Chromatographic separation alkanes using MFI zeolite
Jolimaitre et al. (2002) have reported experimental breakthrough curves for binary and ternary mixtures containing 2-methylbutane (2MB), 2-methylpetane (2MP) and 2,2dimethylbutane (22DMB) obtained in bed packed with MFI crystals at a temperature of 473 K. We have simulated their results, ignoring macropore diffusion resistances. The key input data for 2MP and 22DMB are summarized in Table 2. ; other data can be found in their paper (Jolimaitre et al. 2002).
|
Langmuir parameter |
|
|
Temperature |
473.0 |
[K] |
|
|
|
b2MP |
1.27 ´10-4 |
[1/Pa] |
Pressure |
5 |
[bar] |
|
|
b22DMB |
7.12 ´10-5 |
[1/Pa] |
Ugas |
0.019 |
[m/s] |
|
Diffusivities: |
|
|
Reactor length |
0.795 |
[m] |
|
|
|
D2MP/R2 |
0.5 |
[1/s] |
Cat. voidage |
0.4 |
|
|
|
D22DMB/R2 |
0.0063 |
[1/s] |
Cat. density |
620.8 |
[kg/m3] |
Table 1: Key input parameters for 2MP and 22DMB in MFI zeolite at 473 K. Data from Jolimaitre et al. (2002).
As shown in Fig. 38 and 39 in Krishna and Baur (2003) , the agreement between model and experimental data for binary and ternary mixtures are reasonably good, and remarkable when we consider that only pure component data have been used in the simulations. Subsequently, we pick run 17 of Jolimaitre et al. (2002) as a sample run in order to illustrate the dynamic behaviour of the chromatograph. Following a step injection strategy 2MP and 22DMB is injected into an empty adsorber at t = 0. The inlet concentration of 22DMB is slightly in excess (C22DMB = 6.76 mol/m3 and C2MP = 5.76 mol/m3).
Figure 1: Predicted concentration at the outlet of the adsorber. The symbols denote experimental data adopted from Jolimaitre et al. (2002).
Fig. 1 shows the concentration change at the exit of the chromatograph. The slower diffusing and less adsorbing component 22DMB leaves the adsorber first followed by 2MP. Furthermore, the transient of 22DMB exhibits a "rollup"; a rollup is said to occur if the transient molar concentration in the gas phase exceeds the inlet value, i.e. ci/ci0 >1.
Fig. 2(a) shows the animation of the gas concentration profile along the bed length and Fig. 2(b)-(d) the intra-crystal fractional loadings at z/L= 0.0, 0.5 and 1.
Figure 2: Animation of the pulsed chromatographic operation. (a) concentration in the gas phase along the fixed bed. (b)-(c) intra-crystal fractional loadings at z/L= 0.0, 0.5 and 1. Note that the time between snapshots is varying.
The loading profile at the inlet shows that 2MP is predominantly adsorbed and buffered in the zeolite at the beginning of the injection; see Fig 2 (a). Consequently, the gas concentration of 22DMB is quickly raising and creates a leading front traversing the adsorber. A slower traversing and flatter front of 2MP follows. Since there is a delay between the two fronts, 22DMB of the leading front will be adsorbed as long as the 2MP front did not reach that specific location. Once this happens, 22DMB is displaced by 2MP. The eluted component 22DMB cause the total concentration of 22DMB to be higher than the inlet concentration. Hence, the concentration profile of 22DMB exhibits its characteristic rollup. It is noteworthy that the intra-crystal concentration profiles typically reveal minima and maxima; see Fig 2 (b)-(c).
Further details and reading can be found in Krishna and Baur (2003).
Return to contents or next: chromatographic separation of hexane isomers using MFI zeolite .