The gaussian distribution of θ1/3 — a two-parameter statistical model for approximating RTD's in CSTR's

The normal or Gaussian distribution of Θ^1/3 is shown to give a satisfactory approximation of RTD's in continuous-flow stirred tanks, whether mixing is ideal or not, and its mean Θ_m^1/3 and standard deviation σ are, in each case, determined from the F-curve as a function of Θ^1/3 on linear probability paper. For ideal mixing, Θ_m^1/3 and σ approach respectively the values of 0.89 and 0.32. In this case, the approximation is considered satisfactory for values of the relative time Θ ranging from 0.05, for F = 0.01, to 9.21, for F = 0.9999. As mixing conditions depart more and more for ideality, Θ_m^1/3 assumes progressively lower values than 0.89 whilst the values of σ are all greater than 0.32. For a wide range of non-ideal mixing conditions, the same approximation is satisfactory for values of Θ ranging from 0.125 to 5.0 and even up to 8.5 in some cases.     

Modèle statistique généralisé pour la représentation de la distribution des temps de séjour à l'aide de la loi normale de laplace-gauss

The two-parameter statistical model proposed to approximate residence time distributions in stirred tanks, by means of the normal Laplace-Gauss distribution of θ^1/3, does not always hold. By introducing a third parameter, the exponent c of θ, the quality of representation can be improved such that a linear distribution of residence times is obtained in a normal diagram. The parameters are determined from experimental points by means of a computer; once they are known, it is possible to calculate the moments of the residence time distribution and to determine the derived curve or non-cumulative distribution. The generalized model is also applicable to tanks in series and to plug flow.     

Application of residence time distribution technique to the study of the hydrodynamic behaviour of a full‐scale wastewater treatment plant plug‐flow bioreactor

The hydrodynamic behaviour of a full‐scale wastewater treatment plant (WWTP) bioreactor treating municipal wastewater, situated in Granollers (Barcelona, Spain), has been studied by means of a residence time distribution (RTD) technique using lithium (chloride) as tracer. The bioreactor studied is designed to work as a plug‐flow reactor and it is divided into two independent lanes (1 and 2), each one composed of four compartments in series resulting in a total volume of 3970 m³ per lane. During the RTD experiments, working flow was 1000 m³ h^?1 per lane, which implied an ideal mean residence time of 3.97 h. When a lithium chloride tracer was injected in the bioreactor, both lanes showed a similar highly non‐ideal hydrodynamic behaviour, which had an important effect on the reactor's performance. This global RTD was complemented by means of local RTDs in different locations of the bioreactor in order to determine qualitatively the reactor's mixing regime. Different non‐ideal models (namely axial dispersion, tanks‐in‐series and some simple compartment models) have been tested for the modelling of the experimental RTD. The best model fitting RTD data for Lanes 1 and 2 was a configuration consisting of four mixed tanks in series. The RTD study proposed in this work will permit improvement of the reactor's mixing performance, which is of special interest in future projects including simultaneous removal of carbon, nitrogen and phosphorus. Copyright © 2005 Society of Chemical Industry     

Residence time distribution in a single‐phase rotor–stator spinning disk reactor

A reactor model for the single‐phase rotor–stator spinning disk reactor based on residence time distribution measurements is described. For the experimental validation of the model, the axial clearance between the rotor and both stators is varied from 1.0 × 10^?3 to 3.0 × 10^?3 m, the rotational disk speed is varied from 50 to 2000 RPM, and the volumetric flow rate is varied from 7.5 × 10^?6 to 22.5 × 10^?6 m³ s^?1. Tracer injection experiments show that the residence time distribution can be described by a plug flow model in combination with 2–3 ideally stirred tanks‐in‐series. The resulting reactor model is explained with the effect of turbulence, the formation of Von Kármán and Bödewadt boundary layers, and the effect of the volumetric flow rate. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2686–2693, 2013     

Makro- und Mikromischen im Rührkessel

Macro- and micromixing in stirred tanks. Macromixing time, based on turbulence theory, is calculated theoretically as a function of the specific power input, the kinematic viscosity, the Schmidt number, and the degree of segregation. A comparison of the micromixing time with data for macromixing found in literature reveals that the micromixing time governs the reaction for very small vessel diameters. If the vessel diameter is greater than the critical size than the turbulent macromixing time controls the mixing process. The micromixing time θ_mikro is calculated by means of the experimental results of the turbulent velocity distributions in different stirred vessels for several geometrical arrangements of stirred tanks and different positions of feed input. The calculated data for turbulent mixing times are only valid for geometrically optimized stirred vessels. The necessary mixing time may increase or, if a chemical reaction of 2nd order takes place, the desired yield of the product may decrease considerably if the geometrical conditions are not taken into account.     

