Process to reduce the benzene content of gasoline

Benzene, which is toxic and carcinogenic, is removed from a gasoline or gasoline precursor stream, such as a mixture of a reformate and a naphtha produced in a fluidized catalytic cracking unit, in a process which includes fractionating the precursor stream into a light hydrocarbon stream containing the benzene and a heavy hydrocarbon stream. The light hydrocarbon stream is then admixed with an olefinic hydrocarbon, passed through an alkylation zone and then remixed with the heavy hydrocarbon stream.

FIELD OF THE INVENTION 
The invention relates to a hydrocarbon conversion process having a gasoline 
boiling range feed stream. The invention also relates to a method of 
treating a gasoline stream for the removal of benzene. A limited 
embodiment of the invention relates to the alkylation of benzene with a 
C.sub.3 or C.sub.4 olefin at an olefin to aromatic hydrocarbon ratio above 
1.0:1.0 through the use of an SPA catalyst. 
PRIOR ART 
Large amounts of benzene are present in several of the streams used in 
refineries as blending stocks for gasoline. These include reformates and 
the gasoline produced in an FCC gas concentration unit. Some of this 
benzene has traditionally been removed, as by liquid-liquid extraction, to 
be used as a petrochemical feedstock and for other purposes. However, the 
great majority of benzene present in gasoline precursor streams has been 
allowed to remain in these streams since it is readily abundant and has an 
acceptable octane number. 
The alkylation of benzene is widely practiced commercially. For instance, 
the alkylation of benzene with propylene to form cumene is described in 
U.S. Pat. Nos. 3,132,109 (Cl. 252-435); 3,293,315 (Cl. 260-671); 3,499,826 
(Cl. 203-27); 3,510,534; 3,520,945 and 4,008,290 (Cl. 260-672). These 
references also describe solid phosphoric acid (SPA) catalysts. Another 
catalyst system utilizes boron trifluoride to effect the alkylation of 
benzene with ethylene and propylene. This alkylation process is described 
in U.S. Pat. Nos. 2,995,611 (Cl. 260-671); 3,126,421; 3,238,268 and 
3,894,090. A large number of other catalyst systems are known. Examples 
are found in U.S. Pat. Nos. 2,887,520 and 3,336,410. 
The contacting of gasolines produced by catalytic cracking with an SPA 
catalyst was performed in a process referred to as polytreating. This 
process is described in an article which appears at page 1045 of Vol. 38, 
No. 10 of Industrial and Engineering Chemistry published in October, 1946. 
The conditions employed in the process included a temperature from about 
400.degree. F. to 560.degree. F. and a pressure in excess of 400 psig. The 
objective of the invention was to lower the concentration of various 
olefinic constituents which, by virtue of their low lead susceptibility, 
were considered less desirable. The article specifies aromatic 
hydrocarbons of the gasoline undergo virtually no change during the 
polytreating reactions and this is described as highly desirable. Among 
the reactions listed as occurring during the process are the 
polymerization of olefins, the cyclization of higher olefins to naphthenes 
and the hydrogenation of olefins present in the original gasoline by 
hydrogen produced in other reactions. 
