Chemical Composition Monitoring in a Batch Distillation Process

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Chemical Composition Monitoring in a Batch Distillation Process Using Raman Spectroscopy Hollie C. Struthers, Florian M. Zehentbauer, Ese Ono-Sorhue, and Johannes Kiefer* School of Engineering, University of Aberdeen, Aberdeen AB24 3UE, Scotland, U.K. ABSTRACT: Raman spectroscopy was used to monitor the batch distillation of an ethanol/water mixture in a sieve-plate column. The chemical composition of the top product was determined as a function of process time. Four different experimental runs with systematically varied heating and reflux strategies were studied. The results show that the reflux ratio influences the process timing as well as the product quality, whereas the heating strategy impacts the timing only. In the second part of this work, we demonstrate that the Raman technique can also be applied to hydrocarbon systems that are relevant in the oil and gas industries. For this purpose, hexane isomers and their mixtures were studied. The results show that, even though they exhibit very similar structure, the individual compounds can be identified and, moreover, quantified in a mixture. In conclusion, Raman spectroscopy is a useful and versatile tool for monitoring practical distillation processes.

1. INTRODUCTION The ultimate aim of every process in the chemical and petrochemical industries is to produce a product that meets the required specifications. These specifications are basically defined by the required purity or, in a more general sense, the chemical composition. However, few technical processes facilitate the direct generation of a product that meets all specifications. Typically, byproducts and impurities are present and need to be removed, by means of separation technologies such as distillation. To certify the product quality in terms of its chemical composition, reliable chemical analysis is essential. Currently, this often means a time-consuming procedure in an analytical laboratory, for example, using gas chromatography (GC) or highperformance liquid chromatography (HPLC). Unfortunately, although very accurate, these methods are not feasible for in situ and real-time monitoring of the process. Such online monitoring would ultimately allow the process to be controlled and operated under optimized conditions and, hence, has the potential for achieving a higher product yield with less consumption of raw materials and energy. Therefore, the development of new approaches for process monitoring is desirable. In this context, spectroscopic methods are becoming increasingly popular.1 In particular, techniques based on the speciesspecific absorption of light in the ultraviolet (UV), visible (vis), near-infrared (NIR), and mid-infrared (IR) regions of the spectrum have been brought into practice over the past decade.2 4 Even in complex environments, qualitative process monitoring is possible when spectral features are identified as suitable indicators for the progress of the process in combination with sophisticated data analysis procedures, such as chemometric approaches. In recent years, the application of Raman spectroscopy has been promoted for process analytics. For example, it has been applied to chemical reactors,5,6 crystallization processes,7,8 coating processes,9,10 and spray processes.11,12 Similarly to IR absorption spectroscopy, the Raman method is a vibrational spectroscopy and provides more detailed structural information than absorption spectroscopy in the UV, vis, and NIR spectral regions; r 2011 American Chemical Society

consequently, in principle, it allows individual species to be identified and quantified. IR absorption spectroscopy delivers similar information, but according to quantum mechanical selection rules, it is limited to molecules with a permanent dipole moment. In addition, because water is a very strong absorber in the IR range, the application of IR absorption spectroscopy to aqueous systems is inherently difficult. In contrast, Raman spectroscopy is well-suited for investigating systems containing water. A detailed treatment of the fundamentals of Raman scattering can be found in refs 13 and 14, and practical considerations are discussed in refs 1, 15, and 16, for instance. In this article, we focus on the application of Raman spectroscopy to monitor a batch distillation process in which an ethanol/ water mixture is treated. The ethanol/water system was chosen because the purification of ethanol is important, for example, in distilleries and in the production of ethanol as a solvent or fuel. A series of experimental runs with different operating conditions in terms of the heating strategy and reflux ratio were carried out. In the second part of this article, we analyze hexane isomers and their mixture to demonstrate that Raman spectroscopy can also be a suitable tool for monitoring hydrocarbon systems, which are common in the petrochemical industry.

