Patent Description:
The present invention is based on the steam cracking technology for the production of olefins and other base chemicals, as e.g. described in the article "<NPL>.

Presently, the thermal energy required for initiating and maintaining the endothermic cracking reactions in steam cracking is provided by the combustion of fuel gas in a refractory furnace. The process gas initially containing steam and the hydrocarbons to be cracked is passed through so-called cracking coils placed inside the refractory box, also called radiant zone or section. On this flow path the process gas is continuously heated, enabling the desired cracking reactions to take place inside the cracking coils, and thus the process gas is continuously enriched in the cracking products. Typical inlet temperatures for the process gas into the cracking coils are between <NUM> and <NUM>, outlet temperatures are typically in the range between <NUM> and <NUM>.

In addition to the radiant zone, fired cracking furnaces comprise a so-called convection zone or section and a so-called quench zone or section. The convection zone is usually positioned above the radiant zone and composed of various tube bundles traversing the flue gas duct from the radiant zone. Its main function is to recover as much energy as possible from the hot flue gas leaving the radiant zone. Indeed, only <NUM> to <NUM>% of the total firing duty is typically transferred within the radiant zone to the process gas passed through the cracking coils. The convection zone therefore plays a central role in the energy management in steam cracking, as it is responsible for the beneficial usage of approximately <NUM> to <NUM>% of the heat input into a furnace (i.e. of the firing duty). Indeed, when taking the radiant and convection zone together, modern steam cracking plants make use of <NUM> to <NUM>% of the overall fired duty (based on the fuel's lower heating value or net calorific value). In the convection section, the flue gas is cooled down to temperature levels between <NUM> and <NUM> before leaving the convection section and being released to the atmosphere via stack.

The flue gas heat recovered in the convection zone is typically used for process duties such as preheating of boiler feed water and/or hydrocarbon feeds, (partial) vaporization of liquid hydrocarbon feeds (with or without prior process steam injection), and superheating of process steam and high-pressure steam.

The quench zone is positioned downstream of the radiant zone along the main process gas route. It is composed of one or more heat exchanger units, having the main functions of quickly cooling the process gas below a maximum temperature level to stop the cracking reactions, to further cool down the process gas for downstream treatment, and to effectively recover sensible heat from the process gas for further energetic usage. In addition, further cooling or quenching can be effected via injection of liquids, e.g. by oil quench cooling when steam cracking liquid feeds.

The process gas heat recovered in the quench section is typically used for vaporizing high-pressure (HP) or super-high-pressure (SHP) boiler feed water (typical at a pressure range between <NUM> and <NUM> bar absolute pressure), and for preheating the same boiler feed water, before it being fed to a steam drum. Saturated high-pressure or super-high-pressure steam generated accordingly may be superheated in the convection zone (see above) to form superheated high-pressure or super-high-pressure steam, and from there may be distributed to the central steam system of the plant, providing heat and power for heat exchangers and steam turbines or other rotating equipment. The typical degree of steam superheating achieved in furnace convection zones lies between <NUM> and <NUM> above the saturation temperature (dew point margin). Generally, steam cracking furnaces may operate with high-pressure steam (typically at <NUM> to <NUM> bar) or with super-high-pressure-steam (typically at <NUM> to <NUM> bar). For the sake of clarity in the description of the present invention, high-pressure-steam will be used for the entire pressure range between <NUM> and <NUM> bar, but also beyond this upper limit, since the present invention includes usage of steam at pressures of up to <NUM> bar.

An important part of the process gas treatment subsequent to quench cooling is compression which is typically performed after further treatment such as the removal of heavy hydrocarbons and process water, in order to condition the process gas for separation. This compression, also called process or cracked gas compression, is typically performed with multistage compressors driven by steam turbines. In the steam turbines, steam at a suitable pressure from the central steam system of the plant mentioned, and thus comprising steam produced using heat from the convection section and from quench cooling, can be used. Typically, in a steam cracking plant of the prior art, heat of the flue gas (in the convection zone) and heat of the process gas (in the quench zone) is well balanced with the heat demand for producing a large part of the steam amounts needed for heating and driving steam turbines. In other words, waste heat may be more or less fully utilized for generating steam which is needed in the plant. Additional heat for steam generation may be provided in a (fired) steam boiler.

For reference, and to further illustrate the background of the invention, a conventional fired steam cracking arrangement is illustrated in <FIG> in a highly simplified, schematic partial representation and is designated <NUM>.

The steam cracking arrangement <NUM> illustrated in <FIG> comprises, as illustrated with a reinforced line, one or more cracking furnaces <NUM>. For conciseness only, "one" cracking furnace <NUM> is referred to in the following, while typical steam cracking arrangements <NUM> may comprise a plurality of cracking furnaces <NUM> which can be operated under the same or different conditions. Furthermore, cracking furnaces <NUM> may comprise one or more of the components explained below.

The cracking furnace <NUM> comprises a radiant zone <NUM> and a convection zone <NUM>. In other embodiments than the one shown in <FIG>, also several radiant zones <NUM> may be associated with a single convection zone <NUM>, etc..

In the example illustrated, several heat exchangers <NUM> to <NUM> are arranged in the convection zone <NUM>, either in the arrangement or sequence shown or in a different arrangement or sequence. These heat exchangers <NUM> to <NUM> are typically provided in the form of tube bundles passing through the convection zone <NUM> and are positioned in the flue gas stream from the radiant zone <NUM>.

In the example illustrated, the radiant zone <NUM> is heated by means of a plurality of burners <NUM> arranged on the floor and wall sides of a refractory forming the radiant zone <NUM>, which are only partially designated. In other embodiments, the burners <NUM> may also be provided solely at the wall sides or solely at the floor side. The latter may preferentially be the case e.g. when pure hydrogen is used for firing.

In the example illustrated, a gaseous or liquid feed stream <NUM> containing hydrocarbons is provided to the steam cracking arrangement <NUM>. It is also possible to use several feed streams <NUM> in the manner shown or in a different manner. The feed stream <NUM> is preheated in the heat exchanger <NUM> in the convection zone <NUM>.

