Effects of Hydrogen and 1-Butene Concentrations on the Molecular

Feb 15, 1997 - Alberta, Canada T6G 2G6 .... paratus (Chakravarty, 1993; Lacombe, 1995). Both ... analysis of the samples are provided by Lacombe (1995...
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Ind. Eng. Chem. Res. 1997, 36, 1136-1143

Effects of Hydrogen and 1-Butene Concentrations on the Molecular Properties of Polyethylene Produced by Catalytic Gas-Phase Polymerization James C.-K. Huang,† Yves Lacombe,‡ David T. Lynch, and Sieghard E. Wanke* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6

The effects of the variations in concentrations of hydrogen (0-480 mol/m3) and 1-butene (0380 mol/m3) on the gas-phase copolymerization of ethylene and 1-butene over a MgCl2/SiO2supported Ti catalyst were investigated using a semibatch reactor operated at 70 °C. Polymers were characterized by size exclusion chromatography (SEC), melt flow index, analytical and preparative temperature-rising elution fractionation (ATREF and PTREF), and PTREF-SEC cross-fractionation. Excellent correlations were obtained between the reactor operating conditions and polymer properties; e.g., the average polymerization rate was proportional to 1/(1 + a[H2]0.5), the methyl group concentration in the polymer was proportional to the 1-butene concentration, and the melt flow index varied as Mw-3.4. The most significant finding was that the hydrogen concentration dependence of the termination rate by hydrogen was different for different catalytic sites; the termination rate was first order for the catalytic sites responsible for the formation of copolymer and half-order for the sites responsible for the homopolymer component of the polymer. Introduction Polyethylene (PE) is the most widely used synthetic polymer, and linear low-density polyethylene (LLDPE) is capturing an increasing share of the polyethylene market. Gas-phase fluidized-bed processes for the production of LLDPE, such as the UNIPOL process developed by Union Carbide, have dominated the growth in the LLDPE production for the past 2 decades. The fluidized-bed gas-phase processes, using supported Ziegler-Natta catalysts, are very versatile and offer safety and environmental advantages over most other PE processes (Xie et al., 1994; Amundson, 1988; Schaper, 1990). Large quantities of low-cost LLDPE will continue to be produced using Ziegler-Natta catalysts in spite of the major advances that have occurred in the commercialization of processes based on single-site catalysts (e.g., metallocene catalysts). The desired molecular properties, such as the average molar mass, molar mass distribution, and comonomer content, of the LLDPE produced in gas-phase processes are obtained by the selection of an appropriate catalyst and operating conditions (temperature and hydrogen, ethylene, and comonomer partial pressures). The choice of catalyst and operating conditions for making an LLDPE with the desired properties (e.g., density and melt flow index) is usually based on experience. The recent review by Xie et al. (1994) clearly shows that there is a lack of information in the open literature on many of the fundamental aspects of gas-phase ethylene polymerization required for modeling these processes. In the current paper, we report a systematic study of the effects of 1-butene and hydrogen concentrations on the rates of catalytic polymerization, average molar masses, and 1-butene incorporation during the gasphase polymerization of ethylene. LLDPE samples with * Telephone: (403) 492-3817. Fax: (403) 492-2881. E-mail: [email protected]. † Present address: AT Plastics Inc., P.O. Box 428, Edmonton, Canada T5J 2K1. ‡ Present address: NOVA Chemicals Research & Technology Corp., 2928 16th St. NE, Calgary, Canada T2E 7K7. S0888-5885(96)00473-3 CCC: $14.00

