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Mixing Time (mixing + time)
Kinds of Mixing Time Selected AbstractsUltrasonic Investigation of the Effect of Vegetable Shortening and Mixing Time on the Mechanical Properties of Bread DoughJOURNAL OF FOOD SCIENCE, Issue 9 2009K.L. Mehta ABSTRACT:, Mixing is a critical stage in breadmaking since it controls gluten development and nucleation of gas bubbles in the dough. Bubbles affect the rheology of the dough and largely govern the quality of the final product. This study used ultrasound (at a frequency where it is sensitive to the presence of bubbles) to nondestructively examine dough properties as a function of mixing time in doughs prepared from strong red spring wheat flour with various amounts of shortening (0%, 2%, 4%, 8% flour weight basis). The doughs were mixed for various times at atmospheric pressure or under vacuum (to minimize bubble nucleation). Ultrasonic velocity and attenuation (nominally at 50 kHz) were measured in the dough, and dough density was measured independently from specific gravity determinations. Ultrasonic velocity decreased substantially as mixing time increased (and more bubbles were entrained) for all doughs mixed in air; for example, in doughs made without shortening, velocity decreased from 165 to 105 ms,1, although superimposed on this overall decrease was a peak in velocity at optimum mixing time. Changes in attenuation coefficient due to the addition of shortening were evident in both air-mixed and vacuum-mixed doughs, suggesting that ultrasound was sensitive to changes in the properties of the dough matrix during dough development and to plasticization of the gluten polymers by the shortening. Due to its ability to probe the effect of mixing times and ingredients on dough properties, ultrasound has the potential to be deployed as an online quality control tool in the baking industry. [source] Mixing Time in a Short Bubble ColumnTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2003M Ravinath Abstract Mixing time measurements have been carried out in a 0.2m I.D. short bubble column (Hc/D , 5) with different spargers and for different clear liquid height to diameter (HC/D) ratios. Superficial gas velocity has been varied in the range of 0.01m/s to 0.1m/s. Effect of bulk fluid viscosity on the mixing time has also been studied. The circulation cell model, with two fitted parameters viz. number of circulation cells, S and the inter-cell exchange velocity, Ve, has been used to predict and explain the variation in mixing time and the flow pattern in the short bubble column for different types of spargers. On a effectué des mesures de temps de mélange dans une colonne à bulles courte de 0,2 m de diamètre intérieur (Hc/D , 5) munie de différents aérateurs et pour différents rapports entre la hauteur de liquide clair et le diamètre (HC/D). On a fait varier la vitesse de gaz superficielle dans la gamme de 0,01-0,1 m/s. L'effet de la viscosité en masse du fluide sur le temps de mélange est également étudié. On a utilisé le modèle de zones de circulation, comprenant deux paramètres calés, à savoir le nombre de zones de circulation, S, et la vitesse d'échange entre les zones, Ve, pour prédire et expliquer la variation du temps de mélange et le profil d'écoulement dans la colonne à bulles courte pour différents types d'aérateurs. [source] Enhanced mixing of Newtonian fluids in a stirred vessel using impeller speed modulationTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2009Wei M. Yek Abstract This paper reports on an experimental study of mixing intensification using speed modulation of a six-blade Rushton turbine in a stirred vessel. Mixing times were measured using a non-intrusive technique based on direct visualisation of an acid-base reaction in a Newtonian fluid. The impeller speed modulation was achieved by using two waveforms: a square wave and a sine wave. The amplitude was fixed between a maximum Reynolds number of Remax,=,60 and minimum Reynolds numbers of Remin,=,40 or 30. The wave periods were varied (10, 20, or 40,s) in order to compare the effects of unsteady stirring on mixing performance. It was observed that a square wave protocol with the shortest wave period and the larger amplitude resulted in the shortest time to destroy the observed isolated mixing regions (IMRs), which are known to exist in stirred vessels operating at low Reynolds number. However, the sine wave protocol led to a slow diffusive mechanism in which IMR structures reached an asymptotic volume and remained visible even after several hours. The results are presented and discussed using digital photographs taken at different time intervals during experimentation. Ce papier présente une étude experimentale concernant l'intensification du mélange en modulant la vitesse d'une turbine de type Rushton a six palettes dans une cuve agitée. Les temps de mélanges sont mesurés avec une technique non-intrusive basée sur la visualisation directe d'une réaction acide-base au sein d'un fluide Newtonien. La modulation de la vitesse de la turbine a été realisée en utilisant deux formes d'ondes: une onde carrée et une onde sinusoidale. L'amplitude de chaque onde a été fixée entre un nombre de Reynolds maximal de Remax,=,60 et un nombre de Reynolds minimal de Remin,=,40 ou 30. Les périodes des ondes étaient variées (10, 20 ou 40,s) dans le but de comparer les effets du mélange non-stationnaire sur les performances du systeme. Il a été observé que le protocole suivant l'onde carrée avec la periode la plus courte et l'amplitude la plus large produisait le temps le plus court necessaire pour la destruction des zones de mélanges isolées (IMRs), dont l'éxistense est connue dans les cuves a mélange opérant aux nombres de Reynolds bas. Toutefois, le protocol suivant une onde sinusoidale a conduit a un