BRIEF SUMMARY OF THE INVENTION 
The invention provides a process for treating gasoline streams to remove 
benzene which does not have a major adverse impact on the octane number or 
total aromatics content of the gasoline stream being treated. The process 
may be broadly characterized as comprising the steps of passing a gasoline 
precursor stream comprising less than 5.0 mole percent benzene, 1.0 mole 
percent toluene, at least 5.0 mole percent C.sub.4 to C.sub.6 paraffinic 
hydrocarbons, at least 10.0 mole percent C.sub.8 to C.sub.10 aromatic 
hydrocarbons and also containing C.sub.7 to C.sub.9 paraffinic 
hydrocarbons into a fractionation zone, and separating the gasoline 
precursor stream into a light fraction comprising substantially all of the 
benzene contained in the gasoline precursor stream and a heavy fraction 
comprising substantially all of the toluene and higher boiling 
hydrocarbons contained in the gasoline precursor stream; admixing an 
olefin feed stream comprising olefinic hydrocarbons having from 2 to 4 
carbon atoms per molecule into the light fraction of the gasoline 
precursor stream to form a reaction zone feed stream; passing the reaction 
zone feed stream through an alkylation reaction zone maintained at 
benzene-alkylation promoting conditions and effecting the production of a 
reaction zone effluent stream comprising an aromatic hydrocarbon having 
from 8 to 10 carbon atoms per molecule and C.sub.4 and C.sub.6 paraffinic 
hydrocarbons and which contains less than 1.0 mole percent benzene; 
passing the reaction zone effluent stream into a separation zone and 
effecting the division of the reaction zone effluent stream into a light 
separation zone effluent stream comprising a C.sub.3 or C.sub.4 
hydrocarbon and a heavy separation zone effluent stream comprising 
substantially all of the aromatic hydrocarbons contained in the reaction 
zone effluent stream; and admixing the heavy separation zone effluent 
stream with the heavy fraction of the gasoline precursor stream to form a 
low benzene content gasoline blending stream.

DETAILED DESCRIPTION 
Benzene is present in the gasoline boiling range effluent streams of 
several petroleum refining processes. These processes include catalytic 
reforming, coking, pyrolysis and fluidized catalytic cracking. Some of the 
benzene in gasoline boiling range streams is recovered and purified, as by 
liquid-liquid extraction and fractionation, for use as a petrochemical 
product or feedstock. However, the amount of benzene present in the total 
of the streams used for gasoline production far exceeds the amount 
required to satisfy the demand for benzene. Most of the benzene in 
gasoline boiling range streams is therefore allowed to remain in these 
streams and is eventually used in gasoline. 
Benzene is now coming under close scrutiny as a possible carcinogen or 
leukemogen. Its presence in gasoline therefore presents the possibility 
that service station operators, motorists and others are being exposed to 
harmful benzene concentrations on a regularly recurring basis. It is an 
objective of the invention to provide a process for treating a gasoline 
boiling range hydrocarbon stream to reduce its benzene content. It is 
another objective of the invention that this reduction of the benzene 
content of gasoline precursor streams be accomplished in an economical 
process which has minimal adverse effect on the octane number of the 
gasoline precursor stream. Yet another objective of the invention is to 
remove benzene from gasoline boiling range streams in a manner which does 
not create an oversupply of benzene, thus causing a reduction in its 
market value, and which converts the benzene to a high value product. 
Benzene may of course be eliminated from gasoline precursor streams in the 
same manner it is now partially removed for benzene production. For 
instance, reformates may be fractionated to yield a C.sub.6 to C.sub.8 cut 
which is then fed to a liquid-liquid extraction zone and contacted with a 
solvent selective for aromatic hydrocarbons. The resultant extract stream 
is separated to yield an aromatic hydrocarbon product stream from which 
the benzene may be separated by fractionation. A suitable aromatic 
hydrocarbon extraction process is described in U.S. Pat. Nos. 3,492,222; 
3,642,614 and 3,652,452. If the gasoline precursor stream is derived from 
a catalytic cracking process, it will normally be necessary to hydrotreat 
the precursor stream prior to charging it into the liquid-liquid 
extraction zone. The benzene produced in this manner may be alkylated to 
yield styrene, cumene or a long chain alkylate used to manufacture 
detergents. 
The subject process is used to treat a gasoline precursor stream. The 
phrase "gasoline precursor stream" is intended to refer to a stream 
comprising a mixture of aromatic and paraffin hydrocarbons having boiling 
points between about 90.degree. F. and 410.degree. F. and which is to be 
used to produce gasoline. Gasolines are often produced by blending 
together several different hydrocarbon streams. Some of these streams do 
not contain benzene, and therefore do not require treatment by the subject 
invention. For instance, benzene-free branched chained paraffinic 
hydrocarbon streams, such as those produced by the HF-catalyzed alkylation 
of isobutane, may be admixed into the gasoline precursor stream. However, 
this admixture is preferably done downstream of the subject process in 
order to avoid the unnecessary treating of this material. Likewise, any 
addition of butane or other light hydrocarbons to adjust the volatility of 
the product gasoline is also preferably done downstream of the subject 
benzene removal process. 
