Meeting Government Standards in Blended Fuel Using Vapor Pressure Testing Methods

Fuel blending is a key practice in the fuel industry to help manage costs, meet quality standards, and efficiently use available fuel stocks.
Back blending involves the integration of different 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 final 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 Profits 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 refiners carefully blend gasoline. That blending affords refiners the opportunity for incremental profits, 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 profitability. 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 profits.
Typically, between 2% and 3% butane by volume can be blended into gasoline, depending on the vapor pressure of gasoline before blending. The most profitable 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 significantly to profitability by allowing refiners 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 profits generated.
Better precision leads directly to significantly more blending profit, 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 refinery 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 final vapor pressure is calculated based on these measurements. ASTM D6378 is widely used in the industry due to its accuracy and reliability, particularly for fuels with high vapor pressures.
•  ASTM D5191: Standard Test Method for Vapor Pressure of Petroleum Products and Liquid Fuels (Mini Method)
This is a test method that measures the vapor pressure of gasoline and gasoline-oxygenate blends. It is commonly used to ensure compliance with EPA regulations. A sample is placed in a test chamber, and the vapor pressure is measured at 37.8°C (100°F) after a single expansion. This standard is essential for testing reformulated gasoline (RFG) and ensuring it meets regulatory limits on vapor pressure.
•  ASTM D5188: Standard Test Method for Vapor-Liquid Ratio Temperature Determination of Fuels (Evacuated Chamber and Piston Based Method)
This test method measures the vapor-liquid ratio of fuels at a specified temperature, typically 60°C (140°F). It helps determine the propensity of a fuel to form vapor at high temperatures. The sample is heated to the test temperature, and the volume of vapor formed is measured and compared to the liquid volume. This standard is particularly useful for assessing the performance of fuels in hot climates and ensuring they do not cause excessive vapor lock in engines.

The Challenge of Meeting Vapor Pressure Testing Standards

In back blending, a small amount of high-quality fuel is mixed with a larger quantity of lower-quality fuel to improve its overall quality. This practice allows fuel producers to maximize the use of available resources while ensuring that the final product meets the required specifications.
By carefully controlling the blending process and continuously monitoring the quality, producers can deliver fuel that is both cost-effective and high-performing.
Producers of gasoline and gasoline-oxygenate blends operating in the United States must certify their products for vapor-liquid ratio temperature (commonly referred to as ‘V/L’), according to regional and federal regulations.
The industry-standard test method for vapor-liquid ratio temperatures T(V/L=20) is ASTM D5188. This is one of many fuel-characteristic methods that are required within the standard specification for automotive spark-ignition engine fuel (ASTM D4814).
The vapor-liquid ratio temperature indicates the tendency of a fuel to vaporize, within a vehicle’s fuel system.
Modern vehicles have the highest under-hood operating temperatures of all time. This means that V/L is critical for avoiding fuel vaporization that causes fuel handling issues such as no starts, rough idling, and poor full-throttle acceleration.
V/L has an inverse relationship with vapor pressure – as the vapor pressure of a sample increases, its T(V/L=20) decreases (higher tendency of vapor-lock). The producer’s goal is to blend less expensive, volatile components that increase vapor pressure, making it more profitable, while complying with the V/L specification. The refinery must perform many V/L measurements daily and therefore needs multiple reliable units.
To meet requirements, fuel labs require the most accurate, precise, and robust laboratory analyzer capable of performing T(V/L=20) measurements according to the piston-based method ASTM D5188.
This analyzer must be easy to operate and maintain, with a fast return on investment (ROI) and low cost of ownership.
Additionally, the manufacturer or vendor must be able to provide continued service and calibration.

MINIVAP VP VISION

Vapor Pressure analyzer
The MINIVAP VP Vision from AMETEK Grabner Instruments is designed to meet vapor pressure testing standards. This portable vapor pressure tester offers high precision and a pressure range of 0-2000 kPa. Its closed-system, automatic oiler significantly extends the service life of components such as the piston and valves, reducing the cost of ownership.
Grabner Instruments’ proprietary 2-D calibration ensures accuracy and precision across the full measuring range of the MINIVAP VP Vision. The automatic dead volume calculation program, essential for reliable V/L measurements, ensures precise sample injection volumes.
The analyzer is highly versatile, with the capability to measure both V/L and global vapor pressure methods. Its rapid test program allows automatic performance of dry vapor pressure equivalent (DVPE) and T(V/L = 20) measurements back-to-back.
If a dedicated vapor pressure analyzer fails or is unavailable, the MINIVAP VP Vision can handle vapor pressure measurement tasks until the dedicated analyzer is returned to service, providing system redundancy.
Certified for its robustness and durability, the MINIVAP VP Vision uses Grabner Instruments’ Cockpit™ technology for enhanced networking and global analyzer access. It can measure gasoline, jet fuel, crude oil, LPG, and solvents, maintaining precision across a pressure range of 0-2000 kPa.
Long-term testing demonstrates repeatability of less than 0.13 kPa. The 2D-Calibration (273 points) and precise piston positioning ensure accurate results over the full measuring range. The Sampling Pro™ valve design minimizes cross-contamination, which is crucial when switching from volatile samples to those with a lower vapor pressure.
The analyzer features an intuitive user interface on a 10” touchscreen, simplifying measurement selection and monitoring. Measurement templates can be customized easily, and Grabner Instruments’ Cockpit™ PC software enhances flexibility by consolidating results from multiple locations.
The MINIVAP VP Vision supports all major VP methods for fuels and LPG. Every analyzer incorporates a speed-adjustable shaker to provide rapid equilibrium for crude oil with fuels and LPG. It is certified for operation in various climates and can withstand vibrations and heavy shocks, making it suitable for mobile laboratories, military applications, and harsh environments.

The Cockpit™ PC software allows automatic recognition and configuration of multiple instruments worldwide. Through a secure VPN, instruments can be monitored, diagnosed, serviced, and upgraded remotely. The call home function enables easy support from the Grabner Instruments team.
The triple expansion method used by the MINIVAP VP Vision helps users to ensure they are meeting the government regulations as closely to the standard as possible, maximizing profits and maintaining fuel quality.
The graph shows relevant ASTM standards and their impact on profit margins. The indicated ‘give away range’ demonstrates how much potential profit is given away each year by fuel blenders who adhere to standards that do not meet expectations of the Grabner Instruments triple expansion method.
In addition, the MINIVAP ON-LINE Reid Vapor Pressure (RVP) analyzer can be used for this application. It uses exactly the same principles and methods as the MINIVAP VP Vision, and shares many of the same components, so it can be advantageous to a refinery to use this analyzer for process monitoring, while the VP Vision is kept for laboratory use.
Up to two different sample streams can be connected to one single MINIVAP ON-LINE analyzer, and this process vapor pressure tester fully complies to the industry relevant ASTM standards for gasoline (ASTM D6378), crude oil (D6377) and LPG (D6897).

Conclusion

In the competitive and highly regulated world of fuel blending, ensuring that blended fuels meet government vapor pressure standards is crucial. Vapor pressure testing methods such as ASTM D6378, ASTM D5191, and ASTM D5188 provide the framework for accurate and reliable measurements.
By leveraging precise vapor pressure measurements, fuel blenders can navigate the complexities of regulatory requirements, optimize their operations for maximum profitability, and ensure that their products meet the highest standards of quality and safety.
By supplying advanced testing solutions like the MINIVAP VP Vision, Grabner Instruments supports fuel blenders in achieving compliance, optimizing blending processes, and maintaining high-quality fuel standards.