Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02310216 2000-OS-29
"Alkylaromatic Process Using A Solid Alkylation Catalyst "
FIELD OF THE INVENTION
The invention relates to the alkylation of aromatic compounds with olefins
using solid catalyst.
s BACKGROUND OF THE INVENTION
About thirty years ago it became apparent that household laundry
detergents made of branched alkylbenzene sulfonates were gradually polluting
rivers and lakes. Solution of the problem led to the manufacture of detergents
made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade
io more rapidly than the branched variety.
LABS are manufactured from linear alkylbenzenes (LAB).
The petrochemical industry produces LAB by dehydrogenating linear paraffins to
linear olefins and then alkylating benzene with the linear olefins in the
presence
of HF. This is the industry's standard process. Over the last decade,
is environmental concerns over HF have increased, leading to a search for
substitute processes employing catalysts other than HF that are equivalent or
superior to the standard process. Solid alkylation catalysts, for example, are
the
subject of vigorous, ongoing research.
Solid alkylation catalyst processes tend to operate at a higher molar ratio
20 of benzene per olefin than processes that employ HF. Detergent alkylation
processes that use HF tend to operate at a benzene/olefin molar ratio of 12:1
to
6:1. Solid alkylation catalyst processes tend to run at higher benzene/olefin
ratios, typically 30:1 to 20:1. One reason for this is that solid alkylation
catalysts
tend to be less selective toward producing monoalkylbenzene, and therefore the
2s benzene/olefin molar ratio must be increased to meet increasingly stringent
selectivity requirements. Selectivity is defined as the weight ratio of
monoalkylbenzene product to all products.
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Solid catalysts deactivate with use. An alkylation process employing a
solid alkylation catalyst typically includes means for periodically
regenerating it
by removing the gum-type polymers that accumulate on the surface of the
catalyst and block reaction sites. For a solid alkylation catalyst, therefore,
the
s catalyst life is measured in terms of time in service at constant conversion
between regenerations.
Solid catalyst can be best used in the continuous alkylation of aromatics
where effective and inexpensive means of catalyst regeneration are available.
Solid catalysts used for alkylation of aromatic compounds by olefins,
io especially those in the 6 to 20 carbon atom range, usually are deactivated
by
by-products which are preferentially adsorbed by the catalysts. Such
by-products include polynuclear hydrocarbons in the 10 to 20 carbon atom
range formed in the dehydrogenation of C6 to C2o linear paraffins and also
include products of higher molecular weight than the desired monoalkyl
is benzenes, e.g., di- and tri-alkyl benzenes, as well as olefin oligomers.
Such
catalyst deactivating agents or "poisons" are an adjunct of aromatic
alkylation.
These deactivating agents can be readily desorbed from the catalyst by washing
the catalyst with the aromatic reactant. Thus, catalyst reactivation, or
catalyst
regeneration is conveniently effected by flushing the catalyst with aromatic
2o reactants to remove accumulated poisons from the catalyst surface,
generally
with restoration of 100% of catalyst activity.
Therefore, it is imperative to have means of repeatedly regenerating these
catalysts, i.e., to restore their activity, in order to utilize their
catalytic
effectiveness over long periods of time. It is further desirable to minimize
the
2s additional equipment required for regeneration.
Accordingly, an integrated continuous alkylation process with a method of
removing catalyst deactivation agents or minimizing catalyst deactivation is
sought.
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SUMMARY OF THE INVENTION
In one embodiment, this invention is an integrated process for producing
alkyl aromatics from paraffins and aromatics, for regenerating deactivated
solid
alkylation catalyst, and optionally for preventing catalyst-deactivating by-
products
s from contacting the solid alkylation catalyst. In this invention, the
effluent of a
solid catalyst alkylation reactor producing alkyl aromatics (e.g., detergent-
grade
alkyl aromatics) is separated to produce a relatively low-purity aromatic-
containing (e.g. benzene-containing) stream which is suitable for recycling to
an
on-stream solid catalyst reactor and a relatively high-purity aromatic-
containing
io stream (e.g., benzene-containing) stream which is suitable for passing to
an
off-stream alkylation reactor containing deactivated catalyst which is
undergoing
regeneration. A rectifier provides an economical way of producing the
relatively
low purity aromatic containing stream and maintaining a relatively high molar
ratio of aromatic (e.g., benzene) per olefin in an on-stream solid catalyst
bed,
is thereby helping to retard deactivation and extend the life of the solid
alkylation
catalyst. The bottom stream of the rectifier may pass to an aromatic
fractionation column, to produce the relatively high-purity aromatic-
containing
stream. Although some capital and operating costs are incurred in producing
this relatively high-purity stream, the aromatic column does not needlessly
2o recycle only the relatively high-purity stream to the on-stream alkylation
reactor.
Thus, savings accrue to the extent that the relatively low-purity stream such
as
that obtained from a rectifier, instead of the relatively high-purity
fractionation
column overhead stream, is recycled to the on-stream alkylation reactor.
