Split stream reforming

A reformate richer in BTX content than the reformate obtained from the conventional catalytic reformation of a wide cut naphtha is obtained by splitting the naphtha into two fractions, catalytically reforming the heavy fraction sequentially through a series of at least three catalyst beds and introducing the light fraction into the feed to the last or the penultimate reactor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the catalytic reforming of naphtha. More 
particularly, it relates to the catalytic reforming of light and heavy 
fractions of naphtha. This invention especially relates to the 
fractionation of naphtha and the catalytic reforming of the several 
fractions in a fashion which provides an increased BTX yield. 
2. Description of the Prior Art 
The art of reforming naphtha hydrocarbons boiling in the gasoline boiling 
range has been practiced in one form or another for many years. Over these 
years the reforming process has developed to include regenerative and 
semi-regenerative operations in combination with operations wherein the 
total naphtha charge is passed sequentially through a plurarity of 
separate catalyst beds or separate fractions thereof are passed through 
one or more beds of reforming catalyst under conditions of operating 
temperature, pressure and space velocity considered most suitable for 
achieving desired reforming reactions. More recently reforming processes 
have been developed where the catalyst is regenerated continously. 
When hydrocarbons boiling in the gasoline boiling range come in contact 
with the dual functional catalysts employed in reforming, a number of 
reactions takes place which include dehydrogenation of cycloparaffins to 
form aromatics, dehydrocyclization of paraffins to form aromatics, 
isomerization reactions and hydrocracking reactions. A typical dual 
functional catalyst contains a metallic hydrogenation-dehydrogenation 
catalyst, typically 0.1 to 1.0 weight % Pt which is dispersed on an oxide, 
acidic catalyst such as alumina. These bring about dehydrogenation and 
isomerization of saturated parraffins. When the reforming conditions are 
quite severe, coke formation in the catalyst occurs with consequent 
deactivation of the catalyst. Thus, it is quite apparent that the 
composition of the naphtha charge will necessarily influence the severity 
of the reforming conditions employed to produce a desired product. 
However, the reforming operations, as we know them today, have certain 
built in limits because of reaction kinetics, catalysts available and 
equipment to perform the reforming operation. With the advent of unleaded 
gasoline requirements a renewed interest has been generated to further 
adapt the reforming operation of the production of high octane unleaded 
reformate gasoline product. 
The reforming art has suggested splitting a wide-boiling range petroleum 
fraction, often a full range or wide cut naphtha (100-430.degree. F.), 
into a lighter cut and a heavier cut and separately reforming the two cuts 
using optimium operating conditions to provide a particularly useful 
reformate. Such a split feed reforming process is disclosed in U.S. Pat. 
Nos. 3,432,425 of Bodkin et al., 3,753,891 of Graven et al. and 4,002,555 
of Farnham et al. In all three of these processes the light fractions and 
heavy fractions are separately reformed in parallel fashion although each 
fraction may be processed in a series of catalytic reactors. Bodkin et al. 
reforms the light fraction under more severe conditions then the heavy 
fraction while Graven et al. employs the opposite concept. Farnham uses a 
cascade hydrogen system so that the heavy fraction is reformed at a higher 
pressure than the light fraction. 
Although the naphtha feed is split into two fractions in U.S. Pat. No. 
3,647,679 of Kirk, Jr. et al., parallel reforming is not employed. Rather, 
the light naphtha is catalytically reformed serially in a number of 
reactors with the heavy fraction added to the feed entering the last 
reactor in the series. The light fraction has a boiling range below about 
390.degree. F. while heavy fraction boils in the range of about 
390.degree. to about 415.degree. F. The process is said to upgrade heavy 
naphtha without excessive coke formation and to provide an increased 
amount of reformate of increased octane. 
