U.S. patent application number 09/796218 was filed with the patent office on 2002-10-31 for process and adsorbent for gas drying.
Invention is credited to Cohen, Alan P., Kanazirev, Vladislav I..
Application Number | 20020157535 09/796218 |
Document ID | / |
Family ID | 25167636 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020157535 |
Kind Code |
A1 |
Kanazirev, Vladislav I. ; et
al. |
October 31, 2002 |
Process and adsorbent for gas drying
Abstract
An adsorbent and a process are disclosed for drying gases in a
pressure swing dryer. The adsorbent comprises a pre-formed alumina
particle with a thin layer of colloidal silica disbursed on the
surface of the preformed alumina particle. Surprisingly the thin
layer of colloidal silica does not interfere with the drying
properties of the pre-formed alumina particle, while significantly
improving the physical properties of the pre-formed alumina
particle
Inventors: |
Kanazirev, Vladislav I.;
(Arlington Heights, IL) ; Cohen, Alan P.;
(Highland Park, IL) |
Correspondence
Address: |
JOHN G TOLOMEI, PATENT DEPARTMENT
UOP LLC
25 EAST ALGONQUIN ROAD
P O BOX 5017
DES PLAINES
IL
60017-5017
US
|
Family ID: |
25167636 |
Appl. No.: |
09/796218 |
Filed: |
February 28, 2001 |
Current U.S.
Class: |
95/96 ; 502/407;
502/415; 95/117 |
Current CPC
Class: |
B01D 2253/104 20130101;
B01J 20/08 20130101; B01D 53/047 20130101; B01J 20/103 20130101;
B01J 2220/62 20130101; B60T 17/004 20130101; B01J 20/3293 20130101;
B01D 2257/80 20130101; B01J 20/3236 20130101; B01D 2253/106
20130101; B01J 20/28016 20130101; B01J 20/3078 20130101; B01J
20/28042 20130101; B01J 20/28061 20130101; B01D 53/02 20130101;
B01D 2259/4143 20130101; B01J 20/28019 20130101; B01J 20/3204
20130101; B01D 53/0415 20130101; B01D 2253/342 20130101; B01D
53/261 20130101 |
Class at
Publication: |
95/96 ; 95/117;
502/407; 502/415 |
International
Class: |
B01D 053/02 |
Claims
What is claimed is:
1. A process for the drying of a compressed gas comprising a gas
component and water, said process comprising contacting the
compressed gas stream with an adsorbent to adsorb at least a
portion of the water to produce a dry compressed gas stream,
wherein said selective adsorbent comprises a shaped alumina
particle having dispersed thereon a colloidal silica layer.
2. The process of claim 1 wherein the adsorbent has an attrition
loss of less than 0.04 wt-%.
3. The process of claim 1 wherein the silica layer extends less
than about 50 microns from the surface of the shaped particle
towards the center of the particle.
4. The process of claim 1 wherein the adsorbent comprises a
rehydration loss of less than about 5 wt-%.
5. The process of claim 1 wherein the shaped alumina particle is
selected from the group consisting of spherical, an extrudate, a
pill, a flake or a monolith.
6. The process of claim 1 wherein the gas component is selected
from the group consisting of air, nitrogen, light hydrocarbons and
mixtures thereof.
7. The process of claim 1 wherein the gas component comprises
air.
8. The process of claim 1 further comprising periodically purging
the selective adsorbent to desorb the water.
9. The process of claim 1 wherein the contacting of the compressed
gas with the adsorbent takes place in a pressure swing adsorption
process wherein the adsorbent is depressurized to an effective
desorption pressure and purged to desorb the water.
10. A method for producing an adsorbent for drying compressed gas
comprising: a) coating a pre-formed alumina particle with a coating
solution comprising colloidal silica to provide a coated pre-formed
particle having a narrow layer of colloidal silica disbursed
thereon; and b) drying and activating the coated pre-formed
particle to produce the adsorbent.
11. The method of claim 10 wherein the pre-formed particle
comprises a sphere or an extrudate.
12. The method of claim 10 wherein the coating solution also
comprises an ion selected from the group consisting of sodium,
chloride, hydrogen, and the like.
13. The method of claim 10 wherein the coated pre-formed particle
is activated at an activation temperature greater than about
400.degree. C. (750.degree. F.).
14. A shaped particle having a core consisting of a shaped alumina
carrier having a colloidal silica layer dispersed on the shaped
alumina carrier, at least 60 percent of the colloidal silica being
present in a band extending from the surface towards the center and
having a width of less than about 50 microns.
15. The shaped particle of claim 14 wherein the band of colloidal
silica extending from the surface toward the center comprises a
width of less than about 20 microns.
