Chemistry: analytical and immunological testing – Pyrolysis – combustion – or elevated temperature conversion – Combustion with oxygen containing gas
Reexamination Certificate
2000-05-08
2003-09-23
Warden, Jill (Department: 1743)
Chemistry: analytical and immunological testing
Pyrolysis, combustion, or elevated temperature conversion
Combustion with oxygen containing gas
C436S155000, C374S037000, C073S432100, C073S036000, C073S024050, C073S025030, C073S035040
Reexamination Certificate
active
06623976
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to analysis of potentially hazardous mixtures of substances and specifically to apparatus and method means for combustibility monitoring by determining the concentration of a first critical component in an environment or atmosphere of interest containing a second component that is capable of forming a combustible mixture with the critical first component. The invention is of particular relevance for monitoring gaseous substances and atmospheres containing them.
Combustible gaseous mixtures are those in which a first gaseous component thereof is capable of rapid exothermic reaction with a second component of the mixture by an oxidative process which may, but need not, involve oxygen as the oxidizing agent. Depending upon the relative amounts of the reacting components, the speed of reactions, the amount of heat generated, and the activation required for initiation of the reaction, such mixtures may cause substantial hazards.
Consequently, prevention of the formation of such mixtures, such as by continuous monitoring of an atmosphere of interest, is of general technological importance, and may be mandatory by law. Such monitoring is of particular importance in substantially enclosed environments, such as housings of processors which generate combustible or explosive gas mixtures by evolving hydrogen, possibly but not necessarily in the presence of a surplus of oxygen, or other relatively enclosed spaces where hazardous or potentially hazardous gas mixtures may occur without human intervention, such as methane in the air of mines, or spaces in areas where gaseous combustible chemicals or fuels are produced, processed, distributed or consumed.
The presence of elemental hydrogen in an atmosphere that contains, or may come into contact with, a sufficient amount of oxygen, such as air, is particularly hazardous because of the extremely high specific amount of heat generated upon reaction between hydrogen and oxygen and the extremely low activation energy required for initiation of the reaction. Thus, hydrogen monitoring is needed in various industrial work-places, e.g. in the immediate vicinities of containers deliberately filled with gaseous hydrogen, such as large electrical transformers and nuclear or chemical reactors as well fuel reservoirs whenever liquid or gaseous hydrogen is used as a fuel. The atmosphere within such containers or a surrounding enclosure will become explosive when the amount of gaseous hydrogen as the first component attains 5%, by volume, if oxygen as the second component is present in an at least stoichiometrically equivalent amount or in stoichiometric excess, or when at least 5%, by volume, of gaseous oxygen as the first component is formed in the presence of a stoichiometric equivalent or excess of gaseous hydrogen as the second component.
However, while monitoring of atmospheres of interest that do contain oxygen and may contain hydrogen is of particular interest, the present invention is not limited to this aspect but is applicable wherever a first component (which may be an oxidizable or an oxidizing compound) is capable of forming a “combustible” i.e. exothermally reactive mixture with a second component which provides the complement of the reaction, i.e. is oxidizing for an oxidizable first component, or—in reverse—is oxidizable by an oxidizing first component. While elemental oxygen is a typical “second” component because of its universal appearance in air, other oxidizing substances, notably those having a normally (i.e. under normal conditions of temperature and pressure) gaseous phase, such as fluorine or bromine, are capable of forming exothermally reactive gaseous mixtures with such oxidizable gases as hydrogen as well. Accordingly, the term “combustion” is neither to be understood as a limitation to reactions involving oxygen as the oxidizing component, nor as a limitation to monitoring of oxidizable components of mixtures.
Consequently, the term “critical first component” as used herein is intended to refer generally to that particular and potentially appearing component of an environment or atmosphere of interest which is monitored so as to indicate formation of a combustible mixture with a second component which may be but need not be present in the environment of interest.
Examples of oxidizable and typically gaseous substances of particular interest herein other than hydrogen are low molecular hydrocarbons (methane, ethane, propane, butane, singly or in mixtures) as well as other organic or inorganic substances including low boiling organic compounds, cyanogen, carbon monoxide, hydrogen sulfide etc., which can form combustible mixtures with a reactively complemental component, such as typically oxygen, capable of fast or “explosive” reaction.
The concentrations, or concentration limits, at or within which a given pair of an oxidizable and an oxidizing component forms an explosive mixture are known in the art of chemical processing and do not require specific explanation herein.
PRIOR ART
Various apparatus and method means for determining the concentration of a component of a gaseous mixture are known and operate on various principles of detection, including measurement of thermal conductivity, measurement of the heat of combustion by calorimetry using, for example, a pellistor, determination of oxidizable components of an atmosphere, and methods based on the modification of electrical conductivity of semiconductors caused by the electron donating properties of oxidizable substances, cf. EP-A-0 429 397 or EP-A-0 607 756 and the literature cited therein; such prior art means could, in theory, be used for monitoring of combustible atmospheres.
However, such prior art methods have not proved to be entirely satisfactory for monitoring potentially combustible atmospheres; disadvantages include inherent difficulties of avoiding the risk of igniting the atmosphere of interest, sensitivity to interference by oxygen or other impurities, notably when electrochemical methods or the use of semiconductor are concerned, as well as insufficient stability of the monitored parameters, or lack of precision and/reproducibility and, hence, poor reliability.
Prior art amperometric means for measuring elemental gaseous hydrogen are disclosed, e.g. in U.S. Pat. No. 4,563,249. Continuous monitoring of potentially hazardous hydrogen/oxygen mixtures as disclosed, for example, in U.S. Pat. Nos. 4,906,339 and 4,985,130 operates on the principle of selectively measuring the concentration of either component in the presence of the other and could indicate the formation of an explosive mixture. Electrolytes are required in these methods, however, and if the electrolyte is aqueous, prolonged operation in a dry gaseous environment causes evaporation of water and requires frequent replacement of the electrolyte and precludes unattended operation for prolonged periods of time; non-aqueous electrolytes, on the other hand, are not satisfactory because they cause a rapid drift of the monitoring signal.
Non-amperometric monitoring methods and apparatus such as disclosed in U.S. Pat. Nos. 5,144,831 and 5,255,553 (also termed pulse monitoring herein) while capable of monitoring various substances that were difficult to monitor by prior art methods are operable with binary—or at least pseudo-binary-mixtures and become less reliable when the substance to be monitored is accompanied by interfering components.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, a first general object of the present invention are apparatus and method means for monitoring potentially explosive environments (including atmospheres within an essentially enclosed space) while avoiding or minimizing the above mentioned disadvantages of prior art, yet providing economic and completely safe apparatus and method means for continuously analyzing or monitoring mixtures that are potentially
WO-A-9 602 826 discloses a device and method for measuring the amount of a reactive gaseous component in a gas or gas mixture contained within a voluminous enclos
Hale John Martin
Serratore Dominique
Stehle Gerard Roland
Weber Eugen
Weber Peter and André
Blank Rome Comisky & McCauley LLP
Orbsphere Laboratories Neuchatel SA
Sines Brian
Warden Jill
Weber Peter and André
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