Xylenes separation by crystallization

The mixture of xylidines has been used as a first component of azo-dyes. The chief constituent of the mixture is m-xylidine (4-amino-1,3-xylene). It can be separated by crystallization from glacial ethanoic acid. It is also used for the preparation of azo-dyes. 

Aromatic polyesters had been successfully synthesized from the reaction of ethylene glycol and various aromatic diacids but commercialization awaited a ready inexpensive source of aromatic diacides. An inexpensive process was discovered for the separation of the various xylene isomers by crystallization. The availability of inexpensive xylene isomers allowed the formation of terephthalic acid through the air oxidation of the p-xylene isomer. DuPont produced polyester fibers from melt spinning in 1953, but it was not until the 1970s that these fibers became commercially available. 

The homopolymer of DMP dissolves readily in methylene chloride but precipitates on standing as a crystalline polymer-CH2Cl2 complex, providing a method for distinguishing between block copolymers and mixtures of homopolymers. Random copolymers prepared by methods a and b form stable solutions in methylene chloride. Copolymers with a 1 1 ratio of DMP and DPP prepared by methods c and d also yield stable methylene chloride solutions. Since the NMR spectrum shows that the DMP portion of these materials is present as a block and the solubility in methylene chloride shows that DMP homopolymer is absent, these copolymers have the block structure. They can be separated by crystallization from m-xylene into an insoluble DPP-rich fraction and a soluble DMP-rich fraction, both fractions having the NMR spectra characteristic of block copolymers. A typical 1 1 copolymer prepared by adding DMP to growing DPP polymer yielded 35% of insoluble material... 

Reactive crystallization/precipitation plays a role in a number of industrially relevant processes, such as liquid-phase oxidation of para-xylene to produce technical-grade terephthalic acid, the acidic hydrolysis of sodium salicylate to salicylic acid, and the absorption of ammonia in aqueous sulfuric acid to form ammonium sulfate (135). Reactive crystalhzation/precipitation is also widely applied in the pharmaceutical industry, to facilitate the resolution of the enantiomers (diastereomeric crystallization). Here, the racemate is reacted with a specific optically active material (resolving agent) to produce two diastereomeric derivatives (usually salts) that are easily separated by crystallization ... 

CrystPX A process for separating />xylcne from xylene mixtures by crystallization. Only crystallizers and centrifuges are used. Developed by GTC Technology and offered by that company and Lyondell Chemical. 

Unique adsorption selectivities are employed in the separation of 0, aromatic isomers, a classical problem that cannot be easily solved by distillation, crystallization, or solvent extraction (10). Although xylene [106-42-3] can be separated by crystallization, its recovery is limited because of the formation of eutectic with / -xylene [108-58-3]. However, either jfr-xylene, -xylene, 0-xylene [95-47-6]y or ethylbenzene [100A1 -4] can be extracted selectively by suitable modification of Zeolitic adsorbents. 

The toluic aldehyde/HBF complex is then decomposed by heating between 130 and 180 C in the presence of a solvent (benzene). The BF3. HF and unconverted toluene are recovered and recycled. The o- and p-tohiic aldehydes are separated by crystallization. Purified p-toluic aldehyde is air-oxidized (in solution m acetic add) in the presence of manganese acetate, cobalt acetate and sodium bromide, by the technique employed for p-xylene. This takes place around 200°C, at 2.10 Pa absolute ... 

The oxidation process for m-xylene is based on the liquid-phase oxidation for the production of terephthalic acid. Air oxidation in acetic acid is catalysed by cobalt and manganese salts and bromine and conducted at temperatures ranging from 170 to 230 °C and pressures of 20 to 25 bar. Following the reaction, isophthalic acid is separated by crystallization and the mother liquor is recycled. 

By the methylation of commercial xylene and separation of the mesitylene from the mixture of hydrocarbons by crystallization of the sulfonate. Smith and Cass, J. Am. Chem. Soc. 54, 1603 (1932). 

Method A To a mixture of the 9-(hydroxymethyl)-9,10-dihydroacridine (4mmol) and sea sand (10g) under N2 in refluxing anhyd xylene (30 mL) was added in four portions during a 2-h period, P205 (4g, 28 mmol). The yellow-orange mixture was heated under reflux for an additional 1.5 h, then cooled, and quenched cautiously with a large excess of cold H20. The mixture was filtered to remove the sand, and the aqueous layer was then separated from the filtrate. The sand and the aqueous layer were extracted with hot benzene and the benzene and xylene solutions were combined, dried, and the solvent removed under reduced pressure. The crude product was obtained as a yellow-orange residue which was purified by crystallization (benzene). 

