WO2004029592A1

WO2004029592A1 - Method and device for continuously conducting a fuel preparation process, especially for a diesel engine, by mixing the constituents thereof on-line

Info

Publication number: WO2004029592A1
Application number: PCT/FR2003/002748
Authority: WO
Inventors: Michel Masson; Frédéric POULY; Christophe Langellier
Original assignee: Totalfinaelf France
Priority date: 2002-09-30
Filing date: 2003-09-18
Publication date: 2004-04-08
Also published as: AU2003282172A1; FR2845013B1; FR2845013A1; EP1549931A1

Abstract

According to said method, a mixer (1) is continuously provided with at least two constituents (2-3) at controlled rates of delivery (2a-3a). According to the invention, a portion of the UV spectrum absorbed by the fuel thus prepared is measured on-line with the aid of a UV spectrometer (4) downstream from the mixer(1); the amount of polycylic aromatic hydrocarbons in said fuel is quantitatively determined by mathematical treatment of the measured intensities; the difference between said amount and at least one set-point value is calculated; and said information is reconfigured in the form of electric signals (5-6) in order to control functional parameters such as supply rates for various constituents (2a-3a) as a function of the result thus obtained in order to cancel out the difference between the measured value and the set-point value.

Description

Method and device for continuously carrying out a process for preparing a fuel, in particular for a diesel engine, by in-line mixing of its constituents.

The present invention relates to a method and a device for the preparation of a fuel, in particular for a diesel engine, or of a fuel for a heating installation, by in-line mixing of its various constituents, with optionally incorporation of additives.

It is known that fuels for diesel engines are usually made up of a mixture of various components, generally hydrocarbonaceous, in proportions such that the fuel exhibits certain performance criteria, associated with environmental qualities, in particular during combustion, and responds to precise specifications, relating, for example, to the value of cetane, resistance to cold, sulfur or polyaromatic content.

In an oil refinery, these fuels for diesel engines can be produced by the technique known as in-line mixing, according to which the various liquid constituents, as well as any additives, are introduced simultaneously, and continuously, into a line serving as a mixer. . The flow rate of the various constituents is controlled and controlled by a computer and the duration of preparation of a batch of fuel can extend over several tens of hours.

It is this type of industrial installation that will be referred to more precisely in the remainder of this description, without this implying a limitation of the scope of the invention.

Naturally, the properties of the fuel produced are checked on various occasions during manufacture and analyzes are carried out for this purpose on samples taken at the outlet of the mixer, or in the storage enclosure during filling. From the results of these analyzes, the flow rates of the components of the mixture as well as any additives are adjusted to align the measured values with the set values.

Of course, for each analysis, it is advantageous to limit as much as possible the time interval separating the taking of a sample of the mixture in preparation and obtaining the measured value, and the analyzers used for this purpose. , so-called online analyzers, generally meet this requirement. Thus the Applicant has patented a process and a device (see FR-A-2 747 322) intended to continuously measure the number of cetane in a diesel engine fuel during manufacture. We also know that, by their nature, certain petroleum fractions resulting from the distillation of crude oil contain Polycyclic Aromatic Hydrocarbons (abbreviated as PAH), such as benzo (a) pyrene, naphthalene or even thiophenes, which are reputed compounds dangerous to the health of living beings and found especially in the constituents of diesel fuels. These PAHs tend to generate particles and black smoke after combustion in diesel engines, which are then released into the atmosphere with the exhaust gases from the engine and thus present a potential danger to the health of living beings. It seems that benzo (a) pyrene metabolized in the body is susceptible to general carcinogenic effects for humans, while black particles and fumes can promote allergies or asthma, and provoke if necessary , triggering lung cancer. Other effects on health have also been observed within the framework of the French study program "ERPURS" (Risk Assessment of

Urban pollution for health), and in particular on the reproductive system, the skin and the cardiovascular system.

It is to remedy these potential dangers that the European Commission launched, in conjunction with car manufacturers and oil tankers, the Community program Auto Oil II, which sets targets for fuel quality and automobile pollutant emissions. , to meet the air quality standards by 2010. the European Directive 98/70 / EC, published in late 1998 and took effect on 1 ^January 2000, referring among others to European standard EN 590 on diesel fuel, in particular requires that the PAH content of this fuel is not currently greater than 11% by mass. The updating of the Directive, resulting in increased severity, is under study, for application from 2005, both at European and international ISO level. Today, the measurement of the PAH content in fuels for diesel engines, however, is not subject to an on-line control at the outlet of the mixer, due to the complexity of the method of standard measure, a measure which will nevertheless become essential in the short term for the refiner, given the increased severity expected from the specification.

