Process types of LLDPE

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1. Process types of LLDPE. There are mainly 4 methods for producing polyethylene: high-pressure method, gas-phase method, solution method, and slurry method. However, at present, the gas-phase and solution-process methods are commonly used worldwide to produce LLDPE resin. In the solution process, Dow Chemical’s low-pressure cooling method in the United States and NOVA Chemicals Corporation’s medium-pressure method in Canada hold a dominant position. Both of these processes can be switched to produce LLDPE and HDPE. Dow’s low-pressure solvent process has been used in many plants around the world, but these are all Dow’s own plants. In this process, ethylene, octene-1, and C8–C9 isomeric paraffin solvents are fed together with a modified Ziegler catalyst solution into two stirred reactors connected in series. The reaction was carried out at 395 pounds per square inch and 160°C. In the solution of the second reactor, the polymer content is 10%. The total stay time is 30 minutes. The reactor effluent is flashed at an absolute pressure of 35 pounds per square inch to remove ethylene from the solution. Subsequently, the solvent is removed using a heating/flash step. The polymers are then extruded and granulated. Canada’s NOVA company’s medium-pressure SclairTM solution process was developed by DuPont Canada. In mid-1994, NOVA Chemicals acquired the SclairTM technology along with its worldwide licensing rights, and by using a new generation of non-metallocene catalysts, it developed the SclairⅡTM technology. In gas-phase processes, Univation’s low-pressure gas-phase fluidized bed process, namely the UnipolTM process, is the most common industrial process for producing LLDPE. In this process, ethylene and comonomers (butene-1 or hexene-1) are polymerized in a fluidized bed reactor to produce granular polymers. Its feature is that a carrier-based titanium or titanium-chromium catalyst powder is continuously fed into the fluidized bed reactor, and polymer product particles are continuously removed from the reactor. In the fluidized bed, the growing polymer particles are fluidized by the circulating ethylene/co-monomer stream. The circulating fluid is cooled by an external cooler to remove the heat of reaction. The reactor pressure is approximately 300 pounds per square inch, and the reaction temperature is about 88°C. The UnipolTM process can also be used to produce polypropylene, utilizing Shell’s superhighly active catalyst (SHAC). Furthermore, BP’s low-pressure gas-phase fluidized bed process is very similar to the UnipolTM process. Only the method of feeding the condensate into the fluidized bed is slightly different. BP’s method involves first separating the condensate from the circulating stream, and then atomizing it using nozzles placed within the fluidized bed before feeding it into the fluidized bed layer. UnipolTM does not perform separation; the condensate is sent to the fluidized bed reactor along with the recycle stream. 2. Process Flow There are various processes for producing LLDPE; here, the two main processes, namely the vapor-phase method and the solution method, will be introduced. 1. The Unipol gas-phase process of U.S. United Carbon Company (UCC). This process is similar to the BP gas-phase process, but UCC offers a wider range of products with more varieties; it uses 4 different catalysts to produce various resins with molecular weight distributions ranging from narrow to wide across the full density range, and with melt indices ranging from 0.91 g/10 min to 125 g/10 min. Among various processes, the UCC gas-phase method offers the widest range of products. The BP process uses a catalyst to produce full-density polyethylene, with a melt index ranging from 0.35/10min to 30 g/10min; it features a narrow molecular weight distribution. When producing grades with a wider molecular weight distribution, additives must be added during extrusion granulation, but such grades are limited in number. The plant for the Unipol PE process generally consists of 4 sections: monomer purification, polymerization reaction, resin degassing, and resin granulation. The process flow is shown in Figure 2. (1) Monomer purification: All monomers entering the polymerization reactor (including ethylene and copolymer monomers) must have impurities toxic to the catalyst, such as oxygen, carbon monoxide, carbon dioxide, water, sulfides, methanol, and