Electric heating steam cracking furnace for olefin production

Abstract

The invention provides an electric heating furnace. The electric heating furnace comprises one or more unit chambers. Each unit chamber includes a radiant heating section, one or more process coils disposed within the radiant heating section, and a quench unit for cooling the cracked product from the one or more process coils and producing a quench reaction product. The furnace further includes one or more electrical heating elements disposed within the radiant heating section, the one or more electrical heating elements being arranged to provide radiant energy to the one or more process coils. Further, the electric heating furnace includes: a first area corresponding to a heating area of the one or more electric heating elements; a second area corresponding to a wall area of a wall on which the one or more electrical heating elements are disposed; and a third area corresponding to a surface area of the one or more process coils.

Description

Technical Field

[0001] Embodiments of the present disclosure are generally directed to an electrically heated furnace for large-scale production of bulk chemicals such as ethylene, propylene, and butadiene, as well as aromatics and other products (product gas), from hydrocarbon feedstocks (feed gas). Background Art

[0002] Steam crackers are the heart of petrochemical production facilities because they produce bulk chemicals for downstream processes. However, the energy for steam crackers usually comes from burning fossil fuels, and for world-class petrochemical facilities, the energy input exceeds 900MW. Large-scale production is achieved by including multiple (e.g., 6 to 8) reaction "coils" in a single furnace enclosure and using fossil fuels as energy sources for endothermic reactions that convert hydrocarbon feedstocks into useful products such as olefins. Tubular reactors are preferred structures and there are multiple configurations known in the art. The preheated hydrocarbon feed enters the coil at 500°C to 730°C and is rapidly heated to 750°C to 925°C. Tubular reactors (or cracking coils) are designed to optimize the temperature and pressure distribution along the radiant coils so as to maximize the yield of valuable products by more rapid temperature increases in the inlet section of the cracking coils and low pressure drops in the outlet section. However, the required energy input is very large. If this energy is supplied by burning fuel, large amounts of CO2 are produced ranging from 0.3 to 1.6 tons per ton of ethylene (t / t), depending on the feed to the steam cracker. In addition, due to the high temperatures required for pyrolysis, NOx emissions are produced in the flue gas.

[0003] FIG1 illustrates a typical fuel-fired furnace for thermal cracking of hydrocarbons. The furnace 100 includes a radiant section 10 with floor and / or wall burners 12 and a convection section 14. Disposed in the convection section 14 are heating coils for recovering convection heat from the flue gases, which may be used to preheat hydrocarbon feed 16, generate steam 20 from boiler feed (BF) 18, and may be used to superheat steam feed (steam) 22 into high pressure steam (HP steam) 20. Once the feed is preheated in one or more convection section coils, it may be supplied to the reaction coils disposed in the radiant section 10 of the heater to be rapidly heated, resulting in the desired cracking of the hydrocarbons, producing cracked gas 22. To prevent over-cracking, a transfer line exchanger 24 may be provided to rapidly cool the radiant section effluent, providing a cooled cracked gas product stream 26. Steam drums 28 and other heat recovery devices may also be used to facilitate heating, cooling, or mixing of the hydrocarbon and steam feeds as appropriate for a particular process scheme.

[0004] Various publications related to furnaces for cracking hydrocarbons may include the following. WO2020245016A1 discloses one or more heat-consuming processes (>500°C), at least one of which is electrically heated. The products of the heat-consuming process are transferred to an energy carrier network with a capacity of 5GWh. US5321191A describes a process for pyrolysis in an electric furnace having an integral structure composed of ceramics. WO2021214256A1 and WO2021214257A1 relate to a protection system for an electric reactor in the event that a reaction tube is damaged and combustible gas is released into the reactor enclosure. US20090022635A1 teaches that a furnace for thermal cracking hydrocarbons can be subdivided into multiple discrete sections and that the heat source can be concentrated in the discrete sections. US9908091B2 discloses a furnace for steam reforming, the furnace comprising at least one burner and at least one voltage source connected to a reactor tube. Current passes through the reactor tube to heat the feedstock. US20210254774A1 discloses a steam reformer having a combustion chamber containing a reactor tube. A heating element is placed inside the reactor tube. US20210051770A1 discloses an electrically heatable solid packing device (listed, but not described in detail) for endothermic reactions including propane dehydrogenation. The packed bed is divided into three electrically isolated upper, middle and lower sections. Heating is by means of electrodes mounted in a conductive solid packing. US20210171344A1 discloses a steam reforming reactor having a macrostructure supporting a catalyst coating. An electric current passes through the macrostructure to heat a portion of the catalyst to >500°C.

