EP1828765A1

EP1828765A1 - Method for analyzing hydrocarbon composition of liquid petroleum products

Info

Publication number: EP1828765A1
Application number: EP05825938A
Authority: EP
Inventors: Myriam Reminiac; Pierre Pestiaux; Jean-Claude De Wit
Original assignee: Total France SA
Current assignee: TotalEnergies Marketing Services SA
Priority date: 2004-12-13
Filing date: 2005-12-12
Publication date: 2007-09-05
Also published as: FR2879293B1; FR2879293A1; WO2006064116A1

Abstract

The invention concerns a method for determining respective volume fractions of saturated, olefinic and aromatic hydrocarbons in a liquid petroleum product sample distilling below 315 °C, in operational conditions defined in the ASTM D 1319-03 standard. Said method is different from the normalized method in that the reading of the analysis results and their use is not carried out by a human operator, but by means of a system for acquiring and processing digital images comprising at least one digital camera with charge-coupled device (CCD) and a computer with a computer programme for processing the recorded digital images.

Description

PROCESS FOR THE ANALYSIS OF THE HYDROCARBON COMPOSITION OF LIQUID PETROLEUM PRODUCTS

The present invention relates to the use of a system for acquiring and processing digital images in the context of a method for determining the respective proportions by volume of various organic compounds, in particular of saturated, olefinic and aromatics in petroleum fractions distilling below 315 ° C, as well as a device for carrying out this process.

One of the analysis methods commonly used to characterize the composition of hydrocarbons, especially fuels, is the analysis using the principle of the selective adsorption of fluorescent indicators according to the chemical species, after separation by liquid chromatography (in English: fluorescent indicator adsorption, abbreviated FIA).

The FIA is a standardized method (ASTM D 1319 or its equivalents ISO 3837, IP 356 and NF M07-024) for identifying and quantifying the respective proportions of saturated hydrocarbon fractions (alkanes and cycloalkanes), olefins (alkenes, cycloalkenes and alkanedienes) and aromatic (condensed, aromatic, diene aromatic, aromatic, sulfurous, nitrogenous or oxygenated monocyclic or polycyclic aromatic compounds) in light petroleum fractions distilling below 315 ^° C. This method is based on adsorption chromatography using a gel column. activated silica. The column has, from top to bottom, three distinct areas used for loading, separating and analyzing the product, respectively.

When filling the column, a small amount of silica gel containing a specific indicator is introduced halfway up the separation zone. The liquid hydrocarbon sample to be analyzed is injected at about 30 mm below the upper limit of the silica gel, and the column is eluted with a lower alcohol, preferably with isopropanol, under the pressure of an inert gas ( nitrogen, air). The elution results in the separation of the three hydrocarbon types mentioned above (saturated, olefinic and aromatic) and the separation of the indicator into three fluorescent colors which are specific to these hydrocarbon types, namely yellow, blue and red, which are respectively at the upper limit of the zones containing the saturated, olefinic and aromatic hydrocarbons. In the ASTM D 1319 standard analysis method, when the three fluorescence rings are visible in the analysis zone of the column, an operator raises, from the bottom up, the position of the remarkable boundaries, that is, say the solvent front (Fsoid = wet-dry boundary) and the respective positions of the fluorescent zones (Fyune, Fcc and Frue) visible under UV radiation (λmax 365 nm), then repeat this operation in the reverse order in order to correct the errors due to the advancement of the product in the column during the measurement, that is to say after progression of the solvent front of about 50 mm (F 'sant, F' yellow, F 'bieu and F' red ). It then calculates from the two values recorded for each remarkable limit, the corresponding average, then the respective quantities of the volume fractions of the three families of hydrocarbons.

