Molecular sieve abstract
The present invention relates to the manufacture of novel molecular
sieve adsorbents for the separation of a gaseous mixture of oxygen,
nitrogen and argon. The adsorbent is useful for the separation of
oxygen and/or nitrogen from air. More specifically, the invention
relates to the manufacture of a molecular sieve adsorbent, which
is selective towards nitrogen from its gaseous mixture with oxygen
and/or argon and argon from its gaseous mixture with oxygen.
Molecular sieve claims
What is claimed is:
1. A single stage process for the preparation of crystalline molecular
sieve adsorbent by silver ion exchange, used for selectively adsorbing
nitrogen and argon from a gaseous mixture containing oxygen, said
process comprising the steps of: (a) mixing Zeolite A with an aqueous
solution of silver salt; (b) refluxing the solution at 30-90.degree.
C. for 4-8 hours in dark to obtain a residue; (c) filtering and
washing the residue with water till the residue is free from silver
ions; and (d) drying below 85.degree. C. in air followed by under
reduced pressure to obtain the crystalline molecular sieve adsorbent
having a chemical composition (Ag.sub.2 O).sub.x.(M.sub.2/n O).sub.y.(Al.sub.2
O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2 O.
2. A process according to claim 1 wherein steps (b)-(d) can be
optionally performed in the following steps: (a) mixing Zeolite
A with equal amount of silver salt solution; (b) heating the mixture
at a temperature in the range of 500-575.degree. C. in an inert
atmosphere; (c) washing the residue with water till the residue
is free from silver ions; and (d) drying the mixture at an ambient
temperature under reduced pressure to obtain the crystalline molecular
sieve adsorbent.
3. A process according to claim 1 wherein the value of x varies
from 1.2 to 6.0 moles.
4. A process according to claim 1 wherein the values of y varies
from 0.0 to 4.8 moles.
5. A process according to claim 1 wherein M is a cation selected
from the group consisting of sodium, calcium, potassium or lithium.
6. A process according to claim 1 wherein the zeolite is in the
form of granule, powder or pellets.
7. A process according to claim 1 wherein the aqueous solution
of silver salt solution is selected from silver per chlorate (AgClO.sub.4),
silver acetate or silver nitrate (AgNO.sub.3).
8. A process according to claim 1 wherein the concentration of
silver salt solution is in the range of 0.25%-15% by weight/volume
of zeolite A.
9. A process according to claim 1 wherein the ratio of aqueous
solution of silver salt with zeolite A is 1:80.
10. A process according to claim 1 wherein said molecular sieve
has a high nitrogen adsorption capacity upto 22.3 cc/g at 30.degree.
C. and 765 mm Hg.
11. A process according to claim 1 wherein said molecular sieve
has a high nitrogen adsorption capacity up to 22.3 cc/g at 30.degree.
C. and 765 mm Hg.
12. A process according to claim 1 wherein said molecular sieve
has a selectivity for nitrogen over oxygen of 5-14.6 at 30.degree.
C.
13. A process according to claim 1 wherein said molecular sieve
has an argon adsorption capacity up to 6.5 cc/g at 30.degree. C.
and 765 mm Hg.
14. A process according to claim 1 wherein said molecular sieve
has selectivity for argon in the range of 1.2-2.0 at 30.degree.
C.
15. A process according to claim 1 wherein said molecular sieve
has a low hydroxylation thereby preventing the necessity of slow
heating.
16. A process according to claim 1 wherein said molecular sieve
has a high purification capacity of oxygen greater than 96%.
17. A process according to claim 1 wherein 10 to 100 equivalent
percentages of silver ions is loaded into zeolite in a single step
using a water soluble silver salt selected from silver nitrate perchlorate
and silver acetate.
18. A process according to claim 1 wherein the molecular sieve
adaorbent is dried at a temperature in the range of 20.degree. C.
to 80.degree. C.
Molecular sieve description
FIELD OF INVENTION
The present invention relates to a process for the preparation
of a molecular sieve adsorbent for selectively adsorbing nitrogen
and argon from a gaseous mixture with oxygen.
BACKGROUND AND PRIOR ART REFERENCES
Adsorption processes for the separation of oxygen and nitrogen
from air are being increasingly used for commercial purposes for
the last three decades. Oxygen requirements in sewage treatment,
fermentation, cutting and welding, fish breeding, electric furnaces,
pulp bleaching, glass blowing, medical purposes and in the steel
industries particularly when the required oxygen purity is 90 to
95% is being largely met by adsorption based pressure swing or vacuum
swing processes. It is estimated that at present, 4-5% of the world's
oxygen demand is met by adsorptive separation of air. However, the
maximum attainable purity by adsorption processes is around 95%
with separation of 0.934 mole percent argon present in the air being
a limiting factor to achieve 100% oxygen purity. Furthermore, the
adsorption-based production of oxygen from air is economically not
competitive to cryogenic fractionation of air for production levels
more than 100 tonne oxygen per day. Of the total cost of the oxygen
production by adsorption processes, it is estimated that capital
cost of equipment and power consumption are the two major factors
influencing the overall cost with their share being 50% and 40%
respectively. Along with the factors like process and system design,
the adsorbent is the key component, which can bring down the cost
of oxygen production by adsorption. The adsorbent selectivity and
capacity are important parameters for determining the size of the
adsorption vessels, compressors or vacuum pumps. It is desirable
to have an adsorbent, which shows a high adsorption capacity as
well as selectivity for nitrogen compared to oxygen. The improvement
in these properties of the adsorbent directly results in lowering
the adsorbent inventory of a system and hence the size and power
consumption of the air compressor or vacuum pump. Furthermore, adsorbent
having a high. nitrogen capacity and selectivity can also be used
to produce reasonably pure nitrogen along with oxygen by evacuating
nitrogen adsorbed on the adsorbent. Furthermore, adsorbents having
both nitrogen and argon selectivity over oxygen can be used for
producing high purity (>96%) oxygen from air.
