3
Advances in equipment manufacture now allow the construction of crystallizers and
centrifuges that are larger and more reliable, and which can sustain extended periods of
operation without maintenance efforts. Crystallization units designed with this modern
equipment and operated in an efficient manner are indeed competitive with the adsorption-
based processes.
The world's largest paraxylene producing company makes extensive use of multi-stage
crystallization in all of its units, including those processing dilute paraxylene feedstocks. In
cases where the feedstock is enriched in paraxylene, crystallization methods are far superior
to absorption to produce the purified paraxylene.
2
GTC Technology Corporation offers a highly competitive crystallization technology,
CrystPX
sm
. The technology reduces the number of operating equipment pieces, and produces
very high purity product. Additionally the technology uses equipment, which is readily and
inexpensively procured.
Recent Advances in Paraxylene Crystallization
CrystPX technology is far different from the first generation units dating back to the
1960's, for the following reasons:
·
Equipment reliability has improved, so that maintenance requirements are consistent
with other equipment in the complex
·
Larger-scale equipment can be manufactured now; reducing the number of trains
and overall quantity of rotating equipment, and providing more reliable operation
·
Unit efficiency is improved due to updated configuration and processing scheme
The advantages and relative disadvantages of crystallization for use in PX recovery are
summarized below:
Advantages of Crystallization
Disadvantages of Crystallization
§
Lower installed cost due to the use of more
conventional equipment
§
High recovery of paraxylene requires low
temperature refrigerants
§
Equipment can be easily procured
§
Higher content of mechanical moving
equipment
§
Shorter construction schedule for revamps or
grassroots applications
§
Relative efficiency in purifying the
paraxylene
§
Tolerant of modest quantities of low and high
boiling hydrocarbons in the feed, so that the
feed preparation requirements are relaxed
§
Insensitive to most common feed impurities
§
Highest product purity
§
Convenient to add incremental capacity with parallel units
The flow scheme for the CrystPX paraxylene recovery process, based on a dilute mixed
xylene feed stream is shown in Figure 2 below. This scheme uses two levels of refrigerant to
achieve the highest PX recovery.