Indeed, the measurement, long and complicated, is carried out under very precise conditions in the laboratory, fixed by the standard

EN 12916/2000, equivalent to standard IP-391-01, using the analytical technique of High Performance Liquid Chromatography. The method is in most cases very precise, but requires several manual interventions, which are demanding in know-how and skill, in particular for its implementation with pure compounds, and more precisely with so-called reference PAHs presenting a potential danger to the health of users. In addition, this method is sensitive to the presence of methyl esters of vegetable oils, and in particular rapeseed (abbreviated as EMC) in the fuel analyzed, resulting in an artificial increase in the content of triaromatic compounds. It is therefore necessary, in this case, to measure separately the content of EMC present in the analyzed sample, then to subtract this contribution from the results obtained by the method EN 12916/2000. Two analyzes are therefore necessary, which lengthen the response time of the laboratory, which can go up to several hours between the taking of the sample of the manufacturing in progress and the transmission of the result of the corresponding PAH content.

Other methods and devices have been proposed in the art for measuring PAHs in petroleum products, in particular the Fitzgerald method, which requires a high resolution mass spectrometer. This method gives very detailed results but presents constraints in terms of a complex calibration and its stability over time. In addition, by the very nature of the analysis technique used, the method is unsuitable for online use.

The measurement of PAHs by NIR spectrometry (Near InfraRed) is, in addition to the high purchase cost of the measuring device, complicated to implement. Indeed, it is necessary to establish and maintain unique models of complex calculations for each type of diesel analyzed. However, in an oil refinery, it is common for diesel to come from different units, for example, from distillation atmospheric crude oil, vacuum distillation, visbreaking unit or catalytic cracking. In addition, the model thus calculated is very sensitive to variations and modifications in the production process markets. Another measurement method, called Burdett, named after its designer, is also proposed for measuring PAHs in petroleum products. This non-standardized laboratory method is based on ultra violet spectrometry and measures absorbances at different wavelengths, for example 197, 230 and 260 nanometers (abbreviated to nm). From the intensities thus recorded, after deduction of several response coefficients, the contents of mono-, di- and triaromatic compounds present in the sample analyzed are deduced. The Burdett method dates from 1954 and was initially developed to carry out measurements on oils, then, some twenty years ago, it was adapted for fuels by the use of corrective factors, in particular for diesel fuels. It is a fast, repeatable and simple method to implement.

However, several drawbacks are inherent in the use of this measurement method, among which: - the results which are expressed in moles per 100 grams of fuel must be corrected on the basis of the essential knowledge of the different molecular weights of the families of hydrocarbons present in the sample measured, these different molecular weights being assimilated in practice to the average molecular weight of the sample;

- the UV signal is extremely intense, in particular at 197 nm, and therefore requires significant dilutions, causing difficulties in implementation for online measurement, as well as sources of potential errors for the expression of the results. Indeed, the main difficulty of the method is the adequacy between the concentration to be measured and the length of the optical path of the cell, which requires an adaptation according to the intensity of the signal;

- finally, the results are affected by the presence of sulfur elements in the sample measured. There is therefore a need in the art to have a rapid, reliable and automatic measurement means allowing better control of the PAH content in petroleum products, which is Usable in particular online in the process of preparing a fuel for diesel engine, and which thus responds more effectively to the environmental constraints of today and to come.

It is this problem which the present invention proposes to solve by continuous use, downstream of the mixer, of an analyzer operating on the principle of measurement by absorbance of ultraviolet radiation, usually called "UV spectrometer", under conditions such that the results of the measurement of the PAH content can be known easily and at short notice, as the fuel is prepared.

The invention also aims to provide a method and a device for online mixing of the constituents, and possibly additives, of a diesel engine fuel, in which the measurement of the PAH content of the mixture produced can be rapid, reliable and fully automated and can be used as an automatic adjustment parameter for the fuel preparation unit, for example an in-line mixer.

