Corrosion control methods in supercritical water oxidation and gasification processes

Use of supercritical water (SCW) as a medium for oxidation reactions, conversion of organic materials to gaseous or liquid products, and for organic and inorganic synthesis processes, has been the subject of extensive research, development, and some commercial activity for over 25 years. A key aspect of the technology concerns the identification of materials, component designs, and operating techniques suitable for handling the moderately high temperatures and pressures and aggressive environments present in many SCW processes. Depending upon the particular application, or upon the particular location within a single process, the SCW process environment may be oxidizing, reducing, acidic, basic, nonionic, or highly ionic. Thus, it is difficult to find any one material or design that can withstand the effects of all feed types under all conditions. Nevertheless, several approaches have been developed to allow successful continuous processing with sufficient corrosion resistance for an acceptable period of time. The present paper reviews the experience to date for methods of corrosion control in the two most prevalent SCW processing applications: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).     

Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water

In this work, the denitrogenation of hydrocarbons under supercritical water oxidation environment was investigated in a rotated bomb reactor at 623-723 K and 25-35 MPa over sulfided NiMo catalyst. Quinoline was used as a model nitrogen-containing compound. A high reduction of total nitrogen up to about 85% was obtained. The denitrogenation pathway is composed of two consecutive steps: in situ H₂ generation and the hydrogenation of quinoline. The hydrogenation mechanism of quinoline varies with reaction temperature because of the participation of supercritical water in HDN step. The strong adsorption of quinoline and its hydrogenation intermediates on catalyst surface has an adverse influence on total nitrogen reduction rate.     

Non-catalytic recovery of phenol through decomposition of 2-isopropylphenol in supercritical water

The decomposition of 2-isopropylphenol (IPP) was studied in supercritical water at 723- with a water density of 0- in the absence of catalyst. The main products were phenol, 2-propylphenol (PP), 2-cresol and 2-ethylphenol. The reaction was determined to proceed as follows. At first, the dealkylation and rearrangement of IPP yielded phenol and PP, respectively. Next, the dealkylation of PP lead to the formation of 2-cresol and 2-ethylphenol. The conversion of IPP and the selectivity of phenol increased with the increasing water density, which led to an increase in the yield of phenol. The recoveries of phenol as high as 43% can be obtained in the high water density region at . The rate constant for decomposition of IPP was correlated with a global reaction model for a range of temperatures from 613 to .     

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.     

A process for generating power from the oxidation of coal in supercritical water

A theoretical study of power generation from oxidation of coal by supercritical water oxidation (SCWO) is presented. Two versions of SCWO power plant are compared to two of the most efficient conventional power plant processes: pulverised coal power plants and pressurised fluidised bed power plant. The effects of steam pressure and temperature on produced (W_p), consumed (W_c) and net work (W_N) are calculated in order to compare the efficiency of these power plants for the same steam conditions. Enthalpies have been calculated using residual enthalpies by Peng-Robinson equation of state. Calculated results show that net work in SCWO power plant is 5% higher than in other power plants, due to the fact that no air surplus is necessary for complete combustion and because steam is produced by direct heating. Energetic efficiency of SCWO increases more quickly with temperature than for the other power plants. The effect of steam pressure is different: until 30 MPa power plant efficiencies increase more quickly in SCWO power plants than in conventional plants, but when steam pressures increases beyond 30 MPa, efficiencies decrease in SCWO power plants.     

Fast catalytic oxidation of phenol in supercritical water

The catalytic oxidation of phenol in water over a commercial oxidation catalyst, CARULITE 150, was investigated in a fixed bed flow reactor at 250 atm and temperatures from 380°C to 430°C. The phenol and oxygen concentrations at the reactor entrance varied between 0.070 and 1.24 mmol/l, and 9.60 and 39.6 mmol/l, respectively. The reaction conditions produced phenol conversions and selectivities to CO₂ much higher than those produced by non-catalytic oxidation. The kinetics of phenol disappearance and of CO₂ formation were both roughly first-order, and the activation energies were 31 and 47 kcal/mol, respectively. The catalyst did not undergo continuous deactivation during the catalytic oxidation, but rather maintained a high activity even after several days of continuous operation.     

