Method for cracking hydrocarbons

Abstract

Methods of cracking hydrocarbon feeds, and in particular a method for cracking of a hydrocarbon feed, which method comprisesa) passing the hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, 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, andc) heating the vaporised hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed,characterised in that the hydrocarbon feed is analysed, and the electrical energy provided to the one or more electrically heated sections in the first zone and / or the electrical energy provided to the one or more electrically heated sections in the second zone are controlled based on the results of the analysis.

Description

FIELD AND BACKGROUND OF THE INVENTIONThe 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.

[0004] In many conventional designs heating and vaporisation is done in what is known as the “convection section” of the furnace, which is a section located above the section where cracking occurs. (This latter is generally referred to as the “radiation section”.) The hydrocarbon to be cracked is fed through tubes located in the convection section, and combustion gases from the burners in the radiation section, which are still hot after leaving the radiation section, are used to heat and vaporise the hydrocarbon to be cracked.

[0005] 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.

[0006] Generally, the cracking of different feeds takes place at similar temperatures, typically above 750° C. as already noted, although the optimum 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. In contrast, it takes more energy to heat and vaporise medium and heavy feeds than it does light ones.

[0007] For these reasons, a furnace designed to vaporise and crack a naphtha feed is generally not optimally designed for vaporising and cracking an ethane containing feed. For example, the number of tubes in the convection section required to fully vaporise naphtha tends to be more than are required to full vaporise ethane, but not sufficient to fully vaporise heavier feeds.

[0008] Steam 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.

[0009] A particular advantage of electrical heating is that it can provide more control of the temperature of individual sections of the furnace / reactant tube.

[0010] U.S. Pat. No. 7,288,690 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.

[0011] 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.SUMMARY OF THE INVENTION

[0012] We have now found a method which enables the cracking of different hydrocarbon feeds to be optimally cracked in the same furnace.

[0013] We have now found a method which enables the same reactant tube (or a bundle of parallel tubes) to be utilised for different hydrocarbon feeds, and in particular where both the vaporisation and cracking of each feed may be performed in the same reactant tube. This enables to change from cracking of one feed to cracking of a different feed in the same reactant tube in a manner which provides optimised cracking.

[0014] Thus, in a first aspect there is provided a method for cracking of a hydrocarbon feed, which method comprises

[0015] a) passing the hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, downstream of the first zone, each zone comprising one or more electrically heated sections,

[0016] b) heating the hydrocarbon feed in the first zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and

[0017] c) heating the vaporised hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed,

[0018] characterised in that the hydrocarbon feed is analysed, and the electrical energy provided to the one or more electrically heated sections in the first zone and / or the electrical energy provided to the one or more electrically heated sections in the second zone are controlled based on the results of the analysis.

[0019] A particular feature of this first aspect is that the heating in the first and second zones is controlled based on an analysis of the hydrocarbon feed.

[0020] In one embodiment, the hydrocarbon feed is analysed prior to entry into the first zone. The analysis in this embodiment preferably provides information on the boiling point range of the hydrocarbon feed. This may be an actual boiling point range, or other information indicative thereof, such as a density measurement.

[0021] An alternative embodiment involves analysis to measure the presence of a liquid phase, for example with an ultrasound sensor, on the hydrocarbon feed between the first zone and the second zone. The heating in the first zone may then be adapted accordingly, for example to increase the heating if liquid is observed. The heating in the second zone, for example in earlier sections of the second zone could also be adjusted based on such a measurement. (An increase in the presence of liquid in the hydrocarbon exiting the first zone, in the absence of any other changes, would generally indicate an increase in the boiling point of the hydrocarbon feed, and hence this is an example of a method which provide information indicative of the boiling point range of the hydrocarbon feed.)

[0022] In either embodiment, analysis may be carried out on-line or off-line, and any suitable analysis may be used. In the present invention the electrical energy provided to one or more electrically heated sections is controlled based on the results of the analysis. It will be apparent that the most accurate control of the cracking reaction is obtained when frequently updated analysis results are used to control the electrical energy provided to the one or more electrically heated sections in the first and / or second zones. Preferably the hydrocarbon feed is analysed and the results used for the control of the electrical energy provided to the one or more electrically heated sections in the first zone and / or the electrical energy provided to the one or more electrically heated sections in the second zone at least once every 12 hours. Generally this means that updated analysis results are provided to a control system by which the electrical energy provided is controlled at least once every 12 hours. The control system will then adjust, if required, the electrical energy provided based on the updated analysis results. (If may also be that no adjustment is needed.) Preferably, the hydrocarbon feed is analysed and the results used for the control of the electrical energy provided to the one or more electrically heated sections in the first zone and / or the electrical energy provided to the one or more electrically heated sections in the second zone at least once every 3 hours, such as at least once every hour, and most preferably not less than once every 10 minutes.

[0023] The analyser itself may be operated to provide continuous or regular analyses.