To this end, the subject of the present invention is a process for the preparation of a fuel, in particular for a diesel engine, or of a fuel for a heating installation, by in-line mixing of its constituents, from different streams having controlled flow rates, with possible incorporation of additives, the fuel having to have for sale a specific content of hydrocarbons of polycyclic aromatic types, process in which a mixer is fed continuously with at least two constituents at controlled flow rates, this process being characterized in that:

- A portion of the UV spectrum absorbed by the fuel thus prepared is measured online using a UV spectrometer, downstream of the mixer;

- The content of polycyclic aromatic hydrocarbons in this fuel is determined quantitatively, by mathematical treatment of the measured intensities;

- the difference between this content and at least one set value is calculated;

- and this information is restored in the form of electrical signals, to control, depending on the result thus obtained, functional parameters, such as the feed rates for the various constituents, so as to cancel the difference between the measured value and the set value.

The measurement of the PAH content on the fuel will be carried out in a simple manner using a UV spectrometer, preferably using a double optical beam operating in a wavelength range between 270 and 370 nm and, preferably, between 290 and 330 nm This wavelength range corresponds to the third least intense aromatic absorbance band, thus avoiding, in a spectrometric measurement process, prior dilution of the sample to be measured .

Preferably, the spectrometer will measure a reference, or more precisely its baseline, before each measurement of the PAH content on a fuel sample during manufacture. This reference will be obtained from a pure product of controlled quality, preferably isooctane, which does not absorb in the UV spectral range as defined above, and which is placed in a cell suitable for this purpose. Of course, the automation of the spectrometer measurements on the reference product, then on the fuel sample whose PAH content is to be measured, as well as the opening and closing of the shutter means for the sampling of this sample in the measurement cell, are made using a device programmed and controlled by a computer.

The exploitation of UV spectra by any type of mathematical processing and, preferably, by the technique of deconvolution of spectra makes it possible to extract from them the average contributions of each of the types of aromatic molecules (mono-, di- and triaromatic ), and then to evaluate precisely the content of total polyaromatic compounds, that is to say in compounds containing at least two aromatic rings. This method, like any spectrometric method, is not absolute, and it requires beforehand the fabrication of a model from data obtained by the reference method (EN 12916 or IP 391). Unlike the techniques using an NIR spectrometer, the model thus created is almost universal and can be applied to diesel fuels of various origins obtained by different manufacturing processes. The PAH contents of the sample of the mixture during manufacture are determined continuously, from information supplied in the form of electrical signals by the UV spectrometer. For each measurement, the calculated content is then converted, by means of a calculator, into a deviation in PAH content from a previously fixed reference value. The computer then transforms this difference into an electrical signal, which is sent to a computer programmed to control the relative quantities of the various constituents introduced into the mixer. The measurement system is therefore easy to automate, since it suffices to control it by at least one programmed automatic device, controlled by a computer, to trigger the measurement sequences of the UV spectrometer, as well as the means of closing the sampling loop intended to isolate the product to be analyzed during the measurement time. Of course, the valves for adjusting the flow rates of constituents of diesel fuel to be manufactured, located upstream of the mixer, will also be controlled in a known manner by the computer, so as to reduce the difference obtained between the result of the measurement of the PAH content on a sample during manufacture, and the set value previously set by the operator.

The measurement time to obtain a PAH content and thus act on the valves for supplying various constituents of the mixture is less than an hour, or even less than ten minutes, which constitutes a considerable advantage compared to the techniques usually used and which require at least one hour to obtain a PAH content, for example the reference method EN12916 / 2000, in order to then be able to act on the settings of the feed rates of the mixture during manufacture. The invention also relates to a device for the preparation of a fuel, in particular for a diesel engine, or of a fuel for a heating installation, by in-line mixing of its constituents from different streams having controlled flow rates, with optionally incorporation of additives, the fuel having to have for sale a specific content of polycyclic aromatic hydrocarbons (PAH), this device comprising at least two tanks for the storage of at least two different constituents of the mixture, a mixer supplied by separate lines with adjustable flow rates in the various constituents of the mixture, at least one line for discharging the mixture connected to the mixer, preferably a tank for storing the manufactured product, possibly at least a source of additives with adjustable and automated flow rate, a means for measuring the PAH content of the manufactured product, at least one means for controlling the flow rate of the various feed lines of the mixer, the control means being, on the one hand, controlled by the measuring means and on the other hand, programmed so as to adjust the different flow rates to reduce the difference between the measured value and the value of the target PAH content, this device being characterized in that the measuring means for measuring the PAH content of the mixture comprises:

