METHOD FOR INLINE MEASUREMENT ON SIMULATED MOVING BED UNITS OR HYBRID UNITS FOR SEPARATION BY SIMULATED MOVING BED AND CRYSTALLIZATION, AND APPLICATION TO THE CONTROL AND REGULATION OF SAID UNITS

Abstract

A method for measuring the concentrations of species present at at least one point of a separation unit operating in simulated moving bed (SMB) mode, or a hybrid separation unit employing a step for simulated moving bed (SMB) separation and a step for crystallization, by calibration by inline acquisition of Raman spectra for different mixtures; analysis by inline signal processing of the Raman spectrum.

Claims

1. A method for measuring concentrations of species present at at least one point of a separation unit operating as a simulated moving bed (SMB), or a hybrid separation unit employing a step for simulated moving bed (SMB) separation and a step for crystallization, said method employing: an immersion probe placed at a point of the unit or at a point located on one of the streams entering or leaving said unit (termed the measurement point), a thermocouple placed at a distance between the immersed end of the probe and the thermocouple which is at most 30 cm from the measurement point, a sampling point downstream of the measurement point for analysis by a reference analytical technique during the calibration step, in a manner such as to provide, its Raman spectrum and its temperature simultaneously for each measurement point, said method comprising the following steps: a) calibration by inline acquisition of Raman spectra for different mixtures covering the range of concentrations of the species which are to be quantified and under temperature and pressure conditions which are representative of an industrial unit and sampling, simultaneously in situ at the sampling point, of the moving mixture for analysis by a reference technique, enabling one or more mathematical model(s) to be constructed per constituent as a function of its content; b) analysis by inline signal processing: the Raman spectrum obtained is processed at each measurement point by means of a chemometric mathematical method employing the or said models constructed during the calibration step for each constituent, taking into account the temperature (T.sub.spl) at the measurement point under consideration as well as the range of concentrations C.sub.j of the species present at said measurement point, in order to obtain the concentration C.sub.i of each species present, in which, for each of steps a) and b), the acquisition of each Raman spectrum is carried out by means of the following steps: sending a monochromatic signal through a first optical fibre connected to the immersion probe, originating from a laser source with a wavelength of 785 nm plus or minus 1 nm, retrieving, through a second optical fibre also connected to the immersion probe, a signal corresponding to the Raman effect termed the Raman signal, which is sent to a spectrometer, retrieving the Raman spectrum of the signal under consideration at the output from the spectrometer.

2. The measurement method as claimed in claim 1, in which the total length of the first optical fibre and of the second optical fibre is less than 1000 m, and preferably less than 700 m.

3. The measurement method as claimed in claim 1, in which the spectrometer uses filters defining a cut-off threshold.

4. The measurement method as claimed in claim 1, in which the, or one of the measurement points when there are several, is or are located at the recycling pumps on the recycling circuit.

5. The measurement method as claimed in claim 1, in which two measurement points located at the following sites are used: in the vicinity of the recycling pump on the recycling circuit, and in the vicinity of the feed pump on the feed circuit.

6. The measurement method as claimed in claim 1, in which three measurement points located at the following sites are used: the first point is in the vicinity of the recycling pump on the recycling circuit, the second point is in the vicinity of the feed pump on the feed circuit, and the third measurement point is located in a rectification zone for the raffinate distillation column.

7. The measurement method as claimed in claim 1, in which a hybrid separation unit is used, and a supplemental measurement point is used on the liquid stream at the outlet from the crystallization unit.

8. The measurement method as claimed in claim 1, in which the or said mathematical regression model(s) are constructed by means of an analytical method selected from the DCLS (Direct Classical Least Squares) method, the cross-correlation method, the ICLS (Indirect Classical Least Squares) method, methods of the ILS (Inverse Least Squares) type such as PCA (Principal Components Analysis), MLR (Multiple Linear Regression), PCR (Principal Component Regression) or the Partial Least Squares (PLS) method.

9. The measurement method as claimed in claim 8, in which the analytical method is the Partial Least Squares (PLS) method.

10. The measurement method as claimed in claim 1, in which the reference technique used for the calibration step is gas phase chromatography.