mechanisme de diffusion lent dans lequel les IMRs atteignies un volume symptotique et sont restées visibles après plusieurs heures. Les résultats sont présentés and discutés en utilisant des photographes digitales prises à des intervales de temps differents durant les éxperiences. [source] Effect of radial angle on mixing time for a double jet mixerASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2010P. Manjula Abstract Mixing is one of the common unit operations employed in chemical industries. It is used for blending of liquids, flocculation, homogenization of mixtures, ensuring proper heat and mass transfer in various operations, prevention of deposition of solid particles, etc. Earlier research aspects were focused on experimental estimation of mixing time and proposing suitable correlations for the prediction of mixing time, the recent one being on flow visualization. However, most of the results reported in the literature deal with liquid flow with multi jets, whereas the effect of radial angle on mixing time was not studied. This study describes the effect of radial angle on mixing time as determined by experiment and simulation. A computational fluid dynamics (CFD) modeling is done for a jet mixing tank having two jets for a water,water system. Nozzle configuration for jet1 was fixed on the basis of our earlier studies (2/3rd position, flow rate 9l/m, nozzle angle 45° and nozzle diameter 10 mm). Mixing times were estimated for different jet2 configurations (jet angle 30°, 45° and 60°; radial angles 60°, 120°, 180°) located at different tank heights (2/3rd and 1/3rd from the bottom of the tank). The results obtained for mixing time for jet mixing in a tank with two jets are analyzed and the suitable nozzle angle, radial angle and position are proposed for the jet2 of the jet mixer considered in the present study. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Microporous Silica Hollow Microspheres and Hollow Worm-Like Materials: A Simple Method for Their Synthesis and Their Application in Controlled ReleaseEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2010Mingwei Zhao Abstract Hollow silica microspheres and hollow worm-like materials were synthesized by using a simple method with the aid of 1-dodecyl-3-methylimidazolium bromide (C12mimBr). Hollow silica microspheres were initially produced by utilizing the combination of evaporation and an emulsion template. At a longer mixing time, the microspheres fused to form hollow worm-like silica materials due to the fusion of the emulsion templates. The resultant silica materials were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and nitrogen adsorption/desorption. Both the hollow silica microspheres and the hollow worm-like materials are microporous. On the basis of experimental observations and the resulting products, a plausible formation mechanism is proposed. Preliminary tests demonstrate that the hollow silica microspheres and worm-like materials are capable of being loaded with Rhodamine B and releasing it, thus showing a great potential in controlled delivery applications. [source] Synthesis, Protonation and CuII Complexes of Two Novel Isomeric Pentaazacyclophane Ligands: Potentiometric, DFT, Kinetic and AMP Recognition StudiesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 1 2009Andrés G. Algarra Abstract The synthesis and coordination chemistry of two novel ligands, 2,6,9,12,16-pentaaza[17]metacyclophane (L1) and 2,6,9,12,16-pentaaza[17]paracyclophane (L2), is described. Potentiometric studies indicate that L1 and L2 form a variety of mononuclear complexes the stability constants of which reveal a change in the denticity of the ligand when moving from L1 to L2, a behaviour that can be qualitatively explained by the inability of the paracyclophanes to simultaneously use both benzylic nitrogen atoms for coordination to a single metal centre. In contrast, the formation of dinuclear hydroxylated complexes is more favoured for the paraL2 ligand. DFT calculations have been carried out to compare the geometries and relative energies of isomeric forms of the [CuL]2+ complexes of L1 and L2 in which the cyclophane acts either as tri- or tetradentate. The results indicate that the energy cost associated with a change in the coordination mode of the cyclophane from tri- to tetradentate is moderate for both ligands so that the actual coordination mode can be determined not only by the characteristics of the first coordination sphere but also by the specific interactions with additional nearby water molecules. The kinetics of the acid promoted decomposition of the mono- and dinuclear CuII complexes of both cyclophanes have also been studied. For both ligands, dinuclear complexes convert rapidly to mononuclear species upon addition of excess acid, the release of the first metal ion occurring within the mixing time of the stopped-flow instrument. Decomposition of the mononuclear [CuL2]2+ and [CuHL2]3+ species occurs with the same kinetics, thus showing that protonation of [CuL2]2+ occurs at an uncoordinated amine group. In contrast, the [CuL1]2+ and [CuHL1]3+ species show different decomposition kinetics indicating the existence of significant structural reorganisation upon protonation of the [CuL1]2+ species. The interaction of AMP with the protonated forms of the cyclophanes and the formation of mixed complexes in the systems Cu,L1 -AMP, Cu,L2 -AMP, and Cu,L3 -AMP, where L3 is the related pyridinophane containing the same polyamine chain and 2,6-dimethylpyridine as a spacer, is also reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] A reactive polymer for toughening epoxy resinJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2010Hsu-Chiang Kuan Abstract Epoxy resins are hardly toughened by low weight content of tougheners. In this study, 5 wt % polyurea was adopted