9 replies to this topic

[djack77494]

Hi all,
I'm fairly new to refining and am not sure I fully understand the definition of the term "cut point". I used search engines and other resources I have available, but still don't feel I have a totally acceptable and understandable definition. Would be best if you could define it relative to operation of a crude unit. Your help is appreciated.
Thanks,
Doug

[Zauberberg]

Hello Jack,

Cut point is the temperature on crude oil cumulative TBP curve, which defines the yields of specific cuts (naphtha, kero, gas oil, residue etc.).
For example, if you define your naphtha TBP cut point of 220C, you can read from the curve a cumulative yield (vol% or weight%) of C1-220C fraction which corresponds to the selected 220C. From the operational point of view, this does not mean that your CDU naphtha will have the same end-point: it will be slightly higher, which depends on fractionation efficiency between naphtha and adjacent side product of your atmospheric tower. It also means that initial boiling point of that same adjacent product will probably be lower than 220C. In other words, there is no 100% perfect separation between adjacent cuts.
When looking at crude oil TBP curve with defined cuts, usually there are TBP curves of all individual cuts, and you can see the "overlaps" of adjacent cuts - in forms of distillation tails at frond and rear ends. These tails, amongst other parameters immanent to crude oil species, define your products qualities (flash, freeze, cloud point...), which all together define maximum yields of every specific cut.

I hope this makes the picture somewhat more clearer.

Regards

[abhi_agrawa]

Hello Jack,

Crude oil, gasoline, disel and other refinery feeds and products consists of several of components. They are often characterized by their boiling curve. Like (most) mixtures (ie. other than azeotropes) they have a range of boiling point. The temperature at which they starts to boil is refered to as the IBP (Initial Boiling Point), and the temperature at which they boil off completely is refered to as the FBP (Final Boiling Point). A curve of temperature vs. the volume percent boiled off is refered to as the TBP curve (sometimes instead of volume percent, weight percent is also used).

Let us say that you have a feed with a certain TBP curve, and you would like to split it into several streams like Light Naphtha, Heavy Naphtha and Kerosene etc. So you feed this stream to a fractionator, and cut the feed. So if you Light Naphtha specification is such that the FBP should be (say) 85°C, then in refinery terminology it is said that the cut point of Light Naphtha is 85°C.

Hope this helps,
Abhishek

[djack77494]

Abhi & Zauberberg,

Thank you both for your help. Your responses helped enormously and I now feel I have a grasp on the definition.

Doug

[Zauberberg]

Hello Doug,

Is there some way to visit this refinery in Alaska?
It must be very exciting to run process plants in such non-friendly environment. Did you have opportunities to watch Aurora Borealis?
What's it like, working in Alaska?

[djack77494]

Zauberberg,
Though it's a bit off topic, hopefully our host will not mind me addressing your reply.

I am pretty widely traveled, having been to many countries in North & South America, Europe, Africa, and Asia. I've seen 46 of 50 US states, about half the provinces of Canada, and also about half the countries of Europe and Asia. (I even went through Serbia, though it was Yugoslavia back then.)

In my opinion, Alaska is the best place I have ever lived or visited. (I am not originally from Alaska.) I love the climate of Anchorage, which is on an inlet of the sea and so does not have very harsh winters. Do not be surprized by that. If we were talking about the Alaskan interior or the North Slope, I would be saying very different things. There are many opportunities to see the Aurora Borealis (or Northern Lights), but it is difficult to do so in Anchorage, due to the high concentration of artifical lighting. Fairbanks in mid winter would maximize your chances, though there is a price to be paid.

Almost all engineering opportunities in Alaska are related to North Slope oil production. There is a small amount of work occuring with Cook Inlet gas production and processing, and an even smaller amount of work connected with the two small refineries in the state. At the moment, I am working on an out-of-state project and cannot really advise you regarding a refinery visit. You might want to develop some contacts that could assist you. The refineries are run by Flint Hills in North Pole, AK, and Tesoro in Kenai, AK. Regarding the unfriendly environment, I've not seen a location that didn't have an unfriendly environment. Even producing oil from an Indian oil field, the oil we were looking at was so waxy that the desert heat was way below what was necessary to keep the oil from setting up. There are always environmental challenges; they just come in many different forms. As the saying goes, "that's why they pay us the big bucks".

Hope that helps,
Doug

Hello Doug,
Is there some way to visit this refinery in Alaska?
It must be very exciting to run process plants in such non-friendly environment. Did you have opportunities to watch Aurora Borealis?
What's it like, working in Alaska?

[Zauberberg]

Thanks, Doug

This was the complete answer! When I think of Alaska, at least from what I know after looking at beautiful landscapes available on the internet, I cannot imagine more attractive place to live and work at (except Virgin Islands, maybe - there is also one Hovensa refinery I would like to visit, and open a beach bar after leaving process operations job). It looks like perfect end of ChE career...

Best regards, friend

[dylant]

Abhi (or anyone else):

I am reading your answer and I have question.
How do you determine the IBP, FBP, and TBP of the feeds?

Thanks,
Dylan

Hello Jack,

Crude oil, gasoline, disel and other refinery feeds and products consists of several of components. They are often characterized by their boiling curve. Like (most) mixtures (ie. other than azeotropes) they have a range of boiling point. The temperature at which they starts to boil is refered to as the IBP (Initial Boiling Point), and the temperature at which they boil off completely is refered to as the FBP (Final Boiling Point). A curve of temperature vs. the volume percent boiled off is refered to as the TBP curve (sometimes instead of volume percent, weight percent is also used).

Let us say that you have a feed with a certain TBP curve, and you would like to split it into several streams like Light Naphtha, Heavy Naphtha and Kerosene etc. So you feed this stream to a fractionator, and cut the feed. So if you Light Naphtha specification is such that the FBP should be (say) 85°C, then in refinery terminology it is said that the cut point of Light Naphtha is 85°C.

Hope this helps,
Abhishek

[Zauberberg]

Hello again Doug,

I was making backup of my PC files and I found that picture (cumulative TBP curve) we were talking about. Please find it attached.

Best regards,

[attachment=536:TBP_crude.JPG]

[djack77494]

Thank you, Zauberberg