Process for cracking hydrocarbons

Abstract

The present invention relates to a method for cracking a hydrocarbon feedstock, and in particular provides a method for cracking a hydrocarbon feedstock, comprising the steps of: a) feeding the hydrocarbon feedstock to a reactor tube comprising a first zone and a second zone downstream of the first zone, each zone comprising one or more electric heating sections; b) heating the hydrocarbon feedstock in the first zone of the one or more electric heating sections to vaporize the hydrocarbon feedstock; and c) heating the vaporized hydrocarbon feedstock in the second zone of the one or more electric heating sections to crack the hydrocarbon feedstock, wherein the hydrocarbon feedstock is analyzed, and the electrical energy supplied to the one or more electric heating sections of the first zone and / or the electrical energy supplied to the one or more electric heating sections of the second zone is controlled based on the results of the analysis.

Description

[Technical Field]

[0001] The present invention relates to a method for cracking a hydrocarbon feedstock, and more particularly to a method for cracking a hydrocarbon feedstock in an electrically heated furnace. [Background technology]

[0002] Hydrocarbon cracking is typically carried out in a furnace. For example, in steam cracking, the hydrocarbon feedstock to be cracked is passed into the furnace along with steam, typically through heated reactor tubes. In conventional furnaces, heat is provided by burners located inside the furnace, which generate heat for cracking by burning fuel. Commercial cracking of hydrocarbons is generally carried out at temperatures above 750° C. Before being cracked at such temperatures, the hydrocarbons to be cracked are vaporized, so they are usually heated and vaporized in the convection section of a furnace before being sent to the burner section where the cracking takes place. In many conventional designs, heating and vaporization occurs in what is known as the "convection section" of the furnace, which is located above the section where cracking occurs (commonly referred to as the "radiant section"). The hydrocarbons to be cracked are fed through tubes located in the convection section, and combustion gases from burners in the radiant section, still hot after leaving the radiant section, are used to heat and vaporize the hydrocarbons to be cracked.

[0003] Steam cracking can be performed on a variety of hydrocarbon feedstocks, including "light" hydrocarbon feedstocks such as ethane, "medium" feedstocks such as naphtha, and "heavy" feedstocks such as pyrolysis oil. Generally, cracking of different feedstocks is carried out at similar temperatures, typically above 750°C, as already mentioned, but the optimum temperatures tend to be slightly higher for lighter feedstocks than for heavier feedstocks, and the optimum residence times also tend to be longer for lighter feedstocks. In contrast, medium and heavy feedstocks require more energy to heat and vaporize than lighter feedstocks. For these reasons, furnaces designed to vaporize and crack naphtha feedstocks are generally not optimally designed to vaporize and crack ethane-containing feedstocks. For example, the number of convection section tubes required to fully vaporize naphtha tends to be greater than the number required to fully vaporize ethane, but is not sufficient to fully vaporize heavier feedstocks.

[0004] Steam cracking furnaces based on electrical heating rather than burners have been proposed, and in such designs both the vaporization and cracking steps can use electrical heating. A particular advantage of electrical heating is that it allows for greater control of the temperature of individual sections of the furnace / reactor tube. U.S. Patent Application Publication No. 7,288,690 describes a method and apparatus for steam cracking hydrocarbons using cogeneration with the combustion of fuel to simultaneously produce both thermal energy and mechanical work that is converted into electricity, in which the mixture is first heated using the thermal energy provided by the cogeneration, and then heated to the desired cracking temperature by electrical heating using the electricity provided 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 mixtures of hydrocarbon feedstocks, allowing for varying heating depending on the feedstock being cracked. This document also discloses that preheating can be performed external to the main reactor, providing preheat for each feedstock. Summary of the Invention

[0005] The inventors have now discovered a method whereby the same reactor tube (or bundle of parallel tubes) can be utilized for different hydrocarbon feedstocks, and in particular where both the vaporization and cracking of each feedstock can be performed within the same reactor tube, thereby allowing for the ability to change from cracking one feedstock to cracking a different feedstock within the same reactor tube in a manner that provides optimized cracking. DETAILED DESCRIPTION OF THE INVENTION

