Method for analysis of an effluent gas

Abstract

The invention is directed to a method for determination of a solid content in an effluent gas stream from a device for cracking a hydrocarbon feedstock, the method comprising: a. providing a device for cracking a hydrocarbon feedstock, which comprises one or more furnaces, one or more heat exchangers and one or more stacks, and providing a light scattering measurement system comprising a laser probe, b. operating the device for cracking a hydrocarbon feedstock at least partly in a decoking mode, wherein a decoking feed stream comprising oxygen, nitrogen and / or water is fed to at least one of the one or more furnaces, the decoking feed stream is heated to a decoking temperature in a range from 500°C to 1500°C and an effluent gas stream comprising carbon monoxide and optionally a solid carbon dioxide, nitrogen and / or water is obtained, c. discharging the effluent gas stream through the one or more heat exchangers and the one or more stacks and d. contacting the laser probe with the effluent gas stream, wherein light scattering data of the effluent gas stream is collected and the solid content of the effluent gas stream is derived from the light scattering data.

Description

[0001] Method for analysis of an effluent gas

[0002] Description

[0003] The present invention relates to a method for determination of a solid content in an effluent gas stream from a device for cracking a hydrocarbon feedstock, also referred to as cracker. The invention further relates to a system for carrying out the method.

[0004] Crackers are applied to produce building blocks such as ethylene and propene from a hydrocarbon feedstock for the chemical industry in large scale. Typically, the hydrocarbon feedstock is thermally cracked into smaller molecules by high temperature pyrolysis in presence of steam. The cracked gas is submitted to refinery and separated into desired products by a sequence of separation and chemical-treatment steps. Liquified petroleum gas comprising propane and butane, natural gas liquids comprising ethane, propane and butane, gas oils and naphtha can be used for example as hydrocarbon feedstock. Naphtha contains more than 100 individual components with different boiling points and properties, for example paraffins, isoalkanes, olefins, naphthenes and aromatics.

[0005] During operation of steam crackers, deposition of coke occurs on inner walls of furnaces and downstream of the furnaces. The deposited coke has an insulating effect. The wall temperature in the furnaces increases as well as pressure loss and the cracking yield is lowered. Heat transfer in heat exchangers is reduced and the temperature of the product gas rises. Therefore, the coke has to be removed regularly. The removal can be conducted for example by feeding the cracker with a mixture of steam and air, steam alone or with a mixture comprising steam and hydrogen. In the presence of oxygen, the coke is burnt off. Alternatively, or in addition, the coke can be removed mechanically. However, a mechanical cleaning requires long shut-down periods of the plant and causes excessive material stress.

[0006] DE 3010000 A1 describes a process for thermal decoking of cracked gas coolers, wherein heated air is passed through tubes to be decoked.

[0007] During the decoking cycle, the effluent gas stream contains dust, and the dust content is monitored to comply with environmental regulations. According to the state of the art, the amount of dust in the effluent gas stream is determined manually according to the standard measure DIN EN 13284-1 :2018-02. Samples of the effluent gas streams are extracted, dust comprised in the individual samples is collected and the amount of dust is determined gravimetrically.

[0008] US 2022 / 0064544 A1 focusses on coke and tar removal from a furnace effluent, wherein the furnace effluent from a steam cracker is contacted with a quench liquid and then fed into a centrifugal separator drum. For analysis of particle matter with respect to particle size different measurement principles are known.

[0009] US 4,890,920 A discloses a particle measuring device for particles present in a fluid substance, wherein a particle size distribution and a volumetric density of the particles are measured simultaneously. A solid content of a heterogenous stream, in particular originating from cracking processes for production of chemical building blocks, is not addressed.

[0010] US 2010 / 0043528 A1 is directed to a device and method for continuous measurement of concentrations of tars in a gas flow. Gravimetric measurements as well as a calorimetric measurement system are applied. Part of the main gas stream is removed manually for analysis and a complex set-up is required for analysis of gas streams containing a high amount of water. Further, the gravimetric method is a discontinuous method, and no continuous measurement signal is available. Further, the calorimetric measurement suffers from dependencies on the chemical composition of the detected particles.

[0011] It is an object of the present invention to enable a reliable and continuous monitoring of effluent gas from cracker decoking in an automated manner.

[0012] This object is achieved by a method for determination of a solid content in an effluent gas stream from a device for cracking a hydrocarbon feedstock, wherein the method comprises the following steps: a. providing a device for cracking a hydrocarbon feedstock, which comprises one or more furnaces, one or more heat exchangers, in particular transfer line exchangers (TLE), and one or more stacks, and providing a light scattering measurement system comprising a laser probe, b. operating the device for cracking a hydrocarbon feedstock at least partly in a decoking mode, wherein a decoking feed stream comprising oxygen, nitrogen and / or water is fed to at least one of the one or more furnaces, the decoking feed stream is heated to a decoking temperature in a range from 500°C to 1500°C, in particular from 700°C to 1000°C, more preferably from 700°C to 900°C, even more preferably from 750°C to 900°C, and an effluent gas stream comprising carbon monoxide and optionally a solid, carbon dioxide, nitrogen and / or water is obtained, c. discharging the effluent gas stream through the one or more heat exchangers and the one or more stacks and d. contacting the laser probe with the effluent gas stream, wherein light scattering data of the effluent gas stream is collected and the solid content of the effluent gas stream is derived from the light scattering data.

[0013] The invention is further directed to a system for carrying out the method, wherein the system comprises a device for cracking a hydrocarbon feedstock, which comprises one or more furnaces, one or more heat exchangers, in particular one or more transfer line exchangers (TLE); one or more stacks and at least one light scattering measurement system comprising a laser probe, wherein the laser probe is preferably arranged at a stack of the one or more stacks. The laser probe is more preferably arranged at an outlet of the stack. The device can comprise for example 1 to 15 furnaces, preferably 2 to 4 furnaces. Further, the device can comprise 1 to 15 stacks, preferably 2 to 4 stacks. The effluent gas streams of at least two of the furnaces can be combined in one stack. Steps a., b., c., and d. can be performed in any order, preferably steps a., b., c., and d. are performed in the given order.

[0014] The system according to the present invention can be referred to as automated measuring system. The invention provides an automated method for analysis of off-gases from industrial cracker plants even in presence of varying and large amounts of water and under harsh conditions, in particular at very high temperatures.

[0015] As the laser probe can be arranged directly in the effluent gas stream and no withdrawal of individual sample volumes is required, measurement inaccuracy due to sampling is omitted. As an example, the manual gravimetric method according to the state of the art requires an isokinetic sampling to separate a sample volume from the effluent gas stream, which exhibit a representative solid content. For an isokinetic sampling, the flow velocity in the sampling device has to be equal to the flow velocity of the effluent gas stream, otherwise the solid content in the sample volume differs from the solid content of the actual effluent gas stream. By the method applying the laser probe, erroneous analysis of the effluent gas stream caused by sampling errors is avoided.

