chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657Contents lists available at ScienceDirect Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd Energy saving and capital cost evaluation in distillation column sequences with a divided wall column Massimiliano Errico a,∗ , Giuseppe Tola a , Ben-Guang Rong b , Daniele Demurtas a , Ilkka Turunen c Università degli Studi di Cagliari, Dipartimento di Ingegneria Chimica e Materiali, P.zza D’Armi sn, I-09123 Cagliari, Italy University of Southern Denmark, Institute of Chemical Engineering, Biotechnology and Environmental Technology, DK-5230 Odense M, Denmark c Lappeenranta University of Technology, Department of Chemical Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland b a a b s t r a c t The divided wall column (DWC) to separate three components in a single distillation tower is receiving increasing interest in industrial applications due to the potentiality in energy and capital cost savings. In this work, the DWC configurations for the separation of a four components mixture was considered, and 5 different composition cases were analyzed. After selecting the best simple column (SC) sequence, the hybrid structures obtained by considering a configuration with a DWC replacing the first or the last two SCs of the sequence are considered. To simulate the DWCs a short-cut code was used to get the input data necessary to initialize the rigorous simulations. The results obtained for the hybrid structures were compared with the performance of the best SC sequence from which are derived to evaluate energy and capital cost savings. The Petlyuk and the DWC structures were considered independently in the capital cost evaluation to select the most convenient configuration. A significant energy reduction was achieved with DWC structures, while the saving in capital costs is lower than the 30% value reported in most of the specialized literature. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Divided wall column; Petlyuk column; Multicomponent distillation; Fully thermally coupled structure 1. Introduction Distillation is considered, by far, the prevalent method for separating fluid mixtures in the chemical and petrochemical industry. Despite its huge diffusion this method has the drawback of the high energy consumption. After the energy crisis and with the introduction of more strict environmental regulations, the necessity to define more energy efficient structures becomes the first step in the choice of the design alternatives. Many studies in the last decades focused on the problem of the best separation sequence for a given multicomponent mixture, from the analysis of the space including all the SC configurations (Thompson and King, 1972) to new distillation column arrangements recently proposed (Rong and Turunen, 2006). Among all the possibilities, the thermal coupling technique is considered as the most promising strategy to reach the scope of energy reduction in both design and retrofit cases (Calzon-McConville et al., 2006; Errico et al., 2008). It is well known that the thermal coupling is realized by the substitution of a condenser and/or a reboiler with a two-way liquid and vapour interconnecting streams between the distillation columns. In the case where all the possible thermal couplings are introduced at the same time and the separation is carried out by employing only one condenser and one reboiler, the structure is called fully thermally coupled configuration or Petlyuk column (Agrawal, 2000; Petlyuk et al., 1965). Completely thermal coupling structures were initially patented in the first half of the XX century by different authors (Brugma, 1942; Wright, 1949) and then reconsidered from the point of view of the reduction of the thermodynamic losses related to the separation technique (Petlyuk et al., 1965). The Petlyuk configuration for a 3 component separation is reported Corresponding author. Tel.: +39 070 675 5061; fax: +39 070 675 5067. E-mail address:
[email protected] (M. Errico). Received 28 October 2008; Received in revised form 18 May 2009; Accepted 20 May 2009 0263-8762/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2009.05.006 ∗ 5 different simple column sequences are possible. The first column. C c f L m P r TC V w heat exchanger area [m2 ] bottom component flowrate [kmol/h] annual incremental unit investment cost [$/m2 yr] annual incremental unit investment cost in condenser and reboiler equipment [$/m2 yr] cost of steam and coolant to vaporize and condense respectively 1 kmole of distillate [$/kmol] distillate component flowrate [kmol/h] direct structure column diameter [m] direct–indirect structure fractional plate efficiency feed flowrate [kmol/h] feed component