Surface diffusion, that is, diffusion of adsorbed molecules or atoms on surfaces, has long been investigated (Kapoor et al., 1989). Diffusion of sorbate in zeolite, called zeolitic diffusion, has attracted a special interest recently because of the increasing importance of zeolite in separation and catalysis. One of the most intriguing aspects of both surface diffusion and zeolitic diffusion is the strong dependence of diffusivity (Fickian diffusivity) on sorbate concentration. However, seemingly different concentration dependences have been observed for surface diffusion and zeolitic diffusion, and these dependences have been interpreted by different and unrelated theories (Yang, 1987).Surface diffusivity increases with sorbate concentration. The H 1 0 theory (Higashi, Itoh and Oishi, 1963) stipulates that D / D , = 1/(1 -0), where D is diffusivity, O is fractional surface coverage, and Do is zero surface coverage. The H I 0 theory has been modified by Yang et al. (1973) to account for secondlayer adsorption, and hence a slower rise of diffusivity with 8 . To date, all observed concentration dependences of surface diffusivity follow the increasing trend ( D increases with 0), although the opposite is possible if the sorbate-sorbate bond is stronger than the sorbate-surface bond, according to the model of Yang et al. (1973). The concentration dependence of zeolitic diffusion seems to be more complex and erratic; all possible types of dependence have been observed (Yang, 1987). In most of the published results, an increasing dependence similar to the HIO-type is seen. A popular interpretation of this dependence is based on Darken's theory (1948). However, lingering questions remain regarding the assumptions made in the derivation of the Darken-type theory (Yang, 1987) and, more important, on the origin of the concentration dependence of diffusivity.In this note, we propose aunified model based on the random walk/hopping mechanism. This model is capable of interpreting both increasing and decreasing concentration dependences for both surface diffusion and zeolitic diffusion, and it sheds light on the origin of the concentration dependence for both kinds of diffusion.
NaX zeolite was ion-exchanged to obtain LiX and AgX zeolites. The LiX form was further exchanged to replace 20% of the Li+ cations by Ag+, to obtain a LiAgX zeolite. Equilibrium adsorption isotherms of pure-component N2 and O2 were measured at 25 and 50 °C on these four zeolites. AgX was stable since the N2 isotherm was not affected after prolonged exposure of the zeolite to air at 350 °C. Bonding of N2 was substantially stronger on AgX than on the other zeolites. The high isosteric heat of adsorption (8.4 kcal/mol) and the relatively slow desorption of N2 on AgX indicated some degree of weak π-complexation, which was substantiated by molecular orbital calculation results using model systems. Binary N2/O2 selectivity (or separation factor, α) was calculated by using the ideal adsorbed solution theory. The high N2/O2 selectivities at low total pressures for AgX will result in difficult N2 desorption; therefore, AgX is not suitable for air separation. LiX is presently employed in industry as the sorbent for air separation by pressure-swing adsorption. Comparing LiX with LiAgX, the N2/O2 selectivities were higher for LiAgX at high total pressures and lower for LiAgX at lower pressures, due to a (relative) selectivity reversal. This result, combined with the higher N2 capacity for LiAgX, led to the conclusion that LiAgX can be superior to LiX for air separation.
A model based on kinetic theory is derived for surface diffusion of multilayer adsorbed species on a homogeneous surface. A simple solution is obtained for expressing the concentration dependence of surface diffusivity. For monolayer adsorbate, the solution is reduced to the classical HI0 model. This solution, in principle, requires no fitting parameters and is capable of predicting the unique concentration dependent behavior exhibited by multilayer surface diffusion, where the diffwivity first increases with surface concentration to a maximum followed by a decrease. Seventeen sets of experimental data from the literature are used to test the model, with fair results. It is also shown that heats of adsorption can be calculated from multilayer surface diffusivity data by using this model.
A real-time CNC tool path generation algorithm has been developed for machining IGES surfaces. IGES-based CAD data files can be directly inputted to the CNC system and the tool paths generated in real time can be passed to a motion controller during cutting via a Multibuss II backplane structure. The development of such a real-time tool path generation method has eliminated the need for a tradeoff between the desired surface accuracy and the required memory size for storing off-line generated NC codes. The real-time NC path generation algorithm can properly deal with issues such as trimming lines, gouging detection, and adaptive tool step adjustment. The developed algorithm has been implemented on a multi-processor CNC system and verified through actual cutting tests. The test results show that no violating conditions occurred on machined part surfaces, and the surface contour error of the cut part is less than the given tolerance, which was 0.02 mm in this particular test.
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