[0005] US20210113980 ('980) describes a reactor system including an electrically heated furnace having at least one electrically radiant heating element within the furnace (heat is transferred to the reactor tube by radiation). FIG. 2 herein is a reproduction of FIG. 1 of '980, illustrating that the heating mechanism includes radiant heat and convective heat, the radiant heat being emitted from the hot surface (T3) of the reactor wall or insulation layer and the heating element (T2), and the convective heat being provided by the circulation of steam within the reactor, each of which provides heat to the reactor tube (T1). Summary of the invention

[0006] This summary is provided to introduce a series of concepts that are further described 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 to help limit the scope of the claimed subject matter.

[0007] In one aspect, embodiments disclosed herein relate to an electrically heated furnace. The furnace includes one or more unit chambers. Each unit chamber includes a radiant heating section, one or more process coils disposed in the radiant heating section, and a quenching unit for cooling the cracked products from the one or more process coils and producing a quenched reaction product. The furnace also includes one or more electric heating elements disposed in the radiant heating section, the one or more electric heating elements being arranged to provide radiant energy to the one or more process coils. In addition, the electrically heated furnace includes: a first area, the first area corresponding to the heating area of ​​the one or more electric heating elements; a second area, the second area corresponding to the wall area of ​​the wall on which the one or more electric heating elements are disposed; and a third area, the third area corresponding to the surface area of ​​the one or more process coils.

[0008] Other aspects and advantages of the claimed subject matter will become apparent from the following description and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 (PRIOR ART) illustrates a prior art system for producing ethylene and other bulk chemicals from a hydrocarbon feed using energy supplied by the combustion of fuel gas (and / or liquid fuel).

[0010] Figure 2 (Prior Art) is a furnace system as described in US20210113980, whereby a large amount of electrical energy is supplied to a reactor having multiple coils, and the supply is divided into multiple zones.

[0011] Figure 3 is a diagrammatic representation of an electric heating element used in the embodiments herein, wherein advantageous coil spacing is provided and the coils are grouped in cells / enclosures.

[0012] Figure 4 A heater system of multiple unit chambers according to embodiments herein combined to form a single heater system.

[0013] Figure 5 and Figure 5A is a 3D arrangement of unit cells using a SiC heating element system according to embodiments herein.

[0014] Figure 6 and Fig. 6A is a general arrangement diagram of a furnace system according to embodiments herein.

[0015] Figure 7 and 7A is a schematic diagram showing a layout of two unit cells with a common feed inlet according to embodiments herein. The figure illustrates the components contained in the defined unit cells (power controller / transformer, enclosure heating element and quench device). DETAILED DESCRIPTION

[0016] One method of reducing or even eliminating emissions from large-scale production of olefins is to electrify energy, preferably wherein electricity is supplied by renewable energy. Embodiments herein relate to electric furnaces for the production of petrochemicals such as ethylene, propylene and butadiene, as well as aromatics and other products for downstream processes. More specifically, embodiments herein are directed to electric furnaces for large-scale production of bulk chemicals such as ethylene, propylene and butadiene, as well as aromatics and other products (product gases) from hydrocarbon feedstocks (feed gases).

[0017] Rather than relying on burning fossil fuels to obtain the required energy input, embodiments herein provide a furnace that employs multiple reaction coils in parallel that are heated with electrical resistance. Each coil is housed in a separate enclosure within a cell, with a heating area A1 corresponding to the heating area of ​​the resistance element mounted on the wall of the cell enclosure, a wall area A2 corresponding to the area of ​​the wall on which the element is mounted and a coil area A3 corresponding to the surface area of ​​the reaction coil. The heating area A1 and the wall area A2 are larger than the reaction coil area A3.

[0018] A plurality of unit cells are arranged along an axis and the reaction coils in each unit cell are arranged perpendicular to the axis or at a selected angle from 90 to 45 degrees. By placing the unit cells at an angle such as from 45 to 60 degrees, the footprint of the entire reactor can be reduced.

[0019] There are a variety of methods to supply electrical energy to heat the coils and provide the necessary heat flux to the hydrocarbon feed for the thermal cracking process. These methods may include direct resistive heating by passing an electric current through the reactor coils, inducing an electric current by surrounding the coils with electromagnets, or radiative heating by resistive heating elements adjacent to the coils.

[0020] Embodiments herein may apply any of these methods. Some embodiments include a heating method that requires a resistive heating element on the wall of the enclosure surrounding the reaction tube because the heating method can use an existing reaction coil design without any modification. Electrical heating elements have heat flux limitations at high temperatures. The heating element receives current at an applied voltage, and heat is generated based on the element resistance. The temperature of the element will increase until the electrical energy is dissipated in the form of heat, or the material reaches its temperature limit and fails. Therefore, the resistive heating element will have a limitation on the applied heat flux at a given temperature. As the operating temperature increases, the maximum heat flux that can be achieved decreases.

