Aromatics, stripping

Solvent traces are removed from the aromatics vapors in packing (4), again by reflux. It is, however, packing (5), where the aromatics are stripped off from the solvent that is of crucial importance. Extractive distillation can only be effective if the aromatics content is drastically reduced to 0.1%. Intensive aromatics stripping is crucial for the aromatics yield. 

The solubility of hydrocarbon liquids from the same chemical family diminishes as the molecular weight increases. This effect is particularly sensitive thus in the paraffin series, the solubility expressed in mole fraction is divided by a factor of about five when the number of carbon atoms is increased by one. The result is that heavy paraffin solubilities are extremely small. The polynuclear aromatics have high solubilities in water which makes it difficult to eliminate them by steam stripping. 

Composition is normally expressed by a distillation curve, and can be supplemented by compositional analyses such as those for aromatics content. Some physical properties such as density or vapor pressure are often added. The degree of purity is indicated by color or other appropriate test (copper strip corrosion, for example). 

A schematic of the MGCC process is shown in Figure 9. The mixed Cg aromatic feed is sent to an extractor (unit A) where it is in contact with HF—BF and hexane. The MX—HF—BF complex is sent to the decomposer (unit B) or the isomerization section (unit D). In the decomposer, BF is stripped and taken overhead from a condensor—separator (unit C), whereas HF in hexane is recycled from the bottom of C. Recovered MX is sent to column E for further purification. The remaining Cg aromatic compounds and hexane are sent to raffinate column E where residual BE and HE are separated, as well as hexane for recycle. Higher boiling materials are rejected in column H, and EB and OX are recovered in columns I and J. The overhead from J is fed to unit K for PX separation. The raffinate or mother Hquor is then recycled for isomerization. 

Polymerizations are typically quenched with water, alcohol, or base. The resulting polymerizates are then distilled and steam and/or vacuum stripped to yield hard resin. Hydrocarbon resins may also be precipitated by the addition of the quenched reaction mixture to an excess of an appropriate poor solvent. As an example, aUphatic C-5 resins are readily precipitated in acetone, while a more polar solvent such as methanol is better suited for aromatic C-9 resins. 

A common procedure for the preparation of vinylated alkyds is as foUows. A base alkyd resin is brought to the desired endpoint. The resin is then cooled to about 160°C and often diluted with aromatic thinner. The desired monomer is added, usually at about 20 —60% based on the final product, foUowed by an appropriate amount of a free radical initiator. Alternatively, a premix of the monomer and the initiator is added at a controUed rate over most of the reaction. The reaction is brought to monomer reflux, until the residual monomer content has fallen below a specified level. Residual monomer, if any, is stripped away before the product is diluted in a solvent, filtered, and packaged. 

In general, the sulfolane extraction unit consists of four basic parts extractor, extractive stripper, extract recovery column, and water—wash tower. The hydrocarbon feed is first contacted with sulfolane in the extractor, where the aromatics and some light nonaromatics dissolve in the sulfolane. The rich solvent then passes to the extractive stripper where the light nonaromatics are stripped. The bottom stream, which consists of sulfolane and aromatic components, and which at this point is essentiaHy free of nonaromatics, enters the recovery column where the aromatics are removed. The sulfolane is returned to the extractor. The non aromatic raffinate obtained initially from the extractor is contacted with water in the wash tower to remove dissolved sulfolane, which is subsequently recovered in the extract recovery column. Benzene and toluene recoveries in the process are routinely greater than 99%, and xylene recoveries exceed 95%. 

The overhead temperatures of both the absorber and stripper are kept as low as possible to minimise solvent carryover. A temperature of about 311 K is typically used ia the high pressure absorber. The overhead temperature ia the stripper is set by the boiling poiat of the saturated complex solution and by the operating pressure of the stripper. At a stripping pressure of 0.166 MPa (1.7 atm), a temperature of 378 Kis used. The solvent-rich gas from the stripper is cooled to recover as much solvent as possible by condensation prior to the final aromatics-recovery section. Fiaal solvent recovery is accomphshed by adsorption on activated carbon (95). 

