Myth Busters: What's so Super About Supercritical CO2?

While the Latin root for the word “super” simply translates to “above,” for those of us who grew up with comics, movies, and cartoons of Superman, the word raises our adrenaline and we become “super” excited. Thus, it is no surprise when we talk about supercritical fluids that the average turbomachinery engineer starts to hyperventilate. As engineers, we also love critical things—albeit it is not totally clear why. There are critical speeds, critical loads, critical Reynolds numbers, etc., and they all separate the normal from the abyss. They also often describe situations that are difficult to calculate and interesting to investigate, which may make them cool … and super is even better than critical.

Although super is often used in its original sense—above or beyond—supercritical is not in the abyss but on the other side. A supercritical speed for a rotor is a speed that is (usually) higher than the first critical speed—we learned how to operate at supercritical speeds about 150 years ago … ergo, nothing to worry about. Now, a load beyond the critical load may cause something to break or buckle, so sometimes bad things do happen beyond critical.

SO, WHAT ABOUT THIS sCO₂ THAT EVERYONE IS TALKING ABOUT?

Any gas or gas mixture has a critical point at a critical pressure and temperature. This can be seen on a phase diagram where the fluid’s gas phase line intersects its liquid phase line at the highest gas temperature. For CO₂, the critical point is at about 30° C and 74 bar pressure. Water’s critical point, on the other hand, is at about 374° C and 220 bar; Methane is at -83° C and 46 bar. The critical pressure and temperature for CO₂, unlike many other substances, are well within pressures and temperatures often encountered in typical gas compression applications, such as heat pumps, carbon sequestration, and sCO₂ Rankine power cycles. For any substance, if temperature and pressure exceed the critical point, distinct liquid and gas phases cease existence. (Note: This is only true as long as the pressure stays below solid-forming pressure, which for CO₂ is somewhere in the range of several thousands of bar, and for all practical purposes not relevant for this discussion.) A good example is the atmosphere of Venus, which is 96.5% CO₂ and 3.5% nitrogen. There the surface pressure is 93 bar, and the surface temperature is 462° C, which is well above the critical points of both major constituents, thus making the surface atmosphere of Venus a supercritical fluid.

Above the critical point, fluids are in their dense phase, characterized by high density (comparable to a liquid) but maintain the low viscosity of a gas. In the dense phase, the fluid does not form a free surface as a liquid would, but the fluid is still somewhat compressible— although the compressibility, i.e., density change with pressure, is relatively low. This is quite advantageous for transport purposes in pipelines since longer-distance transport is possible with a smaller power demand for compressors or pumps than transport as a gas. Thus, most long-distance transport of CO₂ for carbon sequestration will likely be performed under dense-phase conditions but will require pipelines rated for 150 bar (2,200 psi) to reliably keep the fluid in the dense phase. It is important to understand that dense-phase conditions do not imply that the fluid is completely incompressible, so depending on pressure and temperature, either pumps or compressors can be used for pipeline transport or injection.

THE SCARY CRITICAL POINT OF FLUIDS

However, for the turbomachinery designer, the critical point of any fluid itself is considered pure unadulterated evil. It is like the juvenile delinquent that lives on your street who you keep your teenage daughter as far away from as possible. We try to avoid operating near the critical point at all costs. The reason is that most physical properties of a fluid, such as density, speed of sound, viscosity, specific heat, etc., change drastically near the critical point, i.e., physical properties have steep gradients with respect to pressure and temperature changes. This makes it nearly impossible to accurately design a compressor or expander for stable and predictable operation near the critical point. Thus, when designing a turbomachine, we usually select operating points at pressures and temperatures far, far away from a fluid’s critical point.

As previously noted, CO₂ is supercritical at pressures and temperatures above the critical point, about 30° C and 74 bar. This is well within the normal experience range for turbomachinery. There are interesting things happening in this region. When CO₂ is kept above its critical temperature and pressure, it acts like a gas yet has the density of a liquid. In this supercritical state, small changes in temperature or pressure cause dramatic shifts in density, making sCO₂ an attractive working fluid to generate power. The high density allows for machines with an extremely high-power density. Since CO₂ is readily available, non-flammable, and non-toxic, it lends itself as a working fluid both for power cycles and high-efficiency heat pumps. Closed-loop Brayton cycles using sCO₂ allow for increased efficiency and a substantially smaller footprint (both physically and environmentally) compared to air Brayton and steam Rankine cycles. This also applies to bottoming cycles for gas turbine waste-heat recovery. Additionally, because the heater and cooler do not involve evaporation or condensation, the temperature differences between the process fluid and heating/ cooling fluids are minimized, which decreases losses in the heat exchangers and improves achievable cycle efficiencies.

So, there are features that make sCO₂ very attractive for several applications, and indeed many organizations are building heat pumps, power cycles, bottoming cycles, and pipelines that take advantage of the behavior of sCO₂.

Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is a recent retiree from Manager of Gas Compressor Engineering at Solar Turbines Inc. in San Diego, CA. He is an ASME Fellow and has published over 200 articles and papers in the turbomachinery field.