Completed Research
Mass Transfer and Mathematical Modeling for Propylene Polymerization Process
Jianjun Li, PhD, 1995
(Thesis: UMI Dissertation Publishing)
The gas solubility (C*) and mass transfer coefficient (kLa) were measured for hydrogen, ethylene, propylene, propane in polypropylene/n-hexane, polypropylene/propylene slurry systems at various pressures (1-60 bar), temperatures (313-353 K), mixing speeds (800-1200 rpm) and solid concentrations (0-40 wt%) in a 4 liter Zipper-Clave reactor operating in a surface-aeration mode. The solubility C* values were calculated using a modified Peng-Robinson equation of state. The solubility values for the systems studied were found to increase with gas partial pressure and temperature for hydrogen while the values decreased with temperature for gaseous ethylene, propane and propylene. The phenomenon of liquid propane and propylene evaporation was confirmed from the fugacity calculations during hydrogen gas absorption. The solubility of hydrogen in both propylene and propane appeared to agree very well with available literature data. The solubilities of different gases in liquid n-hexane followed the order: C*C3H6 > C*C3H8 > C*C2H4 > C*H2 and the gas solubilities in liquid propylene followed the order: C*C2H4 > C*H2 The transient Physical Gas Absorption Technique (TGPAT) was used to obtain the mass transfer coefficients, kLa. The experimental results showed that kLa values increased with mixing speed, slightly increased and then decreased with solid polypropylene powder concentration, while the effects of pressure and temperature were system dependent. Large kLa values were found for hydrogen in propylene at very low pressure which were attributed to the presence of large number of small gas bubbles in that condition. A statistical approach was followed to investigate the effects of pressure, temperature, mixing speed and solid concentration on kLa for gaseous hydrogen, ethylene, and propylene in liquid n-hexane containing polypropylene powder. The statistical models developed were able to correlate the experimental values with a very reasonable confidence level (alpha=0.1). kLa values appeared to reach a maximum around 15 wt% solid concentration and sharply decrease above 30 wt% solids. The effects of pressure and temperature on kLa were found to depend on the gas-liquid system and operating conditions used and were less pronounced when compared with those of mixing speed and solid concentration. A mathematical model which takes into account the effect of kinetics, mass transfer, hydrogen, and electron donor on the performance of propylene polymerization process was developed. The model was capable of predicting the performance of a 16-gallon Pilot Plant agitated reactor within 20%. The prediction of the model showed that mass transfer reduced the performance of propylene polymerization process by as much as 30%, if the mass transfer coefficients of propylene were to drop below 10-3 l/s. This situation could occur in industrial processes due to inadequate mixing conditions. The model predicted that the molecular weight of the isotactic polypropylene decreases with hydrogen partial pressure, increases with propylene concentration and gas/liquid mass transfer coefficient of propylene. The model also showed that polymerization rate and polymer yield decrease with electron donor concentration and increase with hydrogen partial pressure, propylene partial pressure and mass transfer coefficients.