[0024] Where the analyser is operated to provide continuous analyses the results of the analysis may be used to control the electrical energy provided continuously or may be used non-continuously. In the latter case, then suitably an analysis result is taken from the continuous analyser and used for the control at least once every 12 hours, for example at least once every 3 hours, such as at least once every hour, and most preferably not less than once every 10 minutes as already noted.

[0025] Where the analyser / analysis is not continuous, then suitably an analysis is performed at least at the frequency required for use in the control i.e. at least once every 12 hours. It is not necessary for every analysis performed to be used to control the electrical energy provided to the one or more electrically heated sections, although typically it is desirable to do so. It is preferred that analysis is performed at least once every 3 hours, such as at least once every hour. In most preferred embodiments, the analysis may be performed not less than once every 10 minutes.

[0026] Generally, preference is given to analysis methods which can provide a relatively quick result as this then enables the heating to be adjusted rapidly and frequently to any changes in feed composition detected. The analyser can be located upstream or downstream of the first zone (for the first and second embodiments respectively) and operated to provide continuous or regular analyses on the hydrocarbon feed, and which can then be used to control the electrically heated sections in the first and / or second zones. Suitable analysis methods, particularly for the first embodiment, include GC, Near-IR analysis and density measurements.

[0027] The control of the electrical energy to control the electrically heated sections in the first and / or second zones will be adjusted using a suitable process control system i.e. the result of the analyses are passed to process control system for the cracking reaction, and this will adjust the energy inputs accordingly.

[0028] Typically, if the boiling point range of the hydrocarbon feed increases then the electrical energy to the first zone is increased to provide increased heating and to ensure vaporisation. The electrical energy to the second zone may be decreased.

[0029] (It will be apparent that process parameters other than the heating can also be adjusted. For example, hydrocarbon feed rate, and hence residence times in the first and second zones, can also be adjusted depending on the hydrocarbon feed.)

[0030] The method of the first aspect of the present invention takes advantage of the increased controllability of the electrical heating compared to fired (burner based) furnaces. In particular, the amount of heating from an individual electrical heater can be finely controlled between zero and the maximum rating of the heater. Typically, in fact, each heated section in a zone may be heated by several electric heaters. Either way, “fine” adjustments can be made to the heat supplied to each zone and the cracking can therefore be optimised even if the feed composition changes only in a relatively minor amount.

[0031] (As a simple example, if there are four heaters and it is desired to reduce the amount of heat provided by 25%, you could reduce the electrical energy to each to reduce the overall heat by 25%, or you could turn off one of the heaters.)

[0032] 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, convection and induction.

[0033] 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. As used herein, this means that each heating section has one or more specific electric heaters which are adjusted when looking to control the temperature of that section, and that at least some electric heaters associated with a particular section are different and distinct to electric heaters on a different section.

[0034] 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 is reduced by reducing the electrical power supplied to the electric heater(s) of a particular heated section, and vice versa.

[0035] As already noted, it will be apparent that the most accurate control of the cracking reaction is obtained when more frequent analysis results are provided and used to control the electrical energy provided to the one or more electrically heated sections in the first and / or second zones. It is also the case, however, that more frequent analysis may be more advantageous for particular hydrocarbon feed types or operations.

[0036] For example, relatively simple feeds of relatively high purity, such as ethane or propane, may usually vary relatively little with time, and during cracking of such feeds a high frequency of analysis may be unnecessary. (Control based on an analysis provided once every 3 hours or once every hour may be sufficient for accurate control of the electrical energy, even if more frequent analysis would not be detrimental.)

[0037] More complex feeds, such as naphtha or gas oil, in contrast, may vary more significantly and potentially over a relatively short time period. This is especially the case if the hydrocarbon feed can come from different sources or from a single source, such as a refinery, which itself processes a range of different feedstocks to provide the hydrocarbon feed for cracking. (Naphtha feeds obtained from different sources for example will not generally be the same.) In these cases there may be a “natural” variation of the hydrocarbon feed with time, which the analysis according to the present invention can identify and adjust for.

[0038] In one preferred aspect, the method of the first aspect of the present invention is applied to cracking of naphtha, gas oil, pyrolysis oil or other complex hydrocarbon feed (defined herein as a hydrocarbon feed which comprises a mixture of at least two hydrocarbons which are each present in an amount of at least 10 wt %).

[0039] In another preferred aspect, the method of the first aspect of the present invention is applied where the hydrocarbon feed is deliberately changed from one hydrocarbon feed to another.

[0040] The present invention is most advantageous when a relatively large change in the hydrocarbon feed to be cracked can take place. This may happen due to the changing of the feed to one sourced elsewhere, as already discussed, or by a specific change to a different type of feed, such as between naphtha and an ethane containing feed, or between naphtha and a pyrolysis oil, for example.)

