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1 Computers and Chemical Engineering 38 ( Contents lists available at SciVerse ScienceDirect Computers and Chemical Engineering j ourna l ho me pag e: w ww.elsevier.com/locate/compchemeng Innovative dimethyl ether synthesis in a reactive dividing-wall column Anton A. Kiss a,, David J.-P.C. Suszwalak a,b a AkzoNobel Research, Development & Innovation, Process Technology ECG, Zutphenseweg 10, 7418 AJ Deventer, The Netherlands b Ecole Nationale Supérieure de Chimie de Mulhouse (ENSCMu, Mulhouse, France a r t i c l e i n f o Article history: Received 5 September 2011 Received in revised form 15 November 2011 Accepted 18 November 2011 Available online 26 November 2011 Keywords: DWC Reactive distillation Dimethyl ether dehydration Capital and energy savings a b s t r a c t Dimethyl ether (DME is of great industrial interest due to its use as clean fuel for diesel engines or in combustion cells, as a precursor to other organic compounds, as well as a green aerosol propellant that can effectively replace chloro-fluoro-carbons. Conventionally, high purity DME is synthesized by dehydration of methanol produced from syngas, in a process involving a catalytic fixed-bed reactor and a direct sequence of two distillation columns. The key problem of this classic process is the high investment costs for several units that require a large overall plant footprint, as well as the associated high energy requirements. To solve these problems, we propose in this work an innovative DME process based on a reactive dividing-wall column (R-DWC that effectively integrates in one shell a reactive distillation (RD unit with the DWC technology. The double integrated system allows the production of high-purity DME in only one unit, with minimal footprint and significantly lower costs. This study also makes a fair comparison between the reported conventional DME process and the optimally designed process alternatives based on RD and R-DWC, respectively. All processes are optimized in terms of minimal energy requirements, using the state of the art sequential quadratic programming (SQP method implemented in AspenTech Aspen Plus. The results clearly demonstrate that the R-DWC process has superior performances as compared to the conventional or RD process: significant energy savings of 12 58%, up to 60% reduced CO 2 emissions, as well as up to 30% lower capital investment costs Elsevier Ltd. All rights reserved. 1. Introduction The more restrictive environmental regulations led to raising interest in dimethyl ether (DME as clean fuel for diesel engines or in combustion cells, as a precursor to organic compounds, and as a green aerosol propellant that can effectively replace chloro-fluorocarbons (CFC. Consequently, the large-scale production of DME needs to increase capacity in an efficient way in order to meet the accelerated growing market demand (Arcoumanis, Bae, Crookes, & Kinoshita, Dimethyl ether is the simplest aliphatic ether, with notable properties: non-toxic colorless gas, non-corrosive, noncarcinogenic and environmentally friendly (Muller & Hubsch, As a clean fuel, DME produces no SOx and minimal NOx and CO emissions, while its high oxygen content results in soot-free combustion in engines. Remarkable, DME also exhibits low autoignition temperature and it has a higher cetane number (55 60 as compared to petroleum diesel (Arcoumanis et al., 2008; Ciftci, Corresponding author. Tel.: addresses: tonykiss@gmail.com, Tony.Kiss@akzonobel.com (A.A. Kiss. Sezgi, & Dogu, 2010; Pop, Bozga, Ganea, & Natu, 2009; Yaripour, Mollavali, Jam Sh, & Atashi, Currently, DME is produced by the conversion of various feedstock such as natural gas, coal, oil residues and bio-mass into syngas (CO/H 2, followed by a two-step process: methanol synthesis and then methanol dehydration. is produced first from syngas over a copper-based catalyst (Cu/Zn, Cu/Zn/Al, Cu/Zn/Co, and then it is dehydrated over a -alumina catalyst or zeolites in order to produce DME (Chen, Zhang, Ying, & Fang, 2010; Lu, Teng, & Xiao, 2004; Mao, Yang, Xia, Zhang, & Lu, 2006; Moradi, Ahmadpour, Nazari, & Yaripour, 2008; Nie, Liu, Liu, Ying, & Fang, The methanol dehydration step takes place at temperatures of C and pressure up to 20 bar. The current industrial process involves a fixed-bed reactor, followed by a direct sequence of two distillation columns delivering high-purity DME (>99.99 wt% that is virtually odorless (Muller & Hubsch, Fig. 1 (top illustrates the simplified conventional flowsheet for methanol dehydration. The reaction of pure, vaporized methanol is carried out in a fixed-bed catalytic reactor. The outlet of the reactor consists of DME, water and methanol. This is cooled and subsequently distilled in the first tower to yield pure DME. The unreacted methanol is separated from water in a second distillation column and recycled back to the reactor (Muller & Hubsch, /$ see front matter 2011 Elsevier Ltd. All rights reserved. doi: /j.compchemeng

