水溶液全循环法尿素装置蒸发系统的改进

我国中型氮肥厂尿素装置普遍采用水溶液全循环法生产工艺,蒸发系统采用负压操作,尿素溶液经加热蒸发水分,通过闪蒸槽和一蒸分离器将尿液与气相分离,与此同时,尿液中的一部分尿素也被带了出来,气相经一段表面冷凝器冷凝后进入碳铵液槽,碳铵液经解吸系统将其中的二氧化碳和氨解吸出来后送到二段循环系统回收利用,但被带出来的这部分尿素随着解吸废液排出了系统。本文介绍通过简单的改造来回收这部分尿素。1　设备及流程改进在原系统的基础上,在闪蒸槽和一蒸分离器上方各增加一小型填料塔洗涤器,将二段表面冷凝液通入填料塔的上部,使其在填料部…     

尿素蒸发洗涤器的应用

介绍了尿素蒸发洗涤器在尿素蒸发系统的应用情况,对工艺指标的控制及出现的问题进行了分析总结。运行情况表明,解吸液中的尿素含量明显下降,尿素产量增加,系统消耗降低。     

水溶液全循环尿素装置蒸发造粒系统技改综述

传统尿素工艺装置增产节能技术改造须与蒸发造粒工序的技术改造同步，应将蒸发工序设备与前面工序的设备结合起来考虑．既提高蒸发系统的生产能力，又使一段循环系统的热能回收率和生产能力得到提高。     

尿素闪蒸蒸发新工艺简介

1 概况 尿素是由氨和CO_2合成的,第一步生成氨基甲酸铵,然后氨基甲酸铵部分地转化成尿素和水。生成甲铵的步骤是放热的快步骤,该反应可以接近达到平衡。第二步是吸热的慢步骤,CO_2转化率在65％左右达到平衡。     

基于激光光谱法的尿素水溶液液膜多参数测量

在选择性催化还原（SCR）系统中,对尿素水溶液液膜多个参数（厚度、浓度及温度）进行高精度的定量分析,对减少液膜形成、提高SCR系统的催化反应效率、降低NO_x对环境的污染至关重要。基于比尔-朗伯定律,通过结合1420 nm和1488 nm两个波长的半导体激光器,提出一种恒温度条件下尿素水溶液液膜厚度和浓度同步测量,以及恒浓度条件下尿素水溶液液膜厚度和温度同步测量的新方法。并在此基础上,利用标准具验证了该方法的测量精度。结果表明恒温度条件下,该方法同步测量尿素水溶液液膜厚度和浓度的平均测量误差分别为0.82%和3.93%;恒浓度条件下,同步测量尿素水溶液液膜厚度和温度的平均测量误差分别为0.79%和2.58%。     

增设尿素蒸发洗涤回收装置应注意的问题

水溶液全循环法尿素生产装置普遍存在解吸排放废液氨氮超标的问题,部分企业对解吸塔进行了改造,虽使解吸废液含氨量小于0 03%,但含尿素问题仍然存在,直接排放后造成水体污染,影响企业的长期生存和发展。又由于近几年化肥市场不景气,企业普遍亏损,为降低成本,各企业生产装置在没有较大改造的情况下,超设计能力生产使一段蒸发气、二段蒸发气中夹带尿素量增加,使解吸排放的废液中尿素含量达2 0%以上。解决这一问题的办法有两个:一是增设深度水解装置,但因投资费用较大,蒸汽消耗较高,一般企业较难实现;二是增设蒸发气洗涤回收装置,因其投资…     

蒸发流失尿素的回收

对尿素蒸发过程中尿素流失的原因进行了分析，对几种回收方法作了介绍和比较；同时对该厂开发的“一段蒸发尿素回收装置”的试生产情况进行了总结。     

非热等离子体强化TiO2催化尿素分解副产物水解性能的研究

尿素-选择性催化还原技术低温下运行时尿素分解不彻底,易形成缩二脲、三聚氰酸和三聚氰胺等副产物。本研究将TiO₂催化剂与介质阻挡放电等离子体相结合,在程序升温条件下考察了载气中有无O₂时引入等离子体前后TiO₂催化尿素分解副产物水解的性能。结果表明：TiO₂表面缩二脲、三聚氰酸和三聚氰胺分别在43~261℃、217~300℃和199~300℃水解生成NH₃和CO₂,载气中有无O₂对催化水解过程几乎无影响。引入等离子体后缩二脲、三聚氰酸和三聚氰胺水解所需温度显著降低,载气中无O₂时引入等离子体NH₃产率变化不大,副产物仅有少量N₂O和NO,有O₂时NH₃产率显著降低,且生成较多N₂O、NO、NO₂及少量NH₄NO₂和NH₄NO₃。未来需从优化放电条件和催化剂组成等方面解决引入等离子体导致副产物形成等问题。     

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     

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 study on non-equilibrium fraction of NaCl solution circulatory flash evaporation

