8
PX Crystallization
·
The capital cost of the crystallization unit is much less than that for an adsorption unit.
·
The xylene column size is much smaller compared to one used in an adsorption-based
complex, due to the lower fractionation severity required (higher tolerance to C
9
+
material in the crystallization unit).
·
A portion of the equilibrium xylene mixture from the isomerization unit can be sent
directly to the crystallization unit by withdrawing it as a sidecut from the
de-heptanizer column. This results in a notable reduction in the utility requirements for
feed preparation, as this material bypasses the xylene column.
·
Project schedules encompassing all activities can be compressed, because of more
conventional equipment that does not require long lead times.
Toluene Alkylation
·
All of the reformate toluene is directed to a toluene alkylation unit that uses low-cost
methanol feedstock to produce a paraxylene-enriched stream. The xylene fraction can be
directed separately to the second-stage of the CrystPX unit thereby recovering the
paraxylene more efficiently.
·
The aromatics complex can be designed for the lowest benzene/paraxylene ratio because
of negligible benzene production in the GT-TolAlk unit.
Heavy Aromatics Transalkylation
·
Mixed xylenes feed requirements can be reduced by more than 50% and replaced with
toluene and C9+ feedstock.
·
The initial investment for the aromatics complex can be reduced because of lower
feedstock requirements.
·
For producers with limited catalytic reforming capacity or those considering adding new
facilities, this scheme represents the maximum utilization of the BTX molecules in the
reformate/naphtha feedstock to produce paraxylene.
Flexibility
§
This integrated technology gives the greatest flexibility to optimize the disposition of the
aromatic isomers, according to the variation in the price level.
§
In an expansion case, it is easy to add recovery modules for either low-purity or high-
purity feedstock to go directly into the crystallizer. Expansion opportunities with an
adsorption-based complex are more limited.
Table 6 below shows how the cash cost of production of paraxylene and the investment cost
are reduced by using a combined toluene plus xylenes feed source.
