Performing a cracking reaction of hydrocarbons in a moving bed reactor

Abstract

A method for performing a cracking reaction of hydrocarbons having at least two carbons to yield olefins comprises allowing carbonaceous particles to flow downwardly under gravity flow as a particle bed through a reaction zone; maintaining the particles in the reaction zone at a temperature of 700 to 900 °C; feeding a feed gas stream comprising hydrocarbons having at least two carbons through the reaction zone in counter current to the particles flow; withdrawing the carbonaceous particles from the reaction zone; and withdrawing a product gas stream comprising cracking products from the reaction zone. The carbonaceous particles have a surface area of less than 15 m2 / g, preferably less than 8 m2 / g. Use of carbonaceous particles having said low surface area minimizes coke formation.

Description

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

[0002] The present invention relates to a method for performing a cracking reaction of 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 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 hydrogen, methane, hydrocarbons and carbon dioxide, of which 10 to 80% of the gas may be ethylene produced by reactions such as

[0004] C2H6^ C2H4+ H2(1) and

[0005] C3H8^ C2H4 + CH4. (2)

[0006] 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 flue-gas 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.

[0007] It has been suggested to perform cracking in a fluidized bed or moving bed reactor with electrical heating.

[0008] EP 3 945 066 A1 discloses a process to perform steam cracking reaction of hydrocarbons having at least two carbons in a fluidized bed reactor. At least a portion of the particles of the bed are electrically conductive particles, and heating of the fluidized bed to a temperature ranging from 500°C to 1200°C to conduct the steam cracking reaction of the hydrocarbon feedstock is performed by passing an electric current through the fluidized bed.

[0009] 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.

[0010] A concurrent reaction to the cracking reaction is the pyrolysis of the hydrocarbon feed to form coke and hydrogen. It is desirable to minimize coke formation. The invention relates to a method for performing a cracking reaction of hydrocarbons having at least two carbons, comprising allowing carbonaceous particles to flow downwardly under gravity flow as a particle bed through a reaction zone; maintaining the particles in the reaction zone at a temperature of 500 to 1200 °C, in particular 700 to 900 °C; feeding a feed gas stream comprising hydrocarbons having at least two carbons through the reaction zone in counter current to the particles flow; withdrawing the carbonaceous particles from the reaction zone; and withdrawing a product gas stream comprising cracking products from the reaction zone, wherein the carbonaceous particles have a surface area of less than 15 m2 / g, preferably less than 8 m2 / g.

[0011] The cracking operation of the invention involves converting hydrocarbons to olefins, e.g., ethylene and / or propylene, by a thermal (or non-catalytic) dehydrogenation reaction. Dehydrogenation may be accompanied by splitting of large-chain hydrocarbons into smaller molecules.

[0012] According to the idealized reaction scheme, no carbon is formed in the dehydrogenation reaction. During cracking reactions of hydrocarbons, however, coke formation may take place via surface reactions. Accordingly, the surface of the carbonaceous particles flowing through the reaction zone as a particle bed influences coke formation during cracking. In particular, it is believed that large surface volume ratios of the particles lead to a larger energy transfer surface (surface area of the particles). In order to achieve low coke formation, the specific surface area of the carbonaceous particles should be as small as possible. In the method of the invention, the carbonaceous particles have a surface area of less than 15 m2 / g, preferably less than 8 m2 / g, most preferably less than 5 m2 / g.

[0013] The specific surface area is defined as the total surface area of the particles per g (m2 / g) and may be calculated from the particle size distribution or measured by BET adsorption. Herein, the values of the specific surface area are understood to be determined by DIN 66133.

[0014] A low specific surface area of the carbonaceous particles may be achieved by using particles with a low surface-to-volume ratio and / or low porosity. It must be taken into account that the particle size should not be too large, so as not to impair the flowability of the carbonaceous particles and avoid wall effects which may have an influence on the particle bed density. In particular, the ratio of the diameter of the reactor to the particle size should not be below 25, so as to avoid wall effects.

[0015] In one embodiment, the carbonaceous particles have a particle size distribution characterized by a weight average particle diameter (D50) of 1 to 200 mm, preferably 2.5 to 30 mm, as determined by dynamic image analysis.

