Apparatus and method for determining the density of radicals in a gas - Patents.com

Abstract

The present invention further relates to a method for determining the density of radicals of a radical type in a measurement space, the method comprising: a catalytic material (1) that can be brought into contact with the measurement space at least in the region of a first surface (15) of the catalytic material, the catalytic material being suitable for causing an exothermic recombination reaction of radicals of a radical type when the radicals of the radical type are in contact with the first surface; a temperature actuator (2) in thermal contact with the first surface; and a temperature sensor (3) in thermal contact with the first surface, the device being designed to control the temperature actuator by a control signal such that a measured value detected by the temperature sensor is kept at a set value, the control signal being capable of being evaluated to determine the density of radicals of a radical type in the measurement space. The present invention further relates to a method for determining the density of radicals of a radical type in a measurement space.

Description

[Technical field]

[0001] The present invention relates to an apparatus for determining the density of radicals in a measurement volume. A well-known technique for determining the density of radicals is their measurement by optical methods (e.g., H.-P. Dorn et al., Atmos. Meas. Tech., 6, 1111-1140, 2013). Another technique is based on measuring the temperature rise of a catalyst where the surface recombination energy is released by the radical recombination reaction. In the latter technique, the temperature rise is related to the density of radicals and can therefore be inferred to some extent. This technique was used as early as 1943 by WV Smith to measure H and OH radicals (J. Chem. Phys 11, 110f, 1954). In the case of O radicals, for example, Linnett and Greaves (JW Linnett and DGH Marsden, Proc. R. Soc. Lond. A 234, 504-515, 1956; JC Greaves and JW Linnett, Trans. Faraday Soc., 54, 1323-1330, 1958) used this measurement methodology. Haraki et al. improved this general technique as a sensor element for O radicals by compensating for general gas temperature effects with a reference element that is unresponsive to the radicals being measured. Furthermore, the sensor can be used to quantitatively measure radical density (N. Haraki et al., Electrical Engineering in Japan 149(4), 1075-1080, 2004). Thus, in the prior art, temperature changes are measured as a measure of radical concentration. These temperature changes can be as high as more than 1000 ° C, as reported in Haraki et al.

[0002] However, measurement methods based on temperature changes have several drawbacks for radical concentration measurements. First, quantitative measurements of the concentration are complicated by the fact that the recombination rate constant itself is a function of temperature (Fryberg et al., J. Chem. Phys. 32, 622-623, 1960). In addition, different reactions and reaction mechanisms of all potential reactants present in the measurement space may also occur at different temperatures. Significant temperature changes can then alter the mechanical and chemical properties of the catalyst as well as the entire sensor assembly, in particular weakening, deformation, or aging, for example due to multiple temperature cycles. Finally, during heating of the catalytic element, existing adsorbents that limited the number of catalytic sites may desorb, thus obtaining additional catalytic areas during high temperature operation. This makes quantitative estimation very difficult for dynamic processes.

[0003] It was an object of the present invention to provide an apparatus or method which reduces or eliminates at least one of the disadvantages of the prior art. In particular, the object was to determine more accurately the density of radicals in a measurement space.

[0004] This object is solved by an apparatus according to claim 1 or a method according to claim 10.

[0005] The device according to the invention is a device for determining the density of radicals of a radical type in a measurement space. a catalytic material that can be brought into contact with the measurement space at least in the region of a first surface of the catalytic material, the catalytic material being suitable for causing an exothermic recombination reaction of radical-type radicals when the latter come into contact with the first surface; a temperature actuator in thermal contact with the first surface; a temperature sensor in thermal contact with the first surface.

[0006] The apparatus is designed to control the temperature actuator by a control signal such that a measurement value detected by the temperature sensor is maintained at a set value, the control signal being capable of being evaluated to determine a density of radicals of the radical type within the measurement space.