2 A.A. Kiss, D.J.-P.C. Suszwalak / Computers and Chemical Engineering 38 ( Reactive Distillation DME RX DC1 DC2 Fixed-bed reactor Water DME Distillation Reactive DWC RDC DC Water Dividing Wall Column Fig. 2. Path from conventional setup to reactive dividing-wall column (R-DWC. R-DWC DME Water Fig. 1. Simplified DME production processes alternatives: conventional process (top, reactive distillation (mid and reactive dividing-wall column (btm. Different types of solid acid catalysts can be used, such as alumina ( -Al 2 O 3, HZSM-5, silica-alumina, phosphorous-alumina and fluorinated-alumina. Among them -alumina is the preferred one due to its thermal stability, mechanical resistance, high surface area and catalytic properties, even if it produces undesirable side products such as hydrocarbons (Muller & Hubsch, Recent investigations were performed in order to find better catalysts with higher selectivity toward DME and less production of hydrocarbons (Ciftci et al., 2010; Mollavali, Yaripour, Atashi, & Sahebdelfar, 2008; Pop et al., 2009; Xu, Lunsford, Goodman, & Bhattacharyya, 1997; Yaripour, Baghaei, Schmidt, & Perregaard, 2005; Yaripour et al., Note that in the quest of finding alternative methods for DME production, one of the main subjects of recent research is the syngas to dimethyl ether (STD process that allows the DME production from syngas via a one-step synthesis in a single reactor (Hadipour & Sohrabi, 2008; Vakili, Setoodeh, Pourazadi, Iranshahi, & Rahimpour, Additional research studies investigated the possibility of using various reactive distillation (RD alternatives for the methanol dehydration to DME (An, Chuang, & Sanger, 2004; Lei, Zou, Dai, Li, & Chen, The process flowsheet shown in Fig. 1 (mid involves a RD column followed by an ordinary distillation column (DC for methanol recovery. Known also as catalytic distillation when a solid catalyst is used RD is a proven process intensification technique that combines reaction and distillation in one operating unit (Schoenmakers & Bessling, Already implemented in industry for many equilibrium limited reactions, reactive distillation can greatly improve the performances of a process, by reducing the capital investment costs and the energy requirements (Harmsen, 2010; Huss, Chen, Malone, & Doherty, 1999, 2003; Kiss, 2011; Malone, Huss, & Doherty, 2003; Sundmacher & Kienle, Other innovative solutions to overcome the drawback of energy intensive distillation are using thermally coupled distillation columns, dividing-wall columns (DWC, heat-integrated distillation or cyclic distillation (Olujic, Jodecke, Shilkin, Schuch, & Kaibel, 2009; Dejanović, Matijašević, & Olujić, 2010; Dejanović, Matijašević, Jansen, & Olujić, 2011; Maleta, Kiss, Taran, & Maleta, The Petlyuk configuration, consisting of two fully thermally coupled distillation columns (Petlyuk, Platonov, & Slavinskii, 1965, evolved to the practical implementation in a DWC that splits the middle section of a single tower into two sections by inserting a vertical wall in the vessel, at an appropriate position. DWC found great appeal in the chemical process industry as it can separate more components in a single distillation unit, thereby saving the cost of building two columns and cutting the operating costs by using a single condenser and reboiler. In fact, using DWC can save up to 30% in capital investment and up to 40% in operating costs (Isopescu, Woinaroschy, & Draghiciu, Several good reviews and research papers were recently published on this topic, covering the design, simulation, control, optimization and applications of DWC (Asprion & Kaibel, 2010; Dejanović et al., 2010; Kiss & Bildea, 2011; Kiss & Rewagad, 2011; Rangaiah, Ooi, & Premkumar, 2009; Rong & Turunen, 2006; Segovia-Hernandez, Hernandez-Vargas, & Marquez-Munoz, 2007; van Diggelen, Kiss, & Heemink, 2010; Yildirim, Kiss, & Kenig, Remarkable, the DWC technology is not limited to ternary separations alone, but it can be used also in azeotropic separations (Midori, Zheng, & Yamada, 2001, extractive distillation (Kiss & Suszwalak, 2012; Bravo-Bravo et al., 2010, and even reactive distillation processes (Hernandez et al., 2009; Kiss, Pragt, & van Strien, 2009; Kiss, Segovia-Hernandez, Bildea, Miranda-Galindo, & Hernandez, 2012; Mueller & Kenig, In this study we propose a novel process for DME production by methanol dehydration, based on a reactive dividing-wall column (R-DWC as illustrated by Fig. 1 (btm. is fed on top of the reactive zone where the heterogeneous catalyst is located, while DME is produced as top distillate, water as bottom product, and methanol as side stream product that is recycled. Fig. 2 conveniently illustrates the evolution of ordinary distillation to DWC or reactive distillation, and eventually to the double integrated reactive DWC system (Yildirim et al., 2011.