The residence time distribution for ideal laminar flow in helical tube

A theoretical residence time distribution is derived for ideal laminar flow through a helical tube with no diffusion. It is shown that, when the flow pattern is fully developed, the residence time distribution becomes approximately independent of curvature and Reynolds number and may be closely approximated by the expression E θ(0) = 0 for 0 < θ(0) < 0.613, E θ(0) = 0.7050−3.81 for θ0 > 0.613. The conditions of applicability are considered and it is shown that the theory should be a reasonable approximation for liquid phase systems over a limited range of conditions.     

Etude calorimétrique de la rétention de l'eau par les résines novolaques

Moisture retention in various substituted novolake resins has been investigated by differential scanning calorimetric analysis. Water molecules leave the polymer network in a two step drying process. The first one occurs at T > T_g, the second at T < T_g and requires an external energy supply. The energy recorded in the second drying step indicates the presence of a dipolar link.     

Hydrolyse enzymatique du lactose dans un réacteur type réservoir agité

The kinetic study of the lactose enzymatic hydrolysis has been achieved in a continuous stirred tank reactor under isothermic conditions, at temperatures varying between 30° and 50°C and at a pH of 4.5. The use of a spinning basket containing the immobilized enzyme permits to cut in a large way the external transfer resistances. The enzyme originating from the Aspergillus Niger species has been immobilized on SiO₂ balls. The characteristics of a perfect mixing in the reactor have been confirmed in a hydrodynamic study carried out under the same operating conditions. A statistical analysis has been made in order to identify the possible models: it has been found that the Michaelis-Menten model, with a competitive inhibition by the galactose product, adapts itself the best to the experimental data:      

First‐Principles Insight into the Composition‐Dependent Structure and Properties of γ‐Alon

Spinel type Aluminum oxynitride (γ‐alon) solid solution exists in the Al₂O₃‐rich region of AlN–Al₂O₃ systems. The first‐principles calculations facilitate our investigations on composition‐dependence of structure and properties in γ‐alon without the limit of experimental solubility. The constant anion crystal structure of γ‐alon was described as the solid solution of spinel phase γ‐Al₂O₃ and the ideal Al₃O₃N with formula of Al_(8+x)/3O_4?xN_x (0 ≤ x ≤ 1). The unit cell expands with increasing Al₃O₃N composition. The lattice parameter increases nonlinearly with the composition of Al₃O₃N, which deviates from Vegard's law. As x increases, the anions dilate from the Al^IV along (1 1 1) direction, and two new narrow peaks (approximate –12.7 and –0.3 eV) become stronger and the conduction band presents a downward shift to low energy in the TDOS. The absorption edge in the UV region of ε₂(ω) present slight red‐shift of 0.5 eV as x increases. Because the compressibility was improved by expansion of coordination polyhedron, the elastic properties were just slightly enhanced as the nitrogen concentration increases. It is suggested that the notable enhancement of mechanical properties in γ‐alon may be difficult to yield by varying the content of substituted nitrogen atoms. The calculated results provide the basis for understanding the crystal structure and intrinsic properties of γ‐alon with different compositions.     

Density measurements of molten Y3Al5O12 and Y2O3-Al2O3 compounds using an electrostatic levitation technique

The densities of molten Y₂O₃-Al₂O₃ compounds, including yttrium aluminum garnet (Y₃Al₅O₁₂), were determined over a wide temperature range that included an undercooled region, using an electrostatic levitation furnace. The density of the molten Y₃Al₅O₁₂ varied with temperature according to the relationship 3750 − 0.25(T − T_m) (kg/m³) with T_m = 2240 K and for the range of 1300 K ≤ T ≤ T_m, yielding the thermal expansion coefficient ɑ = 6.7×10⁻⁵ K⁻¹. The molar volumes of molten (100-x)Y₂O₃-xAl₂O₃ (x = 0, 33.3, 50, 55, 62.5, 76.5, 81.5, or 100 mol%) were found to vary with the value of x in a linear manner within the superheated temperature range. However, the molar volumes in the undercooled region deviated from those calculated using an ideal solution model owing to attractive interactions between Y₂O₃ and Al₂O₃.     

