Falling film contactors

The fading film MSR is one of the most commonly used devices for gas-liquid reactions (examples are given in gas-liquid reactions section). The liquid flows downward because of gravity in the form of film and gas flows through the open space that lies in the top cover of the housing. The falling film contactor consists in general of a stainless steel plate with open channels, typically 300 pm deep, separated by about 100 pm thick walls. The role of open microchannels is to prevent the breakup of the liquid film. 

In our calculations for organic and aqueous systems we used respectively the values proposed by Alvarez (2) and by Laurent (17) the later s values are comparable with ours (22). These values have been obtained in our laboratory with a cylindrical wetted wall falling film contactor. 

Falling film contactors employ thin liquid films that are created by a liquid feed falling under gravitational puD. The liquid film is in contact with a solid support, which is... 

Figure 8.15 Disassembled and partially assembled helicoidal microchannel falling film contactor with path length 540 mm. From [97],...

A numerical model accounts for the variation in liquid film thickness and back mixing effects. It can be used for a wide range of process conditions in micro-structured falling-film contactors. It enables also investigation of physical and chemical absorption systems with slower reaction kinetics, in which the mass-transfer resistance is concentrated in the liquid phase [143]. 

Fig. 8.2 Liquid-gas contactors, a Horizontal contactor, b vertical (falling film) contactor...

Figure 17.11. Types of contactors for reacting gases with liquids many of these also are suitable for reacting immiscible liquids. Tanks (a) with a gas entraining impeller (b) with baffled impellers (c) with a draft tube (d) with gas input through a rotating hollow shaft, (e) Venturi mixer for rapid reactions, (f) Self-priming turbine pump as a mixer-reactor, (g) Multispray chamber. Towers (h) parallel flow falling film (i) spray tower with gas as continuous phase (j) parallel flow packed tower (k) counter flow tray tower. (1) A doublepipe heat exchanger used as a tubular reactor.

Differential contactors include packed towers, spray towers, and falling-film absorbers, and are often called counterflow contactors. In such devices gas and liquid flow more or less continuously as they move through the equipment. 

The falling-film principle uses the wetting of a surface by a liquid stream, governed by gravitational force, which thus spreads to form an expanded thin film (see Figure 4.28), a concept known from macroscale contactors. Typical films have a thickness of a few ten to a few hundred micrometers [230,248,263]. 

Owing to the complex and often dedicated equipment required to perform gas-liquid phase reactions within research laboratories, this area of synthetic chemistry is somewhat underutilized. Over the past decade, however, numerous research groups have developed an array of continuous flow reactors capable of conducting such reactions in a safe and efficient manner, including microchannel contactors, falling film micro reactors, and packed-bed reactors [68, 69]. 

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. 

Figure 8.13 Disassembled falling film microchannel contactor and schematic showing the gas-liquid contacting. From [92].

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