different properties were produced in a semibatch gasphase reactor at various ethylene, 1-butene, and hydrogen concentrations; the reactor was operated at 70 °C for all polymerizations. The polymers produced were characterized by size exclusion chromatography (SEC), temperature-rising elution fractionation (TREF), TREFSEC cross-fractionation, and melt flow index (MFI). The polymerizations, carried out under well-defined conditions, provided information on the behavior of different catalytic sites as a function of hydrogen and monomer concentrations. Such information is required for the development of improved reactor models for gas-phase olefin polymerization. Different catalytic sites, as proposed by Usami et al. (1986), are responsible to the compositional heterogeneity of LLDPE made with Ziegler-Natta catalysts. Experimental Section Materials. Polymer-grade ethylene (from Matheson), ultrahigh-purity hydrogen (from Linde), and prepurified nitrogen (from Linde) were each passed through a series of Alltech high-pressure gas purifiers containing BASF R3-11 catalyst, Ascarite, and 5A molecular sieves for the removal of oxygen, carbon dioxide, and moisture. Liquefied 1-butene (from NOVA Chemicals) was used without further purification. A supported Ti-based catalyst, prepared in accordance with the procedure described by Karol et al. (1981), was used for all the experiments. Tri-n-hexylaluminum (TNHAL), obtained from Texas Alkyls, was used as the cocatalyst. Sodium chloride, with an average particle size of about 0.5 mm (from Fisher Scientific), was used for the seed bed in the reactor. Equipment and Procedures. The previously described stainless steel, 1-L, stirred reactor was used for all polymerization studies (Lynch and Wanke, 1991). Equipment details and complete operating procedures are described by Huang (1995). In brief, the procedure for the polymerizations began with placing a seed bed of 200 g of NaCl into the open reactor, attaching the reactor to the feed system, and pressure testing the reactor at room temperature with nitrogen at 2 MPa. © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1137

If the reactor was leak-free, it was then immersed into an oil bath at 90 °C and evacuated overnight. The next morning, the reactor was cooled to 70 °C, and ethylene was introduced into the reactor to a pressure of 0.2 MPa prior to injecting 1.0 cm3 of tri-n-hexylaluminum alkyl (TNHAL) with a Hamilton Gastight syringe. A pressure of about 0.2 MPa is the upper threshold at which injections with a syringe can be performed safely. The salt bed was stirred for 5-10 min after the injection of the TNHAL; the stirring was briefly stopped, the catalyst, suspended in heptane, was injected with a syringe, and ethylene was added until the total pressure in the reactor was 0.35 MPa. The pyrophoric catalyst and TNHAL were stored in a glovebox (Vacuum Atmospheres Ltd., Model HE 493); the filling of the syringes was done in the glovebox. After the catalyst had been injected into the reactor, the desired amount of 1-butene was pumped into the reactor with an ISCO 500D high-pressure syringe pump. Finally, the desired amount of hydrogen was added to the reactor, and then, ethylene was added to attain the desired total reactor pressure. The additions of catalyst, 1-butene, and hydrogen and the start of the ethylene flow were done in rapid succession to minimize the amount of polymer formation during the start-up procedure; the time between catalyst injection and ethylene addition to the final desired total reactor pressure was less than 4 min. Ethylene was added continuously to the reactor to maintain the reactor at the desired pressure throughout each run. Polymerization was allowed to proceed for 2 h for each run after the total reactor pressure had reached the desired value. In most runs, 1-butene was also added continuously with the syringe pump, in addition to the initial charge, in an attempt to maintain constant 1-butene concentrations in the gas phase. Periodic gas chromatographic analyses during the runs indicated that the maximum variations in the gas-phase 1-butene concentration during the runs were typically between 5% and 15%. Hydrogen was not added continuously during the experiment because the hydrogen consumption due to chain termination never exceeded 5% of the amount of hydrogen initially charged to the reactor. Characterization Methods. The molar masses of the polyethylenes were obtained by SEC with a Waters 150C GPC equipped with a differential refractometer and four in-series Shodex GPC/AT-800M/S columns. The columns and the detector were maintained at 140 °C, and the solvent, HPLC-grade 1,2,4-trichlorobenzene (from Fisher), was pumped through the columns at 1.0 cm3/min. Polystyrene samples of known molar masses (from TSK Standards), linear paraffins (C20, C40, and C60 from Fluka), and polyethylene reference materials 1475, 1482, 1483, and 1484 (from NIST) were used as standards for the molar mass calibrations. TREF analyses were done using a custom-built apparatus (Chakravarty, 1993; Lacombe, 1995). Both analytical TREF (ATREF) and preparative TREF (PTREF) were done; off-column crystallization was used for both (Wild, 1990). For the crystallization step, samples of polyethylene (4 mg for ATREF and 20 mg for PTREF) were dissolved in o-xylene (1 cm3 of o-xylene per mg of polyethylene) at 125 °C and stirred at 125 °C for 2 h. The dissolved samples were transferred to an Endcal RTE 220 bath/circulator and kept at 125 °C for another 2 h without stirring before starting the programmed cooling crystallization step. The cooling from 125 to -8 °C was done at a rate of 1.5 °C/h. After the

