- Email: [email protected]
Towards a systematic framework for the synthesis of operable process intensification systems - application to reactive distillation systems
Anton A. Kiss, Edwin Zondervan, Richard Lakerveld, Leyla Özkan (Eds.) Proceedings of the 29th European Symposium on Computer Aided Process Engineering June 16th to 19th , 2019, Eindhoven, The Netherlands. © 2019 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/B978-0-12-818634-3.50013-8
Towards a systematic framework for the synthesis of operable process intensification systems - application to reactive distillation systems Yuhe Tiana,b, Iosif S. Pappas a,b, Baris Burnak a,b, Justin Katz a,b, Styliani Avraamidou a,b, Nikolaos A. Diangelakisa,b and Efstratios N. Pistikopoulosa,b,* a Artie
McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, United States b Texas A&M Energy Institute, Texas A&M University, College Station, TX 77843, United States [email protected]
Abstract We present a systematic framework for the design and operability/safety optimization of process intensified systems, with specific focus on reactive distillation processes. This framework is based on a phenomenological process intensification/synthesis approach (i.e., Generalized Modular Representation Framework) which first identifies promising intensified tasks and then translates them to equipment-based flowsheet alternatives. Flexibility analysis is integrated with the synthesis model to ensure that resulting design configurations are operable under varying operating conditions. To systematically account for inherent safety performance, risk assessment criteria are included as process constraints involving failure frequency and consequence severity criteria. A case study on the production of methyl tertiary butyl ether is presented to highlight the potential of the proposed approach in deriving inherently operable and safe intensified reactive separation systems. Keywords: Process intensification, Reactive distillation, Generalized Modular Representation Framework, Operability analysis, Risk assessment
1. Introduction In recent years, process intensification (PI) has attracted burgeoning interest in the chemical engineering research community and the chemical/energy industry due to its potential for drastic improvements in process productivity, efficiency, and profitability (Tian et al., 2018). The integration of multiple processing tasks into a single unit (e.g., combined reaction/separation processes) is one of the major PI pathways towards the development of breakthrough technologies. Reactive distillation (RD), a classic intensified invention, is showing significant energy and cost savings over conventional reactor-distillation counterparts (between 15 and 80 %) (Harmsen, 2007). Recent works for the synthesis of reactive separation systems have been leveraging phenomenological representation methods to systematically generate intensified process options from a lower-aggregated level without the pre-postulation of plausible unit-operation-based flowsheets which may hinder the discovery of novel solutions (Tula et al., 2017; da Cruz and Manousiouthakis, 2017; Demirel et al., 2017). However, these efforts mainly focus on steady-state design
74
Y. Tian et al.
at nominal operating conditions. The operational performances in these units under uncertainty and disturbances are mostly neglected, whereas their highly integrated schemes often decrease the degrees of freedom of the online decision maker, adversely affecting the process safety and limiting the operability of the systems (Baldea, 2015). Therefore, a holistic synthesis approach is required for the delivery of reactive distillation systems with guaranteed operability and safety performances at the early design stage. In this work, we introduce a systematic framework for intensified process synthesis, which is based on the phenomenological Generalized Modular Representation Framework (GMF) with embedded process operability and safety assessment criteria. The rest of the paper is organized as follows. In Section 2, the proposed framework is presented in detail. Section 3 showcases this approach on a distillation-based reactive separation process for methyl tertiary butyl ether (MTBE) production. Finally, conclusions and directions for future work are discussed in Section 4.
2. Synthesis framework for Process Intensification To address the synthesis of operable PI systems, we propose an integrated framework as depicted in Figure 1, with its stepwise procedure described in what follows: Step 1: Process Intensification/Synthesis Representation via Generalized Modular Representation Framework – GMF, originally introduced by Papalexandri and Pistikopoulos (1996), represents chemical processes with two sets of phenomenological building blocks, namely the pure heat exchange module and the multifunctional mass/heat exchange module. Utilizing Gibbs free energybased driving force constraints to characterize mass/heat transfer feasibility, GMF characterizes various (intensified) tasks (e.g., separation, reaction, combined separation/reaction) by optimizing physical and chemical driving forces to exploit the “ultimate” thermodynamic design space. The synergistic integration of multiple phenomena are automatically explored via superstructure optimization (Ismail et al., 2001) without a pre-postulation of plausible (and possibly myriad) tasks.
