Resolved Concentration Profiles Research Articles

Physics informed neural networks reveal valid models for reactive diffusion of volatiles through paper

Predictive models for the transport of volatile organic compounds in paper need to consider the complex interplay of diffusion, adsorption, desorption, or chemical reactions. The relative importance of each of these processes is determined by the polarity of the volatile. Hence, it is challenging to pick a valid theoretical model that correctly predicts transport regardless of the polarity. Here, physics-informed neural networks (PINNs) assess which of five different models correctly describe transport of DMSO as polar and n-tetradecane as apolar model compound: (i) a pseudo first-order adsorption model for an irreversible sorption process, (ii) a first-order kinetics model allowing reversible sorption, (iii) a second-order model with a reversible process, and an effective diffusion model accounting for a constant (iv) and for a variable effective diffusivity (v). Each tested model is given as set of partial differential equations (PDE). Considering the model under testing and experimentally obtained spatially and temporally resolved concentration profiles through stacks of paper sheets, PINNs predict concentration of the volatiles and associated material constants such as sorption constants and effective diffusion coefficients by solving the inverse problem. Our PINNs revealed two models, pseudo first-order sorption and second-order reversible sorption, that correctly predict concentration profiles and polarity-driven differences in sorption times. While a PINN-based picking of valid transport models has important implications for the development of effective methods for controlling emission of volatiles from paper materials, PINNs represent a versatile mathematical tool to validate or refute the capability of PDE-based theoretical models to describe experimental data.

Coupling of Liquid and Surface Chemistry in Urea SCR Systems

Close-coupled selective catalytic reduction (SCR) systems are one method to deal with tightening emission legislation for NOx in internal combustion engines. Due to smaller mixing sections and at unfavourable boundary conditions, however, urea-water solution (UWS) droplets can impact on the SCR catalyst itself. To investigate this phenomenon further, this work develops a modeling capability of this process. Established mechanism for NH3-SCR and HNCO hydrolysis from literature is integrated into DETCHEMCHANNEL and a 2D COMSOL model to simulate the influence in the SCR Channel. Simulations are validated against end-of-pipe experiments from literature and spatially resolved concentration profiles from a hot gas test rig with very good agreement. Finally, a channel simulation is coupled with a model to describe the catalytic decomposition of an urea droplet. The coupled simulation is able to simulate the influence of UWS droplet impact onto a catalyst channel. Fast droplet decomposition causes a peak in NH3 and HNCO in the single channel and thus increases NOx conversion. However, the overall uniformity and efficiency are decreased, which is why droplet impact on the catalyst should be strictly avoided.

Determining effective diffusion coefficients of chlorohydrocarbons in natural clays: Unique results from highly resolved controlled release field experiments

This study aims to precisely determine the effective diffusion coefficients of chlorohydrocarbons in low permeable units under in-situ field conditions. To this end, two controlled release field experiments using TCE and PCE as dense non-aqueous phase liquids (DNAPLs) were conducted in two natural clayey deposits. Several months to years after the controlled DNAPL release, highly resolved concentration profiles were determined for the chlorohydrocarbons that had diffused into the clayey deposits. Effective diffusion coefficients for TCE and PCE were then determined by calibrating a 3D numerical and 1D analytical model, respectively, to the measured high-resolution concentration profiles. The simulations revealed that the effective diffusion coefficients vary by as much as a factor of four within the same low permeability unit being consistent with observed small-scale heterogeneities. The determined chlorohydrocarbon effective diffusion coefficients were further used to determine the equivalent thickness of DNAPL that would completely dissolve in an idealized, parallel-plate fracture by diffusion transport into clayey deposits for the time periods of the controlled release field experiments. The equivalent TCE and PCE DNAPL film thicknesses ranged between 36 and 581 μm, respectively, comparable and exceeding fracture apertures measured in naturally fractured clay rich deposits. Hence, films of DNAPL initially contained within fractures in clay-rich deposits can completely dissolve away within a few months to a few years due to diffusion. This stored contaminant mass poses a risk to adjacent aquifers if it is re-released due to diffusion out of the matrix after source depletion or remediation.

