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

 

[25362]

So a exquisitely detailed split only to get 100.01% w/w...

Brainstorming, are you asking us to do the estimations for you ?

 

[25362]

Anyway, ASTM D3588-98(2003) may be of help.

 

[brainstorming]

25362....10000
I'm asking the way to do so not the values?

Thank you anyway for your contribution

Cheers

 

[Montemayor]

brainstorming:

The answer to this problem is fully detailed and explained step-by-step in David M. Himmelblau's classic text book: "Basic Principles and Calculations in Chemical Engineering"; 6th Edition. This book is used as text in 2nd year engineering university courses. The University of New Brunswick has a WebPage:

http://www.unb.ca/che/che1004/introduction.htm

where the Himmelblau text problems are worked out as examples. This is a very basic and straight-forward Chemical Engineering problem that should be worked out on an Excel spreadsheet in a minimal of time.

 

[mbeychok]

.
Brainstorming:

First, let me say that it is very difficult to beleive that the FCC offgas has no H2S in it. Be that as it may, you can do what you want quite easily by setting up an Excel spreadsheet:

(1) Select a horizontal row upon which to start. Label the first six columns (from left to right) as: 1, 2, 3, 4, 5, and 6.

(2) In row 2 (just beneath the starting row): enter "Component" in column 1, "Mol Wt" in column 2, "Btu/Lb" in column 3, "lbs/hr" in column 4, "lb-mols/hr" in column 5, and "Btu/hr" in column 6.

(3) In column 1 of row 3, enter "H2". In column 2 of row 3, enter the molecular weight of Hydrogen (which is 2). In column 3 of row 3, enter the heating value of Hydrogen in Btu/lb (either the gross heating value or the net heating value, whichever one you want)

(4) In column 1 of row 4, enter "N2". In column 2 of row 3, enter the molecular weight of Nitrogen (which is 28). In column 3 of row 3, enter the heating value of Nitrogen in Btu/lb (either the gross heating value or the net heating value, whichever one you want)

(5) Continue filling in columns 1, 2, and 3 for each of the 12 components in rows 3-14 just as you did for H2 and N2.

(6) In column 4, for each component, multiply your 27 tons/hr by the lbs/ton and then multiply by the wt % and then divide by 100 ... and enter the resulting lbs/hr for each component.

(7) In column 5, for each component, divide the lbs/hr by the molecular weight ... and enter the resulting lb-mols/hr for each component.

(8) In column 6, for each component, multiply the lbs/hr by the Btu/lb heating value ... and enter the resulting Btu/hr for each component.

(7) In column 1 of row 16, enter "Totals".

(8) In column 4 of row 16, sum the values in rows 3 through 14 of that column. That will give you the total lbs/hr which should convert to the 27 tons/hr that you started with (be it long tons, short tons or metric tons ... you didn't tell us which it was.)

(9) In column 5 of row 16, sum the values in rows 3 through 14 of that column. That will give you the total lb-mols/hr.

(10) In column 6 of row 16, sum the values in rows 3 through 14 of that column. That will give you the total BTU/hr.

(11) Divide the total lbs/hr by the total mols/hr, and you will have the overall molecular weight which you then enter in column 2 of row 16.

(11) Divide the total lbs/hr by the total Btu/hr, and you will have the overall heating value in Btu/lb which you then enter in column 3 of row 16. Please note that my term "heating value" is the same as your term "caloric value".

(12) Finally, multiply the total mols/hr by 379.482 scf/lb-mol and you will have the overall scf/hr at standard conditions of 60 [°]F and 1 atmosphere pressure. Then you can multiply scf/hr by 24 and then divide by 1,000,000 ... and you will have the overall MMSCF/D.

As a final note, you should have told us:

(a) Why your FCC overhead gas has no H2S in it.
(b) Whether you wanted the gross caloric value or the net caloric value.
(c) Whether you were giving us short tons/hr or long tons/hr or metric tons/hr.
(d) What the standard temperature and pressure was for your desired MMSCF/D.

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

 

[Montemayor]

Milton:

AMEN!

You outdid yourself not by giving a detailed step-by-step algorithm of how to solve a basic Chem Engineering problem, but by the pearls of wisdom you have so adroitly added at the end. A class act from a class guy.

