Method for carrying out chemical reactions and reactor configurations

Abstract

The present invention relates to a method for carrying out a chemical reaction using a reactor arrangement (100-400), comprising a reaction vessel (1) in which reaction tubes (2) are arranged, to which one or more reactants are supplied at a first mass flow rate during one or more first operating modes and either not supplied or supplied at a second mass flow rate that is at least time-averaged less than the first mass flow rate during one or more second operating modes, and wherein radiant heat is supplied into the reaction vessel (1) by one or more electric heating elements (3) at a first heat flow rate during one or more first operating modes and either not supplied or supplied at a second heat flow rate that is at least time-averaged less than the first heat flow rate during one or more second operating modes. At least a portion of the reactor vessel (1) equipped with one or more heating elements (3) is supplied with a gas atmosphere containing a predetermined volume fraction of oxygen, wherein during one or more first operating modes, the volume fraction of oxygen is adjusted between a first limit value of 500 ppm or more and a second limit value of 10% or less, and during one or more second operating modes, the volume fraction of oxygen is adjusted, at least on a time average, to a value higher than the volume fraction during one or more first operating modes. Corresponding reactor configurations (100-400) are also subject of the present invention.

Description

[Technical Field]

[0001] The present invention relates to a process for carrying out chemical reactions and a corresponding reactor arrangement according to the preambles of the independent claims. [Background technology]

[0002] Many processes in the chemical industry use reactors in which one or more reactants are passed through heated reaction tubes to carry out catalytic or non-catalytic reactions. Heating is particularly used to overcome the activation energy required for the chemical reaction occurring, or, in the case of endothermic reactions, to provide the energy required for the chemical reaction. The reaction can proceed either endothermically overall or, after the activation energy has been overcome, exothermically. The present invention is particularly concerned with strongly endothermic reactions, as further described below.

[0003] Examples of such processes include steam cracking, various reforming processes, especially steam reforming, dry reforming (carbon dioxide reforming), mixed reforming processes, and alkane dehydrogenation processes. In steam cracking, the reactor tube is coiled into the reactor and has at least one reverse bend within the reactor. On the other hand, steam reforming typically uses tubes that run through the reactor without a reverse bend. The present invention may also be used in conjunction with so-called "millisecond" or "single-pass" reactors, which are characterized by very short residence times.

[0004] Further applications of the present invention include reactors for the reverse water gas shift (RWGS) reaction of carbon dioxide and hydrogen to form carbon monoxide and water, reactors for oxygenated dehydrogenations such as the reaction of methanol to formaldehyde and hydrogen, reactors for the decomposition of ammonia to produce gaseous nitrogen and hydrogen, reactors for the dehydrogenation of so-called liquid organic hydrogen carriers (LOHCs) known to those skilled in the art, and reactors for the reforming of methanol and glycerol (unless already included in the term "reforming" above).

[0005] The present invention is suitable for all such process and reactor embodiments. For illustrative purposes only, see the entries "Ethylene," "Gas Production," and "Propene" in Ullmann's Encyclopedia of Industrial Chemistry, e.g., DOI: 10.1002 / 14356007.a10_045.pub2, published April 15, 2009; DOI: 10.1002 / 14356007.a12_169.pub2, published December 15, 2006; and DOI: 10.1002 / 14356007.a22_211, published June 15, 2000.

[0006] The reaction tubes of comparable reactors are conventionally heated using burners, for which purpose they are introduced into a combustion chamber in which a burner is also arranged.

[0007] However, there is currently a growing demand for synthetic products such as olefins, as well as synthesis gas and hydrogen, which can be produced with no or reduced local carbon dioxide emissions. This demand cannot be met by processes that typically use fossil fuels and therefore furnaces. In fact, other processes are ruled out, for example, due to their high costs.

[0008] It has therefore been proposed to supplement or replace the burners in comparable reactors with electrical heating means. In addition to direct electrical heating, for example, in which current is applied to the reaction tubes themselves in a known star circuit, and other types of heating not described in detail here, there is also, in particular, the concept of so-called indirect electrical heating, which is also used in the context of the present invention. Regardless of the specific type of heating or heating concept implemented in the process, a suitably heated reactor is also referred to as a "furnace."

[0009] Such indirect electrical heating can be carried out using electric radiant heating elements ("radiant heaters") suitable for the high temperatures required for the above-mentioned reactions, as described, for example, in WO 2020 / 002326, which are arranged in the furnace so as not to be in direct contact with the reaction tubes. Heat transfer occurs mainly or entirely in the form of radiant heat. Therefore, the terms "indirect heating," "heating by radiant heat," etc. are used synonymously below. The properties of the corresponding heating elements are described below.

[0010] Some further examples of prior art documents are set out below, but no admission is made that these documents anticipate, suggest or are relevant to any aspect of the invention and its embodiments described below.

[0011] WO 2019 / 133215 provides a method and system for optimizing a pyrolysis reaction by measuring temperature from a small insulating skin thermowell. In the system and method disclosed in this document, the upstream temperature and pressure of a pyrolysis reactor are measured through an adiabatic restriction in the inlet manifold of a parallel tube assembly to obtain the upstream absolute temperature and pressure. The downstream temperature of the pyrolysis reactor is also measured after the adiabatic restriction to obtain the downstream absolute temperature. Then, k / k -1 Determine the downstream pressure by multiplying the upstream absolute pressure by the quotient of the downstream temperature divided by the upstream temperature calculated as a power of k, where k is the ratio of the fluid specific heat at constant pressure (Cp) to the fluid specific heat at constant volume (Cv).

[0012] US Patent Application Publication No. 2019 / 002389 discloses a process for continuously preparing tert-butyl esters of ethylenically unsaturated carboxylic acids, which process comprises the following steps: a) reacting an ethylenically unsaturated carboxylic acid with isobutene in the presence of an acidic catalyst to obtain an esterification mixture; b) removing the acidic catalyst; c) removing low-boiling components; and d) feeding a liquid containing the tert-butyl ester to a distillation apparatus and performing purification distillation in the distillation apparatus. In step d), the tert-butyl ester-containing liquid is separated into a gaseous overhead product containing the tert-butyl ester and a liquid bottom product containing the carboxylic acid in a distillation apparatus; d2) the gaseous overhead product containing the tert-butyl ester is at least partially concentrated and the concentrate is partially recycled to the distillation apparatus as reflux; d3) the liquid bottom product containing the carboxylic acid is at least partially recycled to step a); d4) the liquid bottom product containing the carboxylic acid is withdrawn and sent to a heater; a superheated liquid recycle stream is removed from the heater; the superheated recycle stream is lowered into the distillation apparatus; d5) the walls of the distillation apparatus that are in contact with steam, at least in the top region of the distillation apparatus, are heated and / or insulated in at least a subregion. During this process, the tert-butyl ester is separated from the unreacted carboxylic acid. This is accompanied by a particularly low level of polymerization of both the tert-butyl ester and the carboxylic acid.

[0013] EP 1273552 A1 provides a hydrogen production device that supplies at least one material together with air, oxygen, or an oxidant to a reaction section to produce hydrogen through a specific chemical reaction. In this device, for the at least one material, one of two or more predetermined set values ​​is selected according to the required hydrogen production volume to set the supply amount of each material; and for the air, oxygen, or oxidant, the supply amount of air, oxygen, or oxidant is changed and controlled so that the temperature of the reaction section is within a predetermined temperature range.

[0014] EP 4056892 A1 discloses a steam cracking method using a steam cracking system comprising one or more first steam cracking furnace units and one or more second steam cracking furnace units, wherein the first steam cracking furnace unit or each of the plurality of first steam cracking furnace units comprises one or more heated steam cracking furnaces, the second steam cracking furnace unit or each of the plurality of second steam cracking furnace units comprises one or more electric steam cracking furnaces, and the first steam cracking furnace unit or each of the plurality of first steam cracking furnace units comprises means for preheating at least a portion of the combustion air supplied to the heated steam cracking furnace or furnaces to a temperature level of at least 100°C.

[0015] EP 3862076 A1 relates to a reactor for carrying out chemical reactions, which reactor comprises a reaction vessel and one or more reaction tubes. Multiple tube lengths of the one or more reaction tubes each run between a first and a second region in the reaction vessel, and each of the tube lengths can be electrically connected in the first region to a phase terminal of a polyphase AC power supply for heating the tube length. According to this document, the tube lengths are connected to each other in an electrically conductive manner in the second region either as a whole by a single rigid connection element or in groups by multiple rigid connection elements, which connection element(s) are integrally connected to the single or multiple reaction tubes and arranged in the reaction vessel.

[0016] WO 2020 / 002326 describes at least one electrically heated furnace defining a space. The present invention relates to a reactor configuration comprising: at least one reactor tube disposed within the furnace space, the reactor tube having an outlet and an inlet on the outside of the reactor; the furnace further comprising at least one electric radiant heating element suitable for heating to a high temperature in the range of 400 to 1400°C, the heating element being disposed within the furnace so as not to directly contact the at least one reactor tube; multiple inspection ports are present in the furnace wall to allow visual inspection of the condition of at least one reactor tube on each opposing side of the reactor tube during operation, the total number of inspection ports being sufficient to inspect all reactor tubes throughout the entire length and circumference of the furnace; and a furnace heat load of at least 3 MW. Electrically heated processes require a heat flux and temperature profile. In many applications, as the process progresses into the furnace, the heat flux increases but the temperature decreases. As the process progresses toward the outlet, the heat flux decreases but the temperature increases. This requirement can be met by the present invention. This reactor is useful in many industrial-scale high-temperature gas conversion and heating technologies.

[0017] U.S. Patent Application Publication No. 2020 / 299131 discloses that a liquid fuel catalytic partial oxidation (CPOX) reformer and fuel cell system can include a plurality or array of spaced-apart CPOX reactor units, each reactor unit comprising an elongated tube having a gas-permeable wall with an inner surface and an outer surface. The wall encloses an unobstructed gas flow passageway. At least a portion of the wall contains a CPOX catalyst and / or a CPOX catalyst structure. The catalyst-containing wall structure and the wall-bounded open gas flow passageway define a gas-phase CPOX reaction region, and the catalyst-containing wall is gas-permeable, allowing the gaseous CPOX reaction mixture to diffuse into the wall and the hydrogen-rich product reformate to diffuse out of the wall. The liquid fuel CPOX reformer can also include a vaporizer, one or more igniters, and a liquid reformate source. The hydrogen-rich reformate can be converted to electricity in a fuel cell unit integrated with the CPOX reactor unit.

[0018] The object of the present invention is to provide means which make it possible to advantageously operate a reactor of the type described which is indirectly electrically heated using a suitable heating element. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] International Publication No. 2020 / 002326 [Patent Document 2] International Publication No. 2019 / 133215 [Patent Document 3] U.S. Patent Application Publication No. 2019 / 002389 [Patent Document 4] European Patent Application Publication No. 1273552 [Patent Document 5] European Patent Application Publication No. 4056892 [Patent Document 6] European Patent Application Publication No. 3862076 [Patent Document 7] U.S. Patent Application Publication No. 2020 / 299131 [Non-patent literature]

[0020] [Non-Patent Document 1] DOI:10.1002 / 14356007.a10_045.pub2 [Non-patent document 2] DOI:10.1002 / 14356007.a12_169.pub2 [Non-patent document 3] DOI:10.1002 / 14356007.a22_211 Summary of the Invention

[0021] Against this background, the present invention proposes a process for carrying out a chemical reaction and a corresponding reactor arrangement having the features of the independent claims. Embodiments of the invention are the subject of the dependent claims and the following description.

