How to convert MMSCFD of natural gas to lb/hr 2

[Vivaldi SMITH]

Hello,
I am struggling big time converting MMSCFD of natural gas to lb/hr. I would greatly appreciate any help.

 

[MacGyverS2000]

What have you tried? Show us your calcs so far...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[BAretired]

Well, you should not be struggling "big time" over that. I did not recognize the term MMSCFD at first, so I looked it up using Google. It is a Million standard cubic feet per day. You are trying to convert a volume per day to a weight per hour. You know there are 24 hours in a day, right? Do you know the weight per unit volume of natural gas? If so, it should be no problem.

Give it a try and let us see your calculation.



BA

 

[BAretired]

Here is another little gem about natural gas. A very precise weight is given for one cubic foot of compressed gas. It seems to me that it would depend on the pressure, yet no mention was made of that. I did not look any further, but perhaps you could explore the pressure of "compressed natural gas".

BA

 

[waross]

While you are looking up the MMSCFD, check the gas pressure at standard conditions or the weight per cubic pound at standard conditions.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[BAretired]

I wonder if Vivaldi Smith is still "struggling big time" with his unit conversion. Who knows?

BA

 

[waross]

I hope that he wasn't looking for 'pounds per hour' of steam at an unspecified pressure.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Vivaldi SMITH]

My only little concern is to figure if Standard Cubic Feet and Cubic feet will give me an approximation. I understand standard takes into account standard conditions.

 

[BAretired]

My only little concern is to figure if Standard Cubic Feet and Cubic feet will give me an approximation. I understand standard takes into account standard conditions.

I don't know what there is to "figure". The definition of a standard cubic foot follows:

A standard cubic foot defines an amount of gas contained in a volume of one cubic foot at standard temperature and pressure. This standard unit of molecular quantity for gases can be used with the ideal gas law to compute the quantity per unit of volume for other pressures and temperatures. In spite of the label "standard", there is a variety of definitions, mainly depending on the type of gas. Since, for a given volume, the quantity is proportional to the pressure and temperature, each definition fixes base values for pressure and temperature.

The conversion from MMSCFD to pounds/hour is not an approximation. It is precise. The approximation has more to do with whether or not compressed natural gas was stored under standard temperature and pressure.

EDIT: Based on the information provided below, the conversion from volume to weight depends on the chemical composition of natural gas, which is not consistent.

BA

 

[1503-44]

Actually the calculation of the weight of a "standard" cubic foot of natural gas is impossible.
Why? Because there is not a standard definition of the composition of natural gas and therefore it does not have a "standard" molecular weight, nor specific density.

Additionally it must be calculated at "standard" conditions, which are not always "standard".

You can calculate the weight of a cubic foot of a specific natgas, if you know its composition, or if you know its laboratory measured specific density and you can set the "standard" conditions of pressure and temperature.

Any calculation you do concerning natural gas should reference the standard conditions that you used. In the USA, it is usually 14.696 psia and 68°F. A ref temp of 0°C is often used in Europe and sometimes other countries or organizations use 15°C.

Sometimes one just assumes natgas is 100% CH4, methane, but that can at times be a poor approximation. Typical specific densities of Natgas vary between 0.6 and 0.7 referenced to air, again at "standard conditions".


Do you know the composition, molecular weight, or the specific density of your gas stream?
Do you know the temperature and pressure of your gas stream?
Do you know what standard reference conditions are being used?

 

[Vivaldi SMITH]

is about 80.44 C1, 8.71 CO2, 4.92 C2, 3.43 I-But, 0.55 I-But, 0.92 N-But, 0.28 I-Pent, 0.19 N- Pent, 0.18 N2, 0.38 C6+; they all about 1000 PSI and 80 F.

