Conceptual Design of Flowsheet Options Based on Thermodynamic

Article pubs.acs.org/IECR

Conceptual Design of Flowsheet Options Based on Thermodynamic Insights for (Reaction−)Separation Processes Applying Process Intensification Johannes Holtbruegge,* Hanns Kuhlmann, and Philip Lutze Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separations, TU Dortmund University, Emil-Figge-Strasse 70, D-44227 Dortmund, Germany S Supporting Information *

ABSTRACT: This paper presents a systematic conceptual process design approach for the generation of flowsheet options with special focus on process intensification as it considers some integrated reaction−separation and hybrid separation techniques. For the identification of necessary separation steps and possible techniques, an automated tool was developed and implemented in Matlab. The objective of this tool is to find solutions to the design task with respect to predefined boundaries of the final process. In this context, thermodynamic insights that correlate physicochemical and thermodynamic properties of the chemical system with promising techniques are used. The approach is easy to extend by additional techniques, can be embedded into common process design frameworks, and is able to generate a meaningful variety of promising flowsheet options for a given design task. To underline its capabilities, the tool was applied to two design tasks, one of which is the separation of a nonideal fermentation supernatant, while the other represents the production of ethyl lactate. For both, promising flowsheet options were generated and discussed. low likelihood of including the optimal flowsheet option in the pool of candidates. Furthermore, few research efforts have been spent on the inclusion of PI into these methods. Important representatives of knowledge-based methods are the hierarchical heuristic approach presented by Douglas7 and the work of Barnicki et al.,8 which considers PI by providing a loose framework of heuristics for the application of reactive distillation (RD). Design methods based on thermodynamic insights generate flowsheet options by analyzing thermodynamic characteristics of the mixture involved in the design task.6 Thereby, a quick and reliable analysis of flowsheet options is possible by excluding those which are not feasible. However, their drawbacks are the necessity of defining a portfolio of techniques and the required assignment of techniques to thermodynamic characteristics of the mixture. Nevertheless, the integration of PI into these methods is possible. A well-known example of such a method was presented by Jaksland et al.,9 who related physicochemical and thermodynamic properties to select possible techniques for the separation of a given mixture from a database. In addition, rules are used to select possible mass-separating agents (MSAs) when using a MSA-based technique. However, this method does not consider PI. The method of Bek-Pedersen and Gani10 relies on a similar principle and considers hybrid separation techniques by analyzing the driving forces for mass transfer during the development of flowsheet options.

1. INTRODUCTION Global challenges, such as climate change and the progressive scarcity of fossil energy sources, increase the pressure on the chemical industry to develop innovative and sustainable processes that are capable of replacing the current, partly inefficient processes. Process intensification (PI) replaces conventional processes by intensified techniques and can be a key to realizing this objective with a promising short- and longterm prospective.1 Despite all of the benefits arising from applying PI, its use in the chemical industry is still limited because of missing systematic process design approaches considering PI in the generation of flowsheet options.2 Harmsen et al.3 have recognized the high potential of the industrial application of systematic design approaches, even for conventional processes, leading to overall cost savings between 20 and 60%. Therefore, different research groups have worked on the development of systematic process design approaches. Process design approaches can be divided into two main categories. The first category consists of knowledge-based and thermodynamic insights methods, whereas optimization-based methods represent the second category.4 A current trend is the use of hybrid process design approaches which assemble the benefits of the approaches from both categories in a combined approach.5 In the following discussion, the most important approaches are briefly presented. The aim of knowledge-based methods is to systematically reduce the number of flowsheet options using expert knowledge.6 Therefore, heuristic rules or graphical methods (e.g., residue curve maps (RCMs)), commonly organized in a hierarchical structure, are usually applied. These methods enable a strong influence of the engineers’ creativity on the generated flowsheet options. However, their drawback is the © XXXX American Chemical Society

Received: May 29, 2014 Revised: July 26, 2014 Accepted: August 6, 2014

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dx.doi.org/10.1021/ie502171q | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. Framework for the design of intensified processes.

Systematic approaches for the generation of flowsheet options are rarely distributed among engineers as the identification and establishment of an appropriate number of applicable techniques is difficult. Therefore, the approaches that are currently known and able to generate flowsheet options based on a systematic thermodynamic analysis are limited to distillation-based techniques, such as conventional distillation, dividing wall columns, and thermally coupled columns, and detailed addressing of PI is not possible. A first attempt of this systematic generation was presented by Agrawal11 who developed an approach that automatically generates distillation sequences. A few years later, d’Anterroches and Gani12 presented their approach, which is able to synthesize and evaluate distillation sequences relying on the principles presented by Bek-Pedersen and Gani.10 More recently, Rong and Errico13 discussed a new approach that is superior to the older ones as it synthesizes complex distillation sequences for multicomponent separations considering a multiplicity of thermally coupled columns within one flowsheet option. Optimization-based methods rely on the generation of complex superstructures, preferably representing all conceivable flowsheet options.14 Then, interconnections between the involved techniques are enabled or disabled, and operating conditions are optimized simultaneously. Accordingly, this optimization represents a mixed-integer nonlinear programming (MINLP) problem. The advantage of optimization-based methods is the simultaneous rigorous evaluation of different flowsheet options, thereby increasing the likelihood of identifying the optimal one if it was part of the initial superstructure. Furthermore, PI can be easily considered in these methods. Disadvantages arise from the strong dependence of the final solution on the initial superstructure15 and the high computational effort required because handling MINLP problems of this scale is still an issue.16 Examples of optimization-based process design methods were presented by Grossmann15 and Henrich et al.17 A new trend is the use of optimization-based methods for process design at a lower level of aggregation. Within such methods, flowsheet options are generated by connecting mass, component, energy, and momentum balance blocks that are necessary to fulfill the design task. In a subsequent step, the necessary phenomena can be grouped into techniques that represent a flowsheet option. Thereby, the restriction to existing techniques is overcome; phenomena-based methods even offer the potential to create completely new techniques, and PI can be established.18 However, their disadvantage is their high complexity and abstractness, impeding their practical application in the chemical industry. Such methods were presented by Tanskanen et al.19 and Lutze et al.20 As both categories of process design methods have individual benefits and drawbacks, their combination in hybrid design

methods is beneficial.21 These methods usually use expert knowledge or thermodynamic insights to create a certain pool of promising flowsheet options and use mathematical algorithms for their final evaluation and optimization. Therefore, hybrid methods satisfy all requirements to consider PI for fulfilling the design task. One of the first hybrid methods was presented by Mizsey and Fonyo,22 who used hierarchical knowledge-based methods to generate flowsheet options, a bounding strategy to reduce their quantity and optimizationbased methods for the rigorous optimization and identification of the best flowsheet option in terms of the given objective. Figure 1 shows another, well-known and commonly applied hybrid method for process design consisting of four consecutive steps. Within the first step after the definition of the design task, physicochemical and thermodynamic properties of the chemical system are collected and analyzed. In the second step, expert knowledge or thermodynamic insights gained in the first step are used to generate flowsheet options. Subsequently, the possible flowsheet options are reduced further by evaluating their performance. Therefore, the third step applies shortcut or conceptual models to rate their potential in terms of the given objective. In this context, flowsheet options are excluded by identifying either that the flowsheet option is not practical because of limitations arising from the combination of the proposed techniques or that it cannot compete with other options in terms of the objective. The rigorous modeling and optimization of all remaining options is covered by the fourth step of the framework. The objective of this step is to identify the best option and to identify the final structure and operating point that best suit the objective. Thereby, the framework has to struggle with the general problem that a limited quantity of input data is available at the beginning of a new design task. Hence, the framework starts with a low mathematical complexity that is adequate for the corresponding highly qualitative level of information. This is especially beneficial because of the initially large number of potentially feasible flowsheet options that must be handled. With a decreasing number of flowsheet options, the mathematical complexity of the framework is steadily increased, allowing a more precise evaluation of the remaining flowsheet options. Nevertheless, more detailed input data, such as physicochemical and thermodynamic properties, become necessary when reaching the final step of the framework. Therefore, experimental studies become unavoidable when using conceptual or rate-based models. The last two steps can be accompanied by experimental investigations to collect necessary data for rigorous modeling. Marquardt and co-workers2,23 have applied this framework several times and developed shortcut models that allow for a simple evaluation of the flowsheet options. B



