Note: Descriptions are shown in the official language in which they were submitted.
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DESICCANT BASED HONEYCOMB CHEMICAL FILTER AND METHOD OF
MANUFACTURE THEREOF
FIELD OF THE INVENTION
The present invention relates to a chemical filter for removal of
contaminants from air present in industrial or domestic environments. More
particularly, the present invention relates to a desiccant based honeycomb
chemical filter. The present invention also relates to a method for the
manufacture
of a desiccant based honeycomb chemical filter.
BACKGROUND TO THE INVENTION
As stated above, this invention relates to a desiccant based chemical filter,
such as a honeycomb chemical filter, for the removal of contaminants from air
present in industrial or domestic environments. The present invention also
provides
to a method for the manufacture of a desiccant based honeycomb chemical
filter.
The contaminants to be removed are typically gaseous contaminants but
can also comprise contaminants in the form of fine liquid droplets which are
not
easily removable by conventional means.
The term 'desiccant' as used herein is intended to cover both macroporous
and microporous desiccant or desiccants. The desiccant or desiccants based
honeycomb matrix used in the invention is impregnated with different
impregnating
materials. The invention also provides methods for removal of gaseous
contaminants, pollutants and odors from industrial processes using the said
chemical filter. This invention also relates to a process for manufacturing
macroporous /microporous desiccant or desiccants based honeycomb matrix
impregnated with different impregnants for removing gaseous contaminants
contained in air or odour in the air.
It is well-recognized that it is difficult to manage airborne molecular
contaminants due to the difficulty in counting and/or measuring such particles
present in a given area. This difficulty is primarily due to the fact that
gases are
different from particles. The impact of contaminants on productivity as well
as on
human life, if not attended properly, can be severe. The presence of chemical
contaminants even in part per billion (ppb) concentrations can alter the
chemical
properties of the surrounding environment and adversely affect human life and
productivity. Traditional filters have been insufficient in protecting a given
environment from gaseous contaminants, particularly inorganic pollutants such
as
ammonia, amine, ozone, oxides of sulfur, nitrogen oxides, sulphides, and other
CONFIRMATION COPY
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molecular contaminants that remain undetected by particle counters.
The problems in the art will be understood from the following discussion of
some solutions available for removal of volatile organic components that form
a
significant part of airborne molecular contaminants, particularly in an
industrial
environment.
One problem in the art has been ensuring that while odour is controlled,
there is no sacrifice on the humidity level that is desirable for human
comfort and
health. However, maintaining an appropriate level of humidity as well as odour
control presents a particular problem, since existing dehumidifying devices
either
remove too much moisture from the air or are inefficient in removing odours.
Odours in industrial environments, and also in non-industrial environments
such as passenger compartments of transportation vehicles are due to volatile
organic compounds present in the circulating air. These organic compounds are
formed in high levels but at relatively low concentrations from engine
exhaust,
solvents, gas turbines, cogeneration plants, petrochemical plants, and in many
industrial processes where waste gases contain materials such as solvent
vapors,
inks, paints, etc. Volatile organic compounds comprise not only hydrocarbons,
whether saturated, unsaturated, or aromatic, but also contain oxygenated
materials
such as alcohols, esters, ethers, and acids, nitrogen containing compounds
(principally amines), sulfur containing materials (mercaptans and thioethers)
and
halogen-containing materials, especially chlorine-substituted hydrocarbons but
also organic fluorides and bromides. The presence of such compounds in a gas
stream is recognized as a health risk and/or cause the gas stream to have an
unpleasant and undesirable odour.
A traditional method for solvent recovery from humid air comprised an
adsorption system which typically was operated till the solvent concentration
in the
outlet gas stream reached a detectable preset breakthrough level. When this
level
was reached, the flow of gas to the adsorber was stopped. The adsorbent bed
then
contained solvent, other condensable organic contaminants, and some amount of
water dependent on the inlet relative humidity of the solvent laden gas
stream.
This method was subsequently modified by the introduction of a hot inert
gas or steam, either saturated or superheated, which displaces the solvent
from
the adsorbent to produce a solvent/water mixture upon condensation. Typically
two
adsorber beds were used, where one was adsorbing while the other bed
underwent regeneration.
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Further developments in regenerating and recovering solvents from
adsorbent beds involve the use of inert gases or air and low temperature
condensation of the solvent from the regenerating gas/air. However, these
techniques are not directly applicable in all given situations where required.
