A composite formed of small desiccant particles retained in a dark
matrix composed of a porous binder containing a transition metal
oxide with pores to provide moisture transport with respect to the
particles, and metallic fibers to remove the heat of condensation
during dehumidification and provide heat for the removal of moisture
during regeneration. The moisture absorbing properties of the composite
may be regenerated by exposure of the dark matrix to solar radiation
with dehumidification occurring at night.
The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. A composite useful for dehumidifying purposes and comprising
a matrix of a porous binder as a solid web containing a transition
metal oxide having internal surfaces defining pores and a plurality
of desiccant particles embedded in the matrix, the desiccant particles
having diameters in the range of about 1-300 .mu.m, constituting
a major portion of the composite and being selected from the group
consisting of silica gel, zeolite, alumina, activated carbon and
mixtures thereof, the composite being in sheet-like form with exterior
surfaces, a portion of the particles being below the exterior surfaces
and covered by other particles, the transition metal oxide in the
web being capable of transmitting moisture along the pore surfaces
between desiccant particles and between the exterior surfaces and
the desiccant particles.
2. The composite of claim 1 having a major surface and the matrix
exposed over at least a major portion of said surface, the matrix
being a light absorber and being heated when exposed to solar radiation
to cause entrained moisture to be removed by desorption and evaporation.
3. The composite of claim 1 containing a minor amount of metallic
fibers distributed in the matrix for heat transfer to and from the
4. The composite of claim 3 wherein the desiccant is silica gel.
5. The composite of claim 3 wherein the desiccant is zeolite.
6. The composite of claim 1 wherein the transition metal oxide
is manganese dioxide and the binder composition includes a rigid
7. The composite of claim 6 containing a minor amount of metallic
fibers distributed in the matrix for heat transfer to and from the
8. The composite of claim 7 wherein the desiccant is silica gel
with the particles having diameters in the range of about 1-150
9. The composite of claim 8 containing a minor amount of nonmetallic
fibers distributed in the matrix for structural reinforcement.
10. A method of producing a composite in sheet-like form useful
for dehumidifying air comprising the steps of:
providing a liquid binder containing a transition metal oxide,
forming a mixture of the liquid binder composition and a plurality
of desiccant particles embedded in the binder composition and having
diameters in the range of about 1-300 .mu.m, and
treating the binder composition to form a binder in the form of
rigid solid web containing the transition metal oxide and interconnecting
the particles, the web and particles being in sheet-like form having
exterior surfaces with a portion of the particles being below the
exterior surfaces and covered by other particles.
11. The method of claim 10 wherein the forming step includes incorporating
metallic fibers within the binder composition to provide heat transfer
paths to and from the desiccant particles.
12. The method of claim 11 wherein the forming and treating steps
form a first structure and the method further includes the steps
of dividing the first structure into a plurality of second particles,
forming a second mixture of said second particles and a second
liquid binder composition, and
treating the second binder composition to form a second binder
in the form of a rigid solid web interconnecting the second particles.
13. The method of claim 12 wherein the forming of said second mixture
includes incorporating metallic fibers within the second binder
composition to provide heat transfer paths to and from the second
14. The method of claim 13 wherein the forming of said second mixture
includes incorporating heat degradable fibers in said second binder
composition, and said method includes the step subsequent to the
second treating step of heating the second mixture to remove at
least a portion of the degradable fibers to form additional pores
in the composite.
15. The method of claim 14 wherein the forming of said second mixture
includes the step of incorporating metallic rods in the mixture.
BACKGROUND OF THE INVENTION
This invention relates to systems for removing moisture from a
gas and more particularly to systems for dehumidifying air and having
the capability of being regenerated thermally.
Silica gel and other similar desiccants have been used for a variety
of purposes associated with their adsorbent properties. These include
their use in the catalytic processing of organic compositions as
illustrated in U.S. Pat. Nos. 2265389 and 2882243 and moisture
absorption as illustrated in U.S. Pat. Nos. 3141729 and 4341539.
In general, with silica gel, it is often in the form of a multiplicity
of separate ground particles carried on a suitable support. In this
form, the usable surface area of the silica gel for moisture adsorption
is limited by requirements associated with its use. The silica gel
particles are often sized sufficiently large to reduce losses by
attrition and are located in closely packed arrangements to resist
movement. In some instances, they are modified to include channels
to the interior to increase the usable surface area for adsorption.