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into the portfolio is limited (Section 2.2). However, the characteristics of most industrially relevant techniques and those with a considerable potential for PI are implemented. 2.2. Portfolio. The portfolio of available techniques consists of unit operations and processes that are widely applied in the chemical industry. The implemented unit operations for binary separations are distillation, gas stripping (GS), liquid−liquid phase separation (LLPS), and solvent extraction (SE). GS and SE integrated into membrane contactors are also part of the portfolio. Furthermore, different unit operations that have a high PI potential were added. One example is energy-efficient heat pump-assisted distillation (HPAD). On the basis of the work of Kiss,24 heat-integrated distillation, absorption heat pumps, mechanical and thermal vapor recompression, (multistage) vapor compression, compression−resorption heat pumps, thermoacoustic heat pumps, and cyclic distillation were implemented into the portfolio. To offer an intensified technique that is capable of replacing distillation, GS, or LLPS in conventional apparatuses, rotating packed beds (RPBs) were added to the portfolio. To include the possibility of separating binary azeotropic mixtures, special distillation processes, such as azeotropic distillation (AD), extractive distillation (ED), heteroazeotropic distillation (HD), and pressure-swing distillation (PSD) were implemented. Membrane separations have gained increasing interest in terms of PI in recent years as they offer the possibility of separating azeotropic mixtures in an energy efficient way.25 Therefore, common membrane separations were implemented into the portfolio. These membrane separations are pervaporation (PV), vapor permeation (VP), nanofiltration (NF), and reverse osmosis (RO). A remarkable number of unit operations for ternary separations was also implemented into the portfolio. Dividing wall columns (DWCs) represent the integration of two or more distillation steps in one column shell. Therefore, DWCs can also be attributed to PI and were added to the portfolio. In addition to conventional DWCs, DWCs using the principles of the aforementioned special distillation processes to separate ternary mixtures with one binary azeotrope are also part of the portfolio. These include azeotropic, extractive, and heteroazeotropic DWCs, as well as DWCs coupled with a column operating at a different pressure to establish pressure-swing DWCs. A promising process-intensification concept is the use of hybrid separation processes. Therefore, distillation and DWC combined with one of the membrane separations (PV/ VP, NF, or RO), coupled with the distillate, side product, or bottom product are part of the portfolio. The integration of reaction and separation into one apparatus yielding a reactive separation technique can also be considered as a part of PI. From these reactive separation techniques, RD, reactive gas stripping (RGS), reactive liquid−liquid phase separation (RLLPS), reactive solvent extraction (RSE), membrane reactors (MR), and even reactive DWCs (RDWCs) are implemented into the portfolio. A further intensification of reactive separation techniques can be achieved by combining them with other unit operations to create a reactive hybrid separation technique. From this class of techniques, RD and RDWC coupled with one of the membrane separations (PV/VP, NF, or RO) are implemented into the portfolio. The Supporting Information contains a detailed overview of all implemented techniques and provides simplified flowsheets to enhance the comprehensibility of the available portfolio.

A literature survey has shown that especially approaches that systematically generate meaningful flowsheet options in the beginning of the process design framework have been rarely presented. The existing approaches are limited to conventional techniques, such as distillation, and usually miss out PI. Therefore, this work addresses the first two steps of this hybrid process design method and aims at considering a large pool of different intensified techniques during the generation of flowsheet options. We consider various conventional as well as intensified reaction−separation and hybrid separation techniques in an early stage of the process design where a low quantity of input data is available. Our approach is able to handle these circumstances and works at a highly qualitative level of information. The approach systematically proposes flowsheet options considering PI based on thermodynamic insights. In this respect, necessary reaction and separation steps are systematically identified on the basis of the design task, and requirements to possible techniques are formulated. Subsequently, feasible techniques are identified by screening a portfolio for techniques that fulfill these requirements. Flowsheet options are compiled from a portfolio of 67 techniques and configurations. The performance of this approach was demonstrated in a first assessment by proposing flowsheet options for the separation of a fermentation supernatant and by a reactive case study, which is the production of ethyl lactate.

2. GENERATION OF FLOWSHEET OPTIONS BY SYSTEM IDENTIFICATION Within this study, a partially automated tool was developed that aims at generating promising flowsheet options considering PI for a given design task. This section presents the boundaries of this tool (Section 2.1), the portfolio of available techniques that can form a flowsheet option (Section 2.2), a detailed description of the approach to identify and analyze the considered chemical system (Section 2.3), and the procedure that is suggested for the generation of final flowsheet options (Section 2.4). The following sections differentiate among steps, splits, and techniques. Their exact meaning shall be introduced first to provide a better understanding of the subsequent explanations. A flowsheet option consists of all separation steps that are necessary to fulfill the design task. Each step separates at least one pure component from the mixture and consists of several splits between the involved components. Such splits can be sharp or nonsharp. A split between two components is sharp when both components can be obtained in their pure state. Techniques are used to fulfill separation steps. A technique can consist of several sharp or nonsharp splits. 2.1. Boundaries. Because of the huge design space comprising a large number of possible connections of separation steps and available techniques during process synthesis, simplifications are necessary to allow a proper application of the tool. Therefore, the tool is applicable only to fluid mixtures with a maximum of four components, including binary azeotropes. The tool can handle design tasks for conventional separation and reaction−separation systems. For reaction−separation systems, reaction characteristics can have a major impact on the flowsheets. Therefore, the approach is capable of accounting for 11 different chemical equilibriumlimited and consecutive reaction types for which flowsheet options considering PI are deemed beneficial (Section 2.4.1.2). In addition, the number of techniques that were implemented C



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2.3. System Identification and Analysis. The first step of the process design framework covers the system identification and analysis, including the collection of important input data, such as physicochemical and thermodynamic properties, especially limitations caused by phase equilibria. Furthermore, reaction path synthesis is a part of this step if the design task represents the development of a reaction−separation process. In such cases, information on the reaction, such as the chemical equilibrium constant and/or the order of reaction, must be supplied after identifying a promising type of reaction. The reliability of the final solution to the design task already strongly depends on the quality of the input data provided in this step of the design framework. Therefore, accurate physicochemical, thermodynamic, and reaction data must be accessible to use this framework. However, this early stage of the process synthesis is usually connected with a highly qualitative level of information, and those data are often not available with reliable accuracy. Thus, it is permissible to estimate (based on comparable chemical systems or group contribution methods, such as UNIFAC26) or guess these data as a first approximation and to rectify them before initiating the rigorous calculations if promising flowsheet options are generated. To successfully apply the proposed tool, the number of involved components, their specific feed concentrations, their boiling point temperatures (Tb), solubility parameters (δ), molar volumes (Vm), molecular weights (M), dipole moments (μd), kinetic diameters (κ), and partition coefficients between n-octanol and water (KOW) must be specified. The physicochemical pure-component properties are used to calculate property indicators (Table 1) that are employed for

larger than the cutoff and if the molecular weight of each component on the other side of the split is smaller than the cutoff, resulting in a promising separation. The given boundary values for the property indicators are adjustable based on the experience of the user (e.g., by gaining experimental insights for the related separation technique). The default values are recommendations that were chosen based on a literature survey. In addition to the evaluation of property indicators, the detailed consideration of yes/no indicators (Table 2) is an Table 2. Yes/No Indicators for the Identification of Possible Techniques by Screening of the Portfolio

Table 1. Property Indicators for the Identification of Techniques by Screening of the Portfolioa indicator

unit

boundary

description

RBP RSP RMV RMW RDM RKD RKOW MWCO_NF MWCO_RO

(K K−1) (MPa MPa−1) (−) (−) (D D−1) (m m−1) (−) (kg kmol−1) (kg kmol−1)

1.019 1.3 1.9 1.79 1.127 1.127 528 20029 15029

Tbi/Tbj δi/δj Vmi/Vmj Mi/Mj μdi/μdj κi/κj KOWi/KOWj MWCONF MWCORO

indicator

category

description

Fo HV TE TS F MG S avoidMSA LS HBA IBA LBA APS HA deltaT_b150 Side H2O_top T_reb_L T_reb_M T_reb_H deltaT_b10 deltaT_b1020 deltaT_b20

component component component component mixture mixture mixture process process azeotrope azeotrope azeotrope azeotrope azeotrope DWC DWC HPAD HPAD HPAD HPAD HPAD HPAD HPAD

fouling30 high viscosity31 toxic or explosive31 temperature sensitive31 foaming28 miscibility gap contains solids32 avoid MSA-based techniques33 limited space available31 maximum azeotrope intermediate azeotrope minimum azeotrope azeotrope pressure sensitive heterogeneous azeotrope ΔTb ≤ 150 K24 side stream largest product stream24 water removed as distillate24 253.2 K ≤ Treb ≤ 323.2 K24 303.2 K ≤ Treb ≤ 423.2 K24 Treb > 423.2 K24 ΔTb ≤ 10 K24 10 K < ΔTb ≤ 20 K24 ΔTb > 20 K24

additional task in this step. These indicators describe special characteristics of the involved components or the mixture and the resulting requirements to the applied techniques and the final process. Their values are also zero by default and are set to 1 if specific criteria depending on the predefined physicochemical pure-component or mixture and process properties are fulfilled. For example, extremely toxic/explosive (TE) or temperature-sensitive (TS) components as well as ones with a high viscosity (HV) and a tendency to cause fouling (Fo) have to be indicated. In addition, information on the presence of solids (S) and foaming tendency (F) as well as constraints caused by a space limitation (LS) for the final plant can be predefined. Furthermore, the use of MSA-based techniques can be generally avoided (avoidMSA) as they require subsequent recovery steps.33 In addition to the data that are important for calculating the indicators, quantitative information on the existence of binary azeotropes (HBA, IBA, LBA), their composition, boiling point temperature, liquid-phase behavior (HA), and their pressure sensitivity (APS) must be specified. Furthermore, qualitative information on the presence of miscibility gaps (MG) is required by the tool. Further yes/no indicators address the special needs of DWCs and HPADs and are listed in Table 2.

