Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

This work is aimed to develop a multicomponent evaporation model for droplets of urea‐water solution (UWS) and a thermal decomposition model of urea for automotive exhausts by using the selective catalytic reduction systems. In the multicomponent evaporation model, the influence of urea on the UWS evaporation is taken into account using a nonrandom two‐liquid activity model. The thermal decomposition model is based on a semidetailed kinetic scheme accounting not only for the production of ammonia (NH₃) and isocyanic acid but also for the formation of heavier solid by‐products (biuret, cyanuric acid, and ammelide). This kinetics model has been validated against gaseous data as well as solid‐phase concentration profiles obtained by Lundstroem et al. (2009) and Schaber et al. (2004). Both models have been implemented in IFP‐C3D industrial software to simulate UWS droplet evaporation and decomposition as well as the formation of solid by‐products. It has been shown that the presence of the urea solute has a small influence on the water evaporation rate, but its effect on the UWS temperature is significant. In addition, the contributions of hydrolysis and thermolysis to urea decomposition have been assessed. Finally, the impacts of the heating rate as well as gas‐phase chemistry on urea decomposition pathways have been studied in detail. It has been shown that reducing the heating rate of the UWS causes the extent of the polymerization to decrease because of the higher activation energy. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Experimental investigation on evaporation of urea‐water‐solution droplet for SCR applications

The evaporation behavior of urea‐water‐solution (UWS) droplet was investigated for application to urea‐selective catalytic reduction (SCR) systems. A number of experiments were performed with single UWS droplet suspended on the tip of a fine quartz fiber. To cover the temperature range of real‐world diesel exhausts, droplet ambient temperature was regulated from 373 to 873 K using an electrical furnace. As a result of this study, UWS droplet revealed different evaporation characteristics depending on its ambient temperature. At high temperatures, it showed quite complicated behaviors such as bubble formation, distortion, and partial rupture after a linear D²‐law period. However, as temperature decreases, these phenomena became weak and finally disappeared. Also, droplet diminishment coefficients were extracted from transient evaporation histories for various ambient temperatures, which yields a quantitative evaluation on evaporation characteristics of UWS droplet as well as provides valuable empirical data required for modeling or simulation works on urea‐SCR systems. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Modeling the depleting mechanism of urea‐water‐solution droplet for automotive selective catalytic reduction systems

The current work aims to develop a reliable theoretical model capable of simulating the depletion process of urea‐water‐solution (UWS) droplets injected in a hot exhaust stream as experienced in an automotive urea‐based selective catalytic reduction system. A modified multicomponent vaporization model is presented and implemented in the current study to simulate the behavior of UWS droplet in heated environment. Although water depletion is modeled as a vaporization process, urea depletion is modeled using two different approaches: (i) vaporization and (ii) direct thermal decomposition. The suitability of both depletion approaches is assessed in the current study by comparison with experimental data of the decay of a single UWS droplet in a quiescent heated environment. The decay rate of UWS droplet is accurately predicted with the multicomponent vaporization model. The possibility of internal gasification is demonstrated. Based on the complex decomposition behavior of urea, the current study proposes a decomposition mechanism for UWS droplet. The suitability of implementing the rapid mixing approach is assessed through comparison with the diffusion limit approach at various operating conditions and initial UWS droplet sizes. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Investigation on UWS evaporation for vehicle SCR applications

Urea water solution (UWS) droplet evaporation characteristics directly affect the conversion and distribution of NH₃ in urea based selective catalytic reduction (SCR) system. The UWS droplet temperature is very difficult to be measured directly. Whereas, this piece of research work involves the measurement of droplet temperature by an Omega‐K type thermocouple of 127 µm diameter. According to the temperature changes of the droplet, the evaporation process can be divided into four steps. Droplets heat and mass transfer processes are derived theoretically at high exhaust temperature. The UWS droplet has been placed in a continuous observation test system to investigate its diameter and temperature variations in the aforementioned four steps. The results shown that, this unique method of four steps analysis has more explicitly and better described the UWS evaporation process, hence establishing the basis for the subsequent detailed simulation and monitoring. © 2015 American Institute of Chemical Engineers AIChE J, 62: 880–890, 2016     

