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									                        Effect of Particle Size and Stirring Rate on the
                        Signals Obtained from In-line Raman and Near
                          Infrared Measurements in Process Analysis
        C.A. McGill, J. Walsh, A. Nordon and D. Littlejohn, Department of Pure & Applied Chemistry/CPACT,
                        University of Strathclyde, 295 Cathedral Street, Glasgow, UK. G1 1XL.
Introduction                                                                        Results
                                                                                    •The 1st derivative NIR and Raman spectra of toluene and itaconic acid in toluene are shown in Figures 4 and 5.
•In process analytical chemistry, additional research is required                                                                                0.02                                                                                                                          2.50E-03
                                                                                                                                                                                                                                                                               2.00E-03                                                                                   Toluene
on the analysis of heterogeneous systems, e.g. where solid                                                                           0.015                               Toluene
                                                                                                                                                                                                                                                                               1.50E-03                                                                                   Itaconic acid in toluene




                                                                                              1st derivative absorbance
                                                                                                                                                 0.01                    Itaconic acid in toluene

particles are either produced or consumed during a reaction.                                                                                                                                                                                                                   1.00E-03




                                                                                                                                                                                                                                                                Intensity/AU
                                                                                                                                     0.005                                                                                                                                     5.00E-04
                                                                                                                                                    0                                                                                                                          0.00E+00

•Signals obtained by an in-line optical spectroscopic technique                                                             -0.005
                                                                                                                                  1400                           1450          1500        1550         1600
                                                                                                                                                                                                  Wavelength/nm
                                                                                                                                                                                                                  1650      1700      1750       1800                          -5.00E-04 300
                                                                                                                                                                                                                                                                               -1.00E-03
                                                                                                                                                                                                                                                                                                                                         320   340     360     380        400
                                                                                                                                                                                                                                                                                                                                                               Wavenumber/cm-1
                                                                                                                                                                                                                                                                                                                                                                                    420      440       460       480    500


                                                                                                                                           -0.01
may be influenced by particle size, agitation rate and probe                                                                -0.015
                                                                                                                                                                                                                                                                               -1.50E-03
                                                                                                                                                                                                                                                                               -2.00E-03

position.                                                                                                                                  -0.02                                                                                                                               -2.50E-03




•Experiments have been carried out to determine the effect of                               Figure 4. 1st derivative NIR spectra of toluene and itaconic acid                                                                                                Figure 5. 1st derivative Raman spectra of toluene and itaconic
                                                                                                                       in toluene.                                                                                                                                                    acid in toluene.
agitation rate, particle size and probe position on NIR and Raman
signals of itaconic acid particles in toluene in a batch reactor.                    •Itaconic acid signals appear at approx. 1600 nm in the NIR spectrum, and 390 cm-1 in the Raman spectrum.
•This system was studied as part of an investigation of the                          Effect of stirring rate
optimisation and on-line monitoring of the esterification of
itaconic acid, which is sparing soluble in toluene.                                  •Figures 6 and 7 show the same concentration of itaconic acid in toluene (100 g per 700 mL) measured at different stir
                                                                                     rates by NIR and Raman spectrometries.
                                                                                                                                                 0.0155
                                                                                                                                                                               125 rpm                                                                                                              1.50E-03
                                                                                                                                                 0.0135
                                                                                                                                                                               200 rpm
                                                                                                                     1st derivative absorbance




                                                                                                                                                                                                                                                                                                                                               200 rpm       275 rpm        350 rpm




                                                                                                                                                                                                                                                                        1st derivative normalised
                                                                                                                                                 0.0115                        275 rpm                                                                                                              1.00E-03
                                                                                                                                                                               350 rpm

Experimental                                                                                                                                     0.0095                                                                                                                                             5.00E-04




                                                                                                                                                                                                                                                                              intensity/AU
                                                                                                                                                 0.0075                                                                                                                                             0.00E+00
                                                                                                                                                                                                                                                                                                                                  370          380           390            400            410             420          430
                                                                                                                                                 0.0055                                                                                                                                         -5.00E-04
                                                                                                                                                                                                                                                                                                                                                                   Wavenumber/cm-1

•The near infrared spectrometer used was a Foss NIRSystems                                                                                       0.0035

                                                                                                                                                 0.0015
                                                                                                                                                                                                                                                                                                -1.00E-03

                                                                                                                                                                                                                                                                                                -1.50E-03
On-line 6000 (Figure 1), equipped with a transflectance probe                                                                                    -0.0005
                                                                                                                                                        1550     1560      1570       1580       1590  1600     1610      1620     1630   1640    1650

