Note: Descriptions are shown in the official language in which they were submitted.
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PROCESSES FOR THE ISOMERIZATION OF PARAFFINS OF 5 AND 6 CARBON
ATOMS WITH METHYLCYCLOPENTANE RECOVERY
BACKGROUND OF THE INVENTION
[0001] This invention relates to improved processes for the isomerization of
paraffins of 5
and 6 carbon atoms, e.g., to provide isomerate having enhanced Research Octane
Number
(RON) for blending into gasoline pools, and particularly to such processes
using a
deisohexanizer.
[0002] Processes for the isomerization of paraffins into more highly branched
paraffins are
widely practiced. Particularly important commercial isomerization processes
are used to increase
the branching, and thus the octane value of refinery streams containing
paraffins of 4 to 8,
especially 5 and 6, carbon atoms. The isomerate is typically blended with a
refinery reformer
effluent to provide a blended gasoline mixture having a desired research
octane number (RON).
[0003] The isomerization process proceeds toward a thermodynamic equilibrium.
Hence, the
isomerate will still contain normal paraffins that have low octane ratings and
thus detract from the
octane rating of the isomerate. Provided that adequate high octane blending
streams such as
alkylate and reformer effluent is available and that gasolines of lower octane
ratings, such as 85
and 87 RON, are in demand, the presence of these normal paraffins in the
isomerate has been
tolerated.
[0004] Where circumstances demand higher RON isomerates, the isomerization
processes
have been modified by separating the normal paraffins from the isomerate and
recycling them to
the isomerization reactor. Thus, not only are normal paraffins that detract
from the octane rating
removed from the isomerate but also their return to the isomerization reactor
increases the portion
of the feed converted to the more highly desired branched paraffins.
[0005] The major processes for the separation of the normal paraffins from the
isomerate are
the use of adsorptive separation such as disclosed in US 4,717,784 and
4,804,802, and
distillation. The most frequently practiced isomerization processes that
recycle normal paraffins
use a deisohexanizer. A deisohexanizer is one or more distillation columns
where an overhead
containing branched C6 paraffins such as dimethylbutanes (2,2-dimethylbuthane
and 2,3-
dimethylbutane) and lighter components is obtained as the isomerate product
for, e.g., blending
for gasolines, and a side-stream containing normal hexane and similarly
boiling components such
as methylpentanes (2-methylpentane and 3-methylpentane) and methylcyclopentane
is recycled to
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the isomerization reactor. The problem with a deisohexanizer is that the lower
boiling product
stream contains n-pentane which has a low RON value.
[0006] The use of adsorptive separation instead of a deisohexanizer enables n-
pentane to be
removed, thereby providing a high RON motor fuel, often in the range of 91 to
93.
[0007] Separation of linear from branched paraffins has also been proposed,
but membranes
have yet to find a practical, commercial application. US 5,069,794 discloses
microporous
membranes containing crystalline molecular sieve material. At column 8, lines
11 et seq.,
potential applications of the membranes are disclosed including the separation
of linear and
branched paraffins. See also, US 6,090,289, disclosing a layered composite
containing molecular
sieve that could be used as a membrane. Among the potential separations in
which the membrane
maybe used that are disclosed commencing at column 13, line 6, include the
separation of normal
paraffins from branched paraffins. US 6,156,950 and 6,338,791 discuss
permeation separation
techniques that may have application for the separation of normal paraffins
from branched
paraffins and describe certain separation schemes in connection with
isomerization. US
2003/0196931 discloses a two-stage isomerization process for up-grading
hydrocarbon feeds of 4
to 12 carbon atoms.
[0008] Recently, Bourney, et al., in WO 2005/049766 disclose a process for
producing high
octane gasoline using a membrane to remove, inter alia, n-pentane from an
isomerized stream
derived from the overhead of a deisohexanizer. A side cut from the
deisohexanizer is as a sweep
fluid on the permeate side of the membrane. The mixture of the permeate and
sweep fluid is
recycled to the isomerization reactor. In a computer simulation based upon the
use of an MFI on
alumina membrane, example 1 of the publication indicates that 5000 square
meters of membrane
surface area is required to remove 95 mass percent of n-pentane from the
overhead from a
deisohexanizer distillation column. At the flow rate of feed to the permeator
(75000 kg/hr. having
20.6 mass percent n-pentane), the flux of n-pentane used in the simulation
appears to be in the
order of 0.01 gram moles/m2-s at 300 C. The RON of the product with the n-
pentane removed is
said to be 91Ø
[0009] The use by Bourney, et al., of a side cut from the deisohexanizer as a
sweep fluid for
the membrane separation results in recycling valuable high octane compounds
such as
methylpentanes and methylcyclopentane back to the isomerization reactor. In
Table 1, Bourney,
et al., state that the concentration of methylcyclopentane is 9.7 mass
percent. No
methylcyclopentane is in the product stream. Additionally, the sweep stream
contains 4.5 mass
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percent 2,3-dimethylbutane which is also recycled to the isomerization
reactor. As the
isomerization reaction will distribute the isomers toward equilibrium, they
sacrifice per pass yield
of high RON fuel for RON.
[0010] The use of zeolite membranes is suggested as a suitable technique for
separating
linear molecules. See, for instance, paragraphs 0008 and 0032. US 6,818,333
discloses thin
zeolite membranes that are said to have a permeability of n-butane of at least
6.10"7 mol/in2=s=Pa
and a selectivity of at least 250 of n-butane to isobutane.
[0011] Changes in environmental and fuel efficiency regulations can have a
profound effect
on the demand for isomerate of higher octane-ratings. For instance,
requirements to reduce
benzene content of gasolines would necessitate increasing the octane rating of
isomerate and
"once-though" isomerization processes will be required to be retrofitted to a
process that
separates and recycles normal paraffins to the isomerization reactor. Even
existing processes that
use deisohexanizers maybe required to provide isomerate of enhanced octane
rating.
[0012] Accordingly, economically viable, and simple to operate processes to
enhance the
octane rating of deisohexanizer overhead are sought.
[0013] For the purposes of the following discussion of the invention, the
following
membrane properties are defined.
Microporous
[0014] Microporous and microporosity refer to pores having effective diameters
of
between 0.3 to 2 nanometers.
Mesoporous
[0015] Mesoporous and mesoporosity refer to pores having effective diameters
of
between 2 and 50 nanometers.
Macroporous
[0016] Macroporous and macroporosity refer to pores having effective diameters
of
greater than 50 nanometers.
Nanoparticle
[0017] Nanoparticles are particles having a major dimension up to 100
nanometers.
Molecular Sieves
[0018] Molecular sieves are materials having microporosity and may be
amorphous,
partially amorphous or crystalline and may be zeolitic, polymeric, metal,
ceramic or carbon.
Sieving Membrane
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[0019] Sieving membrane is a composite membrane containing a continuous or
discontinuous selective separation medium containing molecular sieve barrier.
A barrier is the
structure that exists to selectively block fluid flow in the membrane. In a
continuous sieving
membrane, the molecular sieve itself forms a continuous layer that is sought
to be defect-free.
The continuous barrier may contain other materials such as would be the case
with mixed
matrix membranes. A discontinuous sieving membrane is a discontinuous assembly
of
molecular sieve barrier in which spaces, or voids, exist between particles or
regions of
molecular sieve. These spaces or voids may contain or be filled with other
solid material. The
particles or regions of molecular sieve are the barrier. The separation
effected by sieving
membranes maybe on steric properties of the components to be separated. Other
factors may
also affect permeation. One is the sorptivity or lack thereof by a component
and the material of
the molecular sieve. Another is the interaction of components to be separated
in the
microporous structure of the molecular sieve. For instance, for some zeolitic
molecular sieves,
the presence of a molecule, say, n-hexane, in a pore, may hinder 2-
methylpentane from entering
that pore more than another n-hexane molecule. Hence, zeolites that would not
appear to offer
much selectivity for the separation of normal and branched paraffins solely
from the standpoint
of molecular size, may in practice provide greater selectivities of
separation.
