Electrically heated steam cracking furnace for olefin production

Abstract

An electrically heated furnace including one or more unit cells, each unit cell including a radiant heating section, one or more processing coils disposed within the radiant heating section, and a quenching unit for cooling decomposition products from the one or more processing coils to produce a quenched reaction product. The furnace also includes one or more electric heating elements disposed within the radiant heating section, the one or more electric heating elements being arranged to provide radiant energy to the one or more processing coils. The electrically heated furnace further includes a first region corresponding to a heating area of ​​the one or more electric heating elements, a second region corresponding to a wall surface area of ​​the wall on which the one or more electric heating elements are disposed, and a third region corresponding to a surface area of ​​the one or more processing coils.

Description

[Technical field]

[0001] SUMMARY OF THE DISCLOSURE Embodiments of the present disclosure relate generally to electrically heated furnaces for the large-scale production of bulk chemicals such as ethylene, propylene, butadiene, as well as aromatics and other products (product gases) from a hydrocarbon feedstock (feed gas). [Background technology]

[0002] Steam crackers are the core of petrochemical production facilities as they produce bulk chemicals for downstream processing. However, the energy source for steam crackers is typically from the combustion of fossil fuels, with energy inputs of over 900 MW in world-scale petrochemical facilities. Large-scale production is achieved by containing multiple (e.g., 6-8) reaction "coils" within a single furnace enclosure, using fossil fuels as the energy source for endothermic reactions that convert the hydrocarbon feedstock into useful products such as olefins. Tubular reactors are the preferred configuration, and there are multiple configurations known in the art. Preheated hydrocarbon feed enters the coil at 500-730°C and is rapidly heated to 750-925°C. Tubular reactors (i.e., cracking coils) are designed to optimize the temperature and pressure profiles along the radiant coil to maximize the yield of valuable products with a more rapid temperature rise at the inlet section of the cracking coil and a low pressure drop at the outlet section. However, the required energy input is very large. When this energy is provided by burning fuels, significant amounts of CO2 are generated, ranging from 0.3 to 1.6 tonnes of CO2 per tonne of ethylene (t / t), depending on the feedstock to the steam cracker. Furthermore, the high temperatures required for pyrolysis result in NOx emissions in the flue gases.

[0003] A typical fuel-fired furnace used for pyrolysis of hydrocarbons is shown in FIG. 1. The furnace 100 includes a radiant section 10 having bed combustors 12 and / or wall combustors 12, and a convection section 14. The convection section 14 includes heating coils for recovering convection heat from flue gases that can be used to preheat the hydrocarbon feedstock 16, generate steam 20 from a boiler feedwater (BF) 18, and superheat the steam feedstock (Steam) 22 to high pressure steam (HP Steam) 20. Once the feedstock has been preheated in one or more convection coils, it is fed to a reaction coil located in the radiant section 10 of the heater where it is rapidly heated to effect the desired cracking of the hydrocarbons and produce cracked gases 22. To prevent over-cracking, a transfer line exchanger 24 can be provided to rapidly cool the radiant section effluent and provide a cooled cracked gas product stream 26. Steam drums 28 and other heat recovery devices can also be used to facilitate heating, cooling, or mixing of the hydrocarbon and steam feedstocks depending on the particular flow scheme.

[0004] Various publications related to hydrocarbon cracking furnaces include: International Publication No. WO2020245016A1 discloses one or more heat-consuming processes (above 500°C), at least one of which is electrically heated; The products of the heat-consuming processes are sent to an energy carrier network with a capacity of 5 GWh; US Patent Publication No. US5321191A describes a pyrolysis process in an electric furnace with a monolithic structure constructed from ceramic; International Publication Nos. WO2021214256A1 and WO2021214257A1 relate to a protection system for an electric reactor in case the reactor tube is damaged and flammable gas is released into the reactor housing; US Patent Publication No. US20090022635A1 teaches that a furnace for pyrolysis of hydrocarbons can be subdivided into a number of separate sectors and that the heat source can be concentrated in the separate sectors. US Patent Publication No. US9908091B2 discloses a furnace for steam reforming, comprising at least one combustor and at least one voltage source connected to the reaction tubes. An electric current is passed through the reaction tubes to heat the feedstock. US Patent Publication No. US20210254774A1 discloses a steam reformer with a combustion chamber comprising reaction tubes. A heating element is installed in the reaction tubes. US Patent Publication No. US20210051770A1 discloses an electrically heatable solid-packed device for endothermic reactions, including the dehydrogenation of propane (described, but not detailed). The packed bed is divided into an electrically insulated upper, middle and lower part. Heating is provided by electrodes installed in the conductive solid packing. US Patent Publication No. US20210171344A1 discloses a steam reforming reactor having a macroscopic structure supporting a catalytic coating. An electric current is passed through the macroscopic structure, heating parts of the catalyst to above 500°C.

