Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
API Publ 4653-1997: Technical and Environmental Review of Oxygenates in Gasoline
A comprehensive analysis of blending properties, environmental fate, and regulatory implications for reformulated gasoline
Scope of API Publ 4653-1997
API Publ 4653-1997, titled “A Review of the Environmental and Technical Issues Related to the Use of Oxygenates in Gasoline”, is a comprehensive publication developed by the American Petroleum Institute. It examines the technical and environmental challenges associated with incorporating oxygenates—primarily methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), and ethanol—into motor gasoline. The document addresses fuel formulation requirements under the Clean Air Act Amendments of 1990, including oxygen content mandates for reformulated gasoline (RFG) in nonattainment areas.
At the time of its release in 1997, MTBE was the dominant oxygenate used in the United States. However, growing evidence of MTBE contamination in groundwater prompted the industry to reassess its use. This publication was intended to consolidate existing data, identify knowledge gaps, and provide guidance to petroleum refiners, environmental managers, and regulators on safe handling, blending, and remediation.
Note: API Publ 4653-1997 is a technical report, not a formal standard. However, its findings have influenced subsequent API recommended practices and federal regulations concerning oxygenate management.
Technical Requirements and Key Findings
Although API Publ 4653-1997 is not a prescriptive standard, it establishes important technical parameters for evaluating oxygenate behavior in gasoline and the environment. The report synthesizes laboratory studies, field investigations, and modeling results to characterize oxygenate properties that affect blending, combustion, transport, and degradation.
Fuel Blending and Performance
Oxygenates are added to gasoline primarily to increase octane ratings and reduce carbon monoxide emissions. The publication provides detailed data on blending octane values, Reid vapor pressure (RVP) effects, and distillation curves. These parameters directly influence engine performance, driveability, and evaporative emissions. The table below summarizes key physical and chemical properties of the four oxygenates reviewed.
| Oxygenate | Chemical Formula | Blending Octane (R+M)/2 | RVP Blending Effect (psi) | Water Solubility (g/L at 25°C) | BOD Half-Life (days) |
|---|---|---|---|---|---|
| MTBE | C5H12O | 109 | −2 to 0 | 51 | ≥100 |
| ETBE | C6H14O | 111 | −2 to 0 | 12 | 30–60 |
| TAME | C6H14O | 105 | −1 to 1 | 20 | 20–40 |
| Ethanol | C2H6O | 130 | +2 to 4 | Miscible | 1–5 |
As the table shows, MTBE provides excellent octane enhancement and minimal RVP increase, making it attractive for RFG. However, its very high water solubility and resistance to biodegradation create significant environmental persistence when released into soil or groundwater. Ethanol, while offering superior octane and rapid biodegradation, substantially raises RVP, exacerbating evaporative emissions and distribution challenges.
Environmental Fate and Transport
A core technical contribution of API Publ 4653-1997 is its analysis of oxygenate behavior in subsurface environments. The report describes how MTBE’s low Henry’s law constant, high water solubility, and low sorption to organic matter cause it to move with groundwater plumes with little retardation. In contrast, ethanol partitions strongly into water and can enhance the dissolution of other fuel hydrocarbons (benzene, toluene, ethylbenzene, xylene) through co-solvency effects.
Tip: Use the data in this publication to prioritize monitoring strategies. MTBE plumes require extensive well networks due to their rapid migration; ethanol releases may necessitate immediate source control to prevent BTEX mobilization.
Implementation Highlights
API Publ 4653-1997 directly influenced several industry and regulatory actions in the late 1990s and early 2000s:
- Leak detection upgrades: Refiners and terminal operators enhanced underground storage tank (UST) monitoring to detect MTBE leaks sooner, prompted by the report’s characterization of MTBE’s high mobility.
- Alternative oxygenate evaluation: The comparative data on ETBE, TAME, and ethanol allowed companies to assess trade-offs and begin transitioning to ethanol before state MTBE bans took effect.
- Remediation technology selection: The report’s review of air stripping, granular activated carbon (GAC), advanced oxidation, and bioremediation guided corrective action plans at release sites.
