Method of preparing benzene and xylenes

A method of preparing benzene and xylenes from catalysates of reforming of gasoline fractions comprising a mixture of aromatic C.sub.6 -C.sub.10 hydrocarbons and non-aromatic hydrocarbons which involves separation of a low-boiling fraction boiling out at a temperature of 90.degree.-108.degree. C. from a reforming catalysate by rectification. The remaining high-boiling fraction is processed in the presence of a hydrogen-containing gas at a temperature within the range of from 450.degree. to 600.degree. C. under a pressure of from 10 to 60 atm on a catalyst. The catalyst consists of 1 to 85% by weight of H-mordenite, 0.1 to 10% by weight of a hydrogenating component as which use might be made of oxides of metals of Group VI of the periodic system, sulphides of these metals, metals of Group VIII of the periodic system, sulphides thereof; the balance being a binder. As a result, a liquid product is formed comprising a mixture of aromatic hydrocarbons which is separated by rectification to give benzene, toluene, total xylenes, a mixture of aromatic C.sub.9 -C.sub.10 hydrocarbons; at least one of the following streams of hydrocarbons prepared by rectification of the liquid product is recycled back to the stage of processing of a high-boiling fraction: toluene, toluene along with a portion of hydrocarbons boiling out at a temperature exceeding the boiling point of toluene. The method according to the present invention makes it possible to increase the benzene yield by 3 to 4 times and the yield of xylenes by 1.2-2 times as compared to the content thereof in the reforming catalysate. The method also enables a substantial simplification in the isolation of the desired products.

The present invention relates to methods of preparing aromatic hydrocarbons 
and, more specifically, benzene and xylenes. 
Said aromatic hydrocarbons are extensively used in different branches of 
industry. Benzene and xylenes comprise the most valuable aromatic 
hydrocarbons. Benzene is widely used for the manufacture of a great 
variety of intermediate products such as cyclohexane, ethylbenzene, cumene 
and the like which are, in turn, useful in the production of synthetic 
materials, fibers, resins, and rubbers. Among xylenes the most utilizable 
are p- and o-isomers of xylene employed for the production of terephthalic 
acid, phthalic anhydride and, therefrom, polyester fibers, resins, 
varnishes, and plastifying agents. 
Aromatic hydrocarbons are prepared mainly by way of catalytic reforming of 
gasoline fractions on aluminoplatinum catalysts. The reforming catalysates 
comprise a mixture of aromatic and non-aromatic, predominantly paraffin, 
hydrocarbons. Aromatic hydrocarbons cannot be recovered from the reforming 
catalysates by a conventional rectification, since they form azeotropic 
mixtures with non-aromatic hydrocarbons. Therefore, preparation of 
aromatic hydrocarbons from reforming catalysates is performed by way of 
extraction thereof with selective solvents such as diethylene and 
triethylene glycols, furfurol, sulpholane and the like. To prepare benzene 
and toluene, a fraction of a straight-run gasoline of 
60.degree.-105.degree. C. is subjected to catalytic reforming. To prepare 
simultaneously benzene, toluene and xylenes subjected to reforming is a 
fraction of 60.degree.-140.degree. C. To prepare mainly xylenes subjected 
to reforming is a fraction of 105.degree.-140.degree. C. If a wider 
fraction is subjected to reforming such as 60.degree.-180.degree. C., then 
besides benzene, toluene and xylenes there is formed a great number of 
other aromatic C.sub.9 -C.sub.10 hydrocarbons. 
The modern bi- and poly-metallic reforming catalysts enable the production 
of catalysates containing 60 to 80% by weight of aromatic hydrocarbons. 
Such are the yields of aromatic compounds in the reforming of wide 
fractions 60.degree.-180.degree. C., 85.degree.-180.degree. C., 
70.degree.-180.degree. C.; resources of these fractions are especially 
vast and constitute 15 to 20% as calculated for petroleum. 
Recovery of aromatic hydrocarbons by extraction from reforming catalysates 
has the following disadvantages. Upon extraction only such aromatic 
hydrocarbons are obtained which are present in the reforming catalysate. 
In reforming catalysates of wide gasoline fractions about 30 to 40% of 
aromatic hydrocarbons constitute aromatic C.sub.9 -C.sub.10 hydrocarbons 
which are practically non-utilized in the organic synthesis, about 20 to 
25% is the share of toluene which also has a quite limited application. At 
the same time, content of benzene in reforming catalysates of wide 
fractions accounts for only 1 to 6% by weight. Therefore, the content of 
most valuable aromatic hydrocarbons, i.e. benzene and aromatic C.sub.8 
(total xylenes) is, as a rule, below 50% by weight of the total amount of 
aromatic hydrocarbons. 
For the reasons discussed hereinabove, to increase the yield of most 
valuable products, the extracted aromatic hydrocarbons are treated in 
additional processes. Toluene is hydrodealkylated to benzene according to 
the reaction: 
EQU C.sub.6 H.sub.5 CH.sub.3 +H.sub.2 .fwdarw.C.sub.6 H.sub.6 +CH.sub.4 
or disproportionated to benzene and xylenes: 
EQU 2C.sub.6 H.sub.5 CH.sub.3 .fwdarw.C.sub.6 H.sub.4 (CH.sub.3).sub.2 +C.sub.6 
H.sub.6 
to obtain the maximum possible yield of xylene, toluene and aromatic 
C.sub.9 hydrocarbons are transalkylated to xylenes: 
EQU C.sub.6 H.sub.5 CH.sub.3 +C.sub.6 H.sub.3 (CH.sub.3).sub.3 .fwdarw.2C.sub.6 
H.sub.4 (CH.sub.3).sub.2 
most valuable isomers of xylenes are p- and o-xylenes. To increase the 
yield thereof, less valuable aromatic C.sub.8 hydrocarbons (m-xylene and 
ethylbenzene) are isomerized to p- and o-xylenes. 
