Reactive Distillation Technology - Process Intensification for Green Chemistry - page 253

Figure 9.4 Modelling RD: equilibrium (left) and non-equilibrium (right) stage.

reaction rate in reference to that of the mass transfer rate - helps to discriminate between
very fast, fast, average and slow chemical reactions. If a fast reaction system is considered,
RD can be satisfactorily described assuming reaction equilibrium. A more physically
consistent way of describing a column stage is the rate-based approach, which takes directly
into account the actual rates of multicomponent mass and heat transfer and of chemical
reactions [6,26,35,39,41,42]. Both approaches can be conveniently simulated in commercial
process simulators, such as Aspen Plus (e.g. RADFRAC distillation unit with RateSep
model), Aspen Custom Modeler [43], ChemCAD, HYSYS, ProSim, Pro II and gPROMS.
It is also important to note that the introduction of an in situ separation function within
the reaction zone leads to complex interactions between VLE, vapour-liquid mass transfer,
catalyst diffusion and chemical kinetics. Such interactions have been shown to lead to the
phenomenon of multiple steady states and complex dynamics, which has been verified in
experimental laboratory and pilot plant units [42,44,45].

9.5 Feasibility and Technical Evaluation

9.5.1 Feasibility Evaluation

The development of a realistic process model is the key step in evaluating the feasibility of
an RD process. Moreover, a successful modelling effort requires a complete understanding
of both the equilibrium and the kinetic limits of the process. Thery et al. [16] give an
overview of the state-of-the-art methods available in the literature for the feasibility
analysis and design of RD processes. A recent approach to the feasibility evaluation of RD,
proposed by Kiss et al. [46], is based on fundamental explanations and industrial
applications of RD reported in the scientific literature (Figure 9.5). In order to perform
this evaluation, some basic information on the chemical process is required, such as: VLE,
stoichiometry of reactions, kinetics and enthalpy of reactions. Note that the boundary
values mentioned in this framework should only be taken as guideline figures and should
not be interpreted as very strict limits of the RD process.
The first step is to check the number of products and the reaction type. If there is only one
product present, the last step of the main reaction is irreversible, and when there are no side