Process gas analyzer and method for analyzing a process gas

Abstract

A process gas analyzer and method for analyzing a process gas carried in a plant section, wherein light from a light source is passed through the process gas and detected via a detector, evaluated in an evaluation unit to produce an analysis result with respect to the absorption in the process gas, where chambers or purging pipes, present between the light source and the plant section and also between the detector and the plant section, are flushed with a purge gas to analyze the process gas, and where the volume flow rate of the purge gas is periodically modulated and the effect of the purge gas on the analysis result is determined based on changes in the detected absorption caused by the modulation and removed from the analysis result to enable a high degree of compensation for measurement errors caused by the purging.

Claims

1. A process gas analyzer for analyzing a process gas carried in a plant section, comprising: a detector; an evaluation unit arranged downstream of the detector; a light source, light from said light source passing through the process gas and being detected by said detector and evaluated with respect to absorption in the process gas in the evaluation unit to produce an analysis result; and a purge gas system including a first chamber provided between the light source and the plant section and a second chamber provided between the detector and the plant section, the first and second chambers being open towards an interior of the plant section and being flushed with a purge gas such that the light passes a total absorption path comprising an absorption path purged by the purge gas and a measuring path in the process gas; wherein the purge gas system further includes a flow rate modulator for periodic modulation of a volume flow rate of the purge gas provided to the purge gas system such that a resulting modulation of the total absorption path purged by the purge gas is less than 100%; and wherein the evaluation unit is configured to determine an effect of the purge gas on the analysis result based on changes in a detected absorption caused by the periodic modulation and to remove said determined effect from the analysis result.

2. The process gas analyzer as claimed in claim 1, wherein the evaluation unit contains a lock-in demodulator which ascertains an amplitude of changes in the detected absorption at a modulation frequency of the volume flow rate.

3. The process gas analyzer as claimed in claim 2, wherein the flow rate modulator for modulating the volume flow rate of the purge gas comprises a variable-speed fan.

4. The process gas analyzer as claimed in claim 2, wherein the flow rate modulator for modulating the volume flow rate of the purge gas comprises a controllable regulator valve arranged in a purge gas feed system which feeds each respective chamber.

5. The process gas analyzer as claimed in claim 2, wherein the flow rate modulator for modulating the volume flow rate of the purge gas comprises a buffer volume which is modifiable via a controller arranged in the purge gas feed system which feeds each respective chamber.

6. The process gas analyzer as claimed in claim 1, wherein the flow rate modulator for modulating the volume flow rate of the purge gas comprises a variable-speed fan.

7. The process gas analyzer as claimed in claim 1, wherein the flow rate modulator for modulating the volume flow rate of the purge gas comprises a controllable regulator valve arranged in a purge gas feed system which feeds each respective chamber.

8. The process gas analyzer as claimed in claim 1, wherein the flow rate modulator for modulating the volume flow rate of the purge gas comprises a buffer volume which is modifiable via a controller arranged in the purge gas feed system which feeds each respective chamber.

9. A method for analyzing a process gas carried in a plant section, comprising: detecting light from a light source via a detector after said light is passed through the process gas; evaluating said light with respect to absorption in the process gas in an evaluation unit arranged downstream to produce an analysis result; flushing chambers included in a purge gas system with a purge gas such that the light passes a total absorption path comprising an absorption path purged by the purge gas and a measuring path in the process gas, the chambers being open towards an interior of the plant section and present between the light source and the plant section and between the detector and the plant section; modulating periodically, by a flow rate modulator, a volume flow rate of the purge gas provided to the purge gas system; determining, by the flow rate modulator, based on changes in the detected absorption caused by said periodic modulation gas, an effect of the purge gas on the analysis result; and removing by the evaluation unit, said effect of the purge gas on the analysis result from the analysis result based on the changes in the detected absorption caused by said periodic modulation; wherein the purge gas system further includes the flow rate modulator for periodic modulation of the volume flow rate of the purge gas provided to the purge gas system such that a resulting modulation of the total absorption path purged by the purge gas is less than 100%.

10. The method as claimed in claim 9, wherein amplitude of the changes in the detected absorption is ascertained at the modulation frequency of the volume flow rate via lock-in demodulation.

11. The method as claimed in claim 10, wherein the flow rate modulator which modulates the volume flow rate of the purge gas comprises a variable-speed fan.

12. The method as claimed in claim 10, wherein the volume flow rate of the purge gas is modulated via a controllable regulator valve arranged in a purge gas feed system.

13. The method as claimed in claim 10, wherein the volume flow rate of the purge gas is modulated via a modifiable buffer volume arranged in a purge gas feed system.

