LDPE studie

16th European Symposium on Computer Aided Process Engineering and 9th International Symposium on Process Systems Engineering W.Marquardt, C. Pantelides (Editors) © 2006 Published by Elsevier B.V. 595 A comprehensive investigation on high-pressure LDPE manufacturing: Dynamic modelling of compressor, reactor and separation units Prokopis Pladis, Apostolos Baltsas and Costas Kiparissides Chemical Engineering Department and Chemical Process Engineering Research Institute Aristotle University of Thessaloniki, P.O.Box 472, 54006 Thessaloniki, Greece Abstract A comprehensive mathematical model is developed for the simulation of high-pressure Low Density Polyethylene (LDPE) plants. Correlations describing the thermodynamic, physical and transport properties of the ethylene-polyethylene mixture are presented and compared with experimental data. Energy balances around the compression units are derived to calculate the energy requirements. A detailed kinetic mechanism is proposed to describe the molecular and structural developments of the free-radical polymerization of ethylene. Based on the postulated kinetic mechanism, a system of differential mass balance equations are derived for the various molecular species, total mass, energy and momentum in the polymerization system. Simulation results show that the proposed mathematical model can be successfully applied to the real-time prediction of reactor temperature profile and polymer melt index. Moreover, model predictions are compared with industrial measurements on reactor and coolant temperature profiles, reactor pressure, conversion, and final molecular properties for different polyethylene grades. Finally, various equations of state (e.g., Sako-Wu-Prausnitz, SAFT, PC-SAFT) are employed to simulate the operation and phase equilibrium in the flash separation units. Keywords: Modeling, LDPE, Compressor, Reactor, Separation, Phase Equilibrium. 1. Introduction Low Density Polyethylene (LDPE) is used in a large number of applications (e.g., packaging, adhesives, coatings, and films), as a result of the wide range of molecular and structural properties. LDPE has been commercially produced in high-pressure reactors for more than 4 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 high temperatures (120-320ºC) and pressures (15003000 bar). 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, (e.g., with different polydispersity, long chain branching and density, 0.915-0.935 g/cm3). A generic flow diagram (Figure 1) of the high-pressure ethylene copolymerization process can be described as follows: Fresh ethylene, after the primary compression, is mixed with the recycled ethylene and comonomer (e.g., vinyl acetate, methyl acrylate, ethyl acrylate, methacrylic acid, etc.), that is then pressurized to the desired reactor pressure in the second compression stage. Polymerization of the monomers is Schematic representation of a high-pressure LDPE tubular reactor process. In the first stage. there are only a few publications that deal with the description of the modeling of the high.596 Ethylene Peroxides Coolant Peroxides Coolant Peroxides Coolant P. In the second stage.. most of the published studies are limited to the modeling of the polymerization reactor. The polymer rich liquid phase from the bottom of the high-pressure separator is directed to the low-pressure separator. Over the past 30 years a great number of papers have been published on the modeling of LDPE tubular reactors (Kiparissides et al. the let down valve drops the pressure of the outlet reactor stream to 150-300 bar. The development of a comprehensive mathematical model for the high–pressure LDPE process should include detailed modeling of the following process units: a) the monomer(s) compression unit.. physical and transport properties of the reaction mixture at the various stages of the process are calculated by using a number of equation of states. Coolant Side Feed 1 Side Feed 2 Products Cooler Primary Compressor Secondary Compressor Coolant Valve . In this study. In addition the energy requirements of compressor units is calculated. The ethylene gas leaving the low-pressure separator is directed to the primary compressor and is mixed with fresh ethylene feed. . A comprehensive mathematical model for the design and simulation of high-pressure .g.and low-pressure separation units. organic peroxides). the pressure of ethylene-polyethylene mixture entering the low-pressure separator is further reduced to about 1. b) the polymerization reactor. The ethylenepolyethylene mixture entering the high-pressure separator is split into a polymer rich liquid phase (containing 70-80% per weight) and an ethylene rich gas phase (containing ethylene and small amounts of wax). However. the thermodynamic. Pladis et al. The monomer conversion per reactor pass can vary from 15 to 35 %. Reactor Zone 1 Reactor Zone 2 Reactor Zone N Feed . The liquid bottom stream leaving the low-pressure separator (containing very low concentration of ethylene) is sent to the extruder where the polymer is pelletized.5 bar. 2005).. Coolant HP Separator Side Feed N-1 Wax Separator Cooler HP recycle Valve Wax Wax Separator Cooler LP recycle LP Separator Wax Dryer Extruder Silo Polyethylene Figure 1. As a result. The separation is performed in two successive stages. initiated by adding a mixture of chemical initiators (e. and c) the product separation system.. 