2. EXPERIMENT AND CALIBRATION 2.1. Chemicals and Sample Preparation. The batch distillation of binary ethanol/water mixtures represents the primary experiment of this work. For every run, 9 L of a mixture containing 80 vol % deionized water and 20 vol % denatured ethanol (Fisher Scientific, purity > 99.5%, water content < 0.01%) were prepared. This particular ratio was chosen for practical interest, as fermentation tolerates ethanol concentrations up to 20%.17 Received: July 13, 2011 Accepted: October 5, 2011 Revised: September 16, 2011 Published: October 05, 2011 12824

dx.doi.org/10.1021/ie2015125 | Ind. Eng. Chem. Res. 2011, 50, 12824–12830

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Figure 2. Experimental Raman spectra of pure ethanol and pure water. The spectral regions utilized for calibration are indicated by arrows.

Figure 1. Schematic diagram of the distillation facility. T1 T4 are thermocouples, and V1 and V2 are valves to control the reflux ratio.

The chemicals stated above were also used for the calibration measurements. Further experiments were carried out on hexane isomers. For this purpose, n-hexane, 2-methylpentane, and 3-methylpentane (all purchased from Fisher Scientific, purity > 99%) were studied as pure substances and also as a ternary mixture containing equal amounts of the three isomers. The mixture was prepared gravimetrically. 2.2. Distillation Column. The laboratory-scale distillation column is depicted schematically in Figure 1. The evaporator was equipped with an electrical heating unit (nominal 4 kW thermal power). The sieve-plate column was made of stainless steel and had a diameter of 50 mm and a height of 765 mm. The number of plates was eight. The vapor at the top was condensed and cooled to about 20 °C in a tube heat exchanger using tap water as the cooling fluid. After the condenser, two magnetic valves were installed: V1 controlled the reflux to the column, and V2 controlled the flow to the product tank. For monitoring purposes, the facility was equipped with 16 Pt(100) thermocouples, four of which are indicated in Figure 1. Thermocouple T1 measured the temperature inside the evaporator; T2 and T3, the temperatures on the first and fifth plates, respectively; and T4, the temperature on the top of the column. Four runs with different heating strategies and reflux ratios were studied. Details are given in the corresponding subsections of section 3. 2.3. Raman Spectroscopy Setup. Raman spectra were recorded using a self-made setup. A continuous-wave diode laser (532 nm, 10 mW) was used as the light source. The Raman signals were collected and collimated with an achromatic lens. Elastically scattered laser light was blocked by an OG550 color glass filter. The spectrally filtered signal was then focused onto an optical fiber with a second achromatic lens. The fiber guided the signal to an imaging spectrograph (entrance slit = 200 μm, grating = 1200 lines/mm, focal length = 163 mm), where it was dispersed, and eventually the signal was detected using an electron-multiplying charge-coupled-device (EM-CCD) camera (Andor). 2.4. Calibration. To calibrate the Raman setup, binary solutions of ethanol and water were prepared in 0.1 mol fraction increments. These ranged from pure water (mole fraction 0) to

Figure 3. Calibration curve for binary ethanol/water mixtures. The diamonds represent experimental data; the dashed line is the best-fit second-order polynomial function.

pure ethanol (mole fraction 1). Raman spectra were recorded for all samples. The spectra of the two pure chemicals are displayed in Figure 2. In the water spectrum, only the broad OH stretching band around 3500 cm 1 can be observed. In the ethanol spectrum, the OH signals are significantly weaker, and the band originating from the CH stretching modes is the predominant feature, appearing around 2900 cm 1. More details on the spectroscopic interpretation of the water and ethanol vibrational spectra are given in our previous work.18,19 For calibration, the ratio of the integrated CH and OH band intensities in the ranges 2800 3150 and 3150 3660 cm 1 (the regions are indicated in Figure 2), respectively, were plotted against the ethanol mole fraction. Figure 3 shows the resulting experimental data, together with the best-fit quadratic function. The nonlinear behavior results from (1) the fact that ethanol exhibits an OH group, so that the OH band is not exclusively from water, and (2) ethanol and water molecules strongly interact with each other through hydrogen bonds and such interactions influence signal intensities and line positions.19

