U.S. patent application number 14/682550 was filed with the patent office on 2015-07-30 for siloxane removal from gases using lignite-enhanced activated carbons and adsorbent media used therefor.
The applicant listed for this patent is Cabot Corporation. Invention is credited to Patton M. Adams, David B. Herrera.
Application Number | 20150209714 14/682550 |
Document ID | / |
Family ID | 50931585 |
Filed Date | 2015-07-30 |
United States Patent
Application |
20150209714 |
Kind Code |
A1 |
Herrera; David B. ; et
al. |
July 30, 2015 |
Siloxane Removal from Gases Using Lignite-Enhanced Activated
Carbons and Adsorbent Media Used Therefor
Abstract
A method of removing siloxanes from a gas stream includes
flowing the gas stream that carries siloxanes through an adsorbent
media to remove at least part of the siloxanes from the gas stream,
wherein the adsorbent media comprises lignite-based activated
carbon. A spent adsorbent media is provided that contains the
lignite-based activated carbon through which a gas stream
containing siloxanes has been at least partially purified, and
which may be regenerated.
Inventors: |
Herrera; David B.;
(Longview, TX) ; Adams; Patton M.; (Longview,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cabot Corporation |
Boston |
MA |
US |
|
|
Family ID: |
50931585 |
Appl. No.: |
14/682550 |
Filed: |
April 9, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13803084 |
Mar 14, 2013 |
9023755 |
|
|
14682550 |
|
|
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|
61738743 |
Dec 18, 2012 |
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Current U.S.
Class: |
95/116 |
Current CPC
Class: |
B01D 2253/25 20130101;
B01D 2257/553 20130101; B01D 2253/102 20130101; C12M 47/18
20130101; Y02E 50/346 20130101; B01D 2253/302 20130101; B01D 53/04
20130101; B01D 2258/05 20130101; B01J 20/20 20130101; Y02E 50/30
20130101; B01D 53/02 20130101 |
International
Class: |
B01D 53/02 20060101
B01D053/02 |
Claims
1. A method of removing siloxanes from a contaminated gas stream,
comprising flowing the gas stream comprising siloxanes through an
adsorbent media to remove at least part of the siloxanes from the
gas stream, wherein said adsorbent media comprises lignite-based
activated carbon.
2. The method of claim 1, wherein the lignite-based activated
carbon has pore sizes and a total pore volume, wherein at least
about 25% of the total pore volume is from pore sizes of from 20
.ANG. to 500 .ANG..
3-4. (canceled)
5. The method of claim 1, wherein the lignite-based activated
carbon has a molasses decolorizing efficiency of at least 50.
6-7. (canceled)
8. The method of claim 1, wherein the lignite-based activated
carbon has a BET surface area of from about 500 to about 650
m.sup.2/g.
9. The method of claim 1, wherein the lignite-based activated
carbon has a pH greater than 7.
10. The method of claim 1, wherein the lignite-based activated
carbon is in powder form or bound form.
11. The method of claim 1, wherein the lignite-based activated
carbon is in the form of granular particles, briquettes, or
extruded pellets.
12. (canceled)
13. The method of claim 1 wherein the lignite-based activated
carbon is steam-activated unwashed lignite.
14. The method of claim 1, wherein the adsorbent media comprises at
least about 50 wt. % lignite-based activated carbon.
15. (canceled)
16. The method of claim 1, wherein the adsorbent media is present
as a homogenous bed of the lignite-based activated carbon.
17. The method of claim 1, wherein the adsorbent media comprises a
blend of the lignite-based activated carbon and at least one
different kind of adsorbent media.
18-20. (canceled)
21. The method of claim 17, wherein the adsorbent media comprises a
blend of the lignite-based activated carbon and at least one
different kind of activated carbon, and wherein at least one of the
lignite-based activated carbon and the at least one different kind
of activated carbon are impregnated with at least one impregnant
that is a salt, an acid, or base.
22-23. (canceled)
24. The method of claim 1, wherein the flowing of the gas stream
through the adsorbent media removes at least about 95 wt. % of the
volatile siloxanes from the gas stream.
25. The method of claim 1, wherein the flowing of the gas through
the adsorbent media removes at least about 95 wt. % of the volatile
siloxanes for a time period that is at least about 2.times. longer
than for adsorbent media containing an equal volume and same bed
geometry of bituminous coal-based activated carbon instead of the
lignite-based activated carbon.
26. (canceled)
27. The method of claim 1, wherein the gas stream at the inlet side
of the adsorbent media contains from about 1 ppm to about 25 ppm
siloxanes,
28-38. (canceled)
39. The method of claim 1, wherein the gas stream comprises
landfill gas emissions.
40. The method of claim 1, wherein the gas stream comprises gas
emissions from anaerobic digestion.
41-48. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims the benefit of U.S.
Provisional Patent Application No. 61/738,743, filed Dec. 18, 2012,
the entire contents of which are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to siloxane removal from gas
streams using lignite-based activated carbons in adsorbent media.
The present invention further relates to spent adsorbent media
having the lignite-based activated carbon and filter beds formed
thereof.
BACKGROUND OF THE INVENTION
[0003] As energy diversification intensifies, alternative sources
of energy such as biogas from landfills and digesters become more
attractive. Landfill (LFG) and digester gas (DG) are both excellent
sources of methane. Waste handlers and municipalities are
increasingly using biogas generated at sewage treatment plants and
solid waste landfills as a biofuel. Biogases can be used as a fuel
to run internal combustion engines or turbines that power equipment
or generate electrical power. However, these biogases can be
contaminated by siloxanes, reduced sulfur compounds, and a variety
of volatile organic compounds (VOC's).
[0004] Siloxanes are used in the manufacture of personal hygiene,
health care and industrial products. As a consequence of their
widespread use, siloxanes are found in wastewater and in solid
residential and industrial waste deposited in landfills. At
wastewater treatment plants and landfills, low molecular weight
siloxanes can volatilize into digester gas and landfill gas.
Although the amount of volatized siloxanes present in such gases
may be small, the siloxanes are problematic. Siloxanes are nuisance
contaminants that accumulate on energy generation equipment,
resulting in increased maintenance and operating costs. When the
biogas is combusted to generate power (such as in gas turbines,
internal combustion engines or boilers), siloxanes are converted to
silicon dioxide (SiO.sub.2), which can deposit in the combustion
and/or exhaust stages of the equipment. The silica deposits can
cause abrasive wear on engine parts and lead to reduced engine
life. The deposits also can inhibit conduction and lubrication, and
also can dislodge and clog lines, or contaminate engine oil. The
siloxanes in biogases thus can significantly increase wear and
other damage in equipment. Manufacturers of gas turbines and
reciprocating engines are increasingly calling for the reduction of
the presence of siloxanes in combustion gases in order to warranty
their products. Waste digesters or landfills for which the biogas
is used as fuel before making any emissions have a need to remove
siloxanes and other materials that may deposit on the burners.
Electrical power generators that use emission catalysts with
combustion of biogases that contain siloxanes also are prone to
incur silica film development with consequent reductions in
catalytic activity, and also have siloxane removal needs.
[0005] Landfills that vent gases directly to the atmosphere can
have emission control needs that go beyond siloxane removal.
Landfill gases that will be vented to the atmosphere may need to be
scrubbed before release to remove not only siloxanes, but also
reduced sulfur compounds (e.g., H.sub.2S) and VOCs, and other
contaminants that may be carried in the gas.
[0006] Sorbent media such as silica gel, layered beds, or
bituminous coal and coconut activated carbon have been proposed for
siloxane removal. Conventional siloxane removal strategies for
biogas purification have tended to rely on multi-stage and/or
multi-layer procedures including hydrogen sulfide (H.sub.2S)
pre-treatment followed by treatment with adsorbent media for
siloxane removal. In U.S. Pat. No. 7,264,648, filter beds formed of
a plurality of different layers of different filter media are
described therein as being useful for siloxane removal. Bituminous
coal and coconut based activated carbon, for example, are widely
used in the industry for removal of many types of contaminants.
