Evaporator four-effect

Liquor containing 5 per cent solids is fed at 340 K to a four-effect evaporator. Forward feed is used to give a product containing 28.5 per cent solids. Do the following figures indicate normal operation If not, why not ... 

Typically, a single-effect membrane caustic evaporator takes around 1.2 tonnes of steam per tonne of caustic soda while a double-effect uses around 0.7 tonne of steam per tonne of caustic soda. An equivalent single-effect diaphragm evaporator uses 4.2 tonnes per tonne. However, a well-run multiple four-effect diaphragm evaporator consumes about 2.1 tonnes of steam to produce one tonne of caustic soda. At a price of US 20 per tonne of steam, a saving of US 4 million per year for a 200 000 tonnes per year plant can be achieved (Fig. 15.6). So what extra equipment is required besides electrolysers ... 

In a four-effect evaporator steam is put into the first effect, where an effect relates to vapor flow, and the heat from that steam is used four times before condensation occurs in a barometric condenser. In theory, a four-effect evaporator will remove four liters of water per kilogram of steam usage, but in actual practice that water removal is about 3.4 1 per kg of steam-Heat losses and the change in the heat of vaporization with temperature account for the difference. 

Since a return of at least 15 percent is required on any investment, three effects are better than four effects, and the four-effect evaporator should receive no further consideration. 

FIGURE 6.3 Quadruple effect evaporator-crystallizer. Pressure and boiling temperature decrease from left to right, temperature decreases about 20-30°C across the four effects. Feed brines to each effect may be directly from brine wells, or may be fed cascade fashion from the filtrate to the salt brine slurry of the previous effect, or a combination of these methods. (Adapted from Kirk-Othmer [10], with permission.)... 

Besides the choice of evaporator type, evaporation process design includes selection of the number of effects to be installed. This choice is primarily a matter of a classical economic balance between the cost of supplying more effects and the benefits they offer in energy consumption. As the preceding section made clear, the number of effects in a caustic evaporator is never very great. For technical and economic reasons, diaphragmcell evaporators usually have three or four effects and membrane-cell evaporators have two or three. 

Control system strategies are the same for both types. Diaphragm-cell evaporators may contain four effects and are more likely to have a mixed-flow configuration. Output is usually controlled by the primary steam flow, and the density control system can switch from startup to normal operation in the manner described above. 

The feed solution of uranyl nitrate hexahydrate, at 1120 g U/1. is provided by evaporation of the 80 to 90 g/1. solvent extraction product solution. This is carried out in a four-effect natural circulation multi-tube evaporator, shown diagramatically in Fig. S.15. A little sulphuric acid is added to the feed, to increase the reactivity of the oxide during the later hydrofluorination reaction. 

Figure 16.19 Process flow sketch of the four-effect evaporative crystallization plant with vacuum...

Figure 11.6 Four-effect falling-film evaporation plant. ...

Each effect is arranged in series and operated so that each succeeding one operates under a higher vacuum (lower pressure). This arrangement allows the juice to be drawn from one vessel to the next and permits it to boil at a low temperature. The concentrated juice (syrup) is removed from the last effect by a pump. Triple-, quadruple-, and quintuple-effect evaporators are used, but the quadruple-effect type is most common. In a four (quadruple )-effect evaporator, one pound of steam evaporates four pounds of water. 

Water is evaporated from purified brine using multiple-effect or vapor recompression evaporators (Figs. 3 and 4). Multiple-effect systems typically contain three or four forced-circulation evaporating vessels (Fig. 4) connected together in series. Steam from boilers suppHes the heat and is fed from one evaporator to the next to increase energy efficiency in the multiple-effect system. 

The above experimenters have used the technique described to obtain flow rate measurements of the liquid wall-film at various mass velocities, tube dimensions, etc., and some typical results from Staniforth and Stevens (S7) are shown in Fig. 7. Also shown are the values of burn-out heat flux obtained at the four different mass velocities indicated. It can be seen that the liquid-film flow rate decreases steadily with increasing heat flux until at burn-out the flow rate becomes zero or very close to zero. We thus have confirmation of a burn-out mechanism in the annular flow regime which postulates a liquid film on the heated wall diminishing under the combined effects of evaporation, entrainment, and deposition until at burn-out, the film has become so thin that it breaks up into rivulets which cause dry spots and consequent overheating. 

The resulting solution is cooled to 0° and decomposed by careful addition of 500 ml. of 102V hydrochloric acid. The mixture is transferred to a separatory funnel, the aqueous phase is separated, and the toluene layer is extracted with two 250-ml. portions of 102V hydrochloric acid. The aqueous extracts are combined and heated under reflux for 15 hours to effect decarboxylation. The hot, dark-colored solution is treated with 10 g. of activated charcoal, filtered, and evaporated to dryness under reduced pressure. The residue is washed into a separatory funnel with 300 ml. of water. The solution is treated with saturated aqueous potassium carbonate solution until it is alkaline to litmus the carbonate solution must be added very carefully to prevent excessive foaming. Solid potassium carbonate is added until a thin slurry is obtained, and the slurry is extracted with four 400-ml. portions of ether. The combined ether extracts are dried for at least 60 minutes over calcined potassium carbonate and then filtered. 

The effect of temperature on the RP-HPLC behaviour of /(-carotene isomers has been extensively investigated and the results were employed for the separation of carotenoids of tomato juice extract. Carotenoids were extracted from food samples of 2g by adding magnesium carbonate to the sample and then extracted with methanol-THF (1 1, v/v) in a homogenizer for 5min. The extraction step was repeated twice. The collected supernatants were evaporated to dryness (30°C) and redissolved in methanol-THF (1 1, v/v). Separations were performed on a polymeric ODS column (250 X 4.6 mm i.d. particle size 5/.an). The isocratic mobile phase consisted of methanol-ACN-isopropanol (54 44 2, m/m). The flow-rate was 0.8 or 2.0 ml/min. The effect of temperature on the retention times of lycopene and four /(-carotene isomers is shown in Table 2.11. The data indicated that the temperature exerts a considerable influence on the retention time and separation of /(-carotene isomers. Low temperature enhances the efficacy of separation. 

The measurement of properties such as the resistivity or dielectric constant of PS requires some kind of contact with the PS layer. Evaporation of a metal onto the PS film-covered silicon sample produces a metal/PS/Si sandwich, which behaves like an MIS structure with an imperfect insulator. Such sandwich structures usually exhibit a rectifying behavior, which has to be taken into account when determining the resistivity [Si3, Bel4]. This can be circumvented by four-terminal measurements of free-standing PS films, but for such contacts the applied electric field has to be limited to rather small values to avoid undesirable heating effects. An electrolytic contact can also be used to probe PS films, but the interpretation of the results is more complicated, because it is difficult to distinguish between ionic and electronic contributions to the measured conductivity. The electrolyte in the porous matrix may short-circuit the silicon filaments, and wetting of PS in-... 

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