3 76 A.A. Kiss, D.J.-P.C. Suszwalak / Computers and Chemical Engineering 38 ( This work explores the RD and R-DWC alternatives, while also making a fair comparison between the reported conventional DME process (Lei et al., 2011 and the optimally designed process alternatives based on RD and R-DWC, respectively. All the processes are optimized in terms of minimal energy requirements, using the state of the art sequential quadratic programming (SQP method implemented in Aspen Plus. 2. Problem statement The conventional process for DME synthesized by methanol dehydration involves a catalytic fixed-bed reactor followed by a direct sequence of two distillation columns. The key problem of this process is the high investments costs for several units that require a large overall plant footprint, as well as the associated energy requirements (Muller & Hubsch, Consequently, significantly better process alternatives are needed in order to reduce the capital and operating costs. Recent studies explored the possibility of using RD for the DME production (An et al., 2004; Lei et al., Although technically feasible, the proposed RD alternatives were non optimal and also not sufficiently attractive economically. To solve these problems, we propose in this work a novel DME process based on a reactive dividing-wall column (DWC that actually integrates in one shell a reactive distillation unit combined with the DWC technology. This double integrated system allows the production of high-purity DME (>99.99 wt% in one processing unit, while offering minimal plant footprint and significantly lower capital investment and operating costs. The head-to-head comparison of the reported conventional system against the optimized RD and R-DWC designs clearly shows that the reactive DWC alternative is the top performer on all decisive factors. 3. Process simulation The alternative DME processes are based on RD columns that integrate the reaction and separation steps into a single operating unit. By combining reaction and separation, one can shift the reaction equilibrium toward product formation by continuous removal of reaction products, instead of using an excess of reactant. The integrated reactive distillation processes were designed according to previously reported process synthesis methods for reactive separations (Mueller & Kenig, 2007; Noeres, Kenig, & Gorak, 2003; Schembecker & Tlatlik, Advanced simulations embedding experimental results were performed in Aspen Plus (Aspen Technology, 2010 using the rigorous RADFRAC distillation unit and explicitly considering three phase balances. The wellknown MESHR equations are governing the process. Note that MESHR is an acronym referring to the type of equations: M mass balance, E equilibrium relationships, S summation equations, H enthalpy balance, and R reaction rate equations. UNIQUAC Redlich Kwong was selected as the most adequate property method in Aspen Plus, and the binary interaction parameters were validated against reported experimental data (Ihmels & Lemmon, 2007; Teodorescu & Rasmussen, 2001; Wu, Zhou, & Lemmon, The residue curves map (RCM and the ternary map of the DME methanol water mixture are presented in Fig. 3. No azeotropes are present in this system, but a small liquid phase split envelope is observed hence the (reactive distillation columns must be modeled using VLLE data. The dehydration of methanol is an equilibrium limited reaction leading to DME and water. As already verified experimentally, no side-reactions occur at the specified conditions (Lei et al., CH 3 OH CH 3 OCH 3 + H 2 O (1 The model of catalytic distillation column includes also the experimentally determined intrinsic kinetic model parameters previously reported by Lei et al. (2011, for the methanol dehydration over an ion exchange resin. Eley Rideal and the equivalent power-law models are both suitable for simulation purposes (Lei et al., The reaction rate is given by: r = kw cat [MeOH] m [H 2 O] n (2 ( Ea k = A exp (3 RT where A is the Arrhenius factor (A = m 3 kg cat 1 s 1, E a is the activation