Process for linear alpha-olefin production

A process for the continuous oligomerization of ethylene to produce linear alpha olefins by oligomerizing ethylene in a polar phase comprising a solution of transition metal catalyst system at oligomerization conditions including a temperature and pressure greater than the critical temperature and pressure of ethylene. The resulting hydrocarbon phase containing oligomers and unreacted ethylene is subjected to physical treatment which tends to render the ethylene a nonsolvent for oligomers and thereby produce a liquid stream rich in unreacted ethylene which may be recycled to the oligomerization reaction zone by pumping.

BACKGROUND OF THE INVENTION 
Linear olefins are one of the most useful classes of hydrocarbons used as 
raw materials in the petrochemical industry and among these the linear 
alpha-olefins--unbranched olefins whose double bond is located at a 
terminus of the chain--form an important subclass. Linear alpha-olefins 
can be converted to linear primary alcohols by hydroformylation (oxo 
synthesis); alcohols of carbon number less than eleven are used in the 
synthesis of plasticizers whereas those of carbon number greater than 
eleven are used in the synthesis of detergents. Hydroformylation also can 
be used to prepare aldehydes as the major products which in turn can be 
oxidized to afford synthetic fatty acids, especially those with an odd 
carbon number, useful in the production of lubricants. Linear 
alpha-olefins also are used in the most important class of detergents for 
domestic use, namely, the linear alkylbenzenesulfonates, which are 
prepared by Friedel-Crafts reaction of benzene with linear olefins 
followed by sulfonation. 
Another important utilization of alpha-olefins is radical hydrobromination 
to give primary bromoalkanes which are important intermediates in the 
production of thiols, amines, amine oxides and ammonium compounds. Direct 
sulfonation of the alpha-olefins afford the alpha-olefin sulfonates, a 
mixture of isomeric alkenesulfonic acids and alkanesulfones, which are 
effective laundry agents even in hard water and at low concentrations. 
Linear alpha-olefins, particularly those of eight carbons and under also 
are used as comonomers in the production of high density polyethylene and 
linear low density polyethylene. 
Although linear olefins are the product of dehydrogenation of linear 
alkanes, the major portion of such products are the internal olefins. 
Preparation of alpha-olefins is based largely on oligomerization of 
ethylene, which has as a corollary that the alpha-olefins produced have an 
even number of carbon atoms. Oligomerization processes for ethylene are 
based mainly on organoaluminum compounds or transition metals as catalyst. 
Using catalytic quantities of, for example, triethylaluminum, the 
oligomerization of ethylene proceeds at temperatures under 200.degree. C. 
to afford a mixture of alpha-olefins whose carbon number follows a 
Schultz-Flory distribution. In the C.sub.6 -C.sub.10 range there is less 
than 4% branched alpha-olefins, but the degree of branching increases to 
about 8% as the chain length is extended to about 18. A modified process, 
the so-called Ethyl process, affords a high conversion of ethylene to 
alpha-olefins with a more controlled distribution but product quality 
suffers dramatically, particularly in the content of branched olefins. 
Thus, in the C.sub.14 -C.sub.16 range linear alpha-olefins represent only 
about 76% of the product. 
A notable advance in the art accompanied the use of transition metals as 
catalysts for ethylene oligomerization. The use of, for example, nickel, 
cobalt, titanium, or zirconium catalysts afforded virtually 100% 
monoolefins with greater than 97% as alpha-olefins, under 2.5% as branched 
olefins, and under 2.5% as internal olefins. Since the catalysts are 
insoluble in hydrocarbons, oligomerization by catalyst systems based on 
transition metals typically is performed in a polar solvent to solubilize 
the catalyst. Ethylene and its oligomers have limited solubility in the 
polar solvents used, which permits a continuous oligomerization process, 
since ethylene can be introduced into the polar phase and oligomerization 
products can be withdrawn as the hydrocarbon phase. 
Ethylene oligomerization affords alpha-olefins with a Schultz-Flory 
distribution which is catalyst dependent and, at least for the catalysts 
of major interest herein, temperature dependent to only a minor degree. A 
class of catalysts having a transition metal component particularly 
attractive as oligomerization catalysts is described in U.S. Pat. Nos. 