Experimental investigation of the evaporation behavior of urea-water-solution droplets exposed to a hot air stream

The behavior of droplets of urea-water-solution (UWS) evaporating under the influence of a hot stream of air was investigated experimentally, under temperatures ranging from 100°C to 400°C. The droplets were suspended on a glass microfiber to minimize the influence of heat conduction, through the fiber, on the evaporation rate of the droplet. The flow rate of air, under all experimental conditions, was measured and these data were used to estimate the average velocity of air around the droplet. Experiments were also conducted on droplets of pure water and the results were compared. The initial mass fraction of urea, in the solution, did not appear to have a significant effect on the evaporation constant, but it did affect a few essential aspects of the evaporation behavior. The evaporation of water droplets was in accordance with the d² law at all temperatures, whereas the evaporation of UWS droplets was ambient-temperature dependent.     

Modeling and simulation of the injection of urea-water-solution for automotive SCR DeNOx-systems

The evaporation of water from a single droplet of urea water solution is investigated theoretically by a Rapid Mixing model and a Diffusion Limit model, which also considers droplet motion and variable properties of the solution. The Rapid Mixing model is then implemented into the commercial CFD code Fire 8.3 from AVL Corp. Therein, the urea water droplets are treated with Lagrangian particle tracking. The evaporation model is extended for droplet boiling and thermal decomposition of urea. CFD simulations of a SCR DeNOx-system are compared to experimental data to determine the kinetic parameters of the urea decomposition. The numerical model allows to simulate SCR exhaust system configurations to predict conversion and local distribution of the reducing agent.     

Distinctive combustion stages of single heavy oil droplet under microgravity

This report presents an investigation on the combustion of single droplets comprised of heavy oil and oil mixtures blending diesel light oil (LO) and a heavy oil residue (HOR). The tests were conducted in a microgravity facility that offered 10 s of free-fall time. Fine wire thermocouples supported the droplets, resulting in a measurement of droplet temperature history. Additional data were the droplet and flame size history. The results identified four distinctive burning stages between ignition and extinction for heavy oil (C class) and HOR-LO blends. They are, in succession, the start-up, inner evaporation, thermal decomposition (pyrolysis) and polymerization stages. The start-up stage denoted an initial transient period, where the LO components burned from the droplet surface and the droplet temperature increased rapidly. The latter three stages featured pronounced droplet swellings and contractions caused by fuel evaporation and decomposition inside the droplet. An evaporation temperature demarcated the start-up stage from the inner evaporation stage, and this temperature corresponded to a plateau in the temperature history of the droplet. Two additional temperatures, termed the decomposition and polymerization temperatures, indicated the ends of the evaporation and decomposition stages. These temperatures were similarly identified by plateaus or inflection points in the time-temperature diagram. The evaporation temperature gradually decreased with increasing the initial LO mass fraction in the droplet, whereas the other two temperatures were almost independent of the oil composition. All three temperatures increased with decreasing initial droplet diameter, but the dependence was very slight. Based on the results, the combustion of heavy oil droplets appears to be dominated by a distillation-like vaporization mechanism, because of the rapid mass transport within the droplets caused by the disruptive burning.     

盐水液滴降压蒸发析盐过程数值模拟

盐水溶液的降压蒸发广泛应用于海水淡化和工业制盐等领域,因此研究盐水在降压过程中的蒸发特性具有重要意义,有助于解决我国水资源缺乏问题。本文通过数值模拟的方法研究了降压环境下盐水液滴蒸发析盐过程,获得了盐析质量和液滴温度随时间的变化。采用的工质为饱和盐水,液滴的初始温度分别为20℃、15℃、10℃;环境压力从0.1MPa降至2000～10000Pa。通过与实验数据相对比,验证了本文模型的有效性。通过该数学模型,分析了影响析盐率和液滴温度变化的主要因素。结果表明:液滴直径越大,在蒸发过程中其析盐率越高,但温度变化越慢;压降速率越快,液滴蒸发速率越快,析盐率越大,温度变化也越快;液滴初始温度越高,蒸发速率越快,液滴表面析盐率越高,但不同初始温度的盐水液滴,在蒸发过程中其最终温度趋于一致。     

