How Membrane-Based Vapor Recovery Works: The Technology Explained
Membrane separation is the core technology of the WHSH-M stage-3 vapor recovery unit. Understanding how it works helps station owners evaluate equipment from any supplier, because the membrane is what determines recovery efficiency, discharge quality and long-term reliability.
The principle: selective permeation
The membrane used is a polymer membrane (PDMS-based) that behaves differently toward hydrocarbons and air. Gasoline vapors (butane, pentane, hexane and heavier hydrocarbons) dissolve into the membrane material and diffuse through it readily. Air (nitrogen and oxygen) permeates far more slowly. This difference is called selective permeation: given the same pressure difference, hydrocarbon molecules cross the membrane many times faster than air molecules.
The membrane module is a spiral-wound element: layers of membrane sheet wrapped around a central collection tube. Vapor flows along the outside of the element under pressure. Hydrocarbon molecules pass through the membrane into the permeate side, where a vacuum pump maintains a negative pressure of roughly -0.05 to -0.09 MPa, and are collected at the center tube. The concentrated hydrocarbon stream is then routed back to the storage tank.
Why a vacuum on the permeate side
The vacuum pump on the permeate side is essential. It maintains the pressure difference that drives permeation, and it pulls the concentrated hydrocarbons out of the module toward the tank. In the tank, the returned stream mixes with saturated tank vapor and re-condenses into liquid — this is where the recovered fuel comes from. In the membrane-only (combined-pump) model, this vacuum pump is the main moving component, which is why the unit is compact, quiet and low in energy use.
What happens to the air
The air that does not permeate the membrane is essentially clean: its hydrocarbon content has been reduced well below the emission limit. It is discharged from the unit. Because the membrane retains hydrocarbons rather than adsorbing them, there is no adsorbent to saturate, no desorption step, and no spent-carbon hazardous waste.
Design figures relevant to the membrane
– Membrane module design life: 10+ years under normal operation
– Recommended inlet pressure: 0.3–0.6 MPa (do not apply reverse pressure)
– Inlet gas: free of particles and liquid oil, temperature ≤45 °C
– Membrane modules are manufactured in-house (annual membrane material capacity above 1.2 million m²; more than 30,000 modules per year)
Practical implications for the station
Because the membrane does the separation without consumables, running costs stay low: no activated-carbon replacement, no desorption steam or hot air, no hazardous waste disposal. The main routine task is checking the vacuum pump oil level and replacing the oil periodically, as described in the operating manual.
The result, in numbers: with normal maintenance, NMHC treatment efficiency above 99%, discharge far below the national emission limit, and a membrane that keeps working for a decade or more.
For more information: +86 135 2699 0215 / lixiang@vohonoilpipe.com
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