[0006] Accordingly, in a first aspect, there is provided a method for cracking a hydrocarbon feedstock, comprising: a) passing a hydrocarbon feedstock into a reactor tube comprising 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 feedstock in a first zone of one or more electric heating sections to vaporize the hydrocarbon feedstock; and c) heating the vaporized hydrocarbon feedstock in a second zone of one or more electrically heated sections to crack the hydrocarbon feedstock; A method comprising: The method is characterized in that the hydrocarbon feedstock is analyzed and the electrical energy supplied to the one or more electrical heating sections of the first zone and / or the electrical energy supplied to the one or more electrical heating sections of the second zone is controlled based on the results of the analysis. A particular feature of this first embodiment is that the heating in the first and second zones is controlled based on analysis of the hydrocarbon feedstock.

[0007] In one embodiment, the hydrocarbon feed is analyzed before entering the first zone. The analysis in this embodiment preferably provides information regarding the boiling range of the hydrocarbon feed. This may be the actual boiling range or other information indicative of the boiling range, such as a density measurement. An alternative embodiment includes analyzing the hydrocarbon feed between the first and second zones to determine the presence of a liquid phase, for example, using an ultrasonic sensor. Heating of the first zone may then be adapted accordingly, for example, by increasing heating if liquid is observed. Heating of the second zone, for example, heating of an earlier section of the second zone, may also be adapted based on such measurements. (An increased presence of liquid in the hydrocarbons exiting the first zone, absent any other changes, generally indicates an increase in the boiling point of the hydrocarbon feed, and thus is an example of a method for providing information indicative of the boiling point range of the hydrocarbon feed.)

[0008] In either embodiment, the analysis may be performed online or offline, and any suitable analysis may be used. In the present invention, the electrical energy supplied to one or more electrical heating 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 analytical results are used to control the electrical energy supplied to one or more electrical heating sections of the first and / or second zones. Preferably, the hydrocarbon feedstock is analyzed, and the results are used to control the electrical energy supplied to one or more electrical heating sections of the first zone and / or one or more electrical heating sections of the second zone at least once every 12 hours. This generally means that the latest analytical results are provided to the control system controlling the supplied electrical energy at least once every 12 hours. The control system then adjusts the supplied electrical energy, as necessary, based on the updated analytical results. (There may be cases where no adjustment is necessary.) Preferably, the hydrocarbon feedstock is analyzed and the results are used to control the electrical energy supplied to one or more electrical heating sections in the first zone and / or the electrical energy supplied to one or more electrical heating sections in the second zone at least once every three hours, for example at least once every hour, and most preferably at least once every 10 minutes. The analytical device itself may be operated to perform continuous or periodic analyses.

[0009] If the analytical device is operated to perform a continuous analysis, the results of the analysis may be used to control the supplied electrical energy continuously or discontinuously, in the latter case preferably the analytical results are obtained from the continuous analytical device and used for control at least once every 12 hours, for example as already mentioned at least once every 3 hours, for example at least once every hour, most preferably more than once every 10 minutes. If the analysis device / analysis is not continuous, then preferably the analysis is performed at least as frequently as necessary for control use, i.e., at least once every 12 hours. Not every analysis performed needs to be used to control the electrical energy supplied to one or more electrical heating sections, although this is usually desirable. It is preferred that the analysis is performed at least once every 3 hours, for example at least once every hour. In the most preferred embodiment, the analysis may be performed more frequently than once every 10 minutes.

[0010] Generally, analytical methods that can provide relatively rapid results are preferred so that heating can be adjusted quickly and frequently in response to any detected changes in feed composition. The analytical device can be located upstream or downstream of the first zone (for the first and second embodiments, respectively) and operated to perform continuous or periodic analysis of the hydrocarbon feed, the results of which can then be used to control the electrical heating sections of the first and / or second zones. Suitable analytical methods, particularly for the first embodiment, include GC, near-infrared analysis, and density measurement. The control of the electrical energy for controlling the electrical heating sections of the first and / or second zones is regulated using a suitable process control system, i.e. the results of the analysis are sent to the cracking reaction process control system which adjusts the energy input accordingly.

