Integrated catalytic reforming and hydrodealkylation process for maximum recovery of benzene

The present invention is an integrated catalytic reforming/hydrodealkylation process that maximizes benzene recovery by incorporating refrigeration and pressure swing adsorption separation units. In the refrigeration separation unit, liquid reformate is used as a sponge oil to recover benzene from a hydrodealkylation purge gas stream, which in the past has been vented. The pressure swing adsorption unit remove impurities from a hydrogen-rich gas stream prior to use in the hydrodealkylation unit.

FIELD OF THE INVENTION 
The present invention relates to an integrated catalytic reforming and 
hydrodealkylation process for maximum recovery of benzene and hydrogen. 
More specifically, the present invention involves the use of a 
process-derived catalytic reformate as a sponge oil in a refrigerated 
separation unit to recover hydrogen and benzene from a hydrodealkylation 
vent stream. 
BACKGROUND OF THE INVENTION 
Hydrocarbons classified as aromatics have enjoyed increasing demand in the 
marketplace due principally to their versatility as gasoline blending 
components. In addition, they can also be used as components in the 
production of various petrochemical compounds. This is particularly true 
in the case of benzene. Benzene represents the building block for the 
direct or indirect manufacture of well over 250 separate products or 
product classifications. Over the past few years, the annual benzene 
demand in the United States alone has ranged from 1.5 to 1.9 billion 
gallons. Worldwide, the annual consumption of benzene has ranged from 3.5 
to 4.2 billion gallons. Historically, the major consumption of benzene has 
been in the production of ethylbenzene, cumene and cyclohexane. The 
principal use of ethylbenzene is to produce styrene by, for example, steam 
hydrogenation. Significant amounts of benzene are also consumed in the 
manufacture of aniline, detergent alkylate, and maleic anhydride. 
At the present time, most of the total aromatics produced in the U.S. come 
from catalytic reforming of hydrocarbons. Typical reforming reactions 
include the dehydrogenation of naphthenes to produce aromatics, 
dehydrocyclization of paraffins directly to aromatics, the hydrocracking 
of long-chained paraffins into lower boiling, normally liquid material, 
and the isomerization of paraffins. In catalytic reforming of 
hydrocarbons, fresh liquid hydrocarbons boiling within the gasoline or 
naphtha boiling range are reacted with hydrogen in the presence of a 
catalyst comprising a Group VIII noble metal on a porous carrier at 
conditions which promote the conversion of naphthenes and paraffins to 
aromatic hydrocarbons. 
Catalytic reforming is primarily an endothermic process effected in a 
plurality of reaction zones having interstage heating therebetween. The 
operation is effected primarily in vapor phase at temperatures of up to 
1200.degree. F. Other operating conditions include a pressure of about 20 
to 1000 psig, a liquid hourly space velocity of about 0.2 to 10, and a 
hydrogen to hydrocarbon mole ratio of about 0.5:1 to 20:1. 
The prior art is replete with catalytic reforming processes using a variety 
of schemes. For example, U.S. Pat. No. 3,664,949 (issued to Petersen et 
al.) discloses a process for reforming a petroleum hydrocarbon feedstock 
that boils within the range of about 120.degree. to 500.degree. F. and is 
selected from a group consisting of virgin naphthas, cracked naphthas, 
catalytic gasolines, coker naphthas, and mixtures thereof. In this 
process, the above-described feedstock is contacted in a reactor system 
consisting of two reactors in the presence of hydrogen and under reforming 
conditions with a catalyst in each reactor comprising a Group VIII noble 
metal and a co-catalytic solid support comprising mordenite. Another 
example of a catalytic reforming process can be found in U.S. Pat. No. 
3,864,241 (issued to Rausch). The Rausch patent discloses a process for 
catalytically reforming a gasoline fraction comprising contacting the 
fraction with a catalytic composite comprising a combination of a platinum 
group component, a tin component, and a halogen component. 
Processes that seek to maximize the production of benzene take at least a 
portion of the catalytic reformate containing alkylaromatics and react it 
in a dealkylation zone in the presence of hydrogen at conditions selected 
to dealkylate alkyl-substituted aromatic hydrocarbons. Thus, toluene and 
mixed xylenes are dealkylated for maximum benzene production, or toluene 
is transalkylated to maximize production of both benzene and mixed 
xylenes. 
