GB2361767A

GB2361767A - Method for determining the raman spectrum of a mixture

Info

Publication number: GB2361767A
Application number: GB0101331A
Authority: GB
Inventors: James Noel Martin Hegarty; Zaid Rawi; Alasdair Ian Thomson
Original assignee: BP Chemicals Ltd
Current assignee: BP Chemicals Ltd
Priority date: 2000-01-24
Filing date: 2001-01-18
Publication date: 2001-10-31
Also published as: GB0001564D0; GB0101331D0

Abstract

A method for determining the Raman spectrum of a mixture, being characterised in that the Raman spectrum of the mixture is frequency drift corrected and/or intensity normalised with respect to a Raman spectral band of sapphire measured under the same conditions and at the same time as the Raman spectrum of the mixture. Laser light source (12) connected to probe (10) by optical fibre (18), passes through the quartz window (22) of the housing (16) and the sapphire window (24) of the sample cell (26) to irradiate the sample of process stream (26). Raman signals from the sapphire window (24) and the process stream (26) are collected by the probe (10) and passed through the optical fibre (20) to the spectrometer/analyser (14) which generates the Raman spectrum of the process stream mixture and the sapphire. Computer (38) frequency drift corrects and/or intensity normalises the Raman spectrum frequency of the mixture with respect to the sapphire Raman band.

Description

1 2361767 METHOD FOR DETERMINING THE RAMAN SPECTRUM OF A NUXTURE The

present invention relates to a method for determining the Raman spectrum of a mixture and for the use of that method in controlling chemical processes.

Raman spectroscopy is based on th& scattering of light by molecules. When a beam of light from a high intensity laser passes through a sample, molecules within the sample scatter a portion of the incident laser light beam. In Raman scattering, the frequency of the scattered light differs from that of the incident light beam by an amount known as the Raman shift. This difference or shift depends on the structure of the molecules in the sample. When the frequency/Raman shift and intensity of each scattered beam is measured, a Raman spectrum is produced. This can be used to provide an accurate analysis of the sample content. Raman spectroscopy has an advantage that it can be used to analyse substances "on-line". In other words, samples need not be removed from the bulk of the material under test for analysis at remote sites. Thus, it may be used in the control of chemical processes.

Various methods and systems are known for determining the Raman spectra of samples such as mixtures of organic compounds. Typically, such systems comprise a probe, which is coupled to a laser source and a spectrometer/analyser, by optical fibres. In use, a laser light beam is transmitted through one of the optical fibres to the probe, and directed onto the sample under test. The light scattered by the sample is collected by the probe and transmitted to the spectrometer/analyser by a second optical fibre. The wavelengths and intensities of the detected beams are measured and recorded to produce the Raman spectrum.

When analysing mixtures using such systems, difficulties may arise because optical conditions such as laser power, optical alignment and coupling change with time. These changes may affect the frequency and intensity of the scattered Raman light, and hence, the accuracy of the analysis. Because of these difficulties, Raman spectroscopy has hitherto been regarded as being difficult to implement. For example, although the composition of a petroleum fraction from a process may be analysed by Raman spectroscopy, such fractions are conventionally analysed using techniques such as gas chromatography and near infra. red spectroscopy.

Various methods have been developed to account for variations in the optical conditions encountered in Raman spectroscopy. For example, Haiming Xiao et a]. (pages 626 to 628, Vol 52, Number 4, 1998 Applied Spectroscopy) describe normalising the intensity of Raman emissions from a toluene sample to a constant internal standard, provided by a diamond Raman signal at 1332 cm-1. J. B. Cooper (Chernometric and Intelligent Laboratory Systems 46 (1999) 231-247) describes normalising: Raman spectra for process control applications.

The use of Raman spectroscopy for the control of a separation process has been described by Zaid Raw] (presentation to Royal Society of Chemistry (RSC) Spectroscopy on Process Analysis meeting 19"' November, 1998, Hull UK).

We have discovered that the Raman spectrum of sapphire can be used to provide a constant standard to which Raman bands from a mixture, such as a mixture of organic compounds and in particular hydrocarbons, can be frequency drift corrected and/or intensity normalised.

