-butanol to obtain a substantially anhydrous
solution of
2 0 silicic acid. The degree of polymerisation of the silicic acid depends on
the actual
conditions used e.g. the time the sodium silicate solution is in contact with
the acid
before addition of the organic solvent, temperature etc.
The silicic acid used in the present invention preferably has an average
molecular
weight in the range of 96 to 10,000 and more preferably of 96 to 3220.
The silicic acids are known compounds and are usually prepared as a mixture of
acids
with a range of different molecular weights and this mixture is suitable for
use in the
present invention.
The silicic acids are combination of silicon, oxygen and hydrogen, linked
together in
the case of polysilicic acids through an oxygen bridge, with terminal -OH
groups.
They have a generic formula of Si~OP(OH)~ where n, p and r can vary from n=1,
p=0,
r--4 in the case of monosilicic acid through to n=8-12, p=12-20, r=8-12 in the
case of
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medium molecular weight silicic acids through to n=20-32, p=36-60 and z=8-20
in
the case of a higher molecular weight polymers.
The membrane can be treated with anhydrous silicic acid and a preferred method
is to
contact the membrane with an anhydrous solution of the silicic acid e.g. by
dipping,
pulling through with vacuum, forming with pressure etc. Preferably the
solution
containing the silicic acid is removed e.g. by evaporation at room temperature
and/or
by heating.
Alternatively cross-linking can be accomplished by treating with an acid or
base or
with acidified or basified water e.g. of pH 2 to 12 preferably of 4 to 10.
The silicic acids used in the present invention can be used in "narrow"
molecular
weight distribution as formed or in a combination of different molecular
weight
ranges.
Greater flexibility can be introduced into the final membranes by treating
them with a
flexibilising agent by adding e.g. a hydroxy terminated polysiloxane into the
silicic
acid solution before treatment of the membrane.
When the membrane is treated by a alkylorthosilicate the membrane is
preferably
treated as above using the alkylorthosilicate in place of the silicic acids.
The membranes treated by this process are improved in terms of their
performance
and membrane strength compared with untreated membranes.
The method of the invention can be used to produce water with very low levels
of
contaminants e.g. below l Oppm from contaminated water..
The degree of purification of the water, i.e. the level of contaminants still
present in
the purified water will depend on the thickness and nature of the zeolite
membrane,
the pressure applied to the water, the time of contact of the contaminated
water with
the zeolite membrane, temperature, etc. At the very low levels of contaminants
obtained, particularly when the exact nature of the contaminants is unknown it
is
convenient to measure the conductivity of the water. De-ionised water
typically has a
conductivity of about 0.1 to 30p,S/cm and distilled water has a conductivity
of about
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1 to 2 ~S/cm. The method of the present invention can produce water with a
conductivity of below these values which shows that there is a very low level
of
contaminants.
The water can be purified by being passed through the membrane by applying a
reduced pressure or vacuum to one side of the membrane and this reduction in
pressure causing the water to pass through the membrane and be separated from
the
impurities.
Owing to the action of the treated membrane, a wide range of contaminated
water can
be purified, e.g. sea water, brackish water, water contaminated with
industrial
chemicals such as hydrocarbons, organic chlorine compounds, metals, detergents
etc.,
biological materials such as urines etc. and water contaminated by human or
animal
use e.g. the so-called "grey" water obtained for example from washing,
showers,
bathwater etc. If desired, by increasing the severity of the treatment
conditions,
substantially pure water or de-ionised water can be obtained.
Even with sea-water which has a relatively high conductivity levels of
conductivity
(30,000 ~,S/cm ) below 30 ~.S/cm can be achieved indicating a very low level
of salt
2 0 in the water.
It is surprising that by treating a zeolite membrane as described above
enables the
membrane to be used to purify contaminated water in a way not possible with
the
untreated zeolite membrane and to obtain such very low levels of conductivity.
The method can be used in a pervaporation process or as a reverse osmosis
process.
In general reverse osmosis processes have a higher flux.
The invention is described in the following Examples in which Example 1 is an
example of the preparation of a membrane using a known method, Example 2 is an
example of preparation of a post-treatment solution, Example 3 is an example
of the
test procedure, Example 4 is an example of the post-treatment procedure and
Examples 5 to 11 are examples of production and testing of treated membranes.
Example 1 Membrane Growth
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The substrate used was a Bekipor (Trade Mark) ST SBL3 filter. This consists of
very
fine 316 stainless steel fibres brought together in a 3-dimensional
labyrinthic
structure. The fibres are arranged randomly in a homogeneous web. This web is
further compacted and sintered to give a very strong metallic bond at each
fibre
crossing. The average pore size is approximately 5.3 microns and the diameter
of the
wire on the surface is 6.5 microns.
