Note: Descriptions are shown in the official language in which they were submitted.
2022982
TRANSALKYLATION PROCESS
FIELD OF THE INVENTION
This invention relates to the transalkylation of
polyalkylated aromatic compounds and more particularly
tv alkylation-transalkylation processes involving
alkylation of a benzene feed stock with a C2-C4
alkylating agent and liquid phase transalkylation of
resulting polyalkylbenzenes, treatment of the alkylation
product in a separation zone, and recycle of at least a
portion of the transalkylation product to the separation
zone.
B
:7
1 n ..
N...,.
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2
BACKGROUND OF THE INVENTION
Processes for the alkylation of aromatic feedstocks
and the use of zeolite molecular sieve catalysts in
aromatic alkylation processes are well known in the art.
Such alkylation processes may be carried out in the
vapor phase, in the liquid phase, or under conditions in
which both liquid and vapor phases exist.
An example of vapor phase alkylation is found in
U.S. Patent No. 4,107,224 to Dwyer. Here, vapor phase
ethylation of benzene over a zeolite catalyst is
accomplished in a down flow reactor. The output from
the reactor is passed to a separation system in which
ethylbenzene product is recovered, with the recycle of
polyethylbenzenes to the alkylation reactor where they
undergo transalkylation reactions with benzene.
An example of an alkylation-transalkylation process
in which the output from the alkylation reaction zone is
passed directly to the transalkylation zone is disclosed
in U.S. Patent No. 3,551,510 to Pollitzer et al. In the
Pollitzer process, alkylation is carried out using an
alkylating agent, characterized as an olefin acting
compound, over a solid phosphoric acid alkylation
catalyst. The olefin acting compound may be selected
from materials such as monoolefins, diolefins,
polyolefins, actylenic hydrocarbons, alkyl halides,
alcohols, ethers and esters. The output from the
alkylation reaction zone, which includes
polyethylbenzenes, is supplied to a transalkylation
reaction zone along with an aromatic substrate, e.g.,
benzene. The transalkylation zone is loaded with an
acid extracted crystalline aluminosilicate catalyst,
specifically mordenite, and is operated in a upflow
mode. Exemplary transalkylation conditions including a
liquid hourly space velocity of 1.0, a pressure of 500
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3
psig and a temperature of 250°C. The output from the
transalkylation zone is supplied to a separation zone
from which a polyalkylarvmatic, e.g., polyethylbenzene,
is withdrawn and recycled to the alkylation reaction
zone.
Another alkylation transalkylation process is
disclosed in U.S. Patent No. 4,008,290 to Ward. Ward,
like the patent to Pollitzer, discloses the use of a
solid phosphoric acid catalyst in the alkylation zone.
.0 In the Ward process, benzene is reacted with propylene
to produce cumene. The output from the alkylation
reactor in Ward is split so that a portion, containing
principally benzene and cumene, is recycled to the
alkylation reactor. Another portion containing
5 principally benzene, cumene, propane and di-and tri-
isopropylbenzene is supplied to a separation zone. In
the separation zone a di- and tri-isopropylbenzene rich
stream is separated and supplied to a transalkylation
zone along with benzene. The transalkylation zone also
.0 contains a solid phosphoric acid catalyst. A cumene
rich effluent is withdrawn from the transalkylation zone
and recycled to the separation zone.
U.S. Patent No. 4,169,111 to Wight discloses an
alkylation-transalkylation process for the manufacture
5 of ethylbenzene employing crystalline aluminosilicates
in the alkylation and transalkylation reactors. The
catalysts in the alkylation and transalkylation reactors
may be the same or different and include low sodium
content zeolites, preferably less than 0.5 weight
0 percent Na20, having silica/alumina mole ratios between
2 and 80 and preferably between 4-12. Exemplary
zeolites include molecular sieves of the X, Y, L, B,
ZSM-5 and Omega crystal types with steam stabilized Y
zeolite containing about 0.2~ Na20 being preferred. The
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4
alkylation reactor is operated in a down flow mode and
under temperature and pressure conditions in which some
liquid phase is present. The transalkylation reactor,
which is described as generally requiring higher
temperatures than the optimum temperature for alkylation
in order to achieve maximum transalkylation efficiency,
is also operated in a down flow mode. In the Wight
procedure, the output from the alkylation reactor is
cooled and supplied to a benzene column from which
benzene is recovered overhead and recycled to the
alkylation reactor. The bottoms fraction from the
benzene column is supplied to an ethylbenzene column
from which ethylbenzene is recovered as the process
product. The bottoms product from the ethylbenzene
column is supplied to a third column which is operated
to provide a substantially pure diethylbenzene overhead
fraction which contains from 10 to 90% preferably 20 to
60% of the total diethylbenzene feed to the column. The
diethylbenzene overhead fraction is recycled to the
alkylation reactor while a side cut containing the
remaining diethylbenzene and triethylbenzene and higher
molecular weight compounds is supplied to the
transalkylation reactor along with benzene. The
effluent from the transalkylation reactor is recycled to
the benzene column.
