FR2708999A1 - Spectrophotometric apparatus with multiplex device and method using the apparatus - Google Patents

Abstract

The invention relates to a spectrophotometric apparatus fitted with a fibre-optic multiplex device including an incident radiation source (1), n forward optical fibres (2) leaving the source in a bundle (12) and each individually connected to one of the n measurement cells (3), and n return optical fibres (4), each starting from one of the n measurement cells, and extending to a photodetection zone (13) essentially consisting of the ends (14) of the n return optical fibres arranged in front of a photodetector (5) mounted on a motorised micrometer movement device (17) so that the photodetector is moved and is successively positioned facing each of the ends (14). The invention also relates to a method using the apparatus, preferably in near-infrared spectrophotometry, both as an analysis means and/or for on-line examination in a manufacturing process for chemical or biochemical petroleum products.

Description

 The present invention relates to a spectrophotometric apparatus provided with an optical fiber multiplex device used to study the optical properties of one or more products, as well as a method using the device.

 Spectrophotometry is the study of the optical properties of a sample at various wavelengths ranging from near-infrared to infrared and visible to ultraviolet. According to the general principle, the measurements are made of variations in the intensity of a radiation of light either reflected or transmitted by the sample to be analyzed, as a function of the wavelength of incident radiation. The data resulting from the measurements can normally be used to determine the sample composition and the relative concentrations of the constituents forming the sample. Usually, the analysis is performed by comparing a spectral diagram of the light intensities of the sample obtained as a function of wavelength with the spectral diagrams of known substances. Comparison methods usually involve mathematical techniques such as the use of the partial least squares method with latent variables.

 Spectrophotometry has a wide variety of applications in scientific research and industry. In the latter case, it can for example be used to control the composition of products used and / or manufactured in chemical, petroleum or biochemical manufacturing processes.

Spectrophotometry has recently gained momentum as a means of analysis and / or on-line control of chemical processes. The term "chemical process" refers to all processes used in the petroleum, chemical, agri-food, pharmaceutical and biochemical industries. In particular, the near-infrared spectrophotometry involving or not the Fourier transform makes it possible to directly give information on the physical, physicochemical and / or mechanical properties of the analyzed products and is used as a means of analysis and on-line control in processes such as those described in the European patent applications
EP-A-0 285 251, EP-A-0 304 232, EP-A-0 304 233, EP-A-0 305 090,
EP-A-0 328 826 and EP-A-0 345 182. In this new development, spectrophotometry often involves the necessity of analyzing one or more products at the various stages of the manufacturing process of the product or products, in particular analyze the raw materials used, possibly the intermediate products, and the final products manufactured. It therefore requires analyzes carried out substantially in the same period of time and at different places of the same industrial production unit.

The typical embodiment of a spectrophotometric analysis in a chemical process comprises the implementation of the following means, represented diagrammatically in FIG.
a source (1) of incident radiation,
- an optical fiber go (2) to guide the radius
incident from the source (1) to a measuring cell
(3),
- a measuring cell (3) arranged at the location of the analysis
the product or products used and / or manufactured in the pro
assigned,
a return optical fiber (4) for guiding the beam
nally reflected or transmitted by the measuring cell (3) to
a photodetector (5), and
a photodetector (5) transforming the reflected radiation or
transmitted in an electrical signal itself converted into data
digitally processed and analyzed by a microcomputer,
the help of numerical relationship, especially correlative,
to calculate from absorbance measurements the
ownership of the product (s) analyzed.

 The spectrophotometer proper (0) generally comprises the source (1) and the photodetector (5) provided with its digital processing means and a microcomputer. The set consisting of the optical fiber go (2), the optical fiber return (4) and the measuring cell (3) is generally known as the single-channel measuring device, and the combination of all these means constitutes a spectrophotometer with a single-channel measuring device.

Since spectrophotometry can currently be used as a means of analysis and / or on-line control of chemical processes using several analysis points simultaneously, it would be conceivable to install a single-channel spectrophotometer for each point of analysis of the process. The
FIG. 2 diagrammatically represents, by way of example, a set of four spectrophotometers of this type for four analysis points, and uses the same references as in FIG. 1. For the sake of economy, it is preferable to use a single spectrophotometer provided with a multi-channel device, which is called a multiplex device.

 However, currently known multiplex device spectrophotometers have several disadvantages. The major disadvantage of these devices is related to the fact that each optical fiber for guiding the radiation comprises at least one cutoff between the radiation source and the measuring cell and / or between the measuring cell and the photodetector.

