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1.
The influence of pulsed electric field (PEF) and subsequent centrifugal osmotic dehydration (OD) on the convective drying behavior of carrot is investigated. The PEF was carried out at an intensity of E = 0.60 kV/cm and a treatment duration of tPEF = 50 ms. The following centrifugal OD was performed in a sucrose solution of 65% (w/w) at 40°C for 0, 1, 2, or 4 h under 2400 × g. The drying was performed after the centrifugal OD for temperatures 40-60°C and at constant air rate (6 m 3/h).
With the increase of OD duration the air drying time is reduced spectacularly. The dimensionless moisture ratio Xr = 0.1 is reached for PEF-untreated carrots after 370 min of air drying at 60°C in absence of centrifugal OD against 90 min of air drying after the 240 min of centrifugal OD. The PEF treatment reduces additionally the air drying time. The total time of dehydration operations can be shortened when OD time is optimized. For instance, the minimal time required to dehydrate untreated carrots until Xr = 0.1 is 260 min (120 min of OD at 40°C and 140 min of drying at 60°C). It is reduced to 230 min with PEF-treated carrots.
The moisture effective diffusivity Deff is calculated for the convective air drying based on Fick's law. The centrifugal OD pretreatment increases drastically the value of Deff. For instance, 4 h of centrifugal OD permitted increasing the value of Deff from 0.93 · 10 -9 to 3.85 · 10 -9 m 2/s for untreated carrots and from 1.17 · 10 -9 to 5.10 · 10 -9 m 2/s for PEF-treated carrots. 相似文献
2.
Layered -titanate materials, Na xM x/2Ti 1−x/2O 2 (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na 0.4Ni 0.2Ti 0.8O 2 were ca. 7×10 3 S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm −1 K −1 at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na 0.4Ni 0.2Ti 0.8O 2 and Na 0.4Co 0.2Ti 0.8O 2 was about 1.9×10 −4 and 1.2×10 −4 K −1 around 700 °C, respectively. 相似文献
3.
Nano-LiFePO 4/C cathode materials were synthesized by a PVB-based rheological phase method, followed by calcination at 550 °C for 10 h in argon. Simultaneous thermogravimetric-differential scanning calorimetry analysis indicates that the crystallization temperature of LiFePO 4 is about 436 °C. In the process of heat treatment, the decomposition of polyvinylbutyral coats carbon on the synthesized LiFePO 4 particles in situ. The resulting LiFePO 4 powders with fine particle sizes and homogeneous carbon network connection were observed by using scanning electron microscopy and transmission electron microscopy. Electrochemical measurements show that the LiFePO 4/C composite cathode delivers a large discharge capacity of 162.3 mAh·g − 1 at the 0.1 C rate, and exhibits a favorable capacity cycling maintenance at lower charge and discharge rate such as 0.5 C rate. 相似文献
4.
Five kinds of rare earth stabilized bismuth oxide ceramics, (Bi 2O 3) 0.75(RE 2O 3) 0.25 (RE=Dy, Y, Ho, Er and Yb), were synthesized by sintering a mixture of Bi 2O 3 and RE 2O 3 at 900–1100 °C and their electrical properties were investigated. The bulk density and the lattice constant linearly increased with an increase in the atomic weight of RE and the ionic radius of RE 3+, respectively. The electrical conductivity at 300 °C slightly increased with the increasing ionic radius of RE 3+, while at 500 and 700 °C, it was constant regardless of the ionic radius of RE 3+. The migration activation energy and the association activation energy showed a maximum value and a minimum value at RE=Er, respectively. 相似文献
5.
Cylindrical pellets of zinc ferrite high temperature desulfurization sorbent have been prepared using a number of formulation recipes and induration conditions. Physical and structural properties of the resultant sorbents were measured, and reactivity and durability screening tests were carried out using a single pellet electrobalance reactor. The formulation variables studied were ZnO to Fe 2O 3 ratio, Fe 2O 3 source, and the addition of inorganic (bentonite) and organic (methocel) binders to the sorbents. Pellet induration conditions ranged from 0.25 hours at 815 °C to 4.0 hours at 1038°C. Stronger pellets having greater attrition resistance resulted when 5% bentonite was added to the formulation recipe and when the pellets were indurated at high temperature for extended times. In contrast, bentonite content was not a significant factor in determining sorbent reactivity and durability, both of which were improved by mild induration conditions. Sorbent regeneration temperature was found to be an important factor in improving reaction durability, as was the addition of 0.5% methocel to the formulation recipe. Pellets containing catalyst-grade Fe 2O 3 were more reactive than those containing pigment-grade Fe 2O 3. This effect, however, was less important than the effect of induration conditions. 相似文献
6.