[0021] The heating elements are supported on the refractory wall and connected to a power source. The resistance heating elements dissipate electrical energy by means of Joule heating and radiation. The electric heating elements radiate energy to the reaction coil and the internal surfaces of the furnace, such as to the wall on which the elements are supported. There are a variety of types of heating elements suitable for the temperatures found in ethylene furnaces, including NiCr, FeCrAl, SiC and MoSi2 types. NiCr is the cheapest but also the most limited to a maximum temperature of 1100°C. FeCrAl is another type of metallic element that can be used up to about 1300°C. SiC elements can be applied up to 1600°C or more, and MoSi2 elements up to 1750°C. These values ​​vary from reference source to reference source but the values ​​quoted are representative. In addition, element aging and appropriate operating margins need to be considered. If the heating element is operated close to its maximum allowable temperature, the service life of the element may be shortened, and a small increase due to operational variations may cause the element to fail. Therefore, in order to obtain the longest service life, it is best to operate the element as far away from its maximum allowable temperature as possible. In practice, this means that the preferred approach is to provide a wall area that is larger than the surface area of ​​the heat sink (reaction coil). In some embodiments, the heating element can be in a rod overbend (ROB) configuration or a rigid silicon carbide rod-type element. For example, a FeCrAl (metal) element can be configured as a rod overbend (ROB) configuration or a rigid silicon carbide rod-type element. Because rigid SiC elements can be used for lengths of 1m to 10m (such as about 5m), they are used in one or more embodiments. Such a length is compatible with the size of the housing for accommodating each reaction coil. Multiple rigid SiC elements can be arranged horizontally in a single span close to and supported on the wall of the furnace enclosure (producing a unit chamber). The electric heating element and the reaction coil will need regular maintenance or replacement. By placing the element on the wall and separating it from the reaction coil, the furnace can be entered for maintenance activities.

[0022] For large heater installations (e.g., 8 or more cells), the individual cells can be configured so that in the event of a single coil rupture, power to the affected cell is immediately shut off, while power to the remaining cells can be shut off in a manner that does not cause a power interruption if necessary.

[0023] The unit chamber is arranged so that an advantageous coil configuration can be used. Unlike US20090022635A1, in which the furnace is divided into sections, each radiation coil of the embodiment herein is enclosed in a single unit chamber with a passage for coil replacement and maintenance between each unit chamber. Carbon deposits or coke are usually formed on the inner wall of the reaction coil. Coking deposits reduce the efficiency of heat exchange, resulting in increased tube wall temperature and increased pressure reduction. In order to keep the tube metal temperature below its maximum allowable design temperature and to limit the pressure increase in the coil that affects the output, or both, regular decoking operations are required. Decoking is usually carried out at a lower temperature and less heat input than the pyrolysis reaction. Conventional furnaces containing multiple reaction coils may require spare heater (e.g., multiple additional coils) capacity to allow the arrangement of decoking operations. According to one or more embodiments herein, the decoking of the unit chamber is greatly simplified due to the fact that each reaction coil is accommodated in a separate unit chamber. Therefore, the need for a spare heater is eliminated. The unit cells disclosed herein also facilitate decoking with steam alone, which takes longer but allows the effluent to pass through the heat recovery section.

[0024] In the case of radiant heating using resistive heating elements, the unit cells can be grouped into groups to form a single heater system. This arrangement increases the available area of ​​the side walls of the enclosure so that it is ideally 1.2 to 3.0 times the area of ​​the radiator, thereby reducing the element temperature required to dissipate a given amount of heat. It is worth noting that operating the resistive heating elements "at the limit" for a long time will shorten the life of the elements and will not provide sufficient design margin when some elements fail. As element technology improves, the available area relative to the reactor (coil) surface area can be less than 1.2 and it can be as low as 0.5. However, for the reasons stated above, the embodiments herein can be applied at a ratio higher than 1.0, and the achievable area ratio will depend on the maximum heat flux of the heating elements, which for metal elements may be different from ceramic SiC elements.

[0025] Each cell includes an air controller that operates in air or in a controlled atmosphere where the oxygen concentration is low enough to avoid explosion risk but high enough to maintain a protective oxide layer on the component surfaces.

[0026] For a world-scale steam cracker, a large amount of tubing is required to achieve the required capacity. The reactor tubes are grouped into "coils". The coil configuration is carefully selected based on cycle time (between decoking operations), residence time, pressure drop, and heat transfer to obtain maximum product yield.

[0027] It is expected to maintain the coil configuration that has been developed for many years and has a long-term proven operating record. The cracking heater for ethylene production can be arranged in series with multiple radiant coils in a single row. In many cases, it is configured to be arranged in an offset or staggered arrangement. It is even possible to use multiple rows of coils as described in Lummus patent application PCT / US2021 / 59542, the coil arrangement of which is incorporated herein by reference. These multiple rows are arranged so that the amount of ethylene products that can be obtained from a single heater system is maximized before the system needs to be replicated (i.e., building a second, third heater, etc.). Typically, multiple coils are grouped together into a single "heater". In this case, a group of multiple coils in the heater can be regarded as a separate "heater system", which has a single enclosure and a unit of ethylene production capacity for the facility (usually in the range of 100 to 300KTA ethylene production capacity per heater). Multiple separate heater systems may be required to achieve the required capacity. In the prior art, the heater system using an electric heating method is only considered to be grouped multiple coils in a single heater system. However, the heating methods proposed in the prior art have not considered the case where heat is provided by electrical means to a large number of individual reaction tubes configured as radiant coils.