[0005] U.S. Patent Publication No. US20210113980 ('980) describes a reactor system that includes an electrically heated furnace with at least one electric radiant heating element within the furnace (heat is transferred to the reactor tubes by radiation). Figure 2 herein is a reproduction of Figure 1 of '980 showing a heating scheme that includes radiant and convective heat, where radiant heat is directed from the hot surface (T3) of the reactor wall or insulation as well as a heating element (T2) and convective heat is provided by the circulation of steam within the reactor, each providing heat to the reactor tubes (T1). Summary of the Invention

[0006] This Summary introduces various concepts that are described in more detail below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0007] In one aspect, the embodiments disclosed herein relate to an electrically heated furnace. The furnace includes one or more unit cells. Each unit cell includes a radiant heating section, one or more processing coils disposed within the radiant heating section, and a quenching unit for cooling decomposition products from the one or more processing coils to generate quenched reaction products. The furnace also includes one or more electric heating elements disposed within the radiant heating section, the one or more electric heating elements being arranged to provide radiant energy to the one or more processing coils. Furthermore, the electrically heated furnace includes a first region corresponding to the heating area of ​​the one or more electric heating elements, a second region corresponding to the wall surface area of ​​the wall where the one or more electric heating elements are disposed, and a third region corresponding to the surface area of ​​the one or more processing coils.

[0008] Other aspects and advantages of the claimed subject matter will become apparent from the following description and the appended claims. [Brief description of the drawings]

[0009] [Figure 1]FIG. 1 (Prior Art) shows a prior art system used to produce ethylene and other bulk chemicals from hydrocarbon feedstocks, powered by the combustion of fuel gases (and / or liquid fuels). [Diagram 2] FIG. 2 (Prior Art) shows a furnace system described in U.S. Patent Publication No. US20210113980, in which a reactor with multiple coils is supplied with bulk electrical energy and the electrical supply is divided into zones. [Diagram 3] FIG. 1 is an illustration of an electric heating element used in embodiments herein with advantageous coil spacing and coils grouped into unit cells / housings. [Figure 4] 1 is a heater system in which multiple unit cells are combined to form a single heater system, according to embodiments herein. [Diagram 5] FIG. 2 is a three-dimensional layout diagram of a unit cell using a SiC heating element system according to embodiments herein. [Figure 5A] FIG. 2 is a three-dimensional layout diagram of a unit cell using a SiC heating element system according to embodiments herein. [Figure 6] 1 is a general layout diagram of a furnace system according to an embodiment herein. [Figure 6A] 1 is a general layout diagram of a furnace system according to an embodiment herein. [Figure 7] FIG. 1 is a schematic diagram illustrating the layout of two unit cells with a common feed inlet, showing the components included in a defined unit cell (power controller / transformer, heating element of the housing, quenching device) according to an embodiment herein. [Figure 7A] FIG. 1 is a schematic diagram illustrating a layout of two unit cells with a common feed inlet, according to embodiments herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] One way to reduce or eliminate emissions from large-scale olefin production is through the electrification of energy sources, preferably powered by renewable energy. Embodiments herein relate to electric furnaces for producing petrochemical products such as ethylene, propylene, butadiene, as well as aromatics and other products for downstream processing. More specifically, embodiments herein are directed to electrically heated furnaces for the large-scale production of bulk chemicals such as ethylene, propylene, butadiene, as well as aromatics and other products (product gas) from a hydrocarbon feedstock (feed gas).

[0011] Rather than relying on burning fossil fuels for the required energy input, embodiments herein provide a furnace that electrically resistively heats multiple reaction coils in parallel. Each coil is housed in a separate unit cell housing having a heating area A1 corresponding to the heating area of ​​a resistive element installed on the wall of the unit cell housing, a wall area A2 corresponding to the area on the wall where the resistive element is installed, and a coil area A3 corresponding to the surface area of ​​the reaction coil. The heating area A1 and wall area A2 are larger than the reaction coil area A3.

[0012] The unit cells are arranged along an axis, and the reaction coil in each unit cell is configured to be perpendicular to this axis or at an angle selected between 90 and 45 degrees. By arranging the unit cells at an angle of 45 to 60 degrees, for example, the total installation space of the reactor can be reduced.

[0013] There are several methods for providing electrical energy to heat the coils and thus provide the necessary heat flux to the hydrocarbon feedstock for the pyrolysis process, including direct resistive heating by passing an electric current through the reactor coil, inducing an electric current by surrounding the coil with an electromagnet, and radiative heating from a resistive heating element adjacent to the coil.

[0014] The embodiments herein can apply any of these methods. Some embodiments include heating methods that require resistive heating elements on the walls of the housing surrounding the reaction tube, as they can be used without modification of existing reaction coil designs. Electrical heating elements are limited in heat flux at high temperatures. The heating element receives an electric current at an applied voltage and heat is generated based on the element resistance. The temperature of the element increases until the electrical energy is dissipated as heat or the material reaches its temperature limit and fails. Thus, resistive heating elements are limited in the heat flux that can be applied at a given temperature. As the operating temperature increases, the maximum achievable heat flux decreases.

