SIMULATION OF AEROSOL FORMATION IN GAS-LIQUID CONTACT DEVICES

In gas-liquid contact devices like absorbers, quench coolers, or condensers, aerosols can be formed by spontaneous phase transitions, initiated by homogeneous or heterogeneous nucleation, if a supersaturated gas phase emerges due to simultaneous heat and mass transfer processes or chemical reactions. Typical examples are the absorption of acid gases, like HCl or SO₃, the condensation of solvents in the presence of inert gases, and the humidification of cold gases by hot liquids.

In this article the basic principles of aerosol formation in contact devices are briefly described. A strategy for modeling and simulation of aerosol formation and particle dynamics is discussed. Simulation results generated with the process tool AerCoDe for the countercurrent absorption of HCl and the humidification of air are presented.     

SIMULATION OF BUBBLE FORMATION AND HEAPING IN A VIBRATING GRANULAR BED

The Eulerian-Eulerian approach is used to simulate flow in a dense granular bed subjected to a slight, vertical, sinusoidal vibration. The bottom of the bed was subjected to a vertical vibration, ASin(ω _v t)J ₀(k _m x), where J represents the profile of the bottom wall and is the first kind of Bessel function. The governing equations of the gas phase and particle phase are described and the results of the model with different vibration frequencies and amplitudes are presented. Bubble formation and upward and downward heaping were observed at different amplitudes and frequencies. The results of the model confirm that there is a change from upward to downward heaps according to the strength of the vibration. In addition, the location of the bubble and the shape of the heaping at the interface of the bed depend on the strength of the vibration and the profile of the bottom of the container.     

多级闪发式海水淡化装置水流特性数值模拟

引　言目前国内外在陆上采用的海水淡化技术主要有蒸馏法和反渗透法两种 .蒸馏法的优点是装置费用随产量增加而明显减少 ,具有规模效应 ,且维护费用较少 ,产水水质很高 .在陆上使用的蒸馏法海水淡化设备一般为大型多级卧式闪发蒸馏装置 ,它由一系列逐级降压的闪发室串联而成 ,过热水在水位差作用下向下一级流动 ,靠逐级降压而保持的过热度使海水不断蒸发 ,产生水蒸气后冷凝成淡水 (图1) [1] .闪发室的结构形状并不复杂 ,但闪蒸过程涉及汽、液两相的流动、相变、热交换过程以及两Fig 1　SketchofMSFdesalinationdevice[1]相之间自由界面的…     

ABSORPTION OF GASES IN GROWING DROPLETS

The situation where a stationary droplet in a supersaturated medium is growing by condensation of a vapor and simultaneously absorbing a gas is theoretically analyzed to study the effect of growth on absorption rate. The absorption rate is assumed to be liquid phase diffusion controlled, and the unsteady state liquid phase diffusion equation with moving boundary has been solved by a similarity technique. The results indicate that for fast droplet growth rates, induced by relatively high degrees of supersaturation, the absorption rate is significantly enhanced, leading to higher solute concentration near the surface than that observed for a similar droplet of finite size. For slow growth rates, the results are not significantly different from the quasi-stationary results.     

A SIMPLIFIED METHOD OF CALCULATING AEROSOL FILTRATION IN FLUIDIZED FILTERS

A simplified model of calculating aerosol filtration in fluidized filters is proposed. Unlike earlier methods (Peters et al., 1982; Ushiki and Tien, 1984) which require compartment-to-compartment calculations, this method assumed that parameters which characterize fluidized bed structure, such as the volume fractions of the dense and bubble phases and the bubble diameter can be considered constant for the entire bed. Analytical solutions of effluent concentration and the total collection efficiency were obtained which correspond to two limiting situations regarding gas mixing in the dense phase. Comparisons between the analytical solutions and experimental data as well as results from more exact compartment-to-compartment calculations indicate that the simplified method gives results of reasonably good accuracy and is adequate for preliminary design calculations.     

