Molecular sieve abstract
A process is described for the manufacture of crystalline molecular
sieve layers with good para-xylene over meta-xylene selectivity's
good para-xylene permeances and selectivities. The process requires
impregnation of the support prior to hydrothermal synthesis using
the seeded method and may be undertaken with pre-impregnation masking.
The crystalline molecular sieve layer has a selectivity (.alpha..sub.x)
for para-xylene over meta-xylene of 2 or greater and a permeance
(Q.sub.x) for para-xylene of 3.27.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px)
or greater measured at a temperature of .gtoreq.250.degree. C. and
an aromatic hydrocarbon partial pressure of .gtoreq.10.times.10.sup.3
Pa.
Molecular sieve claims
What is claimed is:
1. A crystalline molecular sieve layer having a selectivity (.alpha..sub.x)
for para-xylene over meta-xylene of 2 or greater and a permeance
(Q.sub.x) for para-xylene of 3.27.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px)
or greater measured at a temperature of .gtoreq.250.degree. C. and
an aromatic hydrocarbon partial pressure of .gtoreq.10.times.10.sup.3
Pa.
2. A crystalline molecular sieve layer as claimed in claim 1 wherein
the molecular sieve layer is carried on a porous support.
3. A crystalline molecular sieve layer as claimed in claim 1 wherein
the crystalline molecular sieve has been grown from molecular sieve
seeds.
4. A crystalline molecular sieve layer as claimed in claim 1 wherein
the permeance for para-xylene is 5.45.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px)
or greater.
5. A crystalline molecular sieve layer as claimed in claim 1 wherein
the selectivity for para-xylene over meta-xylene is 2.5 or greater.
6. A crystalline molecular sieve layer as claimed in claim 2 which
has a (.DELTA..alpha..sub.px/mx /.DELTA.t) at t=2 hours of greater
than 0.
7. A crystalline molecular sieve layer as claimed in claim 1 measured
at an aromatic hydrocarbon partial pressure of .gtoreq.500.times.10.sup.3
Pa.
8. A crystalline molecular sieve layer as claimed in claim 1 which
is reparated and which has after reparation a selectivity (.alpha..sub.x)
for para-xylene over meta-xylene of 10 or greater and a permeance
(Q.sub.x) for para-xylene of 4.36.times.10.sup.-9 mole(px)/m.sup.2.s.Pa(px)
or greater measured at a temperature of .gtoreq.250.degree. C. and
an aromatic hydrocarbon partial pressure of .gtoreq.10.times.10.sup.3
Pa.
9. A crystalline molecular sieve layer as claimed in claim 7 measured
at a temperature of 360.degree. C. or greater.
10. A crystalline molecular sieve layer as claimed in claim 1 wherein
the crystalline molecular sieve is an MFI type molecular sieve.
11. A process for the manufacture of a crystalline molecular sieve
layer according to claim 1 which process comprises: a) providing
a support having deposited thereon seeds of molecular sieve crystals
of average particle size of 200 nm or less, b) impregnating the
support with an impregnating material before or after deposition
of the seeds of molecular sieve, c) contacting the impregnated support
having seeds deposited thereon with a molecular sieve synthesis
mixture, d) subjecting the impregnated support having seeds deposited
thereon to hydrothermal treatment, whilst in contact with the molecular
sieve synthesis mixture, to form a crystalline molecular sieve layer
on the support, and (e) removing the impregnating material from
the support.
12. A process as claimed in claim 11 wherein a pre-impregnation
masking layer is applied to the support prior to impregnation and
is subsequently removed after impregnation.
13. A process as claimed in claim 11 wherein the molecular sieve
synthesis mixture when formulated for the manufacture of a silicon
containing molecular sieve, comprises a H.sub.2 O:SiO.sub.2 molar
ratio within the range of 7 to 100:1.
14. A process as claimed in claim 11 wherein the impregnating material
is a hydrocarbon resin.
15. A process as claimed in claim 11 wherein the impregnating material
is a hydrocarbon wax.
16. A process as claimed in claim 11 wherein the impregnating material
is an acrylic resin.
17. A process as claimed in claim 11 wherein the average particle
size of the molecular crystal seeds is 100 nm or less.
18. A process as claimed in claim 11 wherein the molecular sieve
seed is deposited substantially as a monolayer.
19. A process as claimed in claim 18 wherein the monolayer seed
layer is deposited via the use of charge reversal and a cationic
polymer.
20. A process as claimed in claim 11 wherein the molecular sieve
seed is present as a seed layer has a thickness of 3 .mu.m or less.
21. A process as claimed in claim 1 wherein the temperature for
hydrothermal synthesis is 100.degree. C. or less.
22. A process as claimed in claim 11 wherein the crystalline molecular
sieve layer is reparated.
23. A process as claimed in claim 12 wherein the pre-impregnation
masking material is polymethylmethacrylate.
24. A process for enhancing the selectivity of a crystalline molecular
sieve layer as claimed in claim 1 which process comprises, (i) exposing
such a crystalline molecular sieve layer to a hydrocarbon stream
comprising at least two components under pressure for a period of
time, such that at least one component is separated from the stream,
and (ii) at some point in time during the exposure increasing the
partial pressure of at least one component of the hydrocarbon stream.
25. A selectivity enhanced crystalline molecular sieve layer prepared
according to claim 24.
26. A process for the separation of at least one component from
a hydrocarbon stream which process comprises exposing a molecular
sieve as claimed in claim 1 to a hydrocarbon stream comprising
at least two components such that at least one component is separated
from the stream.
27. A process for the separation of para-xylene from a mixture
comprising para-xylene and at least one other isomer of xylene,
which process comprises exposing the mixture to a crystalline molecular
sieve layer, as claimed in claim 1 at a temperature of .gtoreq.250.degree.
C. and an aromatic hydrocarbon feed partial pressure of .gtoreq.10.times.10.sup.3
Pa.
Molecular sieve description
This invention relates to crystalline molecular sieve layers, to
processes for their manufacture, and to their use.
Molecular sieves find many uses in physical, physicochemical, and
chemical processes; most notably as selective sorbents, effecting
separation of components in mixtures, and as catalysts. In these
applications the crystallographically-defined pore structure within
the molecular sieve material is normally required to be open; it
is then a prerequisite that any structure-directing agent, or template,
that has been employed in the manufacture of the molecular sieve
be removed, usually by calcination. Numerous materials are known
to act as molecular sieves, among which zeolites form a well-known
class.
In International Application WO 94/25151 is described a supported
inorganic layer comprising optionally contiguous particles of a
crystalline molecular sieve, the mean particle size being within
the range of from 20 nm to 1 .mu.m. The support is advantageously
porous. When the pores of the support are covered to the extent
that they are effectively closed, and the support is continuous,
a molecular sieve membrane results; such membranes have the advantage
that they may perform catalysis and separation simultaneously if
desired. A number of processes are described in WO 94/25151 for
the manufacture of the inorganic layers disclosed therein. WO94/25151
describes the use of a barrier layer which prevents the water in
the aqueous coating suspension used from preferentially entering
the pores of the support to an extent such that the silica and zeolite
particles form a thick gel layer on the support. The barrier layer
may be temporary or permanent; temporary barrier layers are fluids
such as water or glycol. The membranes of WO 94/25151 exhibited
selectivities of para-xylene over ortho-xylene of 20.76 to 60.10
and para-xylene permeances of 1.09.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px)
(10 kg(px)/m.sup.2.day.bar(px)) when measured at low temperature
and pressure.
In International Application WO 96/01683 a structure is described
which comprises a support, a seed layer, and an upper layer, the
seed layer comprising a crystalline molecular sieve having a crystal
size of at most 1 .mu.m, and the upper layer comprising a crystalline
molecular sieve of crystals having at least one dimension greater
than the dimensions of the crystals of the seed layer. There are
a number of processes described in WO 96/01683 for the manufacture
of these layers.
In International Application WO 97/25129 a structure is described
which comprises a crystalline molecular sieve layer on a substrate
and an additional layer of refractory material to occlude voids
in the molecular sieve layer. The structures described in the examples
have para-xylene over meta-xylene selectivities of between 2 to
8.
In International Application WO 96/01686 a structure is described
which comprises a substrate, a zeolite or zeolite-like layer, a
selectivity enhancing coating in contact with the zeolite layer
and optionally a permeable intermediate layer in contact with the
substrate. Examples of these structures are given which have para-xylene
over meta-xylene selectivities of between 1 to 10.
Xomeritikas and Tsapatsis in Chemical Materials, 1999 11 875-878
describe orientated MFI-type zeolite membranes which have been manufactured
using secondary growth a process which requires two successive hydrothermal
growths and produces membranes of 25 to 40 .mu.m thickness. These
membranes exhibited para-xylene over ortho-xylene selectivities
of 18 when measured at a total aromatic hydrocarbon partial pressure
of 27.5 Pa [=15 Pa pX+12.5 Pa oX] and 100.degree. C. and 3.8 at
a total aromatic hydrocarbon partial pressure of 550 Pa [=300 Pa
pX+250 Pa oX] and 100.degree. C., and permeances for para-xylene
of 2.0 to 5.2.times.10.sup.-8 mole/m.sup.2.s.Pa [18 to 48 kg.sub.px
/m.sup.2.day.bar.sub.px ], when tested at temperatures up to 200.degree.
C. and at low hydrocarbon partial pressures. The selectivity decreased
with increasing partial pressure of para-xylene and it was observed
by the authors that the membranes would not be suitable for separation
of xylene isomers at elevated temperatures due to the 20 fold reduction
in flux ratio at 200.degree. C. compared to that observed at 100.degree.
C.
Many commercial petrochemical processes operate at elevated temperature
and pressure. Whilst the molecular sieve layers of the prior art
may exhibit good selectivity and permeance results when tested at
low temperatures, pressures and/or hydrocarbon partial pressures,
this is not repeated when tested at high temperatures and high hydrocarbon
partial pressures. Thus, there is a need for molecular sieve layers
with improved properties for catalytic and/or membrane applications,
especially improved properties at elevated temperatures e.g. >250.degree.
C. and/or elevated hydrocarbon feed partial pressures >10.times.10.sup.3
Pa.