3. DISTILLATION COLUMN MONITORING In this section, we describe the results obtained from the four experimental distillation runs. Before each run began, the evaporator was filled with 9 L of freshly prepared ethanol/water mixture. In the following discussion, a heater setting of 100% means a heat input of 3945 W; the scaling is linear. 3.1. Experimental Run 1. In this particular experiment, the effect of changing the heat input at 0% reflux was investigated. 12825

dx.doi.org/10.1021/ie2015125 |Ind. Eng. Chem. Res. 2011, 50, 12824–12830


Figure 4. Thermal process parameters for experimental run 1. (Top) Experimental temperature data from thermocouples T1 T4. (Bottom) Heater settings of the evaporator during the run.

Figure 5. Development of the top product composition in the vessel for experimental run 1.

The reflux setting meant that no condensate was recycled and all of the mixture flowed into the top product tank. The heater was initially set at 100% (3945 W) and then subsequently cooled. After a period of time, the heat input was then increased again. A detailed scheme is shown in the lower diagram of Figure 4. The upper diagram shows the thermocouple data, which track the temperature evolution in the course of the process at 30-s intervals over a period of 100 min. When the temperature in the evaporator (T1) reached 87 °C after 17.5 min, the heater was lowered to 20% (793 W). The boiling point of ethanol is 78 °C, so no distillate is likely to appear until the temperature at the top of the column (T4) has reached this minimum value. Once the distillate appeared after the condenser, the heat input was raised to 30% (1186 W). Samples for Raman analysis were taken every 5 min from the top product tank. Between samples, the current mixture was swirled thoroughly. The moment the evaporator temperature (T1) reached 92 °C, the heater was switched off. The chemical composition of the top product measured by Raman spectroscopy is plotted as a function of time in Figure 5. The first sample could be taken after about 51 min. Initially, the

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product had a very high ethanol content. Then, the ethanol mole fraction continuously decreased over time. This behavior can be expected, as there was no reflux of distillate to the column. As the temperature rose, more water in the column boiled and eventually entered the top product tank. Therefore, as the experiment proceeded, more water entered the mixture, and the mole fraction of ethanol decreased as a result. After about 86 min, the product composition remained constant at an ethanol mole fraction of 0.55. 3.2. Experimental Run 2. As in experimental run 1, experimental run 2 focused on changing the heat input at 0% reflux to investigate the effects on product composition. The initial setup and conditions were identical to those of experimental run 1. The heat input was initially set at 100% (3945 W) and was lowered after a period of time. In contrast with the first experiment however, the heat input was then lowered further. The procedure used in this experiment is described in the following paragraphs, and a detailed scheme is shown in the lower diagram of Figure 6. The upper diagram shows the thermocouple data, which track the progress of the temperature evolution in the course of the process at 30-s intervals over a period of 100 min. Once the temperature in the evaporator (T1) had reached 87 °C, the heater was lowered to 50% (1973 W), and the time was noted. When the distillate appeared downstream of the condenser, the heater was lowered again to 30% (1186 W). Again, samples were taken every 5 min as in experimental run 1. At 92 °C, the heater was switched off. The chemical composition of the top product that was measured by Raman spectroscopy is plotted as a function of time in Figure 7. The first sample could be taken after about 42 min. Initially, the product had a very high ethanol content, as was the case in experimental run 1. Also, the qualitative behavior of the ethanol mole fraction with time was similar to that of the previous run. As the temperature rose, more water in the column boiled and eventually entered the top product tank. Therefore, as the experiment proceeded, more water entered the mixture, and the 12826




