Classifications

• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
  • C10G9/00—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils
  • C10G9/14—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils in pipes or coils with or without auxiliary means, e.g. digesters, soaking drums, expansion means
  • C10G9/16—Preventing or removing incrustation
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S585/00—Chemistry of hydrocarbon compounds
  • Y10S585/949—Miscellaneous considerations
  • Y10S585/95—Prevention or removal of corrosion or solid deposits

Definitions

• the invention relates to a process for steam cracking of flexible hydrocarbons, that is to say compatible with a wide variety of fillers to be cracked.
• the steam cracking process is the basic process of the petrochemical industry and consists of cracking at high temperature and then brutally cooling a load of hydrocarbons and water vapor.
• the main operational problem results from the deposition of carbonaceous products on the internal walls of the installation. These deposits, consisting of coke or heavy tars of condensed pyrolysis and more or less agglomerated, limit the heat transfer in the cracking zone (coil with pyrolysis tubes) and the indirect quenching zone (effluent quench exchanger), requiring frequent stops to decoker the installation.
• the conventional cycle times (operation between two complete chemical decokings of the cracking zone, in air and / or steam) are either fixed (programmed stops), or variable depending on the coking of the installation, and s typically range from 3 weeks to 12 weeks for fillers such as naphtha and liquefied petroleum gases.
• the erosion of the tubes can be maintained at a very low or zero level, and controlled by the analysis of traces of metals (iron, chromium, nickel) in the recovered powders.
• the efficiency of decoking has been found to depend significantly on the charges and operating conditions (different nature of the coke).
• the light charges: C 3 , C 4 , light Naphtha produce at the start of the reaction zone a catalytic coke much more fragile (5 to 10 times) than the asymptotic coke predominant in the middle and at the end. of the reaction zone. It is therefore desirable for these charges to limit the speed of circulation in this area, to maintain a protective coke layer and / or avoid the risk of erosion of the cracking tubes.
• the geometry of the cracking reactor adapted to a given load, with respect to the prevention of the risks of erosion, is not the same as that adapted to another load having a dilution rate and a nature different coke (for which the appropriate traffic speed profile will be different).
• the applicants therefore propose a new process, flexible steam cracking, compatible with existing steam cracking installations, making it possible to treat various loads according to variable operating conditions, without degrading the thermal balance of the installations, without significant risks of erosion, and with a moderate investment cost.
• a process for steam cracking of hydrocarbon feedstocks in a steam cracking installation comprising at least one steam cracking oven which comprises at least one cracking zone (2) with pyrolysis tubes, connected by a zone of transfer (3) to means (4) for indirect quenching of the effluents from this cracking zone (2), for example a quench exchanger (TLE), and downstream means (6) for treating these cooled effluents, the process comprising the injection of erosive solid particles upstream of the indirect quenching means (4) to eliminate at least part of the carbon deposits located on the internal walls of the installation, the cracking zone remaining in communication with the downstream means (6) during the particle injection phases, the method being characterized in that: a - In the course of a steam cracking cycle, solid erosive particles with an average diameter of between 0.02 and 4 mm are injected, at least ins a point in the transfer zone (3), these particles then circulating in the indirect quenching means, conveyed by a carrier gas
• the overall average rate, [Q + q], of particles injected being determined to limit the increase in the outlet temperature of the indirect quenching means (4) to a value less than 100 ° C per month, and preferably less at 50 ° C per month.
• a steam cracking cycle is generally defined as a period of operation of an oven (or of an area of an oven) between two consecutive long stops for decoking. Between these long-term stops, the oven operates under normal steam cracking conditions.
• a steam cracking cycle (of an area of an oven, or of an entire oven) will be considered as a period of operation, between two consecutive stops of the cracking operation of long duration, by definition of a duration greater than two hours, during which the oven (or part of the oven) remains connected to the downstream sections for processing cracked gases.
• the oven (or the oven area) therefore operates under the conditions of steam cracking, with possibly a few short periods (less than two hours, and generally less than 0.5 hours) during which steam is supplied. water only, without disconnecting the oven from the downstream sections.
• the average rate Q of particles injected into the transfer zone is by definition equal to:
• the cracked gases correspond to the hydrocarbons plus the dilution water vapor.
• the process according to the invention provides for a quantity of particles Q + q suitable for controlling the fouling of the TLE, the quantity q being very insufficient to carry out all of the erosive decoking of the pyrolysis tubes.
• This decoking is therefore carried out mainly by means of chemical gasification.