The set of experimental conditions must be scrupulously respected to obtain reproducibilities and reproducibilities announced in the ASTM D 1319 standard. This standardized method is however dependent on the subjectivity inherent in the recording of the remarkable limits (Fsoivant, Fjaune, Fbieu and Frouge) by a human operator. Indeed, if the determination of the position of the solvent front is generally not problematic, the three fluorescent rings have a certain thickness and it is a question of determining as objectively as possible the point of maximum luminous intensity due to the fluorescence of the yellow ring, the lower limit of the fluorescence of the blue ring and the upper limit of the fluorescence of the red ring, whose color is actually closer to red-brown. In addition, with regard to this red-brown ring, often spread, it is the upper limit of the fluorescence of a double ring that should be identified when the sample analyzed contains oxygenates. A difficulty of assessment therefore appears for the operator when he must locate, on the column, the exact position of the remarkable separation limits as defined above. This difficulty is twofold, since the operator identifies successively and directly on the column, using a pen or a felt pen, twice the separation limits, in accordance with the standard, to compensate the advancement of the solvent front during the measurement time, usually several minutes. Such difficulties of assessment have the consequence that the position of the separation limits is subject to the subjectivity of the operators, resulting in, ultimately, a lower repeatability and reproducibility, that is to say a larger numerical value. This is very detrimental to the operator. It has indeed been demonstrated that an improvement in the reproducibility of a factor two at the level of determining the olefin content of a fuel could generate an economic gain of several tens of thousands of euros in the balance sheet. operating an oil refinery. In addition, the precision of the FIA method is of particular importance in the context of current and future evolution of the specifications of the species and the costs engendered by the resulting manufacturing constraints. Continuing its work on the search for an improvement in the accuracy of the FIA method as standardized, the Applicant proposes to overcome the subjectivity of a visual survey of the boundaries separating the various hydrocarbon zones by a human operator , replacing the pen, the eye and the brain of the operator with a digital image acquisition and processing system.

Such image recording techniques have already been used in methods of analysis, especially in chromatography. Thus, for example, the application EP 0 666 982 teaches a method and a device for controlling the state of separation of different chemical species in thin capillaries, used for example in capillary electrophoresis, in electrokinetic capillary chromatography, or in chromatography on capillary column. It is the measurement and analysis of the light emitted or absorbed by the various components of the sample being separated, after laser irradiation of the capillary, which makes it possible to control the separation process. In addition, the application EP 0 619 476 describes a device for the precise automatic dosing of small amounts of liquid in a medical analysis apparatus from a light source and a photoreceptor.

However, there are no solutions in the state of the art to improve the accuracy of the measurement of the limits separating different hydrocarbon zones in a glass chromatography column, in particular by replacing the assessment. human, by its subjective nature, by an objective, reliable and constant measurement system.

There is therefore a real problem in the art by the absence of a means to improve the accuracy and fidelity of the results of the FIA analysis method, and more generally the absence of a reliable method of determination. the position of a colored interface separating two zones containing for example two different types of hydrocarbons.

Although the problem underlying the present invention has been formulated above using the FIA method as a concrete example, it is obvious to those skilled in the art that a method making it possible to overcome the subjectivity of a visual record of the remarkable limits in FIA, can be used analogously for an objective reading of fluorescence zones on chromatography columns other than that of the type used in the ASTM D 1319 standard.

The subject of the present invention is therefore a process for the determination of the respective volume fractions of various organic compounds in a liquid sample, comprising

(a) separating the organic compounds from the sample by liquid chromatography in the presence of one or more fluorescent indicators capable of associating with said organic compounds and then emitting a specific fluorescence signal indicating the respective positions of the organic compounds ,

(b) recording, by means of the fluorescence signals emitted by the indicator (s), the areas of the chromatography column in which the separated organic compounds are present in step (a), and

(c) calculating the respective volume fractions of the organic compounds from the results of the reading of step (b), characterized in that the reading of the zones of the chromatography column. in step (b) is done by acquisition of digital images by means of at least one digital camera, and in that the calculation of the respective volume fractions of the organic compounds in step (c) is carried out by means of a computer program for processing digital images.

The invention also relates to a device for implementing such a method, comprising at least one column of chromatography to liquid chromatography, and is characterized in that it further comprises _^ one or more camera (s) digital disposed (s) so as to record one or more picture (s) of the digital chromatography column, and a computer and a digital image processing computer program for analyzing the digital image (s) recorded by the digital camera (s).

