Recovery Rate and Emission Performance: What the Numbers Really Mean
Specifications for vapor recovery units quote two kinds of numbers: recovery efficiency and emission concentration. Both are important, but they measure different things, and they are easy to confuse. This article explains what the figures mean and how to compare equipment fairly.
Recovery efficiency
Recovery efficiency is the percentage of the incoming hydrocarbon that is captured and returned as liquid, rather than discharged. For a membrane-based stage-3 unit in normal operation, the NMHC (non-methane hydrocarbon) treatment efficiency is above 99%. In plain terms: of the hydrocarbon vapor entering the unit, more than 99% is recovered; less than 1% leaves with the discharged air.
Why 99% matters: the recovered fraction is product. On a station handling a few hundred thousand liters per month, the annual recovered volume is a real revenue item — recovered fuel can be returned to the tank and resold, and it needs no dewatering because the membrane process returns water-free liquid.
Emission concentration
Emission concentration is the hydrocarbon content of the discharged air, usually expressed in g/m³ or mg/m³. The WHSH-M discharges at a concentration far below the national emission limit. To put it in perspective: typical field measurements at operating stations are around 5 g/m³, well under the 25 g/m³ limit, and the exact value depends on station conditions such as fuel type, vapor concentration and temperature.
Two important caveats when comparing numbers:
1. Same conditions or no comparison. Recovery efficiency and discharge concentration both depend on the inlet vapor concentration, temperature, and throughput. A number measured on a high-concentration vapor stream cannot be compared directly with one measured on a low-concentration stream. Fair comparisons require the same fuel, similar temperature, and similar tank pressure conditions.
2. Efficiency vs. concentration are different guarantees. “Above 99% efficiency” does not by itself tell you the discharge concentration, and vice versa. A good specification states both: the treatment efficiency and the resulting discharge concentration relative to the applicable limit.
How the WHSH-M achieves both
– The membrane retains hydrocarbons and lets air pass, so the discharged stream is inherently low in hydrocarbons.
– The recovered stream is returned to the tank under vacuum, where it re-condenses — no adsorption step, no desorption, no secondary waste.
– The unit’s automatic pressure control keeps the tank near atmospheric, so vapor loads stay within the design range and the system is not overdriven.
The practical takeaway: evaluate a unit on both figures, measured under defined conditions. A unit that recovers above 99% of hydrocarbons while discharging far below the limit is doing the two jobs a stage-3 unit exists to do.
For more information: +86 135 2699 0215 / lixiang@vohonoilpipe.com
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