Abstrict The present invention provides for a process of reducing the polarity
on the internal surfaces of various zeolites having a SiO.sub.2
to Al.sub.2 O.sub.3 ratio of at least about 3 and an average pore
diameter size within the range of from about 4 to about 10 angstroms.
The modified zeolites in accordance with the present invention are
prepared by heating the starting zeolite in an aqueous medium also
containing an acid or a source of ammonium ions to at least partially
dealuminize the zeolite and thereby increase the ratio of silicon
to aluminum present in the tetrahedral structure. At the same time,
the process provides for hydrogen ion exchange with respect to those
zeolites which contain significant amounts of metallic cation in
the structure thereby replacing the bulky metallic cations with
less bulky hydrogen ions and thereby increasing the water adsorptive
capacity of the zeolite. Achievement of the appropriate equilibrium
between reduced surface polarity on the one hand and increased sorptive
capacity of the zeolite on the other hand gives rise to zeolite
materials having an isotherm with a separation factor within the
range of from about 0.07 to about 0.1 which renders the modified
zeolite an ideal desiccant for gas fired air conditioning and dehumidification
equipment.
Claims What is claimed is:
1. In a desiccant cooling system process wherein a moving stream
of air is progressively and sequentially adiabatically dehumidified
by passing it through a desiccant material to produce warm dry air,
cooled to produce cool dry air, and adiabatically humidified to
produce cool nearly saturated air for introduction into the space
to be cooled, the improvement which comprises utilizing as a desiccant
material particles of a modified aluminosilicate having an isotherm
separation factor within the range of from about 0.07 to about 0.1
2. The process of claim 1 wherein said aluminosilicate is modified
by subjecting an aluminosilicate having a silica to alumina ratio
of at least about 3 and an average pore diameter size within the
range of from about 4 to about 10 angstroms to treatment with an
aqueous solution of an acid or a source of ammonium ions to exchange
at least a portion of any metal ions present in the aluminosilicate
with protons and to remove at least a portion of the aluminum present
in the aluminosilicate.
3. The process of claim 2 wherein said acid is an organic or inorganic
acid.
4. The process of claim 3 wherein said acid is ethylenediaminetetracetic
acid.
5. The process of claim 4 wherein said ethylenediaminetetracetic
acid is present in said aqueous solution at a level corresponding
to a molar ratio of acid to aluminum within the range of from about
0.5 to about 1.0.
6. The process of claim 3 wherein said acid is present in said
aqueous solution at a molar concentration within the range of from
about 0.1 to about 20 N.
7. The process of claim 6 wherein said acid is hydrochloric acid.
8. The process of claim 2 wherein said aqueous solution contains
a source of ammonium ions and wherein said protonic ion exchange
is carried out by thermal decomposition of the ammonium exchanged
aluminosilicate.
9. The process of claim 8 wherein said ammonium ion source is present
in said aqueous solution at a molar concentration within the range
of from about 0.5 to about 2.0 N.
10. The process of claim 9 wherein said source of ammonium ions
is ammonium chloride.
11. The process of claim 1 wherein said modified aluminosilicate
has a water adsorption capacity within the range of from about 10
to about 40% weight, based on the dry weight of aluminosilicate.
12. The process of claim 1 wherein said modified aluminosilicate
has a heat of adsorption value within the range of from about 11
to about 13 kcal/mole.
13. The process of claim 2 wherein said aluminosilicate has a silica
to alumina ratio of from about 4 to about 20.
Description BACKGROUND OF THE INVENTION
The present invention relates to desiccants based on modified zeolites
having an isotherm separation factor within the range of from about
0.07 to about 0.1 and their use in gas-fired cooling and dehumidification
equipment.
Desiccant cooling systems employ an open cycle to process water
vapor between a conditioned space and the environment. The use of
thermal energy creates a chemical potential that can be used to
produce a cooling effect. If the air can be made dry enough, an
evaporative cooler will produce air that is as cold as a conventional
electric air conditioner. The overall performance of these systems
relies on the quality of the thermal energy input (availability)
and the environment as both a cold sink and as a source of chemical
potential (unsaturated air). Most solid desiccant cooling cycles
consist of a desiccant dehumidifier, a sensible heat exchanger and
two evaporative coolers. There are two important modes of operation:
1) The ventilation mode where outdoor air is processed to produce
low enthalpy air for the cooled space, and 2) The recirculation
mode where air from the cooled space is processed to maintain low
enthalpy air conditions in the space.
A schematic of the ventilation mode (a) and a psychrometric representation
of the cycle (b) are shown in FIG. 1. Ambient air at (1) is adiabatically
dehumidified by the desiccant (DH). The hot, dry air at (2) is cooled
by the sensible heat exchanger (HE) to create dry cool air at (3).
