旋流塔板的板效率模型

引　言板式塔的操作通常可分为气体为分散相的鼓泡、泡沫工况和液体为分散相的喷射工况 .喷射态操作的塔板 ,虽然其效率一般较低 ,但具有通量大、压降低等重要优点 ,符合现代大工业发展趋势的要求 ,正受到越来越多的重视[1～ 3] .有关鼓泡、泡沫工况下的流体力学和传质过程的研     

FOULING OF SIEVE TRAYS

Fouling due to the deposition of solids on sieve trays was studied in a 153 mm diameter column and a 90 mm diameter simulator. Tests were carried out with various gas-liquid-solid systems to determine the effect of the fouling mechanism on tray performance. For an air-water system containing saturated NaHCO₃, fouling from crystallisation occurs due to water evaporation by dry air at the tray holes. For a CO₂-water system containing K₂C0₃ and saturated KHC0₃ the reaction between C0₂ and K₂CO₃ causes the precipitation of KHCO₃ at the gas-liquid interface. For the air-water system containing insoluble solids, flour and Ca(OH)₂, no fouling was observed for a 48 hour test period. Interesting results on tray performance were also obtained. Test results indicate that (1) trays with larger holes have higher resistance to fouling, (2) fouling does not occur when trays are operated under weeping conditions, (3) fouling rate increases with increasing gas loading, (4) a bed of mesh packing on the sieve tray accelerates the fouling rate. A mathematical model has been developed for predicting the effect of crystallisation fouling on tray pressure drop as a function of time.     

PRODUCTION OF SUB-MICROMETER SIZE O/W EMULSIONS USING A HIGH-PRESSURE WET-TYPE JET MILL

Emulsions composed of sub-micrometer size droplets were obtained by a high-pressure wet-type jet mill. Liquid paraffin as dispersed phase, aqueous sucrose solution as continuous phase, and anionic sodium dodecyl sulfate as emulsifying agent were used as the model emulsification system. Droplet size distribution, Sauter mean diameter (d₃₂), geometric standard deviation of the droplet size distribution (σ_g), and emulsion viscosity (η_e) were investigated under the various combinations of process variables: dispersed phase volume fraction (Φ), dispersed phase viscosity (η_d), continuous phase viscosity (η_c), processing pressure, and number of passages through the wet-type jet mil. d₃₂ and σ_g were correlated with the process variables. For the entire experimental range, maximum droplet diameter was varied with d₃₂ in the range of 1.7-1.9-fold, and a correlation was proposed with K (=η_d/η_c) as a variable. Above a marginal value of d₃₂, at Φ ≥ 0.1, emulsions exhibited Newtonian behavior and could be described well by the Yaron and Gal-Or model. Below the marginal values, emulsions strongly depended on d₃₂ and exhibited a shear-thinning behavior. The relation between η_e and d₃₂ for such emulsions was modeled by use of particle Reynolds number.     

APPLICATION AND FUNDAMENTAL RESEARCH OF ROTATING STREAM TRAY

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旋流塔板上的两相流场

在直径300mm塔内,用激光多普勒测速仪对旋流塔板上的流场进行了实验研究,探明其主要特征,进一步证实了过去对不同用途板型的设计思想.采用两方程k-ε湍流模型,应用有限差分法,对塔板上的气流场进行了数值模拟,计算结果与实验值符合良好.     

TRANSITION TO ANNULAR FLOW IN TWO-PHASE, TWO-COMPONENT VERTICAL FLOW: EXAMINATION OF CORRELATIONS

The available empirical, semi-empirical and analytical correlations and flow pattern maps for predicting the transition to annular flow in upward, two-phase, gas-liquid flow in a vertical tube were tested against a large set of experimental data. These experimental data were taken on the same rig through an observation section (i.d. = 11.7 mm, L = 30.5 cm), preceded by a heated test section ( L/D≃ 52) and a calming length (L/D = 130), all having the same inside diameter as that of the observation section. The total range of variables covered by these data is: 2≤Re_SL≤ 1.3 × 10⁵, 5.6 ≤Pr_L≤ 7000, 0.01 V_SG/V_SL ≤ 7900, 2 × 10^-4≤ρ_G/ρ_Ls0.01, and 19.7 × 10^-3≤ σ ≤72× 10^-3N/m. In total, fifteen correlations and flow-pattern maps were tested with eight liquid-gas combinations covering a very wide range of fluid properties and mass flow rates of both phases. Using the Kutateladze number as the criterion for the transition appears to provide an excellent separation particularly when a lower value than 3.1, as suggested in the model of Taitel et al. (1980) is used (between 1.5 and 2.0). Also using the Weisman and Kang (1981) correlation, the Wallis parameter and the void fraction criteria of Mishima and Ishii (1984) were quite successful in correlating the data.     

