PROCESS AND APPARATUS FOR CRACKING OF THERMALLY UNSTABLE FEEDSTOCK

Abstract

Process for steam cracking of thermally unstable feedstock such as plastic pyrolysis oil, or olefin rich hydrocarbon cuts such as cracked gasoline, coker naphtha or diesel. The invention is also about a heat exchanger suitable for that process.

Claims

1. Process for steam cracking of a thermally unstable feedstock comprising the steps of: (a). preheating the thermally unstable feedstock at a temperature and a pressure wherein most, preferably substantially all, of the feedstock remains liquid, wherein preheating is achieved using convection heat from a steam cracker furnace, (b). combining the preheated feedstock of step (a) with steam to obtain a heated feedstock, (c). heating the heated feedstock of step (b) using an evaporator to produce a gasified feedstock by evaporation and a residue, (d). heating the gasified feedstock of step (c) using convection heat to obtain a heated gasified feedstock, (e). steam cracking of the heated gasified feedstock to obtain cracked gases, (f). recovering and cooling down cracked gases of step (e), wherein heating at step (c) is not performed within a convection section of a steam cracker furnace.

2. Process according to claim 1, wherein heating at step (c) is performed using a dedicated heating fluid such as boiler feed water, steam or a liquid hydrocarbon or mixture of liquid hydrocarbons, preferably chosen among aromatic hydrocarbons, linear, branched and/or cyclic paraffinic hydrocarbons and their mixtures.

3. Process according to claim 2, wherein heating fluid is heated using flue gas within the steam cracking furnace convection section and wherein the heating fluid is optionally in a circulation loop.

4. Process according to claim 1, wherein the residue of step (c) is separated into a liquid residue and a solid residue by gravity drainage or sedimentation, filtration, centrifugation, flocculation, high boiling oil extraction, or a combination of at least two of them.

5. Process according to claim 4, wherein the liquid residue is (i) processed into a plastic pyrolysis oil production process, (ii) mixed with quench oil produced by steam cracker, or (iii) mixed with steam cracker pyrolysis fuel oil.

6. Process according to claim 1, wherein steam from step (b) is obtained by heating steam in the convection section of a steam cracker furnace.

7. Process according to claim 1, wherein solids and optionally gums are removed from the thermally unstable feedstock prior to preheating at step (a).

8. Process according to claim 1, wherein heteroatoms containing impurities are removed from the thermally unstable feedstock prior to preheating at step (a).

9. Process according to claim 1, wherein a stabilizer such as a hindered phenol such as 2,6-di-tert-butyl-4-methylphenol, or a mixture of at least two stabilizers, is added to the thermally unstable feedstock prior to preheating at step (a).

10. Process according to claim 1, wherein the thermally unstable feedstock is selected among optionally hydrotreated plastic pyrolysis oil, optionally hydrotreated biomass pyrolysis oil, optionally hydrotreated vacuum gasoil, crude oil, atmospheric or vacuum distillation residues, or a combination of at least two of them, preferably an optionally hydrotreated plastic pyrolysis oil having a diene value of at least 1 g 12/100 g as measured according to UOP 326, a bromine number of at least 5 g Br2/10 Og 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.

11. Process according to claim 1, wherein the residue at step (c) contains solid and/or liquid, and is separated from the gasified feedstock by gravity drainage or sedimentation, filtration, 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, preferably single or double wall thin film evaporation.

12. Process according to claim 1, wherein the gasified feedstock of step (c) is obtained by evaporation using single or double wall thin film evaporator with or without the use of high boiling oil diluent or spray, or by using single or double wall thin film evaporation, falling film evaporator or kettle type evaporator or within a first shell and tube heat exchanger.

13. Process according to claim 1, wherein the gasified feedstock of step (c) is obtained by evaporation within a first straight-tube shell and tube heat exchanger, preferably within the tubes constituting the first shell and tube heat exchanger, and wherein the tubes optionally contain an anti-fouling device such as rotating or vibrating springs.

14. Process according to claim 13, wherein the first shell and tube heat exchanger contains a purge for removal of the residue.

15. A shell and tube heat exchanger for obtaining by evaporation the gasified feedstock of step (c) of the process according to claim 1, comprising an outer shell and a straight tube bundle within the said outer shell, the tube bundle being connected at one end to an inlet plenum having a tube-side fluid inlet for a first fluid, the straight tube bundle leading at the other end to an outlet plenum having a tube-side fluid outlet for the said first fluid, the tube bundle passing through a sealed compartment comprising a shell-side fluid inlet for a second fluid and a shell-side fluid outlet for the second fluid, wherein the outlet plenum further comprises an outlet purge.

16. A shell and tube heat exchanger according to claim 15, further comprising anti-fouling coating and/or cleaning means arranged within the tube hollows of the bundle, preferably rotating or vibrating springs.

Description

DETAILED DESCRIPTION OF THE INVENTION

Detailed Description of the Figure

[0086] FIG. 1 shows a simplified overview of a possible process scheme according to the invention.

