Enhancement at the gas-liquid interfac

Figure 6. Concentration profiles for mass transfer into a slurry with catalytic particles, with no enhancement at the gas-liquid interface (above) mass transfer and reaction at the interface (below) mass transfer and reaction in the porous particles.

Absorption enhancement at the gas-liquid interface due to chemical reaction on the surface of fine catalyst particles can be neglected (88,89). 

Specially for fast reactions, where enhancement at the gas-liquid interface occurs, knowledge of the true gas liquid specific contact area, a, is desired rather than knowledge of the product kj a only. One method is the measurement of both gas hold-up and the Sauter mean bubble diameter 3. 

A possible criterium for the occurrence of such enhancement at the gas- liquid interface is with... 

Situation 4 very fast reaction, Ha> 3 and Eboundary layer. The hydrogen concentration in the bulk of the liquid falls to zero. Thus, all the catalyst in the bulk is useless. For instantaneous reactions, Ha 3, E=EX and the reaction takes place in a narrow plane located somewhere in the boundary layer the larger Ea0 the closer to the interface the reaction plane. If the limiting enhancement factor E is very high, it is said that the reaction takes place at the gas-liquid interface. Such a case is referred to as surface reaction . 

Figure 2 shows the calculated liquid-film enhancement factor for five buffers as a function of buffer concentration in 0.1 M CaCl2 at pH 5.5 with 1000 ppm SO2 at the gas/liquid interface. 

Table V gives the calculated concentrations of twelve buffers required to get an enhancement factor of 20 in 0.1 M CaCl2 with 10 mM total sulfite at pH 5.0 with 1000 ppm S02 at the gas liquid interface. Relative costs have been caluclated assuming that makeup rates would be proportional to concentration...

Substances that concentrate at the gas/liquid interface and lower surface tension tend to enhance foaming. 

When the catalyst is hydrophobic it may accumulate at the gas/liquid interface (in a similar way as in flotation processes). In case that both the local specific surface area of the catdyst in the film, and consequently the enhancement factor, can be even higher. 

BCR are particularly well suited to carry out reactions in the slow reaction regime of absorption. Due to the high liquid holdup BCR provide for a large liquid volume where the reaction can take place. Also, in slurry reactors where the reaction takes place at the surface of the solid catalyst particles belong to the slow reaction regime. Only a few exceptions are known where absorption enhancement due to the slurry phase reaction has been observed [6, 20 - 22]. Strictly speaking, enhancement and hence transition to the fast reaction regime can only be expected if the diameters of the particle fines are considerably less than the liquid film thickness at the gas/liquid interface. 

Catalytic slurry reactors nowadays are the most important class of slurry reactors by far. Doraiswamy and Sharma [l] presented a list of 34 systems of commercial interest and hundreds of papers have been published in this area. In most applications, solids concentrations are relatively low and as a consequence there is no enhancement of the mass transfer at the gas-liquid interface due to the presence of the catalyst. The reactors can be modelled by the... 

Design a two-phase gas-liquid CSTR that operates at 55°C to accomplish the liquid-phase chlorination of benzene. Benzene enters as a liquid, possibly diluted by an inert solvent, and chlorine gas is bubbled through the liquid mixture. It is only necessary to consider the first chlorination reaction because the kinetic rate constant for the second reaction is a factor of 8 smaller than the kinetic rate constant for the first reaction at 55°C. Furthermore, the kinetic rate constant for the third reaction is a factor of 243 smaller than the kinetic rate constant for the first reaction at 55°C. The extents of reaction for the second and third chlorination steps ( 2 and 3) are much smaller than the value of for any simulation (i.e., see Section 1-2.2). Chlorine gas must diffuse across the gas-liquid interface before the reaction can occur. The total gas-phase volume within the CSTR depends directly on the inlet flow rate ratio of gaseous chlorine to hquid benzene, and the impeller speed-gas sparger combination produces gas bubbles that are 2 mm in diameter. Hence, interphase mass transfer must be considered via mass transfer coefficients. The chemical reaction occurs predominantly in the liquid phase. In this respect, it is necessary to introduce a chemical reaction enhancement factor to correct liquid-phase mass transfer coefficients, as given by equation (13-18). This is accomplished via the dimensionless correlation for one-dimensional diffusion and pseudo-first-order irreversible chemical reaction ... 