However, the size and limited exposed surface area limits their
effectiveness. Also, regeneration of the silica gel often requires
a separate and complex processing operation to remove the adsorbed
One object of the invention is a desiccant having a large surface
area available for moisture adsorption compared to its volume. A
second object of the invention is a desiccant which may be regenerated
within a relatively short time. An additional object is a dehumidification
system utilizing a desiccant capable of being regenerated. Another
object is a desiccant capable of being regenerated by the use of
a low cost source of heat. These and other objects will become apparent
from the following description.
SUMMARY OF THE INVENTION
This invention is directed to a composite formed of a finely divided
desiccant in a porous matrix modified by the presence of a transition
metal oxide. The matrix may include a porous siliceous composition
as a separate binder. Advantageously, the transition metal oxide
such as manganese oxide has a pore structure for transmitting moisture
to the desiccant during dehumidification and from the desiccant
particles during regeneration. Preferably, the composite is particulate
or sheet-like and has the matrix exposed on at least a major portion
of one or more of major surfaces. The matrix is also preferably
dark and absorbs solar energy. The composite advantageously further
includes metallic fibers, rods or the like for heat transfer to
and from the desiccant particles. In this manner, heat of condensation
may be removed and during regeneration, heat may be supplied for
moisture removal. Under these conditions, the composite is particularly
useful for being regenerated by exposure to solar radiation during
which the matrix is heated to an elevated temperature which causes
entrained moisture in the desiccant to be removed by evaporation.
With transition metal oxides such as manganese dioxide which are
dark in color, the temperatures may reach values exceeding 100.degree.
Several advantages are associated with the invention. In the composite,
small particles of the desiccant may be used which otherwise would
be difficult to retain on a support. With small particles of desiccant,
improved surface areas of desiccant are available for moisture adsorption.
The porous matrix provides channels for moisture transport between
the individual particles and between the outer surfaces of the particles.
The incorporation of metal fibers provides improved heat transfer
to and from the interior of the composite for moisture desorption
and readsorption. Structural strength is also improved by the addition
of metal or nonmetallic fibers. With a matrix having significant
radiant energy absorption, the use of low cost solar energy may
be used directly for moisture removal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view partially in cross-section of a desiccant
block as one embodiment of the invention.
FIG. 2 is an enlarged side view of a section of FIG. 1.
FIG. 3 is a side view of a composite particle of the invention.
FIG. 4 is an enlarged view of a section of FIG. 3.
FIG. 5 is a view of a roof mounted dehumidification and solar regenerative
DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT
In accordance with the invention, a composite is provided for dehumidifying
purposes which comprises a matrix of porous binder containing a
transition metal oxide having internal surfaces defining pores and
capable of transmitting moisture along the pore surfaces. Embedded
in the matrix are a plurality of desiccant particles which constitute
a major portion of the composite. Suitably, the particles are composed
of silica gel, zeolite, alumina, activated carbon and the like with
silica gel being preferred. The performance of the composite is
enhanced by the presence in the matrix of a porous transition metal
oxide capable of transferring water along pore surfaces. These oxides
are described in copending application Ser. No. 452361 entitled
"Solid Electrolyte Structure" by Anthony V. Fraioli and
filed Dec. 22 1982 which issued as U.S. Pat. No. 4477541 which
is hereby incorporated herein by reference. In general, the oxide
may be expressed as T.sub.a X.sub.b O.sub.c where T is a transition
metal, X is a transition metal, Al, Sn and/or an alkali metal such
as Li, K, Rb, Cs and the integers a, b and c represent the number
of atoms to balance the formula. Preferably, the transition metal
oxide readily absorbs solar energy as represented by manganese oxide,
nickel oxide, titanium oxide, chromium oxide and the like.
Preferably, the matrix is further characterized by having a plurality
of fibers extending into the interior of the matrix. With metal
fibers such as copper, aluminum or the like, they advantageously
provide heat conducting paths to remove heat of condensation during
dehumidification and to impart heat to aid in the removal of moisture
during regeneration. These may be small lengths or larger shapes
such as nails. With nonmetallic fibers such as graphite, they provide
structural strength to the composite.
As illustrated in FIG. 1 and 2 the composite 10 of the invention
is composed of a plurality of desiccant particles 12 within a matrix
14 composed of a porous binder 16 containing a transition metal
oxide capable of transmitting moisture along the pore surfaces 17.
Preferably the binder 16 also contains a sodium silicate composition
which has been acid neutralized to form a rigid silicate web or
skeleton structure 19. As illustrated, the desiccant particles 12
may form part of larger particles 15 prepared in a two-stage procedure
as described herein.