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mole fraction of ethanol decreased as a result. After about 77 min, the product composition remained constant at 0.54 ethanol mole fraction. 3.3. Experimental Run 3. Experimental run 3 investigated whether the product composition and the temperature profiles are impacted by changing the reflux ratio. The heating strategy employed in experimental run 2 was utilized, and the reflux ratio was set to 50%. The experimental procedure is described in the subsequent paragraphs and depicted in Figure 8. The heater was set to 100% initially. When the evaporator contents reached 87 °C, the heater was turned down to 50% (1973 W). The distillate appeared after the condenser, and the heater was consequently turned down further to 30% (1186 W). Samples were taken every 5 min. When a temperature of 92 °C was reached in the evaporator, the heater was turned off. The chemical composition of the top product measured by Raman spectroscopy is plotted as a function of time in Figure 9. The first sample could be taken after about 36 min. The product initially had an ethanol mole fraction of 0.53 with a tendency to rise. After 41 min, the quality of the top product remained relatively constant between 0.62 and 0.66 ethanol mole fraction. As half of the distillate was being recycled back into the system at this reflux setting, more ethanol was present in the column. This signified that there was more volatile mixture in the column, and

as such, the temperature continued to increase. Consequently, more ethanol was boiled off and entered the top product tank, causing an increase in the mole fraction of ethanol. 3.4. Experimental Run 4. Finally, experimental run 4 examined the effects of changing the reflux ratio using the heating strategy in experimental run 1. The experimental procedure was as follows: Again, the initial heater setting was 100%. A detailed scheme of the heater settings is shown in the lower diagram of Figure 10. The upper diagram shows the thermocouple data, which track the progress of the temperature evolution in the course of the process at 30-s intervals over a period of 100 min. Once the temperature in the evaporator (T1) reached 87 °C, the heater was lowered to 20% (794 W), and the time was observed. When the distillate appeared after the condenser, the heat input was increased to 30% (1186 W). Samples for Raman analysis were taken every 5 min as before. At 92 °C, the heater was switched off. The composition profile of ethanol is illustrated in Figure 11. The first sample could be taken after about 52 min. The product initially had an ethanol mole fraction of 0.52. After 57 min, the quality of the top product remained relatively constant between 0.60 and 0.64 ethanol mole fraction. As half of the distillate was being recycled back into the system at this reflux setting, more ethanol was present in the column. This signified that there was 12827




more volatile mixture in the column, and as such, the temperature continued to increase. Consequently, more ethanol was boiled off and entered the top product tank, causing an increase in the mole fraction of ethanol. 3.5. Comparison of Experimental Runs. At first glance, it would seem that the temperature profiles in the four experiments are very similar. In reality, however, there are large discrepancies regarding the times taken to achieve rectification and the limit temperature of 92 °C. At the beginning, the rise in temperature across the column plates can easily be observed in Figures 4, 6, 8, and 10. When the condensate first appears in the column, new equilibria are formed between the vapor and the liquid. At 87 °C, the heat input was lowered in all experiments, and this is represented by small fluctuations in the temperature profile. Subsequent equilibria were established, and once this was done, the distillate appeared in the condenser, and rectification began. The time taken to achieve this state varied across the four experiments. In the first two procedures, the intervals were approximately 33 and 21 min, respectively. The heating strategies employed in both of these experiments involved reducing the heat input at 87 °C while the column was operating without reflux. In the first experiment, the heat input was lowered to 20% (793 W), whereas in the second, it was lowered only to 50% (1973 W). Consequently, because of a greater heat input at 87 °C, the temperature rise in experimental run 2 was faster than that in experimental run 1, and therefore, the periods in which rectification and 92 °C were reached were significantly reduced. By adjusting the reflux ratio to 50% in the third and fourth experiments, more ethanol was recycled back into the system. Although a cold liquid, ethanol is the driving mechanism, so there was more volatile mixture in the column, which caused the temperature to rise faster. This can be verified by studying the time taken to reach rectification in the latter two procedures. In the same way as for the first two experiments, the heat input was reduced at 87 °C. Experimental run 3 utilized the same heating strategy as used in the second procedure, so the heat input was reduced by only one-half in this instance. Experimental run 4 employed the heating strategy used in experiment one so the heat input was considerably lower at 20% (793 W). A greater heat input and reflux meant that experiment 3 reached rectification in only 24 min and 92 °C in 41 min. Comparatively, experimental run 4 took 38 min for rectification and 47 min to achieve 92 °C. Once the heater had been switched off at this temperature, signaling the end of rectification, a significant temperature drop was displayed in all experiments, particularly from the top column plates.

