Remixing effect

Figure 5.12 Composition profiles for the middle product in the columns of the direct sequence show remixing effects. (From Triantafyllou and Smith, Trans. IChemE, part A, 70 118, 1992 reproduced by permission of the Institution of Chemical Engineers.)...

This remixing that occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast, consider the prefractionator arrangement shown in Figure 11.9. In the prefractionator, a crude split is performed so that Component B is distributed between the top and bottom of the column. The upper section of the prefractionator separates AB from C, whilst the lower section separates BC from A. Thus, both sections remove only one component from the product of that column section and this is also true for all four sections of the main column. In this way, the remixing effects that are a feature of both simple column sequences are avoided4. 

It is unfortunate that research in the area of polymer solutions has been deserted during the past ten years in favor of blend work. With the advent of shear cells, it is expected that research in polymer solutions will become fashionable again. For instance, the phenomenon of shear-induced apparent demixing of high molecular weight polystyrene in semidilute solutions (in DOP for example) is not understood. Kinetics measurements will hopefully permit a close monitoring of the remixing effect after shear cessation as well. 

If, on the other hand, the coal is heated very rapidly - as it is the case in fluidized bed and entrained bed systems - pyrolysis is shifted towards much higher temperatures. At such temperatures, the intermediate products and carbon are so quickly reacted with steam and the entire gasification process is accompanied by such strong remixing effects that the product gas does not contain any tar. 

Although the control and operation strategy for conventional columns is simple, they are inefficient in terms of energy due to the remixing effect (Long and Lee, 2013a). For example, in the direct sequence, the concentration of component B in the first column increases below the feed as the concentration of more-volatile component A decreases. However, moving further down the column, the concentfation of B decreases as the concentration of less-volatile component C increases. Thus, the concentration of B reaches a peak only to be remixed (Smith, 2005). Similarly, with the first column in the indirect sequence, the concentration of B first increases above the feed as that of component C decreases. It... 

A fourth factor is the flow rate of the eluent (mobile phase). A too high rate decreases resolution because there is no time for molecules to diffuse into the pores of the matrix. In contrast, a very slow migration of solvent decreases the resolution by remixing the components by diffusion. The effect of diffusion is minimized if the chromatography is done at low temperature. If short separation times are necessary, pre-packed columns, elevated pressure, or HPLC columns are indispensable. 

Liquid redistributors are installed at frequent intervals in a packed column to remix the liquid, thus counteracting the propagation of maldistribution effects and the natural tendency of liquid to migrate toward the wall. A common design practice is to redistribute the liquid every 20 ft (6-7 m). 

In mixed pigmentations, lecithin additives help to maintain a uniform mixmre in both alkyd and latex paints, ensuring uniform settling in storage. Latex paint redispersion is also made more effective with a water-dispersible lecithin additive than with a synthetic surfactant. Paint specimens with lecithin additives, remixed after 28 months of storage, maintained a very high level of uniform color stability (434). 

When water flows around a partially submerged rock, you usually can see a small whirlpool just behind it downstream. This causes a partial remixing of any separated components. Small rocks provide different size whirlpools than big rocks, and so it is with the particles in an HPLC column. To get the minimum broadening, keep the particle size small and as uniform as possible and use columns of 1-5 mm in diameter to reduce channeling and wall effects. 

The combined effects of solvent efficiency (i.e. the extent to which substances are separated on account of differences in their rates of migration with reference to a particular set of solvent and stationary phase) and column efficiency (i.e. ability to achieve prevention of remixing of the separated substances) is expressed in terms of the resolution, Rs, of the column ... 

The PS introduces a phase difference of effect rotations about the y-axis by +90° and -90°, respectively, thereby unmixing and remixing the amplitudes for taking these paths. The total effect is equivalent to a rotation of the Bloch vector by -0g about the x-axis ... 

Tallman et al. (1984) presented data to suggest that microelectrodes further enhance response by remixing between electroactive zones on the surface. As discussed earlier, in the case of continuous planar electrodes, the diffusion layer increasingly extends out in the cell volume as flow moves down the length of the electrode. In a microarray. significant areas are inactive, and diffusion layers dwindle in thickness over these relaxation zones before moving on to the next active area. The net effect is to maximize the concentration gradient over the entire macroscopic electrode area. This... 

If we want to obtain a greater separation than this, one thing that we can do is use a recycle system, in which a portion of one of the streams is siphoned off and remixed with the feed stream in order for it to be re-separated. The choice of which stream should be re-siphoned depends on the desired properties of the exit streams. The effects of each choice will now be assessed. 

It is worth thinking of this process not as imitation, but as remixing. In today s music business, it has become common for the original raw tracks of music to be remixed in various ways by different producers aiming at different effects. In the same way that listening audiences developed separate criteria for live musical performance and records, remixing has... 

One factor causing remixing of the zones is diffusion during the time it takes to elute the column. P. Just Svendsen (56) avoids this in his design of the column. He can elute with the voltage applied. In this way he can maintain the electrofocusing effect during the entire elution. The apparatus and technique used by Svendsen are described later. 

The LKB 440 ml column is reported as giving better resolution than the 110 ml column. This observation probably confirms the above statement as to remixing in the capillary system after the column. In the 440 ml column, the volume separating two adjacent zones is larger than in the 110 ml column. The capillary system after the column and the flow photometer is the same in both cases. Remixing thus has less effect in the case of the 440 ml coliunn. 

The pavement is heated to temperatures above approximately 90°C using naked flame or infrared heating units (direct heating) or, for more effective in-depth heating, super-heated compressed air heaters (indirect heating). The typical treatment depth ranges from approximately 20 to 100 mm, depending on the procedure followed (repave, remix or reshape). 

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