DE3048216A1 - Gas property investigation by Knudsen flow chamber - is applied to reactive gases - Google Patents

Abstract

In the investigation of gases by the mean molecular speed over a Knudsen flow, the test gas and a reference gas flow from a first known vol. zone to a second known pressure zone, with the amt. of gas in the first zone measured in stationary conditions, the cross-section of the Knudsen flow is altered, the measurement is repeated and the difference in measurements is determined together with their gradient on change of cross-section. Gases which undergo reaction are fed into the chamber, and the density of at least one mixt. component is measured by a selective sensor, esp. a mass spectrometer, in relation to time elapsed since the cross-section change. The main patent determn. of molecule size was with gas speed changes for flow rates about 10 power 13 molecules per sec. The present addn. permits investigation of reactive gases, including those reacting with solids.

Description

Jülich nuclear research facility

Limited liability company Official code: P 30 48 216.7 Method and device for analyzing gases (addendum to patent: 2 720 300) The invention relates to a method for examining gases using the mean molecular velocity over the Knudsen flow, at which one on the one hand a reference gas and on the other hand the gas to be examined in a defined amount per Unit of time into a first zone of known volume and from this first zone through a Knudsen flow with a variable effective cross-section into a second Zone with known pressure can flow, the amount of gas or a proportional one Measures size in the first zone after reaching a first steady state, the effective cross-section of the Knudsen flow changed spontaneously, the measurement for repeats the second steady state reached thereafter and the difference in Values of the two stationary states as well as the gradients of the measured value afterwards to the change in cross-section determined according to patent P 2 720 300, as well as to a device to carry out the procedure.

The method and device according to the main patent are used to measure the molecular size via the molecular speed, which is caused by changing the flow cross-sections can be determined for gas flows in the region of 10 molecules / s. As a possibly an additional sensor in particular Mass spectrometer used, and one of the advantages of the main patent procedure is that perfect assignments of fragment signals are possible. Difficulties arise however, if the gases are not inert.

The aim of the invention is therefore a modification of the method according to the Main patent, so that studies of reactive gases and gas / solid-state reactions are also possible become possible.

For this purpose, the method according to the invention is that mentioned at the beginning Kind characterized in that gases are fed into the chamber which are in the chamber Einyehen reactions and that the gas density of at least one of the components the resulting mixture with a selective sensor, in particular a mass spectrometer measured as a function of the time after the spontaneous change in cross-section. The change in cross-section can be done either by opening another valve or with the help of a step-by-step process to open the valve.

The measurements are made for different pressures as well as different temperatures repeated, so that by evaluating the obtained Data statements about the reaction rate and activation energy as well as reaction order become possible. Measurements with a reactive surface are carried out in such a way that a reactive Surface with a defined temperature in the vacuum chamber provides whose effective area is determined in comparison to a real outflow opening.

The device according to the main patent that can be used for these investigations with a vacuum chamber with a certain volume, which is supplied by an attached supply can be charged with gas and optionally and independently of one another via at least two to be opened and closed valves is connected to a suction pump and a Sensor for determining the gas density prevailing in it, has for the latter Investigations in particular a sample holder and, if necessary, a shield as well as a Sample heating in the vacuum chamber.

In particular, a device is used to investigate chemical reactions is used as illustrated by the accompanying drawing.

This includes a vacuum chamber 1, the valves S, S1 and S2 is connected to a (not shown) vacuum pump system 2.

The gas to be examined is drawn from a storage vessel 3 via a valve 4 let into the vacuum chamber 1. This also contains a sensor 5 for the Gas tight and a temperature measuring device 6 and access to the ionization chamber 7 of the mass spectrometer 8.

The mode of operation of this device is described in the main patent.

In addition, this device according to the invention contains a sample holder 9, which allows the sample to be moved in the chamber and, if necessary, a shielding device as well as a heater 1o formed in particular as a high-frequency heater and a temperature sensor 11 (particularly a thermocouple). When using a The ion source 7 of the device is electrically high-frequency heating by means of a cage 12 shield.

The inventive modification of the method and device according to the main patent for the investigation of reactive gases is based on the following considerations: In a reaction, the gas is consumed and another gas can be produced. In the case of Knudsen-StrömXlr.g (which requires very low pressures), it can predominantly be only a surface reaction act, whereby the surface through the wall of the device or through built-in components such as a manometer tube, sensor surface or a specific introduced area is given. With such a reaction, the behaves Surface as if it opens up to the reactive, the as and an effective Forms cross-section Gf for the Cas consumption. This chemically determined effective cross-section Gas is dependent on the gas density and the temperature of the surface. The chemical conditional effective cross section 6j on the one hand affects the compensation curve, what must be taken into account in the evaluation. This best-fit curve is the density as a function of the time after the spontaneous change in cross-section.

