METHOD FOR IMPROVING FEEDSTOCK FLEXIBILITY OF STEAM CRACKING

Abstract

Process for the purification, treatment and steam cracking of a secondary hydrocarbon stream in combination with a primary hydrocarbon stream, wherein the secondary hydrocarbon stream is vaporized before introduction within the convection section of a steam cracker furnace in combination with at least a portion of the said primary hydrocarbon stream.

Claims

1. Process for the purification and treatment of a secondary hydrocarbon stream comprising: (a). Evaporating the secondary hydrocarbon stream, optionally in the presence of steam, to obtain a secondary gaseous hydrocarbon stream and optionally a secondary residue; (b). Evaporating a primary hydrocarbon stream in the presence of steam to obtain a primary gaseous hydrocarbon stream and optionally a primary residue; (c). Combining the secondary gaseous hydrocarbon stream with the primary gaseous hydrocarbon stream optionally in the presence of additional steam, to obtain a combined hydrocarbon and steam mixture; (d). Thermally cracking the combined hydrocarbon and steam mixture under conditions enabling production of ethylene and propylene; wherein the secondary hydrocarbon stream has a final boiling point different than the primary hydrocarbon stream, and wherein the primary gaseous hydrocarbon stream and secondary gaseous hydrocarbon stream are both substantially gaseous when they are combined.

2. Process according to claim 1, wherein, prior to step (b), the primary hydrocarbon stream is preheated at a temperature comprised from 50 to 300 C. and the secondary gaseous hydrocarbon stream has a final boiling point of at most 650 C., preferably of at most 500 C., more preferably of at most 380 C.

3. Process according to claim 2, wherein the primary gaseous hydrocarbon stream of step (b) is heated before combining with the secondary gaseous hydrocarbon at step (c).

4. Process according to claim 1, wherein secondary hydrocarbon stream is evaporated in the presence of a high boiling oil spray or diluent.

5. Process according to claim 1, wherein the primary hydrocarbon stream does not contain ethane and/or propane as main constituent.

6. Process according to claim 4, wherein the primary hydrocarbon stream contains a butane, a naphtha, a diesel or a crude oil condensate as main constituent.

7. Process according to claim 1, wherein the secondary hydrocarbon stream contains an optionally hydrotreated plastic pyrolysis oil, a hydrotreated biomass pyrolysis oil, an optionally hydrotreated vacuum gasoil, a crude oil, atmospheric or vacuum distillation residues or a combination of at least two of them.

8. Process according to claim 7, wherein the secondary hydrocarbon stream contains an optionally hydrotreated plastic pyrolysis oil having a diene value of at least 1 g I.sub.2/100 g as measured according to UOP 326, a bromine number of at least 5 g Br.sub.2/100 g as measured according to ASTM D1159, wherein the content of the said optionally hydrotreated pyrolysis oil is at least 2 wt %, the remaining part being a diluent such as a hydrocarbon and/or steam.

9. Process according to claim 1, wherein solids are removed from the secondary hydrocarbon stream prior to evaporation step (a), preferably using filtration means.

10. Process according to claim 9, wherein the solids are separated by filtration, sedimentation, centrifugation, flocculation, high boiling oil spray extraction, single or double wall thin film evaporator with or without the use of high boiling oil diluent or spray, or a combination of at least two of them.

11. Process according to claim 1, wherein heteroatom containing impurities are removed from the secondary hydrocarbon stream prior to evaporation step (a).

12. Process according to claim 11, wherein heteroatom containing impurities are removed using an adsorbent or a combination of adsorbents.

13. Process according to claim 1, wherein the primary hydrocarbon stream is preheated within a heat exchanger located within the convection section of the steam cracker, preferably within a FPH bank, and the combined hydrocarbon and steam mixture is injected within a high temperature convection section, preferably within a HTC-1 or a HTC-2 bank, more preferably within a HTC-2 bank, and wherein each of the primary hydrocarbon stream and/or of the secondary hydrocarbon stream can be independently and preferably evaporated using a flash drum, a kettle, a single or double wall thin film evaporator, a falling film evaporator or a combination of at least two of them prior to introduction in the radiation section.