4,689,437, 4,716,138, 4,822,915 and 4,668,8323. Using such catalysts under 
conditions where the Schultz-Flory distribution constant is about 0.65 
affords an oligomerization product whose alpha-olefin distribution in the 
C.sub.8 -C.sub.16 range is particularly desirable from an economic 
viewpoint. That is, the economic value of ethylene oligomers may be 
maximized by having a Schultz-Flory distribution of about 0.65. At these 
operating conditions, the oligomerized reactor effluent contains oligomer 
compounds as well as unreacted ethylene. This unreacted ethylene must be 
recovered and recycled to the reaction zone. Previously, the unreacted 
ethylene was recovered via fractionation and then compressed before 
recycle to the reaction zone. Since compression is expensive, it is 
desirable to introduce at least a portion of recycle into the reaction 
zone without the necessity of compression. In accordance with the present 
invention, this desirable goal is achieved. 
It has been discovered that the separation and recycle of supercritical 
ethylene can substantially reduce the amount of compression required for 
unconverted ethylene. Preferably, up to 90% of the ethylene can be 
recovered from the reactor effluent at supercritical conditions and then 
recycled to the reactor inlet by pumping rather than by compression. Since 
the supercritical separation at preferred operating conditions is not a 
sharp, perfect separation, about 15-25 mol % of the material recovered and 
recycled to the reactor are oligomers. However, one advantage to this 
resulting separation is that these oligomers may be used to aid in 
solubilizing heavy wax buildup in the reaction zone. 
During the oligomerization reaction about 10% of the oligomers have 20 or 
more carbon atoms (C.sub.20+) which are solids at ambient temperature. The 
C.sub.20+ oligomers have limited solubility in the resulting hydrocarbon 
phase of the oligomerization process described above and therefore form a 
separate solid waxy phase. The oligomerization process then becomes a 
four-phase system; a vapor phase of ethylene, a polar solvent phase with 
dissolved catalyst, an immiscible liquid hydrocarbon phase and a solid 
phase of C.sub.20+ hydrocarbons. The formation of solids tends to plug 
the reactor as currently configured, so a continuous process becomes 
interrupted periodically due to the necessity of unplugging the reactor 
and even during process operation, liquid flow is impeded as solids 
accumulate. These solids will be a problem and prevention of solid 
precipitation is highly desirable. This can be effected by increasing the 
solubility of the heavy oligomers in the liquid hydrocarbon phase by 
simultaneously recycling some of the lighter oligomer fractions and 
recycle ethylene to the reaction zone. 
INFORMATION DISCLOSURE 
U.S. Pat. No. 4,689,437 (Murray) and U.S. Pat. No. 5,523,508 (Krawczyk et 
al) disclose processes for the oligomerization of ethylene in the presence 
of a catalyst system containing a transition metal compound, a catalyst 
activator and an organophosphorus sulfonate ligand in a polar solvent such 
as sulfolane at preferred operating conditions including a temperature and 
pressure greater than the critical temperature and pressure of ethylene. 
These patents fail to disclose the separation of ethylene from the 
reaction zone effluent and the subsequent recycle of ethylene in 
accordance with the present invention. 
U.S. Pat. No. 2,391,576 (Katz et al) discloses a process for separating 
hydrocarbon mixtures into a multiplicity of vapor phases. 
U.S. Pat. No. 4,795,854 (Levresse) discloses a device for separating a high 
pressure polyphase mixture of a gas containing liquid particles and 
particularly a mixture of ethylene and polyethylene which comprises a 
cylindrical vertical enclosure into which extends an inlet means for 
supplying the mixture thereto and which is provided at its lower end with 
an outlet for discharging separated liquids; a vertical cyclone in 
communication with the enclosure for receiving separated gases therefrom 
and having an outlet at its upper end for discharging gases separated in 
the cyclone and a liquid outlet at its lower end for discharging separated 
liquids therefrom; and an ejector comprising a nozzle, through which the 
polyphase mixture is fed, a mixing zone connected to the liquid outlet of 
the cyclone and a diffuser section for reducing the speed of the resultant 
mixture and being connected to the inlet means for supplying the polyphase 
mixture to the enclosure. 
BRIEF SUMMARY OF THE INVENTION 
The purpose of the present invention is the production of linear 
alpha-olefins from the oligomerization of ethylene using as a catalyst a 
solution of a transition metal catalyst system in a polar solvent while 
affording a more economical separation of the unreacted ethylene for 
recycle to the oligomerization reaction zone. The hydrocarbon phase 
comprising oligomers and unreacted ethylene is withdrawn from the 
oligomerization reaction zone and subjected to a physical treatment which 
tends to render the unreacted ethylene a nonsolvent for oligomers to 
thereby produce a liquid phase comprising ethylene which may economically 
be recycled to the oligomerization reaction zone. 