[0041] Thus, in a second aspect, the present invention provides a method for transitioning from a first process cracking a first hydrocarbon feed to a second process cracking a second hydrocarbon feed, wherein:

[0042] a) the first process comprises

[0043] a. passing the first hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, downstream of the first zone, each zone comprising one or more electrically heated sections,

[0044] b. heating the first hydrocarbon feed in the first zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and

[0045] c. heating the vaporised first hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed,

[0046] b) the transition comprises stopping the feeding of the first hydrocarbon feed to the reactant tube and starting the feeding of the second hydrocarbon feed to the same reactant tube,

[0047] characterised in that the second hydrocarbon feed is analysed, and that the electrical energy provided to the one or more electrically heated sections in the first zone and the electrical energy provided to the one or more electrically heated sections in the second zone are adjusted during the transition based on the results of the analysis.

[0048] Typically the preferred features of this second aspect, such as of the analysis performed and the frequency with which the results of the analysis are used for the adjustment (control) of the electrical energy provided to the one or more electrically heated sections in the first and second zones, are as already described for the first aspect.

[0049] By the method being a “transition” is generally meant that the second hydrocarbon feed has a significantly different composition and / or boiling point range to the first hydrocarbon feed inducing a change of the heating section where the feed is fully vaporized. Preferably the average final boiling point of the boiling point range of the second hydrocarbon feed is at least 20° C. different (higher or lower) than the average boiling point of the boiling point range of the first hydrocarbon feed. The second hydrocarbon feed may be of a different type to that of the first hydrocarbon feed. For example, cracking feeds may be generally characterised as either “ethane based”, “propane based”, ‘butane based”, “naphtha based” or “pyrolysis based” such as pyrolysis oil obtained from plastic recycling, and hence a change from one to another of these would be a transition.

[0050] More generally (in both the first and second aspects) in the first zone the hydrocarbon feed is heated in one or more electrically heated sections to vaporise the hydrocarbon feed. Typically 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, but the temperature of the hydrocarbon feed at the exit of the first zone is typically less than 600° C.

[0051] In the second zone the vaporised hydrocarbon feed is further heated to crack the hydrocarbon feed. The most preferred temperature for the cracking will be different depending on the hydrocarbon feed, but the temperature of the cracked hydrocarbon product at the exit of the second zone is generally at least 700° C., and more usually (and preferably), at least 750° C.

[0052] Generally speaking the first and second zones each comprise one or more electrically heated sections. Typically at least one of the first and second zones comprises more than one heated section. There may, for example, be at least four heated sections in total.

[0053] In some embodiments, particularly if there is a relatively large change in the hydrocarbon feed composition / boiling point range, the number of heated sections used to provide each of the first and second zones may be adjusted depending on the hydrocarbon feed. For example, for one feed composition the first zone may comprise “n1” heating sections and the second zone may comprise “n2” heating sections, where “n1+n2” equals “n”, and “n” is the total number of heating sections present. But for a different feed the first zone may comprise “m1” heating sections and the second zone may comprise “m2” heating sections, where “m1+m2” is again equal to “n” (i.e. the same total number of sections) but m1≠n1 and m2≠n2.

[0054] This may be achieved by adjusting the temperature in heating sections as required.

[0055] For example, if during a change in hydrocarbon feed, it is desired to increase the number of heating sections in the first zone, the electrical energy input can be adjusted so that the temperature in what was previous the first heating section of the second zone is reduced from a temperature sufficient for cracking to one that is sufficient only to vaporise the feed.

[0056] This may be particularly advantageous in a transition or more generally if changing from a first hydrocarbon feed to a second hydrocarbon feed where the latter is a relatively heavier feed i.e. has a higher boiling point range.

[0057] In the present invention (either the first or second aspects) the first zone may be considered as a “vaporisation zone” whilst the second zone may be considered as a “cracking zone”. The individual heated sections in the second / cracking zone may be considered as “cracking sections”.

[0058] The first / vaporisation zone may perform preheating as well as vaporisation, and the heated sections therein may be considered as preheating and / or vaporisation sections.DETAILED DESCRIPTION OF THE INVENTION

[0059] In a preferred embodiment, applicable to either the first or second aspect, there is provided a quench zone, downstream of the second / cracking zone, in which the product stream from the cracking reaction is cooled.

[0060] In one embodiment the cooling in the quench zone may be by indirect heat exchange, for example with water to generate steam.

[0061] 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 first and second zones and reduces the amount of energy for vaporisation in the first zone.

[0062] 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 U.S. Pat. No. 7,288,690 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.

[0063] 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.

[0064] 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.

[0065] 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.EXAMPLESExample 1

[0066] This Example illustrates the control of the electrical energy provided to one or more heating sections based on analysis of the hydrocarbon feed, and in particular in a transition.

[0067] Cracking is performed in a reactant tube. The tube is 15 meters in length, with an internal diameter of 47 mm and outer diameter of 53 mm. 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.