In another aspect, this invention can be further integrated with a sorptive
removal
2s unit for removing aromatic by-products formed during paraffin
dehydrogenation,
because the high-purity stream of the is also suitable for regenerating an off-
stream sorptive bed in the sorptive removal unit.
One arrangement of the invention can use a single column to provide the
low purity and high purity aromatic-containing streams along with a bottom
3o stream comprising feed aromatics and alkylaromatics. In such an arrangement
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the single column will ordinarily take the high purity stream comprising the
aromatics which in most cases will contain benzene from the single column as a
net overhead stream. The relatively low purity aromatic or benzene-containing
stream will ordinarily be taken as a side cut stream at an intermediate tray
level
s of the single fractionation column. The side cut will ordinarily be taken
from an
intermediate elevation of the rectification section within the fractionation
column.
The bottom stream from the single column will contain the alkylaromatic
products
from the alkylation process along with any heavy alkylate by-product generated
in the alkylation zone in any paraffin recycle components.
io
When applied to a detergent alkylation process, the present invention can
use a rectifier to decrease the cost of recycling benzene to alkylation
reactors
that are producing detergent alkylate. The higher the benzene/olefin molar
ratio
in the on-stream detergent alkylation reactor, the greater is the benefit of
this
is invention. This benefit arises not only because rectification is a more
economical method of separating the alkylation reactor effluent than the
fractionation columns employed in the prior art processes, but also because
rectification produces a recycle stream that is sufficiently, but not overly,
pure
benzene-containing stream for recycling to the on-stream detergent alkylation
2o reactor. Thus, by using less of the relatively high-purity stream when the
relatively low-purity overhead stream suffices, this invention decreases the
costs
of recycling benzene to the alkylation reactor.
In one specific form, the benzene rectifier zone bottom stream passes to a
fractionation column, commonly known as the benzene column, which removes
2s most of the remaining benzene that was in the alkylation reactor effluent
and
produces a benzene column overhead stream having a higher purity than that of
the overhead stream produced by the benzene rectifier. Of course, it is within
the scope of this invention that some of the benzene column overhead stream
may be recycled to the on-stream detergent alkylation reactor, but the benefit
of
3o this invention is greatest when the flow of relatively high-purity benzene
from the
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benzene column overhead to the on-stream alkylation reactor is minimized.
One of the significant ways in which this invention can reduce the costs
associated with recycling benzene to an on-stream alkylation reactor is by
significantly decreasing the size of the benzene column. By removing some of
s the benzene from the on-stream alkylation reactor effluent prior to passing
the
remainder of the reactor effluent to the benzene column, the diameter, height,
and reboiler duty of the benzene column are reduced, because the benzene
throughput through the benzene column has been decreased. Although new
solid catalyst alkylation units can benefit from this advantage, this
advantage has
io far-reaching implications for solid catalyst alkylation processes that are
built by
converting existing HF detergent alkylation processes to solid alkylation
catalysts. This is because enough benzene can be removed from the alkylation
reactor effluent using a benzene rectifier that the remaining benzene in the
benzene rectifier bottom stream is not greater than the benzene content in the
is HF stripper bottom stream in an HF alkylation process. Therefore, with the
use
of a benzene rectifier between the alkylation reactor effluent and the benzene
column, the entire existing fractionation train of an existing HF alkylation
process
can be re-used when the catalyst is switched from HF to a solid alkylation
catalyst, resulting in large savings in investment capital for converting to a
solid
2o catalyst alkylation unit. Additional savings are possible because the
existing HF
stripper of the HF alkylation process can be readily modified and then used as
the benzene rectifier in the solid alkylation process and therefore much of
the
cost of a new benzene rectifier is avoided.
Accordingly, in one embodiment, this invention is a process for producing
2s alkylaromatics. Olefins and feed aromatics are reacted to form
alkylaromatics in
an on-stream alkylation zone in the presence of solid alkylation catalyst at
alkylation conditions. The alkylation conditions are sufficient to at least
partially
deactivate at least a portion of the solid alkylation catalyst in the on-
stream
alkylation zone. An on-stream effluent stream comprising alkylaromatics and
3o feed aromatics is withdrawn from the on-stream alkylation zone. At least a
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portion of the on-stream effluent stream is separated into a relatively impure
stream comprising feed aromatics and depleted in alkylaromatics and relatively
impure stream comprising feed aromatics in a higher purity than that of the
pure
stream and depleted in alkylaromatics and a bottom stream comprising feed
s aromatics and enriched in alkylaromatics. At least a portion of the
relatively pure
stream is recycled to the on-stream alkylation zone. Alkylaromatics are
recovered from the bottom stream. At least a portion of the relatively impure
stream comprising feed aromatics passes to an off-stream alkylation zone
containing at least partially deactivated solid alkylation catalyst. The
relatively
io impure stream contacts partially deactivated solid alkylation catalyst in
the off-
stream alkylation zone to partially regenerate the solid alkylation catalyst
and to
produce at least partially regenerated solid alkylation catalyst in the off-
stream
alkylation zone. An off-stream effluent stream comprising feed aromatics is
withdrawn from the off-stream alkylation zone. Periodically the functions of
the
is on-stream and off-stream alkylation zones are shifted by operating the off-
stream
alkylation zone to function as the on-stream alkylation zone and operating the
on-stream alkylation zone to function as the off-stream alkylation zone.
