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Low Power Static-heating Start-up Procedure for Supercritical Water Oxidation Plants
Abstract
One of the factors against the upscaling of SCW plants is the enormous amount of energy demanded during the start-up phase to reach the stationary state and the autothermic operation phase.
This work proposes a new strategy with a lower energy cost and, consequently, with a smaller initial investment. It consists on statically heating up different sections in an SCWO plant by means of electric heaters distributed throughout the plant. A constant temperature of 400 °C can be reached in a plant containing a mixture of water and ethanol at near critical pressure. Then, when the feed and the oxidant are pumped, the oxidation reactions starts. Such reaction, in turn, releases the heat required for autothermal operation. The experiments that have been carried out have confirmed that the Low Power Static-heating Start-up Procedure (LPSSP) proposed would mean a saving of 46.4% in power consumption when compared to a conventional start-up procedure.
... Studies on energy utilization in SCWO system have been conducted by many research institutions such as Massachusetts Institute of Tech- Zhang et al. Biomass and Bioenergy xxx (xxxx) 106322 nology [75] and University of Houston [38] in USA, University of Cádiz [49,80,81,84,87,88] and University of Valladolid [39,61,68,70] in Spain, Vrije Universiteit Brussel in Belgium [69,73], ENEL GEM in Italy [76], Université de Pau et des Pays de l'Adourin France [78], Tohoku University in Japan [85], Xi'an University of Architecture and Technology [82], Nanjing Tech University [86], Dalian University [77], Shandong University [74], and Chinese Academy of Sciences [41,63,79,89] in China. Generally, major research methods on energy utilization can be classified into theoretical calculation, experimental study, and process simulation. ...
... Despite that, the experimental method is still of importance for the validation of a new concept of process. Benjumea et al. reported a new strategy with a lower energy cost for the start-up of SCWO system, which was evaluated by measuring the electricity demand of the system [87]. Benjumea et al. also proposed a double shell reactor (DSR) to generate a clean thermal fluid and measure detailed temperature profiles of the reactor at different conditions to estimate the potential for power generation [84]. ...
... Benjumea et al. proposed a static-heating start-up strategy to reduce power consumption in preheating [87] (Fig. 10). In their system, electric heating elements are distributed evenly in the heaters, HEs, and reactor to provide sufficient heat transfer areas. ...
Supercritical water oxidation (SCWO) is an effective, promising technology for destroying almost all kinds of organic waste with the advantage of high removal efficiency and low residence time, and without producing pollutant byproduct. The chemical energy of organic pollutants can be considered as an emerging renewable energy source. As a high-temperature and pressure process, an immense energy demand is required in the pressurization and preheating sections. On the other side, the reactor effluent has a great potential for energy recovery because of the great amount of heat released during the SCWO reaction combined with the previous energy input. Thus, SCWO is not only an organic waste treatment process but also an energy conversion and utilization system. The application of SCWO has been extended to the clean, efficient utilization of low-quality fuels such as high-sulfur coal, heavy oils, and biomass instead of conventional combustion, hydrothermal spallation drilling in geothermal energy production, and heavy oil recovery. However, energy consumption cost is still rather high in the actual process due to insufficient equipment performance, unreasonable energy system integration, and selection of an anti-corrosion and salt plugging reactor. Thus, a systematic, in-depth analysis on energy conversion and utilization in SCWO is conducted for the first time in the paper. The mechanism of exergy destruction analyses indicates that reducing enthalpy change and energy level difference are critical to reducing energy consumption. Suggestions on improving energy efficiency are proposed in the preheating stage, chemical exergy release, and reactor design.
... Since the SCWO concept was proposed by Modell in the 1980s, many aspects related to this technology have been studied, including reaction kinetics [22][23][24], corrosion behavior of construction materials [25,26], salt precipitation problem [27], reactor design concepts [28], system design [29] and low-cost start-up strategies [30]. These studies mainly focus on reaction mechanisms of different kinds of organic matters, dissolution and precipitation behaviors of inorganic salts, and corrosion mechanisms of alloys in SCW. ...