Coupling of multivariate curve resolution-alternating least squares and mechanistic hard models to investigate antibody purification from human plasma using ion exchange chromatography

A two steps proposal for the purification of immunoglobulin G from human blood plasma is investigated. The first step is precipitation using cold ethanol based on the Cohn method with some modification and the second step is a chromatographic separation by DEAE-Sepharose FF resin as a weak anion exchanger. The presence of interferent in the region3 of chromatographic fractions, which is co-eluted with IgG, restricts the application of the mechanistic chromatography model. Therefore, multivariate cure resolution-alternating least squares (MCR-ALS) as a soft method is employed on measured absorbance data matrix from eluted fractions to recover pure concentration and spectral profiles. Besides, possible solutions for resolved concentration and spectral profiles are investigated. The reaction-dispersive model as a mechanistic hard model for the column is utilized for the evaluation of the ion exchange chromatography. Using a genetic algorithm as a global optimization method, mobile phase modulator (MPM) adsorption model parameters such as β, kdes,0, and Keq,0, were fitted to the concentration profiles from MCR-ALS as 1.96, 2.87×10−4 m3 mol−1s−1, and 1883, respectively. Furthermore, a new resampling incorporated non-parametric statistics is conducted to assess parameters’ uncertainty. Values of 2.00, 1.10×10−3 m3 mol−1s−1, and 549.80 are estimated median, and values of 0.05, 2.5×10−3, and 691.00 are calculated interquartile range (IQR) for β, kdes,0, and Keq,0, respectively. Finally, results show three and two outliers for β and kdes,0, respectively.

Mechanistic insights into the inhibition of pancreatic lipase by apigenin: Inhibitory interaction, conformational change and molecular docking studies

The inhibition of pancreatic lipase by apigenin, a natural dietary flavonoid, was investigated by various spectroscopic techniques and computational simulation. The results showed that apigenin reversibly inhibited pancreatic lipase activity in a competitive manner with an IC50 value of 0.45 ± 0.03 mM, and its combination with orlistat showed a synergistic effect. Formation of a complex between apigenin and pancreatic lipase driven by hydrogen bonds resulted in the fluorescence quenching of the enzyme through a static manner. The binding constant of apigenin with the enzyme was (5.47 ± 0.01) × 104 L mol−1 at 298 K, and the kinetic process of their interaction was characterized by the resolved concentration profiles and pure spectra of different components from the multivariate curve resolution-alternating least squares algorithm. The inhibition of pancreatic lipase by apigenin was mainly attributed to the tighter structure of the enzyme and the interaction between apigenin and the amino acid residues in the active center of the enzyme to prevent the substrate from being oxidized, as demonstrated by the analysis of circular dichroism spectra and molecular docking. Molecular dynamics simulation further characterized the binding conformation and property of apigenin-pancreatic lipase complex. This study provides novel insights into the mechanism of apigenin as pancreatic lipase inhibitor and the development of apigenin as an anti-obesity nutrient.

Migration of mineral oil in elastomers

This work describes a newly introduced experimental procedure to quantify the diffusion progress of mineral oils locally resolved in NBR. Diffusion of reference oils IRM 901, IRM 902 and IRM 903 in NBR with various acrylonitrile contents was investigated. Classical sorption experiments were performed as a basic characterization and compared to the newly introduced method. Here, elastomer specimens are only being dipped with the bottom in a relatively small reservoir of mineral oil. This provides a determination of locally resolved concentration profiles of mineral oils, and the calculation of diffusion coefficients. These diffusion coefficients follow the same trends like those determined via sorption experiments. Despite differences in the absolute numbers, activation energies of diffusion can be applied as a suitable measure for the compatibility of elastomers and fluids.

