The separation of paraffins is operated with 5A zeolite, because it exhibits good
normal/isoparaffins selectivity. The separation mechanism relies in the steric hindrance of isoparaffins and cyclic hydrocarbons to enter the zeolite porosity. Then
normal paraffins are preferentially adsorbed. The operating temperatures are selected
in order to find a good compromise between the prevention of condensation,
chemical degradation, and mass transfer phenomena.
4.6 Xylene Isomers Separation
Paraxylene is the one of three isomers of xylene presenting the highest commercial
interest due to its use as raw material in the polymerization of terephthalic acid. One
of the polyesters obtained is polyethylene terephthalate (PET) which is widely used
in the fabrication of plastic bottles or synthetic fibers for clothing, among others.
The paraxylene market has experienced high growth rates over the years, and it is
expected that this trend will continue in the following years driven by the Asian and
Middle East regions. Mixed xylenes (a mixture from the different isomers) are
generally obtained from catalytic reforming of naphtha and, to a lesser extent,
from pyrolysis gasoline and toluene disproportionation. Associated with the specifications of the polymerization reaction, paraxylene must be obtained at high purity.
In parallel, the market demand requires associated high yields and capacities. In this
way, the separation of paraxylene from the rest of the isomers becomes the key
operation of the aromatic complex. Typical values are 99.7% for the purity, 96% for
the recovery, and 800,000 ton/year for the capacity (even more) [25].
Due to the difficulty of the xylene separation associated with the very close
boiling temperatures of the different isomers, adsorption and crystallization technologies have been historically implemented. Although paraxylene exhibits a melting
temperature significantly different from those of the other isomers, the presence of a
eutectic mixture [26] avoids high recoveries of paraxylene at temperatures higher
than À53
C. Consequently a considerable energy penalty must be paid in order to
overcome the thermodynamic barrier. As a consequence, in the last years, adsorption
has progressively been consolidated as the main technology in paraxylene separation. The use of cationic exchanged X or Y zeolites associated with high-level
technology allows overcoming the limitation associated with the crystallization
technique. In order to meet the market demands, liquid-phase countercurrent separation is implemented. The advantage of countercurrent configuration arises from the
maximization of the mass transfer driving forces. In this way, the adsorbent potential
is better exploited than in a cyclic process. The main difficulties of the technology lie
in appropriately handling the circulation of both adsorbent and fluid phases. Regarding the first, considering a real circulation is almost unfeasible because of attrition
problems (among others). Thus, as will be later introduced, a simulated circulation
implemented by means of a valve manifold arrangement is systematically adopted.
Concerning the more evident fluid circulation, the main challenge is minimizing the
fluid dispersion in order to keep the theoretical plate number of the column intact.
210
J. Pérez-Pellitero and G. D. Pirngruber
normal/isoparaffins selectivity. The separation mechanism relies in the steric hindrance of isoparaffins and cyclic hydrocarbons to enter the zeolite porosity. Then
normal paraffins are preferentially adsorbed. The operating temperatures are selected
in order to find a good compromise between the prevention of condensation,
chemical degradation, and mass transfer phenomena.
4.6 Xylene Isomers Separation
Paraxylene is the one of three isomers of xylene presenting the highest commercial
interest due to its use as raw material in the polymerization of terephthalic acid. One
of the polyesters obtained is polyethylene terephthalate (PET) which is widely used
in the fabrication of plastic bottles or synthetic fibers for clothing, among others.
The paraxylene market has experienced high growth rates over the years, and it is
expected that this trend will continue in the following years driven by the Asian and
Middle East regions. Mixed xylenes (a mixture from the different isomers) are
generally obtained from catalytic reforming of naphtha and, to a lesser extent,
from pyrolysis gasoline and toluene disproportionation. Associated with the specifications of the polymerization reaction, paraxylene must be obtained at high purity.
In parallel, the market demand requires associated high yields and capacities. In this
way, the separation of paraxylene from the rest of the isomers becomes the key
operation of the aromatic complex. Typical values are 99.7% for the purity, 96% for
the recovery, and 800,000 ton/year for the capacity (even more) [25].
Due to the difficulty of the xylene separation associated with the very close
boiling temperatures of the different isomers, adsorption and crystallization technologies have been historically implemented. Although paraxylene exhibits a melting
temperature significantly different from those of the other isomers, the presence of a
eutectic mixture [26] avoids high recoveries of paraxylene at temperatures higher
than À53
C. Consequently a considerable energy penalty must be paid in order to
overcome the thermodynamic barrier. As a consequence, in the last years, adsorption
has progressively been consolidated as the main technology in paraxylene separation. The use of cationic exchanged X or Y zeolites associated with high-level
technology allows overcoming the limitation associated with the crystallization
technique. In order to meet the market demands, liquid-phase countercurrent separation is implemented. The advantage of countercurrent configuration arises from the
maximization of the mass transfer driving forces. In this way, the adsorbent potential
is better exploited than in a cyclic process. The main difficulties of the technology lie
in appropriately handling the circulation of both adsorbent and fluid phases. Regarding the first, considering a real circulation is almost unfeasible because of attrition
problems (among others). Thus, as will be later introduced, a simulated circulation
implemented by means of a valve manifold arrangement is systematically adopted.
Concerning the more evident fluid circulation, the main challenge is minimizing the
fluid dispersion in order to keep the theoretical plate number of the column intact.
210
J. Pérez-Pellitero and G. D. Pirngruber