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Experimental and Theoretical Studies on the Start-Up Operation of a Multivessel
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Batch Distillation ColumnMultivessel batch distillation is a promising alternative to conventional batch distillation. Earlier studies proved
the feasibility of temperature control in a closed operation mode, that is, when no product is removed until
the end of the process. In this article, the influence of the reflux strategy during the initial start-up period is
investigated in detail. This is a very important task in terms of the development of automation procedures in
an industrial realization. The work is pided into three main parts. First, a rigorous dynamic model is introduced
to describe the behavior of a multivessel batch distillation during start-up. Then, two experiments carried out
on a laboratory scale column were chosen to validate the model. Finally, a sensitivity study is presented
observing composition response while changing the reflux strategy. The results show that the process can be
improved by applying a high reflux ratio and establishing the hydraulics as soon as possible.Introduction
Multivessel batch distillation is a superstructure of all batch 6753
distillation configurations. Mostly, the term is used for processes
with at least four product vessels including the reboiler and the
distillate receiver. In closed operation mode, no product is
removed; that is, if no intermediate heating is applied and at
fixed pressure, the process offers four degrees of freedom:
reboiler duty and liquid reflux streams from the product vessels
(Figure 1).
Multivessel batch distillation was the subject of several
investigations in the past two decades.
1 4
The main objective
was to prove the feasibility of the process and to investigate
different process control strategies. Temperature control as
proposed by Wittgens et al.
2
is especially favorable since
the final product composition is independent of the feed
composition. The temperature of the section can be easily
used as a controlled variable. The set points are set to the
arithmetic mean value of the boiling temperatures of the two
key components to be separated in a column section. Besides
remarks on alternative process control strategies, Wittgens
and Skogestad4
also commented on the start-up period based
on experimental experiences. The authors recommend using
an “override” to guarantee reflux during start-up in terms of
feasibility; i.e., one should avoid emptying the reboiler by
setting L3 g L2 g L1 on a volumetric basis in this specific
case. Furlonge et al.
6
performed dynamic optimization studies
of a multivessel batch distillation column but concluded that
no general guidelines on how the column should be run
optimally can be given. The authors suggest carrying out
optimization studies for each new case, which is, obviously,
very time-consuming. Gruetzmann and Fieg7
analyzed op-
timization potentials in a middle vessel batch distillation by
varying the reflux strategy before temperature control is
activated. The authors stated that early manipulations can
decrease the process duration. Therefore, it is logical to have
a closer look at the start-up period of a multivessel batchdistillation column and conclude more general remarks. Thus,
this article presents a reasonable extension of the previous
work.
The article is pided into sections as follows. First, a
definition of the term “start-up” is given and appropriate ways
to automate this period are discussed. A mathematical model
is presented that is capable of dealing with the start-up process.
Then, the model is validated by comparison with two experi-
ments chosen from a series of experimental studies. Finally,the model is used to investigate the sensitivity of relevant