[0016] The device for cracking a hydrocarbon feedstock preferably comprises or is a steam cracker. The device for cracking a hydrocarbon feedstock is preferably usable or periodically used for thermal cracking of the hydrocarbon feedstock, wherein a cracking product gas stream is obtained. The cracking product gas stream particularly comprises unsaturated hydrocarbons and optionally aromatic hydrocarbons. The cracking product gas stream preferably comprises ethylene and optionally propylene, butadiene, benzene, toluene and / or xylene.

[0017] Each furnace of the device for cracking a hydrocarbon feedstock can be operated in a cracking mode or the decoking mode. During the cracking mode the hydrocarbon feedstock is fed to at least one of the one or more furnaces and the cracking product gas stream is obtained. Preferably, at least one of the one or more furnaces is operated alternately in the cracking mode and in the decoking mode. The decoking mode is in particular characterized in that carbon, preferably comprised in the solid and for example present as a deposit, is oxidized in the at least one furnace and / or in the heat exchanger. In the decoking mode, the decoking feed stream is fed to the at least one furnace. The decoking feed stream in particular comprises less than 20 wt.-%, preferably less than 10 wt.-%, more preferably less than 5 wt.-%, even more preferably less than 1 wt.-%, of hydrocarbons, referring to the total decoking feed stream. Hydrocarbons are understood as organic compounds comprising hydrogen and carbon, in particular consisting of hydrogen and carbon. In the decoking mode, the outlet of the at least one furnace is typically connected to or open to the atmosphere for discharging.

[0018] The cracking mode is in particular characterized in that a split reaction of hydrocarbons is performed in the at least one furnace. In the split reaction typically chains of hydrocarbons are broken off into at least two parts. In the cracking mode, the hydrocarbon feedstock is fed to the at least one furnace, wherein the hydrocarbon feedstock preferably comprises more than 25 wt.-%, more preferably more than 50 wt.-%, even more preferably more than 75 wt.-%, of hydrocarbons, referring to the total hydrocarbon feedstock. In the cracking mode the outlet of the at least one furnace is typically connected to a refinery unit.

[0019] The hydrocarbon feedstock comprises or consists of preferably ethane, propane, butane, liquefied petroleum gas, gasoline fractions, such as light naphtha, for example having a boiling point at 0.1 MPa in a range from 30°C to 150°C, full-range naphtha, for example having a boiling point at 0.1 MPa in a range from 30°C to 180°C, heavy naphtha, for example having a boiling point at 0.1 MPa in a range from 150°C to 220°C, kerosene, for example having a boiling point at 0.1 MPa in a range from 200°C to 260°C, gas oils, such a light gas oil, for example having a boiling point at 0.1 MPa in a range from 200°C to 360°C, and heavy gas oil, for example having a boiling point at 0.1 MPa in a range from 310°C to 430°C, vacuum distillates, for example having a boiling point at 0.1 MPa in a range from 400°C to 560°C, and / or chemically recycled fractions having a boiling point at 0.1 MPa of up to 400°C. The hydrocarbon feedstock can comprise or consists of chemically recycled fractions having a boiling point at 0.1 MPa in a range from 160°C to 400°C. The hydrocarbon feedstock can comprise or consist of gasoline fractions, kerosene and / or gas oils, wherein gas oil typically has a boiling point at 0.1 MPa in a range from 180°C to 350°C. Further, the hydrocarbon feedstock can comprise sulfur, for example in a concentration of more than 0.02 wt.-%, referring to the total hydrocarbon feedstock. The presence of sulfur in the hydrocarbon feedstock enhances the tendency to coking and thus to solid deposition in the device for cracking a hydrocarbon feedstock, in particular in the furnaces and heat exchangers.

[0020] The, in particular thermal, cracking is preferably performed in presence of steam, also referred to as steam cracking. Steam is understood to be water in gaseous form. In a preferred embodiment, the hydrocarbon feedstock is fed to the device for cracking a hydrocarbon feedstock together with steam when at least one of the furnaces is operated in the cracking mode. In particular, a feed mixture, comprising the hydrocarbon feedstock and steam, is fed to the device for cracking a hydrocarbon feedstock when at least one of the furnaces is operated in the cracking mode. A weight ratio of steam to the hydrocarbon feedstock, comprised in the feed mixture respectively, is preferably in a range from 0.1 to 1.0, more preferably from 0.2 to 0.8, especially from 0.3 to 0.7, for example from 0.35 to 0.50.

[0021] The one or more furnaces comprised in the device for cracking a hydrocarbon feedstock are preferably indirectly heated tube furnaces, more preferably fired tubular reactors, even more preferably radiant coils. Tubes of the furnaces are preferably arranged vertically. Tubes of the furnaces preferably have a length in a range from 10 m to 100 m. Further, tubes of the furnaces can comprise fins to enlarge the inner surface of the tubes. The tubes of the furnaces are preferably made of a steel alloy material comprising nickel, chrome and optionally silicon, manganese, niobium, molybdenum, tungsten and / or titanium. The steel alloy material comprises preferably 40 wt.-% or more of nickel and 30 wt.-% or more of chrome, referring to the total steel alloy material. The one or more furnaces can comprise each a firebox with tubes, in particular vertical radiant coils, arranged therein, more preferably centrally arranged therein. A temperature in the firebox outside of the tubes is preferably in a range from 1000°C to 1200°C. Further, one or more furnaces comprised in the device for cracking a hydrocarbon feedstock can be at least in part directly or indirectly heated by electricity. One or more of the furnaces can comprise an electrical heater, wherein the electrical heater in particular comprises electrical resistors in order to generate heat.

[0022] A residence time of the hydrocarbon feedstock in the one or more furnaces is preferably in a range from 0.05 seconds to 1.00 second, more preferably from 0.10 seconds to 0.60 seconds, even more preferably from 0.10 seconds to 0.50 seconds. A cracking temperature, in particular a mean average cracking temperature, particularly over the length of the tubes, during the cracking mode in the furnace is preferably 750°C or more, more preferably in a range from 780°C to 900°C, even more preferably in a range from 800°C to 900°C. The cracking temperature can be measured for example by thermowell or thermocouple or via skin thermocouple. In a preferred embodiment, the cracking temperature increases from an inlet of the furnace to the outlet of the furnace. An inlet temperature can be for example in a range from 500°C to 680°C and an outlet temperature in a range from 775°C to 875°C. An energy supply to the tubes of the one or more furnaces in the cracking mode is preferably in a range from 40 000 kcal / (m2'h) (167.36 MJ / (m2'h)) to 80 000 kcal / (m2'h) (334.72 MJ / (m2'h)), more preferably from 50 000 kcal / (m2'h) (209.2 MJ / (m2'h)) to 70 000 kcal / (m2'h) (292.88 MJ / (m2'h)). A capacity of each furnace is typically in a range from 130 000 t / a to 300 000 t / a of the hydrocarbon feedstock.