In addition, a boiler feed water stream <NUM> is passed through the convection zone <NUM> or, more precisely, the heat exchanger <NUM>, where it is preheated. The boiler feed water stream <NUM> is thereafter introduced into a steam drum <NUM>. In the heat exchanger <NUM> in the convection zone <NUM>, a process steam stream <NUM>, which is typically provided from a process steam generation system located outside the furnace system of the steam cracking arrangement <NUM>, is further heated and, in the example illustrated in <FIG>, thereafter combined with the feed stream <NUM>.

A stream <NUM> of feed and steam formed accordingly is passed through a further heat exchanger <NUM> in the convection zone <NUM> and is thereafter passed through the radiant zone <NUM> in typically several cracking coils <NUM> to form a cracked gas stream <NUM>. The illustration in <FIG> is highly simplified. Typically, a corresponding stream <NUM> is evenly distributed over a number of cracking coils <NUM> and a cracked gas formed therein is collected to form the cracked gas stream <NUM>.

As further illustrated in <FIG>, a steam stream <NUM> can be withdrawn from the steam drum <NUM> and can be (over)heated in a further heat exchanger <NUM> in the convection zone <NUM>, generating a high-pressure steam stream <NUM>. The high-pressure steam stream <NUM> can be used in the steam cracking arrangement <NUM> at any suitable location and for any suitable purpose as not specifically illustrated.

The cracked gas stream <NUM> from the radiant zone <NUM> or the cracking coils <NUM> is passed via one or more transfer lines to a quench exchanger <NUM> where it is rapidly cooled for the reasons mentioned. The quench exchanger <NUM> illustrated here represents a primary quench (heat) exchanger. In addition to such a primary quench exchanger <NUM>, further quench exchangers may also be present.

The cooled cracked gas stream <NUM> is passed to further process units <NUM> which are shown here only very schematically. These further process units <NUM> can, in particular, be process units for scrubbing, compression and fractionation of the cracked gas, and a compressor arrangement including a steam turbine, which may be operated using steam from the steam drum <NUM>, being indicated with <NUM>.

In the example shown, the quench exchanger <NUM> is operated with a water stream <NUM> from the steam drum <NUM>. A steam stream <NUM> formed in the quench exchanger <NUM> is returned to the steam drum <NUM>.

Ongoing efforts to reduce at least local carbon dioxide emissions of industrial processes also extend to the operation of steam cracking plants. As in all fields of technology, a reduction of local carbon dioxide emissions may particularly be effected by electrification of a part of or all possible process units.

As described in <CIT> in connection with a reformer furnace, a voltage source may be used in addition to a burner, the voltage source being connected to the reactor tubes in such a manner that an electric current generated thereby heats the feedstock. Steam cracking plants in which electrically heated steam cracking furnaces are used were proposed for example in <CIT>, <CIT> and <CIT>. Electric furnace technology in other or broader contexts is for example disclosed in <CIT>, <CIT>, <CIT>, <CIT> and <CIT>, or in older documents such as for example <CIT> <CIT>, <CIT> and <CIT>.

<CIT> discloses a method and apparatus for steam cracking hydrocarbons, which method consists in heating a mixture of hydrocarbons and steam to a desired temperature that is high enough to crack the hydrocarbons and transform them into olefins, the method being characterized in that the source of energy needed for heating the mixture is supplied essentially by cogeneration using combustion of a fuel to produce simultaneously both heat energy and mechanical work which is transformed into electricity by an alternator, and in that the mixture is initially subjected to preheating using the heat energy supplied by the cogeneration, and is subsequently heated to the desired cracking temperature by means of electrical heating using the electricity supplied by the cogeneration.

According to <CIT>, a cracking furnace system for converting a hydrocarbon feedstock into cracked gas comprises a convection section, a radiant section and a cooling section, wherein the convection section includes a plurality of convection banks configured to receive and preheat hydrocarbon feedstock, wherein the radiant section includes a firebox comprising at least one radiant coil configured to heat up the feedstock to a temperature allowing a pyrolysis reaction, wherein the cooling section includes at least one transfer line exchanger.

Completely or partly modifying the heating concept of a steam cracking plant, i.e. using heat generated by electric energy completely or partly instead of heat generated by burning a fuel, is a rather substantial intervention. As an alternative, less invasive redesign options are often desired, particularly when retrofitting existing plants. These may for example include substituting a steam turbine used for driving the process gas compressor or a different compressor at least partly by an electric drive. While, as mentioned, such a steam turbine may be partly operated with steam generated by waste heat recovered in the convection section of the cracking furnaces, fired steam boilers must typically be provided additionally to supply sufficient steam quantities. Therefore, substituting a steam turbine used for driving the compressors mentioned at least partly by an electric drive may be suitable to reduce or avoid fired boiler duty and thereby to reduce local carbon dioxide emissions.

As further explained below, however, particularly an electrification of parts of such plants has a significant influence on the heat balance of the overall plant. That is, if steam turbines for driving compressors are substituted by electric drives, the waste heat generated in the plant, which was previously used for driving the steam turbines, cannot be fully utilized anymore. On the other hand, if fired furnaces are substituted by electric furnaces, no waste heat from flue gases, which was previously used for providing steam, heating feeds, etc. is not available anymore.

In other words, substituting any carbon dioxide emitting parts of a steam cracking parts has a massive influence on the overall plant operation and is not simply a matter of exchanging one component against another. An efficient and effective integration of such components into a steam cracking plant is therefore of paramount importance for the overall plant design, in particular regarding energy management. This is therefore the object of the present invention.

The present invention relates, in this connection, particularly to a situation wherein fired steam cracking furnaces are substituted by electrically heated steam cracking furnaces, resulting in substantially less or no steam to be produced and to be available for steam consumers such as steam turbines or other rotating equipment. The present invention particularly relates to a situation wherein a "full electrification" of a steam cracking plant is realized. In such situations, as mentioned, an adapted mode of operation must be found as the conventionally well-balanced steam production and consumption situation is changed almost completely.

Against this background, the present invention proposes a method and a system for steam cracking with the features of the independent claims. Embodiments of the invention are the subject of the dependent claims and of the description that follows.

Before further describing the features and advantages of the present invention, some terms used in the description thereof will be further explained.

