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。     

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 %.     

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.     

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.     

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.     

Flameless incineration of pyrene under sub-critical and supercritical water conditions

Pyrene was used as a typical four-ring polycyclic aromatic hydrocarbon (PAH), to investigate the mechanisms and incineration behaviour of large organic molecules in a batch supercritical water oxidation reactor using hydrogen peroxide as oxidant. The distribution of carbon as gaseous species and organic species in relation to the temperature and pressure, and reaction time was monitored. The results showed that at 200 °C, pyrene was only slightly decomposed but as the temperature increased to 250 °C and then to 280 °C, carbonisation and thermal cracking became prevalent leading to char formation and decomposition of pyrene to phenanthrene, and later naphthalene. Rapid dissolution and oxidation of the char and organic species started occurring from 300 °C. Increasing reaction time resulted in increased formation of carbon dioxide and carbon monoxide. Initially high product formation of phenanthrene at short reaction times was followed by high decomposition of the organic products in solution as the reaction conditions became progressively more severe. Oxygenated organic species such as aldehydes, ketones, phenols, xanthone, and benzoic acid were identified as the temperature and reaction times were increased between 300 and 380 °C. From the analytical results obtained, carbon mass balances were calculated for each experiment. A proposed mechanism for the observed oxidative decomposition of pyrene is also reported.     

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。     

烧结床层的热质分析

基于烧结生产的复杂物理化学过程,建立了烧结床层传热、传质和流动的二维非稳态数学模型,考虑了孔隙率、物料颗粒当量直径等床层结构影响参数的变化,并对气固传热系数进行了修正。通过数值计算,获得了烧结床层的温度场、结构变化和烟气的流场、温度场、浓度场等。烟气出口温度、床层总压降与生产实测值吻合较好,验证了数学模型的正确性。进一步分析了燃料配比、风量和给料温度等操作参数对烧结过程的影响。研究结果表明：燃烧带的厚度、最高温度随着烧结过程的进行而逐渐增加。床层孔隙率、颗粒当量直径的变化主要发生在燃烧带的熔融、冷凝阶段。料层压损最大的是燃烧熔融层,其次是混合料带,最小的是烧结矿层。增加焦粉含量、提高烧结混合料的初温,有利于提高成矿质量;风量过大时,会造成成矿质量下降、生产成本提高。     

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.     

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.     

超临界压力水在垂直上升内螺纹管中的传热特性

在压力22.5～30 MPa,质量流速430～1200 kg·m^-2·s^-1,内壁热负荷284～719 kW·m^-2范围内,对超临界压力水在均匀加热垂直上升内螺纹管内的传热特性进行了实验研究,得到了内螺纹管内超临界压力水的传热特性,分析了压力、热负荷和质量流速变化对内螺纹管壁温及传热系数的影响,探讨了拟临界区的传热机理,并给出了能用于工程实际的传热实验关联式。实验结果表明：垂直上升内螺纹管中超临界水具有良好的传热特性。在低焓值区内螺纹管壁温随焓增平缓增加,而在高焓值区壁温随焓增的升高明显。由于热物性的剧烈变化,超临界水在拟临界焓值区发生了明显的传热强化。压力与热负荷的增大以及质量流速的减小均会导致内螺纹管壁温的升高和传热系数的减小,使得传热强化现象削弱,甚至出现传热恶化。     

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。