Distillation. Absorption - Practical study

The purpose of the present article is to determine the operating conditions and the scheme to be used to separate one or more constituents of a more or less complex mixture by distillation or absorption. It is intended for non-specialists who have a separation problem to deal with (for example, to purify a solvent used in a manufacturing process with a view to recycling), to enable them to pose their problem correctly to suppliers and better understand their choices. The description, selection criteria and sizing of equipment used in distillation and absorption units are beyond the scope of this article. The specific piece of equipment used in these units, the fractionating column, is covered in articles [ , and of this collection (Distillation. Absorption). The article Distillation. Absorption: Control and regulation explains the basic principles of column regulation.

The first question to ask concerns the unit operation to be implemented. By its very nature, distillation is a major energy consumer, and the rise in energy costs since 1973 has led manufacturers to consider other separation methods (liquid-liquid extraction, crystallization, absorption, diffusion through membranes) more systematically than in the past, as substitutes for distillation. In almost all cases, the choice of another unit operation entails an increase in investment costs, which is more or less offset in the long term by the energy savings achieved. However, distillation remains the best choice for many separations.

If different separation methods are possible, the choice is made on the basis of an economic comparison of the processes. For each process, the operating conditions need to be chosen in order to determine consumption levels (electricity, steam, cooling water), and the main equipment needs to be pre-dimensioned in order to assess the cost of the installation. To draw up the diagram and choose the general operating conditions, consider :

• liquid-vapour, liquid-liquid or liquid-solid equilibria;

• physical properties of products ;

• local conditions (available utilities, location, flow rates, economic conditions);

• the robustness and simplicity of the techniques used;

• SAFETY ;

• respect for the environment.

It's all about know-how and experience.

The next step is to determine the operating conditions of the equipment. This defines the number of transfer...