Control of molecular weight distribution of polyethylene in gas-phase fluidized bed reactors

This paper presents a feasibility study of the broadening of polyethylene molecular weight distribution produced using a multisite Ziegler-Natta catalyst in a fluidized-bed reactor. A nonlinear model predictive control algorithm, applied to a validated model of the reactor, is used for the on-line control of the entire molecular weight distribution of the produced polymer. Control of a target chain-length distribution is achieved by selecting a collection of points in the distribution and using them as set points for the control algorithm. An on-line Kalman filter is used to incorporate infrequent and delayed off-line molecular weight measurements. Through simulation the control algorithm is evaluated, under tracking conditions as well as plant-model mismatch. The results demonstrate that the control algorithm can regulate the entire molecular weight distribution with minimum steady state error. However, the efficiency of this approach is highly dependent on the dynamics of hydrogen inside the reactor.

[1]  G. Butler,et al.  Polymerization of 2-allyl-1-methylenecyclohexane by the cyclic polymerization mechanism , 1966 .

[2]  W. Ray.  Periodic Operation of Polymerization Reactors , 1968 .

[3]  J. Henderson,et al.  Polymerization Reactors and Processes , 1979 .

[4]  G. R. Meira.  Forced Oscillations in Continuous Polymerization Reactors and Molecular Weight Distribution Control. A Survey , 1981 .

[5]  G. E. Eliçabe,et al.  Estimation and control in polymerization reactors. A review , 1988 .

[6]  W. Kaminsky,et al.  Comparison of zirconocene and hafnocene catalysts for the polymerization of ethylene and 1-butene† , 1992 .

[7]  Emad Ali,et al.  Optimization-based Tuning of Nonlinear Model Predictive Control with State Estimation , 1993 .

[8]  W. Ray,et al.  Dynamic modeling of product grade transitions for olefin polymerization processes , 1994 .

[9]  Andrzej Stankiewicz,et al.  An industrial view on the dynamic operation of chemical converters , 1995 .

[10]  P. McLellan,et al.  Effects of operating conditions on stability of gas‐phase polyethylene reactors , 1995 .

[11]  José Carlos Pinto,et al.  A survey of advanced control of polymerization reactors , 1996 .

[12]  T. Crowley,et al.  Discrete Optimal Control of Molecular Weight Distribution in a Batch Free Radical Polymerization Process , 1997 .

[13]  Hong Chen,et al.  Ziegler-natta catalysts for olefin polymerization , 1997 .

[14]  S. Al-zahrani,et al.  Optimization and Control of Industrial Gas-Phase Ethylene Polymerization Reactors , 1998 .

[15]  J. Soares,et al.  Use of hydrogen for the tailoring of the molecular weight distribution of polyethylene in a bimetallic supported metallocene catalyst system , 1998 .

[16]  C. Kiparissides,et al.  Supervisory Control of High Pressure LDPE Reactors , 1998 .

[17]  J. Leiza,et al.  Molecular‐weight distribution control in emulsion polymerization , 1998 .

[18]  T. Crowley,et al.  Experimental studies on optimal molecular weight distribution control in a batch-free radical polymerization process , 1998 .

[19]  Nader M. Al-Bastak,et al.  Periodic Operation of a Reverse Osmosis Water Desalination Unit , 1998 .

[20]  A. Penlidis,et al.  A critical examination of polyethylene molecular weight distribution control through the combination of soluble metallocene/methylalumoxane catalysts , 1998 .

[21]  Morimasa Ogawa,et al.  Quality inferential control of an industrial high density polyethylene process , 1999 .

[22]  A. Hamielec,et al.  Recipes for synthesizing polyolefins with tailor-made molecular weight, polydispersity index, long-chain branching frequencies, and chemical composition using combined metallocene catalyst systems in a CSTR at steady state , 1999 .

[23]  E. G. Chatzi,et al.  Recent Developments in Hardware Sensors For the On-Line Monitoring of Polymerization Reactions , 1999 .

[24]  Hiroya Seki,et al.  Optimal grade transition control for polymerization reactors , 2000 .

[25]  Jin Suk Chung,et al.  Control of molecular weight distribution for polyethylene catalyzed over Ziegler–Natta/Metallocene hybrid and mixed catalysts , 2000 .

[26]  J. Scheirs,et al.  Metallocene-based polyolefins : preparation, properties and technology , 2000 .

[27]  J. Pinto,et al.  Retrofitting of industrial olefin polymerization plants: producing broad MWDs through multiobjective periodic operation , 2000 .

[28]  E. Lima,et al.  Dynamic optimization of semicontinuous emulsion copolymerization reactions: composition and molecular weight distribution , 2001 .

[29]  J. Leiza,et al.  Simultaneous control of copolymer composition and MWD in emulsion copolymerization , 2001 .

[30]  A. J. Morris,et al.  Online optimizing control of molecular weight properties in batch free-radical polymerization reactors , 2002 .

[31]  Jichang Wang.  Complex dynamics in a nonlinear chemical system switching between two stable stationary states , 2003 .

[32]  A. Ajbar,et al.  Multivariable control of a simulated industrial gas-phase polyethylene reactor , 2003 .

[33]  Martin Schlegel,et al.  Optimization and Control of Polymerization Processes , 2005 .

[34]  Y. Kissin,et al.  Multi‐center nature of heterogeneous Ziegler–Natta catalysts: TREF confirmation , 2005 .

[35]  A. Konieczny,et al.  Periodic operation of biofilters. A concise model and experimental validation , 2005 .

[36]  M. Ali.  Modeling and control of molecular weight distribution in a liquid-phase polypropylene reactor , 2006 .

[37]  John R. Richards,et al.  Measurement and control of polymerization reactors , 2006, Comput. Chem. Eng..

[38]  W. Ray,et al.  Dynamic modeling of polyethylene grade transitions in fluidized bed reactors employing nickel-diimine catalysts , 2006 .

[39]  Neural Network Based Control of the Acrylonitrile Polymerization Process , 2007 .

[40]  C. Kiparissides,et al.  On the Production of Polyolefins with Bimodal Molecular Weight and Copolymer Composition Distributions in Catalytic Gas-Phase Fluidized-Bed Reactors , 2007 .

[41]  Brian Roffel,et al.  Non-linear model based control of a propylene polymerization reactor , 2007 .

[42]  B. Roffel,et al.  Molecular weight distribution broadening of polypropylene by periodic switching of hydrogen and catalyst additions , 2008 .

[43]  P. L. Silveston,et al.  Periodic Operation of Three — Phase Catalytic Reactors , 2008 .

[44]  J. Pinto,et al.  Control of Polymerization Reactors , 2008 .