Functionalized solvents for olefin isomer purification by reactive extractive distillation

Olefin isomer separations are difficult, energy intensive and thus expensive. An overview is presented to investigate the feasibility of metal-ligand complexes as functionalized solvents applied in a novel separation technology, Reactive Extractive Distillation, for the separation and purification of a-olefins like 1-hexene from other C6-olefin isomers (internal, branched, cyclic and diolefins) and paraffins by using p -complexation. Functionalized metal-ligand complexes were synthesized based on commercial available ligands from hydrometallurgy. In screening experiments they were evaluated for p-complexation with ethylene. The best solvents were selected for evaluation of preferential complexation of 1-hexene relative to other olefin isomers. Di (2-ethylhexyl)phosphoric acid (D2EHPA) and di-nonyl naphthalene sulphonic acid (DNNSA) yield both stable metal-ligand complexes and triple the solubility of ethylene. Next, three different phosphoric acid ligands: di (2-ethylhexyl) phosphoric acid (D2EHPA), di (butyl) phosphoric acid (DBPA) and mono (2-ethylhexyl) phosphoric acid (MEHPA) and two sulphonic acid ligands: di-nonyl naphthalene sulphonic acid (DNNSA) and dodecyl benzene sulphonic acid (DBSA), were investigated for a variety of C6-olefin isomers (1-hexene, 2-methyl-1-pentene, n-hexane, 1,5-hexadiene, trans-2-hexene). The highest selectivities were obtained for silver-DBPA (20 wt% Ag, S/F ¼ 3): 1.23 for 2-methyl-1-pentene and 1.42 for n-hexane.
A thermodynamic model is used to conceptually design a Reactive Extractive Distillation column applying the functionalised solvent silver-D2EHPA. Such a column should be operated at a solvent to feed ratio of around 5 (p ¼ 0.2 bar, T ¼ 311 K) for 99.5% purity and 99% recovery. Under these conditions, the minimum number of equilibrium trays (Nmin) to separate 1-hexene from 2-methyl-1-pentene is approximately 42, a dramatic decrease compared to Nmin of about 310 in the absence of silver.