It is, therefore, highly desirable, for an adsorbent to have good
adsorption capacity and adsorption selectivity for a particular
component sought to be separated.
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 the desired components the better is the adsorbent
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.
The adsorption selectivity of a component results from steric factors
such as the differences in the size and shape of the adsorbate molecules;
equilibrium effect, i.e. when the adsorption isotherms of components
of a 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 the adsorption selectivity
is less than 2 it is difficult to design an efficient adsorption
process.
In the prior art, adsorbent which are selective for nitrogen from
its mixture with oxygen and argon have been reported wherein the
zeolites of type A, X and mordenite have been used after ion exchanging
alkali and/or alkaline earth metal ions. However, the adsorption
selectivity reported for the commercially used zeolite A based adsorbents
for this purpose varies from 3 to 5 and adsorption capacity from
12-15 cc/g at 765 mmHg and 30.degree. C. The efforts to enhance
the adsorption capacity and selectivity have been reported by increasing
the number of exchangeable cations into the zeolite structure by
modifying the chemical composition of the zeolite. The adsorption
selectivity for nitrogen has also been substantially enhanced by
exchanging the zeolite with cations like lithium and/or calcium
in some zeolite types.
Zeolite A having a specific amount of calcium has been commercially
used for oxygen production from air by selectively adsorbing nitrogen.
However, presently used adsorbent has the following limitations:
Low adsorption capacity compared to other commercially used adsorbents.
Low adsorption selectivity. It gives oxygen with only 95% maximum
purity. Sensitivity to moisture. It needs multiple exchange with
calcium salt. The activation of the adsorbent requires much care,
in order to prevent the hydroxylation.
R. V. Jasra et al. reviewed the recent status of pressure swing
adsorption as a process for separating multi component gas mixture
in "Separation of gases by pressure swing adsorption";
Separation Science and Technology, 26(7), pp. 885-930 1991 The
application of a new generation of adsorbents were described in
detail. In "Adsorption of a Nitrogen-Oxygen mixture in NaCaA
zeolites by elution Chromatography", Ind. Eng. Chem. Res. 1993
32 548-552 N. V. Choudary et al. describes the influence of calcium
content on adsorption of nitrogen and oxygen is studied on various
NaCaA zeolite samples. N. V. Choudary et al. describes the adsorption
and desorption of nitrogen, oxygen and argon in mordenite type zeolite
having different Si/Al ratios in `Sorption of nitrogen, oxygen and
argon in mordenite type zeolites`, Indian Journal of Chemistry Vol.
38A January 1999 pp.34-39. The heat of adsorption of nitrogen and
argon in mordenite, NaA and NaX were compared to revels the sorbate
interactions with extra-frame work sodium ions as well as lattice
oxygen atoms.
Reference may be made to J. J. Collins et al in U.S. Pat. No. 3973931(1976)
entitled "Air separation by adsorption", wherein an adiabatic
pressure swing process for air separation by selective adsorption
in atleast two zeolitic molecular sieve beds in which air is introduced
at below 90.degree. F., the coldest gas temperature in the inlet
end is 35.degree. F., delta T atleast 15.degree. F., the inlet end
is heated to maintain the gas at maximum of at least 20.degree.
F. warner than without heating, but below 175.degree. F. The main
drawback is it require heating and temperature control in the air
separation process.
C. G. Coe et al. in U.S. Pat. No. 4481018 (1984) entitled "Polyvalent
ion exchanged adsorbent for air separation", describes the
use of a thermally activated polyvalent ion exchanged faujasite-containing
compositions with selectivity 3.4 to 6.7 at 30.degree. C. for the
separation of air into oxygen and nitrogen. The drawbacks are the
thermal activation process requires very slow heating to prevent
hydroxylation and the selectivity of the adsorbent is only 3.4 to
6.7 at 30.degree. C.
S. Sircar et al in U.S. Pat. No. 4557736 (1985) entitled "Binary
ion exchanged type X zeolite adsorbent", describes the use
of an adsorbent comprises a binary ion exchanged type X zeolite,
in which 5%-40% of the available ion sites are occupied by calcium
and 60%-95% of the available ion sites are occupied by strontium
is used for the adsorption of nitrogen from an air stream at superambient
pressure to produce an oxygen rich product streem. The main drawback
is the preparation of the adsorbent requires multistage cation exchange
process.
S. Sircar in U.S. Pat. No. 4756723 (1988) entitled "Preparation
of high purity oxygen", describes the use of a single stage
pressure swing adsorption method for the production of approximately
95% pure oxygen. The main drawback is the maximum attainable oxygen
purity is only 95%.
C. C. Chao in U.S. Pat. No. 4859217 entitled (1989) "Process
for separating nitrogen from mixtures, thereof with less polar substances",
wherein highly lithium exchanged low silica form of zeolite X containing
more than ninety percent lithium cations are used for the selective
adsorption of nitrogen from less polar gases. These adsorbents were
prepared by lithium exchanging with 4-12 fold excess LiCl.sub.3
solution. The main drawbacks are the adsorbents are highly moisture
sensitive and the lithium exchange requires 4-12 fold excess LiCl.sub.3
solution.
C. G. Coe et al. in U.S. Pat. No. 4943304 (1990) entitled "Process
for the purification of bulk gases by using chabazite adsorbents",
which provides a process for the selective adsorption of one or
more minor constituents from a bulk gas stream using a chabazite.
The main drawback is the known methods for preparing commercially
useful synthetic chabazites are not practical since they suffer
from low yields, poor product purity, long crystallization times
and are difficult if not impractical to scale up.
