Method of Making Alpha-Olefin Oligomers

This invention relates to the oligomerization of alpha-olefins, such as l-decene, and more specifically it relates to a multi-stage, continuous process, including an initial tank reactor and a final tube reactor for controlling the ratio of oligomers in the oligomer product.

U.S. Pat. No. 3,149,178 describes the preparation of a synthetic lubricant by the batch polymerization of alpha-olefins, including l-decene, in a reactor using boron trifluoride together with a promoter such as boron trifluoride.sup.. decanol complex or acetic acid.sup.. boron trifluoride complex to produce dimer, trimer and a residual polymer fraction.

U.S. Pat. No. 3,382,291 describes the preparation of an alpha-olefin oligomer comprising a dimer, trimer and residual fraction for use as a synthetic lubricant by introducing a stream of the l-olefin, such as l-decene, saturated with boron trifluoride and a separate stream of a complex of boron trifluoride and a promoter, such as an alcohol, into a reactor for batch reaction. The patent also describes a continuous process in which a product stream is withdrawn from the reactor at the rate of the influent streams.

U.S. Pat. Nos. 3,763,244 and 3,780,128 describe the batch oligomerization of l-olefins, such as l-decene, in a reaction vessel using an alcohol or water co-catalyst and boron trifluoride bubbled through the reaction liquid to provide a molar excess of the boron trifluoride in addition to the boron trifluoride that complexes with the co-catalyst. A product mixture of dimer, trimer, tetramer and higher oligomers is described.

We have discovered in the oligomerization of l-olefins that the relative proportion of the higher oligomers and the lower oligomers can be optimized for any particular needs over a substantial range. The novel oligomerization process is carried out using boron trifluoride with a complex-forming co-catalyst in a continuous, multi-stage process including a tank reactor in the first stage and a tube reactor in the second stage.

Oligomers of certain l-olefins, particularly l-decene and mixtures of l-decene with l-octene and/or l-dodecene are highly useful as base fluids for preparing lubricants, hydraulic fluids, transmission fluids, transformer fluids, and the like, generically described as functional fluids, by the use of appropriate additives. Alpha-olefin oligomers can, in some instances, also be used as functional fluids without the use of property modifying additives. The alpha-oligomers which are useful either directly or with additives as functional fluids generally have a 210.degree. F. (98.9.degree. C.) viscosity within the range of about one centistoke to about 15 centistokes. Each functional fluid product and generally the base fluid from which it is prepared must conform with established viscosity specifications.

The oligomer product prepared by the multi-stage process described herein is a mixture of the dimer, trimer, tetramer and pentamer with minute amounts of higher oligomers sometimes present. The dimer is generally removed for separate use as a functional fluid, particularly use as a transformer fluid, to avoid volatilization loss from functional fluids comprising the higher oligomers. Therefore, the primary oligomer product for general use is a mixture of the trimer, tetramer and pentamer. But since the different viscosity specifications for the broad range of products for which a trimer, tetramer and pentamer mixture can be used varies over a broad range, the relative proportions of trimer, tetramer and pentamer must vary substantially from functional fluid product to product in order to meet these viscosity specifications. In view of the fact that the amount of pentamer resulting from the oligomerization is generally less than about 15 weight percent and is more difficult to quantify and since the amount of tetramer and the amount of pentamer, in general, increases or decreases similarly in different oligomerization runs at different conditions, we have used the trimer to tetramer ratio as a convenient indicator of the trend of changing product composition from run to run.

Generally, the oligomer composition that is obtained from a prior art polymerization procedure is not the composition that is required to meet the viscosity specifications for a desired product. A separation procedure is required to obtain a fraction having the desired trimer to tetramer ratio but the separation also includes a by-product fraction. It would be advantageous to be able to vary the oligomer composition to make an oligomer product, or different oligomer products in different runs, having the desired composition without a by-product fraction.

When two or more separate functional fluid products are desired, these can be obtained as separate fractions from the crude oligomer product, each fraction having its specific trimer to tetramer ratio to provide the required viscosity. However, the relative amounts of the different fractions obtained on separation is determined by the composition of the crude product resulting from the oligomerization. Since the demand for each product fraction will vary over a period of time, it would be most advantageous to be able to vary their relative rate of production by varying the composition of the crude oligomer product and meet, through fractionation of the crude product, the demand for each individual product without undesired by-product.

The preparation of two functional fluid products from a single oligomer product is now described by way of example. A l-decene oligomer, stripped of monomer, contained 12 percent dimer, 55 percent trimer, 27 percent tetramer and 6 percent pentamer and higher. The oligomer was hydrogenated and separated into a dimer fraction, a hydraulic base fluid fraction having a 210.degree. F. (98.9.degree. C.) viscosity of 3.94 cs. and a motor oil base fluid fraction having a 210.degree. F. (98.9.degree. C.) viscosity of 5.79 cs. The hydraulic base fluid contained 3 percent dimer, 79 percent trimer, 17 percent tetramer and 1 percent pentamer while the motor oil base fluid contained 1 percent dimer, 37 percent trimer, 46 percent tetramer and 16 percent pentamer. The weight ratio of hydraulic base fluid to motor oil base fluid resulting from this separation was about 1.4 to one.