The composite 10 may be formed in sheet-like or block structures
18 for use in dehumidification systems illustrated in FIG. 5. Incorporated
in the matrix 14 are nonmetallic fibers 20 for structural strength,
and metallic fibers or rods illustrated in the form of threads 21
and nails 22 for heat transfer. Advantageously, the fibers 21 extend
between particles 12 and between particles 15. Representative dimensions
for structure 18 are 12 inches by 4 inches by 1 inch.
Suitably, the desiccant particles may be silica gel, zeolite, alumina,
activated carbon and the like. Preferably, the desiccant is silica
gel. In this invention, the desiccant is in the form of small particles
held together in a porous binder composition. With small particles,
increased surface area of the desiccant is possible in a larger
structure. Suitably, the desiccant particles will have an exterior
dimension in the order of about 1-300 .mu.m and preferably about
While binder compositions may be prepared in which the porous transition
metal oxide is present in relatively large amounts (e.g., above
70 wt. %), weight percentages in the order of about 20-80 wt. %
of the binder composition are used. Frequently, a supplemental binder
is required to provide the desired binding properties. A sodium
silicate may be used for this purpose and when acid neutralized
provides a rigid silicate web or skeleton of the desiccant particles
held together. An alumina gel may also be used to form the binder.
In forming the larger structure, the desiccant particles may be
mixed with the binder composition and one or more fibers for strength
and heat transfer. The resultant paste is packed into a form preferably
having larger metal nails or rods in addition to smaller fibers
(e.g., 5 .mu.m in diameter) and then baked and/or chemically treated
to form the desired web-like structure.
In general, the composite contains desiccant in major amounts of
over 50 wt. %, advantageously about 50-95 wt. % and preferably about
80-95 wt. %. The inert components are preferably below about 20
wt. % and especially about 1-10 wt. %. Usually, the fiber content
is about 5-20 wt. % of the binder composition.
In a preferred method of producing the composite, a two stage process
is used. In the first stage, particles of the type illustrated in
FIG. 3 may be used. These are generally in the order of 1-150 .mu.m
in diameter and are mixed with a binder composition containing a
transition metal oxide and a siliceous binder with metallic and
nonmetallic fibers also being present. After being connected to
the web-like structure, the composite is reduced in size to small
particles (e.g., 50-300 .mu.m) which are then formed into paste
mixture with additional binder composition and fibers. Heat degradable
fibers such as cellulose fibers are also included to form larger
pores 23 in FIG. 2 (1-100 .mu.m) when removed. Conversion of the
binder composition to a web-like structure is then accomplished.
In the second stage, larger fibers of 1-2 inches may also be used
in an arrangement containing metallic nails or rods. As illustrated
in FIG. 2 coated particles 24 are dispersed in the matrix.
FIGS. 3 and 4 provide illustrations of a composite particle 30
which is subsequently combined with further binder compositions
and fibers to form the structure of FIG. 1. As illustrated, particle
30 includes desiccant particles 32 with coating 34 of a porous binder
36. Graphite, boron nitride or other similar nonmetallic fibers
37 are added to provide structural strength. These fibers may also
be metal-plated for heat transfer as fibers 38. Pores 39 (e.g.,
0.01-1 .mu.m) are provided in the binder 36 for moisture transfer.
FIG. 4 provides an enlargement of FIG. 3 to illustrate the presence
of the transition metal oxide 41 and supplemental binder 42.
FIG. 5 provides a schematic of a dehumidification and regeneration
system using the composite structures 18 of FIG. 1 in an assembly
50 mounted on the roof 54 of building 52. As illustrated, fan 56
and ductwork 58 60 and 62 act to recirculate air in building 52
past an arrangement 64 of composite blocks 66. Valves 68 and 70
isolate the system from outside air. Advantageously, the dehumidification
is carried out during nighttime hours with regeneration occurring
during daylight hours.
As illustrated in FIG. 5 regeneration may be carried out by opening
upper and lower vents 74 and 76 on collector frame 50 and passing
outside air over the composite blocks 66. In the process, the solar
radiation is the driving force to raise the temperature of the blocks
66 to increase the rate of moisture removal.
The above invention provides a useful desiccant composite, a method
of producing the composite, and a method of using the composite
for dehumidifying moist air and for regenerating the moist composite
using a source of solar energy. The metal fibers incorporated into
the composite provide heat transfer paths to and from the interior
of the composite to remove heat of condensation during dehumidification
and for adding heat during regeneration.
The foregoing description of embodiments of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations
are possible in light of the above teaching.