This air is then adiabatically humidified by the evaporative cooler
(EC) to produce cold, nearly saturated air at (4) that enters the
building. Simultaneously, an equal amount of building air at (6)
is adiabatically humidified to (7). This produces the cold sink
for the dry air (3). The air is then heated by the same heat exchanger
to (8). This is an attempt to recuperate as much of the heat of
sorption as possible from the dehumidification process. The enthalpy
of the air must now be increased to (9) by a thermal energy input
such as natural gas heating. This air is then passed through the
desiccant in order to regenerate it. The warm, humid air that exits
the desiccant at (10) is then exhausted to the atmosphere.
A schematic of the recirculation mode (a) and a psychrometric representation
of the cycle (b) are shown in FIG. 2. This time ambient air at (1)
is adiabatically humidified to (2). This air now becomes the cold
sink for the cycle. It is heated by the sensible heat exchanger
to (3) and then further heated to (4) by an external thermal input
such as natural gas heating. This air is then used to regenerate
the desiccant material. The warm, humid air that exits the desiccant
at (5) is returned to the environment. Simultaneously, room air
at (6) is adiabatically dehumidified by the desiccant to produce
warm, dry air at (7). This air is then cooled with the humidified
and cooled ambient air to create cool, dry air at (8). The air is
then adiabatically humidified to produce the cold, near saturated
air at (9) which is returned to the building.
Both of the operational modes previously described operate on a
continuous basis. That is, both the dehumidification and the regeneration
processes occur at different parts of the cycle simultaneously.
In order to accomplish this, the desiccant is deployed into a rotating
wheel or drum that continuously cycles the desiccant between the
dehumidification and regeneration air streams. The heat exchanger
may be of a rotating or static design and the evaporative coolers
must be distinctly separate units.
Three important interrelated performance parameters that determine
the viability of desiccant cooling systems include:
1. Thermal coefficient of performance (COP).
The thermal COP is the nondimensional ratio of the amount of cooling
output that is produced by a given amount of external energy input.
For comparative purposes, COP is often quoted at Air Conditioning
and Refrigeration Institute (ARI) rating conditions of 95.degree.
F. dry bulb and 75.degree. F. wet bulb outdoor temperatures and
80.degree. F. dry bulb and 67.degree. wet bulb indoor temperatures.
2. Parasitic electric energy efficiency ratio (EER).
The EER is a measure of the amount of cooling, in thermal units
(BTUs), that is produced by a given amount of parasitic electric
energy input for fans, pumps, etc. in electrical units (Watts).
3. Specific cooling capacity (SCC).
This factor is defined as tons of cooling capacity per 1000 cubic
feet per minute of supply air.
A combination of higher COP values and increased SCC values yields
desiccant cooling systems that are more efficient and cost effective
than state of the art devices. It has been determined that one of
the primary factors affecting these values is the identity of the
particular desiccant material employed in the system in terms of
its psychrometric performance in the dehumidifier section of the
system. For both the dehumidification and regeneration processes,
there are two fundamental wavefronts that occur. The first and fastest
wave is primarily a thermal front that is most affected by the total
amount of thermal heat capacity associated with the dehumidifier.
The second and slowest wave is the main concentration wavefront
with strong associated thermal effects. Without getting into the
details of the physical chemistry involved, it has been postulated
that the primary function of an ideal desiccant material in an open
cycle desiccant cooling system should be to produce the sharpest
possible concentration wavefronts for both the dehumidification
and regeneration processes.
Properties of ideal desiccants effecting these characteristics
have been determined to include low heat of adsorption, high water
adsorption capacity, high diffusivity of water, high chemical and
physical stability towards heat, and most importantly, the shape
of the desiccant isotherm.
It has been determined that the ideal shape of the desiccant isotherm
for use in gas fired cooling/dehumidifier systems has a separation
factor ranging from 0.07 to about 0.1 in accordance with the isotherm
equation: ##EQU1## wherein x is the normalized loading fraction
of water, P is the relative vapor pressure of water and R is the
separation factor.
Several adsorption isotherms with different identified separation
factors are shown in FIG. 3. Extreme Brunauer Type I isotherm is
shown by the curve designated 0.01 whereas linear and less than
linear isotherms are shown by the line designated 1.0 and the curve
designated 10.0. The ideal isotherm shape is depicted as the area
between the curves designated 0.07 and 0.1.
Most of the commercially available desiccant materials have not
been developed for the specific purpose of providing space cooling.
In most present day applications, the necessity of achieving efficient
regeneration as well as deep drying of the air has not been a consideration.
The requirement of attaining the very sharp adsorption wavefronts
associated with molecular sieves along with the more efficient regeneration
characteristics of the desiccant are what makes this application
unique commercially. It is not surprising therefore, that none of
the commercially available desiccants match the properties needed.
The isotherms of various commercial and laboratory-developed desiccants
are shown in FIG. 4. The trend is to see linear or nearly linear
(Brunauer Type II) isotherms or extreme (Brunauer Type I) isotherms,
as compared with the nearly ideal isotherm shape (moderate type
I or Langmuir) designated as the "desired shape" for the
purposes of this invention. This isotherm has a separation factor
(R) of about 0.1.
