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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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