One embodiment of the present invention may be characterized as a process 
for the continuous oligomerization of ethylene to produce linear 
alpha-olefins comprising: (a) introducing ethylene at oligomerization 
conditions including a temperature and pressure greater than the critical 
temperature and pressure of ethylene into a liquid polar phase comprising 
a solution of transition metal catalyst system in a polar solvent; (b) 
oligomerizing ethylene in the liquid polar phase to produce oligomers 
having more than 4 carbon atoms and forming a liquid hydrocarbon phase 
separate from the liquid polar phase and comprising unreacted ethylene; 
(c) continually withdrawing and subjecting the liquid hydrocarbon phase to 
physical treatment which tends to render the ethylene a nonsolvent for 
oligomers to thereby form a two-phase system with a first liquid phase 
comprising ethylene in a reduced solvent state and a second liquid phase 
comprising oligomers; (d) recycling at least a portion of the first liquid 
phase comprising ethylene produced in step (c) to provide at least a 
portion of the ethylene in step (a); and (e) recovering the second liquid 
phase comprising oligomers produced in step (c). 
Another embodiment of the present invention may be characterized as a 
process for the continuous oligomerization of ethylene to produce linear 
alpha-olefins comprising: (a) introducing ethylene at oligomerization 
conditions including a temperature and pressure greater than the critical 
temperature and pressure of ethylene into a liquid polar phase comprising 
a solution of transition metal catalyst system in a polar solvent; (b) 
oligomerizing ethylene in the liquid polar phase to produce oligomers 
having more than 4 carbon atoms and forming a liquid hydrocarbon phase 
separate from the liquid polar phase and comprising unreacted ethylene; 
(c) continually withdrawing and subjecting the liquid hydrocarbon phase to 
physical treatment which tends to render the ethylene a nonsolvent for 
oligomers to thereby form a two-phase system with a first liquid phase 
comprising ethylene in a reduced solvent state and a second liquid phase 
comprising oligomers and ethylene; (d) recycling at least a portion of the 
first liquid phase comprising ethylene produced in step (c) to provide at 
least a portion of the ethylene in step (a); (e) fractionating at least a 
portion of the second liquid phase comprising oligomers and ethylene 
produced in step (c) to produce a stream comprising ethylene and a stream 
comprising oligomers; (f) recycling at least a portion of the stream 
comprising ethylene produced in step (e) to provide at least a portion of 
the ethylene in step (a); and (g) recovering the stream comprising 
oligomers produced in step (e).

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is a process for the continuous oligomerization of 
ethylene to produce linear alpha-olefins. The oligomerization of ethylene 
using a solution of a transition metal catalyst system in a polar solvent 
proceeds with the formation of a separate hydrocarbon phase consisting 
largely of linear alpha-olefins formed according to the Schultz-Flory 
distribution. At a Schultz-Flory distribution constant of greater than 
about 0.60 considerable amounts of C.sub.20+ oligomers are formed which 
are not completely soluble in the hydrocarbon phase at process 
temperatures. The oligomerization reaction zone effluent contains 
unreacted ethylene and up to about 90% of the ethylene can be recovered 
from the reactor effluent at supercritical conditions and then recycled to 
the reactor inlet by pumping rather than by compression. 
The oligomerization of ethylene is preferably catalyzed by transition metal 
catalyst systems. A suitable metal catalyst system is described in U.S. 
Pat. No. 4,689,437 which is incorporated herein. A preferred transition 
metal catalyst system is a reaction product of three components; a 
transition metal compound, a catalyst activator and an organophosphorus 
sulfonate ligand. Since transition metal catalyst systems for ethylene 
oligomerization are well known in the art, they need not be further 
discussed herein. 
The oligomerization of ethylene is a liquid phase reaction and the catalyst 
may be either dissolved in a solvent or suspended in a liquid medium. This 
solvent or liquid medium of course needs to be inert to process components 
and apparatus under process conditions. Examples of solvents include 
ethanol, methanol, water, sulfolane (tetramethylenesulfone), ethylene 
glycol, 1,4-butanediol, ethylene carbonate, as well as mixtures of the 
foregoing. In the variant under discussion here solvents which permit 
ready phase separation from oligomer products are preferred in order to 
have a polar solvent phase and a hydrocarbon phase. The most preferred 
solvent for ethylene oligomerization is sulfolane in which the catalysts 
of the present invention are quite soluble but the oligomers are not. 
