Packed columns

Packed Columns
Design and Performance
Authors:
Doaa Shaheen Fedaa Mohamad
Alaa Mohamad Ranea
Supervision:
Dr. Taha ALkhames
Julay/2017
Department of chemical Engineering
College of Engineering
Mutah University

Table of Content
contents
 INTRODUCTION ………………………………………………………….
 Distillation process …………………………………………………….
 Types of Distillation Column………………………………………..
 Tray Column ……………………………………………………….
- Advantageof trayed column…………………………………………….
 Packed Column………………………………………………….
-packed Column Description…………………………………………….
-Component of a packed column……………………………………..
-Packing materials…………………………………………………………….
-Type of packed column……………………………………………………
-Design proceder………………………………………………………………
-Packed bed process design………………………………………………
-Representation of packed column……………………………………
-Method for packing height……………………………………………….
-Absorption /stripping in packed columns…………………………
-Evaluation height based on HTU-NTU model……………………
-Theory……………………………………………………………………………..
-Calculation of enrichment in packed columns………………….
-Applications……………………………………………………………………..
-Usage Examples………………………………………………………………..
-Packed column for distillation………………………………………….
-Packed Hydraulic……………………………………………………………..
-PressureDrop in Packed Beds………………………………………….
-Advantages and disadvantageof packed column……………..
-Packed column vapor-equilibrium curve ……………………..
- Comparison to Tray Columns…………………………………………..
-reference…………………………………………………………………………...

INTRODUCTION
Distillation
is by far the most important separation process in the
petroleum and chemical industries.
It is the separation of key componentsin a mixture
by the difference in their relative volatility,or boiling
points. It is also known as fractional distillation or
fractionation.
Is the key operationin any oil refinery. In processing, the
demand for purer products, coupledwith the need for
greater efficiency, has promoted continuedresearch into
the techniques of distillation.
In engineering terms, distillation columnshave to be
designed with a larger range in capacity than any other
types of processing equipment, with single columns 0.3-10
m in diameter and 3–75 m in height.
Designers are required to achieve the desired product
qualityat minimum cost and also to provide constant
purity of product even though there may be variationsin
feed composition.

A distillationunitshould be considered together with its
associated control system, and it is often operated in
associationwith several other separate units.
The vertical cylindricalcolumn provides, in a compact form
and with the minimum of ground requirements, a large
number of separate stages of vaporisationand
condensation.
Types of DistillationColumn :
Tray Column
Tray Columns utilize a pressure and temperature
differential to separate the products.
For most tray columns, the weir holds a liquidlevel of
each tray.In the Figure shows how the tray works. Liquid
enters from the down-comer of the tray above. The
vapor must overcome this liquid head to move up the
column. On the tray the vapor and liquid are
contacted becomes bubble or froth where the mass
transfer takes place and then above the tray they are
separated where froth flows over the outlet weir and
vapor with the light volatilecompoundis disengaged.

Tray column performs well in high liquid and vapor
loading. Tray have higher pressure drop than packed,
and It also have high resistance to corrosion. There
are five major types of tray column:
1- Bubble Cap Tray
is the most efficient separationdevice but, is also the
most costly. It consists of a number of 'Chimneys' or
'Risers' (small, short pipes set into the tray), through
which the vapour can pass. Fitted over the riser is a
'Cap' which causes the rising vapour to turn through 180
°. This forces the gas to 'Bubble' through the liquid
flowing across the tray. The liquidlevel on the tray is
maintainedbelow the top of the riser to prevent
dumping of liquiddown the tower.

2- Sieve Deck Tray:
is simply a metal plate containingdrilled holes through
which the rising vapour can pass into the liquidflowing
across the tray.
3- Dual flow tray

4- Valve Tray
is similar to the sieve type but, each hole is fitted with a
flapper valve which opens as vapour passes through the
hole. This type is used where vapour velocity is not
constant and the valves prevent liquidfrom dumping
through the holes at times of low gas velocity.