Heat and Moisture Transfer Model for Onion Drying

Onion slices of 3 mm thick were dried in an atmospheric batch tray dryer in order to investigate the influence of air temperature and drying time on parameters such as sample moisture content and drying rate. A model is proposed which takes into account both moisture and temperature distributions in the sample and is in a fair agreement with the experimental data. The models suggested so far by other workers take only the moisture distributions into account for onion drying.     

TRANSITION OF FLOW REGIME IN FLUIDIZED BEDS WITH SLANTING BLADE BAFFLES

A flow model is proposed to investigate the transition of flow regime from bubbling to turbulent fluidization postulating that the flow in the emulsion phase follows the Richardson-Zaki equation.

Void fraction of the whole bed ε_f and the mean velocity of bubbles U_b were measured in fluidized beds of 0.3 and 0.5 m ID, in which slanting blade baffles were positioned. Mo-catalyst, silica gel, sand and glass beads with size between 135-443 μm were fluidized by air.

Void fraction of the emulsion phase ε _e was calculated on the basis of the above model. Correlating ε _e with superficial gas velocity U_ƒ, we found that ε _e was very close to ε _mƒ in the bubbling regime and that e, increased with increasing U_ƒ in the turbulent regime.

Calculated values of the volume fraction of bubble phase δ were correlated with U_ƒ, from which apparent transition point from bubbling to turbulent regime was estimated. Combining information obtained, transition of flow regime in the above type of fluidized beds is discussed     

旋流塔板的板效率

本文试验以水冷却饱和热空气,在φ300mm有机玻璃塔中测定了不同结构的五种旋流塔板和两种筛板的板效率.试验范围为:喷淋密度U=7.17—135m~3/m~2h;空塔气速Ws=0.52—4.21m/s.比较系统地研究了气液负荷、塔板结构对板效率的影响,获得了适宜的结构参数;对喷射区域的试验数据进行了关联;将旋流塔板与筛板的性能作了比较;对四川化工厂的工业试验进行了小结;为旋流塔板的设计提供了基础数据.还介绍了一种简便又易于测准的板效率测定方法.     

NOx removal from the flue gas of oil-fired boiler using a multistage plasma-catalyst hybrid system

The study on removal of NO_x from the flue gas of oil-fired boiler has been carried out using non-thermal plasma cum catalyst hybrid reactor at 150 °C. Propylene (C₃H₆) was used as a reducing agent. A multistage plasma-catalyst hybrid reactor was newly designed and successfully operated to clean up the flue gas stream having a flow rate of 30 Nm³/h. TiO₂ and Pd/ZrO₂ wash-coated on cordierite honeycomb were used as catalysts in the present study. Though the plasma-catalyst hybrid reactor with TiO₂ showed good activity on the removal of NO yet it removed only 50–60% of NO_x because a significant portion of NO oxidized to NO₂. On the contrary, the plasma-catalyst hybrid reactor with Pd/ZrO₂ removed about 50% of inlet NO with a negligible amount of NO oxidation into NO₂. The plasma/dual-catalysts hybrid system (front two units of plasma-Pd/ZrO₂ + rear two units of plasma/TiO₂) proved to be very promising in NO_x removal in the presence of C₃H₆. DeNO_x efficiency of about 74% has been achieved at a space velocity of 3300/h at 150 °C.     

