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Gas

Burning Feed Stock

CHEMICAL PLANTSAMMONIAMETHANOL

UTILITIESTOWN GASS SUPPLIESENERGY INTENSIVE INDUSTRIES

Power Generation

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Fundamentals of Natural Gas Processing

Natural gas has been formed by the degradation of organic matterNatural gas has been formed by the degradation of organic matter accumulated in the past millions of years.

Two main mechanisms:BiogenicThermogenic

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Water

S lf i Sulfur species

Mercury

NORM (Naturally occurring radioactive materials)( y g )

Diluents

Oxygen

1) Non-associated gas that occurs in conventional gas fields

2) Associated gas that occurs in conventional oil fields

3) Continuous (or unconventional) gas.

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Natural gas is colorless, odorless, tasteless, shapeless, and lighter than air

Crude oil–gas–water are directed to a central processing and t t t f ilit ll ll d th il ti l ttreatment facility normally called the gas–oil separation plant (GOSP).

The first step in processing of the produced stream is the separation of the phases (oil, gas, and water) into separate streams.

This takes place in mechanical devices known as:This takes place in mechanical devices known as:Two-phase gas–oil separators when the produced stream

contains no water Three-phase separators when the produced stream contains

water.

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The separators are used to relieve the excess pressure dueThe separators are used to relieve the excess pressure due to the gas associated with the produced crude and, consequently, separating it from the oil.

When water exists in the produced stream, separators are also used to separate the free water from the oil. a so used to sepa ate t e ee wate o t e o .

Once separation is done, each stream undergoes the proper processing for further field treatment

Gas Processing

Flare

Gas sales or reinjection

Crude OilSeparation

Crude OilTreating

Gas Processing Module

Field Storage

Reservoir

jNGL sales

CO2 &/or Sulfur sales

Oil sales

GAS

OIL

Produced Water Treating

Water disposalorReinjection

WATER

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This module is required to meet crude oil sales specification:specification:

1. BS&W (Basic Sediment and Water)

2. Vapor Pressure

3. Salt

4 S lf C t t4. Sulfur Content

A typical crude oil produced from a field in the Middle East before and after treatment

Raw crude oil has the following materials present:Water: present in two forms: emulsions (10%) and free water (30%)Salt: 50,000–250,000 mg/L formation waterGas: dissolved gas (600 scf/bbl crude oil)H2S: 1000 ppm

Treated crude oils have the following final specifications:Water content: 0.3 vol% maximumSalt content: 10 lbs (as NaCl) per 1000 barrels of oilH2S: 70 ppmVapor pressure: 10 psig (4–5 psi RVP)

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Produced water must be treated in order to meetProduced water must be treated in order to meet reinjection or disposal specification:

1. Hydrocarbons

2. Free solids

3 Dissolved solids e g CaCO NaCl BaSO etc3. Dissolved solids e.g. CaCO3, NaCl, BaSO4, etc.

Market sales of natural gas require some specifications set by the consumers regarding the maximum contents allowable for the following:

1) Acidic gases and sulfur) g

2) Oxygen

3) Carbon dioxide

4) Water vapor

5) Liquefiable hydrocarbons

6) Minimum thermal heating content of the gas

Two common standard conditions of temperature and pressure are used:

1) Universal scientific, reported at 32°F and 760mmHg

2) Natural gas industry, reported at 60°F and 14.7 psia

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ProducedGas

Natural Gas Sales

GasStream

Conditioning NGL

Extraction

Stabilization ProductTreating

Fuel

NGLSales

Recompression

Conditioning1) Dehydration2) Sweetening

NGL Extraction1) Adsorption (SCU)2) Absorption (lean oil)3) Condensation

a)Valveb)Turboexpanderc)Refrigeration 

Stabilization1) Demethanizer2) Deethanizer

Product Treating1) Dehydration2) Sweetening

a)H2S, COS, CS2b)CO2

Removal of water is named gas dehydration.

The most common processes are:

1) Absorption (highly concentrated glycol solutions)

2) Adsorption [a solid desiccant such as silicagel, alumina or molecular sieve (zeolite)].

It is used when very low (<1 ppm) effluent water content are required.

3) Condensation)

4) Other - membranes, CaCl2, etc.

