SRC SUPER RADIANT COIL ETHYLENE FURNACES STEAM CRACKING TECHNOLOGY EVOLUTION Inventors : Dr.ing. Maurizio Spoto - Dr.ing. Benedetto Spoto
THE CRACKING FURNACE IS THE CORE OF ETHYLENE BUSINESS The reaction yield increases by reducing both the gas residence time and the hydrocarbon partial pressure. Furnace designers are continuously modifying the geometry of the radiant coil to improve the surface to volume ratio. The resulting higher heat flux and tube metal temperature force the radiant coil to operate close to its metallurgical limit.
Short residence time requires small diameter tubes which causes: Higher Tube Metal Temperature (TMT) Higher pressure drop Higher coking rate Higher carburization and creep rates Selectivity loss during the Run Length (RL) Tube plugging
SRC Technology Objectives: Short residence time with large bore tubes Increase the heat transfer area Enhance the radiant heat transfer Reduce the tube metal temperature Reduce the coking rate Avoid tube plugging
SRC Technology Achievements: High olefins selectivity Low coking rate - Extended furnace run length Lower Tube Metal Temperature Extended coil life (low carburization rate)
Heat transfer in cylindrical tube q rb q cb T wo T wi T f Firebox Radiative Heat Flow Firebox Convection Heat Flow Tube Wall Outside Temperature Tube Wall Inside Temperature Cracking gas Temperature T box Firebox Temperature A Tube heat transfer surface U Overall tube heat transfer coefficient Stefan-Bolzmann constant GS Overall exchange area q b =q rb+ q cb =GS* *(T box 4 - T wo4 )+A o *h o *(T box - T wo ) = U*A*(T wi -T f )
Typical commercial coils Geometry Name Run Length days Coil ID inches Residence time sec UDC > 60 4 5 0.5 0.8 Split Coil 45 60 3 4 0.3 0.4 U-tube 30 45 2 2.5 0.2 0.3 Millisecond 10 20 1 1.5 < 0.2
SUPER RADIANT COIL Heat Transfer Mechanism q C2 q C2 q C1 q r SRC q rb q cb q k Firebox Radiative Heat Flow Firebox Convection Heat Flow Conduction heat flow q c1 Convection heat flow 1 q c2 Convection heat flow 2 q r SRC Inner Radiative Heat Flow Heat Balance : q r = q rb + q cb = q C1 +q C2 q r SRC =q C2
SUPER RADIANT COIL Heat Balance T f T wi TSRC Fluid Temperature Tube Wall inside Temperature SRC Temperature SRC q C2 q C1 Convection Heat Transfer : q C =q C1 +q C2 = A wi *h C1 (T wi -T f ) +A SRC *h SRC *(T SRC -T f ) q r SRC q C2 = qr SRC h SRC > hc 1 hc 1 =Tube inner convection heat transfer coefficient h SRC = SRC outer convection heat transfer coefficient
Heat Transfer Over the Insert (SRC) RADIANT HEAT TRANSFER INSIDE THE TUBE: q r SRC = *(T wi4 -T SRC4 )*A SRC f(ε SRC, ε T, ε g,f SRC,T ) SRC q C1 HEAT BALANCE OVER THE CYLINDRICAL INSERT(SRC): q C2 q r SRC = q C2 q r SRC T SRC A SRC Insert temperature Insert surface F SRC,T View factor ε SRC, ε T, ε g Emissivity (SRC, tube and cracking gas)
SUPER RADIANT COIL Industrial Test Furnace Polimeri Europa Ethylene Plant- Gela (Sicily) Industrial test has been carried out by installing two SRC devices in an industrial furnace @ Gela Ethylene plant An identical couple of tubes in the same furnace and operating under identical conditions have been kept unchanged to act as reference tubes Both couples of tubes receive the same heat flow from the fire-box. Tube wall temperatures have been collected by using a laser pyrometer. The tube wall temperature of the base coil has been on an average 40 C higher than the TMT of the SRC coils.
