EP0542597B1 - Thermal hydrocarbon pyrolysis process using an electric furnace - Google Patents

Description

The invention relates to a process for thermal pyrolysis of hydrocarbons using an electric furnace. This process is in particular intended to produce light olefins, and more particularly ethylene and propylene.

Numerous patents describe processes and reactors for the implementation of these processes. Mention may in particular be made of US Pat. No. 4,780,196 in the name of the applicant which describes a thermal pyrolysis process in the presence of water vapor, known as steam cracking process, implemented in a multichannel ceramic reactor. This process provides good yields of ethylene and propylene. However, the design of the reactor is delicate, the ceramics used for its production are relatively expensive ceramics and it is difficult to maintain a constant temperature throughout the reaction zone, which penalizes the process.

The prior art is illustrated in particular by patents EP-A-323,287, EP-A-457,643, FR-A-1,305,287 and US-A-1,407,339.

One of the most important problems encountered in the implementation of thermal pyrolysis and in particular steam cracking of hydrocarbons is linked to the formation of coke. This formation is largely due to side reactions such as the formation of condensed polycyclic aromatic hydrocarbons, as well as to the polymerization of the olefins formed. This latter reaction stems from the tendency of olefins to polymerize when the temperature is around 500 ° C to 600 ° C; to reduce the importance of this secondary reaction, it is therefore necessary to rapidly cool (often called quenching) the reaction effluents, so as to bring them quickly from the temperature at which the pyrolysis takes place. a temperature below about 500 ° C, generally thanks to an indirect heat exchanger.

• augmentation rapide de la température de la charge jusqu'à la température optimale de pyrolyse pour une charge donnée, et maintien de cette température la plus constante possible dans la zone réactionnelle,
• diminution du temps de séjour de la charge dans la partie réactionnelle,
• diminution de la pression partielle de la charge hydrocarbonée,
• trempe rapide et efficace des effluents de réaction.

Thermodynamic and kinetic studies of the pyrolysis reactions of hydrocarbons therefore lead, in order to increase the selectivity of the reaction towards the production of olefins, to intervene on the following parameters:

• rapid increase in the temperature of the charge up to the optimal pyrolysis temperature for a given charge, and maintaining this temperature as constant as possible in the reaction zone,
• reduction in the charge residence time in the reaction part,
• reduction of the partial pressure of the hydrocarbon charge,
• quick and efficient quenching of reaction effluents.

It is therefore particularly important to minimize the contact time between the reaction products and the hot reactor walls.

• a) un préchauffage de la charge éventuellement diluée par de la vapeur d'eau,
• b) un chauffage à haute température de cette charge ou du mélange charge-vapeur d'eau, dans des fours tubulaires afin de limiter le temps de séjour des hydrocarbures au cours de cette phase de pyrolyse,
• c) une trempe rapide des effluents de réaction.

In terms of technology, these imperatives quickly led to a general process diagram consisting of:

• a) preheating of the charge, possibly diluted with steam,
• b) heating at high temperature of this charge or of the charge-water vapor mixture in tubular ovens in order to limit the residence time of the hydrocarbons during this pyrolysis phase,
• c) rapid quenching of the reaction effluents.

The evolution of thermal pyrolysis and notably steam cracking furnaces has essentially been geared towards obtaining shorter residence times and reduction in pressure drop, which has led manufacturers to reduce the length of tubular reactors, therefore to increase the density of heat flux.

The increase in this last factor can be essentially obtained by increasing the skin temperature of the tubular reactors and / or by reducing the diameter of the tubes (which makes it possible to increase the s / v ratio, s being the exchange surface and v the reaction volume).

Advances in metallurgy on special alloys resistant to increasingly high temperatures (INCOLOY 800H, HK 40, HP 40 for example) have enabled manufacturers of pyrolysis ovens, particularly for steam cracking to increase the operating temperatures of these tube furnaces, the current limits of metallurgy being located around 1300 ° C.

Furthermore, the technique has also evolved towards the use of smaller diameter tubes, placed in parallel in order to maintain a satisfactory capacity, and to remain in a suitable pressure drop range.

Several models of pyrolysis furnace have also been proposed, all tending to increase the density of thermal flux towards the beginning of the pyrolysis tube and to reduce it thereafter, either by using tubular reactors of increasing diameter, or by bringing together at least two pyrolysis tubes in one after a certain length of reaction zone (see for example the article by F. WALL et al published in Chemical Engineering Progress, December 1983, pages 50 to 55); non-cylindrical tubular ovens have also been described, tending to increase the s / v ratio; this is how the patent US-A-3572999 describes the use of tubes of oval section and that the patent US-A-3964873 describes that of tubes whose section is in the shape of a dumbbell.

The technology of thermal pyrolysis reactors and in particular steam cracking has thus evolved, from the use of horizontal tubes of approximately 100 meters (m) in length and internal diameters of the order of 90 to 140 millimeters (mm), up to to the classical technique of vertically hanging tubes of approximately 40 m in length and diameter of around 60 mm operating with residence times of around 0.3 to 0.4 seconds (s), and finally the so-called millisecond technique proposed by PULLMAN-KELLOG (patent US-A-3671198) which uses tubes of approximately 10 m in length, vertical and straight, with an internal diameter of 25 to 35 mm, these tubes being brought to temperatures of 1100 ° C (temperature most often very close to that of the limit of use of the metal). The residence time of the charges is, in this type of oven, of the order of 0.07 s; the pressure drop observed is of the order of 0.9 to 1.8 bar (1 bar is equal to 0.1 megapascal), and the calculation of the ratio of the exchange surface s to the reaction volume v leads to values of the order of 120 m⁻¹.