14. The method as claimed in claim 9, wherein the flow rate modulator which modulates the volume flow rate of the purge gas comprises a variable-speed fan.

15. The method as claimed in claim 9, wherein the flow rate modulator which modulates the volume flow rate of the purge gas comprises a controllable regulator valve arranged in a purge gas feed system.

16. The method as claimed in claim 9, wherein the volume flow rate of the purge gas is modulated via a modifiable buffer volume arranged in a purge gas feed system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For further explanation of the invention reference is made in the following to the figures of the drawings, in which:

(2) FIG. 1 shows an exemplary embodiment of the process gas analyzer in accordance with the invention;

(3) FIG. 2 shows a detail from FIG. 1 with a modifiable buffer volume for modulating the volume flow rate of the purge gas;

(4) FIG. 3 shows a graphical plot of an exemplary calibration of the process gas analyzer in accordance with the invention; and

(5) FIG. 4 is a flowchart of the method in accordance with the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

(6) FIG. 1 shows a schematic illustration of a plant section 1, such as an exhaust gas duct, through which a process gas 2 to be analyzed flows. At two diametrically opposed positions, the plant section 1 has process flanges 3, 4 on which are mounted two measuring heads 5, 6, essentially identical in design, of a process gas analyzer. Each of the two measuring heads 5, 6 contains an optoelectronic element 7, 8 that, in the one case, is a light source 7, such as a laser diode, and in the other case is a detector 8, such as a photodetector. The light 9 generated by the light source 7 is directed through the plant section 1 through which the process gas 2 passes and subsequently strikes the detector 8. The optoelectronic elements 7, 8 are separated by means of windows (not shown) from the interior of the plant section 1 and thereby from the process gas 2, where two purging pipes 10, 11 are provided between the windows and the interior of the plant section 1, which purging pipes 10, 11 are closed at one end by the respective window and connect with their other open end into the interior of the plant section 1. The purging pipes 10, 11 through which the light 9 passes are purged by a purge gas 12 that is driven by a fan 13 and introduced by way of purge gas feeds 14, 15 in each case in the vicinity of the window into the purging pipes 10, 11 and leaves the latter at their open ends. A controllable regulator valve 16 and a flow meter 17 are arranged in the common purge gas feed system. Alternatively, a controllable regulator valve and a flow meter can be present in each of the two purge gas feeds 14, 15. A separate fan can also be provided for each of the two purge gas feeds 14, 15.

(7) FIG. 2 shows a further example in which a buffer volume 19 that can modified by a controller, here a piston/cylinder unit, is present downstream of the fan 13 as part of the purge gas feed system. The volume flow rate of the purge gas 12 towards the purging pipes 10, 11 is modulated by periodic variation of the buffer volume.

(8) Returning to FIG. 1, an evaluation unit 18 that evaluates the wavelength-specific absorption of the light 9 in the process gas 2 to produce an analysis result 20 is arranged downstream of the detector 8. The gas analyzer in question is typically a laser spectrometer, where as known, for example, from DE 10 2012 223 874 B3, the wavelength of the generated light 9 is tuned to a specific absorption line of a gas component of the process gas 2 to be measured and the absorption line is thereby sensed periodically in a wavelength-dependent manner. During the comparatively slow sensing of the absorption line, it is additionally possible to sinusoidally modulate the wavelength of the light at a high frequency and low amplitude. The measurement signal 21 generated by the detector 8 is evaluated directly or after demodulation in the case of an n-th harmonic of the modulation frequency. The evaluation is performed, for example, by fitting the Lorentz profile of an ideal absorption line or the n-th derivative thereof to the curve of the (demodulated where applicable) measurement signal. From the obtained measurement result, the concentration of the gas component to be measured is finally determined as the analysis result 20.

(9) In order to minimize the effect of the purge gas 12 on the analysis result 20, as will be explained in detail in the following with reference to an example, the volume flow rate of the purge gas 12 is modulated with the aid of the regulator valve 16 or alternatively of the fan 13. The modulation is controlled by the evaluation unit 18 which regulates the degree of modulation to a predetermined percentage value on the basis of the measured flow.

(10) The wavelength-dependent decrease in intensity of light 9 on the path from the light source 7 to the detector 8 is described by the Beer-Lambert law:
I=I.sub.0.Math.exp(.sub.MG.Math.l.sub.MG.Math.c.sub.MG.sub.SG.Math.l.sub.SG.Math.c.sub.SG),Eq. 1

(11) where at the position (wavelength) of the absorption line of interest of the component to be measured (sample gas):

(12) I is the detected light intensity,

(13) I0 is the initial intensity of the light emitted by the light source 7,

(14) MG is the absorption coefficient of the sample gas,

(15) lMG is the measuring path in the process gas,

(16) cMG is the concentration of the sample gas,

(17) SG is the absorption coefficient of the purge gas,

(18) lSG is the absorption path purged by the purge gas and

(19) cSG is the concentration of the purge gas.

