Refinery Crude Oil Distillation Process Video Lecture | Chemical Technology - Chemical Engineering

one of the key elements of many process
plants is a distillation system it's
important for operators to understand
how distillation systems work because
it's part of their job to make sure the
systems run properly the purpose of any
distillation system is to separate a
liquid mixture into two or more
components the separation takes place in
a distillation column or tower the
liquid mixture is called feed or charge
and the components into which it
separated are called cuts or fractions
let's take a look at a simplified
representation of a distillation system
in this example the feed is stored in a
tank during operation a pump is used to
move the feed from the tank to a pre
heater in the pre heater the mixture is
heated under pressure to just below its
boiling point the pressure in the tower
is lower than the pressure in the pre
heater so when the feed enters the tower
it starts to boil the vapors from the
boiling liquid which primarily contained
the lighter components in the feed rise
in the tower the remaining liquid which
consists primarily of the heavier
components in the feed moves down the
tower and collects at the bottom some of
this liquid is drawn off as the bottoms
product and some of it's routed to a
device called a reboiler which is
connected to the bottom of the tower the
reboiler is usually a heat exchanger
that's designed to vaporize the lighter
components that remain in the liquid
from the bottom of the tower vapors from
the reboiler or in some cases a mixture
of vapors and liquid re-enter the tower
the vapors then rise up in the tower
these vapors and the heat they contain
are often referred to as boil up the hot
boil up provides heat that's needed for
the distillation process to take place
in the tower the vapors that rise up in
the tower are routed to a condenser the
purpose of the condenser is to cool and
condense the vapors into liquid from the
condenser the liquid flows into a
receiver or accumulator the receiver
provides a reservoir for the liquid part
of the liquid from the receiver is
pumped back
to the top of the tower and part of it
is drawn off as the towers overhead
product well we've just looked at one
simple example of a distillation system
not all distillation systems are the
same but most of them have the basic
equipment that we've just seen at the
heart of a distillation system is the
distillation tower basically a
distillation tower is a series of
distillation processes or stills stacked
on top of each other the process of
distillation is sometimes referred to as
fractional distillation or fractionation
and distillation towers are sometimes
called fractionators fractionation is
distillation that occurs at different
levels in a tower this is a simplified
illustration of the inside of one type
of distillation Tower this tower uses
trays called sieve trays to separate
vapors and liquid the trays are spaced
throughout the tower
they're called sieve trays because they
have many openings in them like a sieve
here's a closer view of some of the
trays we've exaggerated the size of the
openings to make them easier to see the
openings in the trays allow vapors to
rise through the trays on their way up
the tower each tray is also designed to
hold liquid dams or Weir's on each tray
allow liquid to build up on the tray
liquid that overflows the Weir's flows
into downcomers the downcomers channel
the liquid from tray to tray down the
tower when the upward moving vapors and
the downward flowing liquid come in
contact on each tray the vapors transfer
some of their heat to the liquid two
things happen the heavier components of
the vapors become cooler and condense
into liquid and the lighter components
of the liquid boil the vapors then rise
toward the next tray as this process
continues the rising vapors contain a
higher concentration of lighter
components and a lower concentration of
heavier components so this is how the
distillation process is repeated on each
tray as the vapors rise through the
tower now not every distillation tower
is designed in the exact same way
however every tower can be divided into
three basics
the middle section is where the feed
enters the tower and part of the feed
vaporizes this vaporization is commonly
known as flashing so this section of the
tower is often called the flash zone the
section above the flash zone is called
the rectification section in this part
of the tower the concentration of
lighter components increases the section
of the tower below the flash zone is
called the stripping section in this
section the lighter components are
vaporized or stripped from the heavier
liquid if a distillation tower worked
perfectly it would produce pure products
unfortunately that's not the case
the lighter products usually contain
some heavier fractions and the heavier
products usually contain some lighter
fractions this is sometimes referred to
as overlap distillation systems use
several methods to help maximize the
purity of the products one of these
methods is called refluxing the vapors
coming off the top of the tower are
condensed in a condenser and then
collected in a receiver part of the
liquid from the receiver is sent to
storage or to other units in the plant
as the towers overhead product the rest
of the liquid is pumped back into the
top of the tower the liquid that's
reintroduced into the tower is called
external reflux because it consists of
