Molecular sieve abstract
The invention relates to the manufacture of 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 molecular sieve adsorbents useful for the
separation of oxygen-argon gaseous mixture. More specifically, the
invention relates to the manufacture and use of a molecular sieve
adsorbent by cation exchange in zeolites by rare earth cations to
obtain oxygen selective adsorbent from its gaseous mixture with
nitrogen and argon at ambient conditions of temperature and pressure.
Thus prepared adsorbent is useful for the separation and purification
of nitrogen and argon from its mixture with oxygen.
Molecular sieve claims
We claim:
1. A process for preparing a molecular sieve adsorbent for selective
adsorption of oxygen from air, the process comprising (i) exchanging
zeolite X with water-soluble salt of a rare earth metal selected
from the group consisting of cerium, europium, gadolinium and any
mixture thereof; (ii) filtering the mixture, washing the powder
or pellet with hot distilled water till it is free from anions to
obtain an exchanged zeolite; (iii) drying the exchanged zeolite;
(iv) and activating the exchanged zeolite.
2. A process as claimed in claim 1 wherein the zeolite X is used
in powder form has 100% crystallinity or pellet form.
3. A process as claimed in claim 1 wherein the Na cations of zeolite
are exchanged with salts of the rare earth metals selected from
chloride, nitrate and acetate
4. A process as claimed in claim 1 wherein the cation exchange
is carried at a temperature in the range of 30.degree. C. to 90.degree.
C. for a period in the range of 4 to 8 hours.
5. A process as claimed in claim 1 wherein the cation exchange
is carried out at a cation concentration in the range of 0.01 to
0.1 M solution.
6. A process as claimed in claim 1 wherein the exchanged zeolite
is dried in a temperature range of 20.degree. C. to 80.degree. C.
in air or under vacuum.
7. A process as claimed in claim 1 wherein the exchanged zeolite
is activated at the temperature range of 350 to 450.degree. C. for
a period in the range of 3-6 hours followed by cooling under inert
or vacuum.
Molecular sieve description
FIELD OF THE INVENTION
[0001] The present invention relates to a process for the preparation
of molecular sieve adsorbent for selective adsorption of oxygen
from air. The present invention also relates to the use of rare
earth exchanged zeolites as selective adsorbents for separation
of gases having closely related physical properties. More particularly,
the present invention relates to the preparation and use of adsorbent,
which is selective towards oxygen, from a gaseous mixture of oxygen
with argon.
BACKGROUND OF THE INVENTION
[0002] The use of adsorption techniques to separate a gaseous component
from a gaseous stream was initially developed for the removal of
carbon dioxide and water from air. Gas adsorption techniques are
now employed in processes for the recovery of hydrogen from its
mixture with hydrocarbons, and enrichment of oxygen from air.
[0003] The four types of adsorbents widely used include activated
carbon, zeolite molecular sieves, silica gel and activated alumina.
Carbon molecular sieves (CMS), which exhibit very narrow pore size
distribution, facilitates separation of air to recover nitrogen
has provided a secure and growing market for carbon molecular sieve.
[0004] Adsorption processes for the separation of oxygen and argon
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 where the required oxygen purity is between
90 to 95% are being largely met by adsorption based pressure swing
or vacuum swing processes. It is estimated that at present, around
20% 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, adsorption based production of oxygen from air is economically
not competitive to cryogenic fractionation of air for production
levels of more than 200 tons per day.
[0005] Argon gas is mainly used in industries as an inert gas for
creating inert atmosphere. An argon or argon-hydrogen mixture is
employed in the production of high-purity iron. Argon is also used
in welding, cutting, and spraying of metals, depending on the welding
process, the noble gas is used pure, as a mixture, or in combination
with oxygen, hydrogen, or carbon dioxide. Argon/argon-hydrogen mixtures
(>5% H.sub.2) are used as protective gases for plasma welding.
There are many potential applications of argon and it is expected
that this work will lead to increased consumption of argon in the
future.
[0006] For the adsorbents used in adsorption separation processes,
there are two key; characteristics namely adsorption capacity and
adsorption selectivity which need to be considered to assess their
potential. Adsorption capacity of the adsorbent is defined as the
amount in terms of 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 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.
[0007] The adsorption selectivity of a component over the other
is calculated as the ratio of the volumes of gas adsorbed at any
given pressure and temperature. The adsorption selectivity of a
component results from steric 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.
