A systematic synthesis framework for extractive distillation processes
Advancing the chemical engineering fundamentals
Distillation, Absorption & Extraction - II (T2-10b)
Keywords: extractive distillation, process design, CAMD, RBM, MINLP
In extractive distillation an additional entrainer is used to alter
the relative volatility of close-boiling or azeotrope forming
components to facilitate an economic separation in a distillation
process. Compared to a conventional distillation setup, the choice
of the entrainer is an important degree of freedom. Further degrees
of freedom of an extractive distillation setup are the entrainer
flowrate, the energy consumption of the process and the number of
stages of the distillation columns. In this contribution a
systematic synthesis framework for extractive distillation is
proposed, in which the degrees of freedom are fixed successively.
The synthesis framework proposes essentially a three step process.
The framework starts with the generation of entrainer candidates in
the first step. While several heuristics have been developed to find
suitable entrainers for a given azeotropic mixture, this entrainer
design is facilitated here through Computer-Aided Molecular Design
(CAMD). This allows the generation of feasible entrainer candidates
systematically based on thermodynamic group contribution methods.
For the evaluation of these entrainer candidates a number of
methods, such as the selectivity at infinite dilution have been
proposed. The CAMD method is used in this work for the preliminary
screening to reduce the search space with respect to thermodynamic
criteria. In the next step, the Rectification Body Method (RBM) for
extractive distillation is used to further screen the selected set
of entrainers in terms of minimum energy demand and minimum
entrainer flowrates. The minimum entrainer flowrate is determined
from the analysis of the nonlinear pinch equations, the minimum
energy demand from the tailored shortcut models. Thus, the results
from RBM is a smaller set of entrainers as this method employs a
stricter set of constraints than the thermodynamic criteria employed
by CAMD.
Once the entrainer choices have been narrowed down to two or three
candidates through these intermediate screening methods, a rigorous
MINLP optimization is started. Here the entire process is optimized
with respect to an economic merit function. In this last step the
optimal number of stages, the optimal feed stage, reflux policies
and the optimal entrainer flowrate are determined. This stepwise
procedure achieves a better robustness of the MINLP optimization,
since information from the shortcut step can be used for the
initialization of the MINLP optimization model.
This framework is illustrated with an industrially relevant case
study.
See the full pdf manuscript of the abstract.
Presented Monday 17, 16:00 to 16:20, in session Distillation, Absorption & Extraction - III (T2-10b).