[0016] In one embodiment, the carbonaceous particles have an accessible porosity, as determined by mercury intrusion porosimetry, of less than 65%, preferably less than 60%, more preferably less than 50%. The carbonaceous particles may be in the form of fines, or be agglomerated such as pelletised or granulated, if required. Carbonaceous particles may include particles of char or coke, such as calcined petroleum coke (CPC), or graphite particles. Use of bio char may serve to further reduce the carbon footprint of the technology.

[0017] In addition to low surface area and low porosity, specific properties of carbonaceous particles are desired, most importantly, regarding levels of transition metals. The fact that carbonaceous particles such as petroleum coke are by-products of the petroleum industry introduces several distinct disadvantages in this regard. The petroleum cokes vary markedly in nature, and often contain significant levels of impurities. The major impurities include nickel and iron.

[0018] Metal impurities contained in the carbonaceous particles may catalyze the formation of filamentous coke structures. It has been found that Ni and Fe impurities in the carbonaceous particles lead to an increased growth of filamentous carbon structures during coke formation. Filamentous carbon structures are however detrimental to the minimization of coke formation, since they increase the particle surface, thereby increasing coke formation.

[0019] It is thus preferred that the nickel content and iron contents of the carbonaceous particles are limited. In one embodiment, the carbonaceous particles have a nickel content of less than 150 ppm, preferably less than 100 ppm, more preferably less than 20 ppm and an iron content of less than 260 ppm, preferably less than 150 ppm, more preferably less than 50 ppm. The nickel content and iron content may be determined by atomic spectroscopy.

[0020] Depending on the source of the carbonaceous particles, the presence of nickel and iron impurities cannot be altogether avoided. Typically, the carbonaceous particles have a nickel content of at least 1 ppm, such as at least 2 ppm, and an iron content of at least 2 ppm, such as at least 5 ppm.

[0021] The feed gas stream comprises hydrocarbons having at least two carbons. The feed gas stream can comprise any volatile hydrocarbon component. A volatile hydrocarbon may be referred to as a hydrocarbon that can be vaporized at one atmosphere, with little if any residue remaining after achieving final boiling point. Volatile hydrocarbons components can include lower boiling point hydrocarbons. For example, the volatile hydrocarbon component may be a hydrocarbon composition having a final boiling point of not greater than 566 °C. Boiling points as referred to herein are preferably determined according to ASTM D86-07 Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure. Suitable hydrocarbons having at least two carbons include ethane, propane, heavier paraffins and naphthas, in particular ethane and propane, most preferably ethane.

[0022] In an embodiment, the feed gas stream can comprise hydrocarbon rich in ethane. For example, hydrocarbon rich in ethane can be hydrocarbon comprised of 50 wt.-% or more, or 60 wt.-% or more, or 70 wt.-% or more, or 80 wt.-% or more, or 90 wt.-% or more ethane. The ethane can be provided from any source. For example, the ethane can be provided from synthetic or natural sources. In one embodiment, the ethane is provided from a petroleum refining process. In an alternative embodiment, the ethane is provided from a geological source or geological origin, such as natural gas. The method of the invention is suitably carried out in a moving-bed reactor. In a moving-bed reactor, a feed gas and solid material particles flow in countercurrent throughout the reactor. Typically, the process comprises feeding solid material 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.

[0023] For this purpose, the reactor is preferably a vertical elongated reactor. The particles are withdrawn from the reactor at the bottom of the reactor. Flow through the moving bed advantageously takes place homogeneously and uniformly (see for example WO 2013 / 004398, WO 2019 / 145279 and WO 2020 / 200522).

[0024] Suitably, the packed density of the particle bed is in the range of 300 to 7000 g / cm3, preferably 500 to 3000 g / cm3, more preferably 600 to 1500 g / cm3

[0025] The feed gas stream is preferably introduced via the bottom of the reactor, preferably having a temperature of 10 to 200 °C. The carbonaceous particles are preferably introduced via the top of the reactor, preferably having a temperature of 10 to 200 °C.