- Blending guideline revisions: Refineries adjusted blending recipes to minimize product giveaway and RVP excursions when ethanol content increased.
Best Practice: For current RFG blending operations, conduct a site-specific oxygenate feasibility study using the analytical framework from API Publ 4653-1997 to balance octane demand, vapor pressure constraints, and environmental risk.
Compliance Notes and Regulatory Context
Although API Publ 4653-1997 is not a regulatory document, it provided critical technical support for compliance with the Clean Air Act’s RFG requirements (40 CFR Part 80). The publication was frequently cited in EPA regulatory impact analyses and state-level proceedings regarding MTBE bans. Key compliance points derived from the report include:
- Oxygen content mandates: The Clean Air Act required RFG to contain at least 2.0% oxygen by weight. API Publ 4653-1997 gave refiners the blending data needed to meet this requirement with various oxygenates while staying within volatility limits.
- Phase-out of MTBE: By 2006, over 20 states had banned MTBE. The environmental findings in this publication—particularly the detection frequency of MTBE in groundwater—were used to justify these prohibitions and the switch to ethanol.
- Release reporting and corrective action: The solubility and mobility data in the publication informed EPA’s guidance on MTBE cleanup levels (typically 20–40 µg/L in drinking water) and selection of treatment technologies.
Regulatory Caution: While API Publ 4653-1997 remains a foundational reference, operators should consult current federal and state regulations because oxygenate requirements have changed substantially since 1997. Ethanol is now the dominant oxygenate, and renewable fuel standards (RFS) impose additional compliance obligations.
Frequently Asked Questions
Q: Is API Publ 4653-1997 still applicable today given that MTBE use has declined significantly?
A: Yes. The report’s analysis of oxygenate properties, transport mechanisms, and treatment options remains technically valid for any ether oxygenate, including ETBE and TAME. It also provides a valuable methodological baseline for evaluating emerging oxygenates such as iso-butanol or other biobased ethers being considered for future gasoline blends.
A: Yes. The report’s analysis of oxygenate properties, transport mechanisms, and treatment options remains technically valid for any ether oxygenate, including ETBE and TAME. It also provides a valuable methodological baseline for evaluating emerging oxygenates such as iso-butanol or other biobased ethers being considered for future gasoline blends.
Q: Does API Publ 4653-1997 contain any formal test methods or standards?
A: No. This publication is a review document. It references many ASTM and EPA test methods (e.g., ASTM D4815 for oxygenate analysis) but does not prescribe them. It should be used as a technical supplement to formal test standards and API RP 1621 for bulk liquid storage of oxygenates.
A: No. This publication is a review document. It references many ASTM and EPA test methods (e.g., ASTM D4815 for oxygenate analysis) but does not prescribe them. It should be used as a technical supplement to formal test standards and API RP 1621 for bulk liquid storage of oxygenates.
Q: How does the report address ethanol’s co-solvency effect on BTEX?
A: The publication dedicates a substantial section to the co-solvency phenomenon, noting that ethanol can increase the aqueous solubility of benzene, toluene, ethylbenzene, and xylene by factors of 2 to 10 depending on ethanol concentration. This has important implications for risk assessment at ethanol-blend release sites, as BTEX plumes can become more extensive and persistent.
A: The publication dedicates a substantial section to the co-solvency phenomenon, noting that ethanol can increase the aqueous solubility of benzene, toluene, ethylbenzene, and xylene by factors of 2 to 10 depending on ethanol concentration. This has important implications for risk assessment at ethanol-blend release sites, as BTEX plumes can become more extensive and persistent.
Q: Where can I obtain a copy of API Publ 4653-1997?
A: The publication is available from the American Petroleum Institute (API) Publication Services and through authorized technical document retailers. As a historical document, it may also be accessible through university and regulatory libraries that maintain archives of API publications.
A: The publication is available from the American Petroleum Institute (API) Publication Services and through authorized technical document retailers. As a historical document, it may also be accessible through university and regulatory libraries that maintain archives of API publications.