Patent Application
20070259440
The subject matter disclosed herein relates to measurements of methane concentrations in gas mixtures containing carbon dioxide.
Carbon dioxide is used in a number of industrial and commercial processes in which accurate, reproducible, and rugged measurements of methane impurities may be needed. These applications include semiconductor processing methods that make use of supercritical carbon dioxide as a more environmental benign alternative to conventional solvents. In addition, carbon dioxide may be used for carbonation of water and other liquids in the beverage industry, for refrigeration and cooling, as an inert gas in chemical processes or as a shield gas in TIG welding, fire extinguishers, in oil fields as a pumped-in gas for oil extraction and maintaining pressure in the drilled formation, as an additive to medical oxygen as a respiration stimulant, as a propellant in aerosol cans, and for sequestration of coal beds
The use of currently available techniques for characterizing methane concentrations in gas mixtures containing carbon dioxide for routine measurements in field applications can be hindered by various factors. For example, maintenance and calibration issues can be cumbersome and costly. In addition, calibration difficulties, detector drift, and somewhat slow response and recovery times can negatively impact the use of such techniques.
Currently available detection methods for methane in carbon dioxide include non-dispersive infrared spectroscopy (NDIR) which may suffer from lower detection limitations as well as absorption band overlap, laser systems for gas detection that use differential absorption of radiation backscattered from topographic targets, and thermal conductivity detectors (TCD) used in concert with a gas chromatograph to separate individual compounds in a gas based on their partitioning between a mobile and a stationary phase. which suffers from cycle time
In one aspect, trace amounts of methane in carbon dioxide backgrounds are detected by directing a beam of light at a selected wavelength through a gas mixture that includes carbon dioxide and methane. The selected wavelength coincides with a methane absorption feature that is resolvable from an absorption background due to carbon dioxide. An absorption at the selected wavelength I quantified in the gas mixture, and a methane concentration in the gas mixture is determined based on the quantified absorption and a calibration function.
In another interrelated aspect, an apparatus includes a laser light source that emits at a selected wavelength that coincides with a methane absorption feature that is resolvable from an absorption background due to carbon dioxide, a sample cell to contain a gas mixture including trace methane in carbon dioxide and having a path length over which light from the laser light source may be absorbed by the gas mixture, a photodetector positioned to quantify an intensity of light traversing the path length and to output a direct current data signal based on the quantified intensity; and a microprocessor configured to receive and interpret the direct current signal from the photodetector and to determine the methane concentration in the gas mixture based on the direct current data signal.
In optional variations, the gas mixture may be contained within a sample cell that provides the path length. The absorption at the selected wavelength may be quantified with a photodetector that provides a detector output signal to a microprocessor. Light may be generated with a range of wavelengths that includes the selected wavelength. In this variation, the generated light may be tuned across the range of wavelengths and a DC signal from the photodetector may be demodulated in the control unit to yield a 2f signal that is analyzed to determine the absorbance at the selected wavelength. The methane concentration may be less than or equal to approximately 50 ppm and the selected wavelength may be in the range of about 1685 nm to 1700 nm. The methane concentration may be less than or equal to approximately 50 ppm and the selected wavelength may be one of approximately 1654 nm, approximately 1687 nm, approximately 1694 nm, and approximately 1697 nm. The methane concentration may be in the range of approximately 1% to approximately 5% and the selected wavelength may be in the range of about 1630 nm to 1660 nm. The methane concentration may be in a range of approximately 1% to 5%, and the selected wavelength may be one of approximately 1637.4 nm, 1640.4 nm, 1642.9 nm, 1645.5 nm, 1648.2 nm, 1650.9 nm, 1653.7 nm, and 1656.5 nm. The beam of light may be provided from a tunable diode laser than is tuned to provide a range of wavelengths comprising the selected wavelength. The gas mixture and the photodetector may be maintained at a constant temperature within a tolerance of approximately ±1° C.
The path length may be equal to or less than approximately 50 cm. Alternatively, the path length may be less than approximately 40 cm. The laser light source may be modulated based on a modulation signal provided by the control unit and the control unit may be configured to demodulate the direct current signal from the photodetector to generate a second harmonic signal that is analyzed to determine the intensity of light traversing the path length at the selected wavelength. The laser light source may be selected from a vertical cavity surface emitting laser, a horizontal cavity surface emitting laser, a quantum cascade laser, a distributed feedback laser, and a color center laser. A thermally controlled chamber may be provided that encloses one or more of the laser source, the photodetector, and the sample cell.