a

The indicators are calculated as binary ratios between components i and j or for the mixture in case of MWCO.

the generation of flowsheet options in the subsequent steps. The values of all property indicators are set to zero by default. The binary ratios of the boiling point temperature (RBP), solubility parameter (RSP), molecular volume (RMV), molecular weight (RMW), dipole moment (RDM), kinetic diameter (RKD), and distribution coefficient (RKOW) are calculated for all combinations of the involved pure components. If the determined ratio exceeds a certain boundary value (Table 1) for a specific split, the corresponding indicator is set to 1, and a separation by exploiting this property difference is considered promising. The molecular weight of each component is compared to the molecular-weight cutoff (MWCO) of the corresponding membrane separations NF and RO to determine the value of the property indicators MWCO_NF and MWCO_RO. Their value is set to 1 if the molecular weight of each component on one side of the split is D



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this approach considers intensified techniques such as integrated reaction−separation and hybrid separation techniques. Reaction properties are considered in this early stage of the process synthesis, and not only sharp splits but also nonsharp splits (separation of the mixture A,B,C into A,B|B,C) are considered in this approach to enable the suggestion of hybrid separation techniques. An algorithm was developed for this level, allowing its full automation. Level II (Section 2.4.2) represents the investigation of the technical feasibility of the suggested separation steps and techniques. Therefore, RCMs are studied and detailed studies on the availability of suitable membranes and MSAs are performed based on a detailed literature survey. Level III (Section 2.4.3) represents the final generation of flowsheet options. These flowsheet options consist of different combinations of all techniques that were deemed suitable for a certain separation step in level II. In this context, a separation step is considered feasible if at least one suited separation technique is available in the portfolio. Because of the complex interconnections among the different levels, iterations may become necessary during the generation of flowsheet options as more insights become available in the ensuing levels that can influence the decisions made in the previous levels. 2.4.1. Level I: Identification of (Reaction−)Separation Techniques. An automated analysis of the (reaction−)separation problem and the subsequent listing of all potentially feasible separation steps and techniques represent the main part of level I. First, all possible sequences of separation steps are listed for the considered mixture. Then, the focus of the analysis lies in the selection of techniques from the portfolio presented in Section 2.2. In this context, this level is not designed to explicitly guarantee the feasibility of the proposed techniques but rather to quickly provide ideas and intensified options for the given design task. This attribute results in an increased effort to exclude infeasible separation splits and

All indicators are assigned to the techniques implemented into the portfolio. If all indicators that recommend a certain technique are 1, its use in the later process is deemed beneficial. The assignment of indicators to techniques was performed based on a detailed literature survey and experience. The Supporting Information shows a detailed list of all separation techniques with their corresponding indicators, which can speak either for or against the technique. Reaction path synthesis has a major influence on the generated flowsheet options and is one important part of this step when considering reactive systems. For such design tasks, focus is placed on the identification of beneficial integrated reaction−separation techniques in the subsequent step of the process design framework. Therefore, the exact type of reaction is an important input for the generation of flowsheet options, as the choice of integrated techniques strongly depends on this. 2.4. Approach for Generation of Flowsheet Options. The actual generation of flowsheet options can be divided into three levels. Figure 2 shows an excerpt of the hybrid design

Figure 2. Detailed structure of the generation of flowsheet options.

framework that also lists the three levels of the second step. Level I (Section 2.4.1) is the identification of promising separation and reaction−separation techniques based on the indicators and insights gathered in the first step of the design framework. In contrast to most of the published approaches,

Figure 3. Scheme of the approach that was developed and implemented in Matlab to identify separation and reaction steps and techniques to fulfill a given design task. E



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Figure 4. Decision tree to assign potentially feasible techniques to binary zeotropic separation steps.

techniques in the subsequent levels. However, the probability of including the best techniques in their optimal combination into the final flowsheet is increased. The screening of possible splits and techniques in level I is performed according to different decision trees and can be divided into the portfolio screening of separation techniques (Section 2.4.1.1) and reaction−separation techniques (Section 2.4.1.2) for which the underlying decision tree is more complex, as reaction characteristics have a strong influence. Usually, the synthesis of reaction−separation options requires the evaluation of the decision trees for both the reaction−separation and separation techniques, increasing the amount of possible techniques and the workload. Independent of the type of design task (separation, reaction− separation task), level I always assumes that the given feed mixture has to be separated into its pure components. Detailed data on the product specifications cannot be addressed at this early stage of the process design and have to be considered during the final optimization studies. 2.4.1.1. Decision Tree Structure for Nonreactive Systems. The decision tree developed for nonreactive systems enables an automatic screening of the portfolio of separation techniques based on the indicators that were calculated in the previous step during system identification and analysis. A technique is deemed feasible and added to the list of promising ones if the indicators of the separation step and the implemented characteristics of the considered technique match. The decision tree for nonreactive systems is divided into four subdecision trees for binary and ternary, each zeotropic and azeotropic separation steps. In this context, a binary separation step leads to two products, whereas a ternary separation step leads to three products. The lower left part of Figure 3 shows the implementation of the decision trees for nonreactive systems into the overall approach.

The principle of the binary and ternary zeotropic decision trees is rather simple; separation techniques can be directly assigned to steps by evaluating the property and yes/no indicators for the given design task. In the course of this step, distillation, HPAD, GS, LLPS, SE, RPBs, and the membrane separations PV/VP, NF, and RO are the separation techniques that can be suggested as a result of the binary decision tree. Figure 4 shows the decision tree for identifying potentially feasible techniques for binary zeotropic separations. The algorithm checks in a step-by-step procedure the property and yes/no indicators and assigns one or more of the aforementioned separation techniques to a separation step if the corresponding requirements are fulfilled. PV/VP is proposed as a potentially feasible separation technique if the property indicators RMV and RSP are 1. Furthermore, the yes/ no indicators Fo and S must not be equal to 1. This procedure is followed for all decision points, and the result is a list of all separation techniques that are possible for each separation step. The corresponding property and yes/no indicators of each technique that were used to develop the decision trees are given in the Supporting Information. The other decision trees rely on the same principle. The ternary zeotropic decision tree can propose different combinations of distillation with membrane separations and DWCs. In the following, the basis of the more complex binary and ternary azeotropic decision trees is presented in detail. Each of the aforementioned techniques whose separation performance is not limited by the formation of azeotropes (SE, LLPS, GS, and the membrane separations PV/VP, NF, and RO) can be used for azeotropic separations. Therefore, these techniques are always suggested by the decision tree if their property indicators match the chemical system. Conventional distillation is not applicable for these design tasks, whereas F



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Figure 5. Basis for deciding whether a separation step involving an azeotropic mixture can be performed in a simple distillation or if a distillationbased azeotropic separation is necessary.

is not part of the automated analysis, complex azeotropic mixtures are examined in level II of the approach. To enable the use of hybrid separation techniques in the approach, the consideration of nonsharp splits is necessary. In addition to azeotropic separations, nonsharp splits can be beneficial when the driving force for mass transfer within a sharp split is low, resulting in a low energy efficiency of the separation step. This can occur when high product purities have to be reached. In such cases, the combination with a different unit operation exploiting another driving force can result in increased energy efficiency of this separation step. Within this approach, the overall hybrid separation technique performs a sharp split, whereas each of the two individual unit operations performs a nonsharp split in its most beneficial operating range. Figure 5 illustrates the principle of the automated suggestion for the separation of ternary azeotropic separations with one binary azeotrope with the help of three examples. The boiling point temperatures of the pure components are the same within all examples, whereas the boiling point temperature of the binary azeotrope and its composition vary to emphasize the influence of these parameters on the possible splits and separation techniques. The upper left case has a minimum azeotrope and a distillation boundary; the upper right case has an intermediate azeotrope and a distillation boundary, while the lower left case has only a minimum azeotrope. The respective ternary diagrams show two different feed compositions with two potential splits each and the resulting configuration of the distillation-based separation technique. The azeotrope-split black box can be substituted by every technique that is able to overcome an azeotrope. The lower right corner summarizes all splits and corresponding techniques that are able to achieve the separation. Figure 5 shows only distillation-based azeotropic

special distillation-based configurations involving AD, HD, ED, PSD, and hybrid separation processes are capable of separating azeotropic mixtures. Although the structure of these distillationbased azeotropic separations is usually more complex, the developed approach can handle them, as these configurations are part of the portfolio of techniques with an additional yes/no indicator that indicates their ability to separate azeotropic mixtures. To assign a certain distillation-based technique to an azeotropic separation, additional steps are necessary during evaluation of the decision trees. Therefore, the boiling point temperatures of the involved pure components and the azeotrope(s) as well as the feed and azeotropic compositions are considered for the identification of potentially feasible techniques. The aim of level I is to evaluate whether a given separation step in an azeotropic mixture can be performed by distillation, distillation-based azeotropic separations, or none of these. A final evaluation of the technical feasibility requires detailed knowledge about the existence and shape of distillation boundaries and is assigned to level II of this approach. According to the method presented by Luyben,34 this approach uses the boiling point temperatures of the pure components and azeotropes and simple mixing rules to evaluate feasible splits. This evaluation can accurately handle ternary mixtures and the existence of one binary azeotrope. For systems with more than one binary azeotrope, the suggestion of possible distillation-based azeotropic separation techniques is based on the identification of minimum and maximum azeotropes and can omit promising techniques. For a detailed consideration of quaternary or higher mixtures and mixtures with more than one azeotrope or ternary azeotropes, the procedure presented by Thong and Jobson35−37 should be followed. As this procedure G