C6 Permeate Flow Index
[0020] The permeability of a sieve membrane, i.e., the rate that a given
component passes
through a given thickness of the membrane, often varies with changes in
conditions such as
temperature and pressure, absolute and differential. Thus, for instance, a
different permeation
rate maybe determined where the absolute pressure on the permeate side is 1000
kPa rather than
that where that pressure is 5000 kPa, all other parameters, including pressure
differential, being
constant. Accordingly, a C6 Permeate Flow Index is used herein for describing
sieving
membranes. The C6 Permeate Flow Index for a given membrane is determined by
measuring the
rate (gram moles per second) at which a substantially pure normal hexane
(preferably at least 95
mass-percent normal hexane) permeates the membrane at approximately 150 C at a
retentate side
pressure of 1000 kPa absolute and a permeate-side pressure of 100 kPa
absolute. The C6
Permeate Flow Index reflects the permeation rate per square meter of retentate-
side surface area
but is not normalized to membrane thickness. Hence, the C6 Permeate Flow Index
for a given
membrane will be in the units of gram moles of normal hexane permeating per
second per square
meter of retentate-side membrane surface area.
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C6 Permeate Flow Ratio
[0021] The C6 Permeate Flow Ratio for a given sieve membrane is the ratio of
the C6
Permeate Flow Index (n-hexane) to an i-C6 Permeate Flow Index wherein the i-C6
Permeate Flow
Index is determined in the same manner as the C6 Permeate Flow Index but using
substantially
pure dimethylbutanes (regardless of distribution between 2,2-dimethylbutane
and 2,3-
dimethylbutane) (preferably at least 95 mass-percent dimethylbutanes).
SUMMARY OF THE INVENTION
[0022] By this invention improvements are made to isomerization processes for
upgrading
the octane rating of paraffin feedstocks comprising 5 and 6 carbon atoms where
the processes
use deisohexanizers to provide a lower boiling ditnethylbutanes-containing
fraction and a
higher boiling normal hexane-containing fraction. In accordance with this
invention,
membranes are used to recover from the higher boiling normal hexane-containing
fraction
components of higher octane rating such as methylcyclopentane and
dimethylbutanes. These
higher octane rating components can be used for blending with motor fuels. As
only a portion
of the isomerization effluent is subjected to membrane treatment, the surface
area of membrane
required can be reduced, thereby enhancing economic viability of using
membranes.
[0023] Preferably, at least a portion of the normal hexane-containing permeate
from the
membrane separation is recycled for isomerization. As the concentration of
normal hexane in
the permeate is higher than that in the higher boiling fraction, the volume of
recycle is reduced
as compared to recycling the same amount of normal hexane but without the
benefit of the
membrane separation. The ability of the processes of the invention to reduce
the volume of
recycle to the isomerization reactor can provide several advantages. For
instance, the reduced
volume of recycle allows for an increase in feed to the isomerization reactor
for a given
conversion, thus increasing the capacity of the isomerization reactor. Also,
by reducing the
amount of higher-octane components such as methylcyclopentane and
dimethylbutanes that
would otherwise be recycled to the isomerization reactor, the equilibrium
nature of the
isomerization reactions will enable more higher-octane product to be produced
per unit of
feedstock.
[0024] The broad aspects of the processes of this invention comprise:
a. isomerizing a feedstock comprising paraffins having 5 and 6 carbon atoms
wherein at
least 15 mass-percent of the feedstock is linear paraffin under isomerization
conditions
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including the presence of isomerization catalyst to provide an isomerization
effluent containing
linear paraffins but in a concentration less than that in the feedstock,
b. distilling at least a portion, preferably at least 90 mass-percent and most
preferably
essentially all, of the isomerization effluent to provide a lower boiling
fraction containing
dimethylbutanes and lighter paraffins and a normal hexane-containing fraction
containing
normal hexane, methylpentanes, dimethylbutanes and methylcyclopentane,
c. contacting at least a portion, preferably at least 90 mass-percent and most
preferably
essentially all, of the normal hexane-containing fraction from step b with a
retentate-side of a
selectively permeable membrane under conditions including sufficient membrane
surface area
and pressure differential across the membrane to provide a retentate fraction
that has an
increased concentration of methylcyclopentane and dimethylbutanes, and to
provide across the
membrane at a permeate-side, a permeate fraction having an increased
concentration of normal
hexane and methylpentanes, said permeate fraction containing at least 75,
preferably at least
90, mass-percent of the normal hexane contained in the normal hexane-
containing fraction
contacted with the membrane, and
d. withdrawing from step c the retentate fraction.
[0025] Preferably at least a portion, more preferably at least 90 mass
percent, and most
preferably essentially all, of the permeate fraction of step c is recycled to
step a.
[0026] Preferably at least 25, more preferably at least 30, mass-percent of
the
methylpentanes contained in the normal hexane-containing stream contacting the
membrane is
contained in the permeate fraction. In many instances, the concentration of
normal hexane to
the total permeate will be less than 90 mass-percent, e.g., from 25 to 90,
say, 40 to 80, mass-
percent. In some aspects of the processes of the invention, the withdrawn
retentate fraction is
greater than 10, say, 15 to 50, mass-percent of the normal hexane-containing
fraction
contacting the membrane. Thus, the volume of the recycle to the isomerization
of step a is less
than in an identical process except that the normal hexane-containing fraction
is not subjected
to the membrane separation.
[0027] The retentate fraction of step d contains significant amounts of
methylcyclopentane
and thus has an attractive octane rating. Often at least 50, preferably at
least 80, mass-percent
of the methylcyclopentane in the normal hexane-containing fraction contacting
the membrane
is retained in the retentate fraction.
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[0028] The separation of monomethylpentanes from dimethylbutanes is difficult
due to the
proximity of boiling points and thus not only does a deisohexanizer use an
extensive number of
distillation trays, often in the range of 80 trays, but also a large reflux to
feed ratio, e.g., 2:1 to
3:1. Hence, the operation of the deisohexanizer'requires substantial reboiler
heat. By the use
of the processes of this invention, a significant portion of the
dimethylbutanes contained in the
normal hexane-containing fraction remain in the retentate fraction and thus
recovered with the
methylcyclopentane for use in the motor fuel pool. Preferably the distillation
of step b is
operated such that the normal hexane-containing fraction contains
dimethylbutanes, say, at
least 2, and sometimes from 5 to 30, mass-percent of the dimethylbutanes
contained in the
isomerization effluent from step a. Preferably at least 30, more preferably at
least 70, mass-
percent of the dimethylbutanes contained in the normal hexane-containing
fraction, is retained
in the retentate fraction. Thus, the withdrawn retentate can be used as motor
fuel or added to a
pool to provide a motor fuel. Accordingly, for an existing deisohexanizer, the
reflux ratio can
be reduced, sometimes by 10 to 50 percent, resulting in energy savings without
undue loss in
the octane rating of the product.
[0029] Preferably the membrane is a sieving membrane having a C6 Permeate Flow
Index
of at least 0.01, more preferably at least 0.02, and a C6 Permeate Flow Ratio
of at least 1.25:1,
more preferably at least 1.3:1, and often 1.35:1 to 5:1 or 6:1.
The invention also pertains to apparatus suitable for conducting the processes
of this invention.
In its broader aspects, the apparatus of this invention is an apparatus for
isomerization of a
feedstock comprising paraffins having between 5 and 6 carbon atoms to provide
a gasoline
fraction comprising:
a. an isomerization reactor (106) being adapted to receive feedstock at an
inlet and
having an outlet,
b. a dehexanizer (116) having an inlet in fluid communication with the outlet
of
isomerization reactor (106), a lower boiling outlet adapted to remove a lower
boiling fraction
via line (118), a outlet to provide a side-cut fraction and a higher boiling
outlet; and
c. a membrane separator (124) having a feed side inlet in fluid communication
with the outlet to provide a side-cut fraction of the dehexanizer (116), a
feed side outlet in fluid
communication with line (118) from the lower boiling outlet of the dehexanizer
(116), and a
permeate outlet in fluid communication with the inlet of the isomerization
reactor (106).