[0028] There are many different coil designs. Coil configurations can be represented using shorthand notation as an xy arrangement, an xyz arrangement, an xyzw arrangement, or others, where x represents the number of inlet tubes, y represents the number of tubes in the next channel that become the outlet channel (xy) or the second channel xyz, and z is the outlet channel. For example, if the arrangement is 4-1, four inlet tubes feed one outlet tube. The possibilities for a large number of coil designs are discussed in PCT / US2021 / 59542. Coil configurations include a simple x arrangement where x tubes are connected in parallel to form a coil.

[0029] The heater system according to the embodiment herein includes feed preheating, indirect quenching and high pressure steam generation and product cooling. Each unit chamber has a dedicated quenching device to minimize the residence time of the adiabatic section in front of the quench cooler. The quenching system cools the cracking gas to a temperature low enough to freeze the reaction, but high enough to achieve significant heat recovery, ideally in the range of 550°C to 680°C. This is very important because, as mentioned above, there is no flue gas in the electric heater and the required electrical energy should be used to provide the heat of the reaction first. Using electrical energy to (effectively) generate steam or preheat the feed at low temperature will require a larger power supply system. Sometimes when economic (or for other reasons) requires a high degree of steam generation, a steam temperature below 550°C can be used, and in other embodiments as low as 300°C. In these cases, feed preheating can be completed by other means including a separate electric heater.

[0030] In some embodiments, the unit cells are configured so that the longest dimension in the horizontal plane is parallel to the element plane. Along the longest dimension, the tubes are placed in a single row or multiple rows.

[0031] For large heater installations (8 or more cells), the individual cells can be configured so that in the event of a single coil rupture, power to the affected cell is immediately shut off, while power to the remaining cells can be shut off in a manner that does not cause a power outage and maximizes the amount of gas that needs to be vented to a flare system or other safety system.

[0032] In some embodiments, a cell size of approximately 3.0 x 5.0 x 15.0 m can be used, so that a single SiC element can be used on the longest dimension in the horizontal plane. As previously described, this facilitates maintenance and access to the heating element and the reaction coil. The elements can be connected in series (2) so that all electrical connections are located on one side of the cell. Alternatively, a delta or star configuration can be used to connect the element group to a three-phase power supply. This configuration can be selected over a series arrangement, particularly in the case of SiC type elements, because changes in element resistance can lead to accelerated aging. The furnace temperature range is in the range of 1150°C to 1250°C for a heat flux of approximately 50 to 120 kW / m2 (based on wall area). This is far below the maximum capacity of commercially available SiC heating elements. In one or more embodiments, the area of ​​the heating element on the side wall is approximately 1.0 to 1.6 times the surface area of ​​the reaction coil. However, in the case of high heat flux SiC or improved elements, the ratio can be less than 1.2, such as 0.8 to 1.2 or 1.0 to 1.2.

[0033] In other embodiments, a cell size of approximately 3.0 x 10.0 x 15.0 m may be used with metal elements. In this configuration, element heat flux is limited to about 30 to 40 kW / m2 or 33 kW / m2, which enables the use of metal elements at temperatures of about 1200° C. The sidewall area is approximately 3.0 times the surface area of ​​the reaction coil.

[0034] In both examples above, the heating load is about 10MW, which is a convenient amount relative to the power supply. Two 5MVA or one 10MVA transformer can be used to supply each cell. This provides great flexibility in operation, enabling the following purposes: (i) each coil can be operated with different feeds and different intensities; (ii) each coil can be operated on a different decoking cycle; (iii) the power supplied to each cell can be changed based on a predetermined coil outlet temperature; and (iv) each coil can use a traditional steam / air decoking process with appropriate flow distribution control, or each coil can be operated in steam decoking mode only, while the other coils are operated in cracking mode.

[0035] Figure 3 The coil arrangement in each unit cell enclosure 300 is illustrated, as well as the arrangement of multiple unit cells 300 relative to each other. As shown, each coil 302 group is composed of 28 inlet pipes and 4 outlet pipes connected via a common manifold. However, other coil designs are also possible, including the construction of each tube collection consisting of an inlet section and an outlet section, a split coil with a small inlet coil header to a larger outlet coil, or a forged U or W configuration. For illustration, a split coil arrangement is shown. However, any type of coil can be used, including many parallel tubes (referred to as single-pass coils in the industry) and multi-pass serpentine coils (single coils with many series tubes).

[0036] Each coil grouping contained within the cell enclosure is heated by an electric heating device 304, and in some embodiments, SiC or FeCrAl resistive heating elements mounted on the wall of the cell enclosure are used. The cell enclosure may be insulated using known refractory insulation materials and the refractory insulation may be provided with hangers or the like to support the heating elements on the wall.