It is known that volatile organic compounds can be removed from air by -
adsorption such as thermal swing adsorption. Air streams needing treatment can
be found in most chemical and manufacturing plants, especially those using
solvents. At concentration levels from 500 to 15,000 ppm, recovery of the VOC
from steam used to thermally regenerate- activated Carbon adsorbent is
economically justified. Concentrations above 15,000 ppm are typically in the
explosive range and require the use of a hot inert gas rather than air for
regeneration. While, below about 500 ppm, recovery is not economically
justifiable,
environmental concerns often dictate adsorptive recovery followed by
destruction.
Activated carbon is the traditional adsorbent for these applications, and
represents
its second largest use.
U.S. Pat. No. 4,421,532 discloses a process for the recovery of VOCs from
industrial waste gases by thermal swing adsorption including the use of hot
inert
gases circulating in a closed cycle to desorb the VOCs.
One device employed for drying air in confined areas such as a home, a
ship or a building down to 0.001 lbs water per pound of air is discussed in
U.S.
Pat. No. 4,134,743. This document discloses a process and apparatus where the
adsorbent body is a wheel of thin sheets or layers of fibrous material
containing
about 10 to 90% by weight of a freely divided molecular sieve material. The
apparatus includes a means for passing air to be processed in one direction
through the wheel and a means for passing a regenerative air stream
countercurrent to the air to be processed. In addition a cooling stream is
provided =
in a direction co-current with the air stream.
U.S. Pat. No. 4,887,438 discloses a desiccant assisted air conditioning
system for delivering dehumidified refrigerated air to a conditioned space
based on
an adsorbent wheel. Meckler teaches the use of a desiccant wheel coated with
silica gel, or a preferred hygroscopic salt, lithium chloride, to remove
moisture from
air. This citation teaches the use of waste heat from the refrigeration
condenser to
heat the reactivation air and employs a liquid refrigerant injection into the
compressor to increase the pressure ratio in a positive displacement
compressor
to counter the problem of "thermal dumpback". Thermal dumpback is the
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associated heat conducted from the desiccant wheel to the treated air which
occurs following the exposure of the wheel to heated regeneration air. This
associated heat adds to the overall cooling load on the refrigeration system.
U.S. Pat. No. 5,242,473 discloses a gas dehumidifying apparatus which
exhibits improved dehumidification efficiency to provide a treated gas with a
high
dryness level employing two dehumidifier rotors, wherein the second rotor uses
a
synthetic zeolite. Gas to be dehumidified is passed first to a silica gel
coated rotor
and then to the zeolite coated rotor. The rotors are regenerated by supplying
a
stream of heated gas through the second rotor and then the first rotor SO that
the
adsorbent in the first rotor is regenerated at a lower temperature than the
zeolite in
the second rotor. A portion of the treated gas is used to countercurrently
cool the
rotors following regeneration.
Of the techniques for removing volatile organic contaminants in low
concentrations from a gas stream by adsorption, the most common is exemplified
in U.S. Pat. No. 4,402,717. This document teaches an apparatus for removing
moisture and odour from a gas stream comprising a cylindrical honeycomb
structure made from corrugated paper, uniformly coated with an adsorbent and
formed in the shape of a disk or wheel. The multiplicity of adsorbent-coated
parallel
flow passages formed by the corrugations in the paper serve as gas passage
ways
which are separated as a zone for the removal of water and odor causing
components in the gas, and as a zone for the regeneration of the adsorbent.
The
zones for removal and regeneration are continuously shiftable as the wheel is
rotated circumferentially about its centerline. Labyrinth seals separate the
outer
side of the rotary structure from the cylindrical wall of a sealed casing.
One of the problems associated with prior art attempts to use a honeycomb
matrix is the adsorbent that is used. Hydrophilic adsorbents such as silica
gel are
typically chosen for dehumidification applications, but are poor adsorbents
for
volatile organic contaminant removal. One such process combination is
discussed
in U.S. Pat. No. 5,181,942. On the other hand, hydrophobic adsorbents, such as
high silica zeolites are typically recommended for VOC removal applications,
but
are poor adsorbents for dehumidification application. Thus, applications for
both
dehumidification and VOC removal may typically require both types of
adsorbents.
Attempts have been made whereby a single adsorbent can be employed for
both operations. Furthermore, the adsorbent-coated paper and some adsorbent
salts have a limited range of humidity and temperature within which they can
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maintain structural integrity. This failure also limits the regeneration
medium to dry,
moderate temperature gases and air. Contact between the adsorbent and gas
stream and hence adsorbent capacity for VOCs is limited to very thin layers of
adsorbent on the surface of the fiber. This feature also limits the ultimate
life of
5 adsorbent wheel, resulting in frequent wheel replacement.
