EP4634336A1

EP4634336A1 - Cracking method

Info

Publication number: EP4634336A1
Application number: EP23814482.8A
Authority: EP
Inventors: Bernard Descales; Philippe Charles Yves LALANNE MAGNE
Original assignee: Ineos Europe AG
Current assignee: Ineos Europe AG
Priority date: 2022-12-15
Filing date: 2023-11-30
Publication date: 2025-10-22
Also published as: CN120418392A; WO2024126074A1; JP2025541254A

Abstract

The present invention relates to methods of cracking hydrocarbon feeds, and in particular provides a method for cracking a hydrocarbon feed, wherein the method comprises a. passing a hydrocarbon feed to a cracking zone of a reactant tube which cracking zone comprises two or more electrically heated sections in series, and b. heating the hydrocarbon feed in the two or more electrically heated sections of the cracking zone to crack the hydrocarbon feed. wherein the heating applied to the last heated section of the cracking zone is less than the heating applied to one or more preceding heated sections of the cracking zone.

Description

CRACKING METHOD

The present invention relates to methods of cracking hydrocarbon feeds, and in particular in electrically heated furnaces.

Cracking of hydrocarbons generally takes place in a furnace. In steam cracking, for example, the hydrocarbon feed to be cracked, with steam, is typically passed through a reactant tube in the furnace, which tube is heated. In conventional furnaces the heat is provided by burners located on the insides of the furnace, which generate the heat for cracking by combustion of a fuel.

The commercial cracking of hydrocarbons generally takes place at a temperature above 750°C. Prior to cracking at such temperatures the hydrocarbons to be cracked are vaporised, and for this reason the hydrocarbons are usually heated and vaporised in a convection section of a furnace before being passed to a burner section where the cracking occurs.

Cracking furnaces based on electrical heating rather than burners have been proposed. In such designs both the vaporisation and cracking steps can use electrical heating.

A particular advantage of electrical heating is that it can provide more control of the temperature of individual sections of the furnace/reactant tube.

US 7288690 describes a method and apparatus for steam cracking hydrocarbons in which cogeneration using combustion of a fuel is used to produce simultaneously both heat energy and mechanical work which is transformed into electricity, and wherein the mixture is initially subjected to heating 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.

WO 2022/094455 discloses an electrically heated cracking furnace. According to this document different reactant coils may be fed with different hydrocarbons or a mix of hydrocarbon feeds and the heating can be varied depending on the feed to be cracked. This document also discloses that preheating can be provided outside of the main reactor to provide preheating to each feed.

A particular problem with cracking furnaces, both "conventional’' and electrically heated, is that coke tends to be deposited on the inside of the reactant tube. In particular, and in general, during the steam cracking of hydrocarbons coke is formed. This coke deposits on the inside of a reactant tube in the cracking zone or section(s). It acts as a thermal barrier or coating inside the reactant tube, reducing heat transfer to the hydrocarbon passing through the reactant tube.

Coke is generally deposited at highest rates towards the outlet of the cracking section or sections of the reactant tube, although it can be deposited throughout any sections of the reactant tube in which cracking takes place. In general, during operation of a conventional steam cracker the temperature of the reactant tube metal, particularly towards the outlet end, increases as coke deposition occurs. When the temperature starts to approach the temperature rating of the metal used for the reactant tube (maximum permissible tube metal temperature (TMT)) then it is necessary to stop the cracking reaction, and clean (“decoke”) the reactant tube.

Thus, not only does increasing coke deposition reduce heating efficiency, but eventually the reaction must be stopped to decoke, which has a cost associated with lost production time. Many options have been proposed to reduce coking rates, but generally all processes have to be stopped periodically to perform decoking.

For example, although it can depend on feed being cracked and mitigations used, a steam cracking process may only operate for as little as 2-3 weeks before decoking is required, whilst decoking may itself take 24-48 hours.

In theory, since electrical heating provides even and controllable heating then coking rates may be relatively low in electrically heated cracking processes. Nevertheless, coking will still occur.

As noted above, however, coking occurs in sections in which cracking occurs, and coke tends to be deposited in highest rates towards the outlet end of any cracking section or sections.

We have now found that operation time between decoking operations can be increased by reducing the heating applied on the sections of the reactant tube which are more prone to coking/ where temperature is approaching the maximum permissible tube metal temperature, and that this can be performed without any impact on the production rate.

Thus, according to a first aspect there is provided a method for cracking a hydrocarbon feed, wherein the method comprises a. passing a hydrocarbon feed to a cracking zone of a reactant tube which cracking zone comprises two or more electrically heated sections in series, and b. heating the hydrocarbon feed in the two or more electrically heated sections of the cracking zone to crack the hydrocarbon feed, wherein the heating applied to the last heated section of the cracking zone is reduced with time to maintain the temperature of the last heated section within 20°C of a temperature, TE.

