1513 A publiation of CHEMICAL ENGINEERING TRANSACTIONS VOL. 70, 2018 Guest Editors: Timoty G. Walmsley, Petar S. Varbanov, Rongxin Su, Jiří J. Klemeš Copyrigt 2018, AIDIC Servizi S.r.l. ISBN 978-88-95608-67-9; ISSN 2283-9216 Te Italian Assoiation of Cemial Engineering Online at www.aidi.it/et DOI: 10.3303/CET1870253 Energy Effiieny Retrofit of Two-Flow Heat Exanger System Leonid M. Ulyev a, *, Maxim V. Kanisev a, Mikail A. Vasilyev a, Abbass Maatouk b a RusEnergoProekt LLC, Volokolamsk Avenue 2, 125080, Mosow, Russian Federation b National Tenial University Karkiv Polyteni Institute, 2 Kirpiova St., 61002, Karkiv, Ukraine leonid.ulyev@gmail.om Tis paper presents te retrofit of te two-flow eat-exange system wit utility pats in order to optimize te eat reuperation apaity under te tenial limitation onditions. Analytial dependenes of eat load of te existing eat exangers and utilities on te surfae area of te new eat exanger are obtained. Te work sows tat te determination of tenologial parameters for existing eat exangers during te retrofit of te eat exange system is an important task beause tey affet te ost of retrofit. For ase study, two streams problem for eat transfer in eat network at te rude and gas separation units is onsidered in tis paper. Te existing system as tree eat exangers. Te temperature measurements were fulfilled for all eat exangers and te eat loads for eat exangers and utility were alulated. Te installation of one eat exanger at te ool side of te system is proposed in retrofit. Te dependenes of te temperature anges for te ot and old proess stream from te value of te additional surfae were obtained for ea eat exanger. Utility apaity and apaity reovery of termal energy in te system is also analyzed. 1. Introdution Te artile deals wit te optimal plaement of a new eat exange surfae for dual-flow eat exange network wit retrofit of te refinery network. It was determined tat 13 % of all inoming oil to te plant is used for oil refining as fuel pratially at all Russian plants. Tis ratio is observed in almost all oil refineries. In Russia, about 320 Mt of rude oil are proessed, and, onsequently, 41.5 Mt of rude oil are spent on its proessing, and at te prie of 65 USD per barrel, te ost of energy spent on proessing of oil in Russia is ~19 billion USD. It is possible to redue fuel onsumption by an amount of 3 % to 50 % using te metods of proesses integration at te surveyed plants (Smit, 2016), depending on te tenologial onditions and tenial restritions. Previously in te work of Linnoff and Flower (1978) onstrutive metods for generation of energy optimal networks were proposed. Various optimality riteria were onsidered by Flower and Linnoff (1978). Hidmar and Linnoff (1983) proposed a metod for designing integrated eat exanger networks for emialtenologial systems. Tjoe and Linnoff (1986) developed a metod for determining te target values of additional surfae area of eat exange and utility loads in te design of termal networks. Papoulias and Grossmann (1983) proposes a metod of optimization of termal networks by means of mixed integer linear programming, and Duran and Grossmann (1986) uses metods of nonlinear programming. Wan Alwi and Manan (2010) ave developed a new grapi metod for utility targeting and network design for maximum energy reuperation. Most of tese works ave dealt wit grassroots design, but reently, mu attention as been paid to termal integration in eat exange systems of operating enterprises. In (Reisen et al., 1995) te pat metod for te retrofit of eat exange networks was proposed. In te paper (Liu and Luo, 2013) a ybrid geneti algoritm was presented to obtain optimal eat exange systems wit full use of existing eat exangers and teir strutures. In Bonivers et al. (2017a) te bridge analysis metod, wi is based on energy transfer