Evaporation falling film evaporator

Evaporation of the thin symp (15—18% solids) is achieved in multiple-stage evaporators (falling film evaporators, natural or forced circulation evaporators). Mildly alkaline conditions (pH 9) are maintained to prevent sucrose inversion. The boiling temperatures decrease in the range of 130—90 °C. The resultant thick syrup (yield of 25—30 kg/100 kg beet) is once more filtered. The symp contains 68—72% solids and sucrose content is 61—67%. The raw, thin and thick symps have purity quotients of approx. 89, 92 and 93, respectively, i.e. the percentage sucrose on a dry matter basis. 

Concentration and Aroma Recovery. Concentration of juice from deciduous fmit is best carried out using an evaporator that causes as httle thermal degradation as possible and that permits recovery of volatile materials important to the aroma of the fresh fmit, ie, essence. Evaporators that use a high temperature for a short time and operate under a vacuum, such as the APV Crepaco falling film plate evaporator or the Alfa Laval centrifugal... 

Since about 1965, efficient vacuum evaporators have been used in most plants. Second stage evaporators, where the ammonium nitrate is concentrated to more than 99%, are designed to retain only a small volume of melt, have short residence times, and are protected from overheating and contamination by sensitizers. Falling film units are especially suited for this appHcation. 

Fig. 3. Film-type evaporators (a) long-tube vertical, (b) falling film, and (c) horizontal tube. Terms are defined in Figure 1. M represents end view of (a).

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. 

Both rising- and falling-film LTN- evaporators are generally unsuited to salting or severely scahng hquids. However, both are widely... 

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

Wetted-wall (falling-film) column Countercurrent, cocurrent Differential Liquid and/or gas Absorption, rectification, stripping, evaporation... 

Major evaporator designs include forced-circulation, long-tube vertical (both rising and falling film), and calandria-type evaporators. The economics of a particular process will dictate the evaporator style and model best suited to a particular application. Forced-circulation and calandria evaporators are required for processes where crystals are formed. These evaporators are designed to keep crystals suspended in solution to prevent scaling of the equipment. Long-tube vertical evaporators are used to concentrate a liquid that does not have solids present. 

Heat transfer equipment has a great variation in heat transfer area per unit of material volume. Table 4.4 compares the surface compactness of a variety of heat exchanger types. Falling film evaporators and wiped film heat exchangers also reduce the inventory of material on the tube side. Process inventory can be minimized by using heat exchangers with the minimum volume of hazardous process fluid for the heat transfer area required. 

Falling-film vertical evaporators, direct expansion systems, and vacuum freezing techniques may also be used. 

Tampella recovery system, falling film black liquid evaporation, lime kiln modifications... 

In a falling film evaporator (4) a water-paraffin mixture is distilled off and completely pumped back to the reactor. The resulting product is separated into a 60% sulfuric acid fraction and paraffin-containing alkanesulfonic acid (5), which is bleached by hydrogen peroxide (6). In a stirred vessel (7) the alkanesulfonic acid is neutralized by 50% sodium hydroxide solution until the pH is exactly 7. The composition of the neutralized product is also given in Table 2. 

Evaporation can be performed directly from reactors or kettles provided that substances are thermally stable. Such evaporation is time consuming because of the low heat-transfer surface area per unit volume. In the case of temperature sensitive materials, the residence time in the evaporator must be short and the temperature should be as low as possible. Consequently, continuous vacuum evaporators with a short residence time should be used to treat such materials. Falling-film (thin-film) evaporators are suitable to perform such operations. A typical falling-film evaporators is shown in Fig. 7.2-14. Centrifugal evaporators are also commonly used. 

Figure 7.2-14. Falling-film evaporator with centrifugal separator and condenser (by courtesy of Ktihni AG) 1. Falling-film evaporator 2. Separator 3. Condenser F = feed D = distillate E = vent/vacuum R = residue V/C = heating medium W = cooling medium.

Evaporator, agitated falling-film, s.s. heat surface area, 3-6 m 0.55... 

Figure 10.32. Long-tube evaporators (a) Rising film (b) Falling film...

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