As used herein an “electrically heated section” means a section of the reactant tube which is heated directly or indirectly by electrical energy. “Direct” heating may comprise, for example, applying electrical energy directly to the reactant tube. “Indirect” heating may comprise, for example, using one or more heating elements which heat the reactant tube through one or more of radiation and convection and induction.

In general, the heating applied to and hence the temperature of each electrically heated section is individually controllable. Generally, each electrically heated section will have one or more electric heaters associated with that section. Each heating section will typically have one or more specific electric heaters which are adjusted when looking to control the temperature of that section, and where at least some electric heaters associated with a particular section are different and distinct to electric heaters on a different section.

The heating applied to any heated section of the cracking zone can be generally defined on the basis of the electrical energy applied to heat the section, and most conveniently by the power to the one or more electric heaters which heat the section. In general, the heating applied to the last heated section is reduced with time by reducing the electrical power supplied to the electric heater(s) of the last heated section with time.

According to the first aspect of the present invention, the heating applied to the last heated section of the cracking zone is reduced with time to maintain the temperature of the last heated section within 20°C of a temperature, TE.

The “temperature of the last heated section” in this aspect is the temperature of the heated section measured at or close to (within 2 metres of) the downstream end of the last heated section. (The heated sections are generally formed of a metal, so the temperature is usually the metal temperature of the heated section.) The temperature, TE, may be considered as a desired or defined set-point temperature. In general, TE WHI be selected such that the temperature of the reactant tube in the last heated section never exceeds the maximum permissible tube metal temperature (TMT) (i.e. TE will be less than the maximum permissible tube metal temperature), and more typically with a suitable safety margin to ensure this is the case. Subject to maintaining a suitable “safety margin” to the maximum permissible tube metal temperature, however, in general, the temperature, TE, is preferred to be relatively high since higher temperatures favour the cracking process. The temperature, TE, is generally not more than 200°C less than the maximum permissible tube metal temperature, and preferably not more than 100°C less than the maximum permissible tube metal temperature. Whilst the temperature, TE, generally depends on the maximum permissible tube metal temperature, usually the temperature, TE, is above 750°C, and more usually above 900°C. (The maximum permissible tube metal temperature of course depends on the construction of the tubes. Typically for the metals commonly used for cracking furnace tubes the maximum permissible tube metal temperature is at least 1000°C, for example in the range 1000-1200°C.)

Whilst TE may be closer to the maximum permissible TMT if the control of the temperature is “tighter”, generally it is preferred that TE is at least 5 °C below the maximum permissible tube metal temperature, and more preferably at least 10°C below this. More typically, TE is at least 20°C below the maximum permissible tube metal temperature, and most preferably at least 25 °C below the maximum permissible tube metal temperature.

The desired/defined set-point temperature, TE, is usually, and preferably, maintained constant i.e. stays the same, as the heating applied is reduced.

In this first aspect of the present invention, the heating applied to the last heated section of the two or more electrically heated sections in the cracking zone is reduced with time to maintain the temperature of the last heated section within 20°C of the temperature, TE. By “within 20°C” it is meant with a range from TE-20°C to TE+20°C. (An equivalent definition applies when “within” a narrower range from TE.) Generally, the heating applied to the last heated section is reduced with time to maintain the temperature of the last heated section within 10°C, such as within 5°C or even within 2°C of the temperature, TE. (i.e. TE+/-10°C, TE+/-5°C, TE+/- 2°C respectively.) The control of the heating may be made by any suitable process control system. In general, the process control system will measure the temperature of the tube in the last heating section, and reduce heating applied as required to maintain a constant or nearly constant temperature (i.e. within 20°C or within a narrower range from the temperature, TE).

In this first aspect of the present invention the amount of heat applied to the last section is reduced with time because of the build-up of coke deposited in the last heating section. It is not necessary to measure the coke build-up directly, however, since the temperature of the reactant tube in its last section will, in the absence of any other changes, begin to increase as coke formation occurs, and then the heating applied can be reduced to compensate as noted.

The present invention takes advantage of the increased controllability of the electrical heating to allow operation of the cracking reaction to continue even as the last heating section in the cracking zone starts to coke.

The method of the first aspect of the present invention may be applied throughout a cracking process i.e. the temperature of the last heated section can be increased to within 20°C of a temperature, TE, at or soon after the start of the cracking process, and then maintained there until the cracking process is stopped.