It is sometimes required to provide a reformate having particularly 
desirable properties for a special use or a reformate of a specifically 
useful composition, such as a reformate enriched in a 
benzene-toluene-xylene (BTX) fraction. This BTX fraction serves as the 
feedstock for a host of petrochemicals which are eventually transformed 
into fabrics, resins, molded products, films and a variety of other 
household and commercial products. U.S. Pat. No. 4,222,854 of Vorhis, Jr. 
et al. discloses a reforming process for the production of motor gasoline 
and BTX-enriched reformate by fractionating a naphtha feedstock into a 
mid-boiling BTX-precursor fraction, a high-boiling fraction and a 
low-boiling fraction. The BTX-precursor fraction is catalytically reformed 
in a first reforming zone to provide the BTX-enriched reformate while the 
high-boiling and low-boiling fractions are combined and catalytically 
reformed in a second reforming zone to provide motor gasoline. 
It is an object of this invention to provide a reformate enriched in BTX 
content. 
It is another object of this invention to provide an improvement in a 
conventional reforming process wherein the reformate will have a higher 
BTX content than is obtained in conventional naphtha reforming. 
It is a further object of this invention to provide a reformate having an 
enhanced BTX composition utilizing conventional reforming catalysts, 
equipment and operating conditions. 
The achievement of these and other objects will be apparent from the 
following description of the subject invention. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it has been found that a 
reformate enriched in BTX content can be provided by splitting the naphtha 
feed into two fractions, catalytically reforming the heavy fraction in 
serial fashion in at least three catalyst beds and introducing the light 
fraction into the feed to the last or penultimate reactor. 
In particular this invention relates to a process for improving the BTX 
composition when reforming naphtha boiling range hydrocarbons which 
comprises: 
(a) separating a naphtha into a low boiling fraction and a high boiling 
fraction, 
(b) contacting said high boiling fraction sequentially with a reforming 
catalyst situated in at least three reforming catalyst zones under 
effective reforming conditions and in the presence of hydrogen, 
(c) introducing the low boiling fraction as part of the feed to the last or 
the penultimate of the reforming catalyst zones, and 
(d) recovering from said last reforming catalyst zone a product effluent 
having a BTX composition substantially higher than if said naphtha were 
catalytically reformed with substantially the same catalyst under 
substantially the same reforming conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present process relates to a reforming process for converting a naphtha 
to a more useful product. In particular, the reforming process is operated 
in a fashion to improve the BTX yield in the reformate over that obtained 
in conventional reforming of a full range naphtha. The improvements of 
this invention are achieved by employing a split feed. The naphtha is 
split into a heavy fraction and a light fraction with the heavy fraction 
subjected to reforming in a conventional multi-reactor catalytic reforming 
unit. The light fraction is introduced along with the effluent from the 
prior reactor into a later reactor, the last or the penultimate in the 
series of at least three reactors. The improvement in BTX yield is at the 
expense of C.sub.9.sup.+ aromatics. The improvement obtained when 
practicing this invention is not specific to a particular reforming 
catalyst or a particular reformer configuration. Similar advantages can be 
expected from any commercially available reforming catalyst as well as 
from both fixed bed and moving bed systems. 
The hydrocarbon feed or naphtha charge to be processed by the method of 
this invention comprises a mixture of hydrocarbons boiling in the range of 
from about C.sub.5 hydrocarbons up to about 430.degree. F. end point i.e., 
a boiling range of about 100.degree. to about 430.degree. F. This boiling 
range includes naphthas in a light and heavy gasoline boiling range. The 
feed may be a straight run naphtha, a thermally cracked naphtha, a 
catalytically cracked naphtha, a hydrocrackate or blends thereof. 
The naphtha feed must be split into a low boiling (light) fraction and a 
high boiling (heavy) fraction when practicing this invention. The feed is 
separated into the desired fractions by conventional means, such as 
fractionation. The cut point range for the naphtha should be between about 
200.degree. to about 350.degree. F. Preferably, the cut point is 
substantially the mid-boiling point of the naphtha. For example, with a 
125.degree.-390.degree. F. naphtha, the light fraction could boil from 
about 125.degree. to about 250.degree. F. while its companion heavy 
fraction could boil from about 225.degree. to about 390.degree. F. 