[0068] Analysis of the feedstock is performed in real time by an on-line near infrared analyser correlated with an ASTM D86 laboratory analyser to provide a distillation temperature profile, and using a densimeter to measure the feedstock density.

[0069] In the first cracking process the feedstock is a naphtha. The analyses determine that the feedstock has a density of 0.715 g / cm3 and a distillation temperature profile as follows:% volume distilledTemperature (° C.)032103820443052406150796095701128012990149100163

[0070] This naphtha at a flow rate is 250 kg / h is mixed with 75 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. It is determined from the analyses that the feedstock can be fully vaporised by provision of 30.9 kW of electrical power, and that this energy can be provided by the first heater in the reactant tube. Thus, in the first process the first 1 meter of the reactant tube is a vaporisation zone (first zone) in which the naphtha is fully vaporised. The gas temperature at the end of the vaporisation zone of 136° C. The remaining 14 meters are a cracking zone (second zone). The cracked gas temperature at the end of the cracking zone / end of the reactant tube is 820° C., this being a typical cracking temperature for naphtha. The average heat transfer to the reactant tube is 150 kW / m2.

[0071] It is desired to transition to a second cracking process in which the feedstock is a gas oil. Analysis of the gas oil is performed and it is found that it has a density of 0.8233 g / cm3 and a distillation temperature profile as follows:% volume distilledTemperature (° C.)0101.51019620221.330230.440244.150254.260270.470285.180300.590322100379

[0072] This gas oil at a flow rate is 250 kg / h is mixed with 75 kg / h of water steam, and is fed to the reactant tube. The feedstock temperature at the inlet is 120° C. and the pressure is 530 kPaa. It is determined from the analysis that the feedstock can be fully vaporised by provision of 102.5 kW of electrical power, and in this process it is decided to use the first 6 metres of the reactant tube as a vaporisation zone (first zone) in which the gas oil is fully vaporised. The feedstock is fully vaporised in the vaporisation zone, with a gas temperature at the end of the vaporisation zone of 301° C. The remaining 9 meters are a cracking zone (second zone). The cracked gas temperature at the end of the cracking zone / end of the reactant tube is 770° C., this being a typical cracking temperature for gas oil.

[0073] During transition the feeding of naphtha is stopped and the feeding of gas oil is started. As the gas oil starts to feed to the reactant tube the electrical heat energy applied to the reactant tube between 2 and 6 m in length from the inlet is reduced.

Examples

example 1

[0066]This Example illustrates the control of the electrical energy provided to one or more heating sections based on analysis of the hydrocarbon feed, and in particular in a transition.

[0067]Cracking is performed in a reactant tube. The tube is 15 meters in length, with an internal diameter of 47 mm and outer diameter of 53 mm. 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.

[0068]Analysis of the feedstock is performed in real time by an on-line near infrared analyser correlated with an ASTM D86 laboratory analyser to provide a distillation temperature profile, and using a densimeter to measure the feedstock density.

[0069]In the first cracking process the feedstock is a naphtha. The analyses determine that the feedstock has a density of 0.715 g / cm3 and a distillation temperature profile as follows:

% volume distilledTemperature (° C.)03210382044305240...

FIELD AND BACKGROUND OF THE INVENTIONThe 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.

[0004] In many conventional designs heating and vaporisation is done in what is known as the “convection section” of the furnace, which is a section located above the section where cracking occurs. (This latter is generally referred to as the “radiation section”.) The hydrocarbon to be cracked is fed through tubes located in the convection section, and combustion gases from the burners in the radiation section, which are still hot after leaving the radiation section, are used to heat and vaporise the hydrocarbon to be cracked.

[0005] 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.

[0006] Generally, the cracking of different feeds takes place at similar temperatures, typically above 750° C. as already noted, although the optimum 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. In contrast, it takes more energy to heat and vaporise medium and heavy feeds than it does light ones.

[0007] For these reasons, a furnace designed to vaporise and crack a naphtha feed is generally not optimally designed for vaporising and cracking an ethane containing feed. For example, the number of tubes in the convection section required to fully vaporise naphtha tends to be more than are required to full vaporise ethane, but not sufficient to fully vaporise heavier feeds.

[0008] Steam 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.

[0009] A particular advantage of electrical heating is that it can provide more control of the temperature of individual sections of the furnace / reactant tube.

[0010] U.S. Pat. No. 7,288,690 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.

[0011] 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.SUMMARY OF THE INVENTION

[0012] We have now found a method which enables the cracking of different hydrocarbon feeds to be optimally cracked in the same furnace.

[0013] We have now found a method which enables the same reactant tube (or a bundle of parallel tubes) to be utilised for different hydrocarbon feeds, and in particular where both the vaporisation and cracking of each feed may be performed in the same reactant tube. This enables to change from cracking of one feed to cracking of a different feed in the same reactant tube in a manner which provides optimised cracking.

