Distillation boiling point curves

Figure 3. Simulated distillation boiling point curves for virgin bitumen, mal-tenes, and asphaltenes...

As the boiling point curves of the three types of solutions are different, the behaviour of these solutions on distillation will be different. 

Liquid-vapor phase diagrams, and boiling-point diagrams in particular, are of importance in connection with distillation, which usually has as its object the partial or complete separation of a liquid solution into its components. Distillation consists basically of boiUng the solution and condensing the vapor into a separate receiver. A simple one-plate distillation of a binary system having no maximum or minimum in its boiling-point curve can be understood by reference to Fig. 3. Let the mole fraction of B in the initial solution be represented by... 

Table CP5.2c lists some of the restrictions placed on the temperatures selected and the possible range of values for each temperature. Functional relations for the boiling-point curves in terms of weight % distilled (FA, FB, and FC) versus temperature (°F) are...

The physical properties (normal boiling points, densities, and molecular weights) of the 34 pseudo components selected to represent the true boiling-point curves of the feed, distillate, and sidestreams are presented by Cecchetti et al.8 On the basis of these data, a feed composed of 34 pseudo components and having the molar compositions and total flow rate shown in Table 4-13 was selected by Cecchetti et al. The specifications for the theoretical analogue column are also given in this table. 

Thus in binary solutions of type n and type III showing large deviations from Raoult s law with a maximum or a minimum in the boiling point curve, it is not possible to separate the two components by fractional distillation. In both the cases, on complete fractional distillation the mixture can only be separated into a constant boiling mixture of composition C or D and one of the pure constituents, A or B depending upon the composition of the original mixture. 

Describe the distillation behaviour of a binary liquid mixture which exhibits a maximum in their boiling point curves. 

A mixture of two liquids exhibits a minimum in their boiling point curves. Describe what happens during the distillation of the above mixture. 

Lube oil volatility is a measure of oil loss due to evaporation. Noack volatility measures the actual evaporative loss which is grade dependent, and a function of molecular composition and the efficiency of the distillation step. The volatility is generally lower for higher viscosity and higher VI base stocks. The gas chromatographic distillation (GCD) can be used to measure the front end of the boiling point curve and may be used as an indication of volatility, e.g. 10% off at 375°C. 

Assay analyses of whole crudes are done by combining an atmospheric and vacuum distillation run. These two runs when combined will provide a TBP (True Boiling Point Curve). While these batch distillation methods are labor intensive, taking between three to five days, they allow the collection of distillation fractions that can be utilized for testing. While each of the distillations techniques have been standardized by ASTM, cut schemes tend to mimic Refinery product classifications and there is no standardization of the individual inspection formats. Each corporation tends to perform both physical and chemical testing that best meet the needs of their refining operations and product suites. 

The physical distillation data from the D 2892 and D 5236 distillation runs are used to create a True Boiling Point Curve (TBP). These two curves will have a disparity in the area corresponding to the overlap between the 15/5 and Hivac distillation runs. This artifact is due to the Hivac still not being at optimum conditions at the early stages of the run. Once a TBP curve has been created property measurements can then be curve fit. 

The design methods considered for multicomponent mixtures in Chap. 9 were based on a limited number of definitely known components. In some cases, the mixtures are so complex that the composition with reference to the pure component is not known. This is particularly true of the petroleum naphthas and oils which are mixtures of many series of hydrocarbons, many of the substances present having boiling points so close together that it is practically impossible to separate them into the pure components by fractional distillation or any other means. Even if it were possible to determine the composition of the mixture exactly, there are so many components present that the methods of Chap. 9 would be too laborious. It has become customary to characterize such mixtures by methods other than the amount of the individual components they contain, such as simple distillation or true-boiling-point curves, density, aromaticity (or some other factor related to types of compounds), refractive index, etc. 

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