Principles of Membrane Separation

Membranes separate particles and molecules on the basis of their molecular size or solubility by semipermeable membranes. The permeate is the fluid passing through the membrane, and the retentate (concentrate) is the fraction not passing through. 

Both principles are combined in composite membranes, where a dense top layer, which is thin to achieve a high permeate flow, is located on a macropo-rous structure (for mechanical stability without transport resistance). Asymmetric membranes are similar, but the thin layer is not dense and has small pores (e.g., 1 nm), while the underlying porous structure has much bigger pores of, for example, 100 nm. 

Depending on the membrane used and driving force (pressure, concentration, or electric field) involved, the basic equations of the flux through a membrane differ. 

Separation technology Membrane structure (np non-porous p porous) Driving force Involved phases (s solid, 1 liquid, g gas, v vapor) Technical applications 

Microfiltration P dpoTc- 0.1-10 pm Ap ( 3bar) s/1 Separation of solid particles from suspensions separation of bacteria and color particles clarification of apple juice and wine 

---

Figure 3.3.63 The two basic principles of membrane separation (a) by porous membranes and (b) by dense (non-porous) membranes. Adapted from Baerns et al. (2006).

The field of membrane separations is radically different from processes based on vapor-liquid phase separation. Nevertheless, membrane separations share the same goal as the more traditional separation processes the separation and purification of products. The principles of membrane separation processes and their application to different types of operations are discussed in the last chapter. 

The principle of membrane separation is based on the differential passage of solutes or suspensions in a solvent through a membrane achieved by the application of a driving force such as pressure. The material is either retained on the feed side and rejected from passage across the membrane or passed through the membrane. The movement across the membrane depends on the size, charge, activity, and partial... 

The combination of diafiltration and batch concentration can be used to fractionate two macrosolutes whose retentions differ by as little as 0.2. It is possible in principle to achieve separations that are competitive with chromatography. When tanks and other equipment are considered, as well as the floor space they occupy, the economics of membrane separation of proteins may be attractive [R. van Reis, U.S. Patent 5,256,294 (1993)]. 

Naturally, there exist a variety of membrane separation processes depending on the particular separation task [1]. The successful introduction of a membrane process into the production line therefore relies on understanding the basic separation principles as well as on the knowledge of the application limits. As is the case with any other unit operation, the optimum configuration needs to be found in view of the overall production process, and combination with other separation techniques (hybrid processes) often proves advantageous for large-scale applications. 

This section will provide an overview of the principles of hydrogen separation and purification using membranes. More detailed discussions of the theory governing membrane separation processes can be found elsewhere.1... 

Membrane processes are UNIT OPERATIONS. Regardless of what chemicals are being separated, the basic design principles for different types of membrane separations are always similar. 

Mulder, M. Basic Principles of Membrane Technology, Kluwer Academic, Dordrecht, 1997. Mulder, M.H.V. in Pervaporation Membrane Separation Processes, R.Y.M. Huang, ed., Elsevier, Amsterdam, 1991. 

An example of the first type of membrane separator is your everyday vacuum cleaner. Vacuum cleaners work by taking in air laden with dust from your carpet. A filter inside the vacuum then traps the dust particles (which are relatively large) and allows the air to pass through it (since air particles are relatively small). A larger-scale operation that works on the same principle is called a fabric filter or "Baghouse", which is used in air pollution control or other applications where a solid must be removed from a gas. 

See the book by J. Mulder (Basic Principles of Membrane Technology, Springer, 1996) which specifically deals with membrane separation. 

FIG. 2 Schematic illustration of the principles of membrane osmometry. A polymer solution in compartment (a) is separated from pure solvent in compartment (b) by a membrane (c), which will allow solvent (but not solute) to pass through. Solvent diffuses from (b) to (a) until the osmotic pressure is just balanced by the hydrostatic pressure. The difference in heights of the column at equilibrium is the osmotic pressure it. 

Additionally, there are a number of useful electrochemical reactions for desulfurization processes (185). Solar—thermal effusional separation of hydrogen from H2S has been proposed (188). The use of microporous Vicor membranes has been proposed to effect the separation of H2 from H2S at 1000°C. These membrane systems function on the principle of upsetting equiUbrium, resulting in a twofold increase in yield over equiUbrium amounts. 

The aperture impedance principle of blood cell counting and sizing, also called the Coulter principle (5), exploits the high electrical resistivity of blood cell membranes. Red blood cells, white blood cells, and blood platelets can all be counted. In the aperture impedance method, blood cells are first diluted and suspended ia an electrolytic medium, then drawn through a narrow orifice (aperture) separating two electrodes (Fig. 1). In the simplest form of the method, a d-c current flows between the electrodes, which are held at different electrical potentials. The resistive cells reduce the current as the cells pass through the aperture, and the current drop is sensed as a change in the aperture resistance. 

---