Progress in technology and catalysts for gas phase polyethylene processes

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1 Progress in technology and catalysts for gas phase polyethylene processes Wang Dengfei 1,2, Wang Jian 2, Guo Feng 1, Gao Yuxin 1, Du Wei 3 Yang Guoxing 1, 1. PetroChina Daqing Petrochemical Research Center of Petrochemical Research Institute; Daqing City; Heilongjiang Province, ; China; 2. College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing City; Heilongjiang Province, ; China; 3. PetroChina Daqing Oilfield Design and Research Institute, Daqing City; Heilongjiang Province, , China. duwey@163.com Abstract: Polyethylene is one of the most widely used polymers in industry as fiber, film and bottle because it has many advantageous properties such as easy of processing, high chemical resistance, flexibility, low density and adequate mechanical properties and low cost. There are various commercial technologies available to produce all the range of polyethylene grades including high pressure PE processes, low pressure PE processes and bimodal or multi-modal PE processes. In principle, swing units PE plant is able to produce HDPE, MDPE and LLDPE; even if cost-wise it is better to avoid frequent switches between the PE grades with different densities. The gas phase polyethylene production process is the most recently developed low pressure PE process and boasts many distinct advantages when compared with the commercial slurry and solution processes. There are three major kinds of gas phase polyethylene production processes: single fluid-bed reactor configuration, dual cascade reactor configuration and hybrid process. Some of the most important licensors which produce swing units polyethylene are Unipol (Licensed by Univation), Innovene G (Licensed by Ineos), Spherilene (Licensed by Lyondell-Basell) and Borstar (Licensed by Borealis Chemical). These famous licensors and their proprietary processes will be reviewed in detail. This paper also reviews the historical development of swing-unit gas phase polymerization technology as well as the development of catalyst from Ziegler-Natta catalyst to metallocene-based catalyst. A critical review of the challenges and opportunities from an industry view-point is presented. In the end, some suggestions on domestic LLDPE/HDPE Polyethylene development were also given. Keywords: Polyethylene; PE Swing Units; Gas Phase Process; Catalysts 1. Introduction Polyethylene (PE) is the most widely utilized thermoplastic polymer today, which is a homopolymer of ethylene or copolymer of ethylene with up to 20% of other comonomers like 1-butene, 1-hexene, and 1-octene and other vinyl monomers [1-3]. Today, polyethylene can be made by four groups of processes, such as gas phase, slurry phase, solution, or high-pressure processes. Polymerization process has a major effect on the properties of the PE and thereby exerts a direct impact upon the satisfaction of all those involved in the transformation and use of PE in industrial and domestic artifacts. Due to its simple process design, no liquid hydrocarbons, and wide product range with respect to co-monomer (butene-1, hexene-1, and potentially octene-1), gas phase processes have gained many advantages over other processes. In this paper, production of swing units polyethylene in gas phase and their catalysts were emphasized. Some of the most important licensors are Univation, Ineos and Lyondell-Basell. Historically, the Unipol process (licensed by Univation) has dominated licenses for gas phase processes for linear polyethylene, but Innovene G (licensed by Ineos) and Spherilene S&C (licensed by Lyondell-Basell) processes have attracted a significant number of licensees in recent years. 2. Processes Description Univation, formerly named Union Carbide and now part of Dow Chemical, was the first company to commercialize the technology for polyolefin production using fluidized-bed gas-phase reactors [4]. Since polymerization occurs in the gas phase, separation of the unreacted monomer from the polymer product is achieved simply by flashing off the monomer. Any low molecular weight polymer formed remains in the polymer particles and no further separation is necessary. The process only requires a fluidized-bed gas-phase 25

