Devolatilization mechanisms

Devolatilization of concentrated solution may start with an above atmospheric flash separation. This way the solvent and unreacted monomer can be easily and directly recycled. However, downstream all devolatilization equipment is operated under reduced 

The definite proof that, even at such low levels of volatiles, the devolatilization mechanism in vented SSEs is a foaming-boiling one came from the work of Biesenberger and Kessidis (9) in 1982, Mehta et al. (10) in 1984, and Tukachinsky etal. (11) in 1994. 

Yet in spite of all the evidence just presented, it is not impossible that at very low volatile levels the Latinen-type model may have some validity, but this likelihood appears to be small because foaming was observed at volatile concentrations as low as 50 ppm. Moreover, the likelihood further diminishes due to the fact that, as shown in Section 8.5, the diffusivity of small molecules in polymeric melts may drop by orders of magnitude, with dropping concentrations at these levels. 

First we consider ideal solutions. An ideal solution is one where the solute and solvent molecules 1 and 2 have roughly the same size, shape, and force fields. An ideal solution obeys Raoult s law  

Solubility is very frequently expressed in the form of Henry s law, where P is proportional to the weight fraction of the solute 

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R. J. Albalak, Z. Tadmor, and Y. Talmon, Polymer Melt Devolatilization Mechanisms, J. Non-Newtonian Fluid Mech., 36, 1313-1320 (1990). 

Akdogan HI (1999) High moisture food extrusion. Int 1 Food Sci Tech 34 195-207 Albalak RJ (1996) An introduction to devolatrhzation in polymer devolatilization. 1-12 Albalak RJ, Tadmor Z et al (1990) Polymer melt devolatilization mechanisms. Wdey Online Libr 36 1313-1320... 

Sawdust and has 2-step devolatilization mechanisms, with initial devolatilization having very low activation energies, and with subsequent devolatilization being volatile stripping from the char matrix, with higher ... 

These partly competing and overlapping devolatilization mechanisms demonstrate that salty coals can cause severe corrosion problems, which are mostly accompanied by abnormal and unstable ash fusion behavior, if gasified in dry-... 

The finishing reactors used for PET and other equilibrium-limited polymerizations pose a classic scaleup problem. Small amounts of the condensation product are removed using devolatilizers (rotating-disk reactors) that create surface area mechanically. They scale as... 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

In the use of polystyrene, the polymerization reaction is exothermic to the extent of 17 Kcal/mol or 200 BTU/lb (heat of polymerization). The polystyrene produced has a broad molecular weight distribution and poor mechanical properties. The residual monomer in the ground polymers can be removed using efficient devolatilization equipment. Several reviews are worthwhile consulting [42-44],... 

Devolatilization removal of vaporizable material by the action of heat. Dewatering removal of water from coal by mechanical equipment such as a vibrating screen, filter, or centrifuge. 

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.

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

In this chapter, subsequent to an introduction to devolatilization equipment, we review the thermodynamics of polymer solution equilibrium, which determines the maximum amount of volatiles that can be separated under a given set of processing conditions the phenomena associated with diffusion and diffusivity of small molecules in polymeric melts, which affects the rate of mass transfer the phenomena and mechanisms involving devolatilization and their modeling and the detailed and complex morphologies within the growing bubbles created during devolatilization of melts. 

The breakup or bursting of liquid droplets suspended in liquids undergoing shear flow has been studied and observed by many researchers beginning with the classic work of G. I. Taylor in the 1930s. For low viscosity drops, two mechanisms of breakup were identified at critical capillary number values. In the first one, the pointed droplet ends release a stream of smaller droplets termed tip streaming whereas, in the second mechanism the drop breaks into two main fragments and one or more satellite droplets. Strictly inviscid droplets such as gas bubbles were found to be stable at all conditions. It must be recalled, however, that gas bubbles are compressible and soluble, and this may play a role in the relief of hydrodynamic instabilities. The relative stability of gas bubbles in shear flow was confirmed experimentally by Canedo et al. (36). They could stretch a bubble all around the cylinder in a Couette flow apparatus without any signs of breakup. Of course, in a real devolatilizer, the flow is not a steady simple shear flow and bubble breakup is more likely to take place. 

Clearly, this mechanism is more complex than ordinary boiling mechanisms, and any theoretical formulation of devolatilization must take into account this complexity. An initial attempt to formulate semiquantitative elements of this mechanism was made by Albalak et al. (41). They proposed that once a nucleus of a macrobubble is created and the bubble begins to grow, the stretched inner surface of the bubble enhances the rate of nucleation just beneath the soft surface, thus generating new blisters, as shown schematically in Fig. 8.20. 

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