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NREL examines potential of blending ethanol with condensate for flex-fuels and high-octane mid-level blends

A team at the National Renewable Energy Laboratory (NREL), with a colleague at EcoEngineering, has explored the potential of blending ethanol with natural gasoline (condensate) to produce flex-fuels (ASTM D5798-13a) and high-octane, mid-level ethanol blends (MLEBs). A paper on their work is published in the ACS journal Energy & Fuels.

The study addresses two current market conditions: first, more ethanol is produced domestically than can legally be blended in E10 (the ethanol blend wall). Second, as a result of recent increases in crude oil and natural gas production in the US, condensate—a component of natural gas liquids (NGLs) found in rich gas—is produced in abundance and could potentially serve as a lower-cost blendstock. Current US production of condensate is estimated at 1.5 × 108 m3 annually compared to 9.7 × 107 m3 annually 10 years ago.

(Condensate can come from two production sources: lease condensate is recovered from the gas stream at the wellhead; plant condensate remains in the gas until it is processed at gas-processing plants and is often called natural gasoline. There is little chemical difference between lease and plant condensates beyond the definition.)

Condensate exports are prohibited form the US, although, the authors noted, in 2014 two companies applied for an d received approval for condensate exports. Absent further export approvals, however, there is a condensate glut in the US market, resulting in a rapidly dropping price.

With the huge increase in natural gasoline production, the cost has dropped, but the domestic market has not expanded. One area where condensate has traditionally been used, as a component of gasoline blendstocks, is shrinking rapidly. The growth of light domestic crude oil is pushing natural gasoline out of the refinery. Natural gasoline is also coveted as a diluent for heavy Canadian crude oil, although competition from other sources is also increasing.

This project looks at the potential of blending ethanol with natural gasoline to produce Flex-Fuels and high-octane MLEBs. For Flex-Fuel blending, the high vapor pressure of natural gasoline may limit whether the natural gasoline−ethanol blends can meet the requirements in ASTM International (ASTM) D5798-13a, “Standard Specification for Ethanol Fuel Blends for Flexible-Fuel Automotive Spark-Ignition Engines”, for vapor pressure.

The inherently low research octane number (RON) of natural gasoline (well below 80) and high vapor pressure (about 90 kPa), especially compared to conventional BOBs [blendstock for oxygenate blending], may be a limiting factor for MLEBs, particularly for summertime blending. … Another issue is that ASTM D7794-14, “Standard Practice for Blending Mid-Level Ethanol Fuel Blends for Flexible-Fuel Vehicles with Automotive Spark-Ignition Engines”, is currently limited to D4814 fuels (conventional gasoline) blended with D4806 denatured fuel ethanol or D5798 Flex-Fuel, which would prohibit natural gasoline as the sole blendstock.

—Alleman et al.

For the study, the team collected 8 gasoline samples from pipeline companies or ethanol producers around the United States.

The prepared the natural gasoline−ethanol blends volumetrically. For flex-fuels, E51, E60, E70, and E83 blends were targeted to cover the range of allowable ethanol content in D5798-13a. For the mid-range, two MLEBs were made, an E30 and an E40. Ethanol content was verified by D5501-12ε1, a gas chromatography method. The blends were evaluated for RON and vapor pressure. Additionally, heat of vaporization (HOV) was estimated from the sample composition obtained by DHA and ethanol content via D5501 and individual component HOV values.

The natural gasoline samples were 80–95% paraffinic, 5–15% naphthenic, 3% or less aromatics, and the balance olefins. The paraffins were typically pentane and isopentanes. The benzene content ranged from approximately 0.1 to 1.2 wt %; blends of E30 or more would meet US EPA) limits for the benzene content in gasoline. The sulfur content in the natural gasoline ranged between 4 and 146 ppm.

Assuming the lowest ethanol content in Flex-Fuel of 51 vol %, a natural gasoline blendstock would be required to have 20 ppm sulfur or less for the finished fuel to meet the upcoming US EPA Tier 3 gasoline sulfur limit. The research octane number (RON) (ASTM D2699-13) for the natural gasoline ranged from 67 to 72. Vapor pressure (ASTM D5191-13) ranged from 89 to 101 kPa.

A 17 ppm sulfur natural gasoline and a 146 ppm sulfur natural gasoline were selected for blending with ethanol to produce Flex-Fuels and MLEBs. For knock resistance, the results indicate that, even for these low-RON blendstocks, the 91 RON level typical of finished regular gasoline in the United States would be met at approximately 30 vol % ethanol. The impact of ethanol blending on HOV was identical for a conventional BOB and ethanol−natural gasoline blends such that the blend HOV is twice that of the hydrocarbon blendstock at 50 vol % ethanol. For these high-vapor-pressure blendstocks, over 70 vol % ethanol could be blended with natural gasoline while still meeting the class 4 (wintertime) Flex-Fuel minimum vapor pressure requirement of 66 kPa. For blending of class 1 (summertime) Flex-Fuel, a minimum of 74 vol % ethanol was required to meet the 62 kPa maximum allowable vapor pressure. Modeling of vapor pressure using UNIFAC and Wilson equation-based approaches provided good agreement with experimental data.

—Alleman et al.

Resources

  • Teresa L. Alleman, Robert L. McCormick, and Janet Yanowitz (2015) “Properties of Ethanol Fuel Blends Made with Natural Gasoline” Energy & Fuels doi: 10.1021/acs.energyfuels.5b00818

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