Zeolites, both natural and synthetic, have been demonstrated in
the past to have sorbent capabilities for water. Zeolites are crystalline,
hydrated aluminosilicates with three-dimensional framework structure.
The aluminosilicate framework is built up such that they possess
cavities and channels of various dimensions depending upon the type
of zeolite. In the structure of zeolite, Al.sup.3+ substitutes for
Si.sup.4+, and hence develops a net negative charge which is balanced
by different alkali metal and alkaline earth cations. It has been
established that the charge-balancing cations of one type can be
replaced by another (ion exchange), in most cases, without changing
the crystalline structure. Because of small pore size, and presence
of negative charge due to Al.sup.3+ substitution and alkali cations
in the cavities, zeolites have a large affinity towards water molecules.
The net effect is to exhibit an extreme Type I (Langmuir) adsorption
isotherm with water as illustrated in FIGS. 3 and 4. For a constant
polarity on the surface of porous solids, the effect of reduced
pore size is to give an enhanced adsorption at low relative pressure
(extreme Langmuir type isotherm) due to an overlapping of potential
fields from the neighboring walls of the pore. On the other hand,
for a constant pore sized material, increased polarity on the pore
surfaces gives an enhanced adsorption of water at low relative pressures
and vice versa. For zeolites, ion exchange with various alkali cations
effectively reduces the volume of readsorption in accordance with
the size of the ions, but not the affinity between zeolite and water
molecules provided the structure is not distorted by ion exchange.
The effect is again to exhibit an extreme Type I isotherm as illustrated
in FIG. 3.
It is known in the prior art that Zeolite materials may be dealuminized
to increase the ratio of silica to aluminum and may also be subjected
to ion exchange reactions to replace the charge-balancing metal
ions with protons, i.e., hydrogen ions.
For example, U.S. Pat No. 4740292 discloses a dealuminization
process comprising reacting a faujasite type zeolite or zeolite
beta with a strong mineral acid or organic acid at temperatures
up to boiling to extract aluminum. The reference also indicates
that the process also at least partially replaces metal ions present
in the crystalline structure with protons. The purpose of the treatment
is to render mixtures of the zeolites more effective as catalytic
cracking catalysts for hydrocarbon feedstocks.
U.S. Pat. No. 4701431 teaches a process for dealuminization of
zeolite, such as zeolite Y, by treatment with ethylenediaminetetracetic
acid or a derivative thereof to dealuminize the zeolite, followed
by an ion exchange reaction wherein the zeolite cations are at least
partially replaced with rare earth metal cations. Once again the
treatment is said to render those materials suitable as catalysts
for various chemical processes or as sorbents.
Similar processes are disclosed in U.S. Pat. Nos. 3551353 and
4477336.
U.S. Pat. No. 3140251 discloses a process for enhancing the catalytic
activity of aluminosilicates comprising reacting the aluminosilicate
with an ammonium compound as a source of ammonium ions, followed
by heat treatment of the resulting complex to decompose the ammonium
complex to provide the hydrogen ion form of the zeolite.
While these and other prior art disclosures generally recognize
that the properties of zeolite materials may be tailored by altering
the chemical composition to render them more effective in given
catalytic or molecular sieve applications, none of these disclosures
has an objective of altering zeolite materials to render them more
suitable for use as desiccant materials in gas fired cooling and
air conditioning applications or the achievement of ideal desiccant
materials for such applications having an isotherm separation factor
within the range of from about 0.07 to about 0.1.
Accordingly, it is an object of this invention to provide zeolite
materials which have been chemically modified to achieve an isotherm
with a separation factor range of from about 0.07 to about 0.1.
Another object of this invention is to provide modified zeolite
materials which are ideally suited for use as desiccants in gas
fired open space air-conditioning and dehumidification systems.
SUMMARY OF THE INVENTION
The present invention provides for a process of reducing the polarity
on the internal surfaces of various zeolites having a SiO.sub.2
to Al.sub.2 O.sub.3 ratio of at least about 3 and an average pore
diameter size within the range of from about 4 to about 10 angstroms.
The modified zeolites in accordance with the present invention are
prepared by heating the starting zeolite in an aqueous medium also
containing an acid or a source of ammonium ions to at least partially
dealuminize the zeolite and thereby increase the ratio of silicon
to aluminum present in the tetrahedral structure. At the same time,
the process provides for hydrogen ion exchange with respect to those
zeolites which contain significant amounts of metallic cation in
the structure thereby replacing the bulky metallic cations with
less bulky hydrogen ions and thereby increasing the water adsorptive
capacity of the zeolite. Achievement of the appropriate equilibrium
between reduced surface polarity on the one hand and increased sorptive
capacity of the zeolite on the other hand gives rise to zeolite
materials having an isotherm with a separation factor within the
range of from about 0.07 to about 0.1 and a heat of adsorption within
the range of from about 11 to about 13 K cal/mole which renders
the modified zeolite an ideal desiccant for gas fired air conditioning
and dehumidification equipment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a ventilation mode of a desiccant
cooling system (a) and a psychrometric representation of the cooling
cycle (b).