flowrate [kmol/h] allowable vapour velocity [kmol/h m2 ] vapour-handling capacity of condenser and reboiler equipment combined [kmol/h m2 ] indirect structure indirect–direct structure side stream component flowrate [kmol/h] number of stages number of stages above the feed location column top pressure [kPa] heat exchanger duty [kW] feed quality reflux ratio number of stages below the feed location symmetrical structure rectifying section vapour flowrate [kmol/h] stripping section vapour flowrate [kmol/h] working time [h/yr] identification column number above the thermal coupling below the thermal coupling generic component condenser feed stage withdrawal of the liquid stream from the main column minimum prefractionator reboiler thermal coupling stage withdrawal of the vapour stream from the main column wall are separated. 2007). Christiansen et al. reported in Fig. compared to all the possible configurations. 1a and consists of two interconnected columns. The top column pressure was chosen as the minimum pressure. The results obtained are compared in order to identify the most energy saving solution. high modelling tools and dynamic simulations. B. are considered for a three components separation. 1987). and the 5 different composition cases reported in Table 1. in this work.99 for each component.. In fact the DWC is realized including the prefractionator in the main column. The resulting configuration. while the methodology was recently extended also to a higher number of components in the feed mixture (Rong and Turunen. to assure a . The Petlyuk or its equivalent DWC structure. in the second column the mixture of the lightest and the middle components is separated in the upper part of the column.1650 chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657 Nomenclature A b C C C d D DD DI E F f G G I ID m N n P Q q R r S V ¯ V Y Subscript 1. 1a and b respectively. Recently it was estimated that more that 100 DWCs are installed worldwide with a trend of 10 columns built each year (Parkinson.. It has been proved that the Petlyuk configuration requires. reported in Fig. from butane to heptane. The Petlyuk and the DWC configurations can be considered equivalent from the energetic point of view assuming that no heat transfer across the wall occurs (Lestak et al. 1997. performs a preliminary non-sharp separation for the middle boiling component. only Petlyuk/DWCs for the separation of three component mixtures are considered. the lowest total boil-up for a given separation for a three components ideal mixture (Fidkowski and Krolikowski. 2. The liquid reflux from the condenser and the vapour from the reboiler are splitted through on the two sides of the wall. was considered.. more suitable mathematical knowledge. anyway with modern control techniques. the possibility to use a Petlyuk/DWC together with a simple column for the separation of a four components mixture is evaluated considering different design lay-outs. Capital cost savings are expected to be considerable due to the single shell column configuration (Becker et al. 1999). 2006. The principal limitation in employing this structure was the lack in the design and the difficulty in the controllability of the system. The simplified DSTWU model based on the Winn–Underwood Gilliland method was first used to initialize the rigorous simulations performed with the RadFrac model. Often the Petlyuk configuration is associated to the divided wall column (DWC). while in the bottom the middle and the highest components A mixture of 1000 kmol/h of normal paraffin. 1994). 1b. For this reason. The two columns are connected by liquid and vapour countercurrent streams. Parkinson et al. For each composition case. Notwithstanding the evident benefits of this kind of configuration only in the last years it becomes more attractive and its applicability more realistic (Agrawal.0 and considering a molar purity of 0. consists of a single shell column with a partitioning wall that separates the feed location from the side draw of the middle boiling component. The total vapour reduction of these configurations was quantified in the range of 10–50% compared to the classical direct and indirect simple column sequences (Agrawal and Fidkowski. above or equal to the atmospheric one. 3 a b A.. moreover the Petlyuk and the DWC structures are independently considered for the capital cost evaluation. the problem can be easily overcome (Halvorsen and Skogestad. 1998). All the sequences were simulated using Aspen Plus 13. Case study Greek symbols ˛ relative volatility Underwood’s active root in Fig. In particular. according to Thompson and King (1972). 1999). called prefractionator. 2. 2001). 