Fuel Gas BTU Analysis and Control Using Process NMR
This application marketed by NMR Process Systems Inc.
For approximately two years, on-line nuclear magnetic resonance (NMR) has been determining BTU, specific gravity, and composition on a fuel gas feed stream to a refinery’s power co-generation plant. Why NMR? Compared to other analytical techniques NMR is non-invasive, nondestructive and independent of the sample’s physical (solid, liquid, gas) or environmental (temperature, pressure) condition. With design improvements in the magnet, electronic, and data processing components of an NMR spectrometer, “portable” NMRs are being utilized in several at line and/or near line applications. By using process line slipstreams, non-invasive NMR analyses overcomes some of the optical path and contamination restrictions experienced with on-line infrared systems and eliminates the need for solvents, columns, carrier gases, and/or separations required with on-line chromatographic systems. NMR analysis is also real-time with typical detection of any protonated analyte at the 0.1-100 percent level.
NMR Fuel Gas Analysis at Texaco’s Los Angeles Refinery: The unit was installed October, 1995, on the refinery’s co-generation unit’s fuel gas stream as a beta test to assess process NMR’s performance, reliability, accuracy, support and maintenance requirements. Measuring BTU, specific gravity, methane content, hydrogen content, and, hydrocarbon content, the NMR compares favorably with the GC. Unlike the GC, process NMR does not require component speciation to determine, BTU and specific gravity. With respect to maintenance and reliability, the process NMR requires no column changes, minimal calibration and minimal manpower in either operating or maintaining the analyzer. The refinery is currently using 2 4-20 mA output signals from the process NMR, one for BTU and one for specific gravity, in their operations. Further, the analyzer has been registered with the Southern California Air Quality Monitoring District, an environmental agency responsible for monitoring and permitting air emission sources in the greater Los Angeles region.
Fuel Gas Analysis: The NMR measures BTU directly, without speciation, by measuring the carbon-hydrogen bond distribution directly, and determining the CH bond quantity by solving a variant of the ideal gas equation, PV=nRT. Initially, pure methane is used to set a calibration curve. In the calibration experiment, the NMR probe volume and methane temperature are known. The integrated intensity of the methane NMR signal is then measured as a function of methane pressure. The calibration curve below gives an example of the NMR response to the change in methane concentration.
Once the calibration curve is established, a fuel gas spectrum is obtained. Each analysis of the fuel gas includes: the NMR data, stream pressure and stream temperature. Since the NMR probe volume is known, a gas equation is used to determine the total concentration of gas in the probe. Since methane is always present in this fuel gas stream, it is used as an internal reference. The integrated intensity of methane in the fuel gas spectrum is compared to the calibration curve to determine the concentration of methane in the fuel gas. This value is then used to obtain the additional carbon-hydrogen bond distribution which is then used to determine the BTU, specific gravity and composition of the fuel gas. The NMR does not observe any species that does not have a hydrogen in its molecular structure. Non-hydrogen containing species are determined by difference.
A typical fuel gas spectrum is shown below. Typical stream conditions are 50 – 55 psia and 20 – 25 degrees C resulting in a total concentration of less than 6 micro moles of gas in the NMR probe. To improve sensitivity, the NMR averages 32 scans per analysis. However, before each scan, a new sample of gas is purged into the probe. Each analysis therefore is the average of 32 samples. Currently, the refinery is performing a fuel gas analysis every half hour. Further, as required by its environmental permit, a methane spectrum is obtained daily, and the measured methane value is compared to the predicted methane value from the calibration curve at the pressures and temperatures obtained during the daily calibration run. Predicted verses measured methane values must be within ±2.5% for the NMR to be within daily calibration. Examples of the daily calibration and fuel gas analysis reports are also shown below.
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NMR Performance: The graph below shows the comparison BTU values between the NMR and the on-line Bendix 9000 GC. Both systems sample the same fuel gas line, with sampling points separated by approximately one hundred feet. Operationally, the NMR has required less than 3 man weeks of support by plant personnel since its installation. Recalculation of the methane calibration curve has been performed twice and have been related to hardware upgrades from the vendor. Since its installation, the system has experienced no NMR related failure. Pressure transducers have failed twice. Loss of water flow to the enclosure air conditioning units occurred three times.
Comparison of NMR BTU vs. Bendix 9000
On-Line GC BTU
Final Design: The purpose of the Los Angeles installation was to demonstrate that process NMR meets and/or exceeds the requirements for reliability, accuracy, on-stream performance, low maintenance/manpower support, etc., when compared to other analyzer technology. In its current configuation, the NMR cannot quantify hydrogen sulfide which is in the fuel gas in the 5 – 100 part per million concentration level. Detection of H2S is necessary for the refinery’s environmental permit, and therefore, the refinery is unable to utilize the NMR as the primary environmental analyzer on the fuel gas. Alternate detection of H2S within NMR system is being investigated. However, the refinery is using the NMR for on-line validation of the primary GC as well as using BTU and specific gravity from the NMR for process control. Finally, with upgrades in software, the NMR will be fully automated for both process analysis and control as shown in the flow scheme below.