Molecular sieve abstract
A molecular sieve is made by reacting an ammonium-exchanged low
silica X-type zeolite precursor with lithium hydroxide, at a pressure
of about 200 millibar or less, and at a temperature of about 60.degree.
or less. The zeolite precursor is preferably an X-type zeolite,
in which the silicon to aluminum atomic ratio is less than about
1.02. The lithium is provided in an amount which is stoichiometrically
equivalent to the amount of ammonium present. The molecular sieve
is especially useful in separating air into components using PSA
or VPSA processes, and has improved productivity and yield as compared
with materials of the prior art. The advantages of the molecular
sieve enable it to be provided in the form of beads having relatively
large diameter, which reduces the pressure drop across the adsorber
bed, and reduces required energy consumption.
Molecular sieve claims
What is claimed is:
1. A method of making a molecular sieve for use in separating components
of a gas, the method comprising reacting an ammonium exchanged low
silica X-type zeolite precursor with lithium hydroxide at a pressure
of about 200 millibar or less, and at a temperature of about 60.degree.
C. or less.
2. The method of claim 1 wherein the lithium hydroxide is provided
in an amount which is stoichiometrically equivalent to an amount
of ammonium present.
3. The method of claim 1 wherein the zeolite precursor is selected
to have a silicon to aluminum atomic ratio of less than about 1.02.
4. The method of claim 1 wherein the zeolite precursor is selected
to be partially ammonium exchanged.
5. The method of claim 1 wherein the zeolite precursor is selected
to be fully ammonium exchanged.
6. The method of claim 1 wherein the reacting step is performed
for a time sufficient to achieve a substantially complete exchange
of lithium ions for ammonium ions in the zeolite precursor.
7. A method of making a molecular sieve for use in separating components
of a gas, the method comprising reacting an ammonium exchanged zeolite
with lithium cations at a pressure of about 200 millibar or less,
and at a temperature of about 60.degree. C. or less.
8. The method of claim 7 wherein the lithium hydroxide is provided
in a quantity which is stoichiometrically equivalent to an amount
of ammonium present in the zeolite.
9. The method of claim 7 wherein the zeolite precursor is selected
to have a silicon to aluminum atomic ratio of less than about 1.02.
10. The method of claim 9 wherein the zeolite precursor is selected
to be an X-type zeolite.
11. The method of claim 7 wherein the reacting step is performed
for a time sufficient to achieve a substantially complete exchange
of lithium ions for ammonium ions in the zeolite precursor.
12. A method of separating oxygen and nitrogen comprising passing
a mixture including nitrogen and oxygen through an adsorption bed
having a molecular sieve made according to the method of claim 1.
13. A method of separating oxygen and nitrogen comprising passing
a mixture including nitrogen and oxygen through an adsorption bed
having a molecular sieve made according to the method of claim 7.
14. A molecular sieve made according to the method of claim 1.
15. A molecular sieve made according to the method of claim 7.
16. A molecular sieve comprising an ammonium-exchanged zeolite
material in which ammonium ions in the material have been exchanged
with lithium ions, wherein the zeolite material has a silicon to
aluminum atomic ratio of less than about 1.02 the sieve comprising
non-zeolitic material, and wherein the material exhibits an NMR
spectrum having a first peak comprising a main signal and a second
peak comprising a secondary signal corresponding to the non-zeolitic
material, the secondary signal representing a shift of 3 ppm relative
to the main signal.
17. A molecular sieve comprising an X-type zeolite material containing
lithium ions, the sieve also including a non-zeolitic material,
and wherein the material exhibits an NMR spectrum having a first
peak comprising a main signal and a second peak comprising a secondary
signal corresponding to the non-zeolitic material, the secondary
signal representing a shift of about 3 ppm relative to the main
signal.
Molecular sieve description
FIELD OF THE INVENTION
[0001] The present invention relates to the use of a novel molecular
sieve with enhanced performance in the separation of nitrogen and
oxygen by pressure swing adsorption processes. The molecular sieve
is obtained by reacting lithium hydroxide with the ammonium form
of a low silica X-type precursor, resulting in a product with novel
structure and improved adsorption properties.
