Application of Intensified Heat Transfer for the Retrofit of Heat Exchanger Network

Transcription

1 Applcaton of Intensfed Heat Transfer for the Retroft of Heat Exchanger Network Yufe Wang, Mng Pan, Igor Bulatov, Robn Smth, Jn-Kuk Km Centre for Process Integraton School of Chemcal Engneerng and Analytcal Scence The Unversty of Manchester Centre for Process Integraton 1

2 Outlne 1. Introducton.Modellng of shell-and-tube heat exchangers 3.Heurstc rules for HEN retroft wth HTE 4.Case study 5.Concluson Centre for Process Integraton 1

3 1. Introducton Centre for Process Integraton 1

4 Heat exchanger network (HEN) H1 H H3 C1 C C3 C4 Models used for unts n heat- exchanger network (HEN) are very smple Specfed overall U Q = U A T LM HEN desgn neglects the heat-exchanger detals No account of pressure drops No detals of geometry, just overall area Not sutable for many retroft applcatons Centre for Process Integraton 1

5 HEN retroft H1 H H3 C1 C C3 C4 Need an approach for retroft Account for detaled performance of heat exchangers Include pressure drop constrants Allow locatons for approprate use of heat transfer enhancement Centre for Process Integraton 1

6 Research objectves Develop a smple but accurate model for heat-exchanger detals Propose approprate heat transfer enhancement Develop a desgn method sutable for HEN retroft wth heat transfer enhancement Centre for Process Integraton 1

7 . Modellng of shell-and-tube heat exchangers Centre for Process Integraton 1

8 Heat exchangers Double ppe (DPHEX) two pars of concentrc ppes, counter flow - the smplest type Shell and tube (STHEX) a bundle of tubes n a cylndrcal shell, combnng parallel and counter flows - the most wdely used type n the chemcal ndustres Plate and frame (PFHEX) metal plates are used to separate and transfer heat between two fluds - the common typed n the food and pharmaceutcal ndustres Centre for Process Integraton 1

9 Lmts of the exstng STHEX models Lots of equatons and emprcal factors unsutable for HEN modellng, leadng to large scale problems Varous assumptons over or less estmate heat transfer coeffcents and pressure drops A smple but relable model s requred for estmatng STHEX performances! Centre for Process Integraton 1

10 New model of STHEX (tube-sde) Tube-sde heat transfer coeffcent (h ): New adjusted ( n / n ) m p v = ρ ( πd / 4) Re Pr Nu =. 4.3 Re Pr t.8 Re = D v ρ / µ for for [ Re Pr ( D / L )] 1 3 heatng coolng value / 3 1/ 3 Nu =.116 (Re 15) Pr [1+ ( D /L ) / 3 ] Pr (Based on Bhatt and Shah, 1987) = 1 C P µ / k Re 1 < Re < 1 Nu = 1.86 Re h = ( k ) / D Nu Bhatt, M. S., and R. K. Shah, Turbulent and transton convectve heat transfer n ducts, n Handbook of Sngle-Phase Convectve Heat Transfer, S. Kakac, R. K. Shah, and W. Aung, eds., Wley, New York, Chap. 4, Centre for Process Integraton 1

11 New model of STHEX (tube-sde) Tube-sde pressure drop ( P ): (Adopt exstng method of Serth, 7) f =.4137Re Re Re 3 f = 64 / < 3 P f.585 = n p f Lρ g c D v Re P r =.5(n p g c 1.5)ρ v P n =.75 N g S c ρ v n P = P f + P r + P n Serth, R. W., Process heat transfer prncples and applcatons, Elsever Ltd, 7. Centre for Process Integraton 1

12 New model of STHEX (shell-sde) Shell-sde heat transfer coeffcent (h ): Numercal correlatons are proposed: v h = m ρ S hm F z =ρdv h µ For F = s F z 9.3 F z F z Graphcal nformaton s only avalable (Ayub, 5) For < Fz 1 F = F h s =.765F k s.6633 z / 3 D B.553 c ( C µ ) p 1/ 3 Ayub, Z. H., A new chart method for evaluatng sngle-phase shell sde heat transfer coeffcent n a sngle segmental shell and tube heat exchanger. Appled Thermal Engneerng, 5, 41-4, 5. Centre for Process Integraton 1

