Vapor circulation in hydrocarbon conversion processes

A hydrocarbon conversion process wherein a fractionation column overhead vapor stream is circulated through a portion of the process, such as through a purification zone or hydrocarbon recovery zone. Part of the motive force for this vapor circulation is derived from depressurizing the vaporous phase of a mixed-phase reaction zone effluent through an ejector which draws a suction on the overhead receiver of the fractionation column.

FIELD OF THE INVENTION 
The invention relates to hydrocarbon conversion processes in general, 
including processes for the alkylation of aromatic hydrocarbons by the 
introduction of an acyclic side chain, and the hydrodealkylation, 
hydrocdesulfurization and reforming of petroleum streams. The invention 
also relates to the separation of a mixed-phase reaction zone effluent 
stream produced in such a process and to circulation of process vapor 
streams through the use of fluid jet ejectors. 
PRIOR ART 
The wide commercial utilization of hydrocarbon conversion processes has 
resulted in much developmental effort and a very high state of the art. 
Specific processes, such as the alkylation of aromatic hydrocarbons, are 
therefore well developed and are performed using a wide variety of 
catalysts and equipment configurations. The alkylation of aromatic 
hydrocarbons using a volatile catalyst promoter is described in U.S. Pat. 
Nos. 3,894,090 (Cl. 260-671); 3,631,122 and 2,971,992. The latter 
reference also shows the passage of the alkylation zone effluent into a 
vapor-liquid separator, with a vapor stream comprising boron trifluoride, 
unreactive gases and the aromatic hydrocarbon being removed from the 
separator and passed into a countercurrent gas-liquid absorption zone. A 
stream of polyalkylated aromatic hydrocarbons is fed to the top of this 
zone to effect the recovery of the boron trifluoride and aromatic 
hydrocarbon. The unreactive gases are vented from the top of this zone. 
The preferred catalyst system is also described in U.S. Pat. No. 3,126,421 
(Cl. 260-671). In the alkylation process presented by this reference the 
effluents of an alkylation zone and a transalkylation zone are combined 
and passed into a flash drum. The liquid removed from the flash drum is 
passed into a benzene column. The vapor removed from the flash drum is 
cooled and passed into a vapor-liquid separator. The vapor removed from 
this separator is passed into the absorber of a boron trifluoride recovery 
zone comprising an absorber and a stripper. The enriched boron trifluoride 
produced in this zone is recirculated to the alkylation and 
transalkylation zones. This same reference also shows the removal of a 
boron trifluoride-containing overhead vapor from the benzene column. This 
overhead vapor stream is, however, recycled to the alkylation zone and not 
passed into a reaction zone vapor-liquid separator as in the present 
invention. Another feature illustrated by this reference is the production 
of a benzene recycle stream from the overhead vapors removed from the 
benzene column. 
Previously cited U.S. Pat. No. 3,894,090 shows a benzene recycle stream 
withdrawn from the benzene column as a sidecut stream. This reference also 
describes the passage of boron trifluoride into the benzene column to 
prevent the precipitation of relatively non-volatile hydrates of boron 
oxide commonly referred to as borates. 
The use of a jet ejector to pull a vacuum on a fractionation column is a 
well established practice. Very often the motive stream used is high 
pressure steam which is subsequently condensed. 
SUMMARY OF THE INVENTION 
The invention provides a hydrocarbon conversion process wherein part of the 
motive force required for the circulation of a fractionation column 
off-gas stream comprising a volatile chemical, such as hydrogen or boron 
trifluoride, is provided by depressurizing the vapor phase portion of the 
effluent of the reaction zone through an ejector. This ejector is used to 
pump the overhead vapors of the fractionation column into a vapor-liquid 
separation zone into which the alkylation zone effluent is also charged. 
The vapors separated in this zone are then passed into a vapor 
purification zone. The invention thereby provides a hydrocarbon conversion 
process requiring less compressor capacity and having lower operating 
costs than the prior art, which, for instance, mechanically compressed the 
overhead vapors of the fractionation column and passed them into the vapor 
purification zone. 
The invention finds specific application to the processes of benzene 
alkylation, alkylaromatic hydrocarbon hydrodealkylation and 
hydrodesulfurization. The invention may be broadly characterized as a 
hydrocarbon conversion process which comprises the steps of passing a 
mixed-phase reaction zone effluent stream into a first vapor-liquid 
separation zone and forming therein a first liquid hydrocarbon stream 
comprising a hydrocarbon having six or more carbon atoms per molecule and 
a first vapor stream comprising a volatile chemical chosen from the group 
consisting of methane, ethane, ethylene, propane, hydrogen, water, 
hydrogen sulfide, boron trifluoride, carbon tetrachloride, hydrogen 
fluoride and hydrogen bromide; passing at least a portion of the first 
vapor stream through an ejector as a motive stream utilized to pump a 
second vapor stream comprising the volatile chemical chosen from the group 
consisting of methane, ethane, ethylene, propane, hydrogen, water, 
hydrogen sulfide, boron trifluoride, carbon tetrachloride, hydrogen 
fluoride and hydrogen bromide and thereby forming an ejector effluent 
vapor stream; passing the ejector effluent vapor stream into a second 
vapor-liquid separation zone having a lower pressure than the first 
vapor-liquid separation zone; passing the first liquid hydrocarbon stream 
into the second vapor-liquid separation zone; passing a second liquid 
hydrocarbon stream comprising the hydrocarbon having six or more carbon 
atoms per molecule from the second vapor-liquid separation zone to a 
fractionation zone having a lower pressure than the second vapor-liquid 
separation zone; withdrawing from the fractionation zone a net overhead 
vapor stream comprising the volatile chemical chosen from the group 
consisting of methane, ethane, ethylene, propane, hydrogen, water, 
hydrogen sulfide, boron trifluoride, carbon tetrachloride, hydrogen 
fluoride and hydrogen bromide and passing the net overhead vapor stream 
into the ejector as the second vapor stream; withdrawing from the second 
vapor-liquid separation zone a third vapor stream comprising the volatile 
chemical chosen from the group consisting of methane, ethane, ethylene, 
propane, hydrogen, water, hydrogen sulfide, boron trifluoride, carbon 
tetrachloride, hydrogen fluoride and hydrogen bromide, and passing the 
third vapor stream into a vapor purification zone which effects the 
removal of an impurity from the third vapor stream to produce a fourth 
vapor stream having a higher concentration of the volatile chemical chosen 
from the group consisting of methane, ethane, ethylene, propane, hydrogen, 
water, hydrogen sulfide, boron trifluoride, carbon tetrachloride, hydrogen 
fluoride and hydrogen bromide than the third vapor stream; and, 
withdrawing a fractionation product stream comprising the hydrocarbon 
having six or more carbon atoms per molecule from the fractionation zone.

