CFD simulation of a transpiring‐wall SCWO reactor: Formation and optimization of the water film

A two‐dimensional axisymmetric computational fluid dynamics model of a transpiring wall reactor for supercritical water oxidation was developed using the commercial software Fluent 6.3. Numerical model was validated by comparisons with experimental temperature profiles and product properties (total organic carbon and CO). Compared with the transpiration intensity, the transpiring water temperature was found to have a more significant influence on the reaction zone. An assumption that an ideal corrosion and salt deposition inhibitive water film can be formed when the temperature of the inner surface of the porous tube is less than 374°C was made. It was observed that lowering transpiring water temperature is conducive to the formation of the water film at the expense of feed degradation. The appropriate mass flux ratio between the total transpiring flow and the core flow was determined at 0.05 based on the formation of the water film and feed degradation. © 2015 American Institute of Chemical Engineers AIChE J, 62: 195–206, 2016     

Transpiring wall reactor in supercritical water oxidation

Reactor corrosion and plugging problems have hindered the commercialization of supercritical water oxidation (SCWO) for wastewater purification. The use of transpiring wall reactor (TWR) is an effective means to overcome the above two problems by forming a protective water film on the internal surface of the reactor to aviod contacting corrosive species and precipitated organic salts. This work mainly aims to objectively review experimental investigations and numerical simulation results concerning TWR. Subsequent investigations for parameters optimizations of TWR are also proposed in order to ultimately build effective regulation methods of obtaining excellent water film properties. All this information is very important in guiding the structure design and operation parameters optimization of TWR.     

Study on the uniformity of water film in a transpiring wall reactor for supercritical water oxidation

A three-dimensional computational fluid dynamic model of a transpiring wall reactor for supercritical water oxidation has been built to optimize the uniformity of water film. Results show that the temperature and species distributions at the nozzle outlet deviate from the reactor centre. The inner wall of the porous tube near the transpiring water injection tube displays low temperatures, while high temperatures are recorded far from the injection tube. The circumferential temperature distribution on the inner wall of the porous tube is uneven. This phenomenon is due to the uneven injection of the transpiring water, leading to the uneven protection of the water film and local overheating of the porous wall. The injection velocity of the transpiring water significantly decreases when the number of injection tubes is increased, and the circumferential velocity and temperature distributions on the porous wall gradually become even. Moreover, high pressure drops across the porous wall at low porosities are useful for the uniform injection of the transpiring water. This characteristic is also conducive to obtaining a more uniform water film protection.     

Molecular dynamics simulations of supercritical water at the iron hydroxide surface

The adsorption properties of supercritical water confined between electrostatically neutral but hydrophilic surfaces of iron (II) hydroxide were determined through molecular dynamics simulations. Simulations were conducted at temperatures of 715, 814 and 913 K, and at water densities typically found in the heat transport system of the supercritical water cooled nuclear reactor (SCWR). Surface water layer densities were obtained and compared to those of the bulk water. Adsorption coverage was calculated as a function of the number of waters per OH group on the surface. Images of the water molecules configurations are provided along with the density profile of the adsorption layer. The localized adsorption and surface clustering of supercritical water, as seen in this study, would likely produce more localized corrosion phenomena in the water bearing components of the SCWR.     

Dechlorination of 1‐chlorohexadecane and 2‐chloronaphthalene in water under sub‐ and supercritical conditions

Reactions of 1‐chlorohexadecane and 2‐chloron‐aphthalene in water under sub‐ and supercritical conditions have been investigated so as to show the possible use of water for the dechlorinations of these organic chlorides. The reactions were carried out at 275°C to 430°C in a small SUS316 batch reactor under nitrogen atmosphere at the molar organic chloride/water ratio of 1/100. Under subcritical temperatures, hydrolysis reactions occurred catalyzed by H⁺ ions, whereas under supercritical conditions hydrodechlorinations occurred too, in which hydrogens evolved by the reaction of HCI with the metal wall of the reactor participated. The hydrogenation and hydrogenolysis of the primary reaction products also occurred for 1‐chlorohexadecane under supercritical conditions. The dechlorination selectivity was nearly 100% for these organic chlorides irrespective of the reaction conditions employed.     

