Liquid film reaction interfacial area

Consider an extreme case in which there is no resistance to reaction and all of the resistance is due to mass transfer. The rate of mass transfer is proportional to the interfacial area and the concentration of the driving force. An expression can be written for the rate of transfer of Component i from gas to liquid through the gas film per unit volume of reaction mixture ... 

Both reactions are slow compared to the film diffusion in the liquid phase13-15. Hence, the reactions can be assumed to take place predominantly in the bulk phase of the liquid. The rate of mass transfer can be calculated using Equation 7.2. The interfacial concentration can be calculated using Henry Law. Mass transfer coefficients, interfacial area and gas hold-up data are required. Gas hold-up is defined as ... 

It is necessary to distinguish among three rate quantities. We use the symbol NA to represent the flux of A, in mol m-2 s-1, through gas and/or liquid film if reaction takes place in the liquid film, NA includes the effect of reaction (loss of A). We use the symbol (—rA), in mol m-2 s 1, to represent the intensive rate of reaction per unit interfacial area. Dimensionally, (—rA) corresponds to NA, but (— rA) and NA are equal only in the two special cases (1) and (2) above. In case (3), they are not equal, because reaction occurs in the bulk liquid (in which there is no flux) as well as in the liquid film. In this case, furthermore, we need to distinguish between the flux of A into the liquid film at the gas-liquid interface, NA(z = 0), and the flux from the liquid film to the bulk liquid, Na(z = 1), where z is the relative distance into the film from the interface these two fluxes differ because of the loss of A by reaction in the liquid film. The third rate quantity is ( rA)int in mol irT3 s-1, the intrinsic rate of reaction per unit volume of liquid in the bulk liquid. ( rA) and (- rA)int are related as shown in equation 9.2-17 below. 

It is possible that the pores of wetted catalyst particles eire filled with liquid. Hence, by virtue of the low values of liquid diffusivities (ca. 10 cm s" ), the effectiveness factor will almost certainly be less than unity. A criterion for assessing the importance of mass transfer in the trickling liquid film has been suggested by Satterfield [40] who argued that if liquid film mass transport were important, the rate of reaction could be equated to the rate of mass transfer across the liquid film. For a spherical catalyst particle with diameter dp, the volume of the enveloping liquid fim is 7rdp /6 and the corresponding interfacial area for mass transfer is TTdn. Hence... 

Regime 5 - instantaneous reactions at an reaction plane developing inside the film For very high reaction rates and/or (very) low mass transfer rates, ozone reacts immediately at the surface of the bubbles. The reaction is no longer dependent on ozone transfer through the liquid film kL or the reaction constant kD, but rather on the specific interfacial surface area a and the gas phase concentration. Here the resistance in the gas phase may be important. For lower c(M) the reaction plane is within the liquid film and both film transfer coefficients as well as a can play a role. The enhancement factor can increase to a high value E > > 3. 

A reaction occurring in a bulk phase will show an increase in the rate with the area as shown in Fig. 5.3 for a reaction occurring in the film or at the interface, the rate will be linearly dependent on the interfacial area. The interfacial area in a dispersed two-phase liquid-liquid system can be estimated by measuring the rate of a suitable test reaction in a reactor with the known interfacial area (a flat interface, Section 5.3.2.1), and comparing it with the reaction rate in a dispersed system [6, 15]. A convenient reactive system for this purpose is a formate ester and 1-2 M aqueous NaOH. Formate esters are very reactive to hydroxide ion (fo typically around 25 M 1 s 1), so the reaction is complete inside the diffusion film, and the reaction rate is proportional to the interfacial area. A plot of the interfacial area per unit volume against the agitator speed obtained in this way in the author s laboratory for the equipment shown in Fig. 5.12 is shown in Fig. 5.14 [8]. 

As discussed in Sec. 7, the factor E represents an enhancement of the rate of transfer of A caused by the reaction compared with physical absorption, i.e., Kq is replaced by EKq. The theoretical variation of E with Hatta number for a first- and second-order reaction in a liquid film is shown in Fig. 19-25. The uppermost line on the upper right represents the pseudo first-order reaction, for which E = Ha coth (Ha). Three regions are identified with different requirements of liquid holdup 8 and interfacial area a, and for which particular kinds of contacting equipment may be best ... 

Typically, the following values can be considered DNO = 8.6 x 10 9m2/s, kLN0 = 3.9 x 10 3 rn/s, k, = 1.5 x 105m3/mol/s, Cfr = 50mol/m3 [6, 8, 10, 13], which leads to a value HaNO = 651. This indicates a very fast reaction taking place in a thin region of tbe film, adjacent to the gas/liquid interface. Therefore, reaction (12.1) is favored by large gas/liquid interfacial area but not influenced by tbe bulk bquid. 

This form is particularly appropriate when the gas is of low solubility in the liquid and "liquid film resistance" controls the rate of transfer. More complex forms which use an overall mass transfer coefficient which includes the effects of gas film resistance must be used otherwise. Also, if chemical reactions are involved, they are not rate limiting. The approach given here, however, illustrates the required calculation steps. The nature of the mixing or agitation primarily affects the interfacial area per unit volume, a. The liquid phase mass transfer coefficient, kL, is primarily a function of the physical properties of the fluid. The interfacial area is determined by the size of the gas bubbles formed and how long they remain in the mixing vessel. The size of the bubbles is normally expressed in terms of their Sauter mean diameter, dSM, which is defined below. How long the bubbles remain is expressed in terms of gas hold-up, H, the fraction of the total fluid volume (gas plus liquid) which is occupied by gas bubbles. 