Non-equilibrium performance of NaCl solution is different from that of pure water due to boiling point elevation (BPE) of NaCl aqueous solution. It's necessary to conduct experiments for NaCl solution circulatory flash evaporation. Experiments were conducted with flow rates of 400, 600, 800, 1000, and 1200 L·h^− 1, initial water film heights ranging from 100 to 265 mm, initial water film concentrations of 0, 5%,10% and at pressures of 7.4, 12.3, 19.9, and 31.2 kPa. Two different benchmarks using the saturate temperatures of pure water and NaCl solution were chosen to calculate the non-equilibrium fraction (NEF) for NaCl solution circulatory flash evaporation, and comparison between the two methods was also performed. Results showed that the method using the saturate temperature of NaCl solution was reasonable to reveal the essence of NaCl solution flash evaporation. NEF considering BPE augmented to a peak value at first and decreased monotonously when the superheat increased. The NEF value increased with the increasing initial water film height and concentration but decreased with the increasing mass flow rate and flash chamber pressure. Moreover, the NEF curve of NaCl solution was coincident with that of pure water in presented 10% NaCl solution flash evaporation cases.     

Experimental investigation on evaporation interface temperature and evaporation rate of water in its own vapor at low pressures

Evaporative phase transitions are widely present in industrial production and daily life such as thin film processes and crystal growth. The evaporation of the liquid layer and the thermocapillary convection affect each other and restrict each other, making the energy transfer mechanism of the evaporation interface very complicated. To understand the evaporation characteristics of water in its low-pressure pure vapor environment, a series of experimental studies were carried out on the temperature distributions and evaporating rate of water evaporation in the annular pool. The cylinder temperature of the annular liquid pool is controlled between 3℃ and 15℃, and the evaporation environment pressure ranges from 394 Pa to 1467 Pa, when the temperature measurement starts, the depth of water is 10 mm. The results show that the temperature of the vapor side on the liquid-vapor interface is higher than that of the liquid side and there is an obvious temperature jump across the vapor-liquid interface. With the decrease of the pressure ratio, the evaporation rate increases, and the interface temperature jump is enlarged. Meanwhile, with the increase of the distance from the cylinder, the local evaporation rate decreases, thus, the temperature jump decreases. At the same pressure ratio, as the cylinder temperature increases, the heat flux from vapor side decreases, the temperature jump decreases at all measurement points. Within the experimental controlled parameters, the maximum temperature jump obtained in the measurements is 2.56℃. Due to the coupling effect of evaporation cooling and thermocapillary convection, there is a uniform temperature layer with a thickness of about 2 mm under the evaporation interface. The thickness of the uniform temperature layer near the cylinder is always larger than that in the middle of the evaporation interface. In the uniform temperature layer, the thermocapillary convection induced by radial temperature gradient transfers heat from the cylinder to the liquid-vapor interface to compensate for the latent heat of evaporation. Below the uniform temperature layer, the temperature rises rapidly due to heat conduction and buoyancy convection.     

低压蒸汽环境中水蒸发界面温度和蒸发速率的实验研究

蒸发相变广泛存在于薄膜过程及晶体生长等工业生产和日常生活中,液层表面蒸发和热毛细对流相互影响、相互制约,使得蒸发界面能量传递机制变得非常复杂。为了深入了解水在低压纯蒸汽环境中的蒸发特性,对环形液池内水蒸发时的温度分布和蒸发速率进行了一系列实验研究。环形液池壁温控制在3～15℃之间,蒸发环境压力在394～1467 Pa之间变化,开始测量时液层深度为10 mm。结果表明,蒸发界面气相侧温度总是高于液相侧,气液界面存在明显的温度跳跃。随着压比减小,蒸发速率增加,界面温度跳跃随之增大;随着距壁面距离增加,局部蒸发速率降低,温度跳跃值减小;相同压比下,随着壁面温度的升高,气相侧热通量减小,蒸发界面温度跳跃值整体降低;在实验范围内测得的最大温度跳跃值为2.56℃。由于蒸发冷却效应和热毛细对流的耦合作用,蒸发界面下液相侧存在一个厚度为2 mm左右的温度均匀层,且壁面附近温度均匀层厚度大于中间区域厚度。在温度均匀层内,径向温度梯度诱导的热毛细对流将热量从壁面传输至气液界面以补偿蒸发所需汽化潜热;在温度均匀层以下,浮力对流和导热共同作用使得液相温度迅速升高。     

NaCl溶液静态闪蒸的蒸发特性

对不同初始条件下NaCl溶液静态闪蒸过程中蒸发质量的变化规律进行了实验研究,实验过程中溶液初始质量分数为5%～26%,初温为74～104℃,初始液位高度为0.1～0.3 m,过热度为0.79～42 K。结果表明:在以ΔT_s为过程参量时,溶液闪蒸蒸发质量随NaCl初始质量分数的增加明显下降;以ΔT为过程参量时,相同初始液位高度,不同初始质量分数NaCl溶液闪蒸蒸发质量相差较小;增加初始液位高度虽可增加总蒸发质量,但使单位质量溶液闪蒸蒸发质量下降;闪蒸蒸发质量随过热度增大而呈近似线性增加。综合分析静态闪蒸过程中主要过程参量对蒸发质量的影响,在实验基础上,提出蒸发质量的量纲1关联式。     

