Title: Gas monitoring system and method
Abstract: An apparatus and method for monitoring a source gas for detection of phosgene and/or chlorine dioxide therein, in which the source gas is filtered for removal of hydrogen sulfide and/or chlorine and/or hydrogen chloride prior to monitoring of the source gas by a gas sensor specific for phosgene or chlorine dioxide detection. The filter includes a support having Ag.sub.2 O thereon, and when the source gas contains chlorine dioxide, chlorine also is present in the source gas prior to its filtration.
Patent Number: 6,840,084 Issued on 01/11/2005 to Nikolskaya
| Inventors:
|
Nikolskaya; Elena (Vitebsky pr., 63, Apartment 142, St. Petersburg, RU 196 233)
|
| Appl. No.:
|
202590 |
| Filed:
|
July 24, 2002 |
| Current U.S. Class: |
73/23.2; 73/31.05; 340/632; 436/124 |
| Intern'l Class: |
G01N 009/00; G01N027/26; G08B017/00 |
| Field of Search: |
73/23.2,31.05
340/632
436/124
205/779.5
204/412
422/56
|
References Cited [Referenced By]
U.S. Patent Documents
| 4127386 | Nov., 1978 | Stahl et al.
| |
| 4141800 | Feb., 1979 | Breuer et al. | 205/779.
|
| 4205043 | May., 1980 | Esch et al. | 422/56.
|
| 4256543 | Mar., 1981 | Petersen et al. | 205/779.
|
| 4571292 | Feb., 1986 | Liu et al. | 204/412.
|
| 4633704 | Jan., 1987 | Tantram et al. | 73/31.
|
| 6252510 | Jun., 2001 | Dungan | 340/632.
|
| 6284545 | Sep., 2001 | Warburton et al.
| |
| 6548024 | Apr., 2003 | Doncaster et al. | 422/88.
|
| 6670887 | Dec., 2003 | Dungan | 340/632.
|
Primary Examiner: Williams; Hezron
Assistant Examiner: Saint-Surin; Jacques M.
Attorney, Agent or Firm: Yang; Yongzhi, Fuierer; Marianne, Hultquist; Steven J.
Claims
What is claimed is:
1. A monitored gas system, comprising:
a source gas;
a gas sensor constructed and arranged to monitor the source gas for
detection of phosgene and/or chlorine dioxide therein; and
a filter for removal of hydrogen sulfide and/or chlorine from the source
gas prior to its monitoring by the gas sensor, said filter comprising a
support having Ag.sub.2 O thereon;
wherein when said source gas contains chlorine dioxide, chlorine is present
in said source gas prior to filtration of said source gas by said filter.
2. The monitored gas system of claim 1, wherein the gas sensor is arranged
to provide an output signal correlative of the concentration of phosgene
and/or chlorine dioxide therein.
3. The monitored gas system of claim 2, further comprising means for
treating the source gas subsequent to monitoring thereof by the gas
sensor.
4. The monitored gas system of claim 3, wherein said means for treating the
source gas are constructed and arranged to at least partially remove
phosgene and/or chlorine dioxide from the source gas.
5. The monitored gas system of claim 4, wherein said treating means for at
least partial removal of phosgene and/or chlorine dioxide is controlled by
or in response to the output signal correlative of the concentration of
phosgene and/or chlorine dioxide in the source gas.
6. The monitored gas system of claim 5, further comprising a CPU coupled in
signal transmission relationship with the gas sensor to receive said
output signal, and wherein the CPU is coupled in controlling relationship
to said treating means for at least partial removal of phosgene and/or
chlorine dioxide.
7. The monitored gas system of claim 1, wherein the support comprises a
porous glass support.
8. The monitored gas system of claim 7, wherein the porous glass support
comprises a binderless needled glass mat.
9. The monitored gas system of claim 8, wherein the Ag2O is in the form of
a powder dispersed in said binderless needled glass mat.
10. The monitored gas system of claim 1, wherein said gas sensor and filter
are disposed in a unitary housing.
11. The monitored gas system of claim 1, wherein the gas sensor comprises
an electrochemical sensor.
12. The monitored gas system of claim 11, wherein the electrochemical
sensor and filter are disposed in a unitary housing.
13. A gas monitoring system for a source gas, comprising:
a gas sensor constructed and arranged to detect phosgene in said source
gas; and
a filter for removal of hydrogen sulfide and/or chlorine from the source
gas prior to its exposure to the gas sensor, said filter comprising a
support impregnated with Ag.sub.2 O.