• a preponderant erosive decoking of the TLE tubes is therefore associated with a predominant chemical decoking (gasification) of the pyrolysis tubes, with excellent reliability of the overall process: the primary advantage of this new process is to ensure the flexibility of the charges without this will subject the installation to the risk of erosion.
• This process is compatible with existing naphtha steam cracking installations, and is superior in energy terms to the conventional process for cracking heavy loads, on special ovens with direct quenching, which does not produce high pressure steam.
• the quantities of particles circulating in the pyrolysis tubes will generally be limited to a level such that the entire pyrolysis bundle can be kept without modification; the installation according to the method will thus be much less costly than that of the prior method carrying out the erosive decoking of the pyrolysis tubes which required the reinforcement and therefore the replacement of all the elbows.
• the "erosive decoking" part of the process works to assist in limiting coking at the bottleneck, that is to say indirect quenching means, which are in an area with low circulation speed and relatively cold, where the metal is typically found below 400 ° C, which considerably limits the risks of erosion. Furthermore, the control of the process and of the quantities of particles injected is considerably facilitated since it is possible to reliably know the outlet temperature of the quench exchanger, which provides a precise indication of its degree of fouling. This was not the case for the skin temperatures of the pyrolysis tubes which are more difficult to measure, and influenced by the conditions of the flexible operation imposing in fact a "blind" decoking, for the process with complete erosive decoking of the pyrolysis tubes. under flexible conditions.
• This process therefore makes it possible to treat heavy loads in a steam cracking installation designed to crack naphtha, and to be able to frequently change the loads to be cracked according to the "spot" prices of these loads, also to vary the severities, without taking risks. for installation.
• the particle injections can be determined so that the increase in the outlet temperature T of the indirect quenching means is less than 50 ° C per month, for example between 5 and 50 ° C per month , in particular between 10 and 40 ° C per month, and preferably less than 30 ° C per month during a steam cracking cycle.
• a sufficient total quantity of particles is injected, that is to say an overall average rate [Q + q] sufficient to substantially stabilize the outlet temperature of the indirect quenching means (TLE ) during a steam cracking cycle.
• a limited quantity of particles can however be usefully injected into the pyrolysis tubes, more particularly at the start of a steam cracking cycle, to remove a significant part of the filamentary catalytic coke which forms at the start of the cycle, and which is much more fragile.
• the erosive efficiency strongly depends on the speed of circulation of the particles in the exchanger variable according to the types of exchangers and the installations, and on the other hand, the quantity of coke deposited, and its brittleness, strongly depend on the charges used, possible impurities (for example, traces of asphaltenes, or heavy aromatics such as ovalene, coronene for certain hydrotreated distillates), as well as operating conditions (severity of cracking, dilution).
• This imprecision on the quantities of particles required is not a problem since the measurement of the outlet temperature of the quenching means, even under flexible conditions, allows reliable control of the process.
• the particles can be injected continuously, but the flow rates are then very low, which makes controlling the flow delicate.
• the erosive particles are injected sequentially, at fixed or variable intervals of between 0.3 and 72 hours and preferably between 1 and 20 hours (for each of the quench exchangers).
• the injections can be made at regular intervals, for example by modulating the quantity of particles injected to obtain the desired effect of controlling the fouling of the quench exchanger. It is also possible to inject the particles (for example a constant quantity) when the temperature of the indirect quenching means (TLE exchangers ”) exceeds a predetermined value.
• TLE exchangers indirect quenching means
• the instantaneous rate of solid particles during an injection (typically carried out discontinuously during normal steam cracking operation) will be much higher, typically between 0.5 and 20% by weight, and preferably 1 and 10% weight compared to cracked gases.
• the particles which can be used for the process according to the invention mainly comprise two categories of solid particles: - According to a first variant of the process, use is made of substantially non-porous mineral solid particles, consisting of silicon carbide, or of simple or mixed oxides of silicon, aluminum and zirconium.
• These particles are very resistant to attrition and, if they comprise at least a fraction of angular particles, are very effective in removing coke. If these mineral particles are used, it is necessary to recover them, for example in a cyclone, downstream of the quench exchanger, so that they do not pollute the downstream sections for processing the cracked gases and the oil. pyrolysis, generally sold as fuel.
• coke particles can be used: petroleum coke, obtained according to the fluid or chamber process, metallurgical coke, or calcined anthracite, ground to the desired particle size.
• particles comprising at least 20% by weight of angular particles will be used, for example a mixture of two different types of particles.
