Chemical Engineering and Processing: Process Intensification
A new fully thermally coupled distillation column with postfractionator
Introduction
The application of a fully thermally coupled distillation column has been increasing in a variety of plants of the world including North America [1], [2]. Especially, the practical application is active in Europe and Japan, where the demand of energy conservation is larger than other places because most of crude oil consumed there is imported from other countries.
For the cost saving in multi-component distillation, many alternative configurations of complex distillation systems have been proposed and systematically examined to find the best selection. Dünnebier and Pantelides [3] proposed an optimal design technique leading to the configuration of minimum operating and capital costs among various alternatives for three and four component distillation systems. A conventional direct or indirect sequence ternary distillation system was modified by introducing an additional vapor stream connection between the first and second columns to improve column efficiency of the distillation with increased reversibility of distillation in the second column [4]. Also, alternative configurations of a fully thermally coupled distillation column were proposed by eliminating one or two of the four interconnecting streams [5]. Because the vapor transfer between distillation columns causes an operational problem requiring tight pressure control at the columns, a general framework was introduced for the design of thermally coupled distillation columns without any intercolumn-vapor transfers [6].
A systematic method to generate possible distillation configurations for multi-component systems was presented using the state-task network (STN) representation, and a procedure to derive distinct thermally coupled configurations from the basic distillation system was introduced by Agrawal [7]. The design procedure of complex distillation columns with the STN representation is explained in detail elsewhere [8]. A variety of configurations of thermally coupled distillation columns were demonstrated in practical aspects, and the shortcut design procedures for simple distillation columns were presented by Shah [9]. Though many possible configurations of thermally coupled distillation columns have been proposed, the proposed configuration with a postfractionator for ternary separation was not introduced yet. Even the systematic method for the new configurations utilizing the STN representation does not show the proposed configuration. The STN representation connects nodes of feed and products with lines indicating column section, but the proposed configuration has a connection between two lines, which the existing method does not include as a possible configuration.
At total reflux operation, the liquid composition profile of trays in a distillation column matches one of equilibrium distillation lines, in which minimum number of trays having perfect distillation column efficiency is required for the separation. The structure of the distillation system is called minimum-stage distillation-column configuration here. Because the column profile of a fully thermally coupled distillation column is similar to the equilibrium distillation line, high distillation column efficiency is available from the reduced feed tray mixing and remixing of intermediate component. The mixing and remixing are irreversible processes lowering the efficiency [10]. A structural design procedure based on the minimum-stage distillation-column configuration has been applied to various systems of ternary [11], [12], [13], [14] and quaternary systems [15], [16].
When the composition of intermediate component in feed is low, the column profile of a prefractionator including the feed tray composition and that of a main column containing the composition of side product—having high concentration of the intermediate component—are distant in a fully thermally coupled distillation column [13]. Therefore, interlinking between the prefractionator and main column produces large composition mismatch between two interlinking trays unless a large number of trays are utilized. In the large tray number system, the composition difference between adjacent trays is so small that it is easy to find the interlinking trays of close compositions. In case of an industrial application of the fully thermally coupled distillation column in Japan, this problem does not arise because the composition of intermediate component in feed is high.
The fractionation process for the separation of BTX mixture is one of the largest processes in a petrochemical plant [17]. Because the processing amount of the process is large, the effect of energy saving is significant. Currently, a series of simple distillation columns separates the products from the process one at a time, but the beginning two columns can be replaced with a fully thermally coupled distillation column. The gas concentration process separating ethane, propane and butane mixture has a similar structure of separation processes with a large amount of processing capacity.
In this study, a new configuration of the fully thermally coupled distillation column is proposed to solve the large mismatch of compositions in interlinking trays to raise thermodynamic column efficiency by adding a postfractionator to the main column of an original fully thermally coupled distillation column (the Petlyuk column). By examining the column profiles of a prefractionator and main column a way of reducing the composition mismatch is proposed from exploring how the modification of the original fully thermally coupled distillation column affects distillation column efficiency. The procedure of structural design of the proposed system is explained here, and the energy requirement of the system is compared with that of the original fully thermally coupled distillation column. Two industrial examples of the fractionation process handling BTX mixture and the gas concentration process of ethane, propane and butane mixture are employed in the performance evaluation using a commercial design program, HYSYS.
Section snippets
Design procedure
Whereas Fig. 1 demonstrates the original fully thermally coupled distillation column, Fig. 2 describes the proposed system with a postfractionator. The main idea of the proposed modification is that the middle section separated with two dashed lines becomes a postfractionator to raise distillation column efficiency. The design procedures of the proposed column are explained below.
Residue curves of a ternary distillation drawn from the composition at a still of equilibrium distillation
Results and discussion
A new structure of fully thermally coupled distillation column system is implemented to the fractionation process of a naphtha-reforming plant for BTX production and the gas concentration process to produce gas products from gas mixture drawn from crude distillation, naphtha reformation and naphtha-cracking processes in a refinery. The feed and product flow rates of both processes are listed in Table 3, Table 4. In addition, the outcomes of structural and operational design are given in Table 1
Conclusions
A new structure of a fully thermally coupled distillation system requiring less energy than an original fully thermally coupled distillation column is proposed, and its design and performance are explained here. The system has a postfractionator connected to the main column of the original system.
