Unit 3 Natural Gas Liquids (NGL) Recovery Unit

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ModuleNo.12:Gasprocessing

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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) RECOVERYUNIT

UNIT 4 GAS FRACTIONATION PLANT

UNIT 5 SULPHUR RECOVERY UNIT

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TABLE OF CONTENTS

Para Page

3.0 COURSE OBJECTIVE 3

3.1 INTRODUCTION 4

3.2 PROCESS DESCRIPTION 5

3.2.1 Feed Gas Compression and First Chilldown 5

3.2.2 Vapour and Liquid Dehydration 7

3.2.3 Second Chilldown, Expander/Compressor, and Demethanizer 9

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3.0 COURSE OBJECTIVE


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3.1 INTRODUCTION

Associated gases from the GOSP's contain a range of hydrocarbons.

These range from the lightest gas, methane (Cl) to heavier hydrocarbons, (C6 +).

The heavy hydrocarbons can be condensed easily.

The lighter hydrocarbons, propane (C3), and especially ethane (C2), need bothHIGH PRESSURE AND LOW TEMPERATURE to make them condense.

Recall how pressure and temperature affect the boiling points of liquids.

Recall also how pressure and temperature affect the dew points of vapours.

The dew point is the temperature at which a vapour (gas) turns to liquid at a givenpressure.

It is very costly to build equipment to contain gases under high pressure.

THE COLDER WE CAN MAKE THE GAS, THE LESS PRESSURE WE NEED TOCAUSE CONDENSATION.

So, the colder we can make the gas, the less costly the equipment needed tocondense it.

Recall your "Refrigeration" course.

When a gas expands with a pressure drop it gets colder.

We can drop the pressure of a gas stream and also use the drop in pressure tomake it do work.

This is done in a device called an "Expander/Compressor".

Recall you learnt about this device in your "Compressors and Turbines" course.

To obtain the required low temperatures in the NGL recovery plant liquidpropane is used as the refrigerant.

Expander/Compressors are used to obtain lower final temperatures.

The amount of propane refrigerant used is kept low by using the cold outletvapours from the expander and demethaniser to cool the feed gas.

These warmed outlet gases are then recompressed in the compressor side ofthe expander/compressor. This provides the load for the expander.

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3.2 PROCESS DESCRIPTION


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For ease of understanding we will look at the NGL recovery process in three mainstages.

Feed Gas Compression and 1st Chilldown

Vapour and Liquid Dehydration

2nd Chilldown, Expander/Compressor and Demethaniser.

3.2.1 Feed Gas Compression and First Chilldown

Refer to Process Flow Diagram 3-1

Figure 3-1 Feed Gas Compressor & 1 st Chilldown

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Sweet gas from the gas treating plant is the feed gas for this unit.


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The feed gas first passes through the feed gas compressor suction drum. This

removes any amine liquid droplets carried over from the gas treating plant.

The gas is then compressed in a single stage in the feed gas compressor. This

compressor is driven by a gas turbine.

After compression the gas is cooled in the compressor after cooler., This is a fin

fan cooler.

The recycle line for anti-surge protection is taken off downstream of the after

cooler. This protects the compressor from overheating should anti-surge controlbe necessary. It recycles cool gas to the suction side of the compressor, throughthe suction drum.

The cool hydrocarbon liquids from the condensate stripper are spiked into the

line downstream of the after cooler. [Recall the condensate stripper was in the W

and LP gas compression unit].

The gas is further cooled by heat energy exchange with demethaniser column

bottoms product. This occurs in the demethanizer reboiler.

The gas is cooled once more by heat energy exchange with liquid propane

refrigerant.

The condensed liquids and remaining vapours then pass into separator No. 1.

The temperature in this separator is controlled at 17 C.

This is done by controlling the pressure of the liquid propane refrigerant supply.[This is not shown on the PFDj

The vapour passes out of the top of the separator to the vapour dehydrators.

The condensed hydrocarbon liquid is pumped to the coalescer. From there it

goes to the liquid dehydrators.

Any free water that collects in the boot of separator No 1 is dumped into the

water blowdown system.

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3.2.2 Vapour and Liquid Dehydration


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Fluids entering this part of the plant are at approximately 17 C.

Downstream processes make the temperatures much lower.

It is therefore very important to dry the fluids before they are processed in the colder

sections of the plant.

Drying the liquids means there is no risk of hydrates forming. Hydrates are ice-likesolids made by hydrocarbons and water under certain temperatures and pressures.

At the operating pressures in the NGL plant equipment, the hydrates will form at

temperatures as high as 15C.

[At plant start-up a chemical called methanol is injected to prevent the formation ofhydrates].

Refer to Process Flow Diagram 3-2.

Figure 3-2 Vapour/Liquid Dehydration

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This PFD is simplified to make it easier to follow.


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The vapour and liquid dehydrators [driers] are of the molecular sieve type.

These molecular sieves contain solid beds which allow hydrocarbon vapours

and liquids to pass through. Water can not pass through. The water is trappedin the beds.