DESCRIPTION OF THE DRAWINGS 
The Drawing illustrates the preferred embodiment of the invention. For 
clarity and simplicity various subsystems and apparatus associated with 
the operation of the process have not been shown. These items include flow 
and pressure control valves, pumps, temperature and pressure monitoring 
systems, reactor and fractionator internals, etc., which may be of 
customary design. This representation of the preferred embodiment is not 
intended to preclude from the scope of the invention those other 
embodiments set out herein or which are the result of reasonable and 
normal modification of these embodiments. 
Referring now to the drawing, a feed stream comprising ethylene enters the 
process through line 1 and is admixed with a feed stream comprising 
benzene which enters through line 2. The resultant admixture is carried by 
line 1 to the junction with line 3, where it is commingled with a recycle 
vapor stream comprising boron trifluoride. The feed material continues 
through line 4 and is admixed with a portion of a recycle benzene stream 
carried by line 48. Line 4 carries the resultant mixture of benzene, 
ethylene and boron trifluoride to the junction with line 55 where it is 
combined with a recycled portion of the alkylation zone effluent, which 
has been cooled by a means not shown. This produces a combined alkylation 
zone feed stream which is passed into the alkylation reactor 6 through 
line 5. The effluent of the alkylation reactor is removed in line 7 and 
divided into the portion which is recycled through line 55 and a second 
portion which is passed through line 8 into a vapor-liquid separator 9. 
The net effluent stream of the alkylation reactor is separated into a vapor 
stream comprising boron trifluoride and possibly other non-reactive gases, 
such as nitrogen and paraffins which were contained in the ethylene feed 
stream, and which is removed in line 34, and a liquid stream comprising 
benzene, the product ethylbenzene and polyalkylated benzenes. The liquid 
stream is removed in line 11 at a rate controlled by valve 10 which 
preferably is operated by a level control means sensing the liquid level 
in the separator 9. The liquid stream in line 11 is combined with the 
effluent stream of a transalkylation reactor 12 which is carried by line 
13. The effluent stream of the transalkylation reactor comprises benzene, 
ethylbenzene, polyalkylated benzenes and boron trifluoride. Line 14 
carries the resultant admixture of these two streams into a separating 
vessel 15. A liquid phase stream is removed from this separating vessel in 
line 16. A vapor stream comprising boron trifluoride is removed from the 
separating vessel 15 in line 18 and passed through a cooler 19. This 
results in the condensation of readily condensible hydrocarbons, such as 
benzene and ethylbenzene, which are collected as a liquid phase in a 
second separating vessel 20. The uncondensed vapors, including boron 
trifluoride are removed from the second vessel in line 21, and the 
condensate is removed in line 17. The condensate is then combined with the 
liquid stream from the first separating vessel and passed into line 24. 
Material in line 24 is passed into a benzene column 25. Also passed into 
this column is a stream of vaporous boron trifluoride from line 50. The 
boron trifluoride and various relatively volatile hydrocarbons rise 
through the benzene column and are removed as an overhead vapor stream in 
line 27. The overhead vapor stream is passed through overhead condenser 28 
which causes the condensation of benzene and other hydrocarbons having 
similar or higher boiling points. The overhead of the benzene column is 
then passed into an overhead receiver 29 and separated into a liquid phase 
reflux stream removed in line 30 and a vapor stream removed in line 31. 
This vapor stream comprises boron trifluoride and such relatively 
non-condensible light gases as nitrogen, methane or ethane which were 
contained in the ethylene feed stream. A stream of makeup boron 
trifluoride is passed into the process through line 32. The resultant 
boron trifluoride enriched vapor stream is caused to pass through line 33 
by the suction generated in an ejector 38. This suction is created through 
the depressurization of the relatively high pressure vaporous portion of 
the alkylation zone effluent passing through line 34. At least a portion 
of the vaporous effluent is used as the motive stream passed into the 
ejector in line 37. A second portion of the vaporous effluent may be 
bypassed around the ejector through line 35 at a rate controlled by valve 
36. The effluent of the ejector is admixed with any vaporous alkylation 
zone effluent which has been bypassed around the ejector and is then 
passed into the first settling vessel 15 through line 40. In this manner 
the relatively high pressure of the alkylation zone is utilized to remove 
the net uncondensed overhead vapors of the benzene column and to circulate 
these vapors into a first receiving vessel. From the first receiving 
vessel these vapors flows through line 18 and 21 in admixture with the 
vapors released from the liquid passing through line 14. 