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separations that are capable of performing binary separation steps. A decision tree for selecting techniques to separate ternary azeotropic separations based on the same working principle is also implemented into the approach. The different techniques for azeotropic separations are also given in the Supporting Information. In case the decision tree suggests using an MSA-based technique, advantegeous ranges for the physicochemical properties of the MSA are proposed under the premise that a simple recovery of the MSA is necessary in a subsequent separation step. Therefore, RBP, RSP, RMW, and MWCO_NF are considered, leading to distillation-based techniques or membrane separations, as these techniques are capable of performing sharp splits without adding additional MSAs. In addition to this, the most important requirement is that at least one of the involved components is preferentially soluble in the MSA. Therefore, a detailed consideration of potential MSAs is indispensable and is part of level II of this approach in case MSA-based techniques are proposed. 2.4.1.2. Decision Tree Structure for Reactive Systems. The decision tree for reactive systems identifies promising integrated reaction−separation techniques based on the type of reaction and physicochemical and thermodynamic properties by using the introduced indicators (Section 2.3). This decision tree consists of various consecutive steps and is part of the overall approach for process synthesis. Its embedment is illustrated in the lower right part of Figure 3. The decision tree for reactive systems shows whether the integration of reaction and separation into one apparatus is potentially feasible and is thus a promising alternative to the sequential arrangement of reaction and separation. To assess the potential of this integration, the method presented by Schembecker and Tlatlik38 was implemented into the approach. This method evaluates whether the separation of one or more products, byproducts, or solvents is beneficial for the reaction and differentiates between the reaction and transport phases. Generating or adding a transport phase to the reaction phase that is capable of slowly dosing reactants to or removing products from the reaction phase can achieve the aforementioned separation (e.g., removal of products from the liquid reaction phase into the vaporous transport phase by RD). The benefit of an additional transport phase depends on the type of reaction, whereas its general feasibility depends on the system properties. The characteristics of seven consecutive and four equilibrium-limited reactions are implemented into the tool. As integrated reaction−separation techniques are within the focus of this study, the removal of products from the reaction phase is the only function considered in the following description. For consecutive reactions, the removal of a target product from the reaction phase can result in increased selectivity. When considering a chemical equilibrium-limited reaction, transport of the target product into a transport phase causes increased reaction yield. In the first step of the reactive decision tree, separations that have a positive influence on the reaction performance (conversion and/or selectivity) are determined depending on the reaction type. Table 3 summarizes the reaction types that are covered by the approach where reactants (R) react in one or even more reactions to products (P), coproducts (CP), and byproducts (BP). Furthermore, the corresponding beneficial separation(s) using a transport phase are listed. On this basis, possible transport phases and integrated reaction−separation techniques are suggested. Therefore, all relevant component

Table 3. Implemented Reaction Types and Corresponding Beneficial Product Separation(s) ID

reaction 1

reaction 2

separation(s)

Consecutive Reactions C1 C2 C3 C4 C5 C6 C7 E1 E2 E3 E4

R→P R + P → CP R → P + CP R + P → BP R → P + CP P + CP → BP R1 + R2 → P P → BP R1 + R2 → P R + P → BP R1 + R2 → P + CP R + P → BP R1 + R2 → P + CP P + CP → BP Equilibrium-Limited Reactions R⇌P − R ⇌ P + CP − R1 + R2 ⇌ P − R1 + R2 ⇌ P + CP −

P|Ra P|Ra P|Ra P|R1,R2a P|R1,R2a P|R1,R2a P|R1,R2a P|R P|R P|R1,R2 P|R1,R2

CP|Rb

CP|R1,R2b

CP|R CP|R1,R2

a

Beneficial if order of reaction of P in reaction 2 is greater than zero. b Beneficial if order of reaction of CP in reaction 2 is greater than zero.

pairs are evaluated according to their separation indicators, which are subsequently used to screen the portfolio of integrated techniques. The currently available techniques comprise RLLPS, RSE, RD, RGS, and MRs as combinations of a reactor with PV/VP, NF, or RO membranes. The steps that follow the identification of potentially feasible integrated reaction−separation techniques are less automated and require an engineer’s judgment. In the first of these steps, a feasibility assessment of the identified integrated techniques is performed by comparing the operating windows of the reaction, the separation, and the apparatus in which they shall be integrated.38 A premise for their integration is that their operating windows show at least a small intersection. If the operating windows do not match initially, this manual step of the approach aims at moving at least one of the individual operating windows to create an intersection. If no overlapping is feasible, a partially integrated solution (e.g., distillation combined with side reactors) is considered a feasible technique. An important task is the evaluation of the necessity of additional nonreactive separation steps to obtain pure products. For this purpose, the product mixture(s) of the integrated techniques can be considered as nonreactive and are passed to the decision trees for the identification of separation techniques (Section 2.4.1.1), which automatically evaluates potentially feasible techniques. Then, iterations between the second and this level become unavoidable as the operating point of the integrated technique strongly influences the following process. Therefore, detailed insights from level II are applied to rectify the initially suggested separation techniques. However, even if all of the aforementioned requirements are fulfilled, the technical feasibility and economic benefit of the integrated techniques is still not guaranteed. Therefore, the conventional, sequential arrangements of reaction and separation are always suggested by the developed approach. 2.4.1.3. Software Implementation. The automated approach for the assignment of the property and yes/no indicators as well as the identification of (reaction−)separation techniques is implemented in MathWorks Matlab. The identification of techniques is performed by simple logical operators and if/else-loops that reliably find potentially feasible techniques in the portfolio on the basis of the indicators that were calculated from the predefined physicochemical and thermodynamic properties. A GUI that enables simple H



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Figure 6. Results obtained from the process synthesis approach for the notional case study of separating component A from the ternary mixture A,B,C.

application was connected to the algorithm. The Matlab tool has an interface with Microsoft Excel. This interface is used to import the relevant physicochemical properties from an Excel spreadsheet and to export the final values of the property and yes/no indicators as well as the lists of techniques that were assigned to the separation steps into a blank spreadsheet. The principle of the algorithm is shown in Figure 3, which illustrates the different steps that are necessary for the initiation of a new process design task and the implemented decision structure calling different decision trees. 2.4.2. Level II: Detailed Investigation of Separation Steps and Techniques. The objective of this level is the detailed interpretation and refinement of the output generated by level I using more detailed data to remove infeasible separation techniques from the list of techniques that are considered to generate flowsheet options in level III. Level II includes tasks that require more detailed information on the chemical system, membranes, and MSAs that cannot be performed in an automated tool as applied in level I. The tasks that must be executed in this level are the selection of suitable membrane materials and MSAs and the identification of attainable product regions. Membrane- and MSA-based techniques are identified as potentially feasible in level I based on the property differences between the involved components without certain information on the suitability of a membrane material or MSA for a given separation step. Therefore, databases and literature have to be screened and expert knowledge has to be exploited in this level to identify promising candidates. For the identification of suitable MSAs, previously determined physicochemical properties allowing their simple recovery should be considered here. If no suitable membrane material or MSA can be identified in this level for a certain separation step, the corresponding separation technique is removed from the list of possible techniques. Detailed investigations of the membrane performance and/or distribution coefficients in the operating window of its later application are part of the experimental investigations in subsequent steps of the process design. The investigation of attainable product regions is essential for proving whether a sharp split in a certain separation step is feasible. Especially for azeotropic separations performed by distillation-based techniques, the determination of product

regions is important. However, the automated routine in level I proposes the use of distillation-based azeotropic separation techniques without knowing the exact shape of the RCMs. Therefore, thermodynamic insights are applied to determine achievable product compositions. In addition to well-known RCMs for determining the component distribution in distillation processes, the analysis of driving forces for mass transfer10 or membrane rejection, selectivity, and modeling maps for NF membranes39,40 are used to evaluate the feasibility of techniques for a specific separation step. If this level reveals that sharp splits are not possible or necessary, the automated routine from level I is called again to check whether the application of a nonsharp split followed by an additional split is an option. The smart combination of two nonsharp splits leads to a hybrid separation technique that in total is able to perform a sharp separation split. If thermodynamic insights show that neither a sharp nor nonsharp split can be achieved with a technique in a particular separation step, it has to be removed from the list of possible techniques for this step before starting the generation of flowsheet options in level III. 2.4.3. Level III: Generation of Flowsheet Options. The generation of flowsheet options is based on the results of the first two levels. Their outcome is a list of feasible techniques assigned to each separation step that is necessary to fulfill the design task. In this regard, the tree of all possible sequences of separation steps is used as the initial point of the flowsheet generation. This knowledge is used to combine all feasible techniques for each step in each possible way, ending up with the flowsheet options. Thereby, a comprehensive overview of all possible flowsheet options with their corresponding techniques is obtained.

