Sizing a membrane stage starts far from the membrane itself. It starts with the vapor: how much arrives, when it arrives, and what has to leave at the other end. Only after those numbers exist does membrane area become a meaningful quantity. Units that disappoint in the field usually trace their disappointment back to a shortcut somewhere in this sequence.
Start with the load profile, not the average
Vapor loads at fuel sites are spiky by nature: unloading events pile volume onto the system, and between events the load falls to breathing losses. The load profile, with its peaks and its valleys, is the first sizing input. A stage sized to the average underperforms at every peak; a stage sized only for the peak over-invests unless the profile genuinely lives at the peak.
From load to concentration target
The second input is the target: how much hydrocarbon must leave the treated stream. The gap between what arrives and what may leave is the work the membrane must do, and the gap’s width decides the stage count as much as the area. A wide gap at high concentration favors a single pass through generous area; a narrow gap at low concentration pushes toward deeper driving force and tighter control.
Driving force is the quiet variable in the middle of the calculation. The membrane moves hydrocarbon in proportion to the partial pressure difference across it, so the suction on the permeate side and the pressure on the feed side both matter. Sizing that ignores the driving force produces areas that look right on paper and starve in operation.
Area, staging and the assumptions that bind them
Membrane area converts the process numbers into hardware, but only together with the assumptions: the feed condition the area is rated at, the aging the design tolerates, and the upstream cleanliness the calculation presumes. An area sized for a clean, steady feed and installed behind a dirty, spiky one is undersized by definition, whatever the brochure says.
The sizing sequence in order
- Build the load profile: peaks, valleys and duration, not just the average;
- Set the outlet target from the requirement that actually applies to the site;
- Compute the separation work between inlet and outlet conditions;
- Choose stage arrangement and driving force to match the work;
- Convert to membrane area with explicit feed condition and aging assumptions;
- Check the whole result against the peak case before freezing the design.
Sizing inputs and where mistakes hide
| Input | Role in sizing | Common mistake |
|---|---|---|
| Load profile | Sets the duty the stage must track | Sizing to the average and failing at peaks |
| Outlet target | Defines the separation work | Copying a target that does not apply locally |
| Driving force | Decides how fast the work gets done | Ignoring permeate-side suction |
| Membrane area | Converts process to hardware | Area quoted without stated feed conditions |
| Aging allowance | Keeps performance honest over time | Sizing a new membrane for an old duty |
What a good sizing quote looks like
A supplier who shows the load profile, the target, the driving force and the assumptions behind the area is selling a design. A supplier who quotes only square meters is selling a number. Buyers do not need to run the calculation themselves, but they should expect to see it, because the visible assumptions are exactly the ones that will be tested by the site in the first month of operation.
Frequently asked questions
Can membrane area simply be added later if duty grows?
Sometimes, when the housing, pumping and controls were selected with growth in mind. Adding area to a unit sized tight everywhere else means re-opening the whole selection, which is why the growth question belongs in the first sizing conversation.
Why does the peak case matter so much?
Because emissions and complaints both happen at peaks. A stage that handles the average gracefully and the peak badly fails at exactly the moments the site is most visible, which is the failure mode buyers remember.
How much aging allowance is reasonable?
Enough that the stage still meets the outlet target near the end of the membrane’s service life under real feed conditions. The honest number comes from the supplier’s performance history, not from a flat percentage guessed at quotation time.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and manufactures vapor recovery units, membrane separation skids and condensers for fuel depots, terminals and retail sites. Contact us for duty assessment and equipment selection support.