It is recognized that chemical filtration strategies are ideal for use in
control
of airborne molecular contamination. However, the use of honeycomb matrix
based
desiccants has not generally been considered due to the difficulty in ensuring
both
humidity Control and chemisorption.
Filters are generally devices such as a membrane or a layer that is
designed to block or contain objects or substances while letting others
through.
Filtration media are generally of three types ¨ mechanical, biological and
chemical.
Chemical filters are preferred for removal of gaseous contaminants in a given
environment since they provide air purification by both adsorbing and
absorbing
odours by adsorbents/impregnated adsorbents such as potassium permanganate
by controlled oxidizing action.
Gaseous contaminants either could be inorganic or organic. In order to
remove gaseous contaminants, selection of adsorbents to adsorb gas or
reactants
for adsorbing gas/impregnates is very important for the chemical filter media.
As
discussed above, a single chemical filter media may not adequately control
multiple contaminants or classes of AMC.
OBJECTS OF THE INVENTION
One object of the present invention is to provide a desiccant based
honeycomb chemical filter where the desiccant is generated in situ, and
impregnated with oxidizing chemicals, acidic or alkaline chemicals, or
reducing
chemicals to remove/contain gaseous contaminants, where the loading percentage
of the impregnants is significantly higher than thought possible in the art.
The primary object of the present invention is to increase the capability of
removing gaseous contaminants whether, acidic, alkaline, or organic and
increase
chemical filter life by the use of higher amounts of impregnants.
A further object of this invention is to produce a desiccant based
honeycomb matrix with or without active carbon or hydrophobic zeolite for
impregnation with different impregnants/reactants to remove gaseous
contaminants.
Another object of the invention is to provide a desiccant based honeycomb
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matrix where pressure loss is minimized. The desiccant may or may not contain
active carbon or hydrophobic zeolite and may or may not be impregnated with
impregnants for enhanced removal of different contaminants.
A further object of the invention is to provide a desiccant based honeycomb
matrix chemical filter that provides an energy saving, and may be with or
without
active carbon or hydrophobic zeolite, and may be impregnated with different
impregnants or un-impregnated, while also enabling enhanced odour control.
SUMMARY OF THE INVENTION
Accordingly, to overcome the problems encountered in the prior art, the
present invention provides a honeycomb matrix comprising a desiccant prepared
in-situ, for use as a chemical filter, as described herein below.
The present invention provides a desiccant based honeycomb chemical
filter comprising a matrix formed of a substrate having a desiccant generated
in
situ or deposited thereon, said desiccant being selected from the group
consisting
of metal silicates, silica gel, molecular sieves, activated alumina, activated
carbon
or hydrophobic zeolite, and any mixture thereof, said substrate being further
impregnated with one or more of an oxidizing agent, or an alkali metal
hydroxide,
or strong or weak acid(s), or reducing agents.
The substrate comprises an inorganic substance or an organic substance or
a mixture thereof.
In a further embodiment, the substance is in the form of a fiber or a pulp.
In a further embodiment, the inorganic fiber is selected from the group
consisting of glass fiber, Kraft paper, ceramic paper and any combination
thereof,
and is preferably a glass fiber.
In another embodiment, the desiccant is selected from a metal silicate and
silica gel or combination of metal silicate and activated carbon and/or a
hydrophobic desiccant such as zeolites selected from HISIV and ZSM-5
The metal silicate is selected from potassium silicate and sodium silicate,
preferably where the silicate is a neutral grade sodium silicate.
In a preferred embodiment, in the sodium silicate, the ratio of Si02 to Na20
is in the range of 1:2.0 to 1:4.0, preferably in the range of 1:3.
In another embodiment of the invention, the impregnated material is
selected from the group consisting of potassium permanganate, sodium
permanganate, potassium hydroxide, sodium hydroxide, phosphoric acid and
sodium thiosulphate.
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In a further embodiment of the invention, the loading of the impregnated
material in the matrix is in the range of 4% to 40 %, preferably about 20%.
In a further embodiment of the invention, the binder is preferably absent.
In a further embodiment of the invention, the basic weight of the substrate is
in the range of 50 to 150 gsm, preferably in the range of BO to 120 gsm.
The present invention also provides a method for manufacture of a
desiccant based honeycomb chemical filter, comprising the steps of
(i) dehydrating of macroporous or microporous desiccant/desiccants based
honecomb matrix at temperature 100 C or more;
(ii) impregnating the dehydrated desiccant based honeycomb matrix with
suitable impregnants and then drying the said matrix with retaining the
moisture content in the range of 15-30%.
