Molecular sieve abstract
An electrically regenerable gas filter system includes a carbon
fiber composite molecular sieve (CFCMS) filter medium. After a separate
medium-efficiency pre-filter removes particulate from the supply
airstream, the CFCMS filter sorbs gaseous air pollutants before
the air is recirculated to the space. When saturated, the CFCMS
media is regenerated utilizing a low-voltage current that is caused
to pass through the filter medium.
Molecular sieve claims
We claim:
1. An electrically regenerable gas filter method for removing pollutants
from a polluted gas comprising the steps of:
(A) Providing a carbon fiber composite molecular sieve medium comprising
an activated carbon fiber composite material further comprising
a multiplicity of porous carbon fibers bonded with a carbonizable
organic binder to form an open, permeable structure, said composite
defining a porosity in a range of approximately 82-86%;
(B) Passing the polluted gas into said filter medium to sorb pollutants
from the unacceptable gas onto said filter medium and to pass acceptable
gas therefrom; and
(C) Passing an electrical current through said filter medium to
desorb sorbed pollutants from said filter medium.
2. The method of claim 1 further comprising an additional step
of:
(D) directing the desorbed pollutants away from the acceptable
gas using a containment means.
3. The method of claim 2 wherein said containment means comprises
at least one means selected from the group of means consisting of
ducts, dampers, and fans.
4. The method of claim 2 further comprising the step of:
(E) exhausting said desorbed pollutants from said containment means.
5. An electrically regenerable gas filter apparatus comprising:
(A) a carbon fiber composite molecular sieve filter medium for
sorbing pollutants from an unacceptable gas flowing thereinto and
permitting an acceptable gas to flow therefrom comprising an activated
carbon fiber composite material further comprising a multiplicity
of porous carbon fibers bonded with a carbonizable organic binder
to form an open, permeable structure, said composite defining a
porosity in a range of approximately 82-86%; and
(B) a regenerating means for causing an electrical current to pass
through said filter medium to desorb sorbed pollutants from said
filter medium.
6. The apparatus of claim 5 further comprising:
(C) containment means for directing desorbed pollutants away from
acceptable gas.
7. The apparatus of claim 6 wherein said containment means comprises
at least one means selected from the group of means consisting of
ducts, dampers, and fans.
8. The apparatus of claim 6 wherein said containment means comprises
exhausting means for exhausting sorbed pollutants from said apparatus.
Molecular sieve description
CROSS-REFERENCE TO RELATED APPLICATIONS
This invention is related to the invention described in the patent
application entitled Method and Apparatus for Separating Gases Based
on Electrically and Magnetically Enhanced Monolithic Carbon Fiber
Composite Sorbents, Docket No. ERID 0065 filed on even date herewith.
Please refer to T. D. Burchell et al., U.S. patent application
Ser. No. 08/747109 filed Nov. 8 1996 entitled Activated Carbon
Fiber Composite Materal and Method of Making which is a Continuation
in Part of U.S. patent application Ser. No. 08/358857. The entire
disclosure of U.S. patent application Ser. No. 08/747109 is herein
incorporated by reference.
Please further refer to R. R. Judkins and T. D. Burchell, U.S.
patent application Ser. No. 08/601672 filed Feb. 15 1996 the
entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to sorption type gas/air filters,
and more particularly to those which employ carbon fiber composite
molecular sieve (CFCMS) air filter media.
BACKGROUND OF THE INVENTION
Current technologies for removing gaseous air pollutants from residential,
commercial, and industrial environments utilize sorbents such as
granular activated carbons and high-surface area aluminas. When
sorbents become saturated, they are removed from service for recycling
(pressure/temperature swing method) or for waste disposal. The effective
life of each sorbent depends on both the amount of pollutant captured
and the sorptive capacity of that material.
