Alkylation process utilizing side draw vapor as heat source in isostripper

An olefin and isoparaffin are contacted in the presence of an acid alkylation catalyst to form an alkylate-containing alkylation effluent. The effluent is separated with the hydrocarbon phase passing to a prefractionation zone. A side stream from the prefractionation zone is passed in indirect heat exchange relationship with an interheater of an isostripper thereby providing a majority of the heat necessary to reboil the isostripper.

BACKGROUND OF THE INVENTION 
This invention relates to a process for alkylating an alkylatable 
isoparaffinic hydrocarbon with olefinic hydrocarbons. In another aspect, 
this invention relates to a process for the alkylation of isoparaffins 
with olefins in the presence of a hydrofluoric acid catalyst. In yet 
another aspect, this invention relates to an alkylation process wherein 
energy is conserved through the use of waste heat available in a stream 
obtained from the prefractionation zone. In still another aspect, this 
invention relates to an alkylation process wherein a side draw stream from 
the prefractionation zone is passed in indirect heat exchange relationship 
with the interheater of an isostripper. In another aspect, this invention 
relates to an alkylation process wherein the bottoms from the 
prefractionation zone is flashed prior to being charged to the isostripper 
thereby reducing the amount of liquid that needs to be heated and 
vaporized in the isostripper. In another aspect, this invention further 
relates to an alkylation process in which the majority of heat needed by 
an isostripper is supplied by a side draw stream taken from the 
prefractionation zone. 
Alkylation of isoparaffinic hydorcarbons, such as isobutane, isopentane, 
and the like, with olefinic hydrocarbons such as propylene, butylene, 
amylenes, and the like is well known as a commercially important method 
for producing gasoline boiling range hydrocarbons. Generally, the 
alkylation of isoparaffins with olefins is accomplished by contacting the 
reactants with an acid-acting catalyst, settling the mixture to separate 
the catalyst from hydrocarbons, and further separating the hydrocarbon 
stream into its various components including the alkylate product. The 
alkylate is typically a mixture of isomers of heptane, octane, etc., with 
the exact composition depending upon the isoparaffin and olefin reactants 
used. Various types of catalysts have been utilized in this reaction, 
including sulfuric acid, hydrofluoric acid, phosphoric acid, certain 
halosulfonic acids, and aluminum chloride. The preferred catalyst is 
hydrofluoric acid, however, because of the relative ease with which it can 
be used and reused and because of the superior quality of the alkylate 
that is produced. 
The energy requirements for heating the various separation zones and 
streams of an alkylation process are great and it would be desirable, 
therefore, to maintain the energy requirements of the alkylation process 
at a minimum level. This is particularly important where energy is 
valuable and the products for generating the energy are in relatively 
short supply and expensive. 
Accordingly, it is an object of this invention to provide an improved 
alkylation process which conserves energy by utilizing the available heat 
in the process in a more efficient manner. 
Another object of this invention is to provide an alkylation process which 
minimizes the energy requirements. 
It is another object of this invention to provide an alkylation process 
wherein the majority of heat required by the isostripper is supplied by 
the waste heat available in a side draw stream from the prefractionation 
zone. 
Another object of this invention is to provide an improved alkylation 
process in which the isostripper requires little, if any, additional heat 
energy than that obtained from an interheater which is heated by waste 
heat available in the process. 
Other objects, aspects, and the several advantages of this invention will 
be apparent to those skilled in the art upon a study of the disclosure, 
the appended claims, and the drawing. 
SUMMARY OF THE INVENTION 
This invention relates to an alkylation system wherein a side draw stream 
is removed from the prefractionation zone and used for interheating the 
isostripper. Most of the heat required for the isostripper can come from 
the interheater. The use of the waste heat in the side draw stream to heat 
the interheater can result in substantial heat savings as it reduces the 
amount of heat necessary from outside of the process to heat the 
isostripper. 
Additional heat savings can be realized upon using the side draw stream 
from the prefractionation zone and the bottoms from the isostripper as 
heat exchange mediums for preheating a feed stream to a tower, e.g., the 
feed stream to a prefractonation zone. 
In another embodiment of the invention, the load, and thereby the amount of 
heat needed by the isostripper, can be reduced by flashing the bottoms 
fraction of the prefractionation zone with the flashed vapor being 
recycled to alkylation in the remaining liquid or residue being charged to 
the isostripper as feed. The flashing of the bottoms fraction removes some 
of the isoparaffin as vapor and thereby reduces the amount of isoparaffin 
being fed to the isostripper that needs to be heated, vaporized, and 
collected as overhead.

