Supercritical Water-A Medium for Chemistry.
Shaw, R. W.; Thomas, B. B.; Antony, A. C.; Charles, A. E.; Franck, E. U.
孩子老是走神Chem.Eng. News 1991, Dec 23,26.
TOC = Total Organic Content
COD = Chemical Oxygen Demand
Punto critico dell’acqua: 374 °C and 22.1 MPa.
As the temperature of a liquid is incread, it becomes less den due to thermal expansion. Above 380 "C, the density of water decreas very rapidly up to 410 "C. Beyond 410 "C, the rate of decrea of density decreas.
桑叶茶有什么功效The dielectric constant of water decreas with decreasing density. Supercritical water behaves almost as an nonaqueous fluid dissolving nonpolar compounds like alkanes, aromatics, etc. In general, organics are completely miscible in water under supercritical condition, while inorganics like NaCl are practically insoluble. The residual solubility of NaCl in supercritical water is of the order of 100 mg/L. St
rong electrolytes behave as weak electrolytes in supercritical water. Oxygen is also completely miscible in all proportions in supercritical water [Pray et al. (1952) and Japas and Franck (1985)l.
The reduced effect of hydrogen bonding is responsible for the change in solubility properties of supercritical water. Hydrogen bonding is a weak force, and its effect decreas with a decrea in the density of water under supercritical conditions. Cochran et al. (19911, on the
basis of spectroscopic and computer simulation studies, found that the number of hydrogen bonds per water molecule under supercritical conditions is about one third the number at ambient conditions. At constant density, as the temperature is incread, the dielectric constant decreas due to the breakage of hydrogen bonds. Water is still associated, and the association strongly depends upon pressure and temperature under supercritical conditions. This association influences the solubility phenomena, the reaction chemistry, and the corrosion.
In ca of SCWO, the pressure may play a more important role since the density of supercritical water varies greatly with the pressure at a given temperature [Li et al. (1991)l. The pressure effect on the rate constant can be expresd as
where k = rate constant, unit dependent on the overall reaction order; ∆V = volume activation (cm3/mol); R = gas constant, 82.05 (atm*cm3)/(mol*K); T = temperature (K), and P = pressure(atm).
Supercritical water is highly corrosive particularly if halogenated compounds are prent. This necessitates the u of expensive special alloy reactors. One economic solution is to u a normal carbon steel reactor lined with corrosion-resistant alloy. The problem in that ca may be the detection of leaks.
can be solved by the u of a thin inrt of corrosion-resistant metal/alloy (such as titanium, zirconium, or Inconel) not fitting to the wall of the carbon steel pressure vesl. The space between the two is filled with a high-temperature heat transfer fluid. The inrt is designed so that it can expand toward the pressure vesl wall when pressurized. The heat transfer fluid balances the pressure, while its electrical properties can be monitored to indicate leaks.
Another problem faced is the precipitation of salts formed during the reaction resulting in vere plugging of the reactor system within a few minutes of operation.
稀土产业Various reactor designs/modifications have been reported to take care of the problems. Huang et al. (1992) describe a vertical cylindrical reactor with internal rotatable scraper blades for supercritical
water oxidation of wastewater.
The oxidized effluent is withdrawn from the top while aqueous brine slurry is discharged from the bottom. The scraper blades dislodge solid deposits from the bottom.
Huang et al. (1992) introduced the oxidant and wastewater into the upper region of the reactor to establish the downward flow of subcritical fluid through the supercritical zone along the inside surface of the vesl wall. The downward flow of subcritical fluid prevented
the deposition of the precipitate formed during oxidation.
Inorganic salts, insoluble under supercritical conditions, are dissolved in liquid water during the cooling of effluent mixture at the outlet of the reactor. Other insoluble matter can be parated by filtration.
The Modar process is quite similar to the wet air oxidation process. The organic waste (either an aqueous solution or a slurry) is pressurized to the reactor pressure and pumped in the reaction vesl. Oxygen is also pumped to the reactor.