Microkinetic Modeling of the Oxidation of Methane Over PdO Catalysts—Towards a Better Understanding of the Water Inhibition Effect

Water, which is an intrinsic part of the exhaust gas of combustion engines, strongly inhibits the methane oxidation reaction over palladium oxide-based catalysts under lean conditions and leads to severe catalyst deactivation. In this combined experimental and modeling work, we approach this challenge with kinetic measurements in flow reactors and a microkinetic model, respectively. We propose a mechanism that takes the instantaneous impact of water on the noble metal particles into account. The dual site microkinetic model is based on the mean-field approximation and consists of 39 reversible surface reactions among 23 surface species, 15 related to Pd-sites, and eight associated with the oxide. A variable number of available catalytically active sites is used to describe light-off activity tests as well as spatially resolved concentration profiles. The total oxidation of methane is studied at atmospheric pressure, with space velocities of 160,000 h−1 in the temperature range of 500–800 K for mixtures of methane in the presence of excess oxygen and up to 15% water, which are typical conditions occurring in the exhaust of lean-operated natural gas engines. The new approach presented is also of interest for modeling catalytic reactors showing a dynamic behavior of the catalytically active particles in general.

Gold-decorated Fe3O4 nanoparticles for efficient photocatalytic degradation of ampicillin: a chemometrics investigation

Wastewater treatment for lowering down the amount of antibiotic drugs in environment is essential for preventing their harmful effects in the ecosystem and for diminishing the growing bacterial resistance. Here, gold-decorated Fe3O4 nanoparticles (NPs) were used for efficient photocatalytic degradation of ampicillin, investigated by UV/Vis absorbance spectrometry. The absorbance data matrices recorded in the course of reactions were analyzed using multivariate curve resolution-alternative least squares as a power chemometrics method. The resolved concentration profiles were composed of three intermediate species and one final product. The concentration profiles were fitted to a first-order kinetics and for the photodegradation experiments run (1) in the absence of photocatalyst, (2) in the presence of Fe3O4 NPs and (3) in the presence of Au-decorated Fe3O4 NPs, the rate constants of 0.023, 0.041 and 0.055 min−1 were obtained, respectively. We found that ampicillin undergoes fast photocatalytic degradation reaction, so that, in the presence of Fe3O4 NPs and Au-decorated magnetic NPs, it was completely converted to its photodegradation product in a few min (35 and 25 min, respectively). The results obtained by chemometrics analysis of the absorbance data agreed with those obtained by capillary electrophoresis.

Probing local diffusion and reaction in a porous catalyst pellet

In the present work, we report on the direct measurement of spatially resolved concentration profiles inside a single catalyst pellet under reaction conditions using a capillary sampling method. CO oxidation on a platinum-coated, porous alumina cylinder was chosen as test reaction system. CFD simulations were conducted to probe the invasiveness of the method, which turned out to be acceptable for practical application. Spatially resolved CO2 mole fraction profiles inside the particle are presented for different reaction conditions. Phenomena that result from the interplay between diffusion and reaction such as boundary layers, bifurcation, multiple steady states and kinetic oscillations are also shown. The method might allow the knowledge-based optimization of many industrial fixed-bed processes without requiring major changes to the process layout.

Impact of the Support on the Catalytic Performance, Inhibition Effects and SO2 Poisoning Resistance of Pt-Based Formaldehyde Oxidation Catalysts

Formaldehyde emissions in addition to the methane-slip have recently been addressed for natural gas fueled combustion engines and thus require an aftertreatment catalyst system. In this work, two Pt-based catalysts supported on Al2O3 and TiO2–SiO2 were investigated by considering their activity, inhibition and stability towards SO2-poisoning. The catalysts were analyzed by X-ray diffraction, electron microscopy, BET surface area, ex situ and operando X-ray absorption spectroscopy, which revealed differences in the oxidation state of Pt depending on the support material and that slightly reduced Pt sites seem to be the active species for this reaction. Systematic activity tests showed that NOx presence leads to a significant inhibition, possibly due to oxidation of the active species, whereas CO addition promotes formaldehyde oxidation above the onset temperature of CO oxidation. These results were supported by spatially resolved concentration profiles along the channel of a catalyst coated monolith. Addition of small amounts of SO2 to the gas mixture resulted in a complete loss of the low-temperature activity, slightly more pronounced for the TiO2–SiO2 supported catalyst.

Evaluation of long-heating kinetic process of edible oils using ATR-FTIR and chemometrics tools.

