Mitigation of the formation of unwanted polymers in the rectification subsection of a primary fractionator of an ethylene plant

Abstract

Process to mitigate the formation of unwanted polymers in a primary fractionator of a steam cracking plant, comprising: providing a feedstream comprising steam and one or more cracked hydrocarbons, providing a primary fractionator; injecting one or more anti-polymerizing additives having a boiling temperature in the range of 145 to 240 °C into the primary fractionator while passing the feedstream into the primary fractionator; recovering tars, pyrolysis oil, and gazole from the primary fractionator; and recovering vapor-phase effluent at the top of said primary fractionator.

Description

[0001] Mitigation of the formation of unwanted polymers in the rectification subsection of a primary fractionator of an ethylene plant

[0002] Technical Field

[0003] The disclosure relates to a method to limit the formation of unwanted polymers in the primary fractionator at the exit of the cracking furnaces of an ethylene plant. More precisely it relates to a method to limit the formation of unwanted polymers in the rectification subsection of the primary fractionator with the help of anti-polymerization additives.

[0004] Technical Background

[0005] In an ethylene plant, hot cracked gases from furnaces must be cooled down (quenched) for further processing and fractionation. This cooling process takes place in the quench system, which in the case of liquid and mixed feed crackers, consists of a series of transfer line exchangers (TLX or TLE), a primary fractionator or quench oil tower, a quench oil loop, and auxiliary equipment (i.e. , filters, optional fuel oil stripper, etc.). After initial cooling (quenching) in the TLX, cracked products are fed to the distillation tower (primary fractionator or quench oil tower), which separates light products to the top (cracked gas and steam-cracked gasoline) and heavier hydrocarbons to the bottom. A portion of the bottom product is circulated via the quench oil loop back to the Transfer Lineas the quenching medium. The immense heat recovered through the quench oil system is used in heat reboilers, to preheat feeds and to produce dilution steam, which in turn is returned to the cracking furnaces aiding in overall heat recovery. The quench oil system is a column separated into two parts: in the bottom of the column, also known as the quench oil section, the cracked gases are quenched with quench oil. The final boiling point of the steam-cracked gasoline is rectified at the top of the column, also known as the rectification subsection. The rectification subsection aims to let the heavier oil in the quench oil section while maintaining the steam-cracked gasoline with a final boiling point adapted for the extraction of the BTX (i.e Benzene, Toluene, and Xylene) or its use in the gasoline pool.

[0006] Typically, two or three cuts are separated from a primary fractionator via their boiling ranges:

[0007] - At the top, there are the cracked gases that contain steam-cracked gasoline that will be condensed in the following cooling and separation step, the final boiling range of this cut is usually 200°C to 215°C. In some designs, an intermediate cut named pyrolysis gas oil is produced. This product is coming from a lateral draw-off if there is one. This product comes from the bottom of the rectification subsection and is highly concentrated in C10 compounds.

[0008] - And finally, the pyrolysis fuel oil is at the bottom.

[0009] The steam-cracked gasoline from the downstream cooling and separation step is used as a reflux at the top of the primary fractionator. The heavy part of this steam-cracked gasoline and the molecules from the partial condensation of cracked gas have a boiling point between 145°C and 240°C. This fraction of the product contains a lot of reactive molecules that can polymerize. And some of these molecules can cross-link the polymers, accelerating their precipitation and fouling.

[0010] The operation of this Primary Fractionator is complex because it is not operated like a classical distillation tower due to:

[0011] • high proportion of non-condensable, >85 wt.% (light gas and water);

[0012] • the absence of a reboiler and a condenser; and

[0013] • the fact that the quench water tower acts as a condenser and that the cracked gases from TLE bring heat and act as a reboiler.

[0014] It is known that the heavy content of quench oil is not always the same and can fluctuate. This is, in particular, due to the incorporation of quench oil of heavy components such as tars. Those heavy components can originate from the cracked gases themselves or, for a minority, they can be formed with the olefins present in the cracked gases. In particular, in the rectification subsection, olefins can recombine to form heavier components and polymers that are later found in the quench oil, increasing the viscosity of quench oil.

[0015] On the other hand, in the rectification subsection, the presence of olefins in the cracked gas leads to the formation of polymers. Part of those polymers deposit in the rectification subsection and limit the distillation efficiency. The distillation trays are blocked, and the distillation or rectification cannot be performed correctly.

[0016] In the prior art, methods of mitigating viscosity increase in quench oil are described. They involve adding specially formulated antifoulants that prevent heavy components from aggregating and depositing, thereby improving the quench oil properties.

[0017] Other prior art documents describe methods to limit the formation of unwanted polymers in the primary fractionator. The addition of those additives allows for limiting the clogging of the primary fractionator. Those additives also allow for limiting the formation of heavy components leading to an increase in the viscosity of the quench oil. In particular, US5985940 describes a method for mitigating fouling and reducing viscosity in primary fractionators and quench sections of ethylene plants by adding to a hydrocarbon stream a mono and / or a polyalkyl-substituted phenol-formaldehyde resin having a weight average molecular weight of from about 1 ,000 to about 30,000 and at least one alkyl substituent containing from about 4 to about 24 carbon atoms, which alkyl substituent may be linear or branched.

[0018] US2004015032 also describes a method for improving the operational parameters in primary fractionators which are experiencing diminished operation efficiencies due to deposits of polymerized hydrocarbon species. This document is particularly focused on adding a foamreducing amount of a foam-reducing composition at the primary fractionator.

[0019] US20150152338A1 discloses methods for improving the operational parameters in primary fractionators which are experiencing diminished operation efficiencies due to deposits of polymerized hydrocarbon species. The disclosure comprises the step of adding a foam-reducing amount of a foam-reducing composition at the primary fractionator. A reduction in foaming is achieved, whereby the operational efficiency of the process is improved based upon operation parameters including, but not limited to, liquid-gas contact ratio, product top temperature, pressure differentials, gasoline endpoint, or combinations thereof.

[0020] Finally, the prior art also describes additives that can be used to inhibit fouling and the viscosity increase in the primary fractionator.

[0021] In particular, US2004055932 describes a method of inhibiting fouling and viscosity increase in hydrocarbon streams including ethylenically unsaturated monomers. The method includes the step of adding to the hydrocarbon stream an effective amount of one or more quinone methides of the formula: wherein R1, R2, and R3are independently selected from the group consisting of H, -OH, -SH, - NH2, alkyl, cycloalkyl, heterocyclo, and aryl.

[0022] US2020010386A1 describes an additive composition for controlling and inhibiting polymerization of monomers, wherein the composition comprises a combination of (a) a phenol compound comprising catechol compound with (b1) an aliphatic tertiary amine, (b2) oxide-treated derivative of the aliphatic tertiary amine, or (b2) a mixture thereof, wherein the aliphatic tertiary amine contains one or more hydroxyl groups in the alkyl chain of the aliphatic tertiary amine. In another embodiment, the present disclosure also relates to methods for controlling and inhibiting the polymerization of monomers in a primary fractionator (or an ethylene plant), for operating a primary fractionator, and for reducing fouling and polymer deposits in a primary fractionator, and extending a run-length of a primary fractionator or of an ethylene plant.

