Identification of reaction rate parameters from uncertain spatially distributed concentration data using gradient-based PDE constrained optimization

A promising approach to quantify reaction rate parameters is to formulate and solve inverse problems by minimizing the deviation between simulation and measurement. One major challenge may become the non- uniqueness of the recovered parameters due to the ill-posed problem formulation, which requires sophisticated approaches such as regularization. This study investigates the feasibility of using spatially distributed reference data, i.e., concentration distributions of reactive flows, which could be obtained by magnetic resonance imaging (MRI), instead of isolated points or integral values to recover reaction rate parameters. We propose a combined framework of computational fluid dynamics (CFD) and gradient-based optimization methods, which minimizes the difference between the simulated concentration distribution and a given data set by automatic iterative parameter adjustments. The forward problem is formulated as a coupled system of reaction-advection-diffusion equations (RADE), which is solved by the lattice Boltzmann method (LBM). Therefore, a system of non-linear partial differential equations (PDE) acts as optimization constraints, limiting the possible outcomes of the inverse problem. A benchmark test case using a CFD simulation as reference data confirms the validity of the presented method by successfully identifying up to three a priori set reaction parameters reversely. With it, initial relative errors could be reduced from around 150% to 10−3 % in 13 optimization steps corresponding to 37 simulations. Even after reducing the accessible reference data from 2D concentration distributions to 1D outflow concentration distribution or by adding noise signals onto the reference data with a signal-to-noise ratio (SNR) of 5, our framework successfully recovered the parameters with a relative error of ≈ 1%. Both, the chosen LBM and optimization algorithms are implemented in the open-source library OpenLB

Solid Sponges as Monolithic Catalyst Supports for CO₂Methanation – Experimental Realization and Structure Optimization

Solid sponges are applied as catalyst supports in a bench-scale fixed-bed reactor (25 x 100 mm) for CO2 methanation to demonstrate their potential for process intensification. The structure e.g. porosity pore size spatial distribution of the porosity and pore size is optimized to tune the heat transport properties locally and to determine the optimized temperature profiles for process intensification.</p

Optimization of oxygen feed membranes in autothermal steam-reformers

In conventional concepts for autothermal steam-reformers the spatial temperature distribution is disadvantageous. The different rates of the exothermic oxidation and endothermic reforming reactions lead to the problem of temperature hot-spots, which can damage or even destroy the catalyst or other elements of the reformer. This is especially the case, when the needed oxygen is premixed to the feed gas resulting in a high temperature peak in the entrance region. In the past, some first attempts have been made to solve this hot-spot problem.. One idea is, that the oxygen is not premixed to the feed gas but injected at different positions distributed over the length of the reformer. This leads to more but much smaller temperature peaks. Expanding this idea, one can use also a continuous oxygen injection over the length of the reformer. This can for example be realized by an open porous membrane. By varying the permeability of such a membrane we can adjust the spatial oxygen flux distribution. One of our previous works 1 focused on the optimization procedure of the membrane structure to achieve an isothermal behaviour of the adiabatic autothermal steam-reformer. In the present work we investigate the use of such oxygen feed membranes with varying permeabilities, both in experiments and by simulation analysis. The simulation results show that an optimized membrane can be used for a wide range of operation conditions, i.e. mass fluxes and amounts of oxygen flux without being reduced in efficiency. The experimental results are in good agreement with the simulations and indicate the feasibility of the concept of spatial oxygen feed distribution in autothermal steam-reformers to avoid the hot-spot problem

Simplification of a sequential extraction scheme to determine the mobilisable heavy metal pool in soils