As explained in the introduction, one embodiment particularly. Of interest to the process according to the invention concerns its application to the determination of the respective volume fractions of saturated, olefinic and aromatic hydrocarbons in a sample of a liquid petroleum product distilling below 315 ° C., in accordance with ASTM standard D 1319 or its equivalents. The method according to the invention in this application comprises, among the various steps defined in said standard, the recording of the exact position of each of the following four following remarkable limits observed, from bottom to top, under UV light (λmax 365 nm), in the analysis zone of the column used: - solvent front (Fsoivant),

- maximum intensity of yellow fluorescence (yellow),

- lower limit of the blue fluorescence zone (Fbieu) and

- upper limit of the red fluorescence zone (Frouge), characterized in that the survey of the position of these four remarkable limits (Fsoivant, Fjaune, Fbieu and Frouge) is done by acquisition of digital images by means of less a digital camera and the measurement of the distances between these different positions and the calculation of the corresponding volume fractions are done by means of a digital image processing computer program. The device making it possible to implement this method according to the ASTM D 1319 standard comprises, in addition to that used in said standard, one or more digital camera (s) arranged (s) so as to record one or more images ( s) of the analysis zone of column (A), and a computer and a digital image processing computer program for analyzing the digital image (s) of the analysis zone of the column used and to calculate, from four remarkable limits of these images, the respective volume fractions of saturated, olefinic and aromatic hydrocarbons of the sample analyzed. The various steps of the known FIA method referred to above and to which reference is made in the preamble of the independent process claim, carried out under the operating conditions defined in ASTM D 1319, are as follows:

(a) filling, under continuous vibration, a chromatography column comprising, from top to bottom, a loading zone, a separation zone and an analysis zone, up to half the separation zone with activated silica gel,

(b) stopping the vibration and introducing a specific amount of silica gel containing fluorescent indicators for FIA (Fluorescent I indicator adsorption analysis, fluorescent indicator adsorption analysis),

(c) resumption of the vibration and completion of the filling of the column,

(d) injection of 0.75 ml of a hydrocarbon sample, previously cooled to a temperature of between 20 ^° C. and ^{40 °} C., at 30 mm below the upper limit of the activated silica gel bed,

(e) eluting the column with a suitable alcohol, preferably isopropanol, under an inert gas pressure,

(f) after about 1 hour of elution, reading the exact position of the following four observed remarkable limits, from bottom to top, under UV light (λmax 365 nm), in the analysis zone of the column Solvent front (Fsoivant), maximum fluorescence intensity yellow (Fjaune), lower limit of the blue fluorescence zone (Fbieu) and upper limit of the red fluorescence zone (Frouge), (g) distance measurement L _s ( distance between F _so ive and Fjaune), L ₀

(distance between Fjaune and Fbieu) and L _a (distance between Fbieu and Frouge) separating these four remarkable limits, and

(h) Calculation of the volume fractions of saturated (% saturated), olefinic (olefinic%) and aromatic (% aromatic) hydrocarbons according to the following formulas (i) to (iii):

(i)% saturated = (L ₈ / L) x 100

(ii) Olefinic% = (L ₀ / L) x 100 (iii) aromatic% = (L _a / L) x 100 with L == L _s + L ₀ + L _a

The known device, used for FIA analysis according to ASTM D 1319, referred to above and referred to in the preamble of the independent device claim, comprises the following elements:

(A) at least one adsorption chromatography column filled with activated silica gel, comprising, from top to bottom, a loading zone, a separation zone and an analysis zone, as defined by the ASTM standard D 1319,

(B) means for loading a sample of a liquid petroleum product to be analyzed on the column (A),

(C) a means for eluting under pressure the sample with an organic solvent,

(D) at least one lighting source emitting UV radiation with a predominant wavelength of 365 nm and disposed near the analysis zone of the column.

The use of the digital image acquisition and processing system dramatically increases the accuracy of the FIA method and allows total objectivity in the survey and exploitation phase of the experimental results. The Applicant has found that the repeatability and reproducibility values thus obtained (expressed in%) were twice as small, that is to say twice as good, as those specified in the standard.

