Distillation Final Rev

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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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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Next was a series of stills with a

continuous feed that flowed through

the series.

Recovery was low.


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The next progression was to returnthe vapor back to the upstream still.

Still low recovery.


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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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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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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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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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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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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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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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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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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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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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Vapor

Liquid


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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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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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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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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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Vent


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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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Vent


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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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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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Vent

f


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3. Trays

A. Baffle Trays

B. Dual Flow Tray /Ripple Tray

C. Bubble Cap TrayD. Sieve Deck Tray

E. Valve TrayF. Tray advances



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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.



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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.



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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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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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T pes of StagesTypes of Stages


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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.



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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%.



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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%.



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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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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



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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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2. Elimination of stagnantzones

The elimination of stagnant

zones on the tray will

improve tray efficiency.



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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



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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.



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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.



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Down Comer

AdaptorExisting DC

Bolting Bar

Stepped Outlet Weir

Froth Pusher

Existing Tray

Support Ring



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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.



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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.



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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.



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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.



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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.



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A. Random Packing



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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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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.


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

AI

Tower Problem Solving


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1. Foaming

2. Entrainment

3. Weeping / Dumping

4. Flooding

Tower Problem SolvingTower Problem Solving


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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.

Distillation Final Rev

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