Multilayer structure of electric heating furnace using conductive refractory material

Abstract

The invention relates to a system and method for a multi-layer heating system. The heating system may include an electrically conductive refractory layer; an electrically insulating layer; and a thermal insulation layer between the electrically conductive refractory layer and the electrically insulating layer. The conductive refractory layer may operate at significantly higher temperatures than the electrically insulating layer. The heating system may also include a wall of the furnace, and an air gap between the thermal insulation layer and the wall of the furnace.

Description

Technical Field

[0001] The present disclosure relates to systems and methods for transferring thermal energy, and more particularly to heating devices for electric heating processes. Background Art

[0002] High-temperature furnaces can be used for various applications, including but not limited to chemical processes. In particular, electric radiant high-temperature furnaces have been envisioned for steam cracking, steam methane reforming (SMR), ammonia reforming, dehydrogenation, tar cracking, or similar applications. Such processes and the furnaces used to heat such processes have traditionally used combustion. To meet new sustainability and carbon dioxide emission reduction requirements, it may be necessary to replace such flame heaters.

[0003] Recent developments include heaters and furnaces powered by electricity, preferably renewable electricity. Non-combustion heaters are needed to supply heat to such processes. Exemplary configurations of heaters for such applications include conductive heating elements, typically wires or strips of metal suspended on non-conductive refractory materials, such as non-conductive refractory bricks. In these systems, a voltage is applied across the conductive elements, and the resulting current causes the elements to heat up. This is known as impedance or ohmic heating. As the elements heat up, they heat the refractory material. When the system reaches its operating temperature, the heaters radiate heat from the elements and from the refractory bricks into the furnace chamber, where the heat is transferred to the furnace tubes. This allows for the replacement of combustion with an electric heating system.

[0004] However, in the above systems, the allowable operating temperature of the heating elements is limited, and the lifespan decreases as the temperature increases. The maximum operating temperature can vary, but generally for metal elements, the maximum operating temperature should not exceed approximately 1300 °C; and even then, their lifespan is only a few years, and frequent and costly element replacements are required during the lifespan of the system. In many processes, higher temperatures are needed, and a much longer lifespan is highly desirable.

[0005] For electric processes, the applicant has identified a need for systems and methods for heating using heaters that can operate at high temperatures and / or have a longer lifespan. Summary of the Invention

[0006] In an embodiment of the present disclosure, a multi-layer heating system can include a conductive refractory layer; an electrical insulation layer; and a thermal insulation layer located between the conductive refractory layer and the electrical insulation layer. Certain embodiments may also include the wall of a furnace, and an air gap between the electrical insulation layer and the wall of the furnace.

[0007] In certain embodiments, the conductive refractory layer may be capable of operating at a temperature significantly higher than that of the electrical insulation layer. In some embodiments, the significantly higher temperature may be at least 200 °C; in other embodiments, the significantly higher temperature may be at least 500 °C. In certain embodiments, the conductive refractory layer may be less than 4 inches thick; in some other certain embodiments, the conductive refractory layer may be less than 1 inch thick. In certain embodiments, the conductive refractory layer, the electrical insulation layer, and the thermal insulation layer may be held together by one or more ceramic bolts. In certain embodiments, the bolts may be conductive when close to the conductive refractory layer but effective electrical insulators when close to the electrical insulation layer.

[0008] In certain embodiments, the conductive refractory layer and the electrical insulation layer are independently materials that are conductive at higher temperatures but effective electrical insulators at lower temperatures. In certain embodiments, a multi-layer heating system may be disposed within a furnace, and wherein the conductive layer may radiate heat into the interior of the furnace.

[0009] In certain embodiments, the conductive layer may radiate heat to a process fluid. In certain embodiments, the process fluid may be a hydrocarbon. In certain embodiments, the furnace may be a steam cracking furnace. In certain embodiments, the furnace may be a steam methane reforming furnace. In certain embodiments, the conductive refractory layer may be capable of operating at a temperature greater than 1000 °C, preferably greater than 1200 °C, and more preferably greater than 1400 °C. In certain embodiments, the electrical insulation layer may be capable of operating at a temperature less than 900 °C, preferably less than 500 °C, and more preferably less than 300 °C. In certain embodiments, when the multi-layer heating system is operating, the conductivity ratio between the conductive refractory layer and the electrical insulation layer may be greater than 3, more preferably greater than 10. In other words, when the system is operating at the temperatures typically encountered in a furnace during steam cracking (about 800 °C - 900 °C), the resistivity of the conductive layer is significantly less than the resistivity of the electrical insulation layer.