Process Intensification/ Synthesis Representation
Process Operability/ Safety Analysis
Process Synthesis/ Optimization
Verifiable & Operable Intensified Designs
Figure 1: Proposed framework for the synthesis of operable process intensification systems (adapted from Tian et al. (2018)).
Step 2: Process Operability/Safety Analysis – Flexibility test is applied to identify the critical operating conditions when the resulting design configuration is operated under an expected range of the uncertainty. As for the consideration of inherent safety, risk assessment criteria (Nemet et al., 2017), which account for equipment failure frequency and consequence severity at the release of entire intrinsic hazards existing in the process, are included as constraints in the synthesis model to enable systematic generation of inherently safer design options instead of iterative evaluation in a posterior manner. Step 3: Process Synthesis/Optimization – In this step, an integrated GMF-operabilitysafety synthesis strategy is developed in the formulation of a mixed integer nonlinear
Towards a systematic framework for the synthesis of operable process intensification systems - application to reactive distillation systems
75
programming (MINLP) problem, as shown in Table 1. The Generalized Benders Decomposition (GBD) method (Geoffrion, 1972) is utilized for the solution of this optimization problem and is implemented in the General Algebraic Modeling System (GAMS) (Rosenthal, 2016). The primal NLP subproblem is solved with solver CONOPT and the master MIP problem with CPLEX. The optimization results will deliver optimal GMF-based design configurations with desired operability and safety performances. Step 4: Operable Intensified Designs Verified via Steady-State & Dynamic Simulation – The resulting GMF configurations are translated to corresponding equipment-based flowsheet alternatives and validated with steady-state simulation. To integrate steady-state and dynamic operation, “high-fidelity” dynamic models are developed to fully capture and analyze the system dynamics. This allows for further design and control optimization studies (Pistikopoulos et al., 2015; Diangelakis et al., 2017). Closed-loop validation is finally performed, with necessary iterations between different steps to ensure the delivery of consistent and verifiable operable intensification systems throughout the framework. Table 1: Mathematical model for GMF-flexibility-safety synthesis – an indicative list.
Mass balance Energy balance Driving force constraints
GMF Synthesis Model f LI xiLI + f V I xVi I − f LO xiLO − f VO xVO i + ∑ νi × r × Mcat = 0 V I hV I − f LO hLO − f VO hVO + H f LI hLI + f ∑ reac × r × Mcat = 0 i i i i L L sat,L f γi xi Pi νik ΔGi + ∑i ∑k RT + νik ln(φiV xVi Ptot ) ∂∂ nεLk G2i = ln φ V xV P i
i
tot
liq: ∑i Keq,i xi ≤ 1, vap: ∑i xi /Keq,i ≤ 1 1 r = k[∏ aα − Ka ∏ aβ ], k = Aexp(−Ea/RT ) γi = γ(xi , T, P), Pisat = Psat (T ) LL CL ye+1 − ye ≤ 0, ye − [∑ yIL ne + ∑ yee + ∑ yee ≤ 0] Flexibility Analysis max min max f j (Vθ ,Vd ,Vx ,Vz ) ≤ 0 s.t.h(Vθ ,Vd ,Vx ,Vz ) = 0
i
Phase defining Kinetic model Thermodynamic model Structural interconnection
Vθ ∈U(Vθ ) Vz
Consequence severity Risk evaluation Total annualized cost
j∈J f
Risk Assessment Severityi,e = Wi,e × O1e × O2e × O3e /Srisk Riski,e = f reqe, f ail × Severityi,e Objective Function Costob j = Costcooling +Costheating +Costmodule
* Nomenclature: G = Gibbs free energy, O = factors accounting for process conditions, S = limit value for hazardous properties, W = quantity of hazardous substances present, y = binary variable, Vθ = uncertain parameters, Vd = design variables, Vx = state variables, Vz = control variables; Superscripts: LI = GMF module liquid inlet stream, V I = vapor inlet stream, LO = liquid outlet stream, VO = vapor outlet stream, LL = interconnecting liquid stream; Subscripts: e = module, i = component, k = reaction, n = feed stream.
3. Case study: MTBE production In this section, the proposed framework is tested on a reactive separation system for the production of methyl tertiary butyl ether (MTBE) (Ismail et al., 2001). The case study aims to demonstrate the integration of operability/safety criteria at the early synthesis level as well as the potential of systematically deriving multiple intensified design options with different operational and cost performances for decision-making.