diagrams and anges in eat transfer of te eat network neessary to redue energy onsumption, wi are alled "bridges", is publised. In Bonivers et al. (2017b), its grapial interpretation is given. In te paper (Osman at al., 2016) te autors propose to inrease te eat reovery apaity in te eat exange system by anging te temperature of tenologial flows witout a signifiant ange in te topology of te eat exange system. Please ite tis artile as: Ulyev L.M., Kanisev M.V., Vasilyev M.A., Maatouk A., 2018, Energy effiieny retrofit of two-flow eat exanger system, Cemial Engineering Transations, 70, 1513-1518 DOI:10.3303/CET1870253
1514 In Akpomiemie and Smit (2015), a metodology is proposed tat ombines euristi rules and optimization strategy wit te analysis of utility pats for te retrofit of eat exange networks witout anging teir topology and witout inreasing te surfae area of eat exange. In te paper by Nemet et al. (2017) te importane of risk assessment in te safe syntesis of eat exanger networks is noted. It sould be mentioned tat tenial, tenologial and eonomi onstraints ould only be partially taken into aount in programming teniques, and in pin analysis teniques an also be used as euristis. As a rule, in industry during te retrofit of eat exange systems it is required to use te minimum possible number of new devies, even by reduing te eonomi benefit. But if it is possible to make an energy-effiient retrofit projet only by re-linking te existing eat exangers, wen it is implemented, in te eat exange system tere will be a redistribution of temperatures on te devies and teir termal loads. If tese parameters go beyond te passport values, it is neessary to arry out industrial safety expert examination for te system eat exangers to avoid te risk of aidents, sine eat exangers are equipment operating under pressure. As a result, it would seem tat witout an investment event, te rebinding of eat exangers turns into an event wit ig osts and a paybak period, sine te ost of arrying out te industrial safety expert examination of one eat exanger is 4-5 k USD. Terefore, wen arrying out energy-effiient retrofit projets, it is neessary to monitor te tenologial parameters of existing and new devies. 2. Metod In tis paper, effiieny improvement and optimization of two-flow eat exange systems wit utilities are onsidered. Two-stream eat exange systems in wi raw materials in front of a reator or separation system are eated by waste produts are found in almost all emial proesses. For example, tey are found in basi emistry proesses (Tovaznyansky et al., 2010), prodution of benzene (Tovaznyansky et al., 2011), rude oil refining (Ulyev et al., 2013), seondary oil refining proesses (Ulyev et al., 2014), proesses of oke making (Ulyev and Vasilyev, 2015), petroemial proesses (Kapustenko et al., 2015) and in te proesses of gas separation (Ulyev et al., 2016). Figure 1: Grid diagram of te existing two-stream eat exange problem One of te eat transfer subsystems on te reyling oil unit is presented in Figure 1. It onsists of tree seriesonneted eat exangers T-1, T-2 and T-3, one eater (ot utility) H, and one refrigerator (old utility) C. Te inlet temperature of te ot proess stream is equal to t S =287 C, te outlet one is equal to t T = 39 C. Te inlet temperature of ool proess stream is equal to t с S =26 C, and te outlet temperature is equal to t с T= 285 C. Te apaity of te ot utility is te value QHmin=3,617 kw, te old utility is equal to QСmin=6,032 kw, te reuperation apaity of eat energy is QREC=9,592 kw. Stream eat apaity of te ot proess flow is equal to СР = 63 kw/ С g, old СРс = 51 kw/ С. Wen arrying out te retrofit projet of te eat network, te termopysial properties of eat arriers were onsidered onstant, eat losses in te eat exange system were absent, and eat exange surfaes S and eat