Towards the understanding of bubble interactions and coalescence in non-Newtonian fluids: a cognitive approach

The present work provides new insights into the behavior of air bubbles in non-Newtonian fluids. The interactions and coalescence between bubbles rising in non-Newtonian fluids were simultaneously investigated by means of birefringence measurements and particle image velocimetry for a chain of bubbles formed from a submerged orifice. Two aspects are identified for the first time as central to interactions and coalescence: (i) the stress creation by the passage of bubbles, and (ii) their relaxation due to the fluid's memory. This competition displays complex nonlinear dynamics, from periodic phenomena to deterministic chaos. From these fundamental mechanisms, a cognitive model based on behavioral rules has been developed to describe collective behaviors of a group of bubbles. By simulating bubbles as adaptive agents with their fluid via residual stresses, model predictions for consecutive coalescence between a great number of bubbles compare very satisfactorily with the experimental investigation.     

Towards a better understanding of the high-temperature oxidation of MAX phase Cr2AlC

High-temperature oxidation of bulk single crystal, fine and coarse grained polycrystals of Cr₂AlC has been performed from 800 °C to 1500 °C. In the temperature range T = [800;1400] °C, an α-Al₂O₃ scale with a Cr₇C₃ subscale are in-situ formed independently of the initial microstructure on top of the bulk Cr₂AlC. CALPHAD calculations were performed to explain the formation of Cr₇C₃ instead of the expected mixture of carbides, indicating a favourable driving force for Cr₇C₃ formation. The kinetic study of the oxidation is consistent with previous works with an activation energy of about 400 kJ.mol^?1. Oxidizing for a longer time (1000 h) or at higher temperature (1500 °C) leads to the formation of chromia in the former alumina scale, which correspond to the calculated equilibrium phases.     

A comparison of a pseudo-homogeneous non-equilibrium model and a three-phase non-equilibrium model for catalytic distillation

A comparison of computer simulation results of catalytic distillation (CD) obtained from a three-phase non-equilibrium model and a pseudo-homogeneous non-equilibrium model was performed. Simulations were carried out on the CD processes for the production of ethyl cellosolve (EC) and diacetone alcohol (DAA) using both the pseudo-homogeneous non-equilibrium model and the three-phase non-equilibrium model. Similar results for the synthesis of EC were obtained using these two models. However, only the three-phase non-equilibrium model could adequately describe the CD process for the aldol condensation of acetone (Ac) at low reflux flow rates. Hence our results suggest that for a reaction system that is kinetically controlled, a pseudo-homogeneous non-equilibrium may adequately simulate the temperature profile, yield and selectivity for a CD process. However, for a CD process that is sensitive to solid–liquid mass transfer, the three-phase non-equilibrium model is required.     

Towards understanding the bifunctional hydrodeoxygenation and aqueous phase reforming of glycerol

Kinetically coupled reactions of glycerol in water over bifunctional Pt/Al₂O₃ catalysts are explored as a function of the Pt particle size and the reaction conditions. Detailed analysis of the reaction network shows that “reforming” and hydrodeoxygenation require the presence of a bifunctional catalyst, i.e., the presence of an acid–base and a metal function. The initial reaction steps are identified to be dehydrogenation and dehydration. The dehydrogenation of hydroxyl groups at primary carbon atoms is followed by decarbonylation and subsequent water gas shift or by disproportionation to the acid (and the alcohol) followed by decarboxylation. Hydrogenolysis of the C–O and C–C bonds in the alcohols does not occur under the present reaction conditions. Larger Pt particles favor hydrodeoxygenation over complete deconstruction to hydrogen and CO₂.     

Towards an understanding of the heat distortion temperature of thermoplastics

A systematic understanding of the heat distortion temperature (HDT) of amorphous and semi-crystalline polymers is possible through a direct correlation with the modulustemperature behavior. For amorphous polymers, the precipitous drop in modulus at the glass transition temperature makes the HDT a well-defined, reproducible and predictable property. Furthermore, the addition of reinforcing fillers has a negligible effect on the HDT of the amorphous polymer. For semi-crystalline polymers, however, the exact opposite may hold true. The modulus exhibits a “plateau” region between the glass transition and the melting transition. Hence the HDT often is difficult to predict, is sensitive to thermal history and may be greatly increased through the addition of fillers. More importantly, the HDT may not be an accurate measure of the upper use temperature for semi-crystalline polymers in load bearing situations since considerable stiffness may still be retained even upon exceeding the HDT.     