The method when used in a situation comprising in situ generation fo
desiccant comprises the following steps -
(i) treating a substrate material with a desiccant forming material or a =
desiccant selected from the group consisting of metal silicates, silica gel,
molecular sieves (hydrophillic or hydrophobic), activated alumina, activated
carbon, and any mixture thereof, wherein the desiccant forming material is
in the form of a solution with a concentration in the range of 15 ¨40 /0;
(ii) drying the treated material to reduce moisture content thereof to a
range of
15-40%;
(iii) soaking the substrate matrix in water soluble metal salt or salts or
strong or
weak acids to form a hydrogel and obtain a gel matrix;
(iv) washing the gel matrix obtained in step iii to remove excess reactants
and
unwanted by-products;
(v) drying the gel matrix under controlled conditions of temperature to
convert
the hydrogel to an aerogel matrix;
(vi) treated the aerogel matrix with one or more of an oxidizing agent, or
an
alkali metal hydroxide, or strong or weak acid(s), or reducing agents;
(vii) drying the treated aerogel matrix of step (vi) above at a temperature
in the
range of 50 - 140 C to obtain said chemical filter.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a solution for the problems in the art of
ensuring maximum efficiency in adsorption of chemical contaminants using a
honeycomb based chemical filter using a desiccant. Honeycomb matrixes can be
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formed from a variety of substrates such as plastic sheet, metal/aluminum
foil,
organic and/or inorganic fiber substrates which are "paper" like, and which
may be
quite porous. The deposition or loading of a desiccant material on the
substrate is
a function, generally, of the substrate of choice, the amount of desiccant
that is to
be deposited or loaded, and the temperature at which the air/matrix is
intended for
use. Among known techniques for deposition or loading the desiccant on the
substrate to prepare the matrix are coating, impregnation and in situ
synthesis.
Coating and impregnation are chosen where the desiccant powder is
produced in bulk and are intended for industrial applications other than HVAC
air
treatment. In situ synthesis is generally chosen in case of desiccant material
such
as silica gels and metal silicates.
In the desiccant based honeycomb chemical filter of the invention, the
desiccant is generated in situ, and impregnated with oxidizing chemicals,
acidic or
alkaline chemicals to remove/contain gaseous contaminants, and the loading
percentage of impregnants is significantly higher than thought possible in the
art.
In a surprising finding, this has resulted in an increase in the capability of
removing gaseous contaminants whether, acidic, alkaline, or organic and
increase
chemical filter life by the use of higher amounts of impregnants.
The desiccant based honeycomb matrix can be made with or without active
carbon or hydrophobic zeolite, for impregnation with different impregnants/
reactants to remove gaseous contaminants. Another significant feature of the
invention is that pressure loss is minimized in the desiccant based honeycomb
matrix. The desiccant may or may not contain active carbon or hydrophobic
zeolite
and may or may not be impregnated with impregnants for enhanced removal of
different contaminants.
The desiccant based honeycomb matrix chemical filter of the invention also
provides an energy saving, and may be with or without active carbon, and may
be
impregnated with different impregnants or un-impregnated, while also enabling
enhanced odour control.
As stated previously, air to be treated, particularly outside air, generally
contains several gaseous contaminants, e.g. VOCs, odour, and the like. It is
desirable to remove these through the desiccant matrix. In air to be treated,
particularly when the air is pre-cooled to near saturation, such gaseous
contaminants are sometime water soluble and together are condensed in the
macro porous desiccant mainly through capillary adsorption. The prior art
contains
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some references to the use of microporous desiccants. However, these
desiccants
generally have limited capillary adsorption and therefore limited adsorption
of
gaseous contaminants.
The prior art also shows a limited loading capability for both the desiccant
and any impregnants due to the fact that generally the desiccant is coated or
impregnated on the substrate. Thus, adsorption in these cases is a surface
phenomena. This necessarily results in the use of less dense paper, typically
in the
range of 20 to 80 gsm thickness. The requirement of coating of the substrate
is
generally a limiting factor in the substrate choice, and it was considered
unfeasible
to increase the density of the substrate without compromising on the surface
area
required for adsorption.
The problems encountered in the prior art are solved in the present
invention by providing a honeycomb matrix comprising a desiccant prepared in-
situ, for use as a chemical filter, as described herein below.
The invention is achieved by a macroporous or microporous desiccant
based honeycomb matrix. The substrate is made from various inorganic or
organic
substances or combinations thereof. The substance can be a fiber or pulp. The
inorganic fiber is selected from glass fibre, Kraft paper, ceramic paper, etc.
and is
preferably glass fiber. The desiccants that can be synthesized in situ
preferably
comprise metal silicate/ silica gel which provide a high adsorption capacity.
