Less volatile component

In principle, extractive distillation is more useful than azeotropic distillation because the process does not depend on the accident of azeotrope formation, and thus a greater choice of mass-separating agent is, in principle, possible. In general, the solvent should have a chemical structure similar to that of the less volatile of the two components. It will then tend to form a near-ideal mixture with the less volatile component and a nonideal mixture with the more volatile component. This has the effect of increasing the volatility of the more volatile component. 

Consider the sequence of simple columns shown in Fig. 5.12. In the direct sequence shown in Fig. 5.12, the composition of component B in the first column increases below the feed as the more volatile component A decreases. However, moving further down the column, the composition of component B decreases again as the composition of the less volatile component C increases. Thus the composition of component B reaches a peak only to be remixed. ... 

Another characteristic similar to A/ 100 is the Distribution Octane Number (DON) proposed by Mobil Corporation and described in ASTM 2886. The idea is to measure the heaviest fractions of the fuel at the inlet manifold to the CFR engine. For this method the CFR has a cooled separation chamber placed between the carburetor and the inlet manifold. Some of the less volatile components are separated and collected in the chamber. This procedure is probably the most realistic but less discriminating than that of the AJ 100 likewise, it is now only of historical interest. 

Batch distillation (see Fig. 3) typically is used for small amounts of solvent wastes that are concentrated and consist of very volatile components that are easily separated from the nonvolatile fraction. Batch distillation is amenable to small quantities of spent solvents which allows these wastes to be recovered onsite. With batch distillation, the waste is placed in the unit and volatile components are vaporized by applying heat through a steam jacket or boiler. The vapor stream is collected overhead, cooled, and condensed. As the waste s more volatile, high vapor pressure components are driven off, the boiling point temperature of the remaining material increases. Less volatile components begin to vaporize and once their concentration in the overhead vapors becomes excessive, the batch process is terrninated. Alternatively, the process can be terrninated when the boiling point temperature reaches a certain level. The residual materials that are not vaporized are called still bottoms. 

Basic distillation involves appHcation of heat to a Hquid mixture, vapori2ation of part of the mixture, and removal of the heat from the vapori2ed portion. The resultant condensed Hquid, the distillate, is richer in the more volatile components and the residual unvapori2ed bottoms are richer in the less volatile components. Most commercial distillations involve some form of multiple staging in order to obtain a greater enrichment than is possible by a single vapori2ation and condensation. 

Separation by distillation is dependent on the fact that when a Hquid is partially vaporized the vapor and Hquid compositions differ. The vapor phase becomes enriched ia the more volatile components and depleted ia the less volatile components with respect to its equiUbrium Hquid phase. By segregating the phases and repeating the partial vaporization, it is often possible to achieve the desired degree of separation. One measure of the degree of enrichment or the ease of separation is the relative volatiHty defined as ... 

Dephlegmation, or partial condensation, refers to the process in which a vapor stream is cooled to a desired temperature such that a portion of the less volatile components of the stream is removed from the vapor by condensation. 

An important characteristic of pervaporation that distinguishes it from distillation is that it is a rate process, not an equilibrium process. The more permeable component may be the less-volatile component. Perv oration has its greatest iitihty in the resolution of azeotropes, as an acqiinct to distillation. Selecting a membrane permeable to the minor corTiponent is important, since the membrane area required is roughly proportional to the mass of permeate. Thus pervaporation devices for the purification of the ethanol-water azeotrope (95 percent ethanol) are always based on a hydrophihc membrane. 

The composition of the vapour in equilibrium with a miscible liquid mixture at any temperature, e.g. on heating during distillation, will be enriched by the more volatile components. The composition of the liquid phase produced on partial condensation will be enriched by the less volatile components. Such fractionation can have implications for safety in tliat tlie flammability and relative toxicity of the mixtures can change significantly. 

When the mass transfer rates of the two components are equal and opposite the process is said to be one of equimolecular counterdiffusion. Such a process occurs in the case of the box with a movable partition, referred to in Section 10.1. It occurs also in a distillation column when the molar latent heats of the two components are the same. At any point in the column a falling stream of liquid is brought into contact with a rising stream of vapour with which it is not in equilibrium. The less volatile component is transferred from... 

If in a distillation column, for example the molar latent heat of A is / times that of B, the condensation of 1 mole of A (taken as the less volatile component) will result in the... 

The more volatile constituent is transferred under the action of a concentration gradient from the liquid to the interface where it evaporates and then is transferred into the vapour stream. The less volatile component is transferred in the opposite direction and, if tlie molar latent heats of the components are equal, equimolecular counterdiffusion takes place. 

FIGURE 8.39 A schematic illustration of the process of fractional distillation. The temperature in the fractionating column decreases with height. The condensations and reboilings illustrated in Fig. 8.38 occur at increasing heights in the column. The less volatile component returns to the flask beneath the fractionating column, and the more volatile component escapes from the top, to be condensed and collected. 

The formation of droplets, which range from 50 to 200 nm in diameter, gives a very large snrface area from which evaporation may take place rapidly. The desolvation chamber is maintained virtually at ambient temperature by providing snfficient heat to overcome the latent heat of vaporization of the mobile phase. While the volatile components vaporize, the less volatile components, such as... 

As pointed out in the previous chapter, the separation of a homogeneous fluid mixture requires the creation of another phase or the addition of a mass separation agent. Consider a homogeneous liquid mixture. If this liquid mixture is partially vaporized, then another phase is created, and the vapor becomes richer in the more volatile components (i.e. those with the lower boiling points) than the liquid phase. The liquid becomes richer in the less-volatile components (i.e. those with the higher boiling points). If the system is allowed to come to equilibrium conditions, then the distribution of the components between the vapor and liquid phases is dictated by vapor-liquid equilibrium considerations (see Chapter 4). All components can appear in both phases. 

On the other hand, rather than partially vaporize a liquid, the starting point could have been a homogeneous mixture of components in the vapor phase and the vapor partially condensed. There would still have been a separation, as the liquid that was formed would be richer in the less-volatile components, while the vapor would have become depleted in the less-volatile components. Again, the distribution of components between the vapor and liquid is dictated by vapor-liquid equilibrium considerations if the system is allowed to come to equilibrium. 

Similarly, with the first column in the indirect sequence, the composition of Component B first increases above the feed as the less-volatile Component C decreases. It reaches a maximum only to decrease as the more volatile Component A increases. Again, the composition of Component B reaches a peak only to be remixed. 

If a gas or vapor process stream is available at a high pressure and downstream conditions do not require this high pressure, the stream can be expanded to provide useful cooling. The cooling might allow partial condensation for recovery of the less volatile components in a mixture, or to... 

To extract a relatively involatile oil such as eugenol (V) without charring requires a high pressure of steam, although the steam will not be hotter than 100 °C, so we generate a mixture of vapours at a temperature lower than that of the less volatile component. 

Rising temperature of the GC column not only assists transport of less volatile components, it also causes the slow release of the liquid phase from the inner wall of the capillary. As a result of slow thermal degradation, even chemically bonded liquid phases show such column bleed at elevated temperature. It is a characteristic of column bleed that it continuously rises as the temperature of the GC oven is raised and it falls again upon cooling of the system. Of course, the peaks from column bleed observed in the mass spectmm depend on the liquid phase of the GC... 

---