process design

1. Reactive Distillation

2. • Reactive distillation is a state-of-the-art
technology that combines distillation and the
chemical reaction in one single operating
step, thus enabling lower capital investment
and operational benefits. It is highly valuable
for equilibrium-limited reactions such as
esterification.

3. • The reaction takes place in a specific section
of the rectification column. The level of
conversion is increased by continuously
removing the reaction products from the
reaction zone by distillation. Both tray and
packed columns can be used.

4. features
• Fewer distillation columns
• Reduced or no recycle streams
• Utilization of the heat of the reaction for mass
transfer
• Higher product yields
• Lower capital investment
• Lower energy costs

6. • The reaction usually takes place in the liquid
phase or at the surface of a solid catalyst in
contact with the liquid.

7. Application
• Separation of mixtures of p-xylene and m-
xylene, whose normal boiling points differ by
only 0.8C, resulting in an alpha of only 1.029.
• Separation by ordinary distillation is
impractical because, to produce 99 mol% pure
products from an equimolar feed, more than
500 theoretical stages are required.

8. • By reacting the m-xylene with a reactive
entrainer such as tert-butylbenzene using a
solid aluminum chloride catalyst, stage
requirements are drastically reduced

9. Conditions for RD
• The chemical reaction occurs in the liquid
phase, in the presence or absence of a
homogeneous catalyst, or at the interface of a
liquid and a solid catalyst.
• Feasible temperature and pressure for the
reaction and distillation are the same.
• That is, reaction rates and distillation rates are
of the same order of magnitude.

10. • The reaction is equilibrium-limited so that if
one or more of the products can be removed
by distillation, the reaction can be driven to
completion; thus, a large reactant excess is
not necessary for a high conversion.
• This is particularly advantageous when excess
reagent recovery is difficult because of
azeotrope formation.

11. • For reactions that are irreversible, it is more
economical to take the reactions to
completion in a reactor and then separate the
products in a distillation column
• In general, reactive distillation is not attractive
for supercritical conditions, for gas-phase
reactions, and for reactions at high
temperatures and pressures and/or that
involve solids.

12. Reaction Schemes For RD

13. Reactions For RD
• The reaction A to R or A to 2R, where R
has a higher volatility than A.
• In this case, only a reboiled rectification
section is needed.
• Feed A is sent to the column reboiler where
the reaction takes place.
• As R is produced, it is vaporized, passing to the
rectification column, where it is purified.

14. • Overhead vapor from the column is
condensed, with part of the condensate
returned as reflux.
• Chemical reaction may also take place in the
column.

15. Reactions in RD:case 2

16. • The reaction A to R or 2A toR, where A has the
higher volatility.
• In this case, only a stripping section is needed.
• Liquid A is sent to the top of the column, from
which it flows down, reacting to produce R.
• The column is provided with a total
condenser and partial reboiler.

17. • Product R is withdrawn from the reboiler.
• This configuration requires close examination
because, at a certain location in the column,
chemical equilibrium may be achieved, and if
the reaction is allowed to proceed beyond
that point, the reverse reaction can occur.

18. Reactions in RD: case C

19. • The reactions 2A TO R + S or A + B To R + S,
where A and B are intermediate in volatility to
R and S, and R has the highest volatility.
• Feed enters an ordinary distillation column
somewhere near the middle, with R
withdrawn as distillate and S as bottoms.
• If B is less volatile than A, then B may enter
the column separately and at a higher level
than A.

20. Commercial Applications
1. Esterification of acetic acid with ethanol to
produce
ethyl acetate and water
2. Reaction of formaldehyde and methanol to
produce methylal and water, using a solid acid
catalyst [62]

21. .3. Esterification of acetic acid with methanol to
produce methyl acetate and water, using a
sulfuric acid catalyst.
4. Reaction of isobutene with methanol to
produce methyl-tert-butyl ether (MTBE), using
a solid, strongacid ion-exchange resin catalyst.