14. A method of monitoring a source gas for phosgene and/or chlorine
dioxide therein, comprising:
filtering the source gas to remove hydrogen sulfide and/or chlorine
therefrom and produce a filtered source gas, by contacting the source gas
with a filter comprising a support having Ag.sub.2 O thereon; and
exposing the filtered source gas to a gas sensor constructed and arranged
to detect phosgene and/or chlorine dioxide therein;
wherein when said source gas contains chlorine dioxide, chlorine is present
in said source gas prior to filtering thereof.
15. The method of claim 14, wherein the gas sensor is arranged to provide
an output signal correlative of the concentration of phosgene and/or
chlorine dioxide therein.
16. The method of claim 15, further comprising treating the source gas
subsequent to monitoring thereof by the gas sensor.
17. The method of claim 16, wherein said step of treating the source gas
comprises at least partially removing phosgene and/or chlorine dioxide
from the source gas.
18. The method of claim 17, wherein said step of at least partially
removing phosgene and/or chlorine dioxide is controlled by or in response
to the output signal correlative of the concentration of phosgene and/or
chlorine dioxide in the source gas.
19. The method of claim 18, further comprising providing a CPU coupled in
signal transmission relationship with the gas sensor to receive said
output signal, and wherein the CPU is arranged to control the step of at
least partially removing phosgene and/or chlorine dioxide.
20. The method of claim 14, wherein the support comprises a porous glass
support.
21. The method of claim 20, wherein the porous glass support comprises a
binderless needled glass mat.
22. The method of claim 21, wherein the Ag.sub.2 O is in the form of a
powder dispersed in said binderless needled glass mat.
23. The method of claim 14, wherein said gas sensor and filter are disposed
in a unitary housing.
24. The method of claim 14, wherein the gas sensor comprises an
electrochemical sensor.
25. The method of claim 24, wherein the electrochemical sensor and filter
are disposed in a unitary housing.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a gas monitoring system, and
more specifically to a system including a filter and a gas sensor, in
which the filter removes hydrogen sulfide and/or chlorine and/or hydrogen
chloride from a source gas that is provided to the gas sensor for sensing
of other component(s) therein.
2. Description of the Related Art
Gas sensors are used in many applications for the detection of hazardous
gas component(s) in a gas stream or gas environment. These hazardous gas
component(s), hereafter referred to as "target gas," may be of widely
varying types. Their hazardous character may derive from their toxicity to
humans, pyrophoricity, explosive character, flammability, deactivating
character as regards materials used for abatement or reclamation of other
components in the gas mixture.
In many applications, the gas sensor is not strictly selective for the
target gas, and the other component(s) of the gas being monitored may
interefere with or preclude the proper operation of the gas sensor. For
example, gas component(s) other than the target gas can produce the same
signal or response by the sensor, so that the concentration of the target
gas in the gas stream or environment being monitored is misattributed by
the gas sensor.
Such misattribution of the concentration of the target gas can have severe
consequences for the process operation or action that is conducted based
on the sensed concentration of the target gas. For example, vital steps of
an industrial process may be curtailed or unduly prolonged due to the
incorrect sensing of target gas, with consequent adverse effect on the
process economics or safety. Action may be taken based on the
misattributed target gas sensing that is wasteful or even superfluous.
Such undesirable behavior of the gas sensor can be prevented by the use of
gas filters that remove from the gas being monitored by the gas sensor,
those component(s) that would otherwise interfere with the accurate
sensing of the target gas by the gas sensor.
The present invention relates to gas filters for such purpose, and to gas
monitoring systems that comprise such filters.
Phosgene (COCl.sub.2) is a chemical of major industrial importance. The
annual production worldwide is more than 1 million tons, 90% of which is
used in the manufacture of isocyanates and polyurethane and polycarbonate
resins. Phosgene is also extensively used as a synthetic reagent in a wide
variety of organic chemical processes, e.g., the synthesis of numerous
chloride compounds.
Phosgene also is a hazardous chemical compound, since it readily decomposes
in the presence of water to yield HCl and CO.sub.2. Phosgene also is
highly adsorbable, even by such chemically inert materials as
polytetrafluoroethylene (PTFE), in addition to being highly toxic,
irritating and corrosive in character. Inhalation of phosgene can cause
fatal respiratory damage. Due to its colorless, odorless character,
phosgene is a gas that requires, sensitive, accurate and reliable
monitoring in gas streams or environments in which it is or may be
present.