BACKGROUND OF THE INVENTION
[0002] The production of highly concentrated oxygen by the physical
separation of air using Pressure Swing Adsorption (PSA) or by Vacuum
Pressure Swing Adsorption (VPSA) processes is one of the major technical
sources of this industrial gas. The adsorbents used for this application
are zeolitic molecular sieves in the majority of cases, mostly of
the general A-type and X-type.
[0003] The effect of zeolites in the process has heretofore been
assigned to the strong and specific interaction of zeolitic cations
with the quadrupole moment of the nitrogen molecule. This results
in the preferential adsorption of nitrogen from air, allowing the
purified oxygen product to pass through the absorber vessel, to
be collected at the outlet as the desired product.
[0004] Historically, sodium exchanged X-type zeolites and calcium
exchanged A-type zeolites have been utilized as adsorbents for air
separation. Their selectivity and performance, however, are rather
poor by contemporary standards. In recent years, lithium containing
zeolites of X-type structure have gained increasing market share
due to their superior performance in the PSA process for oxygen
purification. For example, U.S. Pat. No. 3140933 discloses the
use of a partially lithium exchanged X-type molecular sieve. Better
separation performance was also reported in U.S. Pat. No. 4859217
when the amount of lithium in an X-type zeolite was equal to or
greater than 88% of all exchangeable cations. More recently, it
has been asserted that equally good or better performance can be
achieved when part of the lithium is replaced by higher valent cations.
For example, U.S. Pat. No. 5417957 teaches that X-type molecular
sieves having lithium in combination with copper, cobalt or chromium
enjoy high performance. U.S. Pat. No. 5419891 describes similar
advantages for X-type zeolites exchanged with combinations of lithium
and zinc. U.S. Pat. No. 5464467 discloses a novel molecular sieve
with enhanced performance and thermal stability that, in addition
to lithium, contains trivalent cations like lanthanum, cerium, aluminum,
or iron. The disclosures of all of the above-cited patents are hereby
incorporated by reference.
[0005] It is generally believed that only a small fraction of the
lithium cations which are present in contemporary adsorbents are
actually involved in the adsorption process, and that only this
small fraction has the required accessibility and vacant coordination
to serve as a selective adsorption site for nitrogen. The strength
of adsorption, and more specifically the relative adsorption selectivity
for different gas molecules depend obviously on a highly localized
structural environment of the lithium cations as provided by the
local supporting structure.
[0006] Numerous attempts have been made to optimize the utilization
of lithium by modifying the supporting structure. Yoshida et al.,
in Microporous and Mesoporous Materials, volume 46 pages 203-209
(2001), describe a substantial increase in nitrogen adsorption capacity
when the characteristic cubic crystal structure of a lithium exchanged
X-type precursor is transformed to a structurally distinct orthorhombic
material. This phenomenon was only observed, however, at extremely
low temperatures. The orthorhombic material was not stable at process-typical
temperatures, and therefore deemed not applicable to technical PSA
or VPSA processes.
[0007] Other recent work to increase the efficiency of lithium
utilization has included straightforward attempts to increase the
volume density of support sites for the lithium cations within the
adsorbent particles. U.S. Pat. No. 5962358 for example, the disclosure
of which is incorporated by reference herein, describes a binderless
formulation of lithium-containing molecular sieves, including A-type
and X-type. Still more recent patents have claimed the benefits
of a low silica X-type zeolite with a silicon to aluminum ratio
of 1.0. This particular zeolite, known as LSX, possesses the maximum
possible number of cationic sites for this specific structure type.
[0008] In most cases, a high degree of lithium exchange is achieved
by conventional methods as described, for example, in Breck, "Zeolite
Molecular Sieves", Wiley, New York, 1973. Because of the unfavorable
exchange selectivity of the lithium ion, the introduction of lithium
into adsorbents is normally quite difficult. Large excesses of lithium
salts and high temperatures, preferably in combination with elevated
pressure, traditionally need to be applied. However, U.S. Pat. No.
5916836 the disclosure of which is incorporated by reference
herein, teaches methods to achieve complete lithium exchange by
reacting the ammonium forms of zeolites with stoichiometric amount
of lithium hydroxide.
[0009] While these approaches may indeed increase the quantity
of lithium which can be incorporated in an adsorbent body, they
do not necessarily increase the amount or fraction of lithium moieties
present in the specific environments necessary to be beneficial
to the adsorption process, nor do they increase the efficiency of
lithium utilization. And in spite of the commercial significance
of these adsorbents, little is known of the identity or characteristics
of the unique environment of lithium which gives rise to its beneficial
performance in the selective adsorption of nitrogen from air.
