Absorption and Stripping (where high
capacity and numerous stages are required)

Distillation towers (from deep vacuum to const pressure)

Heat transfer (refinery fractionators and olefin plant quench
columns)

# Packed Column Design

Designing a randomly packed column is a subtle
blend of art and science. Packed columns are most frequently used to remove
contaminants from a gas stream (absorption). However, packed columns can also be
used to remove volatile components from a liquid stream by contacting it with an inert gas
(stripping). They are also used in distillation applications where the separation is
particularly difficult due to close boiling components. While we'll discuss all of
these applications, we'll focus on absorption. However, the design methods are
similar for any of the scenarios.

The first step in designing a packed tower is more
science than art. The equilibrium data between the contaminant and the solvent (or
the distillation components) is needed for the analysis. If tabulated data for your
system is unavailable and the total amount of the contaminant is small (as it usually will
be), Raoult's Law can be used to estimate the equilibrium data for absorption or stripping
applications. For distillation, equilibrium data can be predicted by selecting the
appropriate thermodynamic model (see Choosing a
Thermodynamic Model for Use in Simulation). The operating line for the tower is
constructed differently depending on whether you're dealing with distillation or
absorption /stripping. Since we're focusing on absorption, we'll use it as an
example. In absorption/stripping, the operating line is constructed differently
depending on whether the contaminated stream can be considered "dilute" or if it
must be treated as a concentrated stream.

Usually, it is safe to treat the stream
as dilute if the contaminant makes up less than 10 mole percent of the stream. For
streams that cannot be considered dilute, the mass transfer coefficients must be evaluated
in terms of the gas and liquid flows. Then, graphical evaluation of several
integral relationships must be completed. This type of evaluation is outside the
scope of this article and a text should be consulted for solving these types of
problems. For this article, we will consider dilute streams which are more common
for packed tower absorption and stripping.

Dilute streams allow the column designer to assume constant mass
transfer and the operating line can be constructed in terms of the simplified balance
shown below:

L out x out + G out y out = L in x in + G in
y in

This relation is used in the following manner:

*Suppose you wish to remove acetone from a gas stream of 10,000
mol/h in a packed column. The inlet gas contains 2.6 mole percent acetone and the
outlet gas stream can contain no more than 0.5 mole percent acetone. Assume a pure
water stream enters the packed tower at a rate of 8,000 mol/h.*

L out x out + G out y out = L in x in + G in
y in

(8000) x out + (10000)(0.005) = (8000)(0)+(10000)(0.026)

x out = 0.02625

Just as in the McCabe-Thiele analysis of
distillation, the equilibrium stages are stepped off between the two lines. Note
that for stripping, the operating line would be on the other side of the equilibrium line.

Once the theoretical number of stages have been determined, you
can proceed with the design of the column by following the three steps that we'll outline
below.

*
Specify the packing type and column dimensions
for a column that will be used to remove chlorine from a gas stream using an organic
solvent. Assume the separation requires 20 theoretical stages. The vapor flow
is 7000 kg/h, the average vapor density is 4.8 kg/m*^{3}. The liquid flow is
5000 kg/h, the average liquid density is 833 kg/m^{3}. The liquid's
kinematic viscosity is 0.48 centistokes (4.8 x 10^{-7} m^{2}/s)

**STEP 1: SELECTING A TYPE AND SIZE OF
COLUMN PACKING**

This is where the art of designing packed columns begins.
Some people believe that there are stringent rules surrounding the choice between random
and structured column packing. You can think of random
column packing as the type that comes in a
sack and it is simply dumped into the column. Structured
column packing may come in bales
or intricate designs that are stacked in specific patterns. This is probably one of
those areas of engineering where past experience in the application is the best
guide. Two "areas of choice" where structured
column packing is used are in very
low pressure drop applications and for increasing the capacity of an existing column.
Since we're considering a new design with no serious pressure drop constraint,
we'll choose the more economical random column packing(for
details see: ceramic column packings,
metal column packings,
plastic column packing).

Below are charts showing both English and Metric unit packing
factors. The most common random packing types are shown here:

Generally, the column diameter to
column packing size
ratio should be greater than 30 for Raschig rings, 15 for ceramic saddles, and 10 for
rings or plastic saddles. The geometry of your
column packing will typically be a function
of the needed surface area and/or allowable pressure drop. If several
column packings meet
your requirements, you'll typically choose the least expensive so long as it has an
acceptable operating life. For our example, we'll choose Pall rings (plastic).
For columns over 24 inches in diameter, No. 2 or 2 inch packing should be examined
first. By looking at our flowrates, the chances of our column having a diameter of
at least 24 inches are good, but we'll verify this later. For now, we'll settle on 2
inch plastic Pall rings for our initial analysis.