1997). However the controllability of the system was proved only for three components separations and up to date no serious attempts have been made to implement more complex systems. 70 0.chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657 1651 Fig.10 0.10 0. The operational costs related to hot and cold utilities are referred to European prices (Errico et al. 2 – TAC values for the SC sequences.25 0. .1 psi was considered and the tray efficiency was neglected. The downcomer area was assumed equal to 10% of the total tray area. 3. 2003).40 0. Table 1 – Composition feed cases. In the case of 4 component separation. 2008). In this way is it possible to use cold water as auxiliary fluid in the overhead condensers. heat exchangers area and energy consumption for the selected best SCs are reported in Table 2. The feed was assumed as saturated liquid at the pressure of 1 atm.40 0.8. Details about the configuration parameters. A minimum temperature approach of 10 ◦ C in the heat exchangers.25 0.25 0. The former is obtained utilizing the first simple column of the sequence followed by a Petlyuk/DWC..10 0. Generation of the alternative configurations Once that the best simple column configuration is identified. The column diameter calculations were performed assuming vapour velocities with a flooding fraction equal to 0. it is possible to define the new structures obtained by substituting 2 simple columns with a Petlyuk/DWC. In the latter case the first 2 simple columns are substituted by a Petlyuk/DWC followed by the last simple column of the sequence. 1 – (a) Petlyuk configuration. The heat exchanger area was evaluated with the usual design formula by considering an average heat exchange coefficient of 1800 kJ/(m2 h ◦ C) and 2100 kJ/(m2 h ◦ C) for condensers and reboilers respectively. Component Case 1 Case 2 Case 3 Case 4 Case 5 Molar fraction A: n-C4 H10 B: n-C5 H12 C: n-C6 H14 D: n-C7 H16 0.10 0. available in the simulator. The capital costs were updated to year 2008 by the Marshall and Swift index (Chemical Engineering.10 0. column diameters.10 0.10 0.25 0.10 0. The flooding velocity was estimated using the correlation given by Fair (1961). The Douglas’ correlations for the capital cost estimation were utilized (Douglas.40 distillate temperature at least of 50 ◦ C. 3 shows the structure considered for each composition case obtained Fig.10 0. 1988) considering that are simple to use and the results comparable to more recent data (Taal et al. Fig.70 0. 2. (b) DWC configuration and (c) three columns model. 2008). The best simple column configuration was identified by utilizing the total annual cost (TAC) as the economic index and the results are reported in Fig.. it is possible to generate two new structures.40 0. 3 simple columns are necessary and from each best simple column sequence selected by using the TAC index. Carbon steel shells and sieve trays 0.10 0. an annual running time of 8000 h/yr and a plant life time of 10 years were also assumed. A single tray pressure drop of 0.6 m spaced are considered for all the columns. 54 215 78 from the best simple column sequences already selected in the previous paragraph.73 656 259 13037.33 480 2. 1989).1652 chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657 Table 2 – Design parameters and energy consumption of the best SC configurations selected. Usually the design procedure for a simple distillation column starts with the choice of a short-cut method to initialize more rigorous calculations. Fenske.50 Case 2 C3 30 15 2..43 606 175 15077.39 18609.12 141 53 Case 4 C3 30 15 2. 4.83 326 320 C1 35 18 0. 2008). This procedure was chosen because allows to identify the complete space that includes all the possible operational points for the Petlyuk/DWC configuration. 1c. (2002). 2007).19 220 3. 1992.95 160 2.73 289 276 Case 5 C3 33 17 1. The application of the Underwood method for the evaluation of the minimum vapour flowrate in the column is the most used methodology and its modification for thermally coupled systems today is a well known procedure (Carlberg and Westerberg. Best SC C1 N Nf R P [kPa] DD [m] Ac [m2 ] Ar [m2 ] Qc [kW] Qr [kW] 40 16 0. Sotudeh and Shahraki.48 211 76 12339. etc.77 790 355 C2 60 27 1. like Aspen Plus.20 110 2.86 C1 40 18 3. (1) can be solved with a common “regula falsi” to obtain the two active roots. Sotudeh and Shahraki. 2001).96 480 2. 