Process Technology: An Introduction - Haan A.B. 2015

8 Absorption and stripping
8.4 Basic characteristics of absorbers

The main purpose of various industrial absorbers is to ensure a large gas-liquid mass transfer area and to create conditions such that a high intensity of mass transfer is achieved. Although small-scale processes sometimes use batchwise operation where the liquid is placed in the equipment and only gas is flowing, continuous absorption is most commonly applied in large-scale industrial processes. There are various criteria for classifying absorbers. It seems that the best one is a widely used criterion that takes into account which of the phases (gas or liquid) is in a continuous or disperse form. Using this criterion, absorbers can be classified into the following groups:

· (1) absorbers in which both phases are continuous:

o — packed columns;

o — wetted-wall columns;

o — contactors with flat surface;

o — laminar jet;

o — disc (sphere) columns;

· (2) absorbers with a disperse gas phase and a continuous liquid phase:

o — tray columns;

o — tray columns with packing;

o — bubble columns;

o — packed bubbles columns;

o — mechanically agitated absorbers;

o — jet absorbers;

· (3) absorbers with a dispersed liquid phase and a continuous gas phase:

o — spray columns;

o — Venturi scrubbers.

In each apparatus, due to different hydrodynamic conditions, various values of the mass transfer coefficients occur in both phases. Therefore, when choosing a given type of absorber, the following criteria should be taken into account:

· (1) the required method of absorption (continuous, semicontinuous);

· (2) the flow rate of the gas and the liquid entering the absorber (e.g. high gas flow rate and low liquid flow rate need different types of equipment than in the case where the two flow rates are of the same size);

· (3) the required liquid hold-up (large or small liquid hold-up is needed);

· (4) which phase controls mass transfer in the absorber (gas phase or liquid phase);

· (5) whether it necessary to remove heat from the absorber;

· (6) the corrosiveness of the absorption systems;

· (7) the required size of the interface (large or small mass transfer area);

· (8) the physicochemical properties of the gas and the liquid (particularly viscosity and surface tension).

Fig. 8.4: Schematic and operating principles of packed, tray and spray towers. Adapted from [9].

Apart from these factors there are many others which influence the selection of equipment. For instance, gas impurities or deposits formed during the absorption process require the application of a given absorber.