[0023] WO2013058997A1 discloses a method of reducing the increase in viscosity and the drop in the heat transfer coefficient that commonly occurs with quenching media which is repeatedly circulated through a hot reaction vessel. The method comprises adding a preserving composition to the quenching media. The composition comprises a) a high-temperature polymerization inhibitor, b) a tar dispersant, and c) a viscosity reducer. The method allows the quenching media to remain effective longer than would otherwise be the case and thereby prevents problems associated with having to devote excessive resources for controlling heat recovery, viscosity increases, product downgrades, or having to operate equipment at temperatures beyond their optimum performance designs.

[0024] The prior art methods described lack efficiency in the rectification subsection of the primary fractionator. Indeed, the anti-polymerization additives added migrate to the bottom of the column, in particular, in the quench oil. There are however some precursors of polymers that migrate to the top of the column where polymers are formed. This leads to the formation of polymers in areas where the concentration of anti-polymerization additive is the minimum.

[0025] There is therefore still a need to improve the primary fractionation section at the exit of the steam cracker. There is still a need to limit the formation of unwanted polymers in the primary fractionator during the processing of the feedstream. In particular, there is a need to limit the formation of unwanted polymers in the primary fractionation section at the top of the column.

[0026] Summary

[0027] In a first aspect, the disclosure provides a process to mitigate the formation of unwanted polymers in a primary fractionator of a steam cracking plant, the process comprising: a) providing a feedstream comprising steam and one or more cracked hydrocarbons, wherein the feedstream is exiting a transfer line exchanger of a steam cracker or is exiting the radiant section of a steam cracker furnace; b) providing a primary fractionator constituted of one column divided into two sections fluidically connected: a quenching section and a rectification subsection wherein said quenching section is located in the lower part of the primary fractionator and the rectification subsection is in the upper part; wherein the rectification section comprises a first rectification subsection and an optional second rectification subsection; c) providing one or more anti-polymerizing additives; e) recovering a vapor-phase effluent at the top of said primary fractionator; and f) recovering tar and pyrolysis oil from a stream withdrawn from the quenching section and / or gazole from a stream withdrawn from the first rectification subsection; the process is remarkable in that at least one anti-polymerizing additive provided in step c) has a boiling temperature in the range of 145 to 240 °C and in that the one or more anti-polymerizing additives are injected, in a step d), into the rectification section of the primary fractionator while passing the feedstream into the primary fractionator.

[0028] It is understood that step d) is performed after step c).

[0029] Indeed, it has been discovered that the boiling temperature of the anti-polymerizing additives is particularly important and that with a boiling point in the range of 145 to 240°C, an antipolymerizing additive concentrates in the part of the column where most of the polymer precursors are concentrated. The concentration of the anti-polymerizing additive is therefore at its maximum where there are the most fouling polymer precursors.

[0030] In an embodiment, one or more anti-polymerizing additives provided in step c) have a boiling temperature in the range of 180°C to 220°C, preferably from 185°C to 209°C; more preferably from 190°C to 200°C. In an embodiment, the anti-polymerizing additives are divided into two types: a first type with a boiling temperature below 210°C, and a second type with a boiling temperature equal to or greater than 210°C; and the anti-polymerizing additives provided in step c) comprise at least one anti-polymerizing additives of the first type and at least one antipolymerizing additives of the second type.

[0031] In an embodiment, one or more anti-polymerizing additives provided in step c) are of a first type with a boiling temperature below 210°C, and step d) comprises injecting one or more antipolymerizing additives of the first type in at least one selected from the quenching section, the first rectification subsection, and the second rectification subsection; with preference the one or more anti-polymerizing additives of the first type have a boiling temperature in the range of from 160°C to 209°C. In an embodiment, the rectification subsection comprises a first rectification sub-section and a second rectification sub-section, wherein when the second rectification subsection is located at the top of the primary fractionator so the first rectification subsection is between said quenching section and said second rectification subsection.

[0032] In an embodiment, a stream in the bottom of the first rectification subsection is withdrawn and recycled on the top tray of said first rectification subsection, and at least 10 wt.% of one or more anti-polymerizing additive of the first type based on the total weight of the anti-polymerizing additives, is added in said stream; and / or at least 10 wt.% of one or more anti-polymerizing additive of the first type based on the total weight of the anti-polymerizing additives is added below the reflux of said stream.

[0033] In an embodiment, one or more anti-polymerizing additives provided in step c) are of a second type with a boiling temperature equal to or greater than 210°C and step d) comprises injecting one or more anti-polymerizing additives of the second type in the second rectification subsection; with preference, the one or more anti-polymerizing additives of the second type have a boiling temperature in the range of from 210 to 235°C.

[0034] In an embodiment, the vapor-phase effluent recovered at step e) is further sent to a separation unit wherein a quench water stream is separated from a hydrocarbon stream with at least a part of said hydrocarbon stream being returned to said first rectification subsection of the primary fractionator as reflux, using a second rectification reflux line, and wherein at least part of the one or more anti-polymerizing additives is added in said second rectification reflux line.

[0035] For example, the vapor-phase effluent recovered at step e) is further sent to a separation unit, wherein a quench water stream is separated from a hydrocarbon stream with at least a part of said hydrocarbon stream being returned to said first rectification subsection of the primary fractionator as reflux using a second rectification reflux line and wherein at least part of one or more anti-polymerizing additive of the first type and / or at least part of one or more antipolymerizing additive of the second type is added in said second rectification reflux line.

[0036] In an embodiment, one or more anti-polymerizing additives are chosen among compounds presenting one or more functional groups selected from a hydroxyl group, an amino group, an ester function, a carbamate function, an ether group, a formyl group, a ketone function, an acidic function, an aromatic function, a phenolic group, a benzoyl group, an acetal group, an aminoxide group, and any combination thereof; with preference, selected from a ketone function, a formyl group, a carbamate function, an acetal group or any combination thereof. In an embodiment, the one or more anti-polymerizing additives are chosen among biphenyl, terpineol, ethyl acetoacetate, tetralin, di-propylene glycol-methyl ether (DPGME), acetophenone, di ethyl amino ethanol, di propylene glycol, propylene glycol, thiophenol, or any isomer of 4-methyl benzenthiolethyl lactate, trans-stilbene, benzaldehyde, benzaldehyde dimethyl acetal, diphenyl carbonate, methyl carbamate, ethyl carbamate, methyl benzoate, ethyl benzoate, ethyl acetoacetate, diethyl aminoethanol, biphenyl, diethanolamine, 3-amino-1 -propanol, terpineol, phenol, methyl phenol with the methyl being in position 2 and / or 3 and / or 4, amino phenol with the methyl being in position 2 and / or 3 and / or 4, methyl amino phenol with the methyl being in position 2 and / or 3 and / or 4, phenyl ethanol, any combination thereof; with preference, the one or more anti-polymerizing additives are or comprise benzaldehyde, acetophenone, or any mixture thereof.

[0037] With preference, the one or more anti-polymerizing additives are or comprise at least one selected from acetophenone, ethyl carbamate, benzaldehyde, methyl benzoate, di propylene glycol, and propylene glycol; with preference, the one or more anti-polymerizing additives are or comprise di propylene glycol and / or propylene glycol.

[0038] In an embodiment, step c) further comprises providing at least one polymerization inhibitor having a boiling point ranging from 195°C to 500°C, preferably said polymerization inhibitor is selected from quinone methides, dinitrophenols like for instance di-nitro-ortho-cresol (DNOC) or di-nitro- sec-butyl-phenol (DNBP), TEMPO compounds like for instance 4- hydroxy-2, 2,6,6- tetramethylpiperidine 1-oxyl, or (2,2,6,6-tetramethylpiperidin-1-yl)oxyl or (2, 2,6,6- tetramethylpiperidin-1-yl)oxidanyl, oximes, or any combination thereof.