2002.76 C1 40 20 1. Instead different approaches are proposed for the evaluation of the minimum and the theoretical number of stages (Kim. For this reason DWCs must be considered as a combination of simple columns connected by thermal couplings (Becker et al. The Petlyuk/DWC configuration can be modelled using the three columns model reported in Fig. Gilliland.00 C1 32 16 0.00 310 1. In this work the procedure followed by Muralikrishna et al. Petlyuk/DWC configurations are not available as a standard unit operation already implemented in the simulator libraries.07 15073.57 C3 42 21 2.05 110 2.86 160 108 Case 3 C3 28 14 1.41 160 2..62 233 345 C2 27 14 1. It is possible to extend this “modus operandi” also to DWCs. Eq.63 286 183 10820.83 480 3. In the first column it is possible to apply the Underwood equations in their original form (Underwood.00 480 2.15 110 2.76 338 593 C2 40 20 2. Modelling Develop a design model to describe the steady state behaviour of a divided wall column is not an easy task. In the most used process simulation packages.85 110 1.10 749 857 Case 1 C2 33 14 2.) is actually under discussion (Triantafyllou and Smith. 1948): ˛i · fi = F · (1 − q) − j (1) i ˛i i ˛i ˛i · d1i = (d1A + d1B + d1C ) · (1 + R1m ) − j (2) By considering that in the prefractionator there is one distributing component (B). even if the application of the traditional short-cut methods (Underwood. . then the identified space can be explored to get the best solution by using a specific objective function. has been considered.97 458 365 C2 45 20 3.50 170 1. 2001.44 16409.71 14687.12 17412. Muralikrishna et al.00 160 1.87 812 212 17182. here briefly resumed. For a separation Fig. 3 – Structures derived from the best SC configurations.42 480 1. 1980) and defining an effective reflux ratio above the minimum. Simulation and results 3. 1975): TAC = C C (1 + R) + C (1 + R) N(1 + R) + E×Y×G Y×G (12) 2. included in the first term of Eq. Anyway at this stage of evaluation. the feed locations. it is possible to calculate the theoretical number of stages by the Gilliland correlation (Gilliland.0. Obviously to adapt the three columns model to the Petlyuk/DWC configuration the following two conditions must be satisfied: 1. the feasible design space must include all the cases with a reflux ratio of the prefractionator higher than the correspondent minimum value: R1 ≥ R1. the thermal coupling flowrates are checked and then . The second condition requires that the number of theoretical plates in the prefractionator is equal to the sum of the stages of the stripping section (below the thermal coupling connection) and of the rectifying section (above the thermal coupling connection) of the second and third column respectively. but is derived from the practical observation that if a DWC is considered. d1A ≥ d2A (5) Once the design space is defined the simplified function for the TAC reported in Eq. The methodology sequence described can be applied also to the other two columns considering the modified Underwood equations so as proposed by Carlberg and Westerberg (1989). the B quantity in the bottom of the prefractionator must be at least equal to the B quantity in the bottom of column 3. The design of the prefractionator ends with the application of the Kirkbride equation (Kirkbride.49 4. 1932) can be utilized to calculate the minimum number of stages for the separation.m (10) 2. V2 = V3 (3) Table 3 – Cost parameters used in Eq. the rectifying vapour flowrate for column 3 must be equal to the stripping vapour flowrate of the column 2. the number of stages for the prefractionator must be higher than the sum of the minimum number of stages corresponding to the stripping and rectifying sections of column 2 and 3 respectively obtained from the Fenske and Kirkbride equations: (r2 + n3 )m ≤ N1 (11) This condition is not a real bond. With some mathematical elaborations (Treybal. (12). Cost parameters C [$/m yr] C [$/m2 yr] C [$/kmol] Y [h/yr] E [%] G [kmol/h m2 ] G [kmol/h m2 ] 2 Value 296. This consideration is valid when a liquid withdrawal is considered.61 × 10−3 8000 90 219. the maximum quantity of component A in the prefractionator distillate flowrate is limited from the A quantity in the feed.chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657 1653 that takes place at infinite reflux ratio. in the case of Petlyuk/DWCs. this approximation seems reasonable for the first selection of trial design parameters. 1940). Both the columns downstream to the prefractionator realize a sharp separation between the light and heavy key component. d1B ≥ d2B (6) It should be noted that this relation was originally developed for simple column configurations but.01 17. b1B = (fB − d1B ) ≥ b3B d1B ≤ fB − b3B (8) 5. 