FIG. 2 is a schematic illustration of a recirculation mode of a
desiccant cooling system (a) and a psychrometric representation
of the cooling cycle (b).
FIG. 3 is a plot showing extreme, ideal and linear isotherms for
desiccant materials.
FIG. 4 is a plot of adsorption isotherms for various identified
desiccant materials.
FIG. 5 is a schematic illustration of a computer-interfaced sorption
measurement apparatus.
FIGS. 6 8 9 10 11 12 13 14 15 16 19 20 21 22 and 23
are plots of water adsorption and desorption isotherms of various
materials described herein.
FIGS. 7 and 18 are plots of X-ray diffraction patterns of various
materials described herein.
FIG. 17 is a plot of isosteric heat of adsorption of a modified
type Y zeolite.
DETAILED DESCRIPTION OF THE INVENTION
As indicated above, zeolite materials which may be modified in
accordance with the process disclosed herein to produce ideal desiccants
are those materials having an initial molar ratio of silica to alumina
of at least about 3 and a pore diameter of from about 4 to about
10 angstroms, more preferably from about 5 to about 8 angstroms.
These are materials which in their "as synthesized" or
"as purchased" form yield an extreme type I isotherm when
the isothermal properties are evaluated, as illustrated in FIG.
3 but which may be chemically and structurally tailored in accordance
with the processes hereinafter described to provide the ideal isotherm
having an isothermal separation factor within the range of from
about 0.07 to about 0.1 as also illustrated in FIG. 3.
The aluminosilicates treated in accordance with the invention include
a wide variety of aluminosilicates, both natural and synthetic,
which have an amorphous or crystalline structure. These aluminosilicates
can be described as a three dimensional framework of SiO.sub.4 and
AlO.sub.4 tetrahedra in which the tetrahedra are cross-linked by
the sharing of oxygen atoms whereby the ratio of the total aluminum
and silicon atoms to oxygen atoms is 1:2. In their hydrated form
the aluminosilicates may be represented by the formula: ##EQU2##
wherein M is a cation which balances the electrovalence of the tetrahedra,
n represents the valence of the cation, w is the moles of SiO.sub.2
and y the moles of H.sub.2 O. The cation may be any or more of a
number of metal ions depending on whether the aluminosilicate is
synthesized or occurs naturally. Typical cations include sodium,
lithium, potassium, silver, magnesium, calcium, zinc, barium, iron,
manganese, and other cations as known in the art.
The cation may also include hydrogen ions, or ammonium ions which
may be subsequently thermally decomposed to form the hydrogen ion
form of the aluminosilicate. The cation may also include mixtures
of two or more of the ions recited above. The degree of hydration
of the aluminosilicate may be such that y in the above formula ranges
from 0 to about 30. Hydrated aluminosilicates wherein y ranges from
about 20 to about 30 are more preferred.
The most preferred aluminosilicate starting materials are crystalline
materials having a silica to alumina ratio of at least about 4
and more preferably from about 4 to about 20. The higher the initial
ratio of silica to alumina within this range, the more heat stable
the zeolite and the less rigorous will be the dealuminization treatment
required to achieve the reduction in polarity needed to approach
the ideal isotherm properties.
Examples of suitable natural and synthetic alumino silicates which
may be employed as the starting material for the purposes of this
invention include synthetic zeolites designated as Zeolites Y, l3Y,
hydrated Na-Y, 100 zeolon Na, 100 zeolon H, and naturally occurring
aluminosilicates such as erionite, mordenite and clinoptilolite.
Dealuminization and hydrogen ion exchange may be accomplished by
dispersing particles of the starting aluminosilicate material in
an aqueous medium containing an acid or a compound providing a source
of ammonium ions, and heating this mixture at a temperature ranging
from about 150.degree. to 220.degree. F. for a period of time sufficient
to exchange as much of the original metal cation as possible with
protons or ammonium ions. Where the ion exchange medium is a protonic
acid, the treatment simultaneously leaches aluminum from the polymorphic
structure thereby reducing the polarity of the aluminosilicate.
Where the ion exchange medium is a source of ammonium ions, the
treatment initially replaces the original cation with ammonium ions.
Subsequent heating of the ammonium exchanged material at temperatures
within the range of 400.degree. to 800.degree. C. and preferably
in a high moisture environment (steam) decomposes at least a portion
of the ammonium form to the hydrogen form. The proton in turn attacks
the aluminum present in the structure, and leaches it out. The steaming
process also tends to solubilize a portion of the SiO.sub.2 to the
amorphous form allowing it to migrate and occupy the positions in
the tetrahedra previously occupied by aluminum. A final calcination
step at temperatures of from 400.degree. to 1000.degree. C. converts
essentially all of the residual ammonium complex to the hydrogen
form.