ABSORPTION OF GASES IN GROWING DROPLETS

The situation where a stationary droplet in a supersaturated medium is growing by condensation of a vapor and simultaneously absorbing a gas is theoretically analyzed to study the effect of growth on absorption rate. The absorption rate is assumed to be liquid phase diffusion controlled, and the unsteady state liquid phase diffusion equation with moving boundary has been solved by a similarity technique. The results indicate that for fast droplet growth rates, induced by relatively high degrees of supersaturation, the absorption rate is significantly enhanced, leading to higher solute concentration near the surface than that observed for a similar droplet of finite size. For slow growth rates, the results are not significantly different from the quasi-stationary results.     

A SIMPLIFIED METHOD OF CALCULATING AEROSOL FILTRATION IN FLUIDIZED FILTERS

A simplified model of calculating aerosol filtration in fluidized filters is proposed. Unlike earlier methods (Peters et al., 1982; Ushiki and Tien, 1984) which require compartment-to-compartment calculations, this method assumed that parameters which characterize fluidized bed structure, such as the volume fractions of the dense and bubble phases and the bubble diameter can be considered constant for the entire bed. Analytical solutions of effluent concentration and the total collection efficiency were obtained which correspond to two limiting situations regarding gas mixing in the dense phase. Comparisons between the analytical solutions and experimental data as well as results from more exact compartment-to-compartment calculations indicate that the simplified method gives results of reasonably good accuracy and is adequate for preliminary design calculations.     

FORMATION OF HYDROBROMIC AND HYDROCHLORIC ACID AEROSOLS IN WET FLUE GAS CLEANING PROCESSES

The formation and behavior of hydrobromic and hydrochloric acid aerosols in a wet flue gas cleaning pilot plant were investigated. The optical three-wavelength extinction (3-WE) method was used to determine mean aerosol droplet diameters and droplet number concentrations. The experimental data are compared with theoretical results of the simulation tool AerCoDe (aerosol formation in contact devices). Results are presented for a raw gas temperature of 200°C and raw gas concentrations up to 260 mg/m³(STP) for HBr, and 2500 mg/m³ (STP) for HCl. Under these conditions aerosol formation for both species is initiated by heterogeneous nucleation. It is shown that during absorption processes HBr is forming essentially higher supersaturated gas phases in comparison to HCl, resulting in higher droplet number concentrations and smaller droplet sizes. For both species the number concentration is a strong function of the maximum degree of saturation, which corresponds to the classical theory of heterogeneous nucleation. HBr aerosol droplets cannot be detected by the 3-WE method directly after the first stage of the flue gas cleaning plant (quench column) because they are smaller than the detection limit ( ≈ 500 nm) of the 3-WE method for nonabsorbing particles. In this case the droplets are enlarged in a second step, and the number concentration is determined after enlargement. Using the actual number concentration as an input parameter, the mean diameter after the first stage can be calculated with the simulation tool AerCoDe.     

简化的Solomon热解模型和旋流煤粉燃烧NO生成的数值模拟

提出一种简化的Solomon热解模型, 用于模拟煤粉燃烧NO生成数值模拟中HCN的释放.用纯双流体模型、k-ε-k_p两相湍流模型、EBU-Arrhenius燃烧模型、六热流辐射模型、双方程热解模型、简化的Solomon热解模型以及NO生成湍流反应二阶矩代数模型对旋流煤粉燃烧器内两相流动、煤粉燃烧、HCN释放以及NO生成进行了数值模拟.模拟结果与文献中实验结果的对比表明,基于简化Solomon热解模型的HCN释放模型预报结果比基于双方程热解模型的HCN释放模型预报结果好.     

