Devolatilization efficiency

Many techniques have also been developed to improve devolatilization efficiency, including steam stripping [114], second fluid-assisted devolatilization [115], supercritical fluid devolatilization [116], and a variety of specially designed... 

Some useful insight can be developed concerning the influence of material properties and process conditions on devolatilization efficiency by considering the special case when the number of bubbles per unit volume of solution is constant. To fix ideas, assume that all bubbles are formed instantaneously when the solution enters the extraction zone and that no bubbles are ruptured until the very end of the process when all rupture simultaneously. Then the rate of formation can be expressed by... 

Alternatively, vents with vent stuffers can be used to prevent product discharging. However, this will reduce the devolatilization efficiency due to the reduction of the available devolatilization cross-section. 

Figure 10 shows the predictions of the computer model for the two reference coals and the CSD and PFD SRC. A temperature of 2500 F and 20% excess air were used for this case. The intercepts of the curves on the Y-axis indicate the pyrolysis (devolatilization) efficiencies of the fuels. The plots show the combustion efficiencies of the CSD and PFD SRC, and the WSB coal approach 100% in 2 seconds residence time, whereas, the marginal KHB coal attains about 97% combustion efficiency. 

Thus, the diffusional transport reduces with 1/Vt. The initial diffusional mass transport will be the highest thereafter, it will reduce with time according to Eq. 5.108. Thus, in order to maintain high devolatilization efficiency, it is very important that the surface through which the volatile is escaping is frequently renewed. This can be achieved by feeding the film into a mixer. 

The improved devolatilization efficiency with surface renewal can be assessed by comparing the residual concentration C after n ideal surface renewals with exposure time between film renewal Xf to the residual concentration C(nX,) with the same total exposure time but without surface renewal by ... 

It is clear from Fig. 5.8 that surface renewal can yield a considerably lower residual concentration compared to the case without surface renewal. The improvement in devolatilization efficiency becomes more pronounced when the ratio Xf/X becomes... 

When the ratio kf/ k-o is very small (< 0.001), the benefits of surface renewal are relatively minor. In actual polymer processing equipment, the surface renewal process will generally not be ideal, because only a fraction of the bulk material will be spread out into a thin film. Therefore, the actual devolatilization efficiency would be expected to be less than predicted by Eqs. 5.109 through 5.111. 

The extraction number is a measure of the overall devolatilization efficiency. The backmixing term disappears when the melt film thickness is zero. This is to be expected because the backmixing is caused by the transport in the melt film. If the film thickness is zero, there can be no transport in the melt film and consequently no backmixing. If the film stage efficiency X, = 1, the backmixing term will disappear as well. This is also easy to understand because In this case the concentration C (z) at which the melt film reenters the melt pool is known and will equal Cg. Thus, the Taylor s series expansion is no longer necessary and the first-order differential equation can be determined directly from the mass balance, Eq. 7.463. 

The devolatilization efficiency of the machine Xx is a function of the individuai stage efficiency X and the extent of surface renewal. In continuous equipment, such as screw extruders, the extent of surface renewal is described by the factor Uf (Eq. 7.459 or 7.467e) in batch devolatilizers, the extent of surface renewal is described by n, the discrete number of surface renewals. The film stage efficiency Xf is a function of the surface-to-volume ratio and the exposure time Xf. It can generally be written as a single function of the ratio Xf/Xf,. Thus, the overall efficiency Xj can be described as a function of the surface renewal factor n, and the ratio Xf... 

Diffusion coefficients are very much temperature dependent. When the polymer is below the melting point, diffusion generally occurs at an extremely low rate. The polymer, therefore, should be above the melting point to increase the rate of diffusion and with it the devolatilization efficiency. Even when the polymer is in the molten state, the diffusion coefficients can often be increased substantially by increasing the temperature of the polymer melt [11]. Further, when the polymer is in the molten state, surface renewal is possible. This greatly enhances the devolatilization process. The extent of surface renewal is a strong function of the screw... 

Fig. 7.3 Hot-melt extrusion devolatilization efficiencies for drying of nonstoichiometric heptane...

Although flash devolatilization has been applied for removal of high-volatility monomers, such as vinyl chloride from poly(vinyl chloride) [86-88) or butadiene from polybutadiene [89), most of the processes described in the literature include the use of a stripping inert gas (usually steam) to improve the devolatilization efficiency. 

There are two main ways to reduce the VOC content in waterborne dispersed polymers post-polymerization and devolatilization. Post-polymerization can only be applied to emulsion polymers. Water-soluble redox initiators yielding hydrophobic radicals have been found to be advantageous for monomer removal by postpolymerization. In suspension polymers the devolatilization is limited by both the diffusion through the particle (because of the usually high Tg of the polymer) and the mass transfer from the particle surface to the aqueous phase (because of the large particle size that results in a relatively small interfadal area). Therefore, the temperature and the polymer particle size play an important role in the devolatilization efficiency. 

The second equality in the above equation is correct for single-flighted screws and negligible flight widths. Hence, backmixing increases with the degree of fill and decreasing values of (j). Ex is the extraction number, and it is a measure of the overall devolatilization efficiency. 

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