5- Baffle Tray
The down-comers require a disengaging area to
separate the liquid from the vapor. This area requires
a minimum distance that normally sets the tray spacing.
The liquidis required to travel across the deck to
the next down-comer

Packed Column:
A packed bed is a hollow tube, pipe, or other vessel that is
filled with a packing material. Packed columns are used for
distillation,gas absorption, and liquid-liquid extraction.
The gas liquidcontact in packed bed column is continuous,
as in a plate column.
The liquidflows down the column over the packing surface
and the gas or vapor, counter-currently ,up the column ,
some gas-absorption column are co-current .
The packing can be randomly filled with small objects like
Raschig rings or else it can be a specificallydesigned
structured packing. Packed beds may also contain catalyst
particles or adsorbents such as zeolite pellets, granular
activated carbon.
Packed column utilize packing to contact between the
phases (liquid-vapor) on the surface. Packed column
performs well at low pressure, low liquidand vapor
loadingthat make packed column have the most efficient
in these terms. At high flow parameters the capacity and
efficiency can significantly reduce, also in heavy fouling
applications and corrosive condition. Packed column
has less pressure drop than tray column and it reduce

foaming since generates thin films instead of fine
droplets for mass and heat transfer. Packed column is
divided by Random, Structured and Grid Packed
Columns which is generate a mass transfer area by
providing a large surface area over (50%) which the
liquidcan transfer heat and mass to the vapor.
The performance of a packed column is very dependet on
the maintenanceof a good liquidand gas distribution
throughout the packed bed
Packed Column Description
In the Figure illustrates a tower with structured
packing.In additionto the packing itself, packed
columnsrequire other internals to assure the
performance ofthe packing. These internals
are:Liquid feed pipes to deliver theSuid to the
liquiddistributors,as seen at the top of the tower
and at the intermediate distributor.Liquid
collectionand mixing as shown below the top bed.
Liquiddraw-off sump and pipe as shown below the
top bed. Liquid redistributors, as presented
between the two beds .Vapor feed pipes as shown

at the vapor inlet nozzle, at the bottom of the tower.
Packing support plates resting on beams and level-led
rings welded to the vessel. Hold-downplates. Incorrect
design or incorrect installationof any of these elements
can lead to tower failure. One of the most critical element,
and often the culprit of tower failures, is the liquid
distributor.
Component of a packed column:

Packing materials:
1-Ceramic: superior wettability ,corrosion resistance at
elevated temperature ,bad strength.
2-metal: superior strength and good wettability.
3-plastic:inexpensive, good strength but may have poor
wettabilityat low liquidrate.

Type of packed column:
1. Random Packed Column:
Random packing is packing of specific geometrical
shapes which are dumped into the tower and orient
themselves randomly. Random packing has more risk
than structured packing and less abilityto handle
maldistributedliquid.
2. Structured Packed Column :
is crimped layers or corrugated sheets which is stacked
in the column. Each layer is oriented at 70° to 90°
to the layer below. Structured packed offers 30%
capacities higher than random packed for equal
efficiency up to 50% higher at the same capacity.

Comparison of type of packing:
3. Grid Packed Column:
Is systematically arranged packing use an open-
lattice structure. This device is composed of panels
that promote mass transfer and enhance entrainment
removal. They have high open area, resulting in
high capacity, low pressure drop, and high tolerance to
fouling.

Design proceder:
Packed bed process design:
There are numerous equationsand correlations that have
been publishedin the technicalliterature for predicting
the pressure drop of the vapor traveling through a packed
bed and for predicting the height equivalentto a
theoretical plate (HETP).There are also numerous rules of
thumb that have been publishedfor use in the process
design of packed beds and which are simpler to use and
probablyas accurate as the equationsand correlations. A
discussion of all the equations,correlations and rules of
thumb would be far beyond the scope of this article.

Representation of packed column:

Method for packing height:
Absorption /strippingin packed columns:

Evaluation height based on HTU-NTU model:

Theory:
The Ergun equationcan be used to predict the pressure
drop along the length of a packed bed given the fluid
velocity, the packing size, and the viscosity and density of
the fluid.
The Ergun equation,while reliablefor systems on the
surface of the earth, is unreliablefor predicting the
behaviorof systems in microgravity. Experiments are
currently underway aboard the InternationalSpace Station
to collect data and developreliable models for on orbit
packed bed reactors