[0007] The inventors have realized that the drawbacks of the prior art can be avoided in a surprisingly simple way by keeping the catalytic element, specifically the above-mentioned first surface of the device according to the invention, at a constant temperature. In this way, the temperature dependence of the recombination rate constant does not come into play, nor does the number of available catalytic sites change due to the temperature-dependent adsorption and desorption of further substances. Nevertheless, the device according to the invention allows the determination of the power delivered by the recombination reaction of radicals of the radical type on the catalytic surface, i.e. by evaluating the control signal to the temperature actuator. For example, the power supplied to the temperature actuator can be monitored. If more power is required to keep the temperature constant, this increase may only correspond to a power that is reduced by the recombination reaction and thus to a decrease in the density of radicals in the measurement space. Conversely, if the density of radicals in the measurement space increases, the power supplied to the catalytic surface by the recombination reaction increases, and therefore less power must be supplied to the temperature actuator to keep the temperature constant.

[0008] The temperature actuator may be configured to heat the first surface, to cool the first surface, or to heat and cool the first surface. For example, the temperature actuator may be an electrically-powered temperature actuator, such as a resistive heater or a Peltier element. In the alternative, the temperature actuator may be a heat exchanger element through which a cooling or heating medium can flow to stabilize the temperature of the first surface. Thus, the temperature actuator is an actuator for adjusting the temperature of the first surface.

[0009] A radical type can be, for example, an oxygen radical that reacts in a recombination reaction to form an oxygen molecule. Catalytic materials that promote this recombination reaction to an oxygen molecule are, for example, salts such as potassium chloride or lithium chloride, especially at high temperatures, metals (for example, Co, Ag, Cu, Pt) and metal oxides (for example, PbO or MoO3) already at low temperatures. It is quite possible to have radicals of several chemical species that react in the measurement space. In this case, we understand the radical type as a collective term that includes radicals of several chemical species. If the catalytic material is a selective catalytic material that does not react some of the radicals present, this non-reacted part does not belong to the radical type.

[0010] The measurement space can be, for example, a gas-filled space or a mostly evacuated space, in the latter case measuring the density of radicals in the residual gas. Depending on the application, the measurement space can be a closed volume or a space that can be in open fluid communication with the process space, the exhaust gas stream, the ambient air, etc.

[0011] In one embodiment of the device, the temperature actuator is an electrically heatable resistive element. The control signal is adapted to control a current through the temperature actuator. For example, the control signal may be a digital signal for controlling a current source. The control signal may be, for example, a voltage applied directly to the resistive element.

[0012] In one embodiment of the device, the temperature sensor is formed by a temperature-dependent electrical resistance element or a thermocouple. The temperature sensor can be designed as a resistance thermometer (RTD) for example in the form of a platinum measuring resistor of 100 Ohms at 0° C. (Pt100).

[0013] In one embodiment of a device combining the above embodiments, a conductive wire having a temperature dependent electrical resistance forms both the temperature actuator and the temperature sensor. In this case, the conductive wire can be coated with a catalytic material or can consist entirely of the catalytic material. The outer surface of the catalytic material forms the first surface on which the radical recombination reaction can occur.

[0014] In one embodiment of a device comprising a conductive wire as a combined temperature actuator and temperature sensor, the conductive wire has a diameter in the range of 5 to 50 microns.

[0015] The inventors have realised that this results in a surface-to-mass ratio of the wire that allows for sensitive measurement of relatively low radical densities with short response times. Even small amounts of energy applied to the surface of the wire result in a measurable change in the temperature of the wire. The relatively low mass ensures low thermal inertia.

[0016] In one embodiment of the device, the conductive wire has a coiled shape, which offers advantages in mechanical stability against shock and vibration and allows for a more compact design with the same sensitivity compared to a non-bending or straight wire.

[0017] In one embodiment, the device further comprises a reference pressure sensor. The reference pressure sensor is designed to detect the pressure in the measurement space. The reference pressure sensor can be in contact with the measurement space in the area of ​​the second surface. The reference pressure sensor uses a pressure measurement principle that operates substantially independent of the density of radicals of the radical type.