Supercritical water oxidation is a promising technology for decomposition of industrial wastewater and sludge. However, the system is operated under high temperature and pressure (usually higher than 500 °C and 25 MPa). Corrosion of component materials and salt deposition may lead to leakage or even the burst of the pressure vessel in the SCWO system, resulting in a high level of worry about the safe operation of the system. In this paper, the safety management and accident- control strategies are introduced according to a commercial-scale SCWO plant in China. The safety management strategy refers to the special design and operation strategies of some facilities. Different types of potential accidents are analyzed and the coping strategies for accidents of different levels of severity are described in detail. The strategies are valuable for the safe operation of commercial SCWO plants and other plants operated in high temperature and high pressure.
... ENN Envirotech Co., Ltd built the maximum processing scale 240 td À1 SCWO plant among The degradation mechanism of the ion exchange resin in the stirred and non-stirred reactor was studied and the oxidation reaction was simulated . University of C adiz 2008-2017 Developing new systems for degrading insoluble organic materials or high concentrations of wastewater (Vadillo et al. 2012); The oxidation kinetics of isopropanol as an auxiliary fuel was studied (Abelleira et al. 2013); Multioxidant injection system, low power static-heating startup procedure, treating nitrogen-containing compounds and a detailed numerical simulation for reactor (Benjumea et al. 2018;Benjumea et al. 2016;Benjumea et al. 2017); Kinetic model of oxygen concentration and simulation of process parameters control in the process for treating wastewater (S anchez-Oneto et al; Vadillo et al. 2011;Xu et al. 2015). Paul Scherrer Institute 2010-2016 Investigating the separation characteristics of inorganic salts, and developing a new type of countercurrent salt separator was designed (Reimer et al. 2016;Schubert et al. 2012;Schubert et al. 2010aSchubert et al. , 2010b. ...
Supercritical water oxidation (SCWO) is a promising technology for treating degradation-resistant organic waste. In this paper, the basic principles and technical advantages of supercritical water oxidation technology are introduced, and the research on and technical characteristics of commercial enterprises are summarized. The typical SCWO process is composed of pretreatment, pressurization, preheating, reaction, cooling, depressurization, gas-liquid-solid separation and oxidant modules. Various SCWO process schemes should be developed to accommodate site conditions, avoid corrosion, prevent salt deposition, reduce building investment and minimize operational expenses. The system could be classified in the form of heat recovery, feed pretreatment, salt separation and reactor. The application range of various process technologies was clarified, and the current research on SCWO processes was summarized.
... However, it is more suitable in tank reactor rather than tube reactor because that the heat generated by combustion could be dispersed in the long tube and have little effect on heating up the feed. Benjumea et al. (2018) proposed a new start-up strategy named (Low Power Static-heating Start-up Process, LPSSP). During their start-up process, a static mixture of water and ethanol was firstly pressured and heated to 400°C in both the heater and the reactor. ...
The decompositon of ethylenediamine (EDA) in supercritical water oxidation (SCWO) were investigated. The experiments were conducted in an isothermal and vertical continuous plug-flow reactor at 375-500°C, 25 MPa, residence time of 2-80 s, and excess oxygen of 150%. It was found that yields of nitrous oxide, nitrogen, and the denitrification rate increased as temperature and residence time increased. The kinetics for denitrification of EDA was described by a rate law involving first order reaction. The results calculated from kinetics equation reconciled the experimental data.
Oxidation of the textiles dyeing wastewater was studied in a continuous-flow reactor that was operated between 400 and 650°C at a fixed pressure of 25 MPa. The total organic carbon (TOC) concentration of the raw wastewater was in the 112.52 mmol/L . Hydrogen peroxide (H2O2) was used as an oxygen source. According to the wastewater and oxidant concentrations, the global rate expression was regressed from the complete set of data under various conditions of the reaction. The reaction rate expression for the oxidation of the textiles dyeing wastewater was determined with the activation energy of 39.908 (±1.99) kJ/mol and the pre-exponential factor of 114 (±7.5) /s; and the reaction orders for the TOC and the oxidant were 0.896±0.01 and 0.061±0.01 at a 95% confidence level.
The corrosion of reactor and salt precipitation are two critical obstacles that inhibiting the development of supercritical water oxidation (SCWO) in to a viable industrial process. A new design of double wall reactor has been developed in which SCWO reaction take place inside an inner tube (alumina or titanium, micro porous). The high pressure air layer between inner surface of reactor and surface of tube could prevent non-soluble materials, corrosion ingredients and inorganic salt depositing and attaching to inner surface, thus critical problems have been overcome. The new reactor performance was evaluated by processing sewage sludge from BeiBei waste water treatment plant. There were no salt plugging or corrosion observed in reactor after one month operation.
