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Raman spectroscopy has proven to be a powerful tool for the highly efficient and automated determination of liquid-liquid equilibria (LLE) in parallel microfluidic flows. However, a stable parallel microfluidic flow regime cannot be established for numerous industrially relevant aqueous-organic LLE systems since they tend to form plug flows. These plug flows have the advantage that inner circulations in the plugs enhance the mass transfer, leading to a much faster equilibration. For moving plugs, the main challenge is that established techniques are not capable to collect sufficient Raman signal for quantification. Therefore, we developed a measurement setup for LLE in microfluidic plug flows. In our setup, a capillary is moved against the flow direction to hold one plug of either the aqueous or the organic phase in the laser focus during the Raman measurement. Full automation is established for the premixing of the components, the calculation of the plug lengths and speeds and the Raman measurements of both phases. The setup and automated measurement procedure are successfully validated by showing excellent agreement with data from the literature for the LLE of the ternary system acetone – toluene – water at t = 25 °C. The developed setup thus enables efficient access to LLE data for industrially relevant mixtures., Fluid Phase Equilibria, 567, ISSN:0378-3812

The spectroscopic quantification of mixture compositions usually requires pure compounds and mixtures of known composition for calibration. Since they are not always available, methods to fill such gaps have evolved, which are, however, not generally applicable. Therefore, calibration can be extremely challenging, especially when multiple instable species, e.g. intermediates, exist in a system. This study presents a new calibration approach that uses ab initio Molecular Dynamics (AIMD)-simulated spectra as to set up and calibrate models for the physics-based spectral analysis method Indirect Hard Modeling (IHM). To demonstrate our approach called AIMD-IHM, we analyze Raman spectra of ternary hydrogen-bonding mixtures of acetone, methanol, and ethanol. The derived AIMD-IHM pure-component models and calibration coefficients are in good agreement with conventionally generated experimental results. The method yields compositions with prediction errors of less than 5% without any experimental calibration input. Our approach can be extended, in principle, to IR and NMR spectroscopy and allows for the analysis of systems that were hitherto inaccessible to quantitative spectroscopic analysis.

The combination of microfluidics and Raman microspectroscopy has proven to reduce the time and amount of materials required to determine liquid–liquid equilibrium (LLE) data. Until now, the experiments have been conducted manually. However, many applications have shown that the highest efficiency and user independence can be reached by automation. Therefore, we developed an automated setup and workflow from calibration to data analysis for the determination of liquid–liquid equilibrium data using Raman microspectroscopy and a microfluidic platform. Pure components are premixed online using a micromixer, resulting in a closed system with the additional advantage of avoiding potential losses of volatile components. In the automated setup, one experiment generates several data points for calibration and LLE data measurements. The automated setup and workflow are successfully validated with respect to both the integrated calibration and the LLE measurements. For this purpose, we studied two ternary systems (c...

Diffusion data in multicomponent liquids are scarce, because these diffusion measurements are time-consuming and laborious. Most diffusion data are therefore available for binary mixtures. While th...

We demonstrated recently that NO concentration measurements are feasible even in the core of largely non-sooting diesel-like jets by combined laser-induced fluorescence (LIF) and spontaneous Raman scattering (SRS). However, the question arises whether previous findings hold also for other diesel-like jets. The current study focuses on fuel effects. In the previous NO measurements, n-heptane was used. It is replaced by pure di-n-butylether (DNBE) and a tailor-made blend of 50% DNBE and 50% n-octanol. These fuels are promising biofuel candidates and lead to an interesting variation of mixing during combustion (MDC). The determination of NO concentrations turns out to be generally feasible with the blend on the jet centerline in the quasi-steady phase of the injection event. The corresponding uncertainty is about ± 28 %. By contrast, some of the NO-LIF measurements in sooting DNBE jets are discarded, primarily due to increased light attenuation. For the remaining NO concentrations with DNBE the corresponding uncertainty is about ± 40 %. For the blend, results indicate that NO formation is very similar to the one in the n-heptane jets. Thus, the net effect of changed volatility and oxygenation is seemingly weak. By contrast, quasi-steady centerline NO concentrations are apparently significantly affected by MDC for pure DNBE. Relatively high NO concentrations are observed in this case, although products of highly fuel-rich fluid parcels are also present there. This study indicates the importance of MDC in such jets.

