|Title||Heat transfer enhancement in a cracking furnace: radiant coil modification|
|Contributor||Petela, G.;Benum, L.; Crowe, J.|
|Rights||This material may be protected by copyright. Permission required for use in any form. For further information please contact the American Flame Research Committee.|
Heat transfer enhancement in a cracking furnace: radiant coil modification G. Petela, L Benum, J. Crowe presented for American Flame Research Committee Annual Meeting, Salt Lake City, Sept. 5 - 7, 2012 Content: 1.0 Intensification Of Heat Transfer in a Cracking Furnace 1.1 Heat transfer mechanism in a furnace radiant section 1.2 Improvement possibilities 1.3 Approach: an engineering furnace model to analyze heat transfer scenarios 2.0 Radiant Coil With Modified /Augmented External Surface 2.1 Coil Geometry » Horizontal circular fins » Vertical fins with: - rectangular cross section - triangular cross section 2.2 Modeling finned coil performance in a furnace radiant section 2.3 Potential Implementation Issues 3.0 Conclusions (at this stage) 2 3 1.0 Intensification Of Heat Transfer in a Cracking Furnace (from Combustion Gas To Feed) 4 coils; 48 burners; 1 coil heated by 12 burners: 6 in cold box, and 6 in hot box Furnace radiant section Burners Feed to radiant coils Coil passes Exiting products Inlet section, "cold" box Outlet section, "hot" box 4 1.1 Heat transfer mechanism in a furnace radiant section Radiation + convection § From a single coil "perspective": REFRACTORY (R) QR-C1 EC1-R QF-C1 QC-C1 Qconv,F-C1 COILS(C) COMBUSTION GASES/FLAME (F) EC1-C reflected C1 rQ E ~ εT4 coil radiation, emissvit Q NET radiative heat flux to coil Qr convection Qconv αQ absorbed 5 § Heat transfer from combustion gases into a feed inside a coil Thermal resistance ? (natural draft furnace) o The highest thermal resistance = "bottle neck " for the overall heat transfer o Good practice : improve the part of the process with the highest thermal resistance Rint.conv~0.0034 mK/W R λ~0.0004 mK/W Rext.conv+rad~0.0078 mK/W 1.2 Improvement possibilities 6 Ø Heat transfer inside the coil Improved convection from COIL → FEED Modified internal coil surface, inserts (issues: cleaning, maintenance, pressure drop, coke formation) Ø External heat transfer into the coil Intensified radiation and ext. convection from FLAME → COIL Luminous radiative flame , high temperature and gas velocity Intensified radiation from REFRACTORY → COIL Modified refractory emissivity and surface To modify a COIL to make it a better heat RECEIPIENT Coil with augmented external surface Method of analysis of numerous options, geometry : CFD? 7 1.3 Approach : to develop an engineering furnace model to analyze heat transfer scenarios Radiant section divided into 33 sections, along coil length For each section: energy and mass balances combustion, heat transfer by radiation + forced convection, feed flow Simulation of furnace operation Bare coils (reference) Coils modified with different types of fins § Natural or balanced draft furnace, 48 gas burners The actual furnace schematics, coils/burners locations § Representation of the furnace radiant section in the model Locations along a coil path where temperatures and heat fluxes are calculated by the model; combustion gas, feed , furnace segments 8 f3 f4 f6 f7 f8 f9 f10 f12 f13 f15 f16 f18 f19 f20 f21 f22 f24 f25 f27 f28 f30 f31 f33 f34 f35 f11 f17 f23 f29 f14 f26 f32 f5 s1 c2 2 3 c3 4 s2 s3 s4 s5 s6 s8 s9 s10 s11 s12 s16 s17 s15 s18 s14 s13 c6 6 c7 7 8 c10 10 s7 c11 11 c14 14 c15 15 16 c18 18 c1919 12 20 c22 22 c23 23 24 c26 26 c27 27 28 s19 s22 s23 s21 s24 s27 s30 s33 s26 s29 s32 s25 s28 s31 c30 30 c31 31 32 c34 34 c35 35 36 c38 38 c39 39 40 c42 42 c43 43 44 f2 Feed Cold box 29 33 41 25 1 5 9 13 17 21 Hot box 37 Bottom terrace Top terrace Middle terrace s20 Exhaust gas 45 BURNERS Product 4 coils; 48 burners; 1 coil heated by 12 burners: 6 in cold box, and 6 in hot box Furnace radiant section Burners Feed to radiant coils Coil passes Exiting products Inlet section, "cold" box Outlet section, "hot" box Calculated temperature of combustion gas and feed, along the coil path Temperature of combustion gases and feed along the coil path successive locations along a coil path 10 20 30 40 t (oC) 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 t1 t2 t2c t3 t3c t4 t5 t6 t6c t7 t7c t8 t9 t10 t10c t11 t11c t12 t13 t14 t14c t2 t15 t15c t16 t17 t18 t18c t19 t19c t20 t21 t22 t22c t23 t23c t24 t25 t26 t26c t27 t27c t28 t29 t30 t30c t31 t31c t32 t33 t34 t34c t35 t35c t36 t37 t38 t38c t39 t39c t40 t41 t42 t42c t43 t43c t44 t45 1 feed cold box combustion gases hot box good qualitative agreement with measurements 9 § Model verification: simulation of the reference operation (with bare/finless process coils) Measured temperature of a coil external surface 8 9 10 11 7 1 coil pass 1 coil pass 2 3 4 5 6 2.1 Coil geometry 10 Lh Ls Lz Lh Lz r z r w Ls = ü Horizontal circular fins ü 2 longitudinal vertical fins, 180o apart , with a) rectangular b) triangular cross section Lz Lh Ls 2.0 Radiant Coils With Modified External Surface 2. 