The present invention is concerned with crystalline molecular sieve
layers which have improved properties compared to crystalline molecular
sieve layers in the art, especially for membrane applications. It
has surprisingly been found that the control of a number of synthesis
parameters for the manufacture of crystalline molecular sieve layers
in conjunction with impregnation of the support onto which the crystalline
molecular sieve layer is to be deposited during its synthesis, results
in crystalline molecular sieve layers with properties, which hitherto
have not been achieved.
The present invention in a first aspect provides a process for
the manufacture of a crystalline molecular sieve layer, which process
comprises: a) providing a porous support having deposited thereon
seeds of molecular sieve crystals of average particle size of 200
nm or less, b) impregnating the support with an impregnating material
before or after deposition of the seeds of molecular sieve, c) contacting
the impregnated support having seeds deposited thereon with a molecular
sieve synthesis mixture, d) subjecting, the impregnated support
having seeds deposited thereon, to hydrothermal treatment whilst
in contact with the molecular sieve synthesis mixture to form a
crystalline molecular sieve layer on the support, and e) removing
the impregnating material from the support.
As examples of porous supports, there may be mentioned porous glass,
sintered porous metals, e.g., steel or nickel, inorganic oxides,
e.g., alpha-alumina, titania, cordierite, zeolite as herein defined,
or zirconia and mixtures of any of these materials. In this context
porous supports include supports which have pores which are occluded;
such supports, whilst having pores which are not suitable for membrane
separation applications, may be used for catalytic applications
or separation processes which are not membrane separation processes
such as for example adsorption or absorption.
The pore size and porosity of the support should be compatible
with the process employed for depositing the molecular sieve seeds.
The porous support may be any material compatible with the coating
and synthesis techniques utilised in the process of the present
invention. For example porous alpha-alumina with a surface pore
size within the range of 0.08 to 1 .mu.m, most preferably from 0.08
to 0.16 .mu.m, and advantageously with a narrow pore size. Ideally
the support should have a relatively high degree of porosity so
that the support exerts an insignificant effect on flux through
the finished product. Preferably the porosity of the support is
30% by volume or greater; ideally and preferably greater than 33%,
and preferably within the range 33 and 40% by volume. The support
may be multilayered; for example, to improve the mass transfer characteristics
of the support; in this context the support may be an asymmetric
support. In such a support the surface region which is in contact
with the molecular sieve seeds may have small diameter pores, while
the bulk of the support, toward the surface remote from the molecular
sieve seeds, may have larger diameter pores. An example of such
a multilayered asymmetric support is an alpha-alumina disk having
pores of about 1 .mu.m average diameter coated with a layer of alpha-alumina
with average pore size of about 0.1 .mu.m. A further example of
a multilayered support is a large pore metal based support which
has an inorganic layer (either metal or non-metal) deposited thereon
of smaller pore size compared to the metal support. It is to be
understood that when the support is a molecular sieve as herein
defined and at least at its surface it has the requisite properties
to function as a molecular sieve seed, in relation to particle size
and crystallinity, then the support surface itself may act as the
molecular sieve seed and deposited molecular sieve seeds may be
dispensed with. Zeolite supports may however also be used in conjunction
with a deposited molecular sieve seeds. Suitable supports include
the composite membranes and layers manufactured according to U.S.
Pat. Nos. 4981590 and 5089299.
It is preferred that the support is such that it is substantially
inert under hydrothermal reaction conditions. It is preferred that
substantially no chemical component of the support participates
in the molecular sieve synthesis and, as a result, becomes incorporated
within the structure of the crystalline molecular sieve layer. This
is particularly advantageous when the crystalline molecular sieve
layer is to function as a catalyst material or to act as the support
for a catalyst material. In these circumstances, incorporation of
unwanted chemical species into the structure of the crystalline
molecular sieve layer may be detrimental to these functions. In
addition, if the crystalline molecular sieve layer is to be used
as a membrane, incorporation of unwanted chemical species from the
support into the layer may adversely affect the permeation properties
of the layer.
The support may be, and preferably is, cleaned prior to deposition
of the molecular sieve seeds. Suitable cleaning techniques include
ultrasonic treatment in water, pentane, acetone or methanol. This
may be followed by a period of drying from a few minutes to 24 hours
under ambient conditions or under temperatures up to 1000.degree.
C., preferably 500 to 700.degree. C. The cleaning regime may comprise
a combination of cleaning steps. Such a combination may be a series
of washing steps with different solvents and/or drying steps. Each
solvent washing step may be utilised in combination with ultrasound.
The molecular sieve seeds may be deposited, and preferably are
deposited, as a discrete layer, or part of a discrete layer, which
comprises molecular sieve seed crystals of average particle size
200 nm or less. Advantageously, the average crystal size of the
molecular sieve seeds in the seed layer is 150 nm or less ideally
within the range 5 to 120 nm and most preferably within the range
25 to 100.
The seed layer may consist substantially of molecular sieve material
only, or it may be a composite layer of the molecular sieve seed
material and intercalating material which may be organic or inorganic.
The particles of the seed layer may be contiguous or non-contiguous;
preferably they are contiguous. The intercalating material may be
the same material as the support. The preferred molecular sieve
seed crystals are colloidal in nature and capable of forming a stable
colloidal suspension.
Colloidal molecular sieve seed crystals may be prepared by processes,
which are well known in the art. Suitable processes are those described
in International Applications; WO93/08125 WO97/03019 WO97/03020
WO97/03021 and WO94/05597 the disclosures of which, in so far as
they refer to the manufacture of colloidal molecular sieve seeds,
are incorporated by reference.
The molecular sieve seed may be applied to the support by techniques
known in the art such as for example sol-gel coating techniques,
spin-coating, wash-coating, spray-coating, brushing, slip-casting
or dip-coating; these processes preferably being undertaken with
a suspension of the colloidal molecular sieve crystals.
The colloidal molecular sieve seed crystals are preferably applied
to the support by spin-coating; the viscosity of the mixture, the
solids concentration and the spin rate inter alia controlling the
coating thickness. The mixture may firstly be contacted with the
stationary support, then after a short contact time the support
is spun at the desired rate. Alternatively, the mixture is contacted
with a support which is already spinning at the desired rate.
When present as a discrete layer, the thickness of the molecular
sieve seed layer is advantageously 3 .mu.m or less, more advantageously
at most 2 .mu.m, preferably 1 .mu.m or less and most preferably
0.5 .mu.m or less. Advantageously, the seed layer is of sufficient
thickness to cover irregularities of comparable scale in the surface
of the support. Advantageously, the seed layer is at most the thickness
of the subsequently deposited crystalline molecular sieve layer.
In one embodiment the seed layer may be deposited and used as a
monolayer. Such a monolayer and its method of deposition is described
in WO97/33684 the disclosure of which in so far as it relates to
the manufacture of a molecular sieve seed monolayer is incorporated
by reference. It is preferred that the molecular sieve seed layer
is one that has substantially a monolayer thickness. It is preferred
that this monolayer is deposited via the charge reversal method
utilising a cationic polymer as described in WO97/33684.
In one aspect of the process of the present invention the support
may be impregnated and placed into the molecular sieve synthesis
mixture without further treatment of the molecular sieve seed layer
after its deposition. Even when submerged in the synthesis mixture,
the particles in the seed layer remain adhered to the support and
facilitate growth of the zeolite layer. However, under some circumstances,
e.g. during stirring or agitation of the synthesis mixture, the
adhesion between the molecular sieve seed layer and the support
may be insufficient and steps may be taken to stabilise the seed
layer.
Therefore, in another aspect of the invention, the molecular sieve
seed layer is stabilised before impregnation or before being placed
into the synthesis mixture. This stabilisation can be achieved in
one aspect by heat-treating the seed layer, e.g. at temperatures
between 30 and 1000.degree. C., ideally greater than 50.degree.
C. and more preferably between 200.degree. C. and 1000.degree. C.
and most preferably greater than 300.degree. C. and between 400.degree.
C. and 600.degree. C., for several hours preferably at least two
hours and most preferably 2 to 10 hours.
The impregnating material may be any material which substantially
remains at its selected location within the support during subsequent
process steps used for deposition of the crystalline molecular sieve
layer e.g. hydrothermal synthesis conditions, and deposition of
the molecular sieve seed layer if this occurs after impregnation,
and which is substantially stable under such process condition,
at least for the time scale of the process.
The impregnation material selected must remain substantially within
the support, and must remain substantially stable, under the deposition
conditions so as not to interfere with the deposition process and
to ensure that a crystalline molecular sieve layer of the desired
quality and properties is obtained in the process.
Ideally the impregnation material should have a viscosity which
enables easy impregnation into the support. The properties of the
impregnation material ideally are such that it may be impregnated
into the support under capillary action, applied pressure or a vacuum.
Furthermore, the impregnation material should be compatible with
the physical properties of the support surfaces to ensure that it
can wet the surfaces of the support and intimately contact with
it.
Water and glycol are not suitable as impregnation material because
they do not remain at any location in the support, selected for
the impregnating material, under hydrothermal synthesis conditions.
The impregnation material should also be capable of being easily
and substantially completely removed from the support after formation
of the crystalline molecular sieve layer. Ideally at least the bulk
of the impregnating material is capable of being removed under an
applied pressure, by washing of the support with a suitable solvent,
via calcination, via melting or any combination of these methods.
It is preferred that the impregnation material is capable of being
removed under calcination conditions which are normally used in
the manufacture of molecular sieve materials such as those used
in zeolite synthesis. It is important that the impregnation material
can easily be removed in order to ensure that as little residual
impregnation material as possible, and preferably no residual impregnation
material, remains which could impair the performance of the crystalline
molecular sieve layer.
The preferred impregnation materials include natural or synthetic
organic resins e.g hydrocarbon resins. In the context of the present
invention hydrocarbon means an organic material which has as its
main components hydrogen and carbon but does not preclude the presence
of one or more heteroatomic species e.g. oxygen or nitrogen or chlorine.
One preferred class of impregnating material are the hydrocarbon
resins which are free of heteroatoms. If a heteroatom is present
it is preferred that it is oxygen or chlorine. Examples of suitable
resins are acrylic resins, PVC resins and the hydrocarbon waxes.