[0008] Adsorption for oxygen and nitrogen production is being widely
used and tremendous research effort is being directed to improve
the adsorption processes for higher adsorption capacity and selectivity.
Adsorbents affect separations by adsorbing one or more components
of the mixture strongly than the others components present in the
mixture. The various interactive forces involved in adsorption process
are van der Waals interactions, acid-base interactions, hydrogen
bond, electrostatic, chelation, and clathration. Therefore, adsorbents
are appropriately modified to enhance the interactions between adsorbent
and adsorbate molecules to improve adsorption capacity and selectivity.
Zeolites which are crystalline inorganic porous materials having
pores with molecular dimensions have largely been used for adsorption
separation. As the extra framework cations of the zeolites are reasonably
mobile, cations exchange in zeolites is one the most common used
techniques for surface modifications with cations of suitable size
and charge to enhance zeolite adsorbate interactions. Literature
on adsorbent development work carried out on zeolites shows that
most of the reported work is confined to alkali and alkaline earth
cations as extra framework cations. Adsorption of nitrogen, oxygen
and argon in zeolites with cations having higher than bivalent is
sparsely reported. As trivalent cations have higher charge density,
they will have higher electrostatic interactions with nitrogen molecules
if these cations are present at sites accessible to nitrogen molecules.
[0009] The principal characteristic of the separation, removal
or concentration of oxygen, nitrogen and argon from the air is that
usually there is no cost for the starting material, which is air.
The cost of the desired gas produced or removed, depends essentially
upon the following other factors.
[0010] (a) Costs of equipment necessary for separating, or concentrating
the gas,
[0011] (b) Costs of energy necessary for operating the equipment,
[0012] (c) When gas with high purity is needed, the cost of the
additional purification step which has to be taken into account.
[0013] Taking the above factors into consideration, various economically
advantageous processes have, heretofore, been proposed. These include,
for example; a process wherein air is liquefied al low temperatures
to separate oxygen or nitrogen making use of difference in the boiling
point between liquid oxygen (-182.9.degree. C.) and liquid nitrogen
(-195.8.degree. C.). The apparatus employed is suited for producing
large amounts of oxygen and nitrogen in the world is based on this
procedure. Disadvantages of the process is that it requires large
amounts of power, large-scale equipment is necessary site specific
and portability is very less, it takes hours for switching on and
switching off the plant. Last two decades, adsorption and membrane
based processes for the separation of oxygen, nitrogen from air
has emerged as potential alternatives.
[0014] A membrane system has been employed for the separation of
oxygen and nitrogen from air. U.S. Pat. No. 5091216 (1992) to
Hayes et al; U.S. Pat. No. 5004482 (1991) to Haas et al. and U.S.
patent application 203862 (2002), to Katz et al disclose the separation
of oxygen and nitrogen from air using polymeric type membranes.
The membrane based systems works at very high pressure. The main
drawbacks of this method is the thin polymeric films are too weak
to withstand the high differential gas pressures required for the
separation and purity of the product gas is only around 50%.
[0015] In the prior art, adsorbent which are selective for nitrogen
from its mixture with oxygen and argon have been reported [U.S.
Pat. No. 5114440 (1992) to Reiss], [U.S. Pat. No. 4481018 (1984)
to Coe et al., U.S. Pat. No. 4557736 (1985) to Sircar et al.,
U.S. Pat. No. 4859217 (1989) to Chao; Chien-Chung, U.S. Pat. No.
5152813 (1992) to Coe et al., U.S. Pat. No. 5174979 (1992) to
Chao; Chien-Chung et al., U.S. Pat. No. 5454857 (1995) to Chao;
Chien-Chung., U.S. Pat. No. 5464467 (1995) to Fitch et al., U.S.
Pat. No. 5698013 (1997) to Chao; Chien-Chung., U.S. Pat. No. 5868818
(1999) to Ogawa et al., U.S. Pat. No. 6030916 (2000) to Choudary
et al.,], [U.S. Pat. No. 4964889 (1990) to Chao; Chien-Chung,
Gerhard,], [U.S. Pat. No. 4943304 (1990) to Coe et al.,] and [U.S.