[0011] Typically, as the boiling range of the hydrocarbon feedstock increases, the electrical energy to the first zone is increased and heating is increased to ensure vaporization, while the electrical energy to the second zone may be decreased. (It will be apparent that process parameters other than heating may also be adjusted. For example, the hydrocarbon feed rate and therefore the residence time in the first and second zones may also be adjusted depending on the hydrocarbon feedstock.) The method of the first aspect of the present invention takes advantage of the increased controllability of electric heating compared to combustion (burner-based) furnaces. In particular, the amount of heat from each individual electric heater can be finely controlled between zero and the heater's maximum rating. Typically, in practice, each heating section within a zone may be heated by multiple electric heaters. In either case, the amount of heat supplied to each zone can be "finely" adjusted, thus optimizing cracking even with relatively small changes in feed composition.

[0012] (As a simple example, if you have four heaters and you want to reduce the amount of heat provided by 25%, you can reduce the electrical energy to each heater to reduce the overall heat by 25%, or you can turn one of the heaters off.) As used herein, "electrically heated section" means a section of a reactor tube that is heated directly or indirectly by electrical energy. "Direct" heating may include, for example, applying electrical energy directly to the reactor tube. "Indirect" heating may include, for example, using one or more heating elements that heat the reactor tube via one or more of radiation, convection, and induction. Typically, the heat applied to each electric heating section, and thus its temperature, is individually controllable. Typically, each electric heating section has one or more electric heaters associated with it. As used herein, this means that each heating section has one or more specific electric heaters that are adjusted when attempting to control the temperature of that section, and that at least some of the electric heaters associated with a particular section are different and distinct from the electric heaters of different sections.

[0013] The heat applied to any heating section of the cracking zone can generally be defined in terms of the electrical energy applied to heat that section, and most conveniently in terms of the power to the electric heater or heaters that heat that section. Generally, by reducing the power supplied to the electric heaters in a particular heating section, the applied heat will decrease, and vice versa. As already mentioned, it will be apparent that the most accurate control of the cracking reaction will be obtained when more frequent analytical results are provided and used to control the electrical energy supplied to one or more of the electrical heating sections of the first and / or second zones. However, there are also cases where more frequent analysis may be more advantageous for certain hydrocarbon feed types or operations.

[0014] For example, relatively simple feedstocks of relatively high purity, such as ethane or propane, may typically exhibit relatively little change over time, and frequent analysis may not be necessary during cracking of such feedstocks. (Control based on analysis performed every three hours or every hour may be sufficient for accurate control of electrical energy, and even more frequent analysis would not be detrimental.) In contrast, more complex feedstocks, such as naphtha or gas oil, may vary more significantly, sometimes over relatively short periods of time. This is especially true when the hydrocarbon feedstock may be obtained from different sources, or from a single source, such as a refinery, that itself processes a variety of feedstocks to provide hydrocarbon feedstock for cracking. (For example, naphtha feedstocks obtained from different sources are generally not identical.) In such cases, the hydrocarbon feedstock may undergo "natural" variations over time, which analysis according to the present invention can identify and adjust for.

[0015] In one preferred embodiment, the process of the first aspect of the present invention is applied to the cracking of naphtha, gas oil, pyrolysis oil or other complex hydrocarbon feedstock (defined herein as a hydrocarbon feedstock comprising a mixture of at least two hydrocarbons, each present in an amount of at least 10% by weight). In another preferred embodiment, the method of the first aspect of the present invention is applied where the hydrocarbon feedstock is intentionally changed from one hydrocarbon feedstock to another. The present invention is most advantageous when relatively large changes in the hydrocarbon feedstock being cracked can occur, which may occur, as discussed above, by changing the feedstock to another source, or by specific changes to different types of feedstock, such as between naphtha and ethane-containing feedstock, or between naphtha and pyrolysis oil.

