Supercritical fluid; How to control and measure flow?

July 23, 2020 Mickael Soobaroyen
Chemical industry

I’ve worked at Bronkhorst France since 10 years now and I must confess, the instrumentation career brings me to discover new applications in various markets even today. Markets like the chemical industry, environmental industry, research applications and so on, for which flow control and measurement solutions are often essential. The applications in which supercritical fluids are used, are often complex because of the fluids state.

It was during one of my visits that I met Jérémy Lagrue, director and founder of SFE Process. I discussed with him the use of Coriolis flow meters in supercritical CO2 processes.

What do we call 'supercritical fluid'?

As an example, supercritical carbon dioxide refers to carbon dioxide that is in a fluid state while also being at or above both its critical temperate and pressure, yielding rather uncommon properties. The density, viscosity and diffusivity of the fluid are then intermediate between those of the liquid phase and those of the gaseous phase.

Supercritical diagram
Supercritical diagram

Supercritical CO2 in extraction processes

Supercritical CO2 is, I believe, the most known supercritical. It is an important commercial and industrial solvent due to its role in chemical extraction in addition to its low toxicity and environmental impact.

Supercritical CO2 can be found in extraction processes, such as algae, oils, flavors and active principles. It is also used in splitting processes, such as drinks fermentation, deodorization of fatty substances in the field of cosmetics and purification processes for polymers.

This inert fluid is interesting because it reaches its supercritical phase at a relatively low pressure (73.8 bara) and a low temperature (31.1 °C.). The relatively low temperature of the process and the stability of CO2 also allows most compounds to be extracted with little damage or denaturing.

Carbon dioxide usually behaves as a gas at standard temperature and pressure (STP) or as a solid called dry ice when frozen. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, it behaves as a supercritical fluid above its critical temperature (31.1°C) and critical pressure expanding to fill its container like a gas but with a density like that of a liquid.

Besides, CO2 offers the advantage of being odorless, non-toxic and non-flammable. It does not alter the product to be extracted or purified.
For environmental reasons, more and more industries tend to use supercritical CO2 in their process because it is positioned as an alternative to organic solvents. Indeed, unlike solvents that are produced from petroleum, CO2 is naturally available and abundant, it is therefore less expensive. However, there are very few solutions to implement it because installations remain expensive.

Jérémy Lagrue, director and founder of SFE Process
Jérémy Lagrue, director and founder of SFE Process

What is SFE Process?

Jérémy Lagrue: “At SFE Process, we’re dealing with applications in high pressure equipment and accessories. The specialty of SFE Process, is the production of special machines or devices for supercritical fluids (like CO2). We supply also consulting, metrology advice, maintenance, and training. SFE has developed an innovative design of high pressure pumps for processes with supercritical fluids, used either to compress liquid CO2 up to 1000 bar or for supercritical recirculation.”

Which problem did SFE want to solve?

“Our customers that are active in the chemical market, such as bio technology or pharmaceutical market, want to inject CO2 into a process of molecule separation or fraction. The goal here is separation of the molecules. To realize this separation, special equipment is necessary. SFE Process manufactures this type of equipment, moreover, they manufacture the pump to generate the flow of this particular fluid. The most important requirements of these pumps are stability, repeatability and accuracy.”

Bronkhorst Coriolis flow meter and SFE Process pump
Bronkhorst Coriolis flow meter and SFE Process pump

Which solution did SFE Process choose?

“I wanted to offer my customers the possibility to establish their mass balance in these chemical processes. Since I’ve worked a long time with many industries and laboratories, I know the importance of the flow parameter in order to determine the efficiency of the process, its production cost, its yield and to make the transition from laboratory scale to industrial scale.”

SFE Process has good experience in supercritical CO2 but they needed to prove the reliability of the equipment and also guaranty that CO2 injection is highly accurate and repeatable.

“The problem was to find an accurate and reliable flow meter capable of guaranteeing the veracity of the results and of course that lends itself perfectly to the use of supercritical CO2. I chose the Coriolis flow meter offered by Bronkhorst. In addition to its design, the reputation of this flow meter and the 3 year manufacturer warranty had influenced my decision making and the tests carried out with this flow meter met my expectations.”

What are the results of this solution?

“The improvement that I experience is that final customers can be sure of the quantity of the fluid they put in their process. SFE Process can justify the good accuracy and repeatability of the pump by way of flow measurement of the Coriolis flow meter. So accuracy has improved.
I’ve integrated the Coriolis flow meter into all the equipment that I offer to users with the fundamental need to build up their mass balance and refer to a reliable flow value.”

Have a look at our video explaining the principle of operation of our Coriolis products