A Systematic Approach towards Synthesis and Design of Pervaporation-Assisted Separation Processes

1534 Research Article Chemie A Systematic Approach towards Synthesis and Design of Pervaporation-Assisted Separation Processes Bettina Scharzec, Thomas Waltermann, and Mirko Skiborowski* DOI: 10.1002/cite.201700079 Dedicated to Prof. Dr.-Ing. Günter Wozny on the occasion of his 70th birthday While membrane processes like pervaporation are considered as key technology towards more sustainable separation processes, they are rarely considered in conceptual design. In order to enable an identification of promising applications, the current article proposes a systematic approach that covers the synthesis of pervaporation-assisted process variants and promotes the use of optimization-based methods for process analysis. It seeks to minimize and guide necessary experiments and includes competitive reference processes as well as further means for process intensification in the evaluation. Keywords: Distillation, Heat integration, Hybrid process, Optimization, Pervaporation, Process design, Vapor recompression Received: July 03, 2017; accepted: July 27, 2017 1 Introduction While the vast majority of fluid separations is still conducted by means of distillation [1, 2], represented by more than 40 000 distillation columns in the US alone [3], alternative separation technologies offer a variety of advantages, especially in terms of energy efficiency and the capability to overcome azeotropes and distillation boundaries. Both of these advantages are of specific importance in the separation of aqueous streams from processing of bio-renewable feedstocks and consequently in the transition to a bio-based economy. The improvement of energy efficiency is, however, also of significant importance in reducing greenhouse gas (GHG) emissions and therefore the avoidance of global warming. In order to meet the ambitious target to limit the global average temperature increase below 2 C above preindustrial levels, the German government specified the objective to reduce Germany s CO 2 emissions by 40 % based on 1990 until 2020, whereas especially the chemical industry is even trying to exceed these goals [4]. In fact, the chemical industry in Germany already had reduced its CO 2 emissions by more than 50 % in the first 10 years after the approval of the Kyoto protocol [5]. In order to increase energy efficiency in the chemical industry even further especially separation processes have to be improved. Based on a survey for the US, Sholl and Lively [6] concluded that about 50 % of the energy in the industrial sector is consumed by separation processes, whereas 80 % of that share relates to thermal separations, such as drying, evaporation, and distillation. Drumm et al. [4] suggest that especially integrated and hybrid separation processes allow for reductions in energy consumption of these separation processes by up to 70 %. Sholl and Lively [6] even state a more provocative claim, proposing that membrane-based separations would use 90 % less energy. Although this is potentially an exaggeration for most applications, membrane separations are not limited by vapor-liquid equilibrium (VLE) and, thus, not limited by azeotropes and distillation boundaries. Furthermore, they provide the potential for highly selective separations at low energy requirements, a compact design through high packing densities, when applied in spiral-wound or hollowfiber modules, as well as a simple scale-up and capacity extension, by numbering up of membrane modules. One of the most prominent examples of a membrane-assisted hybrid separation process is the dehydration of ethanol for which nowadays more than one hundred processes have been installed for solvent dehydration, based on the combination of distillation and pervaporation (PV) [7]. Although PV itself was introduced by Kober in 1917 [8] and the basic concept of the PV-assisted distillation process was first proposed by Binning and James in 1958 [9], the process was Bettina Scharzec, Thomas Waltermann, Dr.-Ing. Mirko Skiborowski mirko.skiborowski@bci.tu-dortmund.de TU Dortmund University, Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separations, Emil-Figge- Straße 70, 44227 Dortmund, Germany. www.cit-journal.com ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549

Chemie Research Article 1535 not implemented before the late 1980s [10]. Whereas Lipnizki et al. [10, 11] provide an elaborate review of the major applications of PV in industry, Baker [12] summarizes that current PV systems are commercially available for basically two applications. The first and most important is the removal of water from concentrated alcohol by means of hydrophilic membranes, whereas the second commercial application of PV is the removal of small amounts of volatile organic compounds (VOCs) from contaminated water by means of hydrophobic membranes. Whereas especially the separation of alcohol-water mixtures has been addressed, other examples for dehydration processes include the purification of glyceride, mineral acids, peroxides, and aggressive solvents, whereas hydrophobic membranes have, e.g., been investigated for the separation of phenol and chlorophenol, methyl ethyl ketone, pyridine, toluene, DMF, or the fermentation products acetone, ethanol, and butanol from diluted aqueous solutions [13]. Yet there are various other applications, which have been investigated or are currently under development. Especially organic-organic separations by organophilic membranes are actively investigated and have been used for solvent separations such as tolueneheptane, methanol-benzene, and methanol-toluene [13], or the separation of isomers, such as o-xylene, m-xylene, p-xylene, and ethyl benzene and dichlorobenzene isomers [14 17]. Besides the separation of aromatic-alicyclic components, such as benzene-paraffin, benzene-olefin, or toluene-paraffin for which despite promising results at lab-scale an industrial scale application is still lacking [18], especially the separation of mixtures with significant differences in polarity is of interest. One such application is the separation of methanol and methyl tert-butyl ether (MTBE), which despite environmental concerns, is still produced on a scale of millions of tons per year [19]. This