Distillation

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Unit P-05-02

Gravity Separation, Distillation and Storage

Distillation

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Distillation - Unit P-05-02

UNIT P-05-02

DISTILLATION

1.0 OBJECTIVES/ INTRODUCTION.........................................................................3

2.0 THE DISTILLATION PROCESS.........................................................4

3.0 COLUMNS....................................................................................6

3.1 DISTILLATION COLUMN................................................................8

3.1.1 Feed Section / Flash Zone...........................................................................8

3.1.2 Enrichment / Rectifying Section..................................................................9

3.1.3 Stripping Section........................................................................................9

3.1.4 Accumulator Zone....................................................................................10

3.2 TRAY COLUMNS.........................................................................10

3.2.1 Bubble Cap Tray.......................................................................................11

3.2.2 Sieve or Perforated Tray...........................................................................12

3.3 SUMMARY.................................................................................13

3.3.1 Nozzles.....................................................................................................13

3.3.2 Trays.........................................................................................................13

3.3.3 Downcomers.............................................................................................13

3.3.4 Weirs........................................................................................................13

3.4 PACKED COLUMNS.....................................................................14

4.0 HEATING / COOLING EQUIPMENT.................................................17

4.1 FEED HEATER.............................................................................17

4.2 REBOILER..................................................................................18

4.3.1 Reboiler Operation....................................................................................21

5.0 OVERHEAD PRODUCT CONDENSING EQUIPMENT..........................24

5.1 FIN FAN COOLER........................................................................24

5.2 WATER COOLED CONDENSERS....................................................24

5.3 REFLUX DRUM...........................................................................25

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1.0 OBJECTIVES/ INTRODUCTION

Objectives

On completion of this unit the trainee will be able to:

Explain the function of columns

Explain the principles of simple distillation

Identify the different types of columns and describe different services.

Identify the major differences between columns fitted with structured packing, random packing & distillation trays

Describe the basic construction of a column

Describe the internal mechanisms in the column and identify their construction, function and principles of operation.

Explain basic column operation & control

Explain the function of multi- staged distillation

List and describe the different types of heat exchangers employed in the distillation system

Introduction

Petroleum refineries and some production facilities use equipment known as

Columns in various processes. In some operating companies Columns are also

called "Towers". In this unit both words are used, there is no difference in the

meaning between them.

Process towers / columns are vertical, circular pressure vessels used to distil and

separate fluid fractions. They vary in diameter from a few centimetres to over ten

metres. They also vary in height from a few metres to over 60 metres. The

diameter depends on the volume of fluid which will be processed in the tower. The

height is determined by the difference in boiling points between the fluids to be

separated and the degree of separation accuracy.

The material that a column is made of depends upon its use, some common

materials are mild steel, copper alloys and stainless steel, some are constructed of

one material and lined with another to save cost. The shell or wall thickness will

depend on the pressure the vessel has to operate at.


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2.0 THE DISTILLATION PROCESS

Distillation and fractionation are important processes in oil refineries and gas plants.

In the distillation process, mixtures of liquids are separated by boiling and

condensing. The liquid mixture is heated and part of the liquid is boiled off

(vapourised). This boiled off vapour is then condensed to a liquid. The condensed

liquid (condensate) is lighter or more pure than the original mixture. The distillation

process is used to purify and separate liquids.

Figure 2-1

Figure 2-1 shows a simple distillation apparatus. A flask, called an evaporator is

filled with a liquid mixture (the feed). The feed mixture contains fluids with different

densities, boiling points and other characteristics. It is heated until it begins to boil

and form vapour (vapourisation), the vapour flows through the overhead pipe and is

cooled in the condenser, and collected in the receiver. This condensate (condensed

liquid) is different than the original feed it is made up mainly of the lighter

components. The remaining liquid in the evaporator flask is heavier than the

original feed. If new feed liquid is added to the flask the process can be repeated.

However, this batch process is not practical on a large scale and a different

approach is required but the basic principle remains the same.

A distillation column is one type of process distillation equipment that is designed to

operate continuously. Feed enters the column and products leave on a continuous

basis. Close control of the temperature and pressure inside the column means that

the product quality can be controlled. To improve the distillation process, a part of

the condensate product is returned from the condenser/receiver to the distillation

tower. This is called reflux. The reflux enters the top of the column and flows down

through the rising vapours. Its purpose is to maintain a desired top temperature in

the column and increase the purity (quality) of the overhead vapours.


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There are three main components needed to operate the distillation process:

The column

Heating equipment

Product condensing equipment

We will discuss these in the following sections.


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3.0 COLUMNS

The main purpose of distillation is to separate a mixture of several components by

taking advantage of their different volatilities and boiling points, the object of the

operation is to obtain the more volatile constituent in pure form.

If the difference in volatility (and in boiling point) between the two constituents is

great, complete separation may be easily accomplished.

Figure 3-1

When the mixture consists of many components, they are drawn off at different

points along the tower.

