EXTRACATIVE FTIR (FOURIER TRANSFORM INFRARED SPECTROSCOPY) MONITORING

GOLDEN currently offers a new state-of-the-art method for monitoring emissions - the Extractive Fourier Transform Infrared Spectroscopy or "FTIR". This method of source monitoring allows the monitoring of several gases at one time utilizing a single instrument. The FTIR will measure any compound that absorbs light energy in the appropriate infrared regions. It will provide a "picture" of the total absorption spectrum of the sample over a broad spectral range. By utilizing a library of spectra the sample may be analyzed for both qualitative and quantitative data, giving the instrument virtually unlimited capabilities.

Background
The Fourier transform is a mathematical process named after Jean Baptiste Joseph Fourier, a French mathematician who lived in the 1800's. He devised a mathematical theorem that expressed a time varying function as a series of sinusoidal terms. The Fourier transform converts time based signal data into a frequency domain spectrum showing the signals frequency content. Infrared (IR) radiation, as used in the FTIR, is essentially light of a long wavelength (beyond what our eyes see). Applying an electrical current across the filament of a light bulb, produces both visible light and infrared light which we sense as heat. Both visible and infrared radiation travel in wave-like motions of certain frequencies or wavelengths. Short wavelengths are visible or ultraviolet light and longer wavelengths are infrared light or radio waves. The plot of various wavelengths and their strengths is called a spectrum. This is what the FTIR produces. When infrared radiation interacts with molecules, the molecules can absorb portions of the radiation causing them to vibrate and/or rotate faster. The exact frequency of radiation that the molecules absorb, causing these vibrations and rotations, is a function of the specific structure of the molecule so it is unique to that molecule. Molecules that absorb infrared radiation show a repeatable pattern of the absorbed frequencies. This spectral pattern is unique to each individual molecule and its intensity is proportional to the concentration of the gas doing the absorption. As a result, FTIR is a unique tool for simultaneously identifying chemical compounds and measuring their respective concentrations.

Specialized Uses for FTIR Monitoring
Monitoring utilizing the FTIR is ideal for process optimization in specialized conditions such as high pressure, high temperature, extreme moisture saturation, and even high carbon dioxide content (>80%). Such sources include reactors, absorbers, Fluidized Catalytic Cracking units and other sources that traditional testing methods produce high biases in the data.

The FTIR is capable of speciating as many as 30 hydrocarbons and other compounds in a single method qualitatively and quantitatively for stack analysis, process, vents, flares, cooling towers, and property lines.

FTIR Design

Figure 1 FTIR Base Unit with IR source, interferometer, power supply, laser control and FTIR electronics.

Figure 2 The Imacc 10 m multi-pass accessory with long path cell, temperature controller, pressure monitor, and IR detector.

The FTIR used is an IMACC extractive system consisting of a Base Unit with a high resolution (0.125 cm-1) interferometer coupled to a multi-pass heated-cell accessory. The Base Unit, shown in Figure 1, contains the interferometer, the power supply, the laser control systems, and the on-board-computer (OBC) which controls the FTIR itself. The FTIR has a "dash-pot" moving mirror which is essentially a graphite piston moving in a precision glass tube. This provides great stability of the instrument because the mirror has only one degree of freedom, that being along the desired direction of travel. However, to correct for any possible variation in the mirror motion caused by vibration or other factors, the fixed mirror is dynamic aligned using the HeNe laser to keep the two mirrors in "perfect" alignment throughout the mirror motion.

The cell accessory, shown in Figure 2, contains the multi-pass cell, the infrared detector, the cell heater controller, and the cell pressure monitor. The cell in the accessory can be either a 1 m to 10m path adjustable cell with an internal cell volume of 2.3 liters. Our first unit has a 10 m cell capable of being heated to 185 C.

The FTIR produces a full spectrum of the infrared (IR) light that passes through it, much like a prism produces a spectrum of visible light. The FTIR is unique, however, in that it produces a whole spectrum from the near-visible to almost micro-waves. It does this simultaneously and with very high signal-to-noise ratio. Buried in the IR spectrum is the absorption "fingerprint" of all gases in the air sample through which the IR beam passes. This is caused by IR radiation interacting with the molecules and the interaction resulting in molecules absorbing specific wavelengths or "colors" of the radiation. The absorption adds energy to the molecule and causes it to vibrate and rotate faster. The vibrations and rotations of molecules are dictated by their structure. This means the patterns of "colors" that are absorbed are also unique to each molecule. The presence of a specific pattern is unequivocal evidence of the presence of a specific compound and the intensity of the absorption is proportional to the concentration of the compound in the path.

The digitized infrared spectrum of the sample in the FTIR gas cell is measured and stored on the FTIR computer hardrive. As discussed previously, features in the spectrum are the "fingerprints of the compounds present. Analysis is performed based on the absorbance spectrum that is the logarithm of the transmission through the FTIR cell.

The absorbance is given by: Ai = ki ci l

The absorbance is obtained by measuring the transmission of light through the cell, then calculating the logarithm. Because absorbance is directly proportional to the absorption coefficient of the gas times the concentration and the path length, the concentration can be obtained by knowing the cell path length and the absorption coefficient. The absorption coefficient is essentially the "reference-standard" of the gas that is recorded on the FTIR hard drive for each molecule. This reference standard has the pattern of the absorption for each gas at a known concentration and a known path length.