to significantly toughen piperidine-cured epoxy, as fracture toughness improved from 0.78 to 1.98 MPa m1/2. We focused on the reactions and morphology evolution of epoxy/polyurea mixture. The polyurea molecular weight was reduced by the exchange reactions of polyurea with epoxy during mixing, as evidenced by gel permeation chromatograph and Fourier transform infrared spectroscopy. As a result, epoxy molecules were chemically bonded with polyurea, improving particle content and interface thickness. Transmission electron microscope observation shows that (a) polyurea in situ formed nanoparticles in matrix which subsequently aggregate into micron-sized particles of thick interface with matrix; and (b) the particles became less stainable with increasing the mixing time, because the reactions promoted high levels of crosslink density of the particles which were thus more resistant to the diffusion of staining chemicals. Longer mixing time improved, obviously, the fracture toughness of epoxy/polyurea composite. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] EFFECT OF COMPOSITION OF GLUTHNIN SUBFRACTIONS ON RHEOLOGICAL PROPERTIES OF WHEATJOURNAL OF FOOD BIOCHEMISTRY, Issue 4 2000S. JOOD ABSTRACT Gluten extracted from defatted flours of cv. Aubaine (extra-strong), Hereward (strong) and Riband (weak) was separated into five different fractions (R2 to R6) by sequential centrifugation and addition of sodium chloride. A seven-minute mixing time was used to carry out fractionation on the basis of depolymerization of glutenin macropolymers (GMP). Depolymerization of GMP occurred at much higher rates in dough of the weak cultivar compared to the strong and extra-strong cultivars. Polypeptide compositions of different ghttenin fractions were determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis under reduced and non-reduced conditions, followed by densitometric scanning of stained patterns. The amount of HMW-glutenin subunits decreased and LMW-glutenin subunits increased correspondingly in each cultivar with the fractionation from R2 to R6. The rheological behavior of the fractions was analyzed by small deformation rheological tests (strain sweep and frequency tests). The high molecular weight fraction (R2) from extra-strong wheat had a higher vahte of G' and a lower tan , value as compared to strong and weak bread-making wheats. The moduli of HMW glutenin fractions (R2 and R3) were frequency independent and promoted the network properties, whereas moduli of LMW glutenin fractions were frequency dependent and gave rise to a plasticizing effect. Therefore, it was concluded from the present studies that HMW-glutenin subunits are not the only factors governing good bread-making quality but their proportions in relation to low molecular weight glutenin subunits is equally important in sinking a balance between viscous and elastic properties essential for bread making performance. [source] IDENTIFICATION OF IMPORTANT PRODUCTION VARIABLES AFFECTING HARD PRETZEL QUALITYJOURNAL OF FOOD QUALITY, Issue 3 2005N. YAO ABSTRACT The objective of this study was to determine the importance of raw material and processing variables that influence pretzel quality by utilizing a screening experiment design. Eleven variables were selected based on preliminary experiments, and a two-level-11-factor (211) fractional factorial experimental design was used to screen the variables. Several responses were measured for dough before and after extrusion, for half-baked and fully baked pretzels. These responses are important indicators of consistency and quality during pretzel processing. Results indicated that flour protein content, the amount of water added to make dough and dough mixing time were important variables influencing dough behavior. Caustic concentration affected brightness of half-baked pretzels but did not influence the color of the final product. Baking time was the most important factor for both half-baked product and final product qualities. The hardness of fully baked pretzels was influenced by baking time, temperature in baking oven zone 1, drying time and drying temperature. The color of final products was significantly influenced by baking time, while both baking time and drying temperature affected the moisture content of the final product. A key observation was that none of the raw material or dough processing parameters, within the range tested, influenced final pretzel quality as defined by pretzel moisture content, hardness or color. [source] Ultrasonic Investigation of the Effect of Vegetable Shortening and Mixing Time on the Mechanical Properties of Bread DoughJOURNAL OF FOOD SCIENCE, Issue 9 2009K.L. Mehta ABSTRACT:, Mixing is a critical stage in breadmaking since it controls gluten development and nucleation of gas bubbles in the dough. Bubbles affect the rheology of the dough and largely govern the quality of the final product. This study used ultrasound (at a frequency where it is sensitive to the presence of bubbles) to nondestructively examine dough properties as a function of mixing time in doughs prepared from strong red spring wheat flour with various amounts of shortening (0%, 2%, 4%, 8% flour weight basis). The doughs were mixed for various times at atmospheric pressure or under vacuum (to minimize bubble nucleation). Ultrasonic velocity and attenuation (nominally at 50 kHz) were measured in the dough, and dough density was measured independently from specific gravity determinations. Ultrasonic velocity decreased substantially as mixing time increased (and more bubbles were entrained) for all doughs mixed in air; for example, in doughs made without shortening, velocity decreased from 165 to 105 ms,1, although superimposed on this overall decrease was a peak in velocity at optimum mixing time. Changes in attenuation coefficient due to the addition