Konstruktion und Test eines Gradientensystems für NMR‐Diffusionsuntersuchungen in Grenzflächensystemen

A z‐gradient system for NMR diffusion measurements with intensive pulsed field gradients was redesigned. The gradient field of an actively screened gradient coil was optimized using finite element analysis. The gradient system was constructed with glass ceramic as coil support material. It does not show any background ¹H NMR signal and has a high current‐to‐gradient conversion factor of 0.37 T m^–1A^–1. The functionality of the system for studying slow diffusion processes in interface systems is demonstrated by observing isotropic and anisotropic diffusion in aqueous solutions of a PEO‐PPO‐PEO triblock copolymer and of methane in two different microporous crystalline absorbencies.     

Numerical investigation of the hydrodynamics of split‐and‐recombination and multilamination microreactors

The hydrodynamics and residence time distribution (RTD) of two microreactors based on the split‐and‐recombination (SAR), and multilamination mixing mechanisms, respectively were investigated. It was found that the design of the distribution manifolds of the SAR mechanism produces an unbalanced flow distribution. For feeding ratios different than one, bypassing and recirculation occur within the SAR manifolds. For equal flow rates the SAR flow behavior can be accurately described by the pure convection model. The manifold used in the multilamination microreactor achieves a homogeneous distribution of flow and its interdigital mixing structure generates an alternated pattern of fluid layers which is maintained for Re < 140. After this point the ordered arrangement is broken and two large segregated zones are formed. In the absence of molecular diffusion both microreactors reach limiting values of scale and intensity of segregation that were found to be independent of the energy applied to the system. © 2012 American Institute of Chemical Engineers AIChE J, 59: 988–1001, 2013     

Die Modellierung von Stofftransportprozessen in Mehrphasenströmungen – am Beispiel des Strahlzonen‐Schlaufenreaktors

The efficiency and selectivity of chemical reactions are influenced by the mixing characteristics of the reactor. Existing models often assume homogeneous mixing on micro scale to calculate the reaction yield. However, neglecting the local hydrodynamic phenomena causes a discrepancy between model calculation and experimental data especially considering mass transfer limited reactions. In two‐phase flows different mass transfer phenomena have to be considered: the diffusion in the gas‐liquid boundary layer and diffusion in the Batchelor layer. The aim of the paper is to describe the mass transfer affecting mechanisms in multi‐phase flows and to discuss the first results of the investigation of local mass transfer phenomena in a two phase flow driven jet‐zone loop reactor.     

Turbulent mixing in the confined swirling flow of a multi‐inlet vortex reactor

Turbulent mixing in the confined swirling flow of a multi‐inlet vortex reactor (MIVR) was investigated using planar laser induced fluorescence (PLIF). The investigated Reynolds numbers based on the bulk inlet velocity ranged from 3290 to 8225, and the Schmidt number of the passive scalar was 1250. Measurements were taken in the MIVR at three different heights (¼, ½, and ¾ planes). The mixing characteristics and performance of the MIVR were investigated using instantaneous PLIF fields and pointwise statistics such as mixture fraction mean, variance, and one‐point concentration probability density function. It was found that the scalar is stretched along velocity streamlines, forming a spiral mixing pattern in the free‐vortex region. In the forced‐vortex region, mixing intensifies as the turbulent fluctuations increase significantly there. The mixing mechanisms in the MIVR were revealed by identifying specific segregation zones. At Re = 8225 the mixing in the free‐vortex region was dominated by both large‐scale structures and turbulent diffusion, while in the forced‐vortex region mixing is dominated by turbulent diffusion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2409–2419, 2017     

Investigation of concentration‐dependent diffusion on frontal instabilities and mass transfer in homogeneous porous media

Mass transfer plays an important role in influencing the efficiency of miscible displacements in solvent‐based processes in enhanced oil recovery. The mass transfer rate because of the pure molecular diffusion is very slow. However, this process can be greatly enhanced by the appearance of frontal instabilities called viscous fingering mechanisms, which are beneficial for improving the mixing and mass transfer between the injected solvent and oil. Instead of a piston‐like displacement, the interface between solvent and oil is very convoluted with intricate finger‐like patterns of the less viscous solvent intruding into the highly viscous oil. This intrusion significantly increases the surface area of contact of the two fluids and leads to more efficient mass transfer and mixing. Experimental measurements on the diffusion coefficients of two miscible fluids indicate that, instead of a constant diffusion coefficient (CDC), a concentration‐dependent diffusion coefficient (CDDC) is more realistic. In the present study, a CDDC relation in which the diffusion coefficient is exponentially proportional to concentration is adopted. Its effect on the development of frontal instabilities is examined through highly accurate nonlinear numerical simulations. The differences between the CDDC case and the widely assumed CDC case are discussed. Furthermore, the enhancement of frontal instabilities on mass transfer when the CDDC is considered is investigated at various mobility ratios and Peclet numbers. The special characteristics for the CDDC case indicate its important role in miscible displacements. Eventually, the relation of breakthrough time to parameters is correlated to accurately predict the breakthrough time in any CDDC scenario.     