SUMMARY OF THE INVENTION
[0010] The present invention relates to the manufacture of a lithium
containing molecular sieve with superior properties for the separation
of air by Pressure Swing Adsorption processes. The manufacturing
process starts from an ammonium exchanged low silica X (LSX) type
zeolite precursor. This zeolite is reacted with a stoichiometric
amount of lithium hydroxide under specific conditions of low pressure
and low temperature. Surprisingly it has been found that if the
treatment with lithium hydroxide takes place at pressures of 200
millibar or lower, and at temperatures of 60.degree. C. or lower,
the resulting lithium containing material exhibits unusual structural
features that are significantly different from the zeolites described
in the prior art. Even more surprisingly it was found that the material
produced using this special sequence of techniques exhibits superior
performance in air separation by Pressure Swing Adsorption (PSA)
or by Vacuum Pressure Swing Adsorption (VPSA).
[0011] The present invention therefore has the primary object of
providing a method of making a molecular sieve that is useful in
the separation of air into components.
[0012] The invention has the further object of providing a molecular
sieve useful in the separation of air into components.
[0013] The invention has the further object of providing a molecular
sieve that exhibits a significantly higher adsorption capacity for
nitrogen at partial pressures higher than about 1000 millibar, as
compared with prior art materials.
[0014] The invention has the further object of providing a molecular
sieve having improved productivity and yield when used in PSA and
VPSA processes.
[0015] The invention has the further object of providing a molecular
sieve which may be provided in the form of relatively large diameter
beads, thereby reducing the pressure drop across the adsorber bed,
and reducing energy consumption of the machinery used to convey
gas through the system.
[0016] The invention has the further object of providing a molecular
sieve which has a relatively fast adsorption rate, and in which
the cycle time of the process is thereby reduced.
[0017] The invention has the further object of providing a molecular
sieve which provides superior performance in PSA processes with
adsorption at super-atmospheric pressures and regeneration at ambient
pressure.
[0018] The invention has the further object of providing a molecular
sieve having a reduced level of vacuum required for efficient regeneration.
[0019] The invention has the further object of providing a molecular
sieve having improved working capacity.
[0020] The invention has the further object of providing an improved
method of separating air into components, through the use of the
molecular sieve described above.
[0021] The reader skilled in the art will recognize other objects
and advantages of the invention, from a reading of the following
brief description of the drawings, the detailed description of the
invention, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIGS. 1a and 1b provide graphs representing X-ray diffraction
patterns relating, respectively, to the novel lithium-containing
material made by the present invention, and the starting material
NH4-LSX (b).
[0023] FIGS. 2a, 2b, and 2c provide graphs of the 29Si-MAS-NMR
spectrum of the novel material (FIG. 2c) compared to the equivalent
spectrum of Li-LSX obtained by conventional ion exchange (FIG. 2b)
and a standard Li-X (FIG. 2a) with a silicon to aluminum ratio of
1.17.
[0024] FIG. 3 provides a graph showing oxygen and nitrogen adsorption
isotherms of a novel material made according to Example 3 of the
present invention and a comparative material made according to the
prior art (Example 4).
[0025] FIG. 4 provides a graph showing the nitrogen working capacity
of a novel material made according to Example 3 of the present invention
and a comparative material made according to the prior art (Example
4).
[0026] FIG. 5 provides a graph showing the performance in a vacuum
swing air separation process, performance being defined by oxygen
yield and oxygen productivity, of a novel material made according
to Examples 1 and 3 of the present invention, and a comparative
material made according to the prior art (Examples 2 and 4) and
measured under the conditions given in Example 5.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The present invention comprises the manufacture of a lithium
exchanged molecular sieve with novel structural features and an
enhanced nitrogen adsorption capacity, that can be used beneficially
in the production of adsorbents with superior performance in the
separation of air by Pressure Swing Adsorption (PSA) or by Vacuum
Pressure Swing Adsorption (VPSA) processes.