• the major part of the injected coke particles have undergone, generally before the possible final grinding to the desired particle size, during their manufacturing process, a temperature at least equal to 850 ° C. (by example calcination at a temperature greater than or equal to 850 ° C.). These particles stabilized at 850 ° C are much less likely to burst when they are introduced into the cracked gases at this temperature. Particle injections are generally carried out during the operation of the installation, under normal steam cracking conditions; the gas carrying the particles in the tubes of the quenching exchanger is then the stream of cracked gases.
• the charge of the furnace is modified during the injection phase of the particles, by replacing it with a lighter charge belonging to the group formed by steam, hydrocarbons with a boiling point below 250 ° C and their mixtures.
• This modification of the charge makes it possible to transport the particles by a carrier gas composed of water vapor alone, or of cracked gases of charges such as naphtha, and to avoid potential condensations of heavy tars.
• This variant of the process can be used each time the pollution of the recovered particles proves to be troublesome.
• this modification will be carried out for short durations: For example, it will be possible to carry out a circulation of water vapor alone for a duration less than two hours, and preferably less than 1 hour and particularly 0.3 hours, this duration including the period in which the particles are injected.
• Such very brief interruptions in steam cracking, the oven remaining connected to the downstream sections, and swept by steam, do not greatly disturb the production of an installation comprising numerous ovens.
• these very brief interruptions do not correspond, according to the definition chosen, to a new steam cracking cycle, and that a steam cracking cycle corresponds to a continuous or substantially continuous operation of the cracking of hydrocarbons, which may include interruptions. very brief cracking, lasting less than 2 hours.
• the carrier gas carrying the particles is therefore either a mixture of hydrocarbons and water vapor (general case), or water vapor alone.
• These non-recovered coke particles have an action of eliminating residual deposits in the lines downstream of the quench exchanger.
• this operation can be modified at the time of particle injections, by increasing by 10 to 50% the speed of circulation of the cracked gases; this can be achieved by momentary increase in the flow of hydrocarbons and water vapor or in the single flow of water vapor.
• the advantage of this arrangement is important because it makes it possible to increase the speed and therefore the erosive effect of the particles, and consequently to reduce the quantity of particles injected; this is particularly useful for unrecovered coke particles, which are trapped downstream in the pyrolysis fuel.
• the particles injected into the transfer zone can be introduced at one or more points where the circulation speed is reduced by at least 25% compared to the circulation speed in the terminal part of the cracking zone .
• the most suitable particle introduction points are generally located on the inlet cone of the quench exchanger. This inlet cone, by definition, is part of the transfer zone, and not of the indirect quenching means, these corresponding to the exchanger itself, that is to say in fact to the circulation tubes of cracked gases, which carry out indirect quenching.
• the quantities of particles circulating in the cracking zone (2) being, according to the invention, insufficient to remove most of the coke formed in this zone, the invention provides for a preponderant chemical decoking, at relatively short intervals, or even continuous .
• This chemical decoking can be carried out according to several variants which have in common the establishment of conditions for accelerated chemical gasification of coke, these conditions being accelerated compared to the normal conditions of steam cracking, where water vapor has a limited action of gasification of coke, in particular by the reaction of gas with water.
• a first variant consists in accelerating the gasification by combustion of the coke by circulation of air or air / water vapor mixtures; this variant is the conventional "air decoking" process, the furnace being disconnected from downstream, and the supply of hydrocarbons interrupted.
• a second known variant consists in interrupting the supply of the hydrocarbon feedstock and in gasifying the coke by circulation of steam alone, or of steam / hydrogen mixtures.
• This "steam decoking" can be carried out either by leaving the oven connected with the downstream, or by disconnecting it, so as not to mix notable quantities of carbon monoxide CO with the cracked gases.
• the active compounds typically contain one or more inorganic salts of elements from the alkali and alkaline earth group, for example a salt of an element from the potassium, sodium, lithium, barium and strontium group.
• Active mineral salts are, for example, precursors of oxides of the elements considered, in particular carbonates, or carbonate precursors such as acetates.
• salt compositions whose melting point is less than 750 ° C., to promote their transfer to the walls of the pyrolysis tubes.
• Compositions similar to eutectic for example an equimolar composition of potassium carbonate and sodium carbonate, is well suited. If we want to inject incompatible compounds with each other at the level of their storage, we can use several flows and several storage.
• These compounds can also be injected upstream of the cracking zone during “steam decoking” phases, previously described, to accelerate gasification (and only during these phases, if there is fear of corrosion in the case of a permanent injection).