76
Y. Tian et al.
3.1. Problem statement MTBE can be produced via the catalytic reaction of methanol (MeOH) and isobutylene (IB4) in the liquid phase using an ion-exchange resin as catalyst (e.g., Amberlyst 15 (Rehfinger and Hoffmann, 1990)). The production task is to obtain liquid MTBE at a rate of at least 197 mol/s with a purity higher than 98 mol%. Raw material availability (i.e., methanol and butene feed streams with given composition, flowrate, and temperature) is taken from Hauan et al. (1995). Additionally, an uncertainty range of ±10 mol/s is considered for the methanol feed flowrate. Given the hazardous properties of the substances in this process, manifold process risks (i.e., toxicity, flammability, explosiveness) need to be considered for a holistic evaluation of inherent safety performance. The synthesis objective is to identify a flowsheet alternative to meet the MTBE production specifications with minimum total annualized cost (TAC) and desired operability/safety performance. 3.2. Synthesis with operability and safety considerations In this case study, a maximum of 10 GMF mass/heat exchange modules and 20 pure heat exchange modules are allowed for the representation/synthesis of this process. The separation and/or reaction task taking place in each mass/heat exchange module is not postulated a priori, but to be determined through the superstructure optimization driven by the minimization of total annualized cost. A nominal GMF configuration without flexibility or safety considerations is first synthesized as a reference design. The resulting MINLP model has 14,594 constraints, 8,098 continuous variables, as well as 734 binary variables. The optimal solution, shown in Fig. 2, features reactive distillation column at a total annualized cost of $1.7 × 106 /y. In this nominal design configuration, two pure heat exchange modules are selected as reboiler/condenser, while two reactive separation modules are used as reaction zone to produce MTBE, two separation modules as stripping section to separate unreacted methanol/isobutylene back to reaction zone, and another one as rectification section to transfer n-butene to distillate.
CW – V
MeOH
L – V
L – V L – V
IB4 + NB4
L – V L – V
L – ST
MTBE
Figure 2: Nominal design.
Flexibility analysis is then applied to test if the nominal GMF configuration can be operated under the uncertainty of methanol feed flowrate, which identifies a critical point at the higher flowrate extreme value. On the other hand, inherent safety performance is improved by reducing the overall process risk by at least 20 % than that seen in the nominal design. The resulting design alternative, as shown in Fig. 3(a), utilizes four mass/heat exchange modules. Thus in this case, the enhancement of process safety is achieved by minimizing the unit size. However, a comparatively more “risky” component is observed in Fig. 3(a) - i.e. the second reactive separation module (numbered from top to bottom) takes up more than 1/3 of overall process risk. To avoid the safety concerns resulting from a single module, the individual module risk is constrained to be less than 30 % of the overall process risk. The new optimal design configuration is illustrated in Fig. 3(b). The afore-mentioned module
77
Towards a systematic framework for the synthesis of operable process intensification systems - application to reactive distillation systems
is bypassed to alleviate its mass/heat transfer burden, thus introducing more degrees of freedom to enhance the system’s flexibility and operability performances. The detailed design and operating parameters can be found in Table 2. CW – V
CW – V
L – V MeOH
MeOH
L – V
L – V
L – V
L – V
L – V
IB4 + NB4
IB4 + NB4 L – V
L – V
L – ST
L – ST MTBE (a)
MTBE (b)
Figure 3: Flexible & inherently safer design configurations. (a) Operable design I, (b) Operable design II 3.3. Steady-state & dynamic simulation and validation Having generated the optimal and operable GMF configurations for the MTBE production (Fig. 3), in this step we identify and validate the corresponding equipment-based flowsheet using steady-state simulation. Each GMF separation module is translated to two distillation trays in Aspen Plus® RADFRAC column (Aspen, 1981-2018), while each GMF reactive separation module to three reactive distillation trays. Therefore, the first operable design is verified as a 13-tray RD column, while the other one as a 10-tray RD column integrated with an additional 3-tray side-column. Table 2 provides a summary of quantitative validation between GMF and Aspen simulation on design/operating variables. “High-fidelity” dynamic models (Schenk et al., 1999) are also developed for the reactive distillation systems (Fig. 3) in gPROMS ModelBuilder® (PSE, 1997-2018), consisting of a system of differential and algebraic equations (DAE) for the description of component molar and energy balances for each tray, the partial reboiler and the total condenser, reaction kinetics, phase equilibrium, etc. The developed dynamic systems will be utilized for the derivation of receding horizon control policies (Pistikopoulos et al., 2015) to guarantee the optimal operation of the proposed designs. Table 2: GMF quantitative validation with Aspen simulation.