Alternatively, the method of the first aspect may be started after the cracking process has started. In particular, as noted, the amount of heat applied to the last section is reduced with time in the present invention because of the build-up of coke deposited in the last heating section during the cracking process. In the absence of the method of the first aspect, the temperature of the last heating section increases due to coke formation. It may be that when the cracking process is started the temperature of the last heating section is below the temperature TE. With time during the cracking process coke formation occurs and the temperature of the last heated section increases. Once the temperature of the last heated section gets at or close to the temperature, TE, then the method of the invention can be started to maintain the temperature within 20°C of the temperature, TE (and typically then being continued until the cracking process is stopped).

Either way, the present invention allows the cracking process to be operated for longer than if the temperature is allowed to increase without applying the method of the first aspect of the present invention and to then approach the maximum permissible tube metal temperature.

The method of the present invention is generally applied for a number of days, such as 7 days or more, and more typically for at least 14 days, such as at least 21 days. (And if applied throughout the cracking process, more typically for at least 30 days, and more usually at least 60 days, although the exact time of operation depends on the feed to be cracked and how quickly coking occurs in the reactant tube.) In embodiments, and preferably, the heating applied may also be adjusted on one or more preceding heated sections of the two or more heated sections in the cracking zone. This may be done, for example, to maintain constant the production rate and the cracking severity. For example, as the heating applied to the last heated section is reduced then additional heating may be applied on the preceding section or sections. In this way the overall heating applied in the cracking zone is not reduced or is reduced less than when simply reducing the heating applied on the last section. This can enable the temperature of the cracked gas stream at the exit of the reactant tube, and hence the product yield and flowrate, to be maintained. For example, the temperature of the cracked gas stream at the exit of the reactant tube, may be maintained within a suitable range, for example within 5°C, such as within 2°C, and most preferably within 1 °C of a desired cracked gas temperature.

(In the present invention reference is made to temperatures of heated sections and to temperatures of hydrocarbon feed/reactants or cracked products. As already noted, “TE” as used in this first aspect is a desired temperature of a heated section, and in particular relates to the temperature, typically the metal temperature, measured on the heated section. Temperature of the cracked gas at the end of the cracking zone may nevertheless also be maintained close to a desired temperature as noted above.)

Thus, in an embodiment, which may be applied as an alternative to the method of the first aspect, but preferably is applied in combination with the method of the first aspect, the heating applied to the last heated section of the cracking zone may be reduced with time whilst maintaining the temperature of the cracked gas at the end of the cracking zone within 20°C of a temperature, Tc-

(In particular, when applied as an alternative to the method of the first aspect, the present invention provides a method for cracking a hydrocarbon feed, wherein the method comprises a. passing a hydrocarbon feed to a cracking zone of a reactant tube which cracking zone comprises two or more electrically heated sections in series, and b. heating the hydrocarbon feed in the two or more electrically heated sections of the cracking zone to crack the hydrocarbon feed, wherein the heating applied to the last heated section of the cracking zone is reduced with time whilst maintaining the temperature of the cracked gas at the end of the cracking zone within 20°C of a temperature, Tc.)

Similar to the temperature, TE, the temperature, Tc, may be considered as a desired or defined set-point temperature, except is this case for the temperature of the cracked gas at the end of the cracking zone. In this embodiment, the temperature of the cracked gas at the end of the cracking zone is maintained within a narrow range whilst the heating applied to the last heated section is reduced. In general, Tc will be selected based on, and in particular will be at or close to, the optimum cracking temperature. Whilst the optimum temperature generally depends on the hydrocarbon being cracked, in general the temperature, Tc, is usually above 500°C, such as in the range 600°C to 900°C.

The optimum temperature for cracking of a particular hydrocarbon feed is generally fairly well defined. Thus, typically Tc is constant with time and the temperature of the cracked gas is maintained within a relatively narrow range around this temperature.

The heating applied to the last heated section is preferably reduced with time whilst maintaining the temperature of the last heated section within 10°C, such as within 5°C or even within 2°C of the temperature, Tc. (i.e. Tc+/-10°C, Tc+/-5°C, Tc+/- 2°C respectively.)

The control of the temperature of the cracked gas within 20°C (or a narrower range) of a temperature, Tc may also be applied throughout a cracking process or may be started after the cracking process has started, in a similar manner to already described for the control of the temperature of the last heated section. In general, however, since it is generally desired that the temperature of the cracked gas is maintained at or close to the optimum for a particular feed to be cracked, it is usual for the temperature of the cracked gas at the end of the cracking zone within 20°C of a temperature, Tc throughout the cracking process. In general, in the first aspect the heating applied on the last heated section is reduced, but the heating applied on the preceding sections is maintained or even increased. Thus, in general, the heating applied to the last heated section of the cracking zone is less than the heating applied to one or more preceding heated sections of the cracking zone.