Due to its hazardous character, the maximum workplace concentration (MWC)
of phosgene during a 40 hour week in a workplace environment is 0.1 parts
per million by volume (ppmv).
Chlorine dioxide (ClO.sub.2) is another chlorine-containing hazardous gas,
whose MWC value also is 0.1 ppmv. ClO.sub.2 is manufactured on a large
scale, as is used as a substitute for chlorine or ozone in many industrial
applications. Its uses include biocidal applications (e.g., in the pulp
and paper industry), disinfection applications (in municipal water
treatment, treatment of medical waste, and food applications), circuit
board cleaning in the electronics industry, treatment of sulfides in the
petroleum industry, and bleaching applications in the textile industry, to
name a few. An advantage of using ClO.sub.2 is that it does not directly
form halogenated byproducts, as is the case when chlorine is employed.
Like chlorine, ClO.sub.2 is a very strong oxidant. ClO.sub.2 also has the
advantage that it does not form dioxins.
ClO.sub.2, however, is not stable, and it therefore is typically produced
at the point of use (POU) location, in the amount that is required.
Chlorine dioxide is a highly reactive gas, readily entering into
disproportion reactions, decomposing to HCl and HClO.sub.3 in the presence
of water, or to ClO.sub.3 and H.sub.2 O in alkaline solution. ClO.sub.2 is
able to react as an oxidative or a reductive agent. It can be oxidized by
strong oxidants such as potassium permanganate but in many instances
reacts as an oxidant itself. Chlorine dioxide is highly adsorbable, e.g.,
by activated carbon. Due to its high toxicity, it is necessary to monitor
chlorine dioxide in an accurate, sensitive and reliable manner.
Electrochemical sensors are widely used for measuring the concentration of
toxic gases (see, for example, Advances in Electrochemistry and
Electrochemical Engineering, Volume 10 (J. Wiley & Sons, 1976). A
potential disadvantage of electrochemical sensors is their
cross-sensitivity to other hazardous gases that may be present in the
stream or environment being monitored for a target gas.
Considering the aforementioned gases COCl.sub.2 and ClO.sub.2 as target gas
species, which are desirably monitored in environments and/or process
streams containing same, it is to be noted that the presence of COCl.sub.2
and/or ClO.sub.2 gas in many applications is accompanied by the presence
of hydrogen sulfide and/or chlorine and/or HCl. The latter gases are less
toxic than phosgene or chlorine dioxide, as shown by their MWC values.
Whereas COCl.sub.2 and ClO.sub.2 each have a MWC value of 0.1 ppmv, the
MWC value of Cl.sub.2 is 1.0 ppmv, the MWC value of HCl is 5.0 ppmv and
the MWC value of H.sub.2 S is 10.0 ppmv.
H.sub.2 S is easily oxidized in the following reaction:
H.sub.2 S.sup.31 +4H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +8H.sup.+ +8e.sup.-
and chlorine is a strong oxidant:
Cl.sub.2 +2e.sup.-.fwdarw.2Cl.sup.- Eo=1.36 volts
In electrochemical sensors for COCl.sub.2, phosgene produces an anode
current. In electrochemical sensors for chlorine dioxide, the ClO.sub.2
gas produces a cathode current by the following reduction reaction
ClO.sub.2 +4H.sup.+ +5e.sup.-.fwdarw.2H.sub.2 O+Cl.sup.- E.sub.o =1.27
volts
In such sensors for phosgene and chlorine dioxide, the sensor response to
H.sub.2 S has the same polarity as the sensor response to phosgene, and
the opposite polarity to the response of the sensor to chlorine dioxide.
Thus, the presence of hydrogen sulfide in an air mixture with phosgene will
produce a false higher response of the sensor to phosgene, and the
presence of hydrogen sulfide in an air mixture with chlorine dioxide will
produce a false lower response of the sensor to chlorine dioxide, even
when the hydrogen sulfide in the respective air mixtures is at a level
below the MWC value.
Correspondingly, in such sensors for phosgene and chlorine dioxide, the
sensor response to chlorine has the opposite polarity to the response of
the sensor to COCl.sub.2 and the same polarity as the response of the
sensor to ClO.sub.2.
Thus, the presence of chlorine in an air mixture with phosgene will produce
a false lower response of the sensor to phosgene, and the presence of
chlorine in an air mixture with chlorine dioxide will produce a false
higher response of the sensor to chlorine dioxide, even when the chlorine
in the respective air mixtures is at a level below the MWC value.
