An Innovative Technology for Natural Gas Sweetening by means of Cryogenic Distillation

Presented at 2015 Annual Conference, Florence 16 - 18 September 2015 by
Laura A. Pellegrini & Stefano Langé, Politecnico di Milano, Oldrich Mikus, Stamicarbon, and Barbara Picutti, Paolo Vergani, Guido Franzoni, Marco Lo Savio & Fabio Brignoli, Tecnimont
The application of traditional chemical absorption of CO2 and H2S by means of alkanolamines is not competitive for natural gas sweetening when the amount of acid gas is high, because the energy required by these processes is proportional to the quantity of acid components to be removed.
On the other hand, the cryogenic distillation technologies can be usefully applied to process natural gas with any amount of acid components, since the costs are incrementally reduced, the higher the acid gas content in the stream.
In the amine washing separation methods, CO2 and H2S are recovered at low pressures (nominally 1-2 bar) and need then to be dried and then compressed for re-injection into underground storage; on the contrary cryogenic distillation technologies discharge the CO2 and H2S mixture as a high pressure liquid, with a commercial advantage when this stream is re-injected for storage into depleted reservoirs or for EOR purposes.
An innovative process based on a "dual-pressure" cryogenic distillation unit is proposed with its two sections (the high-pressure column for the CO2 bulk removal and the low-pressure column for the methane recovery) working across the critical pressure of methane and the SVL locus of the binary CO2-CH4 mixture. This allows a complete separation of CO2 and H2S from methane, while avoiding the CO2 freezing.
A dynamic simulation study has been developed in order to validate the process architecture and to define the unit start-up procedure, while a laboratory experimental campaign is beginning.