[0022] The present invention relates to a process for conducting chemical reactions, utilizing a reactor configuration using reaction tubes disposed within a reaction vessel. One or more reactants are fed (through) the reaction tubes at a first mass flow rate during one or more first modes of operation and not fed (through) the reaction tubes during one or more second modes of operation, or are fed (through) the reaction tubes at a second mass flow rate that is less than the first mass flow rate, at least on a time-averaged basis, during one or more of the second modes of operation. By "on a time-averaged basis," it is meant that even if the flow rate fluctuates over a period of time, the average flow rate over that period is characteristically less in the second mode of operation. The time basis may be, in particular, for one or each of the first modes of operation or for one or each of the second modes of operation, rather than on a corresponding time-averaged basis. Other reactants may be used in one or more second modes of operation than in the first mode of operation, such as air in a decoking mode. Different reactants may be used in the second modes of operation relative to one another.

[0023] In this regard, the one or more first modes of operation may correspond to or be carried out during one or more reaction periods, which are described in more detail below, and the one or more second modes of operation may correspond to or be carried out during one or more heat-up, cool-down, load change, decoking, or standby periods, which are also described below.

[0024] The present invention is particularly advantageous in that it can cope with (severe) load changes. Such load changes, referred to above by the term "load change period", may occur between first operating mode(s) and second operating mode(s), or between different second operating modes. In particular, load changes associated with different power outputs of heating elements place particularly high strain on the latter. As will be explained further below, the present invention improves this situation.

[0025] According to the present invention, radiant heat, obtained by one or more electric heating elements disposed within the reactor vessel, is supplied to the reactor tubes at a first heat flux during one or more first operating modes, and is not supplied to the reactor tubes during one or more second operating modes, or is supplied to the reactor tubes at a second heat flux that is lower, at least on a time-average basis, than the first operating mode during one or more second operating modes. For the meaning of "on a time-average basis," see the explanation above. Similarly, the corresponding time may be, in particular, one or more periods during one or more first operating modes, or one or more periods during one or more second operating modes, rather than the corresponding time-average. As explained below, this allows the reactor tubes to be maintained at temperatures within a range of different temperatures. In particular, the first heat flux can be obtained by operating one or more heating elements at a constant or nearly constant power input or current, and the second heat flux can be obtained by operating one or more heating elements at a lower power input or current (at least on a time-average basis), which may also vary over time. It is not essential to the invention that the heating element(s) provide any heat flux in the one or more second operating modes. Instead, in the one or more second operating modes, it is possible to perform, for example, a so-called "hot steam standby" in which steam is passed through the reactor tubes to keep them at a certain temperature, i.e., at a predetermined temperature or within a predetermined temperature range, but the heating elements are set to low power consumption or current, or optionally not operated at all.

[0026] The reactor tube temperature achieved by utilizing the first heat flow in the reactor tube can be selected to be the same or comparable to that of a calciner or other electrically heated furnace. Because significant temperature gradients sometimes occur in the corresponding reactor tubes (especially with increased coking, resulting in "cold" inlets and "hot" outlets), reactor tube temperatures cover a relatively wide temperature range. When using radiant heating elements, the required heat flow rate necessitates higher temperatures at the heating element to achieve the above reactor tube temperature levels. Simply put, the average heating element temperature generally "elevates" above the average reactor tube temperature level, and this "elevation" between these temperatures is proportional to the heat flux density required by the process during operation. The higher the required average heat flux density, the greater the average temperature difference between the heating element and the reactor tube. However, the local temperature at a specific location on the heating element surface is variable, resulting from a complex thermal equilibrium process at that given location involving all relevant heat transfer mechanisms (radiation, convection, conduction) and Joule heating due to electrical current.

[0027] The present invention can be used as described above, in particular as initially mentioned in connection with the production of olefins and / or other synthesis products by steam cracking or the production of synthesis gas or hydrogen by steam reforming, but is in principle suitable for all reactions of the type in which the feed mixture is passed in the gaseous state through reaction tubes which are heated externally to a suitable temperature level, thereby causing the reaction to take place.

[0028] In particular, the reactor tubes can be introduced into the reactor vessel in any conceivable way, with or without one or more backflow points or reverse bends. In particular, the reactor tubes can be arranged in a single row in a vertically arranged plane and heated by radiant heating elements arranged on both sides of the plane. It is also possible to arrange the reactor tubes in multiple rows in the intermediate region between two planes and correspondingly heat the intermediate region from the outside. In particular, the reactor tubes have a length of 5 to 100 m and / or a diameter of 20 to 200 mm. Furthermore, the individual reactor tubes can be designed in the form of two or more parallel bundles with a smaller tube diameter compared to a single tube. Preferably, a multi-bundle section is arranged near the entrance to the furnace, providing the longest possible reactor tube wall area in this region. In this arrangement, further downstream, several initially parallel bundles are combined into a single common bundle, preferably with a larger tube diameter. In this example, the reactor tubes consist of two or more parallel bundles, in particular a connection section with fittings, and a combined bundle. Conversely, it is also possible in principle to provide a multi-tube design of reactor tubes at the ends or in the intermediate section with intermediate partitions and, if necessary, additional connecting pieces. Generally, in embodiments of the present invention, the tubes may be divided or combined in any conceivable manner. Depending on the type of reaction, the reactor tubes may be filled with suitable catalytic and / or inert materials or may be provided empty.

[0029] In the present invention, the reaction tubes are heated by electrically supplied radiant heat. However, the present invention does not exclude the use of other types of additional heating, such as direct heating, in which the reaction tube itself generates heat as an electrical resistance, induction heating, or heating using a burner if a reaction vessel is added to the reactor configuration. In any case, in addition to radiant heat, a portion of the heat supplied by a suitable heating element may be transferred to the reaction tubes by convection.

[0030] Thus, when reference is made herein to the use of indirect electric heating, i.e. the use of radiant heat supplied by electric heating elements, this does not exclude the presence of additional electric or non-electric heating. In particular, it is also possible to envisage varying the contribution of each type of electric heating, in particular non-electric heating, over time, for example as a function of the supply and price of electricity or the supply and price of non-electric energy sources.

[0031] "Reaction vessel" is understood in this specification to mean an enclosed space that is partially or completely insulated from the outside and that may be lined with a material that is heat-resistant at the aforementioned temperatures. In particular, the reaction vessel is surrounded to a large extent, i.e., at least 90%, 95%, 99%, 99.5%, or 99.8%, by (solid) walls that have insulating properties. These walls may include a rigid, continuous, or impermeable backing, such as a metal sheet, and one or more insulating layers. The quantity indicated as the proportion of a reaction vessel that is "surrounded by insulating walls" may in this respect be understood in particular as the proportion of the overall reactor enclosure, which insulating walls are made of a solid structure that has insulating properties, i.e., are coated with, formed from, or contain insulating material. Openings or access points in the reactor enclosure, which are not usually considered to be completely insulated, may not be included in the quantity of a "largely enclosed" reaction vessel. Any part of the reactor wall that is understood in this specification to be "insulated" has a heat transmittance of 2 W / m 2 K or less, especially 1.5 W / m 2 Less than K, 1 W / m 2 Less than K, 0.5W / m 2 Less than K or 0.2W / m 2The term "heat transmission coefficient" is intended to mean that the numerical value of the associated quantity refers (only) to the conductive heat transfer coefficient in a solid structure (excluding radiative and convective heat transfer components, particularly on the inside and outside of the wall). For example, if a reactor vessel is surrounded by at least x% of its wall area by insulating walls, as indicated above, then x% or less of that wall area may be configured to have the heat transmission coefficient as indicated above. Thus, as noted above, openings or ports in the reactor housing may not be insulating and therefore may have a higher heat transmission coefficient. Alternatively, for example, in the case of permanent openings, the openings may not provide a thermal barrier at all. To provide a thermally insulated reactor wall, as noted above, the reactor wall may be made of, contain, or be coated with insulating materials, such as, but not limited to, ceramic fibers, heat-reflective metal foils, minerals, expandable polymers, or combinations thereof. In particular, different insulating materials may be provided to accommodate different local temperatures and different thermal resistances present.

[0032] As mentioned above, the invention is not limited to the use of strictly one reaction vessel, but can also be used in arrangements with different heated reaction vessels. Further details about the corresponding reaction vessels and their equipment with gas supply devices and, where applicable, their connections to gas extraction devices, stacks, etc. are further described below. In this specification, the terms (existing) "stack" and "chimney" are used synonymously, and both relate to a structure whose (primary) function is to connect a fluid to a safe outlet location, for example to the atmosphere, preferably at a sufficient height above ground level.

[0033] In the context of the present invention, the reaction vessel does not need to be designed to be gas-tight, or at least not completely gas-tight. According to an embodiment of the present invention, the reaction vessel is particularly provided to be sufficiently gas-tight to allow substantial control of the oxygen level inside the vessel. As mentioned herein, a defined oxygen concentration is particularly advantageous at the heating element, and therefore the gas-tightness of the reaction vessel is particularly important in the vicinity of the heating element. Therefore, in the vicinity of the heating element, the reaction vessel wall may be less gas-tight. However, this does not apply to all embodiments of the present invention. For the avoidance of doubt, the gas-tightness may not be suitable for intentionally introduced gas, even if this gas flows under the influence of a pressure difference between the outside and the inside of the reaction vessel, i.e., a pressure difference across the wall.

[0034] "Reaction period" is understood to mean the period or part of that period during which the reaction takes place and the reactants necessary for the reaction pass through the reaction tubes. This period corresponds in particular to the first operating mode(s). During the reaction period, combustible components, in particular hydrocarbons, are usually contained in the process feed gas and therefore pass through the reaction tubes. During periods other than the reaction period, such as regeneration periods or inerting periods (i.e. in particular the second operating mode(s)), such combustible components usually do not pass through the reaction tubes.

[0035] In embodiments of the present invention, the reactor tubes are heated to a reactor tube temperature level during a period before the actual reaction period, maintained at this temperature during the reaction period, and / or, in each case, cooled from this reactor tube temperature level during a cooling period after these reaction periods. The reaction period(s) are in particular periods during which "one or more first operating modes" are carried out in the sense understood herein, while the heating or cooling periods are in particular periods during which "one or more second operating modes" are carried out in the sense understood herein as described above. Heating is in particular carried out after a (temporary) shutdown of the reactor arrangement, for example for cleaning, decoking, repair, and / or maintenance. Cooling is accordingly carried out before the (temporary) shutdown and may comprise active or passive cooling (i.e., "cooling down" the reactor).

[0036] As is generally known, processes of the type described may also include a decoking operation, in particular for removing deposits formed in the reaction tubes after the corresponding reaction period, for example by "burnout" with an oxygen-containing gas or gas mixture. This is particularly true in the case of purely gas-phase reactions without catalysts. Before the corresponding decoking operation, the reaction tubes usually contain no reactants and are in particular pre-cooled or subsequently heated. Not only the decoking operation, but also corresponding periods of standby operation, for example, for adding pure steam to the reaction tubes to avoid (excessive) cooling (so-called "hot steam standby operation"), as well as periods of cooling or heating, are not considered part of the reaction period in the sense of this specification. This means that "one or more first operation modes" are not performed during that period, nor are, for example, maintenance periods or periods for replacing or regenerating the catalyst bed. These periods are periods during which "one or more second operation modes" are performed.

[0037] To summarize the above, during one or more second modes of operation, the reactor tubes may be operated in at least one of a steam standby mode in which steam is flowed through the reactor tubes to maintain a predetermined temperature or a predetermined temperature range, a decoking mode in which steam and air are flowed through the reactor tubes to decoke the reactor tubes, and a temporary cracking mode in which the reactor feed load through the reactor tubes and / or the process gas temperature at the outlet of the reactor tubes are varied over time. The heating elements may or may not be operated depending on the particular mode of operation.

