CBMC simulation

Figure 13.3 Comparison of experimental and CBMC simulations for adsorption of/-butane over MFI [6],...

Figure 13.6 CBMC simulations for linear and branched hexane adsorption over AFI [6].

Often the ratio of Henry coefficients, related to adsorption at zero loading, is used for predicting the selectivity of adsorption for mixtures. The ratio of Henry coefficients for linear and mono-branched alkanes with carbon number n = 5-8 are summarized for various zeolites in Figure 13.10 [15]. The Henry coefficient ratios were 1 for FAU, 2 for BEA, MOR and MFI, 6-9 for TON and 10-14 for MTT. Interestingly, CBMC simulations suggest that the ratio of Henry coefficients, actu-... 

Figure 13.11 CBMC simulations of adsorption isotherm (a) and adsorption selectivity (b) for a 50 50 mixture of n-hexane and 3-methylpentane over MFI at 362°K [6].

Schenk M. et al, Sep. of alkane isomers by exploiting entropy effects during adsorption on silicalite-1 a CBMC simulation study. Langmuir 17 (2001) pp 1SS8-1S70... 

The value of S(Q) at zero Q value cannot be determined experimentally on the same instrument that is used to measure diffusivities there are not enough points at small Q in Fig. 8b. However, the S(Q) scale, which is given in Fig. 8b in arbitrary units, can be renormalized. At infinite dilution, S(0) should be equal to one (hke in a gas), and sorption thermodynamics also imply that the thermodynamic correction factor should be equal to one, so that Eq. 38 will be fulfilled. On the other hand, at high concentrations, F increases while S(0) goes down. In Fig. 8b, r is equal to 6.6 for a concentration of 14 CF4 per u.c. so that S(0) should go down to 0.15. A more quantitative analysis has been recently performed for n-hexane and n-heptane in sihcalite [36] where the inverse of the thermodynamic factor, calculated from S(Q) was found to be in good agreement with configurational-bias Monte Carlo (CBMC) simulations. 

Fig. 10 Loadings of mixture components in silicalite as a function of 2-methylpentane fraction in the gas phase, experiment (total hydrocarbon partial pressure 6.6 kPa, 433 K) and CBMC simulations (total hydrocarbon partial pressure 7.8 kPa, 433 K)...

The loading of n-hexane in mixtures is somewhat higher than it is expected to be if it were proportional to its partial pressure (Fig. 12). On the contrary, the 2-methylpentane loading is somewhat lower. This points to preferential adsorption of -hexane over isohexane in their mixtures in H-ZSM-5 than in silicalite-1. In earlier experimental [50] and CBMC simulation studies [44] of n-hexane/isohexane mixtures in silicalite-1, a slight preferential adsorption of the linear alkane over the branched one has been found. The most prominent explanation for this preference is the molecular siting of these two hydrocarbon molecules. Whereas -hexane exhibits no clear preference for a position in the micropore system of MFI zeolite, the branched isomer is preferentially located at the channel intersections due to entropic reasons [44]. Consequently, 2-methylpentane will be pushed out from silicalite-1 by -hexane. These effects are even stronger for H-ZSM-5, most likely due to the stronger... 

GEMC/CBMC simulations were also successfully applied to investigate the contribution of adsorption at the mobile/stationary phase interface (here the stationary phase is represented as a two-dimensional film) [22] and the influence of analyte overloading (here the analyte concentration is sufficiently high to result in departures from Henry s law behavior) [23] in gas-liquid chromatography. 

The capacity factor can be determined experimentally from retention data and is exactly equivalent to the average number of analyte molecules in the stationary phase divided by the average number in the mobile phase. Thus, this quantity is easily measured in a GEMC/CBMC simulation. 

The GEMC/CBMC simulation methodology applied here has proved very useful for studying structure and retention in complex GLC and RPLC systems. The methodology affords the computation of retention data with sufficient precision (and accuracy for the TraPPE force field) for complex analytes to validate the... 

In summary, we have briefly reviewed some of the interesting aspects of CBMC. It was found that we can efficiently use parallel computers to perform CBMC simulations and that special care has to be taken for the simulation of branched molecules. 

The simulated isotherms for 2-methylalkanes at 300K temperature are shown in figure 4.19. The continuous lines in this figure are fits of the CBMC simulations using the dual-site Langmuir model which will be discussed in section 4.6. 

Available experimental data from the literature confirm the accuracy of the predictions of the CBMC simulations for both linear and branched alkanes. However, in the latter case the number of experimental data are much less as compared to that available for linear alkanes. 

The temperature dependency of the isotherms cire also properly modeled by the CBMC simulations. 

Figure 5.7 Pure component and 50%-50% mixture isotherms at 362K in Silicalite. The open square and circle symbols represent the CBMC simulations for (1) n-Cg and (2) 3MP respectively. The continuous and dashed lines are the dual-site Langmuir (DSL) fits with the parameter values determined only from pure component CBMC simulation data. The dual-site Langmuir parameter values are for n-Cg 0ia = 4, 0ib = 4, kiA = 0.07Pa kiB = 2 x and for...

In chapter 4, we discuss the adsorption of linear and branched alkanes in the zeolite Silicalite. We have used the simulation techniques described in the previous chapters for this. Silicalite has a three dimensional channel structure which consists of straight and zigzag channels that cross at the intersections (see figures 1.1 en 4.1). To compute the adsorption behavior, we have fitted a force field which is able to reproduce the Henry coefficient (adsorption isotherm at low pressure) and the heat of adsorption. From CBMC simulations it turns out that linear alkanes can occupy all channels of Silicalite. For u-Cg en u-C/, the length of the molecule is almost identical to the length of the zigzag channel. In literature, this process is called commensurate freezing and causes an inflection in the adsorption isotherm of these molecules. This effect has also been observed experimentally. 

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