Gas Separation Membranes

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The membrane can be defined essentially as a barrier, which separates two phases and restricts transport of various chemicals in a selective manner. A membrane can he homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, can carry a positive or negative charge or be neutral or bipolar. 

A membrane separation system separates an influent stream into two effluent streams known as the permeate and the concentrate. The permeate is the portion of the fluid that has passed through the semi-permeable membrane. Whereas the concentrate stream contains the constituents that have been rejected by the membrane. The system works on the principle of selective gas permeation.

Polymeric hollow fiber membranes are designed with a thin, permeable wall that separates gases based on their relative permeability rates. Oxygen and water vapor are fast gases which quickly permeate the membrane, allowing nitrogen to flow through the fiber bores as the product stream.

Nitrogen product emerges from membrane units at close to the compressed air feed pressure. In many applications this means that no supplemental product compression is required. Because there are no moving parts in the separation process, membrane units can be rapidly activated when needed and shut down when they are not. 

Different gases pass through certain membranes at significantly different rates, thus permitting a partial separation. The rate of permeation is proportional to the pressure differential across die membrane and inversely proportional to the membrane thickness. The rate of permeation is also proportional to the solubility of the gas in the membrane and also to the diffusivity of gas through the membrane

Gas separation is affected by three key performance attributes of membranes:

• Selectivity towards the gases separated
   
• Membrane flux or permeability
   
• The life of the membrane, maintenance and replacement costs.

Advantages of Membrane System

• Membrane units are most cost effective for relatively low demand applications.
   
• The modular construction used for larger-capacity units results in high-capacity units having near-constant cost per unit of separation/ production capacity.
   
• Compactness and light in weight
   
• Low labour intensity
   
• Modular design permitting easy expansion or operation at partial capacity
   
• Low energy requirements

Applications of Membrane Separation

• Appreciable energy savings
   
• Environmentally benign
   
• Clean technology with operational ease
   
• Replaces the conventional processes like filtration, distillation, ion-exchange and chemical treatment systems
   
• Produces high,quality products
   
• Greater flexibility in designing systems.