[0038] Specifically, in steam standby mode, the heating elements may not be operated, but may be operated in "hot" steam standby. In decoking mode, the heating elements may be specifically operated, and in transient cracking mode, the heating element duty may be varied in response to the reactor feed duty and / or the process gas temperature at the outlet of the reactor tubes.

[0039] In the context of the present invention, "first" and "second" operating modes may refer to general operating modes of a reactor arrangement or reactor vessel. These operating modes may include: (i) one or more modes with or without gas flow through the reactor tubes; (ii) one or more modes of passing a hydrocarbon-containing mixture through the reactor tubes in a cracking operation, particularly for a reactor arrangement configured for steam cracking; (iii) one or more modes of passing steam through the reactor tubes in a standby operation; (iv) one or more modes of passing an air-containing mixture through the reactor tubes, particularly for decoking; or (v) one or more modes involving other start-up and shutdown operations using a gas flow, particularly nitrogen, air, or similar "start-up medium." Such modes may also be designated based on, for example, the detected run time of the reactor equipment or reactor vessel after a particular event since the last decoking cycle.

[0040] Alternatively or additionally, whether the first or second operating mode is active or being performed can also be determined based on data from real-time measurements, such as temperature measurements in the region of the heating element, in or on the reaction tube, or on the temperature of the process gas at the outlet of the reaction tube. Accordingly, it is possible to obtain, for example, current measurements, in particular in the supply lines to the reaction tube (to record the flow rate and current composition), oxygen measurements in the reaction vessel and / or in the connected exhaust stack and / or in the outlet stack, pressure measurements in the reaction vessel, analytical measurements downstream of the outlet of the reaction tube (to determine the product composition and, based thereon, for example, the gap sharpness), measurements of the heating power, the intensity of the applied voltage and / or current, and also pressure measurements upstream and / or downstream of the reaction tube or pressure differences across the reaction tube.

[0041] In embodiments, the present invention may include any process controls, as further explained below. These process controls may be based on or aimed at, among other things: (i) temperature setpoints (e.g., process gas temperature at the reactor tube outlet); (ii) specifications for flow control, especially in the supply lines to the reactor tube (to determine flow rates and gas stream composition); (iii) setpoints for heating power and the intensity of the applied voltage and / or current; and (iv) setpoints for heating or cooling rates. At any time, the acquired measurement data or setpoints can be evaluated over time and considered accordingly, allowing the time rate of change of the measured value to be determined (e.g., average rate of change over a period of time). It is also possible to quantify the time variation of the measured value (e.g., by taking the standard deviation of the measured value over a period of time). Using a time rate of change averaged over a significant period of time may result in a particularly quiet signal.

[0042] In the context of the present invention, the reaction tube is supplied with one or more reactants used in the chemical reaction during the reaction period or "first" mode of operation described herein, and during the heating and / or cooling period or other corresponding "second" mode of operation, one or more reactants are not supplied or are supplied in amounts less than during the reaction period. Typical reactants that are not used during the second mode of operation and therefore used in amounts less than those used in the first mode of operation are, in particular, hydrocarbons. Other reactants may include water (steam), oxygen, and other compounds.

[0043] In the context of the present invention, one or more second operating modes may be implemented by, for example, varying the feed rate, product gas temperature, and / or product gas composition during the reaction period, preferably during sub-periods during which load changes occur, such as changes in the reactor tube temperature level or changes in the current or power to the heating elements, resulting in changes in the heating element temperature.

[0044] According to the invention, a gas atmosphere is provided in at least a part of the reaction vessel in which one or more heating elements are arranged.

[0045] The gas atmosphere provided in at least a portion of the reaction vessel, which is equipped with one or more heating elements, is isolated by the reactor tube wall from one or more reactants supplied to the reaction tube. In other words, the oxygen content adjusted according to the present invention is specifically involved in the "gas space" of the reactor, i.e., the gas atmosphere does not come into contact with the process gas passing through the reaction tube containing one or more reactants, except in the case of a coil rupture. However, this gas space is in direct contact with the outer surface of the heating elements as well as the outer surface of the gas-tight reactor tube wall. Thus, the gas atmosphere surrounds the reaction tube, but is not supplied into the reaction tube.

[0046] The second operating mode may be the aforementioned decok- ing operation. In this operation, a mixture of a decoking gas stream, e.g., air as vapor, is passed through the reaction tubes. The oxygen content of the gas atmosphere supplied to at least a portion of the reaction vessel provided with one or more heating elements is adjusted, in particular independently of the oxygen content of the decoking gas stream. However, in this operation, the gas space outside the reaction tubes may be increased simultaneously or at any time.

[0047] According to an embodiment of the invention, the amount of radiant heat supplied to the reaction tubes by one or more electric heating elements in the reaction vessel may be more than 90%, in particular the entire amount, of the total amount of heat supplied to the reaction tubes in the reaction vessel, i.e., according to such an embodiment of the invention, either no (additional) combustion other than electrical heating is carried out in the reactor, or such combustion supplies only a small amount of heat to the reaction tubes.

[0048] Therefore, in such an embodiment, the oxygen content set in the gas space outside the reaction tubes does not significantly affect the heat input to the reaction tubes. Therefore, oxygen control is primarily used for safety and service life protection, not process control. Furthermore, the energy consumption of an electric furnace is not affected by the oxygen content. This is in contrast to a combustion furnace, where the temperature of the combustion chamber is affected by the oxygen content, which in turn affects the heat input to the reaction tubes. It should also be noted that the oxygen content in the coil box does not significantly affect the pollutant emissions during normal operation of an electric furnace. Only the aging process on the surface of the heating elements is affected. This aging refers to a slow reaction at very low conversion rates.

[0049] The gas atmosphere contains oxygen inclusions, in addition to one or more known inert gases, such as nitrogen or carbon dioxide, or one or more noble gases. According to the present invention, the inclusions are dynamically adjusted during operation to obtain a predetermined oxygen volume fraction value and / or range that varies over time. According to the present invention, this adjustment is performed such that the oxygen volume fraction during one or more first operating modes is adjusted between a first limit value and a second limit value. The first limit value is a volume fraction between 500 ppm and 0.5%, and the second limit value is a value above the first limit value, between 3% and 10%. The first limit value may be a volume fraction between 500 ppm and 0.1%, a volume fraction between 0.1% and 0.2%, a volume fraction between 0.2% and 0.3%, a volume fraction between 0.3% and 0.4%, and a volume fraction between 0.4% and 0.5%, or any continuous combination of two or more of these ranges. The second limit may be a volume fraction of 3% to 4%, a volume fraction of 4% to 5%, a volume fraction of 5% to 6%, a volume fraction of 6% to 7%, a volume fraction of 7% to 8%, a volume fraction of 8% to 9%, a volume fraction of 9% to 10%, or any continuous combination of two or more of these ranges.

[0050] During the one or more second operating modes, the oxygen volume fraction is, according to the invention, set to a higher value, at least on a time-averaged basis (in the sense described above), than in the one or more first operating modes. In this specification, a lower limit value may be used to define a lower threshold and an upper limit value may be used to define an upper threshold for a (feedback) control structure implemented in a control device or system that adjusts the oxygen volume fraction.

[0051] In particular, within the scope of the present invention, the oxygen volume fraction may be adjusted within a range between a third limit value and a fourth limit value during one or more second operating modes, where the third limit value is higher than the first limit value, the fourth limit value is higher than the second limit value, and / or the third limit value is higher than the second limit value. The third and / or fourth limit values ​​may vary during one or more second operating modes, in particular over time. Accordingly, a corresponding threshold-based setting can also be implemented for (higher) oxygen contents during one or more second operating modes. The precisely set value can also be changed continuously or stepwise during one or more second operating modes, for example, to accommodate a gradual increase in temperature and / or a gradual increase in reactant supply.

[0052] In general, within the scope of the present invention, adjusted to the above-mentioned limits, the maximum oxygen content may be specified to be equal to or less than the atmospheric oxygen content, i.e., in particular, less than 20, 15, or 10% by volume. The time-averaged oxygen content at one or more locations in the reaction vessel in one or more second modes is in particular 0.1, 0.5, 1, 2, or 5 percentage points higher than the time-averaged oxygen content in one or more first modes. Preferably, during a transient decomposition operation, or generally during any other second operating mode that supplies the reaction tube with combustible gas components, the time-averaged oxygen content at one or more locations in the reaction vessel is 0.1 to 5 percentage points, or 0.5 to 2 percentage points higher than in one or more first operating modes, but remains below the atmospheric oxygen content or a lower maximum oxygen content due to safety considerations. Preferably, in steam standby operation, decoking operation, or any other second mode of operation in which no combustible gas components are generally supplied to the reaction tubes, the time-averaged oxygen content at one or more locations within the reaction vessel is 1, 2, or 5 percentage points or more higher than in one or more first modes of operation, and may increase to atmospheric oxygen content levels.

[0053] Maintaining the oxygen content between the variation limits according to embodiments of the present invention makes it possible, on the one hand, to increase the durability of the corresponding heating elements and, on the other hand, to ensure a high level of operational safety. A relatively low oxygen content during one or more first operating modes ensures that the formation of a critical (ignition or explosion) atmosphere in the reaction vessel is prevented or combustion is minimized in the event of a reactant leak from the reaction tube. A relatively high oxygen concentration during one or more second operating modes ensures that the heating elements are not particularly susceptible to damage in their protective oxide layer due to thermal expansion effects during these phases, or that defects in the oxide layer are repaired more quickly due to the increased availability of oxygen in the atmosphere due to re-oxidation.

[0054] The time-variable oxygen control maximizes the durability of the heating element on the one hand and ensures safe operation at all times on the other hand. Thus, within the scope of the present invention, various possibilities for dynamic atmosphere adjustment are possible, so that the above requirements are met. In particular, dynamic control may involve dynamically adjusting the lower and / or upper oxygen content limit of the gas atmosphere in the reaction vessel depending on the immediate operating mode (first or second operating mode) and operating conditions. During this adjustment, both lower and upper stop limits, alarm limits, process switching values ​​(especially in the case of floating control) and / or process setpoints (in the case of continuous control) can be changed.

[0055] The heating elements used for indirect heating of the corresponding reaction tubes usually comprise an electrically conductive metallic or non-metallic heating structure of a given shape, for example in the form of a linear or other shaped rod, wire or strip, the metallic heating structure being preferably made of an alloy containing at least the elements Fe, Cr and Al. Alternatively or additionally, the metallic heating structure can also be at least partially made of a nickel-chromium alloy, a copper-nickel alloy or a nickel-iron alloy.

[0056] It is known that in indirect heating of reaction tubes, particularly in steam cracking, extremely high heat flux densities at high temperatures are required for economical operation, and therefore the heating element or heating structure must operate near its upper temperature limit. However, it is precisely near this limiting temperature that the heating element and heating structure become very sensitive to the furnace atmosphere. In particular, a specific minimum oxygen content is advantageous to avoid or delay rapid or gradual deterioration of the heating element or heating structure. For example, when using a metal heating structure containing aluminum, a stable aluminum oxide layer can be formed on the surface of the heating structure, protecting the material from uncontrolled corrosion and other damage mechanisms. Therefore, the present invention is effective in improving the durability of the heating element or its heating structure by using an appropriate minimum oxygen content.

