Distillation pressure sensitivity

Hydrocarbon resin is a broad term that is usually used to describe a low molecular weight thermoplastic polymer synthesized via the thermal or catalytic polymerization of coal-tar fractions, cracked petroleum distillates, terpenes, or pure olefinic monomers. These resins are used extensively as modifiers in the hot melt and pressure sensitive adhesive industries. They are also used in numerous other appHcations such as sealants, printing inks, paints, plastics, road marking, carpet backing, flooring, and oil field appHcations. They are rarely used alone. 

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

Only a fraction of the known azeotropes are sufficientiy pressure-sensitive for the conventional pressure-swing distillation process to work. However, the concept can be extended to pressure-insensitive azeotropes by adding a separating agent which forms a pressure-sensitive azeotrope and distillation boundary. Then the pressure is varied to shift the location of the distillation boundary (85). 

Exploitation of Pressure Sensitivity The breaking of homogeneous azeotropes that are part of a distiUation boundary (that is, into produc ts in different distillation regions) requires that the boundaiy... 

Exploitation of boundaiy curvature for breaking azeotropes is veiy similar to exploiting pressure sensitivity from a mass-balance point of view, and suffers from the same disadvantages. Separation schemes have large recycle flows, and in the case of minimum-boiling azeotropes, the recycle streams are distillates. However, in the case of maximum-boihng azeotropes, these recycles are underflows and... 

Methyl trichlorosilane [75-79-6] M 149.5, b 13,7 /101mm, 64.3 /710.8mm, 65.5 /745mm, 66.1 /atm, d 1.263, n 1.4110. If very pure distil before use. Purity checked by Si nmr, 6 in MeCN is 13.14 with respect to Me4Si. Possible contaminants are other silanes which can be removed by fractional distillation through a Stedman column of >72 theoretical plates with total reflux and 0.35% take-off (see p. 441). The apparatus is under N2 at a rate of 12 bubbles/min fed into the line using an Hg manometer to control the pressure. Sensitive to H2O. [J Am Chem Soc 73 4252 7957 J Org Chem 48 3667 7955.]... 

B.A. Horwitz, Optimize pressure sensitive distillation, Chem. Engng. Progress, April, (1997)47-51. 

The composition of many azeotropes varies with the system pressure (Horsley, Azeotropic Data-Ill, American Chemical Society, Washington, 1983 Gmehling et ah. Azeotropic Data, VCH Publishers, Deerfield Beach, Fla., 1994). This effect can be exploited to separate azeotropic mixtures by so-called pressure-swing distillation if at some pressure the azeotrope simply disappears, such as does the ethanol-water azeotrope at pressures below 11.5 kPa. However, pressure sensitivity can still be exploited if the azeotropic composition and related distillation boundary change sufficiently over a moderate... 

General information on pressure-swing distillation can be found in Van Winkle (Distillation, McGraw-Hill, New York, 1967), Wankat (Ejmilibrium-Stased Separations, Elsevier, New York, 1988), and Knapp and Doherty [Ina. Eng. Chem. Res., 31, 346 (1992)]. Only a relatively small fraction of azeotropes are sufficiently pressure-sensitive for a pressure-swing process to be economical. Some applications include the minimum-boiling azeotrope tetrahydrofuran-water [Tanabe et al. ... 

U.S. Patent 4,093,633 (1978)], and maximum-boiling azeotropes of hydrogen chloride-water and formic acid-water (Horsley, Azeotropic Data-III, American Chemical Society, Washington, 1983). Since distillation boundaries move with pressure-sensitive azeotropes, the pressureswing principle can also be used for overcoming distillation boundaries in multicomponent azeotropic mixtures. 

Liu, Q. Xiao, J. Synthesis of tetrahydrofuran from butanediol by pressure-sensitive reactive distillation. Jisuanji Yu Yingyong Huaxue 2001, 18 (2), 123-126. 

The feed consists of two components to be separated, A and B, that are close boilers or that form an azeotrope with each other. An entrainer E that forms an azeotrope with one of the components is added to the feed. If the component forming the azeotrope is A, the resulting azeotrope AE is separated from B by distillation. The problem is shifted from separating azeotrope AB to separating azeotrope AE. Eor the process to be viable, there should be an economical means for separating AE. If AE has a pressure-sensitive azeotropic composition or is a heterogeneous azeotrope, the methods described in Section 10.1.1 or 10.1.2 could be used to round out the process. 

A breakthrough distillate hydrotreating catalyst, jointly developed by ExxonMobil and AkzoNobel, was recently commercialized and deployed in three refineries. This catalyst has three to ten times the activity of the most active current HDS/HDN catalysts, depending on pressure. This catalyst also exhibits novel and unique pressure sensitivity from 400 psi Hydrogen-2 pressure up to 2,000 psi Hydrogen-2. It represents a major advance in catalyst performance, composition, structure, and morphology. The product is stripped not only of sulfur but also nitrogen and achieves substantial aromatic saturation. 

Figure 6-7 Use of two columns to separate a maximum-boiling homogeneous azeotrope which is pressure sensitive. (M. Van Winkle, Distillation, 1967, by courtesy McGraw-Hill Book Company.)...

With the availability of economical and efficient packings, packed towers are finding increasing use in new distillation processes and for retrofitting existing trayed towers. They are particularly useful in applications where pressure drop must be low, as in low-pressure distillation, and where liquid holdup must be small, such as when distilling heat-sensitive materials whose exposure to high temperatures must be minimized. 

Horwitz, B.A., Optimize Pressure-Sensitive Distillation, Chart Eng. Prog., 93(4),47 (1997). 

Since the iC5/methanol azeotrope is pressure sensitive (79 mol% iC5 at 10 bar and 67 mol% iC5 at 4 bar), it is possible to use a pressure-swing process with two distillation columns, operating at two different pressures, to separate methanol from the C5 components. An alternative separation process for this system is extractive distillation, which is studied in this chapter. 

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