[0023] The one or more furnaces are preferably heated, typically in the cracking mode as well as in the decoking mode, by firing with burners, in particular radiant burners. The burners can consume gaseous and / or liquid fuels for combustion with air. The burners are preferably run with a fuel gas. The fuel gas is fed to the burners, whereas the hydrocarbon feedstock or the decoking feed stream is fed into the interior of the furnace, in particular to a reaction zone of the furnace, preferably into the tubes of the furnace, which is heated by the burners. Flue gas from the burners can be used to preheat the hydrocarbon feedstock and / or the air for combustion. The preheating can be carried out by direct or indirect heat exchange. The optionally preheated hydrocarbon feedstock is then preferably mixed with steam.

[0024] The flue gas of the burners is preferably submitted to a NOx separation step. The term NOx is understood to describe nitrogen oxides comprising or consisting of nitric oxide (NO) and nitrogen dioxide (NO2). The device for cracking a hydrocarbon feedstock can further comprise a NOx removal unit, wherein NOx is preferably removed from the flue gas by selective catalytic reduction. In a preferred embodiment, NOx present in the flue gas of the burners reacts with ammonia in the NOx removal unit.

[0025] The stack, the effluent gas stream is preferably discharged through, can be used, in particular at the same time and / or in the cracking mode, as flue gas stack. Thus, the stack the effluent gas stream is discharged through, can be arranged directly on one of the furnaces. The stack, through which the effluent gas stream is discharged, can be the flue gas stack, respectively. In one embodiment, the effluent gas stream is first led through one of the burners and then discharged through the stack together with the flue gas from the burner. After leaving the furnace in the cracking mode, the cracking product gas stream is preferably cooled to temperatures of 700°C or less, more preferably to 650°C or less, for example to temperatures in a range from 500°C to 700°C, preferably from 550°C to 650°C, to prevent undesired side-reactions. The cracking product gas stream can be cooled by direct or indirect cooling methods. For direct cooling methods liquid hydrocarbons, such as an oil, and / or water can be injected into the cracking product gas stream.

[0026] In a preferred embodiment, the cracking product gas stream and / or the effluent gas stream are cooled in the one or more heat exchangers, in particular with water. The one or more heat exchangers can be indirect heat exchangers or direct heat exchangers. Preferably, the one or more heat exchangers are indirect heat exchangers. The one or more heat exchangers are preferably tube bundle heat exchangers, more preferably each of the one or more heat exchangers comprises at least 50 exchanger tubes. The one or more heat exchangers are more preferably transfer line exchangers (TLE), even more preferably linear transfer line exchangers. A heat exchanger is preferably mounted on top of a furnace, more preferably one heat exchanger is mounted on top of each furnace. Alternatively, the heat exchanger can be mounted along a side of the furnace or underneath the furnace. The one or more heat exchangers can be orientated vertically or horizontally. Preferably each furnace is connected to at least one heat exchanger. Each furnace can be connected to a series of at least two heat exchangers. Accordingly, the cracking product gas stream and / or the effluent gas stream can be cooled stepwise.

[0027] The one or more heat exchangers serve preferably for cooling of the cracking product gas stream and / or the effluent gas stream and for heat recovery in form of high-pressure steam. In particular, steam is generated in the one or more heat exchangers under high pressure in a range from 5 MPa to 15 MPa, more particularly from 8 MPa to 13 MPa. The pressurized stream can be used as energy source in the process.

[0028] The cracking product gas stream, in particular the cooled cracking product gas stream, is preferably led to the refinery unit for separation of desired components or fractions comprised in the cracking product gas stream. The cracking product gas stream, in particular the cooled cracking product gas stream, is preferably submitted to at least one refinery step. In a preferred embodiment, the cracking product gas streams from all furnaces are combined. The at least one refinery step can comprise additional removal of heat, condensation of water and heavy hydrocarbons, compression, washing, drying, thermal separation and / or hydrogenation.

[0029] In the cracking mode, acetylenic, diolefinic and aromatic compounds are typically formed. The cracking product gas stream can comprise for example ethylene (ethene), acetylene, aromatics, butadiene, butene, isobutene, 2-methyl- 1,3-butadiene, 1,3-pentadiene, cyclopentadiene, ethane, fuel oil, hydrogen, methane, naphthalene, propane, propene, tar and / or sulfur.

[0030] Typically, acetylenic, diolefinic and aromatic compounds deposit coke in the device for cracking a hydrocarbon feedstock, in particular on the inner surfaces of the one or more furnaces and / or the one or more heat exchangers. Thus, typically in the cracking mode, the solid, in particular coke, is deposited in the device for cracking a hydrocarbon feedstock, in particular on the inner surfaces of the one or more furnaces and the one or more heat exchangers, more particularly on inner walls of the one or more furnaces and the heat exchangers. Coking, in particular catalytic coking, pyrolytic and / or thermal coking, generally occurs in the cracking mode. During the cracking mode, the solid, particularly coke, is typically deposited in the device for cracking a hydrocarbon feedstock. In particular, the solid is deposited in the one or more furnaces and the one or more transfer line exchangers. By deposition of the solid in the device for cracking of a hydrocarbon feedstock, an outlet temperature of the one or more heat exchangers is typically increased by 100°C or more. The cracking mode is preferably stopped at least for one furnace after an increase of the outlet temperature of at least one heat exchanger to close to the design temperature.

[0031] At the end of the cracking mode, feeding of the hydrocarbon feedstock to the at least one furnace is stopped. The feed of the hydrocarbon feedstock to the at least one furnace is in particular stopped when a temperature of the cracking product gas stream downstream of the one or more heat exchangers exceeds a temperature limit of 700°C, preferably of 650°C. To start the decoking mode, feeding of the decoking feed stream to the at least one furnace is started.

[0032] Preferably, the decoking feed stream is heated to a decoking temperature in a range from 600°C to 1100°C, more preferably from 700°C to 1050°C, even more preferably from 800°C to 1000°C, especially from 800°C to 850°C, in particular in the at least one furnace. Preferably, an absolute decoking pressure in the at least one furnace is in a range from 500 hPa to 2800 hPa, more preferably from 1500 hPa to 2800 hPa, even more preferably from 1500 hPa to 2000 hPa.