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It is evident from the experimental results that, at both reflux settings, a higher heat input caused the rectification stage to be attained quickly, which meant that the overall process was completed sooner. This was further enhanced by increasing the reflux ratio. More volatile liquid was recycled through the system, and as such, the temperature rose. Nevertheless, a large heat input can have serious cost implications for a company, so often, a lower heat input is preferred. Clearly, increasing the reflux ratio is advantageous because it saves time, thereby reducing the economic impact. With regard to the composition, several points are considered. The steady-state mole fractions of ethanol obtained in the first two experiments were both 0.55. This suggests that changing the heat input has little or no effect on the composition attained. A heating strategy that involved no increase in heat input might be preferred for economic purposes. Faster production of the product might be required, however, in which case more heat would be needed. As with any operation, there are many constraints to consider, and a balance must be achieved between cost, quality, time, and scope. When the reflux was increased to 50%, there was a corresponding change in the mole fraction of ethanol. A clear transformation in trends in the concentration graphs (Figures 5, 7, 9, and 11) when the reflux was increased from 0% to 50% can be seen. At 0%, no distillate was being recycled, so more water was present in the column and was subsequently boiled off. Consequently, more water was present in the top product tank, and the ethanol mole fraction decreased with process time. Conversely, at 50% reflux, some of the distillate was being recycled back into the system, which implies more ethanol was in the column. This ethanol was boiled off and entered the top product tank, and as a result, the ethanol mole fraction increased. It is intuitive, therefore, increasing the reflux ratio significantly improved the quality of products.

4. HYDROCARBON MIXTURE ANALYSIS In the previous section, we focused on the Raman spectroscopic analysis of ethanol/water mixtures. Other fluids commonly processed in distillation columns are crude oils, which are basically mixtures of hydrocarbons. Regarding spectroscopy for process monitoring, however, it is a widely held belief, especially in industry, that vibrational spectroscopies such as Raman and IR spectroscopy are not capable of distinguishing between different compounds that exhibit similar chemical structures. To disprove this prejudice, we studied isomers of hexane as a proof-of-concept experiment. Figure 12 displays the experimental Raman spectra of the pure chemicals n-hexane, 2-methylpentane, and 3-methylpentane. The spectra are intensity-normalized. At first glance, they look very similar, as can be expected. However, particularly in the CH stretching region (shown enlarged in Figure 12 for wavenumbers between 2600 and 3100 cm 1), there are clear differences that allow the different isomers to be distinguished. This is in concert with previous work in which we showed that analysis of the CH stretching region facilitates discrimination even of longchain polyunsaturated fatty acids exhibiting marginal structural differences.20 For process monitoring in a petrochemical application, however, it is necessary to extract quantitative information from a spectrum recorded from a mixture of hydrocarbons. In the literature, this has been demonstrated for natural gas.21,22 12828



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Figure 13. Experimental spectrum from a ternary mixture of hexane isomers and the best-fit synthetic spectrum. The difference between the two is indicated underneath.

Figure 12. Raman spectra of pure n-hexane and its isomers 2-methylpentane and 3-methylpentane. The inset shows the enlarged CH stretching region.

