Macromolecules 1996, 29, 3103-3110
3103
A Novel Laboratory Slurry Reactor for Ethylene Polymerization Studies Jacek A. Szymura,† Ivo G. Dalla Lana,* Ryszard Fiedorow,‡ and Piotr A. Zielinski§ Department of Chemical Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada Received June 29, 1995; Revised Manuscript Received January 29, 1996X
ABSTRACT: A novel laboratory slurry reactor useful for studies of polymerization of olefins with solid catalysts such as Cr/silica is described. The operation of this reactor, in which gaseous impurities are minimized by purification of incoming gases and by in situ activation of the catalyst, has enabled the early stages of the homopolymerization of ethylene to be studied without interference from catalyst deactivation effects. By standardizing the conditions for the experimental runs, the reactor demonstrated its usefulness in isolating and assessing the relative physical roles of different silica supports. Four catalysts supported on different silica supports were compared, and their differences in promoting the polymerization were interpreted in terms of porosity, average particle size, and changes in specific surface area. During early polymerization, the fracturing of catalysts prepared by impregnation of the porous silica support ensures access to these original Cr sites, in the absence of additional concealed sites. At low yields, the polymerization process is clearly affected by the physical character of the silica support; however, at higher yields, the evidence suggests that other causes such as chemical effects are responsible for the continuing acceleration in rate. The porous Cr/silica catalyst tested did not fracture instantaneously; hence, current models of nascent polymer do not describe this process. The “hardcore” model of nascent polymer seems applicable to polymerization on the nonporous Cr/Cab-O-Sil catalyst tested.
1. Introduction To produce linear polyethylene, three classes of reactors have been employed: gas-phase, solution, and slurry types.1,2 In many laboratory studies of catalytic olefin polymerization, slurry reactors operated in a semibatch mode have generally been used because of their simpler construction and relative ease of operation (mixing and temperature control).3 Excellent heat transfer is obtained because of the liquid phase surrounding the catalyst-polymer particles and the good mixing/suspension of the particles in this hydrocarbon liquid medium. In the slurry mode, the polyethylene formed generally does not dissolve or swell. In laboratory-scale slurry reactors, often used to characterize catalyst behavior, operation at lower pressures is more convenient and simpler. One should recognize that the higher pressures and temperatures employed in industrial operations will affect the kinetics of the polymerization, and thus, studies involving simpler/different operating conditions in the laboratory reactor should have clearly defined objectives. Although slurry reactors have been extensively used in the laboratory, the majority of earlier studies have involved Ziegler-Natta type catalysts.4-6 Fewer results7,8 are available concerning slurry reactor studies with Phillips type catalysts (Cr/silica). Too often, these earlier studies fail to present important details concerning the reactor construction, its operation, or the conditions of the measurements being made. This paucity of detail makes it difficult to compare the differing experimental results reported by various workers. This paper will describe a novel slurry reactor and some operating details, e.g., catalyst activation, gas purifica* Author to whom correspondence should be addressed. † On leave from Technical University of Bydgoszcz, 85-326 Bydgoszcz, Poland. ‡ On leave from Faculty of Chemistry, A. Mickiewicz University, Poznan, Poland. § Present address: Cominco, Ltd., Trail, B.C., Canada. X Abstract published in Advance ACS Abstracts, April 1, 1996.
0024-9297/96/2229-3103$12.00/0
tion, and polymerization procedures, used to compare the performances of different Phillips-type catalysts. This new reactor operates in an open mode (continuous) for ethylene flows but in a closed mode (batch) for polymer-catalyst particles. More commonly, in earlier studies, ethylene was usually injected into the slurry reactor as needed to maintain the preset operating pressure but was not withdrawn unreacted during the course of a run. At high pressures, the closed mode operation probably is not subject to large swings in the ethylene pressure between consecutive ethylene injections. At low pressures, periodic injections of ethylene would likely result in substantial pressure variations. The use of an open mode for ethylene enables operation at a constant reactor pressure and a constant partial pressure of ethylene over the duration of the polymerization run. Gas phase inerts do not build up in the reactor vessel; however, some of the slurry liquid medium may evaporate into the ethylene gas phase, and the subsequent decline in liquid volume can limit the duration of the run. The kinetics may be interpreted somewhat more simply because the ethylene concentration in the gas phase and, presumably, its corresponding saturation concentration in the liquid phase remain constant even though the reaction rate may be changing with time. The novel reactor was constructed using high-temperature-resistant Vycor glass, which enabled the catalyst to be both pretreated and activated in situ within the reactor vessel and to be observed visually throughout the polymerization process. Careful purification of all gases used in the experiments before their admission to the reactor and contact with the catalyst prevented deactivation and ensured stability of the catalytically active sites throughout the run. Earlier studies9,10 using this new reactor have shown unique transient rate phenomena to be occurring during the early stages of ethylene polymerization using Cr/silica catalysts. This paper describes this novel slurry reactor and discusses conditions which influence © 1996 American Chemical Society
3104
Szymura et al.
