This document provides an overview of distillation columns used for separating liquid mixtures into components based on their volatilities, detailing the mechanisms of fractionating columns, including the types of trays (bubble-cap, sieve, and valve trays) and packed columns. It describes the roles of condensers and reboilers, the dynamics of liquid and gas contact, and the design variables impacting operation efficiency and pressure drops. The document also highlights the trade-offs involved in column design necessary to optimize cost, energy, and product purity.

1. This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 869993.
Fractionating
column

2. Streams in a distillation column
• In a basic distillation column, a feed
stream enters in the middle of the column
and two streams leave.
• The feed tray divides the column into
rectifying section and stripping section.
• The lighter, lower boiling point components
evaporate and travel up the column to
form the top product (distillate).
• The heavier, higher boiling point
components condense and travel down
the column to form the bottom product. Distillation takes place in a column
with cross-flow trays.
(Courtesy of Sulzer)

3. Condenser and reboiler
• Separation is achieved by controlling the
column temperature and pressure profiles.
• Reboiler is a heat exchanger at the bottom of
the column. It boils some of the liquid and the
vapor generated returns to the column at the
bottom of the stripping section.
• Condenser cools and condenses the vapor
leaving the top of the column.
• Reflux drum holds the condensed vapor from
the top of the column so that liquid (reflux) can
be recycled back to the column. A typical distillation column.
Picture: Mbeychok CC BY-SA 3.0

4. Fractionating columns
• A fractionating column or fractional column is an essential item used in
the distillation of liquid mixtures to separate the mixture into its component
parts, or fractions, based on the differences in volatilities.
• There are two main types of industrial fractionating columns (or towers):
• Fractionating columns can have multiple feed streams and/or multiple
product streams (fractions).
Tray (or plate) columns
• Sieve trays
• Bubble-cap trays
• Valve trays
Packed columns
• Random packings
• Structured packings
• Grid paddings

5. Plate column (tray column)
• In plate column, liquid flows downward via gravity
and gas flows upward (countercurrent flow).
• In the simplest case, there is only one feed stream
and two product streams (top and bottom).
• Trays (or plates) divide the column into stages. The
process is a stagewise contact operation.
• The most common types of trays are bubble-cap,
sieve and valve trays.
• Liquid flows through downcomers from one tray to
the lower one.
• Gas-liquid contact occurs in the trays.
Distillation column with trays.
Picture: CC BY-SA 2.5

6. Liquid/gas contact on the tray
• In the tray column, gas and liquid phases are brought
into contact on the trays of the column.
• Liquid flows downwards and vapor rises upwards.
• The liquid entering the tray will contact the gas
exiting the tray. The hotter vapor phase will heat the
incoming liquid phase as it bubbles through the tray,
evaporating the light components which then leave
the tray with the vapor phase.
• The liquid phase will cool the vapor phase and it will
cause the heavier components of the vapor phase to
condense and exit the tray with the liquid phase.
(Courtesy of Sulzer)

7. Examples of trays
• There are many modifications of these on the market and also other types.
Bubble-cap trays Sieve trays Valve trays
- The oldiest type of tray
- Bubble-cap consists of the
cap and the riser. The riser
acts as a vapour passage.
- Bubble-caps are arranged
on a plate.
- The simplest type of trays
- Bubble-caps are replaced
by holes. Diameters may
vary from 1/8 inch to ½ inch.
- Holes are usually made by
punching.
- Relatively new class of tray
- Valves can slide vertically
on the tray floor.
- This provides variable area
for the gas flow depending
upon the flow rate.
(Courtesy of Sulzer)

8. Bubble cap trays
• Bubble cap trays allow the vapor rising up the
column to “bubble through” the downflowing
liquid to create the contacting.
Benefits:
• Provides intimate mixing
• Ability to minimize weeping (the liquid
weeping down into the tray below)
Disadvantages:
• Can be costly to fabricate
• With dirty or high viscosity fluids, the ability of
the cap to flow up and down can be hindered
• Cleaning can be a major challenge Picture: Fabiuccio CC BY-SA 2.5

