3 replies to this topic

[hari2020pkr]

In the vacuum furnace we usually maintain a coil outlet temperature of 386 degrees for reduced crude from CDU. Why cracking not happening here.?

[Bobby Strain]

A bit does.

Bobby

[hari2020pkr]

but usually what is the usual cracking temp of RCO

[Pilesar]

Bottoms of atmospheric columns are generally kept below about 700 F (371 C) because around that temperature thermal cracking begins to increase with coke formers and metals getting into the gas oil. By distilling the bottoms again in a vacuum, additional gas oils can be recovered and thermal cracking kept to a minimum. Flash zone temperatures in a vacuum column are generally a bit higher, but not by much. Cracking increases with the amount of time the material is hot. The compromise is to allow some cracking and recover the optimum quantity and quality of gas oils. 386 C is not an abnormal furnace outlet temperature. But just because you operate at 386 C does not mean that absolutely no cracking occurs.

You can find where cracking becomes noticeable by getting the lab to run a distillation and then plot the results. Around 700 F or so you should see where the curve begins to not follow the expected extrapolation of boiling points and looks like it is trying to level off. The change in slope is due to cracking.

Remember that you are working with a stream that contains thousands of components. Some will be more prone to cracking than others. Not all components which boil at the same temperature are alike. It is convenient to think of crude composition in terms of stable pseudocomponents of a certain boiling point range, but in reality you will have a different actual composition in your products than you do in your feed due to cracking and polymerization. This is the reason that when simulating the vacuum column distillation, it is best practice to force the feed composition to be the sum of the column products. If you were to start with the feed being the measured topped crude, then the simulated products would never match the real world.