14. Process according to claim 13, wherein each of the primary residue and/or the secondary residue is independently blended with steam cracker pyrolysis fuel oil, fuel oil, bunker fuel, atmospheric distillation residue or vacuum distillation residue.

15. Process for decoking of the radiant section and/or of the convection section of a steam cracker running consistently with the process according to claim 1, wherein an oxygen containing gas and steam mixture is introduced in lieu of each of (A) the primary hydrocarbon stream, (B) the secondary hydrocarbon stream, (C) the primary gaseous hydrocarbon stream, (D) the secondary gaseous stream, and (E) the combined hydrocarbon and steam mixture, and their combinations, wherein the oxygen containing gas is preferably air and wherein temperature is maintained sufficient to allow combustion of coke layers without impairing metallurgy integrity.

Description

DETAILED DESCRIPTION OF THE FIGURE

[0077] FIG. 1 shows a simplified overview of a possible process scheme according to the invention.

[0078] A first hydrocarbon containing feedstock (1) is introduced into a filter unit (2) to produce a filtered stream (3). Filtration aims to remove solids and gums, which may be present when the stream contained unsaturated hydrocarbons at the origin and when storage conditions were not appropriate. For instance, excessive heat, oxygen, lack of antioxidant, traces of catalytic metals may result in gums and/or solids formation. Hydrocarbon containing feedstocks particularly prone to gums and/or solids formation include without limitation plastic pyrolysis oil, vacuum gasoil, diesel and heavier cuts from fluid catalytic cracking, oils from biological origin such as castor oil, linen oil or spent cooking oil. Other means to remove solids and gums may be used, such as centrifugation. Filtered stream (3) is then passed through an adsorbent section (4) to remove contaminants such as oxygen, nitrogen and sulfur containing molecules and metals to provide a secondary hydrocarbon stream (5). Filtration (3) and adsorption (4) are preferably conducted at ambient or near ambient temperature since heating may result in lowered stream chemical stability and lower process performance. Other methods for contaminant removal may be appropriate, including e.g. one or two stage hydrotreatment, flocculation and precipitation. The secondary hydrocarbon stream (5) is then heated using steam (6) optionally in combination with e.g. a heat exchanger (not shown) to produce an at least partially vaporized decontaminated hydrocarbon and steam stream (7). It is desirable that the secondary hydrocarbon (5) is at a temperature which is high enough to avoid condensation of steam when direct mixing is envisioned, since steam condensation could lead to hammering issues. If this is not the case, preheating of (5) in a heat-exchanger is necessary before additional heating with steam is achieved. Non-vaporized products (9) from stream (7) are removed in a separation section (8) to produce a secondary gaseous hydrocarbon stream (10). Alternatively, the secondary hydrocarbon (5) is heated using a hot oil (42), which is collected in admixture with the non-vaporized products (9) and may be recycled.

[0079] A primary hydrocarbon stream (11) is heated in a feedstock preheater section (FPH) located in the convection section (12) of a steam cracker furnace (13) to provide a pre-heated hydrocarbon feedstock (14). The pre-heated hydrocarbon feedstock (14) is diluted with stream (15) in a first mixing section M (16) prior to introduction in a first high temperature convection bank (HTC-1) wherein gasification of hydrocarbons produces a primary gaseous hydrocarbon stream (17). Stream (15) may be chosen among steam, the secondary gaseous hydrocarbon stream (10) or a combination of both. In case the secondary gaseous hydrocarbon stream (10) is sent to first mixing box M (16) with or without additional steam, then second mixing box M (18) is not directly connected with the secondary gaseous hydrocarbon stream (10) line (this option is not shown on scheme 1). The first mixing section (17) is also fed with additional steam (36) obtained by heating a steam stream (37) into a dilution steam super heater section (DSSH) located in convection section (12). Primary gaseous hydrocarbon stream (17) is mixed with the secondary gaseous hydrocarbon stream (10) in a second mixing section M (18) prior to introduction in a second high temperature convection bank (HTC-2) wherein the combined stream of (10) and (18) is heated to provide heated stream (19). Heating within HTC-2 bank is shown here as co-current, while it could be considered that counter-current heating would be desirable. Heated stream (19) passes through a nozzle (20) before introduction in the radiant section (21) of steam cracker furnace (13) wherein hydrocarbons contained in heated stream (19) are cracked. Radiant section (21) is heated using fuel gas feeding through line (35) (combustion air inlet not shown). Combustion gases leave radiant section (21) to enter convection section (12) (see, bottom arrow) wherein their heat is used by different heat exchangers, respectively HTC-2, HPSSH-2, HPSSH-1, DSSH, HTC-1, ECO and FPH, before release (see, upper arrow). Resulting cracked gases (22), when leaving furnace (13) are rapidly quenched within a transfer line exchanger (TLE) and directed in line (23) to be processed in a separation section (not shown) wherein olefins of interest, especially ethylene and propylene, are isolated along with other higher molecular weight products such as butadiene and aromatics.