16. The shaped particle of claim 14 wherein the adsorbent has an
attrition loss of less than 0.4 wt-%.
17. The shaped particle of claim 14 wherein the adsorbent comprises
a rehydration loss of less than about 5 wt-%.
18. The shaped particle of claim 14 wherein the shaped alumina
particle is selected from the group consisting of a sphere, an
extrudate, a pill, a flake or a monolith.
19. An air brake dryer assembly having a refillable or replaceable
cartridge containing the shaped particle of claim 14.
Description
FIELD OF THE INVENTION
[0001] The present invention relates, in general, to a process for
drying a compressed gas stream and, more particularly, to a process
for drying a gas stream with a novel solid adsorbent.
BACKGROUND OF THE INVENTION
[0002] Drying of gases to remove water is a common requirement in
providing gases at high pressure for use in chemical processing,
air brake systems in transportation, and shop applications ranging
from pneumatic control to paint spraying. The removal of moisture
from the high pressure gases reduces corrosion and prevents water
buildup and freezing in such systems. Condensation of water in air
brake systems or pneumatic systems can result in inefficiencies and
system failure.
[0003] In fluid drying and purification applications in which a
contaminant is present in the fluid at concentrations of about 3
percent or less, the adsorbent is generally of low value and
frequently removed for disposal. One important drying and
purification application includes drying air, particularly
compressed air. In such applications cost can become a significant
factor in the selection of a drying process. For example, a
once-through process employs an adsorbent which is exposed to the
fluid until the adsorbent is no longer effective in removing the
contaminant Once-through processes require large inventories of
adsorbent and frequent disposal of spent adsorbent An alternative
to this approach is to employ a regenerative adsorption cycle
wherein the adsorbent is periodically regenerated to remove the
contaminant from the spent adsorbent and restore the adsorbent's
capacity for adsorbing the contaminant. Regenerative adsorption
cycles are generally classified according to the manner in which
the desorption takes place Both thermal swing adsorption processes
and pressure swing adsorption processes are generally known in the
art for various types of adsorptive separations. Generally, thermal
swing processes utilize the process steps of adsorption at a low
temperature, regeneration at an elevated temperature with a hot
purge gas and subsequent cooling down to the adsorption
temperature. One process for drying gases generally exemplary of
thermal swing processes is described in U.S. Pat. No. 4,484,933,
issued to Cohen. The patent describes basic thermal swing
processing steps coupled with the use of an auxiliary adsorber bed
for improving the regeneration step Thermal swing processes are
often used for drying gases and liquids and for purification where
trace impurities are to be removed. Often, thermal swing processes
are employed when the components to be adsorbed are strongly
adsorbed on the adsorbent, i.e., water, and thus, heat is required
for regeneration. Thermal swing adsorption processes are
characterized by long cycles which require large amounts of purge
gas to regenerate and cool the adsorbent to return the adsorbent to
adsorption conditions in each cycle.
[0004] Pressure swing adsorption (PSA) provides a means for
adsorption that does not require heat for regeneration. Instead,
regeneration is accomplished by reducing the pressure in the
adsorber bed to below the pressure at which adsorption had
occurred. PSA process typically includes steps of adsorption at an
elevated pressure, desorption to a lower pressure and
repressurization to the adsorption pressure. The process also often
includes a purge step at the desorption pressure to enhance
desorption. PSA processes are characterized by lower adsorbent
differential loadings within each cycle which results in greater
inventories of adsorbent Furthermore, the cycle time of a PSA
process is shorter than a thermal swing adsorption process because
there are no heat transfer related delays while the bed is cooled
to adsorption conditions. Lastly, the amount of purge gas required
for a PSA process can be significantly lower than the amount of
purge gas in a thermal swing adsorption process, because the purge
gas in the PSA process is only required to remove adsorbate from
the adsorbent and not to additionally cool the adsorbent.
[0005] Such PSA processing is disclosed in U.S. Pat. No. 3,430,418
issued to Wagner and in U.S. Pat. No. 3,986,849 issued to Fuderer
et al, wherein cycles based on the use of multi-bed systems are
described in detail As is generally known and described in these
patents, the contents of which are incorporated herein by reference
as if set out in full, the PSA process is generally carried out in
a sequential processing cycle that includes each bed of the PSA
system. Such cycles are commonly based on the release of void space
gas from the product end of each bed in one or more cocurrent
depressurization steps upon completion of the adsorption step. In
these cycles, the released gas typically is employed for pressure
equalization and for subsequent purge steps. The bed is thereafter
countercurrently depressurized and often purged to desorb the more
selectively adsorbed component of the gas mixture from the
adsorbent and to remove such gas from the feed end of the bed prior
to the repressurization thereof to the adsorption pressure.
