EP0252355B1 - Process and furnace for the steam cracking of hydrocarbons for the preparation of olefins and diolefins - Google Patents

Description

The present invention relates to a process for cracking hydrocarbons in the presence of water vapor, intended for manufacturing olefins and diolefins, and in particular ethylene. The present invention also relates to a device consisting of a cracking oven intended for the implementation of this process.

It is known to carry out the steam cracking of liquid hydrocarbons containing from 5 to 15 carbon atoms, such as naphtha, light gasolines and diesel oil, in ovens whose outlet temperature is generally between 750 ° C and 850 _° C, or gaseous hydrocarbons, such as alkanes having 2 to 4 carbon atoms, optionally mixed with methane and / or alkenes having 2 to 4 carbon atoms, in ovens whose outlet temperature is generally between 800 _° C and 880 _° C. In this process, known under the name of cracking or pyrolysis with steam, or also under the name of steam cracking, through the radiant part of an oven a mixture of hydrocarbons and water vapor circulating in a cracking tube arranged in the form of a coil inside this oven, the pressure of this mixture at the outlet of the oven generally being between 120 kPa and 240 k Pa. The hydrocarbons are thus in particular transformed, on the one hand, into a gaseous hydrocarbon fraction comprising in particular olefins containing from 2 to 6 carbon atoms, such as ethylene, propylene and isobutene, and diolefins such as butadiene, and, on the other hand, in a liquid hydrocarbon fraction, called "steam cracking gasoline", comprising hydrocarbons containing from 5 to 12 carbon atoms, as well as undesirable by-products, such as methane.

• Taux pondéral de conversion = 1₀₀ -(% en poids de la fraction hydrocarbonée à la sortie du four, ayant un intervalle de distillation ASTM 45/205_°C).

It is known, in particular, that ethylene is formed at a higher temperature than higher olefins containing at least 3 carbon atoms. It is known, moreover, that these higher olefins undergo at high temperatures in the presence of hydrogen, secondary hydrocracking and condensation reactions, favoring the formation of light hydrocarbons and gasoline. Generally, in such a steam cracking process, using liquid or gaseous hydrocarbons, the yield of olefins, in particular ethylene, and the yield of diolefins, especially butadiene, are determined by the weight ratio of the quantities of olefins or of diolefins. produced at the quantity of hydrocarbons used. The weight conversion rate of the hydrocarbons used is also determined by the following equation:

• Weight conversion rate = 1 ₀₀ - (% by weight of the hydrocarbon fraction at the outlet of the oven, having a distillation range of ASTM 45/205 _° C).

The steam cracking processes, known until now, using in particular liquid hydrocarbons and in particular that described in GB-A 1 165 907 are carried out with the aim, obviously, of obtaining the highest possible yield of olefins and as diolefins, but under conditions which favor the production of ethylene over those of other olefins and diolefins. To obtain this result, steam cracking ovens are generally designed to operate under conditions known as high severity. These conditions are such that the mixture of hydrocarbons and water vapor, circulating in the cracking tube arranged in the form of a coil inside the radiant part of an oven, is subjected to a high temperature. and at low pressure, for a relatively short time.

It is also known that the development of industrial installations for steam cracking gaseous hydrocarbons, such as natural gas consisting mainly of ethane, has led to excess ethylene on the market. It has thus appeared in recent years an urgent need to modify the steam cracking processes of liquid hydrocarbons in order to significantly increase the production of higher olefins and diolefins compared to the production of ethylene. However, in view of the large size of industrial steam cracking installations and the high cost of investment, it is not conceivable that the envisaged modification of the process results in excessively large and costly transformations of the already existing steam cracking units. Furthermore, it is also not economically conceivable that the process of steam cracking of hydrocarbons be modified by accepting a drop, however small it may be, in the yield of olefins and diolefins. Thus, for several years, numerous studies have been undertaken in this field and incessant research efforts have been made both at the laboratory stage and at the industrial stage.

Furthermore, in the steam cracking processes known up to now, using gaseous hydrocarbons generally of a low price, such as natural gas, it is sought to convert the highest possible quantity of gaseous hydrocarbons into olefins. . These processes are therefore carried out with the aim of obtaining a high conversion rate, this conversion rate being defined by the weight ratio of the quantity of hydrocarbons transformed to the quantity of hydrocarbons used. However, this high conversion rate is generally obtained to the detriment of the selectivity in olefins and in particular in ethylene of the steam cracking reaction, this selectivity in ethylene being defined by the weight ratio of the quantity of ethylene manufactured to the quantity of hydrocarbons gaseous transformed. These processes are carried out using steam cracking ovens which are also designed to operate under so-called high severity conditions. However, the processes using these steam cracking ovens can present serious drawbacks, such as significant phenomena of coking inside the cracking tube and a premature aging effect of the steam cracking installations.

Depending on the economic circumstances, steam cracking processes can use relatively higher cost gaseous hydrocarbons, such as liquefied petroleum gas, also called LPG, or ethane, a secondary product resulting from steam cracking of hydrocarbons. liquids, such as naphtha or gas oil. In this case, it is advantageous to seek a steam cracking process having the highest possible ethylene selectivity, in particular a process making it possible to manufacture, for a given quantity of ethylene, the lowest possible quantity of undesirable by-products, such as than methane. In recent years, an urgent need has also appeared to modify the steam cracking processes for gaseous hydrocarbons, in order to significantly increase the ethylene selectivity of steam cracking reactions.

A process and an oven for cracking liquid or gaseous hydrocarbons have now been found in the presence of water vapor, making it possible, in the case of liquid hydrocarbons, not only to very significantly increase the production of proplyene, isobutene and butadiene with respect to the production of ethylene, but also to significantly increase the yield of cracking into olefins and diolefins, and in the case of gaseous hydrocarbons very significantly increase the selectivity for ethylene of the steam cracking reaction and at the same time very significantly reduce the amount of methane produced, while also avoiding the drawbacks mentioned above. The method and the device of the invention can, moreover, be easily adapted to already existing steam cracking installations.

• (a) le temps de séjour moyen du mélange d'hydrocarbures et de vapeur d'eau circulant dans le tube de craquage entre l'entrée et la sortie de la zone de radiation est compris entre 300 et 1800 millisecondes, et
• (b) le volume réactionnel de la première moitié de la longueur du tube de craquage, située vers l'entrée de la zone de radiation, est de 1,3 à 4 fois supérieur à celui de la deuxième moitié de la longueur du tube, située vers la sortie de cette zone.
  • La figure 1 est une illustration schématique d'un four horizontal de vapocraquage, comprenant une enceinte thermique de radiation, également appelée zone de radiation, à travers laquelle passe un tube de craquage disposé sous la forme d'un serpentin.
  • Les figures 2 et 3 sont des graphiques tridimensionnels représentant la répartition du flux thermique à l'intérieur de l'enceinte thermique de radiation d'un four horizontal de vapocraquage, répartition obtenue respectivement selon une puissance de chauffe de type non-homogène et de type homogène.
  • La figure 4 est un graphique représentant l'augmentation de la température de craquage d'un mélange d'hydrocarbures et de vapeur d'eau circulant dans un tube de craquage depuis l'entrée jusqu'à la sortie de la zone de radiation d'un four horizontal de vapocraquage, en fonction du volume réactionnel que traverse le mélange.
  • La figure 5 est un graphique représentant l'augmentation de la température de craquage d'un mélange d'hydrocarbures et de vapeur d'eau circulant dans un tube de craquage depuis l'entrée jusqu'à la sortie de la zone de radiation d'un four horizontal de vapocraquage en fonction du temps de séjour moyen du mélange dans le four.