3. CONCEPTUAL EXAMPLE To illustrate the working principle of the approach presented in Sections 2.3 and 2.4, an example of its application to generate flowsheet options for the separation of component A from a notional ternary mixture (A,B,C) is given. Therefore, the physicochemical properties of component A are compared to those of components B and C to determine the property indicators. Assuming that the automated analysis shows a remarkable difference in boiling point temperatures, solubility parameters, molecular volumes, and partition coefficients causes I



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supernatant of fermentation designed to produce HAc. The following sections present promising flowsheet options considering PI for this design task. 4.1.1. System Identification and Analysis. This section summarizes all of the input data that were collected in the first step of the process design framework and that are necessary to successfully screen the available portfolio of techniques. Table 4 lists the physicochemical pure-component properties that are used to determine the values of the property indicators.

the indicators RBP, RSP, RMV, and RKOW to be set to 1. The insights that the mixture does not contain solids (indicator, S = 0), causes no fouling (Fo = 0), tends not to foam (F = 0), forms no miscibility gaps (MG = 0) or azeotropes (LBA = IBA = HBA = 0) are used to evaluate the yes/no indicators. On the basis of these indicators, the automated tool (level I) identifies potentially feasible techniques. Figure 6 shows that these techniques are distillation (RBP = 1, F = 0, S = 0), SE (RKOW = 1, avoidMSA = 0) and PV/VP (RSP = 1, RMV = 1, Fo = 0, S = 0). The more detailed investigation of these techniques with respect to the chemical system in level II reveals that a sharp split is possible by distillation, and a suitable PV/VP membrane is available to sharply separate the components A and C. However, component B is distributed between both, resulting in a nonsharp split between A and the other two components with this technique. Therefore, a second call of level I is applied to expand the list of potentially feasible techniques by additional techniques that are also capable of performing the nonsharp split A,B|B,C. SE was removed from the list of feasible techniques because no MSA that can be easily recovered was identified in level II. The second screening of the portfolio to identify techniques that can perform a nonsharp split A,B|B,C revealed no additional potentially feasible techniques. Therefore, an additional step to separate the components A and B becomes necessary to obtain pure component A as a product. In this context, the property indicators RBP, RSP, RMV, RMW, RKOW, and MWCO_RO are set to 1. In comparison to the property indicators of the separation of A|B,C, the additional property indicators RMW and MWCO_RO are 1 as the binary ratios between A and B match the required specification. For this additional separation step, distillation, SE, and PV/VP are proposed for the same reasons as mentioned above. In addition, RO (MWCO_RO = 1, RMW = 1, Fo = 0, S = 0) is proposed in level I. The detailed analysis shows that distillation results in a sharp split and that an RO membrane that selectively separates the mixture is available. As in the first pass of level II, no suitable MSA is available, and the identified PV/VP membrane is not able to perform a sharp split, which leads to their exclusion as feasible techniques to separate components A and B. Therefore, three feasible and promising flowsheet options are generated in level III from the techniques identified by evaluating the available indicators. The first option consists of a single distillation column; the second option is a process using PV/VP coupled with distillation, and the third option is the combination of the membrane separations PV/VP and RO.

Table 4. Physicochemical Pure-Component Properties for the Quaternary Mixture Consisting of MeOH, H2O, HAc, and FF physical property

unit

MeOH

H2O

HAc

FF

Tb41 δ41 Vm41 M41 μd42 KOW41

(K) (MPa0.5) (m3 kmol−1) (kg kmol−1) (D) (−)

337.8 29.3 0.040 32.04 1.69 0.191

373.2 47.8 0.018 18.02 1.85 −

391.1 19.0 0.058 60.05 1.74 0.708

434.9 23.6 0.083 96.09 3.60 4.898

The components H2O and FF form a homogeneous minimum azeotrope with an H2O mass fraction of 0.650 g g−1 and a bubble point temperature of 371.1 K.43 The azeotropic composition is not pressure-sensitive. The following additional characteristics of the chemical system were considered to determine the values of the yes/no indicators: • The mixture does not contain solids and does not tend to foam; • No extremely toxic, explosive, temperature-sensitive, viscous components or components that cause fouling are involved; • None of the involved mixtures forms a miscibility gap. A quaternary feed mixture with a MeOH mass fraction of 0.005 g g−1, H2O mass fraction of 0.981 g g−1, HAc mass fraction of 0.012 g g−1, and FF mass fraction of 0.002 g g−1 was used for the generation of possible flowsheet options. The low product concentration is typical for fermentation-based processes. The indicator values for each possible separation step were calculated by the automated routine implemented in Matlab. A detailed list containing their final values is given in the Supporting Information. 4.1.2. Generation of Flowsheet Options. The approach presented in Section 2.4 was used to generate promising flowsheet options for the separation of the quaternary mixture consisting of MeOH, H2O, HAc, and FF. In the following, the results from each level of this step are briefly presented. 4.1.2.1. Identification of Separation Techniques. The automated part of the approach generates a complete list of potentially feasible techniques for each possible separation step based on the indicators calculated in the first step of the framework. As no reaction is involved in this design task, the decision trees for nonreactive systems were used to identify separation techniques. All possible combinations of separation steps to achieve the design task of completely separating the quaternary mixture into its pure components were collected in level I. Subsequently, the portfolio of separation techniques was screened. This screening revealed several potentially feasible separation techniques that can be used to fulfill the design task. These separation techniques include distillation, GS, and SE as well as hybrid alternatives consisting of either distillation or

4. CASE STUDIES Two industrially relevant case studies were used to demonstrate the capabilities of the approach that was presented in Section 2 by proposing flowsheet options that are capable of fulfilling the given design task. In this context, flowsheet options were generated for a nonreactive mixture and a reactive mixture. As a nonreactive mixture, a fermentation supernatant consisting of methanol, water, acetic acid, and furfural was considered. The production of ethyl lactate was used as a case study to practically present the working principle of the reactive decision tree. 4.1. Separation of a Fermentation Supernatant. In the following, results for the application of the developed approach to the separation of a nonreactive, quaternary mixture consisting of methanol (MeOH), water (H2O), acetic acid (HAc), and furfural (FF) are presented. This mixture is the J



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Figure 7. Reduced output of the algorithm that is implemented in Matlab to assign potentially feasible techniques to all possible separation steps for the separation of the quaternary mixture.

Figure 8. Excerpt of the generated flowsheet options for the separation of the quaternary mixture consisting of MeOH, H2O, HAc, and FF.

DWCs combined with PV/VP or NF. Figure 7 shows an excerpt of the separation steps with corresponding potentially feasible techniques for the considered case study. The property differences between HAc and the other components do not allow for its direct separation from the mixture with one of the available techniques. Therefore, no separation was proposed for this step. The sharp separation between MeOH,H2O on the one hand and HAc,FF on the other hand is possible with

several of the implemented techniques. The system identification has shown that the indicators RBP, RMW, RKD, IBA, T_reb_M, and deltaT_b20 are 1. Thus, distillation (RBP = 1), different HPAD techniques (RBP = 1, T_reb_M = 1, deltaT_b20 = 1), and GS (RKD = 1) can be applied as the presence of the intermediate azeotrope does not hinder this particular separation step. When performing this step first, two additional separation steps are necessary. For the separation of K



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from the mixture. The subsequent solvent recovery differs between both options. A distillation is used in the third option, whereas an NF membrane is used in the fourth option. The remaining zeotropic ternary mixture is separated in a DWC in both options, recovering MeOH in the distillate, HAc in the side stream, and FF in the bottom product. In addition to these examples of flowsheet options, various additional options were found. Detailed explanations of the individual techniques that are part of these flowsheets are given in the Supporting Information. 4.2. Production of Ethyl Lactate by Esterification of Lactic Acid with Ethanol. The developed approach was also applied to a case study comprising a chemical reaction. The design task was to develop a process for the production of ethyl lactate. In a first step, a reaction path synthesis was performed, and the chemical equilibrium-limited esterification reaction of lactic acid (HLac) with ethanol (EtOH) to yield ethyl lactate (EtLac) and water (H2O) was selected, as these reaction types offer a huge potential for PI. The following sections present promising flowsheet options considering PI for this design task. 4.2.1. System Identification and Analysis. Again, input data collected in the first step of the framework is summarized first. The production of EtLac occurs according to the following reaction scheme:

