Ind. Eng. Chem. Res. 2008, 47, 1975-1980
1975
Process Intensification for the Retrofit of a Multicomponent Distillation PlantsAn Industrial Case Study Massimiliano Errico,† Ben-Guang Rong,*,‡ Giuseppe Tola,† and Ilkka Turunen‡ Dipartimento di Ingegneria Chimica e Materiali, UniVersita` degli Studi di Cagliari, P.zza D’Armi s.n., 09123 Cagliari, Italy, and Department of Chemical Technology, Lappeenranta UniVersity of Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland
Process intensification is currently considered as the main trend to improve process performance, and one of the major approaches to this regard is to reduce the number of pieces of equipment in the plant. For distillation systems, thermal coupling technique provides such an approach to retrofit the traditional simple column configurations through the elimination of the condenser and/or the reboiler. In this work, starting from the existing plant with the simple columns, two types of retrofit alternatives are proposed. The first includes the possible alternatives by eliminating all the condensers and reboilers involving the submixtures. The second includes all the possible alternatives when only some of the condensers and/or reboilers are eliminated at one time. The latter solution represents less modification to the existing plant. In addition, in order to investigate the capital investment for the modified plant, all the obtained alternatives through the recombination of the column sections have been compared. All the configurations were compared with respect to energy saving and the possibilities to reuse the plant equipment. The optimal solution was found to be the alternative that has the minimum modification to the existing plant. At the same time, it achieves a 23% savings on energy consumption and allows a maximum reuse of the pieces of the existing equipment. 1. Introduction Distillation is by far the most widely used separation method for multicomponent mixtures in chemical and petrochemical plants. Yet, a number of existing distillation plants use the conventional simple column configurations that suffer from highenergy consumptions. As energy is ever becoming a critical issue in fulfilling sustainable development, it is a significant problem to pursue energy savings for such distillation plants in the context of process retrofit. Among the alternative techniques for the retrofit of conventional distillation schemes, recent studies1-3 showed that the thermal coupling technique is one of the most promising options. On the one hand, it is implemented by directly eliminating some condensers and reboilers of the traditional distillation configurations, and on the other hand, it provides the potential to save both energy and capital costs. Moreover, it is shown that, in certain cases, the thermally coupled configurations can outperform traditional simple column configurations in terms of dynamic responses.4 In this work, the retrofit of the separation plant of a light ends stream from a crude distillation unit is studied. The existing distillation plant consists of three simple columns. The feed of normal paraffin with four components from butane to heptane is separated into four product streams with required purity specifications. Starting from a simple column configuration (SC), by applying the thermal coupling technique, it is possible to obtain two types of thermally coupled configurations: one consists of the options where all of the condensers and reboilers associated with submixtures are eliminated through thermal couplings, which represents a maximum modification to the distillation plant. Another consists of a subspace of the options * To whom correspondence should be addressed. Tel.: +358 5 6216113. Fax: +358 5 6212199. E-mail:
[email protected]. † Universita` degli Studi di Cagliari. ‡ Lappeenranta University of Technology.
where only some of the condensers and reboilers associated with submixtures are eliminated through thermal couplings. This subspace provides the important options for retrofit because it represents a class of alternatives with less modification to the plant. As a consequence, one can expect the retrofit target toward both energy saving and less plant modification. Another significant issue for such plant retrofit is the reuse of the existing equipment. This will specifically concern two aspects. First, because some of the condensers and reboilers in the existing plant will be eliminated in the thermally coupled configurations, the remaining heat exchangers will have different heat loads from that in the original plant. As a consequence, it is important to investigate the reemployment of the eliminated heat exchangers in the modified configurations. Second, as will be shown later that some retrofit configurations are obtained by recombination of the column sections of the original plant, how to maximum reuse of the existing columns to construct the new ones in the modified configurations is also significant in terms of capital cost savings. The aim of this paper is to study the retrofit of a real industrial distillation plant taking into consideration both energy savings and capital costs. The target is not just to identify the best solution in terms of total annual cost (TAC) like in the design of a new plant, but to find the solution that matches energy savings with a maximum usage of the existing equipments, such as trays, exchangers, and columns. A detailed comparison in terms of energy consumption, heat exchanger area, and column section reuse is performed for the considered configurations in order to identify the best solution. 2. Generation of the Retrofit Alternative Configurations from the Existing Plant It is known that, for the separation of pure components from an n-component mixture, (n - 1) simple columns are needed. Thompson and King5 reported the following formula to calculate the number of all simple configurations.