When the waste contains a heteroatom which will produce mineral acid, caustic may be injected as a
part of the feed stream to neutralize the acid formed.
In order to ensure rapid completion of the oxidation reaction, part of the effluent from the reactor is mixed with the feed stream to rai its temperature to a sufficiently high value. The remaining part of the effluent is fed to a cyclone. The inorganic salts precipitate out and are parated. The effluent, still at very high temperature and pressure, is ud for energy recovery. The gas stream can be expanded through a turbine to extract the available energy as power. The Modar process has been reported to be energetically lf-sustaining at 2% organic concentration in the feed, and excess energy can be recovered in the form of steam at higher feed organic concentrations.
一日之计在于晨In general the destruction efficiencies of pollutants are of the order of 99-99.9% at 400-500 °C in 1-5 min residence time. Higher destruction efficiencies of pollutants (of 99.9999%) are achievable at 600-650 °C even with residence times of the order of conds. The oxidation end products are CO2 and simple acids (in the ca of halogenated organics), and the final effluent is so innocuous that it can be discharged without any further treatment. The gaous effluent is also clean. Carbon monoxide is at the most a few parts per million. Nitrogen-containing compounds are converted to N2 and N2O under supercritical water oxidation conditions. Nitrous oxide (N2O) can be eliminated by performing the oxidation at higher temperature . If the oxidation temperatures are lower (400-500 °C),
ammonia may form as intermediate which has been found to oxidize to N2at higher temperature ('600 °C).
For this purpo a number of WAO studies have been performed on the aqueous solutions of veral pure compounds (important model pollutants) and wastewaters containing toxic and hazardous compounds. Wet air oxidation of aqueous solutions of phenols and carboxylic acids has been studied in great detail with emphasis on the kinetics and mechanism of wet air oxidation.
The slow rate of oxidation of low molecular weight carboxylic acids is a major limitation of the WAO technique. In view of this the understanding of WAO of low molecular weight carboxylic acid achieves great significance.我的老师200字
The u of homogeneous and heterogeneous catalysts (particularly copper salts) has received a great attention.
It is important to u catalyst systems which are stable and do not get leached away in the solution by reacting with acids prent. This is the ca when CuO is ud as the catalyst.
Copper sulfate and copper nitrate have been ud as homogeneous catalysts for the oxidation of car
亭子怎么画boxylic acids. The heterogeneous catalysts ud include transition as well as noble metals. The various heterogeneous catalysts that have been ud are Cu, Pd, CoO/ZnO (6.5: 82.9, Cu:Mn:La oxides (4:2:1) supported on a spinel support (ZnO and Al203,48.5 and 51.5%, respectively),
乳链菌肽copper chromite, iron oxide, Co:Bi (51) complex oxides, RdCe, and Mn/Ce. Of the various catalysts studied, the multicomponent catalyst systems like Co:Bi, Cu:Co, Cu:Co:Bi, and Ru/Ce were considerably more active than other catalysts (except Mn/Ce catalyst). Co:Bi (51) was found to be the most active one [Imamura et al. (1982a,b 1988)l. Activity of Co:Bi catalyst was due to the prence of basic sites on the catalyst surface on which acetic acid is adsorbed. This is followed by a redox reaction between catalyst and adsorbed acetic acid to induce its decomposition.
The organics in the effluent from a WAO system can be divided into three groups [as by Li et al. (1991): all initial and relatively unstable intermediates except acetic acid (group A), refractory intermediates like acetic acid (group B) and oxidation end products (group C). A schematic pathway is given below:
It has been obrved by the authors that, in addition to acetic acid and other oxidation end products, significant quantities of solid carbonaceous materials are also formed during WAO of veral waste streams. This solid carbonaceous material undergoes oxidation in the same manner as the activated carbon during its wet air oxidative regeneration (ction 3.5). This aspect of formation and subquent oxidation of solid carbonaceous material during WAO of waste streams should be incorporated in the model of Li et al. (1991).