transfer oeffiients K to be fixed and given in Table 1. Te load on te ooling of te ot stream is H = CPH (t S-t T) = 15,620 kw, and te eating of te old stream is HC = CPC (t S-t T) = 13,210 kw, wi is greater tan te reuperation apaity. In tis ase, to redue te value of utilities, it is neessary to inrease te eat reuperation apaity in te system, and for tis, it is neessary to inrease te surfae area of te eat exange, sine te parameters of te existing eat exangers are fixed. Te passport data of te pumping equipment and te existing pressure drops allow to install additional eat exange equipment in te onsidered eat exange system. Te analysis of te equipment loation on te unit as sown tat it is possible to install an additional eat exanger only on te old edge of te eat exange system as sown in Figure 2. Let us onsider ow te temperatures on te eat exangers will ange depending on te eat load of te new T-4 eat exanger. For tis purpose, te termal balanes will be write down for ea eat exanger, assuming tat tere are N devies in te system:
1515 Table 1: Carateristis of eat exangers HE t in, С t out, С CP, kw/ С t In, С t out, С CP, kw/ С S, m 2 K, kw/m 2 С Т-1 287 243 63 161 215 51 214 0.17 Т-2 243 195 63 102 161 51 214 0.16 Т-3 195 134 63 26 102 51 214 0.18 Figure 2: Grid diagram of a two-flow eat exange system wit four eat exangers CP (t N i t N i 1 ) = CP (t i t i+1 ), i = 0 N-1, (1) were t 0 - is te initial temperature of te ot stream, in our ase, equal to 287 C, t 0 is te initial temperature of te old stream, in our ase, equal to 26 C. On te oter and, te load on te eat exanger is defined as: Q i = S i T lni K i, i=0 N-1, (2) and ten for te i-t eat exanger an be reorded: (t i ti+1 S i K i ) (tn 1 t N i 1 ) ln t i t N 1 t i+1 tn i 1 = CP (t i t i+1 ), i = 0 N 1. (3) Taking into aount Eq(1), te ratio for te temperature differene is obtained: t i t N 1 = (t i+1 t N i 1 )e A i, i=0 N-1, (4) were A i = S ik i CP (1 CP CP ). Considering tat N=4, i.e. in eat exange system tere are 4 eat exangers, and making onseutive substitutions in te system of equations Eq(4), te temperature of te old flow at te outlet from te first eat exanger is found: t 4 = t 0 (t 4 t 0 )E, (5) were E = e A 1e A 2e A 3e A 4. Using te expression for te eat balane of te entire system of eat transfer (t 4 t 0 )CP = (t 0 t 4 )CP, (6) te temperature of te ot stream at te outlet of te 4 t eat exanger is found: t 4 = t 0 (1 CP CP )+t 0 (E 1) E CP CP. (7) Next, te temperature differene between eat arriers on te old side of te eat exange system is determined: t m = t 4 t 0 = (t 0 t 0 ) CP CP ECP CP. (8) From te system of equations Eq(1), te eat arrier temperatures at te inlet to te eat exangers and te eat arrier temperatures at te outlet of te eat exangers is determined:
1516 t 1 = b + d t m e A 4, (9) t 2 = b + d t m e A 4e A 3, (10) t 3 = b + d t m e A 4e A 3e A 2, (11) t 4 = b + d t m e A 4e A 3e A 2e A 1, (12) t 1 = b + g t m e A 4e A 3e A 2, (13) t 2 = b + g t m e A 4e A 3, (14) t 3 = b + g t m e A 4, (15) were b = CP Ct 0 CP H t 4, d = CP, g = CP. CP C CP H CP CP CP CP After te temperatures alulating, te apaities of ot and old utilities are determined: Q Cmin = CP (t 4 t 5 ), (16) Q Hmin = CP (t 5 t 4 ). (17) Using te found oolant temperatures at te inlet and outlet of te eat exangers sown in Figure 3, te eat exanger loads are alulated: Q i = CP (t N+1 i t N i )=CP (t i 1 t i ). (18) In tis ase, for tenologial reasons, only sell-and-tube eat exangers an be used for reonstrution. Te ost of installing one setion of te sell-and-tube eat exanger will be determined as (Smit, 2016): CosT = A + B(S), (19) were A - is te ost of installing one setion of te eat exanger, B - is te equivalent of te ost of 1 m 2 of te eat exange surfae area, is an indiator of te nonlinear dependene of te ost, refleting