The present invention firstly relates to a process for the manufacture of olefins and diolefins by cracking liquid or gaseous hydrocarbons in the presence of water vapor, consisting in passing, through a radiation zone of a furnace, a mixture of hydrocarbons and steam, circulating in a cracking tube placed inside this zone, under an oven outlet pressure between 120 kPa and 240 kPa, the cracking temperature of the mixture being, at the entrance to the radiation zone, between 400 and 700 ° C, and at the exit from this zone, between 720 and 880 ° C, process characterized in that

• (a) the average residence time of the mixture of hydrocarbons and water vapor circulating in the cracking tube between the entry and the exit of the radiation zone is between 300 and 1800 milliseconds, and
• (b) the reaction volume of the first half of the length of the cracking tube, located towards the entrance to the radiation zone, is 1.3 to 4 times that of the second half of the length of the tube, located towards the exit of this area.
  • Figure 1 is a schematic illustration of a horizontal steam cracking oven, comprising a thermal radiation enclosure, also called a radiation zone, through which passes a cracking tube arranged in the form of a coil.
  • Figures 2 and 3 are three-dimensional graphs representing the distribution of the heat flux inside the thermal radiation enclosure of a horizontal steam cracking oven, distribution obtained respectively according to a heating power of non-homogeneous type and of type homogeneous.
  • FIG. 4 is a graph representing the increase in the cracking temperature of a mixture of hydrocarbons and water vapor circulating in a cracking tube from the entry to the exit from the radiation zone of a horizontal steam cracking oven, depending on the reaction volume through which the mixture passes.
  • FIG. 5 is a graph showing the increase in the cracking temperature of a mixture of hydrocarbons and water vapor circulating in a cracking tube from the entry to the exit from the radiation zone of a horizontal steam cracking oven as a function of the average residence time of the mixture in the oven.

The cracking temperature of the mixture of hydrocarbons and steam increases along the cracking tube, between the inlet and the outlet of the radiation area of the furnace, i.e. in the direction of flow of the mixture. Preferably, the mixture of hydrocarbons and water vapor is subjected to preheating before it enters the radiation zone of the oven, this preheating can be carried out by any known means, in particular in a zone of convection heating of the oven. . In particular, in the case of liquid hydrocarbons, the cracking temperature of the mixture of hydrocarbons and water vapor is at the entry of the radiation region of the oven between 400 _° C and 650 _° C, preferably between 430 ° C and 580 _° C; it is at the exit of this zone comprised between 720 ° C and 860 ° C, preferably comprised between 760 ° C and 810 ° C. In the case of gaseous hydrocarbons, the cracking temperature of the mixture of hydrocarbons and steam is at the entry of the radiation zone of the furnace between 500 _° C and 700 _° C, preferably between 550 _° C and 660 ° C; it is at the exit of this zone between 800 ° C and 880 ° C, preferably between 810 ° C and 850 ° C.

The method according to the present invention is characterized by an average residence time of the mixture of hydrocarbons and water vapor, circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace. This average residence time can be relatively longer than that usually existing in steam cracking processes for hydrocarbons operating under conditions of high severity. It is generally between 300 and 1800 milliseconds, preferably com taken between 400 and 1400 milliseconds, especially when the hydrocarbons used are gaseous. It is, moreover, between 850 and 1800 milliseconds, preferably between 870 and 1500 milliseconds, and more particularly between 900 and 1400 milliseconds, when the hydrocarbons used are liquid.

Furthermore, the method according to the present invention is also characterized by the reaction volume of the cracking tube which in the first half of the length of the tube, located towards the entrance to the radiation zone, is 1.3 to 4 times greater, preferably 1.5 to 2.5 times greater than that of the second half of the length of the tube, located towards the exit of this area. More particularly, the reaction volume per unit length of the cracking tube decreases continuously or discontinuously from the entry to the exit from the radiation zone of the furnace. In practice, it is preferred to carry out this reduction in a discontinuous manner, in stages along the cracking tube.

It is found that under these conditions that the average residence time of the mixture per unit length of the cracking tube, also called partial residence time, is not constant along the cracking tube from the inlet to the out of the radiation area of the oven, but tends to decrease significantly in the direction of flow of the mixture in the cracking tube. More specifically, the average residence time of the mixture circulating in the first half of the length of the tube, located towards the entrance to the radiation area of the furnace, is 2 to 4 times greater, preferably 2.6 to 3 times greater than that existing in the second half of the length of the tube, located towards the exit of this zone. It is also observed that the apparent surface speed of the mixture of hydrocarbons and water vapor circulating in the cracking tube increases in the direction of flow of the mixture. Thus, this speed is relatively low in the first half of the length of the cracking tube, located towards the entrance to the radiation zone, for example between 30 and 80 m / sec, and higher in the second half of the length of the tube, located towards the exit of the radiation zone, for example between 90 and 150 m / sec. Thus, the method according to the present invention allows the mixture of hydrocarbons and steam to pass relatively slowly through the part of the cracking tube where the temperature is relatively low, and on the contrary more quickly through the part of the cracking tube where the temperature is higher. It therefore makes it possible not only to increase the production of propylene, isobutene and butadiene relative to that of ethylene but also the yield of cracking into olefins and diolefins, in particular when liquid hydrocarbons are used.

It has however been observed that the best results are obtained when the increase in the cracking temperature of the mixture of hydrocarbons and water vapor between the inlet and the outlet of the radiation zone of the furnace is associated with a distribution. uneven thermal power of the oven applied along the tube, distribution such that the thermal power applied to the second half of the length of the tube, located towards the exit of the radiation area, is 1.5 to 5 times greater to that applied to the first half of the length of the tube, located towards the entrance of this zone.

Thus, the cracking temperature of the mixture of hydrocarbons and steam does not increase uniformly along the tube, between the inlet and the outlet of the radiation zone of the furnace. More specifically, the increase in the cracking temperature of the mixture is relatively moderate in the first half of the length of the tube, located towards the entrance to the radiation zone of the furnace, while the increase in the cracking temperature of the mixing is more important in the second half of the length of the tube, located towards the exit of the radiation area of the oven. The regulation of the cracking temperature of the mixture of hydrocarbons and steam, circulating in the tube between the inlet and the outlet of the radiation area of the furnace, is obtained by a graded distribution of the thermal power applied to the pipe. In particular, the thermal power applied to the second half of the length of the tube, located towards the exit from the radiation area of the furnace, is 1.5 to 5 times greater, preferably 2 to 4 times greater than that applied at the first half of the length of the tube, located towards the entrance to this area. By thermal power is meant here the quantity of heat supplied per unit of time and per unit of volume of the oven surrounding the cracking tube.