We have discovered a simple, flexible procedure for controlling the ratio of lower to higher l-olefins, as indicated by the trimer to tetramer ratio, over a significant range during the oligomerization of alpha-olefins. We have discovered that this variation in the trimer to tetramer ratio can be effected in a continuous, multi-stage procedure in which the l-olefin and the catalysts are introduced into and mixed together in a tank reactor for partial polymerization to oligomer and then directed to a tube reactor for further oligomerization. We have discovered that in this procedure the trimer to tetramer ratio is increased by decreasing either or both the overall conversion and the proportion of the total oligomerization taking place in the first reactor and conversely the ratio is decreased by increasing either or both the overall conversion and the proportion of the total oligomerization taking place in the first reactor. Thus, we have discovered as a feature of our invention that the trimer to tetramer ratio is increased at constant conversion by increasing the proportion of the total oligomerization taking place in the tube reactor.

The novel process described herein can be utilized to vary the proportion of lower oligomers and higher oligomers during oligomerization of a broad class of alpha-olefins including those having between about five and about 14-carbon atoms and mixtures thereof. However, since the trimer, tetramer and pentamer are of particular significance in lubricants and other functional fluids, the process is particularly useful in the oligomerization of those alpha-olefins which result in superior functional fluid compositions. For this reason we find that this multi-stage process is more useful in the oligomerization of mixtures of l-decene with up to 50 mol percent of l-octene or l-dodecene or a mixture of these and preferably most useful in the oligomerization of l-decene itself. The alpha-olefins useful herein are the straight chain compounds as well as the branched chain compounds provided that the branch is on the third or fourth carbon atom or higher.

Boron trifluoride is used as a catalyst in our process together with a compound commonly called a promoter or a co-catalyst. The co-catalyst is conventional and can be any compound which complexes with boron trifluoride to form a coordination compound which is catalytically active for the oligomerization reaction. Included in this list of co-catalysts are aliphatic ethers, such as dimethyl ether, diethyl ether and the like; aliphatic alcohols such as methanol, ethanol, n-butanol, decanol, and the like; polyols, such as ethylene glycol, glycerol and the like; water; aliphatic carboxylic acids such as acetic acid, propanoic acid, butyric acid, and the like; esters, such as ethyl acetate, methyl propionate, and the like; ketones, such as acetone and the like; aldehydes, such as acetaldehyde, benzaldehyde, and the like; acid anhydrides, such as acetic acid anhydride, succinic anhydride, and the like; and the like. It is preferred that the above-described co-catalyst have from one to about 10 carbon atoms although higher carbon co-catalyst compounds can be used. The co-catalyst can be used in a catalytic amount such as from about 0.01 to about 3.0 weight percent of the l-olefin, preferably from about 0.1 to about 1.0 weight percent. When the amount is low; the reaction becomes slow and when it is too high, there is no added benefit.

The boron trifluoride, introduced into the first reactor, will complex with the co-catalyst. It is preferred that the reaction liquid contain an excess of boron trifluoride in addition to that complexed with the co-catalyst in order to have a suitable rate of oligomerization. We have found that this can be accomplished in a simple manner by pressuring the first reactor with boron trifluoride gas. By this technique sufficient boron trifluoride gas dissolves in the reaction liquid not only to complex with the co-catalyst but also to provide free boron trifluoride in solution. The pressure of boron trifluoride gas that is maintained in the first reactor can suitably range from about 5 to about 500 psig. (0.352 to 35.2 Kg/cm.sup.2) or higher with a preferred range of about 20 to about 100 psig. (1.41 to 7.03 Kg/cm.sup.2). Alternatively, the boron trifluoride gas can be bubbled through the liquid into the first stage maintained under atmospheric or higher pressure with boron trifluoride gas recycle. Other procedures are known in the art for introducing boron trifluoride into the reaction solution.

The temperature required for suitable oligomerization is conventional and can broadly range from about -20.degree. to about 90.degree. C. with a temperature in the range of about 20.degree. to about 70.degree. C. being preferred. The higher the temperature the greater the rate of catalyst consumption and the lower the rate of reaction while the lower the temperature, the greater the cooling costs with the reaction rate being satisfactory at the lower temperatures.