A vapor stream in line 21 is passed into a promoter recovery zone 22. 
Preferably, this zone comprises a countercurrent absorption column and a 
stripping column. The operation of this preferred recovery zone 
configuration is described in greater detail herein. An off-gas stream 
comprising such impurities as nitrogen, methane and ethane is removed from 
the process in line 56. A purified stream of boron trifluoride vapors is 
removed in line 23 for recycling within the process. A first portion of 
this purified stream is passed into the bottom of the benzene column 
through line 50. The remaining portion passes through line 51 and is 
pressurized in a compressor 54. This pressurized portion is then divided 
between the boron trifluoride stream passed to the alkylation zone through 
line 3 and the boron trifluoride stream passed to the transalkylation zone 
through line 52 
A sidecut stream comprising benzene is removed from the benzene column in 
line 26 and passed through alumina treater 46. This fractionation product 
stream is then recycled through line 47 to provide the recycle benzene 
which is passed into the alkylation zone via line 48 and into the 
transalkylation zone via line 49. A net bottoms stream is removed from the 
benzene column in line 41 and passed into a fractionation zone 42. 
Preferably, this fractionation zone comprises two fractionation columns as 
described in greater detail herein. A net overhead product stream of 
substantially pure ethylbenzene is removed from the first fractionation 
column and forms the product stream removed in line 44. Preferably, a 
second fractionation column separates from the remaining hydrocarbons a 
relatively small by-product stream removed in line 43 and which comprises 
various high boiling aromatic hydrocarbons and polymers. This leaves the 
intermediate hydrocarbons including various bi- and tri-alkylated 
ethylbenzenes which are concentrated into a recycle stream removed in line 
45. This recycle stream is combined with the boron trifluoride from line 
52 and a recycle benzene stream from line 49 and passed into the 
transalkylation reactor through line 53. 
DETAILED DESCRIPTION 
The subject invention may be applied to a wide variety of hydrocarbon 
conversion processes which satisfy a set of criteria based on process 
conditions and methods and independent of the reaction performed in the 
process. This set of criteria includes the existence of a mixed-phase 
reaction zone effluent stream. The reaction zone effluent may be a 
vapor-phase stream as it emerges from the reaction zone and subjected to a 
subsequent cooling operation which causes the partial condensation of 
reaction zone effluent stream before it is passed into the first 
vapor-liquid separation zone. This will commonly be the situation in most 
processes for the hydrocracking or hydrotreating of light vaporizable 
petroleum stocks. In other processes, such as aromatic alkylation 
processes for the production of cumene or gasoline blending components 
which utilize an SPA catalyst, the reaction zone effluent is normally a 
mixed-phase stream as it emerges from the reaction zone. 
The criteria which determine the applicability of the subject invention to 
a particular process also include two pressure differentials. These 
inherent characteristics include a substantially greater pressure in the 
reaction zone and in the first separation zone than in the second 
vapor-liquid separation zone. This is necessary to allow the vapor phase 
motive stream to be depressurized through the ejector into the second 
separation zone. It is also necessary that the pressure in the 
fractionation column be less than that in the second vapor-liquid 
separation zone and in the vapor purification zone since an objective of 
the invention is to transfer the net overhead vapors of the fractionation 
column to a purification zone having a higher pressure than the 
fractionation column. 
The alkylation of aromatic hydrocarbons finds utility in several industrial 
processes. It is performed for instance to effect the production of 
ethylbenzene from benzene and ethylene, with much of the product 
ethylbenzene being subsequently dehydrogenated to produce styrene. In a 
similar manner, isopropylbenzene or cumene may be formed by the reaction 
of propylene and benzene. Cumene is used in the synthesis of phenol, 
acetone, alphamethylstyrene and acetophenone. These cumene-derived 
chemicals are intermediates in the production of resins for plastics 
including nylon. Other aromatic hydrocarbons are possible feedstocks for 
use in the subject invention. These include alkyl-substituted aromatics 
such as toluene, phenols and polycyclic aromatics. However, as the 
preferred embodiment of the invention concerns the alkylation of benzene 
and the prior art examples are often also based on the use of this 
feedstock, the invention will be described mainly in terms of benzene 
alkylation. This is not intended to in any way exclude other aromatic 
feedstocks from use in the invention. 
Furthermore, the invention is one of general application within the broad 
field of hydrocarbon conversion processes. It may be applied to other 
specific reactions including reforming, dehydrogenation, isomerization of 
paraffins and aromatics, hydrodecyclization, hydroformylation, 
hydrodesulfurization, coal gasification and liquefaction, hydrocracking, 
esterification, methanation, hydrogenation and the alkylation of 
paraffinic or olefinic hydrocarbons. Any of these reactions could be 
performed in the reaction zone. In a similar manner the catalyst promoter 
recovery zone could be replaced by other types of purification operations, 
including gas concentration units. The basic concept is therefore a 
hydrocarbon conversion process in which a relatively high pressure 
mixed-phase reaction zone effluent stream is separated into liquid and 
vapor phases, the vapor phase is depressurized through an ejector to 
thereby increase the pressure of a low pressure vapor stream and to pump 
the low pressure stream, or a portion thereof, into an intermediate 
pressure zone in admixture with at least some of the reaction zone 
effluent vapor. 
