Molecular sieve abstract
The present invention relates to binder-free molecular sieve zeolite
granules of lithium zeolite A and lithium zeolite X, a process for
preparing these molecular sieve zeolite granules and their use for
preparing nitrogen or oxygen from air by pressure swing adsorption.
Molecular sieve claims
What is claimed is:
1. A pressure swing adsorption process for producing oxygen or
nitrogen from air, said process comprising:
a) passing the air to a zeolite bed containing molecular sieve
zeolite granules characterised in that said granules consist essentially
of a mixture of zeolites of Li zeolite A and Li zeolite X, said
granules containing 10-35 wt-% of Li zeolite A and 65-90 wt-% of
Li zeolite X wherein the Li zeolite A was obtained by the conversion
of a silicate binder and withdrawing an O2-rich gas from a discharge
zone;
b) counter-currently depressurizing the zeolite bed;
c) repressurizing the zeolite bed with air or a portion of the
O2-rich gas; and,
d) repeating steps a-c to provide a continuous process.
Molecular sieve description
The present invention relates to binder-free molecular sieve zeolite
granules which contain zeolites of the type lithium zeolite A and
lithium zeolite X, a process for preparing these molecular sieve
zeolite granules and their use for preparing nitrogen or oxygen
from air by pressure swing adsorption.
The production of oxygen from air at ambient temperatures (e.g.
-30.degree. C. to +50.degree. C.) is generally performed on an industrial
scale using molecular sieve zeolites. Here, the preferential adsorption
of nitrogen as compared with oxygen is used, i.e. oxygen and argon
from air are collected as product at the discharge point after the
air has passed through a zeolite bed. Desorption of the adsorbed
nitrogen can be performed, for example, by reducing the pressure
in the bed. In this case, the process is called vacuum swing adsorption
(VSA) in contrast to the also known pressure swing adsorption process
(PSA), wherein the nitrogen is desorbed at approximately ambient
pressure. A continuous VSA process is characterised by the following
process steps:
a) passage of air through a zeolite bed (e.g. at ambient pressure)
and withdrawal of O2-rich gas from the discharge zone;
b) reduction of pressure in the bed to, for example, about 100
to 400 mbar, using a vacuum pump, in counterflow to the flow of
air;
c) filling the bed with O2-rich gas in counterflow to the stream
of air or with air in co-current flow with the stream of air to
the adsorption pressure or approximately to the adsorption pressure.
The objective of the various processes is always a high product
rate, with reference to the amount of zeolite combination used,
and a high O.sub.2 yield (ratio of the amount of O.sub.2 in the
product to the amount of O.sub.2 in the quantity of air used). A
high O.sub.2 yield includes a low energy demand (with reference
to the amount of O.sub.2 produced) for the vacuum pump or air compressor.
As a result of the three steps mentioned above, three zeolites
are generally used, i.e. three adsorbers, which are operated in
a cycle.
The economic viability of these types of adsorption units is affected
by the investment such as, for instance, the amount of adsorption
agent and the size of the vacuum pumps and in particular by the
operating costs such as, for example, power consumption by the vacuum
pump and/or the air compressor. Therefore, zeolites have been developed
with which it is possible to achieve high nitrogen adsorptions in
the range between the adsorption pressure and minimal desorption
pressure, so that the amount of zeolite used can be kept at a low
level or even reduced. As described in EP-A 374 631 Ca zeolites
A have been used for this purpose. Further developments in this
area are directed at increasing the selectivity for nitrogen as
compared to oxygen.
Higher selectivity is achieved by using lithium zeolite X (EP-A
297 542). A higher separation factor (N.sub.2 loading to O.sub.2
loading) and a higher N.sub.2 loading are obtained than with Na
zeolite X.
U.S. Pat. No. 5174979 describes granules bonded with clay minerals,
the zeolite fraction consisting of Li zeolite A or Li zeolite X,
wherein the Li.sub.2 O/Al.sub.2 O.sub.3 ratio in the Li zeolite
A granules is between 10 and 70% and the Li.sub.2 O/Al.sub.2 O.sub.3
ratio in the Li zeolite X granules is between 50 and 95% and the
remaining cations are calcium or strontium ions. At an air pressure
of 1 bar (abs.), pure lithium zeolite A granules demonstrate an
N.sub.2 adsorption of only about 0.35 mmol/g. equ.; and the N.sub.2
adsorption on Li zeolite X granules at 0.8 bar (abs.) is about 1.1
mmol/g. equ.
Granules consisting of lithium zeolite A can therefore be improved
by introducing additional calcium or strontium ions in an exchange
process.
In EP-A 0 548 755 it is shown that in the case of lithium zeolite
X, the N.sub.2 adsorption and N.sub.2 /O.sub.2 selectivity does
not decrease substantially, in comparison with a completely exchanged
lithium zeolite X, by introducing calcium and strontium ions as
long as the amount of Na.sub.2 O in the zeolite lattice remains
small. According to FIG. 5 in this document, a zeolite X completely
exchanged with lithium has only about 34% higher "nitrogen
working capacity" than a zeolite X completely exchanged with
calcium ions. According to FIG. 7 in this document, the N.sub.2
/O.sub.2 selectivity of Ca zeolite is in fact about 10% (in relative
terms) better than lithium zeolite X.
In EP-A 297 542 to prepare lithium zeolite X granules, Na zeolite
X powder is bonded with clay, then calcined, moistened again, exchanged
with a LiCl solution, washed with LiOH and finally activated with
a hot stream of gas, i.e. rendered anhydrous.