High service-cycle costs have limited the implementation of many
technologies for air quality improvements as in the case of activated
carbon systems. Major technical and operating problems associated
with granular sorbents include channeling, settling (packing), and
resistance to air flow. With new standards for indoor air quality,
American National Standards Institute (ANSI)/American Society of
Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE)
62-1989R, driving the application of cost-effective systems, lower
life cycle and service cycle costs are needed to meet the demands
of rapidly growing residential and commercial markets.
Gas separation, waste treatment, and catalyst supports are common
industrial uses for active carbons. Settling, channeling, and pressure
loss during operation are major problems associated with typical
granular activated carbons. A process has been developed to produce
a porous carbon fiber carbon composite with a density less than
0.2 g/cc. This structurally rigid composite can be activated to
produce a significant volume of mesopores (2 to 50 nm) and/or micropores
(<2 nm). Furthermore, the rigid structure has macropores in the
range of 10-500 microns which allow for excellent fluid flow through
the sample, resulting in an acceptable pressure drop. The rigid
nature of the composite also eliminates problems due to channeling
and settling, therefore making it a possible candidate for a catalyst
support. The material has a continuous carbon structure and is thus
electrically conductive. The passage of electric current, typically
1-20 amps at 1-5 volts for a small segment of media, causes the
carbon fiber composite molecular sieve to heat, thus electrically
and thermally desorbing sorbed gases.
Carbon fibers are produced commercially from rayon, phenolics,
polyacrylonitrile (PAN), or pitch. The latter type are further divided
into fiber produced from isotropic pitch precursors, and those derived
from pitch that has been pretreated to introduce a high concentration
of carbonaceous mesophase. High performance fibers, i.e. those with
high strength or stiffness, are generally produced for PAN or mesophase
pitches. Lower performance, general purpose fibers are produced
from isotropic pitch precursors. These materials are produced as
short, blown fibers (rather that continuous filaments) from precursors
such as ethylene cracker tart coal tar pitch, and petroleum pitch
prepared from decant oils produced by fluidized catalytic cracking.
Applications of isotropic fibers include: friction materials; reinforcements
for engineering plastics; electrically conductive fillers for polymers;
filter media; paper and panels; hybrid yarns; and as a reinforcement
for concrete.
More recently, interest has developed in activated forms of isotropic
carbon fibers, where high surface areas can be produced by partial
gasification in steam or other oxidizing gases. Activated carbon
fibers have novel properties that make them more attractive than
conventional forms (powders or large-size carbons) for certain applications.
While porosity can be generated in most types of carbon fiber, low
modulus fiber produced from isotropic pitch are particularly suited
for activation because of their unique structure, where the random
packing of small crystallites allows the development of an extensive
pore structure.
Among the possible applications, activated carbon fibers are of
interest for the adsorption and recovery of organic vapors; in environmental
protection; the removal of SO.sub.x and NO.sub.x from flue gas;
the improvement of air quality; and water treatment. Difficulties
in handling and utilizing activated carbon fibers can be surmounted
by their incorporation into composites, such as woven and non-woven
fabrics, felt and paper. This invention provides a rigid, activated
carbon fiber composite material that has an open and permeable structure
and can be produced in single pieces to a given size and shape.
The unique properties of the activated composite made from isotropic
pitch derived carbon fibers, such as; narrow, unimodal pore structure
in the fiber, high-surface area, rapid rates of adsorption and desorption,
the ability to form specific shapes of high permeability and strength,
suggest that, among other applications (notably in environmental
protection), they may be suitable for molecular separation on the
basis of size and shape.
Pressure swing adsorption (PSA) is a known process for separation
of gases from multicomponent gas mixtures. The PSA apparatus relies
on the physical process of adsorption, in which gases are selectively
adsorbed onto a substrate from a gas stream, thus depleting the
stream of one gaseous species. The adsorbed gas is then desorbed
at a lower pressure into a second gas stream thus enriching it with
the desorbed species. The desorption step regenerates the adsorbent
material for reuse during the subsequent adsorption step. It is
widely acknowledged that PSA technology has fully matured and that
further advances in this technology will require the development
of superior adsorbent and molecular sieve materials which have significantly
higher surface areas combined with mean micropore width of 5-10
.ANG..