C. C. Chao in U.S. Pat. No. 4964889 (1990) entitled "Selective
adsorption on magnesium containing clinoptilites", in which
gases having molecular dimensions equal to or smaller than nitrogen
are selectively adsorbed and separated from other gases having molecular
dimensions higher than nitrogen. The main drawback is the particle
size of the commercial clinoptilolite varies and the particle size
of the clinoptilolite will affect the speed and completeness of
the ion exchange reaction.
G. Reiss in U.S. Pat. No. 5114440 (1992) entitled "Process
for the adsorptive-oxygen enrichment of air with mixture of calcium
zeolite A molecular sieve by means of vacuum swing adsorption",
which gives a process for oxygen enrichment of air by means of vacuum
swing adsorption using CaA molecular sieve. The drawbacks of this
adsorbent are low nitrogen adsorption capacity, low selectivity
of nitrogen over oxygen, its preparation needs multistage calcium
exchange and its activation process requires very slow heating to
prevent hydroxylation.
C. G. Coe et al in U.S. Pat. No. 5152813 (1992) entitled "Nitrogen
adsorption with a Ca and/or Sr exchanged lithium X zeolite",
which is directed to a process for separating nitrogen from gas
mixtures containing oxygen, hydrogen, argon or helium by use of
an at least binary exchanged X-zeolite having lithium and calcium
and/or strontium ions in ratio of preferably 5% to 50% calcium and/or
strontium and 50% to 95% lithium. The main drawbacks are the preparation
of the adsorbent requires multistage cation exchange, its activation
process requires very slow heating to prevent hydroxylation and
adsorbent is highly sensitive to moisture.
C. C. Chao et al.in U.S. Pat. No. 5174979 (1992) entitled "Mixed
ion exchanged zeolites and processes for the use thereof in gas
separations", wherein lithium/alkaline earth metal X zeolites
in which the lithium:alkaline earth metal equivalent ratio is from
95:5 to about 50:50 and lithium/alkaline earth metal A zeolites
in which the lithium: alkaline earth metal equivalent ratio is from
10:90 to about 70:30 are found useful for the separation of oxygen
and nitrogen from a gas mixture. The main drawbacks are the preparation
of the adsorbent requires multistage cation exchange, nitrogen selectivity
is only 2-7 and the adsorbent is highly moisture sensitive.
T. R. Gafney et al. in U.S. Pat. No. 5266102 (1993) entitled
"Oxygen VSA process with low oxygen capacity adsorbents",
wherein adsorbents with moderate nitrogen capacity and a high selectivity
is used for the separation by VSA process. The main drawbacks are
the maximum attainable oxygen purity is only 95% and adsorbent with
low nitrogen capacity was used for the separation process.
C. C. Chao in U.S. Pat. No. 5454857 (1995) entitled "Air
separation process", wherein 60 to 89 equivalent percent calcium
exchanged forms of zeolite X having silica/alumina ratio in the
range of 2.0 to 2.4 is used in a temperature range of 50.degree.
C. to -20.degree. C. and pressure range of 0.05 to 5 atmospheres.
The main drawbacks are the preparation of the adsorbent requires
multistage cation exchange and its activation process requirs very
slow heating to prevent hydroxylation.
F. R. Fitch et al in U.S. Pat. No. 5464467 (995) entitled "Adsorptive
separation of nitrogen from other gases", where in type X zeolites
whose charge compensating cations are composed of 95 to 50% lithium
ions, 4 to 50% of one or more of aluminum, cerium, lanthanum and
mixed lanthanides and 0 to 15% of other ions were used for selectively
adsorb nitrogen from gas mixture. The main drawbacks of this adsorbent
are its high affinity towards moisture and its preparation requires
multistage cation exchange from 5 to 10 fold lithium chloride solutions.
C. C. Chao et al. in U.S. Pat. No. 5698013 (1997) entitled "Nitrogen
selective zeolitic adsorbent for use in air separation process",
wherein using 60 to 89 equivalent percent calcium exchanged forms
of zeolite X having SiO.sub.2 /Al.sub.2 O.sub.3 ratio with in the
range of 2.0 to 2.4 as selective adsorption for nitrogen in air
separation process by pressure swing adsorption. The main drawbacks
of this adsorbent are its high affinity towards moisture and its
preparation requires multistage calcium exchange and its activation
process requirs very slow heating to prevent hydroxylation.
T. C. Golden et al. in U.S. Pat. No. 5779767 (1998) entitled
"Use of zeolites and alumna in adsorption processes",
wherein describes a process for the purification of air by adsorption.
The main drawback is this adsorbent is useful only for the adsorption
of carbon dioxide, water, hydrocarbons and nitrogen oxides from
the gas mixtures.
N. Ogawa et al. in U.S. Pat. No. 5868818 (1999) entitled "Adsorbents
for air separation, production method thereof, and air-separation
method using it", describes the use of crystalline zeolite
X having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of not larger
than 3.0 which contains at least 90 mol% lithium cations were used
for the air separation by pressure swing adsorption. The main drawbacks
of this adsorbent are its preparation requires multistage cation
exchange and it is highly sensitive to small amount of moisture.
J. T. Mullhaupt et al. in U.S. Pat. No. 5945079 (1999) entitled
"Oxygen selective sorbents", describes an invention comprises
a process for air separation using oxygen selective sorbent with
enhanced selectivity, loading capacities and oxygen uptake rates
have a transition metal complex in solid form supported on a high
surface area substrate. The drawbacks of this adsorbent are (a)
The adsorption is not physical adsorption and hence it is not completely
reversible, (b) The preparation and handling of the adsorbent is
very difficult and (c) the use of this adsorbent is not commercially
economical.