[0015] The heating elements are supported on a refractory wall and connected to a power source. Resistive heating elements dissipate electrical energy by Joule heating and radiation. Electric heating elements radiate energy towards the reaction coil or furnace surfaces, e.g. the walls on which the elements are supported. There are several types of heating elements suitable for temperatures in ethylene furnaces, including NiCr, FeCrAl, SiC, and MoSi2. NiCr is the least expensive but is the most limited with a maximum temperature of 1100 °C. FeCrAl is also a type of metal element that can be used up to about 1300 °C. SiC elements can be applied up to over 1600 °C, and MoSi2 elements up to 1750 °C. These values ​​vary depending on the reference source, but the quoted values ​​are representative. In addition, the aging of the element and an adequate operating margin must be considered. Operating the heating element at temperatures close to the maximum allowable temperature can shorten the useful life of the element, and even small increases due to operational changes can lead to element failure. Therefore, to maximize the life of the element, it is desirable to operate it as far away from the maximum allowable temperature as possible. In practice, this means that it is desirable to have a wall area that is larger than the surface area of ​​the heat sink (reaction coil). In some embodiments, the heating elements may be in a rod-over-bend (ROB) configuration or in a rigid silicon carbide rod type element. For example, FeCrAl (metal) elements can be provided in a rod-over-bend (ROB) configuration or in a rigid silicon carbide rod type element. Rigid SiC elements are used in one or more embodiments because they are available in lengths of 1 m to 10 m, e.g., about 5 m. Such lengths are compatible with the dimensions of the housing that houses the individual reaction coils. A number of rigid SiC elements can be arranged horizontally in a single span adjacent to and supported on the wall of the furnace housing (forming a unit cell). The electric heating elements and reaction coils require periodic maintenance or replacement. Placing the elements on the wall and away from the reaction coils allows access to the furnace for maintenance activities.

[0016] For larger heater installations (e.g., eight or more unit cells), the individual unit cells can be configured such that, in the event of a single coil rupture, the electrical supply to the affected unit cell is immediately stopped, while the electrical supply to the remaining unit cells can be stopped as needed in a manner that does not disrupt the electrical supply utility.

[0017] The unit cells are arranged to allow for advantageous coil configurations. Unlike US20090022635A1, where the furnace is divided into sectors, each radiant coil of the embodiments herein is enclosed in a single unit cell, allowing for coil replacement and maintenance between each unit cell. Carbon deposits, or coke, typically form on the inner walls of the reaction coils. Coking deposits reduce heat exchange efficiency, causing increased tube wall temperatures and increased pressure drop. Periodic decoking operations are required to keep the tube metal temperature below its maximum allowable design temperature and / or to reduce pressure build-up in the coils, which affects yield. Decoking is typically performed at lower temperatures and with lower heat input than the pyrolysis reaction. Conventional furnaces containing multiple reaction coils may require extra heater capacity (e.g., multiple additional coils) to allow for scheduling of decoking operations. Decoking of a unit cell according to one or more embodiments herein is greatly simplified by the fact that each reaction coil is housed in a separate unit cell, thus eliminating the need for extra heaters. Decoking with steam alone is slow, but the effluent can be passed through a heat recovery section and is facilitated by the unit cell disclosed herein.

[0018] For radiative heating using electrical resistive heating elements, unit cells can be grouped in sets to form a single heater system. This arrangement increases the available area of ​​the housing sidewall, ideally 1.2 to 3.0 times the heat sink area, lowering the temperature of the element required to dissipate a given amount of heat. It is important to note that operating resistive heating elements "at the limit" for extended periods of time shortens the life of the elements and does not provide sufficient design margin in case some of the elements fail. Improvements in element technology have resulted in available area to reactor (coil) surface area ratios below 1.2 and sometimes as low as 0.5. However, embodiments herein may be applied with ratios greater than 1.0 for the reasons discussed above, with the achievable area ratio depending on the maximum heat flux of the heating element, which may be different for metal elements compared to ceramic SiC elements.

[0019] Each unit cell contains an air control device that operates in air or an atmosphere controlled to an oxygen concentration low enough to avoid explosion hazards, but sufficient to maintain a protective oxide layer on the device surfaces.

[0020] A world-scale steam cracker requires a large number of tubes to achieve the required capacity. The reactor tubes are grouped into "coils". To obtain maximum product yield, the coil configuration is carefully selected based on cycle time (between one decoking operation and the next), residence time, pressure drop, and heat transfer.

[0021] It is desirable to maintain coil configurations that have been developed over many years and have a long history of proven operation. Cracker heaters for ethylene production can have multiple radiant coils arranged in a single row in a linear fashion. Often, the two rows are arranged in either an offset or staggered arrangement. Even multiple rows of coils may be used, as described in Lummus patent application PCT / US2021 / 59542, the coil arrangements of which are incorporated herein by reference. Such multiple row arrangements are used to maximize the amount of ethylene product that can be obtained from a single heater system before duplication of the system (i.e., building a second, third heater, etc.) is required. Typically, multiple coils are grouped together as a single "heater." In this context, multiple coils in a heater can be considered as individual "heater systems" with a single housing, and the unit of ethylene production capacity for the facility is typically 100-300 KTA of ethylene production capacity per heater. Multiple individual heater systems may be required to achieve the desired capacity. In the prior art, heater systems employing electrical heating have been considered only as a collection of multiple coils within a single heater system, however, the heating methods proposed in the prior art have not been considered in the context of providing heat by electrical means to a number of individual reactor tubes configured as radiant coils.