[0026] The carbonaceous particles are maintained at a temperature of 500 to 1200 °C in the reaction zone. 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 cracking reaction takes place. Heated carbonaceous particles are continuously removed at the reactor bottom. Thus, a continuous supply of carbonaceous particles for the reaction is provided. The gaseous product is cooled down at the reactor top upon contact with the cold fresh carbonaceous particles. As a consequence, the carbonaceous particles are preheated by the outlet gas before entering the reaction zone and being heated. Likewise, the feed gas is preheated by contact of the carbonaceous 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.

[0027] The product gas stream is preferably recovered via the top of the reactor, preferably having a temperature of 10 to 200 °C. The heated carbonaceous particles are preferably recovered via the bottom of the reactor, preferably having a temperature of 10 to 200 °C. Discharging the particles may be accomplished by conventional discharge means, e.g., by a cellular wheel sluice.

[0028] In one embodiment, the method additionally comprises recycling the carbonaceous particles to the reaction zone.

[0029] The flow velocity of the carbonaceous particles is advantageously in the range of 0.005 to 0.5 cm / s. The flow velocity of the feed gas flow is advantageously in the range of 0.025 to 2 m / s.

[0030] The gas residence time in the reactor is advantageously between 0.1 and 1.5 s, preferably between 0.2 and 1.0 s. The residence time of the solid material particles is preferably between 0.1 and 2.5 hours, preferably between 0.25 and 2.0 hours and more preferably between 0.5 and 1.5 hours. The reaction is preferably carried out at a temperature of 500 to 1200 °C, more preferably at a temperature of 700 to 900 °C. The reaction is preferably carried out at pressures ranging from 1 to 20 bar (abs), more preferably 1 to 15 bar (abs).

[0031] A measure of how much unwanted pyrolysis of hydrocarbons (leading to coke) occurs in the cracking operation is the coking yield. The coking yield is the weight percentage of the hydrocarbon feed that ends up as coke. Hence, the coking yield is understood to relate to the amount of carbon deposited on the carbonaceous particles as they flow downwardly under gravity flow as a particle bed through the reaction zone. The coking yield may be determined by measuring the carbon deposition on the carbonaceous particles by gravimetry of the carbonaceous particles before and after allowing the carbonaceous particles to flow downwardly under gravity flow as a particle bed through the reaction zone and then relating it to the total amount of feed used during the course of the experiments. In one embodiment, the carbonaceous particles withdrawn from the reaction zone exhibit a coking yield of at most 1.8 wt.-%, preferably at most 1 .5 wt.-%, more preferably at most 1 .2 wt.-% or at most 1 .0 wt.-%. For example, the coking yield may be in the range of 0.01 to 1.8 wt.-%, 0.05 to 1.5 wt.-%, 0.1 to 1.2 wt.-% or 0.1 to 1.0 wt.-%.

[0032] Cracking reactions of hydrocarbons having at least two carbons are endothermic. As an example, thermally cracking ethane to produce ethylene involves the endothermic equilibrium reaction:

[0033] C2H6C2H4+ H2AH=+138 kJ mol’1

[0034] Since the cracking process involves endothermic equilibrium reactions, Le Chatelier's Principle suggests that high equilibrium yields of olefin products will be favored by carrying out the process at higher temperatures and lower pressures.

[0035] In a preferred embodiment, thermal energy is provided to the reaction by electrical heating. 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. When the electricity comes from a non -fossil resource, the endothermic reaction can be implemented with negative emission of carbon dioxide.

[0036] The type of electrical heating is not particularly limited. For example, resistive heating elements may be used, which 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. In other embodiments, heat can be provided by inductive heating.

[0037] Since carbonaceous particles are inherently electrically conductive, they may be heated by resistive heating (Joule heating), as described for example in US 2,982,622, WO 2019 / 145279 and WO 2020 / 200522. To this end, the carbonaceous particles may be heated by an electrical potential or voltage applied across at least a portion of the particle bed.

[0038] Hence, in a preferred embodiment the method comprises applying a voltage across the particle bed in the reaction zone to provide direct electric resistance heating. Electrical power may be supplied through a plurality of electrodes that are in an electrically conductive relationship with the particle bed, e.g., immersed in the particle bed. 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 may be supplied through a plurality of electrodes that are in an electrically conductive relationship with the bed, e.g., immersed in the particle bed.

