(652h) New Adsorptive PSA Separation Process for Production of Isopentane and Di-Methyl Branched Paraffins from C5/C6 Light Straight Naphtha Feeds | AIChE

(652h) New Adsorptive PSA Separation Process for Production of Isopentane and Di-Methyl Branched Paraffins from C5/C6 Light Straight Naphtha Feeds

Authors 

Henrique, A. - Presenter, Faculty of Engineering University of Porto
Brântuas, P., Ecole Nationale Supérieure des Mines de St-Etienne
Nouar, F., Ecole Normale Supérieure de Paris
Rodrigues, A. E., LSRE - Laboratory of Separation and Reaction Engineering - Associate Laboratory LSRE/LCM
Maurin, G., University of Montpellier
Serre, C., Université de Versailles Saint Quentin-en-Yvelines
Silva, J. A. C., ESTiG-IPB
The conventional Total Isomerization Process (TIP) for improving the octane rating of light straight-run naphtha (LSRN) (C5/C6 range), is one of the first and most successful processes in the application of adsorption phenomena to industrial processes. The process typically isomerizes straight-chain hydrocarbons (pentanes (C5) and hexanes (C6)) with low research octane number (RON) into their more valuable mono- and di-branched isomers in a thermodynamically controlled reaction. Afterward, linear hydrocarbons non-isomerized are separated (for recycling) from the output stream of the catalytic reactor by adsorption using the molecular sieve zeolite 5A that completely separate linear from branched paraffins. In terms of performance, actual TIP processes can produce a final isomerate mixture with an average RON around 87-89. However, it suffers from the disadvantage that in the final stream, there are still 30% of molecules with low RON, such as the mono-branched isomers of hexane.1,2

In this work, we report a new advanced recycling technology3 (Figure 1) developed to fractionate C5/C6 isomers mixtures into low RON (LRON) and high RON (HRON) fractions by also recycling the mono-branched C6 isomers in TIP processes. For that, a new adsorber unit is used through a combination of two adsorbents: i) a benchmark Zeolite and ii) a Metal-Organic Framework (MOF). The Zeolite separates the linear n-pentane (nC5, RON:61.7) and n-hexane (nC6, RON:24.8) from the branched paraffins. In contrast, the MOF, separates low RON mono-branched hexane isomers (2-methylpentane (2MP, RON:73.4), 3-methylpentane (3MP, RON:74.5)) from the high RON pentane and hexane di-branched isomers (isopentane (iC5, RON:93), 2,3-dimethylbutane (23DMB, RON: 101.7), and 2,2-dimethylbutane (22DMB, RON:91.8)). As a result, the LRON linear and mono-branched C6 isomers are separated through a synergy between size selectivity (zeolite) and host-guest interactions (MOF), and recycled to the catalytic reactor providing a final HRON isomerate product having values higher than 92 RON (Figure 1).

To develop this new technology, and previously to cyclic Pressure Swing Adsorption tests (PSA), several fixed bed breakthrough experiments with equimolar C5/C6 isomers mixtures were carried out on a mixed bed of Zeolite [beads (1.0 to 1.8 mm)] and MOF [shaped beads (1.0 to 1.8 mm)], to select the best operating conditions. The breakthroughs were measured at the temperature range of 373 - 473 K and total hydrocarbon pressure up to 50 kPa. The example shown in Figure 2, is for a breakthrough curve performed at 423 K and 50 kPa, where a clear separation between the HRON (22DMB, 23DMB and iC5) and the LRON isomers (nC6, nC5, 2MP, 3MP) is obtained, resulting in a selectivity (hierarchy) elution order: nC6 >> nC5 >> 2MP ≈ 3MP >> 23DMB > iC5 ≈ 22DMB. For this experiment the calculated productivity of the mixed bed, is 0.59 mol.kgads-1 for a RON product of 92.3.

Figure 3 shows the steady-state effluent concentration profiles of the separation of C5/C6 isomers in a 4 step PSA experiment: I – Pressurization with feed (adsorption); II – High-Pressure feed (adsorption); III – Countercurrent blowdown (desorption) and IV – Countercurrent Low-pressure purge (desorption). As shown in Figure 3, it is clear that at the end of the adsorption steps (I and II) the mass transfer front of the HRON isomers iC5, 22DMB and 23DMB has left the column as the final product. Regarding the other LRON isomers, the majority of their respective mass transfer fronts remains inside the column, with only a very small concentration leaving the column. It is also clear from Figure 3 that at the end of the desorption steps (III and IV), the concentration of the HRON isomers at the effluent of the column is very small which means that the bed is efficiently cleaned. This experiment gives a final product in the end of the adsorption step with a RON around 93. A mathematical model was also developed to simulate the fixed bed and PSA experiments.

As a final conclusion, this works shows the remarkable capacity of a mixed bed of a Zeolite/MOF adsorptive system to separate all C5/C6 isomers by cyclic PSA adsorption processes into distinct HRON and LRON fractions. This result is of significance importance to upgrade actual existing TIP processes.

References:

1 - Holcombe, T. C.; Sager, T. C.; Volles, W. K.; Zarchy, A S. Isomerization Process U.S. Patent 4,929,799, 1990.

2 - Minkkinen, A., Mank, L., Jullian, S., Process for the isomerization o C5/C6 normal paraffins with recycling of normal paraffins, U.S. Patent 5,233,120, 1993.

3 - Serre, C.; Nouar, F.; Rodrigues, A.; Maurin, G; Brântuas, P.; Henrique, A.; Silva, J., Unpublished results, 2020.

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