Two types of model for product quality controls in polymerization processes have been developed.
The first is a macroscale dynamic model for predicting the residence time distribution (RTD) of polymer particles in propylene polymerization over Ziegler-Natta catalyst. The polymerization process is carried out in a slurry reactor which is embedded with a classifier. This dynamic model is possibly the first in this area. From RTD, several important process parameters can be deducted, such as particle size distribution (PSD) of polymer particles, catalyst efficiency (CE) and void fraction of the slurry. The method of lines is applied in order to solve the model numerically. Testing of several finite difference approximation schemes in the age domain found that the 'best' discretization methods is the five points upwind finite difference method.
A comparison of the steady state RTD calculated from the model with that from the analytical equation for a simplified process consisting of only a reactor (without classifier) has shown that the model is quite accurate. The steady state CEs calculated from the model in a similar system is also in very good agreement with that of the steady state model and the plant data, with deviation ranging from 2-10%. Simulation of the model run by changing process variables has been performed. The results of this simulation show that the model represents the process quite satisfactorily and therefore it has the potential to be used as a tool for sensitivity study and as a 'soft sensor' for control purposes.
The second model is a mesoscale dynamic model for predicting the rate of adsorption of polymer surfactant onto the oil/water interface and its effect on the change of interfacial properties. This phenomenon is important in suspension polymerization systems. The kinetic model takes into account the rate of conformation process of adsorbed polymers, which has long been known to affect the adsorption nature of polymer as well as the interfacial tension. The model is an applicable model that its practical form enable it to be embedded in a macroscale model for predicting macro properties of the process, e.g. PSD of droplets and polymer particles product. Testing of the model by varying the rate constant of an individual mechanism, which is mvolved in the adsorption process, has shown that the role of the conformation process should not be neglected. The rate of adsorption from the bulk phase plays an important role during the early stage of adsorption, where a sharp increase in adsorption occurs. The adsorption dynamics of the stage following the early stage adsorption process will be determined by competition between the conformation and desorption rates. The final 'maximum' amount of the adsorbed polymer depends on the competition between the rate of adsorption and conformation, against the rate of desorption.
Verification of the model with experimental data of the adsorption dynamics of poly(vinyl) acetate (PVAs) of various chain lengths and various degrees of hydrophobicities into oil/water interface has been carried out. The interfacial tensions calculated by the model are in very good agreement with those of the data. The adsorption rate constant (ka) is found to be dependent on bulk polymer concentration (Cfc), while the rate constants for conformation (kc) and desorption (kd0) are independent of Cb.
The chain length and, more profoundly, the degree of hydrophobicity of the PVAs affect the interfacial tension. The longer the chain length is, the higher the change of interfacial tension will be; whereas the higher the degree of hydrolysis is, the smaller the change in interfacial tension will be.