Tray vs packed column

Tray vs. Packed Column
Prepared By Eng. Abdelrahman Kamal
For Chemical Engineering Applications

Tray Column
Usage
Operation
Components
eng.abdelrahman.kamal25@gmail.com

Tray vs. Packed Column for Chemical Engineering Applications eng.abdelrahman.kamal25@gmail.com
Plate Column
▪ Plate columns can handle wide range of gas and liquid flow rates.
▪ Plate column provide stage controlled contact.
▪ Plate column operation is considerably smooth.
▪ Plate efficiency can be predicted more accurately
▪ When the liquid cause fouling or deposition of solids, even though the tray
is blocked up, it is easier to clean it.
Introduction

Tray Downcomer
Weir
Plate Column
a c t i v e a r e a
▪ Plate column is composed of several equilibrium stages.
▪ Every Equilibrium Stage has its own VLE (vapour liquid equilibrium)
different
▪ VLE in Equilibrium stages are related to previous equilibrium stage
VLE and affect the next one.
▪ Equilibrium Stage is called tray.
▪ Tray has different designs (bubble cap – valve – sieve)
Introduction
Tray vs. Packed Column for Chemical Engineering Applications eng.abdelrahman.kamal25@gmail.com

Plate Column Anatomy
Basic Components of Tray Column
▪ Inlets:
✓ The feed inlets are located between the rectifying and stripping sections.
✓ Distillation columns can have three or more feed lines.
▪ Outlets:
✓ The overhead outlet is the vapor exit point from the distillation column to pass into the condenser.
✓ The bottoms outlet is used to remove residue from the distillation column and to feed the reboiler.
✓ The side stream outlet is used to remove side product in multicomponent distillation processes
▪ Feed Tray: The location of the feed tray is determined by the ratio of stages above and below the inlet feed
line.
Tray vs. Packed Column for Chemical Engineering Applications

▪ Rectifying Section:
✓ is located above the feed tray.
✓ has a higher concentration of light components than the
stripping section
▪ Stripping Section
✓ is located below the feed tray.
✓ has a higher concentration of light components than the
stripping section.
✓ Temperatures in the stripping section are usually much higher than in the rectifying section

▪ Overhead System
✓ used to condense hot vapors into liquid.
✓ May be:
• Total Condenser
• Partial Condenser
▪ Bottom System
✓ help control overlap and heat balance
✓ May be:
• Reboiler
• Steam Injection
Overhead System
Bottom System

There are three types of trays used in
distillation towers:
• Bubble Cap Tray
• Valve Tray
• Sieve Tray
eng.abdelrahman.kamal25@gmail.comTray vs. Packed Column for Chemical Engineering Applications

Types of Trays
Bubble Cap Tray Valve Tray Sieve Perforated Tray

Types of Trays
a) Bubble Cap Tray
▪ Bubble-cap trays are used commonly in the distillation process.
▪ A weir on the tray maintains a liquid level over the caps.
▪ This vapor comes into direct contact with the liquid on the tray
and transfers some heat energy.
▪ The liquid condenses the heavier components of the vapor,
whereas the lighter components move up the tower.

Types of Trays
b) Valve Tray
▪ Valve trays have simple, movable plates.
▪ Plate movement depends on process flow.
▪ This type of tray is more efficient over a wider range of flow rates
than the sieve tray

Types of Trays
c) Sieve tray
▪ Sieve trays are the simplest type of tray.
▪ A sieve tray is essentially a metal plate with small holes in it.
▪ Vapor comes through the holes, it moves too fast to be stopped by
the liquid level on the tray, but it does slow down significantly as it
passes through the liquid.
▪ The holes range from 1 inch to 1/8th inch in diameter.
▪ Pressure drop through a large number of small holes is greater
than the drop through fewer large holes

Tray Classification according to Pass NO.
Tray Pass
▪ The more paths for liquid flow, the higher the liquid handling capacity
▪ Liquid flow paths change pressure balance, multiple flow paths can increase vapor capacity as well
▪ 1, 2, 4 paths common
▪ 3 used occasionally
▪ 5+ not common
▪ The more passes, the larger the tower minimum diameter
▪ The active area should be a minimum width to allow for manways

Tray Classification according to Pass NO.

▪ Weir:
The function of a weir is to maintain a desired liquid level on the tray
Types:
✓ Low weirs are frequently used in low pressure or vacuum columns.
✓ Notched (rectangular or V-shaped) weirs are commonly used for low liquid loads..
The higher the liquid level, the higher the tray pressure drop, the more liquid hold-up on the tray, which may be
undesirable if the liquid is toxic or hazardous.

▪ Downcomers:
The function of downcomer is to guide liquid flow from an upper tray to a lower tray.
Types:
✓ straight: is widely used as it provides good utilization of column area for downflow and has cost
and simplicity advantage
✓ Sloped: can be used if vapour-liquid disengagement is difficult (e.g. due to foaming) and also
provide a slightly larger active area for vapour-liquid contact, but is also more expensive

▪ Downcomers:
➢ A downcomer must be sufficiently large to allow liquid to flow smoothly without choking.
➢ Sufficient time must also be provided to allow proper vapour disengagement from the down-flowing
liquid, so that the liquid is relatively free of vapour when it enters the tray below.
➢ Inadequate downcomer area will lead to downcomer choking, whereby liquid backs up the downcomer
into the tray above and eventually flood the column.