Pat. No. 6231644 (2001) to Jain et al.] wherein the zeolites of
type A, faujasite, clinoptilolite, chabazite and monolith respectively
have been used. The efforts to enhance the adsorption capacity and
selectivity have been reported by exchanging the extra framework
cations with alkali and/or alkaline earth metal cations and increasing
the number of extra framework cations 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 faujasite type zeolite. These adsorbents have been employed in
processes for the separation or concentration of oxygen by removing
nitrogen selectively from the air. The drawback of these adsorbents
are the maximum attainable oxygen purity by adsorption processes
is around 95%, with separation of 0.934-mole percent argon present
in the air being a limiting factor t achieve 100% oxygen purity.
These adsorbents are also highly moisture sensitive and the adsorption
capacity and selectivity will decay in the presence of moisture.
[0016] U.S. Pat. No. 4453952 (1984) to Izumi et al. discloses
the manufacture of an oxygen selective adsorbent by substituting
the Na cations of zeolite A with K and Fe (II). The adsorbent shows
oxygen selectivity only at low temperature and its preparation requires
iron exchange carried out at around 80.degree. C. using aqueous
salt solutions of metal ions followed by exchange with potassium.
The drawback of this invention is that the potassium exchange in
zeolite leads to lower thermal and hydrothermal stability of the
adsorbent.
[0017] U.S. Pat. No. 3979330 to Munzner et al. discloses the
preparation of carbon containing molecular sieves in which coke
containing up to 5% volatile components is treated at 600-900.degree.
C. in order to split off carbon from a hydrocarbon. The split-off
carbon is deposited in the carbon framework of the coke to narrow
the existing pores. The drawback of this process is deposition on
carbon framework is not uniform and very energy intensive process.
[0018] U.S. Pat. No. 4742040 to Ohsaki et al. discloses a process
for making a carbon molecular sieve having increased adsorption
capacity and selectivity by palletising powder of charcoal containing
small amounts of coal tar as a binder, carbonising, washing in mineral
acid solution to remove soluble ingredients, adding specified amounts
of creosote or other aromatic compounds, heating at 950-1000.degree.
C., and then cooling in an inert gas. The drawback of this process
is energy intensive and tedious and organic compounds are expensive.
[0019] U.S. Pat. No. 4880765 to Knoblauch et al., discloses a
process for producing carbon molecular sieves with uniform quality
and good separating properties by treating a carbonaceous product
with inert gas and steam in a vibrating oven with multi step and
further treating it with benzene at high temperatures thereby narrow
existing pores. Preparation of carbon molecular sieve is a multi-step
process with utmost care at each state to get totally reproducible
carbon molecular sieve. Additionally, the process is very high temperature
process, which results into higher cost of the products.
[0020] U.S. Pat. No. 5081097 (1992) to Sharma et al., discloses
copper modified carbon molecular sieves for selective removal of
oxygen from air. The sieve is prepared by pyrolysis of a mixture
of a copper-containing material and polyfunctional alcohol to form
a sorbent precursor. The sorbent precursor is then heated and reduced
to produce a copper modified carbon molecular sieve. Pyrolysis is
high temperature process making the whole process of preparation
of the adsorbent an energy intensive process.
[0021] U.S. Pat. No. 6087289 (2000) to Choudary et al. discloses
a process for the preparation of a zeolite-based adsorbent containing
cerium cations for the selective adsorption of oxygen from the gas
mixture. Cerium exchange into zeolite is carried out under reflux
conditions using aqueous solution of cerium salt at around 80.degree.
C. for 4-8 hours and repeating the ion exchange process several
times and separation of gases was studied by gas chromatography
in very low-pressure range. The main drawbacks of this adsorbent
are oxygen selectivity being obtained only in the low-pressure region.
Additionally, adsorption was studied only by gas chromatography
in limited pressure range. Thus higher pressure range adsorption
data was not obtained.
[0022] In another approach, chemical vapour deposition technique
was used for controlling the pore opening size of the zeolites by
the deposition of silicon alkoxide [M. Niwa et al., JCS Farady Trans.
I, 1984 80 3135-3145; M. Niwa et al., M. Niwa et al., J. Phys.
Chem., 1986 90 6233-6237; Chemistry Letters, 1989 441-442; M.
Niwa et al., Ind. Eng. Chem. Res., 1991 30 38-42; D. Ohayon et
al., Applied Catalysis A-General, 2001 217 241-251]. Chemcal vapour
deposition is carried out by taking a requisite quantity of zeolite
in a glass reactor, which is thermally activated at 450.degree.