11. Application of the measurement method as claimed in claim 1 to the control and regulation of a xylenes separation unit, the difference between a concentration profile measured by said method and a set concentration profile for at least one of the constituents present in the unit meaning that at least one control parameter which is selected from the group constituted by: the internal flow rates, the feed flow rates, the eluent flow rate, the extract flow rate and the permutation period can be acted upon.

12. A device for the control and regulation of an industrial unit for the separation of xylenes as claimed in claim 11, comprising: two immersion probes, a thermocouple, a Raman spectrometry analysis system, a calibration system comprising a means for sampling downstream of the Raman spectrometry analysis system, a processing system, and a regulation and control loop.

Description

LIST OF FIGURES

[0112] FIG. 1: Diagram of the measurement system in accordance with the invention comprising the laser source, the spectrometer, the immersion probe and a control and regulation loop for the unit.

[0113] FIG. 2: Relative concentrations (in % v/v) obtained by gas phase chromatography (open symbols) and by Raman analysis (solid symbols) under the conditions of Example 1 (prior art).

BRIEF DESCRIPTION OF FIG. 1

[0114] FIG. 1 is a diagrammatic view of the measurement system comprising the laser source, the spectrometer, the immersion probe and a control and regulation loop which means that the Raman spectrum can be used to define a corrective action to be carried out on the unit if required.

[0115] A laser source 1 emits light at 785 nm. This light is guided along an optical fibre 2 to an immersion probe 3. This immersion probe is immersed at a measurement point of the xylenes separation unit 4 at which the concentration of the various constituents is to be determined. The Raman signal emitted at the measurement point is collected by the immersion probe then transmitted with the aid of a second optical fibre 5 to the Raman spectrometer 6. This latter generates the Raman spectrum corresponding to the measurement point. This spectrum is sent to the PC analyser 7.

[0116] At the same time, in a zone in the vicinity of the measurement point, a thermocouple 8 is immersed in the unit in order to acquire the temperature of the zone under consideration (which thus contains the measurement point for the Raman spectrum) to the PC analyser 7.

[0117] From the Raman spectrum and the temperature, the PC analyser determines the concentration of the various species present at the measurement point by using the processing method forming an integral part of the invention. By comparing the concentration values obtained thereby with the reference concentration values, an action is carried out by an actuator 9 on one or more of the operating variables of the process, for example the flow rate of a valve as shown in dashed lines at 10.

[0118] The dashed line indicates an optional element in the present measurement system.

[0119] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0120] In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

[0121] The entire disclosures of all applications, patents and publications, cited herein and of corresponding application No. FR 1662139, filed Dec. 8, 2016 are incorporated by reference herein.

EXAMPLES

[0122] The two examples below are intended to highlight the improvement in the measurement by means of the root mean square error (RMSE) variable when passing from Example 1 (carried out in accordance with the prior art) to Example 2 (in accordance with the invention).

Example 1: (in Accordance with the Prior Art)

[0123] In this example, the inline measurement was carried out using an immersion probe, a thermocouple in the vicinity of said probe, a 785 nm laser source and a simplified method for processing the spectra using a matrix mathematical method (not in accordance with the invention).

[0124] A Raman analyser using a 785 nm excitation laser was used on a xylenes separation unit using paradiethylbenzene as a solvent in order to determine the concentrations of ortho-(OX), meta- (MX), para-xylene (PX), ethylbenzene (EB) and paradiethylbenzene (PDEB).

[0125] The Raman spectrum of the mixture was measured directly on the principal stream from the unit with the aid of an immersion probe. A thermocouple was installed in the vicinity of the immersion probe.

[0126] The Raman spectrum and the temperature were measured at the same point of the unit and were thus sent to the PC analyser simultaneously.

[0127] The temperature of the sample, T.sub.spl, at the measurement point was equal to 175 C., a different temperature to the calibration temperatures T.sub.cal.