[0039] Preferably, a pair of axially spaced electrodes is employed, i.e. , upper and lower electrodes. Alternatively, a radial electrode arrangement may be contemplated with a central electrode extending along the axis of the reactor and a generally cylindrical counter electrode. The spacing between electrodes, i.e., the spacing through which electric current flows, defines the reaction zone.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] In conventional cracking reactors, e.g., a steam cracking furnace configuration, the heating occurs with a combusting stream on one side of a tubular wall partition and the feed stream on the other side. In the process of the invention, heat from the heated solid particles is transferred to the feed gas in order to heat the feed gas in the reaction zone. The moving-bed reactor provides a very good heat transfer between the gas and the particles. In addition, there is no back-mixing, and the residence time can be controlled for both phases. The counterflow operation allows the energy integration of the reactor, and the excellent heat transfer between the gas and the solid particles guarantees a thermally efficient process.

[0047] In order to optimize heat integration achieved by the counterflow of solid particles and gas, the method additionally comprises subsequently guiding the carbonaceous particles through a first heat integration zone located above the reaction zone, the reaction zone, and a second heat integration zone located below the reaction zone; transferring heat from the product gas leaving the reaction zone to the carbonaceous particles in the first heat integration zone by direct heat transfer; and transferring heat from the carbonaceous particles leaving the reaction zone to the feed gas in the second heat integration zone.

[0048] There is an uninterrupted particle flow through the first heat integration zone, the reaction zone and the second heat integration zone.

[0049] Heat exchange in the heat integration zones can be optimized by varying the downward flow velocity of the 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.

[0050] In one embodiment, the feed gas stream comprises a diluent gas. A diluent gas is a gas that is inert under the conditions encountered in the reactor, in particular does not take part in the cracking reaction and does not give rise to side reactions. Diluent gas can be used to moderate the cracking reaction in the reactor. The addition of diluent gas reduces the partial pressure of the hydrocarbon portion of the feed, thereby lowering reactant feed pressure to increase selectivity to olefins.

[0051] In an embodiment, the feed gas volume flow, the proportion of diluent gas and / or the particle mass flow are selected such that the product gas temperature leaves the first heat integration zone at a temperature below 450 °C, such as below 400 °C, and / or the particles leave the second heat integration zone at a temperature of below 450 °C, such as below 400 °C.

[0052] The product gas stream comprising cracking products may be processed to appropriately separate and recover the olefins and, possibly, dienes. For example, one or more distillation towers may be used to separate the product gas stream into two or more fractions, such as a hydrogen fraction, a methane fraction, a C2 fraction, an ethylene fraction, an ethane fraction, a C3 fraction, a propylene fraction, a propane fraction, a C4 fraction, a butadiene fraction, a butene fraction, a butane fraction, and / or a 65+ containing fraction. If desired, a portion or all of the 65+ containing fraction(s) may be recycled for further cracking to produce additional desired products.

[0053] The invention is further illustrated by the following examples and the accompanying figures.

[0054] Fig. 1 shows the impact of the specific surface area on the coking yield in a laboratory reactor filled with carbon particles for ethane and propane cracking experiments. Fig. 2a and 2b are scanning electron microscope images of carbonaceous particles with high Ni and Fe content and low Ni and Fe content, respectively, after being used in a laboratory reactor filled with carbon particles for ethane and propane cracking experiments.

[0055] Example 1

[0056] The influence of the specific surface area of different carbonaceous particles on the coking yield was examined in a laboratory reactor.

[0057] The reactor was a ceramic tube of 60 x 5 mm and had a reaction zone length of 600 mm. The reactor was placed in an electrically heated furnace. The whole assembly was located inside a water-cooled pressure shell, designed for pressures up to 40 bar(g). The cross section of the reactor was 28 mm.

[0058] Ethane and propane as feedstock as well as methane and hydrogen as diluent were supplied from gas bottles. For the hexane feed, the setup was equipped with an addition feed system consisting of a storage tank and an evaporator with high temperature thermostat.

