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7/27/2019 Distillation Final Rev
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Distillation OverviewDistillation OverviewDistillation Overview
1. Introduction2. History of Distillation
3. Types of Stages4. Control Schemes
5. Trouble Shooting6. Industry Incidents
7. Conclusions
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IntroductionIntroduction
Distillation
Is a unit operation in which a mixture of two or more
molecular substances are separated into different
products.
Distillation is one of the most common separation
techniques. Distillation can also be called fractional
distillation or fractionation.
Other types of separation techniques includes,
crystallization and membrane separation.
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IntroductionIntroduction
The separation of components from a
mixture by distillation depends on the
difference in boiling points of the
individual components, sometimescalled relative volatility.
For close boiling mixtures many stages
are required to reach the desired purity.
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Distillation HistoryDistillation History
Early Distillation was basically batch
stills to produce ethanol.
The Crude ethanol was placed in astill and heated, the vapor condensed
for consumption.
Later crude oil was placed in batch
stills to produce lamp oil.
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Distillation HistoryDistillation History
The next progression was tocontinually feed the still and recover
the light product.
Notice the separation that can be
accomplished in one stage.
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Distillation HistoryDistillation History
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Distillation HistoryDistillation History
Next was a series of stills with a
continuous feed that flowed through
the series.
Recovery was low.
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Distillation HistoryDistillation History
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Distillation HistoryDistillation History
The next progression was to returnthe vapor back to the upstream still.
Still low recovery.
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Distillation HistoryDistillation History
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Distillation HistoryDistillation History
The next progression was to place
the stills in a column andinterchange the vapor and liquid to
improve recovery.
To obtain good separation you needmany stills in series.
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Distillation HistoryDistillation History
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Distillation HistoryDistillation History
Early small towers has one or two
large bubble caps per tray.
Early flow measurement was madeby counting the strokes of a positive
displacement pump.
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Distillation HistoryDistillation History
Early still men set their reflux rates
in strokes per minute.
All instrumentation was local.
You ran the units by sound and feel.
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Distillation HistoryDistillation History
A Kerosene Hydrotreaterbuilt in 1952 had one bubble cap
per tray on the Stripper Towerwhich was one meter in diameter.
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Distillation History
1 Bubble cap per Tray
1 m
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Distillation HistoryDistillation History
Later when tray efficiencybecame more important,
flow on the tray beganto be studied and several
types of trays with changedflow patterns were developed.
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Types Of Distillation ColumnsTypes Of Distillation Columns
1. Batch Distillation
2. Continuous Distillation
A. Multiple Feeds and Products
B. Extractive Distillation
C. Reactive Distillation
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Types Of Distillation ColumnsTypes Of Distillation Columns
Batch Distillation processes on a
dis-continuous basis.
Feed is processed in batches and
distilled by selectively removing
the more volatile fractions overtime.
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Types Of Distillation ColumnsTypes Of Distillation Columns
Continuous Distillation has a constant feed
rate and is the most common of the two types.
Additional variations may be utilized in a
Continuous Distillation Column.
The column may have multiple feed points
and products.
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Types Of Distillation ColumnsTypes Of Distillation Columns
The column may have a solvent added to the
system to help increase the separation; thistype of column is named an extractive
distillation column.
The column may have a catalyst bed and
reaction occurring in the column; this type ofcolumn is named a reactive distillation
column.
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Types of StagesTypes of Stages
1. Flash Drum
2. Reboiler / Condenser
3. TraysA. Dual Flow Tray /Ripple Tray
B. Bubble Cap Tray
C. Sieve Deck Tray
D. Valve Tray
E. Down comer Advances
4. Packing
A. Random
B. Structured
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Types of StagesTypes of Stages
1. Flash Drum
The simplest type of distillation device is the flash
drum. It is a single stage and can approach 100% of
equilibrium limits it there is sufficient residence time.
An example of where we utilize a flash drum for
separation in an ethylene plant is in the hydrogen /
methane drum.
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Types of StagesTypes of Stages
1. Flash Drum
An over head receiver is an example of a flash drum.
The separation of a single flash drum is limited.