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DESCRIPTION OF THE FIGURE
[0030] Figure 1 is a schematic representation of processes in accordance with
this invention
using a stabilizer column prior to a deisohexanizer.
DETAILED DESCRIPTION OF THE INVENTION
Isomerization
[0031] Any suitable paraffin-containing feedstock may be used in the processes
of this
invention. Naphtha feedstocks are the most often used as the feedstocks to
isomerization
processes. Naphtha feedstocks comprise paraffins, naphthenes, and aromatics,
and may comprise
small amounts of olefins, boiling within the gasoline range. Feedstocks which
maybe utilized
include straight-run naphthas, natural gasoline, synthetic naphthas, thermal
gasoline, catalytically
cracked gasoline, partially reformed naphthas or raffinates from extraction of
aromatics. The
feedstock essentially is encompassed by the range of a full-range naphtha, or
within the range of
0'to 230'C. Usually the feedstock is light naphtha having an initial boiling
point of 10 to 65'C
and a final boiling point from 75* to 110 C; preferably, the final boiling
point is less than 95*C.
[0032] Naphtha feedstocks generally contain small amounts of sulfur compounds
amounting
to less than 10 mass parts per million (mppm) on an elemental basis.
Preferably the naphtha
feedstock has been prepared from a contaminated feedstock by a conventional
pretreating step
such as hydrotreating, hydrorefining or hydrodesulfurization to convert such
contaminants as
sulfurous, nitrogenous and oxygenated compounds to H2S, NH3 and H2O,
respectively, which can
be separated from hydrocarbons by fractionation. This conversion preferably
will employ a
catalyst known to the art comprising an inorganic oxide support and metals
selected from Groups
VIB(IUPAC 6) and VIII(IUPAC 9-10) of the Periodic Table. Water can act to
attenuate catalyst
acidity by acting as a base, and sulfur temporarily deactivates the catalyst
by platinum poisoning.
Feedstock hydrotreating as described hereinabove usually reduces water-
generating oxygenates
and deactivating sulfur compounds to suitable levels, and other means such as
adsorption systems
for the removal of sulfur and water from hydrocarbon streams generally are not
required. It is
within the ambit of the present invention that this optional pretreating step
be included in the
present process combination.
[0033] The principal components of the preferred feedstock are cyclic and
acyclic paraffins
having from 4 to 8 carbon atoms per molecule (C4 to C8), especially C5 and C6,
and smaller
amounts of aromatic and olefinic hydrocarbons also maybe present. Usually, the
concentration
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of C7 and heavier components is less than 20 mass-percent of the feedstock,
and the concentration
of C4 and lighter components is less than 20, preferably less than 10, mass-
percent of the
feedstock. The mass ratio of Cs to C6 components in the preferred feedstocks
is 1:10 to 1:1.
[0034] Although there are no specific limits to the total content in the
feedstock of cyclic
hydrocarbons, the feedstock generally contains between 2 and 40 mass-percent
of cyclics
comprising naphthenes and aromatics. The aromatics contained in the naphtha
feedstock,
although generally amounting to less than the alkanes and cycloalkanes, may
comprise from 2 to
20 mass-percent and more usually 5 to 10 mass-percent of the total. Benzene
usually comprises
the principal aromatics constituent of the preferred feedstock, optionally
along with smaller
amounts of toluene and higher-boiling aromatics within the boiling ranges
described above.
[0035] In general, linear paraffins constitute at least 15, often from 40,
preferably at least 50,
mass-percent to essentially all of the feedstocks used in the processes of
this invention. For
naphtha feedstocks, linear paraffins are typically present in amounts of at
least to 50, say, 50 to
90, mass-percent. The mass ratio of non-linear paraffins to linear paraffins
in the feedstocks is
often less than 1:1, say, 0.1:1 to 0.95:1. Non-linear paraffins include
branched acyclic paraffins
and substituted or unsubstituted cycloparaffins. Other components such as
aromatics and olefinic
compounds may also be present in the feedstocks as described above.
[0036] The feedstock is passed to one or more isomerization zones. In the
aspects of this
invention where normal hexane is recycled, the feedstock and recycle are
usually admixed prior to
entry into the isomerization zone, but if desired, may be separately
introduced. In any case, the
total feed to the isomerization zone is referred to herein as the
isomerization feed. The recycle
may be provided in one or more streams. As discussed later, the recycle
contains linear paraffins.
The concentration of linear paraffins in the isomerization feed will not only
depend upon the
concentration of linear paraffins in the feedstock but also the concentration
in the recycle and the
relative amount of recycle to feedstock, which can fall within a wide range.
Often, the
isomerization feed has a linear paraffins concentration of at least 30, say,
between 35 and 90,
preferably 40 to 70, mass-percent, and a mole ratio of non-linear paraffins to
linear paraffins of
between 0.2:1 to 1.5:1, and sometimes between 0.4:1 to 1.2:1.
[0037] In the isomerization zone the isomerization feed is subjected to
isomerization
conditions including the presence of isomerization catalyst preferably in the
presence of a limited
but positive amount of hydrogen as described in US 4,804,803 and 5,326,296
The isomerization of paraffins is generally considered a reversible first
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order reaction. Thus, the isomerization reaction effluent will contain a
greater concentration of
non-linear paraffins and a lesser concentration of linear paraffins than does
the isomerization
feed. In preferred embodiments of this invention, the isomerization conditions
are sufficient to
isomerize at least 20, preferably, between 30 and 60, mass-percent of the
normal paraffins in the
isomerization feed. In general, the isomerization conditions achieve at least
70, preferably at least
75, say, 75 to 97, percent of equilibrium for C6 paraffins present in the
isomerization feed. In
many instances, the isomerization reaction effluent has a mass ratio of non-
linear paraffins to
linear paraffins of at least 2:1, preferably between 2.5 to 4:1.
[0038] The isomerization catalyst is not critical to the broad aspects of the
processes of this
invention, and any suitable isomerization catalyst may find application.
Suitable isomerization
catalysts include acidic catalysts using chloride for maintaining the sought
acidity and sulfated
catalysts. The isomerization catalyst may be amorphous, e.g. based upon
amorphous alumina, or
zeolitic. A zeolitic catalyst would still normally contain an amorphous
binder. The catalyst may
comprise a sulfated zirconia and platinum as described in US 5,036,035 and
European application
0 666 109 Al or a platinum group metal on chlorided alumina as described in US
5,705,730 and
6,214,764. Another suitable catalyst is described in US 5,922,639. US
6,818,589 discloses a
catalyst comprising a tungstated support of an oxide or hydroxide of a Group
IVB (IUPAC 4)
metal, preferably zirconium oxide or hydroxide, at least a first component
which is a lanthanide
element and/or yttrium component, and at least a second component being a
platinum-group
metal component.
[0039] Contacting within the isomerization zones may be effected using the
catalyst in a
fixed-bed system, a moving-bed system, a fluidized-bed system, or in a batch-
type operation. A
fixed-bed system is preferred. The reactants may be contacted with the bed of
catalyst particles
in upward, downward, or radial-flow fashion. The reactants may be in the
liquid phase, a mixed
liquid-vapor phase, or a vapor phase when contacted with the catalyst
particles, with excellent
results being obtained by application of the present invention to a primarily
liquid-phase
operation. The isomerization zone may be in a single reactor or in two or more
separate reactors
with suitable means to ensure that the desired isomerization temperature is
maintained at the
entrance to each zone. Two or more reactors in sequence are preferred to
enable improved
isomerization through control of individual reactor temperatures and.for
partial catalyst
replacement without a process shutdown.