Silica gel behaves like a molecular sieve, and may only perform
well when targeting a single contaminant under controlled
conditions. Silica gel also can have low macro-porosity which can
restrict diffusion, and silica gel may be vulnerable to contaminant
spikes or composition variations. "Super Siloxane Adsorbent Silica
gel" by ADCOA has been marketed for use in siloxane removal.
[0007] New strategies for siloxane removal from gas streams are
needed, which can provide high adsorbent capacity for siloxanes
without relying on designs that are complicated and/or costly.
Also, new strategies are needed for single stage or single layer
removal of siloxanes and different companion contaminants from the
same gas streams.
SUMMARY OF THE INVENTION
[0008] A feature of the present invention is a method for the
removal of siloxanes from contaminated gas streams using
lignite-based activated carbon in adsorbent media.
[0009] A further feature of the present invention is a method for
the removal of siloxanes from contaminated gas streams using single
bed layer comprised of lignite-based activated carbon alone or
blended with one or more different sorbent media.
[0010] Another feature of the present invention is a method using
adsorbent media comprising lignite-based activated carbon having a
pore size distribution which can remove siloxanes at a high
adsorbent capacity from contaminated gas streams for which gas flow
is directed through the media or flows through the media.
[0011] An additional feature of the present invention is a method
using adsorbent media comprising a blend of lignite-based activated
carbon and at least one different adsorbent media which can remove
siloxanes at a high adsorbent capacity and at least one other
volatized or gas-borne component from the same gas stream for which
gas flow is directed through the media.
[0012] A further feature of the present invention is spent
adsorbent media comprising the lignite-based activated carbon
through which a gas stream containing siloxane has been at least
partially purified, which may be regeneratable at least in
part.
[0013] Additional features and advantages of the present invention
will be set forth in part in the description that follows, and in
part will be apparent from the description, or may be learned by
practice of the present invention. The objectives and other
advantages of the present invention will be realized and attained
by means of the elements and combinations particularly pointed out
in the description and appended claims.
[0014] To achieve these and other advantages and in accordance with
the purposes of the present invention, as embodied and broadly
described herein, the present invention relates, in part, to a
method of removing siloxanes from a contaminated gas stream,
comprising flowing the gas stream comprising siloxanes through an
adsorbent media to remove at least part of the siloxanes from the
gas stream, wherein the adsorbent media comprises lignite-based
activated carbon.
[0015] The present invention also relates to a spent adsorbent
media comprising lignite-based activated carbon through which a gas
stream containing siloxanes has been at least partially
purified.
[0016] The present invention also relates to a filter bed
comprising the spent lignite-based activated carbon.
[0017] The present invention also relates to an adsorbent media
comprising lignite-based activated carbon and having siloxanes
adsorbed or attached therein.
[0018] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are intended to provide a further
explanation of the present invention, as claimed.
[0019] The accompanying figures, which are incorporated in and
constitute a part of this application, illustrate various features
of the present invention and, together with the description, serve
to explain the principles of the present invention. The features
depicted in the figures are not necessarily drawn to scale.
Similarly numbered elements in different figures represent similar
components unless indicated otherwise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows a flow chart of a process for siloxane removal
according to an example of the present application.
[0021] FIG. 2 shows a flow chart of a process for siloxane removal
according to an example of the present application.
[0022] FIG. 3 shows a bar graph showing average values of siloxane
levels (ppmv) and runtimes to breakthrough (hours) for averaged
data collected for lignite-based activated carbon in a single
(non-blended) type of media (AC1) and a media comprised of a blend
of lignite-based activated carbon and a different carbon media
(AC2) in comparison with a standard (non-lignite) coal activated
carbon (CS1) according to an example of the present
application.
[0023] FIG. 4 shows a bar graph for a digester field test of the
performance of lignite-based activated carbon in a single
(non-blended) type of media (AC1) on siloxane removal (%) based on
inlet and outlet measurements over a time period (days), and a
curve for siloxane concentration (ppbv) over the test period,
according to an example of the present application.
[0024] FIG. 5 shows a bar graph of gas analysis for a landfill test
of the removal performance of a media comprised of a blend of
lignite-based activated carbon and a different carbon media (AC2)
at two different dates with contaminant concentrations (ppm) for
siloxanes, sulfurs, VOCs and overall shown for raw gas, a primary
vessel outlet, and a secondary vessel outlet, according to an
example of the present application.
[0025] FIG. 6 shows extrapolated breakthrough curves with respect
to the number of days to breakthrough for a previous layered bed
media blend in comparison to a media comprised of a blend of
lignite-based activated carbon and a different carbon media (AC2)
according to an example of the present application.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Siloxane removal from gas streams is provided with adsorbent
media comprising lignite-based activated carbon adsorbent media.
The adsorbent media can comprise lignite-based activated carbon
adsorbent as a sole type of media used or in blends with different
kinds of media. These sorbents can be used for biogas purification
or purification of other gas streams which can carry siloxanes and,
if present, other gas-borne contaminants. The lignite-based
activated carbon adsorbent media can be used for removal of
siloxane as the sole targeted contaminant or for concurrent removal
of a combination of contaminants or "catch-all" media that includes
siloxane. The lignite-based activated carbon, whether used alone or
in blends in media, can be used in a single layer to achieve
filtering objectives that include siloxane removal, although its
use in combination with other layers is not excluded. For biogas
purification, the lignite-based activated carbon can cover a
spectrum of contaminants to improve siloxane removal, eliminate or
support H.sub.2S pre-treatment, and/or manage VOC treatment
objectives, or provide other purification effects. These sorbent
media can achieve unexpectedly advantageous performance in removal
of siloxane in particular, and also volatile organic compound
(VOC), and total reduced sulfur compounds if needed or desired.
[0027] Treatment goals in landfill and digester gas purification
may vary but siloxane removal needs are usually or often common to
both applications. Due to possible high concentrations of hydrogen
sulfide (H.sub.2S) and VOC's, competitive adsorption can inhibit
siloxane removal. As indicated, the industry has used multi-layered
or gradient beds inter alia as a type of media for biogas
treatment. The lignite-based activated carbons of the present
application can provide high siloxane removal capacity with minimal
competitive influences from other contaminants without requiring
layered or gradient bed configurations.
[0028] As shown by performance data herein for tests conducted at
multiple landfill and digester sites, the lignite-based activated
carbon adsorbent media of this application has excellent sulfur
removal performance. The test results show, for example, that both
sorbents based on lignite activated carbon itself and as a blend
with other sorbent media in single layer configurations can achieve
up to 100 wt. % removal for siloxanes, greater than 90 wt. %
removal of VOC's, and up to 100 wt. % removal of total reduced
sulfur (TRS). The test results of a field test performed at a
digester described herein have shown a 200% increase in engine oil
life from 1,000 hours to 3,000 hours, greater than 25% increase in
spark plug life, and improved siloxane removal efficiency. The test
results of another field test performed at a landfill described
herein demonstrate significant improvements in media longevity for
the lignite-based activated carbon over a traditional (non-lignite)
coal activated carbon media. These results show that use of the
lignite-based activated carbon can highly effectively remove
siloxanes and other contaminants present in biogas. These test
results further indicate that the lignite-based activated carbon
media can provide a single-media solution without need of
impregnant or multi-layering of diverse media in the bed or
pre-filtering, although these additional features optionally may be
included.
[0029] The media comprised of lignite-based activated carbon
unblended with other media can provide a high degree of
meso-porosity and minimal competitive adsorption with VOC's. The
test results also show that the blends of lignite activated carbon
unblended with other media, such as different activated carbon, can
have the ability to perform as a "catch-all" carbon. Both types of
sorbents having the lignite-based activated carbon can be naturally
catalytic, and can remove hydrogen sulfide with trace oxygen and
moisture present.
[0030] As indicated, the lignite-based activated carbon can be used
to remove siloxanes, and, optionally reduce sulfur compounds and/or
VOCs from biogas. Digester operators may be, but not necessarily,
concerned primarily or only about siloxane removal, and in some
cases also hydrogen sulfide. Landfill operators where gases are
directly vented may need removal of siloxanes, H.sub.2S, and VOC's.
