Whereas the composition of the distillate and bottoms in batch distillation changes over time, a continuous column operates under steady conditions where the composition at a given location does not change over time. This steady state operation is desired in almost all continuous unit operations. Because the composition only depends upon the position in the column, additional product steams can be easily tapped at different heights (not shown) and each tray will have a different composition of compounds. The trays at the top of the column are rich in light boiling compounds while those at the bottom are rich in compounds that only boil at high temperatures.