[0039] In an embodiment, at least one anti-polymerizing additive is added in said steam-cracked gasoline at a concentration of 0.1 to 1000 ppm wt., preferably 1 to 100 ppm wt., even more preferably 10 to 50 ppm wt. based on the total weight of the feedstream.

[0040] In a second aspect, the disclosure provides the use in a process according to the first aspect, of one or more anti-polymerizing additives selected from biphenyl, terpineol, ethyl acetoacetate, tetralin, di-propylene glycol-methyl ether (DPGME), acetophenone, di ethyl amino ethanol, ethyl carbamate, benzaldehyde, methyl benzoate, di propylene glycol, propylene glycol.

[0041] In a third aspect, the disclosure provides an installation for carrying the process according to the first aspect, said installation being remarkable in that it comprises: a steam cracker connected to a transfer line exchanger, and fluidically connected by a transfer line to a primary fractionator ; a primary fractionator constituted of one column divided into at least two sections fluidically connected: a quenching section and a rectification section, wherein said quenching section is located in the lower part of the primary fractionator and the rectification section is in the upper part; wherein the rectification section comprises a first rectification subsection and an optional second rectification sub-section; one or more reflux lines selected from a quenching reflux line and a first rectification reflux line, wherein the quenching reflux line is located in the bottom of said primary fractionator, and withdraws and recycles a stream on the top tray of said quenching section before said first rectification subsection, and wherein the first rectification reflux line is arranged to withdraw and recycle a stream from the bottom of the first rectification subsection to the top tray of said first rectification subsection, before the second rectification subsection when present; a separation unit that is fluidically connected with said primary fractionator to recover the vapor phase effluent of said primary fractionator, and a second rectification reflux line that is fluidically connected to said separation unit to send back to the primary fractionator part of the effluent of said separation unit; the separation unit comprising a quench water tower and a separator drum wherein the separator drum is placed downstream of the quench water tower; wherein said steam cracker is fluidically connected to said primary fractionator; and wherein it comprises at least one additive injection nozzle in the first rectification subsection, and / or in the second rectification subsection when present.

[0042] In a preferred embodiment, the installation further comprises at least one additive injection nozzle in the quenching section.

[0043] In an embodiment, the installation further comprises means to add anti-polymerizing additives in one or more reflux lines selected from the quenching reflux line, the first rectification reflux line, and the second rectification reflux line.

[0044] Definition

[0045] The term DVB stands for divinyl benzene. The term PF stands for primary fractionator. The term TLE (or TLX) stands for transfer line exchanger. The term SCGO stands for steam-cracked gasoil. The term SCG stands for steam-cracked gasoline.

[0046] The term “anti-polymerization”, relating to the function of the additive having an “antipolymerization function”, refers to a chemical function able to inhibit the polymerization of monomers placed under polymerization conditions. The anti-polymerization additive refers to an additive able to inhibit the polymerization of monomers placed under a polymerization reaction. Namely, an anti-polymerization additive is an additive able to reduce the polymerization of monomers placed under polymerization conditions if present at a concentration of at least 1 wt. % in a stream of monomers. Reducing the polymerization refers to a decrease in the mass of the polymer formed of at least 10 wt. % compared with the weight fraction of the polymer formed under the same conditions but without the presence of the anti-polymerization additive.

[0047] The terms “olefin” or “alkene” as used herein relate to an unsaturated hydrocarbon compound containing at least one carbon-carbon double bond. They are sometimes referred to by the symbol “HC=”.

[0048] The term “hydrocarbon” refers to the alkanes (saturated hydrocarbons), cycloalkanes, aromatics, and unsaturated hydrocarbons together.

[0049] As used herein, the terms “C# alcohols”, “C# alkenes”, or “C# hydrocarbons”, wherein “#” is a positive integer, are meant to describe respectively all alcohols, alkenes or hydrocarbons having # carbon atoms. Moreover, the term “C#+ alcohols”, “C#+ alkenes”, or “C#+ hydrocarbons”, is meant to describe all alcohol molecules, alkene molecules, or hydrocarbon molecules having # or more carbon atoms. Accordingly, the expression “C5+ alcohols” is meant to describe a mixture of alcohols having 5 or more carbon atoms.

[0050] The terms "comprising", "comprises" and "comprised of as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements, or method steps. The terms "comprising", "comprises" and "comprised of also include the term “consisting of.

[0051] The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the recited endpoint values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0052] The terms "wt.%", "vol.%", or "mol.%" refer to the weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of the component in 100 grams of the material is 10 wt. % of components. The term “naphtha” refers to the general definition used in the oil and gas industry. In particular, it refers to a hydrocarbon originating from crude oil distillation having a boiling range from 15 to 250°C as measured by ASTM D2887. Naphtha contains substantially no olefins as the hydrocarbons originate from crude oil. It is generally considered that a naphtha has a carbon number between C3 and C11 , although the carbon number can reach in some cases C15. It is also generally admitted that the density of naphtha ranges from 0.65 to 0.77 g / mL.

[0053] The particular features, structures, characteristics, or embodiments may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments.

[0054] Figures

[0055] Figure 1 represents a steam cracking installation together with the primary fractionator and the quench water. The first rectification subsection is the location wherein the fouling was found to be the most important.

[0056] Figure 2 represents the excess pressure drop of the primary fractionator according to example 2.

[0057] Figures 3A to 3D are pictures of the various trays of a Primary Fractionator according to example 1 . Figure 3 A is a picture of the first tray. Figure 3 B is a picture of the sixth tray. Figure 3 C is a picture of the tenth tray. Figure 3 D is the picture of the twelfth tray.

[0058] Figure 4 represents the concentration profile of DVB (divinylbenzene) in the first rectification subsection of the primary fractionator. Tray number 1 is at the top, tray number 4 is at the bottom of the first rectification subsection, and tray 0 is at the top reflux. These numbers of trays are theoretical trays: as physical trays have a partial efficiency, their number is higher. The thick line corresponds to a PGC suction pressure at 0.5 bar(g) and the thin line to a PGC suction pressure at 0.7 bar(g).

[0059] Figure 5 represents the impact of the additive in the formation of unwanted polymers according to example 3.

[0060] Detailed description of the disclosure

[0061] The disclosure relates to a process to mitigate the formation of unwanted polymers in a primary fractionator of a steam cracking plant as well as an installation for carrying out such a process.

