Tray Efficiency

By the measuring the performance of the actual trays eventually through the separation of the materials of interest the tray efficiencies are determined. In the earlier phases of a design this is not very practical. Some kind of estimation is required. The data collected from other columns or the theoretical evidences show the estimates.

The data collected from the actual columns forms the base for the O’Connell correlation. These are conservative for the valve and the sieve trays and are based on the bubble cap trays. The

η_oa = 0.492 ( μ_F α_HK )^-0.245

Based on the constraints that is imposed by flooding the column diameter is measured.

Semi-theoretical models:

The semi-theoretical models which is based on the assumption that there is a well-mixed vapour in between the trays, again the liquid that are present in the downcomers is well-mixed, and the liquid which is on the tray is well mixed as well so that it will be of the same composition as in the liquid in the downcomer which leaves the tray.

For each tray, this is defined in accordance with the separation which is achieved on each tray. This can either be based on the vapor phase or on the liquid phase. It is equal to the changes that happens in the actual concentration divided by the change which will be predicted by the equilibrium condition.

Vaporphase :

Liquid phase :

Theoretical stages:

There are number of overall theoretical stages will include the condenser, the internals, and the reboilers which are required in order to match the process. Achieving the full theoretical stage purely depends on the configuration of the condenser and the reboiler. The composition of the inlet and outlet of the column should be stimulated to match the column compositions in order to achieve the overall theoretical stages which are needed to match the entire process.

Vapor-liquid separation process:

If both the vapor phases and the liquid phases are mixed perfectly for the vapor-liquid separation process, the efficiency of the distillation for the component i on stage j is

And Q is an average value of the product among the mass transfer coefficient, the time of contact in liquid vapour and the area of contact /mole vapour.

On the other hand, if the definition is built on the liquid composition. The efficiency would be

These equations listed above shows the change in the composition on stage in accordance to the change when a thermodynamic equilibrium exists between these phases.  In a general point of view these efficiencies are much less than the unity.

Questions:

• What is vapor-liquid separation process?
• What are included in the theoretical stages?