MeOH and H2O, the indicators RBP, RSP, RMV, RMW, RKD, RKOW, T_reb_M, and deltaT_b20 were set to 1 because of the present system properties. For the binary separation between HAc and FF, the indicators RBP, RMW, RDM, RKD, RKOW, T_reb_H, and deltaT_b20 were set to 1. Therefore, distillation, different HPAD techniques, GS, and SE can be applied to each of these separation steps. SE is suggested for the separation of H2O from the organic components. Hence, an MSA that dissolves the organic components and forms a miscibility gap with H2O is necessary. As mentioned in Section 2.4.1, the automated approach automatically suggests physicochemical properties that are advantageous for the subsequent recovery of the MSA. For this specific separation, a boiling point temperature either lower than 334.5 K or higher than 439.2 K is proposed to recover the MSA by means of distillation. For separation by NF, a solubility parameter smaller than 14.6 MPa0.5 or higher than 38.5 MPa0.5 or a molecular weight higher than 200 kg kmol−1 is suggested. 4.1.2.2. Detailed Investigation of Separation Steps and Techniques. To evaluate the practical feasibility of separation by PV/VP or NF membranes, the literature was screened for successful applications of these techniques for the present separation tasks. The preceding analysis of the physicochemical properties showed that MeOH and H2O have the most similar properties that evaluate the appropriateness of using PV/VP to separate H2O from the mixture. Therefore, their sharp separation by means of this technique is deemed most critical. However, Liu et al.44 found a hydrophilic membrane that is capable of separating H2O from MeOH by a PV/VP mechanism. Thus, PV/VP membranes were considered in the generation of flowsheet options, and an evaluation of its stability with respect to HAc must be performed within the experimental investigations in the subsequent design steps. For the separation of these components by NF, no suitable membrane was found. Therefore, NF must not be considered during the generation of flowsheet options except for the potential solvent recovery step. Its feasibility in this step depends on the final choice of an MSA and must also be evaluated in subsequent design steps. However, the general feasibility of SE was shown in this level. A screening of MSAs that form a miscibility gap with H2O was performed, and their relevant physicochemical properties were taken from the literature.45 For subsequent solvent recovery by distillation, chloroform (Tb = 334.2 K) and n-octanol (Tb = 468.2 K) were identified as suitable MSAs.41 For subsequent recovery by NF membranes, the organic solvents toluene (δ = 8.9 MPa0.5), nheptane (δ = 7.5 MPa0.5), and cyclohexane (δ = 8.2 MPa0.5) were found to be suitable.41 4.1.2.3. Generation of Flowsheet Options. Figure 8 shows an excerpt of the flowsheet options that were developed based on the insights gained in the first two levels. The first two options apply a PV/VP membrane first to separate H2O from the mixture. Afterward, a conventional distillation is applied to separate MeOH as distillate. The first option uses a hybrid configuration consisting of a DWC and a PV/VP separation in the distillate to separate the bottom product from the first column. The membrane is used to separate the azeotropic mixture, while HAc and FF are recovered in the side and bottom products, respectively. The second flowsheet option uses two individual columns instead of a DWC, and H2O is recovered in the hybrid configuration of distillation and membrane, while the second column separates HAc and FF. The third and fourth options use an SE step to separate H2O

HLac + EtOH ⇌ EtLac + H 2O

Table 5 lists the physicochemical pure-component properties that are used to determine the value of the property indicators. Table 5. Physicochemical Pure-Component Properties for the Quaternary Mixture Consisting of EtOH, H2O, EtLac, and HLac physical property

unit

EtOH

H2O

EtLac

HLac

Tb41 δ41 Vm41 M41 μd42 KOW41

(K) (MPa0.5) (m3 kmol−1) (kg kmol−1) (D) (−)

351.4 28.5 0.059 46.07 1.69 0.490

373.2 47.8 0.018 18.02 1.85 −

427.7 23.5 0.115 118.13 3.46 0.661

490.0 15.9 0.074 90.08 −a 0.191

a

No value for the dipole moment of lactic acid was found in any database.

Two homogeneous minimum azeotropes are present in the mixture, thereby limiting the purification into its pure components. A binary azeotrope between EtOH and H2O with an EtOH mass fraction of 0.958 g g−1 and a bubble-point temperature of 351.3 K and an additional binary azeotrope between H2O and EtLac with an H2O mass fraction of 0.683 g g−1 and a bubble-point temperature of 372.4 K are present in this mixture.46 The composition of both azeotropes was found not to be dependent on pressure. To determine the values of the yes/no indicators, the following characteristics of the chemical system were considered: • The mixture does not contain solids and does not tend to foam; • No extremely toxic, explosive, temperature sensitive, or viscous components or components that cause fouling are involved; • None of the involved mixtures forms a miscibility gap. The indicator values were calculated by the automated routine implemented in Matlab. A detailed list containing their L



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Figure 9. Reduced output of the algorithm that is implemented in Matlab to assign possible techniques to reaction and separation steps for the production of EtLac by esterification of HLac with EtOH.

final values for each possible separation step is given in the Supporting Information. An equimolar mixture of EtOH and HLac was assumed as the feed in this first step. 4.2.2. Generation of Flowsheet Options. This section illustrates the use of the approach presented in Section 2.4 to generate promising flowsheet options considering PI for a reactive system, the production of EtLac by esterification of HLac with EtOH. 4.2.2.1. Identification of Separation Techniques. The presented approach enters the decision tree for reactive systems in a first step and identifies beneficial separations for the reaction type E4 (Table 3). On the basis of that, the separations H2O|EtOH,HLac and EtLac|EtOH,HLac were identified as beneficial for shifting the chemical equilibrium and increasing reactant conversions. Consequently, the approach generates indicators for these separations and screens the portfolio of available reaction−separation techniques for promising techniques. Subsequently, the decision tree for the nonreactive systems is entered, and potentially feasible techniques for the separation of the remaining reaction product mixture are screened. The concentrations of the components involved in the product are estimated on the basis of the proposed integrated reaction−separation techniques. For the sequential arrangement of reaction and separation that is always considered, it is assumed that all involved components are present in the product. As shown in Figure 9, none of the available reaction− separation techniques is able to directly separate the product EtLac from the reaction mixture. The separation of the

coproduct H2O is feasible using a transport phase. The system identification has revealed that a difference in boiling point temperatures, solubility parameters, and molecular volumes and weights between H2O and the reactants can be exploited to separate H2O from the reaction mixture. Therefore, the property indicators RBP, RSP, and RMW were set to 1. For this case study, the use of RD (RBP = 1, F = 0, S = 0) or an MR coupled with either PV/VP (RSP = 1, RMV = 1, Fo = 0, S = 0) or NF (RSP = 1, Fo = 0, S = 0) is promising. If an RD column is used for this separation, a subsequent azeotropic separation is necessary, as the coproduct H2O can be removed only together with the reactant EtOH, which form a minimum azeotrope. This issue was identified by the approach, and it has identified the property indicators RBP, RSP, RMV, RMW, and RKOW to be 1. Furthermore, the yes/no indicators LBA, H2O_top, and T_reb_M were set to 1. As the yes/no indicator LBA is 1, the binary azeotropic decision tree is called for this separation. This tree suggests several distillation-based azeotropic separations (distillation and PV/VP, distillation and NF, ED, AD) as well as stand-alone PV/VP or NF. The lower part of Figure 9 shows an excerpt of the possible separation steps that become necessary when using a sequential reaction−separation arrangement. Again, these separation techniques were identified by screening the portfolio using the property and yes/no indicators. This screening has revealed that a sequence of separation steps is necessary to obtain pure products. The reduced number of separation steps necessary when applying integrated instead of conventional techniques underlines the benefits (e.g., reduced M



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this separation has been reported in the literature. Therefore, NF was excluded from the generation of flowsheet options. Because membrane materials to selectively separate H2O from the quaternary mixture are available, the use of an MR coupled with PV/VP membranes is deemed feasible. 4.2.2.3. Generation of Flowsheet Options. Figure 11 shows an excerpt of nine flowsheet options that were developed on the basis of the insights gained in the first two levels of this step. The first option is the sequential arrangement of reaction and separation necessitating a reactor, two distillation steps, and one azeotropic distillation to fulfill the given design task. The following eight options consider PI. Seven of these options make use of an RD column, whereas one configuration using a MR is presented. Options two and three use a combination of RD and ED to fulfill the design task. The only difference between them is the use of a rectifying section in the RD column of the second option. The RRCM in Figure 10 shows only the performance of the reactive section of the RD column. An additional rectifying section increasing the EtOH concentration in the reactive section and moving the concentration of the distillate more toward the azeotrope might be a beneficial option. Therefore, the presented flowsheet options distinguish between RD columns that have only a reactive section and RD columns with an additional rectifying section. The same is valid for the fourth and fifth flowsheet options, which use AD to separate the azeotropic mixture and recycle EtOH into the RD column, maintaining its excess. The following three flowsheet options exploit the separation mechanism of PV/VP to separate the azeotropic mixture in different hybrid configurations with the RD column. The last flowsheet option in the scope of this discussion is the combination of an MR with a conventional distillation column. A PV/VP membrane is applied to remove H2O during the reaction, thereby enabling full conversion of HLac. Subsequently, the remaining zeotropic mixture EtOH/EtLac is separated in a distillation column. The substantial meaningfulness of the obtained flowsheet options considering PI is underlined by different process options that have been investigated and presented in the literature. Gao et al.51 presented a successful integration of this chemical system into a laboratory-scale RD column equipped with a heterogeneous catalyst, establishing an EtLac yield higher than 50%. Pereira et al.52 showed the integration of this chemical system into an MR combined with PV to separate H 2 O from a quaternary mixture. They were able to demonstrate an HLac conversion of 98% using their ratebased model parametrized by using experimental laboratoryscale results.

