Infrared Analyzers Notes

Instrument Technician - Infrared Analyzers

This module provides information on infrared analyzers for instrument technicians.

Objective One

Infrared Analyzers

Infrared (IR) analyzers are spectroscopic instruments used for both qualitative and quantitative analysis.

• Dispersive Infrared (DIR) Analyzers: Qualitative analysis, off-line lab use, broad range IR.

• Non-Dispersive Infrared (NDIR) Analyzers: Quantitative analysis, on-line process, small band IR.

Infrared Absorption

Infrared radiation is electromagnetic radiation (EMR) with a wavelength range of $0.8$ to $1000$ micrometers ($\mu$m), where $1 \mu m = 10^{-6}$ meters. It lies between red light and microwaves on the electromagnetic spectrum. IR radiation has lower energy and a longer wavelength than red light, and higher energy with a smaller wavelength than microwaves.

Before a substance can absorb IR radiation, it must:

1. Consist of two or more covalently bonded atoms.

2. Have molecular bonds with different types of atoms.

Monoatomic substances and homonuclear molecules (e.g., Helium (He), Nitrogen (N2)) do not absorb IR radiation. Nitrogen (N2) is often used as a zero gas for calibrating IR gas analyzers.

Heteronuclear molecules like carbon dioxide (CO2) do absorb IR radiation. The changing polarity of IR radiation interacts with polarized molecular bonds. The bond vibrates at specific frequencies and can absorb IR radiation at the same frequency, increasing the molecule's energy level and temperature. Other vibration types include bending.

Transmittance (T) is inversely proportional to absorption. A low transmittance indicates high absorption. For example, CO2 absorbs IR radiation at approximately $4 \mu m$ and $15 \mu m$ wavelengths. The higher frequency ($4 \mu m$) IR radiation causes the carbon-oxygen bond to stretch, while the lower frequency ($15 \mu m$) IR radiation causes it to bend.

Chemists use IR spectrometers (or DIR analyzers) to match absorption peak wavelengths of unknown substances with those of known bonds for qualitative analysis. Process analyzers perform quantitative analysis by measuring transmittance (T) and absorbance (A) for a narrow band of strongly absorbed IR wavelengths.

$A = log(1/T)$

Absorbance is proportional to concentration according to the Beer-Lambert Law.

Dispersive Infrared (DIR) Analyzers

A dispersive infrared (DIR) analyzer is a type of laboratory IR spectrometer that uses a diffraction grating to separate IR radiation into individual wavelengths.

The main components of a DIR analyzer are:

1. Continuous multi-wavelength IR source.

2. Reference cell (non-IR absorbing gas).

3. Sample cell (IR absorbing sample).

4. Rotating chopper (alternately selects reference and sample IR beams).

5. Monochromator (separates IR wavelengths).

6. Infrared (IR) detector (responds to all IR wavelengths).

Radiation from an IR source splits into two beams: one passes through a reference cell, and the other through a sample cell. A rotating chopper with a hole and a mirror alternately sends pulses of the sample beam IR and the reference beam IR to the monochromator and IR detector. The monochromator, typically a diffraction grating, separates the IR beam into its component wavelengths. By turning the grating, different wavelengths of IR focus on the detector. The detector measures the intensity (I) of the IR radiation and outputs a proportional signal.

The instrument calculates transmittance (T) by dividing the sample detector's signal by the reference signal. The resulting voltage, representing T, moves the pen of a chart recorder. The instrument synchronizes the horizontal movement of the chart paper to the motion of the grating, creating a plot of T against IR wavelength, which is the IR absorption spectrum of the sample.

Non-Dispersive (NDIR) Infrared Analyzers

Non-dispersive infrared (NDIR) analyzers measure the concentration of a single substance in a multicomponent process stream. They are used for continuous quantitative analysis of process gas samples, such as flue gas, for compounds like carbon monoxide (CO). NDIR analyzers do not use dispersive devices like prisms or gratings.

