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An Engineering Tool to Screen and Integrate Biomass Valorization Paths in Multiple-Feedstock Biorefineries
Anton Friedl, Jiří J. Klemeš, Stefan Radl, Petar S. Varbanov, Thomas Wallek (Eds.) Proceedings of the 28th European Symposium on Computer Aided Process Engineering June 10th to 13th, 2018, Graz, Austria. © 2018 Elsevier B.V. All rights reserved. https://doi.org/10.1016/B978-0-444-64235-6.50103-0
An Engineering Tool to Screen and Integrate Biomass Valorization Paths in Multiple-Feedstock Biorefineries Konstantinos A. Pyrgakis, Antonis C. Kokossis School of Chemical Engineering, National Technical University of Athens, Iroon Polytechneiou 9, Athens, GR-15780, Greece [email protected]
Abstract Biorefineries are the most promising route for the sustainable exploitation of lignocellulosic biomass sources into a wide range of fuels and materials. Value chains include all potential processing routes from raw materials to intermediate and endchemicals that are possible to enter the biorefinery site. In this scope, process integration techniques are required to evaluate all potential biorefinery structures and reveal promising multiple-product biorefinery solutions with high efficiencies in use of energy and materials. Provided that biorefinery operations subject to seasonal availability of biomass varieties, the design problem additionally requires screening and planning of appropriate biorenewable feedstocks. This work introduces new concepts and representations that incorporate processes and feedstocks as additional degrees of freedom in integration. The proposed methodology investigates synergies among candidate processes, which benefit the biorefinery instead of operating individually, as well as schedules multiple-feedstock operations. A biomass representation maps all process synthesis options along value chains, while a cascade representation is proposed to simultaneously model direct (heat source-to-sink) and indirect (via steam generationreuse) integration among involved processes of under-construction site. The proposed model (MILP) is explained through real-life biorefinery cases, which involve 15 candidate biorefinery paths and 6 candidate biomass varieties. The model reveals high efficiency biorefining routes and examines preferences on the use of multiple feedstocks minimizing the annual energy cost of under-construction biorefineries.
1. Introduction Biorefineries are mainly related with the use of cheap and abundant lignocellulosic material that consists of sugars and lignin. Lignocellulosic sources are met in agricultural and forestry residues as well as in pulp industry wastes. A range of novel chemistries have been revealed over the recent years that upgrade lignocellulosic biomass into commodity and chemical specialties like polymers, solvents, lubricants, pharmaceuticals, nutraceuticals, cosmetics, surfactants, animal feed and fuels (Hughes et al., 2013). Figure 1 shows a typical lignocellulosic value chain with multiple choices for biomass feedstocks and chemical products that may enter upcoming biorefineries. Since there are not clear technical, commercial and environmental evidences for the selection of biomass varieties and chemistries, engineering tools are required to systematically screen benefits from integration of different processes across same site and detect biomass mixtures that maximize efficiencies in use of energy and materials.
K.A. Kyrgakis and A.C. Kokossis
574
Figure 1. Illustrative biorefinery value chain.
2. Problem description Process integration techniques are used to estimate cost savings by exchanging materials and energy among processes instead of operating individually. Energy integration searches for heat source-to-sink matches maximizing heat recovery within each process. At a next level, Total Site Analysis (TSA) applies for the integration of different processes and estimates steam targets by means of graphical and numerical tools based on the concept of Site Sources and Sinks Profiles (SSSPs). Though there is extensive literature for steam targeting in sites, the conventional concepts appear inefficient to apply for the design of biorefineries, since processes and operations are not fixed and known beforehand; namely, SSSPs cannot directly apply for the infinite candidate sites that emerge through value chains. Beyond selection of processes to integrate, the problem also brings questions about operations and capacities of involved processes, which subject to seasonal availability and variability of biomass. There are several literature studies focusing on the optimization of biomass supply chains (Yue et al., 2014) and planning energy systems (Varbanov and Klemeš, 2011); however the approaches are limited by the use of graphical tools that exclusively address particular synthesis applications and scenarios. The design of sustainable biorefineries requires for multiple processes and products that properly integrate within sites as well as planning of multiple-feedstock operations over the year. Therefore, integration thermodynamics should be combined with mathematical programming to systematically assess energy targets of all potential process combinations so as to maximize benefits from process-toprocess integration. Fundamentals of this concept has been recently presented by Kokossis et al. (2015). This work further presents systems representations that map value chain synthesis options and integration thermodynamics to develop a comprehensive optimization model that can be widely used as a engineering tool for synthesis, integration and planning problems in existing or upcoming industries.