• limiter au maximum la formation de coke, en particulier sur les surfaces chaudes telles que par exemple les parois des gaines entourant les résistances,
• utiliser comme gaz dans l'espace des résistances un gaz ou un mélange de gaz comprenant de préférence un gaz déjà présent dans le mélange de gaz circulant dans l'espace process, ce qui permet d'employer des gaines dont l'étanchéité n'a pas besoin d'être très grande,
• améliorer les échanges thermiques entre le mélange gazeux, contenant au moins un hydrocarbure comprenant au moins deux atomes de carbone ou un mélange d'hydrocarbures contenant au moins deux hydrocarbures dont l'un au moins est un hydrocarbure ayant au moins deux atomes de carbone, et les surfaces chaudes en contact avec ce mélange,
• augmenter la fiabilité du dispositif,
• augmenter les rendements en éthylène et propylène par rapport aux procédés existants.

One of the objects of the invention is to remedy the drawbacks described above. The objectives which it is proposed to achieve and which respond to the problems raised by the prior art are essentially the following:

• limit as much as possible the formation of coke, in particular on hot surfaces such as for example the walls of the sheaths surrounding the resistors,
• use as gas in the space of the resistances a gas or a mixture of gases preferably comprising a gas already present in the mixture of gases circulating in the process space, which makes it possible to use sheaths whose sealing has not no need to be very tall,
• improving the heat exchanges between the gas mixture, containing at least one hydrocarbon comprising at least two carbon atoms or a mixture of hydrocarbons containing at least two hydrocarbons, at least one of which is a hydrocarbon having at least two carbon atoms, and hot surfaces in contact with this mixture,
• increase the reliability of the device,
• increase the yields of ethylene and propylene compared to existing processes.

It is known from patent EP-A-457 643 of the Applicant a process for the synthesis of ethylene, acetylene and benzene products in a reactor operating substantially under conditions making it possible to remedy the drawbacks mentioned above, starting from a charge containing at least 10% methane, for example from 10 to 99% methane and the rest of the charge consisting of 1 to 90% hydrogen.

The present invention proposes a method and a device for its implementation bringing notable improvements compared to the embodiments according to the prior art such as for example an easier, more flexible and better controlled implementation and a lower cost, also both at the investment level and at the utility level. The flexibility of use is linked to the use of electricity, which makes it possible to regulate the heat flow and therefore the temperature profile of the process gas as desired.

More particularly, the invention relates to a process for thermal pyrolysis of hydrocarbons in a reaction zone, of elongated shape in a direction (an axis), comprising a heating zone and a cooling zone, following said heating zone , in which a gas mixture, containing at least one hydrocarbon, is circulated in the heating zone, in a flow direction substantially parallel to the direction (to the axis) of the reaction zone, said heating zone comprising a plurality of electric heating means arranged in sheets, substantially parallel to each other, forming in transverse projection a bundle with a triangular, square or rectangular pitch, said heating means being grouped in successive transverse sections substantially perpendicular to the direction (at axis) of the reaction zone, independent of each other and supplied with electrical energy so as to determine at least two parts in the heating zone, the first part allowing the load to be brought to a temperature at most equal to about 1300 ° C., and the second part, subsequent to the first part, making it possible to maintain the load at a temperature substantially equal to the maximum temperature to which it has been brought in said first part, and in which the effluents from the heating zone are cooled, then the products formed at the end of the reaction zone are collected, the heating means electric being isolated from direct contact with the gas mixture by sheaths into which a gas G, called sheath gas or sealing gas, is introduced, said sheaths having an appropriate permeability and the gas being introduced inside said sheaths at a pressure such that there is diffusion, at least at certain points, of at least part of this gas G from the interior of said sheaths to the exterior of ites sheaths, this gas G can then dilute in said gas mixture, the method being characterized in that the gas mixture comprises at least one hydrocarbon with at least two carbon atoms and less than 10% by volume of methane.

• d'une part, l'espace de réaction ou espace process, à l'extérieur des gaines protégeant les résistances, dans lequel circule le mélange gazeux contenant au moins un hydrocarbure à au moins deux atomes de carbone,
• d'autre part, l'espace des résistances formé par le volume compris entre les résistances proprement dites et les gaines d'isolement, dans lequel on introduit le gaz G.

In carrying out this process, two spaces are defined within the reactor:

• on the one hand, the reaction space or process space, outside the sheaths protecting the resistors, in which the gaseous mixture containing at least one hydrocarbon containing at least two carbon atoms circulates,
• on the other hand, the space of the resistors formed by the volume between the resistors proper and the insulation sheaths, into which the gas G is introduced.

The thermal pyrolysis process of the present invention applies in particular to the thermal pyrolysis of ethane or of a mixture of hydrocarbons containing ethane, in the presence of hydrogen.

In the thermal pyrolysis process of the present invention, the gaseous mixture, circulating in the reaction space, may further contain water vapor. In the latter case, the thermal pyrolysis process is usually described under the term steam cracking. The following description of the process of the present invention is made in connection with this case.

The heating zone is heated by supplying electrical energy through heating means such as electrical resistors, the heat released by the Joule effect in these resistors is transmitted mainly by radiation to the sheaths arranged around the resistors in a non-contiguous manner. These sheaths are usually made of ceramic material or any refractory material supporting the required temperatures and the reducing and / or oxidizing atmospheres of the medium, such as for example certain new metal alloys from the firm KANTHAL SA such as KANTHAL AF, or KANTHAL APM. The gas mixture, containing at least one hydrocarbon, which circulates in the heating zone in a manner substantially perpendicular to the axis of the ducts, is heated essentially by convection and by radiation.

Steam cracking of hydrocarbons, with at least two carbon atoms, is a highly endothermic reaction and requires obtaining, at a high temperature level, of the order of 800 to 1300 ° C, a density of thermal flux. very important. It is necessary that the maximum supply of heat is carried out in the zone where the endothermic cracking and dehydrogenation reactions take place; moreover, taking into account the reactivity of the products formed, such as ethylene and / or propylene, it is necessary to have a contact time controlled, relatively short, followed by rapid quenching, so as to obtain a 'square' type temperature profile and to avoid excessive formation of coke.