Therefore, the prior art method of preparing benzene and xylenes (including 
p- and o-xylenes) from reforming catalysates of wide fractions comprises 
extraction of aromatic hydrocarbons and subsequent performance of a series 
of processes such as dealkylation, disproportionation, transalkylation, 
and isomerization. 
Known in the art is another method of preparing pure aromatic hydrocarbons 
from aromatized gasolines by means of processes of 
hydrocracking-hydrodelakylation, wherein non-aromatic hydrocarbons are 
hydrocracked to gaseous products, while alkylaromatic hydrocarbons are 
dealkylated to benzene which is recovered by rectification. These 
processes are usually performed with or without catalysts at a temperature 
within the range of from 500.degree. to 800.degree. C. 
The method of hydrocracking-hydrodealkylation is used mainly for processing 
of pyrolysis gasolines containing small amounts of non-aromatic 
hydrocarbons (10 to 25% by weight) and the content of benzene is higher 
than that of homologues thereof. Utilization of this process for the 
treatment of reforming catalysates is poorly efficient due to the 
unfavorable composition of the aromatic hydrocarbons in this feedstock. As 
it has been mentioned hereinbefore, more than 30% by weight of aromatic 
hydrocarbons in a reforming catalysate are represented by aromatic C.sub.9 
-C.sub.10 hydrocarbons and dealkylation thereof to benzene is accompanied 
by substantial losses of the liquid product and, accordingly, by a high 
consumption rate of hydrogen as it can be seen from the stoichiometric 
character of the reaction: 
EQU C.sub.6 H.sub.3 (CH.sub.3).sub.3 +3H.sub.2 .fwdarw.C.sub.6 H.sub.6 
+3CH.sub.4 
even upon a selective performance of the process, dealkylation of aromatic 
C.sub.9 -C.sub.10 hydrocarbons proceeds so that about 35 to 40% by weight 
of the feedstock is converted to a gas. The total xylenes (aromatic 
C.sub.8 hydrocarbons) present in the feedstock are converted to benzene 
also with a substantial weight loss (27%). At the same time, xylenes are 
not less valuable than benzene. 
Hydrocracking of paraffin hydrocarbons present in the feedstock also 
necessitates a high consumption rate of hydrogen, as it can be seen from 
the stoichiometric character of the reaction: 
EQU C.sub.7 H.sub.16 +6H.sub.2 .fwdarw.7CH.sub.4. 
all these drawbacks results in great losses of the liquid product, 
considerable gas-formation and high consumption of hydrogen. Thus, upon 
hydrodealkylation of a reforming catalysate of a wide fraction on a 
catalyst containing Pt, Pd, Cr.sub.2 O.sub.3 supported by Al.sub.2 O.sub.3 
more than half of the starting feedstock is converted to gas and hydrogen 
consumption is about 10% of the total amount of the feedstock. 
The above-mentioned disadvantages are responsible for the fact that 
reforming catalysates of wide fractions do not find any commercial 
implementation as a starting feed for the processes of 
hydrodealkylation-hydrocracking. 
Known in the art is still another method of preparing aromatic 
hydrocarbons, wherein from the reforming catalysates there are 
simultaneously obtained an aromatic concentrate and isobutane. According 
to this method, reforming of gasoline is effected under mild conditions, 
whereunder the aromatic hydrocarbons are formed mainly from naphthenic 
hydrocarbons, whereas dehydrocyclization of paraffins to aromatic 
hydrocarbons constitutes below 40%. In accordance with this method, the 
reforming catalysate or a portion thereof after separation of C.sub.5 
-C.sub.6 hydrocarbons by distillation is fed to the hydrocracking zone. 
The hydrocracking is performed at a temperature within the range of from 
340.degree. to 450.degree. C. under a pressure of from 10 to 70 atm using 
a catalyst containing mordenite combined with alumina, Pd and Ni 
components preferably in the form of sulphides. Space velocity of the feed 
supply is varied within the range of from 1.0 to 10 volumes per one volume 
of the catalyst per hour; circulation rate of hydrogen is 3,000 to 20,000 
sft.sup.3 /barrel. 
Under the above-described conditions, paraffin hydrocarbons contained in 
the reforming catalysate are subjected to hydrocracking with the formation 
of paraffin C.sub.2 -C.sub.4 hydrocarbons, mainly isobutane, and 
simultaneously obtained is a concentrate of aromatic hydrocarbons 
contained in the starting feedstock, with a small amount of paraffin 
hydrocarbons. Paraffins C.sub.5 -C.sub.6 are distilled from the liquid 
product and then isomerized in the presence of hydrogen by a separate 
process of isomerization. The propane-butane fraction of the product is 
dehydrogenated in a separate process to give olefins which are then 
employed for alkylation of isobutane. 
This method of preparing aromatic hydrocarbons has certain disadvantages. 
First of all, the aromatic concentrate formed in the process contains 
mainly those aromatic hydrocarbons C.sub.6 -C.sub.10 which are present in 
the reforming catalysate at the same non-favorable ratio therebetween, 
i.e. a low content of benzene and a high content of low-value aromatic 
hydrocarbons: toluene and C.sub.9 -C.sub.10 aromatics. 
Secondly, under the process conditions an exhaustive hydrocracking of 
non-aromatic hydrocarbons is not achieved, whereby separation of pure 
aromatic hydrocarbons by rectification of the resulting aromatic 
concentrate is impossible. In particular, it is impossible to recover 
total xylenes of a required purity grade by rectification of said aromatic 
concentrate. For this reason, the resulting concentrate of aromatic 
hydrocarbons is considered as a high-octane component of an automobile 
gasoline. 
It is an object of the present invention to provide such a method of 
preparing benzene and xylenes which would make it possible to 
substantially increase the yield of said products and to simplify their 
separation. 
It is another object of the present invention to obtain total xylenes 
substantially free from ethylbenzene thus facilitating a further treatment 
and separation of xylenes. 
Still another object of the present invention is to overcome the necessity 
of performing a separate process for isomerization of xylenes. 