[0037] To illustrate the cell concept, two calculations were performed for the radiation heat transfer from a wall mounted heating element (surface 1) to the reactor coil assembly (surface 2) using a simplified two surface resistance model.

[0038] Example 1: Consider a metallic FeCrAl heating element with a maximum heat flux of about 33 kW / m at a furnace temperature of 1200°C. 2 , the absorbed heat flux at the tube metal temperature of 1080℃ is 86kW / m 2 The unit room size is 3m x 5m x 14m, so the side wall area is 2.8 x 10m x 14m = 280m 2 In comparison, the tube area is 105m 2 (Ratio 2.7). If the desired heat duty for the reaction is 9.0 MW, this can be supplied using a wall heat flux of 32.3 MW and an element temperature of 1175°C.

[0039] Example 2: Consider a ceramic SiC heating element with a maximum heat flux of about 70 kW / m at a furnace temperature of 1300°C. 2 , the absorbed heat flux at the tube metal temperature of 1080℃ is 86kW / m 2 The unit size is 2.8m x 5m x 14m, with a side wall area of ​​140m 2 , and the pipe area is 105m 2 (Ratio 1.3). For the same 9.0 MW heat load as Example 1, the wall temperature will be 1237°C

[0040] Therefore, the size of each unit cell can be selected based on the required heat flux at the wall and the heat flux absorbed at the reactor coil.

[0041] refer to Figure 4 The plurality of unit cells 400 are arranged so 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 their inlet tubes 406 placed on either side of the outlet tubes, which are grouped toward the center of the enclosure. Each of the plurality of unit cells 400 is heated using an electric heating device 404. Each unit cell will have a quench unit (for clarity, Figure 4 (not shown), the quenching unit receives the cracked gas from the outlet pipe and rapidly cools it to a temperature lower than the reaction temperature, ie, from the reaction temperature to a temperature lower than 700°C, such as in the range of 550°C to 680°C.

[0042] The cracked gas produced in the reaction coil of each unit cell 400 can be cooled and supplied to a single transfer line dedicated to a group of unit cells, and thereafter supplied to the main transfer line for the entire device for downstream separation. The preferred method for further cooling the cracked gas from the quench unit is in a feed-effluent heat exchanger, commonly referred to as a secondary transfer line exchanger (STLE). STLE has many suitable configurations, and a STLE can cool the cracked gas and provide heat for a single unit cell or multiple unit cells. It is desired that the feed is heated to a temperature as high as possible so that the amount of electricity required for furnace heating is minimal. Before being fed to the reaction coil, the feed can be heated to more than 550°C, preferably more than 600°C, even up to 635°C.

[0043] Figure 5 5A (multiple cells) and 5B (single cell) show a cell 500 using a silicon carbide heating element 504. As an example, the heating element 504 is shown as being arranged horizontally. The heating element 504 can also be arranged vertically or in a different geometry, depending on the specific installation requirements. Figure 5 In FIG. 5 , the row of cells 500 is arranged along an axis 508. Hydrocarbons will flow along this line from the inlet side to the TLE side.

[0044] .Given the high current and physical size of the electrically heated steam cracking furnace, it is advantageous to reduce the length of the cable required to provide power to the heating elements. In some embodiments, a circuit will be selected that enables the cabling to be routed to one side of the unit chamber. In the case of SiC elements that span the entire length of the unit chamber, a circuit configuration that adopts a star or alternatively a triangle arrangement can be used. The advantage of the star circuit is a simplified wiring scheme, in which on one side of the unit chamber, all heating element ends will terminate at a common neutral bus, and all line voltage connections will be located on the other side of the unit chamber, thereby limiting the cable to only one side of the unit. However, similarly, a triangle circuit can be used, using two adjacent elements connected in series to avoid laying line voltage cables to both sides of the unit chamber. Multiple elements can be grouped in a circuit regulated using a voltage control unit. Silicon controlled rectifiers (SCRs) are solid-state switching devices that provide fast, stepless variable power proportional control for resistive loads and are particularly suitable for current applications. A single SCR can be dedicated to currents of 250A to 2100A, depending on the number of elements in each circuit. There will be multiple individually controlled heating circuits in each unit chamber. For example, in a 10 MW unit room, there may be 6 circuits controlling 1.66 MW of power to each circuit.

[0045] Figure 6 and Fig. 6A It is the general arrangement diagram of a group of unit chambers 600 arranged as heater system, and each unit chamber 600 has reaction coil 602.As shown in the figure, in some embodiments, each unit chamber 600 has single special quenching unit 610, which is supplied by steam drum in this case.Effluent is further cooled in STLE heat exchanger 612 relative to the feed entering, dilution steam mixture.STLE 612 can heat the feed from one, two or more unit chambers 600 and cool cracking gas.Except hydrocarbon steam mixture, other fluids such as steam only can also be considered.However, in other embodiments, each unit chamber can have independent quenching system and feed preheating / effluent cooling heat exchanger can be shared between two or more unit chambers, or each unit chamber can have two or feed preheating / effluent cooling heat exchangers.

