Packed Column Sizing

Packed column sizing is a kind of packed bed employed to carry out separation processes like absorption, stripping and distillation. It is a pressure vessel that has a packed section. The columns can be filled with random packing or structured packing sections which are arranged or stacked. In the column, liquids seem to wet the surface of the packing and the vapors pass across this wetted surface where mass transfer occurs. Packing material can be employed instead of arrays to develop separation in distillation columns.

The Advantage of a Lower Pressure Drop

Packing provides the positivity of a lower pressure drop across the column that is beneficial when operating under vacuum. Varied shaped packing materials have different surface areas and void space between the packing. And these factors affect packing performance. Further packing shape and surface area, is the liquid and vapor distribution that goes into the packed bed.

The Theoretical Stages

The amount of theoretical stages needed to make a given separation is calculated employing a particular vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio will not be right and the desired separation will not be achieved. The packing will appear to not be working properly. The height equivalent to a theoretical plate will be greater than expected. The problem is not the packing itself but the mal-distribution of the fluids going into the packed column sizing.

The Role of Distributors and Redistributors

Those columns can include liquid distributors and redistributors that assist to allocate the liquid equally over a section of packing, raising the efficiency of the mass transfer. The design of the liquid distributors employed to bring in the feed and reflux to a packed bed is critical to making packed column sizing at maximum efficiency. Packed column sizing have a consistent vapor-balance curve, unlike traditional tray distillation in which all tray represents a separate point of vapor-liquid balance. Nevertheless, when modeling packed column sizing it is helpful to compute a number of theoretical plates to indicate the separation efficiency of the packed column with respect to more conventional trays.

In design, the number of necessary theoretical balance stages is first decided and then packing height equivalent to a theoretical balance stage called as the height equivalent to a theoretical plate is also decided. The entire packing height needed is the number theoretical stages multiplied by the HETP.

Questions:

• Define packed column sizing.
• State the importance of packed column sizing.