Ethanol-blended gasoline changes the vapour stream that reaches a vapor recovery unit, and the difference is worth understanding before it shows up as an unexplained drop in recovery. Ethanol is more polar and has a stronger affinity for water than the hydrocarbon components of gasoline. As a result the vapour carries a different mix, tends to hold more moisture, and behaves differently as it passes through the condensation and membrane stages of the unit.
What is different about ethanol vapour
In a tank holding ethanol-blended fuel, the vapour space contains hydrocarbon components plus ethanol that has partitioned into the gas phase. Ethanol also draws water into the fuel, and that water can follow the same route into the vapour. The practical effect is that the stream arriving at the unit is not simply gasoline vapour diluted in air; it is a mixture whose composition shifts with temperature, with the blend ratio and with how much water the fuel has absorbed.
Effect on the condensation stage
Condensation does the bulk of the recovery work by cooling the vapour until hydrocarbons liquefy. Ethanol condenses under different conditions from the light hydrocarbon fractions, so the mixture does not behave like a single substance. The condensate that forms is therefore not identical to the fuel in the tank, and the temperature needed to achieve a given recovery changes with the blend.
Effect on the membrane stage
The membrane stage handles the dilute residual after condensation. Ethanol and water both pass through membrane material differently from hydrocarbon molecules, so the separation performance of the module changes when the feed composition changes. A module that performs well on hydrocarbon-only vapour may show a different enrichment behaviour on an ethanol-containing stream, and the practical symptom is usually a recovery rate that no longer matches the commissioning record.
Water as the recurring issue
| Parameter | Hydrocarbon vapour | Ethanol-blended vapour | Practical consequence |
|---|---|---|---|
| Water affinity | Low | Higher, water is drawn into the fuel | More moisture in the vapour stream |
| Condensation behaviour | Behaves as a mixed hydrocarbon stream | Components condense over a wider range | Recovery varies with blend and temperature |
| Membrane response | Predictable enrichment | Shifts with feed composition | Recovery rate drifts from baseline |
| Condensate quality | Mainly light fuel fractions | May carry water and ethanol | Returned liquid needs attention |
Operating adjustments worth considering
- Record the fuel blend in use at the site so that performance can be judged against the right baseline;
- Watch condensate quality, not only the recovery rate, since water in the returned liquid is an early warning;
- Review drain routines on traps and knock-out vessels more often in humid conditions;
- Compare performance at different times of year, because blend changes with season in many markets;
- Keep the commissioning data and re-measure against it, rather than against memory, when performance is questioned.
Common questions
Should the unit be specified differently for ethanol-blended fuel?
The specification has to account for the fuel that will actually be handled, including any seasonal blend change. Where a site handles both conventional and ethanol-blended fuel, the design basis should reflect the range rather than a single case.
Is a lower recovery rate always a fault?
Not always. A change in feed composition can shift performance without anything being wrong with the equipment. That is why comparing against a recorded baseline under known conditions is more useful than comparing against a single figure.
How does water get into the system at all?
Through the fuel, through humid air drawn into the tank as it breathes, and through the vapour stream itself. Once present, water tends to collect at low points and in traps, which is why drainage and low-point inspection are part of routine care.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and manufactures vapor recovery units and membrane separation skids for gasoline stations and fuel depots. Contact us for system sizing, site survey and after-sales service.