EP0345182B1 - Process and apparatus for the fabrication of olefins and diolefins by steam cracking of hydrocarbons, controlled by a system comprising an infrared spectrophotometer - Google Patents

Abstract

The present invention relates to a process and an apparatus for steam cracking a mixture of hydrocarbons comprising passing steam and the mixture of hydrocarbons through at least one heated cracking tube. The process is characterised in that it is controlled by analyzing the mixture of hydrocarbons fed to the cracking tube with an infrared spectrophotometer to determine the absorbances at a number n of wavelengths in the range 0.8 to 2.6 microns and by using the results of this absorbance to determine one or more values V of steam cracking process conditions which will achieve a desired value P of the space time yield of one or more products of the steam cracking reaction.

Description

The present invention relates to a process and an apparatus for cracking hydrocarbons in the presence of water vapor, intended for manufacturing olefins and diolefins, in particular ethylene and propylene. It consists in particular in using an infrared spectrophotometer making it possible to analyze the hydrocarbons supplying a cracking furnace, and in controlling as a function of this analysis in particular the yields of olefins and of diolefins.

It is known to perform the cracking or pyrolysis of hydrocarbons in the presence of water vapor, also called steam cracking, by passing a mixture of hydrocarbons and water vapor through a cracking tube placed in an oven heated to high temperature. The hydrocarbons are subjected to a cracking reaction which transforms them in particular (i) 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 isoprene, (ii) in a liquid hydrocarbon fraction, also called "steam cracking gasoline", comprising in particular hydrocarbons containing from 5 to 12 carbon atoms, (iii) as well as by-products undesirable, such as methane.

Generally, for each type of hydrocarbon to be cracked, the conditions of the cracking reaction are chosen which make it possible to manufacture at least one product or a group of products, such as an olefin, a diolefin or a steam cracking gasoline, with a desired yield and fixed in advance. The term “yield of a product of the cracking reaction” is understood to mean the weight ratio of the quantity produced of this product to the quantity used of hydrocarbons. However, it is common to observe that a cracking furnace is supplied with a mixture of hydrocarbons, the nature and composition of which can frequently vary over time, depending on the origin of these hydrocarbons. It therefore appears necessary to modify the conditions of the cracking reaction as often as the nature and composition of the hydrocarbon mixture changes, if it is desired to manufacture at least one product or a group of products. according to a desired yield. As a result, generally the characteristics of the hydrocarbon mixtures to be cracked must be known and determined as frequently as possible during the cracking reaction. However, these characteristics are generally determined by separate and specific measurements which require for each of them the implementation of a specific apparatus and analytical method, and which require a relatively long time. Also, when the nature and the composition of the mixture of hydrocarbons vary over time, it follows that the conditions of the cracking reaction are modified with a relatively long delay and that it is no longer possible to maintain the yields desired values set in advance. We can easily understand the importance of such drawbacks, when we also know the generally considerable size of an industrial hydrocarbon cracking installation.

A process and an apparatus for steam cracking of hydrocarbons have now been found which make it possible to avoid the drawbacks mentioned above and to manufacture olefins and diolefins with yields which can be fixed in advance at desired values. One of the aims of the present invention is to control the productivity of one or more products of a hydrocarbon steam cracking reaction directly by means of the near infrared absorbance measurements of the mixture of hydrocarbons feeding a tube. cracked. One of the advantages of the present process is to be able to control the steam cracking reaction while avoiding seeking to know and highlight the physical and / or chemical characteristics of the mixture of hydrocarbons to be cracked. More precisely, all the digital data obtained by absorbance measurements of the mixture of hydrocarbons at selected wavelengths in the near infrared can be used for information for monitoring the steam cracking reaction, with a view to '' obtain a desired productivity P in one or more products of this reaction. In particular, the present invention uses an infrared spectrophotometer which, during the cracking reaction, makes it possible to perform a series of measurements in an extremely short time. the results of which make it possible to directly determine the reaction conditions necessary for the manufacture of olefins, diolefins and other products of the reaction with desired yields.

