Exploring Alternative Processes for Ethylene Manufacturing
Funded through Round 1: Open Call in 2011, NOVA Chemicals aimed to reduce the energy and greenhouse gas (GHG) associated with separating ethylene/ethane. Traditional methods of ethylene separation require significant energy and contributes heavily to the emissions associated with ethylene manufacturing; therefore, NOVA sought to explore alternative, lower emitting processes.
Ethane, the main feedstock of ethylene manufacturing, is primarily produced through separation from a natural gas stream at very low temperatures in a process known as “cryogenic distillation”. Distillation is a high energy intensity process because a two-phase mixture (a mixture of saturated liquid and saturated vapor) is needed for separation, and this requires large amounts of energy to produce. The most frequently utilized technology for achieving this separation is called the Gas Subcooled Process (GSP).
More recently, advancements in molecular sieve technology have indicated it may be possible to improve the energy efficiency of separation through a process known as “adsorption”. Adsorption is the adhesion of molecules from a gas or liquid phase onto a solid surface. When one component of a mixture binds more strongly with the adsorbent than the other components, selective separation can be achieved without the need to form a two-phase mixture. In theory, without the need of a two-phase mixture, ethylene manufacturing could be done without distillation, ultimately reducing emissions.
This project was completed in 2015 and specifically explored the pressure swing adsorption (PSA) cycle as a replacement, both on its own and in combination with GSP. PSA utilizes pressure to instigate adsorption, and thus the separation process to form ethane.
Current Technology Proved to be Most Sustainable and Cost Effective
During the testing, the PSA cycle performed well and was able to recover 87 per cent of the ethane with 86 per cent purity. The GSP-PSA hybrid process design also showed a high extra ethane recovery of 84 per cent from ethane not captured initially. While these results show the adsorption methods investigated are technically feasible, however, it was found to not be an attractive technology for implementation due to the energy requirements. If implemented the extra power consumption would be 420 gigawatt hours, which is much larger than the current GSP process power consumption at 168 gigawatt hours. This results in high energy intensity for extra ethane recovery compared to the current GSP. In addition, it would lead to 27.3 million dollars of extra operating cost and increase GHG emissions by 225 kilo tonnes.
What’s next?
Given the high energy intensity, negative environmental impact, high capital and operating cost, and negative profitability index, Nova Chemicals did not pursue this technology further. Despite this, the project proved the technical feasibility and provided insights into the weaknesses and strengths of this technology in ethylene manufacturing.