[0087] A gums and solids 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 used 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 gums and solids depleted thermally unstable feedstock (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, coalescence and precipitation. The thermally unstable feedstock (5) is then heated within a feedstock preheater (FPH) bank (6) of the convection section (8) of a steam cracker furnace (9) to produce a preheated feedstock (7). The preheated feedstock (7) is heated in a first mixing section M (10) by addition of steam (11) obtained by heating colder steam (12) within a diluted steam super heater (DSSH) bank (13) to provide a heated feedstock (14). It is desirable that the final temperature of heated feedstock (14) at the outlet of first mixing section M (10) be above the temperature of steam condensation at a given pressure to avoid any hammering issues. If this is not the case, increasing temperature of steam (11) and/or increasing flowrate of steam (11) and/or lowering flowrate of preheated feedstock (7) and/or lowering pressure in first mixing section M (10) shall be considered. It is preferable to avoid excessive preheating of preheated feedstock (7) prior to mixing with steam (11). Heated feedstock (14) enters a vaporizer (15) to produce a combined gasified feedstock and residue (16). A dedicated heated fluid (42), which has been heated within a medium pre-heater bank (MPH) (43), feeds vaporizer (15) and results in vaporization of heated feedstock (14) and in a colder heated fluid (44), the latter being pumped through pump (45) back to medium pre-heater bank (MPH) (43).

[0088] Combined gasified feedstock and residue (16) is separated in a flash drum (17) into a gasified feedstock (18) and a residue (19), which is withdrawn. Gasified feedstock (18) is heated in a high temperature convection bank (HTC) (20) to provide a heated gasified feedstock (21), which enters radiant section (22) of cracker furnace (9) through nozzle (23), to be cracked.

[0089] Resulting cracked gases (24), when leaving radiant section (22) of furnace (9) are rapidly quenched within a transfer line exchanger (TLE) (25) and directed in line (26) 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. Optionally, cracked gases quenching may also be performed through two successive transfer line exchangers.

[0090] Transfer line exchanger (TLE) (25) allows rapid cooling of cracked gases (24) by means of a heat exchanger wherein water is circulated. Water (27) coming from the bottom of a steam drum (SD) (28) is vaporized in the transfer line exchanger (TLE) (25) and sent back to steam drum (SD) (28) through line (29). Additional water (30) is pre-heated within the convection section (8) in exchanger (ECO) (31), then resulting pre-heated water (32) feeds steam drum (SD) (28). Steam drum (SD) (28) is regulated using purge line (33). Vapor from steam drum (SD) (28) is sent through line (34) into a first high pressure steam super heater bank (HPSSH-1) (35) wherein steam is super-heated. Resulting super-heated steam (36) is then quenched with water stream (37) in a second mixing section M (38) before introduction in second high pressure steam super heater bank (HPSSH-2) (39) through line (40). Second mixing section M (38) enables thermal regulation of convection section (8) through cooling of stream (36). Last, high pressure steam leaves high pressure steam super heater bank (HPSSH-2) (39) through line (41) for further use.

[0091] Radiant section (22) is heated using fuel gas feeding through line (46) (combustion air inlet not shown). Combustion gases leave radiant section (22) to enter convection section (8) (see, bottom arrow) wherein their heat is used by different heat exchangers, respectively HTC, HPSSH-2, HPSSH-1, DSSH, MPH, ECO and FPH, before release (see, upper arrow). Decoking steam and process air connections (not shown) can be added on any of the feeding lines and on various locations to allow simultaneous or sequential on-line decoking of vaporizer (15), flash drum (17) and furnace (9). Suitable connections may be located for instance on the feedstock convection section entering line (5) or on the first mixing section M (10) or on preheated feedstock (7) line that feeds said first mixing section M (10), the steam inlet line (12), the heated feedstock (14) line, the combined gasified feedstock and residue (16) line, the flash drum (17), the gasified feedstock (18) line, the residue (19) line, the heated gasified feedstock (21) line, and the cracked gases (24) line. Although not shown in FIG. 1, location of convection section banks can be changed if the temperature requirements are respected. The only exception is FPH bank that needs to be on the top to benefit from the lowest possible tube wall temperatures. For example, the lowest convection section bank (i.e. with highest tube wall temperature) does not need to be HTC but could instead be HPSSH-2. Alternatively, DSSH can be the lowest convection section bank. The location of convection section banks as shown in FIG. 1 should be considered an illustrative and non-limitative example of the invention.

Example

[0092] The embodiments of the present invention will be better understood by looking at the following example, below.

[0093] A thermally unstable feedstock (1) consisting of plastic pyrolysis oil containing 3 wt % of solid particles having a 10 ?m mean particles diameter may be contacted with a stabilizer to avoid gums formation or buildup. The plastic pyrolysis oil stream has a final boiling point (FBP) of 420? C. 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 plastic pyrolysis oil and treatment conditions.

