3. What is distillation?
ā¢ A process of separating the component
substances from a liquid mixture by selective
evaporation and condensation.
ā¢ Undergoes either complete separation (nearly
pure components) or partial separation
ā¢ Itās a physical separation process and not a
chemical reaction.
4. Vacuum Distillation
ā¢ Vacuum distillation is used for compounds that
have high boiling points (above 200Ā°C).
ā¢ Such compounds often undergo thermal
decomposition at the temperatures required for
their distillation at atmospheric pressure.
ā¢ The boiling point of a compound is lowered
substantially by reducing the applied pressure.
5. Faster processing time
Effective distillation while keeping
the process under the maximum
temperature of the distillation
unitās heater
Effective processing of higher
boiling point solvents without
igniting them or causing thermal
breakdown
Advantages of Vacuum Distillation
6. Vacuum distillation is used for:
ā¢ Liquids to be distilled that have high
atmospheric boiling points.
ā¢ Liquids that chemically change at temperatures
near their atmospheric boiling points.
ā¢ To remove solvents from the mixture without
damaging the product.
7. Laboratory-scale applications
Rotary evaporation
ā¢ Used to remove bulk solvents from the liquid
being distilled.
ā¢ Used by environmental regulatory agencies for
determining the amount of solvents in paint,
coatings and inks
8.
9. ā¢ The rotary evaporator rotates the distillation
flask to enhance the distillation.
ā¢ Rotating the flask throws up liquid on the walls
of the flask and thus increases the surface area
for evaporation.
ā¢ Heat is often applied to the rotating distillation
flask by partially immersing it in a heated bath
of water or oil.
ā¢ The vacuum in such systems is generated by a
water aspirator or a vacuum pump of some type.
10. ā¢ Vacuum distillation columns used in oil
refineries have:
Diameters
ranging up to
about 14 meters
(46 feet),
Heights ranging
up to about 50
meters (164
feet),
Feed rates
ranging up to
about 25,400
cubic meters
per day
(160,000 barrels
per day).
10 to 40 mmHg
of absolute
pressure increases
the volume of
vapor formed per
volume of liquid
distilled.
11. Vacuum distillation is a part of the refining process
that helps to produce petroleum products out of the
heavier oils leftover from atmospheric distillation.
The end product of Atmospheric distillation,
called bottoms, remain at the bottom of the ADU.
To increase the production of high-value petroleum
products, these bottoms are run through a vacuum
distillation column to further refine them.
12. ā¢ Heavies from the atmospheric distillation
column are heated to approximately 400ĖC in a
fired heater and fed to the vacuum distillation
column where they are fractionated into light
gas oil, heavy gas oil and vacuum reside.
ā¢ The distillation column is under a vacuum, or
significantly less than atmospheric pressure of
760 mmHg.
13. ā¢ Superheated steam is injected with the feed and in
the tower bottom to reduce hydrocarbon partial
pressure to 10 mm of mercury or less.
ā¢ Lower partial pressure of the hydrocarbons makes
it even more easier for them to be vaporized, thus
consuming less heat energy for the process.
ā¢ At low pressures, the boiling point of the ADU
bottoms is low enough that lighter products can
vaporize without cracking, or degrading the oil.
14. ā¢ In distilling the crude oil, it is important not to
subject the crude oil to temperatures above 370 to
380 Ā°C because the high molecular weight
components in the crude oil will undergo thermal
cracking and form petroleum coke.
ā¢ The vacuum distillation column internals must
provide good vapor-liquid contact.
ā¢ Ensure to maintain a very low pressure increase
from the column top to the column bottom.
15. ā¢ Refinery vacuum columns often use
distillation trays only when withdrawing
products from the side of the column.
ā¢ The remainder of the column uses packing
material for the vapor-liquid contacting
because such packing has a lower pressure
drop than distillation trays.
ā¢ This packing material can be either structured
sheet metal or randomly dumped packing such
as Raschig rings.
16.
17.
18. Light vacuum gas Oil is sent
to a hydrotreater and then to
a 'catalytic cracking' unit to
obtain smaller chain
hydrocarbons.
Heavy vacuum gas oil is
also sent for cracking using
hydrogen in a
'hydrocracking unit' to
produce smaller chain
hydrocarbons.
19. ā¢ A typical first-phase vacuum tower may produce
gas oils, lubricating oil base stocks, and heavy
residual for propane de-asphalting.
ā¢ A second-phase tower operating at lower vacuum
may distill surplus residuum from the
atmospheric tower, which is not used for lube-
stock processing, and surplus residuum from the
first vacuum tower not used for de-asphalting.
ā¢ Vacuum towers are typically used to separate
catalytic cracking feedstock from surplus
residuum.
20. Vacuum Distillation Unit (VDU)
Process Steps
The reduced
crude oil is
pumped through
a series of heat
exchangers and
a crude furnace
until reaching
the desired
temperature
(350Ā°C ā 390Ā°C)
The reduced
crude oil is
flashed to
separate the
desired
fractions. Light
vapors rise to the
top and heavier
hydrocarbon
liquids fall to the
bottom.
Steam
injection at
the bottom of
the column
improves the
separation of
lighter boiling
components.
The vacuum
column uses a
series of pump
around to
maintain
temperature at
the correct level
at certain points
along the tower.
Light vapour gases
are removed at the
top of the tower,
condensed and
recycled back to the
column as reflux.
Light Naphtha is
drawn off and
excess gases sent to
flare.
Vacuum gas oil
(VGO) and
lubricating oils
are drawn off
and routed for
further
treatment in
Hydrotreating
units.
Vacuum residue from
the bottom is sent to
intermediate storage
or typically to be
further processed in a
FCC or delayed
coking unit.
