Article pubs.acs.org/EF
Optimization of an Analytical Method for the Measurement of Oil Carryover from a Compressor in Compressed Natural Gas Refueling Stations Karine Arrhenius,* Doris Podien, Haleh Yaghooby, and Nijaz Smajovic Department of Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, Brinellgatan 4, Post Office Box 857, SE-501 15 Borås, Sweden ABSTRACT: The aims of the study were to determine the best method for extracting oil absorbed on coalescing filters at compressed natural gas (CNG) refueling stations and to compare the mass spectrometer (MS) and flame ionization detector (FID) for the quantification of the oil recovered in the extracts. Dichloromethane and heptane as solvents gave slightly higher recovery yields than pentane. The preferred extraction method with regard to time and solvent consumption consisted of an ultrasonic extraction, followed by removal of the remaining solvent under a stream of nitrogen. The FID and MS were found to be equally suitable for quantifying oil carryover, if the sample only contained the target oil when the instruments of analysis have been properly calibrated. If the sample is contaminated by compounds other than the target oil, MS and FID will provide different valuable information: MS may give information on the structure of the contaminants, while FID will give a more reliable quantification without proper calibration. The work discusses issues with the reusability of the filters and how to handle the memory effects.
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Existing Methods of Extraction and Measurements of Oil Carryover. Several methods have been proposed for measuring oil carryover in gas, and their complexity often depends upon whether it is necessary to measure the dissolved and aerosol oil phases separately or not. The Institute of Gas Technology (IGT, Des Plaines, IL) has developed a method7 comprising high- and low-pressure sample lines, with each sample line being equipped with a highly efficient coalescing filter. The oil as aerosol is adsorbed on the high-pressure device, while the oil as vapor passes through. Thereafter, the pressure and temperature are lowered and the oil as vapor will condense and become an aerosol adsorbed on the low-pressure device. The oil retained on the filters is then extracted, and the samples are analyzed by gas chromatography with a flame ionization detector (GC/FID). This method has been evaluated as giving reliable results but is estimated to be too complicated and costly to be used as a standard field test protocol.7 To overcome the cost barrier, a simpler method has been developed by Atlanta Gas Light (AGL, Atlanta, GA)8 using a gravimetric collection device for oil in which a tube filled with an adsorbent (60/80 mesh Chromasorb P NAW) is connected to a high-pressure line. In this case, it is not possible to measure oil as aerosol and oil as vapor separately. The oil content is determined as the weight difference of the tube before and after sampling (after heating to remove water) divided by the volume passing over the device. This method has been compared to the IGT method described above. At low levels of oil carryover (for example, when polyglycol oil is used), the gravimetric method has been found to overestimate the oil
INTRODUCTION Gaseous fuels, including biomethane, are one of the accepted alternative fuels in the world today because of their positive environmental impact in terms of carbon footprint and emissions compared to conventional fossil fuels, such as gasoline and diesel. Compressed natural gas (CNG) technology has been developed and applied for decades, and it is becoming a mature and applicable technology. Biogas used as automotive fuel presents even better environmental characteristics than natural gas.1 However, operational disturbances and malfunctions attributable to oil, water, and impurities, such as siloxanes and sulfur compounds (for biomethane), are a known problem.2 Compressor oil can be entrained into the final product; these oil slips reach the natural gas vehicle (NGV) engines and can cause operational problems with pressure regulators and gas injectors. Therefore, the oil levels need to be monitored and controlled by means of oil removal downstream of the compressor.3 Oil is carried by the compressed gas in two forms: as an aerosol, which is formed by mechanical shearing in the compressor, and as a vapor, which is formed during oil vaporization and absorption in natural gas/biomethane.4,5 The use of coalescing filters to remove liquids and aerosols from gases is a well-known, reliable, and proven technology for reducing or even practically completely removing oil aerosol carryover.6 However, compressed gas exiting lubricated compressors also carries oil as a vapor. Dependent upon their composition, oils are more or less susceptible to being partially absorbed (dissolved) in natural gas, mostly when the natural gas is at supercritical state and, thus, acting as a solvent. The vapor oil cannot be filtered by coalescing filters unless the pressure and/or temperature is lowered, causing the vaporized oil to condense and form a very fine aerosol. © XXXX American Chemical Society
Received: December 8, 2014 Revised: March 6, 2015
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DOI: 10.1021/ef5027423 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. Scheme of the sampler used in this study.