The reforming proccess with which this invention is sucessfully employed is 
generally carried out in a plurality of interconnected and sequentially 
arranged reaction zones under conditions selected to promote 
dehydrogenation, dehydrocyclization and isomerization of at least 
C.sub.5.sup.+ hydrocarbons to higher octane products. 
At least three catalytic reaction zones are employed in the process of this 
invention although four reaction zones are preferred. Five or more 
catalytic reaction zones may be employed but are seldom found to be 
economically attractive. The catalyst may be present as a fixed or moving 
bed, the particular type being often determined by the frequency and 
nature of the catalyst regeneration. However, the beneficial results 
achieved by the process of the invention are not specific to the type of 
catalyst bed or regeneration technique employed. 
Suitable reforming conditions include reforming temperatures in the range 
of from about 800.degree. F. to about 1000.degree. F. and more usually 
temperatures of at least 850.degree. F. The reforming pressure employed 
may be as high as about 1000 p.s.i.g., however, it is preferred to employ 
lower operating pressures for economic reasons in the order of about 500 
p.s.i.g. or lower. Pressures as low as 100 p.s.i.g. may be employed to 
advantage in some operations. Liquid hourly space velocities of the 
reactants may also be varied over a relatively wide range of from about 
0.1 up to about 10 but usually not substantially greater than about 4. In 
general, it is preferred to maintain an excess of hydrogen in combination 
with the naphtha being reformed so that the mole ratio of hydrogen to 
hydrocarbon charge employed may be in the range of from about 1 to about 
20, preferably from about 4 to about 12. 
The catalyst employed in the reforming reaction zones may be selected from 
a number of known reforming catalysts of the prior art which include for 
example, a catalyst comprising alumina in the eta or gamma form or 
mixtures thereof in combination with a noble metal. Platinum series metals 
such as platinum, palladium, osmium, irridium, ruthenium, or rhodium 
deposited on a suitable support comprising alumina is preferred. 
Generally, the alumina comprises a major portion of the catalyst and may 
comprise 95% by weight or more of the catalyst. It is contemplated, 
however, combining other components with the alumina such as the oxides of 
silica, magnesium, zirconium, thorium, vanadium, titanium, boron or 
mixtures thereof. In another embodiment the plantinum-alumina complex 
either with or without one or more of the above components such as silica 
etc. may also be promoted with small amounts of halogen such as chlorine 
or fluorine in amounts ranging from about 0.1% up to about 3% by weight. 
Metal promoters such as rhenium, tin, iridium, etc. in amounts ranging 
from 0.01 to 5% by weight may also be employed. However, in a preferred 
embodiment the reforming catalyst carrier material is preferably a high 
surfaced area material, primarily gamma alumina material of at least 200 
and preferably 300 or more square meters per gram. This alumina carrier 
material is impregnated with a platinum type hydrogenating component 
described above in amounts ranging up to about 1% by weight but generally, 
not substantially over about 0.6% by weight. This catalyst may also be 
promoted with one or more of the other catalyst components above described 
and known in the art. A particularly preferred catalyst is composed of 
platinum and rhenium on an alumina support. 
It is to be understood that a naturally occurring or synthetically prepared 
alumina with or without silica may be employed as a carrier material or 
support for the platinum type hydrogenating component. Preferably, the 
platinum-alumina catalyst employed comprises a high surface area material 
such as an eta or gamma base alumina discussed above. Before use, the 
catalyst is reduced in a hydrogen atmosphere under conditions to maintain 
the catalyst in a relatively dry moisture free atmosphere before being put 
on-stream since it has been found that at a given moisture and certain 
related temperature level that a relationship exists which decreases the 
surface area and has a simultaneous deactivating effect on the catalyst. 
Accordingly, it is preferred to employ in the reforming step of this 
invention, relatively dry reforming conditions and this is particularly 
true when employing relatively low pressure reforming conditions. 