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Removal of H2S and CO2

The most common processes are:

1) Chemical absorption – amines, potassium carbonate

2) Physical absorption – SELEXOL, methanol

3) Solid bed – molecular sieves, iron sponge, zinc oxide

4) Other – direct conversion, membrane, extractive distillation

Nitrogen removal

Cryogenic fractionation from methane

Operating temperatures vary with pressure but can be as low as

-185°C [-300°F]

Mercury removal

Mercury removal is done by passing the gas across a bed of sulfur-impregnated activated charcoal where the mercury reacts to form mercuric p g ysulfide.

Effluent levels of Hg should be less than1 pbb.

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WHY EXTRACT NGL?

1)Meet gas sales specification requirement such as hydrocarbon dew point

2)Upgrade the market value of the products

NGL products and their markets are summarized as:

Ethane(C2) – Petrochemical feedstock

Lightest NGL with NBP of -88°C [-127°F]

Recovery can be justified in those areas where a ready petrochemical market and viable transportation exist

Propane (C3) – Petrochemical feedstock and fuel

NBP of -42°C [-44°F]

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Butanes (iC4 and nC4)

Petrochemical feedstock, fuel and gasoline blendingPetrochemical feedstock, fuel and gasoline blending

iC4 is the more volatile isomer NBP of -12°C [11°F] and the most valuable.

nC4 an important feedstock for the manufacturing of monolefins (ethylene, propylene) and the diolefin, butadiene. NBP -0.5°C [31°F]

Natural gasoline (iC5+)

Pentanes and heavier components in a gas stream

primarily consist of straight and branched chain hydrocarbon

Refinery feedstock, Petrochemical feedstock

Commercial NGL extraction processes can be classified as f llfollows:

Absorption – lean oil

Adsorption – short cycle unit, hydrocarbon recovery unit

Condensation – mechanical refrigeration, turboexpander, valve expansionexpansion

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Gas and liquid contracts usually contain the following basic considerations:

GAS

1) Minimum, maximum, and nominal delivery pressure

2) Maximum water content (as a water dew point at a given pressure or as a concentration)

3) Maximum condensable hydrocarbon (hydrocarbon dew point, analysis, etc.)

4) Maximum delivery temperature4) Maximum delivery temperature

5) Allowable concentration of contaminants (H2S, CO2, mercaptanes)

6) Minimum heating value

7) Cleanliness (allowable solid concentration)

Liquid

1) Quality of products (vapor pressure relative or absolute density1) Quality of products (vapor pressure, relative or absolute density, or by standard designation such as Commercial Propane)

2) Specifications such as color, concentration of contaminants, etc.

3) Maximum water content

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Heating value (heat of combustion): 

The amount of heat developed by the complete combustion of a materialmaterial. 

a)The gross or higher heating value 

b)The net, or lower, heating value (normally referred to in Europe)

Sulfur content

This limits the amount of sulfur compounds to prevent corrosionThis limits the amount of sulfur compounds to prevent corrosion, toxicity and odor when the gas is burned.

Typical specification for H2S is 4 ppm [0.25 grain/100scf]

Maximum delivery temperature

A maximum delivery temperature at the transfer point may be specifiedspecified.

It is often about 49°C [120°F].

Water content (dew point)

The mass of water per unit volume of gas or the maximum allowable water dewpoint temperature at specified pressurewater dewpoint temperature at specified pressure

In the U.S. 7 lb H2O/MMscf [110mg/m3] equivalent to a water dewpoint of 32°F at 1000 psia

In Europe and Canada 50-60 mg/m3 [3-4 lb/MMscf]

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Hydrocarbon dewpoint

Hydrocarbon dewpoint specifications vary with climate and buyer.

In north America and Europe values range from -10 to 0°C [15-32°F] at any pressure.

Other

Nitrogen is the most important inert

Its concentration is usually limited to less than 1-2%.

CO2

A maximum 2% is typical, but the CO2 specification is often negotiable.

Liquid produced by simple separation is a mixture whose composition is fixed by equilibrium. This is sometimes called a “slot” cut

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Raw Gas

Condensate Acid Gas

DesulfurizationTail Gas 

Treatment

Acid Gas Tail  Gas

and Water Removal

Acid Gas Removal

Dehydration

Mercury R l

Nitrogen R j i

NGL Recovery

Waste water

condensate

sulfur Offgas to Incinerator

RemovalRejectionNG Recovery

Fractionation Train

Gas Sales Nitrogen Rich Gas

Ethane, Propane, Butanes, Pentane+