**STEP 2: DETERMINE THE COLUMN DIAMETER**

Most methods for determining the size of
randomly packed towers are derived from the Sherwood correlation. A
design gas rate, G, can be determined with the help of the figure below
which is based on correlation from the Sherwood equation:

Each line on the graph is marked with an
acceptable pressure drop in inches of water per foot of packing (numbers in parentheses
are in mm of water per meter of packing). Guidelines are as follows:

Moderate to high pressure distillation = 0.4 to 0.75 in water / ft
packing

= 32 to 63 mm water / m packing

Vacuum Distillation = 0.1 to 0.2 in water / ft packing

= 8 to 16 mm water / m packing

Absorbers and Strippers = 0.2 to 0.6 in water / ft packing

= 16 to 48 mm water / m packing

These guidelines are designed around "flooding pressure drops" documented in
literature. In other words, for most cases, designing with these pressure drops
should help you avoid flooding. In the later stages of design, you may want to
perform a thorough flooding calculation. __Perry's Chemical Engineers' Handbook__
covers this topic well. Since we are designing an absorber, we will design for 42 mm
water / m packing (you could design for a lower pressure drop, but the column will
increase in diameter and most likely cost). First, we'll evaluate the x-axis of the
graph above:

(L/V)(vapor density/liquid density)^{0.5} = (5000/7000)(4.2/833)^{0.5} =
0.0507

Note that 4.2 kg/m^{3} was used for the vapor density. The __average__
vapor density was given as 4.8 kg/m^{3}. However, at the top
of the column, the vapor will be less dense and at it's highest velocity.
This is what you should design for. As a rule of thumb, I reduce the
average vapor density by about 15% for design, however if you can get real
data from a similar tower, certainly do so! Reading the
intersection of the 42 mm water/m packing line and 0.05 on the axis, we find
a value of 1.5 for the y-axis.

From the previous charts, we read a
column packing factor of 24 for 2 inch
plastic Pall rings. All other information is know so we can solve for G as shown on
the y-axis of the graph:

G = [1.5 [(4.2)(833-4.2)]/[(10.764)(24)(0.48)^{0.1}]]^{0.5}
= 4.66 kg/m^{2} s

Now, we solve for the column cross sectional area:

Ax = Vapor Flow / G = 7000 kg/h / [(4.66 kg/m^{2} s)(3600 s/hour)]
= 0.42 m^{2}

and the column diameter is calculated by:

Diameter = [Ax / (PI/4)]^{0.5} = [0.42/(PI/4)]^{0.5} =
0.73 m or 2.4 ft

So our assumption of at least a 24 in column diameter is accurate.
If it had not been accurate, G would be recalculated using a smaller packing which would
also correspond to a larger column packing factor.

**STEP 3: DETERMINE THE COLUMN HEIGHT**

Perhaps the most interesting step in designing a packed
column is deciding how tall to build it. You should first ask yourself "What
stage of the design are we currently working on?" If the design is preliminary,
the general HETP (Height Equivalent to a Theoretical Plate) will work well. If the
design requires a higher degree of accuracy, please consulting the
**column packings
manufacturer** or a book entitled __Distillation Design__ by Henry Kister (McGraw-Hill,
ISBN 0-07-034909-6). __Distillation Design__ contains an exhaustive list of HETP
values based on the components of the system and the type of packing used (Chapters 10 and
11). As for preliminary estimates, the following HETP values should be used:

**SETUP** |
**HETP** expressed as ft (meters) |

**Method** |
**Packing Size (in)** |

Distillation |
1.0 |
1.5 (0.46) |

1.5 |
2.2 (0.67) |

2.0 |
3.0 (0.91) |

Vacuum Distillation |
1.0 |
2.0 (0.67) |

1.5 |
2.7 (0.82) |

2.0 |
3.5 (1.06) |

Absorption/Stripping |
All Sizes |
6.0 (1.83) |

To determine the height of the absorption tower in our example, we
multiple the 20 theoretical stages by 6 ft or 1.83 m. We estimate the height of the
tower to be 120 ft or about 37 meters.

**OTHER NOTES:**

While our example problem focused on absorption, packed
towers are also widely used in distillation. Perhaps the most popular of which is
the well documented vacuum distillation of ethylbenzene and styrene in the Production of Styrene. Distillation
Design covers this application very well. If you're seeking a
qualified packing manufacturer to consult with, please
contact us.
AceChemPack're very well respected in this
field and our experience is unmatched.