 Therefore, for each break, there arises the delicate connection problem of precisely adjusting an end-to-end alignment of the cut ends of the optical fibers. Moreover, each cut and each connection inevitably generate energy losses in the transport of the radiation.

These energy losses are particularly detrimental in Fourier transform near-infrared spectrophotometry where the information received is extremely complex.

 By way of example, FIG. 3 diagrammatically represents a multiplex device spectrophotometer as described above, the references of the Figure being identical to those cited in FIG. 1. In particular, the spectrophotometer comprises an output beam (6) to several optical fibers go (2), in this case four in number. Each of the optical fibers go (2) has a cutoff (7) interrupting the optical fiber. Between the ends of the optical fiber interrupted by the cut (7) is inserted a shutter (8) preventing the transmission of radiation. The shutter (8) has an opening (9) allowing the passage of the radiation selectively through a single cut (7) at a time and therefore through a single optical fiber. The shutter (8) is translational so that the aperture (9) passes from one cutoff (7) to another successively and selectively passes radiation through each of the four forward optical fibers ( 2). Each optical fiber go (2) and each optical fiber return (4) also comprise a connection (10). All the return optical fibers (4) are gathered in a return beam (11) arranged vis-à-vis the photodetector (5).

 The present invention relates to a spectrophotometric apparatus provided with an optical fiber multiplex device no longer having the disadvantages mentioned above, as well as a method for using the apparatus. In particular, the optical fibers of the apparatus and of the multiplex device no longer contain cuts and the problem of end-to-end alignment of the cut ends of the optical fibers no longer arises. The spectrophotometer of the invention can operate for long periods of time with a high degree of accuracy and reproducibility.

The present invention firstly relates to a spectrophotometric apparatus provided with an optical fiber multiplex device and comprising a source (1) of incident radiation, a photodetector (5) and a number n at least equal to 2 of measuring cells ( 3), characterized in that
- n optical fibers go (2) to guide the radius
incidentally go into a beam (12) of the source (1)
and then separate into n fiber optics go indivi
dual, each directly connected to one of the n cells of
measure (3), and
- n optical fiber return (4), each starting from one of the n
measuring cells (3) and for guiding the radiation
reflected or transmitted, go to a photo area
detection (13) consisting essentially of the ends
(14) n optical fiber optics (4) arranged front facing
to the photodetector (5) mounted on a motorized device
micrometric translation (17), so that the
photodetector (5) moves in front of the photodep area
tection (13) and positions itself successively opposite
each of the ends (14).

 Figure 1 schematically shows a spectrophotometric apparatus with a single-channel measuring device.

 Figure 2 schematically shows a set of four spectrophotometric devices, each being provided with a single-channel measuring device.

 Figure 3 schematically shows a multiplex device spectrophotometric apparatus including a shutter and fiber optic connections.

FIG. 4 schematically represents the multiplex device spectrophotometric apparatus according to the present invention and the references cited above appear by way of example in FIG.
Figure 4

 FIG. 5 diagrammatically shows in a cavalier perspective the photodetection zone (13) and the photodetector (5) mounted on the micrometric motorized translation device (17) of the invention.

 Figure 6 shows a graphical representation of the variation of the light energy received by a photodetector as a function of the displacement of the latter relative to the end of a return fiber.

 The spectrophotometric apparatus, as diagrammatically shown in FIG. 4, comprises a source (1). incident radiation whose wavelength can vary and correspond to those of the near infrared, infrared, visible or ultraviolet.

Preferably, the source (1) emits radiation in the infrared and more particularly in the near infrared, that is to say wavelengths ranging from 0.8 to 2.6 .mu.m. The apparatus also generally comprises a photodetector (5) adapted to the type of wavelengths used. The photodetector comprises an active portion (18) generally consisting of a semiconductor, for example indium arsenide when far-infrared radiation is used. The active portion (18) may have a circular flat surface of 0.5 to 5 mm, preferably 1 to 3 mm. Electronic elements are usually provided for measuring the energy transmitted by the photodetector and providing digitized signals to a microcomputer controlling the operation of the spectrophotometer.

