METHOD AND APPARATUS FOR DETECTION OF GASES AS IMPURITIES IN AN INERT GAS

Abstract: A method is described for the quantitative measurement of traces of gas, particularly CO₂ and CH₄ which constitute impurities in impure inert gases such as rare gases and nitrogen, by means of the use of a known analyzer for reducing gases, which uses a bed of mercuric oxide and makes the measurement by an optical method of the mercury vapours produced which are proportional into the concentration of the impurities to be detected. The method of the present invention subjects the impure inert gas to an electric discharge which may be at a high voltage, capable of ionizing the impurities before the impure inert gas is made to pass through the bed of mercuric oxide. The device according to the invention comprises, between a gas-chromatographic separation column and the analyzer, an electrode within a passage through which flows the gas to be analyzed and in which there is an ionizing discharge.

Description: The present invention relates to a method and related apparatus for detection of traces of gas, in particular non-reducing gases such as CO₂ and CH₄, as impurities in an impure inert gas. Various methods and apparatus are known in the art for the measurement, even in low concentrations, of reducing gases, in particular CO and H₂ in various gases and above all in air. See for example US Patents No.3,420,626; 3,871,827 and 3,790,348 (the ′348 Patent). More recently US Patent No. 4,411,867 (the ′867 Patent) issued based on the principle of using a bed of mercuric oxide as taught in US Patent No. 3,420,636. The ′867 Patent describes a very precise apparatus for the measurement of traces of CO and H₂ down to levels of one part per million or even one part per billion (ppb) (1 ppb = 1 in 10⁹). The measuring apparatus comprises a chromatographic column and an optical sensor for the detection of mercury vapour which is released by the reducing action of the traces of CO and H₂ on the bed of mercuric oxide. Furthermore, it is known that in N₂ or the rare gas which is required to have the highest possible purity there are present, not only the reducing gases which are detectable by the ′867 Patent but there are also present other non-reducing gases, and especially CO₂ and CH₄. It is true that the ′348 Patent is also capable of measuring methane and total hydrocarbons present in a gas, and in particular in air, but the disclosed method, which is based on a hydrogen flame detector, does not give precise results at concentrations as low as can be obtained by the method of the '867 Patent. Unfortunately, the '867 Patent can only detect the presence of reducing gases. It is therefore an object of the present invention to provide a method and an apparatus for accurately determining the amount of methane and/or carbon dioxide in an impure inert gas contaminated with trace amounts of said non-reducing gases CH₄ and/or CO₂. This is accomplished by means of a method as defined by the features of claim 1. The apparatus of the present invention comprises a gas-chromatographic separation column, a reducing gas analyzer based on the use of a mercuric oxide bed, followed by a mercury vapour detector, and is characterized by the fact of comprising an ionizer in series between said separation column and said analyzer, with a passage for the impure inert gas, having an electrode therein at a voltage of ± 150-1000 volts. In one particularly preferred embodiment of said apparatus said passage comprises a cylindrical chamber within which there is mounted a coaxial electrode capable of generating a symmetrical discharge to the walls of said chamber. The above and other objects, characteristics and advantages of the method and apparatus of the present invention will become evident to those skilled in the art from the following detailed description and by reference to the attached drawings in which: FIGURE 1 shows a block diagram of the complete apparatus suitable for practicing the method of the present invention; FIGURE 2 shows a cross-section of a preferred ambodiment of the ionizer useful in producing an ionizing discharge in the impure inert gas to be analyzed and which allows a determination of the presence of certain trace impurities; FIGURE 3 is a graph obtained showing the peaks of CO and H₂ detected in an impure inert gas containing traces also of non-reducing gases. This graph was obtained using an analyzing device of the '867 Patent which is to say an apparatus similar to that of in Figure 1 but having no ionizer; and FIGURE 4 shows a graph of the peaks detected from the same gas as used for Figure 3 when analyzed according to the method of the present invention shown in Figure 1. Referring now to Figure 1, there is shown an apparatus 2 suitable for practicing the process of the present invention. The apparatus 2 comprises a container 4 holding an impure inert gas. The container 4 is connected to conduit 6 containing valve 7. The conduit 6 is in fluid communication with a gas-chromatographic column 8 which in turn is in fluid communication with a conduit 10. The conduit 10 discharges to an ionizer 12 which in turn, discharges to a conduit 13. The conduit 13 is in fluid communication with a container 14 containing a bed 15 of mercuric oxide. The container 14 discharges to a conduit 16 which in turn is in fluid communication with an optical sensor 17. The optical sensor 17 optically senses the mercury vapour released from the bed 15 of mercuric oxide. The optical sensor 17 is connected to a chart recorder 18 which records observed values from the optical sensor 17. In operation the valve 7 is opened permitting impure inert gas to flow from the container 4 through the conduit 6 and into the column 8. As is well known in the gas-chromatographic art, in the column 8 the impurities are separated such that they leave in sequence when the impure inert gas leaves the column 8 and enters the conduit 10. In the ionizer 12, the impurities are ionized such that they react with the mercuric oxide in the bed 15 releasing the mercury vapour. The amount of mercury vapour is directly proportional to the amount of the impurity. Since the column 8 has separated the impurities, the optical sensor 17 first senses mercury vapour from the first received impurity and then subsequently senses mercury vapour released from subsequent impurities. The chart recorder 18 produces a chart 18′, practical examples of which are shown in this case in the form of Figures 3 and 4. Referring now to Figure 2 there is shown in cross-section a preferred embodiment of an ionizer 12 capable of internally generating an ionizing discharge which has surprisingly been found to allow the measurement of the presence of traces of non-reducing gases in impure inert gas. Although the mechanism by which this takes place is not completely clear it is believed that the discharge occuring within ionizer 12 provokes a type of cracking of the non-reducing gases such as CO₂ and CH₄ freeing respectively CO and H₂. These gases then react with the mercuric oxide present in the bed 15 releasing mercury vapour. The mercury vapour is then detected and measured by optical sensor 17. Although the ionizing discharge acting on the impure inert gas can be obtained in various ways, for example also by means of high frequency waves, it has been found preferable to obtain this discharge by means of an ionizer 12, the preferred embodiment of which is shown in Figure 2. The ionizer 12 comprises a body 20 with an inlet 21 and an outlet 22 for connection by conduit 10 to the gas-chromatographic separation column 8 and by a conduit 13 to bed 15 and eventually by conduit 16 to the optical sensor 17. Inlet 21 is connected by a passage 22′ with cylindrical chamber 23 in which there is located an electrode 24 which defines, with the internal wall of said chamber 23 an annular space through which impure inert gas is obliged to flow. This gas then reaches outlet 22 through channel 25. The electrode 24 is connected to an external source of high voltage by means of an insulating element 26 which is gastight. The high voltage which must be connected to the electrode 24 in order to obtain the required discharge depends upon the dimensions of the surrounding space that is to say of the ionizing chamber 23, and it also depends on the type of gas to be analyzed. An indicative range of values is approximately from 150 to 1000 volts. The polarity may be either positive or negative with respect to the outer wall connected to ground. The higher values are required for example with the gas nitrogen. The resistance of the electrode 24 should be sufficiently high to limit the value of current which is generated during the discharge. In the embodiment of Figure 2 the discharge preferably takes place in a symmetrical manner about electrode 24 which is within the annular space between the electrode and the wall of chamber 23 and therefore acts on all the gas which flows through this area. As used herein the term impure inert gas refers to a gas or vapour the major component of which does not react with mercuric oxide. Examples of impure inert gases include nitrogen, and the rare gases namely argon, neon, krypton, helium, xenon and mixtures thereof. EXAMPLE 1 The apparatus of Figure 1 was operated using an impure inert gas comprising helium containing the following impurities as manufactured by a speciality gas supplier: CO₂ (2.4 ppm); CO (1.7 ppm); H₂ (1.5 ppm); CH₄ (1.3 ppm). While ionizer 12 was not acting and electrode 24 had no voltage applied. In this case the apparatus behaves as described in the ′867 Patent in which mercuric oxide in the bed 15 is active only towards reducing gases. Chart recorder 18 therefore only indicates the peaks relative to CO and H₂ as shown in Figure 3. EXAMPLE 2 The same gas as used for Example 1 was made to flow through the apparatus 2 of the present invention. This time ionizer 12 had electrode 24 connected to a high voltage source of -430 volts, with a resistance of one megaohm. The wall of chamber 23 was connected to ground. A symmetrical cylindrical discharge takes place in all the gas as it flows on the ionizing electrode 24 which ionization is probably in the form of a plasma. The graph obtained on chart recorder 18 now shows four peaks that is to say again those relative to CO and H₂, in the same position, and furthermore distinctly separated there are another two peaks one for CO₂ and one for CH₄ as shown in Figure 4. In Figures 3 and 4 concentration is shown on the y-axis. The units on the y-axis are arbitrary. The scale of the y-axis of Figure 3 and the scale of the y-axis of Figure 4 are not uniform. In Figures 3 and 4 the x-axis is time which increases from right to left. This is because these figures represent the chart 18′ wherein any given point moves from left to right.

