SCWO process

Work on SCWO process development was performed in the early 1970s and in the 1980s, many of the developments were performed under private contract to industrial or federal agencies. The technology for waste treatment and generation of energy from waste or low grade materials was commercialized by Modar (Natick, Massachusetts). 

Other above-ground continuous flow systems have been designed and operated for SCWO processes. A system developed by ModeU Development Corp. (Modec) uses a tubular reactor and can be operated at temperatures above 500°C. It employs a pressure letdown system in which soHd, Hquids, and gases are separated prior to pressure release. This simplifies valve design and material selection on the Hquid leg. 

The most significant issue, the occurrence of periodic spikes in hydrocarbon and CO measurements in the gaseous effluent, will require further testing to identify the cause and provide a remedy for the problem (see Figure 4-3). The committee considers this problem to be very serious. Resolution must be obtained before the transpiring-wall SCWO process can be seriously considered... 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

Ascertain how well die SCWO process can handle high-solids materials (shredded dunnage). 

The transpiring-waU reactor has the potential to minimize many of the corrosion and deposition problems that have plagued previous SCWO studies. SRCE is developing a proprietary closed-cycle process that recirculates water for SCWO process at full system pressures. The design of the system was developed from previous work with gas turbine and rocket engines. 

In the SCWO process, water is subjected to temperatures and pressnres above its critical point (374.2°C, 22.1 MPa), where it exhibits properties that differ from both liquid water and steam. At the critical point, the liquid and vapor phases of water have the same density. When the critical point is exceeded, hydrogen bonding between water molecules essentially stops. Supercritical water sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. Some organic compounds that are normally insoluble in liquid water become completely soluble (miscible in all proportions) in supercritical water. Some water-soluble inorganic compounds, such as salts, become insoluble in supercritical water. 

These catalysts have been used in air wet-oxidation processes, where catalysts are essential to obtain high efficiencies [9]. In SCWO processes these efficiencies can be obtained by increasing temperature, and therefore they are not considered necessary. Catalysts studies have been performed on synthetic mixtures, and poisoning studies should be made when using wastewater. 

The main advantage of using catalysts in SCWO processes is to reduce the operating temperature. A complete review of catalytic oxidation in supercritical water can be found in reference [8]. 

Studies performed so far using Monel (nickel-copper alloys) have shown its low corrosion-resistance due to problems associated to selective leaching in the SCWO processes. Normally it is not to be considered in the materials selection. 

Owing to reduced salt solubility, the formation of metal oxides and, eventually, the presence of stable solid-matter particles, these are all present in the SCWO processes. These particles can cause equipment-fouling and erosion. However the reduced solubility of salts under supercritical conditions introduces the possibility of a solid fluid separation. 

M. J. Cocero, E. Alonso, D. Vallelado, R. Torio, F. Fdz-Polanco, Optimization of Operational Variables of a Supercritical Water Oxidation (SCWO) Process, Water Sci. Tech. 

Involved in extensive study of SCWO processes Investigated SCWO process for pulp mill sludges Explored kinetics of SCWO of phenol Explored supercritical water reactor Investigated the unique features of supercritical water in terms of density, dielectric constant, viscosity, diffusivity, electric conductance, and solvating ability Explored multistep kinetic model of phenol in SCWO Involved in extensive SCWO study of priority pollutants... 

The SCWO process is able to achieve destruction efficiencies for organic waste comparable with those attained by incineration technology, without the requirement of expensive dewatering equipment. The key to a successful SCWO process is a design that integrates various unit operations. Important design considerations include ... 

A flow chart of a generic SCWO process is shown in Figure 10.4. It illustrates the feed stream of a typical aqueous waste. Oxidants such as air, oxygen, or hydrogen peroxide must be provided unless the waste itself is an oxidant. A supplemental fuel source should also be available for low-heat-content wastes. The streams entering the SCWO reactor must be heated and pressurized to supercritical conditions. Influent streams are frequently heated by thermal contact with the hot effluent. Both influent pressure and back pressure must be provided. The influent streams are then combined under supercritical conditions where oxidation occurs. Certain properties of supercritical water make it an excellent medium for oxidation. Acetic acid is generally considered one of the most refractory by-products of the SCWO process of industrial waste. 

