Velocity profile, bulk polymerization

As a sequel to the simple reactor model described above, two-zone cases for the bulk polymerization of styrene were also studied. Polymerizations in straight, empty tubes give rise to unfavorable temperature and velocity profiles which can lead to hydrodynamic or thermal instabilities. These instabilities may be avoided or postponed by manipulating the wall temperature. 

For the bulk polymerization of styrene using thermal initiation, the kinetic model of Hui and Hamielec (13) was used. The flow model (Harkness (1)) takes radial variations in temperature and concentration into account and the velocity profile was calculated at every axial point based on the radial viscosity at that point. The system equations were solved using the method of lines with a Gear routine for solving the resulting set of ordinary differential equations. 

For viscous solutions, the assumptions of plug flow are not strictly valid. If the velocity profile is not flat, polymer solution near the tube wall will move more slowly than that near the center of the tube. Since the slow-moving polymer near the wall remains in the reactor longer, it will polymerize to a higher conversion (or extent of reaction) than the bulk material. This higher conversion will then compound the viscosity problem. Studies on the effect of this deviation from plug flow in tubular polymerization have to be carried out by Hamer and Ray [4,5]. 

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