te possibility of plaing te eat exange surfae of different sizes in one asing. In tis ase, te values of tese parameters are A = 40,000 USD; B = 1,000 USD; = 0.97. Te maximum surfae area of te eat exange for one setion of te seleted manufaturer is Smax = 250 m 2. Taking into aount te value of te maximum eat exange surfae of one setion, te expression Eq (18) for one eat exange plaement will take te form of: CosT m = A S S max +B(S), (20) were x - Iverson's eiling funtion returning te smallest integer greater tan or equal to x (Graam, 1994). Te ost of ot utilities in te installation inludes te ost of own gas, natural gas from te ity igway, te ost of liquid fuel onsisting of a mixture of fuel oil, gas oil and diesel fuels. Te final ost of ot utility is te value tat is equal to CH = 120 USD for 1 kw a year. Te ost of old utilities inludes te ost of te fres ooling water, ost of eletriity, feed-pump drives, fan motors of air oolers, and it is equal to CC = 25 USD per year. Te present value of te installed equipment is determined by te expression (Smit, 2016): i(i+1) n C d = CosT m, (21) (i+1) n 1 were i - is te annual disount rate, n - is te number of years. Te redued ost of energy in te eat exange system under onsideration is determined by te ratio: CE = Q Hmin C H + Q Cmin C C. (22) 3. Results and disussion Wen te area of te eat exange surfae of te new eat exanger inreases, its termal load will inrease as sown in Figure 3a, and te eat load of existing eat exangers will derease, altoug te total eat reuperation apaity will only inrease. As a result, te apaity of ot and old utilities dereases wit te inrease in te eat exange surfae of te fourt apparatus as sown in Figure 3b. Reduing te eat load of existing devies is mainly due to te inrease in te eat exange surfae of te fourt unit. Wit derease
in loading on te existing devies, tere is te derease in differene of temperatures of eat arriers on tem, and derease in temperature of eat arriers in devies as sown in Figure 4. 1517 Figure 3: Te apaity of termal energy. a) for reuperation: 1 - on te first eat exanger; 2 - on te seond eat exanger; 3 - on te tird; 4 on te fourt; 5 - te total eat reovery apaity in te new eat exange system. b) for utilities onsumed by te proess: 1-old; 2 - ot Te greatest differene of temperatures between eat arriers, sine some size of its surfae, will be observed on te new eat exanger. As a result, all oolant temperatures on te ot side of te new eat exanger will tend to teir boundary values on te ot side, and te temperature of te ot oolant on te old side of te new apparatus to te target value. Figure 4: Cange of temperature of te eat arriers. a) old:1- inlet temperature in te fourt eat exanger; 2- in te 3rd; 3 - in te 2nd; 4 - in te 1st; b) - ot. In depending on te size of te eat exange surfae area of te new eat exanger. 1 - Outlet temperature of te first eat exanger, 2 - from 2nd, 3 - from 3rd, 4 - from 4t In order to determine te required area of te eat exange surfae of te new eat exanger, te disounted values of apex and energy depending on its eat exange surfae will be onstruted as sown in Figure 5. Te inrease in te surfae leads to te monotonous inrease in its ost and inrease in te ost of eat Figure 5: Disounted ost values. 1- te annual ost of energy; 2 - te redued apital osts; 3 - te total present value of te re-onstrution projet exange setions, but te ost of energy due to te inrease in te eat reovery apaity monotonially dereases. As a result, te total present value of te renovation projet will be a non-monotoni funtion. Te minimum value will orrespond to te minimum present osts for te retrofit projet of eat exange system, and te value of te new eat exange surfae area will be optimal for te retrofit projet. In our ase, te optimal value is te eat exange surfae area of 500 m 2.