alkynes, removed. Catalyst beds and molecular sieves, which are commonly used for deoxidation and oxidation, are employed to remove impurities. (2) Polymerization reaction The polymerization reaction takes place in a fluidized bed reactor, the lower part of which is cylindrical, while the upper part consists of an inverted cone and a hemisphere. At the bottom of the reactor there is a gas distribution plate, on which a fluidized bed layer made of powdered resin is present. The catalyst and co-catalyst are directly introduced into the reaction bed from the distribution plate, while a blower supplies circulating gas to keep the bed in a fluidized state, ensuring uniform mixing of the reaction monomers with the catalyst and simultaneously removing the heat generated by the reaction. The reaction heat is removed from the system in the circulating gas cooler. The mass velocity of the gas through the bed should be 3 to 6 times Gmf (the minimum gas flow velocity required for fluidization). The molecular weight regulator—hydrogen—is also introduced into the system along with the monomer from the bottom of the reactor. The properties of the resin are adjusted by the amounts of catalyst, co-catalyst, comonomer, and hydrogen added. The reaction residence time is about 3 hours. (3) Resin degassing: The resin exits the reactor and passes through a special discharge system to remove unreacted monomers ； The resin from which the monomer has been recovered is recycled to the reactor and sent to a degassing chamber, where the hydrocarbons adsorbed in the resin are further removed. A purge gas is introduced from the bottom of the degassing chamber and comes into countercurrent contact with the resin, thereby blowing away the hydrocarbons; at the same time, a small amount of deactivator is introduced to deactivate any remaining active centers on the polymer. (4) Granulation: The degassed resin has large particles removed using equipment such as vibrating screens, and before entering the granulation system, it is first mixed with solid and liquid additives. Unipol’s granulation system is a tightly integrated trinity of a mixer, a melting pump, and a granulator, which allows for approximately a 1/3 reduction in energy consumption compared to similar systems in other processes. The granular slices are carried away using circulating soft water; the water is separated through drying, and then the material is fed into a hopper, from where it is sent to the mixing, storage, transportation, and packaging processes using air. 2. Canadian DuPont medium-pressure solution process (Sclairtech) process flow. This process is the one with the highest production capacity and fastest development among solution-based methods. In 1960, DuPont built the first plant with a capacity of 11 kt/a in Shania, Canada. By 1990, the production capacity using this process had reached 720 kt/a to 780 kt/a, with the largest reactors having a capacity of 300 kt/a. The process flow of DuPont’s medium-pressure solution method (Sclairtech) is shown in Figure 3. (1) Polymerization: After being pressurized, ethylene enters the cooling absorber along with the purified recycled comonomer and solvent (cyclohexane), where it is thoroughly mixed and dissolved as the temperature is reduced. It is pressurized using a feed pump to reach a reaction pressure of 10.79–16.67 Mpa (110–170 kgf/cm2), and the reaction temperature is adjusted via a temperature control system to range from 100–300°C. The conversion rate of ethylene is controlled to around 95% by adjusting the amount of Ziegler-type catalyst added, while the melt index is regulated using hydrogen. Adjust the density of polyethylene using the amount of comonomer. Two (or more) reactors are used, operated under different temperatures and at various hydrogen addition points, to regulate the molecular weight distribution of the product. An deactivator is added at the reactor outlet to terminate the reaction, after which the reaction stream is heated to 300°C to remove catalyst residues through adsorption by Al2O3 adsorbent ； If the improved new catalyst system (ACS) is used, the facility for catalyst removal can be eliminated. Then, the reaction mixture enters a medium-pressure flasher to remove the reactive ethylene, comonomers, and most of the solvent. (2) Post-treatment: After the melt has had volatile substances such as monomers and solvents removed, it is mixed with solid additives and fed