N. V. Choudray et al in No. U.S. Pat. No. 6030916 (2000) entitled
"Process for the preparation of a molecular sieve adsorbent
for selectively adsorbing nitrogen from a gaseous mixture",
describes the preparation of molecular sieve adsorbents containing
yttrium and alkali and/or alkaline earth metals as the extra framework
exchangeable cations, useful for the separation of oxygen and/or
nitrogen from air. The main drawback is the yttrium exchange process
requires several cycles to obtain the adsorbent having high nitrogen
selectivity.
N. V. Choudray et al in U.S. Pat. No. 6087289 (2000) entitled
"Process for the preparation of a molecular sieve adsorbent
for selectively adsorbing oxygen from a gas mixture", describes
a process for the, preparation of a zeolte based adsorbent containing
cerium cations are used for the selective adsorption of oxygen from
a gas mixture. The main drawbacks of this adsorbent are its low
oxygen capacity (around 3 cc/g at 30.degree. C. and 1 atm) and the
oxygen selectivity is only in the low-pressure region.
M. Bulow et al. in U.S. Pat. No. 6143057 (2000) entitled "Adsorbents
and adsorptive separation process", describes the use of an
adsorbent composites composed of microparticulate zeolites at least
90% of whose particles have a characteristic particle dimension
not greater than about 0.6 microns and a macropore inert binder
used for separating nitrogen or carbon dioxide from air. A mixture
of type A zeolite, alpha zeolite, type X zeolite and type Y zeolite
in which the available cation sites are occupied by a mixture of
cations was used as the adsorbent for the selective adsorption.
The main drawbacks of this adsorbent are its preparation involves
several cycles of cation exchange process and the adsorbent is highly
sensitive to moisture.
R. Jain et al. in U.S. Pat. No. 6231644 (2001) entitled "Air
separation using monolith adsorbent bed", describes the use
of monolith bed for separating a first gaseous component from a
gas mixture comprising the first gaseous component and a second
gaseous component comprising passing the gaseous mixture into an
adsorption zone. The main drawbacks of this adsorbent are its high
affinity towards moisture and its preparation requires multistage
ion exchange process.
N. V. Choudary et al. in Indian patent No. 181823 (1995) entitled
"A process for the preparation of a molecular sieve adsorbent
useful in the oxygen enrichment of air", describes the use
of a zeolite A based adsorbent containing clay binders were used
to produce 85-95% pure oxygen by pressure swing adsorption. The
main drawbacks of this adsorbent are the low nitrogen selectivity
over oxygen (3-5 at 30.degree. C. and 1 atm), low nitrogen capacity
(around 15 cc/g at 30.degree. C. and 1 atm), its high affinity towards
moisture and its preparation requires multistage calcium exchange
process.
OBJECTS OF THE INVENTION
The main object of the present invention is to provide a process
for the preparation of a molecular sieve adsorbent for selectively
adsorbing nitrogen and argon from a gaseous mixture with oxygen,
which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a nitrogen
selective adsorbent based on synthetic zeolite.
Yet another object of the present invention is to provide argon
selective adsorbent (compared to oxygen) based on synthetic zeolite.
Still another object of the present invention is to provide an
adsorbent that can visibly refer the decay in adsorption capacity
and selectivity due to moisture by its colour change.
Further object of the present invention is to provide an adsorbent
with increased adsorption selectivity and capacity for nitrogen
from its mixture with oxygen and/or argon.
Still another object of the present invention is to provide an
adsorbent, which is selective to nitrogen and argon over oxygen
and can be used commercially for the separation of air.
Yet another object of the present invention is to provide an adsorbent,
which can be prepared by a single stage cation exchange process.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a "Process for
the preparation of a molecular sieve adsorbent for selectively adsorbing
nitrogen and argon from a gaseous mixture with oxygen", which
comprises of a molecular sieve adsorbent represented by the general
formula,
where the values of x varies from 4.8 to 6.0 y from 0.0 to 1.2
w being the number of moles of water and M is any metal ion having
valancy n.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the present invention provides a single stage process
for the preparation of crystalline molecular sieve adsorbent by
silver ion exchange, used for selectively adsorbing nitrogen and
argon from a gaseous mixture containing oxygen, said process comprising
the steps of: (a) mixing Zeolite A with an aqueous solution of silver
salt; (b) refluxing the solution at 30-90.degree. C. for 4-8 hours
in dark to obtain a residue; (c) filtering and washing the residue
with water till the residue is free from silver ions; and (d) drying
below 85.degree. C. in air followed by under reduced pressure to
obtain the crystalline molecular sieve adsorbent having a chemical
composition (Ag.sub.2 O).sub.x.(M.sub.2/n O).sub.y.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O
An embodiment of the present invention, wherein steps (b)-(d) can
be optionally performed in the following steps: (a) mixing Zeolite
A with equal amount of silver salt solution;. (b) heating the mixture
at a temperature in the range of 500-575.degree. C. in an inert
atmosphere; (c) washing the residue with water till the residue
is free from silver ions; and (d) drying the mixture at an ambient
temperature under reduced pressure to obtain the crystalline molecular
sieve adsorbent.
Yet another embodiment of the present invention, wherein the value
of x varies from 1.2 to 6.0 moles.
Still another embodiment of the present invention, wherein the
values of y varies from 0.0 to 4.8moles.
Yet another embodiment of the present invention, wherein w is number
of moles of water.
Still another embodiment of the present invention, wherein M is
a cation selected from the group consisting of sodium, calcium,
potassium or lithium and most preferably sodium.
Still another embodiment of the present invention, wherein the
zeolite selected is in the form of granule, powder and pellets.
Yet another embodiment of the present invention, wherein the aqueous
solution of silver salt solution is selected from silver per chlorate
(AgClO.sub.4), silver acetate or silver nitrate (AgNO.sub.3).
