Process for separating 1-dodecene and 1-tetradecene from an aluminum alkyl chain growth product

Dodecene-1 and 1-tetradecene can be rectified from a mixture containing 1-dodecene and 1-tetradecene and triethylaluminum ("TEA") which has about the same normal boiling point as 1-dodecene and 1-tetradecene by feeding the above mixture to an intermediate point of a rectification column maintained under sufficient vacuum (e.g. 5-30 torr) such that the temperature at the intermediate feed point is about 250.degree.-260.degree. F. and the overhead distillation temperature is about 190.degree.-240.degree. F. The TEA which normally boils at about the same temperature as 1-dodecene and 1-tetradecene will exist as a dimer permitting part of the 1-dodecene and 1-tetradecene to distill overhead and most of the TEA to exit the bottom of the column.

BACKGROUND 
Alpha-olefins are made in commercial quantities by a process developed in 
the fifties by Karl Ziegler and his coworkers. The so-called Ziegler 
process involves the reaction of triethyl aluminum (TEA) and ethylene at 
temperatures in the range of 200.degree.-350.degree. F. and pressure in 
the range of 2000-5000 psig to yield a mixture of tri-C.sub.2-20+ alkyl 
aluminum having a poisson distribution and C.sub.2-20 olefins. The 
ethylene is flashed from the reaction mixture for recycle and the light 
olefins through 1-decene can be distilled from the mixed aluminum alkyls 
since they have a normal boiling point below the lightest aluminum alkyl 
(viz. TEA). In the past, attempts to distill 1-dodecene and 1-tetradecene 
from the mixed aluminum alkyls resulted in a substantial amount of the TEA 
and other light aluminum alkyls co-distilling with the 1-dodecene and 
1-tetradecene. This light aluminum alkyl represents an economic penalty 
because it must be hydrolyzed which also serves to contaminate the 
.alpha.-olefin product with paraffins, e.g. ethane, butane and hexane. 
This problem was recognized in Roming et al. U.S. Pat. No. 3,227,773 
wherein the patentees state: 
"Some prior art processes have been limited generally to the production 
of C.sub.8-10 olefins, since no practical methods were known for 
completely separating olefins boiling close to C.sub.10-14 olefins from 
the C.sub.2 or C.sub.3 alkyl aluminum remaining after the displacement 
reaction. Thus, the C.sub.12 and higher olefins could not be economically 
distilled overhead from the liquid alkyl aluminum due to the relatively 
low decomposition temperature of said alkyl aluminum and/or the closely 
similar boiling ranges of lower alkyl aluminum and these higher olefins." 
From this it is apparent that a need exists for an efficient method of 
separating C.sub.12-14 .alpha.-olefins from aluminum alkyls containing 
light alkyls such as TEA. It is an object of the present invention to 
provide such a method. 
SUMMARY 
According to the present invention, C.sub.12-14 .alpha.-olefins can be 
distilled from a mixture of olefins and aluminum alkyls containing 
C.sub.12-14 .alpha.-olefins and triethyl aluminum by conducting the 
distillation in a rectification column under sufficient vacuum to maintain 
the rectification section of the column at a temperature in the range of 
about 200.degree.-250.degree. F.

In the drawing an olefin is represented by "C.sup.= " and a subscript 
integer which specifies the number of carbon atoms in the olefin. Thus 
C.sub.2.sup.= is ethylene, C.sub.4.sup.= is butene, C.sub.4-10.sup.= 
represents olefin from butene to decene and C.sub.16+.sup.= represents 
olefins containing 16 or more carbon atoms. Most of the olefins are linear 
.alpha.-olefins although minor amounts of internal and branched olefins 
may be present. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the invention is a process for separating an 
aluminum alkyl chain growth product comprising C.sub.12-14 .alpha.-olefins 
and triethyl aluminum said process comprising: 
(A) feeding said chain growth product to an intermediate point of a 
rectification column, said rectification column being maintained under 
vacuum such that in operation the rectification section above said 
intermediate point is at a temperature in the range of 
200.degree.-250.degree. F. whereby triethyl aluminum entering said 
rectification section is mainly in the form of triethyl aluminum dimer 
thereby increasing its molecular weight and decreasing its volatility, 
(B) distilling at least part of said C.sub.12-14 .alpha.-olefins overhead 
from said rectification section and 
(C) removing a major portion of said triethyl aluminum as a bottoms stream 
from said rectification column. 
The process embodies a vacuum rectification column, either as a separate 
unit or as part of aluminum alkyl chain growth .alpha.-olefin process. The 
process separates at least part of the C.sub.12-14 olefin, e.g. 1-dodecene 
and 1-tetradecene from a mixture which includes TEA, with minimal TEA 
contamination of the distillate and thus, little loss of TEA and minimal 
paraffin contamination of the .alpha.-olefin products. This has been 
considered impossible in the past because of the close proximity of the 
normal boiling points of 1-dodecene, 1-tetradecene and TEA. In fact 
1-tetradecene is reported to have a normal boiling point above that of 
TEA, 250.degree.-260.degree. C. vs. 187.degree. C. Previous attempts at 
such a distillation have resulted in large amounts of TEA in the 
C.sub.12-14 .alpha.-olefin distillate. Removal of this triethyl aluminum 
by hydrolysis results in large losses of aluminum alkyl value as well as 
paraffin contamination of the .alpha.-olefin products placing a severe 
economic penalty on the process. 