The invention will now be described with reference to an exemplary and
non-limiting embodiment where glass fiber is used as the substrate. The
process
comprises laminating water glass treated/ impregnated glass fiber in the shape
of
either a honeycomb or as a block. The matrix is treated with either water
soluble
salts (whether divalent or trivalent) or any acid (whether organic or
inorganic) to
convert the water glass silicate into the active desiccant material with a
pore
diameter in the range of 10 ¨ 70 A , a pore volume of 0.10-0.80gm/cc and
surface
area of 300-700 m2.
The desiccant based honeycomb matrix is generally a flat and/or corrugated
sheet of the fiber glass substrate where the active desiccant has been
synthesized
into the pores. The fiber glass substrate used in the present invention is a
highly
porous material having the fiber diameter of 4-15 micron, fiber length 6-10mm,
thickness in the range of 0.10-0.75 mm. The binder content of the fiber glass
substrate used in the process is not greater than 8% and the preferred binder
used
in making substrate is polyvinyl alcohol or combination with acrylic. It is
advisable
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to use a low content of organic binder in the substrate so that the percentage
of
organic content in the desiccant based honeycomb matrix is also reduced.
Generally, the need for adding an external binder is avoided where the
water soluble silicate desiccant is generated in situ. The silicate material
itself
5 carries out the function of a desiccant once treated with suitable
reactant/s.
However, when the desiccant is to be coated on the matrix, it is advisable to
add
an additional binder material. Water soluble silicates can be used for this
purpose,.
In the alternative commercially available binders may also be used where the
desiccant is coated on the matrix.
10 The basic weight of the substrate used in the process is 50-150 gsm,
preferably between 80 to 120 gsm. The fiber used in the making of such
substrates
can be of electrical grade fiber. The flat sheet that is to be corrugated is
first
passed through the desired concentration of water glass solution (from 15-40%,
preferably 30%), with or without active carbon material or hydrophobic
zeolite. The
sheets are dried in a drying chamber, preferably using infra-red heating to
ensure
that the moisture content is in the desired range of 15-40%, preferably 20%.
While,
other types of water soluble silicates such as potassium silicate can also be
used,
sodium silicate is preferable due to its cost efficiency, high solubility of
its by-
products, higher bonding strength and easy availability.
It is preferable to use neutral grade water soluble silicate, preferably
sodium
silicate to impregnate the fiber glass substrate. It is observed that the
presence of
high alkali content in alkaline grade as compared to neutral grade silicate
necessitates use of a large quantity of reactants. Use of alkaline grade
sodium
silicate also results in a higher rate of gelation than neutral grade sodium
silicate.
The gelling rate adversely affects the characteristics of the desiccants.
The process of in situ synthesis of the desiccant material on the substrate
can be accomplished simultaneously with the deposition of active carbon and/or
hydrophobic zeolite material such as HISIV or ZSM05. The use of active carbon
and/or hydrophobic zeolitic material enhances the level of odour removal and
also
provides a significant enhancement in structural strength of the substrate
itself.
Active carbon (and/or hydrophobic zeolitic material) in the form of powder is
mixed
with water soluble silicate or any suitable binder-inorganic or organic_prior
to the
formation of the desiccant in situ. This enables capture/encapsulation of
active
carbon particles or hydrophobic zeolite within the desiccant, thereby
enhancing
odour control, VOC adsorption, and the structural strength of the matrix
itself.
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The corrugated sheet is produced by passing the silicate (or silicate and
active carbon or hydrophobic zeolite) impregnated flat sheet through a toothed
roller. Moisture content of the silicate or silicate and active carbon or
hydrophobic
zeolite impregnated sheets, grade of water glass, corrugated rolls, the
control of
clearances on the adhesive rolls and the curing heat of the adhesive are
important
in making a single facer unit. The corrugated sheet and the flat sheet are
pressed
together using additional rolls of the same material that is used for
corrugation. It is
preferable to avoid the use of adhesive while making single facer, even with
the
same water glass used for impregnation, in order to avoid powdering or higher
pressure drops after synthesis. This is achieved by keeping higher moisture
content in the flat sheet.
The single facer that is a combination of flat and corrugated sheets can be
converted into either a block or a cylindrical shape. Neutral grade sodium
silicate is
preferably applied to ensure adhesion of the single facers sequentially prior
to
stacking or winding. Application of S102: Na20 in the range of 1:2.0 To 1:4.0
preferably 1:3.0 before winding or stacking of the single facer gives
additional
strength to the honeycomb matrix. The honeycomb matrix that is produced is
then
dried to enhance the adhesion of the sheets. If the ratio of Si02:Na20 is
the
bonding strength is weaker and if ratio of Si02: Na20 is above 4.0 the
adhesive
strength is less.
