Residence Time Distributions in Ozone Contactors

The hydraulics in ozone systems, characterized by the residence time distribution, are investigated numerically as well as experimentally. The complex geometry of the ozone contactors requires the application of computational fluid dynamics (CFD), which in combination with experimental results gives insight in the hydraulic processes. Particle tracking provides a distribution of CT-values (dissolved ozone concentration times residence time) to estimate disinfection precisely and points out dead zones that hamper disinfection. The CFD modeling predicts that small changes in geometry reducing the strength of the recirculation zone can significantly increase the inactivation of micro-organisms.     

Application Of A Single-Bubble Model In Estimation Of Ozone Transfer Efficiency In Water

A single-bubble model of mass transfer in gas-liquid systems enables the estimation of transfer efficiencies under different process conditions. In particular, it can be applied to simulate the effects of bubble size, value of the mass transfer coefficient, kinetics of reactions taking place in water and depth of the contact chamber. The results of such modelling in terms of transfer efficiency are presented for physical and chemical absorption of ozone in water at different hydrodynamic conditions (bubble size, water temperature, water depth in the contact chamber, and initial ozone concentration in the bubbles). The results of computations are compared with some reference data on ozone absorption in water in industrial-scale contact chambers.     

多层带式干燥机风速场的CFD模拟及检验

多层带式干燥机箱内风速分布不均匀是导致干燥效率低、干燥不均匀的主要原因。对多层带式干燥机风速场进行了实验研究，建立了多层带式干燥机箱的CFD理论模型，对其风速场进行了数值模拟，并与实验测试结果进行对比，认为用CFD方法模拟多层带式干燥机箱内风速场是可行的。     

Numerical and experimental investigation of a fluidized bed chamber hydrodynamics with heat transfer

The hydrodynamics and heat transfer of a gas-solid fluidized bed chamber was investigated by computational fluid dynamic (CFD) techniques. A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied to simulate the unsteady state behavior of this chamber. For momentum exchange coefficients, Syamlal-O’Brien drag functions were used. A suitable numerical method that employed finite volume method was applied to discretize the equations. The simulation results also indicated that small bubbles were produced at the bottom of the bed. These bubbles collided with each other as they moved upwards forming larger bubbles. Also, the solid particle temperature effect on heat transfer and hydrodynamics was studied. Simulation results were compared with the experimental data in order to validate the CFD model. Pressure drops and mean gas temperature predicted by the simulations at different positions in the chamber were in good agreement with experimental measurements at gas velocities higher than the minimum fluidization velocity. Furthermore, this comparison showed that the model could predict hydrodynamics and heat transfer behaviors of gas solid fluidized bed reasonably well.     

Modeling and Validating the Effective Hydraulic Detention Time for a 10 mgd Ozone Contactor at the Lake Washington Surface Water Treatment Plant,Melbourne, Florida

Ozonation has been added to the treatment process at the 20 mgd Lake Washington Surface Water Treatment Plant (SWTP), Melbourne, Florida to improve finished water quality, control taste and odor, and provide greater than 0.5-log credit for inactivation of Cryptosporidium. The new ozone contactors (2 parallel 10 mgd trains) are located between the existing Actiflo^TM clarifiers and the filters on a relatively congested site. Prior to construction, a Computational Fluid Dynamics (CFD) study indicated that the proposed contactor design was not optimized to provide the targeted Cryptosporidium log inactivation. Based on the CFD modeling, the design was modified to reduce the baffle heights (increasing end gaps at turns) and the baffle ends were chamfered to improve flow patterns within the contactor. Internal access platforms were rotated from parallel to the flow (along outside walls) to perpendicular to the flow (along baffle walls). These small design changes increased the modeled effective detention time (t₁₀) for disinfection and the corresponding hydraulic efficiency (baffling factor) by 22 %. In addition, the dosing location for the ozone quenching chemical (hydrogen peroxide) was modified to provide extra ozone detention. The ozone contactors were then constructed to incorporate the design changes optimized from the modeling. During the start up of the ozone system, fluoride tracer tests were completed to validate the effective detention time (and baffling factor) calculated by CFD. Furthermore, a series of modifications are planned for the ozonated water sampling regime to take greater advantage of Ct₁₀ credits and reduce the required ozone dose, thereby saving energy and operating costs. This paper discusses several calculation methods for Cryptosporidium inactivation Ct₁₀ reporting in compliance with the Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and the LT2ESWTR Pre-proposal Draft Regulatory Language for Stakeholder Review (USEPA, 2002). The selection of the method will depend upon ease of implementation, modifications required to the ozone-in-water residual sampling system, long-term results of bromate sampling and actual Cryptosporidium bin classification under the LT2ESWTR. The article describes the CFD studies, improvements made to the contactor design, the tracer validation of the design, Ct₁₀ reporting methods and modifications to the ozone residual sampling system that the City is considering.     

