Evaporators Scraped surface

Provided with scraper elements that continuously sweep the heat-transfer 

Stirrer Evaporator with Paddie Type Stirrers driven from the side and with heated semi spherical bottom (for products with unfavourable flow pettern end vdry high viscosity) 

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FIGURE 32.13 Osmotic dehydration with re-concentration of hypertonic solution (1) feeding conveyor (2) osmotic dehydrator (3) redler conveyor (4) heat exchanger (5) scraped surface evaporator (6) thermo-compressor (7) driven wheel flow of hypertonic solution —, vapor =>, high pressure steam -heating steam). 

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. 

In one continuous method (17), the waxy oil charge—for example, in the case of a topped crude—is diluted with an equal volume of propane and the mixture is chilled to about —25° F. by means of heat exchange with cold filtrate in double-pipe scraped-surface equipment. This chilled mixture is then mixed with propane which has been chilled by autorefrigeration to —45° F. to bring the ratio of propane to oil to about 5 to 1. Ninety per cent of the refrigeration required is accomplished in this manner the remaining 10% is obtained by evaporating propane from the diluted mixture. 

Grainer salt is made by surface evaporation of brine in flat pans open to the atmosphere. Heat usually is furnished by steam pipes located a few inches below tlie tank bottom. Crystals form at the surface of the brine and are held tliere temporarily by surface tension. Thus, they grow laterally for awhile and form thin flakes. But, as they grow, they tend to sink and this process imparts a peculiar, hollow pyramid-like structure to them. Such crystals are called hopper crystals. Ultimately, the crystals sink to the bottom where they are scraped to one end of the pan. The crystals are fragile and during handling they break up, finally assuming a flake-like shape. Thus, tlie term flake salt. 

For chilling and crystallizing with an evaporating refrigerant, a 27-in shell with seven 6-in pipes is available (Henry Vogt Machine Co.). In direct contact with the scraped surface is the process fluid which may... 

Increasing solubility because of increased concentration of impurities will result in a similar equilibrium change, although in some cases, the effect could be much greater. In extreme cases, when the residual solvent concentration is reduced to less than a critical value, the substrate could melt or solidify, depending on the melting point and the impurity effect. This condition is often used in laboratory preparations for convenience in changing solvents and is referred to as concentration to dryness. It is obviously not a scalable operation in a stirred vessel. Specialized tubular evaporators with close-clearance or scraped-surface rotors are available for these applications and have been successfully used by the authors for concentration but not for simultaneous crystallization. 

Equation (15.23) applies to a small hi -speed unit known as a Votator. Data for laminar-flow heat transfer in large slow-speed exchangers are given in Ref. 24. Scraped-surface devices for evaporating wiscous liquids are described in Chap. 16. 

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). 

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

A definite advantage of freeze crystallization, important in many food industry applications, is that volatile flavour components that are normally lost during conventional evaporation can be retained in a freeze-concentrated product. In fact, at present, freeze crystallization finds its main application in the food industry, for the concentration of fruit juices, etc. Indirect-contact freezing processes are normally used, e.g. the liquid feedstock is crystallized in a scraped-surface heat exchanger (section 8.2.2) and the resulting ice slurry passes to a wash column where the crystals are separated and washed to recover valuable product. The wash column is the key item in the process. Figure 8.56 shows an example of the Grenco system of freeze crystallization. 

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. 

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

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. 

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