2. Thus we know only H1 and H2 and Ws are left as unknowns. scaled reactor temperature profiles are plotted for three homopolymer polyethylene grades (A. From the steady-state energy balance around the compressor unit. To account for the temperature increase after a compression stage the energy balance calculations around the compressor unit should be derived. respectively: ΔH = H 2 − H 1 = − Ws Normally. This presupposes a thorough understanding of the polymerization kinetics. 2005.g. In a compression process the isentropic work is the minimum shaft work required for compression of a gas from a given initial state to a given discharge pressure: (ΔH )s = − Ws (isentropic) In a non-ideal operation the compression efficiency is defined as follows: η= Ws (isentropic ) (ΔH )s = Ws ΔH Compression efficiencies are usually in the range 70 to 80 percent. and short and long chain branching frequencies). are known. E). The predictive capabilities of the proposed mathematical model are demonstrated by direct comparison of the model predictions with literature experimental measurements and industrial data covering a wide range of operating conditions.The compression of gases is accomplished in high-pressure reciprocating compressors. SAFT. In the present study.5 bar to about 260 bar.A Comprehensive Investigation on High-Pressure LDPE Manufacturing 597 LDPE reactors is presented. In Figure 2. In the secondary compressor system. chain length. copolymer composition. Pladis and Kiparissides. The dynamic model of the separator is able to predict deviations from the theoretical phase equilibrium state as it has been observed in real plant data. The kinetic constants are taken by Kiparissides et al. Tubular Reactor Units. For the thermodynamic calculations SAFT equation of state was employed.. The continuous lines .2700 bar). Figures 2 . Modeling of LDPE Plant Units Compressor Units. control of the polymer chain microstructure during polymerization is of profound importance. (2005). The elementary reactions considered are summarized in Table 1 (Kiparissides et al. The compressor efficiency is used to determine the actual enthalpy change and therefore the actual temperature at the compressor outlet. the pressure is raised from about 1.g. Polymers made by free-radical polymerization are typically mixtures of macromolecules with different molecular structural characteristics (e. the inlet conditions (T1. PC-SAFT). The accurate modeling of primary and secondary compressor units are essential in LDPE production plants. The predictive capabilities of the mathematical model were examined by simulating the operation of an industrial high-pressure LDPE tubular reactor. C. we obtain for the initial (1) and final conditions (2). 1998). a comprehensive kinetic mechanism is postulated to describe the free-radical polymerization of ethylene. the calculation of phase equilibrium and the dynamic operation of high and low-pressure separator units is discussed. The ethylene-polyethylene phase equilibrium is calculated using various equations of state (e. Since the molecular features of the produced polymers are directly related to their enduse properties.5 illustrate some representative simulation and experimental results of the industrial LDPE tubular polymerization reactor. The number of temperature peaks (three) corresponds to the respective initiator injection points.P1) and the discharge pressure P2. Finally. Sako-Wu-Prausnitz. the pressure of the compressed monomer(s) and solvent(s) is raised to the reactor feed operating conditions (2400 .. In the primary compressor system. To accurately predict the performance of the flash separators.k β ' Termination by combination R x + R y ⎯⎯→ D x + y Termination by disproportionation k tcij R x + R y ⎯⎯→ D = x + Dy k tdij represent model predictions (obtained through the on-line parameter estimator modulus of the software) while the discrete points represent the experimental temperature measurements. It is apparent that the model predictions are in a very good agreement with measured temperatures. Pladis et al. In Figure 3-5. C.598 Table 1 : Kinetic Mechanism of Ethylene Polymerization Initiator(s) decomposition k ∗ I i ⎯⎯→ 2R di P.and low-pressure separators and. The phase equilibrium in the separator units is of major importance because it determines the residual amounts of monomer and other gases in the polymer leaving the high. E. Separator Units. 2 …. the predicted final properties are in a good with the experimental measurements. . and long chain branching per 1000 carbon atoms are plotted with respect to the reactor length for Grades A. ethylene conversion. 