DETAILED DESCRIPTION OF THE INVENTION 
This invention relates to an alkylation process involving the contacting of 
an olefin and isoparaffin in the presence of an acid alkylation catalyst 
under such alkylation conditions as to form an alkylate containing 
alkylation effluent. The effluent is passed to a phase separation zone and 
allowed to separate into a hydrocarbon phase and an acid catalyst phase. 
The hydrocarbon phase is then passed to a prefractionation zone wherein 
the hydrocarbon phase is separated into at least three fractions with one 
fraction being an overhead vapor fraction, another being a bottoms liquid 
fraction, and the third being a side draw vapor stream. The side draw 
vapor stream taken from the prefractionation zone is passed in indirect 
heat exchange relationship with the interheater of an isostripper in order 
to provide the majority of heat energy required by the isostripper. Once 
passed in indirect heat exchange relationship with the isostripper, the 
vapor stream can then be recycled to alkylation. The overhead fraction 
from the prefractionation zone can be charged to a depropanizer with the 
bottoms fraction of the prefractionation zone being charged to an 
isostripper as feed. 
The prefractionation zone is preferably operated at high pressure in order 
to obtain a very high temperature side stream therefrom. The higher the 
temperature of the vapor side draw stream, the more heat it can furnish to 
the isostripper thereby reducing the amount of independent heat energy 
necessary to reboil the isostripper. The isostripper, however, is 
preferably run at a low pressure as opposed to the high pressure of the 
prefractionator. 
Additional heat savings can be realized by further utilizing the waste heat 
available in the vapor side draw stream from the prefractionation zone as 
well as the heat available in the bottoms fraction from the isostripper. 
The two streams can be passed in indirect heat exchange relationship with 
a feed stream to one of the the towers in the alkylation system in order 
to preheat said feed stream. In one embodiment of the invention, the vapor 
side draw stream from the prefractionation zone and the bottoms fraction 
from the isostripper are passed, independently, in indirect heat exchange 
relationship with the feed stream to the prefractionation zone thereby 
preheating the feed stream. The vapor side draw stream is passed in heat 
exchange relationship with a feed stream after it has been passed in heat 
exchange relationship with the isostripper and is being recycled to 
alkylation. Upon passing the isostripper bottoms in heat exchange 
relationship with a feed stream, the bottoms stream is then passed to 
further use or to storage as alkylate product. 
In another embodiment of the invention, the amount of heat required by the 
isostripper can be reduced by flashing the bottoms fraction from the 
prefractionation zone. The flashed vapor is then recycled to alkylation 
with the resulting liquid being charged to the isostripper as feed. 
Normally, the bottoms fraction from the prefractionation zone is charged 
in its entirety to the isostripper as feed, however, the flashing allows 
for the removal of some isoparaffin thereby reducing the amount of feed to 
the isostripper. Also reduced is the amount of heat necessary or required 
by the isostripper and that the percentage of isoparaffin removed in the 
flashed vapor does not require heat from the isostripper in order to 
vaporize it and thereby remove it as overhead for recycle to alkylation. 
The process of the invention is applicable to any olefin which can be 
appropriately used in an alkylation reaction and the particular olefin 
used will depend upon the type of alkylate product desired. Examples of 
appropriate olefins are the C.sub.3 -C.sub.7 olefins. The preferred 
olefins, however, are the propylene and butylenes. 
The process of the invention is also applicable to any isoparaffin 
appropriate for alkylation with the particular isoparaffin to be used 
depending on the type of alkylate product desired. Examples of appropriate 
isoparaffins are the C.sub.4 -C.sub.8 isoparaffins with isobutane being 
the preferred isoparaffin reactant. 
The invention is not dependent upon specific reaction conditions as 
appropriate conditions are conventional and well known in the art. 
However, the mole ratio of isoparaffin to olefin is usually maintained 
somewhere in the range of about 4:1 to about 20:1. The volume ratio of 
acid catalyst to hydrocarbon feed can be maintained at about 4:1, but can 
be varied in the range of about 0.5:1 to 6:1. The particular temperature 
at which the reaction is run will depend upon the particular olefin used 
in the alkylation. Sufficient pressure is used to maintain liquid phases. 