[0014] Thus, in a first aspect there is provided a method for cracking of a hydrocarbon feed, which method comprises

[0015] a) passing the hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, downstream of the first zone, each zone comprising one or more electrically heated sections,

[0016] b) heating the hydrocarbon feed in the first zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and

[0017] c) heating the vaporised hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed,

[0018] characterised in that the hydrocarbon feed is analysed, and the electrical energy provided to the one or more electrically heated sections in the first zone and / or the electrical energy provided to the one or more electrically heated sections in the second zone are controlled based on the results of the analysis.

[0019] A particular feature of this first aspect is that the heating in the first and second zones is controlled based on an analysis of the hydrocarbon feed.

[0020] In one embodiment, the hydrocarbon feed is analysed prior to entry into the first zone. The analysis in this embodiment preferably provides information on the boiling point range of the hydrocarbon feed. This may be an actual boiling point range, or other information indicative thereof, such as a density measurement.

[0021] An alternative embodiment involves analysis to measure the presence of a liquid phase, for example with an ultrasound sensor, on the hydrocarbon feed between the first zone and the second zone. The heating in the first zone may then be adapted accordingly, for example to increase the heating if liquid is observed. The heating in the second zone, for example in earlier sections of the second zone could also be adjusted based on such a measurement. (An increase in the presence of liquid in the hydrocarbon exiting the first zone, in the absence of any other changes, would generally indicate an increase in the boiling point of the hydrocarbon feed, and hence this is an example of a method which provide information indicative of the boiling point range of the hydrocarbon feed.)

[0022] In either embodiment, analysis may be carried out on-line or off-line, and any suitable analysis may be used. In the present invention the electrical energy provided to one or more electrically heated sections is controlled based on the results of the analysis. It will be apparent that the most accurate control of the cracking reaction is obtained when frequently updated analysis results are used to control the electrical energy provided to the one or more electrically heated sections in the first and / or second zones. Preferably the hydrocarbon feed is analysed and the results used for the control of the electrical energy provided to the one or more electrically heated sections in the first zone and / or the electrical energy provided to the one or more electrically heated sections in the second zone at least once every 12 hours. Generally this means that updated analysis results are provided to a control system by which the electrical energy provided is controlled at least once every 12 hours. The control system will then adjust, if required, the electrical energy provided based on the updated analysis results. (If may also be that no adjustment is needed.) Preferably, the hydrocarbon feed is analysed and the results used for the control of the electrical energy provided to the one or more electrically heated sections in the first zone and / or the electrical energy provided to the one or more electrically heated sections in the second zone at least once every 3 hours, such as at least once every hour, and most preferably not less than once every 10 minutes.

[0023] The analyser itself may be operated to provide continuous or regular analyses.

[0024] Where the analyser is operated to provide continuous analyses the results of the analysis may be used to control the electrical energy provided continuously or may be used non-continuously. In the latter case, then suitably an analysis result is taken from the continuous analyser and used for the control at least once every 12 hours, for example at least once every 3 hours, such as at least once every hour, and most preferably not less than once every 10 minutes as already noted.

[0025] Where the analyser / analysis is not continuous, then suitably an analysis is performed at least at the frequency required for use in the control i.e. at least once every 12 hours. It is not necessary for every analysis performed to be used to control the electrical energy provided to the one or more electrically heated sections, although typically it is desirable to do so. It is preferred that analysis is performed at least once every 3 hours, such as at least once every hour. In most preferred embodiments, the analysis may be performed not less than once every 10 minutes.

[0026] Generally, preference is given to analysis methods which can provide a relatively quick result as this then enables the heating to be adjusted rapidly and frequently to any changes in feed composition detected. The analyser can be located upstream or downstream of the first zone (for the first and second embodiments respectively) and operated to provide continuous or regular analyses on the hydrocarbon feed, and which can then be used to control the electrically heated sections in the first and / or second zones. Suitable analysis methods, particularly for the first embodiment, include GC, Near-IR analysis and density measurements.

[0027] The control of the electrical energy to control the electrically heated sections in the first and / or second zones will be adjusted using a suitable process control system i.e. the result of the analyses are passed to process control system for the cracking reaction, and this will adjust the energy inputs accordingly.

[0028] Typically, if the boiling point range of the hydrocarbon feed increases then the electrical energy to the first zone is increased to provide increased heating and to ensure vaporisation. The electrical energy to the second zone may be decreased.

[0029] (It will be apparent that process parameters other than the heating can also be adjusted. For example, hydrocarbon feed rate, and hence residence times in the first and second zones, can also be adjusted depending on the hydrocarbon feed.)

[0030] The method of the first aspect of the present invention takes advantage of the increased controllability of the electrical heating compared to fired (burner based) furnaces. In particular, the amount of heating from an individual electrical heater can be finely controlled between zero and the maximum rating of the heater. Typically, in fact, each heated section in a zone may be heated by several electric heaters. Either way, “fine” adjustments can be made to the heat supplied to each zone and the cracking can therefore be optimised even if the feed composition changes only in a relatively minor amount.