Supercritical Water Oxidation (SCWO) processes have been studied by numerous researchers. The effectiveness of this approach to treat a wide variety of wastes has been proved and the kinetics involved in some cases have been described. Phenol is commonly present in industrial wastewaters and it is extremely toxic. Hence, phenol is a model pollutant that has been the subject of numerous studies by SCWO on a laboratory scale. In this work, a pilot-scale SCWO system has been used to compare experimental and predicted conversions in the SCWO of phenol, using the reaction kinetic equations obtained at the laboratory scale. In this context, "PROSIM PLUS" software was employed to develop a simulator for the pilot plant facility, with the reaction kinetic parameters adjusted to represent the experimental data. In this study it was necessary to determine the thermal losses between the experimental reactor and its surroundings. These thermal losses were obtained from tests with pure water and oxidant streams in the absence of chemical reaction. An equation that predicted the effect of flow rate and temperature on the thermal losses was used. Experimental oxidation tests were conducted with initial temperature in the range 380 to 425-° C, at 250 bar and phenol concentrations ranging from 1 to 12 g/l. Good agreement in the simulation was obtained by adjusting the kinetic parameters within their confidence range. This simulator was used to optimize the SCWO of phenol solutions in the pilot plant facility.
Although a great deal of research has been devoted to PCBs destruction by supercritical water oxidation, PCB-contaminated oils, which represent a high percentage of existing PCB wastes in South American countries, have been barely studied. In this paper we present experimental results on the supercritical water oxidation of a real-world mineral transformer oil highly contaminated with PCBs. Process conditions of 539°C, 350% oxygen excess and 241bar allow one to obtain a 99.6% organic matter destruction of the complex mixture of hydrocarbons and PCBs. The PCB concentration in the aqueous effluent was below the detection limit of the chromatographic method used in the analysis, and a toxicity test for Daphnia pulex showed that the aqueous effluent of the process is not ecotoxic. Based on these results, a process flowsheet of a supercritical water oxidation mobile pilot plant was proposed as an alternative to incineration. A preliminary economic assessment of the proposed flowsheet shows that the process is feasible when compared to treatment by incineration.
Supercritical Water Oxidation (SCWO) is a high temperature and pressure process whose operating conditions are beyond the critical point of pure water (Tc = 374 °C and Pc = 221 bar), and is a powerful technology to treat wastewaters in a clean and effective manner. Moreover, the heat released during the oxidation reaction allows the recovery of energy and improves the efficiency of the process. In those cases where the reaction of high concentrated wastewaters generates a high temperature profile in a thermally insulating tubular reactor, a temperature control system is necessary to keep the operation within safety limits. With the aim of controlling the temperature along the reactor, both cooling water and oxidant split injectors have been installed in a 25 kg/h pilot plant. Experimental tests have been carried out to compare the operation of the pilot plant before and after those two improvements. Intermittent water injections provide a quick and located cooling. Each injection reduces the temperature inside the reactor by around 20–40 °C, and the frequency between injections depends on the exothermicity of reactions. When oxidant split is added in two injections, air in defect for the first injection (n ≈0.5) and in excess for the second one (n ≈1.2), the peak of temperature can also be smoothened and the development of the reaction can be controlled. In both cases, the temperature profile along the reactor is effectively controlled and the reaction remains within safe limits that allow a stable and reliable treatment of highly concentrated wastewaters.
Supercritical Water Oxidation (SCWO) is a powerful process for the treatment of organic wastewater. It has already been studied for decades and, nowadays there are several demonstration and industrial plants under construction worldwide. Simulation tools play an important role, both to predict and optimize the operating conditions during the process and to evaluate its economic viability. In the case of SCWO, transitory state models are sparse in the literature. In order to exhaustively know the behavior of the SCWO process during time-dependent applications, a transitory tubular reactor model has been built. That model has been developed by the finite difference method in two dimensions. It takes into account the reaction of wastewater along the reactor and the heat loss from the fluid to the atmosphere in a radial way, both along the pipe and through the isolation material.