Analysis of a Two‐Layer Nozzle‐and‐Diffuser Electroosmotic Micromixer

An electroosmotic micromixer with two‐layer microchannels of a nozzle‐and‐diffuser structure was proposed. Numerical analysis of the flow and mixing was performed using the three‐dimensional Poisson‐Boltzmann and Navier‐Stokes equations with a diffusion‐convection model for the species concentration. A parametric analysis of the microchannels was performed using three geometric parameters, i.e., length of the nozzle section, length of the diffusion section, and width of the nozzle end, to investigate the impact of each parameter on the mixing performance, which was quantified by a quantitative measure based on the mass variance. The numerical results were used to improve the design of the proposed micromixer, leading to a far better mixing performance with a much shorter channel length compared to an existing electroosmotic micromixer.     

Development of Two‐point Dynamic Method for Evaluating Extraction Columns

The steady‐state method by measuring the concentration profile along the column height is an effective way, but it is a time and material consumption method for large extraction columns. In order to investigate the axial‐mixing and mass transfer performances in a large pulsed‐sieve‐plate extraction column with the diameter of 150mm, a two‐point dynamic method with mass transfer based on the diffusion model has been developed. The results proved that the two‐point dynamic method has the advantages of good accuracy, simple boundary equations and flexible sampling position over the traditional single‐point dynamic method. It is a reliable tool for studying the axial‐mixing and the mass transfer performances.     

Adsorption of dyes and humic acid from water using chitosan‐encapsulated activated carbon

The adsorption isotherms and rates of two dyes and humic acid from aqueous solutions onto chitosan‐encapsulated activated carbon (CEAC) beads were measured at 30 °C. Such beads were prepared by mixing different weight percents of cuttlefish‐based chitosan (100%, 80%, 67%, and 55%) and rice‐based activated carbons. It was shown that the isotherms of dyes and humic acid were well fitted by the Freundlich equation. The adsorption capacity and rate could be enhanced when activated carbon was encapsulated with chitosan. Four simplified kinetic models including the pseudo‐first‐order equation, pseudo‐second‐order equation, intraparticle diffusion model, and the Elovich equation were tested to follow the adsorption processes. The adsorption of dyes was best described by the Elovich equation, but that of humic acid was best described by the intraparticle diffusion model. The kinetic parameters of each best‐fit model were calculated and are discussed in this paper. © 2002 Society of Chemical Industry     

Recycling of polypropylene‐based eco‐composites

BACKGROUND: Renewable resources and recyclable thermoplastic polymers provide an attractive eco‐friendly quality as well as environmental sustainability to the resulting natural fibre‐reinforced composites. The properties of polypropylene (PP)‐based composites reinforced with rice hulls or kenaf fibres were investigated with respect to their recyclability. Rice hulls from rice processing plants and natural lignocellulosic kenaf fibres from the bast of the plant Hibiscus cannabinus represent renewable sources that could be utilized for composites. Maleic anhydride‐grafted PP was used as a coupling agent to improve the interfacial adhesion between fillers and matrix. Composites containing 30 wt% reinforcement were manufactured by melt mixing and their mechanical and thermal properties were determined. The composites were then pelletized and reprocessed by melt mixing. Finally, structure/properties relationships were investigated as a function of the number of reprocessing cycles. RESULTS: It is found that the recycling processes do not induce very significant changes in flexural strength and thermal stability of the composites. In particular PP‐based composites reinforced with kenaf fibres are less sensitive to reprocessing cycles with respect to PP‐based composites reinforced with rice hulls. CONCLUSION: The response of PP‐based composites reinforced with rice hulls or kenaf fibres is promising since their properties remain almost unchanged after recycling processes. Moreover, the recycled composites are suitable for applications as construction materials for indoor applications. In fact, the flexural strength and modulus of these materials are comparable to those of conventional formaldehyde wood medium‐density fibreboards. Copyright © 2008 Society of Chemical Industry     