Thus, according to a second aspect of the present invention there is provided a method for cracking a hydrocarbon feed, wherein the method comprises a. passing a hydrocarbon feed to a cracking zone of a reactant tube which cracking zone comprises two or more electrically heated sections in series, and b. heating the hydrocarbon feed in the two or more electrically heated sections of the cracking zone to crack the hydrocarbon feed, wherein the heating applied to the last heated section of the cracking zone is less than the heating applied to a preceding heated section of the cracking zone.

As with the first aspect, the second aspect of the present invention takes advantage of the increased controllability of the electrical heating to allow operation of the cracking reaction to continue even as the last heating section in the cracking zone starts to coke.

In general, the “electrically heated section” is as already defined for the first aspect. In particular, generally each electrically heated section will have one or more electric heaters associated with that section. According to this second aspect, the heating applied to the last heated section of the two or more electrically heated sections in the cracking zone is less than the heating applied to one or more preceding heated sections of the two or more electrically heated sections in the cracking zone.

It should be noted that at the start of a cracking process i.e. when the heating sections in the cracking zone are free or substantially free of coke, then the amount of heating applied to each section to crack the hydrocarbon feed may be equal, or largely equal. The second aspect of the present invention will then particularly be applied during the operation, but after an initial or start-up phase, and in particular when coke deposition has started to occur.

As with the first aspect, the heating applied to any heated section of the cracking zone in this second aspect can be generally defined on the basis of the electrical energy applied to the electric heaters which heat the section, and most conveniently by the power to the electric heaters which heat the section. However, for this second aspect, to provide a comparison of the heating applied between different heated sections which may have a different number of heaters associated therewith, the heating applied to any heated section should be determined as the average power of each heater associated with said section. Thus, in this second aspect of the present invention, reference to the heating applied to the last heated section of the cracking zone being less than the heating applied to a preceding heated section of the cracking zone typically means that:

- each electrically heated section has one or more electric heaters associated with that section such that the heating applied to and the temperature of each electrically heated section is individually controllable, and

- the heating applied to the last heated section of the cracking zone, determined as the average of the amount of heating provided by each electric heater associated with the last section, is less than the heating applied to a preceding heated section of the cracking zone, determined as the average of the amount of heating provided by each electric heater associated with that preceding section.

Again, it will generally be clear to the person skilled in the art if less heating is being applied on the last section than the preceding section. If necessary to provide a “formal comparison” then values of the heating applied to a section may be determined as average power applied to each heater associated with that section over a defined period, for example of 15 minutes.

More generally, the second aspect and the first aspect may be combined, in which case the heating applied to the last heated section of the cracking zone is reduced with time to maintain the temperature of the last heated section within 20°C of a temperature, TE and the heating applied to the last heated section of the cracking zone is less than the heating applied to a preceding heated section of the cracking zone.

In the present invention (either the first or second aspects) a hydrocarbon feed is passed to a cracking zone of a reactant tube and is heated in the two or more electrically heated sections of the cracking zone to crack the hydrocarbon feed. The individual heated sections in this zone may be considered as “cracking sections”.

The hydrocarbon feed is preferably in a vaporised form prior to entry into the cracking zone. In embodiments, there may be provided one or more electrically heated sections, upstream of the two or more sections of the cracking zone, in which the hydrocarbon feed is preheated and/or vaporised.

For example, the reactant tube may comprise a. a vaporisation zone comprising one or more electrically heated sections in which the hydrocarbon feed is heated to vaporise the hydrocarbon feed, and b. a cracking zone, downstream of the vaporisation zone, the cracking zone comprising two or more electrically heated sections in series in which the hydrocarbon feed is heated to crack the hydrocarbon feed.

The method of the present invention then comprises a. passing a hydrocarbon feed to a reactant tube which comprises a vaporisation zone comprising one or more electrically heated sections and a cracking zone, downstream of the vaporisation zone, which cracking zone comprises two or more electrically heated sections in series, b. heating the hydrocarbon feed in the vaporisation zone to vaporise the hydrocarbon feed, and c. heating the vaporised hydrocarbon feed in the cracking zone to crack the hydrocarbon feed.

(And the heating in the last section of the cracking zone being as already defined in the first and second aspects.)

The vaporisation zone may perform preheating as well as vaporisation, and the heated sections therein may be considered as preheating and/or vaporisation sections. The vaporisation zone operates at a temperature sufficient for preheating and vaporisation, but below the temperature at which cracking may occur. In general, the temperature of the hydrocarbon feed at the exit of the vaporisation zone is less than 600°C. The actual temperature required for vaporisation, however, depends on the specific hydrocarbon feed.

