Molecular sieve abstract
A molecular sieve adsorbent for the purification of gas streams
containing water vapor and carbon dioxide. The adsorbent is a combination
of sodium form of a low-silica faujasite, having a residual content
of potassium ions less than about 8.0 percent (equiv.), a low content
of crystalline and amorphous admixtures and crystal sizes generally
within the range of 1-4 .mu.m, and a binder. A process for the adsorbent
preparation which comprises specific parameters of low silica faujasite
synthesis, sodium-potassium ion exchange, blending and granulation.
Molecular sieve claims
We claim:
1. A molecular sieve adsorbent for gas purification which comprises
a sodium form of low-silica faujasite, having a silica:alumina ratio
of about 1.8-2.2 with a residual content of potassium ions less
than about 8.0 percent (equiv.), and a binder, wherein at least
about 80% of said low-silica faujasite by weight has a crystal size
in the range of 1-4 .mu.m.
2. The molecular sieve adsorbent of claim 1 wherein the silica
to alumina ratio is about 2.0 to about 2.2.
3. The molecular sieve adsorbent of claim 1 wherein the silica
to alumina ratio is about 2.0 to about 2.1.
4. The molecular sieve adsorbent of claim 1 wherein the residual
content of potassium ions is less than about 2.5 percent (equiv.).
5. The molecular sieve adsorbent of claim 1 wherein the residual
content of potassium ions is less than about 1.0 percent (equiv.).
6. The molecular sieve adsorbent of claim 1 wherein the low silica
faujasite comprises from about 60 to about 85 percent and the binder
comprises from about 15 to about 40 percent of the adsorbent.
7. The adsorbent of claim 1 wherein the low-silica faujasite comprises
less than 10.0 percent by weight zeolite A, high silica faujasite
X and other crystalline and amorphous admixtures.
8. The adsorbent of claim 1 wherein the low-silica faujasite comprises
less than 2.0 percent by weight zeolite A, high silica faujasite
X and other crystalline and amorphous admixtures.
9. The adsorbent of claim 1 wherein the crystal size of at least
about 80 percent of the low-silica faujasite is in the range of
about 1-3 .mu.m.
10. The adsorbent of claim 1 wherein the crystal size of at least
about 80 percent of the low-silica faujasite is in the range of
about 1-2 .mu.m.
11. The adsorbent of claim 1 wherein the binder is selected from
the group consisting of kaolinite, bentonite, montmorillonite, and
attapulgite clays, silica, alumina, alumosilicates, and cements.
12. The adsorbent of claim 1 wherein the adsorbent has a volume
of macropores with a radius from about 1000 to 10000 .ANG. from
about 0.3 to about 0.5 cm.sup.3 /g.
13. The molecular sieve adsorbent of claim 1 wherein the low silica
faujasite has a volume of macropores with a radius from about 1000
to 10000 .ANG. of at least about 0.3 cm.sup.3 /g.
14. A process for the adsorption of carbon dioxide and water from
a gas stream comprising passing the gas stream containing carbon
dioxide and water over the molecular sieve adsorbent of claim 1
whereby substantial quantities of the carbon dioxide and water are
adsorbed by the molecular sieve adsorbent.
15. A molecular sieve adsorbent for gas purification which comprises
a sodium form of low-silica faujasite, having a silica:alumina ratio
of about 1.8-2.2 with a residual content of potassium ions less
than about 8.0 percent (equiv.), and a binder, wherein the adsorbent
has a volume of macropores with a radius from about 1000 to 10000
.ANG. of at least about 0.3 cm.sup.3 /g.
16. A molecular sieve adsorbent for gas purification which comprises
a sodium form of low-silica faujasite, having a silica:alumina ratio
of about 1.8-2.2 with a residual content of potassium ions less
than about 8.0 percent (equiv.), and a binder, wherein at least
about 80% of said low-silica faujasite by weight has a crystal size
in the range of 1-4 .mu.m and wherein the adsorbent has a volume
of macropores with a radius from about 1000 to 10000 .ANG. of at
least about 0.3 cm.sup.3 /g.
17. A process for the production of a molecular sieve adsorbent
comprising preparing a sodium form of a low-silica faujasite, wherein
the low-silica faujasite has a silica:alumina ratio of about 1.8
to about 2.2 and a residual content of potassium ions less than
about 8.0 percent (equiv.), wherein at least about 80 percent of
the low-silica faujasite by weight has a crystal size in the range
of 1-4 .mu.m; blending the resultant product with a binder and a
peptizer; forming the blended product into a shaped article; and
drying and calcining the article to produce the adsorbent product.
18. The process of claim 17 wherein the low-silica faujasite is
prepared by mixing sodium aluminate, sodium silicate and sodium
and potassium hydroxide, wherein the ratio of the components expressed
as mixtures of oxide are within the following ranges:
19. The process of claim 17 further comprising mixing a powder
of montmorillonite in the amount of about 0.6 to about 1.5 percent,
based on the final weight of the reaction product, with the prepared
low silica faujasite into a reaction mixture under intensive stirring
for about 15 to 30 minutes.
20. The process of claim 17 further comprising ion exchanging the
prepared, sodium form of low-silica faujasite with a sodium chloride
solution.