The concentration of acid present in the aqueous solution may vary
between about 0.1 to about 20 molar, more preferably from 0.1 to
12 molar where the acid is a strong acid. The amount of acid should
be in excess of the amount which would be theoretically required
to exchange all of the original cations present in the aluminosilicate
with protons, even though in many cases it is not possible to exchange
100% of these original cations. Expressed on the basis of moles
of acid per mole of aluminum present in the zeolite, the preferred
acid concentration ranges from about 0.5 to about 1.0 moles of acid
per mole of aluminum.
Where the ion exchange material is a source of ammonium ions, the
concentration of the ion exchange material in aqueous solution may
be within the range of from about 0.5 to 2 molar and is preferably
at least equivalent to the theoretical amount required to exchange
all of the original cations present in the original aluminosilicate
with ammonium ions, more preferably from about 3 to 10 times the
amount so required.
The concentration of aluminosilicate contacted with the ion exchange
solution may generally range from about 1 to about 25% by weight,
more preferably from about 2 to 15% by weight.
In many instances, particularly involving ammonium exchange, a
single ion exchange treatment is not sufficient to produce the desired
degree of exchange and dealumination. The exchange process must
therfore be repeated for a number of cycles, generally ranging from
2 to about 10 total cycles. This may be accomplished by separating
the aluminosilicate from the aqueous medium after each treatment,
washing it with water and then repeating the ion exchange process
with fresh ion exchange solution.
Treatment times will vary as a function of the nature of the starting
zeolite and the identity of the ion exchange material. In general,
each treatment cycle is preferably carried out by heating the ion
exchange material at reflux for a period of from about 1 to about
6 hours, more preferably from about 2 to about 4 hours.
A wide variety of acidic materials may be utilized in the dealuminization/ion
exchange process of this invention. These include amine acids such
as ethylenediaminetetracetic acid (H.sub.4 EDTA) as well as derivatives
thereof such as diethylene triamine pentaacetic acid, nitrilotriacetic
acid and the like.
Representative inorganic acids which can be employed include acids
such as hydrochloric acid, hypochlorous acid, chloroplatinic acid,
sulfuric acid, sulfurous acid, hydrosulfuric acid, peroxydisulfonic
acid (H.sub.2 S.sub.2 O.sub.3), peroxymonosulfuric acid (H.sub.2
SO.sub.5), dithionic acid (H.sub.2 S.sub.2 O.sub.6), sulfamic acid
(H.sub.2 NSO.sub.3 H), amidodisulfonic acid (NH(SO.sub.3 H).sub.2)
chlorosulfuric acid, thiocyanic acid, hyposulfurous acid (H.sub.2
S.sub.2 O.sub.4) pyrosulfuric acid (H.sub.2 S.sub.2 O.sub.7), thiosulfuric
acid (H.sub.2 S.sub.2 O.sub.3), nitrosulfonic acid (HSO.sub.3.NO),hydroxylamine
disulfonic acid ((HSO.sub.3).sub.3 NOH), nitric acid, nitrous acid,
hyponitrous acid, carbonic acid and the like.
Typical organic acids which find utility in the practice of the
invention include the monocarboxylic, dicarboxylic and polycarboxylic
acids which can be aliphatic, aromatic or cycloaliphatic in nature.
Representative aliphatic monocarboxylic, dicarboxylic and polycarboxylic
acids include the saturated and unsaturated, substituted and unsubstituted
acids such as formic acid, acetic acid, bromoacetic acid, propionic
acid, 2-bromopropionic acid, 3-bromopropionic acid, lactic acid,
n-butyric acid, isobutyric acid, crotonic acid, n-valeric acid,
isovaleric acid, n-caproic acid, ocnanthic acid, pelargonic acid,
capric acid, undecyclic acid, lauric acid, myristic acid, palmitic
acid, stearic acid, oxalic acid, malonic acid, succinic acid, glutaric
acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic
acid, alkylsuccinic acid, alkenylsuccinic acid, maleic acid, fumaric
acid, itaconic acid, citraconic acid, mesaconic acid, plutonic acid,
muconic acid, ethylidene malonic acid, isopropylidene malonic acid,
allyl malonic acid and the like.
Representative aromatic and cycloaliphatic monocarboxylic, dicarboxylic
and polycarboxylic acids include 12-cyclohexane-dicarboxylic acid,
14-cyclohexane-dicarboxylic acid, 2-carboxy-2-methylcyclohexaneacetic
acid, phthalic acid, isophthalic acid, terephthalic acid, 18-naphthalenedicarboxylic
acid, 12-naphthalenedicarboxylic acid, tetrahydrophthalic acid,
3-carboxy-cinnamic acid, hydrocinnamic acid, pyrogallic acid, benzoic
acid, ortho, meta and para-methyl, hydroxyl, chloro, bromo and nitro-substituted
benzoic acids, phenylacetic acid, mandelic acid, benzylic acid,
hippuric acid, benzenesulfonic acid, toluenesulfonic acid, methanesulfonic
acid and the like.