Typical catalyst concentrations are in the range of about 10 ppm to about 
1,000 ppm of transition metal. Some of the more active catalysts give 
quite high reaction rates at 40 ppm, and a broader range of catalyst 
concentration is between about 0.1 to about 1,000 ppm. In a preferred mode 
of practicing the present invention catalyst concentrations range between 
about 15 and about 300 ppm. 
In accordance with the present invention, the oligomerization conditions 
include a temperature in the range of about 40.degree. F. (5.degree. C.) 
to about 392.degree. F. (200.degree. C.), with the interval between 
68.degree. F. (20.degree. C.) and 284.degree. F. (140.degree. C.) 
preferred and that between 86.degree. F. (30.degree. C.) and about 
176.degree. F. (80.degree. C.) even more preferred. Oligomerization 
pressures preferably are in the range from about 885 psig to about 5,000 
psig and more preferably in the range of about 885 to about 2,000 psig. 
These pressures are the pressures at which the ethylene is introduced into 
the reactor and at which the reactor is maintained. The critical 
temperature and pressure of ethylene is 40.degree. F. (5.degree. C.) and 
885 psig, respectively, and therefore the oligomer reaction zone is 
operated at conditions above the critical temperature and pressure of 
ethylene. 
As commented on above, the oligomerization process forms oligomers which 
are predominantly linear alpha-olefins having from 4 to over 20 carbon 
atoms and which have low solubility in the polar solvents utilized, 
especially where sulfolane is the solvent for the transition metal 
catalyst systems of our invention. Consequently, oligomer formation is 
accompanied by formation of a separate hydrocarbon phase, at least a 
portion of which is continually removed. The constituents of this 
hydrocarbon phase are ethylene oligomers whose relative proportions 
closely follow a Schultz-Flory distribution. The practice of the present 
invention is particularly applicable to those cases where substantial 
amounts of heavy oligomers are formed, which is a function of the 
Schultz-Flory distribution. By "heavy oligomers" is meant oligomers 
normally a (waxy) solid at process temperatures, and may be considered as 
C.sub.20+ oligomers. These heavy oligomers have a limited, 
temperature-dependent solubility in the hydrocarbon phase. But since the 
temperature also affects oligomer product quality via the selectivity to 
linear alpha-olefins, it is not practical to raise the reaction 
temperature in order to maintain homogeneity. Unless homogeneity in the 
hydrocarbon phase is maintained, reactor (or an ancillary unit) clogging 
results, which is alleviated by the present invention. 
The process of the present invention is practiced in a way typical for 
ethylene oligomerization other than the supercritical recovery of at least 
a portion of the unreacted ethylene from the reaction zone effluent. Thus, 
ethylene is continually fed to a reactor sufficient to maintain ethylene 
pressures between about 885 and about 5000 psig at temperatures between 
about 5.degree. and about 200.degree. C. The transition metal catalyst 
system is present in solution in a polar solvent, preferably sulfolane. 
Oligomerization proceeds with formation of a separate hydrocarbon phase 
resulting from the low solubility of oligomers in the sulfolane. The 
hydrocarbon phase containing oligomers and unreacted ethylene is 
continually removed and subjected to a physical treatment to reduce the 
solvent ability of the supercritical ethylene. 
The reduction of the solvent ability of the supercritical ethylene may be 
achieved by pressure reduction, temperature increase or a combination 
thereof. The key to the separation and recycle of the supercritical 
ethylene is that the capacity of the supercritical ethylene to extract 
heavier components is highly dependent on its density. As the density of 
the supercritical ethylene is decreased (by pressure reduction, 
temperature increase or a combination thereof, the amount of heavier 
material, i.e., oligomers in this case that can remain in the 
supercritical ethylene decreases. In accordance with the present 
invention, it is preferred that the capacity of the supercritical ethylene 
for oligomers is reduced by both decreasing the pressure and increasing 
the temperature. It is preferred that the pressure is decreased in the 
range from about 1350 psig to about 850 psig and that the temperature is 
increased in the range from about 175.degree. F. (80.degree. C.) to about 
400.degree. F. (205.degree. C.). After the reduction of the solvent 
ability of the supercritical ethylene, the reaction zone effluent is 
introduced into a supercritical separation zone wherein an overhead liquid 
stream containing unreacted ethylene is produced, recovered and recycled 
to the reaction zone. Since the separation of unreacted ethylene from the 
reactor effluent is not a sharp separation, about 15-25 mol percent of the 
material recovered and recycled to the reactor is oligomers having up to 
about 14 carbon atoms. These oligomers which are recycled with the 
supercritical ethylene are used to aid in solubilizing heavy wax buildup 
in the oligomerization reaction zone. 