PSA systems typically comprise several adsorption beds, through
which the gas stream is passed, allowing for the near complete separation
of the selected gas species. The adsorbent materials used in a PSA
unit are selected to have the appropriate mean micropore width (typically
in the range of 5-10 .ANG.) to selectively adsorb or sieve the required
gas species and additionally must possess large surface areas. Currently
available adsorbent materials include zeolites, with surface areas
in the range of 10-350 m.sup.2 /g, and activated carbons with surface
areas in the range of 500-1000 m.sup.2 /g. Conventional activated
carbons and carbon molecular sieves are granular in structure. During
operation in a PSA system, granular materials suffer attrition and
can settle resulting in the formation of channels which allow the
fluid stream to bypass the adsorbent.
OBJECTS OF THE INVENTION
Accordingly, objects of the present invention include the provision
of a new and improved regenerable air/gas filter which reduces operating
costs of gaseous air filtration systems by providing superior sorption
capacity and on-site regeneration.
The carbon fiber composite material of the present invention is
comprised of porous carbon fibers bonded to form a monolithic, open
and permeable structure. The carbon fiber composite defines surface
areas greater than 1000 m.sup.2 /g. Several characteristics of the
carbon fiber composite can be altered by altering selected conditions
during the production and activation of the composite.
Further and other objects of the present invention will become
apparent from the description contained herein.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, the foregoing
and other objects are achieved by an electrically regenerable gas
filter apparatus which comprises: a carbon fiber composite molecular
sieve filter medium for sorbing pollutants from unacceptable gas
flowing thereinto and permitting an acceptable gas to flow therefrom;
and a regenerating means for causing an electrical current to pass
through the filter medium to desorb sorbed pollutants from the filter
medium.
In accordance with a second aspect of the present invention, the
foregoing and other objects are achieved by an electrically regenerable
gas filter method for removing pollutants from unacceptable gas
which comprises the steps of: providing a carbon fiber composite
molecular sieve filter medium; passing the polluted gas through
the filter medium to sorb the pollutants from the gas onto the filter
medium and to pass acceptable gas therefrom; and passing an electrical
current through the filter medium to desorb sorbed pollutants from
the filter medium.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing the carbon filter 36 further
comprising a carbon fiber composite molecular sieve medium 37; unacceptable
air 38; filtered to acceptable air 39; and purge or scavenger air
40 and 40' to remove contaminants; and a regenerating means 20 whereby
an electrical current may be caused to pass through the filter medium
to desorb sorbed pollutants from the filter medium. In a preferred
embodiment the regenerating means 20 further comprises an electrical
circuit 21 further comprising a power supply 22 switching means
23 and electrical conductors 24.
In a more specific embodiment, FIGS. 2 and 3 show a heat pump or
air conditioner apparatus which comprises a case 1 containing a
heat transfer coil assembly 2 a room air blower assembly 3 with
a room air discharge 4 a conventional particulate air filter 5
a carbon filter 6 further comprising a carbon fiber composite molecular
sieve medium 7 an outside air and return air damper 8 an exhaust
air damper 9 an exhaust air blower 10 having an exhaust air discharge
11 an exhaust air back draft damper 12 a return air inlet 13
an outside air inlet 14 and associated ductwork well known to one
of skill in the art.
FIG. 2 is a schematic sectional view showing the air conditioning
or heat-pump unit mounted against the wall of a room. Arrows show
air flow through the apparatus in the normal operational mode.
FIG. 3 is a schematic sectional view of the same embodiment as
FIG. 2. Arrows show air flow through the apparatus in the cleaning
or purge mode.
For a better understanding of the present invention, together with
other and further objects, advantages and capabilities thereof,
reference is made to the following disclosure and appended claims
in connection with the above-described drawings.
