E.G. Chatzi

Characterization of the LCST behaviour of aqueous poly(N-isopropylacrylamide) solutions by thermal and cloud point techniques

Abstract A systematic investigation of the phase separation behaviour of aqueous low molecular weight PNIPA solutions was carried out in order to quantitatively investigate the effect of measurement conditions on the reported phase separation temperatures in relation to three techniques, namely, differential scanning calorimetry, optical cloud point and u.v. turbidimetry. The PNIPA concentration was varied in the range 0.5–22 wt%. All three techniques yielded comparable phase separation temperatures, independently of the transition kinetics, provided that the time scale of the experiments is large enough to ensure close to equilibrium conditions.

Recent Developments in Hardware Sensors For the On-Line Monitoring of Polymerization Reactions

2. ON-LINE CONVERSION AND COPOLYMER COMPOSITION MONITORING 61 2.1. Densimetry 63 2.2. Refractive Index Measurements 68 2.3. Gas Chromatography 70 2.4. Calorimetry/Reactor Energy Balances 75 2.5. Ultrasound Measurements 81 2.6. Fluorescence Spectroscopy 85 2.7. Ultraviolet Reflection Spectroscopy 91 2.8. Near-Infrared Spectroscopy 92 2.9. Midrange Infrared Spectroscopy 97 2.10. Raman Spectroscopy 98

Dynamic simulation of bimodal drop size distributions in low-coalescence batch dispersion systems

Abstract Transient drop size distributions in a batch stirred tank are a manifestation of the drop breakage and coalescence mechanisms. A recently developed experimental configuration using a laser diffraction technique suitable for on-line monitoring of drop size distributions, showed increased sensitivity in measuring the small diameter drops. For all experimental conditions tested it was observed that the batch low-coalescing dispersion system assumed characteristic bimodal distributions within very short agitation time. The laser diffraction technique revealed the fine changes of the small size peak of the distribution with sufficiently high resolution. The objective of the present study was to establish a breakage mechanism in order to predict the dynamic behaviour of the bimodal drop size distributions by using the population balance formalism. The proposed model considers breakage of the drops into one daughter and several satellite drops. The volume ratio between the generated drops as well as the number of satellite drops are considered to be proportional to the parent drop volume. The obtained agreement between the experimental and theoretical bimodal drop size distributions is quite satisfactory for a range of experimental conditions.

Drop size distributions in high holdup fraction dispersion systems: effect of the degree of hydrolysis of PVA stabilizer

Abstract Although turbulence-stabilized dispersions are of considerable industrial importance, their behaviour is generally not well understood, especially for the high dispersed phase volume fractions encountered in suspension polymerization reactors. The dispersion is formed by two dynamic processes, namely, drop breakage and coalescence. Breakage mainly occurs in regions of high shear stress near the impeller or as a result of turbulent velocity and pressure fluctuations along the surface of a drop. Droplet coalescence is prevented by the combined action of turbulent agitation and the addition of protective colloids such as poly(vinyl alcohol) (PVA). Depending on the combination of the degree of agitation and the concentration and type of surface active agent, the average droplet size can exhibit a U-shaped variation with respect to the impeller speed or the degree of hydrolysis of the PVA stabilizer. This U-type behaviour has been confirmed both experimentally and theoretically and has been attributed to a manifestation of the balance between breakage and coalescence rates of the monomer drops. Both processes are related to the interfacial tension and its variation with time as a result of the varying polymer concentration and conformation at the monomer/water interface. In the present study, a series of experiments were carried out with a model system of 50% n -butyl chloride in water in the presence of PVA with varying degrees of hydrolysis at different agitation rates. A generalized numerical algorithm incorporating the most comprehensive models describing the breakage and coalescence processes has been used for simulating the steady-state drop size distributions in a high holdup liquid—liquid dispersion system in order to elucidate the breakage and coalescence mechanisms as a function of the droplet interfacial characteristics. It is shown that the model simulations are in satisfactory agreement with measurements of the drop size distributions over the whole range of the experimental conditions investigated.

Use of CFD in prediction of particle size distribution in suspension polymer reactors

Abstract A two-compartment population balance model has been developed for taking into account the large spatial variations of the local turbulent kinetic energy, in order to predict the evolution of droplet sizes in a high holdup (i.e., 47–50 vol%) suspension polymerization system as a function of the most important process conditions. Phenomenological expressions were applied for describing the breakage and coalescence rates as a function of the local energy dissipation rate and physical properties of the system. Computational fluid dynamics (CFD) simulations were used for estimating the volume ratio of the impeller and circulation regions, the ratio of turbulent dissipation rates and the exchange flow rate of the two compartments at different agitation rates and continuous phase viscosities. A satisfactory agreement was obtained between theoretically and experimentally determined drop size distributions.

Steric stabilization in emulsion polymerization using oligomeric nonionic surfactants

Although nonionic surfactants show improved stabilization characteristics in emulsion polymerization as well as superior shear and freeze thaw stability of the final latex, their behaviour is generally not well understood. In the present paper a steric stabilization model is developed for describing particle stabilization in emulsion polymerization systems in the presence of nonionic oligomeric surfactants. The model takes into account the effect of unequal particle sizes on the steric interaction potential and the resulting enhanced heterocoagulation. An additional feature of the model is the incorporation of a possible surfactant lateral migration mechanism, which may be significant in the case of oligomeric nonionic surfactants. The proposed model can simulate reasonably well the experimentally observed conversion profiles and average particle sizes as a function of the surfactant molecular structure (i.e., lengths of stabilizing and adsorbing moieties) and surfactant concentration. As a result of the competing effects of the adsorbed volume fraction and surface coverage on particle stability, an optimum hydrophobicity/hydrophilicity ratio for the oligomeric nonionic surfactants can be derived.

Recent Hardware Sensors Developments for Monitoring Polymerization Reactions

Abstract The availability of process computers and the recent developments of on-line techniques, particularly in combination with fiber optic waveguides, offer quite new possibilities for remote on-line and in-line sensing and have considerably increased the scope of on-line control of product quality in polymerization reactors. In the present work an attempt is made to present, as completely as possible, a comprehensive literature survey of the recent advances made in the development of on-line hardware sensors for monitoring of polymerization reactions, as well as to investigate the robustness and accuracy of the online techniques used to measure continuously monomer conversion, copolymer composition, molecular weight and MWD and particle size distribution in polymer reaction operations. Finally, software-sensing strategies including state estimation and filtering techniques are very briefly reviewed