3. Methodology The conventional Total Site integration tools assume given processes that exchange energy via steam production and reuse. When the case includes candidate processes and infinite combinations to integrate, the graphical tool appears inefficient to estimate steam targets; steam sources and sink profiles would actually behave as variable curves that vary according to process selections and capacities along value chains each time. Kokossis et al. (2015) originally introduced an integration concept that addresses processes as additional variables in integration generating value chain paths combinations assuming paths as binary options. This paper further establishes a cascade
An Eng. Tool to Screen and Integr. Biom. Valoriz.
575
representation that lets processes and their capacities get combined in any way; namely, competitive processes may exist in the design for sake of improved energy targets. Synthesis options for storing and biomass pre-treatment (drying) are also introduced in the process synthesis representation to enable switching and combining biomass feedstocks as well as to plan multiple-feedstock operations over the year. Process portfolios are developed by means of a Biomass Bipartite graph Representation (BBR) that enables the development of mass balances along value chains considering seasonal changes in biomass feedstocks. Heat contents of sources (hot streams) and sinks (cold streams) vary according to selections made by BBR. A Total Site Representation (TSR) that is based on common sense of heat cascading but formulated by means of an extended transshipment model enables recording energy balances among candidate process hot/cold streams and utility levels. The proposed Total Site Cascade is configured by combining temperature intervals of all individual heat cascades - each related with a candidate process - along a super-interval cascade that is used for direct and indirect integration purposes of variable processes. Heat contribution to cascade intervals is adjusted by decisions made by BBR. Direct heat source-to-sink integration (within each process) is described quite similarly to conventional integration at each interval of the proposed cascade that is configured according to minimum temperature difference (𝛥𝛵𝑚𝑖𝑛 ). Indirect integration refers to heat exchange among processes via steam production-reuse and is described by translating excess heat of intervals into generated steam and transferring that energy, through cross-interval heat flows, to lower temperature intervals according to minimum temperature difference of 𝑠𝑖𝑡𝑒 = 2 ∙ 𝛥𝛵𝑚𝑖𝑛 , where 1 ∙ 𝛥𝛵𝑚𝑖𝑛 accounts for steam generation and the site, 𝛥𝛵𝑚𝑖𝑛 1 ∙ 𝛥𝛵𝑚𝑖𝑛 for steam reuse. Hot streams and utilities residuals of the proposed Total Site Cascade can be used to reproduce the sources and sinks profiles of regular SSSPs. Synthesis of paths also copes with matching steam-source with steam-sink processes improving steam savings due to overlap of generated and demanded steam. Processes may act as steam source or sink or both according to processes portfolio in which they are integrated with. While Total Site cascade evaluates steam savings under steady-state biorefinery operations, biomass seasonality imposes changes on process capacities and energy efficiencies in time, since each variety features with different organic content and yields in sugars and lignin capacities; these constitute the main intermediates in lignocellulosic biorefineries (Figure 1) and rule all downstream process capacities.