Examples of suitable acrylic resins are the L R White Resins manufactured
and supplied by the London Resin Co. These are hydrophilic acrylic
resins of low viscosity (typically 8 mPa.s) which are commercially
available in three grades of hardness; LR1280 hard grade, LR1281
medium grade and LR1282 soft grade. These resins may be thermally
or cold cured, with or without the use of an accelerator such as
LR1283.
Suitable hydrocarbon resins include for example the hydrocarbon
waxes such as Exxon ESCOMER.TM. H101 and H231. H101 has a molecular
weight within the range 1600 to 2300 and a viscosity at 121.degree.
C. of approximately 25.5 mPa.s, at 140.degree. C. of approximately
17 mPa.s and at 190.degree. C. of approximately 9 mPa.s. H231 has
an approximate molecular weight of 6590 and a viscosity at 121.degree.
C. of approximately 600 mPa.s.
An example of a suitable impregnating material incorporating PVC
is a PVC plastisol. Such plastisols are well known in the art and
typically comprise PVC in combination with plasticizer, stabiliser
and viscosity depressor.
Further examples of suitable impregnating materials are ethylene-butylene
resins of approximate molecular weight 300 to 10000 or polyisobutylene
resins of approximate molecular weight 500 to 5000.
The molecular sieve seed material may be deposited prior to or
after impregnation of the support; preferably in one embodiment
it is deposited prior to impregnation of the support. In this instance
after impregnation of the support there may be quantities of impregnating
material located on the surface of the molecular sieve seed layer,
which has already been deposited on the support. If this layer of
impregnating material is relatively thin or discontinuous then surprisingly
it may not have an adverse effect on the seeding properties of the
molecular sieve seed layer and need not be removed or if some removal
is desired need not be completely removed. This is especially the
case where the impregnating material is mildly unstable under the
conditions used for subsequent deposition of the crystalline molecular
sieve layer e.g. hydrothermal synthesis conditions, and is slowly
dissolved in the synthesis mixture. Such a material, in accordance
with the requirements of the process of the present invention, has
acceptable stability. Examples of materials which have this property
include, the hydrocarbon waxes, acrylic resins and ethylenelbutene
resins described above. If necessary excess impregnation material
may be removed from the surface of the molecular sieve seed layer
by any suitable means. One suitable means, in the case where a co-solvent
is used for impregnation, is to use the same solvent to clean the
surface of the seed layer. When no co-solvent is used then any suitable
solvent for the resin may be used to clean the seed layer surface.
The thickness of this surface deposited layer of impregnation material
should be less than 1 .mu.m and preferably it should be less than
0.5 .mu.m, and most preferably less than 0.1 .mu.m
The most preferred resins are the hydrocarbon wax resins which
may easily impregnate the support and which are removed from the
support under calcination temperatures that are normally used in
zeolite synthesis, with or without prior melting of the bulk material.
Materials which have been found to be unsuitable as impregnating
materials include some low molecular weight hydrocarbons e.g hexadecane,
silicone oils and polyimide resins. This is believed to be mainly
due to their propensity for relatively rapid removal from the support
under the conditions used for deposition of the crystalline molecular
sieve layer.
Any suitable impregnation material may be used alone or in combination
with other impregnation materials and/or other materials which may
be required to assist in their impregnation. For example PVC resins
may advantageously be impregnated into the support as a solution
in THF; the THF being evaporated prior to deposition of molecular
sieve seed layer and/or crystalline molecular sieve layer. Other
suitable solvents may be used in conjunction with the resins. The
resins may be applied in the molten form under ambient pressure
conditions or under an applied pressure; for example hydrocarbon
waxes are advantageously applied in the molten form.
The impregnation stage may be and preferably is repeated one or
more times to ensure that the pores of the support, which are at
or proximate to the surface for deposition of the molecular sieve
seed layer or crystalline molecular sieve layer, are substantially
filled with impregnating material. Alternatively impregnation may
be undertaken for extended periods of time to achieve the same result
as repeated impregnation stages. In the case of hydrocarbon wax
as impregnating material the impregnation time is typically in the
order of 2 minutes or more at 150.degree. C. under vacuum, ideally
2 to 5 minutes; for the same material an extended impregnation time
is greater than 5 minutes and ideally in the order of 20 minutes
or more under the similar conditions. Wax impregnation may usefully
be, and preferably is, undertaken for one hour or more at 150.degree.
C. under an applied vacuum.
In one embodiment the support is impregnated through surfaces of
the support other than the surface onto which the crystalline molecular
sieve layer is to be deposited. For example a support in the form
of a disk may be impregnated through one side only; the other side
being the surface onto which the molecular sieve seed layer and
crystalline molecular sieve layer are to be deposited. In one embodiment,
the impregnation may be partial in order to fill the pores of the
surfaces other than the surface onto which the crystalline molecular
sieve layer is to be deposited. This partial filling of the pores
of the support is acceptable if it results in improved performance
of the crystalline molecular sieve layer compared to that manufactured
without impregnation. Partial impregnation is particularly suitable
when a molecular sieve seed layer is used and the crystalline molecular
sieve layer is deposited via hydrothermal synthesis utilising a
zeolite synthesis solution which comprises colloidal silica. Surprisingly
the combination of a seed layer and colloidal silica in the synthesis
solution, allows the use of partial impregnation. Impregnation may
be continued until substantially all the pores of the support are
impregnated including pores proximate to the surface of the molecular
sieve seed. In the case of wax impregnation this may be observed
visually by an optical change in the support and the degree of impregnation
can be confirmed by cross-section SEM. In a further embodiment the
support may be impregnated through the molecular sieve seed layer.
After impregnation the nature of the organic resin may be such
that it is advantageous to cure the resin in-situ prior to use of
the impregnated support in the manufacture of a crystalline molecular
sieve layer. This curing ensures that the resin remains in the impregnated
location during subsequent manufacture of the crystalline molecular
sieve layer. Advantageously and preferably the impregnating material
has a melting point at or above the temperature used in the process
for manufacture of the crystalline molecular sieve layer. It is
not essential that the impregnating material is or remains solid
within the support during manufacture of the crystalline molecular
sieve layer. It may become liquid or molten during this manufacture;
this is acceptable if in this physical state the impregnating material
meets the requirements described in detail above i.e. remains stable
and in the desired location within the support.
In a further aspect the process of the present invention may utilise
a further process step which is undertaken prior to impregnation
of the porous support. When used in conjunction with impregnation
this additional process step provides further control in the process
and further improvements in performance and ease of manufacture.
This further process step may be referred to as pre-impregnation
masking and involves deposition of a removable coating onto the
support surface which in due course will receive the crystalline
molecular sieve layer. The pre-impregnation masking step enables
a more accurate and effect impregnation stage to be undertaken.
The pre-impregnation masking is applied to the appropriate surface
of the porous support such that it does not impregnate the support
or only impregnates, to a limited extent, the surface region of
the support. After deposition of the pre-impregnation masking the
support is then impregnated as described above, ideally so that
the impregnating material comes into contact with or close proximity
to the pre-impregnation masking. Once impregnation is completed
the pre-impregnation masking may be removed and the remaining process
steps undertaken in order to manufacture the crystalline molecular
sieve layer.
The pre-impregnation masking may be applied before or after deposition
of a molecular sieve seed layer on the support. When applied to
a support which already has a molecular sieve seed layer deposited
on its surface the pre-impregnation masking offers the additional
benefit of protecting the molecular sieve seed layer surface from
contamination with the impregnating material. When the pre-impregnation
masking is applied to a support which does not have a seed layer
deposited on its surface the seed layer is advantageously applied
after removal of the pre-impregnation masking onto a high quality
impregnated support. Of particular benefit is the use of such a
high quality impregnated support with the monolayer seeding method
described in WO97/33684. When this seeding method is used in conjunction
with pre-impregnation masking good quality crystalline molecular
sieve layers may be produced.
An important factor in pre-impregnation masking is to ensure that
the material used for the masking is able to intimately contact
the surface of the support and is compatible with the impregnating
material and method of impregnation. If contact properties are inadequate
impregnating material may fill spaces which arise between the pre-impregnation
masking and the support; the resultant region of impregnating material
on the support surface prevents subsequent deposition and growth
of the crystalline molecular sieve layer and can thus lead to a
poor quality layer.
The steps required for pre-impregnation masking include; cleaning
of the support surface, coating the support surface with an appropriate
masking material, impregnation of the masked support and removal
of the masking material after impregnation.
The methods used to clean the support surface may be the same as
those indicated above for preparation of the support for impregnation.
A preferred method is to rinse the support in acetone and filtered
ethanol (0.1 .mu.m filter, Anotop.TM. Whatman) followed by drying.
The material used for the pre-impregnation masking may be any material
which can be easily applied to the surface of the support and which
may be readily removed after impregnation without significant disturbance
to the impregnation material. The pre-impregnation masking material
must be compatible with the surface of the porous support so as
to effectively wet and coat substantially the whole of the desired
region for masking. The choice of pre-impregnation masking material
will also depend on the nature of the support e.g. its surface properties
such as polarity. Examples of suitable pre-impregnation masking
materials include organic polymers. Of particular interest for the
masking of inorganic and asymmetric supports such as ceramics, in
particular alpha-alumina, are polar polymeric materials such as
the acrylic polymers and resins. A preferred masking material is
polymethylmethacrylate (PMMA). An example of a suitable PMMA polymer
is CM205 of MW 100000 g/mole with a polydispersity of 1.8. An example
of a polymer which is less suitable for use as a masking material
with asymmetric alpha-alumina supports is polystyrene; it is believed
that this is due to its relatively low polarity. Preferred organic
polymers therefore have a polarity which is greater than that of
polystyrene. The masking material may be applied in a number of
ways. One method is to melt the organic polymer and to apply this
to the surface of the support. A further and preferred method is
to apply the organic polymer from solution in a suitable solvent
for the polymer. In this context a true solution may not be formed
and the solvent simply reduces the viscosity of the masking material
for ease of application. A particularly useful solvent for PMMA
is acetone. Preferably, the PMMA as masking material is applied
as a solution of 1 part PMMA in 3.75 parts acetone. The solution
of masking material is applied to the support and the deposited
material is carefully dried to remove the solvent if used. Too rapid
a drying process may lead to ineffective masking. In the case of
PMMA applied via acetone the solvent is removed by drying at a rate
of 1.degree. C./h to 150.degree. C. Impregnation of the masked support
may be undertaken as described above.