A 50:50 mixture of ethylene and propylene can only
be separated to about 80:20 in a single flash drum.
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Types of StagesTypes of Stages
Vapor
Liquid
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Types of StagesTypes of Stages
2. Reboiler / Condenser
A reboiler is a flash drum with heat exchange. It can
approach 100% of equilibrium limits it there is
sufficient residence time. The three normal types of
reboilers are horizontal thermosyphon, vertical
thermosyphon, and kettle drums.
The best design is for the liquid from the bottom tray
to pass through the reboiler at least once before being
drawn off as tower bottoms.
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Types of StagesTypes of Stages
2. Reboiler / Condenser
A Condensers is a flash drum with heat exchange
There are three major types of condensers;total,
partial,
and hot vapor by-pass.
Each has its best application.
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Types of StagesTypes of Stages
A. Partial Condenser
The partial condenser is best used when there is a large
difference in the overhead vapor compositions. For
example when there is a small amount of methane and
hydrogen mixed in a propylene stream, like in the
propylene towers. The partial condenser condenses the
propylene and leaves the methane and hydrogen as a
vapor to be vented from the overhead receiver.
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Types of StagesTypes of Stages
A. Partial Condenser
This type of condenser works well for most
applications. The system needs to be reviewed to
address the potential build of on non-condensable
gases in the heat exchanger that can reduce the cooling
potential of the exchanger.
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Types of StagesTypes of Stages
Vent
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Types of StagesTypes of Stages
B. Total Condenser
The total condenser is best used when there is a small
difference in the overhead vapor compositions. The
overhead vapors can be condensed at approximately
the same temperature.
This system also needs to be reviewed to address the
potential build of non-condensable gases in the heat
exchanger that can reduce the cooling potential of the
exchanger.
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Types of StagesTypes of Stages
Vent
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Types of StagesTypes of Stages
C. Hot Vapor By Pass Condenser
The hot vapor by pass condenser is best utilized when
there is the potential for large changes of overhead
vapor composition. The vapor by pass can be used to
maintain the pressure in the tower system when the
light components are lower than design. The hot vapor
by pass condenser also has a lower installed cost due to
the heat exchanger being installed on the ground level.
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Types of StagesTypes of Stages
C. Hot Vapor By Pass Condenser
The negatives of the Hot Vapor By Pass Condenser isthat the by pass can be opened too much, increasing
the temperature of the reflux. This reduces the tray
efficiency in the top of the tower, and raises the tower
pressure, which makes hydrocarbons harder to
separate.
This system, because of the physical location of the
exchanger, has even higher potential to build non-
condensable gases in the heat exchanger that can
reduce the cooling potential of the exchanger.
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Types of StagesTypes of Stages
Vent
f
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Types of StagesTypes of Stages
3. Trays
A. Baffle Trays
B. Dual Flow Tray /Ripple Tray
C. Bubble Cap TrayD. Sieve Deck Tray
E. Valve TrayF. Tray advances
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
3. Tray design
Trayed Columns utilize a pressure andtemperature differential to separate the
products. For most trayed columns, the weir
holds a liquid level of each tray.
The vapor must over come this liquid head tomove up the column. On the tray the vapor
and liquid are contacted and then above the
tray they are separated.
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
3. Tray design
Any deviation that develops that restricts the
vapor and liquid from contacting and then
separating will deteriorate the columns abilityto meet design specifications.
Trays utilize the staged contact principle to
separate products.
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
3. Tray design
One of the first trays developed was the dual
flow tray. The liquid and vapor traveled up
and down the column in the same tray opening.
Tray design then moved to sieve decks, to
bubble caps, valve trays, and to directional
flow valve trays. For trays to function theyneed to mix the vapor and the liquid, then
separate the vapor and the liquid. Each
function must be as complete as possible.
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Staged Equipment: the mechanism
Vapor and liquid mixes to form a froth.
Mass transfer occurs within the froth. The froth overflows across a weir.
The vapor disengages and move to the tray
above.
The bulk liquid flows down the downcomer to the
next tray. The steps are repeated in the next tray.
T f StT f St
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Types of StagesTypes of Stages
B. Bubble Cap Tray
Is a tray that has cap over the opening of the traydeck, with a bubbling ring in the bottom of the cap.
