Molecular sieve abstract
The invention relates to the manufacture of novel molecular sieve
adsorbents which are selective towards oxygen from its gaseous mixture
with argon and/or nitrogen. More particularly, this invention relates
to the manufacture of novel molecular sieve adsorbents useful for
the separation of oxygen-argon-nitrogen gaseous mixture.
Molecular sieve claims
What is claimed is:
1. A process for preparing a molecular sieve adsorbent for selectively
adsorbing oxygen from a gaseous mixture comprising oxygen and nitrogen;
oxygen and an inert gas; or oxygen, nitrogen and inert gas said
process comprising:
(a) preparing a mixture of zeolite powder from zeolite X or zeolite
Y, clay and an organic binder;
(b) shaping said zeolite mixture to obtain zeolite adsorbent bodies;
(c) subjecting said adsorbent bodies to calcination; and
(d) subjecting said adsorbent bodies either prior to or after calcination
or both, to cationic exchange in the presence of at least a cerium
salt solution to effect surface modification of said adsorbent bodies
to obtain said molecular sieve adsorbent which is oxygen selective.
2. The process as claimed in claim 1 wherein the zeolite adsorbent
body has the formula
where the value of
x is 0.0 to 0.8
y is from 0.06 to 0.50
a is from 1 to 2
b is from 2 to 3
z is from 2 to 5.5
M is alkali or alkaline earth metal ion, and w represents the moles
of water.
3. The process as claimed in claim 2 wherein M is lithium, sodium,
potassium or calcium.
4. The process as claimed in claim 1 wherein said cation exchange
is carried out in the presence of a cerium salt solution in combination
with lithium, calcium salt solution or a mixture thereof.
5. The process as claimed in claim 1 wherein said clay is bentonite
clay.
6. The process as claimed in claim 1 wherein said clay is present
in an amount of 2 to 40% by weight.
7. The process as claimed in claim 1 wherein said binder is selected
from the group consisting of sodium lignosulfate, starch and polyvinyl
alcohol.
8. The process as claimed in claim 1 wherein said zeolite mixture
is subjected to ball milling to produce powders of particle size
of less than 60 microns.
9. The process as claimed in claim 1 wherein said calcination
is carried out at a temperature of 450.degree. C. to 700.degree.
C.
10. The process as claimed in claim 1 wherein said calcination
is carried out for a period of from 2 to 18 hrs.
11. The process as claimed in claim 1 wherein said cation exchange
is carried out at a concentration of 1 to 10% by weight.
12. The as claimed in claim 1 wherein said cation exchange is
carried out with 1 to 10% by wt. of said aqueous solution at a temperature
of 30 to 100.degree. C. for 4 to 48 hrs.
13. A molecular sieve adsorbent having a composition
where the value of
x is 0.0 to 0.8
y is from 0.06 to 0.50
a is from 1 to 2
b is from 2 to 3
z is from 2 to 5.5
M is alkali or alkaline earth metal ion, and w represents the moles
of water.
14. The adsorbent according to claim 13 wherein M is lithium, sodium,
potassium or calcium.
Molecular sieve description
The invention relates to the manufacture of novel molecular sieve
adsorbents which are selective towards oxygen from its gaseous mixture
with argon and/or nitrogen. More particularly, this invention relates
to the manufacture of novel molecular sieve adsorbents useful for
the separation of oxygen-argon-nitrogen gaseous mixture.
BACKGROUND OF THE INVENTION
Adsorption processes for the separation of oxygen and nitrogen
from air are being increasingly used for commercial purposes for
the last two decades. Presently, 4-5% of the world's oxygen demand
is met by adsorptive separation of air. However, the maximum attainable
oxygen purity by-adsorption processes is around 95%. Separation
of 0.934 mole percent argon present in the air from oxygen being
a limiting factor to achieve 100% oxygen purity by adsorption methods.
However, there are many situations where high purity oxygen (>99%)
is desired. For example, the efficiency of welding and cutting processes
using oxygen is greatly dependent upon the purity of oxygen available.
For these applications, purity of at least 99.5% oxygen is customarily
specified. Furthermore, oxygen-argon separation is also needed for
purification of argon produced during cryogenic separation of oxygen
and nitrogen from air. The crude argon (95-97%) produced in such
processes will have nitrogen and oxygen and is required to be further
purified. Presently high purity argon (99.999%) is produced by catalytic
hydrogen combustion or low temperature oxygen adsorption in a synthetic
zeolite.