2 Advances in Sciences and Engineering reactor, a product discharge system to get the polymer out of the reactor and flash off the monomer, and a purge column to remove any residual monomer and to deactivate the catalyst. The Unipol plant requires the least capital investment among the major polyolefin production processes. Both HDPE and LLDPE can be made using the Unipol process, although this process has found broader acceptance for the production of LLDPE. Some Unipol plants were designed and operated in the swing mode between HDPE, MDPE and LLDPE, but most plants are designed only for LLPDE production [5]. Several other companies have developed and are licensing gas-phase polyethylene technologies. They include the Innovene from BP (now Ineos), and the Lupotech G process (now licensed as Spherilene S process), and Spherilene C process from Basell, and Borstar process from Borealis Chemical. All of them are based on the same principle of using a fluidized-bed gas-phase reactor, although the operating mode and conditions differs among these different processes. Table 1 details the main characteristics of fluidized-bed processes for ethylene polymerization [6-8]. Table 1 Typical reactor conditions for fluidized-bed processes Process Unipol Innovene G Spherilene S&C Borstar Licensors Univation Inoes Basell Borealis Chemical Reactor type 1 or 2 fluidized-bed 1 fluidized-bed Mode of operation Reaction temperature( ) Condensed/Super condensed Condensed 1 or 2 fluidized-bed loop and 1 fluidized-bed Loop: fluidized-bed: 88 Reaction Loop: pressure(bar) fluidized-bed: 20 Residential time(h) Z-N, Cr, Catalysts types Z-N, Cr, Z-N, Post Metallocene, Only Z-N thereof Metallocene metallocene Bimodal 2.1 Single fluid-bed reactor configuration Unipol Ι Process Vent Recovery Reaction System Catalyst Resin Purging Additive Addition Raw Material Handling Pelleting System Figure 1 A simplified process schematic of the Unipol process. Figure 1 shows a simplified process schematic of the Unipol process. Fresh ethylene and co-monomer feed to the unit is passed through a separate series of purifiers where trace quantities of impurities are 26

3 removed. The ethylene and co-monomers are fed to the reaction system. The reaction system consists of a fluid bed reactor, a cycle gas compressor and cooler, and product discharge tanks. Ethylene, co-monomers and a recycle stream from the vent recovery system are fed continuously to the reactor. Polyethylene is removed from the reactor by the discharge tanks and sent to a purge tank where unreacted monomer and dissolved hydrocarbons are stripped from the resin and are sent to the vent recovery system. The purged resin is sent to the pelleting system. The vent recovery system recovers as much hydrocarbons as possible from the streams sent to it. The condensed components are returned directly to the reaction system and the light gases are used as a conveying gas to reduce nitrogen consumption. Solid additives are metered and sent to the pelleting system. The resin, solid additives and liquid additives are mixed, melted and pelleted in the pelleting system. The pellets are dried, cooled and sent to product blending and storage [9]. As is known, reactor capacity increases depend primarily on the ability to remove the heat of polymerization. The traditional non-condensation phase operation has a low space time yield (STY) of fluidized bed reactor because of the limitation of recycle gas stream s capacity of reaction heats removal. Since the condensation phase operation is applied, there exists condensed liquid in recycle stream,so the polymerization heat is removed both by the temperature rise of recycle stream gas phase and the evaporation of the liquid phase,which leading to increasing of the STY. However, the content of liquid phase in recycle stream is limited,so the reactor s production capacity can t be improved furthermore. To solve this problem and increase the liquid phase content,the super condensing process is necessary. At present the super condensing process is not mature yet,and the influences of liquid phase content increasing in recycle stream on fluidized bed operation,resin degassing unit and vent gas recovery unit are lack of deep research, the super condensing technology is not applied widely [10] Innovene G Process Figure 2 A simplified process schematic of the Innovene G process. Figure 2 shows the schematic of the Ineos Innovene G plant [11]. In this process, the catalyst and co-catalyst are fed to a slurry stirred-tank reactor in which pre-polymerization occurs. Pre-polymerization under mild conditions helps to prevent hot spots or the production of fines which is caused by high heat generation and growth stress inside the particles. The pre-polymer is transferred to a dryer where hot nitrogen evaporates the solvent. Then the pre-polymer powder, as a catalyst for the main polymerization reactor, is fed continuously to the fluidized bed reactor. Cyclones are installed to remove fines coming from circulated gas from the top of the reactor. Fines from bottom of the cyclones, which are usually quite active, are returned to the reactor for further polymerization. Circulating gases from top of the cyclones are passed through a heat exchanger(s) and then mixed with a certain quantity of fresh feeds to fix the composition of components in the reactor. Finally, the gases are compressed and returned to the bottom of the reactor. The so-called Condensed mode was introduced by Jenkins et al [12] and involves a liquid hydrocarbon being injected into the bed to remove reaction heat by evaporation. A cooling system in the loop condenses and separates the liquid from the circulating gas, and the gas is then injected back into the bed. Like Unipol possess, Innovene G possess also adopt CM (Condensed Mode) and SCM-T (Super Condensed Mode Technology for the gas phase PE reaction system leading to better heat removal. 27