The advantage of bubble cap trays is that they donot weep, but they are more expensive than other
types of trays.
They are used in palm oil reactive distillation.
T f StT pes of Stages
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Types of StagesTypes of Stages
T pes of StagesTypes of Stages
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Types of StagesTypes of Stages
C. Sieve Deck Tray
A sieve tray is essentially a plate with holes punched
into the plate. The number and size of the holes is
based on the vapor flow up the tower.
The liquid flow is transported down the tower by
down comers, a dam and overflow device on the side
of the plate, which maintains a set liquid level on the
tray.
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
C. Sieve Deck Tray
To maintain the liquid level on
the tray a minimum amount ofvapor traffic up the tower must
be maintained, or the liquid level
on the tray will weep down to thenext tray through the holes
punched on the plate.
Typically sieve deck trays have a
minimum capacity, or downturn,
of approximately 70%.
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
D. Valve Tray
One of the next developments was to add
a variable valve opening to the tray deck.
This valve would open in relation to the
vapor flow. The advantage to this design
was the ability to maintain the liquid level
on the tray deck.
Typically valve deck trays have a
minimum capacity, or downturn, of
approximately 60%.
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
Tray Advancements
The latest in tray
advancements involvethe flow across the tray
and the down comer
design.
T f StT f St
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Types of StagesTypes of Stages
Tray Efficiency is a function of
1. Path Flow Length
2. Elimination of stagnant zones3. Bubbling Promoters
4. Weir Heights
5. Hydraulic Rates - V:L
Types of StagesTypes of Stages
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yp gyp g
1. Path Flow Length
The efficiently of a tray is a function of path flowlength. The path flow lengths less than 500 mm shouldbe avoided.
An advantage of swept back weirs is that it increasesthe path flow length.
On of the reason the multiple down comers trays hasless efficiency is path flow length.
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Types of StagesTypes of Stages
2. Elimination of stagnantzones
The elimination of stagnant
zones on the tray will
improve tray efficiency.
Types of StagesTypes of Stages
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Eddy flows
Caused by the round shape of columns.
Liquid flows in straight directions across a tray.
The rounded edge becomes dead zones
Poor mass transfer and high residence time.
flow directioner to change the flow pattern at the
edge
Types of StagesTypes of Stages
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3. Bubbling Promoters
The clear liquid exiting the downcomer has
some resistance to the vapor, as compared
to the froth developed later on the tray path.
Types of StagesTypes of Stages
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3. Bubbling Promoters
On a tray with long flow path length and
short weir heights, you can develop vapor
and liquid channeling, causing weeping at
the down comer outlet.
Types of StagesTypes of Stages
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Down Comer
AdaptorExisting DC
Bolting Bar
Stepped Outlet Weir
Froth Pusher
Existing Tray
Support Ring
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
4. Weir Heights
The weir height has an effect on the trayefficiency. Recommendations are not to
exceed 100 mm or 1/6 of tray spacing, and50 to 75 is suggested for all services exceptvacuum services.
Types of StagesTypes of Stages
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Types of StagesTypes of Stages
5. Hydraulic Rates
The hydraulic rate has an effect on the trayefficiency. At low rate trays will weep, at high ratesfroth touches the next tray.
When the V:L ratios are not equal molar, efficiencywill decrease.
Types of StagesTypes of Stages
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4. Packing
Random and Structured Packed Columns
generate a mass transfer area by providing a
large surface area over which the liquid can
transfer heat and mass to the vapor.
Packing utilizes a continuous contacting
principle to separate products. A majoradvantage to packed columns is the
reduction in pressure across the column.
Types of StagesTypes of Stages
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4. Packing
Typically the column pressure drop for a
packed column is less than that of a trayed
column because of the percent open area.
Typical percent open area of a trayed
column is 8 to 15%, whereas a packed
column can approach 50%. Liquid
accumulation for a packed column is lower
than that of a trayed column.
Types of StagesTypes of Stages
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4. Packing
Another advantage of packed column isreduced foaming. Packing generates thin
films instead of fine droplets for mass and
heat transfer, reducing entrainment whenfoaming agents are present.