▪ Tray Manway:
➢ Tray manways allow maintenance workers and inspectors to travel from one tray to another.
➢ Manway size should be at least 16 x 20-inch.
➢ For multi-pass trays, one tray manway should be provided for each tray pass, as the central
downcomers restrict access from one side to the other.

Bubble CapValveSieveComparison
LowestHighestMidCapacity
HigherMidLowerPressure Drop
MidHigher [4 to 10.1]Lowest [2.1]Turndown Ratio
HighestMediumMediumEntrainment
LowestHighHighEfficiency
100 / 200% more
than Sieve
20 / 50% more than
Sieve
Cheapest
Cost

Packed Column
Usage
Operation
Components

Packed Column
▪ Packed columns are more suitable for low capacity operations.
▪ Packed columns are particularly useful in the field of vacuum distillation.
▪ For separating heat sensitive materials packed columns are useful
because the liquid hold up* is low.
▪ When corrosion is a problem packing may be the only answer.
▪ Pressure drop per unit length is less in packed column.
▪ Total weight of packed column is less due to use of low weight and high
capacity packing.
Introduction
* Liquid holdup HL , is defined as the fraction of an element of pipe which is occupied by liquid

Packing
Height (Z)
Packing Tower Anatomy
Introduction and Representation Packing Material
➢ Packed beds can be used in a chemical reactor, a distillation
process, or a scrubber, but packed beds have also been used to
store heat in chemical plants.
➢ The purpose of a packed bed is typically to improve contact
between two phases in a chemical or similar process.
➢ The gas liquid contact in a packed bed column is continuous, not
stage-wise, as in a plate column.

Packed Column Principle
✓ Packed towers use specially designed packing materials to
provide more surface area for maximum contact between gases
and liquids.
✓ Liquids wet the surface of the packing as they migrate down the
tower.
✓ Rising vapors come into contact with the wetted surface and
exchange heat.
The performance of a packed column is very dependent on the maintenance of
good liquid and gas distribution throughout the packed bed.

Liquid travels down through feed distributors, hold-down grids,
random and structured packing, support plates, redistributors, liquid
collectors, structured grids, and finally into the bottom section.
Hot vapors move countercurrent to the downward flow of liquid.
Types of packed distillation columns:
▪ Random Packing
▪ Structured Packing
▪ Stacked Packing

Packed Column Accessory
a) Shell
▪ The cylindrical shell of the distillation column is typically metal, carbon
steel, stainless steel, special alloy or nonferrous. .
▪ Columns can also be composed of glass, ceramic, plastic, or wood.
▪ The type of chemicals that will be used in the distillation column will
determine the design material, lining specification, wall thickness,
pressure rating, and temperature rating.

b) 1) Random Packing
▪ Random packing includes discrete pieces of packing that are
randomly dumped or poured into a packed column.
Use:
▪ It provides a surface for good vapor-liquid contact, distillation
efficiency characteristics, and predictable pressure drop.

Random Packing

b) 2) Structured packing
▪ Structured packing has specific geometric shapes, like a mesh.
Use:
▪ It works best in columns requiring high liquid loadings.

Structured Packing

b) 3) Stacked Packing
▪ Stacked packing is uniformly arranged inside a distillation column.
Use:
▪ Packed columns are designed for pressure drops between 0.20
and 0.60 inch of water per foot of packing medium

Packing Material

c) Packing Support
▪ The packing inside a distillation column is supported by packing supports.
▪ It is designed to provide sufficient air-liquid passage and to prevent the packing
from migrating into the lower sections of the column.

d) Bed Limiters and Hold-Down Plates
▪ They are designed to keep fixed bed packing from migrating or fluidizing out of the
section.
▪ Hold-down plates rest on top of the packing, whereas bed limiters are attached to
the inside wall of the column.
▪ Hold-down plates are primarily used on carbon and ceramic packing.
▪ Bed limiters are used on metal or plastic packing

e) Liquid Distribution and Redistribution
▪ It’s so important device where it is used to disperse fluid evenly over the top
of the packing to enhance distribution of liquid through packing.
▪ Poor liquid distribution reduces vapor-liquid contact and promotes channeling
which reduces the operation efficiency of the column.

Liquid Distributer
Orifice Type Spray Nozzle Trough Type

Liquid Distributer
Channel Distributor Ring DistributorPipe Ladder

Packed ColumnTray ColumnComparison
LowerHigherCapacity
SmallerLargerDiameter
More ExpensiveLess ExpensiveCost
More DifficultEasierMaintenance
MoreLessVapour Liquid Contact
Not AvailableAvailableSide Stream
AppropriateNot RecommendedCorrosive Materials
Not recommended for low rateWide rangeSide Stream

Packed ColumnTray ColumnComparison
LowerHigherPressure Drop
LowerHigherMax Temperature
AppropriateNot safeToxic And
Flammable Liquids
Not recommendedAppropriateFouling Liquids
AppropriateNot RecommendedFoaming Systems

References
▪ Process Technology Equipment and Systems, Charles E. Thomas 4th edition
▪ AICHE Resources, aiche.org
▪ NPTEL Website
▪ Chemical Engineering World Website
▪ Separation Processes Website

Author Contacts
Abdelrahman Kamal
Department of Chemical Engineering and Petroleum Refining
Faculty of Petroleum and Mining Engineering, Suez University
Email: eng.abdelrahman.kamal25@gmail.com
Abdelrahman.KMHu@pme.suezuni.edu.eg
Mob: 01159080221
LinkedIn: AbdelRahman Kamal
https://www.linkedin.com/in/abdelrahman-kamal994
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