Low temperature fractionation

Ciyst lliz tion. Low temperature fractional crystallization was the first and for many years the only commercial technique for separating PX from mixed xylenes. As shown in Table 2, PX has a much higher freezing point than the other xylene isomers. Thus, upon cooling, a pure soHd phase of PX crystallizes first. Eventually, upon further cooling, a temperature is reached where soHd crystals of another isomer also form. This is called the eutectic point. PX crystals usually form at about —4° C and the PX-MX eutectic is reached at about —68° C. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point. At all temperatures above the eutectic point, PX is stiU soluble in the remaining Cg aromatics Hquid solution,... 

Molecular sieves have had increasing use in the dehydration of cracked gases in ethylene plants before low temperature fractionation for olefin production. The Type 3A molecular sieve is size-selective for water molecules and does not co-adsorb the olefin molecules. 

The gases from the reactor are then cooled and subjected to a caustic wash to remove unreacted hydrogen chloride. This is then followed by a methanol wash to remove water introduced during the caustic wash. A final purification to remove aldehydes and ethylidene dichloride, formed during side reactions, is then carried out by low-temperature fractionation. The resulting pure vinyl chloride is then stored under nitrogen in a stainless steel tank. 

The removal of water from certain light ends streams may be necessitated by the subsequent processing step, such as isomerization or low temperature fractionation, or it may be required to meet product specifications, such as for LPG. 

This is still a large quantity of oil to absorb the ethylene. Under some circumstances it might be less expensive to separate the ethylene by low-temperature fractionation. 

Sir William Ramsay (1852-1916) and Morris William Travers (1872-1961) discovered it spectroscopically in the low-temperature fractionation of crude argon. 

With the exception of these fractionation pathways, studies of igneous systems chiefly focus on the potential of Li isotopes as geochemical tracers fingerprinting the cycling of Li derived from specific (low-temperature) sources through the solid Earth. The sections below deal with observations of Li isotopes in high-temperature systems, and the mechanisms for low-temperature fractionation processes are discussed after, imder the heading, Planetary surface systems. ... 

As shown in Figures 4b and 4c, the low temperature fraction of Beulah-Zap lignite reveals a completely different type of mass spectrum. From previous studies... 

The development of a commercial mass spectrometer and its application to hydrocarbon gas analysis by the method of Washburn et al. (63) made gas analysis rapid, economical, and, what is even more important, inspired a confidence in the results of routine hydrocarbon gas analysis which was badly lacking. A complex gaseous mixture comprising the atmospheric gases, carbon monoxide, and Ci to C6 hydrocarbons required more than 20 hours of applied time by the previous methods of low temperature fractional distillation coupled with chemical absorption methods. With the mass spectrometer such an analysis is completed in 2 hours or less, about 15 minutes of which is consumed in the... 

Subsequently the gas is precooled in exchanger E-6 and charged to a low temperature fractionator. This tower has a reboiler and a top refluxing system. At the top the conditions are 280psig and —75°F. Freon refrigerant at — 90°F is used in the condenser. The bottoms is recycled to the pyrolysis coil. The uncondensed vapor leaving the reflux accumulator constitutes the product of this plant. It is used to precool the feed to the fractionator in E-6 and then leaves this part of the plant for further purification. 

Fig. 9.22. Low-temperature fractional-distillation column, ( ) Norman s modification of a design by Dobson and Schaeffer, (b) An alternate column design (I. Dobson, Ph.D. thesis. Indiana University, 1967).

Low-temperature fractionation is the preferred method of hydrocarbon separation. Cascade refrigeration is used with propylene and ethylene, the commonly encountered refrigerants. Refrigeration compressors are large and have energy requirements essentially similar to those of the initial product gas compressors. 

Platforming operations make a gas that contains as much as 90 to 95% hydrogen. This gas is usually purified by low temperature fractionation or by washing with liquid nitrogen. 

Figure 2. Conductivity of chlorine trifluoride as a function of temperature. A—before tow temperature fractionation 0—after low temperature fractionation...

Figure 4. Schematic diagram of low temperature fractionation and gas chromatography apparatus...

Figure 18 D/H ratios of several comets compared to the oceans (SMOW), planets, the solar nehula (PSN), and the interstellar medium. Low-temperature fractionation processes increase D/H. Jupiter and Saturn have compositions close to the original nehular composition, hut low-temperature formation of ice caused the enhancements seen in Uranus and Neptune (the ice giants) and comets. The discrepancy between the plotted LP comets and SMOW argues against these comets providing Earth with a major fraction of its water. Other comets, formed in warmer environments, near Jupiter, could he more similar to SMOW (source Huehner, 2002).

Nitrogen is taken from air or from the nitrogen content of natural gas. This is carried out by low temperature fractionation of air, which is preferred when pure oxygen is required as an oxidizing agent in the production of synthesis gas. Alternatively air is employed directly in the production of synthesis gas and the oxygen is removed by the to be oxidized reaction partners. 

Low-temperature fractionation in low salt-ethanol mixtures has been applied to the systematic fractionation of plasma since 1940. There has been a gradual evolution and decisive advance both in respect to the number and specificity of the fractions separated and to the preservation of components in undenatured form. 

Low-Temperature Fractionation. Low-temperature fractionation is used to separate 75% of the ethylene produced in the United States (62),... 

Recent phosgene plants constructed in the Gulf Coast of the United States obtain their CO from synthesis gas by low-temperature fractionation [8]. Early units derived their raw material from coal, directly or indirectly. Most phosgene plants have their own caustic chlorine facilities. Since most phosgene is consumed captively, the hydrogen chloride is recovered in an oxidation unit. 

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