(Figure 2). The transflectance probe allowed transmission data                                                                                                                                      Wavelength/nm



to be collected from the liquid phase, and reflectance data to be                        Figure 6. NIR spectra of itaconic acid in toluene at different                                                                                                  Figure 7. 1st derivative Raman spectra of itaconic acid in toluene at
                                                                                                               agitation rates.                                                                                                                                                 different agitation rates.
collected from the particles.
                                                                                     •At 125 rpm, no particles were passing through the NIR probe region and only toluene was detected.
•The Raman spectrometer used was a Kaiser Optical Systems
HoloProbe 785 (laser excitation wavelength of 785 nm, Figure                         •Different spectra were obtained at the different stirring rates as different amounts of particles were passing through the
3). The probe used for making the measurements collected 180°                        probe regions.
backscatter from the liquid and particulate matter.
                                                                                     •These variations affected the accuracy and precision of calibration models for determination of itaconic acid.
•All measurements were carried out in a 1 litre reaction vessel
                                                                                     Effect of particle size
for concentrations of 30 – 100 g per 700 mL toluene.
                                                                                      •Figures 8 and 9 show the spectra obtained from different particle size ranges of 100 g itaconic acid per 700 mL
                                       Illumination and collection fibres             toluene, agitated at 350 rpm, measured by NIR and Raman spectrometries.
                                                  (37 of each)                                                                                                                                                                                                                                                                   0.006
                                                                                                                                            0.015
                                                                                                                                                                                                                                                                                                                                                               <211 um
                                                                                                                                                                                                                                                                                                     Normalised 1st derivative
                                                                                                                                                                                         <211 um                                                                                                                                 0.004
                                                                                                                                                                                                                                                                                                                                                               >500 um
                                                                                                1sr derivative absorbance




                                                                                                                                                                                         211 - 499 um
                                                                                                                                                 0.01                                                                                                                                                    intensity/AU            0.002
                                                                                                                                                                                         >500 um
                                                                                                                                                                                                                                                                                                                                    0
                                                                                                                                            0.005                                                                                                                                                                                        370         380           390          400                  410          420         430
                                                                      Reflection                                                                                                                                                                                                                                            -0.002                                       Wavenumber/cm-1
                                                                                                                                                                                                 Wavelength/nm
                                                                                                                                                                                                                                                                                                                            -0.004
                                                                                                                                                    0
                                                                     Transmission                                                                   1550       1560     1570    1580      1590      1600   1610    1620   1630     1640   1650                                                                              -0.006

                                                                    Mirror                                                         -0.005
                                                                                     Figure 8. 1st derivative NIR spectra of different particle size ranges of                                                                                            Figure 9. 1st derivative Raman spectra of different particle size
                                                                                                              itaconic acid in toluene.                                                                                                                                   ranges of itaconic acid in toluene.
                                                                                    •For particles <211 mm, the itaconic acid to toluene signal ratio (1608 cm-1:1630 cm-1) is greater, indicating that more
                                           Figure 2. Diagram of NIR                 particles are passing into NIR the probe region than for the larger particle size ranges
                                              transflectance probe.
                                                                                    •This observation is confirmed by the Raman spectra.
 Figure 1. Foss NIRSystems near                                                     NIR probe orientation
      infrared spectrometer.
                                                                                    •The height and orientation of the NIR probe were varied and measurements made of identical mixtures of itaconic
                                                                                    acid in toluene. It was found that both the direction that the probe faced (opening towards or away from the stirrer),
                                                                                    and the height of the probe in the reactor had an effect on the signals obtained. Both height and orientation
                                                                                    influenced the amount of itaconic acid particles sampled by the probes at a particular agitation rate.

                                                                                    Conclusions
                                                                                    •This work has shown that agitation rate, particle size and probe position all have a considerable effect the NIR and
                                                                                    Raman signals from solid itaconic acid particles.
                                                                                    •This introduces serious complications when constructing and transferring calibration models for the analysis of
                                                                                    heterogeneous systems if the amounts and sizes of particles sampled are different owing to variations in probe
                                                                                    position and agitation rate.
             Figure 3. Kaiser Holoprobe Raman spectrometer.                         Future Work
                                                                                    •Investigate the effect of different shaped stirrer blades. •Make measurements in a fast sampling loop.
                                                                                    •Make measurements with the probe entering the side of reactor instead of the top.

								
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