5. In this way it is possible to merge the two columns in an only one. and taking into account that the design space was mapped using a short-cut method. are higher compared to the simple column configurations due to the internal wall. d1A ≤ fA (7) Using the short-cut methodology for the Petlyuk/DWC configuration a Fortran code was compiled to initialize the rigorous simulations performed by means of Aspen Plus 13. The definition of the design space is related to seven bounds summarized as follows: 1. Table 3 summarizes the parameters values used in Eq. Since no distributing components are present only one Underwood’s root is active. (12) was chosen to select the attractive design options for the subsequent rigorous simulations (Happel and Jordan.76 20. 1944) to identify the feed location. the prefractionator distillate flow rate of component A is higher than or equal to the same component flow rate in column 2. the Fenske equations (Fenske. N1 = r2 + n3 (4) 7. All the parameters obtained. the flowrate of C from the bottom of the prefractionator should be equal to or higher than the quantity of the same component in the residue of column 3: b1C = (fC − d1C ) ≥ b3C d1C ≤ fC − b3C (9) 6. (12). Considering that columns 2 and 3 substitute the main column of the Petlyuk configuration. like the number of trays.71 0. no simplified expressions are available and probably for these systems the internal column costs. (12). the prefractionator distillate flowrate of component B is higher than or equal to the same component flowrate in column 2. it is better to have a close number of plates on both sides of the wall to assure a good column stability. 01/3. optimized by sensitive analysis to reach the minimum energy consumption.40 975. 5.75 160 2.00 306. the substitution of the last two columns with a Petlyuk/DWC forces to perform the separation at the highest pressure to assure the possibility to condensate the lightest component in ordinary water cooled condenser.37/1.40/3.90 480 1. in the SC + DWC configuration the first simple column removes the excess of the Table 5 – Design parameters and energy consumption of the DWC + SC sequences.77 790 355 Case 3 SC 40 – 20 – – – – – 1. C) are separated at the same pressure with a penalty for the separation efficiency. For composi- tion case 2. diameters. Composition case 5 is the only one where the SC + DWC configuration is more energy demanding compared to the corresponding best SC.75 160 1. 3.60 5.78 18464. In this way all the three components (A.98 310 3.22 14989. The reason can be attributed to the feed component distribution: in cases 1 and 3 the Petlyuk/DWC column is fed by an equimolar stream.24 16297.69/3.50a 1106 234 14998.80 480 3.26/1. . for this composition case.51 13399. Comparing.23 21780. the best SC sequence with those derived substituting 2 simple columns with a Petlyuk/DWC. For composition cases 1.86 159 87 Case 3 SC 28 – 14 – – – – 1.87a 1694 471 15419. for each composition case.96 480 2.00 14.62 SC 42 – 21 – – – – – 2.37a 1077 549 13617. B.80 5.60 133.10 749 857 Prefractionator/main column.73 335 187 DWC 34 24 12 4 24 14 252. are summarized in Tables 4 and 5. thermal couplings flowrate.94a 1536 1538 16506.00 480 2.61 18489.76 338 593 Case 4 SC 40 – 18 – – – – – 3.55 400 2. 3 and 4 the direct sequence is the best simple configuration. The pressure of the Petlyuk/DWC configuration is defined according to the highest pressure value of the substituted SCs. In composition case 5 the feed contains an equal excess of the lightest and heaviest components and taking into account that the best SC is the direct-indirect. The saving is quantified in about 12.00 936. The results of configuration parameters.68 299 231 Case 5 SC 33 – 20 – – – – – 1.96a 402 140 10657.91 480 2.02 110 2.92a 1428 728 14256.55 DWC 33 22 19 5 27 15 756. This aspect is considered first using the total condenser and reboiler duty to compare. Energy comparison The main advantage expected for these types of configurations is the possibility to achieve an energy load reduction. the first SC remains unchanged and performs the separation of the lightest component at the highest pressure.97 458 365 DWC 32 19 10 6 23 15 86.17a 1586 1586 16845. or equal to the approximation of the evaluation method.85 110 1. heat exchanger area and energy consumption.00 864.84 40 – 16 – – – – – 0.42 480 1.05 160 2. where the best SC sequence is the indirect one.19 220 3. Thus.00 8.40 280.80a 1312 692 15346. instead in case 4 there is an equal excess of component B and C removed from the distillate and the side stream respectively.12 141 53 Case 4 SC 30 – 15 – – – – – 2.00 160 1.1654 chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657 Table 4 – Design parameters and energy consumption of the SC + DWC sequences.62 233 345 Case 5 SC 32 – 16 – – – – – 0.01/4.80 1.83 480 3.5 160 2.1.70 DWC 32 19 9 5 23 13 248.17 110 2.89a 1188 613 14197. DWC + SC Case 1 DWC N Np Nf NL NV Ns LTC [kmol/h] VTC [kmol/h] R P [kPa] DD [m] Ac [m2 ] Ar [m2 ] Qc [kW] Qr [kW] a Case 2 SC 30 – 15 – – – – – 2.00 3.80/5.00 5.37 206 55 34 24 17 4 28 14 162.27 DWC 35 17 6 7 23 13 306.35 Case 2 SC 35 – 18 – – – – – 0.20 406.80 5.53 18114. SC + DWC SC N Np Nf NL NV Ns LTC [kmol/h] VTC [kmol/h] R P [kPa] DD [m] Ac [m2 ] Ar [m2 ] Qc [kW] Qr [kW] a Case 1 DWC 36 20 13 7 25 20 165. 