18 Analytical Instrumentation
MEETING GOVERNMENT STANDARDS IN BLENDED FUEL USING VAPOR PRESSURE TESTING METHODS
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Fuel blending is a key practice in the fuel industry to help manage costs, meet quality standards, and effi ciently use available fuel stocks. Back blending involves the integration of diff erent hydrocarbons into gasoline to optimize its volume and performance. Ensuring the quality of these blends is critical, and advanced analytical techniques play a crucial role in this process. One of the primary challenges in the blending process lies in ensuring that the fi nal blended fuel meets regulatory standards, particularly in terms of vapor pressure. Vapor pressure testing is a crucial method used to determine the volatility of fuels and ensure compliance with environmental regulations.
Fuel Blending and Vapor Lock
Gasoline vapor lock causes engines to stall, most typically at high temperatures or high altitudes. A vapor lock prevents fuel in the line from moving to the engine.
Vapor lock relates closely to gasoline vapor pressure, which is also essential to assess fuel performance and the risk of outgassing. For this reason, strict regulations for vapor pressure have been imposed on petroleum manufacturers to prevent vapor lock and environmental issues.
Testing fuels for their vapor-to-liquid (V/L) ratio temperature is essential, because blending affects the vapor pressure. For example, the vapor pressure of ethanol is much lower than the vapor pressure of gasoline. Adding 10% ethanol to gasoline – as is currently done in the USA – drastically reduces the V/L ratio temperature.
Therefore, a critical vapor-to-liquid ratio, which can cause vapor lock, will be reached at lower temperatures with gasoline-ethanol blends than with regular gasoline
Optimizing Profi ts by Blending Butane
into Gasoline Gasoline vapor pressure is an important factor in preventing fuel vapor lock and in providing easy motor starting in cold weather or at high altitudes.
The regulation of maximum vapor pressure limits requires that PIN AUGUST / SEPTEMBER 2024
MINIVAP ON-LINE - a process monitoring analyzer for the determination of the vapor pressure of gasoline, crude oil, LPG and NPG
refi ners carefully blend gasoline. That blending affords refi ners the opportunity for incremental profi ts, especially when low-cost butane is available to blend into the more expensive gasoline.
Butane has good anti-engine knock qualities but is a major overall contributor to gasoline vapor pressure, such that the maximum amount of butane blended into gasoline is limited by the maximum allowed vapor pressure of the fuel.
The spread between the price of gasoline and butane is a key factor in blending profi tability. For example, if the market price for gasoline is $3 per gallon and the price for butane is $1.5 per gallon, every gallon of butane used to replace a gallon of gasoline in a blend yields $1.5 per gallon in extra profi ts.
Typically, between 2% and 3% butane by volume can be blended into gasoline, depending on the vapor pressure of gasoline before blending. The most profi table time for blending butane into gasoline is during the transition between the summer and winter driving seasons, when sometimes up to 10% of butane by volume can be blended into gasoline. During colder seasons, higher vapor pressure is acceptable.
Apart from exact butane dosing, high-accuracy vapor pressure testing contributes signifi cantly to profi tability by allowing refi ners to use a narrower margin of safety and still guarantee that the gasoline vapor pressure is within limits. As a result, more butane can be added and greater profi ts generated.
Better precision leads directly to signifi cantly more blending profi t, even at a very low butane blending ratio. If the vapor pressure of gasoline increases by 1 psi (7 kPa) through blending, an effective analyzer will allow up to 0.5% more butane to be blended into gasoline, compared with the ASTM D5191 standard method.
As a consequence, a medium-sized refi nery may save more than a million dollars per year and recoup the analyzer’s costs almost immediately.
Test Methods and Standards
Vapor pressure is a measure of a liquid’s tendency to evaporate. In the context of fuels, it indicates the volatility and potential for emissions. High vapor pressure can lead to increased evaporation, contributing to air pollution and environmental hazards.
As a consequence, regulatory bodies like the US Environmental Protection Agency (EPA) impose strict limits on the vapor pressure of fuels, especially during warmer months when volatility is naturally higher.
Ensuring that blended fuels meet these standards is essential not only for regulatory compliance but also for maintaining fuel quality and safety. Vapor pressure testing methods are used to measure the vapor pressure of fuels accurately, providing data that can be used to adjust blending processes and ensure compliance.
Relevant standards for vapor pressure testing include those set by ASTM International and the International Organization for Standardization (ISO). Adhering to these standards ensures that the fuel meets industry and regulatory requirements.
Examples include:
• ASTM D6378: Standard Test Method for Determination of Vapor Pressure (VPX) of Petroleum Products, Hydrocarbons, and Hydrocarbon-Oxygenate Mixtures (Triple Expansion Method) This test method is used to determine the vapor pressure of petroleum products, including gasoline and its components, using a triple expansion method. It is particularly useful for volatile petroleum products. The sample is introduced into a measuring chamber at a controlled temperature, and the pressure is measured after three successive expansions. The fi nal vapor pressure is calculated based on these
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