Surface mass transfer resistance carbonation

Resistances to the mass transfer of oxygen and carbon dioxide (and also of substrates and products) at the cell surface can be neglected because of the minute size of the cells, which may be only a few microns. The existence of liquid films or the renewal of a liquid surface around these fine particles is inconceivable. The compositions of the broths in well-mixed fermentors can, in practical terms, be assumed uniform. In other words, mass transfer resistance through the main body ofthe broth maybe considered negligible. 

The second generation of nonporous membranes was silicon based which displayed increased CO2 permeabilities. In 1965, Bramson et al. commercialized the first nonporous membrane BO [18]. Since the diffusion coefficient of oxygen and carbon dioxide in air is about four orders of magnitude higher than in blood, the gas side mass-transfer resistance was negligible. The major resistance to respiratory gas transfer was due to the membrane and the liquid side concentration boundary layer [19]. Though nonporous membrane BOs reduced blood damage, up to 5.5 m membrane surface area was often required to ensure adequate gas transfer rates. 

There are at least two main sources of resistance to mass transfer (Figure 5.4 [96]) external film mass transfer resistance and intrapartide diffusion that is composed of pore and surface diffusion. The latter diffusion is insignificant in numerous adsorbents but plays an important role in most adsorbents used in RPLC. For particles having micropores, there is an additional mass transfer resistance, the resistance to diffusion through micropores which is often important. This explains why considerable attention is paid in the preparation of stationary phases for FIPLC to avoid the formation of micropores. This explains also why graphi-tized carbon black, which tends to be plagued by a profusion of micropores, has not been a successful stationary phase for HPLC. 

Regarding the solvent used to prepare the catalyst ink, its properties in catalyst ink should be mentioned as it also plays an important role in determining the microstructure and cataljAic activity of the CL. When ionomer such as Nafion solution is mixed with solvent, the mixture may become a solution, a colloid, or a precipitate due to the different dielectric constants of the solvent. When the dielectric constant is more than 10, a solution is formed between three and 10, a colloidal solution is formed and less than 3, precipitation occurs.If the mixture is a solution (i.e., the solution method ), excessive ionomer may cover the carbon surface, resulting in decreased Pt utilization. However, when the mixture is a colloid (the colloidal method ), ionomer colloids adsorb on the catalyst powder and the size of the catalyst powder agglomerates increases, leading to an increased porosity of the CL. In this case, the mass transfer resistance could be diminished because of the continuous network of ionomers throughout the CL, which then improves the proton transport from the catalyst to the membrane. ... 

Removal from the gas exhibits a slight minimum for the liquid flow rate (SLV) that could be an experimental artifact however, there is a significant minimum in the conversion to acid at an SLV of around 0.77 mm/s. Productivity increases with SLV above a velocity of 0.8 mm/s. Minimums in conversion or in reaction rates have been observed previously for SO2 oxidation in water-flushed trickle beds (Mata and Smith, 1981 Haure et al., 1989). They have been attributed to opposing effects of wetting of the carbon surface and increasing mass transfer resistance as the liquid velocity... 

It has been shown that there exists a correlation between the properties of samples obtained via an abrasion technique, to the position within the active carbon particles. Anisotropy of porous structure is due to burn -off of the carbonaceous substance which reduces radially from the outer surface of the particle to its inner core. The drop of burn-off, in turn, is the result of mass transfer resistance caused by the high temperature activation process. 

In certain adsorbents, notably partially coked 2eohtes and some carbon molecular sieves, the resistance to mass transfer may be concentrated at the surface of the particle, lea ding to an uptake expression of the form... 

The possible existence of an interface resistance in mass transfer has been examined by Raimondi and Toor(12) who absorbed carbon dioxide into a laminar jet of water with a flat velocity profile, using contact times down to 1 ms. They found that the rate of absorption was not more than 4 per cent less than that predicted on the assumption of instantaneous saturation of the surface layers of liquid. Thus, the effects of interfacial resistance could not have been significant. When the jet was formed at the outlet of a long capillary tube so that a parabolic velocity profile was established, absorption rates were lower than predicted because of the reduced surface velocity. The presence of surface-active agents appeared to cause an interfacial resistance, although this effect is probably attributable to a modification of the hydrodynamic pattern. 

Thus, when deahng with gas transfer in aerobic fermentors, it is important to consider only the resistance at the gas-liquid interface, usually at the surface of gas bubbles. As the solubihty of oxygen in water is relatively low (cf. Section 6.2 and Table 6.1), we can neglect the gas-phase resistance when dealing with oxygen absorption into the aqueous media, and consider only the liquid film mass transfer coefficient Aj and the volumetric coefficient k a, which are practically equal to and K a, respectively. Although carbon dioxide is considerably more soluble in water than oxygen, we can also consider that the liquid film resistance will control the rate of carbon dioxide desorption from the aqueous media. 

The first hypothesis seems unlikely to be true in view of the rather wide variation in the ratio of carbon dioxide s kinetic diameter to the diameter of the intracrystalline pores (about 0.87, 0.77 and 0.39 for 4A, 5A and 13X, respectively (1J2)). The alternative hypothesis, however, (additional dif-fusional modes through the macropore spaces) could be interpreted in terms of transport along the crystal surfaces comprising the "walls" of the macropore spaces. This surface diffusion would act in an additive manner to the effective Maxwell-Knudsen diffusion coefficient, thus reducing the overall resistance to mass transfer within the macropores. 

We assume that the adsorbent mass used in the kinetic test consists of a sphere of radius R. It may be composed of several microsize particles (such as zeolite crystals) bonded together as in a commercial zeolite bead or simply an assemblage of the microparticles. It may also be composed of a noncrystalline material such as gels or aluminas or activated carbons. The resistance to mass transfer may occur at the surface of the sphere or at the surface of each microparticle. The heat transfer inside the adsorbent mass is controlled by its effective thermal conductivity. Each microparticle is at a uniform temperature dependent on time and its position in the sphere. 

Physical adsorption at a surface is extremely rapid, and the kinetics of physical adsorption are invariably controlled by mass or heat transfer rather than by the intrinsic rate of the surface process. Biporous adsorbents such as pelleted zeolites or carbon molecular sieves offer three distinct resistances to mass transfer the external resistance of the... 

Calculate the maximum drop size allowable so that the final drop after a 2.0-s fall contains on an average 0.1 wt % carbon. Assume that the mass transfer rate of gases at the surface is very great, so there is no outside resistance. Assume no internal circulation of the liquid. Hence, the decarburization rate is controlled by the rate of diffusion of carbon to the surface of the droplet. The diffusivity of carbon in iron is 7.5 x 10" m /s (S7). Hint Can Fig, 5.3-13 be used for this case )... 

It is well known that catalytic activity is strongly dependent on the shape, size, and distribution of the metal particles [6], Furthermore, support also plays a vital role in the performance of a catalyst. Carbon is an excellent support in many ways its high specific surface area is necessary for high metal loading its pore structure is suitable for mass transfer and its high conductivity of graphitization can reduce resistance in electron transportation. In research on catalyst structures, XRD has played an effective role in determining catalyst composition. 

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