Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 675-680
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Fixed-Bed Catalytic Process To Produce Synthetic Lubricants from Decene-I Ajay M. Madgavkar and Harold E. Swlft" Gulf Research and Development Company, Pittsburgh, Pennsylvanla 15230
The oligomerization of a-olefins to form functional fluids is well documented. I n particular, hydrogenated decene-1 oligomers form synthetic lubricants having excellent viscosity-volatility relationships and viscosity indices. Such oligomer fluids are especially adapted for use under exceptionally rigorous conditions. Various types of catalysts have been used to oligomerize olefins, with complexed BF, being the most widely cited catalyst. The use of complexed BF, does cause problems for the commercial production of synthetic lubricants. Complexed BF, cannot be reused and has to be disposed. Both factors can significantly contribute to the product cost. I n search of a way to minimize the cost for producing synthetic lubricants, the use of a fixed-bed catalyst surface with gaseous BF, was investigated. A major goal was to recycle as much BF, as possible and not have any complexed BF, for disposal. I n the novel process reported In this paper, decene-1 can be oligomerized to produce fluids with excellent lubricant properties by use of a siilca-BF,-water catalyst. With this system, much of the BF, can be recycled, thus minimizing BF, consumption and disposal problems.
Introduction The oligomerization of a-olefins to form functional fluids is well documented. In particular, hydrogenated decene-1 oligomers form synthetic lubricants having excellent viscosity-volatility relationships and viscosity indices (Cupples et al., 1977). Such oligomer derived fluids are especially adapted for use under exceptionally rigorous conditions, such as hydraulic fluid or an engine lubricant in jet aircraft involving both very high and very low temperatures. These oligomer fluids are particularly suitable for general use in an arctic environment. Various types of catalysts have been used to oligomerize olefins, with complexed BF, being the most widely cited catalyst. It has been reported that pure BF, is not an active catalyst, except when traces of moisture are present (Brennan, 1976). For less reactive olefins, such as decene-1, a useful degree of oligomerization is achieved only by adding a measurable quantity of an activator such as water or a primary alcohol to BF,. Other BF3 activators, such as esters, ketones, acids, anhydrides, etc., have also been claimed to form good a-olefin oligomerization catalysts (Brennan, 1968). The use of complexed BF, does cause problems for the commercial production of synthetic lubricants. Complexed BF, cannot be reused and creates a disposal problem. Both factors can significantly contribute to the product cost. In search of a way to minimize the cost for producing synthetic lubricants, the use of a fixed-bed catalyst surface with gaseous BF, was investigated. A major goal was to recycle as much BF, as possible and not have any complexed BF, for disposal. The use of BF, in conjunction with various solids has been reported for oligomerizing lower olefins. For example, the dimerization of propylenes to tetramethylethylene was accomplished by using BF, along with silica, clays, and other materials (Davidson, 1953). The polymerization of isobutylene was also reported by use of a BF341ica catalyst system (Child et al., 1963). In another case, a BF,-hydroxycarboxylic acid complex on carbon was used to oligomerize olefins to liquids boiling in the gasoline range (Bohlbro, 1955). I t is believed that none of these systems ever reached commercialization. However, in the commercialized Alkar Process, developed by Universal Oil Products, alkylation of benzene with 0 196-4321/83/ 1222-0675$01.50/0
ethylene, propylene, or butenes is achieved by using a BF,-alumina catalyst with BF3 continually added to the feedstock (Grote, 1958). The Alkar Process is designed to minimize the formation of undesirable olefin oligomers. In the novel process reported in this paper, decene-1 is oligomerized to produce fluids which after hydrogenation possess excellent lubricant properties, while minimizing BF3 consumption and disposal problems. Experimental Section Reactor System and Procedure. A schematic of the reactor unit and ancillary equipment is shown in Figure 1. The Autoclave Engineers' reactor was made from 316 high-pressure stainless steel tubing with dimensions 1/2 in. i.d., 3/4 in. 0.d. and 24 in. length. A thermocouple well of in. 0.d. and 20 in. length was positioned at the center of the reactor. Thus temperature measurements along the length of the reactor were conveniently made by properly locating a thermocouple. The feed was pumped into the reactor with a Milton Roy minipump from a 4-gal steel tank. The moisture content of the feed was measured in a continuous manner with an on-line Parametrics Model 2000 electronic hygrometer. BF3 gas was introduced directly into the reactor by use of a Matheson 8240-0422 electronic mass flow controller unit. The reactor pressure was controlled with a Research Valve and an Ametek Pressure Pilot. The reactor temperature was maintained with the help of a coolant pumped continuously through a cooling jacket. The coolant was a 50-50 solution of ethylene glycol-water externally cooled to about -10 OC in a refrigerated stirred tank and pumped via a high throughput centrifugal pump through the reactor jacket. Finally, the product was collected in a 500-mL steel receiver and the BF, discharged and liquid product collected at various times. A number of batches of fresh silica was usd in the reactor to eliminate possible catalyst deactivation, especially when the effects of reaction variables were studied. However, this proved to be an unnecessary precaution since later results showed no apparent aging of the BF,-silica-water catalyst system. For each fresh batch of silica placed in the reactor the following procedure was used at the start of its first run. The coolant recirculation was initiated and the reactor was 0 1983 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 ?--?