The rapid compression machine (RCM) is a great tool for investigating fuel properties under engine relevant conditions (high-pressures, low temperatures). The most common diagnostics is measuring the pressure over time and determining the ignition delay time (IDT). In this study, for the first time, the OH* luminescence of ethanol/air mixture is measured within an RCM experiment at 15 and 20 bar for Φ = 0.5. Combining the common pressure measurements with the simultaneously recorded high-speed images (up to 74.5 kHz framerate) gives a first insight into understanding the ignition modes and the corresponding pressure traces. At 74.5 kHz, in contrast to findings in literature, the ethanol ignition did not show to be purely homogeneous. Four different propagating fronts of OH* luminescence have been recorded. Besides a flame kernel and a detonation-like ignition front two further fronts prior to main ignition have been observed. The propagating speeds of the fronts have been determined and depend on the overall IDT.

Di-n-butyl ether (DNBE) and n-octanol have very low sooting tendencies in diesel-like combustion, as demonstrated in previous engine studies. This finding is not fully understood for pure DNBE, because it has a very high cetane rating (∼100). In order to investigate the underlying mechanisms, the structure of diesel-type jets is analyzed by a number of optical diagnostics, such as spontaneous Raman scattering (SRS), laser-induced fluorescence (LIF), OH* luminescence imaging, Mie scattering, and shadowgraphy. Pure DNBE and a tailor-made blend of 50% DNBE and 50% n-octanol as well as neat n-heptane are used as the fuel in separate experiments. The jets are probed in a simulated engine-like environment in a high-pressure combustion vessel. In particular, the inner flame structure is analyzed by SRS and LIF. This yields information on the local temperature and the concentrations of O2, CO, and polycyclic aromatic hydrocarbons (PAH). For the first time, O2 is quantitatively detected in the core of a diesel-like flame by resonance-enhanced SRS. Thereby, air entrainment into the inner flame core is assessed. Results show that air entrainment is particularly strong for pure DNBE, explaining its high soot oxidation rate and overall low sooting tendency. High entrainment is primarily attributed to the low heat-release rate of DNBE, which is likely an effect of its high ignitability. Thus, it can be concluded that the high cetane rating of pure DNBE does not only lead to relatively poor pre-combustion mixture preparation and consequently considerable soot formation but seemingly also to particularly strong soot oxidation. Moreover, the jet structure turns out to be very similar for the DNBE/n-octanol blend and neat n-heptane, indicating that the net effect of volatility and fuel oxygenation is weak.

It was demonstrated recently that quantitative temperature and CO measurements can be conducted by spontaneous Raman scattering (SRS) in non-sooting diesel-like biofuel jets in a vessel. In contrast, n-heptane is used as the fuel in the present study. This leads to slightly increased sooting tendency. One-dimensional temperature and CO measurements are generally performed successfully. Fuel-injection pressure is varied and the flame structure is characterized, including the jet core. Effects of pre-combustion mixture inhomogeneity are particularly observed at lower injection pressure (700 bar). Strong deviations from adiabatic-equilibrium conditions in terms of temperature and CO mole fraction are observed in particularly fuel-rich fluid parcels. CO formation is apparently affected by turbulent mixing there. Relatively low temperatures are measured around the flame lift-off region, in particular for the highest injection pressure (1500bar), indicating turbulence-chemistry interaction.