U.S. Patent No. 4,774,377 to Banger et al.
discloses an alkylation/transalkylation process which,
like the above-described Wight process, involves the use
of separate alkylation and transalkylation reaction
zones, with recycle of the transalkylated product to an
intermediate separation zone. In the Banger process,
the temperature and pressure conditions are adjusted so
that the alkylation and transalkylation reactions take
place in essentially the liquid phase. The
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transalkylation catalyst is an aluminosilicate molecular
sieve including X-type, Y-type, ultrastable-Y, L-type,
omega type and mordenite type zeolites with the latter
being preferred. The catalyst employed in the
5 alkylation reaction zone is a solid phosphoric acid
containing material. Aluminosilicate alkylation
catalysts may also be employed and water varying from
0.01 to 6 volume percent is supplied to the alkylation
reaction zone. The output from the alkylation reaction
zone is supplied to first and second separation zones.
In the second reaction zone intermediate aromatic
products and trialkylaromatic and heavier products are
separated to provide an input to the transalkylation
reaction zone having only dialkyl aromatic components,
or diethylbenzene in the case of an ethylbenzene
manufacturing procedure or diisopropylbenzene in the
case of cumene production. A benzene substrate is also
supplied to the transalkylation zone for the
transalkylation reaction and the output from the
transalkylation zone is recycled to the first separation
zone. The alkylation and transalkylation zones may be
operated in a downflow, upflow or horizontal flow
configurations.
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6
SUMMARY OF THE INVENTION
In accordance with the present invention there is
provided an alkylation-transalkylation process involving
alkylation of an aromatic substrate with a C2-C4
alkylating agent coupled with separation to recover a
monoalkylated aromatic product and liquid phase
transalkylation of a polyalkylated product. In one
aspect of the invention, both the alkylation and
transalkylation reactions are carried out in the liquid
phase over molecular sieve aromatic alkylation and
transalkylation catalysts. The output from the
alkylation reaction zone is supplied to a separation
zone which is operated to produce a lower boiling
fraction comprising the aromatic substrate, which may be
recycled to the alkylation reaction zone, and a higher
boiling fraction comprising a mixture of monoalkylated
and polyalkylated aromatics. The higher boiling
fraction is supplied to a second separation zone to
produce a second lower boiling fraction comprising the
desired monoalkylated product and a higher boiling
fraction comprising polyalkylated product.
At least a portion of the polyalkylated fraction
including substantially all dialkylated and trialkylated
aromatics is supplied, along with the aromatic
substrate, to a transalkylation reaction zone containing
a molecular sieve transalkylation catalyst. The
transalkylation zone is operated under liquid phase
conditions to cause disproportionation of the
polyalkylated fraction to arrive at a disproportionation
product having a reduced polyalkylated aromatic content
and an enhanced monoalkylated aromatic content. At
least a portion of the disproportionation product is
supplied to the first separation zone. In a specific
application of the invention directed to the production
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of ethylbenzene or cumene, the output from the
transalkylation zone is supplied to a third separation
zone from which benzene and a monalkyl benzene fraction
(ethylbenzene or cumene) is recovered and recycled to
the separation zone.
