Filter for gas sensor

Details Filter for gas sensor Filter for gas sensor

1999-03-24

2001-09-04

Warden, Jill (Department: 1743)

Chemistry: analytical and immunological testing

Halogen containing

C436S127000, C436S153000, C436S806000, C422S050000, C422S083000, C422S088000, C422S090000, C422S098000

Reexamination Certificate

active

06284545

ABSTRACT:

BACKGROUND OF THE INVENTION
Chlorine dioxide (ClO
2
) is a highly reactive gas that is used as a substitute for chlorine, which is rapidly being phased out of many industries, such as pulp and paper, flour and textiles, due to environmental concerns regarding the formation of dioxins. In addition to being used as a bleaching agent, chlorine dioxide is also used for other purposes such as the disinfection and sterilization of foods and drinking water, and for the treatment of leather. Chlorine dioxide is so highly reactive that it cannot easily be stored in compressed gas cylinders, and must be generated at the point of use. Chlorine dioxide is commonly generated by the electrochemical oxidation of chlorite salts. Further details of the chemistry of chlorine dioxide may be found in standard texts, such as “Chlorine Dioxide—Chemistry and Environmental Impact of Oxychlorine Compounds,” W. J. Masschelein, Ann Arbor Science Publishers Inc, Ann Arbor, Mich.
This high level of reactivity is also reflected in a high toxicity, and the OSHA work place permissible exposure to chlorine dioxide is only 0.1 ppm averaged over an eight hour shift, with a level of 5 ppm considered immediately dangerous to life and health (NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services, June 1997). Thus, it is important to have adequate protection of personnel who are using chlorine dioxide. While most facilities using chlorine dioxide gas use engineering controls to ensure that the ambient concentration of chlorine dioxide is maintained at safe levels, leaks and other emissions unfortunately do occur, posing a risk to personnel in the vicinity. Therefore, it is common practice to use gas detection instruments to monitor for chlorine dioxide and other potentially hazardous gases.
These instruments may be portable instruments intended to provide personal protection, and are typically worn by the personnel to be protected. Alternatively, fixed (e.g. wall mounted) gas detection devices may be employed which monitor the area for the presence of potentially hazardous atmospheres. In a typical application such as a paper mill, instruments may be used to detect chlorine dioxide, hydrogen sulfide, sulfur dioxide and oxygen deficiency. In many cases, multi-gas instruments are available which incorporate sensors for several different types of gases.
Hydrogen sulfide is typically found in many industrial applications, including petroleum-refining operations, coking of coal, purification of natural gas and the evaporation of black liquor in Kraft pulping. For large-scale operations, the hydrogen sulfide is recovered and converted to sulfur dioxide for subsequent conversion to sulfuric acid or elemental sulfur. For smaller scale operations, other pollution control processes are used, such as iron-oxide fire boxes, wet scrubbers containing solutions of oxidants such as chlorine, alkaline potassium permanganate or atmospheric oxygen, or bases such as organic amines, (e.g. ethanolamine) and tripotassium phosphate and sodium carbonate (“Industrial Pollution control Handbook”, H. F. Lund, Ed., McGraw-Hill book Company, New York, 1971; “Pollutant Removal Handbook”, M. Sittig, Noyes Data Corporation, Park Ridge, N.Y., 1973; Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Ed., Vol. 19, Interscience Publishers, New York, 1969). More recently, a number of cobalt, iron and manganese chelate and macrocycle compounds have also been used to catalyze the air oxidation of hydrogen sulfide. Mimoun et al in U.S. Pat. No. 3,956,473 and Deberry et al in U.S. Pat. No. 5,705,135, describe the use of iron chelates in non-aqueous solvents. Anderson et al in U.S. Pat. No. 3,923,645 describe the use of substituted cobalt porphyrins as catalysts for the air oxidation of hydrogen sulfide. Analogously, Verachtert in U.S. Pat. No. 5,244,643 describes the use of transition metal phthalocyanine complexes as catalysts for the air oxidation of hydrogen sulfide and mercaptans in aqueous alkali. Bridges et al in U.S. Pat. No. 5,527,517 have also described the removal of hydrogen sulfide from a gas stream by oxidation of the hydrogen sulfide by aqueous hydrogen peroxide, catalyzed by a silver nitrate or other silver salts.