liquid that was cooled in the condenser
the external reflux is cooler than the
liquid in the top of the tower as the
external reflux cools the top of the
tower vapors made of heavier fractions
condensed liquid made of heavier
fractions flows down the tower and is
referred to as internal reflux meanwhile
the top of the tower is still hot enough
to keep the lighter fractions in vapor
form the vapors are drawn off the top of
the tower and into the condenser
refluxing increases the purity of the
overhead product because condensing the
vapors made of heavier fractions keeps
them out of the stream of vapors that
leaves the top of the tower another
method used to maximize product purity
is called reboiler the bottoms liquid
that's drawn off from the tower is sent
to a heater called a reboiler
the rest of the bottoms liquid is sent
to storage or to other units in the
plant as the towers bottoms product the
reboiler heats the liquid it receives so
that a mixture of vapors and liquid is
formed depending on the system either
vapors or the mixture of vapors and
liquid is then reintroduced into the
tower the hot vapors cause any lighter
fractions in the liquid at the bottom to
vaporize and move up the tower this
reduces the amount of lighter fractions
in the bottoms product the reboiler
provides the major portion of the heat
that's required to make the distillation
process work in the type of system we've
been discussing the heat supplied by
preheating the feed is not sufficient
for proper distillation the reboiler is
needed to provide enough additional heat
to vaporize the lighter fractions in the
tower so that the products meet
specifications although all distillation
towers serve the same basic function
they don't all have the same internal
components one type of tower uses trays
called sieve trays to separate vapors in
liquid that type of Tower is covered in
the tower operation part of this program
in this part will look inside to other
types of distillation towers and see how
their internal components differ let's
start with a tower that uses trays with
bubble caps the holes in each tray are
covered with caps called bubble caps the
slots in these bubble caps disperse the
rising vapors through the liquid on the
tray each bubble cap has many slots and
each tray has many bubble caps to spread
out the vapors this ensures a maximum
contact between vapors and liquid so
that maximum heat transfer can take
place
another type of tower called a packed
tower contains layers of devices called
packing instead of trays with bubble
caps there are many different types of
packing some towers have sections that
are filled with cylindrical rings like
the one Illustrated here it's known as a
rash agreeing another type of packing is
known as a burl saddle the packing
breaks up the liquid so that it flows
over a large amount of surface area this
exposes more of the
liquid to the vapors and increases heat
transfer from the vapors to the liquid
packing can be made for many different
materials including porcelain copper
aluminum and iron the main requirement
is that the material must be compatible
with the liquid in the tower and the
conditions under which the tower is
operated this is a section of another
type of packing from a distillation
tower it's called a packing grid each
layer in the grid has spaces for vapors
to rise through the packing on their way
up the tower liquid flows over these
surfaces which channel the liquid as it
flows down the tower the packing grid
provides a great deal of surface area
where contact between the vapors and the
liquid can take place so heat transfer
is maximized temperature control is
crucial to the operation of a
distillation system correct temperatures
are necessary to vaporize the lighter
components of a liquid mixture while
keeping the heavier components in the
vapors to a minimum if the temperatures
at certain points in the system are
either too high or too low
acceptable products won't be produced
let's go over several points in the
distillation system where temperature is
monitored as we do keep in mind that the
mixture will be referring to is a binary
liquid mixture that is a mixture that
has two components we'll begin with the
temperature at the top of the tower the
temperature at the top of the tower
should be at or slightly above the
boiling temperature of the desired
overhead product at the operating
pressure of the tower if the temperature
at the top of the tower is higher than
it should be more of the heavier
components will vaporize and become part
of the overhead product instead of
flowing down the tower as a liquid on
the other hand if the temperature at the
top of the tower is lower than it should
be less of the lighter components will
vaporize some of the lighter components
will remain as a liquid and flow down
the tower the temperature at the bottom
of the tower is also important the
temperature at the bottom of the tower
is usually slightly below the boiling
point of the heavier component if the
temperature at the bottom of the tower
is too high more of the heavier
components will vaporize and move up the
tower instead of remaining as a liquid
if
the temperature at the bottom of the
tower is too low less of the lighter
components will vaporize and move up the
tower another place where temperature
control is important is at the feed
point the temperature at the feed point
should be within the boiling range of
the mixture the temperature at the feed
point should be close to the temperature