C. in situ under inert gas like nitrogen flow. The vapours of silicon
alkoxide are continuously injected into inert gas stream, which
carries the vapours to zeolite surface where alkoxide chemically
reacts with silanol groups of the zeolite. Once the desired quantity
of alkoxide is deposited on the zeolite, sample is heated to 550.degree.
C. in air for 4-6 hours after which it is brought down to ambient
temperature and used for adsorption. The major disadvantages of
this technique are (i) Chemical vapour deposition, which leads to
non-uniform coating of alkoxide results in non-uniform pore mouth
closure, (ii) The process has to be carried out at elevated temperature
where the alkoxide is likely to be vaporised.
[0023] U.S. Pat. No. 4239509 (1980) to Bligh et al. discloses
a method for purifying crude argon containing argon, oxygen and
nitrogen which comprises the steps of reducing the amount of nitrogen
in the crude argon to between a trace and 0.15% (by volume) and
passing the remaining oxygen and argon, together with residual nitrogen,
through 4A molecular sieve to separate the oxygen and argon. All
the remaining oxygen and nitrogen has to pass through a volume of
4A molecular sieve, which is wholly at or below -250 F. The disadvantages
of this process are adsorption process was carried out at -157.degree.
C. (-250.degree. F.) temperature, and the arrangement of equipment
is complicated for very low temperature separation, which is not
economically acceptable.
[0024] U.S. Pat. No. 4447265 (1984) to Kumar et al. discloses
that argon is recovered from a gas stream comprising the same in
admixture with oxygen and nitrogen, by a vacuum swing adsorption
(VSA) process wherein the mixed gas is passed through an adsorbent
bed having thermodynamic selectivity for adsorption of nitrogen
and unabsorbed portion is then passed through a second adsorbent
bed having kinetic selectivity for retaining oxygen. Both adsorbent
beds are regenerated by vacuum desorption, applied to the first
bed for a longer time period than that of the second bed. The mixed
gas stream fed to the VSA unit may be that obtained from the crude
argon column associated with a cryogenic air separation plant and
waste gas from the VSA unit may be recycled to the main column of
the cryogenic air separation plant, thus enhancing argon recovery.
The disadvantages of this possess are regeneration of the adsorbent
is time consuming process and also for more recovery; of argon required
cryogenic unit otherwise recovery is low.
[0025] U.S. Pat. No. 4529412 (1985) to Hayashi et al. discloses
a process for obtaining high purity argon from air by means of pressure-swing-adsorption.
The air is initially passed through a zeolite molecular sieve-packed
adsorption apparatus and then again passed through carbon molecular
sieve-packed adsorption apparatus, and then subjected to pressure-swing-adsorption
operation, obtaining concentrated argon and high purity oxygen simultaneously.
The drawback of this process is, to adsorption beds are required
and process takes longer time. Moreover, in the process two beds
are required for production thereby increasing the production cost.
[0026] U.S. Pat. No. 4817392 (1989) to Agrawal et al. discloses
a process for the production and recovery of an O.sub.2-lean argon
stream from a gas mixture containing argon and oxygen. The argon-containing
gas mixture is initially treated in a cryogenic separation unit
to produce a crude argon stream having an argon concentration between
80-98%. The crude argon stream is then passed to a membrane based
separation unit where it is separated to produce an O.sub.2-lean
argon stream and an O.sub.2-rich stream. The O.sub.2-rich stream
is recycled to the cryogenic separation unit and the Ar-lean oxygen
stream is recovered as product or further purified. The disadvantage
of this process is that it requires membrane based separation, thereby
increasing the production cost.
[0027] U.S. Pat. No. 5557951 (1996) to Prasad et al. discloses
an apparatus for producing high purity product grade argon from
an argon-containing stream using a cryogenic argon column in combination
with a solid electrolyte ionic or mixed conductor membrane. The
disadvantage of this process is recovery of argon can be achieved
by two processes--one is cryogenic and the other is membrane separation,
thereby increasing the production cost.
[0028] US Patent RE 34 595 (1994) to Chen et al. discloses a process
for purifying argon gas, specially an argon gas stream obtained
by cryogenically separating air, wherein the argon gas is heated
and compressed, and then permeated through a solid electrolyte membrane
selective to the permeation of oxygen over other components of the
gas, and removing oxygen from the argon by selective permeation
of oxygen through the membrane. The purified argon can then be distilled
to remove other components such as nitrogen. A process is provided
for producing a purified argon stream wherein oxygen and nitrogen
are removed from crude bulk argon streams, particularly those produced
by cryogenic, adsorptive or membrane separation of air. The process
comprises separating a heated, compressed crude argon stream containing
nitrogen and oxygen into an oxygen permeate stream and an oxygen-depleted
argon stream by passing the compressed heated argon stream through
a solid electrolyte membrane selective to the permeation of oxygen.