SUPER RADIANT COIL : Process Calculations Process yields Material, heat and momentum balance Coke formation Are calculated using our proprietary kinetic model PYCOS PYCOS PYROLYSIS COIL SIMULATOR
SUPER RADIANT COIL The following two case studies 1 & 2 show that using SRC technology it is possible to increase the furnace capacity of existing furnaces with a dramatic reduction of the tube wall temperature and coking rates. The third case study is the recoil of an existing Selas furnace. The use of the SRC technology allows a very good selectivity increase: Ethylene from 26.61 to 28.39 wt % Propylene from 13.55 to 14.51 wt % Butadiene from 3.57 to 5.26 wt % The calculate economic benefits of the selectivity increase is about 3.200.000 /y The total cost of the recoil is about 2.500.000
Case study 1 : Capacity increase from 9.62 t/h to 10.5 t/h Feed:ethane 99.87 % wt -ethylene 0.13 % wt Fixed firebox size -Tube design temperature:1030 C Coils type unit base LC base HC SRC (60%) SRC (65%) Ethane flow t /h 9,62 10,5 10.5 10,5 Steam dilution ratio Steam /Oil kg/kg 0,4 0,4 0,4 0,4 Fire duty GJ/h 82,54 92,6 90,2 102 Firebox efficiency % 40 38,34 39,28 37,1 Furnace inside radiant surface m 2 119.46 119.46 132 132 Average inside heat flux Kw/m 2 76,8 82,6 74,6 80 Pressure drop at SOR Kg/cm 2 1,67 1,9 1,12 1,12 Pressure drop at EOR Kg/cm 3 2,24 2,57 1,47 1,69 Residence time s 0,58 0,54 0,58 0,58 Ethane conversion % 60 60 60 65 Ethylene yield % 48,9 49 48,8 51,8 Max inner TMT at SOR C 954 955 933 974 Max outer TMT at R.L C 1030 1030 982 1015 Coking rate mm/mth 2,93 2,98 1,86 2,73 Max velovcity at R.L. m/s 300 328 271 304 Run length: R.L. days 62 53 90 90
Case study 2: Capacity increase from 23.0 t/h to 25.3 t/h Feed : Naphtha -Fixed firebox size -Tube design temperature:1100 C unit Case a Case -b Case c Hydrocarbon flow t /h 23 25.3 25.3 Steam dilution Kg/kg 0.5 0.5 0.5 Fired duty GJ/h 162.98 185.11 171.16 Fire box efficiency % 39.62 37.8 41.29 Heat absorbed GJ/h 64.63 70.04 70.74 Furnace inside radiant surface m 2 187.76 187.76 218.56 Average inside heat flux kw/m 2 95.61 103.62 89.9 XOT: cross over temperature C 600 610 610 COT: coil outlet temperature C 829 829 830 Flue gas at cross-over C 1165 1197 1136 Severity: propylene/ethylene ratio Kg/kg 0.55 0.55 0.55 COP: coil outlet pressure Bar, a 1.75 1.77 1.77 Pressure drop Kg/cm 2 1.25 1.41 1.1 Residence time s 0.46 0.44 0.38 Coking rate (last tube) mm/mth 2.6 2.8 1.7 MAX TMT SOR (last tube) C 986 988 958 MAX TMT 30days (last tube) C 1042 1056 986 Run length Days 60 50 152
Case study 3 : Selas recoil - Capacity : 24 t/h Feed : Naphtha -Fixed firebox size -Tube design temperature:1100 C Yields % wt Selas Selas-SRC Delta /kg /a /a H2 0.96 0.95-0.01 0.581 1070899 1059744 CH4 17.62 15.43-2.19 0.2698 9127442 7992987 C2H2 0.26 0.57 0.31 0.76 379392 831744 C2H4 26.61 28.39 1.78 0.95 48536640 51783360 C2H6 4.24 3.92-0.32 0.76 6187008 5720064 C3H4 0.52 0.85 0.33 0.6242 623201 1018694 C3H6 13.55 14.51 0.96 0.6242 16239187 17389713 C3H8 0.48 0.45-0.03 0.6242 575263 539309 C4H6 3.57 5.26 1.69 0.3832 2626606 3870013 C4H8 3.07 4.1 1.03 0.3832 2258734 3016550 C4H10 0.25 0.31 0.06 0.3832 183936 228081 C5 totali 3.28 3.75 0.47 0.3832 2413240 2759040 Benzolo 8.67 7.47-1.2 0.3832 6378900 5496008 Toluolo 4.25 3.82-0.43 0.3832 3126912 2810542 Xiloli+Etb 1.67 1.47-0.2 0.3832 1228692 1081544 Styrolo 1.45 1.07-0.38 0.3832 1066829 787246 C6-C8 Na 1.27 1.29 0.02 0.3832 934395 949110 C9+ 4.12 2.84-1.28 0.3832 3031265 2089513 Residuo 4.16 3.55-0.61 0.1812 1447281 1235059 Total feed [t/year] ( * ) 107435823 110658321 Naphtha price [ /kg] 0.516 99072000 99072000 Run length [days] 47 114 8363823 11586321 Fuel gas [kg/h] 2981 2989.0 8.00 0.2698 17267 Operating hours = 8000 hours/year ( * ) Selectivity economics naphtha flow rate 24 t/h 8363823 11569053 Benefits /a 3205230
SUPER RADIANT COIL:Applications Ethane Main Purpose Large Diameter Tubes Low Tube Wall Temperature Low Coking Rate No Tube Plugging Increase Conversion Increase Capacity Increase Selectivity Low carburization Low Pressure Drop LPG/naphtha/GO Main Purpose Large Diameter Tubes Short Residence Time Low Tube Wall Temperature Low Coking Rate No Tube Plugging Long Operating Cycle High Severity High Selectivity Low Pressure Drop
SUPER RADIANT COIL Conclusions Objectives: Short residence time Increase heat transfer rate Enhance radiant heat transfer Reduce TMT Reduce coking rate Reduce tube plugging SRC achievements: Reduced cross sectional area Higher film coefficient Internal radiant heat transfer Increased Heat Transfer Area Lower TMT Large diameter tube
SUPER RADIANT COIL PATENT GRANTED IN EUROPE USA & RUSSIA