Adsorption studies for the separation of light hydrocarbons.

Abstract

Traditionally, the separation of ethylene from ethane is undertaken using a fractionation sequence. The distillation is performed at low temperatures and elevated pressures in conventional trayed fractionators. For economic feasibility, the separation scheme must be heat integrated to produce the low temperatures needed for separation – as low as 243 K. Low temperature distillation units are expensive to build and are typically only economically feasible for feed streams containing high amounts of ethylene. Adsorption provides a favourable alternative to the traditional low temperature distillation process. The availability of accurately measured adsorption data over a wide range of temperatures and pressures is vital in the design of efficient separation processes. However, reproducible binary adsorption data are not readily available in the literature due largely to the uncertainties involved in measuring adsorption equilibria. This project involved the measurement of adsorption equilibria using two techniques – the gravimetric and the volumetric technique. Particular focus was placed on the design and commissioning of a volumetric apparatus capable of measuring binary adsorption equilibria over a range of temperatures and pressures. The gravimetric apparatus is not capable of measuring multicomponent adsorption equilibria. The Thermodynamic Research Unit (TRU) has extensive capabilities in the field of phase equilibria with specialized expertise in the field of vapour liquid equilibria (VLE). The objective of this project is to develop competence in the field of adsorption equilibria by designing and commissioning new apparatus. This forms part of a larger objective to extend the capabilities of TRU. The volumetric apparatus designed and commissioned in this study uses an innovative gas mixer to prepare binary mixtures for adsorption equilibrium measurements. The measured data were compared to literature to validate the measurement reproducibility of the apparatus and accuracy of measurement techniques used. Adsorption equilibrium data were measured for pure components and a binary system. Pure component adsorption data were measured for methane, ethane and ethylene. The binary system of ethane + ethylene was also investigated. Measurements were performed at pressures up to 15 bar, at temperatures of 298 K and 323 K, on an adsorbent zeolite 13X. The gravimetric and volumetric apparatus both showed good reliability and reproducibility. Uncertainties in temperature and pressure were 0.1 K and 4×10-3 bar for the gravimetric apparatus and 0.03 K and 0.002 bar for the volumetric apparatus respectively. The measured equilibrium data were fitted to the Langmuir, Sips and Vacancy Solution Model (VSM) adsorption models. The regressed parameters were used to predict binary adsorption equilibria. The Langmuir model performed the poorest across the pressure range investigated, with an average absolute deviation (AAD) as high as 5%. The deviation however, was comparable with the experimental uncertainties reported in literature. The Sips model improved upon the Langmuir model with the VSM model generally performing the best with an AAD of approximately 1%. The Extended Langmuir, Extended Sips and VSM all provided good predictions of the binary adsorption equilibria. The Extended Langmuir model performed best with an AAD of 3%. The Extended Sips model performed marginally poorer with an AAD of 3.05%. The VSM model performed satisfactorily with an AAD of 6%, marginally higher than the reported experimental uncertainties of 5%.