The present process permits the distillation of at least part of the 
1-dodecene and 1-tetradecene in an olefin mixture which contains both TEA 
1-dodecene and 1-tetradecene. By "at least part" is meant at least 10 
weight percent and generally at least 20 weight percent of the 1-dodecene 
and 1-tetradecene in the mixture fed to the column. At the same time a 
major amount of the TEA in the feed is rejected into the column bottoms. 
By "a major amount" is meant over 50 weight percent. In practice, it has 
been possible to reject 75-95 weight percent of the TEA in the feed into 
the column bottoms. 
The present process takes advantage of the fact that TEA exists as a dimer 
as long as it is maintained below a critical temperature. The dimer has 
the structure: 
##STR1## 
wherein "et" represents an ethyl group. 
Previous attempts to distill C.sub.12-14 .alpha.-olefins have exceeded this 
critical temperature causing the TEA dimer to disassociate to form TEA 
monomer which readily co-distills with the C.sub.12-14 .alpha.-olefins. 
TEA dimer has a much lower vapor pressure and tends not to pass up through 
a rectification column if the column is maintained under a vacuum 
sufficient to cause at least part of the C.sub.12-14 .alpha.-olefins to 
distill and at the same time maintain the temperature in the rectification 
section of the column below about 250.degree. F., for example in the range 
of 200.degree.-250.degree. F., more preferably 220.degree.-230.degree. F. 
The precise vacuum needed to achieve these conditions may vary somewhat 
with the overall composition being rectified but is generally in the range 
of 5-30 torr, more often 15-25 torr. 
The mixtures that are separated by the present process all contain at least 
some TEA and C.sub.12-14 .alpha.-olefins. Other components may be present 
in the mixture as well. For example a typical TEA chain growth product 
will contain TEA as well as a poisson distribution of tri-C.sub.2-20+ 
alkyl aluminum and possibly even higher alkyl aluminums. Such mixture also 
include ethylene which is conventionally flashed off immediately following 
the chain growth stage. By "flashed off" is meant passed through a 
vapor-liquid separator which permits the more volatile components to 
vaporize but does not have any substantial rectification effect. 
After the ethylene is removed the ethylene-stripped chain growth product 
will contain any C.sub.4-20+ olefins that might have formed in the chain 
growth reactor via incidental displacement as well as C.sub.4-20 
.alpha.-olefins that might have been recycled to the chain growth reactor 
as solvent. 
A suitable feed to the rectification column might include: 
5-75 weight percent aluminum alkyls of which 2-10 weight percent is TEA and 
the remainder higher trialkyl aluminum 
25-95 weight percent .alpha.-olefins of which 5-90 weight percent are 
C.sub.12-14 .alpha.-olefins. 
The rectification column may be any of the several known types including 
those with individual trays or it may be a packed column. The preferred 
rectification column is a packed column using structured packing which has 
a very low liquid hold up. Structured packing is a series fabricated mass 
transfer units stacked one upon another in the column. They consist of 
corrugated stainless steel structures, sometimes perforated, that are 
bonded together in units and installed in the distillation column. One 
such unit is Flexipac.RTM. structured packing manufactured by Koch 
Engineering Co., Houston, Tex. 
The rectification section of the column is the section above the 
intermediate point where the mixture to be separated is introduced. The 
rectification section should have at least one theoretical stage. There is 
no real maximum number of stages in the rectification section but little 
is gained by exceeding three stages. Excellent results have been achieved 
using a rectification column having one theoretical stage in the 
rectification section and four stages below the rectification section. 
Tests were carried out to demonstrate the feasability of distilling 
C.sub.12-14 .alpha.-olefins from a mixture containing TEA and C.sub.12-14 
.alpha.-olefins. The tests utilized a column 2 inch diameter with 13 
actual trays. The column was an Oldershaw column with sieve trays. The 
feed point was three trays down from the top such that the rectification 
section above the feed point had about 1.5 theoretical trays. The column 
was maintained at a pressure of 19-26 torr. 
The feed mixture consisted of: 
______________________________________ 
Component Weight Percent 
______________________________________ 
1-dodecene 12.9 
1-tetradecene 8.1 
1-hexadecene 4.3 
TEA 0.635 
tri-C.sub.4-20+ alkyl aluminum 
56.8 
______________________________________ 
Feed to the column was at the rate of 1.0 ml/min. The column temperature at 
the feed tray was 216.degree.-256.degree. F. and the overhead temperature 
was 190.degree.-233.degree. F. The temperature at the bottom of the column 
was 320.degree.-335.degree. F. and the reboiler temperature was 
340.degree.-346.degree. F. 
