External force based separations

It is useful to provide a list of the basic physical or physiochemical properties, each of which could be a basis for separation it is also useful to list simultaneously the core phenomenon exploiting such a physical or physicochemical property for separation. It is to be noted that this list is not exhaustive rather, it contains the more familiar properties. Table 8 identifies a variety of these basic properties and lists phenomena employing a particular basic property leading to separation. For each basic properly and phenomenon in this table, there are three columns corresponding to three different types of basic separation processes phase-equilibrium-based separation processes membrane-separation processes and external force based separation processes. An entry into these three columns identifies a separation process or processes where the particular basic property is key to separation. References to Tables 1-7, a section in the book or a separate reference have been provided to each entry in these three columns. 

The same time-invariant mode of feed stream introduction and feed-phase flow perpendicular to the force direction is also utilized in separations where any one stream enters the separator, provided at least two product streams are continuously withdrawn from the separator. Crossflow membrane separators and external force based separators studied in Sections 7.2 and 7.3 are particular examples. Of the at least two product streams, one is the depleted feed stream. The other product stream continuously removes selected species from the feed stream (Figure 6.1.9(b)). 

Basic property Phenomenon causing separation Phase equilibrium based separation processes Membrane separation processes External force based and other separation processes... 

There is one basic phenomenon in the three types of separations identified above and described in detail in this chapter. It arises from the force direction being perpendicular to the direction of the bulk flow of the feed-containing phase (phases in one stream in the case of particle separation). Focus on Figures 7.0.l(i) and 7.0.1(m). In a given period of time, a feed species or a protein molecule, or a cell or a particle, moves a certain distance away from the axial line of its introduction, as defined by its trajectory. The latter is shaped by the magnitude of its species-specific/particle-specific velocity perpendicular to the bulk flow and the magnitude of its velocity toward the separator outlet, determined by the convective bulk fluid velocity. The higher the species-specific/particle-specific velocity perpendicular to the bulk flow, the further the end-point of the species/particle trajectory from the point of introduction. This is a successful recipe for multicomponent separation into fractions for both molecular/macromolecular species/ions as well as particles in external force based systems. 

On-line dialysis also separates the analyte from tissue matrix based upon molecular size, but in this case, the sample extract is passed over a membrane filter through which the analyte (and other low molecular weight compounds) is diffused into a second solvent on the other side of the membrane filter. Usually, the second solvent is then concentrated on to an SPE column to minimize the dilution effect that is caused by the dialysis process. Agasoester used on-line dialysis to separate oxytetracycline from muscle, liver, milk, and egg tissue matrix components. A problem encountered with on-line dialysis is the inability of analyte molecules that are bound to proteins in the sample extract to pass through the membrane filter. Problems with membrane clogging are reduced with on-line dialysis compared with ultrafiltration because no external force is being applied to bring the analyte across the membrane filter. 

A simple but often very successful approach in imaging of solid materials is the use of liquids as contrast agents, because they can be investigated by standard techniques. Porous structures like foams, internal cracks, voids or other spatial features can easily be analyzed in this way [116]. Furthermore, the local separation of liquids in pastes depending on external forces has been demonstrated in extrusion processes of PTFE/water pastes [94]. This kind of investigation is based on the assumption that the interaction of the... 

Physical processes separate contaminants from uncontaminated material by exploiting differences in their physical properties (e.g. density, particle size, volatility, by applying some external force (e.g. abrasion) or by altering some physical characteristic to enable separation to occur (e.g. flotation). Depending on the nature and distribution of the contamination within the soil, physical processes may result in the segregation of differentially contaminated fractions (for example a relatively uncontaminated material and a contaminant concentrate based on a size separation) or separation of the contaminants (for example oil or metal particles) from the soil particles. Table 6 summarises the main advantages and disadvantages of physical processes. 

Rate processes, on the other hand, are limited by the rate of mass transfer of individual components from one phase into another under the influence of physical shmuli. Concentrahon gradients are the most common stimuli, but temperature, pressure, or external force fields can also cause mass transfer. One mass-transfer-based process is gas absorption, a process by which a vapor is removed from its mixture with an inert gas by means of a liquid in which it is soluble. Desorption, or stripping, on the other hand, is the removal of a volatile gas from a Hquid by means of a gas in which it is soluble. Adsorption consists of the removal of a species from a fluid stream by means of a solid adsorbent with which it has a higher affinity. Ion exchange is similar to adsorption, except that the species removed from solution is replaced with a species from the solid resin matrix so that electroneutrality is maintained. Lastly, membrane separations are based upon differences in permeability (transport through the membrane) due to size and chemical selectivity for the membrane material between components of a feed stream. 

The physicocheimcal basis for separation is the primary focus of Chapter 3. Separation happens via species-specific force driven relative displacement of molecules of one species in relation to other species into species-specific region in the separation system. Particles of different sizes/ properties similarly undergo relative displacements. To develop this perspective. Chapter 3 (Section 3.1) identifies various external forces and chemical potential gradient based... 

External force field based separation bulk flow perpendicular to force... 

---