Another advantage of the use of neutral grade silicate is the rate of drying.
Moisture loss is affected by ratio of Si02: Na20 because water retention is a
direct
function of alkalinity. Higher ratio of silicate retains less amount of water.
Drying of
more alkaline silicates of the ratio of Si02: Na20 in the range of 2.0-2.6 is
slower
than silicate having Si02: Na20 in the range of 3.0-3.3.
The honeycomb matrix produced above either in block or cylindrical form is
then soaked in water soluble metal salt/salts or strong or weak acid/acids-
inorganic or organic in different proportions and other forms of solution to
produce
silicate hydrogel. The reaction between water glass silicates with metal salts
to
form insoluble metal silicate hydro gel is given in table below:
Na2SiO3 + Al2 (SO4)3 Al2 (SiO3)3 Na2SO4
Na2SiO3 + MgSO4 MgSiO3 + Na2SO4
Na2SiO3 + MgCl2 4 MgSiO3 + NaCI
Na2SiO3 + AlC13 9 Al2 (SiO3)3 + NaCI
Na2SiO3+ 1-1CI 9 H2SiO3 + NaCI
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Washing of the gel matrix is essential to remove by-products and excess
reactants formed during the synthesis of active materials. The acidity of gel
matrix
due to presence of excess reactants deteriorates the construction material
used in
the system. The hydrogel is washed at controlled pH. The gelling pH of the
matrix
or the concentration of reactant/ reactants, temperature, reaction time used
in the
process changes the active material characteristics such as pore size,
porosity,
pore volume and surface area.
The element is further dried under specified conditions to convert the
hydrogel into the aerogel. It is observed that the type of silicate, types of
salts its
pH, concentration, temperature and time during which the gel is aged or
otherwise
treated, greatly affects the gel characteristics such as pore diameter, pore
volume,
surface area, adsorption capacity etc., The other important factors which
affects
gel characteristics are salt contents and surface tension of the liquid medium
as it
is being evaporated from the pores of the gel.
The manufacture of the desiccant based honeycomb matrix chemical filter
of the invention comprises laminating water glass treated/impregnated glass
fiber
substrate (with or without carbon or hydrophobic zeolite ) in the form/shape
of
cylinder or block, treating this element with one or more water soluble
divalent/trivalent salt(s) or organic or inorganic acid(s) ¨ strong or weak,
to convert
the water glass into active material, and then impregnating the active
material with
different impregnants.
The produced aerogel is immersed / impregnated with a solution of
oxidizing agent(s), alkaline solution and weak acid solution or any reactants
suitable for removing gaseous contaminants of different concentration at
different-
temperature for different soak time. When the honeycomb is sufficiently
impregnated with solution, the excess impregnated is drained of and kept in
another vessel after readjusting the concentration.
Time required for full impregnation vanes as a function of the structure of
the adsorbent, temperature and other factors. The honeycomb material is then
placed in an oven and heated till free moisture and water evaporate or are
driven
out of the mixture leaving impregnating material within the pores of honeycomb
silicate aerogel material and/or silicate hydrogel and activated carbon or
hydrophobic zeolite. Preferably, the drying temperature of the impregnated
honeycomb material is in the range of 50-140 C. The exposure time in the
heating
is varied with quality and quantity of materials, heating efficiency and other
factors.
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After preparation, the honeycomb matrix impregnated with impregnants is
preserved until ready for use. The impregnation steps are described below:
a. dehydration of macroporous or microporous desiccant/desiccants based
honeycomb matrix
b. impregnation with impregnant/impregnants
c. drying the impregnated macroporous or microporous desiccant/desiccants
based honeycomb matrix.
In our experiments the preferred impregnants used are potassium and
sodium permanganate, sodium or potassium hydroxide, weak acids such as
phosphoric acid, and a reducing agent such as sodium thiosulphate. The loading
of impregnants depends on various factors such as types of desiccants,
concentration of impregnants, soak time, temperature, number of dips, etc.
Impregnation with Potassium Permanganate and Sodium Permanganate
Macroporous or microporous desiccant (with or without active carbon)
material supported honeycomb matrix is impregnated with potassium/sodium
permanganate solution in a range of 5 to 40%, preferably 15% concentration in
case of potassium permanganate and 30% in case of sodium permanganate, at
10 C to 90 C, most preferably at 70 C for 5 to 60 minutes, most preferably for
15
minutes, to get the maximum impregnant loading without affecting the
mechanical
strength of the matrix and wateriCTC adsorption capacity. Table 1 gives
details of
factors that affect percentage of loading of impregnant (KMn04 vs. NaMn04) and
adsorption capacity of impregnated honeycomb desiccant based matrix.