Synthesis and Electrochemical Properties of BaFe1−xCuxO3−δ Perovskite Oxide for IT‐SOFC Cathode

A series of iron‐based perovskite oxides BaFe_1−xCu_xO_3−δ (x = 0.10, 0.15, 0.20 and 0.25, abbreviated as BFC‐10, BFC‐15, BFC‐20 and BFC‐25, respectively) as cathode materials have been prepared via a combined EDTA‐citrate complexing sol‐gel method. The effects of Cu contents on the crystal structure, chemical stability, electrical conductivity, thermal expansion coefficient (TEC) and electrochemical properties of BFC‐x materials have been studied. All the BFC‐x samples exhibit the cubic phase with a space group Pm3m (221). The electrical conductivity decreases with increasing Cu content. The maximum electrical conductivity is 60.9 ± 0.9 S cm⁻¹ for BFC‐20 at 600 °C. Substitution of Fe by Cu increases the thermal expansion coefficient. The average TEC increases from 20.6 × 10⁻⁶ K⁻¹ for BFC‐10 to 23.7 × 10⁻⁶ K⁻¹ for BFC‐25 at the temperature range of 30–850 °C. Among the samples, BFC‐20 shows the best electrochemical performance. The area specific resistance (ASR) of BFC‐20 on SDC electrolyte is 0.014 Ω cm² at 800 °C. The single fuel cell with the configguration of BFC‐20/SDC/NiO‐SDC delivers the highest power density of 0.57 W cm⁻² at 800 °C. The favorable electrochemical activities can be attributed to the cubic lattice structure and the high oxygen vacancy concentration caused by Cu doping.     

Structural and Activity Investigation into Al<Superscript>3+</Superscript>, La<Superscript>3+</Superscript> and Ce<Superscript>3+</Superscript> Addition to the Phosphomolybdate Heteropolyanion for Isobutane Selective Oxidation

Abstract  

Twelve phosphomolybdate compounds were synthesized via cationic exchange and were of the form: M _x H_3–3x [PMo₁₂O₄₀] (M = Al, La or Ce; 0 ≤ x ≤ 1). These compounds were analyzed by XRD and adsorption isotherm. Aluminum addition causes a primitive cubic phase, while lanthanum and cerium yield body-centered structures. La and Ce addition reduces surface area of phosphomolybdate structure. Temperature-programmed experiments for the selective oxidation of isobutane yielded methacrolein, 3-methyl-2-oxetanone (lactone), acetic acid (not with aluminous compounds), propene (only with aluminous compounds), carbon dioxide and water. The preference for propene rather than acetic acid formation with Al³⁺ may be due to the smaller cation size, or primitive cubic structure. These products form via two distinct reaction processes, labeled categories 1 and 2. Category 1 formation is associated with isobutane forming products on the surface, but reaction rate determined by bulk migration of charged particles. Category 2 formation is concerned with isobutane penetrating deep within the bulk of the substrate and forming products which subsequently desorb in a series of bell-shaped humps. Methacrolein forms via both category 1 and 2, whilst all other products form via category 2 exclusively. Kinetic analysis showed apparent activation barriers for category 1 methacrolein formation range from 67 ± 2 kJ mol⁻¹ to >350 kJ mol⁻¹, and occur in groups with small, medium and large activation barriers. The addition of +3 metal cations to the phosphomolybdate anion increase thermal stability, significantly decreasing deactivation; IR spectroscopy shows that the Keggin structure remains intact during temperature-programmed experiments with the Al, La and Ce salts.     

Le cyclone: Un réacteur chimique. Application à la réaction de décarbonatation de NaHCO3

The purpose of the present paper is the experimental and theoretical study of the behaviour of the cyclone working as a chemical reactor. The model reaction is the decarbonation of NaHCO₃. Thanks to the use of scale-up relationships previously determined, it is possible to derive kinetic constants in good agreement with other published values obtained under quite different conditions. The measurements are performed in a wide range of experimental conditions (particle size, residence time of particles, cyclone wall temperatures, solid flow rate). The paper emphasizes two possible fields of utilization for the cyclone: a device for the measurement of kinetic constants, a processing reactor operating with high throughput of solids.     