4 Advances in Sciences and Engineering Spherilene S process Figure 3 A simplified process schematic of the Basell Spherilene S process. The schematic of the Basell Spherilene S plant is showed in Figure 3 [13]. Catalyst components are mixed and fed directly to a pre-contact vessel, where the catalyst is activated under controlled conditions. The activated catalyst system flows continuously into the gas-phase reactor. A cooler on the circulation gas loop removes the reaction heat. The polymer, in spherical form with particle size ranging from approximately 0.5 mm to 3 mm, is then discharged by a unique reactor outlet system at the base of the reactor enables a highly efficient, continuous withdrawal of product with minimal gas carry-over. The polymer is dried by a closed-loop nitrogen system and with no volatile substances, is sent to liquid and/or solid additives incorporation step before extrusion [14]. 2.2 Dual cascade reactor configuration Unipol ΙΙ Process Union Carbide also developed the dual reactor Unipol-II gas phase process in the 1980s to make bi-modal HDPE resins with superior mechanical properties compared to Unipol-I single-reactor HDPE resins [15]. In early 1994, full density range of PE resins was announced to successfully produce by Unipol II technology. A world scale Unipol-II plant was constructed in 1996 to make differentiated bimodal PE resins. They are produced using at least two reactors in series with heterogeneous Ziegler-Natta catalysts or supported metallocenes. In a Unipol II process, the catalyst is fed into the first reactor where the first polymer fraction is produced. After that the polymer is transferred into the second reactor for the production of the second polymer fraction. Between the reactors there may be a separation unit to remove unreacted monomers, hydrogen from the reaction mixture before the polymer is passed into the second polymerization stage. Normally, the low molecular weight polymer component is produced in the first reactor and the high molecular weight component in the second reactor. In some cases, the reactors may also be operated in parallel mode, not only in series Spherilene C process New generation Spherilene C gas-phase technology can produce linear-low-density polyethylene (LLDPE), medium density polyethylene (MDPE) and high-density polyethylene (HDPE) of narrow, unimodal molecular weight distribution as well as bimodal molecular weight distribution, using only a single Ziegler-Natta titanium-based catalyst family, with full online swing capability without shutdowns. The schematic of the Basell Spherilene C plant is showed in Figure 4 [16]. The comparison of Spherilene C and Spherilene S processes are listed in table 2. Unlike competing technologies Unipol and Innovene G that require a seedbed and associated storage and transfer systems, the reactor in the Spherilene process can be started up with patented catalysts and do not require a polymer seed-bed. The Spherilene technology has a unique and proven ability to operate with two reactors in series. The second reactor can either be installed at plant construction, or added to a single reactor plant at a later stage to extend product capabilities. 28