Oxygen and argon gaseous mixture is difficult to separate due to
closeness in their physical properties. At present, commercially
this is done by cryogenic fractionation techniques. The boiling
points of oxygen (-182.97.degree.), argon (-185.9.degree. C.) and
nitrogen (-195.8.degree. C.) being very low make these processes
highly energy intensive. Thus, it is desired to develop a commercially
attractive separation process for oxygen-argon separation. Adsorption
based process can compete with highly energy intensive cryogenic
fractionation of oxygen/argon mixture if a suitable adsorbent which
is selective towards one of the components and which possesses requisite
adsorption capacity is commercially available.
In the prior art, adsorbents which are selective for argon from
its mixture with oxygen has been reported (PCT Int. Appl. 94. 06.
541. Mar. 31 1994) by impregnation of silver in commercial zeolites.
However, the adsorption selectivity reported for argon is less than
2 in these adsorbents making it commercially unattractive. Oxygen
with purity >99% has been produced (U.S. Pat. No. 4813979
1989) by using carbon molecular sieve adsorbent in which argon is
selectively adsorbed due to its smaller kinetic diameter of 3.40A.degree.
compared to 3.46A.degree. of oxygen. However, there are no reports
on the development of adsorbent which is selective towards oxygen
from its mixture with argon in the literature. The present invention
deals with the development of synthetic zeolite based oxygen selective
adsorbents which can for the first time separate oxygen from a
gaseous mixture of oxygen and argon.
Adsorption processes are also being used on a commercial scale
for the production of nitrogen from air. These processes employ
carbon molecular sieve type adsorbents in which oxygen diffuses
faster than nitrogen resulting in the separation of the two components.
Some efforts to develop zeolite type adsorbents for these applications
have also been reported in the literature wherein the differences
in the diffusion of oxygen and nitrogen have been used to achieve
oxygen adsorption selectivity. It is desired to develop a zeolite
based adsorbent which can result in the oxygen adsorption selectivity
due to difference in equilibrium adsorption of oxygen and air.
The characteristics which are highly desirable, if not absolutely
essential, for an adsorbent to be suitable for selective adsorption
process include adsorption capacity of the adsorbent and adsorption
selectivity for a particular component.
Adsorption capacity of the adsorbent is defined as the amount in
terms of volume or weight of the desired component adsorbed per
unit volume or weight of the adsorbent. The higher the adsorbent's
capacity for adsorbing the desired component the better the adsorbent
is as the increased adsorption capacity of a particular adsorbent
helps to reduce the amount of adsorbent required to separate a specific
amount of a component from a mixture of particular concentration.
Such a reduction in adsorbent quantity in a specific adsorption
process brings down the cost of a separation process.
Adsorption selectivity of component A over B is defined as
where O is adsorption selectivity, X is the adsorbed concentration
and Y is gas-phase concentration. The expression gas-phase concentration
means the amount of unadsorbed component remaining in the gas-phase.
The adsorption selectivity of a component depends on
steric factors such as difference in the shape and see of the adsorbate
molecules;
equilibrium effect, i.e., when the adsorption isotherms of the
components of the gas mixture differ appreciably;
kinetic effect, when the components have substantially different
adsorption rates.
It is generally observed that for a process to be commercially
economical, the minimum acceptable adsorption selectivity for the
desired component is about 3 and when an adsorption selectivity
is less than 2 it is difficult to design an efficient separation
process.
Zeolites which are microporous crystalline aluminosilicates are
finding increased applications as adsorbents for separating mixtures
of closely related compounds. Zeolites have a three dimensional
network of basic structural units consisting of SiO.sub.4 and AlO.sub.4
tetrahedral linked to each other by sharing of apical oxygen atoms.
Silicon and aluminum atoms lie at the center of the tetrahedral.
The resulting aluminosilicate structure which is generally highly
porous possesses three dimensional pores the access to which is
through molecular sized windows. In a hydrated form, the preferred
zeolites are generally represented by the following Formula [I]
where "M" is a cation which balances the electrovalence
of the tetrahedral and is generally referred to as extra framework
exchangeable cation, n represents the valency of the cation, x and
w represent the moles of SiO.sub.2 and water respectively. The cations
may be any one of the number of cations which will hereinafter be
described in detail.
