Cryogenic distillation is the current method for separating propylene/propane mixtures. It is a difficult separation requiring high reflux ratios, high energy input, and tall columns, because of the small difference in the volatilities of propylene and propane. Economical alternatives for the separation of propylene/propane mixtures are highly desirable, particularly for smaller streams where the contained propylene has significant value, but not enough to warrant additional distillation capacity. Adsorption based processes have been explored in recent years as potential replacement of or augment to the incumbent cryogenic distillation process to reduce the energy intensity or debottleneck existing assets. To make the adsorption processes economically advantageous, a highly selective adsorbent is the key since propylene is the most adsorbed component and is recovered in the effluent during adsorbent regeneration.
We have discovered new carbon molecular sieves (CMS) that show a high separation factor for olefin and paraffin, particularly for propane and propylene. The adsorption isotherms of propylene and propane on the CMS were measured at different temperatures that cover the commercial operating range. A temperature dependent, four-parameter Langmuir model was selected to fit the single component isotherm data and the model parameters were obtained by using the Aspen ADSORPTIONTM equilibrium parameter estimation tool. The heat of adsorption of each component was determined from the isotherm models according to the Clausius – Clapeyron equation. The adsorption kinetics of propylene and propane on the CMS were studied by fixed bed adsorption breakthrough experiments. Breakthrough curves were obtained at different feed flow rates. The mass transfer coefficients of propylene and propane were estimated using Aspen ADSORPTIONTM simulations.
The new CMS was experimentally investigated for its propane and propylene separation performance using a fully automated bench-scale PSA unit. The propylene enrichment, recovery and propane purity were measured for each PSA experiment. An Aspen ADSORPTIONTM model was developed for this PSA process and was validated against experimental data. The validated PSA model can then be used for future process optimization and scale-up.