Structures and Properties of Pb(Zr0.5Ti0.5)O3−Pb(Zn1/3Nb2/3)O3− Pb(Ni1/3Nb2/3)O3 Ceramics for Energy Harvesting Devices

Piezoelectric ceramics with large energy density coefficient d₃₃·g₃₃ value have been found suitable for piezoelectric energy harvesting applications. In this study, the phase structures and piezoelectric properties of xPb(Zr_0.5Ti_0.5)O₃?yPb(Zn_1/3Nb_2/3)O₃?(1?x?y)Pb(Ni_1/3Nb_2/3)O₃ (xPZT?yPZN?(1?x?y)PNN) ceramic were investigated with systematically varying PZN and PNN components. The ternary phase diagram of PZT?PZN?PNN system was illustrated and the composition region of morphotropic phase boundary (MPB) was determined. Piezoelectric and dielectric measurements verify that the materials in MPB region all present large d₃₃ and d₃₃·g₃₃ values. In particular, very high d₃₃·g₃₃ coefficients of 20162.2 × 10^?15 m²/N and 21026.3 × 10^?15 m²/N are observed from samples 0.75PZT?0.15PZN?0.1PNN and 0.8PZT?0.05PZN?0.15PNN with compositions located on the rhombohedral phase side near MPB because the dielectric coefficient ε₃₃^T/ε₀ decreases faster than the d₃₃ coefficient at this side.     

Tomography images to analyze the deformation of the cavern in the continuous‐flow mixing of non‐Newtonian fluids

Tomography, an efficient nonintrusive technique, was employed to visualize the flow in continuous‐flow mixing and to measure the cavern volume (V_c) in batch mixing. This study has demonstrated an efficient method for flow visualization in the continuous‐flow mixing of opaque fluids using two‐dimensional (2‐D) and 3‐D tomograms. The main objective of this study was to explore the effects of four inlet‐outlet configurations, fluid rheology (0.5–1.5% xanthan gum concentration), high‐velocity jet (0.317–1.660 m s^?1), and feed flow rate (5.3 × 10^?5?2.36 × 10^?4 m³ s^?1) on the deformation of the cavern. Dynamic tests were also performed to estimate the fully mixed volume (V_{fully mixed}) for the RT, A310, and 3AM impellers in a continuous‐flow mixing system, and it was found that V_{fully mixed} was greater than V_c. Incorporating the findings of this study into the design criteria will minimize the extent of nonideal flows in the continuous‐flow mixing of complex fluids and eventually improve the quality of end‐products. © 2013 American Institute of Chemical Engineers AIChE J, 60: 315–331, 2014     

Structure Stability and Microwave Dielectric Properties of (1 − x) Ba(Mg1/2W1/2) O3 + xBa(Y2/3W1/3)O3 Ceramics

The structure stabilities of double perovskite ceramics‐ (1 ? x) Ba(Mg_1/2W_1/2)O₃ + xBa(Y_2/3W_1/3)O₃ (0.01 ≤ x ≤ 0.4) have been studied by X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman spectrometry in this study. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results showed that all the compounds exhibited face‐centered cubic perovskite structure. Part of Y³⁺ and W⁶⁺ cations occupied 4a‐site and the remaining Y³⁺ and Mg²⁺ distributed over 4b‐site, respectively, and kept the B‐site ratio 1:1 ordered. Local ordering of Y³⁺/Mg²⁺ on 4b‐site and Y³⁺/W⁶⁺ cations on 4a‐site within the short‐range scale could be observed with increasing Y‐doping content. The decomposition of the double perovskite compound at high temperature was successfully suppressed by doping with Y on B‐site. However, Ba₂Y_0.667WO₆ impurity phase appeared when x > 0.1. The optimized dielectric permittivity increased with the increase in Y doping. The optimized Q × f value was remarkably improved with small amount of Y doping (x ≤ 0.02) and reached a maximum value of about 160 000 GHz at x = 0.02 composition. Further increasing in Y doping led to the decrease in Q × f value. All compositions exhibited negative τ_f values. The absolute value of τ_f decreased with increasing Y‐doping content. Excellent combined microwave dielectric properties with ε_r = 20, Q × f = 160 000 GHz, and τ_f = ?21 ppm/°C could be obtained for x = 0.02 composition.     

Structural Evolution and Multiferroics in Sr‐Doped Bi7Fe1.5Co1.5Ti3O21 Ceramics

The ceramics with the composition of Sr_xBi_7?xFe_1.5Co_1.5Ti₃O_21?δ (0 ≤ x ≤ 1, SBFCT) were prepared by a citrate–nitrate combustion method, and the phase evolution with an increasing Sr content was investigated. Pure Aurivillius phase SBFCT with a layer number of n = 6 was obtained when x ≤ 0.25, and then the structure collapsed to 5 layers for x = 0.50, then alternating 4 and 5 layers for x = 0.75, and finally 4 layers for x = 1.00. Meanwhile, secondary phase Sr_1?mBi_mFe_1?iCo_iO_3?γ appeared when x > 0.25, which is antiferromagnetic (AFM) and with low resistivity. Enhanced ferromagnetic and ferroelectric properties were observed from single phase SBFCTs at the room temperature, and the ferromagnetic transition temperature (T_c) increases with the Sr doping level x in the single phase range. The remnant magnetization (2M_r) is 2.27 emu/g and the remnant polarization (2P_r) is 2.89 μC/cm² at an applied electric field of 100 kV/cm for the x = 0.25 sample.     

