Membrane Technology and Applications

Acknowledgments for the first edition. Acknowledgments for the second edition. Historical Development of Membranes. Structure—Permeability Relationships in Solution-diffusion Membranes. Conclusions and Future Directions.

Metal Membranes and Ceramic Membranes. Boundary Layer Film Model.

Determination of the Peclet Number. Concentration Polarization in Liquid Separation Processes. Concentration Polarization in Gas Separation Processes. Cross-flow, Co-flow and Counter-flow.

Reverse Osmosis Membrane Categories. Characterization of Ultrafiltration Membranes.

Concentration Polarization and Membrane Fouling. Membrane Materials and Structure. There are two main flow configurations of membrane processes: In cross-flow filtration the feed flow is tangential to the surface of membrane, retentate is removed from the same side further downstream, whereas the permeate flow is tracked on the other side.

In dead-end filtration the direction of the fluid flow is normal to the membrane surface. Both flow geometries offer some advantages and disadvantages. Generally, dead-end filtration is used for feasibility studies on a laboratory scale. The dead-end membranes are relatively easy to fabricate which reduces the cost of the separation process. The dead-end membrane separation process is easy to implement and the process is usually cheaper than cross-flow membrane filtration. The dead-end filtration process is usually a batch -type process, where the filtering solution is loaded or slowly fed into the membrane device, which then allows passage of some particles subject to the driving force.

The main disadvantage of a dead end filtration is the extensive membrane fouling and concentration polarization. The fouling is usually induced faster at higher driving forces.

Membrane fouling and particle retention in a feed solution also builds up a concentration gradients and particle back flow concentration polarization. The tangential flow devices are more cost and labor-intensive, but they are less susceptible to fouling due to the sweeping effects and high shear rates of the passing flow. Flat plates are usually constructed as circular thin flat membrane surfaces to be used in dead-end geometry modules. Disc tube module is using a cross-flow geometry, and consists of a pressure tube and hydraulic discs, which are held by a central tension rod, and membrane cushions that lie between two discs.

The selection of synthetic membranes for a targeted separation process is usually based on few requirements. Membranes have to provide enough mass transfer area to process large amounts of feed stream. The selected membrane has to have high selectivity rejection properties for certain particles; it has to resist fouling and to have high mechanical stability. It also needs to be reproducible and to have low manufacturing costs.

R m can be interpreted as a membrane resistance to the solvent water permeation. R is related to the type of membrane foulant, its concentration in the filtering solution, and the nature of foulant-membrane interactions. The solute sieving coefficient is defined by the equation: Hydraulic permeability is defined as the inverse of resistance and is represented by the equation: The solute sieving coefficient and hydraulic permeability allow the quick assessment of the synthetic membrane performance.

Membrane separation processes have a very important role in the separation industry. Nevertheless, they were not considered technically important until the mids. Membrane separation processes differ based on separation mechanisms and size of the separated particles. The widely used membrane processes include microfiltration , ultrafiltration , nanofiltration , reverse osmosis , electrolysis , dialysis , electrodialysis , gas separation , vapor permeation, pervaporation , membrane distillation , and membrane contactors.

All processes except electro dialysis are pressure driven. Microfiltration and ultrafiltration is widely used in food and beverage processing beer microfiltration, apple juice ultrafiltration , biotechnological applications and pharmaceutical industry antibiotic production, protein purification , water purification and wastewater treatment , the microelectronics industry, and others.

Nanofiltration and reverse osmosis membranes are mainly used for water purification purposes. Dense membranes are utilized for gas separations removal of CO 2 from natural gas, separating N 2 from air, organic vapor removal from air or a nitrogen stream and sometimes in membrane distillation. The later process helps in the separation of azeotropic compositions reducing the costs of distillation processes.

The pore sizes of technical membranes are specified differently depending on the manufacturer. One common distinction is by nominal pore size. It describes the maximum pore size distribution [4] and gives only vague information about the retention capacity of a membrane. The exclusion limit or "cut-off" of the membrane is usually specified in the form of NMWC nominal molecular weight cut-off, or MWCO , molecular weight cut off , with units in Dalton.

The cut-off, depending on the method, can by converted to so-called D 90 , which is then expressed in a metric unit.

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The form and shape of the membrane pores are highly dependent on the manufacturing process and are often difficult to specify. Therefore, for characterization, test filtrations are carried out and the pore diameter refers to the diameter of the smallest particles which could not pass through the membrane. The rejection can be determined in various ways and provides an indirect measurement of the pore size. One possibility is the filtration of macromolecules often dextran , polyethylene glycol or albumin , another is measurement of the cut-off by gel permeation chromatography.

These methods are used mainly to measure membranes for ultrafiltration applications. Another testing method is the filtration of particles with defined size and their measurement with a particle sizer or by laser induced breakdown spectroscopy LIBS.

A vivid characterization is to measure the rejection of dextran blue or other colored molecules. The retention of bacteriophage and bacteria , the so-called "bacteriachallenge test", can also provide information about the pore size. To determine the pore diameter, physical methods such as porosimetry mercury, liquid-liquid porosimetry and Bubble Point Test are also used, but a certain form of the pores such as cylindrical or concatenated spherical holes is assumed. Such methods are used for membranes whose pore geometry does not match the ideal, and we get "nominal" pore diameter, which characterizes the membrane, but does not necessarily reflect its actual filtration behavior and selectivity.