Biomass seasonality and the use of multiple feedstocks are incorporated by letting input chemicals flows related with chemicals stored in previous seasons. Storage and drying facilities have been employed (i) to store biomass feedstocks for seasons with biomass shortage as well as to compose biomass mixtures that improve biorefinery efficiencies and (ii) to dry woody biomass before entering the biomass fractionation process, which constitutes the core process in lignocellulosic biorefineries (Figure 1). Changes in biomass feedstocks naturally result in changes of intermediates yields, downstream process capacities and process streams heat contents. As a result, selections on biomass varieties have an apparent impact on energy savings and the selection of steamsource/sink-process portfolios. Steam savings are also affected by the number and the temperature levels of applied utilities along the cascade. The size and complexity of the combined process synthesis and integration problem require the use of mathematical programming to optimize multiple-product biorefinery structures and plan multiplefeedstock operations. Portfolios made by BBR are integrated and evaluated by TSR. Mass (BBR) and energy (TSR) balances are constructed as an optimization model
K.A. Kyrgakis and A.C. Kokossis
576
(MILP) that makes selections on processes, biomass mixtures and utilities and plans the annual biorefinery operation to minimize the energy cost of the under-construction site.
4. Systems representations BBRs (Figure 2.a) employ product nodes, units and interconnections to translate unclear value chains into superstructure-like representations for optimization purposes. So, let P={p|p=1,NP} and PR={pr|pr=1,NPR} be the process units and product nodes assigned to each path and chemical of the value chain; T={t|t=1,NT} be the interconnections that make connections among nodes (pr) and units (p); B={b|b=1,NB} be the biomass varieties and SE={se|se=se1,…,se4} be the seasons of the year for biorefinery operation. Also, let variable 𝐹𝑡 be the chemical flowrate of interconnection t. Storing in BBRs is dedicated to interconnections that transfer chemicals from one season to next one. Storing in facilities protected from environmental conditions is also capable to secure drying of woody biomass (50% w/w moisture content) at the rate of 5% w/w per season. For this reason, storages p1-p4 denote (Figure 2.a) free of energy cost drying in storage facilities for seasons se1-se4, respectively; drying in storages cannot reduce moisture lower than 30% w/w. Drying units p6-p9 are next applied to further dry biomass exiting units p1-p4 down to the limit of 15% w/w that is required for operation of the core biomass fractionation process; drying is not required for nonwoody biomass, whose moisture content is below this limit. Mass balances are recorded using flowrates, 𝐹𝑡 , of input-output interconnections at each product node and process unit. Logical constraints are also required to limit intense and impractical fluctuations of process capacities over the year due to equipment sizing and instrumental design limitations. The heat contents of contributed process streams are linearly estimated by the selected operation capacities (flowrates of process input interconnections) and the known (estimated by process simulations) heat contents of streams per product flowrate. For the development of TSR (Figure 2.b), let 𝐶𝑝 be the cascade of each candidate 𝐶
𝐶
𝑇𝑆 , 𝑄𝑊𝑛𝑇𝑆 be the heat loads of hot, cold utilities process p. Also, let 𝑄𝑆𝑚𝑝 , 𝑄𝑊𝑛 𝑝 and 𝑄𝑆𝑚 of cascade 𝐶𝑝 and the Total Site. The Total Site Cascade (TSC) is formulated by the temperature intervals of all candidate cascades. Energy balances are formulated using an extended transshipment model that uses detailed heat exchange options among hot-cold streams and utilities at each interval. Heat flows referring to direct integration of streams of different processes are fixed to zero. Thus, the TSC incorporates all debottleneck problems of candidate processes without affecting each other; accordingly, the utility demands of the TSC correspond to the demands of all individual cascades. A Utility Storages System (USS) has been also set-up to supervise the utilities demands of the TSC. Excess heat from the TSC is extracted by cold utilities and next sent to lowertemperature intervals replacing external steam demands. The excess heat is transferred via energy bridges that pass through the USS, which estimates the energy overlap between generated-demanded steam and thus, minimizes the external utilities needs.