13 New model of STHEX (shell-sde) Shell-sde pressure drop ( P ): f. 15 ( D ) 1 = s Re P f ( D ) = s Re f = 144[f1 1.5 (1 B /Ds)(f1 f) ] fb,%bc = f D s g ρ c D v e p P n.75nsρ = g c v n Baflle cut s specfed for % (Serth, 7) New correlaton for other baffle cuts are proposed: P f = Pf,%Bc(Bc /Bmn c ) n 1 B mn c = % P = Pf + Pn Serth, R. W., Process heat transfer prncples and applcatons, Elsever Ltd, 7. L4 18 Modellng of Intensfed Heat Transfer for the Retroft of Heat Exchanger Networks Centre for Process Integraton 1

14 New model of STHEX U, LMTD, FT and A: U = D [ h D D + ( /D ) ln D k tube + 1 h RDD + D +R D ] 1 Energy balance: m C p (T,outlet T,nlet ) = m C p (T,outlet T, nlet ) LMTD = (T,nlet T,outlet (T ln (T ),nlet,outlet (T T,outlet,outlet T,nlet ) ) T,nlet ) FT = (R 1) R ln 1 S +1ln 1 RS S(R +1 ( R +1) S R +1+ R +1) A = m C p (T,nlet T U FT LMTD,outlet ) A = nπd t L eff Centre for Process Integraton 1

15 Procedure of the new model Input stream and geometry parameters of heat exchanger: T hot, n, T cold, n, Cp hot, Cp cold,µ hot,µ cold, L, D,. Calculate tubesde ( P ) Plan tube correlatons Calculate tubesde (h ) Dttus-Boelter correlaton Calculate shellsde (h ) Chart method Calculate shell-sde ( P ) Smplfed Delaware method Calculate overall heat transfer coeffcent Assume hot stream outlet temperature (T hot, out ) Calculate cold stream outlet temperature (T cold, out ) Calculate LMTD, LMTD correcton factor (F), and heat-transfer area based on tubes (A) Calculate overall heat transfer area wth U No IA A I ε Yes Stop Centre for Process Integraton 1

16 Examples Ten examples are consdered for model valdaton: Heat exchanger geometry: Tube: 14 ~ 3983 Tube passes: ~ 6 Tube length:.4 m ~ 9 m Tube dameter: 15 mm ~ 5 mm Tube pattern: 3º, 45º, 6º, 9º Shell dameter:.489 m ~ 1.9 m Baffle spacng:.978 m ~.5 m Baffle cut: % ~ 4%.. Stream Propertes: Specfc heat (J/kg K): 64 ~ 4179 Thermal conductvty (W/m K):.8 ~.137 Vscosty (mpa s):.17 ~ Densty (kg/m 3 ): 635 ~ 1 Centre for Process Integraton 1

17 Detals of examples Example 1 Example Example 3 Example 4 Example 5 Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Streams Specfc heat C P (J/kg K) Thermal conductvty k (W/m K) Vscosty µ (mpa s) Densty ρ (kg/m 3 ) Flow rate m (kg/s) Inlet temperature T n ( C) Foulng resstance (m K/W) Geometry of heat exchanger Tube ptch PT (m) Number of tubes n t Number of tube passes n p 6 4 Tube length L (m) Tube effectve length L eff (m) Tube nner dameter D (m) Tube outer dameter D (m) Shell nner dameter D s (m) Number of baffles n b Baffle spacng B (m) Inlet baffle spacng B n (m) Outlet baffle spacng B out (m) Baffle cut B c % % 5% % % Inner dameter of tube-sde nlet nozzle D,nlet (m) Inner dameter of tube-sde outlet nozzle D,outlet (m) Inner dameter of shell-sde nlet nozzle D,nlet (m) Inner dameter of shell-sde outlet nozzle D,outlet (m) Shell-bundle dametrc clearance L sb (m) Centre for Process Integraton 1

