TEX-PEP experiments

Figure 6 shows a schematic diagram of the reactor system used for tracer-exchange positron-emission profiling (TEX-PEP) experiments. During these tracer exchange experiments, a constant flow of unlabeled hydrocarbons in a hydrogen carrier stream is fed into the reactor. The -hexane/... 

In order to derive self-diffusion coefficients from the TEX-PEP experiments, a mathematical model is needed to describe the re-exchange process in the zeolite reactor bed. A common way to describe diffusion in packed beds is to use a set of diffusion equations, describing the mass transport in the zeolite bed and inside the crystals [3,38,39]. The model to analyze the TEX-PEP experiment study is basically a modification of the equations by Noordhoek et al. [25]. The process is thought to consist of the transport of molecules via convection and axial diffusion in the space between the crystals, adsorption and desorption at the zeolite crystal surface and diffusion inside the pores... 

The model of Nijhuis et al. [39] expHcitly accounts for adsorption/de-sorption at the crystal boundary, assuming Langmuir adsorption kinetics. As the TEX-PEP experiments are conducted under steady-state conditions, this mechanism can be replaced by a simple first-order adsorption/desorption process... 

Figure 11 shows the self-diffusion coefficients obtained from the TEX-PEP experiments for both alkanes as a function of the gas-phase mixture composition. Evidently, we find that the self-diffusivity of n-hexane is an order of magnitude higher than that of the 2-methylpentane. Indeed, the kinetic diameter of n-hexane (4.3 A) is smaller than that of isohexane (5.0 A) [51]. Moreover, we observe a decrease in mobility with increasing fraction of the branched alkane in the gas phase. Analogous behavior was found for CH4/CF4 mixtures, where the self-diffusivity of both components decreased as the loading of the slower diffusing tetrafiuoromethane increased [52]. 

With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n-hexane/2-methylpentane in large silicalite-1 crystals have been performed. By modeling the experimental tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant were obtained. Slight preference for the adsorption of /t-hexane was found. Diffusivity of -hexane sharply decreases with increasing fraction of its isomer, since the last one occupies channel intersections thus blocking zeolite network. 

Fig. 8 TEX-PEP profiles for labeled 2-methylpentane left) and n-hexane (right) at several detection positions in an equimolar mixture of n-hexane and 2-methylpentane in silicalite-1 at a total hydrocarbon pressure of 6.6 kPa and a temperature of 433 K (note that these two graphs have been obtained from two different experiments in which one of the two hydrocarbons was labeled)...

The current trends identified for n-pentane are strikingly similar to those for n-hexane. TEX-PEP provides the unique possibility to study diffusion of two linear hydrocarbon in two separate sets of experiments. In a first set, n-pentane is labeled and its diffusion coefficients are determined in a mixture with non-labeled n-hexane, while the reverse is done in a second set. The experiments have been performed at a temperature of 433 K and the total hydrocarbon pressure was kept constant at 6.6 kPa by varying the ratio between n-hexane and n-pentane in the gas phase. Figure 19 shows the loadings of both components in the mixture as a function of the n-hexane fraction in... 

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