[0031] (As a simple example, if there are four heaters and it is desired to reduce the amount of heat provided by 25%, you could reduce the electrical energy to each to reduce the overall heat by 25%, or you could turn off one of the heaters.)

[0032] 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, convection and induction.

[0033] 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. As used herein, this means that each heating section has one or more specific electric heaters which are adjusted when looking to control the temperature of that section, and that at least some electric heaters associated with a particular section are different and distinct to electric heaters on a different section.

[0034] 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 is reduced by reducing the electrical power supplied to the electric heater(s) of a particular heated section, and vice versa.

[0035] As already noted, it will be apparent that the most accurate control of the cracking reaction is obtained when more frequent analysis results are provided and used to control the electrical energy provided to the one or more electrically heated sections in the first and / or second zones. It is also the case, however, that more frequent analysis may be more advantageous for particular hydrocarbon feed types or operations.

[0036] For example, relatively simple feeds of relatively high purity, such as ethane or propane, may usually vary relatively little with time, and during cracking of such feeds a high frequency of analysis may be unnecessary. (Control based on an analysis provided once every 3 hours or once every hour may be sufficient for accurate control of the electrical energy, even if more frequent analysis would not be detrimental.)

[0037] More complex feeds, such as naphtha or gas oil, in contrast, may vary more significantly and potentially over a relatively short time period. This is especially the case if the hydrocarbon feed can come from different sources or from a single source, such as a refinery, which itself processes a range of different feedstocks to provide the hydrocarbon feed for cracking. (Naphtha feeds obtained from different sources for example will not generally be the same.) In these cases there may be a “natural” variation of the hydrocarbon feed with time, which the analysis according to the present invention can identify and adjust for.

[0038] In one preferred aspect, the method of the first aspect of the present invention is applied to cracking of naphtha, gas oil, pyrolysis oil or other complex hydrocarbon feed (defined herein as a hydrocarbon feed which comprises a mixture of at least two hydrocarbons which are each present in an amount of at least 10 wt %).

[0039] In another preferred aspect, the method of the first aspect of the present invention is applied where the hydrocarbon feed is deliberately changed from one hydrocarbon feed to another.

[0040] The present invention is most advantageous when a relatively large change in the hydrocarbon feed to be cracked can take place. This may happen due to the changing of the feed to one sourced elsewhere, as already discussed, or by a specific change to a different type of feed, such as between naphtha and an ethane containing feed, or between naphtha and a pyrolysis oil, for example.)

[0041] Thus, in a second aspect, the present invention provides a method for transitioning from a first process cracking a first hydrocarbon feed to a second process cracking a second hydrocarbon feed, wherein:

[0042] a) the first process comprises

[0043] a. passing the first hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, downstream of the first zone, each zone comprising one or more electrically heated sections,

[0044] b. heating the first hydrocarbon feed in the first zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, and

[0045] c. heating the vaporised first hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed,

[0046] b) the transition comprises stopping the feeding of the first hydrocarbon feed to the reactant tube and starting the feeding of the second hydrocarbon feed to the same reactant tube,

[0047] characterised in that the second hydrocarbon feed is analysed, and that the electrical energy provided to the one or more electrically heated sections in the first zone and the electrical energy provided to the one or more electrically heated sections in the second zone are adjusted during the transition based on the results of the analysis.

[0048] Typically the preferred features of this second aspect, such as of the analysis performed and the frequency with which the results of the analysis are used for the adjustment (control) of the electrical energy provided to the one or more electrically heated sections in the first and second zones, are as already described for the first aspect.

[0049] By the method being a “transition” is generally meant that the second hydrocarbon feed has a significantly different composition and / or boiling point range to the first hydrocarbon feed inducing a change of the heating section where the feed is fully vaporized. Preferably the average final boiling point of the boiling point range of the second hydrocarbon feed is at least 20° C. different (higher or lower) than the average boiling point of the boiling point range of the first hydrocarbon feed. The second hydrocarbon feed may be of a different type to that of the first hydrocarbon feed. For example, cracking feeds may be generally characterised as either “ethane based”, “propane based”, ‘butane based”, “naphtha based” or “pyrolysis based” such as pyrolysis oil obtained from plastic recycling, and hence a change from one to another of these would be a transition.

[0050] More generally (in both the first and second aspects) in the first zone the hydrocarbon feed is heated in one or more electrically heated sections to vaporise the hydrocarbon feed. Typically 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, but the temperature of the hydrocarbon feed at the exit of the first zone is typically less than 600° C.

[0051] In the second zone the vaporised hydrocarbon feed is further heated to crack the hydrocarbon feed. The most preferred temperature for the cracking will be different depending on the hydrocarbon feed, but the temperature of the cracked hydrocarbon product at the exit of the second zone is generally at least 700° C., and more usually (and preferably), at least 750° C.