International audienceThis investigation had the purpose of evaluating the reliability of a sequential extraction procedure according to Zelen and Bru?mmer (1989) which is performed routinely to analyse the distribution of heavy metals to different soil phases. Reliability was tested by two hypotheses. According to the first hypothesis an error propagation increased during a morefold extraction of heavy metals from a single soil sample to such an extend that it does not allow a statistical comparison of different analyses. This assumption was confirmed in an interlaboratory study of five participants applying the sequential extraction scheme on two different soils. The heavy metal amounts extracted by the different partners were of the same magnitude, but from the results no statistical correspondence at the 95 % confidence level could be observed. The second hypothesis stated that also weak extraction agents were able to release metals from the more immobile soil fractions, especially if the amount of easily soluble metals was comparatively small. To answer this question the sequential extraction was modified by carrying out selected weak extraction steps several times. As expected the intensified extraction conditions caused a decrease of the element content within the more resistant phases. However, the additional release in the first four extraction steps of Pb, Mn, Fe, and Zn was only in the range of 5 to 10 %. Furthermore, it was observed that a single EDTA extraction (step four within the sequential extraction scheme) was capable to extract the same amount of metals as the first four extraction steps of the original scheme at a variation of about 15 %. From these results it was concluded that the EDTA step alone already represents a reliable pool of mobilisable metals. Thus it can be maintained that such a simple single EDTA extraction can be used to assess the environmental risk from heavy metal contaminated soils or to predict the potential heavy metal release of soil remediation processes

Impact of the pulsed voltage input and the electrode spacing on the enhancement of the permeate flux in a dielectrophoresis based anti-fouling system for a submerged membrane bioreactor

A new electrode configuration based on dielectrophoresis (DEP) for a Submerged Membrane Bioreactors was suggested. The intensification of the permeate flux with the new system was examined experimentally using a pulsed DEP of 10 s on and 15 s off. In comparison with the test without DEP effect, the normalized permeate is 3.5; 3.9 and 4.5 times higher when applying 105 Veff, 144 Veff, and 220 Veff respectively. The membrane working time is found to be 10 times higher when using DEP. The impact of electrodes spacing was also examined experimentally and numerically. It was demonstrated that a large electrodes spacing is not able to intensify the permeate flux, but it could increase the membrane lifespan by stabilising the permeate flux.This publication was made possible by NPRP grant (NPRP7-089-2-044) from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. Also the authors would like to thank the public works authority of Qatar (ASHGAL) for their supply of the activated sludge samples.Scopu

Anaerobic biodegradability of ionic liquid cations under denitrifying conditions

Biodegradability and ecotoxicity of ionic liquids (ILs) are key properties for determining the greenness of IL applications, and have been increasingly investigated during the last few years. Former studies on the biodegradability of ILs were solely focused on the aerobic side. Nevertheless, the anaerobic biodegradation of many compounds plays an important role in the environment. Anaerobic respiration, especially nitrogen reduction, is widespread in the environment and is commonly used for waste water treatment. Therefore, we investigated in this study, whether ILs can be biodegraded under nitrogen reducing conditions. The primary anaerobic biodegradability of nine different imidazolium, pyridinium and dimethylaminopyridinium based IL cations was monitored via HPLC-UV over a time period of 11 months. Only for the 1-(8-hydroxyoctyl)-3-methyl-imidazolium cation (IM18OH), and a degradation could be observed and several metabolites were identified using LC-MS. Co-metabolism is sometimes the only way to degrade difficult substances. However, a possible co-metabolism of the substances by using acetate was not observed. All in all, the biodegradability of the tested ILs seems to be even worse under denitrifying conditions compared to aerobic ones. Nevertheless, the present paper aims to fill the gap concerning the biodegradability of ILs in waste water treatment plants. It gives a first insight into the biological degradation of ILs in the absence of oxygen, and provides further data for an appropriate hazard assessment

A physical explanation of the gas flow diode effect

International audienceGas flow properties in channels with slightly varying cross section have a dependency on the direction of channel perfusion when the gas is in the slip and transitional flow regimes. In the past, it was observed that the flow rate in converging direction is higher compared to the case where the channel diverges alongside. This gas flow diode effect does neither exist in the continuum regime nor in the free molecular regime, and it has its maximum at the same level of gaseous rarefaction as the well-known Knudsen minimum. However, no comprehensive study on the physics of this diode effect is carried out yet. In order to overcome this knowledge gap, the current paper proceeds our previous works by an appropriate experimental study. Here we can show that the diode effect crucially depends on the proportion of inclined walls to the overall channel inner surface. Also the inclination of the wall itself determines the strength of the diode effect meaning that the diodicity increases with the opening angle. Furthermore, we found indication that the diodicity also depends on the molar mass and the internal structure of the impinging gas molecules. Finally, we propose an explanation of the diode effect that is mainly based on the tangential reflection process of gas molecules colliding with the inclined walls of a tapered channel