Superheated-water studies

Meluch et al.10 reported that high-pressure steam hydrolyzes flexible polyurethane foams rapidly at temperatures of 232-316°C. The diamines are distilled and extracted from the steam and the polyols are isolated from the hydrolysis residue. Good results were obtained by using reclaimed polyol in flexible-foam recipes at file 5% level. Mahoney et al.53 reported the reaction of polyurethane foams with superheated water at 200°C for 15 min to form toluene diamines and polypropylene oxide. Gerlock et al.54 studied the mechanism and kinetics of the reaction... 

Superheated water at 100°-240 °C, with its obvious benefits of low cost and low toxicity, was proposed as a solvent for reversed-phase chromatography.59 Hydrophobic compounds such as parabens, sulfonamides, and barbiturates were separated rapidly on poly(styrene-divinyl benzene) and graphitic phases. Elution of simple aromatic compounds with acetonitrile-water heated at 30°-130 °C was studied on coupled colums of zirconia coated with polybutadiene and carbon.60 The retention order on the polybutadiene phase is essentially uncorrelated to that on the carbon phase, so adjusting the temperature of one of the columns allows the resolution of critical pairs of... 

An area of study related to this topic is the use of subcritical, but superheated water as a mobile phase for chromatographic separations [78], These separations use water heated to 100-220°C and pressures up to 50 bar, avoiding problems due to hydrolysis and oxidation, which is common when supercritical water is used. Although this is a new area of investigation, several reports on the hyphenation of HPLC using... 

SUPERHEATED-WATER HPLC-NMR AND HPLC-NMR-MS STUDIES ON PHARMACEUTICALS... 

Most recently, a further study has been performed using superheated-water HPLC with NMR and MS to analyse a mixture of sulphonamides [68]. The chromatography was performed as before with D20-phosphate buffer (pD 3.0) as eluent. A temperature gradient from 160 to 200 °C at 2°C min-1 was employed. A mixture of four sulphonamides, i.e. sulacetamide, sulphadiazine, sulfamerazine and sulfamethazine, was separated in this system with UV, NMR and MS detection. It rapidly became clear from a study of the spectroscopic data that while sulfacetamide and sulfadiazine gave the expected NMR and mass spectra, those for sulfamerazine and sulfamethazine did not. These compounds gave spectra that were 3 and 6 mass units higher than expected,... 

Three main aims have driven these studies the use of temperature as a variable to optimize separations, an interest in improved efficiency, and the potential for green separations methods, such as superheated water chromatography, which can eliminate the organic solvent from the mobile phase. 

In studying the kinetics of spontaneous boiling-up of superheated water use was made of the method of measuring the lifetime (quasi-static method) and the method of pulse superheat of a liquid on a thin platinum wire (dynamic method). 

Abstract A synthetic pure water fluid inclusion showing a wide temperature range of metastability (Th - Tn 50°C temperature of homogenization Th = 144°C and nucleation temperature of Tn = 89°C) was selected to make a kinetic study of the lifetime of an isolated microvolume of superheated water. The occluded liquid was placed in the metastable field by isochoric cooling and the duration of the metastable state was measured repetitively for 7 fixed temperatures above Tn. Statistically, metastability lifetimes for the 7 data sets follow the exponential reliability distribution, i.e., the probability of non nucleation within time t equals. This enabled us to calculate the half-life periods of metastability r for each of the selected temperature, and then to predict i at any temperature T > Tn for the considered inclusion, according to the equation i(s) = 22.1x j Hence we conclude that... 

GC is the most commonly used separation method in the analysis of BTEX from environmental samples. Liquid chromatography (LC) analysis with superheated water or water-dimethylsulfoxide (DMSO) mixmres has also been reported. In both cases a reduction in the dielectric constant of the mobile phase for the separation of nonpolar analytes was studied. The results showed how the rise in temperamre required a decrease in DMSO in order to achieve the same retention time. 

The extraction of polyaromatic hydrocarbons from soil and urban particulates by superheated water was reported in 1994 [17]. Extraction of compounds up to ben-zo[a]pyrene was virtually complete in 15 min at 250°C, with a flow rate of 1 ml mim and a sample of 0.5 g. Good but less complete results were obtained when extracting urban air particulates. The pressure did not influence the extraction behavior, provided it was sufficient to maintain water as a Hquid. The extraction of polychlorinated biphenyls from soil and a river sediment was also found to be complete in 15 min at 250°C [18]. Work with a wider range of compounds showed that extraction was class selective [6, 19], with phenols and Hghter aromatics being extracted at 50 to 150°C, polyaromatic hydrocarbons and lighter ahphatics at 250 to 300°C, but the heavier ahphatics only removed by steam at 250 to 300°C. This selectivity has been compared to other extraction methods [20]. The extraction of agrochemicals from soil has also been studied [6]. 