TABLE 1
A. CONCENTRATION OF IMPREGNANTS
Parameters KMnO4 NaMnat
Cl C2 03 C4 05 06 C1- C2 C3 04 C5 C6
1.% Loading 1.9 2.7 4.8 4.7 - - 4.8 9.6 12.7 15.9 24.0 28.0
7
2.Water 35
32 29 - - - 47 41 48 38 44 43
adsorption/
CTC
B. SOAK TIME OF KMn04 & NaMn04
Parameters KMn0.4 NaMn04
ST1 ST2 ST3 ST4 ST1 ST2 ST3 ST4
1.% Loading 10.2 11.3 10.7 13.2 24.0 23.1 24.8 25.1
2.Water adsorption/CTC 33 32 32 32 44 39 34 33
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C. TEMPERATURE OF KMno4 & NaMn04
Parameters KMnat NaMn04
T1 T2 T3 Ti T2 T3
1.% Loading 9,6 9.5 17.3 25.5 25.8 23.4
2.Water adsorption/CTC 29 27 23
D. KMn04& NaMn04 (Macro vs Micro)
Parameters _ Macroporous Microporous
KMna, NaMn04 KMn04 NaMnat
1.% Loading 10.2 26.5 9.6 19.6
Concentrations, soak time, solution temperature, successive dipping and
surface property of active material play an important role in achieving the
twin
objective loading and adsorption. The method of Impregnation of alkali
preferably
sodium or potassium hydroxide, or of acid preferably phosphoric acid and a
reducing agent such as sodium thiosulphate with micro or macroporous and or
with hybrid desiccant (insoluble metal silicate and activated carbon acid) is
described hereinafter. A slurry of activated carbon is prepared in water glass
solution and honeycomb matrix is wound in the form of a block or a cylinder.
The
honeycomb matrix supported with water glass and activated carbon is treated
with
a salt solution as described above. The desiccant material, whether
macroporous
or microporous (and with or without active carbon), supported honeycomb matrix
is
impregnated with different concentrations of impregnants.
Impregnation with Potassium hydroxide
Macroporous or microporous desiccant (with or without active carbon and/or
hydrophobic zeolitic material) material supported honeycomb matrix is
impregnated with potassium hydroxide solution in the range of 2 to 15%, most
preferably 6%, at 10 C to 50 C, most preferably at 30 C for 5 to 60 minutes,
most
preferably for 15 minutes to get the maximum impregnant loading without
affecting
the mechanical strength of the matrix and water/CTC adsorption capacity. Table
2
gives details of factors affecting the % of loading of impregnant (KOH) and
adsorption capacity of impregnated honeycomb desiccant based matrix.
TABLE 2
A. CONCENTRATION OF IMPREGNANTS ¨ macroporous vs. macroporous with
activated carbon (hybrid desiccants)
Parameters Macro Hybrid
desiccants
Cl C2 C3 Cl C2 C3
1.% Loading 5.8 11.0 10.4 5.2 5.6 6.8
2.Water adsorption/CTC 44 46 49 41 41 49
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B. SOAK TIME OF KOH
Parameters Macro Hybrid desiccants
ST1 ST2 ST3 ST4 ST1 ST2 ST3 ST4
1. /o Loading 8.05 10.1 11.0 10.1 5.9 7.1 8.2
5.9
2.Water adsorption/CTC - 44 44 46 50
C. TEMPERATURE OF KOH
Parameters Macro Hybrid desiccants
Ti T2 T3 T1 T2 T3
1.% Loading 10.3 8.7 7.8 7.5 6.0 6.3
5 Impregnation with Phosphoric Acid
Macroporous or microporous (with or without active carbon) material
supported honeycomb matrix is impregnated with phosphoric acid solution in the
range of 2 to 15%, most preferably 8%, at 10 C to 50 C, most preferably at 30
C
for 5 to 60 minutes, most preferably for 15 minutes to get the maximum
impregnant
10 loading without affecting mechanical strength of the matrix and water/CTC
adsorption capacity. Table 3 gives details of factors affecting percentage
loading of
impregnant (Phosphoric acid) and adsorption capacity of impregnated honeycomb
desiccant based matrix.
