Material transfer - Permanent contact operations

The active part of a permanent contact material exchanger consists of a cylindrical drum in which material is transferred between two phases, without there being any materialized stages in which this transfer takes place. Generally, the two phases exchanging matter are liquids, gases, vapors or even solids. These permanent-contact exchangers are widely used in industry, thanks to their adaptability to production conditions and their much lower cost than an individual-stage system.

The operation of a permanent contact material exchanger requires a contact surface between the two phases being treated. For maximum efficiency, the contact area between the two phases per unit volume of contactor should be as high as possible. This enhances the exchange capacity between the two phases. To achieve this, a number of techniques are used, such as filling the contactor with a packing or using a method such as pulsation or agitation to maintain the dispersion of one phase in the other. Among the most widely used fillings are Raschig rings (glass or ceramic rings) and Berl saddles. Another condition for significant transfer is the existence of a potential difference between the two phases. This is why equilibrium is never achieved in permanent contact devices. Or at least, if it is achieved at one point in the column, the rest of the column is inoperative.

Permanent contact operations are commonly used in absorption, distillation, liquid-liquid extraction, drying, humidification, chromatography and ion exchange.

The transfer of matter is almost always accompanied by a transfer of energy (or heat). We will only consider operations where heat transfer is negligible.

Generally speaking, we operate against the current, with very rare exceptions such as the absorption of a pure gas. Flows are of paramount importance, as these operations are under kinetic control. We will look at the application of simple ideal models and more sophisticated models to the calculation of these operations.