21. The process of claim 20 wherein the sodium form of low silica
faujasite is ion exchanged from about 1 to about 3 times.
22. The process of claim 20 wherein the sodium form of low silica
faujasite is ion exchanged at a temperature from about 60.degree.
C. to about 85.degree. C.
23. The process of claim 17 further comprising ion exchanging the
prepared, sodium form of low-silica faujasite with a solution comprising
sodium chloride and sodium hydroxide.
24. The process of claim 17 further comprising treating the prepared
sodium form of low-silica faujasite and binder with a pore-forming
additive.
25. The process of claim 24 wherein the pore-forming additive is
selected from the group consisting of synthetic and natural materials
including mineral acids, polyalkeneglycols, polyacrylamides, polycarboxylates,
starch, molasses and lignin.
26. The process of claim 25 wherein the peptizer and pore-forming
additives comprise from about 0.5 to about 2.0 percent of the adsorbent
product, based on a total weight of the adsorbent product.
27. The process of claim 17 further comprising blending the low-silica
faujasite with a peptizer prior to mixing with the binder.
28. The process of claim 17 further comprising blending the binder
with the peptizer prior to mixing with the low silica faujasite.
Molecular sieve description
TECHNICAL FIELD
The present invention relates to a novel adsorbent for removal
of water and carbon dioxide from gases, and more particularly, an
adsorbent for purification of air, nitrogen, hydrogen and natural
gas streams. The invention is also a method of preparation of the
adsorbent.
BACKGROUND ART
Carbon dioxide is an undesired impurity in many commercial gas
applications owing to its ability to freeze and form hydrates with
moisture at low temperatures. Formation of solids or solid particles
makes gas processing, operation, transportation and storage quite
difficult or even impossible. For example, cryogenic units for air
separation to produce oxygen and nitrogen demand practically complete
removal of carbon dioxide (1 ppm and less) and water vapor from
air before separation. Refineries place similar requirements on
the quantity of carbon dioxide and moisture present in hydrogen-contaminated
gas flows. Similar requirements are placed on ammonia plants for
nitrogen purity and gas processing plants on the carbon dioxide
content and natural gas dew point prior to ethane and helium recovery
and/or before natural gas liquefaction. Also, petrochemical plants
have to eliminate carbon dioxide and water in monomers: ethylene,
propylene, butadiene, etc. to prevent poisoning of the polymerization
catalysts and deterioration of polymer properties.
Adsorption of carbon dioxide and water vapor is the most common
method of removal of these compounds from gas streams due to high
performance and relatively low capital and operational costs. Two
adsorption techniques are commonly used in commercial gas manufacturing:
temperature swing adsorption (TSA) and pressure swing adsorption
(PSA). Efficiency of both adsorption processes is determined by
the properties of the adsorbent. High adsorption of carbon dioxide
is the most important property of the adsorbent, especially at very
low partial pressures.
Several types of CO.sub.2 adsorbents have been created to meet
industry needs. Because general duty adsorbents, such as alumina,
silica gel and activated carbon, do not have a substantial adsorption
capacity for carbon dioxide, more complex adsorbents have been prepared.
U.S. Pat. No. 3865924 Gidaspow, discloses a carbon dioxide adsorbent,
which constitutes a mechanical mixture of activated alumina and
alkali metal carbonates. U.S. Pat. No. 4433981 Slaugh, discloses
an adsorbent prepared by impregnating alumina with an alkali or
alkaline earth metal oxide or salt decomposable upon calcination.
U.S. Pat. No. 4493715 Hogan, discloses an adsorbent for carbon
dioxide removal from olefins which comprises alkali metal oxides,
hydroxides, nitrates, acetates, etc. placed on an activated alumina.
All such adsorbents employ chemisorption or reversible chemical
reactions to bind carbon dioxide to the metal carbonates or bicarbonates.
The main disadvantage of these adsorbents is low operational reliability
and short life due to the tendency of active components to sinter.
Secondly, the time before water breakthrough on the majority of
to adsorbents is shorter than the time before the carbon dioxide
breakthrough. This results in the need to employ supplemental desiccant
beds.
It is also impossible to use base-containing adsorbents in the
PSA-type units because they form compounds with CO.sub.2 that do
not regenerate under reduced pressure.
A newer process for gas dehydration and carbon dioxide recovery
technology uses molecular sieves, natural and synthetic zeolites.
It is known that synthetic zeolite A and X types are effective adsorbent
of CO.sub.2 and water. For instance, U.S. Pat. No. 3981698 Leppard,
U.S. Pat. No. 4039620 Netteland, U.S. Pat. No. 4711645 Kumar,
U.S. Pat. No. 4986835 Uno, and U.S. Pat. No. 5156657 Ravi,
suggest the use of standard molecular sieves 5A, 10A and 13X as
carbon dioxide adsorbents. These molecular sieves adsorb CO.sub.2
by physical adsorption and are regenerable at ambient temperatures.
However, they do not possess sufficient adsorption capacity for
carbon dioxide. Thus, such adsorbents cannot provide extensive gas
purification, demand an increased loading volume and often require
the use of supplemental adsorbent beds to decrease the water and
carbon dioxide concentration prior to introduction into the zeolite
bed.
