Examples separation mechanism

With regard to mistakes, two separate mechanisms operate. In the rule-based mode, an error of intention can arise if an incorrect diagnostic rule is used. For example, a worker who has considerable experience in operating a batch reactor may have learned diagnostic rules that are inappropriate for continuous process operations. If he or she attempts to apply these rules to evaluate the cause of a continuous process disturbance, a misdiagnosis could result, which could then lead to an inappropriate action. In other situations, there is a tendency to overuse diagnostic rules that have been successful in the past. 

In thinking about performing multidimensional separations within the framework of unified chromatography, we must think about using all available tuning opportunities to maximize the differences in the separation mechanisms in the successive parts of the process. The following is just one example. 

The adsorption of collectors on sulfide mineral occurs by two separate mechanisms chemical and electrochemical. The former results in the presence of chemisorbed metal xanthate (or other thiol collector ion) onto the mineral surface. The latter yields an oxidation product (dixanthogen if collector added is xanthate) that is the hydrophobic species adsorbed onto the mineral surface. The chemisorption mechanism is reported to occur with galena, chalcocite and sphalerite minerals, whereas electrochemical oxidation is reportedly the primary mechanism for pyrite, arsenopyrite, and pyrrhotite minerals. The mineral, chalcopyrite, is an example where both the mechanisms are known to be operative. Besides these mechanisms, the adsorption of collectors can be explained from the point of interfacial energies involved between air, mineral, and solution. 

In a number of cases ITIES can be used to separate the products of a photoinduced electron-transfer reaction. An early example is the work by Willner et al. [7] at the water/toluene interface, who studied the photooxidation of [Ru(bpy)3]2+ in the aqueous phase. The excited state was quenched by hexadecyl- 4,4 bipyridinium, which becomes hydrophobic on reduction and crosses to the toluene phase. There are other examples and mechanisms at the present time their main interest resides in their chemistry, and in the separation of products that can be achieved at the interface. 

First, porous membranes will be discussed. Gases can be separated due to differences in their molecular masses (Knudsen diffusion), due to interaction (surface diffusion, multilayer diffusion and capillary condensation) and due to their size (molecular sieving). All these mechanisms and their possibilities will be discussed. For the sake of simplicity, theoretical aspects are not covered in detail, but examples of separations in literature will be given. The next section deals with nonporous membranes. Here the separation mechanism is solution-diffusion, e.g. solution and diffusion of hydrogen through a platinum membrane. This section is followed by an outline of some new developments and conclusions. 

Figure 6.2 illustrates the separation of n-Csis and non-n-Cs/is in CaA molecular sieves or 5A. The separation mechanism is obvious when the kinetic diameter of the molecules and molecular sieve pore size opening are compared. n-Csjc have kinetic diameters of less than 4.4 A which can diffuse freely into the 4.7 A pores of the CaA molecular sieve, while non-n-Cs/ have kinetic diameters of 6.2A. A commercial example of shape-selective adsorption is the UOP Molex process, which uses CaA molecular sieves to separate Cio-C n-paraffins from non- -parafHns (aromatics, branched, naphthenes). 

Figure 10.5 Summary of the features of separation mechanisms exhibited by zeolite membranes along with common examples. Superscripts 1 high temperature and/or low partial pressure, 2 low temperature and/or high partial pressure.

Not all fibers yield fibers on comminution. Fibrous varieties of quartz (Si02), for example, are formed from tightly bonded, aligned helical fibers that cannot be separated mechanically (Frondel, 1978). Fibrous calcite (CaCOa), when crushed or ground, breaks into equant grains of rhombic shape. The fragments reflect the cleavage characteristics of the mineral. 

According to what may be called the methyl separation mechanism, the polymerization reaction involves the rupture of a carbon-carbon bond of the olefin to give a methyl and an olefin radical which then add to the double bond of another olefin molecule (Kline and Drake, 60). The molecule of isobutylene, for example, behaves as though it were activated in the following manner CHa-+C(CH3)=CH2. The positive and negative signs indicate the relative electronegativities (Kharasch, 61) and are not intended to indicate ionization. The addi-... 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

Since one or more of the interactions in these systems might originate from the stationary phase, only a two- or a one-point interaction between the solute and the selector is necessary for mechanisms (2) and (3) to occur [50]. However, some of the CMPAs used in HPLC [37,40,51,52] have also been used as chiral selectors in CE [53-56], which indicates that at least one of the separation mechanisms between the selector and enantiomers is selective complex formation in the mobile phase in these cases, since there is no stationary phase present in CE. A recent example by Yuan et al. [57] is presented in Eigure 17.1. The authors introduced the use of (R)-A,A,A-trimethyl-2-aminobutanol-bis(trifluoromethane-sulfon)imidate as the chiral selector for enantioseparation in HPLC, CE, and GC. This chiral liquid serves simultaneously as a chiral selector and a co-solvent. 

The second special mechanism that we shall not consider is actually not a separate mechanism but is an electrophilic addition to one of the aromatic double bonds followed by an elimination reaction. An example is shown below ... 

One approach to delivering increased performance in a membrane process is to complement one separation mechanism with another. Vapor-arbitrated pervaporation is an example of this strategy. In bioseparations, as will be covered in a later section, a similar integration of several process enhancements in High-Performance Tangential How Filtration is responsible for dramatic improvement in separation efficiency of protein mixtures once considered unachievable by means of conventional ultrafiltration. 

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