commercially using metallocene catalysts in a variety of different reactor technologies including gas-phase and solution reactors. [3] Metallocene catalysts are organometallic compounds with a transition metal such as Zr or Hf, surrounded by cyclic organic ligands. [3] Metallocene aluminoxane catalysts can produce PE with narrow molecular weight distribution (MWD) and comonomer composition distribution (CCD), which can result in excellent mechanical properties, but limited processability. The MWD of PE produced using metallocene catalysts can be broadened using several techniques. Using a combination of two or more metallocene catalysts in the same reactor can produce PE products with a tailored joint MW and composition distribution that leads to favorable processing and end-use properties. [3] Supporting a single type of metallocene catalyst on a solid support can also result in the formation of two or more types of active catalyst sites, which may have a beneficial influence on polymer properties. [2] Interactions with different activators or scavengers in the reactor system can also result in multi-site behavior. [4] When multiple types of active sites are present in a reactor, each site type instantaneously produces PE of a different MW and composition distribution. For example, in a two-site metallocene system, one site may tend to produce PE with higher molecular weight and higher comonomer incorporation than the other site, resulting in broad orthogonal composition distribution (BOCD). [4,5] The joint MW and composition distribution of ethylene/α-olefin copolymers produced using metallocene catalysts depends on the reactor operating conditions (e.g., temperature and the concentrations of ethylene, comonomer, and hydrogen, which is used as a chain-transfer agent to control average molecular weight). As a result, PE with tailored properties can also be produced using multiple reactors in series or a single reactor containing multiple zones with different operating conditions in each. [6] When designing new product grades and deciding how best to make them, PE producers benefit from mathematical models that predict product properties from reactor operating conditions. [6] The main difficulty associated with model development for PE processes is determining suitable values for the many kinetic A dynamic model is developed to predict detailed chain-length and comonomer incorporation behavior during gas-phase ethylene/hexene copolymerization using a supported hafnocene catalyst. The multi-site catalyst results in a copolymer with a broad orthogonal composition distribution (BOCD) where the high molecular-weight tail has high hexene incorporation. The model relies on gel permeation chromatography measurements obtained using multiple detectors (GPC-4D), so that the composition of the copolymer is determined for different chain-length fractions. Chainlength distributions are discretized into bins so that comparisons can be made between GPC-4D data and model predictions. Parameter estimation is aided by an estimabil...