Supercritical water oxidation (SCWO) is considered to be a green technology, providing an effective route to convert waste materials into simpler, less hazardous products. This article reports the use of a physical modelling approach to assess mixing dynamics inside three different types of reactor where supercritical water (water above 374 °C and 218 atmospheres) is mixed with a second colder, waste containing, effluent flow. Physical or ‘pseudo’ modelling was used to simulate the general flow patterns and mixing regimes in transparent pseudo reactors (to allow visualization). Towns water was used to simulate the supercritical water flow (density 998 kg/m3 and viscosity 1.0 × kg/m.s at 25 °C and 1 bar respectively) and 40%w/w aqueous sucrose solution to simulate the cold aqueous effluent flow (density 1176 kg/m3, viscosity 6.16 × kg/m.s at 25 °C and 1 bar respectively). Flow rates of 100’s ml.min−1 to 1000’s ml.min−1 were used to create a range of Reynolds numbers experienced during mixing at supercritical conditions (in laminar and turbulent regimes). Three types of vertical pipe-in-pipe reactor were simulated using this method (counter current and co-current arrangements). This visual technique allowed the quantification of mixing efficiency, as well as identification of issues such as flow recycling, stagnant zones, and other inconsistencies in the mixing dynamics. An upwards co-current arrangement provided the ‘best’ mixing i.e. with minimal wall contact during the downstream oxidation process.
Supercritical water oxidation (SCWO) has been studied for the past three decades and is now a well-known process. However, the commercial development of this technique is currently delayed due to several drawbacks such as corrosion, salt precipitation, and high costs. In an effort to overcome these constraints several authors have studied and designed new SCWO reactor concepts, but these technical solutions involve the use of special materials and complex designs that increase the process
costs. However, conventional SCWO could be commercialized for certain wastewaters that satisfy certain requirements, e.g., very low salt and chloride contents, and it is necessary to continue studying the SCWO process at high concentrations and on the pilot plant scale. At present, simulations based on the SCWO of real wastewaters at high concentrations on the pilot plant scale are scarce in the literature. Process simulation is a powerful tool to study processes in depth and to make advances in the scale-up process.
Nevertheless, the use of specific chemical engineering software, such as Prosim Plus or Aspen Plus, is not applicable to simulate a complex wastewater. In addition, the use of the kinetic data available in the literature is not straightforward because these kinetic parameters were obtained from experiments conducted under very different conditions (isothermal, low concentration, etc.). In the work described here, the simulation of Biocut 35 cutting fluid SCWO on a pilot plant scale has been conducted satisfactorily. The model was developed using a Microsoft Excel spreadsheet and a kinetic model obtained on the laboratory scale. Fifteen experiments were carried out in order to validate the simulator. These experiments were conducted on a pilot plant scale at a constant pressure of 250 bar and initial temperatures ranging from 388 to 428 �C. The cutting fluid concentration used in these experiments was varied from 19 to 95 g of O2/L. Finally, the simulator was used to check the effect of the operational variables such as wastewater concentration, initial temperature, wastewater flow rate, and thermal insulation.
In order to design and define appropriate dimensions for a supercritical oxidation reactor, a comparative 2D and 3D simulation of the fluid dynamics and heat transfer during an oxidation process has been performed. The solver used is a commercial code, Fluent 6.2®. The turbulent flow field in the reactor, created by the stirrer, is taken into account with a k–ω model and a swirl imposed to the fluid. In the 3D case the rotation of the stirrer can be modelled using the sliding mesh model and the moving reference frame model. This work allows comparing 2D and 3D velocity and heat transfer calculations. The predicted values (mainly species concentrations and temperature profiles) are of the same order in both cases. The reactivity of the system is taken into account with a classical Eddy Dissipation Concept combustion model. Comparisons with experimental temperature measurements validate the ability of the CFD modelling to simulate the supercritical water oxidation reactive medium. Results indicate that the flow can be considered as plug flow-like and that heat transfer is strongly enhanced by the stirring.
After more than three decades since its potential was first recognized, supercritical water oxidation (SCWO) remains an innovative and viable treatment technology for destruction of aqueous based organic wastes. An extensive data base of destruction efficiencies, corrosion data, and salt phase behavior has been developed over the years through the combined efforts of many investigators at both the fundamental research and commercial level. As a result, SCWO technology has been increasingly utilized in a variety of full-scale designs and applications, handling feeds as diverse as polychlorinated biphenyls (PCBs), sewage sludge, spent catalysts, and chemical weapons. This paper reviews the status of current full-scale commercial SCWO facilities around the world, focusing on the unique challenges and design strategies employed by different companies for corrosion and salt precipitation control in each application. A summary of past commercial SCWO activity as well as future plans among the current active SCWO companies is also included.