[0046] Figure 7 and Fig. 7A One or more unit cells 700 are shown sharing a common feed 702 including hydrocarbons 704 and dilution steam 706 .

[0047] Embodiments herein may include unit cells 700, each unit cell comprising: (i) a single reactor coil consisting of a group of reactor tubes with a common feed; (ii) a quench device 710; (iii) an enclosure; (iv) a heating element 712 connected to a power source via a bus bar 714 (or the like); (v) a power controller 716; and (vi) a transformer 718. In addition, multiple unit cells will have a common high voltage power source.

[0048] exist Figure 7 In the illustrated embodiment, the feed 702 and dilution steam are preheated in the first preheating section 720 and then further heated in the second TLE 722 against the quenched cracked gas from the quenching unit 710 of the two cells. Two or more cells may share a common feed and a common feed preheating device. Figure 7 / Fig. 7A In the embodiment shown, the two cells share a common feed with associated preheat and auxiliary transfer line heat exchange, however the primary quench system is part of the cell system. The feed and dilution steam mixture is divided between the two cells and supplied to a reaction coil 726 consisting of a set of reaction tubes, each of which is independently housed in the cell 700 enclosure. The reactants are heated to a temperature of 750°C to 925°C and thermally cracked to produce cracked gas 728 in each set of reactor coils. Fig. 7A In the illustrated embodiment, the feed 704 is heated in a naphtha heater 724 before being mixed with dilution steam 706 and supplied to the first preheating stage 720 .

[0049] Radiative heating is provided by resistive heating elements 712 inside the cell 700 enclosure and preferably mounted on both side walls of the enclosure. These elements are supplied with current from a main supply source (bus A 714) such as a high voltage in the range of 13 kV to 34.5 kV via a power controller 716 and a transformer 718 which steps down the higher supply voltage to a lower and absorbs heat flux at the reactor coils.

[0050] The cracked gas 728 produced in the reaction coil 726 of each unit cell 700 can be cooled and supplied to a single transfer line dedicated to a group of unit cells (quench unit 710), and thereafter supplied to the main transfer line for the entire device 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 (commonly referred to as a secondary transfer line exchanger (STLE)).

[0051] In a radiant coil, the hydrocarbon feed must first be heated to the reaction temperature before the thermal cracking reaction. It is known to use multiple inlet tubes for each outlet tube so that the fluid can be quickly heated (in a single or a smaller number of larger diameter outlet tubes) before high temperature cracking in a short residence time. The short residence time in the reaction section improves the selectivity to the desired product.

[0052] In the case of an electrically heated furnace, there is no flue gas, so any heat recovery is primarily from the product cracked gas 728. It is desirable to maximize the temperature of the feed entering the inlet of the reaction coil 712, as this will minimize the amount of electrical energy required to reach the reaction temperature. Ideally, electrical energy should be used preferentially to generate the endothermic heat of the reaction required to produce the desired products.

[0053] It is also desirable to rapidly quench the reaction products leaving the radiation coil at about 750°C to 925°C. Ideally, the cracked gas 728 should be cooled immediately, but in practice this means that the residence time in the insulation section between the furnace outlet and the quenching system should be short, ideally <10% of the residence time in the radiation coil. Direct and indirect quenching methods have been used, with indirect quenching generally being the preferred method for allowing the production of valuable high-pressure steam. For the reasons described above, the quencher (also referred to as the primary transport line exchanger) is generally arranged as close to the outlet of the reaction coil as possible. Suitable heat exchanger types include Arvos transport line exchangers, bathtub-type, rapid quencher-type or Borsig "tunnel flow" or "linear" transmission line exchangers. Other ultra-high pressure or medium-high pressure exchangers may also be used. Such a system may use boiler feed water 730 to cool the cracked product 728, producing steam 732. A steam drum 734 may be used to collect steam and produce an output stream 736 and a condensate 738.

[0054] For the embodiment furnaces herein, the reaction coils are not grouped into "heater systems", but are contained in individual unit cells. Therefore, the enclosure (furnace box) is divided into discrete sections, and each unit cell has a dedicated power supply and control device. In the embodiments herein, each unit cell can be regarded as a separate control zone, completely independent of the coils contained in each unit cell. The power supplied to each unit cell can be varied based on the measured coil outlet temperature.

[0055] The cell concept is particularly useful when the heating method is radiation heating using electrical resistance elements. For metal elements made of FeCrAl wire, both the coil spacing and the sidewall area can be increased so that the elements can be mounted on the wall and operate at temperatures of 1100°C to 1200°C and 30 to 40 kW / m 2In the prior art, the elements need to be mounted on a single very long sidewall (which can be over 90m long). Conceptually, the single long furnace is essentially divided into several smaller sections, and then each section is 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.