H.sub.2S may be removed via pre-treatment. In that instance, the
media beds may be mainly used for siloxanes and VOC's. The removal
of reduced sulfur compounds (primarily H.sub.2S) can be desirable,
especially at landfills, if no such separate pre-treatment for
hydrogen sulfide or total reduced sulfur (TRS) is applied. TRS can
refer to the sulfur compounds which can include hydrogen sulfide,
methyl mercaptan, dimethylsulfide, and dimethyldisulfide. H.sub.2S
can lead to the generation of SO.sub.2 and SO.sub.3 during a
combustion process and poses the potential for acidic/corrosive
condensation in general. The removal of VOC's also may be needed or
desired. The three aforementioned families of contaminants (i.e.,
siloxanes, reduced sulfur, VOCs) each adsorb or chemsorb
differently. Activated carbon layers have frequently been
characterized by carbon tetrachloride activity (CTC activity). CTC
activity is a measure of micro-porosity most suitable for low
molecular weight organics (i.e. VOC's) and does not adequately
assess the functionality of meso-porosity or catalytic potential.
The broad spectrum of contaminants that can be removed with the
lignite-based activated carbon of the present application can
range, for example, to compounds in the molecular weight
(MW)(g/mol) range of from about 34 to about 400 or more, or from
about 34 to about 300, or from about 40 to about 200, or other
values. CTC activity focuses on microporosity which is limited up
to 20 .ANG.. A majority of siloxanes can be greater than 20 .ANG.
in size and have average MW's greater than 300.
[0031] Treatable biogas can be produced in many different ways,
e.g., domestic waste (landfill), anaerobic waste digester (sewage)
treatment, animal manure (e.g., animal feedlots), decaying
vegetation compost, or any other decomposition process that
generates methane, and the like. As indicated, siloxanes are
present in many sources of biogas. The lignite-based activated
carbon can be deployed as a cost effective method for the
purification of biogas is the application of activated carbon (AC)
upstream of an energy generation process (e.g., internal combustion
engine, reciprocating engine, gas turbine, microturbine, boiler,
catalysts, etc.). Siloxanes can be removed from biogas with the
lignite-based activated carbon without disrupting the methane
content such that resulting cleaned biogas can be used in
combustion equipment with reduced risk of engine fouling or other
damage from siloxanes. The lignite-based activated carbon can
effectively eliminate siloxanes from biogas, for example, to
sharply reduce or completely prevent opportunity for development
and accumulation of silica deposits from siloxanes on energy
generation equipment. The type of gas stream that can be treated
with the lignite-based activated carbon adsorbent media to remove
siloxanes is not limited to biogas. The gas streams containing
siloxanes that can be treated with the lignite-based activated also
can include, for example, industrial process gas streams,
conditioned or ventilation air streams, or other types of gas
streams containing siloxanes. The lignite-based activated carbon
also can be used to filter gas streams associated with other
environments, such as oxidation catalyst beds used in electrical
power generation, siloxane-contaminated gas streams in
semiconductor chip device manufacture, or other situations that can
encounter or generate siloxane-contaminated gas streams.
[0032] The adsorption of the media comprising lignite-based
activated carbon can be dominant, for example, for certain
molecules, such as siloxanes, in meso-sized pores of 20-500 .ANG.
in diameter, or have other pore sizes. According to IUPAC notation,
"microporous" materials have pore diameters of less than 2 nm and
"macroporous" materials have pore diameters of greater than 50 nm;
the mesoporous category can lie in the middle, i.e., a "mesoporous"
material can be a material containing pores with diameters between
2 and 50 nm (20 .ANG. to 500 .ANG.). The lignite-based activated
carbon can have, for example, pore sizes, wherein at least about
25%, or from about 25% to 40%, or from about 30% to about 40%, or
other amounts, based on total pore volume, are from pore sizes of
from 20 .ANG. to 500 .ANG., and wherein total pore volume is
determined based on mL/g carbon material. The mesoporosity of the
lignite-based activated carbon can be indicated by a molasses
decolorizing efficiency of at least 50, or at least 55, or at least
60, or at least 65, or at least 70, or at least 75, or at least 80,
or at least 85, or at least 90, or at least 95, or from 50 to 100,
or from 50 to 90, or other values. Molasses DE (Decolorizing
Efficiency) is a measure of a carbon's ability to remove color from
a standard molasses solution. The test carbon performance can be
compared to a standard at 90% color removal. Coconut based
activated carbon has almost zero molasses DE, and bituminous coal
based activated carbon may be 30-40.
[0033] The lignite-based activated carbon can reduce siloxane
content in gas streams to levels to below 150 ppb, or below 50 ppb,
or below 1 ppb, or below detectible limits. A media bed containing
the lignite-based activated carbon can entrap siloxane, and
optionally also other unwanted gas contaminates, while allowing a
high percentage (e.g., >99.9% by volume) of the original methane
to flow out as cleaned fuel. The lignite-based activated carbon can
be used in a carbon bed filter to remove siloxane, and if present
hydrogen sulfide and VOCs, from a gas stream before the gas reaches
a combustion engine or combustion turbine system. The lignite-based
activated carbons can have enhanced porosity characteristics for
the removal of siloxanes, and optionally other gas stream
contaminants, such as acid gases, volatile organic compounds
(VOC's), or other components, and increased total pore volume and
mesoporosity. In addition, lignite can have high catalytic
activity, neutralizing capacity, and/or alkalinity. Adsorbent media
products based on the lignite-based activated carbon can be
designed primarily for siloxane removal, or can be modified to suit
a variety of treatment goals. Lignite-enhanced activated carbons
include lignite activated carbon, and blends of lignite activated
carbon with a different adsorbent media as part of the recipe.
Although not desiring to be bound to theory, it is theorized that
the siloxanes may selectively adsorb in the meso pores of activated
carbon, therefore, giving lignite activated carbons an advantage
over traditional (non-lignite) coal-based media because a greater
fraction of the porosity in lignite-based activated carbon is
meso-porosity. Traditional media can rely on micro-porosity,
whereas lignite can be characterized by significant
meso-porosity.
[0034] As indicated, the lignite-based activated carbons can be
used alone or as blended with a different type of activated carbon.
This variable can depend on the treatment goal of the end user. In
treatment of biogas produced by anaerobic waste digesters, for
example, focus usually is on siloxane removal and in some cases,
acid gases like hydrogen sulfide. In landfill gas treatments, the
gas treatment needs may vary, but may tend to focus on removal of
siloxanes, hydrogen sulfide, and VOC's. In cases, hydrogen sulfide
may be removed via separate pre-treatment, in which case the filter
beds can be mainly for removing siloxanes and VOC's. Regenerative
systems also may be used, wherein spent lignite-based activated
carbon may be regenerated to recover at least part of the original
siloxane and/or other contaminant(s) removal capacity.
[0035] The lignite-based activated carbon referenced herein can be
activated carbon produced by steam activation of lignite coal.
Lignite coal also is often referred to as brown coal. The lignite
can be types such as xyloid lignite or compact lignite. Lignite can
have a carbon content, for example, of approximately 25-35% by
weight, a moisture content that can be, for example, as high as
about 66% weight, and an ash content which can range, for example,
from about 6% to about 19%, by weight of lignite.
[0036] Unwashed lignite product of an example of the present
application can be highly selective to siloxanes and, if present,
can pick up at least a portion of hydrogen sulfide (H.sub.2S) and
acid gases and volatile organics (VOCs). Unwashed coal is not
washed coal. "Washed coal" refers to a process of using acid to
remove ash content and neutralize the final product. Washing
eliminates catalytic activity or its ability to remove acid gasses
like H2S. Another sorbent product can be the unwashed lignite in a
blend with other activated carbons, and which can pick up
additional contaminants. The porosity in the unwashed lignite can
be, for example, 20-36% of the total pore volume from pore sizes
less than 20 .ANG., 25-34% of the total pore volume from pore sizes
of from 20-500 .ANG., and 40-44% of the total pore volume from pore
sizes greater than 500 .ANG., or other distributions. The porosity
in the unwashed lignite can be, for example, 22-34% of the total
pore volume from pore sizes less than 20 .ANG., 26-33% of the total
pore volume from pore sizes of from 20-500 .ANG., and 40-44% of the
total pore volume from pore sizes greater than 500 .ANG., or other
distributions. The porosity in the unwashed lignite can be, for
example, 23-33% of the total pore volume from pore sizes less than
20 .ANG., 27-32% of the total pore volume from pore sizes of from
20-500 .ANG., and 41-43% of the total pore volume from pore sizes
greater than 500 .ANG., or other distributions. The total porosity
of the media can be, for example, from about 0.9 to about 1.4 mL/g,
or from about 1.0 to about 1.3 mL/g, or from about 1.1 to about 1.2
mL/g, or other values. The lignite-based activated carbon can have
an apparent density of, for example, from about 0.3 to about 0.6
g/mL, or from about 0.35 to about 0.55 g/mL, or from about 0.4 to
about 0.5 g/mL, or other values. The lignite-based activated carbon
can have a BET surface area, for example, of at least 500
m.sup.2/g, or from about 500 m.sup.2/g to about 650 m.sup.2/g, or
from about 525 m.sup.2/g to about 625 m.sup.2/g, or from about 550
to about 600 m.sup.2/g, or other values. Surface area (B.E.T.) is
the total surface area of a solid calculated by the B.E.T.
