You would think that measurements of mass flow would be expressed in units of mass, such as grams/hour, milligrams/second etc. Most users, however, think and work in units of volume. That’s OK, at least when we are talking about the same reference conditions. Let me start with an example:
Mass versus Volume
Imagine you have a cylinder of 1 liter, which is closed by means of a moveable piston of negligible weight. This cylinder contains 1 liter of air at ambient pressure, approximately 1 bar. The weight of this volume of air at 0°C is 1.293 g, this is the mass.
When we move the piston half way to the bottom of the cylinder, then the contained volume of air is only ½ liter, the pressure is approximately 2 bar, but the mass hasn’t been changed, 1.293 g; nothing has been added, or left out.
According to this example, mass flow should actually be expressed in units of weight such as g/h and mg/s. Many users, however, think and work in units of volume. This not a problem, provided conditions under which the mass is converted to volume are agreed upon.
Using density in converting mass flow to volume flow
In order to use density in converting mass flow to volumetric flow, we must pick a set of specific pressure and temperature conditions at which we use the density value for the gas.
The conditions that are agreed upon contain various references, normal reference and standard reference, available in European or American style. What is de difference?
Normal reference, European style
Following the ‘European’ definition, a temperature of 0°C and a pressure of 1,013 bar are selected as ‘normal’ reference conditions, indicated by the underlying letter “n” in the unit of volume used (mln/min or m3n/h). The direct thermal mass flow measurement method is always based on these reference conditions unless otherwise requested.
An example conversion to volumetric units using Normal reference conditions:
The mass flow meter indicates 100 g/h of Air flow.
• Density Air (@ 0°C) = 1.293 kg/m3
• X ln/m Air = 100 g/h / (60 minutes x 1.293 kg/m3)
• Flow = 1.29 ln/m Air
Standard reference, European style
Alternatively, a temperature of 20°C and a pressure of 1,013 bar are used to refer to ‘standard’ reference conditions, indicated by the underlying letter ‘s’ in the unit of volume used (mls/min or m3s/h).
An example conversion to volumetric units using Standard conditions:
The mass flow meter indicates 100 g/h Air flow.
• Density Air (@ 20°C): 1.205 kg/m3
• X ls/m Air = 100 g/h / (60 minutes x 1.205 kg/m3)
• Flow = 1.38 ls/m Air
If the prefix ‘s’ has been used, it refers to the American style.
Standard reference, American style
According to the ‘American’ definition the prefix ‘s’ in sccm, slm or scfh refers to ‘standard’ conditions, 101.325 kPa absolute (14.6959 psia) and temperature of 0°C (32°F).
Please be aware of the reference conditions when ordering an instrument. ‘Normal’ and ‘Standard’ can be relative to each customer.
Why is this important? Because mixing up these reference conditions causes an offset in what the customer expects to see by greater than 7%!
In our daily life we use plastics or polymers in many different forms whether as a disposable product such as packaging film or as a long-lasting component in the automotive industry, in construction or in sports equipment and toys.
Nowadays, plastics are tailor-made for the respective application, depending on the properties desired. In this way, properties such as hardness, mold ability (or formability), elasticity, tensile strength, temperature, radiation and heat resistance can be adjusted as well as the chemical and physical resistance can be adapted to the desired function.
This large variety can be modified within wide limits by the choice of the basic building blocks (macromolecules), the production process and additives. The respective macromolecules are polymers of regularly repeating molecular units. The type of crosslinking and the used additives determine the final properties of the material. In 2016, the world-wide production of plastics for bulk materials and films was over 300 million tons (source: BMBF) of which almost one third was produced in China. Europe and North America follow with slightly less than 20 percent each.
Precise dosing for operational efficiency and minimization of unnecessary waste
Typical additives in the plastics industry are antistatic agents, dyes, flame retardants, fillers, lubricants, colorants, stabilizers and plasticizers. Many of these additives are liquid.
Precise dosing of the additives leads to operational efficiency and the minimization of unnecessary waste.
Additives are frequently added by use of needle valves, which is inexpensive, but always has a risk on malfunction because of fluctuation within the process (e.g., pressure and temperature). In particular the use of plasticizers is increasingly critical since some of these substances are directly absorbed by human beings or accumulate in the food chain.
