Multicomponent Distillation

1
Multicomponent Distillation

• Key and non-key components
• Patterns of change in multicomponent distillation
• Heuristics for separation task selection

2
Key and non-key components

• Key and non-key components
• Patterns of change in multicomponent distillation
• Heuristics for separation task selection

3
Depropanizer example

• Purpose is to remove propane (C3) from higher
  mole mass gases.
• Column operates at 300 psig 15 ideal stages
  feed between 9-10th stage from bottom.
• Feed composition given in Table 1.
• Feed is 66 mol vapour at 300 psig.
• Partial condenser and the reflux rate is 0.9
  moles reflux/mole of feed.

adapted from King, Separation Processes, McGraw
Hill, 1981.
4
Table 1. Depropanizer Example
adapted from King, Separation Processes, McGraw
Hill, 1981.
5
Vapour composition profiles
yi
adapted from King, Separation Processes, McGraw
Hill, 1981.
6
Flow rates in column
adapted from King, Separation Processes, McGraw
Hill, 1981.
7
Liquid composition profiles
xi
adapted from King, Separation Processes, McGraw
Hill, 1981.
8
Temperature profile
adapted from King, Separation Processes, McGraw
Hill, 1981.
9
Typical compositions at minimum reflux ratio
adapted from King, Separation Processes, McGraw
Hill, 1981.
10
Heuristics for arranging columns

• How do you arrange columns in series/parallel in
  order to optimize the efficiency of the
  separation process?
• With 6 component feed and needing 6 pure
  streams then 5 columns are required and there are
  42 possible sequences!!!!
• Some heuristics are given to help start with
  this design. They are only approximations!

adapted from King, Separation Processes, McGraw
Hill, 1981.
11
Heuristics for arranging columns

• Direct sequence, for instance

adapted from King, Separation Processes, McGraw
Hill, 1981.
12
Heuristics for separation task selection

• Of the many differences that may exist between
  the source and the destination of a stream,
  differences involving composition predominate.
  Select the separation tasks first.
• When possible reduce the separation load by
  stream splitting and blending.
• All other things being equal, aim to separate the
  more plentiful components early. If amounts are
  equal, aim to separate into equal parts.
• Remove the corrosive and hazardous materials
  early.

adapted from King, Separation Processes, McGraw
Hill, 1981.
13
Heuristics for separation task selection (cont)

• The difficult separations are best saved for
  the last.
• All other things being equal, stay away from
  separations that require the use of species not
  normally present in the processing. However, if
  a foreign species is used to effect a
  separation, remove that species early, unless it
  enhances downstream separation processes.
• All other things being equal, avoid excursions
  in temperature and pressure, but aim high rather
  than low.

adapted from King, Separation Processes, McGraw
Hill, 1981.