(Brunauer, Emmett, Teller) equation, from nitrogen adsorption or
desorption data obtained under specified conditions; square meters
per gram. The lignite-based activated carbon can have a pH, for
example, of greater than 7, or greater than 7.5, or greater than 8,
or greater than 9, or other values.
[0037] The lignite-based activated carbon can be formed by high
temperature steam activation of unwashed lignite coal.
Lignite-based activated carbon which is commercially available for
other uses can be used or adapted for use in the methods of
siloxane removal from contaminated gas streams of the present
application. The lignite-based activated carbon has high loading
capacity for siloxanes without impregnants or additives. The
steam-activated lignite carbon optionally can be impregnated with a
treating agent to enhance or broaden performance. The impregnation
of the activated carbon can be done by mixing a solid or liquid
chemical with the carbon substrate before, during, or after
activation. The impregnant may be an acid, base, or salt. A large
number of inorganic and organic compounds can be used for the
impregnant. The impregnant can be, for example, magnesium oxide,
sodium bicarbonate, sodium carbonate, sodium hydroxide, potassium
hydroxide, potassium iodide, potassium permanganate, basic copper
and zinc carbonates, copper and zinc sulfates, sodium molybdate,
sodium vanadate, ammonium chloride, ammonium carbonate, silver
nitrate, as well as TEDA (triethylenediamine) or tartaric acid and
salts thereof, which can be used individually or in any
combinations thereof for impregnating the activated carbons for use
in gas filters. As an example, the lignite-based activated carbon
can contain impregnant in an amount of from 0 to about 5 wt. %, or
from 0 to about 1 wt. %, or from 0 to about 0.5 wt. %, or from 0 to
about 0.1 wt. %, or from 0 to about 0.01 wt. %, or below detectible
limits, or other values.
[0038] The lignite-based activated carbons can be made in
particulate form as powders, granules, extruded pellets,
briquettes, agglomerates, or other forms. Granular activated carbon
has a relatively larger particle size compared to powdered
activated carbon. Powdered activated carbon can be an activated
carbon with particles predominantly smaller than 80 mesh in size.
Extruded pellets can be a mixture of powdered activated carbon and
binder, which are fused together and extruded into a cylindrical
shaped activated carbon black. Briquettes can be a mixture of
activated carbon powder and binder that is pressed in a form. The
particulate forms of activated carbon can have a particle size, for
example, of up to about 5 mm, or up to 4 mm, or from about 0.1 mm
to about 4 mm, or from about 0.15 mm to about 3.75 mm, or from
about 0.25 to about 3.5, or from about 0.5 mm to about 3 mm, or
from about 1 mm to about 2.5 mm, or other sizes. At least about
50%, or at least about 60%, or at least about 70%, or at least
about 80%, or at least about 90%, or from about 50% to 100% by
weight, of the granular particles, briquettes, or extruded pellets
used in a media layer or bed can have a size in the range of from
about 2 mm to about 5 mm.
[0039] Adsorbent media can comprise the lignite-based activated
carbon in an amount of at least about 40 wt. %, or at least about
50 wt. %, or at least about 60 wt. %, or at least about 70 wt. %,
or at least about 80 wt. %, or at least about 90 wt %, or at least
about 95 wt. %, or from about 50-100 wt. %, or from about 60-90 wt.
%, or other amounts, of the adsorbent media. The adsorbent media
can be present as a homogenous bed of the lignite-based activated
carbon. The filter bed may be used in a single vessel of a process,
or multiple vessels of a process such as in a train of vessels. As
indicated, the adsorbent media also can comprise a blend of the
lignite-based activated carbon and at least one different kind of
adsorbent media. The at least one different kind of adsorbent media
can be, for example, a different activated carbon such as activated
carbon produced from bituminous coal, coconut shells, wood,
sawdust, rice hulls, peat, petroleum residues, or any combinations
thereof. The at least one different kind of activated carbon can
have a pore size distribution, for example, that comprises a
percentage of pore volume having pore sizes less than 2 nm that is
greater, e.g., at least 3% greater, or at least 5% greater, or at
least 10% greater, or at least 20% greater, or at least 30%
greater, or at least 50% greater, than a percentage of pore volume
in the lignite-based activated carbon that has pore sizes less than
2 nm. The blends of lignite-based activated carbon and different
activated carbon can be proportioned to provide a spectrum of
selectivities for different contaminants in addition to siloxanes
without loss of the siloxane removal performance. The adsorbent
media can comprise a blend of the lignite-based activated carbon
and at least one non-carbon based adsorbent media, such as a silica
gel. The total amounts of non-lignite based activated carbon that
can be used in the blends can be, for example, up to about 60 wt.
%, or up to about 50 wt. %, or up to about 40 wt. %, or up to about
30 wt. %, or up to about 20 wt. %, or up to about 10 wt. %, or up
to about 5 wt. %, or from about 0-50 wt. %, or from about 10-40 wt.
%, or other amounts, of the blend comprising lignite-based
activated carbon.
[0040] The adsorbent media that comprises the lignite-based
activated carbon can be deployed in a variety of restrained forms,
such as packed column or bed, loaded canister, or impregnated
nonwoven or woven fabrics. The impregnated fabrics can be in
various forms, such as in roll, pleated, framed, or discrete filter
pad forms. The fabrics can be organic and/or inorganic fiber
constructions which can provide a substrate to which activated
carbon can be attached by contacting the fabric with a
thermosettable or hardenable impregnating resin in which the
activated carbon is dispersed, and setting the resin. The adsorbent
media may be used in fluidized or moving beds. The size of the
filter bed can vary depending on the particular application and
process conditions. For biogas treatment at a anaerobic digester,
for example, the filter bed may be used in the form of a layer
having a size of from about 90 cm to about 460 cm substantially
uniform depth for a bed diameter of from about 30 cm to about 400
cm, or can have other dimensions. For biogas treatment at a
landfill operation, for example, the filter bed may be used in the
form of a layer having a size of from about 90 cm to about 460 cm
substantially uniform depth for a bed diameter of from about 30 cm
to about 400 cm, or can have other dimensions.
[0041] The lignite-based activated carbon or blend thereof in the
media can be essentially uniform and homogeneous in composition
throughout a filter bed or other configuration of use. For example,
the composition of the media with respect to the amount of
lignite-based activated carbon present, and any additional
different sorbent co-present, each may not vary more than about
.+-.5%, or not more than about .+-.4%, or not more than about
.+-.3%, or not more than about .+-.2%, or not more than about
.+-.1%, between any two locations within the filter bed layer or
other configuration of use. The filter bed can be devoid of
compositional gradients and the like.
[0042] The siloxanes removed by the adsorbent media can be, for
example, a saturated silicon-oxygen hydride formed from atoms of
carbon (C), hydrogen (H), oxygen (O), and silicon (Si), which
includes at least one unit of alternating silicon and oxygen atoms
(e.g., --Si(R.sub.2)--O--Si(R.sub.2)--), wherein each R can be a
hydrogen atom or a hydrocarbon group (e.g., C1-C5 alkyl, such as
methyl). The siloxanes that the lignite-based activate carbon and
blends thereof can remove from gas streams can include volatile
methyl siloxanes. The methyl siloxanes can be linear
methylsiloxanes or methylcyclosiloxanes. These methyl siloxanes can
be, for example, hexamethyldisiloxane (L2, MM),
hexamethylcyclotrisiloxane (D3), octamethyltrisiloxane (L3, MDM),
octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane
(D5), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5),
dodecamethylcyclohexasiloxane (D6), or others, or any combinations
thereof. Common abbreviations used in the industry for these
siloxanes are indicated here and used in the examples herein.
