A new fully thermally coupled distillation column with postfractionator

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Abstract

For the improvement of distillation column efficiency, a new system of a fully thermally coupled distillation column is proposed and its performance is examined with two industrial processes. The system has an extra column called postfractionator and attached to the main column of an original fully thermally coupled distillation column.

The outcome of performance investigation indicates that a 29% energy saving is yielded with fractionation process and a 7% saving with gas concentration process over the original fully thermally coupled distillation column. The design detail of the new system is explained, and the increase of distillation column efficiency is discussed. In addition, some considerations on the new column construction, such as arrangement of distillation column sections and divided wall construction, are discussed here.

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.

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