Distillation

Question 1

Desalting

Answer 1

Desalting is required before adding the feed into the distillation process in order to reduce salt content and remove suspended solids. The crude will require desalting is the salt content is more than 10lb/bbl.
The desalting is performed by rising the crude with 3-10% volume of water. When the water is added, the salt crystals dissolve in the water phase and with the welling of the solids by water, the solids become suspended in the water phase. the water and oil emulsion is then send to a settler/separator for separation either by the addition of chemicals or by electrostatic separation. Electrostatic separation is more efficient as it can get a higher separation at lower temperatures. With one stage separation, about 90-95% efficiency can be achieved and if two stages are used, we can get up to 99% efficiency. Desalting in a naphthene heavy crude is less efficient as naphthene makes a more stable emulsion with water. Wetting agents can also be added to make sure that the solids are trapped by the water and oil blowdown in the separator is reduced.

Question 2

Main components of feed

Answer 2

paraffins, naphtha, aromatica. olefins are only formed during processing.

Question 3

What to do to improve reflux and thermal efficiency

Answer 3

As the liquid is continually drawn to of the side streams in the column, the amount of reflux below the draw pff point reduces. So in order to achieve max reflux we need to remove all the heat at the top of the tower. The problem with this is that the operation would require a tower design which has a high diameter at the top like an inverted cone. In order to prevent this, even out the liquid loading and also improve energy effect, the use of draw off and pump around stream.

• Draw off streams are intermediate liquid streams which are taken out of the volume, cooled down in a heat exchanger and then returned back in to the column. Return of the cooled liquid into the column causes the vapour to condense have increasing reflux.
• For a homogeneous operation pump around streams have to be included at the bottom of the column to stimulate the creation of vapour that rises up.
• Steam is also added at the bottom of the tower to reduce the partial pressure of the hydrocarbons.
• Apart from all these when liquid product side streams are taken out they contain light gases which reduce flash point. Theses gases are removed in a stripping column where heat is added at the bottom. The light gases and steam are then re-injected back into the column at a the vapour zone

Question 4

Vacuum Distillation

Answer 4

Vacuum distillation is performed in order to reduce the boiling point of the heavy feed. If the normal atmospheric volume is operated at boiling temperatures of the heavy crude, thermal cracking of the lighter ends will occur ends giving unwanted products.

To further increase vaporisation partial pressure is reduced by the addition of steam. Steam can be added at the feed point to the furnace or in the column itself. Where the feed is added represents the kind of operation.

• wet operation: steam is added at the feed and in the column.
• damp operation: adding steam in only one place
• dry operation: no steam added.

Steam is added to minimise coke formation, increase inlet tube velocity and reduce hydrocarbon partial pressure.
Low pressures are maintained by a system of steam ejectors.

Question 5

Crude distillation

Answer 5

Desalting takes place first. Before entering column, the feed is heated to enable flashing once it enters the column.Crude temp must be high enough to allow about 80% flashing while 20% is used for internal reflux. Steam is added at the bottom, reflux is gasoline, bottoms goes to the vacuum unit.

Question 6

Products of distillation

Answer 6

Fuel Gas 
Wel Gas 
LSR Gasoline
HSR Naphtha
Light Gas Oil
Heavy Gas Oil 
Residuum