6 and 8% for the reboiler duties respectively.34 17138.28 19015.33 480 2. so as reported in Fig.18/3.29 Prefractionator/main column. 10 and 11% for condenser and 9. the results for the best SC reported in Table 2 with those obtained for the SC + DWC sequence included in Table 4 it is possible to notice that the energy consumption can be considered similar.20 4. then the Petlyuk/DWC column completes the separation.97a 388 201 10838.00 1144.68 16735. by considering first the combination SC + DWC.00 17.62 DWC 35 23 16 7 29 23 396.00 1692.40 420 2.57/3.52 DWC 39 17 10 9 22 13 288.67/3.28 DWC 35 20 13 8 26 21 331.00 540. so the feed to the Petlyuk/DWC is a stream unbalanced in the heaviest component. This zone corresponds to the right side of the wall section in the DWC. There is the only exception of the composition case 4 where a saving of 4% in the condenser duty was achieved. are reported in Table 6 together with those of the Petlyuk configurations. compared to the traditional SC sequence. considering that the main scope of the alternative proposed configurations is to reduce the energy demand of the plant. are reported in Table 6. The results obtained for the capital cost evaluation. For all the composition cases the DWC + SC configuration appears to be not convenient from the energetic point of view.09 DWC 0. Anyway it is not clear how this value was obtained.chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657 1655 lightest component. but no indications are provided about the diameter considered for the zone of the column with the wall (Wolff and Skogestad. so as previously described. DWC capital cost evaluation 5. derived from the direct or from the direct indirect (cases 1. It is possible to notice that in the column section included between the thermal couplings there is a reduction of the diameter value.1. 2001). A method for the capital cost evaluation of this column is actually not well defined. First of all in the case of DWC the presence of the internal dividing wall makes unreliable the classical equations used for the plate cost calculations. Table 7 reports the results of the diameter calculations considering the three sections of the DWC: the stages above.e. while the duty of the reboiler is similar to that of the corresponding best SC sequence. Table 6 – Normalized capital cost of the selected configurations for the Petlyuk and the DWC design.2.2. 4 – Diameter stage distribution in the Petlyuk main column for the selected cases. 2004.2. normalized with respect to the capital cost of the corresponding best SC. Fig. Both the columns’ cost can be evaluated using the classical simplified correlations (Douglas. It is possible to conclude that the pressure and the feed component distribution are the main parameters that affect the energy consumption of the Petlyuk/DWC configurations when are used in substitution of two SCs. thus reducing the convenience to employ the Petlyuk configuration as evidenced in cases 3 and 4. it is possible to notice that even if the DWC allows to reduce the number of equipments. Petlyuk capital cost evaluation The Petlyuk configuration. in most of the cases. Amminudin et al. Lestak and Collins. 1988). From these results it is possible to notice that the Petlyuk configuration is particularly convenient in composition case 1 where the main column is fed at the lowest flowrate compared to the other cases considered. but no indications were provided about the total exchanger area that is the main parameter related to the cost. The main column diameter increases as the feed flowrate increases. A few researchers made the assumption to use the classical cost evaluation formulas considering in the case of the DWC configuration only the stages of the main column. For the composition case 2 the highest pressure column is the SC. .98 capital cost. The energy performances of the DWC + SC configurations are summarized in Table 5 together with the design parameters. the total exchanger area.89 0. By comparing the results reported in Table 2 for the best SCs and in Tables 4 and 5 for the SC + DWC and DWC + SC configuration respectively.. normalized with respect to the corresponding best SC cases. Also some savings in the column shell cost are achievable but it was not evidenced how to allocate the prefractionator in the main column and how to evaluate the corresponding diameter.86 0. the Petlyuk/DWC is forced to operate at the highest pressure with a penalization in the separation efficiency. To this regard the first parameter that we considered for the comparison is the heat exchanger area requested for condensers and reboilers. 