short summary illustrates that although there is a strong focus on the dehydration of alcohols and the removal of VOCs from aqueous streams the potential application range of PV-assisted separation processes is much broader. Nevertheless, the industrial application of PV-assisted separation processes is still limited, what can be attributed to two reasons. On the one hand side, the development of high performance PV membranes with long-term stability is still a challenge, such that membrane fouling, scaling, and the limited availability of suitable materials still restrict the commercialization of membrane modules and, as a consequence, interest on PV at the industrial level [17]. This is especially true for the separation of organic-organic mixtures for which an excellent chemical resistance of the membrane material to the aggressive mixtures and a potential tuning of the membrane properties for a specific application is required. However, several organophilic PV separations have been implemented and commercial membranes are available from Sulzer Chemtech, reported efficient in alcohols-ether separations, and PolyAn GmbH, which supplies molecular surface engineered tailor made polymeric membranes [17]. The second reason relates to the lack of suitable design methodologies, which enable the consideration of PV-assisted processes in an early stage of conceptual design. There have been a multitude of studies illustrating the potential benefits of hybrid pervaporation-distillation processes in terms of energy efficiency [10, 11, 20 22] and shortcut models for a rapid screening of process performance as well as rigorous conceptual and rate-based design models for simulation and optimizationbased design have been proposed [23], but all of these methods consider that an appropriate membrane performance model is available, requiring a preceding experimental investigation and the selection of a suitable membrane. Despite that, most of these methods are limited in applicability taking into account a variety of simplifying assumptions [19]. Most of all, these approaches do not address the question of how to identify a potential application of a PV-assisted process as well as how to derive suitable process configurations [23]. In order to overcome this limitation, the current work proposes a systematic design methodology that starts with an analysis of the potential feasibility of a PV-based process and the generation of suitable process variants by means of a thermodynamic analysis. Furthermore, it proposes an integrated approach that combines experimental analysis with an optimization-based design approach to evaluate the potential benefits of a PV-based process. Thereby the maximum improvement can be evaluated prior to any experimental effort and the experimental effort can be limited to those conditions relevant for the final process design, minimizing the overall effort and costs of process development. In order to provide a meaningful evaluation the process is further benchmarked against an optimized reference process and means for heat integration are considered as well. The general approach and its elements are introduced in Sect. 2, while the application to an exemplary case study, which is the separation of a ternary stream of acetone, isopropanol, and water, referring to the production of acetone by dehydrogenation of isopropyl alcohol, is investigated in Sect. 3. Finally, Sect. 4 presents a summary and conclusion, as well as a short outlook. 2 Methodology While most methodologies for the design of hybrid membrane-assisted processes focus either on variant generation, the identification and experimental characterization of a suitable membrane or process analysis and optimization, Micovic et al. [24] already proposed a four-step design method that tries to combine these aspects. In a first step different process variants are generated based on heuristics and thermodynamic insight. Afterwards an a-priori process analysis is performed, which should be linked to a membrane database and combines optimization methods with suitable process models. The objective of the process analysis is to identify necessary performance characteristics of Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549 ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.com

1536 Research Article Chemie the membrane-assisted process to economically outperform the considered competitors. It is of eminent importance to perform such an analysis on the basis of the integrated process in order to consider the mutual effects of membrane performance on the coupled process units, which cannot be considered by focusing on the membrane separation alone. Based on the identified performance characteristics, which in the case of the investigated organic solvent nanofiltration (OSN) process of Micovic et al. [24] were peremeability and permselecitivity ranges, a screening of suitable membranes is performed and a more detailed characterization of the most promising membrane is pursued in order to determine a rigorous model for process design. In the final fourth step of the methodology the process flowsheet is optimized based on the developed model and a detailed design of the process is determined. In the current article an extension of this methodology is presented, which not only places more focus on the variant generation and a-priori evaluation of potential improvements, but also transfers the ideas to PV-assisted distillation processes, including potential means for energy integration in the process design methodology. Therefore, it combines the ideas of Micovic et al. [24] with the multi-level process design approach of the process synthesis framework [25] by applying models at different levels of detail in the different steps of the process design methodology. An illustration of the proposed process design methodology is given in Fig. 1. The first step of the design approach is also focusing on the synthesis of process variants, taking into account thermodynamic insight on different levels of detail. The main objective at this stage is the identification of a potential feasibility of a PV-assisted separation process and suitable process configurations that are to be analyzed in the subsequent steps. If not already available upfront, also