Ultrafiltration driving potential

Ultrafiltration (UF) is used to filter any large molecules (e.g., proteins) present in a solution by using an appropriate membrane. Although the driving potential in UF is the hydraulic pressure difference, the mass transfer rates will often affect the rate of UF due to a phenomenon known as concentration polarization (this will be discussed later in the chapter). 

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

Reverse osmosis and ultrafiltration are processes for separating liquid solutions. The driving force for these separations comes from a pressure difference. This pressure difference does increase the chemical potentials on the feed side, but it does not dramatically alter the concentrations on this side. As a result, these separations are different than the gas separations described in the previous section, where the feed concentrations are directly proportional to the feed pressure. 

---