INFORMATION DISCLOSURE
2o US-A-5,648,579 (Kulprathinpanja et al.) teaches that solid catalyst used
for the alkylation of aromatic compounds by olefins usually are deactivated by
by-products which are preferentially adsorbed by the solid catalysts, and that
the
deactivating agents can be readily desorbed from the solid alkylation catalyst
by
washing the catalyst with the aromatic reactant.
2s US-A-5,276,231 (Kocal et al.) teaches an alkylaromatic process with
removal of aromatic by-products which are normally formed in paraffin
dehydrogenation by sorbing the aromatic by-products on a sorbent and
contacting the sorbent with liquid benzene to regenerate the sorbent.
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BRIEF DESCRIPTION OF THE DRAWING
The drawing is a process flow diagram of an embodiment of this invention.
DETAILED DESCRIPTION OF THE INVENTION
s This invention is an integrated process for producing alkyl aromatics by
alkylating aromatics with olefins using a solid alkylation catalyst and for
regenerating deactivated solid alkylation catalyst. The feedstocks which are
used in the practice of the invention normally result from the dehydrogenation
of
paraffins. The entire dehydrogenation reaction mixture often is used.
io The polyolefins formed during dehydrogenation are minimized in the
feedstocks
used in the practice of this invention. Consequently the feedstocks are a
mixture
largely of unreacted paraffins, branched monoolefins, and unbranched or linear
monoolefins. These paraffins and monoolefins typically are in the C6-C22
range,
although those in the Cg-C16 range are preferred in the practice of this
invention,
is and those in the Cip-C~4 range are even more preferred. The monoolefins in
the
feedstock are reacted with benzene or alkylated derivatives of benzene that
are
charged to the subject process. Suitable alkylated derivatives of benzene
(alkylaromatics) include, but are not limited to, toluene, xylenes, and higher
methylated benzenes; ethylbenzene, diethylbenzene, and triethylbenzenes;
2o isopropylbenzene (cumene), n-propylbenzene, and higher propylbenzenes;
butylbenzenes; and pentylbenzenes. Thus, the alkylated derivative of benzene
may have one or more alkyl groups, and each alkyl group may have from 1 to 5
or even more carbon atoms.
The most widely practiced alkylaromatic process to which the present
2s invention is applicable is the production of linear alkylbenzenes (LAB).
An LAB process usually charges normal paraffins to the dehydrogenation
reactor. Branched olefins formed in dehydrogenation are usually not removed,
Branched monoolefins in the feedstock are usually present in small
concentrations. As for the monoolefins in the feedstock, unsaturation may
CA 02310216 2000-OS-29
appear anywhere on the monoolefin chain, since there is no requirement as to
the position of the double bond. The monoolefins in the feedstock are reacted
with benzene, since the product of alkylating a monoolefin with an alkylated
derivative of benzene may not be as suitable a detergent precursor as
alkylated
s benzene. Although the stoichiometry of the alkylation reaction requires only
1
molar proportion of benzene per mole of total linear monoolefins, the use of a
1:1 mole proportion results in excessive olefin polymerization and
polyalkylation
creating large amounts of the dialkylbenzenes, trialkylbenzenes, possibly
higher
polyalkylated benzenes, olefin dimers, trimers, etc., and unreacted benzene.
To
io carry out alkylation with the conversion, selectivity, and linearity
required, a total
benzene: monoolefin molar ratio of from 5:1 up to as high as 30:1 is
recommended, with ratios of between about 8:1 and about 20:1 preferred.
The benzene and linear monoolefins are reacted in the presence of a
solid alkylation catalyst under alkylation conditions. These alkylation
conditions
is include a temperature in the range between about 80°C (176°F)
and about
140°C (284°F), most usually at a temperature not exceeding
135°C (275°F).
Since the alkylation is conducted as a liquid phase process, pressures must be
sufficient to maintain the reactants in the liquid state. The requisite
pressure
necessarily depends upon the feedstock and temperature, but normally is in the
2o range of 1480-7000 kPa absolute (200-1000 psi(g)), and most usually 2170-
3550 kPa(g) (300-500 psi(g)).
Solid alkylation catalysts typically have an acid function and are,
therefore, better known as solid acid catalysts. Such solid acid catalysts
include,
materials such as amorphous silica-alumina, crystalline aluminosilicate
materials
2s such as zeolites and molecular sieves, naturally occurring and man-made
clays
including pillared clays, sulfated oxides such as sulfonated zirconia,
traditional
Friedel-Crafts catalyst such as aluminum chloride and zinc chloride, and solid
Lewis acids generally. Solid alkylation catalysts are illustrated which
disclose an
extruded catalyst comprising clay and at least one multi-valent metal; U.S.