of shortening were evident in both air-mixed and vacuum-mixed doughs, suggesting that ultrasound was sensitive to changes in the properties of the dough matrix during dough development and to plasticization of the gluten polymers by the shortening. Due to its ability to probe the effect of mixing times and ingredients on dough properties, ultrasound has the potential to be deployed as an online quality control tool in the baking industry. [source] Experimental and numerical investigation of the precipitation of barium sulfate in a rotating liquid film reactorAICHE JOURNAL, Issue 8 2009Shengchang Guo Abstract Precipitation of nanosized barium sulfate in a rotating liquid film reactor (RLFR) has been investigated experimentally and through simulations based on the computational fluid dynamics technique including the population balance equation coupled with the Navier,Stokes equations, renormalization group k,, model equations, and species transport equations. A comparative experiment was carried out involving conventional precipitation in a flask. The structure of the precipitate was identified by powder X-ray diffraction (PXRD), which showed that the crystals obtained using the RLFR were smaller in size than those obtained in the flask. Transmission electron microscopy (TEM) images demonstrated that the crystals produced by the two different processes had different morphologies. Further detailed experiments involving varying the operating parameters of the RLFR were performed to investigate the effects on crystal size distribution (CSD). Increasing the speed of the rotor in the RLFR in the range 1000,5000 rpm or increasing the rotor-stator gap in the range 0.1,0.5 mm resulted in a decrease in particle size and narrower particle size distributions. The simulation results suggested that turbulent effects and reaction processes in the effective reactor space were directly related to rotor speed and rotor-stator gap. The simulated volume weighted mean diameter and CSD of particles of barium sulfate were almost identical to the corresponding experimental results obtained using TEM and laser particle size analyzer. The effects of other parameters such as the Kolmogorov scale and competition between induction time and mixing time are also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Geometric design of fluid segments in microreactors using dimensionless numbersAICHE JOURNAL, Issue 4 2006Nobuaki Aoki Abstract In microreactors, reactant fluids are split into many fluid segments and then fed into the reactors to shorten mixing time. Two dimensionless numbers are introduced to represent effects of geometric design factors of fluid segments, such as shapes and arrangements, on reactor performance, namely mixing rate and product yield and selectivity: the ratio of reaction rate to diffusion rate and the aspect ratio of the mean diffusion length in the two-dimensional (2-D) directions in the reactor cross section. Methods to determine these numbers are also proposed. To examine the validity of these numbers on estimating the reactor performance, we compare product yields between each pair of reactors having the different geometric design factors but the same dimensionless numbers using computational fluid dynamics simulations. The results show that these numbers serve as the indices for estimating the reactor performance. Using these numbers, design guidelines for geometry of fluid segments are also discussed. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source] Correlating dough elastic recovery during sheeting with flour analyses and rheological propertiesJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 15 2008DanQiu Ren Abstract The dough rheological behaviors of three flours having different chemical and physical properties were measured, as were changes in thickness and snapback (thickness of the machined dough sheet relative to the roll gap), immediately following sheeting. Dough snapback was determined to be a function of processing parameters, reduction ratio, and dough rest time, as well as different flour properties. The predication equation for dough snapback is based on multiple flour properties and sheeting conditions. Higher protein flours normally have stronger dough properties and larger snapback, compared with low protein flours, but also depend on the protein quality. The snapback will increase either with increased protein content, reduction ratio, or reduced rest time. Dough snapback using flour made by blending two flours 50/50 had intermediate values between the two original flours. Among the variables, Mixograph work, reduction ratio, and dough rest time were the main factors affecting the elastic characteristics of the doughs. Minimum snapback occurred with the weakest flour experiencing the longest rest time and the smallest reduction ratio. A linear 7-factor equation was found to predict the snapback of several flours, by combining reduction ratio, dough rest time, Mixograph work, peak height, and mixing time, Alveograph P/L, and protein content. Only three factor combinations of Mixograph work, reduction ratio, and rest time were needed to develop a second order equation for predicting snapback. Copyright © 2008 Society of Chemical Industry [source] Thermo-Mechanical Degradation of LDPE-Based NanocompositesMACROMOLECULAR MATERIALS & ENGINEERING, Issue 7 2007Nadka Tzankova Dintcheva Abstract Thermo-mechanical degradation of LDPE-based nanocomposites was studied by mainly investigating the rheological properties. For all of the investigated processing conditions, the viscosity of the nanocomposites was higher than that of the pure-LDPE matrix, but on increasing the severity of the mixing conditions, the difference between the viscosity of the nano-filled polymer and that of the pure LDPE decreased. The X-ray traces