U.S. Pat. No. 3,197,523 is illustrative of a hydrodealkylation process. In 
this process, a feedstock comprising toluene, mixed xylenes, ethylbenzene, 
mixed diethylbenzenes, and various alkyl-substituted naphthalenes is 
reacted in the presence of a catalyst containing at least one oxide of 
tin, titanium, and zirconium combined with at least one oxide in chromium, 
molybdenum and tungsten at conditions including temperatures of about 
1000.degree. to 1500.degree. F. and pressures of about 300 to 1000 psig. 
U.S. Pat. No. 4,157,355 (issued to Addison) discloses an integrated 
catalytic reforming and hydrodealkylation process wherein a liquid phase 
of a catalytic reforming effluent is passed to a catalytic 
hydrodealkylation zone, the products of which are separated into a 
hydrogen-rich vapor phase and a liquid aromatic-containing phase. The 
hydrogen-rich vapor phase is then recycled to the catalytic reforming 
zone, and the aromatic-containing liquid phase is sent to a fractionator 
wherein a benzene-rich stream is recovered. 
The prior art integrated processes have several disadvantages. First, in 
the prior art process, the hydrogen-rich gas contains some light 
hydrocarbons which are carried forward to the hydrodealkylation unit and 
which will crack to form methane in the hydrodealkylation unit, thereby 
increasing the overall hydrogen consumption. Second, in the prior art 
processes, a significant amount of benzene, hydrogen, and some methane can 
be lost through venting of purge gases in the hydrodealkylation unit. 
Third, the use of a catalytic hydrodealkylation unit has the disadvantage 
of process shutdowns required for catalyst replacement. 
There is a need for an integrated catalytic reforming/hydrodealkylation 
process that maximizes the recovery of benzene and uses hydrogen more 
efficiently. 
SUMMARY OF THE INVENTION 
In the present invention, the problem of introducing a relatively impure 
hydrogen make-up gas (hydrogen make-up gas being defined as the hydrogen 
gas produced in a hydrocarbon reforming process) into the 
hydrodealkylation unit is solved by first passing the hydrogen make-up gas 
through a refrigerated separation unit that uses a process-derived 
reformate as a sponge oil for: 
(1) recovering benzene produced in the thermal dealkylation process of the 
present invention; and 
(2) recovering LPG material (which is defined as liquified petroleum gas 
products which are composed of those readily liquefiable hydrocarbon 
compounds which are produced in the course of conventional refining of 
crude oil) from the hydrogen make-up gas. 
Accordingly, the benefits of the present invention include: 
(1) increased benzene recovery; 
(2) decreased hydrogen consumption in the thermal dealkylation zone due to 
reduced hydrocracking of paraffins that can enter the thermal dealkylation 
unit through the hydrogen make-up gas; 
(3) increased LPG recovery due to a reduction in the amount of LP material 
sent to the thermal dealkylation zone where the LPG material can be 
cracked into lower value hydrocarbons; and 
(4) a smaller thermal dealkylation reactor due to a reduction in the amount 
of LPG material sent to the thermal dealkylation zone. 
The present invention relates to a process for the recovery of benzene 
comprising the steps of: reacting a hydrocarbon charge stock and hydrogen 
in a catalytic reforming reaction zone at reforming conditions sufficient 
to produce a benzene-containing reformate and a hydrogen-containing vapor 
phase; passing the reformate into a stabilizing zone to produce a 
hydrocarbon-containing vapor phase and a benzene-containing, stabilized 
reformate, at least a portion of the benzene-containing, stabilized 
reformate being passed to a fractionation zone to produce a benzene-rich 
product stream and a toluene-rich stream; refrigerating the 
hydrogen-containing vapor phase and the benzene-containing, stabilized 
reformate and admixing the hydrogen-containing vapor phase with at least a 
portion of the benzene-containing, stabilized reformate to form a 
refrigerated admixture; introducing the refrigerated admixture to a 
vapor-liquid separator and withdrawing from the separator a hydrogen-rich 
gas stream comprising light hydrocarbons and a liquid phase stream; 
passing the hydrogen-rich gaseous stream to a first adsorber bed 
containing adsorbent having adsorptive capacity for hydrocarbons at 
effective adsorption conditions; withdrawing from the first adsorber bed a 
substantially hydrocarbon-free, hydrogen-rich gas stream; withdrawing a 
stream rich in hydrocarbons from a second adsorber bed containing 
adsorbent having adsorptive capacity for hydrocarbons, the bed undergoing 
desorption of previously loaded hydrocarbons and is undergoing desorption; 
reacting the toluene-rich stream, in admixture with at least a portion of 
the hydrocarbon-free, hydrogen-rich vapor phase, in a hydrodealkylation 
reaction zone at conditions selected to produce a benzene-containing 
product stream and a vapor-containing purge stream; and recovering the 
benzene-containing product stream. 