70
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Figure 1. Schematic of packed-bed flow system.
sufficiently cooled in anticipation of an exotherm. At this point the silica was exposed to BF, vapors at the present flow rate and pressure for about 30 min. Following BF3 exposure, the decene-1 feed was introduced at the desired flow rate while maintaining the BF, flow. This caused the bed inlet temperature to rise instantaneously due to the exothermic reaction. The rest of the reactor was maintained at about the initial temperature. In time this abnormally high hot zone cooled to a nominally constant (but still above an average reactor temperature) value. The conversion and product composition also appeared to "line-out" along with this thermal gradient along the reactor. This is referred to as a "steady-state" condition. In most cases such a state was obtained after about 20 h on-stream. Most of the key process variable effects were obtained under such steady-state conditions. In subsequent runs with a given batch of silica, no such BF3 exposure was used, both feeds being simultaneously introduced. To study the aspects of BF3 recovery from the product, the above system was modified by going to a packed column instead of the product receiver, as shown in Figure 1. The packed column was a 1 in. diameter glass tube about 24 in. in height and contained 11/4-in.Berl Saddles packed to a height of roughly 18 in. The column was operated under vacuum and heated externally in a controlled manner using heating tape and a Variac controller. In the experiments to evaluate BF3 separation, the reactor pressure was 100 psig with a decene-1 feed rate of 75 cm3/h. Materials. Gulf decene-1 was used in this study. Typically it contained about 98.5 wt % decene-1, the balance being saturates and other olefins. Whenever necessary it was dried with 4-A molecular sieves. Dry BF3 was obtained from Allied Corp. Catalysts. Davison Grade 59 silica (BET surface area of 248 m2/g and average pore radius of 90 A) was used in this study. The silica was calcined at 1000 OF, and after sizing to the desired mesh size it was elutriated to reduce fine particles. Care was taken to minimize moisture absorption by silica from air in its preparation as well as during packing of the reactor. This was achieved by avoiding contact with air and by maintaining a dry nitrogen atmosphere. Later studies revealed that such precaution was not necessary. Different mesh sizes were used in the study and the actual particle sizes employed are reported with the corresponding results. Analysis. Product analyses were carried out with a Varian Model 3700 or a Hewlett-Packard Model 5830A gas chromatograph equipped with thermal conductivity detectors. A 3 f t X in. stainless steel column packed with 5% Dexsil300 on 80/100 Chromosorb P was used. Temperature programming was used with an initial value of
i :301 c 40
0.2 PPM MOISTURE
4
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 677
at 15 ppm water also had a pronounced effect on conversion as shown in Figure 2. At very high BF3 concentrations, the conversion leveled off at about 65%. At a moisture level of less than 10 ppm, the conversion significantly declined, even at a BF3 pressure of 250 psig. Upon increasing the water to 25 ppm and pressure to 250 psig, the conversion rose to 75%. In general, what is considered a desirable product distribution was obtained. These results confirm published reports that pure BF3 is a poor catalyst, except when a small quantity of water is present (Brennan, 1976). In our case, a minimum water level of around 15 ppm seemed essential for an active catalyst. Even with such a catalyst a long reaction time of about 3 h was needed to achieve a high decene-1 conversion. In the absence of water in the decene-1, the BF3-silica catalyst rapidly deactivated from an initial high activity level. The catalyst half-life, which depended mainly on the reaction conditions, was of the order of only 1to 3 days. For example, a fresh batch of 40/50 mesh silica gave a decene-1 conversion of 95% at 250 psig, 11"C, 0.5 LHSV, and 2.31 weight % BF3 in dry decene-1. The product quality was also attractive with selectivities of 17% dimer, 55% trimer, 18% tetramer, and 10% pentramer. Within a few hours the conversion started to decline, while the product distribution was unchanged. As the catalyst activity continued to decrease, significant changes in the product selectivity occurred with more dimer being formed. At the end of 63 h, the conversion dropped to 33% with selectivities of 41% dimer, 51% trimer, 6% tetramer, and 2% pentamer. The production of dimer is an indication of catalyst deterioration. This rapid aging could be temporarily