The term "process steam" shall refer to steam that is added to a hydrocarbon feed before the hydrocarbon feed is subjected to steam cracking. In other terminology, the process steam is a part of a corresponding feed. Process steam therefore takes part in the steam cracking reactions as generally known. Process steam may particularly include steam generated from the vaporization of "process water", i.e. water which was previously separated from a mixed hydrocarbon/water stream, e.g. from the process gas withdrawn from steam cracking furnaces or from a fraction thereof, particularly by gravity separation in vessels/coalescers, deoxygenation units, or using filters.

The "process gas" is the gas mixture passed through a steam cracking furnace and thereafter subjected to processing steps such as quenching, compression, cooling and separation. The process gas, when supplied to the steam cracking furnace, comprises steam and the educt hydrocarbons subjected to steam cracking, i.e. also the "feed stream" submitted to steam cracking is, herein, also referred to as process gas. If a differentiation is needed, this is indicated by language such as "process gas introduced into a steam cracking furnace" and "process gas effluent" or similar. When leaving the steam cracking furnace, the process gas is enriched in the cracking products and is particularly depleted in the educt hydrocarbons. During the subsequent processing steps, the composition of the process gas may further change, e.g. due to fractions being separated therefrom.

The term "high-purity steam" shall, in contrast to process steam, refer to steam generated from the vaporization of purified boiler feed water. High purity steam is typically specified by standards customary in the field, such as VGB-S-<NUM>-T-<NUM> or similar. It typically does not include steam generated from process water, as the latter typically contains some further components from the process gas.

The term "feed hydrocarbons" shall refer to at least one hydrocarbon which is subjected to steam cracking in a steam cracking furnace in a process gas. Where the term "gas feed" is used, the feed hydrocarbons predominantly or exclusively comprise hydrocarbons with two to four carbon atoms per molecule. In contrast, the term "liquid feed" shall refer to feed hydrocarbons which predominantly or exclusively comprise hydrocarbons with four to <NUM> carbon atoms per molecule, "heavy feed" being at the upper end of this range.

The term "electric furnace" may generally be used for a steam cracking furnace in which the heat required to heat the process gas in the cracking coils is predominantly or exclusively provided by electricity. Such a furnace may include one or more electric heater devices that are connected to an electric power supply system, either via wired connections and/or via inductive power transmission. Inside the heater device material, the applied electric current is generating a volumetric heat source by Joule heating. If the cracking coil itself is used as electric heating device, the released heat is directly transferred to the process gas by convective-conductive heat transfer. If separate electric heating devices are used, the heat released by Joule heating is indirectly transferred from the heating device to the process gas, first from the heating device to the cracking coils preferably via radiation and, to a minor extent, via convection, and then from the cracking coils to the process gas by convective-conductive heat transfer. The process gas may be preheated in various ways before being supplied to the cracking furnace.

A "fired furnace" is, in contrast, generally a steam cracking furnace in which the heat required to heat the process gas in the cracking coils is predominantly or exclusively provided by firing a fuel using one or more burners. The process gas may be preheated in various ways before being supplied to the cracking furnace.

The term "hybrid heating concept" may generally be used when, in steam cracking, a combination of electric furnaces and fired furnaces is used. In the context of the present invention, it is preferably foreseen that a single cracking coil is strictly attributed to a fired or to an electric furnace, i.e. each cracking coil is either exclusively heated by electric energy or exclusively by firing.

The term "predominantly" may, herein, refer to a proportion or a content of at least <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>% or <NUM>%.

The term "rotating equipment", as used herein, may relate to one or more components selected from a compressor, a blower, a pump and a generator, such rotating equipment drivable by a source of mechanical energy such as an electric motor, a steam turbine or a gas turbine.

A "multi-stream heat exchanger" is a heat exchanger in which particularly the medium to be cooled is passed through a plurality of passages such as in a "transfer line exchanger" as e.g. mentioned in the Ullmann article mentioned at the outset.

To the knowledge of the inventors, the existing literature on electrically heated cracking furnaces is limited to the design and operation of the electric coil heating section itself. There is little information available regarding integration concepts into full furnace architectures (including preheating and quench sections), nor into wider cracker plant architectures. This is valid with exceptions to the most recent publications mentioned above, i.e. <CIT>, <CIT> and <CIT>.

An efficient and effective integration of electric furnaces into a steamcracker (referred to as "steamcracking arrangement" hereinbelow) is of paramount importance for the overall plant design, in particular regarding energy management. A major difficulty arises from the fact that electrically heated furnaces do not feature a convection zone, as mentioned. This is of such importance since, as it was already mentioned, in fired cracking furnaces <NUM> to <NUM>% of the overall heat input is recovered in the convection zone and can be used for various purposes.

Concepts and solutions provided according to the present invention particularly are intended and suitable to fulfil the following duties or requirements which are necessary for steam cracking arrangements including electric furnace systems.

The present invention proposes new process solutions in terms of furnace design, arrangement and operation for such a setup. In simple words the present invention provides a solution to the following question: "How to balance and distribute heat quantities in a low- to zero-emission steamcracker featuring some, mostly or exclusively electric furnaces?".

The existing prior art contains no example on how to solve these tasks simultaneously, because all fired furnace integration concepts strictly rely on the existence of a convection zone, in which heat is recovered from a hot flue gas stream.

While prior publications may indicate that heat from the process gas stream may be recovered and utilized, e.g. for feed preheating or process steam generation, there is no solution provided how to supply usable process heat to the wealth of other process heat consumers in a steamcracker plant and adjacent chemical complex. While there might be suggestions to not use steam any longer as the primary energy carrier, the mentioned heat supply question is left unanswered, unless one uses electricity for all heating duties in the plant. The latter, rather trivial solution is far from the energetic optimum, because using electricity for heating purposes at low temperatures leads to significant exergy losses. In other embodiments of the prior art, steam generated is strongly superheated, with the intent of electricity generation in a steam turbine combined with a generator system. This is also a questionable solution, since generating electricity from steam originally produced in an electrically heated reactor system again leads to very high exergy losses and non-optimal resource management.

According to the present invention, a method of steam cracking using a steam cracking arrangement including an electric cracking furnace without a convection zone and further including a quench cooling train is provided, wherein a process gas stream is passed at least through the electric cracking furnace and the quench cooling train. Be it noted that, while, in the following description, reference is made to arrangements, devices, streams etc. in the singular, the present invention can likewise comprise embodiments where each of these items can be provided in plurality. In this connection, streams may be combined from different components or may be distributed to different components as necessary.

