Emad M. Ali

Effect of monomer feed and production rate on the control of molecular weight distribution of polyethylene in gas phase reactors

Abstract The control of the polymer molecular weight distribution via altering the monomer to hydrogen molar ratio in fluidized bed reactors is examined. The molar ratio is altered by manipulating the monomer and hydrogen feed rates using nonlinear model predictive controller. The simulation revealed promising results however a trade-off between utilizing both the monomer and hydrogen flows simultaneously and the hydrogen flow exclusively exists. Utilization of the monomer and hydrogen flows together favors the rapid transition to the required MWD but at the expense of higher purge and inconsistent production rate. Exclusive use of the hydrogen intake leads to steady production rate and less consumption of the purge. However, longer time is needed to achieve the desired MWD. The same phenomena are observed when discrete mechanism for product withdrawal is implemented.

Broadening the polyethylene molecular weight distribution by controlling the hydrogen concentration and catalyst feed rates

This paper discusses the control of an industrial gas-phase polyethylene reactor to produce a desired molecular weight distribution (MWD) of the polymer. The controller objective is to regulate online the entire molecular weight distribution by either manipulating the hydrogen content inside the reactor or coordinating the feed rates of two different types of catalysts. In this work, the molecular weight distribution is modeled as a function of the reaction kinetics and hydrogen to monomer ratio. Nonlinear model predictive controller (NLMPC) algorithm is used to maintain the desired molecular weight distribution online. The closed-loop simulations indicated the effectiveness of NLMPC to achieve its goal even in the presence of modeling errors. Moreover, the results showed that, altering the hydrogen concentration solely can produce the required polymer quality provided that an efficient mechanism is available to readily alter the hydrogen composition. Alternatively, the desired MWD can also be guaranteed with proper manipulation of the catalyst feed rates while the other process inputs are kept constant.

Controlling the Polyethylene Molecular Weight Distribution Using Hydrogen Influent

This paper addresses the feasibility of controlling the entire molecular weight distribution of the produced polymer in gas-phase ethylene polymerization reactors. Nonlinear model predictive controller is used to attain the control objective by utilizing the hydrogen feed rate as the only manipulated variable. The use of other manipulated variables is limited to avoid disturbing the process when influential inputs such catalyst and/or monomer inflows are used. The simulation results indicated successful implementation of the control algorithm to achieve the desired molecular weight distribution. The success depends on the improved hydrogen activities inside the reactor through a modified catalyst that is responsive to hydrogen variation and a wider admissible range of hydrogen feed rates.© 2010 ASME