11 replies to this topic

[sri]

I have ASTM D-86 distillation data for Naphtha. Weight percentages of Paraffins, Naphthenes, Aromatics are also known. Sp Gr. is also known.
I would like to know its molecular weight. Is there any way to calculate its molecular weight from the above data.
Thanks

[sri]

Does any corelation exists?

[Andree]

1. you can calculate mean molecular weight additively, but you need to recalculate mass fractions to molar ones

2. if you know specific gravity at normal conditions, you can calculated molecular weigth from:
M = SG * ro_air_norm / V_norm
where ro_norm_air is density of air in normal conds
V_norm is specific volume that 1 mole of gas occupies under normal conditions (approx. 22.4 l/mol)

[ankur2061]

1. you can calculate mean molecular weight additively, but you need to recalculate mass fractions to molar ones

2. if you know specific gravity at normal conditions, you can calculated molecular weigth from:
M = SG * ro_air_norm / V_norm
where ro_norm_air is density of air in normal conds
V_norm is specific volume that 1 mole of gas occupies under normal conditions (approx. 22.4 l/mol)

Andrei,

I think you are making a mistake. The person is talking about Naphtha ( a liquid!!!) not a gas. Where on earth the density of air comes into picture?

Sri,

I do not think there is any general correlation for molecular weight as such just based on what you have stated. If you have weight fractions of each of the components in your naphtha, then just by law of proportions you can estimate your molecular weight.

Example:

Basis: 1 kg-mol of liquid containing components A, B and C

Assume three components A, B and C with molecular weight of M_A, M_B and M_C respectively and weight fraction of 0.5 , 0.3 and 0.2 (Total 0.5+0.3+0.2 = 1.0) respectively. Then molecular weight of the liquid with components A,B and C would be:

Molecular weight = (M_A*0.5)+(M_B*0.3)+(M_C*0.2) / 1

The above is based on the fundamental definition of moles i.e. ,

Moles = Weight / Molecular Weight

Hope this helps in understanding the concept.

Regards,
Ankur.

[Zauberberg]

I think there are some correlations in API Technical Book, there might be some other resources as well. And I think you have to convert D86 to TBP distillation for such purpose. Try to find API's book.

[CMA010]

I think there are some correlations in API Technical Book, there might be some other resources as well. And I think you have to convert D86 to TBP distillation for such purpose. Try to find API's book.

You can use procedure 2B1.1 and 2B2.1 of the API technical data book to calculate the Mw when you have a D86 curve.

[ankur2061]

I think there are some correlations in API Technical Book, there might be some other resources as well. And I think you have to convert D86 to TBP distillation for such purpose. Try to find API's book.

You can use procedure 2B1.1 and 2B2.1 of the API technical data book to calculate the Mw when you have a D86 curve.

Hi,

Is this the equation you are referring to:

MW = 20.486*[exp(0.000165*T_b-7.78712*SG+0.0011582*T_b*SG)]*T_b^1.26007*SG^4.98308

where:
MW = molecular weight of the fraction
T_b = mean average boiling point of the petroleum fraction ®
SG = specific gravity of the cut

Would request for confirmation of the equation from any forum member.

Regards,
Ankur.

[JMW]

I'm pretty sure there is an ASTM standard for calculating molecular weight from the known kinematic viscosity at two temperatures and the density but I can't think whicj standard it is and there are quite a number to search through.

Otherwise, this link might be relevant?

http://www.sciencedi...4d5763b29aa1dd7

[CMA010]

I think there are some correlations in API Technical Book, there might be some other resources as well. And I think you have to convert D86 to TBP distillation for such purpose. Try to find API's book.

You can use procedure 2B1.1 and 2B2.1 of the API technical data book to calculate the Mw when you have a D86 curve.

Hi,

Is this the equation you are referring to:

MW = 20.486*[exp(0.000165*T_b-7.78712*SG+0.0011582*T_b*SG)]*T_b^1.26007*SG^4.98308

where:
MW = molecular weight of the fraction
T_b = mean average boiling point of the petroleum fraction ®
SG = specific gravity of the cut

Would request for confirmation of the equation from any forum member.

Regards,
Ankur.

That's the equation i was referring to.

[sri]

Thanks a lot everyone.

Ankur: Pl let me know How to calculate
mean avg boiling point.
Is it (IBP+FBP)/2 or temp. at which 50% boils.
Thanks a lot

[ankur2061]

Thanks a lot everyone.

Ankur: Pl let me know How to calculate
mean avg boiling point.
Is it (IBP+FBP)/2 or temp. at which 50% boils.
Thanks a lot

Sri,

Checkout the following links to find out more about 'Mean Avg. Boiling Point':

http://www.chemstati.../physprop54.pdf

http://committees.ap.../docs/2b1-3.pdf

Regards,
Ankur.

[sri]

Thanks, Ankur