FACTORS AFFECTING SELECTION OF TRAYS

 Relative Cost of plate will depend upon material of construction used.

   For mild steel, the ratio of cost between plates is

        

Sieve plate     :       valve plate    :        bubble-cap plate

                                   3.0           :            1.5           :              1.0

·        There is little difference in Capacity Rating of the three types (the column diameter required for a given flow rate).

Sieve tray     >      valve tray    >      bubble-cap tray

Operating Range means the range of liquid and vapour flow rates which must be above the weeping conditions and below the flooding conditions. Operating range flexibility comparison is.

Bubble cape tray >   Valve tray   >   Sieve tray

       Sieve plate depends on the vapours flow through the holes to hold the liquid on the plate, and cannot operate at very low vapour flow rates. But with good design, sieve plate gives satisfactory operating range.

The Plate pressure drop will depends on the detailed design of plate but, in general, sieve plate gives the lowest pressure drop, followed by valves, with bubble-caps giving the highest.

Operation of Typical distillation Column:

                                                                        The operation of typical distillation column may by followed by figure. The column consists of a cylindrical structure divided into sections by a series of perforated trays which permit the upward flow of vapour. The liquid reflux flows across each tray, over a weir and down a down comer to the tray below. The vapour rising from the top tray passes to condenser and then through an accumulator or reflux drum and a reflux divider, where part is withdrawn as the overhead product D and the remainder is returned to the top tray as reflux R.

In the bottom there is reboiler which is used to give heat to the system. Liquid from the bottom of distillation column is fed to the reboiler which vaporises the in coming liquid. These vapours in turn move towards the bottom plate interact with the liquid over that plate. Due to which partial condensation of vapours occur. Also partial vaporization of liquid occurs too. That is less volatile component condensed first and more volatile component vaporizes first. This phenomenon occurs on each plate. Causing enrichment on each plate.

A schematic of a typical distillation unit with a single feed and two product streams is shown below.