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NATURAL GAS PROCESSING

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Absorption method or cryogenic expander process (both 90% of NGL production) ... Chill gas to -120oF; common - turbo-expander process ... – PowerPoint PPT presentation

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Title: NATURAL GAS PROCESSING


1
NATURAL GAS PROCESSING
2
PROPERTIES OF NATURAL GAS
  • Density
  • Test method ASTM D1070
  • Important for storage and transportation
  • Compressibility factor for natural gas (Figure
    7.1)
  • Gas Hydrates
  • Presence of free water, NG-water mixture solidify
    at high P and T above freezing point of H2O
  • Hydrates soln of gas in crystalline solids

3
PROPERTIES OF NATURAL GAS
  • Gas hydrates (continued)
  • Correlations predicting gas hydrate formation on
    T-P gradient (Figure 7.2)
  • Use antifreeze (methanol, ethylene glycol) for
    lowering freezing points thus prevent hydrates
    formation (Figure 7.3)
  • Quantity of methanol depends on total P of gas
    (Figure 7.4)
  • Quantity of methanol can be calculated using
    steps in Table 7.9

4
TEST
  • HC content
  • Indirectly by heating value (ASTM D1826) or SG
    (ASTM D1070 or D3588)
  • If high amt of CO2 or N2 present in NG, both
    method cannot give a reliable data
  • Water content
  • Expressed in lb H2O/MCF or Dew point T and P
    (ASTM D1142)
  • Two methods have definite relationship (Figure
    7.5)
  • Acid gas content
  • H2S foul odor, gt 600 ppm is fatal, corrosive to
    all metals 4 ppmv limit
  • During combustion sulfur dioxide is produced
    highly toxic, corrosive, poison catalyst

5
  • Acid gas content (continued)
  • CO2 has no heating value
  • Removal sometimes necessary to increase energy
    content (BTU/ft3) and prevent solidification in
    cryogenic plants
  • Both H2S and CO2 promote hydrate formation (more
    so on CO2)
  • Gas liquids
  • Define i) ethane ii) LPG iii) natural gasoline
  • Ethane max methane 1.5 v/v CO2 0.28 v/v
  • LPG compose of propane and/or butane (limits on
    high-MW HC content isopentane content)
  • Natural gasoline consists of pentane and
    higher-MW HC
  • Other properties
  • Heat of combustion, VP, corrosivity and gas
    residue

6
Natural Gas Plant General Layout
                                                  
                                                  
                                                  
  
7
1. Acid and Gas Removal
  • Sour gas
  • H2S gt 5.7 mg per m3 NG
  • Sweetening NG gas process to remove H2S
  • Specs 3 to 4 mole CO2 H2S 4 ppmv
  • Processes
  • Chemical absorption processes -
    expAlkanolamines, alkaline salt solutions
  • Physical adsorption processes Selexol, Rectisol
  • Pysical chemical process Sulfinol
  • Absorption, distillation, adsoprtion, membrane
    and chemical reaction

8
  • Amine reactions
  • Advantages reverse reaction and recover H2S
  • Girdle Process (Absorption)
  • 95 in US gas sweetening operations
  • Sour gas contacted with amine soln (MEA or DEA)
  • Amine soln is regenerated
  • Solid desiccant process (adsoption)
  • Use iron sponges to remove H2S and CO2
  • Sulfur
  • H2S process to reduce to elemental form S
  • Claus process thermal and catalytic reaction

9
Amine Process Unit
                                                  
                                                  
                                                  
  
10
Sweetening Process Schematic
                                                  
                                                  
                                                  
  
11
2. Oil and Condensate Removal
  • Wellhead separation
  • To process and transport associated NG
  • Separate using equipment at or near wellhead
  • Process and equipment used vary widely
  • Conventional separator
  • Closed tank gravity force to separate oil and
    NG
  • Low-Temperature separator (LTX)
  • Often used for high P gas with light oil or
    condensate
  • Pressure differential to cool wet NG and separate
  • Wet gas ? cooled slightly by heatX ? gas go thru
    high P liquid knockout ? low T (choke
    mechanism) ? gas expand when enters separator ?
    rapid expansion lowers T ? liquid removed ?
    warmed gas thru heatX

12
LTX Unit
                                                  
                                                  
                                                  
  
13
LTX Schematic
                                                  
                                                  
                                                  
  
14
3. Water Removal
  • Water in NG
  • Most free associated water removed by simple
    extraction method at or near wellhead
  • Water vapor in NG solution need more complex
    treatment
  • Process of dehydration of NG absorption or
    adsorption
  • Pipeline specs 7.0 lb H2O/MMSCF
  • Absorption Glycol Dehydration
  • Glycol solution (high affinity to water)
    diethylene glycol (DEG) or triethylene Glycol
    (TEG)
  • TEG/DEG contact wet gas stream (called contactor)
    ? absorb water ? glycol soln sink to bottom ?
    removed
  • Glycol recovery vaporize glycol using special
    boiler
  • New tech addition of flash tank separator
    condensers before boiler to condense methane (90
    99 recovery)

15
  • Solid-Desiccant Dehydration
  • Adsorption process consists of 2 or more
    adsoption tower filled with solid desiccant
  • At least 1 working, 1 regenerating
  • Desiccants activated alumina or granular silica
    gel
  • Wet NG ? pass thru towers from top to bottom ?
    H2O retains on particle surface ? dry NG exits ?
    saturated desiccant heated with heater to
    vaporize water
  • Best suite for large volumes gas under very high
    P

16
Glycol Dehydration Unit
                                                  
                                                  
                                                  
  
17
Glycol Dehydrator Schematic
                                                  
                                                  
                                                  
  
18
Solid Absorbance Dehydration Unit
                                                  
                                                  
                                                  
  
19
Solid Desiccant Dehydration Schematic
                                                  
                                                  
                                                  
  
20
4. Separation of Natural Gas Liquids
  • Natural Gas Liquids (NGL)
  • NG from well contain some NGL
  • Higher value if sold as separate products
  • Physical chemical process - Sulfinol
  • Technique
  • Absorption method or cryogenic expander process
    (both 90 of NGL production)
  • Two basic steps extract liquid from NG ?
    separate to base components
  • Absorption Method
  • NGL is absorb by absorbing oil (termed lean
    before absorption and rich oil after
    absorption)
  • Fed into oil stills ? heated (Tgtbp of NGL)
  • Recovery 75 butane, 85-90 pentane and heavier
    HC

21
  • Modified absorption using refrigerated oil to
    improve efficiency
  • Propane recovery gt90 40 ethane, others 100
  • Cryogenic Expansion Process
  • Ethane often more difficult to extract from NG
    stream
  • Chill gas to -120oF common - turbo-expander
    process
  • External refrigerant cool NG stream ? rapidly
    expand chilled gas (expansion turbine) ? T drop ?
    condense ethane and other HC maintain methane in
    gaseous form
  • Recovery 90 to 95 ethane
  • Energy released from expansion turbine later use
    in extracting ethane process

22
Absorption Tower
                                                  
                                                  
                                                  
  
23
Absorption Process at Ambient T
24
Cryogenic Expander Process
25
5. Natural Gas Fractionation
  • Process Units
  • Deethanizer separates ethane from NGL stream
  • Depropanizer separates propane
  • Debutanizer boils off butanes leaving pentanes
    and heavier HC in NGL stream
  • Butane splitter (Desiobutanizer) separates iso
    and n butanes
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