A 7cm. disc of the metal mesh was placed in a 100m1 flat bottomed petri dish
which
had previously been cleaned by washing with de-ionised water, acetone, toluene
and
finally acetone before being dried in an oven at 90 degrees C. for 3 hours.
(a) Cobalt pre-treatment:
The mesh was placed in a beaker to which was added SOmI of 0.1 M cobalt
nitrate
solution, the beaker was placed in an oven at 90°C to dry, then the
mesh was removed
from the beaker and fired at 250°C for 4 hours. The mesh was removed
from the
furnace and allowed to cool. This procedure was repeated 2 more times to
obtain a
good cobalt oxide coating.
(b) Zeolite Pre-treatment
Zeolite 4A powder was rubbed into the active side of the substrate, which had
already
been cobalt coated as above, using a gloved finger, until no more zeolite will
rub into
the surface, any excess zeoiite was tapped off.
Two solutions A and B were prepared separately
in two S00 ml glass bottles as follows:-
Solution A
24.498 Sodium Aluminate, 3.758 Sodium Hydroxide and 148.608 de-ionised water
were mechanically shaken until dissolved. The Sodium Aluminate had an actual
composition 62.48% A1203, 35.24% Na20, and 2.28% H20.
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Solution B
50.57g Sodium Silicate of composition 14.21% Na20, 35.59% Si02 and 50.20% H20
was dissolved in 148.60g de-ionised water.
Solution A was added slowly to solution B with both stirring and shaking by
hand to
ensure complete and even mixing (it is important that no lumps of hydrogel are
formed). This resulted in a hydrogel having a molar composition
2.01 Na20 : A12O3: 2.0 Si02 : 120.0 H20
100 ml of the hydrogel was slowly poured into a growth vessel containing the
cobalt
oxide treated and zeolite rubbed mesh in a vertical position. The growth
vessel was
placed in a domestic pressure cooker together with a beaker containing the
remaining
hydrogel solution. The pressure cooker was placed in an oven preheated to 100
degrees C. for 5 hours. Subsequently it was removed from the oven and allowed
to
cool for 30 minutes. The growth vessel was removed and the solution poured
away.
The metal mesh was carefully removed with a long flat rod ensuring that the
mesh
was not bent or damaged in any way. The mesh was placed in a glass beaker and
washed three times with 100m1 aliquots of de-ionised water, swirling the
solution
each time to ensure complete removal of residues. The membrane was allowed to
air
dry overnight.
2 5 The surface of the dried coated mesh was subsequently wiped clean with a
clean lens
tissue in order to remove any loose powdery deposits which may have formed on
the
surface. The mesh was inverted and the process repeated
The mesh was reinverted and the top surface cleaned again. It was then washed
with
de-ionised water and left to air dry.
X-ray Analysis showed this to be a Zeolite 4A
Example 2 Preparation of TEOS For Post Treatment of Membrane
The post treatment solution was prepared by placing 40m1 of (TEOS) into a
clean, dry
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beaker, adding 180 ml of deionised water and 180m1 of ethanol. The mixture was
then stirred at 300r.p.m. for a period of 5 mins.
Example 3 Membrane Test Procedure
A membrane is loaded into a pervaporated test cell in an apparatus as shown in
the
accompanying drawing. The apparatus consists of a stainless steel test cell
(1) fitted
with a pressure gauge (2) relief valve (3) magnetic stirrer (4) and
thermocouple (5).
The membrane prepared as in Example 1 was placed on a porous stainless steel
disc
(6) and was sealed into the cell with O ring (7).
The cell could be simultaneously heated and stirred by a heater /stirrer (8).
Vacuum
could be applied through line (9). Vapour removed from the test cell was
condensed
out in cold trap ( 10). Line (9) had a pressure gauge ( 11 ) and relief valve
( 12).
The test cell was filled with an isopropanol/water (IPA/H20) mixture (90/10
wt.
respectively). The membrane was tested at approximately 70°C.
The pressure on the side of the membrane remote from the liquid was reduced to
4
2 0 mbar (0.4 kN). Permeate was collected over periods of 8 hours and weighed,
and
small aliquots were analysed, feed water concentration was monitored
throughout.
Examine 4 Post-treatment Procedure of Membrane
After the initial test of the untreated membrane in the apparatus of Example 3
with
IPA/Water, the cell was emptied, rinsed with 2 x 50 ml aliquots of ethanol and
then
another SOmI aliquot of ethanol was placed into the cell which was then placed
under
vacuum for 30 mins.
The ethanol was removed from the cell and the TEOS post-treatment solution
prepared as .in Example 2 was poured into the cell. The cell was then treated
to 70°C
for a period of 24 hours, with the downstream side under vacuum. After this
period,
the mix was removed, the heat switched off, vacuum removed and then compressed
air was passed over the membrane for a period of one hour.