[0080] Transfer line exchanger (TLE) allows rapid cooling of cracked gases (22) by means of a heat exchanger (not shown) wherein water is circulated. Water (24) coming from the bottom of a steam drum (SD) is vaporized in the transfer line exchanger (TLE) and sent back to steam drum (SD) through line (25). Additional water (26) is pre-heated within the convection section (12) in an exchanger (ECO), then resulting pre-heated water (27) feds steam drum (SD). Steam drum (SD) is regulated using purge line (34). Vapor from steam drum (SD) is sent through line (28) into a first high pressure steam super heater section (HPSSH-1) wherein steam is super-heated. Resulting super-heated steam (29) is then quenched with water stream (31) in a third mixing section M (30) before introduction in a second high pressure steam super heater section (HPSSH-2) through line (32). Third mixing section M (30) enables thermal regulation of convection section (12) through cooling of stream (29). Last, high pressure steam leaves high pressure steam super heater section (HPSSH-2) through line (33) for further use. Dotted square section (A) corresponds to an alternative design of dotted square section (A), in which heated stream leaving first mixing section M (16) is sent to a separation section (38), wherein non-vaporized compounds (39) are removed to prevent downstream fouling of HTC-1 heat exchanger. Similarly, dotted square section (B) corresponds to an alternative design of dotted square section (B), in which non-vaporized compounds (40) present in stream (17) are removed in a separation section (41) prior to introduction in second mixing section M (18). When used, alternative designs (A) and (B) may be present each independently or in combination. Non-vaporized streams (9), (39) and (40) may be recycled or used for other purposes depending on their composition.

[0081] Process air connections (not shown) can be added on any of the feeding lines and on various locations to allow simultaneous on-line decoking of evaporator and furnace. Suitable connections may be located for instance on the primary hydrocarbon stream (11) line, the secondary hydrocarbon stream (6) line, the preheated hydrocarbon feedstock (14) line, the primary gaseous hydrocarbon stream (17) line, the secondary gaseous hydrocarbon stream (10) line, the first mixing section M (16), the second mixing section M (18) and heated stream (19) line, which heated stream (19) being a combined hydrocarbon and steam mixture.

DETAILED DESCRIPTION OF THE INVENTION

[0082] The secondary hydrocarbon stream may contain pyrolysis plastic oil up to 100 wt. %. The other component of said secondary hydrocarbon stream in combination with pyrolysis plastic oil may include any diluent acceptable by a steam cracker containing as few olefins and dienes as possible. A naphtha can be used as diluent. The use of naphtha as diluent is particularly advantaging. In a preferred embodiment, the pyrolysis plastic oil is diluted into naphtha having a boiling range from 15 to 250 C., preferably 38 to 150 C., as measured with method ASTM D2887 to form the hydrocarbon stream at a concentration of 50 wt %, preferably 75 wt %, more preferably 90 wt % of pyrolysis plastic oil that is diluted in the naphtha. Alternatively, a middle distillate (boiling range 180-360 C.) may be used as diluent, preferably a jet fuel or a diesel. Paraffinic middle distillate is preferred. Other suitable secondary hydrocarbon streams include spent cooking oil, animal fats and greases, edible or non-edible vegetable oils, which may be advantageously diluted in preferably paraffinic middle distillate. Last, crude oil and especially crude condensates and preferably atmospheric and vacuum distillation products or residues are suitable secondary hydrocarbon streams.

[0083] With regards to the waste plastic pyrolysis, an example of a pyrolysis process for waste plastics is disclosed in U.S. Pat. No. 8,895,790 and US20140228606.