Heat exchanges are one of the key elements for this type of very endothermic reaction where it is necessary to transfer very large amounts of energy from the resistors to the gas mixture containing at least one hydrocarbon containing at least two carbon atoms called below gas process. During the preliminary study carried out by the applicant on the heat exchanges in a pyrolysis oven built according to the model used in the present invention, it was noticed that the heat exchange from the resistance to the sheath is an exchange essentially radiative, but on the other hand there is little radiative exchange between the sheath and the process gas. In fact, this usually consists essentially of a hydrocarbon-water mixture, a mixture which absorbs little of the radiation emitted by the sheaths. The thermal transfer, between the process gas and the ducts, is therefore in the case envisaged in the present invention mainly a transfer by convection. In such a case, the quality of the heat exchanges will be directly linked to the available exchange surface and to the surface / volume ratio.

Thus, if the exchange surface is relatively small, it will be necessary, in order to obtain a given process gas temperature corresponding to a previously chosen conversion rate, to increase the temperature of the ducts in proportions that are all the more important as this surface is small, which implies an increased risk of coke formation and also the need to increase the temperature of the resistors, which leads to a faster aging of these resistors or even, if the previously chosen conversion rate is very high, the amount of energy to be transferred becomes very large and the risk of deterioration of the resistors increases very greatly.

The walls participate in an important way in the heat exchange, since they are able to absorb the radiation emitted by the sheaths and consequently the temperatures of the sheaths and the walls tend to balance. So he is possible to significantly increase the exchange surface and practically to double it by modifying the design of the device as follows:

whereas, in the initial design, the sheaths protecting the resistors and allowing heat transfer to the process gas were preferably staggered, according to a preferred embodiment of the present invention they will be aligned, which makes it possible to constitute n rows or layers of m resistors in the longitudinal direction (for a total number of resistors equal to nxm), this will form at least one longitudinal zone and most often at least two longitudinal zones each comprising at least one and often several layers of heating elements, each zone being separated from the next by a wall of refractory material. By radiation, the temperature of these walls increases and tends to reach the same value as that of the sheaths surrounding the resistors. These walls will therefore also participate in the convection heating of the process gas. Thus, in this embodiment, the exchange surface being significantly increased, the same process gas temperature can be obtained with a relatively lower temperature of the sheaths and walls, which consequently allows a reduction in the formation of coke. In the present description, the term heating element designates the assembly consisting of a protective sheath and at least one resistance inside said sheath.

In a particular embodiment of the invention, each zone will comprise a single layer of heating elements.

According to these two embodiments, the convective exchanges between the process gas and the walls are greatly increased and they can be further improved by imposing significant speeds on the process gas and by creating zones of turbulence. The increase in the speed of the process gas can for example be obtained by using walls whose shape favors this increase in speed and the appearance of zones of turbulence. Particularly shaped walls are shown without limitation in Figure 1C.

The walls are usually made of refractory material. Any refractory material can be used to make the walls and we can cite as examples not limiting zirconia, silicon carbide, mullite and various refractory concretes.

Since it is not at all necessary to have a seal at the level of the walls, since the composition of the gas is practically identical on each side of the walls, this embodiment induces only a minimal increase in the cost of the oven. Indeed, on the one hand it is not necessary to have specially thick walls, nor a particularly complex embodiment, on the other hand the overall dimensions of the oven increase little because most of the width of this oven is due to the width of the sheaths. For example, the sheaths can have a width of the order of 150 mm, for a wall thickness having a value of the order of 50 mm, which only causes an increase in overall width of the oven. around 30%.

An additional advantage of this embodiment comprising walls is to allow a simpler embodiment of the oven, the vertical walls allowing, in addition to improving the heat transfer by convection, to support the roof of the oven.

Furthermore, it is preferable that each wall includes at least one means for balancing the pressures in the longitudinal zones located on either side of the wall. As an example of a simple but effective means for balancing the pressures, mention may be made of the creation of zones comprising one or more perforations or of porous zones.

According to one of the characteristics of the invention, the electrical resistances which supply heat to the heating zone are supplied independently with electrical energy, either individually, or in transverse rows, or even in small groups, so as to define heating sections along the heating zone and thus be able to modulate the quantity of energy supplied while in the ding of this zone.

The heating zone is usually composed of 2 to 20 heating sections, preferably 5 to 12 sections. In the first part of this zone, the mixture gaseous containing at least one hydrocarbon, previously heated to about 600 ° C, is usually brought to a temperature at most equal to about 1300 ° C, and advantageously between 800 and 1100 ° C (the start of the heating zone is located at the where the charge is introduced).

The modulation of these heating sections is carried out in a conventional manner; the heating elements corresponding to the aforementioned sections are generally supplied by thyristor modulator assemblies. Transformers can be used to adapt the voltages a priori, while the modulators allow fine and continuous adjustment of the injected power.

In order to allow regulation of the assembly, each heating section can be provided with a thermocouple pyrometric rod adapted to the temperature level; these rods are arranged in the spaces where the load circulates, the information is transmitted to the regulator which controls the thyristor modulator.

The length of the first part of the heating zone usually represents from 5 to 50% of the total length of the heating zone, advantageously from 10 to 20%.

The electrical energy supplied to this first part of the heating zone is such that it generates a strong temperature gradient which makes it possible to have a relatively high average temperature of the load, over the entire heating zone, which is favorable to the selectivity in light olefins.

In the second part of the heating zone, the electrical energy supplied to the different heating sections of this zone is modulated so that the temperature variation throughout this zone is small, usually less than about 50 ° C. (+ or - 25 ° C around the setpoint value) and advantageously less than about 20 ° C (+ or - 10 ° C around the setpoint value).