 The measuring cells (3) are suitable for the type of radiation emitted by the source (1). They can be identical to those described in the European patent application EP-A-0 368 560. The number n of measurement cells (3) depends on the number of desired points of analysis, for example the number of desired points of analyzing, for example the number of analysis points and / or online control in a process of manufacturing chemicals. The number n can for example be from 2 to 50, preferably from 2 to 20 and more especially from 2 to 12. In an industrial production unit for example of chemical, petroleum or biochemical products, the measuring cells (3) can be installed in particular on the supply lines of the raw materials, on the mixing chamber and / or reaction of the constituents used, and / or on the outlet pipes of the final product or products manufactured.

 The spectrophotometer proper (0) comprises the source (1) and generally the photodetector (5). From the source (1), part of a beam (12) of n optical fibers go (2), the number n of optical fibers go (2) being identical to the number n of measuring cells (3). The optical fibers go (2) are adapted to the radiation used; they may be for example zirconium fluoride or silica, when the radiation is respectively near infrared lontain (wavelength of 2 to 2.5 Hm) or near infrared (wavelength of 0.8 to 2 hum). They are generally of circular section and may have a diameter ranging from 10 to 500 μm, preferably from 50 to 400 μm, in particular from 100 to 300 μm. They are arranged in a bundle (12), that is to say in the form of a cable comprising the n optical fibers go (2). The beam (12) may itself be of circular section whose diameter depends on the number n of optical fibers (2) and the diameter of said fibers. Thus, for example, if seven optical fibers (2) each having a diameter of 200/250 μm are used, the diameter of the circular section of the beam (12) may be between 1 and 3 mm, preferably between 1.3 and 1.7 mm. A filling material consisting of, for example, a solidifying agent such as a resin can fill the free space between the optical fibers (2) in the beam (12) and hold them fixed in the latter. The end face (15) of the bundle (12) from which the n optical fibers (2) originate is located generally opposite the source (1). Preferably, there is disposed between the source (1) and the end face (15) of the beam an optical focusing interface (16) which distributes the incident radiation over the entire surface of the end face (15) of the beam, such that the radiation is optimally transmitted by the n optical fibers (2).

 The n optical fibers go (2) leave the beam and separate into n individual forward optical fibers which are then each connected directly to a measuring cell (3). The total length of each optical fiber going (2), including the portion included in the beam (12), depends on the distance separating the source (1), that is to say the spectrophotometer itself (0), of the measuring cell (3). The total length may be of the order of a few centimeters to several tens of meters, for example from 5 to 25 meters. Each optical fiber going (2) is preferably a continuous fiber, that is to say an integral fiber having no cut or interruption, so that starting from the end (15) of the beam ( 12), it is directly connected to the measuring cell (3).

 The n back optical fibers (4) are generally of the same nature as the optical fibers go (2). Their number n is also identical to the number n of measuring cells (3), since from each measuring cell (3) part a return optical fiber (4). Each back optical fiber (4) is also, preferably, a continuous fiber, that is to say an integral fiber having no cut or interruption. Thus, starting from the measuring cell (3), it is directly connected to the photodetection zone (13) located near the photodetector (5), itself part of the spectrophotometer itself (0) or located in its vicinity immediate. The total length of each optical fiber return (4) is therefore substantially identical to that of the optical fiber go (2).

 The photodetection zone (13) essentially consists of the ends (14) of the n optical return fibers (4) facing the photodetector (5). The end (14) is generally constituted by the orthogonal section of a return optical fiber (4), that is to say a circular section of identical diameter to that of the optical fiber back (4). The ends (14) are preferably arranged on the flat surface of a solid support, for example the vertical surface (19) of a parallelepiped support (20) as shown schematically in FIG. 5. The ends (14) can sufficiently separate from each other, so that the free space between two consecutive ends (14) is preferably greater than one, in particular greater than 10 times the diameter of the end (14), and more particularly a distance of between 10 and 1000 times the diameter of the end (14).

 The ends (14) may be disposed on the flat surface of a solid support in any arrangement or, preferably, a substantially regular arrangement, i.e. an arrangement in which the distance between two consecutive ends is substantially constant. A regular arrangement, for example in checkerboard, in a circle or more particularly in a rectilinear arrangement as represented by the axis XX 'in FIG. 5, is thus preferred.

