xiong Li

Efficiency of H2O2 and O2 in supercritical water oxidation of 2,4-dichlorophenol and acetic acid

Abstract The efficiencies of hydrogen peroxide and oxygen as oxidants for the destruction of acetic acid and 2,4-dichlorophenol were compared under supercritical water conditions using a batch reactor system. The effects of temperature, water density, and oxidant concentration on the destruction of both compounds were studied. Diluted oxygen (21% O2 and 79% N2) and premixed hydrogen peroxide (32 % aqueous solution) were used as the oxidants. The reaction temperatures were 400, 450, and 500°C. Water densities were fixed at 0.15 g/mL and 0.35 g/mL. The destruction efficiency of hydrogen peroxide was significantly higher than that of oxygen. For acetic acid, the highest conversion of 97.7% was achieved with hydrogen peroxide at 500°C for a reaction time of 10 min. while 64.3% conversion was achieved with oxygen at the same temperature but for a reaction time of 30 min. For 2,4-dichlorophenol, conversions higher than 99.995% were obtained with hydrogen peroxide at 450°C for a reaction time of 2 min as compared to 87.6% conversion with oxygen at 500°C for the same reaction time. The change in supercritical water density from 0.15 g/mL to 0.35 g/mL at 400°C only slightly increased the conversion of both compounds. The conversion of acetic acid was enhanced by 10% at 450°C as hydrogen peroxide supply was increased from 100 to 200% of stoichiometric demand. However, concentration of hydrogen peroxide higher than 300% of stoichiometric demand did not yield higher conversions of acetic acid. The effect of excess oxygen was not conclusive.

Supercritical water oxidation: An engineering update

Abstract This paper reviews the need for innovative treatment technologies and describes a wastewater treatment system capable of completely destroying toxic organic substances and biological sludges. The basic concept of supercritical water oxidation (SCWO), associated engineering research, and technology development are examined. During the last few years a growing body of SCWO knowledge has been assembled. A number of universities, federal agencies, and private companies have participated in both engineering research and technology development. In general, engineering research has focused upon reaction kinetics and mechanisms, salt formation and solubility, mass and heat transfer, transformation product identification, catalysis, corrosion, and additive impacts. As research progressed, technology development has directed its attention to materials of construction, reactor design, heat exchange and recuperative heat recovery, solid-liquid separation, gas-liquid separation, control systems, emuent handling, ash disposal, safety requirements, and process integration. SCWO technology relies on the unique properties of supercritical water to create an excellent reaction medium. The system is capable of operating as a totally enclosed facility, providing complete destruction, and meeting all regulatory emuent requirements. As such, this new technology is an environmentally attractive option. Because of favorable treatability, safety, and economic considerations a growing number private and governmental entities are currently involved in the development of more effective SCWO facilities.

Supercritical water oxidation of acetic acid by potassium permanganate

Abstract Supercritical water oxidation (SCWO) of acetic acid by potassium permanganate (KMnO 4 ) was studied. The experiments were performed in a batch reactor at temperatures and pressures, respectively, ranging from 400 °C to 460 °C and from 275 bar to 350 bar. For comparison purposes, other oxidants, such as oxygen, hydrogen peroxide, CuH 2 O 2 , FeH 2 O 2 and MnSO 4 H 2 O 2 , were studied. Subcritical tests with potassium permanganate were also conducted at temperatures of 250°C, 300°C and 350°C. The order of acetic acid destruction effectiveness was found to be KMnO 4 > Cu-H 2 O 2 > FeH 2 O 2 > MnSO 4 H 2 O 2 > H 2 O 2 > O 2 at a temperature of 400°C, density of 0.3 g/ml and reaction time of less than 10 min. The acetic acid destruction efficiency by potassium permanganate was 77% at a temperature of 400°C, density of 0.3 g/ml and a reaction time of 2.5 min. Under similar conditions, only 40% of acetic acid was destroyed at 250°C. Potassium permanganate was demonstrated to be an effective oxidant for SCWO of acetic acid.

Separation of metal oxides from supercritical water by crossflow microfiltration

Low-shear crossflow microfilters rated for 34.5 MPa and 455°C were evaluated for their ability to remove α-alumina particles from supercritical water. The crossflow microfilters were tested in conjunction with a 2.5 L/h bench-scale apparatus and a 150 L/h pilot plant. The filter elements were made of sintered Stainless Steel 316L tubes with a nominal pore size of 0.5 μm. Process variables included volumetric feed concentration, feed flow rate, temperature, and fluid density and viscosity. The mean particle diameter was about 1.6 μm. Filtration characteristics were similar at supercritical water conditions to those at ambient conditions: Increased shear rates and decreased fluid viscosity resulted in increased filtrate fluxes. Filtrate flow deterioration over time and the required transfilter pressure drop were about 40% less at supercritical water conditions as compared to the performance obtainable at ambient conditions. Higher shear rates delayed the establishment of steady state operating conditions. Filtrate flux could be augmented by pressure swings or periodic increases in the shear rate. Particle separation efficiencies typically exceeded 99.9%. A modified concentration polarization model for turbulent flow yielded steady state filtrate fluxes that were within a factor of two of the experimental results. Back diffusion was modeled as a process-in-series of molecular diffusion and eddy diffusion. The proposed model was consistent with findings of numerical diffusion studies and the theory of concentration polarization as recently presented in the literature. To save on energy costs it might be possible to achieve the benefits of filtration at supercritical conditions at near-critical subcritical conditions: viscosity and mass diffusion coefficients are similar but the density of subcritical water is 3 to 4 times higher than supercritical water thus increasing the filtrate mass flow rate by a factor of 3 to 4 for a given filter.

Separation of Inorganic Salts From Supercritical Water by Cross-Flow Microfiltration

Abstract A cross-flow microfilter capable of operating at elevated temperatures and pressures was evaluated for its ability to remove inorganic salts from supercritical water (SCW). The separation characteristics of molten sodium nitrate were investigated. The overall performance of the cross-flow microfilter and the effects of process variables on the separation efficiency were evaluated. Separation efficiencies up to 85% were observed. An empirical model was developed for the prediction of the filtrate salt concentration and the fluidized cake resistance as a function of the salt solubility and salt flux to the filter. Physical principles governing the separation process were defined.

SEPARATION OF IONIC SPECIES UNDER SUPERCRITICAL WATER CONDITIONS

ABSTRACT The unique characteristics of supercritical water (SCW) offer potentially attractive processing options that can be explored for reaction and separation purposes. While supercritical water oxidation (SCWO) can achieve high organic conversion efficiencies, low and relative solubilities of inorganic species in SCW may be further utilized for in situ separation of potential by-products from the SCWO process effluent. This paper describes a novel method for separating ionic species under SCW conditions. The concept is based on relative solubilities of different ionic species in SCW. Laboratory-scale demonstration tests were conducted with a Nylon monomer manufacturing process wastewater containing sodium hydroxide, sodium borate, carboxylic acids, and water. The process achieved (1) effective destruction (>99%) of organic components in the wastewater; (2) selective precipitation of sodium (>99.5%) as carbonates produced from oxidation of the organic components; and (3) efficient recovery of boron (>9...

A reciprocating pump check valve designed for supercritical water oxidation of sludges

Abstract This paper describes the design and test of a reciprocating pump check valve suitable for supercritical water oxidation of sludges. Various designs of Design and test of the sludge check valve were performed with an American Lewa diaphragm pump (Model HLM-1) which had a capacity of 1.5 g/s (0.2 lb/min