It is found that under these conditions the combination of a non-uniform distribution of the thermal power applied along the cracking tube with a decreasing reaction volume per unit length of the cracking tube has the result of significantly increasing the average residence time of the mixture in the first half of the length of the cracking tube located towards the entrance to the radiation area of the oven. The effect of this combination thus allows the mixture of hydrocarbons and water vapor to pass relatively slowly through the part of the cracking tube where the applied thermal power is the lowest, and on the contrary more quickly through the part of the cracking tube. where the thermal power applied is the greatest. This has the result of considerably increasing both the production of propylene, isobutene and butadiene relative to the production of ethylene, and the yield of cracking into olefins and diolefins, in particular when the liquid hydrocarbons are used in the process. This combination also has the result of increasing the ethylene selectivity of the steam cracking reaction and of significantly reducing the quantity of methane produced, when gaseous hydrocarbons are used in particular. This result is also obtained with an improved thermal radiation efficiency compared to previously known methods, due to a relatively lower average cracking temperature.

The method according to the present invention also provides other advantages. In particular, it makes it possible to reduce the coking phenomena occurring inside the cracking tube. It allows, in in addition, to increase the service life of a steam cracking installation, thus operating at a relatively low average cracking temperature.

The composition of the mixture of hydrocarbons and steam, used in the process according to the invention, is such that the weight ratio of the quantity of hydrocarbons to the quantity of steam is between 1 and 10, preferably between 2 and 6, in the case of gaseous hydrocarbons in particular, and preferably between 3 and 6, when it is in particular liquid hydrocarbons.

The liquid hydrocarbons, used in the mixture with the water vapor, can be chosen from naphtha, consisting of hydrocarbons containing approximately from 5 to 10 carbon atoms, light gasolines consisting of hydrocarbons comprising approximately 5 or 6 carbon atoms, the diesel oil consisting of hydrocarbons containing approximately from 8 to 15 carbon atoms, as well as their mixtures. They can also be used in admixture with saturated and unsaturated hydrocarbons containing from 3 to 6 carbon atoms.

The gaseous hydrocarbons, used in the mixture with water vapor, consist of alkanes comprising from 2 to 4 carbon atoms, in particular ethane, propane or butane, or by their mixtures. These alkanes can optionally be used in admixture with alkenes containing from 2 to 6 carbon atoms and / or methane and / or alkanes containing from 5 to 6 carbon atoms. It is possible, in particular, to use in the process of the invention natural gas or liquefied petroleum gas, also called LPG, or ethane, a secondary product resulting from the steam cracking of liquid hydrocarbons, such as naphtha or diesel.

et

The process of the present invention, using liquid hydrocarbons, is particularly advantageous for increasing the production of higher olefins and diolefins compared to that of ethylene, in particular the production of olefins having 3 or 4 carbon atoms, such as propylene and isobutene and the production of diolefins such as butadiene. This advantage is appreciated, in particular, by defining, on the one hand, a selectivity, Ss, in produced hydrocarbons comprising 3 carbon atoms, and on the other hand a selectivity, S ₄ , in produced hydrocarbons comprising 4 carbon atoms, according to the following equations:

and

Thus, the method makes it possible to carry out the steam cracking of liquid hydrocarbons with a selectivity S ₃ equal to or greater than 0.73 and a selectivity S ₄ equal or greater than 0.51, when the thermal power is applied in a homogeneous manner along of the cracking tube. The selectivities S ₃ and S ₄ can become equal to or greater than 0.78 and 0.57 respectively, when the thermal power is applied in a non-homogeneous manner along the cracking tube, according to the process of the invention .

• (a) le rapport entre la longueur et le diamètre moyen interne du tube de craquage traversant l'enceinte thermique de radiation est compris entre 200 et 600, et
• (b) le diamètre interne du tube de craquage diminue d'une façon continue ou discontinue depuis l'entrée jusqu'à la sortie de l'enceinte thermique de radiation, de telle sorte que le rapport entre les diamètres internes du tube à l'entrée et à la sortie de cette enceinte est compris entre 1,2 et 3.

The present invention also relates to a device allowing the implementation of the hydrocarbon steam cracking process described above, in particular a device consisting of a hydrocarbon cracking oven in the presence of water vapor comprising a thermal radiation enclosure provided with heating means, enclosure through which at least one cracking tube passes through which the mixture of water vapor and hyrdrocarbons to be cracked circulates, device characterized in that:

• (a) the ratio between the length and the mean internal diameter of the cracking tube passing through the thermal radiation enclosure is between 200 and 600, and
• (b) the internal diameter of the cracking tube decreases continuously or discontinuously from the entry to the exit of the thermal radiation enclosure, so that the ratio between the internal diameters of the tube to the input and output of this enclosure is between 1.2 and 3.

The steam cracking furnace according to the present invention comprises a thermal radiation enclosure through which at least one cracking tube passes, arranged in the form of a horizontal or vertical coil. This cracking tube must have a ratio between the length and the mean internal diameter of between 200 and 600, preferably between 300 and 500. In particular, when liquid hydrocarbons are used in this furnace, the mean internal diameter of the cracking tube is preferably equal to or greater than 100 mm, so that the average residence time of the mixture in the cracking tube can be relatively long and that the pressure drops of the mixture circulating in the powerful cracking tube to be weak. However, the mean internal diameter and the length of the tube must remain within ranges of values compatible with the mechanical and thermal stresses to which the materials constituting the cracking tube are subjected. In particular, the average internal diameter of the cracking tube cannot exceed approximately 250 mm. Furthermore, when hydrocarbons gaseous bures are used in this oven, the internal mean diameter of the cracking tube can be between 70 mm and 160 mm, preferably between 80 and 150 mm.

Furthermore, the internal diameter of the cracking tube decreases continuously or discontinuously from the inlet to the outlet of the thermal radiation enclosure of the furnace, that is to say in the direction of the flow of the mixture of hydrocarbons and water vapor. In particular, the reduction in the internal diameter of the cracking tube is such that the ratio between the internal diameters of the tube at the inlet and at the outlet of the thermal radiation enclosure is between 1.2 and 3, preferably included between 1.4 and 2.2, more particularly between 1.4 and 2. In practice, when liquid hydrocarbons are used in this furnace, the internal diameter of the cracking tube at the inlet of the thermal enclosure radiation is preferably between 140 and 220 mm, and that at the outlet of this enclosure is preferably between 70 and 120 mm. Furthermore, when gaseous hydrocarbons are used in this furnace, the internal diameter of the cracking tube at the inlet of the thermal radiation enclosure is preferably between 110 and 180 mm, and that at the outlet of this enclosure is preferably between 60 and 100 mm. These values take into account the fact that one wants to avoid excessively increasing the pressure drops of the cracking tube, in particular in the part where the internal diameter of the tube is the smallest. The decrease in internal diameter can be continuous all along the cracking tube. However, it is preferred to use a cracking tube consisting of a succession of tubes of decreasing internal diameter from the inlet to the outlet of the thermal radiation enclosure of the furnace.

In practice, the cracking tube is arranged in the form of a coil made up of a succession of straight sections connected together by elbows, these straight sections having decreasing internal diameters from the inlet to the outlet of the pipe. thermal radiation enclosure.