The present invention therefore relates to a process for steam cracking a mixture of hydrocarbons consisting in passing steam and the mixture of hydrocarbons through at least one heated cracking tube, a process characterized in that the process is controlled (a) by analyzing the mixture of hydrocarbons feeding the cracking tube using an infrared spectrophotometer to determine n absorbances at n wavelengths ranging from 0.8 to 2.6 · 10⁻⁶ m, (b) using the results of n absorbances to determine at least one V value of one of the conditions of the steam cracking reaction, and (c) by operating the steam cracking to the value or values V thus determined, so to obtain a desired value P fixed in advance of the productivity in one or more products of the steam cracking reaction.

One of the essential characteristics of the present invention is to perform, during the steam cracking reaction, absorbance measurements on the mixture of hydrocarbons feeding the cracking tube using an infrared spectrophotometer operating according to the reflection technique. , or the transmission technique, or even a combination of these two techniques. The absorbance is generally defined, according to BEER-LAMBERT's law, as being the decimal logarithm of the ratio between the intensity Io of the radiation emitted by the infrared spectrophotometer and the intensity I of the radiation transmitted and / or reflected by the mixture of hydrocarbons.

More particularly, it involves performing several times during the steam cracking reaction a series of n absorbance measurements of the mixture of hydrocarbons, at n wavelengths chosen in the near infrared range, ranging from 0.8 at 2.6 · 10⁻⁶ m, preferably from 1.0 to 2.5, and more particularly from 1.4 to 2.5 · 10⁻⁶ m. The number n of absorbance measurements is generally from 2 to 20 approximately, preferably from 2 to 10. The choice of the number n of absorbance measurements is partly related to the precision with which it is then desired to determine the value V of at least one of the conditions of the steam cracking reaction. One can, for example, choose to carry out the absorbance measurements at wavelengths chosen from the following, expressed 10⁻⁶ m, or substantially similar wavelengths:
2.141-2.166-2.181-2.278-2.308-2.347-2.375-2.398-2.439-2.457 and 2.475.

More particularly, the absorbance measurements can be carried out at the following 5 wavelengths, expressed in 10⁻⁶ m, or at substantially similar wavelengths: 2,278-2,308-2,398-2,439 and 2,475.

The wavelengths to be used in the process in order to obtain a desired productivity P in one or more products of a steam cracking reaction can be chosen by statistical methods using factor analyzes and multilinear regressions, during a calibration procedure. The latter may notably consist in varying the nature of the mixture of hydrocarbons to be cracked and the reaction conditions, according to an orthogonal experimental design, carried out in a cracking tube of an industrial production unit or in a cracking of laboratory, in particular of a micropyrolyser, and to choose the wavelengths in the near infrared, so that one can determine with an optimal precision and sufficient to carry out the process a correlative relation binding the productivity P to n results Ri of the n absorbance measurements and at the V values of the reaction conditions. The wavelengths generally chosen are those of which the amplitude of absorbance varies greatly during the calibration procedure.

Another essential characteristic of the present invention is to fix, in an extremely short time, as a function of the absorbance measurements, at least one of the conditions of the cracking reaction so that the productivity of one or more reaction products is equal to a desired value P. The conditions of the steam cracking reaction are those usually known for this type of reaction and can in particular be chosen among the flow rates of water vapor and of mixture of hydrocarbons supplying the cracking tube, the cracking temperature at any point of this tube, in particular at the entry or at the exit of the radiation zone of the furnace, the cracking pressure at any point of this tube, in particular at the exit from the radiation zone of the furnace, as well as the weight ratio between the quantity used of the mixture of hydrocarbons and that of water vapor.