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[0040] Isomerization conditions in the isomerization zone include reactor
temperatures
usually ranging from 40 to 250'C. Lower reaction temperatures are generally
preferred in order
to favor equilibrium mixtures having the highest concentration of high-octane
highly branched
isoalkanes and to minimize cracking of the feed to lighter hydrocarbons.
Temperatures in the
range of from 100 to 200 C are preferred in the present invention. Reactor
operating pressures
generally range from 100 kPa to 10 MPa absolute, preferably between 0.5 and 4
MPa absolute.
Liquid hourly space velocities range from 0.2 to 25 volumes of isomerizable
hydrocarbon feed
per hour per volume of catalyst, with a range of 0.5 to 15 hr-1 being
preferred.
[0041] Hydrogen is admixed with or remains with the isomerization feed to the
isomerization
zone to provide a mole ratio of hydrogen to hydrocarbon feed of from 0.01 to
20, preferably from
0.05 to 5. The hydrogen may be supplied totally from outside the process or
supplemented by
hydrogen recycled to the feed after separation from isomerization reactor
effluent. Light
hydrocarbons and small amounts of inerts such as nitrogen and argon maybe
present in the
hydrogen. Water should be removed from hydrogen supplied from outside the
process,
preferably by an adsorption system as is known in the art. In a preferred
embodiment the
hydrogen to hydrocarbon mol ratio in the reactor effluent is equal to or less
than 0.05, generally
obviating the need to recycle hydrogen from the reactor effluent to the feed.
[0042] Especially where a chlorided catalyst is used for isomerization, the
isomerization
reaction effluent is contacted with a sorbent to remove any chloride
components such as disclosed
in US 5,705,730.
Distillation and Membrane Separation
[0043] The isomerization reaction effluent is subjected to one or more
separation operations
to provide a product fraction of an enhanced octane rating and, optionally, to
remove other
components such as hydrogen, lower alkanes and, especially with respect to
chlorided catalysts,
halogen compounds.
[0044] In a commonly practiced isomerization process, the isomerization is
conducted in the
liquid phase and the isomerization reaction effluent is passed to a product
separator in which a
gaseous overhead containing hydrogen and lower alkane is obtained. At least a
portion of this
hydrogen can be recycled to the isomerization reactor for providing at least a
portion of the
sought hydrogen for the isomerization. The liquid bottoms is passed to a
distillation assembly
(deisohexanizer) to provide a lower boiling fraction containing
dimethylbutanes and a higher
boiling normal hexane-containing fraction. Most often, the deisohexanizer is
adapted to provide
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the normal hexane-containing stream as a side stream and provides a bottoms
stream comprising
normal heptane. The deisohexanizer may be a packed or frayed column and
typically operates
with a top pressure of between 50 and 500 kPa (gauge) and a bottoms
temperature of between 75
and 1700C.
[0045] The composition of the lower boiling fraction from the deisohexanizer
will depend
upon the operation and design of the assembly and any separation processes to
which the
isomerization effluent has been subjected. For instance, if the stream to the
deisohexanizer
contains lights such as C1 to C4 compounds, the deisohexanizer may be adapted
to provide an
overhead fraction containing these lights, and a side-draw fraction containing
C5 compounds and
branched C6 compounds, especially dimethylbutanes. Typically the lower boiling
fraction
contains 20 to 60 mass-percent dimethylbutanes; 10 to 40 mass-percent normal
pentane and 20 to
60 mass-percent isopentane and butane. Depending upon the operation of the
deisohexanizer, the
lower boiling fraction may also contain significant, e.g., at least 10 mass-
percent methylpentanes.
The deisohexanizer may also be adapted to provide a C5-rich stream in addition
to the lower
boiling stream.
[0046] The higher boiling normal hexane-containing fraction also contains
methylpentanes
and methylcyclopentane. As stated earlier, the processes of this invention
permit the
deisohexanizer to be operated more economically resulting in a greater
concentration of
dimethylbutanes in the normal hexane-containing fraction. Often the normal
hexane-containing
fraction will contain 2 to 10 mass-percent dimethylbutanes; 5 to 50 mass-
percent normal hexane;
20 to 60 mass-percent methylpentanes, and 5 to 25 mass-percent
methylcyclopentane. Typically,
the deisohexanizer will be designed to provide a side stream that contains
methyl pentanes,
methylcyclopentane, normal hexane, dimethylbutanes and cyclohexane, and a
bottoms stream that
contains cyclohexane and C7+ hydrocarbons. If the normal hexane-containing
fraction were the
bottom fraction of the deisohexanizer, that fraction would also contain such
heavier
hydrocarbons.
[0047] As stated above, the ability to recover dimethylbutanes from the higher
boiling normal
hexane-containing fraction enables the distillation to be conducted with a
lower reflux ratio. The
reflux ratio used will depend upon the nature of the feed to the column as
well as the design of the
column and thus can vary over a broad range, e.g., from 1.5:1 to 2.5:1 on a
mass basis of reflux to
feed.
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[0048] At least a portion, preferably at least 50, and more preferably at
least 80, mass-percent
to substantially all of the normal hexane-containing fraction from the
deisohexanizer is contacted
with the retentate side of a selective membrane to provide a retentate
fraction of the isomerization
reaction effluent that has a reduced concentration of total linear paraffins,
and to provide across
the membrane at a permeate-side, a permeate fraction having an increased
concentration of total
linear paraffins. The permeate fraction contains at least 75 mass-percent of
the normal hexane,
preferably at least 75 mass-percent of the normal hexane, in the fraction
contacted with the
membrane. The retentate preferably contains at least 50, more preferably at
least 80 to
substantially all of the methylcyclopentane contained in the fraction
contacting the membrane. In
preferred aspects of the invention, the membrane allows at least 25, say, 30
to 90, mass-percent of
the methylpentanes contained in the normal hexane-containing fraction
contacting the membrane
to penneate.
[0049] A pressure drop is maintained across the membrane in order to effect
the desired
separation at suitable permeation rates. The membrane maybe of any suitable
type including
diffusion and sieving, and may be constructed of inorganic, organic or
composite materials. For
diffusion membranes, the driving force is the differential in partial
pressures between the retentate
and the permeate sides. In sieving membranes, the absolute pressure drop
becomes a significant
component of the driving force independent of partial pressures or
concentrations. The preferred
membranes are sieving membranes having a C6 Permeate Flow Index of at least
0.01 and a C6
Permeate Flow Ratio of at least 1.25:1. The sieving membranes are discussed in
more detail
below.
[0050] In the membrane separations, the pressure drop is often in the range of
0.1 to 10,
preferably 0.2 to 2, MPa. In practice, the normal hexane-containing fraction
will be contacted
with the retentate side of the membranes without additional compression to
minimize capital and
operating costs. The temperature for the membrane separation will depend in
part on the nature
of the membrane and on the temperature of the fraction. Thus, for polymer-
containing
membranes, temperatures should be sufficiently low that the strength of the
membrane is not
unduly adversely affected. In most instances, the temperature for the
separation is the
temperature of the deisohexanizer fraction. Often the temperature is in the
range of 25 C to
150 C. Thus, the conditions of the membrane separation may provide for a
liquid or gas or
mixed phase on the retentate side of the membrane. Regardless of the phase of
the fluid on the
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retentate side, the permeate may be a gas. If the fluid on the retentate side
of the membrane is in
the liquid phase, the permeate may be liquid, gaseous or mixed phase.
[0051] Any suitable selectively permeable membrane may be used in the
apparatus and
processes of this invention. The preferred membranes are sieving membranes.
The membranes
used in the processes of this invention are characterized in having high flux,
i.e., having a C6
Permeate Flow Index of at least 0.01. The membranes maybe in any suitable fonn
such as
hollow fibers, sheets, and the like which can be assembled in a separator unit
such as bundled
hollow fibers or flat plate or spiral wound sheet membranes. The physical
design of the
membranes should enable, when assembled in the separator unit, sufficient
pressure drop across
the membrane to provide desirable flux. For hollow fiber membranes, the high
pressure side
(retentate side) is usually at the outside of the hollow fiber. The flow of
the permeate may be co-
current, countercurrent or cross-current with respect to the flow of the fluid
on the retentate side
of the membrane.