energy (133.8 kj mol 1, and m and n are the orders of reaction with respect to methanol and water, respectively (m = 1.51 and n = The reaction takes place only in the liquid phase. It is also worth noting that Lei et al. (2011 determined these data in the same temperature range ( K as the one covered by the simulation studies reported here. The alternative processes described hereafter were optimized in terms of minimal energy requirement by using the state of the art sequential quadratic programming (SQP method implemented in Aspen Plus (AspenTech, Backed by a solid theoretical and computational foundation, the sequential quadratic programming (SQP method has become the most successful method for solving nonlinearly constrained optimization problems (Bartholomew- Biggs, 2008; Boggs & Tolle, 1995a,b; Rodriguez-Toral, Morton, & Mitchell, 2001; Ternet & Biegler, 1998, This can be linked in Aspen Plus to the minimization of the total heat duty of the sequence, constraint by the required purities for DME and water, and using several optimization variables such as: total number of stages, number of reactive stages and location of the reactive zone, length of the dividing-wall, location of feed and side-stream, reflux ratio, boilup rate, liquid and vapor split. The purity target was selected to be over wt% for both DME and water. No hard constraint was set on the purity of the unreacted methanol, as this stream is being recycled in the process. 4. Results and discussion This section describes the results of the three process alternatives considered in this work: conventional process (Lei et al., 2011, reactive distillation column followed by an ordinary distillation column (RDC + DC, and reactive dividing-wall column (R-DWC. For all the alternatives investigated in this work, the same fresh feed stream was used pure methanol at a flowrate of 9 kmol/h. This processing rate was selected such, in order to allow a fair comparison of the alternative processes with the reported conventional one (Lei et al., Conventional DME process The key features of the process can be summarized as follows. High purity DME and water (>99.5 mol% are produced in a conventional process consisting of a fixed-bed gas reactor operated at 403 K and 20 bar, followed by two ordinary distillation columns of 15 stages each operated at 9 and 1 bar, and a reflux ratio of 3.5 and 5, respectively (Fig. 1, top. The first column separates the DME product while the second one recovers the unreacted methanol that is recycled back. Remarkable, the methanol conversion reaches high values of up to 92.91%. For more details the reader is kindly directed to the work of Lei et al. (2011. Note that such systems consisting of reactor separator recycle are prone to exhibit multiple steady state and nonlinear behavior (Kiss, Bildea, Dimian, & Iedema, 2005; Kiss, Bildea, & Dimian, Consequently, the integrated design and control of such systems is of utmost importance. Nevertheless these undesired phenomena can be avoided if the reactor

4 A.A. Kiss, D.J.-P.C. Suszwalak / Computers and Chemical Engineering 38 ( Table 1 Design parameters of an optimal two-column sequence (RDC + DC. Design parameters Value Unit Flowrate of feed stream 9 kmol/h Temperature of feed stream 25 C Pressure of feed stream 10 bar Total number of stages (RDC 32 Stages reactive zone 8 31 Feed stage (RDC 12 Operating pressure (RDC 10 bar Reflux ratio (RDC 0.75 kmol/kmol Distillate to feed ratio (RDC 0.25 kmol/kmol Total number of stages (DC 23 Feed stage (DC 17 Operating pressure (DC 1 bar Reflux ratio (DC 1.7 kmol/kmol Distillate to feed ratio (DC kmol/kmol DME product purity >99.99/99.99 wt%/%mol DME mass flowrate kg/h conversion % Water purity at the bottom >99.99/99.99 wt%/%mol Reboiler duty RDC kw Condenser duty RDC kw Reboiler duty DC kw Condenser duty DC kw inlet is set on flow control and the fresh feed is on self-regulation control (Bildea & Dimian, 2003; Kiss et al., Reactive distillation process The RD column is divided into three sections: a central reactive zone from which