In another embodiment of the invention, a benzene
feed stock and a C2-C4 alkylating agent are supplied to
an alkylation reaction zone containing a molecular sieve
alkylation catalyst and which is operated to produce an
alkylated product comprising a mixture of monoalkyl and
polyalkyl benzenes. In this embodiment of the invention
the alkylation zone may be operated under liquid phase
or vapor phase conditions with the output from the
alkylation zone being subjected to separation steps as
described above. The transalkylation reaction zone is
operated at an average temperature below the average
temperature of the alkylation reaction zone and under
conditions to maintain the benzene in the liquid phase.
In a specific application of this embodiment of the
invention to a procedure employing vapor phase
ethylation of benzene followed by liquid phase
transalkylation, the average temperature of the
transalkylation reaction zone is at least 50°C, and more
preferably 100°C, less than the average temperature of the
alkylation reaction zone.
In yet a further aspect of the invention involving
the alkylation of a benzene feed stock with a C2-Cq
alkylating agent, the alkylation catalyst is selected
from the group consisting of zeolite beta
and zeolite Y and the alkylation reactor is operated
under conditions to maintain the benzene feed stock in
the liquid phase as described previously. The effluent
from the alkylation reactor is subjected to separation
steps along the lines described above and subsequent to
B
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8
separation to recover the desired monoalkylbenzene
product, e.g., ethylbenzene or cumene, at least a
portion of the polyalkylbenzene fraction including
substantially all of the dialkylbenzene content and a
predominant portion of the trialkylbenzene content is
supplied to the transalkylation zone containing a
transalkylation catalyst comprising
zeolite Y. Preferably the alkylation catalyst comprises
zeolite beta.
-
In a further embodiment of the invention directed
specifically to the production of ethylbenzene, in which
the alkylation reaction takes place over an aromatic
alkylation catalyst comprising
zeolite beta, the output from the
alkylation reaction zone is supplied to a benzene
separation zone. A higher boiling fraction comprising
an ethylbenzene polyethylbenzene mixtures is supplied
from the benzene separation zone to an ethylbenzene
separation zone. This zone is operated to produce a
lower boiling product fraction comprising ethylbenzene
and a higher boiling fraction comprising
polyethylbenzene containing no more than 5 wt. %
ethylbenzene. The polyethylbenzene fraction is supplied
along with benzene to a transalkylation reaction zone
which preferably contains a transalkylation catalyst
comprising zeolite Y.
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9
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURES la-lc and 2a-2c are graphs illustrating the
results of transalkylation experiments carried out using
two different zeolite Y catalysts.
FIGURES 3a-3c are graphs illustrating the results
of experimental work carried out in the transalkylation
of diethylbenzene using zeolite Y.
FIGURES 4a-4c are a series of graphs showing
experimental work carried out with a rare earth zeolite.
FIGURES 5a, 5b and 6 are graphs illustrating
further experimental work employing a zeolite Y
catalyst.
FIGURE 7 is a simplified schematic flow diagram
illustrating one embodiment of the invention in which a
polyethylbenzene fraction is subjected to a residue
extraction step prior to transalkylation.
FIGURE 8 is an illustration is a schematic
illustration of a modification of the process of FIGURE
7 in which the output from the transalkylation reactor
is subjected to a separation step prior to recycle.
FIGURE 9 is a simplified schematic illustration of
yet another embodiment of the invention in which the
bottoms fraction from an ethylbenzene column is supplied
directly to a transalkylation reactor with the output of
the transalkylation reactor being supplied to a
downstream separation zone.
FIGURE 10 is a schematic flow diagram showing a
modification of the embodiment of FIGURE 9.
B
2022982
DETAILED DESCRIPTION
The preferred application of the invention involves
liquid phase alkylation over a molecular sieve
alkylation catalyst selected from the group consisting
of zeolite beta and zeolite omega coupled with liquid
phase transalkylation over a molecular sieve
transalkylating catalyst selected from the group
consisting of zeolite y and zeolite omega. An
especially preferred embodiment of the invention
involves the use of zeolite beta as an alkylation
catalyst and zeolite omega as a transalkylation
catalyst. However, as will appear below, other
molecular sieve catalysts can be employed in carrying
out the present invention. Moreover, while a preferred
application of the invention is in the use of liquid-
phase transalkylation in conjunction with liquid-phase
alkylation, the invention can be carried out employing
vapor-phase alkylation, as disclosed, for example, in
the aforementioned patent to Dwyer, coupled with liquid-
phase transalkylation and appropriate recycle of the
transalkylated product to a separation zone.