[0052] Sufficient membrane surface area is provided that under steady state
conditions at
least 75, preferably at least 80, and more preferably at least 90, mass-
percent of the normal
hexane contained in the fraction from the deisohexanizer is contained in the
permeate. The
concentration of normal hexane will depend upon the selectivity of the
membrane. While the
membrane may be highly selective and provide a permeate containing 99 mass-
percent or more of
normal hexane, advantageous embodiments of this invention can be achieved with
lesser purity
permeates. The concentration of normal hexane to the total permeate in these
embodiments will
be less than 90 mass-percent, e.g., from 25 to 90, say, 40 to 80, mass-
percent. The remainder of
the effluent will typically be methylpentanes and some methylcyclopentane and
dimethylbutanes
that pass through the membrane.
[0053] The preferred high flux, sieving membranes permit a portion of branched
paraffins to
permeate. The relative rates of permeation will depend upon the molecular
configuration of the
paraffins. Methyl pentanes will pass more readily through the membrane than
the
dimethylbutanes and cyclopentane.
[0054] Preferably at least a portion of the permeate is recycled to the
isomerization step. The
permeate will contain linear paraffins that will be subjected to isomerization
conditions during the
isomerization step.
Sieving Membranes
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[0055] The preferred sieving membranes may be of various types, for instance,
molecular
sieves, pore-containing ceramic, metal, polymeric or carbon membranes, or
composite
membranes having a highly porous polymeric, metallic, molecular sieve, ceramic
or carbon
support with a thin sieving layer or barrier (molecular sieve), e.g.,
zeolitic, polymeric, metal,
ceramic or carbon, having microporosity.
[0056] The membranes may be continuous or discontinuous. A discontinuous
membrane
comprises an assembly of small particle size microporous barrier whereas a
continuous
membrane comprises a continuous layer of microporous barrier. The membranes
may be formed
of a single material or they may be composites containing microporous barrier
and support and,
optionally, other structure. When making a thin, continuous barrier layer, as
the thickness of the
sieving layer decreases, the difficulties in obtaining a defect-free layer
increase. As the processes
of this invention do not require high selectivity, the membranes can contain
minor defects.
Typically continuous membranes are made by depositing or growing on a
meso/macroporous
structure, a continuous, thin layer of microporous barrier. Discontinuous
assemblies of nano-
sized microporous barrier enable very small permeating thicknesses to be
achieved, but with the
potential of by-pass. Discontinuous membranes use a meso/macroporous structure
with which
the microporous barrier is associated.
[0057] Examples of zeolite barrier include small pore molecular sieves such as
SAPO-
34, DDR, A1PO-14, A1PO-17, A1PO-18, A1PO-34, SSZ-62, SSZ-13, zeolite 3A,
zeolite 4A,
zeolite 5A, zeolite KFI, H-ZK-5, LTA, UZM-9, UZM-13, ERS-12, CDS-1,
Phillipsite, MCM-65,
and MCM-47; medium pore molecular sieves such as silicalite, SAPO-3 1, MFI,
BEA,and MEL;
large pore molecular sieves such as FAU, OFF, NaX, NaY, CaY, 13X, and zeolite
L; and
mesoporous molecular sieves such as MCM-41 and SBA-15. Anumber of types of
molecular
sieves are available in colloidal (nano-sized particle) form such as A, X, L,
OFF, MFI, and SAPO-
34. The zeolites may or may not be metal exchanged.
[0058] Other types of sieving materials include carbon sieves; polymers such
as PIMs
(polymers of intrinsic microporosity) such as disclosed by McKeown, et al.,
Chem. Commun.,
2780 (2002); McKeown, et al.,, Chem. Eur. J., 11:2610 (2005); Budd, et al., J.
Mater. Chem.,
13:2721 (2003); Budd, et al., Adv. Mater., 16:456 (2004) and Budd, et al.,
Chem Commun., 230
(2004); polymers in which porosity is induced by pore-forming agents such as
poly(alkylene
oxide), polyvinylpyrrolidone; cyclic organic hosts such as cyclodextrins,
calixarenes, crown
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ethers, and spherands; microporous metal-organic frameworks such as MOF-5 (or
IRMOF-1);
glass, ceramic and metal shapes into which microporosity has been introduced.
[0059] In composite membranes, a meso/macroporous structure is used. The
meso/macroporous structure serves one or more functions depending upon the
type membrane.
It can be the support for the membrane composite, it can be an integral part
of forining the
microporous barrier, it can be the structure upon which or in which the
microporous barrier is
located. The meso/macroporous structure can be continuous or discontinuous,
and the
meso/macroporosity may thus be channels through the material of the
meso/macroporous
structure or be formed between particles that form the meso/macroporous
structure. Examples
of the latter are the AccuSepTM inorganic filtration membranes available from
the Pall Corp.
having a zirconia layer on a porous metal support wherein the zirconia is in
the form of
spherical crystals.
[0060] The meso/macroporous structure preferably defines channels, or pores,
in the
range of 2 to 500, preferably, 10 to 250, more preferably between 20 and 200,
nanometers in
diameter, and has a high flux. In more preferred embodiments, the C6 Permeant
Flow Index of
the meso/macroporous structure is at least 1, and most preferably at least 10,
and sometimes at
least 1000. The meso/macroporous structure maybe isotropic or anisotropic. The
meso/macropores may be relatively straight or tortuous.
[0061] The meso/macroporous structure may be composed of inorganic, organic or
mixed inorganic and organic material. The selection of the material will
depend upon the
conditions of the separation as well as the type of meso/macroporous structure
formed. The
material of the meso/macroporous structure may be the same or different than
the material for
the molecular sieve. Examples of porous structure compositions include metal,
alumina such
as alpha-alumina, gamma alumina and transition aluminas, molecular sieve,
ceramics, glass,
polymer, and carbon. In preferred embodiments, defects in the substrate are
repaired prior to
providing the barrier or precursor to the barrier. In another embodiment, the
substrate maybe
treated with a silica sol to partially occlude pores and facilitate deposition
of the barrier or
precursor to the barrier. The silica particles will still provide sufficient
space between their
interstices to allow high flux rates. Another technique is to coat the support
with silicon rubber
or other polymer that permits high flux but occludes defects in the support or
in the barrier.
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[0062] If the meso/macroporous structure does not so serve, the membrane can
contain a
porous support for the meso/macroporous structure. The porous support is
typically selected on
the basis of strength, tolerance for the conditions of the intended separation
and porosity.
[0063] The AccuSepTM inorganic filtration membranes available from Pall Corp.
and
similar types of meso/macroporous structures are particularly advantageous
since the
meso/macroporous structure can be thin thereby avoiding undue thicknesses of
molecular sieve
being grown. Further, the zirconia is relatively inert to zeolite-forming
precursor solutions and
synthesis and calcination conditions, making it a preferred meso/macroporous
structure for
these types of sieving membrane.
[0064] High flux is achieved through at least one of the following techniques:
first, using
a larger pore than required for normal alkane to pass; and second, using an
extremely thin pore-
containing layer. Where high flux is achieved using larger, less selective
micropores in the
microporous barrier, adequate separation may be achieved. Often the pores for
these types of
membranes have an average pore diameter of greater than 5.0 A (average of
length and width),
say, 5.0 to 7.0 or 8 A. Preferably, the structures have an aspect ratio
(length to width) of less than
1.25:1, e.g., 1.2:1 to 1:1. For molecular sieve-containing membranes,
exemplary structures are
USY, ZSM-12, SSZ-35, SSZ-44, VPI-8, and Cancrinite. In some instances, a
permeating
molecule in a micropore may assist in enhancing selectivity. For instance, a
normal hydrocarbon
in a pore may decrease the rate at which a branched hydrocarbon can enter the
pore as compared
to another normal hydrocarbon.