With the proven CORI-FILL dosing technology, Bronkhorst offers an easy-to-use setup to ensure the required accuracy and reproducibility. By combining a mini CORI -FLOW with a pump or a suitable valve, fluids can be dosed continuously or as a batch into the reactor with high reproducibility. These systems can be integrated or used as an add-on in already existing processes and production lines.
mini CORI-FLOW flow meter combined with a Tuthill pump
5 Reasons why additive dosing with a Coriolis instrument supports process efficiency for plastic manufacturers
No need for (re)calibration in the field – fluid independent flow measurement and control
Gas and liquid can be measured with the same sensor
Ability to measure undefined or variable mixture
The CORI-FILL™ technology features an integrated batch counter function and enables direct control shut-off valves or pumps
The first variable area (VA) meter with rotating float was invented by Karl Kueppers in Aachen in 1908. The device was patented in Germany that same year. Felix Meyer was among the first to recognize the significance of Kueppers’ work and implemented the process for offering the meter for sale. In 1909, the firm of "Deutsche Rotawerke GmbH" was created in Aachen (Germany). They improved this invention with new shapes of the float and of the glass tube. It didn’t take long for the new device to capture attention in Europe, the United Kingdom, and other areas.
VA flow meters (or purge meters)
Over time, different types of VA flow meters (also called purge meters) have been developed, usually in response to some specific need. Nowadays a purge meter usually consists of a tapered tube, typically made of glass or plastic. Inside this tapered tube there is the ‘float’ which is made either from anodized aluminum or ceramic. The float is actually a shaped weight that is pushed up by the drag force of the flow and pulled down by gravity. The drag force for a given fluid and float cross section is a function of flow speed squared only.
While the meters are still relatively simplistic in design, relatively low cost, low maintenance and easy to install they are used in many kinds of application. Despite these facts, the traditional VA meter has a number of drawbacks. For instance, graduations on a given purge meter will only be accurate for a given substance at a given temperature and pressure. Either way, due to the direct flow indication, the resolution is relatively poor. Especially when they are built into a machine, reading might be hard. Moreover, the float must be read through the flowing medium, so you can imagine that some fluids may obscure the reading.
9 reasons why to use a thermal mass flow meter instead of a traditional purge meter
As for the current century, Bronkhorst has developed a thermal mass flow meter series (MASS-VIEW, as shown in picture 1) which is the digital high-tech alternative to the traditional VA flow meters. Thanks to today’s digital possibilities, many other advantages arise for many industrial processes and chemical plants.
MASS-VIEW flow meter in application
The MASS-VIEW flow meter series operate on the principle of direct thermal mass flow measurement (no by-pass); rather than measuring the volume flow it measures the actual mass flow, without the need of temperature and pressure correction.
The digital OLED display provides an easy direct or relative reading of the actual flow. Herewith parallax errors are excluded.
With this digital mass flow meter it is easily possible to obtain the accumulated flow. This availability of data gives insight in costs, leading to data driven decision making power.
In contrast to the traditional VA meter which need to be mounted in a vertical position, this digital alternative can be mounted in any position.
The flow path is made of sustainable aluminum rather than plastic or glass which is fragile.
The instruments are standard equipped with 0-5V, RS-232 and Modbus-RTU output signals. Note that the traditional VA meters usually do not have any output signal available at all.
As a standard feature, there are 2 built-in relays which indicate an alarm situation. Herewith, external devices can be controlled.
Multi Gas; as opposed to traditional VA meters, which are produced for one particular fluid only, the digital alternative has up to 10 pre-installed gases available as a standard feature.
Multi Range; traditional VA meters usually have a rangeability of 1:10 and one single full scale range only, the digital alternative has a rangeability of 1:100 as well as up to 4 pre-installed flow ranges.
Achieve a stable flow
A VA meter, whether it is a conventional or a digital one, can be equipped with a built-in needle valve. This needle valve enables the user to regulate the flow rate by means of a restriction inside the flow channel. As long as the inlet pressure is stable, the subsequent flow will be stable too. On the other hand, once pressure conditions are susceptible to change, the flow rate will become equally unstable. If this is not desirable, you’ll have to compensate these pressure fluctuations.
Manual control valve
This effect can be eliminated by using a manual control valve like the FLOW-CONTROL series which keeps the pressure drop across the needle valve (delta-P) constant. This is accomplished by a second (normally open) valve, though it is operated by a membrane this time. The operating principle is based on a balance that forms between the pre-pressure, back-pressure and the spring force on the mebrane. A change in the pressure conditions leads to a change of the equilibrium and thus a change in the valve position as well (as shown in the picture below).
Working principle of a pressure compensated control valve
Although Bronkhorsts’ pressure compensation technology is suitable for either gases and liquids, the nice thing about this is that both technologies, the digital VA meters and pressure compensation, lend themselves well to being built together. However, in that case it is applicable for gases only.
Learn more about the different models in the manual constant-flow control series
Traditionally, and in most cases we see, dosing- or metering pumps are believed to be accurate because the theory is that a known pump head displacement will move a known volume over a known time giving a known delivered volume. In practice however it will never achieve a high level of accuracy with deviations of 10-15% being normal. Inaccuracies like this are caused by many changing process conditions, such as:
Wear of components
These factors can each be the cause of an inaccuracy in the expected volume of displacement from a pump head movement. If you then multiple each of those factors you can realise quite large measuring errors that create both inaccuracy and inconsistency.