[0128] The concentrations by volume were obtained using the prior art method:

[00001]$C_j=\frac{P_j\left(T,C_1,\mathrm{.Math.}.Math.,C_5\right).Math.{}_j\left(T,C_1,\mathrm{.Math.}.Math.,C_5\right)}{\underset{i=1}{\overset{5}{.Math.}}.Math.P_i\left(T,C_1,\mathrm{.Math.}.Math.,C_5\right).Math.{}_i\left(T,C_1,\mathrm{.Math.}.Math.,C_5\right)}$

in which: [0129] P.sub.i is the integrated intensity of the Raman band due to the molecule i [0130] .sub.i denotes the effective cross section relating to the molecule i,
in which expression the integrated intensities P.sub.i are obtained from the measured intensities M.sub.j on the Raman spectrum by means of a matrix product in which the coefficients a.sub.ij of the matrix M result from a calibration carried out at the temperature T.sub.spl, the measurement point or at several temperatures around said temperature of the measurement point, the inverse of the effective cross sections .sub.i being a function of the temperature T and the concentrations C.sub.i of the various constituents.

[0131] The calibration was carried out using three different temperatures, namely 100 C., 140 C. and 180 C.

[0132] In the vicinity of the bypass loop, a sampling point allowed an aliquot of the principal stream to be removed from the unit. This aliquot was used for laboratory analysis by gas phase chromatography in order to determine the concentrations of the various constituents.

[0133] The gas phase chromatography method (GC) is a proven method for the analysis of C8-C10 aromatic hydrocarbons, providing reference values for the concentrations of the various constituents.

[0134] Cross-comparisons with the reference method by GC were carried out on a series of samples.

[0135] The results obtained in the low concentration range (<5%) are shown in FIG. 2.

[0136] The correlation between these two sets of values was also evaluated by the root mean square error method (RMSE) defined as follows:

[00002]$R.Math..Math.M.Math..Math.S.Math..Math.E=\sqrt{\frac{\underset{i=1}{\overset{n}{.Math.}}.Math.{\left(y_{\mathrm{Raman}}-y_{\mathrm{GC}}\right)}^2}{n}}$

where y.sub.GC are the concentrations obtained by GC, y.sub.Raman that for Raman and n is the number of concentrations measured. The maximum absolute difference was also recorded.

[0137] The results obtained are recorded in Table 1. The correlation between the GC measurements and the Raman measurements was good (R.sup.2=0.9986). However, the Raman measurements exhibited significant differences with the reference measurements: these differences were particularly large in the case of measurements at low concentrations (<5% v/v).

[0138] Under said conditions, the root mean square difference increased significantly, changing from 0.19% to 0.61% v/v.

[0139] FIG. 2 shows the relative concentrations (in % v/v) obtained by gas phase chromatography (open symbols) and by Raman analysis (solid symbols) under the conditions of Example 1.

TABLE-US-00001 TABLE 1 Statistical correlation data between the GC relative concentrations (in % v/v) and the Raman concentrations under the conditions of Example 1 Linear Maximum Concentration regression absolute measurement coefficient RMSE difference range Compound (R.sup.2) (% v/v) (% v/v) (% v/v) PDEB 0.9996 0.23 0.84 10-100 OX 0.9998 0.15 0.50 5-20 MX 0.9999 0.18 0.58 5-50 PX 0.9992 0.12 0.42 5-40 EB 0.9994 0.29 0.95 5-20 All constituents 0.9996 0.19 0.95 5-100 together OX 0.9510 0.62 1.92 0.05-5 MX 0.9349 0.94 1.89 0.05-5 PX 0.8616 1.07 1.99 0.6-5 EB 0.9678 0.40 1.04 0.05-5 All constituents 0.8357 0.61 1.99 0.05-5 together

Example 2 (in Accordance with the Invention)

[0140] In this example, the inline measurement was carried out using an immersion probe, a thermocouple in the vicinity of said probe, a 785 nm laser source and the method for processing the spectra presented in the invention using a chemometric mathematical method.

[0141] A Raman analyser using a 785 nm excitation laser was used on a xylenes separation unit using paradiethylbenzene as a solvent in order to determine the concentrations of ortho-(OX), meta- (MX), para-xylene (PX), ethylbenzene (EB) and paradiethylbenzene (PDEB).