For example, steam cracking can be performed on different hydrocarbon feeds.

These include “light” hydrocarbon feeds, such as ethane, “medium” feeds, such as naphtha, and “heavy” feeds, such as pyrolysis oils. Thus, the hydrocarbon feed in the present invention may be selected from these and any other suitable hydrocarbon feeds.

It generally takes more energy and a higher temperature to heat and vaporise medium and heavy feeds than it does light ones. In one embodiment, the number of sections and the overall heating applied in a vaporisation zone, when present, may be selected depending on the hydrocarbon feed.

Generally, the cracking of different feeds takes place at similar temperatures, typically above 750°C, although the optimum cracking temperature tends to be slightly higher for lighter feeds than heavier ones, and the optimum residence time also tends to be longer for lighter feeds.

(For avoidance of doubt, in the present invention reference to the temperature of a feed or reactant stream, including reference to cracking temperature, refers to the temperature of the stream. Whilst related to, this will generally be different to (lower than) the (metal) temperature of the reactant tube at the same locations. The term “cracking temperature” is generally used, as in the art, to refer to the temperature of the cracked gas at the outlet of the reactant tube (outlet of the last heated section).)

In a preferred embodiment, applicable to either the first or second aspect, there is provided a quench zone, downstream of the two or more heated sections of the cracking zone, in which the product stream from the cracking reaction is cooled.

In one embodiment the cooling in the quench zone may be by indirect heat exchange, for example with water to generate steam.

In a preferred embodiment the cracked product stream is cooled in the quench zone by indirect heat exchange with incoming (fresh) hydrocarbon feed. This provides preliminary pre-heating of the hydrocarbon feed prior to the vaporisation and cracking zones and reduces the amount of energy for vaporisation in the vaporisation zone.

Whilst the present invention is applied particularly on the last section of the reactant tube in the cracking zone, the heating applied on all sections in the cracking zone may be controlled based on temperature readings of the tube metal temperature. By this means heating can also be adjusted to compensate for coking on other sections, if observed.

The present invention, by maintaining the last heated section (and in fact all sections) of the cracking zone below the maximum permissible tube metal temperature, can allow extended operation before decoking needs to be performed.

As noted, the issue of coking also tend to be worse in the last heated section of a cracking zone of a reactant tube.

It has now been found that reactor operational time can be increased yet further in a reactant tube with electrically heated sections by reversing the operation of the process, and in particular so that the “later” sections of the reactant tube initially used for cracking are subsequently used as “earlier” sections for vaporising the feed and vice versa. The sections initially used for cracking, even if coked, can safely operate at the lower temperatures required for vaporising the feed, whilst the sections initially used for vaporising the feed, since not coked during the initial stage, can be operated to then perform cracking, and at the start are effectively “clean” reactant tube sections for this purpose.

Although it takes a small amount of time to switch the process, this is much less than a normal decoking process, so effectively the number of decoking processes required is halved/the operating time of the reactor can be approximately doubled.

Thus, in a third aspect the present invention provides a method for cracking of a hydrocarbon feed, which method comprises: a) In a first stage: a. passing the hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, which in this first stage is downstream of the first zone, each zone comprising one or more electrically heated sections, b. heating the hydrocarbon feed in the first zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and c. heating the vaporised hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed, b) In a second stage, a. reversing the direction of flow of the hydrocarbon feed to the reactant tube by passing the hydrocarbon feed to the reactant tube and in particular to what was previously the exit of the second zone, such that it passes in the opposite direction through the second zone and the first zone, the first zone being downstream of the second zone in this second stage, b. heating the hydrocarbon feed in the second zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and c. heating the vaporised hydrocarbon feed in the first zone in the one or more electrically heated sections to crack the hydrocarbon feed.

Generally in this third aspect the conditions in the first and second zones in the first stage are as already defined for the equivalent zones in the first and second aspect. In the second stage of this third aspect the conditions are applied to the opposite zones. Thus, in the first stage and in the first zone the hydrocarbon feed is heated in one or more electrically heated sections to vaporise the hydrocarbon feed i.e. the first zone is a vaporisation zone in this stage. The hydrocarbon feed is heated to a temperature which is sufficient to vaporise the feed but insufficient to cause cracking, or at least not significant levels of cracking. The most preferred temperature will be different depending on the hydrocarbon feed, as already noted, but the temperature at the exit of the first zone is typically less than 600°C.