a suitable benchmark process is to be determined in the synthesis step. In the second step of the design methodology, the benchmark process is evaluated based on a simplified performance criteria, such as the minimum energy demand, and the potential benefit of the PV-assisted process variants is evaluated under the assumption of a perfect membrane separation. This provides an estimate of the maximum benefit that can be obtained by the PV-assisted process and either provides sufficient motivation to progress with the examination of suitable membranes or to discard the options prior to any experimental effort. If there is sufficient incentive for further investigations, a suitable membrane has to be identified, which is pursued in a third step in various ways. Performing a literature survey for similar separations, a suitable membrane model that has previously been reported in literature can be identified with minimum effort. If such a model is not available, the results of the previous model-based analysis in step two already allow for a restriction of the experimental design space, such that appropriate experimental conditions for a membrane screening and a further characterization of a suitable membrane material can be identified. A direct interaction between step two and three, which is indicated as well, allows for the determination of necessary performance characteristics in a similar way as reported by Micovic et al. [24]. In the fourth step of the design methodology, the identified membrane model is utilized to determine an optimized process design and compare its economic performance with the considered reference process in terms of total annualized costs (TAC). Finally, additional means for process intensification can be considered. Therefore, in the fifth step the prospect of advanced heat-integrated membrane modules, as proposed by del Pozo Gomez et al. [26], as well as vapor recompression (VRC) for a direct heat-integration of reboiler and condenser of a single distillation column should be investigated. In the following subsections each of the five steps is elaborated in more detail using the well-known dehydration of ethanol, as well as the separation of the ternary mixture of MTBE, n-butene, and methanol (MeOH) as explanatory reference example for the illustration. 2.1 Thermodynamic Analysis and Synthesis of Process Variants In a first step, it needs to be determined in how far PV can aid in the separation of a given mixture. Therefore, as a first step, a thermodynamic analysis of the relevant pure component properties of the given mixture is performed as introduced in the thermodynamic insight approach proposed by Jaksland et al. [27] and extended by Holtbruegge et al. [28]. In accordance with the classification introduced by Jaksland et al. [27], Tab. 1 provides a list of separation techniques, Figure 1. Flowchart for the synthesis and design approach for PV-assisted hybrid processes. www.cit-journal.com ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. 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Chemie Research Article 1537 including among others distillation and PV, as well as the characteristic physical properties, which are considered most relevant to evaluate the potential feasibility of the separation technique for a binary separation. In case there is a distinct difference between the physical properties of a certain pair of components, for which Jaksland et al. [27] consider a list of threshold values, the binary separation by means of the according separation technique is deemed potentially feasible. This means that the split of these components is considered feasible, unless any multicomponent interactions interfere. This is, e.g., the case for distillation boundaries emanating from azeotropes, which are not considered by the comparison of vapor pressure, heat of vaporization, and boiling point differences, but have to be evaluated based on the multicomponent interactions within the mixture. Taking into account the considered case study of ethanol dehydration, a list of relevant physical properties of the two components is listed in Tab. 2. Based on the differences in the physical properties and the evaluation list provided in Tab. 1, vapor-liquid separations by distillation and pervaporation, as well as solid-liquid separations by means of adsorption and crystallization are identified as potentially feasible separation techniques. Since water and ethanol form a well-known azeotrope, it is apparent that a separation by means of a simple distillation column is infeasible. However, the potential feasibility of a hybrid PV-assisted distillation process is easily identified by the (limited) suitability of both separation techniques for the same separation tasks. In fact, besides the well-known PV-assisted distillation for ethanol dehydration [10], hybrid configurations based on the combination of distillation, adsorption, and vapor permeation instead of pervaporation have been investigated in detail and economically optimized by Roth et al. [31]. While the analysis of the pure component physical properties allows for the identification of potentially feasible Table 2. Comparison of properties of ethanol and water [17, 29, 30]. Property Ethanol Water molecular weight M W [g mol 1 ] 46 > 18 boiling temperature W B [ C] 78 < 100 melting temperature W M [ C] 114 < 0 dipole moment m [D] 1.69 ~ 1.85 solubility parameter s [MPA 0.5 ] 26.5 < 48 kinetic diameter d kin [Å] 4.30 > 2.96 separation techniques, it does not provide any insight on how these separation techniques can be combined to synthesize process variants. One option for the identification of process configurations based on different separation techniques for a binary separation is an analysis of a general driving force (DF), as proposed by Gani and Bek-Pederson [32, 33]. In order to identify the most beneficial combination of a variety of available separation techniques, the DF, which is defined as the difference in phase compositions, is plotted over the composition of the main component. For distillation the difference in vapor and liquid composition according to VLE and for adsorption and crystallization solid and liquid phase compositions according to solid-liquid equilibrium (SLE) are considered. For