[0065] High flux can also be achieved using very thin microporous barrier in
either a
continuous or discontinuous membrane. The microporous barrier can, if desired,
be selected
from sieving structures having micropores that are substantially impermeable
to the moiety
sought to be retained on the retentate side. In general, the pores for these
types of membranes
have an average pore diameter of up to 5.5A, for instance, 4.5 to 5.4A. The
aspect ratio of the
pores of these membranes may vary widely, and is usually in the range of 1.5:1
to 1:1. For
molecular sieve-containing membranes, exemplary structures are ZSM-5,
silicalite, ALPO-1 1,
ALPO-31, ferrierite, ZSM-11, ZSM-57, ZSM-23, MCM-22, NU-87, UZM-9, and CaA.
[0066] Membranes comprising a discontinuous assembly of microporous barrier
are
characterized in that the barrier has a major dimension less than 100
nanometers, and the
microporous barrier is associated with a meso/macroporous structure defining
fluid flow pores,
wherein barrier is positioned to hinder fluid flow through the pores of the
meso/macroporous
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structure. A molecular sieve barrier is "associated" with a meso/macroporous
structure when it is
positioned on or in the structure whether or not bonded to the structure.
Hence, nano-sized
particles or islands of molecular sieve are used as barriers for the
membranes. The discontinuous,
microporous barrier is positioned to hinder fluid flow through fluid flow
channels defined by the
meso/macroporous structure. The barrier maybe at least partially occluding the
opening of a
fluid flow channel of the meso/macroporous structure and/or within the fluid
flow channel. Due
to the small size of the particles or islands forming the discontinuous
assembly of microporous
barrier, some selectivity of separation is achievable despite the
discontinuity.
[0067] Typically the size and configuration of the molecular sieve particles
and the size
and configuration of the meso/macropores in the meso/macroporous structure
will be taken into
account in selecting the components for the sieving membranes. With more
spherical
molecular sieve particles, such as silicalite, it is preferred to select a
meso/macroporous
structure having pores that are close to the same effective diameter. In this
manner, the
molecular sieve particles, if placed in, or partially in, the pores of the
meso/macroporous
structure, will provide minimal void space for by-pass. More flexibility
exists with platelets
and irregular shaped molecular sieve particles as they can overlap with little
or no void space.
In some instances a combination of molecular sieve configurations may be
desirable. For
instance, a spherical molecular sieve may be drawn into the pores of a
meso/macroporous
structure with smaller, more plate-like molecular sieve particles being
subsequently introduced.
The complementary functions are that the sphere serves as a support for the
plate-like particles
and the plate-like particles overlap to reduce by-pass. While the molecular
sieves will likely be
different compositions, and thus have different microporosity size and
configuration, the
benefit is enhanced separation without undue loss of permeance.
[0068] Where zeolitic molecular sieves are used, obtaining small particles is
important
to obtaining the high flux in a discontinuous microporous barrier. For many
zeolites, seed
particles are available that are less than 100 nanometers in major dimension.
Most molecular
sieves are made using organic templates that must be removed to provide access
to the cages.
Typically this removal is done by calcination. As discussed later, the
calcination maybe
effected when the template-containing molecular sieves are positioned in a
macropore such that
undue agglomeration is avoided simply by limiting the number of particles that
are proximate.
Another technique for avoiding agglomeration of the zeolite particles during
calcination is to
silate the surface of the zeolite, e.g., with an aminoalkyltrialkoxysilane,
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aminoalkylalkyldialkoxysilane, or aininoalkyldialkylalkoxysilane. The amount
of silation
required will depend upon the size of the zeolite and its composition as well
as the conditions
to be used for calcination. In general, between 0.1 to 10 millimoles of silane
are used per gram
of zeolite.
[0069] Various techniques exist for providing the molecular sieve particles on
or in the
meso/macroporous support in a manner that at least partially occludes the meso-
or macropores
in the support. The specific technique to be used will depend upon the size
and configuration
of the molecular sieve particles, the size and configuration of the
meso/macropores in the
meso/macroporous structure, and the desired placement of the molecular sieve
in or on the
meso/microporous structure.
[0070] Especially where molecular sieve is placed on the surface of a
meso/macroporous structure to occlude at least a portion of the opening of the
pores, the
meso/macroporous structure may be wet with a solution, or suspension, of nano-
sized
molecular sieve. The concentration of molecular sieve in the suspension should
be sufficiently
low that upon drying, the resulting layer of molecular sieve is not unduly
thick.
Advantageously at least a slight pressure drop is maintained across the
meso/macroporous
structure during the coating such that a driving force will exist to draw
molecular sieve to any
pores in the meso/macroporous structure that have not been occluded. Usually
the suspension
will be an aqueous suspension, although suspensions in alcohols and other
relatively inert
liquids can be used advantageously, at a concentration of between 2 and 30,
say 5 and 20, mass
percent. Where a pressure differential is used, the pressure differential is
generally in the range
of 10 to 200 kPa. One or more coats of molecular sieve may be used, preferably
with drying
between coats. Drying is usually at an elevated temperature, e.g., between 30
C and 150 C, for
1 to 50 hours. Vacuum may be used to assist drying. Where zeolites are used as
the molecular
sieve, calcining, e.g., at a temperature of between 450 C and 600 C may, in
some instances,
assist in securing the molecular sieve to the meso/macroporous structure.
Calcining may also
serve to agglomerate the molecular sieve particles and thus reduce voids and
the size of voids.
Calcining, of course, is not essential to the broad aspects of this invention
and is only required
where, for example, template resides in the micropores.
[0071] Where the discontinuous assembly of nano-sized molecular sieve is
located
outside the pores of the meso/macroporous structure, it may be desirable to
bond at least a
portion of the particles to the surface of the structure. This can be
accomplished in a number of
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ways. For instance, the surface of the structure can be functionalized with
hydroxyl groups or
other moieties that would be reactive with a zeolitic molecular sieve. For
polymeric molecular
sieves, the surface may be functionalized with moieties that react, such as
addition or
condensation, with functional moieties on the polymer. These techniques are
well known in
the art for other applications.
[0072] Similar preparation techniques can be used where it is desired to
incorporate at
least a portion of the molecular sieve particles in the pores of the
meso/macroporous structure.
The molecular sieve particles should be of an appropriate size to enter the
meso/macropores. A
pressure differential may be used to draw barrier particles into the pores or
ultrasonication may
be used to aid in getting barrier particles into the pores of the
meso/macroporous support. The
depth of the molecular sieve particles in the pores of the meso/macroporous
structure should
not be so great as to unduly reduce permeance. Often, any surface deposition
of molecular
sieve is removed by, e.g., washing.
[0073] If desired, zeolitic molecular sieves can be grown in situ in the pores
of the
meso/macroporous structure to provide a discontinuous membrane. The synthesis
may provide
discrete particles or islands between other structure such as the
meso/macroporous structure or
other particles.
[0074] An example of using other particles to make discontinuous membranes of
zeolitic molecular sieves, involves providing silica, which may have a
particle size of between
5 and 20 nanometers, in or on the meso/macroporous structure. The silica, due
to the active
hydroxyls on the surface, serves as a nucleating site for a zeolite-forming,
precursor solution,
and layers of zeolite can be grown on and between the silica particles.
[0075] Materials other than silica particles can be used as nucleating sites
including
other molecular sieves or seed crystals of the same zeolite. The surface of
the
meso/macroporous structure can be functionalized to provide a selective
location for zeolite
growth. Some zeolites have self nucleating properties and thus may be used in
the absence of
nucleating sites. Examples of these zeolites are FAU and MFI. In these
situations, it may be
desired to maintain the precursor solution under zeolite forming conditions
for a time sufficient
that growth of the zeolite starts prior to contacting the precursor solution
with the
meso/macroporous structure.
