Supercritical carbon dioxide exhibits anomalous behavior in the vicinity above the critical point. The Irish physical chemist Thomas Andrews (1863) was the first who studied the supercritical behavior of carbon dioxide. He explained his observations of the fluid state above the critical point as follows: “…the surface of demarcation between the liquid and gas became fainter, lost its curvature, and at last disappeared, the tube being then filled with a fluid which, from its optical and other properties, appeared to be perfectly homogeneous [T. Andrews, Jan. 1870]”. 

Unlike what was perceived by Andrews, thermodynamic properties of a supercritical fluid are not homogeneous as confirmed by inelastic X-ray scattering measurements and molecular dynamics simulations, so that liquid- or gas-like regions can be further extended beyond the critical point to the supercritical region. As shown in Figure 1, the transition from gas-like to liquid-like in a supercritical fluid occurs during passing through the coexistence frontier in a so-called “Widom region,”[1] which was first identified experimentally by Nishikawa and Tanaka.[2] 

Another salient feature of supercritical CO2 is that its response functions (i.e., change of isobaric heat capacity, to change in temperature or pressure) increase sharply in magnitude in the neighborhood above the critical point. As shown in Figure 2, the isobaric heat capacity of pure CO2 at 79.25 bara and 35 °C changes by approximately 29.6% as pressure increases by only 0.5 bar.[3] 

The carbon dioxide compression process that involves compressing CO2 in the gas phase to the supercritical phase is ‘transcritical,’ for example, with subcritical low-side and supercritical high-side pressure. Similarly, transcritical CO2 pumping involves pumping subcooled liquid CO2 to high-pressure dense fluid CO2.

As shown in Figure 3, three different pathways can be considered for a transcritical carbon dioxide compression. The choice of these different pathways impacts on the selection of compression equipment, plant layout, operational flexibilities, control methods and the overall efficiency of the transcritical CO2 compression process. 

The Pathway A: Transcritical Compression Only

The Pathway A includes multiple stages of compression, from a low pressure in the gas phase to a high pressure in the supercritical phase and followed by the supercritical after-cooling to dense fluid conditions. This method of transcritical CO2 compression can be achieved using inline (between bearing) multistage centrifugal compressors, integrally geared centrifugal compressors or reciprocating compressors. The heat of compression in the gas phase is removed using interstage coolers.

The maximum flow rate of a low speed, lubricated reciprocating compressor in a transcritical CO2 compression service is limited to approximately 270 am3/min [9,500 ACFM] with two manifolded cylinders at the inlet. Centrifugal compressors may be utilized at higher flowrates. The Pathway A in Figure 3 is based on using an 8-stage integrally geared centrifugal compressor that elevates CO2 pressure from 1 bara to 150 bara. Five intercoolers are used in the subcritical region to keep the inlet temperature of the first six stages at 40° C. An aftercooler is used to cool down the supercritical CO2 at 150 bara and 165° C to the final dense fluid temperature at 40° C. 

The Pathway B: Transcritical Compression and Dense Fluid Pumping

The Pathway B utilizes a similar process as The Pathway A does until the CO2 pressure is elevated above the critical point; then the supercritical CO2 is cooled down to a dense fluid state and is pumped to the final pressure using a multistage centrifugal pump. 

Thermophysical and transport properties of CO2 abruptly change at thermodynamic conditions slightly above the critical point. Therefore, a proper margin must always be maintained from the critical point in the supercritical region.  

The Pathway B in Figure 3 is based on using the same first seven stages of the integrally geared compressor of The Pathway A to reach the discharge pressure of 105 bara. After cooling down the supercritical CO2 to 40 °C, a multistage centrifugal pump is used to increase the pressure of dense CO2 fluid to 150 bara. Over 50% saving in the compression work of the last stage can be achieved by replacing the last supercritical stage of the integrally geared compressor in The Pathway A with a multistage centrifugal pump in The Pathway B, which accounts for approximately 4% of the overall compression work in this example case. 

A large differential pressure (e.g., >80 bar) for the pump is usually required to achieve a meaningful saving in the overall compression power. However, a large differential pressure in the CO2 dense phase results in a high change of density (i.e., large compressibility) and impacts on the hydraulics and selection of dense fluid CO2 pumps. The selection of pump inlet conditions, which defines the inlet density, impacts on the hydraulic performance and rotor-dynamic behavior of CO2 pumps in The Pathway B applications. 

The Pathway C: Subcritical Compression, Liquefaction and Transcritical Pumping

The Pathway C involves subcritical CO2 compression in the gas phase followed by CO2 liquefaction and then pumping subcooled liquid CO2 to a final pressure as a dense fluid. The CO2 liquefaction can be performed either by employing an external refrigeration process using ammonia or light hydrocarbons as refrigerants or by an internal refrigeration process using the CO2 feed as the refrigerant. Unlike The Pathway B, centrifugal pump hydraulics design for The Pathway C is very similar to that of conventional centrifugal pumps, as pumping of subcooled liquid CO2 follows a quasi-isochoric process. For instance, the change of density in the liquid phase of CO2 from 42 bara at -20 °C to 150 bara at -15 °C is approximately 0.5%. 

The Pathway C shown in Figure 3 is based on using the same first five stages of the integrally geared compressor of The Pathway A to reach the subcritical pressure of 42 bara. Using a liquefaction system, CO2 is liquefied and then subcooled liquid CO2 is pumped using a multistage centrifugal pump to the final discharge pressure of 150 bara as a dense fluid. Over 70% savings in the compression work can be achieved by replacing the last three transcritical stages of the integrally geared compressor in The Pathway A with a multistage centrifugal pump in The Pathway C, which accounts for approximately 19% of the overall compression work in this example case. To evaluate the overall energy consumption of The Pathway C, the energy consumption for the CO2 liquefaction process must be added to the compression work. CO2 liquefaction is an energy-intensive process, and lowering energy consumption is a key factor in system design. 

Matt Taher is a Bechtel distinguished technical specialist, who works as a turbomachinery advisor for the LNG technology center of Bechtel Energy Inc. in Houston, Texas. His experience covers various applications and types of turbomachinery in carbon Capture and LNG processing operations. Mr. Taher is a registered professional engineer in the state of Texas and is a Fellow of the American Society of Mechanical Engineers.

REFERENCES

[1] Widom, B., 1972, “Phase Transitions and Critical Phenomena,, Vol. 2 (eds Domb, C. & Green, M. S.).

[2] Nishikawa, K., Tanaka, I., 1995, “Correlation lengths and density fluctuations in supercritical states of carbon dioxide.” Chem. Phys. Lett. 1995, 244, 149-152.

[3] Taher, M., 2022. “Carbon Capture: CO2 Compression Challenges and Design Options,” Proceedings of the ASME Turbo Expo 2022, Rotterdam, the Netherlands, Paper number: GT2022-82209.

Images: Figure 1: Phase diagram of carbon dioxide above and in the vicinity of the critical point showing the dense fluid, liquid-gas-like supercritical and the transitional (Widom) region. Figure 2: Sensitivity of isobaric heat capacity, cp is examined for 0.5 bar change of pressure at a constant temperature of 35 °C across a range of pressures above the critical point of pure CO2 [3]. Figure 3: Three possible transcritical CO2 compression pathways from 1 bara to 150 bara.[3] Credit: Matt Taher, P.E.