[0010] Additional aspects and advantages of these exemplary embodiments and other embodiments are discussed in detail herein. In addition, it is to be understood that both the foregoing information and the following detailed description provide merely illustrative examples of various aspects and embodiments and are intended to provide an overview or framework for understanding the characteristics and features of the claimed aspects and embodiments. Accordingly, these and other objects of the present disclosure, together with advantages and features, will become apparent by reference to the following description and the accompanying drawings. In addition, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. Description of the Drawings

[0011] The accompanying drawings, which are included to provide a further understanding of the embodiments of the present disclosure and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the detailed description, are used to explain the principles of the embodiments discussed herein. No attempt is made to show the structural details of the present disclosure in more detail than is necessary for a basic understanding of the embodiments discussed herein and the various ways in which they may be practiced. By convention, the various features of the drawings discussed below are not necessarily drawn to scale. The dimensions of the various features and elements in the drawings may be enlarged or reduced to more clearly show the embodiments of the present disclosure.

[0012] Figure 1 Schematic view of an exemplary conductive refractory system.

[0013] Figure 2 Flow chart of a conductive refractory system used as a furnace.

[0014] Figure 3 Schematic view of a multi-layer heating system. Detailed Description

[0015] The accompanying drawings include the same reference numerals to indicate the same parts in several views, and the following description is provided as an enabling teaching of exemplary embodiments, and those skilled in the relevant art will recognize that many changes may be made to the described embodiments. It will also be apparent that some of the desired benefits of the described embodiments may be obtained by selecting some features of the embodiments without utilizing other features. Accordingly, those skilled in the art will recognize that many modifications and adaptations to the described embodiments are possible and may even be desirable in some cases. Therefore, the following description is provided as an illustration of the principles of the embodiments rather than a limitation thereof.

[0016] The terminology and phrases used herein are for the purpose of description and should not be regarded as restrictive. As used herein, the term "plurality" refers to two or more items or components. Unless otherwise specified, the terms "comprising", "including", "carrying", "having", "containing", and "involving", whether in the written description or claims, etc., are open-ended terms, i.e., meaning "including but not limited to". Thus, the use of such terms is intended to cover the items listed thereafter and their equivalents as well as additional items. The transitional phrases "consisting of" and "consisting essentially of" are closed or semi-closed transitional phrases with respect to any claim. The use of ordinal terms such as "first", "second", "third", etc. in a claim to modify the claim element itself does not imply any priority, ranking, or order of one claim element relative to another claim element or the chronological order of acts of a method of implementation, but is only used as a label (but for the use of ordinal terms) to distinguish one claim element having a certain name from another element having the same name, to distinguish claim elements.

[0017] Furthermore, although quantitative measurements, values, geometric relationships, etc. may be referred to herein, unless otherwise specified, any one or more (if not all) of these may be absolute or approximate to account for acceptable variations that may occur, such as those due to manufacturing or engineering tolerances, etc.

[0018] As used herein, the term "significantly" means of a size and / or effect / function large enough or important enough to attract attention or have a significant effect / function.

[0019] In an embodiment of the present invention as shown in Figure 1 electricity is directly applied to a conductive refractory material and heat is generated through a conductive refractory system. There may be no separate heating element, such as a metal wire or a ceramic heating element.

[0020] Electrical heating technology can provide heat to a process stream through a conductive refractory system. The heat can be used to increase the temperature of the process stream or provide the heat of reaction required to drive a chemical reaction, or both. For the purposes of this application, reference may be made to "bricks", but the system can be provided in various formats, sizes, and shapes according to the needs of a particular embodiment, such as cylinders, cones, tiles, and other configurations. Using a conductive refractory such as a refractory brick can provide an alternative to: metal wire or metal strip heating elements, or separate ceramic or other types of heating elements that are not an integral component of the refractory system, such as silicon carbide (SiC) or molybdenum carbide (MoC) heating elements. Using a conductive refractory can further eliminate or reduce certain design constraints of those existing systems. Generally, the conductive refractory can be a brick (having a rectangular or square box shape) that does not have metal conduits, but rather current flows through the conductive refractory material. In certain embodiments, the conductive refractory material can be ceramic. In certain embodiments, the conductive refractory material can also be referred to as electro-heated ceramic (EHC).