membrane processes, such as PV, however either experimental data or an appropriate model is required to quantify permeate and retentate composition. Fig. 2 provides an exemplary illustration for the case of ethanol and water, including a derived PV-assisted distillation process configuration. At the azeotropic point, the DF for distillation equals zero, since the liquid and vapor phase have the same composition. All feasible paths to generate specific product purities can be identified by moving along the obtained DF-curves. Bek-Pedersen Table 1. Exemplary selection of separation techniques and considered characteristic pure component properties to evaluate the potential feasibility of certain separation technique for a binary separation. Separation type Separation technique Characteristic physical properties gas separation absorption solubility parameter gas separation membranes critical temperature, van der Waals volume liquid separation micro-/ ultrafiltration kinetic diameter, molecular weight nanofiltration solubility parameter, molecular weight liquid-liquid separation LL-Extraction solubility parameter SC-extraction solubility parameter, critical temperature and pressure vapor-liquid separation distillation vapor pressure, heat of vaporization, boiling point pervaporation molar volume, solubility parameter, dipole moment solid-liquid separation crystallization melting point, heat of fusion at melting point adsorption solubility parameter, kinetic diameter Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549 ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.com

1538 Research Article Chemie a) b) Figure 2. Schematic illustration of a driving-force diagram (a) and a corresponding separation process for the separation of binary mixture similar to ethanol and water (b). potential configurations by combinations with suitable membrane separations. Fig. 4 illustrates the residue curve map for the ternary system of MTBE, n-butene, and MeOH, as well as a feasible PV-assisted separation process, which is derived on the assumption of a MeOH selective membrane. The highly integrated process variant with the membrane located in the side stream of the distillation column has been investigated in detail in several studies [19, 36, 37]. Such a configuration has also more generally been proposed as specifically interesting for the separation of ternary systems, since only a single distillation column in combination with a PV membrane process is required [35, 38]. and Gani furthermore consider that the flowsheet with the largest total DF is also the probably most energy efficient one [34]. While they provide several examples that support this assumption a more detailed evaluation of the energy requirements is to be preferred and will be addressed in the second step of the current method. Fig. 3 further illustrates the three most common PV-assisted distillation processes for the separation of binary mixtures [20, 35]. The first one corresponds to the configuration that was already introduced in Fig. 2 for the ethanol dehydration case and is considered most appropriate for the separation of mixtures with azeotropes at a high composition of the lower boiling component. The second one is considered for the case of non-azeotropic, but close-boiling mixtures, while the third one is considered most appropriate in case the azeotrope is located at intermediate compositions, so that an efficient purification can be accomplished by means of an additional distillation column. Anyhow, all these configurations can be derived from the driving-force analysis. However, for the separation of multicomponent mixtures the analysis becomes more complicated. Either a simplification of the system has to be performed, lumping components together to analyse a simplified binary systems or other means for deriving potential process configurations have to be considered. At least for ternary and quaternary systems, a graphical analysis of the VLE and SLE diagrams provides the means to identify separation limitations and Figure 3. Common PV-assisted distillation processes for the separation of binary mixtures. 2.2 Optimization-Based Process Analysis of Potential Benefits In case potentially feasible PV-assisted process variants were determined in the first step of the methodology, the potential benefit of these process variants is further evaluated under the assumption of a perfect membrane separation and compared with a suitable benchmark process. This provides the maximum potential benefit of such a membranebased process and allows for the quantification of a range of feasible costs that should not be exceeded by the required membrane modules and auxiliary equipment. For PV-assisted distillation processes, especially the characterization of the VLE is essential to determine the minimum energy demand of the process, as well as restrictions concerning the operating limits of the process. The minimum energy demand of the process can be determined as the minimum sum of reboiler heat duties for the implemented distillation columns and the energy required for evaporation of the permeate stream of the PV process. In order to determine the according minimum, it is important to consider the complete process simultaneously to account for the integration of the involved unit operations. Thus, an optimization problem needs to be solved with the objective to minimize the overall energy demand, subject to the product specification and suitable process models that allow for the consideration of the non-ideal thermodynamic models required to accurately describe the VLE. Here, either shortcut models for the distillation columns, such as the pinchbased rectification body method [37, 39] or the feed angle method [40], or alternatively rigorous equilibrium tray column models can be applied [19, 41, 42]. For the evaluation of the minimum energy requirement of the PV process a (nearly) perfect separation into permeate and retentate is assumed and the energy requirement is evaluated based on an energy balance, considering the vaporous permeate stream at suitable permeate pressure. The latter should be as low as www.cit-journal.com ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549