DE-A 1 203 238 discloses granules which consist of Na zeolite A,
in which the SiO.sub.2 binder is converted into zeolite A in an
after treatment step, a so-called aluminising process. The components
called binders are inactive constituents of the granules which bind
the zeolite powder to produce granules (beads or sections of extruded
strands). The N.sub.2 and O.sub.2 loading on the inactive binder
is minimal.
DE-A 3 401 485 describes the preparation of SiO.sub.2 -bonded zeolite
A and zeolite X granules.
According to EP-A 0 170 026 in particular example 2 fracture
resistant granules are disclosed, these consisting of Ca zeolite
A and a SiO.sub.2 binder and being advantageously used in accordance
with EP-A 0 374 631 for the oxygen enrichment of air.
DE-A 1 203 238 in particular example 7 discloses granules which
consist of Na zeolite X in which the SiO.sub.2 binder has been converted
into zeolite A in an aftertreatment step. The disadvantage of zeolite
granules which consist of Na zeolite X and/or Na zeolite A and a
SiO.sub.2 binder is the low fracture strength of these granules,
wherein this is independent of the shape of the granules (beads
or rods).
Treating the SiO.sub.2 binder in zeolite A or zeolite X granules
with solutions of salts of alkaline earth metal cations increases
the fracture strength of the granules. Converting the inactive SiO.sub.2
binder into active Na zeolite A should also increase the adsorption
capacity of the entire granular material. Increasing the fracture
strength of granules made of sodium zeolite A or sodium zeolite
X and a SiO.sub.2 binder by treatment with a solution of a lithium
salt is not possible.
DD 0 154 690 discloses a process for separating oxygen from gases,
wherein binder-free molecular sieve zeolite granules of the type
NaLi zeolite A are used (see page 4 Table 1 and page 5 example
1). In Jzv. Akad. Nauk SSSR, Ser. Khim. 1966 (10), 1869 (CA66:70743f),
the ion exchange of a sodium zeolite to give Li zeolite A and the
X-ray structure of and NMR data for corresponding pellets are also
described.
The object was to provide binder-free, but fracture-resistant,
Li zeolite granules which can be prepared in a technically simple
manner, which can be used for the separation of air in a pressure
swing adsorption process and which ensure a high yield for oxygen
and high product capacity.
The invention provides abrasion-resistant, fracture-resistant,
binder-free molecular sieve zeolite granules which are characterised
in that the granules contain finely distributed zeolites of the
types Li zeolite A and Li zeolite X.
The granules preferably contain 10 to 90 wt. % of Li zeolite A,
preferably 15 to 85 wt. % of Li zeolite A and at the same time 10
to 90 wt. % of Li zeolite X, preferably 15 to 85 wt. % of Li zeolite
X.
In particular, the Li zeolite A preferably has a degree of Li exchange
of 60 to 100%, with reference to exchangeable cations and the Li
zeolite X preferably has a degree of Li exchange of 60 to 100%,
with reference to exchangeable cations.
The Li zeolite A preferably contains up to 20 mol-% of divalent
cations, and the Li zeolite X preferably contains up to 20 mol-%
of divalent cations.
The invention also provides a process for preparing molecular sieve
zeolite granules according to the invention which is characterised
in that powdered zeolites of the type Na zeolite A and Na zeolite
X are mixed with silica sol or other suitable SiO.sub.2 -containing
binders and moulded to give SiO.sub.2 -bonded granules, the granules
obtained in this way are aluminised, wherein the SiO.sub.2 binder
is converted into Na zeolite A, a Li exchange is performed with
the granules treated in this way, wherein 60 to 100% of the exchangeable
cations in the zeolite are exchanged and then the exchanged granules
are subjected to thermal treatment at temperatures of 300 to 650.degree.
C. in order to remove water (so-called activation).
An exchange with divalent cations from the group magnesium, calcium,
barium, strontium, zinc, iron, cobalt or nickel, is preferably also
performed, before or after Li exchange.
Preferred SiO.sub.2 -containing binders are, for instance, waterglasses,
silica sols, silica gels, aerosils or silica fillers. Silica sols
are particularly suitable.
So-called aluminisation in the process according to the invention
is performed as follows:
The granules produced by granulation, in the moist state, are placed
in contact with an aqueous sodium aluminate solution for several
hours at an elevated temperature. The aluminate concentration is
chosen to be as high as possible (preferably 0.5 to 2.0 mol of Al.sub.2
O.sub.3 per liter) in order to keep the volume of treatment solution
small. The amount of aluminate solution is such that there are at
least 0.5 moles of Al.sub.2 O.sub.3 to 1 mole of SiO.sub.2 binder.
More than 0.5 moles of Al.sub.2 O.sub.3 does not cause any problems.
The concentration of caustic in the aluminate solution (treatment
solution) may vary between 1.5 and 10 moles of alkali metal hydroxide
per liter.
Aluminisation is preferably performed (see also DE-A 1 203 238
example 7) in such a way that, in a first step, the granules are
treated with aluminate solution for 0.5 to 15 hours at a temperature
of 25.degree. C. to 60.degree. C. Then, in a second step, treatment
is continued for 2 to 6 hours at temperatures between 70 and 90.degree.
C. The precise treatment times depend mainly on the diameter of
the granules. The smaller the diameter of the granules, the shorter
the treatment times (residence times). After this procedure, the
amorphous SiO.sub.2 binder is entirely converted into Na zeolite
A (crystalline zeolite phase). After optionally then washing the
granules, these may be subjected directly to an ion exchange process.
Activation is preferably performed as follows:
Thermal treatment of the granules takes place at temperatures between
300 and 650.degree. C., this preferably being performed with dry
gases such as, for instance, air or nitrogen. |