Modeling of monothermal ammonia-hydrogen chemical exchange column using a non-equilibrium model

The monothermal NH₃-H₂ chemical exchange process, operating at low temperature and high pressures, is simulated by considering non-ideal behavior of both phases and multicomponent mass transfer at the gas-liquid interface. The hydrodynamics of special ejector tube tray is incorporated in the simulation and validated against actual operating plant data. The column operation is simulated with respect to process parameters to determine the optimum operating window for the column. A higher temperature operation near 270 K has been suggested for better performance of the column. The multicomponent effect is not prominent in the first tower operated at 248 K and 20 MPa but is significant in the second tower operated at 270 K and 10 MPa. In the overall operation, the multicomponent effect cannot be neglected.     

Towards an understanding of the effects of operating conditions on separation by microfluidic distillation

A parametric study on the separation of an acetone–ethanol mixture using a microfabricated distillation chip, with a microchannel 600 μm wide and 400 mm long, was conducted. The aim of the study was to identify the key parameters for optimal design and operation of the microdistillation device. The performance of the microscale separation device was studied by varying the relevant operating parameters, including the temperatures of the heating and cooling regions, the flowrate and composition of the feed solution and the flowrate of the bottom stream. The optimum separation was found to be equivalent to at least 5.4 equilibrium stages (estimated at total reflux conditions). A decrease in the heating temperature, or an increase in the cooling temperature, resulted in poorer separation. In addition, the allowable temperature profile along the microdistillation chip constrained its operation. The feed conditions were found to be critical to the separation performance and the chip seemed to be particularly good at separating a feed mixture high in light component (acetone) at a low flowrate. The purity of the bottom stream could be increased by decreasing the bottom flowrate, but at the expense of the distillate stream purity, and vice versa. By comparing the experimental results with simulation results obtained by CHEMCAD, it was shown that the feed composition had a more significant effect on microdistillation than on conventional distillation columns.     

Dynamic polymorphic phase transitions in a model binary triglyceride system measured by position-sensitive X-ray diffraction methods

X-ray diffraction measurements have been made on a binary mixture of pure tripalmitin and tristearin which undergoes particular polymorphic phase transitions on heating from the solid. The results were obtained using a position-sensitive detector which can record the whole diffraction pattern in time periods less than a minute. This allows many patterns to be recorded during the transformations where heating rates can be up to several^oC/min. As such the transitory existence of the intermediate β' phase has been unambiguously identified. The results serve to exhibit the usefulness of the X-ray apparatus and indicate its potential for future studies in dynamic crystallization and melting. On behalf of Loders-Croklaan     

Sorption properties and phase transitions temperature of freeze-dried strawberry model based on hydrocolloids with a tailored structure

The aim of this work was to investigate the effect of aerated structure on sorption properties and phase transitions temperature of freeze-dried hydrocolloid gels. Sugars and citric acid were added to gels with low-methoxyl pectin (LMP), the mixtures of xanthan gum and locust bean gum, xanthan gum and guar gum, to create freeze-dried strawberry model. Hydrocolloid type changed freeze-dried strawberry model properties, but aeration time was generally insignificant. LMP gels porous structure increased sorption properties and decreased the glass transition temperature. Freeze-dried strawberry models based on hydrocolloids mixture were characterized by more compact structure, which decreased the sorption ability and increased the glass transition temperature.     

EMMS-based modeling of gas–solid generalized fluidization: Towards a unified phase diagram

Hydrodynamic features of gas-solid generalized fluidization can be well expressed in the form of phase diagrams, which are important for engineering design. Mesoscale structure presents almost universally in generalized fluidization and should be considered in such phase diagrams. However, current phase diagrams were mainly proposed for cocurrent upward flow according to experimental data or empirical correlations with homogeneous assumption. The energy-minimization multiscale (EMMS) model has shown the capability of capturing mesoscale structure in generalized fluidization, so EMMS-based phase diagrams of generalized fluidization were proposed in this article, which describe more reasonable global hydrodynamics over all regimes including the important engineering phenomena of choking and flooding. These characteristics were also found in discrete particle simulation under various conditions. For wider range of application, the typical hydrodynamic parameters of the phase diagrams were correlated to non-dimensional numbers reflecting the effects of material properties and operation conditions. This study thus shows a possible route to develop a unified phase diagram in the future.     