The effect of reduction conditions, mainly reduction temperature and duration time on the activity of Ni/γAl 2O 3 catalyst were studied for a methanation reaction in a gradientless Berty reactor. The methanation reaction was investigated using a feed containing CO (6.7 mole%), H 2 (26 mole%) and the balance being nitrogen at a pressure of 30 psig and a fixed temperature of 350°C. The reduction temperature was varying from 250 to 500°C, in order to investigate its effect on the methanation reaction. The methanation activity of the catalyst increased to a maximum by increasing the reduction temperature up to a maximum at 300-350°C and showed a slight negative decline afterward. The second parameter investigated was reduction duration time which was varied from 2 to 16 hours. It was observed that the methanation reaction activity increased by increasing the reduction duration time up to 6 hours. After six hours, there was no increase in activity. Based on the finding of this investigation, a recommended set of reduction conditions is given: reduction temperature of 300°C and a duration time of six hours. 相似文献
7.
Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF 2 and YF 3 as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F 2 and Ca 12Al 14O 32F 2. Liquid CaYAlO 4 mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F 2 at a relatively low temperature results in homogeneous YF 3 distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity. 相似文献
8.
The mean activity coefficients for NaCl in a ternary electrolyte system were determined by the potentiometric method, at 25°C, using a solvent polymeric (PVC) sodium-selective membrane electrode (Na + ISE), containing N,N'-dibenzyl-N,N'-diphenyl-1,2-phenylenedioxydiacetamide as ionophore, and combined with an Ag/AgCl electrode. The potentiometric measurements were performed at the same ionic strengths in different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (where r = m 1/m 2 = 1, 10, 50, 100). The nonideal behavior of the ternary NaCl(m 1) + LiCl(m 2) + H 2O electrolyte system was described based on the Pitzer ion-interaction model for mixed salts over the ionic strength ranging from 0.01 up to about 4 mol/kg. Two- and three-particle Pitzer interaction parameters for a mixed electrolyte system were determined based on potentiometric data, and the critical role of potentiometric selectivity coefficient ( K12) of ISE as limiting factor in the potentiometric measurements was analyzed. 相似文献
9.
This article presents a batch carbonation method with ultrasonication to synthesize monodispersed nanoparticles of calcium carbonate (CaCO 3). The synthesis processes with and without ultrasonication were compared. The results showed that the application of ultrasonication into the synthesis caused a supersaturation of Ca 2+ ions in the synthesis, leading to a rapid nucleation of calcium carbonate and improved the solute transfer as well as. It was also found that the effect of ultrasonication on the grain size of the nanoparticles synthesized was related to other synthesis conditions, such as initial temperature for carbonation, mass fraction of Ca(OH) 2 suspension, and CO 2 flow rate. The initial temperature applied for the carbonation was increased by at least 5°C when ultrasonication was applied. The nanoparticles formed in the presence of ultrasonication became smaller with a narrower particle size distribution at 6-12 wt.% of Ca(OH) 2 suspension or at 1.28 l/h of CO 2 consumed by per gram of Ca(OH) 2 or more. 相似文献
10.
We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NO x trap catalyst using different reductants. H 2 was found to be more effective at regenerating the NO x storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NO x storage catalyst is sulphated in SO 2 under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H 2/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H 2 reduced sample was exposed to a rich condition in a gas mixture containing CO 2 at 650 °C, the storage activity can be recovered. Under these rich conditions the S 2− species becomes less stable than the CO 32−, which is active for storing NO x. Samples which were lean aged in air containing 60 ppm SO 2 at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO 2 is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NO x storage. 相似文献
11.