[0062] The installation

[0063] The disclosure relates to an installation 1 for carrying out the process according to the disclosure, comprising: a steam cracker connected to a transfer line exchanger; and fluidically connected by a Transfer Line 27 to a primary fractionator (3); a primary fractionator 3 constituted of one column divided into two sections fluidically connected: a quenching section 5, and a rectification section 7 wherein said quenching section 5 is located in the lower part of the primary fractionator and the rectification section 7 is in the upper part; wherein the rectification section 7 comprises a first rectification subsection 9 and an optional second rectification sub-section 11 ; one or more reflux lines selected from a quenching reflux line 13 and a first rectification reflux line 15 wherein the quenching reflux line 13 located in the bottom of said primary fractionator withdraws and recycles a stream 31 from the primary fractionator on the top tray of said quenching section 5 before said first rectification subsection 9 and wherein the first rectification reflux line 15 is arranged to withdraw and recycle a stream 33 from the bottom of the first rectification subsection 9 to the top tray of said first rectification subsection 9 before the second rectification subsection 11 when present; a separation unit 17 that is fluidically connected with said primary fractionator 3 by a line arranged to recover the vapor-phase effluent 19 of said primary fractionator 3 and a second rectification reflux line 21 that is fluidically connected to said separation unit 17 to send back to the primary fractionator 3 part of the effluent of said separation unit 17; the separation unit 17 comprising a quench water tower 23 and a separator drum 25 wherein the separator drum 25 is placed downstream of the quench water tower 23; wherein said steam cracker is fluidically connected to said primary fractionator 3; and wherein it comprises at least one additive injection nozzle in the first rectification subsection 9 and / or in the second rectification subsection 11 when present. With preference, it further comprises at least one additive injection nozzle in the quenching section 5.

[0064] In an embodiment, it further comprises means to add anti-polymerizing additives in one or more reflux lines selected from the quenching reflux line 13, the first rectification reflux line 15, and the second rectification reflux line 21.

[0065] Primary fractionator

[0066] The primary fractionator 3 is sometimes named quench oil tower. It is the first tower after the transfer line exchanger (TLE). The main functions of the primary fractionator 3 are to cool the cracked gas from the TLE while recovering heat from the cracked gas via quench oil circulation, to separate the heavier components from the cracked gas, and to adjust the final boiling point of the steam-cracked gasoline. The disclosure is particularly adapted for steam cracker cracking naphtha because naphtha crackers generally have a quench oil tower; whereas ethane crackers do not necessarily have a quench tower. In some cases, revamping a naphtha cracker into an ethane cracker implies that the ethane cracker has a quench oil tower. It is however not always the case.

[0067] A typical primary fractionator is shown in Figure 1. A hydrocarbon feed is conducted via inlet line 35 to a steam cracker furnace 37 having two main sections: a convection section 39 and a radiant section 41. The hydrocarbon feed is introduced into convection coils located in the furnace's convection section 39. Steam is introduced into the convection coils via line 43. Inlet line 35 and associated feed-handling equipment are adapted so that the hydrocarbon feed can comprise Light Hydrocarbon (typically in the vapor phase) and / or Heavy Hydrocarbon (typically in the liquid phase).

[0068] The hydrocarbon feed is heated and vaporized in the convection section 39, e.g., by indirect contact with hot flue gas from the radiant section 41 and by direct contact with the steam introduced into the convection coils via line 43. The mixture of steam and vaporized hydrocarbon feed (the steam cracker feed) is typically preheated in convection coils located proximate to the lower end of the steam cracker furnace's convection section 39. The preheated steam cracker feed is transferred, typically using cross-over piping (not shown), from the outlet of the convection tubes to the inlet of radiant tubes located in radiant section 41 .

[0069] The pre-heated steam cracker feed is introduced into the radiant section 41 , where at least a portion of the steam cracker feed's hydrocarbon is pyrolyzed to produce C2+ olefin. The steam cracker feed is typically in the vapor phase at the inlet of the radiant coils, e.g., >90 wt. % of the steam cracker feed is in the vapor phase, such as > 95 wt. %, or > 99 wt. %. The steam cracker feed in the radiant coils is exposed to a temperature > 400° C to convert at least a portion of the feed's hydrocarbon molecules to C2+ olefins by pyrolysis. Suitable pyrolysis conditions in the radiant section include, e.g., exposing the steam cracker feed to a temperature (measured at the outlet of the radiant coils) > 400° C, e.g., in the range of 400° C to 900° C, and a pressure > 0.1 bar (absolute), for a residence time in the range of from about 0.01 second to 5.0 seconds. For example, the steam cracking conditions can include one or more of (i) a temperature >760° C, e.g., in the range of about 760° C to about 880° C; (ii) a pressure > 0.5 bar (absolute), e.g., in the range of from about 1 .0 to about 5.0 bar, such as in the range of from about 1 .1 to about 2.5 bar; or (iii) a residence time in the range of from about 0.10 to about 2.0 seconds. The furnace's radiant section's effluent (i.e. the feedstream 27) is conducted away via a line and generally comprises unconverted steam cracker feed and pyrolysis products. The pyrolysis products generally include the C2+ olefin, molecular hydrogen, acetylene, aromatic hydrocarbon, saturated hydrocarbon, C3+ diolefin, and typically one or more of aldehyde, acidic gases such as H2S and / or CO2, and mercaptan.

[0070] The furnace's radiant section's effluent (i.e. the feedstream 27) can be quenched (e.g., by contacting with a quench oil boiling in the SCGO boiling range) in a quenching zone (not shown) upstream of primary fractionator s. Conventional primary fractionators and associated equipment can be used, e.g., those described in U.S. Pat. No. 8,083,931. Additional stages for removing heat (such as one or more transfer line heat exchangers) and removing tar (such as tar drums) can be located in or upstream of primary fractionator 3 if desired.

[0071] The primary fractionator 3 includes two sections, as shown in Figure 1 : a quenching section 5 located proximate to the lower end of the primary fractionator 3 and a rectification section 7. The rectification section 7 comprises a first rectification subsection 9 located above the quenching section 5, and an optional second rectification subsection 11 located above the first rectification subsection 9. Steam cracker tar 29 is withdrawn from primary fractionator 3 via a quenching reflux line 13. A quenching reflux line 13 (such as a pump-around loop) withdraws a stream 31 (i.e., the SCGO boiling-range oil) proximate to the lower end of the primary fractionator's quenching section 5 and recycles at least a part of said stream at a location that is typically proximate to the top tray (i.e., the upper end) of the quenching section 5 but below the primary fractionator's first rectification subsection 9.

[0072] Quenching the vapor-phase effluent condenses at least a portion of Steam-Cracked Gasoline (i.e hydrocarbons) present in the vapor-phase effluent. Hydrocarbons and water (i.e. the vapor phase effluents 19 of said primary fractionator 3) are withdrawn from a location proximate to the top of the primary fractionator's second rectification subsection via line 45. The vapor-phase effluent 19 is condensed in the quench water tower 23 and the condensed stream 47 is sent to a separator drum 25 wherein hydrocarbons are separated from water 53. At least a part of the hydrocarbon stream 49 is returned to the first rectification subsection 9 of the primary fractionator 3 as reflux through the second rectification reflux line 21 via pump inlet line. For example, a stream 51 being a part of the hydrocarbon stream 49 is withdrawn and directed to a downstream Steam-Cracked Gasoline treatment unit (not shown). In an embodiment, the installation comprises a valve to maintain the desired amount of Steam-Cracked Gasoline reflux (i.e. the hydrocarbon stream 49) volumetric flow rate. Quenched effluent, primarily in the vapor phase, is conducted via line 45 to a separation unit 17 comprising a quench water tower 23 and a separator drum 25 wherein the separator drum 25 is placed downstream of the quench water tower 23. A line withdraws the bottom effluent (i.e. the condensed stream 47) of the quench water tower 23 and conducts it to the separator drum 25 wherein any remaining water 53 is separated and discharged. The separator drum 25 is preferably a decanter.

