Fractionation methods sieving

Separation depends on the selection of a process in which the behaviour of the material is influenced to a very marked degree by some physical property. Thus, if a material is to be separated into various size fractions, a sieving method may be used because this process depends primarily on the size of the particles, though other physical properties such as the shape of the particles and their tendency to agglomerate may also be involved. Other methods of separation depend on the differences in the behaviour of the particles in a moving fluid, and in this case the size and the density of the particles are the most important factors and shape is of secondary importance. Other processes make use of differences in electrical or magnetic properties of the materials or in their surface properties. 

Excluded from this list is sieving, to which the concept of selectivity is not applicable. For completeness, we have subdivided the FFF family into sedimentation FFF, thermal FFF, flow FFF, and steric FFF to show how the selectivity of each of these subtechniques compares to that of the other fractionation methods. The values reported here differ from S values reported elsewhere (12), which refer to mass rather than size selectivity. 

A sharp separation results in two high purity, high recovery product streams. No restrictions ate placed on the mole fractions of the components to be separated. A separation is considered to be sharp if the ratio of flow rates of a key component in the two products is >10. The separation methods that can potentially obtain a sharp separation in a single step ate physical absorption, molecular sieve adsorption, equiHbrium adsorption, and cryogenic distillation. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the components to be removed ate present in low concentrations. 

The special case involving the removal of a low (2—3 mol %) mole fraction impurity at high (>99 mol%) recovery is called purification separation. Purification separation typically results in one product of very high purity. It may or may not be desirable to recover the impurity in the other product. The separation methods appHcable to purification separation include equiUbrium adsorption, molecular sieve adsorption, chemical absorption, and catalytic conversion. Physical absorption is not included in this Hst as this method typically caimot achieve extremely high purities. Table 8 presents a Hst of the gas—vapor separation methods with their corresponding characteristic properties. The considerations for gas—vapor methods are as follows (26—44). 

The propionamide can be dried over CaO. H2O and unreacted propionic acid were removed as their xylene azeotropes. It was vacuum dried. Material used as an electrolyte solvent (specific conductance less than 10 ohm cm" ) was obtained by fractional distn under reduced pressure, and stored over BaO or molecular sieves because it readily absorbs moisture from the atmosphere on prolonged storage. [Hoover Pure Appl Chem 37 581 I974 Recommended Methods for Purification of Solvents and Tests for Impurities, Coetzee Ed., Pergamon Press, 1982.]... 

The general purification methods listed for xylene are applicable. p-Xylene can readily be separated from its isomers by crystn from such solvents as MeOH, EtOH, isopropanol, acetone, butanone, toluene, pentane or pentene. It can be further purified by fractional crystn by partial freezing, and stored over sodium wire or molecular sieves Linde type 4A. [Stokes and French J Chem Soc, Faraday Trans 1 76 537 1980.]... 

Souders-Brown. The Souders-Brown method (References 1, 2) is based on bubble caps, but is handy for modem trays since the effect of surface tension can be evaluated and factors are included to compare various fractionator and absorber services. These same factors may be found to apply for comparing the services when using valve or sieve trays. A copy of the Souders-Brown C factor chart is shown in Reference 2. 

The second analytical method uses a combustion system (O Neil et al. 1994) in place of reaction with BrF,. This method was used for the crocodiles because they were represented by very thin caps of enamel. The enamel was powdered and sieved (20 mg), pretreated in NaOCl to oxidize organic material and then precipitated as silver phosphate. Approximately 10-20 mg of silver phosphate were mixed with powdered graphite in quartz tubes, evacuated and sealed. Combustion at 1,200°C was followed by rapid cooling in water which prevents isotopic fractionation between the CO2 produced and the residual solid in the tube. Analyses of separate aliquots from the same sample typically showed precisions of 0.1%o to 0.4%o with 2 to 4 repetitive analyses even though yields are on the order of 25%. 

The cumulative mass fraction plot is another method of representing the data. The cumulative mass fraction oversize is the relative mass of particles retained on each sieve. Similarly, the cumulative mass fraction undersize is the relative mass of particles passing through each sieve. Figures 15.24 and 15.25 show these cumulative plots. The cumulative mass fraction oversize is calculated by adding the mass... 

TBT of mark tch , is used which was distiled in vacuum, three times selecting fraction with boiling temperature 7j,=430-432 K at pressure 1,33 Gpa [6]. Obtaited by such method fraction was preserved under molecular sieve 4 A. 

Soil samples were wet sieved into (a) 2-4 mm, (b) 1-2 mm, (c) 0.5-1 mm, (d) 250-500 i m, (e) 125-250 am, (f) 63-125 j,m and (g) <63 j.m fractions. A ferruginous/magnetic fraction (m) was also prepared from the 2-4 mm fraction. Soil fractions were crushed, digested with HNO3/HCI/HF/HCIO4 and then analysed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) for Al, Ca, Cu, Fe, K, Mn, Na, P, S and Zn. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to determine Ag, As, Cd, Pb and Sb because of the lower detection limits by this method. The mineralogy of selected samples was determined by qualitative X-ray diffractometry. 

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