In another embodiment, the present invention is a process for the recovery 
of benzene comprising the steps of: reacting a hydrocarbon charge stock 
and hydrogen in a catalytic reforming reaction zone at reforming 
conditions to produce a benzene-containing reformate and a 
hydrogen-containing vapor phase; passing the reformate into a stabilizing 
zone to produce a hydrocarbon-containing vapor phase and a 
benzene-containing, stabilized reformate, at least a portion of the 
benzene-containing, stabilized reformate being passed to a fractionation 
zone to produce a benzene-rich product stream and a toluene-rich stream: 
refrigerating the hydrogen-containing vapor phase and the 
benzene-containing, stabilized reformate and admixing the 
hydrogen-containing vapor phase with at least a portion of the 
benzene-containing, stabilized reformate to form a refrigerated admixture; 
introducing the refrigerated admixture to a vapor-liquid separator and 
withdrawing from the separator a hydrogen-rich gaseous stream comprising 
light hydrocarbons and a liquid phase stream, the liquid phase stream 
being recycled to the stabilizing zone; passing the hydrogen-rich gaseous 
stream to a first adsorber bed containing adsorbent having adsorptive 
capacity for hydrocarbons at effective adsorption conditions; withdrawing 
from the first adsorber bed a substantially hydrocarbon-free, 
hydrogen-rich gas stream; withdrawing a stream rich in hydrocarbons from a 
second adsorber bed containing adsorbent having adsorptive capacity for 
hydrocarbons, the bed undergoing desorption of previously loaded 
hydrocarbons; reacting the toluene-rich stream, in admixture with at least 
a portion of the hydrocarbon-free, hydrogen-rich vapor phase, in a 
hydrodealkylation reaction zone at conditions selected to produce a 
benzene-containing product stream and a vapor-containing purge stream; 
recycling the vapor-containing purge stream to the refrigeration section 
described hereinabove and admixing the vapor-containing purge stream with 
the benzene-containing, stabilized reformate; and recovering the 
benzene-containing product stream. 
In another embodiment, the present invention is a process for the recovery 
of benzene comprising the steps of: reacting a hydrocarbon charge stock 
and hydrogen in a catalytic reforming reaction zone at reforming 
conditions to produce a benzene-containing reformate and a 
hydrogen-containing vapor phase; passing the reformate into a stabilizing 
zone to produce a hydrocarbon-containing vapor phase and a 
benzene-containing, stabilized reformate, at least a portion of the 
benzene-containing, stabilized reformate being passed to an aromatics 
extraction zone and a fractionation zone to produce a benzene-rich product 
stream and a toluene-rich stream; refrigerating the hydrogen-containing 
vapor phase and the benzene-containing, stabilized reformate at a 
temperature of less than about 40.degree. F. and admixing the 
hydrogen-containing vapor phase with at least a portion of the 
benzene-containing, stabilized reformate to form a refrigerated admixture; 
introducing the refrigerated admixture to a vapor-liquid separator and 
withdrawing from the separator a hydrogen-rich gaseous stream comprising 
light hydrocarbons and a liquid phase stream, the liquid phase stream 
being recycled to the stabilizing zone; passing the hydrogen-rich gaseous 
stream to a first adsorber bed containing adsorbent having adsorptive 
capacity for hydrocarbons at effective adsorption conditions; withdrawing 
from the first adsorber bed a hydrocarbon-free, hydrogen-rich gas stream; 
withdrawing a stream rich in hydrocarbons from a second adsorber bed 
containing adsorbent having adsorptive capacity for hydrocarbons, the bed 
undergoing desorption of previously loaded hydrocarbons; reacting the 
toluene-rich stream, in admixture with at least a portion of the 
hydrocarbon-free, hydrogen-rich vapor phase, in a thermal 
hydrodealkylation reaction zone in the absence of an added catalyst at a 
temperature of at least about 1200.degree. to 1500.degree. F. to produce a 
benzene-containing product stream and a vapor-containing purge stream; 
recycling the vapor-containing purge stream to the refrigeration section 
described hereinabove and admixing the vapor-containing purge stream with 
the benzene-containing, stabilized reformate; and recovering the 
benzene-containing product stream.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENT 
The present invention is an integrated process that begins with catalytic 
reforming of hydrocarbons. Fresh feed charge stocks suitable for use in 
the present invention include liquid hydrocarbons boiling within the 
gasoline or naphtha boiling range, for example, hydrocarbons which exist 
in a liquid state at one atmosphere of pressure and a temperature of about 
60.degree. F., and which have normal boiling points up to about 
425.degree. F. Thus, it is contemplated that suitable charge stocks will 
include, but not by way of limitation, full boiling range naphthas (about 
100.degree. F. to about 400.degree. F.), light naphthas (100.degree. F. to 
200.degree. F.), and heavy naphthas (200.degree. F. to about 400.degree. 