In addition, the use of different heating cross sections, independent of each other, makes it possible to bring, at the level of the second part of the heating zone, the maximum thermal energy to the place where is carried out most of the endothermic reactions, and maintain an almost uniform temperature in the rest of the heating zone.

The length of the heating zone is usually about 50 to about 90% of the total length of the reaction zone.

• elles acceptent une charge (puissance émise par unité de surface) importante qui peut aller jusqu'à 20 W/cm,
• elles peuvent travailler à très haute température,
• elles accusent dans le temps un vieillissement négligeable,
• elles supportent facilement des atmosphères réductrices à des températures élevées.

One obtains, especially under the above heating conditions, a very high heat flux at a high temperature level. This usually implies a particular choice for the material constituting the resistors which must, in addition to being resistant to the atmosphere in which the resistors bathe under operating temperature conditions, be capable of delivering a power per unit area relatively important. As an example of a material which can be used for the production of resistors, mention may be made of silicon carbide, boron nitride, silicon nitride and molybdenum bisilicide (MoSi₂). It is usually preferred to use molybdenum bisilide resistors which have many advantages when used at high temperature:

• they accept a large load (power emitted per unit area) which can be up to 20 W / cm,
• they can work at very high temperatures,
• they show negligible aging over time,
• they easily withstand reducing atmospheres at high temperatures.

In the process of the invention, the heating zone is followed by a cooling (or quenching) zone so as to very quickly lower the temperature of the effluents from the heating zone to around 300 ° C. for example. Areas heating and quenching may or may not be incorporated in the same enclosure, hereinafter referred to as a reactor.

According to one embodiment, a direct quenching is carried out; the reaction effluents leave the heating zone and are very quickly cooled by direct contact with a cooling fluid which is injected into the effluents by means of at least one injector, usually made of ceramic material, disposed at the periphery of the reactor. Hydrocarbon oils or water can be used as the coolant. The total effluents resulting from the mixture are then collected and separated.

According to a preferred embodiment, the reaction effluents from the heating zone are cooled by indirect contact with a cooling fluid, for example by circulating said fluid in sealed conduits inside the cooling zone .

All of these characteristics make it possible, thanks to this process, to cracking the hydrocarbons into ethylene and propylene taking place with a good conversion rate and a high selectivity in these products.

The hydrocarbon feedstocks usable within the general framework of the present invention include saturated aliphatic hydrocarbons, such as ethane, mixtures of alkanes, or petroleum fractions such as naphthas, atmospheric gas oils and vacuum gas oils, the latter can have a final point of distillation of the order of 570 ° C. The petroleum fractions can have, if necessary, undergone a pretreatment such as, for example, a hydrotreatment. These feedstocks can also contain hydrogen in an amount which can range, for example, up to 90% by volume. These charges generally include at least one hydrocarbon having two carbon atoms in its molecule. Fillers are very often used, the majority of which (more than 50% by volume) contain hydrocarbons having at least two carbon atoms in their molecule.

By way of illustration, it is possible to envisage as an petroleum cut, a cut of LPG resulting from the distillation of crude oil or a cut resulting from the steam cracking of naphtha or of gas oil such as for example the C4 cuts.

• la coupe C4 riche en n-butane et isobutane issue de la distillation atmosphérique du brut,
• la coupe C4 totale issue du vapocraqueur,
• la coupe butadiène 1-3 issue de la distillation extractive du butadiène de la coupe C4 totale du vapocraqueur,
• le raffinat issu de la distillation extractive du butadiène de la coupe C4 totale du vapocraqueur, raffinat qui est riche en n-butènes et en isobutène,
• une coupe riche en isobutène.

Among these cuts, it is advantageous to crack thermally, in view of obtaining, among others, acetylene, methylacetylene, and propadiene, the following cuts:

• C4 cut rich in n-butane and isobutane from atmospheric distillation of crude,
• the total C4 cut from the steam cracker,
• the butadiene cut 1-3 resulting from the extractive distillation of butadiene from the total C4 cut of the steam cracker,
• the raffinate from the extractive distillation of butadiene from the total C4 fraction of the steam cracker, a raffinate which is rich in n-butenes and isobutene,
• a cut rich in isobutene.

Given the high level of the reaction temperature (usually between 900 and 1200 ° C) which is required to maximize the yields of acetylenic hydrocarbons, the thermal cracking of these cuts is preferably done using hydrogen as diluent. Therefore, the sealing gas G which is introduced into the sheaths surrounding the resistors will preferably be substantially pure hydrogen; given the use of such a sealing gas, the sheaths will be made of a preferably non-porous material, the leakage of gas G towards the process gas will result from the sealing on each sheath which will be carried out voluntarily imperfectly.

The weight ratio of the dilution water vapor to the hydrocarbon charge varies according to the charges to be treated. It can be from approximately 0.2: 1 to approximately 1.5: 1, generally, the ratio used is of the order of 1: 1 when using diesel under vacuum and of the order of 0, 5: 1 for steam cracking naphtha. Part of the dilution water vapor can be introduced with gas G. This fraction introduced with gas G can then represent up to 100% of the amount of water required for steam cracking. Preferably, this fraction represents from 0 to 50% of this amount.

The charges to be treated have a residence time in the reaction zone, usually from about 2 milliseconds to about 1 second and preferably from about 30 to about 400 milliseconds.

The gas G which is introduced into the sheaths surrounding the resistors is usually a gas free from any hydrocarbon capable of a thermal conversion reaction leading to the formation of coke. This gas is also chosen so that it does not damage the resistors used and does not cause accelerated aging of these resistors. This gas can be steam alone, hydrogen alone, a mixture of gases containing steam and hydrogen. This gas G can also be an inert gas such as nitrogen or a rare gas such as helium or argon. This gas G can also be a mixture of gases containing, in addition to water vapor and / or hydrogen, an inert gas or a rare gas such as, for example, those mentioned above.

