CALIBRATION & VALIDATION PHILOSOPHY AND PROCEDURES FOR TUNEABLE DIODE LASER ANALYZERS IN PROCESS APPLICATIONS

CALIBRATION & VALIDATION PHILOSOPHY AND PROCEDURES FOR TUNEABLE DIODE LASER ANALYZERS IN PROCESS APPLICATIONS Dr. Janardhan Madabushi David Fahle Servomex Company Inc Servomex Company Inc 525 Julie Rivers Rd. Suite 185 525 Julie Rivers Rd. Suite 185 Sugar Land, TX 77598, USA Sugar Land, TX 77598, USA Dr. Christian Heinlein NEO Monitors AS Solheimveien 62A, P.O.Box 384 N-1471 Loerenskog, Norway KEYWORDS Tuneable Diode Laser, Optical Absorption Spectroscopy, Calibration, Validation ABSTRACTGas analyzers based on Tuneable Diode Laser (TDL) Absorption Spectroscopy are nowwidely accepted in the industry as process analyzers because of their reliability and lowmaintenance. TDL analyzers offer a non-contact, optical measurement without moving partswith most manufacturers recommending a calibration frequency of up to one year. Thisextended calibration interval has raised skepticism among industry users because it iscontrary to their experience with traditional analyzers, which require more frequentcalibration, supported by most maintenance protocols which call for frequent validation andcalibration. This paper will discuss the calibration and validation philosophy for TDLanalyzers in process applications. INTRODUCTIONGas analyzers for industrial applications must have high reliability and availability andrequire little maintenance. Analyzers based on tunable diode laser absorption spectroscopy(TDLAS) meet these requirements, because they offer a non-contact, optical measurement Session 8.2: – Page 1 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

without the need for consumables or moving parts. Because of this, TDLAS analyzers can beapplied for true in-situ (cross stack or pipe) measurements for many applications as they donot require regular access.TDLAS became feasible when reliable, tunable laser diodes became available as a directspin-off from the development of light sources for optical fiber communication. These lasershave wavelengths in the near infrared (typically from 1300 nm to 1700 nm, but more recentlyout to 2700 nm), where many of the process critical gases have (overtone) absorption linessufficiently strong to allow measurements of gas concentrations in the low ppm range.The TDLAS technique has become widely accepted in the chemical and petrochemicalindustry by providing in-process measurements of gases such as oxygen (O2), carbonmonoxide (CO) and carbon dioxide (CO2) in hazardous and/or poisonous atmospheres. Inaddition to process applications, this technology is also widely implemented for emissioncontrol of pollutants such as hydrogen fluoride (HF), hydrogen chloride (HCl), and ammonia(NH3) from aluminum smelters, power plants or waste incinerators. This makes it a veryviable tool for use in Continuous Emission Monitoring Systems (CEMS).The non-contact measurement approach on the TDLAS lends to increased stability of themeasurement, gradually enhanced by improvements of electronic components. Accordingly,most manufactures of the TDLAS based analyzers suggest a calibration frequency of 6 to 12months, which greatly contrast to the regular calibration requirements of traditional analyzertechniques. This paper includes a brief description of measurement principle, a typical TDLsystem (Instrumentation), measurement techniques and the calibration and validationphilosophy for TDL analyzers in process applications. MEASUREMENT PRINCIPLEOptical absorption spectroscopy for quantification of gas concentration has been used inindustrial applications for more than 60 years. The technique is based on the Beer-Lambertlaw:T = exp(-Sg(f)NL) (1)T is the transmittance,S is the absorption line strength,g(f) is the line shape function,N is the concentration of absorbing gas molecules,L is the optical path length.Concentration N can be calculated by measuring T and knowing S, g(f), and L. Theconventional way of obtaining quantitative information is to compare the measured responseto that generated by a calibration gas with known content of the molecule in question. Unlikeconventional ultra violet (UV) or infrared (IR) spectrographic instruments, laser-basedmonitors employ the measurement principle known as ‘single line spectroscopy’, where asingle gas absorption line is chosen in the NIR spectral range and scanned by the single-modediode laser. Many detection techniques have been developed for TDLAS and an overview of Session 8.2: – Page 2 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

