Multi-Stage Distillation: Part 04

  • When designing a distillation column from scratch, the only design specifications we will have are the feed and outlet concentrations.
  • We will need to decide upon a column operating pressure and use this to calculate the q -value of the feed stream (see the last lecture's slides).
  • We will also need to choose a reflux ratio.
  • Selecting the reflux ratio and the column operating pressure requires examining the economic trade-off between capital and operating costs.
  • The column operating pressure will require a full optimisation study. However, we can make some educated estimates when it comes to the reflux ratio
  • The minimum reflux ratio corresponds to the first (highest) R where a pinch is formed.
  • We can then just read off the y -intercept of the enrichment operating line to find the minimum reflux ratio. \begin{align*} R_{min}=\frac{x_D}{y(x=0)}-1 \end{align*}
  • The optimum reflux ratio (see C&R vol. 2, pg. 575 for details) is generally around 1.1–1.5 times the minimum reflux ratio.
  • This is where the running costs of the column (heating steam for the boiler, cooling water for the condenser, pumps) roughly balance with the capital costs of the column (number of trays, cost of reboiler and condenser).
  • So we can now calculate a rough reflux ratio, given just the outlet conditions and the q value! We can design binary distillation columns…
  • The next subject on distillation design is plate efficiencies.
  • So far we've only concerned ourselves with an overall plate efficiency, E_O.
  • We can convert from theoretical stages (N) to real stages ( N_{real}) by just dividing by the efficiency. \begin{align*} N_{real} = N / E_O \end{align*}
  • There are expressions for the overall efficiency available in the literature for certain types of distillations (See C&R Vol. 2, Eq. 11.126).
  • However, in general an overall efficiency is a very crude approximation and can only be used for rough calculations.
  • But there are better definitions of the efficiency available…
Multi-Stage Distillation: Part 04