Flash devolatilization

Nauman, E.B., Flash devolatilization, in Encyclopedia of Polymer Science and Engineering, Supplement Volume,... 

These machines are used primarily for removal of solvent [Gras, 1972 Gras and Else, 1975]. Flash devolatilization occurs in a flash dome attached to a barrel. The polymer solution is delivered under pressure and at temperature above the boiling point of the solvent. The solution is then expanded through a nozzle into the flash dome. The foamy material resulting from the flash devolatilization is then transported away by the four screws. In many cases, downstream vent sections are also used to further reduce solvent content. 

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... 

An alternative to a flash devolatilization unit is the oil heated thin film or WFE. In this equipment, the molten polymer/solvent solution is throttled to the WFE comprising a rotating set of blades that draws the melt into a thin film. In this manner, very good heat transfer from the oil heated surface is obtained and the thin film minimizes diffusion distances and allows rapid mass transfer of volatiles out of the melt. Both vertical and horizontal WFE units are in commercial production and are effective for small-to-medium-sized plants with moderate viscosity melts. Larger units require very large motors to strip viscous resins. Like flash devolatilization units, bubble formation and collapse are essential to effective mass transport of solvent from the polymer melt. 

Many standard methods are used to devolatilize materials. Flash devolatilizer or falling strand devolatilizer are synonyms of equipment in which the falling melt is kept below the saturation pressure of volatiles. Styrene-acrylonitrile copolymers devolatilized in flash devolatilizer had a final eoncentration of ethylbenzene of 0.04-0.06. Devolatilization of LLDPE in a single-serew extruder leaves 100 ppm of hydrocarbon solvent. 500 ppm chlorobenzene remains in similarly extruded polycarbonate. It is estimated that if the polymer contains initially 1-2% solvent, 50-70% of that solvent will be removed through the vacuum port of an extruder. These data seem to corroborate the information included in the above... 

Although flash devolatilization has been applied for high volatility monomers removal, such as vinyl chloride from poly(vinyl chloride) [103] and butadiene from polybutadiene [104], devolatihzation of aqueous polymer dispersions is usually carried out using a stripping agent (steam and nitrogen are the most commonly used air can also be used, but explosive vapor mixtures can be produced). Devolatilization of aqueous phase dispersions is a mass-transfer process, which involves the following steps in series ... 

Robust equipment has been developed for the various processing steps, including stirred-tank and tubular reactors, flash devolatilizers, extruder reactors, and extruder devolatilizers. Equipment costs are high based on working volume, but the volumetric efficiency of bulk polymerization is also high. If a polymer can be made in bulk, manufacturing economics will most likely favor this approach. 

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. 

Figure 3.9 Selective Dissolution Single Solvent Flash Devolatilization Miniplant Lynch and Nannian, 1989 ...

A technique that is often employed in devolatilization of polymer solutions is flash devolatilization [34], in this technique, the polymer solution is delivered to a flash point under high pressure and at temperatures above the boiling point of the volatile component. The solution is then expanded through a nozzle large amounts of volatiles can thus be extracted rather quickly. The foamy liquid that results from this operation is often exposed to another devolatilization step to remove residual amounts of volatiles. This second step is generally a conventional melt film devolatilization, where the material in the film is continuously renewed to obtain an effective extraction of the volatiles. 

Char made from flash devolatilization of the coal Is shown along with chars partially burned to produce process heat at 1335and 1600 F. The char combustion temperature is expected to fall within this range. Again, partial combustion results in a char with lower sulfur, higher surface area, adequate volatile matter for use as a pulverized fuel and very similar heating value to the devolatilized char. The relative ignition temperature is increased by partial combustion. 

A common application of flash devolatilization is the removal of 10-15% styrene-ethylbenzene from polystyrene. One commercial process uses Tin = 240°C and Tout = 1.3 kPa (10 torr). Equilibrium calculations for an adiabatic flash predict Tout = 220°C and Wout = 500 ppm actual operation gives about 1200 ppm. 

Flash devolatilization is a relatively mature technology for the removal of volatile components from polymer melts and solutions. It was developed by major... 

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