Chemical Engineering and Processing 67 (2013) 49–59 Contents lists available at SciVerse ScienceDirect Chemical Engineering and Processing: Process Intensification jo u rn al hom epage: www.elsevier.com/locate/cep Control structure selection for four-product Petlyuk column Deeptanshu Dwivedi a , Ivar J. Halvorsen b , Sigurd Skogestad a,∗ a Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway b SINTEF ICT, Applied Cybernetics, N-7465 Trondheim, Norway a r t i c l e i n f o Article history: Received 13 March 2012 Received in revised form 30 June 2012 Accepted 23 July 2012 Available online 31 July 2012 Keywords: Energy efficient distillation Thermally coupled distillation Control structure design Divided wall columns Petlyuk column a b s t r a c t Direct material coupling between column sections in distillation leads to energy efficient systems like the Petlyuk column but with more difficult control problems compared to conventional multicomponent distillation sequences. A control study of a four-product extended Petlyuk column operating close to minimum energy is reported here. We study the “ideal” case with all steady state degrees of freedom available for control, including the vapor split valves, which is required to achieve minimum energy under all conditions. Four decentralized control structures are proposed and tested against a wide range of disturbances. This work demonstrates also the use of the graphical V min tool which can be used to visualize the minimum boilup requirement for Petlyuk arrangements. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Significant energy losses in conventional distillation sequences result from internal remixing. This can be reduced greatly by direct material coupling and by doing easiest split first. Petlyuk et al. [1] proposed such a scheme to separate feed into three products, using a prefractionator. The prefractionator is designed and oper- ated to do the easiest split first. A similar scheme was shown in a patent by Cahn and DiMiceli [2]. Stupin [3] claimed significant energy and capital saving using thermally coupled arrangement with a prefractionator. Such prefractionator arrangements can also be implemented in a single column shell using a dividing wall [4]. The German company BASF reports more than 100 industrial instal- lations [5] of divided wall columns for separation of a feed into three products. The idea of Petlyuk to separate a mixture to three products can be extended to separate a feed mixture into four products in a “four-product extended Petlyuk column”. Such systems may offer further energy savings [6]. While several control studies have been reported on three-product divided wall columns and four- product Kaibel columns, there are no control studies reported on four-product extended Petlyuk columns. Wolff and Skogestad [7] did a steady state study and operability analysis on a three-product Petlyuk column and conclude that the simultaneous specification of both impurities in the side product may be infeasible. Further, the liquid and vapor split ratios between ∗ Corresponding author. Tel.: +47 735 94154; fax: +47 735 94080. E-mail address: skoge@ntnu.no (S. Skogestad). prefractionator and the main column should be manipulated to get the optimal energy benefits. Niggemann et al. [8] conducted simu- lation and experimental studies for separation of a mixture of fatty alcohols into three high-purity products. They reported that the heat transfer across the dividing wall can be a factor in design and operation. Lestak et al. [9] argued that there may be some beneficial regions and the heat transfer across the dividing wall, should help decrease the overall energy consumptions. In non- beneficial regions however, the wall should be insulated. Mutalib et al. [10] reported experimental studies conducted on pilot plant and showed a two point control of the system. Ling and Luyben [11] explained that the liquid split valve must be manipulated and proposed a control structure with the use of four composition loops with the liquid split controlling the heavy key at the top stage of the prefractionator. Kiss and Bildea [12] gave some general control perspectives on dividing-wall columns. Ling et al. [13] suggested a control structure that can avoid remixing of intermediates lead- ing to energy optimal operation. van Diggelen et al. [14] reported a study on dividing-wall columns giving emphasis on the controlla- bility properties and dynamic responses. Some more works on the use of Model Predictive Control have been reported for divided wall columns [15–17]. Olujic et al. [18] reported recent advances on column internals for divided wall columns. Dejanovic et al.[5] reported simple design procedures for separating multicomponent aromatic mixtures into four products using energy efficient multiple partitioned dividing wall arrangements. In this paper we report the very first work on control of a four- product extended Petlyuk column. We study, here the separation of A (methanol), B (ethanol), C (propanol) and D (n-butanol) using the 0255-2701/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cep.2012.07.013