2. determines the flows and compositions of streams in the LDPE plant. Ns k Transfer to Polymer (LCB) R x + D y ⎯⎯→ D x + R y Intramolecular Chain Transfer (SCB) k tpij Rx k ⎯⎯→ bi Rx β scission of secondary and tertiary radicals R x ⎯⎯ ⎯→ D = x −1 + R 1 β scission of internal radicals k R x + Dy ⎯ ⎯→ Dx + R z + D= y−z B k β . N i Chain initiation reaction R∗ + M k k ⎯ ⎯→ R1 Thermal initiation thj 3M ⎯⎯→ R1 Propagation Rx + M p ⎯ ⎯→ R x +1 k Transfer to Monomer tm R x + M ⎯k ⎯→ Dx + R1 Transfer to CTAs tsij R x + S k ⎯⎯→ D x + R 1 . k = 1. . number average molecular weight. on the same time.. a study of the thermodynamic phase equilibrium behavior of an ethylene/polyethylene mixture was undertaken.. . I i = 1. In all cases.. 5 0. . thermodynamic and transport properties of the reaction mixture were calculated and each of the basic process units were successfully modelled. 3.2 0.7 0.1 Grade A Grade C Grade E Grade Aexp. E).9 1. Predicted vs experimental ethylene conversion profiles (Grades A.3 0. Conclusions It is well known that the dynamic behaviour of the complete plant can be completely different from the behaviour of the reactor due to the various recycling streams and different time-scaled process units (Cervantes. simulation of industrial highpressure LDPE plants was developed. The polymer rich liquid phase from the bottom of the high-pressure separator is directed to the low-pressure separator.50 Grade A Grade C Grade E Grade A exp. 2000). The reduction of the amount of off-specification polymer during the grade transition operation is important for the economical operation of continuous polymer plants. the pressure of ethylene-polyethylene mixture entering the low-pressure separator is further reduced to about 1.1 0.0 0. Grade E exp.4 0. As it can be seen the polyethylene of the vapor phase consist of polymer with lower molecular weights compared with the polymer in the other phase.9 1. 0.0 Relative Reactor Length Relative Reactor Length Figure 2. Grade transition operation is essential in continuous polymer plants because many grades of polymers are produced from the same process. the pressure of the reactor outlet stream is reduced to 260 bar and then is directed at the inlet of the high-pressure separator.00 0. C.0 1.7 0. In the present study a comprehensive mathematical model for the design.4 0. In the second stage. In the first stage. Grade E exp.6 0. The separation of LDPE from the unreacted monomer and solvents is carried out in a two-stage process downstream the tubular reactor (Buchelli. and c) the product separation system as well as accurate predictions of the thermodynamic and transport properties of the fluid at the various stages of the process.80 0.1 0.70 0.5 0. 0.6 0. In Figure 6 the Molecular weight distributions (MWD) of the vapor and liquid phases as well as the NAMW calculated at 1500 bar using the Sako-Wu-Prausnitz equation of state.10 599 Relative Reactor Temperature 1.8 0. b) the polymerization reactor.2 0.3 0. Grade C exp.0 0. C. Figure 7 depicts the effect of separator pressure on the number average molecular weight of the polymer that is distributed in the two phases. E). Various equations of state and correlations for the predictions of physical. Figure 3.A Comprehensive Investigation on High-Pressure LDPE Manufacturing 1.40 0.5 bar. The development of a comprehensive mathematical model for the high– pressure LDPE process should include detailed modeling of the following process units: a) the monomer(s) compression unit.60 0.8 0. Predicted vs measured temperature profiles (Grades A.90 Ethylene Conversion 30 25 20 15 10 5 0 0. 2004). Grade Cexp. 983 C.5 0. A. Eng. Kiparissides. Ind. Cervantes A.3 0.8 0.. Grade C exp. Brandolin.0x10 4. . 2592. Kelly. Buchelli. M. A.I. Eng. L. J. References A..0x10 2. 18. Bokis.4 0. C. Effect of pressure on the number average molecular weight of the polymer in the two phases.0 0.9 1.0x10 6. NAMW 1. E).9 1. E). A.0x10 2. Grade E exp. Brown. Pladis and C.0x10 4 P. Kiparissides. 2004. 2005. 2000. Bandoni . Richards.5x10 1.0 0. Congalidis. Nonequilibrium Behavior in Ethylene/Polyethylene Flash Separators. Molecular weight distributions of the vapor and liquid phases calculated using the Sako-Wu-Prausnitz equation of state Figure 7. A. C. 10000 -5 -5 Total Liquid Vapor Number Average Molecular Weight 8000 6000 Liquid Vapor Weight Fraction 8. Chemical Engineering Science. S.5x10 2.2x10 1.1 Relative Reactor Length Relative Reactor Length Figure 4. Call. Large-scale dynamic optimization of a low density polyethylene plant.0 0. Grade C exp. Pladis et al.8 0.1 0..0x10 -5 Figure 5. and J. 3315. 1768. Biegler . Mathematical Modeling of Free-Radical Ethylene Copolymerization in High-Pressure Tubular Reactors.6 0. Ind.3 0. M. Eng.0x10 -6 -6 4000 2000 0 -6 -6 0. Chem. and Y. 1. Chem. S. Baltsas. A comprehensive Model for the Calculation of Molecular Weight and Long Chain Branching Distribution in Free-Radical Polymerizations.5 0. Franjione.2 0. S. Papadopoulos. 1998. A. 44. 53.0x10 4 4 5. C.0x10 3 0.6 0.1 0 0.7 0. 4 6 Long Chain Branching 4 5 4 3 2 1 Grade A Grade C Grade E Grade A exp. Comp. Tonelli. Res. Predicted vs experimental NAMW profiles (Grades A. Res.0 1. 24.P. 7 Grade A Grade C Grade E Grade Aexp. Ye.1 0.4x10 1.600 3. Predicted vs experimental long chain branching profiles (Grades A. Ramanathan. P. J. 43.L.0 100 1000 10000 100000 1000000 Molecular W eight 0 200 400 600 800 1000 1200 1400 1600 1800 Pressure (bar) Figure 6.4 0.7 0.2 0. Chem. Grade E exp.0 1.