Hydrocarbon weight fraction

Mixtures of hydrocarbons are assumed to be athermal by UNIFAC, meaning there is no residual contribution to the activity coefficient. The free volume contribution is considered significant only for mixtures containing polymers and is equal to zero for liquid mixtures. The combinatorial activity coefficient contribution is calculated from the volume and surface area fractions of the molecule or polymer segment. The molecule structural parameters needed to do this are the van der Waals or hard core volumes and surface areas of the molecule relative to those of a standardized polyethylene methylene CH2 segment. UNIFAC for polymers (UNIFAC-FV) calculates in terms of activity (a,-) instead of the activity coefficient and uses weight fractions... 

Iwai. Y.. Ohzono, M., Aria, Y. Gas chromatographic determination and correlation of weight-fraction henry s constants for hydrocarbon gases and vapors in molten polymers. Chem. Eng. Commun., 1985, 34 225-240. 

Paraffin, Synthetic, occurs as a white wax that is very hard at room temperature. It is synthesized by the Fischer-Tropsch process from carbon monoxide and hydrogen, which are cata-lytically converted to a mixture of paraffin hydrocarbons the lower-molecular-weight fractions are removed by distillation, and the residue is hydrogenated and further treated by percolation through activated charcoal. It is soluble in hot hydrocarbon solvents. 

Since the initial work of Smidsrod and Guillet numerous investigators have used I.G.C. to determine physicochemical parameters characterising the interaction of small amounts of volatile solutes with polymers Baranyi has shown that infinite dilution weight fraction activity coefficients, interaction parameters and excess partial molar heats of mixing can be readily determined with this technique. Partial molar heats and free energies of mixing, and solubility parameters of a wide variety of hydrocarbons in polystyrene and poly(methyl methacrylete) have been determined The temperature dependence of the interaction parameter between two polymers has also been studied... 

Circulation is promoted by the difference in static heads of supply liquid and the column of partially vaporized material. The exit weight fraction vaporized should be in the range of 0.1-0.35 for hydrocarbons and 0.02-0.10 for aqueous solutions. Circulation may be controlled with a valve in the supply line. The top tube sheet often is placed at the level of the liquid in the tower. The flow area of the outlet piping commonly is made the same as that of all the tubes. Tube diameters of 19-25 mm diameter are used, lengths up to 12 ft or so, but some 20 ft tubes are used. Greater tube lengths make for less ground space but necessitate taller tower skirts. 

Absorption. Available data suggest that hydrocarbons in the aliphatic EC5-EC8 and aromatic EC5-EC9 fractions may be more readily absorbed by the lungs, gastrointestinal tract, and skin than hydrocarbons in the aliphatic or aromatic hydrocarbons in larger molecular weight fractions. This difference is due to their smaller molecular size and the presumed dependence of absorption of hydrocarbons on diffusion or facilitated diffusion. 

---