甲醇合成水冷反应器的优化设计

采用合适的反应器结构来保证催化剂床层温度的相对恒定,是合成过程高效、稳定运行的关键.简述了首台甲醇合成水冷反应器与以往类似设备在设计的选材及设计过程中的结构优化,使甲醇合成反应取得较高的反应物单程转化率和较低的副产物产率,提高了相同直径甲醇合成水冷反应器的甲醇产量,并去掉了不必要的大接管,减少了对筒体强度的影响,降低了...     

超临界水热燃烧技术研究及应用进展

超临界水热燃烧技术作为一种新型的高效清洁燃烧技术,为实现有机废物处理、稠油资源高效开发、煤基固体燃料清洁转化利用、新型钻井技术开发及劣质燃料品质提升等提供了一条崭新的途径,具有广阔的发展前景。本文概述了超临界水热燃烧的提出、发展历程及其技术优势,评述了不同燃料的水热火焰特性、水热燃烧反应器形式以及水热燃烧技术工程应用方面的研究现状。指出对于特定燃料,水热燃烧反应器具有较低的燃料熄火温度是提高反应器内水热火焰稳定性的关键。水热燃烧反应器开发过程中水热火焰区的结构布置需综合考虑蓄热需求与反应器壁面安全。水热火焰特性与超临界水中传热传质的耦合机制、水热燃烧过程数值模拟、光-超临水-氧气复杂环境下的材料腐蚀特性、水热火焰辅助降解有机废物、生产多元热流体辅助稠油开采、煤基固体燃料的水热燃烧是超临界水热燃烧领域未来研究热点。     

A novel concept reactor design for preventing salt deposition in supercritical water

Reactor plugging and corrosion are the key problems which hinder commercial applications of supercritical water oxidation and gasification, and can be efficiently overcome by preventing salt deposition on internal surface of reactor. In this work the problems caused by salt deposition and the correspondingly main solutions are further reviewed objectively. A novel reactor is designed and manufactured with a feed rate of about 100 L/h for sewage sludge treatment. The reactor combines the characteristics of Modar reactor and transpiring wall reactor for the first time, which is expected to prevent reactor plugging and corrosion as well as to decrease catalyst deactivation rate. The reactor is the core equipment of the first pilot-scale plant for supercritical water oxidation in China. Further optimizations of reactor configuration and operational parameters need plenty of experiments and/or a long-time test with sewage sludge in the subsequent work.     

Application of water vapor and hydrogen-permselective membranes in an industrial fixed-bed reactor for large scale methanol production

Coupling reaction and separation in a membrane reactor improves the reactor efficiency and reduces purification cost in the next stages. In this work a novel reactor consisting two membrane layers has been proposed for simultaneous hydrogen permeation to reaction zone and water vapor removal from reaction zone in the methanol synthesis reactor. In this configuration conventional methanol reactor is supported by a Pd/Ag membrane layer for hydrogen permeation and alumina-silica composite membrane layer for water vapor removal from reaction zone. In this reactor syngas is fed to the reaction zone that is surrounded with hydrogen-permselective membrane tube. The water vapor-permselective membrane tube is placed in the reaction zone. A steady state heterogeneous one-dimensional mathematical model is developed for simulation of the proposed reactor. To verify the accuracy of the model, simulation results of the conventional reactor is compared with the available plant data. The membrane fixed bed reactor benefits are higher methanol production rate, higher quality of outlet product and consequently lower cost in product purification stage. This configuration has enhanced the methanol yield by 10.02% compared with industrial reactor. Experimental proof-of-concept is needed to establish the safe operation of the proposed configuration.     

Study of transpiring fluid dynamics in supercritical water oxidation using a transparent reactor

The transpiring wall reactor (TWR) is considered to be one of the most promising reactors because it minimizes both corrosion and salt precipitation problems that seriously hinder the industrialization of supercritical water oxidation technologies. A transparent reactor is built to study the fluid dynamics of transpiring flow, which are the foundation of reactor design and optimization. The results showed that the transpiring flow is anisotropic with respect to the surface of the transpiring wall due to both the static pressure and viscous resistance. Finally, the novel idea of using air as the transpiring fluid instead of water is presented in an attempt to alleviate current TWR problems such as high energy consumption, high volume of pure water consumption, and temperature fluctuation in the reaction area. A series of experiments and theoretical derivations demonstrate that this novel idea is feasible.     