Please refer to our earlier blog about ‘High Accuracy’.
What can be done to improve the accuracy?
Option 1) Add a flow meter between the pump and the process
By adding a flow meter between the pump and the process, you can take information from the flow meter to adjust the speed of the pump. Traditionally, this would be managed with an analogue output signal, 4….20 mA or similar, from the flow meter into a separate PID controller that compares the real flow signal to the desired flow. Subsequently, the electronic controller can then adjust the speed of the pump to achieve the desired dose or flow.
Using this solution will mitigate the issues in the original solution, however it introduces more:
Slow flow signal due to signal filtering in the PID controller
Slow pump response due to extra control relay
Increased complexity with extra components
Time to achieve stable flow can be long
Additional price of meter and PID controller
Option 2) Direct mass flow measurement with a flow meter with built in PID control
Now we need to discuss the next possible solution, using a direct Mass Flow measurement device with built in PID control
that can drive a pump to achieve the desired dose or flow.
With this solution you do not need to include the pump in the control system, just give a set point demand to the mass flow meter and it will drive the pump to achieve the desired dose or flow. This solution will give you several advantages, such as:
Direct mass flow control of the flow
Mass flow dosing is independent of temperature and pressure, in contrast to the volumetric dosing when only a pump is used;
Accurate delivery mitigating normal pump issues
Alarm functionality of low flow
Preventative maintenance based on pump performance over time
Consistent flow measurement based on actual not assumed numbers
Coriolis mass flow meter in modular dosing system
These advantages can be utilised in many different industries:
Anywhere that liquid is dispensed into a container that will require quality assurance, and commonly the quality control test is carried out on a small percentage of the vials to ensure general compliance. If you use a mass flow meter to control the dose you can achieve 100% QC checking of your product with reduced human input.
If you need to dose additives, performance chemicals or mix liquids then the ability to control the flow of the additive and know what that flow is can be a huge advantage to the outcome of the application.
Pump control can offer accurate dosing solutions for house hold chemicals like detergents and cleaning products.
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 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.
What is SFE Process?
Jérémy Lagrue, director and founder of 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.”
Coriolis flow meter and SFE Process pump
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.”
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.”
In this blog, I would like to share an application of our flow instrumentation at one of our customers in which we needed to deal with high temperature and high pressure. This customer – an energy research organisation - investigates a catalysed chemical reaction of a mixture of hydrocarbon compounds.
Catalysts are being used to accelerate a chemical reaction without actually being consumed. So a small amount of catalyst is sufficient to obtain a large amount of reaction products.
Solid catalysts are often small, highly porous particles, with a large internal surface area in a small volume. This internal surface contains active sites on which the reaction takes place. Gaseous or liquid chemicals diffuse into the pores of these particles, and react at the catalytically active sites to reaction products that diffuse out of the particle. Often, these reactions occur at extreme process conditions.
What were the application requirements?
A simple and reliable solution had to be found to inject a liquid flow at a high pressure. This injection has to take place at 30 to 60 bars, and needs to result in a stable flow without pulsation. Furthermore, the liquid flow needs to be controlled accurately, and during the process it has to be known how much liquid actually has been injected.
Which solution did the customer choose?
The solution comprises a Coriolis mass flow meter that controls a HPLC piston pump at the inlet side of the reactor and an independently operating back pressure controller at the outlet side. The tested Coriolis mass flow meter (mini CORI-FLOW ML120) has proven to be a very stable and accurate mass flow meter. The WADose HPLC pump gives a very stable flow without pulsation. The combination of an HPLC pump and mass flow meter works as a mass flow controller. The control valve of the Coriolis mas flow meter is not necessary, as the pump is used as an actuator.
The pump can handle a liquid viscosity of max. 40 mPa.s at the upstream side. The maximum operating temperature is 70 °C. The temperature of the furnace that contains the reactor tube with small catalyst particles is much higher. The pressure at the reactor tube outlet has to remain at a high value. Beyond the outlet there is a cold trap for water or oil condensation, a back pressure controller with control valve that can handle pressure differences up to 400 bars and an exhaust to atmospheric pressure.
The pressure controller can handle gas and liquid in a very stable controlled flow. Especially at very small flow rates, this pressure controller has a much better control performance than a mechanical pressure reducer. The exhaust is used to remove gas that has been produced at the reaction.
The pump has three control modes: pressure, volumetric (only the speed of the piston is controlled) and mass flow. The latter is a special feature that can be offered, and is convenient from a chemist’s point of
view. As the flow can be controlled directly, the exact number of moles injected to the process is known.
Control and monitoring occurs via the digital interface. The mass flow measure and setpoint, density, temperature and counter value are visible via this single digital interface.
The success of this setup has been demonstrated by a recent order of three additional pumps.
For more details have a look at the application story 'Catalysis at high pressure'.