European Membrane House

2. Membrane Processes and Applications

There are various membrane processes which differ in their molecular separation size and the driving force which has to be expended.

Microfiltration (MF) closely resembles conventional coarse filtration and concerns the separation of particles between 0.1 and 10µm, such as suspended solids (colloids), bacteria and large proteins. MF employs membranes with a porous structure corresponding to low operating pressures in the 0.1 to 2 bar range.

MF is applied for clarification and sterilization purposes, for cell harvesting, separation of oil-water emulsions, etc.

Ultrafiltration (UF) belong to the pressure-driven membrane processes. This technique uses microporous membranes whose pore diameters are between 1-100 nm. Such membranes let through small molecules (water, salts) and adopt the high molecular weight molecules (polymers, proteins, colloids). Operating pressures are typically in the range of 1 to 5 bar for cross-flow application. With a semi-dead end operation mode, the pressures are much lower, around 0.2-0.3 bar.

UF is ideally suited for fractionation, concentration and purification purposes.

Major developments are expected in the area of membrane functionalisation, e.g. by modification with ligands, by a combinaison with enzymes and/or nanoparticles.

Reverse Osmosis (RO) serves to separate components of a solution. It is based on a pressure-driven process, the driving force resulting from the difference of the electrochemical potential on both sides of the membrane. Operating pressures can range from 10 bars up to 100 bars.

A typical RO application is seawater desalination. The major trends for RO for the past 15 years are improved performance and a significant reduction in price.

Nanofiltration (NF) is a pressure-driven membrane process which is preferentially used for the recycling of aqueous solutions. Operating pressures are between 5 and 20 bars.

Recent developments have greatly extended the capabilities of the membranes to withstand aggressive environments. Further progress was also made on improved performance with regard to both permeability and selectivity.

Electrodialysis (ED) is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential.
The membranes are cation- or anion-selective, which basically means that either positive ions or negative ions will flow through. Cation-selective membranes are polyelectrolytes with negatively charged matter, which rejects negatively charged ions and allows positively charged ions to flow through.
By placing multiple membranes in a row, which alternately allow positively or negatively charged ions to flow through, the ions can be removed from wastewater.

ED is used for desalination, demineralisation and the removal of metals.

Pervaporation (PV) is a fractionation process in which a liquid mixture is maitained at atmospheric pressure on the feed side of the membrane, while the permeate is removed as a vapor. Transport is induced by using a vaccum pump at the permeate side or by cooling the permeate vapor to create a partial vaccum.

PV can be a good alternative to more traditional techniques such as vaccum distillation and solvent extraction. The process has advanrages over these techniques with respect to energy saving, process simplicity and lower capital costs.

Gas separation membranes are applied at an industrial scale in several areas: Air separation, hydrogen separation and recovery, natural gas separations, air dehydration, organic vapor recovery, and reduction or elimination of CO₂ emissions from electricity power plants fuelled by coal or gas. 

3. Operating Modes

Membrane separation processes can be operated in Cross-flow or dead-end mode :

• Cross-flow operation is used in nanofiltration and reverse osmosis. In ultra- and microfiltration both operating modes are possible. In this mode, the feed is pumped parallel to the membrane surface and the permeate is withdrawn diagonally to it.

Cross-flow mode induces turbulence at the membrane surface to inhibit the build up of the fouling layer on the membrane surface

• In Dead-end operation the membrane is fed orthogonally, comparable to a "coffee filter".

In this mode, retained particles accumulate to form a cake layer and fouling tendencies are therefore high.