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How much pressure is the TFF membrane under?

TFF operating conditions calculator (TMP, flux, permeability)

From the feed, retentate and permeate pressures, gives the pressure actually driving the membrane (transmembrane pressure, TMP), and from the permeate flow and membrane area, the permeate per m² per hour (flux).

When to use itUse it during an ultrafiltration (UF/DF) run, to watch for membrane fouling or to set the pressure.

Prefilled with an example: 1.5 bar at the inlet and 0.5 bar at the outlet, with 12 L an hour coming through 0.5 m² of membrane. Change any value and the result updates.

bar

The pressure where liquid enters the membrane module.

bar

The outlet pressure on the side that does not pass through the membrane (retentate).

bar

The pressure on the side of the liquid that has passed the membrane. 0 if open to atmosphere.

L/h

Liquid passing through the membrane, per hour.

m²

The effective area of the membrane in use.

Pressure across the membrane (TMP)1 bar
Flux per m² per hour24 LMH
Permeability (flux per bar)24 LMH/bar

The membrane sees an average of 1 bar, and 24 L of permeate comes through each m² per hour. If a small increase in TMP raises flux in proportion, there is still room to go up. If it does not, hold the pressure and increase crossflow instead.

Operating tips (concentration polarization and gel layer)

  • Low pressureMore pressure, more fluxThe membrane's own resistance is in control here, and flux rises roughly in proportion to TMP.
  • High pressureMore pressure, no more fluxOnce a concentrated protein layer forms on the membrane surface (concentration polarization layer or gel layer), raising TMP no longer raises flux and can speed up fouling. The standard fix is not more pressure but stronger flow along the membrane surface (crossflow) to thin the layer.
  • What crossflow doesSweeps the surface and thins the layerKeeping enough flow parallel to the membrane thins the concentrated layer and helps hold flux. Set it as a balance against shear and pressure drop.

How it is calculated

TMP is the average pressure pushing on the membrane, and flux is the liquid coming through each m² of membrane per hour (L/m²/h, or LMH). Permeability is flux divided by TMP, a guide to how easily the membrane passes liquid.

TMP (bar) = (feed pressure + retentate pressure) ÷ 2 − permeate pressure flux (LMH) = permeate flow (L/h) ÷ membrane area (m²) permeability = flux ÷ TMP

Source: Transmembrane pressure TMP = (feed pressure + retentate pressure) ÷ 2 − permeate pressure, and flux = permeate flow ÷ membrane area (LMH). These are the standard TFF operating definitions (see, for example, the Millipore and Cytiva TFF handbooks).

With your numbers

  1. TMP = (feed pressure + retentate pressure) ÷ 2 − permeate pressure = (1.5 + 0.5) ÷ 2 − 0 = 1 bar
  2. flux = permeate flow ÷ membrane area = 12 ÷ 0.5 = 24 LMH
  3. permeability = flux ÷ TMP = 24 ÷ 1 = 24 LMH/bar

Limits of this calculation

  • The best TMP and flux depend on the membrane, the product and its concentration. The result is a snapshot and will change as the retentate concentrates during the run.
  • Mixing pressure units (bar, kPa, psi) or flow and area units throws the result off by orders of magnitude. Flux is in L/m²/h (LMH).
  • How concentration polarization, gel layers and fouling develop depends on the membrane, the flow path and the conditions, and has to be confirmed by measurement. Treat the result as an estimate for learning.

Primary sources

Every calculator gives an estimate for learning, built on published information and primary sources. Real decisions follow the applicable guidance and expert judgment.