Reactive Distillation Technology - Process Intensification for Green Chemistry - page 249

operating conditions is not always the optimal solution, but merely a trade-off [16]. On the
contrary, in a conventional multi-unit flow sheet, the reactors can be operated at the
parameters that are most favourable for the chemical kinetics while the distillation columns
can be operated at the pressures and temperatures that are most favourable for the vapour-
liquid equilibrium (VLE) properties [1]. The use of residue curve maps (RCM) is
recommended, as an invaluable tool for initial screening and flow sheet development of RD.
The pressure and temperature effects are much more pronounced in RD than in
conventional distillation as these parameters affect both the phase equilibrium and the
chemical kinetics [1,18-20]. A low temperature that gives high relative volatilities may
give small reaction rates that require large amounts of catalyst or liquid holdups in order to
achieve the required conversion. In contrast, a high temperature may promote undesirable
side reactions or give a low equilibrium constant that makes it difficult to drive the reaction
to completion [1,16].
RD is typically applied to equilibrium reactions, such as esterification, etherification,
hydrolysis and alkylation. Remarkably, over 1100 articles and 800 US patents on RD were
published in the past 40 years, covering in total over 235 reaction systems [1]. Figure 9.1
(right) provides a convenient overview of these systems, classified into various reaction
types [17]. Most of the reactions types belong to the aA

þ bB ! cC þ dDoraA þ bB ! cC

or aA ! bB þ cC class, with the rest falling into other categories of two- or three-stage

reaction [1].

9.3 Design, Control and Applications

As compared to conventional distillation, RD sets specifications on both product compo-
sitions and reaction conversion. Consequently, the degrees of freedom (DoF) in an RDC
must be adjusted to accomplish these specifications while optimizing an objective function
such as the total annual cost (TAC). These design DoF include pressure-temperature,
number of reactive trays, holdup of the reactive trays, location of reactant feed streams,
number of stripping and rectifying trays, reflux ratio and reboiler duty [1,10].
Note that in conventional distillation the typical design specifications are the concentration
of the heavy key component in the distillate and the concentration of the light key component
in the bottom product. The holdup has no effect whatsoever on the steady state design of a
distillation column, only on the dynamic behaviour. The column diameter is easily deter-
mined from the maximum vapour-loading correlations, after calculating the vapour rates
required to achieve the desired separation. However, the holdup is very important inRDas the
reaction rates depend directly on the liquid holdup and the amount of catalyst on each tray.
Accordingly, RD requires an iterative design procedure, since the liquid holdup must be
known before the column can be designed. This means that a tray holdup is assumed and the
column is designed to achieve the desired conversion and product purities. The column
diameter is then calculated, as well as the required height of liquid on the reactive trays
corresponding to the assumed holdup. Note that liquid heights of over 15 cm are not
recommended due to the hydraulic pressure-drop limitations. If the calculated liquid height
is too large then a smaller holdup is assumed and the calculations are repeated [1].
For a long time it was assumed that reaction and distillation could be favourably combined
in a column enhanced with special internals or with additional exterior volume. However,