[0006] Pressure swing dryers are used on heavy trucks and busses to
dry the compressed air that operates the brakes and other pneumatic
accessories. These dryers use molecular sieve or other desiccants
to remove water from ambient air that has been compressed to
usually about 130 psig (8.8 bar gauge). Some trucks now operate
systems with 12 bar gauge (176 psig) of air pressure. The dryers
operate on a pressure swing cycle. Compressed ambient air is passed
into the air brake dryer unit, wherein liquid water is separated
and water vapor is adsorbed on a desiccant. Dry, compressed air is
removed from the air brake dryer unit and passed to the brake
system. Water is desorbed from the desiccant in a desorption or
depressurization step which typically takes place at atmospheric
pressure, 0 bar (gauge pressure) and may include a purge step at
atmospheric pressure.
[0007] In air brake systems, the compressor runs intermittently in
cycles determined by the filling and depletion of the air in a
supply tank. The compressor pumps until the pressure in the supply
tank reaches the cut out pressure or upper pressure limit,
typically about 1000 kPa (145 psia). Then the compressor rests
until the pressure in the supply tank is reduced by air usage in
the brake systems, other pneumatic equipment and regeneration
(purging) of the dryer, falls to the lower pressure limit reaches
about 860 kPa (125 psia).
[0008] An alumino-silicate gel desiccant, for example, is commonly
used as a drying agent for extracting water vapor from gas. This
desiccant has a strong molecular attraction for water vapor and may
require heating to about 250.degree.to 400.degree. F. (120.degree.
to 205.degree. C.) to fully desorb water vapor. It has been
observed that alumino-silicate gel desiccant also will adsorb VOC's
and may require heating up to about 350.degree. F. (about
175.degree. C.) or more to fully desorb typical VOC's released in a
paint spray booth. Low boiling point hydrocarbons are desorbed at a
lower temperature than higher boiling point hydrocarbons. When
either water vapor or VOC's are directly adsorbed by a desiccant, a
strong dipole bond is formed. One desiccant for adsorbing VOC's is
an alumino-silicate gel containing about 3% alumina
(Al.sub.2O.sub.3) and 97% silica (SiO.sub.2) in the form of hard,
generally spherical beads. Such a desiccant is commercially
available, for example, from Engelhard Corporation of Iselin, N.J.
under the trademark "Sorbead.RTM.".
[0009] Pressure swing dryers contain a desiccant, which alternately
adsorbs water from the compressed air and then desorbs the water
into the purge air. Molecular sieves, silica gel, activated
alumina, and alumino-silicate gels as disclosed hereinabove are all
used commercially as pressure swing desiccants. Other VOC adsorbent
materials such as zeolite also may be acceptable alternatives (see
U.S. Pat. No. 5,968,235). In addition to the ability to adsorb and
desorb water, a pressure swing desiccant should have suitable
physical properties (attrition, abrasion, and dust) and low cost,
Low reactivity with hydrocarbons is also desirable. Silica gel is a
low cost alternative, but generally suffers from breakage when
contacted with liquid water. Activated alumina is also a low cost
alternative, but suffers from high abrasion loss and dust.
Molecular sieves are effective, but cost much more than silica gel
or alumina. The alumino-silicate gel consisting of 3% alumina and
97% silica is more costly than activated alumina and is highly
reactive to unsaturated hydrocarbons than molecular sieves, silica
gel or alumina.
[0010] There is a need for low cost, effective adsorbents for use
in pressure swing adsorption air dryers. A suitable adsorbent must
not only produce low dew points, it must also have acceptable
physical properties, such as low dust and resistance to abrasion
loss and attrition. Air brake drying represents a severe
environment for the operation of a dryer wherein the desiccant is
subject to vibration and large pressure swings between the
adsorption and desorption steps. In addition, the desiccant is
exposed to hot compressed air which may contain oil and other
volatile compounds such as hydrocarbons which could harm brake
cylinders and related equipment. Adsorbents are sought which are
more resistant to attrition and abrasion loss than conventional
materials and which are effective in the removal of water and other
contaminants from the compressed gas to produce dry compressed
gases, i.e., compressed gases having a low dew point.
SUMMARY OF THE INVENTION
[0011] The present invention provides a desiccant and a process for
drying a gas comprising a gas component such as air, light
hydrocarbons, nitrogen, hydrogen, and mixtures thereof and water.
The present invention can be employed to provide improved systems
for drying compressed gases, for such applications as air brake
drying in truck and locomotive brake systems, and in providing dry
compressed gases for shop use. According to the present invention,
activated shaped or pre-formed alumina particles are modified by
depositing a thin dispersed layer of colloidal silica on the
external surface of the alumina particles The colloidal silica
coating drastically reduces the abrasion loss and dusting.