Representative ammonium compounds which can be employed include
ammonium chloride, ammonium bromide, ammonium iodide, ammonium carbonate,
ammonium bicarbonate, ammonium sulfate, ammonium hydroxide, ammonium
sulfide, ammonium thiocyanate, ammonium dithiocarbamate, ammonium
peroxysulfate, ammonium acetate, ammonium tungstate, ammonium molybdate,
ammonium benzoate, ammonium borate, ammonium carbamate, ammonium
sesquicarbonate, ammonium chloroplumbate, ammonium citrate, ammonium
dithionate, ammonium fluoride, ammonium galate, ammonium nitrate,
ammonium nitrite, ammonium formate, ammonium propionate, ammonium
butyrate, ammonium valerate, ammonium lacate, ammonium malonate,
ammonium oxalate, ammonium palmitate, ammonium tartarate and the
like. Still other ammonium compounds which can be employed include
tetraalkyl and tetraaryl ammonium salts such as tetramethylammonium
hydroxide, and trimethylammonium hydroxide. Other compounds which
can be employed are nitrogen bases such as the salts of guanidine,
pyridine, quinoline, etc.
The precise process conditions and concentration of ion exchange
material in solution required to produce the modified zeolites of
this invention having ideal desiccation properties for gas fired
cooling systems will vary depending upon the identity of the starting
zeolite material and its chemical and physical structure. As illustrated
in the following examples, a certain amount of trial and error experimentation
within the parameters set forth above may be required to achieve
a modified material having the ideal isotherm separation factor
of from 0.07 to 0.1 and a relatively low heat of adsorption within
the range of from about 11 to about 13 Kcal/mole.
In the following examples, the polarity on the zeolite surfaces
was reduced by dealuminization of the structure by HCl and H.sub.4
EDTA exchange, and by NH.sub.4.sup.+ exchange followed by steam
treatment. Since the stability of zeolites towards acid depends
on the Si/Al ratio in the structure, the concentrations and the
type of treatments (whether H.sub.4 EDTA or HCl) to be employed
depends on the type of zeolite. The preferred concentration of acid
may be varied from 0.1 to 12 M and may be present at a level of
from about 0.5 to about 1.0 moles per mole of aluminum in the zeolite.
The ideal shape of the water isotherms was achieved both without
calcination and with samples calcined at 600.degree. C., and for
crystalline samples, the x-ray crystallinity was also preserved
up to 1000.degree. C. (1832.degree. C.) The adsorption capacities
of these materials covers a range from 10 to 40% on the weight basis
of the sample, and in some cases, the capacities were greater than
those of the original samples. The amorphous sample was similar
to silica gel as seen in x-ray diffractograms, but the micropore
structure of the parent zeolite was preserved. This invention thus
provides for x-ray amorphous, crystalline and composites of both
amorphous and crystalline materials which give moderate type I isotherms
for water adsorption.
Water sorption isotherms were measured by a volumetric method at
20.degree. or 25.degree. C. using a computer interfaced sorption
apparatus, such as a schematically illustrated in FIG. 5. The molar
quantity of water vapor sorption (n) by the sample can be calculated
on the basis of the ideal gas equation:
wherein .DELTA. p is the pressure difference in the system, Vo
is the total volume of the system excluding the sample volume, R
is the gas constant, and T is the absolute temperature of the vapor.
About 50-70 mg of precalcined sample is degassed at 200.degree.
C. for 5-10 hr prior to the sorption measurements. The constant
volume in which the sample was exposed to water vapor was about
350 ml. The pressure was recorded by a high-accuracy pressure transducer
(MKS Instrument, Inc., Model 390H). The equilibrium conditions were
defined by the pressure diffrence of 0.05 Torr during 240 seconds,
that is, if the pressure difference during 240 seconds <0.05
Torr, equilibrium was assumed. This setting for equilibrium corresponds
to the increase in the sorption amount of <20 micrograms in 240
seconds. For desorption measurements, the equilibrium conditions
are defined by the pressure difference of 0.05 Torr in 360 seconds.
The apparatus is precise and sensitive, inasmuch as the pressure
transducer can detect a pressure difference of +3 micrograms of
water.
Sorption was measured at 20.degree., 30.degree., and 40.degree.
C. for the estimation of the heat of adsorption. The isosteric heat
of adsorption (q.sub.st) is calculated according to the following
equation:
where P, n, q.sub.st and T are the equilibrium pressure, amount
of adsorption, isosteric heats of adsorption and absolute temperature
(K), respectively. The plots of lnP vs. l/T at different amounts
of sorption (n) showed linear relationships, and the value q.sub.st
is calculated from the slope of the plot.