To increase carbon dioxide adsorption capacity, several adsorbents
have been proposed based on various cation exchanged forms of molecular
sieve X and other crystalline structures. Thus, U.S. Pat. No. 3885927
Sherman, discloses a barium cation form of zeolite X in which 90-95%
of the Na.sup.+ ions are replaced by Ba.sup.2+ ions. U.S. Pat. No.
4477267 Reiss, utilizes an adsorbent for hydrogen purification
containing CaX-zeolite. For carbon dioxide removal, U.S. Pat. No.
4775396 Rastelli, describes the use of zinc, rare earth metals,
a proton and ammonium cation exchanged forms of synthetic faujasite
having a silica:alumina ratio in broad range of 2-100. U.S. Pat.
No. 5587003 Bulow, discloses the use of a natural or synthetic
clinoptilolite, which contains as exchangeable cation the ions of
metals of Groups 1A, 2A, 3A, 3B, the lanthanide group and mixtures
of these.
All of these molecular sieve adsorbents are characterized by carbon
dioxide adsorption capacity extended at moderate and high partial
pressures of the admixture to be adsorbed. However, their capacity
to adsorb at low partial pressure of CO.sub.2 (<5 torr) and at
ambient temperatures is not sufficient to provide the purity of
the gas required. In addition, due to the relatively short time
before CO.sub.2 breakthrough, the water capacity of these adsorbents
appears to be only 10-15 percent of potential. This decreases adsorbent
performance in such applications as TSA and PSA air pre-purification
units where carbon dioxide inlet adsorption is very low. Employing
the above mentioned adsorbents in such applications demands gas
chilling to a temperature below about 5.degree. C. In turn, this
results in a substantial increase in operation and capital costs.
U.S. Pat. No. 5531808 Ojo, discloses an adsorbent for carbon
dioxide adsorption comprising a type X zeolite having a silicon
to aluminum ratio in the range of 1.0-1.15. The type X zeolite adsorbent
contains ions of Group 1A, Group 2A, Group 3A, Group 3B, the lanthanide
series and mixtures thereof. It fails to teach any critical quantitative
relationship among various cations in the type X-zeolite crystalline
structure that is necessary to provide high levels of adsorption
capacity of carbon dioxide at low partial pressures and at ambient
temperatures. It also fails to disclose preferable limits for crystalline
purity and crystal sizes. It also does not disclose the adsorbent
macroporosity limits necessary to provide the appropriate kinetics
and dynamics of carbon dioxide adsorption.
A process for preparing low-silica faujasite (LSF) with a silica/alumina
ratio .about.2.0 is disclosed in Kuhl "Crystallization of Low-Silica
Faujasite" Zeolites, vol.7 p.451 (1987). Kuhl discloses that
both sodium and potassium cations should be present to obtain faujasite
crystals with relatively low silica content. The crystallization
process disclosed comprises preparing a sodium aluminate water solution
with addition of sodium and potassium hydroxides, mixing the solution
with sodium silicate, aging the gelled mixture, and filtering and
washing of the crystallization product. Kuhl also describes specific
reagent ratios, temperatures and retention times, which are required
for crystallization of the product. However, it does not specify
the range of crystallization parameters that provide definite size
of the faujasite crystals and a final product with a low content
of admixture crystals of different types. Kuhl also does not disclose
sodium-potassium ion exchange procedures for obtaining LSF with
low residual potassium ion content.
A number of other patents disclose molecular sieve adsorbents having
improved adsorption capacities, especially for the removal of carbon
dioxide from gas mixtures. For example, U.S. Pat. No. 2882244
Milton, discloses a variety of crystalline aluminosilicates useful
for CO.sub.2 adsorption. U.S. Pat. No. 3078639 Milton, discloses
a zeolite X useful for the adsorption of carbon dioxide from a gas
stream. British Patent Nos. 1508928 Mobil Oil, and 1051621
Furtig et al., disclose faujasite-type zeolites having a silica
to alumina ratio from 1.8 to 2.2.
While these products have been useful in the adsorption of carbon
dioxide and water from gas streams, it is important to provide improved
adsorbents. Further, while it has been discovered that low silica
faujasites are useful in the adsorption of carbon dioxide and water
from gas streams, newer low silica faujasites with improved adsorption
capabilities which do not exhibit the limitations of the earlier
products would be helpful.
Accordingly, it is an aspect of the invention to provide an adsorbent
for carbon dioxide and water vapor with enhanced adsorption capacity.
It is a further aspect of the invention to provide an adsorbent
for carbon dioxide useful for adsorption at ambient temperatures
and low partial pressures.
It is a still further aspect of the invention to provide an adsorbent
for carbon dioxide in a gas stream which reduces operation and capital
expenses when used.
It is a still further aspect of the invention to provide an adsorbent
for carbon dioxide and water vapor with improved kinetics and dynamics
of adsorption for both Temperature Swing Adsorption processes and
Pressure Swing Adsorption processes and an aggregate of the two
processes.
It is a still further aspect of the invention to disclose an adsorbent
for carbon dioxide which produces a gas stream containing less than
one part per million of carbon dioxide.
It is a still further aspect of the invention to provide a process
for the production of a low silica faujasite adsorbent for carbon
dioxide.
