Solids mixing in a fluidized bed riser

The solids mixing in a riser with a height of 10 m and 0.186 m inner diameter was investigated by using pneumatic phosphor tracer technique. Considering the shielding effect of the bed material on the light emitted from the phosphor tracer particle, a modified method for the phosphor tracer measurement is proposed. And then the curves of particle residence time distribution were obtained. The experimental results show that the particle diffusion mechanism can be explained by the dispersions of dispersed particles and particle clusters in the axial direction, and as well the core-annulus nonuniform distribution of the solids fraction in the radial direction of the riser. Moreover, based on the experimental results, a two-dimensional dispersion model was established to predict the solids axial and radial diffusion. Furthermore, the effects of superficial gas velocity and solids circulating flux on the axial and radial Peclet number of the particles were discussed; two empirical correlation formulas about the axial and the radial Peclet numbers were given; the calculated values agree well with the experimental results.     

Discrete element simulation of free flowing grains in a four‐bladed mixer

Numerical simulations of granular flow in a cylindrical vessel agitated by a four‐blade impeller were performed using the discrete element method. Velocity, density, and stress profiles within the mixer displayed a periodic behavior with a fluctuation frequency equal to that of the blade rotation. Blade orientation was found to affect flow patterns and mixing kinetics. For an obtuse blade pitch orientation, a three‐dimensional recirculation zone develops in‐front of the blade due to formation of heaps where the blades are present. This flow pattern promotes vertical and radial mixing. No recirculation zone was observed when the blade orientation was changed to an acute blade pitch. The system's frictional characteristics are shown to strongly influence the granular behavior within the mixer. At low friction coefficients, the 3‐D recirculation in front of the obtuse blade is not present reducing convective mixing. Higher friction coefficients lead to an increase in granular temperature which is associated with an increase in diffusive mixing. Normal and shear stresses were found to vary with mixer height with maximum values near the bottom plate. Additionally, a strong dependence between the magnitude of the shear stresses and the friction coefficient of the particles was found. The stress tensor characteristics indicate that the granular flow in our simulations occurs in the quasi‐static regime. At the same time, the averaged pressure was found to vary linearly with bed height and could be predicted by a simple hydrostatic approximation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Analysis and optimization of low-pressure drop static mixers

Various designs of the so called Low-Pressure Drop (LPD) static mixer are analyzed for their mixing performance using the mapping method. The two types of LPD designs, the RR and RL type, show essentially different mixing patterns. The RL design provides globally chaotic mixing, whereas the RR design always yields unmixed regions separated by KAM boundaries from mixed regions. The crossing angle between the elliptical plates of the LPD is the key design parameter to decide the performance of various designs. Four different crossing angles from 90° to 160° are used for both the RR and RL designs. Mixing performance is computed as a function of the energy to mix, reflected in overall pressure drop for all designs. Optimization using the flux-weighted intensity of segregation versus pressure drop proves the existence of the best mixer with an optimized crossing angle. The optimized angle proves to be indeed the LLPD design used in practice: the RL-120 with θ = 120°, although RL-140 θ = 140° performs as good. Shear thinning shows minor effects on the mixing profiles, and the main optimization conclusions remain unaltered. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

A numerical study of mixing in a microchannel with circular mixing chambers

The mixing of fluids in a microchannel is numerically investigated using three-dimensional Navier–Stokes equations. The microchannel has circular mixing chambers that are designed to create a self-circulating flow that operates at low Reynolds numbers. The investigations have been performed on a design that comprises of four circular mixing chambers that are joined together with constriction channels. The study has been carried out in two parts. Firstly, the mixing and the flow field are analyzed for a wide range (1–250) of the Reynolds number. Secondly, the effects of two design parameters, namely, the ratio, w/d, of the width of the constriction channel to the diameter of the circular chamber, and the angle, θ, between the outer walls of the chamber and the connection channel, on the mixing and the flow field have been evaluated. The mixing has been evaluated using a parameter, called mixing index, which is based on the variance of the mass fraction. The mixing index at the end of the device increases rapidly with the Reynolds number. The presence of a flow recirculation zone in the circular chamber is found to be effective in enhancing mixing, especially for larger Reynolds numbers. The mixing performance improves with an increase in θ, and with a decrease in w/d. The characteristics of the pressure drop have also been investigated as a function of the Reynolds number and geometric parameters. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Effects of sand and process water pH on toluene diluted heavy oil in water emulsions in turbulent flow

The presence of sand in heavy oil production is known to enhance oil recovery. Sand can also be detrimental depending on the properties of the sand–water interface. In this process, the water soluble material interacts with both sand and oil droplets and affects emulsion stability. The formation and stability of heavy oil‐in‐water emulsions during turbulent flow using batch process stirred‐tank mixing of oil, sand, and water were investigated at three pH. Size distributions were measured by laser diffraction. High‐speed video photomicrography was used to observe the process during mixing. Results showed that the presence of sand enhanced formation of stable, fine emulsion at basic pH 8.5. When the pH of the water was reduced below 6.5 both sand and droplets surface properties changed, the emulsions became less stable and coalescence was apparent. The sand grains acted as coalescers at low pH and enhanced breakage at high pH. © Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2008 AIChE J, 2009     

劲性搅拌桩芯桩长度的数值分析

劲性搅拌桩是适用于软土地基的一种新型复合桩,桩体由水泥土环桩内包钢筋混凝土芯桩组成。这种桩具有施工方便、造价低廉、单桩承载力高、桩身质量稳定可靠和无污染等多方面的优点,是一种极具开发前景的新型桩。在承载力机理和理论研究方面,针对劲性搅拌桩整体承载力的研究已较为成熟。但关于不同长度的芯桩对劲性搅拌桩整体受力性能及土体的应力应变影响尚缺乏研究。本文以现有的试验数据为基础,借助MIDAS/GTS有限元分析工具对不同芯桩长度的劲性搅拌桩进行的拟合计算;对数值计算结果进行对比分析并得出芯桩长度的合理取值,为劲性搅拌桩的推广和应用提供一些参考。     

超深搅拌桩对软土强度及抗渗性能影响研究

介绍了超深水泥土搅拌桩在软土加固工程中的应用,通过进行室内渗透试验和抗压试验,测定不同土层处桩体的渗透系数及加固体强度,并对试验结果进行分析。试验表明：超深水泥土搅拌桩能够显著降低砂性土层的渗透系数,增强土体的强度。采用超深水泥土搅拌桩加固软土地基是可行的。     

一种通过试拌检验混凝土生产质量稳定性的方法

介绍了一种通过现场试拌来检验混凝土生产质量稳定性的方法,该方法具有简单、直接及周期短的特点,可以被广泛应用.     

复掺粉煤灰和硅灰在自密实混凝土中的应用

复掺粉煤灰和硅灰应用到自密实混凝土中,可以增大新拌混凝土的流动性,改善离析泌水,增加混凝土硬化后的强度,提高其耐久性能。试验研究了粉煤灰的最佳掺量,以及几种矿物质掺合料复掺的复合效应。     

VIC型密炼机用于白炭黑胶料混炼工艺的探讨

温度控制法是目前密炼机排料控制中最常用的方法之一。排料温度不仅影响到混炼胶的性能,而且还影响到产品的使用寿命,特别是对热敏性橡胶和含有白炭黑等特殊材料要求的混炼就更为重要。VIC密炼机具有优良冷却效果和混炼温度可控的优点,本文应用VIC275密炼机对含有白炭黑的胶料进行了混炼实验,实验结果表明其混炼胶性能有了大幅度的提高。