射流鼓泡反应器中液相体积传质系数的测定

羰基合成反应一般采用射流鼓泡反应器,该类反应器气液混合的方式采用射流而非机械搅拌,其主要优点是结构简单、制作简便、维护费用低。研究该类型反应器的传质系数对于其设计、优化及放大操作均具有重要意义。本研究采用缩颈式圆形喷嘴,以动态溶氧法对射流鼓泡反应器内的液相体积传质系数进行测定,考察了表观气速、射流雷诺数对液相体积传质系数的影响。研究发现,随气速增大液相体积传质系数的变化规律为先增大而后保持不变。维持表观气速不变,随雷诺数增加液相体积传质系数增大,但当表观气速小于0.0012 m/s时,雷诺数对传质改善较小。建立了液相体积传质系数的经验关联式,当气体输入功率占总功率56%时,液相体积传质系数最大,气体鼓泡和液体射流的协同作用最强。     

Insight into the electroreduction of nitrate ions at a copper electrode, in neutral solution, after determination of their diffusion coefficient by electrochemical impedance spectroscopy

The electrochemical reduction of nitrate ions at a copper electrode in an unbuffered neutral aqueous solution is studied. Using a two compartment electrochemical cell, three stationary cathodic waves, noted P1, P2 and P3, were evidenced by cyclic voltammetry at −0.9, −1.2 and −1.3 V/SCE, respectively. By comparing the electrochemical response of nitrate and nitrite containing solutions, P1 was attributed to the reduction of nitrate to nitrite. In order to assign P2 and P3 features by determining the number of electrons involved at the corresponding potential, rotating disk electrode experiments at various rotation speeds, combined with linear sweep voltammetry, were performed. Current data analysis at a given potential was carried out using Koutecky-Levich treatment taking into account water reduction. Confident values of the diffusion coefficient D of nitrate ions were assessed by electrochemical impedance spectroscopy for nitrate concentrations of 10⁻³, 10⁻² and 10⁻¹ M. For a nitrate concentration of 10⁻² M, D was found to be 1.31 × 10⁻⁵ cm² s⁻¹ allowing the number of electrons to be determined as 6 for P2 and 8 for P3, in accordance with nitrate reduction into hydroxylamine and ammonia, respectively. The formation of hydroxylamine was confirmed by the observation of its reoxidation at a Pt microelectrode present at the Cu electrode/nitrate solution interface.     

悬浮载体对不同曝气器氧转移特性的影响研究

以中试装置为研究对象,对比曝气器形式、通气量和悬浮载体填充率对曝气设备氧转移特性的影响。结果表明,微孔曝气器标准氧总转移系数(KLas)高于大孔曝气器;微孔与大孔曝气器的KLas随通气量的增加而提高,悬浮载体的增加有助于提高微孔曝气器的KLas,也可显著提高大孔曝气器的KLas。随着通气量增加或载体填充率的增大,微孔曝气器比标准氧转移效率(SSOTE)增大,优化通气量和填充率分别为≤40 m^3/h和≤40%;大孔曝气器的SSOTE显著增加,优化填充率为40%。随着通气量增加,微孔曝气器理论动力效率(Ep)保持不变或降低,且其通气量应≤32 m^3/h;而大孔曝气器Ep显著增加;二者优化填充率均为<40%。     

Residence time distribution and oxygen transfer in a novel constructed soil filter