[0033] Preferably, a ratio of the hourly weight throughput of the decoking feed stream in the decoking mode to the hourly weight throughput of the hydrocarbon feedstock in the cracking mode is from 0.05 to 5.00, more preferably from 0.10 to 3.00, especially from 0.10 to 2.00, referring to one furnace.

[0034] In a preferred embodiment, the decoking mode is upheld for one furnace for a decoking period in a range from 10 hours to 50 hours, more preferably from 20 hours to 40 hours.

[0035] During the decoking mode, the decoking feed stream is converted to the effluent gas stream preferably in the at least one furnace and / or the at least one heat exchanger. Preferably, at least part of the decoking feed stream is led from the at least one furnace to the at least one heat exchanger, in particular to at least one transfer line exchanger, and / or at least part of the decoking feed stream is introduced directly to the at least one heat exchanger, in particular the at least one transfer line exchanger.

[0036] The effluent gas stream is preferably a heterogenous stream. Typically, the effluent gas stream comprises carbon monoxide and the solid and optionally carbon dioxide, nitrogen and / or water. In particular, the effluent gas stream comprises carbon monoxide, the solid and water and optionally carbon dioxide and / or nitrogen. More preferably the effluent gas stream comprises a gaseous phase and the solid. The gaseous phase comprises in particular the carbon monoxide and optionally carbon dioxide, nitrogen and / or water.

[0037] The term solid encompasses semi-solids and / or heavy viscous liquids such as tar, which is a precursor of coke. Tar typically comprises poly-aromatic components, in particular with a high boiling point. The solid is in particular coke. Coke is typically a grey, hard and porous solid. Coke has in general a high carbon content and comprises few impurities. The solid preferably comprises at least 80 wt.-%, more preferably at least 95 wt.-%, even more preferably at least 98 wt.-%, of carbon, referring to the total solid. The solid can comprise aluminum, boron, calcium or heavy metals such as chromium, cobalt, iron, manganese, magnesium, molybdenum, nickel, potassium, silicon, sodium, titanium and / or vanadium. These substances typically originate from the coil material. The solid has a density typically in a range from 1500 kg / m3to 2500 kg / m3, in particular from 1800 kg / m3to 2200 kg / m3, for example of 2000 kg / m3.

[0038] Typically, the effluent gas stream comprises particles of the solid, also referred to as dust, which is, in particular, finely dispersed coke. The particles of the solid are in particular dispersed in the gaseous phase comprised in the effluent gas stream. The particles of the solid can in general be of any shape, structure or density. The effluent gas stream preferably comprises solid carbon particles. A particle size distribution D50 of the particles is, preferably, in a range from 0.1 m to 200 pm, more preferably from 0.1 pm to 100 pm, even more preferably from 0.1 pm to 50 pm, even more preferably from 5 pm to 50 pm. The particle size distribution D50 can be measured according to the analysis method ISO 13320:2020.

[0039] The content, also referred to as concentration, of the solid in the effluent gas stream is determined by means of the laser probe according to the present invention. Preferably, the light scattering data is collected while the device for cracking a hydrocarbon feedstock is operated at least partly in the decoking mode.

[0040] The solid content refers preferably to the dry weight of solid material comprised in a given total volume of the effluent gas stream. The solid content can be determined directly for example by the manual gravimetric method defined in DIN EN 13284-1 : 2018-02. A mean average value in the solid content of the effluent gas stream can be calculated over the duration of the decoking mode when several samples are taken. In particular, a mean average value is calculated for the time period, wherein oxygen is fed to the at least one furnace.

[0041] Preferably, the effluent gas stream comprises the solid, in particular the particles of the solid, in a concentration in a range from 0.01 to 2000.00 mg / m3, more preferably from 0.10 to 1000.00 mg / m3, more preferably from 0.50 to 500.00 mg / m3, even more preferably from 0.50 to 50.00 mg / m3and even more preferably from 1.00 to 20.00 mg / m3, referring to the total effluent gas stream. Accordingly, the effluent gas stream preferably has a solid content, in particular a content of solid particles, of 1 vol.-% or less, more preferably less than 1 vol.-%, even more preferably of 0.1 vol.-% or less, even more preferably of 0.01 vol.-% or less, even more preferably of 0.001 vol.-% or less, referring to the total effluent gas stream. Typically, the effluent gas stream has a solid content, in particular a content of solid particles, of more than 0 vol.-%, referring to the total effluent gas stream.

[0042] The effluent gas stream preferably comprises water, in particular in form of steam. Typically, the effluent gas stream comprises water in varying amounts or proportions, referring to the total effluent gas stream. Preferably, the effluent gas stream comprises water and during data collection in step d., the relative humidity of the effluent gas stream varies with time. In particular, the water content in the decoking feed stream and the effluent gas stream varies with time. Preferably, the water content in the decoking feed stream varies during the decoking mode by a factor of at least 1 .3, more preferably by at least 1 .5, even more preferably by at least 1 .7, even more preferably by at least 2, referring to a mean average value of the water content over the total duration of the decoking mode. In particular, the water content in the decoking feed stream is at least 50 wt.-%, referring to the total decoking feed stream, at at least one point of time during the decoking mode. Preferably, the water content in the decoking feed stream extends or is varied over the whole range from 80 wt.-% to 20 wt.-%, more preferably from 100 wt.-% to 1 wt.-%, referring to the total decoking feed stream. The water content in the decoking feed stream can be varied at least over the whole range from 80 wt.-% to 20 wt.-%, more preferably from 100 wt.-% to 1 wt.-%, referring to the total decoking feed stream. In particular, the water content in the decoking feed stream is varied continuously, for example but not necessarily and at least partly in a linear manner, preferably from 80 wt.-% to 20 wt.-%, more preferably from 100 wt.-% to 1 wt.-%, referring to the total decoking feed stream. Variations between other minimum and maximum water content values, in particular variations, such as a continuous variation, between a decoking feed stream comprising 0 wt.-% of water and a decoking feed stream comprising 100 wt.-% of water, particularly steam, are likewise feasible. The data collection in step d. is reliable over broad ranges and variations of the water content in the decoking feed stream and the effluent gas stream, respectively. Even more preferably, the water content in the decoking feed stream varies from 100 wt.-% to 0 wt.-%, referring to the total decoking feed stream. A variation of the water content in the decoking feed stream results in a variation of the water content in the effluent gas stream. Typically, the variation of the water content in the decoking feed stream results in the respective variation of the water content in the effluent gas stream. The variation of the water content in the given ranges can be decreasing and / or increasing over time, preferably decreasing. Thus, preferably a varying amount of water is fed to the at least one furnace during the decoking mode.

