Final report from Call 1 Project: Tractable equations of state for CO₂ mixtures in CCS

Written by Richard Graham, University of Nottingham, Principal Investigator on UKCCSRC Call 1 Project – Tractable equations of state for CO₂ mixtures in CCS which completed in June 2014

A potential bottle-neck for Carbon Capture and Storage is CO₂ from power plants to the storage location, by pipeline. Key to safe and inexpensive transport is a detailed understanding of the physical properties of carbon dioxide. However, no gas separation process is 100% efficient, and the resulting carbon dioxide contains a number of different impurities. These impurities can greatly influence the physical properties of the fluid compared to pure CO₂. They have important design, safety and cost implications for the compression and transport of carbon dioxide and its storage location.

We have been developing models for the phase behaviour of impure carbon dioxide, under the conditions typically found in carbon capture from power stations, and in high-pressure (liquid phase) and low-pressure (gas phase) pipelines. Models for phase behaviour are known as equations of state (EoS). To optimise their accuracy, EoS need to be calibrated by reference to experimental measurements on carbon dioxide mixtures. New measurements become available very frequently so there is an on-going need to regularly rederive, refine and reparameterise EoS.

We have applied cutting-edge computer algorithms to automatically reparameterise EoS for CCS modelling. Our techniques allow a user to specify their requirements and rederive model parameters matched to their needs. We have also begun to go beyond parameter fitting and, instead, use algorithms that learn mathematical forms for EoS directly and automatically from experimental data, thus fully automating the derivation of EoS. Our new EoS rigorously account for uncertainties in both the modelling and the experiments. This will allow effective control and minimisation of risk in CCS processes, which will improve CCS regulation and safety.

We have fitted our models to literature data for carbon dioxide mixed with nitrogen, oxygen and argon. We have also begun fitting to newly emerging, high precision data on carbon dioxide-hydrogen mixtures from our colleagues in Chemistry at Nottingham. We have also begun working with experts in computational fluid dynamics to incorporate our EoS into modelling software for pipeline ruptures.

We have incorporated our algorithms into computer software with a user-friendly interface. The software enables users to rapidly produce bespoke EoS, tailored to their particular application. It will also enable these models to continually evolve as new measurements become available, ensuring that experimental advances are rapidly converted into improved CCS modelling and, ultimately, better performance and efficiency of real CCS processes. During the project we held a demonstration workshop to train potential industrial and academic users on the use of our software.

Ultimately, our tools will make it straightforward for users to continually and rapidly update their EoS in response to emerging experimental data. The resulting EoS will deliver greater range, certainty and computational efficiency in a wide range of CCS modelling. These EoS will contribute to more effective optimisation of CCS, making the process cleaner, safer more efficient and more economical. Our modelling will also be essential to design codes and safety regulations and will provide key modelling support to the urgently required public engagement and consultation on pipeline transport of CO₂.