Counter Current and Co-current Mass Transfer Operations

Mass transfer processes that involve two fluid streams are frequently carried out continuously by passing the fluid streams either in a counter-current or a co-current fashion in a column device. 

The average driving force for a given situation will be more significant in the counter-current operation than in the co-current process and because of this smaller size equipment is needed for a given set of flow conditions for counter-current flow is more common.

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Example of Counter-Mass Transfer

(i) Distillation in the packed column 

In this case, a vapor stream (thermally created) is moving up against the downward flow of liquid reflux. At a steady state, a state of dynamic equilibrium is set up. The more volatile component under the action of a concentration gradient is transferred from the liquid phase to the interface and then from the interface into the vapor phase. 

The less volatile component is transferred in the opposite direction and phases through the vapor into the liquid. Here equimolar counter diffusion takes place if the molar latent heats of the components are of the same magnitude. 

(ii) Absorption in the packed column

In this case, a solute gas together with a carrier gas moves up against the downward flow of liquid. The solute gas diffuses through the gas phase to the interface and away from the interface into the extract phase. 

(iii) Liquid-liquid extraction in a packed column 

In this case, both streams are liquid. The lighter liquid rises through the denser one. The solute diffuses from the raffinate phase to the interface and away from the interface into the extract phase. 

Examples of Co-current Flow

The co-current flow of gas and liquid streams usually in the downward direction through a packed column is used for catalytic chemical reactions, e. g. in the absorption of hydrogen sulfide in sodium hydroxide solution. 

The co-current flow produces a separation effect equivalent to that obtained with only one theoretical stage so used for such cases only. With co-current flow, there is no flooding and there is no upper limit for permissible phase flow rates. The co-current flow is rarely used in the absence of a chemical reaction. 

Analogies among Heat, Mass, and Momentum Transfer

Depending upon the fluid flow conditions, the mechanism of the transfer process can be classified as molecular and eddy transfer. There are similarities in governing equations for molecular and turbulent transport, and we can say that we have analogies among these transport processes. A great deal of effort has been devoted in the literature to developing analogies among these three transport processes for turbulent transfer. 

General Molecular Transfer Equation

The molecular transfer of momentum, heat, and mass is characterized by the same general equation of the type. 

Rate of a transfer process = Driving force/Resistance 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane