Performing a Cracking Reaction of Hydrocarbons in a Moving Bed Reactor with Improved Dilution

Abstract

In a method for performing a cracking reaction of a cracker feedstock comprising hydrocarbons having at least two carbons, substrate particles are allowed to flow downwardly under gravity flow as a particle bed; the substrate particles are guided through a gas cooling zone located above a reaction zone, and through the reaction zone; and the substrate particles are maintained in the reaction zone at a temperature of 500 to 1200 °C. The cracker feedstock is combined with a diluent gas comprising methane to obtain a feed gas stream, the feed gas stream is fed in countercurrent to the particles flow through the reaction zone to obtain a product gas stream comprising the diluent gas and cracking products; the product gas is guided through the gas cooling zone and cooled by transferring heat from the product gas to the substrate particles by direct heat transfer. The cooled product gas stream is withdrawn from the gas cooling zone; and the substrate particles are withdrawn from the reaction zone. The process gives rise to higher selectivities towards ethylene at identical conversion of ethane with methane as diluent gas, compared to methods known from the prior art using hydrogen as diluent gas.

Description

[0001] Performing a Cracking Reaction of Hydrocarbons in a Moving Bed Reactor with Improved Dilution

[0002] The present invention relates to a method for performing a cracking reaction of a cracker feedstock comprising hydrocarbons having at least two carbons.

[0003] Ethylene and propylene are important feedstocks used in the production of numerous petrochemical intermediate products and end products. They are made by cracking ethane, propane, or sometimes heavier paraffins or naphthas at high temperature. The material to be cracked is diluted with a diluent gas such as an inert gas, usually steam, and heated to 700 to 1000 °C for a very short time. The gas mixture leaving the cracker is typically a mixture of the diluent gas and predominantly hydrogen and hydrocarbons, of which 10 to 80% of the gas may be ethylene produced by reactions that can be represented by simplified net reaction equations such as

[0004] C2H6^ C2H4+ H2

[0005] (1) and

[0006] C3H8C2H4+ CH4

[0007] (2)

[0008] The hot cracked gas leaving the cracker is cooled down quickly in order to prevent unwanted follow-up reactions This is usually done in several steps. In a first step, the cracked gas is cooled down to about 450 °C by heat exchangers. A further cooling step occurs via direct contact between the cracked gas and a high boiling liquid, usually referred to as quench oil. The quench results in a partial condensation of the cracked gas. In this step, a heavy stream rich in Cw+ hydrocarbons is separated from the cracked gas A further cooling step of the cracked gas takes place in a water quench column for primary fractionation, cooling down the gas to around 30 °C. In this step, a C5-9 fraction, commonly referred to as pyrolysis gasoline, is separated from C4minus components.

[0009] While process heat up to 300 to 400 °C can readily be supplied through high-pressure steam, high temperature heating is currently achieved through natural gas firing which is inherently accompanied by CO2emissions. Although the overall CO2emissions of these processes are high, the concentration of CO2in the emitted fluegas from firing is rather low. The effort to absorb CO2and permanently remove it from the atmosphere requires that large volumes of gas be treated, which in itself is energy intensive and thus generates CO2.

[0010] Dilution of the material to be cracked with steam results in water being present in the gas mixture leaving the cracker, i.e. in the product stream. Said water must be removed from the product stream after the reaction, typically via a direct quench, in which the temperature of the product stream is reduced to such an extent that a large proportion of the water condenses. Water removal is unfavorable both in terms of reactor design and operating expenses.

[0011] It has also been suggested to perform cracking in a fluidized bed or moving bed reactor with electrical heating.

[0012] WO 2020 / 200522 relates to a reactor for endothermic high-temperature reactions, for example, cracking or thermal cracking of ethane. The reactor is configured to provide a gravity-driven moving bed in a reaction zone of the reactor interior, which moving bed comprises a large number of solid material particles. The reactor is also configured to guide a feed gas into the reaction zone, wherein, in order to heat the feed gas, the reactor is configured to heat the solid material particles in the reaction zone by generating an electric current in the solid material particles such that, by transferring heat from the solid material particles to the feed gas.

[0013] WO 2023 / 072996 A1 relates to a process for the non-oxidative conversion of saturated Ci+ hydrocarbons into unsaturated C2+ hydrocarbons and hydrogen in the presence of an unsupported carbon-based catalyst having a carbon content of at least 90.0 wt% and a metal concentration which is less than 0.3 wt%. The process comprises the steps of supplying the carbon-based catalyst to a reaction zone and directly heating of the carbonbased catalyst by induction heating, supplying a reaction gas comprising saturated Ci+ hydrocarbons to the heated reaction zone, and subjecting the reaction gas to a non-oxidative conversion in the presence of the carbon-based catalyst thereby converting at least a portion of the saturated Ci+ hydrocarbons into unsaturated C2+ hydrocarbons and hydrogen. Optionally, the a reaction gas may also comprise hydrogen; the molar ratio of this optional hydrogen to saturated C1+ hydrocarbon in said reaction gas may be in the range from about 1 :4 to 0: 1. In some embodiments, an inert gas selected from helium, nitrogen, argon, and mixtures thereof is fed to the reaction zone comprising the carbon-based catalyst The inert gas may be used to dilute the reaction gas.

[0014] There is an ongoing need to provide improved methods for performing cracking reactions in moving bed reactors, in particular methods allowing to attain higher mass fractions of olefins. It is desirable that the method can be implemented without necessitating adjustments in downstream processes.

[0015] Therefore, the present invention relates to a method for performing a cracking reaction of a cracker feedstock comprising hydrocarbons having at least two carbons, comprising allowing substrate particles to flow downwardly under gravity flow as a particle bed; guiding the substrate particles through a gas cooling zone located above a reaction zone, and through the reaction zone; maintaining the substrate particles in the reaction zone at a temperature of 500 to 1200 °C, preferably 800 to 1000 °C; combining the cracker feedstock with a diluent gas comprising methane to obtain a feed gas stream, feeding the feed gas stream in countercurrent to the particles flow through the reaction zone to obtain a product gas stream comprising the diluent gas and cracking products; guiding the product gas through the gas cooling zone and cooling the product gas by transferring heat from the product gas to the substrate particles by direct heat transfer; and withdrawing the cooled product gas stream from the gas cooling zone; and withdrawing the substrate particles from the reaction zone.

[0016] The method of the invention has several advantages. Coke formed as an unwanted by-product in the cracking reaction is deposited on the substrate particles and can easily and continuously be discharged from the reactor via the moving bed. Furthermore, coke deposited on the surface of the reactor wall is continuously abraded by the moving bed. By using a moving bed reactor, coke accumulation in the reactor is therefore avoided, allowing for longer uninterrupted operating periods of the moving bed reactor. The process of the invention gives rise to higher selectivities towards ethylene at identical conversion of ethane with methane as diluent gas, compared to methods known from the prior art using hydrogen as diluent gas (see the experimental results), and thus, the inventive method is an improved method for performing a cracking reaction.

[0017] In a moving bed reactor, a feed gas and solid material particles (herein synonymously referred to as “substrate particles”) flow in countercurrent throughout the reactor. Typically, the process comprises feeding substrate particles to the top of the reactor, allowing the particles to flow downwardly under gravity flow as a compact column, which means that the movement of the moving bed is gravity driven. The feed gas stream is preferably introduced via the bottom of the reactor, preferably having a temperature of 10 to 200 °C. The substrate particles are preferably introduced via the top of the reactor, preferably having a temperature of 10 to 200 °C.

[0018] The substrate particles are guided through the gas cooling zone located above the reaction zone, and subsequently through the reaction zone.

[0019] For this purpose, the reactor is preferably a vertical elongated reactor. Heated substrate particles are continuously removed at the reactor bottom. Thus, a continuous supply of substrate particles for the reaction is provided. Flow through the moving bed advantageously takes place homogeneously and uniformly (see for example WO 2013 / 004398, WO 2019 / 145279 and WO 2020 / 200522).

[0020] The present invention involves a cracking reaction of a cracker feedstock. In an embodiment, the cracker feedstock comprises ethane, propane, naphtha, or mixtures thereof. Ethane is especially preferred. The term “naphtha” encompasses liquid hydrocarbon mixtures produced from natural gas condensates, petroleum distillates, and the distillation of coal tar and peat.

[0021] The method of the present invention involves diluting the cracker feedstock with a diluent gas comprising methane to form a feed gas stream before subjecting the same to the cracking reaction in the reaction zone of the moving bed reactor. In an embodiment, the cracker feedstock is combined with 10 to 45 vol.-% of diluent methane, based on the volume of the cracker feedstock. A diluent gas is a gas that is inert under the conditions encountered in the reactor, in particular does not take part in the reaction and does not give rise to side reactions. A diluent gas can be used to moderate the reaction in the reactor.

