Fractions of Crude Oil & Distillation - Refining
Re-published - @MiserMcMiserson
The author of this article is @MiserMcMiserson from Twitter. I am republishing it with permission, subject to the same disclaimer [see end]. The author otherwise desires to retain anonymity. I felt this thread and podcast were worthy of preserving, consistent with the reasons I’ve created this Substack account.
The thread is part of a long discussion (over 3 hours). Worth listening if you are interested. The images from the folowing article are referenced in the podcast:
What is Crude Oil, and How is it Processed
A daunting question to answer? I think not if you'll permit a little technical discussion. I'm going to explain it so that if you've completed US high school chemistry, you should be able to understand with no issues.If you find yourself not understanding keep listening.
So let's dive in to my visual aids. I'm not going to type everything out, but I will provide labels, and I will assign an arbitrary number to the all of them so they can be easily referenced in the space. These tables, figure, s and graphs hold the key to understand refining.
Let's look at some crude. I have fifteen (15) crude samples in glass jars. I've photographed a few:
Extremely light (42 deg API) on top. On the bottom heavier crude with a lot of precipitated wax at the bottom of the glass.
Samples taken at different depths from production. Production of oil from the ground is not my area of expertise.
A very waxy crude. Nevertheless the API is still pretty light: 35.8 deg API. Evidence that API doesn't tell you everything.
API gravity - Wikipedia
This is a formula for pricing crude oil. Credit to Thomas W. Yeung, P.E.; Crude Oil Quality Group Meeting in Chicago on June 7, 2007, “Latest Refining Advances to Process HACs and BOB.” Supposedly a World Bank formula, but I have never been able to independently verify it.
The other two specs oil is priced against is Sulfur (weight percent) and TAN (total acid number, milligrams if KOH/ gram of oil). Similar to pH, TAN is measured on a log scale and the oil is titratred with the strong base (potassium hydroxide).
Forgot to index! Doh! A plot of the logarithm of TAN (log TAN) vs. API for 0.5 mgKOH/ gram of oil (Credit Yeung). “Fairly acidic crudes.” TAN affects corrosion of the whole front end of a refinery and cannot be run in plants with Carbon Steel only. You have to “metal up.”
A plot of the logarithm of TAN (log TAN) vs. API for 1.0 mgKOH/ gram of oil (Credit Yeung). “Very acidic crudes.”
Enough about TAN. When crude oil is distilled, impurities concentrate into the heavier, higher boiling point fractions of crude oil. How do we measure heavier? A laboratory runs a crude assay for you. The most important part of an assay is the distillation temperature.
Here is a publicly available link to an assay for one of the samples I have, a 33.5° API Crude: BP Thunderhorse in the Gulf of Mexico. Let's take a look at it and the graphs that result because it can teach you a lot. (Source excel link, Stay tuned).
Here is the full page 1 crude assay characterization. Almost everything you need to know about if you want to know if you can run this crude in a particular refinery is right here on this first page. (It's not usually this easy, they must be contracting with a good lab).
If we don't know anything about crude though, the second page tells us more.
I will talk us through this. The particular values aren't really that important, what's important is understanding what they represent: A test which records cumulative liquid volume fractions of crude from zero to one hundred percent at increments of 5 degrees Celsius
THE MOST IMPORTANT SLIDE! Plotting that data yields the following chart. True Boiling Point (TBP) vs. LV% for Thunderhorse Crude Oil (source is excel link above). See the legend on the right. Those are all the petroleum fractions, also known as "cuts"!
Let’s move on to some other crudes! First, the light crudes (remember high API= low density). First up two very light crudes: Eagle Ford WTI (shale oil) and Arab Extra Light (TBP vs. LV%):
Next medium API crudes (27-30 API): Urals from Russia and Arab Heavy TBP vs. LV%. (Arab Heavy is not really "heavy" compared to other crudes).
Now let's look at some heavy crude. Bottom line is that if your API is greater than 24, you are not talking about heavy crude. Check out the affect of the diluent in WCS! Compare and contrast the yield of each of the cuts as the crude goes from light to heavy.
To illustrate this point, compare the two extremes: Very light shale crude oil from Eagle Ford (API=41), with heavy Venezuelan crude (API=16). The VZ crude has already been processed in an upgrader, and you can totally tell because there is very little front end material.
(This will be a good time to pause for questions in the space before we move on.)
Now let's run some crude through a refinery! Here is a block flow diagram of the different process units (aka "units") in a refinery. Every refinery is slightly different based on what crude(s) it was designed to process (source link to follow).
Image Source: Hydrocarbon Processing
Distillation
Process Figures:
Fig 1
Fig 2
Fig 3
Fig 4
Image Source Link.
Distillation Column source link.
So you want to run some Eagle Ford shale oil. Cool. What are you going to fill your heavy ends refinery units with? Well, something... heavy! What about Russian M-100 atmospheric residue? The price was right and it definitely did the trick!
Link to my previous thread about M-100 and other crudes. Look at the graphic but go down this rabbit hole later:
I will explain on the Twitter space why the Russians were doing this to begin with (IMO 2020 + reasons). This concept ties a lot of the previous charts together, and the charts provide us an opportunity to think about suitable crudes to replace it with.
Waxy 35.8 deg API crude, now liquid (took about an hour, maybe because of that light stuff separated and observed earlier), melted with the heat of my hand.
Disclaimer
These graphs are not guaranteed to be correct, and whatever you do with the information you assume your own risk. The author has shared his interpretation, but does not give engineering advice to the public.
I may be tired but is there more or if not, what is the summary point one should take way ?