- a UV spectrometer preferably mounted as a bypass on the mixture discharge line,

- a measuring cell optically connected to the UV spectrometer and to the discharge line, respectively, by lines equipped with sealing means and at least one non-return valve,

- a measurement cell containing a reference product, optically connected to the spectrometer,

- connected to the UV spectrometer, a programmed control means for the functional organs, such as those controlling the measurement sequences of the UV spectrometer, the means for shutting off the measurement cell and the flow control valves for the supply lines of the mixer, depending on the results of sample analysis.

The present invention can also be used to measure the PAHs on the various flows located upstream of the mixer and thus make it possible, depending on the results obtained, to automatically and continuously adjust the parameters of the mixer.

Another application of the invention, in the petroleum refining industry, is the continuous control of the flow rates of the streams in charge of a diesel hydrotreating unit (HDT). In fact, after measuring the PAH content of the reaction products at the outlet of this HDT, the invention makes it possible to adjust the various feed flows to obtain a zero difference between the PAH content measured downstream of this HDT and the corresponding setpoint. Continuous adjustment of the severity of certain crude oil processing units or of the different fractions thereof, aimed at reducing the PAH contents, such as for example an HDT, a catalytic cracker or a visbreaker, also constitutes a application of the invention. Indeed, the measurement of the PAHs downstream of the unit, and the exploitation of the corresponding signal, or of the deviation thereof with a value representative of a setpoint, to control different specific adjustment parameters of the severity of the unit, makes it possible to pilot and control said severity. Other characteristics and advantages of the invention will emerge more clearly from the detailed description which follows of its implementation, with reference to the appended schematic drawings. In these drawings:

FIG. 1 is a diagram of the entire on-line mixing device of the fuel constituents; FIG. 2 illustrates the system for measuring the PAH content on the mixture in progress;

Figures 3 to 8 are graphs illustrating examples of implementation of the invention, which will be explained in connection with these examples. The device represented in FIG. 1 is intended for the production of a diesel fuel having predetermined properties, and in particular a prefixed PAH content, by mixing in line of at least two constituents in a mixer 1 and, optionally, in the line 9 for evacuating this mixture to a tank 10. Each of the feed lines of the mixer is equipped with an automatic flow control valve 2a - 3a in the different constituents of the mixture, coming from storage tanks 2 - 3 These various constituents, at least one of which must have a PAH content less than a set value, may be, for example and without limitation, alone or in mixture (s), a heavy gasoline derived from catalytic cracker, a light fraction from a crude oil distillation tower, a diesel fuel with a high sulfur content, or a diesel fuel with a low sulfur content from a desulphurization unit, or an LCO, abré Viation from the English "Light Cycle Oil", diesel fuel from the catalytic cracking of heavy petroleum fractions and therefore containing a high proportion of unsaturated hydrocarbons and in particular polyaromatics. Several online analyzers, only one of which, 22, is shown in FIG. 1, can be connected in a known manner to the discharge line 9 of the fuel produced by means of a sampling loop 21. These analyzers are connected to a control means 11, preferably a commercial computer, itself connected by lines 5 and 6 to valves 2a and 3a, and programmed so as to reduce the difference between the measured values and the values of setpoint. One of these analyzers can be a cetane comparator, as described by the Applicant in FR-A-2 747 322. According to the invention, the PAH content of the mixture discharged via line 9 is measured online and the flow rates feeding the mixer 1 and optionally injecting additives at different points (not shown) in line 9 are adjusted so as to bring the measured PAH content closer to the desired setpoint for this value.

To this end, a device 4 for measuring the PAH content of the mixture during manufacture is supplied by a sampling loop of known type and consisting of two separate supply lines 12 and discharge 13 of the the sample to be measured, these two lines being provided with closure means 12a and 13a which can be controlled remotely by the computer 11. In addition, a non-return valve 23 is installed downstream of the valve 13a on line 13.

The device 4 is itself connected to the computer 11 and transmits to it a signal proportional to the difference between the measurement carried out on the sample and the value recorded on the baseline, so that the computer can adjust accordingly the flow rates of the various constituents to cancel the difference between the content which will then be calculated and the set value.