[0043] FIG. 1 shows a process for siloxane removal according to an
example of the present application. The process 10 includes a
(bio)gas stream 11 which is filtered with lignite-based activated
carbon as a single media in a single layer in step 12 to provide a
clean or purified gas 13. FIG. 2 shows a process 20 for siloxane
removal wherein the (bio)gas 21 is filtered with a blend of
lignite-based activated carbon with a different sorbent in a single
layer in step 22 to provide a purified gas 23. As indicated, the
gas can be a biogas or any other gas that carries volatized
siloxanes.
[0044] The method of flowing the gas stream through the adsorbent
media having the lignite-based activated carbon can remove
siloxanes from the gas in amounts, for example, of at least about
95 wt. % , or at least about 96 wt. %, or at least about 97 wt. %,
or at least about 98 wt. %, or at least about 99 wt. %, or 100 wt.
% or below detectible limits, or from about 95 wt. % to 100 wt. %,
or from about 96 wt. % to about 99 wt. %, or other amounts of the
volatile siloxanes from the gas stream. The method can remove at
least about 95 wt. % of the volatile siloxanes for a time period
that is at least about 2.times. (i.e., two times or 100%) longer,
or at least about 2.5.times. (i.e., 2.5 times or 150%) longer, or
at least about 3.times. (i.e., 3 times or 200%) longer, than for
adsorbent media containing an equal volume and similar bed geometry
of bituminous coal-based activated carbon instead of the
lignite-based activated carbon. The method can remove at least
about 99 wt. % of the volatile siloxanes for a time period that is
at least about 2.times. longer, or at least about 2.5.times.
longer, or at least about 3.times. longer, than for adsorbent media
containing an equal volume and similar bed geometry of bituminous
coal-based activated carbon instead of the lignite-based activated
carbon. These methods can be applied, for example, to treatment of
a gas stream at the inlet side of the adsorbent media that contains
siloxanes in amounts of from about 1 ppm to about 25 ppm, or from
about 3 to about 20 ppm, or from about 5 to about 15 ppm, or other
amounts of siloxanes.
[0045] As indicated, the gas stream that is treated by the method
can comprise volatile siloxanes, and optionally at least one of
total reduced sulfur compounds and/or volatile organic compounds,
when introduced at an inlet side of the adsorbent media. The
adsorbent media can remove total reduced sulfur compounds from the
gas in amounts, for example, of at least about 95 wt. % , or at
least about 96 wt. %, or at least about 97 wt. %, or at least about
98 wt. %, or at least about 99 wt. %, or 100 wt. % or below
detectible limits, or from about 95 wt. % to 100 wt. %, or from
about 96 wt. % to about 99 wt. %, or other amounts. The adsorbent
media can remove VOCs from the gas in amounts, for example, of at
least about 90 wt. %, or at least about 95 wt. %, or at least about
96 wt. %, or at least about 97 wt. %, or at least about 98 wt. %,
or at least about 99 wt. %, or 100 wt. % or below detectible
limits, or from about 90 wt. % to 100 wt. %, or from about 95 wt. %
to 100 wt. %, or from about 96 wt. % to about 99 wt. %, or other
amounts. The method can remove, for example, at least about 99 wt.
% of the volatile siloxanes, and at least one of i) at least about
99.5 wt. % of the total reduced sulfur compounds and ii) at least
about 95 wt. % of the volatile organic compounds from the gas
stream. These contaminant removal rates can be provided in the
treatment of a gas stream at the inlet side of the adsorbent media
which contains, for example, from about 1 ppm to about 25 ppm
siloxanes, from about 10 ppm to about 150 ppm total reduce sulfur
compounds, and from about 200 ppm to about 500 ppm volatile organic
compounds, or other compositions thereof.
[0046] The present invention also relates to a spent adsorbent
media and filter beds that comprise lignite-based activated carbon
through which a gas stream containing siloxanes has been at least
partially purified and to which siloxanes have been adsorbed or
attached. These spent adsorbent media may be regenerated at least
in part for re-use. The siloxanes can present in the spent media in
an amount, for example, of from about 0.1 wt % to about 60 wt %, or
from about 0.2 wt % to about 60 wt %, or from about 0.3 wt. % to
about 50 wt. %, or from about 0.4 wt. % to about 25 wt %, or from
about 0.5 wt. % to about 10 wt. %, or other values, based on the
weight of the spent adsorbent media.
[0047] The present invention includes the following
aspects/embodiments/features in any order and/or in any
combination: [0048] 1. A method of removing siloxanes from a gas
stream, comprising flowing the gas stream comprising siloxanes
through an adsorbent media to remove at least part of the siloxanes
from the gas stream, wherein said adsorbent media comprises
lignite-based activated carbon. [0049] 2. The method of any
preceding or following embodiment/feature/aspect, wherein the
lignite-based activated carbon has pore sizes and a total pore
volume, wherein at least about 25% of the total pore volume is from
pore sizes of from 20 .ANG. to 500 .ANG.. [0050] 3. The method of
any preceding or following embodiment/feature/aspect, wherein the
lignite-based activated carbon has pore sizes and a total pore
volume, wherein from about 25% to about 40% of the total pore
volume is from pores sizes of from 20 .ANG. to 500 .ANG.. [0051] 4.
The method of any preceding or following embodiment/feature/aspect,
wherein the lignite-based activated carbon having a pore size
distribution of:
[0052] about 20% to about 36% of total pore volume from pore sizes
less than 20 .ANG.,
[0053] about 25% to about 40% of total pore volume from pore sizes
of from 20 .ANG. to 500 .ANG., and
[0054] about 40% to about 44% of total pore volume from pore sizes
greater than 500 .ANG.. [0055] 5. The method of any preceding or
following embodiment/feature/aspect, wherein the lignite-based
activated carbon has a molasses decolorizing efficiency of at least
50. [0056] 6. The method of any preceding or following
embodiment/feature/aspect, wherein the lignite-based activated
carbon has an apparent density of from about 0.3 to about 0.6 g/mL.
[0057] 7. The method of any preceding or following
embodiment/feature/aspect, wherein the lignite-based activated
carbon has a total porosity of from about 0.9 to about 2.0 mL/g.
[0058] 8. The method of any preceding or following
embodiment/feature/aspect, wherein the lignite-based activated
carbon has a BET surface area of from about 500 to about 650
m.sup.2/g. [0059] 9. The method of any preceding or following
embodiment/feature/aspect, wherein the lignite-based activated
carbon has a pH greater than 7. [0060] 10. The method of any
preceding or following embodiment/feature/aspect, wherein the
lignite-based activated carbon is in powder form or bound form.
[0061] 11. The method of any preceding or following
embodiment/feature/aspect, wherein the lignite-based activated
carbon is in the form of granular particles, briquettes, or
extruded pellets. [0062] 12. The method of any preceding or
following embodiment/feature/aspect, wherein at least 50% of the
granular particles, briquettes, or extruded pellets have a size in
the range of from about 2 mm to about 5 mm. [0063] 13. The method
of any preceding or following embodiment/feature/aspect, wherein
the lignite-based activated carbon is steam-activated unwashed
lignite. [0064] 14. The method of any preceding or following
embodiment/feature/aspect, wherein the adsorbent media comprises at
least about 50 wt. % lignite-based activated carbon. [0065] 15. The
method of any preceding or following embodiment/feature/aspect,
wherein the adsorbent media comprises at least about 90 wt. %
lignite-based activated carbon. [0066] 16. The method of any
preceding or following embodiment/feature/aspect, wherein the
adsorbent media is present as a homogenous bed of the lignite-based
activated carbon. [0067] 17. The method of any preceding or
following embodiment/feature/aspect, wherein the adsorbent media
comprises a blend of the lignite-based activated carbon and at
least one different kind of adsorbent media. [0068] 18. The method
of any preceding or following embodiment/feature/aspect, wherein
the adsorbent media comprises a blend of the lignite-based
activated carbon and at least one different kind of activated
carbon. [0069] 19. The method of any preceding or following
embodiment/feature/aspect, wherein the at least one different kind
of activated carbon is activated carbon produced from bituminous
coal, coconut shells, wood, sawdust, rice hulls, peat, petroleum
residues, or any combinations thereof. [0070] 20. The method of any
preceding or following embodiment/feature/aspect, wherein the at
least one different kind of activated carbon has a pore size
distribution comprising a percentage of pores having pore sizes
less than 2 nm that is greater than a percentage of pores in the
lignite-based activated carbon that has pore sizes less than 2 nm.