It has been coupled with enzyme immunoassay for efficient and fast polyaromatic hydrocarbon (PAH) screening in soil [21]. In a number of studies both static and dynamic superheated water extraction has been coupled to solid-phase microextraction [15, 25, 28, 30, 35, 38], sometimes with other analytical methods also coupled. It has been coupled with gas chromatography-mass spectrometry [31], capillary electrophoresis [31], liquid chromatography-mass spectrometry [32] and liquid chromatography-gas chromatography [41]. Sometimes other chemicals are added to the water used, such as acid [42] or phosphate buffer [43]. Different trapping methods for analytical extraction have been examined [44]. 

Very often compounds being extracted by superheated water react in the medium by hydrolysis or otherwise. It is know from other studies involving pure contaminants that they will react, for example chlorinated hydrocarbons are often dechlo-rinated and converted into hydrocarbons. In other cases benign materials are obtained from pollutants. In the extraction of the explosives TNT, RDX and HMX from contaminated soil, decomposition occurs non-dramatically and completely to benign substances [48]. These compounds contain an oxidative reagent within the molecule. Soil obtained from a bomb disposal site contaminated with 120 000 ppm (12%) of TNT, after treatment in a static ceU at 275°C for 1 h, contained only 2 ppm and the water remaining 4 ppm. Dioxins in contaminated soil treated for 4 h at were found to be reduced by 99.4%, 94.5% and 60% at temperatures of 350°C, 300°C and 150°C, respectively [49]. 

Owen [3] studied the evaporation of superheated water droplets in saturated steam at pressures up to 900 kPa. The droplets were superheated by depressurizing the vessel at different rates. He observed that if the excess temperature was below 5°C, the evaporation was on the surface between 5°C and 18°C the droplets started to boU internally, however they did not disintegrate. Above 18°C the droplets flashed, and disintegrated. 

It appears that the results of experimental studies presented in this chapter have amply demonstrated the technical feasibility of plasma-assisted drying. The most promising is the use of a plasma and/or superheated water vapor as the heat source and drying carrier. In addition to the advantages inherent to superheated steam drying (see Chapter 7), the use of steam plasmas may offer the following benefits ... 

He et al. [ 116] and Wan and Shu [117] reported on the influence of calcination and hydrothermal treatment on compositional characteristics and thermal stability of rare earth containing Y zeolites and their performance in catalytic cracking. The alkaline and hydrothermal stability of Y zeolites dealuminated via hydrothermal treatment and by the SiCl4 technique was studied by Lutz et al. [118]. Hydrothermal treatment was found to increase the chemical resistance of Y zeoHte to superheated water at 200°C as well as to alkaline solutions due to the formation of a protective layer of extra-lattice oxidic aluminum species on the external surface of the zeoHte crystals. The removal of this layer by acid leaching resulted in significantly less stable products. 

Groendes and Mesler (1982) studied the saturated film boiling impacts of a 4.7 mm water droplet on a quartz surface of 460 °C. The fluctuation of the surface temperature was detected using a fast-response thermometer. The maximal temperature drop of the solid surface during a droplet impact was reported to be about 20 °C. Considering the lower thermal diffusivity of quartz, this temperature drop implies a low heat-transfer rate on the surface. Biance et al. (2003) studied the steady-state evaporation of the water droplet on a superheated surface and found that for the nonwetting contact condition, the droplet size cannot exceed the capillary length. 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

This comprehensive survey of the title topic is in three parts, the first dealing with the theoretical background and laboratory studies, with 29 references. The second part, with 21 references deals with case histories and experimental studies of industrial vapour explosions. These involved the systems molten titanium-water, molten copper-water, molten aluminium-water, smelt-water, water-various cryogenic liquids, molten salt-water and molten uranium dioxide-liquid sodium. In the third part (with a further 26 references) is discussion of the various theories which abound, and the general conclusion that superheated liquids most likely play a major role in all these phenomena [1]. A further related publication covers BLEVEs and pressure let-down explosions [2],... 

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