The device 4 is shown in detail in FIG. 2. It comprises a UV spectrometer 14 controlled using software by the computer 11, the latter being able to be relocated from the place of measurement. The quartz measuring cell 17 having a dimension of 10 micrometers or, preferably, a measuring cell of the ATR immersion probe type (in English "Attenuated Total Refraction", in French "Réfraction Totale Aténuée") to avoid any dilution of the sample to be measured is supplied by lines 12 and 13 and isolated from the production flow in line 9 by closing the valves 12a and 13a controlled by the computer 11 and the non-return valve 23 on the line 13. The cell 17 is connected to the spectrometer 14 by two optical fibers 15 and 16, one of which being intended to send the UV signal to the sample , while the other is used to transfer the signal, after absorption in the sample, to the spectrometer 14. The use of optical fibers, makes it possible to relocate, if necessary, the UV spectrometer in order to locate it in a place under the conditions more suitable than those recommended by the supplier of the device.

Each measurement of a sample taken automatically as indicated above, and to avoid possible measurement drifts, the spectrometer 14 recalculates its baseline via the calibration cell 20. A UV signal is sent by through the optical fiber 18 in this cell containing a product of pure quality, preferably isooctane, and is then returned, after absorption into the reference product, to the spectrometer 14 by the optical fiber 19. After measurement by the UV 14 spectrometer of the absorption spectrum in the radiation range between 240 nm and 350 nm, and preferably between 290 nm and 330 nm, in the sample and in the reference product, the spectra thus obtained are exploited by mathematical processing from the model programmed in computer 11 to deduce the PAH content of the sample. The computer then controls the opening of the valves 12a and 13a until their next closing for a new analysis. The method and the device according to the invention are simple and reliable in use, require only a short time from the operator and require only a low investment cost.

It will be noted that, unlike the reference method EN12916 / 2000 or IP391, the presence of EMC does not disturb the results obtained with a method and a device in accordance with the present invention.

The invention therefore provides, for the first time, a PAH measurement system which can be used online in an assembly for preparing a mixture of hydrocarbons, in particular a fuel for diesel engines, by online mixing of the constituents of this fuel. Examples The examples which follow are intended to show the various possibilities for measuring polyaromatics in diesel from the method and the device according to the invention. Equipment used

The spectrometer used is a device marketed by the company SECONAM under the name Anthélie Senior. The UV absorption spectra of the samples, representative of the fuel during manufacture, are measured, in accordance with the invention, in a cell with a low optical path or by the use of an ATR probe (Reflection

Total Attenuated) while the reference, or baseline, is measured in an identical cell containing isooctane. Spectrum processing software is also marketed by SECONAM under the name UVPRO. Principle of deconvolution calculation - calibration

Deconvolution is the decomposition of the UV spectrum of a sample into a certain number of reference spectra (or main components), which are 8 in number at the most and are predetermined. These are UV spectra of pure products (5 in number) and spectra calculated by principal component analysis (3 in number), the objective being, from these 8 spectra, to reconstruct by linear combination any which UV spectrum of diesel oil from the following formula:

Spectrum (GO) = α Spectrum (ref 1) + β spectrum (ref 2) + ... η Spectrum

(ref 8) + Err

The difference between the “real sample” spectrum and the “calculated sample” spectrum by linear combination of the 8 reference spectra is equal to “Err”. This coefficient must not exceed 4%, otherwise the calculation is rejected due to atypical sample.

The calibration will allow, after a stepwise multiple linear regression calculation (with Fisher test), to determine the contribution of each of the reference spectra in the expression of the desired result, namely the polyaromatic content. Certain reference spectra contribute to the spectrum of the sample and make it possible to deduce the polyaromatic content therefrom. The determination of the reference spectra and the calibration require the collection of a certain number of diesel fuel samples and the measurement of their polyaromatic content by the standardized method.

The highlighting of the contribution of the diaromatics in the third and last band of absorbance of aromatics (290 to 330 nm) is made all the more easy as, in this zone, the absorbance of polyaromatics is clearly differentiated from that of monoaromatics:

- polyaromatics extend their absorbance up to 330 nm, while this clearly stops around 290 nm for mono-aromatics,

- polyaromatics absorb with much stronger intensity than mono-aromatics with equal content.