Further embodiment of the present invention, wherein the concentration
of silver salt solution is in the range of 0.25%-15% by weight/volume
of zeolite A.
Still another embodiment of the present invention, wherein the
ratio of aqueous solution of silver salt with zeolite A is 1:80.
Yet another embodiment of the present invention, wherein said molecular
sieve having a high nitrogen adsorption capacity upto 22.3 cc/g
at 30.degree. C. and 765 mm Hg.
Further embodiment of the present invention, wherein said molecular
sieve having selectivity for nitrogen over oxygen is 5-14.6 at 30.degree.
C.;
Yet another embodiment of the present invention, wherein said molecular
sieve having argon adsorption capacity upto 6.5 cc/g at 30.degree.
C. and 765 mm Hg.
Further embodiment of the present invention, wherein said molecular
sieve having selectivity for argon in the range of 1.2-2.0 at 30.degree.
C.
Yet another embodiment of the present invention, wherein said molecular
sieve having a low hydroxylation thereby preventing the necessity
of slow heating.
Further embodiment of the present invention, wherein said molecular
sieve having a high purification capacity of oxygen greater than
96%.
Yet another embodiment of the present invention, wherein 10 to
100 equivalent percentages of silver ions is loaded into zeolite
in a single step using any water soluble silver salt selected from
silver nitrate silver perchlorate or silver acetate.
Still another embodiment of the present invention, wherein the
zeolite is. ion exchanged with 80 to 100 equivalent percent silver
ions and activated molecular sieve adsorbent is orange red/brick
red coloured.
Yet another embodiment of the present invention, wherein the molecular
sieve adsorbent is dried at a temperature below 85.degree. C. preferably
at a temperature in the range of 20.degree. C. to 80.degree. C.
in air or under vacuum.
The invention is further explained in the form of the following
embodiments:
Zeolites, which are microporous crystalline alumna-silicates, are
finding increased applications as adsorbents for. separating mixtures
of closely related compounds. Zeolites have a three dimensional
network of basic structural units consisting SiO.sub.4 and AlO.sub.4
tetrahedrons linked to each other by sharing apical oxygen atoms.
Silicon and aluminum atoms lie in the center of the tetrahedral.
The resulting alumino-silicate 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,
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 valancy of the cation and x and w represents the
moles of SiO.sub.2 and water respectively. The cation may be any
one of the numbers of cations, which will hereinafter be described
in detail.
The attributes which makes the zeolites 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.
X-ray powder diffraction data was collected using PHILIPS X'pert
MPD system equipped with XRK 900 reaction chamber. Comparing the
X-ray diffraction data with literature X-ray data checked the crystallinity
of the adsorbent particles. The X-ray diffraction at "d"
values 12.1925 5.489 4.086 3.2818 2.9773 and 2.7215 were used
for comparison.
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. X-ray diffraction data showed
that the starting material was highly crystalline. The zeolite NaA
was mixed with a specified concentration of aqueous silver salt
solutions in the ratio 1:80 and treated at 30-90.degree. C. for
4-8 hours in the dark. The residue was filtered, washed with hot
distilled water, until the washings were free from silver ions (tested
with sodium chloride solution) and dried at room temperature to
80.degree. C. in air and also under vacuum conditions as specified
in the examples. The silver exchange completes in a single step
since the equilibrium of the cation exchange reaction favors the
easy formation of the product. The extent of silver exchange was
determined by Atomic Absorption Spectroscopy.
Oxygen, nitrogen and argon adsorption at 15.degree. C. and 30.degree.
C. was measured using a static volumetric system (Micromeritics
ASAP 2010), after activating the sample at 350.degree. C. to 450.degree.
C. under vacuum for 4 hours as described in the Examples. Addition
of the adsorbate gas was made at volumes required to achieve a targeted
set of pressures ranging from 0.5 to 850 mmHg. A minimum equilibrium
interval of 5 seconds with a relative target tolerance of 5.0% of
the targeted pressure and an absolute target tolerance of 5.000
mmHg were used to determine equilibrium for each measurement point.
The adsorption and desorption are completely reversible, hence it
is possible to remove the adsorbed gases by simple evacuation.
The heat of adsorption was calculated using the following equation
where R is the universal gas constant, .theta. is the amount of
gas adsorbed at a pressure p and temperature T. A plot of Inp against
1/T should be a straight line of slope .DELTA..sub.ad H.sup.0 /R.
The selectivity of two gases A and B is given by the equation,
where V.sub.A and V.sub.B are the volumes of gas A and B adsorbed
at any given pressure P and temperature T.
The important inventive steps involved in the present invention
are that the molecular sieve adsorbent obtained, (i) is prepared
by a single stage ion exchange process using aqueous solution of
any silver salt in a temperature range of room temperature to 90.degree.
C., (ii) is stable during the activation process, hydroxylation
chance is very low and hence the activation process does not requre
very slow heating, (iii) has a brick red/orange red colour afer
the activation, which changes with decay of the adsorption capacity
and selectivity due to the presence of moisture in the feed gas
mixture, (iv) has, adsorption capacity and selectivity for nitorgen
is maximum reported for any zeolite A based adsorbent so far and
(v) has argon selectivity over oxygen, which will be useful for
the production of oxygen with purity higher than 96%.
BRIEF DESCRIPTION OF ACCOMPANIED FIGURES
FIG. 1 represents the adsorption isotherms of nitrogen, argon and
oxygen at 30.degree. C. on the adsorbent obtained from example-4.
FIG. 2 represents the nitrogen adsorption isotherms on zeolites
NaA, CaA and AgA at 30.degree. C.
FIG. 3 represents the oxygen adsorption isotherms on zeolites NaA,
AgA and CaA at 30.degree. C.