[0043] Preferably, all water comprised in the effluent gas stream is present in gaseous form at the laser probe, when the laser probe is in contact with the effluent gas stream. Typically, no water is condensed in the effluent gas stream before reaching the laser probe or at the laser probe.

[0044] Preferably, the decoking feed stream comprises water, oxygen, hydrogen and / or nitrogen. More preferably, the decoking feed stream comprises oxygen and / or water and optionally hydrogen and / or nitrogen. Even more preferably, the decoking feed stream comprises water, oxygen and nitrogen, at at least one point of time in the decoking mode. In a first embodiment, the decoking feed stream comprises or consists of water, oxygen and nitrogen, in particular a weight ratio of water to the sum of oxygen and nitrogen is from 100:1 to 2:8, preferably from 9:1 to 3:7. In a second embodiment, or at another point of time in the decoking mode, the decoking feed stream comprises or consists of water, in particular comprises more than 90 vol .-%, more preferably more than 95 vol.-%, even more preferably more than 99 vol.-%, of water, referring to the total decoking feed stream. In a third embodiment, or at another point of time in the decoking mode, the decoking feed stream comprises or consists of water and optionally hydrogen. In a fourth embodiment, or at another point of time in the decoking mode, the decoking feed stream comprises or consists of oxygen and nitrogen, in particular comprises more than 90 vol.-%, more preferably more than 95 vol.-%, even more preferably more than 99 vol.-%, of the sum of oxygen and nitrogen, referring to the total decoking feed stream. In the fourth embodiment, the decoking feed stream preferably comprises or consists of 4 vol.-% to 40 vol.-% of oxygen and 60 vol.-% to 96 vol.-% of nitrogen, more preferably 10 vol.-% to 30 vol.-% of oxygen and 70 vol.-% to 90 vol.-% of nitrogen, even more preferably 15 vol.-% to 25 vol.-% of oxygen and 75 vol.-% to 85 vol.-% of nitrogen, for example 21 vol.-% of oxygen and 79 vol.-% of nitrogen, referring to the total decoking feed stream.

[0045] In particular, the decoking feed stream comprises less than 20 wt.-%, preferably less than 10 wt.-%, more preferably less than 5 wt.-%, even more preferably less than 1 wt.-%, of hydrocarbons, referring to the total decoking feed stream. Preferably, the decoking feed stream comprises less than 20 wt.-%, preferably less than 10 wt.-%, more preferably less than 5 wt.-%, even more preferably less than 1 wt.-%, of ethane, propane, butane, liquefied petroleum gas and gasoline fractions, referring to the total decoking feed stream.

[0046] Preferably, the decoking feed stream is the only stream that is fed to the at least one furnace during decoking mode.

[0047] In a preferred embodiment, the composition of the decoking feed stream is varied over time in the decoking mode. In a more preferred embodiment, in a first interval of the decoking mode the decoking feed stream comprises or consists of water, oxygen and nitrogen, in particular consists of water, oxygen and nitrogen, and in a second interval of the decoking mode, preferably directly or indirectly following the first interval, the decoking feed stream comprises or consists of oxygen and nitrogen, in particular comprises more than 90 vol-%, more preferably more than 95 vol-%, even more preferably more than 99 vol-%, of oxygen and nitrogen, referring to the total decoking feed stream. In the first interval, preferably the decoking feed stream according to the above defined first embodiment is applied and in the second interval, preferably the decoking feeds stream according to the above defined fourth embodiment is applied. In general, the first interval substantially serves for decoking the at least one furnace and the second interval substantially serves for decoking the at least one heat exchanger.

[0048] A decoking feed stream consisting of steam can be applied to start the decoking mode, in particular before the first interval. Hereby, residual amounts of the hydrocarbon feedstock being present in the at least one furnace and the at least one heat exchanger can be purged. During the decoking mode, the oxygen concentration in the decoking feed stream is preferably increased over time. In another preferred embodiment, the decoking mode is started with the decoking feed stream comprising or consisting of water, oxygen and nitrogen, having a very low content in oxygen and nitrogen, for example less than 10 wt.-% by weight of oxygen and nitrogen, or with water, in particular steam, alone. The decoking mode is preferably continued with increasing amounts of oxygen and nitrogen, in particular until the decoking feed stream comprises up to 70 vol.-% or even up to 100 vol.-% of oxygen and nitrogen, referring to the total decoking feed stream.

[0049] In a preferred embodiment, the decoking feed stream comprises water, oxygen, and nitrogen at least at point of time during the decoking mode. Preferably, the concentration of water in the decoking feed stream is varied over time from 0 vol.-% to 100 vol.-% during the decoking mode, referring to the total decoking feed stream. Preferably, the concentration of air in the decoking feed stream is varied over time from 0 vol.-% to 100 vol.-% during the decoking mode, referring to the total decoking feed stream, wherein the air comprises or consists of preferably 4 vol.-% to 30 vol.-% of oxygen and 70 vol.-% to 96 vol.-% of nitrogen, more preferably 10 vol.-% to 30 vol.-% of oxygen and 70 vol.-% to 90 vol.-% of nitrogen, even more preferably 15 vol.-% to 25 vol.-% of oxygen and 75 vol.-% to 85 vol.-% of nitrogen, for example 21 vol.-% of oxygen and 79 vol.-% of nitrogen, referring to the total air.

[0050] Water, more specifically steam, comprised in the decoking feed stream preferably reacts with the solid, in particular with deposited coke, under formation of carbon monoxide, carbon dioxide and hydrogen in a water gas reaction.

[0051] The effluent gas stream typically comprises 0 vol.-% to 15 vol.-%, more preferably 0.1 vol.-% to 15 vol.-%, even more preferably from 1 vol.-% to 10 vol.-% of the sum of carbon monoxide and carbon dioxide, referring to the total effluent gas stream.

[0052] The concentration of the sum of carbon monoxide and carbon dioxide comprised in the effluent gas stream can be monitored, for example by optical methods, in particular infrared-photometry or by means of a laser, or by means of an electrochemical measurement device. In a preferred embodiment, the operation of the device for cracking a hydrocarbon feedstock, in particular the operation of the at least one furnace, is controlled in the decoking mode based on the concentration of the sum of carbon monoxide and carbon dioxide comprised in the effluent gas stream. More preferably, the oxygen concentration in the decoking fee stream is adjusted based on the concentration of the sum of carbon monoxide and carbon dioxide comprised in the effluent gas stream.