This disclosure may be better understood upon reading the detailed description and by reference to the attached drawings, in which:
Absorption spectroscopy can be used to measure low concentrations of various trace gases in gas mixtures. In general, a light beam of suitable wavelength is passed through a sample of a gas that is contained within a sample cell. As light passes through the gas, some of its intensity is absorbed by trace gas molecules of compounds that absorb at that specific wavelength. The amount of light absorbed is dependent on the concentration (partial pressure) of compounds that absorb at the specific wavelength and can therefore be used to calculate the concentration. This arrangement is suitable when the background gas has no or very weak absorption features in the spectral region being used for the trace gas measurement.
Near infrared radiation generally lacks sufficient photon energy to induce absorption by electronic transitions such as those induced by ultraviolet radiation. Therefore, IR absorption is restricted to compounds with small energy differences in the possible vibrational and rotational states of the molecules. For a molecule to absorb IR radiation, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation interacts with fluctuations in the dipole moment of the molecule. The energy of the incident light radiation is
E=hv (I)
where E is the photon energy, h is Planck's constant and v is the frequency of the light. If E matches the energy necessary to excite a vibrational mode of a molecule, then radiation will be absorbed causing a change in the amplitude of this molecular vibration. The two main types of molecular motion, which includes relative motion between atoms making up the molecule, involve stretching and vibration of inter-atomic bonds.
Stretching transitions require moderate energies and are therefore quite useful to IR absorption spectroscopy. In stretching transitions, the inter-atomic distance changes along bond axes, and the resultant absorbance of IR by gas-phase molecules yield line spectra sufficiently spaced apart to allow detection. In liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions such that they cannot be measured by IR absorption spectroscopy.
The relative positions of atoms in molecules are not fixed, but are rather subject to a number of different vibrations relative to other atoms in the molecule. A specific molecular motion requires a corresponding quantum of activating photon energy. Therefore, an incident photon's energy must be of exactly the right wavelength to be absorbed into the molecule. Thus, if a gas containing a molecule that absorbs and vibrates at a given wavelength λ is illuminated by a beam of light of wavelength λ, some of the incident photons will be absorbed as it passes through the gas. This absorbance Ai,λ is calculated from the beam power incident on the sample P0 and the beam power passing through the sample P as follows:
Ai,λ=−ln(P/P0) (2)
In accordance with Beer-Lambert's Law, the absorbance Ai,λ due to a specific gas-phase compound i at the incident wavelength λ is directly proportional to its concentration Ci in the cell:
Ai,λ=Ciεi,λL (3)
where εi,λ is the extinction coefficient for the compound at the incident wavelength, and L is the path length of the absorption/sample cell.
An analyzer used in connection with the subject matter disclosed here can be used to make measurements of methane in gas mixtures containing a substantial background concentration of carbon dioxide, up to and including a mixture of only methane in carbon dioxide. In general, such an analyzer includes a source of incident light, such as a laser, one or more detectors with sensitivity in the wavelength range of the light source, and one or more absorption cells, each arranged such that the gas provides a path length L though which a beam from the light source passes before reaching the detector. Control electronics, such as a microprocessor, and user accessible input/output channels can also be included. The following is a general description of various examples of such devices and their operation.
In one implementation, a sample of a gas containing methane and carbon dioxide is illuminated by a laser light source that emits light either in a continuous or a pulsed beam. The light source can be a laser such as a tunable diode laser (TDL) or alternatively a fixed wavelength laser light source operating at a specific wavelength chosen to have detectable methane absorption without interference from high concentrations of carbon dioxide. The calibration function can be determined based on analysis of one or more samples of gas containing methane and/or carbon at known concentrations. The samples are analyzed using the analyzer to be used in the above method. Absorption of light at the selected wavelength is quantified for each sample, and a fitting function is applied to relate methane concentration to measured absorption. The calibration function can be a linear relationship or it can alternatively be a more complex mathematical function.
Examples of tunable lasers that can be used are the distributed feedback laser (DFB), the vertical cavity surface emitting laser (VCSEL), and the horizontal cavity surface emitting laser (HCSEL). These lasers can be direct emitters or fiber coupled. Quantum cascade lasers can also be utilized as can other lasers capable of producing a beam of incident light in the desired wavelength range. Additional detail about these types of lasers is available in co-pending U.S. patent application Ser. No. 11/715,599, the disclosure of which is hereby incorporated by reference in its entirety.
An illustrative implementation of an analyzer as disclosed herein is depicted schematically in
A light source 208 that provides light with at least a wavelength where methane absorption can be resolved from a carbon dioxide background generates a continuous or pulsed beam 210 that is directed through the gas volume of the sample cell 202. In the example shown in
If the control unit 220 is electronically connected to the light source 208, it can optionally control the light source. For example, if the light source 208 is a tunable diode laser, such as one of those described in U.S. patent application Ser. No. 11/715,599, the control unit can control the scan rate and also interpret the direct voltage measurements by the photodetector 216 to convert them to modulated 2f values. The control unit can also adjust the modulation amplitude as necessary to improve spectral resolution.
