Solid phosphoric acid catalyzed alkylation of aromatic hydrocarbons

A process for the alkylation of aromatic hydrocarbons using a solid phosphoric acid catalyst in which liquid phosphoric acid is removed from the bottoms stream of the rectification column to which the alkylation zone effluent is charged. The acid is removed by passing the bottoms stream into a settling vessel operated at a lower pressure and temperature than the bottom of the rectification column.

FIELD OF THE INVENTION 
The invention relates to a hydrocarbon conversion process. The invention 
more specifically relates to an improved process for the alkylation of 
aromatic hydrocarbons using a solid phosphoric acid catalyst. The 
invention also relates to a separatory method used to recover 
alkylaromatic hydrocarbons from the effluent of a solid phosphoric acid 
catalyzed alkylation process. 
PRIOR ART 
The alkylation of aromatic hydrocarbons using a solid phosphoric acid 
catalyst is a well developed art which is practiced commercially. One 
commercial application of the process is the alkylation of benzene with 
propylene to form cumene, which is used to produce phenol and acetone. 
Those skilled in the art are therefore familiar with the design and 
operation of the process. 
The prior art is well described in the literature. For instance, a typical 
prior art flow scheme is presented as FIG. 1 of the article beginning at 
page 91 of the March 1976 edition of Hydrocarbon Processing. A portion of 
this flow shceme is also shown in U.S. Pat. No. 3,510,534 (Cl. 260-671). 
In this latter reference the utilization of SPA (solid phosphoric acid) 
for both the alkylation of aromatic hydrocarbons and the oligomerization 
of olefins is described. The process flow described in greatest detail is 
directed to the recovery of cumene from the effluent of a benzene 
alkylation zone. The effluent is passed into a first rectification column 
and flashed. Reflux is provided to this column by passing a portion of the 
recycled benzene into the top of the column. The overhead vapors of the 
first rectifier are passed into a second rectification column. The liquid 
portion of the alkylation zone effluent is separated into two liquid 
phases in the bottom of the first rectification column. A small amount of 
denser phosphoric acid is drained from the bottom of this column, and a 
hydrocarbon bottoms stream is removed at slightly higher elevation. This 
bottoms stream passes through a pressure reduction valve and into a 
benzene recycle column which is operated at a lower pressure than the 
first rectification column. Benzene is removed as the overhead product of 
this column and cumene is removed in the bottoms stream for recovery in a 
second column. It is believed that heretofore the bottoms stream of the 
first rectification column or its equivalent was passed directly into the 
downstream recycle column without the removal of phosphoric acid from the 
bottoms stream. 
This passage of an SPA alkylation zone effluent stream into a first 
fractionation zone, followed by withdrawal of the product alkylated 
aromatic hydrocarbon in a bottoms stream which is passed into a second 
fractionation zone is also shown in U.S. Pat. Nos. 3,499,826 (Cl. 204- 
27); 3,520,944 and 3,520,945. 
BRIEF SUMMARY OF THE INVENTION 
The invention provides a process for the SPA catalyzed production of 
alkylaromatic hydrocarbons wherein corrosion in the fractionation columns 
utilized to recover the alkylaromatic hydrocarbons is reduced. This 
improvement is accomplished by depressurizing the bottoms stream of a 
first fractionation zone and passing the thus cooled bottoms stream into a 
settling vessel operated at quiescent conditions which effect the 
separation of liquid phase phosphoric acid from the bottoms stream. This 
phosphoric acid is thereby removed from the process and the amount of 
corrosion in the carbon steel fractionation column downstream of the 
settling vessel is reduced.

Referring now to the Drawing, a feed stream comprising a mixture of propane 
and propylene enters the process in line 1 and is admixed with a stream of 
recycle benzene from line 2. The resultant admixture is carried by line 3 
to the junction with line 4, where it is commingled with additional 
benzene from line 4. This produces the alkylation zone feed stream carried 
by line 6. This stream is first heated in heat exchanger 5 and then in 
heater 7 prior to being inserted into the bottom of reactor 8. Contacting 
of the alkylation zone feed stream with an SPA catalyst maintained at 
alkylation-promoting conditions effects the reaction of at least a major 
portion of the propylene with benzene to form cumene or isopropylbenzene. 