Surprisingly, this improvement is achieved without seriously
reducing the drying performance of alumina. Furthermore, the
colloidal silica-coated alumina of the present invention has low
reactivity to unsaturated hydrocarbons because the surface coating
of colloidal silica minimizes contact between the unsaturated
hydrocarbons and the deactivated alumina surface.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The process of the invention is intended to be practiced on
feed gas streams which comprise a feed gas and moisture or water;
such feed gas streams include air, nitrogen, light hydrocarbons and
mixtures thereof. The term "light hydrocarbons" is intended to
include hydrocarbons having 1 to 4 carbon atoms per molecule.
[0013] The process of the present invention is useful in gas drying
applications such as in air brake systems for trucks or locomotives
In these systems the gas, such as air, is generally saturated with
water following its compression to some operating pressure to
activate the braking systems of the vehicle or piece of heavy
equipment If this water is not removed or significantly reduced,
the braking systems will be subject to greater corrosion, or
freezing, resulting in higher maintenance costs and reduced
reliability Generally, the feed gas stream employed to activate a
braking system comprises air at water saturation conditions and a
pressure above atmospheric pressure. Preferably the temperature at
which the feed gas stream is available ranges between about
37.degree. C. (100.degree. F.) and about 82.degree. C. (180.degree.
F.). More preferably the temperature at which the feed gas stream
is available ranges between about 37.degree. C. (100.degree. F.)
and about 66.degree. C. (150.degree. F.). Preferably the pressure
at which the feed stream is available is greater than about 790 kPa
(100 psig), and more preferably the pressure at which the feed gas
stream is available ranges between about 790 kPa and about 1.14 MPa
(150 psig), and most preferably the pressure at which the feed gas
stream is available ranges between about 790 kPa and about 869 kPa
(110 psig).
[0014] The water in the feed gas stream, or compressed gas stream,
will be present at saturated conditions which comprise from about
8,000 to about 32,000 ppm-vol depending upon the temperature and
the pressure of the feed gas. Preferably the water content in the
product gas, or dry compressed gas, produced by the dryer will be
reduced by at least about 95 percent, and more preferably the water
content in the product gas will be reduced by about 95 to about
99.9 volume percent of the amount of water in the feed gas.
Preferably the amount of water in the product gas stream will be
less than 1000 ppm-vol, and more preferably the amount of water in
the product gas stream will range from about 50 to about 100
ppm-vol. In some cases the water content of the product gas will be
expressed as the dew point depression from the ambient air
temperature, wherein the dew point depression is the ambient
temperature minus the dew point temperature for any stream. The dew
point temperature for any stream is the temperature at which
condensation will occur. In such cases, the dew point depression
preferably will be greater than about 17.degree. C. (30.degree. F.)
and more preferably will be greater than about 50.degree. C.
(90.degree. F.).
[0015] The term "pressure swing adsorption" as used herein
generally relates to an adsorption process wherein an adsorber bed
is part of an integrated process whereby a continuous adsorber
operation can be obtained while simultaneously regenerating a spent
adsorber bed. In pressure swing adsorption, there exists a
plurality of adsorption zones maintained at an elevated pressure
effective to adsorb water while allowing the bulk of the feed gas
to pass through the adsorber bed. At a defined time, the passing of
the adsorber feed to a first adsorber bed is discontinued and the
first adsorber bed is depressurized by one or more counter-current
depressurization steps wherein the pressure is reduced to a defined
level which permits additional components of the feed gas remaining
in the first adsorber bed to be withdrawn and utilized or vented
The first adsorber bed is depressurized in a countercurrent
depressurization step wherein the pressure in the first adsorber
bed is further reduced by withdrawing desorbed water vapor
countercurrently to the direction of the feed. Finally the adsorber
bed is purged with a purge gas such as dried product gas and
repressurized. The final stage of repressurization is with the feed
gas or the product gas in order to conserve the feed or the product
gas.
[0016] In air brake drying systems, the process is simplified and
generally there are no feed gas conservation steps. Pressure swing
adsorption in air brake drying is generally carried out in a single
adsorbent bed with essentially two process steps. The adsorption
stroke comprises rising pressure adsorption as the feed gas is
introduced and the adsorber bed pressure rises to the pressure of
the feed gas, followed by constant pressure adsorption as the feed
gas is passed through the adsorber bed and a product gas is
collected in a product reservoir. The adsorption stroke is
continued until a pressure set point in the product reservoir is
reached which corresponds to the capacity of the reservoir.