The preferred catalyst system for use in the alkylation zone, and in the 
transalkylation zone if one is present, is one which utilizes a catalyst 
promoter which is vaporous at standard conditions of temperature and 
pressure. This catalyst promoter is preferably a halogen-containing 
compound chosen from the group consisting of boron trifluoride, boron 
trichloride, boron tribromide, hydrogen chloride, carbon tetrachloride, 
hydrogen fluoride, hydrogen bromide, ammonium fluoride, ammonium chloride, 
ammonium bromide and ammonium iodide. More preferably the catalyst 
promoter is a boron halide, with boron trifluoride being especially 
preferred. 
The preferred catalyst system is described in U.S. Pat. Nos. 3,126,421 (Cl. 
260- 671); 3,631,122 and 3,894,090. The reaction zones contain a boron 
halide-promoted catalyst utilizing a solid carrier and maintained in an 
anhydrous condition. Preferably the boron halide is boron trifluoride and 
the carrier is an inorganic oxide. The carrier may be selected from among 
many inorganic oxides including alumina, silica, boria, oxides of 
phosphorus, titanium dioxide, zirconium dioxide, chromia, etc., and 
various naturally occurring inorganic oxides of various states of purity, 
such as clay or diatomaceous earth. Of the above-mentioned inorganic 
oxides, the gamma and theta forms of alumina are most readily modified by 
boron trifluoride, and the use of one or both of these materials is 
preferred. The reaction zones may also or alternatively contain a bed of a 
crystalline aluminosilicate zeolite. 
Modification of the carrier may be carried out prior to or simultaneously 
with the initial passage of the reactants over the carrier. This 
modification is accomplished by the passage of a boron 
trifluoride-containing gas stream over a bed of the carrier material 
maintained at an elevated temperature of from about 300.degree. F. to 
about 500.degree. F. To maintain the catalyst in an active state during 
operation, boron trifluoride is recirculated to each reaction zone at the 
relatively small rates of about 2000 ppm. in the alkylation zone and about 
3500 ppm. in the transalkylation zone. These two zones will be described 
more fully hereinafter. Generally, boron trifluoride is utilized as a pure 
vaporous material by direct passage to the reaction zones while dissolved 
in the aromatic feed stream. Although an organic solvent solution of 
BF.sub.3 may be employed, such a technique is not preferred since BF.sub.3 
complexes with a multitude of organic compounds, and the latter can react 
with either the aromatic or olefinic feed material. Required quantities of 
BF.sub.3 are relatively small, and may be conveniently expressed as grams 
of BF.sub.3 per mole of olefin. The quantity of BF.sub.3 used may be less 
than 1.0 gram per gram mole of olefin and preferably is from about 0.1 to 
1.0 gram per gram mole. 
The preferred boron halide-promoted alkylation process utilizes two 
reaction zones to produce mono-alkylated aromatic hydrocarbons. The first 
reaction zone is used to alkylate the aromatic hydrocarbon, which in the 
preferred embodiment results in the formation of ethylbenzene. The second 
reaction zone is used to transalkylate polyalkylated aromatics produced in 
the first reaction zone. According to the prior art methods of operating 
this process, the liquid phase portion of the effluent streams of the two 
reaction zones are combined and fed into a single fractional distillation 
column, referred to as the recycle benzene column. The alkylation reaction 
zone is normally operated in a downflow manner at a temperature of from 
100.degree. F. to 600.degree. F., with the preferred operating temperature 
being from 250.degree. F. to 450.degree. F. The transalkylation reaction 
zone is normally operated in an upflow manner and maintained at a higher 
temperature of from 350.degree. F. to 450.degree. F., but may vary in 
temperature from about 200.degree. F. to 700.degree. F. The pressure in 
either reaction zone may range from about atmospheric to 1500 psig., 
although it is presently desirable to use a pressure range of from about 
300 psig. to 600 psig. The pressure is preferably chosen to be sufficient 
to maintain the aromatic hydrocarbon compounds in a liquid state. 
To obtain a high selectivity for the production of a mono-alkylated benzene 
in the alkylation zone, it is best to have present from about 1.5 to about 
5 moles of benzene for each mole of ethylene. This is also true for the 
alkylation of other aromatic hydrocarbons and other olefins. The olefins 
are therefore to be completely reacted in the alkylation reaction zone and 
are not present in the effluent of this zone. The excess aromatic 
hydrocarbon also tends to reduce the polymerization of the olefin and acts 
as a heat sink. To maintain the high excess of aromatic material, it is 
common practice to recirculate a large amount of unfractionated reactor 
effluent, which may be up to about 15 times as large as the reactor feed 
stream or net reactor product. The liquid hourly space velocity used in 
the alkylation reaction zone may vary between 0.5 and about 10.0. 
In the transalkylation zone, an excess of unalkylated aromatic hydrocarbons 
over polyalkylated aromatic hydrocarbons is maintained, with the relative 
ratio being from about 1.5 to about 3 moles of unalkylated aromatic 
hydrocarbon per mole of polyalkylated aromatic hydrocarbon. The liquid 
hourly space velocity of the reactants in the transalkylation reaction 
zone is preferably between about 0.2 to about 3. In order to obtain 
essentially complete conversion of polyethylbenzenes, the charge rate to 
the transalkylation reaction zone is generally more than three times that 
of the polyethylbenzene make. This is because the transalkylation 
reactions of polyethylbenzenes proceed at a much slower rate than the 
initial alkylation reaction of benzene. This greater charge rate may be 
obtained by the recycling of a portion of the transalkylation zone 
effluent stream. Operating conditions in either reaction zone may be 
varied to correspond to the type of alkylation step which is effected 
therein and to provide optimum yields. Further details on the preferred 
alkylation process may be obtained by reference to U.S. Pat. Nos. 