18 Detals of examples (contnued) Example 6 Example 7 Example 8 Example 9 Example 1 Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Streams Specfc heat C P (J/kg K) Thermal conductvty k (W/m K) Vscosty µ (mpa s) Densty ρ (kg/m 3 ) Flow rate m (kg/s) Inlet temperature T n ( C) Foulng resstance (m K/W) Geometry of heat exchanger Tube ptch PT (m) Number of tubes n t Number of tube passes n p 4 Tube length L (m) Tube effectve length L eff (m) Tube nner dameter D (m) Tube outer dameter D (m) Shell nner dameter D s (m) Number of baffles n b Baffle spacng B (m) Inlet baffle spacng B n (m) Outlet baffle spacng B out (m) Baffle cut B c % 4.4% 38% % 4% Inner dameter of tube-sde nlet nozzle D,nlet (m) Inner dameter of tube-sde outlet nozzle D,outlet (m) Inner dameter of shell-sde nlet nozzle D,nlet (m) Inner dameter of shell-sde outlet nozzle D,outlet (m) Shell-bundle dametrc clearance L sb (m) Centre for Process Integraton 1

19 Results (New model vs. HTRI/HEXTRAN) Tube-sde: Heat transfer coeffcent (h) Pressure drop (P) HTRI / HEXTRAN h (W/m.K) HTRI HEXTRAN HTRI / HEXTRAN P (kpa) HTRI HEXTRAN New model New model Shell-sde: Heat transfer coeffcent (h) Pressure drop (P) HTRI / HEXTRAN h (W/m.K) HTRI HEXTRAN HTRI / HEXTRAN P (kpa) HTRI HEXTRAN New model New model Centre for Process Integraton 1

20 Modellng of heat exchanger The new model: Fewer equatons and emprcal factors (compared wth the exstng models) Relable estmaton for heat transfer coeffcents and pressure drops (compared wth HTRI and HEXTRAN ) Lmts: No phase change Phase change wll be consdered n future work Sngle segmental baffle Centre for Process Integraton 1

21 3. Heurstc rules for HEN retroft wth HTE L4 43 Modellng of Intensfed Heat Transfer for the Retroft of Heat Exchanger Networks Centre for Process Integraton 1

22 Exstng desgn methods for HEN retroft Yee and Grossmann (1991), retroft desgn Sorsak and Kravanja (4), dfferent exchanger types Ponce-Ortega et al. (8), phase changes Lmts: Lots of topology modfcatons Too much reppng work No account of STHEX geometry modfcatons Yee TF, Grossmann IE. A screenng and optmzaton approach for the retroft of heat exchanger networks. Industry and Engneerng Chemstry Research 1991; 3 (1): Sorsak A, Kravanja Z. MINLP retroft of heat exchanger networks comprsng dfferent exchanger types. Computers and Chemcal Engneerng 4; 8: Ponce-Ortega JM, Jménez-Gutérrez A, Grossmann IE. Optmal synthess of heat exchanger networks nvolvng sothermal process streams. Computers and Chemcal Engneerng 8; 3: Centre for Process Integraton 1

23 Exstng desgn methods for HEN retroft wth HTE Polley et al. (199), potental analyss of heat recovery Zhu et al. (), network pnch approach Smth et al. (9), structural modfcatons and cost- effectve desgn Lmts: Large scale problems No pressure drop restrctons Stream outlet temperatures change Polley GT, Reyes Athe CM, Gough M. Use of heat transfer enhancement n process ntegraton. Heat Recovery Systems and CHP 199; 1(3): Zhu X, Zanfr M, Klemes J. Heat transfer enhancement for heat exchanger network retroft. Heat Transfer Engneerng ; 1(): Smth R, Jobson M, Chen L. Recent development n the retroft of heat exchanger networks. Chemcal Engneerng Transactons 9; 18: 7-3. Centre for Process Integraton 1