The attributes which make them attractive for separation include,
an unusually high thermal and hydrothermal stability, uniform pore
structure, easy pore aperture modification and substantial adsorption
capacity even at low adsorbate pressures. Furthermore, zeolites
can be produced synthetically under relatively moderate hydrothermal
conditions.
Zeolite of type X structure as described and defined in U.S. Pat.
No. 2882244 are the preferred adsorbents for adsorption separation
of the gaseous mixture described in this invention. Zeolite of type
X in hydrated or partially hydrated form can be described in terms
of metal oxide of Formula II
where "M" represents at least one cation having valence
n, w represents the number of moles of water the value of which
depends on the degree of hydration of the zeolite. Normally, the
zeolite when synthesized has sodium as exchangeable cations.
Zeolites as such have very little cohesion and it is, therefore,
necessary to use appropriate binders to produce the adsorbent in
the form of particles such as extrudates, aggregates, spheres or
granules to suit commercial applications. Zeolitic content of the
adsorbent particle vary from 60 wt % to 100 wt % depending on the
type of binder used. Clays such as bentonite, kaolin and attapulgite
are normally used as inorganic binders for agglomeration of zeolite
powders.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide adsorbents
which can be used for the separation of oxygen-argon, oxygen-nitrogen
and oxygen-nitrogen-argon gaseous mixtures.
Yet another object of the present invention is to provide an oxygen
selective adsorbent based on synthetic zeolites.
Yet another object of the present invention is to provide an oxygen
selective adsorbent by modification of surface characteristics of
synthetic zeolites.
Yet another object of the present invention is to provide an adsorbent
with high adsorption selectivity and capacity for oxygen from its
mixture with argon and/or nitrogen.
Yet another object of the present invention is to provide an oxygen
selective adsorbent which can be used commercially.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a chromatogram of mixture of oxygen and argon on CeXP-1.
FIG. 2 is a chromatogram of mixture of oxygen and argon on CeXP-3.
FIG. 3 is a chromatogram of mixture of oxygen and argon on CeXP-4.
FIG. 4 is a chromatogram of mixture of oxygen, nitrogen and argon
on CeXE.
FIG. 5 is a chromatogram of mixture of oxygen and argon on CaCeXP.
DESCRIPTION OF THE INVENTION
According to the present invention, there is provided a molecular
sieve adsorbent having a composition
where the value of
x is from 0.0 to 0.8;
y is from 0.06 to 0.50;
a is from 1 to 2;
b is from 2 to 3;
z is from 2 to 5.5;
M is alkali or alkaline earth metal ion such as lithium, sodium,
potassium, calcium and w represents the number of moles of water.
The initial zeolite, i.e. the starting material of the present
invention may be prepared by any technique known in the art. Typically,
(i) a mixture of Zeolite powder type X as described in U.S. Pat.
No. 2882244 or zeolite Y as described in U.S. Pat. No. 3130007
is prepared with a clay such as herein described and an organic
binder such as herein described, (ii) adsorbent bodies of desired
shape are formed or the adsorbent powder is subjected to cation
exchange with one or more cations and then formed into adsorbent
bodies, (iii) adsorbent bodies so formed are subjected to calcination
(iv) the calcined adsorbent bodies are subjected to cation exchange
with one or more cations if the cation exchange has been done in
step (ii).
The present invention employs the technique of modification of
the surface properties of the adsorbent bodies by cation exchange
with one or more cations to obtain oxygen selective adsorbent from
gaseous mixture of oxygen and argon.
The modification of the surface property, hereinafter referred
to as surface modification is the most critical and important aspect
of the invention. It is the very specific surface modification which
renders the zeolite particularly selective towards oxygen. It has
been surprisingly found that if zeolite at all x-type is treated
with a cerium salt solution and/or a combination of cerium salt
solution with lithium and/or calcium salt solution, it renders the
zeolite particularly selective towards oxygen over argon.