A bifurcation approach to understanding instabilities in gas-fluidized beds using a single phase compressible flow model

It has been shown that the equations of motion and continuity for the particles in a fluidized bed can be related to those of a compressible fluid acted upon by a density-dependent force. In the previous work on the compressible flow equations, the solution structure of fully developed plane (one-dimensional) waves was computed. It was shown that the plane waves can lose stability in the lateral direction. In this work we study the two-dimensional solutions which reveal that bubble-like solutions can evolve both from the plane waves as well as from the uniform state. A representative value of the lateral wavenumber is chosen and the global bifurcation diagram is explored, which consists of a number of distinct one- and two-dimensional branches. The existence of mixed mode and double humped solutions is demonstrated and transient simulations are used to examine the mechanism of density wave development and coalescence.     

Towards a better understanding of the epoxy-polymerization process using multi-objective evolutionary computation

The epoxy-polymerization process can be better understood by investigating the underlying optimization problem involving a number of conflicting objectives and more than 20 decision parameters. A combination of minimization or maximization of objectives, such as the number average molecular weight, polydispersity index and reaction time, are considered in this paper. The first two objectives are related to the properties of a polymer, whereas the third objective is related to productivity of the polymerization process. The decision variables are addition quantities of various reactants, e.g. the amount of addition for bisphenol-A (a monomer), sodium hydroxide and epichlorohydrin at different time steps (modeled in a semi-batch operation), whereas the satisfaction of all species balance equations is treated as constraints. A multi-objective evolutionary algorithm (the elitist non-dominated sorting genetic algorithm or NSGA-II) is used to obtain a set of non-dominated solutions in a single simulation run. The results show a substantial improvement (with about 300% more productivity) over the benchmark condition (reported by performing a one-time addition of reactants in the beginning in a batch process). Importantly, this study brings out a salient aspect of using an evolutionary approach to multi-objective problem solving. The availability of multiple optimal trade-off solutions allows a process engineer to have salient information about the polymerization process. Changes in the distribution of various polymer species in the course of polymerization process as observed among various Pareto-optimal solutions are identified and explained for this purpose. Such information provide important information about optimal operating conditions corresponding to different trade-offs among objectives, which are otherwise difficult to obtain. The systematic approach of starting from the two-objective problems to capture the essential features of interesting optimal operating conditions to finally solving the three-objective problem associated with the epoxy-polymerization problem in discovering the optimal trade-off interactions should motivate further such studies on other chemical process optimization problems. Overall, this paper demonstrates how fundamental optimization principles can be used systematically and reliably to find optimum operating conditions for complex chemical process operations.     

Towards a better understanding of the release mechanisms of caffeine from PLGA microparticles

Poly(lactic-co-glycolic acid) (PLGA)-based microparticles can be successfully used to control the release rate of a drug and optimize the therapeutic efficacy of a medical treatment. However, the underlying drug release mechanisms can be complex and are often not fully understood. This renders system optimization cumbersome. In this study, differently sized caffeine-loaded PLGA microparticles were prepared and the swelling and drug release behaviors of single microparticles were monitored upon exposure to phosphate buffer pH 7.4. Ensembles of microparticles were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, gel permeation chromatography, and optical microscopy. The observed triphasic drug release patterns could be explained as follows. The initial burst release can be attributed to the dissolution of tiny drug crystals with direct surface access. The subsequent second drug release phase (with an about constant release rate) could be attributed to the release of drug crystals in regions, which undergo local swelling. The third release phase (again rapid, leading to complete drug exhaust) could be explained by substantial polymer swelling throughout the systems. Once a critical polymer molecular weight is reached, the PLGA chains are sufficiently hydrophilic, insufficiently entangled and the osmotic pressure created by water soluble degradation products attracts high amounts of water into the system. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48710.     