When both hydrogen sulfide and chlorine are present with the target gas in
a three-component gas mixture, the phosgene sensor or chlorine dioxide
sensor will show a superpositional response, i.e., an algebraic summation
of the responses of the sensor to each gas component.
Hydrogen chloride (HCl) poisons COCl.sub.2 sensors, which typically use
gold working electrodes. It is thought that the Cl.sup.- anion forms
complexes with the gold electrode thereby preventing accurate
determination of ClO.sub.2 concentration. As one example, 10.0 ppmv HCl
distorts a ClO.sub.2 sensor signal by between about 150 and 300 nA.
Hydrogen sulfide, chlorine and hydrogen chloride are also interferent gas
components for other electrochemical gas sensors, e.g., those employed for
monitoring of target gas species such as sulfur dioxide, nitrogen dioxide,
hydrogen, hydrogen chloride and ammonia.
The use of chemically selective filters is known in the art, wherein the
filter effects removal of the interferent gas species from the gas being
monitored, so that the filtered gas subsequently exposed to the gas sensor
produces a concentration sensing for the target gas that is unaffected by
the presence of the interferent gas species, and thereby accurate for the
target gas. For example, hydrogen sulfide filters are described in
Warburton et al. U.S. Pat. No. 6,284,545 and are otherwise known, which
operate by oxidation or adsorption of the hydrogen sulfide component of
the gas mixture containing same, using filters employing manganese
dioxide, potassium permanganate, activated carbon, activated carbon with
manganese dioxide, etc. Such filters are effective in removing hydrogen
sulfide as well as chlorine, but at the same time they also remove
phosgene and chlorine dioxide with very high effectiveness. In
consequence, these filters produce a filtered gas that is
misrepresentative of the concentration of phosgene and chlorine dioxide in
the original source gas (i.e., prior to filtering), producing false lower
sensed concentrations of the target gas. Such false low reading of the
target gas concentration by the gas sensor thus creates a situation of
potential danger to personnel in the vicinity of the source gas as well as
inadequate treatment or processing of gas due to the false lower sensed
concentration of the target gas.
The art therefore is in need of a gas sensing system for monitoring
concentration of phosgene and chlorine dioxide in instances where the
source gas being monitored contains hydrogen sulfide and/or chlorine,
and/or hydrogen chloride.
SUMMARY OF THE INVENTION
The present invention relates generally to an apparatus and method for
monitoring a source gas for detection of phosgene and/or chlorine dioxide
therein, in which the source gas is filtered for removal of hydrogen
sulfide and/or chlorine, and/or hydrogen chloride prior to monitoring of
the source gas by a gas sensor specific for phosgene and/or chlorine
dioxide detection.
In one aspect, the invention relates to a monitored gas system, comprising:
a source gas;
a gas sensor constructed and arranged to monitor the source gas for
detection of at least one of, phosgene and chlorine dioxide therein; and
a filter for removal of at least one of, hydrogen sulfide, chlorine, and
hydrogen chloride, from the source gas prior to its monitoring by the gas
sensor, said filter comprising a support having Ag.sub.2 O thereon;
wherein, when said source gas contains chlorine dioxide, chlorine is
present in said source gas prior to filtration of said source gas by said
filter.
Another aspect of the invention relates to a gas monitoring system for a
source gas, comprising:
a gas sensor constructed and arranged to detect phosgene in said source
gas; and
a filter for removal of at least one of, hydrogen sulfide, chlorine and
hydrogen chloride from the source gas prior to its exposure to the gas
sensor, said filter comprising a support impregnated with Ag.sub.2 O.
A still further aspect of the invention relates to a method of monitoring a
source gas for at least one of phosgene and chlorine dioxide therein,
comprising:
filtering the source gas to remove at least one of hydrogen sulfide,
chlorine and hydrogen chloride therefrom to produce a filtered source gas,
by contacting the source gas with a filter comprising a support having
Ag.sub.2 O thereon; and
exposing the filtered source gas to a gas sensor constructed and arranged
to detect at least one of, phosgene and chlorine dioxide therein; wherein
when said source gas contains chlorine dioxide, chlorine is present in
said source gas prior to filtering thereof.
Other aspects, features and embodiments of the invention will be more fully
apparent from the ensuing disclosure and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a process system employing a gas
sensor and associated gas filter according to the present invention, in an
illustrative embodiment thereof.
