(207a) High Purity p-Xylene Production Via m-Xylene Isomerization Over Pt-HZSM-5 Catalyst : Use of An Extractor-Type Catalytic Membrane Reactor Equipped with a Nanocomposite MFI-Alumina Membrane Tube as Separation Unit

Abstract

Energy efficiency
and energy saving are becoming increasingly important components of government
policies around the world in response to a range of challenges which include
perceptions of resource scarcity, high energy prices, security of energy supply
and environmental protection. One of the energy
saving potentials that has attracted research focus for about a decade now is
the application of extractor-type catalytic membrane reactors (e-CMR) for the
production of p-xylene from mixed xylene. P-xylene
is almost exclusively used as raw material in the production of terephthalic
acid (TPA) and dimethyl terephthalate (DMT), which are then reacted with
ethyleneglycol to form polyethylene terephthalate (PET), the raw material for
most polyester. For industrial application of p-xylene, ultrapure p-xylene
purity is essential in most cases. Advantages of e-CMR over conventional
fixed-bed reactor (FBR) for p-xylene production include increase in p-xylene
yield and m-xylene conversion due to selective extraction of p-xylene from the
reaction zone. This in turn forward-shifts the equilibrium position of the
reaction, allowing more m-xylene to react to produce more p-xylene.

 In view of
the interesting potentials of e-CMR, this study presents further
investigations on the development and evaluation of energy-efficient process
for p-xylene production based on the application of MFI-type zeolite membranes
in e-CMR. In this study
possibility of obtaining ultrapure p-xylene during m-xylene isomerization in an
e-CMR, which has a tubular nanocomposite MFI-alumina membrane as separation
unit, is demonstrated. Unlike "film-like" architectures, in nanocomposite
architectures zeolite crystals are embedded within the pores of the supports.
Also for better understanding of the fundamental behaviour of e-CMR at lower
reaction temperatures during xylene isomerization, the reaction was conducted
from 473 K to 573 K. Before isomerization test, the formation of a
nanocomposite material (namely no formation of continuous MFI film) on the support
was confirmed with scanning electron microscope (SEM) and further
characterization was done by room-temperature
H₂ gas permeation test, room-temperature n-butane/H₂ binary
mixture separation test and xylene isomers separation tests. For m-xylene isomerization tests, the
inside of the membrane tube was packed with 2.18 g of Pt-HZSM-5 catalyst and
the reactor was fed with m-xylene saturated in N₂ gas. For
comparison, an equivalent fixed-bed reactor (FBR) was operated at same
conditions as the e-CMR.

The SEM micrograph shows the formation of
zeolite crystals within the three layers of the support and the
room-temperature H₂ permeance and room-temperature n-butane/H₂
separation factor of the membrane were 0.49 µmol.m^-2.s^-1.Pa^-1 and >100, respectively. Regarding the separation
of xylene isomers (p/m/o: 0.63 kPa/0.27 kPa/0.32 kPa), at 473 K the membrane
displayed a p-xylene permeance of 11.4 nmol.m^-2.s^-1.Pa^-1
with a p-xylene/o-xylene (p/o) separation factor >400. It is
noteworthy to mention that during the xylene isomers separation tests, neither
o-xylene nor m-xylene was detected in the permeate stream. Thus the membrane
could be considered "defect-free". Furthermore, during the separation test no
isomerization products were detected. This suggests that the membrane was inert
to xylene isomerization. At the same time it should be noted that the membrane
underwent several thermal cycles to conduct the separation tests reported.
Nevertheless, the membrane still kept a repeatable separation performance,
indicating a high thermal stability.

Regarding the
isomerization test, in the
permeate only mode (only products in the permeate stream were considered), the
p-xylene yield increased from 2.2% at 573 K reaching a maximum of about 2.7 %
at 473 K with p-xylene purity in the permeate approaching 100% at 473 K. In
combined mode (products in both permeate and retentate streams were
considered), a maximum p-xylene yield of 19.0 % was obtained at 473 K and for
an equivalent FBR operated at the same operating conditions as e-CMR, the
p-xylene yield was 15.8% at 473 K, indicating about 18% in p-xylene yield
compared to the e-CMR. Throughout the reaction temperatures investigated, the
membrane displayed 100% selectivity to p-xylene in permeate-only mode. The results showed
that it is possible to obtain ultra pure p-xylene in e-CMR, especially in the permeate side,
with both p-xylene purity and para-selectivity approaching 100%. This is the
first time p-xylene purity will be reported for e-CMR having nanocomposite
MFI-type membranes as separation unit. With these results, the
possibility of cutting down operational cost via a reduction in energy
consumption during the production of ultra-pure p-xylene could be feasible with
the application nanocomposite MFI-alumina membranes in e-CMR. However,
defect-free nanocomposite MFI-type zeolite membranes with high p-xylene flux
are necessary to make this technology attractive and competitive with existing
technologies.