Separations Using Superheated Water

Green Separation Processes. Edited by C. A. M. Afonso and J. G. Crespo Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30985-3 

Most of the work described below is on a laboratory scale and some of it is directed towards chemical analysis. However, some larger-scale processes are under consideration, mainly for environmental reasons. Chemical reactions, which bring about modification of the extracted compounds, can occur during extraction. In some cases this is an advantage, e.g. when pollutants are destroyed or flavors produced. The work related to these is described in so far as it is publicly available. Some important features of superheated water are first briefly discussed. 

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Reversed-phase separations using superheated water have not found any major applications so far, but a wide range of low molecular mass polar compounds have been separated at modest temperatures with relatively simple and rugged instrumentation. It has potential for wider use in laboratories that wish to reduce consumption of organic solvents. In addition, superheated water should be compatible with on-column preconcentration techniques for trace analysis... 

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,... 

Figure 18-11. Separation of the anticancer drugs 5-fluorouracil (5-FU), methotrexate (MTX), 7-hydroxymethotrexate (7-OH-MTX), and etopoxide (VP-16) using superheated water and a PS-DVB column at 80°C. (Reproduced from reference 75, with permission. Copyright 2001, American Chemical Society.)...

Temperature is an important and often ignored parameter in method optimization. A lack of temperature control can result in poor inter- and intralaboratory reproducibility. Increased temperatures can speed up and alter separations and may improve efficiency and throughput, especially of macromolecules. High-temperature work using superheated water can eliminate organic solvents from the mobile phase, simplifying detection and solvent interferences in detection. At lower temperature the reduction in molecular motion can resolve interconverting chiral and structural analytes. 

T. Teutenberg, O. Lerch. H-J. Gotze, and P. Zinn, Separation of selected anticancer drugs using superheated water as the mobile phase, Anal. Chem. 73 (2001), 3896-3899. 

In reeent years, tire use of elevated temperatures has been reeognised as a potential variable in method development. Witlr inereased temperature, aqueous-organie mobile phases separations ean improve, viseosity deereases and diffusion inereases so baek pressures are redueed. At higher temperatures (usually with superheated water > 100 °C under modest pressures) water alone ean be used as the mobile phase and eair provide unique separation opportunities. The absenee of an organie solvent enables the use in HPLC of alternative deteetors sueh as FID or on-line LC-NMR using deuterium oxide as the eluent. 

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... 

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. 

TABLE 18-2. Pharmaceutical Compounds Separated Using Hot and Superheated Water... 

In continuous steam distillation, an insulated conveying system with superheated steam as carrier is used for providing a countercurrent flow of steam and pulverised plant material. During transport, the oil is transferred into the vapour phase and exits the system with the steam. A cyclonic vessel separates the gas phase from the solid phase. In the last step the gas phase (steam and oil) is condensed, the oil is separated using a Florentine flask and the water recycled to the boiler [27]. 

As demonstrated from the 312 relevant SciFinder references, ecdysteroids have been isolated by RP-HPLC,176-179 HP-NPC,180-182 HP-IEX,183,184 or HP-AC.185 The dominant mode for the separation of ecdysteroids is HP-RPC, with numerous published applications including the use of superheated water as a mobile phase.186... 

Mahoney et a/.87 have described the reaction of polyurethane foam and superheated water at 200 °C for 15 min, which leads to toluene diamines and polypropylene oxide. Hydrolysis of polyurethane and rubber mixtures has been used as a method not only of recovering valuable chemicals from the polyurethane fraction, but also to separate the polymers because rubber is inert to hydrolysis.89 The degradation takes place by contact with saturated steam at 200 °C for 12 h. This process may find particular applications in the treatment of rubber/polyurethane laminations. 

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. 

FI. A distillation column separating ethanol from water uses open steam heating. The bottoms composition is 0.00001 mole fraction ethanol. The inlet steam is pure water vapor and is superheated to 700°F. The pressure is 1.0 atm. The ratio of bottoms to steam flow rate is B/S = 2.0. Find the slope of the bottom operating line. 

Extraction for the analyte from the fuel can be used for sample preparation, which combines the advantages of separating the analyte from the matrix, transferring it to an aqueous phase and may also result in preconcentration. Liquid-liquid extraction procedures present as main advantage for their simplicity. Akinlua and Smith (2010) reported the extraction of trace metals from petroleum source rock by superheated water and the conditions for maximum yield were determined. The optimum temperature for superheated water extraction of the metals from petroleum source rocks was 250°C. The extraction time was 30 min. The leaching of Cd, Cr, Mn and Ni had better yield with superheated water, while V had better yield with acid digestion. 

Other methods for the separation rf fatty acids from glycerine—incorrectly termed saponification processes—are those in which the decomposition of the fat is effected by means cf acid, more e.specially by sulphuric acid finally there is the use of even water itself in place of acids, the saponification being brought about with either superheated water or superheated steam. 

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