24 Heat Exchanger Network Retroft Through Heat Transfer Enhancement Yes Network structure analyss Search exchangers n utlty path(heurstc rule 1) Senstvty analyss(heurstc rule ) Check the pnchng match (Heurstc rule 3) Enhance the canddate exchanger Any other good canddates? (Heurstc rule 4) No Yes Stll need mprovement? Enhance pnchng match (Heurstc rule 5) No Result P The methodology s Heurstc P The objectve of ths methodology s to fnd canddates to be enhanced P The methodology s manly based on network pnch approach and senstvty tables Centre for Process Integraton 1

25 Heurstc methodology (Rule 1) H1 1 N:1 N :11 3 N:13 7 N :19 H 3 N: 1 N:9 8 N :1 H3 3 N:3 5 N:3 9 N:5 3 H4 4 N:4 4 N:15 1 N :7 1 C1 N:14 3 N:5 5 8 C N:18 6 N:1 1 N:1 N:16 4 N:4 5 N:6 6 Steam 4 7 N:7 6 N :17 5 Coolng Water N : N:6 N:8 N : 7 N:8 8 Rule 1(Network structure analyss) P Rule 1 s to fnd the potental canddates to be enhanced. Only those exchangers on a utlty path and on a same stream wth utlty exchanger wll be selected. Centre for Process Integraton 1

26 Heurstc methodology (Rule ) Rule (Senstvty graph) H1 1 N:1 N:1 1 3 N:1 3 7 N:1 9 3 H N: 1 N:9 8 N: 1 H3 3 N:3 5 N: 3 9 N: 5 H4 3 4 N:4 4 N:1 5 1 N: 7 1 C1 N:1 4 3 N:5 5 8 C N:1 8 6 N:1 1 N:1 N:1 6 4 N: 4 5 N:6 6 Steam 4 7 N:7 6 Canddate N:1 7 5 Coolng Water N: 8 N: 6 9 N: 8 1 N: 7 N:8 8 P Rule s to analyze the energy savng potental of each canddate by usng senstvty tables. Centre for Process Integraton 1

27 Reasons for hgh senstvty CP value of the hot stream (assumng the chosen utlty exchanger s a hot utlty) The T mn between hot and cold stream The canddate poston n network Centre for Process Integraton 1

28 Heurstc methodology (Rule 3) Pnchng match Rule 3(check network pnch) H1 1 N:1 N:11 3 N:13 7 N:19 H 3 N: 1 N:9 8 N:1 H3 3 N:3 5 N:3 9 N:5 3 H4 4 N:4 4 N:15 1 N:7 1 C1 Canddate N:14 3 N :5 5 8 C N:18 6 N:1 1 N:1 N:16 4 N:4 5 N :6 6 Steam 4 7 N:7 6 N:17 Canddate 5 Coolng Water N : N:6 N :8 N: 7 N :8 8 P Rule 3 s to check the network pnch whch may nfluence the preformance of canddates. P After checkng the network pnch, the best canddate can be dentfed by the results of senstvty tables. Centre for Process Integraton 1

29 Heurstc methodology (Rule 4) H1 1 N:1 N:11 3 N :13 7 N:19 H 3 N: 1 N :9 8 N :1 H3 3 N:3 5 N:3 9 N:5 3 H4 4 N:4 4 N:15 1 N :7 1 C1 Canddate Canddate N :14 3 N :5 8 5 C N:18 6 N :1 1 N:1 N:16 4 N:4 5 N :6 6 Steam 4 7 N:7 6 N:17 5 Coolng Water N : N:6 N :8 N: 7 N :8 8 Rule 4(Enhance several canddates smultaneously) P Rule 4 s to determne f there are some other exchangers can be enhanced. P Senstvty graphs wll be appled agan. Centre for Process Integraton 1