DETAILED DESCRIPTION OF THE INVENTION
Carbon fiber composite molecular sieve (CFCMS) air filter media
is a semi-permanent activated carbon media which efficiently removes
many organic and inorganic gaseous air pollutants from an airstream.
The media can be installed in existing or new filter banks of industrial,
institutional, commercial and residential air handling and/or exhaust
systems.
Pollutants that are generally known to contribute to unacceptable
indoor and/or outdoor air quality (toxic, odor, irritant, material
damage) are targeted for removal by sorption. Inorganic pollutants
include ozone, radon, and carbon monoxide. Organic pollutant families
include n-alkanes, chlorinated hydrocarbons, ethers, aldehydes,
and aromatic hydrocarbons, and various hydrogen compounds. Here
we define unacceptable air as air not meeting the air purity requirements
of ASHRAE 62.
When the CFCMS media is saturated with pollutant, a low voltage
current is applied through the carbon fibers which causes heating
and desorption of sorbed pollutants from the fibers into the surrounding
purge air thereby regenerating the media. Regeneration can be performed
on-site, either inside the filter bank or externally.
After a separate medium-efficiency pre-filter removes particulate
from the supply airstream, the CFCMS filter sorbs gaseous air pollutants
before the air is recirculated to the space or exhausted. When saturated,
the CFCMS media is regenerated on-site (either inside the filter
rack or externally) utilizing a low-voltage current that is caused
to pass through the filter medium. The voltage is normally less
than about 150 volts AC or DC. Preferred voltage and current depends
upon geometry and size of the CFCMS media, but in a preferred embodiment
would cause heating to a temperature sufficient to desorb sorbed
pollutants, typically 50.degree. C. to 500.degree. C. The pollutants
are thermally desorbed from the CFCMS media and become airborne
in the surrounding purge air which transports the pollutant to exhaust
or disposal either through natural or forced air dispersion. A damper
mechanism may facilitate the flow of air through the filter bank
to atmosphere or to exhaust for disposal during the regeneration
phase.
The subject CFCMS filter media is useful in efforts to meet new,
more stringent air quality standards in an economically justifiable
fashion and avoid disposal problems associated with spent carbon.
Uniquely, the CFCMS filter media is suitable for installation as
a semi-permanent filter with on-site (in-place or external) regeneration.
The CFCMS filter media can be manufactured to fit most standard
filter housings for both retrofit and new construction applications.
Equipment for providing the necessary electrical current for regeneration
and trapping of desorbed pollutants is well-known to the skilled
artisan.
Carbon fibers used to fabricate the CFCMS filter media can be impregnated
with other, conventional sorbents to enable pollutant-specific sorption
or treated/impregnated to enhance sorption of nuclear, biological,
and chemical (NBC) challenge gases, well-known as battlefield threats
to military personnel. Moreover, the CFCMS can be tailored through
judicious selection of carbon filter types, fabrication process
parameters and activation conditions to meet specific sorptive challenges.
Acid gases and alkaline gases are examples of pollutants that may
require special additives for sorption.
Air quality applications for the present invention include residential,
vehicle, industrial, business, and military (nuclear, biological,
and chemical).
The CFCMS is especially useful in the air quality applications
because: the low concentrations of gaseous pollutants generally
experienced requires high performance sorbers; the families of gaseous
pollutants involved are easily sorbed; semi-permanent filter media
configuration reduces life cycle costs; on-site electrical/thermal
regeneration prevents costly downtime; and the filtration system
is adaptable both to retrofit and new construction.
A novel carbon fiber composite material and a method for making
is described herein. The carbon fiber composite material is designed
to controlled porosity. Moreover, the carbon fiber composite material
defines a rigid, open, monolithic structure with high permeability.
The carbon fiber composite of the present invention is comprised
generally of carbon fibers and a binder. The composite is strong
and porous allowing fluids to easily flow through the material.
At the same time, when activated, the carbon fibers provide a porous
structure for adsorption.