Chemie Research Article 1539 a) b) Figure 4. Residue curve map for a pressure of 6 bar (a) and potential PV-assisted distillation process (b) for the separation of the ternary mixture of butene, MTBE, and MeOH into MeOH, MTBE, and the binary butene-meoh azeotrope, using a MeOH selective membrane. possible, while still allowing for condensation by means of cooling water. A suitable pressure value can be determined based on boiling point computations according to VLE. Even in the simple case of ethanol dehydration with a single distillation column and the PV membrane processing the top stream of the distillation column, there is a distinct top vapor composition of the distillation column at which the minimum overall energy demand is determined. This is illustrated in Fig. 5 for the separation of an ethanol water mixture, with 10 mol % of ethanol into purified ethanol (99.5 mol %) and water (99.9 mol %). Here, the minimum overall energy demand is computed according to the previously described guideline, taking into account a rigorous equilibrium tray model with 80 equilibrium trays. VLE is computed on the basis of the non-random twoliquid (NRTL) model and the extended Antoine equation, while specific enthalpies are determined on the basis of DIPPR correlations for the specific heat capacity and heat of vaporization, taking into account the parameters given in the database of Aspen Plus V34.0. The calculations were performed by means of a GAMS model, following a similar initialization and solution approach as described by Skiborowski et al. [19]. Comparing the results obtained under consideration of a perfect PV separation, with 100 mol % water in the permeate, and a nearly perfect separation with only 90 mol % water in the permeate, there is not only little difference in the resulting total energy requirement, but also approximately the same optimal top vapor composition, of 80.4 mol % ethanol. The required 12.2 MW for a 1 kmol s 1 feed stream (or 12.8 MW in case of 90 mol % water in the permeate), is considerably smaller than the 39.6 MW corresponding to the heat of evaporation of the water in the feed, which is the minimum heat that would have to be provided for a stand-alone PV process with a perfect membrane. It is also well below the necessary 27.2 MW required for a heteroazeotropic distillation process with cyclohexane as entrainer, which is selected as reference process. The process configuration with two columns, each with 80 equilibrium trays, and a decanter, is depicted in Fig. 5 and was optimized in accordance with the solution approach described by Skiborowski et al. [41]. According to the evaluation of the energy requirements, there is a large incentive towards the investigation of a suitable PV membrane that allows for such a separation. If such a membrane exists an economic assessment has to be performed to determine if the PV-assisted process also provides an economic potential. The information from Fig. 5, however, already indicates that if the economic assessment will Figure 5. Evaluation of PV-assisted distillation for the separation of an ethanol-water mixture based on a perfect (100 mol % H 2 O) and nearly perfect (90 mol % H 2 O) hydrophilic PV separation. Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549 ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.com

1540 Research Article Chemie result in a column top vapor composition above 88 mol % ethanol, the PV-assisted distillation process will require more energy than the heteroazeotropic reference process. While the latter has been considered as relevant solution by many authors [43 45], Roth et al. [31] suggests distillation assisted by vapor permeation and for higher purities of the ethanol product stream also a polishing step by adsorption, as the most economic solutions for the separation of diluted aqueous ethanol streams. Considering the availability of data, the potential benefit of a PV-assisted process should always be benchmarked with the best known reference process. Only if there is sufficient incentive towards a closer investigation, the effort in determining a suitable membrane and membrane performance model should be continued. 2.3 Identification of a Suitable Membrane Model While the prior step of the methodology performs an analysis of the potential benefits of PV-assisted processes based on the assumption of a perfect performance of a PV membrane (hydrophobic, hydrophilic, or organophilic), the performance of the membrane needs to be quantified more specific to perform an economic assessment in the upcoming steps. Therefore, experimental data concerning available membranes are required, which can either be obtained based on a literature survey, or by conducting own experiments for the identification and characterization of a suitable membrane. The simplest form of performance data that can be obtained from literature is given in form of lists concerning the overall flux and the selectivity or separation factors of certain membranes (or membrane materials) for the separation of binary mixtures, as, e.g., compiled for ethanol-water and butanol-water mixtures in the book of Basile [17]. These values do not only provide an idea of how well the separation by means of the PV membrane works, but can also be used to estimate the required membrane area for a preliminary economic assessment. On the contrary, the results from the evaluation in the previous step can also be used to determine which flux would be acceptable, taking into account the overall permeate stream and an estimated membrane price per area, in order to define a target value for selecting a suitable membrane material. Going one step further, as has been done in the work of Micovic et al. [24] for an OSN assisted distillation process, such an economic assessment by means of an optimizationbased method can further be used to evaluate target values for a non-perfect separation. Here, flux and separation factor, or as in this case a rejection of the membrane are considered as variable parameters in a sensitivity study that for each set of parameters evaluates an optimized process design, which is compared to a reference process. In this way, Micovic et al. [24] determined target values for the membrane screening process for which an economic benefit of the OSN-assisted process is expected. This approach results in an integration