into an extruder and a cutting machine. The granules are carried away by circulating water; after dehydration, they are mixed with hot water to form a slurry, which allows for further removal of solvents from the resin. The resin then enters a stripping unit, where reverse steam stripping is used to reduce the residual solvent content to less than 500 mg/L. It is subsequently dried further and sent to a mixing bin and packaging process using hot air. (3) Solvent recovery: The ethylene, comonomers, and cyclohexane that emerge from the tops of the medium-pressure and low-pressure flashers enter the low-boiling-point tower via the first and second condensers, respectively. The material at the top of this tower then passes through an ethylene column and a comonomer column to enable the recovery of ethylene and comonomers. The material at the bottom of the low-boiling-point tower is sent to the high-boiling-point tower and the resin stripping tower for further processing; cyclohexane is recovered from the top of the high-boiling-point tower, while oily low-polymerized substances are discharged from the bottom of the resin stripping tower. The additional comonomer is fed into the comonomer column, from which isomer 2-butene is also discharged via a side stream. 3. Cost requirements for producing LLDPE: The various processes used to produce polyethylene differ in terms of their reaction mechanisms and technical procedures, which results in different production processes and conditions. As a consequence, there are variations in the consumption of raw materials and utility resources, as well as in the requirements regarding the number and type of equipment needed. Therefore, the investment and costs associated with these different production facilities also vary significantly. The technical and economic comparisons of polyethylene production processes are shown in the tables below. Figure 1: Comparison of investments in various polyethylene production facilities (100 kt/a), in millions of dollars. Production method and characteristics, Investment within the plant area, Investment outside the plant area, Total fixed assets investment, Order of investment magnitude: UCC Company’s Unipol powder product – 18.2, 8.8, 27.0; Rank 1. Dow Chemical’s low-pressure cooled solution method, granular product – 19.1, 12.8, 31.9; Rank 2. DuPont’s medium-pressure insulated solution method, granular product – 24.4, 15.8, 40.2; Rank 3. UCC Company’s Unipol granular product – 28.1, 13.8, 41.9; Rank 4. Philips’ lightly diluted slurry method, granular product – 27.9, 14.4, 42.3; Rank 5. Solvay’s heavily diluted slurry method, granular product – 28.7, 14.2, 42.9; Rank 6. DSM’s low-pressure adiabatic solution method, granular product – 28.1, 15.8, 43.9; Rank 7. BP’s vapor-phase method, granular product – 30.6, 13.8, 44.4; Rank 8. Mitsui Oil Chemicals and Hoechst’s heavily diluted slurry method – 30.5, 15.1, 45.6; Rank 9. Autoclave method for homopolymers and EVA copolymers – 54.4, 20.0, 74.4; Rank 10. High-pressure tubular method for homopolymers, EVA, and EBA – 55.7, 18.9, 74.6; Note: The investment figures in the table correspond to prices in the U.S. Gulf region in mid-1984. Figure 2: Consumption indicators for several typical polyethylene production processes
Item: Unipol gas-phase method, DuPont solution method, slurry method, high-pressure method
HDPE, LLDPE: 0.96, 0.946, 0.919
DSM, Philips: Batch process, tubular process; C=4 copolymerization, C=4 copolymerization, C=4, C=6 – C=4, C=4, C=4, C=4; Homopolymerization, homopolymerization
Ethylene, t/t PE: 0.993, 0.998, 0.940, 0.933, 1.011, 0.999, 0.946, 0.948, 0.923
Copolymer monomers, t/t PE: 0.025, 0.020, 0.078, 0.085, 0.017, 0.080, 0.071, 0.087
Total monomer consumption, t/t PE: 1.018, 1.018, 1.018, 1.018, 1.011, 1.016, 1.026, 1.019, 1.010, 1.026, 1.027
Solvents or diluents, t/t PE: 0.012, 0.012, 0.012, 0.016, 0.005–0.015
Catalysts and chemicals, USD/t PE: 9.92, 9.92, 9.92, 9.92, 6.5, 3.6, 6.35, 9.44, 6.5, 6.6, 6.6
Utilities consumption: Electricity, kW·h/t PE

Figure 3: Cost comparison of producing a representative grade using different processes
Process method, Density (g/cm3), Melt index, Copolymer monomer percentage, Product price (cents/lb)
UCC gas-phase method (powder): 0.918, 1.0, C=4/7.6, 27.22
DuPont medium-pressure insulated solution method (pellets): 0.924, 5.1, C=4/7.0, 28.09
UCC gas-phase method (pellets): 0.918, 1.0, C=4/7.6, 29.75