the products are continuously removed thus overcoming the equilibrium limitations, an upper rectification zone where the DME product is separated as lightest component, and a lower stripping zone where water and the unreacted methanol are separated as heavy components (Fig. 1, mid. High-purity DME is collected at the top of RDC, while a water methanol mixture is obtained as bottom stream that is afterwards fed to a distillation column (DC. Pure water is delivered as bottom stream of DC, while the top distillate consists of methanol and tiny amounts of DME. The top methanol stream of DC is then conveniently recycled back to the RDC unit. The optimization problem for the minimization of the heat duty (Q is defined as follows: Min(Q = f (N T, N F, N R, N RZ, RR, V (4 Subject to y m x m where N T is the total number of stages, N F is the feed stage, N R is the number of reactive stages, N RZ is the location of the reactive zone, RR is the reflux ratio, V is the boilup rate, while y m and x m are vectors of the obtained and required purities for the m products. This design problem is a challenging optimization problem with discrete (N T, N F, N R, N RZ and continuous (RR, V decision variables. Table 1 lists the main design parameters for the optimal sequence using a RDC and DC. These optimized design parameters are a bit different as compared to previous studies (Lei et al., 2011 as the product purity targets are slightly higher (99.99% instead of 99.50% and the operating pressure of the RDC column is also somewhat higher (10 bar instead of 9 bar. The RDC unit consists of 32 stages with the reactive zone located from stage 8 to 31, hosting a macroporous sulfonic acid ion exchange resin (Lei et al., The methanol feed stream is fed close to the top of the reactive zone, on stage 12. Remarkable, eight stages are required for the rectification zone, whereas only one stage is needed for the stripping zone. Within the reactive zone of the RDC, a total load of 15 kg of solid catalyst per stage was used intentionally the same value as in the study of Lei et al. (2011, in order to have a fair comparison between our results and the ones previously reported. The next distillation column (DC used for water separation and methanol recovery has a total number of 23 stages with the feed located on stage 17. Note that the stages are numbered from the top to the bottom, thus stage 1 being the condenser and stage 32 the reboiler. In spite of the quite high investments costs required for 2 columns, 2 reboilers and 2 condensers, this RD process has the key advantage of being flexible as the RDC and DC can be operated at different pressures this not being possible in a DWC configuration. Note that here the RDC is operated at 10 bar while DC is operated at only 1 bar, just as in the conventional process. Fig. 4 plots the temperature and liquid composition profiles along the RDC and DC. Both distillation columns show similar temperature ranges, although they are operated at different pressures. Consequently, the use of a DWC seems to be an attractive alternative providing that working at one pressure maintains the similar temperature profile on both sides of the DWC. High purity (>99.99 wt% DME and water products are obtained whereas the purity of the recovered methanol stream that is recycled is also high (99.9 wt%. The methanol conversion is slightly above 50% as the reaction takes place in liquid phase. Such conversion level is still acceptable since methanol can be recycled as liquid stream hence only an extra pump is needed and no compressor whatsoever. Note that the overall specific energy requirements for this process alternative account for 1.37 kw h/kg DME product Reactive DWC process The reactive dividing-wall column is a highly integrated setup that consists of only one column shell, one reboiler and one condenser. Due to the absence of an off-shelf DWC unit in Aspen Plus, two coupled RADFRAC units were used as the thermodynamically Fig. 3. Residue curve map and ternary diagram of the mixture DME MeOH H 2O at 10 bar.

