No abstract

Effects of mixing on number-and weight-average degree of polymerization in free radical homopolymerization in solution in a semibatch stirred tank-reactor have been modeled by use of the "partially segregated feed" model of Villermaux (1989) and the lumped kinetic treatment of addition polymerization. Mixing conditions are described in terms of the dimensionless time constants OM and 0, for convective and diffusive mixing, respectively. Results show that DP,, is relatively insensitive, while DP, is highly sensitive to mixing conditions. Comparison with ideal mixing results shows thatpolydispersity (DPJDP,,) can rise greatly as mixing becomes increasingly nonideal, depending on the value of dimensionless flow rate and dimensionless initiator rate constant. Time constants of the mixing model need to be expressed as dimensional correlations before the proposedpolymerization model can be compared to experimental measurements. The model may be adapted to melt polymerization in nonideal CSTR such as LDPE.

Pressure drop is an important parameter in the design of gas/liquid cocurrent downflow packed bed reactors. It is needed in calculating the energy required for pumping and compression, as well as in sizing related equipment.It also affects the operating pressure and gas-phase reactant partial pressure. Pressure drop has been used by several workers to correlate gas-liquid and liquid-solid mass transfer. In this study past work in the literature is reviewed and the most significant correlations are examined against measurements from the present work on six nonfoaming gas-liquid systems with a 5.1 cm i.d. glass column packed with spherical glass beads of 1.9 mm diameter to a height of 85 cm, with liquid and gas superficial mass velocities ranging from 3 to 30 kg/m2 s and 0.0005 to 1.0 kg/m2 s, respectively. A new correlation of the Lockhart-Martinelli type is proposed which agrees well with two similar correlations in the literature.

Flow and reaction in a typical commercial scale autoclave LDPE reactor were modeled by a three-dimensional computational fluid dynamic (CFD) k-model in order to shed light on the macrosegregation effects that can occur in these reactors. It is shown that the CFD model predicts significant differences from CSTR behavior. Results are discussed in terms of the effects of macro-and microscale inhomogeneities of concentration and temperature on free radical polymerization kinetics. The observed nonidealities in terms of minima in the initiator consumption curves and multiple steady states are explained on the basis of competing turbulent transport and chemical kinetics. Microsegregation effects are shown to be negligible in comparison to macrosegregation effects. Given the fact that the CFD model is based on reactive scalar and energy balances without adjusted parameters in the three-dimensional flow field of the entire reactor, it is tentatively concluded that commercial-scale LDPE vessel reactors can have significant macrosegregation effects beyond a certain steady-state adiabatic operating temperature that is specific to the initiator being used.