Column pressure (atm) Reflux ratio Reboiler duty (kW) Condenser duty (kW) Module/Tray Number Bottom product flowrate (mol/s) Product purity (MTBE mol/mol) TAC (×104 $/y )
Nominal Design GMF Aspen 5.46 6.00 1.70 2.10 7.5 × 103 6.6 × 103 2.3 × 104 2.2 × 104 7 15 197.0 197.0 0.98 0.98 171.1
Operable Design I GMF Aspen 7.85 7.95 1.70 2.50 8.4 × 103 9.6 × 103 2.3 × 104 2.4 × 104 6 13 197.0 197.0 0.98 0.98 182.4
Operable Design II GMF Aspen 9.48 8.20 1.70 3.30 8.9 × 103 2 × 104 2.3 × 104 3.4 × 104 6 13 197.0 197.0 0.98 0.98 190.2
78
Y. Tian et al.
4. Conclusion In this paper, we have presented a systematic framework to efficiently address the combinatorial PI design space and to integrate steady-state synthesis, dynamic analysis, and operability assessment to deliver validated operable intensification designs. It is shown, through an MTBE reactive separation example, that operability considerations may result in significant structural and operating changes of the process optimal solutions. Ongoing work addresses simultaneous design and control studies on the resulting MTBE reactive distillation configurations to close the loop under dynamic operating conditions.
5. Acknowledgement We acknowledge the financial support from the Texas A&M Energy Institute, Shell Oil Company, RAPID SYNOPSIS Project (DE-EE0007888-09-03), and the NSF (Grant no. 1705423) under the project titled SusChEM: An integrated framework for process design, control and scheduling [PAROC].
References Aspen, 1981-2018. Aspen technology, inc. https://www.aspentech.com/en/products/engineering/aspen-plus. M. Baldea, 2015. From process integration to process intensification. Computers & Chemical Engineering 81, 104–114. F. E. da Cruz, V. I. Manousiouthakis, 2017. Process intensification of reactive separator networks through the ideas conceptual framework. Computers & Chemical Engineering 105, 39–55. S. E. Demirel, J. Li, M. F. Hasan, 2017. Systematic process intensification using building blocks. Computers & Chemical Engineering 105, 2–38. N. A. Diangelakis, B. Burnak, J. Katz, E. N. Pistikopoulos, 2017. Process design and control optimization: A simultaneous approach by multi-parametric programming. AIChE Journal 63 (11), 4827–4846. A. M. Geoffrion, 1972. Generalized benders decomposition. Journal of optimization theory and applications 10 (4), 237–260. G. J. Harmsen, 2007. Reactive distillation: the front-runner of industrial process intensification: a full review of commercial applications, research, scale-up, design and operation. Chemical Engineering and Processing: Process Intensification 46 (9), 774–780. S. R. Ismail, P. Proios, E. N. Pistikopoulos, 2001. Modular synthesis framework for combined separation/reaction systems. AIChE Journal 47 (3), 629–649. A. Nemet, J. J. Klemeš, I. Moon, Z. Kravanja, 2017. Safety analysis embedded in heat exchanger network synthesis. Computers & Chemical Engineering 107, 357–380. K. P. Papalexandri, E. N. Pistikopoulos, 1996. Generalized modular representation framework for process synthesis. AIChE Journal 42 (4), 1010–1032. E. N. Pistikopoulos, N. A. Diangelakis, R. Oberdieck, M. M. Papathanasiou, I. Nascu, M. Sun, 2015. PAROC – an integrated framework and software platform for the optimisation and advanced model-based control of process systems. Chemical Engineering Science 136, 115–138. PSE, 1997-2018. Process systems enterprise. gproms. https://www.psenterprise.com/products/gproms. A. Rehfinger, U. Hoffmann, 1990. Kinetics of methyl tertiary butyl ether liquid phase synthesis catalyzed by ion exchange resin – i. intrinsic rate expression in liquid phase activities. Chemical Engineering Science 45 (6), 1605–1617. R. E. Rosenthal, 2016. GAMS: A user’s guide. M. Schenk, R. Gani, D. Bogle, E. Pistikopoulos, 1999. A hybrid modelling approach for separation systems involving distillation. Chemical Engineering Research and Design 77 (6), 519–534. Y. Tian, S. E. Demirel, M. F. Hasan, E. N. Pistikopoulos, 2018. An overview of process systems engineering approaches for process intensification: State of the art. Chemical Engineering and Processing-Process Intensification 133, 160–210. A. K. Tula, D. K. Babi, J. Bottlaender, M. R. Eden, R. Gani, 2017. A computer-aided software-tool for sustainable process synthesis-intensification. Computers & Chemical Engineering 105, 74–95.
