Enhancement of mass transfer

In addition, experiments of isobutane diffusion reveal that the effective diffusivity is three times higher over a carbon-templated hierarchical zeolite [143]. Similar conclusions were drawn by Groen et al. [144] with hierarchical zeolites synthesized using the desilication method, where a two-order-magnitude improvement was observed in the diffusion of neopentane inside desiUcated ZSM-5 because of the shorter diffusion path length and presence of an accessible network of mesopores. 

---

Confined flows typically exhibit laminar-flow regimes, i.e. rely on a diffusion mixing mechanism, and consequently are only slowly mixed when the diffusion distance is set too large. For this reason, in view of the potential of microfabrication, many authors pointed to the enhancement of mass transfer that can be achieved on further decreasing the diffusional length scales. By simple correlations based on Fick s law, it is evident that short liquid mixing times in the order of milliseconds should result on decreasing the diffusion distance to a few micrometers. 

Given the feasibility of micro-channel devices for en2yme-based oligosaccharide synthesis, enhancement of mass transfer therein conld speed up this reaction. Also, it may be hoped that en2yme degradation may be reduced, for reasons that are not so straightforward. 

The velocity, viscosity, density, and channel-height values are all similar to UF, but the diffusivity of large particles (MF) is orders-of-magnitude lower than the diffusivity of macromolecules (UF). It is thus quite surprising to find the fluxes of cross-flow MF processes to be similar to, and often higher than, UF fluxes. Two primary theories for the enhanced diffusion of particles in a shear field, the inertial-lift theory and the shear-induced theory, are explained by Davis [in Ho and Sirkar (eds.), op. cit., pp. 480-505], and Belfort, Davis, and Zydney [/. Membrane. Sci., 96, 1-58 (1994)]. While not clear-cut, shear-induced diffusion is quite large compared to Brownian diffusion except for those cases with very small particles or very low cross-flow velocity. The enhancement of mass transfer in turbulent-flow microfiltration, a major effect, remains completely empirical. 

From the above it can be concluded that only the reaction with component B may enhance mass transfer of ozone substantially. And only if the Hatta number HaB is much higher than 1. Therefore it can be expected that whenever we have to deal with an enhancement of mass transfer due to chemical reactions, this influences the selectivity of the oxidation process in a negative way. The factor which has to be considered in this respect is the Hatta number for the reaction of ozone with component B (equation 29). HaB increases with increasing value of kB and CBb and with decreasing value of the mass transfer coefficient for ozone, kHq,... 

On the other hand stable cavitation (bubbles that oscillate in a regular fashion for many acoustic cycles) induce microstreaming in the surrounding liquid which can also induce stress in any microbiological species present [5]. This type of cavitation may well be important in a range of applications of ultrasound to biotechnology [6]. An important consequence of the fluid micro-convection induced by bubble collapse is a sharp increase in the mass transfer at liquid-solid interfaces. In microbiology there are two zones where this ultrasonic enhancement of mass transfer will be important. The first is at the membrane and/or cellular wall and the second is in the cytosol i. e. the liquid present inside the cell. 

The variation of efficiencies is due to interaction phenomena caused by the simultaneous diffusional transport of several components. From a fundamental point of view one should therefore take these interaction phenomena explicitly into account in the description of the elementary processes (i.e. mass and heat transfer with chemical reaction). In literature this approach has been used within the non-equilibrium stage model (Sivasubramanian and Boston, 1990). Sawistowski (1983) and Sawistowski and Pilavakis (1979) have developed a model describing reactive distillation in a packed column. Their model incorporates a simple representation of the prevailing mass and heat transfer processes supplemented with a rate equation for chemical reaction, allowing chemical enhancement of mass transfer. They assumed elementary reaction kinetics, equal binary diffusion coefficients and equal molar latent heat of evaporation for each component. 

There is conflicting evidence regarding the extent to which imposed vibrations increase particle to fluid heat and mass transfer rates (G2), with some authors even claiming that transfer rates are decreased. For sinusoidal velocity variations superimposed on steady relative motion, enhancement of transfer depends on a scale ratio A/d and a velocity ratio Af /Uj (G3). These quantities are rather like the scale and intensity of turbulence (see Chapter 10). For Af /Uj < l/2n, the vibrations do not cause reversal in the relative motion and the enhancement of mass transfer has been correlated (G3) by... 