These and further aspects of the invention will be apparent from
the foregoing description of a preferred embodiment of the invention.
SUMMARY OF INVENTION
The present invention is an adsorbent for carbon dioxide, and preferably
water vapor, from gas streams where the adsorbent exhibits high
adsorption capacity at low partial pressures and ambient temperatures.
The adsorbent comprises a sodium form of a low silica faujasite,
wherein the silica to alumina ratio is from about 1.8:1 to about
2.2:1 preferably about 2.0:1 to about 2.1:1 and wherein the content
of the potassium ions present in the low silica faujasite constitute
less than about 8.0 percent, preferably less than about 2.5 percent,
most preferably less than about 1.0 percent of the exchangeable
cations (equivalents). Preferably, the volume of the macro-pores
in the range from about 1000 to about 10000 .ANG. radii is greater
than about 0.4 cm.sup.3 /g, preferably from about 0.4 to about 0.6
cm.sup.3 /g and most preferably from about 0.4 to about 0.5 cm.sup.3
/g.
The present invention also features a process for the production
of the adsorbent product comprising preparing a sodium form of a
low-silica faujasite, wherein the low-silica faujasite has a silica:alumina
ratio of about 1.8:1 to about 2.2:1 and a residual content of potassium
ions of less than about 8.0 percent (equiv.); blending the resultant
product with a binder; forming the blended product into a shaped
article; and drying and calcining the article to produce the adsorbent
product.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the carbon dioxide adsorption isotherms for low-silica
faujasites having differing percentages of residual potassium cations.
FIG. 2 compares the carbon dioxide adsorption of various adsorbents
including the present invention (Example 4) and other adsorbents,
including a molecular sieve 5A (CaA-94.5 percent Ca.sup.+), molecular
sieve 10A (CaX), molecular sieve 13X (NaX) and Example 16 showing
a calcium low-silica faujasite with a potassium ion content of 0.16
percent.
DISCLOSURE OF THE INVENTION
It is known to use low silica faujasites for the adsorption of
carbon dioxide and water vapor from gas streams. It is also known
that the substitution of sodium cations by other metal ions of larger
size causes an increase in the adsorption capacity of standard faujasites
with high silica content. For example, it is known that the calcium
forms of a faujasite X zeolite having a silica/alumina ratio greater
than 2.3:1 is characterized by higher adsorption capacities for
carbon dioxide than sodium forms of these same faujasites.
Conventionally, low silica faujasite crystals are produced having
a sodium:potassium ratio from about 2 to 3:8 to 7. It has been surprisingly
discovered that by substituting sodium for substantially all of
the potassium ions, the carbon dioxide adsorption capacity of the
low silica faujasite increases significantly over conventional low
silica faujasite with conventional ratios of sodium to potassium
ions. It has also been surprisingly discovered that the fewer the
potassium ions that are present in the low silica faujasite crystal
structure and respectively, the more sodium ions that are present,
the better the adsorption capacity for carbon dioxide of the adsorbent
which is produced.
Previously, it had been discovered by Kuhl that the crystallization
of low silica faujasite is typically accompanied by the formation
of high silica faujasites, zeolite A, zeolite P, sodalite and other
crystalline and amorphous admixtures. It has been discovered that
in order to create improved adsorption capacity for the adsorbent,
the impurities that are conventionally present in low silica faujasite
should be reduced as much as possible. Thus, it is preferable that
the low silica faujasite utilized as an adsorbent contain at least
about 90 percent, by weight, of the low silica faujasites, preferably
at least about 95 percent, by weight, and most preferably at least
about 98 percent, by weight.
In addition, it has also been discovered that in order to increase
the adsorption capacity of the high sodium, low silica faujasites,
at least about 80 percent of the low-silica faujasite crystals,
by weight, should have a crystal size within a range of about 1
to 4 .mu.m, preferably 1 to 3 .mu.m and most preferably 1 to 2 .mu.m.
It has also been discovered that the volume of the macropores of
a size with a radii from about 1000 to 10000 angstrom should be
greater than about 0.4 cm.sup.3 /g, preferably from about 0.4 to
about 0.6 cm.sup.3 /g and most preferably from about 0.4 to about
0.5 cm.sup.3 /g.
If these criteria for high sodium, low silica faujasites are satisfied,
the adsorption capacity of the adsorbent is enhanced in dynamic
conditions. Without utilizing these preferred embodiments for the
high sodium, low silica faujasites, the carbon dioxide adsorption
in a flow on a molecular sieve adsorbent is limited by mass transfer
inside adsorbent granules. Thus, the large macropore volume is required
to provide the carbon dioxide molecules fast diffusion from the
external surface of the adsorbent granules to the external surface
of the crystals. Whereas, small size of zeolite crystals diminishes
resistance to superficial diffusion inside the microcrystals.
In addition, the enhanced carbon dioxide capacity of the adsorbent
of the present invention also increases the water adsorption capacity
of the adsorbent up to 40-50 percent over conventional carbon dioxide
adsorbents.
Generally, high sodium, low silica faujasites are created by preparing
a water solution of sodium aluminate, sodium silicate, and sodium
and potassium hydroxide whose ratios are expressed as mixtures of
oxides within the following range:
The reaction mixture should be maintained at a temperature of about
85.degree. to 95.degree. C. over the reaction process of 2-10 hours.
