Breaking Azeotropes and Absorption Tricks

In many mixtures (binary, ternary, etc.), we encounter azeotropes. In this water-isopropanol system, the azeotrope is clear to discern and we might be able to use pressure swing distillation effectively.
A second column, operating at a different pressure to the first is needed to complete the separation. Note that the most volatile component (MVC) changes as the azeotrope is crossed.
A rough diagram of the column setup for pressure swing distillation. Note that both pure components leave via the bottoms products of the two columns due to the change in the MVC. Taken from the PhD thesis of Eva-Katrine Hilmen,Norwegian University of Science and Technology, Dept. Chem. Eng, 2000.
TXY diagram for ethanol-water mixtures at 1 atm.
  • In many azeotropic systems, pressure swing distillation is ineffective.
  • For example, in the ethanol-water system above, the relative volatility for the high concentrations of ethanol is close to $\alpha\approx1$ !
  • What other methods are available for breaking azeotropes?
  • To break this azeotrope, we must either:
    • Modify the VLE behaviour of the mixture using another component.
    • Use an alternative separation technique.
  • Alternative separation techniques may be used to just to completely separate the azeotropic mixture, or just to help distillation get past the azeotrope.
$xy$ diagram for ethanol-water at 1 atm.
A hybrid separation technique using membranes to break the azeotrope. The membrane is used to perform pervaporation to bypass the azeotrope before further distillation is performed (left), or the pervaporation is enough to complete the separation (right). Taken from the PhD thesis of Eva-Katrine Hilmen,Norwegian University of Science and Technology, Dept. Chem. Eng, 2000.
This modern (1989) membrane setup for the dehydration of ethanol is reportedly 50% cheaper than the extractive distillation counterparts. Taken from the PhD thesis of Eva-Katrine Hilmen,Norwegian University of Science and Technology, Dept. Chem. Eng, 2000.
  • The final technique we'll mention is the use of an entrainer.
  • This is an additional component added to the column to alter the VLE behaviour of the mixture.
  • In Extractive distillation the entrainer is a non-volatile component that exits in the bottom product and binds strongly to only one of the components.
  • This technique can be thought of as a combination of liquid-liquid extraction and distillation occurring in one column at the same time.
Extractive distillation.
  • In azeotropic distillation, an entrainer is added which is volatile and exits the column in the top-product.
  • This method is called azeotropic distillation as a new ternary azeotrope is often formed, but this azeotrope will be at lower concentrations than the binary azeotrope.
  • The distillation can then be continued up to high purities, but a recovery strategy must be devised to recover the entrainer.
  • This technique is quite popular for the dehydration of ethanol…
$x$-$y$ diagram of ethanol-water mixtures at 1 atm.
Here a Benzene entrainer is used to purify azeotropic ethanol-water mixtures. A liquid decanter and two additional columns are used to recover the Benzene from the top product of the azeotropic distillation column.
The lab still is capable of breaking azeotropes using two of the methods described.
We can carry out pressure swing distillation using our lab still. It has a water jet vacuum pump (labelled IX) to allow the column to be operated at $\approx0.5$ atm.
A solvent tank is attached to the top of the column to allow an entrainer to be added. This may be used to perform either extractive or azeotropic distillation.
  • In multi-stage absorbtion, we have an operating line of the following form. \begin{align} L'\frac{x_{A,i}}{1-x_{A,i}}+V'\frac{y_{A,j+1}}{1-y_{A,j+1}}&=L'\frac{x_{A,j}}{1-x_{A,j}} + V'\frac{y_{A,i+1}}{1-y_{A,i+1}} \end{align}
  • We also have some equilibrium data, which at typical absorber concentrations can described by Henry's law \begin{align*} y = \mathcal{H} x \end{align*}
  • The solution of these equations is just the same as for multi-stage distillation. We draw the operating line and the equilibrium line and step between the two.
The solution to the absorber design in tutorial 04.

\begin{align} L'\frac{x_{A,i}}{1-x_{A,i}}+V'\frac{y_{A,j+1}}{1-y_{A,j+1}}&=L'\frac{x_{A,j}}{1-x_{A,j}} + V'\frac{y_{A,i+1}}{1-y_{A,i+1}} \end{align}

  • One of the biggest difficulties of absorber design is that you have to alter the operating line a lot.
  • In distillation, the operating line is straight due to the assumption of constant molar overflow.
  • But in absorption, we must plot several points to capture its curvature (see right).
  • Can we make the operating line straight using a change of variables?
\begin{align} L'\frac{x_{A,i}}{1-x_{A,i}}+V'\frac{y_{A,j+1}}{1-y_{A,j+1}}&=L'\frac{x_{A,j}}{1-x_{A,j}} + V'\frac{y_{A,i+1}}{1-y_{A,i+1}} \end{align}
  • Defining two new variables: \begin{align*} X&=\frac{x}{1-x} & Y&=\frac{y}{1-y} \end{align*}
  • We can rewrite the operating line in terms of these variables. \begin{align} L' X_i+V' Y_{j+1}&=L' X_j + V' Y_{i+1} \end{align}
  • Rearranging then gives us a convenient linear form: \begin{align*} Y_{A,j+1}&=\frac{L'}{V'}\left(X_{A,j}-X_{A,i}\right)+ Y_{A,i} \end{align*}
\begin{align*} Y_{A,j+1}&=\frac{L'}{V'}\left(X_{A,j}-X_{A,i}\right)+ Y_{A,i} \end{align*}
  • We can easily convert back from these new variables too: \begin{align*} y &= \frac{Y}{1+Y} & x&=\frac{X}{1+X} \end{align*}
\begin{align*} Y_{A,j+1}&=\frac{L'}{V'}\left(X_{A,j}-X_{A,i}\right)+ Y_{A,i} \end{align*}
  • This form is much more convenient to plot, as it is a straight line on a XY plot.
  • It is also very easy to convert from $x\to X$ or $y\to Y$.
  • The only difficulty is that the equilibrium line is no longer straight. \begin{align*} y &= \mathcal{H} x & Y &= \frac{\mathcal{H} x}{1+\mathcal{H} x} \end{align*}
  • However, if XY graphs for the equilibrium data have been prepared once, it is much more convenient to perform the design in these variables.