Melt crystallization xylene isomers

Separation of a chemical species from a mixture of similar compounds can also be achieved by melt crystallization, which is, for example, an important means of separatingpara- s.yXen.e (p-xylene) from the ortho and meta isomers. -Xylene is crystallized at the top of a vertical column and crystals are moved downward countercurrentiy to Hquid. The Hquid flowing upward is generated by adding heat to melt the crystals at the bottom of the column a portion of the melt is removed as product and the remainder flows up the column to contact the downward-flowing crystals. Effluent mother Hquor, consisting almost entirely of the ortho and meta isomers of xylene, is removed from the top of the column. 

One of the early column crystallizers was that iatroduced for the separation of xylene isomers (see Xylene and Ethylbenzene). In this unit, shown schematically ia Eigure 25, -xylene crystals are formed ia a scraped-surface chiller above the column and fed to the column. The crystals move downward counter-currenfly to impure Hquid ia the upper portion of the column and melted -xylene ia the lower part of the column. Impure Hquor is withdrawn from an appropriate poiat near the top of the column of crystals while pure product, xylene, is removed from the bottom of the column. The pulse unit drives melt up the column as reflux and iato a product receiver. 

Column crystallizers of the end-fed type can be used for purification of many eutectic-type systems and for aqueous as well as organic systems (McKay, loc. cit.). Column crystallizers have been used for xylene isomer separation, but recently other separation technologies including more efficient melt crystallization equipment have tended to supplant the Phillips style crystallizer. 

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 nitration of m-xylene produces a mixture of isomers of nitroxylene, which consists of 80 to 85% 2,4-dimethylnitrobenzene and 15 to 20% 2,6-dimethylnitro-benzene the proportion of 3,5-dimethylnitrobenzene is minor. Separation of this mixture of isomers is effected by distillation and melt crystallization. The corresponding xylidines are produced by reduction of the nitro compounds with Raney nickel. 

The physical properties of substituted benzenes resemble those of alkanes and alkenes of similar shape and molecular weight. Table 13.1 collects some physical properties for a number of common substituted benzenes. Notice the effects of symmetry. -Xylene melts at a much higher temperature than the ortho or meta isomer, for example. Many para isomers have high melting points, and crystallization can sometimes be used as a means of separating the para regioisomer from the others. 

Crystallization is used to separate p-xylene from its C8 isomers. The melting point of p-xylene is 13 C which is significantly higher than the other xylenes or ethylbenzene. Therefore, p-xylene can be selectively crystallized by cooling a C8 mixture. 

---