Transpiring Wall Reactor Research Articles

Energy and exergy assessments of supercritical water oxidation of wet organic wastes under hydrothermal flames with a Y-shape reactor for power generation

A novel Y-shape reactor combined the concepts of transpiring wall reactor (TWR) and hydrothermal flame was proposed for supercritical water oxidation (SCWO) of wet organic wastes. Hydrothermal flames can enhance feed degradation, and the Y-shaped structure enables the efficient gas–solid separation and allows extracting a part of uncooled supercritical fluids for power generation. The proposed reactor and corresponding system were simulated using Aspen Plus 8.2, and simulation results agree well with experimental flame temperatures and reaction products. Exergy analysis results indicate the reactor, heat exchanger 7, heat exchanger 1, and turbine contribute to the main destruction of the system, with exergy distribution coefficients of 18.65%, 17.75%, 11.95%, and 7.64%, respectively. The exergy destruction coefficient for the reactor has been greatly reduced compared with previous TWR-SCWO systems, indicating great improvement in exergy efficiency with the Y-shape reactor. The exergy efficiency can be increased from 24.83% to 40.50% when hot water output is replaced with steam production because of the great reduction in exergy destruction. Lower ratios of fuel flow to feed flow contribute to higher exergy efficiencies on the premise of the safety and economy of the system. The increases of separation efficiency and split coefficient are helpful for increasing system efficiency for more supercritical fluids can be converted into high-grade energy. The increase of feed concentration is beneficial for the increase of system economy even though at a relative low exergy efficiency.

Effects of key parameters on water film properties and temperature field distribution inside transpiring wall reactor

Transpiring wall reactor (TWR) can efficiently weaken corrosion and salt deposition problems during supercritical water oxidation. Its corrosion and salt deposition resistance are mainly affected by water film properties and temperature field distribution. Therefore, the effects of crucial parameters on water film properties and temperature field distribution are investigated via numerical simulation in this work. The results indicate that the upper supercritical oxidation zone is mainly affected by the second layer of transpiration water instead of the third layer. The second, third layer optimal injection locations are at 0.036 m, 0.060 m in this study, respectively. Excessively thick transpiring wall decreases water film coverage rate thereby damaging TWR, conversely makes supercritical zone shrink and thus weakening degradation ability. Annular gap primarily affects the temperature distribution near the transpiring wall. Under the conditions of the work, transpiring wall thickness of 0.002 m and annular gap of 0.004 m are recommended.

Formation and evolution characteristics of hydrothermal flames inside a transpiring wall reactor: A transient numerical investigation

Transpiring-wall reactors with hydrothermal flames are effective in preventing corrosion and salt deposition in supercritical water oxidation systems. To uncover the not-well-understood evolution characteristics of the hydrothermal flame surrounded by a transpiring water film, a 2-D unsteady simulation is firstly conducted in this study. The transient results show that the variations of flow fields and flame shapes are attributed to the competition between inertial forces and buoyant forces. And the quasi-steady results present that high feed concentration may damage transpiring wall and delay the evolution of hydrothermal flames. In addition, a low feed temperature or a big flow area of jet can lead to open hydrothermal flames, which adversely affect the reactor. Furthermore, Froude Number, Fr, shows effectiveness in predicting quasi-steady flame shapes. For large-Fr conditions, the inertial force outweighs the buoyant force and hence closed flames are developed; while for low-Fr cases, open flames are developed.

Experimental investigation on the oxidation of printing ink wastewater under hydrothermal flames

Printing ink wastewater has a strong color and contains high salinity and high concentrations of complex organic compounds, posting a severe threat to the ecological environment and human health. In this work, supercritical water oxidation of printing ink wastewater is performed within a transpiring wall reactor (TWR) to avoid corrosion and salt plugging issues. The high concentration of organic matter in printing ink wastewater is used to form hydrothermal flames for enhanced degradation. Flame temperatures, gaseous and aqueous products characteristics, as well as the anti-corrosion and salt plugging performance of the reactor, are obtained. A remarkable temperature fluctuation due to the multi-component systems indicates the instability of hydrothermal flame. The salt recovery rate of 94.56% and the integrity of the porous tube inner wall indicate that TWR has a good performance in terms of anti-corrosion and salt plugging. Under typical conditions, the removal rates for chemical oxygen demand, total nitrogen, and color are 99.52%, 70.07%, and 97.89%, respectively. Alkanes, nitrogenous compounds, benzodiazepines, and heterocyclic compounds in the aqueous products, and CO, H2, NH3, and CH4 in the gaseous products are the main intermediate products of printing ink wastewater under hydrothermal flames. Ammonia nitrogen, as the main intermediate product, inhibits the complete oxidation of nitrogenous organic compounds. Additionally, higher KNO3 concentrations promote the complete oxidation of nitrogenous organic compounds, while higher NaOH concentrations can increase the pH value and slightly facilitate degradation. In addition, higher feed flow rates prevent deposition and adhesion of additives that occur at high concentrations.