[0084] With regards to the steam cracker, the primary hydrocarbon feedstock fed to the steam cracker can be ethane or propane if the secondary hydrocarbon feedstock mainly contains a light hydrocarbon such as a naphtha. The primary hydrocarbon feedstock is preferably liquefied petroleum gas, naphtha or gasoil. Liquefied petroleum gas (LPG) consists essentially of propane and butanes. Gasoils have a boiling range from about 200 to 350 C., consisting of C.sub.10 to C.sub.22 hydrocarbons, including essentially linear and branched paraffins, cyclic paraffins and aromatics (including mono-, naphtho- and poly-aromatic).

[0085] In particular, the cracking products obtained at the outlet of the steam cracker may include ethylene, propylene and benzene, and optionally hydrogen, toluene, xylenes, and 1,3-butadiene.

[0086] In a preferred embodiment, the outlet temperature of the steam cracking furnace may range from 750 to 1000 C., preferably from 800 to 950 C., more preferably from 810 to 900 C., more preferably from 820 C. to 880 C. The outlet temperature may influence the content of high value chemicals in the cracking products produced by the present process.

[0087] In a preferred embodiment, the residence time in the steam cracker, through the radiation section of the reactor where the temperature is between 650 and 1200 C., may range from 0.02 to 2.0 seconds, preferably from 0.1 to 1.0 seconds.

[0088] In a preferred embodiment, steam cracking is done in presence of steam in a ratio of 0.1 to 1.5 kg steam per kg of hydrocarbon feedstock, preferably from 0.25 to 1.0 kg steam per kg of hydrocarbon feedstock in the steam cracker, preferably in a ratio from 0.30 to 0.7 kg steam per kg of feedstock mixture, to obtain cracking products as defined above.

[0089] In a preferred embodiment, the steam cracker reactor outlet pressure may range from 500 to 2500 mbarg, preferably from 700 to 1000 mbarg, more preferably may be approx. 850 mbarg. The residence time of the feed in the reactor and the temperature are to be considered together. A lower operating pressure results in easier light olefins formation and reduced coke formation. The lowest pressure possible is accomplished by (i) maintaining the output pressure of the reactor as close as possible to atmospheric pressure at the suction of the cracked gas compressor (ii) reducing the pressure of the hydrocarbons by dilution with steam (which has a substantial influence on slowing down coke formation). The steam/feedstock ratio may be maintained at a level sufficient to limit coke formation.

[0090] Effluent from the steam cracker contains unreacted feedstock, desired olefins (mainly ethylene and propylene), hydrogen, methane, a mixture of C.sub.4 (primarily isobutylene and butadiene), pyrolysis gasoline (aromatics in the C.sub.6 to C.sub.8 range), ethane, propane, di-olefins (acetylene, methyl acetylene, 1,2-propadiene), and heavier hydrocarbons that boil in the temperature range of fuel oil (pyrolysis fuel oil). This cracked gas is rapidly quenched to a temperature that may range from 200 to 600 C. to stop the pyrolysis reactions, minimize consecutive reactions and to recover the sensible heat in the gas by generating high-pressure steam in parallel transfer-line heat exchangers (TLE's). In gaseous feedstock-based plants, the TLE-quenched gas stream flows forward to a direct water quench tower, where the gas is cooled further with recirculating cold water. In liquid feedstock-based plants, a prefractionator precedes the water quench tower to condense and separate the fuel oil fraction from the cracked gas. In both types of plants, the major portions of the dilution steam and heavy gasoline in the cracked gas are condensed in the water quench tower at 35-40 C. The gas is subsequently compressed to about 25-35 barg in 4 or 5 stages. Between compression stages, the condensed water and light gasoline are removed, and the cracked gas is washed with a caustic solution or with a regenerative amine solution, followed by a caustic solution, to remove acid gases (CO.sub.2, H.sub.2S and SO.sub.2). The compressed cracked gas is dried with a desiccant and cooled with propylene and ethylene refrigerants to cryogenic temperatures for the subsequent product fractionation: front-end demethanization, front-end depropanization or front-end deethanization.

EXAMPLES

[0091] The embodiments of the present invention will be better understood by looking at the different examples, below.