Additionally, to provide the narrow distribution of low silica faujasite
microcrystals so that their sizes will be in the range of about
1 to 4 .mu.m, preferably 1 to 3 .mu.m and most preferably 1 to 2
.mu.m, fine submicron powders of montmorillonite, such as Rym.RTM.
Kill 10 MB, with particle size of 0.4-0.8 .mu.m in a quantity of
0.6-1.5 percent, by weight should be introduced under intensive
stirring into the reaction mixture after about 15 to 30 minutes.
The composition is then stirred for at least about 2 hours and maintained
at a temperature of about 85.degree. to 95.degree. C. during stirring.
The crystals produced are filtered from the reaction solution. The
crystals obtained from this reaction are then washed with deionized
water to maintain the pH of the crystals in a range from about 10.5
to about 11.
Once these crystals are obtained, the potassium ion content of
the low silica faujasites must be reduced to levels less than about
8 percent, preferably less than 2.5 percent, most preferably about
1.0 percent.
There are several methodologies that may be used to produce this
type of high sodium, low silica faujasite. For example, a powder
exchange can be performed on a belt filter with from one to three
stages of sodium chloride solution feed. The concentration of the
sodium chloride solution should be increased in each stage so that
the equivalent ratio of sodium in solution to potassium in zeolite
reaches 1.5 during the first stage, 2.2 during the second stage,
and 3.0 during the third stage.
In an alternative method, calcined sodium/potassium, low silica
faujasite beads can be ion exchanged in a column with a sodium chloride
solution (10-15 percent by weight) or preferably with a sodium chloride
plus sodium hydroxide solution (7-10 percent NaCl+3-5 percent NaOH),
by weight, at a temperature of about 60.degree. to 85.degree. C.
In either process, the ion exchanged product is washed with deionized
water to remove excess sodium ion. Both of these processes produce
adsorbents with a potassium content of less than about 8.0 percent,
preferably less than about 2.5 percent, and most preferably, less
than about 1.0 percent (equiv.).
The high sodium, low silica faujasite zeolite powder obtained is
then admixed with a binder to produce a final adsorbent product.
The binder can be chosen from mineral or synthetic materials, such
as clays (kaolinite, bentonite, montmorillonite, attapulgite or
other such clay materials), silica, alumina, alumina hydrate (pseudoboehmite),
alumina trihydrate, alumosilicates, cements or other such materials.
The binder comprises about 15-40 percent by weight of the final
adsorbent. The mixture is then kneaded with 18-35 percent, by weight,
water to form a paste which is then aggregated to form shaped articles
of conventional shapes.
In a preferred embodiment certain peptizers and/or pore-forming
ingredients are aggregated with the molecular sieve product and/or
with the binder in an amount from about 0.5 to about 2.0 percent
of the final adsorbent product by weight to produce the extended
macropore volume required. Such additives typically include mineral
acids, surfactants and plasticizers such as, for example, polyalkeneglycols,
polyacrylamides, polycarboxylates, natural organic products such
as starches, molasses, lignin or other such related material. Following
this addition, the shaped product is cured, dried and calcined at
a temperature from about 550.degree. C. to about 650.degree. C.
Utilizing high sodium, low silica faujasite particles produced
by the above-described process creates a product which is particularly
useful for the adsorption of carbon dioxide and water vapor from
gas streams. The preferred types of gas streams in which this type
of high sodium, low silica faujasite crystals can be utilized includes
air, nitrogen, hydrogen, natural gas, individual hydrocarbons and
monomers, such as ethylene, propylene, 1.3 butadiene, isoprene and
other such gas systems.
It has been surprisingly discovered that high sodium, low silica
faujasite with a potassium content less than about 2.5 percent (equiv.),
when utilized in a conventional air pre-purification process, reduces
the carbon dioxide level in the gas stream generally to a range
of 0.3 ppm to about 0.5 ppm which is significantly lower over the
same length of the adsorbent bed than that of conventional low silica
faujasites with a sodium content of about 62 percent to about 90
percent (equiv.). These conventional low silica faujasites reduce
the CO.sub.2 content only to about 2 ppm. Simultaneously the dynamic
adsorption capacity of the high sodium, low silica faujasite for
water is also increased from about 1.5 percent up to about 3.5 percent
(by weight). Therefore, it has been surprisingly discovered that
low silica faujasites with a potassium content less than about 8.0
percent (equivalent), preferably less than 2.5 percent (equivalent),
and most preferably, less than about 1.0 percent (equivalent) are
highly efficient in the adsorption of both carbon dioxide and water
vapor in conventional gas stream operations.
In order to illustrate the present invention and the advantages
thereof, the following examples are provided. It is understood that
these examples are illustrative and do not provide any limitation
on the invention.
