Heat falling film evaporators

The laboratory setup used to study this concept is shown in Figure 71. The distillation column, 30 mm I.D., consisted of a tray section in the bottom and four packed sections, each of the latter loaded with nine cartridges of type SULZER EX. The column was energized by an oil-heated falling film evaporator featuring a hermetic circulation pump. The distillate of the extractive distillation column of chapter 16.6.1 was introduced below the two uppermost SULZER columns, while hexane was introduced one SULZER section below the feed inlet. Between the bottom tray section and the lowermost SULZER section, the diketones were withdrawn as a vapor. 

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. 

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.

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 ... 

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.

This is not a falling-film evaporator because the liquid (juice) is mostly suspended in the vapor, and heat transfer is to the turbulent mixture. The exit velocity of the juice from the tubes is on the order of 20 to 100 meters per second (2). 

The heat of reaction generates 1) process steam in the reactor and 2) 5.5 bar pure steam. The process steam is used to preheat boiler feed water and nitric acid as well as operate the falling film evaporator. The pure steam is fed to the plant steam header295. 

Evaporators in commercial use include circulatory systems, shell and tube heat exchangers and falling film-type evaporators. The falling film evaporator has the advantages of a small working volume and a short residence time. All evaporators produce steam that must be purified, and some of the techniques mentioned above can also be applied to this steam103. 

In UCC s low-pressure and Mitsubishi Kasei oxo processes the reaction products (isononyl aldehyde, etc.) are separated by distillation from the catalyst phase. As already mentioned, in the Ruhrchemie/Rhone-Poulenc s hydrofor-mylation process, the aqueous phase containing the catalyst is removed after the reaction from the organic phase by decantation. Also in this process, the heat from the oxo reaction is recovered in a falling film evaporator incorporated inside the reactor, which acts a reboiler for the /i- butanal/isobutana I distillation column. 

In horizontal tube evaporators, the liquor is usually on the outside of the tubes and the heating medium on the inside. Rather than submerging the tubes, the boiling liquid is sometimes sprayed on the outside of the tubes. This gives a performance approaching that of falling film evaporators. ... 

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