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SCFM as molar flow

  • 1.  SCFM as molar flow

    Posted 01-04-2019 11:58
    I'm studying a Material Balance. Molar flow is showed in SCFM unit, as long as I know SCFM is per Standard Cubic Feet per Minute and it is used for volume flow only. This is stream is vapor phase. Is this correct?
    Thanks you.

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    Paola Hernandez

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  • 2.  RE: SCFM as molar flow

    Posted 01-04-2019 23:56
    Using the ideal gas law molar flow can be shown as SCFM.  At standard condtioins v is proportional to n. Very common in the natural gas industry,  especially in ancient times when orders were in volume and not energy. Hope that helps





  • 3.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-05-2019 00:06
    SCFM is volumetric flow rate of gas: Standard Cubic feet per minute at STP conditions.
    Volumetric Flowrate, V = (N(molar flow rate (moles/minute)/density of gas or vapor)

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    Best Regards
    Venkat Subramanian





  • 4.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-08-2019 07:23
    When you want to get back to moles be sure you use a consistent standard temperature.  In Chemistry studies 0 deg C is the standard. However, in the commercial gas world 15.5 C or 60 F is more commonly used.


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    Denis Fallon
    Engineering Fellow
    Celanese Corp
    Blacksburg VA
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  • 5.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-05-2019 00:10
    Assuming that this stream is in the vapor phase, is a weak assumption. The reason, you would make this assumption is because the way the units of measure was specified. But what would happen should this same stream go through a heat exchanger which would cool this stream below is dew dew point, then part or all of this stream would be in the liquid phase. Therefore, relying on units of measure to determine the phase of the material would be very bad practice.

    The phase of the material is strictly determined by the temperature, pressure and composition of the fluid. If you have a fluid consisting of a single component, e.g. water, then specifying either the temperature or the pressure will uniquely define whether the fluid is in the vapor or liquid phase (there is more that can be said here, e.g. when the fluid is at equilibrium conditions then 2 phases exists. Anyway I think I am getting beyond what your questions was). For more on this topic see the phase rule for the degrees of freedom.







  • 6.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-05-2019 07:31
    Paola, The vapor phase flow you have is volumetric flow of a gas or vapor.  It is being expressed as SCFM and can be converted to moles per minute with the conversion factor of 359 SCF/lb-mol.  The conversion factor is for a pressure of 1 atm and temperature of 0 deg C.   I hope that answers your question.

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    Barry Juran PE
    Consultant
    Philadelphia PA
    Chemical Process Engineer and Sr. Biopharm Specialist
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  • 7.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-05-2019 09:59
    Paola:

    SCFM (or in the metric system, Nm3/hr) is one of those traditional "units" that is useful but misleading. It actually is a mass or molar flow rate. I believe it originated in the compressed air industry, as a mass flow that is roughly the same as the volumetric flow at the compressor inlet. (On a dry day, at a pleasant temperature, near sea level).
    The unit is converted to a mass flow rate using the gas molecular weight, and the "standard" temperature and pressure.  The problem is that these "standard" conditions  vary, so you have to be careful to know what that standard is, for an exact conversion. For scfm, the temperature will most likely be 60, 68, or 70ºF, as an example.

    My preference in a material balance was to always use mass flow (lbs or kg/hr), because molar and volumetric flows vary; but mass in always equaled mass out.

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    Alexander Smith PE
    Littleton MA
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  • 8.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-05-2019 14:56
    It is correct provided by vapor you mean gas only no mist.

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    Fred Pethick
    Porter TX
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  • 9.  RE: SCFM as molar flow

    LIFE MEMBER
    Posted 01-07-2019 08:25
    I think standard conditions for SCFM will be most likely at 60 degrees F.  I know it's strange, but it's tradition.  A person using SCFM really should point that out, but perhaps you may never find that.

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    Joseph Bays PE
    Senior Associate
    Eastman Chemical Company
    Kingsport TN
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  • 10.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-07-2019 08:58
    which is the composition of the stream?

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    Carlos Guardia ING,ESP
    Project Development Engineer
    Sugar Cane Growers Cooperative of Florida
    Belle Glade FL
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  • 11.  RE: SCFM as molar flow

    Posted 01-07-2019 09:33
    There have been some excellent replies. One word of caution is to find out exactly what standard was used. The standards vary depending on the Industry, particularly with  Standard Temperature. I've seen 0 C, 60 F and 70 F all used in the past year.  I've also seen: 1 atm, 1 bar and some  metric units all used for Standard Pressure.

    So basically, anything listed as standard is useless, unless you know exactly what standard was used.

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    James Phillips
    Benton KY
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  • 12.  RE: SCFM as molar flow

    Posted 01-08-2019 10:51
    Ah, you have stumbled upon a big and confusing topic!  For the purpose of your H&M balance just think of SCFM as a number of lb-mols, regardless of the stream conditions.  I never think of SCFM as a measure of volume, but as a measure of moles.  Once I enter this mind set, even liquids can be in SCFM units and all is good.  SCFM in equals SCFM out, which cannot be said for volumes.