High surface area to volume ratios are another distinctive feature of microscale reaction systems. Interfacial areas per unit volume in falling film microreactors have been reported to be as high as 25000 m /m. By comparison, interfacial areas in bubble columns are typically of the order of 1-200 m /m. As a consequence, surface effects that are often neglected in the macroscale become dominant. This makes a tremendous difference in gas-liquid reactions, where mass transfer from the gas to the liquid often limits the rate. 

A different design for three-phase systems was proposed by Kobayashi et al. [120]. The authors immobilized a palladium catalyst on the glass wall of a capillary and operated the microchannel reactor such that an annular flow pattern was obtained, which is characterized by a liquid film on the wall (Figure 16). The hydrogenation of benzalacetone was used as a model reaction to demonstrate the general applicability of this concept. The authors could achieve an effective interaction between H2, substrate, and the palladium catalyst as a result of the large interfacial area and the short diffusion path in the narrow space. 

A further advantage of absorption plus reaction is the increase in the mass-transfer coefficient. Some of this increase comes from a greater effective interfacial area, since absorption can now take place in the nearly stagnant regions (static holdup) as well as in the dynamic liquid holdup. For NHj absorption in H2SO4 solutions, K a was 1.5 to 2 times the value for absorption in water.Since the gas-film resistance is controlling, this effect must be due mainly to an increase in effective area. The values of K a for NH3 absorption in acid solutions were about the same as those for vaporization of water, where all the interfacial area is also expected to be effective. The factors and... 

The mathematical models for different kinds of gas-liquid reactors are based on the mass balances of the components in the gas and liquid phases. The bulk gas and liquid phases are divided by thin films where chemical reactions and molecular diffusion occur. The flux of component i from the gas bulk to the gas film is, and the flux from the liquid film to the liquid bulk is, Vf(. The fluxes are given with respect to the interfacial contact area (A)... 

If we return to the Hatta picture of reaction and diffusion, recall that reaction and diffusion occur only in the film. Reaction also occurs in the bulk liquid phase, of course, but there the concentration of reactants as a function of position is determined by the nature of mixing in that phase. Let us reformulate the problem so that the fraction of liquid phase occupied by the film, a, is defined explieitly. If L is film thickness and interfacial area, then... 

Silica gels and controlled-pore glass, which were covered with thin films of polar phases such as water, ethylene glycol or ionic liquids, were used as polar solid supports. These systems are limited to very polar, usually ionic catalysts and non-polar reaction media in order to prevent catalyst leaching. This in turn, can be limiting to the range of substrates. Existing catalytic processes in common liquid-liquid biphasic systems can be easily transferred to supported liquid-phase conditions. At the same time the interfacial area between the... 

To overcome most of solubilization problems, colloidal surfactant systems (e.g. micelles, liquid crystals, microemulsions, vesicles, emulsions, etc.) are attracting a great deal of attention as alternative reaction media (Walde 1996 Holmberg 1997 Antonietti 2001). Their advantages are they possess micro- and nanostmctures consisting of well-defined hydrophilic and lipophilic domains separated by surfactant films with very large interfacial area, the exchange between chemical species... 

On the other hand, when the reaction occurs in both bulk and film, the reactor should combine good interfacial area generation with efficient bulk liquid utilization. A measure of the efficiency of bulk liquid utilization is the parameter 7bu given by (Kramers and Westerterp, 1963)... 

Falling film Low pressure drop High interfacial area Unstable at high throughput Thick liquid film results in higher mass transfer resistance for three-phase reactions... 

A microstructured film reactor was used for the hydrogenation of nitrobenzene (NB) to aniline (AN) in ethanol at 60 C, 0.1-0.4 MPa hydrogen pressure and residence time of9-17 s [7,11]. Palladium catalyst was deposited as films or particles on the microstructured plate. Confocal microscopy was used to measure the liquid film thickness. With increasing flow rates between 0.5 and 1.0 cm min thicker liquid films between 67 and 92 pm were observed. The k a of this system was estimated to be 3-8 s with an interfacial surface area per reaction volume of 9000-15 000 m m . Conversion was found to be affected by both liquid flow rate and hydrogen pressure, and the reactor operated between the kinetic and mass transfer controlled regimes. 

Select a fast reaction (for example, Co" > lO"" gmol/1) in which the process rate constant is dependent on k, and a, according to Equ. 4.117. The reaction rate constant k may then be obtained as a function of temperature, pH, and catalyst concentration, assuming that one is working with a so-called perfect or model reactor for G L reactions in which the interfacial area, a, is known and the hydrodynamics are simple and also known. Examples of such reactors are the falling film reactor, the liquid jet reactor (Astarita, 1967), and the thin film reactor with rotating drum (Dank-werts and Kennedy, 1954 Moser, 1973) as shown in Fig, 3.43. 

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