The gasoline precursor or feed stream will normally contain about 0.5 to 
5.0 or higher mole percent benzene. It will also contain various C.sub.7 
to C.sub.10 aromatic hydrocarbons including about 1.0 mole percent toluene 
and at least 10 mole percent of C.sub.8 to C.sub.10 aromatic hydrocarbons. 
The total concentration of all aromatic hydrocarbons in the gasoline 
precursor stream may be above 25 mole percent. The gasoline precursor 
stream will also normally contain some C.sub.4 to C.sub.6 paraffinic 
hydrocarbons. These may include butane, isopentane, isohexane and n-hexane 
and will normally be present at a concentration above 5.0 mole percent. 
C.sub.7 to C.sub.9 paraffinic hydrocarbons such as isoheptane and 
iso-octane are also present in many gasoline precursor streams. The 
concentration of these paraffins will normally be above 2.0 mole percent 
and may be above 5.0 or 15.0 mole percent. The exact composition of the 
gasoline precursor stream will depend on its source. It may be formed by 
blending all or a portion of the effluent of several different petroleum 
processing units. Two such effluents are the bottoms product of the 
stripper column used in FCC gas concentration units and stabilized 
reformates which contain C.sub.6 to C.sub.9 aromatic hydrocarbons. 
The feed stream of the subject process is fed into a fractionation zone 
maintained at suitable fractionation conditions. Preferably, this zone 
comprises a single trayed fractionation column which is sized according to 
well known criteria based on the flow rate and composition of the feed 
stream. The conditions used in this zone may be those which are customary 
in the art. A positive pressure of about 5 to 450 psig. is preferred. The 
temperature required is dependent on the pressure and the feed stream 
composition but is preferably within the broad range of from 250.degree. 
F. to 650.degree. F. as measured at the bottoms draw-off point of the 
zone. A precise split between the various components of the feed stream is 
not required. The criteria for operation of the fractionation zone is that 
substantially all of the benzene contained in the feed stream is separated 
into a separate stream, which will be the overhead product of the 
fractionation zone. As used herein, modifiers such as "substantially all" 
are intended to refer to molar percentages above 95%. Preferably, over 98 
mole percent of the benzene is contained in the overhead product stream of 
the fractionation zone, which is also referred to herein as the light 
hydrocarbon stream. Some toluene may also be present in this light 
hydrocarbon stream, but most of the toluene is preferably retained in the 
heavy hydrocarbon stream removed as the bottoms product of the 
fractionation zone. The overhead product of the zone will also contain 
various C.sub.4 to C.sub.6 paraffinic hydrocarbons contained in the feed 
stream, while substantially all C.sub.8 to C.sub.10 aromatic hydrocarbons 
will be contained in the heavy hydrocarbon stream. 
A reaction zone charge stream is formed by admixing one or more olefinic 
hydrocarbons into the light hydrocarbon stream produced in the 
fractionation zone. Suitable hydrocarbons are ethylene, propylene and the 
butylenes. High purity streams of one olefin may be used if available, but 
mixtures of the olefins may also be employed in the invention. Preferably, 
the olefin feed stream is rich in olefins and contains less than 25 mole 
percent non-olefinic hydrocarbons. The preferred composition of the feed 
stream containing the added olefins will be influenced by several factors. 
One of the most important will be the reactions promoted by the catalyst 
employed in the downstream benzene alkylation zone and the effects of 
olefin feed stream composition on the reaction zone product distribution. 
With some catalysts, it may be beneficial to utilize a high purity olefin 
stream or to minimize the presence of light paraffins such as butane. 
