Evaporators Climbing film

C3. Cathro, K. J., and Tail, R. W. F., Heat transfer to liquids boiling inside tubes. I. The climbing-film evaporator, Australian J. Appl. Sci. 8, 279 (1957). 

C9. Coulson, J. M., and McNelly, M. J., Heat transfer in a climbing film evaporator, Trans. Inst. Chem. Engrs. (London) 34, 247 (1956). 

A double-effect climbing-film evaporator is connected so that the feed passes through two preheaters, one heated by vapour from the first effect and the other by vapour from the second effect. The condensate from the first effect is passed into the steam space of the second effect. The temperature of the feed is initially 289 K, 348 K after the first heater and 383 K after the second heater. The vapour temperature in the first effect is 398 K and in the second effect 373 K. The feed flowrate is 0.25 kg/s and the steam is dry and saturated at 413 K. What is the economy of the unit if the evaporation rate is 0.125 kg/s ... 

Fig. 2 (A) Evaporator with calandria (B) climbing film evaporator.

In circulation evaporators a pool of liquid is held within the equipment. Incoming feed mixes with the liquid from the pool, and the mixture passes through the tubes. Unevaporated liquid discharged from the tubes returns to the pool, so that only part of the total evaporation occurs in one pass. All forced-circulation evaporators are operated in this way climbing-film evaporators are usually circulation units. 

Various methods have been used to dehydrate the esterification reaction mixture and promote reaction. Azeotropic distillation is usually involved. Once the mixture is anhydrous it is often heated under superatmospheric presure to accelerate the reaction. Water may be removed from the system with a molecular sieve (457). The silica may be in an emulsified mixture of water and immiscible solvent such as mineral oil and cetyl-oleyl glycol ether and an emulsifying agent. This is dehydrated in a climbing-film evaporator (458). Water can be removed by reaction with a ketal according to Ryznar (459). 

The climbing-film evaporator is the further development ofthe natural circulation evaporator. Before entering the evaporator, the product is preheated to boiling temperature or to a temperature slightly above boiling temperature, which is even better. Evaporation takes place immediately on the product inlet. In this way, the complete tube grid is available for the two-phase heat transition with high k values. 

The climbing-film evaporator is a high-performance evaporator. 

The fact that the vapour has to push and accelerate the liquid drops, however, results in a very high dynamic pressure loss, and this again implies that a large temperature difference has to be provided for. For this reason, climbing-film evaporators are not suited for temperature-sensitive products. For the first time, however, this is an evaporator that can be run in single-pass product flow (Figure 11.4). 

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. 

In the meantime, these advantages are also used for evaporation. The achieved k values are similar as those achieved in a tube climbing-film evaporator. A separate preheater is not required since the heat transfer in the liquid phase is already good. 

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