25355114 Natural Gas Processing

8/8/2019 25355114 Natural Gas Processing

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NATURAL GASNATURAL GASPROCESSINGPROCESSING

&&LNGLNG

By

Muhammad Sharique Khan

26, September

2009


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IntroductionIntroduction

Objectives of Natural Gas Processing &LNG :

Purification of CO2, H2S, H2O, etc

Hydrocarbon recovery like liquefiedpetroleum gas (LPG), ethane.

Upgrading N2 rejection and Heliumrecovery.

Huge reduction in Volume, a factor of 650and decreasing the size and cost ofstorage and transportation.


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Composition of some NaturalComposition of some Natural

GasesGases


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Maximum Allowable ImpuritiesMaximum Allowable Impurities

Levels in Natural gas.Levels in Natural gas.


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PurificationPurification A Purification is the process where the material

removed from the gas stream is discardede.g. water.

Recovery process is one where the material

removed has economical value e.g. helium or

ethane.

Impurities are removed to: Obtain a desired product purity. Prevent the deposition of solids.

Remove an explosion hazard.

Prevent the blanketing of heat transfer surfaces bynon condensable gases.

Prevent Corrosion.


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Classification of Impurity RemovalClassification of Impurity Removal

ProcessProcess

Condensation to a liquid by increase of pressure at constanttemperature

Condensation to liquid or solid by cooling at constant pressure

Absorption by a liquid

Purely physical With chemical reaction

Adsorption by a solid Physical

With chemical reaction

Chemical reaction.

Permeation through a membrane.

Mechanical separation.


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Water Removal- DehydrationWater Removal- Dehydration

Natural feed to baseload plants can contain as much as1600 kg H2O/10

6m3.

The removal of water vapor is accomplished either byabsorption in a liquid or by adsorption on a solid.

The absorption processes generally use diethylene glycolor triethylene glycol as the drying liquid in acountercurrent absorber.


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Glycol Water Absorption ProcessGlycol Water Absorption Process


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Molecular Sieve DehydrationMolecular Sieve Dehydration

ProcessProcess

Sieves Adsorption is used to produce very low water contentin the natural gas due to the greatest affinity for waterdue to their natural structure.

Sieves can effectively remove acid gases like H2S and CO2with water.

Over 75% of all Cryogenic hydrocarbon recovery plants relyon molecular sieve for dehydration.

Dehydration to dew points of 200K(-100-105 F) isaccomplished through the use of molecular sieve.

Disadvantages of Sieves dehydration: High capital costs, limitation of maximum economical throughput Danger of adsorbent contamination and undesirable co adsorption Need for source of clean, dry reactivation gas Possibility of adsorbent dusting


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Molecular Sieve DehydrationMolecular Sieve Dehydration

ProcessProcess


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Two-Stage AdsorptionTwo-Stage Adsorption

&&

PurificationPurification

Four-tower molecular sieve system is used

Two towers (dryers) for removing water from the total inlet naturalgas stream.

Two towers (purifiers) for removing carbon dioxide from theportion of gas being liquefied

Advantages of four-tower, two-stage system

The first-stage absorbers can be efficiently designed for water andH2S removal, taking advantage of the high adsorption capacityof the adsorbent for these impurities.

Second-stage absorbers can be designed specifically for the higherconcentrations and lower adsorptive capacity of CO 2. The adsorbed thushandles a smaller quantity of feed gas, thereby reducing overall system

requirements


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Four-tower Molecular Sieve system forFour-tower Molecular Sieve system forNatural Gas Dehydration & CONatural Gas Dehydration & CO22 removal.removal.


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Removal ofRemoval ofCOCO22 && HH22SS Inlet natural gas typically contains 0.52.0 vol % CO2.

Local freeze out of CO2 occur at low temp.

CO2 content of the feed gas usually kept below 50 ppm.

Hydrogen sulfide is removed for two reasons To eliminate an undesirable component in the gas stream.

To recover the H2S as elemental sulfur.

The H2S and CO2 removal operations may be roughlygrouped into three categories: Processes using a reversible chemical reaction.

Processes using physical absorption.

Processes employing fixed beds of solids.


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Amine system for natural gasAmine system for natural gas

COCO22& H& H22S removalS removal


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Mercury RemovalMercury Removal Mercury concentrations ranging from 0.001 to 180 mg/m3

exist in all natural gas streams.

Mercury is responsible for aluminum corrosion so it shouldbe removed from the feed gas.

Mercury is first converted to organic and inorganic formbecause their presence has no problem in the plantsand removed easily.

Mercury removal in this process is based on the highreactivity of mercury with sulfur and its compounds;less than 10 ng/m3 is obtained with a high spacevelocity.


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Cascade Process FlowsheetCascade Process Flowsheet


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The Classical Cascade CycleThe Classical Cascade Cycle

Typically, a propaneethylenemethane cascadecombination is used with the natural gas being cooledthrough the vaporization of each component in turn.

By vaporizing each refrigerant at two or three pressurelevels, the efficiency of the natural gas cooling processis increased, but so is the complexity of the entireliquefaction cycle.

The rapid increase in power requirement with decreasingtemperature is a key consideration in the design ofnatural gas liquefaction units.


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Expander CycleExpander Cycle


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The Expander CycleThe Expander Cycle

Expander cycle uses the cooling effect obtained byexpansion of the natural gas to liquefy a portion of thenatural gas.

Expansion turbines are used in this process, withliquefaction occurring upon expansion through a JouleThomson valve.

l d l i d f i


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Closed-cycle mixed-refrigerantClosed-cycle mixed-refrigerant

cascadecascade


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Mixed-Refrigerant Cascade CycleMixed-Refrigerant Cascade Cycle

q MCR Cascade cycle based on the same principle as theclassical cascade cycle.

q

q A single refrigeration loop complete with their owncompressor is used .

q

q If composition of feed changes the composition of themixed refrigerant must also be changed to reducethermodynamic irreversibility.

q

q The mixed refrigerant is repeatedly condensed, vaporized,

separated and expanded.q

q Major problem with MCR cycle is that problem occur inhandling two-phase multicomponent mixtures in theheat exchangers.


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Thank youThank you

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25355114 Natural Gas Processing