However, it is preferred to utilize a catalyst which will tolerate various 
amounts of these unreactive light hydrocarbons. This allows the use of 
lower purity gas streams. One such gas stream is that produced as the 
overhead product stream of the stripping column employed in a typical FCC 
gas concentration plant. This gas stream may comprise methane, ethane, 
ethylene, propane, propylene, butane and various butenes. An olefin-rich 
C.sub.3 to C.sub.4 stream derived from the stripping column overhead may 
also be used. 
The reaction zone charge stream is brought into intimate contact with a 
catalyst in an alkylation reaction zone maintained at benzene 
alkylation-promoting conditions. A homogeneous catalyst system may be 
employed if desired. These include hydrofluoric acid systems, sulfuric 
acid systems and various Friedel-Crafts catalysts such as the aluminum 
chloride (AlCl.sub.3) system described in U.S. Pat. No. 3,848,012 (Cl. 
260-671R). The catalyst employed in the alkylation reaction zone 
preferably comprises a fixed bed of solid material. For instance, a 
crystalline aluminosilicate such as described in U.S. Pat. Nos. 3,751,504; 
3,751,506 and 3,755,483 may possibly be employed. Another suitable 
catalyst system employs a gaseous catalyst promoter which is circulated 
through a bed of solid carrier particles. These carrier particles are 
normally inorganic oxides such as the gamma and theta forms of alumina, 
silica, boria and various naturally occurring inorganic oxides including 
clays and diatomaceous earth. The vaporous catalyst promoter is preferably 
a halogen-containing compound such as boron trifluoride, boron 
trichloride, hydrogen chloride, carbon tetrachloride, hydrogen fluoride, 
ammonium fluoride and ammonium chloride. More preferably, the catalyst 
promoter is boron trifluoride. This catalyst system is further described 
in U.S. Pat. Nos. 3,126,421; 3,631,122 and 3,894,090. 
The preferred catalyst for use in the subject process is a solid phosphoric 
acid (SPA) catalyst. One reason for this preference is its propensity to 
produce mono-alkylated aromatic hydrocarbons from benzene and propylene 
compared to most other catalyst systems. Suitable solid phosphoric acid 
catalysts are available commercially. As used herein, the term "SPA 
catalyst" or its equivalent is intended to refer generically to a solid 
catalyst which contains as one of its principal raw ingredients an acid of 
phosphorus such as ortho-, pyro- or tetraphosphoric acid. These catalysts 
are normally formed by mixing the acid with a siliceous solid carrier to 
form a wet paste. This paste may be calcined and then crushed to yield 
catalyst particles, or the paste may be extruded or pelleted prior to 
calcining to produce more uniform catalyst particles. The carrier is 
preferably a naturally occurring porous silica-containing material such as 
kieselguhr, kaolin, infusorial earth and diatomaceous earth. A minor 
amount of various additives such as mineral talc, fullers earth and iron 
compounds including iron oxide have been added to the carrier to increase 
its strength and hardness. The combination of the carrier and the 
additives normally comprises about 15- 30 wt.% of the catalyst, with the 
remainder being the phosphoric acid. However, the amount of phosphoric 
acid used in the manufacture of the catalyst may vary from about 8-80 wt.% 
of the catalyst as described in U.S. Pat. No. 3,402,130. The amount of the 
additives may be equal to about 3-20 wt.% of the total carrier material. 
Further details as to the composition and production of typical SPA 
catalysts may be obtained from U.S. Pat. Nos. 3.050,472; 3,050,473 and 
3,132,109 and from other references. 