These and other objects of the present invention are accomplished by a 
method of preparing benzene and xylenes from reforming catalysates of 
gasoline fractions comprising mixtures of aromatic C.sub.6 -C.sub.10 
hydrocarbons and non-aromatic hydrocarbons, wherein a low-boiling fraction 
is distilled from the reforming catalysate and the remaining high-boiling 
fraction is treated in the presence of hydrogen at an elevated temperature 
and under a pressure within the range of from 10 to 60 atm on a catalyst 
consisting of mordenite, a hydrogenating agent, the balance being a 
binder. In accordance with the present invention, a low-boiling fraction, 
boiling out to a temperature of from 90.degree. to 108.degree. C., is 
separated by distillation, the remaining high-boiling fraction is treated 
at a temperature of from 450.degree. to 600.degree. C., using a catalyst 
consisting of H-mordenite in an amount of from 1 to 85% by weight, 
MoO.sub.3, WO.sub.3, Co, Pt taken either separately or in various 
combinations with each other in an amount of from 0.1 to 10% by weight as 
a hydrogenating agent. The resulting liquid product is separated by 
rectification to give benzene, toluene, total xylenes and a mixture of 
C.sub.9 -C.sub.10 aromatic hydrocarbons. Recycled back to the stage of 
treatment of the high-boiling fraction are hydrocarbons prepared by 
rectification of the liquid product: toluene or toluene along with a 
portion of hydrocarbons boiling above the boiling point of toluene. 
As the reforming catalysates use is made of catalysates of reforming of 
wide gasoline fractions containing a mixture of aromatic C.sub.6 -C.sub.10 
hydrocarbons and non-aromatic hydrocarbons. Such catalysates are obtained 
upon a catalytic reforming of fractions of straight-run gasoline boiling 
out within the range of from 60.degree. to 180.degree. C., for example 
fractions over 85.degree. to 180.degree. C., 70.degree. to 160.degree. C., 
70.degree. to 180.degree. C. and the like. It is preferable that the 
content of aromatic hydrocarbons in said catalysates be above 60% by 
weight. The latter are obtained upon reforming gasolines under severe 
conditions using aluminoplatinum catalysts containing additives of 
rhenium, iridium and other promotors. Severe reforming conditions mean a 
relatively low pressure and elevated temperatures, at which the process of 
reforming to produce highly-aromitized gasolines is performed. 
In accordance with the method of the present invention, a low-boiling 
(head) fraction with the boiling end between 90.degree. and 108.degree. 
C., preferably from 100.degree. to 106.degree. C., is previously 
distilled-off from the starting reforming catalysate. The head fraction 
constitutes 20 to 35% by weight of the reforming catalysate. It contains 
mainly paraffin hydrocarbons, benzene and a minor amount of toluene. The 
remaining high-boiling portion of the reforming catalysate is a starting 
feedstock for the catalytic stage of the process. It contains toluene, 
aromatic C.sub.8, C.sub.9 and possibly C.sub.10 hydrocarbons as well as 
non-aromatic (paraffin) hydrocarbons. The amount of paraffin hydrocarbons 
in said fraction is, as a rule, below 15% by weight. This high-boiling 
portion of the catalysate along with hydrogen or a hydrogen-containing gas 
and a recycled stream of a portion of the liquid product obtained in this 
process is passed, under the above-described conditions, through a 
catalyst bed consisting of H-mordenite, a hydrogenating component and a 
binder. 
An important feature of the present invention is that the reaction stage is 
performed at a temperature within the range of from 450.degree. to 
600.degree. C. in the catalyst bed. At a temperature below 450.degree. C. 
the liquid product contains an increased amount of paraffin C.sub.9 
-C.sub.10 hydrocarbons which hinder separation of xylenes by 
rectification. At a temperature exceeding the above-indicated upper 
temperature limit of the process, stability of the catalyst is impaired. 
As a result, a gas-liquid mixture is obtained which is cooled and the gas 
stream is separated from the liquid product. The liquid product comprising 
a mixture of aromatic hydrocarbons is separated by rectification to give 
benzene, toluene, pure total xylenes and a fraction of aromatic C.sub.9 
-C.sub.10 hydrocarbons. Toluene is recycled to the reaction zone as it is 
or along with the fraction of aromatic C.sub.9 -C.sub.10 hydrocarbons, or 
along with a concentrate of m-xylene and the fraction of aromatic C.sub.9 
-C.sub.10 hydrocarbons. The concentrate of m-xylene is obtained after 
separation of p- and o-xylenes from the total xylenes by conventional 
techniques (crystallization, rectification, molecular-sieve absorption). 
By the term "concentrate of m-xylene" is meant a mixture of aromatic 
C.sub.8 hydrocarbons containing mainly m-xylene and also ethylbenzene and 
a minor amount of the non-recovered p- and o-xylenes. A portion of toluene 
obtained in the process may be withdrawn therefrom when required. However, 
recycle of toluene or at least a portion thereof is an obligatory 
condition of the process. It is preferable to perform recycling of the 
total amount of toluene recovered from the liquid product. 
The fraction of aromatic C.sub.9 -C.sub.10 hydrocarbons separated by 
rectification contains mainly aromatic C.sub.9 hydrocarbons. In all the 
process embodiments, as the desired products there are discharged benzene 
and pure total xylenes. Benzene separated from the liquid product of the 
process usually contains as impurities a certain amount of non-aromatic 
hydrocarbons. If it is necessary to obtain benzene with a purity over 
99.5%, it may be prepared by purification of the separated benzene from 
said impurities using conventional methods of extractive or azeotropic 
rectification. 
The preferred embodiment of the method according to the present invention 
contemplates isolation of pure benzene from two streams: from the liquid 
product of the process and from the low-boiling fraction of the reforming 
catalysate. 
The purity grade of total xylenes recovered from the mixture of aromatic 
hydrocarbons by rectification is 99.5% and above. 