According to a first aspect of the present invention, there is provided a method for determining the Raman spectrurn of a mixture, said method being characterised in that the Raman spectrum of the mixture is frequency drift corrected and/or intensity normalised with respect to a Raman spectral band of sapphire measured under the same conditions and at the same time as the Raman spectrum of the mixture.

The Raman band of sapphire provides a reference for the frequency drift correction and/or intensity normalisation of spectra obtained under fluctuating process conditions. Thus, for example, if there is a drift in laser frequency over time, Raman spectra of mixtures taken at successive time intervals can be compared, by changing the frequencies of the spectra with respect to the frequency of a Raman spectral band of 2 sapphire recorded at the same time as the spectra of the mixtures. Similarly, if the intensity of the laser light source fluctuates, the spectra can be compared after normalising the intensity of the spectra with respect to the intensity of a Raman spectral band of sapphire recorded at the same time as the spectra of the mixtures.

When the mixture comprises hydrocarbons, the Raman band of sapphire at about 644 cm -' is employed. This Raman peak is positioned clear of any hydrocarbon Raman bands.

When the mixture comprises hydrocarbons, the Raman spectra may frequency drift corrected with respect to the frequency of the sapphire Raman band and may be intensity normalised with respect to the intensity of the Raman peak of the H-C-H bending vibration of the hydrocarbons.

A comparison of the spectra of mixtures corrected according to the process of the present invention and taken at time intervals may be used to control processes, for example distillation or chemical processes which produce the mixture.

For the purpose of frequency drift correcting the Raman spectrum of the mixture, it has been found that the Raman spectral band of sapphire may be measured under the same conditions as the Raman spectrum of the mixture by using a sample cell with a sapphire window to contain the mixture whilst measuring its Raman spectral band and focusing the incident laser light into the mixture near to the cell window. This provides a Raman spectrum of the sapphire under the same conditions as the mixture which may be measured with the Raman spectrum of the mixture. The Raman beam from the sapphire window may also be used to intensity normalise the Raman spectrum of the mixture in the cell. When the Raman beam from the sapphire window is used to intensity normalise the Raman spectrum of the mixture, care should be taken to correct for and/or prevent changes relative to the mixture peak intensities due to, for example alignment changes.

A suitable Raman band of the sample may be used for intensity normalisation, for example the H-C-H bending vibration at about 1400-1500 cm-' of a hydrocarbon mixture.

For the purpose of normalising the Raman spectrum of the mixture with respect to intensity, the Raman spectral band of sapphire may be measured under the same conditions as the Raman spectrum of the mixture by using one or more sapphire containing optical components in the optics used to provide the laser light to the mixture 3 and/or to conduct the resulting Raman beams from the mixture to a suitable detector for measuring the spectrum. This provides a Raman spectrum of the sapphire under the same conditions as the mixture which may be measured with the Raman spectrum of the mixture. Such sapphire-containing components include sapphire windows in the sample cell and sapphire lenses. These sapphire-containing components may also provide a Raman spectrum for frequency drift correction of the Raman spectrum of the mixture.

Preferably, the present invention is employed to measure the Raman spectrum of a mixture having little or no Raman signal in the region of 644 cm-1. This ensures that the sapphire band at about 644 cm-' is not obscured, and is easily detectable. Other Raman bands of sapphire may be used in the process of the invention provided that they are not obscured or interfered with by the Raman bands of the mixture.

The mixture may be a liquid and/or a gas. Preferably, the mixture comprises organic compounds such as hydrocarbons. For example, the mixture may comprise saturated and unsaturated hydrocarbons up to an including waxes and polymers.

Preferably, the mixture comprises C, to C7hydrocarbons. In one embodiment, the mixture is a petroleum fraction boiling in the range from -I 'C to 144'C. Such fractions comprise hydrocarbons such as paraffins, aromatics and naphthenes generally in the range C4 to C7. A suitable mixture is naphtha. In another embodiment, the mixture comprises olefins such as C2 to C7 olefins.