EXAMPLE 1
16.8 l of sodium hydroxide and 8.9 l of potassium hydroxide with
a molar ratio of Na.sub.2 O: (Na.sub.2 O+K.sub.2 O) equaling 0.75
were added to 20 liters of a sodium aluminate solution so that the
(Na.sub.2 O+K.sub.2 O):Al.sub.2 O.sub.3 ratio equals 1.62. This
solution was mixed with 16.5 l of a sodium silicate solution in
such quantitative relation to provide a silica/alumina ratio of
2.0 wherein the sum of moles of water to the alkali metal oxides
equals 16.4. The gelled mixture obtained was aged for 1 hour and
crystallized at 92.degree. C. for 4 hours. After 18 minutes of thermotreating,
a fine montmorillonite powder (Rym.RTM. Kill 10 MB with particle
size of 0.4-0.8 .mu.m) was added in an amount equal to 0.6 percent
of the final product weight. The crystals produced were filtered
and washed with deionized water to reduce the pH of the product
produced to about 10.6.
The product was analyzed and found to contain low silica faujasite
crystals of 98 percent purity (X-ray analysis). The crystal size
of the product under electron microscopy analysis was determined
to be from 1-3 .mu.m. The product had a potassium ion content of
about 26 percent (equivalent). A paste was formed and dried at 110.degree.
C. for 3 hours.
8 kg of this dried powder was mixed with 2.0 kg of attapulgite
Minugel 400 moisturized and formed into beads of about 1.6 mm diameter
by means of plant plate granulator. The beads were then dried at
110.degree. C. for 2 hours and calcined first at 250.degree. C.
for 2 hours, then at 350.degree. C. for 2 hours and at 600.degree.
C. for 1 hour.
EXAMPLE 2
The Invention
4.0 kg. of the sodium/potassium low silica faujasite beads obtained
from Example 1 were treated at ambient temperature with 16 liters
of a 1.5N solution of sodium chloride. The product was washed with
80 l of deionized water and then again treated with 8 liters of
2.2N solution of sodium chloride. The product was then washed with
deionized water to remove the chloride ions until a negative measurement
with a 0.028N solution of silver nitrate. Then the operating procedure
of the Example 1 for the adsorbent drying and calcining was repeated.
Elemental analysis of the resulting product showed a 7.5 percent
(Equiv.) residual potassium ion content. The analysis was done by
Inductively Coupled Plasma Atomic Emission Spectroscopy.
EXAMPLE 3
The Invention
2.3 kg. of the beads produced from Example 2 were treated under
ion exchange conditions with 8 liters of a 3.0N solution of sodium
chloride at ambient temperature. The product obtained was washed
with deionized water as it is shown in Example 2. Then the operating
procedure of the Example 1 for drying and calcining was repeated.
The product produced from this exchange operation was analyzed for
potassium and determined to have a potassium content of 2.4 percent
(equiv.).
EXAMPLE 4
The Invention
1.0 kg. of the beads prepared in Example 3 were treated at a temperature
of 80.degree. C. with 2 liters of 3.0N solution of sodium chloride.
The operating procedure of Example 2 for the adsorbent washing,
drying and calcining was repeated. The residual content of the potassium
ion in the adsorbent was 0.3 percent (equiv.).
EXAMPLE 5
Adsorption Equilibrium Test
The samples of Examples 1 through 4 were tested for carbon dioxide
adsorption equilibrium. Adsorption isotherms were measured employing
an adsorption volumetric unit Micromeritics ASAP 2010 at a temperature
of 25.degree. C. The samples were activated preliminarily at 400.degree.
C. for 2 hours. The carbon dioxide partial pressure varied in the
range of 1-100 torr.
The results obtained are shown in FIG. 1.
As is apparent from FIG. 1 the residual content of potassium ion
in the low-silica faujasite has a dramatic impact on the ability
of each of these adsorbents to adsorb carbon dioxide. This Figure
shows that the less potassium ions that are present, the higher
the equilibrium adsorption capacity for carbon dioxide over its
full range of partial pressures. This Figure also confirms that
only adsorbents having a potassium content of less than about 8.0
percent (equiv.) provide a satisfactory adsorption capacity at ambient
temperature and low partial pressures (1-10 torr.). The adsorbents,
according to the present invention (Examples 2 to 4), display adsorption
capacity well above that of the adsorbent according to U.S. Pat.
No. 5531808 (Example 1) which include comparable amounts of both
alkali metal ions: sodium and potassium.
EXAMPLE 6
Comparative
The operating procedure of Example 1 for low silica faujasite synthesis
was repeated with one modification. Sodium and potassium hydroxide
solutions were used so that the final Na.sub.2 O: (Na.sub.2 O+K.sub.2
O) had a ratio of 0.72. The product produced contained 86 percent
of a low silica faujasite under X-ray diffraction. The XRD and chemical
analyses of the product showed significant-amounts (.about.12 percent)
of high silica faujasite and zeolite A. The powder obtained from
this procedure was exchanged three times with a sodium chloride
solution as described in Example 3. The potassium percentage in
the exchange product was 2.2 percent (equiv.). The operating procedure
of Example 1 for mixing the product with an attapulgite binder to
form beads was repeated.
EXAMPLE 7
Adsorption Equilibrium Test
The adsorbent samples of Examples 3 and 6 were tested for carbon
dioxide adsorption at partial pressures of 2 5 8 and 15 torr.
In the process, the testing procedure of Example 5 was employed.
The results are reported in Table 1.