F.) The naphtha feedstock may be preheated via indirect heat exchange with 
one or more high temperature process streams, such as process-derived 
reformate and dealkylation unit effluent. The naphtha feedstock may then 
be introduced into a direct fired heater wherein its temperature is 
further increased to the level needed to provide the design temperature at 
the inlet to the catalyst bed and the reforming reaction zone. 
The catalytic reforming system may function with a plurality of fixed-bed 
zones, with a plurality of stacked zones through which catalyst particles 
flow via gravity, or a combination thereof. 
The precise reforming operating conditions utilized in the reforming 
section of the present invention will depend on the chemical and physical 
characteristics of the naphtha boiling range charge stock as well as upon 
the selected aromatic concentrate. Nevertheless, operating conditions 
suitable for use in the present invention include temperatures in the 
range of about 750.degree. to 1020.degree. F., pressures in the range of 
about 20 to 1000 psig, a liquid hourly space velocity of about 0.5 to 10.0 
(defined as volume of fresh charge stock per hour, per volume of total 
catalyst particles), and a hydrogen to hydrocarbon mole ratio in the range 
of about 1:1 to 15:1. As a practical matter, fixed bed reforming systems 
necessitate lower catalyst bed temperatures from 750.degree. to 
910.degree. F., higher pressures from about 500 to 1000 psig, lower space 
velocities of about 0.5 to 2.5, and higher hydrogen to hydrocarbon mole 
ratios of 4.5:1 to about 8:1. On the other hand, benefits accrue through 
continuous catalyst regeneration reforming in that the operating 
conditions involve higher catalyst bed temperatures of about 950.degree. 
to 1010.degree. F., lower pressures of about 20 to 450 psig, higher space 
velocities of 3.0 to about 8.0, and lower hydrogen/hydrocarbon mole ratios 
of about 0.5:1 to 5.5:1. 
A reforming catalyst suitable for use in the present invention includes, 
but is not limited to, any Group VIII noble metal deposited on a porous 
inorganic oxide support. Examples of Group VIII metals are platinum, 
palladium, rhodium, osmium, ruthenium, and iridium. Suitable inorganic 
oxides include, but are not limited to alumina, silica, zirconia, and any 
combinations thereof. In a preferred embodiment, cojoint catalyst 
modifiers can be used, such as, cobalt, nickel, gallium, germanium, tin, 
rhenium, vanadium, tungsten, zinc, and any mixtures thereof. 
The products of the catalytic reforming unit are a benzene-containing 
reformate and a hydrogen-containing vapor phase. In addition to containing 
benzene, the benzene-containing reformate comprises other aromatics such 
as toluene and mixed xylenes. In addition to containing hydrogen, the 
hydrogen-containing vapor phase comprises light hydrocarbons, such as 
methane, ethane, propane, and butane. The benzene-containing reformate is 
sent to a stabilizing zone to produce a hydrocarbon-containing vapor phase 
comprising a substantial amount of the C.sub.1 -C.sub.2 hydrocarbons and a 
benzene-containing stabilized reformate. The benzene-containing reformate 
also contains liquid C.sub.3 -C.sub.4 hydrocarbons. Suitable operating 
conditions for the stabilizing zone include a pressure of about 100 to 300 
psig and a bottoms temperature of 350.degree. to 550.degree. F. 
In a preferred embodiment, at least a portion of the benzene-containing 
stabilized reformate is passed to an aromatics extraction zone. The 
purpose of the aromatic extraction zone is to separate benzene and other 
aromatics from nonaromatics that are not converted in the reforming zone. 