Other analyzer configurations besides that shown in
The path length of the sample cell can be varied depending on the strength of the specific absorption line of interest or the magnitude of the difference between the absorption line of interest and interfering absorption lines from other gas species present. A cell of insufficient length can not provide sufficient sensitivity while one of excessive length can absorb the entirety of the incident light such that no measurable signal reaches the detector (a situation called saturation).
In some cases, the concentration of methane in a carbon dioxide-rich gas mixture can be very small or not readily distinguishable from other components present in the gas. In such cases, the length of the cell can be increased to increase the sensitivity of the measurement. As equation 3 states, Ai,λ is directly proportional to the path length L over which the laser beam traverses the gas mixture. Thus, a cell that is twice as long will absorb twice as much light etc. Therefore, in some implementations of the analyzers described here, sample cells can be employed having path lengths on the order of many meters or even thousands of meters.
To achieve longer optical path lengths without the use of extremely long sample cells, sample cell configurations within the scope of this disclosure can also include the use of one or more mirrors to reflect the beam such that the beam passes through the sample contained in the sample cell two or more times. In such a multipass configuration, the beam can enter and exit the cell through the same window or through different windows. In some implementations, windowless sample cell configurations can be utilized in which, for example, the laser source and/or the photodetector are contained within the sample cell.
One example of such a multipass sample cell configuration is shown in
The light source used for the absorption measurements disclosed can emit in the infrared (for example in a wavelength range of approximately 800 to 10,000 nm). The analyzer can utilize a laser whose spectral bandwidth is much narrower than the bandwidth of the absorption lines of interest. Such an arrangement allows for single line absorption spectroscopy in which it is not necessary to scan the entire width of the absorption line or even the peak absorption feature of the line. The wavelength of the laser can be chosen to be one at which there is a resolvable difference in the relative absorbance of water molecules and the other components of the gas to be measured. In one implementation, the laser frequency can be scanned (tuned) back and forth across the chosen absorption wavelength while a photodetector positioned at the opposite end of the beam path length quantifies the light intensity transmitted through the sample as a function of wavelength.
With the laser absorption spectrometers described herein, the tunable laser wavelength can be varied by changing the injection current while keeping the laser temperature constant. The temperature can be controlled by placing the laser in intimate contact with a thermoelectric cooler (Peltier cooler) whose temperature is measured with a thermistor and controlled by a feedback circuit. The control unit of a device, system, or apparatus as described herein can provide process control functions to regulate the system temperature.
In some implementations, an absorption spectrometer system can employ a harmonic spectroscopy technique in connection with its TDL light source. Harmonic spectroscopy as used in the disclosed subject matter involves the modulation of the TDL laser (DFB or VCSEL) wavelength at a high frequency (kHz-MHz) and the detection of the signal at a multiple of the modulation frequency. If the detection is performed at twice the modulation frequency, the term second harmonic or “2f” spectroscopy is used. Advantages to this technique include the minimization of 1/f noise, and the removal of the sloping baseline that is present on TDL spectra (due to the fact that the laser output power increases as the laser injection current increases, and changing the laser injection current is how the laser is tuned).
In another implementation, direct absorption spectroscopy can be used. In this implementation, the laser frequency is tuned over the selected absorption transition and the zero-absorption baseline can be obtained by fitting the regions outside the absorption line to a low-order polynomial. The integrated absorbance is directly proportional to the concentrations of absorbing species in the laser path length as well as the line strength of the transition. The absolute species concentration can be obtained without any calibration
Photodetectors used in the disclosed absorption spectrometers depend on the specific wavelengths of the lasers and absorption lines to be measured. One photodetector is an indium gallium arsenide (InGaAs) photodiode sensitive to light in the 1200 to 2600 nm wavelength region. For longer wavelengths, an indium arsenide photodiode, sensitive for wavelengths up to approximately 3.6 μm, can be used. Alternatively, indium antimonide detectors are currently available for wavelengths as long as approximately 5.5 μm. Both of the indium devices operate in a photovoltaic mode and do not require a bias current for operation. These photodetectors, which lack low frequency noise, are advantageous for DC or low frequency applications. Such detectors are also advantageous for high speed pulse laser detection, making them particularly useful in trace gas absorption spectroscopy. Other photodetectors, such as for example indium arsenide (InAs), silicon (Si), or germanium (Ge) photodiodes and mercury-cadmium-telluride (MCT) and lead-sulfide (PbS) detectors, may also be used.
In this implementation, the laser temperature is held constant by a temperature controller board 616 and the current varied for tuning the laser wavelength. The temperature control loop uses a thermistor (not shown) located close to the laser 614 as the temperature input and a thermoelectric cooler 620 mounted as close (thermally) to the laser 614 as possible. TECs and thermistors can be positioned either directly adjacent to the laser diode or externally to the laser diode enclosure. The temperature controller 616 can be used to set the exact laser wavelength such that variation of the driving current can provide the tuning range which can, for example, be in the range of approximately ±0.3 cm−1.