Furthermore, one of the objects of the present invention is to control the steam cracking process by fixing in advance at a desired value P the productivity of one or more products resulting from the cracking reaction. In particular, the productivity can be fixed in an olefin such as ethylene, propylene or butene-1, the productivity in a diolefin such as butadiene, or also the productivity in several reaction products such as "gasoline". steam cracking ". The productivity of one or more reaction products can be defined by the production rate, corresponding to the quantity produced of the product (s) per unit of time. Productivity can also be defined as the yield of the cracking reaction in one or more products. It can also be defined by a ratio between two quantities of manufactured products, such as for example the ratio between the manufactured quantities of ethylene and propylene, or the ratio between the manufactured quantities of hydrocarbons having 3 carbon atoms and of hydrocarbons having 4 carbon atoms. In this case, this ratio is an indication of the selectivity of the cracking reaction between two products or two groups of products.

The method of the present invention consists in particular in determining the value V of at least one of the conditions of the steam cracking reaction directly as a function of the n results R _i from each series of the n absorbance measurements, as well as as a function of 'at least one desired value P of a productivity. Thus, each time a series of n absorbance measurements is carried out, at least one of the conditions of the cracking reaction is fixed at a value V which makes it possible to obtain the desired productivity. In practice, we carry out several series of n absorbance measurements and therefore we determines the value V several times during the steam cracking reaction, preferably periodically over time, for example once a day or an hour, or once every 5 or 15 minutes.

forme dans laquelle P représente une valeur de la productivité en l'un des produits de la réaction, R_i représente une des valeurs des n mesures d'absorbance avec i variant de 1 à n, V_m représente une des valeurs des conditions de la réaction, m représente le nombre des conditions de la réaction contrôlées et a, b_i et c_m représentent des coefficients numériques, négatifs ou positifs, entiers ou décimaux.Furthermore, the value V of one of the conditions of the steam cracking reaction can advantageously be determined by means of a correlative relation linking the condition of the reaction to several variables. These variables are constituted in particular by the n results R _i of the n absorbance measurements, by at least one desired value P of the productivity and possibly by one or more other reaction conditions. The correlative relationship can be established beforehand by means of a multivariate regression carried out on the basis of the productivity values of products obtained under different cracking conditions for various mixtures of hydrocarbons. It can in particular be a linear function of the n results R _i of the n absorbance measurements, of a value P of at least one productivity and possibly of a value V of at least one of the conditions of the reaction. The correlative relation can be, for example, of the general form:

form in which P represents a value of the productivity in one of the products of the reaction, R _i represents one of the values of the n absorbance measurements with i varying from 1 to n, V _m represents one of the values of the conditions of the reaction, m represents the number of controlled reaction conditions and a, b _i and c _m represent numerical coefficients, negative or positive, whole or decimal.

formes dans lesquelles les variables et les paramètres ont les mêmes définitions que précédemment, V₁ représente l'une des valeurs V_m des conditions de la réaction, k_ij représente un coefficient numérique, négatif ou positif, entier ou décimal, R_j représente une des valeurs des n mesures d'absorbance avec j étant différent de i et variant de 1 à n, et P et P′ représentent des valeurs de la productivité en deux produits de la réaction. Cette relation corrélative dépend du type de spectrophotomètre infrarouge utilisé, des conditions dans lesquelles on l'utilise, des n des longueurs d'onde choisies, ainsi que du ou des produits de la réaction de craquage dont on veut fixer par avance la productivité.The correlative relation can also be an algebraic function of these same variables and can contain products or quotients of these variables, for example, in one of the following general forms:

forms in which the variables and the parameters have the same definitions as previously, V₁ represents one of the values V _m of the conditions of the reaction, k _ij represents a numerical coefficient, negative or positive, integer or decimal, R _j represents one of the values of the n absorbance measures with j being different from i and varying from 1 to n, and P and P ′ represent values of the productivity in two products of the reaction. This correlative relationship depends on the type of infrared spectrophotometer used, the conditions under which it is used, the n of the wavelengths chosen, as well as the product or products of the cracking reaction whose productivity is to be fixed in advance.