After impregnation, the pre-impregnation masking material is removed.
A suitable method for removal is washing with a suitable solvent.
In the case of PMMA and other polar masking materials, a suitable
solvent is acetone or the solvent that was used in the application
of the mask. After solvent removal of the masking material the impregnated
support surface that was in contact with the masking material may
be further treated and preferably is further treated with an ammonia
solution, ideally a 0.1M ammonia solution. After this treatment
the impregnated support may be utilised for the deposition of a
seeding layer, preferably using the monolayer technique, and deposition
of a crystalline molecular sieve layer.
The composition of the synthesis solution is selected to provide
the desired molecular sieve or molecular sieve type. When the crystalline
molecular sieve layer comprises silicon in its framework then the
H.sub.2 O to SiO.sub.2 ratio must be within the range of 7 to 100.
Preferred silicon sources include tetraethylorthosilicate (TEOS)
and colloidal silica when the support is partially impregnated.
Preferably, the H.sub.2 O to SiO.sub.2 molar ratio in the synthesis
mixture is within the range of 7 to 70 more preferably 7 to 60.
For certain molecular sieves such as aluminophosphates (ALPO's)
a source of silica is not required.
The composition of the synthesis mixture varies according to the
process; the mixture always contains sources of the various components
of the desired molecular sieve and usually contains a structure
directing agent. A preferred colloidal silica source is an ammonia-stabilised
colloidal silica, e.g., that available from du Pont under the trade
mark Ludox AS-40.
The source of silicon may also be the source of potassium, in the
form of potassium silicate. Such a silicate is conveniently in the
form of an aqueous solution such, for example, as that sold by Aremco
Products, Inc. under the trade mark CERAMA-BIND, which is available
as a solution of pH 11.3 specific gravity 1.26 and viscosity 40
mPas. Other sources of silicon include, for example, silicic acid.
As other sources of potassium, when present, there may be mentioned
potassium hydroxide. Whether or not the synthesis mixture contains
a potassium source, it may also contain sodium hydroxide to give
the desired alkalinity.
The structure directing agent, when present, may be any of those
commonly used in zeolite synthesis. For the manufacture of an MFI
layer, a tetrapropylammonium hydroxide or halide is advantageously
used.
For the manufacture of an MFI type zeolite, especially ZSM-5 or
silicalite-I, the synthesis mixture is advantageously of a molar
composition, calculated in terms of oxides, within the ranges:
M.sub.2 O:SiO.sub.2 0 to 0.7 to :1 preferably 0 to 0.350:1 SiO.sub.2
:Al.sub.2 O.sub.3 12 to infinity :1 (TPA).sub.2 O:SiO.sub.2 0 to
0.2:1 preferably 0 to 0.075:1 H.sub.2 O:SiO.sub.2 7 to 100:1 preferably
9 to 70:1
wherein TPA represents tetrapropylammonium and M is an alkali metal,
preferably sodium or potassium, although it may also be Li , Cs
or ammonia. Other template agents may be used in these ratios. In
the embodiment where pre-impregnation masking is not used its is
most preferred that the M.sub.2 O:SiO.sub.2 molar ratio is within
the range 0.016 to 0.350:1 and preferably that the that the H.sub.2
O:SiO.sub.2 molar ratio is within the range 7 to 60 more preferably
9 to 30:1 and most preferably 9 to 20:1.
In this specification ratios with infinity as the value indicate
that one of the ratio materials is not present in the mixture.
The hydrothermal synthesis is preferably undertaken at a temperature
of between 60 and 180.degree. C. and for a period within the range
1 to 200 hours. In a preferred aspect the process of the present
invention utilises a hydrothermal synthesis temperature of 140.degree.
C. or less, preferably within the range from 60 to 100.degree. C.,
and most preferably within the range 60 to 90.degree. C. When pre-impregnation
masking is used the preferred temperature range is 60 to 100.degree.
C.
In a preferred aspect the process of the present invention utilises
a synthesis time of 4 to 100 hours, in particular 4 to 80 hours
and most preferably 4 to 36 hours. The time of reaction will vary
depending on the temperature used during the hydrothermal synthesis
and may be adjusted accordingly with shorter synthesis times generally
being applicable when higher synthesis temperatures are used.
In the most preferred aspect of the process the hydrothermal synthesis
temperature is approximately 90.degree. C., the hydrothermal synthesis
time is approximately 36 hours, and the H.sub.2 O:SiO.sub.2 molar
ratio in the synthesis mixture is within the range 9 to 20.
The hydrothermal treatment advantageously is undertaken in an autoclave
under autogenous pressure. However, with synthesis temperatures
below 100.degree. C. it is possible to perform the synthesis under
ambient pressure conditions.
After deposition of the crystalline molecular sieve layer the impregnating
material is substantially completely removed by any of the methods
or combination of methods indicated above. The removal method chosen
will depend to some extent on the exact nature of the impregnating
material. The essential requirement is that the removal method is
capable of removing substantially all of the impregnated material.
One suitable method is to utilise the final calcination step in
the molecular sieve synthesis process to remove the impregnating
material.
After crystallisation, the structure comprising the support and
deposited crystalline molecular sieve layer with or without impregnating
material may be washed, dried, and the crystalline molecular sieve
calcined. The calcination conditions preferably comprise slow heating
and cooling to ensure that the structure, and in particular the
crystalline molecular sieve layer, remains intact with the minimum
amount of cracking and/or delamination. Preferably, the structure
is calcined at a temperature of 350 to 600.degree. C., preferably
450 to 550.degree. C. It is preferred that the structure is raised
to the desired calcination temperature at a rate of 0.1 to 6.degree.
C. per minute most preferably 0.2 to 3.degree. C. per minute.
In relation to the processes described herein contacting is to
be understood to include immersion or partial immersion of the support
in the relevant zeolite synthesis mixture.
The crystalline molecular sieve layer may be any known molecular
sieve material; for example it may be a silicate, an aluminosilicate,
an aluminophosphate (ALPO's), a silicoaluminophosphate, a metalloaluminophosphate,
or a metalloaluminophosphosilicate.
The preferred molecular sieve will depend on the chosen application,
e.g. separation, catalytic applications, and combined reaction and
separation, and on the size of the molecules being treated. There
are many known ways to tailor the properties of the molecular sieves,
for example, structure type, chemical composition, ion-exchange,
and activation procedures.
Representative examples are molecular sieves/zeolites which may
be used in the molecular sieve layer include the structure types
AFI, AEL, BEA, CHA, EUO, FAU, FER, KFI, LTA, LTL, MAZ, MOR, MEL,
MTW, OFF, TON and, especially and preferably MFI.
The structure types of the molecular sieve seed and crystalline
molecular sieve layers may be the same or different. Further, if
the structure types are the same, the compositions may be the same
or different. It is preferred that the molecular sieve seeds and
the crystalline molecular sieve layer are both of the MFI structure
type.
Some of the above materials while not being true zeolites are frequently
referred to in the literature as such, and this term will be used
broadly in this specification.
It is preferred that the hydrothermal synthesis stage of the process
is undertaken under such conditions as to prevent the settling,
on the forming crystalline molecular sieve layer, of particles produced
within the synthesis mixture e.g. molecular sieve crystals which
have homogeneously nucleated in the synthesis solution. Contacting
of the support coated with molecular sieve seeds is advantageously
carried out by immersion or partial immersion and with the support
in an orientation and location in the synthesis mixture such that
the influence of settling of crystals formed in the reaction mixture
itself, rather than on the coated surface, is minimised. If support
surface is three dimensional, e.g., a honeycomb, other means may
be used to inhibit settling, for example, agitation, stirring or
pumping.
The process of the present invention provides crystalline molecular
sieve layers with good separation properties especially at high
temperatures .gtoreq.250.degree. C. and preferably .gtoreq.360.degree.
C. and/or hydrocarbon feed partial pressures in the feed of .gtoreq.50.times.10.sup.3
Pa, preferably .gtoreq.100.times.10.sup.3 Pa, most preferably at
500.times.10.sup.3 Pa. Crystalline molecular sieve layers, especially
when in the form of a membrane, have been characterised by means
of a number of analytical techniques. One such technique is the
dye permeation test as described in WO96/01683. Whilst this test
is a good indication as to whether or not unacceptable defects are
present in a crystalline molecular sieve layer, it is a coarse test
and filter, and does not provide any absolute measurable difference
which is quantifiable between different crystalline molecular sieve
layers which pass the test; it is a pass or fail test. Crystalline
molecular sieve layers have been further characterised using x-ray
diffraction, transmission electron microscopy (TEM) and scanning
electron microscopy (SEM). Such techniques have been used to characterise
crystalline molecular sieve membranes in for example WO96/01683.
The crystalline molecular sieve layers of the present invention
when characterised using the dye permeation test or SEM are indistinguishable
from those crystalline molecular sieve layers described in WO96/01683.
However, it has been found that the crystalline molecular sieve
layers of the present invention exhibit different membrane properties,
especially at high temperature and/or hydrocarbon partial pressure,
compared to the prior art. It is possible to characterise the crystalline
molecular sieve layers of the present invention using a simple membrane
test which measures the transport characteristics, such as the selectivity
and mass transport properties of the crystalline molecular sieve
layer. This test enables the crystalline molecular sieve layers
of the present invention to be distinguished from the prior art
layers.
The test method is based on the evaluation of the selectivity and
permeance of the crystalline molecular sieve layer arranged in the
form of a membrane, using for example a mixture comprising para-xylene
and meta-xylene or para-xylene and ortho-xylene; para-xylene and
meta-xylene are particularly suitable to evaluate MFI molecular
sieve membranes. The crystalline molecular sieve layer as a membrane
is first analysed for its capacity to preferentially transport para-xylene
from a mixture comprising para-xylene and meta-xylene on the feed
side of the membrane to the permeate side of the membrane. The permeance
of each isomer is measured simultaneously and the ratio of para-xylene
to meta-xylene permeance provides a selectivity for para-xylene
over meta-xylene. This parameter is dimensionless. The details of
the test and calculations of selectivity and permeance are provided
in the examples below.
