Stills falling-film

Figure 13.30. Molecular distillation and related kinds of equipment, (a) Principle of the operation of the falling film still (Chemical Engineers Handbook, McGraw-Hill, New York, 1973). (b) Thin-layer evaporator with rigid wiper blades (Luwa Co., Switzerland), (c) The Liprotherm rotating thin film evaporator, for performance intermediate to those of film evaporators and molecular stills (Sibtec Co., Stockholm), (d) Centrifugal molecular still [Hickman, Ind. Eng. Chem. 39, 686 (1947)].

A reagent grade acrylic acid, obtained from Fisher Scientific Co. and inhibited with p-methoxyphenol, was employed in this study. The acrylic acid was purified prior to each use with a Falling Film Still. 

What is an average film thickness in an unwiped falling film still ... 

Falling-film stills are constructed of two vertical concentric tubes. The inner tube is heated internally, and the liquid to be distilled flows down the outer wall of the inner tube in a thin film. The outer tube is the condenser, and the annular space is under vacuum. Some thin-film arrangements involve continuous wiping of the film by a rotating blade. 

Falling film flow. The extent of basic modeling of two-phase annular flow is still very limited, because annular flow is the pattern that is least well understood... 

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. 

The rate of distillation depends on how fast material from below the surface can get to the surface. Therefore, a device to keep a fresh surface exposed, a flowing film still is used in commercial apparatus. Flowing films can either fall by gravity or move sideways by centrifugal force, just as long as a thin film is produced. 

The main thrust of a commercial still is to provide a large evaporating surface and a continuous process. Batch stills first were rotated to increase the surface area, but several developments since then have occurred. The falling film method, the centrifugal method, the wiped film method, and the fractionating method are all now common. 

The distilland (sample) is added to the top and allowed to flow by the force of gravity down the surface in a thin film. The still usually consists of two vertical concentric cylinders, one being the evaporator and the other the condenser. Most of the sample is stored at low temperature and only the portion that is in immediate contact with the evaporator is heated and then only for a few seconds (10-50). Most falling films are from 0.1 to 2.0 mm thick. The efficiency is better than that of all but the smallest batch apparatus with f values approaching 1 and having one theoretical plate between the evaporator and the condenser. A value of 5-6 g/secW is reasonable with small units. From 5-10 % of the sample is distilled in one pass. Such a still is shown in Figure 7-3. 

First, the overall mass transfer coefRcient k a of the microreactor was estimated to be 3-8 s [43]. For intensified gas liquid contactors, kj a can reach 3 s while bubble columns and agitated tanks do not exceed 0.2 s Reducing the flow rate and, accordingly, the liquid film thickness is a means of further increasing kj a, which is limited, however, by liquid dry-out at very thin films. Despite such large mass transfer coefficients, gas-liquid microreactors such as the falling film device may still operate between mass transfer and kinetic control regimes, as fundamental simulation studies on the carbon dioxide absorption have demonstrated [44]. Distinct concentration profiles in the liquid, and even gas, phase are predicted. 

The chief problem in a falling-film evaporator is that of distributing the liquid uniformly as a film inside the tubes. This is done by a set of perforated metal plates above a carefully leveled tube sheet, by inserts in the tube ends to cause the liquid to flow evenly into each tube, or by spider distributors with radial arms from which the feed is sprayed at a steady rate on the inside surface of each tube. Still another way is to use an individual spray nozzle inside each tube. 

I = 0.0471 X (2/3) X 998/(2526.5/3600) = 44.7 s Correcting the residence time in the falling-film evaporator (part (a)) for the decrease in liquid rate would increase the residence time to 4 1/2 or 5 seconds, still about an order of magnitude smaller than in an upflow evaporator. 

The author s thin-film apparatus, as developed further by VEB Glaswerk Stiitzer-bach and illustrated in Fig. 196, was adapted from the early types of molecular stills operating with a falling film. It contains two concentric tubes. The inner tube, which is interchangeable by the provision of a ground joint, can be heated by an element a and functions as a surface for evaporation. The outer tube acts as condensing surface. 

Essential larger quantities of material can be treated with a newly developed plate molecular still [5]. It is a combination of plate-falling film evaporator and short path still. In a vacuum chamber heated and cooled plates are fixed alternately Figure 15.12 shows the principle. The feed is equally distributed at the top of the heated plates. The produced vapours are condensed on the cold surfaces. At the bottom of the still distillates and residues are collected separately and discharged. Main advantages no rotating parts and a large evaporation area per volume vacuum chamber. 

Figure 10 Falling film molecular still for the removal of volatiles from lipids. (From Ref. 20.)...

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