Towards a systematic framework for the synthesis of operable process intensification systems - application to reactive distillation systems Yuhe Tiana,b, Iosif S. Pappas a,b, Baris Burnak a,b, Justin Katz a,b, Styliani Avraamidou a,b, Nikolaos A. Diangelakisa,b and Efstratios N. Pistikopoulosa,b,* a Artie
McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, United States b Texas A&M Energy Institute, Texas A&M University, College Station, TX 77843, United States [email protected]
Abstract We present a systematic framework for the design and operability/safety optimization of process intensified systems, with specific focus on reactive distillation processes. This framework is based on a phenomenological process intensification/synthesis approach (i.e., Generalized Modular Representation Framework) which first identifies promising intensified tasks and then translates them to equipment-based flowsheet alternatives. Flexibility analysis is integrated with the synthesis model to ensure that resulting design configurations are operable under varying operating conditions. To systematically account for inherent safety performance, risk assessment criteria are included as process constraints involving failure frequency and consequence severity criteria. A case study on the production of methyl tertiary butyl ether is presented to highlight the potential of the proposed approach in deriving inherently operable and safe intensified reactive separation systems. Keywords: Process intensification, Reactive distillation, Generalized Modular Representation Framework, Operability analysis, Risk assessment
1. Introduction In recent years, process intensification (PI) has attracted burgeoning interest in the chemical engineering research community and the chemical/energy industry due to its potential for drastic improvements in process productivity, efficiency, and profitability (Tian et al., 2018). The integration of multiple processing tasks into a single unit (e.g., combined reaction/separation processes) is one of the major PI pathways towards the development of breakthrough technologies. Reactive distillation (RD), a classic intensified invention, is showing significant energy and cost savings over conventional reactor-distillation counterparts (between 15 and 80 %) (Harmsen, 2007). Recent works for the synthesis of reactive separation systems have been leveraging phenomenological representation methods to systematically generate intensified process options from a lower-aggregated level without the pre-postulation of plausible unit-operation-based flowsheets which may hinder the discovery of novel solutions (Tula et al., 2017; da Cruz and Manousiouthakis, 2017; Demirel et al., 2017). However, these efforts mainly focus on steady-state design
74
Y. Tian et al.
at nominal operating conditions. The operational performances in these units under uncertainty and disturbances are mostly neglected, whereas their highly integrated schemes often decrease the degrees of freedom of the online decision maker, adversely affecting the process safety and limiting the operability of the systems (Baldea, 2015). Therefore, a holistic synthesis approach is required for the delivery of reactive distillation systems with guaranteed operability and safety performances at the early design stage. In this work, we introduce a systematic framework for intensified process synthesis, which is based on the phenomenological Generalized Modular Representation Framework (GMF) with embedded process operability and safety assessment criteria. The rest of the paper is organized as follows. In Section 2, the proposed framework is presented in detail. Section 3 showcases this approach on a distillation-based reactive separation process for methyl tertiary butyl ether (MTBE) production. Finally, conclusions and directions for future work are discussed in Section 4.
2. Synthesis framework for Process Intensification To address the synthesis of operable PI systems, we propose an integrated framework as depicted in Figure 1, with its stepwise procedure described in what follows: Step 1: Process Intensification/Synthesis Representation via Generalized Modular Representation Framework – GMF, originally introduced by Papalexandri and Pistikopoulos (1996), represents chemical processes with two sets of phenomenological building blocks, namely the pure heat exchange module and the multifunctional mass/heat exchange module. Utilizing Gibbs free energybased driving force constraints to characterize mass/heat transfer feasibility, GMF characterizes various (intensified) tasks (e.g., separation, reaction, combined separation/reaction) by optimizing physical and chemical driving forces to exploit the “ultimate” thermodynamic design space. The synergistic integration of multiple phenomena are automatically explored via superstructure optimization (Ismail et al., 2001) without a pre-postulation of plausible (and possibly myriad) tasks.
Process Intensification/ Synthesis Representation
Process Operability/ Safety Analysis
Process Synthesis/ Optimization
Verifiable & Operable Intensified Designs
Figure 1: Proposed framework for the synthesis of operable process intensification systems (adapted from Tian et al. (2018)).