 

[1503-44]

Too many values for Butane.
Is 3.43 actually the value for Propane C3?
I will assume it is C3
Comp quantity MW MWmix(i)
C1 0.8044 16.042 12.90
C2 0.0492 30.068 1.48
C3 0.0344 44.09 1.52
IC4 0.0055 58.12 0.32
NC4 0.0092 58.12 0.53
IC5 0.0028 72.14 0.20
NC5 0.0019 72.14 0.14
C6+ 0.0038 100.198 0.38
N2 0.0018 28.016 0.05
CO2 0.0871 44.01 3.83
H2O 0 18.016 0.00
H2S 0 34.076 0.00
MWmix 21.36

MW(air) 28.964
SG(relative to air) 0.737
Compressibility Factor Z = 0.809 (Benedict-Webb_Rubin)
GasDensity_pcf = P_psia * SG_rel2air * 28.964 / (z * 10.73 * T_degR)
GasDensity_pcf = 4.619 lbs/ft3 at 80F & 1015 psia

You should check that. Its a new spreadsheet.

 

[Vivaldi SMITH]

Replying to 1503-44 (Petroleum). That means for that gas composition 1 MMCFD will give an equivalent density of 3.55 lbs/ft3? . How do i relate that to the mass flow in lb/hr?

 

[1503-44]

Notice the change in lb/ft at 80F and 1015psia above to 4.619 lbs/ft3. Like I said, its a new spreadsheet.

1 MMCFD. What is the reference temp & pressure? 14.696 psia and 68 F?

4.691 lbs/ft3 at 1015 psia and 80F, z = 0.809
convert that to 14.696 psia and 68 F

Ohhh. *%&^$^%#^ I have to run out for an hour. I'll be back soon.

 

[Vivaldi SMITH]

Replying to 1503-44 (Petroleum). Yes correct, the reference pressure is 14.696 psia and 68 F.

 

[1503-44]

Sorry. I had to get to the hardware store before it closed.

1 MMCFD. What is the reference temp & pressure? 14.696 psia and 68 F?

We found previously that mass of 1ft3 at pipe flow conditions was
4.691 lbs/ft3 at 1015 psia and 80F, z = 0.809
convert that to 14.696 psia and 68 F

GasDen1 = 4.691 lbs/ft3
Vol1 = 1ft3
Z1 = 0.80767
P1 = 1015 psia
T1 = 80F = 539.67R
A = P1 * V1 / T1 / Z1 = 2.329

Z2 at 14.696psia and 68F = 1.000
P2 = 14.696 psia
T2 = 68F = 527.67R
B = P2 * T2 / Z2 = 35.9057

Vol2 = A x B = 2.329 x 35.9057 = 83.61 ft3
GasDen2 = 4.691 lbs / 83.61 ft3 = 0.05544 lbs/ft3

Air at 14.696psia and 68F weighs 0.0765 lbs/ft3
SG_gas = 0.05544 / 0.0765 = 0.725

================== Calculate weight directly from quantity of gas at STP ==============================
1 Million Standard Cubic Feet (MMSCF) gas at 14.696psia and 68F (Our Standard Conditions) with a
Specific Gravity of 0.737 relative to air, from our Composition Calculation above, which was close to what I calculated using BWR compressibility. But here we don't have to worry about compressibility factors, because 1MMCFD is at normal pressure and temps where Z=1.

Gas weighs SG * Weight of Air
0.737 x Weight of Air at 14.696psia & 68F
Weight of 1ft3 of air at 14.696psia & 68F = 0.0765 lbs/ft3
Therefore 1 ft3 Gas at 14.696psia and 68F weighs
0.737 * 0.0765 lbs/ft3 = 0.564 lbs/ft3 x 1,000,000
1MM SCF is 564,000 lbs of 0.737 SG natural gas

If that MM SCF of gas passes by in 1 Day, that's 24h
564,000 lbs/24h
Mass flow rate is 2349 lbs/h

Mass flow rate is constant at all pressures and temperatures inside any diameter pipe.

 

[Vivaldi SMITH]

Replying to :1503-44 (Petroleum).
Thanks alot for your time and your help. I appreciate from the bottom of my heart. Have a blessed day and stay safe.