[0016] Accordingly, in a second aspect, the present invention provides a method for transitioning from a first process for cracking a first hydrocarbon feedstock to a second process for cracking a second hydrocarbon feedstock, the method comprising: a) The first process is a. passing a first hydrocarbon feedstock into a reactor tube comprising a first zone and a second zone downstream of the first zone, each zone comprising one or more electrically heated sections; b. heating a first hydrocarbon feedstock in a first zone of one or more electric heating sections to vaporize the hydrocarbon feedstock; and c. heating the vaporized first hydrocarbon feedstock in a second zone of one or more electrically heated sections to crack the hydrocarbon feedstock; Including, b) the transition comprises stopping the supply of a first hydrocarbon feed to a reactor tube and starting the supply of a second hydrocarbon feed to the same reactor tube, The method further comprises: analyzing the second hydrocarbon feedstock; and adjusting the electrical energy supplied to the one or more electrical heating sections of the first zone and the one or more electrical heating sections of the second zone during the transition based on the results of the analysis.

[0017] Typically, preferred features of this second aspect, such as the analysis performed and how often the results of that analysis are used to adjust (control) the electrical energy supplied to one or more electrical heating sections of the first and second zones, are as already described for the first aspect. A method is referred to as a "transition" generally because the second hydrocarbon feedstock has a significantly different composition and / or boiling range from the first hydrocarbon feedstock, causing a change in the heating section where the feedstock is fully vaporized. Preferably, the average final boiling point of the boiling range of the second hydrocarbon feedstock is at least 20°C different from (higher or lower than) the average boiling point of the boiling range of the first hydrocarbon feedstock. The second hydrocarbon feedstock may be of a different type than the first hydrocarbon feedstock. For example, cracking feedstocks may generally be characterized as either "ethane-based," "propane-based," "butane-based," "naphtha-based," or "pyrolysis-based," such as pyrolysis oil obtained from plastic recycling; therefore, changing from one of these to another would be a transition.

[0018] More generally (in both the first and second embodiments), in the first zone the hydrocarbon feed is heated by one or more electrically heated sections to vaporize the hydrocarbon feed. Typically, the hydrocarbon feed is heated to a temperature sufficient to vaporize the feed but insufficient to cause cracking, or at least significant levels of cracking. While the most preferred temperature will vary depending on the hydrocarbon feed, the temperature of the hydrocarbon feed at the outlet of the first zone is typically less than 600°C. In the second zone, the vaporized hydrocarbon feed is further heated to crack the hydrocarbon feed. The most preferred temperature for cracking will vary depending on the hydrocarbon feed, but the temperature of the cracked hydrocarbon product at the outlet of the second zone will generally be at least 700°C, more typically (and preferably) at least 750°C. Generally speaking, each of the first and second zones includes one or more electric heating sections. Typically, at least one of the first and second zones includes two or more heating sections. For example, there may be at least four heating sections in total.

[0019] In some embodiments, the number of heating sections used to provide each first and second zone may be adjusted depending on the hydrocarbon feed, especially when there is a relatively large variation in hydrocarbon feed composition / boiling point range. For example, for a given feed composition, the first zone may include "n1" heating sections and the second zone may include "n2" heating sections, where "n1 + n2" equals "n" and "n" is the total number of heating sections present. However, for a different feed, the first zone may include "m1" heating sections and the second zone may include "m2" heating sections, where "m1 + m2" equals "n" (i.e., the total number of sections is the same), but m1 ≠ n1 and m2 ≠ n2. This may be achieved by adjusting the temperature of the heating section as needed. For example, if it is desired to increase the number of heating sections in the first zone when changing hydrocarbon feedstock, the electrical energy input can be adjusted to reduce the temperature of what was once the first heating section in the second zone from a temperature sufficient for cracking to a temperature sufficient only to vaporize the feedstock.

[0020] This can be particularly advantageous in transitions, or more generally when changing from a first hydrocarbon feed to a second hydrocarbon feed, the latter being a relatively heavier feed, i.e., having a higher boiling range. In the present invention (either the first or second aspect), the first zone can be considered the "vaporization zone," while the second zone can be considered the "cracking zone." Individual heating sections within the second zone / cracking zone can be considered "cracking sections." The first zone / vaporization zone may perform preheating and vaporization, and the heating sections within that zone may be considered preheating and / or vaporization sections.

[0021] In a preferred embodiment applicable to either the first or second aspect, downstream of the second zone / cracking zone there is provided a quench zone in which the product stream from the cracking reaction is cooled. In one embodiment, cooling in the quench zone may be achieved by indirect heat exchange with water, for example to generate steam. In a preferred embodiment, the cracking product stream is cooled in a quench zone by indirect heat exchange with the incoming (fresh) hydrocarbon feed, thereby providing preliminary preheating of the hydrocarbon feed prior to the first and second zones and reducing the amount of energy required for vaporization in the first zone.