Manufacture The xylenes are obtained with benzene (and toluene) from the catalytic reforming of naphtha and separated from the aromatic mixture by distillation. From the mixed isomers, the ortho- can be obtained by distillation because its boiling point is sufficiently different. The meta- and para- are separated by either selective adsorption or by crystallization. 

When the mixture has cooled sometvhat, 75 to 100 cc. of absolute alcohol is added. After all the particles of sodium have reacted, 500 cc. of water is added, the condenser is arranged for downward distillation, and the benzene and unchanged />-chlorobiphenyl are removed by distillation from a steam bath (Note 7). The crude product remaining in the flask is separated by filtration, washed with 100-200 cc. of water, and pressed as dry as possible. The solid is dissolved in 600 cc. of xylene in a 1-1. distilling flask, and the solution is subjected to distillation until 25 to 50 cc. of distillate (xylene and water) has been collected. The solution is cooled somewhat, 1 to 2 g. Norite is added, and then the mixture is boiled for five minutes. The hot solution is filtered rapidly and the filtrate is cooled. The product is collected by filtration, washed with 25 to 50 cc. of cold xylene then with 200 cc. of petroleum ether, and dried. The tribiphenylcarbinol forms small white crystals which melt at 207-208°. The product weighs 57-65 g. (35-40 per cent of the theoretical amount) (Note 8). 

Krupp-Koppers (1) A process for separating / -xylene from its isomers by crystallization. In 1979, eight plants were operating. 

Can a mixture of benzene, toluene, and meta-xylene be separated by cryogenic crystallization Whats the usual (more economic) way ... 

Since the early 1970s p-xylene has grown to become a large volume petrochemical. It is used primarily for the production of polyester fibers, films and resins, such as PET (polyethylene terephthalate) [7]. Demand for p-xylene has increased tenfold since 1970 to about 26xl0 t/year. Almost all of this additional production has been by the UOP Parex process as shown in Figure 7.1. A baseline production ofp-xylene is maintained by crystallization based sites that existed before the SMB adsorptive separation technology was established [8]. 

Because of the large demand for p-xylene, another method is now being used by Amoco to increase the percentage of the para isomer in mixed xylenes. They are heated at 300°C with an acidic zeolite catalyst, which equilibrates the three xylenes to an o,m,p ratio of 10 72 18%. The para isomer is separated by fractional crystallization, whereas the o,m mixture is reisomerized with the catalyst to produce more para product. Theoretically, all the xylenes could be transformed into the desired para isomer. The zeolite catalyst has the following structure. 

The process to separate the four Cg aromatics by distillation is very difficult because of their closeness in boiling points (table 11.3), so that only o-xylene can be separated by distillation. / -Xylene has a unique high melting point, and can be separated by cryogenic crystallization, but this is an expensive process that requires refrigeration. What is desired is an economic separation process that singles out jo-xylene among these four compounds. 

The separation of p-xylene from mixed Cg aromatics can be achieved commercially by crystallizing and centrifuging at temperatures in the range of —50° to —150° F. 

The ultimate in xylene separation is claimed, however, by Hetzner (10), who first distills the mixture to remove o-xylene by taking m-p-xylene and ethylbenzene overhead in a column having about 35 to 60 theoretical plates. It is reported that concentrates containing up to 97% o-xylene have been produced by this process. The m-xylene, p-xylene, and ethylbenzene mixture is selectively sulfonated to remove m-xylene. In this operation, 2 moles of Sulfuric acid (96 to 98%) are added per mole of m-xylene in the mixture to be treated. After separation, the aqueous layer is hydrolyzed at 250° to 300° F. to recover a concentrate containing 90% or more m-xylene. The hydrocarbon layer is cooled to produce p-xylene crystals, which are separated by filtration or centrifugation. The 85 to 90% p-xylene concentrate is reprocessed to recover a final product containing 96% p-xylene. The mother liquor from the p-xylene crystallization contains impure ethylbenzene and is rejected from the system. 

Isomerization of xylenes is always coupled with separation processes. In most cases, p-xylene is removed from the reaction mixture by crystallization or selective adsorption. The recovered o-/m-xylene mixture, in turn, is usually recycled for reequilibration. Because of cost of separation, the highly selective carbonylation of toluene to p-tolualdehyde gained significance. Subsequent reduction gives p-xylene. 

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