of the nanocomposites suggest that intercalation has been achieved during the melt, when less-severe processing conditions were used. At severe processing conditions (longer mixing time, high temperature and shear stress) the thermo-mechanical degradation was accelerated, possibly due to the loss of mass from the organoclay galleries. The variations of the viscosity in the presence of two organo-modified montmorillonite (MMt) clays were compared to the ones observed with a MMt clay at different processing conditions. [source] Reactively and physically compatibilized immiscible polymer blends: stability of the copolymer at the interfaceMACROMOLECULAR SYMPOSIA, Issue 1 2003Charef Harrats Abstract This paper reports on the interfacial behaviour of block and graft copolymers used as compatibilizers in immiscible polymer blends. A limited residence time of the copolymer at the interface has been shown in both reactive blending and blend compatibilization by preformed copolymers. Polystyrene (PS)/polyamide6 (PA6), polyphenylene oxide (PPO)/PA6 and polymethylmethacrylate (PMMA)/PA6 blends have been reactively compatibilized by a styrene-maleic anhydride copolymer SMA. The extent of miscibility of SMA with PS, PPO and PMMA is a key criterion for the stability of the graft copolymer at the interface. For the first 10 to 15 minutes of mixing, the in situ formed copolymer is able to decrease the particle size of the dispersed phase and to prevent it from coalescencing. However, upon increasing mixing time, the copolymer leaves the interface which results in phase coalescence. In PS/LDPE blends compatibilized by preformed PS/hydrogenated polybutadiene (hPB) block copolymers, a tapered diblock stabilizes efficiently a co-continuous two-phase morphology, in contrast to a triblock copolymer that was unable to prevent phase coarsening during annealing at 180°C for 150 minutes. [source] Precision 1H,1H distance measurement via 13C NMR signals: utilization of 1H,1H double-quantum dipolar interactions recoupled under magic angle spinning conditionsMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2004Yoh Matsuki Abstract We applied the POST-C7 DQ-dipolar recoupling pulse sequence to the measurement of 1H,1H distances with high precision. The spectral resolution is enhanced by detecting the 1H magnetization via 13C signals. A least-squares fitting of the build-up curve of the transferred magnetization to the exact numerical simulations yielded a 1H,,1H, distance of 248 ± 4 pm for fully 13C-labeled L -valine. This distance agrees with the neutron diffraction study. The negative transferred magnetization clearly indicates that the direct DQ 1H,1H dipolar couplings have the largest effect. The signal for the magnetization transfer builds up rapidly by the direct 1H,1H dipolar coupling, and decreases to zero at longer mixing time when the relayed magnetization transfer becomes significant. This large intensity change of the signal leads to the high precision in the distance measurement. We inspected factors that limit the effective bandwidth of the POST-C7 recoupling for the 1H and 13C homonuclear spin systems. The spin interactions at times shorter than the cycle time of the C7 sequence were also evaluated to measure the distances. The carbon-detected 2D 1H DQ mixing experiment was demonstrated for the measurement of multiple 1H,1H distances. Copyright © 2004 John Wiley & Sons, Ltd. [source] Genotypic and temperature effects on wheat grain yield and quality in a hot irrigated environmentPLANT BREEDING, Issue 4 2006I. S. A. Tahir Abstract High temperature influences both grain yield and end-use quality of wheat. The objectives of this study were to evaluate the performance of selected wheat genotypes under heat stress and to examine the effects of high temperatures during grain filling on grain yield and end-use quality parameters. Fifteen bread wheat genotypes in 2000/2001 and 18 genotypes in 2002/2003 were evaluated under the optimum and late-sowing conditions of the irrigated hot environment of the Gezira Research Farm, Wad Medani, Sudan. The genotypes comprised released varieties and elite lines from the Sudanese wheat improvement programme. Data collected included grain yield, grain weight and grain end-use quality including protein content, protein composition, SDS sedimentation values (SDSS) and gluten strength as determined by mixograph analyses. High temperatures significantly decreased grain yield by decreasing grain weight. Although genotypes exhibited variation in magnitude of response, results indicated that high temperature during grain filling increased both soluble and insoluble protein contents, SDSS, mixograph peak height (MPH) and the descending slope at 2 min past peak (MDS). In contrast, mixograph peak time (MPT) and the curve width at 2 min past peak (MCW) were significantly decreased. Flour protein correlated positively with SDSS, MPH and MDS and negatively with MCW. MPT correlated negatively with MDS and positively with MCW. Results indicate that high temperature increased both soluble and insoluble protein contents, SDSS and MPH, and hence the gluten strength, but decreased flour mixing time and tolerance and hence the dough elasticity. Variation observed among genotypes suggests that grain end-use quality could be improved under high temperature conditions utilizing the available variability; however, it might require evaluation under various growing conditions. [source] Polymer,nanofiller prepared by high-energy ball milling and high velocity cold compactionPOLYMER COMPOSITES, Issue 3 2008Bruska Azhdar High-energy ball milling using comilling in a solid state by low-temperature mechanical alloying to prepare nickel-ferrite (NiFe2O4) nanopowders and ultrafine poly(methyl methacrylate) (PMMA), dispersing nanoparticles in a polymer matrix, and a