Long thermal oxidative kinetic and stability of four different edible oils (colza, corn, frying, sunflower) from various brands were surveyed with the use of attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) combined with multivariate curve resolution-alternative least square (MCR-ALS). Sampling from the heated oils (at 170°C) was performed each 3h during a 36-h period. Changes in the ATR-FTIR spectra of the oil samples in the range of 4000-550cm-1 were followed as a function of heating time. MCR-ALS was utilized to resolve the concentration and spectral profiles of three detected kinetic components. Three variations in resolved concentration profiles were related to the thermal-deduction of triacylglycerol of unsaturated acid, appearance of hydroperoxides form of triacylglycerols and generation of secondary oxidation products. The kinetic profiles of these species were dependent on the type of oil. The proposed method can define a new way to monitor the oils' quality.

Model for computing kinetics of the graphene edge epitaxial growth on copper

A basic kinetic model that incorporates a coupled dynamics of the carbon atoms and dimers on a copper surface is used to compute growth of a single-layer graphene island. The speed of the island's edge advancement on Cu[111] and Cu[100] surfaces is computed as a function of the growth temperature and pressure. Spatially resolved concentration profiles of the atoms and dimers are determined, and the contributions provided by these species to the growth speed are discussed. Island growth under the conditions of a thermal cycling is studied.

NO Inhibition Effects During Oxidation of Propylene on Cu-Chabazite Catalyst: A Kinetic and Mechanistic Study

Bench-flow reactor and in situ DRIFTS experiments were carried out to elucidate the features of the propylene + NO + O2 reaction system on Cu–SSZ13 (chabazite) monolithic catalyst. Experiments were conducted under both steady state and transient conditions for application-relevant feed conditions. Steady state conversion data of the C3H6 light-off in the presence of excess O2 (5%) shows inhibition by a much smaller amount of NO (500 ppm). Corresponding data in the presence of NO2 reveals a transition in the C3H6 light-off curve; for temperatures below 350 °C, the C3H6 conversion is higher in the presence of NO2 as compared to O2 whereas above 350 °C the C3H6 conversion in the absence of NO2 is higher. The reduction of NO2 to NO and N2 is enhanced by the addition of C3H6, which facilitates the reduction of Cu sites favorable for the NO2 reduction. Spatially resolved concentration profiles using a series of monolith pieces of different lengths provide insight into the reaction pathways. The spatial profiles...

Employment of multivariate curve resolution to liquid chromatography coupled with NMR

NMR spectral data from aliquots at different retention times of an ordinary liquid chromatographic column were resolved into individual concentration and spectral profiles using multivariate curve resolution based on alternative least squares (MCR-ALS) and canonical correlation analysis (CCA). Samples were a number of the reaction product mixtures obtained at different experimental conditions, based on a simple experimental design, and for synthesis of α-amido phosphonate. NMR data from different experiments were augmented and aligned using correlation optimized warping (COW) procedure. Orthogonal projection approach (OPA) was applied to make initial estimates for MCR-ALS. CCA was implemented in three steps; the first step was determining the regions of NMR peak clusters, the second was the rank analysis of each peak cluster, and the third was assignment of peak clusters to different compounds using CCA. Employing both resolution methods, the NMR data from liquid chromatographic column was successfully resolved to spectral and concentration profiles of pure components. From the resolved concentration profiles the optimum experimental conditions with maximum yield of reaction were obtained as air atmosphere and at 25 °C. Due to the fact that there is rotational ambiguity in the obtained results of MCR-ALS, the resolved concentration profiles from the two methods were different. However, both methods resulted in the same optimal experimental conditions.

Resolving batch chromatographic overlapping peaks of flavoring essence using stepwise key spectrum selection

Stepwise key spectrum selection (SKSS) was introduced to resolve batch overlapping peaks from gas chromatography–mass spectrometry (GC–MS) analysis of ten batch tobacco flavoring samples in different storage times. Resolution was implemented on a software platform that embedded the SKSS method. The data from GC–MS analysis of the samples were saved and prepared in ASCII files and then were inputted into the software platform for visual inspections. The data segment with overlapping peaks was precut for subsequent analysis. Spectral background in the data was removed using a linear fitting of the baseline. Four components in the overlapping peaks were automatically detected by the SKSS method. The resolution of the concentration profiles and spectra of the four components was conducted by setting only one parameter, the negative area ratio, as 0.01. The fixed size moving window evolving factor analysis and evolving factor analysis were applied to validate the resolved concentration profiles. The resolved mass spectra were validated by the searched standard through library search at the pure component regions revealed by the resolved concentration profiles. The results showed that the SKSS method could be a simple but powerful tool in resolving batch chromatographic overlapping peaks.

