Thin-film evaporation equipment

Parker, N. Agitated Thin-Film Evaporators-Equipment and Economics, Chemical Engineering, Sept. 13, 1965, p. 179. 

Figure 1. Thin-film evaporator equipped with a fixed blade rotor (Luwa)...

The difference in processing behavior between rotors provided with straight blades and those with pitched blades is shown in Figure 6 by an application referring to solvent recovery from a polymer solution of low initial viscosity. The evaporation capacity of a thin-film evaporator equipped with straight blades decreases considerably as soon as the concentrated polymer reaches a viscosity between 1000 and 2000 P. The greatly increased mean film thickness that characterizes this vis-... 

Mg. 7.1-7 Agitated thin-film evaporator equipped with a rotor with fixed blades... 

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. 

In order to make a multipurpose plant even more versatile than module IV, equipment for unit operations such as soHd materials handling, high temperature/high pressure reaction, fractional distillation (qv), Hquid—Hquid extraction (see Extraction, liquid-liquid), soHd—Hquid separation, thin-film evaporation (qv), dryiag (qv), size reduction (qv) of soHds, and adsorption (qv) and absorption (qv), maybe iastalled. 

Evaporation. Evaporation can be used to separate volatile compounds from nonvolatile components and often is used to remove residual moisture or solvents from soHds or semisoHds. Thin-film evaporators and dryers are examples of evaporation equipment used for this type of appHcation. Some evaporators are also appropriate for aqueous solutions. 

Low-inventory distillation equipment, such as the thin film evaporator, are also available and should be considered for hazardous materials. This equipment offers the additional advantage of short residence time and is particularly useful for reactive or unstable materials. 

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]. 

Parker, N. Chem. Eng., Albany 72, No. 19 (13 Sept. 1965) 179-85. Agitated thin film evaporators. Part 2. Equipment and economics. 

Figure 13.30. Molecular distillation and related kinds of equipment, (a) Principle of the operation of the falling film still (Chemical Engineers Handbook, McGraw-Hill, New York, 1973). (b) Thin-layer evaporator with rigid wiper blades (Luwa Co., Switzerland), (c) The Liprotherm rotating thin film evaporator, for performance intermediate to those of film evaporators and molecular stills (Sibtec Co., Stockholm), (d) Centrifugal molecular still [Hickman, Ind. Eng. Chem. 39, 686 (1947)].

These thin-film evaporators are equipped with rotating elements that create a thin, liquid film of high turbulence along the inner surface of the heated tube (see Figure 1). Consequently, favorable heat and mass-transfer conditions (I), (2) and short residence times result owing to the small holdup (3,4). 

Figure 6. Comparison of evaporation capacity between thin-film machines equipped with straight-blade rotors and the high viscosity machine (angled blades), expressed as a function of the concentrated polymer (polypropylene-heptane-solution)...

Product recovety is some combination of rote methods of using processing tools and inspired application of new and conventional separation tools. For example, if aldehydes make distillation infeasible because of polymerization products and/or vacuum distillation costs, then one can consider utilizing the Cannizzaro reaction to make organic acid salts and ketones from the aldehydes before proceeding. Many bench-scale processing techniques such as thin-film evaporation have been scaled up to handle larger quantities of temperature sensitive materials. Assistance in utilization of these techniques can usually be obtained from the equipment vendors. 

In large-scale applications, waste streams of BTFs may be safely reclaimed by distillation in an explosion-proof distillation unit or still. Most stills available today have reclaim efficiencies of 90 to 99%. Using a high efficiency, thin-film evaporation still, 95% or more of the BTF maybe recovered from the spent solvent sludge. To further increase the amount of reclaimed solvent and to reduce the volume of waste that must be disposed of, stills can be equipped to employ steam stripping. 

The second stage of the hybrid-process is essentially a crystallization capable of crystallizing all solids dissolved in the feed resp. the brine of the RO stage. Parts of these substances tend to heavy scaling and, therefore, the choice of the crystallization equipment is limited. A proven solution is, of course, the agitated thin film evaporator. However, this type of equipment must lead to high specific treatment costs for two reasons ... 

The equipment used in radiochemical plants is, for the most part, similar to that found in other industrial chemical plants. The equipment should be selected for minimum holdup, thus reducing shielding requirements and criticality hazards. For example, thin film evaporators, operating continuously, should be selected instead of pot-type vessels. The method of coupling equipment together and the remote control and handling features of equipment design may be quite different, especially for the remote-maintenance type of plant. 

Process Considerations and Performance The capacity, or performance, of a thin-film evaporator is often reported as a feed mass flow-rate scale factor (unit ma per unit surface area) for a specific process step. Performance is a function of a few process and equipment design variables ... 

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. 

This method, which is also known as rotary evaporation or rovap , is the technique of choice for the removal of large volumes of volatile solvents from solutions, e.g. from extractions and column chromatography (p. 217). Rotary evaporators are now standard communal pieces of equipment in the laboratory and the operating principle is that of a reduced-pressure distillation except that the evaporation flask can be rotated. This rotation reduces the risk of bumping , inherent in all reduced-pressure distillations, and spreads the solution in a thin film on the walls of the flask. This effectively increases the surface area of the solution and increases the rate of evaporation, which is further enhanced by the use of a vacuum. 

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