Multicomponent distillation boiling point

The maximum boiling point is that temperature corresponding to a definite composition of a Iwo-coinponenl or multicomponent system al which the boiling point of the system is a maximum. At this temperature the liquid and vapor have the same composition and the solution distills completely without change in temperature. Binary liquid systems that show negative deviations from Raoult s law have maximum boiling points. See Raoult s I xiw and Van t Hoff I,aw. 

When complex multicomponent mixtures are distilled, particularly those associated with oil refining, it is difficult to characterize them in terms of their components. Instead, they are characterized in terms of their boiling range, which gives some indication of the quantities of the components present. The true boiling point distillation (TBP) is probably the most useful, in which the percent distilled is recorded as a function of the boiling temperature of the mixture. For the TPB distillation, a 5 1 reflux ratio is often used with 15 theoretical stages in a laboratory characterization column (see Section III). 

Feed analyses in terms of component compositions are usually not available for complex hydrocarbon mixtures with a final normal boiling point above about 38°C (100°F) (n-pentane). One method of handling such a feed is to break it down into pseudocomponents (narrow-boiling fractions) and then estimate the mole fraction and K value for each such component. Edmister [Ind. Eng. Chem., 47,1685 (1955)] and Maxwell (Data Book on Hydrocarbons, Van Nostrand, Princeton, N.J., 1958) give charts that are useful for this estimation. Once K values are available, the calculation proceeds as described above for multicomponent mixtures. Another approach to complex mixtures is to obtain an American Society for Testing and Materials (ASTM) or true-boiling point (TBP) curve for the mixture and then use empirical correlations to construct the atmospheric-pressure equihbrium flash vaporization (EFV) curve, which can then be corrected to the desired operating pressure. A discussion of this method and the necessaiy charts is presented in a later subsection Petroleum and Complex-Mixture Distillation. 

To design complex distillation columns, multicomponent methods are used. The true boiling curve is replaced by an approximate stepwise representation as a shown in Figure 12.18. Each step represents a pseudocomponent with a boiling point as indicated and a fraction of the total feed mixture based on the length of the horizontal portion of the step. 

The separation of a binary mixture by distillation may be represented in two-dimensional space while n-dimensional space is required to represent the separation of a multicomponent mixture (i > 2). The graphical method proposed by McCabe and Thiele9 for the solution of problems involving binary mixtures is presented in a subsequent section. The McCabe-Thiele method makes use of an equilibrium curve which may be obtained from the boiling-point diagram."... 

BUBBLE-POINT AND DEW-POINT CALCULATION. Determination of the bubble point (initial boiling point of a liquid mixture) or the dew point (initial condensation temperature) is required for a flash-distillation calculation and for each stage of a multicomponent distillation. The basic equations are, for the bubble point,... 

As a rule the compound to be added is so chosen that it forms an azeotrope of minimum boiling point with one of the components. But it is also possible to select an entrainer forming a binary or ternary minimum azeotrope with both of the components to be separated in the latter case it is necessary for the proportion of the components in the new azeotropes to be different from their initial proportions. Discus.sing extensive investigations of various types of phase diagrams and of the elaboration of column schemes Sharov and Serafiniov [35a] have treated the problems specific to the countercurrent distillation of azeotropic multicomponent mixtures. 

Extractive distillation is not limited to the separation of binary mixtures, but is also capable of removing particular classes of substances from multicomponent inixtiire.s, as for instance benzene from mineral oil fractions. Mixtures of saturated and imsaturated hydrocarbons having closely similar boiling points can be separated by extractive distillation with ketoesters [73]. Recently, the sei)aration of lower hydrocarbons CyCa has been gaining ground [74]. Garner et al. [75] studied the efficiency of packed columns in the extractive distillation of the system iiictliyl cyclohexane-toluene with derived equations for this process. 

Azeotropes are of great importance to distillation and rectification. At the azeotrope gas and liquid have the same concentration y = x) and, in turn, no driving force for interfacial mass transfer exists. Azeotropic mixtures behave in some respects like pure substances. They cannot be fractionated by simple distillation. Azeotropes can exhibit a boiling point minimum (minimum azeotropes) or a boiling point maximum (maximum azeotropes). In multicomponent mixtures saddle point azeotropes with intermediate boiling temperature can also exist. 

Now consider the more general case of the synthesis of all possible ordinary distillation sequences for a multicomponent feed that is to be separated into P final products, which are nearly pure components and/or multicomponent mixtures. The components in the feed are ordered by volatility, with the first component being the most volatile. This order is almost always consistent with that for normal boiling point if the mixture forms nearly ideal liquid solutions, such that Eq. (7.3) applies. Assume that the order of volatility of the components does not change as the sequence proceeds. Furthermore, assume that any multicomponent products contain only components that are adjacent in volatility. For example, suppose that the previously cited mixture of benzene, toluene, and biphenyl is to be separated into toluene and a multicomponent product of benzene and biphenyl. With ordinary distillation, it would be necessary first to produce products of benzene, toluene, and biphenyl, and then blend the benzene and biphenyl. 

In industry many of the distillation processes involve the separation of more than two components. The general principles of design of multicomponent distillation towers are the same in many respects as those described for binary systems. There is one mass balance for each component in the multicomponent mixture. Enthalpy or heat balances are made which are similar to those for the binary case. Equilibrium data are used to calculate boiling points and dew points. The concepts of minimum reflux and total reflux as limiting cases are also used. 

EXAMPLE 11.7-1. Boiling Point of a Multicomponent Liquid A liquid feed to a distillation tower at 405.3 kPa abs is fed to a distillation tower. The composition in mol fractions is as follows n-butane(x = 0.40), n-pentane [xg = 0.25), n-hexanefx = 0.20), n-heptane(xp = 0.15). Calculate the boiling point and the vapor in equilibrium with the liquid. 

Most investigations of azeotropes have dealt with binary or ternary mixtures. The binary pairs of a multicomponent mixture may separately form azeotropes, but these are submerged by die possible azeotropes involving the Aril mixture. It is sometimes possible to utilize binary pair azeotrope information in estimating Are role that a multicomponent azeotn will play in the distillation separation. As an exariqile, the ethanol-water system exhibits a minimum boiling azeotrope. At atmospheric pressure, the boiling points are... 

For multicomponent distillation we can use a true boiling point (TBP) curve to represent the same two cases. A TBP curve is obtained in the laboratory, in principle, by slowly... 

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. 

For nonideal systems, however, this approach does not provide yet sufficient accuracy, especially for multicomponent cases, because of limitations in the available equations of state. Such systems, in addition, have typically higher boiling points at atmospheric pressure and their separation by distillation is carried out at low pressures. These systems are described using the following methodology ... 

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