Oxidation of phenol by molecular oxygen has been propod to be an electrophilic reaction. The reaction between aryloxy radical with oxygen was considered to be the rate-limiting step. They obrved the following order of reactivity for the phenols studied:
Phenol and chlorophenols exhibited an induction period, the length of which depended on the oxygen partial pressure, followed by a fast reaction step. In the ca of methoxyphenols, the induction period was abnt. The methoxyl group favors the formation of aryloxy radical by increasing the electron density on the aryl ring. This results in incread oxidation rate, and
thus there is no induction period. Oxidation of alkyl group occurs much more readily compared to that of the ring resulting in the rapid formation of radicals. Due to this, oxidation of alkylphenols is ch
aracterized by an initial fast reaction period followed by a slow reaction period. Decomposition of cyanide ion occurs even in the abnce of oxygen as per the following equations CN- +2H2O→HCOO- +NH3at higher temperature (2.14)
or
NaCN +H2O → NaOH +HCN at <50 °C (2.15)
Thus under the conditions of high temperature prevailing during WAO, the destruction is mainly by hydrolysis. In the prence of oxygen and even in the prence of traces of Ni or iron oxides, NaCN is oxidized as
2NaCN +02→2NaCNO (2.16)
2NaCNO +1.5 02→Na2CO3 +N2 +CO2(2.17)
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Acrylic acid was found to oxidize directly to carbon dioxide and water.
They assumed the sludge to be consisting of (i) solid matter, X, (ii) soluble nonevaporative matter at 120 "C, Y; (iii) soluble evaporative matter at 120 "C, Z; and water. The weight and COD of X, Y, and Z are state variables and the model can be reprented as
no corrosion (almost generally) appears in the supercritical region the corrosion is most vere between 270 - 390 ºC
SCWO has been applied to a broad range of materials, e.g., aqueous waste streams, sludges, contaminated soils, industrial organic chemicals, plastics, synthetics, paints and allied products, industrial organics, agricultural chemicals, explosives, petroleum and coal products, and rubber and plastic products. It is applicable to the treatment of a range of contaminants
including acrylonitrile wastewater, cyanide wastewater, pesticide wastewater, PCBs, halogenated aliphatics and aromatics, aromatic hydrocarbons, MEK and organic nitrogen compounds [3]. Methods to destroy polychlorinated biphenyls (PCBs) are facing unusually challenging problems due to the very high chemical stability and low water solubility of the compounds (Hutzinger et al., 1983; Bolgar et al., 1995; D. O. Carpenter, 2000). For example, incineration of PCBs generates very harmful products such as polychlorinated dibenzofurans/dioxins (PCDFs/Ds) and becau PCBs themlves result from incineration of chlorinated pollutants and have been ud as fire retardant, this method is inappropriate for PCB destruction.
Supercritical water oxidation (SCWO) of organic compounds is drawing much attention due to attractive features such as cleanness, quickness, and the potential to effectively destroy a large variety of industrial and high-risk wastes.
Some stable products of PCB SCWO such as PCDDs/Fs, formed under certain conditions, are more hazardous than the starting material and their formation is a factor to consider in the design of SCWO reactors.
When recycling paper, de-inking sludge is received as a by-product. This sludge contains about 3% organic material, mainly fibres, and 3% inorganic material, mainly paper filler. The ability of SCWO to destroy the organic material in this sludge completely has been proven elwhere [2,3]. In addition to the destruction of organic matter, the industry has an interest in recycling the paper filler after the SCWO treatment.
wastewater which was produced in a process for amine manufacturing. The waste contains ammonia and short chain amines and gives a nitrogen rich water containing almost as much total nitrogen (Tot-N) as total organic carbon (TOC), about 15 000 to 20 000 mg/l.