    The project should have (and I always preferred to have it on the H&M balance) the conversion between SCF and lb-moles.  This will be a fixed constant number that will depend only on the "standard" conditions (usually about 379.4 SCF/lb-mol for 60F and 14.696 psia).  The confusion is a result of differences between standards for the pressure and temperature, and even if the gas is considered ideal.  The differences can be critical when product specs are given in SCF units.

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    David Coyle
    Technical Manager
    RETIRED
    Houston TX
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  • 13.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-08-2019 13:13
    These replies appear to have assumed that you know the density of the vapor or that it behaves as an Ideal Gas.   An ideal gas can be described with PV = nRT.  However, the term "vapor" was used in the original question, not "gas".  For many real-life chemical engineering problems, vapors may not behave as ideal gases.  Non-ideal vapor behavior becomes more significant as critical temperatures and pressures are approached.  A non-ideal fudge coefficient is often applied to compensate for non-ideal behavior to result in PV = znRT as  a useable equation, with z being a complex function of how close the vapor is to its critical temperature and pressure.  If the vapor is a hydrocarbon, the natural gas industry has published useful graphs for the density of low molecular weight alkane hydrocarbons and methods to approximate the vapor density of gas mixtures.

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    James Diebold
    Principal Scientist
    Community Power Corporation
    Eureka CA
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  • 14.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-29-2019 01:29
    Paola,

    Academically speaking, I believe it is incorrect to specify SCFM ( or any other variation of std. gas flow) as molar flow.

    Std. Gas Flow is just the preferred term to be used in material balances when dealing with gas industry clients or problems. Only true If there is a gas network where only compressors, heat exchangers, splitters, or mixers exist then std. gas flow will conserve as mass flow does. But as soon as you have a reactor or significant liquid drop-out on separators the std. gas flow does not conserve.

    I would not do stoichiometric conversions on that basis and this relationship is not applicable when a liquid/solid phase are introduced in the system.

    Unfortunately, some simulation software (such as HYSYS and UniSim) allow to specify std. gas flow in the molar flow description. However, the reason for this may be that std. gas flow  is not defined as an input variable within the software. So, a practical solution (saving time and resources) was to allow molar flow to be defined as such. This way, the user does not have to use an "Adjuster" to serve the same purpose,  and the simulation company does not have to spend the time and the resources involved in creating a new input variable.

    In other words, it was practical solution not so much academically incline (more business incline). At the end, the programmer will say "if you do not like it, you still can define molar basis as you wish or just add an adjuster if you prefer".

    Note that  younger simulation  companies already corrected this and std. gas flow appears as an input variable.

    Regards,

    Sergio Zerpa



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    Sergio Zerpa PE
    Process Engineer
    Solaris MCI
    Langley BC
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  • 15.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-31-2019 06:46
    As Sergio points out in his reply, the ability to specify molar flow in SCFM units is an unfortunate provision made available in simulators such as HYSYS and Unisim. The ability to do this has helped to perpetuate this kind of confusion, especially when dealing with "vapors".
    Also, as others have pointed out, it is possible to convert between molar and volumetric flow using a factor (e.g molar volume of an ideal gas). However the user must verify that such a factor is applicable to the situation ( i.e. is the gas truly ideal? is there a possibility of liquid drop out? etc). Due to such considerations, conversion factors should only be used with careful evaluation of the circumstances.
    I think molar flow should always be specified in units of moles per time, and volumetric flow in the appropriate volume units per time.

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    Unwana Ufeh
    Process Engineer
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  • 16.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-30-2019 10:50
    SCFM is used to describe a gas that is considered ideal. Standard conditions are usually atmospheric pressure and 60 deg F. From the ideal gas law you can calculate  SCF/mole. This number is 379.46 SCF/lb-mole at the standard conditions I cited.

    This term came about because we measure volumetric flow from the exit of a fan or compressor. That measurement can then be converted to molar or mass flow. This unit is was never meant to be used to describe the flow of a liquid or solid. When this unit is entered in a process simulator it is converted into moles/time using the conversion factor I cited no matter what the state of the fluid.

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    John Braccili
    Wallingford, PA
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  • 17.  RE: SCFM as molar flow

    SENIOR MEMBER
    Posted 01-31-2019 08:41
    In addition to others comments, be careful the right molecular weight or gas density is used. For example carboxylic acids can dimerize in the vapor phase, increasing molecular weight. Degree of dimerization or higher association can be pressure and temperature dependent. There are published methods such as Haydon O'Connell that estimate association off such compounds and help us estimate the corrrect density in Actual Cubic Feet Per Minute.

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    Denis Fallon
    Engineering Fellow
    Celanese Corp
    Blacksburg VA
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  • 18.  RE: SCFM as molar flow

    Posted 02-09-2019 14:36
    In addition, try the material balance by converting gas, liquid in Mass unit like ton per day or kg/hr.  it will be easy and donot confuse you in interchangeability of units. Dont forget to use correct s.g/RD at defined temperature.

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    Shahzeb Hassan ME,BE Process Engineering
    Executive Process engineer, PRL
    Karachi
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