Typically this pressure set point is about 930 kPa (135 psia) and
is reached after about 2 minutes in heavy services or in about 30
seconds under light duty. At this point, the feed gas passing is
discontinued and the adsorber bed is placed in a desorption stroke
wherein the adsorber bed is simultaneously counter-currently
depressurized and purged with a portion of the product gas
withdrawn from the product reservoir. Typically the portion of the
product gas is collected during each cycle in the product
reservoir, or pressurized storage tank, and the desiccant or
adsorbent is disposed in a replaceable or refillable cartridge in
an air brake dryer assembly. At the completion of the desorption
stroke, the pressure swing air dryer is returned to the adsorption
stroke. A cycle time for pressure swing air brake drying means the
length of time of the adsorption stroke plus the length of the
desorption stroke. Typically the cycle times for such pressure
swing air brake dryers ranges from about 60 seconds to about 150
seconds. The pressure swing air brake dryers are fixed beds
containing an adsorbent and are configured with low length to
diameter ratios to accommodate vehicle space limitations. In some
severe service situations for example in locomotives, pressure
swing adsorption air brake systems are provided with 2 adsorption
beds which alternate between the adsorption and desorption strokes
to provide a continuous supply of dry air. An example of an air
brake dryer containing a desiccant is disclosed in U.S. Pat. No.
5,607,500, which is hereby incorporated by reference.
[0017] Pressure swing adsorption systems are more efficient at
higher pressures, typically pressures greater than about 2.2 MPa
(300 psig), In addition, pressure swing adsorption air brake dryers
are generally characterized by short cycles with relatively low
differences in adsorbent loading between the upper and lower
pressure ranges of operation. This leads to the use of larger
amounts of adsorbent in a pressure swing adsorption device than a
thermal swing adsorption process. Furthermore, the amount of purge
gas required for a pressure swing adsorption unit varies inversely
with pressure; the ratio of purge gas to feed gas is reduced as the
pressure increases. At low pressures, the amount of purge gas
approaches the amount of feed gas.
[0018] The adsorbent of the present invention is prepared by
treating shaped alumina particles with colloidal silica prior to
activation. The activated alumina referred to includes alumina
having a surface area usually greater than 100 m.sup.2/g and
typically in the range of 100 to 550 m.sup.2/g. Further, the
activated alumina powder is preferably obtained by rapid
dehydration of aluminum hydroxides, e.g., alumina trihydrate or
hydrargillite in a stream of hot gases. Dehydration may be
accomplished in any suitable apparatus using the stream of hot
gases. Generally, the time for heating or contacting with the hot
gases is a very short period of time, typically from a fraction of
a second to 4 or 5 seconds, for example. Normally, the temperature
of the gases varies between 400.degree.and 1000.degree. C. The
process is commonly referred to as flash calcination and is
disclosed in U.S. Pat. No. 2,915,365 incorporated herein by
reference. However, other methods of calcination may be employed to
dehydrate or partially dehydrate the trihydrate.
[0019] A source of activated alumina is gibbsite which is one form
of alumina hydrate derived from bauxite using the Bayer process.
However, alpha alumina monohydrate, pseudoboehmite or the alumina
trihydrate may be used. Other sources of alumina may also be
utilized including clays and aluminum alkoxides. Also, alumina gel
may be used as a source of the alumina. The shaped or pre-formed
alumina particles are agglomerates having an average pore diameter
less than about 3.5 nm (35 angstroms). The agglomerates may be
formed into spheres, pills, extrudates, flakes, monoliths and the
like. Agglomerates formed in this manner generally comprise alumina
particles having an average particle diameter of about 9,000 to
10,000 nm. The total surface area of a typical alumina particle
will be about 350 m.sup.2/g, while the surface area of the surface
only of the alumina particle is about 12 m.sup.2/g. The adsorbent
of the present invention comprises alumina particles having
colloidal silica disposed on the surface of the alumina particles
without obstructing the remaining surface area within the porous
alumina particle. This phenomena was confirmed by scanning electron
microscopy and energy dispersive x-ray analysis (SEM-EDS). When the
shaped alumina particles which range in size from about 1.0 to
about 3.0 mm are treated with colloidal silica, the silica is
disposed in a narrow layer or band concentrated near the surface of
the shaped alumina particle. Surprisingly, this narrow band of
silica does not interfere with the adsorption properties of the
shaped or pre-formed alumina particle, and actually improves the
physical properties of the adsorbent including attrition loss and
abrasion. The colloidal silica could be applied to the pre-formed
alumina particle by spraying, soaking or any other conventional
means. It is preferred to apply the colloidal silica layer by
soaking the pre-formed alumina particle in a colloidal silica
solution.
