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     

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.     

Effects of structural parameters on water film properties of transpiring wall reactor

Corrosion and salt deposition problems severely restrict the industrialization of supercritical water oxidation. Transpiring wall reactor can effectively weaken these two problems by a protective water film. In this work, methanol was selected as organic matter, and the influences of vital structural parameters on water film properties and organic matter removal were studied via numerical simulation. The results indicate that higher than 99% of methanol conversion could be obtained and hardly affected by transpiration water layer, transpiring wall porosity and inner diameter. Increasing layer and porosity reduced reactor center temperature, but inner diameter's influence was lower relatively. Water film temperature reduced but coverage rate raised as layer, porosity, and inner diameter increased. Notably, the whole reactor was in supercritical state and coverage rate was only approximately 85% in the case of one layer. Increasing reactor length affected slightly the volume of the upper supercritical zone but enlarged the subcritical zone.     

Analysis of the scale up of a transpiring wall reactor with a hydrothermal flame as a heat source for the supercritical water oxidation

Experimental data from a tubular reactor and from a transpiring wall reactor (TWR) are used to analyze the scaling up of SCWO reactors operating with a hydrothermal flame as a heat source. Results obtained with the tubular reactor show that fluid velocity inside the reactor determines the minimum injection temperature at which a stable hydrothermal flame is formed. When the fluid velocity inside of the reactor is lower, the extinction temperature of the hydrothermal flame in that reactor is also lower. Using this reactor, extinction temperatures are always near or above the critical temperature of water. Total TOC removals are possible working with isopropyl-alcohol at temperatures between 650 and 700 °C and residence times of 0.5 s. Results of the TWR show that steady operation with a hydrothermal flame inside is possible even when reagents are injected at subcritical conditions as low as 170 °C. Temperature measurements show that reaction is not initiated in the injector but in the reaction chamber, where fluid velocity is lower than 0.1 s. This was explained by estimating that the flame front velocity of a hydrothermal flame is of the order of 0.1 m/s. Thus, it is expected that the flame is stabilized in the reaction chamber and not in the injector, where fluid velocities are higher than 2 m/s. A previously developed model of the TWR was modified in order to describe the ignition in the reaction chamber and not in the injector. The model reproduces satisfactorily experimental data and it was used to propose the design of scaled up reactors for SCWO with a hydrothermal flame inside.     

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。     

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.     

超临界压力正癸烷在螺旋管中传热与裂解吸热现象的数值模拟

探讨了正癸烷的裂解吸热反应对流动、传热和化学组分分布的影响。结果表明,由于裂解吸热反应把加热热量转换成了燃料分子的化学能,与未考虑热裂解的计算结果相比,考虑裂解反应时出口处流体平均温度降低了约50 K,壁面温度降低了约70 K,对流传热系数提高了10%左右。这一方面是因为裂解引起的密度降低、轴向速度增加,另一方面是由于裂解反应提高了螺旋管截面的径向速度,加强了二次流动,增加了壁面湍动能。正癸烷在内侧温度较高的区域裂解度较高,因此螺旋管内侧温度降低,环向温度分布变均匀;裂解度越大,环向温度分布越均匀。与热结焦有关的烯烃类裂解产物C₂H₄,C₃H₆在温度较高的内侧质量分数较大,表明结焦更可能发生在螺旋管的内侧。     

国外超临界水氧化技术的研究现状

超临界水氧化是水处理技术发展的新方向,但该技术对设备的要求比较高,工业化应用仍有一定的难度。为了克服这一难题,目前的研究工作主要集中在催化剂的选择以及设备防腐蚀等方面。介绍了贵金属类催化剂、过渡金属类催化剂、碱金属盐类催化剂、杂聚酸类催化剂以及碳基类催化剂,在降解不同污染物时的催化效率。在反应器材质和反应器形式的研究中,分别对铁、镊、铬等纯金属以及不同材料的合金在各种条件下的防腐蚀性能作了比较;两种最新的反应器形式:可蒸发壁式反应器和流动式反应器。它们在超临界水氧化中表现出了良好的防腐能力。     