some of these can be found in [1 – 3]. The dual modulation (2f) technique is, in our opinion,the best candidate for an industrial monitor. INSTRUMENTATIONA typical TDLAS system will consist of the following units:• Laser module with temperature controlled diode laser, driving circuit and collimating lens plus window to isolate from the process• Detector module with pre-amplification circuit, photo diode, and focusing lens plus window to isolate from the process• Main electronics with central processing unit (CPU) for signal treatment and calculation of gas concentration• Interface electronics with analog and digital input and output connectionsFigure 1 shows the mechanical layout of such a monitor installed on a stack. Transmitter Flanges Receiver Purge gas Purg e gas Diode inlet inlet Laser Focusing lens Detector Collimating lens Instrument Process window gas Loop cableFIGURE 1. SCHEMATIC DRAWING OF A TDLAS CROSS STACKINSTALLATION MEASUREMENT TECHNIQUESTDLAS is an emerging technology for process measurement and its implementation forprocess measurement has been elaborately described in various publications andpresentations [4 - 6]. To summarize, measurements can be performed by in-situ, bypass loopor extractive installation, but in all cases the concept remains the same: a laser transmitter and Session 8.2: – Page 3 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

receiver (detector) unit are both located or fiber optically coupled at the measurement pointwith mounting flanges and intermediate purge and alignment units. A continuous flow ofpurge gas is utilized to prevent dust from fouling the optical windows, and in case of O2 andH2O measurements, to displace ambient concentrations of these components in flange nozzlesand analyzer optics. The in-situ approach is illustrated in Figure 1 with installation acrosspipes, stacks or ducts, with typical path lengths ranging from 0.5 – 20 meters. This is themost preferred approach because it eliminates the need for sample conditioning system and istherefore ideal for fast and real time measurement.A bypass mode with a slip stream (Figure 2) is employed to increase the optical path length(OPL), when the process pipe does not have sufficient OPL to obtain the desiredmeasurement range. An extractive set up (Figure 3) is typically implemented when sampleconditioning is not conducive for a direct in-situ measurement and some sample pre-conditioning is required to address pressure, temperature or dew point issues.FIGURE 2. SCHEMATIC OF A TDLAS SYSTEM IN BYPASS LOOP SETUP CALIBRATION AND VALIDATIONThe absorption signal peak shape, formed by amplitude and width, is influenced by a range offactors including analyte concentration, optical path length (OPL), pressure, temperature andvariation in stream composition created by background gases. The amplitude is influencedby analyte concentration and OPL, while there are three contributors to the peak width:natural line broadening, Doppler broadening and collision broadening.Collision broadening of the absorption signal dominates industrial processes, depending as itdoes on the collision frequency of the molecules. Collision broadening directly depends onthe process pressure, temperature and composition, so the accuracy and repeatability of themeasurement is always influenced by these parameters [7]. Session 8.2: – Page 4 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

FIGURE 3. SCHEMATIC OF A TDLAS SYSTEM IN EXTRACTIVE SETUPIn industrial applications, line broadening effects must be taken into account in order toachieve the required measurement accuracy. For process measurement, the effect ofpressure and temperature is compensated by live temperature and pressure input to theanalyzer. Broadening due to variation in gas composition is addressed by measuring the linewidth of the absorption signal.EXPERIMENTAL:A typical layout of initial factory calibration is shown in Figure 4. Based on the submittedprocess temperature and pressure data from the end user, a pressure dependence curve and atemperature dependence curve are generated in a controlled environment, using a calibratedstandard gas and the above referenced set up. The pressure and temperature profile isspecific for each transition hence wavelength dependant. This procedure is an importantaspect of final testing, to compensate for pressure and temperature induced line widthbroadening.RESULTS AND DISCUSSIONS:Figure 5 illustrates the relationship between pressure and detector signal. The signalincreases with density at low pressures, while the line broadening mechanism dominates atpressures above ≈ 1 bar, hence the signal decreases at P > 1 bar. This profile is wavelengthdependant; as different absorption lines will have different pressure dependence profile. Inaddition to this the location of maximum in Fig 5 could be moved towards higher pressureby applying higher frequency modulation amplitude. In process application pressure canvary from slightly negative to positive pressure greater than 1 Bar A. Hence each analyzer isprogrammed with the respective pressure profile to enable live pressure compensation,typically controlled by a 4-20mA pressure input. For processes operating at a fixed pressure,live pressure compensation is not required but the magnitude of the process pressure ismanually entered in the instrument configuration for accurate measurement. Session 8.2: – Page 5 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

