Varying Stage Efficiencies and Feed Location

  • In the previous lecture, we discussed column efficiencies and how to use them:
    • The overall column efficiency, EO, is the ratio of ideal trays to real trays in the column.
    • The Murphree tray efficiency, EM, is the effectiveness of a single real tray when compared to a ideal tray.
    • The Murphree point efficiency, EP, is the efficiency of a single point on a tray. This is only useful when considering the flow on a tray in detail.
  • A constant Murphree tray efficiency is relatively easy to use, forming an “effective” VLE line below the true VLE line, which we called the Murphree line.
  • But what is happening at the bottom of the stepping when using the Murphree efficiency?
  • This is the first example of a case where we have different efficiencies for different parts of the column.
  • The bottom stage is usually a re-boiler stage, and it is often assumed to have a Murphree efficiency of EM=1.0 !
  • This is because the re-boilers actually generate vapour from the liquid.
  • On trays, you are contacting vapour and liquid phases to try to get them to come into equilibrium.
  • Generating vapour will typically be more effective at achieving equilibrium concentrations.
  • Let's consider the distillation trays in a real distillation column.
  • The tray's in a column will actually vary in type, from random/structured packing, to sieve/valve/chimney trays.
  • They will vary as the viscosity of the column mixture changes with concentration (compare the heavy components of crude to the light paraffin's).
  • This means the efficiency will vary significantly in the column.
  • The efficiency will also change due to the difference in vapour and liquid flow rates in the stripping and enrichment sections due to the addition of feed.
  • The last bit of ambiguity to clarify in distillation design is which tray is the feed tray?
  • The feed tray is defined as the tray below where the feed enters the column.
  • The liquid falling down from the feed point will land on this tray, and the feed vapour will join the rising vapour from the tray.
  • Therefore, the feed tray is the tray which connects the two operating lines.
Varying Stage Efficiencies and Feed Location