Membranes 102


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Membrane Processes


Among the different membrane separation techniques, pressure-driven processes are simplest in terms of their ability to separate particulates in liquid and gas feed streams according to size. Through utilizing pressure as a driving force for separation, with a membrane acting as a semipermeable barrier, pressure-driven processes are also associated with higher flux compared to their thermal and concentration-based separation counterparts. Types of pressure-driven membrane separation techniques are categorized according to membrane pore size, which, in turn, dictates the degree of separation achieved.These categories are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO).


Microfiltration (MF) lies on the upper end of the spectrum of pressure-driven membrane techniques, with membranes containing the largest pore size of the aforementioned processes. It is capable of separating suspended solids within the range of 0.1-10μm and is often used as a precursor stepto downstream filtration applications in order to achieve the desired degree of separation within a given feed stream. Due to the larger pore size of MF membranes, manyof these processes are capable of being run at lower pressures than those with membranescontainingsmaller pores. Common MF applications involve the separation of large macromolecules in clarification steps, such as in the removal of bacteria from cellular broths and in fat removal processes in the dairy industry.


Within the family of pressure-driven membrane processes, ultrafiltration (UF) lies between microfiltration and nanofiltration in terms of pore size, which can range from 1-100 nm. This size range allows for the concentration of high molecular weight proteins, macromolecules, and other small, suspended solids. In contrast to MF, UF membranes are categorized with respect to their molecular weight cutoff, i.e., their ability to retain a molecule of a given size, rather than by the size of their pores. Nevertheless, the pore size range of UF membranes makes them well-suited for use in a wide variety of ultrafiltration applications across multiple industries. In the automotive industry, UF is used in the recovery of undeposited paint for reuse in the electrocoating process. In the food and beverage industries, it is used in applications ranging from the concentration of whey protein to the clarification of fruit juices.


In contrast to MF and UF, in which solutes are separated according to size, both size and charge play a role in nanofiltration (NF) separation processes. With a pore size between 0.1-10 nm, NF membranes are capable of retaining low molecular weight, uncharged solutes, such as sugars and other organic molecules. NF membranes also retain charged species, such as polyvalent ions and large monovalent ions, whereas smaller monovalent species pass through. Applications for NF membranes range from theremoval of natural organic matter in wastewater treatment, hardness reduction in water purification, and whey demineralization in dairy processing.

Reverse Osmosis

Reverse osmosis (RO) membranes contain the smallest pores of the pressure-driven membrane processes and are capable of retaining all dissolved particles within a feed stream, including monovalent ions. This degree of separation results in a permeate consisting ofa pure solvent, which, in many cases, is water. Separation using RO is accomplished not only through size exclusion but utilizes a diffusive mechanism as well. The necessity of overcoming the osmotic pressure, in addition to the extremely narrow pore size found in RO membranes, results in RO processes requiring higher pressures than those previously mentioned. The most common applications for RO are in the preparation of drinking water and beverage concentration.
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