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    Experimental and Theoretical Studies on the Start-Up Operation of a Multivessel
    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
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