Figure 1 schematically illustrates a horizontal steam cracking furnace comprising a thermal radiation enclosure (1) through which passes a cracking tube arranged in the form of a coil consisting of eight horizontal straight sections connected together by elbows, the sections (2) and (3) having an internal diameter of 172 mm, the sections (4) and (5) an internal diameter of 150 mm, the sections (6) and (7) a diameter of 129 mm and the sections (8 ) and (9) an internal diameter of 108 mm, the inlet and outlet of the cracking tube in the thermal radiation enclosure being in (10) and (11) respectively.

A variant may consist in using a cracking tube which, upon entering the thermal radiation chamber of the furnace, is divided into a bundle of parallel tubes whose internal diameter can be constant and whose number decreases since l entry to the exit of the thermal enclosure, so that the reaction volume constituted by the set of tubes corresponding to the first half of the length of the cracking tube is 1.3 to 4 times greater, preferably 1.5 to 2.5 times greater than that corresponding to the second half of the length of the tube.

The steam cracking oven comprises a thermal radiation enclosure provided with heating means consisting of burners, arranged for example in rows on the floor and / or on the walls of the enclosure. The arrangement, adjustment and / or size of the burners in the thermal enclosure are such that the thermal power can be distributed uniformly along the tube, and the mixture of hydrocarbons and steam is subjected to a temperature which increases rapidly in the first half of the tube, then more slowly in the second half of the tube. However, the maximum heating power must be such that the skin temperature does not exceed the limit compatible with the nature of the metal or alloy constituting the cracking tube.

However, it has been observed that the best results are obtained when the steam cracking furnace comprises heating means constituted by burners whose thermal power increases along the cracking tube, from the inlet to the outlet of the enclosure. thermal radiation, so that the ratio between the thermal power of the burners applied to the first half of the length of the cracking tube, located towards the inlet of the thermal radiation enclosure, and that applied to the second half of the length of the tube, located towards the outlet of this enclosure, is between 40/60 and 15/85, preferably between 33/67 and 20/80. The arrangement, adjustment and / or size of the burners in the thermal enclosure are such that the thermal power increases along the cracking tube from the inlet to the outlet of the enclosure. This increasing profile of the thermal power of the burners applied along the cracking tube can be easily obtained by appropriately adjusting the gas or fuel-gas supply rate of each of the burners. Another way is to have burners of the appropriate size and heating capacity in the thermal enclosure. However, the maximum heating power must be such that the skin temperature does not exceed the limit compatible with the nature of the metal or alloy constituting the cracking tube.

The following nonlimiting examples illustrate the present invention.

Free 1

A steam cracking furnace, as shown diagrammatically in FIG. 1, comprises a thermal radiation enclosure (1) in brickwork, constituted by a rectangular parallelepiped whose internal dimensions are 9.75 m for the length, 1.70 m for the width and 4.85 m for the height. In the enclosure (1), a cracking tube of refractory steel based on nickel and chromium, having an average internal diameter of 140 mm, a thickness of 8 mm and, taking into account the capacity of the enclosure, is placed. (1), a total length of 64 meters, between the inlet (10) and the outlet (11). The relationship between lon and the internal mean diameter of the tube is 457. This cracking tube is arranged in the form of a serpentine, comprising eight straight horizontal sections, of equal length each, connected to each other by elbows. The internal diameter of the sections (2) and (3) located towards the entrance to the thermal enclosure is 172 mm; the following sections (4) and (5) have an internal diameter of 150 mm; then sections (6) and (7) have an internal diameter of 129 mm; the internal diameter of the sections (8) and (9) located towards the outlet of the thermal enclosure is 108 mm.

Furthermore, the internal diameters of the cracking tube at the inlet (10) and at the outlet (11) of the enclosure (1) being 172 mm and 108 mm respectively, the ratio between the internal diameters of the tube to entry and exit is therefore 1.6. Furthermore, the reaction volume of the first half of the length of the cracking tube, corresponding to the straight sections (2), (3), (4) and (5), is 1.84 times greater than the reaction volume of the second half the length of the cracking tube, corresponding to the straight sections (6), (7), (8) and (9).

The thermal radiation enclosure of the steam cracking furnace is provided with burners arranged on the walls of the enclosure, in five horizontal rows, located at equal distance from each other. The thermal power of all of these burners is evenly distributed between these five rows.

In this cracking tube, a mixture of liquid hydrocarbons and water vapor is circulated. The liquid hydrocarbons consist of a naphtha with a density of 0.718, having a distillation range of ASTM 45/180 _° C. and weight contents of 35% in linear paraffins, 29.4% in branched paraffins, 28.3% in compounds cyclic and 7.3% aromatic compounds. The composition of the mixture of naphtha and water vapor used is such that the weight ratio of the quantity of naphtha to the quantity of water vapor is 4. The naphtha is thus introduced into the cracking tube according to a flow rate of 3500 kg / h and water vapor at a flow rate of 875 kg / h.

The cracking temperature of the mixture of naphtha and steam rises from 470 _° C at the entrance to the oven radiation zone up to 775 _° C at the exit from this zone. The evolution of the cracking temperature of the mixture along the cracking tube is described by the curve (a) of FIG. 4, representing on the abscissa the reaction volume (in liters) crossed by the mixture and on the ordinate the cracking temperature (in _° C) of the mixture. Curve (a) shows that the cracking temperature of the mixture increases in its initial part relatively slowly as a function of the reaction volume passed through. The pressure of the mixture is at the outlet of the oven of 170 kPa.

The average residence time of the mixture of naphtha and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 1030 milliseconds. Furthermore, the average residence time of this mixture circulating in the first half of the length of the cracking tube is 2.3 times greater than that in the second half of the length of the tube.

Under these conditions, 580 kg of ethylene, 520 kg of propylene, 105 kg of isobutene, 165 kg of butadiene and 145 kg of ethane are produced per hour. The ethane thus produced in this furnace is then subjected to a secondary stage of steam cracking making it possible to transform it into ethylene according to a weight yield of 85% and thereby improving the overall production of ethylene from the steam cracking installation. It is also found that the production of higher olefins and butadiene are relatively high compared to the production of ethylene. Thus, for a ton of ethylene produced and collected at the outlet of the steam cracking installation, the production of propylene, isobutene and butadiene are 740 kg, 150 kg and 235 kg respectively.

• S₃ = 0,74
• S₄ = 0,53

Furthermore, the selectivity Sa in produced hydrocarbons, comprising 3 carbon atoms, and the selectivity S ₄ in produced hydrocarbons, comprising 4 carbon atoms, are the following:

• S ₃ = 0.74
• S ₄ = 0.53

These two relatively high values show that the steam cracking reaction of naphtha thus carried out promotes the formation of olefins containing 3 to 4 carbon atoms, as well as the formation of butadiene.

Free 2

The operation is carried out in a steam cracking furnace identical to that of Example 1. A mixture of naphtha and steam is circulated in the cracking tube of this furnace identical to that used in Example 1. The flow rates of naphtha and water vapor circulating in the tube are respectively 4800 and 1200 kg / h, this increase in flow rates compared to those of Example 1 can be easily achieved thanks to the fact that the cracking tube used has a relatively low pressure drop.

Under these conditions, the cracking temperature of the mixture of naphtha and water vapor rises from 480 ° C. at the entrance to the radiation zone of the furnace to 775 _° C. at the exit from this zone. The pressure of the mixture is 170 kPa at the outlet of the oven.