It is known that the content of n-nonane, C.sub.9 naphthenes and C.sub.10 
paraffins in total xylenes should not exceed 0.2% by weight. Otherwise, 
pure o-xylene cannot be recovered by rectification of total xylenes. 
The method according to the present invention ensures substantially 
complete hydrocracking of said impurities. Total xylenes obtained in the 
process contain practically no C.sub.10 paraffins, C.sub.9 naphthenes and 
n-nonane. A distinctive feature of the total xylenes as prepared by the 
method according to the present invention resides is that only an 
insignificant amount of ethylbenzene (1.5 to 3% by weight) is present 
therein. The total xylenes contain about 50% by weight of m-xylene and 
about 23 to 26% by weight of each of o-xylene and p-xylene. 
The catalytic stage of the process is performed at a space velocity of the 
starting feedstock along with the recycle of from 1 to 8 volumes per 
volume of the catalyst per hour. 
The ratio between a hydrogen-containing gas and the mixture of feedstock 
with recycle is varied from 600 to 2,000 nl/l. 
The hydrogen-containing gas, as a rule, is recycled in the process to 
maintain the concentration of hydrogen therein of at least 50% by volume. 
The required concentration is ensured for example by adding an appropriate 
amount of fresh hydrogen. The gaseous products consist of paraffin C.sub.1 
-C.sub.4 hydrocarbons; therewith, the yield of C.sub.2 -C.sub.4 
hydrocarbons is as high as 8-10 times of the yield of methane. 
The catalyst employed in the method according to the present invention 
consists of 1 to 85% by weight of n-mordenite, 0.1 to 10% by weight of 
hydrogenating agent, the balance being constituted by a binder. 
H-mordenite is a crystalline alumosilicate of a cubic structure having an 
inlet port diameter of from 6 to 10 A and a molar ratio of SiO.sub.2 
/Al.sub.2 O.sub.3 of 10 and over, generally of from 10 to 30. 
The above-indicated wide range of mordenite content in the catalyst is 
explained by different activity thereof depending on a molar ratio of 
SiO.sub.2 /Al.sub.2 O.sub.3. Thus, common H-mordenite having a molar ratio 
of 10 to 12 is substantially less active as compared to a dealuminated 
mordenite having a high value of the molar ratio of SiO.sub.2 /Al.sub.2 
O.sub.3. For this reason, common mordenite should be contained in the 
catalyst in a greater amount that the dealuminated one. Increasing the 
content of mordenite above the indicated upper limit is inefficient, since 
it does not result in a higher activity of the catalyst and even lowers 
its mechanical strength. The content of mordenite below 1% by weight in 
the catalyst results in a lowered activity thereof in the process below 
the critical level. 
As hydrogenating components in the catalyst according to the present 
invention use is made of metals pertaining to Group IV and Group VIII of 
the periodic system, preferably Mo, W, Co, Pt. They may be present in the 
catalyst in different forms: the Group Vi metal--in the oxide form 
(MoO.sub.3, WO.sub.3) or sulphide form; Group VIII metals--in the 
elemental or sulphide form. It is especially advantageous to use Mo per se 
or in combination with the above-mentioned components. 
With a content of the hydrogenating components of from 0.1 to 10% by 
weight, the Group VIII metals are taken as calculated for the elemental 
condition, and the Group VI metals--as calculated for trioxide. 
When MoO.sub.3 is used, its amount in the catalyst might constitute several 
percent, whereas in the case of platinum the amount of the latter 
constitutes decimal fractions of a percent. 
The catalyst composition might include H-Me-mordenite, wherein Me is at 
least one element selected from the group consisting of rare-earth and 
alkali-earth metals such as Ca, Mg, Ce, La, and commercial mixtures of 
lanthanides. 
Said elements are introduced into the mordenite structure using 
ion-exchange methods; in doing so, the maximal amount of the elements 
should not exceed 50% of the theoretically possible degree of substitution 
of hydrogen cations in n-mordenite. 
The binder (matrix) comprises a material imparting a required mechanical 
strength to the catalyst. As the binder use might be made of amorphous 
alumosilicate, silica gel, alumina. It is preferable to employ alumina as 
the binder. 
The method according to the present invention ensures an exhaustive 
hydrocracking of paraffin hydrocarbons forming azeotropic mixtures with 
aromatic hydrocarbons, whereby separation of benzene and xylenes becomes 
simplified. A complex combination of chemical reactions of transformation 
of aromatic hydrocarbons results, in the final end, in their complete 
conversion to valuable products, i.e. benzene and xylenes. The method 
according to the present invention has made it possible to balance a great 
number of reaction (dealkylation, disproportionation, transalkylation, 
isomerization of aromatic hydrocarbons; hydrocracking of non-aromatic 
hydrocarbons) and to inhibit undesirable reactions of decomposition of the 
aromatic ring. 
The method according to the present invention makes it possible to increase 
the yield of benzene by 3-4 times; that of xylenes--by 1.2 to 2 times as 
compared to the content of these hydrocarbons in the reforming catalysate. 
Separation of said products is also substantially simplified. 
Total xylenes prepared by the method according to the present invention 
have a higher quality than those obtained from catalytic reforming and 
extraction. They contain an insignificant amount (1 to 3% by weight) of 
ethylbenzene and, hence, a greater amount of xylenes thus facilitating 
their subsequent separation and isomerization. 
Furthermore, one of the embodiments of the method according to the present 
invention does not require any additional unit for isomerization of 
m-xylene, since after separation of p- and o-xylenes, m-xylene might be 
recycled back into the process. 
Therefore, the method according to the present invention makes it possible 
to simultaneously solve a series of different problems which hitherto have 
been solved through the use of a whole number of individual processes 
(extraction, dealkylation, disproportionation, transalkylation, 
isomerization of aromatic hydrocarbons). 