Table 10.5 provides performance data regarding the SCWO process. Typical destruction efficiencies (DEs) for a number of compounds are also summarized in Table 10.5, which indicates that the DE could be affected by various parameters such as temperature, pressure, reaction time, oxidant type, and feed concentration. Feed concentrations can slightly increase the DE in supercritical oxidation processes. For SCWO, the oxidation rates appear to be first order and zero order with respect to the reactant and oxygen concentration, respectively. Depending upon reaction conditions and reactants involved, the rate of oxidation varies considerably. Pressure is another factor that can affect the oxidation rate in supercritical water. At a given temperature, pressure variations directly affect the properties of water, and in turn change the reactant concentrations. Furthermore, the properties of water are strong functions of temperature and pressure near its critical point. 

At Sandia National Laboratories, experiments in an SCWO flow reactor provided data on a number of organics, including methanol, phenol, and other industrial chemicals, as well as military munitions (Rice, 1994). Commercial SCWO processes are designed to operate at temperatures typically less than 700°C. The development of SCWO technology depends on understanding the reaction kinetics of a wide variety of compounds at SCWO conditions. Predictive chemistry models, as they become available, will play an important role in finding answers to such design problems as ... 

Table 10.11 shows the different residual sludges and their characteristics after the SCWO process. Based on characteristics presented in Table 10.11, the chemical composition of sludge residue from SCWO can be disposed of at any sanitary landfill. 

Fig. 1 shows a graphical representation of these operational regions. WAO and SCWO processes are often referred to as hydrothermal oxidation technologies (HTOs). The major difference between the processes is that, in SCWO, organics are completely oxidized in a relatively short time (seconds to minutes), whereas in WAO, the reaction may require a longer time (minutes to hours). Furthermore, in WAO, some refractory organics are not completely oxidized because of the lower temperature of operation... 

C), thus requiring a secondary treatment process. As information on WAO technology is readily available from other sources [2-5], the SCWO process is mainly discussed here. 

The SCWO process involves bringing together an aqueous waste stream and oxygen in a heated pressurized reactor operating above the critical point of... 

Figure 2 A generic hydrothermal oxidation (WAO, SCWO) process flow diagram. 

The preheater section of the system mimics a miniature WAO system because the reaction conditions in the preheater are similar to those of a WAO system except that WAO systems need longer reaction times. In the heat-recovery mode of operation, the SCWO uses the heat from the reaction to preheat the influent. As a rule of thumb, if the aqueous waste stream contains about 4 wt.% of organics, the SCWO can be processed under self-sufficient heat conditions. However, for dilute aqueous waste streams, the SCWO process may not be cost-effective because of the additional thermal energy required to maintain the reactor temperature in the 400-650°C range. 

Identifying the products (both intermediates and final products) from the SCWO process is an essential prerequisite for evaluating the environmental impact of the technology. Additionally, identification of products is key to optimizing the process parameters to obtain the desired conversion for the destruction of the pollutant. The intermediate products and their composition depend on the temperature, water density (or pressure), oxidant concentration, concentrations of other additives, if present, reactor surface, and the extent of the conversion. 

In theory, the SCWO process can be operated under closed-system conditions with minimal exhaust release to the atmosphere. Therefore, during the laboratory-scale testing, post treatments are not required if the waste stream is treated under optimized conditions to completely oxidize the organic carbon to carbon dioxide. However, the effluent from the reactor should be treated under EPA guidelines for the waste (e.g., waste model compounds). 

Figure 10 A schematic of an SCWO processing plant for PCB disposal. (Courtesy of Mitsubishi Heavy Industries, Japan.)...

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