unit 3 fractionation & distillation equipment

7/29/2019 Unit 3 Fractionation & Distillation Equipment

1/15

UNITS IN THIS COURSE

UNIT 1 SEPARATORS

UNIT 2 TOWERS AND COLUMNS

UNIT 3 FRACTIONATION AND DISTILLATION EQUIPMENT

UNIT 4 STORAGE TANKS

TABLE OF CONTENTS

ParaPage

3.0 COURSE OBJECTIVE 3

3.1 INTRODUCTION 4

3.2 HEATING EQUIPMENT 5

3.2.1 Thermal Syphon Reboiler 12

3.2.2 Assisted Thermal Syphon Reboiler 13

3.2.3 Fired Heater Reboiler 13

3.2.4 Fractionator Bottom Temperature Control 16

3.2.5 Fractionator Overhead Temperature Control System 17

3.3 OVERHEAD PRODUCT CONDENSING EQUIPMENT 18

3.3.1 Fin Fan Cooler 18

3.3.2 Water Cooled Condensers 18

3.3.3 Reflux Drum 19

3.0 COURSE OBJECTIVE

On completion of this unit the trainee will be able:

Describe the equipment that is used to operate a fractionation and distillation tower.

Describe the types of reboiling equipment used in the operation of fractionationtower.

Describe the types of condensing equipment used in the operation of a fractionationtower.

Discuss the control methods used in the operation of a fractionation tower.

Mo

duleNo.

4:Processvessels

UnitNo.

4-Sto

ragetanks

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 1/19


2/15

3 .1 INTRODUCTION

The distillation process is one of the most important processes in oil refineries andgas plants. In the distillation process, mixtures of liquids are separated into theircomponent parts. The liquid mixture is heated. Part of the liquid is boiled off into avapour. This boiled off vapour is then condensed to a liquid. The condensed liquid

(condensate) is purer than the original mixture. The distillation process is used topurify and separate liquids.

Figure 3-1 Simple Distillation Apparatus

Figure 3-1 shows a simple distillation apparatus. A flask, called an evaporator isfilled with a liquid mixture (the feed liquid). The mixture is heated until it begins to

boil. The vapour that is boiled off is then cooled in the condenser and collected inthe receiver. This condensate is different than the original feed liquid.. It is pure.

When the distillation is finished, the heat is removed. The remaining liquid in theflask is removed. The distillation apparatus is allowed to cool down. New feed liquidis added to the flask. The process can then be repeated. However, this process isnot practical on a large scale.

The oil industry uses fractionation which is a form of distillation. The unit used forthis procedure is, called a distillation tower or a fractionation column. Some peopleuse the term fractionation towers and some use fractionation columns. In this unitwe will use both towers and columns so that you can learn them.

In this process, a part of the product is returned from the condenser to .thedistillation tower. This is called reflux. The product or reflux flows down the columnfrom tray to tray.

The vapour rising through the liquid on each tray is a distillation process. The liquidflowing down the column from one tray to the next tray bellow is reflux for the lowertray. At any point on the tower, a product can be drawn off from one of the trays.

There are two main types of equipment needed to operate fractionating columns.

Heating (vapourising) equipment.

Mo

duleNo.

4:Processvessels

UnitNo.

4-Sto

ragetanks

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 2/19


3/15

Condensing (reflux) equipment.

A fractionation column has the following pieces of equipment:

Heating equipment. (For heating the feed liquid). Bottoms reboiler or fired heater.

Product pump. Reflux pump. Overhead condenser. Reflux accumulator. Temperature, pressure and flow

recorders, and control instrumentation. Valving.

3.2 HEATING EQUIPMENT

A feed heater is a device which gives heat to the incoming feed liquid so as toproduce fractionation. The feed heater provides a large part of the heat needed forthe distillation column.

The heat exchanger is a device which takes heat energy from one process fluid andgives (or transfers) it to another process fluid of a lower temperature. However, thetwo fluids do not mix together. So, it operates as follows.

One fluid flows through tubes inside the vessel (tube side). The other fluid flowsaround the outside of the tubes inside of the vessel (shell side). Heat energy fromthe fluid with the higher temperature is absorbed by the fluid with the lowertemperature.

Heat Exchanger Parts (See Figure 3-2)

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 3/19


4/15

Figures 3-2 Heat Exchanger Parts

Tubes: These are small diameter pipes through which one of the fluids flows.

Tube Sheet: This is a flat circular plate to which the tubes are fitted and secured.

There are two main types of heat exchangers that are used in the fractionationprocess:

a) Shell and Tube

The shell and tube is the most common type of heat exchanger. The fluids

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 4/19


5/15

do not change state in the exchanger.