[0028] The novel molecular sieve is obtained by treating a partially
or fully ammonium exchanged X-type zeolite, most preferably with
a silicon to aluminum atomic ratio of less than 1.02 with a quantity
of lithium hydroxide solution which is stoichiometrically equivalent
to the amount of ammonium present, under specific conditions of
low temperature and low pressure. In the prior art, this exchange
reaction has been performed at elevated temperatures in order to
facilitate the removal of ammonia evolved from the reaction zone,
and thereby to improve the rate and completeness of the reaction.
Temperatures reported for example in U.S. Pat. Nos. 5916837 and
6407025 the disclosures of which are incorporated by reference
herein, have been close to the boiling point of water, and at least
higher than 90.degree. C. If, however, a reduced pressure is applied
in the reaction vessel, the same effect can be achieved at significantly
lower temperatures. By reducing the reaction system pressure to
200 millibar or below, reaction temperatures of 60.degree. C. or
lower can be successfully used.
[0029] This low temperature, low pressure treatment results in
immediate and complete exchange of lithium ions for ammonium ions
in the material. Additionally, during this exchange process, indications
of dramatic structural modifications are observed. Notably, a physical
contraction occurs, reducing the size of the cubic unit cell from
25.20 angstroms to 24.65 angstroms, as illustrated by the peak shift
in the X-ray diffraction patterns shown in FIG. 1.
[0030] It has also been found, surprisingly, that when the treatment
as described is performed under such conditions, for example, at
temperatures below 60.degree. C. and reaction pressures below 200
millibar, the resulting lithium molecular sieve exhibits novel structural
features that are distinctly different from those described in the
prior art.
[0031] The local structure of the novel material appears to be
significantly different as compared to a lithium zeolite of similar
composition, and as produced by conventional ion exchange procedures.
This is demonstrated by the high resolution 29-Si-MAS-NMR spectra
presented in FIGS. 2a-2c, explained in the following paragraphs.
[0032] FIG. 2a provides an NMR spectrum of a lithium X-type zeolite
having a higher silicon to aluminum ratio, namely 1.17 than is
preferred in the present invention. FIG. 2b provides a comparable
NMR spectrum of a prior art lithium LSX-type in which the silicon
to aluminum ratio is less than 1.02 as in the product of the present
invention, but in which the material has not been produced using
the inventive method. FIG. 2c provides the NMR spectrum of a material
produced according to the present invention.
[0033] While the comparative material (FIG. 2b) produced by conventional
ion exchange procedures exhibits only one signal representing a
single species of silicon surrounded by four O-Al-groups, the novel
material (FIG. 2c) exhibits an additional signal with a shift of
3 ppm relative to the main signal. Based on the intensity of this
signal, it is estimated that 10-15% of the silicon present in the
novel material is in this unique form. To those skilled in the art,
this is a clear indication that the extraordinary signal at 3 ppm
displayed by the material made according to the present invention
cannot be attributed to zeolitic silicon having one or more atoms
of silicon in its second shell of coordination. Such zeolite materials
would be expected to display a relative shift of at least 8 ppm.
[0034] For comparison, FIG. 2a shows the NMR spectrum of a lithium
X-type zeolite with a higher silicon to aluminum ratio, namely 1.17
than is preferred in the present invention. This figure shows the
full suite of signals at positions which are characteristic of X-type
zeolites in general, and further demonstrates, clearly, that the
new signal at 3 ppm, in FIG. 2c, does not arise from a zeolitic
material, but instead arises from a different phase entirely.
[0035] Since the X-ray diffraction pattern excludes the presence
of other zeolitic materials that could explain such a sharp second
signal with only 3 ppm relative shift, it is believed that a local
disruption of the zeolite lattice has occurred, and that the presence
of a non-zeolitic phase is the source of the signal at 3 ppm. The
material giving rise to this signal cannot be characterized as zeolitic.
And as concluded above, 10-15% of the total silicon atoms are found
in the novel material produced by this special ion exchange procedure,
and by inference 10-15% of the total lithium.
[0036] Furthermore, surprisingly, it was found that the adsorbent
prepared according to the present invention exhibits a significantly
higher adsorption capacity for nitrogen at partial pressures higher
than 1 bar, while the nitrogen adsorption capacity at partial pressures
lower than 0.5 bar is slightly lower. Both of these are desirable
characteristics. Therefore, the adsorption characteristics of the
novel material disclosed herein are favorable as compared with those
of zeolite adsorbents made by conventional methods of lithium exchange,
i.e., by repeated treatment of X-type zeolites with lithium chloride
solution, or by materials prepared by ammonium replacement by lithium
hydroxide, but at high temperatures and pressures, i.e. those that
do not show the observed unique phase characterized by the NMR signal
at 3 ppm.