crystallization, the samples were stored in a freezer at -15 °C until they were used for TREF elution. Prior to the TREF elution, the crystallized polyethylene was transferred to a TREF column containing 100mesh glass beads (the inside diameters of the ATREF and PTREF columns were 6.0 and 10.9 mm, respectively). The packed column was introduced into the temperature-programmable chamber of the TREF apparatus. The initial temperature of the chamber was -10 or 25 °C depending on the nature of the sample; -10 °C was used for samples with high comonomer content. The elution was done with either 1,2,4-trichlorobenzene or o-dichlorobenzene. For most ATREF runs, the temperature was increased at a rate of 1.0 °C/min from the initial temperature to a final temperature of about 110 °C; the solvent flow rate for these runs was 0.5 cm3/min. Some ATREF runs were done at heating rates of 2.0 °C/min and solvent flow rates of 1.0 cm3/ min. For the PTREF experiments, stepped increases in temperature and an on-off flow of solvent procedure were used to generate fractions for the TREF-SEC cross-fractionation. Eight to 10 temperature steps, of unequal magnitude, were used to increase the chamber temperature from the initial to the final temperature; a typical set of temperature intervals for which PTREF samples were collected for subsequent SEC analysis was 30-50, 50-60, 60-70, 70-80, 80-85, 85-90, 90-95, 95-100, and 100-105 °C. Fractions at elution temperatures < 20 °C were only collected for samples with high 1-butene content, i.e., those produced during runs with total 1-butene charges > 17 g. Insufficient amounts of polymer for molar mass determination were present in the eluent below 30 °C for LLDPE made with 1-butene charges < 14 g. The details of the stop-flow method for the collection of PTREF fractions and subsequent SEC analysis of the samples are provided by Lacombe (1995). Linear paraffins (C40 and C60), polyethylene reference materials with narrow molar mass distributions from NIST (SRM1482, SRM1483, and SRM1484), and seven linear polyethylenes with narrow molar mass distributions prepared in our laboratory by PTREF were used for the TREF calibration. MFIs were obtained with a Kayeness Model 7053 melt flow indexer. Molten polyethylene at 190 °C was extruded for 10 min by a 2.16-kg load through a cylindrical exit 0.80 cm in length and 0.2096 cm in diameter (ASTM Method D 1238-94 Procedure A). MFI measurements were only done for samples with Mw < 2 × 105 because the melt was too viscous under the measurement conditions to extrude appreciable amounts of polymer in 10 min for higher molar masses. Results All the polyethylene samples examined in the current study were produced in the semibatch laboratory reactor under well-defined reaction conditions. The reactor temperature for all polymerization runs was 70 °C, and the sum of the ethylene and 1-butene partial pressures was 1.38 MPa for all runs. The hydrogen partial pressures were varied from 0 to 1.38 MPa in the “hydrogen series”, and the total 1-butene charges to the reactor were varied from 0 to 23.1 g in the “1-butene series”. The range of hydrogen partial pressures corresponds to a range of hydrogen concentrations, assuming ideal gas behavior, of 0-480 mol/m3. The estimated range of 1-butene gas-phase concentrations was 0-380 mol/m3. The amount of 1-butene in the reactor was

1138 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997

Figure 1. Typical activity profiles at various monomer and hydrogen concentrations (T ) 70 °C).

Figure 2. Average polymerization rate as a function of hydrogen concentration.

Table 1. Reactor Operating Conditions for Polymerization Runs (T ) 70 °C for All Runs)

Table 2. Reproducibility of PE Yields and Molar Masses PH2, kPa

amt of 1-butene charged, g PH2, kPa

Ptot, MPa

0 70 138 207 276 414 552 690 1035 1380 690 690 690 690

0

initially

during run

amt of catalyst, g

av PE yield, g

1.38 1.45 1.52 1.59 1.66 1.79 1.93 2.07

5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1

4.2-8.1 0-4.9 2.3-4.9 5.1 3.1-6.2 3.1 2.9 3.1

2.41 2.76 2.07 2.07 2.07 2.07

5.1 5.1 0 2.6 10.3 15.4

0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.06 0.06 0.06 0.06 0.06 0.06 0.06