[0056] SiC heating elements can be advantageously used in the embodiments herein because the box dimensions are adapted to allow, for example: (i) a rigid element that can be mounted horizontally on the furnace wall to span the longest dimension in the horizontal plane. Such an element installation can be removable during operation, or the element can be removed independently of the reaction coil in the event of a major maintenance activity (i.e., the element does not have to be removed in order to remove the coil, or vice versa). In the prior art, a single heater system enclosure is too long to allow a single element to span the longest dimension of the enclosure; (ii) multiple elements mounted horizontally from top to bottom, possibly located in different areas.

[0057] In summary, the desired electric heater system should have the following characteristics: (i) maintain best design practices in terms of coil layout and residence time, and provide flexibility in coil layout; (ii) allow the use of existing heating elements that have been proven and widely used at the temperatures required in ethylene cracking furnaces; (iii) the surface area of ​​the heating elements is larger than the surface area to be heated (maximizing element life); (iv) allow single unit chambers to be isolated and quickly shut down in the event of a coil rupture, thereby providing safe operation without affecting the balance of the reactant system; (v) provide the maximum possible feed inlet (crossover) temperature at the inlet to the radiant section to maximize the amount of electrical energy that needs to be dissipated by the element, as this is the highest value heat source; and (vi) promote rapid quenching of the reaction products to maximize the yield of valuable products.

[0058] As described above, the embodiments herein provide an electric furnace system suitable for large-scale thermal cracking processes. The embodiments herein can maintain the optimal coil layout that has been developed for fuel-fired furnaces, so increased coking rates should not be a problem. In fact, due to the effects of hot spots, electric heat may coke more evenly than open flame heating, so the coking of the electric heating system may actually be slightly lower.

[0059] Embodiments herein also provide a furnace configured as a series of unit chambers, each unit chamber containing a reactor coil with a corresponding manifold, and each coil manifold is aligned perpendicular to the axis. The heating elements can be attached to the refractory wall instead of along the reactor tube, or in the form of coaxial radiators, and the area of ​​the heating element A1 and the refractory area associated with the heating element (A2) are larger than the area (A3) of the coil to be heated. The types of heating elements available are limited to a certain heat flux, which decreases with increasing operating temperature and the operating life of the heating element will also depend on the temperature at which the element is operated. For example, if operating at a furnace temperature of 1200°C, FeCrAl type elements are limited to approximately 30 to 40 kW / m 2 If the absorption heat flux required for the endothermic reaction is >40kW / m 2 , then the surface area of ​​the heating element needs to be higher in order to meet the heat flux and temperature constraints at the required heat load. Relative to the prior art practice of including multiple reaction coils in a single structure, the unit cell arrangement increases the ratio of refractory area to reactor coil area. Therefore, the embodiments herein allow for increasing the tube spacing within each unit cell.

[0060] Embodiments herein also relate to the arrangement of reactor coils within each unit cell, which provides advantages in terms of higher wall area per unit coil area and isolation of the unit cells relative to power supply and in the event of coil rupture. It should be noted that embodiments herein allow the use of different heating methods or combinations of methods. For example, some required heat can be provided by non-radiant heat transfer, such as by passing an electric current directly through the reactor coils. This can be direct current or alternating current, recognizing that the resistance of the reactor tube is relatively low, and if the entire heating load is to be provided by this method, a large current is required. Another method is to provide a portion of the heating load by non-radiant heating.

[0061] Optionally, an inert gas with a reduced oxygen content (<10%) is fed into the reactor enclosure to ensure limited explosion or combustion events in the event of a hydrocarbon release. While such a precaution is also contemplated, an additional advantage of the cell enclosure is that in such an event, the amount of gas that must be released from the enclosure is significantly reduced and the event can be controlled. In an embodiment, power to the affected cell can be stopped immediately, while power to the remaining cells can be reduced more gradually if desired.

[0062] For electric heating according to embodiments herein, the amount of electricity is in the range of 5MVA to 10MVA for the supplied electricity. A single coil assembly has a similar required heat input, in the range of 8MW to 10MW of absorbed power. Therefore, the furnace heat input can be controlled by dividing the enclosure into cells, so that each cell is supplied by a single transformer or a pair of transformers.

[0063] Compared with the grid supply voltage (30kV to 40kV), the resistance element voltage is low (200V to 600V). 200V to 600V is considered low voltage in industry. For illustration, low voltage is used. When medium voltage elements (1000v to 6000v) are available, they can also be used. The cost and quantity of the control device are greatly affected by the current, and when the voltage is in the range of 300V to 600V, the cost and quantity of the control device will be higher. The embodiments herein allow each individual unit room to be supplied by a small amount or even a single step-down transformer. On the low-voltage side of the step-down transformer, the SCR (silicon controlled rectifier) ​​controls the voltage supply to the SiC resistance element circuit. The PLC will monitor the voltage and current in the circuit and calculate the power flowing through the element in real time. The PLC will then be able to adjust the voltage supply to reach a given power demand set point through the SCR. In order to avoid overheating events in the heating element, the PLC will also monitor the temperature of the element and reduce or limit the power set point or eventually trip the circuit. Optionally, a temperature set point may be used by the PLC and the voltage may be varied by the SCRs to achieve the target furnace temperature.