[0037] Without being bound by any theory, it is believed that the
structural disruption and the appearance of the novel phase or environment
occurs during the fast and substantial contraction of the zeolite
structure. This contraction is brought about by the special ion
exchange procedure described herein, and this phase can only be
conserved, or "frozen", if the process is performed at
low temperatures. If higher temperatures are applied, the structure
will have the opportunity to relax or anneal, or the novel phase
may not be formed at all. The improved nitrogen adsorption capacity
and more ideal adsorption characteristics may be attributed to lithium
ions that are exposed in a different way when supported on or incorporated
in this novel phase, and therefore exhibit a different pressure
dependence of interaction than adsorption centers in a completely
relaxed structure. Dynamic adsorption tests confirm the static adsorption
results, and demonstrate improved kinetics for the adsorption and
desorption of nitrogen. These improved kinetic parameters are also
attributable to more exposed or optimally coordinated lithium.
[0038] According to the present invention the increased adsorption
capacity and better adsorption kinetics of the material made according
to the present invention can be utilized for the production of an
adsorbent with superior performance in VPSA (vacuum pressure swing
adsorption) processes and also PSA (pressure swing adsorption) processes
for the separation of oxygen and nitrogen from air.
[0039] There are two characteristic numbers for describing the
performance of zeolites in PSA and VPSA processes, defined as follows:
[0040] Productivity is the amount of oxygen produced per kilogram
of adsorbent per hour, as defined by Equation (1), in which V.sub.P
is the volume of total product per hour, y.sub.02 is the volume
fraction of oxygen in the total product, and m.sub.A is the mass
of adsorbent in the system. Productivity is expressed in normal
liters of oxygen per hour per kilogram of adsorbent.
Productivity=V.sub.P.multidot.y.sub.02/m.sub.A Equation (1)
[0041] The second characteristic is yield, which is the ratio of
the amount of oxygen in the product gas to the amount in the feed
gas, as defined by Equation (2), in which V.sub.F is the volume
of total feed per hour, and y.sub.F is the volume fraction of oxygen
in the feed. Yield is expressed in volume percent.
Yield=V.sub.P.multidot.y.sub.02/V.sub.F.multidot.y.sub.F Equation
(2)
[0042] Productivity is important in determining the required amount
of adsorbent and the required size of the adsorber vessels, and
therefore indicates the investment costs of a commercial installation.
Yield specifies the energy efficiency of the plant operation and
the size of some of the support machinery, for example, air compressors,
air blowers, or vacuum pumps. The use of the novel adsorbent described
in this invention leads to higher Productivity and Yield in PSA
and VPSA processes, and will therefore benefit both investment and
operating costs.
[0043] The following illustrative examples will serve to demonstrate
that the novel adsorbent described in the invention has preferential
adsorption kinetics compared with prior art Li-LSX molecular sieves.
This results in a better Productivity and Yield, especially for
a high product purity. It is therefore possible to use larger diameter
beads of the new adsorbent compared to the prior art Li-LSX zeolite,
while still attaining similar kinetic performance. The use of such
bigger beads results in a lower pressure drop of the adsorber bed,
thereby reducing the energy consumption of the machinery, as for
example, the air blower and the vacuum pump of a VPSA process.
[0044] Another desirable option which is enabled by the faster
adsorption kinetics of the new adsorbent is a reduction of the cycle
time of the processes. It is well known to those skilled in the
art that shorter cycles leads to a higher Productivity, but cycle
time is limited by increasing pressure drop. However, the bigger
beads enabled by the novel adsorbent described in this invention
will relieve the higher pressure drop limit at shorter cycle times.
Therefore the use of the novel lithium adsorbent in VPSA and PSA
processes will allow additional reductions in energy requirements.