The quantitative analysis of liquids using vibrational spectroscopy is more complicated. When the molecules exhibit functional groups such as hydroxyl groups, strong intermolecular interactions such as hydrogen bonds can form, and the vibrational spectrum can significantly change with mixture composition.19 As a consequence, extensive calibration and complex data evaluation would be necessary to achieve quantitative analysis of multiplecomponent systems. Nevertheless, in a binary system such as ethanol/water, this problem can be overcome by applying the simple and straightforward calibration strategy described in the previous sections. For liquids representing hydrocarbon mixtures, it is advantageous that hydrocarbons not exhibit highly polar groups that form hydrogen bonds, but instead interact with each other through van der Waals forces. Such interactions are comparatively weak and typically do not modify the vibrational spectrum significantly. As such, it should be possible to apply a method for extracting species concentrations that was originally developed for gas mixture analysis.22 24 This evaluation and calibration procedure is based on the assumption that the mixture spectrum is basically a sum of the concentration-weighted pure-component spectra. Consequently, the mixture composition can be determined by fitting a synthetic spectrum, in which the concentrations of the individual species are the fitting parameters, to the experimental spectrum recorded in the actual mixture. Details can be found in refs 22 and 23. In the present work, we applied this procedure to a liquid for the first time. The calibration constants, which were required for translating the weighting factors obtained from the fitting to concentration values, were derived from the purecomponent spectra. This is a novel and simpler approach compared to the gas application in which the calibration factors are determined from analyzing a mixture with known composition. However, for the sake of completeness, we must note that utilizing the spectra of the pure components for calibration purposes is possible only under the assumption of high stability and, thus, reproducibility of the Raman setup. This was the case in this work. Figure 13 shows the experimental spectrum of a mixture of 33.4% n-hexane, 33.3% 2-methylpentane, and 33.3% 3-methylpentane and the best-fit synthetic spectrum. The evaluation yields the following composition: 33.8% n-hexane, 32.6% 2-methylpentane,

and 33.6% 3-methylpentane. This is in good agreement with the gravimetric results and highlights the feasibility of Raman spectroscopy for analyzing liquid hydrocarbon mixtures. The difference between the experimental and synthetic spectra is also displayed in Figure 13. The difference spectrum reveals slight deviations from zero in the CH stretching region between 2800 and 3000 cm 1. These deviations are due to statistical signal fluctuations and are not systematic errors.

5. CONCLUSIONS AND OUTLOOK In the first part of this article, we applied Raman spectroscopy to monitor the product composition of a distillation column during batch rectification of an ethanol/water mixture. To the best of our knowledge, this is the first use of the Raman method for this purpose. Four different operating strategies with respect to heating and reflux conditions of the column were studied to characterize the facility. The experimental results revealed that a reflux ratio of 50% has benefits concerning the process time and product quality, whereas the heating strategy impacts the process time only. Raman spectroscopy was thus demonstrated in this study to be a very reliable monitoring technique. This method requires virtually no sample preparation, and the analysis time is short. This is attractive in industry, as it significantly reduces costs. Also, the process is noninvasive so that the sample can be directly analyzed in its packaging. Because water is a weak Raman scatterer, this technique can be applied to aqueous solutions, making it ideal for the ethanol/water mixture investigated. In the second part of the study, we carried out a proofof-concept experiment on hexane isomers and their mixture representing a simplified model system for a hydrocarbon mixture. Our results showed that Raman spectroscopy is a suitable technique to distinguish between the different isomers and, in addition, is capable of determining the chemical composition of the mixture. The experimental approach and the method employed for data evaluation can, in principle, be transferred to more complicated systems, such as to the different fractions of crude oil produced from a distillation column in the petrochemical industry. In summary, Raman spectroscopy was demonstrated to be a useful technology for monitoring distillation processes in both aqueous and hydrocarbon systems. However, for the sake of completeness, we note that, in practical systems, care must be taken because laser-induced fluorescence emission from impurities or aromatic components can cause interference with the Raman signal. Such fluorescence interference can either be avoided using laser sources in the red and near-infrared spectral ranges, 12829