[0018] The inventors have realized that the accuracy of the determination of the density of radicals can be further increased by taking into account the pressure in the measurement space. In particular, the pressure in the measurement space can affect the power flowing from the first surface to the environment via the thermal conductivity of the gas in the measurement space, thus complicating the interpretation of the power balance. If the effective pressure is known, the effect can be corrected. The reference pressure sensor can be configured, for example, as a membrane pressure sensor.

[0019] In one embodiment of the apparatus, the reference pressure sensor is a thermal conductance gauge having a sensing wire, the reference pressure sensor adapted to determine a pressure dependent heat output of the sensing wire, the wire not including a catalytic material.

[0020] In this embodiment, the reference pressure sensor is therefore a Pirani vacuum gauge. In particular, a variant of this embodiment is conceivable in which the reference pressure sensor and the part of the device sensitive to radical recombination reactions are constructed substantially identically, with the difference that the latter part has a surface of catalytic material and the Pirani sensor does not contain catalytic material. In this arrangement, the Pirani sensor is equally sensitive to all perturbations as the part of the device sensitive to radical recombination reactions, so that the difference between the two sensor elements is particularly sensitive to the density of radicals.

[0021] In one embodiment of the device, the device further comprises a reference temperature sensor designed to detect a temperature in the measurement space, the reference temperature sensor being capable of contacting the measurement space in an area of ​​a third surface, the third surface being substantially free of catalytic material.

[0022] The inventors have realized that temperature changes in the environment of the device can interfere with the measurement of the density of radicals. By means of a reference temperature sensor, for example, pre-calibrated interference effects can be extracted from the signal. In this connection, it is important that the reference temperature sensor is as unaffected as possible by radical recombination reactions of the radical type to be measured. This is achieved by ensuring that the third surface of the reference temperature sensor, which is in contact with the measurement space and thus with the radicals when the measurement is performed, does not contain catalytic materials.

[0023] Furthermore, the invention relates to a method according to claim 10. The method according to the invention is used to determine the density of radicals of one radical type in a measurement space. - contacting a first surface of a catalytic material with a gas in the measurement space, the catalytic material being suitable for causing an exothermic recombination reaction of radicals when the radicals come into contact with the first surface; - keeping the temperature of the first surface constant at a target temperature, said keeping being performed by a control loop comprising a temperature actuator in thermal contact with the first surface and a temperature sensor in thermal contact with the first surface; - evaluating a control signal of a control loop for a temperature actuator to determine a density of radicals in the measurement space.

[0024] The inventors have realized that by keeping the temperature of the catalyst surface constant, the drawbacks of the prior art are eliminated in a simple manner. Instead of the temperature rise of the catalyst being observed and evaluated, according to the invention, a control signal that controls a temperature actuator such that the temperature of the first surface remains constant is used as the quantity by which the density of radicals of the radical type is determined.

[0025] In one variation of the method, the temperature actuator is a heating element, in which case the target temperature is higher than the temperature established at the first surface solely by the heating power caused by the radical recombination reaction.

[0026] In this variant, the control loop supplies maximum heating power to the first surface when no radicals are present. As soon as the radical recombination reactions heat the surface, the supplied heating power is reduced and the temperature remains constant. Ideally, at the expected radical density, the required supplied heating power is still slightly positive.

[0027] In a variant of the method, the device according to the invention is used according to one of the claims. In particular, the method according to the invention may also be a method for operating the device according to the invention.