Patent
3o No. 5,034,564 issued to J.A. Kocal which discloses a catalyst comprising a
pillared
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clay and a binder; U.S. Patent Nos. 5,196,574 and 5,344,997, both issued to J.
A.
Kocal, which disclose a fluorided silica-alumina catalyst; U.S. Patent No.
5,302,732
issued to K.Z. Steigleder et al which describes an ultra-low sodium silica-
alumina
catalyst; and U.S. Patent No. 5,491,271 issued to Marinangeli et al. which
s discloses the use of either delaminated or pillared tetrahedrally charged
clays.
The effluent of the alkylation reaction zone, preferrably passes to a
rectifier. A rectifier differs distinctly from a stripper. The differences
between a
rectifier and a stripper are readily apparent by considering distillation
processes
in general. Distillation processes rely on the well-known tendency that when
io liquid and vapor phases contact, the more volatile components tend to
concentrate more in the vapor phase than in the liquid phase. In multi-stage
operation, a liquid descends a vertical distillation column and passes through
a
number of stages in which it is contacted countercurrently by ascending vapor.
The point at which feed is introduced to the distillation column divides the
column
is into two sections. The stripping section is below the feed point, and the
rectifying
section is above the feed point. In the stripping section, the more volatile
component is stripped from the descending liquid. In the rectifying section,
the
concentration of the less volatile component in the vapor is reduced. In
practice,
the stages in which the streams of liquid and vapor contact each other may be
2o trays or packing material. Therefore, in a rectifier the feed is at the
bottom of a
number of stages, in comparison to a stripper where the feed to a stripper is
at
the top of a number of stages. Furthermore, a rectifier reduces the
concentration
of the less volatile component in the vapor, whereas a stripper strips the
more
volatile component from the descending liquid.
2s A rectifier has generally from about 10 to about 20 separation stages and
usually uses sieve trays with a tray efficiency of about 60%. Thus, the
rectifier
generally has from about 15 to about 25 trays, and typically 20 trays. Fewer
than
15 trays could be used, and some or all of the trays could be replaced with a
vapor-liquid contacting media, such as regular-shaped Berl saddles or Raschig
3o rings in a random arrangement or such as structured elements in an ordered
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arrangement. The benzene rectifier usually employs a reboiler, either external
or
internal to the benzene rectifier, a feed preheater, or both. The benzene
rectifier
typically also employs a total condenser, which condenses a vapor or mixture
of
vapors, condensing generally more than 95 wt-%, and more commonly, more
s than 99.5 wt-% of the vapors. A portion of the condensed overhead stream is
typically refluxed to the upper portion of the benzene rectifier. The
remaining
portion of the condensed overhead stream recycles to an on-stream alkylation
reactor. As used herein in the context of a portion of a stream, the term
"portion"
means an aliquot portion or a nonaliquot portion, unless otherwise stated.
io An aliquot portion of a stream is a portion of the stream that has
essentially the
same composition as the stream.
The operating conditions of the benzene rectifier typically include a
pressure of from about 50 to about 70 psi(g) (345 to 483 kPa(g)), although
higher pressures up to the design limit of the vessel may be employed. The
is overhead and bottom temperatures of the benzene rectifier are normally
about
300°F (149°C) such that the benzene rectifier operates with
relatively little
difference between the overhead and bottom temperatures. The benzene
rectifier generally produces a bottom stream that contains a sufficient amount
of
benzene such that the boiling point of the bottom stream is relatively close
to that
zo of the overhead stream. Generally about 50 percent to about 70 percent of
the
benzene entering the rectifier exits with the net overhead stream.
The purity of the relatively impure benzene-containing stream such as that
recovered from the net overhead of the benzene rectifier, is relatively low in
comparison with the relatively high purity stream which may be recovered from
2s the net overhead of a benzene column. The relatively impure benzene-
containing net stream recovered from the overhead of the benzene rectifier
generally has a benzene concentration of from about 80 to about 98 mol-%. For
the benzene rectifier overhead stream, the concentration of paraffins is
generally
from 2 to 20 mol-% and preferably from 2 to 5 mol-%, and the concentration of
3o alkylated benzenes (alkylaromatics) is generally less than 100 wppm. The
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presence of paraffins in contact with the catalyst at normal alkylation
temperatures is believed to not have a significant detrimental effect on the
solid
alkylation catalysts, other than occupying volume in the reactor that could be
producing alkylated benzenes. Accordingly, despite its paraffin content, the a
s relatively impure liquid stream such as that of the benzene rectifier is a
suitable
source of benzene for recycling a portion or an aliquot portion thereof to the
alkylation reactor.