盐水液滴降压蒸发析盐过程传热传质特性

针对单个盐水(NaCl溶液)液滴在降压环境下蒸发析盐的传热传质过程建立了数学模型。模型考虑了多孔盐壳在液滴表面的形成过程,降压过程引起的气流运动,液核通过多孔介质的传质扩散,以及液滴表面的蒸发换热和对流换热。将实验数据与计算结果对比,验证了模型的有效性。通过模型计算获得了液滴表面温度及液滴质量随时间的变化。结果表明盐水液滴在降压环境下蒸发析盐过程的温度变化分为4个阶段:温度骤降阶段、温度回升阶段、平衡温度阶段和温度上升阶段。平衡温度阶段,盐壳界面运动较慢,随蒸发进行,液核尺寸逐渐减小,盐壳界面运动速度加快。理论分析了环境压力对盐水液滴蒸发析盐过程的影响,环境压力越低,平衡温度越低,盐分完全析出时间越短。     

Steady‐state behavior of liquid fuel hydrazine decomposition in packed bed

A general theoretical model is presented to analyze the steady‐state decomposition process of liquid monopropellants in packed beds for thruster systems. Additionally, an experiment studying the decomposition of liquid hydrazine in a packed bed is used to validate this model. The liquid droplet evaporation rate is determined through calculating the gas‐liquid mass transfer for the mixture temperatures lower than the liquid propellant boiling point and solving the gas‐liquid or liquid‐solid heat transfer equations at the temperature exceeding the boiling point. The process of liquid propellant decomposition in packed beds are simulated based on the Naive–Stokes equation for the mixture model integrated with the developed liquid evaporation rate, in which both the heterogeneous catalytic reaction coupled with the diffusion of reactants in the pore of catalyst, and the homogenous decomposition reactions are considered. The calculated results for the axial distribution of the temperature are in good agreement with the experimental data. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1064–1080, 2015     

高温辐射环境中液滴的沸腾效应

建立了液滴在高温对流和辐射环境中的受热和蒸发模型,结合液滴均质沸腾模型,编制了计算程序。以正十二烷液滴为例,考虑液滴的膨胀效应以及液滴与周围气流的热物性变化,数值模拟了高温辐射与对流加热下的液滴升温和蒸发过程。分析了不同对流和高温辐射条件下,液滴内部是否能够发生沸腾。研究表明,液滴在高温辐射和对流加热下,蒸发伴随热膨胀;高温热辐射加热可导致液滴内部温度高于表面温度,升温到一定程度后可达到液滴内部沸腾状态;影响液滴沸腾的因素有液滴半径、辐射温度、环境气流温度等;同时,随着液滴蒸发,高温环境中液滴的沸腾过热度逐渐增大。     

A lumped model for thermal decomposition of urea. Uncertainties analysis and selective non-catalytic reduction of NO

A two-step mechanism for non-catalytic thermal decomposition of urea was investigated. It involves a first order reaction for the thermolysis of urea and a pseudo-first order reaction for hydrolysis of isocyanic acid. Rate constants of these reactions were tuned on experimental measurements of NH₃ and HNCO concentrations at 423-723 K and 1083-1383 K in isothermal flow reactors made of aluminum and ceramic, respectively. A regression analysis by taking the uncertainties in the kinetic parameters into account shown that the resultant pairs of Arrhenius expressions for thermo-hydrolyzing urea were different with a 90% confidence when obtained from the different sets of experimental data. Additional results of ammonia concentration from experiments of urea solution decomposition reported in the literature revealed the reliability of the Arrhenius expressions tuned on the reaction rates obtained in the highest range of temperature in the ceramic reactor. The consistency of these equations was also verified by a comparison between calculated and experimental rates of selective non-catalytic reduction of nitric oxide with urea as reducing agent. A statistical analysis based on the identification of possible tendencies in these residuals again evidences the weakness of the Arrhenius expressions based on the experiments performed at lower temperatures, which was attributed to a non-expected catalytic effect of aluminum reactor walls on reactions rates of urea thermal decomposition.     

