Scraped-film evaporators

Solvent is recovered in two steps. 90% of the solvent is recovered in a scraped film evaporator (FI). The remaining 10% of the volatiles are recovered by a degassing extruder (El) equipped with a pelletizer. The pellets are cooled with water. The cooling water is removed by filtration and the pellets are dried in two centrifugal dryers. 

Distillation equipment for soap-lye and esterification cmde requires salt-resistant metallurgy. The solid salt that results when glycerol is vaporized is removed by filtration or as bottoms from a wiped film evaporator. The Luwa scraped wall... 

For viscous liquids, one way to increase the heat transferred is to improve the heat transfer coefficient by scraping or stirring the fluid adjacent to the wall, as in agitated film or wiped film evaporators. Accommodation of the mechanical devices used to mix the fluid close to the wall requires a fairly large diameter tube, so these devices tend to consist of only a single tube thus, heat transfer area is relatively small. The introduction of moving mechanical parts may lead to maintenance problems. 

In mechanically aided film evaporators, a thin film of material is maintained on the heat transfer surface irrespective of the viscosity. This is usually achieved by means of a rotor, concentric with the tube, which carries blades that either scrape the tube or ride with low clearance in the film. Mechanical agitation permits the evaporation of highly viscous materials or those that have a low thermal conductivity. Because the temperature variations in the film are reduced and residence times are shortened, the vacuum evaporation of viscous thermolabile materials becomes possible. 

Scraped-surface and thin-film evaporators (Figs 4.2 and 4.3)... 

For the agitated-film evaporator, the heat-transfer coefficient may be estimated using Eq. (4.13-4) for a scraped surface heat exchanger. 

Evolving gas from viscous liquids in bubble reactors may give rise to viscous foams and entrainment that will limit the reaction rate, especidly on a larger scale. A spray column may be the first alternative (section 4.6.2.1). If cooling is also required effective measures are costly. For this purpose thin film evaporators (section 4.63.1) or scraped surface evaporators can be effective. For very viscous liquids one may consider batch kneaders or extruders with vapour ports. 

Drum Drying. The dmm or roHer dryers used for milk operate on the same principles as for other products. A thin layer or film of product is dried over an internally steam-heated dmm with steam pressures up to 620 kPa (90 psi) and 149°C. Approximately 1.2—1.3 kg of steam ate requited per kilogram of water evaporated. The dry film produced on the roHer is scraped from the surface, moved from the dryer by conveyor, and pulverized, sized, cooled, and put iato a container. 

Evaporation Retardants. Small molecule solvents that make up the most effective paint removers also have high vapor pressure and evaporate easily, sometimes before the remover has time to penetrate the finish. Low vapor pressure cosolvents are added to help reduce evaporation. The best approach has been to add a low melting point paraffin wax (mp = 46-57° C) to the paint remover formulation. When evaporation occurs the solvent is chilled and the wax is shocked-out forming a film on the surface of the remover that acts as a barrier to evaporation (5,6). The addition of certain esters enhances the effectiveness of the wax film. It is important not to break the wax film with excessive bmshing or scraping until the remover has penetrated and lifted the finish from the substrate. Likewise, it is important that the remover be used at warm temperatures, since at cool temperatures the wax film may not form, or if it does it will be brittle and fracture. Rapid evaporation occurs when the wax film is absent or broken. 

Melting of AlMq3 could be observed with DSC, followed immediately by evaporation or decomposition. Figure 3.17 shows DSC traces of sublimed-scraped samples as well as films deposited directly onto the DSC pans. Tm was reproducible in the... 

Close-clearance scrapers for viscous liquids are included in the review by Uhl [253]. An application of scraped-surface heat transfer to air flows is reported by Hagge and Junkhan [256] a tenfold improvement in heat transfer coefficient was reported for laminar flow over a flat plate. Scrapers were also suggested for creating thin evaporating films. Lustenader et al. [257] outline the technique, and Tleimat [258] presents performance data. The heat transfer coefficients are much higher than those observed for pool evaporation (without nucleate boiling). 

A scraped surface device can process a thin viscous film. Here the focus is on its use as a reactor. Elsewhere are described its use to condition foodstuffs. Section 3.3, as a vertical device for evaporation, Section 4.1 and as a crystallizer. Section 4.6. 

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