• anti-smoking chemical compounds for example dimethyldisulfide and / or phosphorous compounds, in particular phosphates or phosphites, or other compounds which may have an anti-smoking action (neutralization of radicals and / or promotion of the gasification of coke), or reduction of the formation of CO, or an anticorrosion action.
• the effluent of the means can be subdivided indirect quenching during these gasification phases with water vapor, in a minor part which joins the downstream means, and a major part which is withdrawn from the steam cracking effluent circuit.
• the invention also provides a steam cracking installation, for carrying out the method according to the invention, comprising at least one steam cracking oven comprising at least one cracking zone with pyrolysis tubes connected downstream by a transfer line at least to one indirect quenching exchanger for effluents including the inlet cone is part of the transfer line), and downstream treatment means for the effluents connected to said exchanger, characterized in that it comprises:
• the invention also provides an installation such that the particles are introduced into the inlet cone of the quench exchanger, at at least one point, the point or points of introduction being located on the inlet cone, such so that the local passage section of the cracked gases is at least 25% greater than the passage section of the initial part of the transfer zone, which reduces the risks of erosion of the tube plate of the exchanger and particle attrition.
• the installation comprises means for metering and discontinuous injection of coke particles with an average diameter of between 0.07 and 4 mm, which have good erosive efficiency and can be separated easily, connected to the pipe. transfer to introduce all the coke particles injected upstream of the quench exchanger.
• An installation according to the invention advantageously uses, in the cracking zone, pyrolysis tubes connected together by elbows which for the most part at least are conventional non-reinforced elbows, which eliminates a very high installation cost.
• the invention also provides, according to a characteristic variant, an installation which comprises metering and injection means upstream of the cracking zone of chemical compounds gasification catalysts comprising at least one active compound from the group of mineral salts of a element of the group of sodium, potassium, lithium, barium and strontium. These compounds greatly increase the duration of the steam cracking cycle.
• the installation comprises a device for simplified recovery of the coke particles (particles of average diameter between 0.07 and 4 mm introduced into the transfer zone); this device can be installed on at least one line for discharging the cooled steam cracking effluents, comprising at least one oven outlet valve, the device being arranged between the outlet of the indirect quenching means such as a quench exchanger and the valve oven outlet.
• this device for simplified recovery of the coke particles (particles of average diameter between 0.07 and 4 mm introduced into the transfer zone); this device can be installed on at least one line for discharging the cooled steam cracking effluents, comprising at least one oven outlet valve, the device being arranged between the outlet of the indirect quenching means such as a quench exchanger and the valve oven outlet.
• the evacuation line comprises, according to this device, an abrupt change of direction, of the type with simple deflection of an angle comprised between 30 and 180 °, for the evacuation of the greater part at least of the steam cracking effluents, a particle recovery chamber situated at the level of the abrupt change or downstream, connected by a narrowing to a reservoir for receiving the recovered coke particles, and means for maintaining this reservoir in an atmosphere that cannot be condensed under the conditions of the reservoir.
• This device takes advantage of the inertia of the particles to separate them from the gas, for at least part, due to the sudden change of direction.
• This device is much more economical than cyclone type devices, where the flow follows a helical path.
• the installation also includes means for implementing a circulation of non-recovered coke particles, towards the downstream means.
• it may include means for discontinuous introduction of a gas stream, simultaneously with at least some injections of coke particles, to disturb the operation of the gas / solid separation means, and cause the circulation of a portion at least particles of coke injected, towards the downstream means.
• This device is much simpler than an injection of coke particles downstream of the separation means, because it only implements an additional gas introduction, and not additional means for introducing particles.
• This new method according to the invention is very superior to the previous method both from the point of view of the reliability and the elimination of the risks of erosion under flexible conditions, as from the point of view of the investment cost.
• FIG. 1 schematically represents a steam cracking installation according to the invention, comprising several devices relating to different characteristic variants according to the invention.
• FIG. 2 schematically represents two embodiments (FIGS. 2A and 2B) of a part of a steam cracking installation according to one of the characteristic variants of the invention.
• FIG. 1 where there is shown a steam cracking oven (20), delimited by its enclosure, comprising a preheating zone (1), convection, a cracking zone (2) with tubes pyrolysis, located in the radiation zone of the furnace, a transfer zone (3) comprising on the one hand a transfer line located just at the exit of the cracking zone and on the other hand the inlet cone of a quenching exchanger (TLE), the cracked gas circulation tubes in this exchanger constituting means (4) for indirect quenching of the steam cracking effluents coming from the zone (2), through the transfer zone (3).
