Heat-Integrated Water Allocation Network Synthesis for Industrial Parks

Transcription

1 475 A ublication of CMICAL NGINRING TRANSACTIONS VOL Guest ditors: Petar S Varbanov Rongxin Su on Loong Lam Xia Liu Jiří J Klemeš Coyright 17 AIDIC Servizi S.r.l. ISBN ; ISSN The Italian Association of Chemical ngineering Online at DOI: 10.33/CT eat-integrated Water Allocation Network Synthesis for Industrial Parks aodong Song Linlin Liu* Jian Du Institute of Chemical Process System ngineering School of Chemical ngineering Dalian University of Technology Dalian Liaoning China liulinlin@dlut.edu.cn Industrial arks can achieve the cooerative utilization of resource among multile enterrises and have become a major trend of rocess industry. Concerning the combined consumtion issue of water and energy in industrial arks this aer resents a methodology for interlant heat-integrated water allocation network (IIWAN) synthesis to exlore the otential of water and energy conservation in industrial arks. The novel model roosed takes both direct and indirect schemes of cross-lant water reuse into consideration and develos strategies for eat Integration secific to industrial arks. The economically otimal overall IIWAN is ursued by mathematical rogramming under the objective of minimum total annual cost (TAC). Based on suerstructure a non-linear rogramming (NLP) model is formulated containing two sub-networks: water allocation sub-network and heat exchanger sub-network. Regarding the two sub-networks the overall network is synthesized in two solution aroaches resectively: sequential design and simultaneous design. Through the comarison between these two aroaches it is indicated that simultaneous design resents referable result desite the higher requirement on solution rocess. The effectiveness of the roosed methodology is illustrated by a case study of an industrial ark including three lants. 1. Introduction Over the ast decades considerable efforts have been made on the synthesis of water allocation network (WAN) and heat exchanger network (N) which are efficient means to reduce the consumtion of water and energy resectively. In recent years industrial arks have become a major trend due to the advantage in industrial symbiosis among multile lants. Yet most of the works on water and energy integration are carried out either searately or within a single lant which limits the otential of resource conservation. Consequently it is crucial to achieve the cooerative integration of water and energy in the scale of industrial arks. The study on individual synthesis of WAN and N in industrial arks started in the 1990s with mathematical rogramming as the major aroach. Chew et al. (08) classified interlant water integration into two schemes: direct and indirect integration wherein the former directly via cross-lant ielines and the latter via a centralized utility hub. Liu et al. (16) resented a lant-based mode of water allocation in industrial arks to restrict the number of interlant streams and thereby reduce comlexity. As for eat Integration Chang et al. (15) develoed a two-ste methodology for interlant eat Integration via eat Recovery Loo considering additional factors associated with distance between lants. Combining WAN and N whilst utilizing the interactions between these two sub-networks water and energy consumtion can be further reduced. So far heat-integrated water allocation network (IWAN) synthesis has been widely studied and the otimization aroaches mainly follow two routes: sequential design and simultaneous design. The former is a stewise strategy dividing the roblem into two sub-roblems (concerning water and energy resectively) and solving sequentially; while the latter takes the otimization roblem as a whole and all the trade-offs are balanced within the model which is otimized under one objective covering both water and energy. owever most of the works only focus on the IWAN synthesis within a single lant whereas interlant IWAN synthesis in the scale of industrial ark has not been adequately investigated. Boix et al. (11) addressed the water and energy management for an industrial ark involving three lants yet detailed issues such as network Please cite this article as: Song. Liu L. Du J. 17 eat-integrated water allocation network synthesis for industrial arks Chemical ngineering Transactions DOI:10.33/CT