[0022] Coil designs vary widely. Coil configurations are sometimes referred to using shorthand notation as xy-arranged, xyz-arranged, xyzw-arranged, or other, where x refers to the number of inlet tubes, y refers to the number of tubes in the next pass, the outlet pass (xy) or the second pass xyz, and z is the outlet pass. For example, in a 4-1 arrangement, four inlet tubes feed one outlet tube. Numerous coil design possibilities are discussed within PCT / US2021 / 59542. Coil configurations include a simple X arrangement, where X tubes are connected in parallel to form a coil.

[0023] The heater system according to embodiments herein includes feed preheat, indirect quench, and high pressure steam generation and product cooling. Each unit cell has a dedicated quench to minimize residence time in the adiabatic section ahead of the quench cooler. The quench system cools the cracked gases to a temperature low enough to freeze the reaction, but high enough to allow significant heat recovery, ideally in the range of 550-680°C. This is important because, as previously mentioned, electric heaters do not have flue gases and the required electrical energy should be prioritized to provide the heat of reaction. To use electrical energy to (effectively) generate steam or preheat the feed at low temperatures would require a more extensive electrical supply system. In certain cases where high levels of steam generation are required for economic (or other) reasons, the steam temperature may be lower than 550°C, and in other embodiments may be as low as 300°C. In such situations, feed preheating can be accomplished by other means, including a separate electrical heater.

[0024] The unit cells are, in some embodiments, configured such that their longest dimension in a horizontal plane is parallel to the plane of the element, along which the tubes are arranged in a single row or multiple rows.

[0025] For large heater installations (8 or more unit cells), the individual unit cells can be configured so that in the event of a single coil rupture, electrical supply to the affected unit cell is immediately stopped, while electrical supply to the remaining unit cells can be stopped in a manner that does not disrupt the electrical supply utility, minimizing the amount of gas that needs to be vented to a flare system or other safety system.

[0026] In some embodiments, a unit cell size of approximately 3.0 x 5.0 x 15.0 m may be used, with a single SiC element across the longest dimension of the horizontal plane. As previously mentioned, this allows for easy maintenance and access to both the heating element and the reaction coil. The elements may also be connected in series (2) such that all electrical connections are on one side of the unit cell. Alternatively, a delta or wye configuration may be used to connect a group of elements to a three-phase power source. Such a configuration may be chosen in preference to a series arrangement, especially for SiC type elements, as variations in element resistance values ​​may accelerate aging. The furnace temperature is in the range of 1150-1250°C, and the heat flux (based on wall area product) is approximately 50-120 kW / m 2 This is well below the maximum capacity of commercially available SiC heating elements. In one or more embodiments, the area of ​​the sidewall heating element is about 1.0-1.6 times the surface area of ​​the reaction coil. However, for high heat flux SiC or improved elements, the ratio can be less than 1.2, such as 0.8-1.2 or 1.0-1.2.

[0027] In another embodiment, a unit cell size of about 3.0 x 10.0 x 15.0 m can be used with the metal elements. In this configuration, the heat flux of the element is about 30-40 kW / m 2 , or 33 kW / m 2 This allows the use of metal elements at temperatures of about 1200° C. The sidewall area is about 3.0 times the surface area of ​​the reaction coil.

[0028] In both the above examples, the heating load is about 10 MW, which is a convenient amount for power supply. Two 5 MVA or one 10 MVA transformer can be used to supply each unit cell. This provides a great deal of flexibility in operation, allowing: (i) each coil to be operated with different feedstocks and different severities; (ii) individual coils to be operated with different decoking cycles; (iii) the power supplied to each unit cell can be varied based on a pre-determined coil outlet temperature; and (iv) individual coils to run a conventional steam / air decoking process with appropriate flow distribution control. Alternatively, individual coils can be operated with steam only decoking while other coils are operated in cracking mode.

[0029] FIG. 3 shows the arrangement of coils within each unit cell housing 300 and the arrangement of multiple unit cells 300 relative to each other. As shown, each coil 302 group consists of 28 inlet tubes and 4 outlet tubes connected through a common manifold. However, other coil designs are possible, such as configurations where there is a collection of individual tubes with inlet and outlet sections, split coils with a small inlet coil connected to a larger outlet coil, or swaged U- or W-configurations. For illustration purposes, a split coil configuration is shown. However, any type of coil can be used, such as many parallel tubes (known in the industry as a single pass coil) or a multi-pass serpentine coil (a single coil with many tubes connected in series).

[0030] Each coil group contained within the unit cell housing is heated by an electric heating device 304, which in some embodiments uses SiC or FeCrAl electric resistance heating elements mounted on a wall(s) of the unit cell housing. The unit cell housing can be insulated using known fire-resistant insulation, which can include hangers or the like for supporting the heating elements on the wall.

[0031] To illustrate the unit cell concept, two calculations are provided for radiative heat transfer from a heating element installed on the wall (surface 1) to the reactor tube coil assembly (surface 2) using a simplified two-surface resistance model. EXAMPLES

[0032] Example 1: For metallic FeCrAl heating elements, maximum heat flux of about 33 kW / m 2 , furnace temperature 1200℃, tube metal temperature 1080℃, absorption heat flux 86kW / m 2 The dimensions of the unit cell are 3m x 5m x 14m, so the area of ​​the side wall is 2.8 x 10m x 14m = 280m 2 On the other hand, the area of ​​the pipe is 105m 2 (ratio 2.7). If the desired heat load for the reaction is 9.0 MW, it can be supplied using a wall heat flux of 32.3 MW and an element temperature of 1175°C.

