Devolatilization Concepts

Because the reaction processes described previously do not take the reaction to completion, a separate unit operation is required to remove monomer(s) and solvent from the polymer product. This is typically completed by heating the polymer solution and flashing off the unwanted monomer and solvent. There are several concerns such as equilibrium levels, polymer degradation, and mass transfer that must be considered. 

Devolatilization performance is usually measured against the equilibrium amount of volatile in the final polymer. The equilibrium level for the devolatilization conditions used can be calculated using a simplified Flory-Huggins equation for monomer activity in the polymer melt [6]. By equating the partial pressure of the monomer solution to the flash tank partial pressure, the following results  

---

The devolatilization concept has been extended to certain commonly used isotope sources with the additional advantage of making them more stable or storable. [ H/ C]Methyl nosylate (17) has been developed as a substitute for tritiated or carbon-14-labeled methyl iodide, both of which are relatively unstable and difficult to handle (Figure 1.6). Ester 17 is a nonvolatile, easily purifiable solid that is substantially less radiolytically sensitive than the corresponding methyl halides ([ H]methyl nosylate at a specific activity of > 80 Ci/mmol suffered no appreciable decomposition after storage at 39 mCi/mL for 14 weeks at 4 °C in hexane/ethyl acetate ). This derivative is reported to possess similar reactivity to [ H/ C]methyl iodide in a variety of reactions, and to provide greater flexibUity during use in synthesis. 

The importance of these concepts can be illustrated by the extent to which the pyrolysis reactions contribute to gas produdion. In a moving-bed gasifier (e.g., producer-gas gasifier), the particle is heated through several distinct thermal zones. At the initial heat-up zone, coal carbonization or devolatilization dominates. In the successively hotter zones, char devolatihzation, char gasification, and fixed carbon... 

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 ... 

Devolatilizers Devolatilization systems are liquid-limited due to the combination of high liquid viscosity and removal of a component with high relative volatility. Simpson and Lynn [AlChE J., 23 (5), 666-673 (1977)] reported oxygen stripping from water at 98 percent complete, in less than 1 ft of contact. The concept has been employed for residual devolatilization in refineries. 

The various chapters in this book address the topics of material selection, characterization and evaluation as well as membrane preparation, characterization and evaluation. At the expense of neglecting membranes for applications such as controlled release and impermeable barriers, this book focuses on synthetic membranes for separation processes as well as active membranes and conductive membranes. While many of the concepts developed herein can be extrapolated to other applications, the Interested reader is referred elsewhere for specific details (for example, controlled release (25-30), coating and packaging barriers (31-33), contact lenses (34,35), devolatilization (36), ion-selective membrane electrodes (37-42) and membranes in electrochemical power sources (43)). 

Penetrant Concentration-Plasticization Polymer Molecular Structure Relaxation-Controlled Transport Applications of Transport Concepts Barrier Materials Devolatilization Additive Migration Dyeing... 

Equation-of-state approaches are preferred concepts for a quantitative representation of polymer solution properties. They are able to correlate experimental VLE data over wide ranges of pressure and temperature and allow for physically meaningful extrapolation of experimental data into unmeasured regions of interest for application. Based on the experience of the author about the application of the COR equation-of-state model to many polymer-solvent systems, it is possible, for example, to measure some vapor pressures at temperatures between 50 and 100 C and concentrations between 50 and 80 wt% polymer by isopiestic sorption together with some infinite dilution data (limiting activity coefficients, Henry s constants) at temperatures between 100 and 200 C by IGC and then to calculate the complete vapor-liquid equilibrium region between room temperature and about 350 C, pressures between 0.1 mbar and 10 bar, and solvent concentration between the common polymer solution of about 75-95 wt% solvent and the ppm-region where the final solvent and/or monomer devolatilization process takes place. Equivalent results can be obtained with any other comparable equation of state model like PHC, SAFT, PHSC, etc. 

Biesenberger, J. A. (ed.). Devolatilization of Polymers, Hanser, Munich, 1983. Chien, Y.W., Novel Drug Delivery Systems Fundamentals, Developmental Concepts, Biomedical Assessments, Marcel Dekker, Newybrk, 1982. 

---