Synthesis of the carbon fiber composite is generally comprised
of mixing a selected carbon fiber in a water slurry with a carbonizable
organic powder. The desired monolith configuration is molded from
the slurry. The resulting green form is dried and removed from the
mold. The composite is cured prior to carbonization under an inert
gas. Once carbonized the composite is readily machined to the desired
final configuration. The composite material is then activated to
develop the fibers pore structure.
The fiber or fibers selected will depend upon the ultimate use
of the resultant composite. The process described herein will focus
on the production of a carbon fiber composite for use as a molecular
sieve.
For use as an adsorbent or molecular sieve, it is preferable to
use carbon fibers derived from a suitable isotropic pitch precursor.
The manufacture of pitch based fibers is well known in the art
and is briefly described herein. Pitch is conventionally derived
from a heavy petroleum fraction. Fiber forming methods include melt
spinning and melt blowing. During both of these processes, the pitch
is melted to a carefully controlled viscosity then forced through
a number of fine capillaries to produce fibers as the pitch resolidifies.
In the melt spinning process the fiber diameter is controlled by
drawing the fibers down and winding them onto a reel as they form.
The melt blowing process employs a stream of air which draws the
fibers down as it blows them onto a moving belt to form a random
mat of "green" pitch fibers. In both methods, extreme
care must be taken to control the temperature and other conditions.
Once formed, the green fibers are "stabilized", by heating
the fibers in an oxidizing atmosphere, so that they are rendered
thermosetting and will retain their fibrous form at the high temperatures
used in the subsequent carbonization step. After carbonization,
the fiber mats contain about 95% carbon by weight.
In the preferred embodiment, the isotropic pitch precursor is formed
such that the resultant fibers define a diameter of approximately
10-25 .mu.m. The fibers can be in a stabilized or carbonized condition.
The fibers are cut to a selected size. For the preferred embodiment,
it is preferable to cut the fibers to an average length of approximately
400 .mu.m, and can range from 100 to 1000 .mu.m.
The chopped carbon fibers are mixed in a water slurry with a carbonizable
organic powder, such as pitch, thermosetting resin or phenolic resin.
In the preferred embodiment, powdered phenolic resin is utilized.
The preferred forming method is vacuum molding, where the slurry
is transferred to a molding tank and the water is drawn through
a porous mold under vacuum. The material can be molded into any
configuration desired such as a cylinder or plate. Obviously, the
configuration will be determined by the configuration of the mold
into which the slurry is transferred. Other methods of forming can
be utilized such as pressure forming or various forming methods
practiced in the plastics industry.
The resulting green form is dried. In the preferred embodiment,
the form is dried in air at 50.degree. C. Once dried, the form is
removed from the mold.
The dried green form is then cured to produce a cured monolith.
In the preferred embodiment, the composite is cured at approximately
130.degree. C. in air. The resulting composite is carbonized under
an inert gas. Preferably, the composite is carbonized for 3 hours
under nitrogen at 650.degree. C. to pyrolize the resin binder.
The composite formed by the above process defines voids between
the fibers (interfiber pores) which allow free flow of fluid through
the material and ready access to the carbon fiber surface. The voids
range from 10-500 .mu.m in size. Further, the individual carbon
fibers are held in place by the pyrolized resin binder and thus
cannot move or settle due to the flow of gases through the material.
The carbonized bulk density of the composite material is typically
0.3-0.4 g/cm.sup.3. Assuming a theoretical density of 2.26 g/cm.sup.3
(density of a single crystal pure graphite) for the composite of
the present invention, at a density of 0.3-0.4 g/cm.sup.3 the composite
would range from approximately 82-86% porosity.
Following its manufacture, the monolithic carbon fiber composite
is activated. Activation of the carbon fibers is accomplished by
steam, carbon dioxide, oxygen or chemical activation. The resulting
chemical reactions remove carbon and develop pores in the carbon
fibers, which are classified by diameter: micropores (less than
2 nm), mesopores (2-50 nm) and macropores (greater than 50 nm).