If reference is made here to an electric cracking furnace "without a convection zone", this relates to the absence of a zone in which a significant amount of typically more than <NUM> kW of process heat is continuously recovered from a flue gas stream. In other terms, an electric cracking furnace without a convection zone is a cracking furnace without carbon dioxide emission from flue gas streams that are purposely cooled down to continuously recover significant amounts of typically more than <NUM> kW of process heat. The furnace system may, however, feature carbon dioxide emission sources for non-process purposes, e.g. safety-related pilot burners at the outlet of gas evacuation stacks. These provide, however, significantly lower amounts of generally non-recoverable heat.

Generally, therefore, during hydrocarbon cracking operation, preferably a heat amount of not more than <NUM> kW is transferred in the electric cracking furnace as sensible heat to streams other than the process gas stream passed through or withdrawn from the electric cracking furnace according to the present invention. , Such other streams may for example be high-purity steam streams. Expressed differently, said heat transferred in the electric cracking furnace to streams other than the process gas may also be not more than <NUM>% or not more than <NUM>% of the heat transferred to the process gas.

According to the present invention, the quench cooling train is operated to comprise at least two distinct cooling steps, wherein in a first one of the cooling steps at least a part of the process gas stream withdrawn from the electric cracking furnace is cooled against vaporizing boiler feed water at an absolute pressure level between <NUM> and <NUM> bar, particularly between <NUM> and <NUM> bar, more particularly between <NUM> and <NUM> bar, and wherein in a second one of the cooling steps at least a part of the process gas stream withdrawn from the electric cracking furnace is cooled against a superheated mixture of feed hydrocarbons and process steam used in forming the process gas stream which is thereby heated to a temperature level between <NUM> and <NUM>, particularly between <NUM> and <NUM>, more particularly between <NUM> and <NUM>.

According to a particularly preferred embodiment of the present invention, a steam generation arrangement is operated in thermal association with the steam cracking arrangement and may also form part thereof, wherein using the steam generation arrangement at least superheated high pressure steam at a first pressure level of <NUM> and <NUM> bar absolute pressure and at a first temperature level and substantially no steam at a higher temperature level than the first temperature level is generated. The term "substantially no steam" shall, in this connection, particularly refer to a steam amount of less than <NUM>% of the total steam amount generated in the steam generation arrangement.

Further according to this embodiment, the superheated high pressure steam at the first pressure level and the first temperature level is at least in part adiabatically and isenthalpically expanded to a second pressure level below the first pressure level, the second pressure level being particularly, but not necessarily, above <NUM> bar absolute pressure, such that its temperature level is lowered, only by the adiabatic and isenthalpic expansion, to a second temperature level. The first temperature level is selected such that each intermediate temperature level reached at intermediate pressure levels of more than <NUM> bar during the adiabatic and isenthalpic expansion process is between <NUM> and <NUM>, particularly between <NUM> and <NUM>, further particularly between <NUM> and <NUM> above the dew point of steam at the respective intermediate pressure level during the adiabatic and isenthalpic expansion. In other words, the expanded steam is, by selecting the first temperature level according to the present invention, kept at moderate superheating levels, while simultaneously being held with a sufficient distance from the boiling point curve throughout the process of expansion for all intermediate pressure levels above <NUM> bar. The latter is particularly relevant in the case of an expansion starting from a first pressure level of more than <NUM> bar as in such cases the two-phase region may be reached or at least temporarily passed. This is avoided according to the present invention.

Limiting the level of steam superheating according to this embodiment inside the furnace system, i.e. performing a moderate superheating, is very suitable if the steam flow exported from the furnace system is solely intended for supplying process heat to consumers, the term "exported" relating in this connection to a withdrawal from the steam generation arrangement and not, or not necessarily, from an overall system. This steam may also be referred to as "dry" steam as its superheating level is selected essentially to prevent condensation, which may e.g. result in abrasion during steam transport. By mere adiabatic and isenthalpic expansion its pressure can be reduced without phase change after or during the expansion to the pressure and temperature levels required by the heat sink if the temperature levels as indicated above are observed. For any possibly applied adiabatic and isenthalpic expansion down to a minimum pressure, i.e. the second pressure level, the resulting dew point margin of the steam flow at any intermediate pressure level above <NUM> bar during the expansion is in the ranges already mentioned before.

By avoiding strong steam superheating according to the embodiment of the present invention, the availability of quench heat for feed preheating at higher temperature levels (typically more than <NUM>) can be maximized. In embodiments comprising electric steam superheaters, as further explained below, the import of electric energy to the electric cracking furnaces can be minimized.

The present invention differs from all known fired furnace integration systems by the fact that neither a feed preheating nor a steam superheating is performed against flue gas (due to the absence of a convection zone). Contrarily to the electric furnace integration concepts proposed previously, the present invention explicitly foresees to use steam as a primary energy carrier, more specifically as a heat carrier to process heat consumers at various temperature levels. The steam generation and export conditions are specifically designed to suit the intended purpose of heat distribution inside the steamcracker plant and an adjacent chemical complex.

Furthermore, the topologies used in embodiments according to the present invention for feed hydrocarbon, process steam and boiler feed water preheating up to temperature levels of approximately <NUM>, using solely saturated and/or moderately superheated high pressure steam and its resulting condensates, represent an inventive solution for fulfilling these process duties in an electric furnace, in which no additional waste heat from flue gas is available (unlike in fired furnaces). These solutions have the benefit of using a heat medium directly available at the furnace, thereby reducing piping needs, and of minimizing exergy losses by keeping temperature differences in heat exchangers small and preferably performing a subcooling of the condensates formed for maximum heat recovery.

