Falling film column

Wetted-wall (falling-film) column Countercurrent, cocurrent Differential Liquid and/or gas Absorption, rectification, stripping, evaporation... 

Falling-film column. A falling-film column (Figure 24.1(d)) is also an empty vessel with liquid, introduced at the top, flowing down the wall as a film to con-tad an upward-flowing gas stream. Ideal flow for each phase is PF. Since neither liquid nor gas is dispersed, the interfacial area developed is relatively small, and gas-liquid contact is relatively inefficient This type is used primarily in the exper-imental determination of mass transfer characteristics, since the interfacial area is w ell defined... 

We included the term r = 0 to indicate that there is no reaction in the gas phase. The mass transfer rates obviously have opposite signs, and we have to multiply the mass transfer flux by [areaA olume], where the volume is that occupied by that phase. Note that the mass transfer term after dividing out becomes proportional to R. Since the reactor volume is proportional to R while the surface area for mass transfer is proportional to R, the falling film column obviously becomes less efficient for larger reactor sizes. This is a fundamental problem with the falling film reactor in that small tubes give high mass transfer rates but low total production of product. 

Wetted-wall or falling-film columns have found application in mass-transfer problems when high-heat-transfer-rate requirements are concomitant with the absorption process. Large areas of open surface... 

Wetted-wall or falling-film columns have found application in mass-transfer problems when high-heat-transfer-rate requirements are concomitant with the absorption process. Large areas of open surface are available for heat transfer for a given rate of mass transfer in this type of equipment because of the low mass-transfer rate inherent in wetted-wall equipment. In addition, this type of equipment lends itself to annular-type cooling devices. 

The kinetics of the Co + heterogeneously catalysed oxidation of sodium sulfite were measured by Sawicki and Barron [480] in a falling-film column. In contrast with [568] the reaction imder such conditions was found to be of second-order with respect to oxygen concentration and 0.5-order with respect to Co + concentration. 

Packing column (falling-film column) Packed column (Figs. 2-70a and 2-71)... 

Let us consider the case of a wetted wall (falling film) column undergoing gas absorption with the following assumptions ... 

Ammonia is absorbed in a falling film of water in an absorption apparatus and the film is disrupted and mixed at regular intervals as it flows down the column. The mass transfer rate is calculated from the penetration theory on the assumption that all the relevant conditions apply. It is found from measurements that the muss transfer rate immediately before mixing is only 16 pet cent of that calculated from the theory anil the difference has been attributed to the existence of a surface film which remains intact and unaffected by the mixing process. If the liquid mixing process lakes place every second, what thickness of surface film would account for the discrepancy, ... 

For toluene fluorination, the impact of micro-reactor processing on the ratio of ortho-, meta- and para-isomers for monofluorinated toluene could be deduced and explained by a change in the type of reaction mechanism. The ortho-, meta- and para-isomer ratio was 5 1 3 for fluorination in a falling film micro reactor and a micro bubble column at a temperature of-16 °C [164,167]. This ratio is in accordance with an electrophilic substitution pathway. In contrast, radical mechanisms are strongly favored for conventional laboratory-scale processing, resulting in much more meta-substitution accompanied by imcontroUed multi-fluorination, addition and polymerization reactions. 

Figure 5.17 Comparison of performance of a typical laboratory column (LBC) with those of micro-reactor devices falling film micro reactor (FFMR) micro bubble column (MBC I and MBC II) [38].

The laboratory and the micro bubble column show decreasing selectivity with increasing conversion. The falling film micro reactor shows a near-constant selectivity-conversion relationship [3, 38]. 

GL 1] [R 1] [R 3] [P le] The falling film micro reactor has a better selectivity-conversion performance than the two micro bubble columns tested (Figure 5.18) (3, 38]. The micro bubble column with narrow channels has a better behavior at large conversion than the version with wide channels. The behavior of the falling film micro reactor and the micro bubble column with narrow channels is characterized by a nearly constant selectivity with increasing conversion, while the bubble column with wide channels shows notably decreasing selectivity with conversion (similar to the laboratory bubble column). 

GL 1] [R 1] [R 3] [P la-d] Space-time yields higher by order of magnitude were found for the falling film micro reactor and the micro bubble column as compared with the laboratory bubble column [38], The space-time yields for the micro reactors ranged from about 20 000 to 110 000 mol monofluorinated product m h. The ratio with regard to this quantity between the falling film micro reactor and the micro bubble column was about 2. The performance of the laboratory bubble column was of the order of 40-60 mol monofluorinated product m" h. ... 

Figure 5.19 Conversion of the direct fluorination of toluene in different reactor types as a function of the molar ratio of fluorine to toluene (a) and efficiency of these reactors, defined as conversion normalized by the molar ratio of fluorine to toluene, as a function of the molar ratio of fluorine to toluene (b). Falling film micro reactor (FFMR) micro bubble column (MBC) laboratory bubble column (LBC) [38].

GL 1] [R 1] [R 3] [P la-d] For micro-channel processing, an analysis of the content of fluorine actually consumed as a function of the fluorine-to-toluene ratio was made [38]. The curves for two micro reactors and one laboratory bubble column do not show the same trend a decrease of converted fluorine with increasing ratio results for the falling-film micro reactor, whereas the micro and laboratory bubble columns show increasing performance. The two micro reactors use about 50-75% of all fluorine offered, whereas the laboratory tool has an efficiency of only 15%. 

If long distillation time is a problem, one can move to continuous distillation with conventional shell and tube heaters accompanied by a typical column bottom (often called a sump) which is a high temperature holdup, or better yet a short path evaporator (falling film, thin film, or wiped film) with usually a smaller receiver (called an accumulator in this case). The most chemical damage is in the thin liquid film at the heat transfer surface, so the short path evaporators do the least thermal damage. 

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... 

Figure 24.1 Types of tower or column reactors for gas-liquid reactions (a) packed tower, (b) plate tower, (c) spray tower, (d) falling-film tower, (e) bubble column...

Relative Kga valid for all systems controlled by mass transfer coefficient (Kg) and wetted area (a) per unit volume of column. Some variation should be expected when liquid reaction rate is controlling (not liquid diffusion rate). In these cases liquid hold-up becomes more important. In general a packing having high liquid hold-up which is clearly greater than that in the falling film has poor capacity. 

We consider a vertical cylindrical tube of length L and diameter Dq (radius 2Ro) with liquid admitted at the top such that it forms a falling film that coats the walls of the tube. We also add a gas into the top of the tube (cocurrent) or into the bottom (countercurrent). This is a standard unit in extraction processes called a wetted-wall column. 

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