Fractional Distillation Flashcards
Outline the function of the sieve trays.
To ensure the mixing if the rising vapour and condensed liquid. To do this the sieves have tiny holes in them small enough to allow the vapour to pass though and mix with the liquid whilst preventing the liquid from seeping through. Alternatively there may be valves over the holes which open when pressure below the tray is higher than that above it. These are more efficient.
Briefly Outline the process of fractional distillation.
Crude oil is heated in a furnace at about 640K and the resulting vapour is fed into a fractioning column which can have a height of 25-100m. These towers contain 30-40 sieve trays which ensure intimate mixing between the descending liquid and ascending vapour.
What kind of gradient exists in the tower?
A temperature gradient with the top being cooler than the bottom.
What happens when vapour reaches a tray whose temprature is below the vapour’s boiling point?
It partially condenses. As this happens the dissipated latent heat heats more liquid and the more volatile components of the liquid evaporate, joining the remaining vapour and passing up the tower. The less volatile liquid flows across the tray and down a pipe to the tray below.
What does this process result in?
This results in each tray containing products with a relatively narrow boiling point range called a ‘close cut’ of products.
Where are products with high molecular mass found?
At the bottom of the tower.
Where are products with low molecular mass found?
At the top of the tower.
What happens to the high boiling point residue at the bottom?
It is transferred to another column and distiller under vaccum.
Why is the heavier residue distiller in a vaccum?
This reduces the boiling point and ensures the constituents distill at temperatures below their decomposition temperature.
What are the products of the secondary distillation?
From this process lubricating oils and waxes are obtained and the final residue is called bitumen. It also produces a feedstock for catalytic cracking.
What is found at the very top of the fractional distillation tower?
Gasses which have a boiling point lower than 300K. These molecules have chain lengths of 1-4 and account for about 4% of the total product. This may be used as fuel on site or as liquified fuel (L.P.G), it can also be used as a feedstock for chemicals.
What is found just below the gas?
Naphtha, of which there are two types; heavy naphtha and light naphtha. These have a chain length of between 5 and 10.
15-30% of the total product is Light naphtha (bp 300-470K) and 5-10% is heavy naphtha (bp 403-493K). This is used as fuel (petrol) and as a chemical feedstock for making plastics etc.
What is found just below Naptha in the fractional distillation column?
Kerosine consists of molecules which have a chain length of 5-10. It’s boiling point is between 450 and 530K and accounts for 10-15% of the total product. It is used as a feedstock for jet engine fuel, paraffin and domestic oil production.
What is found in Below Kerosine in the fractional distillation tower?
Gas oils and heavy gas oil. This has a chain length between 14 and 20, has a boiling point of 530-620K and accounts for 15-20% of the total product. It is used as diesel fuel, for blending fuel oil and as a chemical feedstock.
What is found below gas oil in the fractional distillation tower?
The residue which consists of molecules with chain lengths greater than 20. This is the feedstock for vaccum distillation.
