Introduction
Two important pillars of the energy transition are the use of hydrogen as energy carrier and the capture and storage of CO2 (CCS: Carbon Capture and Storage). While process measurement of hydrogen is already done for years in for example the (petro-)chemical industry, and measurement of CO2 in the food & beverage industry, it still brings some specific challenges to measure with high accuracy. In this article we give further background about the technical challenges that KROHNE Ultrasonic and Coriolis flowmeters faced on hydrogen and CO2.

Phase diagram
Like most media, CO2 comes in 4 phases: solid, gas, liquid and supercritical. With a critical point close to typical operational conditions, care should be taken regarding phase changes. As an example, CO2 at 58 bar (840 psi) and 20°C (68°F) is a liquid with a density of 780 kg/m3 (49 lbs/cft), reducing the pressure to 57 bar (825 psi) means it is a gas of 190 kg/m3 (12 lbs/cft). This means that a small pressure drop can results in a dramatic change in density, which impacts measurement performance. At elevated pressure and temperature CO2 is in a supercritical phase. While supercritical sounds somewhat elusive, it is something we see in other application as well. For example, methane has its critical point at 46 bar (670 psi) and -83°C (-117°F), meaning typical 60 bar (870 psi) ambient temperature methane applications are also in the supercritical phase. Close to the critical point large density fluctuations are seen, which could make measurement difficult in case process conditions are not stable. 

Hydrogen has its critical point at 13 bar (188 psi) and -240°C (-400°F), as a result it can only become a liquid when cooled significantly. At its critical temperature of -240°C (-400°F) hydrogen needs at least 13 bar (188 psi) to become a liquid, under ambient pressure hydrogen needs to be cooled to -253°C
(-423°F) to liquefy.

Challenges for ultrasonic flowmeters
The ultrasonic principle can be used on single-phase gas, liquid, or supercritical phase, where each phase requires a dedicated meter set-up. A specific challenge for CO2 is the molecular thermal relaxation effect, causing the CO2 molecule to ‘absorb’ the ultrasonic sound signal. The phenomenon is not unique to CO2, however for CO2 the absorption peak is in the frequency range that manufacturers typically use for their ultrasonic transducers. As the frequency of the absorption peak is pressure dependent, manufacturers will select an ultrasonic transducer frequency that is sufficiently separated from the frequency of the absorption peak when sizing a meter. This will ensure performance over a wide pressure range.    

For hydrogen applications the low density and high speed of sound must be considered. The low density of hydrogen makes it more difficult for the ultrasonic signal to enter the medium and to reach the receiving transducer. To manage this challenge manufacturers will select the optimal ultrasonic transducer frequency to maximize acoustic performance. The high speed of sound in hydrogen results in very short transit times of the ultrasonic signal between the transducers. Consequently, the receiving transducer has to be ready in time to receive the signal from the sending transducer.

As there are no large-scale CO2 or hydrogen flow calibration facilities, ultrasonic flowmeters are typically calibrated on water, air or natural gas. Where required a Reynolds-number based calibration can be done, so flow profiles during calibration will be similar to that in the field. Using for example water at 7x higher flowrates gives similar Reynolds numbers to liquid CO2. When necessary, the water-based Reynolds curve can be extrapolated similar to how this is done for LNG flowmeters.

Challenges for Coriolis flowmeters
Coriolis meters provide high accuracy flow measurement on single phase fluids, which can be either in liquid, gas or supercritical phase. Also they continue to measure on multiphase flow. When measuring CO2, care should be taken to avoid large sudden density variations that can occur in process conditions close to the critical point. In case of gas measurement, especially with low density hydrogen, care should be taken that the meter minimum density requirement is met to ensure performance of the flowmeter. In practice this means that a minimum pressure is required. Coriolis meters are normally calibrated on water. Offering a direct mass measurement, the meters are not affected by fluid properties or flow profiles.

Further considerations
Captured CO2 can contain other gasses such as N2 or O2. Close to the phase transition line, this can cause two-phase flow where CO2 is in liquid phase and the other elements are in gas phase. Also, free water in CO2 needs careful attention as this can result in carbonic acid corrosion (Fe + CO2+ H2O -> FeCO3 + H2).

Whether a coriolis or ultrasonic flowmeter is the best option for your application depends on the application requirements. Coriolis meters offer a direct mass measurement and do not require straight inlet piping. Ultrasonic flowmeters offer a negligible low pressure drop and are available in full-bore very large sizes.  

Measurement standard have not fully caught up with the energy transition yet, gaseous CO2 is for example not covered by the European MID MI-002 as it is not a combustible gas. Also approved tables for pressure and temperature correction and conversion from volume to mass are not always available. For these cases KROHNE works actively with local metrology offices such as NMi to find a solution.