Constant and Intermittent Drying Characteristics of Olive Cake

The drying kinetics of olive cake, the solid by-product of the olive oil extraction process, has been experimentally investigated in a small-scale tray dryer using both constant and intermittent (on/off) heating schemes. The parameters investigated include inlet air temperature and intermittency of heat input. The drying kinetics was interpreted through two mathematical models, the Page equation and the Lewis equation. The Page equation was most appropriate in describing the drying behavior of olive cake. A diffusion model was used to describe the moisture transfer and the effective diffusion coefficient at each temperature was determined. The dependence of the effective diffusion coefficient on drying temperature can be adequately explained based on an Arrhenius-type relation. The effective diffusion coefficient varied between 7.6 × 10^-8 and 2.5 × 10^-7 m²/min with an activation energy of 38.55 kJ/mol. Comparison of time evolution of material moisture content due to intermittent and constant drying is also made.     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (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_0.8O₂ were ca. 7×10³ 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_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

Solid state ceramic gas sensors based on interfacing ionic conductors with semiconducting oxides

Solid-state ceramic NO_x sensors based on interfacing an ionic conductor (NASICON) with semiconducting oxides (rare earth perovskite-type oxides) were investigated. NASICON powders were pressed into thimbles 12 mm long with 3 mm inner diameter and 4 mm outer diameter, then sintered at 1270°C in air. A Pt wire was attached to the outer surface of the tubes using a platinum paste. A uniform Au/Pd (60 wt.%) coating, permeable to oxygen but not to NO_x, was sputtered for 40 min on the sensor external surface to allow the exposure of both electrodes to the gas atmosphere without using reference air. Windowless energy-dispersive spectroscopy (EDS) was used to evaluate the chemical composition of the Au–Pd layer before and after the performance of sensing tests. Sodalite powder as an auxiliary phase was tightly packed into the NASICON thimbles with a Pt lead for the electrical contact. To get an in-situ NO conversion to NO₂, a Pt-loaded alumina powder was used as a catalyst and incorporated with the sensor on the top of the auxiliary phase. Nano-sized and chemically-pure rare earth perovskite-type oxide (LaFeO₃, SmFeO₃, NdFeO₃ and LaCoO₃) powders, prepared by the thermal decomposition of the corresponding hexacyanocomplexes, were also used in the electrochemical cells. Each of the tested oxides was packed into the thimbles replacing the sodalite and the Pt-loaded alumina catalyst. Tests were performed also using only the perovskitic oxides. The microstructure of the materials tested was evaluated using scanning electron microscopy (SEM). The NO₂ sensing properties of the prototype sensors were investigated at controlled temperature (in the range 300–600°C) by measuring the electromotive force (EMF) at different NO₂ concentrations (in the range 2–2000 ppm in air). Some measurements were performed at various NO concentrations diluted with Ar. The results obtained showed a promising NO₂ sensing performance when ferrites were used. SmFeO₃ has a lower catalytic effect on NO oxidation than the Pt-loaded alumina catalyst, and has a similar effect to sodalite when used as auxiliary phase. The perovskite-type oxides are more preferable as auxiliary phase than sodalite because they improve the stability of the electrochemical sensor performances.     

Properties and microstructure of sintered incinerator bottom ash

The fraction of incinerator bottom ash with a particle size less than 8 mm produced at a commercial municipal solid waste incinerator was wet milled, dried, compacted and sintered at a range of temperatures to form ceramic materials. The effects of milled ash particle size distribution, powder compaction pressure and sintering temperature were investigated, and the materials formed characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis (TG/DTA). The main minerals present in the milled ash were quartz (SiO₂) and calcite (CaCO₃). Sintered densities of materials produced from ash milled to 95% less than 27 μm increased from 1.38 to 2.63 g/cm³ on increasing the sintering temperature from 1020 to 1080 °C. Firing above 1080 °C caused a rapid decrease in density and sample expansion. The principal crystalline phase present in the high-density material was diopside (CaMgSi₂O₆). This work shows that a significant fraction of incinerator bottom ash can be processed to form sintered materials with properties controlled by ash particle size distribution and sintering conditions.     

AXIAL PHASES DISTRIBUTION AND HOMOGENIZATION IN THREE-PHASE FLUIDIZED BEDS

We have investigated the axial distribution of the liquid phase in a three-phase fluidized bed in which the particles were glass beads of two different diameters, 710-850 μm and 2 mm. Two methods were used to reduce the axial heterogeneity and homogenize the bed: the use of packing elements and the addition of a small quantity of a high molecular weight polymer. The use of packing elements was shown to be very efficient in controlling phases distribution in the bed. The packing consisted of 1 mm thick steel horizontal plates punched and stretched to obtain parallelogram-shaped openings. The addition of a high molecular weight polyisobutylene (BASF Oppanol B246, M_u, =6.15 Lo⁶) on the phase distribution was also studied. Low polymer concentrations up to 397 ppm (based on the liquid phase) were investigated. The addition of the polymer reduced bubble coalescence and allowed for a better distribution of the phases.     