In the first stage the vaporised hydrocarbon feed is then passed to the second zone. In the second zone the vaporised hydrocarbon feed is further heated to crack the hydrocarbon feed i.e. the second zone is a cracking zone in this stage. The most preferred temperature for the cracking will be different depending on the hydrocarbon feed, but the cracked gas temperature at the exit of the second zone is generally at least 700°C, and more usually (and preferably), at least 750°C.

When desired, the process is reversed. In particular, the hydrocarbon feed is then passed into the second zone, in particular into what was previously the exit of the second zone. But in this second stage the heating applied in the second zone heats the feed to temperature which is sufficient to vaporise the feed but insufficient to cause cracking, or at least not significant levels of cracking. Thus, in the second stage the second zone is a vaporisation zone. As noted for the equivalent heating in the first stage, the most preferred temperature will be different depending on the hydrocarbon feed, but the temperature at the exit of the first zone is typically less than 600°C.

In the second stage the vaporised hydrocarbon feed from the second zone is then passed to the first zone. In the first zone the vaporised hydrocarbon feed is further heated to crack the hydrocarbon feed. Thus, the first zone is a cracking zone in this stage. Again, the most preferred temperature for the cracking will be different depending on the hydrocarbon feed, but the temperature at the exit of the first zone is generally at least 700°C, and more usually (and preferably), at least 750°C.

The switch of the direction of flow of the hydrocarbon feed may be performed by any suitable means. Generally speaking the reactor preferably has a single feed system and a single quench system/zone which are used in both the first and second stages. For example, a single feed system, preferably with means for pre-heating the hydrocarbon feed using heat from the product stream, and a single quench zone, may both be provided and a suitable valve system provided which can direct the hydrocarbon feed to either end of the reactant tube from the feed system and from either end of the reactant tube to the quench zone.

Suitable valve systems could include four on-off valves. Two entry valves may be connected to the feed system where opening one passes the hydrocarbon feed to one end of the reactant tube, and opening the other passes the hydrocarbon feed to the other end. Two exit valves may be connected to the quench zone in a similar way. Means can be provided to ensure that only one of the two entry valves and only one of the two exit valves are opened at any one time, and that the respective valves which are open must be at opposite ends of the reactant tube. An example of such a system is shown in Figure 1 , where there are provided four valves (1-4), a reactant tube (5) shown in this case with 5 heated sections and a quench zone (6). Hydrocarbon feed is fed via inlet (7). It passes first to the quench zone (6) where it is heat exchanged with cracked products. The pre-heated feed may be passed via valve 1 to one end of the reactant tube (5), and cracked products can then exit via valve 4 to the quench zone. During this period valves 2 and 3 are closed. To reverse the flow through the reactant tube (5), valves 1 and 4 are closed, and valves 2 and 3 opened.

Another option would be four-way valves at either end of the reactant tube with ports connected to the exit of the pre-heating system, the entrance to the quench zone, the end of the reactant tube of the first zone and the end of the reactant tube of the second zone. In one position the valve connects the feed system with the first zone of the reactant tube, and connects the second zone of the reactant tube with the quench zone. When rotated the valve connects the feed system with the second zone of the reactant tube, and connects the first zone of the reactant tube with the quench zone.

The switch of the direction of flow of the hydrocarbon feed may be performed when desired. Generally, this will be when the level of coking in the last section is considered an impediment to acceptable continued operation of the cracking reaction in the first stage of the method.

The level of coking in the last section during the first stage may, in general, be estimated by any means known in the art. This may include from previous operational experience and/or from measurements performed on the last section, such as temperature measurements, which may indicate reduction in heat transfer coefficient. The acceptable level of coking may depend on the specific cracking process, including factors such as the temperature (which may be related to the hydrocarbon being cracked), the diameter of the reactant tube, and the required minimum heat transfer. Such a determination may, in general, be made by the person skilled in the art based on any typical criteria known to said person skilled in the art. The determination will, for example, usually be based on similar considerations to those made by the person skilled in the art when determining that it is necessary to stop and decoke a reactant tube in a “conventional” process. (Except, of course, that in the present invention the process is not stopped to decoke but is first reversed.)

In some embodiments, the determination may involve reversing the process slightly earlier than it would normally be stopped were the reactant tube to be decoked. This may be done, for example, to ensure that the level of coking is not detrimental to the heating required for vaporisation in the relevant section in the second stage.

In preferred embodiments of the third aspect of the present invention, the method of the first or second aspects is applied to the last heated section, this section then being changed when the flow is reversed. (In particular, therefore, in such embodiments the first and second zones each comprise two or more heated sections.)