 The photodetector (5) is mounted on a micrometric motorized translation device (17). The device (17) allows the translation of the photodetector (5) and in particular of its active portion (18) facing the front formed by the n ends (14) of the optical fiber return. The translation of the active portion (18) of the photodetector can be done, preferably, in a plane substantially parallel to that where the ends (14) are arranged, for example to that of the flat surface (19) of the solid support (20). ) carrying the ends (14). The free space between the plane formed by the n ends (14) and the plane where the active portion (18) of the photodetector is translated corresponds to a shortest possible distance, for example from 0.1 to 2 mm, of preferably from 0.5 to 1 mm. The device (17) can also allow displacements and positioning of the photodetector (5) and in particular of its active portion (18) vis-à-vis successively each of the n ends (14). It can also make it possible to carry out displacements and positioning in a repetitive and cyclic mode. The device (17) may be chosen from devices having a reproducibility in their displacements and positioning of the order of 1 to 50 μm, preferably of 2 to 20 μm, in particular of 2 to 10 μm. I1 finally allows positioning in optimal conditions.

 The optimal positioning of the photodetector is pre-established during calibration tests taking into account that the response of the photodetector is not optimal throughout the surface of its active portion. Particularly in near-infrared spectrophotometry, it is recommended to illuminate the photodetector in the zone collecting the maximum optical flux where the transformation of the light signal into an electrical signal will be optimal. This makes it possible to obtain the maximum incident intensity combined with an optimum degree of accuracy and reproducibility in the measurement.

Thus, by way of example, FIG. 6 shows a typical graphical representation of the variation of the energy (in ordinate) received by an indium arsenide photodetector (5) having an active portion constituted by a circular section of detection of 1.5 mm in diameter and having at its center an optimum optical flux collection area, as a function of the translation expressed in millimeters (abscissa) of said photodetector facing the end (14) of a return optical fiber (4) 200/250 ym in diameter (manufactured by LE VERRE FLUORE (France) under the trade name "IR Guide
SHST 2CEA "type" 200/250 - 10 ") illuminated by near-infrared radiation of 0.8 to 2.5 pm wavelength and 0.5 mm distance from the photodetector. 6 that the optimal positioning of the photodetector is easily identified by the maximum of the curve, thus the calibration tests will make it possible to locate for each end (14) of the n optical fibers (4) the optimal positioning of the photodetector in its translation and To establish a program storing the desired translation with the n optimal positions, the program will then be used to operate the micrometric motorized translation device (17) with excellent reproducibility and accuracy.

The present invention also relates to a method for performing spectrophotometric measurements and calculations for determining from said measurements at least a property of samples passing through n absorbance measuring cells using the spectrophotometric apparatus. of the invention, characterized in that
- incident radiation is selected at one or more
wavelengths in the near infrared, infrared,
visible or ultraviolet,
- the photodetector (5) is translated using the motorcycle device
micrometrically smooth translation (17) facing the ends
(14) and position it successively in front of
each of the ends (14), and
at each positioning of the photodetector (5) vis-à-vis
at one end (14), the absorbance (s) of
the sample passing through the measuring cell (3), and
directly calculates at least one of the properties of the sample.
by the application of a numerical or
lative between said property and the absorbance values
measured.

 The calculated properties or properties of the samples are preferably chosen from the concentrations of the constituent (s) of the samples, the chemical, physical-chemical, physical or mechanical properties of the samples. The calculated properties and the numerical or correlative relations existing between the measured absorbance values and the properties are in particular mentioned in the European patent applications cited above.

The numerical or correlative relationships are preferably determined experimentally by multivariate regression or by any other appropriate mathematical method and depend only on the type of spectrophotometer used, the type of properties calculated and the number of wavelengths employed.

 It is preferred to select an incident radiation having m wavelengths in the near infrared that is to say wavelengths ranging from 0.8 to 2.6 Hm, the number m ranging from 2 to 800, preferably from 2 to 400. The modes of selection of the wavelengths of the incident radiation, the types of numerical or correlative relations and the modes of calculation for each measurement carried out in the n measurement cells (3) are mentioned for example in the European Patent Applications EP-A-0 285 251, EP-A-0 304 232, EP-A-0 304 233, EP-A-0 305 090, EP-A-0 328 826 and EP-A-0 345 182.

The apparatus and the method are particularly useful as means of analysis and / or on-line control of petroleum or petrochemical product units with determined physical properties such as octane number, cetane number, density , viscosity, flocculation threshold, cloud point, pour point, filterability, flash point, thermal stability, distillation properties, chemical compositions by hydrocarbon families and / or the number of of carbon atom, or the content of specific chemicals (oxygenating agents, alcohols, benzene, ...)
They can also be used as means of analysis and / or online control of a chemical or petrochemical manufacturing unit, such as steam cracking of hydrocarbons to produce olefins, or polymer manufacture. They can also be used in the agri-food or pharmaceutical industry, or in biochemical manufacturing processes.