TABLE 1 LSF Adsorption Value, cm.sup.3 /g purity Partial Pressure,
torr Examples % 2.0 8.0 15.0 30.0 3 98 32.6 44.8 51.3 61.4 6 86
26.5 41.8 50.2 61.6
As in Example 7 the adsorbent according to the present invention,
having low silica faujasite of high purity, 98 percent, demonstrates
a higher adsorption capacity at low partial pressures (2-10 torr)
of carbon dioxide than that of the adsorbent with lower LSF purity.
EXAMPLE 8
Comparative
The operating procedure for the low silica faujasite synthesis
of Example 1 was repeated with one exception: the montmorillonite
powder was not added to the gelled crystallizing mixture. The adsorbent,
so prepared, contained low silica faujasite crystals of 4-6 .mu.m
size. The beads, produced with 20 percent of attapulgite were ion-exchanged
with NaCl solution using the same procedure as shown in Example
3.
EXAMPLE 9
Test of Dynamic Capacity
The adsorbents of Examples 3 and 8 were tested for dynamic adsorption
in air purification employing a tube adsorber with adsorbent bed
volume of 100 cm.sup.3 /g at a temperature of 25.degree. C. Air,
having a 6 percent relative humidity and containing 340 ppm of CO.sub.2
was fed through the adsorption unit at the linear velocity of 0.1
m/sec. Breakthrough concentrations were assumed: for water --0.1
ppm, for carbon dioxide --1 ppm. All measurements were carried out
up to the first component breakthrough. Thus, the water capacity
is determined taking into account the time before carbon dioxide
breakthrough occurs.
The results for dynamic water and carbon dioxide capacity of the
samples tested are reported in Table 2.
TABLE 2 Crystal Potassium Dynamic Capacity, % w. Size, Content,
Carbon Example .mu.m % equiv. Water Dioxide 3 1-3 2.4 3.30 0.66
8 4-6 2.2 2.84 0.54
As in Example 9 the adsorbent, according to the present invention,
having crystal sizes predominantly in the range of 1-3 .mu.m, with
other similar characteristics, demonstrated substantially higher
dynamic capacity for both impurities recovered than the adsorbent
with conventional 4-6 .mu.m crystal size.
EXAMPLE 10
Comparative
4 kg of low-silica faujasite powder from Example 1 was exchanged
with 2.2 N and 3.0 N solutions of NaCl as described in Examples
2 and 3. 0.8 kg of the product produced having residual content
of K.sup.+ ions of 2.5 percent (equiv.) was added to 0.2 kg of attapulgite,
Minugel 400 in a worm disintegrator, Strand F4 for 1 hour. The
homogenous blend obtained was mixed with 280 ml of water and the
resulting paste was extruded through a spinneret to produce extrudates
with a diameter of 1.6 mm. The extrudates were cured at ambient
temperature for 24 hours, dried at 110.degree. C. for 2 hours and
calcined at 250.degree. C., 350.degree. C. and 600.degree. C. for
1 hour at each temperature.
The products produced had a macropore (1000-10000 .ANG.) volume
equal to 0.34 cm.sup.3 /g., according to mercury porosimetry evaluation.
EXAMPLE 11
According to the Invention
The operating procedure of Example 10 was repeated except the low
silica faujasite-attapulgite blend was mixed with 220 ml of a 5
percent water solution of polyacrylamide 1500. The procedures for
extrusion, extrudate drying and calcination from Example 10 were
repeated.
The product produced had a macropore volume (1000-10000 .ANG.)
of 0.43 cm.sup.3 /g.
EXAMPLES 12 and 13
Comparative
Low potassium, low silica faujasite powder from Example 10 was
extruded through a spinneret (1.6 mm diameter) with 20 percent of
kaolin Sperse 100 by weight (Example 12) and 20 percent of activated
alumina binders, by weight (Example 13). Binders were peptized preliminarily
with 1.5 percent of poly-(2-carboxyethyl)-acrylate 170 based on
binder content. The remaining extrusion, extrudate drying and calcination
procedures were the same as shown in Example 10.
The adsorbents produced had macropore volume (1000-10000 .ANG.):
Example 12--0.39 cm.sup.3 /g;
EXAMPLE 14
According to the Invention
Activated alumina AP-22 from Porocel, having surface area of 270
m.sup.2 /g and median particle size of 6 .mu.m, was used as a binder.
0.2 kg of the binder was treated with 1 percent of ammonium polycarboxylate
40 and added to 0.8 kg of low potassium, low silica faujasite powder
from Example 10. The blend was extruded, dried and calcined in the
same manner as in Example 10.
A macropore volume of 0.47 cm.sup.3 /g characterized the product
produced.
EXAMPLE 15
Equilibrium and Dynamic Capacity Test
The adsorbents of Examples 10 to 14 were tested for equilibrium
and dynamic adsorption capacity. Carbon dioxide equilibrium adsorption
value was measured at 25.degree. C. and CO.sub.2 partial pressure
1.8 torr employing the instrumentation and method described in Example
5. Water and carbon dioxide dynamic capacities were measured by
means of the testing procedure of Example 9.
The results are reported in Table 3.
