Article pubs.acs.org/IECR
Impact of Addition of a Catalyst or Its Support on Reactor Wall Coating Due to Electrostatic Charging during Fluidization of Polyethylene Andrew Sowinski and Poupak Mehrani* Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis-Pasteur, K1N 6N5 Ottawa, Ontario, Canada ABSTRACT: In this work the degree of fluidized bed electrification and reactor wall coating were investigated where a metallocene catalyst and its silica support were added to a bed of polyethylene resin and fluidized in a carbon steel reactor, 0.1 m in diameter and operating under ambient conditions. Tests were conducted to simulate industrial polyethylene reactors where both polyethylene and catalyst particles are present during the fluidization process. Since majority of the catalyst surface area is occupied by its support, the influence of the support was also examined. The electrostatic charge measurement technique used was similar to that described by Sowinski et al.1 Results demonstrated that both the catalyst and the silica support augmented the fluidized bed reactor wall coating due to their inherent large specific charge, which was partly gained during their passage through the injection tube. This finding demonstrates the possibility that in commercial gas-phase polyethylene fluidized bed reactors the catalyst particles could be one of the sources leading to reactor wall coating formation and growth. Results from both catalyst and its supports were very similar implying that the choice of the catalyst support could have a large impact on the polyethylene reactor electrostatic charging behavior.
1. INTRODUCTION Gas−solid fluidized beds are commonly used in many chemical industries for various processes such as coating, drying, and mixing, and extensively for gas−solid catalytic reactions. In these systems electrostatic charges are typically generated due to continuous particle−particle and particle−container wall contacts, resulting from a phenomenon known as triboelectrification.2 Because of the inherent nature of the motions of the particles in gas−solid fluidized beds, controlling the electrostatic charge generation in such systems has plagued some industries such as manufacturers of polyolefin for numerous years. In the polyethylene production process, typically an alpha olefin fluidizing gas reacts with solid catalyst particles and polymerizes forming polyethylene resin with desired properties. A highly charged fluidized bed reactor (either positive or negative) has been shown to cause the formation of polymer chunks (particle agglomeration) and reactor wall coating, known as “sheeting” in the industry.3 The polymer sheets can vary significantly in size ranging from a few square centimeters to a few square meters and are mostly formed where the drag forces along the reactor wall are at a minimum.3 If the sheets become large enough, they can break off the reactor wall and cover the distributor plate, effectively defluidizing the bed and causing a costly downtime required for reactor cleanup. Over the years, there have been numerous patents published by the industry aimed at measuring and reducing the electrostatic charges within polyolefin gas−solid fluidized bed reactors.4−10 However, the problem still persists since the underlying mechanisms of charge generation and reduction in these reactors are not very well understood. In industrial polyethylene gas−solid fluidized bed reactors more than one type of solid material is present at any given time. Catalyst particles are continuously injected into the reactor, while the polyethylene resin is formed through the © 2015 American Chemical Society
polymerization reaction and removed as a product. Catalyst particles are fine powders that are recirculated through the reactor, if not reacted, until completely surrounded by the polyethylene, and the reaction is terminated. There are numerous types of catalyst that have been used for gas-phase polyethylene reactions, for example, Ziegler−Natta catalysts, chromium based catalysts, and single site catalysts such as metallocene catalysts.11 The catalyst can have one or more active sites where the polymerization reaction can take place. The active catalyst is generally impregnated onto a support, such as metal or silicon oxides, providing numerous active sites and to improve catalyst activity.11 The catalysts vary in size but are generally small in the range of 40 μm.11 The catalyst is normally injected into the reactor using pressured nitrogen through narrow stainless steel tubes situated in the reaction zone above the distributor plate. During the polymerization process, in addition to the polyethylene resin, the catalyst particles would also become electrostatically charged through contacts with the reactor wall and the polyethylene particles, as well as charging through their pneumatic transport during injection and while being recirculated within the system through the recycle line. Therefore, in order to gain a better understanding of the underlying electrostatic charging mechanisms in polyethylene reactors it is essential to investigate the influence of the catalyst on polyethylene particles charging and on reactor wall fouling formation. There have been a few studies12−16 that have investigated the presence of two or more types of particles in a fluidized bed, namely, the bulk particles and a small quantity of fine particles added as an additive. Received: Revised: Accepted: Published: 3981