• a steam cracking oven delimited by its enclosure, comprising a preheating zone (1), convection, a cracking zone (2) with tubes pyrolysis, located in the radiation zone of the furnace, a transfer zone (3) comprising on the one hand a transfer line located just at the exit of the cracking zone and on the other hand the inlet cone of a quenching exchanger (TLE), the cracked gas circulation tubes in this exchanger constituting means (4)
• the effluents from the quench exchanger are conveyed by a line (10) to downstream means (6) for processing the cooled effluents, well known to those skilled in the art, which comprise, for example, direct quenching means, primary fractionation, compression, drying, desulfurization, refrigeration and final fractionation of the constituents of the cracked gases, to typically produce ethylene, propylene, a C 4 cut, a petrol fraction and an oil fraction of pyrolysis.
• the line (10) for discharging the cooled effluents also includes an oven outlet valve (VF) allowing its isolation from the downstream means (6) and passes through a gas / solids separator (S) for the recovery of particles.
• the particles recovered in the separator (S) fall into a receiving tank (12), via a conduit, forming a narrowing, which includes an isolation valve (13).
• Means (21) such as supplying a limited flow of barrier gas (water vapor, nitrogen or fuel gas) keep the receiving tank (12) under an incondensable atmosphere under the conditions of the tank.
• a decoking line (19) is also connected to this conduit, and comprises a valve (VDK) called decoking valve. This line is used during air decoking phases, or in air / steam mixtures, for the evacuation of the coke combustion gases to generally a "decoking pit", not referenced here.
• the particles contained in the reservoir 12 are evacuated and eliminated or recycled by a line 30 to injection means 7.
• a line connected to line 10 and comprising a valve (14), makes it possible, if necessary, to subtract a major part of the gas rich in CO, during particular phases of decoking, with steam alone or by steam mixtures d water / hydrogen, in order to reduce the average CO content of the cracked gases in the downstream treatment means (6), when this particular decoking arrangement is used, according to one of the characteristic variants of the invention.
• the line (10) also includes means (16) for measuring the temperature of the effluents from the quench exchanger, allowing the process according to the invention to be controlled. These means 16 can optionally be connected to the means 7 for metering and injecting solid particles.
• a line (25) allow the supply of a greater flow of gas to disturb the operation of the separation means (S) and allow coke particles to flow towards the downstream means (6), according to a variant of the method for decoking the line (10) and the lines downstream of the line (10).
• the installation therefore also includes means (7) for metering and injecting solid particles, which can be introduced:
• At least 70% by weight of the particles are introduced into the transfer zone (3). It would be possible, without departing from the scope of the invention, to introduce these particles at the limit of the zone (3) at the crossing of the enclosure of the zone. radiation from the oven, or even a few tens of cm upstream of this crossing; this has no advantages, however.
• the particles are injected into the inlet cone of this exchanger, at a level such that the local passage section of the cracked gases is greater by at least 25%, for example from 40% to 400%, relative to the cross section of these gases in the initial part of the transfer zone (3).
• This limitation of the gas speed at the points of introduction of the particles is very beneficial, since it greatly reduces the risks of erosion of the tube plate of the exchanger.
• This tubular plate may also particularly advantageously be protected by an impactor, not shown, located in the inlet cone of the TLE, just downstream of the points of introduction of the particles, for example by a substantially opaque impactor, or opaque to the less than 70% seen from the arrival of gases in this inlet cone.
• a gas permeable impactor consisting of several baffles, or rows of surfaces offset from one another will both protect the tubular plate of the TLE, and improve the distribution of the particles injected, in the different tubes of this exchanger.
• Such an impactor will be shown diagrammatically in FIG. 2.
• the particles are transported from the means (7) to their points of introduction, by pneumatic transport by means of a carrier gas, for example steam, or fuel gas, or nitrogen.
• a carrier gas for example steam, or fuel gas, or nitrogen.
• the installation comprises means (15) for metering and injecting chemical compounds catalyzing the gasification of coke by water vapor.
• dilute aqueous solutions of active mineral salts may be used, for example dilute aqueous solutions of sodium carbonate and potassium carbonate, in particular of compositions close to eutectic such as a composition at 50 mol% of these two carbonates.
• acetates of active compounds from the alkali and alkaline-earth group for example an equimolar composition of sodium acetate, potassium acetate, lithium acetate and barium acetate.
• the installation described in FIG. 1 also provides for injecting by means (15), as a mixture or separately, other types of anti-coking chemical compounds, in particular compounds making it possible to reduce the CO content in the cracked gases, or having anti-coking activities (for example of neutralization of radicals, with or without catalysis of gasification by steam).