In the aromatic extraction zone, the aromatic-containing feed enters an 
extractor and flows upward countercurrently to a stream of lean solvent. 
As the feed flows through the extractor, aromatics are selectively 
dissolved in the solvent, and raffinate of very low aromatic content is 
withdrawn from the top of the extractor. Rich solvent from the extractor 
enters an extractive stripper in which partial stripping of the 
hydrocarbons from the rich solvent takes place. The nonaromatic components 
having volatilities higher than that of benzene under conditions existing 
in the column are essentially stripped from the solvent and removed in the 
overhead stream. This stream is returned to the extractor as recycle for 
recovery of aromatics contained therein, while facilitating purification 
by displacing heavy nonaromatics from the solvent phase by light easily 
stripped nonaromatic hydrocarbons contained in the recycle. The bottoms 
stream from the extractive stripper consists of solvent and aromatic 
components substantially free of nonaromatics. This stream enters the 
recovery column in which the aromatic product is separated from the 
solvent. Because of the large difference in boiling points between the 
solvent used and the heaviest desired aromatic product, this separation is 
handled readily. Lean solvent from the column is returned to the 
extractor. Raffinate from the extractor is contacted with water to remove 
dissolved solvent, and the rich water is returned to the extract-recovery 
column as stripping steam generated via exchange with the hot circulating 
solvent in a water-stripper reboiler. 
In accordance with the present invention, at least a portion of the 
benzene-containing stabilized reformate is sent to a fractionation zone 
from which a benzene-rich product stream and a toluene-rich stream are 
produced. 
An essential feature of the present invention is refrigerating the 
hydrogen-containing vapor phase to remove a substantial amount of any 
aromatics and LPG present therein prior to its use in the 
hydrodealkylation unit using a processed-derived, benzene-containing, 
stabilized reformate as the recovery liquid or sponge oil. Accordingly, in 
the present invention, the hydrogen-containing vapor phase is admixed with 
the benzene-containing, stabilized reformate to form an admixture which is 
subjected to refrigeration. The refrigeration lowers the temperature of 
the hydrogen-containing vapor phase and the benzene-containing stabilized 
reformate admixed therewith to a temperature of between -15.degree. to 
40.degree. F. 
After refrigeration, the resulting admixture is passed to a vapor-liquid 
equilibrium separation zone wherein there is produced a hydrogen-rich gas 
stream and liquid phase stream comprising recovered benzene, 
benzene-containing stabilized reformate, and LPG. This zone is operated at 
conditions that will maximize the absorption of the liquefiable 
hydrocarbons by the benzene-containing, stabilized reformate. Generally, 
the conditions within the separation zone will include a temperature of 
about -15.degree. to 40.degree. F. and a pressure of about 50 to 500 psig. 
The separation zone usually consists of an open vessel that operates in 
the nature of a flash drum. 
The hydrogen-rich gas stream containing at least a portion of the 
benzene-containing, stabilized reformate is removed from the separator and 
passed to an adsorber bed containing adsorbent having adsorptive capacity 
for hydrocarbons at effective adsorption conditions. The adsorber bed is 
preferably part of an integrated pressure swing adsorption (PSA) process 
whereby a continuous adsorber operation can be maintained while 
simultaneously regenerating a spent adsorber bed. 
It is contemplated that the PSA feature of the present invention comprises 
a plurality of adsorption zones maintained at an elevated pressure 
effective to adsorb hydrocarbons while letting the hydrogen pass through 
the adsorber bed. At a defined time, the passing of the adsorber feed to 
one adsorber bed is discontinued and the adsorber bed is depressurized by 
one or more co-current depressurization steps wherein the pressure is 
reduced to a defined level which permits additional hydrogen and light 
hydrocarbon components remaining in the adsorber bed to be withdrawn and 
utilized. Then the adsorber bed is depressured by a countercurrent 
depressurization step wherein the pressure in the adsorber bed is further 
reduced by withdrawing desorbed hydrocarbons countercurrently to the 
direction of the feed. Finally, the adsorber bed is purged and 
repressurized. A suitable purge gas is the co-current depressurization 
hydrogen-rich gas produced from another adsorber vessel. The final stage 
of repressurization is with feed gas or light gases produced during the 
adsorption step. An additional description of a pressure swing adsorption 
process suitable for use in the present invention can be found in U.S. 
Pat. No. 4,461,630 which is herein incorporated by reference. 