As indicated above, the method can be implemented with one or more digital cameras. These are preferably digital cameras provided with a Charge Coupled Device (CCD). The simplest way is, of course, to use a single digital CCD camera. This unique digital camera is then placed at a fixed distance relative to the chromatography column so that the image recorded by it contains all four boundaries Points Fsoivant, Fjaune, Fbieu and F _rO uge located in the analysis zone of the column and spread over a height of approximately 50 to 60 cm (reading zone). The accuracy of this image acquisition step depends the resolution of the recorded images, in particular the number of pixels contained in the CCD matrix, of the shape of the pixels, which should preferably be square, of the diameter of the objective of the digital camera (for maximum light and power separator), the focal ratio that directly affects the amount of light received and, finally, the focal length to achieve the maximum pixel resolution of the digital camera.

Given these parameters and the high cost of CCD digital cameras with a very high number of pixels, it is often advantageous, to reduce the investment costs of the analysis device, not to use a single large number of cameras. pixels, but several digital cameras with lower pixel count. These digital cameras can be placed closer to the chromatographic column than the single digital camera, so as to acquire an image corresponding to only a part of the reading zone. The entire image of the reading zone is then reconstructed from the set of partial images recorded by the different digital cameras.

Specifically, in this embodiment at least two CCD digital cameras are placed at shooting positions aligned along an axis parallel to the axis of the column and located at a distance relative thereto such that the The partial image recorded by each of the digital cameras contains a portion of the remarkable boundaries Fsoivant, Fjaune, Fbieu and F _r0 uge and that the combination of the recorded partial images allows the reconstruction of a single image containing all four remarkable limits Fsoivant,

Fjaune, Fbleu and Frouge,

In another embodiment similar to that described above, a plurality of images are also recorded from a plurality of shooting positions, except that this series of partial images is performed not simultaneously. by a plurality of devices, but successively by a single digital camera which is moved, for example on a rail, parallel along the reading zone. In this embodiment, the acquisition of the digital images is therefore done by means of a single digital CCD camera, which successively records a series of partial images to from a series of shooting positions, aligned along an axis parallel to the axis of the column and situated at a distance from the column such that each of the series of partial images contains a part of the remarkable limits F _so on, Fjaune, Fbieu and Frouge and that the combination of the series of partial images allows the reconstitution of a single image containing all four remarkable limits F _S0 lvant, Fjaune, Fbleu and Frouge.

This embodiment is certainly the most economical because it requires only one digital camera with a relatively small number of pixels. It nevertheless has the disadvantage of being relatively more complex than the two other embodiments described above. Indeed, the simultaneity of the shots may require a double reading as described in the introduction concerning the ASTM D 1319 standard method, and the calculation of average values from the corresponding F and F 'values, respectively determined during the test. first and second statement.

The reconstitution of a single image from the partial images, recorded simultaneously or successively, is possible only if each recorded image partly overlaps the adjacent image. Those skilled in the art will be able to choose the shooting positions so as to obtain such partial recovery. An example of position alignment, shooting is shown in Figure 1. In this figure, three shooting positions are symbolized by three digital cameras (Al, A2 and A3). It can be three separate devices, fixed on the rail (1) and present simultaneously, or a single device that is slid along the rail (1) so as to make him successively adopt three positions different. The UV light source (2) is of course located outside the viewing plane, so as not to be in the image area recorded by the digital camera (s).

In the embodiment shown in FIG. 1, each of the three partial images of the column (3), recorded from the three shooting positions, contains two of the four remarkable boundaries Fsoivant, Fjaune, Fbieu and Frouge. The reconstitution of the overall image can be done here easily by superimposing the remarkable boundary yellow or visible on a fi rst image at the same remarkable boundary yellow or visible on the adjacent image. Other embodiments may, however, be envisaged where the same remarkable limit does not appear on two adjacent images, that is to say, where the overlapping zones do not contain a fluorescence zone. The reconstitution of the images then requires markers other than the fluorescence zones. Such markers may be provided for example by a graduated rule (4) disposed near the column so as to be in the recorded partial image area (see FIG. 1). The reconstitution of a single, global image from the set of images recorded simultaneously or successively is then done with reference images, taken with the same digital camera from the same shooting positions but under one image. visible lighting. Each of these reference images shows, in addition to the chromatography column area, the graduated rule placed near and parallel to the column. The overlap ratio of two adjacent images, determined from images obtained under illumination with visible light, is then applied to the corresponding adjacent images, obtained under UV light.

