Intercambio Ionico Para Principiantes

HETEROGENEOUS AZEOTROPIC DISTILLATION

Heterogeneous azeotropic distillation is a widely practiced process for the dehydration of a wide

range of materials including acetic acid, chloroform, ethanol, and many higher alcohols. The

technique involves separating close boiling components by adding a third component, called an

entrainer, to form a minimum boiling, normally ternary azeotrope which carries the water

overhead and leaves dry product in the bottom. The overhead is condensed to two liquid

phases; the organic, "entrainer rich" phase being refluxed while the aqueous phase is decanted.

Given the non-idealities, the phase splitting, the distillation boundaries present, and the possible

existence of multiple steady states in such a system, columns like these can be extremely

difficult to simulate and to operate. A systematic approach to the problem can save you many

headaches.

When designing such systems, keep the following points in mind:

1.

You need to start with two triangular plots: The residue curve map and the binodal plot. The

residue curve map should have the liquid phase envelope superimposed. The binodal plot

should be at anticipated condenser conditions.

2.

The key feature of feasible heterogeneous azeotropic distillation is that entrainers and top

tray vapor compositions are selected to generate liquid-liquid tie lines which straddle at least

one of the distillation boundaries dividing the two regions containing the two components to

be separated. In this way, the "jump effect" can be exploited, whereby each column is

provided with a feed composition in the required (different) distillation regions. This is why

the condenser must be two-liquid phase and we reflux the organic phase while decanting

the aqueous phase.

3.

Distillation boundaries in homogeneous mixtures cannot be crossed by residue curves, and

for all practical purposes they cannot be crossed by the steady-state liquid composition

profile in a distillation column either. Therefore, in order to isolate two pure components

which lie in two different distillation regions, it is necessary to have two different feed

compositions (one from each of the two regions) and two distillation columns. What Doherty

and Caldarola (1985) show is that such an arrangement is impossible to construct by

external recycles alone.

4.

The distillate and bottoms product must lie on the same residue curve.

5.

The feed, distillate, and bottoms product must be colinear; i.e., all lie on the same straight

material balance line.

6.

All feed to the column (reflux, makeup, and process feed) should be entered near the top of

the column.

7.

The composition of the vapor leaving the top tray must be near but not necessarily at the

ternary azeotrope.

8.

If at all possible (and it generally is possible) the column should be operated such that only

one liquid phase exists on the stages. Since the condenser must be operated in the

heterogeneous region, this means that the vapor coming from the top tray must be in

equilibrium with a single liquid phase, but must condense to two liquid phases. This narrows

the selection of top tray vapor compositions considerably and in fact can only be achieved

by careful manipulation of the condenser operating conditions and/or a mix of the decanter

organic and aqueous phases in the reflux.

For minimum boiling, heterogeneous azeotropic systems, this can be accomplished by

either adding water to the decanter in order to shift the overall composition into the twoliquid

phase region, or by lowering the temperature of the condensate in order to enlarge the

two-liquid phase region enough to encompass the distillate composition.

9.

It is important to note that the liquid boiling envelope is not a simple liquid-liquid binodal

curve at a fixed temperature, but the projection of the heterogeneous liquid boiling surface

onto the composition base plane. This is critical because the top tray equilibrium liquid

composition must lie outside the liquid boiling envelope. It is not enough to lie outside the

condenser binodal plot heterogeneous envelope.

10. The composition of the two-liquid phases generated in the condenser must be taken from tie

lines on a binodal plot calculated at condenser conditions, not top tray conditions.

11. The relative proportions of the two-liquid phases in the condenser, f, is given by the lever

rule;

ye,i-x aq,i

...