[0033] Example 2: Ceramic SiC heating element at furnace temperature of 1300°C with a heat flux of approximately 70 kW / m 2 , tube metal temperature 1080℃, absorbed heat flux 86kW / m 2 The dimensions of the unit cell are 2.8m x 5m x 14m, and the area of ​​the side walls is 105m. 2 Compared to the area of ​​the canal, the area is 140m 2 (ratio 1.3). For the same heat load of 9.0 MW as in Example 1, the wall temperature is 1237°C.

[0034] Therefore, the dimensions of each unit cell can be selected based on the required heat flux at the walls and the absorbed heat flux at the reactor coil.

[0035] Referring to FIG. 4, the plurality of unit cells 400 are arranged such that the longest wall in the horizontal (xy) plane is directly adjacent to the next unit cell, and so on. The individual reaction tubes 402 are arranged in one or more rows, with inlet tubes 406 disposed on either side of the grouped outlet tubes toward the center of the housing. Each of the plurality of unit cells 400 is heated using an electric heating device 404. Each unit cell includes a quench unit (not shown in FIG. 4 for clarity) that receives the cracked gas from the outlet tube and cools it to a temperature below the reaction temperature, i.e., from the reaction temperature to a temperature below 700° C., for example, in the range of 550-680° C.

[0036] The cracked gas produced in the reaction coil of each unit cell 400 can be cooled and fed into a single transfer line dedicated to the group of unit cells, and then fed into the main transfer line of the entire facility for downstream separation. A preferred method for further cooling the cracked gas from the quench unit is a feed / effluent heat exchanger, often called a secondary transfer line exchanger (STLE). There are many suitable configurations of STLEs, and one STLE can cool the cracked gas and heat feed of a single unit cell, or multiple unit cells. It is desirable to heat the feedstock as high as possible to minimize the power required to heat the furnace. The feedstock can be heated to temperatures above 550°C, preferably above 600°C, or even as high as 635°C, before being fed into the reaction coil.

[0037] FIG. 5 (multiple unit cells) and FIG. 5A (single unit cell) show a unit cell 500 that uses silicon carbide heating elements 504. The heating elements 504 are shown in a horizontal arrangement as an example. The heating elements 504 can also be arranged vertically and can be shaped differently depending on the particular installation requirements. In FIG. 5, the row of unit cells 500 is arranged along an axis 508. Hydrocarbons flow along this line from the inlet side to the TLE side.

[0038] Considering the high current and physical size of an electrically heated steam cracking furnace, it is advantageous to reduce the length of cable required to power the heating elements. In some embodiments, an electrical circuit is selected that allows the cable to be routed to one side of the unit cell. For SiC elements that span the entire length of the unit cell, a circuit configuration using a Wye or Delta arrangement can be used. The advantage of a Wye circuit is that all heating element ends terminate to a common neutral bus on one side of the unit cell, while all line voltage connections are on the other side of the unit cell, limiting cable routing to only one side of the unit cell, resulting in a simplified wiring scheme. Similarly, a Delta circuit could be used, but with two adjacent elements connected in series to avoid running line voltage cables on both sides of the unit cell. Multiple elements can be grouped into a circuit that is regulated using a voltage control unit. Silicon controlled rectifiers (SCRs) are semiconductor switching devices that provide fast, stepless, proportional control of power to a resistive load and are particularly well suited for the current application. A single SCR can carry currents from 250A to 2100A, depending on the number of elements in each circuit. Each unit cell has multiple heating circuits that are individually controlled. For example, a 10MW unit cell has six circuits, each controlling 1.66MW of power.

[0039] 6 and 6A are general arrangements of a grouping of unit cells 600 arranged as a heater system, with each unit cell 600 having a reaction coil 602. As shown, in some embodiments, each unit cell 600 has a single dedicated quench unit 610, fed by a steam drum in this case. The effluent is further cooled in STLE heat exchanger 612 against the incoming feed, dilute steam mixture. STLE 612 can heat the feed from one, two, or more unit cells 600 and cool the cracked gas. Instead of the hydrocarbon steam mixture, other fluids such as steam only can be considered. However, in other embodiments, each unit cell can have an individual quench system, the feed preheat / effluent cooling heat exchanger can be shared between two or more unit cells, or there can be two feed preheat / effluent cooling heat exchangers per unit cell.

[0040] 7 and 7A show one or more unit cells 700 sharing a common feedstock 702 including a hydrocarbon 704 and dilution steam 706.

[0041] Embodiments herein can include unit cells 700, each including (i) a single reactor coil consisting of reactor tubes with a common feedstock, (ii) a quench cooler 710, (iii) an enclosure, (iv) heating elements 712 connected to a power source via bus bars 714 (or the like), (v) a power controller 716, and (vi) a transformer 718. Additionally, multiple unit cells share a common high voltage power source.

