Superheated polarity

CO2 extraction has been prevalent for the isolation of essential oils and other natural lipophilic pigments like carotenoids. Hot water and superheated water extraction methods are used for analytical preparation of polar pigments. The technique is commonly referred to as subcritical water extraction because the practitioners of this approach come from SEE backgrounds. 

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

Superheating of the solvent was believed to be responsible of the observed rate enhancement under microwave irradiation in the synthesis of 3,5-disubstituted 4-amino-1,2,4-triazoles when conducted in 1,2-ethylene glycol as (polar) solvent (Eq. 4) [32]. 

It is to be noted that the reactions mentioned so far were performed under homogeneous conditions and, in most cases, using polar solvents, which are efficient absorbers of MW energy. Rate enhancements were attributed to the superheating of the solvent due to the elevated pressures generated in the closed vessels. 

The rate was enhanced up to 2.6-fold for reaction of the 2-isomer and up to 14-fold for the 4-isomer. The product distribution in the final reaction mixtures was always somewhat different when microwave heating was used. The results were explained in terms of efficient interaction of microwaves with a highly polarized reagent molecule adsorbed on the acidic active site. Possible superheating of the active sites was difficult to detect (Sect. 10.3.3). 

These investigations of the use of superheated water with NMR spectroscopy are still at an early stage and have not yet been applied to real problems. Many questions remain to be answered concerning the suitability of this chromatography for thermally labile compounds, and with the current stationary phases available the technique is probably limited to moderately polar compounds. However, the technique is readily implemented and may in time prove to be a useful addition to the armoury of HPLC-NMR methods. 

A number of alternatives to Soxhlet extraction have been described. By pressurized liquid or accelerated solvent extraction, the extraction efficiency can be enhanced. Superheated water extraction, taking advantages of the decreased polarity of water at higher temperature and pressure, has been used for liquid extraction of solid samples as well. 

Direct application of heat via in situ combustion or via superheated steam generation at the surface and injection are other effective methods to boost production, either in mature oil fields or in heavy oil fields where the petroleum is naturally quite viscous. While both formation heating methods achieve production rate improvements by viscosity reduction, the apparent simplicity of the in situ combustion concept is offset by the difficult separation of recovered oil from an aqueous solution containing nitrogen oxides, sulfur oxides, and other polar combustion products. The acids present in the aqueous phase contribute to the stability of the emulsions obtained from the producing wells and are highly corrosive to steel pipes and tanks. 

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