[0057] It has been found that FeCrAl-based heating elements are damaged when exposed to high-nitrogen and low-oxygen atmospheres at high temperatures, resulting in a maximum operating temperature lower than the maximum allowable operating temperature in air. Without being bound by theory, this damage is believed to be related to nitride formation, which prevents the formation of a protective aluminum oxide layer on the heating element surface and causes corrosion that significantly shortens the element's lifespan. The extent and rate at which such damage can occur is related to the concentration of oxygen and oxygen-containing species in the atmosphere in contact with the heating element, as well as the element's temperature. For example, a study reported in the Journal of Mining and Metallurgy, Section B, Vol. 55, p. 55, 2019, found that when FeCrAl material is heated to 1200°C in a 99.996% nitrogen atmosphere (with impurity levels of oxygen and water less than 10 ppm), corrosion proceeds via the formation of localized subsurface nitrided regions composed of AlN phase particles. Conversely, as described for FeCrAl alloys in Surface and Coatings Technology, 2001, Vol. 135, p. 291, no significant differences in morphology were observed between oxide scales obtained by oxidation in air or in gas atmospheres containing 2% or 10% by volume of oxygen.

[0058] Furthermore, without being bound by theory and without limiting the scope of the present invention, it is believed that the oxygen concentration required on the surface of the heating element to prevent accelerated deterioration of the heating element depends on operating conditions such as temperature and the thermal history of the heating element, which determines the thickness and quality of the protective oxide layer. While a very low oxygen concentration (e.g., 100 ppm) may be sufficient to prevent accelerated deterioration under favorable conditions, it is prudent to set a higher oxygen concentration in the furnace atmosphere given the susceptibility of the heating element surface to nitriding and the possibility of uneven oxygen distribution in the furnace, resulting in localized drops in the oxygen concentration below the target concentration. Thus, a practical lower limit for the oxygen concentration in the furnace or reactor atmosphere is considered to be 0.1% by volume, although 500 ppm may be selected. Higher limit concentrations, for example, oxygen concentrations of 0.2% by volume or more, such as 0.5% or 1% by volume, may further expand the safety margin when furnace conditions are less favorable or the oxygen distribution is more significantly uneven. This oxygen concentration may be selected in accordance with the present invention. Because the oxidation rate of typical heating element materials is known to increase with oxygen concentration, it may be beneficial to have a low oxygen concentration near the heating element, as long as a minimum oxygen concentration to prevent nitride corrosion is met, which may depend on temperature and also on the composition of the heating element.

[0059] Given the complexity of the underlying physical mechanisms within such furnaces and the wide range of operating conditions, it is particularly interesting to predict the oxygen control possibilities that change over time and adapt the atmospheric conditions as needed in accordance with the present invention.

[0060] The provision of a gas atmosphere according to the invention is advantageous for the above-mentioned metal alloys, but in principle it is also advantageous for use with other materials, for example based on MoSi or SiC, regardless of the damaging effects observed in each case.

[0061] Consideration of the flammability limits of the feed and product gases is important in determining the maximum oxygen content allowed. Within the flammability limits of all flammable gases, there exists an oxygen concentration, commonly referred to as the limiting oxygen concentration (LOC), below which a flammable mixture cannot form. For example, the LOC of ethylene at 25°C and 1 atmosphere is 10% oxygen. Under these conditions, any mixture of ethylene, nitrogen, and oxygen that does not reach an oxygen concentration of at least 10% cannot produce a self-propagating flame. Combining literature data and temperature control procedures, the LOCs for ethane and ethylene at a typical steam cracking temperature of 830°C can be estimated to be 4.1% and 3.6%, respectively. If the oxygen concentration in the reactor is below these limits, a flammable mixture will not form even if the coil breaks.

[0062] Although there is some uncertainty in calculating the same limit for a complex mixture such as naphtha, the estimated LOC is 4.2% for hexane, and thus ethylene is expected to be the reactant / product with the lowest LOC. Although 830°C is above the autoignition temperature of all of these hydrocarbons, staying below the LOC is expected to prevent shock wave formation even if spontaneous combustion were to occur.

[0063] Based on these observations, oxygen levels for the first and second modes of operation in accordance with the present invention may be selected in any mode of operation in which a combustible component is supplied to the process line. It is further noted that in one or more second modes of operation in which no combustible component is supplied to the process line, the maximum oxygen level may be temporarily increased to atmospheric levels.

[0064] Generally, a heating element used in the context of the present invention may have a base, for example, made of a non-conductive and heat-resistant material (e.g., ceramic), on or within which a heating structure, for example in the form of a heating wire or heating ribbon, is introduced, for example, in a serpentine shape. Alternatively, one or more linear and / or curved heating structures may be used, with a holder connected to the heating element. For example, so-called heating cartridges may be used, which can be fixed by a suitable connection, such as a plug-in or bayonet connection. Typically, multi-phase alternating current (AC), especially three-phase AC, is used for heating, and the heating wires can be connected in groups to the corresponding AC phases, although direct current (DC) heating may also be used. The present invention allows for any grouping, arrangement, and operating mode of the corresponding heating elements and is not limited thereby.

[0065] By using the present invention, i.e., by using a relatively high oxygen content, especially during heating and / or cooling periods or other corresponding "second" operating modes, it is possible to reliably avoid excessive aging of the heating element, even during the corresponding heating and / or cooling periods or other operating modes that are (more) susceptible to this heating and / or cooling. For example, during heating, cracks can form in the metal substrate of the heating element, especially due to differences in the thermal expansion coefficients of the latter and the protective oxide layer formed thereon. The formation of such cracks can be eliminated by increasing the oxygen content, facilitating the rapid regeneration of an intact protective oxide layer. At the same time, during the corresponding heating periods, when steam is being supplied to the reaction tube but flammable reactants such as hydrocarbons are not yet being supplied, it is less important to ensure a low-oxygen atmosphere in the reaction vessel. This is taken into account by the present invention.

[0066] Thus, prior to feeding the (particularly combustible) reactants, especially hydrocarbons, the heating element can be brought to a high temperature for a corresponding heating period, during which a relatively high oxygen content can still be maintained. The oxygen setpoint can then be reduced, either continuously or stepwise, to a level low enough to begin feeding the reactants. Once operation has stabilized, further reductions may be made.

[0067] In one embodiment of the present invention, characterized by a large number of individually controllable adjustment gas injection points, the latter reduction can be achieved, for example, by adjusting the premixing ratio at the injection points close to the wall, so that the content in the region of the reactor tube remains practically unchanged.Accordingly, during cooling after the reaction run, the oxygen content in the reactor vessel can be increased in the reverse procedure.

[0068] As described above, the volume fraction of oxygen can be varied continuously or stepwise during one or more second operation modes, and the volume fraction of oxygen can be adjusted, particularly continuously or stepwise, as a function of the reaction tube temperature, the temperature of the heater(s), and / or the amount of reactant(s) supplied to the reaction tube during one or more second operation modes. In this way, the aforementioned objectives (e.g., avoidance of damage to heating elements, avoidance of an explosive atmosphere) can always be achieved to the maximum extent within the scope of the present invention.

[0069] As mentioned above, during one or more first operating modes, the reactor tube may be maintained at a reactor tube temperature level in at least one zone such that the reactor tube temperature level in this zone varies only within a predetermined range, in particular not more than 10 K, 30 K, or 50 K. The heating element may be energized during one or more first operating modes, in particular with a constant or approximately constant effective current, where approximately constant current refers to operation in which the power consumption fluctuates by no more than 5%, 10%, or 30% during one or more first operating modes. Power consumption here is preferably understood to mean the effective power consumption averaged over a period of at least 10 seconds, 30 seconds, 1 minute, or 5 minutes, in particular to eliminate short-term fluctuations associated with power control operation (e.g., burst control or phase angle control in thyristors).

[0070] On the other hand, the reactor tube temperature may vary significantly during one or more second operating modes, e.g., the heating or cooling periods. During the heating period, the reactor tube may be heated from an initial temperature level, e.g., in the range of -50 to 700°C, to a final temperature level, e.g., in the range of 500 to 1200°C, and vice versa during the cooling period. The temperature of the heating element also varies accordingly, e.g., during the heating period, the initial temperature level is e.g., -50 to 700°C, and the final temperature level is e.g., 600 to 1400°C. In either case, the present invention allows for special consideration of this temperature change by using different oxygen contents. During such heating periods, one or more heating elements installed in the furnace may or may not be electrically energized. For example, the initial heating stage may be performed by simply flowing an externally preheated medium, e.g., steam, air, and / or nitrogen, through the reactor tube in the furnace. Alternatively, the furnace may be at least partially heated by applying electrical current to one or more heating elements but not flowing fluid through the reactor tube. In a preferred embodiment, at least a portion of the heating period involves flowing an externally preheated medium, such as steam, air, nitrogen, and / or a hydrocarbon, while simultaneously applying electrical current to one or more heating elements, thereby incrementally increasing the temperature of at least a portion of the furnace. The cooling period may operate in a similar manner by varying the composition, flow rate, and / or preheat temperature of the fluid medium passing through the reactor tubes and / or varying the electrical current applied to one or more heating elements within the furnace.

[0071] In a more general implementation, the oxygen volume fraction may be varied at least temporarily or intermittently during one or more second modes of operation, while simultaneously varying at least intermittently the second mass flow rate of one or more reactants and / or varying at least intermittently the second radiant heat flux during one or more second modes of operation. The term "intermittently" refers to the fact that variations do not necessarily occur constantly during the corresponding modes of operation, but may be interrupted by periods of no variation. The at least intermittent variation of the oxygen volume fraction during one or more second modes of operation may be implemented in conjunction with at least intermittent variation of the second volumetric flow rate of (at least) one or more reactants and / or at least intermittent variation of the second radiant heat flux during one or more modes of operation. Further embodiments and criteria for implementing the first and second modes of operation have been described above.

[0072] Alternatively, as described above, at least a portion of each reactor tube may be maintained at a reactor tube temperature within a first temperature range during one or more first operating modes and at a reactor tube temperature within a second temperature range during one or more second operating modes. The first temperature range may be, particularly at the surface and / or interior of the reactor tube, 400 to 1500°C, more particularly 450 to 1300°C, 500 to 1200°C, or 600 to 1100°C. The second temperature range may be defined, for example, by a temperature difference from the first temperature range, particularly a temperature difference of at least 1 K, 10 K, 50 K, or 100 K. In some examples, this difference may be small, for example, near the inlet of the reactor tube. In some examples, this difference may be locally negative, for example, when the inlet temperature is increased in the second operating mode. As described above, the second temperature range may also be achieved by appropriate standby operation, particularly using a pure steam flow.

[0073] In the context of the present invention, corresponding heating elements can be arranged in particular in the reactor wall, from which heat is radiated to the reaction tubes. The wall can be straight or curved, for example parabolic. The wall can also combine any wall shape and can have, for example, straight wall sections that can be arranged at an angle or any angle to one another. Providing a gas atmosphere according to the present invention ensures that the aforementioned oxygen content is available in the area where the heating elements are arranged.

[0074] The present invention improves the operational safety of the corresponding reactor vessel by adjusting the proposed upper oxygen concentration limit during one or more first operating modes, especially in the event of damage to the reactor tubes ("coil burst"). While comparable damage may result in the complete disconnection of one or more reactor tubes, the present invention also favors smaller leaks. In the event of comparable damage, flammable gases may suddenly or gradually leak into the reactor vessel, which is essentially sealed for thermal insulation reasons.

[0075] Such damage poses less of a safety risk in conventionally heated reactors than in the configuration of the present invention, in which at least one reaction vessel is primarily electrically heated. This is because, in heated reactors, flammable gases escaping from the reaction tubes, for example in the form of a hydrocarbon / steam mixture, can be converted in a controlled manner by combustion in the reaction vessel or a comparable combustion chamber, or can be safely vented in the exhaust gas stream. Furthermore, the gas chamber surrounding the reaction tubes is essentially already "inert," since the oxygen content has already been significantly reduced by the combustion of the fuel gases generated in the conventional manner. In contrast, in the case of solely electrically heated reactors, the same flammable gases accumulate in the reaction vessel, where they may reach their explosion or detonation limits, for example, at temperatures above their autoignition temperature, even with the oxygen content of air. Even in the case of combustion without explosion or detonation, energy is released by complete or incomplete combustion, which can lead to overheating. Complete or incomplete combustion, along with the volume of gases escaping from the reaction tubes, can cause an undesirable pressure increase, among other things. In the present invention, such pressure buildup is suppressed because the low oxygen concentration and therefore low oxygen accumulation in the reaction chamber limits combustion of the gas mixture.