Research progress of supercritical water oxidation based on transpiring wall reactor

超临界水氧化技术的发展面临着腐蚀和盐沉积两大技术难题,采用蒸发壁反应器是解决这两大技术难题最为有效的方法。本文综述了国内外蒸发壁反应器的结构特点和性能,分析了基于蒸发壁反应器的超临界水氧化技术应用过程中仍然存在的问题,如多孔管的性能、物料的预热、系统能量利用及经济性,并提出了相应的解决办法。     

Catalytic partial oxidation of methane in a novel heat-integrated wall reactor

A novel reactor has been developed and applied in the reaction of partial oxidation of methane to synthesis gas. The reactor consists of a ceramic tube in the inner and outer surface of which a metal catalyst film is deposited. The CH₄/O₂ feed enters into the tube and a large fraction of the heat generated on the wall by methane combustion is transported across the tube wall towards the outer catalyst film, where the endothermic reforming reactions take place. In this way, the temperature in the combustion zone is controlled and hot spots are significantly reduced in magnitude. Initial results presented in this work demonstrate the feasibility of the concept. This revised version was published online in July 2006 with corrections to the Cover Date.     

超临界水条件下生物质气化制氢

生物质制氢是农业废弃物资源化利用的一项很有发展前途的技术。介绍了超临界水条件下生物质的气化制氢技术,论述了温度、压力、停留时间以及反应器对气化产物组成及气化制氢效果的影响,着重阐述了催化剂的影响。分析了目前超临界水气化制氢在有机废弃物资源化应用中存在的主要问题,并展望了超临界水气化制氢的研究前景。     

Hydrogen production from bioethanol reforming in supercritical water

Hydrogen production by reforming and oxidative reforming of ethanol in supercritical water (SCW) at the intermediate temperature range of 500-600 °C and pressure of 25 MPa were investigated at different ethanol concentrations or water to ethanol ratios (3, 20 and 30), with the absence and the presence of oxygen (oxygen to ethanol ratio between 0 and 0.156). Hydrogen was the main product accompanied with relatively low amounts of carbon dioxide, methane and carbon monoxide. Some liquid products, such as acetaldehyde and, occasionally, methanol were present. The ethanol conversion and hydrogen yield and selectivity increased substantially as the water to ethanol ratio and the reaction temperature increased. Ethanol was almost completely reformed and mainly converted to hydrogen giving a H₂/CO ratio of 2.6 at 550 °C and water to ethanol ratio of 30 without carbon formation. Coke deposition was favored at low water to ethanol ratio, especially at high temperatures (≥550 °C). The hydrogen yield improved as the ethanol was partially oxidized by the oxygen added into the feed at oxygen to ethanol ratios <0.071. It was evidenced that the metal components in Inconel 625 reactor wall reduced by a hydrogen stream acted as a catalyst promoting hydrocarbon reforming as well as water-gas-shift reactions while dehydrogenation of ethanol forming acetaldehyde can proceed homogeneously under the SCW condition. However, at high oxygen to ethanol ratio, the reactor wall was gradually deactivated after being exposed to the oxidant in the feed. The loss of the catalytic activity of the reactor surface was mainly due to the metal oxide formation resulting in reduction of catalytic activity of the reactor wall and reforming of carbon species was no longer promoted.     

Optimization of Catalyst Loading for Porous Copper Fiber Sintered Felts Used in Methanol Steam Reforming Microreactors

A novel porous copper fiber sintered felt (PCFSF) has been used as catalyst support to construct a methanol steam reforming microreactor for hydrogen production. The Cu/Zn/Al/Zr catalyst was loaded on the PCFSF by means of a two‐layer impregnation method. The ultrasonic water bath vibration method was employed to investigate the effect of porosity, catalyst loading, and sintering temperature on the loading performance of PCFSFs. The effective catalyst loading, being limited by porosity and pore size, was > 92 % in the lower porosity range of 70–80 %, but significantly reduced in higher porosity ranges >80 %.     