[0020] A colloid is defined as consisting of small particles which
are in suspension which fails to settle out and diffracts a beam of
light. By the term "colloidal silica" it is meant silica particles
ranging in average diameter greater than 4 nm and less than about
500 nm. Preferably, the average diameter of the colloidal silica
particles ranges from about 4 nm to about 200 nm, and most
preferably the average diameter of the colloidal silica particles
ranges from about 4 nm to about 10 nm, A preferred colloidal silica
solution comprises colloidal silica in an aqueous solution. The
colloidal silica solution can comprise other ions selected from the
group consisting of sodium ions, hydrogen ions, chloride ions, and
the like. In the present invention, the colloidal silica does not
penetrate the entire particle, but remains within a thin layer near
the surface of the pre-formed alumina particle. On the other hand,
silicate treating; that is, treating with soluble silica, results
in silica dispersed beyond the surface toward the center of the
pre-formed alumina particle. Such silicate treated materials do not
show the improved physical properties such as attrition loss and
abrasion of the adsorbent of the present invention.
[0021] One of the problems of coating silica on pre-formed
particles of alumina is keeping the coating on the alumina
particle, This ability of the coating to remain on the particle is
measured by attrition loss and by abrasion loss. Attrition loss is
a physical property of the pre-formed alumina particle which is
measured by rubbing or grinding the particles together and
measuring the loss during the grinding process. Accordingly, a
sample of the material is rotated for a set period of time in a
cylindrical drum having a single baffle. Any fines produced during
the test are determined and considered to be a measure of the
propensity of the material to produce fines in the course of
transportation, handling and use. Typically, 100 g of material are
rotated for 1800 revolutions at a rate of about 60 revolutions per
minute. The colloidal silica coated alumina particles of the
present invention surprisingly showed a significant reduction in
attrition loss relative to untreated and silicate treated alumina
particles. The attrition loss after colloidal silica treatment was
less than 0.5 wt-% and more particularly the attrition loss of the
colloidal silica treated alumina was less than 0.2 wt-%. This
represents a 400% improvement over conventionally silicate treated
and untreated alumina.
[0022] Abrasion loss is measured by placing a weighted sample of
material on a screen and tapping the screen for about 30 minutes
while catching any dust produced The amount of dust produced is
determined and expressed as a weight percent loss. Abrasion loss
determined for the colloidal silica treated alumina particles
showed significant improvement over conventional silica treated and
untreated aluminas. The abrasion loss for colloidal silica treated
alumina particles was less than 0.5 wt-%, and more particularly
less than 0.05 wt-%
EXAMPLES
[0023] The following examples are provided to illustrate the
process of the present invention and are not intended to limit the
scope of the claims that follow. The examples are based on
laboratory data of adsorption characteristics of various desiccants
or on process engineering design calculations and adsorption
relationships as noted. Some of the examples presented below are
experiments done with unactivated, pre-formed alumina particles as
described hereinabove. They are formed in a conventional manner by
agglomerating flash calcined alumina as described hereinabove.
Example 1
[0024] (Spray Treatment)
[0025] About 1000 g of unactivated, pre-formed alumina particles of
2.4 to 4 mm in diameter (size 5.times.8 mesh Tyler screen series)
were treated in a 0.3 meter (12-inch) laboratory rotating drum and
sprayed by hand with the following solutions. The method used was
to spray the pre-formed alumina particles for 3-5 minutes with
70-80 cc of the silica solution followed by about 5 minute further
rotation. The alumina particles were then tray activated for 1 hour
at 750.degree. F. (400.degree. C.) in a Grieve oven and analyzed
for attrition according to the American Society for Testing and
Measurement (ASTM) test designation D 4058-87. Abrasion was also
measured to determine the abrasion loss of activated alumina
wherein a 165 g sample is placed on a 595 micron (28 mesh Tyler)
screen and the screen is tapped for 30 minutes. Abrasion loss is
the weight percent of sample lost through the screen.
1TABLE 1 Approx. Abrasion Treatment SiO.sub.2 content Loss, SPM
Attrition, ASTM Sample Solution (VFB*) 202B, wt-% D4058-87, wt-%
1-1 None 0 0.66 0.85 1-2 Water 0 0.62 0.77 1-3 Na.sub.2SiO.sub.3
2.2 1.03 0.89 1-4 Ludox .RTM. Cl 2.1 0.048 0.10 1-5 Ludox .RTM. SM
1.8 0.04 0.21 *volatile free basis
[0026] Ludox.RTM. C1 (DuPont Corp. identified as Ludox.RTM. CL) is
a colloidal silica coated with alumina and containing chloride as
an ion. The Ludox.RTM. CL has a total content of SiO.sub.2 and
Al.sub.2O.sub.3 of about 30 wt-%, and a pH of 4.6. Ludox.RTM. SM
(DuPont Corp. identified as Ludox.RTM. SM-30) is sodium-stabilized
30 wt-% colloidal silica, with a pH of 10.5. The spraying solution
for samples (1-4) and (1-5) was prepared by diluting the original
Ludox.RTM. reagent with an equal mass of water. A weight of 92.7 g
of diluted Ludox.RTM. Cl solution was sprayed over 1000 g
pre-formed alumina particles to make sample (1-4), and 78.6 g of
diluted Ludox.RTM. SM was sprayed over 1000 g of pre-formed alumina
particles to make sample (1-5).
