Pentane-Methanol System

The pressure-swing distillation system is well suited for heat integration because the temperatures in the high-pressure column are much higher than those in the low-pressure column. Therefore, it is often possible to use the hot vapor from the top of the high-pressure column to provide heat in the reboiler of the low-pressure column. Heat-integrated pressureswing systems are discussed in Chapter 6. 

The final pressure-swing azeotropic distillation system presented in this chapter is the separation of pentanes from methanol. This application arises in the production of tert-amyl methyl ether (TAME), which is used as a high-octane gasoline blending component. TAME is produced in a reactive distillation column by the reaction of methanol with the unsaturated five-carbon iso-amylenes (2-methyl-1-butene and 2-methyl-2-butene). 

The process flowsheet has a prereactor, a reactive distillation column, and a methanol-recovery section. A methanol-recovery section is required because the inert C5 components coming in with the reactive iso-amylenes in the C5 fresh feed form azeotropes with methanol. The result is that a significant amount of methanol is present in the distillate from the reactive column. 

The chemically inert components in the C5 feed stream include isopentane, n-pentane, 1-pentene, and 2-pentene. These components are not inert from the standpoint of phase equilibrium. Essentially all of these chemically inert components go out the top of the reactive distillation column as azeotropes with methanol. Isopentane (iCs) is the major C5 component. We illustrate the phase equilibrium by looking at the binary system iCs and methanol. 

The normal boiling points of iCs and methanol are 301 and 338 K, respectively. The reactive column operates at 4 bar, at which the boiling points are 348 and 377 K, respectively. Using UNIFAC physical properties, the binary azeotrope is 25.01 mol% methanol at 4 bar and 339 K. This means that the distillate from the reactive distillation column is a mixture of C5 components and methanol. 

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Wilsak, R.A. Campbell, S.W. Thodos, G. Vapor-liquid equilibrium measurements for the n-pentane-methanol system at 372.7, 397.7, and 422.6 K. Fluid Phase Equilib. 1987, 33, 157-171. 

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Absorption spectra of 2-nitroanisole in supercritical C02, N20, Freon-13, ammonia and C02-methanol mixtures were obtained on a Cary model 1605 spectrophotometer operated in the dual beam mode. The gases used as supercritical solvents were of the highest purity available from the supplier (Matheson) and were further filtered prior to use. The mixed solvent system of C02-methanol was obtained from Scott Speciality Gases (15.4 wt% methanol), and other mixtures were made in the laboratory. Spectra of 2-nitroanisole in n-pentane, methanol, tetrahydrofuran and acetonitrile (Burdick A Jackson) were obtained using quartz cells with a 1-cm light path and with a pure solvent blank in the reference beam. Vapor phase and supercritical fluid spectra were obtained using an air reference. 

Fig. 3 CCC separation of fat-soluble vitamins by cross-axis CPC. Experimental conditions sample, (A) calciferol (3 mg) -1-vitamin A acetate (30 mg)-1-( )-oc-tocopherol acetate (40 mg) and (B) vitamin K3 (3 mg)-1-vitamin Ki (10 mg) solvent system, 2,2,4-trimethyl pentane/methanol (1 1) mobile phase, lower phase. For other experimental conditions, see Fig. 1 caption. SF = solvent front.

The drawback to this as a recovery method is that pentane carries very little methanol out of the system (pentane/methanol azeotrope, 8% w/w methanol) so that if the original mixture of methanol and THF is rich in methanol, a preliminary concentration is desirable to reach the THF/methanol azetrope (31% methanol). Since the relative volatility between methanol and the azeotrope is about 2.0, this should not be a difficult separation. 

MacMillan and Wright [133] identified and measured saturated and unsaturated 1,3- and saturated 1,4-sultones in anionic surfactants by a series of separation maneuvers. Ion exchange treatment separates sultones from the bulk of the ionic surfactant. TLC concentrates the sultones systems for HPLC analysis. They found that pentane-ether is preferable to the usual hexane-ether system and that the addition of a little methanol sharpens the separations. Finally, HPLC using a micro-Porasil column with 90 1 isooctane/ethanol provides quali-... 

The distilling flask, collection flask, and column are continuously evacuated with a high-vacuum system. When the bulk of the pentane and residual hexane have distilled away, the temperature of a silicone oil bath surrounding the distillation flask is raised from room temperature to 125° over about a one-hour period. When the distillation rate diminishes appreciably, the temperature is slowly raised to 150° and maintained there until no more liquid is obtained. The distillation flask is cooled to room temperature before air is admitted to the system. The distilled product weighs 86.5 g. (89% yield checkers report 89 %t) and melts at 32 to 32.5°. In this state of purity, (bromomethyl)-carborane is suitable for most uses, but it may be further purified by crystallization from pentane or methanol. For example, crystals obtained by chilling a solution of 86.5 g. of the car-... 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

Change-transfer complexes of solute-alcohol stoichiometry 1 2 have been reported by Walker, Bednar, and Lumry3 for indole and certain methyl derivatives (M) in mixtures of associating solvents n-butanol and methanol (Q) with n-pentane these authors introduced the term exciplex to describe the emitter of the red-shifted structureless fluorescence band which increases in intensity with the alcohol content of the mixed solvent. The shift of the exciplex band to longer wavelengths as the solvent polarity is increased, described by Eq. (15), confirms the dipolar nature of the complex that must have the structure M+Q2. No emission corresponding to the 1 1 complex is observed in these systems which indicates (but does not prove) that the photo-association involves the alcohol dimer. The complex stoichiometry M+Q determined from (Eqs. 9, 10, and 12)... 

Special interest has been paid to the synthesis of bridged four-membered rings. The smallest system of this type, the bicyclo[1. l.l]pentane skeleton 6, could be prepared by photolysis of 3-diazobicyclo[2.1.1]hexan-2-one in methanol,43 however, the yield was very low ( 2%). 

The TD-GC system can be calibrated by running tubes that are spiked with a known volume and concentration of a standard vapor-gas atmosphere of the analyte or injecting a known concentration and volume of a solution of the analyte by a microliter syringe into the sorbent bed and remove the solvent by purging the tube with helium or nitrogen. The solvent must be tailored to the method, so that it can be evaporated from the tube without removing the analytes. Methanol or pentane are good choices for Tenax TA. 

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