Falling film evaporator, recirculation

The RO treatment removes salts and the concentrate is crystallized by a tube type of falling film evaporator and its condensate is recirculated back to the process. For example, when a stream at McKinley of approximately 660 m /day is RO treated it results in a permeate of about 490 m /day (1-1.3 m /t product). The salt removal is needed because the high conductivity levels dismrb the usability of the wet end chemicals [45,46]. 

Falling-film evaporators, with no recirculation and short residence times, handle sensitive products that can be concentrated in no other way. They are also well adapted to concentrating viscous hquids. 

The problem of film dismpture may be tempered by recirculation of the film to avoid very thin films. Another possibility is the use of a falling film evaporator (only one tube) equipped with a rotor and blades which have the duty to distribute and thin the film. Figure 7.1-7 shows an evaporator with motionless blades fixed on the rotor. The distance between the blade smface and the inside of the tube is approximately 1-2 mm. 

Since membrane-cell evaporators do not produce solids, forced-circulation evaporators are used less frequently. Rising-film and falling-film types appear in a number of plants. The rising-film evaporator depends on natural circulation of caustic from the bottom to the top of the tubes. Falling-film evaporators, as shown in Section 9.3.S.2, depend on pumps to lift caustic to the distribution system at the top. These units generally have better heat-transfer coefficients and less tendency to foul. Recirculated units in particular allow good control of flow to maintain a proper film on the tubes. This also permits the designer to provide more turndown capability. Liquid velocities are lower... 

A modification of the horizontal tube evaporator is the spray-film evaporator as shown in Figure 11-6. This is essentially a horizontal, falling-film evaporator in which the liquid is distributed by recirculation through a spray system. Sprayed liquid falls by gravity from tube to tube. Advantages include ... 

FIG. 11-122 Evaporator types, a) Forced circulation, (h) Siibmerged-tiihe forced circulation, (c) Oslo-type crystallizer, (d) Short-tube vertical, (e) Propeller calandria. (f) Long-tube vertical, (g) Recirculating long-tube vertical, (h) Falling film, (ij) Horizontal-tube evaporators. G = condensate F = feed G = vent P = product S = steam V = vapor ENT T = separated entrainment outlet. 

FIG. 11-125 Flow sheets for seawater evaporators, a) Multiple effect (falling film), (h) Multistage flash (once-through). (c) Multistage flash (recirculating). 

Fig. 3. Triple-effect steepwater evaporator. The third effect (forced circulation) Is shown in background second effect (falling film, recirculating) is middle unit. The first effect vapor head is shown in foreground. (Swenson. Whiting Corp)...

The principal problem with the falling-film LTV evaporator is that of feed distribution to the tubes. It is essential that all tube surfaces be wetted continually. This usually requires recirculation of the liquid unless the ratio of feed to evaporation is quite high. An alternative to the simple recirculation system of Fig. ll-122/i is sometimes used when the feed undergoes an appreciable concentration change and the product is viscous and/or has a high boiling point rise. The feed chamber and vapor head are divided into a number of liqu... 

The main problem associated with falling film units is the need to distribute the liquid evenly to all tubes. All tubes must be wetted uniformly and this may require recirculation of the liquid unless the ratio of feed to evaporation is relatively high. Recirculation can only be accomplished by pumping. Distribution can be achieved with distributors for individual tubes, with orifice plates above the tubes and tubesheet, or by spraying. Updraft operation complicates the liquid distribution. 

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