In its more general aspects, the invention involves
transalkylation coupled with aromatic alkylation
employing C2-C4 alkylating agents which, broadly stated,
can be alkylating agents of the type disclosed in the
aforementioned patent to Pollitzer et al., such as
olefins, alkynes, alkyl halides, alcohols, ethers and
esters. The most widely used alkylating agents are
ethylene and propylene applied in the production of
ethylbenzene and cumene, respectively. The~invention is
especially applicable to the ethylation of benzene under
conditions in a manner in which byproduct xylene yields
are reduced and the invention will be described
specifically by reference to the production of
2022982
ethylbenzene together with the attendant transalkylation
of polyethylbenzenes.
As noted previously, a conventional process for the
production of ethylbenzene involves recycling
polyethylbenzenes, separated from the ethylbenzene
product, to the alkylation reactor where they undergo
transalkylation to yield ethylbenzene. A byproduct of
this procedure is increased xylene yield in the effluent
from the alkylation reactor. The presence of xylenes
complicates downstream processing aid separation steps.
A particular impact of a significant xylene content in
the-product stream is that it often mandates operation
of the distillation column from which the ethylbenzene
is taken overhead in a manner to provide a substantial
ethylbenzene content, oftentimes 15-20% or more, in the
bottom polyethylbenzene fraction. For example,
ethylbenzene produced in accordance with the present
invention can be employed in the production of styrene
by catalytic dehydrogenation. The boiling points of
ortho xylene and styrene are very close, within 1C of
one another. As a practical matter, the ethylbenzene
specifications will call for a very low xylene content,
normally less than 2,000 ppm. In order to meet this
specification, it is normally necessary to operate the
ethylbenzene column under moderate distillation
conditions resulting in a high ethylbenzene content in
the bottoms fraction as described above. The present
invention, by carrying out polyethylbenzene
transalkylation in a separate reactor under relatively
mild liquid phase conditions, minimizes the xylene make
in the manufacturing process. This enables ethylbenzene
recirculation to be reduced by limiting the ethylbenzene
content in the polyethylbenzene fraction to 5 wt.% or
less and, where preferred catalysts are used to further
B
2022982
12
minimize xylene make, down to about 2 wt.% or less
ethylbenzene.
A preferred aspect of the present invention
involves supplying the polyethylbenzene fraction,
including both diethylbenzene and the triethylbenzene
and higher molecular weight compounds to the
transalkylation reactor as contrasted with separating
out a substantial portion of the diethylbenzene for
recycle to the alkylation zone, as disclosed in the
aforementioned patent to Wight, or separating out
trialkylaromatics with transalkylation only of
dialkylbenzene, as disclosed in the aforementioned
patent to Barger. In this respect, depending upon the
configuration of the interface of the transalkylation
reactor and polyethylbenzene or other separation zones,
substantially all of the diethylbenzene and
substantially all or most of the triethylbenzene content
will be supplied to the transalkylation reactor. In
either case, the practical effect of this embodiment of
the invention is that recycle to the alkylation reactor
is limited to benzene and lighter components, e.g.,
ethylene, while most, if not all of the triethylbenzenes
together with diethylbenzenes are retained in the system
ultimately for conversion to benzene and ethylbenzene.
This offers significant advantages over the prior art
processes, not only in terms of reduced xylene make as
described previously, but also in terms of ultimate
product yield.
In experimental work relative to the invention a
number of catalysts were employed in transalkylation
tests carried out in an upflow, flooded-bed reactor,
that is, only a liquid phase was in contact with the
catalyst. The feed employed in this experimental work
was an approximate 1:1 mixture of benzene and the
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13
polyethylbenzene overheads fraction from a commercial
operation employing vapor-phase alkylation of benzene to
produce ethylbenzene. A typical feed employed in the
experimental work had the composition as shown below in
Table I.