When the process consists, in particular, in controlling the process by fixing two or more conditions of the cracking reaction, an appropriate value V must be determined for each of these conditions, in particular by means of correlative relationships as defined previously.

The determination of the value V can advantageously be carried out by means of a computer. The latter has the function of calculating the value V from the variables on which it depends, in particular from the n results R _i of the n absorbance measurements and from at least one desired value P. When the computer is connected directly to the infrared spectrophotometer , the acquisition of n results R _i by the calculator is practically instantaneous, and the complete determination of the value V can take a few minutes, generally less than 2 minutes.

When the value V of a condition of the cracking reaction is thus determined, the process is controlled by operating and conducting the reaction to this value by means known in themselves, in particular using a computer. preferably linked to regulation means capable of maintaining the condition at the determined value V, until a new series of n absorbance measurements is carried out. If the nature and / or the composition of the mixture of hydrocarbons to be cracked have changed in the interval between two successive series of n absorbance measurements, a new value V will then be determined from the last series of measurements carried out and the condition of the cracking reaction will be immediately corrected and fixed at this new value, in order to maintain the productivity in one or more reaction products at the desired value P, fixed in advance.

One of the main advantages of the process of the present invention is that it is able to maintain the productivity of one or more products of the cracking reaction at a constant value, whatever the fluctuations in the nature or the composition of the mixture of hydrocarbons. feeding the cracking tube. In particular, it is quite remarkable to note that, thanks to this process, the corrections of the conditions of the cracking reaction are made in an extremely short time, which makes it possible to avoid any drift, even momentary, of the reaction , towards the production of undesirable products or products obtained with unsatisfactory productivities. This result is obtained in particular thanks to the fact that the process does not comprise any stage consisting in the research or the determination of the physical and / or chemical characteristics of the mixture of hydrocarbons to be cracked. The results of the absorbance measurements can be directly used in the form of digital data in the correlative relations linking these to the desired productivity P and to the values V of the conditions of the steam cracking reaction. It is particularly surprising to note that it is now possible to control a steam cracking process at a given level of productivity and that it can tolerate large variations in the quality of the hydrocarbons supplying the cracking tube, for example as well as liquid hydrocarbons containing about 5 to 15 carbon atoms , such as naphtha, light gasolines and diesel oil, than gaseous hydrocarbons such as alkanes containing from 2 to 4 carbon atoms, optionally in admixture with alkenes containing from 2 to 6 carbon atoms, or with methane and alkanes containing from 5 to 6 carbon atoms, in particular natural gas, liquefied petroleum gas, also called LPG, ethane, propane, butane, or light by-products from the steam cracking of hydrocarbons liquids.

Depending on the nature and the composition of the mixture of hydrocarbons to be cracked, the conditions of the cracking reaction can be corrected instantaneously and fixed at values V comprised within known limits. In particular, the temperature of the reaction mixture at the entrance to the radiation zone of the furnace can be around 400 ° C. to 700 ° C., the temperature of the reaction mixture at the exit from this zone can be around 720 ° C at 800 ° C, the pressure in the cracking tube at the outlet of this zone can be from 120 kPa to 240 kPa, the weight ratio of the quantity of hydrocarbon mixture used to that of water vapor can be from 1 to 6 approximately.

Furthermore, the temperature of the reaction mixture circulating in the cracking tube can increase from the entry to the exit from the radiation zone of the furnace according to a profile such as that described in European patent applications No. 252355 and n ° 252356.