[0022] Among other purposes, dilution methane is present in the feed gas stream to decrease C2+ hydrocarbon partial pressure during the cracking reaction, which shifts equilibrium towards ethylene and propylene production but away from coke production.

[0023] It will be appreciated that the cracker feedstock may inherently comprise small amounts of methane. Such minor amounts of methane impurities (herein also referred to as “intrinsic methane") alone do not achieve the advantages of the invention.

[0024] Cracking reactions are endothermic high-temperature reactions and thus, energy needs to be provided to the reaction occurring in the reaction zone of the moving bed reactor. In moving bed reactors, energy is provided by maintaining the substrate particles at a pre-determined temperature in the reaction zone, e.g. at a temperature of 500 to 1200 °C, preferably 800 to 1000 °C. The reaction zone is a zone, e.g., a zone of a reactor, that is actively heated. This heat is released and transferred to the feed gas. Consequently, the reaction takes place.

[0025] Thus, the feed gas stream is fed through the reaction zone in countercurrent to the substrate particles flow. In the reaction zone, the feed gas is brought in contact with the heated particles and reacted in an endothermic reaction to obtain a product gas. Suitably, the residence time of the feed gas in the reaction zone is below 650 ms. Suitably, the residence time of the substrate particles is in the range of from 0.1 and 2.5 hours, preferably 0.25 and 2.0 hours, more preferably 0.5 and 1 5 hours. The flow velocity of the substrate particles is advantageously in the range of 0.005 to 1 0 cm / s. The flow velocity of the feed gas stream is advantageously in the range of 0.025 to 10 m / s.

[0026] The product gas stream comprises the diluent gas, i.e. methane and cracking products, and may further comprise intrinsic methane, if present, and methane newly formed during cracking (herein also referred to as “newly formed methane”).

[0027] After the cracking reaction yielding the product gas, the method further comprises guiding the product gas through the gas cooling zone located above the reaction zone. As the substrate particles are guided through the gas cooling zone in countercurrent to the feed gas stream before being guided through the reaction zone, the product gas is cooled upon contact with the cold fresh substrate particles by transferring heat from the product gas to the substrate particles by direct heat transfer. As a consequence, the substrate particles are preheated by the product gas before entering the reaction zone and being heated. In said countercurrent operation, the hot product gas is cooled quickly by the substrate particles in the gas cooling zone. Thus, this quick cooling by direct heat transfer between the substrate particles and the product gas can obviate one or more separate quench steps that have hitherto been necessary. This allows for both simplifying reactor design and reducing operating expenses. In other words, the highly efficient heat integration in the gas cooling zone is advantageous as one or more separate quench units for the product gas may be omitted and a heat transport medium is not required.

[0028] Finally, the method of the invention further comprises the steps of withdrawing the substrate particles from the reaction zone, and withdrawing the cooled product gas stream from the gas cooling zone. In an embodiment, the feed gas volume flow and the particle mass flow are selected such that the product gas leaves the gas cooling zone at a temperature below 450 °C, preferably below 300 °C.

[0029] The product gas stream is preferably withdrawn via the top of the reactor, preferably having a temperature of 10 to 200 °C. The heated substrate particles are preferably withdrawn via the bottom of the reactor, preferably having a temperature of 10 to 200 °C. Withdrawing the substrate particles may be accomplished by conventional discharge means, e.g., by a cellular wheel sluice.

[0030] In an embodiment, the method further comprises recycling the substrate particles to the reaction zone.

[0031] Generally, a refractory lining electrically and thermally insulates the moving particles from the reactor shell Typically, refractory rocks advantageously comprising aluminum oxide, zirconium oxide and mixed oxides of aluminum, magnesium, chromium, silicon are used for the refractory lining.

[0032] The reactor may include an outer pressure jacket. For ease of assembly and disassembly, the pressure jacket is preferably split in sections with flanged ends that may be secured by bolts or the like to the flange of an adjacent section of the pressure jacket or the flange of the terminal hoods of the reactor.

[0033] Preferably, the heating means envisaged in the present invention are electrical heating means. The use of electric energy as a heat source instead of heating by combustion of natural gas allows considerable advantages, in particular with regard to the ease of control. The use of electricity offers opportunities for the use of compact, modular, high performance and energy efficient reactors. When the electricity derives from a nonfossil resource, the endothermic cracking operation can be implemented without net emission of carbon dioxide. The heating energy can be generated directly in the bed or in separate heating elements.