基于CFD⁃DEM的水下切粒装置水室内颗粒流动过程数值模拟

基于CFD?DEM耦合方法,研究了颗粒在水室内的流动状态,分析了不同刀盘转速、粒子水通入量和水室出口角度对造粒过程的影响,发现提高刀盘转速、增加粒子水通入量和水室出口倾斜一定的角度都有利于水室内颗粒的排出。进一步研究了颗粒与碎屑在水室内的流动,发现在水室出口处二者的流动基本呈现出一定的分离角度。     

Optimization of Mixing and Mass Transfer in In-line Multi-Jet Ozone Contactors Using Computational Fluid Dynamics

Typical ozone mixing and mass transfer calculations are lumped approaches based on ideal operating conditions and can misrepresent behavior in real-life installations. This article models the effect of local hydrodynamics and mixing on the overall mass transfer of ozone into water with the aid of multiphase computational fluid dynamics (CFD). CFD models were validated with measured data from a pipeline ozone contactor installation which was optimized for more rapid, uniform mixing and mass transfer. Results emphasize the sensitivity of mixing quality to nozzle placement, size, orientation and spacing relative to main pipeline diameter and flows.     

The Role of Mixing in Ozone Dissolution Systems

The mixing efficiency of four alternative ozone dissolution systems, including conventional bubble diffusion and pipeline injection/diffusion reactors, were compared by measuring the variability of ozone residual measurements at the outlet of each reactor. The standard deviation and coefficient of variation of a time series of residual measurements were used to provide a quantitative measure of the level of mixing in the reactor. The results indicate that properly designed pipeline dissolution systems provide significantly improved mixing over oxygen-fed or air-fed bubble diffusion systems. Minimum mixing criteria for bubble diffusion systems cited in the literature may underestimate the level of mixing required to achieve stable ozone residuals downstream of the dissolution chamber of conventional bubble diffusion chambers.     

Investigation of Ozone Loss Rate Influenced by the Surface Material of a Discharge Chamber

Decay characteristics of ozone concentration in oxygen in a chamber with three types of wall material (stainless steel, copper, and aluminum) are measured using the 254 nm photoabsorption method. Effective lifetimes of ozone are estimated from decay curves of ozone concentration. These values depend on the wall material: They are largest for stainless steel and smallest for aluminum. The relationship between effective lifetime and gas pressure is investigated precisely to determine three values. The equivalent diffusion coefficient of ozone in oxygen and the reflection coefficient of ozone at the wall correspond to the loss rate of ozone at the wall. The collisional loss (quenching) rate coefficient of ozone in oxygen is also determined.     

Simulation of a Spray Dryer Fitted with a Rotary Disk Atomizer Using a Three-Dimensional Computional Fluid Dynamic Model

Spray dryers fitted with a rotary disk atomizer are widely used in many industries requiring high throughputs to produce powders from liquid streams. The interaction between droplets or particles and the drying medium within the drying chamber is still not well understood and hence difficult to model reliably. In this article CFD results are presented to describe the behavior of the performance of a spray dryer fitted with a rotary disk atomizer in a cylinder-on-cone chamber geometry. Four different turbulence models, i.e., standard k - ε, RNG k - ε, Realizable k - ε, and Reynolds stress models were tested and compared to simulate the swirling two-phase flow with heat and mass transfer in the chamber. The results of this investigation can provide further insight into turbulent swirling flow modeling. The predicted results, such as, air flow patterns, air velocity and temperature, distributions, particle/droplet trajectories, drying performance etc., are obtained using the CFD code FLUENT6.1. Comparison with available limited experimental data shows that CFD results display reasonable agreement. Predicted results also show that the RNG k - ε model performs better in this specific case.     