(20) With the total absorption path I0=lMG+lSG this gives:
I=I.sub.0.Math.exp(.sub.MG.Math.(l.sub.0l.sub.SG).Math.c.sub.MB.sub.SG.Math.l.sub.SG.Math.c.sub.SG).Eq. 2

(21) In the case of a sinusoidal modulation of the purge gas flow the absorption path lSG purged by the purge gas changes accordingly:
l.sub.SG=l.sub.SG0.Math.(1+M.Math.sin 2ft),

(22) where M (0<M<1) is the normalized amplitude and f is the frequency of the modulation.

(23) Accordingly the following results for the detected light intensity:
I=I.sub.0.Math.exp(.sub.MB.Math.l.sub.0.Math.c.sub.MG+(.sub.MG.Math.c.sub.MG.sub.SG.Math.c.sub.SG).Math.l.sub.SG0+(.sub.MB.Math.c.sub.MB.sub.SG.Math.c.sub.SG).Math.l.sub.SG0.Math.M.Math.sin 2ft).Eq. 4

(24) For the purpose of computational simplification, the extinction or absorbance based on the natural logarithm is used in the following:

(25) $\begin{array}{cc}E=-\ln \frac{I}{I_0}={}_{\mathrm{MG}}.Math.l_0.Math.c_{\mathrm{MG}}-\left({}_{\mathrm{MG}}.Math.c_{\mathrm{MG}}-{}_{\mathrm{SG}}.Math.c_{\mathrm{SG}}\right).Math.l_{\mathrm{SG}0}-\left({}_{\mathrm{MG}}.Math.c_{\mathrm{MG}}-{}_{\mathrm{SG}}.Math.c_{\mathrm{SG}}\right).Math.l_{\mathrm{SG}0}.Math.M.Math.\sin 2\mathrm{ft}.& \mathrm{Eq}.5\end{array}$

(26) The detected extinction E thus has an alternating component having the amplitude AF in addition to a direct component:
AF=(.sub.MG.Math.c.sub.MG.sub.SG.Math.c.sub.SG).Math.l.sub.SG0.Math.M.Eq. 6

(27) The equation for the detected extinction E can thereby be rewritten as follows:

(28) $\begin{array}{cc}E={}_{\mathrm{MG}}.Math.l_0.Math.c_{\mathrm{MG}}-\frac{\mathrm{AF}}{M}-\mathrm{AF}.Math.\sin 2\mathrm{ft}.& \mathrm{Eq}.7\end{array}$

(29) The detected extinction E defined in Eq. 7 therefore consists of a first direct component MG.Math.I0.Math.cMG unaffected by the purge gas, a second direct component AF/M affected by the purge gas and the alternating component having the amplitude AF.

(30) If the volume flow rate of the purge gas 12 is varied by a predetermined low percentage, such as by 10%, the absorption path lSG purged by the purge gas will also change by a sufficiently close approximation of the same percentage, i.e., the normalized amplitude M will have the value 0.1. The amplitude AF of the alternating component can be ascertained directly by evaluating the extinction E at the modulation frequency f. As a result, the second direct component AF/M mentioned above is also known, here AF/M=AF/0.1=10.Math.AF. Finally, the total absorption path I0 and the absorption coefficient MG of the sample gas are also known variables, which means that the concentration cMG of the sample gas can be determined from the detected extinction E or of the detected light intensity I free from effects of the purge gas 12.

(31) In the case of greater changes in the volume flow rate of the purge gas 12, the absorption path lSG purged by the purge gas will not change in a linear manner, i.e., not by the same percentage. In this case, the normalized amplitude M at which the absorption path lSG purged by the purge gas changes is ascertained in the context of a one-off calibration depending on changes of differing magnitudes in the volume flow rate of the purge gas 12. As illustrated by FIG. 3, to this end, given constant process conditions, the associated amplitude values AF10%, AF20%, . . . AF50% of the alternating component AF of the detected extinction E can be ascertained for various normalized modulation amplitudes MVSSG of the purge gas flow, here for example MVSSG=10%, 20%, . . . 50%. Now M can be determined simply with

(32) $\begin{array}{cc}M=\frac{\mathrm{AF}.Math.x}{{\mathrm{AF}}_x}& \mathrm{Eq}.8\end{array}$

(33) where x denotes as small a relative modulation amplitude as possible, such that: MVSSG=M=x. If x=10%, when a modulation of the purge gas flow takes place for, example, with MVSSG=50%, then this gives rise to a resulting modulation M of the absorption path lSG purged by the purge gas of:

(34) $\begin{array}{cc}M=\frac{{\mathrm{AF}}_{50\%}.Math.0.1}{{\mathrm{AF}}_{10\%}}.& \mathrm{Eq}.9\end{array}$

(35) Following the described calibration the method in accordance with the invention can be performed using any desired modulation of the volume flow rate of the purge gas 12, even if the relationship between M and MVSSG is not linear. The purging pipes 10, 11 are not separated from the purge gas feed system. As a result, this ensures that even in the case of a modulation of 100% purge gas 12 is always present in the purging pipes 10, 11 to protect the windows or other optical components.

