Study II VDF Precipitation Polymerization

The base case corresponds to an initial monomer concentration of 3.1 mol L and a pressure of 20.4 MPa, respectively. The fitting to the experimental data in order to estimate the three adjustable parameters was done in terms of conversion versus time (Fig. 6.8) and MWD versus conversion (Fig. 6.9). Inspection of the results depicted in the two figures shows that the agreement is remarkably good, and the final values of the adjustable parameters are given in Table 6.3. Note that a value of 0.25 m g was used for the specific surface area. This value corresponds to a final particle diameter of around 10 pm, which is reasonable for the system under examination. 

The polymerization proceeds in two reaction loci, the continuous phase and the polymer particles. At low conversion, the first locus is dominant and low-molecular-weight chains are predominantly produced. Since no gel effect is operative in supercritical phase, no acceleration is verified in the first part of the conversion versus time curve and the first mode of the MWD is quite narrow, corresponding to a polydispersity of about 1.5. On the other hand, the relevance of the polymerization in the particles is increases rapidly and becomes dominant by the first hour. This behavior explains the increase in polymerization rate and the high molecular weight of the second MWD mode (much lower ter- 

The effect of changing the initial pressure on reaction rate and MWD is shown in Figs. 6.10 and 6.11, respectively. Even though the experimental data exhibit some dependence upon pressure (increasing reaction rate and chain 

As mentioned above, the model was adapted to continuous reactors (CSTR) in order to further vahdate its prediction ability. In particular, the effect of changing the inlet monomer concentration on the MWD was investigated. Simulations were carried out using the identical set of model parameter values already considered for the batch simulations above. However, because of the higher operating temperature in the CSTR reactions (see Table 6.5) with respect to the batch experiments, the binary interaction parameters used in the frame of the Sanchez-Lacombe equation of state have to be updated. This has been done 

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Case Study II VDF Precipitation Polymerization 125 Table 6.3 Model parameter values and corresponding sources used In case study II. 

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