Two common designs of NDIR single gas analyzers are:

1. Dual-beam instruments

2. Single-beam instruments

Dual-Beam

Dual-beam NDIR analyzers have:

1. Two continuous emission IR sources (balanced).

2. One rotating chopper (alternately blocks each IR beam).

3. One sample cell (continuous flow of sample gas).

4. One reference cell (non-IR absorbing gas, e.g., nitrogen).

5. One differential IR detector (output proportional to the difference in IR intensity between sample and reference beams).

6. Readout electronics (converts the difference in IR intensity to a concentration reading).

A semi-circular chopper blade alternately blocks the reference or sample beam. The sample absorbs IR energy from the sample beam, with absorbance increasing as concentration increases. The reference cell contains a non-IR absorbing gas that passes the IR beam without reduction in intensity.

The differential IR detector is selective to the IR wavelengths that the measuring component absorbs. Using two beams and a differential detector compensates for power supply fluctuations or detector changes. However, it cannot compensate for a dirty sample cell window.

Single-Beam

Single-beam NDIR analyzers have:

1. A single IR source.

2. A rotating filter wheel (reference and measurement filters).

3. A sample cell (continuously flowing process gas sample).

4. An IR detector (outputs reference and measurement signal pulses proportional to the intensities of the reference and measurement IR wavelengths).

5. Readout electronics (converts detector signal pulses to a concentration reading).

The rotating filter wheel alternately selects reference and measurement IR wavelengths. Location enables it to alternately send the following two IR wavelengths through the sample.

1. Measure wavelength: Strongly absorbed by the component of interest.

2. Reference wavelength: Not absorbed by any gas sample components.

Interference filters select very narrow bands of wavelengths. The sample absorbs the measurement wavelength, and the reference wavelength is not absorbed. The readout electronics calculate transmittance (T) and absorbance (A) values:

$T = \frac{Measurement \ signal}{Reference \ signal}$

$A = log(\frac{1}{T})$

Changes in the condition of the cell windows equally affect both beams and cancel out.

Objective Two

Non-Dispersive Infrared (NDIR) Analyzer Components

NDIR analyzers use IR sources, sample cells, and IR detectors. Dual-beam analyzers use wavelength-specific detectors (e.g., Luft detector), while single-beam analyzers use wide-band detectors (e.g., pyroelectric detectors).

Sources

A common IR source is a tightly wound coil of nichrome wire, electrically heated to emit IR radiation in the 2-15 $mu$m wavelength range.

Dual-beam NDIR analyzers require two balanced IR beams, accomplished with:

• A pair of identical wire filaments.

• A single filament plus a concave mirror.

A balancing adjustment screw can partially block one of the beams to equalize intensity. The single-filament design is self-balancing.

Sample Cells

NDIR sample cells are typically stainless tubes, gold plated inside, with IR transparent cell windows. The gold surface increases IR transmission. Path length is a critical parameter determining the absorbance range.

According to the Beer-Lambert Law:

$A = a \times b \times c$

Where:

• A = Absorbance

• a = Absorptivity

• b = Path length

• c = Concentration

Optimal path length is calculated to provide optimal precision. Too short = low absorbance, indistinguishable from noise. Too long = high absorbance, invalid measurement. Ideal path length varies from millimeters (high gas concentrations) to meters (low concentrations).

Detectors

Dual-beam NDIR analyzers measure the difference in IR intensity between two beam paths, while single-beam analyzers measure the IR intensities of a measure wavelength and an unabsorbed reference wavelength.

Two types of detectors:

1. Differential IR detectors

2. Wide-band IR detectors

Differential IR Detectors

Differential detectors compare IR intensity in dual-beam NDIR analyzers. They rely on pneumatic changes in gas pressure caused by infrared heating.

• Luft detectors: Change capacitance as detector gas pressure changes.

• Micro flow detectors: Change gas flow rate between sealed chambers as detector gas pressure changes.

Luft and micro flow detectors are specific IR detectors responding only to the IR wavelengths of the target gas. They are filled with a high concentration of the target gas mixed with a non-IR absorbing gas, such as nitrogen.