A reaction zone effluent stream comprising benzene, propane and cumene is 
therefore removed in line 9. 
The reaction zone effluent stream passes through a pressure control valve 
not shown and then into a first rectifier or rectification column 10 which 
is operated at a lower pressure and temperature than reactor. Vapors 
liberated from the reaction zone effluent stream pass upward through 
fractionation trays countercurrent to liquid generated by the addition of 
benzene from line 13 at the top of the rectifier. An overhead vapor stream 
comprising benzene, propane and a small amount of water is removed in line 
11 and heat exchanged against the alkylation zone feed stream. It is then 
passed into a second rectifier 14. The bottoms stream of this column 
contains benzene which is passed to the reactor via line 4. A feed benzene 
stream enters the column through line 15 to be dried. An overhead vapor 
stream comprising water and propane is removed from the second rectifier 
in line 16 and passed through an overhead condenser 17. The resultant 
condensate stream is passed into overhead receiver 18 and separated into 
an aqueous phase removed in line 20 and a liquid propane stream removed in 
line 21. A net propane stream removed from the process in line 22 
comprises the propane entering through line 1. The remainder of the liquid 
propane stream is passed into the rectifier via line 23 as reflux. 
A very small amount of aqueous phosphoric acid is removed from the first 
rectifier in line 12. The hydrocarbonaceous bottoms stream of this column 
is removed in line 24 and comprises benzene and cumene. This bottoms 
stream is subjected to a flashing operation which lowers its pressure and 
temperature by passage through valve 25 into a settling vessel 26. The 
lower temperature and quiescent conditions maintained in the settling 
vessel cause a phosphoric acid phase to form in the settling vessel. This 
additional acid is removed in line 27 and therefore does not enter the 
downstream vessels. 
Substantially all of the hydrocarbons in the rectifier bottoms stream 
continue through line 28 to a benzene recycle column 29. This column is 
operated at conditions which are effective to vaporize substantially all 
of the benzene in the rectifier bottoms stream and form an overhead vapor 
stream comprising benzene. This vapor stream is condensed in overhead 
condenser 31 and passed into overhead receiver 32 via line 30. The 
resultant benzene-rich liquid is removed in line 33, with a first portion 
delivered to the recycle column in line 34 as reflux and a second portion 
entering line 35. A drag stream is removed in line 36 to prevent the 
buildup within the process of hydrocarbons having boiling points between 
benzene and propane. The remainder of the benzene is passed through line 
37 for recycling to the reactor and for reflux to the first rectifier. 
Cumene and other alkylaromatic hydrocarbons are withdrawn from the benzene 
recycle column as a bottoms stream in line 38 and passed into a cumene 
column 39. The operation of this column is effective to cause the 
production of an overhead vapor stream of relatively pure cumene. This 
vapor stream is passed through condenser 42 into overhead receiver 43 via 
line 41. The cumene is withdrawn from the receiver in line 44 and divided 
between the reflux stream carried by line 45 and a net product stream 
carried by line 46. A net bottoms stream comprising polyalkylated aromatic 
hydrocarbons is removed from the cumene column in line 40. 