FORMATION OF OXYGENATED-POLYCYCLIC AROMATIC COMPOUNDS IN AEROSOL FROM THE PHOTO-OXIDATION OF o-TOLUALDEHYDE

The secondary organic aerosol formed during the photo-oxidation of o-tolualdehyde in a large smog reaction chamber has been studied using comprehensive two-dimensional gas chromatography and time-of-flight mass spectrometry. The separation and detection system was coupled directly to an aerosol thermal desorption stage, providing the ability to resolve all gas chromatography amenable species in a single experimental procedure. The organic content of aerosol formed from the photo-oxidation of o-tolualdehyde contained a diverse range of chemical functionalities including mono-aromatic, carboxylic acid and carbonyl which were predicted as photochemical by-products by a comprehensive chemical mechanism. Around 3% of the organic aerosol content resolved was polycyclic-aromatic in nature. This has indicated that the formation of relatively low volatility by-products was possible via the ring-retained channel from a mono-aromatic precursor. Species found included 2,3 naphthalene dicarboxylic acid, and anthraquinone quantitatively accounting for 0.40% (24.78 ng) and 0.075% (4.64 ng) respectively of the total aerosol (6181.4 ng).     

ANALYSIS OF AXIAL MIXING IN A GAS-LIQUID RECIPROCATING PLATE COLUMN

Axial mixing in the continuous phase in a Landau reciprocating-plate column (LRPC) has been investigated for both single-phase and two-phase gas-liquid flow conditions. A hydrodynamic model is proposed in which axial mixing is described as a process consisting of a backflow through the plate plus longitudinal mixing within the stage. The region in the proximity of the plates is almost perfectly mixed, beyond which there is a low-intensity mixing zone that varies in height and degree of mixing depending on phase velocities as well as the plates design and oscillation velocity. The presence of the dispersed phase affects axial mixing in both the well- and poorly mixed regions of each stage in two opposite ways: it decreases the backflow between the stages due to the hindrance effect caused by the presence of gas bubbles, and it increases the axial dispersion coefficient in the second stage by increasing the turbulence and phase entrainment caused by circulation and bubbles rising. The model adjustable parameters were determined from an experimentally measured dispersion coefficient over a wide range of operating conditions using the transient tracer injection method. The predictions of the model compare favorably with experimental data and can be applied for describing axial mixing in the continuous phase in an LRPC with±14% accuracy.     

SIMULATION OF SYNTHESIS OF PALLADIUM NANOPARTICLES IN A JET AEROSOL FLOW CONDENSER

Synthesis of Pd nanoparticles by evaporation of Pd vapor generated by inductive heating of a metal ingot followed by rapid condensation in a jet aerosol flow condenser (JAFC) is simulated. Computational fluid dynamics (CFD) elucidate the detailed temperature and velocity profiles in the complex geometry of the JAFC. A model is developed by interfacing simple jet correlations for temperature and velocity with monodisperse aerosol dynamics. It is shown that coagulation is a dominant process for particle growth. The model predictions for the average particle diameter are within a factor of two from the experimental data without any adjustable parameters.     

Analytic and numerical calculations of the formation of a sulphuric acid aerosol in the upper troposphere

Analytic and numerical calculations are performed on the production of sulphuric acid aerosol in conditions of a very large nucleation event observed in the upper troposphere. The numerical results feature a growing peak in the size distribution whose magnitude is reproduced well analytically, and are consistent with the observed particle number concentration at sizes greater than 25 nm (measured dry diameter), but suggest that most of the aerosol was at unobserved smaller sizes. Because of growth and coagulation, number concentrations of the aerosol rapidly become independent of the number initially nucleated, so that conclusions as to the nucleation process, either homogeneous or ion-induced nucleation, cannot easily be drawn from existing atmospheric observations. The final concentration is very insensitive to the magnitude of the SO₂ source, but, if condensation on, and coagulation with, a remnant background aerosol occurs, such nucleation events will be cut off for source magnitudes less than a specific value. Anthropogenic emissions of SO₂ which exceed this value can produce higher aerosol number concentrations in the atmosphere with consequences for the indirect effect of aerosols on the climate.     