Under these conditions, the average residence time of the mixture of naphtha and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 900 milliseconds. Furthermore, the average residence time of this mixture circulating in the first half of the length of the cracked tube is 2.3 times greater than that in the second half of the length of the tube. Per hour, 640 kg of ethylene, 612 kg of propylene, 122 kg of isobutene, 200 kg of butadiene and 170 kg of ethane are thus produced. The ethane thus produced in this furnace is then subjected to a secondary stage of steam cracking making it possible to transform it into ethylene according to a weight yield of 85% and thereby improving the overall production of ethylene from the steam cracking installation. It is found that the productions of olefins and of diolefins are higher than those of Example 1, because of the gain on the flow rates of raw materials which makes it possible to accomplish the steam cracking furnace of the present invention. Furthermore, it is also observed that the productions of higher olefins and of butadiene are relatively high compared to the production of ethylene. Thus, for a tonne of ethylene produced and collected at the outlet of the steam cracking installation, the production of propylene, isobutene and butadiene are 780 kg, 155 kg and 255 kg respectively.

• Sa = 0,77
• S₄ = 0,56.

Furthermore, the selectivities S ₃ and S ₄ are as follows:

• Sa = 0.77
• S ₄ = 0.56.

These two relatively high values show that for this type of oven and thanks to the process of the invention, the steam cracking reaction of naphtha promotes the formation of olefins having 3 to 4 carbon atoms, as well as the formation of butadiene, to the detriment of the formation of ethylene.

Example 3 (comcaratift

A steam cracking oven comprises a thermal radiation enclosure, identical in shape and size to that of Example 1. In this enclosure, a cracking tube of refractory steel based on nickel and chromium, of a weight, is placed. total substantially identical to that of Example 1, having an internal diameter of 108 mm, a thickness of 8 mm and, taking into account the capacity of the enclosure and the mechanical and thermal stresses of the oven, a total length of 80 meters between the entrance and the exit of the enclosure. The ratio between the length and the internal diameter of the tube is 740. This cracking tube is arranged in the form of a coil comprising eight straight horizontal sections, of equal length each, connected to each other by elbows. The internal diameter of these straight sections is constant and equal to 108 mm. Thus, the internal diameters of the tube at the inlet and at the outlet of the enclosure are identical. Likewise, the reaction volume of the first half of the length of the cracking tube, corresponding to the first four straight sections, is identical to the reaction volume of the second half of the length of the cracking tube, corresponding to the last four straight sections.

The thermal radiation enclosure of the steam cracking furnace is provided with burners arranged on the walls of the enclosure, in five horizontal rows, located at equal distance from each other. The thermal power of all of these burners is evenly distributed between these five rows.

In this cracking tube, a mixture of naphtha and steam is circulated, identical to that used in Example 1. Taking into account the relatively high pressure drops in this cracking tube, the flow rates in naphtha and in steam are 3500 kg / h and 875 kg / h respectively.

The cracking temperature of the mixture of naphtha and steam is 490 _° C at the entrance to the oven radiation zone and 775 ° C at the exit from this zone. The evolution of the cracking temperature of the mixture along the cracking tube is described by curve (b) of FIG. 4, representing on the abscissa the reaction volume (in liters) crossed by the mixture and on the ordinate the cracking temperature (in _° C) of the mixture. Curve (b) shows that the cracking temperature of the mixture increases in its initial part relatively rapidly as a function of the reaction volume passed through. The pressure of the mixture is at the outlet of the oven of 170 kPa.

The average residence time of the mixture of naphtha and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 830 milliseconds.

Under these conditions, 588 kg of ethylene, 501 kg of propylene, 95 kg of isobutene, 147 kg of butadiene and 155 kg of ethane are produced per hour. The ethane thus produced in this furnace is then subjected to a secondary stage of steam cracking making it possible to transform it into ethylene according to a weight yield of 85% and thereby improving the overall production of ethylene from the steam cracking installation. It is noted that the productions of olefins and of diolefins are lower than those of Example 2 and that the productions of propylene, isobutene and butadiene compared to the production of ethylene are relatively lower than those observed in the examples 1 and 2. Thus, for a tonne of ethylene produced and collected at the outlet of the steam cracking installation, the productions of propylene, isobutene and butadiene are 696 kg, 132 kg and 204 kg respectively.

• Ss - 0,70

S₄ - 0,48.Furthermore, the selectivities S ₃ and S ₄ are as follows:

• Ss - 0.70

S ₄ - 0.48.

These two values are lower than those obtained in Examples 1 and 2.

In addition, the maximum capacity loss of such a steam cracking furnace is approximately 35%, for an unchanged volume of the thermal radiation enclosure and for substantially identical mechanical and thermal stresses of the furnace, in comparison with the furnace. described in Example 1.

Example 4 (comcarative)

A steam cracking oven comprises a thermal radiation enclosure, identical in shape and size to that of Example 1. In this enclosure, a cracking tube of refractory steel based on nickel and chromium, of a weight, is placed. total substantially identical to that of Example 1, having an internal diameter of 140 mm, a thickness of 8 mm and, taking into account the capacity of the enclosure and the mechanical and thermal stresses of the oven, a total length of 64 meters between the entry and exit of the enclosure. The ratio between the length and the internal diameter of the tube is 457. This cracking tube is arranged in the form of a coil comprising eight horizontal straight sections, of equal length each, connected to each other by elbows. The internal diameter of these straight sections is constant and equal to 140 mm. Thus, the internal diameters of the tube at the inlet and at the outlet of the enclosure are identical. Likewise, the reaction volume of the first half of the length of the cracking tube, corresponding to the first four straight sections, is identical to the reaction volume of the second half of the length of the cracking tube, corresponding to the last four straight sections.

The thermal radiation enclosure of the steam cracking furnace is provided with burners arranged on the walls of the enclosure, in five horizontal rows, located at equal distance from each other. The thermal power of all of these burners is evenly distributed between these five rows.

In this cracking tube, a mixture of naphtha and water vapor is circulated, identical to that used in Example 1. The naphtha is introduced therein at a rate of 3500 kg / h and the vapor of water at a flow rate of 875 kg / h.

The cracking temperature of the mixture of naphtha and water vapor rises from 500 _° C at the entrance to the radiation zone of the furnace up to 775 _° C at the exit from this zone. The pressure of the mixture is at the outlet of the oven of 170 kPa.

The average residence time of the mixture of naphtha and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 900 milliseconds.

• S₃ = 0,715
• S₄ = 0,500.

Under these conditions, 585 kg of ethylene, 506 kg of propylene, 101 kg of isobutene, 156 kg of butadiene and 150 kg of ethane are produced per hour. The ethane thus produced in this furnace is then subjected to a secondary steam cracking step making it possible to transform it into ethylene according to a weight yield of 85% and thereby improving the overall production of ethylene of the steam cracking installation. It can be seen that the propylene, isobutene and butadiene productions are relatively low. Thus, for a ton of ethylene produced, and collected at the outlet of the steam cracking installation, the productions of propylene, isobutene and butadiene are respectively 710 kg, 140 kg and 219 kg. Furthermore, the selectivities S ₃ and S ₄ are as follows:

• S ₃ = 0.715
• S ₄ = 0.500.