2 476 structure and interlant distribution mode were not discussed. Zhou et al. (12) resented the design of interlant water-allocation and heat-exchange network in a mixed integer nonlinear rogramming (MINLP) model based on multi-scale state-sace suerstructure involving various scenarios of regeneration and redistribution. Still relevant works are quite few and the methodologies are yet to be further exlored. In this work a methodology for the economically otimal design of IIWAN is resented to reduce water and energy consumtion in industrial arks. Based on suerstructure aroach a mathematical model is resented and strategies for interlant integration secific to industrial arks are develoed. It's worth noting that to simlify the model and reduce the comutation load all the discrete variables are eliminated and the roblem is formulated as an NLP model rather than an MINLP model. 2. Problem statement Given an industrial ark including several lants each lant containing a set of water generating and consuming units (assimilated as water sources and sinks) with secific flowrate temerature and contaminant concentration the objective is to synthesize an economically-otimal IIWAN for the industrial ark with minimum total annual cost whilst meeting all the rocess requirements. Regeneration oeration is available with required oerating temerature and fixed removal ratio of contaminant. 3. Suerstructure The suerstructure for IIWAN synthesis in industrial ark is shown in Figure 1. The solid blocks denote the single lants in the ark and the dotted blocks denote the water allocation and eat Integration subsystems within each lant. As can be seen lants are correlated by inter-lant water streams and a shared regeneration unit enables the centralized oeration in the ark. The water allocation subsystem is revised from the lantbased WAN suerstructure in our recent work (Liu et al. 16). All the ossible intra-lant matches between water sources (including fresh water) and sinks (including waste effluent) are taken in as well as inter-lant reuse oortunities in both direct and indirect scheme. Figure 1: IIWAN suerstructure for industrial ark

3 As for eat Integration subsystem heat exchange is allowed among the streams within and entering/leaving the same lant. Secifically streams of direct interlant reuse articiate in the N of target lant. In each lant the water streams to the same target unit (sinks/effluent/central regenerator) are non-isothermally mixed into one single stream before entering the N to make the most of direct heat exchange and reach the minimum number of streams involved in eat Integration subsystem. Consequently utility consumtion is lowered and the N structures are relatively less intricate. Detailed design of Ns is based on the modified stage-wise N suerstructure. The existence and role (hot/cold/byass) of the otential streams in N are identified in the synthesis rocedure. Note that the inlet stream of waste effluent shall not be heated by hot utility if identified as cold stream since only the uer temerature limit is required for effluent discharge. 4. Mathematical model Based on the suerstructure an NLP model is accordingly formulated to solve the roblem. Comared with the generally used MINLP model continuous variables u=f/(f+δ) and z=q/(q+δ) (δ is an infinitesimal number) are used to indicate the existence of otential water streams and heat exchangers resectively instead of binary variables which significantly lowers comutation comlexity and facilitates the search for more desirable result. The model is divided into the following three sections. The objective function is the minimum TAC of the overall IIWAN which is the summation of the two subsystems. IIWAN WAN N obj TAC min TAC TAC (1) 4.1 Water allocation subsystem The model for water allocation subsystem is basically revised from our recent work (Liu et al. 16) including flowrate balance for water sources and sinks contaminant concentration limit for water sinks flowrate and contaminant load balance for inter-lant water streams flowrate and contaminant load balance for centralized regenerator and uer/lower bounds for flowrate. The TAC of water allocation subsystem consists of the cost of fresh water regeneration and cross-lant ieline as shown in q(2). 477 WAN fresh water regeneration ieline TAC cost cost cost reg fw fw ca indout o indout f j uc AW reg f AF reg f AW j c f var l 3.6 v AF D indout indin dis u u j SK fixed dir SP ' SP ' l u ' 2 ' (2 ) where SP is the set of single lants in the ark; SK is the set of water sinks in lant ; f denotes the flowrate of water stream; uc denotes unit cost; Ddis denotes inter-lant distance; AW denotes annual working hour. 4.2 eat Integration subsystem The model for eat Integration subsystem mainly includes overall heat balance for each stream heat balance at each stage in suerstructure assignment of initial temerature in suerstructure feasibility constraints for temeratures and constraints for heat transfer aroach temerature. The TAC of eat Integration subsystem consists of the cost of heat exchangers (fixed charge + area cost) and utilities given by q(3). N heat exchangers utilities TAC cost cost fixed hc Uc hcu he zh c k zc zh h c k c h area he hc Ah c k area h c k he area area h S c CS SP k ST he Uc hcu he Ac A h c h U Uc CU hcu uc qc uc q h c h (3) where S and CS are the sets of hot and cold streams in the N of lant ; ST denotes the stages in N

