Molecular sieve abstract
This invention relates to the manufacture of a molecular sieve
adsorbent for selectively separating methane from its gaseous mixture
with nitrogen. More particularly, the invention relates to the manufacture
of novel molecular sieve adsorbents useful for the separation of
methane-nitrogen gaseous mixture.
Molecular sieve claims
What is claimed is:
1. A process for preparing a molecular sieve adsorbent for selectively
adsorbing methane from a gaseous mixture comprising methane and
nitrogen, said process comprising:
(a) preparing a mixture of zeolite powder from a zeolite of the
formula
where m is from 2 to 8 n is from 0 to 30 the value of n depends
on the value of m and degree of hydration.
and, clay and an organic binder;
(b) shaping said zeolite mixture to obtain adsorbent bodies;
(c) subjecting said adsorbent bodies to calcination; and
(d) subjecting said adsorbent bodies either prior to or after calcination
or both, to cationic exchange in the presence of an alkali metal
solution to affect surface modification of said adsorbent bodies
to obtain said molecular sieve adsorbent of the formula
where the value of
a is from 0.1 to 1
b and c are from 0 to 8
and x is from 2 to 8
M1 is potassium or cesium or a mixture thereof and M2 is a rare
earth metal ion, and n represents the moles of water.
2. The process as claimed in claim 1 wherein the alkali metal
solution is a salt solution of potassium or cesium or a mixture
thereof.
3. The process as claimed in claim 1 wherein said clay is bentonite
clay.
4. The process as claimed in claim 1 wherein said clay is present
in an amount of 2 to 40% by weight.
5. The process as claimed in claim 1 wherein said binder is selected
from the group consisting of sodium lignosulfate, starch and polyvinyl
alcohol.
6. The process as claimed in claim 1 wherein said zeolite mixture
is subjected to ball milling to produce powders of particle size
of less than 60 microns.
7. The process as claimed in claim 1 wherein the adsorbent bodies
are dried at room temperature for about 6 to 12 hrs. followed by
oven drying at a temperature of 110.degree. C. for 6to 18 hrs.
8. The process as claimed in claim 1 wherein said calcination
is carried out at a temperature of 500 to 750.degree. C.
9. The process as claimed in claim 1 wherein said calcination
is carried out for a period of from 2 to 12 hrs.
10. The process as claimed in claim 1 wherein said cation exchange
is carried out at a concentration of 1 to 10% by weight of the cation
in aqueous solution.
11. The process as claimed in claim 10 wherein said cation exchange
is carried out at a temperature of 30 to 100.degree. C. for 4 to
48 hrs.
12. The process as claimed in claim 1 wherein said cation exchanged
adsorbent bodies are thoroughly washed with hot water.
13. The process as claimed in claim 12 wherein the adsorbent bodies
are oven dried at 110.degree. C. for 6 to 8 hours.
14. A molecular sieve adsorbent of the formula
where the value of
a is from 0.1 to 1
b and c are from 0 to 8
and x is from 2to 8
M1 is potassium or cesium or a mixture thereof and M2 is a rare
earth metal ion, and n represents the moles of water which is methane
selective.
Molecular sieve descriptionThis invention relates to the manufacture
of a molecular sieve adsorbent for selectively separating methane
from its gaseous mixture with nitrogen. More particularly, the invention
relates to the manufacture of novel molecular sieve adsorbents useful
for the separation of methane-nitrogen gaseous mixture.
BACKGROUND OF THE INVENTION
The mixture of methane and nitrogen gases is found in a variety
of situations and the mixture must be separated before using the
individual component gases. For example, much of the natural gas
resources are not readily usable due to high nitrogen (above 10%
by volume) content as the commercially usable natural gas must have
at least 90% methane gas. Furthermore, the nitrogen content of a
reservoir increases with time the reservoir is in use. Methane-nitrogen
mixture is also found in fire damp where 27-50% of methane is found.
Separation of these gases is difficult due to closeness in their
physical properties. At present, commercially this is achieved by
energy intensive cryogenic techniques. The application of adsorption
based separation of gases by pressure swing adsorption process is
being increasingly used now. For example, separation of nitrogen
and oxygen from air is in wide prevalence all over the world. Adsorption
based processes can compete with highly energy intensive cryogenic
separation of methane/nitrogen mixture if a suitable adsorbent which
is selective towards one of the components and having adsorption
capacity is commercially available.