improved by changing reaction conditions such as increasing pressure, decreasing space velocity, or temperature. However, this response did not last long and a continued catalyst deactivation occurred. After a few process changes, the catalyst reached a stage where additional changes had no effect on performance. A detailed mechanistic study of catalyst decay was beyond the scope of this research; however, some information was obtained to better understand this deactivation. By studying conversion-time as well as selectivity-time relationships, it appeared that the deactivation could be explained due to a gradual loss of active sites from silica. Pore plugging of silica due to the adsorption of heavy oligomers can cause this. If this is the case, it should be possible to detect the presence of polymers on the silica. Some deactivated silica was analyzed by recovering the hydrocarbons from this silica by solvent treatment and analyzing for the percentage of heavy oligomers (C, plus). The deactivated silica was successively treated with water, methylene chloride, and tetrahydrofuran, and the organic fractions were then combined and concentrated. This fraction was analyzed by gel permeation chromatography (GPC) using a column set of lo4 A, lo3 A, 500 A, and 50 A with tetrahydrofuran as the mobile phase. The resultant chromatogram of a calibration standard containing less than 1% of C40+oligomers was also analyzed (see Figure 3). By comparing chromatograms it can be seen that the fraction obtained from deactivated silica had substantial amounts of C40+ oligomers. In fact, roughly 17% of the olefins were C40+, with a high molecular weight tail up to 9000. This molecular weight estimate was based on polystyrene standards. The solvent-treated silica was analyzed to be free of carbon, indicating that the solvent treatment was successful in removing all hydrocarbons. The surface areas and pore size distributions of the washed and fresh silica were the same,
Figure 3. Chromatograms of (A) product containing less than 1% C+ , oligomer and (B) product from deactivated silica.
indicating that extensive catalyst aging resulted in no adverse effects. Thus, deactivation due to pore plugging from heavier oligomers appears to be a plausible explanation of catalyst deactivation. By maintaining a low concentration of water in the feed, little catalyst deactivation was observed with the highly active BF3-silica catalyst. For example, with 40150 mesh silica, at 250 psig, 27 "C, 4 LHSV, 3.1 wt % BF,, and about 30 ppm water in decene-1, a steady-state decene-1 conversion of 87% was achieved. In another experiment, a steady-state conversion of around 89% was achieved with 40/50 mesh silica at 150 psig, 11"C temperature, 5 LHSV, 42 ppm water, and 3.14 wt % BF3. The higher activity of this catalyst system was further exemplified by achieving a lined-out conversion of 70% at 10 LHSV, 85 psig, and 11"C. These results were all obtained under steady-state lined-out conditions after several hours into the run. There was no indication of catalyst deactivation as long as the feed contained water. Some of the experiments were run for three weeks without any indication of activity loss. Subsequent pilot plant studies confirmed that a very long life can be achieved with the BF3-water-silica catalyst. Excellent product selectivities were also obtained from all of these experiments. A determination of the role of water is also beyond the scope of this paper; however, a comment of its possible role or roles is appropriate. The silica most likely enhances activity because of the high concentration of surface hydroxyl groups, which could result in the formation of a catalytically active BF3-OH complex. The water could serve to replenish such groups if such surface groups were lost as BF3-H,0 complexes. The water may also act to displace oligomers from the surface, which otherwise would continue to grow resulting in the observed catalyst deactivation. This is an area where basic surface studies could greatly aid in understanding this catalyst system. B. Effects of Process Variables. The process variables studied were reaction pressure, temperature, LHSV,
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Figure 4. Effect of reactor pressure on activity and selectivity of BF,-silica-water catalyst system. 100,
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Figure 6. Effect of moisture concentration on activity and selectivity: 20/30 mesh silica; 125 psig; 3 LHSV; 27 "C;3.23 wt % BF3 in decene-1. Table I. Results of BF, Removal Studies Using Berl Saddles Column Degasser System
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Figure 5. Effect of temperature on activity and selectivity.
