Written By: Souad LOUSDAD
We have already mentioned « Atmospheric Distillation” in the previous article An Introduction to Crude Oil Refining we said that it ranges with the “Separation Processes” as the first process the crude oil goes through.
Before we dive in the details, let’s start with simple questions to clarify the global idea:
Why is the “Atmospheric Distillation” the first in the whole refinery process? Can we for example place it in the middle? Can we not consider it once in a while, maybe?
Well, “Atmospheric Distillation” is the first and the most fundamental step in the refining process because it aims to separate crude oil into fractions for further processing by other processing units, that means it cannot be placed somewhere else in the refinery and definitely not set aside.
Now, let’s get to the real business: What is the deal of “Atmospheric Distillation”?
After being extracted, the crude oil goes through some treating processes such as desalting in order to avoid any complication (Corrosion, impurities…) in the process of distillation or further.
The objective of crude distillation is to fractionate/separate crude oil into light-end hydrocarbons.
It is carried out in a tower/column, known as Atmospheric Tower, Pipe Still, Crude Unit, Crude distillation Unit, CDU, Fractional Distillation Column at a pressure slightly above the atmospheric (around 1-3 atmosphere), and a temperature below 380°C (T°˂ 380°C) to avoid undesirable thermal cracking at higher temperature.
When pressure in the distillation column increases, the separation of two Components will be difficult because the volatility decreases and that is why the pressure in our case is maintained slightly above the Atmospheric: To raise the boiling point of the fractions and separate them, also if the pressure is below atmospheric (vaccum) the temperatures decrease and the boiling points of the estimated fractions cannot be reached.
The distillation column consists of a vertical shell where separation of the liquid components is done, column internals referred to as trays or plates to facilitate contact between the vapor phase and the liquid phase, two sections:
The trays between the bottom of the column and the feed tray called stripping section, aiming to concentrate the heavier component in the liquid phase and the trays between the feed tray and the top of the column called rectifying section aiming to concentrate the lighter component in the vapor phase.
About the process: After being extracted, stored in a tank, the crude oil is pumped from storage and the desalting water is then injected just upstream the desalter where salts (Magnesium, Calcium and Sodium Chlorides…) are dissolved.
read also Wet Crude Treatment Plant
in the next articles we will discuss Vacuum Distillation , but now we have to understand that the desalted crude enters then a surge drum to flash off the remaining liquid components in the crude and because of the drop in pressure, they vaporize, taken off overhead the drum, the liquid crude from the bottom of the drum goes through a furnace, pre-heated to the temperature of 280°C (depending on the type of petroleum and the quality of the end products desired) where its vapors condense at different temperatures.
The two – phase heated crude enters the fractionation tower in a lower section called “the flash zone”.
As the liquid heats, the distillate vapors move up the tower counter current to a cooler liquid reflex stream:
Supposing we have a mixture of two liquids A and B to separate and A is more volatile than B.
As we go from tray to tray, the vapor contains more components from A than B and the liquid contains more components from B:
“the vapor richer in A rises from the feed let’s say from tray 3, to tray 4 which is cooler so some components from B will condense leaving the vapor rising from tray 4 to tray 5 and the liquid portion falls from tray 3, to tray 2 which is hotter and it all goes around again, the liquid falls from tray 2 to 1.
Now, the top vapor leaving tray 5 is condensed by cooling it and some is fed back to the top of the column (reflux) while the rest leaves as the top product in order to keep the temperature differential between the trays also some of the bottom product is heated (reboil) and fed back to the bottom tray and the remaining of it leaves as bottom product, this step ensures that temperature falls from tray to tray going up the column.”
Note that in crude oil distillation, we are considering not just two components but a complex mixture of various components.
Each fraction meets the temperature that corresponds to its own particular boiling range; it condenses and turns to a liquid.
In this way, the whole mixture of crude oil is separated on the plates and the cuts are drawn off.
Some of the light product that bubbles through the liquid fraction on each tray is present and can be removed by passing steam through the withdrawn liquid fraction in a stripper, the mixture steam is returned back to the column while the liquid cut goes for further treatment.
To sum up:
The less dense a substance is, the lighter it is, the lower temperature it needs to evaporate and that is why as we go from the bottom to the top of the column:
Temperature decreases, volatility increases and the boiling points drop.
Also, the substances tend to flow easily (viscosity decreases) and become more flammable.
Flash zone vapor rises up through the column, where it is contacted by cooler liquid flowing down due to gravity from the top trays of the column.
At every tray, the liquids and vapors are in equilibrium so; we have a number of different vapor-liquid equilibria at different stages of the column with varying temperature and pressure conditions.
This means that the hydrocarbon composition also varies for different trays with the variation in temperature and pressure:
The lighter cuts diffused into the upper part of the tower are the refinery gases.
Next in order of volatility come the intermediate fractions:
Gasoline, heavy and light naphtha, kerosene, light and heavy gasoil…
Finally, the heaviest fraction condensed in the lower part of the column is called the atmospheric residue and this one pursues the next separation refinery process: the vaccum distillation.
Resources:
– Petroleum refining (separation processes).
– Student’s Guide to Refining.
– Refining Crude Oil.
-Design of Distillation Column Control Systems.