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High feedstock flexibility
With the ability to vary the toluene to heavy aromatic ratio in the feed, the TransPlussm (heavy aromatics transalkylation) process offers a high degree of flexibility and profit-maximizing potential. TransPlus can effectively coprocess a high level of C9+ aromatics with either toluene or benzene. Fresh feed composition can be varied from 100% toluene to 100% C9+ aromatics. This gives refiners increased latitude to optimize the disposition of the C9+ aromatics stream by enabling incremental production of mixed xylenes while reducing the heavy aromatics content in gasoline in order to meet reformulated fuel requirements or distillation specifications.
Longer cycles and enhanced product selectivity
Unlike transalkylation processes that have relatively strict feed specifications, TransPlus utilizes a unique catalyst that has a higher tolerance to the heavy aromatic impurities that cause catalyst aging. As a result cycle lengths of several years have been attained using the fully regenerable catalyst.
In addition, the higher selectivity of the catalyst allows for flexibility to vary the fraction of the C9+ aromatics in the feed, giving operators the option to change the benzene/xylene product ratio to meet market conditions or product demand. Refiners can also adjust feed composition based on the pricing differential between toluene and C9+ aromatics.
TransPlus has been a solid performer since 1997, when an existing transalkylation unit at CPC’s Lin-Yuan petrochemical plant was converted to TransPlus. As a result, CPC realized a 30% increase in xylene yield and a 20% reduction in utility consumption.
Capital savings: retrofit or grassroots
The higher weight hourly space velocity (WHSV), lower hydrogen-to-hydrocarbon mole ratio and higher conversion of C9+ aromatics, relative to competitor processes, make it possible to readily retrofit the TransPlus process into existing units that have limited reactor volume or compressor capacity. This can result in lower-cost debottlenecking alternatives. These characteristics can also lower the capital investment for grassroots installations.
Other potential benefits include:
Simple fixed-bed process
The process flow (shown) is typical for a vapor-phase reaction in a fixed-bed reactor. Toluene feed and C9+ aromatics, combined with a hydrogen-rich recycle gas, is preheated and passed through a catalyst bed. In the catalyst bed, de-ethylation and disproportionation/transalkylation occur at moderate temperature and pressure to produce equilibrium-mixed xylenes and benzene and/or toluene, depending on the feedstock. By-product yields are small.
Reactor effluent is cooled by heat exchange and the liquid products are separated from the recycle gas. The separated liquid is stripped to remove the light ends and then fractionated to recover the benzene product and the mixed xylenes.