Packing has been used successfully in low-
pressure applications, less than 150 psig.
Types of StagesTypes of Stages
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A. Random Packing
Types of StagesTypes of Stages
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yp g
B. Structured Packing
Non-staged Equipment: the mechanism
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Bulk liquid is broken
into fine droplets whenin contact withpackings.
Provides large surfacearea for effective heatand mass transfer.
Appears as acontinuous froth acrossthe height of the
packed bed.
Non-staged Equipment: the mechanism
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The introduction of vapor and liquid to the
packing is very important. Trays will normallyeventually equalize whatever mal-distribution is
developed by the vapor and liquid feeds.
Packing will enhance whatever mal-distribution
is developed by the introduction of the vapor andliquid feeds.
Concepts
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Concepts
1. Up till this point we have discussed
hardware.
2. Now let us discussed why the
hardware might work.
3. In distillation the hardware was
developed and then a study was
conducted to understand why it might
work.
Concepts
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1. Hydrocarbons separate better at lower pressure.
The nature of hydrocarbons is such that they separatebetter at lower pressure. Normally the pressure of a
tower is set as low as possible based on the ability of the
over head condenser to condense the over head vapor.
The preferred cooling medium is water and the tower
pressure is raised until the over head vapor can becondensed at 90 degrees F / 45 degrees C.
Concepts
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1. Hydrocarbons separate better at lower pressure.
A balance must be done between the construction costand the utility cost. The ability to separate determines
the number of trays, which is a construction cost.
To utilize lower pressure can require refrigeration,
which is a utility cost. Low pressure DeMethanizers
with Methane Refrigeration can be shown to be costeffective. Some C2 Splitters run at lower pressure than
other designs utilizing this lower pressure separation
ability.
Concepts
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2. Heat is a form of energy caused by the motion of
molecules.
Heat is a form of energy. The motion of molecules
causes heat energy. When energy is added to a
substance, the motion of the molecules increases.
When energy is added to a substance and the
temperature of the substance increases, this is calledsensible heat. When energy is added to a substance and
the substance changes phases from a liquid to a gas, that
energy is called latent heat.
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Concepts
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2. Heat is a form of energy caused by the motion of
molecules.
The availability and large latent heat value are the
reasons we use water as a heat carrier. The large latentheat value can be utilized in reboilers.
Because we do not use the latent heat in pumps andcompressors, the temperature and pressure change are
the energy carriers.
Concepts
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6. Equilibrium is the state which systems will establish, if
given enough time.
A vessel filled with two hydrocarbons is said to be at
equilibrium if the number of light molecules escaping
from the liquid is equal to the number returning to theliquid.
In one vessel because of the transfer of molecules from theliquid to the vapor, total separation of two similar
hydrocarbons cannot be obtained.
Concepts
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6. Equilibrium is the state which systems will establish, if
given enough time.
ConceptsConceptsHydro
Carbon
Formula Molecular
Weight
Boiling
Point (F)
Density
(ft3/lb)
Flammability
Limits
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Carbon Weight Point (F) (ft3/lb) Limits
Low/High
Hydrogen H2
2.0159 -426.130 188.25 2.3 / 4.00
Methane CH4
16.043 -258.71 23.65 5.0 / 15.0
Ethylene C2H
428.054 -154.71 13.53 2.7 / 36.0
Ethane C2H
630.070 -127.46 12.62 2.9 / 13.0
Propylene C3H
642.081 - 53.83 9.02 2.0 / 11.7
Propane C3H
844.097 - 43.73 8.61 2.0 / 9.5
Iso Butane C4H
1058.123 10.78 6.53 1.8 / 8.5
Tower Balances
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A. Mass Balance
B. Energy Balance
C Composition Balance
Mass Balance
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All vessels will eventually reach a material
balance, mass in equals mass out. For distillationtowers the material balance is;
Mass In = Mass Out + Accumulation
Mass Balance
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Sources of material into the tower normally are
the feed points.
Sources of material out of the tower includeOverhead Vapor, Overhead Liquid, Side Draws
and Tower Bottoms flows.