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Figure 3-2


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Inside the towers, the liquids and vapors separate into components or fractions

according to weight and boiling point. The lightest fractions, including gasoline and

liquid petroleum gas (LPG), vaporize and rise to the top of the tower, where they are

condensed back to liquids. Medium weight liquids, including kerosene and diesel oil

distillates, stay in the middle. Heavier liquids, called gas oils, separate lower down,

while the heaviest fractions with the highest boiling points settle at the bottom.

These tar like fractions, called residue, are literally the "bottom of the barrel."

The fractions now are ready for piping to the next process or plant within the

refinery. Some components require relatively little additional processing to become

asphalt base or diesel fuel. However, most fractions that are destined to become

high-value products require much more processing. This type of process is usually

confined to refinery and chemical plants and is not common on production facility.

If the boiling points of the constituents of a mixture differ only slightly, complete

separation cannot be achieved in a single distillation process. The product from the

first distillation column is redistilled once or twice to produce the desired products.

The example in Figure 3-3 shows the fractionation section of an LPG Plant.

Figure 3-3


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A simplified drawing of a distillation process is shown in Figure 3-4, it can be divided

into three sections, the column, heating system and overhead/reflux system.

Figure 3-4

3.1 Distillation Column

For better understanding of the process the distillation tower can be divided into

sections or zones. Each zone performs a different task within the overall process.

3.1.1 Feed Section / Flash Zone

The tower usually has one inlet nozzle to direct the inlet streams into the correct

level of the tower (Flash Zone) see Figure 3-5. The feed is heated and flashes or

separates as it enters the tower. Vapours rise up the tower and heavier

hydrocarbon liquid flows to the lower sections


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3.1.2 Enrichment / Rectifying Section

This is the area above the feed nozzle and flash zone. The light vapours from the

flash zone rise up the tower. Condensed hydrocarbons from the cooler top section

of the tower flow down through the enrichment/rectifying section and come into

contact with the rising vapour. The contacting of liquid and vapour causes some of

the heavier fractions in the vapour to condense and lighter fractions in the liquid to

evaporate.

Figure 3-5

3.1.3 Stripping Section

This section is located below the feed nozzle and flash zone. The light vapour

components are stripped (separated) from heavier liquid hydrocarbons by the hot

vapours that rise up through the column from the accumulator zone.


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3.1.4 Accumulator Zone

The heaviest liquids collect in this area at the bottom of the tower. Some of the

liquid is heated by a reboiler to maintain the temperature in the bottom in the

bottom and throughout tower. When the fluid returns to the column from the

reboiler the lighter fractions flash off and make their way back up the column.

Excess liquid leaves the tower through the bottom outlet line to further processing

or to storage tanks. The excess liquid is hot and usually exchanges its heat with the

incoming feed to the tower in a feed/effluent heat exchanger.

3.2 Tray Columns

There are several method of ensuring a close contact between the materials in a

column; in this section we will discuss tray columns. A tray is a metal plate that is

installed in a horizontal position. They are installed at different levels inside the

column. The number of trays and the space between them depends on the fluid

flow characteristics and temperature profile inside the column, these factors are

taken into account at the equipment design stage.

Figure 3-6

Some columns have only two or three trays. Very tall columns can have several

hundred trays. The trays are held in position by support rings. The trays can be

removed for maintenance / replacement.


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There are several types of trays used in the petroleum industry, the two most

commonly used trays are the bubble cap tray and the valve cap tray.

3.2.1 Bubble Cap Tray

The bubble cap tray has metal caps covering the holes in the tray. Figure 3-7 shows

a bubble cap tray. Hot vapours flow up the column as liquid flows down the column.

The hot vapours pass up through slots in the bubble caps. The bubble caps slow

down the flow of the hot vapours and make them pass through the liquid on the

tray. This increases the contact between the fluids.

Figure 3-7

The liquid held on the tray covers the slots in the bubble caps. The level of the

liquid on the tray is controlled by a dam or weir. The vapour passes out of the slots

as small bubbles. Small bubbles contact more of the liquid on the tray.


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The liquid on the top tray of the column flows through the downcomer to the tray

below. The downcomer must go below the surface of the liquid on the tray below.

Bubble cap trays are a very efficient way of allowing the lighter vapours to filter

through the heavier liquids as they flow up the tower. This contact between the

different fluids in the tower is an important part of the distillation process.

Figure 3-8

3.2.2 Sieve or Perforated Tray

The sieve or perforated tray has many small holes in it. Vapour flowing up the

column and the liquid flowing down the column pass through the holes. The vapour

and the liquid come into contact as they pass through the holes. These trays are

not used in processes that are difficult to control.


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Figure 3-9

Figure 3.10 shows how the weir controls the liquid level on the tray. The downcomer

directs excess illiquid to the next lower tray. It is immersed in the liquid level on the

lower tray to prevent vapours passing up the downcomer.

Figure 3-10

3.3 Summary

3.3.1 Nozzles

Short sections of pipe welded to the column at one end and flanged at the other.

Inlet and outlet pipe work, sight glasses, instrumentation equipment etc. are bolted

to the flanged ends.

3.3.2 Trays

The trays are used to give maximum fluid contact inside the tower. This produces

the most efficient end product. Two common types are bubble cap and sieve trays.