December March 26, March 27, March 27,
5, 2014 2015 2015 2015 DOI: 10.1021/ie5047686 Ind. Eng. Chem. Res. 2015, 54, 3981−3988
Article
Industrial & Engineering Chemistry Research
cylinder to enable the injection of the powders into the fluidized bed. Linear low-density polyethylene (PE) produced using a metallocene-based catalyst in an industrial gas-phase fluidized bed reactor was used as the fluidizing particles. The resin had a particle density of 918 kg/m3 and an approximate size range of 20−1500 μm with a volume-based mean particle diameter of 700 μm. The particle size distribution was measured using a Malvern Mastersizer 2000. The same metallocene catalyst used to produce the polyethylene resin (deactivated) and its support were utilized for the binary mixture experiments. The catalyst was supported on dried silica and composed of Al and Zr metal atoms. The catalyst and the silica support added to the bed of polyethylene resin for the amount of 0.1% of the mass of the polyethylene particles used. The catalyst and its support had different particle size ranges of 30−110 and 20−75 μm with densities of 2100 (true) and 460 kg/m3 (bulk), respectively. In all experimental runs, fluidization was performed for a period of 60 min with dry air at a velocity 1.5 times the minimum fluidization velocity (representing bubbling flow regime). Minimum fluidization velocity was determined experimentally through bed pressure fluctuation measurements. For all experimental runs, the fluidization gas relative humidity (RH) and temperature were 0% and 23 ± 0.6 °C, respectively. The room temperature and relative humidity were 23 ± 0.5 °C and 10−20%, respectively. Prior to placing the polyethylene particles in the fluidization column, the injection tube was extended to the entire column diameter to reduce the chance of any clogging of the tube by the particles. After the polyethylene resin was loaded into the column, the injector was partially withdrawn from the system, and a fraction of the additive was loaded into the sample holder and injected into the bed using pressurized extra dry air from a cylinder. Then the injection tube was partially withdrawn once more and more additive was loaded and injected into the bed. This process was continued until the injection tube was flush with the column wall. The aforementioned injection procedure aimed at distribution of the powder in a radial direction within the bed. The injector unit was thoroughly cleaned after each injection. For each experiment a new batch of polyethylene resin was used as received. All experiments were repeated at least three times to ensure the reproducibility of results. In each trial, the initial charge of the PE resin was measured in a bench-scale Faraday cup prior to placing the particles inside the fluidization column. It was anticipated that both catalyst and its support particles would become charged just by passing through the injection tube. Thus, in a separate experiment, samples of the same particles were passed through the injector unit and directly injected into a bench-scale Faraday cup to determine the charge of these particles at the point of their entry into the fluidization column and prior to mixing with the PE particles. Samples of the same particles were also placed directly in the bench-scale Faraday cup to measure the initial charge of these particles prior to their injection. For this study the primary focus was on the fluidization column wall coating magnitude and specific charge, as well as the charge and mass of the entrained particles (Fines). Since the injected powders had a much smaller particle size distribution in comparison to the mean particle diameter of the PE resin, it was anticipated that some of the injected powders would be entrained from the bed. Hence, it was important to determine the mass and charge of the entrained particles during fluidization. Upon the completion of the
However, no literature was found that reported examining magnitude of bed electrification and reactor wall coating for a binary mixture of a polyethylene resin and its catalyst. Having previously developed an online Faraday cup measurement technique1,17 that is able to provide information on the charge distribution of the particles within a fluidized bed and most importantly the degree of reactor wall coating, it was therefore very valuable to apply the same measurement technique and to investigate the influence of a two particle type fluidization system. Hence, the aim of this work was to study the influence of addition of a metallocene catalyst to a bed of polyethylene resin and investigate the extent of reactor wall coating and particle charging. Since the catalyst support occupies the majority of the catalyst as the active catalyst sites occupy only a small fraction of the total catalyst surface area, it is likely that the catalyst support also plays a role in particles charging within the fluidization reactor. Thus, a binary mixture system of the catalyst support added to a bed of polyethylene resin was also examined.