TABLE I
Component Wt.%
Non-aromatics 0.032
Benzene 50.241
Toluene 0.000
Ethylbenzene 6.117
p + M-Xylene 0.000
Styrene 0.063
o-Xylene 0.066
Cumene 3.973
n Propylbenzene 7.816
m + p Ethyltoluene 2.053
1,3,5-Trimethylbenzene 0.128
o-Ethyltoluene 0.356
1,2,4-Trimethylbenzene 0.536
1,2,3-Trimethylbenzene 0.401
m-Diethylbenzene 14.808
o + p-Diethylbenzene 7.328
Butylbenzenes 1.653
Heavies 4.429
In the experimental work, the average pressure was
about 300 psia with a pressure
drop across the reactor
ranging from about 5 to 15 psi. The temperature profile
across the reactor was r elatively constant with an
endotherm from the inlet to the outlet of less than 10C
and usually less than 5 C. The experimental runs were
initiated at relatively low temperatures, usually less
than 100C and progressi vely increased as described
~~ 2022982
14
later. The space velocity was maintained relatively
constant at a value of 6 hr-1 (LHSV) based on the total
hydrocarbon feed. Diethylbenzene conversions and
selectivity to ethylbenzene were measured as a function
of catalyst age (duration of the run) along with the
production of various other components including
xylenes.
In a first test run, the catalyst used was a
commercially available zeolite Y (identified herein as
Catalyst A) in which the inlet temperature was
progressively increased up to about 235°C and stabilized
there with an average temperature increase through the
reactor of only 1° or 2°C. The results of this
experimental work are illustrated in FIGURES la-lc in
which percent diethylbenzene conversion C, percent
selectivity to ethylbenzene, S, ortho xylene production
O, in ppm, and temperature, T, °C are plotted as curves
11, 12, 14 and 16, respectively versus the catalyst age
A, in hours, on the abscissa. As can be seen from an
examination of the data presented in FIGURE 1, the
diethylbenzene conversion stabilized in about the 32-37%
range for a reactor temperature of about 237°C with the
catalyst showing very little deactivation over the
duration of the run. The selectivity to ethylbenzene
was virtually 100%. During the run, O-xylene production
stabilized at about 400 to 500 ppm.
Another test run was carried out using an
experimental zeolite Y identified herein as catalyst B.
The results of this run are set forth in FIGURES 2a-2C
in which curves 18, 19, 21 and 22 are graphs of
diethylbenzene conversion, C, selectivity to
ethylbenzene, S, parts per million o-xylene, O, and
temperature, T, respectively plotted as a function of
catalyst Age A. In this experiment, the catalyst was
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run for nearly 400 hours with the temperature, after
initialization, increasing slightly with time to a final
value of about 240°C. As can be seen diethylbenzene
conversion was relatively good, mostly in the 30-40%
5 range at relatively moderate temperatures. Selectivity
to ethylbenzene was greater than 90% and during most of
the run was virtually at 100%. The o-xylene content of
the product stream stabilized at about 900 ppm.
Yet another test run was carried out employing a
10 zeolite Y catalyst identified herein as catalyst ~.
The results here in terms of diethylbenzene conversion,
selectivity and as a function of time and temperature
are set forth in FIGURES 3a-3c. In FIGURE 3 curves 24,
25, 27 and 28 are graphs of diethylbenzene conversion,
15 selectivity to ethylbenzene, o-xylene content (ppm), 0,
and temperature, T, °C as a function of catalyst age on
the abscissa. As shown in FIGURE 3, diethylbenzene
conversion was, on balance, slightly better than for
catalysts _A and _B, and fell generally into the 40-50%
range at reactor temperatures ranging from about 210° to
about 236°C. Selectivity to ethylbenzene was more than
90% over much of the run at virtually 100%. O-xylene
content stabilized at about 800-900 ppm. The catalyst
showed very ~_ittle deactivation over the life of the
run.
A rare earth zeolite Y identified herein as
catalyst _D was employed in yet another test. The
results for catalyst _D are set forth in FIGURES 4a-4c
with curves 30, 32, 33 and 35 representing graphs of
diethylbenzene conversion, selectivity to ethylbenzene,
ppm o-xylene and temperature, respectively, as a
function of catalyst age. Catalyst D showed relatively
good results including diethylbenzene conversion in the
40-50% range. Initial selectivity was about 100%, with
B
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16
selectivity falling off slightly to about 90% toward the
end of the run. While good conversion and selectivity
were achieved, the reaction temperature was
substantially higher than for zeolite Y of FIGURES 1-3;
rising to about 270°C at the conclusion of the run,
about 210 hours.