FIG. 2 is a schematic representation of a gas sensor and filter unit
according to one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF
The present invention provides a gas sensing system useful for monitoring
phosgene and/or chlorine dioxide, as a target gas, in a source gas
containing at least one of, hydrogen sulfide, chlorine and hydrogen
chloride in mixture with such target gas. The gas sensing system utilizes
a gas filter that is highly selective for hydrogen sulfide and/or chlorine
and/or hydrogen chloride in such source gas, and is substantially
non-interactive with the target gas species. When the source gas is an air
mixture containing the target gas and at least one of, hydrogen sulfide,
chlorine and hydrogen chloride, the gas filter is effective even in the
presence of moisture, over a wide range of humidity of the source gas.
The gas filter in the gas sensing system of the invention in one embodiment
includes an inert high porosity support with finely divided silver (I)
oxide in the support. When contacted with the source gas containing
phosgene and/or chlorine dioxide, in combination with at least one of
hydrogen sulfide (H.sub.2 S), chlorine (Cl.sub.2), and hydrogen chloride
(HCl), the concentration of at least one of H.sub.2 S, Cl.sub.2 and HCl is
reduced from the source gas to yield a sensing gas mixture whose
concentration of the target gas species is substantially unchanged from
the source gas.
The gas filter in accordance with the invention includes a high porosity
support for penetration of the source gas, and an active agent on the
support. The gas filter has a high removal capacity for the interferent
hydrogen sulfide and/or chlorine and/or hydrogen chloride species present
in the source gas, and is effective to remove substantially all of the
interferent gas species from the source gas. The filter is substantially
non-interactive with the target gas species in the source gas, whereby the
concentration of the target species in the source gas after filtration is
substantially equal to the concentration of the target gas species in the
source gas prior to filtration treatment of the source gas to provide the
sensing gas mixture.
The active agent in the gas filter of the invention comprises a silver (I)
compound which forms insoluble compounds with both sulfide and chlorine
ions, and is substantially non-reactive with phosgene and with chlorine
dioxide, whereby an associated phosgene sensor or associated chlorine
dioxide sensor provides an accurate and reproducible sensing of the
respective target gas species. Table 1 below sets out illustrative silver
(I) compounds and their solubility product coefficient values, K.sub.sp.
TABLE 1
SOLUBILITY PRODUCT COEFFICIENT
SALT K sp
Ag.sub.2 S 1 .times. 10.sup.-49
AgCl 1.6 .times. 10.sup.-10
Ag.sub.2 O 2.0 .times. 10.sup.-8
The active agent and the support of the gas filter of the invention are
preferably selected to provide a support with suitably high surface for
extended lifetime operation in conjunction with the gas sensor, so that
interferent species are removed from the source gas for sensing by the gas
sensor, during the entire operating life of the sensor. The active agent
and the support of the gas filter are also preferably selected to provide
high stability of the filter during the gas monitoring operation, e.g.,
over a wide range of relative humidity when the source gas comprises
ambient air in mixture with the target gas and the interferent gases, and
the ambient air contains moisture (a non-zero relative humidity). The
silver compound and the support are desirably selected to provide a high
and enduring level of association (adhesion) of the silver compound to the
support. In one preferred aspect of the invention, the silver compound is
hydrophobic in character, e.g., a hydrophobic silver salt.
The silver compound is suitably applied to the support by impregnation of
the silver compound from a solution of the compound, with which the
support is contacted, followed by drying of the contacted support to
evaporate the solvent and yield the silver compound-impregnated support.
When the silver compound is hydrophilic in nature, the impregnation
solution may be an aqueous solution of the silver compound, e.g., a silver
salt, but the resulting gas filter should be used in services where the
source gas is anhydrous (moisture-free) since the presence of water in the
source gas in such case will result in the H.sub.2 S/Cl.sub.2 /HCl removal
capability of the filter varying with the relative humidity of the source
gas.
A preferred silver compound in the gas filter of the invention is Ag.sub.2
O, which is water-insoluble and hydrophobic, and therefore stable over a
wide relative humidity range for filtering hydrogen sulfide, chlorine and
hydrogen chloride from source gas that contains moisture.
