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Distillation tower in Refinery:

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. Also atmospheric distillation typically provides most of the feed for the other process units in the refinery.

Refining of crude oils essentially consists of primary separation processes and secondary conversion processes. The petroleum refining process is the separation of the different hydrocarbons present in the crude oil into useful fractions and the conversion of some of the hydrocarbons into products having higher quality performance.

Atmospheric and vacuum distillation of crude oils is the main primary separation processes producing various straight runproducts, e.g., gasoline to lube oils/vacuum gas oils. Distillation of crude oil is typically performed either under atmospheric pressure and under a vacuum. Low boiling fractions usually vaporize below 400°C at atmospheric pressure without cracking the hydrocarbon compounds.

Therefore, all the low boiling fractions of crude oil are separated by atmospheric distillation. A crude distillation unit (CDU) consists of pre-flash distillation column. The petroleum products obtained from the distillation process are light, medium, and heavy naphtha, kerosene, diesel, and oil residue.

FCC Distillation Column

Crude oil obtained from the desalter at temperature of 250 °C–260 °C is further heated by a tube-still heater to a temperature of 350 °C–360 °C. The hot crude oil is then passed into a distillation column that allows the separation of the crude oil into different fractions depending on the difference in volatility. The pressure at the top is maintained at 1.2–1.5 atmso that the distillation can be carried out at close to atmospheric pressure, and therefore it is known as atmospheric distillation column.

petroleum distillation tower

The vapors from the top of the column are a mixture of hydrocarbon gases and naphtha, at a temperature of 120 °C–130 °C. The vapor stream associated with steam used at bottom of the column is condensed by the water cooler and the liquid collected in a vessel is known as reflux drum which is present at the top of the column. Some part of the liquid is returned to the top plate of the column as overhead reflux, and the remaining liquid is sent to a stabilizer column which separates gases from liquid naphtha. A few plates below the top plate, the kerosene is obtained as product at a temperature of 190 °C–200 °C. Part of this fraction is returned to the column after it is cooled by a heat exchanger. This cooled liquid is known as circulating reflux, and it is important to control the heat load in the column. The remaining crude oil is passed through a side stripper which uses steam to separate kerosene.

The kerosene obtained is cooled and collected in a storage tank as raw kerosene, known as straight run kerosene that boils at a range of 140 °C–270 °C. A few plates below the kerosene draw plate, the diesel fraction is obtained at a temperature of 280 °C–300 °C. The diesel fraction is then cooled and stored. The top product from the atmospheric distillation column is a mixture of hydrocarbon gases, e.g., methane, ethane, propane, butane, and naphtha vapors. Residual oil present at the bottom of the column is known as reduced crude oil(RCO). The temperature of the stream at the bottom is 340 °C–350 °C, which is below the cracking temperature of oil.

Simulation helps in crude oil characterization so that thermodynamic and transport properties can be predicted.Dynamic models help in examining the relationships that could not be found by experimental methods (Ellner & Guckenheimer, 2006). By using modeling and simulation software, 80% of the time can be saved rather than constructing an actual working model. Also it saves cost. Moreover, a model can provide more accurate study of the real system.

The various components of crude oil have different sizes, weights and boiling temperatures; so, the first step is to separate these components. Because they have different boiling temperatures, they can be separated easily by a process called fractional distillation. The steps of fractional distillation are as follows:

  1. You heat the mixture of two or more substances (liquids) with different boiling points to a high temperature. Heating is usually done with high pressure steam to temperatures of about 1112 degrees Fahrenheit / 600 degrees Celsius.
  2. The mixture boils, forming vapor (gases); most substances go into the vapor phase.
  3. The vapor enters the bottom of a long column (fractional distillation column) that is filled with trays or plates. The trays have many holes or bubble caps (like a loosened cap on a soda bottle) in them to allow the vapor to pass through. They increase the contact time between the vapor and the liquids in the column and help to collect liquids that form at various heights in the column. There is a temperature difference across the column (hot at the bottom, cool at the top).
  4. The vapor rises in the column.
  5. As the vapor rises through the trays in the column, it cools.
  6. When a substance in the vapor 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.).
  7. The trays collect the various liquid fractions.
  8. 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
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