Method and system for steamcracking

This application is the national phase of, and claims priority to, International Application No. PCT/EP2022/055878, filed Mar. 8, 2022, which claims priority to European Application No. EP 21161780.8, filed Mar. 10, 2021.

The invention relates to a method and a system for steam cracking.

The invention is based on the steam cracking technology for the production of olefins and other base chemicals, as e.g. described in the article “Ethylene” in Ullmann's Encyclopedia of Industrial Chemistry, online publication 15 Apr. 2009, DOI: 10.1002/14356007.a10_045.pub2.

A method and apparatus for steam cracking hydrocarbons is disclosed in US 2006/116543 A1. This 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.

In US 2020/172814 A1, a cracking furnace system for converting a hydrocarbon feedstock into cracked gas comprising 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.

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 550 and 750° C., outlet temperatures are typically in the range between 800 and 900° C.

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 35 to 50% 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 40 to 60% 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 90 to 95% 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 60 and 140° C. 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 30 and 130 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 150 and 250 K above the saturation temperature (dew point margin). Generally, steam cracking furnaces may operate with high-pressure steam (typically at 30 to 60 bar) or with super-high-pressure-steam (typically at 60 to 130 bar). For the sake of clarity in the description of the invention, high-pressure-steam will be used for the entire pressure range between 30 and 130 bar, but also beyond this upper limit, since the invention includes usage of steam at pressures of up to 175 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 inin a highly simplified, schematic partial representation and is designated.

The steam cracking arrangementillustrated incomprises, as illustrated with a reinforced line, one or more cracking furnaces. For conciseness only, “one” cracking furnaceis referred to in the following, while typical steam cracking arrangementsmay comprise a plurality of cracking furnaceswhich can be operated under the same or different conditions. Furthermore, cracking furnacesmay comprise one or more of the components explained below.

The cracking furnacecomprises a radiant zoneand a convection zone. In other embodiments than the one shown in, also several radiant zonesmay be associated with a single convection zone, etc.

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

In the example illustrated, the radiant zoneis heated by means of a plurality of burnersarranged on the floor and wall sides of a refractory forming the radiant zone, which are only partially designated. In other embodiments, the burnersmay 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 streamcontaining hydrocarbons is provided to the steam cracking arrangement. It is also possible to use several feed streamsin the manner shown or in a different manner. The feed streamis preheated in the heat exchangerin the convection zone.

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

A streamof feed and steam formed accordingly is passed through a further heat exchangerin the convection zoneand is thereafter passed through the radiant zonein typically several cracking coilsto form a cracked gas stream. The illustration inis highly simplified. Typically, a corresponding streamis evenly distributed over a number of cracking coilsand a cracked gas formed therein is collected to form the cracked gas stream.

As further illustrated in, a steam streamcan be withdrawn from the steam drumand can be (over)heated in a further heat exchangerin the convection zone, generating a high-pressure steam stream. The high-pressure steam streamcan be used in the steam cracking arrangementat any suitable location and for any suitable purpose as not specifically illustrated.

The cracked gas streamfrom the radiant zoneor the cracking coilsis passed via one or more transfer lines to a quench exchangerwhere it is rapidly cooled for the reasons mentioned. The quench exchangerillustrated here represents a primary quench (heat) exchanger. In addition to such a primary quench exchanger, further quench exchangers may also be present.

The cooled cracked gas streamis passed to further process unitswhich are shown here only very schematically. These further process unitscan, 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.

In the example shown, the quench exchangeris operated with a water streamfrom the steam drum. A steam streamformed in the quench exchangeris returned to the steam drum.

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 EP 3 075 704 A1 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 WO 2020/150244 A1, WO 2020/150248 A1 and WO 2020/150249 A1. Electric furnace technology in other or broader contexts is for example disclosed in WO 2020/035575 A1, WO 2015/197181 A1, EP 3 249 028 A1, EP 3 249 027 A1 and WO 2014/090914 A1, or in older documents such as for example DE 23 62 628 A1, DE 1 615 278 A1, DE 710 185 C and DE 33 34 334 A1.

US 2006/116543 A1 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 US 2020/172814 A1, 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.

The 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 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.

According to one embodiment of the invention, a method of steam cracking includes providing a steam cracking arrangement that has an electric cracking furnace without a convection zone; and a quench cooling train. The method further includes passing a process gas stream at least through the electric cracking furnace and the quench cooling train. The quench cooling train is operated in 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 is cooled against vaporizing boiler feed water at an absolute pressure level between 30 and 175 bar; and 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 350 and 750° C.

According to another embodiment of the invention, a system for performing a method of steam cracking includes a steam cracking arrangement, having an electric cracking furnace without a convection zone; and a quench cooling train. The system is configured to pass a process gas stream at least through the electric cracking furnace and the quench cooling train. The quench cooling train comprises means to perform at least two distinct cooling steps, wherein a first one of the cooling steps is adapted to cool at least a part of the process gas stream withdrawn from the electric cracking furnace against vaporizing boiler feed water at an absolute pressure level between 30 and 175 bar; and a second one of the cooling steps is adapted to cool at least a part of the process gas stream withdrawn from the electric cracking furnace 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 350 and 750° C.

Before further describing the features and advantages of the 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-010-T-00 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 40 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 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 50%, 60%, 70%, 80%, 90% or 95%.

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. WO 2020/150244 A1, WO 2020/150248 A1 and WO 2020/150249 A1.

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 furnacesto 60% 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 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 invention proposes new process solutions in terms of furnace design, arrangement and operation for such a setup. In simple words the 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.