30 Heurstc methodology (Rule 5) Rule 5(enhance both pnchng match and canddate H1 1 N:1 Pnchng match N:11 3 N:13 7 N:19 H 3 N: 1 N:9 8 N:1 H3 3 N:3 5 N:3 9 N:5 3 H4 4 N:4 4 N:15 1 N:7 1 C1 C N:18 6 N:1 1 Canddate N:1 N:16 4 N:4 5 N:14 3 N:5 8 N:6 5 6 Steam 4 7 N:7 6 N:17 5 Coolng Water N: 8 N:6 9 N:8 1 N: 7 N:8 8 P Rule 5 s to enhance pnchng match to release the canddate to recover more heat Centre for Process Integraton 1

31 4. Case study Centre for Process Integraton 1

32 Case study Centre for Process Integraton 1

33 Case study Stream data and geometry of heat exchangers, 4, 6 and 8 Heat Exchanger Heat Exchanger 4 Heat Exchanger 6 Heat Exchanger 8 Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Fluds H C H C3 H3 C3 H9 C3 Specfc heat C P (J/kg K) Thermal conductvty k (W/m K) Vscosty µ (mpa s) Densty ρ (kg/m 3 ) Flow rate m (kg/s) Inlet temperature T n ( C) Foulng resstance (m K/W) Geometry of heat exchanger Tube ptch PT (m) Number of tubes n t Number of tube passes n p 1 Tube length L (m) Tube effectve length L eff (m) Tube conductvty k tube (W/m K) Tube pattern (tube layout angle) Tube nner dameter D (m).... Tube outer dameter D (m) Shell nner dameter D s (m) Number of baffles n b Baffle spacng B (m) Inlet baffle spacng B n (m) Outlet baffle spacng B out (m) Baffle cut B c % % 4% % Inner dameter of tube-sde nlet nozzle D,nlet (m) Inner dameter of tube-sde outlet nozzle D,outlet (m) Inner dameter of shell-sde nlet nozzle D,nlet (m) Inner dameter of shell-sde outlet nozzle D,outlet (m) Shell-bundle dametrc clearance L sb (m) Maxmum pressure drops n shell and tube sde (KPa) Centre for Process Integraton 1

34 Case study Tube-sde heat transfer enhancement for heat exchangers, 4, 6 and 8 Example Example 4 Example 6 Example 8 Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde Shell-sde Tube-sde H C H C3 H3 C3 H9 C3 New model Pressure drop P (KPa) Flm coeffcent h (W/m K) Overall heat transfer coeffcent U (W/m K) HTRI Pressure drop P (KPa) Flm coeffcent h (W/m K) Overall heat transfer coeffcent U (W/m K) Heat transfer enhancement of tube sde Pressure drop P (KPa) Flm coeffcent h (W/m K) Overall heat transfer coeffcent U (W/m K) Centre for Process Integraton 1

35 Case study Optmzaton of HEN retroft For a gven number of enhanced exchangers Increase energy savng Subject to maxmum pressure drop constrants n streams Centre for Process Integraton 1

36 Case study Exchanger U (kw/m K) Intal stuaton Enhancement stuaton Area (m ) T ln ( C) Duty (kw) U (kw/m K) Area (m ) T ln ( C) Duty (kw) (hot utlty) Centre for Process Integraton 1

37 Case study Summary: Overall heat transfer coeffcents of enhanced exchangers ncrease Pressure drops restrctons are satsfed No topology modfcatons for HEN No many geometry modfcatons for exchangers, just enhanced wth tube-nserts Based on the new method, up to 3.4% reducton of heat duty s acheved (65.4 MW to 63.4 MW) Centre for Process Integraton 1

38 5. Concluson Centre for Process Integraton 1

39 Concluson New model of heat exchanger Tube-sde heat transfer coeffcents and pressure drops Shell-sde heat transfer coeffcents and pressure drops Retroft of HEN wth heat transfer enhancement Increase overall heat transfer coeffcents of enhanced exchangers Satsfy pressure drop constrants Increase energy savng Centre for Process Integraton 1

40 Acknowledgement Fnancal support from Research Councls UK Energy Programme (EP/G674/1; Intensfed Heat Transfer for Energy Savng n Process Industres) s gratefully acknowledged. Centre for Process Integraton 1