A model for aerosol-phase densification of particles during their synthesis by spray pyrolysis is presented. The model was used to describe the evolution of the specific surface area of PdO powders synthesized at temperatures between 400 to 800 °C (residence times 27.9 to 22.7 s). Surface areas and grain sizes ranged between 56 m 2/g and 4 nm at 400 °C to 3.2 m 2/g and 40 nm at 800 °C The characteristic coalescence lime was determined as: τ= 1.7 x 10 13T d 3 exp(1.3 x 10 5/RT) [for lattice diffusion] and t = 2.7 x 10 19T d4exp (1.6 x 10 5/RT) [for grain boundary diffusion] (τ is in seconds, Tin degrees Kelvin, R is 8.314J/(mol.K)and d is in meters), but the data and model did not allow identification of the specific mechanism. The model provides a general approach for correlating changes in particle surface area with reactor operating conditions that is applicable to a wide variety of materials. 相似文献
12.
This article discusses a mechanism for preparing perovskite powders, 0.75Pb(Ni 1/3Nb 2/3)O 3-0.25PbTiO 3 (PNN-PT), using a semichemical method (SCM).Precursors were prepared by adding aqueous Ni(Ac) 2 solutions to an alcohol slurry of PbO, Nb 2O 5, and TiO 2. The TG-DTG and DSC analysis of the precursors and XRD analysis of the powders at different thermal treatment temperatures showed that the reaction mechanisms in this method differ from those in the conventional mixed-oxide method. The aqueous Ni(Ac) 2 solution reacted with PbO to form Pb(Ac) 2 · Pb(OH) 2 · H 2O and Ni(OH) 2, which decomposed to form nascent PbO and NiO, thereby improving the reactivity and distribution of PbO and NiO. Pb 3Nb 2O 8 and NiNb 2O 6 formed and were easily converted into the perovskite phase during the thermal treatment process. At a thermal treatment temperature of 850°C, the content of the perovskite phase reached 98%. Pyrochlore-free PNN-PT ceramic was obtained after 2 h of sintering at 1100°C, and its dielectric properties were found to be excellent at temperatures ranging between -55 and 120°C. 相似文献
13.
A dense functionally gradient SiC/SiO 2 coating has been developed to improve the oxidation resistance of carbon at elevated temperatures. SiC was coated on the surface of a graphite substrate by a reaction between thermally evaporated silicon and carbon at 1400 °C. The SiO 2 layer was deposited by exposing the SiC coated specimens next to a bed of Si powder in a flowing H 2–H 2O gas ( PH2O=2.6×10 −2 atm) at 1400 °C. The formed SiC/SiO 2 layers were dense and had gradient compositions with good adhesion to the carbon substrate. However, as the coating thickness increased, the coating layer became cracked and delaminated from the substrate due to thermal stress. The specimens with the continuous SiC/SiO 2 layer showed a remarkably improved oxidation resistance up to 1200 °C. 相似文献
14.
The residence times of the components of two- and three-component mixtures of fine (195 µm), coarse (1315 µm) and very coarse (5040 µm) sands were measured in a pilot-scale cascading rotary dryer. The effects of mixture composition and air velocity (0-5.4 m s -1) were determined. With no air flowing through the drum, the residence times of the individual components were almost the same as that of the overall mixture. Increasing the gas velocity caused a large decrease in residence time. In contrast, particle size had very little effect. The spread of residence times increased with air velocity, peaking between 2 and 4 m s -1; composition had very little effect on the spread. The residence time of the overall mixture could be calculated using the particle transport model of Matchett and Baker if both the modified drag coefficient Φ d and the particle Reynolds number Rep were based on the superficial air velocity and the mass-average particle diameter. 相似文献
15.
A new composite membrane is fabricated by incorporating tungsten trioxide hydrate into Nafion to be employed as a candidate electrolyte for proton exchange membrane fuel cells (PEMFCs) operated above 100°C. Thermal behavior and proton conductivity of the composite membrane are studied by means of thermogravimetric/differential thermal analyis (TG/DTA) and AC impedance measurements, respectively. These results demonstrate that the thermal stability of the composite membrane has no appreciable change when compared with the native Nafion membrane. The proton conductivity of the composite membrane is found to be better than that of the native Nafion membrane at high temperature and lower relative humidity. When the composite membrane is used as an electrolyte in H 2/O 2 PEMFC under the operating conditions of 110°C, 1.36 atm gas pressure, and 70% relative humidity, the observed current density value at 0.4 V is 1.5 times higher than that of the cell employing native Nafion membrane as an electrolyte. 相似文献
16.
Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity ( De) was determined by a correlation based on the air velocity ( v), the air temperature ( T) and the dimensional moisture content of the carrot particles ( X/ Xo). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min. 相似文献
17.
The weight and water loss of 6 mm thick pineapple slabs (one of a six part of slice) were analyzed during osmotic dehydration in sucrose solution at different temperatures (50, 60 and 70°C), sucrose concentrations (50, 60 and 70°Bx) and pH's (6, 7 and 8), in 3 3 experimental design. These results were fitted to a modified Azuara equation to obtain water and sucrose diffusivity results at equilibrium condition. Mean result of water diffusivity was 1.717 × 10 -5 cm 2/s and sucrose diffusivity varied from 2.0 to 4.6 × 10 -5 cm 2/s. The results of water loss at equilibrium in pineapple slabs varied between 0.6 to 0.67 g/g of initial sample weight. The results of sucrose gain at equilibrium varied between 0.15 and 0.21 g/g of initial sample weight. The results from mathematical modeling were compared to experimental results with r2 = 0.94. 相似文献
18.
Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl 2 vapor is deposited. TPR shows peaks for Co 2+ ions at 695–705°C and for Co 3O 4 at 385–390°C, but a peak in the 220–250°C region appears to indicate Co 2+ oxo-ions. The catalysts have been tested for the selective reduction of NOx with iso-C4H10 under O2-rich conditions and in the absence of O2, both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: oxo-ions or Co3O4 clusters first oxidize NO to NO2, which is chemisorbed as NOy (y≥2) and reduced. In this modus operandi catalyst SUB shows the highest N2 yield 90% near 390°C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co2+ ions and has poor activity below 400°C, excels at T>430°C. This and the observation that, at high temperature, NO is reduced in O2-free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co2+ ions without intermediate formation of NO2. The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO2 formation. 相似文献
19.
Lithium fluoride powder (LiF) is a white powder with a density of 2.64 gr/cm 3 and a melting point of 848°C. This powder has several applications such as flux, glaze, soldering, and aluminum melting process, but one of the most important uses of this powder is its application in dosimetry. The commercial powders currently used for this purpose have average sizes of 5 to 10 micrometers; the objective of this research is to produce LiF powder with nano-metric particle size. In this study, the reaction of LiOH + HF → LiF + H 2O has been selected from among several reactions that were able to produce LiF powder, and some precipitation parameters such as temperature, time, agitation type, and supersaturation degree have been controlled. The morphology, phase analysis, and particle size distribution of the resulting powders were analyzed by SEM, XRD, and LPSA. Finally, lithium fluoride nano-powder was synthesized at a temperature of 25°C, pH about 2-3, reaction time less than 1 s, and agitation by ultrasonic bath. 相似文献
20.
Fresh mango ( Mangifera indica) slices were osmotically dehydrated using four treatment variables: treatment time, temperature, sugar concentration and slice thickness. Treatment times for 2, 4 and 6 mm slice thickness were 3, 5 and 7 h at temperatures of 20, 30 and 40°C and sugar concentrations of 40, 50 and 60% (w/w). The responses variables tested were weight reduction (WR), sugar gain (SG), final moisture content ( Mf) and overall product acceptability (OA). A fractional factorial design (3 level-4 parameter) with 27 runs was used as the experimental design.
A Response Surface Methodology (RSM) was performed to analyze and predict the optimum conditions of the mango osmotic dehydration process. All treatment variables significantly affected WR, while SG and Mf were significantly affected only by thickness. On the other hand, none of the variables showed no significant effect on the OA.
The optimum conditions were determined with the following response limiting values: at least 30% WR, at least 8% in SG, Mf of not exceeding 70% were based on previous studies and panel evaluation results. The optimum conditions generated were: treatment time of 6 h, temperature of 35°C, sugar concentration of 65% (w/w) for 5 mm slice thickness. Results of validation tests were fairly acceptable since the Coefficient of Variation (CV) of the responses were less than 15% except for sugar gain with a 26.5% CV. 相似文献
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