The process used in the extraction of various hydrocarbons is distillation, or fractionation , and the process happens in a Distillation or Fractionation Column.
So how does the distillation process work?
The various components or fractions in crude oil, that make up gas, petrol, diesel, heating oils, naphtha, ethylene, polyethylene and bitumen / asphalt, have different sizes, weights and boiling temperatures.
These distillates (or fractions) need to be separated.
Fractional distillation is useful for separating a mixture of substances with small differences in boiling points, and is the most important step in the refining process.
Because of their different boiling temperatures, they can be easily separated by the distillation process. The steps of distillation are as follows:
The mixture of two or more substances (liquids) with different boiling points to a high temperature is heated with high pressure steam to about 1112 degrees Fahrenheit / 600 degrees Celsius.
The mixture boils, forming vapor (gases); most substances go from liquid into the vapour phase.
The vapor enters the bottom of the distillation column which is filled with trays or plates. The trays have many holes in them which allow the vapor to pass through. The trays increase the contact time between the vapour and the liquids in the column and this helps to collect liquids that form at various heights in the column.
There is a temperature difference across the column from being hot at the bottom, to cool at the top.
The vapour rises in the column.
As the vapour cools as it rises through the trays in the column.
When a substance in the vapour reaches a height where the temperature of the column is equal to that substance's boiling point, it will condense to form a liquid. (The substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column.).
The trays collect the various liquid distillates.
The collected liquid fractions may pass to condensers, which cool them further, and then go to storage tanks, or they may go to other areas for further chemical processing.
This image from http://worldenergyblog.com shows where the different fractions are siphoned off, which co-incide with their different boiling points.
Parts of a Distillation Column
The feed, containing the mixture to be separated usually enters around the middle of the column.
It can be in any state from a cool liquid to a superheated vapour.
Liquid and vapour travel in opposite directions in the column.
Liquid travels down, while vapour travels up.
At each distillation stage, some of the vapour is condensed collected on the distillation tray.
At the same time, some of the liquid travelling down the column is evaporated by the tray.
This is the section above the feed.
This is also where the the more volatile components of both the liquid and vapour are concentrated.
This is the section below the feed.
This is also where the the less volatile components of both the liquid and vapour are concentrated.
Both the Rectifying and Stripping Stages have liquid outlets at intervals up the column which allow for the withdrawal of different distillates or products having different boiling points or boiling ranges.
The overhead vapour, containing the most volatile elements of the feed, move from the top of the column to the compressor, where water is often used as a cooling media to return it to a more stable liquid state.
From the condenser, Reflux is fed back into the column, travels down the column where it is reheated and the process starts again.
The condenser also produces Overhead Product, which meets defined specification for a product.
In the Reboiler, steam evaporates the bottom liquid, and sends it back up the column.
The Bottom Liquid, contains the least volatile elements of the feed, and flows from the base of the column to the reboiler.
The Bottom Product meets defined specification for a product
Considerations for the piping designer.
Considerations for the piping designer before starting your piping design on a Column.
From the basic layout and engineering specifications:
Minimum access, platform width, walkways and headroom requirements.
Handling Facilities for the column internals, line blinds, relief valves, manhole covers.
Max. rise of ladders.
Requirements of piping systems.
Min. line size and number and location of hose stations.
Access requirements for valves and instruments.
From Design Standards:
Details of ladder dimensions
Ladder and platform position - step through or side step landings.
Toe plate, hand rail and safety gate details.
From P&ID and Tower Technical Specification:
Process data showing interconnected equipment and piping
Pipe sizes, specs and components
Steam tracing and insulation thickness
Tower elevations and differences in related equipment levels.
From the Plot Plan:
The physical location of a column and it relationship to other equipment.
Main pipe rack or pipe run
From the Instrument Standards:
The location of instrument connections on the column for gauges, level controllers and level alarms
Location of pressure and temperature connections.
From the column fabrication drawings:
Diameter and height of the column.
Dimensions and details of column internals.
Process Piping Connections
Pump, exchanger, drum drawings showing details of of process equipment adjacent to, or supported on the column itself.
- The terms Column / Tower, Fractionation / Distillation, Fractions / Distillates are interchangable depending on the region, project or site on which you are working.
- Related Articles:
Section - 8A: General Guidelines for Equipment and Piping Location, Spacing, Distances and Clearances
Section - 8B: Introduction to Vessels and Vessel Orientation
Section - 8C: Vertical Vessel Orientation
- Special thanks to James O. Pennock for his superb editorial input.
About the Author
Anton Dooley is a Piper with over 25 years experience covering process plant engineering, design & training. He is the founder of pipingdesigners.com