5. Case studies 5.1. Case 1 The biorefinery value chain of Figure 3 involves 6 candidate biomass varieties (wheat straw, rice, barley, miscanthus, softwood and hardwood) and 17 chemical products that need to be examined in order to reveal energy promising biorefinery solutions. Each
An Eng. Tool to Screen and Integr. Biom. Valoriz.
577
Figure 2. Systems Representations of BBR (a) and TSR (b). path from one chemical to another denotes a chemical process. The paths have been optimized in the course of a biorefinery study in Hungary based on real-life seasonality data. Hungary offers options for wheat, barley and hardwood. The biorefinery size is bounded by the installation capacity of the biomass fractionation process at 22.500 kg/hr of dry matter. Wheat straw was preferred instead of barley and hardwood due to its low sugars content, which result in lower operation capacities and energy cost for the energy intensive sugar-based paths compared to the lower energy intensive lignin-based paths. Barley is used at wheat depletion completing the annual biomass needs, while hardwood is excluded. The biorefinery includes the biomass fractionation process as well as the xylitol, isoprop./butanol and poly-urethanes production processes. The utilities are estimated to 58 MW (hot) and 40 MW (cold), while the generated steam replaces 6.5% of external steam demands. In Case 2, the proposed model was applied to examine preferences on biomass varieties in building biorefinery efficiencies. 5.2. Case 2 The model was applied for multiple times by additionally using inter-cut equations to constrain selections about biomass varieties by excluding each time the varieties that selected in previous runs. Table 1 presents the results according to the preferences obtained for the varieties. Miscanthus has been selected as the prime biomass variety for minimizing energy cost, while softwood holds the last position. High trends have been obtained for xylitol, isoprop./butanol and poly-urethanes production processes, while itaconic acid enters the biorefinery site sharing the xylose (C6 sugars) intermediate with isoprop./butanol process at the use of rice or barley. The percentages reflect to the selected process operation capacities on the basis of their maximum capacities. The maximum capacities account for operations, when processes do not share upstream (input) chemicals with other competitive processes. Drying facilities are also included at the use of woody biomass. The results of Table 1 could play a significant role in making quick decisions on selecting and planning production lines and multiple-feedstock operations according to biomass availability at each season.
K.A. Kyrgakis and A.C. Kokossis
578
Figure 3. Understudied biorefinery value chain. Table 1: Ranking of biomass varieties with respect to energy cost
Ranking biomass varieties (1)Miscanthus (2)Rice (3)Wheat (4)Barley (5)Hardwood (6)Softwood
Splitting intermediates among competitive products Biomass to Xylose to Glucose to Lignin to Dryers
Fractionation process
Xylitol
100% 100%
100% 100% 100% 100% 100% 100%
100% 100% 100% 100% 100% 100%
Itaconic Isoprop./ Polyacid Butanol Urethanes
13% 13%
100% 87% 100% 87% 100% 100%
100% 100% 100% 100% 100% 100%
6. Conclusions This paper presents systems representations to model process-to-process integration and select biorefinery portfolios that subject to seasonal availability of biomass varieties. Total Site integration is re-stated as a process synthesis tool that addresses processes and biomass feedstocks as additional degrees of freedom in integration procedure. A biomass representation maps all value chain process synthesis options and the use of multiple feedstocks, while an extended transshipment model is used to evaluate energy savings from process-to-process integration. The proposed methodology was used to reveal high energy efficient process portfolios, assess preferences on biomass varieties and plan multiple-feedstock operations minimizing the annual biorefinery energy cost.
Acknowledgments The authors acknowledge joint financial support by Alexander S. Onassis Public Benefit Foundation (Greece) and the FP7 KBBE Grant BIOCORE (FP7-241566).