[0042] In the embodiment shown in FIG. 7, the feedstock 702 and dilution steam are preheated in the first preheat section 720 and then further heated in the secondary TLE 722 against the quenched cracked gas from the quench unit 710 from both unit cells. Two or more unit cells can share a common feedstock and a common feedstock preheating device. In the embodiment shown in FIG. 7 / 7A, two unit cells share a common feedstock with associated preheating and heat exchange of the secondary transfer line, but the primary quench system is part of the unit cell system. The mixture of feedstock and dilution steam is split between the two unit cells and fed to the reaction coil 726, which consists of a group of reaction tubes each individually housed in the housing of the unit cell 700. The reactants are heated to a temperature of 750-925° C. and pyrolyzed to produce cracked gas 728 in each set of reactor coils. In the embodiment shown in FIG. 7A, the feedstock 704 is mixed with the dilution steam 706 and heated in the naphtha heater 724 before being fed to the first preheat section 720.

[0043] Radiative heating is provided by resistive heating elements 712 within the housing of the unit cell 700, preferably mounted on two side walls of the housing. The elements are supplied with current from a high voltage mains supply (Bus A 714), such as in the range of 13 kV to 34.5 kV, via a power supply controller 716 and a transformer 718 that steps down the high supply voltage to a lower voltage, and are fed by the heat flux absorbed by the reactor coil.

[0044] The cracked gas 728 produced in the reaction coil 726 of each unit cell 700 can be cooled and fed into a single transfer line dedicated to the group of unit cells (quench unit 710) and then fed into the main transfer line for the entire facility for downstream separation. Some embodiments provide for further cooling of the cracked gas from the quench unit 701 in a feed / effluent heat exchanger 722, often referred to as a secondary transfer line exchanger (STLE).

[0045] In the radiant coil, the hydrocarbon feedstock must first be heated to reaction temperature prior to the pyrolysis reaction. It is known to use multiple inlet tubes for each outlet tube to rapidly heat the fluid (in a single or a few large diameter outlet tubes) with a short residence time before cracking at high temperature. The short residence time in the reaction section improves selectivity to the desired products.

[0046] In the case of an electrically heated furnace, there is no flue gas, so any heat recovery comes primarily from the cracked gas 728. To minimize the amount of electrical energy required to reach reaction temperature, it is desirable to maximize the temperature of the feed entering the inlet of the reaction coil 712. Ideally, the electrical energy should prioritize the endothermic heat of reaction required to produce the desired products.

[0047] It is also desirable to rapidly quench the reaction products, which exit the radiant coil at about 750°C to 925°C. Ideally, the cracked gases 728 should be cooled instantly, but in practice this means that the residence time in the insulation between the furnace outlet and the quench system should be short, ideally less than 10% of the residence time in the radiant coil. Direct and indirect quenching methods have been used, with indirect quenching generally being the preferred method, which can generate valuable high pressure steam. For the reasons stated above, the quench cooler (also called the primary transfer line exchanger) is usually located as close as possible to the outlet of the reaction coil. Suitable heat exchanger types are the Arvos transfer line exchanger, bathtub type, rapid quench type, or Borsig "tunnel flow" or "linear type" transfer line exchangers. Other ultra-high pressure or medium pressure exchangers may also be used. In such a system, boiler feed water 730 may be used to cool the cracked products 728 and generate steam 732. A steam drum 734 can be used to recover steam and produce an export stream 736 and a condensate 738 .

[0048] In the embodiment furnaces herein, the reaction coils are not grouped into a "heater system" but are instead housed in individual unit cells. The housing (furnace box) is therefore divided into separate sections with dedicated electrical supplies and controls for each unit cell. In the embodiments herein, each unit cell can be considered an individual control zone, completely independent of the coils contained within each unit cell. The power supplied to each unit cell can be varied based on the measured coil outlet temperature.

[0049] The unit cell concept is particularly useful when the heating method is radiative heating with electrical resistance elements. In the case of metal elements made of FeCrAl wire, by increasing the coil spacing and the area of ​​the side walls, the elements can be mounted on the wall and the temperature can be increased to 1100-1200 °C and the heat flux to 30-40 kW / m 2 In the prior art, the elements had to be mounted on a single very long wall (perhaps over 90m long). Conceptually, the single long furnace is essentially divided into several smaller sections, each of which is then rotated 90 degrees so that the longest dimension of the coil assembly in the horizontal xy plane (z is the vertical height) is perpendicular to the axis.

[0050] In the embodiments herein, SiC heating elements can be advantageously used because the dimensions of the box are suitable to allow, for example, (i) a rigid element that can be installed horizontally on the furnace wall, covering its longest dimension in a horizontal plane. Installation of such an element allows the element to be removed independently of the reaction coil during operation - or in case of major maintenance work (i.e., the element does not need to be removed to remove the coil and vice versa). In the prior art, a single heating system enclosure would be too long to allow a single element to cover the longest dimension of the enclosure. and (ii) multiple elements installed horizontally from top to bottom, possibly in separate zones.

[0051] In summary, a desirable electric heater system would have the following characteristics: (i) maintain best design practices for coil placement and residence time, providing flexibility in coil placement; (ii) allow the use of available heating elements that are proven and widely used at the temperatures required for ethylene cracking furnaces; (iii) have a heating element surface area that is greater than the surface area being heated (maximizing element life); (iv) provide safe operation by allowing a single unit cell to be isolated and shut down quickly in the event of a coil rupture without affecting the balance of the reactant system; (v) provide the highest possible feed inlet (crossover) temperature at the inlet to the radiant section, minimizing the amount of electrical energy that needs to be dissipated by the element since this is the highest value heating source; and (vi) promote rapid quenching of the reaction products, maximizing the yield of valuable products.