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Examine 5
A membrane produced by the method of Example 1 was treated under pervaporation
conditions described on Example 3 and the results shown in Table 1 below
Table 1
Isopropanol/water mixture at 70~C
Time on Stream Feed Water Permeate WaterPermeate
Water Flux
(~
(hours) % weight % weight kg/mz/day
1 11.10 90.11 147.75
1.5 6.00 96.47 88.00
2.0 4.36 94.81 72.87
4.0 2.94 81.14 29.74
5.5 0.47 30.57 4.91
The membrane was then post treated as in Example 4, the membrane was re-tested
under pervaporation conditions as in Example 3 and the results shown in Table
2.
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Table 2
Isopropanol/water mixture
at 70C
Time on Stream Feed Water Permeate WaterPermeate
Water Flux (J)
(hours) % weight % weight kg/m2/day
0.5 7.36 98.62 102.82
0.8 5.27 100.00 99.40
1.3 4.01 99.69 63.05
1.8 2.60 98.28 30.96
3.2 0.75 89.06 10.51
3.5 0.63 85.00 1.13
Example 6
A membrane produced by the
method as described in
Example 1 was treated under
2 pervaporation conditions
0 described in example 3
and the results shown in
Table 3
Table 3
IsopropanoUwater mixture
at 70C
Time on Stream Feed Water Permeate WaterPermeate
Water Flux (J)
(hours) % weight % weight kg/m2/day
1.0 9.42 87.46 156.34
1.5 - 3.33 86.31 65.84
2.0 2.10 76.84 45.81
4.0 1.15 46.30 12.65
5.5 0.37 18.88 1.36
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The membrane was then post treated as in Example 4, the membrane was re-tested
under pervaporation conditions as in example 3 and the results shown in Table
4.
Table 4
Isopropanol/water mixture at 70°C
Time on Stream Feed Water Permeate WaterPermeate
Water Flux
(J)
(hours) % weight % weight kg/m2/day
0.5 10.16 98.83 103.04
0.83 6.86 100.00 99.40
1.33 4.91 99.53 62.95
1.83 3.39 97.43 30.69
3.17 0.40 88.48 10.44
3.5 0.28 85.00 1.13
Examines 7-11 Various different water samples were purified using the membrane
of the invention.
Examule 7
The membrane used was prepared as in Example 6 and after post treatment was re-
tested under pervaporation conditions as in Example 3 and the results shown in
Table
5.
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Table S
Tap Water at 70°C
Time on Stream Permeate Water Flux (J)
(hours) kg/mZ/day
1 269.6
2 270.2
3 274.1
4 274.7
279.2
6 280.2
Conductivity
Tap Water 660 ~.S/cm
Tap Water Permeate 2.2 ~,S/cm
Example 8
The membrane used was prepared as in Example 6 and after post treatment was re
tested under pervaporation conditions as in Example 3 and the results shown in
Table
6.
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17 _
Table 6
De-ionised Water at 70°C
Time on Stream Permeate Water Flux (J)
(hours) kg/mZ/day
1 394.9
2 368.1
3 346.3
4 339.4
Conductivity
De-ionised Water 1.8 ~,S/cm
De-ionised Water Permeate 0.8 ~.S/cm
Example 9
The membrane used was prepared as in Example 5 and after post treatment was re-
tested under pervaporation conditions as in Example 3 and the results shown in
Table
7
Table 7
Soapy Water at 70°C
Time on Stream Permeate Water Flux (J)
(hours) kg/m2/day
1 129.1
2 154.1
3 - 172.4
4 182.1
5 183.9
6 182.2
7 180.7
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Conductivity
Soapy Water 771 ~,S/cm
Soapy Water Permeate 24.8 p,S/cm
Examule 10
The membrane used was prepared as in Example 5 and after post treatment was re
tested under pervaporation conditions as in Example 3 and the results shown in
Table
8.
Table 8
Urine at 70°C
Time on Stream Permeate Water Flux (J)
(hours) kg/mz/day
1 91.9
2 77.1
3 77.2
4 71.9
5 72.3
6 70.1
Conductivity
Urine 12,600 ~,s/cm
Urine Permeate 25.6 ~.s/cm
Examule 11
The membrane used in Example 6 and after post treatment was re-tested under
pervaporation conditions as in Example 3 and the results shown in Table 9.
CA 02306847 2000-04-18
PCT/GB98/03251
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Table 9
Synthetic Sea Water at 70°C
Time on Stream Permeate Water Flux (J)
(hours) kg/m2/day
1 96.1
2 97.0
3 96.0
4 103.7
5 104.6
6 91.9
Conductivity
Sea Water 31,000 ~,S/cm
Sea Water Permeate 25.2 ~,S/cm