1518 4. Conlusions Te teoretial analysis of eat transfer proesses for two-stream systems wit te presene of ot and old utilities is arried out. It is sown tat te total disounted ost of te eat network retrofit is a nonmonotoni funtion of te additional eat exange surfae. In tis ase, te minimum annual ost is observed wen installing 500 m 2 additional eat exange surfae, wi orresponds to te eat exanger, wi onsists of two setions. It sould be noted tat te developed metod ould be used to optimize two-stream eat network wit utilities in all industries. Referenes Akpomiemie M.O., Smit R., 2015, Retrofit of eat exanger witout topology modifiations and additional eat transfer area, Applied Energy, 159, 381 390. Bonivers J.-C., Srinivasan B., Stuart P.R., 2017a, New analysis metod to redue te industrial energy requirements by eat-exanger network retrofit: Part 1 Conepts, Applied Termal Engineering, 119, 659 669. Bonivers J.-C., Alva-Argaez A., Srinivasan B., Stuart P.R., 2017b, New analysis metod to redue te industrial energy requirements by eat-exanger network retrofit: Part 2 Stepwise and grapial approa, Applied Termal Engineering, 119, 670 688. Duran M.A., Grossmann I.E., 1986, Simultaneous optimization and eat integration of emial proesses, AICE J, 32, 123 138. Flower J.R., Linnoff B., 1978, Syntesis of eat exanger networks 2. Evolutionary generation of networks wit various riteria of optimality, AICE J, 24, 642 654. Graam R.L., Knut D.E., Patasik O., 1994, Conrete Matematis. A Foundation for Computer Siene, Seond Edition, Addison-Wesley, Amsterdam. Kapustenko P.O., Ulyev L.M., Ilenko M.V., Arsenyeva O.P., 2015, Integration Proesses of Benzene-toluenexylen Frationation, Hydrogenation, Hydrodesulpurization and Hydrotermoproessing Installation of Benzene Unit, Cemial Engineering Transations, 45, 235 240. Linnoff B, Flower J.R., 1978, Syntesis of eat exanger networks: I. Systemati generation of energy optimal networks, AICE J, 24, 633 642. Linnoff B., Hidmars E., 1983, Te Pin Design Metod for Heat Exanger Networks, Cemial Engineering Siene, 38, 745 763. Liu X.-W., Luo X., Ma H., 2013, Studies on te retrofit of eat exanger network based on te ybrid geneti algoritm, Applied Termal Engineering, 61, 785 790. Nemet A., Klemeš J.J., Moon I., Kravanja Z., 2017, Syntesis of safer eat exanger networks, Cemial Engineering Transations, 56, 1885 1890. Osman A., Abdul Mutalib M.I., Sigidi I., 2016, Heat reovery enanement in HENs using a ombinatorial approa of pats ombination and proess streams temperature flexibility, Sout Afrian Journal of Cemial Engineering, 21, 37 48. Papoulias S.A., Grossmann I.E., 1983, A strutural optimization approa in proess syntesis III. Total proessing systems, Computers and Cemial Engineering, 7, 723 734. Reisen van J.L.B., Grievink J., Polley G.T., Vereijen P.J.T., 1995, Te plaement of 2-strem and multi-stream eat-exangers in an existing network troug pat-analysis, Comput. Cem. Eng, 19, 143 148. Smit R, 2016, Cemial Proess Design and Integration, 2nd Edition, Wiley & Sons Ltd, Ciester, UK. Tovaznyansky L., Kapustenko P., Ulyev L., Boldyryev S., 2011, Heat integration improvement for benzene ydroarbons extration from oke-oven gas, Cemial Engineering Transation, 25, 153 158. Tovaznyansky L., Kapustenko P., Ulyev L., Boldyryev S., Arsenyeva O., 2010, Proess integration of sodium ypopospite prodution, Applied Termal Engineering, 30, 2306 2314. Ulyev L.M., Kapustenko P.A., Melnykovskaya L.A., Neyporenko D.D., 2013, Te Preise Definition of te Payload Tube Furnaes for Units of Primary Oil Reforming, Cemial Engineering Transation, 35, 247 252. Ulyev L.M., Kapustenko P.O., Neiporenko D.D., 2014, Te Coie of te Optimal Retrofit Metod for Setions of te Catalyti Reforming Unit, Cemial Engineering Transations, 39,169 174. Ulyev L.M., Vasilyev M.A., 2015, Heat and Power Integration of Proesses for te Refinement of Coking Produts, Teoretial Foundation of Cemial Engineering, 49, 676 687. Ulyev L., Vasilyev M., Maatouk A., Dui N., Kusanov A., 2016, Total Site Integration of Ligt Hydroarbons Separation Proess, Cemial Engineering Transation, 52, 1 6. Wan Alwi S.R., Manan Z.A., 2010, STEP A new grapi tool for simultaneous targeting and design of a eat exanger network, Cemial Engineering Journal, 162, 106 121.