Step 2: Process Operability/Safety Analysis – Flexibility test is applied to identify the critical operating conditions when the resulting design configuration is operated under an expected range of the uncertainty. As for the consideration of inherent safety, risk assessment criteria (Nemet et al., 2017), which account for equipment failure frequency and consequence severity at the release of entire intrinsic hazards existing in the process, are included as constraints in the synthesis model to enable systematic generation of inherently safer design options instead of iterative evaluation in a posterior manner. Step 3: Process Synthesis/Optimization – In this step, an integrated GMF-operabilitysafety synthesis strategy is developed in the formulation of a mixed integer nonlinear
Towards a systematic framework for the synthesis of operable process intensification systems - application to reactive distillation systems
75
programming (MINLP) problem, as shown in Table 1. The Generalized Benders Decomposition (GBD) method (Geoffrion, 1972) is utilized for the solution of this optimization problem and is implemented in the General Algebraic Modeling System (GAMS) (Rosenthal, 2016). The primal NLP subproblem is solved with solver CONOPT and the master MIP problem with CPLEX. The optimization results will deliver optimal GMF-based design configurations with desired operability and safety performances. Step 4: Operable Intensified Designs Verified via Steady-State & Dynamic Simulation – The resulting GMF configurations are translated to corresponding equipment-based flowsheet alternatives and validated with steady-state simulation. To integrate steady-state and dynamic operation, “high-fidelity” dynamic models are developed to fully capture and analyze the system dynamics. This allows for further design and control optimization studies (Pistikopoulos et al., 2015; Diangelakis et al., 2017). Closed-loop validation is finally performed, with necessary iterations between different steps to ensure the delivery of consistent and verifiable operable intensification systems throughout the framework. Table 1: Mathematical model for GMF-flexibility-safety synthesis – an indicative list.
Mass balance Energy balance Driving force constraints
GMF Synthesis Model f LI xiLI + f V I xVi I − f LO xiLO − f VO xVO i + ∑ νi × r × Mcat = 0 V I hV I − f LO hLO − f VO hVO + H f LI hLI + f ∑ reac × r × Mcat = 0 i i i i L L sat,L f γi xi Pi νik ΔGi + ∑i ∑k RT + νik ln(φiV xVi Ptot ) ∂∂ nεLk G2i = ln φ V xV P i
i
tot
liq: ∑i Keq,i xi ≤ 1, vap: ∑i xi /Keq,i ≤ 1 1 r = k[∏ aα − Ka ∏ aβ ], k = Aexp(−Ea/RT ) γi = γ(xi , T, P), Pisat = Psat (T ) LL CL ye+1 − ye ≤ 0, ye − [∑ yIL ne + ∑ yee + ∑ yee ≤ 0] Flexibility Analysis max min max f j (Vθ ,Vd ,Vx ,Vz ) ≤ 0 s.t.h(Vθ ,Vd ,Vx ,Vz ) = 0
i
Phase defining Kinetic model Thermodynamic model Structural interconnection
Vθ ∈U(Vθ ) Vz
Consequence severity Risk evaluation Total annualized cost
j∈J f
Risk Assessment Severityi,e = Wi,e × O1e × O2e × O3e /Srisk Riski,e = f reqe, f ail × Severityi,e Objective Function Costob j = Costcooling +Costheating +Costmodule
* Nomenclature: G = Gibbs free energy, O = factors accounting for process conditions, S = limit value for hazardous properties, W = quantity of hazardous substances present, y = binary variable, Vθ = uncertain parameters, Vd = design variables, Vx = state variables, Vz = control variables; Superscripts: LI = GMF module liquid inlet stream, V I = vapor inlet stream, LO = liquid outlet stream, VO = vapor outlet stream, LL = interconnecting liquid stream; Subscripts: e = module, i = component, k = reaction, n = feed stream.
3. Case study: MTBE production In this section, the proposed framework is tested on a reactive separation system for the production of methyl tertiary butyl ether (MTBE) (Ismail et al., 2001). The case study aims to demonstrate the integration of operability/safety criteria at the early synthesis level as well as the potential of systematically deriving multiple intensified design options with different operational and cost performances for decision-making.