筛孔塔板气液两相流动的速度场模拟

<正>由于气体的搅动,筛孔塔板上的流动是极其复杂的气液两相流。目前两相流理论尚不成熟,未见运用此理论对塔板流速场的研究报道,Zhang和Yu的研究虽考虑了气相对液相流动的影响,但由于利用单相流理论,计算结果与实际情况有较大偏差。本文利用Elghobashi等建立的两相流双流体模型,建立了塔板上气液两相流二维流速场计算模型。在此基础上,通过调整模型参数,考虑了液层高度对塔板上返流区二维流型的影响,建立了模型参数值与液层高度的关系,实现了考虑液层高度影响的二维流动——拟三维流动的模拟。计算结果与Porter的实验结果基本相符。     

ABSOLUTE ON-LINE MEASUREMENT OF THE MAGNETIC MOMENT OF AEROSOL PARTICLES

We developed a method of on-line measurement of the magnetic moment of aerosol particles. For this purpose we determine the deposition of the particles in a filter consisting of magnetic nickel screens. Deposition is governed by the dipole force magnetic particles experience near the magnetized wires of the screens. The number concentration at the in-and outlet of the filter is measured by optical particle counters. The so called single fiber efficiency is derived directly from the measured data as a function of particle size, particle magnetic moment and fiber magnetization. The relationship between this dimensionless value and the magnelic moment is obtained by a computer simulation using trajectory analysis. The method is tested with equally sized iron/iron oxide particles in the size range from 40 to 170nm.     

APPLICATION OF THE NRTL EQUATION TO SELECTED VLE DATA FOR THE SIMULATION OF THE CONTACT SULFURIC ACID PROCESS

The success of flowsheeting systems in modeling chemical processes is fundamentally dependent upon their ability to adequately represent the physical properties and thermodynamic quantities of the chemical system of interest. The nonideal H₂SO₂ and H₂O system poses particular difficulties both representing the physical properties and in modeling the adsorption-reaction step involved in the formation of sulfuric acid by the contact process. This paper proposes the use of suitable nonideal correlation equations such as Wilson and Renon's NRTL for the reduction of specially formulated VLE data to develop a correlation that overcomes the difficulties associated with calculating the mass and energy balances for this system. The approach presented here enables the user to make maximum use of the flowsheeting capabilities without adding user supplied subroutines. The results indicate improved agreement over previous modeling approaches for the calculation of the absorber energy balances in which the absorption-reaction step must be taken into account. The improvement is seen in closer agreement between plant and model outlet stream temperatures.     

Aerosol formation in gas-liquid contact devices—nucleation, growth and particle dynamics

In gas-liquid contact devices like absorbers, scrubbers, quench coolers or condensers, aerosols can be formed by spontaneous phase transitions, initiated by homogeneous or heterogeneous nucleation, if special process operation conditions lead to a metastable, i.e. a supersaturated state in the gas phase. Aerosol formation can impact severely the mass separation efficiency of gas-liquid contactors. This is demonstrated by experiments performed in semi-technical plants.The paper is aimed to identify strategies for understanding and describing the complex aerosol behaviour in gas-liquid contact devices.Operation conditions are identified under which supersaturation can arise, and the fundamentals of modelling aerosol formation and growth in gas-liquid contactors are discussed.The SENECA code developed by the authors allows to simulate aerosol formation and behaviour in contact devices as well as in multistage gas cleaning processes. Experimental results show that most of all important features of aerosol behaviour in flue gas cleaning and in condensation processes can be predicted with good accuracy by SENECA.     

利用流程模拟软件优化脱乙烷塔运行

独石化乙烯装置脱乙烷塔运行状态长期存在偏离设计值,导致其塔顶物流中夹带过量的丙烯,一方面造成丙烯产品损失,另一方面对乙炔反应器和轻烃裂解炉的运行造成不利影响。为解决此问题。利用HYSYS流程模拟软件对该塔进行了双操作参数作用下的灵敏度分析,找出最佳操作值并进行应用,将塔顶物流中丙烯含量（m01）自0．8％降低至约0．2％。