FIGURE 4. TYPICAL SCHEMATIC FOR INITIAL FACTORY CALIBRATION 25 Measured 20 Fitted curve S n l (a . u its) ig a rb n 15 10 5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Pressure (Bar absolute)FIGURE 5. PRESSURE DEPENDENCE OF THE ABSORPTION SIGNALFigure 6 illustrates the relationship between temperature and detector signal. Temperatureaffects absorption line width and absorption strength, and different absorption lines will havedifferent temperature dependence profile. Each analyzer is programmed with the respectivetemperature profile to enable live temperature compensation, typically by a 4-20mAtemperature input. Session 8.2: – Page 6 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

20 Signal (arb. units) 15 10 5 Measurement Fitted curve 0 0 50 100 150 200 250 Temperature (C)FIGURE 6. TEMPERATURE DEPENDENCE OF THE ABSORPTION SIGNAL Three gas types that occur frequently in industrial processes and have a large influence on theline width are H2O, CO2 and hydrocarbons such as CH4. The concentration of H2O fromwaste incinerators and boilers may typically vary from 10 to 25 vol%, but in some cases itcan be as high as 40 vol%. Figure 7 illustrates the influence of H2O concentration on the peakamplitude of the 2f signal for HCl at 270 °C: It is clear that variations in gas composition caninfluence the measurement and must be compensated for. The broadened peak will decreasein amplitude and increase in width but overall area under the curve remains the same. Linewidth broadening due to variation in gas composition can be compensated by measuring orcalculating the integrated area of the absorption peak. 1 Measured Fitted curve 0.8 HCl Signal, normalised 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 40 H2O conc. (%vol.)FIGURE 7. EFFECT OF H2O CONCENTRATION ON 2f PEAK SIGNAL OF HClPeak area is calculated using the peak height and the line width of the 2f signal. Thistechnique is also known as Line Width Compensation (LWC) [4]. Session 8.2: – Page 7 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

40 % 80 % 100 % 10 %H2S 2f-signal 0 1 2 3 4 5 6 7 Sample (corresponds to wavelength)FIGURE 8. VARIATIONS IN 2F SIGNAL FOR H2S AT PERCENTCONCENTRATIONSFigure 8 illustrates self-induced band broadening, where large variation in the analyteconcentration can alter the line width of the absorption peak. Measuring the peak amplitudealone will result in a large systemic error, but measuring the integral 2f signal using LWCcorrects the systemic error in the measurement (Figure 9).FIELD APPLICATION – CALIBRATION:Having established the factory calibration procedure for a TDLAS analyzer, the next logicalquestion is how these analyzers can be calibrated in the field. It is easy to calibrate analyzersinstalled in the extractive mode (Figure 3). To calibrate the analyzer when installed in thein-situ mode and by-pass modes the analyzer is removed from the process and a benchcalibration is performed using a calibration cell and span gas. The calibration is typicallyperformed at ambient temperature and pressure, which entails resetting the OPL to the lengthof the calibration cell, and restoring temperature and pressure to ambient values. Since eachanalyzer is programmed with its unique temperature and pressure profile it is feasible tocalibrate the analyzer at ambient temperature and pressure. This answers one of the mostcommonly asked questions for calibration: namely, how can an ambient temperaturecalibration of the analyzer still read accurately when installed in a process at 400 °C? Session 8.2: – Page 8 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

120 Measured concentration (%) 100 80 60 40 20 y = 1.0024x + 0.1203 R2 = 0.9996 0 0 20 40 60 80 100 120 H2S concentration in % set with mass flow controllersFIGURE 9. LWC COMPENSATED DETECTOR RESPONSE FOR H2SIn some instances the analyzer cannot be easily removed from the process. In such instancesit may be possible to rescale the TDLAS response to the expected concentration, if there issome legitimacy, based on historical data, for the expected reading. While this practice is notgenerally recommended, this feature can be used with the acceptance of the risks, when theanalyzer is used mainly to trend the concentration of the analyte and an absolutemeasurement is not necessary. Situations like this also bring up the need for in-situvalidation of the analyzer.FIELD APPLICATION – IN-SITU VALIDATION:Very few TDLAS analyzers are installed at grade level. Most in-situ installations are up on astack or at higher elevations in the plant. Thus it is not very practical to frequently removethe analyzer from the process to verify its performance. The extended calibrationfrequency does help in this regard but most plants have operating procedures that call forroutine validation of the analyzer’s measurement to verify performance. Subsequentcalibration may be initiated based on the results from this validation. The concept ofvalidation is also important in most CEMS applications [8].Validation is performed either by comparison against a validated reference method or withanalyte spiking. This section will highlight validation of TDLAS by the analyte spiking; alsoknow as the bump test method, in process applications.Once again it is easy to validate analyzers installed in the extractive mode by introducing thespike gas at the sample inlet. For in-situ applications a different approach is taken. Typically Session 8.2: – Page 9 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