In the practice of this invention the heavy naphtha fraction passes through 
the entire sequence of catalytic reforming reactors while the light 
fraction only is processed in the last or the last two reactors in the 
series. The heavy stream contacts the reforming catalyst for a much longer 
period than the light stream. Hence, at a given severity the degree of 
reforming reaction for the heavy fraction is much higher than the light 
fraction. As compared to conventional reforming of a wide cut naphtha, the 
process of this invention results in a higher conversion of feed to heavy 
aromatics due to the combination of longer contact time and improved 
reformability of the heavy stream. It is thought that the heavy aromatics 
undergo further dealkylation reactions, which results in an improved BTX 
yield. 
Having provided a general discussion of the process of this invention, 
reference is made to FIG. 1 which depicts a flowplan of an embodiment of 
the invention. A wide-cut straight run naphtha having a boiling range of 
about 125.degree.-390.degree. F. is passed through line 2 to fractionator 
4. The naphtha is split in fractionator 4 into a light naphtha fraction 
having a boiling range of about 125.degree.-250.degree. F. which passes 
from the top of the fractionator through line 6 and a heavy naphtha 
fraction having a boiling range of about 225.degree.-390.degree. F. which 
passes from the bottom of the fractionator through line 8. The heavy 
naphtha is then conveyed to the multi-reactor reforming unit by pump 10 
and line 12. Recycle hydrogen from line 14 is combined with the heavy 
naphtha in line 12. 
The reforming unit consists of a series of four furnaces, 16a, 16b, 16c and 
16d, four fixed bed reactors, 20a, 20b, 20c and 20d, piping 18a, 18b, 18c 
and 18d, conducting the reaction mixture from each furnace to its 
corresponding reactor and piping 22a, 22b and 22c, conducting the reactor 
effluent from each reactor to the next downstream furnace. Each reactor 
contains a fixed bed of reforming catalyst, platinum-rhenium on alumina or 
some other suitable reforming catalyst. The heavy naphtha is passed 
through reactors 20a, 20b, 20c and 20d in serial fashion under effective 
reforming conditions. Typically, the heavy naphtha passes through furnace 
16a, line 18a, reactor 20a and line 22a. The latter delivers the reaction 
mixture to the next reactor in line. 
Returning to the light naphtha recovered from fractionator 4 through line 
6, this stream is conveyed by pump 24 and line 26 to line 22b which is 
conducting the effluent from the second reactor (reactor 20b) to the third 
furnace (furnace 16c). The light naphtha is combined with the effluent 
stream from the second reactor 20b. The combined mixture is then passed 
through the final two reactors (reactors 20e and 20d) under effective 
reforming conditions. Where the light naphtha is of a quality which does 
not require as much reforming as is provided by the last two reactors of 
the reforming unit, it may be combined with the reaction mixture in line 
22c so as to only pass through the last furnace and the last reactor 
thereby only being reformed to the extent provided by reactor 20d. 
The total product from the reforming unit is then passed by line 28 to 
cooler 30. From the cooler it is passed by line 32 to separator 34. In 
separator 34, hydrogen-rich gaseous material is separated from the 
reformed naphtha product and withdrawn through line 36 for recycling to 
reactor 20a by means of compressor 38. This hydrogen rich stream supplies 
hydrogen required for reforming. A portion of the gaseous material may be 
removed from line 36 through line 40 and make-up hydrogen may be added 
through line 42 to maintain the required hydrogen quality. The reformed 
naphtha product, having a significantly enhanced BTX content, is recovered 
from separator 34 through line 44. 
In an optional embodiment which further improves the BTX yield, the recycle 
hydrogen gas is split with a portion of it supplied through line 14 to the 
feed to the first reactor 20a and a portion of it supplied through line 46 
to the feed to the third reactor 20c. The feed to the third reactor is the 
effluent from the second reactor 20b plus the light naphtha stream. The 
split hydrogen recycle scheme will result in a lower average partial 
pressure thereby further improving BTX yield. However, this advantage is 
obtained at the expense of somewhat shorter reformer cycle length. 
The following example illustrates the practice of this invention. 
