Basics of CO2 flow measurement

Carbon Capture and Storage (CCS) is expected to play an important role in the energy transition. By means of installing an add-on capture installation, CCS can reduce the CO2 emission of large emitters, such as production of grey hydrogen and gas or coal fired power plant. Capturing CO2 can also reduce emissions of hard to abate processes such as waste incineration and the calcination process in cement production.

Since permanent storage of CO2 in depleted gas fields is done by specialist companies, there is a point where the ownership of CO2 is transferred from the company that captured the CO2 to the company that will store the CO2. At this point highly accurate measurement is required. While process measurement of CO2 is already done for years in for example the food & beverage and oil & gas industry, measuring CO2 with high accuracy is not completely straightforward. In this article some of the specific challenges are explained.      

Physics

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 a small pressure drop can results in a dramatic change in density, which impacts measurement performance.

At elevated pressures and temperatures 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 small changes in pressure and temperature cause large density fluctuations, which could make measurement difficult in case process conditions are not stable. 

Challenges for ultrasonic flowmeters

The ultrasonic principle can be used on single-phase gas, liquid, or supercritical phase. Transducer design and frequency must be adjusted to the phase of the medium because the propagation of the acoustic signal depends on the density. With pure CO2 the phase transition area is well defined, however if impurities are included this may not be the case. In that case, to ensure stable measurement, the condition should be far enough away from the transition area between two phases. Ultrasonic flowmeters for process measurement are available with one, two or three paths, also highly accurate multipath ultrasonic flowmeters are available.

A specific point of attention 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 in a different frequency range than the absorption peak to ensure performance over a wide pressure range. 

As there are limited possibilities to calibrate flowmeters on CO2, ultrasonic flowmeters are typically calibrated on water, air or natural gas. Where required a Reynolds-number based calibration can be done, so conditions during calibration will be similar to that in the field as much as possible. Using for example water at (~8x) higher flowrates gives similar Reynolds numbers to liquid CO2 at lower flowrates. Another possibility is increasing the water temperature to decrease viscosity. When necessary, the water-based Reynolds curve can be extrapolated like this is done for Custody Transfer LNG flowmeters that are being calibrated on water.

Challenges for Coriolis flowmeters

Coriolis meters provide high accuracy flow measurement in liquid, gas or supercritical phase. Most Coriolis meters are also able to continue measurement on a (multiphase) combination of phases and indicate when a shift from liquid to gas phase occurs. Albeit this might reduce the measurement accuracy. When measuring CO2, care should be taken to avoid large sudden density variations that can occur in process conditions close to the critical point and the gas-liquid phase conversion. Coriolis meters are normally calibrated on water. Offering a direct mass measurement, the meters are not affected by fluid properties or flow profiles.

Challenges for differential pressure flowmeters

DP flowmeters such as flow nozzles, Venturi tubes or orifice plates have been used to measure fluid flow for over 100 years. DP flow measurement is unaffected by the acoustic attenuation of CO2 and covers a wide range of applications in terms of temperature, pressure and flow range.

For example, performance testing of the world's largest CO2 compressors is carried out using calibrated venturi metering sections . This volumetric measurement principle requires additional inline pressure and temperature correction for mass flow measurements.  Additional process information about changes in the fluid state can be obtained by using an additional pressure recuperation tap with the appropriate energy recovery calculation. DP flowmeters can be calibrated with liquids or gases.

Challenges for vortex flowmeters

Vortex flowmeters can handle a wide range of applications – whether the medium is a conductive or non-conductive liquid, an industrial gas, liquid gas or any kind of steam.

The measurement of gaseous CO2 is challenging when it comes to measuring and outputting derived flows. Fluctuating process conditions while measuring compressible media affect the density, which is used to calculate normalized flow rates or mass flow rates.

Vortex flowmeters are equipped with an integrated temperature sensor and can have an integrated pressure sensor as an option. Hence, volumetric flow rate and process conditions are measured at one single point. Varying process conditions can be determined and the density can directly be compensated for temperature and pressure fluctuations.

Vortex flowmeters with integrated temperature and pressure compensation are used in the early stages of CCS, for example to precisely record the amount of gaseous CO2 after the separation process in cement production.

Further considerations

Captured CO2 can contain other gases such as N2 or O2. This can cause multiphase flow where CO2 is in liquid phase and 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, ultrasonic or differential pressure 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 and very large sizes.  

Measurement standards 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. The expertise provided by KROHNE can fill any of the gaps that are still not covered by standards. Also, for these cases KROHNE can work actively with local metrology offices such as NMi.