You deserve more than your share of stars for this one. Brainstorm owes you a dinner of your choosing.

 

[mbeychok]

.
Art:

Thanks for your comments. What I forgot to say in my response to Brainstorm was that it is just as easy and perhaps even faster to make that simple tablulation on a sheet of paper as it is to make up an Excel spreadsheet.

Regards,

Milt

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

 

[TD2K]

It might be faster Milt but you set up a SS and then you have it the next time you need it.

 

[brainstorming]

Hello everybody
I agree with you that the question posted is considered to be simple and basic, but your responses are complicated and there is another way which is much simpler and better.

To convert from MTPD to MMSCFD, a conversion factor of 35.9146 is used as follow :
MTPD / 35.9146 = MMSCFD

Also, 1 MMSCFD equals to 1000 Btu, by this above MMSCFD can be converted to Btu by multiplying by 1000

Also, methane calorific value can be found by same way as above after calculating its flow from its percentage.

I hope that you can answer simple questions by simple answers not vice versa.

Thank you to all

Cheers

 

[Montemayor]

It is a prize fool who wastes his time by asking an engineering question that he is convinced he can answer. It is a worse fool who presents an illogical and totally erroneous answer, convinced that he alone is correct.

Hopefully young engineers will profit by carefully studying the erroneous and illogical answers “Brain” is presenting. His proposed answers defy the laws of mathematics and science combined.

No one can convert a mass flow rate to a calorific value without first identifying the type of mass. Neither can anyone convert a mass flow rate to volumetric rate without identifying its type - and, consequently, the number of moles involved. These are high school chemistry basics.

Although no less an authority than the GPSA lists Methane as having a gross calorific value of 1,009.7 Btu per Scf, Mr. Brain asserts that “1 MMSCFD equals to 1,000 Btu” – this time victimizing high school math.

We have received a classical, detailed, engineering explanation to a basic question from Milt Beychok in the quality akin to a Rolls-Royce. Now we receive a note of “gratitude” in the class of a soap-box cart.

Milt, I thank you for your splendid contributions to us all, but most of all for your class act.

 

[mbeychok]

.
Brainstorming:

As pointed out by Art Montemayor, your answers are completely wrong!!

(1) Methane has a gross caloric value of approximately 1000 Btu per scf at 60 [°]F and 1 atmosphere ... not per MMscf! And your gas only contains 27.48 wt% methane ... the N2 in your gas has no caloric value, the Hydrogen caloric value is not the same as methane, nor are the caloric values of the other hydrocarbons in your gas.

(2) Your original message stated 27 t/hr without telling us whether your ton was a long ton, short ton or metric ton. Now your latest posting refers to metric tons per day.

(3) I don't know where you got your conversion of MTPD/35.9146 = MMSCFD (nor do I care to know). Such a conversion would only apply to a gas of a specific molecular weight. Such a conversion cannot be applied to all gases because most gases have different molecular weights. Nor does your conversion define the reference temperature and pressure for the SCFD.

(4) You still haven't defined whether you are using gross or net caloric values.

(5) Nor have you explained why your FCC offgas gas has no H2S in it.

It is commendable to seek simple answers but not at the expense of being simple-minded! If you are indeed a graduate chemical engineer, I would strongly urge you to go back to school and take some refresher courses.

Cheers!

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

 

[NeedAHoliday]

Brainstorming,

Having just read this thread I feel I need to provide comment.

An engineer that questions and extends himself and others through his/her queries/responses has the potential to be exceptional as you will always be seeking knowledge, understanding and improvement. An engineer that questions but disregards others knowledgable answers as overly complicated without understanding the fundamentals is inept. An engineer that queries nothing and is willing to accept everything he reads or is told as gospel is dangerous.

I don't believe you are within the dangerous category but you are certainly running close to the inept area in my humble opinion. It staggers me that you so easily dismiss the exceptional response of Milton Beychok when it clearly and simply defines the solution to your query, while you single handedly debase the fundamentals of chemistry and physics.

Furthermore you dismissed a response that most would not even take the time to explain, as it is something that a fist/second year chem eng student should be capable of defining and evaluating. If you truly believe that Milton's response was complicated then I fear for the future of our profession and reputation.