It is usually preferred to use a gas G containing water vapor and / or hydrogen and most often a gas containing a proportion of water vapor and / or hydrogen by volume greater than about 50%. Most often, it is recommended to use a G gas containing water vapor.

The sheath permeability must be sufficient to allow, at least at certain points, the diffusion of at least part of the gas G introduced into the space of the resistors towards the process space. It would not go beyond the scope of the invention if the sheath permeability is such that it allows the diffusion of all the gaseous compounds, contained in the gas G introduced into the space of the resistors, towards the process area. This permeability can result from a tightness on each sheath produced voluntarily imperfectly and / or from the use of a material constituting the sheaths having an open porosity allowing the passage of at least part of the gas G, that is in other words, a permeable material. Most often, it is recommended to use a permeable material.

Thus, according to a preferred embodiment of the invention, the sheaths, insulating the electrical heating means from direct contact with the gas mixture containing at least one hydrocarbon, are made of a porous material whose porosity is sufficient to allow the diffusion of 'at least part of the gas G through said sheaths. These sheaths are thus preferably made of a porous material having an open porosity of at least about 1% and at most about 40% by volume relative to the volume of the wall, and usually from about 5% to about 30 %.

The use of gas G, containing water vapor and / or hydrogen, which diffuses at least in part towards the process space, provides several advantages. It does not complicate the separations downstream of the pyrolysis oven since the water vapor is a compound present in the process space and the hydrogen can be a compound present in the process space as a product of the cracking reaction. and possibly also as a component of the load.

Furthermore, according to the present invention, since it is no longer desirable to seek as perfect seals as possible between the process space and the resistance space, the cost of producing the furnace is reduced while also decreasing thermo-mechanical constraints at the sheath connections, which increases the reliability of the entire device.

In the case of the use of ceramic sheaths, it is well known to those skilled in the art that there are many varieties of ceramic, and in particular silicon carbide, which come from the quality of the constituent powder and from the conditions of very different sintering. Without wanting to go into detail, it can however be noted that one of the quality criteria is linked to the lower persistence of porosity after sintering. It is well known that if part of this porosity is found to be closed, that is to say without effect on the overall tightness of the material, another part, not insignificant, especially for the most common silicon carbide, is an open porosity and that in particular there is, at high temperature, diffusion of at least part of the gas G (or at least one of the constituents of the gas mixture) through this material. We therefore understand that when a gas such as steam and / or hydrogen is used as the sealing gas, it is not necessary to use sheaths of ceramic material, and in particular of silicon carbide, as waterproof as possible, that is to say of very high quality and therefore very high price.

The use of ceramic sheaths, in particular of silicon carbide, of medium quality, having an open porosity of at least about 1% by volume (for example about 20% by volume), is thus not only possible but even desirable, which lowers the cost of making the oven. Furthermore, the very existence of this open porosity creates on the surface of the ceramic sheath on the process space side a partial pressure of the gas G introduced into the space of the resistors insulating in a way the surface of the ceramic from the process gas which, without wishing to be bound by any theory, explains the significant reduction in the formation of coke since it usually usually forms on the surface of the sheaths and on the contrary the products formed will be in a less favorable local atmosphere to the formation of coke.

By open porosity, in the description of the present invention, is meant the porosity consisting of microcavities included in the solid ceramic pieces considered, the open adjective signifying that there is free passage on the one hand between most of said microcavities, and on the other hand between said microcavities and the internal and external surfaces of the parts considered; the concept of free passage must also be considered according to the nature of the environment and the physical conditions in which the ceramic is found. For example, for small molecules such as hydrogen or helium, free passage will be easy, all the more so if there is a pressure difference between the two surfaces of the part ceramic. In this case, the part is said to be permeable, for example with hydrogen, or not waterproof.

By closed porosity, the description of the present invention designates the porosity consisting of microcavities which do not communicate with the surface of the part. In this case, this closed porosity only causes an overall decrease in the density of the part.

The method according to the invention can be implemented in a device comprising a reactor (1) of elongated shape along an axis, preferably of square or rectangular section, comprising at a first end supply means (5) for mixing gaseous, containing at least one hydrocarbon, at the opposite end of the means for discharging (10) the effluents produced and between these two ends of the means for supplying cooling fluid, said reactor comprising in a first part (side of the first end ) a plurality of electrical heating means (3) surrounded by sheaths (4), said substantially parallel means being arranged in substantially parallel sheets, perpendicular to the axis of the reactor so as to define between the sheaths and / or the sheets formed by these ducts spaces or passages for the circulation of gas mixtures and / or effluents, said heating means and said ducts being adapted to heat said successive cross-sectional passages, independent and substantially perpendicular to the axis of the reactor, said reactor further comprising control and modulation heating means connected to said heating means, and comprising in a second part ( 8) (opposite end side) adjacent to the first part of the cooling means (9) of the effluents connected to said cooling fluid supply means, said device comprising means for introducing, at an appropriate pressure, a gas G said sheath gas or sealing gas, preferably containing water vapor and / or hydrogen, inside the sheaths (4) and in that said sheaths have sufficient permeability to allow, at least at certain points, the diffusion of at least part of this gas G from the interior of said sheaths towards the exterior of said sheaths, this gas G then diluting in said gas mixture.

The means for introducing gas G at an appropriate pressure are those known to those skilled in the art. They may also comprise means for regulating and controlling the pressures prevailing inside and outside said sheaths.

Said cooling means are means adapted to cool by direct contact or by indirect contact the effluents leaving the heating zone.

The sheaths surrounding the resistors, usually in non-contiguous fashion, can be arranged in a superimposed or staggered fashion and can form in transverse projection a beam with triangular, square or rectangular pitch.

The total number of layers comprising heating means and the number of heating means in each sheath and per layer are not decisive in the process; they are obviously a function of the size, the heating means, the sheaths which surround them and, where they exist, the walls separating the sheets. The heating elements can be identical to each other or different, both in size and in heating power. For example, a heating element may include inside the sheath from 1 to 5 resistors and most often from 1 to 3 resistors.