At the beginning of each measurement cycle, the current is held to zero to read the signal produced by the photodetector without laser input and thereby provide the zero for that measurement cycle. This zero can vary a small amount due to slight changes in the detector dark current and the electronic noise so it is advantageous to measure it during each detector cycle. Following determination of the zero, the current is rapidly increased to the laser threshold current. This current is then increased over the remainder of the cycle until the peak current is reached. The beam created from this signal is directed through the sample cell 622 and onto the detector 624 which can be a photodiode array or other comparable detector. The output current from the detector is first amplified by a preamplifier 626. The output of the preamplifier is split and sent to a bandpass filter 630 and a lowpass filter 632. The bandpass filter 630 is a narrowband filter that singles out the 2f signal at 15 KHz and directs it to a lock-in amplifier 634 whose reference is set at 15 KHz from a signal provided by the microprocessor. The lock-in amplifier 634 further amplifies the signal and directs it to an A-D board 636 and back into the microprocessor 602. The lowpass filter 632 provides the detector output except the 2f signal. This signal provides the microprocessor 602 with the zero for the system and is also a diagnostic tool.
The signal is developed and recorded by the microprocessor 602 for each cycle of the analyzer. The processor determines the concentration of the gas sample of interest by computing the absorbance of the gas as a ratio between the zero and the measured value of absorbance at the peak of the absorbance line. The absorbance is a function of the gas pressure and temperature in the sample cell 622 which are measured by appropriate means 642 and 644, respectively, whose outputs are supplied to the A/D board 636. The absorbance can be adjusted by a pressure/temperature calibration matrix stored in the microprocessor memory 644. This matrix is developed on an analyzer-by-analyzer basis. Alternatively, one or more corrective calculations can be performed based on measured temperature and pressure in the sample cell.
Once the corrected absorbance value is determined, the concentration can be computed using equation 3. In one implementation, this concentration can be converted into units of, for example lbs/mmscf, averaged four times, and sent to the outputs once per second. Outputs that can be included in this system are a 4-20 mA current loop 646, a visual display 650 and RS-232 or comparable serial ports 652 and 654. Power for the system is provided by an appropriately chosen power supply 656.
The chart of laser current vs. time 700 shown in
The specific absorption transitions for measurement of CH4 in various background gases which contains CO2 are summarized in Table 1. However, other wavelength ranges may be utilized provided that methane molecules absorb light at a greater level than do background gas molecules. Selection of an individual absorption line may be completed using a Figure of Merit (FOM), which is defined as the absorption of 1 ppmv methane divided by the total background absorption. All methane lines satisfying an FOM of approximately greater than or equal to 1×10−6 are suitable for sensitive detection of methane.
Spectrometers described herein can be used to accurately and repeatedly measure low levels, for example at concentrations of less than approximately 1% or alternatively less than approximately 5% of methane in carbon dioxide. Measurement of concentrations of methane in carbon dioxide gas in a range of approximately 1% to 5% methane can be performed at a wavelength of approximately 1645 nm using TDLAS spectroscopy with an absorption path length that is less than or equal to approximately 40 cm and at approximately atmospheric pressure. Specific methan transitions near 1645 nm are listed in table 1. As can be seen from
It is also possible to quantify methane in carbon dioxide at concentrations less than or equal to approximately 50 ppm. The technique has been found to be successful at wavelengths between 1685 and 1700 nm with an absorption path length of less than or equal to approximately 50 cm as shown in the absorbance chart 900 in
The subject matter described herein can be embodied in systems, apparatus, methods, and/or articles depending on the desired configuration. In particular, various aspects of the subject matter described herein can be realized in digital electronic circuitry, integrated circuitry, specially designed application specific integrated circuits (ASICs), computer hardware, firmware, software, and/or combinations thereof. These various implementations can include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which can be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device.
These computer programs (also known as programs, software, software applications or code) include machine instructions for a programmable processor, and can be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the term “machine-readable medium” refers to any computer program product, apparatus and/or device (e.g., magnetic discs, optical disks, memory, programmable logic devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions as a machine-readable signal. The term “machine-readable signal” refers to any signal used to provide machine instructions and/or data to a programmable processor.
Although a few variations have been described in detail above, other modifications, additions, and implementations are possible are within the scope and spirit of the disclosed subject matter. Features implementing functions can also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flow depicted in the accompanying figures and/or described herein do not require the particular order shown, or sequential order, to achieve desirable results.
The present patent application claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application Ser. No. 60/793,402, filed on Apr. 19, 2006, and entitled “Measurement of Low Levels of CH4 in CO2”, the entire disclosure of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 60793402 | Apr 2006 | US |