As shown in Table 1, the coefficient of the solubility product (K.sub.sp)
for Ag.sub.2 O is far higher than for Ag.sub.2 S (by about 40 orders of
magnitude) and for AgCl (by 2 orders of magnitude). The removal of
hydrogen sulfide, chlorine and hydrogen chloride with an Ag.sub.2 O-based
filter is highly effective, while at the same time the silver oxide active
agent does not adsorb or otherwise interact with the target gas
(COCl.sub.2 or ClO.sub.2). Further, the reaction products of the reaction
of Ag.sub.2 O with hydrogen sulfide, chlorine and hydrogen chloride are
non-reactive with the target gas, do not clog the filter, and include
reaction product species (e.g., AgCl) that are also useful for hydrogen
sulfide removal. Silver chloride is reactive with hydrogen sulfide (see
Table 1 hereinabove, showing a K.sub.sp difference for Ag.sub.2 S and AgCl
of about 40 orders of magnitude). The filter thus effects the following
reactions:
Ag.sub.2 O+H.sub.2 S=Ag.sub.2 S+H.sub.2 O
2Ag.sub.2 O+Cl.sub.2 =2AgO+2AgCl
Ag.sub.2 O+2HCl=2AgCl+H2O
The support for the filter of the present invention in a preferred
embodiment is a needled glass mat without binder. A particularly preferred
binderless needled glass mat for such purpose is commercially available as
ecoMat Type G 300 (Johns Manville Sales GmbH), which provides high
efficacy in use of Ag.sub.2 O as the active removal agent, and is
completely inert to both COCl.sub.2 and ClO.sub.2, other components of the
gas mixture and the electrolyte. The mat is a highly porous glass material
with a square meter weight of more than 300 grams/m.sup.2, providing a
highly gas-penetrable structure for the target gas.
In a preferred embodiment using the binderless needled glass mat, the
active agent is Ag.sub.2 O in the form of a finely divided powder spread
on the high surface area needles in the mat, to provide a thin layer of
silver oxide fine particles. The thin layer of silver oxide fine particles
has very good adhesion to the needled glass mat, providing a high
filtering capacity for hydrogen sulfide, hydrogen chloride and chlorine.
Adhesion to the needled glass mat of the removal reaction products
(Ag.sub.2 S and AgCl) is also high.
A particularly complex issue faced in the development of the filter of the
present invention relates to the ratio of the MWC of the gases present in
the source gas to be analyzed.
##STR1##
The objective is to reliably measure low concentration (<0.1 ppmv) of
the target gas (phosgene or chlorine dioxide) in presence of harmful gases
which concentrations could be, for example, about one order (Cl.sub.2),
more than 1.5 order (HCl), or even two orders (H.sub.2 S) of magnitude
higher. The present invention achieves such objective by providing high
gas sensor sensitivity to the target as, high kinetic rates for removal of
hydrogen sulfide, chlorine and hydrogen chloride from the source gas, and
high filter capacitance for hydrogen sulfide, chlorine and hydrogen
chloride.
The phosgene or chlorine dioxide sensor in the practice of the present
invention is of any suitable type, such as an electrochemical sensor
including an assembly of working, reference and counter electrodes or
working and counter electrodes, wherein the successive electrodes are
separated from one another by separator elements, and disposed in a
housing containing an electrolyte.
The hydrogen sulfide/chlorine/hydrogen chloride filter of the invention is
advantageously deployed in proximity to the sensor, and in a preferred
aspect, the H.sub.2 S/Cl.sub.2 /HCl filter is beneficially integrated with
the electrochemical sensor for phosgene or chlorine dioxide monitoring of
the source gas. The interferent species filter for H.sub.2 S/Cl.sub.2 /HCl
removal may be deployed downstream of a dust filter, to remove
particulates that could otherwise interfere with the proper operation of
the interferent species filter and the gas sensor. In such arrangement,
the source gas flows sequentially through the dust filter, interferent
species filter and the electrochemical sensor. The dust filter may be
disposed in a unitary assembly of the interferent species filter and the
gas sensor, to provide an integrated gas sensing assembly, as hereinafter
more fully described.
The dust filter, when employed, should be appropriately designed with
respect to the flow impedance characteristics of such filter, since the
dust filter may serve to alter the sensitivity of the gas sensor. The
porosity of the dust filter therefore is a design parameter and should be
appropriately selected to provide a desired sensitivity in the gas sensor,
since increasing the porosity of the dust filter increases the sensitivity
of the COCl.sub.2 or ClO.sub.2 sensor. Additionally, the interferent
species filter should be designed to avoid uptake of any target gas
species, and therefore such filter is desirably formed of materials of
construction, as to the housing and filtering medium, and associated flow
circuitry thereof. Such dust filter/interferent species filter arrangement
is advantageously optimized with respect to dust filter porosity,
thickness of the support in the interferent species filter, and quantity
of active agent (e.g., Ag.sub.2 O) on the support in the interferent
species filter.
