Fundamental dewatering properties of wastewater treatment sludges from filtration and sedimentation testing

The prediction and optimisation of the performance of dewatering devices such as filters and thickeners requires the laboratory measurement of fundamental material characteristics. This work presents the application of recently developed methods for the extraction of meaningful phenomenological properties from filtration and sedimentation testing of wastewater sludges, which has previously been limited due to theoretical and experimental constraints. The results for a sample of digested wastewater treatment sludge described a material with weak network strength that was quite permeable at low solids concentrations and the gel point was about 1 v/v%. The sample compressed to high solids concentrations at moderate pressures (up to 40 v/v% at 400 kPa) but became extremely impermeable at these high concentrations. The solids concentration dependencies of the material characteristics had a form that was expected for highly compressible materials such as wastewater sludges, but had not been reported previously. The extracted material characteristics were then used to predict the test results to show that the characterisation was valid.     

Segregation and mixing effects in the riser of a circulating fluidized bed

Segregation and mixing effects of binary mixtures of particles having difference in sizes and densities were studied in 0.1016 m-diameter riser of a circulating fluidized bed at gas velocities between 2.01 and 4.681 m/s and solids circulation rate between 12.5 and 50 kg/m² s. Two groups of bed materials (three quartz sand-spent fcc catalyst mixtures with different initial mass % of sand and two coal-iron mixtures, one with almost same sizes but with different densities and the other having both different sizes and densities) were used. Using local axial mass % of heavier/coarser particles and their mean sizes the extent of segregation was evaluated. The influence of operating conditions like superficial gas velocity and solids circulation rate on segregation was examined and found that with their increase segregation effects generally tend to decrease and a uniform mixture conforming to initial composition of the mixture results. Using the data available in the literature and those of the present authors an empirical correlation to obtain the extent of segregation in CFBs has been proposed.     

A perspective on vibration-induced size segregation of granular materials

Segregation of particulate mixtures is a problem of great consequence in industries involved with the handling and processing of granular materials in which homogeneity is generally required. While there are several factors that may be responsible for segregation in bulk solids, it is well accepted that nonuniformity in particle size is a fundamental contributor. When the granular material is exposed to vibrations, the question of whether or not convection is an essential ingredient for size segregation is addressed by distinguishing between the situation where vibrations are not sufficiently energetic to promote a mean flow of the bulk solid, and those cases where a convective flow does occur. Based on experimental and simulation results in the literature, as well as dynamical systems analysis of a recent model of a binary granular mixture, it is proposed that “void-filling” beneath large particles is a universal mechanism promoting segregation, while convection essentially provides a means of mixing enhancement.     

循环射流混合槽内液液两相混合特性数值模拟

循环射流混合槽作为一种高效的混合装置在化工过程强化处理技术中具有潜在的工业应用前景。由于缺乏对其内多相体系流动和混合行为的研究,制约了循环射流混合反应器的优化设计与工业化应用。本文选取水和二甲基硅油两相体系,采用计算流体力学软件ANSYS Fluent V16.1中Eulerian-Eulerian多相流模型和SST k-?湍流模型,对两种不同加料方式下循环射流混合槽内液液两相射流中心线速度、离析强度、拉伸率等参数进行研究。研究结果表明：分散相浓度（α_d）增大射流卷吸能耗增大,在l/s<0.4内α_d=1.80%和2.86%量纲为1的射流中心线速度衰减趋势与α_d=6.00%相比减弱51%和21%;在低分散相浓度时,量纲为1的射流中心线速度随Re的增大衰减趋势变化小,在l/s<0.24内Re=6346、9519和12692量纲为1的射流中心线速度衰减趋势与Re=3173相比分别减弱2.60%、2.87%和12.69%。离析强度随混合时间的增大而减小,随周向角度增大呈W形变化趋势。在相含率和雷诺数相同时,对称球状较圆柱状加料达到混合时间减少65.5%;不同喷嘴之间的拉伸率随迹线长度的增大而增大,jet1和jet9位置处的拉伸率与其余喷嘴相比较大;相同喷嘴之间拉伸率随Re的增大而增大,Re=6346、9519和12692的拉伸率与Re=3173相比分别提高289%~320%、418%~454%和607%~667%。     

