PCSIR Internship
Pilot department
Types of distillation column packing
Submitted by: Huda shah
Chemical engineering
Intern, U.E.T
Submitted to: Sir Fayaz
Junior Engineer
PACKING
The main stipulations that need to be met for packing are that it should meet the
following conditions:
1. Allow for a large surface area for the interface between liquid and gas.
2. The packing should be configured in a way that allows the least resistance to
the gas flow, preferably by having an open arrangement.
3. It should enhance steady liquid flow on the exterior of the packing.
4. It should enhance a steady gas and vapour distribution across the crosssection of the distillation column.
Numerous types of packing exist in various shapes and sizes to meet various
demands. However, they can be split into two main categories:
1. Packing with a consistent layout, termed structured packing. Examples of
structured packing include stacked rings, proprietary structured packing, as
well as grid packing.
2. Random packing in the form of proprietary shapes, saddles and rings.
Random packing is arbitrarily unloaded into the distillation column and takes
up a random order within the column.
Packing with a consistent layout, for example, grids have an advantage in that they
provide a much larger surface area for substances to flow through. This allows
higher flow rates of gases where a minimal pressure drop is important such as in
cooling towers
STRUCTURED PACKING
Structured packing usually consists of alternating layers of mesh, gauze or fine
corrugated sheets as illustrated in the image below. These are typically made of a
wide range of materials including ceramics, graphite, plastic or metal alloys. The
material is arranged with a regular geometry to obtain a large surface area with a
large void fraction. Structured packing usually results in smaller pressure drops
and overall improved efficiency with shorter bed heights when compared to
random packing. However, they are typically more costly to construct, and the time
of construction is also lengthier (Pilling & Holden, 2009).
The main advantage of structured packing when compared to random packing is
that the separation efficiency, typically expressed in the height of packing
equivalent to a theoretical equilibrium plate (HETP) is usually less than 0.5 m and
the low-pressure drop of 100 Pa/m.
Structured packing is more favoured than random packing for the following cases:
1. In column revamps: in scenarios to enhance the capacity of the column and
minimise the reflux ratio.
2. High vacuum distillations.
3. For separations that need numerous stages, for example, in the separation of
isotopes due to the more favourable HETP.
Structured packing is typically more expensive than random packing. However,
this comes at the advantage of an improved efficiency when compared to random
packing
RANDOM PACKING
1. RASCHIG RINGS
Raschig Rings were the first kind of packing used in mass transfer processes, and
they consist of a hollow cylindrical tube with a height to diameter ratio of 1:1.
They display an exceptional strength to weight ratio and are much more resistant to
fouling than other types of packing (The Pall Ring Company, 2020).
2. PALL RINGS
Pall rings are a progression from the Raschig rings and have the same cylindrical
shape and ratio. However, they have two rows of punched out holes with webs
from the centre which drastically enhances the execution of the packing with
regards to the pressure drop, throughput and efficiency (The Pall Ring Company,
2020).
PRESSURE DROP VERSUS CAPACITY
They are most appropriate when a low-pressure drop is required in high capacity
scenarios and can be customised in a wide variety of sizes for optimum
performance for separation. They create a large amount of randomness, and the
high surface area to volume ratio enhances mass transfer. Pall rings provide a
consistent ratio of free and blocked pathways regardless of the orientation.
MECHANICAL STRENGTH
The internal structure with the web-like design results in a mechanically robust
type of packing that is appropriate for use in packed beds that are deep.
Metal pall rings can be made of stainless steel alloys or other metals and provide
the advantage that they are capable of handling higher temperature separations and
are highly resistant to fouling and are also mechanically robust. They are typically
used in separations with H2S, NH3 and SO2 as well as absorption and stripping,
steam stripping, quench towers and direct contact cooling.
3. BERL SADDLES
The ceramic Berl saddles are an improvement from both the Pall rings and Raschig
rings with regards to an enhanced flow distribution. The shape of the saddles has
both the external and internal surfaces exposed, and the distribution of vapourliquid is the same on both sides. When comparing Berl saddles to the previous two
types of packing, they produce a lower pressure on the surrounding column walls
(Ultimo Engineers, sa).
Berl saddles typically consist of chemical grade porcelain which is chemically
resistant to many substances apart from Hydrogen Fluoride and strong alkalis. It is
most appropriate for sulphuric acid applications.
PACKING SIZE
The rule of thumb is that the biggest size of random packing appropriate for the
dimensions of the column should be used. However, an upper limit of 50 mm
should be put in place. It should be noted that small-sized packing is considerably
more expensive. Packing sizes greater than 50 mm do not show any cost-benefit
when considering the reduced mass transfer efficiency. This is because of larger
packing results in substandard liquid distribution (Sinnott, 2005, 592).
Recommended size ranges are:
 For a column diameter smaller than 0.3 m diameter, use a packing size
smaller than 25 mm.
 For a column diameter between 0.3 m and 0.9 m, use a packing size of
between 25 mm and 38 mm accordingly.
 For a column diameter above 0.9 m, use a packing size between 50 mm and
75 mm.