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

AbstractCorrosion 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.

Effect of operating parameters on water film properties of transpiring wall reactor

Transpiring wall reactor can effectively minimize reactor corrosion and salt deposition problems, which seriously hinder the commercialization of supercritical water oxidation. In this work, methanol was selected as organic matter, and the influences of key operating parameters on water film properties and organic matter removal were investigated mainly via numerical simulation. The results show that methanol conversion could be higher than 99% and hardly affected by various feed and transpiration water parameters. The highest value of reactor center temperature significantly increased by about 300 K as feed preheating temperature changed from 673 K to 823 K. Decreasing feed flow rate, feed concentration, feed preheating temperature and transpiration water temperature, and increasing transpiration intensity reduced water film temperature but raised coverage rate, which are beneficial to forming excellent water film with good corrosion and salt deposition resistance. These findings can provide significant reference for the optimization of reactor operating parameters.

Effect of evaporation on the energy conversion of a supercritical water oxidation system containing a hydrothermal flame

Hydrothermal flame is an effective solution to avoid coking and salt plugging in the preheating section of a supercritical water oxidation (SCWO) system with a transpiring wall reactor (TWR). An SCWO system with an evaporation module (SCWOE) is proposed in this work to concentrate the wastewater and promote the formation of a hydrothermal flame. The SCWOE system is simulated using Aspen Plus, and the simulation model is validated by comparing with the experimental data on the migration of organics in evaporation. The introduction of the evaporation module greatly reduces the exergy input. The exergy destruction mainly comes from the TWR, electric heater, and heat exchangers. The highest exergy destruction in the TWR appears in the reaction section, and the total exergy destruction in the TWR due to the transpiring water injection reaches 89.9%. The increase in concentration ratio (α) and feed concentration (ω) can lower the exergy input and improve the energy efficiency, and an energy self-sufficient rate of 79.7% occurs at ω = 15% and α = 2. Moreover, the involatile property of organics is conducive to improving energy self-sufficient rate, and the volatilization rate of phenol in wastewater must be controlled in this system.

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

AbstractA 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.

Experimental study on the mixing characteristics inside an inner preheating transpiring-wall reactor for supercritical water oxidation

The mixing characteristics between bulk flows and transpiring water are critical to feed degradation and water film formation inside a transpiring-wall reactor (TWR). In this study, a supercritical water oxidation system with an inner preheating TWR is established, and special experiments are designed by adding organic matter (glycerol) to the transpiring water to evaluate the mixing degree between the bulk flows and transpiring water in the hydrothermal flame zone. Although the chemical oxygen demand concentration of the reactor effluent increases significantly due to the addition of glycerol to the transpiring water, a certain amount of glycerol is entrapped into the flame zone and degraded. The increase in jet velocity with increased heat source flow or temperature is favorable for mixing and enhances jet entrainment. Moreover, increasing the transpiring coefficient and transpiring water temperature can increase the radial velocity of transpiring water, which also benefits mixing. Possible applications utilizing the mixing characteristics are further proposed.

Combined effects of protective film and oxidant on the performance of the supercritical water oxidation system with a film protective reactor: A simulation study

Supercritical water oxidation (SCWO) is a promising technology for treating organic waste and recovering energy. Water film protective reactor (WFPR), which is also referred to as transpiring wall reactor, provides a solution to corrosion and salt plugging issues. Three novel film protective reactors (FPRs) using air, nitrogen, and hydrogen peroxide as protective fluids were proposed to improve the corrosion and cost issues in WFPRs. Detailed optimized process parameters and flow field characteristics within the FPRs were obtained using Aspen Plus and computational fluid dynamics, respectively. The anti-corrosion performance and prevention of salt plugging in the FPRs were analyzed, and economic analyses for SCWO systems with different FPRs were conducted. Results indicate that the WFPR and hydrogen peroxide FPR present good solubility for inorganic salts but a high probability of corrosion. An SCWO system with the WFPR or nitrogen FPR is more competitive for industrial plants compared with other SCWO systems.