of steps two and three of the method, which was already indicated in Fig. 1. In case the membrane screening is successful and the membrane performance is promising for the determined reference conditions, the membrane performance needs to be characterized in more detail over the range of expected feed compositions and temperatures, as well as permeate pressure. Here again the results from the previous optimization-based evaluation, or a new refined evaluation considering the performance at reference conditions, provides an idea of the range of operating conditions under which the membrane performance needs to be characterized. The best case scenario is that such an experimental characterization and modeling of the membrane performance for the considered operating range has already been performed and reported on. However, even if this information is missing in the current method the necessary effort is only spent once the potential benefit has been quantified and the time and cost consuming experiments are justified. For the considered ethanol-water separation a huge variety of membranes, as well as membrane models are available for direct consideration [17, 31, 37, 46]. 2.4 Comparison of Optimized Process Candidate Flowsheets After a suitable membrane was identified and the separation performance was characterized for the expected operating range, the process design model that was used for the evaluation in step 2 is refined, exchanging the (nearly) perfect PV separation with the determined performance model. Furthermore, sizing and costing correlations are added and the process configuration is transformed into a superstructure that allows for the modification of feed and side stream locations as well as the decimation of non-used parts of the process, such as equilibrium trays in a distillation column or membrane stages in a membrane network [23]. A specific approach for PV-assisted distillation processes was proposed by Skiborowski et al. [19] and was already applied to the case study of ethanol dehydration providing a comparison with a pressure-swing distillation process. Therefore, a detailed elaboration on the optimization approach and superstructure model is spared in the current contribution and the interested reader is instead referred to the article of Skiborowski et al. [19]. Note, that even further detailed process models can be used in order to derive a more accurate design of the process, at an increased information requirement [23, 47]. In order to illustrate this step of the current method, the results of the optimization-based design approach for the PV-assisted process configuration with a six-stage membrane network [19] is compared to those obtained for the heteroazeotropic distillation process with cyclohexane as entrainer [41]. For both processes a feed stream with 0.5 kmol s 1 and the compositions and product specificawww.cit-journal.com ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. 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Chemie Research Article 1541 tions introduced in Sect. 2.3 is considered. Furthermore, the membrane model for the PVA-PAN membrane Mol1140 (GFT) derived by Bausa and Marquardt [37], which was utilized by Skiborowski et al. [19], is also considered for the evaluation. Despite the possible energy savings of the PV-assisted distillation process (cf. Fig. 5), the heteroazeotropic distillation process offers a higher economic potential with TAC of 2.9 Mio a 1, which is significantly smaller than the TAC of 3.8 Mio a 1 that result for the membrane-assisted process. While the energy requirements of the optimized heteroazeotropic distillation process (total heat duty of 14.0 MW) and the PV-assisted distillation process (total heat duty of 8.0 MW) are in line with the process analysis (cf. Fig. 5), the considered operation at a permeate pressure of 30 mbar necessitates the use of expensive cooling brine for permeate condensation, which results in a major share of the TAC of 1.29 Mio a 1. This indicates the importance of considering an economic evaluation of the process alternatives and highlights the significance of the operating conditions of the PV process. A more selective membrane and an operation at higher permeate pressure can result in significant economic savings and should be considered for a potential re-evaluation of the process variant. membrane-heat-exchanger configuration has been proposed by Lutze et al. [50]. Alternatively, the top vapor could be compressed and used for an internal vapor recompression to be integrated with the reboiler of the distillation column. Consequently, this option, which requires only wellknown standard equipment, should be considered as well. An investigation of the potential for process improvement is presented in the subsequent case study. An illustration of the integrated membrane-heat-exchanger and the direct VRC design is provided in Fig. 6. a) b) 2.5 Investigation of Additional Means for Process Intensification While PV-assisted hybrid separation processes are already considered as a potential means for process intensification, these processes as well as the considered reference processes might further be intensified. However, the different process variants do not necessarily possess the same potential for intensification and the consequently an additional step is added to the process evaluation. While various means for heat integration of distillation processes exists [48], the potential for energy integration for PV processes is significantly smaller due to the low temperature levels at which permeate condensation is performed. Nevertheless, different ideas for heat integration of PV-assisted processes have been proposed. One specifically interesting idea is the implementation of a heat integrated PV module, which is heated by an internal steam source, which can be a compressed vapor stream from the connected distillation column. Such a configuration was first proposed by Del Pozo Gomez et al. [26, 49], who considered alcohol dehydration by means of the purifying the distillate stream via PV, partially using the compressed top vapor stream of the column as heat source for an integrated membrane and heat exchanger module. Such a module does not only provide an efficient means for heat integration, but can also result in significant savings in membrane area requirements due to an increased DF, by means of an at best constant operating temperature. A similar idea for an integrated Figure 6. Illustration of additional means for process intensification: heat-integrated membrane modules (a) and direct vapor recompression (b). 