B) Kettle Type

The kettle type is also called a reboiler. The fluid on the shell side gets hotterand changes from liquid to vapour. The shell is made larger to handle theincrease in volume when the liquid becomes a vapour.

The heat exchanger is designed to have the following qualities:

Maximum heat transfer between fluids.

Minimum restriction of fluid flow.

To be easy to clean and inspect.

To have good thermal expansion capabilities.

To be efficient and easy to operate.

The following factors make a heat exchanger transfer heat efficiently:

Having a maximum heat transfer surface.

Clean heat transfer surface.

The speed of the fluid through the heat exchanger. If the fluid moves slowly itgives more time for the transfer of heat.

Turbulent flow of the fluids. (Turbulence helps the heat to transfer)

The properties of the fluids and the heat exchanger material also determinesthe amount of heat energy transfer. E.g., some metals transfer heat more easilythan others.

Head End: This is a small shell attached to the tube sheet. It changes the directionof the fluid flow from one set of tubes to another.

Tube Bundle: This is the term used for the complete unit of the tubes, tube sheetand baffles.

Partitions: These are flat piece of plate (in the channel) that is used to direct thefluid flow through the tubes.

Baffles: This is a flat plates used to direct the fluid flow around the shell side of thetubes. They are also used to support the tube bundle

There are two types of baffles:

Segmented (see figure 3-2)

Disc and Doughnut. (see figure 3-3)

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 5/19


6/15

Figure 3-3 Disc and Doughnut Baffle

Types of heat exchanger

Different processes require different types of heat exchangers. Some of the mostcommon are:

Figure 3-4 Single Pass Exchanger

Single Pass: The fluid on the tube side only goes through the heat exchangeronce.

Multi Pass: The fluid on the tube side parses through the heat exchanger two ormore times.

Fixed Tube Sheet: The tube sheet and bundle are bolted or secured at both endsof the heat exchanger. This type allows very little thermal expansion. This type isonly good when there are small temperature differences between the fluids.

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 6/19


7/15

Figure 3-5 "U" Tube Heat Exchanger

"U" Tube Heat Exchanger: The back end of the tube bundle is connected by "U"shaped tubes (not a head). This type allows for large thermal expansion. This typeis good for high temperature differences between the fluids. However, the tubes inthis type are difficult to clean. (See figure 3-5)

Floating Head: The front tube sheet is secured / bolted to the heat exchanger shell.This type allows for large thermal expansion. These are good where there is a hightemperature difference between the fluids. The tubes in this type are easily

removed for cleaning.

Heat Exchanger Operation

The start-up and shutdown of a heat exchanger must be performed correctly. Thiswill minimise thermal expansion. On start-up the colder fluid must always be fed tothe heat exchanger first. On shutdown the hot fluid must be stopped first. Do notopen the hot fluid flow to the heat exchanger when the cold fluid is isolated. Thismay cause hydraulic expansion of the trapped cold fluid. This can cause seriousdamage to the heat exchanger.

The flow through the heat exchanger must always be controlled on the downstream

side of the heat exchanger. So, the flow control valve must always be locateddownstream of the heat exchanger. The heat exchanger always operates flooded.This will minimise corrosion and erosion.

The temperature increase across the heat exchanger should always be monitored.When the temperature increase across the heat exchanger begins to go down, itwill indicate that the heat exchanger is becoming dirty or plugged.

Some heat exchangers can be back flushed / backwashed. This means that theflow through the heat exchanger can be reversed to wash out the dirty material. Ifthis does not work the tube bundle must be removed and cleaned. A high pressurewater wash or a chemical solution are used to do this.

Checks should always be made on the low pressure fluid outlet to ensure that thetubes or gaskets are not leaking. If there are any leaks, one fluid will mix with the

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 7/19


8/15

other fluid. Fluid will leak from the high pressure side to the low pressure side.

Fin Fan Cooler

Figure 3-6 Fin Fan Cooler

The fin fan cooler is another type of heat exchanger. These are used a lot in theplants. The hot fluid flows into an inlet header. It then flows through finned tubes.Finned tubes have large flat surfaces which give better and quicker transfer ofheat. The liquid flows out of the heat exchanger into the discharge header. Air isforced up and over the tubes by fans. The waste heat energy passes into the air.The amount of heat exchange is controlled by the following:

Fan Speed. (Faster flow of air cools the fluid more quickly).

Louvre Control. (The amount of air flowing can be changed).

Fan Blade Pitch. (The fan can push a little or a lot of air)

Fluid volume through the tubes. (Smaller volumes of fluid cool more quickly).

Warm air recycle. (The warmed air may get sucked back into the air inlet. Thisshould be avoided at the design stage)

3.2.1 Thermal Syphon Reboiler

Modu

leNo.