(36) Although changing process conditions, such as pressure, temperature or volume flow rate of the process gas 2 in the plant section 1, affect the absorption path lSG purged by the purge gas 12, they are however largely compensated for by the method in accordance with the invention. If, for example, the pressure increases in the plant section 1, then the absorption path lSG purged by the purge gas is reduced, where in a first approximation the modulation-dependent change in lSG also changes to the same extent and M thus remains constant.

(37) As already mentioned, the volume flow rate of the purge gas 12 can be modulated in almost any manner to subsequently determine the effect of the purge gas 12 on the analysis result 20 based on the variations in the detected absorption caused by the modulation and calculate the effect from the analysis result 20. For example, a rectangular modulation of the volume flow rate of the purge gas 12 causes the absorption path lSG purged by the purge gas 12 to periodically switch between the values lSG1=lSG0 and lSG2=lSG0.Math.(1+M). Accordingly, two values E1 and E2 for the detected extinction E are obtained in each modulation period:
E1=.sub.MG.Math.l.sub.0.Math.c.sub.MG(.sub.MG.Math.c.sub.MG.sub.SG.Math.c.sub.SG).Math.l.sub.SG0Eq. 10
and
E2=.sub.MG.Math.l.sub.0.Math.c.sub.MG(.sub.MG.Math.c.sub.MG.sub.SG.Math.c.sub.SG).Math.l.sub.SG0(.sub.MB.Math.c.sub.MG.sub.SB.Math.c.sub.SG).Math.l.sub.SG0.Math.M.Eq. 11

(38) AF can be determined from the difference of the values E1 and E2:
E1E2=AF=(.sub.MG.Math.c.sub.MG.sub.SG.Math.c.sub.SG).Math.l.sub.SG0.Math.M,Eq. 12

(39) where the following applies to the value E1:

(40) $\begin{array}{cc}E1={}_{\mathrm{MG}}.Math.l_0.Math.c_{\mathrm{MG}}-\frac{\mathrm{AF}}{M}& \mathrm{Eq}.13\end{array}$

(41) and finally to the concentration cMG of the sample gas:

(42) $\begin{array}{cc}{c}_{\mathrm{MG}}=\frac{1}{{}_{\mathrm{MG}}.Math.l_0}.Math.\left(E1+\frac{\mathrm{AF}}{M}\right)=\frac{1}{{}_{\mathrm{MG}}.Math.l_0}.Math.\left(E1+\frac{E1-E2}{M}\right).& \mathrm{Eq}.14\end{array}$

(43) As previously mentioned above, when there is a small modulation of the volume flow rate of the purge gas 12 of, for example, 10% the absorption path lSG purged by the purge gas 12 changes by the same percentage, i.e., M=0.1. In this case, the following results for the concentration cMG of the sample gas:

(44) $\begin{array}{cc}{c}_{\mathrm{MG}}=\frac{1}{{}_{\mathrm{MG}}.Math.l_0}.Math.\left(11.Math.E1-10.Math.E2\right).& \mathrm{Eq}.15\end{array}$

(45) FIG. 4 is a flowchart of a method for analyzing a process gas 2 carried in a plant section 1. The method comprises detecting light 9 from a light source 7 via a detector 8 after the light is passed through the process gas 2, as indicated in step 410.

(46) Next, the light is evaluated with respect to absorption in the process gas 2 in an evaluation unit 18 arranged downstream to produce an analysis result 20, as indicated in step 420.

(47) Chambers 10, 11 present between the light source 7 and the plant section 1 and between the detector 8 and the plant section 1 and open towards an interior of the plant section 1 are now flushed with a purge gas 12, as indicated in step 430. Next, the volume flow rate of the purge gas 12 is modulated periodically, as indicated in step 440.

(48) The effect of the purge gas 12 on the analysis result 20, based on changes in the detected absorption caused by the periodic modulation, is now determined, as indicated in step 450. Next, the effect of the purge gas 12 on the analysis result is removed from the analysis result 20, as indicated in step 460.

(49) While there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods described and the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.