The condition defined by equation (8) is met by adjustment of (Qg(3)) nd (T(3)). The pressures at the second stripping flow inlet and that of the outlet for solute (C) must be made equal, or close to equal, to prevent cross-flow. Scott and Maggs [7] designed a three stage moving bed system, similar to that described above, to extract pure benzene from coal gas. Coal gas contains a range of saturated aliphatic hydrocarbons, alkenes, naphthenes and aromatics. In the above theory the saturated aliphatic hydrocarbons, alkenes and naphthenes are represented by solute (A). 

The main product, benzene, is represented by solute (B), and the high boiling aromatics are represented by solute (C) (toluene and xylenes). The analysis of the products they obtained are shown in Figure 12. The material stripped form the top section (section (1)) is seen to contain the alkanes, alkenes and naphthenes and very little benzene. The product stripped from the center section appears to be virtually pure benzene. The product from section (3) contained toluene, the xylenes and thiophen which elutes close to benzene. The thiophen, however, was only eliminated at the expense of some loss of benzene to the lower stripping section. Although the system works well it proved experimentally difficult to set up and maintain under constant operating conditions. The problems arose largely from the need to adjust the pressures that must prevent cross-flow. The system as described would be virtually impossible to operate with a liquid mobile phase. 

Fig. 25. Evolution of the tack of polychloroprene-aromatic hydrocarbon resin blends as a function of the resin content. Tack was obtained as the immediate T-peel strength of joints produced with 0.6 mm thick styrene-butadiene rubber strips placed in contact without application of pressure. Peeling rate = 10 cm/min.

Solvents are recovered from the oil stream through distillation and steam stripping in a fractionator. The stream extracted from the solvent contains high concentrations of hydrogen sulfide, aromatics, naphthenes and other hydrocarbons, and is often fed to the hydrocracking unit. 

Example 5. Glycolysis of Polyurethanes with Propylene Oxide after Pretreatment with Ethanolamine.55 A rigid polyurethane foam (ca. 100 g) was dissolved in 30 g ethanolamine by heating. Excess ethanolamine was stripped, leaving a clear solution. Infrared and GPC analysis indicated that the clear solution obtained contained some residual polyurethane, aromatic polyurea, aliphatic polyols, aromatic amines, and N,N -bis(f -hydroxyethyljurea. Next the mixture was dissolved in 45 g propylene oxide and heated at 120°C in an autoclave for 2 h. The pressure increased to 40 psi and then fell to 30 psi at the end of the 2-h heating period. The product was a brown oil with a hydroxyl number of485. 

Grob and Zurcher [36,53-55] have carried out very detailed and systematic studies of the closed loop gas stripping procedure and applied it to the determination of parts per billion of 1-chloroalkanes in water. Westerdorf [56] applied the technique to chlorinated organics and aromatic and aliphatic hydrocarbons. 

Grob et al. [220-223] have carried out very detailed and systematic studies of the closed-loop gas stripping procedure and applied it to the determination of xg/l of 1-chloroalkanes in water. Westerdorf [224] applied the technique to chlorinated organics, and aromatic and aliphatic hydrocarbons. Waggot and Reid [225] reported that a factor of major concern in adapting the technique to more polluted samples is the capacity of the carbon filter, which usually contains only 1.5-2 mg carbon. They showed that the absolute capacity of such a filter for a homologous series of 1-chloro-n-alkanes was 6 xg for complete recovery. 

Drizo A variation of the glycol process for removing water vapor from natural gas, in which the water is removed from the glycol by stripping with a hydrocarbon solvent, typically a mixture of pentanes and heavier aliphatic hydrocarbons. The process also removes aromatic hydrocarbons. Last traces of water are removed from the triethylene glycol by stripping with toluene in a separate, closed loop. Invented in 1966 by J. C. Arnold, R. L. Pearce, and H. G. Scholten at the Dow Chemical Company. Twenty units were operating in 1990. U.S. Patent 3,349,544. 

Horvath et al. sintered the contents of a capillary column packed with 6 pm oc-tadecylsilica by heating to 360 °C in the presence of a sodium bicarbonate solution [101]. These conditions also strip the alkyl ligands from the silica support, thus significantly deteriorating the chromatographic properties. However, the performance was partly recovered after resilanization of the monolithic material with dimethyloctadecylchlorosilane allowing the separation of aromatic hydrocarbons and protected aminoacids with an efficiency of up to 160,000 plates/m. 

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