The present invention also relates to an apparatus specially designed to be able to implement the method described above. The apparatus comprises, on the one hand, a hydrocarbon steam cracking oven essentially comprising a thermal enclosure provided with heating means and crossed by at least one cracking tube, and, on the other hand, an infrared spectrophotometer capable of operating in at least one zone of the near infrared range ranging from 0.8 to 2.6 · 10⁻⁶ m approximately and intended to carry out absorbance measurements of the hydrocarbon mixture feeding the cracking tube. The heating means of the thermal enclosure of the cracking furnace are generally constituted by burners whose arrangement in the enclosure, the size and the adjustment can be chosen or adapted at will, so that the thermal power applied along of the cracking tube is distributed in a more or less homogeneous manner, in particular as described in European patent applications No. 252355 and No. 252356. The cracking tube can be arranged horizontally or vertically through the thermal enclosure, in particular in the radiation area of the furnace. It can have a reaction volume which is constant or which varies between the first and second halves of the length of the cracking tube, from the entry to the exit from the radiation zone of the furnace, as described in the applications for European Patent No. 252355 and No. 252356.

The absorbance measurements of the mixture of hydrocarbons feeding the cracking tube are carried out using the infrared spectrophotometer described above. The latter can be of the Fourier transform infrared spectrophotometer type. It can also be advantageously combined with a calculator intended to determine the value V of at least one of the conditions of the cracking reaction, by virtue of a calculation program containing at least one of the correlative relations linking this condition to variables on which it depends.

The steam cracker oven can also be combined with a process computer and control systems which allow these conditions to be fixed and automatically adjusted to the determined V values. Advantageously, the process computer can also include the calculator program for calculating the value V.

The infrared spectrophotometer can be placed near the supply line of the furnace with a mixture of hydrocarbons or the enclosure for storing this mixture, or alternatively a distance more or less distant from them. It can be equipped with information transmission means such as optical fibers adapted to this particular type of analysis. In this case, these measurements are advantageously carried out directly in real time, that is to say online on the supply line of the furnace with a mixture of hydrocarbons, or on the enclosure for storing this mixture. It is also possible to install a system for taking samples of the mixture of hydrocarbons to be cracked, comprising either a manual device essentially consisting of an airlock provided with taps, or an automatic device controlled by a programmable automaton. This system can be, in this case, arranged on the supply line of the furnace with a mixture of hydrocarbons, or on the enclosure for storing this mixture. Absorbance measurements can also be performed in non-real time, that is to say in deferred time.

The present invention is particularly useful in industrial steam cracking installations having a considerable size and production capacity. Indeed, thanks to this process, any difference in productivity caused by fluctuations in the nature and composition of the mixture of hydrocarbons to be cracked is significantly reduced, if not eliminated, thus avoiding the manufacture of either undesirable products or of products obtained. with unsatisfactory productivity.

The following nonlimiting examples illustrate the present invention.

Example 1 Manufacture of ethylene by steam cracking reaction of a naphtha of variable composition with an ethylene yield fixed at 22% .

A steam cracking reaction of a naphtha is carried out in an oven essentially comprising a thermal radiation radiation enclosure, consisting of a rectangular parallelepiped having an internal length of 9.75 m, an internal width of 1.70 m and a height internal 4.85 m. In this thermal enclosure is placed a cracking tube of refractory steel based on nickel and of chromium having a total length of 80 m, an internal diameter of 108 mm and a thickness of 8 mm. The cracking tube has the shape of a serpentine comprising 8 straight horizontal sections, of equal length each, connected to each other by elbows.

The oven's thermal radiation enclosure is fitted with burners arranged on the walls of the enclosure, in 5 horizontal rows, located at equal distance from each other. The thermal power of all of these burners is evenly distributed between these 5 rows.

The cracking tube is supplied, on the one hand, with water vapor at a constant flow rate of 900 kg / h and, on the other hand, with a naphtha of variable composition over time, at a constant flow rate of 2800 kg / h. Over a 24-hour period, the composition of the naphtha used varies so that its weight content in paraffins increases from 72% to 68%, its weight content in naphthenic compounds from 20% to 23%, its weight content in aromatic compounds 8% to 9%, and its density from 0.713 to 0.719.

