L

Feed

-Vacuum

Product out

Figure 4. Falling film tubular.

-Vacuum

Product out

Figure 4. Falling film tubular.

further enhanced by generating flash vapor at this point. The falling film evaporator does have the advantage that the film is "going with gravity" instead of against it. This results in a thinner, faster moving film and gives rise to even shorter product contact time and a further improvement in the value of HTC.

The rising film unit normally needs a driving force or temperature difference across the heating surface of at least 25°F to establish a well-developed film, whereas the falling film evaporator does not have a driving force limitation. This permits a greater number of evaporator effects to be used within the same overall operating limits, in other words, if steam is available at 2.5 psig corresponding to 220°F and the last effect boiling temperature is 120°F, the total available AT is equal to 100°F. This would limit a rising film evaporator to four effects, each with a AT of 25°F. It would be feasible, meanwhile, to have as many as ten or more effects using the falling film technique.

Rising-Falling Film Tubular. As illustrated by Figure 5, the rising-falling film evaporator has the advantages of the ease of liquid distribution of the rising film unit coupled with lower head room requirements. The tube bundle is approximately half the height of either a rising or falling film evaporator and the vapor-liquid separator is positioned at the bottom of the calandria.

Forced Circulation. The forced circulation evaporator (Fig. 6) was developed for processing liquors that are susceptible to scaling or crystallizing. Liquid is circulated at a high rate through the heat exchanger, boiling is prevented within the unit by virtue of a hydrostatic head maintained above the top tube plate. As the liquid enters the separator, where the absolute pressure is slightly less than in the tube bundle, the liquid flashes to form a vapor.

The main applications for a forced circulation evaporator are in the concentration of inversely soluble materials,

Steam

Steam

Feed H

Steam

Steam

Feed H

Vacuum

Product out

Vacuum

Product out

Wiped Film. The wiped or agitated thin film evaporator depicted in Figure 7 has limited applications and is confined mainly to the concentration of very viscous materials and the stripping of solvents down to very low levels. Feed is introduced at the top of the evaporator and is spread by wiper blades on to the vertical cylindrical surface inside the unit. Evaporation of the solvent takes place as the thin film moves down the evaporator wall. The heating medium normally is high-pressure steam or oil. A high-temperature heating medium generally is necessary in order to obtain a reasonable evaporation rate since the heat-transfer surface available is relatively small as a direct result of its cylindrical configuration.

The wiped film evaporator is very satisfactory for its limited applications. However, in addition to its small surface area, it also has the disadvantage of requiring moving parts such as the wiper blades, which, together with the bearings of the rotating shaft, need periodic maintenance. Capital costs in terms of dollars per pound of solvent evaporated also are very high.

Figure 5. Rising-falling film tubular.

Liquor head to prevent boiling at heating surface

Figure 5. Rising-falling film tubular.

_ Vapor outlet

_ Vapor outlet

Low temperature1 rise across calandria

4 Concentrated liquor outlet

Calandria

Dilute lr- liquor inlet

Circulation pump giving high liquor velocities over heating surface

Figure 6. Forced circulation.

which results in the deposition of solids. In all cases, the temperature rise across the tube bundle is kept as low as possible, generally in the region of 3-5°F. This results in a recirculation ratio as high as 200-330 lb of liquor per pound of water evaporated. These high recirculation rates result in high liquor velocities through the tubes, which helps to minimize the buildup of deposits or crystals along the heating surface. Forced circulation evaporators normally are more expensive than film evaporators because of the need for large-bore circulating pipework and large recirculating pumps generally of the axial flow type. Operating costs of such as unit also are considerably higher.

Plate Evaporators

The plate equivalent of the tubular evaporator is available in four configurations: rising-falling film, falling film, Par-vap, and Parflash plate evaporators. All have been developed by APV to handle the concentration of products that have varying characteristics.

Plate-type evaporators now have been sold commercially for nearly 30 years, and during that time over 1600 units manufactured by APV have been installed for concentrating hundreds of different products.

Feed inlet

Rotor assembly

Vaporizing section

Feed inlet

Rotor assembly

Vaporizing section

External separator

Product discharge

Figure 7. Wiped film.

External separator

Product discharge

Product

Figure 7. Wiped film.