Global direct methanol fuel cells market will reach $367 M by 2025: Zion Market Research

Systematical corrosion tests of austenitic stainless steel 316L exposed to sewage sludge SCWO (supercritical water oxidation) were conducted in a batch stirred reactor with hydrogen peroxide as oxidant. Experiment conditions such as temperature, oxidation coefficient, pH value, corrosion medium, were chosen mainly keeping in mind the place and environment of reactions (i.e. surrounding transpiring wall). The exposed samples were ultimately analyzed by weight measurement, scanning electron microscopy in conjunction with energy dispersive spectroscopy, and X-ray diffraction analysis. The results show that severe pitting corrosion occurred as the sample was exposed to complicated environments, and different oxides including Fe3O4, FeCr2O4 and MoO3 were found on the sample surface. The corrosion rate at all test conditions (360–450 °C pH = 5.2–10.05, oxidation coefficient of 0–2.0, sewage sludge or its SCWO reactor effluent) was in the range of 0.12–0.66 mm/y, and it increased as temperature and OC increased at supercritical conditions. Moreover, potential corrosion mechanism of 316L in sewage sludge SCWO is proposed, and influences of operating parameters on 316L corrosion properties are summarized. 316L and reactor effluent could be considered as transpiring wall material and transpiring water in sewage sludge SCWO with transpiring wall reactor, respectively.

Optimization of structural parameters of an inner preheating transpiring-wall SCWO reactor

A computational fluid dynamics model of an inner preheating transpiring wall reactor for supercritical water oxidation was proposed to optimize the structural parameters. Results showed that higher feed degradation efficiencies and lower inner surface temperatures of the porous wall were obtained at higher reactor diameters. Although the increase in reactor length promoted feed degradation, the increased inner surface temperatures of the porous wall were unsuitable for water film formation. A pyknic-type reactor with a lower length/diameter (H/D) value at a constant volume was more beneficial for water film formation and feed degradation. A higher feed concentration or feed flow required a lower H/D value to ensure an ideal water film formation and feed degradation. We proposed the use of sectional heat load as a new parameter for reactor design based on the similarity between the boiler furnace and the transpiring wall reactor. The sectional heat load in a certain range was calculated to serve as guide in the design of transpiring wall reactors.

Simulation of a Transpiring Wall Reactor for Supercritical Water Oxidation: Characteristics of Water Film

Reactor corrosion and plugging problems severely hinder commercial application of supercritical water oxidation. A transpiring wall reactor can overcome these two problems by forming a protective water film on the inner surface of a porous transpiring wall to isolate corrosive substances and inorganic salt. This work proposed water film coverage rate and inorganic salt concentration in water film as evaluation indexes of water film characteristics. A computational fluid dynamics model of a new transpiring wall reactor was built to explore the characteristics of water film under key operating parameter conditions by numerical simulation together with experiment validations. The results show that water film coverage rate raised either with transpiration intensity and transpiration water temperature increasing or with feedstock flow rate and feedstock preheating temperature decreasing. Inorganic salt concentration in water film declined as transpiration intensity and transpiration length increased. Two equat...

Numerical modeling of an inner preheating transpiring-wall reactor for supercritical water oxidation

Numerical modeling of an inner preheating transpiring-wall reactor for supercritical water oxidation

Corrosion characteristics of 316L as transpiring wall material in supercritical water oxidation of sewage sludge

Systematical corrosion tests of austenitic stainless steel 316L exposed to sewage sludge SCWO (supercritical water oxidation) were conducted in a batch stirred reactor with hydrogen peroxide as oxidant. Experiment conditions such as temperature, oxidation coefficient, pH value, corrosion medium, were chosen mainly keeping in mind the place and environment of reactions (i.e. surrounding transpiring wall). The exposed samples were ultimately analyzed by weight measurement, scanning electron microscopy in conjunction with energy dispersive spectroscopy, and X-ray diffraction analysis. The results show that severe pitting corrosion occurred as the sample was exposed to complicated environments, and different oxides including Fe3O4, FeCr2O4 and MoO3 were found on the sample surface. The corrosion rate at all test conditions (360–450 °C pH = 5.2–10.05, oxidation coefficient of 0–2.0, sewage sludge or its SCWO reactor effluent) was in the range of 0.12–0.66 mm/y, and it increased as temperature and OC increased at supercritical conditions. Moreover, potential corrosion mechanism of 316L in sewage sludge SCWO is proposed, and influences of operating parameters on 316L corrosion properties are summarized. 316L and reactor effluent could be considered as transpiring wall material and transpiring water in sewage sludge SCWO with transpiring wall reactor, respectively.