72.2 48.2 46.0 37.0 43.3 50.9 38.2 40.6 36.3 48.5 32.2 69.7 55.1 68.5 49.3

1.2 1.5 0 1.1 6.8 7.7

calculated by subtracting the amount of 1-butene incorporated into the polymer from the amount charged to the reactor; the 1-butene concentration was taken as the amount of 1-butene in the reactor divided by the gas-phase volume in the reactor; no correction was made for the absorption of 1-butene by the polymer. Absorption of 1-butene by the LLDPE made in the reactor can be substantial (Hutchinson and Ray, 1990); hence, the actual gas-phase 1-butene concentrations were lower than the estimated values. The ranges of reactor operating variables used for producing the various polyethylene samples are summarized in Table 1. The amounts of catalyst used and polymer produced in each 2-h run are also listed in Table 1. The effects of the variations in the operating conditions on the rates of polymerization and polymer properties are presented below. Kinetic Behavior. Typical reaction rate profiles, as measured by the rate of ethylene addition to the semibatch reactor, are shown in Figure 1. In general, increases in the hydrogen concentration caused decreases in the instantaneous polymerization rates. The effect of hydrogen concentration on the average rates of polymerization is shown in Figure 2. The reproducibility of the total PE yield for repeat experiments was relatively poor (see Table 2). The largest variation in yields was a factor of 2, as shown by the last two rows in Table 2. The possible causes of the relatively poor reproducibility in total PE yield are variation in the quantity of injected catalyst (it is difficult to ensure

70

138

690

total C4H8 fed, g

PE yield, kg/g catal

9.3 13.2 5.1 6.3 7.2 10.0 7.4 7.6 8.1 8.6 10.0 8.2 8.2

2.26 2.57 1.36 1.37 2.22 1.51 1.40 1.61 1.81 1.67 1.23 1.35 0.61

10-3Mn

10-3Mw

106 113

612 570

71 56

260 228

37 43 17 16

158 168 67 68

homogeneous catalyst concentration in the catalyst-inheptane suspension), encapsulation of catalyst in crevices and fittings in the reactor internals, and impurities in the reactor resulting in catalyst poisoning. The most probable cause of rate variations was the irreproducibility of the catalyst injections. However, repeat reactor runs produced polymers with essentially the same average molar masses even if the total polymer yield differed significantly (see Table 2). Two relations for the dependence of the rate of polymerization presented by Keii (1986) were used to fit the data in Figure 2. The results of the fit are given by

Rp )

Rp0 1 + 0.0820[H2]0.5

Rp ) Rp0 - 44.3[H2]0.5

(1) (2)

where Rp0 is the rate of polymerization in the absence of hydrogen; it is equal to 1207 (g of PE) h-1 (g of catalyst)-1 in the current study. Equation 1 fits the data well at low hydrogen concentrations but predicts rates which are too high at high hydrogen concentrations (solid line in Figure 2). Equation 2 overestimates the rates at low hydrogen concentrations but fits the rates at high concentrations reasonably well (dashed line in Figure 2). The trend of the fits by eqs 1 and 2 suggests that a linear combination of eqs 1 and 2 would result in an improved fit. Equation 3, and the corresponding line in Figure 2, shows this fit.

Rp )

0.596Rp0 1 + 0.0820[H2]0.5

+ 0.404(Rp0 - 44.3[H2]0.5) (3)

Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1139

Figure 3. Average polymerization rate as a function of 1-butene concentration.

Figure 4. Variation in average molar masses as a function of hydrogen concentration.

The fit by eq 3 describes the observed trend in the rate well; this is an indication that different catalytic sites respond differently to hydrogen. Such differences in rate behavior for different catalytic sites should be taken into account in the modeling of polymerization reactors. The effect of changes in 1-butene concentration, while keeping the sum of the 1-butene plus ethylene concentration approximately constant, is shown in Figure 3. It is surprising that the average polymerization rate is rather insensitive to changes in the 1-butene-to-ethylene ratio since the reactivity of 1-butene for polymerization is much lower than that of ethylene (Quijada and Wanderly, 1986). The increased solubilities of both ethylene and 1-butene in copolymers formed in the presence of 1-butene are a possible cause of the relative insensitivity of the polymerization rate to the ethylene/ 1-butene ratio. Molar Masses. The molar masses, melt flow indices, and short-chain branching concentrations were determined. The effect of hydrogen concentration on the number-average (Mn) and mass-average (Mw) molar masses is shown in Figure 4. Mw and Mn decreased by over an order of magnitude when the hydrogen concentration was increased from 0 to about 500 mol/m3. The polydispersity (Mw/Mn) was relatively constant at 4.3 ( 0.6 for hydrogen concentrations below 250 mol/m3, and it increased to 5.8 and 7.2 at hydrogen concentrations of 360 and 490 mol/m3. This result suggests that different catalytic sites may respond differently to chain termination by hydrogen. Changes in the 1-butene-toethylene ratios do not have a large effect on molar masses as shown in Figure 5. The polydispersity also