The following examples are illustrative of the invention.
EXAMPLE 1
Two grams of zeolite 13Y (SiO.sub.2 /Al.sub.2 O.sub.3 =4.8) were
heated at 90.degree.-100.degree. C. (194.degree.-212.degree. F.)
with 40 ml of 0.25 and 0.5 M HCl respectively for 4 hr. After 4
hr., the samples were washed 5 times by centrifugation with 35 ml
of deionized water. After the last wash, the samples were dried
overnight at 60.degree. C. (140.degree. F.). Samples were calcined
at 200.degree.-500.degree. C. (392.degree.-932.degree. F.) for 20
hr. and degassed at 200.degree. C. (392.degree. F.) for 4-10 hr
prior to water adsorption measurements at 20.degree. (68.degree.
F.) or 25.degree. C. (77.degree. F.) The adsorption capacities were
found to be 27 and 18-20% for 0.25 and 0.5 M HCl treated samples
respectively as compared to 32% water absorption capacity of the
original sample. The water adsorption isotherms of the 0.5 M HCl
treated sample which was degassed at 300.degree. C. (572.degree.
F.) along with the ideal curve are given in FIG. 6. X-ray data indicated
that the sample treated with 0.25 M HCl was a composite of both
the crystalline and amorphous phase, whereas the sample treated
with 0.5 M HCl was completely amorphous and was similar to silica
gel. Scanning electron micrographs showed that the amorphous product
preserved the crystal morphology of the original sample.
EXAMPLE 2
Six grams of hydrated NaY (SiO.sub.2 /Al2O.sub.3 =5.3) were slurried
in 135 ml of deionized water. To the zeolite-water slurries, various
amounts of H.sub.4 EDTA corresponding to H.sub.4 EDTA/Al molar ratio
of X=0.5 0.6 0.75 0.9 and 1.0 were separately added and refluxed
for 2 hr (Method A). In another set of experiments, the zeolite-water-H.sub.4
EDTA slurry was centrifuged and decanted after refluxing for the
first 2 hr. To this solid 135 ml of water was added and the mixture
was refluxed for an additional 2 hr. (Method B). After refluxing,
the solids were washed 4 times with deionized water using a centrifuge.
The supernantant solutions were collected for the determination
of Na, Al, and Si. Solids were dried at 60.degree. C. overnight.
XRD patterns of products prepared by Method B using X=0.5 0.75
and 0.9 are given in FIG. 7. Samples prepared by Method A also gave
exactly the same XRD patterns for X=0.5 and 0.9 as shown in FIG.
7 and gave slightly more crystalline products for X=0.75 compared
to the sample prepared using Method B. The sample prepared using
X=0.5 yielded a crystalline product whereas the sample using X=0.9
yielded a completely amorphous product. Calcination of samples X=0.5
at 400.degree. C. (752.degree. F.) prepared by either Method did
not change their X-ray crystallinity.
As may be seen from Table 2 the amount of Al and Na extraction
increased with increasing X. Some Si was also released which remained
constant with increasing X. These elements were extracted more from
samples prepared by Method B than in the case of Method A. The largest
difference was found for X=0.5. The extraction of Na and Al by Method
B for X=1.0 corresponds to 91 and 89% respectively of the total
amount of these ions originally present in the zeolite.
Water sorption isotherm of the original Na-Y zeolite starting material
is given in FIG. 8. As expected, this sample exhibited an extreme
Type I isotherm. Isotherms of samples treated with X=0.5 are given
in FIGS. 9-12. These samples yielded isotherms with separation factors
in the range of 0.07-0.1. Samples prepared by Method B had larger
sorption capacity compared to samples prepared by Method A. It is,
however, evident that the sample prepared by Method B was more sensitive
to thermal treatment than the sample prepared by Method A (FIGS.
11 and 12).
Water Isotherms of the sample X=0.75 and X=0.9 are given in FIGS.
13 14 15 and 16 respectively. As in the case of X=0.5 samples
prepared by Method B gave higher sorption capacity. For X=0.75
the sample prepared by Method A gave slightly better shape than
sample prepared by Method B.
Scanning electron micrographs of the original Na-Y sample before
and after treatment with H.sub.4 EDTA (X=0.9) by Method B and by
Method A shows that the surface morphology of these two samples
are very similar, indicating that the crystal-like morphology is
maintained in the treated sample, even though the latter sample
is amorphous to X-ray.
Isosteric heat of the sample, X=0.9 prepared by Method A is given
in FIG. 17. This sample gave a low heat of adsorption which is close
to latent heat of vaporization. (9.7 Kcal/mole) and heat of liquefication
(10.6 Kcal/mole). Estimation of heat of adsorption from BET `c`
constant for other samples also gave a reasonably low heat of absorption.