The oxygen-depleted argon stream is then fed to a distillation column
to separate nitrogen from the oxygen-depleted argon stream to form
the purified argon stream and a nitrogen waste system.
OBJECTS OF THE INVENTION
[0029] The main object of the present invention is to provide a
process for the preparation of molecular sieve adsorbent for selective
adsorption of oxygen from air, which obviates, the drawbacks as
detailed above.
[0030] Still another object of the present invention is to provide
an oxygen selective zeolite based adsorbent.
[0031] Still another object of the present invention is to provide
an adsorbent, which can be prepared by the exchanging rare earth
cations especially cerium, europium and gadolinium in ziolite X.
[0032] Yet another object of the present invention is to provide
oxygen selective adsorbent by a simple post-systhesis modification
of zeolite X.
[0033] Yet another object of the present invention is to provide
an adsorbent, which can be regenerated by desorption of oxygen by
controlling equillibrium adsorption pressure.
[0034] Yet another object of the present invention is to provide
an adsorbent, which is selective towards oxygen over argon with
high selectivity and can be commercially for the separation and
purification of argon.
SUMMARY OF THE INVENTION
[0035] The present invention provides a process for the preparation
of molecular sieve adsorbent for selective adsorption of oxygen
from air, by exchanging powder and pellet form of sodium zeolite
X, with an aqueous solution of rare earth cations such as cerium,
europium and gadolinium, at elevated temperature. The dry zeolite
X, containing 20 to 95% rare earth cations of the total exchangeable
sodium cations, after activation at high temperature and vacuum
were subjected to adsorption studies for oxygen, nitrogen and argon
using a static volumetric system of an adsorption equipment supplied
by Micromeritics Corporation USA (Model ASAP 2010). Adsorption capacities
and selectivity for rare earth exchanged zeolite for oxygen, nitrogen
and argon was measured at 15.degree. C. and in the pressure range
of 0.5 to 760 mmHg. From these data adsorption isotherm were plotted
and pure component selectivity of gases were calculated. This invention
provides a process to prepare zeolite adsorbent having selectivity
for oxygen over nitrogen and argon.
[0036] Accordingly, the present invention provides a process for
preparing a molecular sieve adsorbent for selective adsorption of
oxygen from air, the process comprising
[0037] (i) exchanging zeolite X in powder or pellet form with water-soluble
salt of a rare earth metal selected from the group consisting of
cerium, europium, gadolinium and any mixture thereof,
[0038] (ii) filtering the mixture, washing the powder or pellet
with hot distilled water till it is free from anions to obtain an
exchanged zeolite;
[0039] (iii) drying the exchanged zeolite;
[0040] (iv) and activating the exchanged zeolite.
[0041] In an embodiment of the present invention, zeolite X in
powder form having 100% crystallinity and spherical pellet forms
can be used for the preparation of the surface modified molecular
sieve adsorbent.
[0042] In another embodiment of the present invention, Na cations
of zeolite were exchanged with salts or rare earth ions 10 to 100
equivalent percentage (Cerium Europium and Gadolinium) are loaded
using any water-soluble salts of chloride nitrate and acetate.
[0043] In still another embodiment of the present invention, the
cation exchange can be carried at a temperature in the range of
30.degree. C. to 90.degree. C. for a period in the range of 4 to
8 hours.
[0044] In still another embodiment of the present invention, the
cation exchange can be carried out at a cation concentration in
the range of 0.01 to 0.1 M solution.
[0045] In still another embodiment of the present invention, the
exchanged zeolite may be dried in a temperature range of 20.degree.
C. to 80.degree. C. in air or under vacuum conditions.
[0046] In still another embodiment of the present invention, the
exchanged zeolite may be activated in the temperature range of 350
to 450.degree. C. for a period in the range of 3-6 hours followed
by cooling under inert or vacuum condition.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0047] In the drawings accompanying this specification.
[0048] FIG. 1 represents the adsorption isotherms of nitrogen,
argon and oxygen at 15.degree. C. on the zeolite X pellets.
[0049] FIG. 2 represents the adsorption isotherms of nitrogen,
argon and oxygen at 15.degree. C. on the cerium exchanged zeolite
X pellets.
