In the Research Centre for Gas Innovation (RCGI), scientists develop two innovative methods to physically separate carbon dioxide (CO2) from methane (CH4).

The main component of natural gas is methane. The remainder is composed of other hydrocarbons – such as ethane, propane or butane – or even gases, such as CO2. It is generally assumed that methane accounts for something about 80% of the gas, but this composition varies widely according to the source from which it has been obtained. There are wells in which the CO2 content is 50%, including pre-salt wells, which requires “cleaning” the gas for using it, since the component of interest is CH4. Furthermore, the world signed an agreement for emission reduction efforts in Paris, in the latest Conference of the Parties, which makes the separation processes of the CO2 contained in natural gas even more relevant, along with its management.

One of the ways of dealing with this problem is to physically separate CO2 from natural gas. A group of scientists from the Research Centre for Gas Innovation (RCGI), led by Professor José Carlos Mierzwa, from the Escola Politécnica of the USP, studies two devices for this end: a ceramic membrane and a supersonic separator, in which gas is injected at high pressure and develops speed close to that of sound before the CO2 removal.

“In the case of the membranes, we are talking about a molecular-scale filtering. It is a kind of molecular sieve, being that the membranes have a diameter that allows CO2 to go through, but not the other components present in natural gas. To produce a separation module, different membranes are arranged as a bundle in the centre of which a steel tube is inserted; at the top of it, there is a CO2 outlet. The ‘clean’ gas, leaves at the other end,” explains Mierzwa.

According to him, this technology has already been tested by large companies in the oil and gas sector, including Shell, yet most of them use polymer membranes. “We chose to work with ceramic membranes because they allow operating at higher temperatures with a smaller risk of degradation. Among the materials we know from the researches being conducted, ceramics are the ones with a greater separation capacity and greater efficiency to this end.”

Although the technology is already being studied, it has not yet been developed for practical applications. Hence, some first order issues shall be addressed by the group – which also includes two other professors, a post-doc researcher and three scientific initiation students.

“The first question is: how much CO2 will we manage to separate per area of membrane, in a given unit of time? This answer, correlated to a previous analysis of the gas meant to be purified, by which one knows the percentage of CO2 it contains, will allow knowing the number of modules necessary to filter the gas at different levels of occurrence. We already know that the more CO2 the gas contains, the greater the membrane area necessary to filter it will be. But we have to experimentally determine the separation capacity of these membranes. In some cases, a larger number of stages may be necessary to thoroughly clean the gas, that is, making it undergo different modules.”

At the end of the five-year duration of the project, Mierzwa and his team shall deliver the prototype of a separation module, tested and optimised.

Supersonic separator – Another way envisioned by the professor and his team is to separate CO2 using the gases thermodynamic properties, by means of a device in which carbon dioxide is condensed.

“When the gas expands, it cools down. Thus, if we have a great difference in pressure between the two sides of the equipment, the gas temperature is going to greatly fall when going through. The aim here is to promote a variation in the gas pressure that allows reaching a temperature at which only the CO2 is condensed, seeing that its condensation temperature is higher than that of methane and of other gases that might be present in the composition. We can then separate it.”

For this, the scientists have designed a tubular-shaped separator, with the inlet a little narrower than the outlet and a small narrowing along the way, which makes the gas lose pressure energy and gain speed energy, expanding and causing a fall in temperature – and the consequent CO2 condensation. Mierzwa says that the idea is to manipulate these variables, in the light of the thermodynamic properties of the gases, until the point of the CO2 condensation temperature is found. The gas must enter the separator at a pressure a hundred times as high as that of air and the process will occur at a speed close to that of sound. “It will have an initial 14°C temperature and leave the separator at something about – 80°C to – 40° C. We are still going to simulate these values in the computer before we start building a prototype.”

The professor explains that the challenge, in this case, is to remove the liquid CO2 from it. “We first have to think of an outlet. We are envisioning a thermodynamic purger, the condensed CO2 removal of which will occur whenever the volume of the liquid pushes a movable device upwards, the floater, which will be closing the outlet. After that, we are talking of a process conducted at an extremely high speed, during which there is a reduction in pressure and soon afterwards a recovery of pressure, in the very system. I have to remove the gas at the time interval at which the pressure is reduced, which is accounted for in nanoseconds. Our idea is that the gas enters the device already at a rotational movement, thus allowing the centrifugal force to move the liquid CO2 to the equipment walls, where it is easier to collect it.”

At the end of the project, the group of engineers shall deliver a fluid dynamic modelling, jointly developed with the members of another RCGI project, a prototype of the separator and also of the whole separation system, which includes a gas compressor and a purifier. This is because, both in the case of the separator and that of the membranes, before the separation, the gas will have to undergo a pre-treatment process for removing water and H2S (sulphydric gas). “It is necessary to remove the moisture and acid gases that hinder the processes and may deteriorate the equipment.”

Mierzwa stresses that this project has an interface with several others in the RCGI portfolio, which adds to 29. “There is a project on biomethane, coordinated by Professor Suani Teixeira Coelho, which this separation technology shall contribute with. She has already manifested her interest.”