unit 4 gas fractionation plant
DESCRIPTION
Gas Fractionation PlantTRANSCRIPT
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UNITS IN THIS COURSE
UNIT 1 GAS COMPRESSION SYSTEMS
UNIT 2 AMINE GAS SWEETENING UNIT
UNIT 3 NATURAL GAS LIQUIDS (NGL) RECOVERY UNIT
UNIT 4 GAS FRACTIONATION PLANT
UNIT 5 SULPHUR RECOVERY UNIT
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TABLE OF CONTENTS
Para Page
4.0 COURSE OBJECTIVE 3
4.1 INTRODUCTION 4
4.2 PROCESS DESCRIPTION 5
4.3 COLUMN TRAYS 7
4.4 REBOILER FUNCTION 10
4.5 REFLUX FUNCTION 11
4.6 PROCESS EQUIPMENT 12
4.6.1 Columns 12
4.6.2 Overhead Accumulators (Reflux Drums) 12
4.7 ROTATING EQUIPMENT 13
4.7.1 Feed Pumps 13
4.7.2 Reflux Pumps 13
4.8 OVERHEAD PRODUCT CHILLER 13
4.9 OVERHEAD PRODUCT CONDENSERS 13
4.10 PROCESS CONTROL INSTRUMENTATION 13
4.11 LOWER COLUMN TEMPERATURE CONTROL 15
4.12 BOTTOM PRODUCT LEVEL CONTROL 16
4.13 COLUMN PRESSURE CONTROL 16
4.14 COLUMN TOP TEMPERATURE CONTROL 18
4.15 TOP PRODUCT LEVEL CONTROL 18
4.16 LOGIC INSTRUMENT SYSTEMS 18
4.16.1 Pump Logic 18
4.16.2. Overhead Accumulator Level Logic 18
4.17 PROPANE AND BUTANE SWEETENERS 19
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4.0 COURSE OBJECTIVE
On completion of this unit the trainee will be able to:
Describe and identify the most important pieces of equipment used in a gas fractionation plant.
Describe the functions of those pieces of equipment.
Follow the flows through a Process Flow Diagram.
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4.1 INTRODUCTION
The gas fractionating plant that we will look at is an onshore plant.There is also an offshore plant. The basic principles on how the plants operate are the same:
The feed to the onshore gas fractionating plant is from. the two places.
- The LNG recovery plant - Liquid Natural Gas
- The Crude Oil Refinery - Liquid Petroleum Gas
The feed consists of:
- LNG - C2 C3, C4 and C5 +- LPG - C3, C4, C5 + + Mercaptans (RSH)
The gas fractionating plant separates ethane, propane and butane from heavier natural gasoline components. They are separated by a process called distillation.
The sulphur compounds, (mercaptans) are then removed from the liquid propane and the liquid butane.
The distillation takes place in three columns operating in series (one after the other).
the deethanizer - removes C2, ethane
the depropanizer - removes C3, propane
the debutanizer - removes C4, butane.
See figure 4-1 Simplified Process Flow Diagram
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4.2 PROCESS DESCRIPTION
Liquid LNG/LPG feed is pumped into the deethanizer column.
The deethanizer column separates the ethane component from the feed.
The ethane *vapour product leaves the top of the column and is cooled in a chiller. The kettle type chiller uses propane liquid as refrigerant.
The ethane vapour/liquid mixture then flows into an overhead accumulator vessel.
Ethane vapour leaves the top of the overhead accumulator.
Some ethane liquid is pumped back into the top of the column as reflux. This controls the temperature at the top of the column. (Reflux will be explained later).
Heat for distillation is supplied to the column by steam circulating through a kettle type reboiler at the bottom of the column.
Bottoms product flows from the deethanizer to the depropanizer. The pressure difference between the two columns causes the bottoms product to flow.
The depropanizer. column separates the propane from the feed.
The propane vapour product leaves the top of the column. It is cooled in an overhead condenser.
The overhead condenser contains fin-fans. The overhead product is cooled by heat exchange with ambient air.
The propane liquid flows into an overhead accumulator vessel.
A pump sucks the liquid propane from the bottom of the overhead accumulator.
Some liquid propane is pumped back into the top of the column as reflux. This controls the temperature at the top of the column.
The rest of the propane liquid product is pumped through sweeteners to remove any sulphur impurities. Then it goes to storage.
Heat for distillation is supplied to the column by steam circulating through a kettle type reboiler at the bottom of the column.
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Figure 4-1 Gas Fractionation Plant.
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Bottom product flows from the depropanizer to the debutanizer because of the pressure difference between the two columns.
The debutanizer column separates the butane from the feed.
The debutanizer operates the same as the depropanizer.
The only difference is the lower operating pressures and the higher temperatures.
Butane liquid product goes through sweeteners. Then it goes to storage.
Pentane plus, (natural gasoline liquid) , also goes to storage from the bottom of the debutanizer column.
4.3 COLUMN TRAYS
The large fractionating columns use trays.