The pressure of the reaction mixture leaving the radiation zone of the furnace is approximately 165 kPa. The cracking temperature T at the outlet of this zone is variable during this manufacture and is determined so that the yield of ethylene is constantly equal to 22%. The cracking temperature at the entrance to the radiation zone of the oven, initially close to 550 ° C., undergoes slight variations over time due to those of the exit temperature T.

Once every 15 minutes, a sample of naphtha feeding the cracking tube is analyzed using an infrared spectrophotometer "INFRAALYZER 500" ®, sold by BRAN-LUEBBE (United States of America). At each analysis, a series of 5 absorbance measurements is carried out using a technique combining transmission and reflection and corresponding to the BEER-LAMBERT absorption law, at the following 5 wavelengths, expressed in 10⁻⁶ m :
2,278-2,308-2,398-2,439 and 2,475. The results of these 5 measurements are respectively noted R₁, R₂, R₃, R₄ and R₅.

avec T exprimé en degré Celsius. Lorsque la valeur de la température de craquage T est ainsi déterminée, l'ordinateur de procédé fixe aussitôt la température de craquage à la sortie de la zone de radiation du four à cette valeur.These results are transmitted to a "SOLAR 16/65" ® process computer sold by BULL (France), directly connected to the infrared spectrophotometer. The process computer is, in particular, provided with a program making it possible to calculate the value of the cracking temperature T at the exit from the radiation zone of the furnace as a function of the results R₁, R₂, R₃, R₄ and R₅ of the 5 absorbance measurements, of the ethylene yield, RE, desired and fixed in advance at 22%, by application of the following correlative relation:
$$\text{T = 509 + 11.63 RE + 326 R₁ - 448 R₂ + 688 R₃ - 692 R₄ + 611 R₅}$$

with T expressed in degrees Celsius. When the value of the cracking temperature T is thus determined, the process computer immediately sets the cracking temperature at the exit from the radiation zone of the oven to this value.

During the 24 h period when the composition of naphtha varied as indicated above, it is observed that the cracking temperature T itself varied in a range from 788 ° C to 806 ° C. During this same period, it can be seen that the production of ethylene has remained constantly at 616 kg / h and that consequently the yield of ethylene has been maintained at 22%, despite variations in the composition of naphtha.

Example 2 Manufacture of ethylene and propylene by steam cracking reaction of a naphtha of variable composition, with a ratio between the yield of propylene and that of ethylene fixed at 0.6 .

The cracking reaction is carried out in an oven identical to that described in Example 1.

The cracking tube is supplied, on the one hand, with water vapor at a constant flow rate of 964 kg / h and, on the other hand, with a naphtha of variable composition over time, at a constant flow rate of 3000 kg / h. In a 24 hour period, the composition of the The naphtha used varies so that its content by weight of paraffins increases from 68% to 76%, its content by weight of naphthenic compounds increases from 23% to 19%, its content by weight of aramatic compounds increases from 9% to 5% and its density from 0.719 to 0.697.

The pressure of the reaction mixture leaving the radiation zone of the furnace is approximately 165 kPa. The cracking temperature T at the outlet of this zone is variable during this manufacture and is determined so that the ratio between the yield of propylene and that of ethylene is constantly equal to 0.6. The cracking temperature at the entrance to the radiation zone of the oven, initially close to 550 ° C., undergoes slight variations over time due to those of the exit temperature T.

Once every 15 minutes, a sample of naphtha feeding the cracking tube is analyzed using an infrared spectrophotometer "INFRAALYZER 500" ^R , sold by BRAN-LUEBBE (United States of America). At each analysis, a series of 5 absorbance measurements is carried out using a technique combining transmission and reflection and corresponding to the BEER-LAMBERT absorption law, at the following 5 wavelengths, expressed in 10⁻⁶ m :
2,278-2,308-2,398-2,439 and 2,475. The results of these 5 measurements are respectively noted M₁, M₂, M₃, M₄ and M₅.

