Separation of ethylene from refinery dry gas via forming hydrate in w/o dispersion system

• Recovering ethylene from refinery dry gases using a new absorption–hydration method.
• Two-stage separations for CH4 + C2H4 + N2 + H2 gas mixtures were performed.
• Separation factor for ethylene increases with decreasing temperature (>269.2 K).
• The morphology of water/diesel system was measured by PVM and FBRM probes.
• The absorption–hydration method is a promising technology for separating gas mixtures.

A so-called absorption–hydration hybrid method was adopted for recovering ethylene from refinery dry gas using water in diesel emulsion under hydrate formation conditions. Span 20 was used to disperse water or hydrate in diesel. Two (CH4 + C2H4 + N2 + H2) quaternary feed gas mixtures were synthesized in order to simulate a two-stage separation of refinery dry gases. The influences of temperature, pressure, feed gas composition, initial gas–liquid volume ratio, and initial water cut in liquid on the separation efficiency were systematically investigated. A pseudo separation factor (S) was defined to evaluate the separation selectivity of ethylene over other components. The experimental results show that ethylene can be enriched in hydrate slurry phase with high selectivity and high recovery ratio. The operation pressure could be reduced remarkably by decreasing temperature to below the ice point. Both separation factor and recovery ratio of ethylene increase with the decrease of temperature; however, when temperature is decreased to 269.2 K or so, the trend changes inversely. In order to reveal why S decreases with decreasing temperature at lower temperature range, in situ PVM (Particle Video Microscope) and FBRM (Focused Beam Reflectance Measurement) particle analyses were performed to investigate the variation law of the morphology of water/diesel dispersion system with temperature before and during hydrate formation. The experimental results show that the average ice particle size increases with decreasing temperature and leads to the decrease of the hydrate formation rate and the conversion ratio of water into hydrate.