[0022] More generally, the method of the present invention can be applied to the cracking of any hydrocarbon feedstock that can be cracked using similar processes and methods. These include, for example, those discussed in the aforementioned U.S. Patent Application Publication No. 7,288,690 and WO 2022 / 094455. The present invention can be used to crack halogenated hydrocarbons, including the cracking of dichloroethane. Preferred cracking processes to which the present invention can be applied are processes for the cracking of hydrocarbons to produce olefins. Suitable hydrocarbon feedstocks for cracking, particularly for producing olefins, include ethane, propane, butane, naphtha, gas oil, gas condensate, pyrolysis oil, and mixtures thereof.

[0023] A particularly preferred cracking process to which the present invention can be applied is the steam cracking of hydrocarbons, especially the steam cracking of the hydrocarbon feedstocks mentioned above. Other than the requirements defined in this invention, general process conditions such as feed flow rates, ratios of reactants such as steam, residence time, and cracking temperature are generally similar to conventional processes. Similarly, feed systems and downstream systems such as quench systems and / or heat exchange of reactant and feed streams are all present and applicable as in conventional cracking processes. As already mentioned, the "electrically heated section" can be heated directly or indirectly by electrical energy. Typically, the reaction tube / its heating section is placed in a furnace or heated chamber. A gas, preferably an inert gas, can be fed into the chamber. Examples of suitable electrically heated furnaces can be found in the already mentioned WO 2022 / 094455 or WO 2020 / 002326. [Example]

[0024] ( Example 1 ) This example illustrates the control of electrical energy supplied to one or more heating sections based on an analysis of the hydrocarbon feedstock, particularly in transition. Cracking takes place in a reactor tube, 15 meters long, with an inner diameter of 47 mm and an outer diameter of 53 mm. Heating is provided by a set of independently controlled electric heaters, one for each meter of tube. The metal temperature of the tube is measured by a thermocouple.

[0025] Feedstock analysis is performed in real time by an online near-infrared analyzer that correlates with an ASTM D86 lab analyzer to provide a distillation temperature profile and a density meter to measure the density of the feedstock. In the first cracking process, the feedstock is naphtha. Analysis determined that the feedstock has a density of 0.715 g / cm3 and the distillation temperature profile is as follows: [Table 1]

[0026] Naphtha with a flow rate of 250 kg / h is mixed with 75 kg / h of steam and fed to the reactor tube. The temperature of the feedstock at the inlet is 128°C, and the pressure is 530 kPa. Analysis determined that 30.9 kW of power is required to completely vaporize the feedstock, and this energy can be supplied by the first heater in the reactor tube. Therefore, in the first process, the first meter of the reactor tube becomes the vaporization zone (first zone), where the naphtha is completely vaporized. The gas temperature at the end of the vaporization zone is 136°C. The remaining 14 meters are the cracking zone (second zone). The temperature of the cracked gas at the end of the cracking zone / end of the reactor tube is 820°C, which is a typical cracking temperature for naphtha. The average heat transfer to the reactor tube is 150 kW / m 2 is. It is desirable to move to a second cracking process using gas oil as the feedstock. The gas oil was analyzed and found to be 0.8233 g / cm 3and the distillation temperature profile was found to be as follows: [Table 2]

[0027] This gas oil, with a flow rate of 250 kg / h, is mixed with 75 kg / h of steam and fed to the reactor tube. The temperature of the feedstock at the inlet is 120°C, and the pressure is 530 kPa. Analysis determined that the feedstock could be fully vaporized by supplying 102.5 kW of power, and it was decided that the first 6 meters of the reactor tube would be used as the vaporization zone (first zone) to fully vaporize the gas oil. The feedstock is fully vaporized in the vaporization zone, and the gas temperature at the end of the vaporization zone is 301°C. The remaining 9 meters are the cracking zone (second zone). The temperature of the cracked gas at the end of the cracking zone / end of the reactor tube is 770°C, which is a typical cracking temperature for gas oil. During the transition, the naphtha supply is stopped and the gas oil supply is started. When the gas oil starts to be supplied to the reactor tube, the electrical heat energy applied to the reactor tube between 2 and 6 m from the inlet is reduced.