2,887,520 and 3,631,122. 
The preferred catalyst system is troubled by the formation of boron oxide 
hydrates in the alkylation zone by the reaction of small amounts of water 
contained in the feed stream with the boron trifluoride. If left 
untreated, these non-volatile compounds settle on the internal surfaces of 
the downstream benzene column and its associated reboiler. This decreases 
the efficiency of the column and would eventually require the operation of 
the process to be terminated to allow removal of these deposits. A 
preferred method of preventing the deposition of the boron oxide hydrates, 
commonly referred to as borates, in the benzene column is to form a 
volatile complex containing the borates by the addition of additional 
boron trifluoride to the bottom of the benzene column. The complex is 
concentrated by fractionation into a benzene recycle stream removed from 
the benzene column as a sidecut. The benzene recycle stream is then passed 
through a bed of activated alumina which selectively removes the complex. 
This is described in greater detail in U.S. Pat. No. 3,238,268. An 
alternative method in which an olefin-acting compound, such as ethylene or 
butylene, is passed into the bottom of the benzene column is described in 
U.S. Pat. No. 3,631,122. 
The liquid phase portions of the reaction zones which remain at the reduced 
pressure of the low pressure separator are passed into a benzene column, 
which is operated under conditions effective to recover essentially all of 
the remaining benzene from the combined reaction zone effluent. This 
column is preferably operated with a bottom pressure of about 15 psig. and 
with about a 5 psig. pressure drop through the column. The liquid 
temperature at the bottom of the column will be maintained at about 
350.degree. F. to ensure complete removal of the benzene from the 
alkylated benzene being withdrawn as a bottoms liquid product. The 
temperature at the top of the column will be about 200.degree. F. under 
these conditions. As used in this description, references to the benzene 
column are intended to refer generically to the fractionation column in 
which unreacted aromatic hydrocarbons are separated as a top product. This 
aromatic may be toluene, etc., but for convenience is assumed to be 
benzene. 
The bottoms product of the benzene column is preferably passed into a 
second distillation column which is commonly referred to as an 
ethylbenzene column when ethylbenzene is being produced. Conditions 
suitable for operation of the second column include a pressure of about 10 
psig. and a bottom temperature of about 425.degree. F. An overhead vapor 
which is essentially pure ethylbenzene is removed as the net overhead 
product at a temperature of about 280.degree. F. The design of these last 
two fractionation columns and their associated equipment, such as 
fractionation trays, is well within the expertise of those skilled in the 
art, and they may be of conventional design. The ethylbenzene column is 
located within the fractionation zone 42. This zone may contain only the 
ethylbenzene column and produce a bottoms stream of polyalkylated benzenes 
which are recycled to the transalkylation zone as in U.S. Pat. No. 
3,126,421. Alternatively, the bottoms of the ethylbenzene column may be 
passed into a third fractionation column. Heavy polymers, etc., referred 
to as tar would be removed as the bottoms stream of this third column, and 
the polyalkylated benzenes would be removed as an overhead vapor stream. 
This fractionation system is shown in U.S. Pat. No. 2,995,611. 
A net overhead gas stream is removed from the benzene column. This gas 
stream contains various light hydrocarbons, such as saturate C.sub.2 to 
C.sub.5 paraffins, which were dissolved in the liquids fed to the benzene 
column and also boron trifluoride dissolved in these liquids. It also will 
contain some boron trifluoride which was passed into the column as 
previously described to prevent borate deposition. This gas stream must be 
passed into a promoter recovery zone in which the boron trifluoride is 
recovered for reuse and to facilitate the discharge of the off-gas from 
the process. A suitable recovery zone configuration for use with the 
preferred catalyst system is shown in U.S. Pat. No. 3,126,421. The gas 
stream is passed upward through an absorption tower having about 12 trays 
countercurrent to a lean oil, and the boron trifluoride transfers into the 
lean oil. The resultant rich oil is then stripped to recover the boron 
trifluoride in a 24-tray stripping column. The lean oil may be 
polyalkylated benzenes such as are formed in the process. This lean oil 
preferably contains about 15-20% dimethoxybenzene (DMB). DMB forms a weak, 
thermally unstable complex on contact with the boron trifluoride and is 
easily recovered upon heating the lean oil in the stripper. The gases 
removed from the absorber column are passed through a caustic 
neutralization media to produce an environmentally acceptable effluent. 
Other types of equipment or different lean oils or additives may be used 
to recover other catalyst promoters. 
The absorber in the promoter recovery zone is preferably operated at a 
higher pressure than is desired for use at the top of the benzene column. 
Therefore, the net off-gas of the benzene column which is to be passed 
into the promoter recovery zone, as well as that portion to be recycled to 
the reaction zone as in the prior art, must be compressed to be passed 
into the absorber. The additional compressor capacity required to do this 
increases both the capital cost and utility cost of the process. It is an 
objective of this invention to provide a process for the alkylation of 
aromatic hydrocarbons wherein a compressor is not necessary to pass the 
off-gas of the benzene column into a catalyst promoter recovery zone 
operating at a higher pressure than the benzene column. 