In particular, the method of the first or second aspect may be operated until the coking level is such that temperature of the last heated section of the cracking zone cannot be maintained sufficiently below the maximum permissible tube metal temperature. At this point the first stage is stopped and the second stage is started, with the control of the last heating section then being switched to what is now the last section in the reverse direction of flow.

More generally, the methods of the present invention may be applied for cracking on any hydrocarbon feed which can be cracked in similar processes and methods. These include those discussed, for example, in US 7288690 and WO 2022/094455 already noted. The present invention may be used to crack halogenated hydrocarbons, including cracking of dichloroethane. Preferred cracking processes to which the present invention can be applied are processes for cracking of hydrocarbons to produce olefins. Suitable hydrocarbon feeds for cracking, and in particular to produce olefins, include ethane, propane, butane, naphtha, gasoil, gas condensate, pyrolysis oils, and mixtures thereof. A particularly preferred cracking process to which the present invention can be applied is the steam cracking of hydrocarbons, and in particular of the hydrocarbon feeds noted above.

Other than the requirements defined in the present invention, the general process conditions, such as the feed flow rates, ratios of reactants, such as steam, residence times and cracking temperatures and the like are largely as for conventional processes. Similarly the feeding systems and downstream systems, such as quench systems and/or heat exchange of reactant and feed streams may all be present and applied as for conventional cracking processes.

As already noted, the “electrically heated section” can be heated directly or indirectly by electrical energy. Typically, the reactant tube/heated sections thereof are provided inside a furnace or heating chamber. A gas, preferably an inert gas, may be provided inside the chamber. Example of suitable electrically heated furnaces can be found in WO 2022/094455 already noted, or WO 2020/002326.

Examples

Cracking is performed in a reactant tube. The tube is 15 meters in length, with an internal diameter of 47mm and outer diameter of 53mm. The heating is provided by a set of independently controlled electrical heaters with one heater provided every meter of tube. The tube metal temperature is measured by thermocouples. The maximum tube metal temperature is 1100°C. The coke layer is back calculated from classic heat transfer calculation based on the tube metal temperature and the cracked gases properties.

The feedstock is a naphtha with a density of 0.715 g/cm3 and a distillation temperature profile as follows:

Naphtha at a flow rate is 200 kg/h is mixed with 100 kg/h of water steam, and is fed to the reactant tube. The feedstock temperature at the inlet is 128°C and the pressure is 530 kPaa. The first 1 meter of the reactant tube is a vaporisation zone in which the naphtha is fully vaporised. The remaining 14 meters are a cracking zone. The feedstock is cracked at a tube exit temperature (outlet gas temperature) of 820°C and an average heat transfer to the reactant tube of 150kW/m2.

During operation the tube metal temperature at the outlet increases as coke is deposited. After 30 days of run, the following conditions are reached:

Comparative Example 1

This Comparative Example illustrates a process where no mitigation is applied. In particular, as already noted the maximum tube metal temperature is 1100°C. The process described above is continued beyond 30 days. Coke continues to be deposited. After 45 days the coke layer is 8mm thick at the end of the reactant tube (i.e. at 15m length), the tube metal temperature at the end of the reactant tube reaches 1080°C and it is necessary to stop the process to clean the reactant tube.

Example 2

This Example illustrates a process where the heating applied to the last heated section of the cracking zone is reduced with time to maintain the temperature of the last heated section to enable extended operation.

In this Example 2, a temperature of 1055 °C was selected as the temperature, TE, and after 30 days of operation as described above, and in particular when the temperature at the outlet reached 1060°C, the power applied to different heaters was adjusted to decrease the tube metal temperature in the outlet (where the coke layer is thickest), and in particular to reduce the temperature to the temperature of 1055 °C. To compensate for this the power in earlier sections is increased so the overall power applied is maintained.

The following conditions are adopted:

This enables operation to continue with the same overall energy input and the same cracked gas temperature at the reactant tube outlet, whilst maintaining the tube metal temperature lower at the outlet, and in particular further from the maximum tube metal temperature. This enables extended operation.

With time, adjustments can be continued in a similar manner to reduce the heating applied in the last section and maintain the tube metal temperature at the outlet at or close to 1055 °C to further extend operation. In particular, the heating applied to the last heated section may be reduced with time to maintain the temperature of the last heated section within 20°C of this temperature, and preferably within 5°C of this temperature.

Example 3

This Example exemplifies a process where the flow of hydrocarbon through the reactant tube is reversed.

In this Example 3, the operation according to Comparative Example 1 was initially repeated. As in Comparative Example 1 , the tube metal temperature at the outlet continued to increase until, after 45 days, the coke layer is 8mm thick at the end of the reactant tube and the tube metal temperature at the end of the reactant tube reaches 1080°C.