[0050] FIG. 3 represents the adsorption isotherms of nitrogen,
argon and oxygen at 15.degree. C. on the europium exchanged zeolite
Z pellets.
[0051] FIG. 4 represents the adsorption isotherms of nitrogen,
argon and oxygen at 15.degree. C. on the gadolunium exchanged zeolite
X pellets.
DETAILED DESCRIPTION OF THE INVENTION
[0052] The present invention provides a process for the preparation
of an oxygen selective adsorbent, which has oxygen adsorption selectivity
over nitrogen and argon. Furthermore, this adsorbent displays higher
interaction with oxygen compared to nitrogen and argon as observed
from heats adsorption values determined I the in Henry region.
[0053] Zeolites, which are microporous crystalline aluminosilicates,
are finding increased applications as adsorbents for separating
mixtures of compounds having closely related molecular properties.
[0054] 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.
[0055] The zeolite NaX powder and pellet [Na.sub.86(AIO.sub.2).sub.86(SiO.-
sub.2).sub.106wH.sub.2O] was used as the starting material. X-ray
diffraction data showed that the starting material was highly crystalline.
A known amount of the zeolite NaX powder and pellet [Na.sub.86(AIO.sub.2).sub.86(SiO.sub.2).sub.106wH.sub.2O]
was refluxed with 0.01 M rare earth (Ce, Eu and Gd) acetate and
chloride solution taken in 2 litre round bottomed flask with zeolite
X (powder or pellet) to rare earth solution ratio 1:80 at 80-120.degree.
C. for 4 h. Zeolite samples having different amount of rare earth
were prepared by subjecting repeated rare earth cation exchange
into the zeolite. Zeolite sample were filtered after reflux and
washed with distilled water until free from chloride as tested by
AgNO.sub.3 solution. In case of acetate, excess wash with not (60.degree.
C.) water was done. Extent of rare earth exchange into zeolite X
was determined from the concentration of rare earth cations in original
solution and the filtrate. Rare earth cations were analysed by using
0.01 M EDTA solution with xylenol orange tetra sodium salt indicator
at pH=6 employing sodium acetate buffer.
[0056] Oxygen, nitrogen and argon adsorption at 15.degree. C. was
measured using a static volumentric system (Micromeritics, USA.
ASAP 2010), after activating the sample at 350.degree. C. to 450.degree.
C. under vacuum for 4-8 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 100 to 760 mmHg. A minimum equilibrium
interval of 5 seconds was used to determine equilibrium for each
measurement point.
[0057] The pure component selectivity of one gas over other (A
and B) was determined by the equation,
.alpha..sub.A/B=[V.sub.A/V.sub.B].sub.P,T
[0058] where V.sub.A and V.sub.B are the volumes of gas A and B
adsorbed at equilibrium pressure P and temperature T.
[0059] Structural analysis of the zeolite samples was done by X-ray
diffraction wherein the crystallinity of the zeolites are measured
from the intensity of the well-defined peaks at 2 theta values of
6 10 11 8 15.5 20 23.4 26.8 30.5 31 32 and 33.8 X-ray
powder diffraction was measured using PHILIPS X'pert MPD system
equipped with XRK 900 reaction chamber.
[0060] The important inventive steps involved in the present invention
are that the molecular sieve adsorbent, formation of oxygen selective
species inside the zeolite cavities (i) by exchanging with lanthanide
aqueous solution and in addition to cation exchange by forming non-stoichiometric
oxide of cerium/europium/gadolinium which can selectively; interact
with oxygen molecules (ii) the process lies in providing a new technique,
in addition to conventional cation exchange, of introducing sorbate
specific metal oxide in the micropores of the zeolites for developing
new adsorbents. The non-stochiometric oxides of these rare earths
like cerium and europium can react with oxygen in a reversible manner
and reversibly changes the oxidation state thus acting as chemisorption-assisted
adsorption. High heats of adsorption values observed also are indicative
chemisorption type interactions with oxygen molecule.
[0061] The adsorbtive capacity of the catalyst was verified evaluated
by adsorbing nitrogen, oxygen and argon gases on exchanged zeolites
having 99.9% purity at 15.degree. C. and in the pressure range of
0.5 to 800 mmHg and then calculating the adsorption selectivity
of gases at 15.degree. C. and 100 and 760-mmHg pressures.