[0053] In the decoking mode, additional mechanical removal of the solid from the furnace and / or the heat exchanger can be performed. By the mechanical removal the cleaning of the device for cracking a hydrocarbon feedstock ca be further improved. The mechanical removal can be carried out for example by means of a high-pressure water instrument, in particular under a water pressure in a range from 30 MPa to 70 MPa, or by blasting of water and / or sand. Before the mechanical removal, the at least one furnace and / or the at least one heat exchanger are preferably cooled, more preferably to less than 400°C, even more preferably to less than 200°C. Preferably, the effluent gas stream is led from an outlet of the at least one furnace to a heat exchanger of the one or more heat exchangers, in particular a transfer line exchanger of the one or more transfer line exchangers, and then to a stack of the one or more stacks, and the laser probe is arranged at the stack. More preferably, the device for cracking a hydrocarbon feedstock comprises a device for solid separation and the effluent gas stream is led from the heat exchanger, in particular the transfer line exchanger (TLE), through the device for solid separation and then to a stack of the one or more stacks and the laser probe is arranged at the stack.

[0054] Preferably, the effluent gas stream is submitted to a solid separation step. The solid separation can imply dry and / or wet separation, in particular a water quench and / or application of a cyclone. Preferably, the device for cracking a hydrocarbon feedstock comprises the device for solid separation. The device for solid separation is preferably a cyclone. The device for solid separation is more preferably arranged upstream of the one or more stacks.

[0055] At least part of the effluent gas stream is typically released to the atmosphere. Preferably, only part of the effluent gas stream is released to the atmosphere and more preferably only part of the effluent gas stream is released to the atmosphere after the solid separation step. Alternatively, or additionally, the effluent gas stream can be partly recycled into the at least one furnace. Due to this recycle, the solid can be burned in the at least one furnace.

[0056] Due to the position of the laser probe at the stack, in particular in case the device for solid separation is present upstream of the stack, the laser probe is better protected from erosion caused by the solid particles. One or more laser probes can be provided. The laser probe is preferably arranged at the stack, wherein the laser probe can be located upstream and / or downstream of the stack, preferably at a stack inlet and / or at a stack outlet. In particular, the laser probe is preferably arranged downstream of the at least one heat exchanger and more preferably downstream of the device for solid separation.

[0057] The laser probe preferable comprises a light source, more preferably a monochromatic light source, even more preferably a laser, in particular a helium neon laser or a diode laser. Light emitted from the light source is preferably filtered, collimated and focused across a sample path where the effluent gas stream is present. The sample path has preferably a length in a range from 5 mm to 200 mm, more preferably from 20 mm to 100 mm, even more preferably from 40 mm to 70 mm. In the course of passing across the sample path, the light is potentially scattered by the solid, in particular by the particles of the solid, which is or are present in the effluent gas stream and in the sample path. The scattered light is typically collected by the laser probe, in particular by a lens of the laser probe, and focused onto a detector device. The laser probe measures preferably a scattered light intensity. In particular, an intensity distribution of the light is gathered by the detector device. The light scattering data is preferably the detected intensity distribution.

[0058] The laser probe, in particular the light source, can be operated for example at a wavelength in a range from 520 nm to 700 nm, such as from 550 nm to 700 nm. Preferably, the laser probe, in particular the light source, is operated at a wavelength in a range from 600 nm to 700 nm, in particular from 640 nm to 660 nm. The light scattering data is collected preferably by a dynamic light scattering measurement, in particular by application of a photoelectric detector. For the dynamic light scattering measurement temporal fluctuations can be analyzed using the intensity or photon auto-correlation function, also known as photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS). For collection of the light scattering data of the effluent gas stream, an electrical output signal of the laser probe, in particular related to the intensity of the scattered light, is preferably registered. In particular, the backward and / or forward scattered light is registered with detectors. The intensity of the scattered light is a measure for the number of particles in the gas stream. Preferably, the light scattering data is collected continuously and in-situ in the effluent gas stream. In-situ is understood to indicate that the laser probe is arranged in the flow path of the effluent gas stream. In particular, no samples are withdrawn from the effluent gas stream for the solid content measurement. Accordingly, the measurement is carried out preferably online in the effluent gas stream. The operation of the device for cracking a hydrocarbon feedstock, in particular the operation of the at least one furnace, can be controlled in the decoking mode based on the light scattering data and respectively based on the solid content of the effluent gas stream.

[0059] Preferably, the effluent gas stream has a temperature in a range from 100°C to 800°C, more preferably from 170°C to 500°C, even more preferably from 180°C to 450°C, even more preferably from 190°C to 400°C, even more preferably from 200°C to 350°C, at the laser probe, so when the laser probe is in contact with the effluent gas stream. Measuring the effluent gas stream at the given temperatures ensures a working environment in which the effluent gas stream is handled above the dewpoint of the effluent gas stream, in particular above the dewpoint of water comprised in the effluent gas stream. In turn, condensed water, together with the solid present in the effluent gas stream, tends to form highly viscous and sticky mixtures, which could interfere with the optics of the laser probe. Thus, the given temperatures support the reliable function of the laser probe, in particular over changing, also drastically changing, humidity levels or water contents of the effluent gas stream, respectively, during the measurement. Preferably, the effluent gas stream has an absolute pressure in a range from 800 hPa to 1500 hPa, more preferably from 900 hPa to 1200 hPa at the laser probe, so when the laser probe is in contact with the effluent gas stream.

[0060] Preferably, the effluent gas stream has a flow rate in a range from 10.000 m3 / h to 60.000 m3 / h, more preferably from 20.000 m3 / h to 60.000 m3 / h, at the laser probe, so when the laser probe is in contact with the effluent gas stream. The effluent gas stream passes the laser probe preferably with a velocity in a range from 90 m / s to 200 m / s.

[0061] Preferably, a calibration library comprising stored light scattering data from a calibration gas stream and stored solid content data of the calibration gas stream is provided, the light scattering data of the effluent gas stream is compared with the calibration library and the solid content, in particular the current solid content, of the effluent gas stream is obtained. The calibration library is preferably an electronic database. In particular, a calibration curve according to the following formular is applied as calibration library, wherein c is the solid content, more specifically the solid concentration, in the effluent gas stream, I is the output signal, for example in mA, of the laser probe, in particular of the detector, and ko, ki and k2 are calibration coefficients: c = ko + krl + k2'l2

[0062] The calibration library is preferably built based on calibration measurements. The calibration measurements are preferably performed based on parallel measurements with a manual gravimetric reference method, preferably performed according to DIN EN 13284-1 : 2018-02. The calibration measurements can be performed for example according to DIN EN 13284-2:2018-02.