The paraffins present in the net overhead stream of the benzene rectifier
generally have from 5 to 22 carbon atoms. A first source of paraffins in the
io benzene rectifier overhead stream is the paraffins that accompany the
monoolefin-containing feedstock. Such paraffins typically have the same
number of carbon atoms as that of the monoolefins in the feedstock. Paraffins
also enter the benzene rectifier overhead stream with the benzene-containing
charge stream. The paraffins in this benzene charge stream have boiling points
is that are generally close to that of benzene.
A net bottom stream of the benzene rectifier, having a molar ratio of
benzene per alkylaromatic of about 7:1, may pass to a benzene column.
The benzene column can remove the remainder of the benzene using typically
from 45 to 55 sieve trays, usually about 50 sieve trays. The benzene rectifier
zo bottoms stream enters at or around sieve tray 30, as numbered from the top
of
the benzene column. Makeup benzene, which need not be previously dried,
may also be fed to the benzene column. The benzene column usually employs
a reboiler as well as a total condenser for the overhead stream, which
refluxes
liquid to the top of the benzene column. The operating conditions of the
2s benzene column include a pressure of about ) 170 kPa absolute (10 psi(g)),
an
overhead temperature of about 93°C (200°F), and a bottom
temperature of
about 232°C (450°F). The benzene column produces a net overhead
stream
which has a benzene concentration of usually more than 95 mol-%, preferably
more than 99.9 mol-%, and more preferably more than 99.99 mol-%. The
3o benzene column net overhead stream may also contain a small concentration
of
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paraffins of generally less than 5 mol-%, preferably less than 0.1 mol-%, more
preferably less than 100 wppm, and even more preferably less than 10 wppm.
In the benzene column net overhead stream, alkylated benzenes
(alkylaromatics), if any, are generally present at lower concentrations than
that of
s paraffins. Thus, in accordance with this invention the purity of the benzene
stream recovered from the overhead of the benzene column is generally greater
than that of the benzene stream recovered from the overhead of the benzene
rectifier.
The net overhead stream of the benzene column may contain paraffins
io having from 5 to 22 carbon atoms. The particular paraffins present in the
benzene column net overhead stream depend primarily on the paraffins in the
monoolefinic feedstock, the benzene-containing charge, and the purge stream,
if
any, of an aromatic by-products removal zone, if used.
In accord with this invention, a portion such as an aliquot portion of the net
is overhead liquid stream of the benzene column is passed to an off-stream
alkylation reactor containing solid alkylation catalyst that is undergoing
reactivation or regeneration. It is believed that the purity of the benzene
that is
used for regeneration of the solid alkylation catalyst is an important
variable, in
combination with the regeneration temperature, for insuring that the
regenerated
2o catalyst is returned to an acceptable level of activity for alkylating
reactions.
Without being bound to any particular theory, it is believed that the presence
of
paraffins in contact with the alkylation catalyst at the relatively high
temperatures
employed during regeneration has a detrimental effect on the catalyst.
Therefore, it is believed that a relatively pure stream, such as the net
overhead
2s liquid stream of the benzene column, with its lower paraffin concentration
relative
to a relatively impure stream such as the net overhead liquid stream of the
benzene rectifier, is a suitable stream for regenerating deactivated solid
alkylation catalyst.
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Thus in a preferred embodiment, the net overhead liquid stream of a
benzene column is passed to a bed of solid alkylation catalyst which is
undergoing regeneration. The effluent of the reactor that is undergoing
regeneration contains benzene, paraffins, alkylated benzenes, and heavy
s components desorbed from the catalyst. Although this effluent stream from
the
off-stream alkylation reactor could be passed to a benzene rectifier in the
same
manner as the alkylation reactor effluent during normal operation, it is
preferred
that this regeneration effluent stream passes to the benzene column. Thus, the
benzene column may be fed not only with benzene from the bottom of the
io benzene rectifier, and makeup benzene, but also benzene from the alkylation
reactor that is undergoing regeneration.
In one commonly employed arrangement, the bottom stream of the
benzene column passes to a paraffin column which produces an overhead liquid
stream containing unreacted paraffins, which normally is recycled as a recycle
is stream to the dehydrogenation zone, and a bottoms stream containing the
product alkylate and any higher molecular weight side product hydrocarbons
formed in the selective alkylation zone. This bottoms stream is passed into a
rerun column which produces an overhead alkylate product stream containing
the detergent alkylate and a bottoms stream containing polymerized olefins and
2o polyalkylated benzenes (heavy alkylate).
In another embodiment, this invention is an integrated process for
producing alkyl aromatics by dehydrogenating linear paraffins to linear
olefins
and then alkylating benzene with the linear olefins in the presence of a solid
alkylation catalyst, for regenerating deactivated solid alkylation catalyst,
and in
2s addition for preventing catalyst-deactivating by-products from contacting
the solid
alkylation catalyst.
