Intercambio Ionico Para Principiantes
Enviado por LuigiPetrozza • 4 de Diciembre de 2013 • 2.450 Palabras (10 Páginas) • 206 Visitas
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
f =
ye,i -xorg,i
12. The vapor composition in equilibrium with a (liquid) point on a residue curve must lie on the
tangent to the residue curve. For homogeneous systems, each residue curve has an
associated vapor boil-off curve, and they are related by this tangency condition. However,
in the heterogeneous region there is only one vapor boil-off curve, namely, the vapor line.
Therefore, the vapor composition in equilibrium with a point on a heterogeneous residue
curve lies at the intersection of the tangent to the residue curve and the vapor line.
Please note that in the two-liquid phase region, the vapor line lies very close to the
heterogeneous distillation boundary. However, these two curves coincide exactly only when
the distillation boundary is linear. In all other cases, the vapor line must lie to the convex
side of the distillation boundary as required by the tangency condition.
All liquid compositions lying on the liquid boiling envelope will produce equilibrium vapor
compositions on the vapor line. As the liquid composition moves away from the liquid
boiling envelope, into the homogeneous region, the corresponding equilibrium vapor
composition moves off the vapor line. However, the cusp in the vapor surface on either side
of the vapor line is normally quite steep.
13. Within certain ranges of column specifications, heterogeneous azeotropic distillation
systems exhibit multiple steady states. If these steady states are in close proximity, they
frequently cause erratic behavior in the column. This of course makes the column hard to
control.
14. Many heterogeneous azeotropic distillation
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