Equilibrium Based Separation
Separation operations based on equilibrium selectivity can be classified according to the separating agent that is used (matter or energy) and the nature of the phases in contact:
- Distillation is the most commonly used basic operation. The feed is normally liquid, but refrigerated distillation can also be used to fractionate a gaseous mixture. The second phase is generated at the bottom of the column by heating (vapor reflux) and at the top by cooling (liquid reflux). The other operations, necessarily more complex, are generally used only when the difference in volatility between the components to be separated is not sufficient to achieve separation by distillation in satisfactory conditions.
- It is also possible in some cases to generate a solid phase by cooling. In it a component or fraction is concentrated that needs to be separated. The main refinery application of crystallization processes is in dewaxing.
- Adding a solvent can separate components whose volatilities are close but which belong to different chemical families. For instance, liquid-liquid extraction is used to separate aromatics from paraffins in a liquid feed. The same principle is implemented in the case of a solvent absorption operation that is used for a gaseous feed.
- Extractive distillation combines the effect of a phase change by energy input with the effect of solvent addition to separate a component on the basis of its volatility and its chemical nature.
- Difficult separation operations such as isomer separation (paraxy-lene/metaxylene, n-/isoparaffins), and deep purification can be carried out by using an adsorbent. The feed may be gaseous or liquid.
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