Vapor-liquid equilibrium data of fuel mixtures are of major importance for both fuel production as well as combustion. However, the measurement of vapor-liquid equilibrium data usually requires significant experimental effort. The experimental effort is particularly high if multicomponent mixtures are of interest, as experimental effort rises strongly with a rising number of components. In this work, we efficiently characterize the vapor-liquid equilibrium of a quaternary model biofuel and its binary subsystems. For this purpose, we employ the recently developed milliliter-scale Raman Spectroscopic Phase Equilibrium Characterization (RAMSPEQU)-setup. Vapor pressures are collected at T = 283–333 K, and isothermal pTx-data for mixtures at T = 303.2 K resulting in a pressure range of p = 2.8–81.9 kPa. The PCP-SAFT equation of state is used for thermodynamic modeling. Our binary data agrees well with experimental data from literature. The quaternary phase behavior is predicted with very good accuracy using PCP-SAFT with parameters adjusted to pure substance and binary mixture data only. The milliliter-scale setup allows us to characterize the phase equilibria with just 22 ml (binary) and less than 105 ml (quaternary) of the respective mixtures. The agreement of predicted and experimental quaternary phase equilibrium data indicate the reliability of the employed method for multicomponent vapor-liquid equilibrium measurements.

Maintaining low NOx emissions over the operating range of diesel engines continues to be a major issue. However, quantitative measurements of nitric oxide (NO) are lacking especially in the core of diesel jets even if optical measurement techniques like laser-induced fluorescence (LIF) are used. NO may be present in the jet core due to enhanced fuel/air premixing. Close to the flame axis, severe light attenuation occurs. However, the diesel-like jets investigated in this study are largely non-sooting, so that light attenuation is reduced. n-heptane is used as diesel-surrogate fuel in the current investigations, which are performed in a combustion vessel. Quantitative NO measurements are conducted based on the knowledge of attenuation, temperature, the estimated equivalence ratio, and a calibration procedure. Attenuation turns out to be the most important factor in terms of measurement uncertainty. To avoid interfering LIF emissions by PAHs (polycyclic aromatic hydrocarbons) and O2, line imaging with a relatively high spectral resolution is conducted. Thereby, a higher accuracy can be achieved than by state-of-the-art planar imaging set-ups. Results show that NO initially forms throughout the jet cross-section. Quasi-steady NO concentrations measured in the jet core are compared to detailed kinetic simulations, taking the residence time of the fluid parcels in the main reaction zone into account. The residence-time “corrected” NO concentrations seemingly show a significant amount of prompt NO and are essentially consistent with simulation results for a corresponding equivalence ratio that was previously determined by Raman scattering.

Spectral analysis of mixtures often faces challenges due to nonlinear effects such as peak shifts or strong background signals. Nonlinear mixture effects can be effectively treated by the Indirect Hard Modeling (IHM) Method. In IHM, mixture effects are captured by adapting hard models of pure component spectra when fitting a mixture model. However, IHM requires a suitable background treatment, which can become laborious. Background signals do not arise from the components of interest but often superimpose their spectra. In statistical methods for spectral analysis, background treatment is often conducted by derivatives of a spectrum. Derivatives effectively damp broad background signals. Standard IHM is not applicable to derivatives of spectra as the negative parts of a derivative spectrum cannot be modeled by pseudo-Voigt peaks which are always positive. In this work, we propose First-Derivative Indirect Hard Modeling (FD-IHM). FD-IHM uses the analytical derivatives of the peak functions. The analytical derivatives are fitted to numerical derivatives of the spectra. Thereby, we combine background treatment by first derivatives with the IHM method to treat nonlinear effects. The presented FD-IHM is validated using Raman spectra of ethanol/acetone mixtures. To introduce a variety of background signals, we used fluorescence dye, scattering bodies (yeast) and various background light sources. Classical IHM allows us to predict the test sets with a root-mean-square error of prediction (RMSEP) ranging from 0.60 wt% to 2.06 wt%, but careful manual background treatment had to be applied. With FD-IHM, we reduce the RMSEP error by 21%–73% without any background treatment. Thus, FD-IHM allows for both, efficient and accurate analysis of spectra with large background signals.