Enhancement of mass transfer between dispersed phases. 

On the enhancement of mass transfer, Tamir [5] studied the absorption of acetone into water with a similar method, i.e. using a partition. The results they obtained were under suitable operating conditions and with appropriate structural parameters, the runs without partition yield absorption rates higher than those with partition by over 4 times. 

Enhancement of mass transfer of the substrate from the organic phase varying agitation and by the addition of a surfactant... 

For sufficiently large electrodes with a small vibration amplitude, aid < 1, a solution of the hydrodynamic problem is possible [58, 59]. As well as the periodic flow pattern, a steady secondary flow is induced as a consequence of the interaction of viscous and inertial effects in the boundary layer [13] as shown in Fig. 10.10. It is this flow which causes the enhancement of mass-transfer. The theory developed by Schlichting [13] and Jameson [58] applies when the time of oscillation, w l is small in comparison with the time taken for a species to diffuse across the hydrodynamic boundary layer (thickness SH= (v/a>)ln diffusion timescale 8h/D), i.e., when v/D t> 1. Re needs to be sufficiently high for the calculation to converge but sufficiently low such that the flow does not become turbulent. Experiment shows that, for large diameter wires (radius, r, — 1 cm), the condition is Re 2000. The solution Sh = 0.746Re1/2 Sc1/3(a/r)1/6, where Sh (the Sherwood number) = kmr/D and km is the mass-transfer coefficient,... 

Experimental data have been presented on the enhancement of mass transfer into an aqueous sulfite solution in a stirred cell, due to the presence of a dispersed liquid 1-octene phase. Also, the experimental data for O2 mass transfer enhancement to hexadecane and for CO2 mass transfer enhancement due to toluene droplets can be reasonably well described, which indicates that in different liquid-liquid systems the dispersed-phase distribution is similar for different organic droplets in water. 

The enhancement of mass transfer in the solid/liquid system is a frequent stirring operation. It should be remembered, that many salts must be dissolved in the liquid, to prepare a salt solution or to initiate a chemical reaction. In order that the dissolution process proceeds rapidly, the whole surface of the solid particles must be wetted as completely as possible by the liquid and the liquid flow should be turbulent, so that the boundary layer on the liquid side is small and the transfer of the dissolved material to the bulk of the liquid proceeds rapidly. 

In regime 2, the reaction is fast enongh to keep the bulk liquid phase concentration of the gaseous reactant essentially zero but not fast enough to occur substantially in the liquid film. There is no enhancement of mass transfer due to reaction. Diffusion and reaction take place in series fashion. 

Liquid and vapor flow countercurrently through openings between and within packing elements. At low vapor rates, there is relatively little disturbance of liquid by the vapor, and mass transfer proceeds in a fashion similar to that in a wetted wall column. At higher rates, there is considerable interaction between the phases, with vapor flow causing increases in liquid turbulence and holdup. In the so-called loading zone, there is an enhancement of mass transfer but, as rates are increased further, flooding occurs. 

The influence of the pore characteristics of activated carbon on their dynamic properties has been extensively studied. Breakthrough curves obtained with p-nitrophenol and various activated carbons exhibit different shapes due to differences in pore size distribution microporoiis activated carbons induce a slow intraparticular diffusion resulting in flattened curves whereas more meso- and macroporous adsorbents possess a sharper curve because of an enhancement of mass transfer [58]. The adsorption of trihalomethanes on granular and fibrous activated carbon also shows adsorption capacities proportional to the micropore volume [59]. 

The treatment centres around the relative rates of gas-liquid mass transfer and reaction. The reaction rate controls the overall rate if it is very slow but often it is fast, so that the overall rate is mass transfer controlled. Very fast reactions can influence the diffusion process, causing enhancement of mass transfer above the purely physical rate. This enhancement is itself a function of the reaction rate. In extreme cases Cinstantaneous reaction), mass transfer again controls the overall rate of the process. 

---