The present invention can be performed using virtually any adsorbent 
material in the adsorber beds that has a preferential capacity for 
hydrocarbons as compared to hydrogen. Suitable adsorbents known in the art 
and commercially available include crystalline molecular sieves, activated 
carbons, activated clays, silica gels, activated aluminas and the like. 
It is often desirable when using crystalline molecular sieves that the 
molecular sieve be agglomerated with a binder in order to ensure that the 
adsorbent will have suitable physical properties. Although there are a 
variety of synthetic and naturally-occurring binder materials available 
such as metal oxides, clays, silicas, aluminas, silica-aluminas, 
silica-zirconias, silica-thorias, silica-beryllias, silica-titanias, 
silica-alumina-thorias, silica-alumina-zirconias, mixtures thereof and the 
like, clay-type binders are preferred. Examples of binders which may be 
employed to agglomerate the molecular sieve without substantially altering 
the adsorptive properties of the zeolite are attapulgite, kaolin, volclay, 
sepiolite, polygorskite, kaolinite, bentonite, montmorillonite, illite, 
and chlorite. The choice of a suitable binder and methods employed to 
agglomerate the molecular sieves are generally known to those skilled in 
the art and need not be further described herein. 
The PSA cycle used in the present invention preferably includes the steps 
of adsorption, at least one co-current depressurization step, 
countercurrent desorption, purge and repressurization. Thus, cycle steps 
are typically described with reference to their direction relative to the 
adsorption step. The cycle steps wherein the gas flow is in a concurrent 
direction to the adsorption step are known as "co-current" steps. 
Similarly, cycle steps wherein the gas flow is countercurrent to the 
adsorption step are known as "countercurrent" steps. During the adsorption 
step, the feed stream is passed to the adsorber bed at an elevated 
adsorption pressure in order to cause the adsorption of the hydrocarbons 
and produce a hydrocarbon-free, hydrogen-rich gas stream. During the 
co-current depressurization steps, the pressure in the depressurizing bed 
is released and the effluent obtained therefrom, which is rich in 
hydrogen, is passed in a countercurrent direction to another adsorber bed 
undergoing purge or repressurization. Typically, more than one co-current 
depressurization step is used wherein a first equalization step is 
performed after which a purge step is initiated wherein the adsorber bed 
is further co-currently depressured to provide a purge gas that is 
relatively impure with respect to the adsorbed component and thus is 
suitable for use as a purge gas. Optionally, a portion of hydrogen-rich 
adsorption effluent gas having a reduced concentration of hydrocarbons or 
an externally supplied gas can be used to supply the purge gas. Upon the 
completion of the co-current depressurization step, if employed, the 
adsorber bed is countercurrently depressurized to a desorption pressure in 
order to desorb the hydrocarbons. Upon completion of the desorption step, 
the adsorber bed is purged countercurrently with purge gas typically 
obtained from another bed. Finally, the adsorber bed is repressurized, 
first, typically with equalization gas from other adsorber beds and then 
with feed or product gas to adsorption pressure. Other additional steps 
known to those skilled in the art, such as a co-purge step wherein the 
adsorber bed is co-currently purged of the less strongly adsorbed 
components at an elevated pressure such as the adsorption pressure with a 
purge stream comprising hydrocarbons, can be employed. 
The adsorber bed may suitably be operated at a pressure in the range of 
about 50 to 500 psig. The operating temperature for the adsorber bed can 
range from -20.degree. to 150.degree. F. These operating condition ranges 
are suitable for both adsorption and desorption. Additional adsorber bed 
operating conditions, such as cycle times and rates of depressurization, 
are not critical to the present invention and may readily be selected by a 
person skilled in the art. 
In accordance with the present invention, a hydrocarbon-free, hydrogen-rich 
gas stream exits the PSA unit and is fed to a hydrodealkylation unit along 
with the previously mentioned toluene-rich stream. The hydrocarbon-free, 
hydrogen gas stream has a hydrogen purity of at least about 99 mol %. 
Accordingly, hydrocarbon-free is defined as a hydrocarbon content of less 
than about 1 mole % hydrocarbon, preferably less than about 0.1 mole % 
hydrocarbon, most preferably less than about 0.01 mole % hydrocarbon. In 
the hydrodealkylation unit, alkylaromatics contained in the toluene-rich 
stream are converted to benzene and light hydrocarbons (methane and 
ethane), while paraffins and naphthenes are hydrocracked. In a preferred 
embodiment, the hydrodealkylation unit is operated in the absence of an 
added catalyst, i.e., thermally. 