Chemometrics and intelligent laboratory systems 217, 104419 (2021). doi:10.1016/j.chemolab.2021.104419, Published by Elsevier Science, Amsterdam [u.a.]

Experimental liquid–liquid equilibrium (LLE) data are indispensable for many applications ranging from extraction column design to water partitioning of organics in the environment. However, conventional LLE experiments are time-consuming and need large sample volumes. Therefore, a measurement setup is presented for the time and material efficient determination of LLE data. The measurement setup combines the advantages of microfluidics and Raman microspectroscopy: The small dimensions of the used H-cell microchannel lead to rapid equilibration and small sample consumption; Raman microspectroscopy allows for rapid in situ quantification of all components. The measurement setup has successfully been validated by measuring the LLE of the ternary system cyclohexane–methanol–toluene. Excellent agreement with the literature data has been achieved. Thus, the developed setup allows for the efficient determination of liquid–liquid equilibria in multicomponent mixtures.

Experimental vapor-liquid equilibrium data are of major importance for the chemical industry. However, the measurement of vapor-liquid equilibrium data still requires significant experimental effort. Therefore, we present a novel setup and measurement procedure for the rapid determination of isothermal vapor-liquid equilibria using only milliliter samples. The compositions of both liquid and vapor phases are analyzed using Raman spectroscopy. The measurement setup is successfully validated by reproducing vapor pressures of four pure substances and binary vapor-liquid equilibrium data of methyl tert -butyl ether (MTBE) and 2,2,4-trimethylpentane ( iso -octane) at T = 318.1 K as test system from literature. The vapor pressures of MTBE, ethanol, iso -octane and toluene agree with literature data within the measurement uncertainties. The measured binary vapor-liquid equilibrium data are modeled using the PCP-SAFT equation of state. Phase equilibrium data calculated from PCP-SAFT are compared to experimental data from literature. The data deviate by less than 1% in terms of pressure and vapor phase composition for given temperature and liquid phase composition demonstrating the reliability of the presented setup for vapor-liquid equilibrium measurements.

Quantitative spatially resolved measurements of temperature and species are lacking particularly in the core of combusting diesel jets. Major problems are light attenuation and interfering light emissions. However, these factors are reduced in non-sooting diesel-like jets, as demonstrated in the present work, because light is not attenuated by soot and interfering LIF (laser-induced fluorescence) from PAHs (polycyclic aromatic hydrocarbons) is substantially lower. The current results show that thermometry by SRS (spontaneous Raman scattering) excited by a UV (ultraviolet) laser is therefore feasible even in the core of a non-sooting diesel-like jet in a combustion vessel. Two diagnostic approaches are assessed. The first one is based on the spectral band shape of the Stokes (red-shifted) ro-vibrational SRS from N 2 , whereas the ratio of integrated ro-vibrational Stokes to anti-Stokes (blue-shifted) N 2 -SRS bands is exploited in the second one. It turns out that the first method is advantageous in terms of light attenuation by molecular species, the influence of interfering emissions, and resulting single-shot capability. However, these investigations also show that the anti-Stokes N 2 -SRS signal can be used for quantification of light attenuation. This is particularly attractive because this SRS band at ∼235 nm nearly coincides with a LIF emission from NO at ∼237 nm, leading to improved attenuation correction of NO-LIF. Furthermore, the recorded spectra indicate that additional quantitative species measurements by SRS are feasible in the non-sooting jet. For instance, the mole fraction of CO is quantified in this work for the first time in the jet core.

