Falling-Film Evaporator

The pressure drop is lower and evaporation tubes can be designed up to lengths of 20 m. 

A Product B Vapour C Concentrate D Heating steam E Condensate 

The advantage of the falling-film evaporator over the climbing-film evaporator is that of a moderate pressure drop. Thanks to the low pressure drop, it is very suitable for a multi-effect evaporator or for plants with vapour recompression. 

A falling-film evaporator can be operated at low temperature differences, therefore, it is very suitable for temperature-sensitive products moreover, it can be operated with minimum filling volume and in single pass. For these reasons, the falling-film evaporator has been used with great success in the food industry for the evaporation of milk and whey. 

A Product 1 Head B Vapour 2 Heating body C Concentrate 3 Heating body base D Heating steam 4 Mixture channel E Condensate 5 Separator 

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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. 

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.

A Corynebacterium propinquum microbial cell catalyst was employed to convert acrylonitrile to ammonium acrylate, where the final concentration of product was 10-20% and the concentration of unconverted acrylonitrile was <30 ppm [81], The ammonium acrylate solution was concentrated to 40-60% by falling film evaporation, the resulting solution acidified, and the acrylic acid extracted with diethyl ether at 0-10 °C to obtain high-purity acrylic acid. 

The hydroformylation reaction is highly exothermic, which makes temperature control and the use of the reaction heat potentially productive and profitable (e.g, steam generation). The standard installation of Ruhrchemie/Rhone-Poulenc s aqueous-phase processes is heat recovery by heat exchangers done in a way that the reboiler of the distillation column for work-up of the oxo products is a falling film evaporator... 

For the concentration of fruit juice by evaporation, it is proposed to use a falling-film evaporator and to incorporate a heat-pump cycle with ammonia as the medium. The ammonia in vapour form enters the evaporator at 312 K and the water is evaporated from the juices at 287 K. The ammonia in the vapour—liquid mixture enters the condenser at 278 K and the vapour then passes to the compressor. It is estimated that the work required to compress the ammonia is 150 kJ/kg of ammonia and that 2.28 kg of ammonia is cycled/kg water evaporated. The following proposals are made for driving the compressor ... 

Figure 14.23. Single-effect falling-film evaporator...

Heat-assisted evaporation strategies, such as the falling-film evaporator, plate evaporator, forced-film evaporator, and centrifugal forced-film evaporator have been developed and are used to remove vyater from solutions of small peptides, such as antibiotics. But most recombinant proteins are heat labile and may not survive this strategy. 

Figure 1 Flowsheet of the RCH/RP hydroformylation process 38 1 Continuous flow stirred tank reactor,424 2 Phase separator, 3 Stripping column, 4 Distillation column, 5 Heat exchanger, 6 Falling film evaporator, 7 Liquid vapor separator.

Fig. 4. Flow diagram of the Ruhrchemie/Rhone-Poulenc process (137) 1, continuous-flow, stirred tank reactor 2, phase separator 3, stripping column 4, distillation column 5, heat exchanger 6, falling film evaporator 7, liquid-vapor separator.

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