The glycol and the gas do not reach equilibrium on each tray. A tray efficiency of 25% is commonly used for design. That is, if one theoretical equilibrium tray is needed, four actual trays are specified. In bubble cap towers, tray spacing is normally 24 in.
The more trays the greater the dew-point depression for a constant glycol circulation rate and lean glycol concentration. By specifying more trays, fuel savings can be realized because the heat duty of the reboiler is directly related to the glycol circulation rate. Figure 8-8 shows how the number of trays can have a much greater effect on dew-point depression than the circulation rate.
The additional investment for a taller contactor is often easily justified by the resultant fuel savings. Most contactors designed for 1 Ib/MMscf gas are sized for 6 to 8 trays.