2.7.1 and various types were classified as dry,

2.

7.1 NATURAL GASNaturalgas is a colourless, shapeless, and odourless in its pure form. ‘Compared toother fossil fuels, natural gas is cleaner burning and emits lower levels ofpotentially harmful by products into the air. Natural gas is a combustiblemixture of hydrocarbon gases’ (Natgas, 2013a).

Natural Gas is use inresidential, commercial and Industrial.   2.7.2 CLASSIFICATION OF NATURAL GASSaeidand William (2012), natural gas can be classified according to:a)      Proportion of Hydrocarbons: Heavierhydrocarbons than methane and various types were classified as dry, wet andcondensate. Dry gas is purely methane while if other hydrocarbons are present,it is termed as wet. Condensate gas is a gas that has liquid hydrocarbon andformed a liquid phase during production in the reservoir (Saeid and William,2012).

b)      Liquid Contents: Thisclassification classified the gas as either Lean or Rich gas. These termsreferred to the amount of recoverable liquids. It is applied to ethane andother heavier components regarded as valuable components (Saeid and William,2012).c)      Sulfur Contents: Thisclassification classified natural gas as either sweet or sour. A sweet gas is agas that has small or traces amount of H2S content while, sour gasis a gas that has large quantity of H2S (Saeid and William, 2012).2.8 NATURAL GAS PROCESSINGSaeidand William (2012), natural gas processing involves the separation of naturalgas associated hydrocarbons, Acid gasses, water and conditioning of theseparated liquids produced from Gas well.

Natural gas processing philosophy dependson the purity of the gas needed and the usage or project consideration. Gasprocessing extent defends on the processing objectives, type or source of thegas, location and size of the gas field (Saeid and William, 2012).2.8.

1 WATER REMOVALThewater in the natural gas is removed by dehydration which involves twoprocesses, Absorption or Adsorption. Absorption process is a process where adehydrating agent is used to remove the water from the gas stream, whileAdsorption is a process where a condensed water vapour is collected on thesurface by using solid- desiccant (Natgas, 2013b).Glycol Dehydration: Thisis an absorption process in which a liquid desiccant Glycol (dehydrator) is useto absorb the water vapour from the gas stream. This agent (Glycol) has achemical affinity of water. Diethylene Glycol (DEG) or Triethylene Glycol (TEG)solutions are usually used to remove the water from the gas stream when incontact with the water. The absorbed water is then vaporized and the glycolsolution recovered and reuse. The boiling point of glycol is 400OFwhile water is 212OF (Natgas, 2013b).Solid-Desiccant: Thisis an adsorption process of removing water from the natural gas stream.

Itinvolves using two adsorption towers filled with a Solid desiccant. Thedesiccant used is either Activated Alumina or Granular Silica Gel. The wetnatural gas is passes through the top of the towers and made contact with thedesiccant material which was arranged as a desiccant bed.

The water is adsorbedand the dry gas stream leaves the tower from the bottom (Natgas, 2013b).Toregenerate the water saturated desiccant, a gas at high temperature is passedthrough the desiccant bed and this vaporises the water from the desiccant andthe desiccant is ready for reuse (Natgas, 2013b).2.8.

2 SEPARATION OF NATURAL GASLIQUIDSNatgas(2013b), natural gas as produced contains some natural-gas-liquids which havesome economic value. To separate these liquids, two steps are followed:·        Extraction of these liquids and·         Separation of the variousnatural-gas-liquids  into components Natural-Gas-LiquidsExtraction: Two techniques are used for theextraction of natural-gas-liquids, Absorption method and cryogenic expanderprocess. These are the major techniques use in natural-gas-liquid extraction.Ø  Absorption Method: AbsorbingOil which has affinity of natural-gas-liquids is to absorb natural-gas-liquids.This process is similar to the absorption process for dehydration. Theabsorbing oil enters into an absorption tower and it is termed as leanabsorption oil.

After passing through the absorption tower, the oil will absorbthe natural-gas-liquids. The mixture is termed as rich absorption oil. The richabsorbing oil contains natural-gas-liquids such as propane, butanes, pentanesand other heavier hydrocarbons (Natgas, 2013b).Therich absorbing oil is heated to a temperature above the boiling points of thenatural-gas-liquids and below the boiling point of the absorbing oil. Highrecovery of butanes, pentanes and other heavier molecules are achieved.