According to the inventive concept at least a part of the motive force 
required to pass the net benzene column overhead gas into the promoter 
recovery zone is derived by depressurizing the vaporous phase of the 
alkylation zone effluent stream. This vaporous phase is comprised of the 
unreactive saturated paraffins contained in the olefin feed stream and 
boron trifluoride. In comparison, the effluent of the transalkylation zone 
will normally and preferably contain very little vapor, and therefore 
preferably is not used in the same manner as the alkylation zone effluent. 
As previously specified, the alkylation zone effluent preferably has a 
pressure ranging from about 300-600 psig. The pressure specified for the 
top of the benzene column was about 5 psig. There is therefore normally 
available a pressure differential of about 280 to 580 psig. between the 
low pressure stream to be pumped and the high pressure stream fed to the 
ejector. The ejector itself may be of customary design and can be chosen 
by those skilled in the art. Preferably, the outlet of the ejector will be 
at a pressure above about 50 psig. The relatively high pressure 
vapor-liquid separator in which the effluent of the alkylation zone is 
separated into vapor and liquid fractions may also be of customary design 
and is referred to herein as the high pressure separator or the first 
vapor-liquid separation zone. At least a portion of the vapor stream 
removed from this separation zone is used as the high pressure stream fed 
to the ejector. A second portion may be bypassed as desired to control the 
operation of the ejector. 
The liquid stream removed from the first vapor-liquid separation zone 
preferably comprises most of the C.sub.6 + hydrocarbons in the alkylation 
zone. It therefore contains the aromatic hydrocarbon and the product 
alkylaromatic hydrocarbon. It will also contain an equilibrium 
concentration of lower boiling hydrocarbons, such as ethane, and boron 
trifluoride and some polyalkylated aromatic hydrocarbons. Preferably, this 
liquid stream is admixed with the effluent of the translkylation zone and 
passed into a second vapor-liquid separation zone. Referring to the 
Drawing, this zone may be comprised only of the low pressure separator 15 
or it may contain both the low pressure separator 15 and a third 
vapor-liquid separator 20. The second vapor-liquid separation zone is 
preferably operated at a pressure in the range of about 20-65 psig. Lower 
pressures may be used, but this pressure range allows the liquids to be 
transferred into the benzene column without the use of a pump. The 
pressure in this zone should also be greater than in the promoter recovery 
zone or its equivalent. Heat may be recovered from the reaction zone 
effluents through the use of a heat exchanger in line 14. 
The vapor phase stream removed from the low pressure separtor 15 contains 
boron trifluoride and low boiling point hydrocarbons which are released 
from the entering liquid stream due to the lower pressure in the 
separator. It will also contain the boron trifluoride and light 
hydrocarbons contained in the effluent of the ejector and an equilibrium 
concentration of less volatile hydrocarbons such as the aromatic 
hydrocarbon. It is preferred that this vapor phase stream is cooled, as to 
about 100.degree. F., and then passed into the third vapor-liquid 
separator. This has such advantages as allowing the low pressure separator 
15 to be operated at a relatively warm temperature which produces a liquid 
phase effluent stream having a proper temperature for passage into the 
benzene column. Cooling the vapor phase effluent of the low pressure 
separator 15 produces a liquid phase containing the aromatic hydrocarbon. 
This valuable material is therefore not lost to the catalyst promoter 
recovery zone. The remaining uncondensed vapors are then passed into the 
promoter recovery zone. These vapors include boron trifluoride and light 
hydrocarbons removed from the top of the benzene column and which have 
been recycled without the use of a compressor, thereby achieving an 
objective of the invention. 
In accordance with this description, the preferred embodiment of the 
invention may be characterized as a process for the alkylation of aromatic 
hydrocarbons which comprises the steps of passing a mixed phase reaction 
zone effluent stream which comprises a volatile catalyst promoter and two 
different aromatic hydrocarbons into a first vapor-liquid separation zone 
and forming therein a first liquid hydrocarbon stream comprising the 
aromatic hydrocarbons and a first vapor stream comprising the catalyst 
promoter; passing at least a portion of the first vapor stream through an 
ejector as a motive stream utilized to pump a second vapor stream 
comprising the catalyst promoter and thereby forming an ejector effluent 
vapor stream; passing the ejector effluent vapor stream into a second 
vapor-liquid separation zone having a lower pressure than the first 
vapor-liquid separation zone; passing the first liquid hydrocarbon stream 
into the second vapor-liquid separation zone; passing a second liquid 
hydrocarbon stream comprising the aromatic hydrocarbons and the catalyst 
promoter from the second vapor-liquid separation zone to a fractionation 
column having a lower pressure than the second vapor-liquid separation 
zone; withdrawing from the fractionation column a net overhead vapor 
stream comprising the catalyst promoter and passing the net overhead vapor 
stream into the ejector as the second vapor stream; withdrawing from the 
second vapor-liquid separation zone a third vapor stream comprising the 
catalyst promoter, and passing the third vapor stream into a vapor 
purification zone which effects the removal of imputities from the third 
vapor stream to produce a fourth vapor stream having a higher 
concentration of the catalyst promoter than the third vapor stream; 
passing at least a portion of the fourth vapor stream into a reaction zone 
maintained at alkylation promoting conditions in admixture with an 
olefinic hydrocarbon and a feed aromatic hydrocarbon; and withdrawing a 
fractionation product stream comprising a product alkylaromatic 
hydrocarbon from the fractionation column. As previously stated, the 
aromatic hydrocarbons in the reaction zone effluent will normally include 
the excess benzene or other feed aromatic hydrocarbon, the product 
aromatic hydrocarbon and any by-product aromatic hydrocarbons such as 
polyalkylated aromatics. 