Carbon black used as a fluidization aid in gas phase elastomer polymerization: I. Carbon black-monomer interactions

The adsorption of butadiene on carbon black is a function of surface properties, high reinforcing, high surface area carbon black adsorbs more butadiene than low reinforcing big particle size carbon black. It is found that the functional groups on modified carbon blacks, oxidized or sulfur modified carbon blacks significantly reduces the butadiene concentration in headspace. Chemisorption of butadiene on the carbon black surface strongly holds butadiene on the surface, which prevents desorption of butadiene from the carbon black surface. These modified carbon blacks result in a very low concentration of butadiene in the headspace, usually 50-100 times lower than unmodified carbon black.     

Catalytic vapour phase epoxidation of propene with nitrous oxide as an oxidant II. Development of a kinetic model

The vapour phase epoxidation of propene with nitrous oxide (N₂O) was experimentally investigated in a fixed bed reactor using a CsO_x/FeO_y/SiO₂ catalyst over a broad range of residence times. The influence of feed composition on the conversion and product distribution was determined for the reactants propene, propylene oxide (PO), propionaldehyde (PA), and N₂O. The experimental results were used to derive a formal kinetic model to describe the reactions in a network deduced in the first part of this publication [Thömmes, T., Zürcher, S., Wix, A., Reitzmann, A., Kraushaar-Czarnetzki, B., 2007. Catalytic vapour phase epoxidation of propene with nitrous oxide as an oxidant: I. Reaction network and product distribution. Applied Catalysis A 318, 160-169]. Self-inhibition of the propene conversion was observed, and an inhibition of the PO conversion through PO isomerisation products. The N₂O concentration has almost no effect on the conversion of propene and PO, but the PA conversion is accelerated significantly through N₂O. Propene related and N₂O related PO selectivities display opposed dependencies from reactant concentration. The modelling results indicate that coke is predominantly formed from oxygenated products. It is the key issue for future developments to prevent the fast consecutive conversion of PO.     

Fractionation of poly(ethylene-co-vinyl acetate) in supercritical propylene: Towards a molecular understanding of a complex macromolecule

A commercial low-density polyethylene copolymer, poly(ethylene-co-vinyl acetate) (EVA), synthesized via the high-pressure free-radical polymerization process, was fractionated with supercritical propylene by isothermal increasing pressure profiling and critical, isobaric, temperature rising elution fractionation (CITREF™). Extensive characterization of the fractions by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) in combination with low-angle laser light scattering (LALLS), and differential scanning calorimetry (DSC) was used to map not only the molecular-weight and chemical composition distributions of the parent copolymer, but also its short-chain branch (SCB) and long-chain branch (LCB) distributions. Fractionation by increasing pressure profiling confirmed the broad molecular-weight distribution and the narrow acetate-branch distribution expected for this random copolymer but revealed the presence of a small amount (∼ 2 wt %) of low molecular-weight amorphous species containing a high level of alkyl SCBs (80 branches/1000 C). The LCB density estimated from the Zimm-Stockmayer relationship using the GPC data monotonically increases with increasing molecular weight above 60,000 g/mol, in agreement with the kinetics of free-radical polymerization. CITREF™ was found to fractionate this copolymer by crystallinity, which is influenced by both the alkyl SCBs and the acetate branches. Up to 18% difference in total branch density (<5% in crystallinity) between EVA molecules was identified using CITREF™. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2015–2030, 1997     

Thermal properties,phase transitions,and morphology of a conjugated polymer

A highly conjugated polymer was synthesized from α,α′-dibromo-p-xylene, 1,4-bis(2-hydroxy-benzoyl)benzene, and benzyl bromide via a modified Williamson ether synthesis under phase transfer conditions. Based on thermal analysis, polarized light microscopy, electron diffraction, and wide angle X-ray diffraction experiments, the heat capacities in the solid and liquid states have been determined. The glass transition temperature is at 358 K with a heat capacity increase of 164 J/(K mol). Two different ordered crystal forms with different crystal unit cells were observed. The more ordered crystal shows a low transition temperature and the less ordered one a high transition temperature. This is mainly due to the effect of entropy change. These crystals exhibit various crystal morphologies with different birefringences. Transition kinetics are expressed by an Avrami equation. Two crystals show different Avrami exponents with crystallization temperature, which corrspond to the morphological change. Competition between crystal growths is also discussed.