These two values are lower than those obtained in Example 1.

Free 5

• - 5 % de la puissance thermique totale sur la première rangée de brûleurs, située dans le haut de l'enceinte, au voisinage de l'entrée du tube de craquage,
• - 10 % sur la deuxième rangée de brûleurs, située immédiatement en dessous de la première,
• - 15 % sur la troisième rangée de brûleurs, située immédiatement en dessous de la deuxième,
• - 30 % sur la quatrième rangée de brûleurs, située immédiatement en dessous de la troisième, et
• - 40 % sur la cinquième rangée de brûleurs, située immédiatement en dessous de la quatrième, au voisinage de la sortie du tube de craquage. Ainsi, le rapport entre la puissance thermique des brûleurs appliquée à la première moitié de la longueur du tube, située vers l'entrée de l'enceinte, et celle appliquée à la deuxième moitié de la longueur du tube, située vers la sortie de cette enceinte, est de 22,5/77,5.

The operation is carried out in a steam cracking furnace identical to that of Example 1, except that the thermal power of all the burners is not evenly distributed between the five rows of burners, but is distributed in the same way next :

• - 5% of the total thermal power on the first row of burners, located at the top of the enclosure, near the entrance to the cracking tube,
• - 10% on the second row of burners, located immediately below the first,
• - 15% on the third row of burners, located immediately below the second,
• - 30% on the fourth row of burners, located immediately below the third, and
• - 40% on the fifth row of burners, located immediately below the fourth, near the outlet of the cracking tube. Thus, the ratio between the thermal power of the burners applied to the first half of the length of the tube, located towards the inlet of the enclosure, and that applied to the second half of the length of the tube, located towards the outlet of this pregnant, is 22.5 / 77.5.

The heat flux table measured inside the thermal radiation enclosure of the furnace is, under these conditions, represented in FIG. 2 by the surface inscribed in the three-dimensional graph connecting by the three coordinate axes, the length L of the thermal enclosure, the height H of this enclosure and the thermal flux F. FIG. 2 shows, in particular, that the maximum of the thermal flux of radiation is located in the lower part of the thermal enclosure, corresponding to the second half the length of the cracking tube located towards the outlet of the radiation thermal enclosure.

In this cracking tube, a mixture of liquid hydrocarbons and water vapor is circulated. The liquid hydrocarbons consist of a naphtha of density 0.690, having a distillation range ASTM 45/180 _° C and weight contents of 38.2% in linear paraffins, of 36.9% in branched paraffins, of 17.1% in cyclanic compounds and 7.8% in aromatic compounds. The composition of the mixture of naphtha and water vapor used is such that the weight ratio of the quantity of naphtha to the quantity of water vapor is 4. The naphtha is thus introduced into the cracking tube according to a flow rate of 3500 kg / h and water vapor at a flow rate of 875 kg / h.

The cracking temperature of the mixture of naphtha and water vapor rises from 435 _° C at the entrance to the radiation zone of the furnace up to 775 ° C at the exit from this zone. The evolution of the temperature of the mixture along the cracking tube is described by the curve (a) of FIG. 5 representing on the abscissa the average residence time (in milliseconds) of the mixture circulating in the cracking tube from the inlet up to the exit from the radiation area of the oven and on the ordinate the cracking temperature (in _° C) of the mixture. Curve (a) shows that the cracking temperature of the mixture increases in its initial part relatively slowly as a function of the average residence time of the mixture in the cracking tube and that in particular most of the residence time of the mixture is performed at a relatively low cracking temperature, in particular at a temperature below 700 _° C. The pressure of the mixture is at the outlet of the oven of 170 kPa. Given the distribution of the heat flux in the radiation enclosure, the thermal power applied to the second half of the length of the cracking tube, located towards the exit of the radiation area, is 3.4 times that applied to the first half of the tube, located towards the entrance to this area.

The average residence time of the mixture of naphtha and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 1180 milliseconds. Furthermore, the average residence time of this mixture circulating in the first half of the length of the cracking tube is 2.6 times greater than that in the second half of the length of the tube.

Under these conditions, 620 kg of ethylene, 590 kg of propylene, 110 kg of isobutene, 180 kg of butadiene and 150 kg of ethane are produced per hour. The ethane thus produced in this furnace is then subjected to a secondary stage of steam cracking making it possible to transform it into ethylene according to a weight yield of 85% and thereby improving the overall production of ethylene from the steam cracking installation. It is also found that the production of higher olefins and butadiene are relatively high compared to the production of ethylene. Thus, for a tonne of ethylene produced and collected at the outlet of the steam cracking installation, the productions of propylene, isobutene and butadiene are 790 kg, 147 kg and 240 kg respectively.

• Sa = 0,79
• S₄ = 0,57.

Furthermore, the selectivity Sa in produced hydrocarbons, comprising 3 carbon atoms, and the selectivity S ₄ in produced hydrocarbons, comprising 4 carbon atoms, are the following:

• Sa = 0.79
• S ₄ = 0.57.

These two relatively high values show that the steam cracking reaction of naphtha promotes the formation of olefins having 3 to 4 carbon atoms, as well as the formation of butadiene.

Example 6

The operation is carried out in a steam cracking oven identical to that of Example 5. A mixture of naphtha and steam identical to that used in Example 5 is circulated in the cracking tube of this oven. flow rates of naphtha and water vapor circulating in the tube are respectively 4800 kg / h and 1200 kg / h, this increase in flow rates compared to those of Example 5 can be easily achieved thanks to the fact that the tube cracking agent used has a relatively low pressure drop.

Under these conditions, the cracking temperature of the mixture of naphtha and water vapor rises from 445 _° C. at the entrance to the radiation zone of the furnace up to 775 _° C. at the exit from this zone. The evolution of the cracking temperature of the mixture along the cracking tube is described by curve (b) of FIG. 5, representing on the abscissa the average residence time (in milliseconds) of the mixture circulating in the cracking tube from the inlet to the outlet of the radiation area of the oven and on the ordinate the cracking temperature (in _° C) of the mixture. Curve (b) shows that the cracking temperature of the mixture increases in its initial part relatively slowly as a function of the average residence time of the mixture in the cracking tube and that in particular most of the residence time of the mixture is performed at a relatively low cracking temperature, in particular at a temperature below 700 _° C. The pressure of the mixture is 170 kPa at the outlet of the oven.

Under these conditions, the average residence time of the mixture of naphtha and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 1020 milliseconds. Furthermore, the average residence time of this mixture circulating in the first half of the length of the cracking tube is 2.6 times greater than that in the second half of the length of the tube. 750 kg of ethylene, 770 kg of propylene, 110 kg of isobutene, 180 kg of butadiene and 200 kg of ethane are thus produced per hour. The ethane thus produced in this furnace is then subjected to a secondary stage of steam cracking making it possible to transform it into ethylene at a weight yield of 85% and thereby improving the overall production of ethylene from the steam cracking installation. It is further noted that the productions of olefins and of diolefin are higher than those of Example 5, due to the gain on the flow rates of raw materials which the steam cracking furnace of the present invention makes it possible to achieve. Furthermore, it is also observed that the productions of higher olefins and of butadiene are relatively high compared to the production of ethylene. Thus for a tonne of ethylene produced and collected at the outlet of the steam cracking installation, the production of propylene, isobutene and butadiene are 837 kg, 158 kg and 260 kg respectively.