Claims

1. 1. A method for cracking a hydrocarbon feedstock, comprising: a) passing the hydrocarbon feedstock into a reactor tube comprising 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 feedstock in the first zone of the one or more electric heating sections to vaporize the hydrocarbon feedstock; and c) heating the vaporized hydrocarbon feedstock in the second zone of the one or more electric heating sections to crack the hydrocarbon feedstock; A method comprising:

10. The method of claim 9, wherein the hydrocarbon feedstock is analyzed and the electrical energy supplied to the one or more electrical heating sections of the first zone and / or the electrical energy supplied to the one or more electrical heating sections of the second zone is controlled based on the results of the analysis.

2. 10. The method of claim 1, wherein the hydrocarbon feed is analyzed before entering the first zone, the analysis providing information regarding the boiling point range of the hydrocarbon feed.

3. 3. The method of claim 1 or claim 2, wherein the hydrocarbon feed is analyzed to determine the presence of a liquid phase between the first zone and the second zone.

4. 4. The method of any one of claims 1 to 3, wherein a change of the hydrocarbon feedstock to be cracked is made, the change comprising either changing the feedstock to a different feedstock of the same type, or to a different type of feedstock.

5. 1. A method for transitioning from a first process for cracking a first hydrocarbon feedstock to a second process for cracking a second hydrocarbon feedstock, comprising: a) the first process a. passing the first hydrocarbon feedstock into a reactor tube comprising 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 feedstock in the first zone of the one or more electric heating sections to vaporize the hydrocarbon feedstock; and c) heating the vaporized first hydrocarbon feedstock in a second zone of the one or more electric heating sections to crack the hydrocarbon feedstock; Including, b) the transition comprises stopping the supply of the first hydrocarbon feedstock to the reactor tube and starting the supply of the second hydrocarbon feedstock to the same reactor tube, the second hydrocarbon feedstock is analyzed, and the electrical energy supplied to the one or more electrical heating sections of the first zone and the electrical energy supplied to the one or more electrical heating sections of the second zone are adjusted during the transition based on the results of the analysis.

6. 6. The method of claim 5, wherein the second hydrocarbon feed is analyzed prior to entering the first zone, the analysis providing information regarding the boiling range of the hydrocarbon feed.

7. 7. The method of claim 5 or claim 6, wherein the second hydrocarbon feed is analyzed to determine the presence of a liquid phase between the first zone and the second zone.

8. 8. The method of any one of claims 5 to 7, wherein the average final boiling point of the boiling range of the second hydrocarbon feed differs from (is higher or lower than) the average boiling point of the boiling range of the first hydrocarbon feed by at least 20°C.

9. The method of any one of claims 5 to 8, wherein the second hydrocarbon feedstock is of a different type than the first hydrocarbon feedstock.

10. 10. The method of any one of claims 1 to 9, wherein the hydrocarbon feedstock is analyzed and the results of the analysis are used to control the electrical energy supplied to the one or more electrical heating sections at least once every 12 hours.

11. 11. A method according to any one of the preceding claims, wherein the analysis is performed by an online analytical device, operated to provide continuous or periodic analysis of the hydrocarbon feedstock, preferably more than once every 10 minutes.

12. The method of any one of claims 1 to 11, wherein the analysis is selected from GC, near-infrared analysis and density measurement.

13. The process of any one of claims 1 to 12, wherein in the first zone the hydrocarbon feed is heated such that the temperature at the outlet of the first zone is less than 600°C.

14. A process according to any one of the preceding claims, wherein in the second zone the vaporised hydrocarbon feedstock is heated such that the temperature at the outlet of the second zone is typically at least 700°C, preferably at least 800°C.

15. 15. The method of any one of claims 1 to 14, wherein a quench zone is provided downstream of the second zone in which the product stream from the cracking reaction is cooled by indirect heat exchange with an incoming (fresh) hydrocarbon feed before the feed is passed to the first and second zones.

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