The sampler, schematically presented in Figure 1, consists of a NGV1 standard connection, 1/2 in. tubing, a manometer, three ball valves (Oasis Engineering, Ltd., New Zealand), a 12.5 L composite CNG bottle, and two EU37/25 filter housings with a 100-25-BX filter connected in series after a spray nozzle, with a 0.3 mm diameter hole. The sampler is connected to the dispenser via the NGV1 connection. A refueling process is started and stopped manually (if necessary) when the pressure in the bottle has reached 180 bar [which corresponds to about 2 m3 (norm.) of sampled gas]. The gas is then led through the coalescing filters by opening the two other ball valves. As the gas passes through the hole of the nozzle, the pressure drops, resulting in a temperature drop, and the oil is trapped on the filter. Sampling is stopped when the pressure in the bottle reaches 100 bar (equivalent to 1 m3 sampled gas). At least, three samples are taken at each refueling station. The extract is then analyzed by GC and either a flame ionization detector (FID) or a mass spectrometer (MS). This work presented here describes the procedure to extract oil from the filters and practical aspects as reusability of the filters. The work also compares results obtained with a FID and with a MS. Finally, we discuss memory effects and the need to clean the sampler between samplings.
carryover. The difference is explained by the authors as probably due to the fact that a part of the increase in weight is not solely due to oil. It may be caused by heavier hydrocarbons naturally occurring in natural gas (also present in biomethane). At higher levels of oil carryover, the gravimetric method has been found to underestimate oil carryover, showing that this device is probably less effective in capturing oil vapors than the cryotrap/coalescing filter used in the reference method (IGT). The German Association for Gas and Water (DVGW, Germany) has initiated two projects8 for determination of oil and particle in gas refueling stations. In the first project, they have developed a gravimetric method to determine the oil and particle contents. The modular sampling equipment included a filter system where the pressure drop was very limited and a gas tank allowing for sampling even when there is no access to a vehicle. The chosen filter was a Zander gas filter TB 20 CE. In the second project, a number of refueling stations across Germany were visited to determine oil and particle contents in CNG. New Method for Measuring Oil Carryover. Recently, we developed a sampling and analytical method to measure the oil carryover from a compressor.9 This method is based on the fact that, when the pressure is drastically reduced on an adsorbent (which implies that the temperature of the gas also drops), the oil condenses as droplets and deposits on the adsorbent. The method, presented in detail elsewhere,9 is only described in brief here.
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EXPERIMENTAL SECTION
Extraction Procedure. The coalescing filters that were used are Parker (Balston 100-25-BX) filters made of borosilicate glass microfibers with fluorocarbon resin binders. In its principal B
DOI: 10.1021/ef5027423 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. Chromatograms for the method blanks obtained with (a) dichloromethane and (b) pentane. At the end of the first extraction, at least 30 mL of solvent was left on the filters. An apparatus has been developed to recover the remaining solvent. This apparatus consists of a EU37/25 filter housing, where the filter is introduced directly following the ultrasonic extraction; therefore, the solvent left on the filter cannot evaporate to air. Nitrogen is introduced on the top of the filter house to force the solvent out of the filter. The solvent is recovered on the bottom of the filter housing. Analysis of the Extract. The analyses of the extract were performed on GC coupled with a FID and a MS. GC (Agilent, 7890A) was equipped with a split/splitless injector (injector temperature of 400 °C) and two fused-silica capillary columns (VF-5HT Ultimetal, 30 m, 0.25 mm inner diameter, 0.1 μm film, Agilent Technologies). The temperature program was an initial temperature of 35 °C for 5 min, a rate of 10 °C/min, and a final temperature of 400 °C for 5 min; for the mass selective detector (Agilent, 5975C), the ion source temperature was set to 230 °C. The MS was operated in the electron impact (EI) ionization mode at 70 eV. The GC/MS/FID was calibrated by diluting known amounts of oil (from 0.5 to 10 mg) in dichloromethane.