References Hughes SR, Gibbons WR, Moser BR, Rich JO., InTech, 2013, http://dx.doi.org/10.5772/54804. Kokossis AC, Tsakalova M, Pyrgakis K., Computers & Chemical Engineering, 2015, 81, 40-56. Yue D, You F, Snyder SW., Computers & Chemical Engineering, 2014, 66, 36-56. Varbanov PS, Klemeš JJ., Computers & Chemical Engineering, 2011, 35, 1815-1826
An Engineering Tool to Screen and Integrate Biomass Valorization Paths in Multiple-Feedstock Biorefineries Konstantinos A. Pyrgakis, Antonis C. Kokossis School of Chemical Engineering, National Technical University of Athens, Iroon Polytechneiou 9, Athens, GR-15780, Greece [email protected]
Abstract Biorefineries are the most promising route for the sustainable exploitation of lignocellulosic biomass sources into a wide range of fuels and materials. Value chains include all potential processing routes from raw materials to intermediate and endchemicals that are possible to enter the biorefinery site. In this scope, process integration techniques are required to evaluate all potential biorefinery structures and reveal promising multiple-product biorefinery solutions with high efficiencies in use of energy and materials. Provided that biorefinery operations subject to seasonal availability of biomass varieties, the design problem additionally requires screening and planning of appropriate biorenewable feedstocks. This work introduces new concepts and representations that incorporate processes and feedstocks as additional degrees of freedom in integration. The proposed methodology investigates synergies among candidate processes, which benefit the biorefinery instead of operating individually, as well as schedules multiple-feedstock operations. A biomass representation maps all process synthesis options along value chains, while a cascade representation is proposed to simultaneously model direct (heat source-to-sink) and indirect (via steam generationreuse) integration among involved processes of under-construction site. The proposed model (MILP) is explained through real-life biorefinery cases, which involve 15 candidate biorefinery paths and 6 candidate biomass varieties. The model reveals high efficiency biorefining routes and examines preferences on the use of multiple feedstocks minimizing the annual energy cost of under-construction biorefineries.
1. Introduction Biorefineries are mainly related with the use of cheap and abundant lignocellulosic material that consists of sugars and lignin. Lignocellulosic sources are met in agricultural and forestry residues as well as in pulp industry wastes. A range of novel chemistries have been revealed over the recent years that upgrade lignocellulosic biomass into commodity and chemical specialties like polymers, solvents, lubricants, pharmaceuticals, nutraceuticals, cosmetics, surfactants, animal feed and fuels (Hughes et al., 2013). Figure 1 shows a typical lignocellulosic value chain with multiple choices for biomass feedstocks and chemical products that may enter upcoming biorefineries. Since there are not clear technical, commercial and environmental evidences for the selection of biomass varieties and chemistries, engineering tools are required to systematically screen benefits from integration of different processes across same site and detect biomass mixtures that maximize efficiencies in use of energy and materials.
K.A. Kyrgakis and A.C. Kokossis
574
Figure 1. Illustrative biorefinery value chain.
2. Problem description Process integration techniques are used to estimate cost savings by exchanging materials and energy among processes instead of operating individually. Energy integration searches for heat source-to-sink matches maximizing heat recovery within each process. At a next level, Total Site Analysis (TSA) applies for the integration of different processes and estimates steam targets by means of graphical and numerical tools based on the concept of Site Sources and Sinks Profiles (SSSPs). Though there is extensive literature for steam targeting in sites, the conventional concepts appear inefficient to apply for the design of biorefineries, since processes and operations are not fixed and known beforehand; namely, SSSPs cannot directly apply for the infinite candidate sites that emerge through value chains. Beyond selection of processes to integrate, the problem also brings questions about operations and capacities of involved processes, which subject to seasonal availability and variability of biomass. There are several literature studies focusing on the optimization of biomass supply chains (Yue et al., 2014) and planning energy systems (Varbanov and Klemeš, 2011); however the approaches are limited by the use of graphical tools that exclusively address particular synthesis applications and scenarios. The design of sustainable biorefineries requires for multiple processes and products that properly integrate within sites as well as planning of multiple-feedstock operations over the year. Therefore, integration thermodynamics should be combined with mathematical programming to systematically assess energy targets of all potential process combinations so as to maximize benefits from process-toprocess integration. Fundamentals of this concept has been recently presented by Kokossis et al. (2015). This work further presents systems representations that map value chain synthesis options and integration thermodynamics to develop a comprehensive optimization model that can be widely used as a engineering tool for synthesis, integration and planning problems in existing or upcoming industries.