By limiting the steam usage to process heat purposes only and setting steam parameters accordingly, the steam system can be operated flexibly (in relation to pressure and temperature) and can further be used as temporary energy buffer, e.g. by varying the steam superheating and/or pressure levels during operation. This is facilitated by the fact that the produced steam is not used for power generation in steam turbines, which are less tolerant with regard to variations of steam conditions than steam-based heat exchangers. The variation of electric energy import can be realized in different ways for the various embodiments, e.g. by modifying the setpoint of controlled outlet temperatures of specific heat exchangers. In the embodiment shown in <FIG>, for example, which is further explained below, such a variation can be realized by reducing the outlet temperature of the steam-supplied heat exchanger X2, what will result in increasing total electric energy import to other heat exchangers and/or coil heating in order to maintain the same chemical production load of the furnace. In embodiments with electric steam superheating, the variation can be done in straightforward manner by varying the duty.

According to the present invention, therefore, preferably no steam generated by the one or more steam generation arrangements is used in steam turbine drives delivering shaft powers of more than <NUM> MW, and preferably not in steam turbines or other rotating equipment as defined above at all. In other words, according to the present invention no steam turbines and at least no steam turbines delivering shaft powers of more than <NUM> MW, are used which are supplied with steam from the steam generation arrangement(s).

The superheated high pressure steam at the first pressure level and at the first temperature level does preferably not include steam generated from process water and preferably includes only steam generated from boiler feed water. The superheated high pressure steam is therefore preferably high-purity steam as defined above. The superheated high pressure steam is preferably not used in forming the one or more process gas stream, i.e. it does not participate in the steam cracking reactions.

In other words, according to the present invention only a moderately superheated high-purity steam flow is generated, as mentioned, and exported at a corresponding pressure level, i.e. the first pressure level, and for any adiabatic and isenthalpic expansion down to a minimum pressure, i.e. the second pressure level, the resulting dew point margin of the expanded steam flow is in the ranges already mentioned before.

According to the present invention, as the quench cooling train, preferably a quench cooling train comprising a primary quench exchanger and a secondary quench exchanger is used, the primary quench exchanger being used to perform at least a part of the first one of the cooling steps and the secondary quench exchanger being used to perform at least a part of the second one of the cooling steps or vice versa. Corresponding embodiments of the present invention are particularly further explained with reference to the appended drawings.

According to the present invention, a multi-flow heat exchanger in which heat is transferred from the process gas stream withdrawn from the electric cracking furnace to a boiler feed water stream and/or a steam stream used in forming the superheated high pressure steam and/or an electric steam superheater may be used in the steam generation arrangement. Furthermore, at least a part of the feed hydrocarbons used in forming the superheated mixture of feed hydrocarbons and process steam, i.e. the process stream then to be cracked, may be preheated using at least a part of the process gas stream withdrawn from the electric cracking furnace in a multi-flow heat exchanger which is then referred to as a feed-effluent exchanger.

As the quench cooling train, a quench cooling train comprising an arrangement with three or four quench exchangers arranged in series in the process gas stream may be used according to the present invention, of which at least one may be provided as the multi-flow heat exchanger just mentioned. Of this series, the first and second quench exchangers may be the primary and secondary quench exchangers described before. Heat may be transferred in a third and, if existing, in a fourth quench exchanger of such a series of three or four quench exchangers to a boiler feed water stream and/or to a steam stream used in forming the superheated high pressure steam. Alternatively, the last quench exchanger in a series of three or four quench exchangers may be used to preheat at least a part of the feed hydrocarbons used in forming the superheated mixture of feed hydrocarbons and process steam, particularly in a mixture already including process steam, particularly when an electric steam superheater is provided in an embodiment of the invention. The last quench exchanger in a series of three or four quench exchangers is also be referred to as a "tertiary" quench exchanger hereinbelow and the second last quench exchanger in a series of four quench exchangers as an "intermediate" quench exchanger. Be it noted that this specific denomination performed here for easier reference only.

Partly repeating the above, the superheated high pressure steam at the first pressure level and at the first temperature level does preferably not include steam generated from process water and/or only includes steam generated from boiler feed water, such that the superheated high pressure steam at the first pressure level and at the first temperature level is provided as high-purity superheated high pressure steam. Furthermore, as mentioned above already as well, preferably no steam generated by the one or more steam generation arrangements is used in steam turbine drives delivering shaft powers of more than <NUM> MW.

As also mentioned, the steam cracking arrangement is operated, according to a particularly preferred embodiment of the present invention, in different operating modes, using differing electric energy amounts, which becomes possible as a result of the flexibility of steam generation and use according to the invention. In this way, the present invention can also be used for stabilizing an electric grid.

For further details in relation to the steam cracking system provided according to the present invention and preferred embodiments thereof, reference is made to the explanations relating to the inventive method and its preferred embodiments above. Advantageously, the proposed arrangement is adapted to perform a method in at least one of the embodiments explained before in more detail.

Summarizing again what was said above, the present invention proposes novel concepts which ensure that all duties or requirements listed above are fulfilled for steamcracker furnaces in the context of highly electrified steamcracker designs.

The solution to limit the superheating of superheated high pressure steam provided according to an embodiment of the invention particularly breaks with the current state-of-the-art in current steamcracker designs based on fired furnaces and turbine-driven large rotating machinery. This technological choice represents a very efficient solution in the context of highly electrified steamcracker designs.

Indeed, the current practice of producing highly superheated high pressure steam in the furnace section (where dew point margins are typically higher than <NUM> at the furnace outlet) is driven by the abundance of thermal waste energy in the furnace's convection section and its possible use in steam turbines for driving compressors and pumps. Reduced pressure steam taken from turbine extractions or turbine outlets is furthermore used for providing process heat at various levels.

In highly electrified cracker separation trains, the use of electric compressor drives instead of steam turbines leads to a reduction of exergy losses in the steamcracker plant. Furthermore, there is no more efficient use for highly superheated high pressure steam in the separation train. Hence, by reducing the level of superheating, the present invention leads to the use of a large portion of the thermal energy recovered in the quench section for the necessary preheating of the feed hydrocarbon/process steam mixture, either in a direct feed-effluent heat exchanger or indirectly via superheated high pressure steam generation and use of that steam in feed preheating steps.

By maximizing the use of quench heat usage for feed preheating, the total import of electric energy to the furnace is reduced, thereby reducing the furnace's operational cost, facilitating the furnace integration into electrical grid, and reducing the overall exergy loss in the furnace section.