Luft detectors create changes in capacitance when a thin metal membrane moves toward or away from a fixed capacitor plate. A sensitizing gas sealed inside the detector expands when it absorbs IR, causing the membrane to move. The membrane movement depends on the reduction in IR intensity of the sample beam compared to the reference beam.

Micro flow detectors alternately allow and block both the sample and reference beams. The detector's sample and reference chambers connect through a narrow tube with a micro flow sensor. The instrument relates the gas flow rate between the detector chambers to the concentration of the IR absorbing gas in the sample cell.

Wide-Band IR Detectors

Wide-band (non-specific) IR detectors respond to a wide range of IR wavelengths.

Two types:

1. Pyroelectric detectors: Convert IR energy into electrical current.

2. Photoconductive detectors: Change electrical resistance when exposed to IR. Resistance decreases as IR intensity increases.

Photoconductive IR detectors are the most sensitive and are used in laboratory DIR analyzers. Several types of NDIR analyzers use wide-band IR detectors, including single-beam IR analyzers and dual-beam filter analyzers.

Single-beam NDIR analyzers send different wavelengths of IR radiation through a sample cell. The detector outputs a current (pyroelectric) or a change in resistance (photoconductive). Readout electronics calculate the sample's absorbance from the intensity measurements, which is proportional to concentration.

Dual-beam NDIR analyzers use wide-band detectors by mounting a filter in front of the IR source to pass the target wavelength. A rotating chopper blade allows only one beam at a time to go through the cells. Readout electronics converts these measurements into absorbance values according to the equation:

$A = log(\frac{1}{T}) = log(\frac{Reference \ intensity}{Sample \ intensity})$

If the sample follows the Beer-Lambert Law, absorbance is proportional to concentration:

$A = abc = constant \times C$

Objective Three

Infrared Analyzer Filtering Techniques

Process gas samples often contain interfering gases that absorb some of the same IR wavelengths as the target gas. To minimize the effect of these gases, instrument manufacturers use IR analyzer filtering techniques such as positive and negative filtering.

Positive Filtering

Positive filtering removes the interfering IR wavelengths from the sample beam of a dual-beam NDIR analyzer. A gas filter cell in the sample beam path contains high concentrations of the interfering gas.

For example, to measure carbon monoxide (CO) concentration in stack gases containing carbon dioxide (CO2), the differential IR detector is filled with CO, the filter cell with CO2, and the reference cell with nitrogen (N2).

Negative Filtering

Negative filtering reduces the effects of interfering IR absorbing gases on single-beam NDIR analyzer measurements. Single-beam negative filtering NDIR analyzers use rotating gas filter wheels (correlation cells) made up of two sealed gas-filled cells. They are also known as gas filter correlation (GFC) analyzers.

One correlation gas cell (reference filter cell) contains a high concentration of the IR absorbing target gas, while the second gas cell (measurement filter cell) contains a non-IR absorbing gas, such as N2. A non-specific, wide-band IR detector measures the total energy from all the IR it receives.

For example, to measure CO concentration in stack gases, the reference filter correlation cell is filled with 100% CO, and the measurement filter correlation cell is filled with 100% nitrogen.

When the reference filter cell is in the beam path, it removes the IR wavelengths that the CO absorbs. The detector responds to the background IR absorbing gases. When the measurement filter cell is in the beam, it does not remove any IR wavelengths. The detector responds to the background and CO IR absorbing gases.

This filtering technique does not require the identification of any interfering IR absorbers in the background gas. It also minimizes effects of the following operational changes:

changes in IR source intensity,
reduction in beam intensity caused by dirty cell windows,
changes in IR detector sensitivity and
changes in interfering background gas composition.

Negative filtering GFC analyzers have been applied to in-situ measurements.

