Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing gas sample
Reexamination Certificate
1999-03-06
2002-04-09
Warden, Jill (Department: 1743)
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing gas sample
C422S080000, C422S081000, C422S082090, C422S068100, C422S082050, C356S437000, C356S440000, C356S432000
Reexamination Certificate
active
06368560
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the quantitative detection of concentrations of gases, and more particularly to methods and apparatus for detecting concentrations of a gas based on its reaction with mercuric oxide.
Reduction gas detectors operate on the principle of flowing a gas stream to be analyzed through a heated bed of mercuric oxide (HgO). Gases in the stream that can be oxidized (referred to as “reducing gases”), react with the mercuric oxide to produce free mercury vapor as shown in the following general reaction:
X+HgO→XO+Hg
In this equation, X represents a reducing gas species and Hg is present as free mercury vapor. The mercury vapor produced in this reaction can be detected by its absorption of ultraviolet (UV) light within a sample cell forming a part of an ultraviolet photometer. An example of a reduction gas detector can be found in U.S. Pat. No. 4,411,867 of Ostrander, incorporated herein by reference.
Reactions with mercuric oxide are not specific to any particular gas species and a large number of reducing gases can react with mercuric oxide to produce mercury vapor. Gas measurement apparatus intended for quantitative measurements of specific gas species must therefore incorporate some process for isolating the gas species to be measured. One such apparatus is a gas chromatograph, which time-separates the gas sample into individual species. More particularly, this separation is obtained using a long tube or “column” through which flows a gas stream. The exit gas flow from the column is connected to the reduction gas detector and an apparatus for injecting a precise volume of sample gas into the gas stream is located upstream of the column. The column itself is packed with a granular substance which has the characteristic of separating the different gases comprising the sample based on their molecular size or other chemical properties. In the case of columns containing molecular sieve materials, small molecules such as H
2
will flow through the column faster than large molecules such as CO. It will therefore be appreciated that the difference in such properties cause each species or element of the sample to move through the column and into the detector at different times, and the gas species are detected as a series of Gaussian-shaped concentration “peaks.” Starting from a single sample injection onto the column, each peak arrives at the detector in a characteristic time and the peak itself is essentially comprised of a single gas species. The height of each peak, or the integrated area under each peak, is representative of the concentration of the gas species.
In the prior art, reduction gas detectors have typically been operated at temperatures of 150-300° C. in order to promote the desired reactions with mercuric oxide. The sample cell as well as the mercuric oxide bed were heated in this temperature range in order to prevent mercury from condensing on the interior surfaces of the sample cell. As is well known to those skilled in the art, mercury vapor is quite condensable and adheres to relatively cool surfaces. Mercury condensation within the sample cell can result in slow equilibration of the sample cell to changing mercury concentrations and therefore slow time response of reduction gas detectors. Additionally, ultraviolet sample cells include quartz (i.e. pure SiO
2
) windows which allow ultraviolet radiation to be transmitted through the cell. Mercury condensation on the quartz windows reduces the optical transmission of the cell due to absorption of the ultraviolet radiation by mercury condensation on the windows. This results in reducing signals for UV light sensors in the photometer, and correspondingly higher noise levels.
In general, gas detectors used in conjunction with gas chromatography must have relatively fast response times in order to accurately follow the concentration peaks created by the chromatography column. Additionally, typical gas chromatography flow rates are in the range of 20-60 cc/minute which are much lower than the 500-2000 cc/min flow rates associated with other gas measurement techniques (e.g. continuous analyzers). Gas chromatography detectors therefore preferably have small internal volumes in order to minimize concentration equilibration times to rapidly changing gas concentrations, and to reduce condensation of the flowing gas species as described previously.
Sample cells of the prior art, when embodied as a continuous sampling analyzer, were, of necessity, quite large in order to accommodate the large gas flows through the detectors. The large diameters of the prior art continuous sampling analyzer cells also transmitted relatively large quantities of ultraviolet radiation, which was desirable to reduce noise levels in the detector output signal. Sample cells of the prior art for chromatography detectors were smaller than those used for continuous sampling detectors but were still limited to a minimum diameter of 0.15 cm and a maximum length of 10 cm which were the dimensions that could still transmit adequate amounts of ultraviolet light through the passageway of the cell. That is, the diameter of the passageway of the cell was kept fairly large and the length of the cell was kept fairly short, so that a sufficient amount of light from the ultraviolet source could travel through the cell and still be detected by the ultraviolet (UV) sensor. This is because ultraviolet sources are non-coherent and, therefore, the amount of light impinging upon the UV detector is directly proportional to the diameter of the cell passageway and is inversely proportional to the square of the length of the cell. Hence, short, large diameter cells were the norm in the prior art.
The temperature of prior art chromatography detector cells were maintained at the same temperature as the HgO beds which, in practice, was in the range of 265-285° C. Based on this relatively high temperature, the optical windows of the cell were constructed of relatively long quartz rods (approximately 5 cm in length) in order to isolate the hot cell from the temperature-sensitive ultraviolet lamp and light sensor. The amount of UV light transmitted through these rods is also quite dependent on temperature of the rod and, therefore, minor changes in rod temperature affect the amount of light impinging on the UV sensor. Minor variations in convective cooling of the rods of the prior art heated detector cells therefore introduced variations in light transmitted through the cell which were not due to mercury vapor concentration. The net affect of these variations was to increase drift and noise in the output of the light sensor.
It will therefore be appreciated that the performance of the prior art chromatography cell was limited by: a) the relatively large cell diameter and short length required for transmission of suitable levels of UV light; b) the relatively large condensation surface area of the cell due to its diameter and length; and c) the relatively high cell temperature which necessitated the requirement for optical windows comprised of quartz rods which added thermally-induced drift and noise to the detector output.
Since the sensitivity of mercury detection is directly proportionate to cell length, the ideal sample cell would be infinitely long and have zero diameter, zero internal volume, and zero internal surface area when one ignores other factors such as the amount of light in gas that could travel down the passage way of such an ideal sample cell. Additionally, the optical cell windows, if heated, would ideally be infinitely thin and therefore not prone to produce thermal convection errors.
SUMMARY OF THE INVENTION
A preferred embodiment of the present invention is directed to an improved photometer for detecting mercury vapor in a low flow-rate carrier gas. As such, it is well suited for gas chromatography for species that can be reduced in a heated mercuric oxide bed.
The sample cell of the improved photometer of the present invention is long and thin, as compared to sample cells of the prior art. T
Hartman Steven J.
McDowell Chuck
O'Harra, II Dale G.
Ostrander Clinton R.
Oppenheimer Wolff & Donnelly LLP
Sines Brian
Trace Analytical, Inc.
Warden Jill
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