An important feature of the method according to the present invention is 
that it enables avoiding separated reforming of narrow and wide gasoline 
fractions in order to produce aromatic hydrocarbons and a high-octane 
component and makes it possible to obtain both types of products at a 
single high-capacity plant of catalytic reforming so that a portion of the 
catalyst employed at this plant could be treated according to the method 
of the present invention. 
The method according to the present invention is technologically simple and 
can be performed in the following manner. 
A reforming catalysate is fed into a rectification column, wherein the 
low-boiling portion of the catalysate is distilled-off. The high-boiling 
portion of the catalysate serving as a starting feedstock for the 
catalytic stage of the process is discharged from the column bottom. In 
some cases, where a reforming catalysate contains, besides aromatic 
C.sub.6 -C.sub.10 hydrocarbons, also aromatic C.sub.11.sup.+ hydrocarbons, 
it is advisable to additionally separate the high-boiling fraction from a 
heavy residue containing mainly aromatic C.sub.11.sup.+ hydrocarbons which 
causes coking of the catalyst. In this case the high-boiling fraction is 
fed into a second column, wherein it is distilled and discharged from the 
column top, whereas the heavy residue is withdrawn from the bottom. The 
high-boiling fraction of the catalysate containing aromatic C.sub.7 
-C.sub.10 hydrocarbons is further delivered into the reaction unit of the 
process. 
The process according to the present invention is exothermal. It is 
effected in a reactor unit consisting of a number of series-connected 
reactors or in a shelf-type reactor, wherein the catalyst is placed on a 
number of shelves. In between the reactors or shelves the reaction mixture 
is cooled by removing excessive heat by admission of cold hydrogen and/or 
a portion of the feedstock. The feedstock for the catalytic stage and a 
portion of the liquid product of the process to be recycled are evaporated 
in heat-exchangers, mixed with the recycled hydrogen-containing gas and 
fresh hydrogen-containing gas (generally it is the hydrogen-containing gas 
from the catalytic reforming process). 
The mixture is heated in a furnace to a required temperature. The heated 
mixture is passed through a cascade of series-mounted reactors or a 
shelf-type reactor. At the outlet the mixture is cooled in 
heat-exchangers, condensed and delivered to a gas-separator unit, wherein 
the gaseous phase is separated from the liquid one. The gaseous phase 
contains hydrogen and gaseous hydrocarbons. To reduce the supply rate of 
fresh hydrogen, the recycled hydrogen-containing gas is usually purified 
from the gaseous hydrocarbons by means of absorption. The liquid product 
of the process is further delivered from the gas-separator unit into a 
stabilization column, wherein dissolved light hydrocarbons C.sub.1 
-C.sub.4 are separated from the liquid product. The stable liquid product 
is then passed into a number of rectification columns, wherein 
successively separated are: benzene fraction, toluene, total xylenes and 
aromatic C.sub.9 -C.sub.10 hydrocarbons. Toluene is recycled into the 
process, either separately or along with aromatic C.sub.9 -C.sub.10 
hydrocarbons. In the case, where a concentrate of m-xylene is recycled 
along with toluene and aromatic C.sub.9 -C.sub.10 hydrocarbons, p- and 
o-xylenes are previously separated by conventional methods from total 
xylenes in special units. O-xylene is usually recovered by rectification 
and p-xylene--by crystallization or molecular-sieve adsorption. The 
m-xylene concentrate remaining after separation of p- and o-xylenes 
contains a certain amount of ethylbenzene and non-removed portion of p- 
and o-xylenes. 
Pure benzene from the benzene fraction produced in the process is obtained 
by conventional methods of azeotropic or extractive distillation. 
In the azeotropic distillation, as the azeotrope-forming agent use is 
generally made of acetone; in the extractive distillation of benzene use 
is made of conventional solvents such as N-methylpyrrolidone, 
N-formylmorpholine and the like. 
If the reforming catalysate employed in the process contains a substantial 
amount of benzene which is present in the low-boiling fraction, it is 
economically efficient to recover pure benzene from that fraction as well. 
As an embodiment of benzene isolation in this case there might be 
intermixing of the benzene fraction from the process with the low-boiling 
fraction of the reforming catalysate and subsequent recovery of pure 
benzene from this mixture by means of extraction with selective solvents. 
As selective solvents for the extraction glycols, sulpholane, 
N-methylpyrrolidone and the like might be used. 
The total xylenes produced by rectification of the liquid product are 
sufficiently pure and can be used in other processes without any 
additional purification. 
The catalyst for the process is prepared in a conventional manner 
characteristic of the preparation of zeolite catalysts with a binder. The 
starting Na-mordenite is converted to the H-form by treating same with an 
acid such as hydrochloric acid. Another method contemplates treatment of 
Na-mordenite with solutions of ammonium salts such as ammonium chloride. 
In this case, NH.sub.4 -mordenite is formed which is converted to 
H-mordenite upon calcination. The catalyst is prepared by intermixing a 
paste of aluminum hydroxide with a finely-divided powder of H-mordenite or 
NH.sub.4 -mordenite and with a solution of a compound of the hydrogenating 
component such as ammonium molybdate. After the production of a uniform 
plastic mass, the latter is moulded by extrusion; the resulting extrudates 
are dried at a temperature within the range of from 50.degree. to 
130.degree. C. and calcined in the air current. 
The hydrogenating component can be also introduced by impregnating the 
calcined composition of H-mordenite and a binder. 
When use is made of H-Me-mordenite, then alkali-earth and rare-earth metals 
are incorporated by way of ion-exchange. Usually NH.sub.4 -mordenite is 
employed for such ion-exchange. 
Another method of preparation of the catalyst might reside in the 
introduction of mordenite into a hydrosol alumina followed by the 
formation of gel beads in an oil medium. Generally, the catalyst might 
have the form of extrudates, tablets, beads, or irregular-shape granules. 
To reduce the excessive activity, the calcined catalyst is treated, as a 
rule, with sulphur or sulphur compounds (hydrogen sulphide, 
sulphur-organic compounds) in a stream of hydrogen or in a stream of 
hydrogen and the feedstock at a temperature within the range of from 
300.degree. to 450.degree. C.