We present a method to calibrate Raman measurements of reactive systems without relying on thermodynamic models. Raman spectroscopy is able to detect multiple species even in reactive mixtures. Classical calibration methods require the knowledge of concentrations of all species as input. Typically, concentrations of intermediates cannot be prepared and fixed independently and are therefore calculated using thermodynamic models. The quality of the results thus depends on the accuracy of the thermodynamic model. In this paper, we present a method based only on stoichiometric balances and electroneutrality. By avoiding the use of thermodynamic models in the calibration step, the risk of overfitting spectroscopic data to a thermodynamic model is avoided. The presented method is demonstrated for the reactive system monoethanolamine, water, and CO2 and is validated by a thermodynamic model taken from the literature. While the model-free calibration is demonstrated for Raman spectroscopy in this work, the approach is generic should thus be applicable to most spectroscopic techniques.

Diffusion is slow. Thus, diffusion experiments are intrinsically time-consuming and laborious. Additionally, the experimental effort is multiplied for multicomponent systems as the determination of multicomponent diffusion coefficients typically requires several experiments. To reduce the experimental effort, we present the first microfluidic diffusion measurement method for multicomponent liquid systems. The measurement setup combines a microfluidic chip with Raman microspectroscopy. Excellent agreement between experimental results and literature data is achieved for the binary system cyclohexane + toluene and the ternary system 1-propanol + 1-chlorobutane + heptane. The Fick diffusion coefficients are obtained from fitting a multicomponent convection-diffusion model to the mole fractions measured in experiments. Ternary diffusion coefficients can be obtained from a single experiment; high accuracy is already obtained from two experiments. Advantages of the presented measurement method are thus short measurement times, reduced sample consumption, and less experiments for the determination of a multicomponent diffusion coefficient.

Diffusion in liquids can still be predicted only with high uncertainty due to a lack of sufficient experimental data. Diffusion experiments are complex and time-consuming. Furthermore, the determination of the concentration dependence of the diffusion coefficients requires usually several experiments even for binary mixtures. A powerful model identification framework based on two fast experimental techniques is presented here. Raman inter-diffusion experiments in combination with a novel incremental identification technique establish the concentration dependence directly from the data without requiring a priori specification of the model structure. In regions where this technique is sensitive to error noise, it is complemented with NMR intra-diffusion measurements. Models describing the concentration dependence are identified in two steps. Based on the combined data suitable model candidates are proposed and initialized through basic curve fitting in the first identification step. A statistically sound dynamic optimization step yields the final model parameters. The methodology is exemplarily used to determine the diffusion coefficient in the mixture ethyl acetate–cyclohexane in the full concentration space.

Even though multicomponent mixtures are ubiquitous in mass transfer processes, mutual diffusion data in these systems are scarce. This is due to the fact that established diffusion measurements are laborious. In particular, several experiments are required to determine multicomponent diffusion coefficients at a single composition. In order to overcome these deficiencies, a diffusion experiment employing one-dimensional Raman spectroscopy is presented. Concentrations of the individual species are resolved with high spatial and temporal resolution during the diffusion process. In this work, it is shown that these data allow determining the ternary Fick diffusion matrix from a single isothermal diffusion experiment. The experimental procedure is further optimized using model-based experimental design techniques, where general design guidelines for diffusion experiments are derived. Thereby, precision of the diffusion measurements can be improved by an order of magnitude. The theoretical calculations are validated by experimental results for the system n-propanol + 1-chlorobutane + n-heptane. © 2006 American Institute of Chemical Engineers AIChE J, 2006