Oxidation of oily sludge in supercritical water was studied in batch reactor under the conditions of reaction temperature from 390 to 450°C, pressure up to 25MPa, and time from 1 to 10min. The oily sludge oxidation undergoes a parallel-consecutive reaction pathways, in which it first decomposes to intermediates of aliphatic ketones, aldehydes and carboxylic acids with conjugated double bonds and via low molecular mass organic acids to the final product carbon dioxide. A 4-lump kinetic model was proposed to describe supercritical water oxidation of oily sludge. The experimental data obtained were used to estimate the six kinetic constants and the corresponding activation energies in the model. The model testing results revealed that the model predictions were in good agreement with the experimental results. The model helps us get good insight into the performance of the batch reactor that would be useful for optimization of supercritical water oxidation of oily sludge.
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).
Binary Ni-Cr alloys (0 to 45 mass% Cr) were exposed to an aqueous solution resulting from the oxidation of dichloromethane (CH2Cl2) in the presence of hydrogen peroxide (H2O2) at 40 MPa and temperatures between ∼100°C and 415°C. The composition of the reaction medium after complete oxidation of the chlorinated hydrocarbon was 0.12 m hydrochloric acid (HCl), 0.06 m oxygen (O2), and 0.06 m carbon dioxide (CO2). Of the alloys investigated, NiCr25 performed best in experiments for 120 h under these conditions, Aspects of the complex corrosion kinetics for binary Ni-Cr alloys are compared with the predictions of a mathematical model. Results from other investigations that cover temperatures up to 500°C at a comparable pressure are reviewed in light of the new findings.
In this paper, an energetic study of the supercritical water oxidation (SCWO) process for diluted wastewater is presented. An empirical correlation for the calculation of reaction heat is proposed, in terms of C, H and O content in the waste, in order to determine the energy released in the oxidation reaction and to propose a suitable energy integration of the plant. The energy study of the process has been performed using the chemical plant simulation software aspen plus. This simulation has been carried out for a plant with a treatment capacity of 2 m3/h of diluted wastewater. This study has revealed that the minimum heating value required in the feedstream for energy self-sufficient operation is 930 kJ/kg. This value is equivalent, for instance, to a water stream containing 2% w/w n-hexane, 2.4% w/w 1-hexanol or 3.2% w/w hexanoic acid, depending on the oxidation degree of the compound. Operational variables used in the simulation, such as reaction temperature, pressure and oxidant excess, have been fixed in terms of experimental results obtained in the pilot plant of the Chemical Engineering Department at the University of Valladolid (Spain). From these results, removal efficiencies greater of 99.95% are reached for reaction temperature above 650°C, P=23.0 MPa, residence time lower than 50 s and oxygen excess slightly greater than the stoichiometric value.
The destruction of industrial wastewaters by supercritical water oxidation (SCWO) has been studied intensively in the last two decades due to the powerful and promising advantages of this technology. However, the SCWO process is not yet commercially established due to several drawbacks that limit its application as a general treatment, process costs being one of those limitations. In an effort to enhance the viability of SCWO as a commercial process, a study was performed in a pilot plant (25 kg/h) used to treat industrial oily wastes by SCWO, and a simulation was carried out to evaluate the viability of energy production on an industrial scale. The SCWO pilot plant effluent is good for producing hot water or steam by recovering heat of waste organics. Both alternatives are evaluated for a SCWO industrial plant design with 1000 kg/h, with it being possible to recover a maximum of 118 kW, that is, 71% of the energy content of the wastewater.
BACKGROUND: Supercritical water oxidation (SCWO) is a promising technology that respects the environment, destroys wastes and allows energy recovery. This process has been applied to many model compounds and real wastewaters at laboratory scale. However, SCWO treatments at pilot plant scale of real wastewaters are scarce. The application of this technology to industrial wastewaters has drawbacks such as corrosion, salt deposition and high cost, so industrial scale‐up has been delayed.
RESULTS: In a first stage, for safety reasons the feasibility of SCWO applied to flammable industrial wastewaters was evaluated at laboratory scale in an isothermal plug flow reactor with low concentrations (3–10 g COD L ⁻¹ ), at a constant pressure of 250 bar and at different temperatures in the range 350–500 °C. In a second stage, experiments were conducted with much higher concentrations (20–90 g COD L ⁻¹ ) in a SCWO reactor at pilot plant scale. Experiments at pilot plant scale demonstrated the possibility of working under autothermal conditions and the results were used to estimate the treatment costs for a SCWO plant with a capacity of 1 m ³ h ⁻¹ .