4:Processvessels

UnitNo.

4-Stora

getanks

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 8/19


9/15

Figure 3-7 Thermal Syphon Reboiler

The thermal syphon reboiler only uses convection to produce circulation. Thebottom product flows to the bottom of the reboiler by gravity. The addition of heatcauses some of the liquid in the reboiler to vapourise and the remaining liquidexpands.

The mixture of vapour and expanded hot liquid in the reboiler has a much lowerrelative density than the bottom liquid in the fractionation tower. Therefore it isunder lower pressure. Remember that fluid flows from high to low pressure.Therefore, a thermal syphon flow is produced. The flow through a thermal syphonreboiler is shown in Figure 3-7.

3.2.2 Assisted Thermal Syphon Reboiler

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 9/19


10/15

Figure 3-8 Assisted Thermal Syphon Reboiler

The assisted thermal syphon reboiler receives all the flow from the bottom tray. Theflow through the reboiler is produced by the difference in the static head of the

liquid to the reboiler and the head of the liquid vapour mixture in the tower. Thevapour passes up the tower through the chimney in the bottom tray.

3.2.3 Fired Heater Reboiler

Large fractionation towers use fired heaters for reboilers. Figures 3-9 and 3-10show examples of horizontal and vertical fired heaters. (Horizontal and verticalrefers to the tubes in the heater).

Figure 3-9 Horizontal Fired Heater

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 10/19


11/15

Figure 3-10 Vertical Fired Heater

The flow through the fired heater must be positive to keep the tubes full of liquid.This prevents overheating of the tubes in the heater. The fractionator bottomspump circulates all or nearly all of the bottom product through the reboiler. Theremust be a positive flow through all the tubes in the reboiler. The controls must belinked so that the flow of fuel gas to the burners will be stopped if the flow fails to

the tubes.

In some operations the fired heater will supply heat to more than one fractionator.Figure 3-11 shows the flow through a large fired heater type reboiler.

Modu

leNo.

4:Processvessels

UnitNo.

4-Stora

getanks

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 11/19


12/15

Figure 3-10 Fired Heater Reboiler Flow

3.2.4 Fractionator Bottom Temperature Control

Figure 3-11 Typical Bottom Temperature Control System

The pressures and temperatures in the fractionation column must be closelycontrolled. If the temperatures and pressures are not closely controlled the productwill not be pure. If the bottom temperature is too hot, some of the bottom productwill be up the tower. If the bottom temperature is too cold some of the overhead

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 12/19


13/15

product from the top of the tower will condense out in the bottom product.

Figure 3-11 shows a typical bottom temperature control system for a fractionator.The Temperature Recorder Controller (TRC) controls the amount of steam whichflows through the reboiler. If a fired heater were used for a reboiler, the TRC wouldcontrol the flow of fuel gas to the burners.

There is another method of controlling the bottom temperature. The heat input tothe reboiler is not changed. The temperature at the top of the tower is controlled byChanging the reflux flow. This method controls the top temperature and will alsocontrol the bottom temperature. This method of control is shown in Figure 3-12.

3.2.5 Fractionator Overhead Temperature Control System

Figure 3-12 Typical Overhead Temperature Control System.

3.3 OVERHEAD PRODUCT CONDENSING EQUIPMENT

A lot of contact between the liquid and vapour in the top of the tower is needed topurify the overhead product, and to condense any bottom product that is flowing up

the tower. Therefore, some or all of the overhead product is cooled and condensedin a heat exchanger.

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 13/19


14/15

The overhead condenser may use any of the following as a cooling medium.

3.3.1 Fin Fan Cooler

Figure 3-13 Fin Fan Coolers

This type of heat exchanger was described earlier in this unit (section 4-2). Air isforced across the tubes by fans. That is why it is called a "Fin Fan Cooler". Thespeed and operation of the fans, is part of the control system to regulate theoverhead product temperature. Louvres on the coolers are also used to control the

overhead product temperature. Louvres do this by opening and closing like manydoors. They allow more or less air to pass through the cooler.

3.3.2 Water Cooled Condensers

The overhead product temperature may be controlled by regulating the flow ofcooling water through the condenser. This method may be used to condense all orpart of the overhead product.

The overhead product may contain light gases with a high vapour pressure, such asmethane and ethane. These gases will not condense. They are pressure controlledfrom the reflux accumulator. They are recovered as sales gas or fuel gas. Thismethod is shown in Figure 3-14

ModuleNo.

4:Pr

ocessvessels

UnitNo.

4-Storagetanks

Page 14/19


15/15

Figure 3-14 Partial Condenser

3.3.3 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 tostorage as finished product.

Page 15/19

unit 3 fractionation & distillation equipment

Documents