The dehydrogenation section will preferably be configured substantially in
the manner shown in the drawing of US-A-5,276,231. A feed stream containing
paraffins combines with recycled hydrogen and recycled unreacted paraffins
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CA 02310216 2000-OS-29
from the alkylation section. This forms a reactant stream which is heated and
passed through a bed of a suitable catalyst maintained at the proper
dehydrogenation conditions of temperature, pressure, etc. Dehydrogenation
catalysts are well known in the dehydrogenation art, as exemplified by US-A-
s 3,274,287; US-A-3,315,007; US-A-3,315,008; US-A-3,745,112; and US-A-
4,430,517, and need not be described here in great detail. The effluent of
this
catalyst bed or reactor effluent stream is usually cooled, partially
condensed, and
separated to provide an effluent that is passed to the alkylation section.
A common variant of this embodiment includes the selective
io hydrogenation of diolefins that are normally present in the dehydrogenated
product stream. It is well known that diolefins are formed during the
catalytic
dehydrogenation of paraffins. Selective diolefin hydrogenation converts
diolefins
to monoolefins, which are the desired product of the dehydrogenation section,
and produces a selective diolefin hydrogenation product stream. Selective
is diolefin hydrogenation is taught in US-A-4,520,214 and US-A-5,012,021.
An aromatics removal zone eliminates or significantly reduces the
aromatic by-products in the feedstock to the selective alkylation zone in the
present embodiment for the production of alkylated aromatic compounds.
Removal of the aromatic by-products reduces the deactivation rate of solid
2o alkylation catalyst and, thereby, produces a significantly higher yield of
linear
alkylated aromatic compounds.
It is well known that aromatic by-products are formed during the catalytic
dehydrogenation of paraffins. These aromatic by-products are believed to
include alkylated benzenes, naphthalenes, other polynuclear aromatics,
zs alkylated polynuclear hydrocarbons in the C~p-C15 range, indanes, and
tetralins,
and may be viewed as aromatized normal paraffins. Typically, from about 0.2 to
about 0.7 weight percent, and generally no more than 1 weight percent, of the
feed paraffinic compounds to a dehydrogenation zone form aromatic by-
products. It is believed that these by-products are formed at least to a small
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CA 02310216 2000-OS-29
extent at suitable dehydrogenation conditions in the presence of most, if not
all,
commercially available dehydrogenation catalysts. In processes without
removal of aromatic by-products, the concentration of aromatic by-products in
the dehydrogenation effluent stream can typically accumulate to 4-10 weight
s percent, which leads to rapid deactivation of solid alkylation catalyst.
This embodiment of the invention selectively removes at least a portion of
the aromatic by-products in the dehydrogenated product stream using at least
one aromatics removal zone. The aromatics removal zone is preferably located
between the dehydrogenation zone and the selective alkylation zone because
io the aromatic by-products are preferably selectively removed prior to
entering the
selective alkylation zone. Suitable aromatics removal zones for this
embodiment
of the invention include sorptive separation zones Where the aromatics removal
zone is a sorptive separation zone, our invention can be practiced in fixed
bed or
moving sorbent bed systems, but the fixed bed system is preferred. The flow of
is the stream containing the aromatic by-products through the sorptive
separation
zones is preferably performed in a parallel manner so that when one of the
sorbent beds or chambers is spent by the accumulation of the aromatic
by-products thereon, the spent zone may be by-passed while continuing
uninterrupted operation through the parallel zone.
2o Suitable sorbents may be selected from materials which exhibit the
primary requirement of selectivity for the aromatic by-products and which are
otherwise convenient to use. Suitable sorbents include, for example, molecular
sieves, silica, activated carbon activated charcoal, activated alumina,
silica-alumina, clay, cellulose acetate, synthetic magnesium silicate,
2s macroporous magnesium silicate, and/or macroporous polystyrene gel. It
should
be understood that the above-mentioned sorbents are not necessarily equivalent
in their effectiveness. The choice of sorbent will depend on several
considerations including the capacity of the sorbent to retain aromatic
by-products, the selectivity of the sorbent to retain the aromatic by-products
3o which are more detrimental to solid alkylation catalysts, and the cost of
the
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CA 02310216 2000-OS-29
sorbent. The preferred sorbent is a molecular sieve, and the preferred
molecular
sieve is 13 X zeolite (sodium zeolite X).
Those skilled in the art are able to select the appropriate conditions for
operation of the sorbent without undue experimentation. For example, a fixed
s bed sorptive separation zone containing 13 X zeolite may be maintained at a
temperature generally from about 20°C to 300°C (68°F to
about 572°F) and
preferably from about 100°C to 200°C (212°F to about
392°F), a pressure
effective to maintain the stream containing the aromatic by-products in a
liquid
phase at the chosen temperature. and a liquid hourly space velocity from about
io 1 hr' to about 10 hr' and preferably from about 1 hr' to about 3 hr'. The
flow
of the stream containing the aromatic by-products through the sorptive
separation zone may be conducted in an upflow, downflow or radial-flow manner.