CONCLUSION: Results demonstrated the technical feasibility of using a SCWO process to treat flammable industrial wastewater at pilot plant scale due to the absence of operational drawbacks related to the flammability of this wastewater, such as plugging, pressurization or preheating problems and uncontrolled reactions (explosion, etc.). The economic feasibility was demonstrated, especially bearing in mind the energy recovery optimization. Copyright © 2011 Society of Chemical Industry
The oxide scales of 316 stainless steel (316 SS) have been examined after exposure to supercritical water (SCW) with 2.0% H2O2 for up to 250 h. The exposed samples were analyzed using weight measurement, scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). It was found that mass gain of all samples increased with increasing temperature and exposure time. Higher temperature SCW resulted in rougher surfaces and thicker oxide scales. Duplex layer oxide structures with Ni-enrichment at the oxide/metal interface developed on all samples exposed to SCW, which were identified as Fe2O3/Fe3O4 + spinel/Cr2O3/Ni-enrichment/316 SS from the outer to inner layer. The possible oxidation mechanisms are also discussed.
Supercritical water oxidation (SCWO) is an effective technology for treatment of organics and organic components of aqueous wastes. Commercialization of SCWO processes has been hindered by concerns about corrosion and scale buildup/fouling which, when present, must be accommodated by system design and/or operational procedures. Salts are formed during SCWO when acidic solutions are neutralized to reduce corrosion and may also be present in the waste stream itself. Because salts have low solubility in supercritical water (SCW), they precipitate. Precipitated salts often form agglomerates and coat internal surfaces, thereby inhibiting heat transfer from/to exterior surfaces. When scale buildup is left uncontrolled, plugging of transport lines and/or the reactor can occur. The required cleaning can result in substantial and costly downtime in the SCWO process. General principles and research relevant to SCWO have been reviewed elsewhere. A review of the many technologies available to control scale during SCWO is given in the companion paper by Marrone et al. [J. Supercrit. Fluids (in press)]. Presented here is a review of fundamental principles and research pertinent to the precipitation of salts and scale control at the elevated temperatures and pressures found in an SCWO reactor. First, SCWO is introduced and the physics leading to scale buildup during SCWO is discussed. Next, the phase diagrams of model salt–water systems at relevant conditions are presented. Then, the many phenomena which complicate modeling of heat transfer in SCW (buoyancy, rapidly varying thermophysical properties, etc.) are reviewed and a set of correlations to calculate heat transfer coefficients is provided. Finally, the limited number of controlled experimental studies on scale buildup during SCWO are reviewed.
Despite the potential of supercritical water oxidation (SCWO) as a viable technology for organic waste destruction, its commercial development has been hindered by the problems of corrosion and salt precipitation/solids buildup. The extremely low solubility of polar inorganic salts in the supercritical water environment causes salts present in the feed, or formed during reaction, to precipitate inside the reactor. If left unchecked, these salts can rapidly accumulate on reactor walls or process surfaces and form plugs, causing expensive and frequent downtime of the SCWO system. Other solids such as oxides exhibit low solubility in water over the range from ambient to supercritical conditions and, although they have much less tendency to adhere to process surfaces, may still hinder operations if not accommodated. Many wastes will have a combination of salt-type and oxide-type solids, and may have an intermediate tendency to stick to process surfaces. Many of the companies that have attempted to commercialize the SCWO technology over the past two decades have developed innovative approaches to dealing with the corrosion and salt precipitation/solids buildup problems. These are often the distinguishing features of each company's SCWO process. This paper objectively reviews several commercial approaches that have been developed and/or used to control salt precipitation and solids buildup in SCWO systems. The approaches reviewed consist of specific reactor designs and operating techniques, and include the following: reverse flow tank reactor with brine pool, transpiring wall reactor, adsorption/reaction on a fluidized solid phase, reverse flow tubular reactor, centrifuge reactor, high velocity flow, mechanical brushing, rotating scraper, reactor flushing, additives, low turbulence/homogeneous precipitation, crossflow filtration, density separation, and extreme pressure operation. Recent commercial SCWO applications utilizing these approaches are also discussed. A companion paper by Hodes et al. (J. Supercrit. Fluid., see this volume) reviews fundamental principles and research pertinent to scale control in SCWO processes.