For example, samples with X=0.5 0.75 and 0.9 prepared by Method
A gave the net heat of adsorption of 2.7 2.1 and 1.9 Kcal/mole,
respectively. These values are close to the heat of liquefaction.
EXAMPLE 3
About eight grams of Na-Y zeolite (SiO.sub.2 /Al.sub.2 O.sub.3
=5.3) was slurried in 1 N NH.sub.4 Cl solution equivalent to five
times the cation exchange capacity (total Na content) of the zeolite.
The slurry was divided and portions were refluxed from one to four
times. Each reflux treatment was performed for a period of two hours
followed by separating the solids by centrifugation. A fresh solution
of NH.sub.4 Cl was added after each reflux treatment. The excess
salts were removed by washing with deionized water and centrifugation
at the end of each batch experiment. The NH.sub.4.sup.+ exchanged
samples were placed separately in a tubular furnace and heated at
600.degree. C. for 4 hr. in a steam environment. To exchange a very
high amount of NH.sub.4.sup.+ ions, the sample refluxed four times
was steamed at 650.degree. C. for 4 hr. followed by refluxing in
NH.sub.4 Cl solution for 24 hr. and steaming again at 600.degree.
C. for 4 hr. After steaming, all samples were further calcined at
650.degree. C. for 4 hr. in air.
X-ray powder diffraction (XRD) analysis indicated that the crystallinity
of all of the NH.sub.4.sup.+ exchanged samples calcined at 600.degree.
C. in the presence of steam was preserved, whereas samples calcined
in air progressively lost their crystallinity as the number of reflux
treatments increased. XRD patterns of a sample prepared by refluxing
twice with NH.sub.4 Cl solution and calcined in air or steam are
compared in FIG. 18.
Water adsorption isotherms of the original Na-Y and modified samples
are compared in FIG. 19. The extreme Type I water isotherm of the
original sample has progressively been converted to Type IV by way
of moderate Type I isotherms. The achievement of a family of curves
from an extremely hydrophilic material without destroying its crystallinity
is a well known example of the reduction of hydrophilicity (polarity)
in the original material as a result of decationation (removal of
Na.sup.+) and dealumination (removal of Al from the framework).
This process, however, produced some defects and secondary pores
which are apparent from increased adsorption ("hump")
at relative pressure .gtoreq.0.75. These secondary pores were filled
near saturation vapor pressure.
Water isotherms of samples prepared by refluxing once and twice
with NH.sub.4 Cl solution are compared with ideal isotherms in FIGS.
20 and 21. These crystalline samples yielded close to ideal isotherms
at a relative pressure up to about 0.2 with high sorption capacities
(33% by weight) and high thermal stability.
EXAMPLE 4
Four grams of zeolites 100 zeolon Na (SiO.sub.2 /Al.sub.2 O.sub.3
=11.2) and 100zeolonH (SiO.sub.2 /Al.sub.2 O.sub.3 =11.2) were boiled
with 40 ml of 2 M, 4 M, 6 M and 12 M hydrochloric acid (HCl) respectively
for 4 hr. After 4 hr, the slurries were centrifuged, decanted and
washed 5 times with 40 ml deionized water. Washed samples were dried
at 60.degree. C. (140.degree. F.) overnight and calcined at 200.degree.-1000.degree.
C. (392.degree.-1832.degree. F.) for 4-20 hr. Most of the samples
remained crystalline but part of it was rendered amorphous. The
isotherm of the sample treated with 12 HCl and degassed at 200.degree.
C. 392.degree. F.) for at least 4 hr is given in FIG. 22 and shows
ideal shape. About 17% increase in total absorption capacity as
compared to the original sample was observed. This is attributed
to the increase in adsorption volume due to exchange of smaller
H.sup.+ ion for larger Na.sup.+ ions as well as voids created as
a result of dealumination after the acid treatment.
EXAMPLE 5
Four grams of erionite (SiO.sub.2 /Al.sub.2 O.sub.3 =7.9) from
Shoshone, California was treated with 80 ml of 0.5 and 0.75 M HCl
respectively for 4 hr. After 4 hr, the samples were washed 5 times
with deionized water by centrifugation and dried at 60.degree. C.
(140.degree. F.). Samples dried at 60.degree. C. were calcined at
200.degree.-400.degree. C. before the adsorption measurement. Water
adsorption isotherm of the sample treated with 0.5 M HCl and calcined
at 400.degree. C. (752.degree. F.) is given in FIG. 23 and shows
ideal shape. Again the total absorption capacity was found to increase
by 25% as compared to the original sample. XRD analysis showed that
this sample remained largely crystalline.
Accordingly, microporous materials have been developed which exhibit
ideal water adsorption isotherms for use as desiccants in cooling
and dehumidification equipment. These materials have large sorption
capacity, low heat of asorption, high rate of absorption and high
thermal and chemical stability. |