(632g) Non-Classical Gas Hydrate Formation for Propane/Propene Separation

Separation of propane/propene mixtures is one of the most important and challenging processes in the chemical industry [1]. At present, this separation typically relies on cryogenic distillation, which is highly energy and capital intensive due to the very similar properties of propane and propene [2, 3]. It has been reported that the purification of propene and ethene alone account for 0.3% of global energy use [1]. Efforts have been made to develop low energy routes for propane/propene separation such as extractive distillation [4], adsorption [5] and membrane separation [6].

Hydrate-based gas separations show significant potentials in this field, especially with advantages of convenient operation, low energy consumption and low investment costs [7]. To date, hydrate-based separation research has mainly focused on the recovery of CO₂, CH₄, H₂ and N₂, as well as the water desalination. The application to the separation of propane/propene has not been investigated systematically.

Our work aims to achieve the hydrate-based propane/propene separation under moderate operating conditions to substantially reduce process costs. To this effect, a stirred high-pressure, low-temperature apparatus was developed to conduct batch separation experiments. Operating parameters such as reactor pressure, temperature and stirring speed can be controlled precisely and monitored in real time. Each batch experiment allows the propane/propene concentration in the feed gas, the gas phase after hydrate formation and later that in the hydrate phase to be measured using a gas chromatography.

Instead of bulk water, a non-classical form of water, Dry Water (a water-in-air reverse Pickering system), was used in this study to form hydrates as it allows for a considerable acceleration of hydrate formation due to the large available gas/liquid interface area. With the presence of Dry Water, the operating window for propane/propene separation was effectively shifted towards milder conditions and we were able to facilitate hydrate formation and gas separation with temperatures above -10°C and pressures below 5 bar.

Separation experiments were conducted with a feed gas mixture of 50mol-% propene in propane, a key industrial mixture and a byproduct from steam cracking [8]. We demonstrate that propane can be captured and concentrated in the hydrate phase whereas propene dominates in the gas phase. By measuring the effluent gas concentration captured in the hydrate phase of a previous experiment and using this as the feed gas composition for the next batch experiment, we effectively simulated a number of equilibrium stages that allowed the enrichment of propane from 50mol-% to nearly 90mol-% within five steps.

In our presentation we will elaborate on the details of our experimental work and will also present an Aspen Model for the proposed hydrate-based separation process using our experimental equilibrium data. We will show economics and energy analysis of the designed multi-stage separation process and compare this with conventional cryogenic distillation and PSA process solutions. In doing so we will demonstrate the feasibility and competitiveness of this non-classical hydrate-based propane/propene separation strategy.

References

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