 They are particularly suitable for spectrometers operating in the infrared and more particularly in the near infrared.

 The following non-limiting examples illustrate the present invention.

Example 1
A spectrophotometric apparatus equipped with an optical fiber multiplex device identical to that shown schematically in Figure 4 except that the number n of optical fibers and measuring cells is 5 instead of 4, is used to measure the light engine octane number (MON CLR) of a fuel base leaving a catalytic reformer unit, this base being also called catalytic reformate.

A spectrophotometer (0) manufactured by BOMEN is used
HARTMANN & BRAUN (Germany) under the commercial reference "MB 160" S. It comprises in particular a source (1) of incident radiation of wavelength varying in the near infrared (0.8 to 2.6 pm).

Five optical fibers go (2) to guide the incident radiation from the source in the form of a beam (12) consisting of a cable of 1.5 mm in diameter. An adaptation head comprising an optical focusing interface (16) makes it possible to fix the beam (12) facing the source (1). Each of the five optical fibers go (2) is directly connected to a measuring cell (3) manufactured by STEEM (France) under the commercial reference "Cell of
IR measurement "type" 24220 ".

 The catalytic reformate passes successively through the measuring cells (3).

 Five return optical fibers (4) for guiding the transmitted radiation are fed at the rate of one per measuring cell (3) and are directed towards a photodetection zone (13) arranged in close proximity to the spectrophotometer (0).

The optical fibers go (2) and return (4) are made of zirconium fluoride and have a diameter of 200/250 pm. They each have a total length of 10 m. -They are made by LE
GLASS FLUORINE (France) under the trade name "IR Guide SHST 2
CEA "(R) type" 200/250 - 10 ".

 In the photodetection zone (13) as diagrammatically shown in FIG. 5 and using 5 reverse optical fibers instead of 4, the ends (14) of the return optical fibers (4) are arranged on a solid support (20) in parallelepipedal steel 100 mm long, 8 mm wide and 80 mm high. Each end (14) constituted by a circular surface corresponding to the orthogonal section of the back optical fiber is fixed to this support by a hole at half height in the width direction of said support. The ends (14) are arranged at mid-height of the support in a horizontal alignment of axis XX 'and are separated from each other by a distance of 20 mm.

The photodetector (5) of the "MB 160" spectrophotometer of BOMEN - HARTMANN & BRAUN has been removed from its initial position to be placed in the photodetection zone (13) in the immediate vicinity of said spectrophotometer and fixed on a motorized stage (17) of micrometric displacement manufactured by PHOTON CONTROL (England) under the trade name "PTS 1000 - 150" 3. The board (17) is provided with an interface sold by PHOTON CONTROL (England) under the trade name "MIC - DI"@ . As illustrated in
5, the photodetector (5) is fixed on the mobile carriage (21) of the plate so that the active portion (18) of said photodetector moves along the axis YY 'parallel to the axis XX' of the alignment of the ends (14) of the return optical fibers.

The free space separating the surface of the active portion (18) of the photodetector (5) from each end (14) is a distance of 0.5 mm. An electrical connection (22) connects the photodetector (5) to the original photodetection system "MB 160" g.

The motorized movement of the photodetector is previously calibrated by positioning the active portion (18) of the photodetector successively opposite each of the ends (14) so that positioning is considered optimal when the photodetector (5) receives from each end (14) a stationary maximum of light energy as shown in FIG.
Figure 6 by the maximum of the graphical representation. The motorized movement is then stored in a computer program and makes it possible to return the photodetector cyclically towards the first of the ends (14) when it finishes facing the last of the ends ( 14). The motorized stage "PTS 1000 - 150" allows a reproducibility in displacements and positions with a precision of the order of 5 pm.

 The catalytic reformate is passed successively through the measuring cells (3). The absorbances are measured and the MON CtR index is calculated using this spectrophotometric apparatus and according to the method described in EP-A-0 285 251, with the sole difference that instead of selecting 15 frequencies, 200 is selected. frequencies ranging from 4800 cm1 to 4000 cm1, each being distant from the next 4 cm ~ 1 (4800 cm1, 4796 cm -1, 4792 cm -1, 4788 cm -1, 4000 cm -1).