[0142] The Raman spectrometer used in the context of the invention was a dispersive Raman spectrometer equipped with a toroidal incident mirror in order to improve the quality of the image on the detector by correcting optical aberrations, in particular astigmatism. The application used 4 pathways (8 fibres). The spectrometer used rejection filters in order to cut off the Rayleigh beam. In particular, a holographic transmission grating was used in order to simultaneously collect all of the Raman data over a spectral range of 100 cm.sup.1 to 3450 cm.sup.1 without any temporal displacement of the optical elements such as the diffraction gratings, while retaining a very good spectral resolution (less than 1.5 cm.sup.1/pixel). This example was thus entirely in accordance with the invention.

[0143] The Raman spectrum of the mixture was measured directly on the principal stream from the unit with the aid of an immersion probe. A thermocouple was installed in the vicinity of the immersion probe.

[0144] The Raman spectrum and the temperature were measured at the same point of the unit and were thus sent to the PC analyser simultaneously. The temperature of the sample, T.sub.spl, at the measurement point was equal to 175 C., a temperature included in the range of temperatures selected for the calibration.

[0145] These data were processed using the method described in the present invention. Cross comparisons with the gas phase chromatography, GC, analytical technology were carried out on a series of samples in a manner similar to that described for Example 1.

[0146] The calibration step of the method was based on the production of a calibration base containing more than a hundred mixtures covering a wide range of concentrations of species which were to be quantified over a certain range of temperatures and of a mathematical model connecting the Raman spectra of these mixtures with the concentrations obtained by a reference analytical technique: gas phase chromatography. In addition, in contrast to the cases described in the prior art, the spectra for this calibration base were recorded under conditions which were representative of industrial operation. In this manner, inline data acquisition could be employed to integrate effects such as the hydrodynamic dispersion in the line into the calibration operation. In addition, the development of different mathematical models as a function of the range of concentrations measured and the temperature meant that the precision of the method could be very significantly improved.

[0147] The results obtained are summarized in Table 2.

[0148] The root mean square error was reduced over the whole of the tested range.

[0149] In the case of the high concentration range (between 5% and 100% v/v), the RMSE reduced from 0.19% v/v in Example 1 to 0.12% v/v in the present case. This reduction in the root mean square error is due to a substantial reduction in the maximum difference observed, which dropped from 0.95% v/v in Example 1 to 0.50% v/v in this example.

[0150] The reduction in the root mean square error was even more significant in the case of the low concentration range (less than 5% v/v) where the RMSE reduced from 0.61% v/v in Example 1 to 0.041% v/v in the present case. This reduction in the root mean square error is due to a substantial reduction in the maximum difference observed, which dropped from 1.99% v/v in Example 1 to 0.14% v/v in this example.

[0151] Using a novel analysis method based on the measurement of the Raman spectrum and the temperature of the sample at the measurement point as follows: [0152] i) by carrying out an inline calibration under conditions which are representative of the temperature and pressure (for example at five temperatures between 40 C. and 180 C. under 10 bar), [0153] ii) by providing a signal processing method which takes into account a wide range of frequencies of the spectrum measured as well as the temperature, [0154] iii) by adapting the range of frequencies used to the composition of the feed to be analysed by changing the processing method as a consequence;
is at the origin of the excellent agreement between the reference measurements and the Raman measurements.

TABLE-US-00002 TABLE 2 Statistical correlation data between the relative concentrations (in % v/v) GC and the Raman concentrations under the conditions of Example 2. Linear Maximum Concentration regression absolute measurement coefficient RMSE difference range Compound (R.sup.2) (% v/v) (% v/v) (% v/v) PDEB 1.0000 0.12 0.14 10-100 OX 0.9999 0.15 0.50 5-20 MX 0.9999 0.16 0.46 5-70 PX 1.0000 0.11 0.21 5-80 EB 1.0000 0.07 0.19 5-50 All constituents 0.9999 0.12 0.50 5-100 together PDEB 1.0000 0.022 0.03 0.05-5 OX 0.9998 0.072 0.14 0.05-5 MX 0.9988 0.045 0.13 0.05-5 PX 0.9996 0.036 0.05 0.05-5 EB 0.9996 0.042 0.09 0.05-5 All constituents 0.9996 0.041 0.14 0.05-5 together

[0155] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

[0156] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.