[0073] Unwanted polymer formation

[0074] During the steam cracking of naphtha, not only ethylene, propylene, and butadiene are produced. Other monomers such as styrene, indene, di-vinyl benzene, alpha-methyl styrene, and other higher ring compounds or monomers, can be produced. Those compounds are prompt to polymerize in the primary fractionator 3. The polymers hence formed are very heavy and also cross-linked. This was found particularly true when divinyl benzene is present. The unwanted polymers get deposited at a very rapid rate on the tray surfaces. It is in the part of the column where the compounds such as styrene (boiling temperature 145°C), indene (boiling temperature 183°C), di-vinyl benzene (boiling temperature 195°C), alpha-methyl styrene (boiling temperature 166°C) accumulate where there is the most formation of unwanted polymers. The deposition of the unwanted polymers leads to fouling and to an increase in ‘column pressure drop’ along with a reduction in ‘fractionation efficiency’ of the column.

[0075] The process and the additives

[0076] The disclosure relates to a process to mitigate the formation of unwanted polymers in a primary fractionator of a steam cracking plant, the process comprising: a) providing a feedstream 27 comprising steam and one or more cracked hydrocarbons, wherein the feedstream is exiting a transfer line exchanger of a steam cracker or is exiting the radiant section 41 of a steam cracker furnace 37; b) providing a primary fractionator 3 constituted of one column divided into two sections fluidically connected: a quenching section 5 and a rectification section 7 wherein said quenching section 5 is located in the lower part of the primary fractionator 3 and the rectification section 7 is in the upper part; c) providing one or more anti-polymerizing additives having a boiling temperature in the range of 145 to 240 °C; d) injecting the one or more anti-polymerizing additives into the rectification section 7 of the primary fractionator 3 while passing the feedstream 27 into the primary fractionator 3; e) recovering a vapor-phase effluent 19 at the top of said primary fractionator 3; and f) recovering tar 29 and pyrolysis oil from a stream 31 withdrawn from the quenching section

[0077] 5 and / or gazole 55 from a stream 33 withdrawn from the first rectification subsection 9.

[0078] In a preferred embodiment, the rectification subsection 7 comprises a first rectification sub-section 9 and a second rectification sub-section 11 , wherein when the second rectification subsection 11 is located at the top of the primary fractionator 3 so the first rectification subsection 9 is between said quenching section 5 and said second rectification subsection 11 .

[0079] The process of the disclosure is also further remarkable in that said at least one anti-polymerizing additive is added at a concentration of 0.1 to 1000 ppm wt., preferably 1 to 100 ppm wt., even more preferably 10 to 50 ppm wt. based on the total weight of the feedstream 27.

[0080] In a preferred embodiment said at least one anti-polymerizing additive, and optionally at least one polymerization inhibitor, is added in said first rectification subsection 9 and / or in said second rectification subsection 11 when present.

[0081] In other words, wherein the said second rectification subsection 11 is present, said at least one anti-polymerizing additive, and optionally at least one polymerization inhibitor, can be added in said second rectification subsection 11 or in said first rectification subsection 9 or in both said second rectification subsection 11 and said first rectification subsection 9.

[0082] In a preferred embodiment, a stream 31 in the bottom of said primary fractionator 3 (i.e. in the bottom of the quenching section 5) is withdrawn and recycled on the top tray of said quenching section 5 before said first rectification subsection 9, and wherein said at least one antipolymerizing additive is added in said stream 31 or wherein at least part of said at least one antipolymerizing additive is added below the quenching reflux line 13.

[0083] In a preferred embodiment, said vapor-phase effluent 19 recovered at step e) is further sent to a separation unit 17 wherein a quench water stream 53 is separated from an hydrocarbon stream 49 with at least a part of said hydrocarbon stream 49 being returned to said first rectification subsection 9 of the primary fractionator 3 as reflux to a second rectification reflux line 21 and wherein at least part of said at least additive is added in said second rectification reflux line 21 .

[0084] In a preferred embodiment, one or more anti-polymerizing additives provided in step c) have a boiling temperature in the range of 180°C to 220°C, preferably from 190°C to 200°C.

[0085] It has also been discovered that divinylbenzene (DVB) is one of the polymer precursors present in the heavy part of the steam-cracked gasoline. DVB has a boiling point of 195°C and concentrates in a concentration bulge located in the lower part of the first rectification subsection 9. Having one or more anti-polymerizing additives with a boiling point that is close to the one of DVB allows to specifically target a reduction of polymerization from DVB.

[0086] It has also been found that the anti-polymerizing additives can be divided into two types: a first type with a boiling temperature below 210°C, and a second type with a boiling temperature equal to or greater than 210°C.

[0087] In a preferred embodiment, the anti-polymerizing additives provided in step c) comprise at least one anti-polymerizing additive of the first type and at least one anti-polymerizing additive of the second type. Indeed, it has been found that the use of anti-polymerizing additives with different boiling points is beneficial. With preference, the at least one anti-polymerizing additive of the first type is injected in a location different from the at least one anti-polymerizing additive of the second type.

[0088] In a preferred embodiment, one or more anti-polymerizing additives provided in step c) are of a first type with a boiling temperature below 210°C. For example, one or more anti-polymerizing additives provided in step c) are of a first type with a boiling temperature ranging from 145 to below 210°C; preferably, from 160°C to 209°C; more preferably, from 170°C to 205°C; and most preferably from 175°C to 202°C.

[0089] In an embodiment, the one or more anti-polymerizing additives provided in step c) are of a first type with a boiling temperature below 210°C, and step d) comprises injecting one or more antipolymerizing additives of the first type in the second rectification subsection 11 of the primary fractionator and / or in the first rectification subsection 9.

[0090] Indeed, it was found advantageous that anti-polymerizing additives of a first type are injected in the primary fractionator and / or in the second rectification subsection 11 to limit the polymerization in this subsection and decrease the polymer molecular weight in the bottom of the rectification section.

[0091] In an embodiment, a hydrocarbon stream 49 is withdrawn from the separation unit 17 and recycled in the first rectification subsection 9 (below said second rectification subsection 11 when present) and at least 10 wt.% of one or more anti-polymerizing additive of the first type based on the total weight of the anti-polymerizing additives, is added in said hydrocarbon stream 49.

[0092] In an embodiment, a hydrocarbon stream 49 is withdrawn from the separation unit 17 and recycled in the first rectification subsection 9 (below said second rectification subsection 11 when present) Y1 and at least 10 wt.% of one or more anti-polymerizing additive of the first type based on the total weight of the anti-polymerizing additives, is added below the inlet of the second rectification reflux line 21.

[0093] In a preferred embodiment, one or more anti-polymerizing additives provided in step c) are of a second type with a boiling temperature equal to or greater than 210°C. For example, one or more anti-polymerizing additives provided in step c) are of the second type with a boiling temperature ranging from 210°C to below 240°C; preferably, from 210 to 239 °C or from 215°C to 236°C; more preferably, from 220°C to 235°C; and most preferably from 225°C to 233°C.

[0094] In an embodiment, one or more anti-polymerizing additives provided in step c) are of a second type with a boiling temperature equal to or greater than 210°C and step d) comprises injecting one or more anti-polymerizing additives of the second type in the second rectification reflux line 21.

[0095] For example, a N1-(4-Methylpentan-2-yl)-N4-phenylbenzene-1 ,4-diamine is an anti-polymerizing additives of the second type.

