The feed to the column can be liquid, gas or gas and liquid at
equilibrium. Inside the column there are always two phases: one gas phase and one liquid phase. The liquid phase flows downward through the column via gravity, while the gas phase flows upward. These two phases come in contact in correspondence of holes, valves or bubble caps that fill the area of the plates. Gas moves to the higher plate through these devices, while the liquid moves to the lower plate through a downcomer. The liquid is collected to the bottom of the column and it undergoes
evaporation through a
reboiler, while the gas is collected to the top and it undergoes
condensation through a
condenser. The liquid and gas produced at the top and at the bottom are in general recirculated. In the simplest case, there are just one feed stream and two product streams. In the case of the
fractionating column, there are instead many product streams. ==Notes==