BP gas-phase method (pellets): 0.918, 1.0, C=4/7.6, 30.18
Dow Chemical low-pressure cooled solution method (pellets): 0.930, 1.0, C=8/4.9, 30.20
DSM low-pressure adiabatic solution method (pellets): 0.920, 4.4, C=4/8.0, 31.00
Mitsui Oil Chemicals, Hoechst batch slurry method: 0.940, 0.2, C=4/2.3, 31.13
Philips lightly diluted slurry method: 0.935, 35.0, C=4/5.1, 31.83
Solvay heavily diluted slurry method: 0.926, 1.0, C=6/5.7, 32.28
High-pressure batch method: 0.938, 1.0, VA/18.0, 39.11
High-pressure tubular method: 0.936, 2.0, VA/18.0, 42.42
Based on investment and consumption indicators, and using a representative product grade as an example, the production costs of various processes are compared, with the results listed above. From the table, it can be seen that the gas-phase method and the medium-pressure solution method have the lowest costs, followed by the slurry method and the low-pressure solution method, while the high-pressure method has the highest cost. In actual production and operation, a production facility cannot always manufacture one specific grade of product; it is necessary to switch between different grades based on market demand. However, when changing the grade of catalyst, the larger the reactor, the longer the residence time, and the more catalysts that need to be replaced, the more transition material of different grades there will be, and the longer the operating time lost due to this change. As a result, the increase in product cost becomes greater. Under these conditions, the cost increase for particles produced by the vapor phase method is relatively large, while the cost increase for those produced by the solution method, the Philips ring reactor method, and the vapor phase powder method is smaller. The cost of products manufactured by the high-pressure method remains high. 4. The impact of processing technology on LLDPE quality: The development of new technologies and processes not only improves the performance of products but also reduces manufacturing costs, thereby fostering competition and mutual substitution among polymers. Changes in aspects such as catalyst systems, co-catalysts, comonomers, reactors, and polymerization media affect the molecular structure of the polymer, as well as factors such as the crystallinity of the resin, its degree of branching, the distribution of comonomers, and its density, relative molecular mass, and molecular weight distribution (MWD). These structural factors in turn determine the ultimate properties of the polymer, including mechanical strength, optical properties, purity, rheological behavior (processability), stability (against heat, ultraviolet light, etc.), thermal properties, and electrical properties. By using a low-pressure process to produce bimodal copolymers and terpolymers with a wide MWD of LLDPE, resins can be obtained that have processing and performance characteristics similar to those of conventional high-pressure LDPE. LDPE has a higher amount of branched structures, with long branches being predominant, whereas LLDPE contains only short branches; the number of these branches determines the crystallinity and density of the polymer. Improving processing properties will facilitate the expansion of LLDPE’s application areas into those that could not be accessed before due to differences in properties such as transparency and melt strength. In recent years, among the new technologies for LLDPE production, apart from the bimodal process, the most notable development has been that of metallocene and non-metallocene single-center catalyst technologies, which have led to the creation of large quantities of LLDPE that is easy to process and possesses high performance. The LLDPE resins developed using these new technologies are known as second-generation LLDPE resins. As can be seen from the above, whether the production technology and processes are advanced has a decisive impact on product quality and costs. At present, China’s LLDPE production technology is largely based on processes imported from abroad prior to the early 1990s. Due to insufficient digestion and absorption of these technologies, the quality and grade of the products produced are low, the range of available varieties is limited, the products contain many impurities, their quality is inconsistent, and their processability is poor. Therefore, many domestic processing enterprises prefer to pay a high price to import foreign materials rather than use similar domestic products.