continuous centrifuge which separates crystals from liquid. The liquid filtrate, which is
fairly high in paraxylene content, is recycled to the first stage. The crystals are washed
with toluene in the centrifuge, discharged from the centrifuge, and melted.
The paraxylene stream is then fed to a final distillation step to yield high-purity paraxylene.
Crystallization has often been misunderstood or mischaracterized as no longer being
suited for paraxylene recovery. Process units designed with modern, reliable equipment
and operated in an efficient manner are quite competitive with the adsorption-based
processes. These improvements are especially evident in cases, where selective toluene
conversion processes are integrated with mixed xylenes-based routes. The world's
largest paraxylene producing company makes extensive use of multistage crystallization
in all of its units.
Technology Review of Crystallization-Based Paraxylene Complex
Lower investment cost
Relative efficiency in purifying the paraxylene
Tolerant of modest quantities of low and high boiling
hydrocarbons in the feed
Insensitive to most common feed impurities
Convenient to add incremental capacity with parallel units
Lower recovery of paraxylene requires higher recycle
High usage of mechanical moving equipment
Performance Parameters, Crystallization-Based System
Xylenes feedstock required
2000 USGC ISBL
No toluene or C
conversion unit; EB-dealkylation type of isom. catalyst
Taking advantage of the efficiency of the new toluene conversion processes and a low-
cost paraxylene recovery method, the lowest-cost configuration for producing paraxylene
using toluene and mixed xylenes feedstock is shown in Figure 5.