# Using Mass Flow for Chemical Reaction Control

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# Using Mass Flow for Chemical Reaction Control

Mass flow rate is the volumetric rate that is standardised to a specific set of temperatures, and pressure conditions so it can be used for the process control in chemical reactions, fermentation, etc.

There are specialised devices that measure and control mass flow rates of gas that goes through a device as it passes through a flow channel with temperature and pressure of the gas stream, and the information in combination with known gas properties is used to calculate a mass flow rate.

## How to use instantaneous temperature and pressure readings to calculate mass flow:

Every Alicat device is loaded with gas tables thar contain values from 3-dimensional gas properties data. These tables chart NIST-traceable gas compressibility and viscosity data against pressure and temperature across the entire usable range of the mass flow device. The mass flow rate shown on screen is the volumetric flow rate of the gas as if it was flowing at standard temperature and pressure (STP) conditions.

The simple steps are as follows:

• The device uses the actual temperature and pressure of the gas to calculate the instantaneous volumetric flow rate

• The device uses an algorithm to calculate what the flow rate would be if the gas were at 25°C and 1 atm

• The device outputs a standardised mass flow rate

## Standardised conditions

Regardless of whether flowing is at 23°C and 20 PSIA, or -10°C and 56 PSIA, or 42°C and 14 PSIA… the mass flow rate on display will always remain the same value. All measurements are converted to a standardised volumetric flow rate using STP conditions. Although these are adjustable, the default STP conditions are 25°C and 1 atm (14.6959 PSIA).

As an example, when using argon a standardised volumetric flow rate will be calculated for Argon at 23°C and 20 PSIA. However, at 25°C and 1 atm, argon has the following properties:

• Absolute viscosity: 226.23990

• Density: 1.63387

• Compressibility: 0.0003656

Despite what temperature or pressure you are flowing the gas, the value shown will always be as if the gas was flowing at 25°C and 1 atm.

Annual calibration of the instruments and controllers are vital to ensure that the algorithm is functioning correctly and the reference volume is correctly calibrated.

## Units of Mass Flow

Mass flow rate is a standardised volumetric flow rate therefore the units are characteristically expressed as such. Examples include SLPM (standard litres per minute), SCCM (standard cubic centimetres per minute), and SCFH (standard cubic feet per hour).

These are volumes per time, not units of mass. However, a mass flow rate can easily be converted to a true mass flow rate using the following equation:

True mass flow = (mass flow rate)(gas density at STP or NTP).

For this calculation all you need is the mass flow rate and the gas density.

Mass Flow instruments that output a true mass flow measurement directly are the Coriolis instruments from Alicat. The Coriolis mass flow measurement is the utilisation of the Coriolis effect. A tube is electromagnetically actuated and functions as the moving frame for reference. The fluid entering the device travels along the moving tube and experiences a small deflection from its intended path. The sensors measure the magnitude of the deflection as a vibrational phase shift between the various points in the tube. The deflection is dependent on the mass of the fluid which allows the Coriolis mass flow instruments to provide precise mass flow measurements, despite the fluid’s properties, composition, and temperature. A single temperature sensor is included to measure the temperature of the tube as the physical properties can change as a function of temperature.

## What is the Benefits of Using Mass Flow?

Due to the standardisation of a mass flow rate, it can confidently extract numbers of particles through a system without any temperature or pressure fluctuation concerns. It is typical to use mass flow devices for applications that demand high accuracy, precision measurement, and precise control of specific amounts of gas.

Once again using the Argon example, in a situation where you need to flow Argon through a mass flow controller at exactly 5 SLPM. When you begin flowing, the temperature and pressure of the gas are 23°C and 20 PSIA, then the furnace is turned on nearby and the temperature climbs up to 27°C. The device will always correct the values to STP conditions, 25°C and 1 atm, and the exact correct number of particles will be flowed. This is particularly important in mass balanced chemical reactions that may be controlled in a process.

There are a variety of benefits from measuring and controlling mass flow, some include bioreactor outgassing, fuel cell membrane testing, and natural gas monitoring.

## How Does Density Impact Mass Flow Rate?

The density of the gas that is being flowed has a direct impact on the mass flow rate. When it comes to laminar differential pressure devices, it is vital to know the exact composition of the flowing gas so that precise density corrections can be made.

Most differential pressure meters and controllers include 98+ pre-loaded gases that contain gas density information. One advantage of calculating and reporting at standard conditions is that it is simple to apply correction factors to the flow data if the wrong gas happens to be picked.

The measurement of mass flow can also depend on density. Coriolis mass flow meters and controllers don’t use known gas properties to calculate flow rates and as a result can measure fluid densities themselves. This means the Coriolis flow meters accurately output true mass flow rates, even when flowing process fluids of unknown composition.

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