As indicated above, in one embodiment of the method of the invention, the partial images are recorded from three shooting positions arranged so that a first partial image contains Fsoivant and Fj _NE, a second partial image contains Fjaune and Fbieu and a third partial image contains Fbieu and Frouge. In another embodiment, partial images are recorded from four shooting positions arranged so that each partial image contains only one of the four remarkable boundaries Fsoivant, Fjaune, Fbieu and Frouge.

One can also consider any combination of these two embodiments where some digital cameras record partial images containing two or three remarkable limits and other digital cameras record partial images containing a single remarkable limit.

Given the very high sensitivity of digital cameras to CCD, it is necessary to take a number of steps to ensure good image quality. The different protocols for calibration and elimination of background noise are familiar to those skilled in the art using CCD sensors, and do not need to be described in detail here. Examples of measures that guarantee good image quality include the cooling of CCD sensors by a Peltier module in order to eliminate the formation of so-called "thermal" charges and the measurement of background noise.

A particular problem related to the use of CCD sensors in the context of an FIA analysis is the indispensable UV lighting placed near the column. Preferably, an individual UV light source placed near each of the outstanding boundaries will be used. To prevent a possible disturbance of the fluorescence signals by the excitation radiation emitted by the UV source, it is advisable to place between the column and the digital camera (s) a UV filter, opaque to the radiation emitted by the UV light source but permeable to fluorescent light emitted by the remarkable limits.

It is also possible to use band pass filters selectively passing only the yellow, blue and red light. These filters are placed between the column and the digital camera recording an image containing Fjaune respectively Fbieu and F _r0 UGE. The use of such band-pass filters is of course contemplated that where the partial recorded by the digital camera image contains a single fluorescent remarkable limit Fjaune, Fbieu or F _rO UGE.

The processing of the digital images comprises a first step of determining the X, Y coordinates of the recording points of the light maxima on the different rings. For this purpose, from the digital recordings, the image processing system determines a straight line P1-P2, substantially vertical and parallel to the axis of the chromatography column, where the intensity of the fluorescence of each of the fluorescent rings from the column towards the CCD sensors is maximum. To determine this line P1-P2, the fluorescence intensity recorded by a horizontal line of pixels AB, at a first strongly fluorescent zone, and the fluorescence intensity recorded by a horizontal line of pixels CD, are considered. level of a second strongly fluorescent zone. The pixel, located on the first horizontal line AB, having received the maximum fluorescence signal, and P2 the pixel, located on the second horizontal line CD, having received the maximum fluorescence signal. The intensity of the recorded fluorescence is then plotted along the straight line passing through P1-P2 (generally substantially perpendicular to the horizontal lines AB and CD and thus substantially parallel to the axis of the chromatography column) as a function of the height of the the column.

FIG. 2 shows the evolution of the fluorescence intensity as a function of the distance with respect to the lower end of the chromatography column, along the line P1-P2, the low values of abscissa corresponding to the part bottom of the column analysis area and the high abscissa values corresponding to the upper part of the analysis area. Three "peaks" of fluorescence emission are observed. These peaks are, from left to right in the figure, or from the bottom to the top of the column, a yellow fluorescence peak, a blue fluorescence peak and a red fluorescence peak. In accordance with the FIA standard, the four remarkable points used to calculate the fractions of the different types of hydrocarbons are, from left to right (or from bottom to top of the column):

• the solvent front (F _N ivant)

• the maximum of the yellow fluorescence peak (yellow) • the lower limit (or left limit) of the blue fluorescence peak

(Fbleu), and

• the upper limit (or right limit) of the red fluorescence peak

(Frouge) •

From these four remarkable points, we measure the distance separating them, two by two:

Ls = distance between Fsoivant and F j _aU do,

Lo = distance between Fjaune and Fbieu and

L _a ⁼ distance between Fbieu and Frouge •

The value of the ratio of each of these distances L _s , L ₀ and L _a to the total distance L separating F ^ vant and Frouge, corresponds to the volume fraction of the corresponding hydrocarbon type: fraction of saturated hydrocarbons (% saturated) = (L _s / L) x 100 fraction of olefinic hydrocarbons (% olefins) = (L _o / L) x 100 fraction of aromatic hydrocarbons (% aromatics) = (L _a / L) x 100 Examples

Tables 1 to 4 show the FIA analysis results obtained for the same automobile fuel sample under the measurement conditions defined above: Table 1 gathers the results of 6 successive measurements made strictly according to ASTM D 1319 by the same operator (comparative results).