5 78 A.A. Kiss, D.J.-P.C. Suszwalak / Computers and Chemical Engineering 38 ( Table 2 Design parameters of an optimal reactive dividing-wall column (R-DWC. Design parameters Value Unit Flowrate of feed stream 9 kmol/h Temperature of feed stream 25 C Pressure of feed stream 10 bar Number of stages 35 Stages reactive zone 8 31 Feed stage 8 Wall position (from/to stage 8 31 Distillate to feed ratio 0.25 kmol/kmol Reflux ratio 2.45 kmol/kmol Operating pressure 10 bar DME product purity >99.99/99.99 wt%/%mol DME mass flowrate kg/h conversion % Water purity (bottom product >99.99/99.99 wt%/%mol Reboiler duty kw Condenser duty kw y m and x m are vectors of the obtained and required purities for the m products. The design problem is a complex optimization problem with both discrete (N T, N F, N R, N RZ, N DWS, N DWC, N SS and continuous (RR, V, F SS, r V, r L decision variables. Fig. 5 plots the temperature and liquid composition profiles in the R-DWC, while the key parameters of the optimal design are presented in Table 2. Remarkable, the temperature difference between the two sides of the wall is very low less than 15 C such conditions being easily achievable in the practical application with little heat transfer expected and negligible effect on the column performance (Dejanović et al., 2010; Yildirim et al., The R-DWC unit has 35 stages, with the reactive zone located from stages 8 Fig. 4. Temperature and composition profiles along the RDC and DC units (reactive distillation in a two-column sequence. equivalent of the R-DWC. This method has already proven its applicability in the simulation of DWC systems (Hernandez et al., 2009; Kiss et al., 2009; Mueller & Kenig, The main condition in integrating two distillation columns is that similar operating conditions should be applied. The previous Aspen Plus model for the RDC + DC sequence is used as the starting point for the R-DWC simulation, providing initial estimates for the number of trays, feed tray locations, liquid and vapor split and size of the reactive zone. The optimization problem for the minimization of the R-DWC reboiler heat duty is defined as: Min(Q = f (N T, N F, N R, N RZ, N DWS, N DWC, N SS, RR, V, F SS, r V, r L Subject to y m x m (5 where N T is the total number of stages, N F is the feed stage, N R is the number of reactive stages, N RZ is the location of the reactive zone, N DWS is the number of dividing-wall stages, N DWC is the location of the dividing-wall, N SS is the stage of the side-stream withdrawal, RR is the reflux ratio, V is the boilup rate, F SS is the flowrate of the side stream product, r L and r V are the liquid and vapor split, while Temperature / [ C] Molar frac on / [-] Temperature side sec on Temperature R-DWC Stage / [-] RD side of DWC Side product sec on DME Water Stage / [-] Fig. 5. Temperature and composition profiles along the reactive DWC (dashed line used for the side product section, while continuous line used for the main DWC section.

6 A.A. Kiss, D.J.-P.C. Suszwalak / Computers and Chemical Engineering 38 ( Table 3 Head-to-head comparison of conventional vs alternative DME processes. Key performance indicators Conventional process Reactive distillation (RDC + DC Reactive DWC Total investment cost (TIC $126,075 $135,260 $96,531 Total operating costs (TOC $31,233 $32,186 $13,988 Total annual costs (TAC $43,840 $45,712 $23,641 Specific energy requirements (kw h/ton DME CO 2 emissions (kg CO 2/h ton DME to 31 on the feed side, and a common stripping section (stages 32 to 35 as well as a common rectifying zone (stages 1 to 7. The methanol stream is fed on stage 8, at the top of the reactive zone the feed side of the DWC acting as the RD zone where the solid acid catalyst is present. High purity (>99.99 wt% DME is delivered as top distillate, while similar high-purity water is obtained as bottom product. The unreacted methanol is collected as side product, and then recycled back to the process mixed with the fresh feed stream of methanol. The profiles are very similar to the reactive distillation system previously described, with sharp modifications in the temperature and the composition profiles around the feed location between stages 5 and 10. On the side product part, the methanol concentration remains almost constant on a large range of stages from stages 10 to 20 thus indicating that the side stream location has only a minor effect on the purities of the products. While the profiles of the RD and R-DWC processes are similar, the key difference is the higher stripping section required in case of R-DWC for methanol recovery. Nonetheless, the methanol conversion is about 50% but with much lower specific energy requirements of only 0.56 kw h/kg DME Economic evaluation The total investment costs (TIC, total operating costs (TOC and total annual costs (TAC are calculated for all cases in order to perform a fair comparison. The equipment costs are estimated using correlations from the Douglas textbook to the price level of 2009, as described by Dejanović et al. (2011. The value of was used for the Marshall & Swift Equipment Cost. For a carbon steel column, the estimated cost in US$ is given by the relation: C shell = f p ( d c h c (6 where f p is the cost factor (equal to in this case, d c is the