Trays are the most efficient method of fractionating liquid hydrocarbons.
The trays contain holes in them. The holes are covered by "Bubble Caps" or by "Valves".
The bubble cap trays provide effective fractionation.
Most modern fractionating columns contain valve trays. The valves act like check valves. They allow the vapour to rise through the trays, but they stop the liquid from flowing downward through the trays.
The function of any type of tray is to make sure there is close contact between the vapour rising up the column and the liquid flowing down the column.
The liquid on the trays is "scrubbed" (cleaned) by the vapours as they rise up the column.
We will use bubble cap trays as our example because it is easier to see how they operate. See Figure 4-2.
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Figure 4-2 Fractionator in Action
You can see how the liquid flows down from tray to tray, and how the rising vapours, bubble through that liquid on each tray.
The action on the trays performs the following:
1. The rising vapours strip (remove) the lighter hydrocarbons from the down flowing liquid.
1. The down flowing liquid strips the heavier hydrocarbons from the rising vapours.
End results:-
1. Produces an acceptable bottom product to specification (contains no light ends)
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1. Produces an acceptable overhead product to specification (contains no heavy ends).
1. Note that the number of trays in a column depends a tot on the difference in the
vapour pressures of the products being separated.
If the. vapour pressures of the products are nearly the same. It is difficult to separate the products. If it is difficult to separate the products, the column needs more trays.
The depropanizer has 40 trays, and the debutanizer has 30 trays.
Figure 4-3 show a typical valve tray.
Figure 4-3 Typical Valve Tray
You will note that this valve tray has "calming sections".
These devices combine the functions of weirs and down comers.
Down flowing liquid fills the tray, flows over the weir of the calming section, and fills the trough.
The liquid remains in the trough for a time before passing out through the slots in the bottom of the trough.
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These slots are positioned below the liquid level on the tray below. Just like a normal down comer.
Because the liquid "rests" in the calming section troughs for some time the entrained vapours have time to escape from the liquid.
Liquid leaving the calming sections is no longer bubbling and agitated. It is calm with no vapour in it.
This feature makes these valve trays very efficient.
4.4 REBOILER FUNCTION
The liquids at the bottom of the column have a low boiling point. The basic function of the reboiler is to reboil the liquids at the bottom of the column so they change back to vapour. This gives the heat energy needed to make the column work.
The reboiler drives the column.
Steam enters the tube side of the reboiler heat exchanger. We control the bottom temperature of the column by controlling the amount of steam entering the reboiler. This controls the amount of heat energy going into the column.
The bottom temperature also controls the purity of the bottom product.
The fraction of low boiling material in the bottom product depends on the bottom temperature. See Figure 4-4
To make the diagram easier the bottom product is shown to be taken from the bottom of the column. In many cases the bottom product is taken from the reboiler.
How the temperature at the top of the column is controlled is a bit more complicated. This will be explained later in this unit.
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Figure 4-4 Fractionating Column Reboiler
4.5 REFLUX FUNCTION
The basic function of the reflux is to keep liquid on the trays.
We control the temperature at the top of the column by controlling the temperature and flow of the reflux liquid .
The pressure in the column is related to the vapour pressure of the overhead product.
The vapour pressure is proportional to the temperature of the overhead product.
The higher the temperature of the overhead product the higher it's vapour pressure.
The higher the vapour pressure the higher the pressure in the column.
The control of the temperature of the overhead product therefore controls the pressure in the column.
It is that simple.
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4.6 PROCESS EQUIPMENT
4.6.1 Columns
The columns must be the right size to do their job correctly.
They must be tall enough to give the correct temperature range for the overhead product to be separated from the feed.
The columns must be large enough to hold the volume of liquid that passes across the trays.
The deethanizer column has a larger diameter than the depropanizer.
This is because all the feed flows through the deethanizer The ethane is separated from the feed in the deethanizer
So, less feed flows from the deethanizer to the depropanizer.
The propane is removed from the feed in the depropanizer.
o the feed that enters the debutanizer has no ethane or propane.
So the debutanizer column has the smallest diameter because it handles the smallest amount of liquid feed..
It is what is in the feed to each column that decides the size of the columns.
The size (dimension) of the columns will vary from plant to plant.
The space between the feed trays must be large enough to hold the vapour that flashes from the feed on the feed tray.
4.6.2 Overhead Accumulator (Reflux Drums)
The overhead accumulators provide storage for the column reflux liquid.
They also act as 2 phase separators for the overhead product.
These vessels are designed and sized to slow down the flow of the incoming stream of overhead product vapour and liquid.
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They are also designed to hold the product long enough for the liquid and vapour to separate by gravity.
These vessels ensure the liquid overhead product which flows out of the depropanizer and debutanizer columns has no vapour in it. Also the vapour overhead product which leaves the deethanizer. column has no liquid in it.