4 478 suerstructure; q denotes the heat load exchanged in each unit; A denotes the area of heat exchanger. Note that the cost of hot utility and heater related to waste effluent streams is excluded from TAC N in ractical solution rocess since it is invalid as stated in Section Interconnections between the two subsystems As mentioned in Section 3 according to the suerstructure the water streams to the same target unit are remixed into one stream which articiates in the N to reach the required temerature before entering the target unit. The energy balance of the re-mixing is shown by q(4) - q(7) from which the temeratures after mixing can be calculated and then assigned to the inlet temeratures of the streams in N in the order of sinks-effluent-regenerator as shown in q(8). Whilst the outlet temeratures of the streams in N equal to the required temerature of corresonding units. mixc c sr ex i i i T f T f i SR SP (4) dirin dirin c dir ' ' ' T f T ' ' f i SR SP SP (5) T F T f f T f T f T i SR j SK SP (6) mixsk sk sr in fw fw imd dirin imi cr j j i i j j j j i mixe e ime sr ime cr T fi f Ti fi f T i SR SP i i e (7) where SR is the set of water sources in lant ; T mixsk j T mixe T mixc are the inlet temeratures of the re-mixed stream to sinks effluent and centralized regenerator. mixsk T s S j SK SP s j SK in mixe Tss T s S SP s SK 1 mixc T s S SP s SK 2 j (8) where S is the set of otential streams to be involved in the N of lant Ts in s is the inlet temerature of otential heat-exchange stream s in lant ; SK is the number of sinks in lant. When it comes to the simultaneous design of the two subsystems the outlet temerature of re-mixing nodes (i.e. inlet temerature in eat Integration) is unknown before the N synthesis. The method from Yan et al. (16) is adoted to identify the role (hot/cold/byass) of the streams in N and allocate the corresonding rational value to their inlet temeratures given by q(9) and q(10) which cooerate with the feasibility constraints of temeratures to guarantee the validity of the identification. in out in out Thh Tss Tcc Tss 0 h S c CS s S SP h c s (9) Ts Th Tc Ts h S c CS s S SP h c s (10) in in in out s h c s where Th in h and Tc in c are the inlet temeratures of hot stream h and cold stream c in the N of lant. 5. Case study To illustrate the alication of the roosed method a theoretical industrial ark containing three lants is investigated. The rocess data are listed in Table 1. The centralized regenerator features a fixed removal ratio of 90 % and shall be oerated at C cost arameters α ca reg β reg and α o reg are set at and resectively. Pieline cost arameters α fixed l and α var l are set at 2 and 70. Fresh water (0.375 $/t) is sulied at C. ffluent shall be discharged at no higher than C. Inlet and outlet temeratures of cold utility are 10 C and C with unit cost 189 $/(kw y); the temerature of hot utility (low ressure steam) is 1 C with unit cost 377 $/(kw y). eat exchanger cost arameters α fixed he α area he and β area he are set at 0 10 and 0.6; overall heat transfer coefficient is fixed at and 0.5 kw/(m 2 C) for matches with/without steam. AW AF and Ddis are 0 h/y 10 % and 180 m. The mentioned otimization aroaches namely sequential design and simultaneous design are emloyed for the synthesis of the overall network. The roblem is imlemented in GAMS using BARON as solver.

