Basics of Devolatilization

Applications of microwave heating are drying, continuous curing of polymers (rubbers, filled polyethylenes, etc.), preheating for compression or transfer molding, bonding, etc. 

In devolatilization, one or more volatile components are extracted from the polymer. The polymer can be either in the solid state or in the molten state. Two processes occur in the devolatilization process. First, the volatile components diffuse to the polymer-vapor interface then the volatile components evaporate at the interface and are carried away. Thus, the first part of the process is a diffusional mass transport and the second part a convective mass transport. If the diffusional mass flow rate is less than the convective mass flow rate, the process is diffusion-controlled. In polymer-volatile systems, the diffusion constants are generally very low, and, therefore, in many polymer devolatilization processes the process is diffusion-controlled. 

The important relationship in diffusional mass transport is Pick s law. It states that in a one-dimensional diffusion, the positive mass flux of component A is related to a negative concentration gradient. It can be written as  

Pick s law is valid for constant densities and for relatively low concentrations of component A in component B. The term binary mixture is used to describe a two-component mixture. A binary diffusivity is the diffusion constant of one component of a binary mixture. The diffusional mass transport is driven by a concentration 

For a binary system of constant density, where a low concentration component A is diffusing through the other component, the equation of continuity for component A can be written as  

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There are three basic types of devolatilization equipment that have been used for the commercial manufacture of polystyrene wiped film evaporators, devolatilizing extruders and flash evaporators. In wiped film evaporators, the polymer solution is fed into a vessel under vacuum. The solution is moved into thin films along the vessel walls by a set of rotating blades. These blades continue to move the polymer through the vessel while continually renewing the surface area. The tank walls are heated to supply the required energy for devolatilization. These units are typically mounted vertically with the polymer solution fed at the top. At the bottom is a melt pool where a gear pump transfers the melt to the next unit operation, typically pelletization. 

The melt polymerization process involves the base-catalyzed transesterification reaction of BPA with diphenyl carbonate (Fig. 8). A small amount (less than 0.01% molar) of basic catalyst such as Na, Li, K, or tetralkylammonium hydroxide or carbonate is used during the initial stages of the reaction. The reaction is performed under vacuum at 180-300°C. At later stages of the reaction, the temperature and the vacuum are increased (less than ImmHg) to remove phenol and drive the product to high molecular weight. Subsequently, the polymer becomes very viscous, and special devices, such as devolatilizing extruders, are required to ensure complete removal of phenol. 

The text that follows outlines what is known and what needs to be understood about the fundamentals of the combustion of coal and carbon particle in turbulent fluidized beds. Since combustion is affected by the hydrodynamic behavior of the bed, certain aspects of hydrodynamics are described very briefly. The aim of the review is to summarize the present understanding of the basic mechanism of coal or carbon in turbulent fluidized beds. Devolatilization also is described briefly. 

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