avec T exprimé en degré Celsius.These results are transmitted to a "SOLAR 16/65" ® process computer sold by BULL (France), directly connected to the infrared spectrophotometer. The process computer is, in particular, provided with a program making it possible to calculate the value of the cracking temperature T at the outlet of the radiation zone of the furnace as a function of the results M₁, M₂, M₃, M₄ and M₅ of the 5 absorbance measurements, of the ratio (RP / RE) between the yield of propylene and that of ethylene, desired and fixed in advance at 0.6, by application of the following correlative relation:
$$\text{T = 994 - 357 RP / RE + 161 M₁ - 222 M₂ + 341 M₃ - 342 M₄ + 303 M₅}$$

with T expressed in degrees Celsius.

When the value of the cracking temperature T is thus determined, the process computer immediately sets the cracking temperature at the exit from the radiation zone of the oven to this value.

During the 24 hour period when the composition of naphtha varied as indicated above, it is observed that the cracking temperature T itself varied in a range from 800 ° C to 785 ° C. During this same period, it can be seen that the ratio between the yield of propylene and that of ethylene has remained constantly at 0.6, despite variations in the composition of naphtha.

Example 3 Manufacture of ethylene at a constant flow rate of 0.640 T / h and of propylene at a constant flow rate of 0.370 T / h by the steam cracking reaction of a naphtha of variable composition .

The cracking reaction is carried out in an oven identical to that described in Example 1. The cracking tube is supplied with water vapor and with naphtha of variable composition over time. In a 24 hour period, the composition of the naphtha used varies so that its weight content in paraffins increases from 76% to 72%, its content by weight in naphthenic compounds from 19% to 20%, its content by weight in aromatic compounds from 5 % to 8% and its density from 0.697 to 0.713.

The pressure of the reaction mixture leaving the radiation zone of the furnace is approximately 165 kPa. The cracking temperature T at the exit from the radiation zone of the furnace and the flow rate Q of supply of the naphtha tube are variable during this manufacture and are determined so that the production rates of ethylene and propylene are respectively and constantly equal to 0.640 T / h and 0.370 T / h. The cracking temperature at the entrance to the radiation zone of the furnace, initially close to 550 ° C., undergoes slight variations over time. made of those of the output temperature T. The water vapor supply rate varies over time so that the weight ratio of the quantity of hydrocarbon mixture used to that of water vapor is constantly 3.

Once every 10 minutes, a sample of naphtha feeding the cracking tube is analyzed using an infrared spectrophotometer "INFRAALYZER 500" ® sold by BRAN-LUEBBE (United States of America). At each analysis, a series of 5 absorbance measurements is carried out using a technique combining transmission and reflection and corresponding to the BEER-LAMBERT absorption law, at the following 5 wavelengths, expressed in · 10⁻⁶ m = 2.278-2.308-2.398-2.439 and 2.475. The results of these 5 measurements are respectively noted L₁, L₂, L₃, L₄ and L₅.

$${\text{Q}}_{\text{P}}{\text{= Q}}_{\text{E}}\text{(2,552 - 0,0028 T + 0,0775 Q + 0,451 L₁ - 0,6206 L₂ + 0,954L₃ - 0,958 L₄ + 0,847 L₅)}$$

avec T exprimé en degré Celsius et les débits Q, Q_E, Q_P en tonne/heure.These results are transmitted to a "SOLAR 16/65" ® process computer sold by BULL (France) directly connected to the infrared spectrophotometer. The process computer is in particular provided with a program making it possible to calculate the value of the flow rate Q of supply of naphtha to the cracking tube, and of the cracking temperature T at the outlet of the radiation zone of the furnace as a function results L₁, L₂, L₃, L₄ and L₅ of the ethylene production rate Q _E fixed at 0.640 T / h and the propylene production rate Q _P fixed at 0.370 T / h by applying the following two correlative relationships:
$${\text{100 Q}}_{\text{E}}\text{= Q (- 36.4 + 0.086 T - 2.65 Q - 28 L₁ + 38.5 L₂ - 59.2 L₃ + 59.5 L₄ - 52.56 L₅)}$$

$${\text{Q}}_{\text{P}}{\text{= Q}}_{\text{E}}\text{(2.552 - 0.0028 T + 0.0775 Q + 0.451 L₁ - 0.6206 L₂ + 0.954L₃ - 0.958 L₄ + 0.847 L₅)}$$

with T expressed in degrees Celsius and the flow rates Q, Q _E , Q _P in tonnes / hour.