Mass Balance
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Sources of accumulation include Overhead
Receiver or Reflux Drum, Tower BottomsReservoir, and level of hydrocarbons on the trays.
Two of the accumulations are straight forward;
the Overhead Receiver and the Bottoms
Reservoir.
Mass Balance
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The third accumulation, the level on the trays, is
much more difficult to quantify. It is by far the
largest accumulation in the tower.
To build the inventory in the C2 Splitter Tower
takes 8 hours, but less than one hour for the over
head receiver to be filled.
Mass Balance
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The tower delta P is a guide to how much level is
on the trays. The tower delta P is based on the
amount of reflux that is added to the tower.
The reflux is made in the reboiler, therefore the
level on the trays is determined by reboiler. A
stable reboiler heat input will stabilize the tower.
Mass Balance
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One of the first tower control schemes was the
Material Balance that was developed in 1930s.A field operator looking at sight glasses
controlled the overhead and bottoms levels.
Mass Balance
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Today with level controls, the Material Balance
Control Scheme can be utilized if the productspecifications are lax.
The Material Balance Control Scheme does not
address the accumulation of level on the trays,
the tower energy, and composition balance.
Mass Balance
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For most high purity product this is not the
best control scheme due to the large number oftrays in product fractionators.
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Energy Balance
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All vessels will eventually reach an energy balance,
energy in equals energy out. For distillation towers
the energy balance is;
Energy In = Energy Out + Accumulation
The main source of energy into the tower normally is
the reboiler. The feed can also be a source of energy
if it is preheated.
Energy Balance
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The main sources of energy out of the tower are the
Overhead Condenser and the Tower Bottoms Flow.
Smaller sources include Overhead Vapor, OverheadLiquid, Side Draws and Tower Feed if it is cooled.
Sources of accumulation include, Tower BottomsReservoir, and level of hydrocarbons on the trays.
Two of the accumulations are straightforward; the
Overhead Receiver and the Bottoms Reservoir.
Energy Balance
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The third accumulation, the level on the trays, is
much more difficult to quantify. It is by far the
largest energy accumulation in the tower.
Three things happen on a tray when a reflux move ismade.
The first is the level on the tray changes.
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Energy Balance
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If there are 100 trays in the column like in the C2
Splitter, it takes five hours for the change to be seen
in the tower bottoms.
If there are 250 trays like in a Polymer Grade C3Splitter, it takes 12 hours and thirty minutes for the
change to be seen in the tower bottoms.
Energy Balance
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The Energy Balance Control Scheme was developed
in 1960s. Instrumentation was improving and flow
meters were becoming more accurate.
A current orifice plate flow meter can approach1.0% accuracy, if they are installed correctly and
calibrated.
Energy Balance
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An average field orifice plate flow meter is normally
considered to have 2.5% accuracy.
The latest flow meters, which uses the vibrations of a
tube as the fluid flows by can approach 0.01%accuracy.
Energy Balance
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Because of the improved instrumentation, the heat
input to the tower could be measured and controlled.
This led to the Tower Energy Balance Control
Scheme. The energy to the reboiler was measuredand used as a control point. The energy was
removed in the overhead condenser.
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Energy Balance
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To be consistently above the sell specification gives
away product, reducing your cash margin. The main
problem with the energy balance control scheme is
that is does not take into account the compositionbalance.
Energy Balance PI
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Energy Balance
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A later variation of the energy balance control
scheme was the Delta T Energy Balance.
At constant pressure the temperature can be an
indication of the composition and the product flowcan be controlled by the difference in the two
temperature points.
Energy Balance
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For example in a Benzene Toluene Splitter the lower
temperature point will began to increase as the
toluene concentration increases.
This will reduce the delta T between the temperaturepoints. The product flow can then be decreased,
which will increase the reflux to the tower, resulting
in the toluene composition being decreased at theBenzene Product.
Energy Balance
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The advantage of a delta T over a single point is that
the delta T takes the pressure deviations out of the
control scheme. This control scheme works, but
ignores the composition balance.