Because the composition profiles of tray liquid in the proposed system are closer to residue curves than the original fully thermally coupled distillation column, its distillation column efficiency is
Acknowledgments
Financial supports from the Korea Energy Management Corporation and the Korea Science and Engineering Foundation (Grant no. R01-2003-000-10218-0) are gratefully acknowledged.
References (23)
Structural design and operation of a fully thermally coupled distillation column
Chem. Eng. J.
(2002)Rigorous design of extended fully thermally coupled distillation columns
Chem. Eng. J.
(2002)- et al.
Application of a fully thermally coupled distillation column for fractionation process in naphtha reforming plant
Chem. Eng. Process
(2004) - et al.
Operation of integrated three-product (Petlyuk) distillation columns
Ind. Eng. Chem. Res.
(1995) - et al.
Reduce costs with dividing-wall columns
Chem. Eng. Prog.
(2002) - et al.
Optimal design of thermally coupled distillation columns
Ind. Eng. Chem. Res.
(1999) - et al.
Improved direct and indirect systems of columns for ternary distillation
AIChE J.
(1998) - et al.
New thermally coupled schemes for ternary distillation
AIChE J.
(1999) Thermally coupled distillation with reduced number of intercolumn vapor transfers
AIChE J.
(2000)Synthesis of multicomponent distillation column configurations
AIChE J.
(2003)
Conceptual Design of Distillation Systems
Cited by (32)
Basic concepts and elements in the design of thermally coupled distillation systems
2021, Sustainable Design for Renewable Processes: Principles and Case StudiesSystematic design of the integrating heat pump into heat integrated distillation column for recovering energy
2016, Applied Thermal EngineeringCitation Excerpt :Distillation process costs for the degradation of energy and the rejection of low quality energy leading to a low thermodynamic efficiency [1]. Consequently, several technologies have been proposed in recent decades to reduce the energy consumption and improve economic performance [2–6]. As shown in Fig. 1, the vapor recompression column (VRC), also called heat pump column, is a well-known energy-saving technology especially for the separation of close-boiling mixtures [7].
Design process of LNG heavy hydrocarbons fractionation: Low LNG temperature recovery
2014, Chemical Engineering and Processing: Process IntensificationCitation Excerpt :The composition and conditions of the feed (stream 1), feed to demethanizer (stream 6) and feed to the FTCDC (stream 10′) are summarized into Table 1, while the process performance is captured in Table 3. Design of FTCDC/DWCs has been studied using basic equations [21,22] and using commercial simulators [23]. Investigators generally use Fenske–Underwood–Gilliland (FUG) equations as part of shortcut method [22,23] for initialization followed by rigorous simulation with simulators such as HYSYS or Aspen Plus.
Optimization of structural and operational variables for the energy efficiency of a divided wall distillation column
2012, Computers and Chemical EngineeringCitation Excerpt :Yildirim, Kiss, and Kenig (2011) reviewed the current industrial applications of DWCs and related research activities, including column configuration, design, modeling and control aspects. A number of researchers have recently reported on the design, control, and, most importantly, the energy saving aspects of a DWC (Chu, Cadoret, Yu, & Ward, 2011; Dejanovic, Matijaševic, & Halvorsen, 2011; Dejanovic, Matijaševic, & Olujic, 2011b; Diggelen, Kiss, & Heemink, 2010; Errico, Tola, Rong, Demurtas, & Turunen, 2009; Kim, 2005, 2006; Kiss & Bildea, 2011; Kiss & Rewarad, 2011; Kiss & Suszwalak, 2011; Long, Lee, & Lee, 2010; Premkumar & Rangaiah, 2009; Segovia-Hernández, Hernández-Vargasa, & Márquez-Muñoz, 2007; Tamayo-Galván, Segovia-Hernández, Hernández, Cabrera-Ruiz, & Alcántara-Ávila, 2008). Ho, Ward, and Yu (2011) gave analytical expressions to quantify the potential energy saving of a DWC based on remixing.
Modified simple column configurations for quaternary distillations
2012, Computers and Chemical EngineeringCitation Excerpt :Additional information about the complete subspace of thermally coupled structure can be founded in specific works (Agrawal, 2003; Rong & Kraslawski, 2002). Once that the possibility in both operational and capital costs reduction was proved (Caballero & Grossmann, 2004; Rev, Emtir, Szitkai, Mizsey, & Fonyo, 2001), many researchers start studying different applications for industrial retrofit (Errico, Rong, Tola, & Turunen, 2008; Kim, 2006) or improving the understanding of the dynamic behaviour of the structures and their controllability (Hernandez & Jimenez, 1999; Wang, Wong, & Yu, 2008). A lately introduced principle called Process Intensification allows considering distillation, energy and capital cost reduction, greenhouse gas emissions, as a single topic.
Optimal design of multiple dividing wall columns based on genetic programming
2011, Computer Aided Chemical Engineering