application, the microfibers capture the fine liquid droplets suspended in the gas and cause the droplets to run together to form large drops within the depth of the filter cartridge. The large drops are driven by the gas flow to the downstream surface of the filter cartridge, from which the liquid drains by gravity. This process is called “coalescing”. In this study, the filters were used to retain the oil that condensed as the result of the pressure drop created when the gas is forced through a 0.3 mm diameter hole at high pressure. The oil that was chosen for the recovery test study is Rarus SHC 1025 (Exxon Mobil Corporation), which is a synthetic lubricant commonly used in compressors in Sweden. The coalescing filter was introduced into a 500 mL measuring cylinder, which was subsequently filled with a solvent covering the whole filter. The oil was extracted using an ultrasonic bath for 30 min. The number of extractions needed is discussed in the Results and Discussion. The solvents used in this study were dichloromethane (Merck, Lichrosolv; purity ≥ 99.9), heptane (Sigma-Aldrich, Chromasolv; purity ≥ 96.0), and pentane (Merck, Emplura; purity ≥ 99.0). The extract was then concentrated to approximately 10 mL by evaporation in a rotary evaporator (Stuart, RE300). C
DOI: 10.1021/ef5027423 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Recovery Results in Percentage with Different Solvents recovery (%)
extraction 1
extraction 2
extraction 3
total
dichloromethane dichloromethane/heptane (1:1, v/v) heptane pentane
74 79 75 68
20 24 23 18
2 2 3 1
96 105 101 87
Table 2. Recovery Results with Different Extraction Procedures recovery (%) solvent
method
extraction 1
extraction 2
total
dichloromethane dichloromethane
two ultrasonic extractions (1) ultrasonic extraction (2) nitrogen flush two ultrasonic extractions (1) ultrasonic extraction (2) nitrogen flush
74 69
20 20
94 89
68 72
20 15
88 87
pentane pentane
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RESULTS AND DISCUSSION Blank Filters. To evaluate whether the materials in the filter is free from interference under the conditions for the procedure, method blanks have been analyzed. A non-field exposed filter sample has undergone the procedure in a manner identical to that used for field samples. Two solvents have been tested: dichloromethane and pentane. The chromatograms obtained are presented in Figure 2. With dichloromethane, when using a MS and extraction of specific ions for quantification, the method blank was found not to contain the target analytes at a detectable level. When using a FID, the method blank was found to contain analytes at a level less under the lowest quantifiable level by at least a factor of 2. With pentane, when using either a MS with extraction of specific ions for quantification or a FID, the method blank was found not to contain the target analytes at a detectable level. Recovery Tests. The filters were spiked with a known amount of compressor oil (typically 20 mg) and left on overnight. Two tests were performed with each solvent, and the results are the average value. Three extractions were performed. After the first and second extractions, the extract was removed from the measuring cylinder, 250 mL of solvent was added to the measuring cylinder containing the filter, and a second (or third) extraction was performed. The results from the recovery tests using different solvents are presented in Table 1. The results show that two extractions are necessary to recover the oil quantitatively. Different solvents can be used. With pentane, the recovery yields are slightly lower than with the other solvents tested, but concentrating the extract using a rotary evaporator takes about half the time needed when evaporating the other solvents. Removing the solvent remaining on the filters under a gas stream, after the first extraction, was tested as an alternative for the second extraction to reduce the volume of extraction solvent. The solvent left over on the filters was removed under a flow of pure nitrogen. The extract collected using this method was subsequently concentrated to approximately 10 mL using a rotary evaporator. The results of the tests are presented in Table 2. The results show that the two methods (two extractions versus one extraction and removing the remaining solvent under a flow of nitrogen) give raise to comparable results in terms of recovery yields (around 90%); the second method is
less time-consuming and requires less solvent but requires a special apparatus. Reusability of the Filter. Nine filters were spiked with 1 mg (filters A1, A2, and A3), 5 mg (filters B1, B2, and B3), and 20 mg (filters C1, C2, and C3) of the target oil. They were subjected to the entire analytical procedure, including a third extraction. The filters were then stored for 1 week in a clean and dry environment. The filters were then spiked again with 5 mg of the target oil and subjected to the entire analytical procedure. The results of the recovery tests to assess the reusability of the filters are presented in Table 3: Table 3. Recoveries Obtained on Filters That Were Previously Spiked with Different Amounts of Oil recovery (mg) filters
extraction 1
extraction 2
total
series A series B series C
3.5 ± 0.3 3.6 ± 0.3 4.2 ± 0.4
1.1 ± 0.2 1.0 ± 0.2 1.2 ± 0.2
4.6 ± 0.5 4.6 ± 0.5 5.4 ± 0.6
The results show that the recovery is slightly but significantly higher on filters that had previously been spiked with 20 mg of oil compared to filters that were spiked with 1 and 5 mg of oil. The results tend to show that filters that have once been exposed to relatively high amounts of oil should be discarded; otherwise, they may lead to an overestimation of the oil carryover. Comparison MS/FID. Results for oil carryover obtained with a MS have been compared to results obtained with a FID for eight samples obtained at CNG refueling stations. The oils used in the compressors were formulated with either polyaliphatic olefins (PAOs) or alkylated naphthalenes (ANs) as base stock. When quantifying with a FID, the integration was performed on a defined time interval, with the interval being determined from the chromatograms for the standard solutions. When quantifying with a MS, some specific ions characteristic for each oil were extracted and quantified. Oils including lubricants often appear in GC as an unresolved complex mixture, i.e., as a large background/ platform.10 This is the case, for example, for oils that have PAOs as stock base. However, this is not the case for some of the oils formulated with ANs,11 which appear as a limited number of peaks relatively well separated from each other. AN D
DOI: 10.1021/ef5027423 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels oils are most easily prepared by Friedel−Crafts alkylation of naphthalene with an alkylating agent, often an olefin. The alkylation of naphthalene with an olefin results in a mixture of molecules. For oils with ANs, the specific ion was m/z 155, which gave raise to two distinct peaks, and for oils with PAOs, the targeted ion was m/z 57, which appears as an unresolved complex mixture. The chemical structure of the two ions is presented in Figure 3.