Each dehydration unit is a dual system. There are 2 vessels for vapours and 2

vessels for liquids.

While one vessel is in action drying the liquids, the second vessel is being

regenerated.

An automatic system controls the valves around the vessels. This automatic

system also switches the vessels from drying to regeneration on a fixed timebasis.

Hot dry residue gas (methane) is used to regenerate the dehydrators.

The process flow is as follows:

The vapours from separator No 1 flow downward through the vapour

dehydrator where any water is trapped in the molecular sieve bed.

The dried gases are filtered in the vapour dehydrator outlet filter. This removes

any fine solids picked up by the gas from the molecular sieve.

The gases then flow on to the second chilldown.

The flow of the hydrocarbon liquids is controlled by the level controller on

separator No 1 .

The hydrocarbon liquids pumped from separator No 1 first pass through a

coalescer.

The coalescer removes any free water in the liquid hydrocarbon stream. This

prevents free water reaching the molecular sieves. This water is dischargedthrough the water blowdown system.

The hydrocarbon liquids then flow downward through the liquid dehydrator

where any remaining water is trapped in the molecular sieve bed.

The dried hydrocarbon liquids then flow to the demethanizer column (tower).

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3.2.3 Second Chilldown, Expander/Compressor, and Demethanizer


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This section of the NGL recovery plant is more complex.

If we take the two flows (vapour and liquid) step by step it will be easy tounderstand.

Refer to Process Flow Diagram 3-3.

Figure 3-3 Second Chilldown & Demethanisation

In the vapour stream the dry sweet gas from the vapour dehydrator is chilled by

heat energy exchange at three points.

Exchanger No 1.

uses cold vapours from the expander outlet K.O. drum plus cold vapours

from the demethanizer overhead.

Exchanger No 2

uses cold liquids from separator No 2.

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Chiller No 1


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uses propane refrigerant

As the vapour stream cools some of the hydrocarbons condense. The

condensed liquids flow to separator No 2 where they separate from the gas.

These liquids flow from the bottom of separator No 2, through exchanger No 2and on to the demethanizer column.

The remaining vapours flow from the top of separator No 2. These vapours are

further cooled at exchanger No 3. This uses the cold vapours from theexpander outlet K.O. drum plus the cold vapours from the demethanizeroverhead.

As the temperature of the vapours fails more liquids are condensed. These

liquids are removed in separator No 3. From there they flow to thedemethanizer column.

The vapours from separator No 3 flow to the expander inlet drum. Here, any

condensed liquids are removed. These liquids join the liquids flowing fromseparator No 3 to the demethanizer column.

The vapours are expanded in the turbo-expander with a pressure drop of

approximately 400 psi. This expansion causes the turbo expander to spin.

The vapours are cooled by their expansion.

Liquids produced in the cooling/expansion are separated in the expander outlet

knockout drum. These liquids are pumped to the top of the demethanizer

column.

The chilled vapours leaving the top of the expander outlet K.O. drum combine

with the vapours from the demethanizer overhead and flow back through thechilldown train. They are used to cool the vapour feed coming in to exchangersNo 3 and No 1.

After cold recovery at exchangers No 3 and No 1 these vapours pass through

the shell side of the propane sub-cooler. Here, they are used to cool thepropane refrigerant.

The vapours, now called "residue gas", are then compressed in therecompressor that is driven by the turbo-expander.

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The residue gas finally exits the NGL unit to be used as fuel gas at


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approximately 300 psi.

The demethanizer is a 26 tray multiple feed stripper column. It strips the

methane gas from the feed liquid.

The coldest feed comes from the turbo-expander outlet K.O. drum. This

liquid is fed into the top tray; No 26.

The next coldest feed is the liquid from separator No 3 and the expander

inlet drum. This is fed into tray No 23, lower down the column.

The next coldest feed is the liquid from separator No 2. This is fed into

tray No 13.

The warmest feed is the liquid from the liquid dehydrators. This is fed into

tray No 7 near the bottom of the column.

The demethanizer has 2 reboilers:

The main reboiler takes heat energy from the feed gas

compressor discharge gases.Warm liquid from the bottom tray is pumped through thereboiler.80% of the stripping vapour needed by the column isprovided by this reboiler.

The trim reboiler provides the remaining 20% of the stripping vapour. This trim

reboiler uses hot oil to produce heat energy.

The two phase flow stream from the main reboiler returns to the demethanizercolumn at the bottom. It flows into a section called the "Product Surge Volume"section.

The trim reboiler also takes its feed from this section of the column, and returns

its two phase flow back to, this section.

The two phase NGL product separates in this bottom section of the

demethanizer column.

The vapour phase rises up through an internal chimney pipe into the main body

of the tower. This is the stripping vapour.

The liquid that remains is the NGL product.

The NGL product is pumped to the Gas Fractionating Plant.

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Unit 3 Natural Gas Liquids (NGL) Recovery Unit

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