column diameter (calculated using the internals-sizing procedure from Aspen Plus and h c its height (tangent-to-tangent considering a tray-spacing of 0.6 m. For heat exchangers (e.g. condensers and reboilers the next expression was used to calculate the equipment cost (US$: ( C hex = c x A 0.65 (7 280 where c x = for condensers and for kettle reboilers, while A is the heat transfer area (m 2. Also, a price of 600 US $/m 2 was used for the sieve trays cost calculations. The following utility costs were considered: US $0.03/t cooling water and US $13/t steam. For the TAC calculations, a plant lifetime of 10 years was considered. Note that the price of the catalyst in the columns was not accounted for as the ion exchange resins are inexpensive, and the same amount of catalyst was used in all cases described. Moreover, the accuracy of the correlations is in the range of acceptable and realistic ± 30%. Clearly, this accuracy is less important when comparing design alternatives since the error is consistent in all cases. Table 3 provides a head-to-head comparison of the key performance economic indicators, while Fig. 6 conveniently illustrates the energy requirements and costs of the three processes considered. Note that the data for the conventional process was calculated based on results reported by Lei et al. (2011. While the feed flowrate is the same for all three cases, the conversion is different: about 90% for the conventional process, while 50% for the alternatives based on reactive distillation. This explains the differences observed between the TOC values and the energy requirements. Remarkable, the reactive DWC process is the most efficient in terms of energy requirements allowing energy savings of 11.6% and 58.6% as compared to the conventional and RD processes, respectively. Moreover, the R-DWC process is also the least expensive in terms of capital investment and operating costs, leading to the lowest total annual costs. Although the RD process has slightly higher costs as compared to the conventional process, the former could be preferred for its low footprint and milder operating conditions e.g. lower temperature levels Carbon dioxide emissions The energy requirements are closely linked to the CO 2 emissions, but only when no heat integration is considered. When part of the process heat is reused instead of primary energy, then the Specific energy use / [kw.h/kg DME] Cost / [US $] , , ,000 80,000 60,000 40,000 20, Conven onal RDC+DC R-DWC Conven onal RDC+DC R-DWC TIC TOC TAC Fig. 6. Comparison of different DME production processes in terms of key performance indicators: energy requirements, total investment, operating and annual costs.

7 80 A.A. Kiss, D.J.-P.C. Suszwalak / Computers and Chemical Engineering 38 ( CO 2 emissions are lower as compared to the figure expected from the energy data. Moreover, the absolute amount of CO 2 emissions is industrially relevant to the carbon credits. The CO 2 emissions were calculated according to method described by Gadalla, Olujic, de Rijkeb, and Jansens (2006: [CO 2 ] emissions = ( Qfuel NHV ( C% (8 100 where = 3.67 is the ratio of molar masses of CO 2 and C, NHV is the net heating value, and C% is the carbon content dependent on the fuel. For natural gas, NHV is 48,900 kj/kg and the carbon content is 0.41 kg/kg. Hence the amount of fuel used can be calculated as follows: Q fuel = Q ( proc TFTB T 0 (h proc 419 (9 proc T FTB T stack where proc (kj/kg and h proc (kj/kg are the latent heat and enthalpy of the steam, T FTB (K and T stack (K are the flame and stack temperature, respectively. The hourly rate of emissions for the conventional, RD and R-DWC process are kg/h, kg/h and 8.21 kg/h of CO 2 emissions, respectively. Table 3 also lists the specific amount of CO 2 emissions per ton of DME product. As these emissions are closely linked to the amount of energy required, it comes as no surprise that the R-DWC alternative is again in the pole position with the lowest carbon footprint. 5. Conclusions The novel integrated designs presented in this paper clearly demonstrate that reactive distillation is a feasible process intensification alternative to produce high purity DME (>99.99 wt% by methanol dehydration, using solid acid catalysts such as macroporous sulfonic acid ion exchange resins. The state of the art sequential quadratic programming (SQP method implemented in AspenTech Aspen Plus is a very effective tool in finding the optimal process design in terms of minimum energy requirement, constraint by the required purities for DME and water, and using several discrete and continuous optimization variables: total number of stages, feed stage, number of reactive stages, location and length of the dividing-wall, location of the side-stream, reflux ratio, boilup rate, liquid and vapor split. The reactive distillation process (RDC + DC presents similar performance to the conventional industrial process, as well as similar operating and total annual costs. Therefore, the RD process alone might not justify investments in revamping existing DME plants, but in case of building a new plant the RD alternative could be the preferred choice due to the lower footprint and milder operating conditions. 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