When the values of the cracking temperature T and the supply rate Q of naphtha to the cracking tube are thus determined, the process computer immediately sets the temperature of cracking at the outlet of the radiation area of the furnace and the naphtha feed rate of the cracking tube at these values.

During the 24 hour period during which the composition of naphtha varied as indicated above, it is observed that the cracking temperature T itself varied in a range from 789 ° C to 795 ° C, while the flow rate d The naphtha supply Q of the cracking tube varied in a range from 2.8 T / h to 2.9 T / h. During this same period, it can be seen that the ethylene production rate remained constantly at 0.640 T / h and the propylene production rate remained constantly at 0.370 T / h, despite variations in the composition of naphtha.

Claims (8)

1. Process for steam cracking a hydrocarbon mixture, consisting in passing steam and the hydrocarbon mixture through at least one heated cracking tube, the process being characterised in that it is controlled by (a) analysing the hydrocarbon mixture feeding the cracking tube using an infrared spectrophotometer to determine n absorbance readings and n wavelengths ranging from 0.8 to 2.6 × 10⁻⁶ m, (b) using the n absorbance results to determine at least one value V of one of the conditions of the steam cracking reaction, and (c) performing the steam cracking at the value or values thus determined, so as to obtain a desired preset value P of the productivity with respect to one or more products of the steam cracking reaction.

2. Process according to Claim 1, characterised in that the condition of the steam cracking reaction is chosen from the flow rates of water and of the hydrocarbon mixture feeding the cracking tube, the cracking temperature or the cracking pressure at any point in this tube, and also the weight ratio of the quantity of the hydrocarbon mixture employed to that of steam.

3. Process according to Claim 1, characterised in that the productivity with respect to one or more products of the reaction is defined by a yield, by a flow rate or by a ratio between two quantities of products or of groups of products manufactured.

4. Process according to Claim 1, characterised in that the number n of absorbance readings is from 2 to 20.

5. Process according to Claim 1, characterised in that the absorbance readings are performed at wavelengths, expressed in  × 10⁻⁶ m, or substantially similar wavelengths, chosen from 2.141, 2.166, 2.181, 2.278, 2.308, 2.347, 2.375, 2.398, 2.439, 2.457 and 2.475.

6. Process according to Claim 1, characterised in that the value V is determined by means of a linear or algebraic correlation relationship relating the condition of the cracking reaction to the n results R_i of the n absorbance measurements, to at least one desired value P of a productivity with respect to one or more products of the reaction and, if appropriate, to one or more other conditions of the reaction.

7. Apparatus for carrying out the process according to Claim 1, characterised in that it comprises, on the one hand a hydrocarbon steam-cracking furnace essentially containing a heating chamber equipped with means of heating and traversed by at least one cracking tube, and on the other hand an infrared spectrophotometer capable of operating in at least one range of the near infrared region extending from 0.8 to 2.6 × 10⁻⁶ m and intended for performing absorbance measurements of the hydrocarbon mixture feeding the cracking tube.

8. Apparatus according to Claim 7, characterised in that the infrared spectrophotometer is combined with a computer intended for determining the value V of at least one of the conditions of the cracking reaction, by means of a calculation programme containing a linear or algebraic correlation relationship relating the condition of the reaction to the n results R_i of the n absorbance measurements, to at least one desired value P of a productivity with respect to one or more products of the reaction and, if appropriate, to one or more other conditions of the reaction.