Below, we examine the specific case of separation of hexane isomers using MFI fixed bed adsorber. Consider the separation of nC6, 3MP and 22DMB. We first need to understand the pure component and mixture sorption behaviours. The pure component isotherms of hexane isomers nC6, 3MP and 22DMB into MFI zeolite at 362 K are obtained from configurational-Bias Monte Carlo (CBMC) simulations (Vlugt, T.J.H. et. al. , 1998); see Table 1.
|
Component |
Dual Langmuir Parameters |
|||
|
Site A |
Site B |
|||
|
bi,A /[Pa-1] |
Qi,sat,A /[molecules per unit cell] |
bi,B /[Pa-1] |
Qi,sat,B /[molecules per unit cell] |
|
|
nC6 |
0.06319 |
4.0 |
1.703 ´10-3 |
4.0 |
|
3MP |
0.04756 |
4.0 |
2.272 ´10-5 |
2.3 |
|
22DMB |
0.01085 |
4.0 |
|
|
|
nC5 |
0.00815 |
4.0 |
4.194 ´10-4 |
4.273 |
Table 1: Pure component data used for the case study. The unit of the parameter K for the isotherms is [1/Pa]. The saturated loadings are given in [molecule/unit cell].
Our program provides an option to display the pure component and mixtures isotherms. Fig. 1 presents the loadings for a specified pressure over the entire composition space. Fig. 1(a) shows the loading for the pure component and Fig. 1(b) displays the mixture behaviour based on Ideal Adsorbed Solution (IAS) predictions.
Figure 1: (a) Pure component isotherms for nC6,3MP and 22DMB in MFI at 362 K. (b) Component loadings for the mixture obtained based on IAS theory predictions. The total pressure is specified in the top of the figure.
For mixture loadings Qmix < 4, the isomers have practically the same sorption strength. However, the loadings of the mono- and di-branched isomers reduce to very low values when Qmix > 4; see Fig. 1(b). The reason for this "exclusion" of the 3MP and 22DMB is because of configurational entropy effects that tend to favour the linear isomer. For example, 3MP molecules prefer to locate at the intersections between the straight channels and zig-zag channels for Qmix < 4 (R. Krishna et. al. , 1998; T.J.H. Vlugt et. al. , 1999; R. Krishna and D. Paschek, 2000). The normal alkane can be located anywhere within the MFI matrix. At Qmix = 4, all the intersection sites are fully occupied. The 3MP demands an extra "push" to locate within the channel interiors. 3MP suffers a penalty from configurational entropy considerations because these molecules "pack" less efficiently within the MFI matrix; this penalty causes 3MP to be virtually excluded from the MFI matrix near saturation loadings Qmix = 8.Further details and reading can be found in Krishna and Baur (2003).
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