[0034] Electrical heating is selected from inductive heating, microwave heating, and resistive heating. Resistive heating encompasses direct resistive heating and indirect resistive heating. The different concepts of supplying heat may be combined. For indirect resistive heating, indirect resistive heating elements may be used. The indirect resistive heating elements can take the shape of a wire, ribbon, sheet or strip and can be straight, meandering or coiled. Such a heating element converts electricity into heat through the process of Joule heating.

[0035] Direct resistive heating is especially preferred. This means the electric current passed through the electrically conductive bed generates thermal energy. To this end, the method comprises applying a voltage across the particle bed in the reaction zone to provide direct electric resistance heating.

[0036] For this purpose, the particle bed are electrically conductive, such that an electric current can flow through the particle bed. The substrate particles comprise electrically conductive particles through which an electric current flows. The particle bed must contain a sufficient proportion of electrically conductive particles to ensure macroscopic electrical conductivity of the particle bed and allow for the passage of an electrical current through the bed. The proportion of electrically conductive particles in the particle bed can be in the range of from 5 to 100 wt.-%.

[0037] Electrically conductive particles may consist of a material which is inherently conductive or of a material which may be rendered conductive by the reactive deposition of an electrically conducting material, e.g., carbon, or by the adsorption or absorption of a polar liquid such as water.

[0038] The electrically conductive particles themselves may be chemically active or may be essentially catalytically inert. The electrically conductive particles may be admixed with non-conductive particles. The non-conductive particles may be chemically active particles.

[0039] In an embodiment, the substrate particles are selected from carbonaceous, metal, carbide ceramic particles, and composites thereof. Such substrate particles are electrically conductive particles. They may be in the form of fines, or be agglomerated such as pelletized or granulated, if required.

[0040] Carbonaceous particles include particles of char or coke, in particular graphite particles. Metallic particles may be metallic throughout or have a metallic coating over a non-metallic core. Metallic particles include conductive pre-reduced oxidic particles. Carbide ceramic particles include silicone carbide (SiC), tungsten carbide (TiC), zirconium carbide (ZrC), and mixtures thereof.

[0041] In an embodiment, the carbonaceous particles have a nickel content of less than 150 ppm and an iron content of less than 260 ppm.

[0042] This may involve a plurality of electrodes, i.e. two or more electrodes, that are in an electrically conductive relationship with the particle bed and which electrodes are spaced apart from one another. The spacing between electrodes, i.e., the spacing through which electric current flows, defines the reaction zone. An electrically conductive relationship may be achieved by causing the particles to touch an electrode or flow along an electrode, e.g. by electrodes being immersed in the particle bed.

[0043] The shape of the electrodes is not particularly limited. Preferably, the electrodes are designed to achieve a uniform current distribution within the particle bed. Preferably, the electrodes take the shape of a grid or of rods. Suitable electrode assemblies are described in WO 2019 / 145279 A1 .

[0044] When rods are used, each electrode preferably comprises a number of rods distributed across the cross section of the particle bed. Electrode rods that run to a point are particularly advantageous. Preferably, the upper and lower electrode rods run to a point on the side toward the heated zone. The tip may be conical or wedge-shaped. Correspondingly, the end of the rod may take the form of a dot or a line. The rod electrodes are connected to the hood in an electrically conductive manner and are jointly supplied with electrical power via the hood.

[0045] Preferably, the electrodes take the form of a grid. For grid form, various configuration variants are conceivable, for example grids in honeycomb form composed of advantageously regular polygons, rectangular grids formed from parallel bars, grids in the form of spokes or grids composed of concentric rings. Particular preference is given to grids in the form of spokes and grids composed of concentric rings. The diameter of the openings in the grid is at least 1.5 times of the diameter of the substrate particles.

[0046] A voltage is applied across at least two of the plurality of electrodes. A voltage may be applied by connecting electrodes to opposite polarity of an electrical energy source, e.g., either the positive or negative polarity of an electrical energy source.

[0047] The plurality of electrodes may comprise two or more subsets of electrodes, with a voltage being applied between pairs of subsets, the electrodes in each subset being at substantially the same electric potential, e.g. by being electrically connected with each other.

[0048] The applied voltage generates an electric current flux within the spacing between opposite polarity electrodes. The resistance of the discrete yet interconnecting electrically conductive particulates to electrical current results in the particulates being heated. The region traversed by the electric current is heated by direct electric resistance and defines a confined zone in which a chemical reaction can occur. The confined zone is referred to as “reaction zone”. Direct electric resistive heating (Joule heating) is, for example, described in detail in US 2,982,622, WO 2019 / 145279 and WO 2020 / 200522.

