Distillation Column Design

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Distillation Column Design

• Syed Zaheer Abbas

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Distillation

• Process in which a liquid or vapour mixture of
  two or more substances is separated into its
  component fractions of desired purity, by the
  application and removal of heat

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• The design of a distillation column can be
  divided into the following steps
• Specify the degree of separation required set
  product specifications.
• Select the operating conditions batch or
  continuous operating pressure.
• Select the type of contacting device plates or
  packing.
• Determine the stage and reflux requirements the
  number of equilibrium stages.
• Size the column diameter, number of real stages.
• Design the column internals plates,
  distributors, packing supports.
• Mechanical design vessel and internal fittings.

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• The separation of liquid mixtures by distillation
  depends on differences in volatility between the
  components. The greater the relative
  volatilities, the easier the separation.
• In the section below the feed, the more volatile
  components are stripped from the liquid and this
  is known as the stripping section. Above the
  feed, the concentration of the more volatile
  components is increased and this is called the
  enrichment, or more commonly, the rectifying
  section.

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Reflux considerations

• The reflux ratio, R, is normally defined as
• R (flow returned as reflux)/(flow of top
  product taken off)
• The number of stages required for a given
  separation will be dependent on the reflux ratio
  used.

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Total reflux

• Total reflux is the condition when all the
  condensate is returned to the column as reflux
  no product is taken off and there is no feed.
• At total reflux the number of stages required for
  a given separation is the minimum at which it is
  theoretically possible to achieve the separation.

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Minimum reflux

• As the reflux ratio is reduced a pinch point will
  occur at which the separation can only be
  achieved with an infinite number of stages. This
  sets the minimum possible reflux ratio for the
  specified separation.

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Optimum reflux ratio

• Practical reflux ratios will lie somewhere
  between the minimum for the specified separation
  and total reflux.
• For many systems the optimum will lie between 1.2
  to 1.5 times the minimum reflux ratio.

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Flooding and Weeping
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Feed-point location

• The precise location of the feed point will
  affect the number of stages required for a
  specified separation and the subsequent operation
  of the column. As a general rule, the feed should
  enter the column at the point that gives the best
  match between the feed composition (vapor and
  liquid if two phases) and the vapor and liquid
  streams in the column.

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Dew points and bubble points

• Dew points and bubble points can be calculated
  from a knowledge of the vapor-liquid equilibrium
  for the system. In terms of equilibrium
  constants, the bubble point is defined by the
  equation

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Choice b/w Tray Packed Column

• Plate column are designed to handle wide range of
  liquid flow rates without flooding.
• For large column heights, weight of the packed
  column is more than plate column.
• Man holes will be provided for cleaning in tray
  Columns. In packed columns packing must be
  removed before cleaning.
• When large temperature changes are involved as in
  the distillation operations tray column are often
  preferred.
• Random-Packed Column generally not designed with
  the diameter larger than 1.5 m and diameters of
  commercial tray column is seldom less than 0.67m

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Selection of Trays
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DESIGNING STEPS OF DISTILLATION COLUMN

• Calculation of Minimum number of stages, Nmin
• Calculation of Minimum Reflux Ratio Rm.
• Calculation of Actual Reflux Ratio.
• Calculation of theoretical number of stages.
• Calculation of actual number of stages.
• Calculation of diameter of the column.
• Calculation of weeping point, entrainment.
• Calculation of pressure drop.
• Calculation of the height of the column.

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• Calculation of Minimum no. of Plates
• The minimum no. of stages Nmin is obtained from
  Fenske equation which is,
• Nmin LN(xLK/xHK)D(xHK /xLK)B
• LN (aLK/HK) average

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Theoretical no. of Plates
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Problem

• A butane-pentane splitter is to operate at 8.3
  bar with the following feed composition

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• For a specification of not more than 1 mol of the
  light key in the bottom product and not more than
  1mol of the heavy key in the top product, and a
  reflux ratio of 2.5, make a stage-by-stage
  calculation to determine the product composition
  and number of stages required.

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• To estimate the dew and bubble points, assume
  that nothing heavier than the heavy key appears
  in the tops, and nothing lighter than the light
  key in the bottoms.