The plate, or tray column is the most widely used type of distillation column. The number of trays, or stages in the column is dependent on the desired purity and difficulty of separation. The number of stages also determines the height of the column.
The plate column is a chemical equipment used to carry out unit operations where it is necessary to transfer mass between a liquid phase and a gas phase. In other words, it is a particular gas-liquid contactor. The peculiarity of this gas-liquid contactor is that the gas comes in contact with liquid through different stages; each stage is delimited by two plates (except the stage at the top of the column and the stage at the bottom of the column).
Some common applications of plate columns are distillation, gas-liquid absorption and liquid-liquid extraction. In general, plate columns are suitable for both continuous and batch operations.
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 move 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.
How does a plate column works?
The feed enters a plate column towards the middle of the column. Concentration differences cause the less volatile components to transfer from the vapor stream to the liquid stream. The vapor exiting the condenser contains the most volatile components, while the least volatile components exit through the reboiler in the liquid stream.
After the feed mixture enters the column, which is demonstrated by the green arrows in the animation below, liquid flows down the column and across the trays in either crossflow or countercurrent flow. A reboiler at the bottom separates the stream into a vapor stream that returns to the column and a liquid product stream. The vapor stream flows upward through the trays, and contacts the down-flowing liquid stream, allowing the separation to take place. At the top of the column, the vapor is condensed in a condenser. The condensed stream which is shown in blue, is split into an overhead product stream, known as the distillate, and a reflux stream that returns to the top of the column.
Most cost efficient distillation column for diameters greater than 0.6 m.
The liquid/vapor contact in the cross-flow of plate columns is more effective than the countercurrent-flow in packed columns.
Cooling coils can easily be added to the plate column (cryogenic applications).
Can handle high liquid flow rates cost-effectively.