[0063] The manual gravimetric reference method preferably comprises a step of extracting a sample gas stream from the effluent gas stream for a determined period of time, wherein the flow rate of the sample gas stream is controlled and the extracted volume is measured. Preferably, the sample gas stream is extracted by means of a gas pump. The total mass of solids comprised in the sample gas stream is preferably determined gravi metrically . In a preferred embodiment, the solids comprised in the sample gas stream are separated from gaseous components of the sample gas stream, preferably by filtration. More preferably, a filter is pre-weight, used for filtration of the sample gas stream, dried and re-weighed. The filter is dried preferably for at least 1 hour at a drying temperature of at least 160°C. From the total mass of solids comprised in the sample gas stream and the total volume of the sample gas stream, the solid content of the effluent gas stream can be calculated. A sample time for the sample gas stream is preferably 20 minutes or more, more preferably 30 minutes or more. The flow rate of the sample gas stream is preferable in a range from 1 m3 / h to 10 m3 / h. A diameter of the, preferably circular, filter is for example in a range from 50 mm to 150 mm. A filter material of the filter retains preferably 99.5 wt.-% of particles with a diameter of 0.3 pm. In a preferred embodiment the filter material comprises or consists of quartz fibers. A pressure drop of the filter is preferably in a range from 3 kPa to 10 kPa for a velocity of the sample gas stream of 0.5 m / s.

[0064] Preferably, at least 5, more preferably at least 10, measurements are performed with the manual gravimetric reference method. A total sampling time over all parallel measurements performed with the manual gravimetric reference method is preferably 7.5 hours or more, more preferably a sample time is the same for all measurements. In particular, the manual gravimetric reference method is performed at the device for cracking a hydrocarbon feedstock.

[0065] Embodiments of the invention are illustrated in the figures and further described in the following.

[0066] The figures show:

[0067] Figure 1 an overview of a system comprising a device for cracking a hydrocarbon feedstock, Figure 2 a schematic representation of an exemplified concentration profile of the decoking feed stream during decoking mode and

[0068] Figure 3 an exemplified calibration curve of the laser probe.

[0069] Figure 1 shows an overview of a system 2 comprising a device for cracking 3 a hydrocarbon feedstock 4 from which an effluent gas stream 1 is emitted. The device for cracking 3 the hydrocarbon feedstock 4 comprises a furnace 5, a stack 7, a transfer line exchanger (TLE) as heat exchanger 9 and a device for solid separation 19. Further, a light scattering measurement system 11 is provided with a laser probe 13.

[0070] In a cracking mode, the hydrocarbon feedstock 4 and water in form of steam 6 are fed to the furnace 5. The furnace 5 is heated by combustion of a fuel gas 8 and a flue gas is emitted through a flue gas stack 10, which is arranged on the furnace 5.

[0071] In a decoking mode, a decoking feed stream 15, which comprises steam 6 and / or decoking air 16, respectively, is fed to the furnace 5. The decoking feed stream 15 is heated and the effluent gas stream 1 comprising a solid is obtained. The effluent gas stream 1 is led from an outlet 17 of the furnace 5 to the heat exchanger 9. From the heat exchanger 9 the effluent gas stream 1 is led through the device for solid separation 19 and then to the stack 7, where the laser probe 13 is arranged. Part of the effluent gas stream 1 is discharged through the stack 7 to the atmosphere.

[0072] The effluent gas stream 1 is contacted with the laser probe 13 and light scattering data of the effluent gas stream 1 is collected continuously online by means of the light scattering measurement system 11 . The laser probe 13 can be arranged at a first position 21 or at a second position 23 at the stack 7. The first position 21 is located upstream of the stack 7 and the second position 23 is located downstream of the stack 7. Both positions, the first position 21 and the second position 23, are located downstream of the heat exchanger 9 and the device for solid separation 19.

[0073] Figure 2 shows a schematic representation of an exemplified concentration profile of the decoking feed stream during the decoking mode. The mass ratio in the decoking feed stream on an ordinate 27 is shown over the time of the decoking mode on an abscissa 25. A first graph 29 with a solid line represents the mass ratio of water and a second graph 31 with a dotted line represents the mass ratio of air in the decoking feed stream, respectively. In this illustrative example, the decoking feed stream consists of steam at the beginning of the decoking mode and at the end of the decoking mode, the decoking feed stream consists of air. Here, the water content in the decoking feed stream is varied continuously, and partly in a linear manner, from 100 wt.-% to 0 wt.-%, referring to the total decoking feed stream.

[0074] Figure 3 is further described below in the context of the example.

[0075] Examples and comparative examples A tube cracking furnace was employed to crack a hydrocarbon feedstock in the presence of steam. The applied hydrocarbon feedstock was a gasoline fraction having a boiling point at 0.1 MPa in a range from 40°C to 180°C, also referred to as naphtha. The temperature at the furnace outlet was 850°C and the resulting product gas was cooled in a heat exchanger to a temperature of less than 450°C. After a running time of several months the feed of the hydrocarbon feedstock was stopped and a decoking feed stream comprising water, oxygen and nitrogen, respectively, was fed to the furnace and further to the heat exchanger.

[0076] Initially, a decoking feed stream comprising 1 .00 t / h of steam and 0.08 t / h of air, comprising 21 vol.-% of oxygen and 79 vol.-% of nitrogen, referring to the total air, was applied per cracking tube. The throughput of air was increased and the throughput of steam was reduced over 10 hours until a decoking feed stream comprising 70 vol.-% of air was obtained. The decoking feed stream comprising 70 vol.-% of air, referring to the total decoking feed stream, was maintained for further 6 hours. Subsequently, the throughput of steam was stopped and only air was fed to the furnace and the heat exchanger in an amount of 1 .3 t / h per cracking tube for 15 hours The temperature at the furnace outlet was 850°C, wherein the temperature at the heat exchanger was less than 400°C.

[0077] An offline measurement of the solid content, namely the dust concentration, in the effluent gas stream was performed according to DIN EN 13284-1 : 2018-02 during a decoking cycle as described above. In parallel to the offline measurements, online measurements were effectuated by means of a laser probe Dusthunter SP100, purchased from Sick. The measurements were performed at a total pressure of 1009.1 hPa and at an average temperature of 220°C over all measurements. The offline as well as the online measurements were performed at the stack. The stack was arranged at the device for cracking a hydrocarbon feedstock downstream of a cyclone, which was applied as device for solid separation.