[0071] 21. The method of any preceding or following
embodiment/feature/aspect, wherein at least one of the
lignite-based activated carbon and the at least one different kind
of activated carbon are impregnated with at least one impregnant
that is a salt, an acid, or base. [0072] 22. The method of any
preceding or following embodiment/feature/aspect, wherein the
adsorbent media comprises a blend of the lignite-based activated
carbon and at least one non-carbon based adsorbent media. [0073]
23. The method of any preceding or following
embodiment/feature/aspect, wherein the adsorbent media comprises a
blend of the lignite-based activated carbon and silica gel. [0074]
24. The method of any preceding or following
embodiment/feature/aspect, wherein the flowing of the gas stream
through the adsorbent media removes at least about 95 wt. % of the
volatile siloxanes from the gas stream. [0075] 25. The method of
any preceding or following embodiment/feature/aspect, wherein the
flowing of the gas through the adsorbent media removes at least
about 95 wt. % of the volatile siloxanes for a time period that is
at least about 2.times. longer than for adsorbent media containing
an equal volume and same bed geometry of bituminous coal-based
activated carbon instead of the lignite-based activated carbon.
[0076] 26. The method of any preceding or following
embodiment/feature/aspect, wherein the flowing of the gas through
the adsorbent media removes at least about 95 wt. % of the volatile
siloxanes for a time period that is at least about 3.times. longer
than for adsorbent media containing an equal volume and same bed
geometry of bituminous coal-based activated carbon instead of the
lignite-based activated carbon. [0077] 27. The method of any
preceding or following embodiment/feature/aspect, wherein the gas
stream at the inlet side of the adsorbent media contains from about
1 ppm to about 25 ppm siloxanes, [0078] 28. The method of any
preceding or following embodiment/feature/aspect, wherein the gas
stream comprises volatile siloxanes, total reduced sulfur compounds
and volatile organic compounds, when introduced at an inlet side of
the adsorbent media. [0079] 29. The method of any preceding or
following embodiment/feature/aspect, wherein the flowing of the gas
stream through the adsorbent media removes at least about 95 wt. %
of the volatile siloxanes from the gas stream. [0080] 30. The
method of any preceding or following embodiment/feature/aspect,
wherein the flowing of the gas stream through the adsorbent media
removes at least about 95 wt. % of total reduced sulfur compounds.
[0081] 31. The method of any preceding or following
embodiment/feature/aspect, wherein the flowing of the gas stream
through the adsorbent media removes at least about 90 wt. % of the
volatile organic compounds from the gas stream. [0082] 32. The
method of any preceding or following embodiment/feature/aspect,
wherein the flowing of the gas stream through the adsorbent media
removes at least about 99 wt. % of the volatile siloxanes from the
gas stream. [0083] 33. The method of any preceding or following
embodiment/feature/aspect, wherein the flowing of the gas stream
through the adsorbent media removes at least about 99 wt. % of the
volatile siloxanes, and at least one of i) at least about 99.5 wt.
% of the total reduced sulfur compounds and ii) at least about 95
wt. % of the volatile organic compounds from the gas stream. [0084]
34. The method of any preceding or following
embodiment/feature/aspect, wherein the flowing of the gas through
the adsorbent media removes at least about 95 wt. % of the volatile
siloxanes for a time period that is at least about 2.times. longer
than for adsorbent media containing an equal volume and same bed
geometry of bituminous coal-based activated carbon instead of the
lignite-based activated carbon. [0085] 35. The method of any
preceding or following embodiment/feature/aspect, wherein the
flowing of the gas through the adsorbent media removes at least
about 95 wt. % of the volatile siloxanes for a time period that is
at least about 3.times. longer than for adsorbent media containing
an equal volume and same bed geometry of bituminous coal-based
activated carbon instead of the lignite-based activated carbon.
[0086] 36. The method of any preceding or following
embodiment/feature/aspect, wherein the gas stream at the inlet side
of the adsorbent media contains from about 1 ppm to about 25 ppm
siloxanes, from about 10 ppm to about 150 ppm total reduce sulfur
compounds, and from about 200 ppm to about 500 ppm volatile organic
compounds. [0087] 37. The method of any preceding or following
embodiment/feature/aspect, wherein the siloxanes comprise linear
siloxanes, cyclic siloxanes, or any combinations thereof. [0088]
38. The method of any preceding or following
embodiment/feature/aspect, wherein the siloxanes comprise
polymethylpolysiloxane, polymethylcyclopolysiloxane, or any
combinations thereof. [0089] 39. The method of any preceding or
following embodiment/feature/aspect, wherein the gas stream
comprises landfill gas emissions. [0090] 40. The method of any
preceding or following embodiment/feature/aspect, wherein the gas
stream comprises gas emissions from anaerobic digestion. [0091] 41.
The present invention also relates to a spent adsorbent media
comprising lignite-based activated carbon through which a gas
stream containing siloxanes has been at least partially purified.
[0092] 42. The spent adsorbent media comprising lignite-based
activated carbon of any preceding or following
embodiment/feature/aspect. [0093] 43. The present invention also
relates to a filter bed comprising the spent lignite-based
activated carbon of any preceding or following
embodiment/feature/aspect. [0094] 44. The present invention also
relates to an adsorbent media comprising lignite-based activated
carbon of any preceding or following embodiment/feature/aspect and
having siloxanes adsorbed or attached therein. [0095] 45. The
adsorbent media of any preceding or following
embodiment/feature/aspect, wherein the siloxanes are present in an
amount of from about 0.1 wt % to about 60 wt % , based on the
weight of said adsorbent media. [0096] 46. The adsorbent media of
any preceding or following embodiment/feature/aspect, wherein the
siloxanes are present in an amount of from about 0.2 wt % to about
60 wt %, based on the weight of said adsorbent media. [0097] 47.
The adsorbent media of any preceding or following
embodiment/feature/aspect, wherein the lignite-based activated
carbon has a pore size distribution of:
[0098] about 20% to about 36% of total pore volume from pore sizes
less than 20 .ANG.,
[0099] about 25% to about 40% of total pore volume from pore sizes
of from 20 .ANG. to 500 .ANG., and
[0100] about 40% to about 44% of total pore volume from pore sizes
greater than 500 .ANG.. [0101] 48. The adsorbent media of any
preceding or following embodiment/feature/aspect, wherein said
siloxanes comprises hexamethyldisiloxane,
hexamethylcyclotrisiloxane, octamethyltrisiloxane,
octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane,
decamethyltetrasiloxane, dodecamethylpentasiloxane,
dodecamethylcyclohexasiloxane, or any combinations thereof.
[0102] The present invention will be further clarified by the
following examples, which are intended to be only exemplary of the
present invention. Unless indicated otherwise, all amounts,
percentages, ratios and the like used herein are by weight.
EXAMPLES
[0103] Lignite-based activated carbon was field tested as an
adsorbent sorbent at three biogas-generating locations that
included a waste landfill at one site and anaerobic digesters of
sewage treatment facilities at two different geographical
sites.
[0104] Two different forms of lignite-based activated carbon were
tested. In a first form, referred to herein as "AC1", activated
carbon obtained as steam-activated unwashed lignite coal was used
as the adsorbent media. AC1 had a high degree of meso-porosity. AC1
had a pore size distribution of about 20% to about 36% less than 20
.ANG., about 25% to about 40% from 20 .ANG. to 500 .ANG., and about
40% to about 44% greater than 500 .ANG.. "AC2" was a blend of about
50 wt. % AC1 and about 50 wt. % coconut-based activated carbon.