[0096] In an embodiment, the vapor-phase effluent 19 recovered at step e) is further sent to a separation unit 17 wherein quench water stream 53 is separated from a hydrocarbon stream 49 with said hydrocarbon stream 49 being returned to said first rectification subsection 9 of the primary fractionator 3 as reflux using a second rectification reflux line 21 and at least part of one or more anti-polymerizing additives is added in the second rectification reflux line 21 . For example, at least part of one or more anti-polymerizing additives of the first type and / or at least part of one or more anti-polymerizing additives of the second type is added in the second rectification reflux line 21.

[0097] The below considerations are true whether one or more anti-polymerizing additives are from the first or the second type:

[0098] Said at least one anti-polymerizing additive can have an anti-polymerization function obtained by one or more functional groups. Therefore, it is preferred that one or more anti-polymerizing additives are among compounds presenting one or more functional groups selected from a hydroxyl group (-OH), an amino group (-NH2), an ester function (R-C(=O)-O-R', where R stands for any group (typically hydrogen or organyl) and R' stands for organyl group), a carbamate function (R2NC(O)OR), an ether group (R-O-R', where R and R' represent the alkyl or aryl groups), a formyl group (R-CH=O), a ketone function (R-C(=O)-R', where R and R’ stand individually for any organyl group), an acidic function, an aromatic function, a phenolic group, a benzoyl group, an acetal function (R2C(OR')2, where R and R' stand individually for hydrogen or an organyl group), an aminoxide group (-NO), and any combination thereof.

[0099] In a preferred embodiment, said at least one anti-polymerizing additive is chosen among compounds presenting one or more functional groups selected from an amino group, an aminoxide, a ketone function, a formyl group, a carbamate function, an acetal group, and any combination thereof.

[0100] Thus at least one anti-polymerizing additive is selected from amines, alcohols, alkanolamines, labile C-C, esters, carbamates, ether, aldehydes, ketones, acids, acetates, aromatics, phenolic compounds, benzoates, labile hydrogen, and combinations thereof. With preference, at least one anti-polymerizing additive is selected from amines, alcohols, alkanolamines, esters, carbamates, ether, aldehydes, ketones, acids, acetates, aromatics, phenolic compounds, benzoates, and combinations thereof.

[0101] Said anti-polymerization function should be active to inhibit the crosslinking of diene compounds such as divinyl benzene (DVB). Depending on their specific structure, said additives interact with the polymerization reactions. The specific molecules of said additives are selected regarding their availability to react with free radicals involved in the polymerization process or oxidation leading to the polymerization process. The effect of the addition of such an additive is a decrease in the fouling kinetic due to a decrease in the polymerization kinetic or in the molecular weight of the polymers formed.

[0102] In an embodiment, one or more anti-polymerizing additives are chosen among biphenyl, terpineol, ethyl acetoacetate, tetralin, di-propylene glycol-methyl ether (DPGME), acetophenone, di ethyl amino ethanol, di propylene glycol, propylene glycol, thiophenol, or any isomer of 4-methyl benzenthiolethyl lactate, tetralin, trans stilbene, benzaldehyde, benzaldehyde dimethyl acetal, diphenyl carbonate, methyl carbamate, ethyl carbamate, methyl benzoate, ethyl benzoate, ethylacetoacetate, diethylaminoethanol, biphenyl, diethanolamine, 3-amino-1 -propanol, terpineol, phenol, methyl phenol with the methyl being in position 2 and / or 3 and / or 4, amino phenol with the methyl being in position 2 and / or 3 and / or 4, methyl amino phenol with the methyl being in position 2 and / or 3 and / or 4, phenylethanol such as 1 -phenylethanol, or any combination thereof.

[0103] With preference, the one or more anti-polymerizing additives are or comprise at least one selected from biphenyl, terpineol, ethyl acetoacetate, tetralin, di-propylene glycol-methyl ether (DPGME), acetophenone, di ethyl amino ethanol, ethyl carbamate, benzaldehyde, methyl benzoate, di propylene glycol, propylene glycol. More preferably, the one or more anti-polymerizing additives are or comprise at least one selected from acetophenone, ethyl carbamate, benzaldehyde, methyl benzoate, di propylene glycol, and propylene glycol. Even more preferably, one or more antipolymerizing additives are or comprise di propylene glycol, propylene glycol, or any mixture thereof; and / or at least one anti-polymerizing additive is or comprises benzaldehyde acetophenone, or any mixture thereof.

[0104] For example, the anti-polymerizing additives provided in step c) comprise at least one anti-polymerizing additives of the first type selected from propylene glycol, benzaldehyde, acetophenone, or any mixture thereof; and at least one anti-polymerizing additives of the second type being di propylene glycol.

[0105] Without wishing to be limited by theory, the one or more anti-polymerizing additives may be active to inhibit DVB crosslinking by consuming DVB (e.g., causing incorporation thereof into a soluble polymer or otherwise reacting with DVB), preventing its incorporation into radical polymerization. For example, benzaldehyde may react with DVB and thus effectively remove it from the process stream, and alkyl benzoates appear to suppress DVB reactivity by another mechanism. Addition reactions including, but not limited to, the Diels-Alder Reaction may occur to remove DVB from the process. Another possible mechanism is the inhibition of the polymerization reaction, the antipolymerization action of the additive. By anti-polymerization action, it is meant that the monomers being in conditions to polymerize are not polymerized when a compound having an antipolymerization action is present.

[0106] The herein-disclosed additive may comprise more than one chemical component active to inhibit DVB crosslinking and having a boiling point near that of DVB. For example, in embodiments, the additive comprises one such chemical component having a boiling point below that of DVB (i.e., less than 195°C), and another such chemical component having a boiling point above that of DVB (i.e., greater than 195°C). Such an additive mixture may be operable to span vapor pressure regions and thus provide broad coverage for retarding DVB reactivity. Such mixtures containing species having boiling points that are higher and lower than DVB may, for example, provide broad coverage in the condensation areas and distillation columns. For instance, the distillation range of the additive mixture can range between 145 to 250°C preferably from 175 to 220°C.

[0107] An example of such a mixture comprises benzaldehyde and N1-(4-Methylpentan-2-yl)-N4- phenylbenzene-1 ,4-diamine (CAS 793-24-8). Another example of such a mixture comprises benzaldehyde and 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (CAS No.: 2226-96-2). In a preferred embodiment, step c) further comprises providing a polymerization inhibitor, and step d) comprises injecting the one or more polymerization inhibitors into the primary fractionator while passing the feedstream into the primary fractionator.

[0108] The additives may further enhance overall performance when used with a conventional polymerization inhibitor. In embodiments, step c) further comprises providing a polymerization inhibitor having a boiling point greater than 195°C such as ranging from 195°C to 500°C; with preference, greater than 200°C, greater than 250°C, or greater than 300°C and / or at most 500°C. Such polymerization inhibitors include, without limitation, those comprising quinone methides (e.g., 4-benzylidene-2,6-di-tert-butylcyclohexa-2,5-diene-1-one (CAS 7078-98-0)), dinitrophenols (e.g., DNOC (di-nitro-ortho-cresol) or DNBP (di-nitro-sec-butyl-phenol)), TEMPO compounds (e.g., 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl), oximes (e.g., alternative retarder), or any combination thereof. Polymerization inhibitors are commercially available.

