MMSCFD – Molar Flowrate Or Volumetric ... - Interesting Thing To Think

Yesterday, one of my ex-co-worker asked me regarding MMSCFD. Or I’d say he wanted to discuss rather than asked. We are actually kind of newbie in Chemical & Process Engineering, despite our 3-5 years of experience. (For those who do not know, MMSCFD stands for Million Standard Cubic Feet per day. It is a molar flowrate unit for gas. For simple explanations, you may check wikipedia and Dictionary  from University of California).

The thing he confused was how could Volumetric Flow was used for Molar Flow Rate. Molar flow is molar flow, volumetric flow is volumetric flow. How could those two units diffuse and confuse some people?

Well, the confusing part is Standard Cubic Feet (SCF). It is like volume unit. And in fact, it is. But, with a big Red Note: at certain Pressure and Temperature. Omit word “standard” from SCF and you will fall to plain Cubic Feet, which is volume unit without any note.

How in the world is volumetric unit used for molar unit? Those two qualities are different! Well, for ideal gas, it is not that different. According to Avogadro’s Law, any given samples of ideal gas, at the same temperature, pressure and volume, contain the same number of molecules. Thus, the number of molecules or atoms in a specific volume of gas is independent of their size or the molar mass of the gas (Reference). As the number of molecules directly shows mole quantity of component, hence at the same temperature and Pressure, any ideal gases at the same volume contain the same amount of mole quantity.

To give a simple example, at 0 oC temperature and 1 atm pressure, one m3 of ideal gas shall consist of approximately 44.615 mole of molecule. No matter what the gas is, as long as it is ideal gas, it shall contain 44.615 mole of molecule. Whether it is Nitrogen, Hydrogen, Helium, or any ideal gas at 0 oC and 1 atm, they all shall consist of 44.615 mole of molecule.

And the reverse is also correct. It means, at 0 oC and 1 atm 44.615 mole of ideal gas shall be occupying 1 m3 of volume. Linier extrapolation is also correct here. 1 mole of ideal gas shall occupy approximately 0.022414 m3 of volume, or equals to 22.414 Liter of volume.

Hence, for ideal gas, molar unit and volume unit are interchangeable. But there is a big note here: Temperature and Pressure shall also be informed when volume unit is used to depict molar quantity. And that is what Standard in Standard Cubic Feet for. That is also what Normal in Normal Meter Cubic for. The term Standard and Normal show certain Pressure and Temperature. But what Pressure and Temperature are Standard and Normal term pointing at? Unfortunately, it is not conclusive. There are some – if not broad – definitions of standard condition might be pointed by term Standard & Normal. The following table is taken from Wikipedia, showing broad definition of Standard Pressure & Temperature.

Well, I don’t find any exact reference, but for my daily work at my office, I am conveniently using definition Standard as 1 atm and 60 oF, while Normal as 1 atm and 0 oC. But every engineer is permitted using any definition, as long as he use the commonly used definition and noting the standard / normal condition he/she points at.

For British unit, the term Standard is commonly used. The combination commonly used is  14.696 psia or 14.73 psia for pressure, and 60 oF, 68 oF, or 70 oF for temperature. Despite its name “british”, it is commonly used in North America.

For Metric Unit (SI Unit), the term Normal is commonly used. The combination could be 101.325 kPa or 100 kPa for pressure, and 0 oC, 20 oC, or 25 oC for temperature. This metric unit – and so is Normal term – is commonly used in Europe and other country using metric unit. However, in Indonesia, the country where I live, both Metric and British unit are used, depending on each company preference.

This broad definition is what make some people confuse – and they even criticize it. Art Montemayor in one of Cheresources discussion criticized this concept of using Standard Volume flow as molar flow unit. He proposed the consistent unit usage, i.e. never use volumetric flow unit anymore for molar flow. At the least, he expected that there should be clear definition for term Standard. (He expected it to Hysys developer though).

Well, in one thing I agree with Art Montemayor. There should be clear definition for term Standard and Normal. And this definition shall be bold-noted on each document containing SCF or Nm3 unit. But for the other thing I also disagree with Art Montemayor. The use of standard volumetric flow is not a bad engineering, nor it is a fallacy. There is a solid science in that, i.e. Avogadro Law and Ideal Gas Law. In fact, Process / Chemical Engineering is not the only field using that type of “confusing” unit. Astronomy is also using this kind of interchangeable unit (time vs leght to name it). And as long as the scientific basic is solid, and as long as there is a strong international / industrial convention about it, we could still use the unit.

On the next post I’d like to share the use of kind-of-confusing unit in daily life and astronomy.

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