Example 1: Secondary Hydrocarbon Stream Pretreatment: Solids and Gums Removal Using Cellulose Containing Fibers, Activated Charcoal and Diatomite Followed by Filter Press Separation

[0092] A pyrolysis plastic oil containing 3 wt % of solid particles having a 10 m mean particles diameter may be contacted with (i) from 0.1 to 0.5 wt % activated charcoal with specific surface area greater than 50 m.sup.2/g, (ii) from 0.1 to 1 wt % of cellulose fibers, and (iii) from 0.1 to 10 wt % of diatomite. Contacting is preferably done during at least 1 minute and up to several days if the mixture is not heated. A stabilizer may be added to the mixture to avoid gums formation or buildup. A suitable stabilizer may be an antioxidant such as 2,6-di-tert-butyl-4-methylphenol (BHT) or derivatives such as Irganox products (BASF). The amount of stabilizer to be added may be determined by those skilled in the art and may range from 0,001 wt % to 0.1 wt %, depending e.g. on the nature of the pyrolysis plastic oil and treatment conditions. Albeit not mandatory, the resulting mixture may be heated at from 50 C. to 100 C. for 1 to 30 minutes. The mixture can then be filtered through a filter press to produce (i) a solid cake that is discharged and (ii) a solids and gums depleted liquid phase which should contain less chlorine and less metals than pyrolysis plastic oil starting material.

[0093] Depending on the contents in impurities, the solids and gums depleted liquid phase may be used as secondary hydrocarbon stream in a steam cracker such as the one described in the present invention, pure or diluted. Depending on the steam cracker technology, the liquid phase may be diluted with e.g. a VGO, a gas oil, diesel, a naphtha, a LPG, butane, propane, ethane or combinations thereof.

Example 2: Steam Cracking of 2.5 t/h Plastic Pyrolysis Oil as Secondary Hydrocarbon Stream in Combination with 9.5 t/h Naphtha as Primary Hydrocarbon Stream

[0094] A 2.5 t/h vaporized purified plastic pyrolysis oil stream as secondary gaseous hydrocarbon stream (10) with a steam to oil ratio (SOR) of 0.7 kg/kg can be combined with a 9.5 t/h naphtha stream as primary gaseous hydrocarbon stream (17) with a steam to oil ratio (SOR) of 0.7 kg/kg. The plastic pyrolysis oil stream has a final boiling point (FBP) of 420 C. The combined mixture can be introduced in the HTC-2 section of a steam cracker at a temperature of 377 C. and a pressure of 4.1 barg. The mixture leaves HTC-2 section at 578 C. and 3.7 barg and passes through nozzle (20) wherein pressure drops down to 1.9 barg, then enters radiant section (21) wherein steam cracking occurs. The radiant section contains a radiant box heated with 1562 kg/h of fuel gas at 43 C. burning with 9.1% air excess introduced at 7 C. The low heating value (LHV) of fuel gas is 12368 kcal/kg at 25 C. Fired heat is 19.3 Gcal/h at 25 C., absorbed heat is 8.5 Gcal/h, box efficiency based on fired heat is 44.1%, convection duty is 9.3 Gcal/h, for an overall efficiency of 92.37%.

[0095] Naphtha (11) (9.5 t/h, 113 C., 4.8 barg) is first pre-heated in the FPH section to provide stream (14) at 119 C. and 4.7 barg which is then introduced in the first mixing section M (16), combined with 6.65 t/h of dilution steam (36) issued from DSSH section at 474 C. and 4.7 barg and introduced at 204 C. and 4.7 barg in HTC-1 section. Dilution steam (36) from DSSH section results from the feeding of 6.65 t/h steam (37) at 218 C. and 4.8 barg in said DSSH section. Naphtha and steam mixture (17) leaves HTC-1 section at 398 C. and 4.1 barg as primary gaseous hydrocarbon stream (17).

[0096] Boiler feed water (26) (14.4 t/h, 116 C., 127 barg) is heated in the ECO section and feeds steam drum (SD) at 226 C. and 126 barg. Steam drum (SD) feeds HPSSH-1 section with steam (14.1 t/h, 327 C., 126 barg) and affords heated steam at 368 C., which is desuperheated with Boiler feed water (0.05 t/h, 116 C., 127 barg) before entering HPSSH-2 section to be superheated at the target temperature. HPSSH-2 section produces high pressure steam at 482 C. and 126 barg. Steam drum purge drain (34) releases approximatively 0.28 t/h. After leaving the radiant box at 826 C. and 1.6 barg, cracked gases are quenched in TLE section to provide 20.1 t/h of a steam and hydrocarbon combined stream at 364 C. Combustion gases (32081 kg/h) are released from the furnace stack at 144 C.