FIG. 4 represents the argon adsorption isotherms on zeolites NaA,
CaA and AgA at 30.degree. C.
The following examples are given by way of illustration and therefore
should not be constructed to limit the scope of the present invention.
EXAMPLE-1
The zeolite NaA powder [(Na.sub.2 O).sub.6.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. A known amount of the sample
was activated at 350.degree. C. under vacuum and adsorption measurements
were carried out as described earlier. The adsorption capacity for
nitrogen is only 7.5 cc/g at 30.degree. C. and 765 mmHg and selectivity
for nitrogen over oxygen is only around 3 in the pressure range
studied, the values are given in table 1. The heat of adsorption
values, given in table-2 and the shape of desorption curve shows
that the adsorption is physisorption and completely reversible.
EXAMPLE-2
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the NaA powder was
mixed with 0.1M calcium chloride solutions in the ratio 1:80 and
refluxed at 80.degree. C. for 4 hours. The solution was decanted
and the residue was mixed with fresh calcium chloride and the procedure
was repeated four more times to ensure the complete replacement
of the sodium ions with calcium ions. The residue is filtered, washed
with hot distilled water, until the washings are free from chloride
(tested with silver nitrate solution) and dried in air at room temperature
(30.degree. C.). Elemental analysis shows the chemical composition
of the adsorbent is (CaO).sub.5.8.(Na.sub.2 O).sub.0.2.(Al.sub.2
O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2 O. X-ray diffraction
data shows that the material is highly crystalline. A known amount
of the sample was activated at 350.degree. C. under vacuum and adsorption
measurements were carried out as described earlier. The adsorption
capacity for nitrogen is 15.5 cc/g at 30.degree. C. and 765 mmHg
and selectivity for nitrogen over oxygen is 3 to 5.5 in the pressure
range studied, values are given in table-1. The heat of adsorption
values, given in table-2 and the shape of desorption curve shows
that the adsorption is physisorption and completely reversible.
EXAMPLE-3
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 25.0 g of the molecular sieve
NaA powder was mixed with 0.1M AgNO.sub.3 solutions in the ratio
1:80 and refluxed at 80.degree. C. for 4 hours. The residue is filtered,
washed with hot distilled water, until the washings are free from
silver ions (tested with sodium chloride solution) and dried in
air at room temperature (28.degree. C.). Elemental analysis shows
the chemical composition of the adsorbent is (Ag.sub.2 O).sub.5.6.(Na.sub.2
O).sub.0.4.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became blackish brown after activating
at 350.degree. C. The adsorption capacity for nitrogen is 21.4 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is 4.9 to 10.3 in the pressure range studied, values are
given in table-1 which are very high compared to the commercially
using zeolite A based adsorbent. The heat of adsorption values,
given in table-2 and the shape of desorption curve shows that the
adsorption is physisorption and completely reversible.
EXAMPLE-4
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 25.0 g of the molecular sieve
NaA powder was mixed with 0.1M AgNO.sub.3 solutions in the ratio
1:80 and refluxed at 80.degree. C. for 4 hours. The residue is filtered,
washed with hot distilled water, until the washings are free from
silver ions (tested with sodium chloride solution) and dried in
air at room temperature (30.degree. C.). Elemental analysis shows
the chemical composition of the adsorbent is (Ag.sub.2 O).sub.5.8.(Na.sub.2
O).sub.0.2.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 380.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became brick red after activating
at 380.degree. C. The adsorption isotherms are given in FIG. 1.
The adsorption capacity for nitrogen is 22.3 cc/g at 30.degree.
C. and 765 mmHg and selectivity for nitrogen over oxygen is 5.2
to 14.6 in the pressure range studied, values are given in table-1
which are very high compared to the commercially using zeolite A
based adsorbent. The heat of adsorption values, given in table-2
and the shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-5
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 25.0 g of the molecular sieve
NaA powder was mixed with 0.1M AgNO.sub.3 solutions in the ratio
1:80 and refluxed at 80.degree. C. for 4 hours. The residue is filtered,
washed with hot distilled water, until the washings are free from
silver ions (tested with sodium chloride solution) and dried in
air oven at 60.degree. C. Elemental analysis shows the chemical
composition of the adsorbent is (Ag.sub.2 O).sub.5.4.(Na.sub.2 O).sub.0.6.(Al.sub.2
O.sub.3).sub.6 (SiO.sub.2).sub.12.wH.sub.2 O. X-ray diffraction
data shows that the material is highly crystalline. A known amount
of the sample was activated at 380.degree. C. under vacuum and adsorption
measurements were carried out as described earlier. The colour of
the sample became brick red after activating at 380.degree. C. The
adsorption capacity for nitrogen is 20.7 cc/g at 30.degree. C. and
765 mmHg and selectivity for nitrogen over oxygen is 4.7 to 12.0
in the pressure range studied, values are given in table-1 which
are very high compared to the commercially using zeolite A based
adsorbent. The heat of adsorption values, given in table-2 and the
shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-6
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the molecular sieve
NaA powder was mixed with AgNO.sub.3 solutions containing 12.0 g
AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in air oven at 40.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.5.2.(Na.sub.2 O).sub.0.8.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 400.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became orange red after activating
at 400.degree. C. The adsorption capacity for nitrogen is 20.1 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is 4.6 to 12.7 in the pressure range studied, values are
given in table-1 which are very high compared to the commercially
using zeolite A based adsorbent. The heat of adsorption values,
given in table-2 and the shape of desorption curve shows that the
adsorption is physisorption and completely reversible.