Although both liquid and vapor phase operations can be used in many
sorptive separation processes, liquid phase operation is preferred for the
sorptive
is separation zone because of the lower temperature requirements and because
of
the higher sorption yields of the aromatic by-products that can be obtained
with
liquid phase operation. Therefore, the temperature and pressure of the
sorptive
separation zone during sorption of the aromatic by-products are preferably
selected to maintain in a liquid phase the stream from which the aromatic
2o by-products are selectively removed. However, the operating conditions of a
sorptive separation zone can be optimized by those skilled in the art to
operate
over wide ranges which are expected to include the conditions in the reaction
zones of our invention and its variants. Therefore, this embodiment of our
invention includes a sorptive separation zone contained in a common reaction
zs vessel with the dehydrogenation zone, the selective diolefin hydrogenation
zone,
the selective alkylation zone or the selective monoolefin hydrogenation zone.
Following an appropriate processing period the sorbent, is regenerated
by removing the sorbed aromatic by-products from the sorbent. There are
numerous methods of regenerating the sorbent using and any suitable
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CA 02310216 2000-OS-29
regeneration method may be used, including altering the temperature and
pressure of the sorbent and treating the sorbent with a relatively pure stream
such as that obtained from the benzene column overhead stream to displace or
desorb the sorbed aromatic by-products. The flow direction of the benzene
s column overhead stream through the sorptive separation zone may be upflow or
radial flow, but the preferred direction is downflow. The phase of the benzene
column overhead stream mixture through the sorptive separation zone may be
liquid phase and/or vapor phase.
An effluent stream is withdrawn from the aromatics removal zone which
io contains benzene, a purge hydrocarbon such as pentanes where the zone was
purged with a purge hydrocarbon prior to being contacted with the benzene-
containing stream, and aromatic by-products produced during dehydrogenation.
This effluent stream is typically passed to a desorbent fractionation column,
which produces a heavy bottom stream comprising the aromatic by-products. If
is a benzene column overhead stream containing any feed paraffins passed to
the
aromatics removal zone, these feed paraffins would be present in the effluent
stream, and would ultimately appear in the heavy bottom stream of the
desorbent column. This is because the aromatics by-products and the paraffins
generally have the same carbon number, and hence both the aromatic by-
2o products and the paraffins co-boil at approximately the same temperature.
Because the paraffins, which can potentially be converted into desirable
alkylated aromatics, and the aromatic by-products, which cannot be readily
converted into the desired alkylaromatics, are recovered in the same stream,
the
rejection of the aromatic by-products from the desorbent column results also
in
2s the rejection of the paraffins. The higher the concentration of paraffins
in the
benzene column overhead stream to the aromatics removal zone, the greater is
the loss of these paraffins from the desorbent column with the aromatic by-
products. Accordingly, it is preferred that the regeneration stream for the
aromatics removal zone be a relatively pure stream such as that from the net
30 overhead liquid of the benzene column rather than a relatively impure
stream
CA 02310216 2000-OS-29
such as the net overhead liquid of the benzene rectifier because of its higher
purity. For this reason the relatively pure stream such as the benzene column
overhead stream, contains preferably less than 0.1 mol-% paraffins, more
preferably less than 100 wppm paraffins, and even more preferably less than
s 10 wppm paraffins.
The desorbent column also produces a net overhead stream which
contains the lighter components, namely benzene and a purge compound such
as pentane. This net overhead stream passes to a fractionation column which
separates the purge compound from the benzene. In the case where the purge
io compound is pentane, this separation zone is a depentanizing fractionation
column, which produces a net overhead stream comprising pentanes and a net
bottom stream comprising benzene. The net overhead stream is recovered for
use in purging the aromatics removal zone, and the net bottom stream is
recycled to the solid catalyst alkylation zone. In this manner, some of the
is benzene requirements for the solid catalyst alkylation zone is supplied by
the
fractionation column downstream of the desorbent column associated with the
aromatics removal zone, which in the aforementioned case is a depentanizer.
A complete operation of the process can be more fully understood from a
process flow for a preferred embodiment. Referring now to the drawing, a line
20 212 charges a paraffin feed stream comprising an admixture of C1o-C,5
normal
paraffins. (which will usually include recycle paraffins from line 174) to a
dehydrogenation section 210, that contacts the paraffins with a
dehydrogenation
catalyst in the presence of hydrogen at conditions that convert a significant
amount of the paraffins to the corresponding olefins. The product of the
2s dehydrogenation section comprises monoolefins, unreacted paraffins, and
aromatic by-products and passes via line 214 to an aromatic by-product removal
zone having a bed 230 on-stream for removing aromatic by-products and bed
220 off-stream for regeneration of the sorbent. Valve 222 is open and valve
226
is closed. The dehydrogenation section product passes through lines 218 and
30 224 and open valve 222 to enter on-stream bed 230, which removes aromatic
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CA 02310216 2000-OS-29
by-products. The effluent of bed 230 flows through lines 232, 234, and 258 to
depentanizer column 280, valve 240 being open and valve 250 being closed.