the spike gas is introduced into a fixed volume gas cell situated outside the process duct butalong the optical path as depicted in Figure 10.FIGURE 10. TYPICAL OPTION FOR INTRODUCING SPAN GAS FORVALIDATIONThe gas cell is a stainless steel chamber, typically about 10 cm long, with sealed windows onboth sides (axial), to contain the spike gas. During normal operation a spectroscopically inertgas is present in the gas cell; i.e. if oxygen is being measured then the inert gas in nitrogen.Likewise if NH3 is the measured component dry instrument grade air is used as the inert gas.To validate, spike gas containing a significantly higher concentration of the analyte isintroduced to bump the analyzer response. The analyte concentration in the spike gas isdictated by the process OPL, process measurement range and the OPL of the validationchamber.For example, consider the following situation where, process OPL is 1 m (~ 40”),measurement is 0 – 6 % O2 and the chamber OPL is 0.01 m. The absorption signal from thegas cell should always be stronger than the signal from the process in order to achieve areliable validation check. The path length of the gas cell is only 0.01 m and the maximumanalyte concentration that can be introduced into the chamber is 100%. The effectiveconcentration of the spike gas is calculated using equation (2). CS = [S] x LC (2) LP Session 8.2: – Page 10 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

CS is the Effective Spike Concentration,[S] is the concentration of the spike gas,LC is the length of the gas cell andLP is the process pathlength.Therefore 100% O2 in 0.01 m gas cell corresponds to 1 % O2 for 1 meter process pathlength.Thus it is obvious that the validation check will not be reliable for this process. However, ifthe process OPL is much shorter, 0.2 m (~ 8”), then 100% O2 on 0.01 m OPL corresponds to5 % O2 for 0.2 meter process pathlength. In this situation it is viable to perform a reliablevalidation check using 100% O2 as the spike gas.In CEMS applications the measurement range is usually much lower. For example if weconsider measurement of ammonia slip in DENOX scrubbers, the typical range is 0 – 20 ppmNH3 and the process OPL is 2 to 3 meters. To achieve an effective spike concentration of 10ppm with the 10 cm chamber, the required spike gas concentration will be: [S] = 10 ppm x 2.5 m = 2500 ppm. (3) 0.01 mThus for such applications, if needed, it is possible to use smaller chamber with higher spikegas concentration. One approach is to use a small sealed glass cuvette containing highconcentration of NH3 as the validation chamber. To initiate validation, the cell is flipped intothe optical path using a servomotor. The cell has a very short OPL of 2 mm and typicallyfilled with 5% to 10% NH3. This yields an effective spike concentration of 4 ppm to 8 ppmfor a process OPL of 2.5 m. The entire validation assembly can be integrated inside thereceiver unit. This approach eliminates the requirement for a spike gas and inert gas, hencemore economical to the end user.It is important to realize that the validation is performed along with the process measurement.The TDLAS analyzer calculates the CS as follows: CS= RS – RP (4)RS is response from the spike gasRP is the response from the process gas just before the initiation of the validation.Typical equilibration time for the validation will depend on the methodology employed; thefixed volume gas cell method will have a longer equilibration time as compared to the sealedglass cuvette method. It is evident from equation (4) that the process should be fairly stableduring this entire duration, to obtain meaningful validation results.Zero check is initiated when the gas cell is filled with the inert gas or when the sealed glasscuvette is not in the optical path. It is performed either by subtracting the process reading justbefore the initiation of the validation from the current process reading. CZ = RPC - RP (5)CZ is the zero check responseRPC is the current response from the process Session 8.2: – Page 11 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