For a better understanding of the present invention some specific Examples 
are given hereinbelow by way of illustration. 
EXAMPLE 1 
A catalysate of reforming of a fraction of straight-run gasoline containing 
72% of aromatic C.sub.6 -C.sub.10 hydrocarbons is separated, by 
rectification, into a low-boiling fraction with the boiling end of 
105.degree. C. and a high-boiling (bottoms) fraction. 
The yields of the low-boiling and high-boiling fractions are 28 and 72% by 
weight respectively as calculated for the reforming catalysate. 
The low-boiling fraction has the following composition, percent by weight: 
______________________________________ 
non-aromatic hydrocarbons 
77.0 
benzene 19.3 
toluene 3.7. 
______________________________________ 
The high-boiling fraction has the following composition, percent by weight: 
______________________________________ 
non-aromatic hydrocarbons 
8.9 
toluene 22.6 
C.sub.8 -aromatics 34.8 
C.sub.9 -aromatics 30.5 
C.sub.10 -aromatics 3.2. 
______________________________________ 
As the starting feedstock for the reaction stage use is made of the 
high-boiling stage of the reforming catalysate. The experiment is 
conducted using a catalyst of the following composition, percent by 
weight: MoO.sub.3 6.9; H-mordenite (SiO.sub.2 /Al.sub.2 O.sub.3 =12) 69.8; 
alumina 23.3. The catalyst extrudates have the following dimensions: 
length 4 mm, diameter 3 mm. Bulk weight is 0.5 g/cm.sup.3. The experiment 
is conducted in a direct-flow apparatus with circulation of a 
hydrogen-containing gas. Temperature in the catalyst bed is varied within 
the range of from 500.degree. to 510.degree. C., pressure is 35 atm, space 
rate of the feed supply (along with the recycle) is 2 hr.sup.-1 ; 
circulation ratio of the gas is 1,800 nl/l of the feed and recycle; supply 
rate of fresh hydrogen is 200 nl/l of the feed and recycle. Content of 
hydrogen in the circulated gas is 75% by volume. 
The catalyst is previously sulphidized with dimethylsulphide by adding same 
to the starting feedstock at a temperature of from 380.degree. to 
400.degree. C. under the pressure of 35 atm, whereafter the temperature is 
brought to 500.degree.-510.degree. C. 
Toluene is added to the fresh feedstock until the yield of toluene, as 
calculated for the starting feed, becomes equal to the amount of toluene 
added to the fresh feedstock. Such stationary conditions of the process 
are achieved at the ratio of the fresh feedstock to the recycled toluene 
of 0.63 and 0.37 part by weight respectively, where the total is assumed 
to be equal to 1. Under these conditions, the yield of liquid product 
C.sub.5.sup.+ as calculated for the mixture of fresh feedstock with the 
recycle, is 88.0% by weight, including: benzene 13.0, toluene 37.0, xylene 
29.2; C.sub.9 -C.sub.10 -aromatics 7.8; C.sub.5 -C.sub.9 non-aromatics 
1.0. The content of non-aromatic C.sub.9 hydrocarbons in the liquid 
products is 0.1% by weight, including n-nonane 0.02% by weight. 
The gaseous products have the following composition, percent by weight: 
______________________________________ 
hydrogen 
9.3 
methane 9.2 
ethane 57.1 
propane 17.9 
butanes 6.5. 
______________________________________ 
Benzene with a purity of 95.5% is isolated by rectification of the 
catalysate along with toluene, total xylenes, and a fraction boiling above 
the boiling point of toluene (C.sub.9 -C.sub.10 aromatic hydrocarbons). 
Continuously supplying toluene, as a recycle, to the catalytic zone, there 
are produced (as calculated for 100% of a fresh high-boiling fraction of 
the catalysate): 
______________________________________ 
benzene 21.5% (20.5% calculated for pure benzene); 
total xylenes 46.4% 
C.sub.9 -C.sub.10 -aromatics 
12.5. 
______________________________________ 
The total xylenes recovered by rectification from the liquid products of 
the process have a purity of 99.6% by weight and the following isomeric 
composition, percent by weight: 
______________________________________ 
ethylbenzene 
2.0 
p-xylene 22.0 
o-xylene 26.4 
m-xylene 49.6. 
______________________________________ 
These results remain practically unchanged during 400 hours of the catalyst 
operation. 
Benzene formed in the process is mixed with the low-boiling fraction of the 
reforming catalysate and pure benzene is isolated from the mixture by 
extraction with a selective solvent (diethylene glycol). There are 
obtained, as calculated for the starting reforming catalysate, 19.8% by 
weight of benzene and 33.3% by weight of total xylenes. 
EXAMPLE 2 
Process conditions, catalyst and composition of the starting stock are the 
same as in the foregoing Example 1. Recycled is toluene along with 
aromatic C.sub.9 -C.sub.10 hydrocarbons. Experimentally found ratio of the 
fresh feedstock and recycle at the reactor inlet is: 
______________________________________ 
fresh feedstock 0.55 
toluene 0.315 
C.sub.9 -C.sub.10 -aromatics 
0.135 
Total: 1.0. 
______________________________________ 
The yield of the liquid product, as calculated for the mixture of the 
feedstock and recycle, is 88.0% by weight including: benzene 9.5; toluene 
31.5; total xylene 32.8; C.sub.9 -C.sub.10 -aromatics 13.5; C.sub.5 
-C.sub.9 non-aromatics 0.7. The content of non-aromatic C.sub.9 compounds 
is 0.1% including n-nonane 0.02%. 
The gaseous products have the following composition, percent by weight: 
______________________________________ 
hydrogen 
8.4 
methane 9.5 
ethane 59.4 
propane 16.3 
butane 6.4. 