It has been found that the crystalline molecular sieve layers of
the present invention have good para-xylene over meta-xylene selectivity
and permeance, especially at high temperatures and aromatic hydrocarbon
partial pressures.
Accordingly the present invention also provides a crystalline molecular
sieve layer having a selectivity (.alpha..sub.x) for para-xylene
over meta-xylene of 2 or greater and a permeance (Q.sub.x) for para-xylene
of 3.27.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px) (30 kg(px)/m.sup.2.day.bar(px))
or greater measured at a temperature of .gtoreq.250.degree. C. and
an aromatic hydrocarbon partial pressure of .gtoreq.10.times.10.sup.3
Pa.
The selectivity and permeance are calculated and determined as
described below. The crystalline molecular sieve layers of the present
invention are defined in terms of their selectivity and permeance
properties for para-xylene separations. However, the present invention
is not limited to crystalline molecular sieve layers only when used
for para-xylene separations; the layers may be used for other separations
and/or applications such as catalysts and sensors e.g. gas sensors.
For .alpha..sub.x and Q.sub.x the subscipt x denotes the total aromatic
hydrocarbon partial pressure in kpa on the feed side of the layer;
thus x has a minimum value of 10 or greater, preferably 50 or greater,
more preferably 100 or greater and most preferably 500 or greater.
Ideally, x is within the range 10 to 1000 more preferably 100 to
1000 and most preferably 500 to 1000. Preferably the aromatic hydrocarbon
partial pressure is .gtoreq.100.times.10.sup.3 Pa, more preferably
.gtoreq.500.times.10.sup.3 Pa. Preferably, the temperature is .gtoreq.360.degree.
C. and most preferably .gtoreq.4000 and ideally within the range
250.degree. C. to 600.degree. C., most preferably within the range
360.degree. C. to 600.degree. C. It is preferred for all layers
of the present invention that the performance levels are attained
in the presence of hydrogen.
It is preferred that the para-xylene over meta-xylene selectivity
(.alpha..sub.x) of the membrane layer is 2.5 or greater, more preferably
3 or greater, more preferably 5 or greater and most preferably 8
or greater. Ideally, it is within the range 2 to 30000 preferably
8 to 3000 and most preferably 8 to 100. The para-xylene permeance
(Q.sub.x) is preferably 5.45.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px)
(50 kg(px)/m.sup.2.day.bar(px)) or greater, and more preferably
7.63.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px) (70 kg(px)/m.sup.2.day.bar(px))
or greater. Ideally, it is within the range 3.27.times.10.sup.-8
to 5.4.times.10.sup.-6 mole(px)/m.sup.2.s.Pa(px) (30 to 5000 kg(px)/m.sup.2.day.bar(px)),
more preferably within the range 7.63.times.10.sup.-8 to 3.3.times.10.sup.-6
mole(px)/m.sup.2.s.Pa(px) (70 to 3000 kg(px)/m.sup.2.day.bar(px)).
It will be appreciated that the structure comprising a crystalline
molecular sieve layer and a support may be of any shape, and may
be, for example, planar, cylindrical, especially cylindrical with
a circular cross-section, or may be a honeycomb structure. For clarity,
however, the following description will refer to the structure as
if it were planar, and references will be made to the plane of a
layer.
The products of the invention may additionally be characterised
by X-Ray Diffraction (XRD) among other techniques. For this purpose
a conventional X-Ray diffraction technique may be used, where the
supported layered structure in the shape of a disk is mounted in
a modified powder sample holder and a conventional .theta./2.theta.
scan is performed. The intensities of the zeolite reflections thus
measured are compared to the intensities of reflections of a randomly
oriented powder of a zeolite of the same structure and composition.
If one or more sets of reflections related to one or more specific
orientations of the crystal are significantly stronger than the
remaining reflections as compared to the diffractogram of a randomly
oriented powder, this indicates that the orientation distribution
in the sample deviates from random. This is referred to as a crystallographic
preferred orientation or CPO. It is preferred that the crystalline
molecular sieve layers of the present invention are MFI structure
type molecular sieves and exhibit a strong combined 011/101 reflection
which is indicative of 011/101 CPO as measured by X-Ray Diffraction
(XRD).
The thickness of the crystalline molecular sieve layer is advantageously
less than 3 .mu.m, more advantageously less than 2 .mu.m, and preferably
1 .mu.m or less most preferably 0.5 .mu.m or less. Advantageously,
the thickness of the layer, and the crystallite size of the crystalline
molecular sieve, are such that the layer thickness is approximately
the size of the longest edges of the crystals, giving essentially
a monolayer. In such a monolayer the crystals are orientated such
that the crystalline molecular sieve layer exhibits a columnar appearance
when viewed in cross-section by SEM. In such a structure the majority
of the inter-crystal grain boundaries are oriented substantially
perpendicular to the plane which approximates to the interface between
the support and crystalline molecular sieve layer. The crystalline
molecular sieve layer contains substantially no crystals which are
orientated such that the plane of their grain to grain interfaces
are parallel to the support/crystalline molecular sieve layer interface;
without wishing to be bound to any theory, the inventors believe
that such interfacess may reduce the performance of the membrane.
It is preferred that the combined thickness of the molecular sieve
seed layer and the crystalline molecular sieve layer is 3 .mu.m
or less, preferably 2 .mu.m or less, and most preferably 1 .mu.m
or less.
Advantageously, in the hydrothermally deposited crystalline molecular
sieve layer, the crystals are contiguous, i.e. substantially every
crystal is in contact with one of its neighbours, although not necessarily
in contact with one of its neighbours throughout its entire length.
Although it is desired that the crystalline molecular sieve layers
of the present invention are crack free as determined by the dye
test. It is acceptable to have cracks which may be reparated. It
is also acceptable for the surface of the crystalline molecular
sieve layer to exhibit a significant degree of surface cracking.
It is surprising that although the crystalline molecular sieve layers
of the present invention may exhibit an extensive surface cracked
topography, they still exhibit good selectivity, and permeance even
without the use of reparation techniques.
The crystalline molecular sieve layers of the present invention
may be treated to further improve or stabilise their properties.
In one aspect, whilst intact layer regions are of good quality,
there may be regions of the layer which are cracked or where there
may be pinholes present. If these cracks and pinholes are of such
quantity and dimensions that they have a disproportionate effect
on membrane performance then it is useful to reparate the layer.
Suitable reparation techniques are described in for example WO96/01682
WO96/01686 and WO97/25129 the disclosures of which are incorporated
by reference. The preferred method of reparation is that described
in WO96/01686. If the crystalline molecular sieve layer of the present
invention has no pinholes or cracks which disproportionately effect
the layer performance it may still be advantageous to treat the
crystalline molecular sieve layer to maintain its performance. In
this context a suitable treatment is the selectivity enhancing layers
described in WO96/01686. Such selectivity enhancing layers may at
the same time also reparate defective crystalline molecular sieve
layers. Such selectivity enhancing layers provide mechanical stability
to the crystalline molecular sieve layers during use.
An alternative reparation method involves the use of a hydrolysed
crystallisation solution. In this method a hydrolysed synthesis
mixture, identical or similar to that used to deposit the crystalline
molecular sieve layer, is applied to the surface of the crystalline
molecular sieve layer on the support. Any suitable application method
may be used; one such method is spin-coating at for example 8000
rpm. After deposition of the hydrolysed synthesis mixture the surface
of the crystalline molecular sieve is further treated with an ammonia
solution e.g. 0.1 M ammonia to clean the surface. The treated and
ammonia cleaned crystalline molecular sieve layer is then exposed
to moisture at elevated temperature, ideally in a closed autoclave
at 100.degree. C. for 24 hours. After exposure to moisture the crystalline
molecular sieve is calcined. A suitable calcination regime is heating
to 400.degree. C. in air for 6 hours using a heat-up and cool-down
rate of 2.degree. C. per minute. This reparation method is particularly
suitable for reparation of crystalline molecular sieve layers which
have been manufactured using the pre-impregnation masking method.
When a membrane layer is reparated the end result is typically
a modification of the selectivity and permeance properties of the
layer. Typically there is a reduction in the permeance and an increase
in the selectivity. It has surprisingly been found that when crystalline
molecular sieve layers manufactured by the process of the present
invention are reparated they possess high selectivity for para-xylene
over meta-xylene and in addition retain acceptably good para-xylene
permeance properties.
Accordingly the present invention in a further embodiment provides
a reparated membrane comprising crystalline molecular sieve and
having a selectivity (.alpha..sub.x) for para-xylene over meta-xylene
of 10 or greater and a permeance (Q.sub.x) for para-xylene of 4.36.times.10.sup.-9
mole(px)/m.sup.2.s.Pa(px) (4 kg(px)/m.sup.2.day.bar(px)) or greater
measured at a temperature of .gtoreq.250.degree. C. and an aromatic
hydrocarbon partial pressure of .gtoreq.10.times.10.sup.3 Pa.
Preferably, the reparated membrane layer has a para-xylene over
meta-xylene selectivity (.alpha..sub.x) of 12 most preferably 17
and ideally 60 or 100 or greater. Preferably, the selectivity is
within the range 10 to 30000 more preferably 10 to 3000 and most
preferably 10 to 200. The subscript x has the same values and ranges
as indicated above for non-reparated membranes.
Preferably the reparated membrane layer has a para-xylene permeance
(Q.sub.x) of 5.12.times.10.sup.-9 mole(px)/m.sup.2.s.Pa(px) or greater
(4.7 kg(px)/m.sup.2.day.bar(px)), more preferably 7.08.times.10.sup.-9
mole(px)/m.sup.2.s.Pa(px) or greater (6.5 kg(px)/m.sup.2.day.bar(px))
more preferably 8.1.times.10.sup.-9 or greater, and most preferably
1.09.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px) or greater (10 kg(px)/m.sup.2.day.bar(px)).
Preferably the permeance is within the range of 6.54.times.10.sup.-9
to 5.4.times.10.sup.-6 mole(px)/m.sup.2.s.Pa(px) (6 to 5000 kg(px)/m.sup.2.day.bar(px)),
and most preferably within the range 7.0.times.10.sup.-9 to 3.3.times.10.sup.-6
mole(px)/m.sup.2.s.Pa(px).
