10 replies to this topic

[roychua]

Hi, I am new to the oil industry, and there are some help I need on crude oil specifications.
I have received a description of a blended crude / fuel oil, that has a IBP of 50 celsius and FBP of 360 celsius.

This is something new to me, because the other specifications I have received, have FBP in the range of 500 celsius and above, which makes me wonder about what happens after the oil has been boiled at a temperature higher than its FBP in a distillation process?

Is the resultant oil (heated at above its FBP) still in liquid state?
And, am I right to say that the oil will no longer boil but it can and it will only crack after it has reached its FBP?

If a oil's FBP is at 360 celsius, can I continue a distillation test beyond 360c ?

Thank you!

[Qalander (Chem)]

Dear roychua Hello/Good Morning,

I assume your query relates to Vaccum distillation aspect; since beyond certain temperatures (usually above 360~365 degrees celsius) Atmospheric distillation residues cracking starts and coke, as well as smaller chain cracked/ unsaturated lighter H-C fractions, get generated.

This is a general reply, since your query has been 'unclear'.

Hope this helps
Regards
Qalander

[djack77494]

Laboratory determinations of distillation curves for heavier hydrocarbon fractions switch to distillation under vacuum conditions to determine the boiling characteristics of the fraction while still minimizing any cracking. The procedures used adjust the results back to the effective temperatures at atmospheric pressure. When the FBP is reached, 100% of the fraction will have vaporized and there will be no liquid phase remaining.

[Andrei]

I will try an even more general answer about my understanding on FBP.
FBP is the temperature when all the oil liquid fractions were vaporized. As we all know the boiling point of any substance is a function of the pressure. When I see a lab bulletin with a TBP curve with FBP 360 ºC, I would expect that this is at atmospheric pressure.
According to ASTM-2892 lab atmospheric distillation can be conducted as high as 350 ºC. A low residence time has to be used in the chromatograph column to minimize the cracking processes. Plus, there are activity tests that can be conducted to determine a thermal cracking curve of a crude that can be used to correct the atmospheric distillation curve.
Depending on each crude characteristics there can be significant cracking occurring beyond 300ºC. There are crudes though with little cracking occurring below 400 ºC. Accuracy of an atmospheric distillation curve will be significantly affected by cracking. In this case, distillation is continued under vacuum conditions, but the results are converted to atmospheric conditions.
So, according to my understanding if you pass a FBP of 360ºC there shouldn't be any liquid left, everything should be vapor, regardless of any cracking process.
FBP (Final Boiling Point) can be for some crudes above the temperature where cracking occurs but it is not related directly to the cracking process itself, it is only a measurement of the boiling temperature of the final fraction of the crude. How do you reach in the lab to FBP avoiding cracking it is a completely different issue.

[Zauberberg]

My guess is that you have D86 analysis with you, because these are performed at ambient pressure and usually are limited at 350-360C final boiling temperature. D86 is quite common for crude oil distillation analysis on a day-to-day basis, because it is quick, inexpensive, and it gives basic information about the crude. However, do not rely on it in the case you need accurate crude distillation data; TBP method with correction curves is there for that purpose.

This value (FBP) depends on the method applied - that's the bottom line. Coupled with a volume % of sample distilled at a given D86 FBP temperature, it gives you a very rough estimate of distribution between the atmospheric residue and distillate products in crude (CDU) distillation tower.

[roychua]

Dear brothers and sisters,

Thank you for all the kind explanation. I have certainly understood a lot more about the characteristics of crude and the distillation processes. However, about the specifications I have received, I am not sure about which method of distillation is used. I was only provided with the following distillation range information:

% recovered at 50 Deg C: 0.25
% recovered at 75 Deg C: 3.5
% recovered at 100 Deg C: 7.5
% recovered at 125 Deg C: 12.0
% recovered at 150 Deg C: 18.0
% recovered at 175 Deg C: 24.0
% recovered at 200 Deg C: 28.5
% recovered at 250 Deg C: 33.0
% recovered at 275 Deg C: 37.0
% recovered at 300 Deg C: 41.5

Total Distillate: 46.5
Distillation Loss: 1.0
Residue: 51.5

IBP: 50 Deg C
FBP: 360 Deg C

Does it mean that:
1) If we heat the oil to a temperature beyond 360 deg C, what is left should only be solid residue, since all the liquid should have been evaporated?