Theprocess can be improved and aimed at a particular in natural-gas-liquid (Natgas,2013b).Ø  The Cryogenic Expansion Process:This process is used to extract lighter hydrocarbons such as ethane. Cryogenicprocesses are use which requires dropping the gas temperature to around -120OF.Turbo expander process is the most effective process, in this process, externalrefrigerants are used to cool the gas and expansion turbine is used to expandthe cryogenic gas. This sudden temperature drop condensed the lighterhydrocarbons in the gas stream and methane remained in a gaseous form. Higherpercentage of ethane recovery is achieved in this process (Natgas, 2013b).

2.8.3 NATURAL GASLIQUID FRACTIONATIONNatgas(2013b), after extraction of the natural-gas-liquids, the mixed stream willundergo a fractionation process.

The fractionation of the differentnatural-gas-liquid hydrocarbons is achieved based on the boiling pointdifference of each natural-gas-liquid in the stream. The following processesand steps are the fractionation steps for the extraction.Ø  DeethanizerØ  DepropanizerØ  DebutanizerØ  ButaneSplitter or DeisobutanizerThesesteps separate Ethane, Propane, Butane and iso, Pentanes and heavierhydrocarbons are left in the natural-gas-liquid stream (Natgas, 2013b).2.8.4 SULFUR ANDCARBONDIOXIDE REMOVALEIA(2006), sulphur removal is achieved by passing the gas stream through an Aminesolution inside a tower.

The solution absorbs sulphur compound and the gasstream is passes through the next section after desulphurization in a series offilter tubes. Separation of other components is achieved by gravity andcentrifugal force as the velocity of the gas stream reduces in each unit.Theremoval of sulphur from natural gas is called sweetening. Natural gas that hassulphur is referred to as sour gas, while non-sulphur natural gas is referredto as sweet gas.

FIGURE2.5: GENERALISE NATURAL GAS PROCESSING FLOW DIAGRAM(ChemEngineering,2016)2.9 NATURAL GAS COMPRESSIONNaturalgas compression is use for gas lift, gas gathering, gas processing, gastransmission, gas distribution and gas transportation to a long distance forthe purpose of maintaining a high gas pressure. Basically, centrifugalcompressor and reciprocating compressor are used in gas compression.Centrifugal compressor uses gas turbines or electric motor to drive thecompressor, while reciprocating compressor uses electric motor or gas enginesto drive the compressor. In selecting a compressor, factors such as compressorcost, capital cost, maintenance cost, life-cycle, energy cost and spare partsare normally considered (Saeid and William, 2012).

 2.7.1 NATURAL GASNaturalgas is a colourless, shapeless, and odourless in its pure form. ‘Compared toother fossil fuels, natural gas is cleaner burning and emits lower levels ofpotentially harmful by products into the air.

Natural gas is a combustiblemixture of hydrocarbon gases’ (Natgas, 2013a). Natural Gas is use inresidential, commercial and Industrial.   2.7.2 CLASSIFICATION OF NATURAL GASSaeidand William (2012), natural gas can be classified according to:a)      Proportion of Hydrocarbons: Heavierhydrocarbons than methane and various types were classified as dry, wet andcondensate. Dry gas is purely methane while if other hydrocarbons are present,it is termed as wet.

Condensate gas is a gas that has liquid hydrocarbon andformed a liquid phase during production in the reservoir (Saeid and William,2012).b)      Liquid Contents: Thisclassification classified the gas as either Lean or Rich gas. These termsreferred to the amount of recoverable liquids. It is applied to ethane andother heavier components regarded as valuable components (Saeid and William,2012).c)      Sulfur Contents: Thisclassification classified natural gas as either sweet or sour. A sweet gas is agas that has small or traces amount of H2S content while, sour gasis a gas that has large quantity of H2S (Saeid and William, 2012).2.8 NATURAL GAS PROCESSINGSaeidand William (2012), natural gas processing involves the separation of naturalgas associated hydrocarbons, Acid gasses, water and conditioning of theseparated liquids produced from Gas well.

Natural gas processing philosophy dependson the purity of the gas needed and the usage or project consideration. Gasprocessing extent defends on the processing objectives, type or source of thegas, location and size of the gas field (Saeid and William, 2012).2.8.1 WATER REMOVALThewater in the natural gas is removed by dehydration which involves twoprocesses, Absorption or Adsorption. Absorption process is a process where adehydrating agent is used to remove the water from the gas stream, whileAdsorption is a process where a condensed water vapour is collected on thesurface by using solid- desiccant (Natgas, 2013b).Glycol Dehydration: Thisis an absorption process in which a liquid desiccant Glycol (dehydrator) is useto absorb the water vapour from the gas stream.