Manufacture of a model for the measurement of diaromatics and more (2 aromatic cycles and more) The mathematical exploitation of UV spectra is preferably obtained by any method of correlative calculation between measured spectra and contents obtained by another method of determination and preferably by the so-called deconvolution method.

The manufacture of the model and its validation were carried out on a hundred samples whose diaromatic contents were previously measured by the reference method IP391, version 95. The diesel oils used are of all types, with a large majority of diesel oils for engines (or diesel base), and they come from different production sites. The contents of diaromatics and more, for these samples, are between 0 and 50%. In addition, some of these samples contain EMHV (Methyl Ester of Vegetable Oil) and in particular EMC (Methyl Ester of Rapeseed).

The model thus obtained can be used for any type of diesel for the measurement of diaromatics and more. Results achieved

Validation of the mathematical model (deconvolution)

The PAH contents (diaromatics and more) of the samples were measured in accordance with the measurement process of the invention, then calculated from the mathematical model previously determined.

Figures 3 and 4 are two graphs illustrating the correlation between the diaromatic contents or more of the above samples measured by the reference method (contents plotted on the abscissa) and the contents obtained by the process of the invention (contents plotted or ordered), for contents of diaromatics or more comprised respectively between 0 and 50% by weight and between 0 and 25% by weight. We observe in these figures a good correlation between the contents obtained by IP391, version 95 and those measured in accordance with the invention, whatever the level of contents (Figure 3: R ² = 0.9926 for contents of diaromatics and more) , between 0 and 50% and (Figure 4: R ² = 0.9888 for contents of diaromatics and more between 0 and 25%).

The UV method as described in the present invention therefore appears to be a reliable alternative for determining the content of diaromatics and more in gas oils. Validation of the UV method The UV method has been successfully validated by the ASTM method

D 6122-01 (Standard Practice for Validation of Multivariate Process Infrared Spectrometers). This standard developed for infrared spectrometry has been directly used for ultraviolet spectrometry. Repeatability of the method

(*) The repeatability of the EN12916 / 2000 or IP391-01 method is

0.13 (X + 2.6) with X, (PAH content) between 0 and 26% by mass, ie repeatability in this range between 0.3 and 3.7. The repeatability obtained with the method and the device according to the invention is approximately half that obtained with the reference standard. The method by online UV spectrometry is therefore, by its simplicity and its automation, more repeatable than the reference method, using high performance liquid column chromatography.

Contents of diaromatics and triaromatics and more The invention makes it possible to distinguish, in addition to the overall measurement of diaromatics and more, as presented above (FIGS. 3 and 4): the contents of diaromatics, that is to say of PAH corresponding to the molecules containing 2 aromatic rings (see FIGS. 5 and 6) and

- the contents of triaromatics and more, ie molecules containing 3 and more aromatic rings (see Figures 7 and 8). The implementation of the present invention allows the measurement of

PAHs containing two or more aromatic rings. It also makes it possible to distinguish certain PAHs by measuring diaromatics (two aromatic cycles) and triaromatics and more (3 aromatic cycles and more). In addition, the implementation, according to the invention, of the automatic control of a mixer of Diesel-type fuels, from the PAH content measured, do not pose any particular difficulty for those skilled in the art. The result obtained is a better monitoring of the PAH content, thanks to the use of a rapid, effective, inexpensive means of reaction and compensation for any measured drift of the polyaromatic content in a process for the manufacture of diesel fuel by in-line mixing of its constituents. In addition, the device used in accordance with the invention makes it possible to avoid the drawbacks encountered in the art, such as the greater or lesser participation in the results of the measurements of the presence of EMC or sulfur in the sample analyzed.

Although the invention has been described above in its application to the preparation of a fuel, in particular for a diesel engine, it will be clear to those skilled in the art that it applies identically to the preparation of a fuel. for heating installations, by in-line mixing of its constituents from different flows having controlled flow rates, with the possible addition of additives.

Claims

1. Process for the preparation of a fuel, in particular for a diesel engine, or of a fuel for a heating installation, by in-line mixing of its constituents from different flows having controlled flow rates, with optionally the incorporation of additives, the fuel having to have a specific content of polycyclic aromatic hydrocarbons for its marketing, process in which a mixer (1) is fed continuously with at least two constituents (2-3) at controlled flow rates (2a-3a) , this process being characterized in that:

- Online measurement is carried out using a UV spectrometer (4) of a portion of the UV spectrum absorbed by the fuel thus prepared, downstream of the mixer (1);

- the difference between this content and at least one set value is calculated;

- And this information is restored in the form of electrical signals (5-6), to control, depending on the result thus obtained, functional parameters, such as the feed rates of the various constituents (2a-3a), so as to cancel the difference between the measured value and the set value.