In the preferred embodiment, the composite is steam activated in
a steam/nitrogen atmosphere. The preferred activation conditions
are: 800.degree.-950.degree. C., steam at a partial pressure of
0.1-0.9 atmospheres and for durations of 1-3 hours. Burn off is
calculated from the initial and final weights. Up to approximately
60% burn off, the surface area increases with burn off. The main
concern with a higher burn off is the reduction in the strength
of the composite. Using the manufacturing process described herein,
the reduction of crush strength of the composite is almost linear
with burn-off, rather than the strong exponential dependance more
typically exhibited by thermally oxidized carbons. At 56% burn-off,
the BET N.sub.2 surface area was 1670 m.sup.2 /g. A crush strength
of approximately 1 MPa is retained after activation. The resultant
fibers in the composite define a high micropore volume, a low mesopore
volume and no macropores.
The activation conditions can be varied by changing the activation
gas, its concentration, the flow rate, the temperature and the optional
presence of a catalyst to influence total surface area and pore
size distribution. Further, the use of post activation treatments
can be implemented. For example, further heating or the introduction
of chemicals could affect the pore size distribution and surface
chemistry.
Once carbonized or activated, the composite can be machined to
any desired shape, forming a monolithic carbon fiber composite.
The resultant activated carbon fiber composite is ideal for use
as an adsorbent or molecular sieve in the Pressure Swing Adsorption
(PSA) process. It has a very high surface area, a narrow micropore
distribution centered around mean pore widths of 5-10 .ANG., a high
micropore volume, low mesopore volume, a high gas adsorption/desorption
rate, and a permeable macrostructure through which fluid can easily
pass. Further, because the carbon fiber composite is a monolith,
it overcomes the settling problems associated with beds of granular
activated carbon and carbon molecular sieves and thus bypass flows
and pressure drops are eliminated. Moreover, the mean micropore
size is controllable through the activation process thus allowing
for the carbon fiber composite to be tailored for adsorption or
sieving of specific gas molecules. The density and void size of
the carbon fiber composite can be altered by varying the fiber length,
binder content and molding conditions. The most likely initial applications
are in the field of gas separations.
Although use of the carbon fiber composite as an adsorbent in the
PSA process has been emphasized, with variations to process described
above, there are several alternative uses. Carbon fibers derived
from coal tar pitch, rayon, polyacrylonitrile (PAN) or heavy oils
such as oil shale residue and refinery residue can be utilized in
the production of the composite. Further, the fibers can be vapor
grown. The fibers or a blend of different carbon fibers can be utilized
to control the characteristics of the resultant carbon fiber composite.
More specifically, the strength, thermal conductivity, pore size
distribution, density and electrical properties are examples of
the characteristics that can be modified or controlled with the
appropriate carbon fiber or blend of carbon fibers.
By selecting a different carbon fiber and altering selected parameters
in the production process, the carbon fiber composite can be modified
for use in a variety of applications. For example, the carbon fiber
composite can be utilized in gas purification applications and specifically
air purification, especially in confined spaces where the higher
rates of adsorption on activated carbon fiber composites offers
potential space savings. Further, a composite with a bulk density
greater than 1 gm/cm.sup.3 and an active surface area greater than
1000 m.sup.2 /g can be utilized for gas storage. For example, the
composite could be utilized as a CH.sub.4 or H.sub.2 storage medium
or as a CH.sub.4 purification medium for CH.sub.4 from various sources,
including land fill gases or coal bed gas. Further, a mesoporous
carbon fiber composite is suitable for use in a liquid phase applications
or as a catalyst support.
From the foregoing description, it will be recognized by those
skilled in the art that a carbon fiber composite, and a method for
making offering advantages over the prior art has been provided.
Specifically, the carbon fibers in the composite provides a high
surface area and is in a rigid, permeable, monolithic form.
While there have been shown and described what are at present considered
the preferred embodiments of the invention, it will be obvious to
those skilled in the art that various changes and modifications
can be made therein without departing from the scope of the inventions
defined by the appended claims. |