The first-stage tank reactor serves several important functions. These include the intimate mixing of the various components, the dissolution of boron trifluoride gas, and the control of the temperature of the reaction liquid during the exothermic oligomerization reaction. In the present process the reactant alpha-olefin and the catalysts are continuously introduced into the first stage tank reactor with reactor liquid being continuously withdrawn at a rate that will maintain a constant liquid level in the tank reactor and fed to the tube reactor. In order to provide a uniform distribution of the components in the tank reactor it is desirable to provide some means to add the various components into the reactor and rapidly disperse them in order to provide a substantially homogeneous reaction medium. This can be conveniently accomplished by providing an externally driven stirrer. Another method involves the agitation of the entire reactor and contents by rocking or vibration, but this method is not particularly suitable with large reactors. Also bubbling boron trifluoride gas into the reactor provides some mixing of the reactor contents.

The expression tank reactor as used herein refers to a reactor in which the major dimensions are similar so that substantial homogeneity of composition and temperature can be maintained in the reaction fluid. Thus, it is preferred that the reactor itself be so dimensioned that the major dimension of the reactor differs from the minor dimension by a factor no greater than about ten to one, for example, height to diameter or vice versa. More preferably, it is desired that the major and minor dimensions of the reactor vary no more than by a factor of about five to one and most desirably by a factor no greater than about two and one-half to one for most effective functioning in accordance with the process herein. It is understood that the reactor will operate at a suitable liquid level for effective functioning thereof.

The exothermic oligomerization reaction is conveniently controlled in the first reactor by the diluting effect of the reactor liquid on the incoming l-olefin. If greater dilution for temperature control is desirable, the reactor liquid volume can be increased, the inflow rate can be decreased, or an inert diluent can be added to the reactor with the l-olefin feed. Temperature control is also desirably accomplished by heat exchange means associated with the first reactor. Various factors which affect the degree of oligomerization taking place in this first stage reactor include the average time which the reaction liquid is in the reactor as measured by the liquid volume in the reactor divided by the flow rate from or into the reactor and the reaction rate, which is itself dependent on such factors as the concentration of the catalyst, the concentration of l-decene and the temperature.

A stream of reaction liquid is continuously removed from the tank reactor and introduced into the tube reactor. Preferably the tube reactor is so designed, that is, with such length and capacity that the process can be operated to give complete conversion, when desired. By tube reactor we mean a reactor having a length much greater than its cross-sectional dimension such that the composition of the flowing, reacting liquid will differ in different portions of the tube. That is, if this tube reactor possesses a circular cross-section, then its length could be as low as about 15 times its diameter, provided that back-mixing can be avoided in the flowing stream, but preferably its length is at least about 100 times the diameter and most preferably at least about 1,000 times the diameter. The higher ratios are preferred to decrease back-mixing of the reaction liquid flowing through the tube reactor and to improve heat transfer to the cooling liquid. The same general principle holds true whether the tube reactor possesses an oval, square or other cross-sectional configuration. Although the temperature in the tube reactor can be the same, higher or lower than the first reactor, it is most practical to operate the two reactors at the same or a similar temperature.

We have discovered that the trimer to tetramer ratio decreases with increasing conversion in both reactors but that this decrease is less in the tube reactor than in the tank reactor. We have also discovered that for any specific conversion that the trimer to tetramer ratio is higher in the tube reactor than in the tank reactor. Thus, we have discovered that we can control the trimer to tetramer ratio over a substantial range by varying the percent of overall conversion of l-olefin to oligomer that is accomplished in each stage. At substantially complete overall conversion, the trimer to tetramer ratio is increased as the percent of conversion in the tank reactor is decreased and the conversion in the tube reactor is increased. Furthermore, the trimer to tetramer ratio can be further increased at low conversion in the tank reactor if the amount of conversion in the tube reactor is also reduced. This procedure of less than complete overall conversion is less preferred when substantial unreacted l-olefin must be separated out and recycled unless the demand for the very high trimer to tetramer ratio warrants this additional expense.

The relative amount of conversion undertaken in the first-stage tank reactor can be controlled by several techniques used alone or in combination. A convenient way, particularly in a relatively large reactor, is by raising or lowering the liquid level in the tank reactor. This can be done without changing the input or outflow rates. When the reactor liquid level (i.e., volume) is increased at constant inflow, the percent conversion in the tank reactor increases and when it is decreased, the percent conversion decreases. The conversion in the first stage can also be controlled by changing the flow rate through the first reactor at constant liquid volume, the higher the flow rate, the lower the conversion. This technique possesses the potential disadvantage that it changes the flow rate through the second reactor and may affect the overall conversion particularly if the flow rate is increased. The percent conversion in the first reactor can also be controlled by reducing the reaction rate and this can be done by reducing the amount of co-catalyst, and consequently the amount of catalyst complex, below the optimum level. However, this technique may require the addition of co-catalyst to the input of the tube reactor for efficient utilization of the tube reactor.