[0027] Sodium metasilicate Na.sub.2SiO.sub.3.multidot.5 H.sub.2O is
a technical grade reagent available from Alfa Aesar. Forty grams of
the granular product was dissolved in 100 g water. The pre-formed
particles (1000 gm) were sprayed with 108 g of sodium metasilicate
solution to make sample (1-3).
[0028] The abrasion loss for the water treated control sample (1-2)
is practically the same as the untreated particles of sample (1-1).
The abrasion loss of the alumina particles treated with both forms
of colloidal silica samples (1-4) and (1-5) are improved by more
than one order of magnitude, while the abrasion loss with the
sodium silicate sample (1-3) actually became worse, indicating that
the silicate coating may be contributing to the abrasion loss. The
samples treated with colloidal silica were significantly reduced in
attrition loss under the ASTM test of 1800 revolutions of the test
drum.
[0029] In Example 1, the treatment with colloidal silica has
apparently produced a narrow layer of silica around the alumina
particle that protects the outer surface from loss of material by
attrition and abrasion. It is surprising that sodium silicate is
distributed more evenly through the alumina core and is ineffective
in producing an outer layer with the associated benefits.
Example 2
[0030] (Rehydration of Silica Treated Pre-formed Particles)
[0031] The pre-formed alumina particles treated with solutions in
Example 1 were further analyzed by thermal gravimetric analysis
(TGA) a few hours after the treatment, stored at room temperature
in closed glass vials, and re-analyzed after 11 days. A
differential TGA plot of weight loss vs. temperature gives insight
into the alumina phases that are present in the sample. Of
particular interest is the phase change or "rehydration" of the
alumina to aluminum hydroxide [Al(OH).sub.3]. Aluminum hydroxide
gives a water loss at 270.degree. C., whereas alumina does not.
Based on the area under the peak of the differential TGA plot at
about 300.degree. C., rehydration was estimated as follows:
2TABLE 2 Rehydration Effects on Alumina Particles Conversion to
AL(OH).sub.3 with time Conversion to Conversion to Approx.
SiO.sub.2 Al(OH).sub.3, %, Al(OH).sub.3, %, Sample content (VFB*)
Same day After 11 days 1-2 0 1.0 7.0 1-3 2.2 2.0 7.5 1-4 2.1 3.8
3.8 1-5 1.8 1.0 3.8 *volatile free basis
[0032] The data show that treatment with colloidal silica in
samples (14) and (1-5) enhanced the stability of the pre-formed
alumina particles and resulted in a significantly reduced tendency
for rehydration, relative to the untreated and silicate treated
alumina.
Example 3
[0033] (Stability Towards Rehydration of Activated Alumina
Particles)
[0034] The pre-formed alumina particles produced in Example 1 were
activated at 750.degree. F. (400.degree. C.) in an oven. Then 70 g
of activated particles were added to 90 g deionized water and
stored at room temperature for a prolonged period of time. Small
samples were withdrawn and analyzed by TGA to determine the degree
of rehydration. The results are shown in Table 3.
3TABLE 3 Rehydration Stability Rehydration Rehydration Rehydration
Sample after 24 h after 90 h after 240 h 1-1 5.1 9.7 15.0 1-2 4.0
9.5 14.2 1-3 5.5 8.5 12.0 1-4 3.3 7.6 12.6 1-5 4.2 8.8 12.4
[0035] The data on the activated particles in Table 3 show that all
the silica-coating materials, including sodium silicate and both
colloidal silicas, reduced the rehydration tendency of alumina.
After 240 hours in water, the untreated (1-1) and water-treated
(1-2) lost more than 14 wt-% of the alumina to rehydration, but the
silica-treated particles lost only 12 wt-%, an improvement of about
15% after 240 hours, which is unexpected The expected result would
have been that only the SiO.sub.2-coated portion of the coated
pre-formed alumina particle would resist re-hydration. The expected
improvement in re-hydration would be just about 3 5 wt-% for the
20-micron penetration of SiO.sub.2 into the particle observed in
Example 5 below.