[0109] As it is understood from the above, the cracked gas exiting the cracker is cooled in two steps:

[0110] In a first step, by quenching the hot cracked gas is performed with quench oil in the quenching section 5 of the primary fractionator. This step cools down the cracked gas to a temperature in the range of 130°C to 150°C.

[0111] In a second step, by the vaporization of the gasoline reflux in the first and second rectification subsections (9, 11) of the Primary Fractionator. This cools the cracked gas down to ~100°C.

[0112] The heaviest fraction of the effluent (Pyrolysis Fuel Oil or PFO) is drawn off from the bottom of the quenching section 5 by the quenching reflux line 13. The bottom temperature is in the range of 120°C to 220°C depending on the installation design.

[0113] In the Quench Oil loop formed by the first rectification reflux line 15, the stream is cooled down to a temperature between 70°C and 130°C.

[0114] The operating pressure of the Primary Fractionator is between 0.6 barg - 1 barg depending of the Cracked Gas Compressor (CGC) suction pressure.

[0115] The process according to the disclosure is also further remarkable in that steam-cracked gasoline (SCG) circulates in both the first rectification subsection 9 and the second rectification subsection 11 and in that said at least one anti-polymerizing additive is added in the steam-cracked gasoline before entering the first rectification subsection 9 and before entering the second rectification subsection 11 . For instance, said steam-cracked gasoline circulates in said first rectification subsection 9, and said at least one anti-polymerizing additive is added in the steam-cracked gasoline before entering said first rectification subsection 9.

[0116] There is a combined effect of the temperature range in the first and second rectification subsections (9, 11) and of the concentration of the additives in the bulge of DVB in the column with the maximum concentration of DVB. Thanks to the choice of additives, the local concentration of additives is the highest where the need for the additives is also the highest. Usually additives efficiency is limited because their concentration is much lower than reactive monomers concentration. In the case of the disclosure, a high concentration of additives favours the reaction between them and the active free radicals. The free radicals are then no more available for polymerization and the unwanted polymerization is decreased.

[0117] Examples

[0118] Example 1

[0119] The primary fractionator from plant A had to stop due to a sharp pressure drop increase. During operation, the pressure drop of the tower is continuously monitored. This pressure drop is the main indication of fouling. As operating conditions and throughput fluctuate the pressure drop is compared to the tower model. Figure 2 shows the excess pressure drop increase over time before the operations needed to be stopped. During the stop for cleaning, this primary fractionator column was opened and inspected. This tower has 12 trays in the first rectification subsection. Tray numbers are from 1 (top tray) to 12 (bottom tray of the first rectification subsection). The overhead of this Primary fractionator temperature is 106°C on average and the temperature below the first rectification subsection is 136°C. It appeared that the column trays at the top of the column were relatively clean compared with the lower trays. Tray fouling was more and more important from the top to lower trays. The picture 3a shows the first tray, the picture 3b shows the sixth tray, the picture 3c shows the tenth tray and picture 3d shows the twelfth tray. This example illustrates that the usual additives used (here N (1 ,3 dimethyl butyl) N' phenyl p phenylene diamine - CAS 793-24-8) in the first rectification subsection of the primary fractionator are not active enough in the lower part of the first rectification subsection of the column. Additionally, fouling of the column mainly appears at the bottom of the first rectification subsection of the column.

[0120] It is therefore required to identify additives that will be efficient in this part of the column.

[0121] Example 2 - Simulation of the presence of DVB in the first rectification subsection of the primary fractionator. This example demonstrates that polymerization precursors are mainly located in a specific place of the first rectification subsection of the primary fractionator.

[0122] Pro II simulations of a primary fractionator located downstream of an ethylene plant were performed. This unit was particularly followed because of the gradual increase in pressure drop observed over time. The pressure drop increase is linked with many parameters such as the concentration of polymers precursors or fouling precursors. It is known that the main fouling precursor are styrenes, indenes and di vinyl benzene (DVB). However, DVB is the most important and strongest fouling precursor due to its high impact on polymer precipitation. With its structure containing two olefinic groups, DVB is a very efficient cross-linking agent decreasing the molecular weight of polymer precipitation. So, the concentration profile of DVB was there particularly studied inside the first rectification subsection of the column. Due to refluxing liquid hydrocarbons in the tower, the fouling location is usually in the lower part of the concentration bulge of the most impacting molecules.

[0123] The result of the simulation for ethane cracking under two different operating conditions is displayed in Figure 4. It appears that the concentration of DVB is maximum in the middle of the first rectification subsection of the column. This is due to the ebullition temperature of DVB being in the range of 195°C.

[0124] This simulation confirms the link between fouling and DVB concentration bulge location: fouling appears in the middle of the first rectification of the column and lowers the concentration bulge of DVB. Anti-polymerization additives shall therefore be chosen among compounds that will stay in this part of the column. Ideally, the anti-polymerization additive should have the same boiling temperature as the DVB in order to concentrate in the same area in the first rectification subsection.

[0125] Therefore, the boiling point of the additives was used to identify such additives.

[0126] Boiling point of anti-polymerization additives and their efficiency as additives.

[0127] To determine the best additives for limiting polymerization in the first rectification subsection, various products were tested.

[0128] The reference example comprises no additive (untreated).

[0129] In a batch of 0,5 litre of crude styrene, 1000 ppm wt. of DVB and 1000 ppm of the additives were added. The mixture was put at 120°C for 30 min. The mixture is then cooled down at room temperature and the content of DVB is measured. The mass of the polymer formed is also weighted. The results obtained are shown in Figure 5.

[0130] The highest is the change in DVB concentration in combination with a lower polymer formation compared to the reference, the highest has the component the ability to limit unwanted polymerization. This is indeed due to the reaction of the additive with the DVB and / or with the growing polymer. The best result is obtained with benzaldehyde. Other additives like ethyl acetoacetate, acetophenone, or tetralin lead also to significant changes in the DVB concentration.

[0131] On the other hand, the variation of the polymer growth can either be due to the styrene polymerizing itself or to the reaction of DVB with styrene. It appears that for all the cases, the polymer growths stay relatively small. This indicates that the additives used always have some anti-polymerization effect. From Figure 5 it can be seen that the best results in the variation of the polymer growth are obtained with ethyl carbamate, methyl benzoate, benzaldehyde, di propylene glycol, and propylene glycol.

[0132] The boiling points of the various additives used are presented in the following table. The additives studied have a boiling point in the range of DVB. They will therefore be in the same area of the column.

[0133] Table 1 : boiling point of the various additives studied This example illustrates that the choice of additives with anti-polymerization activities suitable for the present disclosure shall meet the following two criteria:

[0134] First, it should have an anti-polymerization function. Anti-polymerization functions include a chemical function enabling the termination of the radical polymerization reaction and / or a function able to slow down the radical polymerization i.e. inducing an equilibrated reaction during the radical polymerization reaction.

[0135] Second, it should have a boiling temperature in the same range as the boiling temperature of the polymerization precursors such as DVB.