Hard modeling Multivariate Curve Resolution using LASSO: Application to Ion Mobility Spectra

Multivariate Curve Resolution (MCR) aims to blindly recover the concentration profile and the source spectra without any prior supervised calibration step. It is well known that imposing additional constraints like positiveness, closure and others may improve the quality of the solution. When a physico-chemical model of the process is known, this can be also introduced constraining even more the solution. In this paper, we apply MCR to Ion Mobility Spectra. Since instrumental models suggest that peaks are of Gaussian shape with a width depending on the instrument resolution, we introduce that each source is characterized by a linear superposition of Gaussian peaks of fixed spread. We also prove that this model is able to fit wider peaks departing from pure Gaussian shape. Instead of introducing a non-linear Gaussian peak fitting, we use a very dense model and rely on a least square solver with L1-norm regularization to obtain a sparse solution. This is accomplished via Least Absolute Shrinkage and Selection Operator (LASSO). Results provide nicely resolved concentration profiles and spectra improving the results of the basic MCR solution.

Effects of H 2O and CO 2 addition in catalytic partial oxidation of methane on Rh

The autothermal catalytic partial oxidation (CPO) of methane was performed at short contact times (∼8 ms) over three Rh-based catalysts: 5 wt% Rh/ α-Al 2O 3, 5 wt% Rh/2 wt% γ-Al 2O 3/ α-Al 2O 3, and 5 wt% Rh/2 wt% Ce/2 wt% γ-Al 2O 3/ α-Al 2O 3. The effects of H 2O addition (10%, 20%, and 40% of the total feed) and CO 2 addition (20% and 35%) were studied over a wide range of inlet C/O ratios (0.75–1.2) by means of the capillary sampling technique. Over Rh/ α-Al 2O 3 samples, spatially resolved concentration profiles revealed that the rate of CH 4 reforming is independent of the concentration of H 2O and CO 2. Differences in the product distribution followed the behavior expected from water gas shift (WGS) chemistry: in H 2O-rich tests, the production of H 2 and CO 2 increased at the expense of H 2O and CO, while the opposite was observed in CO 2-rich tests. CPO experiments with a simultaneous feed of H 2O and CO 2 (40% H 2O, 20% CO 2, 20% CH 4, C/O = 1) provided direct evidence that H 2O is the preferential co-reactant of CH 4 in reforming and that CO 2 reforming is absent. Addition of a γ-Al 2O 3 washcoat to the catalyst significantly enhanced the rate of steam reforming while revealing the limits of Rh WGS activity. Products were equilibrated for nearly all cases examined over the Ce-promoted catalyst, indicating that WGS kinetics, either forward or reverse, have an important role in the CPO mechanism, particularly with H 2O and CO 2 co-feed. The experimental results clearly illustrate the flexible nature of CPO process. They show that the H 2/CO can be varied within a wide range of values in a one-step process, and syngas production is sustainable under highly diluted conditions.

Retrospective Search for Evidence of the 1957 Windscale Fire in NE Ireland Using 129I and Other Long-Lived Nuclides