3.3.3 Downcomers

The downcomers direct the fluid from one tray to the next lower tray in the tower.


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3.3.4 Weirs

The weirs hold the correct level of liquid on each tray. The rising vapours must pass

through the liquid as it flows up the column. The contact between the liquids and

the rising vapours improves the distillation process.


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3.4 Packed Columns

A simple method of providing close contact between the liquids is to fill the column

with packing material.

Figure 3-11


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Packing provides a large surface area that improves the contact between fluids in

the column. Two common types of packing are "Raschig Rings" and "Beryl

Saddles".

The packing is made of a material that will not react with the liquids and vapours in

the process, ceramic, plastic and various metals are common. The packing is

supported near the bottom of the column by a metal grating. Other metal grating

separate layers of packing, a top grating holds the packing in place.

Figure 3-12


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Figure 3-13


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4.0 HEATING / COOLING EQUIPMENT

4.1 Feed Heater

A feed heater is a device which gives heat to the incoming feed liquid so as to

produce fractionation. The feed heater provides a large part of the heat needed for

the distillation column. The heat required can be generated using an external heat

medium or recovered from the outgoing bottom product from the tower, or a

combination of both methods.

A feed/effluent exchanger is a device which takes heat energy from the bottom

product leaving the tower and transfers it to the incoming feed.

The shell and tube is the most common type of heat exchanger. The fluids do not

change state in the exchanger.


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4.2 Reboiler

The kettle type heat exchanger is a common type of reboiler.

Figure 4-1

The bottom product from the tower flows to the reboiler where it heated. Part of

the liquid is vapourised and returns back to the tower. The hot vapour rises up

through the trays and exchanges heat with the down coming liquid. In the reboiler

the liquid that is not vapourised overflows a weir and leaves the reboiler under level

control. The reboiler shell is made larger enough to handle the increase in volume

when the liquid becomes a vapour.


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Figure 4-2

The level in the reboiler is the same as the liquid level in the bottom of the tower.

In this type of reboiler, the liquid flows by gravity from the tower to the reboiler. It

is helped by the convection of the heated liquid in the reboiler. The level controller

on the reboiler also controls the liquid level in the tower. The reboiler also serves as

another stage of fractionation for the bottom product.


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4.3 Thermal Siphon Reboiler

The thermo siphon reboiler uses convection to produce circulation. The bottom

product flows to the bottom of the reboiler where it is heated. The addition of heat

causes some of the liquid in the reboiler to rise and flow back to the column. The

rising liquid causes a convection flow that draws more liquid into the bottom of the

reboiler. This type of circulation is called thermo-siphon, vapour is also produced in

the process. See Figure 4-3.

Figure 4-3

Another reboiler piping line up, see Figure 4-4 receives all the flow from the bottom

tray. The flow through the reboiler, the heated liquid/ vapour mixture flows into the

lower section of the column. The vapour passes up the tower through the chimney

in the bottom tray.


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Figure 4-4

4.3.1 Reboiler Operation

The start-up and shutdown of a reboiler or any other heat exchanger must be

performed correctly to minimise thermal expansion.

On start-up the colder fluid must always be fed to the heat exchanger first.

On shutdown the hot fluid must be stopped first.

It is important to maintain the pressure in the tube / shell side of heat exchanger to

prevent vapours forming in the equipment. When flow is to be controlled it must be

done on the downstream side.

The temperature increase across the heat exchanger should always be monitored

closely. A temperature increase or decreases across the heat exchanger may

indicate a problem with the heat exchanger, e.g. dirty or plugged, tube leak etc.


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4.4 Fired Reboiler Heater

Large fractionation towers use fired heaters for reboil. Figures 4-5 and 4-6 show

examples of horizontal and vertical tube heaters. (Horizontal and vertical refers to

the orientation of the tubes in the heater).

Figure 4-5


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Figure 4-6


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The flow through the fired heater must be maintained to keep the tubes full of liquid

and prevent overheating. The column bottoms pump circulates a high percentage

of the bottom product through the reboiler. Heat is produced by the combustion of

fuel in the combustion chamber, the control instruments are linked so that the flow

of fuel to the burners will be stop if the flow to the tubes falls below a minimum

rate.


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5.0 OVERHEAD PRODUCT CONDENSING EQUIPMENT

Contact between the liquid and vapour in the top of the tower is crucial to

purification of overhead product. Some or all of the overhead product is condensed

in the overhead condenser and is collected in the overhead accumulator.

Overhead condensers use various cooling mediums. Common ones are:

5.1 Fin Fan Cooler

Air is forced across the tubes by fans. The speed of rotation or pitch of the fans

blades can be used to control the overhead product temperature. Louvers are also

used for temperature control. By opening or closing louvers more or less air is

allowed to pass through the cooler.

Figure 5-1

5.2 Water Cooled Condensers

The overhead product temperature may be controlled by regulating the flow of

cooling water through the condenser. This method may be used to condense all or

part of the overhead product.


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5.3 Reflux Drum

The reflux drum (reflux accumulator) receives the condensed overhead product.

From the reflux drum the product is pumped back to the tower as reflux or it goes

to storage as finished product.

Figure 5-2


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