甲烷水蒸气重整制氢反应及其影响因素的数值分析

本文通过建立包含动量、能量、质量以及化学反应的多物理场耦合数值模型, 以多孔介质模型表征催化剂层, 对工业转化炉管中的甲烷水蒸气重整制氢过程进行了详细分析。计算得到了转化炉管内甲烷重整过程反应物及产物气体的速度、温度及浓度场分布, 以此分析了甲烷重整制氢过程的反应特性, 并阐明了转化炉管的壁面温度、原料气入口水碳比以及入口速度对甲烷转化率的影响。结果表明:水蒸气重整在转化炉管的入口区域反应迅速, 沿着气体流动方向, 反应速率由于反应物浓度的不断降低而减小, 导致混合气体流动速度和温度也逐渐趋于稳定;水碳比和转化管壁面温度的增加以及原料气体入口流速的降低, 都会提高甲烷的转化率。本文所得到的结论对于优化实际生产中甲烷水蒸气重整制氢反应的工况条件具有一定的参考和借鉴意义。     

管形对水平管降膜圆周膜厚和Nusselt数的影响

针对海水淡化系统的水平管降膜蒸发,建立了二维数值模型,分析了光滑圆管和6种不同截面形状的蛋形管外降膜流动及传热特性。采用VOF方法考察了不同管型对管外液膜分布和传热特性的影响。数值结果表明:蛋形管外液体沿周向流动较圆管稍快,且可获得更均匀更薄的液膜;液膜厚度随半轴比ε增大而减小,随周向先逐渐减小后迅速增加,圆管和蛋形管的液膜最小值分别出现在周向相对坐标0.69和0.70~0.84附近。蛋形管的膜内量纲1温度较圆管的小,其热边界层厚度较薄,具有更好的传热性能。拟合数据得到,ε为2.4的蛋形管具有最好的传热性能,其Nusselt数可达0.32,较圆管的高出12.68%。最后,将数值模拟结果与文献中的数据进行了对比,验证了数值模型的可行性和合理性。     

垂直U型管两相流流型和传热研究——(Ⅱ)U型管废热锅炉传热恶化

为查明垂直U型管废热锅炉爆管原因,在研究垂直U型管弯管段空气-水两相流流型转变的基础上,本文在高压电加热水回路试验装置上对其进行了热态试验研究,压力106×10~5Pa、质量流速800—2000kg/m~2·s、热负荷80—330kW/m~2.得出了沿U型管弯管段壁温分布特性.从两相流流型变化规律分析了U型管传热恶化的机理.根据热态试验结果修正了U型管两相流流型图.确定了U型管可靠操作条件.     

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.     

Temperature control of the water gas shift reaction in microstructured reactors

Microchannel reactors offer unique possibilities for temperature control of chemical reactions due to the strong coupling of channel and wall temperatures. This may be applied to all chemical reactions which require a certain temperature profile to achieve an optimum yield. For the reformation of hydrocarbons for fuel cell applications a low CO concentration of the product gas is desired. In conventional systems, this is achieved by sequentially processing the reformate through a high and low temperature water gas shift reactor because increased temperature enlarges the reaction rate while lower temperature shifts the equilibrium to the desired small CO concentrations. However, for every gas composition arising during the reaction process an optimum temperature exists at which the reaction rate is highest. We will demonstrate that this optimum temperature profile to a good approximation can be achieved in a single step WGS reactor by controlling the temperature via cooling gas flowing in counter current to the reformate. Furthermore, the effect of water addition (steam injection) is analysed for a conventional two-step adiabatic reactor system and the possible size reduction in an integrated heat-exchanger reactor under comparable conditions is validated. Finally, the effect of diffusion limitations at various channel dimensions is investigated applying a two-dimensional model which allows a trade-off between pressure drop or respective reactor size and performance when dimensioning a real system in future.     

THEORETICAL SIMULATION OF HIGHLY EFFECTIVE COOLING WITH AN EVAPORATING ANNULAR FLOW IN A VERTICAL TUBE

A new concept is proposed for the highly effective cooling of a polymer electrolyte membrane fuel cell (PEMFC) using the downward annular two-phase flow of high-speed air and subcooled water in a small vertical tube. Numerical simulations based on the two-phase flow boundary layer model are performed to investigate the heat and mass transfer characteristics of the annular flow with uniform heat flux at the tube wall. The coupled heat transfer due to evaporation and convection and the effects of various relevant parameters on the temperature profiles on the wall and of the gas core are studied. It is shown that annular two-phase flow of air and subcooled water in a small vertical tube can provide high heat transfer rate through the evaporation of the water film, while still maintaining low wall temperature. This cooling method is found to be encouraging for use in the highly effective cooling of PEMFC.     