The feeds for the experimental work depicted in
FIGURES 1-4 conformed generally to the composition shown
in Table I. However, the feed for the first test run
(catalyst A) was free of ortho xylene and the feed for
the second run (catalyst B) contained about 0.02% para
and meta xylene. '
Additional experimental work under the above-
identified conditions were carried out employing three
additional catalysts; catalyst E, a cation exchange
resin available from Rohm and Haas under the designation
Amberlyst*15, catalyst F, a superacidic alumina
available from Harshaw-Filtrol under the designation
3998 and catalyst G a nickel modified mordenite
available from Union Carbide under the designation
Ni-Cn904G* Catalyst E showed little diethylbenzene
conversion and no ethylbenzene production up to the time
the experiment was terminated, at about 50 hours and a
temperature of 155°C, due to experimental difficulties.
Catalyst _F produced diethylbenzene conversions ranging
from about 10 to 20% at temperatures ranging from about
300°-450°C with selectivity to ethylbenzene for the most
part being less than 50%. Catalyst G was run for 100
hours at temperatures ranging up to 350°C and showed
almost no diethylbenzene conversion.
The zeolite Y catalysts identified above as
Catalyst A and B were also used in down flow trickle bed
reactors where a substantial gas phase was present.
Fresh and- regenerated catalysts were used. This
'" Trademark
B
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17
experimental work was carried out at pressures of about
330 psig, nominal space velocities of about lOhr-1
(LHSV) and average reactor temperatures of about 300°C
in the case of fresh catalyst A, about 300°-400°C in the
case of fresh catalyst H and about 200°C in the case of
the regenerated catalysts. For fresh catalyst A,
initial diethylbenzene conversion was about 24% but this
fell off rapidly after a few hours. The catalyst was
then regenerated and under the less severe temperature
conditions of about 200°C, initial diethylbenzene
conversion was high, about 60% but this, again, reduced
to only a few percent after about 24 hours.
When employing fresh catalyst B the initial
diethylbenzene conversion was over 50%, but this fell to
about 20% after about 5 hours and then decreased further
to only a few percent. The regenerated catalyst B, when
run at the lower temperature of about 200°C, showed an
initial diethylbenzene conversion of about 58% which
declined to about 27% after 29 hours, at which time the
run was terminated.
Yet additional experimental work was carried out
employing the zeolite Y identified above as catalyst B
in which the feed was a relatively pure diethylbenzene
mixed in approximately equal parts with benzene. Unlike
the feed stock employed in the experiment work of
FIGURES 1 through 4, the pure diethylbenzene feed stock
contained only very small amounts of material
susceptible to cracking or other conversion reactions,
e.g., deethylation, to produce xylenes and was also free
of xylenes. The make up of the feed stock in this
experimental work is set forth below in Table II.
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18
TABLE II
Components Wt.%
Non Aromatics 0.01
Benzene 56.58
Toluene 0.09
Ethylbenzene 0.01
Xylenes 0.0000
n-PR-BZ 0.02
m,p-ethyltoluene 0.03
o-ethyltoluene 0.01
124 trimethylbenzene
sec-BU-BZ 0.47
123 Trimethylbenzene
m,Diethylbenzene 27.62
L5 o,p-diethylbenzene 14.27
n-BU-BZ 0.35
Heavies 0.54
In this test run, the inlet and outlet pressures
were held at 310 and 305 psig, respectively. The
average temperature of the reactor was increased
approximately linearly with time from an initial value
of about 198° to a final value of about 298°C. The
space velocity was generally held within the range of
about 5.8-6.Ohr-1 (LHSV) with the exception of about
two-thirds of the way through the test where it fell to
about 5.1 before recovering to the higher value.
The results of this test run are set forth in
FIGURES 5 and 6. In FIGURE 5a, curve 38 is a graph of
temperature, T, versus catalyst age A in hours on the
abscissa. In FIGURE 5b curves 40 and 41 are graphs of
percent selectivity to ethylbenzene and percent
ethylbenzene conversion, respectively. Curve 42 is a
graph of the total xylene make, X, expressed in ppm,
based upon the amount of ethylbenzene produced. FIGURE
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19
6, shows the relationship between ethylbenzene
conversion and temperature. Curve 43 is a graph of
ethylbenzene conversion, C, on the ordinate versus
temperature, T, on the abscissa.