[0028] Exemplary embodiments of the invention are explained in further detail below with reference to the figures. [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a schematic diagram of one embodiment of an apparatus. [Diagram 2] FIG. 2 is a schematic diagram of a portion of one embodiment. [Diagram 3] FIG. 2 is a perspective view of a wire according to one embodiment. [Figure 4.a)] FIG. 13 is a partial side view showing the placement of resistive wires for one embodiment of the device. [Figure 4.b)] FIG. 13 is a partial top view showing the placement of resistive wires for one embodiment of the device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] In FIG. 1, the measurement space 4 is shown in the upper section as a schematic cross-section, in this case in open hydrodynamic communication with the environment. Schematically shown as small circles are radicals 5, which can react in a recombination reaction to form molecules, which are also shown as two small circles. The device according to an embodiment is shown in the measurement position. The catalytic material 1 of the device has a first surface 15 in contact with the measurement space 4. The radicals 5 can react at the first surface in a recombination reaction. This reaction is promoted by the catalytic material. Thereby, the first surface absorbs part of the energy released in the reaction. The catalytic material is not consumed in the reaction. In thermal contact with the catalytic material are a temperature actuator 2 and a temperature sensor 3. The device is designed to control the temperature actuator by a control signal such that the measured value detected by the temperature sensor is maintained at a set value, the control signal being able to be evaluated to determine the density of radicals of the radical type in the measurement space. A control circuit 6, which can perform the task of keeping the temperature constant, is shown diagrammatically in dashed lines. The control means 8 receive the temperature signal from the temperature sensor 3, compare it with a predefined target temperature 7 and control the temperature actuator 2 with a control signal so that the temperature measured by the temperature sensor remains at the target temperature 7 or moves towards the target temperature if a deviation is measured. The control means can be, for example, a PID controller, but also simpler or more complex forms of control means can perform the task of keeping the temperature constant. From the control signal to the temperature actuator and the time change of the control signal, respectively, information about the density of radicals in the measurement space and the time change of the density of radicals in the measurement space can be determined.

[0031] FIG. 2 shows an electrical circuit diagram of a partial embodiment of the device. In this embodiment, an electrically heatable resistive element 13 forms both the temperature actuator and the temperature sensor of the device. A power supply 11 supplies a current for heating the resistive element. The resistive element 13, shown here in cross section, is coated with a catalytic material 1 having a first surface 15 towards the outside, where a radical recombination reaction can take place. This reaction further leads to heating of the resistive element 13. The resistive element has a temperature-dependent resistance. From the voltage measured by the voltmeter 12, which drops with the current supplied by the current source 11, the instantaneous resistance and therefore the temperature of the resistive element 13 can be determined. If the temperature decreases, the current and therefore the power supplied increases again until the target temperature is reached. If the temperature rises above the set point, the current is reduced accordingly. The applied current, or the control signal specifying this current, can be evaluated as an indication of the density of the type of radicals to be measured. Such a control loop can be easily realized, for example, by a bridge circuit (Wheatstone's bridge circuit).

[0032] FIG. 3 shows diagrammatically a wire 14 in a coiled form that can be used as an electrical resistance element in one embodiment of the device as a temperature actuator and temperature sensor, the coiled wire being coated with or consisting of a catalytic material.

[0033] FIG. 4 shows in partial view 4.a) a schematic side view of two coiled resistive wires with electrical connections passing through a bushing shown as a cross section in an insulated manner. On the left side of the figure, a connection diagram for connection to a current source and a voltage measuring device is shown diagrammatically. The resistive element 13 shown in the following FIG. 4.a) has the role of temperature sensor and temperature actuator of the device. It is coated with a catalytic material (here represented by grey hatching) which forms a first surface 15. The reference pressure sensor 16 is formed by a second resistive wire, also coiled (here shown in black). In this embodiment, the reference pressure sensor is formed as a Pirani sensor. The resistive element 13 and the reference pressure sensor can therefore be formed with the same geometry and material in a resistive wire, the only difference being that one of the wires is coated with catalytic material and the other is not. The material of the resistive wires can be, for example, platinum. A common ground connection (center feedthrough) of both resistive wires is provided. This has the advantage that the ions in the measurement space are less disturbed by the arrangement when a current flows through the resistive wires during operation. The feedthrough can be designed, for example, to be vacuum-tight. During operation, the right-hand region of the feedthrough protrudes into the measurement space.