At this point, however, rather than stopping the process the feed of naphtha and steam to the reactant tube was reversed so that what was previously the outlet became the inlet and what was previously the inlet became the outlet.

The conditions just before and just after the switch are shown below:

As can be seen, after the switch the “new” outlet has no deposited coke and the tube metal temperature at the outlet can be the same as the temperature at the very start of operation. Thus, operation can be continued for approximately another 45 days before it is necessary to stop the process, even in the absence of any other mitigation steps (such as reducing the heating applied to the last section).

It will be apparent that applying the steps of both Example 2 and Example 3 together will allow extended run times in excess of 90 days.

Claims

1. A method for cracking a hydrocarbon feed, wherein the method comprises a. passing a hydrocarbon feed to a cracking zone of a reactant tube which cracking zone comprises two or more electrically heated sections in series, and b. heating the hydrocarbon feed in the two or more electrically heated sections of the cracking zone to crack the hydrocarbon feed, wherein the heating applied to the last heated section of the cracking zone is reduced with time to maintain the temperature of the last heated section within 20°C of a temperature, TE.

2. A method according to claim 1 wherein the heating applied to the last heated section of the cracking zone is reduced with time to maintain the temperature of the last heated section within 10°C, such as within 5°C or even within 2°C of a temperature, TE.

3. A method according to claim 1 or claim 2 wherein TE is a temperature above 750°C and/or wherein TE is not more than 200°C less than the maximum permissible tube metal temperature of the metal of the reactant tube.

4. A method according to any one of the preceding claims wherein TE is at least 20°C below the maximum permissible tube metal temperature, and more preferably at least 25°C below the maximum permissible tube metal temperature.

5. A method according to any one of the preceding claims wherein the heating applied to the last heated section of the cracking zone is less than the heating applied to a preceding heated section of the cracking zone.

6. A method according to any one of the preceding claims wherein the heating applied to the last heated section of the cracking zone is reduced with time whilst maintaining the temperature of the cracked gas at the end of the cracking zone within 20°C of a temperature, Tc.

7. A method for cracking a hydrocarbon feed, wherein the method comprises a. passing a hydrocarbon feed to a cracking zone of a reactant tube which cracking zone comprises two or more electrically heated sections in series, and b. heating the hydrocarbon feed in the two or more electrically heated sections of the cracking zone to crack the hydrocarbon feed, wherein the heating applied to the last heated section of the cracking zone is less than the heating applied to a preceding heated section of the cracking zone.

8. A method according to claim 7 wherein the amount of heat applied to the last heated section of the cracking zone is reduced with time.

9. A method according to any one of the preceding claims which comprises a. passing a hydrocarbon feed to a reactant tube which comprises a vaporisation zone comprising one or more electrically heated sections and a cracking zone, downstream of the vaporisation zone, which cracking zone comprises two or more electrically heated sections in series, b. heating the hydrocarbon feed in the vaporisation zone to vaporise the hydrocarbon feed, and c. heating the vaporised hydrocarbon feed in the cracking zone to crack the hydrocarbon feed.

10. A method according to claim 9 wherein the vaporisation zone operates at a temperature sufficient for vaporisation but less than 600°C.

1 1. A method according to any one of the preceding claims wherein the cracking of the hydrocarbon feed takes place at a gas temperature above 750°C.

12. A method for cracking of a hydrocarbon feed, which method comprises: a) In a first stage: a. passing the hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, which in this first stage is downstream of the first zone, each zone comprising one or more electrically heated sections, b. heating the hydrocarbon feed in the first zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and c. heating the vaporised hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed, b) In a second stage, a. reversing the direction of flow of the hydrocarbon feed to the reactant tube by passing the hydrocarbon feed to the reactant tube and in particular to what was previously the exit of the second zone, such that it passes in the opposite direction through the second zone and the first zone, the first zone being downstream of the second zone in this second stage. b. heating the hydrocarbon feed in the second zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and c. heating the vaporised hydrocarbon feed in the first zone in the one or more electrically heated sections to crack the hydrocarbon feed.

13. A method according to claim 12, which is also a method according to any one of claims 1 to 11 , and wherein the last heated section is changed when the flow is reversed.

14. A method according to claim 13 wherein the first stage is operated until the temperature of the last section cannot be maintained sufficiently below the maximum permissible tube metal temperature, and then the first stage is stopped and the second stage is started.

15. A method according to any one of the preceding claims wherein the hydrocarbon feed is selected from ethane, propane, butane, naphtha, gasoil, gas condensate, pyrolysis oils, and mixtures thereof.