[0049] Preferably, electrodes are employed that are spaced apart along the longitudinal axis of the reactor. The longitudinal axis of the reactor is preferably orientated vertically. The electrodes may be made of a metal, graphite or any other suitably conductive material Various composite refractory materials, for example containing graphite or metal, may be employed. Electrical power is supplied through the plurality of electrodes that are in an electrically conductive relationship with the bed, e.g., immersed in the particle bed, as outlined above.

[0050] The potential or voltage may arise from an A.C. or D.C. source. Advantageously, a potential difference (voltage) of 1 volt to 10 000 volts, preferably of 10 volts to 5000 volts, more preferably of 50 volts to 1000 volts, is applied. The electrical field strength between the hoods is advantageously in the range of 1 V / m to 100 000 V / m, preferably 10 V / m to 10 000 V / m, more preferably 20 V / m to 5000 V / m, especially 30 V / m to 1000 V / m.

[0051] The specific electrical conductivity of the bed is advantageously from 0.001 S / cm to 1000 S / cm, preferably from 0.01 S / cm to 300 S / cm, especially from 0.1 S / cm to 100 S / cm.

[0052] This advantageously results in an electric current density in the bed of 0.01 A / cm2to 100 A / cm2, preferably from 0.05 A / cm2to 50 A / cm2, especially from 0.1 A / cm2to 10 A / cm2.

[0053] The resistive thermal energy generated by passing a current through the particles may be supplemented by other heat sources. In embodiments, additional heat can be provided to the pyrolysis by preheating the feed gas that is designed to be flowed through the particle bed. In still other embodiments, additional heat can be provided by inductive heating. Optionally, additional resistive heating elements may be used.

[0054] In order to optimize heat integration achieved by the counterflow of solid particles and gas, in an embodiment, the method additionally comprises guiding the substrate particles through a heat integration zone located below the reaction zone; guiding the feed gas through the heat integration zone and in the reaction zone; and transferring heat from the substrate particles leaving the reaction zone to the feed gas in the heat integration zone.

[0055] This results in the feed gas being preheated by contact of the substrate particles leaving the reaction zone. Thus, the thermal energy is substantially stored in the particle bed, and the present method does not suffer from limitations of heat exchange of the product gas against the feed gas. Heat recovery rates of more than 95% of the sensible heat may be accomplished by the present method.

[0056] There is an uninterrupted particle flow through the gas cooling zone, the reaction zone and the heat integration zone.

[0057] Heat exchange in the heat integration zone can be optimized by varying the downward flow velocity of the substrate particles and / or the volume flow of the feed gas to the reactor. The mass flow of solid and gas are adjusted to have the same heat capacity. With this measure heat is recovered and loss through hot gas / solid streams leaving the reactor are minimized.

[0058] In an embodiment, the gas volume flow and the particle mass flow are selected such that substrate particles leave the heat integration zone at a temperature of below 450 °C, preferably below 300 °C.

[0059] In an embodiment, the product gas stream comprises, in addition to the cracking products, components that are higher-boiling in respect to the cracking products, the method comprising

[0060] - fractionating condensing the product gas stream in the gas cooling zone to deposit the higher-boiling components at least partially on the surface of the substrate particles, and

[0061] - returning the higher-boiling components with the substrate particles guided to the reaction zone.

[0062] The higher-boiling components are hydrocarbons which may be uncracked components, i.e. they may originate from incompletely reacted feed such as naphtha. The higher-boiling components may also be hydrocarbons which are formed in the cracking reaction, e.g. as cracking reaction side products. For example, if the cracking product is ethylene, components that are higher-boiling in respect to ethylene may be higher hydrocarbons. These components are condensed in the gas cooling zone via fractional condensation and deposited on the surface of the substrate particles. Afterwards, said components are returned to the reaction and again subjected to cracking conditions in the reaction zone and may undergo the cracking reaction. This advantageously allows for both reducing contamination of the cracking product stream leaving the reactor and for enabling higher conversions of the cracker feedstock.