In a further aspect the process of the present invention produces
crystalline molecular sieve layers which may be characterised by
a further aspect of their separations performance. It has been found
that the selectivity of para-xylene over meta-xylene is not constant
with time during use but surprisingly increases in a specific way,
which is beneficial. This effect may be used to attain, maintain
or improve the desired permeance and selectivity performance. Without
being bound by any theory it is believed that these crystalline
molecular sieve layers have a morphology and structure which lends
itself to this effect. However, these morphology and structure differences
cannot be distinguished from prior art crystalline molecular sieve
layers due to the limitations of available analytical techniques.
It is believed that these differences allow the crystalline molecular
sieve layers of the present invention to preserve their selectivity
performance during use and to allow this selectivity performance
to be easily improved. When the crystalline molecular sieve layers
manufactured by the process of the present invention are exposed
to a mixed hydrocarbon stream e.g. an aromatics stream it is believed
that some component or components of the stream reduce the detrimental
effects of non-selective pathways through the crystalline molecular
sieve layer whilst having little or no effect on the selective pathways.
This is in effect a form of reparation which occurs during use of
the crystalline molecular sieve layer and which may be controlled
during use. As would be expected the permeance of individual components
of the hydrocarbon mixture reduces with time of exposure. However,
it is surprising that for some components the reduction is significantly
more than for others. This selective reduction is believed to account
for the improved selectivity. The reduction in permeance of key
components is not detrimental to the overall performance of the
crystalline molecular sieve layer if at the same time there is a
consequential improvement in selectivity. The selectivity improvement
is particularly noticeable for the separation of para-xylene from
an aromatics stream. This effect may be observed by using the xylenes
separation test described above to provide a plot of selectivity
for the desired component e.g. para-xylene against time. This plot
will show that the selectivity for para-xylene over meta-xylene
increases with time. If this data plot is modified to express the
first differential (.DELTA..alpha..sub.Px/Mx /.DELTA.t), (where
.alpha.=the selectivity for para-xylene over meta-xylene at a given
time t), averaged over the first two hours of testing, then the
value of this differential at t=2 is >0 i.e. it is increasing.
Thus the present invention in a further aspect provides a membrane
comprising a crystalline molecular sieve layer, which membrane has
a (.DELTA..alpha..sub.Px/Mx /.DELTA.t) at t=2 hours of greater than
0.
It has further been observed that as time of exposure to the hydrocarbon
feed is extended then the selectivity either remains constant or
may slowly and gradually decrease. It has been found that this effect
may be prevented or reversed by controlling the hydrocarbon partial
pressure in the feed to the crystalline molecular sieve layer during
use. If the hydrocarbon partial pressure in the feed to the crystalline
molecular sieve layer is increased this surprisingly has been found
to improve the selectivity for selected components of the feed.
Accordingly in a further aspect the invention provides a process
for enhancing the selectivity of a crystalline molecular sieve layer
for the separation of at least one component from a hydrocarbon
stream which process comprises: a) exposing a crystalline molecular
sieve layer to a hydrocarbon stream comprising at least two components
under pressure for a period of time, such that at least one component
is separated from the stream, and b) at some point in time during
the exposure increasing the partial pressure of at least one component
of the hydrocarbon stream.
The increase in hydrocarbon partial pressure, which increase is
on the feed side of the crystalline molecular sieve layer, may be
a gradual increase which occurs throughout the separation cycle
or it may be a gradual increase for a proportion of the cycle or
it may be a stepped increase in pressure or a combination of these.
The increase in pressure may be applied one or more time during
the process if desired. This process may be utilised to enhance
the performance of crystalline molecular sieve layers so that they
meet the desirable performance targets of a selectivity for para-xylene
over meta-xylene of 2 or greater and a permeance for para-xylene
of 3.27.times.10.sup.-8 mole(px)/m.sup.2.s.Pa(px) (30 kg(px)/m.sup.2.day.bar(px))
or greater at a temperature of .gtoreq.250.degree. C. and an aromatic
hydrocarbon partial pressure of .gtoreq.10.times.10.sup.3 Pa.
The preferred crystalline molecular sieve layers for use in this
process are those prepared by the process described above for the
manufacture of crystalline molecular sieve layers.
The preferred hydrocarbon stream is an aromatics stream and most
preferably is an aromatics stream which comprises a mixture of xylenes
with other aromatic components.
Processes suitable for operation in accordance with this aspect
of the invention are described in for example WO93/08125 WO97/03019
WO97/03020 WO97/03021 and WO94/05597 and as described below.
The invention also provides a structure in which the support, especially
a porous support, has crystalline molecular sieve layers according
to the invention on each side, the layers on the two sides being
the same or different; it is also within the scope of the invention
to provide a layer not in accordance with the invention on one side
of the support, or to incorporate other materials in the support
if it is porous.
A catalytic function may be imparted to the crystalline molecular
sieve layers of the invention either by bonding of a catalyst to
the support or the free surface of the crystalline molecular sieve
layer, or its location within a tube or honeycomb formed of the
structure, by its incorporation in the support, e.g., by forming
the support from a mixture of support-forming and catalytic site-forming
materials or in the seed layer or crystalline molecular sieve layer
itself. If the support is porous a catalyst may be incorporated
into the pores, the catalyst optionally being a zeolite. For certain
applications, it suffices for the structure of the invention to
be in close proximity to, or in contact with, a catalyst, e.g. in
particulate form on a face of the structure.
The crystalline molecular sieve layer may be configured as a membrane,
a term used herein to describe a barrier having separation properties,
for separation of fluid (gaseous, liquid, or mixed) mixtures, for
example, separation of a feed for a reaction from a feedstock mixture,
or in catalytic applications, which may if desired combine catalysed
conversion of a reactant or reactants and separation of reaction
products. The crystalline molecular sieve layer may be removed from
the support on which it is formed for use as a membrane or catalyst.
This may be achieved by methods known in the art including for example
dissolution of the support. It is preferred that the crystalline
molecular sieve layers of the present invention are supported on
a porous support in use ideally the support used for their manufacture.
Conversions which may be effected include isomerizations, e.g.,
of alkanes and alkenes, conversion of methanol or naphtha to alkenes,
hydrogenation, dehydrogenation, e.g., of alkanes, for example propane
to propylene, oxidation, catalytic reforming or cracking and thermal
cracking.
The present invention accordingly also provides a process for the
separation of a fluid mixture which comprises contacting the mixture
with one face of a structure according to the invention in the form
of a membrane under conditions such that at least one component
of the mixture has a different steady state permeability through
the structure from that of another component and recovering a component
of mixture of components from the other face of the structure.
The present invention accordingly also provides a process for the
separation of a fluid mixture which comprises contacting the mixture
with a structure according to the invention in one embodiment in
the form of a membrane under conditions such that at least one component
of the mixture is removed from the mixture by adsorption. Optionally
the adsorbed component is recovered and used in a chemical reaction
or may be reacted as an adsorbed species on the structure according
to the invention.
The invention further provides such processes for catalysing a
chemical reaction in which the structure is in close proximity or
in contact with a catalyst.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with a structure
according to the invention which is in active catalytic form under
catalytic conversion conditions and recovering a composition comprising
at least one conversion product.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with one face of
a structure according to the invention, that is in the form of a
membrane and in active catalytic form, under catalytic conversion
conditions, and recovering from an opposite face of the structure
at least one conversion product, advantageously in a concentration
differing from its equilibrium concentration in the reaction mixture.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with one face of
a structure according to the invention that is in the form of a
membrane under conditions such that, at least one component of said
feedstock is removed from the feedstock through the structure to
contact a catalyst on the opposite side of the structure under catalytic
conversion conditions.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting one reactant of a bimolecular
reaction with one face of a structure according to the invention,
that is in the form of a membrane and in active catalytic form,
under catalytic conversion conditions, and controlling the addition
of a second reactant by diffusion from the opposite face of the
structure in order to more precisely control reaction conditions.
Examples include: controlling ethylene, propylene or hydrogen addition
to benzene in the formation of ethylbenzene, cumene or cyclohexane
respectively.
The crystalline molecular sieve layers of the present invention
have particular utility in the separation of para-xylene from mixtures
comprising paraxylene and at least one other isomer of xylene. Accordingly
the present invention also provides a process for the separation
of para-xylene from a mixture comprising para-xylene and at least
one other isomer of xylene, which process comprises exposing the
mixture to a crystalline molecular sieve layer according to the
present invention at a temperature of .gtoreq.250.degree. C. and
an aromatic feed partial pressure of .gtoreq.10.times.10.sup.3 Pa.
In this embodiment the aromatic feed is a feed which comprises isomers
of xylene optionally with other aromatic hydrocarbons e.g. ethylbenzene.
Alternative pressures and temperatures as indicated above in respect
of the crystalline molecular sieve layers may also be used in this
process.
The following Examples, in which parts are by weight unless indicated
otherwise, illustrate the invention:
EXAMPLES 1 to 4
Preparation of Alumina Porous Support
Porous alumina supports were cleaned as follows: 1. Ultrasonicate
in water for 10 minutes. 2. Heat treat in air overnight at 700.degree.
C. 3. Ultrasonicate in pentane for 10 mins. 4. Remove and dry in
air for 10 minutes. 5. Ultrasonicate in acetone for 10 minutes.
6. Remove and dry in air for 10 minutes. 7. Ultrasonicate in methanol
for 10 minutes. 8. Remove and dry for approximately 2 hours at 110.degree.
C. and cool to room temperature.
Preparation of Colloidal Seeds
Silicalite colloidal seeds of .about.50 nm particle size were prepared
according to the general method as described in WO93/08125.
Deposition of Seed Layer
The colloidal seeds were deposited on the cleansed alumina supports
by spin-coating, a colloidal suspension of 0.5% by weight of .about.50
nm sized MFI crystals with a pure silica composition and as prepared
above.
A porous alpha-alumina disk of diameter 25 mm, thickness 3 mm,
pore size 80 nm, and .about.33% porosity by volume, was machined
and polished on one face. The disk was then placed in the specimen
chuck of a CONVAC Model MTS-4 spinner and brought up to a spinning
speed of 4000 rpm. Once this spinning speed had been reached 2 ml
of the seed solution was applied to the centre of the disk and spinning
was continued to a total of 30 seconds. The coated disk was placed
in an oven and heated up to a temperature of 425.degree. C. or 450.degree.