feed moisture content, and BF, concentration. Although a certain minimum value of each variable was necessary for obtaining high conversion and attractive product distribution, it was found that these were most sensitive to pressure and temperature. The effect of pressure on the catalyst conversion and selectivity was obtained using 40J50 mesh silica at 5 LHSV, 2 "C average temperature, 3.14 wt % BF,, and 40 ppm water in decene-1. The results obtained are shown in Figure 4. At atmospheric pressure very little conversion was achieved. However, activity rapidly increased with pressure and at 85 psig about 86% conversion was obtained. Although conversion was strongly dependent on pressure over the range studied, product selectivity remained fairly constant. What is considered a good product distribution was obtained in all of these experiments. Reaction temperature had a significant effect on catalyst selectivity. These experiments were conducted with 20130 mesh silica at 125 psig, 3 LHSV, 40 ppm water in decene-1, and 3.23 wt % BF,. The four temperatures examined were 14, 25, 36, and 44 "C. These temperatures were the hot zones in each of the experiments. The average temperature across the bed was about 8 to 10 O C lower. It is assumed that most of the conversion took place within this hot zone. The results obtained are shown in Figure 5. The reaction was sensitive to temperature even within such a narrow zone, with activity declining at higher temperatures. The selectivity to dimer fraction showed the maximum sensitivity, rising from a low of 17% at 14 "C to about 51% at 44 "C. The selectivity to the desired trimer fraction declined correspondingly,dropping from 65% to about 45 7'0.
degasser pressure, in. Hg
degaser skin temp, "C
8 20 20 10 10 10
23 99 100 100 100 100
BF, concn, ppm
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27 686 27686 27686 27686 27686 27 686
6 80 3 46 61 2 1298 1051 804
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Tetramer as well as pentamer selectivities also decreased with increasing temeprature, though at lower rates. Although the presence of water was essential to maintain catalyst activity, an increase beyond a minimum value of around 20 to 30 ppm had little effect as shown in Figure 6. A similar trend was found for BF3 concentration in the feed. The minimum value of BF3 in the feed depended on other variables such as pressure and temperature. However, under most conditions about 1%BF3 was found to be adequate for maximum conversion and selectivity. An increase beyond this had no noticeable effect on either conversion or selectivity. Feed rate had a limited influence on conversion and product distribution within a reasonable range of values. That is, the effects were exaggerated only at very low (0.25 LHSV) or very high (10 LHSV) values. In general, a higher conversion and higher selectivity to heavier oligomers were obtained at lower feed rates, the opposite occurring at higher rates. This is consistent with the expected seriestype mechanism of decene oligomerization. C. Recovery and Analysis of BF, from Product. A major cost advantage for a fixed-bed BF, catalyst system compared to existing BF,-liquid cocatalysts is the potential to recover BF3 from the crude product for recycle. In addition, costs associated with expensive treatment facilities for fluoride removal and in some cases biotreatment schemes for cocatalyst removal could be eliminated. Thus, it was essential to demonstrate that a simple system could be discovered for efficient BF, recovery and that the recovered BF3 maintain its original activity. Recovery of BF3 was demonstrated with the apparatus as previously described in the Experimental Section. A number of experiments were conducted with varying degrees of vacuum and skin temperatures of the degasser column. Representative conditions and a summary of results, obtained at the 70 to 90% decene-1 conversion level, are given in Table I. The results indicate that up to 99% BF, removal from the reactor product is possible.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 679
Table 11. Comparison of Recycled BF, Activity with Fresh BF, reaction type batch (fresh BF,) semicontinuous (fresh BF,) batch (run no. 1) (recycled BF,) batch (run no. 2) (recycled BF,)
pressure, Ps& i! 125 125 100/56' 125/102a
temp, "C 23 17 16 16
time, h 2.0 2.0 2.0 2.0
% selectivity
conv, %
88.94 80.76 74.84 88.37
C*O 33.37 36.64 32.59 30.79
c 30 52.65 55.19 56.94 57.01
'40
12.68 7.71 9.73 11.56
C 50 1.30 0.36 0.74 0.65
' Denotes starting and final reactor pressures. Table 111. Yields and Properties of Oligomers and Finished Products yields and properties of synthetic fluids
yield and properties of oligomers after hydrogenation dimer
trimer
bottoms
13.7
44.2
25.6
2087 285 16.59 3.69
9073 914 35.06 6.10
yielda
_50.26 5.41 1.72
0 100 210
_-
viscosity index flash point, "F fire point, "F pour point, OF
310 350