[0076] Thus, the present invention is particularly preferred for indirectly electrically heated reactors where the process gas temperature is close to or above the autoignition temperature of the components contained in the process gas, particularly hydrocarbons.

[0077] The proposed measures provide a controlled atmosphere containment vessel, which contributes to the maintenance of a protective oxide surface on the heating elements and to the safety-related protection of electrically energized, high-temperature reactors. In particular, the use of the present invention also increases the durability of the heating elements, which are protected by an increased oxygen content, especially during heating and cooling, load changes, or decoking or standby periods. Thus, within the scope of the present invention, a reactor vessel may be provided that operates in particular with fully electric heating. That is, the heating of at least the reactor tubes in this reactor vessel is advantageously carried out primarily or exclusively by electric heating, i.e., using at least 90%, 95%, or 99% of the heat introduced therein, in particular the entire heat introduced therein, by electric heating means. The heat input from the gas mixture passing through one or more reactor tubes is not taken into account here. This proportion therefore relates in particular to the heat transferred from the outside to the wall of one or more reactor tubes in the reactor vessel, or to the heat generated in the reactor vessel at said wall or catalyst bed.

[0078] In certain embodiments of the invention, also referred to below as the "first group of embodiments," one or more gases or gas mixtures used to provide the gas atmosphere can be supplied to the reaction vessel during one or more first operating modes and / or one or more second operating modes, or during any stage thereof, while a portion of the gas atmosphere is simultaneously evacuated from the reaction vessel. This results in a continuous flow, in particular within the reaction vessel, which also makes it possible to avoid, for example, heat accumulation and localized enrichment or depletion of gas components. In this way, by appropriately adjusting the supply amounts, it becomes easier to control, in particular, the oxygen content of the gas atmosphere.

[0079] In this first group of embodiments, one or more outlet openings (hereinafter, for the sake of simplicity only, the singular form is used in some cases) leading to the reaction vessel, which in particular can establish a connection with an exhaust stack, for example an (emergency) exhaust stack, are permanently open during one or more first operating modes and / or one or more second operating modes or during any phase thereof. This means that the one or more outlet openings obstruct the mechanical resistance to the flow of fluids into or out of the reaction vessel, excluding possible constrictions in the flow cross-section. The one or more openings are therefore open at least during the reaction period.

[0080] In this case, the stack opening or the connection to the stack or another outlet opening also serves to vent excess gases, or in particular flammable hydrocarbons, in the event of damage to the reaction tube. In this case, the stack can be equipped with structural elements (so-called velocity seals or confusers) to prevent backflow (e.g., by free convection) into the reaction vessel, especially in the region of the stack wall.

[0081] In other embodiments, also referred to below as the "second group of embodiments," the outlet opening or outlet openings (hereinafter, the singular form is used in part for the sake of simplicity) leading to the reactor vessel, in particular the stack opening or the connection to the stack, can be designed to open only above a certain pressure level, for example by closing the outlet opening via a pressure flap or bursting disk or equivalent valve. In this case, the outlet opening is normally closed, i.e., below the certain pressure level, but serves to vent excess gas, or in particular flammable hydrocarbons, in the event of damage to the reactor tube and a corresponding pressure increase due to the release of the corresponding stack cross-section. In this case, it is possible to provide an intermittent or permanent opening when a certain pressure level is reached. In this context, "permanent" opening is understood to mean, in particular, an irreversible opening. Therefore, in this embodiment, there is no resealing after the pressure subsequently drops below the certain pressure level due to gas release. On the other hand, in the case of "intermittent" opening, resealing may also be performed.

[0082] In the case of opening at a predetermined pressure level, one or more outlet openings may be provided, for example, with one or more spring-loaded or loaded flaps with an opening resistance defined by a spring or load characteristic, so that they can only open at a corresponding pressure, or more precisely, at a pressure difference across the opening. For possible embodiments, see, for example, International Patent Application PCT / EP2022 / 059330, in particular Figures 6A-6D and the corresponding description on page 28, which is incorporated herein by reference to the fullest extent permitted by law. In addition to the above-described use of rupture disks or (mechanical) pressure relief valves, known per se, it is also possible to detect pressure values, for example by means of sensors, and, once a predetermined threshold is exceeded, to activate an opening mechanism of any type, for example, a pyrotechnic mechanism or an electric actuator. This makes it possible to create openings of a sufficiently large cross-sectional area within a short response time, as required. During normal operation, the openings remain closed in the described manner.

[0083] In this case, i.e. in the second group of embodiments, the stack openings that are closed during normal operation can be bypassed to the stack via corresponding openings in the bypass line in order to remove the gas atmosphere or to flush the reaction vessel with water. In this way, the use of fluidic technical devices in the bypass line allows for a particularly controlled, for example time-controlled, withdrawal.

[0084] Generally, the recovery of gas from the reaction chamber can have the effect of changing the composition and / or cooling the gas atmosphere. The gas recovered from the reaction chamber can be cooled and / or regenerated and reused (recycled) to provide the gas atmosphere. In the cooling process, heat integration can be performed, i.e., the heat recovered from the gas can be transferred to additional steam and / or steam in the steam system, especially in a heat exchanger. For supplying one or more gases or gas mixtures used to provide the gas atmosphere during one or more first operating modes and / or one or more second operating modes or during any phase thereof, gas supply means, in particular in the form of or comprising supply nozzles or supply openings, may be provided and used, as well as a gas reservoir connected thereto, which may in particular be designed to be controllable by means known in fluid technology.

[0085] The supply and / or extraction can be continuous or discontinuous, in particular controlled based on the desired oxygen content according to the first and second limit values ​​used in accordance with the invention, and / or any other set value or limit value, for example the third and fourth limit values ​​used in one embodiment of the invention.

[0086] In other words, in the context of the present invention, during one or more first operating modes and / or one or more second operating modes, or at any stage thereof, one or more gases or gas mixtures used to provide the gas atmosphere may be continuously or discontinuously fed into the reaction vessel, and at least a portion of the gas atmosphere may further be withdrawn from the reaction vessel, said withdrawal being carried out simultaneously with the feeding or at least partially delayed from the feeding.

[0087] Within the scope of the present invention, a subatmospheric pressure level can be provided in the reaction vessel during one or more first operating modes and / or one or more second operating modes, or at any stage thereof. This pressure can be achieved by adjusting the supply and withdrawal, especially in the case of simultaneous supply and withdrawal in the manner described, in particular in the case of embodiments including a permanently open connection from the reaction vessel to the (emergency) stack, or by other measures as provided above in connection with the first group of embodiments. In this case, a static negative pressure is created in the reaction vessel due to the temperature increase in the stack and in the reaction vessel and the resulting decrease in the density of the trapped gas volume. For example, it is also possible in this context to use a fan to induce air ("suction") until a corresponding static negative pressure is created.

[0088] By operating the reaction vessel at a pressure level below atmospheric pressure during one or more first operating modes and / or one or more second operating modes, or during any stage thereof, it is possible to ensure that undesirable components that may be harmful, corrosive or flammable are prevented from escaping from the reaction vessel at all times, although air or secondary air inflow may occur, which can be limited by a sufficiently gas-tight design and / or compensated for by appropriate controls.

[0089] Therefore, when the reactor is operated at sub-atmospheric pressure levels, it is preferred to provide a particularly tight reactor wall to prevent uncontrolled air, and therefore oxygen, from entering the reactor. In one embodiment, the furnace wall has a relative air infiltration rate of 0.5 Nm per furnace inner wall surface area and per average pressure difference (absolute value) between the reactor interior and the surrounding air (at the same altitude). 3 / (h×m 2 × mbar), less than 0.25 Nm 3 / (h×m 2 × mbar), or less than 0.1 Nm 3 / (h×m 2 × mbar), where Nm 3is a standard cubic meter at 0°C and standard atmospheric pressure. The furnace interior wall surface area is defined herein as the sum of the hot surface areas of the thermal box or reactor insulation, which bound the inner box volume in all directions (i.e., sides, top, and bottom), but does not include the surface area of ​​radiant heating elements or other structures that protrude from the insulation into the inner box volume. These values ​​are selected to allow a reasonable inert gas feed rate (to minimize utility consumption and convective heat losses in the exhaust stack) while maintaining the resulting oxygen concentration inside the reactor below a specified upper limit. In preferred embodiments, the average pressure difference (absolute value) between the reactor interior and the ambient air (at the same altitude) is less than 10 mbar, less than 5 mbar, or less than 3 mbar, depending primarily on the exhaust stack design (e.g., height, diameter, insulation) and the provision of any fans or similar devices. As a general design rule, higher reactor wall airtightness is preferred when low oxygen limits are set, and / or when operating costs are minimized, and / or when increasing the absolute pressure differential across the reactor wall relative to the environment.

[0090] However, in an alternative which may be used particularly in relation to the second group of embodiments described above, it is also possible to establish super-atmospheric pressure levels in the reaction vessel during one or more of the first operating modes and / or one or more of the second operating modes, or during any stage thereof. Thus, as explained, it is preferable to be able to provide super-atmospheric pressure levels, provided that the stack opening into the reaction vessel is closed or that the opening only opens above a predetermined pressure level.

[0091] In particular, the gas atmosphere can be provided by supplying one or more gases or gas mixtures used to provide the gas atmosphere to the reaction vessel, without simultaneously removing part of the gas atmosphere from the reaction vessel, as described above in the embodiments. In this case, the corresponding gas or gas mixture can be supplied up to a pressure level above atmospheric pressure, but below the opening pressure of the outlet openings mentioned or described above. Advantageously, a corresponding design makes it possible in particular to reduce the amount of gas required, since the gas atmosphere can be supplied only at the beginning of a reaction stage or intermittently and then maintained without further measures.

[0092] However, in embodiments in which a gas or gas mixture is supplied that simultaneously provides a gas atmosphere and simultaneously withdraws part of the gas atmosphere from the reaction vessel, it is also possible to set pressure levels above atmospheric pressure during one or more first operating modes and / or one or more second operating modes, or during any phase thereof. This is preferably possible by providing a suitably controlled and / or dimensioned bypass line to ensure a corresponding pressure level in the reaction vessel. See above. In other words, even if the outlet opening is permanently open, or even if, for example, the flow rate of the outlet opening is adjustable, it is possible to set pressure levels above atmospheric pressure in the reaction vessel by appropriately adjusting the amount of gas supplied and / or the amount of gas exiting through the outlet opening.

[0093] In the reactor vessel, during one or more first operating modes and / or one or more second operating modes, or during any stage thereof, it is possible to prevent the inflow of ambient air, which would increase the oxygen content in an uncontrolled manner, particularly if the supply is controlled to provide a pressure level above atmospheric pressure in the reactor vessel. In this embodiment, measurement of the oxygen content may not be necessary, since there is no possibility of a subsequent increase in oxygen.

[0094] In this specification, the term "subatmospheric pressure level" refers to any pressure lower than the local atmospheric pressure prevailing at a nearby ambient location around the furnace periphery during the operating time considered, in particular a pressure that is at least 1, 3, 5, 10, 50 or 100 mbar lower than this atmospheric pressure. Correspondingly, the term "superatmospheric pressure level" refers to any pressure higher than the local atmospheric pressure prevailing at a nearby ambient location around the furnace periphery during the operating time considered, in particular a pressure that is at least 1, 3, 5, 10, 50 or 100 mbar higher than this atmospheric pressure.