[0097] Decoking may be realized by adding process air connection aside the feeding lines of the hydrocarbons to be purified, such as next to the secondary feedstock vaporizer (evaporator/heat exchanger) to allow simultaneous on-line decoking of evaporator and the furnace. Decoking of vaporizer and furnace reaction coils may be typically achieved with a furnace coil decoking temperature range of 800-1000 C. and a vaporizer decoking temperature range of 300-450 C.

Example 3: Steam Cracking of 2.5 t/h Plastic Pyrolysis Oil as Secondary Hydrocarbon Stream in Combination with 11.5 t/h Naphtha as Primary Hydrocarbon Stream

[0098] A 2.5 t/h vaporized purified plastic pyrolysis oil stream as secondary gaseous hydrocarbon stream (10) with a steam to oil ratio (SOR) of 0.7 kg/kg can be combined with a 11.5 t/h naphtha stream as primary gaseous hydrocarbon stream (17) with a steam to oil ratio (SOR) of 0.7 kg/kg. The plastic pyrolysis oil stream has a final boiling point (FBP) of 420 C. The combined mixture can be introduced in the HTC-2 section of a steam cracker at a temperature of 394 C. and a pressure of 4.8 barg. The mixture leaves HTC-2 section at 600 C. and 4.4 barg and passes through nozzle (20) wherein pressure drops down to 2.1 barg, then enters radiant section (21) wherein steam cracking occurs. The radiant section contains a radiant box heated with 1865 kg/h of fuel gas at 43 C. burning with 9.1% air excess introduced at 7 C. The low heating value (LHV) of fuel gas is 12368 kcal/kg at 25 C. Fired heat is 23.1 Gcal/h at 25 C., absorbed heat is 9.5 Gcal/h, box efficiency based on fired heat is 41.1%, convection duty is 11.8 Gcal/h, for an overall efficiency of 92.07%.

[0099] Naphtha (11) (11.5 t/h, 113 C., 5.7 barg) is first pre-heated in the FPH section to provide stream (14) at 124 C. and 5.6 barg which is then introduced in the first mixing section M (16), combined with 8.05 t/h of dilution steam (36) issued from DSSH section at 490 C. and 5.6 barg and introduced at 208 C. and 5.6 barg in HTC-1 section. Dilution steam (36) from DSSH section results from the feeding of 8.05 t/h steam (37) at 218 C. and 5.8 barg in said DSSH section. Naphtha and steam mixture (17) leaves HTC-1 section at 413 C. and 4.8 barg as primary gaseous hydrocarbon stream (17).

[0100] Boiler feed water (26) (16.1 t/h, 116 C., 127 barg) is heated in the ECO section and feeds steam drum (SD) at 240 C. and 126 barg. Steam drum (SD) feeds HPSSH-1 section with steam (15.8 t/h, 327 C., 126 barg) and affords heated steam at 374 C., which is desuperheated (i.e. cooled down) with boiler feed water (0.54 t/h, 116 C., 127 barg) before entering HPSSH-2 section to be superheated at the target temperature. HPSSH-2 section produces high pressure steam at 482 C. and 126 barg. Steam drum purge drain (34) releases approximatively 0.32 t/h.

[0101] After leaving the radiant box at 826 C. and 1.6 barg, cracked gases are quenched in TLE section to provide 23.5 t/h of a steam and hydrocarbon combined stream at 404 C. Combustion gases (38291 kg/h) are released from the furnace stack at 154 C.

