Platetype Evaporators

To effectively concentrate an increasing variety of products that differ by industry in such characteristics as physical properties, stability, or precipitation of solid matter, equipment manufacturers have engineered a full range of evaporation systems. Included among these are a number of plate-type evaporators.

Plate evaporators initially were developed and introduced by APV in 1957 to provide an alternative to the tubular systems that had been in use for half a century. The differences and advantages were many. The plate evaporator, for example, offers full accessibility to the heat-transfer surfaces. It also provides flexible capacity merely by adding more plate units, shorter product residence time (resulting in a superior quality concentrate), a more compact design with low headroom requirements and low installation cost.

These APV plate evaporation systems are available in four arrangements—rising-falling film, falling film, Para-vap, and Paraflash—and may be sized for use in new produce development or for production at pilot plant or full-scale operating levels.

Rising-Falling Film Plate

The principle of operation for the rising-falling film plate evaporator (RFFPE) involves the use of a number of plate packs or units, each consisting of two steam plates and two product plates. As shown in Figure 21, these are hung in

Figure 21. Rising-falling film plate evaporator in final stages of fabrication.

a frame resembling that of a plate heat exchanger. The first product passage is a rising pass and the second, a falling pass. The steam plates, meanwhile, are arranged alternately between each product passage.

The product to be evaporated is fed through two parallel feed ports and is equally distributed to each of the rising film annuli. Normally, the feed liquor is introduced at a temperature slightly higher than the evaporation temperature in the plate annuli, and the ensuing flash distributes the feed liquor across the width of the plate. Rising film boiling occurs as heat is transferred from the adjacent steam passage with the vapors that are produced helping to generate a thin, rapidly moving turbulent liquid film.

During operation, the vapor and partially concentrated liquid mixture rises to the top of the first product pass and transfers through a "slot" above one of the adjacent steam passages. The mixture enters the falling film annulus where gravity further assists the film movement and completes the evaporation process. The rapid movement of the thin film is the key to producing low residence time within the evaporator as well as superior heat-transfer coefficients. At the base of the falling film annulus, a rectangular duct connects all of the plate units and transfers the evaporated liquor and generated vapor into a separating device. Steam and condensate ports connect to all the steam annuli (Fig. 22).

The plate evaporator is designed to operate at pressures extending from 10 psig to full vacuum with the use of any

Vapor .

Steam Inlet Steam section section section

Discharge section

Steam section

Steam spacers

Vapor .

Steam Inlet Steam section section section

Discharge section

Steam section

Steam spacers

Gaskets

Concentrate

Condensate outlet

Figure 22. Rising-falling film plate evaporator arranged for one complete pass.

Gaskets

Concentrate

Condensate outlet

Figure 22. Rising-falling film plate evaporator arranged for one complete pass.

number of effects. However, the maximum pressure differential normally experienced between adjacent annuli during single effect operation is 15 psig. This, and the fact that the pressure differential always is from the steam side to the product side, considerably reduces design requirements for supporting the plates. The operating pressures are equivalent to a water vapor saturation temperature range of 245°F downward and thus are compatible with the use of nitrile or butyl rubber gaskets for sealing the plate pack.

Most rising-falling film plate evaporators are used for duties in the food, juice, and dairy industries where the low residence time and 100-200°F operating range temperatures are essential for the production of quality concentrate. An increasing number of plate evaporators, however, are being operated successfully in both pharmaceutical and chemical plants on such products as antibiotics and inorganic acids. These evaporators are available as multi-effect and/or multi-stage systems to allow relatively high concentration ratios to be carried out in a single pass, nonrecirculatory flow.

The rising-falling film plate evaporator should be given consideration for various applications.

• That require operating temperatures between 80 and 210°F

• That have a capacity range of 1000-35,000 lb/h water removal

• That have a need for future capacity increase since evaporator capabilities can be extended by adding plate units or by the addition of extra effects

• That require the evaporator to be installed in an area that has limited headroom

• Where product quality demands a low time-temperature relationship.