[0095] In an embodiment of the present invention, the reactor wall does not include an access port for visually inspecting the interior space of the reactor vessel that is open to the atmosphere, or includes only an access port for visually inspecting the interior space of the reactor vessel that is hermetically sealed with a transparent material, particularly a heat-resistant transparent material. That is, in an embodiment of the present invention, the reactor wall in the form of an (open) access port is particularly designed to prevent heat and / or gas leakage, particularly to allow for the controlled adjustment of the gas atmosphere in the reactor. In an embodiment, a glazed and sealed viewing window, i.e., an access port for visually inspecting the interior space of the reactor vessel that is hermetically sealed with a transparent material, is provided. The window is preferably provided with a movable insulating cover or blind on the outside to limit heat loss when the window is not being used for observation. In an embodiment of the present invention, a camera may be provided that allows observation of the reaction tube but maintains the hermetic seal, i.e., behind the transparent window or inside the reactor. In the latter case, an optional cable may be routed from the hermetic opening to the reactor wall.

[0096] In embodiments of the present invention, openings in the reactor wall may be omitted, particularly since electric heating provides heat in a much more controlled manner compared to burners, thereby reducing or eliminating the need to monitor the temperature of the reactor tubes.

[0097] In summary of the above, during one or more first modes of operation and / or one or more second modes of operation, or during any stage thereof, the gas atmosphere may be provided by injecting one or more gases or gas mixtures used to provide the gas atmosphere into the reaction vessel, with or without simultaneous withdrawal of a portion of the gas atmosphere from the reaction vessel.

[0098] For the sake of clarity, it should be emphasized again that operation at subatmospheric pressure levels is possible, especially if there is a (relatively) large area connection (i.e., low flow-related pressure loss) between the reactor vessel and the stack outlet, and if a sufficiently tall stack is filled with hot (i.e., light) gas. In this case, the pressure drop due to the flow is less than the geodetic pressure difference between the hot gas and the cold outside air across the stack height, resulting in a negative pressure difference between the internal gas atmosphere and the outside air at the same geodetic height. Also, as mentioned above, it is possible to provide subatmospheric pressure levels using fans. Fans can be located in the main stack line as well as in the bypass line.

[0099] Conversely, pressure levels above atmospheric pressure will occur if the connection between the reactor vessel and the stack outlet (in normal operation) is completely closed or dimensionally reduced, for example via a bypass line, so that the pressure loss is greater than the geodetic pressure difference between the hot gas and the cold ambient air that occurs across the height of the stack or bypass line.

[0100] Thus, in the first and second groups of embodiments, the invention can be practiced at both sub-atmospheric and super-atmospheric pressure levels in the reactor vessel during one or more first modes of operation and / or one or more second modes of operation, or during any stage thereof. In the first group of embodiments, sub-atmospheric pressure levels can also be provided by appropriately sizing the outlet opening and / or by using a fan.

[0101] According to a particularly advantageous embodiment, the process according to the invention comprises the use of a plurality of gases or gas mixtures to provide, during one or more first modes of operation and / or one or more second modes of operation, or during any stage thereof, a gas atmosphere comprising a first gas or gas mixture having a first volume fraction of oxygen and a second gas or gas mixture having a second volume fraction of oxygen that is lower than the first volume fraction, which may be used as described below.

[0102] In one embodiment of the present invention, during one or more first operating modes and / or one or more second operating modes, or during any phase thereof, at least a portion of the first gas or gas mixture is supplied to at least a first zone of the reaction vessel, while at least a portion of the second gas or gas mixture is supplied to at least one second zone of the reaction vessel separately from the first zone. This embodiment allows, in particular, to adjust the spatial distribution of the oxygen content in a particularly advantageous manner according to local requirements. It is also possible to supply the first and second zones simultaneously, in particular in an adjustable amount in each case, or not simultaneously. For example, if the air intake (and therefore the oxygen inflow) is too high at subatmospheric pressure levels and only nitrogen or another inert gas is supplied, it is also possible to supply the gas or gas mixture to only one zone, for example, at least intermittently. It is also possible to ensure a defined air intake during one or more first operating modes and / or one or more second operating modes, or during any phase thereof, via adjustable or non-adjustable inlet openings, such as vents, flaps, or closable holes. To make the amount of incoming ambient air adjustable in this way, the corresponding inlet openings can be designed to be able to open in particular with a variable or adjustable flow cross section, whereby, within the meaning of the present invention, the corresponding inlet adjustment can be understood as an additionally set supply of mixed gas, i.e. ambient air.

[0103] In this context, it is also possible to supply a gas or gas mixture only to one region (e.g., by a supply means provided only at a specific point on the reactor wall or by an air inlet opening as described above) during one or more first operating modes and / or one or more second operating modes, or during any phase thereof. The supply "to" the corresponding region or regions is carried out in such a way that the corresponding gas or gas mixture (or a portion thereof) reaches these region(s), for example, from below or from the side. This allows the gas or gas mixture to flow into the region due to a set flow in the reactor vessel caused by thermal effects or solely due to an inflow impulse. Supply into these regions is also possible. However, in another embodiment of the invention, clean "instrument" air is used instead of the air leaking into the reactor. The advantage of using clean air is that it introduces less dust, moisture, and contaminants that may affect the lifetime of the components.

[0104] In particular, the heating element can be arranged in at least one first zone of the reaction vessel and the reaction tubes in at least one second zone of the reaction vessel, with the described gas supply or intake of ambient air resulting in particular in a relatively increased oxygen content in the region of the heating element (to avoid aging / damage in the described manner) and a relatively decreased oxygen content in the region of the reaction tubes (to minimize reactive conversion of components that may escape).

[0105] In particular, because the first and second regions are not separated from each other by any kind of separation device, such an arrangement can be used when the corresponding first and second gases or gas mixtures can be continuously supplied through the corresponding elements. Continuous supply and recovery in such cases maintains a concentration gradient, whereas intermittent supply may lead to mixing over time. Therefore, this embodiment of the present invention is advantageously used in the former case.

[0106] In addition to or instead of the above-described separate supply configuration embodiment, during one or more first operating modes and / or one or more second operating modes, or during any phase thereof, at least a portion of the first gas or gas mixture and at least a portion of the second gas or gas mixture can be fully or partially premixed outside the reactor and supplied to the reactor in a fully or partially premixed state. Such an embodiment is particularly suitable when there is no continuous flow through the reactor. This alternative arrangement can minimize concentration gradients within large-volume reactors, especially when distributed metering is performed at the bottom and / or sidewalls and / or ceiling of the reactor. The advantage of intentionally increasing the oxygen concentration in the heating element region, enabled by the above-described design, is traded off in this example for significantly improved homogeneous distribution and reduced risk of undesirable local imbalances (e.g., localized significant oxygen deficiency at some heating elements or significant oxygen concentration near the reactor tubes).

[0107] It is also possible to combine corresponding measures, such as separately supplying premixed and non-premixed gases during one or more first operating modes and / or one or more second operating modes, or during any stage thereof. In this case, for example, nitrogen can be supplied to the center of the reactor while a nitrogen / air mixture is supplied to the wall of the reactor. This also allows for adequate oxygen enrichment near the heating element, while at the same time minimizing concentration gradients due to partial premixing.

[0108] In principle, in various embodiments of the present invention, the reactor vessel can be fed in a wide variety of locations, in particular at multiple positions.

[0109] The first gas or gas mixture may be or contain air, a gas mixture richer or leaner than air, or oxygen, and the second gas or gas mixture may be or contain a gas mixture leaner than air, nitrogen, carbon dioxide, or another inert gas. In principle, the first gas or gas mixture may contain more than 1%, 5%, or 10% oxygen by volume. It is also possible to obtain the corresponding gas or gas mixture using known processes, such as air separation. The term "inert gas" is understood herein to mean a gas that does not participate as a reactant in an oxidation reaction, especially under the conditions prevailing in the reaction vessel. As mentioned above, it is also possible to supply only one gas or gas mixture, in which case it has the composition described above, especially for the second gas or gas mixture.

[0110] In either case, the actual volume fraction of oxygen in at least one region of the reaction vessel is detected during and / or at the start and / or end of one or more first operating modes and / or during and / or at the start and / or end of one or more second operating modes, or during and / or at the start and / or end of any stage thereof, and the supply of one or more gases or gas mixtures used to provide the gas atmosphere is adjusted or controlled based on that detection, in particular by changing the relative and / or absolute amount. The detection may be carried out in predetermined cycles or (quasi-)continuously, in particular during one or more first operating modes and / or one or more second operating modes, or during any stage thereof.

[0111] In embodiments of the invention where there is continuous flow through the reaction vessel, detection of oxygen content can preferably be performed downstream of the exhaust from the reaction vessel (e.g., in the stack or bypass line, etc.). Additionally or alternatively, oxygen content can be measured at one or more locations within the reaction vessel. Any suitable method for measuring oxygen content can be used, such as a tunable laser diode, a zirconium oxide probe, gas chromatography, paramagnetic materials, etc.

[0112] If the pressurization of the reaction vessel is intermittent, the oxygen content can similarly be measured in the corresponding purge gas outlet line and / or in the reaction vessel itself.

[0113] In all embodiments of the present invention, if the oxygen concentration exceeds the maximum allowable level during one or more first operating modes and / or one or more second operating modes, or during any stage thereof, any kind of safety-related function can be initiated. If the oxygen level falls below the minimum allowable level, operational measures may be initiated to re-establish the desired oxygen content in the reactor. Too low an oxygen concentration is not considered a safety concern, but as mentioned above, it may affect the life of the heating element.

[0114] Unacceptable leakage of gas from the reaction tubes can also be detected, in particular by pressure measurement sensors in the reaction vessel, and in this way it is possible, for example, to immediately prevent or stop the injection of reactants on the basis of a corresponding switching signal.

[0115] It is also possible to continuously measure the content of one or more reactants (especially expressed as carbon monoxide) in the purge stream in order to detect even the slightest damage to the reactor tubes (leakage without a sudden or measurable pressure increase). Unacceptable values ​​also allow the reactant feed to be stopped quickly.

[0116] Using suitable measurement methods (e.g. laser, gas chromatography), the content of, for example, hydrocarbons or their combustion products can additionally or alternatively be measured in the region of the reaction vessel using the same sensors in all the designs described.

[0117] In embodiments of the present invention, leaks may be detected specifically by the presence of moisture, since reactor tubes typically contain large amounts of steam.

[0118] Thus, more generally, the invention may involve determining a value indicative of gas leakage from one or more reactor tubes based on pressure and / or hydrocarbon measurements and / or moisture detection, and initiating one or more safety measures if the value exceeds a predetermined threshold.

[0119] Furthermore, in certain embodiments, the present invention provides means for allowing preheating of the conditioning gas(es) prior to free flow into the reaction vessel, particularly by heat exchange with gas withdrawn from the reaction chamber.

[0120] In other words, the gas or gas mixture, or at least one of the two or more gases or gas mixtures, used to provide the gas atmosphere may be preheated during one or more first modes of operation and / or one or more second modes of operation, or during any stage thereof, before being supplied to the reaction vessel.Embodiments of the invention may include waste heat recovery, in particular preheating by heat exchange with gases exiting the reaction vessel.

[0121] In particular, when the corresponding gas or gas mixture is injected near the wall, it is advantageous to preheat the gas before introducing it into the injection device, for example by first passing it through a sufficient length of pipework through the coil box, i.e., the interior of the reaction vessel, thus avoiding unwanted cooling of the heating element by the cooling conditioning gas, which could impair the target power output of the device.