______________________________________ 
The liquid product is separated by rectification to: benzene, toluene, 
total xylenes, aromatic C.sub.9 -C.sub.10 hydrocarbons. Toluene and a 
fraction boiling at a temperature above the boiling point of xylenes is 
recycled to the process in the above-mentioned ratio to the fresh 
feedstock. The yield of benzene and total xylenes, as calculated for the 
fresh feedstock (high-boiling fraction of the reforming catalysate), is 
17.3 and 59.6% by weight respectively. 
The total xylenes have the following composition, percent by weight: 
______________________________________ 
ethylbenzene 
1.6 
p-xylene 22.6 
o-xylene 25.4 
m-xylene 50.4. 
______________________________________ 
Taking into account separation of benzene from the head fraction of the 
reforming catalysate, the yield of benzene is 17.8% by weight and that of 
total xylenes is 42.8% by weight as calculated for 100% of the catalysate. 
EXAMPLE 3 
Use is made of a catalysate of reforming of a straight-run gasoline 
(85.degree.-180.degree. C.), wherefrom a light fraction is previously 
distilled boiling-out up to 103.degree. C. The high-boiling fraction has 
the following hydrocarbon composition, percent by weight: 
______________________________________ 
non-aromatic hydrocarbons 
9.2 
toluene 27.3 
C.sub.8 -aromatics 46.0 
C.sub.9 -aromatics 15.6 
C.sub.10 -aromatics 1.9. 
______________________________________ 
The catalyst is employed containing, percent by weight: MoO.sub.3, 5.0; Co, 
1.0; H-mordenite, 47; alumina, 47. Conditions are the same as in Example 1 
hereinbefore, except that the space velocity of supply of the feedstock 
and recycle is 3 hr.sup.-1 and the content of hydrogen in the recycled gas 
is 7% by volume. Recycled in combination are toluene, m-xylene (with the 
impurity of ethylbenzene) and the fraction of aromatic hydrocarbons 
boiling at a temperature above the boiling point of xylenes. 
Experimentally found ratio between the fresh feedstock and recycle is: 
______________________________________ 
fresh feedstock 0.33 
toluene 0.33 
m-xylene 0.18 (0.17 m-xylene 
and 0.01 ethylbenzene 
C.sub.9 -C.sub.10 -aromatics 
0.16 
Total: 1.0 
______________________________________ 
The yield of liquid product C.sub.5.sup.+, as calculated for the passed 
mixture, is 93.2% by weight including: 
______________________________________ 
benzene 7.4 
toluene 33.0 
o-xylene 9.5 
p-xylene 8.5 
m-xylene with ethylbenzene 
18.0 
C.sub.9 -C.sub.10 -aromatics 
16.0 
C.sub.5 -C.sub.9 -non-aromatics 
0.8. 
______________________________________ 
The content of non-aromatic C.sub.9 hydrocarbons in the liquid product is 
0.08% by weight including 0.01% by weight of n-nonane. With the account of 
continuous separation of p- and o-xylenes from the total xylenes produced 
in the process and recycle of toluene along with the concentrate of 
m-xylene and fraction of aromatic C.sub.9 -C.sub.10 hydrocarbons, there 
are obtained, as calculated for the high-boiling fraction of the reforming 
catalysate, percent by weight: 
______________________________________ 
benzene 22.4 
o-xylene 28.6 
p-xylene 25.7. 
______________________________________ 
Example 3 illustrates the possibility of carrying out, under the 
above-mentioned conditions along with other reactions, the reaction of 
isomerization of xylenes. 
EXAMPLE 4 
The starting feedstock, catalyst and process conditions are the same as in 
the foregoing Example 1, except that the temperature in the catalyst bed 
is maintained within the range of from 440.degree. to 450.degree. C. The 
experiment being conducted with the use of a fresh feedstock, the yield of 
the liquid product is 89% by weight, including: benzene 7.3; toluene 25.7; 
total xylenes 33.1; C.sub.9 -C.sub.10 -aromatics 15.8; C.sub.5 -C.sub.10 
-non-aromatics 7.1. The content of non-aromatic C.sub.9 hydrocarbons in 
the catalysate is 0.29% by weight including 0.05% by weight of n-nonane; 
the content of i-C.sub.10 is 0.06% by weight. With the above-indicated 
content of the non-aromatic hydrocarbons separation of pure total xylenes 
by rectification is rather difficult. The Example illustrates the minimal 
temperature at which carrying-out the process is still tolerable. 
When the process is conducted under the same conditions, except that the 
catalyst bed temperature is varied within the range of from 590.degree. to 
600.degree. C., the catalyst operation is not as stable as required. After 
only 70 hours of operation the content of C.sub.9 paraffin hydrocarbons is 
about 0.3% by weight, including 0.06% by weight of n-nonane. This example 
illustrates the upper limit of the process temperature. 
EXAMPLE 5 
Use is made of a catalysate of reforming of a straight-run gasoline 
(85.degree.-180.degree. C.) fraction containing 63% by weight of aromatic 
hydrocarbons. After distilling-off the low-boiling fraction boiling-out up 
to 102.degree. C., a high-boiling fraction (68.5% by weight of the 
catalyst) is obtained having the following composition, percent by weight: 
______________________________________ 
non-aromatics 
13.5 
toluene 20.5 
C.sub.8 -aromatics 
33.2 
C.sub.9 -aromatics 
29.2 
C.sub.10 -aromatics 
3.6. 
______________________________________ 
Use is made of a catalyst containing, percent by weight: MoO.sub.3, 7.5; 
H-mordenite (SiO.sub.2 /Al.sub.2 O.sub.3 =20), 2.5; alumina, 90.0. 