Figure 5. Variation in average molar masses as a function of 1-butene concentration.

Figure 6. Fitting of hydrogen termination kinetics for the whole polymer.

remained fairly constant at 4.4 ( 0.7 for the variations in the 1-butene concentration from 0 to 380 mol/m3. The dependence of Mn on hydrogen concentration is usually correlated by (Jaber and Ray, 1993)

1 ) a + b[H2]n Mn

(4)

The order with respect to [H2] is usually taken as 0.5 or 1.0. The first-order dependence describes our data of Mn as a function of [H2] much better than a half-order dependence. The solid line in Figure 6 shows the fit with n ) 1.0 (a ) 1.02 × 10-5; b ) 2.35 × 10-7 m3/mol); the dotted line is the fit with n ) 0.5 (a ) -1.00 × 10-5; b ) 5.27 × 10-6 (m3/mol)0.5). At the lower hydrogen partial pressures, there appears to be a trend toward n < 1.0; this aspect of the effect of hydrogen on Mn will be discussed below in the section on PTREF-SEC crossfractionation. Molar masses are frequently measured as part of polyolefin characterizations in research studies; however, it is not common to use molar mass determinations for quality control in the commercial production of LLDPE. The MFI is the property commonly used for commercial quality control. It is difficult, according to Xie et al. (1994), to relate the MFI to polymerization conditions, and the correlation between the MFI and molar masses is usually poor for a variety of reasons. In the current study, an excellent correlation was observed between Mw and MFI (see Figure 7). Bremner et al. (1990, 1991) formulated an approximate relationship between MFI and Mw which states that the reciprocal of the MFI should be propor-

1140 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997

Figure 7. Correlation of melt flow index with mass-average molar mass.

Figure 9. Effect of 1-butene concentration on the ATREF profiles.

Figure 10. ATREF profile for LLDPE made at high 1-butene concentrations ([1-C4H8] ) 380 mol/m3).

Figure 8. Effect of hydrogen concentration on the ATREF profiles.

tional to Mw raised to the power of 3.4-3.7. The lines in Figure 7 show these suggested dependencies of MFI on Mw; the solid line is given by eq 5 and the dotted line by eq 6. The fit by eq 5 is better than that by eq 6.

( ) ( )

Mw 1 ) 8.52 × 10-8 MFI 1000

3.4

Mw 1 ) 1.87 × 10-8 MFI 1000

3.7

(5) (6)

Our results show that the proposed correlation provides an excellent fit for a family of polymers produced with the same catalyst under similar reactor conditions. Methyl Group Concentrations. The concentration of CH3 groups in the copolymers was determined by ATREF. ATREF profiles for the hydrogen and 1-butene series are shown in Figures 8 and 9. The profiles display the typical bi- and multimodal patterns common for LLDPE. High 1-butene concentrations yield a polymer with a multimodal ATREF profile; this is more evident in Figure 10 in which the bottom profile from Figure 9 is plotted on an expanded scale. The ATREF elution temperature, Tel, is related to the methyl group concentration of the polymer. The meth-

ylene sequence length (MSL) calibration method of Bonner et al. (1993) was used for converting Tel to the methyl group content. The following calibration equation, relating Tel and MSL, was obtained using the previously mentioned commercial and laboratory-prepared standards.

Tel )

[

]

ln(MSL) 1 + 0.006 583 374.1 MSL

-1

(7)

For linear polyethylene, the relationship between the methylene sequence length and methyl group concentration, expressed as CH3/1000 carbons, is given by eq 8.