1997). It is known that employing a DWC for the separation of a three components mixture allows saving one condenser and one reboiler.2. 3 and 4). 5. but on the other hand simplified correlations are not available and the main producers are reluctant to give this type of information. In the other configurations. it was chosen to limit the capital cost analysis only to the sequences where an appreciable energy saving was achieved (i. the Petlyuk/DWC configuration has the potential of a capital cost reduction. To allocate the prefractionator in the main column is necessary to add the area of the prefractionator to the corresponding area of the main column. below the wall and in the divided wall section. utilizing the columns diameter and the stages number reported in Table 4 for the prefractionator and the main column respectively. The second parameter considered for the capital cost evaluation is the sum of the costs associated to the distillation column internals and shell. The column height for the prefractionator is calculated without considering vapour and liquid disengagement. 5. Anyway. anyway the DWC is fed with an excess of heaviest component that makes not suitable to employ this configuration. 4 shows the diameter profile in the main column of the Petlyuk obtained from the simulations for the composition cases selected. configurations SC + DWC for composition cases 1. Capital cost comparison Moreover the possibility of energy saving. In most of the specialized literature a reduction of about 30% of the capital cost was estimated (Kolbe and Wenzel. is the combination of two columns. To this regard the column capital cost of the Petlyuk and DWC configuration must be considered separately. SC + DWC Case 1 Case 3 Case 4 Petlyuk 0. the prefractionator and the main column connected by thermal couplings. is higher than of the simple column configurations. 3 and 5). The results obtained for the total Fig.99 0. in this way it is possible to define the diameter required for the DWC. From the results it is evident the possibility to take advantage from the construction of a single diameter column. 1995.99 1. . Brugma. (1980).. Christiansen. 2006. 1999. S.. I. 72: 639–644.H.M. 1961. R. B.-G. B. Structural design of extended fully thermally coupled distillation columns. Part A. Design and optimization of fully thermally coupled distillation columns.75 Thus from the results it is possible to conclude that the DWC layout outperforms or equalizes at least (case 3) the Petlyuk disposition. K. 1997. N. M. D. U. Fractionation of straight-run Pennsylvania gasoline. and Stehlik. Int Chem Eng. Are thermally coupled distillation columns always thermodynamically more efficient for ternary distillation? Ind Eng Chem Res. (Marcel Dekker. N.. J. C. Chem Eng.256.. Ind Eng Chem Res. 2002.. S. and Lien. 47(6): 1975–1980. The most promising cases correspond to Petlyuk/DWC fed with equimolar of equal excess of lightest and middle component mixtures. Sotudeh. S. Happel. More operable fully thermally coupled distillation configurations for multicomponent distillation.W. 1997. pp. direct-indirect sequences) it is convenient to use first a SC and then to perform the last separation by using a Petlyuk/DWC arrangement. Dividing-wall columns find greater appeal. and Turunen. G. F. Part A.. Part A. Patent No. and Wenzel. Douglas. (2008).91 Case 3 3. and Turunen. 108(1): 68–74. Underwood’s method for petlyuk configuration.. 77: 543–553. Process intensification for the retrofit of a multicomponent distillation plant—an industrial case study. R. 2006.. Kolbe. From the obtained results it can be concluded that the DWC outperforms both the SC configuration and the Petlyuk structure. The most promising structures are first selected on the basis of the lowest energy consumption... 1942. S. 1972. Novel distillation concepts using one-shell columns. H.G. Ind Eng Chem Res.W. Thong. 1940. 3. Chem Eng. 78: 454–464. C. 23: 1819– 1835.B.e. 115(4): 78. A. B.97 1. Rong. 1965. N. 2001. Chem Eng Technol. Fair. G. 33(4): 643–653. and Shahraki. AIChE J. Thermodynamically optimal method for separating multicomponent mixtures. Partitioned distillation columns-why.. Chemical Process Economics (2nd edition).M. Chem Eng. A. G. and Shah. Kreis. R. D’Aquino. Rosales-Zamora. 106(4): 32–35. Conclusions The DWC configurations for the separation of a four components mixture with 5 different feed composition cases are studied. Rong. and Jordan. Advanced distillation saves energy & capital. 1997. and Slavinskii. J. 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