For Milton a star you most deserve, for actually taking the time to explain the solution to a query that should be the bread and butter of a chem eng professional and in persisting with trying to further brainstorming knowledge even after brainstorming's response.

Brainstorming, one bit of credit I give you is that you should never just accept what is written without first understanding what it is relating to you, I beleive this is what you were attempting. You just didn't quite succeed from what I have read. Keep questioning everything and everyone, Milton and Montemayor included, just do your homework next time so as not to put your feet in your mouth.

I leaveoff with a quote from Albert Einstein:

"I never teach my pupils; I only attempt to provide the conditions in which they can learn"

The conditions were set.....

K.

 

[brainstorming]

Regardless of the question and answers discussed in this post, I realize that experience and knowledgeable people should not be best people but there are some who have bad attitude and not polite in their behaviours.
What I have got from your non professional answers gave me bad impression about this site where it should be filtered from such category of people.

Even though whatever question is posted by any person, it can be answered or discussed with those people having good attitude and professional skills not the opposite as shown in this post.

I hope that this site will be the best site for professional people who can share their knowledge, good attitude and professional contributions.

Cheers

 

[NeedAHoliday]

Fair enough point.

K.

 

[25362]

I think brainstorming shouldn't take exception since he/she antagonised the experts, after presenting an erroneous approach to the issue in hand, by saying

I hope you can answer simple questions with simple answers not viceversa

Without even trying to be patronising or paternalistic, I believe brainstorming should -without compounding her/his errors- give considered answers to the points in mbeychok's highly professional post of May 18th.

 

[TD2K]

I don't think I've ever seen any put their foot this good in their mouth and have the chutzpah to lecture the group.

 

[brainstorming]

The conversion factor of 35.9146 is for converting MTPD tp MMSCFD for ethane gas.

e.g. 2880 MTPD of ethane gas is 80 MMSCFD

FCC contains sulphur reachning up to about 100 ppmw H2S and 10 ppmw COS

Cheers

 

[TD2K]

I'm not quite sure why you would even propose using a conversion factor for ethane gas (MW 30) for an FCCU gas stream that for your composition has a MW of about 4.5 (depending how you characterize the C5+ fraction for a MW). Converting from t/hr to MMscfd is solely a function of how many moles per unit time you have which is clearly MW dependent. That's nearly a 7x error there.

Assuming your 1000 BTU number is based on the heating value of NG which is primarily methane (1000 BTU/scf is a pretty common typical value used for NG), why would you then use this heating value if your t/hr to MMscfd factor is based on ethane which has a significantly higher heating value than 1000 BTU/scf? And I'm ignoring for this discussion the total mixup you did in even trying to go from a volumetric flow (MMscfd) to BTU with a single conversion number and the lack of a time unit associated with your BTU number.

Rules of thumb or quick conversion methods have their place, don't get me wrong. But as engineers it is is critical that we understand the basis behind these 'quick' conversions so you know whether they are applicable for your case.

As an example, a quick way to convert from deg C to deg F is to double the deg C value and add 30. It gets you in the ballpark for room temperatures (15 deg C gives you 60 deg F versus the correct value of 59F). But if someone asked you what -250C was in F, it's going to give you a significant error (-470F versus -418F) and of course, -470F is less than absolute zero so it's an impossible temperature. And if someone asked you what 150 deg Rankin was in deg Kelvin, that formula would be totally useless.

 

[25362]

I've tried to understand brainstorming's equivalence factor.
It appears to me that it cannot a conversion via densities. It is rather a conversion via net calorific values.

Excuse my using of many significant figures.

The gas has a NCV of about 1254 Btu/SCF, (60^oF and 1 atm), after making assumptions on C5+ and a calculation that brainstorming dismissed as obvious.

Ethane has a NCV of 20432 Btu/lb (Perry VI table 9-30).

1000 metric ton of ethane would be equivalent in net calorific value to: 20432 Btu/lb[×]2204623 lb/ton[÷]1254 Btu/SCF = 35.9[×]10⁶SCF/ton.

Thus the equivalence factor: 1 MT/D ethane ~ 35.9 MM/D SCF gas.

If I am right in my assumption, question #1 in brainstorming's first post is misleading.

Brainstorming is kindly asked to confirm or refute.