Energy consumption and exergy analyses of a supercritical water oxidation system with a transpiring wall reactor

A supercritical water oxidation system with a transpiring wall reactor was simulated by Aspen Plus, and simulation model was validated by comparisons with experimental reactor outlet temperatures and product properties (total organic carbon and CO). Energy and exergy analyses were conducted to reduce energy consumption and exergy loss of the system. It is indicated that the system’s energy efficiency and exergy efficiency are 97.73% and 13.28% at typical operating conditions, respectively. The exergy loss of electric heater, heat exchanger and reactor account for 39.89%, 26.64%, and 17.23% of the system’s total exergy loss, respectively. The process optimization is conducted by preheating the middle branch of transpiring water with the heat of the reaction products to reduce energy consumption, and the net electric cost is reduced from 7.43 ¥/h to 5.96 ¥/h. It can be observed that when the split coefficient for split 2 equals to 0.6, the minimum electricity input is required for the system. When the feed concentration is increased from 2wt.% to 10wt.%, net electric cost per COD significantly decreases from 14.05 ¥/kg to 1.31 ¥/kg, which indicates that higher feed concentrations are beneficial for reducing the cost of energy consumption. What is more, when feed flow rate increases from 6kg/h to 16kg/h, net electric cost per COD increases from 3.21 ¥/kg to 4.61¥/kg, which shows that higher energy consumption costs will be required at higher feed flow rates.

Characteristics analysis of water film in transpiring wall reactor

Transpiring wall reactor can significantly overcome reactor material corrosion and salt deposition problems by forming a protective water film on its inner porous wall. In this work, a simplified physical model of transpiring wall reactor was built to analyze the process of heat and mass transfer between the water film and bulk fluid. Furthermore, two quantitative correlations between the thickness of water film and main operating parameters were proposed for the first time using theoretical analysis and mathematical deduction method. For a given transpiring wall reactor, appropriately increasing the flow rate and inlet temperature of transpiration water is conducive to maintain the growth of the water film thickness. A positive linear relation exists between the mass flow rate of transpiration water and the thickness of water film under constant feedstock parameters conditions. These quantitative correlations together with some meaningful conclusions help to guide the optimization of operating parameters as well as the design of transpiring wall reactor.

Experimental Study on the Ignition and Extinction Characteristics of the Hydrothermal Flame

AbstractAn inner‐preheating transpiring‐wall reactor with a hydrothermal flame was designed and tested to avoid plugging in the preheating section. Hot water was used as auxiliary heating fluid to preheat the fuel to the ignition temperature. This work focused on the flame characteristics (i.e., the ignition and extinction processes, the ignition and extinction temperatures) inside the inner‐preheating transpiring‐wall reactor. The result shows that higher fuel concentrations and fuel flows permit lower ignition temperatures. The results of the nozzle configuration experiments show that lower velocities provide resistance to extinction.

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

Experimental study on the operating characteristics of an inner preheating transpiring wall reactor for supercritical water oxidation: Temperature profiles and product properties

A new process to generate multiple thermal fluids by supercritical water oxidation (SCWO) was proposed to enhance oil recovery. An inner preheating transpiring wall reactor for SCWO was designed and tested to avoid plugging in the preheating section. Hot water (400–600°C) was used as auxiliary heat source to preheat the feed to the reaction temperature. The effect of different operating parameters on the performance of the inner preheating transpiring wall reactor was investigated, and the optimized operating parameters were determined based on temperature profiles and product properties. The reaction temperature is close to 900°C at an auxiliary heat source flow of 2.79kg/h, and the auxiliary heat source flow is determined at 6–14kg/h to avoid the overheating of the reactor. The useful reaction time is used to quantitatively describe the feed degradation efficiency. The outlet concentration of total organic carbon (TOCout) and CO in the effluent gradually decreases with increasing useful reaction time. The useful reaction time needed for complete oxidation of the feed is 10.5s for the reactor.