(228d) Separation of Complex Mixtures of Partially Hydrogenated Organic Compounds and Isomers By Distillation

Liquid Organic Hydrogen Carriers (LOHCs) are novel media for the storage and carrying of energy. Amongst others, N-ethylcarbazole is a very promising candidate which is currently investigated with respect to its application as LOHC[1]. During the hydrogenation or dehydrogenation reaction the formation of several partially hydrogenated, stable intermediates has been proved experimentally, e.g. tetra-, hexa- and octa-hydro-N-ethylcarbazole[2]. Experimental studies by GC-MS, shown in this presentation, reveal that furthermore plenty of diastereomers are found turning the LOHC system into a complex multicomponent mixture. However, only several physicochemical property data for N-ethylcarbazole are available in a sufficient extend. To determine physicochemical properties, e.g. melting points or vapor pressures of the partially and fully hydrogenated isomers, those have to be separated from the mixture and purified.

In the current contribution the separation of a mixture of N-ethylcarbazole isomers by batch distillation is shown on lab scale (2-250 ml) as well as on pilot plant scale (5‑15 l). Because of the different isomers having close boiling points a high number of separation stages is required. Published vapor pressure data for N-ethylcarbazole show that the vapor pressures of the components have to be considered to be rather low. Therefore the distillation was performed under vacuum conditions preventing distillation temperatures above 250°C at which thermal decomposition of the molecules is possible. Basic studies are performed using a lab scale micro spinning band column with a stage number of up to 50 per meter to fractionate a mixture of perhydrogenated N-ethylcarbazole isomers. The fractions are characterized by GC-MS and NMR. The purities of the fractions are up to x = 99%. For further process design, modeling and scale up vapor pressures are determined using different methods e.g. a GC retention time method or a transpiration method. For the scale-up a pilot scale distillation column with a Sulzer structured gauze packing DX with the height of 2 meter is chosen with respect to the separation of close boiling mixtures. The presentation shows results for basic plant data obtained by experiments with binary test mixtures recommended for close boiling substances with low vapor pressures, e.g. cis/trans-decaline, focusing on the stage efficiency of the packing. Concluding, the separation of complex LOHC mixtures, based on the results of the test mixture experiments and model based process optimization using Aspen Plus^®,is discussed.