2. THE TRAY TYPE TOWER
This is also a tall, cylindrical column. Inside, a series of trays are placed, one above the other. The trays are used to bring the rising vapour and falling liquid into intimate contact. Tray towers do the same job as packed towers but they are very much more efficient in the separation process than packed towers and, they are also more costly. There are various types of tray in use and the type selected depends upon the degree of product purity required, the type of fluids, fluid velocity and other process parameters of the system.
The types of tray used in distillation columns are as follows:
THE SIEVE TRAY is simply a metal plate containing drilled holes through which the rising vapour can pass into the liquid flowing across the tray. Figure: 11
THE VALVE TRAY is similar to the sieve type but, each hole is fitted with a flapper valve which opens as vapour passes through the hole. This type is used where vapour velocity is not constant and the valves prevent liquid from dumping through the holes at times of low gas velocity. Figure: 12
THE BUBBLE-CAP TRAY is the most efficient separation device but, is also the most costly. It consists of a number of 'Chimneys' or 'Risers' (small, short pipes set into the tray), through which the vapour can pass. Fitted over the riser is a 'Cap' which causes the rising vapour to turn through 180 °. This forces the gas to 'Bubble' through the liquid flowing across the tray. The liquid level on the tray is maintained below the top of the riser to prevent dumping of liquid down the tower. Figure: 13
Each of the above trays also has a 'WEIR' that maintains the liquid level on the tray. As the liquid flows over the weir, it enters a 'DOWNCOMER' - (a short pipe), that carries the liquid down to the tray below. The downcomer outlet is below the surface of the liquid on the tray below, acting as a seal to prevent gas from bypassing the tray above.