CH3 2000 ) 1000 C MSL + 2

(8)

Equation 8 was used to convert methylene sequence lengths to methyl group concentrations for the LLDPE samples even though this relationship may not be valid for LLDPE. If the intramolecular methylene sequence lengths are all equal, then estimates of methyl group concentrations using eq 8 may be too high by as much as a factor of 2. For the random intramolecular distributions of methylene sequence lengths, as found in LLDPE made with Ziegler-Natta catalysts, eq 8 appears to be a reasonably good correlation since the

Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1141

Figure 11. Methyl group concentration as a function of 1-butene concentration.

Figure 12. Distribution of PE in various ATREF temperature ranges as a function of hydrogen concentration.

correlation between ATREF elution temperature and methyl group concentration reported by Wild et al. (1982) is very similar to the correlation obtained by a combination of eqs 7 and 8 (Lacombe, 1995). We plan to further investigate the relationship between the methylene sequence length and methyl group concentration since the crystallization-dissolution separation used in TREF depends on methylene sequence lengths (Bonner et al., 1993). The average methyl group content, obtained by integration of the ATREF profiles along with the above calibration equations, was found to be relatively independent of hydrogen concentration. The CH3 content for the hydrogen series (constant 1-butene) was 6.3 ( 1.3 CH3 per 1000 C over the range of hydrogen concentrations from 0 to 490 mol/m3. However, increases in 1-butene concentration, as shown in Figure 11, had a marked effect on the CH3 content. The CH3 content of the LLDPE varied linearly with 1-butene concentration. The mass fractions of PE in various ATREF temperature ranges can be obtained by integration of the ATREF profiles. The results of such integrations are summarized in Figures 12 and 13 for the hydrogen and 1-butene series. The mass fractions of the polymers in the various ATREF temperature ranges were essentially constant for all hydrogen concentrations in the 25-490 mol/m3 range (Figure 12). Exploratory experiments at higher hydrogen concentrations (600 and 720 mol/m3) indicate that the mass fraction in the high-temperature PTREF fraction decreased to less than 0.25. However, the ethylene/1-butene ratios were not the same in these high hydrogen concentration runs. In the absence of hydrogen, the mass fraction in the high-temperature

Figure 13. Distribution of PE in various ATREF temperature ranges as a function of 1-butene concentration.

Figure 14. Mass-average molar masses for various PTREF cuts as a function of hydrogen concentration (see eq 9 for definition of Rw,i, j).

ATREF ranges was higher than in the presence of hydrogen. The response to 1-butene was considerably different (Figure 13). Increases in the 1-butene concentration resulted in large decreases in the mass fraction of the high-temperature ATREF peak; the lowtemperature fractions had corresponding increases. Mass fractions in the intermediate temperature ranges, such as 85-90 °C, initially increased and then decreased with increasing 1-butene concentration (Figure 13). TREF-SEC cross-fractionation experiments were done to obtain information on the molar masses as a function of TREF elution temperature. PTREF-SEC Cross-Fractionation. Polymer samples were separated by PTREF into 8-10 fractions, and the molar masses of PE in each fraction were measured by SEC. For comparison of trends, the molar masses were normalized. For the hydrogen series, molar masses in a specific temperature fraction were normalized with respect to the molar mass of the PE produced at a hydrogen concentration of 25 mol/m3; i.e.,

Rw,i, j ) Mw,i, j/Mw,i,1

(9)

where Mw,i, j is the mass-average molar mass for the ith PTREF temperature fraction at hydrogen concentration j, and Mw,i,1 is the mass-average molar mass for the ith PTREF temperature fraction at a hydrogen concentration of 25 mol/m3. The normalized values Rw,i, j are plotted as a function of hydrogen concentration in Figure 14. The values of Mw,i,1 for the 20-50, 70-80, 85-90, and 95-100 °C ranges were 103 300, 159 800, 198 100, and 244 600, respectively. The interesting, and unexpected, result shown in Figure 14 is that the trend in Mw for the 95-100 °C PTREF fraction is significantly

1142 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 Table 3. Parameters (Equation 4) for Describing Termination with Respect to Hydrogen for Various PTREF Fractions n)1

n ) 0.5

PTREF fraction, °C

105a

m3/mol

corr coeff.a

105a

107b, (m3/mol)0.5

corr coeffa

30-50 50-60 60-70 70-80 80-85 85-90 90-95 95-100

4.14 4.09 2.77 2.33 1.94 1.69 1.52 1.36

4.96 3.61 3.08 1.95 1.69 1.24 0.60 0.30

0.94 0.94 0.95 0.98 0.97 0.96 0.87 0.84

0.03 0.93 0.07 0.51 0.36 0.48 0.87 1.00

109.8 81.4 69.5 45.0 39.0 29.0 14.6 7.6

0.82 0.85 0.86 0.93 0.93 0.94 0.93 0.96

a

107b,

Corr coeff ) correlation coefficient (r2).