[0063] The recovery of the various olefin products from cracking products is usually carried out by fractional distillation using a series of distillation steps or columns to separate out the various components. The unit which separates hydrocarbons with one carbon atom (Ci) and lighter fraction is referred to as "demethanizer”. The unit which separates hydrocarbons with two carbon atoms or less (C2n™s) from the heavier components is referred to as “deethanizer”. The unit which separates the hydrocarbon fraction with three carbon atoms or less (Caminus) from the heavier components is referred to as “depropanizer”. The unit which separates the hydrocarbon fraction with four carbon atoms or less (C^inus) from the heavier components is referred to as “debutanizer”.

[0064] The residual heavier components having a higher carbon number fraction (C5+) may be used as gasoline or recycled back to the cracker.

[0065] The various fractionation units may be arranged in a variety of sequences in order to provide desired results based upon various feedstocks. To that end, a sequence which uses the demethanizer first is commonly referred to as the “front-end demethanizer” sequence. Similarly, when the deethanizer is used first, it is commonly referred to as the “front-end deethanizer" sequence. And, when the depropanizer is used first, it is commonly referred to as “front-end depropanizer” sequence. In the conventional front-end demethanizer sequence, the product gas containing hydrocarbons having one to five or more carbon atoms per molecule (Ci to Cs+) first enters a demethanizer, where methane and lighter fractions (including hydrogen) are separated as an over-head stream. The demethanizer operates at relatively low temperatures, typically ranging from about -100 °C to about 25 °C. Hydrogen contained in the front-end demethanizer over-head stream may be removed in order to separate the over-head stream into a methane rich stream and a lighter fractions rich stream (hydrogen rich stream), as described in detail below.

[0066] The heavy ends exiting the demethanizer consist mainly of C2 to C5+ molecules. These heavy ends then are routed to a deethanizer where the C2 hydrocarbons are taken over the top and the C3 to C5+ compounds leave as bottoms. The C2 components leaving the top of the deethanizer may be fed to an acetylene converter or acetylene removal unit. As some methane remains dissolved in the heavy ends exiting the demethanizer and ends up in the C2 components leaving the deethanizer, the C2 components stream may be subsequently sent to a demethanizer for removal of the remaining methane.

[0067] Hence, in a preferred embodiment, the method comprises recovery of the cracking products from the product gas by a series of distillation steps including at least one demethanizer, in which methane and lighter fractions, including hydrogen, are separated as an over-head stream. Such an existing demethanizer also serves for separating the diluent methane. Thus, this embodiment advantageously enables the separation of diluent methane without additional effort and is therefore advantageous in terms of reactor design and operating expenses

[0068] The over-head stream from the demethanizer comprises methane and hydrogen as the main components. The ratio of methane and hydrogen in the over-head stream may vary depending on the cracking operation, respectively the cracker feedstock (see for example Ullmann's Encyclopedia of Industrial Chemistry, Ethylene 5 1.3 Commercial Cracking Yields, DOI: 10.1002 / 14356007.a10_045.pub3 for different cracking yields depending on different cracker feedstocks) but the methane content is generally in the range of from 40 to 95 wt.-%, preferably 90 to 95 wt.-% of methane, with the remainder being mainly hydrogen.

[0069] In an embodiment, the method comprises recycling the over-head stream or a partial stream thereof or a methane-containing fraction thereof as the diluent gas. Recycling of the over-head stream avoids addition of neat methane to the cracker feedstock. A purge of a sub-stream of the over-head stream may be applied to avoid accumulation of methane and / or hydrogen in the gas cycle.

[0070] In an embodiment, the over-head stream is separated into a methane rich stream and a lighter fractions rich stream (hydrogen rich stream). Preferably, the methane rich stream has a methane content of at least 96 wt.-%, more preferably at least 98 wt.-%, most preferably at least 99 -wt.%, in particular at least 99.9 wt.-%. Preferably, the lighter fractions rich stream (hydrogen rich stream) has a hydrogen content of at least 90 wt -%, more preferably at least 95 wt.-%.

[0071] Separation of hydrogen and methane can be achieved by pressure swing adsorption, cryogenic methods or by means of a selective membrane unit.

[0072] In an embodiment, the method comprises separating the over-head stream by means of a selective membrane unit or a series of selective membrane units, to obtain a methane rich stream, and a lighter fractions rich stream (hydrogen rich stream), and recycling the methane rich stream as the diluent gas. Separation by means of a selective membrane unit may be carried out by passing the over-head stream over a selective membrane unit comprising a membrane selected from polymer membranes, inorganic membranes, carbon membranes, metal membranes, proton-conducting ceramic membranes, and combinations thereof. Separation by means of a selective membrane unit is, e.g., described in detail in WO 2022 / 112497 A1.