Development of Bigels Based on Stearic Acid–Rice Bran Oil Oleogels and Tamarind Gum Hydrogels for Controlled Delivery Applications

In the last few decades, different types of gels have been widely studied as potential drug delivery carriers. In this paper, we propose the synthesis of an oleogel, a tamarind gum hydrogel, and bigels for applications as drug delivery matrices. The oleogel was prepared by mixing stearic acid and rice bran oil, whereas the hydrogel was prepared by mixing tamarind gum with a hydroethanolic solution. Hydrogel‐in‐oleogel and oleogel‐in‐hydrogel bigels were prepared by mixing the hydrogel and the oleogel. The suitability of the formulations for controlled drug release applications was thoroughly examined using microscopy, Fourier transform infrared (FTIR) spectroscopy, as well as mechanical, electrical, thermal, drug release, and antimicrobial studies. An alteration in the microarchitecture of the bigels is observed when the oleogel and the hydrogel are mixed in varying proportions. The associative interactions within the formulations increase with the increase in the hydrogel content. The bigels exhibit the presence of stearic acid melting endotherm (associated with the oleogel) and water evaporation endotherm (associated with the hydrogel). This study suggests that the hydrogel has lowest bulk resistance compared to the other formulations. The structural breakdown of the bigels is composition‐dependent, and the bulk electrical resistance is mainly governed by the oleogel phase. An increase in the diffusion of the moxifloxacin HCl from the formulations is observed with the increase of the hydrogel proportion, which in turn increases the rate of release of the drug. The proposed formulations also exhibit good antimicrobial efficacy. The analysis of these properties suggests that specific formulations can be tailored by need‐based applications of the drug release rate.     

An Analysis of Mixing in a Typical Experimental Set‐up to Measure Nnucleation Rates of Precipitation Processes

Mixing in a typical experimental setup to measure nucleation rates in precipitation processes was assessed. To determine these rates as a function of the driving force for concomitant polymorphs, it is necessary to perform these experiments at constant supersaturation. Therefore, the mixing time must be shorter than the time for the first nuclei to appear. For fast precipitation processes complete mixing has to be achieved within milliseconds. The mixing performance of a wide angle Y‐mixer was studied to see whether this is possible. An analysis of characteristic mixing times as a function of the average energy dissipation rate showed that turbulent dispersion of the feed streams determined the rate of the mixing process. The characteristic time for turbulent dispersion was of the same order as an arbitrarily set residence time in the Y‐mixer. However, CFD simulations of the flow showed large variation in the spatial distribution of the dissipation rate and revealed unsatisfying macromixing.     

Purification process of phosphoric acid by vertical zone‐melting technique

The efficiency of vertical zone‐melting technique as a function of molten‐zone length, number of zone passes, molten‐zone velocity and phosphoric acid concentration on the purification process of phosphoric acid is studied. The results showed that adopting a relatively long molten‐zone length in the initial passes to effect a rapid movement of impurities and a short molten‐zone length in the later passes to obtain greater purification; additional zone‐passes cannot be made as impurities redistribution approaches limiting distribution; for stirring mechanism is not adopted in the present study and as diffusion is the only mixing process, slow molten‐zone velocity 3–6 mm h^?1 is better; the purification efficiency increases with phosphoric acid concentration, and 94–99.8% concentration of phosphoric acid is better. © 2012 Canadian Society for Chemical Engineering     

Thermo‐physical properties of n‐pentane/bitumen and n‐hexane/bitumen mixture systems

The gravity drainage as a result of viscosity reduction is the main governing mechanism of the solvent‐aided thermal bitumen recovery processes. Therefore, the density and viscosity of the diluted or heated bitumen are essential to predict the oil production rate. In this paper, we report thermo‐physical properties of n‐pentane/bitumen and n‐hexane/bitumen mixtures. The density and viscosity of Athabasca bitumen diluted with n‐pentane and n‐hexane were measured at different temperatures (30 to 190 °C), pressures (2 to 8 MPa), and solvent mass fractions (0.05 to 0.5). Various correlations and mixing rules proposed in the literature were examined to calculate the density and viscosity of the diluted bitumen. This study proposes appropriate mixing rules and generalized parameters for predicting the density and viscosity of solvent‐bitumen systems. Our findings will find applications in the design and simulation of heavy oil and bitumen solvent‐aided thermal recovery processes.     