The reaction or alkylation zone is maintained at benzene-alkylation 
promoting conditions. A general range of these conditions includes a 
pressure of from about 50 to 1200 psig. and a temperature of from about 
60.degree. F. to 850.degree. F., and their selection is dependent on the 
catalyst system employed. With SPA catalyst the pressure is preferably 
from 300 to 1000 psig. and the temperature is preferably within the range 
of from 300.degree. F. to 600.degree. F. The liquid hourly space velocity 
of the reactants may range from about 0.5 to 2.5. It is preferred that the 
reaction zone charge stream is a mixed-phase stream when an SPA catalyst 
is used in the reaction zone. To insure this, the olefin feed stream may 
comprise light paraffins having the same number of carbon atoms per 
molecule as the olefin consumed in the process. The configuration and 
apparatus of the reaction zone may be that which is customarily used with 
the catalyst system selected for use in the process. With SPA catalysts, 
upward flow through vertical beds of catalyst is preferred. It is 
preferred that the added olefin comprises propylene or butene when an SPA 
catalyst is employed. 
A stoichiometric excess of the olefinic hydrocarbon is maintained within 
the reaction zone by the admixture of a sufficient quantity of the olefin 
feed stream with the benzene-containing light hydrocarbon stream produced 
in the fractionation zone. That is, the olefin to benzene ratio in the 
reaction zone charge stream is maintained above 1.0:1.0. As benzene may 
not be the only aromatic hydrocarbon present in the reaction zone charge 
stream, the reaction charge stream should have a minimum light olefin to 
total aromatic hydrocarbon ratio of 1.0:1.0. Preferably this ratio is 
above 1.4:1.0, and more preferably, it is above 1.5:1.0 as this is 
believed necessary to achieve the alkylation of 95 mole percent of the 
benzene present in the reaction zone feed stream. However, a very large 
excess of the olefin leads to the production of polyalkylated aromatics 
boiling above the normally accepted gasoline boiling point curve end 
points. The aromatic hydrocarbon to olefin ratio is therefore preferably 
below 2.0:1.0, and more preferably below 1.8:1.0. 
The reaction zone effluent stream will contain residual benzene, the 
C.sub.8 to C.sub.10 product of the alkylation reaction and other 
hydrocarbons such as the C.sub.4 to C.sub.6 paraffins which were in the 
light hydrocarbon stream. The reaction zone effluent stream is preferably 
cooled by indirect heat exchange and then passed into a separation zone. 
This separation zone may take different forms depending on the composition 
of the reaction zone effluent stream and the desired composition of the 
effluent of the process. For instance, any C.sub.2 and C.sub.3 
hydrocarbons present in the reaction zone effluent stream will normally 
have to be entirely absent from the liquid product while some C.sub.4 
hydrocarbons are normally acceptable in gasolines. Consideration must also 
be given to the concentration of dissolved light olefins which can be 
tolerated in the heavy or liquid effluent stream of the separation zone. 
The apparatus used in the separation zone may therefore range from a 
single vapor-liquid separator or knock out vessel to a rectified 
stabilizer or debutanizer column. A simple vapor-liquid separator could be 
operated at a pressure slightly less than that used in the reaction zone 
and a temperature of from about 100.degree. F. to 150.degree. F. A 
stabilizer would be operated at the customary conditions for this widely 
practiced separation. The separation zone is preferably operated at 
conditions effective to remove substantially all hydrogen, methane, ethane 
and propane from the reaction zone effluent stream. These materials will 
be concentrated into a light separation zone effluent stream, which may 
also contain some C.sub.4 hydrocarbons depending on the composition of the 
olefin feed stream. This stream may contain appreciable amounts of the 
olefin consumed in the reaction zone, and therefore all or a portion of it 
my be recycled for use in the process by admixture into the light 
hydrocarbon stream. The recycled portion of the light hydrocarbon stream 
may be passed through a purification zone to remove excessive amounts of 
unreactive light paraffins. The toluene and heavier aromatics and 
paraffins in the reaction zone effluent stream will be concentrated into a 
heavy separation zone effluent stream. 
The heavy separation zone effluent stream is then admixed with the heavy or 
second hydrocarbon stream produced in the fractionation zone. The 
resultant combined stream is then transferred to a final blending system 
wherein it is adjusted to meet standards, such as octane number and 
volatility, which have been established for the desired gasoline. 