uniaxial high-velocity cold compaction process using a cylindrical, hardened steel die and a new technique with relaxation assists have been studied. The focus has been on the particle size distributions of the nanocomposite powder during the milling and on the surface morphology of the nanocomposite-compacted materials after compaction with and without relaxation assists. Experimental results for different milling systems are presented showing the effects of milling time and material ratio. It was found that a longer mixing time give a higher degree of dispersion of the nanopowder on the PMMA particle surfaces. Furthermore, with increasing content of NiFe2O4 nanopowder, the reduction of the particle size was more effective. Different postcompacting profiles, i.e. different energy distributions between the upper and lower parts of the compacted powder bed, lead to different movements of the various particles and particle layers. Uniformity, homogeneity, and densification on the surfaces in the compacted powder are influenced by the postcompacting magnitude and direction. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by reducing the expansion of the compacted volume and by reducing the different opposite velocities, giving the compacted composite bed a more homogeneous opposite velocity during the decompacting stage and reducing the delay time between the successive pressure waves. POLYM. COMPOS., 2008. © 2007 Society of Plastics Engineers [source] Reactive blending of poly(ethylene terephthalate)(pet)/ poly(ethylene 2,6-naphthalate)(pen).POLYMER ENGINEERING & SCIENCE, Issue 8 2002I: Effect of mixing conditions on chain structure Reactive blending of poly(ethylene terephthalate)/poly(ethylene naphthalene 2,6dicarboxylate) with addition of 2,2'-bis(1,3-oxazoline) (BOZ) has been studied under various mixing conditions for the different compositions. The transesterification level, the sequence length of both PET and PEN short blocks, and the degree of randomness were estimated using1H NMR. The results indicate that both mixing time and temperature are the primary factors controlling the transesterification, while the chain extender BOZ can significantly accelerate the transesterification between PET and PEN at 275°C. The composition also, to some extent, influences the transerification level as the mixing time is increased. As a consequence of transesterification proceeding, the sequence structures of the reactive blends are also markedly changed, which corresponds to a transfer from an initial block structure to a multiblock structure with higher randomness. The change in the microstructure of the reactive blends has also been analyzed by a Bernoullian statistics model. The effect of the BOZ on the intrinsic viscosity of the reactive blends is discussed. [source] The mixing time of Glauber dynamics for coloring regular treesRANDOM STRUCTURES AND ALGORITHMS, Issue 4 2010Leslie Ann Goldberg Abstract We consider Metropolis Glauber dynamics for sampling proper q -colorings of the n -vertex complete b -ary tree when 3 , q , b/(2lnb). We give both upper and lower bounds on the mixing time. Our upper bound is nO(b/ log b) and our lower bound is n,(b/(q log b)), where the constants implicit in the O() and ,() notation do not depend upon n, q or b. © 2010 Wiley Periodicals, Inc. Random Struct. Alg., 2010 [source] Rapid mixing of Gibbs sampling on graphs that are sparse on averageRANDOM STRUCTURES AND ALGORITHMS, Issue 2 2009Elchanan Mossel Abstract Gibbs sampling also known as Glauber dynamics is a popular technique for sampling high dimensional distributions defined on graphs. Of special interest is the behavior of Gibbs sampling on the Erd,s-Rényi random graph G(n,d/n), where each edge is chosen independently with probability d/n and d is fixed. While the average degree in G(n,d/n) is d(1 - o(1)), it contains many nodes of degree of order log n/log log n. The existence of nodes of almost logarithmic degrees implies that for many natural distributions defined on G(n,p) such as uniform coloring (with a constant number of colors) or the Ising model at any fixed inverse temperature ,, the mixing time of Gibbs sampling is at least n1+,(1/log log n). Recall that the Ising model with inverse temperature , defined on a graph G = (V,E) is the distribution over {±}Vgiven by . High degree nodes pose a technical challenge in proving polynomial time mixing of the dynamics for many models including the Ising model and coloring. Almost all known sufficient conditions in terms of , or number of colors needed for rapid mixing of Gibbs samplers are stated in terms of the maximum degree of the underlying graph. In this work, we show that for every d < , and the Ising model defined on G (n, d/n), there exists a ,d > 0, such that for all , < ,d with probability going to 1 as n ,,, the mixing time of the dynamics on G (n, d/n) is polynomial in n. Our results are the first polynomial time mixing results proven for a natural model on G (n, d/n) for d > 1 where the parameters of the model do not depend on n. They also provide a rare example where one can prove a polynomial time mixing of Gibbs sampler in a situation where the actual mixing time is slower than npolylog(n). Our proof exploits in novel ways the local tree like structure of Erd,s-Rényi random graphs, comparison and block dynamics arguments and a recent result of Weitz. Our results extend to much more general families of graphs which are sparse in some average sense and to much more general interactions. In particular, they apply to any graph for which every vertex v of the graph has a neighborhood N(v) of radius O(log n) in which the induced sub-graph is a tree union at most O(log n) edges and where for each simple path in N(v) the sum of the vertex degrees along the path is O(log n). Moreover, our result apply also