Devolatilization section

The engineering analysis and design of these operations addresses questions which are different than those addressed in connection with the shaping operations. This is illustrated in Fig. 1 which is a flow sheet, cited by Nichols and Kheradi (1982), for the continuous conversion of latex in the manufacture of acrylonitrile-butadiene-styrene (ABS). In this process three of the nonshaping operations are shown (1) a chemical reaction (coagulation) (2) a liquid-liquid extraction operation which involves a molten polymer and water and (3) a vapor-liquid stripping operation which involves the removal of a volatile component from the molten polymer. The analysis and design around the devolatilization section, for example, would deal with such questions as how the exit concentration of... 

Solid curve. Calculated values for styrene from devolatilization data (this study) with 4-1/4 L/D screw B devolatilizing section... 

L = length of devolatilizing section, L N = rotational speed of screw, /B... 

The pressure profile in the vented extruder is such that pressure first reaches a low value (because of the change in screw geometry) and then actually falls below atmospheric pressure (caused by a vacuum applied at the vent) in the devolatilization section. Figure 7-12 shows the altered screw geometry at the vent openings, which starve flow in the given section. 

Trace devolatilization with the help of a stripper agent has a greatly enhanced efficiency. An important recent result is that, when it forms, foam grows until a limiting volume is reached, regardless of initial volatile content and presence of stripper gas [66], and thus the devolatilization section in vented extruders should... 

DESIGN PROBLEM VII DESIGN OF A DEVOLATILIZATION SECTION FOR A SINGLE-SCREW EXTRUDER... 

Finally, the devolatilization efficiency of the machine, Xt, is a function of the individual stage efficiency, X, and the extent of surface renewal. In the extruder, the extent of surface renewal is described by f in Eq. 8.174. The film stage efficiency, Xf, is a function of the surface-to-volume ratio and the exposure time, tf. In general Xi is a function of nt and the ratio t /to, where to is the residence time in the devolatilization section of the extruder. 

The solution is carried out by two methods. Because of the modular nature of twin-screw extruders, experiments were carried out in advance to determine what length of devolatilization section was required to reduce the level of MMA to 0.1%, which corresponds to a fractional separation, Fs, of about 0.85. Because the single-screw extmder is cheaper to build, it is desirable to determine the length of the DV section and the processing conditions (i.e., pQ, N, and f) required to accomplish the same reduction of MMA in PMMA as done in the SWCOR extruder. The first approach is based on dimensional analysis. In the second approach we use the penetration or diffusion theory summarized in Section 8.5.2. 

It is likely that some devolatilization occurred during the extrusion step, possibly aided by the acetic acid, since it is doubtful that there could be sufficient residence time in the final 180°C tower section to drive the conversion to 99.5%. 

Equilibrium between Monomer and Polymer. A monomer-with-polymer equilibrium is quite different from the polymer-with-condensation-product equilibrium discussed in Section 13.1.1. If the condensation product is removed from the reaction mixture, a condensation polymer increases in molecular weight. If the monomer is removed when it is in equilibrium with the polymer, the polymer depolymerizes to re-form the monomer. At temperatures suitable for long-term use, the equihbrium will be shifted toward stable polymer. However, at fabrication temperatures and at the high temperatures common in devolatilization, the production of monomer and low-molecular-weight ohgomers can be significant. 

Single-screw and double-screw extruders are normally used for polymer melts to accomplish the deaeration or devolatilization of residual volatiles. Devolatilization in an extruder is effected through formation of the venting zone inside the chamber by carefully designed upstream and downstream screw sections. 

Large diameter, melt-fed extruders are commonly used for the final devolatilization and pelletization of LDPE and PE copolymers in resin manufacturing plants. A full description of this type of extruder and process is provided in Section 15.3. Simulation of these processes is complicated by the multiple flights used in the design and the high H/W aspect ratios of the channels. The processes can be simulated from the feed hopper to discharge, however, since they are not required to convey solids and melt resin. This section will show the requirements and difficulties for simulating these processes. 

In industrial practice, high production postreactor streams, as well as compounding and reactive processing operations, need to be devolatilized. The devolatilization process significantly affects the manufacturing cost and is critical to the quality of the product. The equipment is complex and costly and also involves the recovery of the volatiles. Todd et al. (3) and Mehta (4) reviewed, in some detail, the commercial equipment used for devolatilization, which we briefly summarize later in this Section. 

In devolatilizing systems, however, Ca 1 and the bubbles deform into slender S-shaped bodies, as shown in Fig. 8.12. Hinch and Acrivos (35) solved the problem of large droplet deformation in Newtonian fluids. They assumed that the cross section of the drop is circular, of radius a, and showed that the dimensionless bubble surface area, A, defined as the ratio of the surface area of the deformed bubble A to the surface area of a spherical bubble of the same volume, is approximated by (36) ... 

Fig. 8.14 PS-styrene sample extruded at 180°C into atmospheric pressure. The micrograph shows the smooth lateral surface and part of the cross section there is no evidence of huhhles. [Reprinted by permission from R. J. Albalak, Z. Tadmor, and Y. Talmon, Scanning Electron Microscopy Studies of Polymer Melt Devolatilization, AIChE J., 33, 808-818 (1987).]...

Fig. 10.57 Typical 4 LID Co-TSE barrel segments, (a) Feed throat (b) melting (c) downstream feed section (d) mixing (e) devolatilization (f) pumping and limited pressurization. [Courtesy of P. Andersen Coperion Wemer and Pfleiderer, Ramsey, NJ.]...

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. 

As addressed in the previous section, there is a wide range of material variants available for barrels with replaceable inserts. Processing-related functions such as heating and cooling can be integrated into the outer barrel, which is generally made of heat-treated steel. The outer barrel can also incorporate elements for devolatilization, side intake, etc., so the actual insert has become a simple component that can therefore be made from a variety of different material, such as ... 

Above the bed in a fluidized-bed gasifier, the cross-sectional area of the reactor is increased to produce a disengaging zone where the superficial gas velocity is below fluidization velocity.598 This allows the entrained sand particles to fall back down and maintain the bed inventory over time. This larger cross-sectional area zone, or freeboard, can be extended to obtain the total desired gas-phase residence time for complete devolatilization. Eroded bed material or fine char and ash particles that escape the reactor can be collected in a cyclone and either returned to the bed or removed from the system. 

The GPPS and HIPS reactor sections each contain several polymerization reactors in series, two-stage devolatilization and a pelletizing line. The devolatilization equipment is designed to deliver polystyrene product with a concentration of residual total volatile material (TVM) of less than 100 wt-ppm. Common equipment includes sections for feed preparation, SM recovery, water removal and bulk-resin handling. 

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