Foaming devolatilization

P. S. Mehta, L. N. Valsamis, and Z. Tadmor, Foam Devolatilization in Multi-Channel Corotating Disk Processors, Polym. Process Eng, 2, 103-128 (1984). 

S. T. Lee, A Fundamental Study of Foam Devolatilization in Polymer Devolatilization, R. J. Albalak, Ed., Marcel Decker, New York, 1996, Chapter 6. 

Fig. 9.52 Separation efficiency of a three-chamber co-rotating disk devolatilizer of 450°F PS melt containing 1500-3000 ppm styrene, fed at 42-lb/h into 0.54-in-wide chambers at 50-torr absolute pressure, as a function of disk speed and with flow rate as a parameter. Broken curves show calculated residence times. [Reprinted by permission from P. S. Mehta, L. N. Valsamis, and Z. Tadmor, Foam Devolatilization in a Multichannel Co-rotating Disk Processor, Polym. Process. Eng., 2, 103-128 (1984).]...

Todd [37] proposed an equation to describe devolatilization in co-rotating twin screw extruders based on the penetration theory discussed in Section 5.4 and Section 7.6. The equation contains the Peclet number (see Eq. 7.371), which represents the effect of longitudinal backmixing. The Peclet number must be measured or estimated to predict the devolatilizing performance of an extruder. Todd selected a Peclet number of 40 to correlate predictions to experimental results. A similar approach was followed by Werner [38], A visualization study was made by Han and Han [39], particularly to study foam devolatilization, They found substantial entrainment of the bubbles in a circulatory flow region in a partially filled screw devolatilizer. Collins, Denson, and Astarita [40] published an experimental and theoretical study of devolatilization in a co-rotating twin screw extruder. The experimentally determined mass transfer coefficients were about one-third those predicted by the mathematical model. They concluded, therefore, that the effective surface area for mass transfer is substantially less than the sum of the areas of the screws and barrel. 

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. 

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. 

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. 

In devolatilization with viscous polymeric melts, it is difficult, of course, to carry out similar experiments and prove indirectly that free-streaming nuclei may play a similar role, but microscopic particles originating from the monomers and catalyst systems are likely to be found in the polymeric product. Moreover, it is well known that the addition of fine powders and solid particles induces foaming. Therefore, the Biesenberger-Lee proposition seems plausible. 

The preceding observations on the microscopic features of polymer melt devolatilization are not unique to the PS-styrene system, or to strand devolatilization. Similar, though somewhat less rich, features of blister-covered macrobubbles were observed with low-density polyethylene (PE), high-density PE and polypropylene (PP) systems (40,41). Furthermore, Tukachinsky et al. (11) discovered macrobubbles covered with microblisters in a 50-mm-diameter vented SSE, with PS showing more oblong shapes as a result of shearing. The onset of foaming with the application of vacuum was quicker with increased frequency of screw rotation, and the separation was more efficient. 

A. Tukachinsky, Y. Talmon, and Z. Tadmor Foam-enhanced Devolatilization of Polystyrene Melt in a Vented Extruder, AIChE J., 40, 670-675 (1994). 

S. T. Lee and J. A. Biesenberger, Fundamental Study of Polymer Melt Devolatilization. IV Some Theories and Models for Foam-enhanced Devolatilization, Polym. Eng. Sci., 29, 782-790 (1989). 

Devolatilizing Devolatilization in a co-rotating disk chamber can be achieved by spreading the melt on the disk surfaces in a chamber under high vacuum, and collecting the foamed film in a circulating pool at the channel block where bubble rupture takes place. The partly devolatilized melt can then be fed into another chamber in series, and so on. Fig. 9.50 shows a setup of three consecutive devolatilizing chambers. 

Devolatilization is the process of removing unwanted gases (volatiles) from the melt. If these gases were to exit through the die with the melt, undesired foaming and/or surface... 

Devolatilization (7) This is a two-step operation under high vacuum, to remove lights components such as unreacted styrene and diluent, which is enhanced with the addition of a foaming agent in the... 

Hash devolatilization is a simple and effective method to remove the majority of solvent and unreacted monomers from the polymer solution. Product from the reactor is charged to a flash vessel and throttled to vacuum conditions whereby the volatile solvent and monomers are recovered and condensed. In the process, the polymer melt cools, sometimes considerably, due to the evaporation of volatiles. The polymer product is pumped from the bottom of the flash vessel with a gear pump or other suitable pump for viscous materials. Critical to operation of the flash devolatilization unit is prevention of air back into the unit that reduces stripping ability and potentially allows oxygen into the unit that can discolor products or pose a safety hazard if low autoignition temperature solvents are used. Often one flash devolatilization unit is insufficient to reduce the residual material to a sufficient level and thus additional units can be added in series [61]. In each vessel, the equilibrium concentration of volatile material in the polymer melt, is a function of the pressure and temperature the flash unit operates at, with consideration for the polymer solvent interaction effects described by the Hory-Huggins equation. Flash devolatilization units, while simple to operate, may be prone to foam development as the superheated volatiles rapidly escape from the polymer melt. Viscous polymers or polymers with mixed functionalities... 

The use of a foamed PEN prepolymer, combined with a devolatilization step prior to solid state polymerization, provides a particularly fast and productive solid state polymerization process for a PEN polymer. Us-... 

Flash devolatilization is a relatively inexpensive process. It can be applied to most polymers, but there will be a limit at low residual volatiles where the mixture will no longer foam. A foaming agent can be used, but it is more common to use an extruder for the final stages of devolatilization. 

---