• other types of anti-coking chemical compounds in particular compounds making it possible to reduce the CO content in the cracked gases, or having anti-coking activities (for example of neutralization of radicals, with or without catalysis of gasification by steam).
• DMDS dimethyldisulfide
• soluble phosphorus compounds in an appropriate solvent such as water, hydrocarbons, hydrocarbon / alcohol, for example benzyldiethylphosphite, whose activity is established. It is also possible, without limitation, to use active phosphorus compounds, from the group of organic compounds (triethylphosphite, triphenylphosphite,
• FIG. 1 also includes other means making it possible to establish conditions for accelerated gasification of the coke in the cracking zone (2): these means include introduction means (for example the valve (18) for decoking air (AIR), and means for interrupting the supply of hydrocarbon (for example the valve (17)), allowing the circulation of decoking water vapor alone (optionally added with hydrogen by means not shown).
• introduction means for example the valve (18) for decoking air (AIR)
• means for interrupting the supply of hydrocarbon for example the valve (17)
• the installation includes means for introducing a hydrocarbon charge (HC), and means for introducing dilution water vapor (H2O) in the cracked area. It also includes means making it possible to increase the volume flow rate of cracked gases in the quench exchanger from 10% to 50%, at the time of the particle injections, for example means (24) for supplying steam additional. One could also increase during the injections, the flow of hydrocarbons. This increase in the volume flow of the particles increases the speed, therefore the erosive effect, which makes it possible to reduce the quantities injected. This is particularly useful when injecting unrecovered and / or non-recycled coke. It is also possible to seal 4 to 30% of the cracked gas circulation tubes of the quench exchanger in order to increase the circulation speed and the erosive efficiency.
• HC hydrocarbon charge
• H2O dilution water vapor
• FIG. 2A represents a quenching exchanger with cracked gas cooling tubes (4), with its inlet cone into which we introduce during a steam cracking cycle an average quantity, Q, of solid particles coming from means (7) for metering, transporting and injecting solid particles.
• the impactor (23) disposed just downstream of the points of introduction of the particles is constituted by two levels of impaction surfaces, offset, so that it is both gas permeable and at least 70% opaque , and preferably substantially 100%, seen from the inlet pipe of the cracked gases.
• This impactor provides very effective additional protection of the tube plate against erosion, and also distributes the particles more evenly in the different tubes of the exchanger.
• the cracked gases are conveyed by the line (10), comprising means (16) for measuring the temperature of the effluents from the quench exchanger.
• These means (16) effectively provide information on the degree of fouling of the exchanger and allow process control, by modulating the quantities of particles injected or the injection frequencies, so that the increase in the temperature of outlet of the exchanger does not exceed 100 ° C per month, and preferably 50 ° C per month.
• this temperature drift will be limited below 30 ° C. per month, or the quantities of particles adapted to be injected so that the outlet temperature of the exchanger remains substantially constant.
• the cooled steam cracking effluents discharged by line 10 pass through the chamber
• the recovered particles fall into the receiving tank (12), through a narrowing comprising a valve (13); means (21) make it possible to inject an inert gas (more precisely a very low condensation point), that is to say a gas from the group of water vapor, fuel gas, nitrogen, or condensing temperature less than or equal to 100 ° C, at atmospheric pressure. Thanks to the narrowing in accordance with a characteristic of the invention, the inert gas acts as a barrier, going up into the chamber (11), which makes it possible to keep the particles recovered in the tank (12) without condensation, by keeping this tank at a sufficient temperature.
• an inert gas more precisely a very low condensation point
• the decoking line (19), comprising the decoking valve (VDK), which is used during air decoking phases of the cracking zone (2) is connected directly or indirectly to the line (10), upstream of the oven outlet valve (VF) to allow the evacuation of air decoking effluents.
• the assembly formed by the chamber (11), the reservoir (12) and their narrowing connection conduit is disposed substantially at the connection between the evacuation line (10) and the decoking line (19).
• Figure 2-B represents the same part of the installation but with a variant in the simplified solid particle recovery device: the recovery chamber (11) is not crossed by the flow of cracked gases flowing in the line (10), but is located immediately after the sudden change of direction (for example at a distance not exceeding 1.5 m, and preferably less at 0.8 m) The solid particles conveyed by the flow tend to continue straight, without making a sudden change of direction, so that they are collected in the chamber (11) and recovered in the reservoir (12).
• the recovery chamber (11) is not crossed by the flow of cracked gases flowing in the line (10), but is located immediately after the sudden change of direction (for example at a distance not exceeding 1.5 m, and preferably less at 0.8 m)
• the solid particles conveyed by the flow tend to continue straight, without making a sudden change of direction, so that they are collected in the chamber (11) and recovered in the reservoir (12).
