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FACTORS AFFECTING DISTILLATION COLUMN OPERATION
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The
performance of a distillation column is determined by many factors,
for example:
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feed
conditions
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state
of feed
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composition
of feed
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trace
elements that can severely affect the VLE of liquid
mixtures
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internal
liquid and fluid flow conditions
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state
of trays (packings)
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weather
conditions
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Some
of these will be discussed below to give an idea of the complexity
of the distillation process.
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Feed
Conditions
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The
state of the feed mixture and feed composition affects the operating
lines and hence the number of stages required for separation.
It also affects the location of feed tray. During operation, if
the deviations from design specifications are excessive, then
the column may no longer be able handle the separation task. To
overcome the problems associated with the feed, some column are
designed to have multiple feed points when the feed is expected
to containing varying amounts of components.
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Reflux
Conditions
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As
the reflux ratio is increased, the gradient of operating line
for the rectification section moves towards a maximum value of
1. Physically, what this means is that more and more liquid that
is rich in the more volatile components are being recycled back
into the column. Separation then becomes better and thus less
trays are needed to achieve the same degree of separation. Minimum
trays are required under total reflux conditions, i.e.
there is no withdrawal
of distillate.
On
the other hand, as reflux is decreased, the operating line for
the rectification section moves towards the equilibrium line.
The ‘pinch’ between operating and equilibrium
lines
becomes more pronounced and more and more trays are required. This
is easy to verify using the McCabe-Thiele method.
The
limiting condition occurs at minimum reflux
ratio, when an infinite number of trays will be required
to effect separation. Most columns are
designed to operate between 1.2 to 1.5 times the minimum reflux
ratio because this is approximately the region of minimum
operating costs (more reflux means higher reboiler duty).
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Vapour
Flow Conditions
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Adverse
vapour flow conditions can cause
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foaming
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entrainment
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weeping/dumping
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flooding
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Foaming
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Foaming
refers to the expansion of liquid due to passage of vapour
or gas. Although it provides high interfacial liquid-vapour
contact, excessive foaming often leads to liquid buildup on
trays. In some cases, foaming may be so bad that the foam
mixes with liquid on the tray above. Whether foaming will
occur depends primarily on physical properties of the liquid
mixtures, but is sometimes due to tray designs and condition.
Whatever the cause, separation efficiency is always reduced.
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Entrainment
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Entrainment
refers to the liquid carried by vapour up to the tray above
and is again caused by high vapour flow rates. It is detrimental
because tray efficiency is reduced: lower volatile material
is carried to a plate holding liquid of higher volatility.
It could also contaminate high purity distillate. Excessive
entrainment can lead to flooding.
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Weeping/Dumping
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This
phenomenon is caused by low vapour flow. The pressure exerted
by the vapour is insufficient to hold up the liquid on the
tray. Therefore, liquid starts to leak through perforations.
Excessive weeping will lead to dumping. That is the liquid
on all trays will crash (dump) through to the base of the
column (via a domino effect) and the column will have to be
re-started. Weeping is indicated by a sharp pressure drop
in the column and reduced separation efficiency.
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Flooding
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Flooding
is brought about by excessive vapour flow, causing liquid
to be entrained in the vapour up the column. The increased
pressure from excessive vapour also backs up the liquid in
the downcomer, causing an increase in liquid holdup on the
plate above. Depending on the degree of flooding, the
maximum capacity of the column may be severely reduced. Flooding
is detected by sharp increases in column differential pressure
and significant decrease in separation efficiency.
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Reboiler
Temperature
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As a reboiler slowly
plugs on the tube side, the bottoms temperature goes down and this leads to low
vapor flow. Online monitoring of the heat transfer coefficient in control system
can identify this problem early on along with the monitoring of bottoms
temperature. A self cleaning
heat exchanger when used in place of a reboiler can mitigate this
problem.
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Reduction in
Vacuum
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A slight drop in vacuum
causes the boiling point(s) to go up, leading to low vapour flow, loss of
production and and off spec products.
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Wet Feed
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If a column is typically
fed a dry hydrocarbon feed but the feed becomes contaminated with water then at
some point, when the water/ hydrocarbon mixture enters the tower the water can
vaporize so rapidly that it causes an over pressure event. This can and will
damage trays. This effects column efficiency and product quality.
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Column
Diameter
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Most
of the above factors that affect column operation is due to vapour
flow conditions: either excessive or too low. Vapour flow velocity
is dependent on column diameter. Weeping determines the minimum
vapour flow required while flooding determines the maximum vapour
flow allowed, hence column capacity. Thus, if the column diameter
is not sized properly, the column will not perform well. Not only
will operational problems occur, the desired separation duties
may not be achieved.
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State
of Trays and Packings
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Remember
that the actual number of trays required for a
particular separation
duty is determined by the efficiency of the plate, and the packings
if packings are used. Thus, any factors that cause a decrease
in tray efficiency will also change the performance of the column.
Tray efficiencies are affected by fouling, wear and tear and corrosion,
and the rates at which these occur depends on the properties of
the liquids being processed. Thus appropriate materials should
be specified for tray construction.
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Weather
Conditions
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Most
distillation columns are open to the atmosphere. Although many
of the columns are insulated, changing weather conditions can
still affect column operation. Thus the reboiler must be appropriately
sized to ensure that enough vapour can be generated during cold
and windy spells and that it can be turned down sufficiently during
hot seasons. The same applies to condensers.
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These
are some of the more important factors that can cause poor distillation
column performance. Other factors include changing operating conditions
and throughputs, brought about by changes in upstream conditions
and changes in the demand for the products. All these factors,
including the associated control system, should be considered
at the design stages because once a column is built and installed,
nothing much can be done to rectify the situation without incurring
significant costs. The control of distillation columns is a field
in its own right, but that's another story. COSTELLO can provide
engineering design for new columns and support/ troubleshooting for existing columns.
Our engineers have extensive experience in column instrumentation and controls.
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