Part 4: Atmospheric Distillation

Posted by Doug Mason, M.Sc., Organic Chemistry on Sep 30, 2015 3:00:00 PM

 

Distillation Towers

 

Part 4 in the series on how the chemical industry is solving challenges in the industrial sector.

 

The first major process in the refining of petroleum is the distillation of the crude oil at atmospheric pressure. The products of atmospheric distillation include:

  • LPG and lighter gases
  • Straight-run gasoline
  • Naphtha
  • Gas Oil
  • Fuel Oil (kerosene)
  • Atmospheric Residuum (bottoms/asphalt)

Atmospheric Distaillation drawing showing the seperation of crude oil

 

Distillation is a process by which a desired product is separated from other components of a feedstock by taking advantage of the difference in the boiling points of the various components of the feedstock. As the feedstock is heated, the boiling temperature of some of its lighter components is reached. The lighter components are flashed-off as vapor from the heavier components.

 

The feed entering into the distillation column is first heated to the point where it is a mixture of liquid and vapor. As the heated feed enters the tower, the liquid falls to the tower bottom and the vapor rises to the top of the tower. To provide for effective heat transfer in the tower, some means must be used to promote contact between the rising vapors and the falling liquids. The tower trays serve this purpose. Inside the tower are a series of trays that have perforations in them. These perforations allow the vapors to rise. The trays typically are constructed with a device called a bubble cap. The bubble cap will force the vapor to “bubble” through a layer of collected liquid that is several inches deep on each tray.

 

Inside of the Atmospheric distillation tower

 

As the vapor rises through the liquid, it transfers some of its heat to the liquid. This heat transfer causes the vapor to cool slightly and some of the heavier components of the vapor to condense into a liquid. The reverse is also true. As the liquid on the tray gains heat from the vapor, some of its lighter components are driven off as a vapor. The vapor rises up to the next tray and the process is repeated.

 

Proper operation of a distillation tower (fractionator) depends on the transfer of heat from the vapor to the liquid phase. Tower trays, particularly those in the lower portion of the tower, have a tendency to become fouled. The degree of fouling can be high enough to interfere with the operation of the tower and the quality of the separated products. When this happens, the tower must be cleaned.

 

Tray fouling leads to a poor contact between vapors rising through the tower and the liquid. A fouled overhead condenser can reduce the amount of refluxing that takes place - which can reduce the quality of the overhead products. Fouled reboilers increase the cost of fuel required to operate the unit.

 

The type of fouling found in the atmospheric distillation towers is primarily a mixture of organic and inorganic deposits that can be heavy in iron content. The lower tower trays will have heavy organic deposits, primarily waxes and asphaltenes. As we move to kore and more heavier crudes that are sour (high in hydrogen sulfide content), we also encounter pyrophoric iron scales inside the upper region of the tower that must be addressed to prevent a potential fire before vessel entry before maintenance work can begin.

 

The atmospheric tower decontamination process during a shutdown is a critical path item as maintenance work is dependent on its expedient success. Outage time costs the refiner hundreds of thousands of dollars in lost revenue from lost production time. The tower must be degassed of vapors, cleaned of residual liquids, solid organic deposits must be removed, levels of dangerous hydrogen sulfide must be neutralized and combustible iron sulfide scales must be addressed. All of this must be successfully completed in the least amount of time possible.

 

In our next posting, we will discuss, in detail, the decontamination process and how to improve the results while reducing the time required for vessel entry and maintenance work.

 

Read Previous Posts in the Series:

Part 1: What are the advantages and limitations of chemical cleaning?

Part 2: Crude Unit Decontamination – Desalter

Part 3: Crude Unit Decontamination – The Preheat Train

 

 

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Topics: Degassing, Atmospheric Distillation