EXAMPLE 1
[0045] An adsorbent according to the present invention was prepared
as follows. 2.1 dry metric tons of NH4-LSX powder were slurried
in 11 metric tons of water. The slurry was then transferred into
a vacuum proof, vigorously agitated vessel. The gas pressure in
the vessel was reduced to 150 millibar and the temperature adjusted
to 45.degree. C. 6.2 metric tons of a lithium hydroxide solution
containing 5.0 weight % LiOH were then added within 10 minutes under
continuous stirring. The temperature, pressure and stirring conditions
were held constant for another 20 minutes. After that, the slurry
was filtered and the material washed with 10 tons of demineralized
water per ton of solids. The product was finally dried at 120.degree.
C. The composition of the starting material and the resulting product
are listed in Table 1. Equilibrium adsorption data for nitrogen
are given in Table 2. The NMR spectrum for the material prepared
according to Example 1 is depicted in FIG. 2c.
EXAMPLE 2
[0046] This Example comprises prior art technology, and is used
for purposes of comparison with the present invention.
[0047] The Li-LSX was prepared as follows. 5.0 dry kilograms of
conventional Na,K-LSX zeolite were treated with 12 kilograms of
a lithium chloride aqueous solution containing 10 weight % LiCl
in a stirred vessel for 2 hours at 95.degree. C. and ambient pressure.
Afterwards, the material was filtered and washed with demineralized
water at a ratio of 10 kilograms of water per kilogram of solid.
These steps had to be repeated three times with fresh lithium chloride
solution to achieve a degree of lithium exchange equivalent to that
in Example 1. The material was dried at 120.degree. C., as in Example
1. The composition of the starting material and the resulting Li-LSX
are given in Table 1. Adsorption equilibrium data for nitrogen are
provided in Table 2. The comparative NMR spectrum for the material
prepared according to Example 2 is depicted in FIG. 2b.
1TABLE 1 Chemical composition of powders from Examples 1 and 2.
Values reported are in weight %, normalized to analysis after an
ignition pretreatment at 900.degree. C. Prior Novel Adsorbent Art
Zeolite (Example 1) (Example 2) Starting Product Starting Product
Material.sup.1 Material Material Material SiO.sub.2 54.0% 47.8%
42.3% 47.7% Al.sub.2O.sub.3 45.3% 40.2% 35.5% 40.1% Li.sub.2O --
11.4% -- 11.5% Na.sub.2O 0.3% 0.2% 21.2% 0.5% K.sub.2O 0.4% 0.4%
1.0% 0.2% (NH4).sub.2O 22.0% -- -- -- .sup.1(NH.sub.4).sub.2O is
decomposed during ROI 900.degree. C. procedure; therefore, the sum
exceeds 100%.
[0048] The data in Table 1 show that the superficial chemical composition
of the novel adsorbent is equivalent to that of the prior art zeolite.
2TABLE 2 Adsorption equilibrium data for powders from Examples
1 and 2 (Nitrogen adsorption at equilibrium, expressed in normal
liters per kilogram) Novel Adsorbent Prior Art Zeolite (Example
1) (Example 2) 300 millibar 10.7 11.5 500 millibar 16.1 16.7 700
millibar 20.5 20.9 1000 millibar 26.0 26.0 1400 millibar 31.8 31.3
2000 millibar 38.4 37.3 3000 millibar 46.3 44.6
[0049] The data in Table 2 show that the nitrogen adsorption capacity
of the novel adsorbent is significantly higher than that of the
prior art zeolite in the regions of higher applied pressure, specifically
at applied pressures above 1000 millibar. The data further show
that the amount of adsorbed nitrogen on the novel adsorbent is lower
than that on the prior art zeolite in the regions of lower applied
pressure, specifically at applied pressures below 1000 millibar.
Both of these characteristics are desirable.
EXAMPLE 3
[0050] The novel adsorbent of the present invention was incorporated
into beads typical of those utilized in industrial applications
as follows. 4.0 dry kilograms of the material produced according
to Example 1 were mixed thoroughly with 1.0 kilogram of a conventional
attapulgite binder for 1 hour. After adding the required quantity
of water to induce granulation, the mixture was granulated using
an intensive mixer to produce spherical beads of 1 to 4 millimeter
diameter. The beads were activated by careful heating in a flow
of 20000 liters (STP) per hour of dry nitrogen at 550.degree. C.
for 3 hours. Nitrogen working capacities of this material are shown
in Table 3. The nitrogen working capacity is defined as the difference
in the amount of nitrogen adsorbed at 0.8.times. adsorption pressure
and the amount of nitrogen adsorbed at 0.8.times. desorption pressure.