Physical agglomeration behavior in preparation of cellulose‐N‐methyl morpholine N‐oxide hydrate solutions by simple mixing

The different melting temperatures of N‐methyl morpholine N‐oxide (NMMO) hydrates in the cellulose–NMMO hydrate solution may be explained by the rather different crystal structures of NMMO hydrates, which are determined by the amount of the hydrates. The preparative process of cellulose–NMMO hydrate solution may result in cellulose structural change from cellulose I to cellulose II, depending on the amount of the hydrate. Mixtures of cellulose and NMMO hydrate in a blender was changed from the granules to slurry with increasing mixing time at 60–70°C, which is below the melting point of the NMMO hydrate. In the case of 15 wt % cellulose–NMMO hydrate granules, which were made by mixing for 20 min, the melting points of various NMMO hydrates were obtained as 77.8°C (n = 0.83), 70.2°C (n = 0.97), and 69.7°C (n = 1.23), respectively, depending on the hydrate number. However, the melting points of cellulose–NMMO hydrate slurry and solution were shifted lower than those of cellulose granules, while the mixing time of slurry and solution are 25 and 35 min, respectively. These melting behaviors indicate instantaneous liquefaction of the NMMO hydrate and the diffusion of the NMMO hydrate into cellulose during mixing in a blender. When cellulose was completely dissolved in NMMO hydrate, the crystal structure of cellulose showed only cellulose II structure. In the cellulose–NMMO products of granules or slurry obtained by high‐speed mixing, which is a new preparation method, they still retained the original cellulose I structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1687–1697, 2004     

Sub‐stage PCA modelling and monitoring method for uneven‐length batch processes

For the characteristic of multistage of batch processes, a new PCA‐based sub‐stage division algorithm is proposed. This algorithm is based on the fact that production transition can be detected by analysing the loading matrixes and principal component matrixes, which reveal objectively evolvement of underlying process behaviours. Sub‐stage PCA models for each stage are built, and then extended to monitor the batch processes with uneven‐length durations by choosing the right sub‐stage model according to the principle of minimum similarity distance of principal component matrixes. With the proposed method, multi‐stage batch processes with durations of uneven‐length can be monitored effectively.     

Mixing of Shear‐Thinning Fluids with Dual Off‐Centred Impellers

Mixing times for inelastic shear‐thinning fluids in stirred tanks have been experimentally investigated using a combination of two off‐centred impellers operating in both co‐ and counter‐rotating modes. A colour‐discolouration technique based on fast acid‐base reaction was used for the determination of the mixing times as well as to reveal the possible presence of caverns and dead regions. A statistical plan of experiments allowed determining the effects of the impeller position, the rotational speed, the flow behaviour index, the impeller type and their mutual interactions. A stronger influence of the impeller position on mixing times was observed for both rotating modes with fluids exhibiting pronounced shear‐thinning. It was also found that segregated regions could be readily destroyed by dual off‐centred impellers as compared with the single centred impeller configuration. Mixed flow impellers were shown to be less efficient in terms of mixing times than radial flow impellers. Results obtained under the best operating conditions were compared to steady stirring experiments showing the potential and drawbacks of the proposed scenarios.     

Isocyanate crosslinking in two‐component waterborne coatings

Two‐component isocyanate curing films (2C NCO) yield a unique combination of properties, such as excellent mechanical and chemical resistances and very good film formation. For waterborne 2C NCO curing applications, dedicated polyols and polyisocyanates have been developed. Film properties such as drying rate, hardness development and conversion of isocyanate groups were found to depend strongly on the hydroxyl group content of the polyol, and with that the amount of polyisocyanate crosslinker which is added to a formulation. Tack‐free time of a film does not, however, correlate to the conversion of isocyanate groups and, hence, to crosslink density. The crosslink density and chemical resistances increase with hydroxyl group content, although not all chemical resistances increase (coffee resistance) along this trend. This is explained with hydrolysis of isocyanates resulting in more hydrophilic urea links. Due to early vitrification of the curing film, diffusion of crosslinker and, hence, mixing of polyol and crosslinker are impaired. As a result, with increasing hydroxyl group concentration, an increasing part of the crosslinker cannot react with hydroxyl groups and can only undergo hydrolysis. © 2018 Society of Chemical Industry     