3 Case Study: Separation of an Acetone- Isopropanol-Water Mixture In order to demonstrate the potential of the proposed methodology the complete set of steps is subsequently applied for the separation of the ternary mixture of water, isopropyl alcohol (IPA), and acetone, which relates to the production of acetone by dehydration of IPA. The feed composition and the product specifications for the separation are listed in Tab. 3 and are equivalent to the specifications used by Koch et al. [47] and Skiborowski et al. [19]. Table 3. Specifications for acetone, IPA, and water separation. Flow rate [mol s 1 ] x acetone [mol mol 1 ] x IPA [mol mol 1 ] Feed 20.53 0.438 0.052 0.510 Acetone Purity 0.995 Product Recovery 0.995 IPA Purity 0.995 Product Recovery 0.950 x water [mol mol 1 ] Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549 ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.com

1542 Research Article Chemie 3.1 Thermodynamic Analysis and Synthesis of Process Variants As first step of the methodology an analysis of the characteristic pure component properties, which are summarized in Tab. 4, is performed. Based on the significant difference in boiling points distillation is evaluated as a potentially feasible separation technique for the separation of all three components. Furthermore, taking into account the differences in solubility parameters and molecular weight, PV is evaluated as potentially feasible separation technique for the separation of water from the other two organic components. Table 4. Properties of acetone, IPA, and water [17, 29, 30]. Property Acetone IPA Water molecular weight M W [g mol 1 ] boiling temperature W B [ C] solubility parameter s [MPA 0.5 ] kinetic diameter d kin [Å] 58 ~ 60 > 18 56 < 82 < 100 19.9 ~ 11.8 < 48 4.69 ~ 4.70 > 2.25 For the derivation of process variants the ternary residue curve map is illustrated in Fig. 7. The thermodynamic properties of the mixture are modeled by means of the NRTL activity coefficient model and the Redlich-Kwong equation of state in combination with the extended Antoine equation. Analyzing the residue curve map of the ternary system a) b) it becomes eminent that the distillation boundary limits the separation by means of distillation, such that hybrid process configurations are required in order to cross the distillation boundary and break the binary IPA-water azeotrope. However, as indicated in Fig. 7 a sequence of two distillation columns would allow for the separation of acetone and a fraction of the water prior to breaking the azeotrope. On the basis of this information in total five different process variants are derived and illustrated in Fig. 8. The first three variants are based on the sequential separation of the mixture, first separating acetone by means of distillation and then using either an extractive distillation process (1), or a PV-assisted distillation, with one (2) or two distillation columns (3), in order to separate the IPA-water mixture. The extractive distillation process utilizes ethylene glycol (EG) as entrainer, which was identified as effective solvent for the separation of propyl alcohols from water [51], while the specific distillation-based process configuration was screened as most energy efficient in a previous shortcut screening [52]. The two PV-assisted distillation processes consider a hydrophilic membrane for which a variety of options are available. Using a similar membrane also a simultaneous separation of the ternary mixture can be performed in a PV-assisted distillation process in which the membrane process is connected via a side stream to the distillation column (4). In this configuration the composition profile inside the distillation column needs to cross the distillation boundary (SDB), such that acetone is produced as top product and IPA as bottoms product of the distillation column. The last process variant (5) is based on a similar configuration, which, however, considers a hydrophobic membrane to separate the IPA. In that case water is obtained as bottoms product in the distillation column, whereas the composition profile in the distillation column does not need to cross the distillation boundary. 3.2 Optimization-Based Process Analysis of Potential Benefits Figure 7. Residue curve map for a pressure of 1 atm (a) and possible sequence of separation steps (b) for the separation of the ternary mixture of acetone, IPA, and water. The potential benefits of the generated PV-assisted process variants are further assessed in comparison with a reference process based on extractive distillation with EG. The maximum possible benefit is obtained for a perfect membrane separation for which the overall minimum energy demand is evaluated on the basis of the reboiler heat duties and the necessary heat of evaporation of the permeate stream. The miniwww.cit-journal.com ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549

Chemie Research Article 1543 Figure 8. Generated flowsheet variants for the separation of acetone, IPA, and water. mum energy demand of each process variant is evaluated by means of an optimization based on rigorous mass balances, equilibrium conditions, summation constraints, and enthalpy balances (MESH) models for the distillation columns and ideal separators for a perfect membrane separation. All models are set up and solved in GAMS, taking into account the solution approaches described by Skiborowski et al. [19, 41] and using SNOPT as nonlinear programming (NLP) solver. The results of this evaluation are illustrated in Fig. 9. The major share of the required energy demand for each process variant is attributed to the separation and purification of the acetone product stream. The corresponding energy demand of 890 kw for the separation by means of distillation, which is equivalent for process variants (1) (3) is indicated individually in the diagram. The extractive distillation process requires in total 1.177 MW, which means that at least 287 kw are required