Ag.sub.2 O as a preferred active agent for the interferent species filter
forms insoluble salts to bind sulfide anion without hazardous side
reaction products, providing a stable selective filter with high
capacitance for hydrogen sulfide. Concurrently, the difference between
K.sub.sp for Ag.sub.2 O and AgCl enables the efficient removal of chlorine
and hydrogen chloride from the source gas, and provides a means for
achieving high sensitivity and selectivity for the target gas in source
gas mixtures including the target gas, hydrogen sulfide, hydrogen chloride
and chlorine.
Referring now to the drawings, FIG. 1 is a schematic representation of a
process system employing a gas sensor and associated gas filter according
to the present invention, in an illustrative embodiment thereof.
The FIG. 1 process system 10 includes a supply 12 of the source gas. The
supply 12 may include a process unit that generates the target gas in
mixture with at least one of, hydrogen sulfide, chlorine and hydrogen
chloride, as a multicomponent gas mixture. Alternatively, the supply 12 of
the source gas may be a gas environment that is subject to ingress or
contamination by the target gas in mixture with interferent gas species
(H.sub.2 S and/or HCl and/or Cl.sub.2). The source gas, containing
phosgene or chlorine dioxide, in addition to at least one of hydrogen
sulfide, chlorine and drogen chloride, flows from supply 12 in line 14 to
the abatement processing unit 16 in which the source gas is treated to
remove the phosgene or chlorine dioxide therefrom.
A phosgene-depleted, or chlorine dioxide-depleted, stream is discharged
from the abatement processing unit 16 in line 18, and may be passed to a
further downstream process or final disposition, as required.
A side stream of the source gas from line 14 is flowed in line 20, under
the action of motive fluid driver 22, through dust filter 23, interferent
species filter 24 and gas sensor 26, being returned to line 14 downstream
of gas sensor 26, as shown. The dust filter 23 removes particulates from
the source gas, and the interferent species filter 24 removes hydrogen
sulfide and/or chlorine and/or hydrogen chloride from the dust-depleted
source gas, to provide an interferent-free gas mixture comprising the
phosgene or chlorine dioxide component, to the gas sensor 26.
The gas sensor 26 monitors the concentration of the target gas (phosgene or
chlorine dioxide) in the side stream and generates a corresponding
response signal correlative to the sensed concentration of the target gas
species. The response signal is transmitted in signal transmission line 28
to central processing unit (CPU) 30, which in turn generates a
corresponding control signal that is transmitted in control signal line 32
to the abatement processing unit 16. The control signal in line 32 may be
employed to modulate the gas processing operation in abatement processing
unit 16 to abate the target gas species.
For example, if phosgene is the target gas species in the source gas, and
such target gas species is abated by chemical reaction thereof with a
chemical reagent in the abatement processing unit 16, the amount of the
chemical reagent may be modulated in response to the sensed concentration
of the phosgene in the source gas, to effect substantially complete
removal of the phosgene from the gas stream treated in abatement
processing unit 16. In other abatement operations, the process conditions
(e.g., temperatures, pressures, flow rates, retention time) in the
abatement-rocessing unit 16 may be modulated to effect the desired
reduction in the concentration of the target gas species in the effluent
stream being treated.
FIG. 2 is a schematic representation of an integrated gas sensor and filter
unit 50 according to one embodiment of the invention.
The integrated gas sensor and filter unit 50 comprises a housing 52 formed
of a suitable material of construction, e.g., nonporous ceramic, polymer,
etc. defining therewithin an interior volume. The interior volume of the
housing includes an electrolyte compartment 53 containing a suitable
electrolyte, and an electrode assembly including a counter electrode 54, a
reference electrode 58 and a working electrode 62, wherein the counter and
reference electrodes are separated by separator member 56, and the
reference and working electrodes are separated by separator member 60.
Overlying the electrode assembly is an interferent species filter 64 for
removing hydrogen sulfide and/or chlorine and/or hydrogen chloride from
the source gas flowed therethrough. A dust filter 66 is joined to the
housing 52 at the upper end of the housing walls, as shown, being sealed
to the top edges of the walls by bond 68. The bond 68 is formed of a
suitable adhesive or sealant medium, and joins the dust filter 66 to the
housing 52 in a leak-tight manner, so that source gas flowed through the
filter enters the interferent species filter 64 and is prevented from
bypassing the filtration and sensing elements in the housing interior
volume.