This agent (Glycol) has achemical affinity of water. Diethylene Glycol (DEG) or Triethylene Glycol (TEG)solutions are usually used to remove the water from the gas stream when incontact with the water. The absorbed water is then vaporized and the glycolsolution recovered and reuse. The boiling point of glycol is 400OFwhile water is 212OF (Natgas, 2013b).Solid-Desiccant: Thisis an adsorption process of removing water from the natural gas stream.

Itinvolves using two adsorption towers filled with a Solid desiccant. Thedesiccant used is either Activated Alumina or Granular Silica Gel. The wetnatural gas is passes through the top of the towers and made contact with thedesiccant material which was arranged as a desiccant bed. The water is adsorbedand the dry gas stream leaves the tower from the bottom (Natgas, 2013b).

Toregenerate the water saturated desiccant, a gas at high temperature is passedthrough the desiccant bed and this vaporises the water from the desiccant andthe desiccant is ready for reuse (Natgas, 2013b).2.8.2 SEPARATION OF NATURAL GASLIQUIDSNatgas(2013b), natural gas as produced contains some natural-gas-liquids which havesome economic value.

To separate these liquids, two steps are followed:·        Extraction of these liquids and·         Separation of the variousnatural-gas-liquids  into components Natural-Gas-LiquidsExtraction: Two techniques are used for theextraction of natural-gas-liquids, Absorption method and cryogenic expanderprocess. These are the major techniques use in natural-gas-liquid extraction.Ø  Absorption Method: AbsorbingOil which has affinity of natural-gas-liquids is to absorb natural-gas-liquids.This process is similar to the absorption process for dehydration. Theabsorbing oil enters into an absorption tower and it is termed as leanabsorption oil. After passing through the absorption tower, the oil will absorbthe natural-gas-liquids.

The mixture is termed as rich absorption oil. The richabsorbing oil contains natural-gas-liquids such as propane, butanes, pentanesand other heavier hydrocarbons (Natgas, 2013b).Therich absorbing oil is heated to a temperature above the boiling points of thenatural-gas-liquids and below the boiling point of the absorbing oil. Highrecovery of butanes, pentanes and other heavier molecules are achieved. Theprocess can be improved and aimed at a particular in natural-gas-liquid (Natgas,2013b).Ø  The Cryogenic Expansion Process:This process is used to extract lighter hydrocarbons such as ethane. Cryogenicprocesses are use which requires dropping the gas temperature to around -120OF.

Turbo expander process is the most effective process, in this process, externalrefrigerants are used to cool the gas and expansion turbine is used to expandthe cryogenic gas. This sudden temperature drop condensed the lighterhydrocarbons in the gas stream and methane remained in a gaseous form. Higherpercentage of ethane recovery is achieved in this process (Natgas, 2013b).2.

8.3 NATURAL GASLIQUID FRACTIONATIONNatgas(2013b), after extraction of the natural-gas-liquids, the mixed stream willundergo a fractionation process. The fractionation of the differentnatural-gas-liquid hydrocarbons is achieved based on the boiling pointdifference of each natural-gas-liquid in the stream. The following processesand steps are the fractionation steps for the extraction.Ø  DeethanizerØ  DepropanizerØ  DebutanizerØ  ButaneSplitter or DeisobutanizerThesesteps separate Ethane, Propane, Butane and iso, Pentanes and heavierhydrocarbons are left in the natural-gas-liquid stream (Natgas, 2013b).2.8.

4 SULFUR ANDCARBONDIOXIDE REMOVALEIA(2006), sulphur removal is achieved by passing the gas stream through an Aminesolution inside a tower. The solution absorbs sulphur compound and the gasstream is passes through the next section after desulphurization in a series offilter tubes. Separation of other components is achieved by gravity andcentrifugal force as the velocity of the gas stream reduces in each unit.

Theremoval of sulphur from natural gas is called sweetening. Natural gas that hassulphur is referred to as sour gas, while non-sulphur natural gas is referredto as sweet gas.FIGURE2.5: GENERALISE NATURAL GAS PROCESSING FLOW DIAGRAM(ChemEngineering,2016)2.9 NATURAL GAS COMPRESSIONNaturalgas compression is use for gas lift, gas gathering, gas processing, gastransmission, gas distribution and gas transportation to a long distance forthe purpose of maintaining a high gas pressure. Basically, centrifugalcompressor and reciprocating compressor are used in gas compression.

Centrifugal compressor uses gas turbines or electric motor to drive thecompressor, while reciprocating compressor uses electric motor or gas enginesto drive the compressor. In selecting a compressor, factors such as compressorcost, capital cost, maintenance cost, life-cycle, energy cost and spare partsare normally considered (Saeid and William, 2012). 

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