Comparing Positive and Negative Filtering

Objective Four

Infrared Analyzer Process Applications

Process non-dispersive infrared (NDIR) analyzers analyze a single gas component in a mixture of gases. The instrument manufacturer sets up both dual-beam and single beam analyzers to analyze specific gases such as carbon monoxide (CO), carbon dioxide (CO2), ammonia (NH3), methane (CH4) and ethylene (C2H4).

Process NDIR instruments are used in the following two types of sampling situations:

1. Analysis of conditioned samples extracted from the process gas.

2. In-situ analysis of stack gas using GFC analyzers.

Extracted Carbon Monoxide Stack Sample Analysis

Carbon monoxide (CO) forms during incomplete combustion of hydrocarbon fuels. Dual-beam NDIR analyzers require positive filtering techniques to eliminate the IR interference that CO2 and H2O cause on the IR absorbance of CO.

To condition an extracted sample for analysis requires:

• Reducing the dew point of the water vapor.

• Removing particulate matter.

A typical extractive sample system conditions moist stack gas for analysis by NDIR analyzer, including a particulate filter, heated line, cooler, and in-line filter.

The dual-beam NDIR analyzer uses positive filtering with a filter cell filled with concentrated CO2 gas and Isobutane (methylpropane, H3C-CH [CH3]2) as a suitable replacement gas for water vapor for high levels.

In-Situ Carbon Monoxide Stack Sample Analysis

By sending an IR measurement beam across a stack, you can measure CO in-situ (directly), without sample extraction and conditioning. Gas filter correlation negative filtering single beam NDIR analyzers are ideally suited to in-situ stack gas analysis. A chopper interrupts IR radiation from the source, A beam splitter lets the IR beam pass through it to the stack and also reflects the beam returning from the stack which is measured using a wide-band IR detector.

Fourier Transform Infrared Analyzers

Fourier transform infrared (FTIR) analyzers measure the IR absorbance spectrum of a sample in seconds. They are a type of non-dispersive IR analyzer that uses the phenomenon of interference to measure IR absorbance over a wide range of wavelengths.

An interferometer creates interfering mixtures of IR radiation to send through the sample. The sample modifies the IR creating an
interferogram which transforms into the sample absorption spectrum. Using a computer, the interferogram is converted from a time based measurement to a frequency based measurement by applying a mathematical method known as a Fourier transform. . Manufacturers program FTIR analyzers with the absorption frequencies of tested compounds, which allows both frequency identification (qualitative analysis) and concentration value calculation through absorbance (quantitative analysis).

FTIR analyzers identify and measure concentrations of process gas components, stack gas environmental pollutants, and process liquids. Liquid and gas samples require different sample cell path lengths, with longer lengths for gases.

Calibration and Troubleshooting

Changes in a GFC analyzer may cause the calibration of the instrument to gradually drift over time. Typical changes:

• Weakening of the IR source intensity (Figure 31A).

• Blocking the passage of IR when the sample cell windows become dirty (Figure 31B).

• Decrease in detector sensitivity (Figure 31C).

You must routinely check and adjust the analyzer's zero and span calibration. The ideal zero gas should be that of a typical process sample, but with the measured components removed. Some instruments use sealed calibration gas cells or precision optical filters that they insert into the IR sample beam path.

Typical troubleshooting steps:

1. Disconnect the analyzer from the sample handling system.

2. Run zero and span calibration checks (either gases or automated cells).

Modern NDIR analyzers take advantage of microprocessors, which can identify some types of analyzer faults when they occur and provide you with the diagnostic information. NDIR analyzer faults that require action would be:

replacing faulty IR sources,
cleaning dirty sample cells and cell windows and

• Repalcing faulty IR Detectors

Temperature and Pressure Compensation

Temperature and the pressure of the sample gas affect gas analyzer concentration readings because they change the density of the gas in the sample cell. Older NDIR analyzers used simple temperature and pressure control by:

1. Placing the sample cell inside a temperature controlled, heated metal block.

2. Venting the cell to the atmosphere.

Recent instruments use microprocessors to correct instrument concentration readings for measured changes in sample gas temperature and pressure.