[0064] The inventors also realized that in order to account for the difference between the absorbed heat flux (e.g. 90 kW / m2) and the maximum heat flux of the element (e.g. 33 kW / m2 for metal elements and 70 kW / m2 for SiC), a larger wall area is required so that the coil spacing will need to be very large.

[0065] A controlled atmosphere will be required to address the possibility of a coil rupture. A safer approach is to isolate individual cells from the rest of the system by isolating each coil in the cell.

[0066] For example, if an induction coil is placed around each reactor tube, there will be insulation between the induction coil and the reactor tube, rather than electrical connections to the induction coil around each individual reactor tube on the wall of the enclosure. High frequency alternating current is required. The enclosure is relatively cool, but still needs to contain an inert atmosphere to limit combustion in the event of a coil rupture.

[0067] If current is passed through each reaction coil, multiple electrical connections into the enclosure are again required.There are a variety of power supply configurations that can be used with the present invention.

[0068] Each cell can be isolated from the others for a variety of reasons. If a hydrocarbon leak occurs due to a leak or rupture, adjustments or maintenance can be made without significantly reducing the overall capacity of the plant.

[0069] The cells may also provide a decoking schedule. Each cell may be arranged so that more individual decoking cycles (up to a limit of one per cell) are run, but with less capacity reduction for each decoking operation. One cell may be in decoking mode while the other cells of a single heater may crack hydrocarbons. This may be accomplished by steam decoking only and all effluents passing through a recovery section. While the heaters (all cells or coils or preselected cells / coils) are decoking, the effluents may be vented to the atmosphere through a decoking drum.

[0070] Unless defined otherwise, 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, apparatus, methods, processes, and compositions belong.

[0071] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0072] As used herein and in the appended claims, the words "comprising," "having," and "including" and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

[0073] "Optionally" means that the subsequently described event or circumstance may or may not occur. The description includes instances where the event or circumstance occurs and instances where the event or circumstance does not occur.

[0074] When the word "about" or "approximately" is used, the term may mean that the value may vary by up to ±10%, up to 5%, up to 2%, up to 1%, up to 0.5%, up to 0.1%, or up to 0.01%.

[0075] Ranges can be expressed as from about one particular value to about another particular value, inclusive. When such a range is expressed, it should be understood that another embodiment is from the one particular value to the other particular value, as well as all the particular values ​​within the range and combinations thereof.

[0076] While the present disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the following claims.

Claims

1. An electric heating furnace, comprising: One or more units, each unit comprising: a radiant heating section, one or more process coils disposed within the radiant heating section, and a quench unit for cooling cracked products from the one or more process coils and producing a quenched reaction product; one or more electrical heating elements disposed within the radiant heating section, the one or more electrical heating elements being arranged to provide radiant energy to the one or more process coils, The electric heating furnace comprises: a first area corresponding to a heating area of ​​the one or more electric heating elements; a second area corresponding to a wall area of ​​a wall on which the one or more electric heating elements are arranged; and a third area corresponding to a surface area of ​​the one or more process coils.

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

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 feedstock.

4. The device 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, wherein the one or more electric heating elements are horizontally arranged along one or more walls of the radiant heating section, wherein each of the first ends is arranged on a first side of the one or more walls and each of the second ends is arranged on a second side of the one or more walls.

5. 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, wherein the one or more electric heating elements are arranged horizontally along one or more walls of the radiant heating section, wherein the second end of a first electric heating element is electrically connected to the first end of a second electric heating element.

6. The device 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 adjust the voltage from the power controller to an operating voltage for the one or more electric heating elements.

8. The device of claim 1, wherein the one or more electrical heating elements are made of a material selected from the group consisting of NiCr, FeCrAl, SiC, and MoSi2.

9. The device according to 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. The apparatus of claim 1, further comprising an oxygen controller configured to maintain an oxygen concentration in the radiation section low enough to prevent explosions and high enough to maintain a protective oxide layer on surfaces of the one or more electric heating elements.

11. The device 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 electrical heating elements are arranged in the radiant section at an angle of 45 to 90 degrees relative to the one or more process coils.

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

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

15. The apparatus of claim 1, wherein a first unit chamber among the one or more unit chambers is operable in a decoking mode or a maintenance mode, and a second unit chamber among the one or more unit chambers is operable in a pyrolysis mode.

16. A device for cracking hydrocarbons, comprising the electric heating furnace according to claim 1.

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