The overhead distillate was analyzed by gas chromatography and found to 
consist of: 
______________________________________ 
Weight Percent 
______________________________________ 
1-dodecene 34.6 
1-tetradecene 12.1 
TEA 0.43 
______________________________________ 
The bottoms were analyzed and found to contain: 
______________________________________ 
Weight Percent 
______________________________________ 
1-tetradecene 6.4 
1-hexadecene 5.8 
TEA 0.72 
tri-C.sub.4-20+ alkyl aluminum 
80.4 
______________________________________ 
This bench test demonstrated the operability of the process for distilling 
1-dodecene and 1-tetradecene from a mixture which contained TEA while 
rejecting most of the TEA to the bottoms stream. 
The use of the separation process in a typical aluminum alkyl chain growth 
.alpha.-olefin process can be described with reference to the drawing. In 
the drawing, ethylene is shown as C.sub.2 =, butene as C.sub.4 =, and so 
forth. 
Ethylene and TEA are fed to the chain growth reactor 1 through conduits 2 
and 3 respectively. Recycle aluminum alkyls including TEA plus a broad 
range of olefins are also fed to reactor 1 through recycle conduit 4. A 
stoichiometric excess of ethylene is used. A useful ethylene/TEA mole 
ratio is about 4-10/1. Chain growth reactor 1 is maintained under chain 
growth conditions, typically 200.degree.-300.degree. F. at 2000-3500 psig 
for a 20-60 minute residence time. 
Growth product is transferred via conduit 5 to flash separator 6 operating 
at a lower pressure, e.g. 400-700 psig, causing ethylene to vaporize. This 
ethylene is pumped back to chain growth reactor 1 as part of the ethylene 
feed. 
The liquid phase from flash unit 6 comprises residual ethylene, C.sub.4-20+ 
.alpha.-olefins, TEA and poisson tri-C.sub.4-20+ alkyl aluminums. This 
liquid is transferred via conduit 7 to flash distillation unit 9 wherein a 
further pressure drop causes vaporization of C.sub.4-10 .alpha.-olefins. 
Optionally flash distillation unit 9 can be a series of two or more 
separate flash distillation units each sequentially at a lower pressure to 
remove C.sub.4-10 .alpha.-olefins in stages. In either case, the residual 
liquid phase from unit 9 comprises C.sub.12-20+ .alpha.-olefins, TEA and 
a poisson distribution mixture of tri-C.sub.4-20+ alkyl aluminums. This 
liquid is fed via conduit 10 to an intermediate point in rectification 
column 11. Rectification column 11 is packed with low hold-up fabricated 
stainless steel structured packing to give the equivalent of one 
theoretical tray above the intermediate feed point and 4 trays below the 
feed point. The pressure in rectification column 11 is reduced to about 
15-20 torr causing C.sub.12-14 .alpha.-olefins to be rectified overhead. 
The rectification section temperatures range from about 
250.degree.-260.degree. F. at the intermediate feed point to about 
190.degree.-240.degree. F. overhead. 
The bottoms stream from column 11 comprises C.sub.14-20+ .alpha.-olefins, 
TEA and the poisson distribution tri-C.sub.4-20+ alkyl aluminum. This 
bottom stream is conveyed via conduit 12 to ethylene displacement reactor 
13. Ethylene is also fed via conduit 14 to displacement reactor 13 at a 
stoichiometric excess over that required to displace all alkyls in the 
tri-C.sub.4-20+ alkyl aluminum. Ethylene feed of 5-10 moles per mole of 
tri-C.sub.4-20+ alkyl aluminum in the feed has been found to be 
satisfactory. 
Displacement reactor 13 is maintained under displacement conditions of 
about 450.degree.-700.degree. F. and 200-500 psig. Displacement is rapid 
and residence times of 0.1-2 seconds are adequate. 
Effluent from displacement reactor 13 comprises ethylene, C.sub.4-20+ 
.alpha.-olefins, TEA and minor amounts of higher alkyl aluminums. This 
effluent is transferred via conduit 15 to distillation unit 16 which 
removes mainly TEA and C.sub.2-14 olefins overhead. The bottoms from unit 
16 comprise mainly C.sub.16+ olefins and some residual higher alkyl 
aluminums. This can be recovered by air oxidation and hydrolysis followed 
by distillation to recover C.sub.16+ .alpha.-olefins and optionally 
C.sub.16+ alcohols. 
The TEA and C.sub.2-14 olefins from unit 16 are cooled and conveyed via 
conduit 17 to ethylene flash separator 18. Ethylene vapor is removed and 
recycled to chain growth reactor 1. Bottoms from separator 18 are 
transferred via conduit 19 distillation unit 20 which distills mainly 
C.sub.4-8 .alpha.-olefins overhead. Bottoms from unit 20 comprises mainly 
TEA and C.sub.10-14 .alpha.-olefins and is recycled via conduit 4 wherein 
TEA forms part of the TEA feed and the C.sub.10-14 .alpha.-olefins 
function as solvent in the chain growth reactor. 
The various .alpha.-olefin streams taken overhead from units 9, 11 and 20 
are fed to a product distillation section (not shown) wherein the various 
.alpha.-olefin cuts are separated for sales.