76
Y. Tian et al.
3.1. Problem statement MTBE can be produced via the catalytic reaction of methanol (MeOH) and isobutylene (IB4) in the liquid phase using an ion-exchange resin as catalyst (e.g., Amberlyst 15 (Rehfinger and Hoffmann, 1990)). The production task is to obtain liquid MTBE at a rate of at least 197 mol/s with a purity higher than 98 mol%. Raw material availability (i.e., methanol and butene feed streams with given composition, flowrate, and temperature) is taken from Hauan et al. (1995). Additionally, an uncertainty range of ±10 mol/s is considered for the methanol feed flowrate. Given the hazardous properties of the substances in this process, manifold process risks (i.e., toxicity, flammability, explosiveness) need to be considered for a holistic evaluation of inherent safety performance. The synthesis objective is to identify a flowsheet alternative to meet the MTBE production specifications with minimum total annualized cost (TAC) and desired operability/safety performance. 3.2. Synthesis with operability and safety considerations In this case study, a maximum of 10 GMF mass/heat exchange modules and 20 pure heat exchange modules are allowed for the representation/synthesis of this process. The separation and/or reaction task taking place in each mass/heat exchange module is not postulated a priori, but to be determined through the superstructure optimization driven by the minimization of total annualized cost. A nominal GMF configuration without flexibility or safety considerations is first synthesized as a reference design. The resulting MINLP model has 14,594 constraints, 8,098 continuous variables, as well as 734 binary variables. The optimal solution, shown in Fig. 2, features reactive distillation column at a total annualized cost of $1.7 × 106 /y. In this nominal design configuration, two pure heat exchange modules are selected as reboiler/condenser, while two reactive separation modules are used as reaction zone to produce MTBE, two separation modules as stripping section to separate unreacted methanol/isobutylene back to reaction zone, and another one as rectification section to transfer n-butene to distillate.
CW – V
MeOH
L – V
L – V L – V
IB4 + NB4
L – V L – V
L – ST
MTBE
Figure 2: Nominal design.
Flexibility analysis is then applied to test if the nominal GMF configuration can be operated under the uncertainty of methanol feed flowrate, which identifies a critical point at the higher flowrate extreme value. On the other hand, inherent safety performance is improved by reducing the overall process risk by at least 20 % than that seen in the nominal design. The resulting design alternative, as shown in Fig. 3(a), utilizes four mass/heat exchange modules. Thus in this case, the enhancement of process safety is achieved by minimizing the unit size. However, a comparatively more “risky” component is observed in Fig. 3(a) - i.e. the second reactive separation module (numbered from top to bottom) takes up more than 1/3 of overall process risk. To avoid the safety concerns resulting from a single module, the individual module risk is constrained to be less than 30 % of the overall process risk. The new optimal design configuration is illustrated in Fig. 3(b). The afore-mentioned module
77
Towards a systematic framework for the synthesis of operable process intensification systems - application to reactive distillation systems
is bypassed to alleviate its mass/heat transfer burden, thus introducing more degrees of freedom to enhance the system’s flexibility and operability performances. The detailed design and operating parameters can be found in Table 2. CW – V
CW – V
L – V MeOH
MeOH
L – V
L – V
L – V
L – V
L – V
IB4 + NB4
IB4 + NB4 L – V
L – V
L – ST
L – ST MTBE (a)
MTBE (b)
Figure 3: Flexible & inherently safer design configurations. (a) Operable design I, (b) Operable design II 3.3. Steady-state & dynamic simulation and validation Having generated the optimal and operable GMF configurations for the MTBE production (Fig. 3), in this step we identify and validate the corresponding equipment-based flowsheet using steady-state simulation. Each GMF separation module is translated to two distillation trays in Aspen Plus® RADFRAC column (Aspen, 1981-2018), while each GMF reactive separation module to three reactive distillation trays. Therefore, the first operable design is verified as a 13-tray RD column, while the other one as a 10-tray RD column integrated with an additional 3-tray side-column. Table 2 provides a summary of quantitative validation between GMF and Aspen simulation on design/operating variables. “High-fidelity” dynamic models (Schenk et al., 1999) are also developed for the reactive distillation systems (Fig. 3) in gPROMS ModelBuilder® (PSE, 1997-2018), consisting of a system of differential and algebraic equations (DAE) for the description of component molar and energy balances for each tray, the partial reboiler and the total condenser, reaction kinetics, phase equilibrium, etc. The developed dynamic systems will be utilized for the derivation of receding horizon control policies (Pistikopoulos et al., 2015) to guarantee the optimal operation of the proposed designs. Table 2: GMF quantitative validation with Aspen simulation.