3. Methodology The conventional Total Site integration tools assume given processes that exchange energy via steam production and reuse. When the case includes candidate processes and infinite combinations to integrate, the graphical tool appears inefficient to estimate steam targets; steam sources and sink profiles would actually behave as variable curves that vary according to process selections and capacities along value chains each time. Kokossis et al. (2015) originally introduced an integration concept that addresses processes as additional variables in integration generating value chain paths combinations assuming paths as binary options. This paper further establishes a cascade
An Eng. Tool to Screen and Integr. Biom. Valoriz.
575
representation that lets processes and their capacities get combined in any way; namely, competitive processes may exist in the design for sake of improved energy targets. Synthesis options for storing and biomass pre-treatment (drying) are also introduced in the process synthesis representation to enable switching and combining biomass feedstocks as well as to plan multiple-feedstock operations over the year. Process portfolios are developed by means of a Biomass Bipartite graph Representation (BBR) that enables the development of mass balances along value chains considering seasonal changes in biomass feedstocks. Heat contents of sources (hot streams) and sinks (cold streams) vary according to selections made by BBR. A Total Site Representation (TSR) that is based on common sense of heat cascading but formulated by means of an extended transshipment model enables recording energy balances among candidate process hot/cold streams and utility levels. The proposed Total Site Cascade is configured by combining temperature intervals of all individual heat cascades - each related with a candidate process - along a super-interval cascade that is used for direct and indirect integration purposes of variable processes. Heat contribution to cascade intervals is adjusted by decisions made by BBR. Direct heat source-to-sink integration (within each process) is described quite similarly to conventional integration at each interval of the proposed cascade that is configured according to minimum temperature difference (𝛥𝛵𝑚𝑖𝑛 ). Indirect integration refers to heat exchange among processes via steam production-reuse and is described by translating excess heat of intervals into generated steam and transferring that energy, through cross-interval heat flows, to lower temperature intervals according to minimum temperature difference of 𝑠𝑖𝑡𝑒 = 2 ∙ 𝛥𝛵𝑚𝑖𝑛 , where 1 ∙ 𝛥𝛵𝑚𝑖𝑛 accounts for steam generation and the site, 𝛥𝛵𝑚𝑖𝑛 1 ∙ 𝛥𝛵𝑚𝑖𝑛 for steam reuse. Hot streams and utilities residuals of the proposed Total Site Cascade can be used to reproduce the sources and sinks profiles of regular SSSPs. Synthesis of paths also copes with matching steam-source with steam-sink processes improving steam savings due to overlap of generated and demanded steam. Processes may act as steam source or sink or both according to processes portfolio in which they are integrated with. While Total Site cascade evaluates steam savings under steady-state biorefinery operations, biomass seasonality imposes changes on process capacities and energy efficiencies in time, since each variety features with different organic content and yields in sugars and lignin capacities; these constitute the main intermediates in lignocellulosic biorefineries (Figure 1) and rule all downstream process capacities.
