Naphthalene purification, crystallization

Since crystallization is a purely physical separation process, neither solid waste nor wastewater is produced. The comparatively small flows of waste air due only to changes in the contents of the plant are taken to the central waste-gas combustion plant. All impurities in the crude naphthalene are concentrated in the residual oil from stage 1 and can be economically utilized in further processing stages. This naphthalene purification process was installed in the Castrop-Rauxel works of VFT, and has produced > 60 000 t/a of pure naphthalene consistently and without problems for more than 10 years. 

In the Sulser-MWB process the naphthalene fractions produced by the crystallisation process are stored in tanks and fed alternately into the crystalliser. The crystalliser contains around 1100 cooling tubes of 25-mm diameter, through which the naphthalene fraction passes downward in turbulent flow and pardy crystallises out on the tube walls. The residual melt is recycled and pumped into a storage tank at the end of the crystallisation process. The crystals that have been deposited on the tube walls are then pardy melted for further purification. Following the removal of the drained Hquid, the purified naphthalene is melted. Four to six crystallisation stages are required to obtain refined naphthalene with a crystallisation point of 80°C, depending on the quaHty of the feedstock. The yield is typically between 88 and 94%, depending on the concentration of the feedstock fraction. 

Figure 20-8 shows the features of a horizontal center-fed column [Brodie, Aust. Mech. Chem. Eng. Trans., 37 (May 1979)] which has been commercialized for continuous purification of naphthalene and p-dichlorobenzene. Liquid feed enters the column between the hot purifying section and the cold freezing or recovery zone. Crystals are formed internally by indirect coohng of the melt through the walls of the refining and recoveiy zones. Residue liquid that has been depleted of product exits from the coldest section of the column. A spiral conveyor controls the transport of solids through the unit. 

Commercial tetralin contains naphthalene as the principal impurity and this interferes with the preparation of tetralin-1-hydroperoxide or with use of the hydrocarbon as hydrogen donor in hydrogen-transfer reactions. An early purification procedure is uninviting fractionation extraction in turn with mercury (to remove sulfur impurities), with mercuric acetate solution (to remove olefins), and with sulfuric acid fractionation. More recently Bass sulfonated the crude hydrocarbon with coned, sulfuric acid and added ammonium chloride to precipitate ammonium tetralin-6-sulfonate. The salt was crystallized until pure and hydrolyzed by steam distillation from sulfuric acid solution. Distillation from sodium gave material showing no ultraviolet bands characteristic of naphthalene. 

Among the preparative routes to the octaliii mixtures, the acid-catalyzed dehydration of 2-decaloP and the metal-amine reduction of naphthalene appear most satisfactory. Apart from the purification method described in this preparation, pure A -io-octalin has also been obtained by reaction of the octalin mixture with nitrosyl chloride. After separation of the adducts by fractional crystallization, the pure A - -octalin has been regenerated from its nitrosyl chloride adduct. ... 

Because of its more extended aromatic system naphthalene is expected to be more acidic than benzene, but the difference is hardly observed in synthetic experiments. Treatment of naphthalene with BuLi TMEDA in hexane results in the formation of comparable amounts of the 1- and 2-lithio compounds and a dilithio derivative (possibly 1,8-) so that this method is not interesting from a synthetic point of view. If the metallation is carried out with the ternary mixture BuLi f-BuOK, TMEDA in hexane at temperatures in the region of — 20 °C, subsequent addition of dimethyl disulfide affords a mixture of 1-methylthio-, 2-methylthio- and l,8-bis(methyl-thio)naphthalene. The mono- and disubstitution products can be separated by distillation. The favourable ratio of about 15 85, together with the fact that the 2-isomer is solid at room temperature (whereas the 1-isomer is a liquid), permits an easy purification of the predominant product by crystallization. For practical reasons (difficult separation from the products) the use of an excess of naphthalene is avoided (compare the metallation of benzene, Exp. 3), and consequently yields (based on BuLi) are not optimal, since part of the base may react with TMEDA. 

Naphthalenesulfonic acid 106e 145 Naphthalene (100 g) is melted in a flask fitted with a thermometer, dropping funnel, and stirrer, and to it is added concentrated sulfuric acid (160 g) dropwise during 15 min, the temperature being kept at 160°. Stirring is continued for a further 5 min at that temperature, then the solution is poured into cold water (120 ml), and the crystals that separate are collected and pressed on porous plate. For purification, portions (10 g) of the crude product are dissolved in water (5 ml), filtered hot, and treated hot (above 70°) with concentrated hydrochloric acid (about 2 ml). These processes yield about 160 g of the sulfonic acid trihydrate it may be dried on porous plate or placed in a desiccator above sodium hydroxide. 

Basically the same idea was used by Brodie (1971) to create the Brodie Purifier of Union Carbide Corp. (GB-PS 1968), which features a horizontally scraped surface crystallizer with a vertical purification section. The performance of the apparatus depends strongly on the size of the crystals entering the purification section. Walas (1988) reports that as much as 24h residence time is needed to create the correct crystal size. He also presents data about p-dichlorobenzene purification. Mullin (1988) reports that in addition to the p-dichlorobenzene, large-scale production has also been installed for naphthalene. Figure 7.20 shows a diagram of the Purifier. 

Production of Pure Naphthalene without Residues—Replacement of Chemical Purification by Optimized Multiple Crystallization (Example from VFT)... 

Under argon, 269 mg 3-benzyloxy-l,6,8-trimethoxy-naphthalene-2-carboxylic acid 3-bromo-4,5,7-trimethoxy-naphthalen-2-yl methyl ester was dissolved in 1.59 mL THF (0.25 M), from which residual water had been removed with n-butyllithium and o-phenanthroline), and cooled to 5 to -55°C. w-Butyllithium (203 fiL, 0.437 mmol) was then added dropwise. After the solution was stirred for 2 h, the reaction mixture was quenched by the addition of saturated aqueous NH4CI, allowed to warm to ambient temperature, and diluted with EtOAc. After the usual workup, the cmde product was purified by radial thin-layer chromatography (hexane/EtOAc, 9 1, 8 2, 7 3, and then 5 5) to give 130 mg l-(3-benzyloxy-l,6,8-trimethoxy-naphthalen-2-yl)-l-(3-hydroxymethyl-l,6,8-trimethoxy-naphthalen-2-yl)-methanone as a white solid, in a yield of 55%. Further purification was executed by crystallization from cyclohexane-dichloromethane, m.p., 220-224°C. 

Figure 8.57. Comparison of fractional and emulsion crystallization cycles for the purification of naphthalene. After Holed, 1965)...

The continuous Brodie crystallizer (Figure 9.7) consists of three horizontal, scraped-wall chillers, in cascade formation, and one vertical purification column. The crystals are produced in the upper part of the crystallizer (recovery section) they then pass through the middle enrichment zone (refining section) and arrive at the base of the apparatus as molten product. Brodie crystallization is applied for the purification of naphthalene, for example, by Nippon Steel Chemical, Tobata/ Japan. 

Crystallization is mainly used for separation as an alternative to distillation, if the involved compounds are thermally unstable (e.g., acrylic acid), have a low or practically no vapor pressure (like salts), if the boiling points are similar, or if the system forms an azeotrope. Crystallization is used for the production and purification of various organic chemicals ranging from bulk chemicals (p-xylene and naphthalene) to fine chemicals like pharmaceuticals (e.g., proteins). Further examples of industrial crystallization processes are sugar refining, salt production for the food industry, and silicon crystal wafer production. 

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