TABLE 3
15 A. CONCENTRATION OF IMPREGNANTS - macroporous vs. macroporous and
activated carbon (Hybrid desiccants)
Parameters Macro Hybrid desiccants
Cl 02 C3 Cl C2 03
1.% Loading 4.5 6.4 10.2 6.26 6.0
10.5
2.Water adsorption/CTC 46 37 37
B. SOAK TIME OF PHOSPHORIC ACID
Parameters Macro Hybrid desiccants
ST1 ST2 ST3 ST4 ST1 ST2 ST3 ST4
1.% Loading 8.3 7.8 8.5 8.8 8.4 8.4 11.2 8.4
2.Water adsorption/CTC - 30 30 29 31
C. TEMPERATURE OF PHOSPHORIC ACID
Parameters Macro Hybrid desiccants
T1 T2 T3 T1 T2 T3
1.% Loading 8.3 6.7 8.5 10.2 , 8.2 9.6
2.Water adsorption/CTC 38 36 35
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D. KOH (Macro vs Micro)
Parameters macroporous microporous
1.% Loading 11.5 6.5
Impregnation with Sodium Thiosulphate
Macroporous or microporous (with or without active carbon or hydrophobic
zeolite) material supported honeycomb matrix is impregnated with sodium
thiosulphate solution in the range of 5 to 45%, preferably 15 A., at 10 C to
50 C,
most preferably at 30 C for 5 to 60 minutes, preferably for 15 minutes to get
maximum impregnant loading without affecting the mechanical strength of the
matrix and water/CTC adsorption capacity. Table 4 gives details of factors
affecting
% of loading of impregnant (Sodium thiosulphate) and adsorption capacity of
impregnated honeycomb desiccant based matrix:
TABLE NO. :4
A. CONCENTERATION OF IMPREGNANTS ¨ macroporous vs. macroporous and
activated carbon (Hybrid desiccants)
Parameters Macro Hybrid desiccants
Cl C2 C3 C4 Cl C2 C3 C4
1.% Loading 7.4 10.1 17.5 18.0 8.5 6.3 8.1
B. SOAK TIME OF SODIUM THIOSULPHATE SOLN.
Parameters Macro Hybrid desiccants =
ST1 ST2 ST3 ST4 ST1 ST2 ST3 ST4
1.% Loading 18.9 16.6 15.2 18.3 13.5 6.1 10.4 7.5
C. TEMPERATURE OF SODIUM THIOSULPHATE SOLN.
Parameters Macro Hybrid desiccants
T1 T2 13 T1 T2 13
1.% Loading 15.4 9.7 15.4 5.6 8.6 12.0
The present invention provides several significant advantages over prior art
methods. Some of-these are summarized below.
1. While macroporous material is generally preferred, potassium permanganate
can also be impregnated to a large loading percentage on microporous material.
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2. There is an enhancement in the extent of loading due to several factors,
some of
which are given below:
(a) The GSM of the substrate is significantly higher than that used in the
prior art.
The GSM generally ranges from 50 to 150 gsm, preferably between 80 to 120
gsm. This enables higher desiccant loading on the substrate leading to a
larger
impregnate loading of the impregnate;
(b) The formation in situ of the desiccant is done such that it is pH neutral
and is
generally minimally affected by the chemical nature of the gas, whether acidic
or
basic.-Binders are preferably avoided and not deemed necessary for the
formation
of the desiccant preparation and the desiccant materials thus obtained are all
active materials. This also results in the final increase in surface density
of the
substrate of from 400 to 750 gsm.
(c) The extent of loading is enhanced also due to a combination of different
factors
such as soak time, temperature and concentration. For example, permanganates,
usually Na permanganate achieves a loading of greater than 30%. Additionally
the
necessary moisture content is maintained, higher adsorption capacity of the
desiccant since it is synthesized in situ, additional advantages include the
fact that
the desiccant can be bacteriostatic, and non-flammable.
(d) Desiccant can be combined with active carbon or hydrophobic zeolite ,
preferably by mixing the carbon powder or a hydrophobic desiccant such as
zeolitic material selected from HISIV or ZSM-05 with a suitable binder,
preferably
inorganic prior to formation of the desiccant in situ. This enables
capture/encapsulation of the carbon particles within the desiccant. This
enables an
enhancement in odour control, VOC adsorption, and in the structural strength
of
the matrix itself.
3. The invention enables if required, a sacrifice of the surface area of the
matrix to
enable multiple fluid geometries, and allows lower pressure drops without any
significant material sacrifice of the performance.
4. Up to 100 % efficiency is achieved even despite reduced contact time when
run
at higher face velocities of 400 to 600 fpm [ 2 to 3 m/sec].