[0062] The following examples are given by way of illustration
and therefore should not be construed to limit the scope of the
present invention.
EXAMPLE-1
[0063] 1.0 gm of zeolite NaX pellet, [Na.sub.2O).sub.86(AI.sub.2O.sub.3).s-
ub.86.(SiO.sub.2).sub.106.wH.sub.2O], was activated at 350.degree.
C. temperature under vacuum at 10.sup.-3 mmHg and adsorption measurements
were carried out for N.sub.2O.sub.2 Ar having 99.9% purity at
15.degree. C. using volumetric system (Micromeritics ASAP 2010C)
operating at 760 mmHg pressure with equilibrium interval of 5 seconds.
Adsorption capacity for N.sub.2O.sub.2 and Ar is 9.74 cc/g, 3.31
cc/g and 3.29 cc/g respectively at 15.degree. C. temperature and
760-mmHg pressures. Selectivity for nitrogen over oxygen is 2.9;
selectivity for nitrogen over argon is 2.96 and selectivity for
oxygen over argon is 1.0 at 15.degree. C. at 760-mmHg pressures.
EXAMPLE-2
[0064] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.301M Cerium acetate solutions in the ratio 1:80 and refluxed at
80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The Cerium content in dry zeolite amount is 25% of the total replaceable
sodium cations. This ziolite was activated at 350.degree. C. temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.17 gm. The adsorption measurement was carried out at 15.degree.
C. temperature and 760-mmHg pressures. The adsorption capacity for
oxygen is 2.4 cc/g at 15.degree. C. temperature and 760 mmHg and
selectivity for oxygen over argon is 1.0 selectivity for nitrogen
over oxygen is 3.3 and nitrogen over argon is 3.3 at the 100-mmHg
pressure.
EXAMPLE-3
[0065] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Cerium acetate solutions in the ratio 1:80 and refluxed at
80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The cerium content in dry zeolite amount is 84% of the total replaceable
sodium cations. This zeolite was activated at 350.degree. C. temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 1.4 gm. The adsorption measurement was carried out at 15.degree.
C. temperature and 760-mmHg pressures. The adsorption capacity for
oxygen is 3.7 cc/g at 15.degree. C. temperature and 760 mmHg and
selectivity for oxygen over argon is 8.0 selectivity for nitrogen
over oxygen is 0.4 and nitrogen over argon is 3.5 at the 100-mmHg
pressure.
EXAMPLE-4
[0066] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Cerium chloride solutions in the ratio 1:80 and refluxed at
80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from chloride
ions and then dried in air at room temperature (28.degree. C.).
The cerium content in dry zeolite amount is 28% of the total replaceable
sodium cations. This zeolite was activated at 3500C temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.58 gm. The adsorption measurement was carried out at 15.degree.
C. temperature 760-mmHg pressures. The adsorption measurement was
carried out at 15.degree. C. temperature and 760 mmHg and selectivity
for oxygen over argon is 3.0 selectivity for nitrogen over oxygen
is 1.3 and nitrogen over argon is 4.0 at the 100-mmHg pressure.
EXMAPLE-5
[0067] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Cerium chloride solutions in the ratio 1:80 and refluxed at
80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from chloride
ions and then dried in air at room temperature (28.degree. C.).
The cerium content in dry zeolite amount is 93% of the total replaceable
sodium cations. This zeolite was activated at 350.degree. C. temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.53 gm. The adsorption measurement was carried out at 15.degree.
C. temperature and 760-mmHg pressures. The adsorption capacity for
oxygen is 3.1 cc/g at 15.degree. C. temperature and 760 mmHg and
selectivity for oxygen over argon is 3.5 selectivity for nitrogen
over oxygen is 1.4 and nitrogen over argon is 5.0 at the 100-mmHg
pressure.
EXMAPLE-6
[0068] 25.0 g of the molecular sieve NaX powder was exchanged with
0.014M Cerium acetate solutions in the ratio 1:80 and refluxed at
80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The cerium content in dry zeolite amount is 74% of the total replaceable
sodium cations. This zeolite was activated at 350.degree. C. temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.13 gm. The adsorption capacity for oxygen is 4.6 cc/g at 15.degree.
C. temperature and 760 mmHg and selectivity for oxygen over argon
is 4.0 selectivity for nitrogen over oxygen is 1.1 and nitrogen
over argon is 4.2 at the 100-mmHg pressure.