The number of heating elements determines the maximum electrical power available for a given reaction volume and also influences the residence time of the load; it will be chosen according to the admissible charge flow, taking into account these parameters.

It is possible, in the context of the present invention, to produce the entire reactor heating zone and quenching zone either in one-piece form, or even by contiguous juxtaposition of various elements of identical shape, which are assembled together by any usable means like, for example, using clamps.

The electric heating means which can be used in the context of the present invention are preferably heating resistors capable of being used up to temperatures of the order of 1500 ° C; it is preferred to use resistors in molybdenum bisilicide, for example hairpin resistors.

The sheaths which surround the resistors, so as to avoid direct contact between the gas mixtures of the load and the resistors, are preferably of tubular shape. These sheaths of refractory material are usually either ceramic or sintered metal. Ceramics such as mullite, cordierite, silicon nitride, silicon carbide, silica or alumina can be used; silicon carbide is the preferred material because it has good thermal conductivity. In the case where the sheets are separated by walls, the material chosen to make these walls may be the same as that used for the sheaths, but it is often different, in particular for reasons of cost of manufacturing the oven.

The distance between the heating elements and the ducts depends on the section of the heating element. For heating means of which the maximum diameter of the circle encompassing them is equal to d, ducts are generally used tubular or cylindrical with diameter D usually from about 1.2 xd to about 8 xd, and most often about 1 , 5 xd to about 4 x d.

The heating elements are arranged in parallel layers substantially perpendicular to the direction of flow of the charge (process gas), preferably substantially aligned, so that the distance between two neighboring ducts is as small as possible, while taking into account allowable pressure drop requirements; the distance between the sheaths of two neighboring plies or that between the sheaths of a ply and the nearest wall in the case where the plies are separated by walls is usually the same as that between two consecutive sheaths in a given ply. This distance will usually be such that the passages formed between the sheaths or between the sheaths and the nearest wall, passages in which the gas mixture containing hydrocarbons circulates will have a dimension of approximately 1 to approximately 100 mm, and most often d '' about 5 to about 40 mm.

According to one embodiment of the invention, the free spaces or passages defined above, intended for the circulation of process gas, are at least partially occupied by linings, usually made of ceramic, preferentially heat conductors. It is thus possible, for a given type of reactor, to reduce the residence time of the charge in this reactor while homogenizing the flow of the gaseous mixture and better distributing the dissipated heat. These linings may have various shapes and be presented for example in the form of rings (Raschig, Lessing or Pall rings), saddles (Berl saddles), bars, closed cylindrical tubes.

• Les figures 1A, 1B, 1C et 1F représentent une coupe longitudinale d'un réacteur suivant un plan perpendiculaire à l'axe des gaines. Dans le cas de la figure 1B, ce réacteur contient un garnissage. Dans le cas des figures 1C et 1F, ce réacteur comporte des parois séparant une ou plusieurs nappes de gaines contenant les moyens de chauffage électrique.
• Les figures 1D et 1E représentent une coupe longitudinale d'un réacteur suivant l'axe des gaines.
• La figure 2 illustre un détail de réalisation de la zone de chauffage dans un plan identique à celui des figures 1D et 1E.

The invention will be better understood by the description of some embodiments, given purely by way of illustration but in no way limiting, which will be made of it below with the aid of the appended figures, in which similar bodies are designated by the same figures and letters of reference.

• Figures 1A, 1B, 1C and 1F show a longitudinal section of a reactor along a plane perpendicular to the axis of the ducts. In the case of FIG. 1B, this reactor contains a lining. In the case of FIGS. 1C and 1F, this reactor has walls separating one or more layers of sheaths containing the electrical heating means.
• Figures 1D and 1E show a longitudinal section of a reactor along the axis of the sheaths.
• FIG. 2 illustrates a detail of embodiment of the heating zone in a plane identical to that of FIGS. 1D and 1E.

In FIG. 1A, there is shown, according to one embodiment, a vertical reactor (1) of elongated shape and of rectangular section, comprising a distributor (2) making it possible to supply the reactor with an inlet orifice (5) in reaction gas mixture. The latter, which contains a mixture of water vapor and at least one hydrocarbon, has been preheated in a conventional preheating zone, not shown in the figure, preferably by convection. The reactor comprises a plurality of electric heating means (3) surrounded by sheaths (4) arranged in parallel sheets and forming in a plane (plane of the figure) a bundle with square pitch. These layers define transverse heating sections substantially perpendicular to the axis of the reactor defined according to the direction of flow of the charge.

These heating sections are supplied with electrical energy, independently, by virtue of a pair of electrodes (6a, 6b in FIGS. 1D and 1E), thermocouple pyrometric probes (7 in FIGS. 1D and 1E) are housed in the spaces where the charge circulates between the sheaths (4) and make it possible to regulate automatically the temperature of each heating section, by a conventional regulator and modulator device not shown in the figure.

In the first part of the heating zone, the ducts are heated so that the temperature of the charge rapidly rises from 600 ° C (preheating temperature) to approximately 900 ° C; this heating zone generally represents approximately 15% of the total length of the heating zone; the gas mixture then circulates in the second part of the heating zone where the temperature is generally maintained at a constant value substantially equal to that reached at the end of the first heating zone, that is to say approximately 900 ° C. To this end, the electrical power supplied to several heating sections which constitute the second part of the heating zone is modulated; a temperature variation of not more than about 10 ° C is thus obtained around the setpoint value. The length of this second heating zone represents approximately 85% of the total length of the heating zone.

On leaving the heating zone, the reaction effluents are cooled in a cooling zone (8). They are brought into contact with a quenching agent such as water introduced via injectors (9), quenching, arranged at the periphery of the reactor (1) and connected to an external source of water not represented. All of the effluent gases are cooled to a temperature of approximately 500 ° C. and collected by an outlet orifice (10) at the end of the reaction zone (1).