in the case of arbitrary external fields and provide the first FPRAS for sampling the Ising distribution in this case. We finally present a non Markov Chain algorithm for sampling the distribution which is effective for a wider range of parameters. In particular, for G(n, d/n) it applies for all external fields and , < ,d, where d tanh(,d) = 1 is the critical point for decay of correlation for the Ising model on G(n, d/n). © 2009 Wiley Periodicals, Inc. Random Struct. Alg., 2009 [source] The simple random walk and max-degree walk on a directed graphRANDOM STRUCTURES AND ALGORITHMS, Issue 3 2009Ravi Montenegro Abstract We bound total variation and L, mixing times, spectral gap and magnitudes of the complex valued eigenvalues of general (nonreversible nonlazy) Markov chains with a minor expansion property. The resulting bounds for the (nonlazy) simple and max-degree walks on a (directed) graph are of the optimal order. It follows that, within a factor of two or four, the worst case of each of these mixing time and eigenvalue quantities is a walk on a cycle with clockwise drift. © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2009 [source] The evolution of the mixing rate of a simple random walk on the giant component of a random graphRANDOM STRUCTURES AND ALGORITHMS, Issue 1 2008N. Fountoulakis Abstract In this article we present a study of the mixing time of a random walk on the largest component of a supercritical random graph, also known as the giant component. We identify local obstructions that slow down the random walk, when the average degree d is at most O(), proving that the mixing time in this case is ,((n/d)2) asymptotically almost surely. As the average degree grows these become negligible and it is the diameter of the largest component that takes over, yielding mixing time ,(n/d) a.a.s.. We proved these results during the 2003,04 academic year. Similar results but for constant d were later proved independently by Benjamini et al. in 3. © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2008 [source] Mixing in time and space for lattice spin systems: A combinatorial viewRANDOM STRUCTURES AND ALGORITHMS, Issue 4 2004Martin Dyer The paper considers spin systems on the d -dimensional integer lattice ,d with nearest-neighbor interactions. A sharp equivalence is proved between decay with distance of spin correlations (a spatial property of the equilibrium state) and rapid mixing of the Glauber dynamics (a temporal property of a Markov chain Monte Carlo algorithm). Specifically, we show that if the mixing time of the Glauber dynamics is O(n log n) then spin correlations decay exponentially fast with distance. We also prove the converse implication for monotone systems, and for general systems we prove that exponential decay of correlations implies O(n log n) mixing time of a dynamics that updates sufficiently large blocks (rather than single sites). While the above equivalence was already known to hold in various forms, we give proofs that are purely combinatorial and avoid the functional analysis machinery employed in previous proofs. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2004 [source] A bounding chain for Swendsen-WangRANDOM STRUCTURES AND ALGORITHMS, Issue 1 2003Mark Huber The greatest drawback of Monte Carlo Markov chain methods is lack of knowledge of the mixing time of the chain. The use of bounding chains solves this difficulty for some chains by giving theoretical and experimental upper bounds on the mixing time. Moreover, when used with methodologies such as coupling from the past, bounding chains allow the user to take samples drawn exactly from the stationary distribution without knowledge of the mixing time. Here we present a bounding chain for the Swendsen-Wang process. The Swendsen-Wang bounding chain allow us to efficiently obtain exact samples from the ferromagnetic Q-state Potts model for certain classes of graphs. Also, by analyzing this bounding chain, we will show that Swendsen-Wang is rapidly mixing over a slightly larger range of parameters than was known previously. © 2002 Wiley Periodicals, Inc. Random Struct. Alg., 22: 43,59, 2002 [source] Mixing Time in a Short Bubble ColumnTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2003M Ravinath Abstract Mixing time measurements have been carried out in a 0.2m I.D. short bubble column (Hc/D , 5) with different spargers and for different clear liquid height to diameter (HC/D) ratios. Superficial gas velocity has been varied in the range of 0.01m/s to 0.1m/s. Effect of bulk fluid viscosity on the mixing time has also been studied. The circulation cell model, with two fitted parameters viz. number of circulation cells, S and the inter-cell exchange velocity, Ve, has been used to predict and explain the variation in mixing time and the flow pattern in the short bubble column for different types of spargers. On a effectué des mesures de temps de mélange dans une colonne à bulles courte de 0,2 m de diamètre intérieur (Hc/D , 5) munie de différents aérateurs et pour différents rapports entre la hauteur de liquide clair et le diamètre (HC/D). On a fait varier la vitesse de gaz superficielle dans la gamme de 0,01-0,1 m/s. L'effet de la viscosité en masse du fluide sur le temps de mélange est également étudié. On a utilisé le modèle de zones de circulation, comprenant deux paramètres calés, à savoir le nombre de zones de circulation, S, et la vitesse d'échange entre les zones, Ve, pour prédire et expliquer la variation du temps de mélange et le profil d'écoulement dans la colonne à bulles courte pour différents types d'aérateurs. [source] Effect of radial angle on mixing time for a double jet mixerASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2010P. Manjula Abstract Mixing is one of the common unit operations employed in chemical industries. It is used