New theory on dense gases and liquids could aid carbon capture

The transport of dense gases and liquids is becoming increasingly relevant in relation to carbon capture. Research published in The Journal of Chemical Physics is helping us understand more about how this can be done most efficiently.

Gas must be compressed before it can be transported over long distances. This can be done either by increasing the pressure of the gas or by converting it into a liquid. In order for this to occur safely and efficiently, we need to understand as much as possible about how the gas behaves before and during transportation.

Dense gases are affected by changes in pressure and temperature, and there has not been a fundamental theory for different dense gases and liquids until now.

“I am developing a theory to describe the transport properties of dense gases and liquids,” said Ph.D. research fellow Vegard Gjeldvik Jervell from the thermodynamics group at NTNU and the Porelab Centre of Excellence.

His supervisors are professors Øivind Wilhelmsen and Morten Hammer from the thermodynamics group at NTNU’s Department of Chemistry, both of whom are also affiliated with Porelab.

Actually, it is somewhat surprising that this research community has made such significant progress in describing the transport properties of dense gases and liquids. This task is very difficult, because it requires knowledge of how the molecules interact with each other under a wide range of conditions.

“For the past 50 years, experts in the field have claimed that developing a collision theory for liquids is impossible,” said Wilhelmsen.

That certainly isn’t the case—but why is it so beneficial to have a common theory that explains how gases behave during transportation?

“The basis for the existing methods relies on experiments, which can be both challenging and expensive,” explained Jervell.

The theory will become even more important when we decide to start capturing CO₂ from many different emission sources, as that would involve transportation on a massive scale.

Inaccurate models

Wilhelmsen was recently contacted by a gas transportation company, which led to an “aha” moment.

“The company wanted to understand how the gas behaved during transport. The software they had paid a lot of money for wasn’t very accurate, especially when mixtures of gas were involved,” explained Wilhelmsen.

The new theory reduces the need for expensive experimental work.

Wilhelmsen realized that many of the answers the company was looking for could be provided by the thermodynamics group, because they already had the theoretical foundation in place and would soon also have the necessary tools. This meant the research group could do the work in a much simpler way than the company could achieve on its own.

“In some cases, the model even provides more precise answers than can be achieved through experiments,” said Hammer.

The model will not completely replace the laboratory, of course, but the researchers know in what areas the model excels and have a solid understanding of where additional experiments are needed.

“The theory is very accurate for dense mixtures of gas, an area where other models struggle. At the moment, however, it is not yet accurate enough for liquids at low temperatures,” explained Hammer.

Jervell has taken a thorough approach and investigated many dense mixtures of gas.

“We have built the theory from scratch. We started with the molecular interactions and developed the theory to the point where properties could be measured in the lab,” said Jervell.

Using the new theory, they can provide much more insight into the properties of various dense gases.

A versatile theory

“We can now predict with greater certainty what will happen under different conditions. Since the theory is built on a solid foundation, we can rely on it to provide accurate answers even in areas where experiments haven’t been done,” said Jervell.

This is especially important regarding mixtures of gas, because conducting experiments for all possible combinations is simply too time-consuming. The model can already provide insights into the viscosity of gases under various conditions, and also provides information about their thermal conductivity and diffusion rate.

“It is a truly versatile theory,” concluded Wilhelmsen.