DETAILED DESCRIPTION 
SPA (solid phosphoric acid) catalysts find utility in a number of chemical 
conversion processes which are performed commercially. These processes 
include the oligomerization, often called polymerization, of olefins to 
form motor fuel, tetramer for detergent manufacture and C.sub.7, C.sub.8, 
C.sub.9 and C.sub.12 olefins for use in petrochemical processes and also 
the alkylation of aromatic hydrocarbons. The aromatic hydrocarbons which 
may be alkylated with an SPA ctalyst include benzene, toluene, xylenes, 
ethylbenzene, normal propylbenzene, isopropylbenzene and other cyclic 
compounds. Higher molecular weight and polycyclic aromatic hydrocarbons 
may also be alkylated using a solid phosphorus-containing catalyst. The 
alkylating agent may be an olefin-acting compound such as an alcohol, 
ether or ester including alkyl halides, alkyl sulfates and alkyl 
phosphates. Preferably, the alkylating agent is a mono- or di-olefin 
having from 2 to 8 carbon atoms per molecule. The preferred mono-olefins 
include ethylene, propylene, 1-butene, 2-butene and isobutylene. These 
olefins may be used as relatively pure streams containing a single 
hydrocarbon species. Alternatively, a mixture of two or more olefins or of 
olefins and paraffins may be used as the non-aromatic feed stream to the 
process. Typical products include cumene, ethylbenzene and cymene 
(isopropyl toluene). 
The subject invention is practiced with a reaction zone containing a solid, 
phosphorus-containing catalyst. Preferably, the catalyst is one commonly 
referred to as an SPA catalyst. Suitable SPA 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 tetra-phosphoric 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 additive 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. 
It is known in the art that the passage of aromatic hydrocarbons through an 
alkylation zone tends to leach chemically combined water out of an SPA 
catalyst. This is acknowledged in U.S. Pat. Nos. 3,510,534 and 3,520,945, 
the latter of which is directed to the control of the state of hydration 
of the catalyst. The water content of the catalyst is important since 
dehydration causes the SPA catalysts to deteriorate by powdering and 
caking, while excess water causes the catalysts to soften and eventually 
form a sludge which would plug the reactor. Water is therefore injected 
into the feed stream to maintain the catalyst at the proper state of 
hydration by replacing the water leached from the catalyst. The rate of 
this injection is used to control the catalyst hydration level, and the 
feed streams are therefore maintained as dry as practical prior to the 
water injection point. This results in the total water content of the feed 
being essentially the same as the amount injected. Typical water injection 
rates are from about 100 ppm. to 2000 ppm. in aromatic hydrocarbon 
alkylation operations. A preferred water addition rate during the 
production of cumene is from about 200 to 300 ppm. of the combined feed to 
the reaction zone. 
The water which has been leached from the catalyst and the excess water 
added to the feed stream are contained in the reaction zone effluent 
stream. This water contains phosphorus from the catalyst and is therefore 
phosphoric acid of some varying strength. The acid is present at very low 
concentrations in the reaction zone effluent and is apparently dissolved 
in the much larger aromatic hydrocarbon stream. However, the reaction zone 
effluent is normally cooled as by flashing, and at the resultant lower 
temperature a separate aqueous phase of phosphoric acid is formed. 
Experience has shown that the hot reaction zone effluent material is not 
corrosive, but that the cooler, two liquid phase effluent material is 
fairly corrosive to carbon steel. For this reason at least the lines and 
vessels immediately downstream of the reaction zone are normally made of 
stainless steel. The acid therefore collects in the bottom of the first 
vessel, which in a process flow similar to that shown in the Drawing is 
the first rectification column. It is for this reason that the previously 
cited patents show acid being withdrawn from the first fractionation 
column into which the reaction zone effluent is charged. This first 
fractionation column is normally a rectifier and is normally used in 
conjunction with either a second rectifier, an absorber or a depropanizer. 
This first fractionation column and any other column associated with it 
are referred to herein as the first fractionation zone. 
A bottoms stream containing the alkylaromatic hydrocarbon product of the 
process is normally removed from the first fractionation zone and passed 
into a second fractionation zone. Since this bottoms stream often contains 
benzene it is a common practice to first remove the benzene and other 
hydrocarbons boiling below cumene in a first column referred to as a 
benzene recycle column. The cumene or other alkylaromatic hydrocarbon 
product is then recovered in a second column referred to as a cumene 
column. Differing column arrangements may be utilized to perform the 
product recovery. These columns are referred to herein as the second 
fractionation zone. 