[0073] The C2 components from which methane has been removed are then sent to a C2 splitter which produces ethylene as the light product and ethane as the heavy product. The C3 to 5+ stream leaving the bottom of the deethanizer is routed to a depropanizer, which sends the C3 components overhead and the C4 to C5+ components below.

[0074] The C3 product may be hydrotreated to remove C3 acetylene and dienes before being fed to a C3 splitter, where it is separated into propylene at the top and propane at the bottom.

[0075] Hence, in a preferred embodiment, the method comprises recovery of the product streams from the product gas by a series of distillation steps including a separation of saturated and unsaturated C2 hydrocarbons in a C2 splitter and / or separation of saturated and unsaturated C3 hydrocarbons in a C3 splitter.

[0076] The C4 to C5+ stream is fed to a debutanizer, which produces C4 components at the top with the balance of C5+ components leaving as bottoms. Both the C4 and the 65+ streams may be separately hydrotreated to remove undesirable acetylenes and dienes.

[0077] In conventional front-end deethanizer sequences, the product gas containing Ci to C5+ components first enters a deethanizer. The light ends exiting the deethanizer consist of C2 and Ci components along with any hydrogen (C2minus fraction). These light ends are fed to a demethanizer (C2minus demethanizer) where the hydrogen and Ci are removed as light ends and the C2 components are removed as heavy ends. The C2 stream leaving the bottom of the demethanizer may be fed to an acetylene converter and then to a C2 splitter which produces ethylene as the light product and ethane as the heavy product. The heavy ends exiting the deethanizer which consist of C3 to C5+ components are routed to a depropanizer which sends the C3 components over-head and the C4 to Cs+components below. The C3 product is fed to a C3 splitter where it is separated into propylene at the top and propane at the bottom, while the C4 to C5+ stream is fed to a debutanizer which produces C4 compounds at the top with the balance leaving as bottoms to be used for gasoline or to be recirculated as feed into the cracking process. As with the front-end demethanizer sequence, the C3, C4, and Cj+ streams may be separately hydrotreated to remove undesirable acetylenes and dienes.

[0078] In conventional front-end depropanizer sequences, the quenched and acid-free gases containing hydrocarbons having from one to five or more carbon atoms per molecule (Ci to C5+) first enter a depropanizer. The heavy ends exiting the depropanizer consist of C4 to Cs+ components. These are routed to a debutanizer where the C4 components and lighter species are taken over the top with the rest of the feed leaving as bottoms which can be used for gasoline or other chemical recovery. These streams may be separately hydrotreated to remove undesired acetylenes and dienes. The tops of the depropanizer, containing Ci to C3 components, may be fed to an acetylene converter and then to a demethanizer system, where the Ci components and any remaining hydrogen are removed as an over-head. The heavy ends exiting the demethanizer system, which contains C2 and C3 components, are introduced into a deethanizer wherein C2 components are taken off the top and C3 compounds are taken from the bottom. The C2 components are, in turn, fed to a C2 splitter which produces ethylene as the light product and ethane as the heavy product. The C3 stream is fed to a C3 splitter which separates the C3 species, sending propylene to the top and propane to the bottom.

[0079] The saturated C2 hydrocarbons and / or the saturated C3 hydrocarbons or a partial stream thereof may be recycled as feed into the cracking process.

[0080] In an embodiment, the feed gas stream comprises a further diluent gas other than methane, e.g. hydrogen, helium, nitrogen, argon, and mixtures thereof.

[0081] The invention is further illustrated by the following examples and the accompanying figures.

[0082] Fig. 1 depicts simulated ethylene mass fractions for dilution with hydrogen vs. methane.

[0083] Fig. 2 depicts measured conversion of ethane and selectivity towards ethylene for dilution with hydrogen vs. methane.

[0084] Examples

[0085] Simulation Results

[0086] Cracking of ethane at dilution with different diluent gases was simulated (Plug Flow Tubular Reactor (PFTR), Chemical Reaction Engineering Chemical Kinetics (CRECK) mechanism, simulation carried out with Matlab). Simulation 1 was carried out at a dilution with 25 vol.-% of hydrogen. Simulation 2 was carried out at a dilution with 25 vol.-% of methane. The ethylene mass fraction results obtained for simulation 1 and 2 are depicted in Fig. 1 . As can be seen from Fig. 1, the maximum obtainable ethylene mass fraction is higher for methane dilution (66.13 wt.-%) compared to hydrogen dilution (64.75 wt.-%). Example 1

[0087] Experiments for cracking of ethane to yield ethylene were carried out in a moving-bed reactor at dilution rates of 20 vol.-% hydrogen (runs 1 and 2, comparative) vs. methane (runs 3 and 4, according to the invention). Nitrogen was used as internal standard. The experimental conditions and results (ethane conversion, selectivity towards ethylene, calculated yield of ethylene) are shown in table 1 .

