Based on estimates of Nq and tray spacing, the height of a column between the top tray and the bottom tray is computed. By adding another 4 ft above the top tray for removal of entrained liquid and 10 ft below the bottom tray for bottom surge capacity, the total column height is estimated. If the height of greater than 250 ft, two or more columns arranged in series may be preferable to a single column. However, perhaps the tallest column in the world,