Comparative Example 1: Steam Cracking of 2.5 t/h Plastic Pyrolysis Oil and 9.5 t/h Naphtha as Primary Hydrocarbon Stream

[0102] A mixture of 2.5 t/h of purified plastic pyrolysis oil and 9.5 t/h naphtha stream (90 C., 5.0 barg) can be preheated at 122 C. and 4.9 barg in the FPH section of a steam cracker. The plastic pyrolysis oil stream has a final boiling point (FBP) of 420 C. The combined mixture can then be mixed with dilution steam (8.4 t/h, 458 C., 4.9 barg) then partially vaporized in HTC-1 section (352 C., 4.1 barg) and heated and vaporized in HTC-2 section. The mixture leaves HTC-2 section at 565 C. and 3.7 barg and passes through nozzle (20) wherein pressure drops down to 1.9 barg, then enters radiant section (21) wherein steam cracking occurs. The radiant section contains a radiant box heated with 1635 kg/h of fuel gas at 43 C. burning with 9.1% air excess introduced at 7 C. The low heating value (LHV) of fuel gas is 12368 kcal/kg at 25 C. Fired heat is 20.2 Gcal/h at 25 C., absorbed heat is 8.8 Gcal/h, box efficiency based on fired heat is 43.7%, convection duty is 9.9 Gcal/h, for an overall efficiency of 92.62%.

[0103] Boiler feed water (26) (14.1 t/h, 116 C., 127 barg) is heated in the ECO section and feeds steam drum (SD) at 213 C. and 126 barg. Steam drum (SD) feeds HPSSH-1 section with steam (13.8 t/h, 327 C., 126 barg) and affords heated steam at 371 C., which is desuperheated with boiler feed water (0.25 t/h, 116 C., 127 barg) before entering HPSSH-2 section to be superheated at the target temperature. HPSSH-2 section produces high pressure steam at 482 C. and 126 barg. Steam drum purge drain (34) releases approximatively 0.28 t/h. After leaving the radiant box at 827 C. and 1.6 barg, cracked gases are quenched in TLE section to provide 20.4 t/h of a steam and hydrocarbon combined stream at 364 C. Combustion gases (33596 kg/h) are released from the furnace stack at 139 C.

Comparative Example 2: Steam Cracking of 13.1 t/h Naphtha as Sole Hydrocarbon Stream

[0104] A 13.1 t/h naphtha stream (113 C., 4.7 barg) can be preheated at 118 C. and 4.6 barg in the FPH section of a steam cracker. The combined mixture can then be mixed with dilution steam (6.55 t/h, 491 C., 4.6 barg) then vaporized in HTC-1 section (375 C., 3.9 barg) and heated in HTC-2 section. The mixture leaves HTC-2 section at 591 C. and 3.5 barg and passes through nozzle (20) wherein pressure drops down to 1.9 barg, then enters radiant section (21) wherein steam cracking occurs. The radiant section contains a radiant box heated with 1702 kg/h of fuel gas at 43 C. burning with 9.1% air excess introduced at 7 C. The low heating value (LHV) of fuel gas is 12368 kcal/kg at 25 C. Fired heat is 21.0 Gcal/h at 25 C., absorbed heat is 9.1 Gcal/h, box efficiency based on fired heat is 43.1%, convection duty is 10.4 Gcal/h, for an overall efficiency of 92.47%.

[0105] Boiler feed water (26) (14.5 t/h, 116 C., 127 barg) is heated in the ECO section and feeds steam drum (SD) at 223 C. and 126 barg. Steam drum (SD) feeds HPSSH-1 section with steam (14.2 t/h, 327 C., 126 barg) and affords heated steam at 374 C., which is desuperheated with boiler feed water (0.42 t/h, 116 C., 127 barg) before entering HPSSH-2 section to be superheated at the target temperature. HPSSH-2 section produces high pressure steam at 482 C. and 126 barg. Steam drum purge drain (34) releases approximatively 0.28 t/h.

[0106] After leaving the radiant box at 827 C. and 1.6 barg, cracked gases are quenched in TLE section to provide 19.6 t/h of a steam and hydrocarbon combined stream at 364 C. Combustion gases (34953 kg/h) are released from the furnace stack at 143 C.

[0107] From the examples and comparative examples, it appears that energy efficiency of steam cracking using the process according to the invention is equivalent to conventional steam cracking, while avoiding fouling issues in convection sections, which occur when feedstock is not fully vaporized and/or when containing reactive species such as olefins, diolefins and impurities like some oxygen containing hydrocarbons, e.g. peroxides, epoxides resulting from direct oxidation of olefinic functional groups in e.g. unsaturated fatty acids.