[0078] For the offline measurement, seven samples of the effluent gas stream were withdrawn at the stack. The solid was collected in a gas filter. For the samples, where the effluent gas stream contained water, the total solid content was determined based on the sum of the solid comprised in condensed water comprised in samples and the solid collected in the gas filter after condensation. The temperature of the effluent gas stream was monitored with a thermocouple according to DIN ISO 8756:1996-10, the flow rate was determined by means of a pitot tube according to EN ISO 16911-1 :2013 and the moisture content according to DIN EN 14790:2017-05 with silica gel. The water content as well as the solid content in the effluent gas stream decreased with progress of the decoking cycle. One sample was taken at an air to steam ratio in the effluent gas stream of 0.15 kg / kg, two samples were taken at an air to steam ratio in the effluent gas stream of 0.30 kg / kg, two further samples were taken at an air to steam ratio in the effluent gas stream of 1 .00 kg / kg and two further samples were taken, when no water was comprised in the decoking feed stream.

[0079] With the offline and online measurements, a calibration curve of the form c = ko + krl was established and thus the measured light scattering data was convertible into the solid content of the effluent gas stream. Figure 3 shows the obtained calibration curve and thus a joint representation of the offline measurement data and the respective online measurement data in each of the data points 32, respectively. On an abscissa 33 the output signal of the laser probe in the effluent gas stream measured online is given in mg / m3(wet) representing scattered light (SL) data and on an ordinate 35 the solid content in the effluent gas stream measured offline, respectively, is given in mg / Nm3(wet), referring to the wet effluent gas stream. Lines 39 represent the 95% tolerance interval. Based on the given data sets the light scattering measurement system was calibrated within the 95% confidence interval 37, even over a broad range of water content in the effluent gas stream.

[0080] List of reference numerals

[0081] 1 effluent gas stream

[0082] 2 system

[0083] 3 device for cracking

[0084] 4 hydrocarbon feedstock

[0085] 5 furnace

[0086] 6 water I steam

[0087] 7 stack

[0088] 8 fuel gas

[0089] 9 heat exchanger

[0090] 10 flue gas stack

[0091] 11 light scattering measurement system

[0092] 13 laser probe

[0093] 15 decoking feed stream

[0094] 16 decoking air

[0095] 17 outlet

[0096] 19 device for solid separation

[0097] 21 first position

[0098] 23 second position

[0099] 25 abscissa in figure 2

[0100] 27 ordinate in figure 2

[0101] 29 first graph

[0102] 31 second graph

[0103] 32 data point

[0104] 33 abscissa in figure 3

[0105] 35 ordinate in figure 3

[0106] 37 confidence interval

[0107] 39 tolerance interval

Claims

Claims1. Method for determination of a solid content in an effluent gas stream (1) from a device for cracking (3) a hydrocarbon feedstock (4), wherein the method comprises the following steps: a. providing a device for cracking (3) a hydrocarbon feedstock (4), which comprises one or more furnaces (5), one or more heat exchangers (9), in particular transfer line exchangers (TLE), and one or more stacks (7), and providing a light scattering measurement system (11) comprising a laser probe (13), b. operating the device for cracking (3) a hydrocarbon feedstock (4) at least partly in a decoking mode, wherein a decoking feed stream (15) comprising oxygen, nitrogen and / or water is fed to at least one of the one or more furnaces (5), the decoking feed stream (15) is heated to a decoking temperature in a range from 500°C to 1500°C, in particular from 700°C to 1000°C, and an effluent gas stream (1) comprising carbon monoxide and optionally a solid, carbon dioxide, nitrogen and / or water is obtained, c. discharging the effluent gas stream (1) through the one or more heat exchangers (9) and the one or more stacks (7) and d. contacting the laser probe (13) with the effluent gas stream (1), wherein light scattering data of the effluent gas stream (1) is collected and the solid content of the effluent gas stream (1) is derived from the light scattering data.

2. Method according to claim 1, wherein a calibration library comprising stored light scattering data from a calibration gas stream and stored solid content data of the calibration gas stream is provided, the light scattering data of the effluent gas stream (1) is compared with the calibration library and the solid content of the effluent gas stream (1) is obtained.

3. Method according to claim 1 or 2, wherein at least one of the one or more furnaces (5) is operated alternately in a cracking mode and in the decoking mode.

4. Method according to any of claims 1 to 3, wherein the decoking feed stream (15) comprises water, oxygen and nitrogen.

5. Method according to any of claims 1 to 4, wherein the effluent gas stream (1) comprises the solid in a concentration in a range from 0.01 to 2000.00 mg / m3, preferably from 0.10 to 1000.00 mg / m3, referring to the total effluent gas stream (1).

6. Method according to any of claims 1 to 5, wherein the laser probe (13) is operated at a wavelength in a range from 600 nm to 700 nm, in particular from 640 nm to 660 nm.

7. Method according to any of claims 1 to 6, wherein the effluent gas stream (1) has a temperature in a range from 100°C to 800°C, preferably from 170°C to 500°C, more preferably from 200°C to 350°C, at the laser probe (13).

8. Method according to any of claims 1 to 7, wherein the decoking feed stream (15) comprises less than 20 wt.-% of hydrocarbons, referring to the total decoking feed stream (15).

9. Method according to any of claims 1 to 8, wherein the effluent gas stream (1) comprises water and during data collection in step d., the water content in the decoking feed stream (15) varies over the whole range from 80 wt.- % to 20 wt.-%, preferably from 100 wt.-% to 1 wt.-%, referring to the total decoking feed stream (15).

10. Method according to any of claims 1 to 9, wherein all water comprised in the effluent gas stream (1) is present in gaseous form at the laser probe (13).

11. Method according to any of claims 1 to 10, wherein the light scattering data is collected continuously and in-situ in the effluent gas stream (1).

12. Method according to any of claims 1 to 11, wherein the light scattering data is collected by a dynamic light scattering measurement.

13. Method according to any of claims 1 to 12, wherein the effluent gas stream (1) is led from an outlet (17) of the at least one furnace (5) to a heat exchanger of the one or more heat exchangers (9), in particular a transfer line exchanger of the one or more transfer line exchangers, and then to a stack of the one or more stacks (7), and the laser probe (13) is arranged at the stack (7).

14. Method according to any of claims 1 to 13, wherein the device for cracking (3) a hydrocarbon feedstock (4) comprises a device for solid separation (19) and the effluent gas stream (1) is led from the heat exchanger (9), in particular the transfer line exchanger, through the device for solid separation (19) and then to a stack of the one or more stacks (7) and the laser probe (13) is arranged at the stack (7).

15. System (2) for carrying out the method according to any of claims 1 to 14, wherein the system (2) comprises a device for cracking (3) a hydrocarbon feedstock (4), which comprises one or more furnaces (5), one or more heat exchangers (9), in particular one or more transfer line exchangers (TLE); one or more stacks (7) and at least one light scattering measurement system (11) comprising a laser probe (13), wherein the laser probe (13) is preferably arranged at a stack of the one or more stacks (7).

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