Hydrocarbons do not have a very strong Raman spectrum in the same region as that of the sapphire Raman band at about 644 cm-'. Hydrocarbons such as naphtha have been found to have a H-C-H bending vibration at about 1400-1500 cm-' (that is, peak about 1450 cm-1) which can be used in place of the sapphire Raman band to intensity normalise the Raman spectrum of the mixture.

The mixture may be contained in process streams of a chemical process or purification process. For example, the mixture may be contained in a stream emerging from or feeding into a distillation column. In another example, the mixture may be contained in a stream emerging from a reactor, such as a steam cracker. In yet a further example, the process of the present invention may be used in blending processes such as blending of refinery processes streams, for example in the production of aviation spirit.

The method of the present invention may be used to control a chemical, separation or blending process by determining the Raman spectra of a mixture contained 4 in a process stream of the process at time intervals and adjusting the process conditions to maintain the spectra within a pre-determined range of parameters. Preferably, the spectrum may be analysed using Chemometric analysis to determine the concentrations of one or more organic compounds in the mixture. Chemometrics is performed using multivariate computer models to predict quantitative information from Raman spectra, the models having been calibrated with respect to reference mixtures who's compositions have been determined, for example, by gas chromatography. The process conditions may be adjusted to maintain the concentrations of organic compounds thus determined within pre- determined ranges.

Preferably, a-plurality of such process streams are analysed using the method of the present invention. This may be achieved by having a plurality of sample cells each with a sapphire window, through which the process streams to be analysed are passed.

A source of laser light and a detector for the Raman beams are optically connected to a probe which may be moved between the cells to obtain the Raman spectra from the process streams. The frequency drift correction and/or intensity normalisation of the present invention reduces variations caused between successive analyses of samples in any particular sample cell.

Apparatus for use in the method of the present invention may comprise a source of laser light, means for directing the laser light onto the mixture and onto sapphire, and means for detecting scattered Raman signals from the mixture and the sapphire.

A sample cell may be used to contain the mixture- Preferably, a flow cell is used through which a process stream comprising a mixture to be analysed is passed.

The laser light source may be capable of generating a laser beam having a wave length of 488 to 1064 nm, preferably, about 785 rim.

Means for directing incident laser light onto the mixture may be connectable to the laser light source and/or detector by one or more optical fibres. The optical fibres may have a diameter of from 50trn to I mm. Preferably, the optical fibres have a diameter up to 200[Lm. The optical fibres may be up to 2000m, preferably up to I 00m in length, such that the laser source and/or detector may be located at a remote location from the mixture. This is useful, for example, if the mixture is contained in a process stream of a chemical or separation process. The conditions of such a process, and/or the mixture itself, may be a potentially hazardous to the laser equipment and/or operator. As a further safety measure, all or part of the apparatus may be contained in an inert atmosphere (for example in a purged box) to allow the equipment to be operated safely in a potentially explosive atmosphere. This is useful, for example, when the mixture sample is flammable. The box may also minimise the exposure of an operator to laser 5 light.

Means for directing the incident laser light onto the sample may comprise a probe. Advantageously, the probe may also be used to collect the scattered light and direct the light to a spectrometer with a detector. The probe may be of a known design. Advantageously, the probe is moveable between a plurality of cells of different mixtures, preferably by motorised means. This allows a number of mixtures to be analysed using a single apparatus. An example of a suitable motorised means is a motorised platform.

The probe may comprise sapphire-containing optical components. For example, in one embodiment, one end of the probe is provided with a sapphire lens. In use, the probe is employed to direct light from the laser source onto the sample. As light passes through the probe, it comes into contact with the sapphire lens. Some of the light is scattered by the sapphire, whilst the remainder passes through the coating towards the sample. Thus, scattered light detected by the detector comprises Raman bands arising from both the mixture and the sapphire. This may be used in the method of the present invention to frequency drift correct and/or intensity normalise the Raman spectrum of the mixture with respect to the Raman band of the sapphire.

Alternatively or additionally a sample cell for the mixture may be used which has a sapphire window. Incident laser light comes into contact with the sapphire window of the cell. Some of this light is scattered by the sapphire, whilst the remainder passes through the window towards the mixture. The detector, detects Raman bands produced by both the sample and the sapphire, particularly if the laser beam is focused near the sapphire window. The Raman beam from the sapphire window may then be used to frequency drift correct and/or intensity normalise the Raman spectrum of the mixture in the cell.