10.1021/ie070544a CCC: $40.75 © 2008 American Chemical Society Published on Web 02/19/2008
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Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008
Sn )
[2(n - 1)]! n!(n - 1)!
(1)
The plant configuration considered in our case for the separation of the mixture reported in Table 1 is shown in Figure 1. This configuration can be classified as a direct-indirect sequence where the lightest component is removed first, followed by removal of the heaviest component, and finally separation of the two remaining components. The sequence can be represented by the following separation task: A/BCD; BC/D; B/C. Other four simple column configurations are possible for this separation, the direct (A/BCD; B/CD; C/D) and the indirect (ABC/D; AB/C; A/B) sequences, the indirect-direct (ABC/D; A/BC; B/C) sequence, and the distributed sequence (AB/CD; A/B; C/D). These configurations were already considered and compared in terms of energy consumption and TAC in our previous work.6 It was shown that, for this existing distillation plant, the thermally coupled configurations give significant savings on both energy consumption and TAC. It must be indicated that the previous work only considered such thermally coupled configurations where all the possible thermal couplings have been introduced, which require maximum modifications to the conventional simple column configurations. However, from the existing plant configuration, it is possible to generate the feasible thermally coupled configurations with a lower number of thermal couplings. These configurations have the distinct feature to require small modifications to the existing distillation plant, which is favorable for the reduction of the capital investment, and they have the potential to allow energy savings similar to those with all possible thermal couplings introduced. The total number of configurations from a simple column sequence, for a mixture of four or more components, was derived by Rong and Kraslawski.3 n-3
Cn )
(n - 2)!
+1 ∑ j ) 1 j!(n - 2 - j)!
(2)
It is easy to see that, for the plant design of Figure 1, there are three possible configurations as shown in Figure 2. Figure 2a shows the configuration by eliminating the reboiler of the first column associated with the submixture BCD. Figure 2b is obtained by eliminating only the condenser of the second column associated with the submixture BC, and in Figure 2c, both the condenser and the reboiler are substituted with thermal couplings. Parts a and b of Figure 2 form the subset of thermally coupled configurations where the thermal coupling is not introduced simultaneously for all the submixtures. All the configurations shown in Figure 2 form the family of the original partially thermally coupled (OPC) schemes. By defining a column section as a portion in a distillation column that is not interrupted by entering streams or heat flows,7 every section was numbered in all the figures. By examining the OPC schemes, the thermodynamically equivalent structures (TES) are obtained. In Figure 2b, the liquid reflux flow rate of the second column is provided from the third column by condenser B in section 5; thus, the rectifying section 5 can be moved above section 3 of the second column. Similar considerations can be
Figure 1. Plant configuration.
applied to the reboiler D in Figure 2a or to the condenser B and the reboiler D at the same time in Figure 2c. The five thermodynamically equivalent configurations are shown in Figure 3. Specifically, Figure 3a is derived from configuration 2a, Figure 3b is derived from configuration 2b, and parts c, d, and e of Figure 3 are derived from 2c. These configurations have the same energy consumption as the corresponding OPC. However, through rearrangement of the column sections, they provide the possibility to mitigate the uneven distribution of vapor and liquid flows within the OPC. For this reason, it is possible to obtain a better column equipment design with improved hydraulic performance8 and more operable structures with respect to the vapor transfer between the columns.9 3. Simulation and Results All the configurations shown in Figures 1-3 were simulated for the required molar purity of 85% for the components A and B, 89% for C, and 98% for D. All the simulations were performed with the rigorous RADFRAC model in Aspen Plus 12.0 in order to obtain the data to evaluate capital costs and energy consumptions. The Wilson equation was adopted for the calculation of the activity coefficients of the liquid phase, whereas ideal behavior was assumed for the vapor phase. The real plant shown in Figure 1 consists of three columns. The first column has 28 trays with the diameter of 2.5 m, the second one has 42 trays and the diameter of 3 m, and the third one has 34 trays and the diameter of 2 m. Single-pass sieve trays are used with a downcomer area, expressed as a fraction of the total tray area, equal to 0.1. The tray spacing is 0.61 m. The simulation of the existing plant was checked for temperature profiles, product flow rates, molar purities, condenser and reboiler duties, and column diameters. Good agreement was obtained between the simulated results and the plant data; a comparison between the relevant parameters is reported in Table 2. The difference between these values is