[0052] As discussed above, embodiments herein provide an electric furnace system suitable for large-scale pyrolysis processes. Since embodiments herein can maintain the optimal coil arrangement developed for fuel-fired furnaces, increased coking rates should not be a concern. In fact, coking may actually be somewhat lower with an electric heating system because electric heating may be more uniform than the coking limitations of combustion heating due to hot spots.

[0053] The embodiments herein further provide a furnace configured as a series of unit cells, each including a reactor coil with a corresponding manifold, each coil manifold aligned perpendicular to the axis. The heating elements can be mounted on the refractory wall, rather than along the reactor tube, or in the form of coaxial radiating sheets, and the area of ​​the heating element A1 and the area of ​​the refractory associated with the heating element (A2) are greater than the area of ​​the coil being heated (A3). Available heating element types are limited to a certain heat flux that decreases as the operating temperature increases, and the service life of the heating element also depends on the temperature at which the heating element is used. For example, FeCrAl type heating elements have a heat flux of about 30 kW / m when operated at a furnace temperature of 1200°C. 2 ~40kW / m2 The heat flux required for endothermic reactions is limited to 40 kW / m 2 above 1000 rpm, the surface area of ​​the heating elements must be increased to meet the heat flux and temperature constraints at the required heat load. The unit cell arrangement increases the ratio of refractory area to reactor coil area compared to prior art that includes multiple reactor coils in a single structure. Thus, embodiments herein allow for increased tube spacing within each unit cell.

[0054] The embodiments herein also relate to the arrangement of the reactor coils in the individual unit cells, which provides advantages in terms of a larger wall area per unit coil area and in terms of isolation of the unit cells with respect to electrical supply and in the event of coil rupture. It should be noted that the embodiments herein may use different heating methods or combine methods. For example, part of the required heat may be provided by non-radiative heat transfer, such as passing an electric current directly through the reactor coil. This may be direct or alternating current, but due to the relatively low electrical resistance of the reactor tubes, a large current is required to provide the full heating capacity in this manner. An alternative is to provide part of the heating capacity by non-radiative heating.

[0055] Optionally, an inert gas with reduced oxygen content (less than 10%) is supplied to the reactor housing to ensure that explosion or combustion events are limited in the event of a hydrocarbon release. While such precautions are anticipated, the housing of the unit cells also has the advantage that the amount of gas that must be vented from the housing during such an event is significantly reduced, thus limiting the event. In an embodiment, power to the affected unit cell can be immediately removed, while power to the remaining unit cells can be more gradually reduced as needed.

[0056] For electric heating according to the embodiments herein, the power output is in the range of 5-10 MVA for the input power. For a single coil assembly, the required heat input is comparable, with absorbed power in the range of 8-10 MW. Thus, the heat input to the furnace can be controlled by dividing the housing into unit cells, with each unit cell being supplied by a single transformer or a pair of transformers.

[0057] The voltage of the resistive element is low (200-600V) compared to the grid supply voltage (30-40kV). 200-600V is considered low voltage in the industry. Low voltage is used for illustration purposes. Medium voltage elements (1000-6000V) can also be used if available. The cost and number of controllers is highly affected by the current and can be much higher when the voltage is in the 300-600V range. In embodiments herein, the individual unit cells can be powered by a few or even a single step-down transformer. On the low voltage side of the step-down transformer, an SCR (silicon controlled rectifier) ​​controls the voltage supply to the circuit of SiC resistive elements. A PLC monitors the circuit voltage and current and calculates the power flowing through the element in real time. The PLC can then adjust the voltage supply through the SCR to achieve a given power demand set point. To avoid an overheating event in the heater element, the PLC also monitors the temperature of the element and reduces or limits the power set point or ultimately trips the circuit. Optionally, the temperature setpoint can be used by the PLC to vary the voltage at the SCR to achieve the target furnace temperature.

[0058] The inventors also calculated the absorption heat flux (e.g., 90 kW / m 2 ) and the maximum heat flux of the element (e.g., 33 kW / m for a metal element). 2 , 70kW / m for SiC 2 They also recognized that the coil spacing would need to be significantly larger to account for the difference in axial length (A) and axial length (B), which would require a larger wall area.

[0059] Given the possibility of a coil rupture, a controlled atmosphere is required. A safer approach is to isolate each coil within a unit cell from the rest of the system.

[0060] For example, if an induction coil is placed around each reactor tube, there will be insulation between the induction coil and the reactor tubes, and electrical connections to the induction coil will be around each reactor tube, not in the enclosure walls. High frequency alternating current is required. The enclosure will be relatively cool, but is still required to contain an inert atmosphere to limit combustion in the event of coil rupture.

[0061] Passing electrical current through each reaction coil again requires multiple electrical connections entering the housing. There are several electrical supply configurations that can be used with the present invention.

[0062] Each unit cell may be isolated from the others for a variety of reasons, including the release of hydrocarbons due to a leak or rupture, or to allow adjustments or maintenance to be carried out without significantly reducing the capacity of the entire plant.