[0076] For example, one method to form a barrier layer is to place a zeolitic
molecular
sieve precursor liquid on a meso/microporous structure. The precursor is
permitted to crystallize
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under hydrothermal crystallization conditions, after which the membrane is
washed and heated to
remove residual organic material. The molecular sieve material resides
primarily in and occludes
the pores of the porous substrate.
[0077] The molecular sieve may be of any suitable combination of elements to
provide
the sought pore structure. Aluminum, silicon, boron, gallium, tin, titanium,
germanium,
phosphorus and oxygen have been used as building blocks for molecular sieves
such as silica-
alumina molecular sieves, including zeolites; silicalite; A1PO; SAPO; and boro-
silicates. The
precursor includes the aforementioned elements, usually as oxides or
phosphates, together with
water and an organic structuring agent which is normally a polar organic
compound such as
tetrapropyl ammonium hydroxide. Other adjuvants may also be used such as
amines, ethers and
alcohols. The mass ratio of the polar organic compound to the building block
materials is
generally in the range of 0.1 to 0.5 and will depend upon the specific
building blocks used. In
order to prepare thin layers of molecular sieves in the membranes, it is
generally preferred that the
precursor solution be water rich. For instance, for silica-alumina molecular
sieves, the more ratio
of water to silica should be at least 20:1 and for aluminophosphate molecular
sieves, the mole
ratio should be at least 20 moles of water per mole of aluminum.
[0078] The crystallization conditions are often in the range of 80 C to 250 C
at pressures
in the range of 100 to 1000, frequently 200 to 500, kPa absolute. The time for
the crystallization
is limited so as not to form an unduly thick layer of molecular sieve. In
general, the
crystallization time is less than 50, say, 10 to 40, hours. Preferably the
time is sufficient to form
crystals but less than that required to form a molecular sieve layer of 200
nanometers, say, 5 to 50
nanometers. The crystallization may be done in an autoclave. In some
instances, microwave
heating will effect crystallization in a shorter period of time. The membrane
is then washed with
water and then calcined at 350 to 550 C to remove any organics.
[0079] Especially with some zeolitic molecular sieve materials, making
particles less
than 100 nanometers is troublesome. Moreover, even with the use of seed
crystals, the particle
size may be larger than desired. Another embodiment in making a discontinuous
barrier
membrane is to synthesize the zeolite in open regions between particles
(substrate particles)
having a major dimension less than 100 nanometers. Accordingly, the major
dimension of the
microporous barrier can be less than 100 nanometers. The substrate particles
serve as a
nucleating site for the zeolite formation and thus are selected from materials
having capability
of nucleating the growth of the zeolite. Examples of such materials are
silica, especially silica
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having a major dimension of between 5 and 50 nanometers and other zeolites
having major
dimensions less than 100 nanometers. The use of fumed silica as the substrate
particle is
particularly useful for making an A1PO microporous barrier.
[0080] The growth of the zeolite on the substrate particle may occur before or
after the
substrate particle is used in forming the membrane composite.
[0081] Advantageously, the growth of the zeolite on the substrate particles
occurs while
drawing the synthesis liquor through the composite. This technique helps
ensure that the
growth occurs not as a layer on top of the particles, but in the interstices
between the particles.
The pressure drop increases as the zeolite growth occurs, and the pressure
drop can be used as
an indicator when adequate zeolite formation has occurred.
[0082] Polymeric molecular sieves can be synthesized in the meso/macroporous
structure. One method for synthesizing a small polymeric molecular sieve is to
functionalize
nano-particles and/or the meso/macroporous structure with a group that can
react with an
oligomer such as through a condensation or addition reaction. For instance,
the functional
groups may provide a hydroxyl, amino, anhydride, dianhydride, aldehyde, amic
acid, carboxyl,
amide, nitrile, or olefinic moiety for addition or condensation reaction with
a reactive moiety of
an oligomer. Suitable oligomers may have molecular weights of 30,000 to
500,000 or more
and may be reactive oligomers of polysulfones; poly(styrenes) including
styrene-containing
copolymers; cellulosic polymers and copolymers; polyamides; polyimides;
polyethers;
polyurethanes; polyesters; acrylic and methacrylic polymers and copolymers;
polysulfides,
polyolefins, especially vinyl polymers and copolymers; polyallyls;
poly(benzimidazole);
polyphosphazines; polyhydrazides; polycarbodiides, and the like.
[0083] The synthesis in situ of the molecular sieve, whether it be inorganic
or organic,
can be under suitable conditions. A preferred technique involves conducting
the synthesis
while drawing the reactant solution, e.g., the precursor solution or oligomer
solution through
the meso/macroporous structure. This technique provides the benefit of
directing the reactant
solution to voids that have not been occluded as well as limits the extent of
growth of the
molecular sieve as no fresh reactant will be able to enter the reaction site
once the molecular
sieve has occluded the meso- or macropore.
[0084] The molecular sieve on polymer support membranes or polymeric supports
themselves may also be pyrolyzed in a vacuum furnace to produce a carbon
membrane. For
such membranes containing molecular sieves, the pore structure of the carbon
support is
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preferably of sufficient diameter to minimize the resistance to the flow of
fluids with the
molecular sieve structure doing the separation. The temperature of the
pyrolysis will depend
upon the nature of the polymer support and will be below a temperature at
which the porosity is
unduly reduced. Examples of polymeric supports include polyimides,
polyacrylonitrile,
polycarbonates, polyetherketones, polyethersulfones and polysulfones, and
prior to pyrolysis,
the supports have pores or openings in the range of 2 to 100, preferably 20 to
50, nanometers.
[0085] Continuous membranes may be prepared by any suitable technique.
Typically, the
thickness of the microporous barrier will be related to the duration of the
deposition or growth of
the microporous barrier on the meso/macroporous structure. The microporous
barrier may be
formed by reducing the pore size of an ultrafiltration membrane (effective
pore diameters of 1 to
100 nanometers) or a microfiltration membrane (effective pore diameters of 100
to 10,000
nanometers) by, e.g., organic or inorganic coating of the channel either
interior of the surface, or
preferably, at least partially proximate to the opening of the channel. The
deposited material
serves to provide a localized reduction of the pores or openings through the
support to a size
which permits the desired sieving without unduly reducing the diameter of the
remaining pore
structure in the support. Examples of vapor depositable materials include
silanes, paraxylylene,
alkylene imines, and alkylene oxides. Another technique for reducing pore size
is to deposit a
coke layer on the meso/macroporous structure. For instance, a carbonizable gas
such as methane,
ethane, ethylene or acetylene can be contacted with the structure at
sufficiently elevated
temperature to cause coking. The preferred porous supports are ultrafiltration
membranes having
pore sizes of between 1 and 80, preferably between 2 and 50, nanometers.
[0086] For zeolitic, continuous membranes, one fabricating technique involves
contacting
the surface of the meso/macroporous structure with precursor for molecular
sieve and growing
the molecular sieve for a time sufficient to achieve the sough film thickness.
The procedures
disclosed above can be used to synthesize the molecular sieve. In some
instances, it maybe
desirable to occlude, e.g., with a wax, the meso/macropores of the support to
prevent undue
growth of zeolite in those pores. The wax can subsequently be removed.
[0087] Various techniques are available to enhance the selectivity of high
flux
membranes. Numerous techniques exist to cure defects in continuous or
discontinuous
membranes. As the membranes need not exhibit high C6 Permeate Flow Ratios to
be useful for
many applications, any technique that increases resistance to flow through the
defects will serve
to improve membrane performance. For instance, a silica sol overlay coating
may be used to
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occlude interstitial openings between the molecular sieve crystals or
remaining large pores in the
support regardless of how the membrane is prepared.
[0088] Another technique to occlude large pores is to provide on one side of
the barrier
layer a large, reactive molecule which is not able to permeate the micropores
of the barrier and on
the other side a cross linking agent. The major defects, and to some extent
the minor defects
become filled with the large, reactive molecule and are fixed by crosslinking.