The solids flow in the CFB-riser quantified by single radioactive particle tracking

Single radioactive particle tracking was used to measure the overall solids residence time (and its distribution) in the riser of a CFB, operating at superficial air velocities (U) of 1 to 9 ms^− 1 and solids circulation fluxes (G) between 20 to 600 kg m^− 2s^− 1.The results demonstrate that the particle motion and mixing differ according to the operating mode of the riser, with a fairly constant velocity throughout the riser achieved in the dilute or dense riser flow, but with a significant amount of back-mixing for intermediate values of U and/or G. This back-mixing is due to the core-annulus mode of particle flow. Whereas experimental results and theoretical predictions are in fair agreement for the dilute and dense riser flow, the core-annulus regime needs to account for a U and G dependent slip factor (φ), in excess of the commonly proposed value φ = 2, especially at U-U_TR < 2 ms^− 1.Moreover, the previously published riser operation diagram is confirmed by the experiments, although a further analysis of the core-annulus regime is needed.     

Prediction of defluidization time of alkali composition at various operating conditions during incineration

This investigation presents a simple model to describe defluidization time. It includes such operating parameters as operating temperature, gas velocity and number of bed particles. The proposed model also uses the plastic-viscous flow mechanism and the average convective solids mass flow rate to evaluate the adhesive and segregation forces. Mass and force balance were employed to develop the model. Then, the thickness of the coating layer was calculated by measuring the density of the agglomerate, to elucidate the coating characteristics of eutectics on the particle surface. The resulting predictions were based on a regression of our previous results. Comparing the experimental data with the predicted defluidization time reveals an error of between − 20% and + 20%, when the proportion of Na in the bed material was between 0.8 and 1. These data represent close agreement.     

FBR for catalytic propylene polymerization: Controlled mixing and reactor modeling

Particle mixing and segregation have been studied in a small-scale fluidized-bed reactor (FBR) under pressure. The solids mixing is relatively faster than the residence time of catalyst particles in the case of a polymerization process, but smaller particles accumulate in the upper zone. Semibatch propylene polymerization experiments showed that the vertical temperature gradients are caused mainly by catalyst segregation. At low gas velocities, segregation and mixing can differ under reacting conditions compared to nonreacting conditions due to particle–particle interactions. Catalyst concentration gradients caused by incomplete mixing are strengthened remarkably by the exothermic reaction even at low polymerization rates. These observations do not represent an industrial situation. The FBR has therefore been equipped with a draft tube and cone to control vertical solids mixing. The internal solids circulation rate is a nonlinear function of the gas velocity. Strongly reduced segregation, elutriation, and entrainment observed were compared to experiments without a draft tube. Temperature profiles observed during polymerization can be controlled by the solids circulation rate. Hydrogen injections led to an instantaneously increased polymerization rate, probably due to the reactivation of dormant sites. Irreversible deactivation rates of dormant and active sites seem to be the same. Moreover, hydrogen appeared to be very effective for widening the molecular-weight distribution. A compartment model developed describes the temperature profile in the reactor and related molecular-weight distribution of the polymer.     

Flow patterns in high-velocity fluidized beds and pneumatic conveying

Four flow patterns are identified for gas-solids vertical upward flows. Homogeneous dilute phase flow is characterized by the absence of both radial and axial solids segregation. Heterogeneous dilute phase flow (also called core-annulus flow) is characterized by the absence of axial solids segregation, with solids carried upward in the core and travelling downward near the outer wall due to the formation of particle streamers. Collapsed flow with a lower dense region and an upper dilute region is also referred to as the fast fluidization regime. In this case, the flow structure in the upper dilute region is similar to that in heterogeneous dilute phase flow, while the lower dense region resembles that in a turbulent fluidized bed. Dense phase flow can be reached when the riser is completely occupied by a relatively dense suspension with little axial density variation and no net solids downflow near the riser wall. The transition from fast fluidization to dense phase flow is still not clearly defined.     