[0063] The unit cells can also accommodate a decoking schedule. Each unit cell can be set to run more individual decoking cycles (up to one per unit cell), but with less capacity reduction for each decoking operation. One unit cell can be in decoking mode while other unit cells of a single heater are cracking hydrocarbons. This can be accomplished with steam-only decoking, with all waste liquid passing through the recovery section. When a heater (all unit cells or coils, or preselected unit cells / coils) is decoking, waste water can be vented to the atmosphere via the decoking drum.

[0064] Unless otherwise defined, all technical and scientific terms used have the same meaning as commonly understood by one of ordinary skill in the art to which these systems, devices, methods, processes, and compositions belong.

[0065] Unless the context clearly dictates otherwise, the singular forms "a," "an," and "the" include plural referents.

[0066] As used herein, the words "comprise," "have," and "include," as well as all grammatical variations thereof, are intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

[0067] "Optionally" means that the subsequently described event or circumstance may or may not occur. The description includes cases where the event or circumstance occurs and cases where it does not occur.

[0068] When the word "approximately" or "about" is used, the term can mean that the value can vary by ±10%, 5%, 2%, 1%, 0.5%, 0.1%, or 0.01%.

[0069] Ranges may be expressed as from about one particular value to about another particular value, inclusive. When such a range is expressed, it is to be understood that another embodiment is from the one particular value to the other particular value, and also includes all particular values ​​and combinations thereof within the range.

[0070] While the present disclosure includes a limited number of embodiments, those skilled in the art having the benefit of this disclosure will appreciate that other embodiments may be devised that do not depart from the scope of the present disclosure, which scope should therefore be limited only by the scope of the appended claims. [Prior art documents] [Patent documents]

[0071] [Patent Document 1] International Publication No. WO2021214256A1 [Patent Document 2] International Publication No. WO2021214257A1 [Patent Document 3] U.S. Patent Publication No. US20090022635A1 [Patent Document 4] U.S. Patent Publication No. US9908091B2 [Patent Document 5] U.S. Patent Publication No. US20210254774A1 [Patent Document 6] U.S. Patent Publication No. US20210051770A1

Claims

1. An electrically heated furnace, One or more unit cells, each unit cell comprising: a radiant heating section, one or more processing coils disposed within the radiant heating section, and a quenching unit for cooling a decomposition product from the one or more processing coils to produce a quenched reaction product. one or more electric heating elements disposed within the radiant heating section, the one or more electric heating elements being arranged to supply radiant energy to the one or more treatment coils. Including a unit cell, The electric heating furnace includes a first region corresponding to a heating area of ​​the one or more electric heating elements, a second region corresponding to a wall surface area of ​​a wall on which the one or more electric heating elements are disposed, and a third region corresponding to a surface area of ​​the one or more process coils. Electric heating furnace.

2. 10. The apparatus of claim 1 further comprising a feed inlet heat exchanger for preheating a reaction feed comprising one or more hydrocarbons and diluent steam to produce a preheated reaction feed.

3. 3. The apparatus of claim 2, further comprising a transfer line heat exchanger configured to indirectly exchange heat between the quenched reaction product and the preheated reaction feed.

4. 2. The apparatus of claim 1, wherein the one or more electric heating elements have a first end connected to a power source and a second end connected to a common ground terminal, the one or more electric heating elements being horizontally disposed along one or more walls of the radiant heating section, each of the first ends being disposed on a first side of the one or more walls and each of the second ends being disposed on a second side of the one or more walls.

5. 2. The apparatus of claim 1, wherein the one or more electric heating elements have a first end connected to a power source and a second end connected to a common ground terminal, the one or more electric heating elements are arranged horizontally along one or more walls of the radiant heating section, and the second end of a first electric heating element is electrically connected to the first end of a second electric heating element.

6. The apparatus of claim 1 , further comprising a power controller configured to receive power from a power grid.

7. The apparatus of claim 6 , further comprising a transformer configured to regulate a voltage from the power controller to an operating voltage of the one or more electric heating elements.

8. The one or more electric heating elements are selected from the group consisting of NiCr, FeCrAl, SiC and MoSi ２ The device of claim 1 , wherein the device is made of a material selected from the group consisting of:

9. The apparatus of claim 8 , wherein the one or more electric heating elements are made of SiC and have a length between 1 m and 10 m.

10. 10. The apparatus of claim 1, further comprising an oxygen controller configured to maintain an oxygen concentration in the radiant portion low enough to prevent explosion and high enough to maintain a protective oxide layer on a surface of the one or more electric heating elements.

11. The apparatus of claim 1 , wherein the first area and the second area are each larger than the third area.

12. The apparatus of claim 1 , wherein the one or more electric heating elements are positioned in the radiant section at an angle of between 45 degrees and 90 degrees relative to the one or more treatment coils.

13. The apparatus of claim 1 , wherein the one or more processing coils are fluidly connected to a common feed header.

14. The apparatus of claim 1 , wherein a plurality of the one or more unit cells are fluidly connected to a common quench unit.

15. 10. The apparatus of claim 1, wherein a first unit cell of the one or more unit cells can be operated in a decoking mode or a maintaining mode, while a second unit cell of the one or more unit cells can be operated in a cracking mode.

16. A hydrocarbon cracking apparatus comprising the electrically heated furnace according to claim 1.

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