基于不同类型溶液蒸气压特性的太阳能界面蒸发实验研究

太阳能界面蒸发可实现高效太阳能海水淡化和蒸发式污水处理,但目前的研究大多局限于纯水或NaCl溶液。实际脱盐或废水处理中溶质会与此不同,导致溶液蒸气压变化并影响蒸发性能。本文首先分析了溶液表面蒸气压曲线类型,将其分为上凸型、下凹型和直线型。针对这几种蒸气压曲线进一步选取[EMIM][OTf]、[EMIM][Ac]和NaCl水溶液作为代表溶液,在不同辐照强度和浓度下进行了实验研究,并与纯水的蒸发作对比。实验结果表明：低浓度下[EMIM][OTf]水溶液展现出了良好的蒸发性能, 主要原因是由于其蒸气压处于上凸区间;当溶液浓度升高或辐照强度提升时,[EMIM][OTf]溶液的蒸发速率提升较NaCl水溶液小,主要原因在于蒸发过程的浓度极化导致气液界面处的[EMIM][OTf]浓度升高,蒸气压相差较小;不同工况下[EMIM][Ac]水溶液的蒸发速率均较慢,纯水的蒸发速率最快,体现了蒸气压对蒸发性能的关键影响,原因是低蒸气压导致高蒸发温度,并带来更多的能量损失。     

多种类型液滴蒸发综述

液滴蒸发过程是伴随着复杂变化但又没有统一且充分认知的的过程,是目前一个重要的研究热点,在许多科学应用中起到关键作用。本文介绍了液滴蒸发的历史研究过程,综述了3种不同类型液滴,即纯液滴、二元混合溶液液滴和聚合物溶液液滴蒸发过程的研究成果,分析了液滴蒸发过程中和蒸发结束后沉积物的影响因素,简述了液滴的研究成果在实际生活中的应用。现有研究表明,不同类型液滴的蒸发过程受到多种因素的影响,比如溶液中的纳米粒子、环境温度和压力等。这些因素还会影响到沉积物的图案和大小。目前,研究人员已经研究出典型的液滴蒸发过程（接触线固定和接触角固定模式）,讨论出液滴蒸发基本理论。对一些常见的二元及多元溶液,研究人员已经发现它们与纯溶液蒸发过程的不同之处,并且已经在科学界进行了大量的研究及讨论,建立出数学模型。最后重点介绍了液滴蒸发在医学领域的研究成果、应用和未来发展的方向,比如通过生物液滴蒸发后的沉积物的纳米层,跟正常沉积物对比结果来检测疾病等。最后对液滴蒸发理论的现状、潜力和未来发展需求进行了总结和展望。     

氧化铝纳米流体液滴瞬态蒸发速率的演化特性分析

以氧化铝纳米流体液滴为研究对象,本文建立了基于任意拉格朗日-欧拉（ALE）法的液滴蒸发瞬态模型,对液滴蒸发过程中蒸汽浓度、纳米颗粒浓度、温度等进行多物理场耦合,并考虑了Marangoni流对液滴蒸发的影响,同时研究还结合蒸发实验可视化结果,分析了氧化铝纳米流体液滴的瞬态蒸发速率随时间的演化规律,讨论了颗粒体积分数和基板温度对蒸发模式的影响。结果表明,在液滴蒸发过程开始时,纳米流体液滴保持定接触半径蒸发模式,气液界面面积逐渐减小,瞬态蒸发速率也呈逐渐减小的趋势;当颗粒体积分数增大至26%时,瞬态蒸发速率曲线达到驻点;蒸发接近完全时,由于Marangoni流影响了内部流场、强化了内部传热,且液滴在已沉积在基板上的颗粒表面形成液膜,瞬态蒸发速率迅速增大。     

Al2O3纳米流体液滴蒸发特性的数值模拟研究

纳米流体液滴蒸发现象在电子设备冷却、喷墨打印以及医学检测等领域都有广泛应用。为了研究水基Al₂O₃纳米流体液滴的蒸发特性,建立了纳米流体液滴蒸发的二维瞬态模型,考虑了纳米颗粒输运行为以及液滴内部流动的影响,并采用任意拉格朗日-欧拉法（ALE）捕捉气液运动界面。基于所建立的模型,分析了水基Al₂O₃纳米流体液滴内部Marangoni流、纳米颗粒初始浓度以及基板温度对纳米流体液滴蒸发特性的影响规律。结果表明,液滴内部Marangoni流会影响气液界面温度分布和蒸发速率。由于液滴内部纳米颗粒浓度分布和气液界面温度发生变化,纳米流体液滴的蒸发速率随着纳米颗粒初始浓度和基板温度升高而增加。