Understanding the Si licon Issue

Transcription

1 Understanding the Si licon Issue Lately, there has been a great deal of discussion within the petroleum and biofuels industry about the dangers of silicon in gasoline and ethanol. What s fueling the recent chatter? Silicon is a contaminant and, depending on where the contamination occurs along the supply chain, the repercussions can range from engine problems for gasoline end users, to possible poisoning of naphtha hydrotreater catalysts at the refinery level. These issues have attracted the attention of ASTM to the point where the standard setting body has added a warning about silicon contamination in the Workmanship Section of the gasoline and ethanol specifications. ASTM warnings, specifications, and test methods. ASTM Workmanship Section of D4814 states, Manufacturers and blenders of gasoline and gasoline-oxygenate blends shall avoid gasoline blending stocks (for example, purchased used toluene solvents) or denatured fuel ethanol (for example, improperly recycled ethanol) contaminated by silicon-containing materials, or both. Silicon contamination of gasoline and gasoline-oxygenate blends has led to fouled vehicle components (for example, spark plugs, exhaust oxygen sensors, catalytic converters) requiring parts replacement and repairs. 1 The ethanol specifications have similar warnings. See the workmanship section in each the following specifications for warnings about silicon: Section ASTM D4806 Standard Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel 2 Section ASTM D4814 Standard Specification for Automotive Spark-Ignition Engine Fuel Section ASTM D5798 Standard Specification for Fuel Ethanol (Ed75-Ed85) for Automotive Spark-Ignition Engines 3 Despite these warnings, no ASTM method for testing silicon in gasoline and ethanol existed until now. The recently approved and published ASTM D Standard Test Method for Silicon in Gasoline and Related Products by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry paves the way for ASTM-approved and accurate silicon measurement. As discussed below, the Signal Analyzer from XOS enables this method and is now available to make these measurements. 15 Tech Valley Drive East Greenbush, New York USA pg. 1

2 This new test method falls under the jurisdiction of ASTM D02 on Petroleum Products and Lubricants and is the direct responsibility of Subcommittee D02.3 on Elemental Analysis. 4 Kishore Nadkarni, Ph.D., of Millennium Analytics, Inc., and chairman of D02.03, notes that D7757 will fill industry needs and that it is already being used successfully for determining silicon concentration in gasoline, gasoline-oxygenate blends, denaturants, and hydrocarbon blend components and denatured fuel ethanol. 5 ASTM DO2 has been a proponent of this test method, and work is underway to take the confirmatory step of adding D7757 as a referenced method in the ASTM gasoline and ethanol specification even though its use is already approved by ASTM for measuring silicon in these fuels. Damage associated with silicon contamination. Silicon contamination of gasoline leads to silica deposits on vehicle components such as spark plugs, catalytic converters, and oxygen sensors. 6 Improper feedback or lack of feedback from failing oxygen sensors can lead to incorrect control of engine air/fuel mixture, which may cause issues such as: Rough engine idle, missing, and pinging Poor fuel economy Increased emissions Stalling or not starting Catalytic converter damage 7,8,9 New O2 sensor (left) and fouled O2 sensors (middle and right). Silicon dioxide forms a whitish deposit similar to the sensor in the middle. Silicon contamination in petroleum refineries may also have a negative impact, including poisoning of naphtha hydrotreater catalysts. When silicon-contaminated naphtha is processed in the hydrotreater, silicon compounds are irreversibly adsorbed onto catalyst surfaces. Over time, this results in reduced desulfurization activity and decreased catalyst life. Ultimately, the catalyst cannot be regenerated. 10 Where does the silicon come from? Recycled toluene. In a 2007 United Kingdom (UK) incident, toluene used to wash silicon chips and other electronic components during manufacturing was recycled and used in gasoline blending to increase gasoline octane. This resulted in silicon-contaminated gasoline at the pump and thousands of automobile failures. 11 Ethanol. In a 2009 United States (US) incident, silicon contamination occurred with the addition of fuel ethanol to gasoline. Again, the result was widespread vehicle failure. In this instance, it is not clear whether the ethanol was recycled from the cosmetics industry, where it may have come in contact 15 Tech Valley Drive East Greenbush, New York USA pg. 2