A bottoms stream containing oligomers and unreacted ethylene is removed 
from the supercritical separation zone and subjected to conventional 
fractionation to recover a stream containing product oligomers and a 
stream containing unreacted ethylene which is also recycled to the 
reaction zone. 
Detailed Description of the Drawing 
The drawing is a preferred embodiment of the present invention and is a 
simplified flow diagram in which such details as pumps, instrumentation, 
heat exchange and heat-recovery circuits, compressors and similar hardware 
have been deleted as being non-essential to the understanding of the 
techniques involved. The use of such miscellaneous equipment is well 
within the purview of one skilled in the art of petroleum refining and 
petrochemical production techniques. 
Referring now to the drawing, a fresh feed ethylene stream is introduced 
into the process via line 1 and is admixed with a first recycle stream 
containing previously unreacted ethylene and oligomers transported via 
line 15 and a second recycle stream containing unreacted ethylene 
transported via line 12. The resulting admixture is introduced by line 2 
into oligomerization reaction zone 3 and a resulting product stream 
containing oligomers and unreacted ethylene is removed from 
oligomerization reaction zone 3 via line 4 and introduced into 
heat-exchanger 5. The resulting heated effluent from heat-exchanger 5 is 
transported via line 6 and introduced into supercritical separation zone 
7. An overhead stream containing unreacted ethylene and oligomers is 
removed from supercritical separation zone 7 via conduit 13 and introduced 
into heat-exchanger 14. The resulting cooled effluent from heat-exchanger 
14 is transported via line 15 and provides the first recycle stream 
described hereinabove. A bottom stream is removed from supercritical 
separation zone 7 via line 8 and is introduced into fractionation zone 9. 
An overhead stream containing unreacted ethylene is removed from the 
overhead of fractionation zone 9 and transported via line 12 which 
provides the second recycle stream described hereinabove. A bottom stream 
containing oligomers is removed from fractionation zone 9 via line 10 and 
recovered. 
The process of the present invention is further demonstrated by the 
following illustrative embodiment. This illustrative embodiment is, 
however, not presented to unduly limit the process of this invention, but 
to further illustrate the advantages of the hereinabove-described 
embodiment. The following results were not obtained by the actual 
performance of the present invention but are considered prospective and 
reasonably illustrative of the expected performance of the invention based 
upon sound engineering calculations. 
Illustrative Embodiment 
A fresh feed ethylene stream in an amount of 981 lb mole/hr is introduced 
into the process and admixed with a first recycle stream from the 
supercritical separation zone in a total amount of 1154 lb mole/hr with 
784 lb mole/hr of ethylene, 213 lb mole/hr butene and the balance (157 lb 
mole/hr) as heavier oligomers. 
A second recycle stream from the oligomer product fractionation zone in an 
amount of 196 lb mole/hr of ethylene is also recycled and admixed with the 
fresh feed ethylene and the first recycle stream, and introduced into an 
oligomerization reaction zone maintained at a temperature of 140.degree. 
F. and a pressure of 1500 psia. The details of the combined feed stream to 
the reaction zone and the product stream from the reaction zone are 
presented in Table 1. 
The reaction zone effluent is depressured to a pressure of 1335 psia, 
heated to 365.degree. F. and introduced into a supercritical separation 
zone. An overhead liquid stream from the supercritical separation zone is 
removed and pumped to the oligomerization reaction zone as the first 
recycle stream. The bottoms stream from the supercritical separation zone 
is fractionated to produce an overhead stream containing unreacted 
ethylene in an amount of 196 lb mole/hr which is returned to the 
oligomerization reaction zone as the second recycle stream. A linear alpha 
olefin oligomer product stream in an amount of 256 lb mole/hr is recovered 
from the fractionation zone and has the characteristics presented in Table 
1. 
The foregoing description and illustrative embodiment clearly illustrate 
theadvantages encompassed by the method of the present invention and the 
benefits to be afforded with the use thereof. 
TABLE 1 
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Stream Analysis 
Linear Alpha 
Reaction Zone 
Reaction Zone 
Olefin Product 
Feed Effluent Stream 
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Ethylene, lb mole 
1961 980 90 
1-Butene, lb mole 
212 303 59 
1-Hexene, l mole 
90 149 38 
1-Octene, lb mole 
38 76 25 
1-Decene, lb mole 
16 41 16 
1-Dodecene, ln mole 
7 23 10 
1-Tetradecene lb, mole 
3 14 7 
1-Hexadecene, lb mole 
1 8 4 
HEAVIES, lb mole 
3 13 7 
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