According to another mode not illustrated, the effluents can be cooled by circulating through sealed conduits arranged in the zone (8) through which the quenching agent flows, these conduits being connected to the external source of the quenching.

According to the embodiment shown diagrammatically in FIG. 1B, the reactor, identical to that shown diagrammatically in FIG. 1A, comprises in the space where the charge circulates a lining (20), advantageously made of ceramic material, which is retained by a grid ( 21) at the end of the heating zone. The sheaths (4) are arranged in parallel layers and form in a plane (plane of the figure) a beam with triangular pitch (staggered arrangement).

In FIG. 1C, there is shown, according to one embodiment, a horizontal reactor (1) of elongated shape and of rectangular section, which differs from the reactor shown in FIG. 1A only in that it is substantially horizontal, that it comprises sheaths arranged in parallel plies and forming in a plane (plane of the figure) a bundle with square pitch, and in that these plies are separated from each other by walls (22) advantageously made of ceramic material. These walls have a shape, adapted to create turbulence, comprising cells at each sheath (4).

The embodiment shown diagrammatically in FIG. 1F differs from that shown diagrammatically in FIG. 1C only in that several layers of heating elements are situated between two walls (22).

FIG. 1D represents, for a horizontal reactor, the same elements as those described in connection with FIG. 1A; there is shown, moreover, a protective housing (11) comprising an orifice (12) through which gas G containing for example steam is introduced and an orifice (13) provided with a valve (24) making it possible to regulate the flow of this gas G. This box (11) is fixed to the metal frame of the reactor (1) and surrounds all the electrical resistances and sheaths containing them, with the exception of the ends of the electrical resistances where the electrical energy is supplied. The resistors (3), in a pin, are positioned in the sheaths (4) using washers (18), for example made of ceramic fiber, comprising passages (23) allowing gas G, for example steam d water, to enter the space between the resistors and the sheaths.

Figure 1E shows the same elements as those described in connection with Figure 1A; there is shown, moreover, the protective boxes (11) provided with orifice (12) and (13) allowing the circulation in the boxes of the gas G containing for example water vapor which penetrates into the space of the resistances through the holes (23) of the washers (18) ensuring the positioning of the resistances. The orifices (13) are provided with valves (24) allowing easier regulation of the flow of the gas G containing for example water vapor. These boxes (11) are fixed to the metal frame of the reactor and surround all the electrical resistances and sheaths containing them, with the exception of the end of the electrical resistances through which the electrical energy is supplied. The circulation of the gas G is carried out in slight overpressure compared to the pressure of the process gas within the reactor, thus ensuring a perfectly controlled atmosphere and a better diffusion of this gas G towards the process space.

The absolute pressure difference between the space of the resistors and the process space, or overpressure, will preferably be such that the pressure in the space of the resistors is at least 0.1% higher and more often than not minus 1% at the pressure in the process space. It is not necessary to have a very high overpressure and most often the pressure in the space of the resistors remains lower than 2 times the pressure in the process space.

FIG. 2 shows a detail of an embodiment of the heating zone according to the invention. Resistors (3) of cylindrical shape are used as the means of electric heating. These resistors comprise at each of their ends cold zones and a part of the central zone which is the hot zone representing for example around 68% of the total length.

A reactor of rectangular section is produced, the walls of which are made of insulating refractory concrete (14) and by a metal frame (15). A circular hole is drilled in two opposite side walls, through which a sheath (4), for example made of ceramic, is passed, with a diameter twice that of the electrical resistance (3). The sheath (4) is positioned by means of a cable gland system (16) acting in a groove at the level of the metal frame on a braid of refractory material (17), for example a braid of ceramic material. The positioning of the resistance (3) in the sheath (4) is carried out by means of washers (18), for example made of ceramic fiber, comprising orifices (23) allowing the passage of the gas G, containing for example steam of water, introduced into the housing (11) through the conduit (12) into the space of the resistors (24).

The hot zone of the resistor (3) is positioned so that it does not enter the opening through the wall of insulating concrete. It is not essential to use a braid (17) at the level of the cable gland since the latter has, within the framework of the invention, the role of positioning means and that it does not have the main purpose to ensure as perfect a seal as possible between the inside and the outside of the reactor. This gland can also advantageously be replaced by a simpler means of positioning the sheaths such as for example simple washers made of refractory material.

There are thus a number of heating resistors sheathed in walls, for example of ceramic material, in successive horizontal rows, these rows preferably being aligned so that, on the side walls of the oven, they form a bundle square or rectangular pitch. A housing (11), of which only protrude the ends of the resistors and / or their electrical supply (6), is traversed by a stream of gas G containing for example water vapor.

Example

Using a horizontal reactor with indirect quenching, the length of the pyrolysis zone is 2.21 meters and rectangular section of 1.4 x 3.72 m. The heating means of this reactor are constituted by electrical resistors in pin, in molybdenum bisilicide (MoSi₂); these resistors are surrounded by ceramic sheaths, arranged concentrically with respect to the center of the circle including the resistors.

These sheaths are made of silicon carbide and have an open porosity of 15% by volume. Each sheath, closed at one end, surrounds 2 pin resistors (Figure 1C and 1D). These sheaths are arranged perpendicular to the direction of flow of the load (vertically), in parallel sheets, and form in perpendicular projection a beam with square pitch. The length of each branch of the pin of the electrical resistance is 1.4 m and the diameter of the resistance is 9 mm. The ceramic sheaths have a length of 1.4 m, an outside diameter of 150 mm and an inside diameter of 130 mm; the distance Eg (Figure 1C) separating two neighboring sheaths is 20 mm. The sheath layers are separated by a refractory concrete wall based on electrofused alumina. The distance Ee (Figure 1C) between the ducts and the walls or dimension of the passages is 10 mm. The walls have in their thinnest part a thickness Ep (Figure 1C) of 15 mm.