2 Modeling finned coil performance in a furnace 11 § Assumptions: - added fins constitute 30% of coil mass; - furnace capacity, feed and product as in the reference operation; § Simulation of heat transfer in the furnace radiant section using the developed model : Calculated parameters: - Temperature distribution of combustion gases along the coil - Feed temperature in the coil - Furnace performance indicators (fuel consumption, radiant section efficiency) Effect of fins - trends Pyrolysis furnace: temparature distributions in combustion gases and in the feed Bare coils vs. coils with 2 rectangular fins (all coils vs. coils in hot box only) Location along coil path 5 10 15 20 25 30 35 T (OC) 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 BARE coils - combust. gas BARE coils - combust. gas BOTH BOXES - coils with 2 rectangular fins HOT BOX - coils with 2 rectangular fins BARE coils - combust. gas BOTH BOXES - coils with 2 rectangular fins HOT BOX - coils with 2 rectangular fins feed ------- - coils with circular fins § Temperature distribution of combustion gases along the radiant coil: 1) bare coils (REF), 2) coils with 2 vertical rectangular fins, 3) coils with circular fins ΔT2 ΔT3 § Calculated performance parameters : furnace with bare and finned coils rz rw Vertical fins Lh Lz Lh Ls r z r w Ls = Lh Lz Lh Ls r z r w Ls = Lz Lh Ls CIRCULAR fins in HOT & COLD boxes 0.92 58.6 32.4 (2) VERTICAL TRAINGULAR fins in HOT & COLD boxes ~0.80 ~68 32.4 Operational parameters BARE COILS (REF) Total fuel, kg/s ~1.0 Furnace eff, % ~53.7 mass FIN /massCOIL % 0 (2) VERTICAL RECTANGULAR fins in HOT & COLD boxes ≥ 0.80 ~67 32.4 Ø Simulation results - effect of fins: Longitudinal vertical fins with rectangular cross section - higher efficiency , (~10%), than radial/circular horizontal fins of the same mass Longitudinal vertical fins with triangular cross section - marginally better than with rectangular cross section 14 challenging manufacturing process triangular geometry is compromised Ø Fin with rectangular vs. triangular cross section 15 § Fin thermal efficiency , eff Rectangular fin as fin size increases : eff ↑ … eff max … eff ≈ constant Triangular fin has to be within a strict size limit to be effective as fin size increases : eff ↑ … eff max …. eff ↓ with space constraints, fin with rectangular cross section is a safer choice § Manufacturing process of a finned coil: rectangular fin - relatively simple triangular fin - fin geometry approximated small efficiency advantage is alleviated 16 Approximation of temperature distribution in a triangular and rectangular fin, 0.00 0.01 0.02 0.03 0.04 x (m) Tx (oC) 800 900 1000 1100 1200 1300 Lz=45mm, Ls=20mm Lz=45mm, Ls=20mm Lz=45mm, Ls=7mm RECTANGULAR FINS Tcoil = 900oC, Tsurr =1350oC TRIANGULAR FIN x o Lower probability to overheat a tip of a rectangular fin fin with rectangular cross section - more suitable for this particular application 17 Ø Optimization of rectangular fin geometry - limited by two constraints: weight (possibility of coil creep or deformation) length (spacing between coils in the furnace) Thermal fin efficiency vs. fin dimensions m2 fins/mcoil, % 10 20 30 40 50 60 70 10 20 30 40 50 60 70 x, (Qcoil with 2 fins/Qbare coil) 1.00 1.05 1.10 1.15 1.20 1.25 Efficiency of rectangular fins , (Ls=const), for a coil in the 108 furnace 2FINS_rect_optx_var Lz Ls=12mm Ls=9mm ~17% ~20% Ls=16mm ~22% (1) Lz=40mm ... m2 fins/mcoil = 44%, Ls=16mm, eff~22% (3) Lz=30mm ... m2 fins/mcoil = 17%, Ls= 9mm, eff ~ 17% (2) Lz=35mm ... m2 fins/mcoil = 30%, Ls=12mm, eff ~ 20% Fig.5B Efficiency of rectangular fins , (Ls=const), for a coil in the 108 furnace ; coil OD=0.1072m, ID=0.0883m, Lh=3.0m, ρ=7860 kg/m3, λ=0.0303 kW/(mK) Lh Ls Lz 18 2.3 Potential issues for a finned coil implementation: - Deformation? - Buckling? - Overheating ? - Excessive Stress ? Distribution / Concentration ? FEA analysis of a finned coil : ANSYS simulations of stress distribution . 19 Buckling Total deformation: BARE tube - 181mm FINNED tube - 193 mm Buckling predicted from the total strain energy in the part ↓ the fins are at least 5 times too thick to buckle Can thermal expansion at the fin end make a finned tube to buckle? 20 q Max. Temperature Normal Stress (oC) (MPa) BARE tube FINNED tube 21 Maximum Von-Mises Stress (MPa) BARE tube FINNED tube 3.0 CONCLUDING (at this stage) Effect of addition of external fins to a process coil surface was analyzed, using the engineering model of heat transfer in a radiant section of Joffre furnace v Coil modification, (vertical fins with rectangular cross section), intensified heat transfer: fuel savings increased efficiency (≤ 12%) of the radiant section, and/or furnace capacity v Manufacturing simplicity & efficiency rectangular fin v Preliminary FEA results for a finned coil: Total deformation under weight - not excessive No possibility of buckling Stress distribution/concentration - elevated v Testing of a finned coil: Pilot plant - for a thermal performance and stress level Field - for material integrity 22 NOVA permission to present is gratefully acknowledged 23 ** Patent applications : CA 2,738,273 ‘Furnace coil with protuberances on the external surface', April 28, 2011 CA 2,735,533 ‘Furnace coil fins', March31, 2011.