Macromolecules, Vol. 29, No. 9, 1996
Table 1. Physical Properties of Silica Supports silica
BET surface area (m2/g)
total pore volume (cm3/g)
Davison Crosfield Ketjen A Ketjen B PQCS Cab-O-Sil
277 286 666 385 359 163
1.57 1.56 1.36 1.17 2.11 nonporous
the early transient phenomena and which are essential to ensure reproducible yield versus time measurements (kinetics). Yield curves were obtained for four different silica-based Cr catalysts. Earlier views of the nascent polymerization process have been reported by Wagner and Karol,11 Wagner and Niegisch,12 and Webb et al.,13 among others. Using our results, we will also attempt to relate to the three catalyst fragmentation mechanisms summarized by Webb et al.13 In a detailed paper, they describe three distributions of catalyst fragments within individual growing polymer particles: (1) a “hard-core” model in which the central catalyst particle remains unfragmented; (2) a “uniform-site” model in which fragmentation creates multiple catalyst particles uniformly distributed within the growing polymer particle; and (3) an “expanding core” model in which the fragments are convected outward within the growing polymer particle. In a series of papers, Ferrero and Chiovetta14,15 develop a mathematical description of another model, which may be considered to be a combined “hard-core/ uniform-site” model, in which uniform fragments originate from the periphery of the hard core by an exfoliation process. Since this paper will analyze our results only in terms of these possible model forms, we will not review earlier published evidence concerning the reality of these model forms.
Figure 1. Cross-sectional view of 100 cm3 Vycor reactor vessel (arrows indicate inlet and outlet ports).
2. Details of Equipment and Supplies 2.1. Catalysts. The earlier results9,10 suggested that the physical properties of the silica supports were responsible for the early transient behavior. To confirm such views1,10 and to define clearly the roles of mixing and experimental factors influencing the polymerization results, Cr catalysts were prepared using a number of silica supports differing in their morphological character. Six supports with a broad range of surface areas and total pore volumes, as listed in Table 1, were used. The catalysts were prepared with Cr contents ranging from 0.5 to 5 wt % by impregnating the silica supports with aqueous solutions of chromium(VI) oxide, followed by air drying at 105 °C. 2.2. Novel Reactor. Figure 1 shows a cross-sectional view of the Vycor reactor of volume, 100 cm3. A second reactor of the same diameter, 4 cm, but of increased length to provide an expanded volume of 150 cm3 and a larger droplet disengaging volume above the agitated slurry was used to extend the duration of a run and yields of polymer beyond those obtainable in the smaller reactor. The catalyst was pretreated or activated safely in this vessel at temperatures up to 800 °C and at near-atmospheric pressure. Figure 2 shows a simplified flowsheet of the polymerization laboratory setup. A 100 cm3 Pyrex storage vessel identical in shape to the reactor enabled purification and storage of the hexane liquid required for the slurry mixture. The storage and reactor vessels were connected to the reaction system through two 4-way valves, 4-2 and 4-1, respectively. A third 4-way valve, 4-3, made it possible to perform “catalyst activation” and “polymerization runs” sequentially, in situ. During the activation of the catalyst, the Vycor vessel was heated using a small cylindrical oven (upper limit of 1000 °C), regulated with a CN-2010 Omega controller which had
Figure 2. Simple flowsheet of laboratory equipment used for experimental polymerization studies. A: Oxygen trap; B: carbon dioxide trap; C: moisture trap (molecular sieve); G: gas chromatograph; M: mass flowmeter; P: roughing vacuum pump; R: rotameter; S: soap bubble-meter; 2, 3, 4, and 5 denote 2-, 3-, 4-, and 5-way valves, respectively. Slurry reactor connects at X1 and hexane storage vessel at X2. temperature-programming capability. The temperature was monitored with a type K thermocouple inserted in the well shown in Figure 1. During a constant temperature polymerization run, the reactor was immersed in a water bath, usually at 25 °C. Mixing of the slurry within the reactor was facilitated by the internal bubbling action of the incoming ethylene flow and by vigorous mechanical shaking of the reactor vessel provided by an external rotating eccentric contactor, providing from 150 to 400 lateral displacements per minute (rpm). The ethylene feed was maintained at the same constant flow and pressure throughout all runs. Because of the good reproducibility of the yields obtained in replicate runs, the liquid hexane in the reactor was assumed to be saturated with dissolved ethylene at the standardized reaction conditions. The total volumetric flow of (unreacted ethylene plus n-hexane evaporated) was measured continuously using a rotameter, R, or periodically, with a soap bubble-meter, S. To expel gas impurities or contaminating residual gases from the system, N2 purging could be augmented by evacuation with a roughing vacuum pump, P.
Macromolecules, Vol. 29, No. 9, 1996 2.3. Feed and Exit Streams. All gases used in this study were purchased from Linde. The possible presence in these gases of impurities such as water vapor or oxygen would deactivate active sites on the catalyst surface. Thus, operating with a continuous feed of ethylene containing such impurities would gradually diminish the catalyst activity and introduce a noncontrolled factor influencing the measured experimental yields. To prevent such decay in activity, the ethylene (minimum purity 99.5% as purchased) was purified by passing sequentially through water vapor, oxygen, and carbon dioxide traps (Alltech), and the nitrogen (minimum purity 99.9995%, containing