A reaction taking place on the liquid-solid interface could ther fore result in appreciable rate enhancement only if a significant number of solid particles are present at distances from the gas-liquid interface less than 10 cm. This in turn would require particle diameters no more than 10 cm, an unrealistically low value. It therefore appears that, whenever the reaction takes place at the liquid-solid interface, no significant rate enhance ment will be observed for the gas-liquid mass transfer process the latter will essentially proceed in the slow-reaction regime. 

An analysis of chemical desorption has recently been published (Chem.Eng.Sci., 21 0980)), which is based on a number of simplifying assumptions the film theory model is assumed, the diffusivities of all species are taken to be equal to each other, and in the solution of the differential equations an approximation which is second order with respect to distance from the gas-liquid interface is used this approximation was introduced as early as 1948 by Van Krevelen and Hoftizer. However, the assumptions listed above are not at all drastic, and two crucial elements are kept in the analysis reversibility of the chemical reactions and arbitrary chemical mechanisms and stoichiometry.The result is a methodology for developing, for any given chemical mechanism, a highly nonlinear, implicit, but algebraic equation for the calculation of the rate enhancement factor as a function of temperature, bulk-liquid composition, interface gas partial pressure and physical mass transfer coefficient The method of solution is easily gene ralized to the case of unequal diffusivities and corrections for differences between the film theory and the penetration theory models can be calculated. 

Slurry reactors are popular in industry where the solids either take part in the reaction or act as catalyst.Many aspects of these reactors,particularly for catalytic systems,have been discussed at length in 1iterature(1,2).Catalytic slurry reactors are also reviewed in this proceedings by Hofmann(3).However,there are still aspects which have not been treated in the literature in sufficient detail.Firstly,until recently little attention has been paid to slurry reactors involving reactive solids.Secondly, it is often assumed that steps of diffusion of the dissolved gas from the gas-liquid interface to the bulk liquid phase bulk liquid phase to the solid catalyst surface and surface reaction are steps in series.This leads of course to a specific gas absorption rate which is always smaller than k.A. While this is a representative picture in a majority of cases of industrial relevance,we can conceive situations,where the catalyst particle size may be smaller than the diffusion film(liquid film next to gas-liquid interface) thickness.We may then have steps of the transport of the dissolved gas from the gas-liquid interface and reaction on the catalyst particle in parallel,that is,while the dissolved gas diffuses it reacts on the catalyst surface.This is then in many ways analogous to normal gas-liquid reactions and may lead to the enhancement of specific gas absorption rate so that it exceeds k.A. This point is also relevant to reactive solid systems indeed in an earlier paper,Ramachandran and Sharma(4) had shown that the specific rate of absorption accompanied by an instantaneous reaction in a slurry containing sparingly soluble fine particle size was considerably smaller than the film thickness. Finally,there is substantial information in the literature on the combined effect of solid particles on k a.However,the information... 

Particles smaller than the liquid film thickness.The liquid film thickness in gas-liquid reactors may vary between 5-100 microns. Thus for a gently stirred system,the average diameter of the particles may considerably be less than the thickness of the liquid film( d < 0.1 6) and we can conceivably have the step of dissolution of solid particles in parallel to that of diffusion of dissolved gas from the gas-liquid interface to the bulk liquid-phase.Depending on the relative rates of diffusion and chemical reaction,the entire amount of dissolved gas may react in the film leading to considerable enhancement of gas absorption rate.Further,the concentration of dissolved reactive species in the liquid film may be uniform(in which case the simultaneous dissolution of solid particles in the film will not affect the specific rate of mass transfer),or may be zero at a location close to the gas-liquid interface. 

Ionic liquids at the gas-liquid and solid-liquid interface have been extensively studied by a variety of surface analytical techniques. The most prominent technique for surface orientational analysis proves to be SFG. Other vibrational spectroscopic and surface-sensitive techniques such as surface-enhanced Raman spectroscopy (SERS) and total internal reflection Raman spectroscopy (TIR Raman) have been employed for studying surface processes these techniques, however, have not been applied yet specifically for the study of ionic hquids. 

---