TDLAS analyzers utilizing the 2f technique for signal processing is immune from baselineoffsets hence do not require a zero calibration [4]. Thus it is also possible to perform a zerocheck by momentarily turning off the laser modulation.This approach of in-situ validation is not yet endorsed by the USEPA for CEMSmeasurement. However a preliminary performance specification for ammonia CEMS hasbeen drafted for further review [8]. CONCLUSIONSMeasurement by TDLAS can be performed by in-situ, bypass loop or extractive installationhence they are gaining acceptance in the process industry. The in-situ approach is the mostcost effective mode of use because it eliminates the need for a sample conditioning systemand provides real time response. TDLAS analyzers are predominantly employed in this modeto exploit this beneficial feature. While there are several publications on product developmentand its application to the process industry, there is limited information on calibration andvalidation of these analyzers in the field.The non-contact measurement approach and improvements of electronic components lends toincreased stability of the measurement, prompting most manufactures of the TDLAS basedanalyzers to suggest a calibration frequency of 6 to 12 months. This is a benefit because theanalyzer has to be dismounted from process for calibration. Built in temperature and pressurecompensation profiles conveniently permit TDLAS analyzers to be calibrated at ambienttemperature and pressure conditions regardless of the actual process measurement conditions.Despite the extended calibration frequency most users prefer routine validation of theanalyzer’s measurement to verify performance. Fixed volume gas cells or sealed glasscuvette containing the target gas can be used for validation. These apparatus are usedexternal to the process stream hence do not interfere with or alter the process chemistry. As aconsequence there is a practical limitation on the size of the gas cell. Feasibility of in-situvalidation is dictated by the process pathlength, process measurement range and thepathlength of the validation gas cell. CEMS applications typically have measurements in theppm or very low % range hence they are an ideal candidate for this validation technique. ACKNOWLEDGEMENTSThe authors wish to thank Chris Lawrenson (Coda Communications, Poole, UK.) for proofreading the manuscript. REFERENCES[1] Schiff, H.I., Mackay, G.I., Bechara. J.: "The use of tuneable diode laser absorption spectroscopy for atmospheric measurements", in Chemical Analysis Series, Vol. 17 (Wiley, New York 1994). Session 8.2: – Page 12 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

[2] Ye, J., Ma, L.S., Hall, J.L.: J. Opt. Soc. Am. B 15, 6 (1998).[3] Feher, M., Martin, P.A.: Spectrochim. Acta, Part A 51, 1579 (1995).[4] Linnerud, I., Kaspersen, P., and Jæger, T.: Gas monitoring in the process industry using diode laser spectroscopy. Applied Physics B 67, 297 – 305 (1998).[5] Amerov, A., Fiore, R., Langridge, S.: “ Process Gas Analyzer for the Measurement of Water and Carbon Dioxide Concentrations”, ISA Analysis Division 54th Symposium Proceedings, Instrumentation, Systems and Automation Society, Houston Texas 2009, Session 7.2 pp1-14.[6] Zhou, X., Liu, X., Feitisch, A.: “Advanced TDL Gas Analyzers for Petrochemical Process Industries”, ISA Analysis Division 53rd Symposium Proceedings, Instrumentation, Systems and Automation Society, Calgary, Canada 2008, Session 3, Paper 3, pp1-12.[7] Hobby, J., Gaskin, I., Kovacich, R., Lopez, M., Alizadeh, B.: “A Comparative Analysis of the Errors in the Measurement of Oxygen Between in-situ and Extractive Methods”, ISA Analysis Division 54th Symposium Proceedings, Instrumentation, Systems and Automation Society, Houston Texas 2009, Session 2.1 pp1-14.[8] www.epa.gov/ttn/emc/prelim/pps-001.pdf: PPS-001: Preliminary Performance Specifications for Ammonia Continuous Emission Monitors (CEMs), pp1-18 Session 8.2: – Page 13 – © Copyright 2010, International Society of Automation. All rights reserved. ISA 55th Analysis Division Symposium 2010, New Orleans, LA.

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CALIBRATION & VALIDATION PHILOSOPHY AND PROCEDURES FOR TUNEABLE DIODE LASER ANALYZERS IN PROCESS APPLICATIONS Document Transcript