BACKGROUND: The constructed soil filter (CSF), also known as soil biotechnology is a system for water renovation, which makes use of formulated media, culture of soil micro‐ and macro‐organisms, additives and plantation to purify water and wastewater. The process gives benefits in terms of applicability across very small to large scale, natural aeration, absence of moving parts, no biological sludge generation, odor free green aesthetic ambience. RESULTS: Residence time distribution (RTD) studies were carried out using laboratory scale CSF. Pulse potassium bromide tracer tests were carried out to determine RTD, and the Peclet number found to be 9–13 for a 2 m bed, and 2–3 for a 0.30 m bed with oxygen transfer of 0.08 h^?1. CONCLUSION: The two‐channel dispersion model for flow behavior shows a good fit to the experimental data, indicating a reactor Peclet number 9–13 for a 2 m bed and 2–3 for a 0.3m bed. Oxygen transfer studies carried out using various methods gave an oxygen transfer coefficient of about 0.08 h^?1. Wastewater purification studies indicate overall COD removal rate of around 50 mg L^?1 h^?1, suggesting that highly aerobic conditions are prevalent in the CSF system. Copyright © 2009 Society of Chemical Industry     

Study of geometry and operational conditions on mixing time, gas hold up, mass transfer, flow regime and biomass production from natural gas in a horizontal tubular loop bioreactor

A horizontal tubular loop bioreactor (HTLB) was used for production of biomass from natural gas. Hydrodynamic characterizations (mixing time and gas hold up) and mass transfer coefficients were considered in the HTLB (L=2.2 m, H=0.4 m and D=0.03 m) as functions of design parameters, i.e., horizontal length to diameter ratio (L/D) and volume of gas-liquid separator (S) as well as operational parameters, i.e., superficial gas and liquid velocities (U_sG, U_sL). In addition, flow regime in different gas and liquid flow rates was investigated. It was observed from experimental results that U_sL has remarkable effects on gas hold up and k_La due to its influence on mixing time. The volumetric mass transfer coefficients for oxygen (k_La_O2) and methane (k_La_CH4) were determined at different geometrical and operational factors. In average, the amount of oxygen consumption for metabolism is approximately 1.4 times higher than that of methane. In bubble flow regime, the HTLB was used for biomass production, too. A gas mixture of 50% methane and 50% oxygen (based on results of dry cell weight, optical density and doubling time) was the best gas mixture inlet for biomass production. The empirical correlations for mixing time, gas hold up and k_La in terms of U_sG, U_sL, L/D and volume of gas-liquid separator were obtained and expressed separately.     

三相循环流化床中的气液相界面积和传质系数

采用溶氧法测量了三相循环流化床中液相溶氧浓度的轴向分布,并按轴向扩散模型处理实验数据,优化得到气液体积传质系数kLa,同时用光纤探头测量了体系中的气含率和气泡大小分布,计算得到了气液相界面积a和气液传质系数kL,并研究了主要操作条件(表观气速、表观液速和固含率)对气液传质系数的影响规律.     

Mass transfer performance and correlations for CO2 absorption into aqueous blended of DEEA/MEA in a random packed column

The mass transfer performance of CO₂ absorption into blended N,N‐diethylethanolamine (DEEA)/ethanolamine (MEA) solutions was investigated using a lab‐scale absorber (H = 1.28 m, D = 28 mm) packed with Dixon ring random packing. The mass transfer coefficient K_Ga_v, the unit volume absorption rate Φ, outlet concentration of CO₂ (y_CO2), and the bottom temperature T_bot of CO₂ in aqueous DEEA/MEA solutions were determined over the feed temperature range of 298.15–323.15 K, lean CO₂ loading of 0.15–0.31 mol/mol, over a wide range of liquid flow rate of 3.90–9.75 m³/m²‐h, by using inert gas flow rate of 26.11–39.17 kmol/m²‐h and 6–18 kPa CO₂ partial pressure. The results show that liquid feed temperature, lean CO₂ loading, liquid flow rate, and CO₂ partial pressure had significant effect on those parameters. However, the inert gas flow rate had little effect. To allow the mass transfer data to be really utilized, K_Ga_v and y_out correlations for the prediction of mass transfer performance were proposed and discussed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3048–3057, 2017     

Measurement of mass transfer coefficient in an airlift reactor with internal loop using coalescent and non‐coalescent liquid media

