Select thermodynamic models for process simulation
A Practical Guide to a Three Steps Methodology

Example 3-12: Separation of n-butane from 1,3-butadiene at 333.15 K using vapour-liquid equilibrium

Find a component that will enhance the separation between these two components (n-butane and 1,3-butadiene).

Analysis:

The physical property used for separation by vapour-liquid equilibrium is the volatility of the components. The vapour pressures of the pure components are the most fundamental piece of information related to volatilities. They can be found in many databases. For example, at 60°C, the vapour pressures of the two components are image image and image image respectively (source DIPPR).

These two values are fairly close to each other. Taking into account non-idealities, we can determine (using the NRTL activity coefficient model, for example) that the infinite dilution activity coefficients are very close to one (image), indicating an almost ideal mixture. The separation of the two components by distillation will be very difficult.

In order to improve the separation, a solvent can be found in which the activity coefficients of both components are quite different. This solvent should, in addition, remain in the liquid phase to be effective, in other words have a low vapour pressure.

Solution:

Using a large database of vapour pressure data, we find that acetonitrile (CH3CN), for example, has a boiling pressure at 333.15 K of imageimage, much lower than that of the two components to be separated, which means that it will preferentially stay in the liquid phase, where the non-idealities are stronger.

The two activity coefficients at infinite dilution of acetonitrile in butane and butadiene are respectively image and image. In other words, butadiene forms a much less non-ideal mixture with acetonitrile than n-butane. Butane has in fact no polar nature at all, while the double bonds of the butadiene result in a polarity that makes it more similar to acetonitrile. The consequences on the volatilities is made visible in figure 1, which shows that the resulting distribution coefficient of n-butane and 1,3 butadiene are both large (i.e. they are preferentially in the vapour phase), and different (i.e. they can now be separated). The acetonitrile distribution coefficient is small (<1), indicating that it remains preferentially in the liquid phase.

image Figure 1: Acetonitrile as an extractive solvent for the n-butane + 1,3-butadiene mixture. The plot shows the distribution coefficient of the components at 333.15 K and the bubble pressure.