The mechanism of foam devolatilization in partially filled screw devolatilizers

Abstract

An experimental investigation was conducted into elucidating the mechanism of foam devolatilization. For the investigation, we have constructed two separate apparatuses of Plexiglas—one simulating the partially filled flow channel in a single‐screw devolatilizer, and the other simulating the partially filled, closed‐chamber (often referred to as the “C‐chamber),” in a twin‐screw devolatilizer. The test fluids employed were aqueous solutions of polyacrylamide having various concentrations. During the preparation of the test fluids, we controlled the amount of air entrapped in the liquid phase by varying the level of vacuum applied. The entrapped air stayed as fine gas bubbles dispersed in the polymer solution, and mixtures of the polymer solution and air bubbles were subjected to devolatilization experiments. In the use of a single‐screw devolatilizer, we have observed that: 1. The streamlines show circulatory flow patterns forming a singular point at a position slightly below the free surface, where small gas bubbles are trapped initially and become stationary, and 2. The small gas bubbles trapped at the stationary position coalesce later to form large gas bubbles, which then move slowly toward the free surface and are removed under vacuum from the system. In the use of twin‐screw devolatilizers, we have found that the degree of fill, the rotational direction of the screws, and the degree of intermeshing, (i. e., partially or fully intermeshing) greatly influence the amount of free surface available for the removal of volatiles, as well as the flow patterns in the liquid pool, and thus the devolatilization efficiency.

Bubble nucleation in polymeric liquids. I. Bubble nucleation in concentrated polymer solutions

“…4. Schematic of the layout of apparatus for light scattering experiment: (1) high pressure optical cell, (2) He-Ne laser light source, (3) laser exciter, (4) photomultiplier, (5) photomultiplier, (6) high voltage power supply, (7) piezoelectric pressure transducer, (8) pressure transducer chamber, (9) charge amplifier, (10) oscilloscope, (11) oscilloscope, (12) signal processing device and differential amplifier, (13) data acquisition and control system, (14) electronic interface, (15) solenoid valve, (16) metering valve, (17) stop valve, (18) solenoid valve, (19) pressure regulator, (20) helium gas supply.…”

Experimental Studies and Modeling of Devolatilization of Highly Viscous Solution in High-Speed Disperser

“…Polymer devolatilization processes include mainly diffusion devolatilization for concentrated polymer solutions and foam devolatilization for polymer solutions with high concentrations of volatiles (Han and Han, 1986). Many mathematical models describing the above two devolatilization processes can be found in the literature.…”

Effects of shear stress on the behavior of volatile bubbles in the high‐density polyethylene melt

“…Bubble evolution widely exists in non-Newtonian fluids, such as melt, emulsion, fluid gel, and crystal-melt suspension in the plastic, food, and pharmaceutical industries, [1][2][3][4] which is especially important for the polymer processing of foaming, extrusion, and devolatilization. Taking polymer devolatilization as an example, as the concentration of volatiles decreases, it can be divided into three periods [5] : flashing devolatilization, foam devolatilization, and diffusion devolatilization. Among them, foam devolatilization, which occurs with bubble nucleation, growth, coalescence, breakup, and collapse, is the most complicated process.…”