Modeling of High and Low Pressure Separator Units in High Pressure LDPE Plants
Systematic methods and tools for managing the complexity
Process Simulation & Optimization - I (T4-9a)
Keywords: LDPE, Separators, Phase equilibria, High pressure, Modeling
Low Density Polyethylene (LDPE) has been commercially produced in high-pressure reactors for more than five decades. Two reactor technologies (i.e., tubular and autoclaves) are employed in the high-pressure polymerization of ethylene. The polymerization of ethylene is typically carried out at very high temperatures (120-320ºC) and pressures (1500-3000 atm). Thus, in the presence of a mixture of initiators (e.g., peroxides, azo compounds), ethylene can be polymerized via a free-radical mechanism. A large variety of LDPE grades is usually produced from a single reactor line, with varying degree of polydispersity, branching and density (0.915-0.935 g/cm3).
In a high pressure LDPE process, fresh ethylene, after its primary compression, is commonly mixed with the recycled ethylene stream from the flash separators. The combined stream is pressurized to the desired pressure, in the second compression stage, and is then fed to the reactor. Polymerization of the monomer is initiated by adding free-radical initiators (usually organic peroxides). Due to the short reactor residence time (30-90 s), the monomer conversion is relatively low, between 15 and 35 %.
The separation of unreacted ethylene from the LDPE is performed in two successive separation stages. In the first stage, the reactor let down valve drops the pressure of the outlet stream to 150-300 atm which is then directed to the high-pressure separator. The ethylene-polyethylene mixture entering the high-pressure separator is split into a polymer rich liquid phase (containing 70-80% per weight polymer) and an ethylene rich gas phase (containing ethylene and amounts of wax). The polymer rich liquid phase from the bottom of the high-pressure separator is directed to the low-pressure separator. In the second flash separator, the pressure is further reduced to about 1.5 atm. The ethylene gas and waxes stream, leaving the low-pressure separator, is fed to the primary compressor while the liquid bottom stream is sent to the extruder where the polymer is pelletized.
In this work, the dynamic modeling of high and low-pressure separator units is addressed. The phase equilibrium of the ethylene-LDPE mixture is calculated using various equations of state models (e.g., Sako-Wu-Prausnitz, SAFT, PC-SAFT). The capability of the proposed model to describe the phase-equilibrium of ethylene-polyethylene mixture and the dynamic operation of flash separator is demonstrated by comparing model predictions with available experimental data.
Presented Monday 17, 11:15 to 11:33, in session Process Simulation & Optimization - I (T4-9a).