Various types of catalysts can be utilized in the process with sulfuric 
acid, hydrofluoric acid, phosphoric acid, certain halosulfonic acids, and 
aluminum chloride being examples of appropriate catalysts. Acid acting or 
acid alkylation catalysts are the more commonly used catalysts 
hydrofluoric acid being the preferred catalyst because of the relative 
ease with which it can be regenerated and reused and because of the 
superior quality of the alkylate that is produced. 
Accordingly, one of the preferred embodiments of the invention involves the 
contacting of an olefin stream of propylene and butylenes with isobutane 
in the presence of an HF alkylation catalyst. The alkylation effluent is 
then passed to an HF alkylation phase separator in order to allow the 
effluent to separate into a hydrocarbon phase and an HF catalyst phase. 
The hydrocarbon liquid phase from the HF alkylation phase separator is 
then pumped, indirectly heated, and charged to a high pressure 
prefractionator. Overhead vapor from the prefractionator is condensed and 
the yield portion is fed to the depropanizer of a depropanizing-HF 
stripping operation to yield bottoms liquid propane from the stripper and 
to yield from the depropanizer a bottoms liquid isobutane which is 
recycled to alkylation. Bottoms liquid from the prefractionator is charged 
to a low pressure isostripper. A high temperature vapor side draw from the 
prefractionator indirectly reboils the mid-portion of the isostripper, 
then indirectly preheats the prefractionator feed, is further cooled, and 
then recycled to alkylation. Bottoms alkylate liquid from the isostripper 
can be used to further indirectly heat the feed to the prefractionator 
before being passed to storage for further use. 
A better understanding of the invention will be obtained upon reference to 
the following description of the drawing. The following description and 
disclosed embodiments of the invention are not intended to limit the 
invention in any way, however, and are only given for illustration. 
Referring to the FIGURE, an alkylation process is shown wherein liquid 
olefin feed and liquid isobutane feed (11) along with subsequently 
recovered recycle isobutane (12) and cooled system recycled HF catalyst 
(9) are charged to reactor (10) to produce alkylate gasoline. The 
particular type of reactor is not important and the various appropriate 
reactors are well known in the art. The preferred reactor, however, is the 
riser-reactor as disclosed in U.S. Pat. No. 3,213,157. The reaction 
emulsion from (10) passes to phase separation (13), the separated lower 
liquid HF catalyst phase being recycled (9) via cooler (14) to reactor 
(10), and the hydrocarbon phase being passed by indirect heating means 
(16) and (17) to prefractionator (20) by means of conduit (21). Overhead 
yield (22) from tower (20) is indirectly heated at (62) and charged to 
depropanizer (30). Overhead (61) from depropanizer accumulator (32) is 
charged to HF stripper (40). Separated HF (33) from accumulator (32) and 
HF also from (33') is returned (not shown) to alkylation (10). Overhead 
(41) from HF stripper (40) is charged via condenser (43) to accumulator 
(32) along with overhead (44) from depropanizer (30). Product propane, 
after treating, is recovered at (42). 
Bottoms (25) from prefractionator (20) is charged to isostripper (50). In 
one embodiment of the invention, bottoms (25) from (20) can be flashed, at 
(102) with the remaining liquid (100) being charged to isostripper (50) 
and the flashed vapors (101) being condensed and added to the overhead 
from tower (50), as shown. The overhead and flashed vapor are then 
recycled together. 
Overhead from tower (50) is pumped via (51) to line (12) as recycled 
isobutane. Bottoms (52) from tower (50) can be passed to storage or used 
to indirectly heat at (17) the feed (21) to tower (20). Stream (52) is 
then further cooled and recovered as alkylate product. In some operations, 
normal butane vapor is removed at (55). If this is not done, the stream 
(52) is conventionally denormal-butanized, not shown, yielding isopentanes 
and heavies as the alkylate product. 
A side draw vapor stream (23) from tower (20) is used to heat interheater 
(56). The location of the interheater, used to heat the isostripper, is 
located below feed (25) and above inlet from fired reboiler. In this 
embodiment, the interheater is located at a mid-locus in isostripper (50). 
The partially cooled stream (24) from interheater (56) is cooled at (16), 
in heating indirectly feed to tower (20), and is further cooled and added 
as liquid to the recycled isobutane (12). 