Figure 5. Comparison of results obtained with FID and MS using specific ions (y = 0.8964x; R2 = 0.996). The red dot is an outlier. Figure 3. Structure of the specific ions for (A) AN oils and (B) PAO oils.
retention time and additional spectral data as well as an enhanced ability to separate coeluting peaks based on unique ions.12 At one of the refueling stations, peaks that did not correspond to compounds of the compressor oil used at the station eluated at the same time as the peaks for the target oil. The results from the MS (12 ppmM) thus gave more accurate information on the carryover from the compressor oil, while the results from the FID (27 ppmM) gave information about the total contamination of the gas (at least regarding compounds eluating in the same time interval). Generally, FID is more reliable without proper calibration because most of the compounds, independent of the nature of the compounds, will give a similar response. However, in this particular case, the results obtained by integrating the signal over a defined interval of time with a MS and a FID were in good agreement (25 ppmM with a MS and 27 ppmM with a FID). These results also suggest that the investigator may study the whole chromatogram when using a MS or a FID, to be able to observe any contamination of the gas by compounds other than the compressor oil. Memory Effects. The risks for memory effects were assessed in the following matter: the stations were visited so that the type of oil was not the same 2 consecutive times. If the previous station was using an AN oil, the station visited thereafter was using a POA oil. At all stations, both types of oils were quantified. The memory effects were found not to be negligible when using AN oils. Up to 5 ppmM of AN oil was found at stations only using POA oils, while not more than 2 ppmM POA oil was found at stations using AN oils. The contamination was logically the highest after sampling at stations having a higher oil carryover. The contamination occurred most likely when the buffer tank is emptied at the end of all samplings performed at one station. A special device was then constructed to clean the buffer tank between samplings. The device consisted of a NGV1 nozzle, a filling line, and a DIN 477 connector on the other side of the line, which can be connected to a bottle of nitrogen, allowing the sampler to be flushed with nitrogen.
The linearity of response was found to be equally good for FID and MS (R2 > 0.99), as shown in Figure 4.
Figure 4. Comparison of the linear calibration curves obtained with (A) GC/MS (y = 1 × 107 − 4 × 106; R2 = 0.9901) and (B) GC/FID (y = 1 × 108 + 2 × 107; R2 = 0.9913).