for the separations in the subsequent sequence of distillation columns. By replacing this sequence with a perfect hydrophilic PV membrane, process variant (2) or (3) could reduce this additional share to 137 kw. The simultaneous separation of all three products in a side stream configuration with a hydrophilic PV membrane results, however, in an increased energy requirement of overall 1.364 MW. While the side stream configuration provides a variety of advantages over a simple sequence of the acetone column followed by stand-alone PV process, it has about the same minimum energy requirement for which the evaporation of water results in a heat requirement of 455 kw, considering a permeate pressure of 50 mbar. This configuration can therefore be excluded from further evaluation. PV-assisted process variant (5), which is the side stream configuration with a hydrophobic membrane for which only the small share of IPA needs to be evaporated as permeate stream, is most promising with a minimum energy demand of only 993 kw. 3.3 Identification of a Suitable Membrane Model In order to quantify the performance of the remaining process variants on a more detailed level, including a technoeconomical evaluation, a suitable membrane needs to be identified and characterized concerning its separation performance. For the latter, additional information concerning the operating ranges of interest for the PV process can be derived from the previous evaluation of the PV-assisted process variants under the assumption of a perfect membrane. This information proofs to be of particular interest for the PV-assisted process variant with the membrane process connected to the distillation column via the side stream. For the remaining process variant with the hydrophobic membrane the liquid feed stream to the membrane, corresponding to the internal liquid stream inside the column, does only contain minor amounts of acetone (0.05 mol %). By removing the necessary amount of IPA the liquid stream composition changes from an initial IPA composition of 40 mol % to a remaining composition of 38 mol %. Consequently a membrane screening and characterization could focus on this small range of feed-retentate compositions. Based on a literature survey it becomes obvious that a variety of hydrophobic membranes have been investigated for the separation of IPA and water [10, 53, 54]. A detailed experimental study of a commercially available membrane for this system has been performed by Koch and Górak [55], who also provide a suitable model for the characteriza- Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549 ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.com

1544 Research Article Chemie in fugacities between feed and permeate side, as proposed by Koch et al. [47]. Figure 9. Evaluation of the potential benefit of the PV-assisted process configurations in comparison with the extractive distillation reference process. tion of the separation performance of the investigated polymeric membrane PERVAPÔ 1210. For this study the parametrization of the model provided by Koch et al. [47] is further used for the techno-economical evaluation of process variants (2) and (3). The PV-assisted process variant (4) was furthermore investigated by Skiborowski et al. [19] based on the same membrane model. A techno-economical evaluation and comparison with the extractive distillation process validated the drawbacks of this process variant [56], which was already discarded according to the preceding process analysis in step two of the current method. The investigations of hydrophobic (or organophilic) pervaporation membranes are yet rather limited compared to those of hydrophobic membranes. Consequently no adequate membrane model was identified based on a literature survey. However, organophilic membranes have been investigated for the separation of similar mixtures, such as the separation of 2-butanol from water [57, 58]. In order to evaluate the incentive for developing a suitable membrane the subsequent process optimization of the PV-assisted process variant (5) is performed with a hypothetical process model, which provides an inverse, yet less efficient performance compared to the model of the hydrophilic PERVAPÔ 1210 membrane provided by Koch et al. [47]. Instead of the original set of parameters a constant permeance is considered for all components (Q IPA = Q Acetone = 20 mol h 1 m 2 bar 1 and Q H2O = 0.2 mol h 1 m 2 bar 1 ), while the DF is modeled by the difference 3.4 Comparison of Optimized Process Candidate Flowsheet The true potential of the PV-assisted process configurations is further evaluated in terms of a techno-economic analysis. For a fair comparison a minimization of the TAC for each process variant is performed by means of an optimizationbased design approach, for which a superstructure model based on the MESH equations for each distillation column is combined with a multi-stage membrane model that implements the previously determined performance model. The models are completed by sizing and costing equations, taking into account the same formulations used by Skiborowski et al. [19]. The optimized flowsheet of the extractive distillation process, which uses EG as entrainer, is illustrated in Fig. 10, while Tab. 5 provides an overview of the cost distribution of the process. It becomes obvious that the acetone purification in col. 1 dominates the economic performance with a share of more than 2/3 of the whole process costs. Since the PV-assisted process variants (2) and (3) both include the same acetone recovery column they offer only the potential for an improvement in comparison to the remaining three column sequence. Both process variants are investigated by an optimization of a superstructure model with a maximum number of 100 equilibrium trays per column and a membrane network with a maximum of five consecutive stages with potential inter-stage heating to the maximum membrane temperature of 100 C. Each membrane stage is operated at 50 mbar, such that condensation of the permeate with cooling water is feasible. However, considering a Figure 10. Optimized flowsheet of process variant (1) extractive distillation process. www.cit-journal.com ª 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. Tech. 2017, 89, No. 11, 1534 1549