4.7 ROTATING EQUIPMENT
4.7.1 Feed Pumps
4.7.2 Reflux Pumps
These are fixed-speed centrifugal pumps. One pump acts as "online" main feed pump. The second pump acts as "spare" or stand by pump.
These are fixed-speed centrifugal pumps. One pump acts as "online' main reflux pump and is capable of meeting total reflux demand under normal operating conditions.
The second pump acts as "spare" or stand by pump. Both pumps can be used if the amount of reflux needed is very high.
4.8 OVERHEAD PRODUCT CHILLER
This is used on the deethanizer. column. It is a kettle type heat exchanger.
Propane liquid is used on the shell side as refrigerant.
4.9 OVERHEAD PRODUCT CONDENSERS
These are used on the depropanizer and debutanizer columns. These condensers are of the fin-fan type. Each contains four fans.
There are also louvers which limit air flow into the cooler. These can be set by hand.
4.10 PROCESS CONTROL INSTRUMENTATION
We will concentrate on the depropanizer and debutanizer columns.
See Figures 4-5 and 4-6.
Process Flow Diagrams for the depropanizer and debutanizer
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Figure 4-5
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Figure 4-6
4.11 LOWER COLUMN TEMPERATURE CONTROL
The temperature of the lower column is controlled by the amount of heat energy going into the reboiler
The temperature controller on the lower column is on tray 5 in the depropanizer and tray 3 in the debutanizer. It acts on cascade flow control to regulate the supply of steam to the reboiler.
The master temperature controller gives the set point value to the slave flow controller.
If the temperature in the column increases the flow of steam into the reboiler will decrease.
If the temperature in the column decreases the flow of steam into the reboiler will increase.
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4.12 BOTTOM PRODUCT LEVEL CONTROL
Depropanizer bottom product is transferred to the debutanizer under level control.
The product flows because of the pressure difference between the two columns. (Fluid flows from higher pressure to a lower pressure).
Debutanizer bottom product is transferred to storage under level control.
4.13 COLUMN PRESSURE CONTROL
All the overhead vapour product passes through an air fan condenser where it is cooled and partly condensed.
Column pressure is the vapour pressure exerted by the uncondensed vapour.
The more cooling there is, the more the vapour condenses.
If more vapour is condensed, then there is less vapour to exert vapour pressure.
Less vapour pressure means less column pressure.
The column pressure is controlled by cooling the overhead product vapour. See Figure 4-7.
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Figure 4-7 Column Pressure control
There is a pressure controller (PIRC) on the column top product vapour line.
This controls column pressure by split range.
The controller output acts on the fans in the column overhead condenser.
The controller also acts on a vent valve to flare.
When the controller output is 0 to 50%, column pressure is controlled by controlling the fans.
(More fans = more cooling, less fans = less cooling).
When the controller output is more than 50% the flare vent valve starts to open.
During normal operations column pressure is controlled by regulating the fans with the flare vent valve closed.
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4.14 COLUMN TOP TEMPERATURE CONTROL
The temperature at the top of the column is not directly controlled.
The top temperature is indirectly controlled by the volume of reflux f low.
The ratio of reflux flow to column feed flow is also control maintains top product purity.
4.15 TOP PRODUCT LEVEL CONTROL
The level of the top product in the overhead accumulator is regulated by cascade flow control of the top product flow to the sweeteners.
4.16 LOGIC INSTRUMENT SYSTEMS
Logic systems protect the fractionating columns, so that they shut down if there is a serious problem.
4.16.1 Pump Logic
Pump suction valves must be open before a pump motor can be started.
Closing a suction valve while the pump is running causes the motor to shut off automatically.
4.16.2. Overhead Accumulator Level Logic
The overhead accumulators are equipped with logic instrument systems that protect them from total loss of liquid level.
When the level of liquid reaches or falls below 5% the reflux pumps stop automatically. This allows liquid to re-accumulate in the accumulators.
Reflux pumps can only be restarted manually by the operator.
The logic systems cannot be changed or closed down by the operator.
The fault that triggered the logic shut-down has to be corrected before the process can be started again.
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4.17 PROPANE AND BUTANE SWEETENERS
Recall that the propane and butane products go through a process which removes sulphur impurities before going to storage.
This process is called sweetening (removal of acidic impurities).
The sulphur impurities are removed by passing the propane or butane through molecular sieves.
The molecular sieves allow the propane and butane to pass through but they adsorb (stop) the impurities.
When the molecular sieves are full of impurities they have to be cleaned.
This cleaning is called regeneration. It is carried out by reverse flowing hot, clean, fuel gas through the sieves.
The hot gas is followed by cool gas to cool down the sieves.
The sieves are then refilled with propane or butane liquid.
Some of the liquid turns to vapour which pressurises the sieve vessel back to operating pressure.
We normally have banks of two sieves. One on stream, one regeneration. See figure 4-8.
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Figure 4-8 Gas Sweetening Unit Schematic
Computers control the order in which the valves open and close when the molecular sieves are regenerated and go onstrearn.
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