The configuration of the reactor and the conversion conditions which are 
maintained within the reactor may be chosen from the great variety which 
are customary for the specific process being performed. For example, the 
inventive concept may be applied to a hydroprocessing operation utilizing 
a fixed-bed reactor containing one or more beds of particulate catalyst. 
The reactor, or reaction zone if two or more reactors are utilized, may 
also be a moving-bed reactor. As used herein, the term "hydroprocessing" 
is intended to encompass three related fields of petroleum refining 
technology. The first is hydrotreating, wherein small amounts of 
undesirable materials including sulfur and nitrogen contained in various 
organic molecular structures are removed from the charge stock with very 
little molecular cracking. The second field is hydrocracking, wherein at 
least 50% of the charge stock is cracked into components having a lower 
molecular weight, such as the production of a naphtha from a heavy 
distillate or the production of LPG from a naphtha. The field of 
hydrorefining is between these two extremes and results in molecular 
changes to up to about 10% or more of the feed together with impurity 
removal. 
The method of manufacture and the composition of the catalyst used in the 
reaction zone during the hydroprocessing operation is not critical. The 
catalyst may therefore be any suitable commercially-available catalyst or 
one of proprietary nature. These catalysts are normally formed as a sphere 
by an oil drop method, such as that demonstrated in U.S. Pat. No. 
2,774,743, or extruded or pelleted. Basic to the manufacture of a large 
majority of all hydroprocessing catalysts is the incorporation of a metal 
of Group VIII of the Periodic Table on a refractory inorganic oxide 
carrier by coprecipitation or impregnation. The more commonly used metals 
from this group are iron, cobalt, nickel, platinum and palladium. In 
addition, metals from Group VI-A such as chromium, molybdenum or tungsten 
are also often incorporated into the catalyst. The carrier material may be 
natural or synthetic and will normally be selected from alumina, silica or 
zirconia or a combination of any of these materials, particularly an 
alumina in combination with one or more of the other oxides. A preferred 
catalyst composition is from 1-20 wt.% cobalt and from 0.1-10 wt.% 
molybdenum supported on alumina spheres. A more detailed listing of 
suitable catalyst composition and manufacturing techniques may be obtained 
by reference to U.S. Pat. Nos. 3,203,889; 3,254,018; 3,525,684 and 
3,471,399. 
The conversion conditions necessary for a hydroprocessing operation are 
determined by such factors as the composition of the charge stock, the 
catalyst used in the reactor and the desired result of the process. A 
broad range of hydroprocessing conditions includes a temperature of from 
about 250.degree. F. to 1000.degree. F., a pressure of from 300 to 4000 
psig., and a liquid hourly space velocity of 0.1 to about 8.0. The exact 
reactor temperature required is determined by such factors as the initial 
activity and prior use of the catalyst. As a general rule, the preferred 
operating pressure will increase with the boiling point of the material 
being processed. More specific examples of hydroprocessing conditions are 
contained in the above-listed U.S. Patents. In all hydroprocessing 
operations, hydrogen is circulated through the reactor at a rate of about 
500 to about 10,000 standard cubic feet per barrel of charge material. 
An example of a hydroprocessing operation which may be conducted in the 
reaction zone of the subject invention is the hydrogenation of 
petroleum-derived feed streams containing polymer-forming olefinic 
materials. These olefinic materials may include olefins, conjugated 
di-olefins such as butadiene and styrene, and indenes. These compounds are 
often found in pyrolysis liquids, distillates from fluid cokers, coke oven 
light oils and coal gasification side-product liquids. Satisfactory 
conversion conditions for a low temperature hydrogenation reactor loaded 
with the preferred palladium-containing catalyst include a temperature in 
the broad range of from 250.degree. F. to 500.degree. F. Preferably, the 
reactor is operated at 270.degree. F. to 400.degree. F. On-stream 
hydrogenation conditions also include a pressure in the broad range of 
from 100 psig. to about 1200 psig. and a molar excess of hydrogen, 
typically in the range of from 500 to 2000 standard cubic feet per barrel 
of combined charge. The preferred catalyst comprises spheres of lithiated 
alumina containing from 0.05 wt.% to about 5.0 wt.% palladium. Preferably, 
the catalyst consists of 1/16-inch alumina spheres containing about 0.4 
wt.% palladium and about 0.5 wt.% lithium. Further instruction as to the 
hydrogenation of olefinic feed streams may be obtained by reference to 
U.S. Pat. Nos. 3,161,586; 3,215,618; 3,537,981 and 3,537,982. 
The inventive concept may also be used in the hydroprocessing of heavy 
petroleum stocks such as residual oils or black oils. These feed materials 
often have boiling points, as determined by the appropriate ASTM 
distillation procedure, above about 600.degree. F. at 1 atmosphere of 
absolute pressure. They will normally contain appreciable amounts of 
asphaltenes, sulfur and various metals, such as iron, nickel and vanadium. 
The fluid stream passed through the reaction zone may therefore be formed 
by the admixture of hydrogen and the various petroleum streams, such as an 
atmospheric tower bottoms, a vacuum tower bottoms (vacuum residuum), a 
topped crude oils, a coal oil extract, or a shale oil or heavy oil 
recovered from tar sands. 
