Batch distillation:

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Batch distillation

• In many instances processes are carried out in
  batches, and it is more convenient to distil each
  batch separately. In these cases the whole of a
  batch is run into the boiler of the still and, on
  heating, the vapour is passed into a
  fractionation column. As with continuous
  distillation, the composition of the top product
  depends on the still composition, the number of
  plates in the column and on the reflux ratio
  used. When the still is operating, since the top
  product will be relatively rich in the more
  volatile component, the liquid remaining in the
  still will become steadily weaker in this
  component. As a result, the purity of the top
  product will steadily fall.

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• 1. This method of operating a batch still
  requires a continuous increase in the reflux
  ratio to maintain a constant quality of the top
  product.
• 2. An alternative method of operation is to work
  with a constant reflux ratio and allow the
  composition of the top product to fall.

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• One of the added merits of batch distillation is
  that more than one product may be obtained. Thus,
  a binary mixture of alcohol and water may be
  distilled to obtain initially a high quality
  alcohol. As the composition in the still weakens
  with respect to alcohol, a second product may be
  removed from the top with a reduced concentration
  of alcohol. In this way it is possible to obtain
  not only two different quality products, but also
  to reduce the alcohol in the still to a minimum
  value. This method of operation is particularly
  useful for handling small quantities of
  multi-component organic mixtures, since it is
  possible to obtain the different components at
  reasonable degrees of purity.

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Operation at constant product composition
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Operation at constant reflux ratio

• If the same column is operated at a constant
  reflux ratio R, the concentration of the more
  volatile component in the top product will
  continuously fall. Over a small interval of time
• dt , the top-product composition with respect to
  the more volatile component will change from xd
  to xd dxd, where dxd is negative for the more
  volatile component. If in this
• time the amount of product obtained is dDb, then
  a material balance on the more volatile component
  gives

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Plate columns

• In order to translate these quantities into an
  actual design the
• following factors should be considered
• (a) The type of plate or tray.
• (b) The vapour velocity, which is the major
  factor in determining the diameter of the
  column.
• (c) The plate spacing, which is the major factor
  fixing the height of the column when the number
  of stages is known.

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Types of trays

• It should provide intimate mixing between the
  liquid and vapour streams, that it should be
  suitable for handling the desired rates of vapour
  and liquid without excessive entrainment or
  flooding, that it should be stable in operation,
  and that it should be reasonably easy to erect
  and maintain.
• The arrangements for the liquid flow over the
  tray depend largely on the ratio of liquid to
  vapour flow.

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• Types of flow
• (a) Cross-flow. Normal, with a good length of
  liquid path giving a good opportunityfor mass
  transfer.
• (b) Reverse. Downcomers are much reduced in area,
  and there is a very long liquid
• path. This design is suitable for low
  liquidvapour ratios.
• (c) Double-pass. As the liquid flow splits into
  two directions, this system will handle high
  liquidvapour ratios.

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• Sieve or perforated trays These are much simpler
  in construction, with small holes in the tray.
  The liquid flows across the tray and down the
  segmental downcomer.

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• bubble-cap tray This is the most widely used
  tray because of its range of operation. The
  individual caps are mounted on risers and have
  rectangular or triangular slots cut around their
  sides. The caps are held in position by some form
  of spider, and the areas of the riser and the
  annular space
• around the riser should be about equal.

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• Valve trays These may be regarded as a cross
  between a bubble-cap and a sieve tray. The
  construction is similar to that of cap types,
  although there are no risers and no slots.
• It may be noted that with most types of valve
  tray the opening may be varied by the vapour
  flow, so that the trays can operate over a wide
  range of flowrates. Because of their flexibility
  and price, valve trays are tending to replace
  bubble-cap trays.

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Factors determining column performance

• (a) Liquid and vapour velocities.
• (b) Physical properties of the liquid and vapour.
• (c) Extent of entrainment of liquid by rising
  vapour streams.
• (d) The hydraulics of the flow of liquid and
  vapour across and through the tray.

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Operating ranges for trays

• For a given tray layout there are certain limits
  for the flows of vapour and liquid within which
  stable operation is obtained.

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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.

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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.

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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.

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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.

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General design methods

• Bubble cap trays
• Bubble-cap trays are rarely used for new
  installations on account of their high cost and
  their high pressure drop. In addition,
  difficulties arise in large columns because of
  the large hydraulic gradients which are set up
  across the trays. Bubble cap trays are capable of
  dealing with very low liquid rates and are
  therefore useful for operation at low reflux
  ratios.
• Sieve trays
• With the sieve tray the vapour passes vertically
  through the holes into the liquid on the tray,
  whereas with the bubble cap the vapour issues in
  an approximately horizontal direction from the
  slots. With the sieve plate the vapour velocity
  through the perforations must be greater than a
  certain minimum value in orderto prevent the
  weeping of the liquid stream down through the
  holes.

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• Valve trays
• The valve tray, which may be regarded as
  intermediate between the bubble cap and the sieve
  tray, offers advantages over both.
• Advantages claimed for valve trays include
• (a) Operation at the same capacity and efficiency
  as sieve trays.
• (b) A low pressure drop which is fairly constant
  over a large portion of the operating range.
• (c) A high turndown ratio, that is it can be
  operated at a small fraction of design capacity.
• (d) A relatively simple construction which leads
  to a cost of only 20 per cent higher than that of
  a comparable sieve tray.

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Plate efficiency

• The ratio n/np of the number of ideal stages n to
  the number of actual trays np represents the
  overall efficiency E of the column.
• For a single ideal tray, the vapour leaving is in
  equilibrium with the liquid leaving, and the
  ratio of the actual change in composition
  achieved to that which would occur if equilibrium
  between yn and xn were attained is known as the
  Murphree plate efficiency EM.

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Packed columns for distillation

• In bubble cap and perforated plate columns, a
  large interfacial area between the rising vapour
  and the reflux is obtained by causing the vapour
  to bubble through the liquid. An alternative
  arrangement, which also provides the necessary
  large interfacial area for diffusion, is the
  packed column, in which the cylindrical shell of
  the column is filled with some form of packing.
  In packed columns the vapour flows steadily up
  and the reflux steadily down the column, giving a
  true countercurrent system in contrast to the
  conditions in bubble cap columns, where the
  process of enrichment is stage wise.

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The method of transfer units
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