Xylene isomerization is a reaction of the type A $ B. In this case,
reaction cannot occur near the extract point if high purity is
required, otherwise the reverse reaction will pollute the product
and purity will always be below 99%. To overcome this situation,
reactors are inserted between the adsorption columns far from
the extract point [9,10]. However, since the minimum concentration
required in the extract for this new configuration is about
70 wt%, a much simpler configuration can be employed. Keeping
the catalyst and adsorbent mixed inside the columns, it may produce
a high enough p-xylene concentration stream to be further
processed by the crystallization unit. This approach involves simpler
operation and allows the direct contact between catalyst
and adsorbent resulting in more efficient p-xylene withdraw as it
is formed to overcome the thermodynamic equilibrium
constraints.
One of the most employed SMB based technologies for p-xylene
separation is UOP’s Parex. The studied aromatic complex uses this
technology consisting of 24 adsorbent beds with length and diameter
of 1.14 and 4.12 m respectively, p-diethylbenzene as desorbent,
particle diameter of 0.062 cm, and a time switch of
1.15 min [3]. The SMBR unit will keep the geometric characteristics
of the Parex unit, i.e. 24 adsorbent beds, with the possibility to
modify the location of inlets and outlets in order to use the appropriate
number of columns in each zone since column configuration
plays an important role when dealing with different product concentrations
[11]. Moreover, p-diethylbenzene cannot be used since
it isomerizes into o-diethylbenzene and m-diethylbenzene over
acid catalysts; toluene, which has been used in the industry, is
used as desorbent [10].