[0021] Advantages of these conductive refractory systems can include, but are not limited to, the following:

[0022] ● Better simplicity and lower cost. The conductive refractory system does not utilize separate conductive wires of various materials (metals, ceramics, etc.), thus eliminating those conductive wires that are one of the most expensive components in heating technology and require frequent replacement, simplifying the system and reducing costs, especially the frequent replacement costs.

[0023] ● Higher heat flux and higher temperature. The conductive refractory system can operate at temperatures up to approximately 2000 °C, while metal heating elements are limited to approximately 1300 °C.

[0024] ● Longer lifespan. Compared to metal wires and metal strips operating near the material temperature limit, the conductive refractory material can have a longer lifespan.

[0025] ● Higher processing temperature. The conductive refractory system can be used to heat the process stream to a higher temperature.

[0026] ● Smaller furnace. The conductive refractory system can achieve a higher flux, and a furnace equipped with such can utilize a larger portion of the wall area, and thus reduce the total surface area and overall furnace size required for a given duty.

[0027] ● Higher voltage. The conductive refractory system can operate at voltages up to 13 kV, while metal heating elements can operate at much lower voltages of approximately 690 V. In certain embodiments, power can be provided at voltages greater than 1000 volts, preferably greater than 4000 volts, and most preferably greater than 10000 volts.

[0028] ● Cheaper electrical kits. When operating at a higher voltage, the furnace requires a much lower amperage, so the electrical equipment required can be significantly cheaper. The electrical equipment can include, but is not limited to, step-down transformers, control elements, switchgear, conductors, connectors, and other devices.

[0029] Deploying these conductive refractories in a furnace is not so simple. The internal temperature of the conductive refractory can be much higher (hotter) than the radiation surface and must be kept below the refractory limit of approximately 2000 °C. In addition, the conductive refractories are at a very high potential (possibly exceeding 10 kV), and they must be deployed in a way that prevents stray currents or short circuits, which is both an operability and a safety issue. Even more complex is that it is extremely difficult to find materials that can act as effective electrical insulators at these high temperatures.

[0030] Certain embodiments of the present invention relate to a multi-layer heating wall that can be deployed inside a furnace enclosure to address these challenges.

[0031] Figure 2 Flow chart for a conductive refractory system for use in a furnace. In certain embodiments, a hydrocarbon feed can be fed into the furnace at a temperature of, for example but not limited to, approximately 650 °C. Electrical energy is supplied to a multi-layer heating system inside the furnace. The hydrocarbon can leave the furnace at a temperature of, for example but not limited to, approximately 850 °C.

[0032] Figure 3 Schematic diagram of a multi-layer heating system utilizing conductive refractories for furnace applications. One or more multi-layer heating systems can be provided inside the furnace. The multi-layer heating system can be referred to herein as a "wall". In certain embodiments, the multi-layer heating system can be self-supporting. In certain embodiments, the wall can be constructed as follows:

[0033] 1) The first layer can be a layer of conductive refractory. This is the layer where heat is generated by resistive heating and whose surface can radiate heat into the furnace. The radiation (or outer) surface of the conductive refractory can operate at a temperature above approximately 100 °C. Since heat is generated resistively throughout the volume of the conductive refractory layer, the temperature at the back (or inner) surface can be even higher (hotter) and can approach the material limit of approximately 2000 °C.

[0034] 2) The second layer can be a layer of insulating refractory. The temperature at the interface between the conductive refractory and the insulating refractory can be the highest (the hottest place) on the wall. The temperature can drop rapidly away from the conductive refractory through the insulating refractory.

[0035] 3) The third layer can be an electrical insulation layer. In some embodiments, the electrical insulation layer can be high-purity alumina, but other materials are possible. Since the electrical insulation layer can be attached to the rear side of the thermal insulation layer, it can operate at a much lower temperature than the conductive refractory material. Through the arrangement of the layers described herein, the system can allow the conductive refractory material to operate at a very high temperature while keeping the electrical insulation layer at a significantly lower temperature.

[0036] 4) The fourth layer can be an air gap. This air gap can provide an additional layer of protection and safety for the system by preventing any stray currents or short circuits between the walls of the furnace and other components.

[0037] 5) The fifth layer can be the wall of the furnace. This can be the outer wall of the furnace. The material of construction is not critical, but it preferably contains a layer that provides effective thermal insulation to minimize heat loss from the furnace.

[0038] Optionally, ceramic bolts can be used to clamp the layers of the wall together. The bolts will conduct electricity in the hot region of the conductive refractory material but will be electrically insulating in the cold region of the electrical insulation layer.