Hydrothermal flames in supercritical water oxidation: investigation in a pilot scale continuous reactor

Hydrothermal flames at 25 MPa supercritical water environment were investigated using a 4800 ml reaction tower, in which the sapphire windows were fitted for optical access. Down flowing hydrothermal flames were observed for oxidation of 2-propanol when the reactor was fed with inlet organic concentration higher than 2 vol% and air ratio higher than 1.8. Flame temperature, as high as 1100 °C, was measured by means of a thermocouple and the temperature was found to be strongly influenced by air ratio. Effective and stable oxidation of organics with TOC removal rate of 99.9% was achieved. Dioxins were also decomposed with a ratio higher than 99.9%, within 1 min reaction time in this reactor configuration.     

Ion exchange resins destruction in a stirred supercritical water oxidation reactor

Spent ion exchange resins (IERs) are radioactive process wastes for which there is no satisfactory industrial treatment. Supercritical water oxidation offers a viable treatment alternative to destroy the organic structure of resins, used to remove radioactivity. Up to now, studies carried out in supercritical water for IER destruction showed that degradation rates higher than 99% are difficult to obtain even using a catalyst or a large oxidant excess. In this study, a co-fuel, isopropanol, has been used in order to improve degradation rates by initiating the oxidation reaction and increasing temperature of the reaction medium. Concentrations up to 20 wt% were tested for anionic and cationic resins. Total organic carbon reduction rates higher than 99% were obtained from this process, without the use of a catalyst. The influence of operating parameters such as IERs feed concentration, nature and counterions of exchanged IERs were also studied.     

超临界水氧化技术研究及进展

介绍了超临界水的特性 ,并对超临界水氧化技术的研究及应用进行了综述 ,提出了超临界水氧化技术的工程应用开发中存在的问题     

Upgrading of asphalt with and without partial oxidation in supercritical water

Supercritical water and supercritical water partial oxidation treatments were applied to the upgrading of asphalt. Asphalt was converted at 613-673 K, 0-0.5 g/cm³ water density under argon or air atmosphere. Under an argon atmosphere and 0.5 g/cm³ water density, both the asphaltene conversion and desulfurization increased with increasing temperature. At 673 K, the asphaltene conversion and the yield of CO₂ increased with an increasing water density. Water apparently participated in the reaction and its hydrogen was used for capping the free radicals generated during the upgrading of asphalt resulting in an increased yield of maltene. Under an air atmosphere at 673 K, asphaltene conversion was lower but desulfurization was higher than those obtained in an argon atmosphere.     

Phenol oxidation over CuO/Al2O3 in supercritical water

Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al₂O₃ as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO₂ formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO₂ formation than did bulk MnO₂ or bulk TiO₂. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.     

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.     

超临界水中乙酸氧化的研究进展

超临界水氧化技术是一种快速彻底降解废水中有机物质的新型处理技术。在超临界水氧化有机物中 ,乙酸被认为是一种中间产物 ,乙酸氧化是反应速率的控制步骤 ,其氧化动力学的研究对反应器设计具有重要意义。大部分研究都集中在动力学参数和反应条件如温度、压力、密度和停留时间上。最近的研究发现加入二氧化锰等催化剂 ,可缓和反应温度、压力条件 ,以达到高效节能的目的。综述了目前在超临界水氧化乙酸动力学方面的研究进展。对连续平推流或间歇反应器中实验数据用幂指数曲线拟合得到的动力学方程表明 ,乙酸氧化为一级反应     

超临界水的性质及氧化反应原理

介绍了超临界水的性质及其氧化分解有机废物的反应原理,并对超临界水氧化法进行了评价分析,指出了其广阔的应用前景.     