3 with silicon-containing antifoam agents commonly used in cosmetics, or if the ethanol was manufactured using silicon-containing antifoam agents. 12 In both the UK and US incidents, silicon was introduced accidentally as a contaminant in a gasoline blending component. As these examples demonstrate, multiple points along the supply chain are vulnerable to silicon contamination. Antifoam agents. Silicon contamination due to antifoam agents is not exclusive to the ethanol industry. Antifoam agents are often used to minimize foaming at the coker, and they also may be used in crude oil extraction. These antifoam agents form breakdown products that end up in the naphtha fraction either through crude distillation or cracking in the coker. 13 Silicon oils are excellent antifoam agents. Known as polymerized siloxanes, silicon oils can simply be described as a chain of alternating silicon and oxygen atoms that form a backbone to which organic side chains (R) attach. The siloxane monomer unit or building block, depicted in the following figure, forms the backbone of the polymerized siloxane, where n is the number of monomer units in the siloxane polymer. Polydimethylsiloxane (PDMS), commonly used in antifoam agents, breaks down into cyclic siloxanes. These compounds are what is primarily found in contaminated gasoline and ethanol. Octamethylcyclotetrasiloxane (also known as D4) is one of the predominant cyclic siloxanes found in these contamination issues. 14,15 Combustion of D4 (see graphic below 16 ) in an automotive engine forms silicon dioxide (SiO 2 ), or silica, which forms a hard, white deposit on spark plugs, oxygen sensors, and other components. 17 The silicon path to engine trouble Silicon Monomer (found in silicon oil) PDMS (commonly used in antifoam agents) SiO 2 Cyclic Siloxane or D4 (found in contamination cases) Combustion of D4 results in silica deposits The silicon monomer (first compound) is the building block of PDMS (second compound). PDMS, a common silicon oil, is used in antifoam agents. PDMS breaks down into cyclic siloxanes such as D4 (third compound), and these compounds are the silicon contaminants found in gasoline and ethanol. D4 combusts in the gasoline engine, forming silicon dioxide (fourth compound). 15 Tech Valley Drive East Greenbush, New York USA pg. 3

4 How much silicon does it take to contaminate fuel? Unfortunately, no one knows for certain. In the 2009 US incident, the contaminated fuel ethanol contained more than 100 parts per million (ppm) of silicon, leading to more than 10 ppm of silicon found in the gasoline-ethanol blend samples taken from the affected gasoline stations. 18 Now, there is an ASTM-approved test method for measuring silicon in gasoline and ethanol. The XOS Signal Analyzer is D7757 compliant and, because it is based on the same proven technology platform as the Sindie 7039 Sulfur Analyzer, it is easy to use and delivers fast, accurate measurements. Testing for silicon. 19,20 ASTM D7757 is the only ASTM-approved test method for the determination of silicon in gasoline and ethanol. The scope of the method is for mg/kg (weight ppm) of silicon in the following fuels: Naphthas Gasoline RFG Ethanol Ethanol-fuel blends Toluene D7757 is a monochromatic wavelength dispersive x-ray fluorescence (MWDXRF) method, and, like other MWDXRF methods, sample preparation is minimal, and the measurement is non-destructive, with a typical analysis time of five to ten minutes per sample. Method calibration is a weighted linear regression, based on five calibration standards. To account for the matrix differences in gasoline, ethanol, and gasoline-ethanol blends, it is recommended to set up an isooctane calibration curve and/or an ethanol calibration curve (if needed) and use correction factors to account for matrix differences. Matrix correction factors are provided in the test method. A recent interlaboratory study was performed to determine test method precision of ASTM D7757 using Signal Analyzers from XOS. This study included six laboratories with participants from petroleum refineries and research labs, a third-party test lab, a government contractor, and an automobile manufacturer. The participants analyzed 26 samples in duplicate, and the sample set was comprised of gasoline, gasoline with 10% ethanol, naphtha, toluene, E85, and E100. Table 1 shows the calculated values. Table 1 Precision Values, All Sample Types Si, mg/kg (ppm) Repeatability r, mg/kg (ppm) Eq. 1 values Reproducibility R, mg/kg (ppm) Eq. 2 values The pooled limit of quantification (PLOQ) was estimated to be 3 mg/kg. 15 Tech Valley Drive East Greenbush, New York USA pg. 4

