Calorific value of FCC offgas 6

[brainstorming]

I would like to know that if I have 27 t/h FCC offgas from refinery with 1.1% H2, 13.31% N2, 1.54% CO, 27.48% CH4, 22.94% C2H4, 23.35% C2H6, 2.11% C3H6, 0.4% C3H8, 4.61% C4H8,0.27% iC4H10, 0.27% nC4H10 and 2.63% C5+ (all wt%)
Then how I can find out the following:
1-Flow of FCC in MMSCFD
2-Calorific value of this gas (MBTU and MW)
3-Calorific value of methane only in the gas (MBTU and MW)

Any help in calculating such values is appreciated.

Cheers

 

[TD2K]

25362, which equivalence factor are you talking about? Brain's "35.9146 is for converting MTPD tp MMSCFD for ethane gas"?

For 2880 MTPD of ethane gas I also get 80 MMscfd but it's a straight conversion from mass to moles to scf (I used 379.6 scf per lbmole). There's no equivalence on a heat content basis if that is what you are trying to work from.

 

[25362]

TD2K, I may be wrong, and it wouldn't be the first time. The mass-to-volume conversion for ethane at 60 deg F and 1 atm., following NIST, is 12.514 CF/lbm.

Multiplying by 2204.6 lb/metric ton one gets 27.6, not 35.9 MM CF/1000 ton. As I said, I may be dead wrong. Kindly show me where is my mistake.

Assuming the density was not the basis for conversion, I tried to see whether the conversion was done on a NCV basis.

The selected gas was ethane, I don't know why. Probably ethylene would be better, since the MW of the gas is near that of ethylene, but the conversion factor would then have to change. It would then be by pure chance that the conversion factor based on NCV, is in conformity with the value brought by brainstorming.

As I said, I may be wrong, and would appreciate somebody shows me the correct answer.

 

[mbeychok]

25362:

Let's first define:

1 MT/D = 1 metric ton/day = 2205 lb
1 MMSCF = 1,000,000 SCF = 10⁶ SCF
1 mol = 1 lb-mol of any ideal gas = 379.5 SCF at 60 [°]F and 1 atmos.

Then 1 MMSCF is equal to:

10⁶SCF x 1 mol/379.5 SCF x 30 lb/mol x 1 MT/2205 lb = 35.9 MT

Thus, the conversion factor: MT/35.9 will give you MMSCF

I would point out that your answer was not 1 MT = 35.9 MMSCF. It was 1000 MT = 35.9 MMSCF which equates to 1 MMSCF = 27.9 MT. The difference between your answer and the ones that Brainstorming, TD2K and I got is due to the caloric values that you used rather than using a straight mass to volume conversion.

The key error made by Brainstorming still remains that he cannot use the conversion factor for ethane as being applicable to his total FCC offgas.

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[TD2K]

I'm getting confused here myself

23562, I used the following approach:

2880 metric tons/day = 2,880,000 kg/day = 6,350,400 lb/hr

Taking ethane as 30.07 MW gives me 211,190 lbmole/day

1 lbmole occupies 379.6 scf (slightly different from Milton's and I don't claim it's right, it's just the one that sticks in my head) gives me 80,170,000 scf/day. I won't quibble between that number and 80 MMscfd.

This is just a straight mass to volume conversion, I'm not implying any correction for heats of combustion.

 

[mbeychok]

25362:

By using your ethane density of 12.514 CF/lb at 60 [°]F and 1 atm, and multiplying that by 2205 lb/MT, one gets that 1 MT = 27,593 SCF. (Note: MT is metric ton, not 1000 tons)

Thus, 1 MMSCF = 1,000,000/27,593 = 36.2 MT which is within 1% of 35.9 MT. (Note: this is an equality)

Therefore the conversion factor: MT/36.2 will give you MMSCFD. (Note: this is a conversion factor)

I think the fact that an equality and a conversion factor are the inverse of each other has led to some confusion.

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[25362]

Now that the confusion created by the notation which I failed to interpret has been cleared by mbeychok, we are still left with the unanswered query: why ethane ?

Brainstorming, can you shed some light ?

 

[brainstorming]

That was just a mistake I have made quick estimate and check that is using ethane conversion factor to get MMSCFD from MTPD and check order of magnitude that for the FCC off gas. Keeping in mind the MW difference between the two gas.

Cheers

 

[UmeshMathur]

As a ChE of 39 years standing, I found this entire discussion quite amusing as we have questions that are naive, even for a freshman ChE student, being answered by noted authorities such as Milton and Art.

This was followed by non-sequitors and more than one riposte that can only be described as hilarious. Finally, we had another series of learned replies by Milton, a man of almost superhuman patience.

NeedAHoliday's remarks may have been pungent, but brainstorming should take the spirit of his comments to heart. A cavalier attitude to process calculations can have serious consequences, as many industrial-scale design errors have proven. This kind of computation has been routine for second year ChE students for 75 years at least.