氨在超临界水中氧化的平衡热力学研究

城市污泥中含有大量的蛋白质等含氮有机物,而氨是含氮有机物超临界水氧化的中间产物。文章使用热力学方法研究了氨在超临界水中氧化构成的平衡体系。采用Peng-Rob inson状态方程和最小自由能法计算了反应体系中各组分的平衡组成。结果表明,当反应体系达到平衡时,氨在超临界水中可以被完全氧化;其中大部分的氨转化成了氮气,只产生少量的NO和NO2,没有N2O的生成。提高温度和过氧量可以增加NOx的平衡产量,但NOx的选择性最高不超过2%。比较热力学平衡计算结果和前人实验结果发现,N2O只是氨在超临界水中氧化的中间产物。氨在超临界水中的降解效果主要受动力学因素影响。     

Partial oxidation of n-hexadecane and polyethylene in supercritical water

In this study, we show the results of partial oxidation experiments of n-hexadecane (n-C₁₆) and polyethylene (PE) in supercritical water (SCW). The experiments were carried out at 673 or 693 K of reaction temperature and 5 or 30 min of reaction time using a 6 cm³ of a batch type reactor. Water density ranged from 0.1 to 0.52 g/cm³ (water pressure: 20–40 MPa). The loaded amount of oxygen was set to 0.3 of the ratio of oxygen atom to carbon atom. Some experiments were made using CO instead of oxygen for the partial oxidation of n-C₁₆ and PE to explore the effect of water gas shift reaction. In the results of partial oxidation of n-C₁₆, the yield of CO and some compounds containing oxygen atoms, such as aldehydes and ketones increased with increasing water density. Moreover, 1-alkene/ n-alkane ratio in the products decreased with increasing water density. The 1-alkene/n-alkane ratio was lower than that of pyrolysis in SCW. Also for the case of PE experiments, in dense SCW (0.42 g/cm³), the 1-alkene/n-alkane ratio in partial oxidation was lower than that in SCW pyrolysis. In the case of CO experiments for n-C₁₆ and PE, 1-alkene/n-alkane ratio was a little lower than that of pyrolysis in SCW. These results show that the yield of n-alkane, which is a hydrogenated compound, was higher through water gas shift reaction in SCW and also through partial oxidation in SCW. Therefore, these results suggest the possibility of hydrogenation of hydrocarbon through partial oxidation followed by the water gas shift reaction.     

Partial oxidation kinetics of m-hydroxybenzyl alcohol with noble metal catalysts in supercritical carbon dioxide

Partial oxidation of m-hydroxybenzyl alcohol was studied over several supported noble metal catalysts in a temperature range from 373 to 413 K, up to 2 MPa of oxygen pressure and 20 MPa of carbon dioxide pressure. The major product detected was m-hydroxybenzaldehyde. A charcoal supported palladium catalyst gave the highest yield of the aldehyde. For high temperature above 393 K and high oxygen pressure above 0.5 MPa, total oxidation was observed, which caused the selectivity of m-hydroxybenzaldehyde to decrease. Supercritical carbon dioxide medium seemed to improve heat dissipation of the reaction to allow the partial oxidation of m-hydroxybenzyl alcohol to occur under mild conditions. The partial oxidation of benzyl alcohol over a charcoal supported palladium catalyst was also examined for comparison and the major product formed was benzaldehyde. The conversion of benzyl alcohol and the selectivity to benzaldehyde was higher than those for the case of partial oxidation of m-hydroxybenzyl alcohol.     

Non-catalytic conversion of wheat straw,walnut shell and almond shell into hydrogen rich gas in supercritical water media

Agricultural wastes as lignocellulosic biomasses are known as the major resources of bioenergy. These valuable resources can be converted into useful environmental friendly fuels and chemicals. Wheat straw, walnut shell and almond shell are the main agricultural wastes in Kurdistan province, Iran. This study investigates the hydrogen-rich gas production via gasification of these biomasses in supercritical water media. Experiments were performed first, in the base case condition using a stainless steel batch micro reactor system. Then, the effect of reaction time on the total gas yield and yield of hydrogen, were investigated. It was seen that the total gas yields and gasification efficiencies increased by increasing the reaction time to 30 min and then the total gas yield was approximately remained constant. Among three used feed stocks, wheat straw with higher amount of cellulose and lower amount of lignin had the highest total gas and hydrogen yields in shorter reaction times. The maximum hydrogen yields of 7.25, 4.1 and 4.63 mmol per gram of wheat straw, almond shell and walnut shell occurred at 10, 15 and 20 min of reaction time, respectively.     

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。