Fractionation methods molecular-sieve

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.]... 

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. 

Dimethoxymethane (methylal) [109-87-5] M 76.1, b 42.3 , d 0.860, n 1.35626, n 1.35298. The main impurity is MeOH, which can be removed by treatment with sodium wire, followed by fractional distn from sodium. The solvent is kept dry by storing in contact with molecular sieves. Alternatively, technical dimethoxymethane was stood with paraformaldehyde and a few drops of H2SO4 for 24h, then distd. It could also be purified by shaking with an equal volume of 20% NaOH, leaving for 30min, and distilling. Methods of purification used for acetal are probably applicable to methylal. 

B. Saturated Hydrocarbons. Purging and freeze-pump-thaw methods are effective in degassing paraffins. Distillation with the elimination of the first fractions produces dry solvent. Molecular sieves may also be used to remove water. Distillation from sodium-benzophenone ketyl is highly effective in obtaining very low levels of water and oxygen. Commercial grades of hydrocarbons are often contaminated with olefins, which can be eliminated by several washings... 

Benzene can be purified by storing the solvent over molecular sieve 4A, filtering and finally fractionally distilling. The water content determined by the Karl Fischer method is 5 - 1(T3M... 

The classical method for the preparation of dry ethanol is to reflux it over calcium oxide, then distil or fractionally distil it. Ethanol can be dried to a water content appraoch-ing 0.005% if adequate precautions are taken to exclude water during the distillation. It has been found more convenient to take only common precautions and obtain ethanol containing not more than 0.05% water. The final drying can be accomplished with Type 3A or 4A molecular sieve or Drierite. 

Techniqttes for the separation of eth benzene in aromatic Cg cuts, particularly super-fractionation and possibly adsorption on molecular sieves, have been covered by spedbe analyses (see Sections 4.Z3 and 4.3.2.1). Thus, only chemical methods need to be examined, especially s>nthesis, which is a decisive hictor at the industrial level, and which involves the alkylation of benzene with ethylene. 

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