2. Method according to claim 1, characterized in that the portion of the UV absorption spectrum measured is between 270 nm and 370 nm and, preferably, between 290 nm and 330 nm.

3. Method according to claim 1 or 2, characterized in that the spectrometer measures a reference absorption on a pure product before each measurement on the fuel.

4. Method according to claim 3, characterized in that the pure product is isooctane.

5. Method according to one of the preceding claims, characterized in that the mathematical exploitation of the UV spectra comprises a method of correlative calculation between measured spectra and contents obtained by another method of determination.

6. Method according to claim 5, characterized in that the correlative calculation method is deconvolution.

7. Method according to one of the preceding claims, characterized in that the taking of fuel samples, their analysis by ultraviolet spectrometry and the transmission of control signals for the adjustment and regulation of the feed rates of the mixer are carried out by at least one automatic programmed device

8. Device for the preparation of a fuel, in particular for a diesel engine, or of a fuel for a heating installation, by in-line mixing of its constituents from different flows having controlled flow rates, with optionally incorporation of additives, the fuel having to have, for its marketing, a specific content of polycyclic aromatic type hydrocarbons (PAH), this device comprising at least two tanks (2-3) for the storage of at least two different constituents of the mixture, a mixer ( 1) supplied by separate lines (7-8) with adjustable flow rates in the various constituents of the mixture, at least one discharge line (9) of the mixture connected to the mixer (1), preferably a tank (10) for storage of the manufactured product, possibly at least one source of additives with adjustable and automated flow rate, a means of measurement (4) of the PAH content of the manufactured product, at least one control means (11) of the flow rate of the various feed lines (7-8) of the mixer (1), the control means (11) being, on the one hand, controlled by the measuring means (4) and, on the other hand, programmed so as to adjust the different flow rates to reduce the difference between the measured value and the value of the target PAH content, this device being characterized in that the measuring means (4) intended to measure the PAH content of the mixture comprises: - a UV spectrometer (14) preferably mounted as a bypass on the discharge line (9) of the mixture,

- a measurement cell (17) optically connected (15-16) to the UV spectrometer (14) and to the discharge line (9), respectively, by lines (12-13) equipped with shutter means (12a - 13a) and at least one non-return valve (23),

- a measurement cell (20) containing a reference product, optically connected (18-19) to the spectrometer (14), - connected to the UV spectrometer (14), a programmed control means (11) of the functional organs, such as those controlling the measurement sequences of the UV spectrometer, the shutter means (12a - 13a) of the measurement cell (17 ) and the flow control valves (2a - 3a) of the feed lines to the mixer (1), depending on the results of sample analysis.

9. Device according to claim 8, characterized in that the measuring cell (17) of the sample is connected to the spectrometer (14) via optical fibers (15 and 16).

10. Device according to claim 8 or 9, characterized in that the measurement cell (20) of the reference product is connected to the spectrometer (14) via optical fibers (18 and 19).

11. Device according to one of claims 8 to 10, characterized in that the measurement cells have a reduced optical path.

12. Device according to claim 11, characterized in that the measurement cell is an ATR (Total Attenuated Refraction) probe.

13. Use of the method according to one of claims 1 to 7 or of the device according to one of claims 8 to 12 for the manufacture of a fuel by online mixing of its constituents, in which the measurements of the aromatic hydrocarbon contents polycyclics are made downstream of the mixer.

14. Use of the method according to one of claims 1 to 7 or of the device according to one of claims 8 to 12 for the manufacture of a fuel by online mixing of its constituents, in which the measurements of the aromatic hydrocarbon contents Polycyclics are performed upstream of the mixer on the different feed streams of said mixer.

15. Use of the method according to one of claims 1 to 7 or of the device according to one of claims 8 to 12 for the continuous control of a diesel hydrotreatment unit.

16. Use of the method according to one of claims 1 to 7 or of the device according to one of claims 8 to 12 for the continuous adjustment of the operating severity of a hydrotreating unit, of a catalytic cracker or a visbreaker for heavy hydrocarbon charges.