EXMAPLE-7
[0069] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Cerium acetate solutions in the ratio 1:80 and refluxed at
50.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The cerium content in dry zeolite amount is 20% of the total replaceable
sodium cations. This zeolite was activated at 350.degree. C. temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.22 gm. The adsorption measurement was carried out at 15.degree.
C. temperature and 760-mmHg pressures. The adsorption capacity for
oxygen is 2.2 cc/g at 15.degree. C. temperature and 760 mmHg and
selectivity for oxygen over argon is 1.5 selectivity for nitrogen
over oxygen is 2.2 and nitrogen over argon is 3.2 at the 100-mmHg
pressure.
EXMAPLE-8
[0070] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.1M Cerium acetate solutions in the ratio 1:80 and refluxed at
80.degree. C. for 4 hours. The hot solution was filtered washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The cerium content in dry zeolite amount is 30% of the total replaceable
sodium cations. This zeolite was activated at 350.degree. C. temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.15 gm. The adsorption measurement was carried out at 15.degree.
C. temperature and 760-mmHg pressures. The adsorption capacity for
oxygen is 3.2 cc/g at 15.degree. C. temperature and 760 mmHg and
selectivity for oxygen over argon is 2.0 selectivity for nitrogen
over oxygen is 2.4 and nitrogen over argon is 3.8 at the 100-mmHg
pressure.
EXAMPLE-9
[0071] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Europium acetate solutions in the ratio 1:80 and refluxed
at 80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The europium content in dry zeolite amount is 52% of the total replaceable
sodium cations. This zeolite was activated at 3500C temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.59 gm. The adsorption measurement was carried out at 15.degree.
C. temperature and 760-mmHg pressures. The adsorption capacity for
oxygen is 2.3 cc/g at 15.degree. C. temperature and 760 mmHg and
selectivity for oxygen over argon is 1.7 selectivity for nitrogen
over oxygen is 1.1 and nitrogen over argon is 2.7 at the 100-mmHg
pressure.
EXMAPLE-10
[0072] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Europium acetate solutions in the ratio 1:80 and refluxed
at 80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The europium content in dry zeolite amount is 67% of the total replaceable
sodium cations. This zeolite was activated at 350.degree. C. temperature
under vacuum (10.sup.-3 mmHg) and the weight of sample after activation
was 0.52 gm. The adsorption measurement was carried out at 15.degree.
C. temperature and 760-mmHg pressures. The adsorption capacity for
oxygen is 2.6 cc/g at 15.degree. C. temperature and 760 mmHg and
selectivity for oxygen over argon is 2.3 selectivity for nitrogen
over oxygen is 1.3 and nitrogen over argon is 3.1 at the 100-mmHg
pressure.
EXMAPLE-11
[0073] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Gadolinium acetate solutions in the ratio 1:80 and refluxed
at 80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The gadolinium content in dry zeolite amount is 82% of the total
replaceable sodium cations. This zeolite was activated at 35.degree.
C. temperature under vacuum (10.sup.-3 mmHg) and the weight of sample
after activation was 0.59 gm. The adsorption measurement was carried
out at 15.degree. C. temperature and 760-mmHg pressures. The adsorption
capacity for oxygen is 3.2 cc/g at 15.degree. C. temperature and
760 mmHg and selectivity for oxygen over argon is 4.0 selectivity
for nitrogen over oxygen is 1.3 and nitrogen over argon is 5.0 at
the 100-mmHg pressure.
EXAMPLE-12
[0074] 25.0 g of the molecular sieve NaX pellet was exchanged with
0.01M Gadolinium acetate solutions in the ratio 1:80 and refluxed
at 80.degree. C. for 4 hours. The hot solution was filtered, washed
with hot distilled water, until the washings are free from acetate
ions and then dried in air at room temperature (28.degree. C.).
The gadolinium content in dry zeolite amount is 88% of the total
replaceable sodium cations. This zeolite was activated at 350.degree.
C. temperature under vacuum (10.sup.-3 mmHg) and weight of sample
after activation was 0.66 gm. The adsorption measurement was carried
out at 15.degree. C. temperature and 760-mmHg pressures. The adsorption
capacity for oxygen is 2.8 cc/g at 15.degree. C. temperature and
760 mmHg and selectivity for oxygen over argon is 2.0 selectivity
for nitrogen over oxygen is 3.0 and nitrogen over argon is 6.0 at
the 100-mmHg pressure. |