5 479 Table 1: Process data for case study Plant Water Source Flowrate (t/h) Contaminant concentration (m) T ( C) Water Sink Flowrate (t/h) Contaminant concentration uer limit (m) T ( C) 5.1 Sequential design Sequential aroach synthesizes the IIWAN in two stes. First water allocation sub-network is designed based on the WAN art of the suerstructure and mathematical model under the objective of minimum TAC WAN. And an economically otimal WAN is obtained with the TAC WAN of $/y. On the basis of this WAN the inlet temeratures of the re-mixed streams to each target unit are obtained by q.(4)-q.(8). Comaring to the corresonding temerature secifications the streams in demand for heat exchange are identified with determinate role flowrate and temerature which are to be involved in Ns. 4 hot streams and 12 cold streams are incororated from water streams. Then the otimal heat exchanger sub-network with minimum TAC N ( $/y) is designed based on stage-wise N suerstructure. Thereby an IIWAN for the industrial ark with the TAC IIWAN of $/y is eventually obtained. 5.2 Simultaneous design Sequential design is able to reduce the resource consumtion and minimize the cost of the two sub-networks resectively but the interaction between the two subsystems is not fully exlored. Simultaneous design treats the whole system as one single roblem and directly minimizes the overall TAC IIWAN. Accordingly the economically-otimal IIWAN with the minimum TAC IIWAN of $/y is obtained as shown in Figure 2. Detailed cost terms of the two aroaches are listed in Table 2. Comared to sequential design it is clear that simultaneous design resents referable results with a reduction of % on TAC. This is mainly due to the instinct limitation of sequential aroach that the internal trade-offs are not adequately rocessed esecially when Interlant Integration is involved. Although simultaneous design features higher fresh water consumtion yet the significantly lower utility consumtion (esecially for Plant 2) still leads to better overall erformance which imlies that the otimal WAN might not be the most suitable one for eat Integration. Yet it should be noted that simultaneous design features much heavier comutation loadand requires more aroriate initial oints to search for solution. Table 2: Results of case study Sequential design Simultaneous design Cost terms (M$/y) Plant 1 Plant 2 Plant 3 Total Plant 1 Plant 2 Plant 3 Total Fresh water Regeneration Cross-lant ieline TAC WAN subsystem eat exchangers ot utility Cold utility TAC N subsystem TAC overall IIWAN

6 480 SR 72 SR1-2 SR SR SK 72 0 SK1-2 SK SR2-1 SR SR SK2-1 SK Plant C Plant C C SK CR SR 72 Fresh SR SK Waste CR Sl-Ind 2. Fresh Source number flowrate t/h concentration m temerature C SR SK3-1 9 SK Sink number flowrate t/h concentration m temerature C SR Centralized Regenerator SR SR SK SK SK SK Flowrate t/h Temerature C Temerature C eat transfer load kw 9665 C Plant CR Sl-Ind 2739 SR SR SR Sl-Dir Figure 2: Otimal IIWAN design of industrial ark for case study 6. Conclusions This aer addresses a methodology for the otimal design of IIWAN in the scale of industrial ark. A combined suerstructure secific to industrial arks is resented caturing all the intra-lant and inter-lant ossibilities for integration in which synthesis strategies are incororated. An NLP model is corresondingly formulated which is more comutational friendly comared to conventional MINLP model. The effectiveness of the method is validated by a case study. Comaring the results v two aroaches it can be concluded that simultaneous design surasses sequential design with TAC lower by %. The method can romote the symbiotic system in industrial arks and contribute to the sustainable develoment of rocess industry. Acknowledgments The authors gratefully acknowledge the financial suort from Natural Science Foundation of China (No ) and the Fundamental Research Funds for Central Universities of China (DUT16RC(4)07). References Boix M. Montastruc L. Pibouleau L. Azzaro-Pantel C. Domenech S. 11 co-industrial Parks for Water and eat Management Comuter Aided Chemical ngineering Chang C. Wang Y. Feng X. Zhang P. 15 A Two Ste Methodology for Inter-Plant eat Integration Design Chemical ngineering Transactions Chew I.M.R. Tan R. Ng D.K.S. Foo D.C.Y. Majozi T. Gouws J. 08 Synthesis of Direct and Indirect Interlant Water Network Industrial & ngineering Chemistry Research Liu L. Wang J. Song. Du J. Yang F. 16 Synthesis of water networks for industrial arks considering inter-lant allocation Comuters and Chemical ngineering Yan F. Wu. Li W. Zhang J. 16 Simultaneous otimization of heat-integrated water networks by a nonlinear rogram Chemical ngineering Science Zhou R. Li L. Dong. Grossman I.. 12 Synthesis of Interlant Water-Allocation and eat-xchange Networks Part 1: Fixed Flowrate Processes Industrial & ngineering Chemistry Research