[0122] In particular, the injector can be located directly at the end of the heated pipe run, or the heated conditioning gas can be initially removed from the coil box and introduced into the pipeline (preferably an insulated pipe) and then become an external injector. Alternatively, an external heat source can be used to preheat the conditioning gas(es) (electricity, steam, hot oil, hot water, etc.).

[0123] The gas injection means used in the corresponding embodiment of the present invention may therefore comprise one or more preheating devices and one or more injection devices, "injection" in this context being intended to refer in particular to the release of a gas or gas mixture into the reaction vessel via the corresponding injection device.

[0124] In other words, in particularly preferred embodiments of the present invention, means may be provided to transfer sensible heat within or from within the reaction vessel to a corresponding gas or gas mixture.

[0125] The invention further proposes a reactor arrangement for carrying out chemical reactions, specific embodiments of which are expressly referred to in the corresponding independent claims.

[0126] Further embodiments of corresponding reactor arrangements may be configured in particular for carrying out the process of any of the embodiments described above, and explicit reference is made to the above descriptions.

[0127] The features and advantages of the invention and its advantageous embodiments are explained again below.

[0128] The proposed concept of a nearly completely sealed reactor vessel charged with a specific gas atmosphere allows for a reduced oxygen content compared to the external atmosphere. As can be utilized in accordance with the present invention, the conversion rate of the leaking hydrocarbons in the event of failure of one or more reactor tubes, and therefore the additional volume expansion rate (as a result of the heat of reaction input), correlates to a first approximation with the oxygen partial pressure. This correlation is summarized in Table 1 below, where xO2 is the oxygen mole fraction and V reak is the volume-to-inertia ratio associated with the reaction. The values ​​shown below are examples only and are not generally valid quantitative information.

[0129] The maximum oxygen content within the reaction vessel (ie, the second limit value used in particular in accordance with the present invention) can be specified based in particular on the dimensions of the existing stack.

[0130] [Table 1]

[0131] Maximum allowable pressure in the reaction vessel p max is derived from the mechanical stability of the respective chamber or the surrounding containment vessel. max is the pressure p in the event of a pipe rupture or other corresponding safety concern. box This value must be equal to or greater than the volume of the associated chamber, V Box , exhaust stack outlet diameter D stack , and depends on the oxygen mole fraction. p max ≧p box =f(V Box ,D stack ,xO2)

[0132] This requirement leads to design criteria for sizing the stack outlet. This relationship is explained below with reference to Figure 5. For example, based on a maximum allowable pressure rise of 20 mbar, as shown by dashed lines 51 and 52, the reaction-related volume increase must be at most about 10 mbar to allow the use of a 500 mm diameter stack (dashed line 51). 3 / sec, which gives a maximum oxygen content of approximately 1%. Looking at it from the other side, if you want a maximum oxygen content of 1%, then the diameter of the exhaust stack must be at least 500mm accordingly.

[0133] To be able to use a 900 mm diameter exhaust stack (dashed line 52), the volumetric flow rate must be approximately 42 m 3 / sec or less, resulting in a maximum oxygen content of approximately 4%. Conversely, similar to the explanation above, if the maximum oxygen content is 4%, the exhaust stack diameter must be at least 900mm accordingly.

[0134] The lower the oxygen content in the reaction vessel, the smaller the volume increase. Therefore, the diameter of the stack outlet through which the additional volume must be dispersed can be reduced. The crucial factor for effectively limiting the oxygen content is always a sufficiently good seal against the environment to prevent or minimize the uncontrolled intrusion of oxygen-containing air in a sufficient manner, especially under conditions of subatmospheric pressure inside the reaction vessel. However, as explained, a complete seal is not necessary in this case.

[0135] The invention will now be further described with reference to the accompanying drawings, which show embodiments of the invention with reference to and in comparison with the prior art. [Brief explanation of the drawings]

[0136] [Figure 1] FIG. 1 is a schematic diagram of a reactor arrangement for conducting chemical reactions in accordance with one embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram of a reactor arrangement for conducting chemical reactions in accordance with one embodiment of the present invention. [Figure 3] FIG. 1 is a schematic diagram of a reactor arrangement for conducting chemical reactions in accordance with one embodiment of the present invention. [Figure 4] FIG. 1 is a schematic diagram of a reactor arrangement for conducting chemical reactions in accordance with one embodiment of the present invention. [Figure 5] 1 is a schematic diagram of an operational mode according to one embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0137] In the figures, structurally or functionally corresponding elements are designated by the same reference numerals and will not be described repeatedly for the sake of clarity. When components of an apparatus are described below, the corresponding description in each case also refers to the processes performed using those components, and vice versa.

[0138] In the reactor arrangement shown in Figure 1, generally designated 100, shown in greatly simplified form, a reactor tube 2 designed in the manner described above is arranged in a reactor vessel 1 designed as described above. A heating element 3 of the type also described above is arranged in the wall of the reactor vessel 1, and heats the reactor tube 2 indirectly by radiant heat.

[0139] In the example shown, gas supply means 4 are arranged at the bottom of the reaction vessel 1, which allow gases or gas mixtures with different oxygen contents to be supplied as indicated in the drawing by arrows 4.1 and 4.2. In the embodiment shown in the drawing, these gases or gas mixtures are supplied separately, so that in particular a gas or gas mixture 4.1 with a higher oxygen content than the gas or gas mixture 4.2 can be supplied in the region of the reaction tube 2 in order to increase the oxygen content in the region of the heating element 3.

[0140] Gas extraction means 5, here in the form of a permanent opening in the stack opening to the stack 6, make it possible to achieve a continuous flow in the reaction vessel 1 with the advantages explained above, simultaneously with the supply via the gas supply means 4. This allows the reaction vessel 1 to operate at a sub-atmospheric pressure level, since the density of the hot gas atmosphere in the stack is lower than that of ambient air. The air inlet is indicated by a curved arrow without a symbol.

[0141] The reactor arrangement 200 shown in FIG. 2 differs from this arrangement 100 essentially in that the gases or gas mixtures 4.1 and 4.2 have already been mixed externally to form a gas mixture 4.3, which is supplied to the reaction vessel 1 by gas supply means 4.

[0142] As explained above, all of the illustrated embodiments can also be operated or provided with only a single gas or gas mixture supplied, either intermittently or constantly.

[0143] The reactor arrangement 300 shown in FIG. 3 differs from the designs described above in that the stack opening is closed by a rupture disk 7 or another suitable means, which opens the stack cross section only when a certain reactor pressure is exceeded. A bypass connection to the stack 6, which can be specifically adjusted and / or dimensioned, is established by gas extraction means, herein designated 5. Thus, with the advantages described, it becomes possible to set pressure levels above atmospheric pressure in the reactor 1. The gas or gases used to provide the desired oxygen content in the reactor 1 can be premixed or supplied separately, as shown here by dashed arrow 4.3 for illustrative purposes. Undetermined gas losses from the reactor 1 are indicated by curved arrows.

[0144] A further embodiment of the reactor arrangement 400 shown in Figure 4 does not have any permanently open gas extraction means, so that no flow-through fraction can be set here, and the reactor vessel 1 can preferably be pressurized with a suitable gas atmosphere at the start or at regular time intervals. As is conventional, the reactor vessel 1 is operated at a pressure level in particular above atmospheric pressure.

[0145] FIG. 5 shows an operation mode according to one embodiment of the present invention, which illustrates the adjustment of a specific oxygen content in a gas atmosphere in a reaction vessel, such as reaction vessel 1 in the embodiment described above.

[0146] The diagram in FIG. 5 shows the maximum temperature 501 of heating elements, such as the above-mentioned heating element 3, expressed in °C, the average temperature 502 of these heating elements, the maximum temperature 503 of reactor tubes, such as the above-mentioned reactor tube 2, the average temperature 504 of these reactor tubes (left vertical axis), and the absorption heating power 505 of reactor tube 2, expressed in kW (right vertical axis), versus the operating time of the corresponding reactor configuration in days (horizontal axis). The absorption heating power of the reactor tubes is strongly correlated with the power input of the heating elements. This is because the power electrically absorbed by the heating elements is converted into heat and transferred to the reactor tubes as absorption heating power almost entirely, i.e., at least 85%, 90%, or 95% of the power input, simply subtracting heat losses to the environment. The absorption heating power figures shown here are for groups of three reactor tubes heated collectively by multiple heating elements. The present invention is equally relevant to cracking furnaces with a different number of reactor tubes and different amounts of absorption heating power. In particular, the present invention is intended for use in large electrolytic cracking furnaces with cumulative absorbed thermal power exceeding 3 MW, 5 MW, 10 MW, and 20 MW.

[0147] The diagram of the exemplary sequence from standby 510 (the "second" mode of operation as understood herein) to cracking operation 520 (the "first" mode of operation), then decok- ing operation 530 (the "second" mode of operation), then standby operation 510, then cracking operation 520, then decok- ing operation 530, then standby operation 510 is simplified, particularly in that only a single decok- ing step is shown for the partial steps listed as examples in Table 2, and in that a linear increase curve for the maximum temperature in the reactor tube during the cracking operation is shown. In practice, when the control system is designed to operate with as constant a product gas composition as possible, e.g., a constant propylene to ethylene ratio in the product gas, the maximum temperature will typically increase at a monotonic, but not constant, rate over time. The plot showing the constant heat absorption rate of the reactor tube during such an operation is also simplified, since some adjustments may be made here depending on the selected control strategy and the coke layer growth inside the reactor tube. Additionally, although it is preferred to timely decoke the process tube after the completion of the cracking operation period to reduce the risk of coil damage associated with the presence of an inner coke layer, the transition from the cracking operation 520 to the decoking operation 530 may also include temporary or intermittent intermediate operation periods similar to the standby operation 510.

[0148] It is also possible to adjust the control during the cracking operation to maintain the product gas temperature as constant as possible, which results in a more time-varying product gas composition and thus a wider adjustment range for the absorption heating power of the reaction tube.

[0149] Depending on the operating mode of the furnace ("first" or "second" operating mode), the upper and lower limits of oxygen content in the atmosphere within the reaction vessel can change significantly at or within a short time interval when the operating mode of the furnace changes from passing hydrocarbons or other reactants through the reaction tubes (reaction operation 520, "first" operating mode) to not passing hydrocarbons (standby 510 or decoking 530 or other start-up or shutdown, "second" operating mode).

[0150] This is particularly advantageous if the upper limit of the oxygen content is lowered beforehand to at least the maximum allowable oxygen content for operation using hydrocarbons (i.e., the maximum value to safeguard against a possible coil burst scenario) before switching to the reaction mode of operation 520. The lower limit is preferably changed at the same time in order to continue to maintain a large enough control window, for example, for setting gas atmosphere adjustments. Therefore, if the upper limit is lowered before switching to the reaction mode 520, it is preferable to also lower the lower limit.

[0151] On the other hand, in the reverse modification starting from the reaction operation 520, it is preferable to increase both limit values: the upper limit value can be increased to an oxygen content corresponding to that of ambient air, and the lower limit value is preferably increased to a value well above the minimum oxygen content favorable for the (temporary) operation of the heating element.

[0152] Because strong changes in heating element temperature primarily occur during switching operations after shutdown or before starting the hydrocarbon feed, the increased oxygen content during operation without hydrocarbons can be advantageously utilized here. This is particularly true because the heating power required during operation without hydrocarbons is significantly lower, significantly impacting the heating element temperature. As shown in Figure 5, when switching from reaction mode 520 to one of the other modes 510 and 530, the heater power 505 and maximum reactor tube temperature 503 decrease significantly. The heating element temperatures 501 and 502 are determined by complex relationships (e.g., the geometrical arrangement from the reactor tube to the heating element). However, in general, it can be said that the temperature difference between the heating element (heat source) and the reactor tube (heat sink) is strongly correlated with the heating power required at the reactor tube. Therefore, there is a relatively large difference in heating element temperature between reaction mode 520 (high temperature and high heating power result in a significant increase in heating element temperature) and the other modes 510 and 530 without hydrocarbon flow (lower coil temperature and lower heating power result in a significant decrease in heating element temperature). The actual heater temperature may vary up or down depending on the furnace geometry and heater design without limiting the scope of this application.

