Material transfer - Ideal compartmentalized distillation

This article presents one of the oldest unit operations, distillation, which is used in a wide range of industrial applications: primarily petroleum, but also chemicals, pharmaceuticals, agri-food...

It is approached as a compartmentalized operation, comprising either a single stage (simple distillation), or several stages; the circulation of the two liquid and vapor phases is then countercurrent (rectification).

The actual operation of the stages is highly complex. However, it can be approached with acceptable accuracy using the theoretical or ideal stage model.

An ideal floor has two essential functions:

• the effluent phases are in thermodynamic equilibrium;

• the physical separation (disengagement) of the two effluent phases is perfect: there is no mechanical entrainment of one phase in the other.

This definition goes beyond distillation, and can be applied to the other unit operations discussed in the following articles (liquid-liquid extraction, gas-liquid extraction, etc.). However, in the particular case of distillation, where material and heat transfers are concomitant, we also assume that the stage is adiabatic and isobaric. Although these two assumptions are highly simplifying, they are not essential. The fundamental advantage of this model is that it makes it possible to size a unit without having to rely on either kinetic data (thermodynamic equilibrium reached) or flow data: the absence of entrainment and material transport between stages means that the inter-stage balance relationships (operating relationships) are particularly simple.

Its disadvantages are linked to the assumptions; an ideal number of floors is determined but, due to the nature of the model, no indication can be given on the following points:

• relationship between theoretical and actual number of stages ;

• dimensioning of real floors.

These points can only be clarified by resorting either to best practice (experience), or to less simple models (the case of non-ideal compartmentalized operations). ). However, based on a reasoned extrapolation of existing installations, this model often provides an accurate estimate of the sizing of separate-compartment units. It is universally used for preliminary design studies.

Two types of problem can be tackled with the ideal stage model: sizing a new plant and adapting an existing one. Distillation is presented here from the first point of view.

The search for solutions is systematically based on graphical...