It will be recognized that the integrated gas sensor and filter unit 50 is
schematically illustrated for ease of description, and does not show the
electrical leads to the electrode elements in the housing or other
ancillary structure, but based on such description, the integrated gas
sensor and filter unit 50 may be readily constructed by those skilled in
the art, to effect gas sensing operation that is accurate and reproducible
for monitoring of the target gas (phosgene or chlorine dioxide) species in
the source gas.
The features and advantages of the invention are more fully shown by the
following non-limiting examples.
EXAMPLE 1
An interferent species filter is fabricated from a needled glass mat
(ecoMat Type G 300; Johns Manville Sales GmbH). The mat is heated at a
temperature of 300.degree. C. in air for 3 hours to clean the surface.
Disks having a diameter of 8 mm are punched from the mat, to provide
support members each having a mass of from about 13.0 mg to about 17.5 mg
(average 15.2 mg.+-.15%).
A quantity of 330 mg of fine silver (I) oxide powder is introduced to a
glass vessel, and 1.0 milliliter of acetone and 0.2 milliliter of water
are added. The vessel then is closed after a magnetic stirrer element is
placed in the powder/solvent mixture, and the vessel contents then are
mixed on a magnetic stirrer for 3 minutes to homogenize the suspension,
and thereafter the suspension is continually stirred to maintain a
homogeneous suspension composition.
Ten of the glass mat disks are placed on an elevated polymer net,
comprising a polyethylene net that is disposed on a Petri cap. The
suspension of Ag.sub.2 O then is pipetted and 5 drops are introduced onto
each needled glass disk from the pipette, following which 4-5 drops of
pure acetone is introduced from a separate pipette onto each needled glass
disk, to spread the fine silver oxide more uniformly throughout the full
volume of the needled glass disk.
The disks then are dried on the elevated polymeric net for 30 minutes in
ambient air at room temperature in a dark (light-free) environment,
following which the disks are heated in an oven at 45.degree. C. for 2
hours.
The impregnated disk then is placed in a sensor cap, and the disk is
covered with a high porosity PTFE dust filter, glued at its perimeter to
the sensor cap as shown in FIG. 2 hereof.
The foregoing construction in application to a phosgene gas sensor assembly
yielded the results shown in Table 2 below.
TABLE 2
Gas sensor
Gas species, without inter- Gas sensor with Interferent species
concentration ferent species interferent species filter capacitance,
in ppmv filter, nA/ppmv filter, nA/ppmv ppmv h
0.33 ppm COCl.sub.2 960 710 --
10 ppm H.sub.2 S 350 0 60
1 ppm Cl.sub.2 -1100 0 2
1 ppm HCl 600 0 0.5
A corresponding construction in application to a chlorine dioxide gas
sensor assembly yielded the results shown in Table 3 below.
TABLE 3
Interferent
Gas species, Gas sensor without Gas sensor with species filter
concentration interferent species interferent species capacitance,
in ppmv. filter, nA/ppmv filter, nA/ppmv ppmv h
1 ppm ClO.sub.2 -990 -990 --
10 ppm H.sub.2 S 300 0 13
1 ppm Cl.sub.2 -300 0 >1
10 ppm HCl 260 0 >10
Table 4 below shows the influence of the exposure of the integrated
filter/sensor unit to 10 ppmv of H.sub.2 S, to 1 ppmv Cl.sub.2 and 1 ppmv
HCl for both phosgene and chlorine dioxide sensors.
TABLE 4
Sensor after Sensor after
Sensor exposure to Sensor after exposure to
response to hydrogen exposure to hydrogen
target gas, sulfide, chlorine, chloride,
Sensor type nA/ppm nA/ppm nA/ppm nA/ppm
COCl.sub.2 sensor 1200 940
COCl.sub.2 sensor 960 830
COCl.sub.2 sensor 850 810
ClO.sub.2 sensor -580 -620
ClO.sub.2 sensor -910 -1310
ClO.sub.2 sensor -600 -580
Although the invention has been variously disclosed herein with reference
to illustrative embodiments and features, it will be appreciated that the
embodiments and features described hereinabove are not intended to limit
the invention, and that other variations, modifications and other
embodiments will suggest themselves to those of ordinary skill in the art.
The invention therefore is to be broadly construed, consistent with the
claims hereafter set forth.
*