An experiment with the high-boiling fraction and recycle of toluene and 
aromatic C.sub.9 -C.sub.10 hydrocarbons is conducted at the temperature of 
480.degree. C., space rate of the feedstock and recycle supply of 2 
hr.sup.-1, circulation ratio of the gas of 1,200 nl/l of the feed and 
recycle and fresh hydrogen supply rate of 300 nl/l of the feed and 
recycle. The content of hydrogen in the recycled gas is 62% by volume. The 
mixture of feedstock and recycle at the reactor inlet contains 0.555 part 
by weight of the fresh feedstock, 0.325 part by weight of toluene and 0.12 
part by weight of aromatic C.sub.9 -C.sub.10 hydrocarbons, the total being 
assumed as 1. The yield of liquid C.sub.5.sup.+ product is 85.4% by weight 
including: benzene 8.2; toluene 32.5; total xylenes 29.7; aromatic C.sub.9 
-C.sub.10 hydrocarbons 12.0; non-aromatic C.sub.5 -C.sub.9 hydrocarbons 
3.0. The content of non-aromatic C.sub.9 hydrocarbons is 0.2% by weight 
including 0.03% by weight of n-nonane. 
EXAMPLE 6 
Use is made of a catalyst containing, percent by weight: MoO.sub.3 7.5; 
H-mordenite ((SiO.sub.2 /Al.sub.2 O.sub.3 =20) 1; alumina 91.5. 
Process conditions, feedstock and proportions of the fresh feedstock, 
toluene and aromatic C.sub.9 -C.sub.10 hydrocarbons at the reactor inlet 
are the same as in the foregoing Example 5. The liquid product yield is 
88.9% by weight including: benzene 7.1; toluene 34.3; xylene 30.2; C.sub.9 
-C.sub.10 aromatics 13.2; C.sub.5 -C.sub.9 non-aromatics 4.1. The content 
of non-aromatic C.sub.9 hydrocarbons is 0.28% by weight including 0.06% by 
weight of n-nonane. The results of this Example show that the content of 
C.sub.9 non-aromatic hydrocarbons, especially of n-nonane, is at the 
extreme of the tolerable limit for separation of the total xylenes by 
rectification. Furthermore, toluene and aromatic C.sub.9 -C.sub.10 
hydrocarbons are formed in an amount exceeding the amount thereof added to 
the fresh feedstock, i.e. a higher ratio between the recycle and feedstock 
is required. The Example illustrates the minimal content of H-mordenite in 
the catalyst. 
EXAMPLE 7 
As the feedstock use is made of a high-boiling fraction of the reforming 
catalysate as described in Example 1 hereinbefore. 
The catalyst is employed consisting of the following components, percent by 
weight: Pt (platinum) 0.25; H-mordenite (SiO.sub.2 /Al.sub.2 O.sub.3 =12) 
75; the balance being represented by alumina. The experiment is conducted 
at a temperature of from 480.degree. to 490.degree. C. under a pressure of 
25 atm, circulation ratio of the hydrogen-containing gas of 1,200 nl/l of 
the feed and recycle (the content of hydrogen is 65% by volume), supply 
rate of fresh hydrogen of 200 nl/l of the feed and recycle. 
The experiment is conducted with toluene recycle: 0.62 part by weight of 
fresh feedstock and 0.38 part by weight of toluene. At a space rate of the 
mixture of 2 hr.sup.-1 the liquid product yield is 88.4% by weight 
including: benzene 11.3; toluene 38.0; xylenes 27.8; aromatic C.sub.9 
-C.sub.10 hydrocarbons 10.3; non-aromatic C.sub.9 -C.sub.10 hydrocarbons 
1.0. The content of C.sub.9 non-aromatics in the liquid product is 0.15% 
by weight including 0.03% by weight of n-nonane. 
EXAMPLE 8 
Use is made of a catalyst containing, percent by weight: MoO.sub.3 5.0; Ni 
1.0; H-Cl-mordenite 69; alumina 25. The content of Ce (cerium) in 
mordenite is 3.2% by weight which corresponds to the degree of 
substitution of hydrogen ions of about 22%. The high-boiling fraction of 
the reforming catalysate is employed, the fraction composition being the 
same as in Example 1 hereinbefore. Experiment with toluene recycle is 
conducted at a temperature of from 500.degree. to 510.degree. C. under a 
pressure of 35 atm, circulation ratio of the gas of 1,200 nl/l of the feed 
and recycle and the supply rate of fresh hydrogen of 200 nl/l of the feed 
and recycle. The content of hydrogen in the recycled gas is 60% by volume. 
Proportions of the fresh feedstock and toluene are 0.65 and 0.35 part by 
weight respectively. At a space rate of the mixture of 1.5 hr.sup.-1, the 
liquid product yield is 87.8% by weight including: benzene 12.1; toluene 
35.0; xylenes 29.1; aromatic C.sub.9 -C.sub.10 hydrocarbons 10.8; 
non-aromatic C.sub.5 -C.sub.9 hydrocarbons 0.8. The content of n-nonane in 
the products is 0.03% by weight. 
EXAMPLE 9 
A catalyst is used containing, percent by weight: MoO.sub.3 5.0; WO.sub.3 
0.5; H-Ca-mordenite 70; alumina 24.5. The content of Ca in mordenite is 
1.44% by weight which is equivalent to the degree of substitution of 
hydrogen ions of about 16%. The feedstock of Example 1 is used. An 
experiment with recycle of toluene (0.35 part by weight of toluene and 
0.65 part by weight of fresh feedstock) is conducted at a temperature of 
from 530.degree. to 540.degree. C. under a pressure of 50 atm, space rate 
of supply of the feed and recycle of 1.5 hr.sup.-1 ; circulation ratio of 
the gas of 1,000 nl/l of the feed and recycle, fresh hydrogen supply rate 
of 200 nl/l of the feed and recycle. The content of hydrogen in the 
circulated gas is 60% by volume. The liquid product yield is 84.5% by 
weight including: benzene 12.8; toluene 35.0; total xylenes 27.2; aromatic 
C.sub.9 -C.sub.10 hydrocarbons 8.8; non-aromatic C.sub.5 -C.sub.9 
hydrocarbons 0.7. The content of C.sub.9 -non-aromatics is 0.14% by 
weight, including 0.02% by weight of n-nonane.