C. at a heat-up rate of 0.3.degree. C./min and held at the terminal
temperature for 6 hours. After 6 hours the coated disk was cooled
at a rate of 0.5.degree. C./min until the disk reached room temperature.
Impregnation
A petri dish was partially filled with H101 hydrocarbon wax, which
had been melted at 150.degree. C. in a vacuum oven. The porous support,
with seed layer deposited thereon, was placed on a holder in the
wax filled petri dish such that only the surface of the support
which was free of deposited seeds was submerged in the wax. This
ensured that the seed layer did not come into contact with the wax.
The vacuum pump was switched on and after 2 minutes it was switched
off at a vacuum of <50 mbar. The oven was brought to atmospheric
pressure and the impregnated wax was allowed to crystallise within
the pores of the support. This impregnated support was now ready
for deposition of a crystalline molecular sieve layer.
Preparation of Hydrothermal Synthesis Solutions
A solution was prepared of 0.92 g NaOH (98.4% purity) in 138.14
g of water. Into this solution was dissolved 7.12 g of tetrapropylammonium
bromide (TPABr: Fluka). To this mixture was added 76.66 g of colloidal
silica solution (Ludox AS 40 supplied by Du Pont) and the resulting
mixture was stirred with a magnetic stirrer for 2 to 10 minutes.
The resulting molar composition was as follows: 0.22 Na.sub.2 O:0.52
TPABr:10 SiO.sub.2 :200 H.sub.2 O
Hydrothermal Synthesis
The impregnated support with seed coating was mounted in a holder
with the spin-coated face pointing downwards, near the surface of
the synthesis mixture in an autoclave. The autoclave was closed,
placed in an oven, and heated during 30 minutes to the crystallisation
temperature and maintained at that temperature for the period specified
in the following Table. The oven was then allowed to cool to room
temperature. After cooling, the disk was removed and washed in demineralized
water until the conductivity of the last washing water was .ltoreq.5
.mu.S/cm. The disk was then dried in an oven at 125.degree. C. After
drying the resulting structure was calcined and tested for para-xylene
separations performance. The calcination conditions were sufficient
to remove substantially all the impregnating material.
Selectivity Enhancing Coating
One crystalline molecular sieve layer was treated using the procedure
described in Example 3 of WO96/01686 to provide a selectivity enhancing
layer on the crystalline molecular sieve layer. The resulting structure
was also tested for para-xylene separation performance.
Para-xylene Separation Test
A simplified diagram of a unit used to test the crystalline molecular
sieve membrane layers is shown in FIG. 1. Hydrogen feed (1) and
aromatics feed (2), are mixed, preheated and vaporised inside a
sand bath (7). A hydrogen sweep (3) is also preheated in the sand
bath (7). The hydrogen feed (1) combined with aromatics feed (2)
flow into the feed side (8) of a stainless steel cell (11) containing
the crystalline molecular sieve layer on a porous support (9). The
hydrogen sweep (3) flows into the same stainless steel cell but
into the sweep side (10). This cell is designed such that selected
components from the aromatics feed pass through the membrane from
the feed side into the sweep side at process conditions. A product
stream labelled retentate (4), which is the feed depleted of select
components, and permeate (5), which is the sweep enriched with selected
components from the feed, separately, but simultaneously, flow out
of the stainless steel cell. The permeate (5) is analysed by an
on-line chromatograph (GC) (6), and the composition of the permeate
is used in conjunction with the permeate flow to calculate the flow
of each individual component through the membrane.
Following is a detailed description of the testing procedure.
1. A molecular sieve membrane on a porous support is mounted into
a metal (steel) cell and sealed with a graphoil o-ring. It is preferable
to have the surface of the steel cell passivated so that it does
not induce catalytic cracking and coking reactions in the test.
The catalytic activity of the cell and the membrane assembly can
be assessed by measuring the level of cracking products in the permeate.
It is also preferable to pretreat the graphoil o-ring so that it
does not outgass carbonaceous materials which have the potential
of fouling the membrane and reducing observed xylenes flows through
the membrane. One procedure for pretreating graphoil o-rings is
by heating up under air at 450.degree. C. for 3 h followed by cooling
to room temperature. It should be noted that the graphoil o-ring
is applied directly to the zeolite layer or any selectivity-enhancing
coating or reparation layer if applied.
2. The cell with the membrane mounted inside is then heated to
a temperature of at least 250.degree. C. and ideally between 360
and 400.degree. C. A suitable heating rate is .about.2.degree. C./min.
While the membrane is being heated, hydrogen is flowed across the
feed and sweep side of the membrane. Flow rates for tests with a
.about.2.5 cm diameter membrane sealed with a graphoil gasket which
exposes an area of 2.91 cm.sup.2 to the feed are: 100 ml/min at
100.times.10.sup.3 Pa absolute on the feed side 100 ml/min at 100.times.10.sup.3
absolute on the sweep side
It should be noted that the feed side is the side of the membrane
structure sealed by the graphoil gasket (i.e., the side on which
the crystalline molecular sieve layer is deposited). In this steady
state there is no .DELTA.p across the membrane.
For the .about.2.5 cm diameter membrane, a liquid hydrocarbon mixture
which, is inter alia composed of para-xylene and meta-xylene isomers
is introduced at a rate of 33 ml/h into the hydrogen flowing on
the feed side of the membrane. The line carrying the mixture to
the cell passes through a hot zone in order to ensure that the feed
is vaporised and to bring the mixture to the temperature at which
the test is to be conducted. The pressure on the feed side is then
increased by at least 50.times.10.sup.3 Pa, ideally at least 100.times.10.sup.3
Pa absolute. This provides a .DELTA.p across the membrane of at
least 50.times.10.sup.3 Pa and ideally at least 100.times.10.sup.3
Pa.
At the testing temperatures, the hydrogen partial pressure on the
feed side is approximately equal to the hydrogen partial pressure
in the flowing hydrogen sweep stream. With this testing procedure,
hydrogen transference through the membrane is minimised and there
is said to be hydrogen balance.
The composition of the aromatic hydrocarbon mixture used in the
examples was nominally 70% meta-xylene (mX), 20% para-xylene (pX),
5% ethylbenzene (EB), and 5% trimethyl-benzene (TMB) by weight;
variation in this composition is acceptable. In the context of the
present invention reference to aromatic hydrocarbon partial pressure
is to the partial pressure of a mixture of meta-xylene (mX), para-xylene
(pX), ethylbenzene (EB), and trimethyl-benzene (TMB). It is preferable
that the oxygen level in these mixtures be low to prevent chemical
reactions which can lead to coking. This can be done by degassing
the mixtures, or by formulating the mixtures from oxygen free solvents.
The composition of the hydrocarbons in the permeate stream is measured
with an FID detector in a gas chromatograph. The integrated area
for each component is used to deduce the flux of each component;
the integrated area can be related to the mass fraction of a component
in the permeate by a calibration procedure in which a known concentration
of mixture components is passed through the gas chromatograph.
The values selected for characterising the membrane may be taken
at a number of pressures and temperatures. Any membrane when tested
at a temperature of at least 250.degree. C. and a pressure of at
least 10.times.10.sup.3 Pa, which has the required permeance and
selectivity is a membrane according to the present invention. The
performance of the membrane is monitored with time. The test reading
may be taken at any time after the membrane is at the required temperature
and pressure and after introduction of the hydrocarbon feed. It
may be desirable to delay the reading until the membranes selectivity
properties are relatively stable. The performance readings are taken
as the maximum values for Q and .alpha. attained during the test.
As indicated above membranes prepared according to the process of
the present invention may exhibit an improvement in selectivity
properties during the initial stages of use; these improvements
can be rapid or may take extended periods of time to stabilise.
This effect is referred to as selectivation. This initial increase
in performance followed by a period of relative stability is illustrated
in FIG. 2(b), which shows the selectivity properties beginning to
plateau after approximately 6 hours. This effect may not be observed
with reparated membranes. The maximum performance values for these
membranes may occur at the start of the test; performance values
for these membranes are typically taken early in the test in the
first few hours of testing. The exact point in time at which the
test reading is taken will therefore vary with the temperature and
test pressures used. What is important is that the test result is
taken at the maximum performance values and in the case of membranes
which exhibit selectivation, when the maximum plateau is reached.
Ideally this test result is taken at least 1 hour after introduction
of the hydrocarbon feed. In these experiments, the test readings
were taken at various times between 1 to 20 hours after introduction
of the hydrocarbon feed. For the reparated membrane the test result
was taken at 1 hour. Hydrogen flow rates are measured in permeate
and retentate. It is preferred that the performance of the membranes
is achieved at hydrogen balance.
The permeance of hydrocarbon component A is calculated as follows:
##EQU1## Area=Membrane area exposed to feed by graphoil gasket Permeance
is expressed in SI units as mole(px)/m.sup.2.s.Pa(px) where mole(px)
refers to moles of para-xylene, and Pa(px) refers to the paraxylene
partial pressure in Pa (an example of a non-SI unit often used in
industry is kg(px)/m.sup.2.day.bar(px) which is equal to 1.09.times.10.sup.-9
mole(px)/m.sup.2.s.Pa(px).
Under certain circumstances, the transfer of hydrocarbons through
the membrane from feed to sweep is low enough that the partial pressure
of hydrocarbons in the sweep is negligible (note that hydrocarbons
are not added to the sweep so any hydrocarbons present in the permeate
must flow through the membrane). In such circumstances, one may
opt to neglect the partial pressure of hydrocarbon A in the sweep
and calculate the permeance of A using the partial pressure of hydrocarbon
A in the feed as the total transmembrane pressure driving force.
The error in such approximation is equal to the ratio of the partial
pressure of A in the sweep to the partial pressure of A in the feed.