Comparison sorbent (CS) used were non-lignite coal-based activated
carbon (60 CTC activity) in 3 mm diameter pellets (CS1) or 4 mm
diameter pellets (CS2), and a commercial activated carbon and
graphite blend (CS3). CS1 was obtained as Norit.RTM. RB 30M and CS2
was Norit.RTM. RB 40M steam activated extruded bituminous coal
and/or coconut based carbons from Norit Americas Inc., Marshall
Tex., USA. CS3 was SAG.TM. (Selective Active Gradient) material
produced by Applied Filter Technology, Evans Ga., USA.
[0105] Table 1 is an overview of the biogas studies that were
performed.
TABLE-US-00001 TABLE 1 Sorbent Biogas Product Treatment Equipment
Variables Source Used Goal Protected Monitored Digester #1 a) AC1
Siloxane Microturbine VOC's, H.sub.2S, b) AC2 removal siloxanes c)
CS1 or CS2 Digester #2 a) AC2 Siloxane IC Engine Siloxanes b) SAG
removal Landfill #1 AC2 Siloxane Oxidation VOC's, removal Catalyst
reduced sulfur species, siloxanes
Example 1
[0106] In a proof of concept study performed at Digester #1, gas
samples were taken from a 2-vessel (series configuration) carbon
adsorption system using TEDLAR.RTM. bags. Samples were taken from
the outlet and tested for siloxane content via gas chromatography
with atomic emission detection (GC-AED). Both linear (L) and cyclic
(D) siloxanes were reported. Testing was performed on three media:
standard media (CS1)(3 mm pellets, 60 CTC activity), AC1, and AC2.
The filter bed dimensions used were 122 cm depth and 30 cm
diameter. This was a dual vessel arrangement, wherein both vessels
had the same size and were arranged in series. The standard
operating conditions for the adsorption system in these tests are
shown in Table 2.
TABLE-US-00002 TABLE 2 System Operating Conditions Pressure (psig):
90 Flow Rate (scfm): 25 Temperature (.degree. F.): Ambient Relative
Humidity (OEM specified): <25% Total Media Volume
(ft.sup.3)/Mass (lbs) CS1: 6.9 ft.sup.3/220 lbs AC2: 6.4
ft.sup.3/160 lbs AC1: 6.4 ft.sup.3/160 lbs
[0107] The waste water treatment plant (WWTP) operated the carbon
adsorption system on a monthly change out pattern. The unit is
semi-continuous; therefore, uptime or skid hours were recorded at
time of change out.
[0108] As indicated, three activated carbon products were tested
for siloxane removal capability. The 3 mm coal-based pellet (CS1)
has been the standard media as specified by the original equipment
manufacturer (OEM). Outlet gas samples were analyzed through gas
chromatography--mass spectrometry (GC/MS) for all common siloxanes,
as well as on total silicon (Si). On average, AC1 and AC2 products
achieved 22% and 34%, respectively, less breakthrough on siloxanes
and total Si than the standard media, respectively (FIG. 3). In
addition, it was observed that AC1 and AC2 products logged 38% more
runtime hours than the standard media. In this respect, the bar
graph in FIG. 3 shows average values of siloxane levels (ppmv) and
runtimes to breakthrough (hours) for averaged data collected for
lignite-based activated carbon in a single (non-blended) type of
media (AC1) and a media comprised of a blend of lignite-based
activated carbon and a different carbon media (AC2) in comparison
with a standard (non-lignite) coal activated carbon (CS1).
[0109] In a field test performed at Digester #1, the siloxane and
other contaminant removal performance of AC1 was evaluated. Gas
samples were taken from a 2-vessel (series configuration) carbon
adsorption system using Summa canisters. A sampling assembly was
used for diverting a small volume of the biogas, which included a
tap line with a valve which could be opened to feed a gas sample
into a Summa canister via a quick connect valve matching the tap
line to the cannister. The gas sample collected in the Summa
canister was analyzed to measure its contaminant contents. Samples
were taken from the inlet and outlet. Analysis was performed on
siloxane and VOC content via gas chromatography-mass spectrometry
(GC/MS). H.sub.2S levels were also monitored using a Type L2
Oda-Log data logger. Sampling frequency for siloxanes and H.sub.2S
was weekly and bi-weekly for VOC's. Analysis for siloxanes followed
compendium method TO-15; analysis for VOC's followed TO-15+TIC. The
standard operating conditions for the adsorption system are
provided in FIG. 2. Change out frequency was modified to better
evaluate performance. Determination of change out was dictated by
the following criteria: siloxane breakthrough of a single siloxane
species exceeds 150 ppbv in two consecutive sample periods, or a
substantial spike in siloxane breakthrough is observed that may be
critical to downstream equipment. The filter bed dimensions used in
the field test were the same as indicated for the proof of concept
study.
[0110] Although proof of concept was established, it is common for
low but significant false positive values on D5 and D6 siloxanes to
occur when using TEDLAR.RTM. bags. A silicone-based lubricant on
the valve stem of TEDLAR.RTM. bag or in the sampling line may
contribute a significant background level of siloxanes. Therefore,
for the field tests, sampling was converted to Summa canisters to
reduce this variability as well as to better support the increased
frequency of testing.
[0111] As indicated, the field test experiment at Digester #1
provided findings for AC1. The emphasis of this test was to observe
performance on siloxane removal. Therefore, in addition to
monitoring siloxane concentration, H.sub.2S and VOC levels were
monitored to evaluate competitive effects. H.sub.2S was observed to
breakthrough first at 336 hours, followed by VOC breakthrough at
1098 hours, and lastly siloxanes (D4 only) at 2054 hours. The
average inlet concentrations for all monitored biogas contaminants
during this study are presented in Table 3.
TABLE-US-00003 TABLE 3 Biogas Contaminants (ppmv) VOC VOC H.sub.2S
(TO - 15) (TO - 15 + TIC) L3 L4 D3 D4 D5 120 7 25 0.09 0.07 0.04
0.49 0.62
[0112] With respect to siloxane removal, the average siloxane
concentration was 1.3 ppmv but has been as high as 3.2 ppmv.
Complete removal of siloxanes was achieved with an average removal
efficiency of 97.3% based on sample analysis, as shown in FIG. 4.
Fluctuations in removal efficiency below 100% are attributed to
values considered below GC/MS detection limits and the "L2
phenomena" (Wheless, E. et al., Siloxanes in landfill and digester
gas update. In Proc. 27th SWANA Landfill Gas Symp., San Antonio,
Tex., March 22-25; Solid Waste Assoc. North America: Silver
Springs, Md.; 2004). The "L2 Phenomena" is the appearance of L2 in
the outlet stream when it is not present in or greatly exceeds the
inlet stream. There was no detection of L2 at the inlet for the
duration of this test but L2 would appear randomly between 10 and
60 ppbv at the outlet.
[0113] AC1 was in operation for 2054 hours before the observation
of breakthrough. According to the OEM, this greatly exceeds the
longest observed media life for their traditional media (CS1, 60
CTC), which was 952 hours. At 2054 hours, siloxane D4 showed
complete breakthrough and siloxane L3 showed minimal breakthrough.
However, a check sample was taken to confirm breakthrough; the
check sample showed 20% D4 breakthrough with all other siloxanes
remaining non-detect or below GC/MS detection limits. Given that
the breakthrough value exceeded 150 ppbv for two consecutive
samples, the media was changed out to protect the downstream
microturbine.
[0114] Based on time-to-breakthrough, a total siloxane loading
between 2.1% and 2.3% by weight is estimated. Based on consolidated
testing summaries by Ajhar, M. et al., Bioresource Techn., 101
(2010), 2913-2923, this loading capacity greatly exceeds the
typical loading capacities for traditional activated carbons and
rivals silica gel as observed in historical field testing (see
Table 4 below). The historical operating pattern for this digester
was 600-960 hours using non-lignite coal-based activated carbon.