5 The calculated values in Table 1 are derived from the following equations: Repeatability (r) = * X^ (Eq. 1) Reproducibility (R) = * X^ (Eq. 2) where X is the silicon concentration in mg/kg silicon. For more information about the test method or the interlaboratory study, test method D7757 and research report RR:D may be obtained from ASTM at XOS Signal Analyzer XOS, widely known for its Sindie 7039 Sulfur Analyzer, manufactures the Signal Analyzer for the testing of silicon in petroleum products and related biofuels. The Signal is the only analyzer that is ASTM D7757 compliant. XOS can help you meet your testing needs. Call ext. 407 to speak with one of our silicon experts or signal@xos.com for more information. Silicon analysis in petroleum and biofuels 15 Tech Valley Drive East Greenbush, New York USA pg. 5

6 1 ASTM Standard D4814, 2011b. Standard Specification for Automotive Spark Ignition Engine Fuel. ASTM International. West Conshohocken, PA DOI: /D B (19 Sept. 2012) 2 ASTM Standard D4806, 2011a. Standard Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark Ignition Engine Fuel. ASTM International. West Conshohocken, PA DOI: /D A (19 Sept. 2012) 3 ASTM Standard D5798, Standard Specification for Ethanol Fuel Blends for Flexible Fuel Automotive Spark Ignition Engines. ASTM International. West Conshohocken, PA DOI: /D (19 Sept. 2012) 4 ASTM Standard D7757, Standard Test Method for Silicon in Gasoline and Related Products by Monochromatic Wavelength Dispersive X ray Fluorescence Spectrometry. ASTM International. West Conshohocken, PA DOI: /D (19 Sept. 2012) 5 Silicon in Gasoline. Standardization News. Sept./Oct. 2012, ASTM Standard D David McGuffin. What Would a Car With a Bad Oxygen Sensor Do? ehow. bad oxygen sensor do_.html (19 Sept. 2012) 8 Paul Novak. Bad Oxygen Sensor Symptoms of a TBI Motor. ehow. symptoms tbi motor.html (19 Sept. 2012) 9 Jason Medina. Common Symptoms of a Bad Oxygen Sensor. ehow. 19 Dec symptoms bad oxygen sensor.html (19 Sept. 2012) 10 J.M. Britto, M.V. Reboucas, and I. Bessa. Troubleshoot Silicon Contamination on Catalysts. Hydrocarbon Processing. Oct. 2010, Ronald Tharby. Avoid Silicon Based Antifoams in Manufacturing Ethanol. Fuels & Lubes International Quarter 4, Tharby J.M. Britto, M.V. Reboucas, and I. Bessa J.M. Britto, M.V. Reboucas, and I. Bessa Tharby Octamethylcyclotetrasiloxane. ChemSpider. Structure html?rid=d6ed70ff fa8c 473d 9200 cca8a72af9a0 (19 Sept. 2012) 17 Richard Van Noorden. Desperately Seeking Silicon. Chemistry World Online. 5 Mar (19 Sept. 2012) 18 Tharby ASTM Test Method D ASTM Research Report RR:D , Interlaboratory Study to Establish Precision Statements for ASTM D7757, Standard Test Method for Silicon in Gasoline and Related Products by Monochromatic Wavelength Dispersive X ray Fluorescence Spectrometry. ASTM International. West Conshohocken, PA, (19 Sept. 2012) 15 Tech Valley Drive East Greenbush, New York USA pg. 6