Column pressure (atm) Reflux ratio Reboiler duty (kW) Condenser duty (kW) Module/Tray Number Bottom product flowrate (mol/s) Product purity (MTBE mol/mol) TAC (×104 $/y )
Nominal Design GMF Aspen 5.46 6.00 1.70 2.10 7.5 × 103 6.6 × 103 2.3 × 104 2.2 × 104 7 15 197.0 197.0 0.98 0.98 171.1
Operable Design I GMF Aspen 7.85 7.95 1.70 2.50 8.4 × 103 9.6 × 103 2.3 × 104 2.4 × 104 6 13 197.0 197.0 0.98 0.98 182.4
Operable Design II GMF Aspen 9.48 8.20 1.70 3.30 8.9 × 103 2 × 104 2.3 × 104 3.4 × 104 6 13 197.0 197.0 0.98 0.98 190.2
78
Y. Tian et al.
4. Conclusion In this paper, we have presented a systematic framework to efficiently address the combinatorial PI design space and to integrate steady-state synthesis, dynamic analysis, and operability assessment to deliver validated operable intensification designs. It is shown, through an MTBE reactive separation example, that operability considerations may result in significant structural and operating changes of the process optimal solutions. Ongoing work addresses simultaneous design and control studies on the resulting MTBE reactive distillation configurations to close the loop under dynamic operating conditions.
5. Acknowledgement We acknowledge the financial support from the Texas A&M Energy Institute, Shell Oil Company, RAPID SYNOPSIS Project (DE-EE0007888-09-03), and the NSF (Grant no. 1705423) under the project titled SusChEM: An integrated framework for process design, control and scheduling [PAROC].
References Aspen, 1981-2018. Aspen technology, inc. https://www.aspentech.com/en/products/engineering/aspen-plus. M. Baldea, 2015. From process integration to process intensification. Computers & Chemical Engineering 81, 104–114. F. E. da Cruz, V. I. Manousiouthakis, 2017. Process intensification of reactive separator networks through the ideas conceptual framework. Computers & Chemical Engineering 105, 39–55. S. E. Demirel, J. Li, M. F. Hasan, 2017. Systematic process intensification using building blocks. Computers & Chemical Engineering 105, 2–38. N. A. Diangelakis, B. Burnak, J. Katz, E. N. Pistikopoulos, 2017. Process design and control optimization: A simultaneous approach by multi-parametric programming. AIChE Journal 63 (11), 4827–4846. A. M. Geoffrion, 1972. Generalized benders decomposition. Journal of optimization theory and applications 10 (4), 237–260. G. J. Harmsen, 2007. Reactive distillation: the front-runner of industrial process intensification: a full review of commercial applications, research, scale-up, design and operation. Chemical Engineering and Processing: Process Intensification 46 (9), 774–780. S. R. Ismail, P. Proios, E. N. Pistikopoulos, 2001. Modular synthesis framework for combined separation/reaction systems. AIChE Journal 47 (3), 629–649. A. Nemet, J. J. Klemeš, I. Moon, Z. Kravanja, 2017. Safety analysis embedded in heat exchanger network synthesis. Computers & Chemical Engineering 107, 357–380. K. P. Papalexandri, E. N. Pistikopoulos, 1996. Generalized modular representation framework for process synthesis. AIChE Journal 42 (4), 1010–1032. E. N. Pistikopoulos, N. A. Diangelakis, R. Oberdieck, M. M. Papathanasiou, I. Nascu, M. Sun, 2015. PAROC – an integrated framework and software platform for the optimisation and advanced model-based control of process systems. Chemical Engineering Science 136, 115–138. PSE, 1997-2018. Process systems enterprise. gproms. https://www.psenterprise.com/products/gproms. A. Rehfinger, U. Hoffmann, 1990. Kinetics of methyl tertiary butyl ether liquid phase synthesis catalyzed by ion exchange resin – i. intrinsic rate expression in liquid phase activities. Chemical Engineering Science 45 (6), 1605–1617. R. E. Rosenthal, 2016. GAMS: A user’s guide. M. Schenk, R. Gani, D. Bogle, E. Pistikopoulos, 1999. A hybrid modelling approach for separation systems involving distillation. Chemical Engineering Research and Design 77 (6), 519–534. Y. Tian, S. E. Demirel, M. F. Hasan, E. N. Pistikopoulos, 2018. An overview of process systems engineering approaches for process intensification: State of the art. Chemical Engineering and Processing-Process Intensification 133, 160–210. A. K. Tula, D. K. Babi, J. Bottlaender, M. R. Eden, R. Gani, 2017. A computer-aided software-tool for sustainable process synthesis-intensification. Computers & Chemical Engineering 105, 74–95.