[0052] Generally speaking the first and second zones each comprise one or more electrically heated sections. Typically at least one of the first and second zones comprises more than one heated section. There may, for example, be at least four heated sections in total.

[0053] In some embodiments, particularly if there is a relatively large change in the hydrocarbon feed composition / boiling point range, the number of heated sections used to provide each of the first and second zones may be adjusted depending on the hydrocarbon feed. For example, for one feed composition the first zone may comprise “n1” heating sections and the second zone may comprise “n2” heating sections, where “n1+n2” equals “n”, and “n” is the total number of heating sections present. But for a different feed the first zone may comprise “m1” heating sections and the second zone may comprise “m2” heating sections, where “m1+m2” is again equal to “n” (i.e. the same total number of sections) but m1≠n1 and m2≠n2.

[0054] This may be achieved by adjusting the temperature in heating sections as required.

[0055] For example, if during a change in hydrocarbon feed, it is desired to increase the number of heating sections in the first zone, the electrical energy input can be adjusted so that the temperature in what was previous the first heating section of the second zone is reduced from a temperature sufficient for cracking to one that is sufficient only to vaporise the feed.

[0056] This may be particularly advantageous in a transition or more generally if changing from a first hydrocarbon feed to a second hydrocarbon feed where the latter is a relatively heavier feed i.e. has a higher boiling point range.

[0057] In the present invention (either the first or second aspects) the first zone may be considered as a “vaporisation zone” whilst the second zone may be considered as a “cracking zone”. The individual heated sections in the second / cracking zone may be considered as “cracking sections”.

[0058] The first / vaporisation zone may perform preheating as well as vaporisation, and the heated sections therein may be considered as preheating and / or vaporisation sections.DETAILED DESCRIPTION OF THE INVENTION

[0059] In a preferred embodiment, applicable to either the first or second aspect, there is provided a quench zone, downstream of the second / cracking zone, in which the product stream from the cracking reaction is cooled.

[0060] In one embodiment the cooling in the quench zone may be by indirect heat exchange, for example with water to generate steam.

[0061] 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 first and second zones and reduces the amount of energy for vaporisation in the first zone.

[0062] 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 U.S. Pat. No. 7,288,690 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.

[0063] 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.

[0064] 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.

[0065] 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.EXAMPLESExample 1

[0066] This Example illustrates the control of the electrical energy provided to one or more heating sections based on analysis of the hydrocarbon feed, and in particular in a transition.

[0067] Cracking is performed in a reactant tube. The tube is 15 meters in length, with an internal diameter of 47 mm and outer diameter of 53 mm. 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.

[0068] Analysis of the feedstock is performed in real time by an on-line near infrared analyser correlated with an ASTM D86 laboratory analyser to provide a distillation temperature profile, and using a densimeter to measure the feedstock density.

[0069] In the first cracking process the feedstock is a naphtha. The analyses determine that the feedstock has a density of 0.715 g / cm3 and a distillation temperature profile as follows:% volume distilledTemperature (° C.)032103820443052406150796095701128012990149100163

[0070] This naphtha at a flow rate is 250 kg / h is mixed with 75 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. It is determined from the analyses that the feedstock can be fully vaporised by provision of 30.9 kW of electrical power, and that this energy can be provided by the first heater in the reactant tube. Thus, in the first process the first 1 meter of the reactant tube is a vaporisation zone (first zone) in which the naphtha is fully vaporised. The gas temperature at the end of the vaporisation zone of 136° C. The remaining 14 meters are a cracking zone (second zone). The cracked gas temperature at the end of the cracking zone / end of the reactant tube is 820° C., this being a typical cracking temperature for naphtha. The average heat transfer to the reactant tube is 150 kW / m2.

[0071] It is desired to transition to a second cracking process in which the feedstock is a gas oil. Analysis of the gas oil is performed and it is found that it has a density of 0.8233 g / cm3 and a distillation temperature profile as follows:% volume distilledTemperature (° C.)0101.51019620221.330230.440244.150254.260270.470285.180300.590322100379

[0072] This gas oil at a flow rate is 250 kg / h is mixed with 75 kg / h of water steam, and is fed to the reactant tube. The feedstock temperature at the inlet is 120° C. and the pressure is 530 kPaa. It is determined from the analysis that the feedstock can be fully vaporised by provision of 102.5 kW of electrical power, and in this process it is decided to use the first 6 metres of the reactant tube as a vaporisation zone (first zone) in which the gas oil is fully vaporised. The feedstock is fully vaporised in the vaporisation zone, with a gas temperature at the end of the vaporisation zone of 301° C. The remaining 9 meters are a cracking zone (second zone). The cracked gas temperature at the end of the cracking zone / end of the reactant tube is 770° C., this being a typical cracking temperature for gas oil.

[0073] During transition the feeding of naphtha is stopped and the feeding of gas oil is started. As the gas oil starts to feed to the reactant tube the electrical heat energy applied to the reactant tube between 2 and 6 m in length from the inlet is reduced.