One of the obstacles that is inhibiting the development of supercritical water oxidation (SCWO) into a viable industrial process, is the problem of corrosion. A bench scale stainless steel flow reactor for supercritical water oxidation studies was constructed. Corrosion of the reactor was studied under pressure of 400 bars and at temperatures of 250, 375 and 420 °C. The concentrations of various metals in the effluent were monitored by flame atomic absorption spectrometry. Higher corrosion rates were observed at 375 °C, or near the critical temperature. Addition of hydrogen peroxide also significantly increases the corrosion of stainless steel. Exposure of the reactor to open air between experiments is also found to be a contributing factor to the corrosion of SCWO experiment.
Supercritical water oxidation (SCWO) oxidizes organic and biological materials virtually completely to benign products without the need for stack gas scrubbing. Heavy metals are recovered as stabilized solid, along with the sand and clay that is present in the feed. The technology has been under development for twenty years. The major obstacle to commercialization has been developing reactors that are not clogged by inorganic solid deposits. That problem has been solved by using tubular reactors with fluid velocities that are high enough to keep solids in suspension. Recently, system designs have been created that reduce the cost of processing sewage sludges below that of incineration. At 10 wt-% dry solids, sludge can be oxidized with virtually complete recovery of the sludge heating value as hot water or high-pressure steam. Liquid carbon dioxide of high purity can be recovered from the gaseous effluent and excess oxygen can be recovered for recycle. The net effect is to reduce the stack to a harmless vent with minimal flow rate of a clean gas. Complete simulations have been developed using physical property models that accurately simulate the thermodynamic properties of sub-and supercritical water in mixtures with O 2 , N 2 , CO 2 , and organics. Capital and operating cost estimates are given for sewage sludge treatment, which are less costly than incineration. The scenario of direct recovery of energy from sludges has inherent benefits compared to other gasification or liquefaction options.
Final disposal of sludge continues to be one of the more pressing problems for the wastewater treatment industry. Present regulations for municipal sludge have favored beneficial use, primarily in land application. However, several agencies and entities have warned of potential health risks associated with these methods. Hydrothermal oxidation provides an alternative method that addresses the health concerns associated with sludge disposal by completely converting all organic matter in the sludge to carbon dioxide, water, and other innocuous materials. A hydrothermal oxidation system using HydroProcessing, L.L.C.'s HydroSolids process has been installed at Harlingen, Texas to process up to 9.8 dry tons per day of sludge. Based on a literature review, this system is the largest hydrothermal oxidation system in the world, and the only one built specifically to process a sludge. Start up of Unit 1 of two units of the HTO system began in April 2001. Early results have indicated COD conversion rates in excess of 99.9%. Harlingen Waterworks System estimates that the HydroSolids system will cost less than other alternatives such as autothermal thermophilic aerobic digestion and more traditional forms of digestion that still require dewatering and final disposal. The Waterworks intends to generate income from the sale of energy in the form of hot water and the use of carbon dioxide from the HydroSolids process for neutralization of high pH industrial effluent. The Waterworks also expects to generate income from the treatment of septage and grease trap wastes.
Salt precipitation and scale control in supercritical water oxidation-part A: fundamentals and research
- M Hodes
- P A Marrone
- G T Hong
- K A Smith
- J W Tester
M. Hodes, P.A. Marrone, G.T. Hong, K.A. Smith, J.W. Tester, Salt precipitation and
scale control in supercritical water oxidation-part A: fundamentals and research,
J. Supercrit. Fluids 29 (2004) 265-288, http://dx.doi.org/10.1016/S08968446(03)00093-7.
- M Benjumea
M. Benjumea et al.
The Journal of Supercritical Fluids 135 (2018) 218-224
- Q Wang
- Y Lv
- R Zhang
- J Bi
Q. Wang, Y. Lv, R. Zhang, J. Bi, Supercritical water oxidation kinetics of textiles
dyeing wastewater, Adv. Mater. Res. 634-638 (2013) 302-306.
Supercritical water oxidation of sewege sludge-an update
- J O'regan
- S Preston
- A Dunne
J. O'Regan, S. Preston, A. Dunne, Supercritical water oxidation of sewege sludge-an update, 13th Eur. Biosolids Org. Resour. Conf. Work, Manchester, UK, 2008.