• Where suspended solid level is low and feed can be passed through 50 mesh screen

A "junior" version of the evaporator is available for pilot plant and test work and for low-capacity production. If necessary, this can be in multieffect-multistage arrangements such as the system illustrated in Figure 23.

Falling Film Plate

Incorporating all the advantages of the original rising— falling film plate evaporator system with the added bene

Figure 23. Three-effect, four-stage, "junior.

fits of shorter residence time and larger evaporation capabilities, the falling film plate evaporator has gained wide acceptance for the concentration of heat sensitive products. With its larger vapor ports, evaporation capacities typically are up to 50,000-60,000 lb/h.

The falling film plate evaporator consists of gasketed plate units (each with a product and a steam plate) compressed within a frame that is ducted to a separator. The number of plate units used is determined by the duty to be handled.

As shown in Figure 24, one important innovation in this type of evaporator is the patented feed distribution system. Feed liquor first is introduced through an orifice (1) into a chamber (2) above the product plate where mild flashing occurs. This vapor/liquid mixture then passes through a single product transfer hole (3) into a flash chamber (4), which extends across the top of the adjacent steam plate. More flash vapor results as pressure is further reduced and the mixture passes in both directions into the falling film plate annulus through a row of small distribution holes (5). These ensure an even film flow down the product plate surface where evaporation occurs. A unique feature is the ability to operate the system either in parallel or in series, giving a two-stage capability to each frame. This is particularly advantageous if product recirculation is not desirable.

In the two-stage method of operation, feed enters the left side of the evaporator and passes down the left half of the product plate, where it is heated by steam coming from the steam sections. After the partially concentrated product is discharged in the separator, it is pumped to the right side of the product plate where concentration is completed. The final concentrate is extracted while vapor is discharged to a subsequent evaporator effect or to a condenser.

Paravap

The operating principle of the Paravap represents an application of the thin film, turbulent path evaporation pro

Figure 24. Typical product and steam plate unit for falling film plate evaporator.

cess and in many cases, replaces the wiped or agitated film evaporator. Since it has no mechanical moving parts within the heat transfer area, costly maintenance repairs common to wiped film systems are eliminated. It is especially designed to concentrate liquids with high solids contents or non-Newtonian viscosity characteristics (Fig. 25).

With feed liquor and the heating medium of steam, hot water or hot oil directed into alternate passages within a plate heat exchanger, boiling begins as the liquid contacts the heated plates. The small plate gap and high-velocity flow pattern (Fig. 26) atomizes the feed and provides a greater liquid surface area for mass transfer. Since the vapor carries the feed in the form of minute particles, the apparent viscosity within the heat exchanger plate pack is

Figure 25. Single-effect R86 Paravap.

Product Heating or cooling liquid

Figure 26. Small plate gap and high-velocity flow pattern.

very low. The final product, however, may be extremely viscous after separation from the vapor.

Advantages include low residence time to minimize thermal degradation, low rates of fouling, and economical operation.

Typical applications: concentrating soap to final product consistency; concentrating apple puree from 25 to 40° brix, grape puree to 50° brix, cherry puree to 60° brix, fruit juices to 95% solids, and sugar or corn syrup solutions to over 97%; solvent stripping duties such as hexane from oil (See also Figs. 25 and 27.)

Paraflash

Operating under a suppressed boiling, forced circulation principle, the APV Paraflash is used to concentrate products subject to fouling too excessive for film evaporators. Unlike the Paravap, vaporization does not occur within the heat-exchanger plate pack. Instead, liquor flashes as it enters a separator, crystallization takes place, and a suspended slurry results. Suppressed boiling combined with high liquid velocities deters scaling on the heat-transfer surface, minimizes cleaning downtime, and promotes longer production runs.

The Paraflash can be used in single or multiple effects for such products as grape juice (tartrate crystals), coffee, wheat starch, distillery effluent, and brewer's yeast (suspended solids). (See Fig. 27).

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