Atmospheric distillation
Also known as:
Atmospheric tower, pipe still, topping unit, crude unit, primary distillation, crude distillation unit, CDU, fractional distillation
Atmospheric distillation is the first and most fundamental step in the the refining process. The primary purpose of the atmospheric distillation tower is to separate crude oil into its components (or distillation cuts, distillation fractions) for further processing by other processing units.
Atmospheric distillation typically sets the capacity limit for the entire refinery. All crude oil processed must first go through atmospheric distillation. Also atmospheric distillation typically provides most of the feed for the other process units in the refinery.
The design and operation of the atmospheric distillation tower will limit the type of crude that the refinery can process, further limiting the volume and quality of feed to other process units.
How it works
Crude with high salt content will typically be processed through a desalter before going to distillation, to remove salts that could cause corrosion in the distillation tower.
Crude oil is then heated in a furnace to about 700-750 degrees F (400C). The heated crude is injected into the lower part of the distillation column (the flash zone) where much of it vaporizes.
As the vapors rise through the tower they pass through a series of perforated trays or structured packing. As the vapors cool, their components will condense back into liquid at different levels in the tower based on their boiling point.
A portion of the vapors reaches the top of the column where is it cooled through heat exchangers and air coolers and partly converted back into liquid, a portion of which is fed back into the distillation column as a reflux stream to contact with the rising vapors, helping to cool them. This effect of counter-current flows of rising vapors meeting falling cooler liquids allows equilibrium conditions to be established throughout the column. The lighter (less-dense) hydrocarbons will condense at higher points in the distillation tower, heavier hydrocarbons will condenser lower down. This results in separation of the hydrocarbons based on the different temperatures at which they boil/condense.
Hydrocarbons are drawn off of the tower at different heights to get a set of streams of different boiling points. These different streams are called distillation cuts or fractions. These individual streams are then sent to other units for further processing or to finished product blending.
The heaviest fractions of the crude have boiling points above 750 F and do not vaporize. These fractions remian liquid and are drawn off at the bottom of the tower as atmospheric bottoms. These are sent to the vacuum distillation for further fractionation under a vacuum.
Feed
The primary input to a atmospheric distillation unit is crude oil. This can be a single crude oil or a mixture of several different crude oil grades. It is common for refineries to run crudes in a blend, and to change the blend as often as every day. Refineries with multiple distillation units will typically dedicate each to running a specific quality of crude.
For a specific crude oil grade, the typical volumes and qualities of these distillation cuts are detailed in the crude assay. The quality of the crude, especially its composition of different fractions, will largely determine the mix of fractions coming out of distillation.
A refinery may also re-process a mixture of unfinished or off-spec products (called slops) by blending with fresh crude oil and sending the blend to the distillation unit.
Outputs
The outputs from distillation are distillation cuts. Typically, a crude distillation unit will have the following distillation cuts:
refinery gas - comprised of methane and ethane. This stream remains a gas and is used as fuel for the refinery.
light ends - this stream contains primarily propane and butane. It is sent to the gas plant for further separation.
light straight run naphtha - sold as a feed stock for petrochemicals, blended into gasoline, or upgraded through isomerization and then blended into gasoline
heavy naphtha - mostly upgraded through the reformer to increase its octane, but sometimes blended directly into gasoline
atmospheric gasoil - used to make diesel or converted to gasoline through upgrading in the FCC
atmospheric bottoms - this cut contains all of the hydrocarbons that do not vaporize in the atmospheric distillation tower. It is typically fed to the vacuum distillation unit for further separation.
Economics
The atmospheric distillation typically sets the overall capacity of a refinery to process crude oil. So without the distillation unit in operation a refinery is effectively unable to operate.
Understanding the constraints that limit distillation capacity and working to debottleneck them are typically key to increasing refinery throughput and profitability. Maintaining high unit reliability in the distillation unit is key to keeping overall refinery throughput high.
The efficiency of stream fractionation in the distillation unit can be an important driver of overall plant yield and profitability. Poor separation of fractions can result downgrading of valuable material into low value streams and/or lower quality of high valued material due to incomplete separation of lower quality material.
Distillation can also pose a limit on the type of crude oil being processed. Distillation is designed around a specific crude diet, with some amount of flexibility. Moving too far from the design crude quality can limit throughput. Common quality limitations created by distillation include:
Overhead limit - The amount of light ends (refinery gas, LPG, naphtha) that can be handled can be a limit on how much light crude can be processed through distillation
TAN limit - Processing of high acid crude can be limited by the distillation units ability to handle the acid either through metallurgy in the front end of the unit that can withstand highly acidic crude or neutralization at the front end that reduces crude acidity before distillation