Devolatilization equipment

There are three basic types of devolatilization equipment that have been used for the commercial manufacture of polystyrene wiped film evaporators, devolatilizing extruders and flash evaporators. In wiped film evaporators, the polymer solution is fed into a vessel under vacuum. The solution is moved into thin films along the vessel walls by a set of rotating blades. These blades continue to move the polymer through the vessel while continually renewing the surface area. The tank walls are heated to supply the required energy for devolatilization. These units are typically mounted vertically with the polymer solution fed at the top. At the bottom is a melt pool where a gear pump transfers the melt to the next unit operation, typically pelletization. 

Although there are clearly some specific advantages with the wiped film evaporators, they have not been widely applied for commercial polystyrene production. Reasons for this are most likely the high equipment and maintenance costs associated with these types of units. 

The second type of equipment used for volatile removal from polystyrene is the devolatilizing extruder. In these devices, an extruder is equipped with one or more pressure let-down sequences where vacuum is applied. In these devices, polymer surfaces are constantly being renewed, giving excellent mass transfer. Another advantage with the devolatilizing extruder is the ability to add and mix additives after devolatilization. This is especially useful if the additive has a 

The types of heat exchangers used in these processes can vary widely. In probably the simplest form, a standard shell and tube type exchanger can be 

Alternatively, a third, low boiling-point additive such as water or inert gas can be added to strip the residual volatiles, which (a) provides more mass transfer area, (b) reduces diffusion distance for the molecules that we wish to remove, (c) increases the driving force for the separation because of the lower concentration of the volatile in the bubbles, and (d) the vaporization of the stripping agent offsets some of the heat generated by viscous dissipation. Of course, after separation we have to deal with a dilute mixture of the volatile in the stripping agent, which may need to be separated for recovery and/or environmental reasons. 

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. 

In industrial practice, high production postreactor streams, as well as compounding and reactive processing operations, need to be devolatilized. The devolatilization process significantly affects the manufacturing cost and is critical to the quality of the product. The equipment is complex and costly and also involves the recovery of the volatiles. Todd et al. (3) and Mehta (4) reviewed, in some detail, the commercial equipment used for devolatilization, which we briefly summarize later in this Section. 

Dilute polymer solutions containing relatively large amounts of volatiles are devolatilized in ordinary, relatively low-cost, single or multiple stage flash tanks. The flash tank is fed via a preheater that superheats the solution. The vapors of the foamingboiling solution are removed at the top of the tank by a vapor takeoff system, and the concentrated solution is removed at the bottom via a gear pump. 

As viscosity increases with decreasing volatile content, the flash tank becomes inefficient as bubbles are entrapped and redissolved upon discharge. The falling-strand devolatilizer, shown schematically in Fig. 8.2, was developed to answer this problem, and represents an improvement over the ordinary flash tank. Here the polymer solution is pumped at high superheat into thin strands that fall gravitationally into the vacuum tank. Free of hydrostatic or shear-induced pressure fields, the bubbles nucleate, grow, coalesce, and rupture so that the volatiles are released before they get trapped in the melt of the cachepot. 

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A twin-screw extmder is used to reduce residual monomers from ca 50 to 0.6%, at 170°C and 3 kPa with a residence time of 2 min (94). In another design, a heated casing encloses the vented devolatilization chamber, which encloses a rotating shaft with specially designed blades (99,100). These continuously regenerate a large surface area to faciUtate the efficient vaporization of monomers. The devolatilization equipment used for the production of polystyrene and ABS is generally suitable for SAN production. 

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],... 

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. 

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 GPPS and HIPS reactor sections each contain several polymerization reactors in series, two-stage devolatilization and a pelletizing line. The devolatilization equipment is designed to deliver polystyrene product with a concentration of residual total volatile material (TVM) of less than 100 wt-ppm. Common equipment includes sections for feed preparation, SM recovery, water removal and bulk-resin handling. 

It is well known that most polymers leaving the polymerization reactor contain various but small amounts of unreacted monomer, solvents, water, and/or various reaction by-products. The presence of these volatiles in the polymer is undesirable. Their concentrations may range from several parts per million to several tens of percentage. Their separation from bulk polymer is necessary to improve polymer properties, to recover monomer and solvents, to meet health and environmental regulations, to eliminate odors, and/or to increase the extent of polymerization. This process of devolatilization is nsnally performed above the glass transition temperature of the polymer. The reader is referred to Albalak [21] for detailed discussion of the theory of devolatilization and various devolatilizing equipments. 

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. 

There exists a wide variety of devolatilization equipment. According to Biesen-berger [4], the processes can be classified into two main categories non-rotating or still equipment, and rotating equipment, in which devolatilization is enhanced by mechanical agitation. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Many types of equipment have been developed to improve the evaporation of solvent in order to provide energy savings. The most widely used techniques for devolatilization are the falling strand devolatilizer (FSD), the thin-film evaporator and the vented extruder [113]. 

Biesenberger, J.A., Devolatilization of Polymers Fundamentals, Equipment, Applications. Hanser Publishers, Munich, Vienna, New York, 1983. 

The continuous mass process is divided into 4 steps rubber solution in styrene monomer, polymerization, devolatilization and compounding. In 1970 N. Platzer (40) drew up a survey of the state of the art. Polymerization is divided into prepolymerization and main polymerization for both steps reactor designs other than the tower reactors shown in Figure 2 have been proposed. Main polymerization is taken to a conversion of 75 to 85% residual monomer and any solvent are separated under vacuum. The copolymer then passes to granulating equipment, frequently through one or more intermediate extruders in which colorant and other auxiliaries are added. 

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. 

Devolatilization is a mass transport operation. The molecules of the volatile components dissolved in the matrix of the polymeric melt must diffuse to liquid-vapor interfaces, and then be removed and collected. All devolatilization processes, irrespective of the complexity of the equipment in which they take place, are represented schematically by Fig. 8.1. 

As volatile levels drop further, yielding very concentrated polymer solutions, the viscosity increases to a level that requires rotary equipment for forward pumping of the solution, which imparts surface renewal and often entraps vapor bubbles, for improved mass and heat transfer as well. There is a wide variety of rotary equipment available, from advanced ribbon devolatilizers, vertical-cone devolatilizers, and disk-ring devolatilizers for moderately viscous solutions, to single and twin screw devolatilizers and thin-film... 

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