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Item-typ:Veröffentlichung, En route towards a comprehensive dimensionless representation of precipitation processesPrecipitation of (nano-) particles is a unit operation for the preparation of suspensions. Despite its widespread use, no general understanding is established of how different operating conditions, such as mixing rates or reactant concentrations, affect the precipitation outcome (e.g. the particle size distribution). We obtain overarching relations between the operating conditions and the precipitation outcome by deriving dimensionless numbers governing the precipitation process. In particular, we consider the interdiffusion of two chemical compounds reacting to a sparingly soluble salt by coupling the reaction–diffusion equations for the solute concentrations, to a population balance equation for the evolution of the dispersed phase including nucleation and growth. We vary the relevant process parameters for different chemical systems and uncover three dimensionless numbers, which fully determine the precipitation outcome under the assumption of fast chemical reactions. Two of these dimensionless numbers are Damköhler numbers, which put the relevant mixing kinetics in relation to the relevant time scales for the nucleation and the growth kinetics, respectively. The third dimensionless number corresponds to a dimensionless solid concentration. Simple functional expressions for the Damköhler numbers valid for any chemical system allow estimating whether the precipitation product is controlled by mixing, or solely by the kinetics of solid formation. In the mixing-controlled regime, scaling laws relate the mean particle sizes and the number of particles to the Damköhler number.Wissenschaftlicher ArtikelBand:42871 79 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, CO2 methanation and reverse water gas shift reaction. Kinetic study based on in situ spatially-resolved measurements(Elsevier, 2020-07-15); ; Kopyscinski, JanThe reaction kinetics for the CO2 methanation and reverse water gas shift reaction over an ordered-mesoporous Ni/Al2O3 catalyst were determined. For the parameter estimation and model discrimination, the kinetic data were obtained by means of spatially-resolved measurement in a catalytic plate reactor. In detail, ~21,000 high-resolution gas composition data were gathered along the reactor axis using a movable sampling capillary connected to a mass spectrometer. Additionally, the catalyst surface temperature was determined via infrared thermography. The influence of reaction temperature (320–420 °C), total pressure (1.2–7.3 barabs), and GHSV, as well as possible inhibition of products such as CH4 and H2O, were investigated. A one-dimensional model of the reactor was developed describing the conservation of mass in the bulk gas and catalyst phase. The Bayesian approach was used to estimate the kinetic parameters of 20 proposed Langmuir-Hinshelwood rate expressions for the CO2 methanation that were derived based on three different mechanisms (i.e., direct dissociation, hydrogen assisted dissociation, and hybrid mechanism). Two kinetic models reflected the measured data very well. The most probable models suggest that the rate determining step includes the reaction of an oxygenated complex (COH* or HCOO*) with an active site (*) or an adsorbed hydrogen (H*). Furthermore, water was assumed to be adsorbed as a hydroxyl species (OH*), while methane did not influence the reaction. Temperature- and time-resolved Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements confirmed the presence of both adsorbed surface intermediates.Wissenschaftlicher ArtikelBand:39045 55 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Pore-scale analysis of axial and radial dispersion coefficients of gas flow in macroporous foam monoliths using NMR-based displacement measurements(Elsevier, 2020-02-06); ; ; ; A micro-scale analysis of mass transport in ceramic foams that are used as catalyst supports in gas phase reactions is of high interest. Although the effects of flow rate and foam parameters on the radial and axial dispersion are known for liquid flows, no pore-scale experimental analysis has been yet reported to correlate the mechanical and diffusional dispersion of gas flows to the geometry of open-cell foams. Here, a spatially resolved Pulsed Field Gradient NMR method is applied to determine dispersion coefficients of thermally polarized gas along axial and transversal directions of open-cell foams. The comparative study of three commercial foam samples with different morphologies shows the effect of open porosity, window size, and flow rate on gas dispersion. Additionally, the influence of mechanical and diffusional dispersion at each flow rate is investigated for individual samples. By observing the transition from diffusional dispersion to mechanically driven dispersion of gas, it is found that diffusional dispersion plays an important role, even at higher flow rates after a transition from Darcy to Darcy-Forchheimer regime occurs. The measured values for dispersion coefficients of methane can be directly used in pseudo-heterogeneous models for the methanation reaction.Wissenschaftlicher ArtikelBand:388132 98 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Chitosan functionalized iron nanosheet for enhanced removal of As(III) and Sb(III): Synergistic effect and mechanism(Elsevier Science, 2019-12-13); ; ; ; Neutral trivalent arsenic (As(III)) and antimony (Sb(III)) species are more toxic and harder to remove from wastewater environment than their pentavalent counterparts. Herein, a natural polysaccharide chitosan (CS) functionalized iron nanosheet using an in-situ doping method was designed, aiming to attract As(III) and Sb(III) from aqueous solution. The surface area of the optimum sample was 111.8 cm2/g, a good dispersion of the iron nanocomposite was observed by the TEM and element mapping characterization. In the batch adsorption experiment the factors of doping contents of CS and solution properties (pH, co-existing anions and humic acid) were studied systematically. The result showed that 0.5 wt% CS functionalized iron nanosheet had higher removal capacity, affinity, selectivity and reusability for Sb(III) than As(III). The optimum adsorption were achieved at the adsorbent dosage of 0.4 g/L at a wide pH values, and the maximum adsorption capacity were 108.6 and 138.8 mg/g for As(III) and Sb(III) calculated from Langmuir non-linear fitting, respectively. Both As(III) and Sb(III) removal were independent on pH values, indicating electrostatic attraction was not the dominate removal mechanism. The detailed removal mechanism was confirmed by a synergetic interaction of As-Fe/Sb-Fe complexes and hydrogen bonding using the combined characterization of FTIR and XPS spectra. This study demonstrated that the CS functionalized iron nanosheet with the properties of environmental friendly, low-cost and facile preparation could have potential applications in water purification.Wissenschaftlicher ArtikelBand:38285 97 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Quantitative modeling of precipitation processes(Elsevier, 2022-09-15); 109 78