Any suitable detector may be employed in the present invention, suitably in a spectrometer/analyser. For example, a holographic grating spectrometer may be used to disperse the Raman signals from the mixture and sapphire, the dispersed light then being focused onto a CCD camera. Alternatively, a fourier-transform spectrometer with a high- 6 power 1064 nm laser may be used to generate and analyse the Raman spectra, An acousto-optic modulator spectrometer may also be used. This disperses the Raman signals using an acoustic wave passing through an optical crystal. Other methods known in the art may also be used for analysing the Raman spectra.

These and other aspects of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure I is a schematic part cross-section diagram of an apparatus for use in the process of the present invention; Figure 2 shows a Raman spectrum of naphtha obtained using a sample cell with a sapphire window; Figure 3 shows the Raman spectrum of Figure 2 normalised with respect to firequency of the sapphire band; Figure 4 shows the Raman spectrum of Figure 3 normalised with respect to the intensity of the H-C-H Raman band at about 1400 - 1500 cm- I; Figure 5 shows the benzene concentration of a mixture'of hydrocarbons determined according to the present invention and Figure 6 shows in schematic form, the apparatus of Figure I for measuring the spectra of three process streams.

Referring to Figure 1, the apparatus comprises a probe (10), a source of laser light (12) and a spectrometer/analyser ( 14). The probe (10) and source of laser light (12) are located within a protective, air-purged housing (16). The laser, probe and spectrometer/analyser may comprise a modified Kaiser Optical Systems Inc. HoIoProbe spectrometer which uses a red semiconductor laser, an optical probe head, a holographic grating for spectral analysis and a digital camera for detection. The probe (10), is optically coupled to the laser source (12) and spectrometer/analyser (14) by optical fibres (18) and (20) respectively. The spectrometer/analyser (14) is located remotely from the purged housing (16), the optical fibre (20) being greater than 100m long. The probe (10) is positioned in use to direct laser light (32) through a quartz window (22) in the purged housing and through a sapphire window (24) in a sample cell (26) onto a process stream (28) comprising a mixture to be analysed. A suitable cell was provided by Specac Ltd. In use, Raman beams produced by the mixture and the sapphire windows are collected by the probe (10) and passed through the optical fibres (20) to the remotely-located spectrometer/analyser (14). The quartz (22) and sapphire (24) windows are separated by a small air gap (30) of less than 0.5 mm. Since the air gap may be contaminated with flammable gases from the process plant, the small size of this gap acts as a flame arrestor 7 in the event that such flammable gases are ignited. The probe may have sapphire containing optical components, represented generally as (40), to generate a sapphire Raman spectral band for frequency drift correcting and/or intensity normalising the Raman spectrum of the process stream according to the present invention.

The sample cell has an inlet (34) for process stream to be analysed and has an outlet (36) for returning the analysed process stream to the process.

In use, the process stream to be analysed passes through the inlet (34) of the sample cell (26), through the sample cell and is returned to the process through outlet (36). Laser light from the laser (12) is passed through the optical fibre (18) to the probe (10), through the quartz window (22) of the housing (16) and the sapphire window (24) of the sample cell (26) to irradiate the sample of process stream (28). Raman signals from the sapphire windows and the process stream are collected by the probe (10) and passed through the optical fibre (20) to the spectrometer/analyser (14) which generates the Raman spectrum of the process stream mixture and the sapphire. A computer (38) &equency drift corrects and/or intensity normalises the Raman spectrum frequency of the mixture with respect to the sapphire Raman band.

The apparatus of Figure I may be used in a separation process for the distillation of a petroleum fraction such as naphtha. Figure 2 shows a Raman spectrum of a naphtha process stream obtained using a sample cell with a sapphire window. The spectral band of sapphire of the cell window, which may be used to normalise the naphtha Raman spectrum for intensity and/or frequency according to the method of the present invention is shown as (50) in Figure 2 at a shift of 643.8 cm-1. Figure 3 shows the same spectrum of Figure 2 frequency drift corrected with respect to the frequency of the Raman band of the sapphire window to 644 cm-1. It has also been found that the Raman spectrum of naphtha can be normalised for intensity with respect to a H-C-H bend vibration at 1400-1500 cm-1, shown as (5 1). The intensity Raman spectrum normalised with respect to the H-C-H bend vibration at about 1400-1500 cm- ' is shown in Figure 4.

