Energy conservation in heavy-hydrocarbon distillation

A process for producing one or more individual xylene isomers from feed streams containing C8 aromatics and C9-and-heavier aromatics, comprising:

• (a) a distillation process comprising two or more xylene columns separating C8-aromatics streams from a C9-and-heavier aromatics streams contained in at least one higher-boiling and at least one lower-boiling feed stream, wherein the at least one higher-boiling feed stream has a higher content of C9-and-heavier aromatics than the at least one lower-boiling feed stream, comprising distilling the at least one higher-boiling feed stream in at least one first low-pressure xylene column at a low pressure to separate a first C8-aromatics stream from a first C9-and-heavier aromatics stream, distilling the at least one lower-boiling feed stream in at least one second xylene column at an elevated pressure to separate a second C8-aromatics stream from a second C9-and-heavier aromatics stream, and condensing an overhead stream from the at least one second xylene column by exchanging heat with one or more of a reboiler of the at least one first low-pressure xylene column;
• (b) a xylene-isomer separation process to recover one or more xylene isomers from one or both of the first and second C8-aromatics streams by injecting the one or both aromatics streams and a desorbent stream into a moving-bed adsorption process to obtain a first mixture of the one or more xylene isomers and desorbent and a second mixture of non-equilibrium C8 aromatics and desorbent;
• and,
• (c) a desorbent-recovery process to separate the first mixture of step (b) by distillation in an extract column with a reboiler having an operating pressure of at least 300 kPa to obtain the one or more individual xylene isomers and a desorbent stream returned to the xylene isomer separation process, wherein the bottoms stream of the at least one second xylene column of step (a) further exchanges heat to supply a reboiler of a raffinate column.