2. EXPERIMENTAL SETUP AND PROCEDURE The overall experimental setup and procedure has been detailed elsewhere17 with the exception of inclusion of a powder injection unit. As seen in Figure 1, the system consisted of a 0.1
Figure 1. Schematic of the fluidization setup including the powder injection unit.
m in diameter carbon steel fluidization column housing two Faraday cups. The Faraday cups allowed the measurement of the charge, mass, and size distribution of the particles in the bulk of the bed, those attached to the column wall, and those entrained. The addition of powders (catalyst and its support) to the fluidized bed was achieved by using a custom-built injector unit (Figure 1). The injector was situated approximately 0.025 m above the distributor plate and consisted of a 25.4 mm in diameter stainless steel sample holder, two ball valves, and a 6.35 mm in diameter stainless steel injection tube. The injection tube diameter was similar to that of a commercial process. The sample holder was attached to an extra dry air gas 3982
DOI: 10.1021/ie5047686 Ind. Eng. Chem. Res. 2015, 54, 3981−3988
Article
Industrial & Engineering Chemistry Research fluidization period, first the bulk of the bed particles was dislodged into the bottom Faraday cup by quickly opening the modified knife-gate valve that acts as a distributor plate. The particle mass and charge were then measured. This was followed by the removal of the particles that remained adhered to the column wall by passing a stream of air into the column from the top. The wall particles were characterized into two groups of loosely bound (those that could be easily dislodged from the column wall by the air stream) and tightly bound (those that still remained adhered to the column wall upon the passage of air stream and thus required to be scraped off of the column wall). As depicted in Figure 2, the combination of the two regions defined the total column wall coating (wall).
m% of the deactivated catalyst was injected into the bed of polyethylene resin, and the mixture was then fluidized (referred to as PE-C).
3. RESULTS AND DISCUSSION 3.1. Initial Conditions and Proposed Charging Mechanisms. For all experiments, the initial net q/m of the polyethylene particles was found to be relatively constant (−0.045 ± 0.016 μC/kg), and the particle size distribution was similar. Therefore, the experiments were assumed to start from similar initial conditions. Table 1 presents the net specific charge of the catalyst particles and the catalyst support prior to being loaded into the Table 1. Net q/m of the Added Powders Prior to Entering and upon Leaving the Injector Unit
catalyst support (CS) catalyst (C)
before injection q/m (μC/kg)
after injection q/m (μC/kg)
−11.55 ± 1.53
−186.50 ± 59.30
−40.80 ± 5.98
−83.54 ± 33.13
injector unit and after passing through the injector and at the onset of entering the fluidization column. Results show that both powders were charged negatively by passing through the injection tube, with the catalyst support gaining a net specific charge of approximately −175 μC/kg and the catalyst gaining approximately −40 μC/kg. During the fluidization of the mixture of polyethylene resin and any of the two powders, numerous contacts occur between different materials. The particles, polyethylene, and added powder (being catalyst or its support) come into contact with the column wall and other particles. With each contact, there is the potential of contact charging between the surfaces. The magnitude of the charging is dependent on numerous parameters (contact pressure, contact time, surface impurities, etc.) and thus cannot be readily measured or predicted for each contact. The work function of the materials in contact gives an indication to the possible magnitude and direction of charge being transferred. In this study, the work functions of all the materials in the system have been estimated from previous literature18 and are summarized in Table 2. The work function
Figure 2. Schematic showing particles collected from different regions of the fluidized bed.
The qualitative analysis of the magnitude of wall fouling was conducted by taking images of the particle layer on the fluidization column inner wall from the bottom of the column upon the completion of fluidization period and removal of the bulk particles as described in detail in Sowinski et al.1 The mass and charge of the loosely bound particles were measured and their charge-to-mass ratio (q/m) was calculated. The net charge of the tightly bound particles was not measured as they could not be removed without any handling, which would have undoubtedly altered their charge. However, samples were taken for particle size distribution analysis. The mass of the tightly bound particles was determined through a mass balance based on knowing the mass of the particles initially placed into the fluidization column and those collected during the fluidization (fines, bulk, and loosely bound). The mass collected in all regions was normalized to the initial mass of the resin used. With the particle size distribution analysis, the median particle diameter was used as a measure of the overall size of the particles, and it was normalized (DP50,N) with respect to the average median particle diameter of the initial sample of resin. To be able to understand the complicated charging phenomena existing within a gas−solid fluidized bed reactor consisting of a multicomponent particle mixture, a comprehensive study was necessary, which was achieved by the following experiments: (a) the polyethylene resin was fluidized without the presence of any additive (referred to as PE); (b) 0.1 m% of the catalyst support was injected into the bed of polyethylene resin, and the mixture was then fluidized (referred to as PE-CS); and (c) 0.1
Table 2. Estimated Work Function and Pseudotriboelectric Series of the Materials Present in the Fluidized Bed Used in This Work (Values Obtained from Masuda et al.18) material
work function (eV)
carbon steel catalyst catalyst support polyethylene
4.5