As indicated by the data set forth in FIGURE 5,
xylene make remained low throughout the test run. No
xylene was produced until the temperature was increased
to about 260°C (which generally corresponded to the
reduction in space velocity to about 5.1 hours-1 as
l0 reported previously). Percent conversion remained good
until the temperature was increased above 280°C. As
indicated in FIGURE 6, ethylbenzene conversion appears
to remain above 50% over a temperature range of about
200°-290°C with the optimum range appearing to be about
210° to 280°C.
With further reference to the drawings, FIGURE 7
through 10 illustrate schematic flow diagrams
illustrating different embodiments of the invention. It
will be assumed for purposes of discussion that the
invention is applied in the production of ethylbenzene
by reaction of ethylene with benzene and that the
alkylation reaction as carried out in a flooded-bed
liquid-phase alkylation reactor employing zeolite beta
or zeolite Y ~~s the alkylation catalyst.
However, as noted previously and as discussed in greater
detail below, the alkylation step can be conducted as a
vapor- phase reaction employing a catalyst such as
silicalite or zSM-5.
Referring first to FIGURE 7, a feed stream 50
containing ethylene and benzene supplied via lines 51
and 52, respectively, is passed first to a dehydrator
54, where the water content is reduced to a level of
about 100 ppm or less, preferably about 50 ppm or less,
and then to an alkylation reaction zone 56. The
B
1
2022882
alkylation reactor which may comprise a plurality of
series connected adiabatic reactors with interstage
infection of ethylene and also interstage cooling,
normally will be operated at an average temperature of
5 about 220°C and under sufficient pressure, about 600
psia or above, to maintain the benzene in the liquid
phase and at least about 2 mole percent of ethylene
solublized in the benzene. As an alternative to using
adiabatic reactors, one or more isothermal reactors can
10 be employed with suitable cooling~means used to maintain
a substantially constant temperature (little or no
temperature differential) from the inlet to the outlet
of the reactor. The effluent stream from the alkylation
reactor is supplied to a prefractionation column 58
15 which is operated to provide a light overheads fraction
including benzene which is supplied via line 59 to the
alkylation reactor input and a heavier liquids fraction
containing benzene, ethylbenzene and polyethylbenzenes.
The output from the prefractionation zone 58 is
20 supplied via line 60 to a benzene separation zone 61.
The overhead fraction from column 61 contains the
remaining benzene which is recycled via line 62 to the
alkylation reactor input. The heavier bottoms fraction
from column 61 is supplied via line 64 to an
ethylbenzene separation zone 65. The overheads fraction
from column 65, of course, comprises ethylbenzene which
is supplied to storage or to any suitable product
destination. By way of example, the ethylbenzene may be
used as a feed stream to a styrene plant in which
styrene is produced by the dehydrogenation of
ethylbenzene. The bottoms fraction containing
polyethylbenzenes, heavier aromatics and preferably only
a small amount of ethylbenzene, no more than 5% as
discussed previously, is supplied to polyethylbenzene
B
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21
separation zone 68. The bottoms fraction of column 68
comprises a residue. The overhead fraction from column
68, containing polyethylbenzene, triethylbenzene
(usually in relatively small quantities) and a minor
amount of ethylbenzene is supplied to a transalkylation
reaction zone 72. By minimizing the amount of
ethylbenzene recovered from the bottom of column 65, the
ethylbenzene content of the transalkylation feed stream
is kept small in order to drive the transalkylation
reaction in the direction of ethylb'enzene production.
The transalkylation zone is operated at a space velocity
(LHSV) based upon benzene and alkylbenzenes which is less than
the space velocity in the primary reaction zone based upon
benzene. Preferably, the transalkylation space velocity is
less than one half the space velocity of the primary
alkylation zone. The space velocity in the transalkylation
zone may be within the range of 1-10 LHSV. The trans-
alkylation zone is operated at a temperature from 50° to 300°C.