Rising/Falling Film Plate. The original plate type evaporator, the rising-falling plate system basically is a flexible, multiduty unit engineered for medium production runs of heat-sensitive products under sanitary conditions and at the lowest possible capital investment.

Operating on a single-pass rising and falling film principle, as shown in the exploded view of Figure 8, the evaporator consists of a series of gasketed plate processing units within a compact frame. As a thin layer of feed liquor passes over the rising and falling film section in each evaporation unit, it is vaporized on contact with adjacent steam heated plates and is discharged with its vapor to a vapor-liquid separator. All evaporation is accomplished in a matter of seconds within the plate pack. The product then is extracted and the vapor passed to a condenser, the next evaporator effect or a mechanical compressor. Product quality is given maximum protection against thermal degradation by means of high heat-transfer rates, low liquid holding volume, and minimum exposure to high temperatures.

The plate evaporator has a number of advantages over its tubular counterpart. Since it is designed to be erected on a single level with minimum headroom requirements, in many cases it will fit within an existing building with overhead restrictions as low as 12 or 13 ft. It also can be arranged as single or multiple effects without extensive building modifications or structural steel supports and can handle expanded duties by the addition of more plate units. Systems are available for evaporation rates to 35,000 lb/h with efficient in-place cleaning of all stainless, steel product contact surfaces.

Falling Film Plate. The success of the rising-falling film plate evaporator prompted the development of the falling film plate system. This design has several advantages over its predecessor, including even lower residence times and higher evaporation capabilities. While the rising-falling film plate unit is restricted to a maximum of30,000-35,000

lb/h of water removal, the falling film plate evaporator with its larger vapor ports can accommodate 59,00060,000 lb/h of evaporation. One of this unique design features as illustrated in Figure 9 is that each side of the plate may be used independently of the other, thus permitting two pass operation within the same frame for even shorter residence time and improved product quality.

Paravap. Especially designed to concentrate foaming liquids or those with high solids content or non-Newtonian viscosity characteristics, the APV Paravap replaces the wiped film evaporator in many cases. It successfully concentrates corn syrups and soap to better than 97-98% total solids or strips hexane and other solvents from vegetable oils and similar products. Figure 10 shows this type of unit in a simple schematic.

Note that the key element in the system is a plate heat exchanger (Fig. 11). Although specifically designed for liquid-liquid applications, it has been found that if a fluid is allowed to vaporize within the plate pack, the small plate gap and corrugated plate pattern create high vapor velocities (Fig. 12). This causes atomization of the liquid within the high-velocity vapor stream, resulting in a greater liquid surface area for mass transfer and enabling low residual solvent concentrations to be realized. Although the final product may be extremely viscous after separation from the vapor, the apparent viscosity within the plate pack is very low since only droplets are being transported in the vapor.

Paraflash. Similar to its tubular counterpart, the APV Paraflash (Fig. 13) is a suppressed boiling forced-circulation evaporator used mainly for concentrating products subject to fouling that is too excessive for a film evaporator. While this system uses a plate heat exchanger, vaporization is not allowed to take place within the exchanger itself. Boiling is suppressed either by a liquid static head above the heat exchanger or by the use of an

Steam section

Inlet section

Steam section r^ Steamspacer Discharge section Steam section

Steam in

Condensate outlet

Steam section

Inlet section

Steam in

Condensate outlet

Joint gaskets

Figure 8. Rising-falling film plate evaporator.

Feed in Vapor and condensate discharge to separator

Joint gaskets

Figure 8. Rising-falling film plate evaporator.

Steam section

Feed

Product from separator

Vapor and product to separator

Steam section

Feed

Product from separator

Vapor and product to separator

Condensate outlet

Figure 9. Falling film plate evaporator.

Condensate outlet

Final product to separator orifice piece in the discharge line. From a heat-transfer standpoint, the forced-circulation Paraflash is far more efficient than a tubular unit but suffers from a smaller equivalent diameter when large crystal sizes are present.

Vacuum

Steam

Steam

Product

Figure 10. Paravap evaporator.

Product

Figure 10. Paravap evaporator.

Homemade Pet Food Secrets

Homemade Pet Food Secrets

It is a well known fact that homemade food is always a healthier option for pets when compared to the market packed food. The increasing hazards to the health of the pets have made pet owners stick to containment of commercial pet food. The basic fundamentals of health for human beings are applicable for pets also.

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