Falling-film reactors

Model Reactions. Independent measurements of interfacial areas are difficult to obtain in Hquid—gas, Hquid—Hquid, and Hquid—soHd—gas systems. Correlations developed from studies of nonreacting systems maybe satisfactory. Comparisons of reaction rates in reactors of known small interfacial areas, such as falling-film reactors, with the reaction rates in reactors of large but undefined areas can provide an effective measure of such surface areas. Another method is substitution of a model reaction whose kinetics are well estabUshed and where the physical and chemical properties of reactants are similar and limiting mechanisms are comparable. The main advantage of employing a model reaction is the use of easily processed reactants, less severe operating conditions, and simpler equipment. 

With a reaction enthalpy of A RH = -170 kJ/g mol the sulfonation with S03 is strongly exothermic. As the color of the acid is dependent not only on the residence time but also to a considerable extent on the reaction temperature, it is necessary to have an effective thermal dissipation. This applies to all of the reactors listed in Table 13. The falling film reactors, of which there are various designs, have the advantage that a very short residence time can be realized [152]. 

As a rule, sulfonation takes place continually in a cascade or a falling film reactor (Table 14) at about 50-70°C. The S03 is steadily diluted to a concentration of 5-10 vol % with air or an inert gas. The LAB conversion reaches a value between 92% and 98% [156,157]. Mixing of the already formed alkyl-benzenes with fresh S03 leads to undesired highly sulfonated byproducts. In order to prevent these side reactions, all processes operate concurrently. 

The process is shown schematically in Fig. 1. During the first stage of the process, a-olefins are sulfonated with diluted S03 gas in a falling film reactor. The optimum molar ratio of S03/olefin varies from 1.0 to 1.2. As the sulfonation reaction is very rapid and exothermic, the reaction temperature can rise to... 

In summary, mild sulfonation of detergent range 10 in a falling film reactor followed by direct neutralization and hydrolysis leads to an IOS system rich in sodium p-hydroxysulfonates and having low concentrations of residual sul-tones, inorganic sulfate, and free oil. 

Reaction time (short residence time falling film reactors are required to attain top quality anionic surfactants)... 

The Ballestra Sulfurex multitube falling film reactor (MTFFR)... 

The Meccaniche Moderne monotube falling film reactor... 

Neutralization of organic acid within 1 min after sulfonation reactor. A separate aging step is not needed under commercial reactor conditions Short residence time falling film reactor required to avoid thermal breakdown of R0S03H... 

Falling film reactor system Mole ratio S03/Me-ester 1.2 1... 

Reaction temperature in short residence time reactor 90°C, cooling water — 80-85°C in lower part and 40-50°C in upper part Aging reactor post-falling film reactor required residence time 0.5 h at 90-95°C, plug flow conditions... 

Sulfonation of the common feedstocks proceeds with a highly exothermic instantaneous initial reaction, followed by a fast but not instant step that is also highly exothermic. The second reaction does not always proceed to completion (e.g., LAB, FAME) in the lower zone of a short residence time falling film reactor (FFR). For these organic feedstocks aging under well-defined conditions of temperature and reaction time is required. 

The detergent industry requires process equipment having high operation flexibility, low energy demand, low operation cost, consistent production yield, and, of course, ecological optimization with respect to effluents and air pollution control. To comply with these requirements, the continuous S03/gas sulfonation and double-step neutralization are the basic principles applied in multitube falling film reactor and Neutrex neutralization (Fig. 5). 

The gaseous S03 stream (previously diluted with dry air to a concentration ranging from 7% to 2.5% volume) is then fed to the sulfonation/sulfation section which is based on a multitube falling film reactor having a number of tubes proportional to the plant production capacity (Figs. 8 and 9). The S03 gas is fed to the upper part of the reactor and distributed equally to each reaction tube. 

Yeong, K. K., Gavriilidis, A., Zape, R., Hessel, V., Catalyst preparation and deactivation issues for nitrobenzene hydrogenation in a microstructured falling film reactor, Catal. Today 81, 4 (2003) 641-651. 

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. 

Figure 12-8 Sketch of a falling film reactor with positions and Rj indicated.

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. 

Example 12-1 An aqueous solution containing lO ppm by weight of an organic contaminant of molecular weight 120 is to be removed by air oxidation in a l-cm-diameter falling film reactor at 25°C. The liquid flows at an average velocity of 10 Clll/sec and forms a film 1 mm thick on the wall, while the air at 1 atm flows at an average velocity of 2 cm/sec. The reaction in the liquid phase has the stoichiometry A + 2O2 products with a rate r K... 

The concentration profile for a reactant A which must migrate from a drop or bubble into the continuous phase to react might be as shown in Figure 12-10. There is a concentration drop around the spherical drop or bubble because it is migrating outward, but, as with a planar gas-liquid interface in the falling film reactor, there should be a discontinuity in at the interface due to the solubility of species A and a consequent equilibrium distribution between phases. 

As with the falling film reactor, the rate of mass transfer to the catalyst goes as R, while the size of the reactor goes as R, so this reactor becomes very inefficient except for very small-diameter tubes. However, we can overcome this problem, not by using many small tubes in parallel, but by allowing the gas and liquid to flow (trickle) over porous catalyst pellets in a trickle bed reactor rather than down a vertical wall, as in the catalytic wall reactor. 

In a falling-film reactor composed of several vertical tubes, on the heated walls of which the reaction medium streams.70... 

On the other hand, sensitized oxidation of highly concentrated sensitizer-substrate mixtures have been successfully developed in falling film reactors (for example, the synthesis of 2-hydroxy-5H-furanone (Eq. 48 [82, 83]) [12]. 

Figure 6,20 Examples of photochemical reactors (a) for batch production the lamp L is placed in the middle of the sample holder S, separated by a filter F and a thermostatted vessel T through which the coolant is circulated, (b) The falling film reactor uses a central lamp L surrounded by a filter F. The sample Sff) falls slowly as a thin film on the inner wall of the reactor, and the photoproducts are collected at the bottom...

Figure 14. Increase of the intrinsic viscosity during the polycondensation reaction of polyester as a function of the residence time. Comparison between free-falling-film reactor (13) and agitated thin-film machine...

Falling-film reactors have a liquid reactant flowing down the walls of a tube with a gaseous reactant flowing up or down (usually countercurrent). This reactor is particularly advantageous when the heat of reaction is high. The reaction surface area is minimal, and the total reaction heat generated can be controlled. 

An example of a reaction performed in a falling-film reactor is the sulfonation of dodecyl benzene. 

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. 

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