Characteristics which are highly desirable, if not absolutely essential,
for an adsorbent to be suitable for selective adsorption process
include adsorption capacity of the adsorbent and adsorption selectivity
for a particular component.
Adsorption capacity of the adsorbent is defined as the amount in
terms of volume or weight of the desired component adsorbed per
unit volume or weight of the adsorbent. The higher the adsorbent's
capacity for the desired adsorbing component the better the adsorbent
is as the increased adsorption capacity of a particular adsorbent
helps to reduce the amount of adsorbent required to separate a specific
amount of a component from a mixture of particular concentration.
Such a reduction in adsorbent quantity in a specific adsorption
process brings down the cost of a separation process.
Adsorption selectivity () of component A over B is defined as
where X is the adsorbed concentration and Y is gas-phase concentration.
The
expression gas-phase concentration means the amount of unadsorbed
component remaining in the gas-phase. The adsorption selectivity
of a component depends on
steric factors such as difference in the shape and size of the
adsorbate molecules;
equilibrium effect, i.e., when the adsorption isotherms of the
components of the gas mixture differ appreciably;
kinetic effect, when the components have substantially different
adsorption rates.
It is generally observed that for a process to be commercially
economical, the minimum acceptable adsorption selectivity for the
desired component is about 3. Where the adsorption selectivity is
less than 2 the separation process is not likely to be effective.
In the prior art, methane-selective adsorbent prepared by impregnating
molybdenum oxide on activated carbon has been reported. Kinetic
separation of methane/nitrogen mixture has also been examined using
a naturally occurring zeolite clinoptilolite. The authors have earlier
developed faujasite type zeolite based adsorbent for methane-nitrogen
separation as disclosed in Indian Patent No. 437/Bom/95 dated Oct.
13 1995.
The present invention deals with the development of methane selective
adsorbents with a new chemical composition based on a different
zeolite structure having enhanced adsorption capacity and high adsorption
selectivity.
Zeolites which are microporous crystalline aluminosilicates are
finding increased application as adsorbents for separating mixtures
of closely related compounds. Zeolites have a three dimensional
network of basic structural units consisting of SiO.sub.4 and AlO.sub.4
tetrahedra linked to each other by sharing of apical oxygen atoms.
Silicon and aluminum atoms lie at the center of the tetrahedra.
The resulting aluminosilicate structure which is generally highly
porous possesses three dimensional pores the access to which is
through molecular sized windows. In a hydrated form, the preferred
zeolites are generally represented by the following Formula [I]
where "M" is a cation which balances the electrovalence
of the tetrahedra and is generally referred to as extra framework
exchangeable cation, n represents the valency of the cation, x and
w represent the moles of SiO.sub.2 and water respectively. The cations
may be any one of the number of cations which will hereinafter be
described in detail.
The attributes which made them attractive for separation include,
an unusually high thermal and hydrothermal stability, uniform pore
structure, easy pore aperture modification and substantial adsorption
capacity even at low adsorbate pressures. Furthermore, zeolites
can be produced synthetically under relatively moderate hydrothermal
conditions.
Pentasil type zeolites as described and defined in U.S. Pat. No.
3574539 are the preferred adsorbents for adsorption separation
of the gaseous mixture described in this invention. Zeolite of type
mordentie in hydrated or partially hydrated form can be described
in terms of the following metal oxide of Formula II
where "M" represents at least one cation having valence
n, w represents the number of moles of water the value of which
depends on the degree of hydration of the zeolite. Normally, the
zeolite when synthesized has sodium as exchangeable cation.
Zeolites as such have very little cohesion and it is, therefore,
necessary to use appropriate binders to produce the adsorbent in
the form of particles such as extrudates, aggregates, spheres or
granules to suit commercial applications. Zeolitic content of the
adsorbent particles vary from 60 wt % to 98 wt % depending on the
type of binder used. Clays such as bentonite, kaolin, or attapulgite
are normally used inorganic binders for agglomeration of zeolite
powders.
BRIEF SUMMARY OF THE INVENTION
It is therefore, an object of the present invention to provide
an adsorbent which adsorbs methane selectively from its mixture
with nitrogen.
Yet another object of this invention is to provide a methane selective
adsorbent by modification of surface characteristics of synthetic
zeolites.
Yet another object of the present invention is to provide an adsorbent
with enhanced adsorption selectivity and capacity for methane from
its mixture with nitrogen.
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