of the feed tray the temperature of the
feed tray depends on its physical
location in the tower for example the
lower the feed tray is in the tower the
higher its temperature will be if the
temperature at the feed point is higher
than it should be more of the heavier
components will vaporize and move up the
tower instead of moving down the tower
as a liquid on the other hand if the
feed point temperature is too low less
of the lighter components will vaporize
and more lighter components will end up
in the bottom of the tower while the
temperatures at various points in a
distillation system are important the
relationships between the temperatures
are also important the temperature
decreases is the material moves higher
in the tower the gradual decrease in
temperature from the bottom of a
distillation tower to the top is called
the temperature gradient the temperature
gradient is measured in terms of the
difference between the temperature at
the bottom of the tower and the
temperature at the top of the tower in
order to maintain the proper temperature
gradient temperatures at critical points
in the system must be controlled
temperatures in a distillation system
are typically controlled in three ways
one way is to control the temperature of
the feed mixture by using a pre heater
this regulates the temperature at the
feed point at the bottom of the tower
temperature is controlled by the amount
of heat that is added by the reboiler
this added heat is referred to as boil
up the temperature at the top of the
tower is controlled by the amount or the
temperature of the cool liquid that's
pumped back into the tower from the
overhead receiver this is called the
reflux rate increasing the reflux rate
decreases the temperature at the top of
the tower some distillation systems
contain equipment known as pump arounds
the purpose of a pump around is to
remove hot liquid from the tower and
palm
through a cooler the cooled liquid is
then reintroduced at a higher level in
the tower a pump around helps control
the temperature of the internal reflux
since pressure affects the boiling
temperature of a liquid it's an
important factor in the distillation
process if precious in a distillation
system are not correct product purity
could decrease tower pressure is often
controlled by a pressure control valve
located on the overhead receiver this
valve controls pressure by releasing
vapours and any non condensable gases
that have collected in the receiver in
some cases a vacuum system draws gases
from the receiver to control tower
pressure another pressure that's
important to an operator is the towers
differential pressure the towers
differential pressure is the difference
between the pressure at the bottom of
the tower and the pressure at the top of
the tower this difference in pressure is
caused by the flow of vapors in the
tower without vapor flow there is no
differential pressure generally if the
rate at which the vapors move up the
tower decreases the differential
pressure will also decrease and if the
rate at which the vapors move up the
tower increases the differential
pressure will increase changes in the
differential pressure may indicate that
a problem exists for example an increase
in differential pressure could be an
indication that the feed rate is too
high too much feed entering the tower
will overload it if this happens the
differential pressure will increase and
the tower will be unable to make the
desired separation in this case it might
be necessary to decrease the feed rate
now changes in the towers differential
pressure may be caused by other problems
for example if the differential pressure
increases the boil up rate may be too
high in other words the reboiler may be
returning too much vapor or vapor liquid
mixture to the tower this problem can be
corrected by reducing the boil up rate
another problem that may affect the
towers differential pressure is a
decrease in condenser efficiency if the
condensers tubes are plugged or there's
not enough cooling water flowing through
the condenser the
pressure will increase as a result the
flow of vapors from the tower to the
condenser will decrease and so will the
vapor flow up the tower this means that
the tower top pressure will increase and
the differential pressure will decrease
if a condenser problem is suspected the
condenser should be checked and
corrective actions should be taken if
necessary now changes in the towers
differential pressure may have other
causes in addition to the ones we've
identified so it's important to
carefully evaluate any situation before
corrective action is taken one of these
methods is called refluxing the vapors
coming off the top of the tower are
condensed in a condenser and then
collected in a receiver part of the
liquid from the receiver is sent to
storage or to other units in the plant
as the towers overhead product the rest
of the liquid is pumped back into the
top of the tower the liquid that's
reintroduced into the tower is called
external reflux because it consists of
liquid that was cooled in the condenser
the external reflux is cooler than the
liquid in the top of in this topic we
looked at two key factors in the
distillation process temperature and
pressure remember that although we
discussed them separately temperature
and pressure work together both of these
factors must be correct for the
distillation process to work properly
now try to answer some practice
questions to check your understanding of
what we've gone over