That pattern translates to 0.57-0.91% siloxane loading on carbon by
weight, which correlates well with Table 4. Table 5 presents the
estimated loading for measured siloxanes at the time of D4 initial
breakthrough when using AC1. The data collection for the field test
was stopped at the initial breakthrough of D4. The values presented
here are understated since carbon was not taken to full
breakthrough or saturation. L3, L4, D3, and D5 did not breakthrough
before the initial breakthrough of D4. The time to breakthrough for
the other siloxane species would have been on an order of at least
over about 50% greater, or about 100% greater than for D4.
TABLE-US-00004 TABLE 4 Media Type Coconut Graphite Silica General
AC AC Gel AC Performances 0.4-1.0 0.6-1.5 1.0-2.2 0.2-1.5 (% by
wt)
TABLE-US-00005 TABLE 5 Siloxane: L3 L4 D3 D4 D5 Loading Capacity (%
by wt.): >0.6 >0.6 >1.4 >2.7 >3.2
[0115] With respect to general observations on H.sub.2S/VOC removal
and competitive effects, AC1 was evaluated for preferential
adsorption to siloxanes while minimizing the competitive effects of
VOC's and total reduced sulfur compounds. H.sub.2S breakthrough
occurred after approximately 336 hours, achieving approximately
3.3% loading by weight. H.sub.2S is not physically loaded onto
activated carbon. It is chemically and catalyticaly converted by
various reaction pathways. AC1 has been tested to demonstrate a
high alkalinity and high catalytic activity (2.26 meq/g
neutralizing capacity).
[0116] VOC breakthrough occurred in 1098 hours suggesting an
overall loading capacity of 5.3% by weight. VOC content was a
mixture of alkanes, ketones, and aromatics. An interesting finding
was continued complete removal of ketones and aromatics (BTEX) even
after complete breakthrough of all other VOC's was observed.
Benzene and toluene breakthrough occurred between 1500 and 1700
hours although xylene removal remained at 100% at time of change
out. Ultimately, given the observed loading capacities, siloxane
breakthrough would have been 216 hours (if sulfur-driven), 600
hours (if VOC-driven), or 1008 hours (if BTEX-driven). Siloxane
breakthrough occurred in 2054 hours suggesting greatly reduced
competitive effects when using AC1.
[0117] The results of the Digester #1 field tests show that the AC1
and AC2 products have high siloxane removal capacity with minimal
competitive influences from other contaminants. AC1, for example,
demonstrated a 215% improvement in bed life at the digester.
Example 2
[0118] In a field test conducted at Digester #2, AC2 was installed
in the primary vessel of a lead/lag system. When tested, CS2 (4 mm
pellets; 60 CTC activity) were installed in the secondary vessel.
Gas samples were taken via TEDLAR.RTM. bag at the inlet and outlet
to measure siloxane concentration.
[0119] AC2 was investigated for siloxane removal performance at a
digester site that historically used the segmented activity
gradient (SAG) or layered bed methodology. Prior to testing AC2, a
baseline was established using 4 mm activated carbon pellets with
60 CTC activity (CS2). Once removal efficiency was established, AC2
was installed in the primary vessel with CS2 remaining in the
secondary vessel. The filter bed dimensions used were 275 cm depth
and 183 cm diameter. This was a 3-vessel lead-lag system with two
vessels operating and a third on standby. All vessels had the same
dimensions and bed depth. Five historical data points for SAG
media, one data point for CS2, and five data points for the AC2
combination were evaluated. The five data points for SAG
performance was collected approximately once a month over four
consecutive months. The AC2 data was collected approximately once a
month over five consecutive months. The CS2 data point was
collected approximately one month after the last data point was
collected for SAG and one month before the first data point was
collected for AC2.
[0120] Table 5 presents the results for siloxane removal
performance of the tested media. Inlet and outlet siloxane
concentrations (ppm) measured before and after flowing through the
media are indicated from which the %Removal was calculated.
TABLE-US-00006 TABLE 5 Data Point Inlet Outlet % Removal SAG 1 0.63
0.09 86% 2 1.1 0.04 96% 3 2.3 0.1 96% 4 0.31 0.08 74% CS2 1 0.2
0.02 90% AC2 1 3.9 0.05 99% 2 0.68 0.14 79% 3 1.6 0.04 98% 4 2.8
0.04 99% 5 1.9 0.03 98%
[0121] On average, the addition of AC2 enhanced siloxane removal.
The average removal efficiency for SAG was 93% whereas the
introduction of AC2 improved performance to 97%. Variability on the
outlet was also reduced. AC2 controlled outlet concentrations of
siloxanes to a variability of .+-.85 ppb; SAG media variability was
.+-.253 ppb. Although not desiring to be bound to any theory, it is
theorized that AC2 offers greater homogeneity for the entire vessel
volume compared to layering. During the AC2 trial, engine oil life
was observed to be increased by a factor of three. In addition, it
was noted that spark plugs were noticeably "cleaner" or
residue-free. Both of these are indicative of improved siloxane
removal. Specifically, a 200% increase in engine oil life from
1,000 hours to 3,000 hours, and greater than 25% increase in spark
plug life, and improved siloxane removal efficiency were
observed.
[0122] The results of the Digester #2 field tests show that the AC2
product has high siloxane removal capacity with minimal competitive
influences from other contaminants.
Example 3
[0123] In field tests performed at Landfill #1, AC2 was installed
in a two-vessel lead/lag system. The filter bed dimensions used
were 152 cm depth and 275 cm diameter. Samples were taken from the
inlet, in-between, and outlet locations via TEDLAR.RTM. bag.
Analysis was performed on siloxane, total reduced sulfur (TRS), and
VOC content via GC/MS. Sampling frequency was weekly. AC2 was
evaluated as an alternative to SAG methodology. A lead/lag system
was utilized where the vessel change outs occur approximately every
12 days. After 12 days, the secondary vessel is switched to primary
position, the primary vessel is then changed out with fresh media,
and placed in the secondary position. At time of change out, both
TRS and siloxane species were trending at 100% removal; VOC removal
was 92% at change out. The plant was shut down at 20 days into the
field test for reasons unrelated to the field test. 20 days of at
least 99% siloxane removal was observed prior to the shutdown
(inlet concentration=4-8 ppm), 100% sulfur removal (TRS) (inlet
concentration=100-160 ppm), and approximately at least 95% VOC
removal (inlet concentration=25-100 ppm). Gas analysis showed 100%
removal of both siloxanes and TRS at the time of plant shut down.
Based on the results observed up until shutdown, the bed life
forecast was 30-35 days as an estimate based on breakthrough on the
lead vessel, and the lag vessel showed 0% breakthrough.
[0124] Breakthrough had been observed at the outlet of the primary
vessel; essentially, H.sub.2S had completely broken through with
VOC's and siloxanes showing initial signs of breakthrough, as shown
in FIG. 5.
[0125] There were insufficient historical records to make a
comparative assessment to previous media. However, the performance
observed in the field test indicates that AC2 is a viable
alternative to the SAG methodology. Extrapolation of primary vessel
breakthrough data indicates a potential 30% improvement in media
longevity and performance compared to the normal operating pattern,
such as shown in FIG. 6.
[0126] The present invention can include any combination of these
various features or embodiments above and/or below as set forth in
sentences and/or paragraphs. Any combination of disclosed features
herein is considered part of the present invention and no
limitation is intended with respect to combinable features.
[0127] Applicants specifically incorporate the entire contents of
all cited references in this disclosure. Further, when an amount,
concentration, or other value or parameter is given as either a
range, preferred range, or a list of upper preferable values and
lower preferable values, this is to be understood as specifically
disclosing all ranges formed from any pair of any upper range limit
or preferred value and any lower range limit or preferred value,
regardless of whether ranges are separately disclosed. Where a
range of numerical values is recited herein, unless otherwise
stated, the range is intended to include the endpoints thereof, and
all integers and fractions within the range. It is not intended
that the scope of the invention be limited to the specific values
recited when defining a range.
[0128] Other embodiments of the present invention will be apparent
to those skilled in the art from consideration of the present
specification and practice of the present invention disclosed
herein. It is intended that the present specification and examples
be considered as exemplary only with a true scope and spirit of the
invention being indicated by the following claims and equivalents
thereof.
* * * * *