EXAMPLE 4
[0051] This Example represents the prior art, and is given for
comparison with Example 3.
[0052] A prior art zeolite was incorporated into beads typical
of those utilized in industrial applications as follows. 4.0 dry
kilograms of the material produced according to Example 2 were mixed
thoroughly with 1.0 kilogram of a conventional attapulgite binder
for 1 hour. After adding the required quantity of water to induce
granulation, the mixture was granulated using an intensive mixer
to produce spherical beads of 1 to 4 millimeter diameter. The beads
were activated by careful heating in a flow of 20000 liters (STP)
per hour of dry nitrogen at 550.degree. C. for 3 hours. Nitrogen
working capacities of these materials are shown in Table 3. The
nitrogen working capacity is defined as the difference in the amount
of nitrogen adsorbed at 0.8.times. adsorption pressure and the amount
of nitrogen adsorbed at 0.8.times. desorption pressure.
[0053] The regeneration of the adsorbent is a critical step in
VPSA processes. A deep vacuum in the desorption step facilitates
good regeneration, with a low nitrogen retention, and therefore
a high available working capacity for the following adsorption step.
However, a consequence of a deep vacuum is a high energy consumption.
FIG. 3 compares the nitrogen adsorption isotherms at 25.degree.
C. for the adsorbent of the present invention and the prior art
Li-LSX zeolite of Examples 3 and 4. It is clear that the novel material
adsorbs more nitrogen at pressures above 1000 millibar, and less
nitrogen at pressures below 1000 millibar. Thus, the slope of the
adsorption versus applied pressure curve is greater for the novel
material than for the prior art zeolite, at pressures above 1000
millibar. Thus, the nitrogen adsorption observed for the novel adsorbent
does not decrease as rapidly with increasing pressure, as does that
for the prior art Li-LSX zeolite. The novel adsorbent therefore
provides superior performance in PSA processes with the adsorption
at super-atmospheric pressures and regeneration at ambient pressure.
Furthermore, it is not necessary in VPSA processes to reach such
deep vacuum for the efficient regeneration of the novel adsorbent
if the regeneration step were done at sub-atmospheric pressure.
3TABLE 3 Nitrogen Working Capacities for PSA Processes, expressed
in normal liters per kilogram Novel Adsorbent Prior Art Zeolite
(Example 3) (Example 4) Process at 3 bar 12.64 12.23 Process at
4 bar 16.60 15.88
[0054] Table 3 provides a comparison of the nitrogen working capacity
of the adsorbent from Example 3 made according to the present invention,
and the prior art zeolite from Example 4 for PSA processes operating
at 3 bar and 4 bar and 40.degree. C. These values are derived from
the adsorption isotherms of FIG. 3. The novel adsorbent shows superior
performance at 3 bar adsorption pressure and even greater advantage
at 4 bar.
[0055] It will be apparent to those skilled in the art that the
magnitude of these differences in the performance of the adsorbent
of the present invention, as compared with the prior art zeolite
are significant, and will have substantial benefit in commercial
applications.
[0056] FIG. 4 compares graphically the nitrogen working capacity
for the adsorbent of the present invention as a function of the
desorption pressure at 40.degree. C. Adsorption pressures are 1.3
bar and 1.5 bar for the two examples depicted. The nitrogen working
capacity is defined as the difference of the amount of nitrogen
adsorbed at 0.8.times. adsorption pressure and the amount of nitrogen
adsorbed at 0.8.times. desorption pressure. It is clear that the
novel adsorbent provides a higher working capacity across the entire
pressure range as compared to the prior art zeolite. Thus, the novel
adsorbent provides superior performance for any given desorption
pressure. The novel adsorbent has the further advantage that it
does not require as deep a vacuum to achieve good regeneration if
the system is operated at constant working capacity.