A kinetic model of a fixed bed reforming unit was employed to demonstrate 
the practice of this invention as compared to the conventional reforming 
of a wide cut straight run naphtha. In this computer simulation a first 
portion of a wide cut naphtha was split into a light fraction and a heavy 
fraction. The properties of the straight run naphtha and the light and 
heavy fractions thereof are shown in Table 1 below. The reforming unit 
employed in this simulation was a four reactor fixed bed reforming unit 
employing a catalyst of platinum-rhenium supported on gamma-alumina. The 
catalyst loading per bed and the operating conditions are shown in Table 2 
below. In the computer simulation, operation A was a conventional 
reforming process where the entire wide cut naphtha was introduced into 
reactor no. 1 and reformed sequentially in the four reactors. In operation 
B, the reforming process, in accordance with the subject invention, was 
employed where the heavy fraction was introduced into reactor no. 1. and 
the light fraction was introduced into reactor no. 3 (this is the 
operation shown in FIG. 1 and discussed herein above). 
Two evaluations were made, one at a reformate octane (R+O) level of 96 and 
other at 98 octane. The yield analysis for these four runs is shown in 
Table 3 below. The advantages of operating a reforming unit in accordance 
with the subject invention are clearly evident. At the 96 octane level the 
BTX yield for the conventional process is 29.9% by weight while for the 
process of the invention it is 32.4% by weight. Similarly, at the 98 
octane level, the conventional process BTX yield is 32.1% by weight 
compared to 34.3% by weight for the process of the invention. 
TABLE 1 
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FEED PROPERTIES 
WIDE CUT LIGHT HEAVY 
NAPHTHA FRACTION FRACTION 
______________________________________ 
Specific Gravity 
0.7349 0.696 0.763 
Paraffins, wt % 
68.5 80.3 60.9 
Naphthenes, wt % 
18.7 15.6 20.8 
Aromatics, wt % 
12.8 4.1 18.3 
ASTM, D-86 
IBP 167 139 257 
10 195 158 278 
30 228 172 289 
50 265 185 299 
70 295 205 314 
90 331 234 341 
EP 376 256 384 
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TABLE 2 
______________________________________ 
OPERATING CONDITIONS 
OPERATION A B 
______________________________________ 
No. of Reactors 4 4 
Catalyst Pt-- Re Pt-- Re 
on on 
Alumina Alumina 
Catalyst Loading, Vol % 
RX 1 9.5 9.5 
RX 2 25 25 
Rx 3 30 30 
Rx 4 30 30 
Reactor Pressure, psia 
350 350 
Recycle Ratio, Overall 
8.0 8.0 
Lt Stream Inlet 1st 3rd 
LHSV, OVERALL 1.5 1.5 
Mol % Lt in Feed 42 42 
______________________________________ 
Note: 
A conventional reforming process 
B reforming process of invention 
TABLE 3 
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OPERATION A B A B 
______________________________________ 
OCTANE (R + O) .rarw. 96 .fwdarw. 
.rarw. 98 .fwdarw. 
Yield 
Benzene, wt % 2.5 2.6 2.9 2.9 
Toluene, wt 9.4 9.7 10.5 10.7 
Xylene, wt 18.0 20.1 18.7 20.7 
Total BTX, wt % 
29.9 32.4 32.1 34.3 
Total Arom., wt % 
45.7 45.8 47.7 47.6 
______________________________________ 
Note: 
A Conventional reforming process 
B Reforming process of invention 
FIG. 2 presents the relationship between C.sub.5.sup.+ yield and octane 
(R+O) for the light naphtha (290-.degree. F.) and the heavy naphtha 
(290+.degree. F.) catalytically reformed in accordance with conventional 
reforming scheme. The yield-octane relationship of both streams indicates 
that the decline in yield for the heavy stream, as the reformate octane 
increases, is much smaller than that for the light stream. Thus the 
difference in yield selectivity between the two streams is throught to be 
the reason why higher BTX yield is obtained when practicing the process of 
this invention. Also, this difference in yield selectivity may be further 
exploited to achieve higher reformate yield.