1. Refinery: Separation units
Distillation

2. Distillation, the most common of all refinery separation processes, uses differences in boiling points to separate hydrocarbon mixtures.

3. Distillation unit First step in crude oil processing
Flashed: When a mixture is heated enough to vaporize part of it, the vapor will have a higher concentration of low boiling point materials than the liquid, while the liquid will be richer in high boiling point materials than the vapor.

4. Continuous distillation columns
Classified according to:
Nature of the feed that they are processing:
binary column - feed contains only two components;
multi-component column - feed contains more than two components.
Number of product streams they have:
multi-product column - column has more than two product streams.
Where extra feed exits when used to help with the separation:
extractive distillation - where the extra feed appears in the bottom product stream;
azeotropic distillation - where the extra feed appears at the top product stream.
Type of column internals:
tray column - trays of various designs used to hold up the liquid to provide better contact between vapour and liquid;
packed column - packings are used to enhance vapour-liquid contact.

5. Main Components of Distillation Columns
A vertical shell where separation of liquid components is done.
Column internals e.g.trays/plates and/or packings which are used to enhance component separations.
A reboiler to provide the necessary vaporization for the distillation process.
A condenser to cool and condense the vapour leaving the top of the column.
A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column.

6. Atmospheric distillation column
Crude oil is pumped through a series of heat exchangers to raise temp = 290oC
Further heated in furnace (direct-fired heater) until 400oC
Vaporised product charged to flash zone of atmospheric column

7. Preheat Heat exchanger train

8. Atmospheric distillation column
Crude oil enters the atmospheric distillation column on a tray very near the bottom of the column.
The column comprises of a series of trays, liquid sidedraws, pump around, a condenser, and a reboiler
The crude oil is heated to oC. The liquid falls to the bottom and the vapor rises. Heavier hydrocarbons condense more quickly and settle on lower trays and lighter hydrocarbons remain as a vapor longer and condense on higher trays.

9. Stripping stream to reduce the partial pressure of hydrocarbons
Circulation pump
(Pump around)
Side drawn stripper
Stripping stream to reduce the partial pressure of hydrocarbons

10. Atmospheric distillation column
Contains fractionation trays
5-8 trays required for each sidestream product
Each of the light ends from sidestreams are further stripped in a column of 4-5 trays with steam introduced at the bottom of the tray

11. Side drawn stripper

12. Products of Atmospheric distillation unit:
Fuel gas,
gasoline,
diesel,
jet fuel,
kerosene,
light gas oil,
fuel oil, and
heavy gas oil

13. Inside a distillation column…
Vapor rising up the column meets cooler downflowing liquid, which causes some of the impurities to condense. Eventually, the vapor that exits the top of the column is of the desired purity
Each tray collects a liquid that corresponds to a particular boiling range
At the top of the tower, vapor is condensed and recycled
At steady state there are two flows: hot, rising vapor and cooler, downward flowing condensate.

14. Inside a distillation column…
As the vapor passes through the liquid on the tray, some of it condenses and is left behind.
The trays are equipped with overflow weirs and downcomers, so that when the liquid reaches a certain level, it overflows down to the tray immediately below it.
The temperature on this tray is higher than the tray above. This causes some of the lighter components in the liquid mixture that arrives from the tray above to be re-vaporized and again rise.
Other components stay behind in the liquid state, so that the condensate on the tray is more pure.
This cycle is repeated over and over again on the trays inside the column: condensates vaporize, rise, condense, fall, re-vaporize, and rise, again and again. Each time, the condensates left on a tray are a little more pure.