Devolatilization process

Since the total pressure (p = p,) of the devolatilization process is usually known, computation of weight fraction (w,) of solvent remaining in the polymer at the limit of phase equilib-... 

To summarize, the kinetics of the silanization reaction are strongly influenced by the efficiency of the devolatilization process. The degree of devolatilization mainly depends on processing conditions (e.g., rotor speed and fill factor), mixer design (e.g., number of rotor flights, size of the mixer), and material characteristics. The diffusion coefficient of the volatile component in the polymeric matrix is of minor influence. 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

The ambient pressure has a complex effect on the devolatilization process. For one thing, in most practical situations an increase in ambient pressure will tend to increase the coal particle heating rate for a given reactor temperature. 

When devolatilization processes are conducted in screw extruders, the screw channels are only partially filled with the polymeric solution to be stripped of the volatile component (see Fig. 5) while the unoccupied portion of the screw channel serves to carry away the evaporated liquid. Because the barrel has a component of motion Vbz in the down channel direction, the solution is caused to flow from the extruder inlet to the outlet, which, in this case, is out of the plane of the paper. The crosschannel component of the barrel motion, Vtx, has two effects. First, it causes a circulation of the fluid in the nip and because of the continual... 

A specific expression for the mass transfer rate in Eq. (11) was first developed by Latinen (1962) in a classic paper that showed how penetration theory can be applied to the analysis of devolatilization processes in single-screw extruders. The derivation presented here parallels that by Latinen but differs in some respects for reasons of clarity. 

Insofar as devolatilization processes are concerned, the three types of... 

Collins et al. (1 3) have suggested that this same concept can be applied to polymer devolatilization processes, except that the HTU might be more appropriately termed the LTU when screw extruders are used since these need not be vertical. By analogy, the following expressions can be written ... 

Char oxidation dominates the time required for complete burnout of a coal particle. The heterogeneous reactions responsible for char oxidation are much slower than the devolatilization process and gas-phase reaction of the volatiles. Char burnout may require from 30 ms to over 1 s, depending on combustion conditions (oxygen level, temperature), and char particle size and reactivity. Char reactivity depends on parent coal type. The rate-limiting step in char burnout can be chemical reaction or gaseous diffusion. At low temperatures or for very large particles, chemical reaction is the rate-limiting step. At... 

These facts can be used to develop impregnation or devolatilization processes or to promote the compatibility of different polymers [94]. 

Another method of introducing heat to fluids in rotating devices involves the generation of eddy currents by rotation through a stationary magnetic field. This approach was successfully used in a polymer devolatilization process (47). 

The industrial manufacture of polymers is rarely taken to completion, and this requires unreacted monomer to be removed from the product—when its viscosity is highest. This devolatilization procedure is notoriously difficult because it usually involves the vacuum stripping of a stirred vessel s contents for many hours. Just as the SDR promotes the removal of the volatile component of a condensation reaction, it is also effective in dramatically accelerating the devolatilization process. 

Devolatilization is a mass transport operation. The molecules of the volatile components dissolved in the matrix of the polymeric melt must diffuse to liquid-vapor interfaces, and then be removed and collected. All devolatilization processes, irrespective of the complexity of the equipment in which they take place, are represented schematically by Fig. 8.1. 

Fig. 8.1 Schematic representation of the devolatilization process. The hatched area represents the polymer melt being devolatilized, which is almost always subject to laminar flow. The bubbles shown are created by the boiling mechanism and by entrapped vapors dragged into the flowing/ circulating melt by moving surfaces.

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. 

The term foaming comes from the fact that the melt is very viscous and, when the devolatilization process begins, the melt fills up with bubbles that appear as foam. Sometimes, as in foaming processes, low boiling-point additives are added to enhance the process. 

As mentioned earlier, entrained free air cannot survive indefinitely in a liquid, but it can still play a role in the devolatilization process in rotary machinery, where the moving surface can drag free air into the melt, forming small bubbles that can serve as nuclei for... 

The morphology remaining in a blister-inhabited area subsequent to the depletion of the volatile is shown in Fig. 8.19. The large circular tracks are those of miniblisters, while the smaller ones are those of the microblisters. Not all macrobubbles show the presence of blisters. Clearly, in the devolatilization process, after the volatiles are depleted... 

The mass transfer of low molecular weight molecules in concentrated polymers plays an important role in many polymer processing steps. During the formation of the polymer, the rate of polymerization is sometimes influenced by the diffusion of low molecular weight species. After the reaction step, a devolatilization process is usually used to remove volatile residuals from the polymer via solvent diffusion. Many other processes, such as the distribution of additives in polymer, involve the diffusion of low molecular weight molecules in concentrated polymer solutions. 

Further experimental data and further model comparisons relate to the rapid pyrolysis of different coals. In the absence of air, this experimental device heats and converts small coal particles (10-200 pm) in gas and distillates. Figure 20 shows a very satisfactory agreement between experimental data relating to a bituminous coal and model results at 1,260 K. It is noteworthy that despite the strong differences between carbon deposit and bituminous coal, the characteristic times for the dehydrogenation processes are practically the same. Further data on this subject, as well as a detailed model for the analysis of the pyrolysis and devolatilization process of coal particles, are available in a recent paper (Migliavacca et al., 2005). 

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