• Sa = 0,84
• S₄ = 0,61.

Furthermore, the selectivities Sa and S ₄ are as follows:

• Sa = 0.84
• S ₄ = 0.61.

These two relatively high values show that the steam cracking reaction of naphtha thus carried out promotes the formation of olefins containing 3 to 4 carbon atoms, as well as the formation of butadiene, to the detriment of the formation of ethylene.

Example 7 (comparative)

The operation is carried out in a steam cracking oven comprising a thermal enclosure, a cracking tube and burners, identical to those of Example 3 (comparative). The thermal power of all the burners is also as in Example 3 (comparative), distributed homogeneously between the five rows.

The thermal flux table measured inside the thermal radiation enclosure of the furnace is, under these conditions, represented in FIG. 3 by the surface inscribed in the three-dimensional graph connecting by the three coordinate axes, the length L of the thermal enclosure, the height H of this enclosure and the thermal flux F. FIG. 3 shows, in particular, that the maximum of the thermal flux of radiation is located in the upper part of the thermal enclosure, corresponding to the first half the length of the cracking tube located towards the entrance to the thermal enclosure.

In this cracking tube, a mixture of naphtha and steam is circulated, identical to that used in Example 5. Taking into account the relatively high pressure drops in this cracking tube, the flow rates in naphtha and in steam are 3500 kg / h and 875 kg / h respectively.

The cracking temperature of the mixture of naphtha and water vapor rises from 495 _° C at the entrance to the radiation zone of the furnace up to 775 _° C at the exit from this zone. The evolution of the cracking temperature of the mixture along the cracking tube is described by curve (c) of FIG. 5, representing on the abscissa the average residence time (in milliseconds) of the mixture circulating in the cracking tube from the inlet to the outlet of the radiation area of the oven and on the ordinate the cracking temperature (in _° C) of the mixture. Curve (c) clearly shows that the cracking temperature of the mixture increases in its initial part rapidly as a function of the residence time of the mixture in the cracking tube, and that in particular a significant part of the residence time of the mixture is achieved at a relatively high cracking temperature, in particular at a temperature above 700 _° C. The pressure of the mixture is at the outlet of the oven of 170 kPa. Given the distribution of the heat flux in the enclosure, the thermal power applied to the second half of the length of the cracking tube is identical to that applied to the first half of the length of the tube.

The average residence time of the mixture of naphtha and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 840 milliseconds.

Under these conditions, 635 kg of ethylene, 545 kg of propylene, 90 kg of isobutene, 140 kg of butadiene and 170 kg of ethane are produced per hour. The ethane thus produced in this furnace is then subjected to a secondary stage of steam cracking making it possible to transform it into ethylene according to a weight yield of 85% and thereby improving the overall production of ethylene from the steam cracking installation. It is further noted that the productions of olefins and diolefins are lower than that of Example 6 and that the productions of propylene, isobutene and butadiene compared to the production of ethylene are relatively lower than in Examples 5 and 6. Thus, for a tonne of ethylene produced and collected at the outlet of the steam cracking installation, the productions of propylene, isobutene and butadiene are 700 kg, 115 kg and 180 kg respectively.

• Sa = 0,700
• S₄ = 0,465.

Furthermore, the selectivities S ₃ and S ₄ are as follows:

• Sa = 0.700
• S ₄ = 0.465.

These two values are lower than those obtained in Examples 5 and 6.

In addition, the maximum capacity loss of such a steam cracking furnace is approximately 35%, for an unchanged volume of the thermal radiation enclosure and for substantially identical mechanical and thermal stresses of the furnace, in comparison with the furnace. in example 5.

Example

A steam cracking furnace, as shown diagrammatically in FIG. 1, comprises a thermal radiation enclosure (1) identical to that described in Example 1. In this enclosure, a cracking tube of refractory steel based on nickel is placed. and of chromium, of dimensions different from that described in Example 1; it has an average internal diameter of 108 mm, a thickness of 8 mm and, taking into account the capacity of the enclosure (1), a total length of 80 meters, comprised between the inlet (10) and the outlet (11 ). This cracking tube is arranged in the form of a serpentine, comprising eight horizontal straight sections, of equal length each, connected to each other by elbows. The internal diameter of the sections (2) and (3) located towards the entrance to the thermal enclosure is 135 mm; the following sections (4) and (5) have an internal diameter of 117 mm; then sections (6) and (7) have an internal diameter of 99 mm; the internal diameter of the sections (8) and (9) located towards the outlet of the thermal enclosure is 81 mm.

Furthermore, the internal diameters of the cracking tube at the inlet (10) and at the outlet (11) of the enclosure (1) being 135 mm and 81 mm respectively, the ratio between the internal diameters of the tube to entry and exit is 1.7. Furthermore, the reaction volume of the first half of the length of the cracking tube, corresponding to the straight sections (2), (3), (4), and (5), is 1.95 times greater than the reaction volume of the second half of the length of the cracking tube, corresponding to the straight sections (6), (7), (8) and (9).

• - 5 % de la puissance thermique totale sur la première rangée de brûleurs, située dans le haut de l'enceinte, au voisinage de l'entrée du tube de craquage,
• - 10 % sur la deuxième rangée de brûleurs, située immédiatement en dessous de la première,
• - 20 % sur la troisième rangée de brûleurs, située immédiatement en dessous de la deuxième,
• - 25 % sur la quatrième rangée de brûleurs, située immédiatement en dessous de la troisième, et
• - 40 % sur la cinquième rangée de brûleurs, située immédiatement en dessous de la quatrième, au voisinage de la sortie du tube de craquage. Ainsi, le rapport entre la puissance thermique des brûleurs appliquée à la première moitié de la longueur du tube, située vers l'entrée de l'enceinte, et celle appliquée à la deuxième moitié de la longueur du tube, située vers la sortie de cette enceinte, est de 25/75.

The thermal radiation enclosure of the steam cracking furnace is provided with burners arranged on the walls of the enclosure, in five horizontal rows, located at equal distance from each other. The total thermal power is distributed among the five rows of burners as follows:

• - 5% of the total thermal power on the first row of burners, located at the top of the enclosure, near the entrance to the cracking tube,
• - 10% on the second row of burners, located immediately below the first,
• - 20% on the third row of burners, located immediately below the second,
• - 25% on the fourth row of burners, located immediately below the third, and
• - 40% on the fifth row of burners, located immediately below the fourth, near the outlet of the cracking tube. Thus, the ratio between the thermal power of the burners applied to the first half of the length of the tube, located towards the inlet of the enclosure, and that applied to the second half of the length of the tube, located towards the outlet of this pregnant, is 25/75.

In this cracking tube, a mixture of ethane and water vapor is circulated. The composition of the mixture of ethane and water vapor used is such that the weight ratio of the amount of ethane to the amount of water vapor is 2.25. Ethane is thus introduced into the cracking tube at a rate of 1800 kg / h and water vapor at a rate of 800 kg / h.