In accordance with this description, the preferred embodiment of the 
invention may be characterized as a process for reducing the benzene 
content of a gasoline precursor stream which comprises the steps of 
passing a gasoline precursor stream comprising from 0.5 to 5.0 mole 
percent benzene, 1.0 mole percent toluene, 5.0 mole percent of C.sub.4 to 
C.sub.6 paraffinic hydrocarbons, 10 mole percent of C.sub.8 to C.sub.10 
aromatic hydrocarbons and also comprising C.sub.7 to C.sub.9 paraffinic 
hydrocarbons and having a boiling point range between 90.degree. F. and 
410.degree. F. into a fractionation zone operated at fractionation 
conditions and separating the gasoline precursor stream into a first 
hydrocarbon stream comprising C.sub.4 to C.sub.6 paraffinic hydrocarbons 
and substantially all of the benzene contained in the gasoline precursor 
stream and a second hydrocarbon stream comprising toluene and 
substantially all of the C.sub.8 to C.sub.10 aromatic hydrocarbons 
contained in the gasoline precursor stream; forming a reaction charge 
stream having an olefin to aromatic hydrocarbon ratio from about 1.5;1.0 
to 1.8:1.0 by admixing a sufficient quantity of an olefin feed stream 
comprising propylene into the first hydrocarbon stream; contacting the 
reaction zone charge stream with an SPA catalyst in an alkylation reaction 
zone maintained at benzene alkylation-promoting conditions, and effecting 
the production of a reaction zone effluent stream which comprises an 
aromatic hydrocarbon having 9 carbon atoms per molecule and C.sub.4 to 
C.sub.6 paraffinic hydrocarbons and which contains less than 1.0 mole 
percent benzene; passing the reaction zone effluent stream into a 
separation zone and effecting the division of the reaction zone effluent 
stream into a light separation zone effluent stream comprising a C.sub.3 
hydrocarbon and a heavy separation zone effluent stream comprising 
substantially all of the aromatic hydrocarbons contained in the reaction 
zone effluent stream; and admixing the heavy separation zone effluent 
stream and the second hydrocarbon stream. 
The subject process may be integrated into an existing of new refinery. 
This requires some changes in the operation of other refinery units, but 
reduces the overall costs of benzene removal. One embodiment of this 
integrated process is utilized in a refinery having both a catalytic 
reforming unit and a fluidized catalytic cracking (FCC) unit. The effluent 
of the reforming zone is customarily cooled and then debutanized in a 
rectified fractionation column. This produces an overhead vapor stream 
substantially free of C.sub.5 and C.sub.6 hydrocarbons. In this embodiment 
of the integrated benzene removal process, the operation of this column is 
changed to produce an overhead stream containing substantially all of the 
benzene originally in the reformate. The prior art debutanizer is thereby 
utilized as part of the fractionation zone which produces the light 
hydrocarbon stream which is passed into the alkylation zone. The C.sub.7 
-plus bottoms product of this column is bypassed around the alkylation 
zone. This reduced the total amount of fractionation required to reduce 
the benzene content of the final gasoline product. 
In this embodiment, the net bottoms stream of the stripping column used in 
the gas concentration unit associated with the fluidized catalytic 
cracking unit is passed into the fractionation zone which produces the 
light hydrocarbon stream passed into the benzene alkylation zone. The 
effluent of the benzene alkylation zone is debutanized to produce an 
overhead stream comprising C.sub.3 and C.sub.4 hydrocarbons from the 
stripping column bottoms and the hydrogen, methane and ethane which was 
present in the overhead stream of the column now utilized as a reformate 
dehexanizer. This debutanizer overhead stream may be handled in three 
ways. Preferably, it is passed into another fractionation column and most 
of the C.sub.3 and C.sub.4 hydrocarbons are removed. The remaining lighter 
material may then be passed into either a refrigerated deethanizer column 
or the primary absorber of the gas concentration unit. Alternatively, the 
debutanizer overhead stream may be passed directly into the primary 
absorber.