Ideal separation process

Figure 2.15. Idealized separation process in a gas chromatograph, 3 components 2 major and 1 minor.

With this respect exclusion chromatography basically diflers from all other modes of chromatography in that the analytes are not retained by the column packing and, therefore, do not need any special displacer or additional portions of the mobile phase, in order to be eluted from the column. Dilution of fractions separated in accordance with the size exclusion mechanism is no more unavoidable. (Dilution can be minimized to the diffusion effects at the front and tail of the analyte zone.) The absence of any supplementary matter in the frontal exclusion chromatography process relates ISE to the above-defined ideal separation process. 

A starting solution that is 1 M in component A and 1 M in component B should be resolvable in a hypothetical ideal separation process in many different ways, for example, giving, from 1.0 L of the initial mixture, two fractions, 0.5 L each, one of them being 2 M in component A and the other being 2 M in analyte B, that is, resulting in the concentration enhancement of each of the components of the initial mixture by a factor of 2. 

Assume now that in our imaginary experiment 1 L of the initial mixture is 1 M in component A and only 0.1 M in component B. With the existing tendency of levehng out the concentration gradients, the ideal separation process could result in a 0.9 L firaction of component A, the concentration of which would be increased by 10%, and a 0.1 L fraction of component B, the concentration of which may rise to 10-fold. This is basically what happens when small amounts of an electrolyte having smaller ions, such as HCl or NH4CI, have to be removed from the dominating salt (see, e.g.. Fig. 12.8). 

As the analytes are not retained by the sorbent, no excess mobile phase is involved in the size exclusion process. Therefore, separation of the components automatically results in their selCconcentration, according to the notion of the ideal separation process. 

FIGURE 3.10 Idealized separation process with two major components and one minor component. 

Reversible Processes. Distillation is an example of a theoretically reversible separation process. In fractional distillation, heat is introduced at the bottom stiUpot to produce the column upflow in the form of vapor which is then condensed and turned back down as Hquid reflux or column downflow. This system is fed at some intermediate point, and product and waste are withdrawn at the ends. Except for losses through the column wall, etc, the heat energy spent at the bottom vaporizer can be recovered at the top condenser, but at a lower temperature. Ideally, the energy input of such a process is dependent only on the properties of feed, product, and waste. Among the diffusion separation methods discussed herein, the centrifuge process (pressure diffusion) constitutes a theoretically reversible separation process. 

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

Sidestream compressors are used in refrigeration processes where, for economy, the refrigerant is flashed off at different pressure levels. Ideally, separate compressors could be used to successively compress the gas back up to the condensing pressure level. The pressure ratio for each stage is low enough to enable this to be done with only one or two impellers in each section. Because of this, compressors can be made with all sections in one casing so that mixing of the streams takes place intern.ll I v. 

Using chemical separation processes can require significant amounts of energy. What makes aluminum so ideal for recycling ... 

Power consumption in distillation is an important consideration. In the ideal limit, as for any separative process, it is only necessary to introduce an amount of energy equivalent to the entropy of mixing,... 

Density separation processes exploit the differences in densities (p) of the particles to be separated. Fly ash is comprised primarily of two components aluminosilicate ash particles (p 2.1-2.4g/cm3) and unbumed carbon particles (p 1.8-2.0 g/cm3) with minor amounts of iron-rich spinel, commonly referred to as magnetite (p 3.0-4.5 g/cm3). Under ideal conditions, particles can be efficiently separated based on differences in particle densities alone. However, in continuous density separation processes, particle size differences induce separation inefficiencies that is, smaller, high-density particles behave similarly to larger, low-density particles. For this reason, narrowing the size... 

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