Xylenes by crystallization

Maruzen (2) A process for purifying p-xylene by crystallization, using ethylene as the direct coolant. Developed by Maruzen Gas Oil Company, United States. Now probably superseded by the Parex (1) process. 

PROABD MSC [melt static crystallization] A process for purifying /r-xylene by crystallization. Used in conjunction with MSTDP. Piloted in France from 1994 to 1996 and proposed for installation in India in 1997 and in Bulgaria in 1998. 

Figure 16.12. Humble two-stage process for recovery of p-xylene by crystallization. Yield is 82.5% of theoretical. ML = mother liquor, PX =p-xylene Haines, Powers and Bennett, 1955).

The best approach to improving separations is to work toward reactions that achieve 100% yields at 100% conversions. Frequently, this will require more selective catalysts. The previous chapter contained an example moving in this direction. Toluene was disproportionated to benzene and xylenes using a silica-modified zeolite catalyst.23 After removal of benzene and unchanged toluene by distillation, the xylene remaining was a 99% para-isomer. It was clean enough to put directly into the process of oxidation to terephthalic acid. This avoided the usual separation of xylenes by crystallization or by a molecular sieve. There are times when an equilibrium can be shifted by removal of a product or by-product continuously to give 100% conversion. The familiar esterification with azeotropic removal of water or removal of water with a molecular sieve is an example. 

Fig. 4.14. Separation of p-xylene by crystallization. ARCO two-step process.

In a conventional scheme comprising the separation of ethylbenzene and o-xylene by distillation, and.of a large fraction of p-xylene by crystallization or nearly completely by adsorption, a mother liquor with a high m-xylene content remains after these operations. It can be upgraded as a solvent or employed in high octane gasolines. Depending on market requirements, however, this Ca cut can be used to boost the production of o-xylene and p-xylene by catalytic isomerization. 

This technique, whose industrial development is recent, serves to increase the availability of benzene and of mixed xylenes, at the expense of toluene. Combined with the separation of p-xylene by crystallization or adsorption, or with isomerization, it can be used to produce additional quantities of o- and p-xylenes without increasing the reformate tonnage to be treated. 

In a somewhat similar manner i-xylene can be separated from a mixture of m- and /i-xylene this binary system forms a eutectic. Carbon tetrachloride produces an equimolecular solid compound with /i-xylene, but not with 0- or m-xylene. Egan and Luthy (1955) reported on a plant for the production of pure -xylene by crystallization meta-para- xylene mixtures in the presence of carbon tetra-chloride. Up to 90 per cent of the para- isomer was recovered by distillation after splitting the separated solid complex. The meta- isomer was recovered by fractionally crystallizing the CCU-free mother liquor. Perfect separation of /i-xylene is not possible, because the ternary system CCU/m-xylene/CCU -xylene forms a eutectic, but fortunately the concentration of the complex CCI4 /i-xylene in this eutectic is very low. Several commercial clathration processes for the separation of m-xylene from Cg petroleum reformate fractions using a variety of complexing agents have been operated (Sherwood, 1965). 

The crystallization point of p-xylene is markedly higher than that of the other Cg-aromatics it is therefore possible to separate p-xylene by crystallization. Since p-xylene can be better adsorbed than the accompanying Cg-aromatics, adsorption can also be used in its recovery. 

The Chevron process has proved particularly effective in producing p-xylene by crystallization. Crystallization is carried out by direct cooling with CO2. Distilla-... 

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. 

Raw Materials. Eor the first decade of PET manufacture, only DMT could be made sufficiently pure to produce high molecular weight PET. DMT is made by the catalytic air oxidation of -xylene to cmde TA, esterification with methanol, and purification by crystallization and distillation. After about 1965, processes to purify cmde TA by hydrogenation and crystallization became commercial (52) (see Phthalic ACID AND OTHER... 

Large-scale recovery of light oil was commercialized in England, Germany, and the United States toward the end of the nineteenth century (151). Industrial coal-tar production dates from the earliest operation of coal-gas faciUties. The principal bulk commodities derived from coal tar are wood-preserving oils, road tars, industrial pitches, and coke. Naphthalene is obtained from tar oils by crystallization, tar acids are derived by extraction of tar oils with caustic, and tar bases by extraction with sulfuric acid. Coal tars generally contain less than 1% benzene and toluene, and may contain up to 1% xylene. The total U.S. production of BTX from coke-oven operations is insignificant compared to petroleum product consumptions. 

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. 

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. 

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

Production of p-xylene via p-xylene removal, i.e., by crystallization or adsorption, and re-equilibration of the para-depleted stream requires recycle operation. Ethylbenzene in the feed must therefore be converted to lower or higher boiling products during the xylene isomerization step, otherwise it would build up in the recycle stream. With dual-functional catalysts, ethylbenzene is converted partly to xylenes and is partly hydrocracked. With mono-functional acid ZSM-5, ethylbenzene is converted at low temperature via transalkylation, and at higher temperature via transalkylation and dealkylation. In both cases, benzene of nitration grade purity is produced as a valuable by-product. 

Likewise, the value for hexane cracking at 538°c, the a-value, is proportional to the intrinsic value for toluene disproportionation and xylene isomerization, and is much easier to obtain since it is not affected by crystal size or diffusive alteration (5 ). 

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]. 

By means of In-situ observation of crystal growth under very high pressure, a defect healing process was studied with p-xylene crystals In p-, m-xylene mixture. Crystals, partially melted by decrease In pressure, were repressurized to grow and to heal their melted surfaces. It was found that the half-melted crystals rapidly grow and recover their rectangular growth shape within a few minutes. 

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 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. 

Xylene [106-42-3] can be purified by crystallization or adsorption. When a typical reformate-derived Cg aromatic mixture is cooled, y>-xylene crystallizes first. Most plants employing crystallization operate at —60 to —75° C, depending on feed composition (37). The process is limited by a eutectic temperature below which o- or / -xylene also crystallize. The solubility ofy>-xylene in the remaining Cg aromatic mixture over the range of —60 to —75°C is 9.6 to 6.2%. 

When naphtha or naphthenic gasoline fractions are catalytically reformed, they usually yield a Cx aromatics stream that is comprised of mixed xylenes and ethylbenzene. It is possible to separate the ethylbenzene and o-xylene by fractionation. It is uneconomic to separate the m- and p-xylenes in this manner because of the closeness of their boiling points. To accomplish the separation, a Werner-type complex for selective absoiption of p-xylene from the feed mixture may be used. Or, because of the widely different freezing points of the two xylene isomers, a process of fractional crystallization may be used. To boost the p-xylene yield, die filtrate from the crystallization step can be catalytically isomerized. 

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... 

---