Energy Balance PI
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TI
TI
DT
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Energy Balance PI
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TI
TI
DT
Composition Balance
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All vessels will eventually reach a composition
balance, composition in equals composition out. Fordistillation towers the composition balance is;
Composition In = Composition Out + Accumulation
Composition Balance
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For the Propylene Tower a propylene balance can
be developed. The propylene in the feed must equal
the propylene that leaves the tower plus theaccumulation of propylene in the tower.
Sources of accumulation include, Tower Bottoms
Reservoir, Overhead Receiver and level of
hydrocarbons on the trays.
Composition Balance
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Two of the accumulations are straightforward; the
Overhead Receiver and the Bottoms Reservoir.
The third accumulation, the level on the trays, is
much more difficult to quantify. It is again thelargest accumulation in the tower.
Composition Balance
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In the 1970s analyzers were becoming reliable and
control schemes that utilized them became common.
A composition balance could be maintained byutilizing the mass flows and the analyzer results
leading to better distillation control.
Composition Balance
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The tower feed and composition are analyzed and
used as a feed forward control for the product.
The product rate and composition are analyzed and
used as a feed back control.
The sum of these two is used to balance the tower
considering all three balances.
Composition BalancePI
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AI
Tower Problem Solving
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1. Foaming
2. Entrainment
3. Weeping / Dumping
4. Flooding
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Do simple checks first.
1. Ensure that levels are accurate.
2. Calculate column pressure drop and then
measure pressure drop.
3. Survey column temperature profile. Review
survey temperature reading to operationsreadings.
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Verify Tower Operations.
1. Sample Feeds and Products.
2. Calculate mass balance to within
2% accuracy, if not calibrate flow meters.
3. Survey heating and cooling temperatures.
4. Have engineers simulate these results. If noproblems are identified consider scanning the
column.
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In distillation towers there are actually two accumulators.
The first is normally obvious, the over head receiver, the
second is the bottom section of the tower.
These accumulators are used to stabilize the operation of the
tower and down stream operations. This internal surge
drum creates an inventory to act as a buffer.
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If this internal level is allowed to rise above the reboiler
return, stripping inlet, or feed inlet, flooding can occur.
There is an inherent error built into sight glass and level
instrumentation. The sight glass and level instrumentation
contain non aerated liquid, called clear liquid, which is not a
true indication of the condition of the liquid within the
tower.
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The liquid within the tower will have two levels, a clear
liquid level below the aerated liquid level. Because the
aerated level will have lower specific gravity than the clear
liquid within the instrumentation, the tower level will be
higher than the instrumentation indicates.
If the level in the tower is higher than the feed or reboiler
return, entrained liquid can be carried to the next stage
causing flooding.
Tower IncidentsTower Incidents
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Attached is a list of tower incidents that was found in the
literature from HZ Kister Recent Trends in Distillation
Tower Malfunctions
1. Fouling, plugging and Coking issues
2. Tower Bottoms and Reboiler Return issues
3. Packing Liquid Distributors issue
4. Intermediate Draws5. Assembly Mishaps
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1. Fouling, plugging and Coking issues
A. Coking
B. Precipitation - salts
C. Scale, corrosion productsD. Solids in feeds
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1. Fouling, plugging and Coking issues location
A. Packing beds and Distributors
B. Trays, active areas and down comers
C. Draw linesD. Instrument lines
E. Feed lines
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2. Tower Bottoms and Reboiler Return issues
A. High liquid levels
B. Impingement by vapor inlets
C. Vapor Mal-distributionD. Water induced pressure surges
E. Leaking reboiler draw
F. Gas entrainment in liquid bottoms
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3. Packing Liquid Distributors issues
A. Distributor Overflow
B. Plugging
C. Fabrication mishapsD. Feed entry problems
E. Damage
F. Poor hole pattern
G. Poor irrigation quality
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4. Intermediate Draws
A. Leakage at draw
B. Restriction of vapor - choking of draw line
C. Plugging
ConclusionsConclusions
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Distillation is one of the major unit operations in
processing plants. It is energy intensive and hasopportunities to be optimized. Product recovery and
purity can be improved by understanding the principles
of distillation.
These principles need to be understood in advance of
operating and trouble shooting a distillation column forthe operator or problem solving to be effective.