LASER-BASED FLOW MEASUREMENTS OF DILUTE VERTICAL PNEUMATIC TRANSPORT

Dilute vertical pneumatic transport in a vertical lifter was studied using the sophisticated measurement techniques of laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The vertical lifter consisted of a lower fluidized silo, an upper receiving tank, and a connecting vertical transport pipe made of clear glass. The experimental study was performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Particle diameter, axial particle, and tangential particle velocities, as well as root mean square velocities, were measured simultaneously for different flow conditions. In addition, overall solid mass fluxes were obtained using weighing cells. Smooth and spherical zirconium oxide (ZrO₂) solids were applied with two different particle size distributions. Measurements were performed using different flow rates of air. The air inlet condition was varied in order to study its effect on the flow behavior. The particle diameter measurements show that no axial or radial segregation by size occurs for this transport condition. The results show that the particle velocity is independent of the particle size as well. The axial velocity profiles at different heights are almost identical and flat, which indicates fully developed turbulent pipe flow. The turbulent velocity measurements show that turbulence is mainly caused by the velocity gradients, and not by particle-particle collisions in dilute flow. The solid mass flux measurements show the importance of optimum inlet condition and how this influences the mass flux.     

Effect of ethanol content on supercritical carbon dioxide extraction of caffeine from tea stalk and fiber wastes

This study presents the effect of ethanol content on supercritical carbon dioxide extraction of caffeine from tea plant wastes. Tea stalk and fiber wastes of Turkish tea plants that have no economical value were evaluated as raw material throughout the caffeine extraction experiments. These wastes were supplied from tea factory marked “Çaykur” in the east blacksea region. They were separately ground, sieved and dried at 105 °C temperature in an oven. Parameters affecting caffeine leaching from tea wastes were determined to be, ethanol flow rate, extraction time, extraction temperature, carbon dioxide flow rate, process pressure and particle size. The maximum yield of caffeine from tea stalk wastes and fiber wastes were 14.95 mg/g tea stalk and 18.92 mg/g tea fiber, respectively. When the supercritical extraction conditions used of ethanol as cosolvent have been compared with the conditions of used only carbon dioxide, approximately the same yield has been reached at 2 h extraction period instead of 7 h. Beside of saving of the time and the amount of carbon dioxide, the supercritical extraction yield with cosolvent increase had been recorded as 62.5% and 63.1%, respectively, in comparison with the chloroform extraction as conventional method of tea stalk and fiber wastes.     

Gravity separation of bidisperse suspensions: Light and heavy particle species

Gravity separation of bidisperse suspensions containing particle species lighter and heavier than the suspending fluid in a vertical tube has been studied both theoretically and experimentally. The suspension was relatively dilute having a total solids volume concentration of no more than 16%. In this dilute range, lateral segregation of light and heavy particles into clusters and formation of fingering flow structure do not occur. The settling velocities of both particle species are retarded.Five models published by Lockett and Al-Habbooby, Mirza and Richardson, Masliyah, Selim et al. and Patwardhan and Tien, were used to predict the settling velocities of the heavy and the light particle species in bidisperse suspensions. The last three of these models give good predictions for the experimental data obtained in this study.     

Control of particle size distribution for the synthesis of small particle size high solids content latexes

A polymerization process to synthesize bimodal latexes with maximum particle diameters below 350 nm and solids content above 65 wt% has been developed.The process is based on an iterative strategy to determine the optimal particle size distribution that gives the maximum packing factor for a given range of particle sizes and at a given solids content. The calculated optimal bimodal PSD was experimentally obtained in a seeded semi-continuous emulsion polymerization reaction as follows: in the first step, a polymer seed latex was loaded in the reactor and grown, under monomer starved conditions, until a given particle size. At this point a fraction of the same seed was added to the reactor and the feed was continued until the desired particle size distribution and solids content were achieved. The point at which the seed was added again to the reactor and the amount of seed required were determined by the iterative strategy and depended on the competitive growth rate ratio of large and small particles that is an input for the iterative strategy.Implementation of the solution obtained from the iterative strategy, and for the first time in the open literature, led to the production of a coagulum free and stable bimodal latex with 70 wt% of solids content and particle sizes below 350 nm.     

