Falling film evaporator design

The crude alcohol is distilled in falling film evaporators. These units are designed to provide very low residence time and also low absolute pressures. This assures low product and catalyst decomposition losses while minimizing alcohol recycle, thereby minimizing alcohol losses to the heavy ends bleed. 

Han, J. Fletcher, L.S. Falling film evaporation and boiling in circumferential and axial grooves on horizontal tubes. Ind. Eng. Chem. Process Design Dev. 1985, 24, 570-575. 

The crude aldehyde is fractionally distilled into n- and isobutanal in a conventional aldehyde distillation unit. The reboiler of this n/iso column is designed as a heatabsorbing falling film evaporator incorporated in the oxo reactor, thus providing a neat, efficient method of recovering heat by transferring the heat of reaction in the reactor to cold n-butanal, which subsequently heats the n/iso column. The preferred hydroformylation temperature is 110-130 °C and is therefore used for the production of process steam. Whereas other oxo processes are steam importers, the RCH/RP process including the distillation of n-/isobutanol exports steam. No special pretreatment or even purification steps are necessary for the catalyst. This reduces the environmental burden still further. 

Feed to a falling film evaporator is usually introduced under the liquid level maintained at the top of the tubes, so that a reservoir of rather low velocity liquid is available for liquid distribution to the many vertical tubes. In falling film evaporator and re-boiler design, equal fluid distribution among the tubes and film initiation are very important factors. For this reason, a number of sophisticated and very effective hydraulic distributing devices have been developed to handle different types of process fluids.f In order... 

The flow maldistribution problem in evaporators is most severe when the major part of the pressure drop occurs in the two-phase heated region and cannot be prevented in some cases (reboilers and evaporators). However, in most heat exchangers with such problems, the economic penalty for maldistribution is small, and deterioration of the thermal performance is slightly affected [115]. The flow instabilities can be controlled by restricting the liquid circulation [2], Falling film evaporators are particularly prone to flow maldistribution. This effect can be reduced through design modifications (the top tube plate must be exactly horizontal, addition of inserts into the top of each tube must be secured, etc.) [120]. 

Falling-film evaporators may be used for simple and countercurrent distillation a < well as for short-path evaporation (see chap. 5.4.4). Heating is mostly done via the outer wall (Figs. 198, 200, 201, 202, 203, 211) but the opposite design is also employed (Figs. 196, 199, 210, 212). 

Falling film evaporator Evaporation rate < design [liquid maldistribution] / steam trap malfunction, see Section 5.1/steam flowrate too small. 

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... 

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. 

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. 

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