The first part of the heating zone, 34 cm long, includes 20 resistance layers, each layer comprising 2 sheaths; in this zone, the load, preheated to 600 ° C, is brought to 900 ° C. This zone is thermally regulated by means of thermocouples arranged in the spaces where the charge circulates.

The second part of the heating zone, adjacent to said first part, is 1.87 m long; it consists of 20 layers of 11 sheaths, arranged in the same way as in the first part of the heating zone. This zone is made up of 5 heating sections, independently regulated, ensuring that the temperature in this zone is maintained at 900 ° C plus or minus 10 ° C.

The effluent gases are firstly cooled to 500 ° C by indirect exchange with the gases in the feed; other temperature exchangers then make it possible to lower their temperature to approximately 350 ° C.

As the charge, naphtha with a density d 20/4 = 0.715 and whose boiling range is between 38 and 185 ° C. diluted with water in a weight ratio of water vapor / charge of 0.5 is used. : 1. This mixture is preheated to 600 ° C and cracked at 900 ° C in the reactor described above. The absolute pressure of the gas mixture in the reactor is kept substantially constant and equal to 0.170 MPa. Resistors of substantially pure water are introduced into the space so as to obtain and maintain in this space an essentially constant absolute pressure equal to 0.175 MPa.

The same charge was cracked in an installation conforming to that described in Example 1 of US Pat. No. 4,780,196 comprising a multichannel pyrolysis zone, made of silicon carbide, each channel having a square section of 10 mm side and having a length of 3 m. The operating conditions are such that the feed is introduced into this reactor at the temperature of 600 ° C. and the effluents at the end of the pyrolysis are at 900 ° C. In this installation, heating is provided by a heat transfer fluid.

After cooling to room temperature, in the case of the process according to the invention, a weight yield of ethylene of 39.6% and a weight yield of propylene of 16.4% are obtained. In the case of the use of the multichannel reactor described in patent US-A-4780196, a weight yield of ethylene of 38.5% and a weight yield of propylene of 15.0% are obtained.

In the case of the process according to the invention, there is a maximum initial coking speed of 10 g x h⁻¹ x m⁻. In the case of the use of the multichannel silicon carbide reactor described in patent US-A-4780196, the maximum initial coking speed observed is 15 g x h x¹ x m⁻.

The process according to the invention therefore makes it possible to obtain the ethylene-propylene assembly with an improved yield of approximately 14% and to reduce the initial maximum coking speed by approximately 33%.

Claims (12)

1. A process for the thermal pyrolysis of hydrocarbons in a reaction zone of elongate shape in one direction (one axis), comprising a heating zone and a cooling zone following on from said heating zone, wherein a gaseous mixture containing at least one hydrocarbon with at least two carbon atoms is circulated in the heating zone in a flow direction substantially parallel to the direction (axis) of the reaction zone, said heating zone comprising a plurality of electric heating means arranged in layers substantially parallel to each other and forming a transversely projecting bundle of triangular, square or rectangular pitch, said heating means being grouped in successive transverse sections which are substantially perpendicular to the direction (axis) of the reaction zone, which are independent of each other and which are supplied with electric energy in such a way as to define at least two parts in the heating zone, the first part enabling the charge to be brought to a temperature which is at least equal to about 1300°C, and the second part which follows on from the first part enabling the charge to be kept at a temperature which is substantially equal to the maximum temperature to which it was brought in the first part, and wherein the effluents from the heating zone are cooled, and the products formed at the end of the reaction zone are collected, the electric heating means being insulated from direct contact with the gaseous mixture containing at least one hydrocarbon by casings in which a gas G, known as the casing gas or sealing gas, is introduced, the casings being of appropriate permeability and the gas introduced inside said casings at a pressure such that diffusion takes place, at least at certain places, of at least one part of the gas G from inside the casings to the outside thereof, the gas G then being able to be diluted in said gaseous mixture, the process being characterized in that the gaseous mixture comprises at least one hydrocarbon having at least two carbon atoms and less than 10 % by volume of methane.
2. A process according to Claim 1, wherein a first part of the heating zone is heated to a maximum temperature which is at least equal to about 1300°C, and wherein the second part, which follows on from the first part, is heated in such a way that the temperature variation along the entire second part of the heating zone is less than about 50°C, and preferably less than about 20°C.
3. A process according to Claim 1 or wherein the pressure of the gas in the casings is greater by at least 0.1 % than the pressure outside said casings.
4. A process according to one of Claims 1 to 3 wherein the casings which insulate the electric heating means from direct contact with the gaseous mixture are made of a porous material of sufficient porosity to permit diffusion of at least a part of the gas G through said casings.
5. A process according to Claim 4 wherein the casings are made of a porous ceramics material with an open porosity of at least about 1 % and moreover about 40 % by volume.
6. A process according to one of Claims 1 to 5 wherein the heating means are insulated from direct contact with the gaseous mixture by cylindrical casings whose diameter is about 1.2 to about 4 times the maximum diameter of the circle which encloses said heating means.
7. A process according to one of Claims 1 to 6 wherein the size of the passages in which the gaseous mixture circulates is about 1 to about 100 mm.
8. A process according to one of Claims 1 to 7 wherein the reaction zone comprises at least two longitudinal zones, each longitudinal zone comprising at least one layer of heating elements and being separated from the following one by a wall of a refractory material.
9. A process according to one of Claims 1 to 8 wherein the electric heating means comprise molybdenum bisilicide resistances.
10. Aprocess according to one of Claims 1 to 9 wherein the gaseous mixture also contains water vapour.
11. A process according to one of Claims 1 to 9 wherein the gaseous mixture contains ethane and hydrogen.
12. A process according to one of Claims 1 to 9 wherein the gaseous mixture contains a C4 cut with, preferably, hydrogen.

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