The unreacted large
molecule component can then be removed as well as unreacted crosslinking
agent. The large
molecule may be an oligomer or large molecule.
[0089] For discontinuous membranes, solid may be provided in at least a
portion of the
voids between particles or islands of microporous barrier and between the
microporous barrier
and the meso/microporous structure.
[0090] One generic technique for enhancing the selectivity of a sieving
membrane is to
agglomerate adjacent particles of molecular sieve to reduce or substantially
eliminate voids
between the particles and between the particles and walls of the pore
structure in the
meso/macroporous structure. Because the particles are nano-sized and the
number of adjacent
particles can be relatively few, the agglomeration can occur while still
retaining desirable
Permeant Flow Rates. For polymeric molecular sieves that are thermoplastic,
the
agglomeration can occur by heating to a temperature where agglomeration occurs
but no so
high as to lose either its microporous structure or its ability to provide the
desired occlusion of
the meso- or macropore of the meso/macroporous structure. Agglomeration can
also be
accomplished by calcining zeolitic molecular sieves. Calcining tends to
agglomerate small
zeolite particles, especially particles that are neither silated nor otherwise
treated to reduce the
tendency to agglomerate. The temperature and duration of the calcining will
depend upon the
nature of the zeolitic molecular sieve. Usually temperatures of between 450 C
and 650 C are
employed over a period of between 2 and 20 hours.
[0091] The agglomeration technique may be used with respect to molecular sieve
particles that are on the surface of the meso/macroporous structure as well as
those within the
pores of the structure. Most preferably, agglomeration is used when the
molecular sieve
particles are located within the meso- or macropores of the meso/macroporous
structure such
that the major dimension of the agglomerate is less than 200, preferably less
than 100,
nanometers. The agglomeration maybe effected with or without a pressure
differential across
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the membrane. Preferably a pressure differential is used to assist in reducing
voids through
which fluid can by-pass the molecular sieve.
[0092] Another generic technique where the discontinuous assembly of barrier
defines
voids is to at least partially occlude at least a portion of the voids by a
solid material therein.
Preferably the solid material is a polymer or inorganic material. The solid
material may simply
reside in the void or it may adhere or be bonded to the molecular sieve or
meso/macroporous
structure. The solid material may be a particle or oligomer that may be
preformed and then
introduced into the voids or it may be formed in situ.
[0093] In one aspect, the solid material provides a "mortar" with the
microporous
barrier particles. The mortar is typically a suitable polymeric material that
can withstand the
conditions of the separation. Representative polymers include polysulfones;
poly(styrenes)
including styrene-containing copolymers; cellulosic polymers and copolymers;
polyamides;
polyimides; polyethers; polyurethanes; polyesters; acrylic and methacrylic
polymers and
copolymers; polysulfides, polyolefins, especially vinyl polymers and
copolymers; polyallyls;
poly(benzimidazole); polyphosphazines; polyhydrazides; polycarbodiides, and
the like.
Preferred polymers are those having porosity such as PIMs (see WO 2005/012397)
and
polymers in which porosity has been induced by pore forming agents. These
polymers have
pores that maybe 0.3 or more, preferably at least 1, nanometer in major
dimension and hence
allow for fluid flow to and from the barrier particles.
[0094] It is not necessary that all particles be encased in the mortar. Often
the average
thickness of the mortar layer is less than 100 nanometers, and is preferably
no more than the
major dimension of the particles. If too much mortar is used, a mixed membrane
structure may
result, and flux unduly penalized. Hence, the mass ratio of barrier particles
to mortar often is
in the range of between 1:2 to 100:1, preferably between 3:1 to 30:1.
[0095] The mortar and particles may be admixed, e.g., in a slurry, and then
placed in
association with the microporous structure, or may be provided after
deposition of the particles.
The polymer may be formed in situ at the region containing the barrier
particles. The barrier
particle may be inert to the polymerization or may have active sites to anchor
a polymer. For
instance, the particle may be functionalized with a reactive group that can
bind with the
polymer or with monomer undergoing polymerization, say, through a condensation
or addition
mechanism such as discussed above.
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[0096] A concern is that the mortar occludes the micropores of the molecular
sieve.
With highly porous polymer such as the PIMs, the effect of any occlusion can
be attenuated.
Often, the amount of polymer used for the mortar and its molecular weight and
configuration is
such that insufficient polymer is present for encapsulating all the molecular
sieve particles.
Frequently, the mass ratio of polymer to molecular sieve is between 0.01:1 and
0.3:1. The
weight average molecular weight of the polymer is sometimes in the range of
20,000 to
500,000, preferably, between 30,000 and 300,000.
[0097] The mortar may be other than polymeric. For example, where the
molecular
sieve is a zeolite, a silicon tetraalkoxide can react with the zeolite and can
through hydrolysis
form a silica framework or mass between the molecular sieve particles. Usually
a dilute
aqueous solution of silicon tetraalkoxide is used, e.g., containing between
0.5 and 25 mass
percent silicon tetraalkoxide, to assure distribution. The functionalization
of the zeolite with
silicon tetraalkoxide also is useful as a cross-linking site with organic
polymer, especially those
containing functional groups such as hydroxyl, amino, anhydride, dianhydride,
aldehyde or
amic acid groups that can form covalent bonds with organosilicon alkoxide.
Also, the same or
different zeolite may be grown between the zeolite particles and the zeolite
particles and the
meso/macroporous structure using the techniques described above.
[0098] Yet another approach to reducing bypass is to use two or more sized
particles in
forming the barrier-containing layer. If, for example, the microporous barrier
particles are
generally spherical with a nominal major dimension of 60 nanometers, the
regions between the
particles can be sizable and enable bypass. Incorporating configurationally
compatible
particles in these regions can hinder fluid flow and thus result in a greater
portion of the fluid
being directed to the barrier particles for the selective separation. The
configuration of the
barrier particles will depend upon the type of barrier particle used. A
microporous zeolitic
molecular sieve particle having a major dimension of less than 100 nanometers
will likely have
a defined configuration due to its crystalline structure. Some zeolites tend
to have a platelet-
type configuration whereas others, such as A1PO-14, have a rod-like structure.
Similarly,
polymeric, ceramic, glass and carbon molecular sieve particles may have
configurations that
are not readily changed. Hence, the configuration of the open regions between
particles can
vary widely.
[0099] Sometimes, the configurationally compatible particles are selected to
achieve at
least partial occlusion of the region. Thus, for spherical barrier particles
rod shaped or much
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smaller configurationally compatible particles may be desired. The
configurationally
compatible particles may be of any suitable composition given the size and
conditions of
operation. The particles may be polymeric, including oligomeric; carbon; and
inorganic such
as fumed silica, zeolite, alumina, and the like.
Detailed Description of the Drawing
[00100] With reference to Figure 1, a linear paraffin-containing feedstock is
supplied to an
isomerization unit via line 102. Hydrogen is provided via line 104. The
combined stream passes
to isomerization reactor 106. The effluent from isomerization reactor 106 is
directed via line 108
to stabilizer column 110. In stabilizer column 110, lights are removed as an
overhead via line
112. The lights maybe used for any suitable purpose including for fuel value.
The bottoms from
stabilizer column 110 are passed through line 114 to deisohexanizer 116. An
overhead is
provided via line 118 from deisohexanizer 116. A bottoms stream from
deisohexanizer 116 is
removed via line 120. A normal hexane-containing side stream from
deisohexanizer 116 is
passed via line 122 to the retentate side of membrane separator 124. A stream
having a lesser
concentration of linear paraffins is removed from separator 124 via line 128.
This stream will
contain an increased concentration of methylcyclopentane. As shown, line 128
directs the
retentate fraction for combination with the overhead in line 118. The permeate
fraction is
recycled via line 126 to isomerization reactor 106.
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