Another method for reducing the conversion in the first reactor involves the introduction of an inert diluent into the first reactor. The inert diluent preferably is easily separated from the products and unreacted l-olefin and can suitably be a hydrocarbon such as pentane, hexane, heptane and the like. This inert diluent technique is less preferred in the sense that the diluent must be separated out and it involves a less efficient use of the equipment. On the other hand, as noted above, an inert diluent can desirably help control the reaction temperature in the first stage by its diluting effect. The inert diluent desirably is not used in an amount greater than 80 weight percent of the total reaction liquid and preferably no greater than about 50 weight percent of the total reaction liquid. We are able to carry out the process very effectively with no inert diluent.

In carrying out our process the percent of total conversion carried out in the tank reactor can be as low as about 15 percent, however, a conversion of at least about 30 percent is preferred and at least about 40 percent is more preferred for effective mixing and heat dissipation in the first stage. The percent conversion in the first stage can be as high as about 90 percent of the total conversion with a maximum of about 80 percent being preferred for the production of less viscous product at high conversion with still lower conversions accomplished in the first stage in order to increase the trimer to tetramer ratio. The first stage can include two or more tank reactors in series, particularly when relatively high conversions in the first stage are desired. When lower first stage conversion is desired with a multiple tank reactor configuration, the additional first stage tank reactors can be bypassed.

In general, it is preferred to operate the process at substantially complete overall conversion for most efficient operation. However, if very high trimer to tetramer ratios are desired, the overall conversion can be reduced down to as low as 50 percent or lower with unreacted l-olefin separated and recycled. When possible, it is usually preferred to obtain the desired trimer to tetramer ratio by adjusting the proportion of the overall conversion carried out in each reactor in accordance with this invention at a relatively high overall conversion.

In general the dimer is substantially removed from the oligomer product in order to avoid vaporization loss from the functional fluid during use. However, in some uses such as those involving low temperatures or a closed system, a substantial amount of the dimer can be left in the final product provided that the product viscosity specification can be met. But since this dimer will affect the product viscosity, the overall trimer to tetramer ratio to be obtained by the present invention must take into consideration the dimer, if any, that will be left in the final product. The minor amount of pentamer and higher oligomer, if any, is, in general, left in the product. Although the pentamer plus fraction generally is less than about 15 weight percent of the total oligomer product, its effect on product viscosity is significant. The pentamer itself comprises the great preponderance of this fraction with a very small quantity of hexamer and possibly heptamer being present.

The oligomer product is hydrogenated to stabilize it and protect it from oxidative degradation either before or subsequent to dimer removal, but preferably before dimer removal since the hydrogenated dimer is itself useful. Conventional hydrogenation catalysts such as palladium, platinum, nickel and the like at a suitable elevated temperature and pressure for hydrogenation, all conventional in this field, are satisfactory.

A first stage reactor having an effective volume of 580 cc. was equipped with an externally-driven bladed stirrer which vigorously agitated the reactor contents. This reactor contained internal cooling coils. The outlet line from this reactor led directly to a coiled tube reactor one-fourth inch (6.4 mm) in internal diameter and 30 feet (9.15 m.) in length and having a volume of 290 cc. This tube reactor was immersed in a temperature controlled bath.

The l-olefin used in this series of experiments was a purified l-decene which contained about two percent non-reactive impurities. A stream of this l-decene and a second stream of l-decene containing 0.6 weight percent n-butanol was introduced into the stirred tank reactor to fill about 60 percent of the effective reactor volume. This reactor was pressured with boron trifluoride gas at a constant pressure of 50 psig. (3.52 Kg/cm.sup.2). During steady state operation the temperature in the stirred tank reactor was maintained at 110.degree. F. (43.3.degree. C.) and the temperature in the tube reactor was maintained at 120.degree. F. (48.9.degree. C.). The reaction rate was varied for each experiment by varying the flow rate of the two feed streams at a substantially constant overall liquid feed rate of about 1,300 .+-. 100 cc. per hour thereby, varying the overall amount of n-butanol introduced into the reactor. The following Table I sets out the results of a series of experiments carried out in this manner showing the product analysis from each reactor in each run and the respective conversions.

It is noted from these experiments, in comparing 4a with 1b and comparing 6a with 3b, for example, that the trimer to tetramer ratio is substantially higher and the pentamer content substantially lower, both factors serving to decrease product viscosity, at a particular total conversion in utilizing the multi-stage process of this invention in comparison with the trimer to tetramer ratio and the pentamer content obtained from a continuous tank reactor at the same overall conversion. Thereby the present reactor combination provides a very useful method of varying the component proportions within the oligomer and thereby the product viscosity, by controlling the percent of conversion taking place in each stage and by controlling the amount of total conversion.

It is to be understood that the above disclosure is by way of specific example and that numerous modifications and variations are available to those of ordinary skill in the art without departing from the true spirit and scope of the invention.