Page 164 - Design of Simple and Robust Process Plants

5.3 Combination of Process Functions 149
The application are found in equilibrium reactions, as continued removal of the
product(s) shifts the reaction to completion and limits consecutive reactions. At
least one product need to be have a higher or lower volatility, which then permits
the feeds to achieve product removal by distillation.
The advantages of this, compared with the conventional set-up of reaction followed
by distillation, include lower energy consumption, higher selectivity, and lower capi-
tal cost.
The limitations of these systems are equilibrium reactions, reasonable reaction
rates at distillation conditions, top and bottom temperatures within operational
range, and hold-up in the column for catalyst baskets. Neither feed nor products
should inhibit the catalyst, and the reaction kinetics should be available for design.
At low conversion rates, a reaction vessel might be installed at the front of a reac-
tive distillation; alternatively, reaction vessel(s) should be equipped with rectification
column(s) or dephlegmator(s)
At high conversion rates, the reaction and distillation are performed in a single
column (for an update on reactive distillation and technology, see Schoenmakers,
2000; the development of a synthesizer and designer tool, as part of a European
research activity, are also discussed in this article). Modeling has been described
elsewhere (Alejski and Duprat, 1996; Zheng et al., 1992; Bollyn and Wright, 1998;
Higler et al., 1998), and can be executed in commercial flowsheeting software.

5.3.1.2 Reactive extrusion
This is a technique which has been studied for several decades, and which resulted
in a number of industrial applications. The understanding of the technology has
been developed and reported extensively (Ganzeveld and Janssen, 1993; Ganzeveld
et al., 1994; van der Goot and Janssen, 1997; Janssen, 1998). The technology is to be
used not only as a tail end reactor but also for co-polymerization reactions in order
to obtain new product properties, as applied by large-scale producers of polymers.
The smaller-scale polymer industry might apply reactive extrusion to modification
reactions in order to achieve specific product properties. The technique is applied to
free radical as well as anionic polymerization, examples of which include the poly-
merization of styrene and styrene-butylmethacrylate, grafting of maleic anhydride
onto high-density polyethylene, urethanes, and co-polymerization of methacrylates.
The most important constraints to the application of this technology are reaction
time and the reaction enthalpy. The reaction time should be on the order of min-
utes, as longer time spans will make the process expensive, as the machinery to
carry out extrusions are expensive. The preferred adiabatic temperature rise is on
2
the order of 150 C, while the heat transfer coefficient should be 400 W/m K. The
stability of reactive extrusion has been studied, (Janssen, 1998).
The growing interest in reactive extrusion is based on its capability to operate
without solvents yet providing a high conversion. The extruder might be equipped
with a stripping section to remove any unreacted monomer. Another advantage is
the larger flexibility that an extruder provides compared with conventional reactor
systems.