Calculation of enrichment in packed columns:
With plate columns, the vapourleavingan idealplate is
richer in the more volatilecomponentthan the vapor
entering the plate, by one equilibriumstep. PETERS(70)
suggests that this same enrichment of the vapour will
occur in a certain height of packing, which is termed the
height equivalentof a theoretical plate (HETP).
As all sections of the packingare physicallythe same, it is
assumed that one equilibriumstage is represented by a
given height of packing. Thus the required height of
packing for any desired separation is given by HETP × (No.
of ideal stages required).
This is a simple method of representation which has been
widely used as a method of design. Despite this fact, there
have been few developments in the theory. MURCH gives
the following relationshipsfor the HETP from an analysis
of the results of a numberof workers.
Columns 50–750 mm diameterand packed over heights of
0.9–3.0 m with rings, saddles, and other packingshave
been considered. Most of the results were for conditions
of total reflux, with a vapour rate of 0.18–2.5 kg/m2s
which corresponded to 25–80 per cent of flooding. The
relationship:

Applications:
In most applications,the purpose of a packed bed is to
provide intimate contacting of the upward flowing vapor
and the downward flowing liquidin separation processes
such as distillationand absorption.
Usage Examples:
Packed beds are most commonly used in air pollution
control, but they are also used in the chemical,
petrochemical, food, pharmaceutical,paper, and
aerospace industries. The beds shown below are used to
absorb and eliminateethylene gas from a sterilization
chamber. The water-soluble ethylene gas is hydrolyzedto
ethylene glycol.

The packed bed absorption column shown below removes
acidic fumes such as H 2 SO 4, HCl, HNO 3, and HF from an
inlet gas stream. Packed bed absorption is commonly used
when dealingwith corrosive substances such as these.

Packed column for distillation:
In bubblecap and perforated plate columns, a large
interfacialarea between the rising vapour and the reflux is
obtainedby causing the vapour to bubblethrough the
liquid.
An alternativearrangement, which also provides the
necessary large interfacialarea for diffusion, is the packed
column, in which the cylindricalshell of the column is filled
with some form of packing.
A common arrangement for distillationis
as indicatedin the Figure, where the
packing may consist of rings, saddles, or
other shaped particles, all of which are
designed to provide a high interfacialarea
for transfer.
In packed columns the vapour flows steadily up and the
reflux steadilydown the column, giving a true
countercurrent system in contrast to the conditionsin
bubblecap columns, where the process of enrichment is
stagewise.

Type of distillation:
1-Batch Columns
In batch operation, the feed to the column is introduced
batch-wise. That is, the column is charged with a 'batch'
and then the distillation process is carried out. When the
desired task is achieved, a next batch of feed is introduced.
2-Continuous Columns
In contrast, continuous columns process a continuousfeed
stream. No interruptionsoccur unless there is a problem
with the column or surrounding process units. They are
capableof handlinghigh throughputs and are the most
common of the two types. We shall concentrate only on
this class of columns

Use of Packing in Distillation:
Use of packing in mass transfer has its origins in theearly
1800s for simple applicationssuch as alcoholdistillation,
and in sulfuric acid plant absorbers.
Glassballs, coke or even stones were used as packing ma-
terials. Nevertheless packings for distillation were
notestablisheduntil the 1930s with the use of
regularshape materials such as ceramic Raschig rings
andBerl saddles, as well as the availabilityof
distillationcalculationssuch as the McCabe}Thieleand
Pon-chon}Savaritmethods.
Early in the second half of thecentury, the use of packing
for distillation wentthrougha transformation, producing
the second-generation packings(seeTable 1).
Regular and im-proved shape of packings, such as pall
rings, becameavailablewith larger open areas that
permitted a sub-stantialincrease both in capacity and
column efRcien-cy.
In the 1960s Sulzer introduced the wire-meshpackings
with very high efRciency (low height equiva-lentto a
theoretical plate, HETP), resulting in a newtransformation
in the use of packings.

In the 1970s and 1980s all major mass-transfer equipment
manu-facturers developed structured packings.
Comparedto the traditionaltray columns spectacular
improve-ments in plantcapacity were achieved, but also
someprojects were pitfalls, when the expected beneRts
didnotmaterialize.
Manufacturers started realizing thatliquiddistributors had
to be improved, but there was no coherent understanding,
nor correlations, that could lead to a safe distributor-
column system design.
Many manufacturers returned to trays, producing new
improved designs, using the area under thedowncomer for
vapourSow: these trays are offered with new names that
indicatetheir increased vapourSow capacity(Maslow,
Super rack, etc.).
The need for good distributionand its effect on the
columnefRciency are now well understood, allowing safe
design and efficient applicationsfor random and
structured packing’s in large industrialcolumns.