Among the embodiments shown, the variants in which the primary quench exchanger is used in steam generation offer the benefit of fastest cracked gas cooling and reaction quenching (high heat transfer coefficient by boiling water), whereas the variants with the primary quench exchanger being designed as feed-effluent exchanger offer the benefit of minimum electric energy import.

The moderate superheating in the given range according to an embodiment of the invention further allows a straightforward and flexible heat supply to process heat consumers, as the distribution to consumers at different temperature levels can simply be done by monophasic, adiabatic and isenthalpic expansion of the moderately superheated steam exported by the furnaces, without need for letdown stations for entire steam levels involving additional boiler feed water injection for desuperheating and/or turbine stages.

As mentioned above, the preheating at lower temperatures reduces piping volumes and allows maximum heat recovery by subcooling steam condensates.

In terms of dynamic behavior, the possibility to balance and buffer changes in electricity import with the steam system facilitates the integration of such furnace systems in industrial complexes preferably supplied with renewable electricity.

Further features and embodiments of the present invention are listed hereinbelow. All these features and embodiments can be combined with the features and embodiments described hereinbefore and hereinafter without limitation, as far as being encompassed by the scope of the claims and as far as technically feasible or sensible.

The present invention and embodiments thereof are further explained in connection with the appended drawings.

<FIG> was already described at the outset.

In <FIG>, a steam cracking arrangement <NUM> according to an embodiment of the present invention, used in implementing a method of steam cracking according to an embodiment of the present invention, and optionally being part of a system according to the present invention is illustrated. As in the subsequent Figures showing steam cracking arrangements as well, method steps of the method may be realized by corresponding process units or devices used and explanations relating to method steps may therefore likewise relate to such process units and devices and vice versa. Repeated explanations are omitted for reasons of conciseness only and mixed language describing the arrangements or systems and the methods according to the embodiments of the present invention is used for clarity. If components are described in the singular, this does not exclude that such components are provided in plurality. The steam cracking arrangement <NUM>, such as the other steam cracking arrangements shown below, may be part of a system <NUM> according to an embodiment of the invention which may include a plurality of further components and whose possible system boundaries are very schematically illustrated in <FIG> only.

In <FIG>, solid arrows indicate hydrocarbon feed, process steam, process gas, or cracked gas streams and streams formed therefrom, such as hydrocarbon fractions. Finely dotted arrows indicate liquid boiler feed water streams, while dashed arrows indicate saturated high-purity steam streams, and dash-dotted arrows indicate superheated high-purity steam streams. Condensate streams are indicated with double-dash dotted arrows.

The steam cracking arrangement <NUM> includes using an electric steam cracking furnace <NUM>, as generally described before, also referred to as an "electric coilbox". No convection zone is present.

Process steam PS, particularly at a temperature level of about <NUM> is mixed in a mixing nozzle M with a stream of feed hydrocarbons HC which is preheated in a heat exchanger X1. A process stream PR thus formed is further heated in a heat exchanger X2 to a temperature level of particularly about <NUM>. The heat exchangers X1 and X2 can also be combined, particularly if the process steam PS is added upstream of the heat exchanger X1.

Four quench exchangers <NUM>, <NUM>, 22a and <NUM> are arranged in series in a process gas pathway downstream of the electric steam cracking furnace <NUM>, forming a quench cooling train <NUM> of the steam cracking arrangement <NUM>. As mentioned, and for reference purposes only, the first and second quench exchangers <NUM>, <NUM> in this series may be the primary and secondary quench exchangers described before. The last quench exchanger <NUM> in the series may also be referred to as a tertiary quench exchanger and the second last quench exchanger 22a in the series as an intermediate quench exchanger. Alternatively, the quench exchanger <NUM> and the quench exchanger 22a may both be referred to as secondary quench exchangers.

The process stream PR is, before being additionally heated in an electric heater E1 to a temperature level of particularly about <NUM> and supplied to the electric steam cracking furnace <NUM> as a feed stream, preheated in the quench exchanger <NUM>. The process stream is, as a cracked gas, and now indicated PE for clarity, withdrawn from the cracking furnace <NUM> and passed through the quench exchangers <NUM>, <NUM>, 22a and <NUM>. The process stream PE effluent from the electric steam cracking furnace <NUM> is withdrawn from the electric steam cracking furnace <NUM> at a temperature level of particularly about <NUM>, from the quench exchanger <NUM> at a temperature level of particularly about <NUM>, from the quench exchanger 22a at a temperature level of particularly about <NUM> and from the quench exchanger <NUM> at a temperature level of particularly about <NUM>.

Thereafter the process stream PE may be, as only shown in <FIG>, be subjected to any type of processing which includes, according to an embodiment of the present invention, compression in a compressor <NUM>, particularly a process gas compressor, which is driven by an electric motor M. As to further details, reference is made to the explanations above. Particularly a separation train is provided in which all or essentially all compressors are driven electrically.

A steam generation arrangement <NUM> is provided and includes a steam drum <NUM> and other components used in generating steam. Generally, if throughout the present description, reference is made to a component belonging to one arrangement or group of components primary described with a certain function, this does not exclude that this component is not also part of a different arrangement or group of components having an additional or different function, as typical for a plant comprising interconnected parts. For example, the quench exchanger <NUM>, the quench exchanger <NUM> and the quench exchanger <NUM> are described here as being part of the cooling train <NUM>, but they may also be integrated into the steam generation arrangement <NUM>.

Boiler feed water BF, as also illustrated with dotted arrows, is heated in a heat exchanger X3 to a temperature level of particularly about <NUM> and in the quench exchanger <NUM> to a temperature level of particularly about <NUM> before being supplied to the steam drum <NUM> from which a stream of boiler feed water BF is also passed to the quench exchanger <NUM> to be evaporated. Saturated steam SS, as also illustrated with dashed arrows, which is formed in the steam drum and which may be provided at a temperature level of particularly about <NUM> and a pressure level of particularly about <NUM> bar absolute pressure, may in part be used to operate the heat exchangers X2, X3 and X1 wherein in the heat exchanger X2 a condensate CO is formed which is subcooled in the heat exchangers X3 and X1.