In this work the sulfite oxidation (SOM), dynamic pressure‐step (DPM) and gassing‐out (GOM) methods were compared for volumetric mass transfer coefficient measurement in an airlift reactor with internal loop. As a liquid phase both, non‐coalescent and coalescent media were used. Among the methods discussed here, the mass transfer coefficient (k_La) values obtained by the DPM appear as the most reliable as they were found to be independent of oxygen concentration in the inlet gas, which confirmed the physical correctness of this method. The difference between data measured using air and oxygen was not higher than 10%, which was comparable to the scatter of experimental data. It has been found that the sulfite oxidation method yielded k_La values only a little higher than those obtained by the DPM and the difference did not exceed 10%. Up to an inlet gas velocity (U_GC) of ?0.03 m s^?1 the GOM using oxygen as a gas medium gave k_La values in fact identical with those obtained by the DPM. At higher flows of the inlet gas, the GOM yielded k_La values as much as 15% lower. The enhancement in oxygen mass transfer rate determined in non‐coalescent media was estimated to be up to +15%, when compared with a coalescent batch. The experimental dependence of k_La vs the overall gas hold‐up was described by an empirical correlation. 1 Copyright © 2004 Society of Chemical Industry     

MODELING DISSOLVED OXYGEN CONCENTRATION FOR OPTIMIZING AERATION SYSTEMS AND REDUCING OXYGEN CONSUMPTION IN ACTIVATED SLUDGE PROCESSES: A REVIEW

Aeration accounts for 30% to 75% of the total energy consumption in activated sludge processes (ASPs). This percentage can be significantly reduced since most aeration systems are not optimized for unsteady influent flow rates and oxygen requirements. Reconfiguration, replacement, and the application of optimal dissolved oxygen (DO) control strategies for current aeration systems within the facility and model-based optimization of DO in wastewater treatment plants can lead to impressive increased energy efficiency and savings and improved stability of the system. These measures increase the operational lifetime of the aeration equipment and improve effluent and activated sludge quality. This article provides a review of two critical nonlinear time-varying parameters that characterize the DO concentration dynamics in an ASP: the oxygen uptake rate (OUR), related to microorganism activity, and the volumetric oxygen mass transfer function, represented by the oxygen transfer rate (OTR). Second, the physico-chemical, geometric, and dynamic factors and aerator type affecting the oxygen mass transfer coefficient (K _L a) are thoroughly discussed. The article concludes with model-based optimization, explaining the usefulness of accurate DO models in wastewater treatment, and provides examples for plant-wide or water chain cycle–focused optimizations.     

Investigation of gas properties, design, and operational parameters on hydrodynamic characteristics, mass transfer, and biomass production from natural gas in an external airlift loop bioreactor

An external airlift loop bioreactor (EALB) was used for production of biomass from natural gas. The effect of riser to downcomer cross sectional area ratio (A_r/A_d), volume of gas-liquid separator, superficial gas velocity (U_sgr), and physical properties of gases and their mixtures [υ_g (μ/ρ) and D_g] were investigated on mixing time, gas hold-up, and volumetric gas liquid mass transfer coefficients (k_La). It was found that A_r/A_d has remarkable effects on gas hold-up and k_La due to its influence on mixing time. Kinematic viscosity (υ_g) showed its significant role on mixing time, gas hold-up and k_La when different gases used (mixing time changes directly whereas gas hold-up and k_La change indirectly). Moreover, it was found that diffusion coefficient of gas in water (D_g) has remarkable effect on k_La. The volumetric mass transfer coefficients for methane and its mixtures with oxygen (three different mixtures) were determined at different geometrical and operational factors. In average, the rate of oxygen utilization is approximately 1.8 times higher than that of methane. A gas mixture of 25 vol% methane and 75 vol% oxygen was the best gas mixture for biomass production in the EALB. The correlations developed for predicting the mixing time, gas hold-up, and k_La in terms of U_sgr, A_r/A_d, volume of gas-liquid separator, and gas phase properties have been found to be encouraging.