Simulation of a Spray Dryer Fitted with a Rotary Disk Atomizer Using a Three-Dimensional Computional Fluid Dynamic Model

Abstract

Spray dryers fitted with a rotary disk atomizer are widely used in many industries requiring high throughputs to produce powders from liquid streams. The interaction between droplets or particles and the drying medium within the drying chamber is still not well understood and hence difficult to model reliably. In this article CFD results are presented to describe the behavior of the performance of a spray dryer fitted with a rotary disk atomizer in a cylinder-on-cone chamber geometry. Four different turbulence models, i.e., standard k ? ε, RNG k ? ε, Realizable k ? ε, and Reynolds stress models were tested and compared to simulate the swirling two-phase flow with heat and mass transfer in the chamber. The results of this investigation can provide further insight into turbulent swirling flow modeling. The predicted results, such as, air flow patterns, air velocity and temperature, distributions, particle/droplet trajectories, drying performance etc., are obtained using the CFD code FLUENT6.1. Comparison with available limited experimental data shows that CFD results display reasonable agreement. Predicted results also show that the RNG k ? ε model performs better in this specific case.     

A Bench-Scale,Two-Phase Reactor For Ozone Treatment: Feasibility Studies For High Strength Wastes

A batch reactor was designed to study the effects of ozone on a complex industrial waste. The objective of the batch reactor was to allow a large mass of ozone to be applied to the waste without losses associated with gas sparging and assure better accuracy than expected from continuously bubbled gas systems. The reactor consisted of two parts: a 478.6 mL cylindrical section for holding effluent and a 1.078 L spherical section for the ozone/oxygen gas mixture. Ozone concentrations were measured at ambient temperature and pressure using the UV absorption method. Ozone diffusion into a reactive test solution in the static condition (during ozone charging of the spherical chamber) was limited to 0.5 mg of the 1,500 mg passed through the spherical chamber.

The batch reactor was shown to be capable of 100% ozone mass transfer during the contacting operation. The unit was shown to be a suitable device for evaluation of the effects on high ozone demand solutions.     

Study on ozone bleaching of cotton fabric – process optimisation, dyeing and finishing properties

In this work the role played by the presence of moisture and pH during ozone bleaching of grey cotton fabric and the design features of the ozone application chamber are reported. The results are explained with the help of a simplified model. The dyeing and resin finishing properties of ozone-bleached fabric are compared with those of hydrogen peroxide-bleached fabric.     

Is Coefficient of Variation a Realistic Index for Characterizing Mixing Efficiency in Ozone Applications?

ABSTRACT

The coefficient of variation (COV) of ozone residual is often used to compare the mixing performance of different ozone contacting systems. Multiphase mass transfer CFD modeling is performed and compared with experimental data to investigate the correlation between mass transfer efficiency, a corresponding full cross-sectional spatial COV, a corresponding “grab sample” temporal COV, and a comprehensive uniformity index for mixing for varying sidestream ozone doses. Typical sampling methodology for ozone residual is reviewed and general guidelines for better representative sampling are suggested.     

A computational study of combustion in compression ignition natural gas engine with separated chamber

The combustion process and fluid flow in a compression ignition natural gas engine with separated chamber are studied by coupling Multi-dimensional Computational Fluid Dynamic (CFD) code FIRE with detailed chemical kinetic mechanism. The calculated data are first validated by experimental data. Subsequently, Multidimensional simulations at a baseline condition are carried out to investigate combustion process, fluid flow, mixture formation and NO emission distribution in the chamber. Ultimately, parametric studies are conducted to illustrate the effect of some parameters on engine performance. The results show that calculated data are in good agreement with the experimental data. It is revealed that auto-ignition of natural gas occurs in separated chamber when the local mixture temperature reaches around 1300 K and engine performance has a strong dependency on some parameters, such as injection timing, composition of natural gas and initial temperature.     