Packed Hydraulic
Packed tower operated under vapor-liquid counter
current conditions and becoming increasingly important
in environmental protection technologies. Among these,
packing has received the greatest attention owing to
their good performance. That why the knowledge of the
hydrauliccharacteristic are essential for design of packing
tower to get the best optimizing performance of packing
for maximizing theoretical stages per height of column,
minimizing pressure drop per theoretical stage of
separation, and maximizing the operating range of the
column.
The operation area of packing is limited by the
maximum loading which depend on the characteristic
of phase, the type and geometry of the internal.
In the Figure show the operating area of packed column:

The upper limit is calledflooding point and the lower
limit dewetting point. Reaching the loadingpoint, the
down flowing liquidphases holdbackby the up flowing gas
phase result a higher liquid in the bulk and increase
the gas pressure drop then gives the floodingpoint.
It make the liquidcan not flow downwards. While in the
other side the vapor passes as single phase without
contact to the liquid on the column.
A strong segregation of the phases and a rapid increase
of the pressure drop. Furthermore increase the loading
and decrease of the efficiency The area between loading
point and flooding point is the operationarea.
The vapor can not pass the column without contact with
liquid.Where the vapor is dispersed, some liquidis hold
back. Because of high turbulenceof both phases result a
good mass transfer and high separation efficiency.
The lower limit is the minimum liquid flow or limit
of wetting of the internals. With decreasing wetting of
the internal, the mass transfer is reduced and separation
efficiency of the column decreases.
The lower limit is influenced by physical properties of
the mixture to be separated as well as from material and
geometry of the internals.

To control these limitations is used hydraulic
mechanism. The study on hydraulic on packing included
the pressure drop over the dry and wetted (irrigated)
packing as well as dynamic (free draining) liquidhold up.
1. Pressure drop:
2. Liquidholdup
The liquid holdup is the fraction of liquid held up in packed
column. The volume of liquid holdupvolume is often needed
for calculating packed bed support beam loadings as well as
for determining how much liquid drains to the bottom of a
tower when the vapor rate is stopped.
3. Liquidrate
At lower liquid rates, irrigation to the bed is poor result poor
efficiency. When liquid is well distributed in the column
result the minimum wetting rate of the packing. Below
minimum wetting the falling liquid film breaks up, some of the
packing surface unwets, and the efficiency drops. When liquid
distributor is poor, it will take more liquid to wet the entire
packing bed.
4. Vaporrate
When vapor rate increases, column operation moves into the
loading region. Efficiency improves because of the greater liquid
holdup, but this improvement is short-lived. As the flood point is
approached, the efficiency passes through the maximum and the
drops because of excessive entrainment.

Pressure Drop in PackedBeds:
The dry-bed pressure gradient is given by the follow-ing
equation:
dry-column line (which is a function of the dragonly). Equation
[3] allows calculation of the packingfactor,Fp, by measuring
the slope of the dry-packing pressure-drop data. As the vapor
rate increases, the slope of the constant liquid rate lines
increase; this increase is also proportional to the liquid rate.
The initial departure from the dry-line slope indicates
interaction between the vapor and liquid, and represents a

loading point. Efficient mass-transfer operations can be
achieved only above the loading point.
For any given liquid rate, as the vapor rate further increases,
the pressure-drop line slope in-creases rapidly until the line
becomes near vertical.
At this point the Sow and Pare unstable, and the bed
dissuaded; the vapourSow does not allow the liquid to Sow
down the bed and there is massive entrainment of liquid in
the vapor phase and mass transfer is no longer viable. For
most packing’s, bedSooding occurs between1 and 2 inches of
water-pressure drop per foot of packing.
Pressure drop astounding seems to be a function of the
packing size. Kisser cited Zen and laterStrigle and Bukovina
observations indicating thatSooding (Pfl) is higher for smaller
size packing’s, and proposed a correlation to determine the
pressure dropat Sodding as a function of the packing factor.