 A series of 20 absorbance measurements at these frequencies is made for each of the measuring cells (3) through which the same catalytic reformer passes. The MON CLR index is calculated for each measurement.

 For each measuring cell (3) the average (MON) and the standard deviation of the computed MON CLR indices are calculated. The results are summarized in Table 1.

Table 1: Averages and standard deviations of the MON CLR index

<tb> n0 <SEP> of <SEP> the <SEP> cell <SEP>
<tb> of <SEP> metric <SEP> 1 <SEP> 2 <SEP> 3 <SEP> 4 <SEP> 5
<tb><SEP> MON <SEP> 78.6 <SE> 78.8 <SE> 78.7 <SE> 78.7 <SE> 78.6
<tb><SEP> 0.029 <SEP> 0.019 <SEP> 0.023 <SEP> 0.026 <SEP> 0.024
<Tb>
From this table, it results
- the maximum deviation of the average of the MON CtR indices between the 5
measuring cells
MON max - MON min = 78.6 - 78.6 = 0.3
and
- the average standard deviation calculated for all 5 cells of
measured
a = 0.0242
These values demonstrate that the absorbance measurements made according to the present invention are of excellent reproducibility. In particular, it can be deduced from these series of measurements made on the measuring cells that the use of the optical fiber multiplex device of the present invention does not substantially degrade the values of the MON CtR index of the fuel base analyzed. .

Example 2 (comDarative)
The same spectrophotometric apparatus is used as in Example 1, except that the optical fiber multiplex device is not used and is replaced by a single-channel device having a go optical fiber (2). and a return optical fiber (4) connecting the source (1) and the photodetector (5) left in its original place in the "MB 160" spectrophotometer to a single measuring cell (3), as diagrammatically shown in FIG. Figure 1. The types of optical fibers (2) and (4) and measuring cell (3) used are identical to those used in Example 1.

 The same catalytic reformate as used in Example 1 passes through the single measuring cell (3) of the present apparatus. Under the same conditions as in Example 1, a series of absorbance measurements is carried out on the catalytic reformate passing through the measuring cell (3) and for each measurement the MON CtR index is calculated as in FIG. Example 1

The average (MON) and the standard deviation of the 20 MON CtR indices thus calculated are as follows:
MON = 78.7
a = 0.024
As a result of this comparative example, the results of measurements made on a spectrophotometric apparatus equipped with a single-fiber optical fiber device are not significantly different from those obtained in Example 1 with the same apparatus but equipped with a multiplex device. optical fiber according to the present invention. It appears in particular that the presence of the multiplex device of Example 1 does not degrade the values of the index
MY CtR of the fuel base, compared with a single-channel device.

Example 3 (comparative)
The same spectrophotometric apparatus is used as in Example 1 except that the optical fiber multiplex device is replaced by a 12-fiber optic multiplex device manufactured by GUIDED WAVE, Inc. (Canada) under the trade name "12 Channel Optical Multiplexer Module (MUX)". The optical fibers go (2) and return (4) are of the same nature as in Example 1, but are 12 in number for the one and 12 for the return. They are connected to 12 measuring cells (3) which are of the same type as in Example 1. The multiplex device comprises in particular a shutter (8) arranged in the spectrophotometric apparatus as shown schematically in FIG. 3, with the exception that the number n of optical fibers go (2) and return (4) and measuring cells (3) is 12 instead of 4. The 12 optical fibers return (4) are joined in a return beam (11) which is connected to the photodetector (5) left in its place of origin in the "MB 160" OR spectrophotometer.

 The types of optical fibers (2) and (4) and measuring cells (3) are identical to those used in Example 1.

 The same catalytic reformate as used in Example 1 passes successively through the 12 measuring cells (3) of the present apparatus. Under the same conditions as in Example 1, a series of absorbance measurements is carried out on the catalytic reformate passing through each of the 12 measurement cells (3) and the MON CtR index is calculated for each measurement.

 For each measuring cell (3) the average (MON) and the standard deviation of the calculated MON CtR indices are calculated. The results are collated in Table 2.