plant size, investment, and operating costs) when using integrated techniques for this case study. 4.2.2.2. Detailed Investigation of Separation Steps and Techniques. To evaluate the feasibility of RD as an integrated reaction−separation technique for this design task, a reactive residue curve map (RRCM) was established for this system according to the procedure presented by Ung and Doherty.47 Figure 10 shows the calculated RRCM for the quaternary

Figure 10. RRCM for the quaternary reactive mixture consisting of EtOH, H2O, EtLac, and HLac at atmospheric pressure.

mixture at atmospheric pressure as a function of the transformed concentrations of EtOH (XEtOH) and HLac (XHLac). The calculation of the vapor−liquid equilibria was based on the UNIQUAC gE-model to compute the activity coefficients in the liquid phase and on the assumption of ideal gas-phase behavior.46 Data on the chemical equilibrium of the esterification reaction were also found in literature.48 The diagram reveals the general feasibility of integrating the esterification reaction into an RD column. The objective of the RD column design is to fully convert the heavy-boiler HLac to recover high-purity EtLac at the bottom of the column. This is feasible when operating the column with a high excess of EtOH in the feed. The product regions indicated in the RRCM impressively show this possibility. However, an azeotropic mixture consisting of EtOH and H2O is recovered as distillate. Therefore, RD is deemed suitable as an integrated reaction− separation technique for this esterification reaction and is applied for the generation of flowsheet options. Because an azeotropic separation of EtOH and H2O is necessary, different additional techniques for the azeotropic separation step were investigated. The separation of the azeotropic mixture EtOH/H2O by using AD or ED is wellestablished in industry. Usually, cyclohexane (in AD processes) or ethylene glycol (in ED processes) is used as MSA, and both techniques can be considered for the generation of flowsheet options.49 In addition, promising membrane materials for performing azeotropic separation via PV/VP or NF membranes were explored. The separation of EtOH/H2O mixtures via PV/ VP mechanisms has been addressed several times in the past, and various membrane materials are available for their separation.50 No successful application of NF membranes for

5. CONCLUSIONS Process intensification is an effective lever to achieve a general improvement in energy efficiency and sustainability, thereby allowing the chemical industry to contend with future challenges. However, the design of intensified processes is challenging because a large number of techniques and interconnections between them are available. In this context, both the identification of promising processes as well as the final optimization to find the best-suited operating point are challenging. In recent years, several approaches that try to systematize this procedure have been presented. A common approach is the hybrid framework for process design that consists of four consecutive steps with increasing mathematical complexity to identify the optimal solution to a given design N



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Figure 11. Excerpt of the generated flowsheet options for the production of EtLac by esterification of HLac with EtOH.

task. In the first step of this approach, a system identification and analysis of physicochemical and thermodynamic properties is performed. In the second step, promising flowsheet options capable of fulfilling the design task are generated. In the third step, these flowsheet options are evaluated with shortcut or conceptual models to exclude infeasible or unattractive options from the detailed investigations. The last step consists of the rigorous optimization of the remaining flowsheet options with respect to a predefined objective. Especially the first two steps of this framework need additional refinement to enable a quick and systematical generation of promising flowsheet options and to consider process intensification in these options. Therefore, a partly

algorithmic approach to support the development of promising flowsheet options was presented in this work. Within this approach, the generation of flowsheet options is based on the identification of property and yes/no indicators that are a function of the physicochemical and thermodynamic properties as well as the predefined boundaries of the design task. A high complexity was found for the generation of flowsheet options. Therefore, this second step of the framework was divided into three levels: (I) the identification of (reaction−)separation techniques based on the indicators, (II) their detailed investigation, and (III) the generation of flowsheet options. The calculation of the indicators in the first step as well as the identification of separation steps and potentially feasible O



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separation or integrated reaction−separation techniques in level I of the second step of the framework were fully automated by developing a capable algorithm implemented in Matlab. A simple decision tree structure that correlates indicators with different techniques that are implemented in a portfolio is used for this task. Because of this simple decision tree algorithm, the portfolio can be easily extended by additional techniques if the corresponding indicators that recommend this technique are accessible. The algorithmic approach works at a highly qualitative level of information, guaranteeing a quick identification of separation steps and techniques for the design task. Therefore, a more detailed investigation of the possible steps in level II is necessary to evaluate their feasibility. Therefore, manual tasks, such as the analysis of (reactive) residue curve maps or membrane selectivity maps is necessary in level II. Finally, flowsheet options based on the techniques that were identified feasible are generated in level III. The capability of this approach was illustrated by considering a nonreactive and a reactive case study. The nonreactive case study covered the separation of a nonideal quaternary mixture consisting of methanol, water, acetic acid, and furfural into its pure components. Promising flowsheet options considering process intensification were identified for this example by using the developed approach. The reactive case study covered the production and purification of ethyl lactate by esterification of lactic acid with ethanol. The use of a transport phase for product removal to increase the reactant conversion within this chemical equilibrium-limited reaction was identified as beneficial by comparing the physicochemical properties of the reactants and the products. Therefore, reactive distillation and membrane reactors were proposed as promising techniques to integrate reaction and separation instead of using the sequential arrangement of reaction and separation steps. The presented approach can be easily used to evaluate the general feasibility and the prospective benefits of intensified compared to conventional processes. It can provide a deeper understanding of intensified techniques and processes, thereby contributing to their broader industrial-scale application. Although promising flowsheet options can be generated, some future work still remains. A first approach is the extension of the portfolio of techniques by additional separation and reaction−separation techniques. Furthermore, the number of considered components and reactions should be enlarged. To further automate this routine, a link to solvent and membrane databases allowing their systematic selection can be established. Application to additional case studies can underline the capabilities of this approach for generating promising flowsheet options considering process intensification.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 (0) 231/755-6002. Fax: +49 (0) 231/755-3035. Notes

The authors declare no competing financial interest.

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NOMENCLATURE

Latin Letters

KOW = partition coefficient in n-octanol and water (−) M = molecular weight (kg kmol−1) MWCO = molecular-weight cutoff (kg kmol−1) Tb = boiling point temperature (K) Vm = molar volume (m3 kmol−1) Xi = transformed molar concentration of component i (mol mol−1) Greek Letters

δ = solubility parameter (MPa0.5) μd = dipole moment (D) κ = kinetic diameter (m) Abbreviations

AD = azeotropic distillation BP = byproduct CP = coproduct DWC = dividing wall column ED = extractive distillation EtLac = ethyl lactate EtOH = ethanol FF = furfural GS = gas stripping HAc = acetic acid HD = heteroazeotropic distillation HLac = lactic acid H2O = water HPAD = heat pump-assisted distillation LLPS = liquid−liquid phase separation MeOH = methanol MINLP = mixed-integer nonlinear programming MR = membrane reactor MSA = mass-separating agent NF = nanofiltration P = product PI = process intensification PSD = pressure-swing distillation PV = pervaporation R = reactant RCM = residue curve map RD = reactive distillation RDWC = reactive dividing wall column RGS = reactive gas stripping RLLPS = reactive liquid−liquid phase separation RO = reverse osmosis RPB = rotating packed bed RRCM = reactive residue curve map RSE = reactive solvent extraction SE = solvent extraction UNIFAC = universal quasichemical functional group activity coefficients UNIQUAC = universal quasichemical VP = vapor permeation

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information shows the portfolio of separation and integrated reaction−separation techniques currently considered by the developed tool. This overview contains a sketch of the operating principles, the indicators used to evaluate the appropriateness of a technique for a given separation step and the reaction and transport phases for integrated techniques. Furthermore, it contains a list of the calculated indicators for each separation step possible for the considered case studies, giving detailed insights into the generation of flowsheet options. This material is available free of charge via the Internet at http://pubs.acs.org. P



