Distillation technology has been evolving for many decades for a variety of
reasons, with the most important ones being energy efficiency and cost. As
a part of the evolution, semicontinuous distillation was conceived, which has
the advantages of both batch and continuous distillation. The economic benefits
of this intensified process compared to batch and continuous distillation
were expounded in many of the previous studies. Semicontinuous distillation
of ternary mixtures, which is the main focus of this thesis, is carried out in a single
distillation column with a tightly integrated external middle vessel and the
operation is driven by a control system. The system operation does not include
any start-up or shut-down phases of the column and has three periodically repeating
operating modes.
In the status quo design procedure, called the ‘sequential design methodology,’
an imaginary continuous distillation system design was used to design the
semicontinuous distillation system. In this methodology, dynamic simulations
of the process were used to find the values of the controller tuning parameters
based on the design of the continuous system. Afterwards, black-box optimization
was used to find better controller tuning parameter values that minimized
cost. However, after analyzing the dynamics of the system for different cases,
it was found that the heuristics used in this design methodology yielded suboptimal
designs. Therefore, the primary goal of the thesis is to improve these
heuristics by incorporating more knowledge of the system and thereby develop
a better design methodology.
Firstly, the setpoint trajectories generated by the ideal side draw recovery arrangement
for side stream flowrate control, which was standard in most semicontinuous
distillation studies, was modified. In this thesis, the performance of
the status quo as compared to the modified version, based on the criteria, cycle
time and cost for different case studies, was presented. Results showed that the
modified-ideal side draw recovery arrangement for side stream flowrate control
performed better with a 10-20% lower separating cost while maintaining
product purities. Furthermore, to reap more cost benefits, dynamic optimization
was used to seek the flow rate trajectory that minimized cost. However, it
was found that the additional cost savings, which is in addition to the benefits
gained by using the modified version, were at the most 2% from different case
studies.
Subsequently, the impact of changing the imaginary continuous distillation
system design on the nature of the semicontinuous distillation limit cycle, specifically,
its period was studied. Results revealed the necessity for a new design
procedure, and thus the back-stepping design methodology was proposed. This
design methodology was used to find better limit cycles of zeotropic ternary
semicontinuous distillation using the aspenONE Engineering suite. The proposed
methodology was applied to three different case studies using feed mixtures
with different chemical components. A comparison with the sequential
design methodology for the two case studies indicates that the new method outperforms
the state-of-the-art by finding limit cycles that were 4% to 57% lower
in terms of cost. Furthermore, the designs obtained from this procedure were
guaranteed to have feasible column operation with stable periodic steady-state
behaviour.
Semicontinuous distillation design using the design methodology with heuristic
components involves guessing, checking and then using black-box optimization
to find the values of the design variables to meet some performance criteria.
Furthermore, mathematical guarantees of either local or global optimality
of the designs obtained from the design procedure do not exist. Therefore, to
address these issues, in this thesis, the application of using the shooting method
for designing the semicontinuous distillation process was demonstrated using
two case studies, which involve the separation of hexane, heptane and octane.
This method has the potential to be combined with gradient-based optimization
algorithms for optimization of the process design in the future. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25334 |
| Date | January 2020 |
| Creators | Madabhushi, Pranav Bhaswanth |
| Contributors | Adams, Thomas, Chemical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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