Processes are described for separating 3,4- and 4,4-dimethylbiphenyl from a mixture comprising at least 3,3-, 3,4- and 4,4-dimethylbiphenyl. In the processes, the mixture is cooled to produce a crystallization product comprising at least of the 4,4-dimethylbiphenyl from the feed mixture and a first mother liquor product. The first mother liquor product is distilled to produce a bottoms stream enriched in 4,4-dimethylbiphenyl as compared with the first mother liquor product and an overhead stream deficient in 4,4-dimethylbiphenyl as compared with the first mother liquor product. The overhead stream is then cooled to produce a second crystallization product comprising at least part of the 3,4-dimethylbiphenyl from the overhead stream and a second mother liquor product.

Disclosed is a process for producing mixed xylenes and C.sub.9+ hydrocarbons in which an aromatic hydrocarbon feedstock comprising benzene and/or toluene is contacted with an alkylating agent comprising methanol and/or dimethyl ether under alkylation conditions in the presence of an alkylation catalyst to produce an alkylated aromatic product stream comprising the mixed xylenes and C.sub.9+ hydrocarbons. The mixed xylenes are subsequently converted to para-xylene, and the C.sub.9+ hydrocarbons and its components may be supplied as motor fuels blending components. The alkylation catalyst comprises a molecular sieve having a Constraint Index in the range from greater than zero up to about 3. The molar ratio of aromatic hydrocarbon to alkylating agent is in the range of greater than 1:1 to less than 4:1.

Disclosed is a process for producing mixed xylenes and C.sub.9+ hydrocarbons in which an aromatic hydrocarbon feedstock comprising benzene and/or toluene is contacted with an alkylating agent comprising methanol and/or dimethyl ether under alkylation conditions in the presence of an alkylation catalyst to produce an alkylated aromatic product stream comprising the mixed xylenes and C.sub.9+ hydrocarbons. The mixed xylenes are subsequently converted to para-xylene, and the C.sub.9+ hydrocarbons and its components may be supplied as motor fuels blending components. The alkylation catalyst comprises a molecular sieve having a Constraint Index in the range from greater than zero up to about 3. The molar ratio of aromatic hydrocarbon to alkylating agent is in the range of greater than 1:1 to less than 4:1.

Increased paraxylene production through the use of a split feed reforming process, wherein hydrotreated naphtha is split into light, middle and heavy fractions. Each fraction is reformed separately to generate streams containing aromatic compounds. These streams can further be processed and can undergo dealkylation, transalkylation, disproportionation, isomerization, and separation steps to maximize paraxylene production. In addition, some streams are recycled or recombined in order to maximize paraxylene production.

Increased paraxylene production through the use of a split feed reforming process, wherein hydrotreated naphtha is split into light, middle and heavy fractions. Each fraction is reformed separately to generate streams containing aromatic compounds. These streams can further be processed and can undergo dealkylation, transalkylation, disproportionation, isomerization, and separation steps to maximize paraxylene production. In addition, some streams are recycled or recombined in order to maximize paraxylene production.

A process and related system for producing para-xylene (PX). In an embodiment, the process includes (a) separating a feed stream comprising C.sub.6+ aromatic hydrocarbons into a toluene containing stream and a C.sub.8+ hydrocarbon containing stream and (b) contacting at least part of the toluene containing stream with a methylating agent in a methylation unit to convert toluene to xylenes and produce a methylated effluent stream. In addition, the process includes (c) recovering PX from the methylated effluent stream in (b) to produce a PX depleted stream and (d) transalkylating the PX depleted stream to produce a transalkylation effluent stream. The transalkylation effluent stream includes a higher concentration of toluene than the PX depleted stream. Further, the process includes (e) converting at least some ethylbenzene (EB) within the C.sub.8+ hydrocarbon containing stream into toluene and (f) flowing the toluene converted in (e) to the contacting in (b).

A process and related system for producing para-xylene (PX). In an embodiment, the process includes (a) separating a feed stream comprising C.sub.6+ aromatic hydrocarbons into a toluene containing stream and a C.sub.8+ hydrocarbon containing stream and (b) contacting at least part of the toluene containing stream with a methylating agent in a methylation unit to convert toluene to xylenes and produce a methylated effluent stream. In addition, the process includes (c) recovering PX from the methylated effluent stream in (b) to produce a PX depleted stream and (d) transalkylating the PX depleted stream to produce a transalkylation effluent stream. The transalkylation effluent stream includes a higher concentration of toluene than the PX depleted stream. Further, the process includes (e) converting at least some ethylbenzene (EB) within the C.sub.8+ hydrocarbon containing stream into toluene and (f) flowing the toluene converted in (e) to the contacting in (b).

Para-xylene is separated from a mixture of xylenes and ethylbenzene by a separation process. An ortho-selective adsorbent is used to reduce the ortho-xylene concentration of the xylenes, prior to contact of the xylenes and ethylbenzene with a para-selective adsorbent. The stream rich in ortho-xylene may be isomerized in the liquid phase to increase the amount of para-xylene therein. The para-xylene-depleted stream may be treated in the vapor phase to remove the ethylbenzene and then subjected to isomerization in the liquid phase to produce a stream having a higher than equilibrium amount of para-xylene.

Para-xylene is separated from a mixture of xylenes and ethylbenzene by a separation process. An ortho-selective adsorbent is used to reduce the ortho-xylene concentration of the xylenes, prior to contact of the xylenes and ethylbenzene with a para-selective adsorbent. The stream rich in ortho-xylene may be isomerized in the liquid phase to increase the amount of para-xylene therein. The para-xylene-depleted stream may be treated in the vapor phase to remove the ethylbenzene and then subjected to isomerization in the liquid phase to produce a stream having a higher than equilibrium amount of para-xylene.

A method for retrofitting a system for recovering paraxylene. The system is retrofitted with a pressure swing adsorption unit and a second isomerization reactor. The retrofit lowers the variable cost of the plant, while providing the opportunity to maintain existing equipment and furnace and refrigeration duty.