A remaining part of the saturated steam SS is superheated in the quench exchanger 22a, forming (moderately) superheated high pressure steam SU, as also illustrated with dash-dotted arrows. Parameters of the superheated high pressure steam SU have been extensively described before. In the embodiment shown, this may have a temperature of about <NUM> and an absolute pressure of about <NUM> bar. In a steam utilization arrangement, which is denoted <NUM> for reference purposes only, the superheated high pressure steam SU is used for heating purposes but preferably not substantially for driving rotary equipment. Herein, the superheated high pressure steam SU is adiabatically and isenthalpically expanded using expansion units <NUM>, <NUM>, <NUM>, forming high pressure steam HP, medium pressure steam MP and low pressure steam LP which is supplied to heat consumers <NUM>, <NUM>, <NUM>. Steam (high-pressure or super-high-pressure steam) exported from all furnaces may be collected in a corresponding steam header, i.e. a large-volume piping system which distributes the steam over the plant to the different consumers. The supply connection to the lower pressure steam headers is made from this highest pressure header. In conventional plants, such a steam header is operated at approx. constant pressure (for operation of the turbines), which is slightly below the steam export pressure at the furnace outlet. According to embodiments of the present invention, the pressure level of the highest pressure steam header can be varied more extensively, to achieve an advantageous buffer effect.

Summarizing the explanations to <FIG> and the steam cracking arrangement <NUM> shown, the process gas PE is in a first step (in the quench exchanger <NUM>) rapidly and effectively cooled against vaporizing boiler feed water BF, similarly to the state-of-the-art in fired furnaces. In a second step (in the quench exchanger <NUM>), the process gas PE is cooled in a feed-effluent exchanger against the process gas PR which is preheated before being fed to the electric cracking furnace <NUM>. In the embodiment shown in <FIG>, a quench exchanger 22a can be provided to cool down the process gas PE while moderately superheating a portion of the saturated steam SS generated in the quench exchanger <NUM>.

In <FIG>, a further steam cracking arrangement <NUM> according to an embodiment of the present invention is illustrated. Generally, the explanations relating to the steam cracking arrangement <NUM> according to <FIG> likewise apply to the steam cracking arrangement <NUM> according to <FIG> and only differences will be explained below.

In the steam cracking arrangement <NUM> according to <FIG>, the quench exchanger 22a is omitted and an electric steam superheater E2 is provided instead. The process gas PE is withdrawn here from the quench exchanger <NUM> at a temperature level of particularly about <NUM>.

In <FIG>, a further steam cracking arrangement <NUM> according to an embodiment of the present invention is illustrated. Generally, the explanations relating to the steam cracking arrangement <NUM> according to <FIG>, based on the explanations for the steam cracking arrangement <NUM> according to <FIG> apply to the steam cracking arrangement <NUM> according to <FIG> and only differences will be explained below.

In the steam cracking arrangement <NUM> according to <FIG>, again no quench exchanger 22a is present and an electric steam superheater E2 is provided instead. In the steam cracking arrangement <NUM> according to <FIG>, also the electric heater E1 is omitted. Furthermore, the process gas stream PR heated in the heat exchanger X2 is further heated in the quench exchanger <NUM> and the steam drum <NUM> is connected with the quench exchanger <NUM>.

The process gas PE effluent from the electric steam cracking furnace <NUM> is withdrawn from the quench exchanger <NUM> at a temperature level of particularly about <NUM>. The process stream PE is withdrawn from the quench exchanger <NUM> at a temperature level of particularly about <NUM>.

In the embodiment shown in <FIG>, therefore, the first two quenching steps are inverted, meaning that the effluent process gas PE is first cooled against the feed process gas PR to be preheated, and then against evaporating boiler feed water BF. In such an embodiment there is no need for an electric feed preheater, as sufficiently high preheating temperatures can be reached in the quench exchanger <NUM>. The high pressure steam to be exported is again moderately superheated, wherein both variants from <FIG> and <FIG> can be used for superheating the steam.

All three embodiments shown in <FIG> are specifically designed for electric cracking furnaces <NUM> operating with light (gaseous) feedstocks, most preferably consisting mostly of ethane. Therefore, all these embodiments feature a quench exchanger <NUM> which, in accordance with today's industrial practice, further cools the cracked gas to temperature levels down to <NUM> while particularly preheating the boiler feed water fed to the steam drum <NUM>.

Moreover, the initial preheating (at temperature levels below <NUM>) of hydrocarbon feed HC and process steam PS after mixing to form the process stream is done by using saturated steam in the heat exchanger X2. The resulting high-pressure condensate CO can further be used in other preheating steps mentioned.

In the steam cracking arrangement <NUM> according to <FIG>, again no quench exchanger 22a is present and an electric steam superheater E2 is provided instead. Instead of a part of the saturated steam SS, a part of the superheated steam SU is now provided to the heat exchanger X3. The process stream PR may therefore particularly be heated in the heat exchanger X2 to a temperature level of particularly about <NUM> such that less heat is withdrawn in the quench exchanger <NUM> and the process stream PE effluent cooled therein is withdrawn therefrom at a temperature level of particularly <NUM>.

The embodiment of <FIG> particularly illustrates that alternatively to the embodiments shown before moderately superheated steam SU can also be used for securing the initial preheating of the hydrocarbon feed HC and process steam PS after forming the process stream PR.

In <FIG>, a further steam cracking arrangement <NUM> according to an embodiment of the present invention is illustrated. Generally, the explanations relating to the main components of the steam cracking arrangement <NUM> according to <FIG> apply to the steam cracking arrangement <NUM> according to <FIG> as well but a number of differences are present and will be explained below.

In the steam cracking arrangement <NUM> according to <FIG>, process steam PS at a temperature level of particularly about <NUM> is mixed in a mixing nozzle M with feed hydrocarbons HC, as above, to form a process stream PR at a temperature level of particularly about <NUM>. The process stream PR is further heated in the quench exchanger <NUM> to a temperature level of particularly about <NUM> and in the quench exchanger <NUM>, as before, to a temperature level of particularly about <NUM> before being supplied to the electric steam cracking furnace <NUM>. The process gas PE effluent is withdrawn from the electric steam cracking furnace <NUM> at a temperature level of particularly about <NUM>, from the quench exchanger <NUM> at a temperature level of particularly about <NUM>, from the quench exchanger <NUM> (no further quench exchanger 22a is present) at a temperature level of particularly about <NUM> and from the quench exchanger <NUM> at a temperature level of particularly about <NUM>.