[0039] The systems and methods described herein can be used to heat a process stream or provide heat for an endothermic chemical reaction with or without a catalyst. For example, when used in a furnace such as Figure 2 , a pressurized gas can be fed into the system via an inlet and directed to or near the multi-layer wall, particularly the conductive layer. Heating of the conductive layer will supply heat to the process stream, and then the hot product gas can be directed to the outlet of the system.

[0040] According to another embodiment, there is provided a conductive refractory material system comprising a) a multi-layer heating wall including (i) a conductive refractory layer, (ii) an electrical insulation layer, and (iii) a thermal insulation layer located between the conductive refractory layer and the electrical insulation layer, wherein the resistivity of the conductive refractory layer is significantly less than the resistivity of the electrical insulation layer; and b) at least two connectors electrically connected to the multi-layer wall and a power source, wherein the power source is sized to heat at least a portion of the multi-layer wall to a temperature of at least 1000 °C by passing an electric current through the conductive layer.

[0041] In one embodiment, at least a portion of the inner surface of the conductive refractory layer is in contact with at least a portion of the outer surface of the thermal insulation layer. In another embodiment, the inner surface of the thermal insulation layer is in contact with at least a portion of the outer surface of the electrical insulation layer.

[0042] According to one embodiment, the conductive layer comprises metal oxide particles, metal nitride particles, metal carbide particles, metal sulfide particles, metal silicide particles, metal boride particles, multiferroic compound particles, mixed ceramic particles, chalcogenide glass particles, or combinations thereof. In other embodiments, the conductive layer has a resistivity of about 10 -5 ohm·m to about 10 -8 ohm·m at 20 °C.

[0043] Examples of metal oxide particles include, but are not limited to, doped and undoped particles of tin oxide, iron oxides (ferrous oxide or iron oxide), zinc oxide, manganese oxide, lead oxide, nickel oxide, cobalt oxide, silver oxide, antimony oxide, and copper oxide (CuO), chromium oxide. Mixtures of metal oxide particles are also suitable.

[0044] Examples of metal nitride particles include, but are not limited to, doped and undoped particles of tantalum nitride, titanium nitride, vanadium nitride, and zirconium nitride. Mixtures of metal nitride particles are also suitable.

[0045] Examples of metal carbide particles include doped and undoped particles of tungsten carbide, niobium carbide, titanium carbide, vanadium carbide, molybdenum carbide, silicon carbide, zirconium carbide, boron carbide, and titanium silicon carbide. Mixtures of metal carbide particles are also suitable.

[0046] Examples of suitable metal sulfide particles include doped and undoped particles of copper sulfide, silver sulfide, iron sulfide, nickel sulfide, cobalt sulfide, lead sulfide, and zinc sulfide. Mixtures of metal sulfide particles are also suitable.

[0047] Examples of suitable metal silicide particles include doped and undoped particles of chromium silicide, molybdenum silicide, cobalt silicide, vanadium silicide, tungsten silicide, and titanium silicide. Mixtures of metal silicide particles are also suitable.

[0048] Examples of suitable metal boride particles include doped and undoped particles of chromium boride, molybdenum boride, titanium boride, zirconium boride, niobium boride, and tantalum boride. Mixtures of metal boride particles are also suitable.

[0049] Examples of particles of multiferroic compounds include, but are not limited to, doped and undoped particles of bismuth ferrite (BiFeO3), bismuth manganite (BiMnO3), and rare earth iron oxides (MFe2O4, where M is a rare earth element, such as LuFe2O4). Mixtures of particles of multiferroic compounds are also suitable.

[0050] Examples of mixed ceramic particles include particles having a mixture of metallic or metalloid elements. Suitable examples include, but are not limited to, silicon carbide and beryllium oxide, silicon carbide and aluminum nitride, copper oxide (CuO) and aluminum oxide, aluminum nitride and glassy carbon, and doped and undoped particles of Si-Ti-C-N ceramics. Examples of chalcogenide glass particles include vitreous materials based on As-Ge-Te and Se-Ge-Te.

[0051] In one embodiment, the conductive layer has a thickness of less than about 6 inches, or preferably less than about 3 inches, or less than 2 inches, or most preferably less than 1 inch. In another embodiment, the conductive layer has a thickness of from about 1 inch to 6 inches, or from about 1.5 inches to about 3 inches.