Hydrogen sulfide gas is of particular concern in those locations where both hydrogen sulfide and chlorine dioxide may be found. Typical locations include pulp and paper mills, water treatment plants, etc. Hydrogen sulfide is a toxic gas, the OSHA permissible exposure limit having a ceiling value of 10 ppm and a level considered immediately dangerous to life and health (IDLH) of 100 ppm (NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services, June 1997). Thus, although hydrogen sulfide is a highly toxic gas, it is less toxic than chlorine dioxide.
In electrochemical gas sensors, the response to hydrogen sulfide is typically the eight-electron oxidation hydrogen sulfide to sulfuric acid.
H
2
S+4H
2
O→H
2
SO
4
+8H
+
+8e
−
The response of an electrochemical sensor to chlorine dioxide is typically either the one electron reduction to hydrogen chlorite:
ClO
2
+H
+
+e
−
→HClO
2
or the five electron reduction to chloride ion:
ClO
2
+4H
+
+5e
−
→2H
2
O+Cl
−
depending on the electrodes and electrolytes used in the sensor. Since hydrogen sulfide is less toxic than chlorine dioxide, a concentration of hydrogen sulfide, within the permissible exposure levels, maybe larger than the permissible exposure levels for chlorine dioxide. Furthermore, the hydrogen sulfide will give a much larger response from the electrochemical gas sensor per unit concentration (eight electrons for hydrogen sulfide versus one electron for chlorine dioxide).
In view of the ease with which hydrogen sulfide is oxidized, electrochemical sensors are often designed to have a reduced sensitivity to hydrogen sulfide compared to the response expected based on gas diffusion of hydrogen sulfide. Despite the efforts of sensor designers, there is still usually a significant response from the sensors to hydrogen sulfide.
However, the most serious problem from a safety perspective is that the response to hydrogen sulfide in an electrochemical sensor is an oxidation reaction, whereas the response to chlorine dioxide in an electrochemical sensor is a reduction reaction. Exposure of a chlorine dioxide detection instrument to hydrogen sulfide alone will usually give a negative response. The response to hydrogen sulfide is in the opposite polarity to the response to chlorine dioxide, and thus the sum of the responses of chlorine dioxide and hydrogen sulfide will be less than that of the same concentration of chlorine dioxide on its own.
Hydrogen sulfide also responds on other electrochemical gas sensors; unfiltered sensors for carbon monoxide, sulfur dioxide, nitrogen dioxide, chlorine, hydrogen, hydrogen chloride and ammonia from, for example, City Technology Ltd., one of the largest gas sensor manufacturers, all give a responses to hydrogen sulfide (Product Data Handbook, Vol. 1. Issue 4, Safety, City Technology Ltd., Portsmouth, United Kingdom, June 1997).
Many sensors incorporate chemical filters in an attempt to reduce the cross sensitivity to hydrogen sulfide; however chemical filters cannot be used for all types of gas sensors, since the filter must scrub out the unwanted gas, but still let the analyte gas pass through to the sensor electrodes. The use of chemical filters within sensors is well known in the prior art; for example, Tantam and Chan in U.S. Pat. No. 4,633,704 describe the use of a soda lime filter to prevent hydrogen sulfide from giving a response on a carbon monoxide sensor.
Carbon filters are also commonly used to protect gas sensors from hydrogen sulfide, as is illustrated by Kiesele et al in U.S. Pat. No. 5,865,973, Xu in U.S. Pat. No. 5,803,337 and Martell et al in U.S. Pat. No. 5,744,697. The use of carbon filters is restricted to only a few types of gas sensors, since activated carbon absorbs a wide range

Affiliated with

Also associated with

Rating

Filter for gas sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Filter for gas sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Filter for gas sensor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2494747