for blending of liquids, flocculation, homogenization of mixtures, ensuring proper heat and mass transfer in various operations, prevention of deposition of solid particles, etc. Earlier research aspects were focused on experimental estimation of mixing time and proposing suitable correlations for the prediction of mixing time, the recent one being on flow visualization. However, most of the results reported in the literature deal with liquid flow with multi jets, whereas the effect of radial angle on mixing time was not studied. This study describes the effect of radial angle on mixing time as determined by experiment and simulation. A computational fluid dynamics (CFD) modeling is done for a jet mixing tank having two jets for a water,water system. Nozzle configuration for jet1 was fixed on the basis of our earlier studies (2/3rd position, flow rate 9l/m, nozzle angle 45° and nozzle diameter 10 mm). Mixing times were estimated for different jet2 configurations (jet angle 30°, 45° and 60°; radial angles 60°, 120°, 180°) located at different tank heights (2/3rd and 1/3rd from the bottom of the tank). The results obtained for mixing time for jet mixing in a tank with two jets are analyzed and the suitable nozzle angle, radial angle and position are proposed for the jet2 of the jet mixer considered in the present study. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Effect of the geometry on the performance of the MaxblendÔ impeller with viscous Newtonian fluidsASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Yoann Guntzburger Abstract Experimental and numerical investigations have been carried out to assess the effect of the vessel geometry (number of baffles) and the shape of the Maxblend impeller (configuration of the bottom paddle and angle of the upper grid) with viscous Newtonian fluids in the laminar and lower turbulent mixing regimes. Two parameters have been explored namely the power consumption of the impeller and the mixing time. Videos of the discoloration process have also been taken to get access to the mixing patterns. A nominal 50-l vessel has been used in the experiments. The number of baffles has been varied from 1 to 4, and the bottom paddle has been modified by making openings to allow flow passage. Finally, the effect of using a straight grid in the upper part of the Maxblend has been studied under the same conditions. It is shown that the number of baffles does not have a significant effect on the power consumption, the mixing evolution, and the mixing time regardless of the flow regime. Making openings in the bottom paddle allows for the destruction of the segregated zones at the bottom of the tank at the expense of a very slight increase in power consumption and mixing time. Finally, the power and mixing time are both increased with a straight grid Maxblend. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] CFD study of mixing characteristics of bubble column and external loop airlift reactorASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2008S. Roy Abstract In the present work, a computational fluid dynamics study has been carried out to bring out the mixing characteristics of bubble column and external loop airlift reactor. A comparison of mixing time for two reactors has been presented on the basis of the same reactor volume as well as the total power input. The CFD model was validated by simulating flow, both in bubble column and external loop airlift reactor. An agreement was observed between the predicted and the experimental data available in the published literature. The validated CFD model has been extended for the simulation of the mixing time for both the reactors. The CFD predictions of mixing time show good agreement with the experimental values published in literature. A systematic numerical study was then carried out to bring out the mixing characteristics of both the reactors. Copyright © 2008 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Development of a simultaneous liquid,liquid extraction and chiral derivatization method for stereospecific GC-MS analysis of amphetamine-type stimulants in human urine using fractional factorial designBIOMEDICAL CHROMATOGRAPHY, Issue 9 2008W. R. Wan Aasim Abstract A stereospecific gas chromatography,mass spectrometry analysis method for amphetamine-type stimulants in human urine was recently developed. For maximum efficiency, liquid,liquid extraction and chiral derivatization of the analytes using (R)-(,)- , -methoxy- , -(trifluoromethyl)phenylacetyl chloride were performed simultaneously. The effects of (1) use of saturated sodium chloride in 2.0 m sodium hydroxide, (2) extraction solvent volume, (3) percentage of triethylamine, (4) derivatization reagent volume, (5) sample mixing time, (6) incubation temperature and (7) incubation time on method sensitivity and variability were assessed using a two-level, eight-run Plackett,Burman design followed by a fold-over design. The use of saturated sodium chloride solution and the derivatization reagent volume were significant factors (ANOVA, p < 0.01). The saturated sodium chloride solution decreased sensitivity whereas an increased volume of derivatization reagent increased sensitivity. Calibration curves for all analytes were linear between 5 and 500 µg/L, with correlation coefficients of >0.99. Detection limits were ,2.3 µg/L and quantitation limits ,7.7 µg/L. Reproducibility was good, with relative standard deviation values at <20%. Recovery exceeded 100% for most analytes. The experimental design enabled easy and rapid identification of significant factors using a minimal number of samples. This method has good potential for studies requiring rapid and sensitive stereospecific quantification of amphetamine-type stimulants. Copyright © 2008 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||||||||