• FIGS. 2-A and 2-B are much more economical than a conventional cyclone recovery.
• the conventional cyclone recovery mode with very high recovery efficiency, will be preferred when injecting mineral particles (therefore non-combustible), which can cause significant pollution of the downstream processing means (6) and in particular of pyrolysis; in such a case, almost complete recovery of the particles is desirable.
• the simplified recovery systems such as those presented in FIGS. 2-A and 2-B are perfectly suited to the characteristic variants of the process using coke particles. Indeed, it is considerably less annoying to pollute the pyrolysis fuel with coke particles, which are combustible. It should also be emphasized that the efficiency of these simplified recovery devices is closely linked to the characteristics of the process according to the invention, and in particular to the preponderant injection (70% minimum) or total injection into the transfer line.
• the particles injected into zone (3), and particularly at a point on the cone at a slow speed of circulation of the cracked gases, are both more effective for decoking, and appreciable recoverable, for example at 60% by simplified means, particularly for coke particles of appreciable diameter, between 0.07 and 4 mm provided for this characteristic variant of the invention.
• the interpretation of these results can be as follows: the coke particles introduced at the entrance to zone (2) impact, crossing this zone, with a large number of elbows, at very high speed (120 to 200 m / s typically). These particles then burst into a dust, not very effective for the erosion of the exchanger and very difficult to separate from the gas.
• the invention is therefore characterized by steam cracking jointly implementing decoking essentially by erosion of the quench exchangers (TLE) and essentially chemical decoking of the tubes of the cracking zone, with simple and reliable means for controlling the process, and economic means of implementation.
• TLE quench exchangers
• the method according to the invention which provides for injecting at least 70% by weight of the particles into the transfer zone, these particles then circulating at reduced speed (relative to the speed in the pyrolysis tubes), with wall temperatures low and in substantially straight tubes without bends, no longer poses a significant risk of erosion.
• solid particles which can be stored in a large capacity tank, of "new" particles, or in a tank in particular of smaller capacity containing a dose of particles which have already circulated in a part of the installation, are dosed, for example by weighing, and sent discontinuously, sequentially to different parts of the installation (for example sequentially in the inlet cones of the different quench exchangers).
• the powders are injected in doses, discontinuously.
• a dose may typically comprise from 2 to 300 kg of particles, and preferably from 5 to 100 kg of particles.
• the value of Q (Quantity of particles injected into the transfer zone, compared to the cracked gases during a steam cracking cycle) is 300 ppm: as an average value 3 Kg / h of particles per 10,000 Kg / h of cracked gas.
• the value of q is 12 ppm (0.12 Kg / h of particles, as an average value over the entire steam cracking cycle, for 10,000 Kg / h of cracked gas).
• This example is therefore in accordance with the invention for the injection of the particles, Q representing 96% of [Q + q].
• this condition is not necessarily sufficient: the operator must in fact, depending on the load treated, adapt the quantities of particles so that the fouling of the quench exchanger remains moderate (more precisely than the increase in effluent temperature is less than 100 ° C per month, and preferably 30 ° C per month, or even is zero).
• the operator will monitor the outlet temperature of the exchanger, by observing the temperature indicator (16), and may have to modify the quantities of particles injected, in particular Q. It could for example increase Q by injecting doses of particles of more than 30 kg, and / or by increasing the frequency of the injections, or on the contrary decrease Q if the value used appears excess. This observation can typically be made once a day, for a known load, and at shorter intervals during each change of operating conditions.
• Another possible operation consists in injecting a dose as soon as the temperature reaches a predetermined value (for example 430 ° C, if the admissible limit temperature is 450 ° C). These operations can be carried out by the operator, manually or automated.
• the powder doses can be conveyed by pneumatic transfer, using a gas carrying a boiling temperature not exceeding 100 ° C at atmospheric pressure, typically water vapor, fuel gas (methane or methane / hydrogen) or nitrogen, in the diluted or dense phase according to known techniques.
• a gas carrying a boiling temperature not exceeding 100 ° C at atmospheric pressure typically water vapor, fuel gas (methane or methane / hydrogen) or nitrogen, in the diluted or dense phase according to known techniques.
• the operator may, at the time of particle injections, temporarily increase the flow of cracked gases, for example from 10 to 50% volume, in order to increase the speed and erosive efficiency of the particles.