The cracking temperature of the mixture of ethane and water vapor rises from 585 _° C at the entrance to the radiation zone of the furnace up to 846 ° C at the exit from this zone. The pressure of the mixture leaving the oven is 170 kPa. Taking into account the distribution of the thermal flux in the enclosure, the thermal power applied to the second half of the length of the cracking tube, located towards the exit of the radiation zone, is 3 times greater than that applied to the first half the length of the tube, located towards the entrance of this area.

The average residence time of the mixture of ethane and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 640 milliseconds.

Under these conditions, 850 kg of ethylene and 55 kg of methane are produced per ton of processed ethane. It can be seen that the selectivity for ethylene is therefore 85%.

Example 9 (comparative)

The operation is carried out in a steam cracking oven comprising a thermal enclosure, a cracking tube and burners, identical to those of Example 3 (comparative). The thermal power of all the burners is, as in Example 3 (comparative), evenly distributed between the five rows.

In this cracking tube, a mixture of ethane and water vapor is circulated, identical to that used in Example 8. The ethane is introduced therein at a rate of 1800 kg / h and the vapor water at a flow rate of 800 kg / h.

The cracking temperature of the mixture of ethane and water vapor rises from 636 _° C at the entrance to the radiation zone of the furnace up to 846 ° C at the exit from this zone. The pressure of the mixture leaving the oven is 170 kPa. Given the distribution of the heat flux in the enclosure, the thermal power applied to the second half of the length of the cracking tube is identical to that applied to the first half of the length of the tube.

The average residence time of the mixture of ethane and water vapor circulating in the cracking tube between the inlet and the outlet of the radiation zone of the furnace is 585 milliseconds.

Under these conditions, 805 kg of ethylene and 71 kg of methane are produced per tonne of transformed ethane.

It is noted that the selectivity for ethylene is 80.5%, a value lower than that of Example 8 and that the quantity of methane produced has increased compared to that of Example 8.

Example 10

The procedure is exactly as in Example 8, except that instead of using ethane, a mixture of gaseous hydro carbides is used comprising 76% by weight of ethane, 19% by weight of propane and 5% by weight of propylene. The pressure at the outlet of the oven is 175 kPa instead of 170 kPa. The cracking temperature of the mixture is at the entry of the radiation zone of 575 _° C instead of 585 ° C, and at the exit of this zone of 848 ° C instead of 846 _° C. The average residence time the mixture of gaseous hydrocarbons and water vapor circulating in the cracking tube between the entry and the exit of the radiation zone is 665 milliseconds instead of 640 milliseconds.

Under these conditions, 785 kg of ethylene and 120 kg of methane are produced per ton of the mixture of gaseous hydrocarbons transformed. It is found that the selectivity for ethylene is 78.5%.

The procedure is exactly as in Example 9 (comparative), except that instead of using ethane, a mixture of gaseous hydrocarbons is used identical to that used in Example 10. The pressure the mixture at the outlet of the oven is 175 kPa, instead of 170 kPa. The cracking temperature of the mixture is at the entry of the radiation zone of 610 _° C instead of 636 _° C and at the exit of this zone of 848 _° C instead of 846 _° C. The residence time of the mixture of gaseous hydrocarbons and water vapor circulating in the cracking tube between the entry and the exit of the radiation zone is 610 milliseconds instead of 585 milliseconds.

Under these conditions, 750 kg of ethylene and 195 kg of methane are manufactured per ton of the mixture of gaseous hydrocarbons transformed. It is found that the selectivity for ethylene is 75%, a value lower than that of Example 10 and that the quantity of methane produced has considerably increased.

Claims (10)

1. A process for the preparation of olefins and diolefins by cracking liquid or gaseous hydrocarbons in the presence of steam, wherein through a radiation zone of a furnace a mixture of hydrocarbons and steam, circulating in a cracking tube disposed within said zone, is caused to flow at a furnace outlet pressure in the range of 120 to 240 kPa, with the cracking temperature of the mixture at the inlet of the radiation zone ranging from 400 to 700_°C and at the outlet of said zone from 720 to 880°C, characterized in that

(a) between the inlet and the outlet of the radiation zone the mean residence period of the mixture of hydrocarbons and steam circulating in the cracking tube ranges from 300 to 1800 milliseconds, and

(b) the reaction volume of the first half of the cracking tube length closer to the inlet of the radiation zone is 1.3 to 4 times greater than that of the second half of the tube length which is located closer to the outlet of said zone.

2. The process according to claim 1, characterized in that the increase of the cracking temperature of the mixture of hydrocarbons and steam is connected with a nonhomogeneous distribution of the thermal capacity of the furnace effective alongside the cracking tube, which distribution is such that the thermal capacity effective on the second half of the tube length closer to the outlet of the radiation zone is 1.5 to 5 times greater than that effective on the first half of the tube length closer to the inlet of said zone.

3. The process according to claim 2, characterized in that the thermal capacity effective on the second half of the tube length is 2 to 4 times greater than that effective on the first half of the tube length.

4. The process according to claim 1, characterized in that the mean residence period of the mixture of hydrocarbons and steam circulating in the cracking tube ranges from 850 to 1800 milliseconds between the inlet and the outlet of the radiation zone if liquid hydrocarbons are employed.

5. The process according to claim 1, characterized in that the mean residence period of the mixture of hydrocarbons and steam circulating in the cracking tube ranges from 400 to 1400 milliseconds between the inlet and the outlet of the radiation zone if gaseous hydrocarbons are employed.

6. The process according to claim 1, characterized in that the composition of the mixture of hydrocarbons and steam employed is such that the weight ratio of the amount of hydrocarbons to the amount of steam is in the range of 1 to 10.

7. The process according to claim 1, characterized in that the hydrocarbons employed are liquid hydrocarbons selected among naphta, the light petrols, the gas oil as well as among their mixtures with the saturated hydrocarbons having 3 to 6 carbon atoms, or also are gaseous hydrocarbons consisting of alkanes having 2 to 4 carbon atoms or of the mixtures thereof, which alkanes optionally are employed as a mixture with alkenes having 2 to 6 carbon atoms and/or methane and/or alkanes having 5 to 6 carbon atoms.

8. An apparatus composed of a furnace for the cracking of hydrocarbons in the presence of steam, which furnace comprises a thermal radiation container equipped with heating means, through which container at least one cracking tube extends in which the mixture of steam and hydrocarbons to be cracked is circulating, characterized in that

(a) the ratio between the length and the mean inner diameter of the cracking tube extending through the thermal radiation container ranges from 200 to 600, and

(b) the inner diameter of the cracking tube decreases in a continuous or non-continuous manner from the inlet to the outlet of the thermal radiation container such that the ratio between the inner diameters of the tube at the inlet and at the outlet of said container is in the range of 1.2 to 3.

9. The apparatus according to claim 8, characterized in that the heating means consist of burners the thermal capacity of which alongside the cracking tube from the inlet to the outlet of the thermal radiation container increases such that the ratio between the thermal capacity effective on the first half of the cracking tube length closer to the inlet of the thermal radiation container and that effective on the second half of the tube length closer to the outlet of said container ranges from 40/60 to 15/85.

10. The apparatus according to claim 8, characterized in that the cracking tube is composed of a sequence of tubes having inner diameters which are decreasing from the inlet towards the outlet of the thermal radiation container.

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