Boiler feed water BF is provided to the steam drum <NUM> which is connected with the quench exchanger <NUM>. Saturated steam SS may be generated at a pressure level of about <NUM> bar absolute pressure and at a temperature level of about <NUM>. This is superheated, forming superheated steam SU with the parameters given above, in an electric heater E2.

The embodiment shown in <FIG> includes a further option for securing the initial preheating of the hydrocarbon feed HC and process steam PS after forming the process stream PR, where the quench exchanger <NUM> is designed as a feed-effluent exchanger. This possibility can be also combined with embodiments such as for example shown in <FIG>, <FIG> and <FIG>.

In the steam cracking arrangement <NUM> according to <FIG>, no quench exchanger <NUM> is present and an oil quench <NUM> is used instead. Boiler feed water BF is therefore heated in heat exchanger X3 only, particularly to a temperature level of about <NUM>, before being passed to the steam drum <NUM>. A further heat exchanger X4 is provided, heating the feed hydrocarbons further before being mixed with the process steam PS in the mixing nozzle M. The process steam PS is likewise, in a further heat exchanger X5, heated before. The heat exchangers X2, X4 and X5 are operated with saturated steam SS and condensate streams are collected before being, as described before, used in the heat exchangers X1 and X3.

In the steam cracking arrangement <NUM> according to <FIG>, process steam PS is initially provided at a temperature level of particularly about <NUM>. The temperature level of the process stream PR downstream of the heat exchanger X2 is particularly about <NUM>. Heating in the electric heater E1 is particularly performed to a temperature level of about <NUM>. The process gas PE effluent is withdrawn from the electric cracking furnace <NUM> at a temperature level of particularly about <NUM>, from the quench exchanger <NUM> at a temperature level of particularly about <NUM>, from the first quench exchanger <NUM> at a temperature level of particularly about <NUM>, from the quench exchanger 22a at a temperature level of particularly about <NUM> and from the oil quench <NUM> at a further suitable temperature level. The saturated steam generated in the steam drum <NUM> is provided at a pressure level of particularly about <NUM> bar absolute pressure and at a temperature level of particularly about <NUM>. The superheated high pressure steam SU downstream of the quench exchanger 22a is provided at a pressure level of particularly about <NUM> bar absolute pressure and at a temperature level of particularly about <NUM>.

In the steam cracking arrangement <NUM> according to <FIG>, process steam PS is successively admixed to the feed hydrocarbons HC in a first and a second mixing nozzle M <NUM>, M2, where the process steam PS admixed in the second mixing nozzle M2 is further heated in a further electric heater E3.

As alternative process variants, <FIG> and <FIG> show exemplary embodiments of the present invention as applied for an electric furnace <NUM> operating on liquid feedstock and heavy liquid feedstock, respectively. In such embodiments, there is no quench exchanger <NUM>, analogously to fired liquid feedstock furnaces. The feed preheating section is typically more complex, featuring e.g. additional feed preheating steps (see <FIG> and <FIG>, incl. electric process steam superheater usage for heavy liquid feedstocks) and/or one or more process steam superheating steps in multiflow heat exchangers. Nevertheless, the embodiments shown in <FIG> and <FIG> are straightforward adaptations of the embodiment shown in <FIG>. Consequently, the variants presented by the embodiments shown in <FIG> can analogously be applied to liquid feed furnaces as shown in <FIG> and <FIG>, as they were applied to the gas feed furnace of <FIG>.

Similar to the steam cracking arrangement <NUM> according to <FIG> again, the quench exchanger 22a is omitted and an electric steam superheater E2 is provided instead. As an exemplary variant, <FIG> shows a process variant for a heavy liquid feed furnace analogous to the gas feed variant shown in <FIG> (with the quench exchanger <NUM> designed as feed-effluent exchanger).

In <FIG>, a Mollier (enthalpy/entropy) diagram with an entropy s in kJ/(K*kg) displayed on the horizontal axis and an enthalpy h in kJ/kg displayed on the vertical axis is shown for water. With a point <NUM>, a moderate superheating as used according to embodiments of the present invention is indicated while with a point <NUM>, a high superheating as used according to the prior art is indicated. An adiabatic and isenthalpic expansion performed according to the present invention and embodiments thereof, characteristic of a state change in valves or reducers when the steam is intended to be used for heating purposes only, is displayed with an arrow starting from point <NUM> while polytropic expansion performed according to the prior art and not according to the present invention, characteristic of a state change in steam turbines when the steam is intended to be first used for mechanical purposes prior to its use for heating purposes, is displayed with an arrow starting from point <NUM>.

According to the present invention, by mere isenthalpic expansion the pressure can be reduced without phase change to the pressure and temperature levels required by the heat consumer. An exemplary temperature evolution curve <NUM> of such an isenthalpic state change (featuring a supporting point at <NUM> and <NUM> bar absolute pressure) is shown in <FIG> for a pressure range between <NUM> and <NUM> bar absolute pressure, altogether with corresponding most preferred curve envelopes <NUM> and <NUM> (with + <NUM> and + <NUM> dew point margins). In <FIG>, an absolute pressure in bar is indicated on the horizontal axis and a temperature in °C is indicated on the vertical axis.

Claim 1:
A method of steam cracking using a steam cracking arrangement (<NUM>-<NUM>) including an electric cracking furnace (<NUM>) without a convection zone (<NUM>) and further including a quench cooling train (<NUM>), wherein a process gas stream is passed at least through the electric cracking furnace (<NUM>) and the quench cooling train (<NUM>), characterized in that the quench cooling train (<NUM>) is operated to comprise at least two distinct cooling steps arranged in either order, wherein in a first one of the cooling steps at least a part of the process gas stream withdrawn from the electric cracking furnace (<NUM>) is cooled against vaporizing boiler feed water at an absolute pressure level between <NUM> and <NUM> bar and wherein in a second one of the cooling steps at least a part of the process gas stream withdrawn from the electric cracking furnace (<NUM>) is cooled against a superheated mixture of feed hydrocarbons and process steam used in forming the process gas stream which is thereby heated to a temperature level between <NUM> and <NUM>.