• the operator may, at the time of particle injections, interrupt the supply of hydrocarbons, to inject the particles into a current water vapor only, with a possibly modified flow rate.
• a circulation of coke particles can be implemented (from 5 to 200 ppm and preferably from 10 to 100 ppm relative to to cracked gases) in the downstream part of line 10 which joins the downstream means (6).
• these coke particles can be introduced into the transfer zone (3) and pass, at least for a substantial fraction of the separation means without being collected, using a stream of gas disturbing the operation of the separation means and limiting the effectiveness of these means. This gas flow disturbing the operation of the separation means (S, 11) is introduced by the line (25) and prevents the recovery of at least part of the coke particles injected into the transfer zone (3).
• the operator can carry out this evacuation without stopping the steam cracking cycle, by closing the valve (13), by opening the valve (22), possibly with additional means not shown (pneumatic, mechanical such as a screw, or lock), or simply by gravity discharge.
• the particles recovered will be transferred to various points of the installation, for storing and / or reprocessing them before recycling them.
• a second mode of decoking consists in decoking with water vapor alone, known to those skilled in the art, this vapor possibly being able to be supplemented with hydrogen. According to a characteristic variant of the invention, it is possible to subtract a major part of the decoking water vapor stream, charged with CO and CO2, from the network for collecting cracked gases towards the means (6), in order to reduce the content of CO and CO2 in these collected cracked gases.
• the process with erosion coke removal at the cracking zone and the quench exchanger is used to achieve substantially continuous operation.
• the implementation of this process requires the change of all the elbows of the cracking zone of each of the ovens, and their replacement by special reinforced elbows (modified geometry, increased thickness, possibly change of materials).
• a layer of coke is allowed to form, for example by steam cracking for 48 hours, under naphtha charge, then by injecting amounts of erosive particles with an average diameter of less than 150 micrometers, at the entrance to the zone. (2) in an amount sufficient to remove at least most of the coke formed.
• This quantity can be modulated, for example raised to 5500 ppm when heavy loads are cracked (diesel, vacuum distillate), and also modulated from the indications provided by the pyrometers measuring the skin temperatures of the tubes.
• the particles are recovered by cyclones at the outlet of the TLE, sieved to remove the coarse coke fragments which may be present, and at least in part, recycled, possibly after the elimination of the very fine particles.
• the cracking zone is chemically decoked, as soon as the skin temperatures reach their limit value, by an air / water vapor mixture, or water vapor alone.
• the cycle times that are established are typically: 50 to 70 days on naphtha charge, 40 to 60 days on diesel charge, and 25 to 45 days on heavy diesel charge or vacuum distillate of medium quality.
• This installation does not carry out a substantially continuous steam cracking, but makes it possible to operate with great security with regard to technological problems (erosion), under conditions of flexibility of the loads.
• the equipment for equipping an existing oven is much more economical than in Example 2, where changing the elbows involves an additional cost greater than 30%.
• Example 3 With additional means for injecting chemical compounds: • 80 ppm compared to the cracked gases of chemical compounds (percentage by weight of alkaline elements K + Na), diluted to form an aqueous solution with 96% water of equimolar composition of potassium carbonate and sodium carbonate, atomized in the charge at the outlet of convection (HC + H2O at 500 ° C) • 80 ppm of benzyldiethylsulfite added to the hydrocarbon charge, by in relation to this charge.
• This installation according to the invention, makes it possible to operate in flexible mode, with cycle times generally exceeding 60 days for the loads considered.
• the particle injections are carried out during the normal steam cracking operation.
• the efficiency of the particles can be increased by increasing, by means of injections, the volume flow rate of cracked gases by 20 to 30% by addition of additional water vapor.
• the decoking of the pyrolysis tubes in zone (2), for examples 4 and 5, can be carried out with steam, or with air / steam mixtures, or else use chemical additives which are gasification catalysts, either during steam cracking either during decoking phases with water vapor.
• the invention therefore makes it possible to operate with significant flexibility of the charges, in a manner compatible with existing installations, in particular by retaining existing quench exchangers which give a favorable energy balance, and this in an economical and technologically reliable, which could not be achieved by any of the known methods.

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• 1994
  • 1994-12-26 FR FR9415743A patent/FR2728578A1/fr active Granted
• 1995
  • 1995-12-22 US US08/860,249 patent/US5972206A/en not_active Expired - Lifetime
  • 1995-12-22 WO PCT/FR1995/001717 patent/WO1996020255A1/fr active IP Right Grant
  • 1995-12-22 EP EP95943261A patent/EP0800564B1/fr not_active Expired - Lifetime
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