An experimental study on the effect of liquid content and viscosity on particle segregation in a rotating drum

The segregation phenomenon of wet granular materials was experimentally studied in a quasi-2D rotating drum. The mono-disperse systems and binary-mixture systems (with 4 mm and 2 mm glass beads and 40% filled volume fraction) were used. All the experiments were controlled so the Froude number of the rolling regime was 2.79 × 10^− 4. The effects of the volume and the viscosity of the liquid added to the granular system on the segregation index and angle of repose in the rotating drum were investigated and are discussed in this paper.The experimental results indicate that the volume and viscosity of the added liquid have significant effects on the wet granular flow. The results demonstrate that the segregation index decreases with an increase of the repose angle of the wet granular materials, regardless of the volume or viscosity of the added liquid.     

LASER-BASED FLOW MEASUREMENTS OF DILUTE VERTICAL PNEUMATIC TRANSPORT

Dilute vertical pneumatic transport in a vertical lifter was studied using the sophisticated measurement techniques of laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The vertical lifter consisted of a lower fluidized silo, an upper receiving tank, and a connecting vertical transport pipe made of clear glass. The experimental study was performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Particle diameter, axial particle, and tangential particle velocities, as well as root mean square velocities, were measured simultaneously for different flow conditions. In addition, overall solid mass fluxes were obtained using weighing cells. Smooth and spherical zirconium oxide (ZrO₂) solids were applied with two different particle size distributions. Measurements were performed using different flow rates of air. The air inlet condition was varied in order to study its effect on the flow behavior. The particle diameter measurements show that no axial or radial segregation by size occurs for this transport condition. The results show that the particle velocity is independent of the particle size as well. The axial velocity profiles at different heights are almost identical and flat, which indicates fully developed turbulent pipe flow. The turbulent velocity measurements show that turbulence is mainly caused by the velocity gradients, and not by particle-particle collisions in dilute flow. The solid mass flux measurements show the importance of optimum inlet condition and how this influences the mass flux.     

A method for pomegranate seed application in food industries: Seed oil encapsulation

During the industrial processing of pomegranate, large volumes of industrial wastes (seeds, peels, leaves) are produced, which have a wide range of nutritional values. In this work, a new method for pomegranate seed application in food industries was developed based on the extraction of seed oil and its subsequent encapsulation by spray drying. Skimmed milk powder was used as encapsulating agent. Ratio of core to wall material, feed solids concentration, inlet air temperature, and drying air flow rate were the factors investigated with respect to encapsulation efficiency using a central composite design. The resulting microcapsules were evaluated in terms of moisture content, particle size, bulk density, and hygroscopicity. The optimum operating conditions were found to be: ratio of core to wall material, 1/9; feed solids concentration, 30% (w/w); inlet air temperature, 187 °C; drying air flow rate, 22.80 m³/h. Under these conditions, the maximum encapsulation efficiency was about 95.6%.     

DEM-CFD simulation of the particle dispersion in a gas-solid two-phase flow for a fuel-rich/lean burner