[0153] Table 2 outlines typical conditions for the three main operating modes 510, 520, 530 described in FIG.

[0154] [Table 2]

[0155] In particular, targeted changes in atmospheric conditioning, i.e. oxygen content setting and other process settings, can be made even for short periods before and after the changeover process.

[0156] A possible option for preparing the reactor vessel for hydrocarbon feed in reaction mode 520 is, for example, to bring the heating element to the highest possible temperature in a special hot steam standby in operation mode 510 (low temperature at the inlet to the reactor tube and high temperature at the outlet), thereby specifying a high target range for oxygen content, and then gradually lowering the target range, at least to a level below the upper limit safe limit, to allow hydrocarbon or reaction operation 520.

[0157] Immediately after switching from the cracking operation 520 to hydrocarbon-free operation, the oxygen content can be increased immediately (preferably within less than 1 hour, 30 minutes, 10 minutes, or 1 minute), especially after closing the hydrocarbon valve, for example during the switching process at the end of the cracking cycle or cracking operation 520. Preferably, after the subsequent decoking operation 530 is completed, a special hot steam standby with increased heating power can be set again in the operating mode 510 in order to maximize the heating element temperature as much as possible, at least intermittently, with a high oxygen concentration in the reactor vessel.

[0158] As detailed above, in the context of the present invention, the oxygen content may be adjusted as a function of an instantaneous measured value and / or target value, and also or alternatively as a function of a measure of the temporal change or temporal variation of such measured value and / or target value.

[0159] In particular, it is possible to provide non-monotonic control relationships, such as hysteretic control, i.e., different types of control during heating or cooling of the heating element (as the risk of defect formation in the outer oxide layer during heating and cooling may be different), such hysteretic control is applicable in particular to the temperatures of the heating element and other components, the temperatures of the process gases as well as partial and / or total flow rates, electrical heating power / voltage / current.

[0160] In particular, such adjustments can be made during transient changes, such as load changes during gap operation, standby operation, or decoking operation. An important example is the ramp-up of the hydrocarbon flow rate after the start of feeding, during which the heating power increases significantly and, as a result, the temperature of the heating element also increases. In the present invention, during the ramp-up of the hydrocarbon load, the oxygen concentration range is always below the upper limit of hydrocarbon operation specified by safety regulations, but the oxygen concentration range is maintained at a relatively high level. This transient ramp-up process can be detected by data on the time increase of the heater temperature, heater power, hydrocarbon flow rate, etc., and can be used accordingly as a control influence variable for modified atmospheric adjustment. Also, for example, the flow rate immediately after reaching the normal design value can be used to detect the occurrence or completion of a transient change of the described type.

[0161] Therefore, it may be practical to initially adjust the oxygen content to a trend upward during ramp-up, and then decrease that value continuously, stepwise, or all at once.

[0162] This is particularly important when "clean" reactor tubes are ramped up to the start of each cracking or reaction cycle. The risk of coil bursting is particularly low when ramping up uncoked reactor tubes. Therefore, in this example, a relatively high upper oxygen concentration limit can be selected, which can then be replaced with a relatively low upper oxygen concentration limit after stable, continuous cracking operation has been achieved and a new coke layer has begun to form in the tubes. This optimizes the stability of the regulation during continuous full-load operation.

[0163] Also, since the risk of reactor tube rupture tends to increase with increasing coke layer thickness, it is possible to further reduce the oxygen content towards the end of the run time. At the same time, the temperature of the heating elements also increases, but not rapidly. Therefore, the risk of defect formation in the protective oxide layer is low, and there are no particularly strict requirements for the oxygen content related to the life of the heating elements.

[0164] It is preferable to ensure that the oxygen content of the controlled added control gas stream remains below the upper limit at all times, particularly during cracking operations, to avoid, for example, local increases in the average oxygen content in the reactor vessel, in regions of the reactor vessel where mixing is slower, resulting in an oxygen content that exceeds the currently specified upper limit.

[0165] Analogous to the described variations of the adjustment parameters in the cracking operation, similar adjustments can be made for the standby or decok- ing operation. For example, especially in the decok- ing operation, the adjustments can be variably adjusted in parallel with the process control, which changes over time (as mentioned above, the decok- ing cycle consists of various steps providing, inter alia, different flow rates and heating capacities). The same applies to the heating of the reaction vessel, for example during commissioning. In this case, it is possible to make variable atmosphere adjustments over time, for example depending on a specified temperature gradient for heating.

Claims

1. A method for conducting a chemical reaction using a reactor assembly (100-400), comprising: A reaction tube (2) is provided in a reaction vessel (1), - supplying one or more reactants to said reaction tubes (2) at a first mass flow rate during one or more first modes of operation and either not supplying them to said reaction tubes (2) during one or more second modes of operation or supplying them to said reaction tubes (2) at a second mass flow rate that is less than said first mass flow rate, at least on a time-averaged basis, during said one or more second modes of operation; - supplying radiant heat to the reaction tubes (2) by one or more electric heating elements (3) within the reaction vessel (1) at a first heat flow rate during said one or more first operation modes, and either not supplying radiant heat to the reaction tubes (2) during said one or more second operation modes, or supplying radiant heat to the reaction tubes (2) at a second heat flow rate that is lower than the first heat flow rate, at least on a time-averaged basis, during said one or more second operation modes; - providing a gas atmosphere with an oxygen content adjusted to a predetermined volume fraction in at least a part of the reaction vessel (1) provided with the one or more heating elements (3); - adjusting, during said one or more first operating modes, said volume fraction of oxygen between a first limit value of not less than 500 ppm and a second limit value of not more than 10%; - during said one or more second modes of operation, setting said volume fraction of oxygen to a value that is higher, at least on a time-averaged basis, than the volume fraction during said one or more first modes of operation. A method characterized by:

2. 2. The method according to claim 1, wherein the gas atmosphere supplied to at least a part of the reaction vessel (1) provided with the one or more heating elements (3) is separated from the one or more reactants supplied to the reaction tube (2) by a wall of the reaction tube.

3. 3. The method according to claim 1 or 2, wherein the amount of radiant heat supplied to the reaction tubes (2) by the one or more electric heating elements (3) in the reaction vessel (1) is more than 90% of the total amount of heat supplied to the reaction tubes (2) in the reaction vessel (1).

4. 4. The method of claim 1, wherein the volume fraction of oxygen during the one or more second operating modes is adjusted in a range between a third limit and a fourth limit, the third limit being higher than the first limit, the fourth limit being equal to or greater than the second limit, and / or the third limit being higher than the second limit.

5. The method of any one of claims 1 to 4, wherein the volume fraction of oxygen is varied over time at least intermittently during the one or more second modes of operation.

6. 6. The method of claim 5, wherein the second mass flow rate of the one or more reactants is at least intermittently varied over time and / or the second heat flow rate of the radiant heat is at least intermittently varied over time during the one or more second operating modes.

7. 7. The method of claim 6, wherein at least intermittently varying the volume fraction of oxygen during the one or more second modes of operation is performed in consideration of at least intermittently varying the second mass flow rate of the one or more reactants and / or at least intermittently varying the second heat flow rate of the radiant heat during the one or more second modes of operation.

8. 8. The method according to any one of claims 1 to 7, wherein at least a portion of each of the reaction tubes (2) is maintained at a reaction tube temperature within a first temperature range during the one or more first operation modes and at a reaction tube temperature within a second temperature range lower than the first temperature range during the one or more second operation modes, the first temperature range being in particular 400 to 1500°C or 500 to 1200°C.

9. The reaction tube (2) operates in at least one of the following modes during the one or more second operation modes: - a steam standby mode in which the reaction tube (2) is maintained at a predetermined temperature or a predetermined temperature range by passing steam; - a decoking mode in which the reaction tubes (2) are decoked by passing steam and air through them; and a transient decomposition mode in which the reaction feed load through the reaction tube (2) and / or the process gas temperature at the outlet of the reaction tube (2) are varied over time.

9. The method according to claim 7 or 8, wherein the method is operated at

10. 1. A method of performing the following: - feeding continuously or discontinuously into the reaction vessel (1) one or more gases or gas mixtures used to provide said gaseous atmosphere, in particular - at least partly simultaneously or at least partly delayed, withdrawing at least part of said gas atmosphere from said reaction vessel (1); The method according to any one of claims 1 to 9.

11. The method according to any one of the preceding claims, wherein a subatmospheric or superatmospheric pressure level is provided in the reaction vessel (1).

12. 2. The method according to claim 1, wherein one or more gases or gas mixtures are used to provide the gas atmosphere, the latter in particular comprising a first gas or gas mixture having a first volume fraction of oxygen and a second gas or gas mixture having a second volume fraction of oxygen lower than the first volume fraction, which are fed simultaneously or separately to the reaction vessel, in particular at least a portion of the first gas or gas mixture is fed to at least a first region of the reaction vessel (1) and at least a portion of the second gas or gas mixture is fed separately from the first gas or gas mixture to at least a second region of the reaction vessel (1) and / or a gas or gas mixture is used to feed the second region of the reaction vessel but not to the first region of the reaction vessel, the heating element (3) being arranged in the at least one first region and the reaction tube (2) being arranged in the at least one second region of the reaction vessel (1).

13. 13. The method according to claim 12, wherein at least a portion of the first gas or gas mixture and at least a portion of the second gas or gas mixture are mixed outside the reaction vessel (1) and supplied in a mixed state to the reaction vessel (1).

14. 14. The method of claim 12 or 13, further comprising detecting an actual volume fraction of oxygen in at least one region of the reaction vessel and / or stack, bypass, or purge line connected to the reaction vessel during and / or at the start and / or end of a reaction period and / or heating period and / or cooling period and / or decoking period and / or waiting period, and adjusting or controlling the supply of the one or more gases or gas mixtures used to provide the gaseous atmosphere based on said detection.

15. A reactor arrangement (100-400) for carrying out a chemical reaction, comprising a reaction vessel (1), a reaction tube (2) arranged in said reactor vessel (1), and the following: - supplying one or more reactants to said reaction tubes (2) at a first mass flow rate during one or more first modes of operation and either not supplying them to said reaction tubes (2) during one or more second modes of operation or supplying them to said reaction tubes (2) at a second mass flow rate that is less than said first mass flow rate, at least on a time-averaged basis, during said one or more second modes of operation; - supplying radiant heat to the reaction tubes (2) by one or more electric heating elements (3) within the reaction vessel (1) at a first heat flow rate during said one or more first operation modes, and either not supplying radiant heat to the reaction tubes (2) during said one or more second operation modes, or supplying radiant heat to the reaction tubes (2) at a second heat flow rate that is lower than the first heat flow rate, at least on a time-averaged basis, during said one or more second operation modes. and means arranged for the purpose of The means is configured and provided to: - providing a gas atmosphere with an oxygen content adjusted to a predetermined volume fraction in at least a part of the reaction vessel (1) provided with the one or more heating elements (3); - adjusting the volume fraction of oxygen between a first limit value of not less than 500 ppm and a second limit value of not more than 10% during the one or more first operating modes; - during said one or more second modes of operation, setting said volume fraction of oxygen to a value that is higher, at least on a time-averaged basis, than the volume fraction during said one or more first modes of operation. A reactor arrangement characterized by:

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