Claims

Claims1 . Process to mitigate the formation of unwanted polymers in a primary fractionator of a steam cracking plant, the process comprising: a) providing a feedstream (27) comprising steam and one or more cracked hydrocarbons, wherein the feedstream (27) is exiting a transfer line exchanger of a steam cracker or is exiting the radiant section (41) of a steam cracker furnace (37); b) providing a primary fractionator (3) constituted of one column divided into two sections fluidically connected: a quenching section (5) and a rectification section (7) wherein said quenching section (5) is located in the lower part of the primary fractionator (3) and the rectification section (7) is in the upper part; wherein the rectification section (7) comprises a first rectification subsection (9) and an optional second rectification subsection (11); c) providing one or more anti-polymerizing additives; e) recovering a vapor-phase effluent (19) at the top of said primary fractionator (3); and f) recovering tar (29) and pyrolysis oil from a stream (31) withdrawn from the quenching section (5) and / or gazole (55) from a stream (33) withdrawn from the first rectification subsection (9); characterized in that at least one anti-polymerizing additive provided in step c) has a boiling temperature in the range of 145 to 240 °C and in that the one or more anti-polymerizing additives are injected, in a step d), into the rectification section (7) of the primary fractionator (3) while passing the feedstream (27) into the primary fractionator (3).

2. The process according to claim 1 is characterized in that the rectification subsection (7) comprises a first rectification subsection (9) and a second rectification subsection (11), wherein when the second rectification subsection (11) is located at the top of the primary fractionator (3) so the first rectification subsection (9) is between said quenching section (5) and said second rectification subsection (11).

3. The process according to claim 1 or 2 is characterized in that one or more anti-polymerizing additives provided in step c) have a boiling temperature in the range of 180°C to 220°C, preferably from 190°C to 200°C.

4. The process according to claim 1 to 3 is characterized in that the anti-polymerizing additives are divided into two types: a first type with a boiling temperature below 210°C, and a second type with a boiling temperature equal to or greater than 210°C; and in that the anti-polymerizingadditives provided in step c) comprise at least one anti-polymerizing additives of the first type and at least one anti-polymerizing additives of the second type.

5. The process according to any one of claims 1 to 4 is characterized in that one or more antipolymerizing additives provided in step c) are of a first type with a boiling temperature below 210°C, and that step d) comprises injecting one or more anti-polymerizing additives of the first type in at least one selected from the quenching section (5), the first rectification subsection (9), and the second rectification subsection (11); with preference, the one or more anti-polymerizing additives of the first type have a boiling temperature in the range of from 160°C to 209°C.

6. The process according to claim 5 is characterized in that a stream (15) in the bottom of the first rectification subsection (9) is withdrawn and recycled on the top tray of the first rectification subsection (9) and in that, at least 10 wt.% of one or more anti-polymerizing additive of the first type based on the total weight of the anti-polymerizing additives, is added in said stream (15); and / or in that at least 10 wt.% of one or more anti-polymerizing additive of the first type based on the total weight of the anti-polymerizing additives is added below the reflux of said stream (15).

7. The process according to any one of claims 1 to 6 is characterized in that one or more antipolymerizing additives provided in step c) are of a second type with a boiling temperature equal to or greater than 210°C and that step d) comprises injecting one or more anti-polymerizing additives of the second type in the second rectification subsection (11); with preference the one or more anti-polymerizing additives of the second type have a boiling temperature in the range of from 210 to 235°C.

8. The process according to any one of claims 4 to 7 is characterized in that the vapor-phase effluent (19) recovered at step e) is further sent to a separation unit (17) wherein a quench water stream (53) is separated from a hydrocarbon stream (49) with at least a part of said hydrocarbon stream (49) being returned to said first rectification subsection (9) of the primary fractionator (3) as reflux, using a second rectification reflux line (21) and wherein at least part of the one or more anti-polymerizing additives is added in said second rectification reflux line (21).

9. The process according to any one of claims 1 to 8 is characterised in that one or more antipolymerizing additives are chosen among biphenyl, terpineol, ethyl acetoacetate, tetralin, dipropylene glycol-methyl ether (DPGME), acetophenone, di ethyl amino ethanol, di propylene glycol, propylene glycol, thiophenol, or any isomer of 4-methyl benzenthiolethyl lactate, trans stilbene, benzaldehyde, benzaldehyde dimethyl acetal, diphenyl carbonate, methyl carbamate, ethyl carbamate, methyl benzoate, ethyl benzoate, ethyl acetoacetate, diethyl aminoethanol,biphenyl, diethanolamine, 3-amino-1-propanol, terpineol, phenol, methyl phenol with the methyl being in position 2 and / or 3 and / or 4, amino phenol with the methyl being in position 2 and / or 3 and / or 4, methyl amino phenol with the methyl being in position 2 and / or 3 and / or 4, phenylethanol, any combination thereof.

10. The process according to any one of claims 1 to 9 is characterised in that the one or more anti-polymerizing additives are or comprise at least one selected from acetophenone, ethyl carbamate, benzaldehyde, methyl benzoate, di propylene glycol, and propylene glycol; with preference one or more anti-polymerizing additives are or comprise di propylene glycol and / or propylene glycol.11 . The process according to any one of claims 1 to 10 is characterised in that the one or more anti-polymerizing additives are or comprise benzaldehyde, acetophenone, or any mixture thereof.

12. The process according to any one of claims 1 to 11 is characterized in that step c) further comprises providing at least one polymerization inhibitor having a boiling point ranging from 195°C to 500°C, preferably said polymerization inhibitor is selected from quinone methides, dinitrophenols, TEMPO compounds, or any combination thereof.

13. The process according to any of the preceding claims wherein said at least one antipolymerizing additive is added in said steam-cracked gasoline at a concentration of 0.1 to 1000 ppm wt., preferably 1 to 100 ppm wt., even more preferably 10 to 50 ppm wt. based on the total weight of the feedstream.

14. An installation (1) for carrying the process according to any of the claims 1 to 13 said installation being characterized in that it comprises: a steam cracker connected to a transfer line exchanger; and fluidically connected by a transfer line (27) to a primary fractionator (3); a primary fractionator (3) constituted of one column divided into at least two sections fluidically connected: a quenching section (5), and a rectification section (7) wherein said quenching section (5) is located in the lower part of the primary fractionator and the rectification section (7) is in the upper part; wherein the rectification section (7) comprises a first rectification subsection (9) and an optional second rectification subsection (11) ; one or more reflux lines selected from a quenching reflux line (13) and a first rectification reflux line (15) wherein the quenching reflux line (13) located in the bottom of said primary fractionator withdraws and recycles a stream (31) on the top tray of said quenching section (5) before said first rectification subsection (9) and wherein the first rectification reflux line(15) is arranged to withdraw and recycle a stream (33) from the bottom of the first rectification subsection (9) to the top tray of said first rectification subsection (9), before the second rectification subsection (11) when present; a separation unit (17) that is fluidically connected with said primary fractionator (3) to recover the vapor-phase effluents (19) of said primary fractionator (3) and a second rectification reflux line (21) that is fluidically connected to said separation unit (17) to send back to the primary fractionator (3) part of the effluent of said separation unit (17); the separation unit (17) comprising a quench water tower (23) and a separator drum (25) wherein the separator drum is placed downstream of the quench water tower (23); wherein said steam cracker is fluidically connected to said primary fractionator; and wherein it comprises at least one additive injection nozzle in the first rectification subsection (9) and / or or in the second rectification subsection (11) when present.

15. The installation according to claim 14 is characterized in that it further comprises at least one additive injection nozzle in the quenching section (5) and / or in that it further comprises means to add anti-polymerizing additives in one or more reflux lines selected from the quenching reflux line (13), the first rectification reflux line (15), and the second rectification reflux line (21).

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