双螺杆挤出共混物分散相粒子的分散和分布——捏合段数量的影响

采用啮合同向双螺杆挤出机不同的螺杆组合形式，研究了熔融塑化区中捏合段数量对双螺杆挤出聚合物共混物表层和内层分散相粒子粒径及其分布的影响。结果表明，增大塑化区中螺杆前段的剪切作用和物料停留时间可明显减小分散相粒子的尺寸；在塑化区域增加剪切捏合段段数对表层分散相粒子尺寸没有明显作用，但可有效减小内层分散相粒子尺寸；对于在塑化区内采用前后两段捏合段的螺杆组合，增大后段剪切作用和物料停留时间比在前段增加更为有效，可同时显著降低共混物分散相表层和内层粒子尺寸，改善其分布特性。     

二氯乙烷裂解管式反应器二维模拟

建立了二氯乙烷在管式反应器中进行气相热裂解的二维模型 ,模型考虑了二氯乙烷热解生成氯乙烯的主反应和生成焦前体的副反应以及气体密度变化对裂解反应的影响 .模拟计算表明 ,二氯乙烷和氯乙烯的浓度沿径向分布平坦 ;但是管内近壁面处由于存在边界层 ,始终存在着明显的径向温差 ;近管壁处始终是裂解的高速率区 ,副反应也主要发生在管壁区 .表明确定最优的炉管管径时必须考虑提高裂解速率与降低结焦速率之间的平衡 .在距进口量纲 1管程 0 .3左右的管壁处裂解速率达到最高 ;副反应速率的最大点位于出口管壁处 .与工业数据比较后发现 ,炉管出口的转化率、选择性、出口压力和温度等数据与模型预测值一致 ,表明模型具有较高的可信度     

超临界水氧化技术中盐沉积问题的研究进展

超临界水氧化技术在处理高浓度难降解有机废水时具有去除率高、反应速度快、无二次污染等独特的优势,但存在盐沉积引起的反应器堵塞问题。本文针对国内外盐沉积问题研究的技术现状进行系统综述,归纳了盐沉积问题的研究方法,总结了部分盐在超临界水中的溶解度以及沉积和分离特性,阐述了盐沉积理论及从源头控制盐沉积途径,介绍了避免盐沉积引起反应器堵塞的技术方法,并对后续的研究进行了展望。指出盐沉积问题的解决还需进一步研究盐形成和沉积机理,建立不同盐类混合物的相图,研究盐沉积动力学和多组分系统的相行为,考察多组分盐之间的相互作用机制。这些信息有利于研究人员掌握超临界水氧化技术中盐沉积问题的基础知识和发展方向,有助于在实际工程应用中指导反应器结构设计和优化运行条件。     

Estimation of heating time in tubular supercritical water reactors

Reactor efficiency and product distribution in supercritical water (SCW) reactors is greatly influenced by the design of the heating section of these reactors. However, little experimental or theoretical work is available to estimate the rate of heat transfer in such systems. In the present study, CFD modeling of the heat transfer in tubular SCW reactors has been performed. Effects of various operating parameters; i.e. reactor temperature and pressure, flow rate, reactor diameter, and the external heating mechanism, on the heating time constant, the temperature profile along the reactor, and reactor residence time are investigated. Based on numerical simulations, a semi-theoretical model is proposed to estimate the heating time constant as a function of reactor operating conditions. Results of this study provide useful insights for designing continuous supercritical water reactors as well as for the analysis of experimental data obtained from such systems.     

Corrosion behavior of ferritic/martensitic steel P92 in supercritical water

The corrosion behavior of a ferritic/martensitic steel P92 exposed to supercritical water (SCW) at 500–600 °C and 25 MPa was investigated by means of gravimetry, scanning electron microscope/energy dispersive X-ray spectroscopy and X-ray diffraction. A dual-layered oxide scale, which was mainly composed of an outer magnetite layer and an inner magnetite/spinel-mixed layer, formed on P92. The initial oxide scale was rather porous, while the porosity decreased with an increase of exposure time. Oxidation rates at three different temperatures followed the parabolic law. The oxidation at 600 °C was so severe that cracks occurred along grain boundaries in the oxide scale. A probable corrosion mechanism for P92 exposed in SCW was proposed based on the above observations, focusing on oxide formation by oxygen absorption without any metallic dissolution.