Bottoms (31) from depropanizer (30) are cooled by indirectly heating at 
(62) the feed (22) to depropanizer (30). The stream (31) is further cooled 
and condensed and also added to (12) as recycled isobutane. 
A calculated example is herewith given in order to illustrate one set of 
possible operating conditions in accordance with the invention. 
______________________________________ 
Calculated Operation 
1. Operating Conditions (Specific Operation): 
(10) HF Alkylation Reactor: 
Pressure, psig., 115 
Temperature, .degree. F., 90 
HF/Total H/C vol. ratio 4:1 
IC.sub.4 /olefin mol. ratio 
20:1 
(20) Prefractionator: 
Top Zone: 
Pressure, psig., 230 
Temperature, .degree. F., 155 
Side Draw: 
Temperature, .degree. F., 200 
Bottom Zone: 
Pressure, psig., 235 
Temperature, .degree. F., 210 
(30) Depropanizer: 
Top Zone: 
Pressure, psig., 240 
Temperature, .degree. F., 116 
Bottom Zone: 
Pressure, psig., 245 
Temperature, .degree. F., 200 
(40) HF Stripper: 
Top Zone: 
Pressure, psig., 270 
Temperature, .degree. F., 125 
Bottom Zone: 
Pressure, psig., 272 
Temperature, .degree. F., 135 
(50) Isostripper: 
Top Zone: 
Pressure, psig., 90 
Temperature, .degree. F., 142 
Mid-Zone: 
Pressure, psig., 92 
Temperature, .degree. F., 150 
Bottom Zone: 
Pressure, psig., 95 
Temperture, .degree. F., 325 
II. Flow Rates (Specific Operation): 
(11) Feed Olefins & Isobutane, B/H 
543 
Composition (of blend), 
Vol. % 
Propane 2.7 
Isobutane 50.2 
Normal Butane 4.0 
Butylenes 43.1 
Total 100.0 
(12) Recycle Isobutane, B/H, 4,674 
Composition, Vol. % 
Propane 1.1 
Isobutane 94.3 
Normal Butane (plus) 
4.6 
(21) Feed to Prefractionator, B/H 
5,196 
(22) Feed to Depropanizer, B/H 404 
(23) Vapor Sidedraw (measured as liquid), B/H 
1,553 
Temperature, .degree. F., 200 
Pressure, psig., 232 
Vol. % iC.sub.4, 93 
(24) Cooled return of sidedraw, B/H 
1,553 
Temperature, .degree. F., 95 
Pressure, psig., 220 
(25) Bottoms from Prefractionator, B/H 
3,214 
(51) Isostripper Overhead, B/H 2,805 
Vol. % iC.sub.4 94 
(52) Alkylate Yield (contains nC.sub.4), B/H 
409 
RON clear of DC.sub.4 alkylate 98 
(41) Propane Yield, B/H 15 
(31) Depropanizer Bottoms, B/H 389 
Vol. % iC.sub.4 95 
______________________________________ 
The liquid hydrocarbon phase from the HF alkylation settler is pressured by 
the pump to the high pressure prefractionator (20) which is reboiled, for 
example, with low pressure steam. A vapor side draw, containing "waste 
heat", and rich in isobutane is removed from the prefractionator and is 
used to indirectly heat the mid-portion of the isostripper. This is the 
main source of heat for the isostripper. 
By operating the prefractionator at a relatively high pressure and thereby 
recovering the side draw vapor at relatively high temperature, and using 
this vapor as the main heat source for the low pressure isostripper, about 
1,500,000,000 Btu/day can be saved as compared with use of low pressure 
prefractionation and low pressure isostripper. This is equivalent to a 
savings of $3,750/day using natural gas at $2.50/1000 SCF (Btu of 
1,000/SCF of gas). Less fired reboiling of the isostripper is required by 
the system of the invention than in the prior operation. The 
prefractionator is preferably operated in the range of about 70 to 200 
psi, above the pressure in the isostripper, allowing, thereby, sufficient 
temperature in the prefractionator side draw and bottoms stream to effect 
desired interheating and feed temperature for the isostripper; the 
prefractionation is preferably operated in the range of about 175 to about 
300 psig, and at a temperature (depending on alkylate effluent 
composition) to effect vaporization to produce overhead yield and side 
draw of about 30 to about 50 volume percent of the feed to the 
prefractionator. 
Reasonable variations and modifications which will become apparent to those 
skilled in the art can be made in the present invention without departing 
from the spirit and scope thereof.