Packed towers

Packed towers
BY SUJA.S
2017027051

Packed towers-introduction
A common apparatus used in gas absorption, distillation and liq-liq extraction.
It consists of cylindrical column (tower) equipped with gas inlet and distributing
space at the bottom, a liquid inlet and distributor at the top, gas and liquid outlets at
top and bottom, and a supported mass of inert solid shapes called tower packing .
Absorption can be
A “physical” absorption  process occurs due to solubility and vapour-pressure
relationships or,
A chemical absorption  chemical reactions between absorbed substance and the
absorbing medium
packing increase the area of contact between gas and liquid this results in increased
mass transfer between phases

REPRESENTATION OF PACKED COLUMN
Co2 + air
Water
Carbonated water

Design and construction
The packing support is typically a screen, corrugated to give it strength , with a large open
area so that flooding does not occur at the support.
The liquid inlet
The inlet liquid , which maybe pure solvent or a dilute solution of solute in the solvent and
which is called “WEAK LIQUOR” is distributed over the top of the packing by the distributor
and, in ideal operation , uniformly wets the surfaces of the packing.
The distributor
The distributor is a set of perforated pipes.
In large towers spray nozzles or distributor plates with overflow weirs are more common.

Orifice type
Spray type
Trough type
Pipe type
Notched
Chimney
Type
Liquid
distributers

The gas inlet
The solute containing gas or rich gas enters the distributing space below the packing and flows upward
through the interstices in the packing counter current to the flow of liquid .
The packing
 The packing provides a large area of contact between the liquid and gas and encourages initimate
contact between the phases.
 The solute in the rich gas is absorbed by the fresh liquid entering the tower, and dilute , or lean, gas
leaves the top.
 The liquid is enriched in solute as it flows down the tower , and a concentrated liquid , called “STRONG
LIQUOR” leaves the bottom of the tower through liquid outlet.

Flow through packed tower
Two types of flow in a column
a. cross flow
b. counter flow
Packed towers are used for continuous
Countercurrent contacting of gas and liquid
absorption.

Types of tower packings
1. Those that are dumped at
random into the tower.
2. Those that are known as
structured or ordered packings.

DUMPED PACKING or RANDOM PACKING :
Dumped packing consists of units 6 to 75mm (1/4
to 3 in) in major dimension.
Packings smaller than 25mm are used mainly in
laboratory or pilot plant columns.
Dumped tower packings are made of cheap , inert
materials such as clay, porcelain , or various plastics .
Thin walled metal rings of steel or aluminium are
sometimes used .
High void spaces and large passages for the fluids
are acheived by making the packing units irregular
or hollow , so that they interlock into open
structures with the porosity or void fraction of 60 to
90 percent.

 Ceramic berl saddles and raschig rings are older types of packing that are not much used now , although they
were big improvement over ceramic spheres or crushed stone when first introduced.
 Intalox saddles are somewhat like berl saddles , but the shapes prevents pieces from nesting closely together ,
and increases the bed porosity.
 Super intalox saddles are a slight variation with scalloped edges ; they are available in plastic or ceramic form .
 Pall rings are made from thin metal with the portions of the wall bent inward od from plastic with slots in the
wall and stiffening ribs inside .
 Hy-pak metal packing and flexirings are similar in shape and performance to metal pall rings.
 Beds of pall rings have over 90 percent void fraction and lower pressure drop than most other packings of the
same nominal size.
•In stacked packing the units are 50 to 200mm (2 to 8in) in size.
•They are much less commonly used than dumped packings .

STRUCTURED PACKINGS or ORDERED
PACKING:
Structured packings with ordered geometry
evolved from the stedman packing of the late
1930s but they found few industrial uses
until the sulzer packing was developed in about
1965.
Early structured packings were fabricated
from wire gauze most current ones are made of
sheets perforated corrugated metal,with
adjacent sheets arranged so that liquid spreads
over their surfaces while vapour flows through
channels formed by the corrugated.
The channels set an angle of 45degree

Types of packing
(a) Raschig rings
(b) Pall rings
(c) Berl saddle ceramic
(d) Intalox saddle ceramic
(e) Metal Hypac
(f) Ceramic, super Intalox
(g) cross partition rings

PACKING MATERIAL
PROPERTIES
Properties of tower packing:
 Low weight per unit volume
 Large surface area per unit volume of
packing
 Chemically inert to the fluids being
processed
 Lightweight but strong
 Good distribution of fluids
 Good wettability

The design of a packed column will involve the
following steps:
Select the type and size of packing.
Determine the column height required for the speciﬁed
separation.
Determine the column diameter (capacity), to handle the
liquid and vapour ﬂow rates.
Select and design the column internal features: packing
support, liquid distributor, redistributors. Example
DESIGN OF PACKED
TOWER:

Requirements:
The principal requirements of a packing are that it should:
Provide a large surface area: a high interfacial area between the gas
and liquid.
Have an open structure: low resistance to gas ﬂow.
Promote uniform liquid distribution on the packing surface.
Promote uniform vapour gas ﬂow across the column cross-section.
Packings with a regular geometry: such as stacked rings, grids and
proprietary structured packings.
Random packings: rings, saddles and proprietary shapes.

The diameter of a packed absorption tower depends on the quantities of gas and liquid
handled, their properties, and the ratio of one stream to the other.
The height of the tower, and hence the total volume of packing, depends on the magnitude of
the desired concentration changes and on the rate of mass transfer per unit of packed volume.
Calculation of the tower height, therefore,rest on
material balances,
enthalpy balances, and
estimates of driving force and mass-transfer coeffcients.

DETERMINATION OF
COLUMN OR TOWER
HEIGHT:
 NTU (number of transfer units)
 HTU (height of transfer unit)
 HETP (height equivalent to a theoretical plate)

 When packings are used instead of trays, the same enrichment
of the vapour will occur over a certain height of packings, and this
height is termed the height equivalent to a theoretical plate
(HETP).
 Therefore, the required height of packings for any operation is
given by = HETP × N

 In industrial practice, the HETP concept is used to convert empirically the number of theoretical
trays to packing height.
 Most data have been derived from small-scale operations and they do not provide a good guide to
the values which will be obtained on full-scale plant.
 This method had been largely replaced by the Method of Transfer Units.
 In practice, packed columns are often analyzed on the basis of equivalent equilibrium stages using a Height
Equivalent to a Theoretical Plate (HETP):
 For a specified operation, in packed tower, the height of packing is to be determined and in tray tower,
numbers of ideal trays are determined. The ratio between packing height to number of trays required for
the operation is called height equivalent to theoretical plate (HETP).
 HETP varies with size and type of packing, flow rate of gas and liquid, concentration of solute, physical and
transport properties as well as equilibrium relationship and uniformity of liquid and gas distribution.

Comparison between HTU / NTU and HETP
The NTU and the HTU should not be confused with the HETP and the number of
theoretical equilibrium stages n, which can be calculated with the Kremser
Equation:
When the operating and equilibrium lines are not only straight but also parallel,
NTU = n and HTU = HETP. Otherwise, the NTU is greater than or less than n.

ADVANTAGES:
Amine absorbers use counter-current flow through a
trayed or packed tower to provide intimate mixing
between the amine solution and the sour gas.
In packed towers also known as air stripping towers,
the contaminated water flows downwards through a
packing, counter-current to an air flow which strips
the VOCs into the gas phase and discharges them
through the top of the tower. The treated water is
collected at the bottom of the tower.

Packed towers

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