The objective of this study was to numerically investigate the particle dispersion mechanisms in the gas-solid two-phase jet for a fuel-rich/lean burner by means of coupling the discrete element method (DEM) with the computational fluid dynamics (CFD). The DEM was employed to deal with the particle-particle and the particle-wall interaction in the computation of solid flow; while gas flow was computed by CFD based on the commercial software package Fluent. The particles with various Stokes numbers equal to 0.1, 0.5, 1, 2 and 3 (corresponding to particle diameter 10.8, 24.17, 34.18, 48.38 and 59.21 μm, respectively) in the gas-solid fuel-rich/lean jet were investigated in this study. The particle-particle collision was simulated and its effect on the fuel-rich/lean separating performance was evaluated. The results show that the particle-particle collision occurred more frequently with the increasing of Stokes numbers from 0.1 to 3. The particle dispersion became more uniform between the fuel-rich side and the fuel-lean side for particles with small Stokes number; while for particles at St > 1, a better fuel-rich/lean separating performance was achieved. The efficiency of the DEM-CFD coupling method was validated by the corresponding experiments, and a good agreement between the simulation and experiments was achieved as a result of the particle-particle collision.     

Macro- and micromixing in a novel sonochemical reactor using high frequency ultrasound

This paper deals with the influence of ultrasound on macro- and micromixing in a new developed sonochemical reactor. Unprecedented piezoelectric transducer arrangement with a high frequency of 1.7 MHz has been used in this novel reactor. Macromixing quality has been investigated visually and the Dushman reaction (iodide-iodate) coupled with a neutralization reaction have been examined in order to characterize micromixing quality. In addition, the effect of liquid viscosity on the segregation index has been studied. The results show that this new developed reactor can establish reasonable macro- and micromixing inside the reactor. Moreover, the performance of this reactor has been compared with a stirred tank reactor equipped with a Rushton turbine impeller. It is found that with the same input electrical power, the obtained segregation index for stirred tank reactor is approximately 10% more than proposed new ultrasound reactor, which means the sonoreactor works more efficiently.     

Percolation segregation of binary mixtures under periodic movement

Three strain rates of 1.0, 0.5, and 0.25 Hz were selected for studying percolation segregation in binary mixtures of urea (spherical) and potash (angular). Mixed binary samples prepared from three mean coarse sizes with their corresponding three and two fines sizes for potash and urea, respectively. Herein, three coarse mean sizes 3675, 3075, and 2580 µm and three mean fine sizes 2180, 1850, and 1550 µm were selected for tests. Percolation segregation in mixed binary sample was quantified using the primary segregation shear cell (PSSC-II). Based on experimental results, the segregated fines mass, normalized segregation rate (NSR) and segregation rate of fines for binary mixtures increased with increasing strain rate from 0.25 Hz to 1.0 Hz. The NSR decreased with decreasing strain rate from 1.0 Hz > 0.5 Hz > 0.25 Hz for size ratios 1.7, 2.0, and 2.4 (p < 0.05). At these three strain rates, for size ratio 2.0, the NSR of coarse size 3675 µm with fines size 1850 µm was smaller than the NSR of coarse size 3075 µm with fine size 1550 µm in the binary mixtures (p < 0.05). At three strain rates of 1.0, 0.5, and 0.25 Hz, the NSR for potash was higher (53%, 56%, and 46%) than the NSR for urea for the same size ratio (p < 0.05).     

Instability-induced clustering and segregation in high-shear Couette flows of model granular materials

Controlling unwanted segregation of components within a particle mixture is a longstanding goal in particle processing technologies. We investigate flow-induced clustering and consequent segregation of cohesionless particulate mixtures flowing rapidly in high-shear Couette geometries, comparing results from particle-dynamic (PD) simulations, kinetic theory and experiments. Using spheres and disks and a simplified plug instability as a surrogate for the variety of coherent flow structures, we find that density, velocity, and granular temperature gradients, and possibly, initiation of vorticity, influence the onset and nature of segregation. For equal density particles, simulations and experiments show that there exists a critical solids fraction at which the direction of segregation is reversed and streamwise diffusivity drops significantly, which appears to correspond to a point where the thermal diffusion flux no longer dominates over other terms. Results compare favorably to those from binary kinetic theories with one exception. We find that the 1D steady-state theoretical solutions do not capture the flux reversal observed in PD simulations of the equal density mixtures. Finally, we illustrate how local density variations can severely affect particle size distribution measurements.