[0057] Overall, the resulting working capacity is higher for the
novel adsorbent than for the prior art Li-LSX zeolite. At an adsorption
pressure of 1.2 to 1.6 bar, and preferably 1.3 to 1.5 bar, an optimum
desorption pressure in terms of energy consumption was found to
be 360 to 490 millibar, and preferably 390-460 millibar. To reach
the equivalent performance of a VPSA plant with a prior art Li-LSX
zeolite, it would be necessary to decrease the regeneration pressure
by an additional 30 millibar, which would require a substantially
higher energy consumption. The advantages of using the novel adsorbent
of the present invention for VPSA processes for air separation are
most apparent when the feed temperature is in a preferred range
of about 20-60.degree. C.
EXAMPLE 5
[0058] The beaded materials of Example 3 and 4 were tested for
their performance in a VPSA pilot plant. The adsorption pressure
was 1.3 bar, desorption pressure was 0.4 bar, and the cycle time
was 26 seconds. The results are shown in FIG. 5. These results were
monitored by parallel simulation and data reduction studies. When
determined at 92% oxygen purity, the data reduction showed an increase
of 2.0% in Productivity and 2.5% in Yield using the novel adsorbent
of the present invention. These large increases would result in
significant cost reductions and/or product quality improvements
in commercial installations.
[0059] The novel adsorbent of the present invention shows superior
adsorption kinetics compared to a prior art Li-LSX molecular sieve.
This results in a better Productivity and Yield, especially for
a high product purity. It is therefore possible to use beads of
the new adsorbent having an increased diameter compared to the prior
art Li-LSX zeolite. The use of such bigger beads will result in
a lower pressure drop across the industrial adsorber bed, thereby
decreasing the energy consumption of the machinery, e.g. the air
blower and the vacuum pump in a VPSA process. The faster adsorption
kinetics of the novel adsorbent will also permit a reduced cycle
time of the processes. And it is well known that shorter cycles
lead will increase Productivity against unit pressure drop limitations.
Due to the possibility of using bigger beads the higher pressure
drop at shorter cycles can be overcome. Therefore use of the adsorbent
of the present invention will lead to further reductions in specific
energy consumption when applied in VPSA and PSA processes.
EXAMPLE 6
[0060] This Example presents a calculation of the economic benefit
achieved by the present invention.
[0061] An air stream is fed at a pressure of 1.3 bar into a VPSA
plant with 2 adsorber vessels. Each adsorber bed contains 51000
lbs. of the adsorbent described in this invention. The adsorber
vessels are additionally filled with an appropriate amount of desiccant
adsorbent in the bottom of the vessel in order to remove water and
carbon dioxide from the feed air prior to flowing through the new
adsorbent. The desorption pressure is 400 millibar and the time
to complete a process cycle for one adsorber is 60 seconds. One
process cycle consists of pressurization, production, pressure equalization,
evacuation and purge. The production flow rate is 100 tons per day
and the oxygen purity is 93.5%. The air consumed by the PSA plant
is 21700 Nm.sup.3/h. The specific energy consumption is 0.30 kWh
per Nm.sup.3 of pure oxygen.
[0062] A VPSA plant, using the same amount of prior art Li-LSX
zeolite and operated under the same conditions produces only 96.8
tons per day at an oxygen purity of 93.5% and an air consumption
of 21650 Nm.sup.3/h. The specific energy consumption increases
by 5% to 0.315 kWh per Nm.sup.3 of pure oxygen product when the
system is charged with the prior art zeolite. That means that the
VPSA unit operating with the new adsorbent can produce 3.3% more
enriched oxygen with the same amount of adsorbent and air utilized,
with a 5% reduction in specific energy consumption. This considerable
advantage in specific energy consumption, in addition to the pure
benefit in recovery, is based on the more linear type of nitrogen
isotherm of the new adsorbent as described in this invention if
compared to prior art zeolite (FIG. 3). This characteristic in combination
with the enhanced diffusivity of the new adsorbent, leads to an
easier release of nitrogen from the adsorbent to the vacuum pump,
i.e. a steeper evacuation curve and therefore lower energy consumption.
[0063] The VPSA process with the new adsorbent described in this
invention will produce 1170 tons per year more oxygen at 93.5% purity,
and will save 123.5 MW per year. Overall, the plant will save approximately
$88000 in operating costs per year if a power cost of $0.05 per
kWh and a product price of $0.10 per Nm.sup.3 of oxygen are applied.
[0064] The reader skilled in the art will recognize that the invention
may be modified, within the scope of the above disclosure. Such
modifications should be considered within the spirit and scope of
the following claims. |