In a great majority of all hydroprocessing operations, the effluent of the 
reaction zone is separated for the recovery of the valuable hydrogen which 
it may contain from the products and by-products of the process. 
Typically, the effluent of the reaction zone will be cooled to effect at 
least a partial condensation of the heavier hydrocarbons and then passed 
into a vapor-liquid separation zone. Liquid hydrocarbon streams formed in 
this manner are then often passed into a fractionation column wherein the 
hydrocarbons are purified or separated. The fractionation column which 
receives the liquid hydrocarbons may be referred to as a stripper, 
stabilizer, debutanizer, deethanizer, etc. The hydrocarbon stream fed to 
the fractionation column will contain the heavier, more readily 
condensible hydrocarbons and also various amounts of dissolved lighter 
hydrocarbons or condensed gases. One function of this fractionation column 
will be to remove some or all of the low boiling hydrocarbons and 
dissolved gases from the feed stream and thereby form streams of differing 
composition which may be removed as sidecuts or bottoms steams from the 
column. Although these light materials may be considered impurties when 
present in the heavier hydrocarbon streams removed from the fractionation 
column, they may have a large economic value which justifies their 
recovery and separation. For this reason, these streams are often passed 
into a purification or recovery zone which may have several different 
forms. 
The net vapor stream removed from the fractionation column may be passed 
into an absorption zone via the second vapor-liquid separation zone in 
accordance with the inventive concept. This absorption zone may be 
operated in a manner similar to those used in the gas concentration unit 
associated with many fluidized catalytic cracking units. The vapor stream 
which is withdrawn from the second vapor-liquid separation zone may 
accordingly be passed into the bottom of a cylindrical multi-stage 
absorption column wherein it passes upward countercurrent to a descending 
stream of naphtha, gas oil or other suitable absorption liquid. 
Preferably, the absorption zone comprises a vertically-disposed trayed 
absorption tower having a total of twenty or more contact stages. The 
absorption zone is maintained at conditions selected to cause the 
absorption of at least a portion of components of the vapor steam which it 
is desired to recover. In general, for hydrocarbons, these conditions 
include a pressure of from about 100 to 500 psig. or higher and a 
temperature of from about 60.degree. F. to 150.degree. F. A preferred 
temperature range for the effluent streams from the absorption zone is 
80.degree. F. to 140.degree. F. The relatively elevated pressure 
maintained within the absorption zone facilitates the absorption and 
recovery of such hydrocarbons as ethane, propane, butane and pentane. 
These hydrocarbons become components of a rich liquid stream which is 
removed from the bottom of the absorption column and transferred to a 
stripping column wherein, at conditions of a greater temperature and/or 
lesser pressure, they are recovered. The separation and recovery of light 
hydrocarbons from a gas stream in this manner is well known and is 
described in greater detail in U.S. Pat. Nos. 3,907,669; 4,009,097 and 
4,010,010. 
The net gas stream removed from the second vapor-liquid separation zone 
during the hydroprocessing of a sulfur-containing feed stream may be 
subjected to a purification operation designed to remove hydrogen sulfide 
or other acidic gases either before or subsequent to its passage into an 
absorption zone. As an alternative, the acid gases may be removed and the 
resultant purified gas stream recycled directly to the reaction zone. This 
acid gas removal operation may comprise contacting with an aqueous caustic 
solution in the customary manner well known to those skilled in the art. 
The removal of acid gases and light hydrocarbons from the gas stream 
withdrawn from a second vapor-liquid separation zone will normally effect 
the production of a vapor stream having a sufficiently high hydrogen 
concentration for use as a recycle gas in the hydroprocessing reaction 
zone. 
The inventive concept may also find application in a process for the 
hydrodealkylation of alkylaromatic hydrocarbons. A process for the thermal 
dealkylation of toluene is presented in U.S. Pat. Nos. 3,284,526 and 
3,291,849. The reaction zone may also contain a catalyst as described in 
U.S. Pat. Nos. 3,751,503 and 3,291,850. These processes typically utilize 
a hydrogen-rich recycle stream which is derived from the reaction zone 
effluent. The liquid phase portion of the reaction zone effluent is passed 
into a fractionation column to separate the product benzene from 
unconverted toluene which is to be recycled to the reaction zone and to 
remove light hydrocarbons such as ethane or propane, which are the result 
of cracking reactions which occur in the reaction zone, from the product 
stream. It is undesirable to pass significant quantities of light 
paraffinic hydrocarbons to the reaction zone. For this reason, gas steams 
which are to be passed into the reaction zone have been contacted with a 
lean oil in an absorption zone under conditions which remove the 
paraffinic hydrocarbons and purify the hydrogen-containing gas stream. The 
subject invention may be adapted to this hydrodealkylation process by 
utilizing it to pass a recycle gas stream comprising vapors from the 
fractionation column into a similar purification zone. 
The fractionation column utlized in the subject invention is preferably a 
trayed column containing twenty or more trays and operated at a pressure 
less than 100 psig. This pressure is to be measured at the top of the 
fractionation column. The column is preferably operated at a reflux to 
feed ratio of from 0.5 to about 3.0, and the liquid feed stream from the 
second vapor-liquid separation zone may enter at either the top or an 
intermediate point of the fractionation column. As used herein, the term 
"net overhead vapor stream" or its equivalent is intended to refer to the 
vapor stream removed from the overhead system of a fractionation column 
after the condensation of hydrocarbons used as reflux liquid and/or an 
overhead liquid product. Two or more columns may be utilized as an 
integrated fractionation zone associated with the specific conversion 
process being performed.