Strives for spontaneous cross-sectional enlargement the Intensity of the sensor signal from the stationary initial value 1a to a new stationary one End value 1e.

The transition is exponential and follows the relationship where d = 1 Ip I, the difference between the initial and final value, is the mean molecular velocity, V is the volume of the chamber and t is the time.

The time counting begins with the spontaneous opening of the second valve. # 0 is the effective cross section of the jump valve, which is actuated for all measurements. The effective cross section of the second valve can be set to a value between set permanently and thus specifies the change in intensity d I. This valve remains open during the measurement.

The relationship (1) applies to non-reactive gases.

If the gas reacts, write instead of (1) A semi-logarithmic evaluation of (1) and (2) only gives matching values for one and the same gas if #r is # 0. If this is not the case, a different evaluation method is therefore required. This is through the relationship (3) given. As can be seen, the intensities only need to be known in relative units. The result according to (3) is independent of #x. This has the advantage that the effective cross section of 6rs does not have to be known. The cross section of 6rK only needs to be kept constant. The semi-logarithmic evaluation according to (3) results in a straight line, the steepness of which is proportional to the mean molecular velocity of the gas molecules, regardless of the effective values of #k and #t for reacting gas results in straight lines according to equations (13, (2) and (3). From the steepness of the straight lines one can determine the ratio of # K / # 0 or # r / # 0. If the measurements are carried out at different gas temperatures , the slopes are to be corrected to a temperature value with the aid of the mean molecular velocity.

Is the steepness of the straight line from (3) for a gas with an edging Molecular weight is known at a temperature, so is also known when the volume is known of the vacuum chamber # 0 and thus also the effective reaction area #r.

Reactive gases admitted into the chamber react, producing product gases. The various gas components can be detected from the mass spectrometry measurements, but no distinction can be made between which is the reacting gas and which is the product gas. The changes in intensity after the jump valve opens spontaneously are markedly different from a non-reacting gas, a reacting gas and a product gas. The relationship applies This makes it possible to distinguish the reaction gas from a product gas in a gas mixture.

It turns out that e.g. H2S already at room temperature and one Pressure of approx. O.6. 10 mbar with oxygen and particularly strong with nitrogen dioxide reacts, whereby S02 and Sx-SO2 complexes are formed.

In one example, the evaluation b Oxygen is shown. The oxygen of the highest purity required for the experiment was filtered out of the air by means of a heatable silver cylinder. The measurements shown are at a pressure of o.45 mbar. When the ionization manometer is switched on, the oxygen reacts on the hot parts of the tube.

In FIG. 2 the evaluation according to (3) is plotted semi-logarithmically. The measuring points with and without reaction give one and the same straight line at given pressures. The steepness is 0.487 s. The volume 3 of the chamber was 39.76 dm. The gas temperature 297 This gives gi.n value for 2 1.74 cm In Fig. 3, the measurements according to (1) and (2) are shown. The steepnesses of the straight lines with oxygen reaction differ considerably from those of the non-reactive gas. The slope for the non-reacting oxygen is -1 o, 589 s 1, from the ratio o 589 0.487 = 1.21 follows # K / # 0 = 0.21 or #K = 0.37 cm². The steepness at 1.4. 10-6 mbar and the reaction is 0.609s-1 and thus # r / # 0 = O, (+ 5 ~ 10 At .17 mbar and o, o4 or for the = o, o7 cm2 reaction follows accordingly #r = 0.17 cm².

If the effective reaction area and the pressure are known, then is also the speed of reaction is known. Is the reaction area at different Temperatures and different pressures are known, so is the activation energy and the reaction order is known.

Claims (4)

Jülich nuclear research facility limited liability company Claims Process for the investigation of gases on the basis of the mean molecular velocity via the Knudsen flow, which is a reference gas on the one hand and a reference gas on the other the gas to be examined in a defined amount per unit of time in a first zone of known volume and from this first zone by a Knudsen current variable effective cross-section flow into a second zone with known pressure leaves, the amount of gas or a proportional size in the first zone after reaching a first steady state measures the effective cross-section of the Knudsen flow changed spontaneously, the measurement for the second steady state reached thereafter repeated and the difference in the values of the two stationary states as well as the Gradients of the measured value determined after the change in cross section Patent P 2 720 300, characterized in that gases are fed into the chamber, the reactions in the chamber go in and that one the gas density of at least one of the components of the resulting mixture with a selective one Sensor, in particular a mass spectrometer, as a function of time the spontaneous change in cross-section measures. 2. The method according to claim 1, characterized in that the Changes the cross-section by means of a valve that can be opened in stages. 3. The method according to claim 1 or 2, characterized in that one provides a reactive surface with a defined temperature in the vacuum chamber, whose effective area is determined in comparison to a real outflow opening. 4. The method according to any one of claims @ to 3, characterized in that that reactive gases are admitted into the chamber and the dictiir change of the starting gas and product gas determined.