Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 119

Two main types of azeotropes exist, i.e., the homogeneous azeotrope, where a single liquid phase is in the equilibrium with a vapor phase; and the heterogeneous azeotropes, where the overall liquid composition which form two liquid phases, is identical to the vapor composition. Most methods of distilling azeotropes and low relative volatility mixtures rely on the addition of specially chosen chemicals to facilitate the separation.

The five methods for separating azeotropic mixtures are: (i) extractive distillation and homogeneous azeotropic distillation where the liquid separating agent is completely miscible, (ii) heterogeneous, (iii) azeotropic distillation, or more commonly, azeotropic distillation where the liquid separating agent (the entrained) forms one or more azeotropes with the other components in the mixture and causes two liquid phases to exist over a wide range of compositions – this immiscibility is the key to making the distillation sequence work, (iv) distillation using ionic salts in which the salts dissociate in the liquid mixture and alters the relative volatilities sufficiently that the separation become possible, (iv) pressure-swing distillation in which a series of column operating at different pressures are used to separate binary azeotropes which change appreciably in composition over a moderate pressure range or where a separating agent which forms a pressure-sensitive azeotrope is added to separate a pressure-insensitive azeotrope, and (v) reactive distillation where the separating agent reacts preferentially and reversibly with one of the azeotropic constitutes; the reaction product is then distilled from the non-reacting components, and the reaction is reversed to recover the initial component, (v) simple distillation in which a multi-component liquid mixture is slowly boiled in a heated zone and the vapors are continuously removed as they form and, at any instant in time, the vapor is in equilibrium with the liquid remaining on the still; because the vapor is always richer in the more volatile components than the liquid, the liquid composition changes continuously with time, becoming more and more concentrated in the least volatile species.

A simple distillation residue curve is a means by which the changes in the composition of the liquid residue curves on the pot changes over time. Residue curve map is a collection of the liquid residue curves originating from different initial compositions. Residue curve maps contain the same information as phase diagrams, but represent this information in a way that is more useful for understanding how to synthesize a distillation sequence to separate a mixture.

All of the residue curves originate at the light (lowest boiling) pure component in a region, move toward the intermediate boiling component, and end at the heavy (highest boiling) pure component in the same region. The lowest temperature nodes are termed as unstable nodes, as all trajectories leave from them, while the highest temperature points in the region are termed stable nodes, as all trajectories ultimately reach them. The point that the trajectories approach from one direction and end in a different direction (as always is the point of intermediate boiling component) is termed saddle point. Residue curves that divide the composition space into different distillation regions are called distillation boundaries.

The separation of components of similar volatility may become economical if an entrainer can be found that effectively changes the relative volatility. It is also desirable that the entrainer be reasonably cheap, stable, non-toxic, and readily recoverable from the components. In practice, it is probably this last criterion that severely limits the application of extractive and azeotropic distillation. The majority of successful processes, in fact, are those in which the entrainer and one of the components separate into two liquid phases on cooling if direct recovery by distillation is not feasible.

A further restriction in the selection of an azeotropic entrainer is that the boiling point of the entrainer be in the range 10 to 40°C (18 to 72°F) below that of the components. Thus, although the entrainer is more volatile than the components and distills off in the overhead product, it is present in a sufficiently high concentration in the rectification section of the column.

See also: Azeotropic Distillation, Distillation.