Modeling of vaporization and cracking of liquid oil injected in a gas-solid riser

Vaporization and cracking of liquid oil injected in a gas-solid riser (fluid catalytic cracking riser reactor) was computationally studied in this work. Evaporation of a single drop injected in a stream of gas-solid mixture was analyzed first. A model for simulating evaporation of a drop considering heat transfer from the gas phase as well as from the solid particles was developed. The model relates the evaporation rate of droplet with rate of collisions of solid particles, specific heat capacities of solid and liquid, latent heat of vaporization, relative velocity of gas and liquid and temperatures of three phases. The understanding gained from such a model was then extended to simulate evaporation of liquid drops injected in FCC risers. The Eulerian-Lagrangian approach was used to simulate simultaneous evaporation and cracking reactions occurring in FCC riser reactors. A commercial CFD code, FLUENT (of Fluent Inc., USA) was used. Four and ten lump models were used for simulating cracking reactions. Appropriate user defined functions were developed to implement heterogeneous kinetics and heat transfer models in FLUENT. A special algorithm was developed to calculate accumulated coke on catalyst particles. A boiling point range was considered for simulating realistic oil feedstock. The model was first evaluated by comparing predicted results with published industrial data. The simulations were then carried out to understand influence of key design and operating parameters on performance of FCC riser reactors. The parameters studied included; initial oil droplet distribution, catalyst inlet temperature, catalyst to oil ratio and thermal cracking. The approach, model and results presented here would be useful for optimization of FCC operation, cost to benefit analysis of new FCC nozzles and related decision-making.     

Maximum Spreading of Urea Water Solution during Drop Impingement

Droplet impingement of urea water solution (UWS) is a common source for liquid film and solid deposits formed in the tailpipe of diesel engines. In order to better understand and predict wetting phenomena on the tailpipe wall, this study focuses on droplet spreading dynamics of urea water solution. Impingement of single droplets is investigated under defined conditions by high‐speed imaging using shadowgraphy technique. The experimental studies are complemented by numerical simulations with a phase‐field method. Computational results are in good agreement with experimental data for the advancing phase of spreading and the maximum and terminal spreading radius, whereas for the receding phase notable differences occur. For the maximum spreading radius, an empirical correlation derived for glycerol‐water‐ethanol mixtures is found to be valid for millimeter‐sized UWS droplets as well. A numerical simulation for a much smaller droplet however indicates that this correlation is not valid for the tiny droplets of UWS sprays in technical applications.     

尿素水溶液液滴的蒸发特性

在石英管式炉上通过挂滴法来观察单个尿素水溶液（urea-aqueous-solution,UAS）液滴的具体蒸发过程,比较了不同环境温度以及不同初始直径大小下液滴的蒸发特性。结果表明,尿素溶液液滴在100～1300 ℃的温度范围内呈现出了不同的蒸发行为。在较高的温度下,液滴的蒸发行为较为复杂,如气泡的产生、液滴的变形以及发生微爆的现象;但是,随着环境温度的降低,这些现象就变得非常微弱甚至消失。同时,还定量分析了稳态蒸发常数与温度、液滴初始直径之间的变化关系,发现在初始直径为2.5 mm、温度在100～600 ℃之间变化的情况下,稳态蒸发常数从0.02075 mm2/s增加到了0.23953 mm2/s,增大了10倍左右。此外,还对气流流速为0.25～1.25 m/s范围内的液滴蒸发特性作了实验研究。当液滴周围有强迫气流存在时,液滴与气体间的换热方式由导热转变为对流换热,从而增强了液滴表面的传热传质能力,促进了液滴的蒸发。     