A new method based on Raman spectroscopy is presented for noninvasive, quantitative determination of the spatial polymer distribution in alginate beads of approximately 4 mm diameter. With the experimental setup, a two-dimensional image is created along a thin measuring line through the bead comprising one spatial and one spectral dimension. For quantitative analysis of the Raman spectra, the method of indirect hard modeling was applied to make use of the information contained in the entire recorded spectra. For quantification of the alginate signals from within the beads, a calibration curve acquired from sodium alginate solutions was used after it was shown that only negligible differences occur between signals from alginate solutions and alginate gels. The distribution of alginate over the bead gel matrix was acquired with high spatial (51 μm) and time (12 s) resolution. The inhomogeneous distribution obtained using the new measuring technique is qualitatively in excellent agreement with data from the literature. In contrast to known measuring techniques, correct quantitative information about the spatial polymer distribution within the matrix was derived. It gave an alginate mass fraction of approximately 0.045 g/g at the edges and 0.02 g/g in the center of the beads. Next to the determination of mere polymer concentrations, the excellent time resolution of the presented method will enable investigation of the dynamic process of gel formation and it will also serve as a basis for investigation of mass transfer of small diffusing molecules in alginate matrices.

We present an indirect hard modeling (IHM) approach for the quantitative analysis of reactive multicomponent mixtures with intermolecular interaction. It can be used when it is not possible to obtain calibration data in the composition region of interest. The goal of this work, specifically, is to analyze reactive systems, although the validation of the method is done with nonreactive systems. Compared to conventional hard modeling, the new approach reduces the manual work required for modeling and renders unnecessary the assignment of bands in mixture spectra to individual components. It is based on parametric models of the pure component spectra that are made just flexible enough to fit the spectra of the unknown mixtures, and it only requires small calibration data sets that may lie in different regions of the composition space. The application to infrared (IR) and Raman spectra of multicomponent systems is discussed.

The measurement of diffusion coefficients in multicomponent liquid mixtures is still an open question. A new technique aimed at overcoming this deficiency is presented. It is based on Raman spectroscopy, which has been extended to obtain liquid concentration data of all components in a mixture simultaneously with high temporal and spatial resolution. The following data analysis uses model-based methods. This provides a very flexible framework, allowing substantial reduction of experimental effort and time and the direct estimation of the diffusion coefficient as a function of concentration. Furthermore, the experimental procedure is improved using optimal experimental design techniques. This new methodology is validated for binary mixtures.

The efficiency of SI engines at part load can be improved by lean burn and EGR. However, the maximum dilution rate of the air/ fuel mixture and therefore the potential of fuel consumption reduction is limited by increasing cyclic combustion variations. This article reports on investigations of the suitability of various ignition and intake systems for increasing the maximum charge dilution.

Optical measurement techniques are more and more widely used to investigate the mixing processes in small and medium scale mixers. Most of these techniques (including LIF based techniques) suffer from principle problems when investigating and quantifying reacting processes. Within this work a Raman scattering test-bench has been set-up to investigate dynamic processes in more than one dimension. While the Raman scattering approach allows investigating concentrations of several components simultaneously in reacting and non-reacting fluids with high accuracy, it is normally restricted to one-dimensional measurements, which is not sufficient for characterization of mixing processes. So the technique was improved to a pseudo two-dimensional multipoint technique. This article deals in detail with the advantages, disadvantages and limitations of different approaches. Furthermore the technical details of the developed Raman scattering set-up are described.

Coherent Anti-Stokes Raman Spectroscopy (CARS) is frequently the method of choice for non-intrusive temperature measurements in combustion systems. The temperature determination requires a comparison of measured spectra with theoretically calculated ones. Conventional gradient-based least squares fitting of experimental data with a library of theoretical spectra usually leads to sensible results, as long as the spectrum shapes behave well and the noise level is low.

Diffusion in liquids can still be predicted only with high uncertainty due to the lack of sufficient experimental data. Diffusion experiments are complex and time-consuming. Furthermore, the concentration dependence of the diffusion coefficients requires usually several experiments even for binary mixtures. The possibility to extract this information from one short Raman diffusion experiment is explored here. A general identification framework is provided which does not require the a priori specification of a diffusion coefficient model structure but establishes the concentration dependence directly from the data. The methodology is used to determine the diffusion coefficient in the mixture ethyl acetate–cyclohexane in a wide concentration range.