Although SCWO has been demonstrated to be effective for the destruction of most organic compoun
ds, little success has been achieved in the complete destruction of ammonia or nitrogen in highly nitrogen containing wastes. However it is known that it is possible to destroy ammonia if the ratio of total organic carbon (TOC) and total nitrogen (Tot-N) in the wastewater is high [11,12]. If this ratio is low it is not possible to destroy all ammonia with oxygen but it is shown elwhere [13,14,15] that it is feasible to destroy the ammonia using nitric acid for the oxidation.
Effluent from the pulping mill generated after cooking of wood or other suitable raw material is termed as black liquor due to its color. It is highly organic in nature and contains organic matter in the form of suspended solids, colloids, BOD, COD, sulfur compounds, pulping chemicals ud, organic acids, chlorinated lignins, resin acids, phenolics, unsaturated fatty acids, terpenes, etc. Cyenides are prent in discharges from the electroplating industry, extraction of metals, coke furnaces, petroleum refineries, etc. Sodium cyanide is ud in manufacture of pharmaceutical, agrochemical, and dye intermediates. The effluents from the industries contain unreacted cyanides. Among nitriles, effluent from acrylonitrile manufacturing plant has attracted attention becau of its high toxicity due to the prence of acrylonitrile, acetonitrile, acrolein, inorganic cyanides, and ammonium sulfate along with large concentration of other organics.
Special Engineering Requirements of SCWO Processing Systems:
Unless catered for by careful engineering design, the high-temperature environment within SCWO reactors and processing systems can prent significant reliability and performance problems, as discusd below.
Experience has shown that corrosion rates can be rapid when treating wastes containing halogens, such as chlorine. Corrosion-resistant alloys such as Hastelloy C-276 and Inconel 625 do not provide adequate protection against chloride attack under the oxidizing conditions found in SCWO systems. In recent years, SCWO reactors have been built using liners fabricated from titanium alloys. The h
ave shown incread resistance to chloride attack. However, the reactors are limited to approximately 650 C maximum reaction temperature due to mechanical strength limitations of the pressure vesl wall.
The aqueous solubility of salt decreas sharply at supercritical pressures when the temperature ris above the critical temperature. If salts are prent in the waste feed, or formed during processing, they will precipitate from solution wherever local temperatures exceed the critical temperature. Other relatively insoluble solid compounds, such as carbonates and metal oxides, are also commonly formed during SCWO processing. Undissolved solids are often prent in the waste stream. Unless the solids are effectively transported through the supercritical regions or otherwi removed from the process, accumulations will form and total plugging of the reactor can occur. Furthermore, significantly higher corrosion rates have been obrved beneath deposited solids. It is esntial to control any tendency for solids to accumulate.
Wastes containing halogenated species and solids are practically a "fact-of-life" for SCWO waste treatment systems and most other waste treatment technologies. Traditional tubular or vat-type SCWO reactors have been incapable of addressing the problems in realistic plant applications.
Summit Rearch has developed a novel SCWO reactor designed to handle corrosive species and solids in a straight-forward manner utilizing proven engineering principles. (Transpiring-Wall SCWO Reactor)
In traditional SCWO processing systems the entire water effluent stream is depressurized and subquently dispod of or treated for re-u by the process at ambient pressure. We have developed a proprietary clod-cycle process that parates and recovers water for the process at full system pressure. (Clod-Cycle SCWO Process )
Traditional SCWO processing systems utilize shell & tube type heat exchangers which are prone to
plugging and corrosion. Our system utilizes an open quench-cooled heat exchanger where cooling is achieved by directly mixing cooled liquid effluent with the hot reactor byproducts. This ensures that solids are heavily diluted and flushed from the system. It is also a more compact heat exchanger for high latent heat load applications. The cooled liquid effluent can be treated with caustic and other additives to control corrosion, effluent pH, solids, and composition of all effluent streams. (Clod-Cycle SCWO Process)
