What Is A Packed Column?

The Engineer's Perspective
The Engineer's Perspective

Table of Contents

What Is A Packed Distillation Column?

A packed distillation column is a cylindrical vessel filled with packing materials that enhance continuous mass transfer between two feed fluids (liquid-liquid or gas-liquid) by effectively increasing the contact surface area.

Typically, one of the fluids preferentially wets the packing, flowing as a film over its surface while the second fluid flows over the remaining volume of the vessel.

A packed distillation column can be filled with random individual packings (e.g. Pall Rings and Berl Saddles), can be structured, grid packed, or filled with catalysts or adsorbents such as zeolite or activated carbon.

Components of a Packed Column

Components of a Packed Column
Components of a Packed Column, Source: Saiptech Packed Tower Technology

The main components of a packed column are:

  • Packings promote contact between two fluids inside flowing either countercurrently or co-currently. The packings can be made of metal, plastic, or ceramic. They can also either be in random or structured arrangement.
  • Spray distribution system spreads the liquid feed evenly across the top of the packing.
  • Packing support allows the liquid being sprayed to drain away from the packing and acts as an inlet where the gas can pass through the tower’s section.
  • Demister prevents entrained liquid droplets from leaving the tower alongside the gas stream.
  • Vessel or tower holds the rest of the components, usually cylindrical and made of steel. Depending on the intended use, the vessel can be coated to resist corrosion and other harmful effects.

How Do Packed Distillation Columns Work?

Packed Column Flow Orientation
Packed Column Flow Orientation, Source: Coulson and Richardson’s Chemical Engineering

In gas/vapor – liquid systems, the liquid is the wetting fluid while the gas/vapor rises through the column making close contact with the downcoming liquid through the packings’ surface. In distillation set-ups such as in fractional distillation, the more volatile component of a mixture progressively transfers to the vapor phase while the less volatile component condenses out into the liquid. For liquid-liquid extraction, the solute is transferred to the solvent as the fluids come in contact with each other in the surface of the packings.

The following terms are commonly used in the operation of a packed column:

  • Loading point of a column is when gas velocity is high enough to restrict the flow of liquid. Beyond this point, the pressure drops at a much faster rate when all the liquid is carried off by the gas (flooding point).
  • Channeling occurs when low fluid flow rates cause maldistribution through the packings. If the liquid is not distributed evenly over the packings, it tends to flow toward the wall. This is analogous to weeping in tray columns where liquid leaks through the perforations or valves due to insufficient countercurrent gas flow.
  • Gas holdup is the amount of gas/vapor retained at the column for a certain time. This determines the intensity of contact between the gas and liquid.
  • Void volume is the volume of the empty column minus the volume occupied by the solid packing materials.

How Do You Calculate Pressure Drop In A Packed Tower?

Pressure drop is the pressure difference between any two points along the packed column. It accounts for mechanical forces such as the gravitational and frictional forces. It essentially dictates the efficiency of mass transfer. 

While there are various approaches to determining the pressure drop, the most accurate method is through the manufacturer’s own literature whenever the data is available.

Pressure drop inside a packed column is greatly affected by the packing factor — the packing’s surface area per unit volume (a) divided by the cube of the packing void fraction (ε): 

                                                                            Fp = a/ε3

which has a dimension of 1/ft or 1/m. However, packing factors nowadays do not correspond to a/ε3 anymore but are rather determined by the manufacturer so as to match a generalized pressure drop calculation. This is due to more efficient packing shapes that can have a higher surface area without causing too much pressure drop especially for structured packings.

Packed Tower vs. Tray Column

The main difference between a packed column and a tray column is the mode by which the two fluids interact. Tray columns use a system of trays and weirs while packed columns use packing materials to promote fluid contact. Between the two, pressure drop is much lower in packed columns. To give an idea of the scale difference, the pressure drop across a structured packed column is only one-sixth of that across a tray column of the same height. 

A thorough discussion between packed and tray columns is discussed here

Advantages & Disadvantages of Packed Columns

In summary, these are the advantages and disadvantages of packed columns:

Advantages & Disadvantages of Packed Columns
Advantages & Disadvantages of Packed Columns


1. What is the difference between capillary column and packed column?

While both packed columns and capillary columns are used in gas chromatography, packed columns are used more in distillation, gas absorption, and liquid-liquid extractions.

In packed columns, the stationary phase is situated in the cavity of the packed column. On the other hand, the stationary phase flows over the inner surface of the cavity of the capillary column.

2. Why is a packed column used in fractional distillation?

Packed columns are used in fractional distillation when access is limited (e.g. small diameter distillation columns) and if corrosion control/resistance is essential. Although, packings are still not recommended for large diameter columns or where a higher turn down ratio is required.

3. How do you find the height of a packed tower?

There are 2 common theoretical approaches — equilibrium stage analysis (through HETP) and mass transfer analysis (through HTU & NTU).

HETP is the height of the packing equivalent to a theoretical plate in which enrichment of the gas occurs. The theoretical number of plates is determined via the McCabe-Thiele Method. There are HETP values for every type of packing to be used in the column.

Meanwhile, the height of a packed tower can also be based on HTU (height of transfer units) x NTU (number of transfer units). The HTU measures the efficiency of packings to enable mass transfer. While, the NTU is equivalent to the number of theoretical plates in a tray column.


  1. Billet,R. and Schultes, M. “Prediction of Mass Transfer Columns with Dumped and Arranged Packings – Updated Summary of Calculation Method of Billet and Schultes.” Trans IChemE, Vol.77, Part A, September 1999. https://www.researchgate.net. Accessed 30 May 2022
  2. Coker, A.K. “Ludwig’s Applied Process Design for Chemical and Petrochemical Plants.” Vol. 2, 4th Ed., 2010. www.sciencedirect.com. Accessed 2 June 2022.
  3. Coulson, J.M. Coulson and Richardson’s Chemical Engineering Vol 2 5th Ed. Butterworth-Heinemann, 2002.
  4. Doa, Shaheen. “Packed Columns Design and Performance.” July 2017.
  5. Foust, et. al. “Principles of Unit Operations.” 2nd Ed. John Wiley & Sons, 2008-10.
  6. Jenkins, Scott. “Facts at your Fingertips: Distillation Trays and Packing.” Chemical Engineering, 1 March 2020, https://www.chemengonline.com/distillation-column-trays-packing/. Accessed 3 June 2022.
  7. Mach Engineering. “Designing a Random Packed Column.” https://www.machengineering.com/designing-a-random-packed-column/. Accessed 2 Jun 2022
  8. Perry, R.H. Perry’s Chemical Engineer’s Handbook 7th Ed. McGraw-Hill, 1997
  9. Saiptech Packed Tower Technology, https://saiptech.en.made-in-china.com. Accessed 30 May 2022
  10. University of Michigan. “Distillation Columns.” Visual Encyclopedia of Chemical Engineering Equipment, n.d, https://encyclopedia.che.engin.umich.edu/distillation-columns/. Accessed 30 May 2022


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