Biomass seasonality and the use of multiple feedstocks are incorporated by letting input chemicals flows related with chemicals stored in previous seasons. Storage and drying facilities have been employed (i) to store biomass feedstocks for seasons with biomass shortage as well as to compose biomass mixtures that improve biorefinery efficiencies and (ii) to dry woody biomass before entering the biomass fractionation process, which constitutes the core process in lignocellulosic biorefineries (Figure 1). Changes in biomass feedstocks naturally result in changes of intermediates yields, downstream process capacities and process streams heat contents. As a result, selections on biomass varieties have an apparent impact on energy savings and the selection of steamsource/sink-process portfolios. Steam savings are also affected by the number and the temperature levels of applied utilities along the cascade. The size and complexity of the combined process synthesis and integration problem require the use of mathematical programming to optimize multiple-product biorefinery structures and plan multiplefeedstock operations. Portfolios made by BBR are integrated and evaluated by TSR. Mass (BBR) and energy (TSR) balances are constructed as an optimization model
K.A. Kyrgakis and A.C. Kokossis
576
(MILP) that makes selections on processes, biomass mixtures and utilities and plans the annual biorefinery operation to minimize the energy cost of the under-construction site.
4. Systems representations BBRs (Figure 2.a) employ product nodes, units and interconnections to translate unclear value chains into superstructure-like representations for optimization purposes. So, let P={p|p=1,NP} and PR={pr|pr=1,NPR} be the process units and product nodes assigned to each path and chemical of the value chain; T={t|t=1,NT} be the interconnections that make connections among nodes (pr) and units (p); B={b|b=1,NB} be the biomass varieties and SE={se|se=se1,…,se4} be the seasons of the year for biorefinery operation. Also, let variable 𝐹𝑡 be the chemical flowrate of interconnection t. Storing in BBRs is dedicated to interconnections that transfer chemicals from one season to next one. Storing in facilities protected from environmental conditions is also capable to secure drying of woody biomass (50% w/w moisture content) at the rate of 5% w/w per season. For this reason, storages p1-p4 denote (Figure 2.a) free of energy cost drying in storage facilities for seasons se1-se4, respectively; drying in storages cannot reduce moisture lower than 30% w/w. Drying units p6-p9 are next applied to further dry biomass exiting units p1-p4 down to the limit of 15% w/w that is required for operation of the core biomass fractionation process; drying is not required for nonwoody biomass, whose moisture content is below this limit. Mass balances are recorded using flowrates, 𝐹𝑡 , of input-output interconnections at each product node and process unit. Logical constraints are also required to limit intense and impractical fluctuations of process capacities over the year due to equipment sizing and instrumental design limitations. The heat contents of contributed process streams are linearly estimated by the selected operation capacities (flowrates of process input interconnections) and the known (estimated by process simulations) heat contents of streams per product flowrate. For the development of TSR (Figure 2.b), let 𝐶𝑝 be the cascade of each candidate 𝐶
𝐶
𝑇𝑆 , 𝑄𝑊𝑛𝑇𝑆 be the heat loads of hot, cold utilities process p. Also, let 𝑄𝑆𝑚𝑝 , 𝑄𝑊𝑛 𝑝 and 𝑄𝑆𝑚 of cascade 𝐶𝑝 and the Total Site. The Total Site Cascade (TSC) is formulated by the temperature intervals of all candidate cascades. Energy balances are formulated using an extended transshipment model that uses detailed heat exchange options among hot-cold streams and utilities at each interval. Heat flows referring to direct integration of streams of different processes are fixed to zero. Thus, the TSC incorporates all debottleneck problems of candidate processes without affecting each other; accordingly, the utility demands of the TSC correspond to the demands of all individual cascades. A Utility Storages System (USS) has been also set-up to supervise the utilities demands of the TSC. Excess heat from the TSC is extracted by cold utilities and next sent to lowertemperature intervals replacing external steam demands. The excess heat is transferred via energy bridges that pass through the USS, which estimates the energy overlap between generated-demanded steam and thus, minimizes the external utilities needs.
5. Case studies 5.1. Case 1 The biorefinery value chain of Figure 3 involves 6 candidate biomass varieties (wheat straw, rice, barley, miscanthus, softwood and hardwood) and 17 chemical products that need to be examined in order to reveal energy promising biorefinery solutions. Each
An Eng. Tool to Screen and Integr. Biom. Valoriz.