Effect of ejector configuration on hydrodynamic characteristics of gas-liquid ejectors

Ejectors are gas-liquid contactors that are reported to provide higher mass transfer rates than conventional contactors. Detailed experiments were performed and computational fluid dynamics (CFD) modeling studies were undertaken to understand the hydrodynamic characteristics of the ejector geometry. The CFD model provides a basis for quantifying the effects of operating conditions on the ejector performance. CFD studies shows that there is an optimum ratio of nozzle area to throat area (area ratio), at which the liquid entrainment rate is the highest. This can lead to substantial economic benefit in the industrial practice. The liquid entrainment rate correlates with pressure difference between the water surface in the suction chamber and the throat exit for a wide variety of ejector geometries and operating conditions.     

Ozone for Inactivation of Aerosolized Bacteriophages

Because of increasing incidence of virus-containing aerosols, ozone was potentially considered to be a promising method to inactivate airborne viruses. In this investigation, bacteriophages MS2, phi X174, phi 6, and T7 are under evaluation. The effects of ozone concentration, contact time, different capsid architecture of virus and relative humidity (RH) on inactivating airborne viruses by ozone were evaluated in a laboratory test chamber. It was observed that the survival fraction of airborne virus decreased exponentially with increasing ozone dose. Airborne viruses required ozone doses of 0.34 to 1.98 and 0.80 to 4.19 min-mg/m ³ for 90% and 99% inactivation, respectively. For all four tested, the ozone dose for 99% inactivation was 2 times higher than that for 90% inactivation. At airborne phase with a short contact time, viruses with more complex capsid architectures were observed to be less susceptible to ozone inactivation than those with simple ones. For all tested viruses at the same inactivation, the required ozone concentration at 85% RH was lower than that at 55% RH, possibly because the generation of more radicals from ozone reacting with water vapor at the higher RH. In summary, it was concluded that ozone is highly effective for the inactivation of airborne virus.     

Killing Effect of Ozone on House Dust Mites,the Major Indoor Allergen of Allergic Disease

This study examined whether ozone could kill house dust mites (HDMs), one of the most common causes of allergic diseases, and if an ozone application might be helpful in the environmental control of allergic patients. The experiments were performed in a small chamber (50 cm³), in which the continuous contact between the gaseous ozone and 40–60 live HDMs could be maintained during the reaction time (temperature=25°C and relative humidity=75%). Within the ozone range of 0.19–10.62% (v/v), the higher concentration dose resulted in a more rapid inactivation of the live HDMs. The CT value of ozone showed a linear relationship with the inactivation efficiency (%) of the live HDMs. From our results, it was found that a CT value 400 mg-min/L was required to obtain an almost 100% removal efficiency of the 40–60 live HDMs.     

紫外老化箱风道及试验箱结构的优化设计

本文就BUV2000C型紫外老化箱进行CFD数值模拟仿真,对其试验室箱体及风道截面温度分布图及流场轨迹图进行了分析,发现温度偏差较大,箱体内空气流场紊乱。在此基础上,提出一种风道及试验箱排风口结构的改进设计方案,用仿真软件建立试验箱及风道模型后对其进行气体流场及温度分布模拟,得到了温度偏差较小的模拟结果。为BUV2000C型老化箱的优化设计提供了有价值的参考。     

Enhancement of Wastewater Disinfection Efficiency in Full-Scale Ozone Bubble-Diffuser Contactors

Tracer and disinfection tests were performed with the ozone bubble-diffuser contactors at the Belmont and Southport Advanced Wastewater Treatment Plants operated by the City of Indianapolis, Department of Public Works. The objective of the study was to develop a better understanding for the role of hydrodynamics and contactor design on the disinfection efficiency achieved in these contactors. Tracer tests were performed at varying gas and wastewater flow rates. The results indicated that high backmixing occurred within each chamber of the over-under ozone bubble-diffuser contactor trains. The addition of three baffles to one of the contactor trains resulted in a decrease in overall contactor backmixing. Low contactor backmixing was observed at high wastewater flow rates combined with high or medium gas flow rates for both the modified and original trains. Monitoring of effluent fecal coliform concentrations for both the original and modified contactor trains revealed lower average concentrations in the modified train effluent as compared to the original train.