The polyethylbenzene fraction withdrawn overhead through
line 70 is mixed with benzene supplied via line 73 and
then supplied to the transalkylation reactor 72. The
mol ratio of benzene to polyethylbenzenes should be at
least 1:1 and preferably is within the range of 1:1 to
4:1. The output from the transalkylation reactor
containing benzene, ethylbenzene and diminished amounts
of polyethylbenzenes is supplied via line 75 to the
benzene column 61.
B
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In the process depicted in FIGURE 7, the alkylation
reaction is carried out in the liquid phase with
dehydration of feed to the alkylation reactor. As noted
previously, the invention may be carried out employing
vapor-phase alkylation followed by liquid phase
transalkylation and in such reactions, depending upon
the catalyst employed, significant quantities of water
may be included in the feed to the alkylation reactor.
In this case, it may be necessary to separately
accomplsih dehydration of the feed to the
transalkylation reactor. Such dehydration may take
place at any point upstream of the transalkylation
reactor, and if necessary, dehydration~should~be
accomplished with respect to the fresh benzene feed
supplied via line 73 as well as with respect to the
20
30
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polyethylbenzne component produced during the alkylation
reaction.
FIGURE 8 discloses a modification of the process
disclosed in FIGURE 7 in which the transalkylation
reactor output is subjected to further treatment prior
to recycle to the separation system. The embodiment of
FIGURE 8 is particularly useful in those cases in which
relatively high conversion is achieved in the
transalkylation reactor. In the embodiment of FIGURE 8,
the alkylation reactor and separation system is
l
identical to that of FIGURE 7 and like components are
indicated by the same reference characters. However,
the output from the transalkylation reactor is supplied
to a secondary separation zone 71 which may take the
form of a distillation column which is operated in a
manner to produce a bottom purge stream withdrawn via
line 78 and a recycle stream withdrawn via line 80 and
supplied to the benzene column.
The purge stream containing heavy hydrocarbons is
withdrawn from the system, thus providing a partially
single pass system in which high molecular weight
hydrocarbons are not recirculated.
FIGURE 9 illustrates yet another embodiment of the
invention in which the polyethylbenzene fraction
recovered from the ethylbenzene column is directly
passed to a transalkylation reactor. In FIGURE 9, the
same system components as shown in FIGURES 7 and 8 are
designated by like reference numerals. As shown in
FIGURE 9, the output from the ethylbenzene column 65 is
mixed with benzene supplied via line 82 and supplied to
the transalkylation reactor 84. Here, the entire
polyethylbenzene fraction is subjected to
transalkylation. The conditions employed in reactor 84
may be the same as described above with the ratio of
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benzene to polyethylbenzenes ranging from about 1:1 to
4:1.
It will be recognized that the procedure depicted
in FIGURE 9 is similar to that of FIGURE 8 except that
the entire bottoms fraction from the ethylbenzene column
is subjected to the transalkylation reaction. Limiting
the ethylbenzene content of the input to the
transalkylation reactor to no more than 5%, preferably
2% or less is especially significant here in
l0 establishing conditions promoting the transalkylation
reaction. The output from the transalkylation reactor
is applied via line 85 to a post transalkylation
separation zone 86 which may take the form of a
distillation column operated to produce an overhead
fraction that is comprised predominantly of benzene and
ethylbenzene and a bottoms fraction, composed
predominantly of C9 and C10 hydrocarbons such as
ethyltoluene, cumene, butylbenzene etc., which is
eliminated from the recycle stream by purge line 88.
The overheads fraction is recycled through line 89 to
the benzene column similarly as described above.
The embodiment of FIGURE 10 is similar to that of
FIGURE 9 except that the transalkylation reactor output
is split, with a portion being directly supplied to the
benzene column 61 via line 92 and the remainder to the
separation zone 86 which is operated as described above.
The configuration of FIGURE 10 provides a means for
maintaining a low concentration of C9 and C10
hydrocarbons in the system and reduces the energy costs
of operating column 86. Typically about 60%~or more of
the transalkylation reactor output is recycled directly
to the benzene column 61 with the remainder being
directed to the separation zone 86.
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Having described specific embodiments of the
present invention, it will be understood that
modification thereof may be suggested to those skilled
in the art, and it is intended to cover all such
modifications as fall within the scope of the appended
claims.