EXAMPLE-7
The zeolite NaA granules [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the molecular sieve
NaA granules was mixed with AgNO.sub.3 solutions containing 14.0
g AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in hot air oven at 80.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.5.7.(Na.sub.2 O).sub.0.3.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became brick red after activating
at 350.degree. C. The adsorption capacity for nitrogen is 22.1 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is 5.3 to 14.4 in the pressure range studied, values are
given in table-1 which are very high compared to the commercially
using zeolite A based adsorbent. The heat of adsorption values,
given in table-2 and the shape of desorption curve shows that the
adsorption is physisorption and completely reversible.
EXAMPLE-8
The zeolite NaA granules [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the zeolite NaA
granules was mixed with AgClO.sub.4 solutions containing 18.0 g
AgClO.sub.4 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in hot air oven at 80.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.5.6.(Na.sub.2 O).sub.0.4.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became Yellow orange after activating
at 350.degree. C. The adsorption capacity for nitrogen is 22.3 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is 5.2 to 14.2 in the pressure range studied, values are
given in table-1 which are very high compared to the commercially
using zeolite A based adsorbent. The heat of adsorption values,
given in table-2 and the shape of desorption curve shows that the
adsorption is physisorption and completely reversible.
EXAMPLE-9
The zeolite NaA granules [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the zeolite NaA
granules was mixed with silver acetate solutions containing 13.0
g silver acetate, in the ratio 1:80 and refluxed at 80.degree. C.
for 4 hours. The residue is filtered, washed with hot distilled
water, until the washings are free from silver ions (tested with
sodium chloride solution) and dried in hot air oven at 80.degree.
C. Elemental analysis shows the chemical composition of the adsorbent
is (Ag.sub.2 O).sub.5.4.(Na.sub.2 O).sub.0.6.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became Orange red after activating
at 350.degree. C. The adsorption capacity for nitrogen is 21.3 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is 4.9 to 13.8 in the pressure range studied, values are
given in table-1 which are very high compared to the commercially
using zeolite A based adsorbent. The heat of adsorption values,
given in table-2 and the shape of desorption curve shows that the
adsorption is physisorption and completely reversible.
EXAMPLE-10
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the molecular sieve
NaA powder was mixed with AgNO.sub.3 solutions containing 8.5 g
AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in vacuum oven at 80.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.4.8.(Na.sub.2 O).sub.1.2.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became yellow orange after activating
at 350.degree. C. The adsorption capacity for nitrogen is 18.8 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is 4.8 to 13.4 in the pressure range studied, values are
given in table-1 which are very high compared to the commercially
using zeolite A based adsorbent. The heat of adsorption values,
given in table-2 and the shape of desorption curve shows that the
adsorption is physisorption and completely reversible.
EXAMPLE-11
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the molecular sieve
NaA powder was mixed with AgNO.sub.3 solutions containing 7.0 g
AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in vacuum oven at 80.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.4.2.(Na.sub.2 O).sub.1.8.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became orange red after activating
at 350.degree. C. The adsorption capacity for nitrogen is 9.1 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is only around 3.5 in the pressure range studied, the values
are given in table-1. The heat of adsorption values, given in table-2
and the shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-12
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O was used as the starting material. 10.0 g of the molecular sieve
NaA powder was mixed with AgNO.sub.3 solutions containing 6.0 g
AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in vacuum oven at 80.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.3.6.(Na.sub.2 O).sub.2.4.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became yellow orange after activating
at 350.degree. C. The adsorption capacity for nitrogen is 8.8 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is only around 3.1 in the pressure range studied, the values
are given in table-1. The heat of adsorption values, given in table-2
and the shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-13
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material 10.0 g of the molecular sieve
NaA powder was mixed with AgNO.sub.3 solutions containing 4.0 g
AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in vacuum oven at 80.degree. C. X-ray
diffraction data shows that the material is highly crystalline.
Elemental analysis shows the chemical composition of the adsorbent
is (Ag.sub.2 O).sub.2.4.(Na.sub.2 O).sub.3.6.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became orange red after activating
at 350.degree. C. The adsorption capacity for nitrogen is 8.7 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is only around 3.7 in the pressure range studied, the values
are given in table-1. The heat of adsorption values, given in table-2
and the shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-14
The zeolite NaA powder [Na.sub.12.(AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the molecular sieve
NaA powder was mixed with AgNO.sub.3 solutions containing 2.0 g
AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in vacuum oven at 80.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.1.2.(Na.sub.2 O).sub.4.8.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became brick red after activating
at 350.degree. C. The adsorption capacity for nitrogen is 5.9 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is only around 3.4 in the pressure range studied, the values
are given in table-1. The heat of adsorption values, given in table-2
and the shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-15
The adsorbent obtained by the method as described in Example-5
were further treated with 0.1M AgNO.sub.3 solutions in the ratio
1:80 and refluxed at 80.degree. C. for 4 hours. The residue is filtered,
washed with hot distilled water, until the washings are free from
silver ions (tested with sodium chloride solution) and dried in
air at room temperature (28.degree. C.). Elemental analysis shows
the chemical composition of the adsorbent is (Ag.sub.2 O).sub.5.8.(Na.sub.2
O).sub.0.2.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraciton data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out at 15.degree.
C. and 30.degree. C. as described above. The colour of the sample
became brick red after activating at 350.degree. C. The adsorption
capacity for nitrogen is 22.1 cc/g at 30.degree. C. and 765 mmHg
and selectivity for nitrogen over oxygen is 5.1 to 14.2 in the pressure
range studied, values are given in table-1 which are very high
compared to the commercially using zeolite A based adsorbent. The
heat of adsorption values, given in table-2 and the shape. of desorption
curve shows that the adsorption is physisorption and completely
reversible.