5 Figure 4 A simplified process schematic of the Basell Spherilene C process. Table 2 The comparison of Spherilene C and Spherilene S processes Technology Configuration Catalysts Products Spherilene S single fluid bed reactor Avant Z and Avant C LLDPE, MDPE, HDPE (Unimodal with Avant Z and broad MWD with Avant C catalyst) swing products Spherilene C two luid bed reactors in series only Avant Z LLDPE, MDPE, HDPE (Unimodal and bimodal with only Avant Z catalyst) swing products 2.3 Hybrid process Hybrid technologies development is another area of active research and development. Utilizing the principles of fluidized bed reactor, the process has been expanded to production of a wider range of polyethylene and extended to production of polypropylene. The Borstar polyethylene process is used when producing bimodal and unimodal linear low density (LLDPE), medium density (MDPE) and high density (HDPE) polyethylene using loop and gas-phase low pressure reactors in series. All products can be produced in one cycle [17]. Currently, Ziegler-Natta catalysts are used, but there is a potential to use single-site catalysts latter. Fig.5 illustrates a schematic representation of an industrial Borstar polyethylene process [18,19]. The Two loop reactors and one fluidized-bed reactor (FBR) are consisted in this process, where the polymerization firstly takes place in supercritical propane, followed by a degassing step, then in gas-phase. In order to to improve the polymer particle morphology and to enable the high activity catalysts to reach an appropriate size for the forthcoming main polymerization, a single-loop reactor is used for prepolymerization at relatively mild conditions (64bar to 80bar, 60 C to 70 C). The main loop reactor is often operated at 64bar to 80bar and 85 C to 95 C. The FBR typically is operated at 85 C and approximately 20bar. In the main loop reactor, ethylene polymerization occurs in the presence of high hydrogen concentration (usually without a comonomer) and low molecular weight polymer is formed. In the FBR, ethylene-1-butene copolymerization is conducted in the presence of relatively low hydrogen concentration and high comonomer concentration and high molecular weight and high comonomer composition product is gained. The flash is used to remove all the propane and hydrogen discharged from the loops. So the slurry-phase loops and gas-phase FBR are run at independent conditions. The high solubility of hydrogen in supercritical propane allows the reactor to be operated at extremely wide range of hydrogen concentrations. Moreover, the lower solubility of the amorphous polyethylene in supercritical propane than other solvents (i.e., isobutene, n-hexane) prevents the loop reactor from fouling. The finishing in an FBR also allows one to add varying levels of 1-butene comonomer without worrying about solubility problem. These characteristics make the Borstar polyethylene process a true versatile process [19]. 29

6 Advances in Sciences and Engineering Figure 5 Schematic representation of an industrial Borstar polyethylene process 3. Catalysts Technologies for Gas Phase Polyethylene Processes There are four major families of catalysts for ethylene polymerization: Ziegler-Natta, Phillips, metallocene and late-transition metal catalysts. Most commercial HDPE and LLDPE resins are made with heterogeneous Ziegler-Natta catalysts nowadays. Phillips catalysts are very important for the production of HDPE, but are not used for LLDPE manufacture. Metallocene catalysts can make both HDPE and LLDPE, but metallocene resins are very different from the ones made with either Ziegler-Natta or Phillips catalysts. Polyethylene made with some metallocene catalysts may also have significant number of LCBs, but their branching structure is completely distinct from that of LDPE resins. The market share of metallocene resins is still small, but has been increasing steadily since the 1990s. Resins made with late transition metal catalysts have had no commercial applications to date and will not be discussed any longer in this paper. The main characteristics of coordination catalysts for ethylene polymerization are listed in Table 3. Table 3 Main characteristics of coordination catalysts for olefin polymerization Type Physical state Examples Polymer type Heterogeneous TiCl 3, TiCl 4 /MgCl 2 Non-uniform Ziegler-Natta Homogeneous VCl 4, VOCl 3 Phillips Heterogeneous CrO 3 /SiO 2 Non-uniform Homogeneous Cp 2 ZrCl 2 Metallocene Heterogeneous Cp 2 ZrCl 2 /SiO 2 Ni, Pd, Co, Fe with diimine Late-transition metal Homogeneous and other ligands 4. Conclusion Each process has its advantages depending on the producer and their product goals and intellectual property considerations. Though every process production of polyethylene varies for various possible catalyst combinations, comonomers selection, transfer agents and polymerization post-treatments, at the heart of all industrial polyethylene processes is the system used to initiate polymer chain growth. Latest findings in polymer science are used to introduce new catalyst and process innovations for producing the required PE, and in many cases also resulted in better production economy. Examples of these new findings were single-site catalysts, condensed mode technology in the gas phase process, which allows retrofitting a reactor and increasing the production capacity up to 60%, use of supercritical propane in a loop reactor, and several new multi-reactor processes (e.g. Unipol II, Spherilene, Advanced Sclairtech, Borstar). It is believed that catalyst and process innovations will go hands by hands and the control over the polymer structure and the ability to tailor material properties will be increased. 30

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