Improving Aqueous Biphasic Hydroformylation of Unsaturated Oleochemicals Using a Jet‐Loop‐Reactor

In two case studies, the reaction performance of the aqueous biphasic hydroformylation of two industrially relevant oleochemicals, namely methyl 10‐undecenoate (case 1) and methyl oleate (case 2), is significantly improved by the use of a Jet‐Loop Reactor concept. Based on previously reported studies, only the two green and benign co‐solvents, 1‐butanol and isopropanol are applied, respectively, in the absence of any additional auxiliary. Both reactions benefit highly from using this special piece of equipment, specifically designed for improving gas–liquid–liquid mixing to create large interfacial areas with no moving internals. In case 1, the loading of the co‐solvent 1‐butanol is successfully reduced. For the first time significant yields (>40% after 1 h) are obtained in the absence of any co‐solvent, which is very beneficial, since aldehyde products and substrate form a pure product phase enabling straightforward separation. In case 2, the loading of the substrate methyl oleate is successfully increased from 6 to 30 wt% still showing satisfying productivity. At 15 wt%, the yield of the desired internal aldehydes in the jet‐loop reactor is increased by a factor of five compared to a stirred tank reactor after 3 h. Practical Applications: The production of aldehydes from hydroformylation of olefins is highly relevant for the chemical industry, since these can undergo numerous subsequent reactions, to form for instance alcohols, amines, and carboxylic acids. Generally, aldehydes from oleochemicals can serve as platform chemicals for gaining access to bifunctional molecules, which are interesting as polymer precursors. Performing hydroformylation with a water‐based solvent system enables efficient product separation from the aqueous catalyst phase for the realzation of more sustainable processes. By using the Jet‐Loop Reactor, the performance of the reaction system can be greatly improved addressing its practical relevance.     

Study on the release behavior and mechanism by monitoring the morphology changes of the large‐sized drug‐LDH nanohybrids

Mg‐Al layered double hydroxide (LDH) nanohybrids intercalated with ibuprofen (IBU) with particle sizes of 150–530 nm have been synthesized through hydrothermal and coprecipitation treatment in aqueous solution without any organic solvent. The in vitro drug release properties of as‐prepared IBU‐LDH nanohybrids are systematically studied and the kinetic simulation to the release profiles suggests that the release processes of the larger nanohybrids are mainly controlled by intraparticle diffusion. The morphology changes from thick sheet‐like into thin margin‐curved platelets for the larger nanohybrid particles, induced by the hydrophobic IBU anions aggregations located in the edge region of interlayer via ion‐exchange diffusion process, is firstly observed during the release process. Based on the SEM, HRTEM, XRD, FTIR, and UV‐vis analyses of the samples recovered at different release time, a release mechanism model of the as‐prepared IBU‐LDH nanohybrids is tentatively proposed along with their morphology changes during the whole release process. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Mixing and Crystallization Kinetics in Gas‐liquid Reactive Crystallization

A study of gas‐liquid reactive crystallization for CO₂‐BaCl₂‐H₂O system was performed in a continuous flow crystallizer. The influences of mixing on the crystallization kinetics of barium carbonate crystals were investigated. The mixing parameters are stirrer speed, feed concentration, gas‐flow rate, pH of solution, addition rate of NaOH solution, and mean residence time. Under pH‐stat operation, the crystallization mechanism can be assessed by the addition rate of NaOH solution, which acts as an indicator for the absorption rate of carbon dioxide. Assuming a size‐independent agglomeration mechanism, the nucleation rate, growth rate and agglomeration kernel can be obtained, simultaneously, at steady state, by the method of moments. Evidence shows that feed concentration, feed rate, gas‐flow rate, and stirrer speed have a significant influence on the nucleation rates and mean particle sizes. This shows the effect of micromixing. The crystallization mechanism tends to be reaction limited when the feed concentration of barium chloride solution is higher than 5 mM, while at lower stirrer speeds and feed concentrations, the mechanism tends to be both mixing and reaction controlled. The growth rate depends on the mean supersaturation value and the pH of the solution and the mass‐transfer resistance cannot be completely eliminated in this work. For a monodispersal collision model, in the viscous sub‐range of turbulence, the agglomeration kernel can be expressed as β ∝ d^{3 –1/4}, showing a low efficiency of collision. The result is also demonstrated by the agglomeration kernel expression. Comparison with a liquid‐liquid‐mixing reactive crystallization system is also discussed.