Boiling point difference

Removal of maleic and fumaric acids from the cmde malononitrile by fractional distillation is impractical because the boiling points differ only slightly. The impurities are therefore converted into high boiling compounds in a conventional reactor by means of a Diels-Alder reaction with a 1,3-diene. The volatile and nonvolatile by-products are finally removed by two vacuum distillations. The by-products are burned. The yield of malononitrile amounts to 66% based on cyanogen chloride or acetonitrile. 

All separations are based on a difference in some property. The separation of the compounds given in Table 4-1 is done by distillation. It is based on the fact that compounds with different vapor pressures will have different compositions in the vapor and liquid phases. The magnitude of this difference, and hence the ease of separation, is directly related to the difference in the vapor pressures. This can be determined from the boiling-point differences. Among the six groups of compounds... 

As a general rule difficult or expensive separations should be performed last, since by that time less total material will be involved. Consider Table 4-1, which gives the product mix obtained in a cracking furnace of an ethylene plant and the normal boiling points of the compounds. Suppose it is desired to separate the six groups listed in the table using distillation. The separation of ethylene from ethane and propylene from propane will be the most difficult because they have the smallest boiling-point differences. Therefore, these steps should be performed last. 

Since members of a homologous series have incremental boiling point differences and if the amount of any homolog in the moving gas phase is related to vapor pressure at the temperature of the experiment, plots of log k vs. carbon number should also be a straight line. (The enthalpy of vaporization increases monotonically with carbon number.) This in fact is observed in gas-liquid equilibrium separation systems. It is the basis of retention index systems pioneered by Kovats for qualitative identification. 

Separation processes. These processes involve separating the different constituents into fractions based on their boiling-point differences. Additional processing of these fractions is usually needed to produce final products to be sold within the market. 

A basic guideline for choosing a desorbent is to match the chemical properties of the extracted components and the desorbent, along with appropriate selection of boiling point differences to allow recovery from the feed components after the... 

The third desorbent characteristic is that the desorbent material must be easily separated from the two Sorbex process products extract and raffinate. The adsorbent chamber s composition profile produces extract and raffinate streams comin-gled with desorbent. In order for the process to be economical, the separation of the feed components from the desorbent (achieved through fractionation) is set by the boiling point differences between the species. Depending on the selectivity possessed by the desorbent over that of the feed normals, the subsequent desorbent rates needed to flush feed normal paraffins from the adsorbent s selective volume and the resulting extract or raffinate streams from the Sorbex chambers could contain in some cases more than 50% desorbent. High concentration of desorbent demonstrates the importance of the desorbent characteristics when selecting a desorbent. 

Pure isomers are often used as starting products for fine chemicals (e.g., different drugs). The separation of isomers entails great difficulties because they often have boiling points differing by only a fraction of a degree and they have closely similar solubilities in many solvents. Solvent extraction processes for the separation of isomers, therefore, have to rely more on chemical reactions than on nonspecific physical interactions between the solute and the solvent. 

It is interesting to note that boiling points of alcohols and phenols are higher in comparison to other classes of compounds, namely hydrocarbons, ethers, haloalkanes and haloarenes of comparable molecular masses. For example, ethanol and propane have comparable molecular masses but their boiling points differ widely. The boiling point of methojqmiethane is intermediate of the two boiling points. 

THF well dissolves oxygen from the air and the unwanted peaks are observed in the area of high retention volumes (Section 16.4.5). THF is highly hydroscopic and it readily absorbs large amounts of moisture. As a result, even the well stored THF eluents may contain the non-negligible amount of water, which may affect retention volumes of polymers both in the SEC and in coupled modes of polymer HPLC [28,267,268]. Azeotropic mixture of THF with water contains about 4.5wt.% of water and its boiling point differs less than 3°C from the boiling point of dry THF at the atmospheric pressure. 

Recovery and separation of the products were rather simple considering that the boiling point difference between the two major components is almost 100 C. Alternatively, a cold acetic anhydride scrubber was used, in which all condensable products were washed out of the effluent gas. A typical effluent contained acetaldehyde (41%), acetic acid (58%), ethyl acetate (0.8-1.0%), EDA (0.1%), and methane (trace). 

Separation of the aromatics from each other and from other hydrocarbons by distillation is not economical because of the limited boiling-point differences and the formation of azeotropic mixtures. Instead, extractive or azeotropic distillation and liquid-liquid extraction are applied.234,235 The latter process is by far the most often used technique. The three processes are applied according to the aromatic content of the gasoline source. p-Xylene, the most valuable of the isomeric xylenes, is isolated by freezing (crystallization) or solid adsorption. 

It is uninformative to refer to liquid phase as being selective, since all liquid phases are selective to varying degrees. Selectivity refers to the relative retention of two components and gives no information regarding the mechanism of separation. Most separations depend upon boiling point differences, variations in molecular weights of the components, and/or the structure of the components. 

Removal of maleic and fuinaric acids from the crude malononitrile hy fractional distillation is impractical because the boiling points differ... 

Fractional distillation, using the ordinary distillation apparatus, is employed for separating substances whose boiling points differ by at least 40°. The mixture is distilled slowly and the distillates are collected in separate receivers. For example, a mixture of benzene (B.P. 80-2°) and nitrobenzene (B.P. 210-9°) can be separated by collecting the distillate which comes over at about 80° in one receiver and at about 210° in another receiver. By careful redistillation of these two fractions the two receivers will ultimately contain pure benzene and pure nitrobenzene respectively. 

Most of the earlier data on redistribution reactions or equilibria reported in the literature are of more or less qualitative character. One of the exceptions is the work of Calingaert (41-51) where in a series of papers entitled The Redistribution Reaction, the laws of probability were applied to a quantitative interpretation of the random redistribution of interchangeable substituents. In these studies, the respective redistribution products were determined quantitatively by fractional distillation of the equilibrated mixture. Although this method was fairly successful in the cases studied by Calingaert, there were many other systems to which it could not be applied due to too small boiling-point differences of the redistribution products or rapid rearrangement under the conditions of the distillation. 

In fact, we do have alkyl fluoride enthalpy-of-formation data for liquid -heptyl and H-octyl fluoride. Reference 1 cites, - 385 and - 439 kJmoL1 derived from the compendium by M. S. Kharasch, Bur. Stand. J. Res., 2, 359 (1929). A major problem—other than that the difference in enthalpies of formation is unreasonable—is found in Kharasch s work. He gives two -octyl fluoride heat-of-combustion values (one from the bromide, one from the iodide, and acknowledges the samples have a boiling point difference of 0.60°) that differ by 19.2 kJmoT1. The original study of the enthalpies of formation of n- and isopropyl fluoride is found in J. R. Lacher, A. Kianpour and J. D. Park, J. Phys. Chem., 69, 1454 (1956). Early in this paper, the authors... 

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