The benzene recycle column is typically operated at a lower pressure and 
temperature than the column producing the bottoms stream which is fed to 
it. In the prior art the bottoms stream of the first fractionation column 
is flashed into the benzene recycle column, and is therefore cooled to a 
lower temperature. This lower temperature causes another small amount of 
the phosphoric acid to drop out of solution in the benzene recycle column. 
To avoid the high cost of stainless steel, the benzene recycle column is 
often made of carbon steel. The phosphoric acid therefore corrodes the 
fractionation trays located in this column. This slowly reduces the 
efficiency of the corroding trays, and may reduce the efficiency of other 
trays by causing the accumulation of corrosion products or debris on the 
surface of the trays. It is an objective of this invention to provide a 
method of reducing the amount of corrosion such as this by lowering the 
amount of phosphoric acid which is passed into the benzene recycle column. 
According to the inventive concept the alkylaromatic hydrocarbon-containing 
bottoms stream of the first fractionation zone is flashed to a lower 
pressure at a point prior to the second fractionation zone. The liquid 
phase material remaining after the flashing operation is then retained for 
some time in a quiescent settling zone which is maintained at conditions 
which allow phosphoric acid to settle out by gravity and to be decanted. 
This settling operation may be aided by the provision of a coalescing 
means of either a mechanical or electrostatic type. The preferred type of 
settling zone is a settling vessel similar to that depicted in the drawing 
and which is commonly used as for overhead receivers. Other types of 
vessels, including those having separate facilities for handling the vapor 
phase formed by the flashing operation may be used. 
The flashing operation performed prior to the settling vessel preferably 
reduces the pressure of the bottoms stream to the lowest pressure which 
still provides an adequate pressure differential between the settling 
vessel and the second fractionation zone to transfer the bottoms stream to 
the second fractionation zone without the use of a pump. A lower pressure 
and temperature may be used however if suitable pumping means are 
supplied. The flashing operation preferably reduces the temperature of the 
bottoms stream by 80.degree. F. or more. No control system need be used to 
regulate the hydrocarbon flow from the settling vessel, and it may be 
directly coupled to the second fractionation zone. In the preferred 
embodiment the pressure in the settling vessel is higher than that in the 
second fractionation zone only by the pressure drop associated with the 
flow of the bottoms stream through the connecting lines and any elevation 
differential. 
The conditions of temperature and pressure maintained in the first 
fractionation zone and also in the second fractionation zone are 
interrelated and variable. The first fractionation zone is preferably 
operated at a pressure at least 100 psig. higher than the second 
fractionation zone and at a temperature over about 100.degree. F. above 
that used in the second fractionation zone. A broad range of conditions 
for the first fractionation zone include a bottoms temperature of about 
350.degree. F. to 500.degree. F. and a top pressure of about 300 to about 
600 psig. or higher. A broad range of conditions for use in the second 
fractionation zone includes a top pressure of about 10 to 150 psig. and a 
bottoms temperature of about 300.degree. to 450.degree. F. 
The reaction zone is maintained at alkylation-promoting conditions which 
include a pressure of about 300 to 1000 psig. and a temperature of about 
300.degree. to 600.degree. F. The liquid hourly space velocity of 
reactants may range from about 0.5 to 2.5. It is preferred that an excess 
of the aromatic hydrocarbon be present in the reaction zone. The mole 
ratio of the aromatic hydrocarbon to the olefin should be within the broad 
range of 3:1 to 20:1. A ratio of about 8:1 is preferred for the production 
of cumene. It is preferred that the reactant stream be mixed-phase through 
the reactor. The feed stream therefore preferably contains some unreactive 
light paraffins having the same number of carbon atoms per molecule as the 
olefin. In the production of cumene it is preferred that the amount of 
propane in the reaction zone feed stream be at least equal to the amount 
of propylene in this stream. This may be accomplished by using a dilute 
propylene feed stream or by recycling propane. 