2) And, am I correct to say that if the remaining solid residue is heated further, there can only be cracking?

[Andrei]

roychua,

You are right about the first statement, but not so right about the second one. The solid residue is not really crack-able, it is more like an inert. The material prone to cracking consists of big molecules from the heavy fractions but still liquid. Among the first to brake are the Sulphur and Nitrogen bonds, and also bonds with small branches and appendices. That's why an increased H2S or nitrogen compounds as well as overall increased gas make is a clear indication that cracking is occurring.

[Zauberberg]

Does it mean that:
1) If we heat the oil to a temperature beyond 360 deg C, what is left should only be solid residue, since all the liquid should have been evaporated?

2) And, am I correct to say that if the remaining solid residue is heated further, there can only be cracking?

1) No, what is left is liquid residue, which cannot be distilled by the selected laboratory method without excessive cracking (read my previous post). Do you think that you have 51.5% of solids in your crude oil?

2) Read the answer under (1)

[Qalander (Chem)]

Does it mean that:
1) If we heat the oil to a temperature beyond 360 deg C, what is left should only be solid residue, since all the liquid should have been evaporated?

2) And, am I correct to say that if the remaining solid residue is heated further, there can only be cracking?

1) No, what is left is liquid residue, which cannot be distilled by the selected laboratory method without excessive cracking (read my previous post). Do you think that you have 51.5% of solids in your crude oil?

2) Read the answer under (1)

Dear roychua, Here I must say that if the word 'ash' is not used than we cosider the residue to be the contents lefts undistilled at the temperatures employed.
(may be liquid/semi-solid)
This is what was correctly ponted out by my friend Zauberberg,however probably my friend Andrei took this otherwise.
Now having said all that it is logically assumed that contents left if sujected to more jeating at ambient conditions(with no vaccume or hydrogen atmosphere or catalysis) are bound to go cracking and coking reactions being hydrocabons.
Hope this helps.
Best regards
Qalander

[Andrei]

As a matter of fact nobody asked for an API gravity of this oil.
Just to explain from where I am comming from, I am used to this kind of distillation curve for a heavy oil with API around 8 to 9. For this kind of crude I am not surprised by a 51% rezidue.
In the case presented by Roychua probably the situation is different. What I understand from his numbers is that all the residue has a boiling range of 300 to 360 ºC. You can call it residue, but this is not a residue to me, it is really heavy crude and most important crack-able. I am calling residue something that is with virtually no value, most probably I will have to spend some money just to discard it.
In my work we start to call a residue a material with BP beyond 1100 ºF (593 ºC) and that is most likely to be solid.

[Qalander (Chem)]

As a matter of fact nobody asked for an API gravity of this oil.
Just to explain from where I am comming from, I am used to this kind of distillation curve for a heavy oil with API around 8 to 9. For this kind of crude I am not surprised by a 51% rezidue.
In the case presented by Roychua probably the situation is different. What I understand from his numbers is that all the residue has a boiling range of 300 to 360 ºC. You can call it residue, but this is not a residue to me, it is really heavy crude and most important crack-able. I am calling residue something that is with virtually no value, most probably I will have to spend some money just to discard it.
In my work we start to call a residue a material with BP beyond 1100 ºF (593 ºC) and that is most likely to be solid.

Dear All, I do reallize/ agree that
1) Certain heavy crude(s) do contain an admixture of sludge/ mud materials.
2) Which is neither valuable hydrocarbon nor any real value asphaltene whatsoever; since distilling becomes non-feasible/un-ecomical.
3)Instead may involve costly disposal arrangements.
Probably similar situation is prevalent in above crude's case.
Hope this summarises.
Best Regards
Qalander