Table 2: Movennes and varpits of the MON CtR index

<tb> n0 <SEP> of <SEP> the <SEP>
<tb> cell <SEP> due <SEP> 1 <SEP> 2 <SEP> 3 <SEP> 4 <SEP> 5 <SEP> 6 <SEP> 7 <SEP> 8 <SEP> 9 <SEP> 10 <SEP > 11 <SEP> 12
<tb> measure
<tb><SEP> MON <SEP> 78.3 <SE> 78.9 <SE> 79.4 <SE> 79.8 <SE> 79.3 <SE> 76.8 <SE> 79.5 <MS> 78.9 <SEP> 76.1 <SEP> 80.4 <SEP> 78.9 <SEP> 78.9
<tb><SEP> 0.28 <SEP> 0.36 <SEP> 0.78 <SEP> 0.27 <SEP> 0.28 <SEP> 0.29 <SE> 0.29 <SEP> 0, 50 <SEP> 0.49 <SEP> 0.36 <SEP> 0.42 <SEP> 0.50
<Tb>
From this table, it follows:
- the maximum deviation of the average of the MON CtR indices between the 12
measuring cells
MON max - MON mini = 4.3
and
- the average standard deviation calculated for all 12 cells of
measured
a = 0.4
These values show that the measurements obtained with an optical fiber multiplex device comprising a shutter are of lower quality than those of Example 1. The average standard deviation is in the present Comparative Example much higher than that of the Example 1 and to that of Comparative Example 2. The presence of the optical fiber multiplex device of the present Comparative Example substantially degrades the values of the MON CLR index, which is not the case in Example 1 and in FIG. Comparative Example 2

Claims (10)

 each of the ends (14).  tection (13) and positions itself successively opposite  photodetector (5) moves in front of the photodep area  micrometric translation (17) so that the  to the photodetector (5) mounted on a motorized device  (14) n optical fiber optics (4) arranged front facing  detection (13) consisting essentially of the ends  reflected or transmitted, go to a photo area  measuring cells (3) and for guiding the radiation  - n optical fiber return (4), each starting from one of the n  measure (3), and  dual, each directly connected to one of the n cells of  and then separate into n fiber optics go indivi  incidentally go into a beam (12) of the source (1)  - n optical fibers go (2) to guide the radius 1. spectrophotometric apparatus provided with an optical fiber multiplex device and comprising a source (1) of incident radiation, a phodetector (5) and a number n at least equal to 2 measuring cells (3), characterized in that 2. Apparatus according to claim 1, characterized in that the source (1) emits incident radiation whose wavelength or wavelengths are chosen in the near infrared. 3. Apparatus according to claim 1 or 2, characterized in that the motorized micrometric translation device (17) has a reproducibility in its movements and positioning with an accuracy of the order of 1 to 50 pm. 4. Apparatus according to any one of claims 1 to 3, characterized in that an optical focusing interface (16) is arranged between the source (1) and the end face (15) of the beam (12) from which the n optical fibers go (2). 5. Apparatus according to any one of claims 1 to 4, characterized in that the ends (14) of the n back optical fibers (4) are arranged on the flat surface (19) of a solid support (20) and the active portion (18) of the photodetector (5) moves in a plane substantially parallel to said planar surface of the solid support. 6. Apparatus according to claim 5, characterized in that the ends (14) are arranged in a rectilinear arrangement on the flat surface (19) of the solid support (20), the free space between two ends (14) consecutive on said flat surface being equal to a distance greater than once the diameter of the end (14), preferably a distance between 10 and 1000 times said diameter. 7. A method for performing spectrophotometric measurements and calculations for determining from said measurements at least one property of samples passing through n absorbance measuring cells using the spectrophotometric apparatus according to any one of Claims 1 to 6, characterized in that  - incident radiation is selected at one or more  wavelengths in the near infrared, infrared,  visible or ultraviolet,  the photodetector (5) is translated using the motorcycle device  micrometrically smooth translation (17) facing the ends  (14) and position it successively in front of  each of the ends (14), and  at each positioning of the photodetector (5) vis-à-vis  at one end (14) the absorbance (s) of  the sample passing through the measuring cell (3) and  directly calculates at least one of the properties of  the sample by applying a numerical relationship or  correlative between the property and the values  of absorbance measured. 8. Method according to claim 7, characterized in that m wavelengths are selected in the near infrared, m being a number ranging from 2 to 800, preferably from 2 to 400. 9. The method of claim 7 or 8, characterized in that the calculated properties or properties of the samples are selected from the concentrations of the constituent or constituents of the samples, the chemical, physico-chemical, physical or mechanical properties of the samples. 10. Use of the spectrophotometric apparatus according to any one of claims 1 to 6 as a means of analysis and / or online control in a process for manufacturing petroleum, chemical or biochemical products.