TABLE 3 ADSORPTION CAPACITY Macropore Equilibrium Dynamic Volume,
Carbon Dioxide Water Carbon Dioxide Example cm.sup.3 /g mmol/g %
w. % w. cm.sup.3 /g % w. 10 0.34 1.33 5.9 2.48 2.47 0.49 11 0.43
1.29 5.7 3.42 3.42 0.68 12 0.39 1.34 5.9 3.09 3.07 0.61 13 0.36
1.32 5.8 3.16 2.66 0.53 14 0.47 1.34 5.9 3.48 3.65 0.72
Table 3 shows that regardless of the type and chemical composition
of the binder, the adsorbent dynamic capacity is controlled by its
macroporosity. The larger the macropore volume, the higher the dynamic
capacity for adsorption of carbon dioxide. Due to enhanced carbon
dioxide adsorption capacity, the adsorbents are capable of adsorbing
significant quantities of water vapor before CO.sub.2 breakthrough.
This leads to an increase of the dynamic water capacity up to 40-50%
from potential.
As is apparent, the efficiency of the adsorbents, according to
the present invention, is 45 percent better than the prior art adsorbent
performance. Contrary to the prior art, the present invention adsorbents
can be employed in PSA and TSA units independently, without a supplemental
desiccant bed.
EXAMPLE 16
Comparative
This Example compares calcium-exchanged sodium LSF adsorbents with
those of the invention. Carbon dioxide adsorption ability of Group
2A forms of low-silica faujasite as claimed in U.S. Pat. No 5531808
was compared with adsorbents of the present invention.
0.5 kg of the sodium low silica faujasite beads from Example 4
having 0.3 percent of residual potassium ions, were exchanged with
5 l of 1N solution of calcium chloride at ambient temperature. The
ICP analysis showed that final product contained: Ca.sup.2+ --65
percent, Na.sup.+ --35 percent and K.sup.+ --0.16 percent.
EXAMPLE 17
Isotherm Adsorption Test
The adsorbent of Example 16 as well as other conventional adsorbents,
standard molecular sieves 5A (U.S. Pat. No 3981698), 10A (U.S.
Pat. No 4986835) and 13X (U.S. Pat. No 5156657) were tested
in a carbon dioxide adsorption equilibrium. The adsorbent according
to the present invention of Example 4 was also tested. Isotherms
of adsorption were measured employing the instrumentation and procedure
of Example 5.
The results obtained are reported in FIG. 2.
It is clear from FIG. 2 that the adsorbent of the present invention
displays carbon dioxide adsorption capability well above prior art
adsorbents. This improvement is shown over the full range of carbon
dioxide partial pressures including very low pressures. The results
also show that the substitution of sodium cations in the low-silica
faujasite structure by larger calcium cations leads to an appreciable
loss of adsorption capacity.
EXAMPLE 18
Dynamic Capacity Test
The adsorbents of Examples 14 and 16 along with the prior art adsorbents:
molecular sieves 5A, 10A and 13X were tested in air purification
for water and carbon dioxide adsorption. The technique and method
of Example 9 were employed. The results are reported in Table 4.
TABLE 4 Molecular DYNAMIC CAPACITY, % Example Sieve Water Carbon
Dioxide 14 NaLSF (0.3% K.sup.+) 3.48 0.72 16 CaNaLSF (65% Ca.sup.2+)
1.52 0.39 5A (94.8% Ca.sup.2+) 2.40 0.46 10A (CaX) 1.64 0.41 13X
(NaX) 1.43 0.37
The adsorbents, according to the present invention, in comparison
to previous adsorbents, display a superior performance in the dynamic
process of air purification. Their capacity for water vapor and
carbon dioxide adsorption of 1.7-2.0 times exceeds the adsorption
characteristics of conventional adsorbents. As in Example 15 the
results of Table 4 confirm that the adsorbent, according to the
invention, provides reliable and extensive purification of gases
without the use of a supplemental desiccant bed. This provides an
opportunity for a substantial decrease in capital investments and
operational costs using the present adsorbent in commercial PSA
and TSA gas purification units.
Accordingly, the invention provides a highly effective, reliable
and simple adsorbent for carbon dioxide and water vapor that can
be used in TSA and/or PSA gas purification plants to increase commercial
performance. The adsorbent can be used at new or existing plants.
Furthermore, the combination of high sodium, low-silica faujasite
with very low content of residual potassium ions has a number of
advantages: (1) it provides a high level of carbon dioxide and water
vapor adsorption for gas purification; (2) it allows the effective
adsorption of carbon dioxide at ambient temperatures and low partial
pressures of carbon dioxide that reduces capital and operation costs
in air pre-purification and inorganic gas manufacturing; (3) it
provides satisfactory dynamics for both Pressure Swing Adsorption
and Temperature Swing Adsorption processes; (4) it allows gas purification
units in one bed without the supplemental use of desiccants, such
as silica gel, activated alumina, molecular sieves 3A, 4A, etc.
(5) it provides reliable and efficient technology for adsorbent
preparation and manufacturing.
The adsorbent can be formed as spheres, beads, cylinders, extrudates,
pellets, granules, rings, multileaves, honeycomb or in monolith
form.
While the invention has been described in terms of various preferred
embodiments, these should not be construed as limitations on the
scope of the invention. Many other variations, modifications, substitutions
and changes may be made without departing from the spirit thereof. |