Accordingly, the present invention provides a process for the preparation
of a molecular sieve adsorbent for selectively adsorbing oxygen
from a gaseous mixture consisting of oxygen, nitrogen and/or an
inert gas such as argon said process comprising:
(a) preparing in any known manner a mixture of zeolite powder with
conventional clay and organic binder;
(b) shaping said zeolite mixture to obtain adsorbent bodies of
desired shape;
(c) subjecting adsorbent bodies to calcination; and
(d) subjecting said adsorbent bodies either prior to or after calcination
or both, to cationic exchange in the presence of at least a cerium
salt solution to effect surface modification of said adsorbent bodies
to obtain said molecular sieve adsorbent which is oxygen selective.
While the aforesaid surface modification may be carried out at
a wide range of temperature and concentration, excellent results
are obtained if the surface modification supply is carried out with
1 to 10% by weight of the salt solution at a temperature of 30 to
100.degree. C. for 4 to 48 hrs.
The adsorbent bodies are prepared from a mixture of zeolite of
type X and clay powder with an addition of an organic binder like
sodium lignosulfonate or starch or polyvinyl alcohol. Bentonite
type clay preferably about 2 to 40% by weight is normally used for
aggregation of zeolite powder. As the clay remains as an inert component
in the adsorbent body and do not display any adsorption properties,
the adsorption capacity and selectivity of the adsorbent body decreases
in proportion to the amount of the clay added in the body.
In a typical process for producing adsorbent pellets, zeolite powder
of type X or type Y was mixed with desired quantity of clay. A known
quantity of an organic binder like sodium lignosulfonate was added
to this mixture which was then subjected to ball milling for some
specified period to have powder particles less than 60 microns.
The powder thus obtained was formed into bodies using a pan granulator
or an extruder. The particles prepared by the above described method
were first dried in air at room temperature (28 to 32.degree. C.)
for about 6 to 18 hours followed by oven drying at 110.degree. C.
for 6 to 8 hours. The dried particles were subjected to air calcination
at 450 to 700.degree. C. for 2-18 hours followed by aforesaid surface
modification.
The quantity of exchangeable cations in the adsorbent particles
after the above treatment is determined by digesting the known amount
of adsorbent particles in hot hydrochloric acid and then making
the aqueous solution. The quantitative estimation of the cations
in the aliquot solution is done by Atomic Absorption Spectroscopic
measurement.
The loss of crystallinity in the adsorbent particles, if any, was
checked by comparing the X-ray diffraction data with literature
X-ray data. The X-ray diffractions at `d` values 14.465 8.845
7.538 5.731 4.811 4.419 3.946 3.808 3.765 3.338 3.051 2.944
2.885 2.794 and 2.743A.degree. were used for comparison. Water
adsorption capacity data on the above treated adsorbent particles
were also compared with a standard zeolite NaX. Water adsorption
capacity was measured using a Mcbain-Bakr quartz spring balance.
Oxygen/argon/nitrogen adsorption capacity and selectivity were
measured by elution chromatography. In this technique, the adsorbent
sample was ground and sieved to obtain 60-80 mesh particles and
packed in a thoroughly cleaned 6.times.600 mm stainless steel column
which was placed in an oven of a gas chromatography. In those cases
where the starting material was zeolite powder, it was first pressed
in to pellets in a hydraulic press to obtain compact particles and
then ground and sieved to obtain 60-80 mesh particles. The adsorbent
was activated by subjecting it to programmed heating from ambient
to 400.degree. C. at the heating rate of 2.degree. C./minute and
held at 400.degree. C. for 12 hours with the flow of 60 ml/minute
of ultra-high purity hydrogen. Alter the activation, the column
temperature was brought down to ambient temperature and the hydrogen
gas flow was reduced to 30 ml/minute. A 0.5 mL pulse of gas mixture
consisting of oxygen, argon, nitrogen and helium in hydrogen was
injected in to the adsorbent column using a sampling valve, and
the retention times of gases measured. The procedure was repeated
at 40 50 and 60.degree. C. The column was equilibrated for at least
1 hour at each temperature before injecting the gas mixture. The
corrected retention times were obtained by subtracting the helium
retention time from those of oxygen, argon and nitrogen. In those
cases where separation did not take place, retention times were
measured by injecting individual gases. To check whether there was
any contribution to measured retention time due to possible oxidation-reduction
reactions, measurements without hydrogen were also done by using
helium rather than hydrogen. The measured retention times were found
within experimental errors indicating the absence of oxidation-reduction
reactions.
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