Industrial & Engineering Chemistry Research taking a loss in signal intensity according to the lower scattering cross section,16 or be corrected for using a polarization-resolved detection scheme that takes advantage of the different polarization properties of fluorescence emission and Raman scattering.25,26 The latter option, however, might not be a suitable generic solution to the problem, as the Raman signals generated in liquids can be depolarized to a certain extent as well.27 Future work in our laboratory will focus on inline measurements combined with real-time data analysis. This will allow an efficient control procedure to be implemented and, hence, the column to be operated under optimized conditions. Another possibility for future activities is to record spectra directly inside the column. This will facilitate a more detailed analysis of the process using Raman spectroscopy for simultaneous determination of the local chemical composition and the temperature.28

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +44 1224 272495. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was funded by the School of Engineering and the College of Physical Sciences of the University of Aberdeen. The authors gratefully acknowledge technical assistance from Janet Walker. J.K. acknowledges support from the Erlangen Graduate School in Advanced Optical Technologies (SAOT) and the German Research Foundation (DFG). ’ REFERENCES (1) Bakeev, K. A. Process Analytical Technology, 2nd ed.; WileyBlackwell: New York, 2010. (2) Roggo, Y.; Chalus, P.; Maurer, L.; Lema-Martinez, C.; Edmond, A.; Jent, N. A review of near infrared spectroscopy and chemometrics in pharmaceutical technologies. J. Pharm. Biomed. Anal. 2007, 44, 683–700. (3) Ojeda, C. B.; Rojas, F. S. Process analytical chemistry: Applications of ultraviolet/visible spectrometry in environmental analysis: An overview. Appl. Spectrosc. Rev. 2009, 44, 245–265. (4) Minnich, C. B.; Buskens, P.; Steffens, H. C.; B€auerlein, P. S.; Butvina, L. N.; K€upper, L.; Leitner, W.; Liauw, M. A.; Greiner, L. Highly Flexible Fiber-Optic ATR-IR Probe for Inline Reaction Monitoring. Org. Process Res. Dev. 2007, 11, 94–97. (5) Knopke, L. R.; Nemati, N.; Kockritz, A.; Bruckner, A.; Bentrup, U. Reaction monitoring of heterogeneously catalyzed hydrogenation of imines by coupled ATR-FTIR, UV/Vis, and Raman spectroscopy. ChemCatChem 2010, 2, 273–280. (6) Assirelli, M.; Xu, W. Y.; Chew, W. Reactor kinetics studies via process Raman spectroscopy, multivariate chemometrics, and kinetics modeling. Org. Process Res. Dev. 2011, 15, 610–621. (7) Cornel, J.; Lindenberg, C.; Mazzotti, M. Quantitative application of in situ ATR-FTIR and Raman spectroscopy in crystallization processes. Ind. Eng. Chem. Res. 2008, 47, 4870–4882. (8) Hu, Y. R.; Liang, J. K.; Myerson, A. S.; Taylor, L. S. Crystallization monitoring by Raman spectroscopy: Simultaneous measurement of desupersaturation profile and polymorphic form in flufenamic acid systems. Ind. Eng. Chem. Res. 2005, 44, 1233–1240. (9) M€uller, J.; Knop, K.; Thies, J.; Uerpmann, C.; Kleinebudde, P. Feasibility of Raman spectroscopy as PAT tool in active coating. Drug Dev. Ind. Pharm. 2010, 36, 234–243. (10) Bogomolov, A.; Engler, M.; Melichar, M.; Wigmore, A. In-line analysis of a fluid bed pellet coating process using a combination of near infrared and Raman spectroscopy. J. Chemom. 2010, 24, 544–557. (11) Weikl, M. C.; Beyrau, F.; Kiefer, J.; Seeger, T.; Leipertz, A. Combined coherent anti-Stokes Raman spectroscopy and linear Raman