Figure 16. Fitting of hydrogen termination kinetics for hightemperature PTREF fractions.

Figure 15. Fitting of hydrogen termination kinetics for lowtemperature PTREF fractions.

different from that of the other fractions. The sensitivity of Mw to hydrogen, at high hydrogen concentrations, is much lower for the 95-100 °C fraction than for the other PTREF fractions. This is an indication that hydrogen-transfer mechanisms are different for the catalytic sites responsible for homopolymer formation (TREF fractions eluted at high temperatures) vs those sites responsible for copolymer formation. To further investigate the hydrogen termination kinetics, the relationships between Mn of the various PTREF fractions and hydrogen concentration were examined. The Mn-[H2] data were fit, by linear regression, to eq 4 for first- and half-order hydrogen dependencies; the parameters of the fits are summarized in Table 3. The results in Table 3 clearly show that hydrogen termination kinetics for the PTREF fractions eluted at temperatures e 85 °C are described well by first-order kinetics, while the fractions eluted at 90100 °C are described well by half-order kinetics. The first-order fit is slightly better than the half-order fit for the PTREF fraction eluted at 85-90 °C. Figures 15 and 16 show plots of the data and regression lines for five PTREF fractions; the parameters listed in Table 3, with the higher correlation coefficient, were used for plotting the lines. The trend in hydrogen termination kinetics shown by these results is clear; the hydrogen dependence of the chain termination kinetics decreases from unity for the low-temperature PTREF fractions to one-half for the high-temperature PTREF fractions. Hence, the catalytic sites responsible for the copolymerization of ethylene and 1-butene are more sensitive to hydrogen termination (first order) than those sites responsible for ethylene homopolymerization (halforder).

Different functional dependencies on concentrations for different catalytic sites must be incorporated into reactor models if these models are to be used for predicting, rather than correlating, the reactor behavior and polymer properties. Unfortunately, a significant amount of experimental work is required for each catalyst-monomer(s) system to determine the kineticproperties-reactor condition relationships. However, extensive PTREF studies may not be required if deconvolution of ATREF profiles, by such methods as proposed by Soares and Hamielec (1995), can be used to describe the behavior of different catalytic sites. We plan to examine these deconvolution methods with the objective of obtaining insight into the behavior of different catalytic sites. Summary and Conclusions The systematic experiments of this study have identified and quantified some of the complex behavior encountered in ethylene-1-butene copolymerization over a titanium catalyst supported on magnesium chloride and silica. The results showed that the reactor operating conditions can be correlated well with the properties of the whole polymer (e.g., the Mn, Mw-[H2], MFI-Mw, and [CH3]-[1-C4H8] correlations), but more importantly, information on the kinetic behavior of different catalytic sites was obtained from the crossfractionation studies. This is the first study, to the best of our knowledge, that provides quantitative evidence that the different catalytic sites have different functional forms for chain termination by transfer to hydrogen. Using lumped kinetics to fit the dependence of Mn on hydrogen concentration may not result in satisfactory fits if various sites follow different kinetics. A description of the overall polymerization and termination kinetics by simple lumped-parameter rate functions may not be sufficient for predictive reactor models. A detailed understanding of the kinetics of the various catalytic sites would be very valuable for optimization of operating conditions and for providing guidance for catalyst modification. Acknowledgment The support of this work by the Natural Sciences and Engineering Research Council of Canada and NOVA Chemicals Ltd. is gratefully acknowledged. We thank N. Bu for measuring the molar masses.

Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1143

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Received for review July 31, 1996 Revised manuscript received September 26, 1996 Accepted September 30, 1996X IE9604738

X Abstract published in Advance ACS Abstracts, February 15, 1997.