Falling-film reactor types

Co-current flow of the organic feedstock and the SOj/air is realised in all falling-film reactor types and the Chemithon Jet Impact Reactor. 

Before we deal with these situations, it is instructive to consider a fixed-area version of a membraneless reactor, the falling film reactor. We cannot think of many applications of this reactor type because one usually benefits considerably by using configurations where the surface area is as large as possible, but the falling film reactor leads naturally to the description of many variable-area multiphase reactors. 

A commercial 3 kW laser of this type (60 cm long, vertically mounted) has been used to build a falling film reactor capable of converting 10 g or more in 10-20 h [7]. At least at present, however, these light sources are rather expensive and require considerable care for their maintenance consequently, they cannot be considered for adoption by an organic photochemistry laboratory requiring a versatile tool for preparative applications. 

Microstructured devices for fluid-fluid systems exist in a number of configurations. They can be roughly classified into three types based on the contacting principles [ 1] micromixer, microchannels, and falling film microreactors. The first two types of devices are used for all fluid-fluid applications while the microstructured falling film reactor is used only for gas-liquid systems. Depending on the application, these microstructured devices can also be used in series, for example. 

DifTerent types of microreactors, ranging from single-channel to multi-channel designs, and even more complex falling film reactors, have been investigated for carrying out photochemical reactions. Reported channel dimensions of photomicroreactors range from 10 to 1000 pm. 

A typical example of a falling-film reactor (i.e., Ballestra multitube type film reactor) is shown in Figure 5.12, whereas Figure 5.13 refers to a process scheme of the LAB-SO3 sulfonation section. 

There is hardly any difference in product quality between the various types of falling-film reactors from the various equipment suppliers. This is not so surprising, because ... 

Figure 4.10.11 shows examples of gas-liquid reactors. Gas is usually dispersed in the liquid by a bubble column, tray column, or a stirring reactor with pressurized gas. Liquid is dispersed in the gas by means of a jet type washer or a spray tower. Liquid in the form of a thin film is exposed to a gas by a falling film reactor or a tricHe reactor with filter elements. Details of the interplay of chemical reaction and mass transfer are given in Section 4.4. 

A falling film reactor is essentially a vertical cylinder, where liquid flows downward in a thin film along the wall, and gas flows in the core. Relatively high mass transfer coefficients are obtained both in the liquid and in the gas phases. The liquid phase can be cooled effectively via the wall. Therefore this type of reactor is preferred for very rapid exothermic gas liquid reactions. There are two variations, one consists of a tube bundle, the other consists of one cylinder with a rotor. 

The Stainicaibon process is described in Figures 3—7. The synthesis section of the plant consists of the reactor, stripper, high pressure carbamate condenser, and a high pressure reactor off-gas scmbber. In order to obtain a maximum urea yield pet pass through the reactor, a pressure of 14 MPa (140 bar) and a 2.95/1 NH —CO2 molar ratio is maintained. The reactor effluent is distributed over the stripper tubes (falling-film type shell and tube exchanger) and contacted by the CO2, countercurrendy. This causes the partial NH pressure to decrease and the carbamate to decompose. 

Toyo Engineering-AGES Process. The synthesis section of the ACES process (Fig. 8) consists of a reactor, a stripper, two carbamate condensers, a scmbber and operates at 17.5 MPa (175 bars). The reactor is operated at 190°C with a NH /CO2 ratio of 4.0 (mol/mol). Liquid NH is fed directly into the reactor by a centrifugal ammonia pump. Gaseous CO2 is sent from the centrifugal CO2 compressor to the bottom section of the falling-film type stripper. 

Fig. 2. High speed photos of organic film—high velocity air dynamics in falling film sulfonation process (a) and (b) are vertical flat plate organic—air dynamics where (a) shows BAB—air at top, (b) BAB sulfonic acid—air at bottom of reactor (c) simulated AUied-type concentric reactor inner cylindrical...

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. 

Reactor type Falling film micro reactor Mini heat exchange channel width depth 1500 pm 500 pm... 

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].

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...

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.

The reactor effluent, which contains unconverted NH3 and C02, is stripped in a falling-film type shell and tube exchanger at reactor pressure. The stripping agent, CO2, flows upward, which is countercurrent to the effluent stream. The stripper tubes are make of 25-22-2 stainless steel, which has lasted almost 30 years in some plants. The stripped-off NH3 and CO2 are then partially condensed and recycled to the reactor. The heat from this condensation is used to make 4.5 bar steam. Some of this steam can be used in downstream sections of the plant while some is sent to the turbine on the CO2 compressor108,110. 

Points 2 and 4 are the main ones governing the choice of reactor type. The high gas/liquid ratio restricts the choice to types d, e, i, and k in Fig. 19-26. Due to the high rate of heat transfer needed, the choice is a falling film or tubular reactor. 

The contact-free photoreactors include the vertical falling film design (Fig. 8-5), the inchned flat bed system (Fig. 8-6) and the batch photoreactor (Figs. 8-7 and 8-8). These reactor configurations do not have a quartz/water interface (as in situations A, B, C, Fig. 8-3), so that the problems related to fouling and film formation are effectively circumvented. Such photoreactor types are even suited for the UV treatment of suspensions and cloudy wastewaters (cf Kim and Thomanetz, 1995). 

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