Apparatus as in Figure I was used to measure the Raman spectra of a heads process stream from the distillation of naphtha. The spectra were frequency drift corrected with respect to the sapphire band from sapphire windows in the sample cell.

The spectra were also intensity normalised with respect to the H-C-H bend vibration at 8 1400-1500 em-'. From the normalised spectra, the concentrations of benzene and other components were determined by chemometrics. Figure 5 shows a graph of the determined benzene concentration over several days. This data could be used to control the distillation conditions, for example, to optimise the recovery of the desired process stream with low benzene content.

The apparatus of Figure 1 may be used to measure the Raman spectra of a plurality of process streams, for example the feed, head and base process streams of a distillation column. Figure 6 shows in schematic form, the apparatus of Figure 1 for measuring the spectra of three process streams. Common components are commonly numbered.

Three flow cells (26) are provided each with a sapphire window (not shown).

The probe (10) is mounted on a motorised platform (50) which is controlled by a controller (52) receiving signals from a modem (54) and optical fibres (56). The motorised platform (50) allows the probe to be positioned over each of the three sample cells in turn to measure the Raman spectra of the process streams flowing through each cell.

In operation, process streams flow through the sample cells (26). The motorised platform (50), actuated by the controller (52), moves the probe (10) into position over a first cell (26). The laser source (12) produces an incident laser beam (785 nin), which is transmitted by the optical fibres (16), and directed by the probe (10) into the flow cell (26) through a quartz window in the housing (16) and a sapphire window in the cell (26). The incident beam is scattered by the sample in flow cell, and also by the sapphire window (not shown). Resultant Raman beams from the process stream and the sapphire windows are collected by the probe (10) and passed through optical fibres (20) to the spectrometer/analyser (14) which is connected to a computer (38). The spectra are frequency drift corrected and/or intensity normalised according to the method of the present invention and then the concentrations of components in the process stream mixture calculated by chemometrics using software in the computer (38). The probe is then re-positioned over a second cell and the process repeated. The apparatus is housed in an air-purged cabinet (16), and has a safety shut-off control (60) and power supply (61).

9 Using this apparatus, it has been possible to determine the Raman spectra of the feed and heads process streams of a distillation column for the tailoring of naphtha. It is believed that the spectra of the base stream could also be determined if a laser having a suitably long wavelength to overcome laser induced fluorescence were to be used. The spectra were frequency drift corrected with respect to the Raman band of the sapphire at about 644 cm-1 and intensity normalised with respect to the H-C-H bending vibration at 1400-1500 cm-' of the naphtha hydrocarbon mixture. By measuring these spectra at time intervals of ever 2-4 minutes it was possible to determine the concentration of benzene and other components in the heads product stream. Using these data and analysis of the base stream by conventional gas chromatography, it will be possible to control the distillation column to obtain the optimum recovery of the heads process stream with acceptable benzene concentration.

Claims

Claims: 1. A method for determining the Raman spectrum of a mixture, said

method being characterised in that the Raman spectrum of the mixture is frequency drift corrected and/or intensity normallsed with respect to a Raman spectral band of sapphire measured under the same conditions and at the same time as the Raman spectrum of the mixture. 5
2. A method as claimed in claim I in which the mixture comprises hydrocarbons.
3. A method as claimed in claim 2 in which the Raman spectral band of sapphire at about 644cm-' is used for frequency drift correction.
4. A method as claimed in claim 2 or claim 3 in which the Raman spectrum is intensity normalised with respect to the H-C-H bending vibration at about 1400-1500 cm-1 of the 10 hydrocarbon mixture.
5. A method as claimed in any one of the preceding claims in which the mixture comprises C, to C7hydrocarbons, naphtha and/orC2 to C7olefins.
6. The use of the method as claimed in any one of the preceding claims in a chemical, separation or blending process.

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