Table 2 shows the results obtained according to ASTM D 1319 by 6 different operators (comparative results). Table 3 collates the results obtained for 6 successive measurements made by the same operator according to the invention, that is to say by using three identical digital cameras arranged as close as possible to the remarkable limits, the fluorescence rings being illuminated individually. by a UV lamp.

Finally, Table 4 groups together the results of the measurements made under conditions identical to those in Table 3, except that they were carried out not by the same operator, but by 6 different operators. In Tables 1 and 3, it can be seen that the use of the method according to the invention comprising the acquisition and processing of digital images makes it possible to divide the repeatability by two. The reproducibility values of Table 2 (comparative) are divided by three when using an image acquisition and processing system according to the invention (see Table 4).

Moreover, it is perfectly possible, given the speed of measurements by digital CCD cameras, to be able to overcome the error of reading due to the advancement of the analyzed product in the chromatography column during the duration of and can therefore be content with a single shot or a series of successive shots which the skilled person will determine the appropriate number to obtain significant average values. Table 1

Repeatability according to the state of the art (comparative results)

Table 2

Reproducibility according to the state of the art (comparative results)

Table 3

Repeatability according to the invention

Table 4

Reproducibility according to the invention

The present invention thus makes it possible to improve the repeatability and reproducibility of the FIA analysis according to ASTM D 1319 or its equivalents, with a view to determining the composition of a petroleum product. in its various hydrocarbon components. The gain on these two parameters is substantially equal to or greater than 2, an improvement substantially equal to at least 50% of the values indicated in the standard.

Claims

A method for determining the respective volume fractions of various organic compounds in a liquid sample, comprising

(b) reading, by means of the fluorescence signals emitted by the indicator (s), the areas of the chromatography column in which the organic compounds separated in step (a) are located, and (c) the calculation of the volume fractions. respective organic compounds from the results of the reading of step (b), characterized in that the reading of the zones of the chromatography column in step (b) is carried out by acquisition of digital images by means of at least one digital camera, and in that the calculation of the respective volume fractions of the organic compounds in step (c) is done by means of a computer program for digital image processing.

The process according to claim 1 for determining the respective volume fractions of saturated, olefinic and aromatic hydrocarbons in a sample of a liquid petroleum product distilling below 315 ° C, according to ASTM D 1319 or its equivalents, comprising among the various steps defined in said standard, the reading of the exact position of each of the following four following remarkable limits observed, from bottom to top, under UV light (λmax 365 nm), in the analysis zone of the column used :

solvent front (Fsoivant),

- maximum intensity of yellow fluorescence (yellow),

- lower limit of the blue fluorescence zone (Fbieu) and

- upper limit of the red fluorescence zone (Frouge), characterized in that the survey of the position of these four remarkable limits (Fsoivant, Fjaune, Fbieu and Frouge) is done by acquisition of digital images by means of least a digital camera and that the measurement of the distances between these different positions and the calculation of the corresponding volume fractions are done by means of a computer program for processing digital images.

3. Method according to claim 1 or 2, characterized in that the digital camera comprises a charge transfer photodetector device (CCD).

4. Method according to claim 2 or 3, characterized in that the acquisition of digital images is done by means of a single digital CCD camera, placed at a fixed distance from the column so that the recorded image contains all of the four remarkable boundaries Fsoivant, Fjaune, Fbieu and Fπmge.

5. Method according to claims 2 and 3, characterized in that the acquisition of digital images is done by means of at least two CCD digital cameras, placed at shooting positions aligned along an axis parallel to the axis of the column and located at a distance from the column such that the partial image recorded by each of the digital cameras contains a portion of the remarkable limits Fsoivant, Fjaune, Fbieu and Frouge and that the combination of partial images recorded allows the reconstitution of a single image containing all four remarkable boundaries Fsoivant, r yellow ₅ rbleu and r red.

