Aromatic concentrates, separation

So, Sulfolane and Carom, ca 1997, are two current rival processes. Sulfolane has a slight advantage over Carom ia energy consumption, while Carom has 6—8% less capital for the same capacity Sulfolane unit. In 1995, Exxon (37) commercialized the most recent technology for aromatics recovery when it used copolymer hoUow-fiber membrane ia concentration-driven processes, pervaporation and perstraction, for aromatic—paraffin separation. Once the non aromatic paraffins and cycloparaffins are removed, fractionation to separate the C to C aromatics is relatively simple. 

The benzene leaves the olefins plant fractionator mixed with the other gasoline components so it is handled the same way as a refinery stream. An aromatics concentrate is made and run through one of the two separation processes you just read about, solvent extraction or extractive distillation. 

Separation of toluene from the other components can be by solvent extraction or extractive distillation, just as described in the benzene chapter. The boiling points of benzene and toluene are far enough apart that the feed to separation unit of choice can be split (fractionated) rather easily into benzene concentrate and a toluene concentrate. Alternatively, the separation unit can be thought of as aromatics recovery unit. Then an aromatics concentrate stream is fed to the solvent extraction unit, and, the aromatics outturn can be split into benzene and toluene streams by fractionation. Both schemes are popular. 

Extractive distillation, that is, fractional distillation in the presence of a solvent, is used to recover aromatic hydrocarbons from, say, reformate fractions in the following manner. By means of preliminary distillation in a 65-tray prefractionator, a fraction containing a single aromatic can be separated from the reformate, and this aromatic concentrate is then pumped to an extraction distillation tower near the top, and aromatic concentrate enters near the bottom. A reboiler in the extractive distillation tower induces the aromatic concentrate to ascend the tower, where it contacts the descending solvent. 

Trace analysis by IR spectroscopy, involving pre-concentration, separation, and computer techniques has been reported by Hannah et al. (1978). The term trace analysis is used to refer to concentrations in the ppm range up to 1%. In instances where interference is at its minimum, analysis may be performed in a straightforward manner by using difference techniques. However, there are many cases in which analysis may be complicated by the fact that the trace material is structurally similar to the matrix material. Moreover, the presence of other trace compounds is intolerable if their spectra interfere with that of the compound under investigation. In this case it is often necessary to use pre-concentration or separation techniques. This method is illustrated by analyses of aromatic isomers, gasoline additives, drugs, and polymer additives. The different aspects of trace analyses by infrared spectroscopy are discussed by Smith (1986). 

The aromatic hydrocarbon content of diesel fuel affects the cetane number and exhaust emissions. One test method (ASTM D-5186) is applicable to diesel fuel and is unaffected by fuel coloration. Aromatics concentration in the range 1-75 mass% and polynuclear aromatic hydrocarbons in the range 0.5-50 mass% can be determined by this test method. In the method, a small aliquot of the fuel sample is injected onto a packed silica adsorption column and eluted with supercritical carbon dioxide mobile phase. Mono- and polynuclear aromatics in the sample are separated from nonaromatics and detected with a flame ionization detector. The detector response to hydrocarbons is recorded throughout the analysis time. The chromatographic areas corresponding to the mononuclear aromatic constituents, polynuclear aromatic constituents, and nonaromatic constituents are determined, and the mass-percent content of each of these groups is calculated by area normalization. 

Xylenes. Mixed xylenes are generally obtained by the catalytic reforming of petroleum fractions (see Section 6.2.1.7, above) or by the recovery of the Cg fraction from an aromatic concentrate (pyrolysis gasoline) stream. An increasingly important source of mixed xylenes is from the disproportionation of toluene. The separation of mixed xylenes into para, meta, and ortho isomers can be accomplished by several methods (e.g., fractional crystallization). 

This technique is similar with extractive distillation, but the solvent is an ionic salt. For example, acetone distillates easier from a solution with methanol in the presence of a concentrated solution of calcium chloride. The salt can be also a complex organic molecule. An example is the use of sulfonates of alkyl-aromatics to separate more heavy organic molecules, as phenols. 

For the refiner, the reduction in benzene concentration to 3% is not a major problem it is achieved by adjusting the initial point of the feed to the catalytic reformers and thereby limiting the amount of benzene precursors such as cyclohexane and Cg paraffins. Further than 3% benzene, the constraints become very severe and can even imply using specific processes alkylation of benzene to substituted aromatics, separation, etc. 

SULPHONATION OF AROMATIC AMINES If aniline is treated with excess of concentrated sulphuric acid and the resulting mixture, which contains aniline sulphate, is heated at 180° until a test portion when mixed with sodium hydroxide solution no longer liberates aniline, p-aminobenzenesulphonic acid or sulphanilic acid is formed this separates as the dihydrate upon pouring the cooled mixture into water. The reaction prohahly proceeds as follows ... 

Many spices are processed (2) to produce essential oils, oleoresins, essences, tinctures, extracts, resinoids, etc. These processes separate nonflavor components and further concentrate the aromatic or pungent principles of the spices. Such products allow a wider variety of uses and appHcations of the vital spice components. 

The carbon black (soot) produced in the partial combustion and electrical discharge processes is of rather small particle si2e and contains substantial amounts of higher (mostly aromatic) hydrocarbons which may render it hydrophobic, sticky, and difficult to remove by filtration. Electrostatic units, combined with water scmbbers, moving coke beds, and bag filters, are used for the removal of soot. The recovery is illustrated by the BASF separation and purification system (23). The bulk of the carbon in the reactor effluent is removed by a water scmbber (quencher). Residual carbon clean-up is by electrostatic filtering in the case of methane feedstock, and by coke particles if the feed is naphtha. Carbon in the quench water is concentrated by flotation, then burned. 

Highly pure / -hexane can be produced by adsorption on molecular sieves (qv) (see Adsorption, liquid separation) (43). The pores admit normal paraffins but exclude isoparaffins, cycloparaffins, and aromatics. The normal paraffins are recovered by changing the temperature and/or pressure of the system or by elution with a Hquid that can be easily separated from / -hexane by distillation. Other than ben2ene, commercial hexanes also may contain small concentrations of olefins (qv) and compounds of sulfur, oxygen, and chlorine. These compounds caimot be tolerated in some chemical and solvent appHcations. In such cases, the commercial hexanes must be purified by hydrogenation. 

Xylene Isomeri tion. The objective of C-8-aromatics processing is the conversion of the usual four-component feedstream (ethylbenzene and the three xylenes) into an isomerically pure xylene. Although the bulk of current demand is for xylene isomer for polyester fiber manufacture, significant markets for the other isomers exist. The primary problem is separation of the 8—40% ethylbenzene that is present in the usual feedstocks, a task that is compHcated by the closeness of the boiling points of ethylbenzene and -xylene. In addition, the equiUbrium concentrations of the xylenes present in the isomer separation train raffinate have to be reestabUshed to maximize the yield of the desired isomer. 

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