Example 4
[0036] (BET Surface Area)
[0037] The pre-formed alumina particles treated in Example 1 and
activated at 750.degree. F. were tested for BET surface area. The
results, given in Table 4, show that the surface area was retained
in spite of the silica coating. This shows that the coating did not
form an impermeable envelope around the pre-formed alumina
particles.
4 TABLE 4 Treatment Approx. SiO.sub.2 BET Surface Sample Solution
content (VFB*) Area, m.sup.2/g 1-2 Water 0 359 1-3
Na.sub.2SiO.sub.3 2.2 (not measured) 1-4 Ludox .RTM. Cl 2.1 356 1-5
Ludox .RTM. SM 1.8 366 *Volatile free basis
Example 5
[0038] (Location of Silica Species)
[0039] The location of the silicate species in the treated
particles of Example 1 was investigated by SEM-EDS scanning
electron microscopy and the silica content was determined within a
particle by energy dispersive analysis by x-rays. Measurements were
made across the interior of the particle to determine the silica
distribution of a sectioned particles of about 3.5 mm in diameter.
The presence of silica as SiO.sub.2 in weight percent was plotted
against the distance from the outer surface of the particles in the
FIGURE. The nomenclature here is the same as in Example 1. These
data show that a colloidal silica layer was formed on the outer
surface by the treatment with colloidal silicas. The Ludox.RTM. C1
produced a silica layer that penetrated only about 20 microns into
the particle. The Ludox.RTM. SM deposited more SiO.sub.2 on the
outside, and its concentration also trailed off at about 20
microns, but a small amount penetrated farther into the particle to
about 140 microns. The sodium silicate was more evenly distributed
in the outer 140 microns of the particle.
Example 6
[0040] (Soaking vs. Spraying Colloidal Silica)
[0041] In another preparation, a sodium stabilized colloidal silica
solution (30% silica) (available from Nalco Chemical as Nalco.RTM.
1130) was applied to 3.5 to 6.7 mm (3.times.6 mesh) pre-formed
alumina particles by spraying and soaking The amount of colloidal
silica applied is shown in Table 5 below:
5TABLE 5 Treatment Treatment Silica Added, Abrasion Loss, Sample
Solution Method wt-% (VFB*) SPM 202B, wt-% 6-1 Water Soak 0 0.35
6-2 Nalco .RTM. 1130 Soak 0.8 0.17 6-3 Water Spray 0 1.33 6-4 Nalco
.RTM. 1130 Spray 2.2 0.15 *Volatile free basis
[0042] Sample (6-3) was prepared by spraying 900 gm of pre-formed
alumina particles with 78.7 gm water. Sample (6-4) was prepared by
spraying 900 gm of pre-formed alumina particles with 62.1 gm
diluted Nalco.RTM. colloidal silica solution. The diluted colloidal
silica solution was prepared by mixing Nalco.RTM. 1130 with an
equal mass of deionized water The soaking was done with water (1000
gm pre-formed alumina particles with 500 gm water for sample (6-1))
and colloidal silica (800 gm pre-formed alumina particles with 400
gm liquid consisting of 50 gm undiluted Nalco.RTM. 1130, 30%
colloidal silica solution and 350 gm water for sample (6-2)). In
both cases, the soaking solution just covered the pre-formed
alumina particles placed in a beaker. After allowing the pre-formed
alumina particles to stand without stirring for about 25 minutes,
the liquid was decanted. Both the sprayed and soaked pre-formed
alumina particles were further de-watered by being spread over a
screen tray and immediately activated at 400.degree. C.
(750.degree. F.) for one hour. The results show that merely soaking
in water is effective in reducing abrasion loss, compared to
spraying with water. However, spraying or soaking with colloidal
silica are significantly more effective in reducing abrasion loss
by more than 200% over treatment with water.
Example 7
[0043] (Dust Measurement)
[0044] Pre-formed alumina particles having an average diameter of
between 1.4 mm and 2.8 mm (7.times.12 mesh) were treated by
spraying with water, diluted with 30% colloidal silica, and sodium
silicate, and then activated at 750.degree. F. As in Example 6, the
diluted colloidal silica solution was prepared by diluting the 30%
colloidal silica solution of Example 6 with an equal mass of water.
The dust level of the samples was reduced significantly by spraying
with colloidal silica compared to spraying with water.
Unexpectedly, sodium silicate treatment actually increased the dust
level of the particles.
6TABLE 6 Solution Treatment Silica Added, Relative Sample Treatment
Method wt-% (VFB*) Dust 7-1 Water Spray 0 466 7-2 30% Colloidal
Silica Spray 1.6 80 7-3 Na.sub.2SiO.sub.3 Spray No data 753
*Volatile free basis
* * * * *