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spectroscopy for simultaneous temperature and multiple species measurements. Opt. Lett. 2006, 31, 1908–1910. (12) Braeuer, A.; Engel, S. R.; Dowy, S.; Luther, S.; Goldl€ucke, J.; Leipertz, A. Raman mixture composition and flow velocity imaging with high repetition rates. Opt. Express 2010, 18, 24579–24587. (13) Long, D. A. Raman Spectroscopy; McGraw-Hill International Book Company: London, 1977. (14) Schrader, B. Infrared and Raman Spectroscopy; VCH Verlagsgesellschaft: Weinheim, Germany, 1995. (15) Smith, E.; Dent, G. Modern Raman Spectroscopy: A Practical Approach; Wiley: New York, 2005. (16) Kiefer, J. Dual-wavelength Raman spectroscopy approach for studying fluid-phase equilibria using a single laser. Appl. Spectrosc. 2010, 64, 687–689. (17) Riegel, E. R.; Kent, J. A. Kent and Riegels’ Handbook of Industrial Chemistry and Biotechnology, 11th ed.; Springer Science and Business Media: New York, 2007. (18) Kiefer, J.; Frank, K.; Schuchmann, H. P. Attenuated total reflection infrared (ATR-IR) spectroscopy of a water-in-oil emulsion. Appl. Spectrosc. 2011, 65, 1024–1028. (19) Noack, K.; Kiefer, J.; Leipertz, A. Concentration dependent hydrogen bonding effects on the dimethyl sulfoxide vibrational structure in the presence of water, methanol and ethanol. ChemPhysChem 2010, 11, 630–637. (20) Kiefer, J.; Noack, K.; Bartelmess, J.; Walter, C.; D€ ornenburg, H.; Leipertz, A. Vibrational structure of the polyunsaturated fatty acids eicosapentaenoic acid and arachidonic acid studied by infrared spectroscopy. J. Mol. Struct. 2010, 965, 121–124. (21) Brunsgaard Hansen, S.; Berg, R. W.; Stenby, E. H. Highpressure measuring cell for Raman spectroscopic studies of natural gas. Appl. Spectrosc. 2001, 55 (1), 55–60. (22) Kiefer, J.; Seeger, T.; Steuer, S.; Schorsch, S.; Weikl, M. C.; Leipertz, A. Design and characterization of a Raman-scattering-based sensor system for temporally resolved gas analysis and its application in a gas turbine power plant. Meas. Sci. Technol. 2008, 19 (8), 085408. (23) Schorsch, S.; Kiefer, J.; Steuer, S.; Seeger, T.; Leipertz, A.; Gonschorek, S.; Abr€oll, B.; K€aß, M. Entwicklung eines EchtzeitanalyseSystems zur Charakterisierung von Brenngasgemischen in Gasturbinenkraftwerken. Chem. Ing. Tech. 2011, 83, 247–253. (24) Eichmann, S. C.; Weschta, M.; Kiefer, J.; Seeger, T.; Leipertz, A. Characterization of a fast gas analyzer based on Raman scattering for the analysis of synthesis gas. Rev. Sci. Instrum. 2010, 81, 125104. (25) Egermann, J.; Seeger, T.; Leipertz, A. Application of 266-nm and 355-nm Nd:YAG laser radiation for the investigation of fuel-rich sooting hydrocarbon flames by Raman scattering. Appl. Opt. 2004, 43, 5564–5574. (26) Saunders, J. E. A.; Davy, M. H. Broadband 308 nm vibrational Raman spectroscopy of gaseous species using a potassium phthalate liquid filter and polarization fluorescence suppression. Rev. Sci. Instrum. 2010, 81, 013108. (27) Kiefer, J.; Fries, J.; Leipertz, A. Experimental vibrational study of imidazolium-based ionic liquids: Raman and infrared spectra of 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-ethyl-3methylimidazolium ethylsulfate. Appl. Spectrosc. 2007, 61, 1306–1311. (28) Knauer, O. S.; Lang, M. C.; Braeuer, A.; Leipertz, A. Simultaneous determination of the composition and temperature gradients in the vicinity of boiling bubbles in liquid binary mixtures using one-dimensional Raman measurements. J. Raman Spectrosc. 2011, 42, 195–200.

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Chemical Composition Monitoring in a Batch Distillation Process

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