577
Figure 2. Systems Representations of BBR (a) and TSR (b). path from one chemical to another denotes a chemical process. The paths have been optimized in the course of a biorefinery study in Hungary based on real-life seasonality data. Hungary offers options for wheat, barley and hardwood. The biorefinery size is bounded by the installation capacity of the biomass fractionation process at 22.500 kg/hr of dry matter. Wheat straw was preferred instead of barley and hardwood due to its low sugars content, which result in lower operation capacities and energy cost for the energy intensive sugar-based paths compared to the lower energy intensive lignin-based paths. Barley is used at wheat depletion completing the annual biomass needs, while hardwood is excluded. The biorefinery includes the biomass fractionation process as well as the xylitol, isoprop./butanol and poly-urethanes production processes. The utilities are estimated to 58 MW (hot) and 40 MW (cold), while the generated steam replaces 6.5% of external steam demands. In Case 2, the proposed model was applied to examine preferences on biomass varieties in building biorefinery efficiencies. 5.2. Case 2 The model was applied for multiple times by additionally using inter-cut equations to constrain selections about biomass varieties by excluding each time the varieties that selected in previous runs. Table 1 presents the results according to the preferences obtained for the varieties. Miscanthus has been selected as the prime biomass variety for minimizing energy cost, while softwood holds the last position. High trends have been obtained for xylitol, isoprop./butanol and poly-urethanes production processes, while itaconic acid enters the biorefinery site sharing the xylose (C6 sugars) intermediate with isoprop./butanol process at the use of rice or barley. The percentages reflect to the selected process operation capacities on the basis of their maximum capacities. The maximum capacities account for operations, when processes do not share upstream (input) chemicals with other competitive processes. Drying facilities are also included at the use of woody biomass. The results of Table 1 could play a significant role in making quick decisions on selecting and planning production lines and multiple-feedstock operations according to biomass availability at each season.
K.A. Kyrgakis and A.C. Kokossis
578
Figure 3. Understudied biorefinery value chain. Table 1: Ranking of biomass varieties with respect to energy cost
Ranking biomass varieties (1)Miscanthus (2)Rice (3)Wheat (4)Barley (5)Hardwood (6)Softwood
Splitting intermediates among competitive products Biomass to Xylose to Glucose to Lignin to Dryers
Fractionation process
Xylitol
100% 100%
100% 100% 100% 100% 100% 100%
100% 100% 100% 100% 100% 100%
Itaconic Isoprop./ Polyacid Butanol Urethanes
13% 13%
100% 87% 100% 87% 100% 100%
100% 100% 100% 100% 100% 100%
6. Conclusions This paper presents systems representations to model process-to-process integration and select biorefinery portfolios that subject to seasonal availability of biomass varieties. Total Site integration is re-stated as a process synthesis tool that addresses processes and biomass feedstocks as additional degrees of freedom in integration procedure. A biomass representation maps all value chain process synthesis options and the use of multiple feedstocks, while an extended transshipment model is used to evaluate energy savings from process-to-process integration. The proposed methodology was used to reveal high energy efficient process portfolios, assess preferences on biomass varieties and plan multiple-feedstock operations minimizing the annual biorefinery energy cost.
Acknowledgments The authors acknowledge joint financial support by Alexander S. Onassis Public Benefit Foundation (Greece) and the FP7 KBBE Grant BIOCORE (FP7-241566).
References Hughes SR, Gibbons WR, Moser BR, Rich JO., InTech, 2013, http://dx.doi.org/10.5772/54804. Kokossis AC, Tsakalova M, Pyrgakis K., Computers & Chemical Engineering, 2015, 81, 40-56. Yue D, You F, Snyder SW., Computers & Chemical Engineering, 2014, 66, 36-56. Varbanov PS, Klemeš JJ., Computers & Chemical Engineering, 2011, 35, 1815-1826