Suitable hydrodealkylation operating conditions include a temperature of 
about 1000.degree. to 2000.degree. F., a pressure of about atmospheric to 
1000 psig, preferably 50 to 750 psig. The make-up hydrogen rate to the 
dealkylation zone can be maintained slightly in excess of that required to 
dealkylate the alkyl aromatics. 
Benzene is recovered from the hydrodealkylation unit by passing the 
hydrodealkylation effluent to a vapor-liquid equilibrium separator where 
the effluent separates into a liquid benzene product and a 
vapor-containing gas stream containing hydrogen, light hydrocarbons, and 
benzene. At least a portion of the vapor-containing gas stream is a purge 
stream while the remainder is recycled to the THDA reactor. 
In a preferred embodiment, the vapor-containing purge stream is recycled 
back to the refrigerated separation unit where it is admixed with the 
previously-mentioned benzene-containing, stabilized reformate. In the 
refrigerated separation unit, the benzene contained in the 
vapor-containing purge stream is absorbed by the benzene-containing, 
stabilized reformate. A liquid phase containing the benzene-containing, 
stabilized reformate and benzene recovered from the vapor-containing purge 
stream exits the refrigerated separation unit and is recycled to the 
stabilizing zone. A hydrogen-rich gas stream containing the hydrogen and 
light hydrocarbons recovered from the vapor-containing purge stream exits 
the refrigerated separation unit and is passed on to the previously 
mentioned PSA unit where the hydrogen is separated from the light 
hydrocarbons in the manner previously described hereinabove. 
Referring to the figure, a naphtha feedstock 2 is charged into a catalytic 
reforming unit 4 along with a hydrogen stream 6. This produces a 
benzene-containing reformate stream 8 and a hydrogen-containing vapor 
phase stream 10. The reformate stream 8 is then passed into a stabilizing 
zone 12 wherein there is produced a hydrocarbon-containing vapor phase 
stream 14 and a benzene-containing, stabilized reformate stream 16. The 
hydrocarbon-containing vapor phase stream 14 is directed to an overhead 
condenser 50. Exiting the top of the overhead condenser 50 is a fuel gas 
stream 54. Exiting the bottom of the overhead condenser 50 is a liquid 
reflux stream 52 and an LPG product stream 56. At least a portion of said 
benzene-containing, stabilized reformate stream 16 is first routed to an 
aromatic extraction zone 27 and then to a fractionation zone 17 to produce 
a benzene-rich product stream 25 and a toluene-rich stream 32. 
The remainder of the benzene-containing, stabilized reformate 16 is 
directed to a refrigerated separation unit 20 where the 
benzene-containing, stabilized reformate 16 is refrigerated to a 
temperature of less than about 40.degree. F. and admixed with said 
hydrogen-containing vapor phase 10. This refrigerated admixture 21 is then 
sent to a vapor-liquid separator 23 to produce a hydrogen-rich gas stream 
22 and a liquid phase stream 19. This liquid phase stream 19 is recycled 
to the stabilizing zone 12. 
The hydrogen-rich gas stream 22 is then directed to a pressure swing 
adsorption unit 24 having a first adsorber bed 24a and second adsorber bed 
24b. The hydrogen-rich gas stream 22 is passed to the first adsorber bed 
24a which contains adsorbent containing adsorptive capacity for 
hydrocarbons at effective adsorption conditions. A hydrocarbon-free, 
hydrogen-rich gas stream 28 is withdrawn from the first adsorber bed 24a. 
A stream rich in hydrocarbons 26 is withdrawn from the second adsorber bed 
containing adsorbent having adsorptive capacity for hydrocarbons, said 
second bed 24b undergoing desorption of previously loaded hydrocarbons. 
At least a portion of the hydrocarbon-free, hydrogen-rich ga stream 28 is 
then admixed with the toluene-rich stream 32 and reacted in a thermal 
hydrodealkylation unit 30 at conditions selected to produce a 
benzene-containing product stream 36 and a vapor-containing purge stream 
34. The vapor-containing purge stream 34 containing benzene, hydrogen and 
light hydrocarbons generated in the hydrodealkylation unit 30 is then 
recycled to the refrigerated separation unit 20 where it is admixed with 
the benzene-containing, stabilized reformate 16.