All the above correlations have been regressed for metallic
random packing’s (Pall Rings and IMPT).For column design, it is
well-accepted practice toassumeSooding at 1 inch of water per
foot of packing pressure drop and design the packing for an
operation at 80%Sood.
However, when reliable packing-factor information is available,
the use of the calculated Pfl, using one of the eqns [4A], [4B] and
[5], is a more accurate approach.
In the Figure presents a family of pressure drop-lines at constant
liquid Sow as a function of the vapour Sow.

Advantages of packed column:
 packed columns are more suitablefor handling
foaming system.
 for corrosive liquids, a packed column will usually be
cheaper than the equivalentplatecolumn
 packing should alwaysbe considered for small
diameter columns, say less than 0.6 m , where plates
would be difficult to install , and expensive.
 Low pressure drop required.
 Small diameters possible.
 Low capital, operating, and maintenancecost.
 Simple construction.
 Can handle corrosive materialsdue to corrosion-
resistant packing.
 Reduces backmixing in comparison to spray columns.
 Better mass transfer than in spray columns.
Disadvantages:
 Fewer stages compared to other columns.
 Channeling, which must be controlled by redistributing
liquid.
 Cannot handle extremely high or low flow rates.
 Cannot handle liquids with high viscosities.
 Need to be preferentially wetted to avoid reduction of the
interfacial area to volume ratio

Packed column vapor-equilibriumcurve:
Packed columns have a continuousvapor-equilibrium
curve, unlike conventionaltray distillationin which every
tray represents a separate point of vapor-liquid
equilibrium.However, when modelingpacked columnsit is
useful to compute a number of theoreticalplates to
denote the separation efficiency of the packed column
with respect to more traditionaltrays. In design, the
number of necessary theoretical equilibriumstages is first
determined and then the packing height equivalentto a
[[Theoretical plat packing height required is the number
theoretical stages multipliedby the HETP.
Packed bed reactors:
Packed bed reactors can be used in chemical reaction.
These reactors are tubularand are filled with solid catalyst
particles, most often used to catalyze gas reactions.[2] The
chemical reaction takes place on the surface of the
catalyst. The advantageof using a packed bed reactor is
the higher conversion per weight of catalyst than other
catalytic reactors. The conversion is based on the amount
of the solid catalyst rather than the volume of the reactor.

References:
 https://www.slideshare.net/alsyourih/design-of-
packed-columns
 Coulson and Richardson’s/ CHEMICAL ENGINEERING
VOLUME 2/FIFTH EDITION /Particle Technologyand
SeparationProcesses
 ENGINEERING DESIGN GUIDELINE/ KLM Technology
Group Practical Engineering Guidelinesfor Processing
Plant Solutions
www.klmtechgroup.com
 II/DISTILLATION/Packed Columns: Design and
Performanc
 http://encyclopedia.che.engin.umich.edu/Pages/Sepa
rationsChemical/Absorbers/Absorbers.html
 https://en.wikipedia.org/wiki/Packed_bed


Packed Columns
Design and Performance

Dedication:
To those who contributed to our success
To all who taught us something new
And fed our thought of science and knowledge
To all who stood beside us and helped us
In all difficulties
To our professors and lecturers at the university
To the cup of empty dishes to hold me a drop of love
To whom we wish to give us a moment of happiness
To the one who took the thorns from Derby to guide me
through the flag
To the big heart
‫الى‬‫من‬‫ساهم‬‫في‬‫وصولنا‬‫لطريق‬‫النجاح‬
‫الى‬‫كل‬‫من‬‫علمنا‬‫شيئا‬‫جديدا‬
‫وغذى‬‫فكرنا‬‫بالعلم‬‫والمعرفه‬
‫الى‬‫كل‬‫من‬‫وقف‬‫بجانبنا‬‫وساعدنا‬
‫في‬‫كل‬‫المصاعب‬
‫الى‬‫اساتذتنا‬‫وداكترنا‬‫في‬‫الجامعه‬
‫إلى‬‫من‬‫جرع‬‫الكأس‬‫فارغا‬‫ليسقيني‬‫قطرة‬‫حب‬
‫إلى‬‫من‬‫ت‬ّ‫ل‬‫ك‬‫أنامله‬‫ليقدم‬‫لنا‬‫لحظة‬‫سعادة‬
‫إلى‬‫من‬‫حصد‬‫األشواك‬‫عن‬‫دربي‬‫ليمهد‬‫لي‬‫طريق‬‫العلم‬
‫إلى‬‫القلب‬‫الكبير‬

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