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(24) Kiss, A. Advanced Distillation Technologies: Design, Control and Applications; John Wiley and Sons Ltd: Chichester, U.K., 2013. (25) Holtbruegge, J.; Wierschem, M.; Steinruecken, S.; et al. Experimental Investigation, Modeling and Scale-up of Hydrophilic Vapor Permeation Membranes: Separation of Azeotropic Dimethyl Carbonate/Methanol Mixtures. Sep. Purif. Technol. 2013, 118, 862. (26) Fredenslund, A.; Jones, R.; Prausnitz, J. Group-Contribution Estimation of Activity Coefficients in Nonideal Liquid Mixtures. AIChE J. 1975, 21 (6), 1086. (27) Pennington, D.; Yue, P. A Screening-Level Prototype for the Synthesis or Analysis of Separation Systems to Support Identification of Inherently Cleaner Chemical Processes. Clean Technol. Environ. Policy 2000, 2 (2), 82. (28) Woods, D. Rules of Thumb in Engineering Practice; Wiley-VCH: Weinheim, Germany, 2007. (29) Melin, T.; Rautenbach, R. Membranverfahren; Springer: Berlin, 2007. (30) Kiss, A.; Suszwalak, D. Enhanced Bioethanol Dehydration by Extractive and Azeotropic Distillation in Dividing-wall Columns. Sep. Purif. Technol. 2012, 86, 70. (31) Fowler, R. Higee - A Status Report. Chem. Eng. 1989, 456, 35. (32) Kister, H. What Caused Tower Malfunctions in the Last 50 Years? In Distillation and Absorption 2002; Darton, R., Ed.; VDIVerlag: Berlin, 2002, 5. (33) Tylko, M.; Barkmann, S.; Sand, G., et al. Synthesis of Reactive Separation Processes. In Integrated Reaction and Separation Operations; Schmidt-Traub, H., Górak, A., Eds.; Springer: Berlin, 2006, 7. (34) Luyben, W. Distillation Design and Control Using Aspen Simulation; John Wiley and Sons Ltd: Hoboken, 2013. (35) Thong, D.; Jobson, M. Multicomponent Homogeneous Azeotropic Distillation 1. Assessing Product Feasibility. Chem. Eng. Sci. 2001, 56 (14), 4369. (36) Thong, D.; Jobson, M. Multicomponent Homogeneous Azeotropic Distillation 2. Column Design. Chem. Eng. Sci. 2001, 56 (14), 4393. (37) Thong, D.; Jobson, M. Multicomponent Homogeneous Azeotropic Distillation 3. Column Sequence Synthesis. Chem. Eng. Sci. 2001, 56 (14), 4417. (38) Schembecker, G.; Tlatlik, S. Process Synthesis for Reactive Separations. Chem. Eng. Process. 2003, 42 (3), 179. (39) Schmidt, P.; Köse, T.; Lutze, P. Characterisation of Organic Solvent Nanofiltration Membranes in Multi-component Mixtures: Membrane Rejection Maps and Membrane Selectivity Maps for Conceptual Process Design. J. Membr. Sci. 2013, 429, 103. (40) Schmidt, P.; Lutze, P. Characterisation of Organic Solvent Nanofiltration Membranes in Multi-component Mixtures: Phenomena-Based Modelling and Membrane Modelling Maps. J. Membr. Sci. 2013, 445, 183. (41) Gani, R.; Hytoft, G.; Jaksland, C.; Jensen, A. An Integrated Computer Aided System for Integrated Design of Chemical Processes. Comput. Chem. Eng. 1997, 21 (10), 1135. (42) Lide, D. CRC Handbook of Chemistry and Physics, 90th ed.; CRC Press: Boca Raton, 2009. (43) Curtis, R.; Hatt, H. Equilibria in Furfural-Water Systems under Increased Pressure and the Influence of Added Salts Upon the Mutual Solubilities of Furfural and Water. Aust. J. Sci. Res., Ser. A 1948, 1, 213. (44) Liu, X.; Sun, Y.; Deng, X. Studies on the Pervaporation Membrane of Permeation Water from Methanol/Water Mixture. J. Membr. Sci. 2008, 325 (1), 192. (45) Smallwood, I. Handbook of Organic Solvent Properties; Elsevier Science: Burlington, MA, 1996. (46) Delgado, P.; Sanz, M.; Beltrán, S. Isobaric Vapor−liquid Equilibria for the Quaternary Reactive System: Ethanol+Water+Ethyl Lactate+Lactic Acid at 101.33 kPa. Fluid Phase Equilib. 2007, 255 (1), 17. (47) Ung, S.; Doherty, M. Vapor-liquid Phase Equilibrium in Systems with Multiple Chemical Reactions. Chem. Eng. Sci. 1995, 50 (1), 23.

REFERENCES

(1) Jansens, P.; Górak, A.; van Suijdam, H.; Schoenmakers, H. Road Map for Separations: A Dutch and German Perspective. Chem. Ing. Tech. 2005, 77 (8), 1242. (2) Skiborowski, M.; Harwardt, A.; Marquardt, W. Conceptual Design of Distillation-based Hybrid Separation Processes. Annu. Rev. Chem. Biomol. Eng. 2013, 4 (1), 45. (3) Harmsen, G.; Hinderink, P.; Sijben, J., et al. Industrially Applied Process Synthesis Method Creates Synergy between Economy and Sustainability. In AIChE Symposium Series 323; Malone, M. F., Trainham, J. A., Carnahan, B., Eds.; Elsevier: Amsterdam, 2000. (4) Li, C.; Wozny, G.; Suzuki, K. Design and Synthesis of Separation Process Based on a Hybrid Method. Asia-Pac. J. Chem. Eng. 2009, 4 (6), 905. (5) Li, X.; Kraslawski, A. Conceptual Process Synthesis: Past and Current Trends. Chem. Eng. Process. 2004, 43 (5), 583. (6) Lutze, P.; Gór ak, A. Reactive and Membrane-Assisted Distillation: Recent Developments and Perspective. Chem. Eng. Res. Des. 2013, 91 (10), 1978. (7) Douglas, J. A Hierarchical Decision Procedure for Process Synthesis. AIChE J. 1985, 31 (3), 353. (8) Barnicki, S.; Hoyme, C.; Siirola, J. Separations Process Synthesis. In Encyclopedia of Chemical Technology; Kirk, R. E., Othmer, D. F., Eds.; Wiley-VCH: New York, 2003. (9) Jaksland, C.; Gani, R.; Lien, K. Separation Process Design and Synthesis Based on Thermodynamic Insights. Chem. Eng. Sci. 1995, 50 (3), 511. (10) Bek-Pedersen, E.; Gani, R.; Levaux, O. Determination of Optimal Energy Efficient Separation Schemes based on Driving Forces. Comput. Chem. Eng. 2000, 24 (2−7), 253. (11) Agrawal, R. Synthesis of Multicomponent Distillation Column Configurations. AIChE J. 2003, 49 (2), 379. (12) d’Anterroches, L.; Gani, R. Group Contribution Based Process Flowsheet Synthesis, Design and Modelling. Fluid Phase Equilib. 2005, 228−229, 141. (13) Rong, B.-G.; Errico, M. Synthesis of Intensified Simple Column Configurations for Multicomponent Distillations. Chem. Eng. Process. 2012, 62, 1. (14) Grossmann, I.; Daichendt, M. New Trends in OptimizationBased Approaches to Process Synthesis. Comput. Chem. Eng. 1996, 20 (6−7), 665. (15) Grossmann, I. Mixed-integer Programming Approach for the Synthesis of Integrated Process Flowsheets. Comput. Chem. Eng. 1985, 9 (5), 463. (16) Westerberg, A. A Retrospective on Design and Process Synthesis. Comput. Chem. Eng. 2004, 28 (4), 447. (17) Henrich, F.; Bouvy, C.; Kausch, C.; et al. Economic Optimization of Non-sharp Separation Sequences by Means of Evolutionary Algorithms. Comput. Chem. Eng. 2008, 32 (7), 1411. (18) Freund, H.; Sundmacher, K. Towards a Methodology for the Systematic Analysis and Design of Efficient Chemical Processes: Part 1. From Unit Operations to Elementary Process Functions. Chem. Eng. Process. 2008, 47 (12), 2051. (19) Tanskanen, J.; Pohjola, V.; Lien, K. Phenomenon Driven Process Design Methodology: Focus on Reactive Distillation. Comput. Chem. Eng. 1995, 19, 77. (20) Lutze, P.; Babi, D.; Woodley, J.; Gani, R. Phenomena Based Methodology for Process Synthesis Incorporating Process Intensification. Ind. Eng. Chem. Res. 2013, 52 (22), 7127. (21) Rippin, D. Introduction: Approaches to Chemical Process Synthesis. In AIChE Symp. Ser.; Siirola, J. J., Grossmann, I. E., Stephanopoulos, G., Eds.; Elsevier: Amsterdam, 1990. (22) Mizsey, P.; Fonyo, Z. Toward a More Realistic Overall Process SynthesisThe Combined Approach. Comput. Chem. Eng. 1990, 14 (11), 1213. (23) Marquardt, W.; Kossack, S.; Kraemer, K. A Framework for the Systematic Design of Hybrid Separation Processes. Chin. J. Chem. Eng. 2008, 16 (3), 333. Q



Article

(48) Delgado, P.; Sanz, M.; Beltrán, S.; Núñez, L. Ethyl Lactate Production via Esterification of Lactic Acid with Ethanol Combined with Pervaporation. Chem. Eng. J. 2010, 165 (2), 693. (49) Stichlmair, J.; Fair, J. Distillation: Principles and Practices; WileyVCH: New York, 1998. (50) Roth, T.; Kreis, P. Rate Based Modelling and Simulation Studies of Hybrid Processes Consisting of Distillation, Vapour Permeation and Adsorption for the Dehydration of Ethanol. In Comput.-Aided Chem. Eng.; Jeżowski, J., Thullie, J., Eds.; Elsevier: Amsterdam, 2009; Vol. 26, pp 815−819. (51) Gao, J.; Zhao, X.; Zhou, L.; Huang, Z. Investigation of Ethyl Lactate Reactive Distillation Process. Chem. Eng. Res. Des. 2007, 85 (4), 525. (52) Pereira, C.; Silva, V.; Pinho, S.; Rodrigues, A. Batch and Continuous Studies for Ethyl Lactate Synthesis in a Pervaporation Membrane Reactor. J. Membr. Sci. 2010, 361 (1−2), 43.

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Conceptual Design of Flowsheet Options Based on Thermodynamic

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