Results obtained with the MS have been compared to results obtained with a FID, as presented in Figure 5, with the red dot being an outlier. The results show a good correlation between the results obtained using these two detectors. On average, the results obtained with the FID are 10% higher than the results obtained with the MS, which typically provides greater selectivity and sensitivity compared to GC/FID. The quantification with FID is more likely to be influenced by the instrument background. Moreover, the GC/MS method provides the ability to confirm compounds based on both the
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CONCLUSION Compressor oil at CNG/biogas refueling stations can be entrained into the vehicle gas; these oil slips reach the engines and can cause operational problems with pressure regulators E
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(4) Fornof, W. P. Compressor oil carryover and its effect on the pneumatic system. SAE Tech. Pap. Ser. 1999, DOI: 10.4271/1999-013768. (5) Adams, R.; Home, D. B. Compressed Natural Gas (CNG) Transit Bus Experience Survey; National Renewable Energy Laboratory (NREL): Golden, CO, 2010; Report NREL/SR-7A2-48814. (6) Mead-Hunter, R.; King, A. J. C.; Mullins, B. J. Aerosol-mist coalescing filtersA review. Sep. Purif. Technol. 2014, 133, 484−506. (7) Czachorski, M.; Kina, R. Validation Testing of a Gravimetric Method To Measure CNG Compressor Oil Carryover; Gas Research Institute (GRI): Chicago, IL, 1998; Report GRI-98/0228. (8) Graf, F.; Riedl, J.; Kröger, K.; Reimert, R.; Meyer, J. Monitoring CNG quality in Germany. Proceedings of the 24th World Gas Conference; Buenos Aires, Argentina, Oct 5−9, 2009. (9) Arrhenius, K.; Yaghooby, H.; Klockar, P. Development and Validation of Methods for Test of CNG Quality Inclusive of Oil Carryover; Svenskt Gasteknikt Center (SGC): Malmö, Sweden, 2013; SGC Report 290. (10) Gough, M. A.; Rowland, S. J. Characterization of unresolved complex mixtures of hydrocarbons in petroleum. Nature 1990, 344, 648−650. (11) Hourani, M. J.; Hessel, E. T.; Abramshe, R. A.; Liang, J. Alkylated naphthalenes as high-performance synthetic lubricating fluids. Tribiol. Trans. 2007, 50, 82−87. (12) Dodds, E. D.; McCoy, M. R.; Rea, L. D.; Kennish, J. M. Gas chromatographic quantification of fatty acid methyl esters: Flame ionization detection vs. electron impact mass spectrometry. Lipids 2005, 40, 419−428.
and gas injectors. Therefore, the oil levels need to be monitored and controlled. Recently, we developed a method to measure the oil carryover at CNG refueling stations, in which the oil is absorbed on coalescing filters. The study presented here was performed to optimize the extraction and quantification of oil absorbed on the filters. The aim of the optimization procedure was to improve the extraction efficiency with the minimum solvent and time consumption. The recommended method uses an ultrasonic extraction, followed by the removal of the solvent remaining on the filters using a stream of pure nitrogen. The extract is then concentrated prior to analysis. In the optimization of ultrasonic extraction, different solvents were compared, regarding the extraction efficiency by comparing the recoveries. Dichloromethane and heptane were found to give higher recovery yields than pentane, but the evaporation time was found to be much longer with heptane. Blank levels from method blanks were found to be lowest with pentane. The results of the tests performed to assess the possibility to reuse the filters show that filters that have once been exposed to relatively high amounts of oil should be discarded; otherwise, they may lead to an overestimation of the oil carryover. The study presented here also showed that the analysis can be performed by GC coupled to either a MS or a FID. Results obtained with both detectors show good agreement. On average, the results obtained with the FID are 10% higher than results obtained with the MS, with the quantification with FID being more likely to be influenced by the instrument background. The results presented in this study also suggest that the investigator must perform a critical visual control of the obtained chromatogram to be able to observe any contamination of the gas by other compounds than the compressor oil.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +46-0-10-516-57-28. E-mail: karine.arrhenius@sp. se. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This paper is written under the frame program co-operation program Energy gas technology coordinated by Svenskt Gastekniskt Center AB (SGC), and the authors acknowledge the funding of this programme financed by the Swedish Energy Agency and industrial partners: E.ON Gas Sverige AB, Ö resundskraft AB, Fordonsgas Sverige AB, Scania CV AB, Borås Energi och Miljö AB, Processkontroll GT, Keolis Sverige AB, and Nobina Sverige AB.
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REFERENCES
(1) Plombin, C.; Hugosson, B.; Landahl, G. Biogas as Vehicle FuelA European Overview; Trendsetter: Stockholm, Sweden, 2003; Trendsetter Report 2003:3. (2) Thomason, L. The Achilles Heel of Natural Gas Vehicles: The Symptoms, Diagnosis and Prevention of Oil Carryover; Natural Gas Vehicle Institute (NGVi): Las Vegas, NV; http://www.ngvi.com/ Documents/ Oil%20Carryover%20%20Symptoms%20Diagnosis%20Prevention_ NGVi.pdf. (3) Novosel, D. Best Practice for Controlling Content of Oil, Water and Sulfur in CNG at Refueling Station Level; Svenskt Gasteknikt Center (SGC): Malmö, Sweden, 2013; SGC Report 291. F
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