B. GANDHAM and C. R. MAHENDAR, Saudi Aramco, Dhahran, Saudi Arabia
Polyethylene refers to a group of thermoplastic homopolymers made up of molecular chains consisting of multiple repeating ethene monomers. Polyethylene and its variants are the most commonly used commodity plastics. Polyethylene is typically used for plastic bags, food and drink containers, and medical applications such as knee joint kits. Polyethylene’s chemical structure consists of repeated monomers made from carbon and hydrogen atoms (FIG. 1).
There are three major processes used to produce high-density polyethylene (HDPE): slurry phase, gas phase and solution phase. One key aspect of these processes is that the reactors are operated at much lower pressure than those in low-density polyethylene (LDPE) production processes. In addition to producing HDPE, the gas and solution processes can also produce linear low-density polyethylene (LLDPE).1 For these reasons, they are referred to as swing processes (FIG. 2). These processes are described here:
Slurry phase process: Catalysts and polymers formed during production remain suspended in a liquid medium but never dissolve. The polymerization reaction takes place in a continuously stirred tank reactor or tubular/loop reactor.
Solution phase process: The catalyst and resulting polymer remain dissolved in a solvent that must be removed to isolate the polymer. The polymerization reaction takes place in a continuously stirred tank reactor.2
Gas phase process: No catalyst is used in this process. The ethylene monomer and supporting catalyst are blown into the reactor. The polymerization reaction takes place in a fluidized bed reactor (FIG. 3).
HDPE classification. HDPE is classified into unimodal, bimodal and multimodal grades, and the ethylene distribution of the co-monomer injection will vary in each of the reactors, directly impacting the melt flow index (MFI) and density. These two parameters play a major role in the mechanical properties of the end products. HDPE finds use in numerous applications and industries where strong impact resistance, excellent tensile strength, low moisture absorption, and chemical and corrosion resistance characteristics are required. Due to these properties, it is popularly used for manufacturing sanitary pipes as it has a tough chemical structure and is conveniently malleable. It has also gained popularity across the packaging industry and is increasingly being used to produce various products such as bottle caps, food storage containers and bags. Moreover, HDPE has also been certified as a food grade polymer, resulting in multiple applications in the food industry.
Uni/bimodal polyethylene density. Because the co-monomer is mainly incorporated into the long polymer chains, bimodal polyethylene has a higher density compared to unimodal polyethylene, resulting in the same amount of short chain branches but no inverse comonomer distribution (FIGS. 4 and 5). For that reason, bimodal polyethylene grades exhibit higher stiffness and enhanced stress crack resistance.
HDPE is a strong, moderately stiff plastic that has a highly crystalline structure, as well as being relatively inexpensive and possessing excellent process abilities. HDPE plastic has several characteristics that make it an ideal material for packaging and manufacturing applications. It is tougher than standard polyethylene, acts as a powerful barrier against moisture and remains solid at room temperature (TABLE 1).
HDPE is resistant to insects, rot and other chemicals, and does not create any harmful emissions during its manufacture or consumption. Furthermore, HDPE leaks no harmful chemicals into the soil or water. The author’s company expects the global HDPE market to exhibit moderate growth over the next 5 yrs (FIG. 6).
Takeaway. Technology selection represents a crucial decision in the early stages of a project and can have a substantial impact on the project’s economic viability. Targeted grades must be produced based on an extensive market survey, particularly for polymer projects. Therefore, a disciplined technology evaluation methodology is needed to ensure that the most appropriate, efficient and cost-effective solution is selected. New technologies can provide benefits and this should be weighed against the increased risk resulting from inaccuracies in, or insufficient, design data and prolonged startup duration.
In-house technology development may be the only choice, and an in-depth evaluation of company resources, expertise and equipment (pilot plants) should be carried out. A realistic schedule should be compiled, including all key development activities following the research and development stage gate model. HP
LITERATURE CITED
Future Market Insights, “Low density polyethylene market size and forecast 2024–2034,” April 2024, online: https://www.futuremarketinsights.com/reports/low-density-polyethylene-market.
Böhm, L. L., J. Berthold, H. F. Enderle M. and Fleissner, “Polyethylene: Polymer with future: Metalorganic catalysts for synthesis and polymerization,” 1999, online: https://doi.org/10.1007/978-3-642-60178-1_1.
BHEEMESH GANDHAM earned a BS degree in chemical engineering from Jawaharlal Nehru Technological University in Hyderabad, India, and an MBA from Symbiosis International University in Pune, India. Gandham is a certified Project Management Professional (PMP) from the Project Management Institute (PMI) and is a part of Saudi Aramco’s project management team. He has 19 yrs of experience in the petrochemicals industry working with manufacturing and technology companies. Gandham has been involved in the operations and technical services of brownfield/greenfield projects from conceptualization to commissioning, and product development for the petrochemicals sector.
CHOPPADANDI RAJA MAHENDAR earned a BS degree in chemical engineering from Jawaharlal Nehru Technological University in Hyderabad, India, and a PMP certification from the PMI. He has more than 22 yrs of experience in the polyolefins, petrochemicals, chemicals and pharmaceuticals sectors. Mahendar has been involved in the operations and technical services of brownfield/greenfield projects from conceptualization to commissioning, and product development for the petrochemicals sector.