[0059] The laboratory reactor was designed for isothermal measurements over a well-defined length of a carrier particle bed. The onset of the reaction zone was precisely defined with the preheating to reaction temperature of 850 °C at very short residence times (~ few tenth milliseconds). The end of the reaction was defined with an argon quench which cooled down the reactants to ~ 400 °C and froze the state-of-reaction for subsequent analysis, thus setting a similar temperature profile to a moving bed reactor used for thermal cracking.

[0060] The instrumentation of the lab reactor comprised an eight-fold-multi-thermocouple in a ceramic shield of 10 x 2 mm on the central axis of the reaction tube in the carbon bed, offline gas sampling after the quench and offline PAH measurements.

[0061] Experiments were performed using carbonaceous carrier materials with different specific surface areas. Experimental conditions were set that led to yields and selectivities comparable to state-of-the-art of conventional steam crackers. Specifically, the experiments were performed at a temperature of 850 °C, a pressure of 1.1 bara, and a residence time of 450 ms. For all experiments, 40 vol.-% dilution with inert gas was set.

[0062] The determination of the coking yield was based on measuring the carbon deposition on the carbon bed due to the reaction by gravimetry of the carbon bed before and after the cracking experiment and then relating it to the total amount of feed used during the course of the experiments.

[0063] The results are shown in Fig. 1. It is evident that low specific surface areas allow for reduced coking yields. Example 2

[0064] The influence of the content of Ni and Fe in carbonaceous particles on the formation of filamentous carbon structures during propane and hexane cracking was examined. As substrate materials, C99 (2 to 4 mm; 165 ppm Ni; 290 ppm Fe; 6.75 m2 / g) for propane and Carbolux SK (2 to 4 mm; 5 ppm Ni; 10 ppm Fe; 3.4 m2 / g) for hexane were compared.

[0065] Experiments were carried out in an identical setup to that described for Example 1 . The experiments were performed at a temperature of 850 °C, a pressure of 1.06 bara, and a residence time of 490 ms.

[0066] The results are shown in Fig. 2a (C99) and 2b (Carbolux SK). It is evident that carbonaceous particles with relatively high Ni and Fe contents form a significant number of filamentous carbon structures, which increase the particle surface. On the other hand, carbonaceous particles with relatively low Ni and Fe content do not form filamentous carbon structures.

Claims

Claims1 . A method for performing a cracking reaction of hydrocarbons having at least two carbons to yield olefins, comprising allowing carbonaceous particles to flow downwardly under gravity flow as a particle bed through a reaction zone; maintaining the particles in the reaction zone at a temperature of 700 to 900 °C; feeding a feed gas stream comprising hydrocarbons having at least two carbons through the reaction zone in counter current to the particles flow; withdrawing the carbonaceous particles from the reaction zone; and withdrawing a product gas stream comprising cracking products from the reaction zone, wherein the carbonaceous particles have a surface area of less than 15 m2 / g, preferably less than 8 m2 / g.

2. The method according to claim 1 , wherein the carbonaceous particles have a particle size distribution characterized by a weight average particle diameter (D50) of 1 to 200 mm, preferably 2.5 to 30 mm.

3. The method according to claim 1 or 2, wherein the carbonaceous particles have an accessible porosity, as determined by mercury intrusion porosimetry, of less than 65 %.

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

6. The method according to any one of the preceding claims, additionally comprising subsequently guiding the carbonaceous particles through a first heat integration zone located above the reaction zone, the reaction zone, and a second heat integration zone located below the reaction zone; transferring heat from the product gas leaving the reaction zone to the carbonaceous particles in the first heat integration zone by direct heat transfer; and transferring heat from the carbonaceous particles leaving the reaction zone to the feed gas in the second heat integration zone.

7. The method according to any one of the preceding claims, wherein the feed gas stream comprises a diluent gas.

8. The method according to claim 6 or 7, wherein the feed gas volume flow, the proportion of diluent gas and / or the particle mass flow are selected such that the product gas temperature leaves the first heat integration zone at a temperature below 450 °C and / or the carbonaceous particles leave the second heat integration zone at a temperature of below 450 °C.

9. The method according to any one of the preceding claims, further comprising recycling the carbonaceous particles to the reaction zone.

10. The method according to any one of the preceding claims, wherein the carbonaceous particles withdrawn from the reaction zone exhibit a coking yield of at most 1.8 wt.-%.

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