乙醇溶液液滴降压蒸发过程传热传质特性

针对单个乙醇溶液液滴在降压环境下蒸发的传热传质过程建立了数学模型。模型基于液相的能量守恒和 传质扩散理论,利用经典拓展模型计算液滴的质量蒸发率,并引入活度系数考虑液滴表面的蒸气分压。采用液 滴悬挂法进行实验,分别记录了乙醇溶液液滴和乙酸溶液液滴在降压蒸发过程中的液滴内温度变化。将实验数 据与计算结果对比,验证了模型的有效性。通过模型计算获得了液滴内部温度分布以及浓度分布随时间的变化。 结果表明:快速降压阶段空气流动较快,加之乙醇工质易挥发,液滴表面温度下降迅速,液滴内部温差和乙醇 浓度梯度较大;压力稳定后,空气流速为零,液滴内部温差和乙醇浓度梯度逐渐减小。由于液滴内部的热扩散 速率大于传质扩散系数,内部温度随时间的变化比浓度随时间的变化更快。     

Droplet evaporation and solute precipitation during spray pyrolysis

The process of spray pyrolysis was investigated theoretically using a model that describes the evolution of the droplet size, solvent vapor concentration in the carrier gas, and both droplet and gas temperatures along the reactor axis. The model also accounts for solute concentration profiles and solute precipitation in the solution droplets. The model was used to describe the evaporation of sodium chloride aqueous solution droplets in diffusion dryers and hot-wall reactors as a function of reactor residence time, droplet size (a few microns), solution molality (up to 2 M), droplet concentration (10⁶–10⁷ cm⁻³), relative humidity of the carrier gas (0–50%) and reactor wall conditions. Decreasing initial droplet size and solution molality accelerated droplet evaporation and resulted in smaller droplets at the onset of solute nucleation. Decreasing droplet concentration and carrier gas inlet relative humidity as well as increasing wall temperature (up to 350°C) or axial wall temperature gradient (up to 100°C cm⁻¹) increased the droplet evaporation rate, but did not change appreciably the droplet size at the point of precipitation for a given droplet size and solute concentration. Thus, control of droplet size at the onset of solute nucleation by varying process parameters other than the solution concentration and initial droplet size is limited.     

Effect of preheating on liquid fuel evaporation on combustion in a model combustion chamber

A combustion model for a sprayed liquid fuel is developed, starting from the assumption that combustion of an individual drop is impossible, and the droplets are completely entrained in the moving medium. The model gives the a flame length close to that observed experimentally. It is established that the preheat time of the droplet is roughly the same as its evaporation time, and the temperature change in the air basically affects the droplet preheating and only weakly affects the droplet evaporation.Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 6, pp. 45–52, November–December, 1991.     

Modeling a hollow micro-particle production process

The process to be modeled produces micro-hollow particles based on spray drying technology. This process involves droplet formation, solvent evaporation, formation of the impermeable outer layer, and decomposition of a blowing agent. The objective of this work is to develop a fundamental model that describes the formation of the hollow particles starting from a single droplet. This model is then used to predict the time to skin formation and total drying time. The effect of parameter uncertainties and varying operating conditions are investigated.     

Modelling the morphological evolution of nanosuspension droplet in constant-rate drying stage

A novel mathematical model of constant-rate stage of nanosuspension droplet drying is proposed. In contrast to previously published literature studies, the developed model considers two morphologically different periods of the constant-rate drying: before the shell formation and after the shell formation; the latter was named “transition period”. The point of initial “locking” between nanoparticles on the droplet surface and beginning of the shell formation is associated with theoretical maximum of solid volume fraction. It is postulated that shrinking and thickening shell of nanoparticles occurs fast, and thus the shell virtually remains submerged in the liquid during the overall transition period. Because of the submerged shrinking shell, in the transition period, the evaporation process still takes place from the droplet surface and the drying rate remains unchanged as it was before the shell formation. Correspondingly, the droplet temperature retains at the level of equilibrium evaporation temperature. The developed theory was successfully validated by the published experiment of silica nanosuspension droplet drying. Finally, the developed model proposes a simple morphology criterion based on comparison between the calculated droplet volume at the end of transition period and the corresponding volume of solid final particle with the given porosity.