Most of the olefinic and paraffinic hydrocarbons entering depentanizer 280
through line 258 are heavier than pentane, and therefore exit via the bottom
of
s depentanizer 280 through line 278. Preferably, the concentration of C~-minus
paraffins in the bottom stream in line 278 is small. The hydrocarbons in line
278
combine with the benzene-containing net overhead liquid stream from benzene
rectifier 150, which flows through line 164.
The combined stream of olefins, paraffins, and benzene flows through
io lines 116, 122, and 286 via open valve 126 to alkylation reactor 110 to
alkylate
benzene with olefins. The on-stream reactor effluent passes through open valve
13 and line 136. Open valve 134 and line 138 combine with the on-stream
effluent of off-stream reactor 120 with the on-stream effluent. The combined
stream flows through line 142, is heated in heat exchanger 130, flows through
is line 146, is further heated in heat exchanger 140, flows through line 152,
and
enters benzene rectifier 150. Heat exchanger 130 supplies heat from the
benzene rectifier overhead vapor stream in line 144. The benzene-containing
overhead vapor stream in line 148 of the benzene rectifier 150 is further
condensed in condenser 160, flows through line 154, and enters overhead
2o receiver 170. A small net stream of light, uncondensed hydrocarbons is
withdrawn from receiver 170 via line 156. Line 170 supplies liquid to the
benzene rectifier 150 as reflux via line 166 and recycle to the on-stream
alkylation reactor 110 via line 164.
The benzene rectifier bottom stream flows through line 158 to benzene
2s fractionation column 180. In this depicted process arrangement, the benzene
fractionation column 180 is a separate vessel from the benzene rectifier 150.
A
benzene-containing makeup stream enters benzene column 180 through line
114. The bottom stream of the benzene column 180 flows through line 172 to
conventional product recovery facilities 190. The streams withdrawn from
3o recovery facilities 190 include a paraffin recycle stream 174, a heavy
alkylate
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CA 02310216 2000-OS-29
stream 178, and an alkylaromatic product stream 176. After condensing the net
liquid overhead stream from benzene column 180 flows via line 168 to off-
stream
bed 220 and to solid alkylation catalyst undegoing regeneration in off-stream
reactor 120.
s Accordingly, one portion of the stream in line 168 flows through lines 268
and 228 to bed 220, valve 272 being open and valve 266 being closed. Thus,
with bed 220 being off-stream there is no flow through line 216, and with bed
230
being on-stream there is no flow through line 264. The effluent flows through
lines 236, 244, and 256 to desorbent column 270, valve 242 being open and
io valve 250 being closed. Because bed 220 is off-stream with valve 250
closed,
there is no flow through line 248, and because bed 230 is on-stream with valve
238 closed there is no flow through line 246. Desorbent column 270 produces a
bottom stream 276 comprising aromatic by-products which is withdrawn from the
process and can be used for fuel. The desorbent column also produces an
is overhead stream 274 comprising benzene and pentanes which flows to
depentanizer 280. Depentanizer column 280 produces an overhead stream 260
comprising pentane, which is routed to storage facilities (not shown) for
maintaining a pentane inventory available for purging sorptive bed 230 when
that
bed is taken off-stream.
2o The other portion of the benzene-containing stream in line 168 flows
through lines 252, 254, and 288, and into off-stream reactor 120. Valve 262 is
open and valve 284 is closed. With reactor 120 being off-stream and reactor
110 being on-stream, there is no flow through line 282 nor is there flow
through
line 124. The benzene entering reactor 120 washes heavy by-products from the
2s alkylation catalyst that cause the catalyst to deactivate. Thus, the
effluent from
off-stream reactor 120 contains benzene as well as these by-products, which
can
include polynuclear hydrocarbons, polyalkylated aromatics, and olefin
oligomers.
The by-products in effluent of off-stream reactor 120 tend to concentrate in
the
bottom streams of benzene rectifier 150 and benzene column 180, and are
3o ultimately recovered by product recovery facilities 190 in the heavy
alkylate
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CA 02310216 2000-OS-29
stream 178. As an alternative to flowing to benzene rectifier 150, the
effluent of
off-stream reactor 120 could instead bypass the benzene rectifier 150 and flow
directly to benzene column 180.
The arrangement of the lines and valves upstream and downstream of the
s reactors 110 and 120 permit the functions of the on-stream reactor 110 and
the
off-stream reactor 120 to be periodically shifted. This shifting of functions
is
performed when the catalyst in the on-stream reactor 110 becomes sufficiently
deactivated as to render continued on-stream operation impractical or
uneconomical, or when the catalyst in the off-stream reactor 120 becomes
to sufficiently reactivated as to render it to be practicably or economically
operated
on-stream, or both. This shifting of functions can be accomplished by opening
the valves 128 and 284 which are closed and closing the valves 126 and 262
which are open. In an analogous manner, the arrangement of the lines and
valves upstream and downstream of the beds 220 and 230 also permit the
Is functions of the on-stream bed 230 and the off-stream bed 220 to be
periodically
shifted. on-stream bed 230 functions as the off-stream bed 220 and the off-
stream bed 220 functions as the on-stream bed 230.
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