EXAMPLE-16
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 10.0 g of the molecular sieve
NaA powder was mixed with AgNO.sub.3 solutions containing 12.0 g
AgNO.sub.3 in the ratio 1:80 and refluxed at 80.degree. C. for
4 hours. The residue is filtered, washed with hot distilled water,
until the washings are free from silver ions (tested with sodium
chloride solution) and dried in hot air oven at 120.degree. C. Elemental
analysis shows the chemical composition of the adsorbent is (Ag.sub.2
O).sub.5.7.(Na.sub.2 O).sub.0.3.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraciton data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became greenish black after activating
at 350.degree. C. The adsorption capacity for nitrogen is 14.5 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen only around 4.9 in the pressure range studied, the values
are given in table-1. The heat of adsorption values, given in table-2
and the shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-17
The zeolite NaA pellets was used as the starting material. 10.0
g of the molecular sieve NaA pellets was mixed with AgNO.sub.3 solutions
containing 12.0 g AgNO.sub.3 in the ratio 1:80 and refluxed at
80.degree. C. for 4 hours. The residue is filtered, washed with
hot distilled water, until the washings are free from silver ions
(tested with sodium chloride solution) and dried in vacuum oven
at 60.degree. C. Elemental analysis shows the chemical composition
of the adsorbent is (Ag.sub.2 O).sub.5.4.(Na.sub.2 O).sub.0.6.(Al.sub.2
O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2 O. X-ray diffraction
data shows that the material is highly crystalline. A known amount
of the sample was activated at 450.degree. C. under vacuum and adsorption
measurements were carried out as described earlier. The colour of
the sample became brownish black after activating at 450.degree.
C. The adsorption capacity for nitrogen is 22.1 cc/g at 30.degree.
C. and 765 mmHg and selectivity for nitrogen over oxygen is 5.1
to 14.2 in the pressure range studied, values are given in table-1
which are very high compared to the commercially using zeolite A
based adsorbent. The heat of adsorption values, given in table-2
and the shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-18
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 5.0 g of the molecular sieve
NaA powder was mixed thoroughly with 5.0 g AgNO.sub.3 and heated
up to 575.degree. C. under helium in a programmable tabular furnace
with a heating rate of 1.degree. C./min and the temperature was
maintained at 575.degree. C. for 4 hours. The material is washed
with hot distilled water, until the washings are free from silver
ions (tested with sodium chloride solution) and dried in air at
room temperature (28.degree. C.). Elemental analysis shows the chemical
composition of the adsorbent is (Ag.sub.2 O).sub.5.5.(Na.sub.2 O).sub.0.5.(Al.sub.2
O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2 O. X-ray diffraction
data shows that the material is highly crystalline. A known amount
of the sample was activated at 350.degree. C. under vacuum and adsorption
measurements were carried out as described earlier. The colour of
the sample became brick red after activating at 350.degree. C. The
adsorption capacity for nitrogen is 21.3 cc/g at 30.degree. C. and
765 mmHg and selectivity for nitrogen over oxygen is 5.0 to 12.8
in the pressure range studied, values are given in table-1 which
are very high compared to the commercially using zeolite A based
adsorbent. The heat of adsorption values, given in table-2 and the
shape of desorption curve shows that the adsorption is physisorption
and completely reversible.
EXAMPLE-19
The zeolite NaA powder [Na.sub.12 (AlO.sub.2).sub.12.(SiO.sub.2).sub.12.wH.sub.2
O] was used as the starting material. 25.0 g of the molecular sieve
NaA powder was mixed with 0.05M AgNO.sub.3 solutions in the ratio
1:80 and refluxed at 80.degree. C. for 4 hours. Decanted and the
procedure was repeated once again to ensure the complete ion exchange.
The residue is filtered, washed with hot distilled water, until
the washings are free from silver ions (tested with sodium chloride
solution) and dried in air at room temperature (28.degree. C.).
Elemental analysis shows the chemical composition of the adsorbent
is (Ag.sub.2 O).sub.5.6.(Na.sub.2 O).sub.0.4.(Al.sub.2 O.sub.3).sub.6.(SiO.sub.2).sub.12.wH.sub.2
O. X-ray diffraction data shows that the material is highly crystalline.
A known amount of the sample was activated at 350.degree. C. under
vacuum and adsorption measurements were carried out as described
earlier. The colour of the sample became brick red after activating
at 350.degree. C. The adsorption capacity for nitrogen is 21.9 cc/g
at 30.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is 5.1 to 13.1 in the pressure range studied, values are
given in table-1 which are very high compared to the commercially
using zeolite A based adsorbent. The heat of adsorption values,
given in table-2 and the shape of desorption curve shows that the
adsorption is physisorption and completely reversible.
The adsorption capacity and selectivity of all the 19 samples are
enumerated in the Table-1 and the colour of the sample after activation
and heat of adsorption for nitrogen, oxygen and argon are given
in Table-2.
ADVANTAGES OF THE INVENTION
The adsorption capacity towards nitrogen is found to be 22.3 cc/g
at 30.degree. C. and 765 mmHg, which is the maximum reported for
any zeolite A based adsorbent so far.
The adsorption selectivity towards nitrogen is found to be 5.3
tol4.6 at 30.degree. C. in pressures range studied, which is the
maximum reported for any zeolite A based adsorbent so far.
The adsorbent shows argon selectivity (around 2 at 30.degree. C.)
over oxygen, which is not generally observed in the case of zeolite-based
adsorbents, and will be useful for the production of oxygen with
purity higher than 96%.
Selectivity of nitrogen as well as argon over oxygen, which is
not generally observed in other commercial molecular sieve adsorbents.
Brick red colour of the activated form of the adsorbent, which
is sensitive to moisture, is helpful to find out easily whether
the adsorbent is active.
The preparation of the adsorbent is a very easy single step process
compared to multistage cation exchange process commercially used.
Since the chance for the hydroxylation is very low, the activation
process of the adsorbent does not require much care compared to
the commercially used ones. |