Olefin/paraffin separation by reactive absorption
A novel solvent for gas absorption was investigated for use in ethylene-ethane separations by reactive absorption. Due to the reactive interactions between olefins and transition metals, selection of the absorption solution required identifying a suitable transition metal salt, coordinating ligand, and solvent. In order to select an appropriate metal-salt:ligand pair, the results from a previous study using bench-top electrochemistry were compared to equilibrium data from a stirred-cell autoclave apparatus. Metal-salt:ligand pairs that were tested included combinations of cuprous chloride and cuprous bromide salts with pyridine, benzylamine, and aniline. Results from both techniques showed qualitative agreement for ethylene capacity, and the solution with the best ethylene capacity consisted of cuprous chloride with aniline as ligand. The presence of the copper salt was found to increase the ethylene absorption capacity up to 0.6 mol/L more than the physical solubility of ethylene in a similar solution prepared without the copper salt. The mixed gas selectivity, ethylene:ethane, for the metal solution was found to be over 12:1 at optimum conditions. The kinetics of the absorption reaction were measured in a stirred cell after calculating the mass transfer coefficient for physical absorption over a range of stirrer speeds. The observed rate was compared to predictions from five different reaction rate models and was found to show good agreement with the instantaneous, reversible model. Following the equilibrium and kinetic study, a small-scale continuous process was designed consisting of two one-inch packed columns for absorption and regeneration of a 50% ethylene / 50% ethane molar mixture. The process was operated over a wide range of flow conditions over a period of six weeks, and the solution was found to be very robust showing no significant reduction in ethylene capacity or selectivity for the duration of the experiments. The flow study also confirmed the instantaneous, reversible behavior of the complexation reaction.