[0034] FIG. 4.b) shows a top view of the arrangement of the resistive element 13 and the reference pressure sensor 16 from the side.

[0035] List of References 1. Catalyst materials 2 Temperature Actuator 3 Temperature Sensor 4 Measurement space 5 Radicals (one type of radical) 6 Control Circuit 7 Target temperature 8. Control Means 10 equipment 11 Current source 12 Voltage measuring device 13 Resistive element, electrically heatable 14 Coiled Wire 15 First Surface 16 Reference pressure sensor

Claims

1. A device (10) for determining the density of radical-type radicals (5) in a measurement space (4), - A catalyst material (1) that can be brought into contact with a measurement space in at least a region of the first surface (15) of the catalyst material, and is suitable for inducing an exothermic recombination reaction of radical-type radicals when the radical-type radicals come into contact with the first surface, - A temperature actuator (2) that is in thermal contact with the first surface, - The device comprises a temperature sensor (3) that is in thermal contact with the first surface, The apparatus (10) is designed to control the temperature actuator by a control signal so that the measured value detected by the temperature sensor is maintained at a set value, and the control signal can be evaluated to determine the density of the radical-type radicals in the measurement space.

2. The apparatus (10) according to claim 1, wherein the temperature actuator (2) is an electrically heatable resistive element (13), and the control signal is adapted to control the current passing through the temperature actuator.

3. The apparatus (10) according to claim 1, wherein the temperature sensor (3) is formed by a temperature-dependent electrical resistance element or a thermocouple.

4. The apparatus (10) according to claims 2 and 3, wherein a conductive wire having temperature-dependent electrical resistance forms both the temperature actuator (2) and the temperature sensor (3), and the conductive wire is coated with the catalyst material (1) or consists of the catalyst material (1).

5. The apparatus (10) according to claim 4, wherein the conductive wire has a diameter in the range of 5 to 50 microns.

6. The apparatus (10) according to claim 4, wherein the conductive wire has a coiled shape (14).

7. The apparatus (10) according to any one of claims 1 to 3, further comprising a reference pressure sensor adapted to sense the pressure in the measurement space, wherein the reference pressure sensor can be brought into contact with the measurement space within a region of a second surface, and the reference pressure sensor uses a pressure measurement principle that is substantially unaffected by the density of the radical-type radicals.

8. The apparatus (10) according to claim 7, wherein the reference pressure sensor is a thermal conduction vacuum gauge having a sensing wire, the reference pressure sensor is adapted to determine the pressure-dependent thermal output of the sensing wire, and the wire does not contain a catalytic material.

9. The apparatus (10) according to any one of claims 1 to 3, further comprising a reference temperature sensor (16) adapted to detect the temperature in the measurement space, wherein the reference temperature sensor can be brought into contact with the measurement space within a region of the third surface, and the third surface is substantially free of the catalyst material.

10. A method for determining the density of radical-type radicals (5) in a measurement space (4), - The first surface (15) of the catalyst material is brought into contact with the gas in the measurement space, wherein the catalyst material is suitable for causing an exothermic recombination reaction of the radicals when the radicals come into contact with the first surface. - Maintaining the temperature of the first surface at a constant target temperature (7), which is performed by a control circuit comprising a temperature actuator that is in thermal contact with the first surface and a temperature sensor that is in thermal contact with the first surface. A method comprising: evaluating the control signal of the control circuit to the temperature actuator in order to determine the density of the radicals in the measurement space.

11. The method according to claim 10, wherein the temperature actuator (2) is a heating element, and the target temperature (7) is higher than the temperature established on the first surface solely by the heating force due to the radical recombination reaction.

12. The method according to any one of claims 10 or 11, wherein the apparatus (10) according to any one of claims 1 to 9 is used.

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