Claims: A method of accurately determining the amount of methane and/or carbon dioxide present in an impure inert gas contaminated with trace amounts of methane and/or carbon dioxide, said method comprising the steps of: I. passing the impure inert gas through a gas-chromatographic separation column to produce treated impure gas; and then II. subjecting the treated impure gas to an ionizing discharge thereby cracking the methane and/or carbon dioxide into hydrogen and/or carbon monoxide and other components thereby producing ionized gas; and then III. contacting the ionized gas with mercuric oxide thereby producing mercury vapour in an amount directly proportional to the amount of methane and/or carbon dioxide present in the impure inert gas; and then IV. determining the amount of methane and/or carbon dioxide present by determining the amount of mercury vapour produced. A method of claim 1 characterized by the fact that said ionizing discharge of step II is obtained by passing impure inert gas to be analyzed around an electrode capable of generating an ionizing discharge. An apparatus for performing the method of claim 2, comprising a gas-chromatographic separation column (8), and an analyzing device for reducing gases based on the use of a bed (15) of mercuric oxide connected to a detector (17) of mercury vapour, characterized by the fact of further comprising, in series between said column (8) and said analyzer (15,17), an ionizer (12) with a passage (21-22) through which the impure inert gas to be analyzed is obliged to flow, within which there is an electrode (24) maintained at a voltage, positive or negative with respect to ground, which has a value of from 150 to 1000 volts. An apparatus of claim 3, characterized by the fact that said passage (21-22) is within a body (20) which has mounted, with a suitable insulating and gastight insulator (26) said electrode (24); said passage (21-22) comprising a cylindrical chamber (23) within which there is coaxially placed said electrode (24). An apparatus of claim 4, in which said electrode (24) is connected by said insulator (26) to an external source of voltage having a value from 150 to 1000 volts while the walls of said chamber (23) are connected to ground.

Inventors: SUCCI, MARCO
Applicants: SAES GETTERS S.P.A.
Application year: 1988
Main classification: G01N33
IPC codes: G01N30, G01N27, G01N21, G01N31