[0094] The resulting mixture can then be filtered through a candle filter (2) to produce (i) a solid cake that is discharged and (ii) a solids and gums depleted liquid phase (3) which may pass through at least one bed of impurities adsorbent (4) to remove polar constituents that are containing heteroatoms and metals. Suitable adsorbent beds include commercial adsorbents which may be selected through preliminary validation tests to check their adsorption capability with regards to the nature of the feedstock to be processed. When leaving adsorption section (4) the resulting impurities depleted thermally unstable feedstock (5) should contain less chlorine, less oxygen containing impurities and less metals than plastic pyrolysis oil starting material (1).

[0095] Albeit not mandatory, and depending on steam cracker technology, the impurities depleted thermally unstable feedstock (5) may be diluted with another thermally unstable feedstock e.g. a cracked naphtha or diesel or with other feedstock such as VGO, a gas oil, diesel, a naphtha, butane or any combinations thereof.

[0096] The thermally unstable feedstock under the form of the above-described solids and gums depleted liquid phase (5) can be introduced at 50? C. in the feedstock preheater (FPH) (6) bank of a steam cracker (9) at a rate of 12 t/h and 3.4 barg to produce preheated feedstock (7) at 92? C. 8.4 t/h of diluted steam (12) at 190? C. and 3.9 barg is heated in DSSH bank (13) to produce heated diluted stream (11) at 396? C. and 3.7 barg. Heated diluted steam (11) is introduced in first mixer M (10) wherein it combines with preheated feedstock (7) to produce 20.4 t/h of steam containing heated feedstock (14) at 177? C. and 3.4 barg. The latter is directly sent in vaporizer (15) to produce combined gasified feedstock and residue (16) at 268? C. and 2.9 barg, which is separated in flash drum (17) into 1.2 t/h of residue which is drawn off through residue purge line (19) and into gasified feedstock, which is directed in line (18) at 2.8 barg for further processing.

[0097] Alternatively, vaporizer (15) and flash drum (17) are combined in one single equipment by using a thin film evaporator or preferably a straight tube shell and tube heat exchanger equipped with a purge line at the bottom of the outlet plenum, as described above. When necessary, a couple of straight tube and shell evaporators are connected in parallel, wherein one is in operation while the other is being cleaned.

[0098] Vaporizer (15) is fed with a loop of 11 t/h of water at 308? C. and 126 barg (42) coming from the outlet of MPH bank (43), to produce above-mentioned stream (16) and colder water at 200? C. and 126.8 barg (44). Colder water (44) is then pumped back using pump (45) to MPH bank at 127 barg.

[0099] Gasified feedstock (19) is heated in HTC bank (20) to produce heated gasified feedstock (21) at 540? C. and 2.5 barg which is then cracked in radiant section (22) coils, wherein furnace delivers 32 t/h of flue gas at 1100? C. to HTC bank (20) within convection section (8). Heated gasified feedstock (21) is sent to radiant section (22) coils through nozzle (23). Furnace coil outlet stream (24) temperature is kept at 809? C., coil outlet pressure is 0.6 barg, while the residence time is set at 0.3 s. The TLE outlet stream (26) temperature is 363? C. and pressure 0.56 barg. Flue gas leaving HTC bank (20) at 789? C. then heat 12.8 t/h of high pressure steam (40) at 365? C. and 125.1 barg within HPSSH-2 bank (39) to generate high pressure steam (41) at 480? C. and 124.7 barg. Flue gas leaving HPSSH-2 bank (39) at 669? C. heat HPSSH-1 bank (35). High pressure steam (40) comes from second mixer M (38) and results from combination of 100 kg/h of quench water (37) at 127 barg and 116? C. with 12.7 t/h of high pressure steam (36) at 370? C. and 125.4 barg. High pressure steam (36) is delivered by HPSSH-1 bank (35) by heating of high pressure steam (34) at 329? C. and 125.6 barg coming from steam drum (28).

[0100] Flue gas leaving HPSSH-1 bank (35) at 591? C. then transfer heat to DSSH bank (13) and leaves the latter at 500? C., then transfer heat to MPH bank (43) and leaves MPH bank (43) at 350? C. Flue gas at 350? C. further transfers heat to economizer ECO bank (31) and leaves at 144? C. Economizer ECO bank (31) is fed with 21 t/h of cold boiler feed water at 116? C. and 127 barg and leaves ECO bank (31) through line (32) at 200? C. and 126 barg to feed steam drum (28). Last, flue gas at 144? C. transfers heat to feedstock preheater FPH bank (36) and finally leaves convection section through a stack at 113? C.

[0101] The radiant section contains a radiant box heated with 1.5 t/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 18.5 Gcal/h at 25? C., radiant section absorbed heat is 8.4 Gcal/h, box efficiency based on fired heat is 45%, convection duty is 8.8 Gcal/h, for an overall efficiency of 95.5%.