Continuous Distillation Column Video Lecture | Mass Transfer - Chemical Engineering

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
preheater
in the preheater the mixture is heated
under pressure to just below its boiling
point the pressure in the tower is lower
than the pressure in the preheater 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 vapours
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 boiler 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 into the top of
the tower and part of it is drawn off as
the towers overhead product
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 down comers the downcomers channel
the liquid from tray to tre 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 a 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 component 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 PAC 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
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
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 condense the 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 plan
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 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
vaporize and more lighter components
will end up in the bottom of the tower
the temperature decreases as 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
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one way is to control the temperature of
the feed mixture by using a preheater
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
pump it 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
vapors 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
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 condensers 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
the condenser problem is suspected the
condenser should be checked and
corrective actions should be taken if
necessary
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