The accident at Windscale in October 1957 resulted in the release to the atmosphere of a large quantity of radioactivity. The presented work is a retrospective search for evidence of contamination from the accident in the northeastern region of Ireland. A lake yielding a high-resolution sedimentary record was identified near the northeast coast of Ireland. This site was used to reconstruct the history of radionuclide input to the region, based on the analysis of a set of cores extracted from the lake. A chronology for sediment accumulation within the lake was established using radioisotopic dating techniques (including 270Pb). High-resolution gamma and alpha spectrometry techniques were used to quantify concentrations of 137Cs, 239,240Pu and 241Am, all of which were released during the accident. The primary radioactive component of the release was 131I (T1/2 = 8 days), but this short-lived isotope has long since decayed. However, 129I (T1/2 = 1.57 x 10(7) years) was also released during the accident, and in a known ratio to 131I. Recent advances in accelerator mass spectrometry now make it feasible to measure 129I at ultra-trace level and thereby retrospectively reconstruct 131I deposition. Clearly resolved concentration profiles for 137Cs, 239,240Pu and 241Am in the lake cores reflect known historical fallout trends. The data suggest that any contamination from the Windscale fire that might have reached this catchment has been overwritten by input from the testing of nuclear weapons in the atmosphere. A time-series for 129I in lake sediment shows that concentrations in recent sediments are approximately 10 times greater than concentrations recorded in strata corresponding to the period of maximum fallout of other radionuclides from atmospheric testing of nuclear weapons (1964). These recent increases in 129I are attributed to increased emissions from the nuclear industry. The study yields no evidence of any enhancement in radioisotope concentrations, over and above global fallout, in strata dated to 1957, and we conclude that contamination from the Windscale fire had negligible impact on the northeastern region of Ireland.

The Influence of Tungsten on the Chemical Composition of a Temporally Evolving Nanostructure of a Model Ni-Al-Cr Superalloy

The influence of W on the temporal evolution of gamma' precipitation toward equilibrium in a model Ni-Al-Cr alloy is investigated by three-dimensional atom-probe (3DAP) microscopy and transmission electron microscopy (TEM). We report on the alloys Ni-10 Al-8.5 Cr (at.%) and Ni-10 Al-8.5 Cr-2 W (at.%), which were aged isothermally in the gamma + gamma' two-phase field at 1073 K, for times ranging from 0.25 to 264 h. Spheroidal-shaped gamma' precipitates, 5-15 nm diameter, form during quenching from above the solvus temperature in both alloys at a high number density (approximately 1023 m-3). As gamma' precipitates grow with aging at 1073 K, a transition from spheroidal- to cuboidal-shaped precipitates is observed in both alloys. The elemental partitioning and spatially resolved concentration profiles across the gamma' precipitates are obtained as a function of aging time from three-dimensional atom-by-atom reconstructions. Proximity histogram concentration profiles (Hellman et al., 2000) of the quaternary alloy demonstrate that W concentration gradients exist in gamma' precipitates in the as-quenched and 0.25-h aging states, which disappear after 1 h of aging. The diffusion coefficient of W in gamma' is estimated to be 6.2 x 10-20 m2 s-1 at 1073 K. The W addition decreases the coarsening rate constant, and leads to stronger partitioning of Al to gamma' and Cr to gamma.

Monitoring of chemical reactions within microreactors using an inverted Raman microscopic spectrometer.

An inverted Raman microscope spectrometer has been used to profile the spatial evolution of reactant and product concentrations for a chemical reaction within a microreactor operating under hydrodynamic flow control. The Raman spectrometer was equipped with a laser source at wavelength of 780 nm, confocal optics, a holographic transmission grating, and a charge-coupled device (CCD) detector. The microreactor consisted of a T-shaped channel network etched within a 0.5 mm thick glass bottom plate that was thermally bonded to a 0.5 mm thick glass top plate. The ends of the channel network were connected to reagent reservoirs that were linked to a syringe pump for driving the solutions by hydrodynamic pumping within the channels. The microchannels were 221 micro m wide and 73 micro m deep. The synthesis of ethyl acetate from ethanol and acetic acid was investigated as a model system within the microreactor as Raman scattering bands for each reactant and product species were clearly resolved. Raman spectral intensities of each band were proportional to concentration for each species and hence all concentrations could be quantitatively measured after calibration. By scanning specific Raman bands within a selected area in the microchannel network at given steps in the X-Y plane, spatially resolved concentration profiles were obtained under steady-state flow conditions. Under the flow conditions used, different positions within the concentration profile correspond to different times after contact and mixing of the reagents, thereby enabling one to observe the time dependence of the product formation. Raman microscopy provides a useful complementary technique to UV/VIS absorbance and fluorescence methods for the in situ monitoring and analysis of chemical reaction species having their lowest S(0)-S(1) absorption bands too far in the UV to be of use, due to their probable overlap with the bands from other reactant, product and solvent molecules.