[0052] In another embodiment, the electrically insulating layer comprises alumina, magnesia, silica, silicon nitride, silicon oxynitride, silicon carbide, or a combination thereof. In one embodiment, the electrically insulating layer has a resistivity greater than 10 ohm·m at 20 °C, such as about 10 9 ohm·m to about 10 25 ohm·m. In some embodiments, when the system is operating at temperatures typically encountered in a furnace during steam cracking (about 800 °C - 900 °C), the electrically insulating layer is capable of having a cold side (inner surface) temperature of less than 400 °C, preferably less than 250 °C, and most preferably less than 150 °C.

[0053] According to another embodiment, the thermal insulation layer comprises zircon, zirconium alloy, titanium nitride, titanium carbide, titanium nitride alloy, titanium carbide alloy, alkali metal titanate, silicone resin, silica fiber, glass fiber, ceramic fiber, or a combination thereof. In some embodiments, when the system is operating at temperatures typically encountered in a furnace during steam cracking (about 800 °C - 900 °C), the temperature drop across the thermal insulation layer is greater than 200 °C, preferably greater than 500 °C, and most preferably greater than 1000 °C.

[0054] When the conductive refractory system includes an air gap between the multi-layer wall and the wall of the furnace, additional thermal and electrical insulation between the multi-layer wall and the wall of the furnace can be obtained. The presence of the air gap helps to avoid excessive heat loss to the surrounding environment and also prevents stray current from reaching the outer wall of the furnace. When the system is operating at temperatures typically encountered in a furnace during steam cracking (about 800 °C - 900 °C), the temperature of the multi-layer wall can reach up to about 2000 °C at least in some of its parts, but by using an air gap between the multi-layer wall and the wall of the furnace, the temperature of the wall of the furnace can be maintained at a significantly lower temperature, such as less than about 300 °C or even less than about 100 °C.

[0055] It should be noted that the conductive refractory system of the present invention may include any suitable number of power supplies and any suitable number of connectors (one or more) connecting the power supply to the conductive layer of the multi-layer wall.

[0056] Although the foregoing relates to embodiments of the present disclosure, other and additional embodiments of the present disclosure may be devised without departing from the basic scope thereof, and the scope of the present disclosure is determined by the claims.

Claims

1. A multi-layer heating system, comprising: A conductive refractory layer; An electrical insulation layer; And A thermal insulation layer between the conductive refractory layer and the electrical insulation layer, Wherein the resistivity of the conductive refractory material is significantly less than the resistivity of the electrical insulation layer.

2. The multi-layer heating system according to claim 1, further comprising a wall of a furnace and an air gap between the electrical insulation layer and the wall of the furnace.

3. The multi-layer heating system according to any one of the preceding claims, wherein during operation, the temperature of the conductive layer is significantly higher than the temperature of the electrical insulation layer by at least 100 °C.

4. The multi-layer heating system according to any one of the preceding claims, wherein the conductive refractory layer is less than 2 inches thick.

5. The multi-layer heating system according to any one of the preceding claims, wherein the conductive refractory layer, the electrical insulation layer and the thermal insulation layer are held together by one or more ceramic bolts.

6. The multi-layer heating system according to claim 5, wherein the bolts are conductive when close to the conductive refractory layer but are effective electrical insulators when close to the electrical insulation layer.

7. The multi-layer heating system according to any one of the preceding claims, wherein the conductive refractory layer and the electrical insulation layer each comprise different materials that are conductive at higher temperatures but are effective electrical insulators at lower temperatures.

8. The multi-layer heating system according to any one of the preceding claims, wherein the multi-layer heating system is disposed within a furnace and wherein the conductive layer radiates heat to the interior of the furnace.

9. The multi-layer heating system according to any one of the preceding claims, wherein during operation, the conductive layer radiates heat to a process fluid.

10. The multi-layer heating system according to claim 9, wherein the process fluid is a hydrocarbon.

11. The multi-layer heating system according to claim 8, wherein the furnace is a steam cracking furnace.

12. The multi-layer heating system according to claim 11, wherein the furnace is a steam methane reforming furnace.

13. The multi-layer heating system according to any one of the preceding claims, wherein during operation, the radiation surface of the conductive refractory layer has a temperature greater than 1000 °C.

14. The multi-layer heating system according to any one of the preceding claims, wherein during operation, the electrical insulation layer has a temperature less than 900 °C.

15. The multi-layer heating system according to any one of the preceding claims, wherein the conductive layer has a resistivity of about 10 -5 Ohm·m to about 10 -8 Ohm·m at 20°C.

16. The multi-layer heating system according to any one of the preceding claims, wherein the electrical insulation layer has a resistivity greater than 10 Ohm·m at 20 °C.

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