29C - Moisture Removal

8/7/2019 29C - Moisture Removal

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The World of Energy

29.3. Moisture Removal

Chapter 29 LNG Technology Gas Treating


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Dehydration Unit

Dehydration is the removal of waterfrom the produced natural gas and is

accomplished by various methods:

Ethylene glycol

Triethylene Glycol dehydration

(TEG) and diethylene glycol (DEG) Dry-bed dehydrators using solid

desiccants


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Molecular Sieve

Molecular sieves areadsorbents composed ofaluminosilicate crystallinepolymers (zeolites).

They efficiently removelow concentrations ofpolar or polarizablecontaminants such asH2O, methanol, H2S,CO2, COS, mercaptans,sulfides, ammonia,aromatics and mercury

down to traceconcentrations.

Molecular sieve productsmanufactured in variousforms: beads, granulesand extrudates, includingstandard pellets and

TRISIVTM pellets. The molecular sieve type,

size and particle shapeselected for a particularcustomer are determinedby the application.


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Molecular Sieve vs Activated Alumina


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An Open Cycle Molecular Sieve Dehydration


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Molecular Sieve Applications

Natural gas dehydration Normal parameters: water saturated, 30-200F, 100-1500

psig. Regeneration is via dry or wet gas, yielding LNG orpipeline specifications, respectively.

Natural gas mercury removal (Combines with dehydration, one system, no additional

out.

Natural gas/LPG desulfurization H2S, mercaptans, COS and sulfides can be removed. Effluent

with H2O


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SE

RVICE

REGE

NERATION

GUARD

HOT

DRY GASDRY

SWEET GAS

WET GASWET

SWEET GAS H2O freezes and

blocked equipments

H20 is adsorbed by

Molecular Sieve (SilicaAlumina) catalyst

Regenerated by hotgas heating

H2O inlet : saturated

H2O oulet : 1 ppm Catalyst life : 4 ~ 6

years

Typical H2O (Moisture) Removal Unit


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Adsorption: Equilibrium & MTZ


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Feed Vapor Dryer & Molecular Sieves


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All LNG Plants use Molecular Sieve adsorption technology.The only technology can meet outlet moisture content below1 ppm (minus 110 Deg. C dew point)

Other dehydration technology such as Glycol Unit, Silica Gelor Activated Alumina cannot meet low moisture outlet

Adsorption/Desorption Scheme:

Pressure Swing Desorption

Heated Desorption (heat regenerated). Suitable for Gas Turbine

Plant. Various type of Molecular Sieve

Beads (Spherical). Sturdy, resistant to crushing and friction.High pressure drop, less surface area.

Pellet (Cylindrical). More surface area, higher efficiency. More

flow void space, less pressure drop. Clover Leaf (Triple Cylindrical). Highest surface area, high

efficiency. Prone to crushing and friction.

Dehydration Technology for LNG


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Causes of Molecular Sieve Aging

Causes of Aging Insoluble water

Liquid entrainment (hydrocarbon, water) High water partial pressure at high

temperature (above 150C)

Amine carryover

CO2 , Carbonic acid formation

H2CO3 + (Na+, Ca++, K+) Na2CO3 -NaHCO3 - CaCO3 - K2CO3

Decomposition of mercaptans during

regeneration Presence of chlorides

Mercury


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Multibed System: Dehydration


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Adsorption Mechanisms and Adsorbents

Adsorption is one of the many separationtechnologies used in treating natural gas. Adsorptioncan be broken down into several different

categories: Chemisorption (usually irreversible; no

regeneration) Mercury removal with promoted activated carbon, H2S

polishing with ZnO.

Physisorption (reversible adsorption on the surfaceof a material) Water or hydrocarbon removal with silica gel

regeneration by heat.

Size exclusion (the molecules of the contaminant fitinto the pores of the adsorbent, the rest passes by.) Removal of N2 or CO2 with Molecular Gate

regeneration with heat or pressure swing.


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Schematic of Adsorption Cycle

29C - Moisture Removal

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