Top-spray fluid bed coating: Scale-up in terms of relative droplet size and drying force

Top-spray fluid bed coating scale-up experiments have been performed in three scales in order to test the validity of two parameters as possible scaling parameters: The drying force and the relative droplet size. The aim was to be able to reproduce the degree of agglomeration as well as the mechanical properties of the coated granules across scale. Two types of placebo enzyme granule cores were tested being non-porous glass ballotini cores (180–350 μm) and low porosity sodium sulphate cores (180–350 μm). Both types of core materials were coated with aqueous solutions of Na₂SO₄ using Dextrin as binder. Coating experiments were repeated for various drying force and relative droplet size values in three top-spray fluid bed scales being a small-scale (Type: GEA Aeromatic-Fielder Strea-1), medium-scale (Type: Niro MP-1) and large-scale (Type: GEA MP-2/3). The tendency of agglomeration was assessed in terms of particle size fractions larger than 425 μm determined by sieve analysis. Results indicated that the particle size distribution may be reproduced across scale with statistical valid precision by keeping the drying force and the relative droplet size constant across scale. It is also shown that none of the two parameters alone may be used for successful scaling. Morphology and microscope studies indicated that the coating layer is homogenous and has similar structures across scale only when both the drying force and the relative droplet size were fixed. Impact and attrition tests indicated that it is possible to produce granules with similar attrition and impact strength across scale and that the two types of mechanical properties are inversely related.     

LIQUID DISPERSING FEATURES IN INNER CHAMBER OF SINGLE VST CAP

By means of photography with high-frequency photoflash and a computer-aided image analyzing system, liquid dispersing features, such as droplet-diameter distribution in the inner chamber of a single vertical sieve tray cap are probed in a square cap of 100 mm×100 mm section, which is set over a Φ70 mm orifice in the tray of a 500 mm×500 mm square column. Photos of the internal cap are taken, illuminated by photoflash from a slit in the upper cover of the cap. Data provided by image analyses show that the distribution of liquid droplets in the cap during operation possesses upper limit log-normal distribution. The coefficients in the upper limit log-normal distribution function are satisfactorily modeled through energy analysis and correlation. Especially, maximum diameter (upper limit of free-floating liquid droplet diameter) of stable liquid droplets, valued 6.426 mm, is derived from theoretical method.     

Electrical resistivity and thermopower of (La1−xSrx)MnO3 and (La1−xSrx)CoO3 at elevated temperatures

Electrical resistivity and Seebeck (S) measurements were performed on (La_1−xSr_x)MnO₃ (0.02x0.50) and (La_1−xSr_x)CoO₃ (0x0.15) in air up to 1073 K. (La_1−xSr_x)MnO₃ (x0.35) showed a metal-to-semiconductor transition; the transition temperature almost linearly increased from 250 to 390 K with increasing Sr content. The semiconductor phase above the transition temperature showed negative values of S. (La_1−xSr_x)CoO₃ (0x0.10) showed a semiconductor-to-metal transition at 500 K. Dominant carriers were holes for the samples of x0.02 above room temperature. LaCoO₃ showed large negative values of S below ca. 400 K, indicative of the electron conduction in the semiconductor phase.     

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WH_U) and down-pumping (WH_D) modes. Glass beads of 50 ∼ 150 μm diameter and density 2500 kg · m^-3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = P_G/P_U) than those with WH_D or WH_U as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WH_U has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.     

Electrical properties of (Bi2O3)0.75(RE2O3)0.25 ceramics (RE=Dy, Y, Ho, Er and Yb)

Five kinds of rare earth stabilized bismuth oxide ceramics, (Bi₂O₃)_0.75(RE₂O₃)_0.25 (RE=Dy, Y, Ho, Er and Yb), were synthesized by sintering a mixture of Bi₂O₃ and RE₂O₃ 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 RE³⁺, respectively. The electrical conductivity at 300 °C slightly increased with the increasing ionic radius of RE³⁺, while at 500 and 700 °C, it was constant regardless of the ionic radius of RE³⁺. The migration activation energy and the association activation energy showed a maximum value and a minimum value at RE=Er, respectively.