| The factors that affect distillation column operation |
FACTORS AFFECTING DISTILLATION COLUMN OPERATION
The performance of a
distillation column is determined by many factors, for example:
feed
conditions
- state of
feed
- composition
of feed
- trace
elements that can severely affect the
VLE of
liquid mixtures
internal liquid and
fluid flow conditions
state of trays (packings)
weather conditions
Some of these will
be discussed below to give an idea of the complexity of the distillation
process.
Feed Conditions
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.
Reflux Conditions
 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 ration, 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).
Vapour Flow
Conditions
Adverse vapour flow
conditions can cause
-
foaming
-
entrainment
-
weeping/dumping
-
flooding
-
Foaming
- 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.
-
Entrainment
- 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.
-
Weeping/Dumping
- 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.
-
Flooding
- 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.
Column Diameter
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.
State of Trays
and Packings
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.
Weather
Conditions
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 condensors.
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.
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