Thus, it follows that if the partial pressure of A in the sweep
is much lower than the partial pressure of A in the feed, the error
is low. Using the flow rates given here to test the membranes described
in this invention, the partial pressure of each hydrocarbon in the
sweep is less than five percent of the partial pressure of the same
hydrocarbon in the feed. This is the result of having deliberately
set the flow rates to attain low transfer of hydrocarbons from feed
to sweep during testing. The total transfer of hydrocarbons from
feed to sweep was kept at less than five percent the amount of hydrocarbons
in the feed. It is preferable that this amount be less than one
percent of the amount of hydrocarbons in the feed. Under these conditions,
the partial pressure of hydrocarbons in the sweep were neglected
in the calculation of permeance, and the permeances reported here
were calculated using the partial pressure of hydrocarbons in the
feed as the total transmembrane pressure driving force. The results
are quoted as Q.sub.x (Q.sub.x =pXy permeance in mole(px)/m.sup.2.s.Pa(px)
and .alpha..sub.x (.alpha..sub.x =pXy/mXy selectivity) with x indicating
the total aromatic hydrocarbon partial pressure in kPa. The test
parameters used in these experiments are as indicated below:
Q.sub.100 and .alpha..sub.100
For Examples 1 to 3 these values were measured under the following
conditions:
Temperature = 360.degree. C. Feed rate =.gtoreq. 100 ml/min Sweep
rate =.gtoreq. 100 ml/min
The composition of feed and sweep gases, in kPa partial pressure
of the gases was as follows:
Feed/ Component of Partial Pressure Sweep Composition kPa Comments
Feed para-xylene 25 Total aromatic hydrocarbon meta-xylene 65 partial
pressure = 100 kPa ethyl-benzene 5 tri-methyl- 5 benzene Hydrogen
100 no net flow of H.sub.2 from feed Sweep Hydrogen 100 to sweep
or vice versa
For Example 4 these values were measured under the following conditions:
Temperature = 400.degree. C. Hydrogen Feed rate =.gtoreq. 120 ml/min
Sweep rate =.gtoreq. 200 ml/min
The composition of feed and sweep gases, in kPa partial pressure
of the gases was as follows:
Feed/ Component of Partial Pressure Sweep Composition kPa Comments
Feed para-xylene 20 Total aromatic hydrocarbon meta-xylene 70 partial
pressure = 100 kPa ethyl-benzene 5 tri-methyl- 5 benzene hydrogen
120 no net flow of H.sub.2 from feed to sweep or vice versa Sweep
hydrogen 115
Q.sub.500 and .alpha..sub.500
These values were measured under the following conditions:
Temperature = 360-400.degree. C. Feed rate =.gtoreq. 200 ml/min
Sweep rate =.gtoreq. 200 ml/min
The composition of feed and sweep gases, in kPa partial pressure
of the gases:
Feed/ Component of Partial Pressure Sweep Composition kPa Comments
Feed para-xylene 125 total aromatic hydrocarbon meta-xylene 325
partial pressure = 500 kPa ethyl-benzene 25 tri-methyl- 25 benzene
hydrogen 1200 no net flow of H.sub.2 from feed Sweep hydrogen 1200
to sweep or vice versa
It must also be pointed out that because of the low total transfer
of hydrocarbons from feed to sweep, the partial pressure of hydrocarbons
in the feed is constant across the membrane surface. Because of
this and the fact that the partial pressures of hydrocarbons in
the sweep are negligible and uniform across the sweep side of the
membrane, the partial pressure difference of each hydrocarbon across
the membrane is constant across the entire membrane area. Therefore,
the permeances reported here are considered point permeances to
distinguish them from permeances one can observe in large-area membranes
where the concentrations in both feed and sweep sides are allowed
to vary across the total membrane area (i.e., the transmembrane
pressure difference varies across the membrane area). Such is the
case of a large membrane module, where, if one applies the equation
of permeance as written, the permeance obtained would be an average
permeance in the membrane module. One may refer to this permeance
as an integrated or module permeance which would be different than
the point permeances provided here. The importance of differentiating
between a point permeance and an average or module permeance is
that a point permeance is the parameter one must use in engineering
the design of a membrane module. An average or module permeance,
on the other hand, only applies to that specific membrane module
under the testing conditions used.
The selectivity of a component A over a component B is calculated
as follows: ##EQU2##
The selectivities and permeance values for the layers according
to the present invention are provided in Table 1.
Examples 1 and 2 illustrate the effect of the additional step of
reparation in the process of the present invention. Example 2 is
reparated and exhibits exceptionally high selectivity for para-xylene
over meta-xylene whilst maintaining an acceptable permeance. The
additional examples illustrate the good selectivities and permeances
observed with the layers of the present invention.
Supported crystalline molecular sieve layers prepared as described
above were monitored for the separation performance with time. FIGS.
2(a) and 2(b) illustrate the results. FIG. 2(a) shows that after
the initial period of stabilisation with time, the permeances of
the components through the layer reduces with exposure to the feed.
In FIG. 2(a) a stepped increase in the feed partial pressure for
hydrogen and for the xylenes components of the feed is identified
at 1000 minutes of exposure. In FIG. 2(b) it can bee seen that this
stepped increase in partial pressure has provided a stepped increase
in selectivity for para-xylene over meta-xylene and trimethylbenzene.
In addition FIG. 2(b) illustrates a surprising improvement in the
selectivity of the layer to para-xylene during use.
TABLE 1 Molecular Sieve Test Time Seed Layer Layer Combined Q.sub.x
10.sup.-8 Temp on oil Thickness Thickness Thickness mole.sub.px
/ Example .degree. C. hours .mu.m .mu.m .mu.m m.sup.2.s.Pa.sub.px
.alpha..sub.x Comments 1 360 6 0.5 0.5 1 Q.sub.100 = 5.8 .alpha..sub.100
= 3.2 Selectivation 2 360 1 0.5 0.5 1 Q.sub.100 = 0.81 .alpha..sub.100
= 103 No Selectivation 3 360 6 0.5 0.5 1 Q.sub.100 = 5.8 .alpha..sub.100
= 2.8 Selectivation 4 400 6 0.5 0.5 1 Q.sub.100 = 9.6 .alpha..sub.100
= 5.4 Selectivation
EXAMPLE 5
Support Cleaning
An .alpha.-alumina disk, 2.5 mm diameter, 3 mm thickness, with
bulk 3 .mu.m pore size, and several intermediate layers, with a
top layer of 100 nm pore size (available from Inocermic GmbH, Hermsdorf,
Germany) was cleaned by rinsing in acetone and filtered ethanol
(0.1 .mu.m filter, Anotop.TM., Whatman) and dried.
Pre-impregnation Masking
A solution of 1 part by weight PMMA (polymethylmetacrylate distributed
by Polykemi AB, Ystad, Sweden as CM 205 MW 100000 g/mole, polydispersity
1.8) in 3.75 parts by weight acetone, was passed through a 0.1 .mu.m
pore filter (Anotop.TM., Whatman), and was brought onto the top
surface of the support by using a syringe, filter and needle. The
deposited solution was then carefully dried by first drying at room
temperature overnight and then heating with a rate of 1.degree.
C./h to 150.degree. C.
Support Impregnation
Hydrocarbon wax (H101 wax) was impregnated from the back of the
masked support support for 1 hour at 150.degree. C. under an applied
vacuum.
Removal of Pre-impregnation Masking
After impregnation the PMMA coating was removed by washing in acetone,
ethanol and the support was then washed with a filtered (0.1 .mu.m
filter, Anotop.TM., Whatman) 0.1 M ammonia solution.
Deposition of Molecular Sieve Seeds
The impregnated support was soaked in a 0.4% aqueous cationic polymer
solution (Redifloc 4150 Eka Nobel AB, Sweden pH adjusted to 8.0
by the addition of ammonia) which was prepared from distilled and
filtered (0.1 .mu.m filter, Anotop.TM., Whatman) water and filtered
through a 0.8 .mu.m filter (Acrodisc.TM., Pall) immediately before
use. The soaking time was 10 minutes. The support was subsequently
rinsed 4 times with a filtered (0.1 .mu.m filter, Anotop.TM., Whatman)
0.1 M ammonia solution. The support was immersed for 10 minutes
in a sol containing 1% .about.60 nm silicalite-1 crystals. The pH
of the sol was 10.0. The support was subsequently rinsed 4 times
with a filtered (0.1 .mu.m filter, Anotop.TM., Whatman) 0.1 M ammonia
solution.
Hydrothermal Synthesis
The seeded supports were treated in a hydrolyzed synthesis mixture
with the molar composition 3TPAOH:25SiO.sub.2 :1500H.sub.2 O:100EtOH
which was heated during 30 h in silicone oil at a temperature of
100.degree. C. After cooling, the membrane was rinsed in 0.1 M ammonia
solution. The resultant membrane was calcined.
The calcined membrane showed the following performance when tested
as above but without hydrogen balance, and averaged over t=1-2 hours
and a sweep flow of 200 ml/min: Q.sub.100 =1.1.times.10.sup.-7 mole/m.sup.2.s.Pa
(102 kg(px)/m.sup.2.day.bar(px)) .alpha..sub.100 =13.2
The test was then continued at higher pressure, and the system
was adjusted to obtain hydrogen balance which was obtained at 19.5
hours; the performance averaged over t=19.5-20.5 hours was: Q.sub.500
=2.75.times.10.sup.-8 mole/m.sup.2.s.Pa (25 kg(px)/m.sup.2.day.bar(px))
.alpha..sub.500 =4.8
EXAMPLE 6
A membrane was prepared as in the Example 10 with the exception
that the crystallization time was 72 hours
Reparation of Membrane
A hydrolyzed synthesis mixture, as used for crystallizing the layer
(see Example 10), was applied to the disk by spincoating at 8000
rpm. While still spinning, the surface was cleaned using 0.1 M ammonia.
The treated membrane was put on a holder above the liquid level
in a 65 ml autoclave containing 10 ml water. The closed autoclave
was held at 100.degree. C. for 24 hours.
The treated membrane was held at 400.degree. C. in air for 6 hours,
heat-up and cool-down rate was 2.degree. C./minute
The resulting membrane showed the following performance after 2
hours exposure to the hydrocarbon stream: Q.sub.100 =1.06.times.10.sup.-7
mole/m.sup.2.s.Pa (97 kg(px)/m.sup.2.day.bar(px)) .alpha..sub.100
=17.4
This membrane did not show selectivation. |