Examples

example 1

[0066]This Example illustrates the control of the electrical energy provided to one or more heating sections based on analysis of the hydrocarbon feed, and in particular in a transition.

[0067]Cracking is performed in a reactant tube. The tube is 15 meters in length, with an internal diameter of 47 mm and outer diameter of 53 mm. 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.

[0068]Analysis of the feedstock is performed in real time by an on-line near infrared analyser correlated with an ASTM D86 laboratory analyser to provide a distillation temperature profile, and using a densimeter to measure the feedstock density.

[0069]In the first cracking process the feedstock is a naphtha. The analyses determine that the feedstock has a density of 0.715 g / cm3 and a distillation temperature profile as follows:

% volume distilledTemperature (° C.)03210382044305240...

Claims

1. A method for cracking of a hydrocarbon feed, which method comprisesa) passing the hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, 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, andc) heating the vaporised hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed,wherein the hydrocarbon feed is analysed, and electrical energy provided to the one or more electrically heated sections in the first zone and / or electrical energy provided to the one or more electrically heated sections in the second zone is controlled based on the results of the analysis.

2. The method according to claim 1 wherein the hydrocarbon feed is analysed prior to entry into the first zone, and the analysis provides information on a boiling point range of the hydrocarbon feed.

3. The method according to claim 1 wherein the hydrocarbon feed is analysed to measure a presence of a liquid phase between the first zone and the second zone.

4. The method according to claim 1 wherein a change in the hydrocarbon feed to be cracked takes place which involves either a changing of the feed to a different feed but of the same type or a change to a different type of feed.

5. A method for transitioning from a first process cracking a first hydrocarbon feed to a second process cracking a second hydrocarbon feed, wherein:a) the first process comprisesa. passing the first hydrocarbon feed to a reactant tube which reactant tube comprises a first zone and a second zone, downstream of the first zone, each zone comprising one or more electrically heated sections,b. heating the first hydrocarbon feed in the first zone in the one or more electrically heated sections to vaporise the hydrocarbon feed, andc. heating the vaporised first hydrocarbon feed in the second zone in the one or more electrically heated sections to crack the hydrocarbon feed,b) the transitioning comprises stopping the passing of the first hydrocarbon feed to the reactant tube and starting a feeding of the second hydrocarbon feed to the same reactant tube,wherein the second hydrocarbon feed is analysed, and electrical energy provided to the one or more electrically heated sections in the first zone and electrical energy provided to the one or more electrically heated sections in the second zone is adjusted during the transitioning based on results of the analysis.

6. The method according to claim 5 wherein the second hydrocarbon feed is analysed prior to entry into the first zone, and the analysis provides information on a boiling point range of the hydrocarbon feed.

7. The method according to claim 5 wherein the second hydrocarbon feed is analysed to measure a presence of a liquid phase between the first zone and the second zone.

8. The method according to claim 5 wherein an average final boiling point of a boiling point range of the second hydrocarbon feed is at least 20° C. different (higher or lower) than an average boiling point of a boiling point range of the first hydrocarbon feed.

9. The method according to claim 5 wherein the second hydrocarbon feed is a different type than the first hydrocarbon feed.

10. The method according to claim 1 wherein the hydrocarbon feed is analysed and the results of the analysis are used for the control of the electrical energy provided to the one or more electrically heated sections at least once every 12 hours.

11. The method according to claim 1 wherein the analysis is performed by an on-line analyser and operated to provide continuous or regular analyses, not less than once every 10 minutes, on the hydrocarbon feed.

12. The method according to claim 1 wherein the analysis is selected from GC, Near-IR analysis and density measurements.

13. The method according to claim 1 wherein in the first zone the hydrocarbon feed is heated such that a temperature at an exit of the first zone is less than 600° C.

14. The method according to claim 1 wherein in the second zone the vaporised hydrocarbon feed is heated such that a temperature at an exit of the second zone is generally at least 700° C.

15. The method according to claim 1 wherein there is provided a quench zone, downstream of the second zone, in which a product stream from a cracking reaction is cooled by indirect heat exchange with incoming (fresh) hydrocarbon feed prior to the feed being passed to the first and second zones.

16. The method according to claim 5 wherein the hydrocarbon feed is analysed and the results of the analysis are used for the control of the electrical energy provided to the one or more electrically heated sections at least once every 12 hours.

17. The method according to claim 5 wherein the analysis is performed by an on-line analyser and operated to provide continuous or regular analyses, not less than once every 10 minutes, on the hydrocarbon feed.

18. The method according to claim 5 wherein the analysis is selected from GC, Near-IR analysis and density measurements.

19. The method according to claim 5 wherein in the first zone the hydrocarbon feed is heated such that a temperature at an exit of the first zone is less than 600° C.

20. The method according to claim 5 wherein in the second zone the vaporised hydrocarbon feed is heated such that a temperature at an exit of the second zone is generally at least 700° C.