Control system design for linear light polyethylene production fluid bed reactor

Master's thesis in the field of chemical engineering (simulation and process control)

Abstract

Control system design for a fluidized bed reactor for linear polyethylene production

Jam petrochemical LLDPE unit under Basel license and Spherilene technology with annual production capacity of 300,000 tons of product is one of the largest polymer units in Iran. In this research, the dynamic behavior and control of the linear light polyethylene production series reactors of Jam Petrochemical have been investigated. For this purpose, a kinetic model with two active sites for the copolymerization reactions of ethylene with 1-butene in the presence of Ziegler-Natta catalyst was used, and to write the mass and energy balance equations, these two reactors were approximated with homogeneous mixing reactors (CSTR). For control purposes, the important parameters of reactors, including temperature and level, are controlled by traditional PID controllers and negative feedback system. In this study, it has been shown that the proposed control system has made the process stable, and then the efficiency of the proposed control system has been investigated to eliminate the known disturbances of the system. By studying the effect of disturbances on the dynamic behavior of the process and the stability of the system, it was found that the temperature of the cooling water has the greatest effect on the output of the system. Keywords: fluidized bed reactor, polyethylene, feedback control, dynamic model, polymerization kinetics

Polyethylene is one of the simplest and cheapest polymers. This material is obtained from the polymerization of ethylene and is briefly shown as PE. Ethylene molecule is C2H4 (has a C=C double bond). In the process of polymerization, the double bond of each monomer is broken and instead a simple bond is created between the carbon atoms of the monomers, and the product created is a macromolecule. Polyethylene is a solid, odorless, waxy, semi-transparent and inactive material that is usually produced in the form of granules. Therefore, polyethylene can be converted into a wide range of ethylene derivatives. Polyethylene is one of the most stable and neutral polymers and has high resistance to chemicals. Having many properties has made polyethylene to be used in a wide range of products. Its most common use is in the production of packaging films [1].

History of polyethylene production

Polyethylene was first accidentally synthesized by the German chemist Hans von Pacma. In 1898, while heating diazomethane, he synthesized a white waxy compound, which was later named polyethylene [2]. The first industrial synthesis method was carried out by Eric Fawcett and Reynolds Gibson. These two scientists obtained a wax-like substance by heating a mixture of ethylene and benzaldehyde under high pressure. The cause of this reaction was the presence of oxygen-containing impurities in the used devices, which acted as polymerization initiators. In 1935, Michael Perrin developed this method and synthesized polyethylene under high pressure, which was adopted as the basic method for the industrial production of polyethylene in 1939. Since then, with the removal of obstacles, many advances have been made in the field of polymer systems and polymer manufacturing, and all of this has led to the fact that the production of polymers has become a huge industry today [3].

Figure 1-1. The molecular structure of ethylene and polyethylene

Types of polyethylene

Polyethylenes are a family of resins that are obtained through the polymerization of ethylene gas (C2H4). Polyethylene contains a very simple structure, so that it is simpler than all commercial polymers. A polyethylene molecule is a long chain of carbon atoms with two hydrogen atoms attached to each carbon atom. Sometimes, instead of hydrogen atoms in the molecule (polyethylene), a long chain of ethylene is connected to carbon atoms, which are called branched polyethylene or light polyethylene (LDPE), because its specific weight is due to occupying the volume.Sometimes, instead of hydrogen atoms in the molecule (polyethylene), a long chain of ethylene is connected to carbon atoms, which are called branched polyethylene or light polyethylene (LDPE), because its specific weight has decreased due to occupying more volume. In this type of polyethylene, ethylene molecules are randomly connected to each other and create a very irregular molecular structure. Its specific weight is between 0.910 and 0.925 and it is produced under high pressure and temperature and often using vinyl free radical polymerization. Of course, Ziegler-Natta polymerization can also be used to prepare it [4]. When there are no branches in the molecule, it is called linear polyethylene (HDPE). Linear polyethylene is harder than branched polyethylene, but branched polyethylene is easier and cheaper to make. The molecular structure of this polymer is very crystalline. Linear polyethylene is a product with a molecular weight of 200,000-500,000, which is polymerized under pressure and relatively low temperatures. Its specific gravity is between 0.941 and 0.965 and it is mostly prepared by a difficult process called Ziegler-Natta polymerization. There is also a type of polyethylene whose specific weight is between the specific weight of these two polymers, i.e. in the range of 0.926 to 0.940 and it is called semi-heavy or medium polyethylene [4].

Polyethylene with a molecular weight between 3 and 6 million is called ultra-high molecular weight polyethylene or UHMWPE and is produced by metallocene catalyst polymerization. do The mentioned material is more difficult to process, but its properties are excellent. When this polymer becomes completely cross-linked through radiation or the use of chemical additives, the mentioned polyethylene will no longer be thermo-soft. This material will be a true heat hardener with good tensile strength, electrical properties and impact strength over a wide range of temperatures when cured during or after molding. This material is used to make very strong fibers to replace Kevlar (a type of polyamide) in bulletproof vests, and also its large plates can be used instead of ice skating fields [4]. By copolymerizing ethylene monomer with a branched alkyl monomer, a copolymer with short hydrocarbon branches is obtained, which is called poly It is called linear ethylene with low specific weight or LLDPE and it is often used to make products such as plastic films. The classification of polyethylenes is based on their density, which is involved in the amount of density, the size of the polymer chain and the type and number of branches in the chain.

High density polyethylene (HDPE):

This type of polyethylene has polymer chains without branches, so the intermolecular force in the chains is high and its strength is higher than other polyethylenes. is The reaction conditions and the type of catalyst used in the production of polyethylene HDPE are effective. To produce unbranched polyethylene, polymerization method with Ziegler-Natta catalyst is usually used. This material is obtained from the polymerization of ethylene and is briefly shown as PE. Ethylene molecule is C2H4 (has a C=C double bond). In the process of polymerization, the double bond of each monomer is broken and instead a simple bond is created between the carbon atoms of the monomers, and the product created is a macromolecule. Polyethylene is a solid, odorless, waxy, semi-transparent and inactive material that is usually produced in the form of granules. Therefore, polyethylene can be converted into a wide range of ethylene derivatives. Polyethylene is one of the most stable and neutral polymers and has high resistance to chemicals. Having many properties has made polyethylene to be used in a wide range of products. Its most common use is in the production of packaging films [1].

History of polyethylene production

Polyethylene was first accidentally synthesized by the German chemist Hans von Pacma. In 1898, while heating diazomethane, he synthesized a white waxy compound, which was later named polyethylene [2]. The first industrial synthesis method was carried out by Eric Fawcett and Reynolds Gibson. These two scientists obtained a wax-like substance by heating a mixture of ethylene and benzaldehyde under high pressure.