9. Sieve trays
• Downcoming liquid and rising vapor go
through the same hole
Benefits:
• Provides intimate mixing
• Low cost
• Ease of maintenance
Disadvantages:
• Potential for weeping, especially at low
operating rates
• Greater possibility of plate-to-plate mixing
can lower the efficiency
Picture: Hipple 2017, 164

10. Valve trays
• Valve tray is like a hybrid between the sieve
tray and bubble cap tray.
• Valve trays can be fixed or floating.
• Movable valves provide higher capacity and
a wider operating range
(Courtesy of Sulzer)
Fixed valves from Sulzer
Rectangular float valve
A tray with V-Grid, sloped outlet weir and push valves.
(Courtesy of Sulzer)

11. Packed column
• In a packed column, the gas and liquid flow up and
down in continuous (not stagewise) way.
• Column is filled with suitable packing materials to
provide a large contacting area between the phases.
• Packing materials can be divided into three
categories:
• random packings (just dumped into the shell)
• structured packings (arranged or stacked packings)
• grid packings
• In addition to actual packing materials, tower
internals consist of gas and liquid distributors and
redistributors, packing supports and liquid collectors.
(Courtesy of Sulzer)
Installation of packing.

12. Examples of packing materials
• Packing materials can be made of plastics, ceramic, metals or carbon. Different
materials are suitable for different process conditions.
Random packings Structured packings Grid packings
(Courtesy of Sulzer)

13. Column internals
• In packed columns, the liquid is collected from the walls and redistributed to the
centre of the column by liquid distiributors. Support grids are also needed.
Liquid distributors Collectors Support grids
(Courtesy of Sulzer)

14. Comparison of columns
Tray column (Plate column) Packed column
Stagewise contact operation Continuous contact operation
Higher pressure drop Lower pressure drop  suitable for vacuum
distillation
Better at handling higher liquid
loadings
Easier separation of close boiling components 
Higher separation efficiency (HETP)
Higher capacity  smaller columns
Better handling of « dispersed
solids»
 Less blocking and easier cleaning
Better handling of corrosive chemical systems
 Non-metallic (plastics, grahite, etc.) materials
can be used
Better handling of non-miscible
liquids
Better handling of foaming chemical systems

15. Design variables
• There are numerous design variables that need to be chosen.
1. Method of contacting between the liquid and gas flows
• Tray or packed column, type of trays or packing materials
2. Types of heat exchangers at the top and bottom of the column
• Condenser at the top and reboiler at the bottom (partial or total)
3. Amount of reflux returned to the column
• Affects the number of trays or height of packing required.
• The bigger the reflux ratio, the shorter the column needs to be
4. Diameter of the column
• Affected by the flow rates and the densities and viscosities of the streams
5. Height of the column
• Primarily determined by the relative volatility, reflux ratio and the purity required
6. Efficiency of contacting between the liquid and vapor flows in the column
• Affected by contacting system and the physical properties of the liquid and vapor

16. Pressure drop and column design
• Higher liquid and gas flows affect larger pressure drop.
• Selected column types and internals affect the pressure drop seen.
• Pressure drops with different internals from small to large:
1. Structured packing
2. Loose packing
3. Sieve trays
4. Valve trays
5. Bubble caps
• There is no one correct column design. It is a trade-off between many factors,
like cost and capital, energy demands, required purity of the top and bottom
products.
• These are discussed more detailed in Level 3.
Small pressure drop
Large pressure drop

17. This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 869993.
References
Dutta, B. K. 2007. Principles of mass transfer and separation processes. New Delhi: Prentice-Hall,
pp. 172-192, 241-243, 319-321.
Hipple, Jack. 2017. Chemical Engineering for Non-Chemical Engineers. American Institute of
Chemical Engineers, pp. 149-168.
Pilling, M. & Holden, B. S. 2009. Choosing trays and packagings for distillation. CEP Magazine.
September 2009, 44-50.
Videos:
• Multistage distillation column demonstration: https://youtu.be/gYnGgre83CI (17:03)
• How a distillation column works: https://youtu.be/M7AL7-44YTc (5:59)
• Distillation towers, reboilers & condensers: https://youtu.be/ex_hScKbYhM (20:24)