Devolatilization Zone

Task - Removal of water, residual monomers, and solvents 

The vent opening should be as large as possible in order to avoid high gas speeds, which can lead to product being drawn into the vent. In practice, three vent inserts are used to ensure effective devolatilization on the one hand and to prevent product from discharging into the vent on the other hand (see Fig. 4.15)  

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. 

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The concentration of the volatUe component at the exit of the devolatilization zone can be computed using Eqs. (26) and (32) along with an equation developed around a mass balance across a differential element [Eq. (11), for example]. 

The velocity of the combustion front is determined by the volatile matter content of the raw coal, Wv. If the volatile matter content of the coal is high the frontal velocity will be slowed. With low volatility coals the frontal velocity will be relatively fast for the same excess energy production in the com-bustion/gasification regions of the bed. The velocity vc is set by the requirement that the volatiles content of the coal ahead of the devolatilization zone be essentially that of the raw coal. From an analysis of the energy balance in the region near the front of the devolatilization zone, it can be shown that a particular root of the relation below establishes v (3) ... 

After wetting, dispersion, and pre-comminution of the fibers, the melt is devolatilized to achieve a perfect pumping and constant material flow within the die. Comminution must be completed prior to the devolatilization zone to prevent fibers from being separated and deposited in the devolatilization area. 

Even though there is always a residence time distribution, it is practical to specify a mean residence time. The mean residence time is calculated from the volumetric flow rate V and the free volume Vfree of the screw. In addition, the volumetric degree of fill f must be taken into account [3]. With this in mind, different processing zones are considered individually (e.g., partially filled devolatilization zones and completely filled zones for pressure buildup). 

High temperature - fuel rich devolatilization zone Production of reducing species NOx decomposition zone Char oxidizing zone... 

The molecular diffusivity is independent of concentration and may be treated as a constant at the average melt temperature in the devolatilizing zone. 

The glass fibers that were used were a 3/16-inch long standard grade from Owens Corning Fiberglas and were fed into the sixth barrel. The glass fibers were compounded into the melt, and a devolatilization zone with a vacuum vent was used in the eighth barrel. Samples were collected for analysis of the extruded... 

Polycondensation reactions are generally mass transfer limited and good devolatilization of the low molecular components is necessary for the reaction to proceed. Special polycondensation extruders have been developed with a succession of several devolatilization zones. 

The effectiveness of the devolatilization operation is strongly dependent on the actual length of the devolatilization zone Lg. In actual extrusion the length Lg can be considerably longer than the length of the extraction section of the screw L. Thus, the length Lg can extend into the pump section of the extruder. This is determined by the filled length of the pump section Lp,. 

In conical twin-screw extruders, a restrictive zone, follows the plastication or melting zone. The restrictive zone seals off the plastication zone from the devolatilization zone. Mixing will occur in the restrictive zone compressing the polymer in the narrower... 

With screw profile B2, all the chemicals of the free radical grafting (PP, MA, styrene and peroxide) are fed to the extruder through the main hopper and the PA6 is added separately down-stream through another hopper. The barrel temperature is set at 200 °C between the first and second hoppers and 240 °C for the rest of the extruder. The devolatilization zone remains at port 1. Unlike screw profile Bl in which the free radical grafting and the reactive compatibilization are more or less overlapped, they become two independent sequential operations in screw profile B2. 

The only difference between screw profiles B2 and B3 is that in the latter case, the devolatilization zone is moved up-stream prior to the feeding location of the PA6. This allows for the removal of non-reacted residual monomers before the PA6 is fed to the extruder. 

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