The invention may be further illustrated by this example of the preferred 
embodiment based on the production of cumene by the alkylation of benzene 
with propylene. For clarity, reference will be made to lines and vessels 
shown in the Drawing. The feed stream to the reaction zone is derived from 
a bottoms stream of rectifier 14 which comprises about 4,890 mph (moles 
per hour), of which about 67 mol.% is benzene and 26 mol.% is propane and 
a recycle stream from line 2 comprising about 2,980 mph of benzene and 
some propane. About 133 lb/hr. of water is injected into this mixture to 
maintain the proper state of hydration of the catalyst. The propylene feed 
stream enters the process at the rate of about 852 mph. The combined feed 
stream contains about 9,195 mph of which 6275 mph is benzene. It is split 
into two identical streams, each of which is passed into the bottom of a 
reactor at a pressure of about 550 psig. and a temperature of about 
383.degree. F. The reactors each contain four catalyst beds of sufficient 
overall volume to provide a WHSV of about 1.25 hr..sup.-1. The catalyst 
used is a standard SPA catalyst. 
The mixed-phase effluents of the two reactors are cooled from about 
437.degree. F. to about 395.degree. F. by being lowered in pressure from 
about 500 psig. to about 271 psig. and are then combined. The resultant 
alkylation zone effluent enters the first rectifier 10 at a rate of about 
8419 mph. Also fed to the first rectifier is a 519 mph benzene-rich stream 
from line 13 at a temperature of about 120.degree. F. The first rectifier 
is operated with a bottom temperature of 394.degree. F. at 271 psig. The 
overhead vapor stream is removed at a temperature of approximately 
379.degree. F. and cooled to about 310.degree. F. in heat exchanger 5 
before being passed into the second rectifier. The first rectifier 
contains eight fractionation trays and the second rectifier contains 28 
fractionation trays. A very small acid stream of about 1 gal/day is 
removed from the bottom of the rectifier. 
The benzene feed stream enters the second rectifier at the eighteenth tray 
from the bottom at about 746 mph and at a temperature of approximately 
80.degree. F. The overhead vapor of this column has a temperature of about 
123.degree. F. and is cooled to about 100.degree. F. in the overhead 
condenser. The overhead receiver is maintained at 250 psig. About 7 mph of 
water and 57.7 mph of liquid hydrocarbons are removed from the receiver. 
About 92 mol.% of this stream is propane with the rest being ethane, 
isobutane and hydrogen. The reflux stream has a flow rate of about 2,482 
mph. A stabbed-in reboiler is used in the second rectifier to provide a 
251.degree. F. bottoms stream. 
The net bottoms stream is flashed from approximately 394.degree. F. and 271 
psig. to about 50 psig. and passed into the settling vessel 26 as a two 
phase stream having a temperature of about 291.degree. F. This temperature 
reduction causes an additional small amount of phosphoric acid to come out 
of solution in the hydrocarbon stream. It is removed from the vessel at a 
rate of less than 1 gal/day. The bottoms stream of the first rectifier is 
then passed into a 36-tray recycle column. This column is operated with a 
bottoms temperature of about 418.degree. F. A largely benzene overhead 
vapor stream is removed at about 40 psig. and condensed at a temperature 
of 120.degree. F. The overhead liquid is divided between reflux and a 
recycle stream. A drag stream of about 4 mph is withdrawn to remove 
unreactive hydrocarbons, and the remainder of the recycle stream forms the 
previously specified rectifier reflux and reactor feed streams. 
A 750 mph bottoms stream of the recycle column is passed into the 45-tray 
cumene column. The overhead vapor stream of this column has a pressure of 
approximately 20 psig. at a temperature of about 374.degree. F. It is 
condensed to form reflux and a 716 mph net cumene product stream. The 
cumene column is operated with a bottoms temperature of about 478.degree. 
F., and a net bottoms stream of impurities is removed at the rate of about 
33.6 mph.