[0088] Table i . ] Direct electrical power

[0089] [2] Mean temperature in the reaction zone

[0090] [3] Maximum temperature in the reaction zone

[0091] [4] Residence time in the reaction zone

[0092] [5] Conversion of ethane

[0093] [6] Selectivity towards ethylene

[0094] [7] Yield of ethylene (calculated)

[0095] * comparative example

[0096] The results of ethane conversion X(Ethane) and selectivity towards ethylene S(Ethylene) are also depicted and plotted in Fig. 2. As can be seen from Fig. 2, higher selectivities towards ethylene are obtained at identical conversion of ethane with methane as diluent gas, in comparison to hydrogen as diluent gas.

Claims

Claims1. A method for performing a cracking reaction of a cracker feedstock comprising hydrocarbons having at least two carbons, comprising allowing substrate particles to flow downwardly under gravity flow as a particle bed as a compact column; guiding the substrate particles through a gas cooling zone located above a reaction zone, and through the reaction zone; maintaining the substrate particles in the reaction zone at a temperature of 500 to 1200 °C, preferably 800 to 1000 °C; combining the cracker feedstock with a diluent gas comprising methane to obtain a feed gas stream, feeding the feed gas stream in countercurrent to the particles flow through the reaction zone to obtain a product gas stream comprising the diluent gas and cracking products; guiding the product gas through the gas cooling zone and cooling the product gas by transferring heat from the product gas to the substrate particles by direct heat transfer; and withdrawing the cooled product gas stream from the gas cooling zone; and withdrawing the substrate particles from the reaction zone.

2. The method according to claim 1 , wherein the cracker feedstock is combined with 10 to 45 vol.-% of methane, based on the volume of the cracker feedstock.3 The method according to claim 1 or 2, wherein the substrate particles are selected from carbonaceous, metal, carbide ceramic particles, and composites thereof.4 The method according to claim 3, wherein the carbonaceous particles have a nickel content of less than 150 ppm and an iron content of less than 260 ppm.

5. The method according to any one of the preceding claims, wherein the cracker feedstock comprises ethane, propane, naphtha, or mixtures thereof, preferably ethane.

6. The method according to any one of the preceding claims, comprising applying a voltage across the particle bed in the reaction zone to provide direct electric resistance heating.

7. The method according to any one of the preceding claims, wherein the feed gas volume flow and the particle mass flow are selected such that the product gas leaves the gas cooling zone at a temperature below 450 °C, preferably below 300 °C.8 The method according to any one of the preceding claims, additionally comprising guiding the substrate particles through a heat integration zone located below the reaction zone; guiding the feed gas through the heat integration zone and in the reaction zone; and transferring heat from the substrate particles leaving the reaction zone to the feed gas in the heat integration zone.

9. The method according to claim 8, wherein the gas volume flow and the particle mass flow are selected such that substrate particles leave the heat integration zone at a temperature of below 450 °C, preferably below 300 °C.

10. The method according to any one of the preceding claims, further comprising recycling the substrate particles to the reaction zone.11 . The method according to any one of the preceding claims, wherein the product gas stream comprises, in addition to the cracking products, components that are higher-boiling in respect to the cracking products, the method comprising- fractionating condensing the product gas stream in the gas cooling zone to deposit the higher- boiling components at least partially on the surface of the substrate particles, and- returning the higher-boiling components with the substrate particles guided to the reaction zone.

12. The method according to any one of the preceding claims, comprising recovery of the cracking products from the product gas by a series of distillation steps including at least one demethanizer, in which methane and lighter fractions including hydrogen are separated as an over-head stream.

13. The method of claim 12, comprising recycling the over-head stream or a partial stream thereof or a methane-containing fraction thereof as the diluent gas.

14. The method of claim 12 or 13, comprising separating the over-head stream by means of a selective membrane unit or a series of selective membrane units to obtain a methane rich stream, and a lighter fractions rich stream, and recycling the methane rich stream as the diluent gas

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