Ionic transport in AgI‐HgS‐As2S3 glasses: Critical percolation and modifier‐controlled domains

Electrical measurements, dc and ac, show that (AgI)_x(HgS)_0.5‐x/2(As₂S₃)_0.5‐x/2 glasses, 0.0 ≤  x ≤ 0.6, exhibit drastic changes in ionic conductivity σ_i with silver iodide additions. The ionic transport increases by 13 orders of magnitude with increasing silver content from ~0.002 to ~23 at.%, and the activation energy decreases from 1.05 to 0.35 eV. Two distinctly different ion transport regimes above the percolation threshold concentration, x_c ≈ 30 ppm, were distinguished. The critical percolation regime at low silver content (≤ 2‐5 at.% Ag) is characterized by a random distribution of silver‐related entities and obeys a power‐law composition dependence of σ_i. The ion transport parameters depend on the host network connectivity, represented by the average coordination number <n₀>, via the critical fictive temperature T₀; the calculated T₀ value is comparable to the glass transition temperature for the glassy (HgS)_0.5(As₂S₃)_0.5 host matrix. In contrast, in the modifier‐controlled domain, the silver‐related entities are nonrandomly distributed. The high Ag⁺ ionic mobility results from interconnected tetrahedral (AgI_2/2S_2/2)_n chains in the silver iodide content range 0.2 < x ≤ 0.5, and from 2D layers (Ag_3/3I_3/3)_n or 3D mixed tetrahedral subnetwork (AgI_3/3S_1/2) in the range x > 0.5.     

Reduced power consumption compared to a traditional stirred tank reactor (STR) for enzymatic saccharification of alpha-cellulose using oscillatory baffled reactor (OBR) technology

Enzymatic saccharification of pure α-cellulose was conducted in oscillatory baffled (OBR) and stirred tank (STR) reactors over a range of mixing intensities requiring power densities (P/V) from 0 to 250 Watts per cubic metre (W/m³). Both reactor designs produced similar saccharification conversion rates at zero mixing. Conversion increased with increasing mixing intensity. The maximum conversion rate occurred at an oscillatory Reynolds number (Re_o) of 600 in the OBR and at an impeller speed of between 185 and 350 rpm in the STR. The OBR was able to achieve a maximum conversion rate at a much lower power density (2.36 W/m³) than the STR (37.2–250 W/m³). The OBR demonstrated a 94–99% decrease in the required power density to achieve maximum conversion rates and showed a 12% increase in glucose production after 24 h at 2.36 W/m³.     

Simultaneous biofiltration of H<Subscript>2</Subscript>S,NH<Subscript>3</Subscript> and toluene using cork as a packing material

Simultaneous removal of ternary gases of NH₃, H₂S and toluene in a contaminated air stream was investigated over 185 days in a biofilter packed with cork as microbial support. Multi-microorganisms including Nitrosomonas and Nitrobactor for nitrogen removal, Thiobacillus thioparus (ATCC 23645) for H2S removal and Pseudomonas aeruginosa (ATCC 15692), Pseudomonas putida (ATCC 17484) and Pseudomonas putida (ATCC 23973) for toluene removal were used simultaneously. The empty bed residence time (EBRT) was 40–120 seconds and the inlet feed concentration was 50-180 ppmv for NH₃, 30–160 ppmv for H₂S and 40–130 ppmv for toluene, respectively. The observed removal efficiency was 45–100% for NH₃, 96–100% for H₂S, and 10–99% for toluene, respectively. Maximum elimination capacity was 5.5 g/m³/hr for NH₃, >20.4 g/m³/hr for H₂S and 4.5 g/m³/hr for toluene, respectively. During long-term operation, the removal efficiency of toluene gradually decreased, mainly due to depositions of elemental sulfur and ammonium sulfate on the cork surface. The results of microbial analysis showed that nearly the same population density was observed on the surfaces of cork chips collected at each sampling point. Kinetic model analyses showed that there were no particular evidences of interactions or inhibitions among the microorganisms.