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.     

Spouting with a porous draft-tube

Particle motion and gas distribution have been measured in 0.3 m spouted beds with porous and solid wall draft-tubes. Particle velocity in the annulus varied with gas rate and bed geometry. Narrow particle residence time distributions were observed under all conditions. Annular gas flow with the porous tubes exhibited a characteristic maximum in the lower part of the bed. A computer model has successfully simulated this phenomenon. The porous tube had a higher annular airflow than the solid tube at low separation distances, and a lower pressure drop for a given annular flow. A composite porous and solid wall tube offers improved performance.     

Experimental study of kerosene–water two‐phase flow in a vertical pipe using hot‐film and dual optical probes

The local parameters for kerosene–water upward flow are measured in a vertical pipe of 77.8 mm inner diameter at 4200 mm from the inlet (L/D = 54) using hot‐film and dual optical probes. The effect of superficial water velocity and volumetric quality on radial distribution of two‐phase flow parameters is investigated. The results show the following: (i) the profiles of volume fraction and drop frequency are very similar, and increasing superficial water velocity at low volumetric qualities (<18.6%) change the profile from a convex shape with peak at the pipe centreline to uniform then to concave shape with peak near the wall; (ii) the profiles of drop cut chord change from a parabolic shape with peak at centreline for low superficial water velocities to a flat shape at higher superficial water velocity, and the area‐averaged drop diameter decreases with higher superficial water velocities for all volumetric qualities; (iii) velocity profiles for both phases have shapes similar to single phase flow, flatter at higher values of superficial water velocity and volumetric quality and centreline peaked at low superficial water velocities and volumetric qualities; (iv) the slip velocity decreases with radial distance having a peak at centreline and zero values near the wall; (v) introducing kerosene drops into single phase water flow results in a sharp increase in turbulent intensity, particularly at low water velocity, and the difference between the single phase and two‐phase flow turbulence intensities decreases with higher superficial water velocities and (vi) the results show that interfacial area concentration increased with higher volumetric quality and higher number of bubbles thereby increases the contact area between the two phases. © 2012 Canadian Society for Chemical Engineering     

Modeling of naphtha pyrolysis in swaged coils

Pyrolysis of naphtha in uniform diameter and swaged reactors has been modeled. Pyrolysis and coking models available for naphtha cracking were used to calculate the reactor profiles of pressure, process gas temperature, tube metal temperature, conversion and the product yields. For the swaged coil, not only was the inlet pressure and maximum tube wall temperature in the clean condition lower than for a uniform diameter reactor, but the increase in the inlet pressure and maximum tube wall temperature due to coke deposition was also less. Swaging the reactor can result in a significant increase in the run length between decokings.     

EXPERIMENTAL INVESTIGATION OF HIGHLY EFFECTIVE COOLING WITH AN EVAPORATING ANNULAR FLOW IN A SMALL VERTICAL TUBE

An experimental study was performed to investigate the heat transfer characteristics of the non-boiling annular two-phase flow of nitrogen gas and subcooled water through a small vertical heated tube with uniform wall heat flux. The experimental results were compared with the numerical calculations based on the two-phase flow boundary layer model. The experimental data show that the annular two-phase flow of nitrogen gas and subcooled water in a small vertical tube can provide quite high heat transfer ability, while wall temperatures lie in a low range even if at quite high wall heat fluxes. In particular, the highest values of the wall temperatures are insensitive to the change of flowing and heating conditions. This characteristic is quite suitable for highly effective cooling of polymer electrolyte membrane fuel cells.     

EXPERIMENTAL INVESTIGATION OF HIGHLY EFFECTIVE COOLING WITH AN EVAPORATING ANNULAR FLOW IN A SMALL VERTICAL TUBE

An experimental study was performed to investigate the heat transfer characteristics of the non-boiling annular two-phase flow of nitrogen gas and subcooled water through a small vertical heated tube with uniform wall heat flux. The experimental results were compared with the numerical calculations based on the two-phase flow boundary layer model. The experimental data show that the annular two-phase flow of nitrogen gas and subcooled water in a small vertical tube can provide quite high heat transfer ability, while wall temperatures lie in a low range even if at quite high wall heat fluxes. In particular, the highest values of the wall temperatures are insensitive to the change of flowing and heating conditions. This characteristic is quite suitable for highly effective cooling of polymer electrolyte membrane fuel cells.