6. Method according to claims 2 and 3, characterized in that the acquisition of digital images is done by means of a single CCD digital camera, which successively records a series of images from a series of images. Shooting positions, aligned along an axis parallel to the axis of the column and located at a distance from the column such that each of the series of partial images contains part of the remarkable limits Fsoivant, Fjaune, Fbieu and Frouge and the combination of the series of images allows the reconstitution of a single image containing all four remarkable limits Fsoivant, Fjaune, Fbieu and Frouge.

7. Method according to claim 5 or 6, characterized in that the reconstitution of a single image from all the partial images recorded simultaneously or successively is done with reference images taken with the same digital camera. from the same shooting positions but under visible lighting, which reference images show, in addition to the column area of chromatography, a graduated rule placed near the column and parallel thereto.

8. Method according to claim 5 or 6, characterized in that partial images are recorded from three shooting positions arranged so that a first partial image contains Fsoivant and Fjaune, a second partial image contains Fjaune and Fbieu and a third partial image contains Fbieu and Frouge-

9. A method according to claim 5 or 6, characterized in that partial images are recorded from four shooting positions arranged so that each partial image contains only one of four remarkable limits Fsoivant, Fjaune, Fbieu and Frouge.

10. Method according to any one of claims 2 to 9, characterized in that a UV filter, opaque to radiation emitted by the UV light source, but permeable to fluorescence light emitted by the remarkable limits, is placed between the digital camera (s) and the column.

11. The method of claim 1, characterized in that the acquisition of digital images is performed by a single shot or a single series of shots.

12. Device for implementing the method according to any one of the preceding claims, comprising at least one chromatography column for liquid chromatography, characterized in that it further comprises, one or more digital camera (s) arranged. (s) to record one or more digital image (s) of the chromatography column, and a computer and a digital image processing computer program, for analyzing the digital image (s) recorded by the or the digital camera (s).

13. Device according to claim 12 for the determination of the respective volume fractions of saturated, olefinic and aromatic hydrocarbons in a sample of a liquid petroleum product distilling below 315 ° C. according to ASTM D 1319, characterized in that it includes, in addition to the one used in that standard, one or more digital cameras arranged to record one or more images of the analysis area of the column (A), and a computer and a computer program for processing digital images, for analyzing the digital image (s) of the analysis zone of the column used and for calculating, from four remarkable limits of these images, the respective volume fractions of saturated, olefinic and aromatic hydrocarbons of the sample analyzed.

14. Device according to claim 12 or 13, characterized in that the digital camera comprises a charge transfer photodetector device (CCD).

15. Device according to claim 13 and 14, characterized in that it comprises a single CCD digital camera, placed at a fixed distance from the column (A) so that the image recorded by this camera numerical contains all the remarkable limits necessary for calculating the volume fractions of the different types of hydrocarbons.

16. Device according to claim 13 and 14, characterized in that it comprises at least two CCD digital cameras, placed at the shooting positions aligned along an axis parallel to the axis of the column (A) and located at a distance from the column such that the image recorded by each of the digital cameras contains a part of the remarkable limits necessary to calculate the volume fractions of the different types of hydrocarbons, and that the combination of all the recorded images allows the reconstruction of a single image containing all the remarkable limits necessary to calculate the volume fractions of the different types of hydrocarbons.

17. Device according to claim 13 and 14, characterized in that it comprises a single CCD digital camera, fixed on a rail parallel to the axis of the column so as to be movable along this rail and successively recording a series of images from a series of shooting positions.

18. Device according to claim 16 or 17, characterized in that it further comprises a graduated rule placed near the column and parallel thereto and a visible light source for illuminating the column and the ruler.

19. Device according to any one of claims 13 to 18 characterized in that it further comprises at least one UV filter, opaque to the radiation emitted by the UV radiation source, placed between the digital camera (s). (s) and the column.

20. Device according to one of claims 13 to 19, characterized in that an individual UV light source is placed near each of the remarkable limits.